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The age and character of alteration and mineralization at the Buckhorn Gold Skarn, Okanogan County, Washington,.. Scorrar, Brendan Alfred 2012

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The Age and Character of Alteration and Mineralization at the Buckhorn Gold Skarn, Okanogan County, Washington, USA by  Brendan Alfred Scorrar  B.Sc., The University of Alberta, 2008  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in The Faculty of Graduate Studies (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  August 2012  © Brendan Alfred Scorrar 2012  Abstract Located in Okanogan County, Washington, USA the Buckhorn mine is one of the largest gold skarns in North America (2.15 Mt at 14 ppm Au). Buckhorn is hosted in Permian Anarchist Group metasedimentary rocks and Jurassic Elise Formation metavolcanic rocks. Monzodiorite comagmatic with the metavolcanic rocks is Jurassic in age (192.4 Ma) confirming the correlation. Two suites of granitoids intrude the local stratigraphy. The Middle Jurassic (170.4 Ma) post-accretionary Buckhorn Intrusive Suite is genetically related to skarn alteration and gold mineralization and comprised of a granodiorite stock, marginal diorite, and several generations of dikes. The Eocene Roosevelt Intrusive Suite (50.5 Ma) is comprised of a small (~0.03 km2) granodiorite stock that post-dates skarn alteration and gold mineralization. Skarn alteration is zoned from dominantly magnetite-garnet in the proximal Magnetic Mine, to equal portions of magnetite-garnet-pyroxene in the Gold Bowl, and pyroxene dominated in the distal Southwest Ore-Zone. The Southwest Ore-Zone contains the majority of the gold mineralization and consists of massive calcic, Fe-rich, reduced skarn alteration along a low-angle shear zone at the contact between the carbonate metasedimentary rocks and the overlying metavolcanic rocks. Skarn alteration is divided into three categories based on the prograde mineralogy: pyroxene, garnet and magnetite skarn. Pyroxene skarn is further subdivided based on the retrograde mineralogy: amphibole-pyroxene, magnetite-pyroxene, and epidote-pyroxene skarn. Gold mineralization occurs in fractures in and intercrystalline space between skarn minerals and is intimately associated with bismuth. Re-Os geochronology of molybdenite bearing skarn confirms the Middle Jurassic age of skarn alteration and gold mineralization (162.8165.5 Ma). Based on the mineralogy, the hydrothermal fluids that caused prograde alteration were between 430-500° C, fO2=-25 to -20, fS2=-8 to -4.5, and near neutral pH. Fluids responsible for retrograde alteration were cooler (300-430° C), more reduced (fO2<-26), and had lower sulfur fugacity (fS2=-15 to -6). Gold mineralization occurred at the end of retrograde alteration. Gold was transported as bisulfide complexes in even cooler (241-300°C) and more reduced (fO2=-42 to -36) fluids than earlier retrograde alteration. Gold was scavenged by a bismuth melt and precipitated due to cooling and local retrograde oxidation reactions.  ii  Table of Contents  Table of Contents  Abstract. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . ii Table of Contents. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .iii List of Tables .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . vi List of Figures. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . viii List of Abbreviations . .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . xii Acknowledgements. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . xiv Chapter 1.	 Introduction . .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 1 1.1	 Rationale for Study.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 1.2	Background.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 1.3	 Thesis Objectives.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 1.4	 Thesis Organization .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  1 2 4 4  Chapter 2.	 Regional and Mine Scale Geology of the Buckhorn Mountain Gold Skarn. .  .  .  .  .  . 6 2.1	Introduction .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  6 2.2	 Regional Geology.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  6 2.2.1	 Quesnel Terrane.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 7 2.2.1.1 Late Paleozoic Assemblages.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  7 2.2.1.2 Early Mesozoic Volcanogenic Assemblages.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  8 2.2.1.3 Jurassic Sedimentary Assemblages.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 8 2.2.1.4 Post-Accretionary Cretaceous Assemblages.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  9 2.2.1.5 Post-Accretionary Paleogene Assemblages.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 9 2.2.1.6 Late Triassic to Early Jurassic Plutonism .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 10 2.2.1.7 Middle to Late Jurassic Plutonism .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 10 2.2.1.8 Cretaceous Plutonism.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 11 2.2.1.9 Paleogene Plutonism.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 11 2.3	 Buckhorn Geology .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 12 2.3.1	 Metasedimentary Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 16 2.3.2	 Metavolcanic Rocks .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 19 2.3.3	 Metamorphism and Skarn Alteration in the BMS and BMV.  .  .  .  .  .  .  .  .  .  .  .  .  . 24 2.3.4	 Intrusive Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 26 2.3.4.1 Buckhorn Intrusive Suite (BIS) .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 26 2.3.4.1.1	 Buckhorn Granodiorite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 26 2.3.4.1.2	 Mafic Diorite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 29 2.3.4.1.3	 Buckhorn Diorite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 31 2.3.4.1.4	 Early Diorite Dikes.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 33 2.3.4.1.5	 Granodiorite dikes.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 36 2.3.4.1.6	 Quartz Porphyry Dikes.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 38 2.3.4.2 The Roosevelt Intrusive Suite (RIS) .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 40 2.3.4.3 Alteration in the Intrusive Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 42 2.3.4.3.1	Deformation .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 42 2.4	Conclusions .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 45 iii  Chapter 3.	 U-Pb Geochronology . .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 47 3.1	Introduction .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 47 3.1.1	Background .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 47 3.1.2	 Sample Preparation.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 48 3.1.3	 Analytical Method and Data Reduction.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 48 3.2	 Samples and Results .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 51 3.2.1	 Buckhorn Mountain Volcanic Sequence (BMV) .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 51 3.2.2	 Buckhorn Granodiorite .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 57 3.2.3	 Early Diorite Dikes.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 67 3.2.4	 Buckhorn Diorite .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 73 3.2.5	 Granodiorite dikes .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 77 3.2.6	 Roosevelt Granodiorite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 81 3.2.7	 Samples with Inconclusive Results .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 85 3.3	 Re-Os Geochronology of Molybdenite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 88 3.3.1	 Introduction and Methods.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 88 3.3.2	 Molybdenite in Skarn Altered Diorite.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 89 3.3.3	 Molybdenite in Skarn Altered Metasedimentary Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 89 3.3.4	 Results of Re-Os Geochronology.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 90 3.4	 Summary and Discussion.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 91 Chapter 4.	 Skarn Alteration and Gold Mineralization . .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 94 4.1	 Categories of Gold Skarns .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 94 4.2	Introduction .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 95 4.3	 Pyroxene Skarn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  101 4.3.1	 Amphibole-Pyroxene Skarn .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 101 4.3.2	 Magnetite-Pyroxene Skarn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  106 4.3.3	 Epidote-Pyroxene Skarn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 109 4.3.4	 Microprobe Analysis of Pyroxene and Amphibole from Pyroxene Skarn .  .  .  .  .  .  .  112 4.3.4.1 Introduction and Methods.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  112 4.3.4.2 Mineral Chemistry .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 113 4.3.5	 Discussion and Summary of Microprobe Results.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  116 4.4	 Garnet Skarn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  116 4.5	 Magnetite Skarn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 122 4.6	 Gold Mineralization in Non Skarn-Altered Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 125 4.7	 Discussion and Conclusions.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  127 4.7.1	 Skarn Alteration .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  127 4.7.1.1 Zonation and Paragenesis of Skarn Alteration .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  129 4.7.1.2 Physicochemical Conditions of Skarn Alteration.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  131 4.7.2	 Gold Mineralization .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  134 4.7.2.1 Physicochemical Conditions of Gold Mineralization.  .  .  .  .  .  .  .  .  .  .  .  .  .  135 4.7.3	 Comparison to Nickel Plate.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 139 Chapter 5.	 Geochemistry of Skarn Alteration at Buckhorn .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 142 5.1	Introduction .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  142 5.2	 Relationship between Major and Trace Elements, Skarn Alteration and Gold-Bismuth Mineriv  alization	.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  142 5.3	 Relationship between Gold Mineralization and Sulfide Forming Elements.  .  .  .  .  .  .  .  .  146 5.4	 High and Low Base Metal Gold Mineralized Populations. . . . . . . . . . . . . . . . . . 148 5.5	 High Field-Strength Elements as a Proxy for Protolith.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 151 5.6	 Gold/Copper and Gold/Silver Ratios for Gold Bearing Skarns.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  154 5.7	 Discussion and Conclusions.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 157 Chapter 6.	 Summary. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 160 6.1	 Exploration Implications.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  161 6.2	 Future Research.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  163 Chapter 7.	 Conclusions .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 165 7.1	 Host Rocks.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  165 7.2	 Skarn Alteration .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 166 7.3	 Gold Mineralization.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 167 7.4	 Comparison to Other Gold Skarns.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  168 References .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 170 Appendix A. Microprobe Analysis. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 180 Appendix B. Four Acid Digestion ICP-MS Geochemical Analysis. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 188  v  List of Tables  List of Tables  Table 3.1: Summary of analysis quality.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 50 Table 3.2: Isotope ratios and age estimates for sample BS064, porphyritic intrusion comagmatic with the BMV.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 54 Table 3.3: Isotope ratios and age estimates for BS048.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 62 Table 3.4: Isotope ratios and age estimates for BS074.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 63 Table 3.5: Summary of isotope ratios and age estimates for BS057.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 66 Table 3.6: Isotope ratios and age estimates for sample BS046 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 71 Table 3.7: Isotope ratios and age estimates for sample BS075 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 72 Table 3.8: Isotope ratios and age estimates for sample BS060 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 76 Table 3.9: Isotope ratios and age estimates for sample BS059 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 80 Table 3.10: Isotope ratios and age estimates for sample BS076.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 84 Table 3.11: Isotope ratios and age estimates for sample BS062.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 87 Table 3.12: Re-Os isotope data for molybdenite samples from Buckhorn .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 91 Table 4.1: Table of diamond drill holes and skarn samples selected for petrographic analysis.  .  .  .  .  . 97 Table 4.2: Mineral compositions at Buckhorn as determined by microprobe analysis, performed by Gaspar (2005). In this thesis, minerals will be referred to by their category, except where the distinction can be made based on optical properties..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 99 Table 4.3: Correlation matrix of major elements in amphibole and gold. Major elements determined by EMP, gold by ICP-MS. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 115 Table 4.4: Characteristics of the skarn classifications.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  128 Table 4.5: Summary of physicochemical conditions of skarn alteration and gold mineralization.  .  .  138 Table 5.1: Summary of diamond drill holes and skarn samples selected for geochemical analysis.  .  .  142 Table 5.2: Base 10 correlation matrix of select elements from geochemical analysis of skarn alteration at Buckhorn .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 144 Table 5.3: Summary of gold skarns compared based on the Au/Ag and Au/Cu ratios .  .  .  .  .  .  .  . 155 Table A.1: Microprobe analysis of pyroxene.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 180 Table A.2: Microprobe analysis of amphibole .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 186 Table B.1: Major element and mineralizing element geochemical analysis of Buckhorn skarn alteration and host rocks .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  188 Table B.2: Trace element geochemical analysis of Buckhorn skarn alteration and host rocks .  .  .  .  .  193 Table B.3: Trace element geochemical analysis of Buckhorn skarn alteration and host rocks (Part 2).  197 vi  Table B.4: Rare earth element geochemical analysis of Buckhorn skarn alteration and host rocks .  . 201  vii  List of Figures  List of Figures  Figure 1.1: (A) Simplified terrane map of the Canadian cordillera modified from Cauldron and Nelson (2011). (B) Simplified geologic map of the southern Canadian cordillera modified from Gaspar (2005) after Tripper et al. (1981), Wheeler and McFeely, (1991), Nelson and Colpron (2007), and Colpron and Nelson (2011)..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  3 Figure 2.1: Geologic map of Buckhorn Mountain with sample locations highlighted. Based on data from this study (Areas 1 & 2), and modified from Kinross Gold Corporation maps. Coordinate system: UTM NAD27, Zone 11U. Map legend on following page..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 13 Figure 2.1 Continued: Legend for the Geologic map of Buckhorn.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 14 Figure 2.2: Geologic cross-section of the South-West Ore Zone and Roosevelt Mine area looking NNE. Based on data from this study and Kinross Gold Corporation drilling.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 15 Figure 2.3: Examples of BMS metasedimentary rocks that have been variably hornfels altered that demonstrate its texture, mineralogy, and character and variation in alteration and deformation (A, B, and C hand samples; A1 and B1 PPL photomicrograph; C1 XPL photomicrograph). See text for further discussion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 2.10: Examples of the Buckhorn Granodiorite that demonstrate its texture, mineralogy, and character and variation in alteration and deformation (A and C outcrop; A1 hand sample; A2 and C1 PPL photomicrograph; B XPL photomicrograph). See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  . 28 Figure 2.15: Examples of the Granodiorite dikes that demonstrate the intrusive style, texture, mineralogy, character and variation in alteration and deformation. (A and B1 hand sample; B outcrop; B2 PPL photomicrograph). See text for further discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 37 Figure 3.1: Linear cumulative probability plot of sample BS057. The analyses affected by subtle lead loss and inheritance are highlighted. The colour scheme will be used in subsequent figures..  .  .  .  .  .  .  . 51 Figure 3.2: BSE image of a representative zircon and linear cumulative probability plot for sample BS064 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 53 Figure 3.3: Concordia and weighted average diagrams for sample BS064 .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 57 Figure 3.4: BSE image of representative zircons from samples BS048 (A) and BS074 (B).  .  .  .  .  .  . 59 Figure 3.5: Concordia and weighted average diagrams for BS048 and BS074. Rejected analyses coloured according to the legend in Figure 3.1. Only concordant data was used for the age calculations and plotted on the Concordia diagrams. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 60 Figure 3.6: Linear cumulative probability plots for samples BS048 and BS074.  .  .  .  .  .  .  .  .  .  .  .  . 61 Figure 3.7: BSE image of a representative zircon and linear cumulative probability plot for BS057 .  . 64 Figure 3.8: Concordia and weighted average and diagram for BS057 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 65 Figure 3.9: BSE images of representative zircons from sample BS046 (A) and BS075 (B).  .  .  .  .  .  . 68 Figure 3.10: Concordia and weighted average diagrams from sample BS046 and BS075.  .  .  .  .  .  .  . 69 Figure 3.11: Linear cumulative probability plots for sample BS046 and BS075 .  .  .  .  .  .  .  .  .  .  .  . 70 viii  Figure 3.12: BSE image of a representative zircon and linear cumulative probability plot for BS060.  . 74 Figure 3.13: Concordia and weighted average diagrams for sample BS060.  .  .  .  .  .  .  .  .  .  .  .  .  .  . 75 Figure 3.14: BSE image and linear cumulative probability plot for sample BS059 .  .  .  .  .  .  .  .  .  .  . 78 Figure 3.15: Concordia and weighted average diagrams for Granodiorite dike sample BS059.  .  .  .  . 79 Figure 3.16: CL images and ages of all the concordant zircon crystals from sample BS076. Ablation lines are highlighted with white lines.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 82 Figure 3.17: Tera-Wasserburg plot with weighted average diagram for sample BS076 .  .  .  .  .  .  .  .  . 83 Figure 3.18: BSE (A) and CL (B) image of the zircon recovered from sample BS062.  .  .  .  .  .  .  .  .  . 86 Figure 3.19: Examples of molybdenite bearing skarn. The photograph (A), PPL photomicrograph (A2), and RL photomicrograph (A3) show sample BS068 an example of Px-Ep endoskarn. This sample was dated by Re-Os geochronology in molybdenite at 165 ± 0.7 Ma (Table 3.12). The photograph (B), PPL photomicrograph (B2), and RL photomicrograph (B3) show sample BS067 an example of garnet skarn from the BMS below the SWOZ. The sample was dated to 162.8 ± 0.7 Ma by Re-Os geochronology of molybdenite (Table 3.12). See text for further discussion .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 90 Figure 3.20: Summary of isotopically determined Middle Jurassic ages at Buckhorn. The range of possible ages for skarn alteration and gold mineralization is denoted by the gold box. Error bars are 2σ, See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 92 Figure 4.1: Geologic map of Buckhorn Mountain with the skarn petrography, skarn geochemistry and ReOs geochronology sample locations highlighted. Based on data from this study and modified from Kinross Gold Corporation maps. See for map legend..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 98 Figure 4.2: Geologic map and cross-section of Buckhorn Mountain with the schematic skarn zonation highlighted. Pyroxene skarn occurs throughout the SWOZ. The thickness of skarn alteration on crosssection is exaggerated by a factor of 2 to more clearly illustrate zoning. Based on data from this study and modified from Kinross Gold Corporation maps. See and Figure 2.2 for map and cross-section legends.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Figure 4.3: Examples of amphibole-pyroxene skarn. The photograph (A), PPL photomicrographs (B, C), RL photomicrograph (B2, C2, D, E), BSE image (B3), and EDS elemental map (E) show the mineralogy, texture and paragenesis of the skarn alteration and gold mineralization. See text for further discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Figure 4.4: Examples of amphibole-pyroxene skarn. The PPL photomicrographs (A, B, C1), photograph (C) and XPL photomicrograph (D) show the mineralogy, texture and paragenesis of the retrograde alteration and the various types of deformation. See text for further discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  106 Figure 4.5: Examples of magnetite-pyroxene skarn. The photograph (A), PPL photomicrographs (A2, B, C3), RL photomicrograph and BSE image (C2) show the mineralogy, texture and paragenesis of the skarn alteration and gold mineralization. C2 and C3 are of particular importance because they shown gold mineralization inter-grown with a late stage of amphibole alteration. See text for further discussion..  .  .  108 Figure 4.6: Examples of epidote-pyroxene skarn. The photographs (A, B, C), PPL photomicrographs (A2, B2, C2) and XPL photomicrograph (A3) show the mineralogy, texture and paragenesis of the prograde ix  and retrograde alteration. C and C1 show an exceptional endo-skarn sample with significant molybdenite mineralization..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  111 Figure 4.7: Composition of pyroxene from the SWOZ. Hedenbergite is the most abundant type of pyroxene at Buckhorn and is characteristic for reduced gold skarns (Meinert et al. (2005)). There is no compositional difference between pyroxene from gold-rich rocks (red triangles) and barren rocks (blue triangles). See text for further discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  114 Figure 4.8: Classification of retrograde amphiboles from the Buckhorn Gold Skarn based on the criteria of Leake et al. (1997). The amphiboles associated with gold mineralization have elevated Mg and decreased Al, corresponding to an increase in Si, when compared to amphiboles in non mineralized skarn. This change in composition is suggestive of a cooling and oxidizing trend (Blundy and Holland, 1990; Spear, 1981; Holland and Blundy, 1994)..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  115 Figure 4.9: Examples of garnet skarn. The photographs (A, B), PPL photomicrographs (C, D, E, F), XPL photomicrograph (C2), and RL photomicrograph (G) show the mineralogy, texture and paragenesis of the prograde and retrograde alteration and gold mineralization. See text for further discussion. .  .  .  .  .  119 Figure 4.10: Examples of garnet skarn. The PPL photomicrograph (A) show several generations of retrograde garnet veins. The RL photomicrographs (B, C, D) show the specific settings of gold mineralization. The photograph (E), PPL photomicrograph (F) and RL photomicrograph (G) show the mineralogy, texture and paragenesis of the skarn alteration in the molybdenite bearing sample. See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  121 Figure 4.11: Examples of magnetite skarn. The photograph (A) PPL photomicrographs (B, C, E, H), and RL photomicrographs (D, F, G) show the mineralogy, texture and paragenesis of the skarn alteration and gold mineralization. See text for detailed discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  124 Figure 4.12: Examples of gold-bearing skarn veins in hornfels altered rock. The photographs (A, B), PPL photomicrograph (C), PPL photomicrographs (D, F), and RL photomicrographs (D2, E) show the mineralogy, texture and paragenetic sequence of the skarn veins and gold mineralization. See text for further discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  126 Figure 4.13: Paragenetic sequence of skarn alteration, deformation, and gold mineralization at Buckhorn 130 Figure 4.14: Temperature versus log fO2 diagram showing the stability fields of major skarn silicate, oxide, and sulfide minerals. The grey box indicates the range of possible conditions for the formation of prograde alteration at Buckhorn. Stability field for prograde alteration at Nickel Plate shown with the diagonal lines. Note that the stability field for the Buckhorn skarn extends to more oxidised conditions. Modified from Einaudi et al. (1981)..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  131 Figure 4.15: Log fO2 versus log fS2 diagram showing the stability fields of major calc-silicate and sulfide prograde minerals at T=+500°C, XCO2=0.1, and P (fluid) =0.5 Kbar. Stability field for skarn alteration at Buckhorn shown as the shaded area. Stability field of the Nickel Plate shown with the diagonal lines. Note that the stability field for the Buckhorn skarn extends to more oxidised and sulfur rich conditions. Nickel Plate stability field from Ettlinger (1992). Mineral stabilities from Ettlinger (1992) and Bowman (1998) ..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 132 Figure 4.16: Log fO2 versus log fS2 diagram showing the stability fields of major sulfide and bismuth minerals formed during retrograde alteration at T=300°C. Stability field for skarn alteration at Buckhorn x  shown as the shaded area. Stability field of Nickel Plate shown with the diagonal lines. Note that the stability field for the Buckhorn skarn extends to more oxidised and sulfur rich conditions. Nickel Plate stability field from Ettlinger (1992). Mineral stabilities from Ettlinger (1992), Barton and Skinner (1979) and references therein..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 133 Figure 4.17: Phase diagram of Au-Bi at 1 bar that demonstrates the eutectic point at 241°C. Modified from Tooth et al. (2008).  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  137 Figure 4.18: Log fO2 versus pH diagram showing the stability fields of Au, Bi, and Fe phases and aqueous complexes at the conditions of retrograde alteration and gold mineralization at Buckhorn (pH=6-8, log fO2=-36 to -42). Modified from Tooth et al. (2008)..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  138 Figure 5.1: Plots of bismuth versus select major and trace elements (Fe, Ca, Mn, Al, Ti, and Zr) that demonstrate the geochemical character of the BMS, BMV, and skarn alteration. See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  145 Figure 5.2: Graphs of Au vs. Ag, Bi, As, Pb, Cu, Mo, S, and Co. The graphs show that there is a moderate correlation between Au-Ag, a strong correlation between Au-Bi, and no correlation between gold and the other elements. Legend on Figure 5.1. See text for further discussion .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 147 Figure 5.3: Graphs of Au/Ag vs Au, Bi, As, and base metals (Pb, Cu, and Mo). The graphs show two distinct populations. See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 149 Figure 5.4: Graphs of Au/Ag vs Au, Bi, As, and base metals (Pb, Cu, and Mo). Plots are the same as Figure 5.3, but coloured according to the skarn type. The graphs show that the two distinct populations occur in all skarn types. See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  150 Figure 5.5: Immobile HFSE (Zr, Ti and total HREE content versus depth for three skarn intercepts through the SWOZ. The contact between the volcanic and carbonate rocks is picked based on a decrease in the immobile elements. Horizontal black lines mark the interpreted contacts between the BMV and the upper carbonate member of the BMS.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 153 Figure 5.6: Plot of Au (ppm)/Cu (%) vs Contained Au (Kg) for a selection of gold, porphyry copper, copper, iron, and lead-zinc skarns. Coloured points from Table 5.3, remainder from Meinert (1989). See text for further discussion..  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 157 Figure 5.7: Plot of Au/Ag vs Contained Au (Kg) for a selection of gold skarns. See text for further discussion. .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  157  xi  List of Abbreviations Amph: Amphibole And: Andradite garnet Asp: Arsenopyrite Bdi: Buckhorn diorite BGdi: Buckhorn Granodiorite Bi: Native bismuth Bi Min: Bismuth bearing mineral other than native bismuth and bismuthinite Bio: Biotite BIS: Buckhorn Intrusive Suite Bis: Bismuthinite BMS: Buckhorn Mountain Sequence BMV: Buckhorn Mountain Volcanic Sequence BSE: Back scatter electrons Ca: Calcite Chl: Chlorite CL: Cathodoluminescence microscopy Cpy: Chalcopyrite EDiD: Early diorite dike EDS: Energy-Dispersive X-Ray Spectroscopy EMP: Electron microprobe Ep: Epidote Felds: Feldspar Fg: Fine-grained Fol: Foliation GDiD: Granodiorite dikes Hbl: Hornblende Hed: Hedenbergite pyroxene HFSE: High field strength elements HREE: Heavy Rare Earth Elements (Gadolinium to Lutetium) Hyd Fe-Sil: Hydrated Iron Silicate minerals such as Ekmanite (Fe,Mg,Mn)3(Si,Al)4O10(OH)22(H2O), Ferrostilpnomelane K(Fe,Mg)8Si10Al2O24 (OH)3-2(H2O), Greenalite Fe2Si2O5(OH)4, and Minnesotaite (Fe,Mg)3Si4O10(OH)2 Ilm: Ilmenite K-Spar: Potassium feldspar LA-ICP-MS: Laser Ablation Inductively Coupled Plasma Mass Spectrometry Mag: Magnetite Mo: Molybdenite Pie: Piemontite Plag: Plagioclase feldspar Po: Pyrrhotite xii  PPL: Px: Py: QP dike: Qtz: RGDi: RIS: RL: SEM: Sphal: SWOZ: XPL:  Plane polarized light in microscopy Pyroxene Pyrite Quartz Porphyry Dike Quartz Roosevelt Granodiorite Roosevelt Intrusive Suite Reflected plane polarized light in microscopy Scanning electron microscope Sphalerite Southwest Ore-Zone Cross polarized light in microscopy  xiii  Acknowledgements The research presented in this dissertation would not have been possible without the contribution, support, and encouragement of many people. I would like to thank Dr. Craig Hart, my MSc supervisor, for his insight and guidance. His thorough understanding of mineral deposits was fundamental for this research. This work benefited from his constructive criticisms and improvements of the manuscript. I am also grateful for the help I received from my committee members Dr. Greg Dipple and Dr. Gerald Ray. Their knowledge of skarn systems is extensive and thorough and their insight was invaluable for my research. I would like to thank Kinross Gold Corporation, Kettle River Operations. This project would not have succeeded without their financial and logistic support, data provided, unrestricted access to the property, and samples collected. I would like to acknowledge Kinross geologists Rod Willard, Joshua Ellis, John May, Michael Olson, Akira Rattenbury, Ian Olsieg and others for sharing their thoughts about and encouraging and supporting me during my time at Buckhorn. Special thanks are due to Peter Cooper, Chief Geologist at Buckhorn, for helping set up this project and giving me the opportunity. I also acknowledge my colleagues at the University of British Columbia and the Mineral Deposits Research Unit who helped make this project a success through their friendship and the constant stream of geologic discussions they provided. Technical support from Dr. Jim Mortensen and the PCIGR staff (U-Pb Geochronology), Dr. Robert Creaser (Re-Os Geochronology), Dr. Mati Raudsepp, Edith Czech and Jenny Lai (electron microprobe and SEM) is gratefully acknowledged. Thank you to the MDRU staff of Arne Toma, Karie Smith, Manjit Dosanjh, Fanny Yip, and Lily Qian. Finally I would like to thank my family who have given me the love and support I needed to complete the project. Thank you Mom, Dad, and Grace for your help from a far. Super thank you Allie for you help, motivation, understanding, and love on a daily basis.  xiv  Chapter 1.	 Introduction 1.1	 Rationale for Study Skarn deposits that contain gold occur throughout the world in a variety of geologic settings, but the most well-understood are associated with reduced plutons that were emplaced into shallow levels of the earth’s crust (Meinert, 2000). These reduced gold skarns are typified by the Nickel Plate deposit (Billingsley and Hume, 1941; Ettlinger et al., 1992; Ray and Dawson, 1994). Other examples of reduced gold skarns include Fortitude, USA (Meinert, 2000; Theodore et al., 1973; Wotruba et al., 1986), El Valle - Boinas and Carles, Rio Narcea district, Spain (Cepedal et al., 2000; Martin-Izard et al., 2000), and Buckhorn, USA (Gaspar, 2005). These deposits are characterized by early widespread hornfels alteration, intense calcic pyroxene-garnet metasomatic alteration, a reduced sulfide assemblage (Po>Py), and an Au-Bi-Te-As geochemical signature (Meinert, 2000), however, due to their relative rarity the variations between them are poorly understood. Further research of reduced gold skarns is needed to better understand the geologic and geochemical characteristics of the skarn alteration and gold mineralization and determine the controls on alteration and mineralization. For example the Au-Bi-Te-As geochemical signature in gold skarns is well documented (Cepedal et al., 2006; Ettlinger et al., 1992; Meinert et al., 2005), but the relationship between gold mineralization and the major and trace elements is poorly documented. Additionally, the mechanism for gold precipitation in reduced gold skarns is ambiguous, and the role of bismuth and tellurium in the gold mineralization process is not established. It is also unclear if there is a continuum between reduced and oxidised gold skarns in terms of mineralogy and/or conditions of formation. To this end the Buckhorn gold skarn is the subject of this research to determine the geologic, geochemical, and geochronological characteristics of the deposit. This research will lead to (1) a better defined deposit model for the Buckhorn gold skarn, (2) a better understanding of the variations in the classification, and (3) will aid in exploration for similar deposits in both new and mature districts. These goals are accomplished through a detailed petrographic study of the host rocks and local intrusive rocks complemented by a U-Pb geochronological study to determine their character and ages. The mineralogical and textural characteristics of skarn alteration are investigated with detailed optical petrography followed by targeted scanning electron microscopy (SEM) of the skarn alteration and electron microprobe (EMP) analysis of certain prograde and retrograde minerals. Geochemical 1  analysis of the skarn alteration complements the mineralogical and textural characterization, and ReOs geochronology of rare molybdenite hosted in the skarn alteration is used to constrain its age.  1.2	 Background The Buckhorn gold skarn is located in Okanogan County in north-central Washington (48° 57’ N, 118° 59’ W) (Figure 1.1). It originally contained approximately 2.15 Mt at 14 ppm gold, for about 1 Moz contained gold (Cooper et al., 2008). It is owned and operated by Kinross Gold Corporation and is being mined underground at a rate of approximately 1,000 tons per day, for an expected 7 year mine life from 2008 to 2015 (Cooper et al., 2008). Skarn alteration has been known to occur at Buckhorn since the early 1900’s (Umpleby, 1911), but gold mineralization was not discovered until 1988 and the Southwest Ore-Zone (SWOZ), the largest ore body, was not discovered until 1992 (Cooper et al., 2008). Buckhorn has been the focus of numerous government, academic and industry studies through the years including the work of Broughton (1943), McMillen (1979), Moen (1980), Hickey(1990, 1992), Jones (1992), Gaspar (2005), Gaspar et al. (2008), and Cooper et al. (2008). The early studies (Pre 1988) focused on the mine scale stratigraphy and the iron-copper skarn mineralization located in the Magnetic Mine area. Hickey’s work in the early 1990’s focused on the skarn alteration and gold mineralization in the Gold Bowl, which currently makes up a subordinate portion of the economic gold mineralization. Buckhorn has been the subject of one academic study, Gaspar (2005), since the discovery of the SWOZ, but it was completed before the majority of the diamond drilling and any underground mining took place. For his PhD, Gaspar (2005) focused on the mineral chemistry and through microprobe analysis determined the composition of the skarn alteration minerals. Gaspar (2005) also completed an early description of the skarn alteration and gold mineralization. With the benefit of greater access to the SWOZ than the previous studies, due to additional drilling and underground mining, the current work is able to provide a more detailed and comprehensive study of the geologic, geochemical, and geochronological characteristics of the skarn alteration and gold mineralization.  2  AA AG  Bridge River Cache Creek  CD  Cadwallader  HA  Harrison Lake Chilliwack  YA  Yakutat  CK  CG  Chugach  OK  Okanagan  PR  Pacific Rim  ST  Stikinia  CR  Crescent  QN  Quesnellia  YT  Yukon-Tanana  SM  Slide Mountain  CG 58°N  AX  Arctic Alaska  NAp  AG  Angayucham, Tozitna  NAc  ST  QN  54°N  NAc 0  100  200  300  km  NAp  Prince Rupert  54°N  de  WR SM  North America platform North America craton & cover  co  Edmonton  for  CA  CC  mp  50°N  lex  CD MT BR  WR  Buckhorn Gold Skarn  Fault  58°N  AX  ic  AA  of  CA  YT  Kootenay  NAb  NWT  BC  Cassiar  CA  62°N  SM  CG  Ancestral North America  N Alaska  Juneau  Yukon  YT CC  n ra ille rd Co  Coast complex  YA  YT CA  ton  KS  Whitehorse  ma  tio  n  plu  Wrangellia  AX  Yellowknife  SM  ST  t  WR  QN  KS  as  Alexander  CG  NAc  NAb  Co  AX  WR  NAp  YT SM  Alberta  Insular  62°N  Methow  MT  TERRANES Outboard  A  limit  Volcanic rocks  BR CC  NAb  rn easte  Vr  66°N  Alas ka  Intermontane  Plutonic rocks  Gd  116°W  NAp  Post-Accretionary Gneiss Dome  124°W  132°W  140°W  Legend Pr  Inuvik  Vancouver  PR  NP Nickel Plate Gold Skarn  132°W  CR  NAb QN  NAp  Calgary  SM  50°N  OK OK  MT CK BR  USA  Victoria  124°W  116°W  Vr SM QN  Ashcroft  Penticton Group  Nicola  Spences Bridge Nicola  Harper Ranch  Ladybird  Ladybird  Pre- to SynAccretion- Okanagan ary Plutons Batholith  Anarchist Penticton Group  NP  Coryell  LadyCoryell bird  Attwood  BR  Anarchist  MT  Coryell Mt Roberts Hall  Coryell Brooklyn  Nicola Kobau  Nelson Batholith  Pr  Knob Hill  Pre- to SynAccretionary Pluton  Elise Rossland NAb Monzonite  49° N  Elise  He  rro  Middle Jr Pluton Kobau  nC ree  Eocene Volcanic Rocks  k  Anarchist  Gd  B  Keller Butte  20km  Devils Elbow  Perm Metasedimentary rocks NAp  120° W  Figure 1.1: (A) Simplified terrane map of the Canadian cordillera modified from Cauldron and Nelson (2011). (B) Simplified geologic map of the southern Canadian cordillera modified from Gaspar (2005) after Tripper et al. (1981), Wheeler and McFeely, (1991), Nelson and Colpron (2007), and Colpron and Nelson (2011).  3  1.3	 Thesis Objectives This thesis aims to answer questions regarding the character, genesis, and age of skarn alteration and gold mineralization at Buckhorn that past studies left unanswered. It does this by focusing on two main questions. (1) What is the geologic and geochemical character of skarn alteration and gold mineralization and what is their relationship to the host rocks, intrusive rocks, deformation, and metamorphism? (2) How old are the host rocks and intrusive rocks? How old is the deformation, skarn alteration, and gold mineralization? The study also resolves confusion regarding the correlation of Buckhorn rocks with the regional stratigraphy. Answering these questions helps construct an accurate deposit model for Buckhorn, which is compared to Nickel Plate and other gold skarns to determine how Buckhorn fits into the gold skarn deposit model. The refined Buckhorn deposit model can be used to improve exploration targeting for similar deposits around the world.  1.4	 Thesis Organization This thesis is arranged in seven chapters. Chapter 1 introduces the study and provides a brief history of the Buckhorn gold skarn. Chapter 2 summarizes the tectonic evolution and geology of the southern Canadian cordillera, with particular attention paid to the Quesnel terrane that hosts Buckhorn. This summary is followed by a detailed description of the Buckhorn geology that focuses on the spatial and temporal relationships between the geologic units, metamorphism, deformation, and both skarn alteration and gold mineralization. This chapter provides context for the remainder of the thesis and correlates the Buckhorn rocks with the regional stratigraphy with the help of the geochronology presented in Chapter 3. Chapter 3 presents a U-Pb geochronology study that aims to confirm the age of the metavolcanic host rocks and local intrusive rocks and to constrain the age of deformation and skarn alteration at Buckhorn. Chapter 3 also includes a Re-Os geochronology study of molybdenite-bearing skarn alteration to further constrain the age of skarn alteration and gold mineralization. Chapter 4 presents detailed and thorough thin section and hand sample descriptions of the skarn alteration and gold mineralization at Buckhorn. Preliminary descriptions were done by Gaspar 4  (2005), but this study greatly benefits from additional drilling, and underground mining that has taken place since that study. This chapter focuses on characterizing the skarn alteration assemblages and settings of gold mineralization, and determining their relationship to each other, deformation and the local intrusions. Major element microprobe analysis of pyroxene and amphibole is included and complements the geologic characterization of the skarn alteration, and aims to determine any relationship between mineral composition and gold mineralization. Chapter 5 characterizes the major, trace and Rare Earth Element geochemical signature of the host rocks, skarn alteration and gold mineralization, thus complementing the geologic descriptions and mineral geochemistry presented in Chapter 4. This chapter also includes a geochemical comparison of Buckhorn and other skarn deposits around the world to document the geochemical variation present and distinguish different types of skarn deposits. Chapter 6 draws conclusions from the aforementioned work. It also presents ideas for future research and explains the implication regarding exploration for similar deposits. Chapter 7 contains a summary of the theses. The chapters are followed by Appendix A and B that contain the results of the microprobe mineral analysis and the geochemical analysis respectively.  5  Chapter 2.	 Regional and Mine Scale Geology of the Buckhorn Mountain Gold Skarn 2.1	 Introduction The following work summarizes the regional geology and tectonic evolution of the southern Canadian cordillera, with particular attention paid to the Quesnel Terrane, which hosts the Buckhorn gold skarn. This provides geologic context for the remainder of the thesis and helps correlate the Buckhorn geology with the regional stratigraphy. The summary of the regional geology is followed by a detailed description of the Buckhorn geology that focuses on the relationships between the geologic units, deformation, alteration, and both skarn alteration and gold mineralization. Particular attention is paid to the regional geology because, while character of the mine scale geology is generally agreed upon, a consensus on its place in the regional stratigraphy has not been reached  2.2	 Regional Geology The Canadian cordillera is made up of tectonic blocks known as terranes that were accreted on to the North American continental margin during the Mesozoic (Figure 1.1). The tectonic evolution of the Canadian cordillera has been the subject of several review papers, (Monger and Price, 2002; Nelson and Colpron, 2007), which are summarized below. The Canadian cordillera originated with the breakup of the supercontinent Rodinia in the Neoproterozoic, which lead to the development of a passive margin on the attenuated western margin of the North American craton (Monger and Price, 2002). The passive margin existed from about the Cambrian until the Middle Devonian, when a convergent inter-plate boundary formed off shore with subduction under the North American craton (Monger and Price, 2002). The Middle Devonian plate margin was characterized by a string of island arcs separated from North America by a back arc basin (Monger and Price, 2002). Sedimentary and volcaniclastic assemblages were deposited into this back arc basin, and they dominate the late Paleozoic rocks in the Quesnel Terrane (Nelson and Colpron, 2007). Accretion of the island arcs onto the North American craton started during the Middle Triassic and was completed by the Early Jurassic (Monger and Price, 2002). In the Quesnel Terrane the Middle Triassic is characterised by voluminous arc-related volcanic rocks with coeval and cogenetic plutons (Nelson and Colpron, 2007). Plutonism continued after accretion and new arcs were built on both the original continent and the newly accreted material (Monger and Price, 2002). The arcrelated plutonic activity in the southern Canadian cordillera persisted until the Late Cretaceous when 6  there was a marked decline in activity (Nelson and Colpron, 2007). The tectonics of the southern Canadian cordillera changed in the Eocene from a compressional to transtensional environment. This changed resulted in strike-slip faults and volcanism that accompanied crustal extension, the exhumation of metamorphic core complexes, and the formation of volcano-sedimentary basins (Nelson and Colpron, 2007). 2.2.1	Quesnel Terrane The Quesnel Terrane covers approximately 85,000 km2, an area about the size of Ireland, from northern Washington State to the central Yukon (Figure 1.1).The Quesnel Terrane is made up of late Paleozoic to middle Mesozoic volcanic and sedimentary strata and plutonic rocks. On the east the Quesnel Terrane structurally and stratigraphically overlies the pericratonic Cassiar and Kootenay terranes and the dominantly oceanic Slide Mountain Terrane (Monger et al., 1991). Structurally to the west of the Quesnel Terrane is the Cache Creek Terrane, composed of rocks of an oceanic affinity (Monger et al., 1991). The pre- to syn-accretionary strata of the Quesnel Terrane can be split into three main divisions: late Paleozoic assemblages, Early Mesozoic volcanic assemblages, and Jurassic sedimentary assemblages (Monger et al., 1991). Post-accretionary strata formed in the Cretaceous and Paleogene (Souther, 1991). The post-accretionary rocks include metamorphic core complexes that were exhumed in the Eocene (Kruckenberg et al., 2008). Plutonic activity in Quesnel Terrane can be split into four main magmatic episodes: Late Triassic to Early Jurassic, Middle to Late Jurassic, Cretaceous and Paleogene (Ghosh, 1995, and references therein). The description of the Quesnel Terrane that follows is compiled from the work of numerous authors (Beatty, 2003; Cheney et al., 1994; Dostal et al., 2001; Hoy and Dunne, 1997; Monger et al., 1991; Mortimer, 1987). 2.2.1.1	Late Paleozoic Assemblages The late Paleozoic strata of the Quesnel Terrane in southern British Columbia and northern Washington are relatively poorly understood and have many local names. Monger et al. (1991) split them into the Okanagan and Harper Ranch Subterranes on the basis of the lithological assemblages. The Harper Ranch Group is the most complete example of the Harper Ranch Subterrane and is predominately made up of carbonate, siliciclastic and volcaniclastic rocks (Beatty, 2003; Monger et al., 1991). The Harper Ranch Group likely formed in an island arc and can been correlated with a number of other units including the Attwood and Anarchist Groups and the Mount Roberts Formation 7  (Beatty, 2003; Cheney et al., 1994; Hoy and Dunne, 1997). In Washington the Anarchist is made up of slate, metasiltstone, metasandstone, metalimestone, and metaconglomerate, with a Permian faunal assemblage (Rinehart and Fox, 1972). The Okanagan Subterrane is made up of limestone, greenstone, chert and ultramafic bodies that were likely formed in an oceanic or marginal basin setting (Monger et al., 1991). Hoy and Dunne (1997) include the ophiolites and greenstones of the Knob Hill and Kobau Groups of the Greenwood area in the Okanagan Subterrane. Although treated as separate units the relationship between the two Subterranes is unclear (Monger et al., 1991). 2.2.1.2	Early Mesozoic Volcanogenic Assemblages Unconformably overlying the late Paleozoic strata is an extensive assemblage of early Mesozoic volcanogenic rocks (Monger et al., 1991). This assemblage is characterized by the Nicola Group which is composed of Late Triassic and Early Jurassic volcanic and sedimentary rocks that are intruded by comagmatic plutons (Monger et al., 1991). Geochemical analysis shows that the Nicola Group volcanic rocks become more alkaline to the east indicating that they formed in a west-facing volcanic island arc (Monger et al., 1991). The Nicola Group also has increasing sedimentary input to the east, locally grading into a sedimentary sequence of argillite and siltstone (Monger et al., 1991). The Nicola Group can be correlated with the Triassic Brooklyn Formation in the Greenwood area (Beatty, 2003) and the Jurassic Elise Formation in the Nelson-Rossland area (Cheney et al., 1994; Monger et al., 1991; Mortimer, 1987a). The Elise Formation is composed of mafic pyroclastic and epiclastic rocks, dominated by augite porphyry flows and lesser mafic tuff (Hoy and Dunne, 1997). Some authors do correlate the Triassic and Jurassic rocks and instead split the Jurassic volcanic rocks into a separate group (Beatty, 2003; Hoy and Dunne, 1997). Regardless of the exact correlations, the early Mesozoic strata of the Quesnel Terrane are dominated by arc-related volcanic and sedimentary rocks. 2.2.1.3	Jurassic Sedimentary Assemblages Early to Middle Jurassic marine sedimentary rocks unconformably overly portions of the Nicola Group rocks and Early Mesozoic plutons (Monger et al., 1991). Lacking volcanic rocks, the sedimentary rocks are dominantly conglomerate, sandstone, shale and siltstone indicating that they were deposited in a post-arc marine basin (Hoy and Dunne, 1997; Monger et al., 1991). Facies analysis indicates that the sediments were deposited in a submarine fan with an eastern sediment source that included eroding granodiorite, diorite and syenite presumably from Early Mesozoic plutons (Monger et al., 1991). These sedimentary rocks have been classified as the Hall Formation in east and the Ashcroft Formation in the west (Hoy and Dunne, 1997; Monger et al., 1991). Ranging in age from Early 8  Pliensbachian to Bajocian (190-168 Ma) the Hall and Ashcroft Formations represent the last marine incursion onto the Quesnel Terrane (Monger et al., 1991). 2.2.1.4	Post-Accretionary Cretaceous Assemblages In the southern Quesnel Terrane, the Spences Bridge Group dominates the Cretaceous postaccretionary assemblage (Souther, 1991). Made up of basaltic to rhyolitic lavas intercalated with volcaniclastic rocks the Spences Bridge Group is located near the western margin of the Quesnel Terrane (Thorkelson and Smith, 1989) where it unconformably overlies the Triassic Nicola Group and earlier Mesozoic plutonic rocks (Irving and Thorkelson, 1990). The Spences Bridge Group is Early Cretaceous (104 Ma) and was deposited in a terrestrial environment during a time of east dipping subduction (Irving and Thorkelson, 1990; Souther, 1991). 2.2.1.5	Post-Accretionary Paleogene Assemblages In south-central British Columbia, Eocene volcanism in the Quesnel Terrane is characterized by the widespread calc-alkalic Kamloops Group and the alkaline Penticton Assemblage (Souther, 1991). The Kamloops Group is composed of volcanic and sedimentary rocks preserved in complex basins that are the surface expression of high angle reverse faults. Geochemical analysis of the Kamloops Group rocks indicates that they formed in a subduction related continental arc setting. A similar setting is proposed for the Penticton Group rocks to the south-east (Souther, 1991). The Penticton Group is primarily made up of highly alkaline lavas and breccias of the Marron Formation, which are equivalent to the 54 Ma to 48 Ma volcanic rocks of the Sanpoil Formation in north-east Washington State (Holder, 1989; Souther, 1991). The Eocene volcanism was coeval with regional extension related to dextral, transcurrent movement on the Northern Rocky Mountain Trench fault system (Holder, 1989, and references therein; Souther, 1991). The volcanism and extension is also coeval with the age of exhumation of the Okanogan gneiss dome, one of many metamorphic core complexes in the Canadian cordillera (Kruckenberg et al., 2008). These metamorphic core complexes are comprised of Mesozoic and older, poly-deformed, high grade paragneiss and variably deformed Cretaceous and Eocene granitoid rocks (Gabrielse, 1991). The granitoids predominately occur in normal faults and shear zones associated with the core complexes. These faults and shears form a regional network that accommodated significant extension and formed the basins that host the Eocene volcanic rocks (Gabrielse, 1991).  9  2.2.1.6	Late Triassic to Early Jurassic Plutonism Early Mesozoic plutonism in the Quesnel Terrane ranges from the Late Triassic to Early Jurassic (Ghosh, 1995). It is pre-accretionary and represents the roots of the Nicola Arc, and ranges from alkalic to calc-alkalic in composition (Ghosh, 1995). Examples include the Bromley batholiths, Mount Riordan stock and the Hedley intrusions; the latter is associated with local gold skarn mineralization (Ray and Dawson, 1994). Plutons of this age are also prospective for porphyry mineralization. Porphyry deposits associated with pre-accretionary plutonism include the Highland Valley and Copper Mountain mining districts, which are associated with the Guichon Creek Batholith and the Copper Mountain Stock respectively (Mortensen et al., 1995; Woodsworth et al., 1991). In other mining districts such as the Greenwood and Nelson-Rossland areas the Late Triassic to Early Jurassic intrusions are volumetrically minor and more mafic in composition (Fyles, 1990; Hoy and Dunne, 2001). Examples include a microdiorite from the Greenwood area that are interpreted as the intrusive equivalent of the Brooklyn Formation, and has a K-Ar age of 206±8 Ma (Church, 1986). In the Nelson-Rossland area examples include the Katie and Shaft monzogabbro plutons that are interpreted as sub-volcanic intrusions, comagmatic with the Early Jurassic Elise Formation (Hoy and Dunne, 2001). The plutons in the Nelson-Rossland area have not been dated, but they are expected to be Sinemurian (197 Ma to 190 Ma) the same age as the Elise formation (Hoy and Dunne, 1997). 2.2.1.7	Middle to Late Jurassic Plutonism Middle to Late Jurassic plutonism in the Quesnel Terrane was syn- to post-accretionary, and it represents the continued subduction of the Cache Creek Ocean, and the obduction and onlap of the eastern side of the Quesnel Terrane onto the North American craton (Hoy and Dunne, 1997). The syn- to post-accretionary plutons display a general compositional trend through time from alkali through calc-alkalic to two mica granitoids (Woodsworth et al., 1991). Located north of Nelson, BC, the Kuskanax Suite intrusions represent the earliest Middle Jurassic magmatism (Ghosh, 1995). The Kuskanax is a homogenous leucocratic quartz-pyroxene monzonite batholith that has a minimum age of 173 ± 5 Ma (Woodsworth et al., 1991, and references therein). The widespread and economically important Nelson Suite intrusions are slightly younger than the Kuskanax suite (Woodsworth et al., 1991). The Nelson Intrusive Suite is made up of the Nelson, Bonnington and Trail batholiths, as well as numerous smaller stocks including the Rossland monzonite (Hoy and Dunne, 2001). The Nelson Intrusive Suite is made up of granodiorite with lesser diorite and granite, and ranges from coarse-grained equigranular to porphyritic (Ghosh, 1995; and references therein). These rocks were 10  emplaced during the Middle Jurassic (173 Ma to 160 Ma) in a continental arc setting and have undergone considerable crustal contamination (Ghosh, 1995; Hoy and Dunne, 2001). The Middle to Late Jurassic intrusive rocks are associated with a number of types of mineralization including porphyries, skarns and polymetallic veins along the borders of the Nelson Batholith, as well as molybdenite skarn and gold-copper veins that are related to the Rossland monzonite (Hoy and Dunne, 2001). 2.2.1.8	Cretaceous Plutonism Cretaceous plutonism is abundant in the southern Canadian cordillera predominately forming west of the Quesnel Terrane and to a lesser extent within it (Woodsworth et al., 1991). Plutonism in the Quesnel Terrane is characteristically more felsic and peraluminous than the intrusions to the west and the dominant rock types are biotite or biotite-muscovite granite and granodiorite (Driver et al., 2000, and references therein). Examples of Cretaceous plutons in the south-eastern Quesnel Terrane include the White Creek, Bayonne, Okanagan Range and the Whatson batholiths (Ghosh, 1995; Hurlow and Nelson, 1993; Woodsworth et al., 1991). In the south-western Quesnel Terrane Cretaceous plutonism is represented by the Verde and Summers Creek stocks, which are leucogranites that cut the Early Cretaceous Spences Bridge Group, and have been interpreted as its hypabyssal equivalent (Ghosh, 1995; Woodsworth et al., 1991). 2.2.1.9	Paleogene Plutonism Paleocene and Eocene plutonism is widespread in southern British Columbia and northern Washington State (Holder and McCarley Holder, 1988; Woodsworth et al., 1991). In British Columbia the most areally extensive are the Lady Bird Suite and the younger Coryell plutons. Rocks of the Ladybird Suite were emplaced between 62 and 52 Ma, possibly from a crustal source, and occur as late kinematic to post-kinematic, I-type biotite leucogranite plutons and batholiths (Carr, 1992). Rocks of the Ladybird suite are compositionally similar to the Keller Butte Suite of the Colville Batholith in Washington, and the two are correlated based on composition and tectonic setting (Carr, 1992; Holder, 1989; Holder and McCarley Holder, 1988). The Devils Elbow Suite was emplaced after the Ladybird/Keller Butte Suite and has an intermediate composition between the earlier Ladybird/Keller Butte Suite and the later Coryell Batholith (Holder and McCarley Holder, 1988). The Devils Elbow Suite has not been correlated with equivalent rocks in British Columbia, but Holder (1989) suggested that it is equivalent to the lower Sanpoil Formation and possibly the Marron Formation. The youngest major Eocene intrusive suite is the Coryell batholiths that are characterized by high-level alkaline porphyritic syenites and lesser granite, diorite and monzonite (Woodsworth et al., 1991). The Coryell 11  batholiths are genetically related to the wide spread Marron Formation rocks and equivalent to the Herron Creek Intrusive Suite in Washington State (Holder, 1989; Woodsworth et al., 1991).  2.3	 Buckhorn Geology The geology of the Buckhorn gold skarn has been described by numerous authors, (Cheney et al., 1994; Gaspar, 2005; Hickey, 1990, 1992; Jones, 1992; McMillen, 1979; Stoffel, 1990a, b) and is generally agreed upon. However, there are still aspects that are poorly understood including the timing of skarn alteration relative to the various intrusions, the age of deformation and its timing relative to skarn alteration, and the correlation of mine units with the regional stratigraphy. The understanding of the area is complicated by the fact that the Buckhorn host rocks have all undergone several stages of metamorphism, deformation and skarn alteration, which locally makes identifying their original character impossible. To address these points the following section presents a thorough description of the different mine scale geologic units followed by a summary of the metamorphism, deformation, and briefly of skarn alteration. The character of the skarn alteration is thoroughly described in Chapter 4. The Buckhorn gold skarn is hosted in metamorphosed carbonate and clastic sedimentary rocks that are unconformably overlain by mafic volcanic flows and intruded by several felsic granitoid stocks and associated dikes (Figure 2.1 and Figure 2.2). This host stratigraphy occurs elsewhere in the southern Quesnel Terrane, for example in the Nelson-Rossland, Greenwood and Hedley areas (Cheney et al., 1994; Church, 1986; Fyles, 1990; Hoy and Dunne, 1997; Ray and Dawson, 1994). The Buckhorn stratigraphy is fault-bound on all sides, on the east by a major NNE normal fault that is the western boundary of the Eocene rocks that fill the Toroda Creek half graben (Suydam and Gaylord, 1997). To the west and north the Chesaw thrust places ophiolite rocks of the Permian Knob Hill Group are over the younger pelitic Buckhorn rocks (Cheney et al., 1994; Fyles, 1990). The southern edge of the Buckhorn rocks is defined by a detachment fault at the edge of the Okanogan metamorphic core complex (Cheney et al., 1994).  12  Area 1  BS074 BS070  BS048 BS060 BS068 FM  MS  MM  BS059 GB  BS063 BS062  BS064  BS047 F NL  SWOZ BS067  BS046  BS065  Area 2  BS057 BS076 RM  TCF  A BS075  BS061  A’  N  Legend U-Pb Geochronology and Intrusive Rock Samples Re-Os Geochronology Samples  1km  Figure Figure: 2.1: Geologic mapmap of Buckhorn Mountain with sample locations highlighted. Based on data Geologic of Buckhorn Mountain. Based on data from this study from this study1(Areas 1 &modified 2), and modified from Gold Kinross Gold Corporation maps. Coordinate (Areas & 2) and from Kinross Corporation maps. Co-ordiante system:system: UTM NAD27, Zone 11U. Map legend on following page. UTM NAD27, Zone 11U. Map legend on following page  13  Sedimentary and Volcanic Rocks  Intrusive Rocks Roosevelt Intrusive Suite  Volcanic Rocks Challis Suite/Penticton group  Roosevelt granodiorite  Buckhorn Mountain Volcanic Sequence (BMV)  Buckhorn Intrusive Suite  Volcaniclastic rocks  Quartz porphyry dikes  Volcanic flows and conglomerates  Granodiorite dikes  Buckhorn Mountain Sequence (BMS) Carbonate rocks  Buckhorn diorite  Mixed clastic-carbonate rocks  Buckhorn granodiorite  BMV Intrusive Rocks  Metarhyolite  Equigranular intrusive  Clastic rocks  Porphyritic intrusive  Linear Features Normal fault  Deposit Outline  Thrust fault  Outline of area mapped for this study  Shear zone Gradational Contact  Mineralized Rocks Skarn  Locations Southwest Ore-Zone (SWOZ) Gold Bowl Ore-Zone (GB) North Lookout Fault (NLF) Footwall Mylonite (FM)  Toroda Creek Graben Fault (TCF) Magnetite Mine (MM) Roosevelt Mine (RM) Mike’s Skarn (MS)  Figure Continued: for the map of(Fig Buckhorn Figure:2.1 Legend for theLegend Geologic mapGeologic of Buckhorn X.x)  14  A’  355,500 m E  355,000 m E  354,500 m E  A  1750m 300˚  NLF SWOZ  RM  Base of drilling  120˚ TCF  1250m Skarn Eocene Volcanic rocks Volcanic flows Carbonate rocks Clastic sedimentary rocks  Roosevelt Gdi Qtz porphyry dike Granodiorite dike Buckhorn diorite Buckhorn granodiorite  Normal fault Inferred fault Shear zone  Co-ordiante system: UTM NAD27, Zone 11U  250m  15  Figure 2.2: Geologic cross-section of the South-West Ore Zone and Roosevelt Mine area looking NNE. Based on data from this study and KinInterpretive drilling geologic cross-section of the South-West Ore Zone and Roosevelt Mine area looking NNE. Based on data from rossFigure. Gold Corporation this study and Kinross Gold Corporation drill-core.  2.3.1	Metasedimentary Rocks Metasedimentary rocks are the oldest rocks in the local stratigraphy and have a thickness of at least 500 m. Locally known as the Buckhorn Mountain Sequence (BMS), the rocks are dominantly argillite, siltstone and lesser sandstone with rare conglomerate. The clastic sedimentary rocks are intercalated with volcaniclastic sedimentary rocks and capped by a carbonate dominated member (Figure 2.2). Clastic rocks in the BMS were originally siltstones and sandstones, argillite with lesser black shale (McMillen, 1979). Greenschist facies metamorphism and later intense hornfels alteration masks the original sedimentary textures and minerals (Figure 2.3). The metasedimentary rocks are currently finely laminated to massive and range in colour from very dark brown to green to white (Figure 2.3). They are variably altered to epidote-zoisite-amphibole or biotite-chlorite-sericite assemblages, both with minor remnant quartz and lesser feldspar grains. The rocks have minor (<1%) disseminated magnetite and sulfides, predominately pyrrhotite and chalcopyrite, which may be associated with alteration of mafic minerals to amphibole. The least abundant, but most recognizable sedimentary rocks in the BMS are the conglomerates, which mostly occur in the Gold Bowl area where they dip gently to the south-east. Based on drilling, the thickness of the conglomerates ranges from several metres up to 10’s of metres. The conglomerates are heterolithic and texturally variable, and can be clast or matrix supported. The clasts are rounded to rarely subangular and range in size from 1 to >10 cm. The clasts are typically chert, but may also be igneous, volcanic or carbonate rocks. When altered, the clasts are usually replaced by an epidote-zoisite assemblage and mafic minerals in the matrix by chlorite (Figure 2.4).  16  1cm  1cm  1cm  A  C  B  0.5mm  250μm  0.5mm  low angle fol 1  fo low angle  Qtz  l2  Qtz  ol ng f  ppi ly di  p Stee  Amph-chl  A1  B1  C1  low angle fol  Figure 2.3: Examples of BMS metasedimentary rocks that have been variably hornfels altered that Figure: Examples Buckhornmineralogy, Mountain Sequence metasedimentary rocks that in are alteration variably hornfels (hand samples demonstrate itsoftexture, and character and variation andaltered deformation (A, A,B,C and photomicrographs A1,B1,C1). Sample A displays amphibole-chlorite and later vein controlled epidote-zoisite B, and C hand samples; A1 and B1 PPL photomicrograph; C1 XPL photomicrograph). See text for alteration. The PPL photomicrograph, A1, shows that it has been recrystallized and has a granoblastic texture. Sample B is further stronglydiscussion. altered to biotite and lesser chlorite-sericite. Visible in the PPL photomicrograph, B1, are two nearly parallel foliations defined by the alignment of the micaceous minerals. Sample C is strongly altered to amphibole and lesser chlorite and sulfides (Py+Po>Cpy+Sphal). In the XPL photomicrograph, C1, it is seen that the S1/S2 foliation is crenulated by a later foliation, S3, that is at a high angle to it.  17  1cm  1mm  Qtz  Amph-chl  Po Amph-chl Ep Ep  A  A1  Figure 2.4: Examples of thesample BMS conglomerate that demonstrate its(A1) texture, mineralogy, and charFigure: Hand (A) and PPL photomicrograph of the acter and variation in alteration deformation. hand sample; A1 PPLThe photomicrograph) See conglomerate unitand of the Buckhorn(AMountain Sequence. clasts text for further discussion. and matrix are variable altered to epidote, amphibole and chlorite. The upper carbonate member of the BMS is a dark grey-blue to white calcic marble that is conformable with the underlying clastic sedimentary rocks (Figure 2.5). It is up to about 300 m thick, and crops out on the west side of Buckhorn Mountain, covering an area of approximately 4.5 km2. It underwent nearly complete recrystallization during contact metamorphism followed by deformation and has a wide range of crystal sizes from 10 μm to over 3 cm. Smaller crystals usually occur in intensely strained portions whereas the very large crystals are in massive zones. A minor component of the upper carbonate member is fossiliferous, with crinoid stems being the most abundant fossil, however, fusulinids, ostracods, bivalves and coral have also been reported by other authors (Gaspar, 2005; Hickey, 1990; McMillen, 1979).  18  1mm  1cm  A1  low angle fol  1cm  low angle fol  A  B  Figure Examples of upper the upper carbonate of the BMS that demonstrate its texture, Figure:2.5: Examples of the carbonate unit member of the Buckhorn Mountain Sequence. Sample A is fine mineralogy, character and variation alteration andphotomicrograph deformation (A and hand that sample; grained andand shows a low angle foliation in S1/S2. A1, a XPL of A,Bshows the A1 XPL photomicrograph). See size textvariation for further discussion. foliation is defined by grain from about 600μm to 10μm. Sample B is a hand sample of very coarse grained, up to 3cm, marble with a granoblastic texture.  The BMS is correlated with the Permian Anarchist Group rocks based on a Late Paleozoic age determined for corals from a graphitic, calcareous, pyritic, quartz-sericite phyllite that were examined by W. J. Sando of the U.S. Geological Survey (McMillen, 1979). More recently this same age was determined from fusulinids by Dr. Sarah Fowell of the University of Alaska at Fairbanks (M. Deal personal communication, 2011). As described in the regional geology section, the Anarchist Group is equivalent to other late Paleozoic strata in southern British Columbia including the Harper Ranch Group in the Kamloops area, the Attwood Group in the Greenwood area, and the Mount Roberts Formation in the Nelson-Rossland area (Beatty, 2003; Cheney et al., 1994; Hoy and Dunne, 1997; Monger et al., 1991). 2.3.2	Metavolcanic Rocks Rocks of the Buckhorn Mountain Volcanic Sequence (BMV) unconformably overlie the BMS and are the uppermost host rock package. They cover 1.7 km2 on the top and east side of Buckhorn Mountain (Figure 2.1). The nature of the contact with the underlying BMS has been disputed in the past. Some authors, (Hickey, 1990, 1992; McMillen, 1979), describe the contact as a thrust fault, 19  while others, (Cheney et al., 1994; Gaspar, 2005), consider the contact to be a disconformity. Current core logging and local underground mapping suggests that the contact is a disconformity, which matches with the stratigraphy at nearby locations such as the Nelson-Rossland area. The BMV is mostly made up of sub-horizontal porphyritic flows and autobreccias with minor tuffs, dikes, sills, and plugs. The porphyritic flows of the BMV are massive and blocky in outcrop. They are typified by coarse grained augite phenocrysts in a fine grained to aphanitic mafic groundmass. The augite crystals range in size from 0.1-1 cm and are altered to amphibole and chlorite. The groundmass is very fine grained to aphanitic and altered to chlorite, amphibole and epidote. There are rare oval to spherical amygdules in some flows that are filled with epidote and later chlorite. The flows usually contain a trace amount of chalcopyrite and sphalerite that may be associated with epidote and/or zoisite alteration (Figure 2.6). 1cm  50cm  A  A1 1cm  0.5mm Px-chl  Px-amph  Metasomatic skarn alteration low angle fol  Groundmass Chl  A2  1mm  Metasomatic skarn alteration  low angle fo l  Ep  B  B1  Figure 2.6: Examples of the pyroxene porphyry flow member of the BMV that demonstrate its Figure: Pyroxene porphyry flow member of the Buckhorn Mountain Volcanic Sequence at a variety of scales. Its texture, mineralogy, and character and variation in alteration and deformation (A outcrop; A1 and B blocky and massive nature can be seen in outcrop (A) and hand sample (A1). The PPL photomicrograph (A2) hand sample; PPL photomicrograph; B1 XPL photomicrograph). text and for further discussion. shows that theA2 pyroxene phenocrysts are variably altered to amphibole and/orSee chlorite occasional amygdules are presently filled with epidote and chlorite alteration. The hand sample (B) and XPL photomicrograph (B1) a deformed example show the S1/S2 foliation and the texturally destructive nature of metasomatic skarn alteration. 20  Autobreccias form a subordinate amount of the BMV. They are monolithic, only containing clasts from volcanic flows, and clast supported with minor fine grained matrix. Minor flow banding of the groundmass around the clasts has been noted. A small proportion of the autobreccias contain clasts from more than one type of flow, as seen in Figure 2.7. The clasts are subangular to subrounded and range in size from 2 mm to greater than 10 cm. The clasts and matrix have a similar composition, but the groundmass is typically more altered than the clasts. Pyroxene phenocrysts are nearly completely altered to chlorite and amphibole.  21  1cm  1mm Clasts of variable composition Groundmass  A 1cm  A1 1mm Deformed and altered groundmass  Less altered clasts  B  B1  Figure Examples of autobreccia BMV that demonstrate its texture, mineralogy, and charFigure:2.7: Flow conglomerate memberinofthe the Buckhorn Mountain Volcanic Sequence: hand acter and variation alteration and deformation (A andTypical B hand and B1 PPL sample photomisamples (A&B) andinPPL photomicrographs (A1&B1). ofsamples; the flowA1 conglomerate, A is undeformed moderate chlorite and amphibole alteration. A1 is a photomicrograph crograph). See textwith for further discussion. of sample A and shows that the clasts are of different compositions and variably altered. It Thebe intrusive members of the BMV range in composition from and can also seen that the groundmass is similar in composition to thediorite clasts.to Asmonzodiorite seen in sample B, some of the conglomerate is moderately deformed with strong biotite (brown) and typically occur in sill-, dike- and plug-like bodies. Based on texture, two varieties have been identichlorite (green) alteration of the groundmass and clasts. B1, a photomicrograph of sample B shows the groundmass is more highly deformed when compared to theforms as fied: thethat equigranular variety and the porphyritic variety.and Thealtered equigranular variety typically clasts. plug like intrusions that more commonly occur on the west side of Buckhorn Mountain, where they 22  intrude through upper carbonate member of the BMS. The equigranular variety is predominately composed of 0.4-1.5 mm, subhedral plagioclase (70 %) crystals. Mafic minerals make up about 30 % of the rock, and are dominantly pyroxene with lesser amphibole, which are both pervasively altered to chlorite and lesser biotite. Accessory minerals include quartz, potassium feldspar and up to 1 % magnetite. The rocks have also undergone minor carbonate alteration (Figure 2.8). The porphyritic variety is less abundant and less well documented than the equigranular variety. It has a similar mineralogy to the equigranular variety, but with more potassium feldspar (15 %) and quartz (5 %). The porphyritic variety is distinguished based on texture, having plagioclase and altered mafic phenocrysts that make up about 50 % of the rock, with the remainder being very fine grained chlorite-feldspar-quartz groundmass. The porphyritic intrusive BMV also has minor epidote alteration of the mafic minerals, but no magnetite (Figure 2.8). 1cm  1cm  B  A 0.5mm  Ca  0.5mm  Plag Bio  Fg calcite rich matrix  A1  Chl Qtz  Chl  Plag  B1  Ep/Zo  K-Spar  Figure 2.8: Examples of intrusive rocks comagmatic with the BMV that demonstrate its texture, Figure: Hand samples (A&B )and PPL photomicrographs (A1&B1) of the intrusive members of the Buckhorn mineralogy, and character in alteration andgreen deformation (A in and B hand sample; A1 Mountain Volcanic Sequence.and Thevariation equigranular variety is dark and massive hand sample (A). In thin and B1 PPL photomicrograph). (A, A1) The equigranular variety and (B, B1) the porphyritic varisection (A1) is seen to be predominately made up of plagioclase with mafic minerals altered to chlorite and lesser andfurther calcite. discussion. The porphyritic variety is paler green in hand sample (B) and has fine grained calcite ety. Seebiotite text for rich matrix and rare epidote/zoisite alteration as seen in thin section (B1).  23  Samples from a pyroxene porphyry flow and an equigranular and porphyritic intrusion were collected for U-Pb zircon geochronology and the latter sample was dated at 192.4 ± 1.0 Ma (Figure 3.3). This age, combined with major and trace element geochemistry from Gaspar (2005) indicates that the BMV is equivalent to the Jurassic Elise formation of the Nelson-Rossland area as described by Hoy and Dunne (1997). This resolves the long-standing disagreement over their age and place in the regional stratigraphy, as past correlations of the BMV have included a number of different units in southern British Columbia and northern Washington: McMillen (1979), Hickey (1990, 1992) and Stoffel (1990a) correlate them with the Permian-Triassic Kobau or Knob Hill Group; McMillen (1979) also correlated a portion of the BMV with the Triassic Brooklyn Formation, whereas Cheney et al. (1994) and Gaspar (2005) correlate them with the Jurassic Elise Formation. 2.3.3	Metamorphism and Skarn Alteration in the BMS and BMV Siliciclastic rocks of the BMS and volcanic rocks of the BMV are affected by chlorite and epidote alteration, indicative of greenschist facies metamorphism (Figure 2.3, Figure 2.4, Figure 2.6, Figure 2.7, and Figure 2.8). The metamorphism in the BMS has been mostly obscured by subsequent hornfels alteration resulting from contact metamorphism. In the BMV the greenschist facies metamorphism is characterised by the alteration of clinopyroxene to amphibole, epidote and chlorite. In the flows and autobreccias, the alteration preferentially affects the pyroxene phenocrysts and matrix (Figure 2.6 and Figure 2.7).This greenschist facies metamorphism occurred before the Middle Jurassic as it is overprint by hornfels alteration during contact metamorphism related to the emplacement of the Buckhorn Intrusive Suite (BIS). The contact metamorphism caused extensive hornfels alteration of the BMS rocks that is characterised by fine grained biotite growth in the siliciclastic sedimentary rocks and amphibole and pyroxene growth in the calcareous sedimentary rocks (Figure 2.3). In the dominantly carbonate rocks the contact metamorphism caused recrystallization of limestone into marble (Figure 2.5). Contact metamorphism and related hornfels alteration is less prominent in the BMV, and is characterized by similar mineralogical changes as in the clastic sedimentary rocks of the BMS (Figure 2.6). This type of hornfels alteration is typical for isochemical contact metamorphism in skarn systems (Meinert et al., 2005). Several stages of deformation affected the host metasedimentary rocks after the greenschist and contact metamorphism. This deformation produced two low angle and one steeply dipping foliation that occur in local shear zones (Figure 2.3). The deformation was previously thought to predate the BMV and intrusive rocks (McMillen, 1979), but this study documents the foliations in all rock Meso24  zoic and Paleozoic rock types. A complete description of the deformation and how it manifests in the different rock types is described below in section 2.3.4.3.1 Following the onset of deformation, metasomatic skarn alteration locally overprints the isochemical hornfels alteration in the BMS and BMV. The skarn alteration is characterized by variable prograde pyroxene, garnet and magnetite and retrograde amphibole, epidote and pyrrhotite alteration. Skarn alteration is more intense along shear zones in the BMV and BMS, most notable along the contact between the upper carbonate member of the BMS and the overlying BMV (Figure 2.2). The location skarn alteration is also spatially associated with the Granodiorite dikes. For example in the Mike’s Skarn area skarn alteration only occurs in the BMS rocks adjacent to the Granodiorite dikes, and not in the more distal BMS rocks (Figure 2.9) (G.E. Ray, Personal Communication 2010). The skarn alteration may be massive or foliated in hand sample, and in thin section is seen to be brittlely and/or ductilely deformed, suggesting that skarn alteration was coeval with the deformation.  N  Legend Clastic BMS  Fault  Carbonate BMS  Thrust fault  Buckhorn diorite Granodiorite Dike Skarn  353,500 m E  5,424,600 m N  50m  Co-ordiante system: UTM NAD27, Zone 11U  Figure: Simplified geologic map of Mike’s skarn. Skarn and gold mineralization is clearly related to the granoFigure 2.9: Simplified geologic map of the Mike’s Skarn area. Skarn alteration is spatially related to diorite dikes in the south-east corner of the map area. The clastic sediments of the BMS distal to the dikes the Granodiorite dikes in the skarn south-east the map. The clasticfrom metasedimentary rocks distal have not been metasomatically alteredcorner or goldofmineralized. Modified mapping done by G.E. Ray to2010. the dikes have not been skarn altered. Modified from mapping done by G.E. Ray (2010)  Gold mineralization is spatially associated with the skarn alteration, although a significant portion of the skarn alteration is not gold mineralized. The gold mineralization is associated with bismuth mineralization and can be petrographically shown to be part of the retrograde alteration assemblage. The mineralogical, textural and geochemical characteristics of skarn alteration and gold mineralization are the subject of Chapters 4 and 5. 25  The last stage of deformation at Buckhorn is characterised by late NNE oriented normal faults that cut the skarn alteration, deformation, and metamorphism. The two most prominent of these faults are the North Lookout fault, which offsets skarn alteration and gold mineralization, and the Toroda Creek Graben Fault, which forms the eastern margin of the Buckhorn stratigraphy. These faults affect all rock types at Buckhorn and are discussed, along with the foliations in more detail in section 2.3.4.3.1. 2.3.4	Intrusive Rocks Intrusive rocks at Buckhorn can be split into two distinct intrusive suites: The Buckhorn Intrusive Suite (BIS) and the Roosevelt Intrusive Suite (RIS). The Middle Jurassic BIS is more voluminous than the Eocene RIS, and rocks of the BIS have undergone similar alteration and deformation to the host rocks, whereas the RIS rocks are less altered and deformed and have only been subjected to the final stage of deformation. The relationship between the two intrusive suites with mineralization is disputed. Early authors, (Hickey, 1990, 1992; McMillen, 1979) suggested that mineralization was related to the BIS, while Gaspar (2005) stated it was related to the RIS. This study resolves this ambiguity. 2.3.4.1	Buckhorn Intrusive Suite (BIS) The BIS includes several distinct rock types: Buckhorn Granodiorite, Mafic Diorite, Buckhorn Diorite, Early Diorite dikes, Granodiorite dikes, and Quartz Porphyry dikes. The orientation of some of the dikes make be influenced by Paleozoic syn-sedimentray NNE striking normal faults that are inferred from drilling below the SWOZ (Figure 2.1 and Figure 2.2). 2.3.4.1.1  Buckhorn Granodiorite  The Buckhorn Granodiorite is the dominant intrusive phase in the Buckhorn area. It outcrops on the northeast side of Buckhorn Mountain and into southern British Columbia covering an area of approximately 19 km2 (Figure 2.1) (Gaspar, 2005; Massey et al., 2005; Schuster and Carutherers, 2005). The Buckhorn Granodiorite has an intrusive contact with both the BMS and BMV. The Buckhorn Granodiorite is a grey-weathering massive, heterogeneous, fine to coarse grained, heterogranular to rarely porphyritic granodiorite pluton. It contains about 30 % quartz, 40 % plagioclase, 15 % potassium feldspar, 10 % hornblende, 5 % biotite and a trace amount of magnetite, pyrite and zircon. Quartz occurs as clear, anhedral interstitial crystals. Potassium feldspar is subhedral and plagioclase is nearly euhedral with polysynthetic twinning. Biotite forms as 2-3 mm euhedral 26  hexagonal crystals and hornblende forms smaller (1mm) subhedral crystals. Trace euhedral magnetite crystals occur with biotite and potassium feldspar (Figure 2.10). The intensity of alteration that affects the Buckhorn Granodiorite is variable. Much of it is relatively unaltered with only minor chlorite-epidote alteration of the mafic minerals and sericite alteration of the plagioclase. Potassic alteration also occurs and is characterized by secondary biotite and octahedral magnetite (Figure 2.10). Locally there is intense alteration of the mafic minerals to chlorite and lesser amphibole, and of the plagioclase and lesser potassium feldspar to sericite. Where cut by the steeply northeast dipping shear zones the Buckhorn Granodiorite is intensely altered to a quartz-amphibole-chlorite assemblage (Figure 2.10). Skarn alteration is rare but does locally occur in the Buckhorn Granodiorite where it is cut by the later Granodiorite dikes (Figure 2.11). This skarn alteration is less intense that in the Early Diorite dikes, BMV or BMS and is characterized by the development of prograde clinopyroxene and retrograde amphibole. The Buckhorn Granodiorite was originally assumed to be Cretaceous (Hickey, 1990, 1992; McMillen, 1979), but U-Pb dating done in Chapter 3 confirms that it was emplaced during the Middle Jurassic with an age of 170.4 ± 1.1 Ma. (Figure 3.5) This allows its correlation with several plutons of similar age and composition in the southern Canadian cordillera: The Cahill Creek Pluton (168.9 ±9 Ma) in the Hedley district (Ray and Dawson, 1994, and references therein); The Nelson (167 Ma), Trail (169±3 Ma) and Bonnington (167 Ma) Plutons and the Rossland Monzonite (167.5±0.5 Ma) in the Nelson-Rossland area (Hoy and Dunne, 2001, and references therein) (Figure 1.1). It was originally proposed as the mineralizing intrusive (Hickey, 1990, 1992; Jones, 1992; McMillen, 1979), but based on the skarn alteration around cross-cutting dikes, it is now shown to predate the skarn alteration.  27  25cm  1cm  A  A1  2mm  0.5mm Hbl  Mag Qtz  Kspar  Bio  Zrn  Hbl-chl Kspar  Qtz  Plag  A2  B 1mm  Amph-Chl  Kspar ste  ep  C  Plagsericite  ly  Plag  Qtz  dip  pin  gf ol  Po  C1  Figure: Buckhorn Granodiorite at a variety of scales. At outcrop scale (A & C) its massive or rarely foliated  Figure 2.10: Examples of the Buckhorn Granodiorite that demonstrate its texture, mineralogy, and nature and weathering style can be seen. The hand sample (A1) and PPL photomicrograph (A2) show a character and variation in alteration deformation (A and C outcrop; sample; A2mafic and typical undeformed and weakly alteredand example with chlorite, secondary biotiteA1 andhand magnetite of the C1 PPL photomicrograph; B XPLofphotomicrograph). See photomicrograph text for further discussion. minerals and mild sericite alteration the plagioclase. The XPL B is of a more altered sample ,BS074, with intense sericite and abundant chlorite alteration of the plagioclase and mafic minerals respectively. The PPL photomicrograph C1 of sample BS057 shows the intense quartz-amphinole-chlorite alteration that exsists in samples affected by the steeply dipping S3 foliation.  28  10cm  Skarn altered Buckhorn granodiorite  1cm  A granodiorite dike  B  A2  0.5mm  0.5mm  Plag  Px  Amph  A3  Plag  Qtz-Felds groundmass  Qtz  Kspar  Mag-hem  Px-amph  B2  Gar  Po  Amphchl  Figure 2.11: Example of a Granodiorite dike cross-cutting the Buckhorn Granodiorite that demonFigure: Example of a granodiorite dike cutting the Buckhorn granodiorite. Weak skarn strates the mineralogy and texture of the local skarn alteration (A/B outcrop; A2 hand sample; A3 alteration can be seen in outcrop (A), hand sample (A2) and PPL photomicrograph (A3). and B2 PPL photomicrograph). See text for further discussion. The skarn alteration is characterized by minor (>5 % of rock) prograde clinopyroxene and magnetite, and retrograde 2.3.4.1.2 Mafic Diorite amphibole and hematite. The granodiorite dike locally has significant (~3%) sulfide content (Po>>Py>Cpy), which causes its rusty appearance in The (B). Mafic Diorite is a volumetrically minor and poorly rockalteratype found outcrop In PPL photomicrograph (B2) it is seen that inunderstood addition tointrusive the sulfide granodiorite is also weakly altered with rarelocal (~1intrusive %) pyroxeneintion the the Buckhorn area. It dike was identified duringendoskarn a geochemical study of the rocks and its amphibole and trace (<0.1 %) garnet. extent is not known. It is defined based on its high titanium and low silica content compared to the other intrusive rocks (M. Deal, personal communication 2011). The Mafic Diorite occurs as dikes that cross cut the Buckhorn Granodiorite, however, it has not been found in contact with any other intrusive rocks, so its age cannot be further constrained. 29  The Mafic Diorite is black to very dark green in hand sample with up to 40 % euhedral white plagioclase crystals that are 400 μm to 2.5 mm in length. Amphibole (5 %) forms 100 μm, and rarely 500 μm, anhedral to subhedral crystals. Primary clinopyroxene (4 %) forms 100 μm anhedral to subhedral crystals. Ilmenite is the only primary opaque mineral and makes up about 1 % of the rock, as irregular wormy grains spatially associated, or possibly intergrown, with primary clinopyroxene and amphibole (Figure 2.12). Amphibole is the most abundant alteration mineral (50 %) replacing the larger primary amphibole crystals with many smaller fibrous crystals and rarely pseudomorphing the primary amphibole (Figure 2.12). Plagioclase is also affected by amphibole alteration that preferentially alters the edges of the crystals giving them a rounded appearance (Figure 2.12). Sericite alteration of the plagioclase also occurs, and is typically pervasive and weak, affecting less than 5 % of a plagioclase crystal (Figure 2.12). Pyrrhotite is the most abundant sulfide with less chalcopyrite and pyrite; together the sulphides make up a trace amount of the rock (Figure 2.12). The Mafic Diorite is locally altered to epidote in centimetre scale bands, and rarely hosts minor prograde pyroxene skarn alteration at its contact with skarn altered metasedimentary rocks. In general the Mafic Diorite appears massive and undeformed in hand sample, but plagioclase and other primary minerals have minor fractures. The age of the Mafic Diorite is unknown. Its age can be constrained by its cross-cutting relationship with the Buckhorn Granodiorite, which requires that it was emplaced after 170.4 ± 1.1 Ma (Figure 3.5), and the presence of skarn alteration suggests that it is older than the Granodiorite dikes at 167.5 ± 0.8 Ma (Figure 3.15). Based on these ages constrains the Mafic Diorite is interpreted as an early dike phase of the BIS.  30  1cm  1mm  Plag Ilm  B1 Px-Amph-Chl  A Plag  250µm  B  Plag  250µm  Px  Px Amph  Ilm  Amph Ilm  B1 500µm Cpy  B2 500µm  Cpy Chl  Chl Amph  Amph Plag  Po  Plag  Po Px  Px C  C1  Figure 2.12: Examples of the Mafic Diorite that demonstrate its texture, mineralogy, and character of alteration and deformation. (A hand sample; B and C PPl photomicrograph; B1 XPL photomiFigure X.x: Examples of the mafic diorite (hand sample A; PPl photomicrograph crograph; B2 and C1 RL photomicrograph). See text for further discussion.  B; XPL photomicrograph B1; RL photomicrograph B2). (A) the mafic diorite is typically black to darkDiorite green in hand sample with 2.3.4.1.3 Buckhorn  The Buckhorn Diorite is a massive mafic intrusive phase that covers 0.8 km2 and crops out on the northeast side of the Buckhorn Mountain between the Buckhorn Granodiorite and the Magnetic Mine skarn (Figure 2.1). It has a gradational contact with the Buckhorn Granodiorite to the north and east (Figure 2.1). On its southwestern side the Footwall Mylonite, a major steeply northeast dipping shear zone, puts the Buckhorn Diorite in contact with the BMS. It has an intrusive contact with metasedimentary and metavolcanic rocks along its southern margin. The Buckhorn Diorite is cross 31  cut by the Quartz Porphyry dikes, however its timing relative to the Early Diorite dikes is unknown. While it is labelled as diorite it is compositionally a quartz-monzodiorite, but for the sake of consistency with older work it will continue to be referred to as the Buckhorn Diorite. The Buckhorn Diorite is a coarse-grained heterogranular to porphyritic quartz monzodiorite made up of coarse-grained amphibole (25 %) and biotite (10 %), fine grained plagioclase (15 %) and alkali feldspar (5 %) in a matrix of fine grained quartz (7 %), plagioclase (25 %) and alkali feldspar (13 %). The Buckhorn Diorite has undergone similar alteration to the Buckhorn Granodiorite. Amphibole and biotite are altered to secondary hydrothermal amphibole, chlorite and epidote. Trace euhedral magnetite is now altered to hematite, and occurs with the secondary mafic minerals (Figure 2.13). Current mapping shows a gradational contact between the Buckhorn Diorite and the Buckhorn Granodiorite (Figure 2.1). As well, the mineralogical differences, more abundant mafic minerals and less quartz when compared to the Buckhorn Granodiorite, are those expected in the margin of a zoned pluton (Pitcher, 1997). U-Pb zircon geochronology also supports the classification as a marginal phase, showing that the Buckhorn Diorite 169.0 ± 0.9 Ma, (Figure 3.13) is the same age as the Buckhorn Granodiorite. Early workers (Hickey, 1990, 1992; McMillen, 1979) drew a similar conclusion from their work and labelled the Buckhorn Diorite as a border phase of the Buckhorn Granodiorite, but that genetic relationship was later questioned (Gaspar, 2005). However based on mapping and geochronology from this study, the Buckhorn Diorite is classified as a marginal phase of the Buckhorn Granodiorite.  32  1cm  0.5mm Plag Amph2 Mag Amph1 Qtz-Felds  A  A1  Figure 2.13: Examples the Buckhorn Diorite thatsample demonstrate its PPL texture, mineralogy, and charFigure:ofBuckhorn diorite in hand (A) and photomiacter of alterationcrograph and deformation. (A hand sample; A1 PPL photomicrograph). (A1). Its hetrogranular to porphyritic nature can beSee text for further discussion seen at both scales. In thin section (A1) minor secondary mag-  netite is clearly associated with amphibole alteration, and Early Dioriteplagioclase Dikes occasional phenocrysts are sericite altered. The Early Diorite dikes are lithologically similar to the Buckhorn Diorite and are distinguished  2.3.4.1.4  from the Buckhorn Diorite on the basis of their porphyritic and endoskarn altered nature (G.E. Ray, personal communication, 2009). They appear to be volumetrically the smallest of the BIS rocks, and their intrusion style is poorly defined. They form predominately in dikes, but may also form as sills and small plugs. They were not found in contact with other intrusive rocks, and their distribution is unknown. The Early Diorite dikes are porphyritic and texturally distinct from the Buckhorn Diorite with coarse-grained lath-shaped plagioclase phenocrysts that form about 30 % of the rock. The plagioclase phenocrysts have abundant, very fine grained inclusions of amphibole and/or pyroxene. Coarse grained clinopyroxene phenocrysts are also present, forming about 15 % of the rock. The remaining 55 % of the rock is made up of sub-micron scale feldspar/quartz groundmass with very fine grained amphibole and/or pyroxene inclusions similar to those in the plagioclase phenocrysts (Figure 2.14). The Early Diorite dikes are the most intensely endoskarn altered rocks in the Buckhorn area. They often host pervasive garnet, amphibole and epidote alteration and minor gold mineralization 33  (Figure 2.14). Less altered examples are characterized by moderate amphibole alteration of pyroxene phenocrysts and sericite alteration of the plagioclase phenocrysts (Figure 2.14). The Early Diorite dikes predate skarn mineralization and have been dated by U-Pb zircon geochronology to 169.4 ± 1.3 Ma (Figure 3.10). Based on the mineralogical similarities with the Buckhorn Diorite, and U-Pb geochronology they are interpreted as an early dikes phase of the BIS.  34  1cm  25cm  Garnet Skarn  Early Diorite dike  A1  A 1mm Qtz  1cm  Fg plag  Cpx Plag  Zrn Ep vein  A2  B  Figure 2.14:Figure: Examples ofdiorite the Early Diorite that the intrusive Early dikes at a dikes variety of demonstrate scales. In outcrop (A) thestyle, texture, mineralogy, character and variation in alteration and deformation.(A outcrop; A1dike-like and B hand sample; proximity to prograde garnet skarn and its generally vertical A2 PPL photomicrograph). text for The further discussion. contact can be See observed. hand sample (A1) and thin section (A2) were taken from the red circle in “A”. Late epidote veins and the plagioclase (white) and pyroxene (dark green) phenocrysts are seen in the hand samples(A1&B), the groundmass of the latter is less altered and it lacks epidote veins. The PPL photomicrograph shows strong chlorite and amphibole alteration of the clinopyroxene phenocrysts and sericite alteration of the plagioclase. Rare quartz and trace zircon crystals occur.  35  2.3.4.1.5  Granodiorite dikes  The Granodiorite dikes have a NNE or WNW orientation and occur throughout the Buckhorn area (Figure 2.1) (Gaspar, 2005). The Granodiorite dikes cross-cut the BMS, BMV, and the Buckhorn Granodiorite, but they have not been found in contact with the other types of dikes, so the relative timing could not be determined. The Granodiorite dikes are mineralogically and texturally similar to the Buckhorn Granodiorite, but contain quartz phenocrysts (Figure 2.15). Most of the dikes are massive with minor chlorite alteration of the mafic minerals, however, a few intensely deformed examples have been documented. When altered, the quartz phenocrysts, and very rarely feldspar grains, are the only original minerals to survive, and the rest of the rock is made up of very fine grained to aphanitic recrystallized quartz and lesser biotite, muscovite, and chlorite. The Granodiorite dikes host minor skarn alteration, which is characterized by a pyroxene-garnet-amphibole-pyrrhotite-chalcopyrite mineral assemblage (Figure 2.11). In addition to being skarn altered, the Granodiorite dikes are also spatially related to the location of skarn alteration and gold mineralization in other rock types. As documented in sections 2.3.4.1.1 and 2.3.3, where the Granodiorite dikes cross cut the Buckhorn Granodiorite (Figure 2.11) or the BMS (Figure 2.9) there may be local skarn alteration. The Granodiorite dikes were dated at 167.5 ± 0.8 Ma (Figure 3.15), confirming their association with the Buckhorn Granodiorite. Based on their spatial association with local skarn alteration their age also provides a maximum age for skarn alteration at Buckhorn.  36  20cm  1cm  fol ngle a w Lo  A1 ing  fol  1mm  Ste e  ply -di  pp  A  Low angle fol Bio  1cm  Qtz  A2  Plag  1mm Plag Qtz  B  Amph  B1  Figure 2.15: Examples of the Granodiorite dikes that demonstrate the intrusive style, texture, mineralogy, character and variation in alteration and deformation. (A and B1 hand sample; B outcrop; B2 PPL photomicrograph). See text for discussion.(B,B1,B2) granodiorite dikes. The undeFigure: Examples of undeformed (A)further and deformed  formed hand sample (A) is mildly sericite and chlorite altered with a massive texture. The outcrop of a deformed granodiorite dike (B) shows the low angle foliation and the hand sample (B1) shows the strong biotite alteration. The PPL photomicrograph (B2) shows that the biotite and other micaceous minerals are aligned to define two foliations at a high angle to each other. As seen in B1 and B2 , quartz and rare feldspar phenocrysts are the only original minerals remaining. 37  2.3.4.1.6  Quartz Porphyry Dikes  The Quartz Porphyry dikes are the most abundant dikes in the Buckhorn area, they occur in NNE and WNW striking swarms (Figure 2.1). The Quartz Porphyry dikes cross-cut the BMS, the BMV, the Buckhorn Granodiorite and the Buckhorn Diorite. Relationships with the Early Diorite dikes or the Granodiorite dikes are not apparent, so the relative timing of the different types of dikes is uncertain. The Quartz Porphyry dikes are white to tan coloured, with distinctive 0.5-1.5 mm subhedral quartz phenocrysts. The groundmass is made up of 10-20 μm anhedral quartz crystals and lesser plagioclase, potassium feldspar and muscovite (Figure 2.16). The Quartz Porphyry dikes are pervasively altered with minor carbonate and muscovite-epidote-chlorite alteration. They are locally skarn altered, hosting a garnet-epidote assemblage (Figure 2.17). Gaspar (2005) reported that gold was recovered during heavy mineral separation indicating that they may be mineralized. The skarn alteration and mineralization is very rare and has only been documented in a small number of drill holes (~5). The Quartz Porphyry dikes were dated to 163.57±0.8 Ma (Gaspar, 2005), and are the youngest of the Jurassic intrusive rocks.  38  1m  1cm  Foliated marble  Foliated QP dike  A1  A Musc  0.5mm  0.5mm Amph-chl  Qtz Musc  ing  ipp  ly d  ep  ste  Fract’d Qtz  fol  A2  low angle fol  Def’d Qtz w/ und extinct  A3  Figure 2.16: Examples of the Quartz Porphyry dikes that demonstrate the intrusive style, texture, Figure: Foliated and altered dike at in a variety of scales. sharp, steeply outcrop; dipping, intrusive with mineralogy, character and QP variation alteration and The deformation.(A A1 handcontact sample; A2 the A3 upper marble member of the BMS seen at outcrop scale (A). At hand sample scale (A1) the white and XPL photomicrograph). Seecan textbefor further discussion. to tan colour and very fine grained nature of the QP dikes is evident. The XPL photomicrographs, A2 and A3, show strong muscovite alteration that is aligned to define the foliations. They also show deformed and possibly fractured quartz phenocrysts, and weak amphibole and later chlorite alteration.  39  1cm  QP Dike  Ep  Gar  A 1mm  QP Dike  B  Cal Gar  Ep  Amph-chl  Figure skarnporphyry alteration(QP) in the Quartz Porphyryofdike demonstrate alteration the texture, Figure:2.17: SkarnExample altered of quartz dike. The intensity thethat garnet-epidote is seen in both handand sample (A) and thin section (B). The PPLsample; photomicrograph also shows mineralogy, character variation in skarn alteration.(A hand B PPL photomicrograph). minor after amphibole and calcite alteration. See textchlorite for further discussion. 2.3.4.2	The Roosevelt Intrusive Suite (RIS) Originally mapped as part of the Buckhorn Granodiorite or border diorite (Hickey, 1990, 1992; McMillen, 1979), the Roosevelt Intrusive Suite (RIS) is made up of the Roosevelt Granodiorite and the Pink Granite. The Roosevelt Granodiorite is a small massive intrusion (0.03 km2) on the east side of Buckhorn Mountain that intrudes the Buckhorn Granodiorite, BMS, and BMV (Figure 2.1). The Roosevelt Granodiorite is similar to the Buckhorn Granodiorite, but it is darker in hand sample with minor mafic clusters that may be xenoliths but do not have chilled or resorbed margins (Figure 2.18). The Roosevelt Granodiorite appears equigranular in hand sample, but in thin section it is clearly porphyritic with abundant plagioclase, amphibole and biotite phenocrysts in a fine-grained feldspar and quartz matrix. The rock also has minor primary magnetite (<1 %). 40  The Roosevelt Granodiorite is less altered than the nearby Buckhorn Granodiorite, having only minor secondary biotite and mild secondary amphibole and chlorite alteration of the primary mafic minerals. The Roosevelt Granodiorite was dated at 50.5 ± 3.0 Ma (Figure 3.17). The Eocene age suggests that it is the intrusive equivalent of the Sanpoil volcanic rocks that fill the adjacent Toroda Creek half graben (Gaspar, 2005; Suydam and Gaylord, 1997). Based on petrography, geochemistry and age data, the Roosevelt Granodiorite is correlated with the Devils Elbow Suite Intrusions that are part of the Colville Igneous Complex (Holder and McCarley Holder, 1988; Suydam and Gaylord, 1997). 50cm  1cm  A1  A 1mm  0.5mm Bio  Mag w/ amph alt  Plag  Amph alt  Fg felds/qtz  Plag  Hbl Hbl  A2  2nd Bio  Mag w/ amph alt  A3  Figure: 2.18: Roosevelt granodiorite. outcrop (A) and hand sample its massivethe andintrusive undeformed nature is Figure Examples of the InRoosevelt Granodiorite that (A1) demonstrate style, texture, observed. An example of a mafic cluster is visible at the top left corner of the hand sample (A1). The PPL mineralogy, and character of alteration and deformation.(A outcrop; A1 hand sample; A2 PPL phophotomicrograph (A2) shows the primary biotite, hornblende and plagioclase phenocrysts and fine grained tomicrograph; A3 in XPL See textAlteration for further discussion. primary magnetite the photomicrograph). quartz-feldspar groundmass. minerals including secondary biotite,  magnetite with amphibole and minor sericite alteration are also visible. The XPL photomicrograph (A3) more clearlyThe shows theGranite porphyritic of the rock and mineralogy groundmass. Pink has nature only been found in athe small number of ofthe diamond drill holes in the Roosevelt  Mine area, and no outcrops were found there during detailed mapping. Gaspar (2005) found the Pink Granite in contact with the Buckhorn Granodiorite in a drill hole and described it as medium-grained pink coloured equigranular granite with minor mafic content.  41  2.3.4.3	Alteration in the Intrusive Rocks During the development of hornfels alteration in the metasedimentary and metavolcanic rocks, the BIS rocks underwent various types of alteration. The first stage is characterized by secondary biotite, magnetite and lesser potassium feldspar (Figure 2.10).The second stage is characterized by the development of amphibole and lesser plagioclase (Figure 2.10, Figure 2.12, and Figure 2.13). The final stage of alteration is characterized by the replacement of hornblende by chlorite, amphibole and epidote and the replacement plagioclase by sericite (Figure 2.10, Figure 2.12, Figure 2.13, and Figure 2.16). Based on the mineralogical characteristics the three stages are classified as potassic, sodic-calcic and propylitic alteration. All three types of alteration are unevenly developed in the BIS rocks, and are best preserved in the Buckhorn Granodiorite and the Buckhorn Diorite (Figure 2.10, Figure 2.13, and Figure 2.14). Skarn alteration is locally developed in the BIS rocks, and is the most intense in the Early Diorite dikes and the Buckhorn Diorite, but has been documented in every rock type. Skarn alteration is dominantly composed of pyroxene-garnet prograde alteration and amphibole-epidote-pyrrhotite retrograde alteration (Figure 2.11 and Figure 2.17). Most of the skarn alteration is texturally destructive, leaving only small patches with igneous textures, and masking any earlier alteration. Skarn alteration in the BIS is spatially related to the emplacement of the Granodiorite dikes. The RIS is much less altered than the other units in the Buckhorn area. The Roosevelt Granodiorite has undergone minor chloritic, sericite and potassic alteration, which is characterized by chlorite and sericite alteration of the mafic minerals and feldspars, along with minor secondary biotite development (Figure 2.18). The relative timing of the three alteration assemblages is unclear but likely progressed from potassic to sericite and chloritic alteration. Gaspar (2005) described the Pink Granite as being even less altered than the Roosevelt Granodiorite, having only undergone deuteric alteration. 2.3.4.3.1  Deformation  Deformation in the Buckhorn area is characterized by two low angle and one steeply northeast dipping foliations, and NNE normal faults. The low angle foliation and steeply dipping foliations occur in shear zones throughout the Buckhorn area, in all rock types except the Eocene RIS. The relative timing of skarn alteration and the foliations is ambiguous suggesting that they are coeval (Figure 4.4). The NNE normal faults post date the foliations and occur in all rock types, cutting all of the aforementioned metamorphism, alteration and deformation. 42  The two low angle foliations, and the corresponding shear zones, are the earliest, most prominent, and most abundant. Both low angle foliations are nearly flat lying and are sub-parallel to each other, and the second foliation may crenulate the first foliation (Figure 2.19). The exact orientation of these foliations has not been determined, so when only one low angle foliation is visible it is impossible to determine which one it is. The most prominent low angle shear zone occurs near the contact between the upper carbonate member of the BMS and the overlying BMV. This shear zone is nearly flat lying, cross cuts the contact, and affects both the carbonate and the volcanic rocks. The steeply northeast dipping foliation and shear zones are less abundant than the low angle variety. The most prominent of the steeply dipping shear zones is the Footwall Mylonite, which occurs between the SWOZ and the Gold Bowl, and also forms the southwestern edge of the Magnetic Mine (Figure 2.1). Elsewhere the northeast dipping shear zones cut the Buckhorn Granodiorite and related rocks. In the clastic metasedimentary, volcanic, and intrusive rocks the foliations are defined by the alignment and deformation of metamorphic phyllosilicate minerals, whereas in the carbonate rocks the foliations are denoted by grain size variation and compositional bands (Figure 2.3, Figure 2.6, Figure 2.15, Figure 2.16, Figure 2.5, and Figure 2.19). In the intrusive rocks the foliations may locally be accompanied by increased propylitic alteration (Figure 2.10). The foliations have similar characteristics in the skarn alteration, where they are defined by the alignment and deformation of amphibole (Figure 4.4).  43  1cm  1mm  low  ang  le fo  l2  low angle fol 1  1 le fol  ng low a low ang  le fol 2  A  1cm  20cm  Metasomatic skan alteration l  le fo  ng ow a  l  A1  low angle fo l  C B Figure: Low angle S1 S2 foliations a variety of demonstrates scales. The S1their andcharacteristics S2 foliations at a Figure 2.19: Examples of and the two low angle at foliations that mayofbescale distinct indifferent hand sample and thin section (A and A1),of orfoliated if the sample is more and variety and in rock types. (A and C hand samples metasedimentary highly deformed they be indistinguishable (C).metasedimentary Sample C also shows texturaly metavolcanic rocks; A1 PPLmay photomicrograph of foliated rock; Bthe outcrop of a folinature ateddescructive Granodiorite dike).of metasomatic skarn alteration. In outcrop (B) the S1/S2 foliation often forms a very planar surface. The final stage of deformation to affect the BMS is characterized by steeply-dipping NNE  oriented normal faults. The normal faults usually have low displacement (<30 m), and are more abundant on the east side of the Buckhorn area (Figure 2.1). The NNE normal faults cut all of the 44  rocks at Buckhorn, and postdate all of the metamorphism, alteration and deformation. The faults are subparallel to the Toroda Creek Graben Fault that forms the eastern margin of the Buckhorn rocks. The North Lookout Fault is the most important of the NNE normal faults as it offsets skarn alteration in the SWOZ (Figure 2.2). Several NNE striking normal faults can be inferred from drill hole data below the SWOZ, however some of these faults do not appear to offset the adjacent volcanic rocks (Figure 2.2), suggesting that some of the faults were active prior to volcanism during sedimentation in the late Paleozoic (Gaspar, 2005). Similar syn-sedimentary faults have been recognized in the Late Paleozoic strata of Nelson-Rossland area (Hoy and Dunne, 1997) and the Hedley district (Ray and Dawson, 1994).  2.4	 Conclusions The Buckhorn host rocks can be correlated with a number of other units of the Quesnel Terrane in southern British Columbia and northern Washington. The BMS is age equivalent to the Harper Ranch, Attwood, and Anarchist Groups, as well as the Mount Roberts Formation. All of these units are made up of carbonate, siliciclastic and volcanic rocks indicative of an arc environment, which is the expected depositional setting for late Paleozoic rocks in the Quesnel Terrane (Beatty, 2003; Cheney et al., 1994; Hoy and Dunne, 1997). Unconformably overlying the BMS is the BMV, which is correlated with the Elise formation. The BMV and the Elise formation have the same age (Figure 3.3), chemistry (Gaspar, 2005), and are composed of arc-related volcanic rocks that are characteristic for early Mesozoic rocks in the Quesnel Terrane (Hoy and Dunne, 1997). Prior to the emplacement of the BIS in the Middle Jurassic the host metasedimentary and metavolcanic rocks underwent greenschist facies metamorphism. Intrusion of the BIS started with the Buckhorn Granodiorite (170.4 Ma), followed by the Mafic Diorite, Early Diorite dikes (169.4 Ma), Buckhorn Diorite (169.0 Ma), Granodiorite dikes (167.5 Ma), and finally the Quartz Porphyry dikes (163.3 Ma). Contact metamorphism related to the Buckhorn Granodiorite caused hornfels alteration in the host rocks that over print the greenschist facies metamorphism and obscured the original depositional textures in the BMS and to a lesser extent in the BMV. Two low angle and one steeply dipping foliations where developed in shear zones during the emplacement of the BIS, locally affecting all rock types. The shear zones were planes of higher permeability that focused the flow of skarn forming fluids, and partially controlled the location of skarn alteration. 45  Skarn alteration is related to the intrusion of the Granodiorite dikes, and has a diverse mineralogy that is made up of variable amounts or prograde pyroxene, garnet and magnetite, followed by retrograde amphibole, epidote, and pyrrhotite alteration. The association of skarn alteration with the Granodiorite dikes is similar to the Nickel Plate deposit in the Hedley Mining camp where dikes and sills acted as conduits for the mineralizing fluids (Ettlinger et al., 1992).The skarn alteration is coeval with the three foliations and both overprints foliated rock and is itself foliated. The skarn alteration also overprints the preceding stages of metamorphism and hornfels alteration, and affects all of the BIS rocks. The Roosevelt Granodiorite of the RIS intruded the Buckhorn Granodiorite, Buckhorn Diorite, BMS and BMV in the Eocene (50.5 Ma). Following potassic, sodic-calcic and propylitic alteration the RIS was cut by the NNE normal faults, which also form the eastern margin of the Buckhorn rocks and offset skarn alteration in the SWOZ.  46  Chapter 3.	 U-Pb Geochronology 3.1	 Introduction The aim of this study is to confirm the age of the metavolcanic host rocks and local intrusive rocks at the Buckhorn gold skarn, and to constrain the age of deformation and skarn alteration at the deposit. The outcome of this work will aid in the proper correlation of the host rocks with the regional stratigraphy, and has implications for the genesis of the Buckhorn gold skarn. Age dating was accomplished through the determination of isotopic ratios of U-Pb in zircon by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The U-Pb isotopic system in zircon was chosen for its high closure temperature, about 900° C (Cherniak and Watson, 2001), which allows it to retain its magmatic age through metamorphism and a give reliable crystallization age for igneous rocks. The high closure temperature is necessary because the area experienced regional extension and intrusion of several granitoid stocks during the Eocene, which could have reset isotopic systems with lower closure temperatures. LA-ICP-MS was chosen for its high spatial resolution, which makes it possible to avoid the inclusions and fractures in the zircons, and its short analysis time that allows a larger number of samples to be analysed (Kosler and Sylvester, 2003). 3.1.1	Background The Buckhorn gold skarn is hosted in a package of clastic and carbonate metasedimentary rocks, known as the Buckhorn Mountain Sequence (BMS), which are overlain by a package of metavolcanic rocks known as the Buckhorn Mountain Volcanic Sequence (BMV) (Gaspar, 2005). These rocks host several igneous stocks and associated dikes. Skarn alteration was preferentially developed along the foliated contact between the BMV and BMS and is spatially related to the Granodiorite dike phase of the Buckhorn Intrusive Suite (BIS) (Figure 2.2 and Figure 2.11). A detailed description of the Buckhorn area stratigraphy, including descriptions of the rock types and their geologic relationships, is in Chapter 2. Five igneous rock types have previously been dated: (1) the Buckhorn Granodiorite, (2) the Quartz Porphyry dikes, (3) the Granodiorite dikes, (4) the Roosevelt Granodiorite and (5) the Pink granite (Gaspar, 2005). The ages of four additional igneous rock types were previously unknown: (6) the Buckhorn Diorite, (7) the Early Diorite dikes, (8) the Mafic Diorite, and (9) an intrusive equivalent of the BMV. Samples were collected from all of the aforementioned intrusive rocks as well as two examples of intrusions comagmatic with the BMV, except the Pink granite. Samples were also collected from a 47  pyroxene porphyry flow member of the BMV and from a conformable rhyolite layer in the BMS to constrain the age of these host rocks (Figure 2.1 and Table 3.1). 3.1.2	Sample Preparation The samples were run through a crushing and grinding circuit that reduced the samples to approximately silt-sized fragments. They were then processed on a Wilfley® Concentrating Table to remove the least dense minerals, and separated using Methylene Iodide to remove minerals with a density less than 3.32 g/cm3. The dense minerals were then run through a Frantz® Magnetic Separator to remove the magnetic minerals. Zircons were picked by hand from the least magnetic separates with the aid of a binocular microscope. 25 to 55 of the largest, clearest and most inclusion free zircon grains from each sample were chosen for analysis. Depending on the number of grains selected, two or three samples were mounted in an epoxy cylinder with approximately 10 internationally recognized standards (Plešovice Zircon, Sláma et al., 2008), and 10 in-house standards. The cylinders were then polished and carbon-coated and taken to the scanning electron microscope (SEM) for imaging with back scatter electrons (BSE). Inclusions detected during BSE imaging were avoided during the laser ablation. Before the isotopic analysis the mounts were re-polished and washed with dilute nitric acid for ten minutes and then rinsed with high purity water. 3.1.3	Analytical Method and Data Reduction The analyses were performed at the Pacific Centre for Isotopic and Geochemical Research (PCIGR), at the University of British Columbia, employing the standard PCIGR methods as described by Tafti et al. (2009) and summarized below. Following imaging the mounts were loaded into a New Wave “Supercell” ablation chamber and ablated using a New Wave 213 nm Nd-YAG laser. The samples were then analyzed using a Finnigan Element2, single collector, double-focusing, magnetic sector high resolution ICP-MS. Two zircon reference standards were used, an internationally recognized standard, Plešovice Zircon (Sláma et al., 2008) with an age of 337 Ma, and an internal standard with an age of 197 Ma. Twenty or more analyses were performed per sample using a 15 to 30 μm spot with 45 % laser power. High quality portions of each grain, free of alteration, inclusions, or inherited cores were selected for analysis. Line scans were done rather than spot analyses in order to minimize within-run elemental fractionation. Background levels were measured with the laser shutter closed for ten seconds, followed by data collection with the laser firing for approximately 35 seconds. 48  Corrections for mass and elemental fractionation were made by bracketing analyses of unknown grains with replicate analyses of the Plešovice zircon standard. A typical analytical session at the PCIGR consists of four analyses of the Plešovice standard zircon, followed by two analyses of the in-house zircon standard, five analyses of unknown zircons, two standard analyses, five unknown analyses, etc., and finally two in-house zircon standards and four Plešovice standard analyses. The 197 Ma in-house zircon standard was analysed as an unknown in order to monitor the reproducibility of the age determinations on a run-to-run basis. The time-integrated signals were analysed using GLITTER software (Griffin et al., 2008; van Achterbergh et al., 2001), which automatically subtracts background measurements, propagates all analytical errors, and calculates isotopic ratios and ages. The time-integrated data was carefully examined to identify and avoid portions of the signal that reflected lead loss, the analysis of older inherited cores or altered zones in the zircon or a combination of all three. Final interpretation and plotting of the analytical results employed the ISOPLOT v. 4.13 software of Ludwig (2009). The ages reported are concordant ages and weighted average 206Pb/238U ages. These ages were chosen as they are the most accurate and precise for Phanerozoic rocks. Errors for the calculated isotopic ratios are given at the 1 sigma level and errors on calculated ages are given at the 2 sigma level. Analyses were excluded from the final interpretation with ISOPLOT if they fell into one of five categories discussed in Campbell et al. (2006).These categories are as follows: analysis that exhibit (1) internal age zoning, (2) discordant ages, (3) indication of the zircon being a xenocryst, (4) indication of the zircon being affected by post crystallization lead loss, or (5) evidence of subtle inheritance. The objective criteria modified from Campbell et al. (2006) were used to reject analyses and return the most accurate and precise age possible. Internal age zoning was detected by analysing the time integrated signals in GLITTER prior to interpretation in ISOPLOT. It was performed by choosing portions of the signal with consistent  Pb/238U and  206  207  Pb/235U ratios, and portions with mixed signals were discarded. Analyses were  deemed discordant if their 207Pb/235U age divided by their 206Pb/238U age was greater or less than 1 ± 0.1. Approximately 8 % of the analyses were discordant. If an analysis returned an age that was clearly older than the crystallization age of the rock it was deemed a xenocryst. With the exception of sample BS076, this was easily determined because the xenocrysts were usually at least 180 million years older than the average age determined for the rock. The specifics of BS076 are discussed later in the chapter. 49  Evidence for post crystallization lead loss and/or subtle inheritance affecting the zircons was present in analyses from each sample. The subtle lead loss or inheritance was objectively detected using linear cumulative probability plots. Linear cumulative probability plots are designed to show data with a normal distribution in a straight line with a positive slope. Different populations will appear as straight lines separated by inflection points. Figure 3.1 shows a linear cumulative probability plot for sample BS057 that shows the analysis of zircon that have been affected by both subtle lead loss and inheritance. Following dating, select zircons were imaged using cathodoluminescence microscopy (CL). This was done on samples where complex inheritance was detected in the age results. Imaging with CL showed more complex growth patterns than was detected with BSE imaging and aided the interpretation of those samples. In retrospect, due to the complexity of the growth patterns detected by post dating CL imaging and the inheritance and leads loss detected in the age dates, it would have been beneficial to image all the zircons from all the samples with CL prior to dating. Unfortunately a CL imaging apparatus was not available to use at the time of dating. Table 3.1: Summary of analysis quality Unit/ Sample no.  Pb/238U Age (Ma)  206  ±2σ(Ma)  Analyses Used (n)  Total Analyses (N)  Grains Excluded MSWD  Xenocrysts  Discord  Lead Loss  Inheritance  Intrusion comagmatic with the BMV BS064  193.5  1.2  37  55  1.4  4  1.7  12  20  2  2 2  10  4  Buckhorn Granodiorite (BGdi) BS048  170.4  BS074  167.8  1.5  8  20  0.91  BS057  165.99  0.97  15  20  0.86  6 2  8  1  3  1  2  1  Early Diorite (EDiD) BS046  168.15  0.7  14  20  0.32  BS075  169.3  1.5  14  20  1.2  3 3  0.86  15  20  1.05  1  0.72  17  20  0.98  1  2.9  2  20  0.025  12  6  Total  134  215  20  17  32  12  Percentages  62.3%  9.3%  7.9%  14.9%  5.6%  3  Buckhorn Diorite (BDi) BS060  168.94  1  3  Granodiorite dikes (GDiD BS059  167.51  2  Roosevelt Granodiorite (RGDi) BS076  50.8  50  171  BS057: Buckhorn Granodiorite (Altered)  169 Inflection Points  Age (Ma)  167 165  Legend  163  Subtle inheritance Discordant Concordant  161  Subtle Lead Loss  159  99.9  99  95  90  70 80  50  20 30  10  5  1  .1  157  Probability Figure 3.1: Linear cumulative probability plot of sample BS057. The analyses affected by subtle lead  Figure X.2 Linear cumulative probability plot of colour sample scheme BS057 with by subtle figures. lead loss and loss and inheritance are highlighted. The willanalyses be usedaffected in subsequent inheritance highlighted. Colour scheme will be used for subsequent linear cumulative probability plots.  3.2	 Samples and Results 3.2.1	Buckhorn Mountain Volcanic Sequence (BMV) Gold mineralization and skarn alteration at Buckhorn are hosted in the metavolcanic rocks of the BMV and the metasedimentary rocks of the BMS. As described in Chapter 2 the BMV is predominately made up of sub-aerial porphyritic flows and autobreccias, with minor tuffs, and dike- and plug-like comagmatic intrusions (Figure 2.6, Figure 2.7, and Figure 2.8). Rocks of the BMV have not been previously isotopically dated and there is a long standing disagreement over their age. McMillen (1979), Hickey (1990, 1992), and Stoffel (1990a) correlate them with the Permian-Triassic Kobau or Knob Hill Group; McMillen (1979) also suggests a portion of the BMV may be equivalent to the Triassic Brooklyn Formation, where as Cheney et al. (1994) and Gaspar (2005) correlate them with the Jurassic Elise Formation. Determining the age of the BMV resolves this disagreement. Three samples were collected for dating; two from different intrusive rocks comagmatic with the BMV, and a third from a pyroxene porphyry flow. Sample BS063 was selected from a plug-like body of the equigranular 51  variety of intrusive BMV that cross cuts the upper carbonate member of the BMS. Sample BS064 was selected from a dike-like body of the porphyritic variety of the intrusive BMV that cross-cuts the BMV flows. Sample BS065 was selected from a pyroxene porphyry flow. Only the porphyritic intrusive sample, BS064, yielded zircons. The zircons are subhedral to nearly euhedral. Most grains are about 100 μm, but about 10 % are 200 to 350 μm. BSE imaging shows that both size fractions have inclusions and fractures, but growth zoning and inherited cores are not visible (Figure 3.2). Sample BS064 yielded a concordant age of 192.4 ± 1.0 Ma and a weighted average 206Pb/238U age of 193.5 ± 1.2 Ma (Figure 3.3), based on thirty-seven of the fifty-five zircons analysed. Ten of the eighteen zircons excluded from the age calculations were affected by subtle lead loss, four by subtle inheritance, and four were discordant (Table 3.2 and Figure 3.2). Analyses of zircons from both size fractions are included in the results, and there was no correlation between the size of the zircon and its age. The concordant data provide the first isotopically determined age for the BMV, which allows the BMV to be compared to age equivalent rocks in southern British Columbia. In the Nelson area several plutons and other small plugs, dikes and sills have been interpreted as comagmatic with the Elise Formation. These comagmatic intrusions have not been dated, but they are expected to be approximately the same age as the Elise Formation, which was deposited in the late Sinemurian (197190 Ma) (Hoy and Dunne, 1997). As explained previously (Chapter 2) the BMV is mineralogically, texturally and geochemically similar to the Elise Formation. Based on the overlap of depositional age of the Elise Formation and the intrusive age of sample BS064, it is the likely that the porphyritic intrusive sample of the BMV is equivalent to the intrusive equivalents of the Elise Formation, and the BMV is therefore equivalent to the Elise Formation.  52  212 208  BS064: Intrusive Volcanic  204  Age (Ma)  200 196 192 188 184 180  99.9  99  95  99.99  Probability  90  70 80  50  20 30  5  10  1  .1  176  Figure 3.2: BSE image of a representative zircon and linear cumulative probability plot for sample BS064 Figure X.17 BSE image of a representative zircon and linear cumulative probability plot for sample BS064  53  Table 3.2: Isotope ratios and age estimates for sample BS064, porphyritic intrusion comagmatic with the BMV Table 3.2 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb /238U  ± 1σ  Discord.  Comments  Discordant  207  206  207  235  206  238  207  206  207  235  206  54  BMV  BS064-1  0.0421  0.0034  0.1719  0.0140  0.0297  0.0007  0.30  0.1  0  161.1  12.14  188.7  4.52  0.854  BMV  BS064-2  0.0539  0.0023  0.2263  0.0099  0.0310  0.0005  0.36  368.1  92.04  207.1  8.21  196.5  3.04  1.054  BMV  BS064-3  0.0646  0.0101  0.2658  0.0418  0.0289  0.0014  0.30  761.9  298.16  239.3  33.54  183.5  8.67  1.304  BMV  BS064-4  0.0540  0.0018  0.2246  0.0078  0.0302  0.0004  0.36  368.6  72.76  205.7  6.46  191.7  2.35  1.073  BMV  BS064-5  0.0493  0.0054  0.1985  0.0218  0.0295  0.0009  0.29  160.8  235.82  183.9  18.46  187.4  5.86  0.981  BMV  BS064-6  0.0507  0.0013  0.2066  0.0056  0.0290  0.0003  0.36  225.5  57.88  190.7  4.72  184.2  1.74  1.035  Pb Loss  BMV  BS064-7  0.0505  0.0015  0.2037  0.0064  0.0294  0.0003  0.36  218.6  68.01  188.2  5.42  186.8  2.05  1.007  Pb Loss  BMV  BS064-8  0.0524  0.0022  0.2090  0.0091  0.0290  0.0004  0.35  301.4  92.99  192.7  7.61  184.4  2.76  1.045  Pb Loss  BMV  BS064-9  0.0517  0.0026  0.2049  0.0105  0.0284  0.0005  0.34  272.7  110.3  189.3  8.83  180.7  3.1  1.048  Pb Loss  BMV  BS064-10  0.0494  0.0018  0.1950  0.0075  0.0291  0.0004  0.35  166  84.81  180.9  6.4  185.2  2.46  0.977  Pb Loss  BMV  BS064-11  0.0466  0.0095  0.2230  0.0459  0.0323  0.0016  0.25  26  426.22  204.4  38.12  205.1  10.23  0.997  Inheritance  BMV  BS064-12  0.0506  0.0063  0.2017  0.0254  0.0290  0.0011  0.29  222.8  264.53  186.6  21.46  184.6  6.64  1.011  Pb Loss  BMV  BS064-13  0.0523  0.0062  0.2328  0.0279  0.0305  0.0011  0.31  299.8  248.15  212.5  22.97  193.8  7.02  1.096  BMV  BS064-14  0.0502  0.0025  0.2116  0.0110  0.0317  0.0006  0.35  202.6  112.77  194.9  9.2  201.3  3.59  0.968  BMV  BS064-15  0.0498  0.0017  0.2127  0.0076  0.0313  0.0004  0.35  186.3  77.91  195.8  6.38  198.6  2.41  0.986  BMV  BS064-16  0.0468  0.0032  0.1945  0.0136  0.0313  0.0007  0.32  38  156.28  180.5  11.55  198.4  4.32  0.910  BMV  BS064-17  0.0496  0.0047  0.2116  0.0206  0.0326  0.0010  0.30  175.8  208.87  194.9  17.24  206.8  6  0.942  BMV  BS064-18  0.0477  0.0022  0.2019  0.0096  0.0311  0.0005  0.34  81.5  106.56  186.7  8.11  197.3  3.12  0.946  BMV  BS064-19  0.0518  0.0014  0.2144  0.0063  0.0301  0.0003  0.35  275.2  61.85  197.2  5.24  191  1.94  1.032  BMV  BS064-20  0.0540  0.0037  0.2259  0.0159  0.0307  0.0007  0.34  370  147.71  206.8  13.17  194.8  4.64  1.062  BMV  BS064-21  0.0474  0.0017  0.1940  0.0073  0.0301  0.0004  0.34  69.4  84.55  180  6.23  191.2  2.46  0.941  BMV  BS064-22  0.0507  0.0018  0.2133  0.0081  0.0305  0.0004  0.34  225  81.96  196.3  6.78  193.9  2.53  1.012  BMV  BS064-23  0.0514  0.0017  0.2193  0.0074  0.0308  0.0004  0.35  259.9  72.35  201.4  6.2  195.7  2.3  1.029  BMV  BS064-24  0.0517  0.0024  0.2105  0.0102  0.0288  0.0005  0.35  270.8  103.8  193.9  8.53  183  3.04  1.060  Discordant  Inheritance  Pb Loss  Table 3.2 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb /238U  ± 1σ  Discord.  207  206  207  235  206  238  207  206  207  235  206  Comments  55  BMV  BS064-25  0.0410  0.0065  0.1893  0.0304  0.0304  0.0012  0.25  0.1  73.26  176  25.94  192.9  7.78  0.912  BMV  BS064-26  0.0493  0.0021  0.2089  0.0091  0.0299  0.0005  0.35  161.4  95.03  192.6  7.63  190  2.81  1.014  BMV  BS064-27  0.0570  0.0050  0.2474  0.0220  0.0330  0.0010  0.33  490.7  181.69  224.5  17.88  209.1  6.14  1.074  BMV  BS064-28  0.0530  0.0046  0.2028  0.0180  0.0301  0.0009  0.33  327.2  187.13  187.5  15.2  191.4  5.47  0.980  BMV  BS064-29  0.0546  0.0066  0.2100  0.0258  0.0298  0.0012  0.31  395.9  251.45  193.6  21.62  189.2  7.21  1.023  BMV  BS064-30  0.0492  0.0016  0.2039  0.0072  0.0311  0.0004  0.35  156.9  76.4  188.4  6.05  197.2  2.36  0.955  BMV  BS064-31  0.0507  0.0031  0.2029  0.0129  0.0296  0.0006  0.33  225.2  137.32  187.6  10.88  188  3.9  0.998  BMV  BS064-32  0.0488  0.0023  0.2108  0.0101  0.0306  0.0005  0.34  138.2  104.71  194.2  8.45  194.3  3.1  0.999  BMV  BS064-33  0.0475  0.0030  0.2040  0.0131  0.0307  0.0007  0.33  72  142.93  188.5  11.06  195.1  4.15  0.966  BMV  BS064-34  0.0482  0.0023  0.2020  0.0101  0.0302  0.0005  0.33  106.9  110.68  186.8  8.55  192  3.13  0.973  BMV  BS064-35  0.0507  0.0022  0.2039  0.0091  0.0300  0.0005  0.35  227.3  96.43  188.4  7.68  190.3  2.93  0.990  BMV  BS064-36  0.0486  0.0019  0.2016  0.0082  0.0302  0.0004  0.34  128.4  89.32  186.5  6.94  191.7  2.61  0.973  BMV  BS064-37  0.0530  0.0030  0.2149  0.0127  0.0289  0.0006  0.35  329.4  124.96  197.6  10.6  183.9  3.67  1.074  BMV  BS064-38  0.0500  0.0022  0.2086  0.0095  0.0299  0.0005  0.34  197  98.45  192.4  7.96  189.7  2.9  1.014  BMV  BS064-39  0.0533  0.0018  0.2222  0.0080  0.0294  0.0004  0.35  340  75.77  203.8  6.68  186.6  2.32  1.092  BMV  BS064-40  0.0497  0.0027  0.2008  0.0113  0.0298  0.0006  0.34  182  122.82  185.8  9.58  189.2  3.57  0.982  BMV  BS064-41  0.0439  0.0054  0.1950  0.0242  0.0302  0.0011  0.29  0.1  160.26  180.8  20.54  191.6  6.81  0.944  BMV  BS064-42  0.0436  0.0038  0.1967  0.0173  0.0313  0.0008  0.29  0.1  68.65  182.3  14.7  198.4  5.04  0.919  BMV  BS064-43  0.0478  0.0029  0.2059  0.0130  0.0307  0.0006  0.32  90.2  140.66  190.1  10.97  195.1  3.86  0.974  BMV  BS064-44  0.0493  0.0022  0.2072  0.0098  0.0311  0.0005  0.34  162.3  102.94  191.2  8.23  197.3  3.11  0.969  BMV  BS064-45  0.0440  0.0036  0.1845  0.0154  0.0314  0.0008  0.30  0.1  78.94  171.9  13.19  199.5  4.91  0.862  Discordant  BMV  BS064-46  0.0506  0.0026  0.1976  0.0103  0.0294  0.0005  0.35  221.3  112.41  183.1  8.71  186.7  3.31  0.981  Pb Loss  BMV  BS064-47  0.0499  0.0018  0.2182  0.0083  0.0324  0.0004  0.34  192.1  81.5  200.4  6.9  205.3  2.63  0.976  Inheritance  BMV  BS064-48  0.0505  0.0019  0.2085  0.0082  0.0304  0.0004  0.34  219.8  84.46  192.3  6.9  192.8  2.57  0.997  BMV  BS064-49  0.0507  0.0023  0.2127  0.0100  0.0315  0.0005  0.35  226.2  100.79  195.8  8.35  199.8  3.17  0.980  Inheritance  Pb Loss Pb Loss  Table 3.2 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb /238U  ± 1σ  Discord.  207  206  207  235  206  238  207  206  207  235  206  BMV  BS064-50  0.0506  0.0018  0.2128  0.0082  0.0302  0.0004  0.34  220.3  82.15  195.9  6.84  191.9  2.49  1.021  BMV  BS064-51  0.0520  0.0032  0.2017  0.0128  0.0295  0.0006  0.33  284.9  135.04  186.6  10.78  187.2  3.87  0.997  BMV  BS064-52  0.0575  0.0041  0.2323  0.0171  0.0294  0.0008  0.35  511.6  151.1  212.1  14.12  187  4.68  1.134  BMV  BS064-53  0.0510  0.0020  0.2121  0.0088  0.0299  0.0004  0.35  240.9  88.68  195.3  7.37  190  2.67  1.028  BMV  BS064-54  0.0531  0.0023  0.2204  0.0098  0.0299  0.0005  0.35  333.3  93.71  202.3  8.18  189.7  2.87  1.066  BMV  BS064-55  0.0488  0.0023  0.2013  0.0098  0.0318  0.0005  0.34  137.7  106.85  186.3  8.32  201.8  3.3  0.923  192.9  2.7  193.5  1.2  Weighted average (95% confidence level)  Comments  Discordant  56  0.034  data-point error ellipses are 2σ  BS064: Comagmatice BMV Intrusive 210  206Pb/238U  0.032  190  0.030  0.028  Concordia Age = 193.4 ±1.0 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 0.25, Probability (of concordance) = 0.62  170 0.026 0.08  0.12  0.16  0.20  0.24  0.28  0.32  207Pb/235U  box heights are 2σ  230  BS064: Comagmatice BMV Intrusive  Pb206/U238  220  210  200  190  180  170  Mean = 193.5±1.2 [0.63%] 95% conf. Wtd by data-pt errs only, 18 of 55 rej. MSWD = 1.4, probability = 0.066 (error bars are 2σ)  160  Figure 3.3: Concordia and weighted average diagrams for sample BS064 Figure X.18 Weighted average and concordia diagrams for sample BS064 3.2.2	Buckhorn Granodiorite The Buckhorn Granodiorite covers an area of approximately 19 km2 northeast of the Buckhorn gold skarn (Figure 2.1). The Buckhorn Granodiorite is the largest component of the BIS, which also includes the Buckhorn Diorite and several generations of dikes. To determine if there is significant variability in age within the stock and to more precisely date Buckhorn Granodiorite, three samples were evaluated. Sample BS048 was collected about 85 m from the intrusive contact, and approximately 10 m from a steeply-dipping northeast oriented shear zone. Sample BS074 was collected near the center of the stock, about 1.7 km from its margin, and has intense sericite and chlorite alteration. Sample BS057 is intensely deformed and altered and was collected from near the Roosevelt Mine. The  57  deformation in sample BS057 is dominated by the steeply north-east dipping foliation that occurs throughout the entire BS057 outcrop for over 10 m. Sample BS057 is also affected by intense sericite and chlorite alteration. Samples BS048 and BS057 will also provide a maximum age for the deformation event responsible for the steeply dipping foliation and the Footwall Mylonite. The zircons from samples BS048 and BS074 are clear, near euhedral elongate crystals about 100 μm wide and up to 300 μm long. A few inherited cores and inclusions were detected by BSE imaging, and were avoided during analysis. A few concentric growth zones and fractures were also visible with BSE imaging; the latter was avoided during analysis to minimize the potential of lead loss (Figure 3.4). BS048 gives a concordant age of 170.4 ± 1.1 Ma and a weighted average  Pb/238U age of 170.4 ±  206  1.7 Ma (Figure 3.5). These ages are based on twelve of the twenty zircons analysed. Two of the zircons analysed were xenocrysts, and have ages of about 350 Ma and 1.05 Ga, the remaining six had experienced subtle lead loss (Table 3.3 and Figure 3.5). Sample BS074 gives a concordant age of 167.8 ± 1.5 Ma and an identical weighted average 206Pb/238U age of 167.8 ± 1.5 Ma (Figure 3.5). Twelve of the twenty analyses were excluded from the age calculations. Two zircons, with ages of approximately 1.4 and 1.8 Ga, are interpreted as xenocrysts. Two of the zircons had discordant ages, and eight had been affected by subtle lead loss (Table 3.4 and Figure 3.5). In contrast to the other granodiorite samples, the zircons from BS057 are mostly subhedral and near equant crystals that range from 100 to 200 μm. Clear in transmitted light, growth zoning was rarely visible and a single core was detected with BSE imaging. Zircons from sample BS057 have inclusions that are up to 20 μm (Figure 3.7). The analysis gives a concordant age of 166.0 ± 1.0 Ma and a weighted average 206Pb/238U age of 166.0 ± 1.0 Ma (Figure 3.8). These ages are based on fifteen of the twenty zircons analysed. Three of the excluded zircons had experienced subtle lead loss, one was discordant and the fifth was affected by subtle inheritance (Table 3.5 and Figure 3.7). These results more precisely and accurately constrain the maximum age of the Buckhorn Granodiorite compared to the 169.4 ± 2.3 Ma age determined by Gaspar (2005). They also clearly indicate that the development of the steeply dipping shear zones occurred after 170.4 Ma and that the deformation event responsible for them persisted until at least 166.0 Ma. These results are also important because they show that the BIS experienced alteration and deformation on a large scale, and not only in meter-scale shear zones as previously thought (Gaspar, 2005). The difference in absolute ages between the three samples can be explained in two ways. The first explanation is that samples BS074 58  and BS057 are younger phases of granodiorite, rather than truly part of the Buckhorn Granodiorite stock. The intensity of alteration and deformation in the two younger samples makes their positive identification as dikes uncertain. However, their ages and the geochemistry of BS074, which deviates from the expected zoning of the stock (M. Deal, personal communication 2011), suggest that samples BS057 and BS074 are Granodiorite dikes. The second possibility is that, as the ages overlap at the 2 sigma level, all three samples represent the same intrusion, and the difference in chemistry is due to the intensity of alteration in sample BS074. This conclusion is supported by the fact that intrusive contacts were not identified during mapping of the sample locations.  A  B  Figure 3.4: BSE image of representative zircons from samples BS048 (A) and BS074 (B)  Figure X.2 BSE image of representative zircons from BS048 (A) and BS074 (B)  59  data-point error ellipses are 2σ  0.0285  data-point error ellipses are 2σ  0.0285  BS074: Buckhorn Granodiorite  BS048: Buckhorn Granodiorite  180  180 0.0275  206Pb/238U  206Pb/238U  0.0275  170 0.0265  0.0255 160  0.0245 0.13  0.15  0.17  0.0255  Concordia Age = 170.4 ±1.1 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 1.8, Probability (of concordance) = 0.18  0.19  0.21  170 0.0265  Concordia Age = 167.8 ±1.5 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 0.024, Probability (of concordance) = 0.88  160  0.0245 0.14  0.23  0.16  0.18  box heights are 2σ  182  0.20  0.22  207Pb/235U  207Pb/235U  box heights are 2σ  BS048: Buckhorn Granodiorite  178  BS074: Buckhorn Granodiorite  178  Pb206/U238  Pb206/U238  174 174  170  166  162  158  170  166  162  Mean = 170.4±1.7 [1.0%] 95% conf. Wtd by data-pt errs only, 6 of 18 rej. MSWD = 2.0, probability = 0.022 (error bars are 2σ)  158  Mean = 167.8±1.5 [0.87%] 95% conf. Wtd by data-pt errs only, 10 of 18 rej. MSWD = 0.91, probability = 0.50 (error bars are 2σ)  154  Figure diagrams for for BS048 andand BS074. Rejected analyses colFigure3.5: X.4Concordia Concordiaand andweighted Weightedaverage average diagrams BS048 BS074. Rejected analyses oured according to the used thecalculations age calculations colours according thelegend legendininFigure Figure3.1. X.1.Only Onlyconcordant concordantdata datawas used forfor age and and plotted on the Concordia diagrams. plotted on the Concordia diagrams.  60  177 BS048: Buckhorn Granodiorite  175  Age (Ma)  173 171 169 167 165  95  99  99.9  95  99  99.9  Probability 173  99.99  90 90  70 80  50  20 30  5  10  1  .1  163  BS074: Buckhorn Granodiorite  171  Age (Ma)  169 167 165 163 161  70 80  50  20 30  5  10  1  .1  .01  159  Probability  Figure 3.6: Linear cumulative plots for plots samples and BS074 Figure X.2 Linearprobability cumulative probability form BS048 sample BS048 and BS074  61  Table 3.3: Isotope ratios and age estimates for BS048 Table 3.3 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  207  206  207  235  206  238  207  206  ± 1σ  Pb / U  ± 1σ  Pb /238U  ± 1σ  Discord.  Comments Pb Loss  207  235  206  BGdi  BS048-a  0.0515  0.0016  0.1936  0.0061  0.0262  0.0003  0.37  262.5  68.08  179.7  5.15  166.9  1.85  1.08  BGdi  BS048-b  0.0440  0.0027  0.1651  0.0095  0.0265  0.0005  0.31  34.3  130.51  155.2  8.31  168.3  3.02  0.92  BGdi  BS048-c  0.0440  0.0012  0.1842  0.0044  0.0268  0.0002  0.36  188.7  53.21  171.7  3.79  170.4  1.44  1.01  BGdi  BS048-d  0.0440  0.0025  0.1903  0.0098  0.0266  0.0005  0.34  212.8  111.86  176.9  8.34  169  2.87  1.05  BGdi  BS048-e  0.0440  0.0012  0.1881  0.0050  0.0276  0.0003  0.36  136.3  58.77  175  4.24  175.2  1.63  1.00  BGdi  BS048-f  0.0440  0.0015  0.1778  0.0059  0.0267  0.0003  0.35  89.1  75.2  166.2  5.05  169.7  1.93  0.98  BGdi  BS048-g  0.0440  0.0020  0.1664  0.0076  0.0264  0.0004  0.34  0.1  77.59  156.3  6.57  167.8  2.56  0.93  BGdi  BS048-h  0.0491  0.0012  0.1769  0.0044  0.0258  0.0002  0.36  150.8  55.9  165.4  3.82  164.4  1.44  1.01  BGdi  BS048-i  0.0440  0.0019  0.1865  0.0073  0.0273  0.0004  0.35  180  86.03  173.7  6.22  173.5  2.32  1.00  BGdi  BS048-j  0.0440  0.0013  0.1811  0.0050  0.0263  0.0003  0.36  195.5  61.04  169  4.3  167.4  1.63  1.01  BGdi  BS048-k  0.0440  0.0012  0.1784  0.0045  0.0264  0.0002  0.36  124.7  55.91  166.7  3.84  167.7  1.48  0.99  BGdi  BS048-l  0.0440  0.0017  0.1811  0.0065  0.0270  0.0003  0.35  152.3  80.04  169  5.61  172  2.1  0.98  BGdi  BS048-m  0.0440  0.0015  0.1770  0.0060  0.0273  0.0003  0.35  47.9  76.49  165.5  5.14  173.6  2.02  0.95  BGdi  BS048-n  0.0440  0.0009  0.3990  0.0078  0.0560  0.0004  0.36  318  39.13  340.9  5.67  351.4  2.37  0.97  Xenocryst  BGdi  BS048-o  0.0480  0.0014  0.1762  0.0054  0.0262  0.0003  0.34  100.8  68.5  164.8  4.63  166.8  1.71  0.99  Pb Loss  BGdi  BS048-p  0.0440  0.0014  1.8026  0.0505  0.1772  0.0015  0.31  1051.7  36.9  1046.4  18.29  1051.5  8.33  1.00  Xenocryst  BGdi  BS048-q  0.0478  0.0013  0.1697  0.0048  0.0262  0.0003  0.35  88.8  64.56  159.1  4.15  166.9  1.66  0.95  Pb Loss  BGdi  BS048-r  0.0440  0.0022  0.1639  0.0084  0.0262  0.0004  0.33  0.1  7.31  154.1  7.36  166.9  2.75  0.92  Pb Loss  BGdi  BS048-s  0.0440  0.0017  0.1795  0.0064  0.0267  0.0003  0.36  132  79.62  167.6  5.53  169.5  2.13  0.99  BGdi  BS048-t  0.0479  0.0011  0.1763  0.0042  0.0262  0.0002  0.36  93.8  54.04  164.8  3.59  166.6  1.38  0.99  168.6  2.9  170.4  1.7  Weighted average (95% confidence level)  Pb Loss  Pb Loss  62  Table 3.4: Isotope ratios and age estimates for BS074 Table 3.4 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios Pb / Pb 207  206  ± 1σ  Pb / U 207  235  Age estimates (Ma)  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  ± 1σ  Pb / U  ± 1σ  Pb /238U  206  238  207  206  207  235  206  ± 1σ  Discord.  Comments  BGdi  BS074-a  0.1145  0.0026 5.1927  0.2526  0.3377  0.0042  0.26  1871.3  40.18  1851.4  41.42  1875.5  20.4  0.99  Xenocryst  BGdi  BS074-b  0.0415  0.0036 0.1463  0.0128  0.0263  0.0007  0.31  0.1  0  138.6  11.31  167.2  4.48  0.83  Discordant  BGdi  BS074-c  0.0493  0.0018 0.1821  0.0068  0.0266  0.0004  0.36  160.5  83.1  169.9  5.86  169.5  2.23  1.00  BGdi  BS074-d  0.0493  0.0013 0.1827  0.0048  0.0264  0.0003  0.36  161  58.85  170.4  4.15  168.2  1.57  1.01  BGdi  BS074-e  0.0499  0.0022 0.1798  0.0082  0.0257  0.0004  0.35  191.9  100.81  167.9  7.09  163.6  2.59  1.03  Pb Loss  BGdi  BS074-f  0.0934  0.0013 3.1816  0.0720  0.2483  0.0017  0.31  1496  25.99  1452.7  17.47  1429.9  8.91  1.02  Xenocryst  BGdi  BS074-g  0.0496  0.0013 0.1776  0.0047  0.0258  0.0003  0.37  177.5  58.31  166  4.02  164.4  1.56  1.01  Pb Loss  BGdi  BS074-h  0.0509  0.0025 0.1904  0.0096  0.0262  0.0005  0.35  237.4  108.8  176.9  8.14  166.7  2.9  1.06  BGdi  BS074-i  0.0511  0.0016 0.1800  0.0060  0.0253  0.0003  0.36  243.5  72.5  168  5.12  160.7  1.9  1.05  Pb Loss  BGdi  BS074-j  0.0507  0.0016 0.1844  0.0058  0.0259  0.0003  0.36  227.7  69.1  171.8  4.98  164.9  1.85  1.04  Pb Loss  BGdi  BS074-k  0.0500  0.0021 0.1750  0.0074  0.0259  0.0004  0.36  192.9  92.92  163.7  6.37  165.1  2.43  0.99  Pb Loss  BGdi  BS074-l  0.0479  0.0019 0.1783  0.0073  0.0265  0.0004  0.34  91.5  93.16  166.6  6.31  168.4  2.29  0.99  BGdi  BS074-m  0.0497  0.0012 0.1781  0.0043  0.0261  0.0002  0.37  179.2  53.07  166.5  3.69  165.9  1.45  1.00  BGdi  BS074-n  0.0472  0.0016 0.1712  0.0059  0.0259  0.0003  0.35  59.8  78.68  160.5  5.14  164.5  1.98  0.98  Pb Loss  BGdi  BS074-o  0.0504  0.0028 0.1817  0.0105  0.0256  0.0005  0.35  215.1  125.24  169.5  8.99  163.1  3.24  1.04  Pb Loss  BGdi  BS074-p  0.0549  0.0032 0.2034  0.0122  0.0266  0.0006  0.35  407.5  125.37  188  10.3  169.4  3.5  1.11  Discordant  BGdi  BS074-q  0.0486  0.0019 0.1782  0.0071  0.0270  0.0004  0.35  127.8  88.97  166.5  6.14  171.9  2.4  0.97  BGdi  BS074-r  0.0482  0.0024 0.1707  0.0086  0.0260  0.0004  0.34  108.7  111.75  160  7.41  165.2  2.79  0.97  BGdi  BS074-s  0.0486  0.0028 0.1828  0.0106  0.0264  0.0005  0.31  130.3  128.38  170.5  9.12  168.2  3.02  1.01  BGdi  BS074-t  0.0472  0.0023 0.1653  0.0082  0.0258  0.0005  0.35  58.7  111.55  155.3  7.11  164.3  2.81  0.95  168.1  3.9  167.8  1.5  Weighted average (95% confidence level)  Pb Loss  63  171  BS057: Buckhorn Granodiorite  169  Age (Ma)  167 165 163 161 159  99.9  99  90  95  70 80  50  20 30  5  10  1  .1  157  Probability  Figure 3.7: BSE image of a representative zircon and linear cumulative probability plot for BS057 Figure X.5 Representative BSE image of a representative zircon and linear cumulative probability plot for sample BS057  64  data-point error ellipses are 2σ  180  BS057: Buckhorn Granodiorite 0.028  206Pb/238U  0.027  170  0.026  160  0.025  Concordia Age = 166.0 ±1.0 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 0.031, Probability (of concordance) = 0.86  0.024 0.13  0.15  0.17  0.19  0.21  207Pb/235U  box heights are 2σ  178  BS057: Buckhorn Granodiorite 174  Pb206/U238  170  166  162  158  154  Mean = 165.99±0.97 [0.58%] 95% conf. Wtd by data-pt errs only, 0 of 15 rej. MSWD = 0.86, probability = 0.60 (error bars are 2σ)  150  Figure 3.8: Concordia and weighted average and diagram for BS057 Figure X.6 Weighted average and concordia diagrams for sample BS057  65  Table 3.5: Summary of isotope ratios and age estimates for BS057 Table 3.5  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Rock Type  Sampleanalysis  BGdi  BS057-1  0.0573  BGdi  BS057-2  0.0484  BGdi  BS057-3  BGdi  Pb / Pb  Pb / U  ± 1σ  Discord.  Comments  0.0028 0.2102  0.0105  0.0265  0.0005  0.36  501.1  102.77  193.7  8.79  168.6  0.0015 0.1733  0.0055  0.0259  0.0003  0.34  117  70.6  162.3  4.71  164.8  2.99  1.149  Discordant  1.77  0.985  0.0480  0.0029 0.1739  0.0106  0.0253  0.0005  0.34  97  136.52  162.8  9.2  161.1  3.31  1.011  BS057-4  0.0503  0.0019 0.1748  0.0069  0.0258  0.0004  0.35  208.9  87.08  163.6  5.97  164.4  2.26  0.995  BGdi  BS057-5  0.0517  0.0018 0.1936  0.0070  0.0261  0.0003  0.35  271.6  78.33  179.7  5.95  165.8  2.05  1.084  BGdi  BS057-6  0.0496  0.0015 0.1772  0.0053  0.0261  0.0003  0.34  175.7  66.87  165.6  4.59  165.9  1.67  0.998  BGdi  BS057-7  0.0480  0.0013 0.1737  0.0048  0.0259  0.0003  0.35  BGdi  BS057-8  0.0491  0.0011 0.1799  0.0040  0.0262  0.0002  0.36  96  63.55  162.6  4.15  165.1  1.55  0.985  153.4  49.59  167.9  3.43  166.7  1.3  1.007  BGdi  BS057-9  0.0495  0.0016 0.1804  0.0058  0.0263  0.0003  0.34  170.8  71.75  168.4  5  167.5  1.82  1.005  BGdi BS057-10  0.0490  0.0016 0.1734  0.0056  0.0257  0.0003  0.35  145.6  72.78  162.3  4.88  163.3  1.81  0.994  BGdi BS057-11  0.0498  0.0019 0.1804  0.0071  0.0256  0.0004  0.35  184.5  86.22  168.4  6.07  163.1  2.21  1.032  BGdi BS057-12  0.0493  0.0029 0.1793  BGdi BS057-13  0.0494  0.0020 0.1664  0.0109  0.0263  0.0005  0.32  162.4  133.33  167.5  9.4  167.3  3.26  1.001  0.0067  0.0249  0.0004  0.35  164.6  90.15  156.3  5.87  158.3  2.23  0.987  BGdi BS057-14  0.0469  0.0016 0.1677  0.0058  0.0259  0.0003  0.34  44.9  78.8  157.5  5.03  164.8  1.9  0.956  BGdi BS057-15  0.0481  0.0018 0.1675  0.0062  0.0250  0.0003  0.33  104.2  83.65  157.3  5.41  159.3  1.98  0.987  BGdi BS057-16  0.0476  0.0029 0.1694  0.0104  0.0263  0.0006  0.34  80.2  137.21  158.9  9.02  167.3  3.45  0.950  BGdi BS057-17  0.0481  0.0013 0.1767  0.0050  0.0267  0.0003  0.35  103.4  64.01  165.2  4.34  170  1.68  0.972  BGdi BS057-18  0.0483  0.0023 0.1828  0.0089  0.0267  0.0004  0.33  116.1  108.05  170.4  7.63  169.8  2.69  1.004  BGdi BS057-19  0.0507  0.0016 0.1805  0.0057  0.0263  0.0003  0.34  225.1  69.55  168.5  4.9  167.3  1.79  1.007  BGdi BS057-20  0.0484  0.0017 0.1770  0.0062  0.0266  0.0003  0.34  117  78.92  165.5  5.37  169.2  1.98  0.978  165.7  2.6  165.99  0.97  206  Weighted average (95% confidence level)  ± 1σ  Pb / U  Age estimates (Ma)  ± 1σ  207  207  235  206  238  Rho  Pb / Pb 207  206  ± 1σ  Pb / U  ± 1σ  207  235  Pb /238U 206  ± 1σ  Pb Loss  Pb Loss Pb Loss Inheritance  66  3.2.3	Early Diorite Dikes The Early Diorite dikes were identified during mapping (G.E. Ray, Personal communication 2009). They are plagioclase- and pyroxene-porphyritic diorite dikes that are so-named because they predate the prograde skarn alteration. Many of the Early Diorite dikes are intensely skarn-altered to a garnet-epidote and lesser pyroxene-amphibole assemblage. Because of the intensity of alteration two samples of the Early Diorite dikes were collected to ensure a dependable age could be determined. Sample BS046, is moderately skarn altered to pyroxene-amphibole-epidote and is about 1 m from intense garnet skarn alteration; sample BS075 is weakly skarn altered to the same mineral assemblage and is about 3 m from garnet skarn alteration. A date from this rock type will provide the first isotopic age for the Early Diorite dikes and a maximum age for skarn alteration. Zircons from the two samples are similar. They are clear in transmitted light, near euhedral crystals with moderate concentric zoning and no inherited cores visible with BSE imaging. The zircons average between 100 and 200 μm in length and about 100 μm in width, and have a few inclusions and fractures (Figure 3.9). The more intensely endoskarn altered sample, BS046, has a concordant age of 168.2 ± 0.7 Ma and a weighted average 206Pb/238U age of 168.2 ± 0.7 Ma (Figure 3.10). These results are based on fourteen of the twenty zircons analysed. The results from three zircons were excluded because the crystals are xenocrysts reflecting crustal contamination by the country rocks during emplacement. The three xenocrysts have concordant ages, two at approximately 1.85 Ga, and the third at about 1.1 Ga. Two analyses were excluded because the zircons experienced subtle lead loss, and the last was affected by subtle inheritance (Table 3.7 and Figure 3.11). The less endoskarn altered sample, BS075, has a concordant age of 169.4 ± 1.3 Ma and a weighted average 206Pb/238U age of 169.3 ± 1.5 Ma (Figure 3.10). These ages are based on fourteen of the twenty zircons analyzed. Of the rejected zircon analyses, three were discordant and three had been affected by subtle inheritance (Table 3.7 Figure 3.11). The ages of the two skarn-altered Early Diorite dikes agree within error of each other and within the age of the Buckhorn Granodiorite. This work provides an age for a previously undated rock type, which can be used along with its geologic relationships to show that the Early Diorite dikes are likely a mafic dike phase of the Buckhorn stock. Also, as the Early Diorite dikes are usually skarnaltered, their emplacement must have predated skarn alteration and gold mineralization and their age of 168.2 ± 0.7 Ma can therefore be used as a maximum age of mineralization.  67  A  BB  Figure 3.9: BSE Figure images zirconszircons fromfrom sample and BS075 X.7ofBSErepresentative images of representative samplesBS046 BS046 (A) (A) and BS075 (B) (B)  68  data-point error ellipses are 2σ  BS075: Early Diorite Dike  0.0285  172  0.0270  206Pb/238U  data-point error ellipses are 2σ  BS046: Early Diorite Dike  180  0.0275  206Pb/238U  0.0274  0.0266 168 0.0262  170 0.0265  0.0255 160 164  0.0258  0.0254 0.155  0.165  0.175  Concordia Age = 168.18 ±0.74 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 4.0, Probability (of concordance) = 0.045  0.185  0.195  Concordia Age = 169.3 ±1.3 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 1.8, Probability (of concordance) = 0.18  0.0245  0.0235 0.13  0.205  150 0.15  207Pb/235U  Pb206/U238  Pb206/U238  BS075: Early Diorite Dike  176  169 167 165  159  0.23  180  171  161  0.21  box heights are 2σ  184  BS046: Early Diorite Dike  173  163  0.19  207Pb/235U  box heights are 2σ  175  0.17  172 168 164 160  Mean = 168.15±0.70 [0.42%] 95% conf. Wtd by data-pt errs only, 3 of 17 rej. MSWD = 0.32, probability = 0.99 (error bars are 2σ)  156 152  Mean = 169.3±1.5 [0.91%] 95% conf. Wtd by data-pt errs only, 6 of 20 rej. MSWD = 1.2, probability = 0.25 (error bars are 2σ)  148  Figure diagrams from BS046 andand BS075 Figure3.10: X.10Concordia Concordiaand andweighted weightedaverage average diagrams forsample samples BS046 BS075  69  BS046: Early Diorite Dike  171  Age (Ma)  169  167  165  95  99  99.9  95  99  99.9  Probability  175  99.99  90 90  70 80  50  20 30  5  10  1  .1  .01  163  BS075: Early Diorite Dike  173  Age (Ma)  171 169 167 165 163 161  70 80  50  20 30  5  10  1  .1  159  Probability  Figure 3.11: Linear probability plots samplefrom BS046 and BS075 Figurecumulative X.8 Linear cumulative probability plotsfor of analyses samples BS046 and BS075  70  Table 3.6: Isotope ratios and age estimates for sample BS046 Table 3.6  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Rock Type  Sampleanalysis  EDiD  BS046-1  0.0510  0.0015 0.1869  0.0055 0.0264  0.0003 0.35  240.4  65.24  174  4.74  167.7  1.72  1.038  EDiD  BS046-2  0.0516  0.0009 0.1869  0.0033 0.0264  0.0002 0.36  269.1  38.96  174  2.83  167.9  1.09  1.036  EDiD  BS046-3  0.0507  0.0012 0.1804  0.0043 0.0258  0.0002 0.36  227.5  53.03  168.4  3.72  164.3  1.4  1.025  Pb loss  EDiD  BS046-4  0.0495  0.0017 0.1817  0.0065 0.0259  0.0003 0.35  173.4  79.82  169.6  5.62  164.9  2.05  1.029  Pb loss  EDiD  BS046-5  0.0502  0.0010 0.1871  0.0038 0.0266  0.0002 0.36  205.6  44.52  174.2  3.21  169.3  1.21  1.029  EDiD  BS046-6  0.0496  0.0012 0.1859  0.0047 0.0269  0.0002 0.35  174.8  56.18  173.2  4.02  170.9  1.52  1.013  EDiD  BS046-7  0.0486  0.0016 0.1812  0.0061 0.0266  0.0003 0.35  129  74.76  169.1  5.2  169.2  1.94  0.999  EDiD  BS046-8  0.0505  0.0008 0.1846  0.0028 0.0264  0.0002 0.37  217.1  33.82  172  2.41  168.1  0.94  1.023  EDiD  BS046-9  0.0487  0.0011 0.1764  0.0039 0.0263  0.0002 0.36  133.8  50.27  165  3.4  167.3  1.29  0.986  EDiD BS046-10 0.0509  0.0010 0.1835  0.0038 0.0263  0.0002 0.36  235.9  46.21  171  3.29  167.5  1.26  1.021  EDiD BS046-11 0.0479  0.0014 0.1768  0.0054 0.0265  0.0003 0.35  92.3  69.79  165.3  4.64  168.8  1.75  0.979  EDiD BS046-12 0.1105  0.0015 5.0777  0.1468 0.3267  0.0025 0.26  1806.9  24.33  1832.4  24.52  1822.2  12.17  1.006  Xenocryst  EDiD BS046-13 0.0791  0.0019 2.0061  0.0777 0.1866  0.0022 0.30  1174.6  46.6  1117.6  26.23  1103.1  11.66  1.013  Xenocryst  EDiD BS046-14 0.0479  0.0013 0.1758  0.0047 0.0266  0.0003 0.35  95  61.53  164.4  4.06  169.2  1.56  0.972  EDiD BS046-15 0.0492  0.0010 0.1786  0.0038 0.0264  0.0002 0.36  158  47.42  166.8  3.26  168.2  1.26  0.992  EDiD BS046-16 0.0504  0.0013 0.1870  0.0049 0.0266  0.0003 0.36  211.1  57.51  174.1  4.16  169.4  1.56  1.028  EDiD BS046-17 0.0504  0.0011 0.1796  0.0040 0.0263  0.0002 0.36  211.3  49.07  167.7  3.42  167.5  1.33  1.001  EDiD BS046-18 0.0497  0.0011 0.1792  0.0039 0.0263  0.0002 0.37  182.4  48.79  167.3  3.38  167.1  1.29  1.001  EDiD BS046-19 0.0488  0.0014 0.1803  0.0054 0.0265  0.0003 0.35  139.3  66.96  168.3  4.64  168.4  1.75  0.999  EDiD BS046-20 0.1112  0.0012 5.1297  0.1063 0.3353  0.0019 0.28  1819.4  18.8  1841  17.61  1864  9.33  0.988  169.9  1.8  168.15 0.7  Pb / Pb 207  206  Weighted average (95% confidence level)  ± 1σ  Pb / U 207  235  ± 1σ  Pb / U 206  238  ± 1σ  Rho  Pb / Pb 207  206  ± 1σ  Pb / U 207  235  ± 1σ  Pb /238U 206  ± 1σ  Discord.  Comments  Inheritance  Xenocryst  71  Table 3.7: Isotope ratios and age estimates for sample BS075 Table 3.7 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios Pb / Pb 207  206  ± 1σ  Pb / U 207  235  ± 1σ  Age estimates (Ma) Pb / U 206  238  ± 1σ  Rho  Pb / Pb 207  206  ± 1σ  Pb / U 207  235  ± 1σ  Pb /238U 206  ± 1σ  Discord.  EDiD  BS075-a  0.0486  0.0023 0.1767  0.0085 0.0274  0.0004 0.34  130.5  107.46 165.3  7.29  174.1  2.79  0.949  EDiD  BS075-b  0.0503  0.0021 0.1872  0.0079 0.0267  0.0004 0.34  209.7  92.94  6.71  170.1  2.39  1.024  EDiD  BS075-c  0.0466  0.0026 0.1661  0.0094 0.0262  0.0005 0.33  26.4  128.88 156  8.15  166.6  3.03  0.936  EDiD  BS075-d  0.0474  0.0025 0.1708  0.0091 0.0265  0.0005 0.34  66.4  121.66 160.1  7.92  168.4  3.02  0.951  EDiD  BS075-e  0.0588  0.0030 0.2163  0.0111 0.0264  0.0005 0.37  560.5  106.32 198.9  9.25  168  3.17  1.184  EDiD  BS075-f  0.0508  0.0019 0.1893  0.0074 0.0269  0.0004 0.35  232.9  85.9  6.3  171.3  2.33  1.028  EDiD  BS075-g  0.0507  0.0030 0.1839  0.0110 0.0265  0.0006 0.35  229.1  130.62 171.4  9.41  168.4  3.43  1.018  EDiD  BS075-h  0.0508  0.0016 0.1931  0.0062 0.0271  0.0003 0.35  233.1  71.34  179.2  5.31  172.1  1.96  1.041  EDiD  BS075-i  0.0507  0.0012 0.1933  0.0048 0.0273  0.0003 0.37  225.9  54.89  179.4  4.1  173.6  1.55  1.033  EDiD  BS075-j  0.0492  0.0017 0.1857  0.0065 0.0272  0.0003 0.35  157.7  78.19  172.9  5.56  172.7  2.1  1.001  EDiD  BS075-k  0.0526  0.0014 0.1892  0.0052 0.0266  0.0003 0.37  311.4  59.61  175.9  4.44  169  1.69  1.041  EDiD  BS075-l  174.2  176.1  Comments Inheritance  Discordant  Inheritance  0.0497  0.0020 0.1767  0.0072 0.0259  0.0004 0.35  179.7  90.64  165.2  6.2  165.1  2.31  1.001  EDiD BS075-m 0.0487  0.0017 0.1802  0.0066 0.0266  0.0003 0.35  134.3  81.82  168.2  5.65  169.1  2.15  0.995  EDiD  BS075-n  0.0505  0.0027 0.1839  0.0098 0.0266  0.0005 0.36  219.7  117.4  171.4  8.43  169.4  3.18  1.012  EDiD  BS075-o  0.0503  0.0035 0.1834  0.0127 0.0255  0.0006 0.32  206.8  151.92 171  10.93  162.5  3.5  1.052  EDiD  BS075-p  0.0561  0.0046 0.1942  0.0159 0.0252  0.0007 0.34  456.9  170.9  180.2  13.54  160.5  4.46  1.123  Discordant  EDiD  BS075-q  0.0442  0.0032 0.1638  0.0118 0.0273  0.0006 0.31  0.1  67.94  154  10.33  173.6  3.82  0.887  Discordant  EDiD  BS075-r  0.0562  0.0033 0.1927  0.0114 0.0259  0.0006 0.37  458.6  125.01 178.9  9.69  164.7  3.49  1.086  EDiD  BS075-s  0.0505  0.0016 0.1886  0.0061 0.0272  0.0003 0.36  219.4  71.7  175.4  5.25  173  2.01  1.014  EDiD  BS075-t  0.0489  0.0017 0.1815  0.0064 0.0269  0.0003 0.35  143.5  78.61  169.3  5.48  170.9  2.05  0.991  171.6  3.4  169.3  1.5  Weighted average (95% confidence level)  Inheritance  72  3.2.4	Buckhorn Diorite The Buckhorn Diorite is a fine-grained equigranular diorite that predominately occurs along the southwest margin of the Buckhorn Granodiorite stock. The Buckhorn Diorite has not been previously dated. It was originally labelled as a border phase of the Buckhorn Granodiorite (Hickey, 1990, 1992; McMillen, 1979), but that genetic relationship was later questioned (Gaspar, 2005). Current mapping shows a gradational contact between the two units (Figure 2.1) and a date will help to correlate it with the Buckhorn Granodiorite and distinguish it from the Early Diorite dikes. The Buckhorn Diorite sample contained similar size zircons to the Buckhorn Granodiorite, but more were subhedral. The grains were near equant with poorly developed crystal faces; they ranged from 100 to 200 μm, and contained few inclusions. Fewer growth zones were visible with BSE imaging than in the Buckhorn Granodiorite samples, and few cores were detected (Figure 3.12). The sample had a concordant age of 169.0 ± 0.9 Ma and a weighted average 206Pb/238U age of 169.9 ± 0.9 Ma (Figure 3.13). Five of the twenty analyses were excluded from the age calculations. One analysis was of a zircon that experienced subtle lead loss, three were affected by subtle inheritance, and the final was discordant (Table 3.8 and Figure 3.12). This work establishes the first age for the Buckhorn Diorite. The age falls within error of the Buckhorn Granodiorite, which supports the idea that it is a border phase of the granodiorite stock, and adds to the understanding of the genesis of the BIS. The age of the Buckhorn Diorite also falls within error of the Early Diorite dike samples; therefore the two diorites cannot be distinguished based on their age. Fortunately as was shown in Chapter 2 they are easily distinguished based on texture.  73  BS060: Buckhorn Diorite  173  Age (Ma)  171  169  167  165  99.9  99  95  99.99  Probability  90  70 80  50  20 30  5  10  1  .1  .01  163  X.11 BSE of a representativezircon zircon andand linear linear cumulativecumulative probability plot forprobability sample BS060 plot for BS060 Figure 3.12: BSEFigure image of aimage representative  74  data-point error ellipses are 2σ  0.0285  180  BS060: Buckhorn Diorite  206Pb/238U  0.0275  170 0.0265  0.0255 160  0.0245 0.14  0.16  Concordia Age = 168.97 ±0.90 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 5.8, Probability (of concordance) = 0.016  0.18  0.20  0.22  207Pb/235U  box heights are 2σ  178  BS060: Buckhorn Diorite  Pb206/U238  174  170  166  162  Mean = 168.94±0.86 [0.51%] 95% conf. Wtd by data-pt errs only, 5 of 20 rej. MSWD = 1.05, probability = 0.40 (error bars are 2σ)  158  Figure 3.13: Concordia andWeighted weightedaverage averageand diagrams fordiagrams sample BS060 Figure X.12 concordia for sample BS060  75  Table 3.8: Isotope ratios and age estimates for sample BS060 Table 3.8  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Rock Type  Sampleanalysis  BDi  BS060-1  0.0508  BDi  BS060-2  0.0512  BDi  BS060-3  BDi  Pb / Pb  Pb / U  ± 1σ  Rho  Discord.  Comments  0.0012 0.1952  0.0045  0.0271  0.0002  0.37  233  51.42  181  3.84  172.5  0.0013 0.1889  0.0047  0.0266  0.0002  0.36  247.7  55.18  175.7  4.01  169.3  1.46  1.049  Inheritance  1.51  1.038  0.0504  0.0011 0.1914  0.0044  0.0269  0.0002  0.36  214.1  50.87  177.8  3.72  170.9  1.4  1.040  BS060-4  0.0508  0.0012 0.1871  0.0045  0.0269  0.0002  0.37  233.5  53.74  174.1  3.86  171.3  1.49  1.016  BDi  BS060-5  0.0504  0.0016 0.1813  0.0058  0.0263  0.0003  0.36  212.8  70.92  169.2  4.95  167.4  1.87  1.011  BDi  BS060-6  0.0503  0.0009 0.1868  0.0036  0.0269  0.0002  0.37  209.7  42.54  173.9  3.05  171.2  1.2  1.016  BDi  BS060-7  0.0506  0.0017 0.1893  0.0063  0.0265  0.0003  0.36  BDi  BS060-8  0.0505  0.0013 0.1852  0.0049  0.0264  0.0003  0.36  220.6  74.33  176  5.41  168.6  2.01  1.044  215.6  58.81  172.5  4.18  167.9  1.58  1.027  BDi  BS060-9  0.0509  0.0015 0.1839  0.0054  0.0264  0.0003  0.36  237.9  65.38  171.4  4.64  167.9  1.76  1.021  BDi  BS060-10  0.0501  0.0018 0.1804  0.0065  0.0263  0.0003  0.36  201  80.31  168.4  5.58  167.2  2.11  1.007  BDi  BS060-11  0.0522  0.0029 0.2074  0.0117  0.0269  0.0005  0.35  293.8  121.4  191.3  9.82  170.8  3.3  1.120  BDi  BS060-12  0.0485  0.0028 0.1833  BDi  BS060-13  0.0473  0.0017 0.1711  0.0106  0.0268  0.0005  0.34  123.7  129.21  170.9  9.1  170.5  3.3  1.002  0.0064  0.0262  0.0003  0.35  65.3  85.92  160.4  5.54  166.9  2.12  0.961  BDi  BS060-14  0.0496  0.0012 0.1869  0.0047  0.0269  0.0002  0.36  174.3  55.75  174  3.98  171.2  1.52  1.016  BDi  BS060-15  0.0497  0.0014 0.1808  0.0054  0.0260  0.0003  0.36  181  66.28  168.8  4.6  165.5  1.75  1.020  BDi  BS060-16  0.0469  0.0016 0.1768  0.0063  0.0265  0.0003  0.35  46  81.67  165.3  5.41  168.4  2.07  0.982  BDi  BS060-17  0.0521  0.0011 0.1928  0.0043  0.0267  0.0002  0.37  291.2  48.32  179  3.62  169.6  1.38  1.055  BDi  BS060-18  0.0480  0.0012 0.1748  0.0043  0.0267  0.0002  0.35  99.2  56.75  163.6  3.69  170.1  1.45  0.962  BDi  BS060-19  0.0505  0.0013 0.1782  0.0048  0.0258  0.0003  0.36  217.7  59.72  166.5  4.11  164  1.58  1.015  BDi  BS060-20  0.0490  0.0021 0.1744  0.0076  0.0262  0.0004  0.34  145.6  97.27  163.2  6.52  166.5  2.44  0.980  171.6  3.1  168.94  0.86  206  Weighted average (95% confidence level)  ± 1σ  Pb / U  Age estimates (Ma)  ± 1σ  207  207  235  206  238  Pb / Pb 207  206  ± 1σ  Pb / U 207  235  ± 1σ  Pb /238U 206  ± 1σ  Inheritance  Discordant  Inheritance  Pb Loss  76  3.2.5	Granodiorite dikes The Granodiorite dikes are typically mineralogically identical to the Buckhorn Granodiorite except coarser grained and containing quartz phenocrysts. The Granodiorite dikes are documented to control the location of skarn alteration in the Buckhorn Granodiorite and BMS (Figure 2.9 and Figure 2.11).The dikes may also be altered and deformed as is seen in sample BS059 chosen for dating (Figure 2.15). This example shows a low angle foliation, and a steeply dipping foliation, both defined by intense biotite alteration (Figure 2.15). In addition to dating the Granodiorite dikes this sample will also provide a maximum age for the prograde skarn alteration and the development of the foliations. Sample BS059 contains many clear, elongate, near euhedral zircons that are 150 to 400 μm in length and 100 to 200 μm wide. Concentric growth zoning, inclusions and a few inherited cores were visible with BSE imaging (Figure 3.14). The sample gives a concordant age of 167.5 ± 0.8 Ma and a weighted average 206Pb/238U age of 167.5 ± 0.7 Ma (Figure 3.15). These ages are based on seventeen of the twenty zircons analysed. Of the three zircons excluded, one was a 2 Ga xenocryst, and two were affected by subtle lead loss (Table 3.9 and Figure 3.14). The concordant data provide a reliable age for the emplacement of the Granodiorite dikes that control the location of skarn alteration and gold mineralization. The date is similar to that indicated by the earlier work of Gaspar (2005), but more precise. The date also provides a maximum age for deformation events responsible for the low angle and the steeply dipping foliation. The low angle foliations were originally thought to predate the Buckhorn Granodiorite and only affect the Paleozoic BMS host rock package (McMillen, 1979).  77  BS059: Granodiorite Dike  170  Age (Ma)  168  166  164  162  160  99.9  99  95  90  70 80  50  20 30  5  10  1  .1  .01  158  Probability  X.13 and BSE image of a representative zirconprobability and linear cumulative plot for sample BS059 Figure 3.14: BSEFigure image linear cumulative plotprobability for sample BS059  78  data-point error ellipses are 2σ  0.0276  BS059: Granodiorite Dike 0.0272  172  206Pb/238U  0.0268  0.0264  0.0260 164 0.0256 Concordia Age = 167.54 ±0.76 Ma (2σ, decay-const. errs included) MSWD (of concordance) = 4.7, Probability (of concordance) = 0.031  0.0252  0.0248 0.15  0.17  0.19  0.21  0.23  207Pb/235U  box heights are 2σ  174  BS059: Granodiorite Dike  Pb206/U238  170  166  162  158  Mean = 167.51±0.72 [0.43%] 95% conf. Wtd by data-pt errs only, 2 of 19 rej. MSWD = 0.98, probability = 0.47 (error bars are 2σ)  154  Figure 3.15: Concordia andWeighted weightedaverage averageand diagrams fordiagrams Granodiorite dikeBS059 sample BS059 Figure X.14 concordia for sample  79  Table 3.9: Isotope ratios and age estimates for sample BS059 Table 3.9 Rock Type  Sampleanalysis  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios Pb / Pb 207  206  ± 1σ  Pb / U 207  235  Age estimates (Ma)  ± 1σ  Pb / U  ± 1σ  Rho  Pb / Pb  ± 1σ  Pb / U  206  238  207  206  207  235  ± 1σ  Pb /238U 206  ± 1σ  Discord.  Comments Pb Loss  GDiD  BS059-a  0.0510  0.0010 0.1759  0.0037  0.0250  0.0002  0.36  241.40  46.16  164.50  3.18  159.40  1.19  1.032  GDiD  BS059-b  0.0491  0.0011 0.1793  0.0043  0.0263  0.0002  0.35  152.80  52.87  167.40  3.66  167.20  1.37  1.001  GDiD  BS059-c  0.0501  0.0015 0.1905  0.0060  0.0265  0.0003  0.35  201.00  69.57  177.10  5.14  168.70  1.82  1.050  GDiD  BS059-d  0.0506  0.0013 0.1805  0.0047  0.0266  0.0002  0.35  224.40  57.09  168.40  4.02  169.00  1.53  0.996  GDiD  BS059-e  0.0482  0.0016 0.1753  0.0059  0.0264  0.0003  0.35  107.00  75.93  164.00  5.13  168.20  1.97  0.975  GDiD  BS059-f  0.0480  0.0016 0.1740  0.0060  0.0258  0.0003  0.35  97.90  79.03  162.90  5.21  164.40  1.95  0.991  GDiD  BS059-g  0.0494  0.0012 0.1785  0.0044  0.0263  0.0002  0.34  167.50  54.28  166.80  3.75  167.20  1.41  0.998  GDiD  BS059-h  0.0506  0.0020 0.1863  0.0077  0.0260  0.0004  0.34  223.30  90.55  173.50  6.61  165.20  2.35  1.050  GDiD  BS059-i  0.0497  0.0011 0.1812  0.0041  0.0259  0.0002  0.36  179.60  50.21  169.10  3.53  165.00  1.31  1.025  GDiD  BS059-j  0.0500  0.0012 0.1867  0.0047  0.0263  0.0002  0.35  192.60  55.24  173.80  4.00  167.60  1.45  1.037  GDiD  BS059-k  0.1240  0.0023 6.1630  0.2682  0.3701  0.0041  0.26  2014.30 33.08  1999.20 38.01  2029.90 19.45  0.985  GDiD  BS059-l  0.0504  0.0017 0.1789  0.0061  0.0260  0.0003  0.34  214.10  74.91  167.20  5.23  165.50  1.89  1.010  GDiD BS059-m 0.0503  0.0010 0.1835  0.0036  0.0265  0.0002  0.36  207.80  43.23  171.10  3.10  168.60  1.18  1.015  GDiD  BS059-n  0.0487  0.0011 0.1778  0.0042  0.0263  0.0002  0.36  131.10  52.22  166.10  3.58  167.10  1.36  0.994  GDiD  BS059-o  0.0505  0.0013 0.1831  0.0049  0.0262  0.0002  0.35  217.20  58.32  170.70  4.17  166.60  1.54  1.025  GDiD  BS059-p  0.0481  0.0009 0.1781  0.0036  0.0266  0.0002  0.35  102.20  45.79  166.40  3.14  169.20  1.20  0.983  GDiD  BS059-q  0.0506  0.0013 0.1837  0.0049  0.0263  0.0002  0.34  222.70  58.10  171.20  4.17  167.40  1.52  1.023  GDiD  BS059-r  0.0505  0.0016 0.1844  0.0059  0.0255  0.0003  0.35  217.50  70.47  171.90  5.08  162.60  1.82  1.057  GDiD  BS059-s  0.0493  0.0011 0.1870  0.0045  0.0267  0.0002  0.35  163.80  52.58  174.10  3.80  169.70  1.39  1.026  GDiD  BS059-t  0.0539  0.0019 0.1966  0.0072  0.0263  0.0004  0.37  367.40  77.49  182.20  6.09  167.30  2.21  1.089  169.60  1.90  167.51  0.72  Weighted average (95% confidence level)  Xenocryst  Pb Loss  80  3.2.6	Roosevelt Granodiorite The Roosevelt Granodiorite is found in the Roosevelt Mine area approximately 500 m south east of the center of the SWOZ. It is a plagioclase, hornblende and biotite porphyritic granodiorite with a fine-grained quartz and feldspar matrix. Although, it has only been found in the Roosevelt Mine area and is areally minor, Gaspar (2005) suggested that it was the mineralizing intrusion. A sample was collected to confirm its age. The sample contained a more diverse zircon population than the other rock types. The zircons were commonly subhedral or equant, and were usually moderately to intensely fractured. The zircons fell into two size categories. About 20 % of the grains were 200 to 300 μm with the remaining 80 % being 100 to 150 μm. Growth zoning and inherited cores were generally not visible with BSE imaging, but a few inclusions were found. Investigation with cathodoluminescence microscopy (CL) after dating revealed that the grains had more diversity than detected with BSE imaging. CL imaging showed that grains had concentric growth zones and up to two stages of core growth as seen in Figure 3.16. Analysis of the Roosevelt Granodiorite gives a concordant age of 50.5 ± 3.0 Ma and a weighted average 206Pb/238U age of 50.5 ± 2.9 Ma (Figure 3.17). These ages are based on the youngest two grains of the twenty analysed. Six of the grains had discordant ages and the rest were xenocrysts or suffered from inheritance. The xenocrysts have a wide range of ages that fall into six age divisions: 57 Ma, 67 Ma, 82 Ma, 168 Ma, 390 Ma and 1.5 Ga (Table 3.10, Figure 3.16, and Figure 3.17). The reported age agrees within error with the previously determined age of 52.7 ± 1.1 Ma (Gaspar, 2005). It is clear from the inherited ages and the CL images (Figure 3.16) that the Roosevelt Granodiorite has had a more complex melting and emplacement history than the other igneous rock present at Buckhorn. Based on the inherited ages, the Roosevelt Granodiorite likely incorporated portions of the Paleozoic and Proterozoic country rock and Middle Jurassic BIS during emplacement. The zircons with Late Cretaceous and Early Paleogene ages were possibly incorporated from Keller Butte Suite rocks found adjacent to the nearby Toroda Creek half Graben (Holder and McCarley Holder, 1988; Suydam and Gaylord, 1997). A similar range of inherited ages was reported with the first dating of the Roosevelt Granodiorite (Gaspar, 2005). The cores visible with CL imaging are not always inherited. For example Figure 3.16 shows a grain that appears to have an inherited core, but had a concordant age of 50 Ma. These pseudo-cores are proposed to be the result of multiple stages of zircon growth and corrosion as described in Corfu et al. (2003). A correlation between the size of the  81  zircon and its age, or the quality of its analysis was not detected. These results confirm the Eocene age of the Roosevelt Granodiorite, as well as further describe its complex genesis.  100 µm Figure 3.16: CL images and ages of all the concordant zircon crystals from sample BS076. Ablation lines are highlighted with white lines  82  BS076: Roosevelt Granodiorite  data-point error ellipses are 2σ box heights are 2σ  Mean = 50.5±2.9 [5.7%] 95% conf. Wtd by data-pt errs only, 4 of 6 rej. MSWD = 0.025, probability = 0.87 (error bars are 2σ)  85  0.12 Pb206/U238 Age (Ma)  75  207Pb/206Pb  0.10  65  55  45  0.08  35  0.06  0.04 0  40  80  120  160  238U/206Pb  Figure 3.17: Tera-Wasserburg plot with weighted average diagram for sample BS076  83  Table 3.10: Isotope ratios and age estimates for sample BS076 Table 3.10 Rock Type  Sampleanalysis  RGDi RGDi  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios Pb / Pb  Age estimates (Ma)  Pb / U  ± 1σ  BS076-1  0.0484 0.0026 0.0718  0.0037  0.0105 0.0002 0.33  117.5  119.71 70.4  3.51  67.1  1.15  1.049  Xenocryst  BS076-2  0.0509 0.0019 0.0747  0.0027  0.0103 0.0001 0.38  235  82.56  73.2  2.53  66.3  0.87  1.104  Discordant  RGDi  BS076-3  0.0959 0.0013 3.5121  0.0851  0.2548 0.0018 0.29  1546  25.06  1529.9  19.15  1463.2  9.19  1.046  Xenocryst  RGDi  BS076-4  0.0974 0.0016 3.7752  0.1173  0.2687 0.0023 0.27  1574.4  30.25  1587.5  24.93  1534.2  11.66  1.035  Xenocryst  RGDi  BS076-5  0.0491 0.0015 0.1664  0.0051  0.0248 0.0003 0.35  154.1  69.1  156.3  4.46  157.8  1.71  0.990  Xenocryst  RGDi  BS076-6  0.0496 0.0010 0.1784  0.0039  0.0265 0.0002 0.37  177.8  48.1  166.7  3.31  168.5  1.32  0.989  Xenocryst  RGDi  BS076-7  0.0488 0.0012 0.1797  0.0045  0.0265 0.0002 0.36  138.7  56.73  167.8  3.91  168.7  1.51  0.995  Xenocryst  RGDi  BS076-8  0.0503 0.0091 0.0526  0.0092  0.0078 0.0004 0.30  210.7  370.74 52  8.88  50.2  2.59  1.036  RGDi  BS076-9  0.0484 0.0011 0.0585  0.0013  0.0090 0.0001 0.36  119.2  50.99  57.7  1.21  57.5  0.47  1.003  Xenocryst  RGDi BS076-10 0.0465 0.0010 0.0572  0.0012  0.0089 0.0001 0.38  23.7  49.94  56.5  1.15  57  0.45  0.991  Xenocryst  RGDi BS076-11 0.0561 0.0115 0.0605  0.0121  0.0075 0.0004 0.29  455.8  400.83 59.6  11.58  47.9  2.82  1.244  Discordant  RGDi BS076-12 0.0461 0.0058 0.0540  0.0067  0.0079 0.0003 0.29  2  279.06 53.4  6.42  50.7  1.79  1.053  RGDi BS076-13 0.0419 0.0125 0.0487  0.0144  0.0084 0.0005 0.21  0.1  385.41 48.3  13.89  53.9  3.26  0.896  Discordant  RGDi BS076-14 0.0626 0.0101 0.0984  0.0155  0.0106 0.0006 0.34  694.7  309.97 95.3  14.28  68.3  3.61  1.395  Discordant  RGDi BS076-15 0.1000 0.0010 3.6196  0.0597  0.2612 0.0014 0.33  1624.2  18.92  1553.8  13.11  1496.2  7.22  1.038  Xenocryst  RGDi BS076-16 0.0605 0.0008 0.5243  0.0075  0.0625 0.0004 0.39  620  26.96  428  5.02  390.5  2.13  1.096  Xenocryst  RGDi BS076-17 0.0519 0.0025 0.0928  0.0044  0.0129 0.0002 0.36  282.2  105.76 90.1  4.04  82.4  1.42  1.093  Xenocryst  RGDi BS076-18 0.0476 0.0017 0.0873  0.0031  0.0128 0.0002 0.35  76.8  83.76  2.88  82.2  1.04  1.034  Xenocryst  RGDi BS076-19 0.0572 0.0048 0.0626  0.0051  0.0078 0.0002 0.35  499.1  175.46 61.6  4.86  50  1.41  1.232  Discordant  RGDi BS076-20 0.0312 0.0073 0.0353  0.0082  0.0084 0.0004 0.18  0.1  0  35.2  8.03  53.8  2.24  0.654  Discordant  53  10  50.5  2.9  207  206  ± 1σ  Weighted average (95% confidence level)  207  235  Pb / U 206  238  ± 1σ  Rho  Pb / Pb  ± 1σ  207  206  Pb / U 207  235  85  ± 1σ  Pb /238U 206  ± 1σ  Discord.  Comments  84  3.2.7	Samples with Inconclusive Results Two samples, BS047 and BS062, of Quartz Porphyry dike were collected to confirm the age of the unit and constrain the timing of deformation and skarn alteration. Based on cross-cutting relationships and the previous dating done by Gaspar (2005), the Quartz Porphyry dikes are the youngest of the Jurassic intrusions. They usually cross-cut skarn mineralization, but as shown in Chapter 2 may be skarn altered. Sample BS062 has at least one of the two low angle foliations (Figure 2.16). Sample BS062 contained a single zircon that was large enough for analysis. The grain was approximately 75 by 200 μm and was subhedral. Minor zoning was noted from BSE imaging, but a core was not visible. However, an inherited core was clearly visible with CL imaging performed after dating (Figure 3.18). Three ablation lines were done on the grain; they were affected by the inherited core and returned ages of approximately 177 Ma, 1795 Ma and 1829 Ma (Table 3.11). The first age is interpreted as a mixing age between the core and the rim, and the latter two as the age of the core. Similar age xenocrysts were found when Gaspar (2005) determined a weighted average 206Pb/238U age of 163.6 ± 0.8 Ma for the Quartz Porphyry dikes. Sample BS047 contained five zircons, but they were not large enough for dating. Sample BS061 was collected from a rhyolite layer that is conformable with the BMS rocks (Figure 2.1). It was chosen to provide a date for the BMS, which has not been dated isotopically, and is one of the host rocks for the gold mineralization and skarn alteration. The sample did not contain any zircons and therefore could not be dated. The Permian ages assigned based on the faunal assemblage remains the best age for the BMS. Sample BS099 was selected for U-Pb geochronology from the Mafic Diorite, which is presumed to be an early dike phase of the BIS. The sample did not contain any zircons, so it could not be dated. In the absence of contradictory evidence the Mafic Diorite will continue to be presumed to be Middle Jurassic in age and an early dike phase of the BIS.  85  A  BB  Figure 3.18: BSEFigure (A) and image ofimage the zircon from BS062 X.18 CL BSE (B) (A) and CL (B) of singlerecovered zircon retrieved fromsample sample BS062  86  Table 3.11: Isotope ratios and age estimates for sample BS062 Table 3.11  U/Pb LA-ICP-MS data and calculated ages Isotopic ratios  Age estimates (Ma)  Rock Type  Sampleanalysis  QP  BS062-aa  0.0470  0.0009 0.1793  0.0038 0.0279  0.0002 0.36  QP  BS062-bb  0.1050  0.0024 4.3908  0.2007 0.3211  QP  BS062-cc  0.1080  0.0023 4.9746  0.2293 0.3280  Pb / Pb 207  206  ± 1σ  Pb / U 207  235  ± 1σ  Pb / U 206  238  ± 1σ  Pb / U  47  46.28  167.5  3.23  177.3  0.0040 0.27  1713.7  40.75  1710.6  37.8  0.0040 0.26  1765.3  38.88  1815  38.97  ± 1σ  Rho  Pb / Pb 207  206  207  235  ± 1σ  Pb /238U  Discord.  Comments  1.32  0.9447  Inheritance  1795  19.55  0.9530  Xenocryst  1828.6  19.27  0.9926  Xenocryst  206  ± 1σ  87  3.3	 Re-Os Geochronology of Molybdenite 3.3.1	Introduction and Methods Prior to this study there was uncertainty regarding the age of gold mineralization at Buckhorn. Early authors correlated the skarn alteration and gold mineralization with the Middle Jurassic Buckhorn Intrusive Suite (Hickey, 1990, 1992) whereas the more recent work (Gaspar, 2005) suggests that alteration and mineralization are related to the Eocene Roosevelt Intrusive Suite. Local small scale mapping and petrography (Figure 2.9 and Figure 2.11), combined with U-Pb geochronology showed that the skarn alteration is spatially related to the Granodiorite dikes and has a maximum age of 167.5 ± 0.8 Ma (Figure 3.15), however a definitive minimum age was not determined. Re-Os geochronology was performed on two samples of rare molybdenite mineralization that postdates the skarn alteration, the resulting ages were used to define the minimum age of skarn alteration and gold mineralization. The Re-Os geochronology was performed by Dr. Roberta A Creaser’s lab at the University of Alberta following their protocol as outlined in Lawley et al. (2010) and summarized below. Molybdenite concentrates were generated using modified mineral separation techniques that minimized contact with metal. Samples were pulverized in a porcelain disc mill and magnetic minerals were removed using a Frantz isodynamic magnetic separator. Molybdenite was separated using a combination of heavy liquids and flotation with high purity water. Final molybdenite concentrates were handpicked under a microscope as a last step in quality control. Molybdenite is the preferred mineral for Re-Os geochronology because of its tendency to incorporate large concentrations of Re and insignificant non-radiogenic Os at the time of crystallization (Stein et al., 2001). This property allows a Re-Os model age to be calculated from the simplified isotope equation, t = ln(187Os/ 187Re + 1)/λ, where t = model age, and λ = 187Re decay constant, 1.666 × 10–11 yr–1; (Smoliar et al., 1996). 187Re and 187Os were measured using isotope dilution mass spectrometry following the methods of Selby and Creaser (2004). Mass-dependent fractionation during analysis was corrected by using a mixed double spike (Markey et al., 2007). Samples were equilibrated with the Re-Os tracer using the Carius tube method. Purified Re and Os concentrates were then loaded onto barium-coated Pt filaments and the respective isotopic concentrations were measured using negative thermal ionization mass spectrometry (Creaser et al., 1991). Total procedure blanks are on the order of <5 pg for Re and <2 pg for Os. Re-Os model age errors are reported at 2σ and account for uncertainties in mass spectrometry measurements, spike and standard Re-Os isotope compositions, and the Re-Os decay constant of Smoliar et al. (1996). 88  3.3.2	Molybdenite in Skarn Altered Diorite The skarn sample with the highest molybdenite content is sample BS068, an example of epidote-pyroxene skarn with a Buckhorn Diorite protolith. BS068 was collected from a diamond drill hole, D09-513, north of the Magnetic Mine (Figure 2.1). The sample is made up of intense prograde pyroxene and retrograde epidote alteration with abundant late molybdenite and minor pyrrhotite and chalcopyrite. The molybdenite occurs in intercrystalline space between the silicate skarn minerals or in vein-like bands and locally makes up approximately 1% of the rock (Figure 3.19). Chalcopyrite and pyrrhotite make up a trace amount of the rock and occur with quartz in intercrystalline space between skarn minerals (Figure 3.19). Based on its location between prograde and retrograde minerals the molybdenite formed towards the end of retrograde alteration. Sample BS068 was not gold mineralized and molybdenite was not found in contact with the other sulfides, so the timing of molybdenite alteration relative to the typical sulfide alteration and gold mineralization is unknown. However, as gold mineralization was documented to also occur towards the end of retrograde alteration the molybdenite is assumed to be approximately coeval with gold mineralization. 3.3.3	Molybdenite in Skarn Altered Metasedimentary Rocks A second sample of molybdenite-bearing skarn, BS067, was collected from a diamond drill hole, D08-484, below the SWOZ. Sample BS067 contains less molybdenite than sample BS068, but the former is closer in character to typical gold-bearing skarn in the SWOZ. Sample BS067 is made up of garnet and lesser pyroxene prograde alteration with moderate amphibole and minor hydrated Fe-silicate retrograde alteration (Figure 3.19). Unlike the typical skarn sample BS067 contains a trace amount of molybdenite that occurs as 20 to 200 μm anhedral crystals. The molybdenite preferentially occurs along quartz crystal boundaries in intercrystalline space between prograde and retrograde minerals. The molybdenite rims rare pyrrhotite in the sample, and therefore postdates the majority of retrograde alteration (Figure 3.19). The sample did not contain any gold grains so the relative timing of molybdenite and gold is unknown, but as noted with sample BS068, based on its place in the paragenesis molybdenite is presumed to be approximately coeval with gold.  89  1cm  250μm  Mo Qtz  Hyd Fe-Sil Mo  Px-Ep Px  Ep A2  A Ep  250μm  1cm  G  Hyd Fe-Sil  Px-Amph Mo  Mo 165.5 ± 0.7 Ma Px 250µm  Hyd Fe-Sil  B  A3 Px-Amph  125µm  Po  Mo  Qtz  G G  Amph Qtz Mo 162.8 ± 0.7 Ma  B2  B3  FigureFigure 3.19: Examples of molybdenite bearingbearing skarn. The photograph (A), PPL(A), photomicrograph X.x: Examples of molybdenite skarn. The photograph PPL (A2), and RL photomicrograph (A3) show sample BS068 an example of Px-Ep endoskarn. photomicrograph (A2), and RL photomicrograph (A3) show sample BS068 an This sampleexample was datedofby Re-Os geochronology in molybdenite at 165 ± 0.7 Ma (Table 3.12). The Px-Ep endoskarn. This sample was dated by Re-Os geochronology in photograph (B), PPL photomicrograph (B2), and RL photomicrograph (B3) show sample molybdenite at 165 ± 0.7 Ma. The photograph (B), PPL photomicrograph (B2), BS067 an example ofRL garnet skarn from the BMS belowsample the SWOZ. wasofdated to 162.8 and photomicrograph (B3) show BS067The ansample example garnet skarn ± 0.7 Ma by Re-Os of molybdenite (Table 3.12).was Seedated text fortofurther fromgeochronology the BMS below the SWOZ. The sample 162.8 ±discussion 0.7 Ma by Re-Os geochronology of molybdenite. See text for further discussion 3.3.4	Results of Re-Os Geochronology The results of the Re-Os geochronology on the molybdenite in samples BS068 and BS067 are shown in Table 3.12. The analysis returned a model age of 165.5 ± 0.7 Ma for sample BS068 and 162.8 ± 0.7 Ma for sample BS067. Analyzed Re and Os concentrations in sample BS068 were 58.58 ppm and 101.65 ppb respectively, and 24.87 ppm and 42.47 ppb for sample BS067 (Table 3.12).  90  Table 3.12: Re-Os isotope data for molybdenite samples from Buckhorn Sample no.  Location  Re (ppm)  ± 2σ  187 Re (ppm)  ± 2σ  187 Os (ppb)  ± 2σ  Common Os (pg)  Model Age ( Ma)  BS068  Epidote-Pyroxene endoskarn with Buckhorn Diorite protolith  58.58  0.15  36.82  95  101.65  0.08  4  165.5  ± 2σ with λ (Ma)  BS067  Garnet Skarn with BMS protolith  24.87  0.08  15.63  47  42.47  0.07  33  162.8  0.7  0.7  3.4	 Summary and Discussion The current dating has added to the understanding of the host rocks and intrusive rocks at the Buckhorn gold skarn in several ways. It has confirmed the Middle Jurassic age of the BIS, with a more precise age for the Buckhorn Granodiorite at 170.4 ± 1.7 Ma. The dating has added to the understanding of the BIS by showing that the Buckhorn Diorite, 168.9 ± 0.9 Ma, is within error of the age of the Buckhorn Granodiorite and is therefore likely a marginal phase of the pluton. It also determined that there are four generations of dikes. The Mafic Diorite was not dated, but based on geochemistry, petrography and mineralogy is clearly distinct from the other dikes. The Early Diorite dikes (168.2 ± 0.7 Ma) were originally thought to predate the pluton, but they are shown to be the statistically the same age as the Granodiorite dikes and the Buckhorn Diorite. A more precise age was determined for the Granodiorite dikes of 167.5 ± 0.7 Ma. As skarn alteration was shown to occur in the halos of the Granodiorite dikes (Figure 2.9 and Figure 2.11), their new age provides a precise maximum age for skarn alteration and gold mineralization. A new age was not determined for the Quartz Porphyry dikes as insufficient zircons were present, but based on the cross-cutting relationships and the age (163.6 ± 0.8 Ma) determined by Gaspar (2005) they are interpreted as the youngest expression of the Middle Jurassic BIS. The new dates also confirm the Eocene age of the porphyritic Roosevelt Granodiorite, at 50.5 ± 2.9 Ma. The Re-Os geochronology provides the first age dates for sulfide alteration at Buckhorn and helps bracket the age of gold mineralization. In both samples the molybdenite forms after the prograde and retrograde alteration, so the dates provide definitive minimum ages for the skarn alteration at both sample locations. The paragenetic relationship between gold and molybdenite is unknown as the two minerals were not found in contact with each other, but as shown in Chapter 4 both gold and molybdenite occur towards the end of retrograde alteration and are therefore assumed to be approximately coeval. Based on this assumption the ages determined for the molybdenite also provide minimum ages for the gold mineralization. When combined with the U-Pb geochronology of local 91  intrusive rocks the Re-Os dates constrain the age of the skarn alteration and gold mineralization at Buckhorn to a 4.7 million year window in the Middle Jurassic between 167.5 Ma and 162.8 Ma (Figure 3.20). Based on these ages, the petrography and the small scale mapping presented in Chapter 2, it is clear that the skarn alteration and gold mineralization at Buckhorn is genetically related to the Buckhorn Intrusive Suite. Moly in Skarn-Alt BMS Quartz Porphyry Dikes Moly in Skarn-Alt BH Diorite Granodiorite Dikes BH Diorite Early Diorite Dikes BH Granodiorite 167.5 ± 0.8 Ma 174  170 Time (Ma)  162.8 ± 0.7 Ma 166  Figure X.x: Summary of Middle Jurassic ages at Buckhorn. Ages determined isotopically or estimated based on geolFigure 3.20: Summary of isotopically determined Middle Jurasogy. The range of possible ages of skarn alteration and gold sic ages at Buckhorn. The range of possible ages for skarn alteramineralization is denoted by the gold box. The Results from tion and gold mineralization is denoted by the gold box. Error the Re-Os Molybdenite geochronology study provide the bars are 2σ, See text for further discussion. minimum age and the age of the granodiorite dikes is used as the maximum age. Error bars are 2σ.  162 U-Pb Zircon (This Study) U-Pb Zircon Gaspar (2005) Re-Os Molybdenite Range of possible ages of skarn alteration and gold mineralization  The U-Pb dating has also constrained the age of the onset of deformation. Prior to this study it was unclear when the deformation events affecting the Buckhorn area occurred. The low angle foliations are most intensely developed in the host BMS rocks leading some authors to suggest that their development predated the emplacement of the BMV and the intrusive rocks (McMillen, 1979). The poly-deformed Granodiorite dike, sample BS059, however, shows that the low angle and steeplydipping foliations formed after the emplacement of the Granodiorite dikes at 167.5 ± 0.72 Ma. This contrasts earlier work that proposed that only the high angle foliation post-dated the igneous rocks and affected the skarn alteration (Gaspar, 2005; McMillen, 1979). Another significant outcome of the present work is the new age of the BMV. The BMV rocks have previously been correlated with a number of units with a wide range of ages including the Permian Kobau Formation (Hickey, 1990, 1992; McMillen, 1979), the Triassic Brooklyn Formation  92  (McMillen, 1979), and the Jurassic Elise Formation (Cheney et al., 1994; Gaspar, 2005). Based on geochemical and geologic similarities Gaspar (2005) concluded that they were likely equivalent to the Jurassic Elise Formation, but said that they were also compositionally similar to varieties of the Permian Attwood Group and the Triassic Brooklyn Formation. Based on the Early Jurassic age of 193.5 ± 1.2 Ma for an intrusion comagmatic with the BMV it is now clear that the BMV, is equivalent to the Jurassic Elise Formation.  93  Chapter 4.	 Skarn Alteration and Gold Mineralization 4.1	 Categories of Gold Skarns Gold skarns can be categorized as reduced, oxidised, magnesian, or metamorphic. These categories are defined based the mineralogy of the prograde and retrograde alteration, the character of the intrusive and host rocks, and the regional tectonic setting (Meinert et al., 2005). Reduced gold skarns are the highest grade and best studied of the gold skarn categories (Meinert et al., 2005). Their prograde mineralogy is dominated by Fe-rich pyroxene (>Hd50), but can contain significant grossular-andradite garnet in proximal zones. The retrograde mineralogy is more diverse and is composed of K-feldspar, scapolite, vesuvianite, apatite, amphibole, and abundant sulfide minerals dominated by pyrrhotite and arsenopyrite (Meinert et al., 2005).The reduced gold skarns typically form from clastic-rich protoliths and are associated with diorite-granodiorite plutons and dikesill complexes (Meinert, 2000). These intrusive rocks are typically emplaced at shallow depths (<5km) and arc related (Meinert, 2000). Other features of reduced gold skarns include their Au-Bi-Te-As geochemical signature, early and/or distal hornfels alteration, and lack of economic concentrations of metals other than gold (Meinert et al., 2005). Oxidized gold skarns are lower grade but have produced more gold than reduced gold skarns (Meinert, 2000). They are characterized by a prograde mineral assemblage composed of Fe poor garnet with lesser pyroxene, and a retrograde assemblage that is made up of abundant K-feldspar and quartz, with lesser pyrite, pyrrhotite and minor but ubiquitous chalcopyrite, sphalerite, and galena (Meinert et al., 2005). Oxidized gold skarns form in similar host rocks as the reduced gold skarns, but are associated with more oxidized and silicic plutons that were emplaced at a similar depth and in a similar tectonic setting (Meinert, 2000). Oxidized gold skarns have the same Au-Bi-Te-As geochemical signature, early and/or distal hornfels alteration, and lack of economic concentrations of metals other than gold (Meinert, 2000). Magnesian gold skarns are less abundant that reduced and oxidised gold skarns. They are distinguished from the latter two on the basis of their magnesium rich mineralogy, which is dominated by forsterite, spinel, and serpentine (Meinert et al., 2005). Magnetite is the most abundant spinel mineral and most of the magnesian gold skarns are mined for their Fe content in addition to gold (Meinert et al., 2005). Magnesian gold skarns form from dolomitic protoliths and can form in the  94  same tectonic setting, associated with similar intrusive rocks to the reduced and oxidised gold skarns (Meinert, 1998; Meinert et al., 2005). Metamorphic gold skarns are distinguished from the other categories of gold skarn primarily on the basis of their tectonic setting (Meinert et al., 2005). Metamorphic skarns occur in orogenic belts where the skarn alteration is associated with both plutonism and high-temperature and high-pressure metamorphism. This contrasts the skarns in the aforementioned categories that have a clear genetic relationship with relatively shallow Phanerozoic plutons that intruded into previously unmetamorphosed sedimentary rocks (Meinert et al., 2005). Some of the metamorphic gold skarns occur in Precambrian terranes and are not associated with intrusive rocks (Meinert et al., 2005). The metamorphic gold skarns typically have a reduced mineralogy made up of Fe-rich almandine-spessartine garnet, hedenbergite pyroxene, and Fe-rich amphibole. The mineralogy may be controlled by Fe-rich protoliths such as Fe formations, komatiites, and metabasalts (Meinert et al., 2005). These deposits typically have the same Au-As-Bi-Te geochemical signature as the reduced, oxidised, and magnesian gold skarns (Meinert et al., 2005).  4.2	 Introduction Skarn alteration at Buckhorn dominates at three locations: The Southwest Ore-Zone (SWOZ), the Gold Bowl Ore Zone and the Magnetic Mine (Figure 2.2). The SWOZ is two sub-parallel, subhorizontal zones located along the upper and lower contact of the upper carbonate member of the BMS (Figure 2.4). The majority of the skarn alteration in the SWOZ forms as a relatively thin (115m) layer replacing both the metasedimentary and metavolcanic rocks along the upper contact. A subordinate portion of the SWOZ replaces both carbonate and clastic metasedimentary rocks along the lower contact of the carbonate member. In addition to these locations, a minor amount of skarn is developed in lenses within the BMS below the SWOZ (Figure 2.4). Skarn alteration in the Gold Bowl is thicker, up to 200 m, and patchy compared to the SWOZ. The Gold Bowl skarn alteration occurs in clastic metasedimentary and igneous rocks in the hanging wall of the footwall mylonite. The Magnetic Mine skarn has a larger areal extent than the Gold Bowl skarn and occurs in metasedimentary rocks at the contact with the southwest margin of the Buckhorn Diorite (Figure 2.2). In addition to these locations, lesser amounts of skarn alteration occur elsewhere in the Buckhorn area, such as at Mike’s Skarn and in alteration halos around Granodiorite dikes (Figure 2.2).  95  Gold mineralization at Buckhorn is predominately hosted in skarn altered rocks. The majority of economic gold mineralization occurs in the upper and lower SWOZ, with additional economic mineralization in the Gold Bowl (Cooper et al., 2008). Gold mineralization is unevenly distributed through the skarn, and significant portions of skarn alteration in the SWOZ and Gold Bowl are not gold mineralized. Gold has also been discovered in non skarn-altered rocks, although not in economic abundances. The Magnetic Mine does not host economic gold mineralization, but was mined during the first half or the 20th century for small amounts of iron, copper, silver, gold and magnesium (Moen, 1980). Gold mineralization also occurs in some of the minor skarn occurrences in the BMS, such as at Mike’s Skarn, but none of these are currently considered to be economic. To better understand the skarn alteration and gold mineralization at Buckhorn detailed optical petrography was performed on 55 samples from the SWOZ and Gold Bowl (Figure 4.1 and Table 4.1). To complement the petrography, back-scattered electron (BSE) imaging was done on 7 skarnaltered samples and major element microprobe analysis was performed on pyroxene and amphibole from 5 skarn-altered samples. Diamond drill holes were logged and small scale underground mapping was also performed in the SWOZ. This work has five main goals: (1) to define the character of the skarn alteration assemblages, including the zonation and paragenesis; (2) to determine the relative timing of skarn alteration, deformation, and gold mineralization; (3) to describe the character of gold-bearing, non skarn-hosted rocks; (4) to define different settings of gold mineralization and to determine their relative age relationships; and (5) to describe the differences and similarities between Buckhorn and other gold skarns. In particular, to compare Buckhorn to the Nickel Plate deposit in the Hedley mining district, one of the largest and best studied reduced gold skarns located approximately 90 km to the northwest (Figure 1.1). The descriptions that follow are first divided by the three major skarn assemblages and then further subdivided based on retrograde alteration mineral assemblages. With the benefit of greater access to the SWOZ than the previous studies, due to additional drilling and underground mining, the current work is able to provide a more detailed and comprehensive description of the skarn alteration and gold mineralization at Buckhorn.  96  Table 4.1: Table of diamond drill holes and skarn samples selected for petrographic analysis Range of Gold Grade (ppm)  Drill Hole  Location  Samples  Types of Skarn Alteration  Analysis Performed  D07-323  SWOZ  BS007-09  Pyroxene and Magnetite  Petrography  0.2 - 21.7  D07-325  SWOZ  BS020-21  Pyroxene  Petrography  11.1 - 27.4  D07-326  SWOZ  BS010-12  Pyroxene  Petrography  0.1 - 31.8  D07-327  SWOZ  BS005-06  Pyroxene  Petrography  <0.1 - 2.8  D07-329  SWOZ  BS002-04  Pyroxene and Magnetite  Petrography  9.7 - 319.8  D07-358  SWOZ  BS018-19  Pyroxene  Petrography  0.6 - 20.5  D07-369  SWOZ  BS082-84  Pyroxene, Garnet, and Magnetite  Geochemistry and Petrography  <0.1 - 62.0  D07-394  SWOZ  BS022-23  Pyroxene and Garnet  Petrography  <0.1 - 0.1  D08-409  SWOZ  BS024  Garnet  Petrography  36.0  D08-410  SWOZ  BS090-94  Pyroxene, Garnet, and Magnetite  Geochemistry, Microprobe Analysis, and Petrography  <0.1 - 51.1  D08-413  SWOZ  BS016-17, BS085-88  Pyroxene, Garnet, and Magnetite  Geochemistry and Petrography  <0.1 - 117.5  D08-414  SWOZ  BS013-15  Magnetite and Pyroxene  Petrography  <0.1 - 172  D08-420  SWOZ  BS025-28  Pyroxene  Petrography  0.6 - 6.9  D08-443  GB  BS077-79, BS089  Pyroxene and Garnet  Geochemistry, EDS, and Petrography  <0.1 - 53.7  D08-480  SWOZ  BS030  Garnet  Petrography  0.1  D08-484  SWOZ  BS067  Molybdenite bearing Garnet  Re-Os Geochronology and Petrography  D09-490  SWOZ  BS033-34  Pyroxene  Petrography  D09-513  SWOZ  BS068  Molybdenite bearing Epidote-Pyroxene  Re-Os Geochronology and Petrography  D09-536  North of GB  BS080-81  Pyroxene, Garnet, and Magnetite  Geochemistry and Petrography  <0.1 - 0.7  D10-569  Below SWOZ  BS095-96  Skarn veins  Geochemistry and Petrography  <0.1 - 24.1  D10-595  Below SWOZ  BS097-98  Skarn veins  Geochemistry and Petrography  <0.1 - 3.5  30.0 - 67.2  97  Legend 355,000 m E  354,000 m E  5,425,000 m N  FM  Diamond Drill With Skarn Samples for Petrography Diamond Drill With Skarn Samples for Petrography and Geochemistry Re-Os Geochronology Samples  MM  5,424,000 m N  GB  5,423,000 m N  F  N  NL  SWOZ  RM 250m  Figure: Geologic of Buckhorn Mountain with the skarn,skarn skarn Figure 4.1: Geologic map map of Buckhorn Mountain with the skarn petrography, geochemistry and Re-Os geochronology locations highlighted. Based on data from this study and modigeochemistry andsample Re-Os geochronology sample locations highlighted. fied from Kinross Gold from Corporation maps. and See for map legend. Based on data this study modified from Kinross Gold Corporation maps. See Figure X.x for map legend.  98  Table 4.2: Mineral compositions at Buckhorn as determined by microprobe analysis, performed by Gaspar (2005). In this thesis, minerals will be referred to by their category, except where the distinction can be made based on optical properties. Mineral Category  End Member Minerals at Buckhorn  Pyroxene  Diopside(64)  Diopside  CaMgSi2O6  Hedenbergite  CaFeSi2O6  Grossular(70)  Grossular  Ca3Al2(SiO4)3  Andradite(99)  Andradite  Ca3Fe2(SiO4)3  Clinozoisite  Ca2Al3(SiO4)3(OH)  Epidote  Ca2(Fe,Al)3(SiO4)3(OH)  Hedenbergite(93) Garnet Epidote  Clinozoisite (XFe=14) Epidote (XFe=33)  Amphibole  Hydrated Fe-silicate  Tremolite  Ca2Mg5(Si8O22)(OH)2  Fe-Actinolite  Ca2Fe5(Si8O22)(OH)2  Fe-hornblende  Ca2Fe4(Al,Fe)(Si7AlO22)(OH)2  Fe-Tschermakite  Ca2Fe3AlFe(Si6Al2O22)(OH)2  Hastingsite  NaCa2Fe4(Al,Fe)(Si6Al2O22)(OH)2  Grunerite  Fe7(Si8O22)(OH)2  Ekmanite  (Fe,Mg,Mn)3(Si,Al)4O10(OH)2-2(H2O)  Ferrostilpnomelane Greenalite Minnesotaite Bismuth phases  Native Bi Bismuthinite  K(Fe,Mg)8Si10Al2O24 (OH)3-2(H2O) Fe2Si2O5(OH)4 (Fe,Mg)3Si4O10(OH)2 Bi Bi2S3  Joseite  Bi4TeS2  Pilsenite  Bi4Te3  Sulphotsumoite  Bi3Te2S  Tetradymite  Bi2Te2S  Joseite-B  Bi4Te2S  Tsumoite  BiTe  unknown Bi Sulfide Gold  End Member Mineral Formula  Gold Electrum  (Bi,Pb)6AuS5 Au(100) Au(88)Ag(12)  XFe = 100*(Fe3+/( Fe3+ + Al))  99  Gar+Mag>Px  355,000 m E  FM  M  FM  Legend  5,425,000 m N  Gar+Mag=Px Px>Gar+Mag  ag ne  tic  M  in  e  Highest concentration of: Garnet Skarn Magnetite Skarn Ep-Px Skarn Px Skarn occurs throughout SWOZ  Gold Bowl  5,424,000 m N  A  SWOZ  NL F  Gold Bowl F NL  SWOZ  5,423,000 m N  355,000 m E  A` N RM 250m  NLF  A  Skarn Metacarbonate BMS  100m 100m  SWOZ  N  A`  BMV BDi  Metaclastic BMS  Figure 4.2: Geologic map and cross-section of Buckhorn Mountain with the schematic skarn zonation highlighted. Pyroxene skarn occurs throughout the SWOZ. The thickness of skarn alteration on cross-section is exaggerated by a factor of 2 to more clearly illustrate zoning. Based on data from Figure: map and Buckhorn Mountain schematic this studyGeologic and modified fromcross-section Kinross GoldofCorporation maps. Seewith andthe Figure 2.2 forskarn map and zonation highlighted. The Thickness of skarn alteration on cross-section is exaggerated by a cross-section legends. factor of 2 to more clearly illustrate zoning. Based on data from this study and modified from Kinross Gold Corporation maps. See Figure X.x and X.x for map and cross-section legends. Co-ordiante system: UTM NAD27, Zone 11U  100  4.3	 Pyroxene Skarn Pyroxene skarn is the most abundant skarn type at Buckhorn, composing the majority of the SWOZ and a significant portion of the Gold Bowl. Pyroxene skarn is more prevalent near the west and south margins of the SWOZ and near the base of the Gold Bowl, but does occur throughout both ore bodies (Figure 4.2). Pyroxene skarn formation is texturally destructive; making identification of the protolith difficult to impossible. Based on the intensity and type of retrograde alteration the pyroxene skarn can be classified into three sub-categories: (1) Amphibole-Pyroxene, (2) MagnetitePyroxene, and (3) Epidote-Pyroxene. 4.3.1	Amphibole-Pyroxene Skarn Amphibole-pyroxene skarn is the most abundant variety of pyroxene skarn at Buckhorn. This type of skarn has a prograde assemblage composed of greater than 90 % pyroxene along with minor garnet (<10 %) and magnetite (<10 %). Retrograde alteration is dominated by amphibole, which replaces 20 to 80 % of the pyroxene. Other retrograde and accessory minerals such as epidote, hydrated Fe-silicates, calcite and quartz occur in minor amounts, however, in rare locations calcite may compose up to 40 % of the rock. Sulfide minerals are part of the retrograde assemblage and usually compose up to about 10 % of the rock. Pyrrhotite is the most abundant sulfide mineral (5-10 %), and is accompanied by lesser pyrite and chalcopyrite, and trace sphalerite, gold, native bismuth and other Bi-minerals. In hand sample the amphibole-pyroxene skarn is moderate to dark green depending on the amphibole content, with the darker specimens corresponding to higher amphibole contents (Figure 4.3 and Figure 4.4).  101  500µm  1cm  Gar  Px  Po>Py  HydFe-Sil  B2 Amph A 250μm  B3  100μm  Bi Po Py2 Py1  B  Po  Gar  Py2  Px-Amph  Bis Au  B3 100μm  Au/Bi in Amph  Amph  Ca Ca  C2  Au intergrown with Bi  Px  Bis 100µm  Au Bis  Ca  ph Am  Px-Amph  Au  C 50μm  Bi-Au  B2  250μm  Px-Amph  Py1  C2 Cpy  Bis D  E  Au indicated by EDS  X.x: Examples of amphibole-pyroxene skarn. The photograph (A), PPL FigureFigure 4.3: Examples of amphibole-pyroxene skarn. The photograph (A), PPL photomicrographs (B,(B2, C) , RL andelemental BSE image (B3)(E) show (B, C),photomicrographs RL photomicrograph C2,photomicrograph D, E), BSE image (B2, (B3),C2, andD)EDS map show the mineralogy, texture and paragenesis of the skarn alteration and the mineralogy, texture and paragenesis of the skarn alteration and gold mineralization.gold See text for See text for further discussion. furthermineralization. discussion.  102  Pyroxene occurs as 100 μm to 1 mm, subhedral to euhedral stubby prismatic crystals (Figure 4.3 and Figure 4.4). Pyroxene typically forms in a decussate, polygonal or web texture with poikiloblastic calcite and minor quartz. When present, garnet occurs as near euhedral, anisotropic, concentrically and/or hexagonally zoned crystals that rarely include pyroxene (Figure 4.3 and Figure 4.4). The garnet crystals are typically larger than pyroxene, and range from 200 μm to 2 mm in diameter. Garnet crystals tend to be concentrated in discrete masses, rather than evenly distributed through the skarn (Figure 4.3 and Figure 4.4). Magnetite occurs as subhedral to anhedral near equant 50 μm crystals that are found as discrete grains or masses irregularly distributed through the pyroxene (Figure 4.3 and Figure 4.4). The paragenesis of prograde minerals is consistent for all the amphibole-pyroxene skarn samples collected. Pyroxene was the first mineral to form followed by garnet and magnetite. The relative timing of garnet and magnetite alteration in amphibole-pyroxene skarn is ambiguous because they are not in contact with each other, but their relative timing can be interpreted from other varieties of skarn in which garnet formed before magnetite (Figure 4.3 and Figure 4.4). Amphibole is the most abundant retrograde mineral and occurs as elongate to fibrous, subhedral to anhedral crystals that are typically smaller than the prograde pyroxene. Amphibole alteration replaces and overgrows pyroxene and in many cases destroys the prograde texture. Amphibole alteration continues through the retrograde alteration, and some of the later stages of amphibole alteration are intergrown with gold mineralization (Figure 4.3). Epidote is the next most abundant retrograde silicate mineral. Most of the epidote occurs as anhedral aggregates replacing pyroxene and/or garnet, however, some of the epidote crystals are subhedral hexagonal or rectangular crystals. Sulfide minerals form a minor portion of the retrograde alteration. Pyrrhotite is the most abundant sulfide mineral, and it is accompanied by lesser amounts of chalcopyrite, sphalerite and two types of pyrite/marcasite. Pyrrhotite occurs in fractures and/or cleavage planes through the prograde minerals or in the intercrystalline space between prograde minerals (Figure 4.3 and Figure 4.4). A lesser amount of pyrrhotite is intergrown with amphibole and/or epidote (Figure 4.3 and Figure 4.4). Chalcopyrite and sphalerite occur together in trace amounts in the same settings as pyrrhotite. In one example gold-bearing chalcopyrite was documented by the use of an elemental map generated by Energy-Dispersive X-Ray Spectroscopy (EDS) (Figure 4.3). The gold-bearing chalcopyrite is optically indistinguishable from the barren chalcopyrite, and the specific location of gold in the chalcopyrite is unknown. It is unclear how widespread the gold-bearing chalcopyrite is, but geochemical analysis of the skarn alteration pre103  sented in Chapter 5 suggests that it is a relatively rare phenomenon. The most abundant style of pyrite is a very fine grained pyrite/marcasite phase that forms with a colloform texture and partially replaces some of the pyrrhotite grains (Figure 4.3). The other style of pyrite is characterized by skeletal grains that occur in similar settings to the other sulfides (Figure 4.3). Retrograde alteration in the amphibole-pyroxene skarn has a consistent paragenesis. Amphibole is the first retrograde mineral to form, followed by epidote and sulfide minerals. The majority of sulfide minerals formed contemporaneously with the retrograde silicate alteration and before the gold mineralization. The skeletal style of pyrite was the first sulfide to form followed by pyrrhotitechalcopyrite-sphalerite and finally the fine grained colloform pyrite/marcasite (Figure 4.3). Gold mineralization occurs as 5 to 100 μm gold grains preferentially located in areas with more intense amphibole alteration. Gold may be intergrown with a generation of amphibole that is typically more fibrous and coarser-grained that the other amphibole alteration in the rock (Figure 4.3). The gold may also occur in intercrystalline space between prograde and retrograde minerals, or rarely in fractures in prograde minerals (Figure 4.3). Much of the gold is finely intergrown with bismuth. The intergrowth with bismuth occurs on the micron scale, and typically with a symplectic texture (Figure 4.3). Bismuthinite and other Bi-minerals typically rim the gold and native bismuth grains (Figure 4.3). Gold mineralization in amphibole-pyroxene skarn occurs after the sulfide mineralization. A volumetrically minor amount of hydrated Fe-silicate alteration, (Table 4.2), occurs in two stages after gold mineralization. The hydrated Fe-silicate alteration is patchy and irregularly distributed. It fills intercrystalline space between and partially replaces prograde and retrograde minerals. This stage of hydrated Fe-silicate alteration forms as the alteration product of magnetite, pyroxene, and amphibole (Figure 4.3 and Figure 4.4). A later stage occurs as less than 1 mm veins that typically contain calcite. The veins cut the replacement style of hydrated Fe-silicate alteration and the prograde and earlier retrograde alteration, and formed after gold mineralization (Figure 4.3 and Figure 4.4). In addition to the hydrated Fe-silicate veins there are also several generations of sulfide, sulfide-calcite and calcite veins (Figure 4.3). The relative timing of these veins is ambiguous, but they appear to postdate gold mineralization. Deformation of the amphibole-pyroxene skarn is separated in the three end member categories. Most of the amphibole-pyroxene skarn is massive and not foliated in hand sample, but brittle deformation is evident at the thin section scale. The brittle deformation occurs in the prograde minerals 104  and the retrograde minerals are generally unaffected (Figure 4.3). In some locations retrograde alteration is more intense in the brittle fracture zones, and the fractures in prograde pyroxene may host gold mineralization (Figure 4.3). A lesser amount of the amphibole-pyroxene skarn appears foliated in hand sample but in thin section the minerals are not ductilely deformed. This pseudo-foliation is proposed to be the result of the skarn alteration overprinting a foliated protolith. In hand samples of the pseudo-foliated amphibole-pyroxene skarn a low angle foliation is defined by variation in the skarn mineralogy and grain size. In thin section it is clear that the pseudo-foliated skarn experienced the same brittle deformation that affected the prograde minerals in other samples, but no foliation is present (Figure 4.4). Finally, a moderate amount of the amphibole-pyroxene skarn was foliated during or after the retrograde alteration. The foliation is more prevalent in amphibole-rich skarn where it is defined by the deformation and alignment of amphibole (Figure 4.4). The foliated areas may have higher sulfide content than usual, but the sulfide minerals do not show signs of deformation. Evidence for ductile deformation is also seen in calcite grains that can have deformed twin lamella and in numerous skarn minerals (pyroxene, quartz, garnet) that have undulatory extinction (Figure 4.3 and Figure 4.4).  105  Hyd Fe-Sil  1mm  250μm Ep  Hyd Fe-Sil/Ca vein  Px Hyd Fe-Sil  Gar  Po/Py  Py -P oM  ag  -C py  Amph  Px  A  1cm  Ca  Ep  fo lia tio n  500μm  B  Ps ue do  Px  C  Au-Bi  ph  Am  C2  250μm Po/Mag  Amph D Figure: Examples amphibole-pyroxene Thephotomicrographs PPL photomicrographs (A,phoFigure 4.4: Examples of of amphibole-pyroxene skarn.skarn. The PPL (A, B, C1), B, C1), photograph (C) and XPL photomicrograph (D) show the mineralogy, tograph (C) and XPL photomicrograph (D) show the mineralogy, texture and paragenesis of the texture and paragenesis of the retrograde alteration andfor the various types of retrograde alteration and the various types of deformation. See text further discussion. deformation. See text for further discussion. 4.3.2	Magnetite-Pyroxene Skarn Magnetite-pyroxene skarn occurs as a lesser component throughout the pyroxene skarn. It has similar mineralogy to the amphibole-pyroxene skarn but with higher magnetite content. There is a continuum of compositions between the two end members. The magnetite-pyroxene skarn typically has a prograde assemblage made up of between 10 and 30 % magnetite, 30 and 60 % pyroxene and trace garnet. Pyroxene is variably retrograde altered to amphibole, which accounts for between 30 and 50 % of the magnetite-pyroxene skarn assemblage. Trace amounts of other retrograde minerals such as epidote and hydrated Fe-silicates also occur. Sulfide minerals (<5 %) occur with the other retro106  grade minerals and are dominated by pyrrhotite with lesser pyrite, chalcopyrite and trace gold, native bismuth and Bi-minerals. Calcite is locally a major component of the rock (40 %). Due to the high magnetite content, the rock is black to mottled black-green in hand sample (Figure 4.5). The magnetite-pyroxene skarn is made up of the same minerals as the amphibole-pyroxene skarn, but the minerals developed with different characteristics. Pyroxene occurs as 100 μm to 1 mm euhedral to subhedral crystals that form in a range of textures from decussate, to polygonal or web texture with poikiloblastic calcite (Figure 4.5). Magnetite crystals range in shape from near euhedral cubic crystals to irregular anhedral shapes that form interstitial to pyroxene (Figure 4.5). In the magnetite-pyroxene skarn, garnet occurs as anhedral crystals between well formed pyroxene crystals and may partially include magnetite (Figure 4.5). The paragenetic sequence of prograde minerals in the magnetite-pyroxene skarn is different than the amphibole-pyroxene skarn. Pyroxene was the first mineral to form followed by magnetite and then garnet. All three prograde minerals appear to have formed at the same time for much of the prograde alteration (Figure 4.5). Retrograde alteration in the magnetite-pyroxene skarn is similar to the amphibole-pyroxene skarn. Pyroxene was altered to amphibole, which replaced between 5 and 90 % of the pyroxene. Trace epidote locally formed as an alteration of pyroxene. A minor amount of sulfide minerals filled fractures and intercrystalline space between the prograde and earlier retrograde minerals. The sulfide minerals formed with similar relative abundances to the amphibole-pyroxene skarn (Po>Py>Cpy) (Figure 4.5). The paragenesis of retrograde alteration in the magnetite-pyroxene skarn for both the silicate and sulfide minerals is the same as the amphibole-pyroxene skarn. Similar to the amphibole-pyroxene skarn, gold mineralization in magnetite-pyroxene skarn follows the main period of sulfide formation and is accompanied by native bismuth and Bi-minerals (Figure 4.5). Gold, bismuth and Bi-minerals are intergrown with amphibole and fill intercrystalline space or fractures in prograde minerals (Figure 4.5). In magnetite-pyroxene skarn the amphibole that is intergrown with gold is typically more fibrous and coarser-grained than the rest of the amphibole in the rock. The fracture-hosted style of gold mineralization primarily occurs in magnetite, which has abundant fractures. The three styles of gold mineralization may be proximal to each other (<200 μm) and have a similar mineral assemblage of gold + bismuth ± Bi-minerals.  107  The magnetite-pyroxene skarn was subjected to less intense alteration after gold mineralization than the amphibole-pyroxene skarn. There is minor hydrated Fe-silicate alteration of some of the magnetite, but unlike the amphibole-pyroxene skarn, hydrated Fe-silicate veining is absent (Figure 4.5). The magnetite-pyroxene skarn is also less deformed that the amphibole-pyroxene skarn. It is massive in hand sample and thin section, and lacks any pseudo-foliations, true foliations or signs of ductile deformation, but it was brittlely deformed after the prograde alteration. The magnetite and to a lesser extent pyroxene crystals are locally intensely fractured and these fractured zones may correlate with increased retrograde alteration (Figure 4.5). 1mm  1cm  Po  Px  Amph  Gar A 1mm  Mag  A2  500μm Mag  Mag Ca  Px-Amph  Hyd Fe-Sil  Px-Amph B  Po-Py  50μm  100μm  C2 Ca  C  Mag Px  Po-Py  Au/Bi  Au/Bi C3  Mag  C2  Amph C3  FigureFigure: 4.5: Examples of magnetite-pyroxene skarn.skarn. The photograph (A), PPL Examples of magnetite-pyroxene The photograph (A),photomicrographs PPL photo(A2, B,micrographs C3), RL photomicrograph and BSE image (C2) show the mineralogy, texture (A2, B, C3), RL photomicrograph and BSE image (C2) show theand paragenesis of mineralogy, the skarn alteration mineralization. andalteration C3 are of particular importance because textureand andgold paragenesis of theC2 skarn and gold mineralizathey shown gold mineralization inter-grown with a late stage of amphibole alteration. See text for tion. C2 and C3 are of particular importance because they shown gold mineralfurtherization discussion. inter-grown with a late stage of amphibole alteration. See text for further discussion. 108  4.3.3	Epidote-Pyroxene Skarn Whereas there is a continuum in composition between amphibole-pyroxene skarn and magnetite-pyroxene skarn the epidote-pyroxene skarn is distinct from the other varieties of pyroxene skarn. It makes up a lesser component of the pyroxene skarn and usually occurs close to the outer margin of the skarn alteration, especially towards the northern and eastern margins of the SWOZ and above it in the BMV (Figure 4.2). The prograde assemblage is comprised entirely of pyroxene that was intensely retrograde altered to epidote-clinozoisite, which makes up between 20 and 40 % of the rock. Calcite and hydrated Fe-silicate content is variable, but less than 5 %. Sulfide alteration and gold mineralization are generally absent, but have been documented in rare locations. Because of the high epidote and pyroxene content the epidote-pyroxene skarn tends to be pale to bright green in hand sample, but is locally mottled pink-green. The pink colour is due to the local abundance of pink epidote, which may be the manganese-rich variety called piemontite (Figure 4.6). In addition to occurring as exoskarn, epidote-pyroxene skarn occurs as endoskarn in the Early Diorite dikes and the Buckhorn Diorite (Figure 4.6) Pyroxene occurs as 50-200 μm near euhedral blocky crystals many of which have clearly visible pyroxene cleavages. Pyroxene usually forms with a polygonal texture leaving little intercrystalline space, but may also form with a decussate texture (Figure 4.6). Retrograde alteration is dominated by the intense development of epidote-clinozoisite. Epidote is more abundant than clinozoisite and the two mineral occur in a 9:1 ratio. In some samples a significant portion (20 %) of the epidote-clinozoisite is a probably piemontite the manganese-rich end member. The possible piemontite is pinkish in hand sample and has pink-violet interference colours and pinkish pleochroism in thin section (Figure 4.6). The majority of the epidote-clinozoisite occurs as anhedral crystals replacing pyroxene, with a lesser amount pseudomorphing pyroxene. Rare calcitequartz veins also occur and cut the prograde and retrograde skarn. Locally epidote-clinozoisite formed during an early stage of vein fill with euhedral crystal terminations pointing towards the center of the veins (Figure 4.6). The calcite – quartz ± epidote veins are less than a millimetre wide and are discontinuous and irregularly shaped (Figure 4.6). Typically the epidote-pyroxene skarn lacks any sulfide alteration or gold mineralization, but was affected by the hydrated Fe-silicate alteration that postdates sulfide alteration and gold mineralization in other skarn varieties. The hydrated Fe-silicate minerals tend to form in the intercrystalline space between pyroxene and/or epidote crystals, likely replacing  109  one of these minerals or possibly intercrystalline calcite (Figure 4.6). Hydrated Fe-silicate minerals rarely form in veins that cut the calcite – quartz ± epidote veins (Figure 4.6). Sulfide bearing epidote-pyroxene skarn is rare, but locally formed from a Buckhorn Diorite protolith. It contains the same prograde and retrograde alteration assemblages as the other examples of epidote-pyroxene skarn, except with abundant sulfide minerals (minor molybdenite and trace chalcopyrite and pyrrhotite). The molybdenite occurs in intercrystalline space between the silicate skarn minerals or in vein-like bands and makes up approximately 1 % of the rock (Figure 4.6). The traces of chalcopyrite and pyrrhotite occur with quartz in intercrystalline space (Figure 4.6). Based on its location between prograde and retrograde minerals the molybdenite formed towards the end of retrograde alteration. However, the molybdenite-bearing sample was not gold mineralized and molybdenite was not found in contact with the other sulfides, so its timing relative to the typical sulfide alteration and gold mineralization is uncertain. The molybdenite was dated by Re-Os geochronology to constrain the age of skarn alteration at Buckhorn and returned an age of 165.5 ± 0.7 Ma (Figure 3.19 and Table 3.12). The results of this study are discussed in Chapter 3. The epidote-pyroxene skarn is characteristically massive in hand sample and does not show any signs of foliation or pseudo-foliation. The prograde pyroxene is typically less fractured compared to the other pyroxene skarn varieties, although, there are several generations of brittle calcite – quartz ± epidote veins that developed after most of the retrograde alteration (Figure 4.6).  110  1cm  1cm  Px  Ca Px-Ep  Ep  Pie  A 500μm  B 500μm  Ep-Pie  Ep-Pie  Ca  Ca  Px  Qtz  A3  Px  Qtz A2  A3 1cm  1mm  Px  Mo  Qtz  Qtz  Ep  Px-Ep  Ca Hyd Fe-Sil  C  B2 250μm  Hyd Fe-Sil Mo Px  Ep C2  X.x: Examples of epidote-pyroxene skarn. The photographs C), PPL FigureFigure 4.6: Examples of epidote-pyroxene skarn. The photographs (A, B, C), (A, PPLB,photomicro(A2,photomicrograph B2, C2) and XPL(A3) photomicrograph (A3) show theand minergraphsphotomicrographs (A2, B2, C2) and XPL show the mineralogy, texture paragenesis texture and paragenesis ofCthe prograde retrogradeendo-skarn alteration.sample C and with of the alogy, prograde and retrograde alteration. and C1 showand an exceptional C1 show an exceptional endo-skarn sample with significant molybdenite minersignificant molybdenite mineralization. alization.  111  4.3.4	Microprobe Analysis of Pyroxene and Amphibole from Pyroxene Skarn 4.3.4.1	Introduction and Methods The major element composition of prograde pyroxene and retrograde amphibole was investigated with an electron microprobe (EMP) to further characterize the pyroxene skarn alteration and to document any relationship between variations in mineral composition and gold grade. Five samples from drill hole D08-410 through the SWOZ were chosen for analysis because their pyroxene skarn alteration has high pyroxene and amphibole content that allowed uncomplicated analysis and their wide spread of gold grades (0.2 to 51.1 ppm) gives a greater chance of detecting any variation in mineral chemistry related to gold mineralization. This study builds on the work of Gaspar (2005), which contained an extensive microprobe study that determined the composition of the skarn alteration minerals, but did not document the relationship between the minerals analyzed and gold mineralization. After detailed optical petrography the samples selected for major element analysis by EMP were carbon coated and BSE imaged with the SEM. The BSE imaging was performed to select the specific pyroxene and amphibole crystals that would be analysed. Electron-probe micro-analyses of pyroxene were done on a fully automated CAMECA SX-50 instrument, operating in the wavelength-dispersion mode with the following operating conditions: excitation voltage, 15 kV; beam current, 20 nA; peak count time, 20 s; background count-time, 10 s; spot diameter, 5 µm. Data reduction was done using the ‘PAP’ Φ(ρZ) method (Pouchou and Pichoir,  1985). For the elements considered, the following standards, X-ray lines and crystals were used: albite, NaKα, TAP; kyanite, AlKα, TAP; diopside, MgKα, TAP; diopside, SiKα, TAP; diopside, CaKα, PET; rutile, TiKα, PET; synthetic magnesiochromite, CrKα, LIF; synthetic rhodonite, MnKα, LIF; synthetic fayalite, FeKα, LIF; synthetic Ni2SiO4, NiKα, LIF. Electron-probe micro-analyses of amphibole were also done on a fully automated CAMECA SX-50 instrument, operating in the wavelength-dispersion mode with the following operating conditions: excitation voltage, 15 kV; beam current, 20 nA; peak count time, 20 s (40 s for F, Cl); background count-time, 10 s (20 s for F, Cl); spot diameter, 5 µm. Data reduction was done using the ‘PAP’ Φ(ρZ) method (Pouchou and Pichoir, 1985). For the elements considered, the following standards, X-ray lines and crystals were used: synthetic phlogopite, FKα, TAP; albite, NaKα, TAP; kyanite,  AlKα, TAP; diopside, MgKα, TAP; diopside, SiKα, TAP; scapolite, ClKα, PET; orthoclase, KKα, 112  PET; diopside, CaKα, PET; rutile, TiKα, PET; synthetic magnesiochromite, CrKα, LIF; synthetic rhodonite, MnKα, LIF; synthetic fayalite, FeKα, LIF. The structural formulas of both amphibole and pyroxene were calculated from the corrected chemical data and are located in Appendix A. 4.3.4.2	 Mineral Chemistry The range of compositions determined for prograde pyroxene from the SWOZ is shown in Figure 4.7 and as expected from the work of Gaspar (2005) they are dominantly hedenbergite with lesser augite. This range of pyroxene compositions is characteristic of reduced gold skarns (Meinert et al., 2005), and indicates that the samples occurred more distal to the source of skarn forming fluids compared to diopside pyroxene that Gaspar (2005) noted at the eastern margin of the SWOZ and Hickey (1990) noted in the Gold Bowl. Figure 4.7 also demonstrates that there is no relationship between pyroxene composition and gold grade, which is consistent with the petrography that shows that the gold mineralization postdates the pyroxene alteration (Figure 4.3). The compositions of amphiboles formed during retrograde alteration are plotted in Figure 4.8, which shows that the amphiboles range from ferrohornblende to ferroactinolite. The work confirms the compositions determined by Gaspar (2005), and matches with the range expected for gold skarns (Meinert et al., 2005). Unlike pyroxene there is a relationship between gold grade and amphibole composition. Amphiboles from gold rich samples have higher magnesium numbers (Mg/(Mg + Fe2+) and more silicon in the mineral formula, which corresponds with a decrease in iron and aluminumtitanium respectively. These changes in amphibole chemistry are also seen in the correlation matrix (Table 4.3) where there is a positive correlation between gold-silicon and gold-magnesium, and a negative correlation between gold-aluminum and gold-iron. The amphibole intergrown with gold mineralization is petrographically distinct from the rest of the amphibole alteration in a specific sample and because of the difference in composition they are assumed to reflect different alteration events. Based on the position of gold mineralization in the skarn paragenesis the barren samples are presumed to represent an earlier stage of retrograde alteration that was locally followed by a later stage of amphibole alteration and gold mineralization. The change in amphibole composition from the barren to gold mineralizing stage is suggestive of a cooling and/or oxidising trend (Blundy and Holland, 1990; Holland and Blundy, 1994; Spear, 1981). The change in amphibole composition was likely affected by both factors, as the temperature of skarn formation  113  typically decreases with time, and oxidising conditions occurred at the end of skarn alteration at Buckhorn as indicated by the presence of hydrated Fe-silicate minerals (Meinert et al., 2005). CaSiO3  Diopside Hedenbergite Augite 70 (MgSiO3)  70 (FeSiO3)  X.x: Composition of pyroxene the Hedenbergite SWOZ. Hedenbergite is the Figure Figure 4.7: Composition of pyroxene from thefrom SWOZ. is the most abundant type of most abundantand type of pyroxene at andskarns is characteristic pyroxene at Buckhorn is characteristic forBuckhorn reduced gold (Meinert et for al. (2005)). There is reduced gold skarns (Reference). There is no compositional difference no compositional difference between pyroxene from gold-rich rocks (red triangles) and barren rocks between pyroxene gold-rich rocks (red triangles) and barren rocks (blue (blue triangles). text forfrom further discussion. MgSiO3 See FeSiO 3 triangles).  114  Classification of Calcic Amphiboles 1  Tremolite  Magnesiohornblende  Tschermakite  0.9 0.8  Legend Gold (ppm)  0.7  >0.51 0.21-0.51 <0.21  Mg#  0.6  Actinolite 0.5  co o  lin  0.4  ga  Mineral Amphibole  0.3  nd  /or  ox id  izin  0.2 0.1  Ferroactinolite  g  Ferrohornblende  Ferrotschermakite  0 8  7.5  7  6.5  6  5.5  Si in formula Figure:4.8: Classification of retrograde amphiboles fromfrom the Buckhorn GoldGold SkarnSkarn basedbased on the Figure Classification of retrograde amphiboles the Buckhorn on the criteriaofofLeake Leakeetetal. al.(1997). (1997). The criteria Theamphiboles amphiboles associated associated with with gold gold mineralization mineralization have have elevated elevated Mg Mgdecreased and decreased Al, corresponding an increase Si, when compared to amphiboles in mineralnon and Al, corresponding to an to increase in Si, in when compared to amphiboles in non mineralized skarn. Thisin change in composition is suggestive of a and cooling and oxidizing trend and ized skarn. This change composition is suggestive of a cooling oxidizing trend (Blundy (Blundy and Holland, 1990; Spear, 1981; Holland and Blundy, 1994). Holland, 1990; Spear, 1981; Holland and Blundy, 1994).  Table 4.3: Correlation matrix of major elements in amphibole and gold. Major elements determined by EMP, gold by ICP-MS.  Correlation  Au(ppm) SiO2(%) TiO2(%) Al2O3(%) Cr2O3(%) FeO(%) MnO(%) MgO(%) CaO(%) Na2O(%) K2O(%) F(%) Cl(%) H2O(%) Au(ppm) 1.00 0.77 ‐0.49 ‐0.55 0.22 ‐0.97 ‐0.31 0.95 0.37 0.15 ‐0.48 ‐0.44 ‐0.66 0.47 SiO2(%) 0.77 1.00 ‐0.40 ‐0.94 ‐0.12 ‐0.74 ‐0.03 0.83 0.74 ‐0.36 ‐0.76 ‐0.50 ‐0.62 0.33 TiO2(%) ‐0.49 ‐0.40 1.00 0.32 0.08 0.67 0.65 ‐0.59 ‐0.05 0.31 ‐0.09 0.84 0.67 ‐0.29 Al2O3(%) ‐0.55 ‐0.94 0.32 1.00 0.31 0.54 ‐0.09 ‐0.68 ‐0.80 0.47 0.81 0.31 0.49 ‐0.11 Cr2O3(%) 0.22 ‐0.12 0.08 0.31 1.00 ‐0.24 0.41 0.11 0.31 0.70 ‐0.55 ‐0.70 ‐0.56 0.89 FeO(%) ‐0.97 ‐0.74 0.67 0.54 ‐0.24 1.00 0.44 ‐0.97 ‐0.28 ‐0.19 0.45 0.44 0.69 ‐0.42 MnO(%) ‐0.31 ‐0.03 0.65 ‐0.09 0.41 0.44 1.00 ‐0.39 0.45 ‐0.21 ‐0.28 ‐0.16 0.37 ‐0.06 MgO(%) 0.95 0.83 ‐0.59 ‐0.68 0.11 ‐0.97 ‐0.39 1.00 0.41 0.10 ‐0.60 ‐0.41 ‐0.65 0.30 CaO(%) 0.37 0.74 ‐0.05 ‐0.80 0.31 ‐0.28 0.45 0.41 1.00 ‐0.38 ‐0.82 ‐0.64 ‐0.18 0.15 Na2O(%) 0.15 ‐0.36 0.31 0.47 0.70 ‐0.19 ‐0.21 0.10 ‐0.38 1.00 0.06 0.19 0.26 ‐0.24 K2O(%) ‐0.48 ‐0.76 ‐0.09 0.81 ‐0.55 0.45 ‐0.28 ‐0.60 ‐0.82 0.06 1.00 0.45 0.20 0.02 F(%) ‐0.44 ‐0.50 0.84 0.31 ‐0.70 0.44 ‐0.16 ‐0.41 ‐0.64 0.19 0.45 1.00 0.14 ‐0.69 Cl(%) ‐0.66 ‐0.62 0.67 0.49 ‐0.56 0.69 0.37 ‐0.65 ‐0.18 0.26 0.20 0.14 1.00 ‐0.68 H2O(%) 0.47 0.33 ‐0.29 ‐0.11 0.89 ‐0.42 ‐0.06 0.30 0.15 ‐0.24 0.02 ‐0.69 ‐0.68 1.00  115  4.3.5	Discussion and Summary of Microprobe Results The microprobe analysis shows that the pyroxene and amphibole alteration at Buckhorn is comprised of hedenbergite-augite and ferrohornblende-ferroactinolite respectively, and that these compositions fit within the range expected for reduced gold skarns (Meinert et al., 2005). The analysis also showed that there is no correlation between pyroxene composition and gold grade, but that several changes in the amphibole composition correlate with gold grade. There is a positive correlation between gold and increased magnesium and silicon content in amphibole, which corresponds with a decrease in iron and aluminum. These compositional changes in amphibole can result from cooler and/ or more oxidised conditions, both of which occurred at the time of gold mineralization at Buckhorn.  4.4	 Garnet Skarn Garnet skarn is less abundant than pyroxene skarn, making up an approximately equal portion of the SWOZ as magnetite skarn. Garnet skarn is more abundant near the northern and eastern margins of the SWOZ, but does occur throughout it (Figure 4.2). Garnet skarn also makes up a significant portion of the Gold Bowl and the Magnetic Mine. In addition to occurring as exoskarn, garnet skarn occurs as endoskarn in the Buckhorn or Early Diorite (Figure 4.9) Garnet is the dominant prograde mineral in garnet skarn and makes up between 40 and 80 % of the rock. Pyroxene is the next most abundant prograde mineral (average 15 %), and varies between 5 and 40 %. Magnetite content is usually below 1 %, but is locally up to 30 % of the garnet skarn (Figure 4.9). Calcite locally forms about 15 % and garnet skarn may contain trace amounts of quartz and feldspar. Amphibole is the dominant retrograde mineral and it is concentrated in areas with high pyroxene content. Amphibole alteration ranges from less than 10 % to greater than 90 % of the pyroxene having been altered to amphibole. Rare retrograde epidote alteration also occurs as a trace amount. The sulfide content (1 %) is typically lower than the pyroxene skarn. Pyrrhotite is the most abundant sulfide, with lesser amounts of pyrite, chalcopyrite and trace arsenopyrite. Several generations of veins cut the garnet skarn, calcite veins are the most abundant, but there are also rare calcite ± hydrated Fe-silicate ± garnet veins. The garnet skarn is red-brown in hand sample with minor patches of dark green amphibole, pale green pyroxene and rare black magnetite (Figure 4.9). Pyroxene occurs as either an aggregate of 20 to 100 μm stubby, blocky crystals or as similarly sized inclusions in garnet. When not included in garnet the pyroxene crystals typically form with a polygonal to web texture. Garnet occurs as 100 μm to 5 mm anhedral to euhedral crystals that have 116  a wide range of optical characteristics. Based on the optical characteristics at least four generation of garnet have been identified (Figure 4.9 and Figure 4.10), which is similar to the work of Gaspar et al. (2008). The first two generations of garnet formed during prograde alteration and the last two formed during retrograde alteration. The first generation garnets are typically isotropic, anhedral and darker in plane polarized transmitted light (Figure 4.9 and Figure 4.10). The next generation of garnet forms as subhedral to anhedral crystals, or rarely as epitaxial overgrowth on the first generation of garnet. The second generation garnets are characteristically anisotropic with sector and polysynthetic twinning and oscillatory zoning (Figure 4.9). Both the first and second generation of garnet may form the majority of a sample, and both occur as minor components of pyroxene and magnetite skarn. The third and fourth generations of garnet formed during retrograde alteration and are discussed according to their place in the paragenesis. When present, magnetite occurs as 20 to 250 μm subhedral crystals that form in the intercrystalline space between garnet and pyroxene, and rarely partially impinging on garnet crystals (Figure 4.9). Arsenopyrite is rare and occurs as 50 μm near euhedral inclusions in Garnet. The texture of the prograde minerals varies from polygonal to web texture with intercrystalline calcite or less commonly quartz or feldspar (Figure 4.9). The paragenesis of prograde alteration in garnet skarn is similar to pyroxene skarn. Pyroxene was the first mineral to form followed by garnet and magnetite (Figure 4.9). In rare instances, arsenopyrite predates garnet. As seen in Figure 4.9, some of the magnetite formed after the other prograde minerals were brittlely deformed. Retrograde silicate alteration in the garnet skarn is dominated by the alteration of pyroxene to amphibole and lesser epidote, as well as two stages of retrograde garnet growth. Garnet is deemed to be part of the retrograde assemblage if it overgrows the second generation of garnet or if it occurs in veins that cut either of the first two stages of garnet. The retrograde garnet that overgrows the second generation of prograde garnet is characteristically isotropic and forms in near euhedral crystals (Figure 4.9 and Figure 4.10). The vein style of retrograde garnet has a more diverse character than overgrowth style, and more than one generation of retrograde garnet veins occurs. The garnet veins have about 300 μm of dilation and may contain isotropic and/or anisotropic garnets with anhedral to nearly euhedral crystal shapes (Figure 4.10). The intensity of amphibole alteration is variable and it postdates the overgrowth style of retrograde garnet, but predates the garnet veins. Locally, the retrograde amphibole alteration is nearly complete with only a trace amount of pyroxene remaining, but elsewhere it is essentially absent. Epidote alteration forms in two settings. Most of the epidote 117  alteration partially pseudomorphs garnet crystals, but a lesser amount forms in veins that cut garnet and pyroxene rich areas. The epidote alteration only forms a trace amount of the garnet skarn, and is more abundant in areas with minimal amphibole alteration. Sulfide alteration follows epidote alteration and is dominated by pyrrhotite with lesser chalcopyrite and trace pyrite. It forms with the same paragenesis as in the pyroxene skarn, pyrrhotite and chalcopyrite together followed by minor pyrite alteration of the pyrrhotite (Figure 4.9 and Figure 4.10). Unlike the pyroxene skarn the early stage of skeletal pyrite does not occur. A trace amount of cobaltite was found in late calcite-garnet-sulfide veins that cut retrograde alteration.  118  1cm  1cm  B  A 1mm  1mm  Gar3  Gar3 Ca  Ca  Gar1  Gar1 Gar2 Au/Bi  Gar2 Px-Amph  Px-Amph  Au/Bi C  Px-Amph  250μm  500μm  Gar2  Hyd Fe-Sil  Ca  Hyd Fe-Sil Ep  Mag Gar  D 1mm  125μm  Cpy  C2  Bi/Au  Gar  Px E Gar  Po-Py  Asp  Gar  Ca  Px-Amph Qtz  F  G  Figure 4.9: Examples of garnet skarn. The photographs (A, B), PPL photomicrographs (C, D, E, Figure: Examples of (C2), garnet skarn. The photographs(G) (A,show B), PPL F), XPL photomicrograph and RL photomicrograph thephotomicrographs mineralogy, texture and (C, D, E, F), XPL photomicrograph (C2), and RL photomicrograph (G) See show paragenesis of the prograde and retrograde alteration and gold mineralization. textthe for further mineralogy, texture and paragenesis of the prograde and retrograde alteration discussion. and gold mineralization. See text for further discussion. The paragenesis of retrograde alteration in the garnet skarn is similar to the pyroxene skarn. Amphibole is the first retrograde mineral to form followed by epidote and sulfide minerals. The timing of 119  the overgrowth style of retrograde garnet is unclear, but it probably formed approximately contemporaneously with amphibole and epidote. The vein style of retrograde garnet alteration cuts the rest of the retrograde alteration and is therefore younger (Figure 4.10 A). Gold mineralization occurs after the sulfide alteration and is accompanied by native bismuth and Bi-minerals. The majority of gold occurs in intercrystalline space between prograde and earlier retrograde minerals as well as in fractures in prograde minerals (Figure 4.10). Rarely gold mineralization is hosted entirely inside calcite that fills the remainder of the fractures and intercrystalline space. A trace amount of gold and Bi-minerals are intergrown with an early stage of hydrated Fe-silicate alteration that also fills intercrystalline space (Figure 4.10). The gold mineralization may also be hosted in calcite-garnet-sulfide veins that cut the earlier skarn alteration (Figure 4.10). Cobaltite is the most abundant sulfide in the late veins, and it is spatially associated with gold mineralization, however the gold mineralization postdates the cobaltite and occurs in brittle fractures within it (Figure 4.10). A single sample of garnet skarn contained molybdenite (Figure 4.10). The sample has the same prograde and retrograde mineral assemblage as the typical garnet skarn, but contains a trace amount of molybdenite that occurs as 20 to 200 μm anhedral crystals. The molybdenite preferentially occurs along quartz crystal boundaries hosted in intercrystalline space between prograde and retrograde minerals. A minor amount of molybdenite rims rare pyrrhotite in the sample, and therefore molybdenite postdates the majority of retrograde alteration. The sample did not contain any gold grains so the relative timing of molybdenite and gold is uncertain. However, molybdenite and gold are presumed to be approximately coeval based on their places in the paragenetic sequence. As discussed in Chapter 3 a sample of the molybdenite-bearing garnet skarn was evaluated by Re-Os geochronology to constrain the age of skarn mineralization. The molybdenite in the garnet skarn returned an age of 162.8 ± 0.7 Ma, which confirms the temporally relationship between the skarn alteration at Buckhorn and the emplacement of the BIS (Figure 3.19 and Table 3.12). Garnet skarn appears massive and undeformed at the hand sample scale, but has clearly been brittlely deformed when viewed in thin section. Many of the prograde garnet crystals are affected by micron scale brittle fractures that are randomly oriented and usually filled by calcite. These fractures affect most of the prograde minerals including garnet, pyroxene and magnetite, but they predate some of the prograde magnetite that fills fractures in garnet (Figure 4.9 and Figure 4.10). The retrograde  120  minerals are typically undeformed, although they are rarely fractured and are cut by the rare calcitegarnet-sulfide veins (Figure 4.9 and Figure 4.10). Qtz GarV2  500μm  Gar1  GarV1  Px-Amph  250μm  Hyd Fe-Sil  Gar Bi/Au  Ep Px 50µm  Gar  A  B  Bi Min/Au  250µm  Bi  Px  Px-Amph  Au  Gar Ep  Co Bis  Au  Cpy  Ca Gar  Bi Min  D  C  1cm  250µm  Px-Amph  Hyd Fe-Sil Gar  Mo  Px-Amph  Mo  Qtz Gar  E  F  125µm  Po  Amph Qtz  Gar Mo  G Examplesofofgarnet garnet skarn. The PPLphotomicrograph photomicrograph show several FigureFigure: 4.10: Examples skarn. The PPL (A) (A) show several generations generations of retrograde garnet veins. The RL photomicrographs (B, C, D) show of retrograde garnet veins. The RL photomicrographs (B, C, D) show the specific settings of gold the specific settings of gold mineralization. The photograph (E), PPL photomicromineralization. The photograph (E), PPL photomicrograph (F) and RL photomicrograph (G) show graph (F) texture and RL and photomicrograph (G)skarn showalteration the mineralogy, texture and para- sample. the mineralogy, paragenesis of the in the molybdenite bearing genesis of the skarn alterartion in the molybddenite bearing sample. See text for See text for further discussion. further discussion. 121  4.5	 Magnetite Skarn Magnetite skarn is less abundant than pyroxene skarn in the SWOZ and like the garnet skarn it predominately occurs towards its northern and eastern sides (Figure 4.2). Magnetite skarn also forms a significant portion of the Gold Bowl and Magnetic Mine. The magnetite skarn has a consistent prograde and retrograde mineral assemblage compared to the pyroxene skarn and therefore is not divided into sub-categories. Magnetite is the dominant prograde mineral (60 and 85 %) in magnetite skarn. Pyroxene is the next most abundant prograde mineral (<5 %), but may locally composed up to 25 % before retrograde alteration. Garnet rarely occurs (0-5 %). Amphibole is the dominant retrograde mineral; it replaces about 75 % of the prograde pyroxene, and forms 5-15 % of the rock. Retrograde alteration to epidote and hydrated Fe-silicate minerals locally occurs (>1 %). Calcite is abundant 10-20 %, and sulfide minerals (~5 %) are dominated by pyrrhotite with trace pyrite and chalcopyrite. Similar to pyroxene skarn and garnet skarn, gold mineralization is accompanied by native bismuth and Bi-minerals. In hand sample the magnetite skarn is black with rare patches of white calcite, green pyroxene, dark green amphibole and red-brown garnet (Figure 4.11). The minor amount of pyroxene that is present in magnetite skarn occurs as 500 μm to 1.5 mm blocky, elongate, subhedral to nearly euhedral crystals (Figure 4.11). Garnet occurs as near euhedral crystals, about 200 μm across, in masses or isolated crystals (Figure 4.11). Magnetite forms as 100 μm to 2 mm equant, euhedral to subhedral crystals. Some magnetite crystals have rare 20 μm to 100 μm calcite and very rarely similarly sized pyrrhotite inclusions. The prograde minerals typically form with a polygonal texture, but where calcite is more abundant they locally have a web texture with intercrystalline calcite (Figure 4.11). Like the other skarn types, pyroxene is the first prograde mineral to form, followed by garnet, which forms at about the same time as pyroxene. Magnetite is the final prograde mineral to develop, and its formation overlaps with the end of pyroxene and garnet alteration. Retrograde alteration in magnetite skarn is less intense than in the pyroxene skarn. Retrograde amphibole alteration is patchy and preferentially replaces pyroxene along crystals boundaries and fractures. Amphibole forms as fibrous crystals that are finer-grained than pyroxene and about 200 to 500 μm in length. A late stage of fine-grained amphibole may be locally intergrown with gold and bismuth mineralization (Figure 4.11). Epidote occurs as blocky aggregates of 10-30 μm crystals that replace garnet and pyroxene (Figure 4.11). Pyrrhotite, lesser pyrite and trace chalcopyrite occur in 122  fractures in the prograde minerals and to a lesser extent in the intercrystalline space between them. The sulfide minerals may rarely also occur as inclusions in magnetite. Pyrite is the first sulfide mineral to form and occurs as ~100 μm anhedral crystals (Figure 4.11). Pyrrhotite and chalcopyrite formed next as anhedral space filling crystals. There is minor late pyrite alteration after pyrrhotite as described in pyroxene skarn. Gold occurs with native bismuth and Bi-minerals in fractures in magnetite. The gold and bismuth typical occur in distinct grains that range in size from less than 20 μm up to 200 μm, but may be intergrown (Figure 4.11). A minor amount of gold occurs as small grains (1 μm) that are intergrown with fine-grained amphibole (Figure 4.11). Gold mineralization intergrown with amphibole typically occurs in samples with high amphibole content and in the same samples that also contain the more abundant fracture-hosted style of gold mineralization. Very rarely bismuth and Bi minerals occur without gold. In theses rocks the bismuth and Bi-minerals occur in fractures in the magnetite and intergrown with amphibole, the same settings as in the gold mineralized rocks. Magnetite skarn has rare hydrated Fe-silicate alteration that occurs along the edges of pyroxene and amphibole crystals as well as in veins that cut the prograde and other retrograde minerals (Figure 4.11). The magnetite skarn appears massive, non-foliated and generally undeformed in hand sample, but in thin section the prograde minerals show signs of brittle deformation. Magnetite, pyroxene and garnet crystals are typically pervasively fractured. This fracturing occurs before the retrograde alteration and the amphibole crystals are not affected (Figure 4.11). The fractures have minor dilation (<30 μm) and are randomly oriented. They are filled by sulfide minerals and calcite and form a crackle breccia (Figure 4.11).  123  250μm  1cm  Amph Mag  Ca  Ca  Gar  Amph  Po  Mag  A Ca  500μm  500μm  Mag  Bi-Min Cpy  Px-Amph  B Amph  Mag  Au  Gar Ca  Po D  C  250μm  250μm  Ca  Amph  Ep  Bi Min Mag  Mag Amph  Au-Bi intergrown with Amph  E Ca  50μm  Cpy  Px  Bi Min  Py2  Au  F  250μm  Po  Hyd Fe-Sil Mag  Py1 Amph  Mag  Px G  H  FigureFigure: 4.11: Examples skarn. The photograph (A) (A) PPLPPL photomicrographs (B, C, E, Examplesofofmagnetite magnetite skarn. The photograph photomicrographs H), and (D, F, G) show(D, theF,mineralogy, texture and paragenesis of the skarn (B,RL C, E,photomicrographs H), and RL photomicrographs G) show the mineralogy, texture and paragenesis the skarn alteration anddetailed gold mineralization. alteration and gold of mineralization. See text for discussion. See text for detailed discussion.  124  4.6	 Gold Mineralization in Non Skarn-Altered Rocks The great majority of gold mineralization occurs in skarn-altered rocks, but in rare locations (~3) significant gold mineralization has been found in rocks that are essentially not skarn-altered. The gold-bearing non skarn-altered rocks that have been identified to date all occur close to the SWOZ in hornfels-altered siliciclastic metasedimentary rocks of the BMS. In these cases, skarn alteration is limited to millimetre scale veins with centimetre scale halos that cut hornfels altered rocks. While these occurrences are rare and do not impact the economics of the deposit, they provide insight into the progression of skarn alteration and gold mineralization at Buckhorn. The non skarn-altered rocks that host the small skarn veins are identical to the isochemical hornfels altered metasedimentary rocks in the Buckhorn area. The majority of the rock is made up of 100 to 500 μm subhedral quartz, feldspar, amphibole, and pyroxene. Lesser, similarly-sized epidote and finer-grained hydrated Fe-silicate minerals also occur (Figure 4.12). The mineralogy is variable from sample to sample, and is indistinguishable from samples of non-gold mineralized hornfels (Figure 4.12). The vein hosted skarn alteration is composed of pyroxene, amphibole, epidote, chlorite, minor sulfides (pyrrhotite, chalcopyrite, pyrite, sphalerite, and cobaltite), and gold (Figure 4.12).The alteration occurs in the veins and in the vein halos, but with less intensity, and follows the same paragenetic sequence as described in the amphibole-pyroxene skarn section. Gold is accompanied by bismuth mineralization in the form of native bismuth, bismuthinite and other Bi-minerals. Gold and bismuth form a much greater portion of the vein hosted skarn than the typical skarn, and locally gold and bismuth minerals make up close to 1 % of the rock. Gold occurs within the skarn veins and skarn alteration halos in intercrystalline space between and fractures in pyroxene, amphibole and epidote, as well as intergrown with amphibole (Figure 4.12). In samples with extremely high gold content (~1% Au/Bi by visual estimate), gold may also be intergrown with epidote and pyroxene, and also along internal boundaries in calcite crystals (Figure 4.12). Gold mineralization in non skarn-altered rock is relatively undeformed, with slight brittle deformation at the mineral scale, and several generations of brittle veins. The orientation and relative timing of the different skarn veins could not be determined, as the samples have ambiguous cross-cutting relationships. The gold bearing non skarn-altered rocks lack ductile deformation (Figure 4.12).  125  1cm  1cm  Gold-bearing skarn vein Barren veins A 1mm  1mm  Qtz/Felds  Bi/Au  Px-Amph Bi/Au  Amph  C 125µm  B  Bi-Min/Au  D2  Po  Amph Vein  Sx  Ca 250µm  Cpy  Px  Ep  Halo D  Qtz/Felds  Ep  Px-Amph  Sphal  Co  Vein  Px Ca  D2  Bi/Au  Amph  E  Ca  500µm  Bi/Au Ep  Chl  Px F  Figure 4.12: Examples of gold-bearing skarn veins in hornfels altered rock. The photographs (A, B), Figure: Examples gold-bearing skarn veins in and hornfels altered rock. The(D2, E) show PPL photomicrograph (C), of PPL photomicrographs (D, F), RL photomicrographs photographs PPL photomicrograph (C), PPLveins photomicrographs (D, F), See the mineralogy, texture (A, andB),paragenetic sequence of the skarn and gold mineralization. and RL photomicrographs (D2, E) show the mineralogy, texture and paragetext for further discussion. netic sequence of the skarn veins and gold mineralization. See text for further discussion.  126  4.7	 Discussion and Conclusions 4.7.1	Skarn Alteration Skarn alteration at Buckhorn is divisible into three major alteration assemblages based on their prograde mineralogy: pyroxene skarn, garnet skarn, and magnetite skarn. The descriptions presented in this work are a detailed and accurate characterization of the skarn alteration assemblages and the variation within them. These descriptions are summarized in Table 4.4.  127  Table 4.4: Characteristics of the skarn classifications Pyroxene Skarn  Garnet Skarn  Magnetite Skarn  Amphibole-Pyroxene  Magnetite-Pyroxene  Epidote-Pyroxene  Prograde Mineralogy  Px > Mag - Gar  Px - Mag > Gar  Px  Gar > Px > Mag  Mag > Px > Gar  Retrograde Alteration Intensity  High  Moderate  High  Low  Low  Retrograde Mineralogy  Amph > Sx > Ca > Hyd Fe-Sil > Ep  Amph > Ca > Sx > Hyd Fe-Sil - Ep  Ep > Hyd Fe-Sil > Amph  Amph > Ca > Gar > Sx > Ep > Hyd Fe-Sil  Amph > Ca > Sx > Hyd Fe-Sil > Ep  Sulfide Mineralogy  Po > Py > Cpy > Sphal  Po > Py > Cpy  (Mo > Po - Cpy)  Po > Py > Cpy > Aspy > (Co - Mo)  Po > Py > Cpy  Sulfide Content (%)  5-10%  <5%  Trace  1%  5%  Ore Mineralogy  Bi > Bis > Au  Bi > Bis > Au  None  Bi > Bis > Au  Bi > Bis > Au  Setting of Gold Mineralization  Intercryst > Intergrown > Fracture > Base Metal  Fracture > Intercryst > Intergrown  None  Intercryst > Fracture > Intergrown  Fracture > Intercryst > Intergrown  Range of Gold Grade (ppm)  <0.1 - 117.5  <0.1 - 51.1  <0.1  <0.1 - 53.7  <0.1 - 22.5  Vein Assemblage  Ca > Ca-Hyd Fe-Sil>Sx  Ca  Ca-Qtz > Ep  Ca > Ca-Hyd Fe-Sil > Gar - Sx  Ca-Hyd Fe-Sil  Protolith  Metavolcanic - Metacarbonate - Metasedimentary  Metavolcanic - Metacarbonate - Metasedimentary  Metavolcanic - Metacarbonate - Metasedimentary  Metavolcanic > Metacarbonate > Metasedimentary  Metacarbonate > Metavolcanic > Metasedimentary  Styles of Deformation  Fractures / Foliations / Pseudofoliations  Fractures  Fractures  Fractures  Fractures  Size of Gold Grains  ()=very rare minerals  None  128  4.7.1.1	Zonation and Paragenesis of Skarn Alteration The skarn alteration at Buckhorn varies from garnet- and magnetite-dominated in the Magnetic Mine area, to roughly equal proportions of garnet, magnetite and pyroxene in the Gold Bowl, and finally pyroxene-dominated in the SWOZ (Figure 4.2). This zonation, from garnet- to pyroxenedominated, has been noted at other gold skarns where it has been interpreted to represent an oxidised to reduced trend from proximal garnet to distal pyroxene (Meinert et al., 2005). The skarn zonation pattern at Buckhorn suggests that the mineralizing fluids were sourced from the Buckhorn Granodiorite and Buckhorn Diorite near the Magnetic Mine and travelled to the south forming the Magnetic Mine, the Gold Bowl and ultimately the SWOZ. The same zonation can be seen at a smaller scale in the SWOZ where the northern and eastern margins have more abundant garnet and magnetite skarn than the southern and western margins. This secondary zonation is likely due to the presences of the Footwall Mylonite that channelled skarn forming fluids and allowed them to travel a greater distance with less interaction with the host rocks. In addition to being zoned in space, prograde mineralogy in the SWOZ is also zoned in time. As the most distal part of the Buckhorn skarn system, skarn alteration in the SWOZ was likely originally composed entirely of pyroxene. However, as skarn alteration progressed, the zones of garnet and magnetite skarn expanded outward and overprinted the pyroxene alteration. This overprinting is reflected in the paragenetic sequence documented in samples collected from the SWOZ where pyroxene predates garnet and magnetite alteration (Figure 4.13). In addition to forming at the metre scale, skarn alteration also forms in veins at the centimetre scale. These skarn veins have the same alteration assemblages, follow the same paragenesis, and have the same mineralization style as the more typical metre scale amphibole-pyroxene skarn alteration. They are found cutting prograde and retrograde skarn, as well as non skarn-altered rocks. Based on these observations the vein-hosted skarn is interpreted as the youngest and/or most distal stage of skarn alteration and gold mineralization at Buckhorn. Similar conclusions have been made regarding skarn veins in other skarn deposits (Einaudi, 1977; Ewers and Sun, 1989; Meinert et al., 2005). The timing of skarn alteration relative to deformation has been disputed in the past (Gaspar, 2005; Hickey, 1990, 1992; McMillen, 1979), but petrography shows that much of the skarn alteration was affected by brittle and/or ductile deformation. In the SWOZ most of the prograde minerals and many of the retrograde minerals are brittlely and ductilely deformed respectively. The brittle 129  deformation manifests as fractures in the prograde minerals and the ductile deformation as a foliation in the retrograde minerals. The foliation is relatively flat-lying and is presumably equivalent to the low angle foliation in the host rocks. This conclusion is supported by work presented in Chapter 2 and Chapter 3 that showed that the deformation events responsible for the low angle foliations are younger than the Granodiorite dikes which acted as fluid conduits for skarn forming fluids (Figure 2.10, Figure 2.11, and Figure 3.15).  Pyroxene Garnet Magnetite Amphibole Epidote  Minerals  Calcite Quartz Arsenopyrite Pyrite Pyrrhotite Chalcopyrite Sphalerite Molybdenite Cobaltite Other Bi-Minerals Native Bismuth Au Hydrated Fe-Silicates  Ore End of Deformation Prograde Retrograde  Legend  Veins  End of skarn alteration  Time  more abundant less abundant interpreted  Figure 4.13: Paragenetic sequence of skarn alteration, deformation, and gold mineralization at Buckhorn  130  4.7.1.2  Physicochemical Conditions of Skarn Alteration  The physicochemical conditions of prograde skarn alteration at Buckhorn can be constrained by the prograde mineralogy. The absence of graphite and pyrite and the presence of hedenbergite, andradite and magnetite in the prograde assemblage limits the log fO2 conditions to between -26 and -20 (Figure 4.14). Figure 4.15 further constrains the minimum fO2 to -25 based on the lack of graphite. The presence of pyrrhotite rather than pyrite and arsenopyrite rather than lollingite constrains the log fS2 conditions of the prograde alteration to between -4.5 and -8 (Figure 4.15). Figure 4.14 also constrains the minimum temperature of prograde skarn alteration to approximately 430°C based on the reactions of andradite + CO2 = magnetite + quartz + calcite + O2 and hedenbergite + O2 + CO2 = magnetite + quartz + calcite. The maximum temperature of prograde alteration is limited to 500°C based on maximum temperature estimates from other skarn systems (Meinert, 2000). The figures show that Buckhorn formed under more oxidized (higher fO2) and more sulfur rich (higher fS2) conditions than Nickel Plate (Figure 4.15) (Ettlinger et al., 1992). -15  log fO2  P=1kbar XCO2=0.1 pH=7  -20  possible progresion of skarn alteration conditions  Hm Qtz Ad Ca  M  SO 2 H2 S Hm g Ma  -Q Ad Hd  ag Ad -Q tz -C a  a M o P  400 Temperature (°C)  tz -Q o d A d-W H  te phi  Gra  -25 Py g-  tz-  g Ma  Qt  z-  g Ma  Hd Mt-Qz-Ca  Fa  500  600  Figure 4.14: Temperature versus log fO2 diagram showing the stability fields of major skarn silicate, oxide, and sulfide minerals. The grey box indicates the range of possible conditions for the formation of prograde alteration at Buckhorn. Stability field for prograde alteration at Nickel Plate shown with the diagonal lines. Noteversus that the field for the Buckhorn skarn extends more oxidised Figure X.x: Temperature logstability fO2 diagram showing the stability fields oftomajor conditions. Modified from al. (1981). skarn silicate, oxide, andEinaudi sulfideet minerals. The grey box indicates the range of posible conditions for the formation of prograde alteration at Buckhorn. Stability field of the Nickel Plate shown with the diagonal lines. Note that the stability field for the Buckhorn skarn extends to more oxidised conditions. Nickel Plate stability field from Ettlinger 131 (1992).. Modified from Einaudi et al. (1981).  Py Hm  Qt  -P y o W d A  Ma  g  ag  log fS2  -8  Ad-Qtz Lo-M ag Asp  Hd-Wo  Hd  Ad-Qtz-Mag  Asp Lo  ag Ah-M QtzAd -Ah Q tz Wo  Ad-Qtz-Po Hd  -12  y  Po  -6  -10  Py  Hm Mag  Py Po  z-A Ad h-P  M  -4  Graphite  -2  -24  -22  -20 log fO2  -18  Figure X.x=versus log fO log fSshowing the stability of 2 versus 2 diagram Figure 4.15: Log fO log fS2 diagram theshowing stability fields of majorfields calc-silicate and 2 majorminerals calc-silicate and sulfide prograde minerals at=0.5 T=+500°C, XCO2=0.1, sulfide prograde at T=+500°C, XCO =0.1, and P (fluid) Kbar. Stability field for skarn 2 and P(fluid)=0.5 Kbar. Stability field for skarn alteration at Buckhorn shown alteration at Buckhorn shown as the shaded area. Stability field of the Nickel Plate shown with the as the shaded area. Stability of the Nickel skarn Plate extends shown with theoxidised diago- and diagonal lines. Note that the stability fieldfield for the Buckhorn to more nal lines. Note thatPlate the stability stabilityfield fieldfrom for the Buckhorn skarn extends to sulfur rich conditions. Nickel Ettlinger (1992). Mineral stabilities from Etmore oxidised and sulfur rich conditions. Nickel Plate stability field from tlinger (1992) and Bowman (1998). Ettlinger (1992). Mineral stabilities from Ettlinger (1992) and Bowman (1998). The conditions of retrograde skarn alteration at Buckhorn can also be constrained by the mineralogy. The lack of garnet and pyroxene (andradite and hedenbergite) indicates that retrograde alteration occurred at temperatures below 430°C (Figure 4.14), and the hydrated Fe-silicate assemblage of grunerite, greenalite, and minnesotaite (Table 4.2) indicates a temperature of approximately 300°C for the later stages of retrograde alteration (Rasmussen et al., 1998). The log fO2 and log fS2 conditions of retrograde alteration and gold mineralization can be constrained by the sulfide mineral assemblage. The presence of arsenopyrite rather than lollingite and chalcopyrite rather than bornite limits the 132  log fS2 to between approximately -6 and -15 (Figure 4.16). Figure 4.16 also provides the maximum log fO2 of -26 based on the lack of hematite in the retrograde alteration assemblage. Figure 4.14 and Figure 4.16 demonstrate that the mineralogical changes from prograde to retrograde alteration can be explained by a cooling and reducing trend. These figures also show that similar to the prograde alteration the retrograde alteration at Buckhorn formed under more oxidized (higher fO2) and more sulfur rich (higher fS2) conditions than the retrograde alteration at Nickel Plate (Ettlinger et al., 1992).  Py -5 Bn-Py Cp Bis Bi  log fS2  -10  Asp -15  Po-Lo Po  Hm L As o p-M ag  -20  Mag  Lo Asp-Fe  -25 Fe  -45  -40  log fO2  -35  -30  -25  Figure loglog fOfS logshowing fS2 diagram showing theofstability fields of 2 versus Figure 4.16: Log fO2 X.x: versus diagram the stability fields major sulfide and bis2 major sulfide and bismuth minerals at T=300°C, XCO2=0.1, and muth minerals formed during retrograde alteration at T=300°C. Stability field for skarn alteration P(fluid)=0.5 Kbar. area. Stability fieldfield for skarn alteration at Buckhorn at Buckhorn shown as the shaded Stability of Nickel Plate shown with the diagonal lines. shown as the shaded area. Stability field of the Nickel Plate shown Note that the stability field for the Buckhorn skarn extends to more oxidised and sulfur rich condiwith the diagonal lines. Note that the stability field for the Buckhorn tions. Nickel Plate stability field from Ettlinger (1992). Mineral stabilities from Ettlinger (1992), skarn (1979) extends to references more oxidised and sulfur rich conditions. Nickel Plate Barton and Skinner and therein. stability field from Ettlinger (1992). Mineral stabilities from Ettlinger (1992), Barton and Skinner (1979) and references therein. 133  4.7.2	Gold Mineralization Gold mineralization in skarn alteration at Buckhorn has been known since the late 1980’s, but the precise location of the gold grains within the skarn alteration had not been determined. This work shows that gold occurs in five distinct settings at the Buckhorn gold skarn: (1) in intercrystalline space between prograde and retrograde minerals, (2) in fractures in prograde minerals, (3) intergrown with retrograde minerals, (4) in skarn veins through skarn and non skarn-altered rocks, and (5) in chalcopyrite mineralization. The first four styles of gold mineralization postdate all of the prograde and most of the retrograde alteration. In all cases, gold is accompanied by significant bismuth mineralization, which may be intergrown with gold, or occur in the same setting. The bismuth mineralization occurs as native bismuth, bismuthinite and other Bi-minerals (Table 4.2). The majority of the bismuth mineralization is made up of equal proportions of native bismuth and bismuthinite, and the remainder is composed of a minor amount of other Bi-minerals. The fifth style of gold mineralization is very rare and has only been documented in a single sample. Unlike the other styles, gold does not exist as free gold, but occurs in chalcopyrite. The gold bearing chalcopyrite predates the first four styles of gold mineralization. The first two styles of gold mineralization, intercrystalline and fracture-hosted, are similar and likely formed at the same time. In both styles, gold precipitated in pre-existing space, either in fractures in prograde minerals or in intercrystalline space between the prograde and/or retrograde minerals. Gold is accompanied by calcite and rare quartz in both settings, and in some samples it is enclosed in these gangue minerals. Pyrrhotite, chalcopyrite and pyrite also occur in similar settings, but they are petrographically shown to predate gold mineralization. Intercrystalline and fracture-hosted gold are the dominant form of mineralization in magnetite and garnet skarn, and also from a moderate portion of the mineralization in pyroxene skarn. The third setting for gold mineralization, gold intergrown with retrograde minerals, is less abundant than the first two settings and probably younger, but the relative timing is uncertain. Amphibole is by far the most common retrograde mineral intergrown with gold, but locally gold is also intergrown with hydrated Fe-silicate minerals. This setting of gold mineralization is more abundant in pyroxene skarn, but does occur in all styles of skarn alteration. The amphibole that is intergrown with gold may be finer- or coarser-grained than the rest of the amphibole in the rock, but is typically visually distinct. The hydrated Fe-silicates are usually coarser-grained, but optically similar, to the rest of the hydrated Fe-silicates in the rock. In both cases the gold-bearing  134  retrograde minerals postdate deformation. More than one of the first three styles of mineralization can occur in the same sample, and this is prevalent in samples with high gold grade (>30ppm). The vein style of gold mineralization is much rarer than the first three styles and was only found in garnet skarn and non-skarn altered rocks. It is presumed to be the result of the youngest and/or most distal expression of mineralization, so there should be no reason why it could not also occur in magnetite or pyroxene skarn. The skarn veins are made up of calcite ± quartz ± amphibole ± epidote ± pyroxene ± garnet ± sulfides, and the skarn alteration in the halos has a similar mineralogy. Gold in the vein style of mineralization is confined to skarn alteration in the vein and the vein halo, and occurs in any or all of the three aforementioned settings. Gold mineralization in chalcopyrite has only been documented in a single sample of amphibolepyroxene skarn, but as the gold-bearing chalcopyrite cannot be optically distinguished from barren chalcopyrite it may be more prevalent than currently known. The gold-bearing chalcopyrite formed during retrograde alteration before the first four styles of mineralization. The location of gold grains within the chalcopyrite is unclear, but their presence was indicated by an elemental map produced by Energy-Dispersive X-Ray Spectroscopy (EDS) (Figure 4.3). 4.7.2.1  Physicochemical Conditions of Gold Mineralization  The physicochemical conditions of gold mineralization at Buckhorn can be constrained by the retrograde calc-silicate, sulfide and bismuth mineralogy. The temperature of gold mineralization can be estimated at approximately 300°C based on the stability of the hydrated Fe-silicate mineral assemblage that is locally intergrown with gold (Rasmussen et al., 1998). This temperature is higher than that minimum temperature of 241°C estimated from eutectic point on the Au-Bi phase diagram (Figure 4.17). The Bi/Te+Se+S (RBi/Te) ratio of bismuth minerals can be used as a proxy for the Redox conditions of a system with RBi/Te>1 characteristic of a reduced environment and RBi/Te <1 characteristic of an oxidised environment (Ciobanu et al., 2005, and references therein). The bismuth mineralogy at Buckhorn has approximately equal proportions of native bismuth (Bi), which is typically intergrown with gold, and bismuthinite (Bi2S3), which typically rims the gold and native bismuth (Figure 4.3). This mineral assemblage and texture suggests that gold precipitated in a reduced environment followed by an increase in oxidation state. This change in bismuth mineralogy matches the change in iron sulfide mineralogy from pyrrhotite to pyrite/marcasite that indicates log fO2 conditions from approximately -36 to -42 (Figure 4.18).The bismuth mineralogy also indicates a similar change in the 135  sulfur fugacity, from low fS2 during gold and native bismuth mineralization to higher during bismuthinite mineralization (Figure 4.16). This texture is also indicative of cooling as bismuthinite is stable under cooler conditions (Tooth et al., 2011). The pH of alteration and gold mineralization in skarns is generally near neutral to slightly acidic, buffered by the presence of calcite. The intimate relationship between gold and bismuth suggests that gold was collected in a bismuth melt. Bismuth collection or bismuth scavenging as it is also known, occurs in deposits where a bismuth melt is stable and coexists with a hydrothermal fluid (Tooth et al., 2008). In these situations the gold concentration in the bismuth melt is several orders of magnitude higher than in the fluid (Tooth et al., 2008). Evidence for bismuth scavenging is seen in the petrography where gold and bismuth mineralization is associated with retrograde minerals formed in oxidation reactions (Fe2+ bearing pyroxene replaced by F3+ bearing amphibole, epidote, and hydrated Fe-silicate minerals and pyrrhotite replaced by pyrite/marcasite). Bismuth scavenging preferentially occurs in reduced, acidic, and low sulfur conditions; like those that occurred during retrograde alteration and gold mineralization at Buckhorn (Tooth et al., 2008). Bismuth scavenging has been suggested to aid in the formation of gold skarns and other deposits including epithermal and volcanic hosted massive sulfide deposits (Cepedal et al., 2006; Cook and Ciobanu, 2004; Törmänen and Koski, 2005). At the conditions that occurred during retrograde alteration and gold mineralization gold was likely transported as bisulfide complexes before being scavenged by a bismuth melt (Figure 4.18). Bisulfide transportation of gold has been proposed previously at Buckhorn (Gaspar, 2005) and also at other gold skarns (Meinert, 2000). Gold precipitation from a bismuth melt was caused by cooling and/or local oxidation reactions during retrograde alteration, both of which occurred during gold mineralization at Buckhorn (Tooth et al., 2011). Gold precipitation due to cooling results in gold-bismuth grains being rimmed by bismuthinite as bismuthinite is more stable under cooler conditions (Tooth et al., 2011). Such textures (gold-bismuth rimmed by bismuthinite) occur at Buckhorn and are more prevalent in the fracture and intercrystalline styles of gold mineralization (Figure 4.3). Gold precipitation caused by local oxidation reactions resulted in the intergrowth of gold-bismuth with retrograde minerals (Tooth et al., 2011), and this texture is characteristic of the intergrowth style of gold mineralization at Buckhorn (Figure 4.3, Figure 4.5, Figure 4.10, and Figure 4.11). The gold mineralization hosted in skarn veins  136  was precipitated by both mechanisms and is contains the fracture-hosted, intercrystalline, and intergrowth styles of gold mineralization (Figure 4.12). The physicochemical conditions of skarn alteration and gold mineralization are now determined and summarized in Table 4.5. The system is progressed to cooler, lower oxygen fugacity, and lower sulfur fugacity conditions from prograde to retrograde alteration and gold mineralization.  gold-melt  400 melt 300  maldonite-melt  T (°C)  maldonite-gold  eutectic point (241° C)  200  bismuth-melt  maldonite-bismuth 100 gold-bismuth Au  20  40  60  80  Bi  Mole % Bi Figure 4.17: Phase diagram of Au-Bi at 1 bar that demonstrates the eutectic point at 241°C. ModiFigure X.x:etPhase diagram of Au-Bi at 1 bar that demonstrates the eutectic fied from Tooth al. (2008) point at 241°C. Modified from Tooth et al. (2008).  137  -10 -15  T=300°C P=500 bar log aBi=-4.3 log aS=-1 NaCl=5%  AuCl2-  log fO2 (aq)  -20  AuOH(aq)  -25  -30  Bi(OH)3 (aq) BiCl63Hm Mag  Bis  -35  Py Bi-melt  Po  -40  Au(HS)2-  AuHS(aq) 2  4  6  8  10  12  pH Figure 4.18: Log fO2 versus pH diagram showing the stability fields of Au, Bi, and Fe phases and X.x: log fOof pH diagram the aqueous complexes Figure at the conditions retrograde alteration showing and gold mineralization at Buckhorn 2 versus stability fields of Au,from Bi, and Fe phases and aqueous (pH=6-8, log fO2=-36 to -42). Modified Tooth et al. (2008). complexes at the conditions of retrograde alteration and gold mineralization at Buckhorn (pH=6-8, logfO2=35 to -40). Modified from Tooth et al. (2008). Table 4.5: Summary of physicochemical conditions of skarn alteration and gold mineralization Prograde Retrograde Gold Mineralization Temperature (°C) 430 to 500 300 to 430 241 to 300 f O2 -25 to -20 < -26 -42 to -36 f S2 -8 to -4.5 -15 to -6 -15 to -6 pH 6 to 8 6 to 8 6 to 8  138  4.7.3	Comparison to Nickel Plate The Nickel Plate deposit in the Hedley mining district is the largest gold skarn in Canada, and was the only known economic gold skarn until the early 1970’s (Meinert, 2000). Production from the deposit totalled 2.3 Moz of gold at an average grade of 5.4 g/t Au, this includes almost 1.3 Moz of gold that was mined underground at a grade of 14 g/t (Ray et al., 1996). Similar to the Buckhorn gold skarn, the Nickel Plate deposit lies in the accreted Quesnel terrane. The Nickel Plate deposits is located in the Late Triassic Nicola Group, which is made up of arc related volcanic and sedimentary rocks, the latter of which are dominant in the Hedley area (Ray et al., 1996). As discussed in Chapter 2, the metavolcanic rocks of the BMV that host part of the Buckhorn gold skarn are correlated with the Nicola Group. The Nickel Plate deposit is hosted in the Late Triassic Hedley formation of the Nicola Group, which consists of limestone and thinly bedded, turbiditic, calcareous siltstone and lesser amounts of calcareous argillite, tuff and conglomerate (Ray et al., 1996). While the Hedley formation is younger than the Permian Anarchist Group rocks that host the Buckhorn gold skarn it has a similar assemblage of lithologies and probably formed in a comparable depositional environment. Similar to the Buckhorn area, the Hedley mining district was intruded by a variety of alkaline to subalkaline, intermediate to shallow depth, gabbro to granodiorite bodies. The intrusions in the Hedley area coincide with the end of volcanism in the Nicola Group, and the oldest of these intrusions, the Hedley intrusive suite, is genetically linked to local gold skarn alteration (Ray et al., 1996). The Hedley intrusive rocks form a number of major stocks as well as sills and minor dikes that are quartz diorite to gabbro in composition and have a reduced oxidation state (Ray et al., 1996). Ranging in age from 212 to 194 Ma, the mineralizing intrusions in the Hedley area are notably older than those at Buckhorn, but they are compositionally similar and have a similar intrusive style. In broad terms, the skarn alteration at Nickel Plate is similar to that at Buckhorn. The prograde assemblage is predominately made up of iron-rich pyroxene and garnet, while the sulfide alteration is dominated by pyrrhotite (Ray et al., 1996). However, the skarn alteration at Nickel Plate differs from the skarn alteration at Buckhorn in several ways. At Nickel Plate the skarn contains significant scapolite alteration, which is absent from the Buckhorn skarn assemblage (Ray et al., 1996). The scapolite at Nickel Plate formed with the lower temperature gold-sulfide mineralization and is presumed to be the by-product of calcic-sodic metasomatism of diorite (Ettlinger et al., 1992). Ray et al. (1990) suggest that the scapolite alteration at Nickel Plate indicates that chloride complexes may have been important for the transportation and precipitation of gold in the system. The lack of scapolite 139  alteration and the physicochemical conditions for gold mineralization at Buckhorn suggest that while chloride complexes may have been important for gold mineralization in some skarn systems they are not necessary to produce an economic deposit. Other major differences between the two skarns exist in the opaque mineral assemblage. The Nickel Plate skarn has abundant arsenopyrite, trace chalcopyrite and negligible magnetite (Ettlinger et al., 1992), whereas Buckhorn has nearly the opposite abundances of these minerals. The presence of magnetite at Buckhorn suggests that parts of it formed under more oxidising conditions than the Nickel Plate skarn (Figure 4.15). The higher chalcopyrite content at Buckhorn is also similar to socalled oxidised gold skarns (Meinert et al., 2005). Another difference between the Nickel Plate and Buckhorn gold skarns exists in the intensity of retrograde alteration. Retrograde alteration to hydrous phases is relatively minor at the Nickel Plate deposit, (Ettlinger et al., 1992), and is characterized by minor amounts of ferrowollastonite and epidote, which locally replace prograde pyroxene. The retrograde alteration at Buckhorn is more intense than at Nickel Plate, and it features a different mineral assemblage consisting of ferroactinolite, epidote and hydrated Fe-silicates (Table 4.4). Gold mineralization at both deposits is accompanied by various bismuth minerals, but the specific mineralogy is different for each deposit. At Nickel Plate, hedleyite (Bi2+xTe1-x), native bismuth and maldonite (Au2Bi) are the most abundant bismuth phases (Ettlinger et al., 1992). At Buckhorn the bismuth minerals tend to be sulfur and/or tellurium bearing and include native bismuth, bismuthinite (Bi2S3), joseite (Bi4TeS2), and pilsenite (Bi4Te3) among others (Gaspar, 2005). The greater sulfur and tellurium content suggests that the mineralization at Buckhorn occurred at more oxidised conditions (Ciobanu et al., 2005, and references therein). The setting of gold mineralization at Nickel Plate is also different from that at Buckhorn. At Nickel Plate the majority of gold and bismuth mineralization is contemporaneous with arsenopyrite and occurs as inclusions within it (Ettlinger et al., 1992). This differs from the Buckhorn gold skarn where gold mineralization postdates the sulfide deposition and occurs in intercrystalline space between and fractures in prograde and retrograde minerals, or intergrown with retrograde minerals. Comparison of the Buckhorn and Nickel Plate gold skarns indicates that despite the many similarities there are significant differences. The two deposits share many large scale characteristics including host rocks and mineralizing intrusions, they also have the same general skarn alteration and 140  gold mineralization assemblage. However, at the smaller scale the Buckhorn gold skarn has a more oxidised skarn assemblage, and a more sulfur-rich ore mineral assemblage. The mineralogy and mineral chemistry at Buckhorn support its classification as a reduced gold skarn similar to other deposits around the world. However, the differences between Buckhorn and Nickel Plate clearly show that there is significant variability within this classification. This has implications for exploration strategies for similar deposits that are discussed in Chapter 6.  141  Chapter 5.	 Geochemistry of Skarn Alteration at Buckhorn 5.1	 Introduction In order to geochemically characterize the skarn alteration and gold mineralization at Buckhorn seventy-nine skarn samples and eight hornfels hosted skarn-vein samples were collected from eight drill holes through the SWOZ and Gold Bowl. Twenty-eight samples of the sedimentary, volcanic and igneous host rocks were also collected to provide geochemical context for the skarn samples. The samples came from 0.24 to 1.28 metre composite pulps from diamond drill holes and cover the three major classifications of skarn alteration: pyroxene skarn, garnet skarn and magnetite skarn (Figure 4.1 and Table 5.1). The samples were analysed by ICP-MS after four acid digestion for 55 elements, including major, trace and rare earth elements (REE). The complete results of the analysis can be found in Appendix B. The combination of four acid digestion and ICP-MS was chosen for the near complete digestion and low detection limit that allows a precise and accurate geochemical characterization of the skarn alteration. Table 5.1: Summary of diamond drill holes and skarn samples selected for geochemical analysis Drill Hole  Location  Number of skarn samples  Samples  Types of skarn alteration  Range of Gold Grade (ppm)  Protoliths  D07-369  SWOZ  22  BS082-84  Pyroxene > Garnet > Magnetite  <0.1 - 62  Metavolcanic and carbonate and clastic metasedimentary rocks  D08-410  SWOZ  21  BS090-94  Pyroxene > Garnet > Magnetite  <0.1 - 51.1  Metavolcanic and carbonate metasedimentary rocks  D08-413  SWOZ  15  BS016-17, Pyroxene > Garnet > Magnetite BS085-88  <0.1 - 117.5  Metavolcanic and carbonate metasedimentary rocks  D08-443  GB  6  BS077-79, BS089  Pyroxene > Garnet  <0.1 - 53.7  Clastic metasedimentary rocks and/or diorite  D09-536  North of GB  15  BS080-81  Pyroxene - Garnet > Magnetite  <0.1 - 0.7  Clastic metasedimentary rocks and/or diorite  D10-569  Below SWOZ  0  BS095-96  Skarn veins  <0.1 - 24.1  Clastic metasedimentary rocks  D10-595  Below SWOZ  0  BS097-98  Skarn veins  <0.1 - 3.5  Clastic metasedimentary rocks  5.2	 Relationship between Major and Trace Elements, Skarn Alteration and Gold-Bismuth Mineralization Geochemical analysis provides insight into the character of skarn alteration and gold mineralization. The relationship between elements in skarn alteration was investigated with a correlation matrix (Table 5.2). The correlation matrix shows that there is a positive correlation between gold and bismuth (r=0.92) and gold and silver (r=0.56), as well as a negative correlation between gold and aluminum(r=-0.5), but no correlation between gold and any other elements. The correlation matrix 142  also demonstrates the relationship of other elements. In addition to correlating with gold, silver also correlates with bismuth (r=0.64), lead (r=0.80), and zinc (0.68). The association of silver, lead, and zinc has been recognized in other skarns (Gemmell et al., 1992) and numerous other deposit types including epithermals (Simmons et al., 2005), porphyries (Sillitoe, 1997) and so-called intrusion related deposits (Thompson et al., 1999). There is also a correlation between aluminum and some of the high field strength elements (HFSE). Aluminum correlates with titanium (r=0.89), zirconium (r=0.80), and niobium (r=0.84) (Table 5.2), similarly, potassium correlates with sodium (r=0.62), rubidium (r=0.87), and strontium (r=0.54) (Table 5.2). The correlation of these sets of elements suggests that they acted in a similar manner during skarn alteration likely due to their similar chemical properties. To further investigate this relationship and geochemically distinguish the different host rocks and categories of skarn alteration a selection of major and trace elements were plotted versus bismuth (Figure 5.1). Bismuth was chosen as a proxy for gold as it has a strong correlation with gold and was precipitated with it (r=0.92, Table 5.2), but has greater range in abundances than gold. The geochemical characterization of host rocks and skarn alteration complements the mineralogical and textural characterizations that were presented in Chapters 2 and 4 and illuminates several trends. The hornfels altered host rocks have a variable geochemical character, but generally have lower iron and manganese and higher aluminum contents than the skarn alteration (Figure 5.1). The carbonate host rocks have a different geochemical character, with higher calcium and lower iron, manganese, aluminum, and titanium content compared to the other host rocks and skarn alteration (Figure 5.1). The Quartz Porphyry dikes were also analysed and have the same general geochemical characteristics as the metasedimentary and metavolcanic rocks, but with lower bismuth content (Figure 5.1). There is significant overlap in the geochemistry of the different skarn alteration assemblages, but some general trends can be identified. Garnet skarn tends to have higher calcium, manganese, and zirconium content, whereas magnetite skarn has high iron content and has low amounts of the other elements. The geochemical signature of pyroxene skarn is more variable, and overlaps with both magnetite and garnet skarn (Figure 5.1). The negative correlation between bismuth and aluminum is visible (r=-0.52), and the correlation is stronger in pyroxene skarn than in the other categories of skarn.  143  Table 5.2: Base 10 correlation matrix of select elements from geochemical analysis of skarn alteration at Buckhorn Log 10 Au Ag Bi Fe Ca P Mg Mn Cr Ti Al Na K S Cu Pb Zn As Mo W Co Ni Rb Sr Zr Nb Sn Sb  Au 1.00 0.56 0.92 0.16 0.09 ‐0.15 0.16 ‐0.18 0.10 ‐0.40 ‐0.50 ‐0.10 ‐0.21 0.16 0.08 0.49 0.13 0.15 ‐0.29 ‐0.13 0.44 ‐0.02 ‐0.11 ‐0.10 ‐0.45 ‐0.47 ‐0.18 0.36  Ag 0.56 1.00 0.64 ‐0.06 0.19 ‐0.03 ‐0.01 ‐0.13 ‐0.32 ‐0.08 ‐0.17 ‐0.13 0.02 0.25 0.45 0.80 0.68 0.04 0.16 0.03 0.32 0.29 0.13 0.17 ‐0.16 ‐0.28 ‐0.05 0.31  Bi 0.92 0.64 1.00 0.22 ‐0.04 ‐0.14 0.18 ‐0.27 0.07 ‐0.41 ‐0.52 ‐0.13 ‐0.20 0.19 0.03 0.62 0.29 0.29 ‐0.22 ‐0.11 0.53 0.04 ‐0.02 0.03 ‐0.50 ‐0.49 ‐0.25 0.46  Fe 0.16 ‐0.06 0.22 1.00 ‐0.42 ‐0.27 ‐0.03 ‐0.12 0.41 ‐0.55 ‐0.59 ‐0.13 ‐0.31 0.62 0.17 ‐0.26 ‐0.39 ‐0.04 ‐0.15 ‐0.03 0.38 0.05 ‐0.32 ‐0.50 ‐0.54 ‐0.39 0.04 ‐0.08  Ca 0.09 0.19 ‐0.04 ‐0.42 1.00 ‐0.15 ‐0.28 0.51 ‐0.17 0.12 0.22 ‐0.47 ‐0.30 ‐0.29 0.00 0.14 0.04 ‐0.08 0.11 0.33 ‐0.24 ‐0.17 ‐0.33 ‐0.12 0.27 0.00 0.27 ‐0.22  P ‐0.15 ‐0.03 ‐0.14 ‐0.27 ‐0.15 1.00 ‐0.30 0.15 0.17 0.74 0.66 0.36 0.49 ‐0.04 ‐0.06 0.04 0.23 0.20 0.30 0.04 0.16 0.47 0.47 0.54 0.54 0.62 0.26 0.33  Mg 0.16 ‐0.01 0.18 ‐0.03 ‐0.28 ‐0.30 1.00 ‐0.33 ‐0.20 ‐0.30 ‐0.33 0.16 ‐0.11 0.11 ‐0.03 0.01 0.09 ‐0.07 ‐0.32 ‐0.42 ‐0.07 ‐0.24 0.05 ‐0.03 ‐0.36 ‐0.29 ‐0.61 0.14  Mn ‐0.18 ‐0.13 ‐0.27 ‐0.12 0.51 0.15 ‐0.33 1.00 0.22 0.25 0.33 ‐0.16 ‐0.30 ‐0.23 ‐0.33 ‐0.13 0.00 ‐0.10 0.20 0.02 ‐0.10 0.03 ‐0.38 ‐0.23 0.37 0.25 0.23 ‐0.29  Cr 0.10 ‐0.32 0.07 0.41 ‐0.17 0.17 ‐0.20 0.22 1.00 ‐0.08 ‐0.07 0.11 ‐0.09 0.22 ‐0.12 ‐0.44 ‐0.46 0.17 ‐0.07 0.15 0.39 0.18 ‐0.12 ‐0.07 ‐0.07 ‐0.05 0.40 0.03  Ti ‐0.40 ‐0.08 ‐0.41 ‐0.55 0.12 0.74 ‐0.30 0.25 ‐0.08 1.00 0.89 0.27 0.51 ‐0.29 ‐0.07 0.03 0.29 0.11 0.49 0.15 ‐0.12 0.33 0.41 0.54 0.85 0.71 0.30 0.11  Al ‐0.50 ‐0.17 ‐0.52 ‐0.59 0.22 0.66 ‐0.33 0.33 ‐0.07 0.89 1.00 0.16 0.39 ‐0.34 ‐0.11 ‐0.09 0.17 0.01 0.36 0.14 ‐0.29 0.22 0.33 0.49 0.80 0.84 0.40 0.04  Na ‐0.10 ‐0.13 ‐0.13 ‐0.13 ‐0.47 0.36 0.16 ‐0.16 0.11 0.27 0.16 1.00 0.62 0.09 0.03 ‐0.08 0.10 ‐0.05 0.05 ‐0.06 0.08 0.28 0.53 0.34 0.11 0.10 ‐0.15 0.20  K ‐0.21 0.02 ‐0.20 ‐0.31 ‐0.30 0.49 ‐0.11 ‐0.30 ‐0.09 0.51 0.39 0.62 1.00 0.09 0.24 0.09 0.27 ‐0.14 0.32 0.05 ‐0.06 0.31 0.87 0.54 0.34 0.36 0.20 0.15  S  Log 10 Au Ag Bi Fe Ca P Mg Mn Cr Ti Al Na K S Cu Pb Zn As Mo W Co Ni Rb Sr Zr Nb Sn Sb  Cu 0.08 0.45 0.03 0.17 0.00 ‐0.06 ‐0.03 ‐0.33 ‐0.12 ‐0.07 ‐0.11 0.03 0.24 0.61 1.00 0.08 0.09 ‐0.31 0.21 0.27 0.10 0.34 0.15 ‐0.10 ‐0.04 ‐0.15 0.29 ‐0.08  Pb 0.49 0.80 0.62 ‐0.26 0.14 0.04 0.01 ‐0.13 ‐0.44 0.03 ‐0.09 ‐0.08 0.09 ‐0.11 0.08 1.00 0.75 0.12 0.08 ‐0.04 0.13 0.05 0.21 0.29 ‐0.11 ‐0.15 ‐0.26 0.34  Zn 0.13 0.68 0.29 ‐0.39 0.04 0.23 0.09 0.00 ‐0.46 0.29 0.17 0.10 0.27 ‐0.06 0.09 0.75 1.00 0.14 0.31 ‐0.07 0.10 0.29 0.38 0.42 0.16 0.02 ‐0.22 0.31  As 0.15 0.04 0.29 ‐0.04 ‐0.08 0.20 ‐0.07 ‐0.10 0.17 0.11 0.01 ‐0.05 ‐0.14 ‐0.21 ‐0.31 0.12 0.14 1.00 ‐0.05 0.12 0.61 0.19 0.04 0.17 0.06 ‐0.07 ‐0.10 0.14  Mo ‐0.29 0.16 ‐0.22 ‐0.15 0.11 0.30 ‐0.32 0.20 ‐0.07 0.49 0.36 0.05 0.32 0.09 0.21 0.08 0.31 ‐0.05 1.00 0.27 0.04 0.35 0.24 0.21 0.50 0.28 0.43 ‐0.01  W ‐0.13 0.03 ‐0.11 ‐0.03 0.33 0.04 ‐0.42 0.02 0.15 0.15 0.14 ‐0.06 0.05 0.11 0.27 ‐0.04 ‐0.07 0.12 0.27 1.00 0.05 0.18 0.08 0.06 0.15 ‐0.04 0.53 ‐0.15  Co 0.44 0.32 0.53 0.38 ‐0.24 0.16 ‐0.07 ‐0.10 0.39 ‐0.12 ‐0.29 0.08 ‐0.06 0.43 0.10 0.13 0.10 0.61 0.04 0.05 1.00 0.62 0.11 0.00 ‐0.23 ‐0.37 0.05 0.13  Ni ‐0.02 0.29 0.04 0.05 ‐0.17 0.47 ‐0.24 0.03 0.18 0.33 0.22 0.28 0.31 0.44 0.34 0.05 0.29 0.19 0.35 0.18 0.62 1.00 0.39 0.25 0.22 0.01 0.27 ‐0.04  Rb ‐0.11 0.13 ‐0.02 ‐0.32 ‐0.33 0.47 0.05 ‐0.38 ‐0.12 0.41 0.33 0.53 0.87 0.15 0.15 0.21 0.38 0.04 0.24 0.08 0.11 0.39 1.00 0.66 0.21 0.22 0.07 0.21  Sr ‐0.10 0.17 0.03 ‐0.50 ‐0.12 0.54 ‐0.03 ‐0.23 ‐0.07 0.54 0.49 0.34 0.54 ‐0.10 ‐0.10 0.29 0.42 0.17 0.21 0.06 0.00 0.25 0.66 1.00 0.30 0.35 0.07 0.55  Zr ‐0.45 ‐0.16 ‐0.50 ‐0.54 0.27 0.54 ‐0.36 0.37 ‐0.07 0.85 0.80 0.11 0.34 ‐0.33 ‐0.04 ‐0.11 0.16 0.06 0.50 0.15 ‐0.23 0.22 0.21 0.30 1.00 0.67 0.37 ‐0.11  Nb ‐0.47 ‐0.28 ‐0.49 ‐0.39 0.00 0.62 ‐0.29 0.25 ‐0.05 0.71 0.84 0.10 0.36 ‐0.33 ‐0.15 ‐0.15 0.02 ‐0.07 0.28 ‐0.04 ‐0.37 0.01 0.22 0.35 0.67 1.00 0.36 0.14  Sn ‐0.18 ‐0.05 ‐0.25 0.04 0.27 0.26 ‐0.61 0.23 0.40 0.30 0.40 ‐0.15 0.20 0.19 0.29 ‐0.26 ‐0.22 ‐0.10 0.43 0.53 0.05 0.27 0.07 0.07 0.37 0.36 1.00 ‐0.12  Sb 0.36 0.31 0.46 ‐0.08 ‐0.22 0.33 0.14 ‐0.29 0.03 0.11 0.04 0.20 0.15 0.09 ‐0.08 0.34 0.31 0.14 ‐0.01 ‐0.15 0.13 ‐0.04 0.21 0.55 ‐0.11 0.14 ‐0.12 1.00  0.16 0.25 0.19 0.62 ‐0.29 ‐0.04 0.11 ‐0.23 0.22 ‐0.29 ‐0.34 0.09 0.09 1.00 0.61 ‐0.11 ‐0.06 ‐0.21 0.09 0.11 0.43 0.44 0.15 ‐0.10 ‐0.33 ‐0.33 0.19 0.09  144  A  30  B  30  Ca (%)  Fe (%)  45 20  10  15  1  10 100 Bi (ppm)  1000  C  9 Al (%)  Mn (ppm)  4,500 3,000  1  10 100 Bi (ppm)  1000  1  10 100 Bi (ppm)  1000  1  10 100 Bi (ppm)  1000  D  6  1000  E  60 F Zr (ppm)  Ti (%)  10 100 Bi (ppm)  3  1,500  0.75  1  0.5  40 20  0.25  1  10 100 Bi (ppm)  1000  Legend Hornfels Altered BMS  Horfels Hosted Skarn Vein  Carobnate BMS  Pyroxene Skarn  Hornfels Altered BMV  Garnet Skarn  Quartz Porphyry Dike  Magentite Skarn  Figure 5.1: Plots of bismuth versus select major and trace elements (Fe, Ca, Mn, Al, Ti, and Zr) that demonCAPTION strate the geochemical character of the BMS, BMV, and skarn alteration. See text for further discussion.  145  5.3	 Relationship between Gold Mineralization and Sulfide Forming Elements Gold content of the skarn and hornfels hosted skarn veins was plotted against silver, bismuth, and a selection of sulfide forming elements (S, Cu, Pb, Zn, As, Mo, and Co) to determine their relationship. Figure 5.2 illustrates the positive correlation between and gold-bismuth (r=0.92, Table 5.2), where the samples plot along a single trend. The relationship between gold and silver is more complex (Figure 5.2). The silver analysis falls into two populations, those with high Au/Ag ratios (>4) form a tight trend, whereas those with lower Au/Ag ratios have a more diffuse pattern. Figure 5.2 also demonstrates the lack of correlation between gold and the sulfide forming elements that was noted with the correlation matrix (Table 5.2). There is also no relationship between metal content and the skarn alteration categories, and the hornfels hosted skarn veins plot in the same area as the rest of the skarn alteration. The correlation between gold and silver suggests that some of the gold occurs as electrum, which fits with microprobe analysis performed by Gaspar (2005). The two populations defined by the difference in Au/Ag ratios have not been previously noted and their character is investigated further in section 5.4.  146  A  B  Bi (ppm)  Ag (ppm)  1000 10  1 Ag Ag Ag u/ u/ Au/ A A 0.1 1/1 1 4/ 10/1  0.1  100 10 1  1 10 Au (ppm)  100  0.1  C  1  10  100  10  100  10  100  10  100  Au (ppm)  D  Pb (ppm)  1000  As (ppm)  100 10  100  10  1  1 0.1  1  Au (ppm)  10  0.1  100  E  1  Au (ppm)  F 100  Mo (ppm)  Cu (ppm)  1000 100  10  1  10 0.1  1  Au (ppm)  10  100  0.1  G  1  Au (ppm)  H 1000  Co (ppm)  S (%)  1  0.1  100  10  0.1  1  Au (ppm)  10  100  0.1  1  Au (ppm)  Figure 5.2: Graphs of Au vs.vs. Ag,Ag, Bi,As, As,Bi,Pb, S, and Co.graphs The graphs showthere that isthere is a Figure: Graphs of Au Co,Cu, Cu, Mo, Pb, and S. The show that a modmoderateerate correlation between Au-Ag, a strong correlation between and no correlation correlation between Au-Bi and Au-Ag, but none of the Au-Bi, other elements. Legend onbetween gold and the other elements. Legend on Figure 5.1. See text for further discussion preceding page, see text for further discussion. 147  5.4	 High and Low Base Metal Gold Mineralized Populations Au/Ag ratios were plotted versus select elements to further investigate the character of the two gold mineralized populations at Buckhorn (Figure 5.3 and Figure 5.4). In Figure 5.3 and Figure 5.4 the distribution of samples falls into two categories. Points on the Au/Ag vs gold, bismuth and arsenic graphs plot throughout the diagrams, whereas, the points on the Au/Ag vs base metal (Pb, Cu, and Mo) graphs predominately plot along the X and Y axes. Based on the latter graphs there are two interpreted populations of mineralized samples, one with high Au/Ag ratios and low base metal content, which plots along the x-axis, and a second with low Au/Ag ratios and higher base metal content that plots along the y-axis. The two populations are less distinct on the Au/Ag vs gold graph, but they appear as two sub-vertical arrays at high and low Au/Ag ratios (Figure 5.3 and Figure 5.4). The two are referred to as the low and high base metal populations. The low base metal population is characterized by Au/Ag ratios greater than four and low base metals content. This population contains samples that have a wide range of gold grades (4.7 to >100 ppm, Figure 5.3 and Figure 5.4), come from all categories of skarn alteration including the vein hosted style (Figure 5.3 and Figure 5.4), and come from both the SWOZ and Gold Bowl (Table 5.1). In contrast, the high base metal population has Au/Ag ratios below four and higher base metal content (Figure 5.3 and Figure 5.4). Similarly, the high base metal population occurs over a wide range of gold grades (<0.06 to >50 ppm), in all categories of skarn alteration and in both the SWOZ and Gold Bowl (Table 5.1, Figure 5.3 and Figure 5.4). Based on the metal content and petrographic analysis the low base metal population is interpreted as the main gold mineralizing event, whereas the high base metal population is interpreted as the earlier sulfide phase of retrograde alteration.  148  A  B 3,000  Bi (ppm)  Au (ppm)  100  50  0  1,500  0  5 Au/Ag  0  10  0  C  5 Au/Ag  10  5 Au/Ag  10  5 Au/Ag  10  D  Pb (ppm)  As (ppm)  2000  1000  0  0  5 Au/Ag  500  250  0  10  0  E  F 200  Mo (ppm)  Cu (ppm)  5,000  2,500  0  0  5 Au/Ag  10  100  0  0  Legend Au (ppm) >31.1 15.6-31.1 4.7-15.6 0.06-1.0  Rock Type Skarn Hornfels Hosted Skarn Vein  Figure X.x: Graphs of Au/Ag vs. Au, Bi and As as well base metals (Pb,vsCu Figure 5.3:asGraphs of Au/Ag Au,and Bi,Mo). Theand graphs As, base show metalstwo (Pb,dinstinct Cu, and populaMo). The tions (base and base metal graphs showmetal-bearing two distinct populations. See poor). textdiscussion. for further discussion. text forSee further  <0.06  149  A  B 3000  Bi (ppm)  Au (ppm)  100  50  0  0  5 Au/Ag  0  10  C Pb (ppm)  As (ppm)  1000  0  0  5 Au/Ag  0  E  5 Au/Ag  10  5 Au/Ag  10  F 200  Cu (ppm)  Mo (ppm)  5,000  2,500  0  10  300  0  10  5 Au/Ag  D  600  2000  0  1500  0  5 Au/Ag  10  100  0  0  Figure X.x: Graphs of Au/Ag vs. Au, Bi and As as well Figure 5.4: Graphs of Au/Ag vs Au, Bi, As, and base as base metals (Pb, Cu and Mo). Plots are the same as Pyroxene Skarn metals (Pb, Cu, and Mo). Plots are the same as Figure figure X.x, but coloured according to the skarn type. Garnet Skarn 5.3, coloured to the skarn type. The graphs Thebut graphs showaccording that the two dinstinct populations show that the two distinct populations occur in all skarn Magnetite Skarn (base metal-bearing and base metal poor) occur in See types. text for further discussion. Hornfels Hosted Skarn Veins types. all skarn Legend  150  5.5	 High Field-Strength Elements as a Proxy for Protolith Based on its position in the mine scale stratigraphy, skarn alteration at Buckhorn is presumed to replace both metavolcanic and carbonate metasedimentary rocks (Figure 2.2). However, in most locations due to the intensity of skarn alteration and its texturally destructive nature it is not possible to visually or petrographically determine the protolith of individual skarn samples. In order to geochemically determine the protolith of specific skarn samples the abundance of select high fieldstrength elements (HFSE) (Ti, Zr and total HREE (Gd-Lu)) were used as proxies for the protolith. These elements were chosen because they are typically immobile in hydrothermal systems (Bau, 1991; MacLean and Kranidiotis, 1987). Major elements were not useful for determining the protolith because during hydrothermal alteration they are typically mobile (Humphris and Thompson, 1978; Lentz and Gregoire, 1995). Figure 5.5 shows a plot of Ti, Zr and total HREE vs depth for three skarn intercepts through the SWOZ; the ore body where the stratigraphy is best known. As seen in Figure 5.5 there is a noticeable decrease in these elements within the skarn alteration and gold mineralization. This decrease is most clearly seen in hole D08-413, but it is present in all three holes as marked by the different horizontal black lines. The metavolcanic rocks have higher abundances of these immobile elements than the carbonate rocks (Gaspar, 2005), so this drop is interpreted to mark the contact between the metavolcanic rocks and the underlying carbonate rocks. The proposed contact is different depending on which element is used as a proxy for protolith. In two of the three holes (D07-369 and D08-413) the proposed contacts are within 1.5 metres of each other, but in the remaining hole, D08-410, the differences between them is greater. The difference may be due to local mobility of the HFSE or local deviation from the expected volcanic rocks over carbonate rocks stratigraphy. The graphs also show local perturbations in the immobile element content, such as points A, B and C, which are interpreted as variations in the protolith lithology. The elevated immobile element content at points A and C is probably due to local siliciclastic-rich layers in the upper carbonate member of the BMS, whereas, the presence of Quartz Porphyry dike is clearly responsible for the locally high immobile element content at point B. These graphs also demonstrate that, while the thickness of skarn alteration in the metavolcanic and the carbonate rocks is variable, the skarn alteration affects both rock types to a similar extent. Figure 5.5 also shows that protolith may partially control the type of skarn alteration that forms. In the samples analysed, garnet skarn solely forms from a metavolcanic protolith and magnetite skarn forms 151  predominately from a carbonate protolith. Pyroxene skarn is the most abundant variety analyzed and forms equally from both protoliths (Figure 5.5). The analysis also shows that both protoliths host gold mineralization to some degree, but that in two of the three drill holes the mineralized interval is almost entirely in the carbonate protolith (Figure 5.5).  152  Zr (ppm) 10 40 Ti (%) 0.1 0.4  Zr (ppm) 10 40 Ti (%) 0.1 0.4  Total HREE (ppm) 5 15 25  Zr (ppm) 10 40 Ti (%) 0.1 0.4  Total HREE (ppm) 5 15 25  Total HREE (ppm) 5 15 25  400 280 375  290 385 425  300  395 Depth (ft)  A  405  450  310  C B 415 D07-369  D08-410  Figure5.5: X.x:Immobile ImmobileHFSE HFSE (Zr, content Figure (Zr,TiTiand andtotal totalHREE) HREE content versus depth for three skarn intercepth through the versus depth for three skarn intercepts through the SWOZ. SWOZ. Thebetween contact the between volcanic and carbonate The contact volcanic and carbonate rocks isrocks is picked based on decreases in the immobile elements. picked based on a decrease in the immobile elements. HoriHorizontal blackmark linesthe areinterpreted the interpreted contact between zontal black lines contacts between the the BMV and the upper carbonate memeber of the BMS BMV and the upper carbonate member of the BMS  320  D08-413  >10 ppm Au pyroxene skarn garnet skarn magnetite skarn quartz porphyry dike metavolcanic rocks carbonate rocks metasedimentary rocks  153  5.6	 Gold/Copper and Gold/Silver Ratios for Gold Bearing Skarns In addition to determining the geochemical character of skarn alteration and gold mineralization at Buckhorn, the geochemical analysis was also used to compare Buckhorn to other skarn deposits around the world including other gold skarns (Table 5.3). To do this the Au (ppm)/Cu (%) and Au (ppm)/Ag (ppm) ratios were plotted against contained gold for numerous skarn deposits. As Figure 5.6 shows there are notable differences between the types of skarns in terms of their Au/Cu ratio. Gold skarns typically have higher Au/Cu ratios, but not necessarily higher gold content compared to copper skarns and porphyry copper skarns. The graph also shows that it is difficult to distinguish skarn deposit types on the basis of Au/Cu ratios, as there is significant overlap between the iron skarns, lead-zinc skarns and copper skarns. The Au/Cu ratios were also plotted in an attempt to distinguish reduced from oxidized gold skarns. Oxidised gold skarns characteristically have minor ubiquitous chalcopyrite mineralization, which is not typically a feature of reduced gold skarns (Meinert et al., 2005). The graph shows that gold skarns can have variable Au/Cu ratios that range from about 1 to 1000 Au (ppm)/Cu (%) and that there is no relationship between the Au/Cu ratio and the reduced-oxidised gold skarn distinction. The Au/Ag ratio from the same set of gold skarns was also plotted against contained gold to try and geochemically distinguish reduced from oxidised gold skarns. Figure 5.7 shows that gold skarns may have a wide range of Au/Ag ratios, and that similar to the Au/Cu ratios, there is no relationship between the oxidised-reduced gold skarn distinction and the Au/Ag ratio. Figure 5.6 and Figure 5.7 also show that while there are significant differences between Buckhorn and the Nickel Plate gold skarn as described in Chapter 4, in terms of Au/Cu and Au/Ag ratios the deposits are similar.  154  Table 5.3: Summary of gold skarns compared based on the Au/Ag and Au/Cu ratios Name  Location  Tonnage (t x 1000)  Gold (ppm)  Contained Gold (kg)  Accessory Metals  Au/Ag  Au/Cu (ppm/%)  Skarn Oxidation State  References  Latitude  Longitude  Nambija, District, Ecuador  4°3'54"S  78°46'59"W  8300-10400  15.0  ~140,000  Ag (~2.5ppm), Cu (0.01%)  6.00  1500  Oxidised  Markowski et al (2006), Fontbote et al (2004), Meinert (1998)  La Luz/Siuna, Nicaragua  13˚44’19"N  84°47'06"W  27700  3.4  94,762  Ag(1.2ppm), Cu(0.44%)  2.85  7.78  Oxidised  Venable (1994), Theodore et al., (1991)  Fortitude, Nevada, USA  40°32'54"N 117°07'57"W  10900  7.2  77,968  Ag (28.2ppm), Cu (0.79%)  0.24  9.05  Reduced  Meinert (2000), Wotruba et al. (1986), Theodore et al. (1973)  Nickel Plate, BC, Canada  49˚21’55"N  120°02'04"W  13400  5.3  71,020  Ag (1.74ppm), Cu (0.029%)  3.04  183  Reduced  Ray et al. (1993), Ettlinger et al. (1992)  El Valle - Boinas, Spain  43˚16’56"N  6°18'19"W  14200  4.3  61,202  Ag(12.27ppm), Cu(0.41%)  0.35  10.50  Reduced  Cepedal et al. (2000), Martin-Izard et al. (2000a)  Red Dome, Australia  17°07'00"S  144°24'00"E  15000  2.6  39,000  Ag(5.25ppm), Cu(0.46%), Zn(1%)  0.50  5.65  Oxidised  Ewers and Sun(1989), Theodore et al., (1991)  Buckhorn, Washington, USA  48˚57’ N  118˚ 59’ W  2147.3  14.0  30,051  Ag (3.42ppm), Cu (0.09%)  4.09  156  Reduced  Cooper et al. (2008), Gaspar (2005), This Study  Phoenix, BC, Canada  49˚05’36"N  118°35'42"W  26956  1.1  29,652  Ag(7.1 ppm), Cu(0.9%)  0.16  1.22  Oxidised  Church (1986)  Beal, Montana, USA  45°57'08"N 112°52'56"W  14800  1.5  23,100  Ag (1.5ppm)  1.00  Reduced  Meinert (2000), Wilkie (1996)  McCoy, Nevada, USA  40°19'51"N 117°13'34"W  15600  1.4  22,464  Ag(0.1ppm), Cu(0.1%)  14.40  Oxidised  Brooks (1994), Meinert (2000), Meinert (1989)  1˚24’07"N  2400  7.2  17,280  Ag(0.1ppm)  72.00  Reduced  Theodore et al., (1991), Meinert (1989), Bowles (1984)  Bau, Malaysia  110°08'45"E  14.40  155  Name  Location  Tonnage (t x 1000)  Gold (ppm)  Contained Gold (kg)  Accessory Metals  Au/Ag  Au/Cu (ppm/%)  Skarn Oxidation State  References  1.06  Reduced  Richards et al (2003), Winter (2001)  37.90  Reduced  Martin-Izard et al (2000b), Meinert (2000)  Oxidised  Dennis et al., (1989), Theodore et al., (1991)  Reduced  Blake et al., (1984), Meinert (1989)  Oxidised  Meredith-Jones, (2010) ( MINFILE No 093A 121)  Latitude  Longitude  Nui Phao, Vietnam  21°38'52"N  105°39'55"E  87900  0.2  16,701  WO3(0.19%), CaF2(7.95%), Cu(0.18%), Bi(0.09%)  Carles, Spain  43˚21’43"N  6°14'45"W  5300  3.0  15,900  Ag(5.84 ppm), As(0.2-2.2%), Cu(0.42%)  2.72  Mt. Hamilton, Nevada, USA  39°15'14"N 115°33'27"W  12617  1.0  12,164  Ag(17.2ppm)  0.06  Minnie-Tomboy, Nevada, USA  40°31'21"N 117°07'23"W  3900  2.8  10,920  Ag(9ppm), Cu(0.3%)  0.31  QR, BC, Canada  52°40'08"N 121°47'11"W  2372  4.6  10,800  Ag(1.05ppm)  4.34  Junction Reef, Australia  33°37'28"S  148°59'45"E  2370  3.2  7,683  Ag(0.5ppm), Cu(0.06%)  0.65  54  Reduced  Gray et al., (1995)  Nixon Fork, Alaska, USA  63° 14' 18" N  154° 45' 59"W  167.618  39.4  6,608  Ag(6.7ppm), Cu(0.6%)  5.88  65.70  Oxidised  Newberry et al., (1997), Bundtzen and Miller (1997)  Surprise, Nevada, USA  40°37'05"N 117°01'25"W  1530  2.8  4,238  Ag(23.1ppm), Cu(0.85%)  0.12  3.26  Oxidised  Theodore et al., (1991)  Marn, Yukon, Canada  64° 29' 37" N  272.155  8.6  2,330  Ag(17.12ppm), Cu(1%), W(0.1%)  0.50  8.56  Reduced  Brown and Nesbitt (1987)  138° 47' 38"W  9.33  156  Au (ppm)/Cu (%)  1000 BH  100  Gold Skarns Oxidized Reduced Unknown Porphyry Cu Skarns  NP  10  Copper Skarns Iron Skarns Pb-Zn Skarns NP = Nickel Plate BH = Buckhorn  1 0.1 10  100  1000  10,000  100,000 1,000,000  Contained Au (Kg)  Au (ppm)/Ag (ppm)  Figure 5.6: Plot of Au (ppm)/Cu (%) vs Contained Au (Kg) for a selection of gold, porphyry copper, copper, iron, and lead-zinc skarns. Coloured points from Table 5.3, remainder from Meinert (1989). See text for further discussion.  100 10  BH NP  1 0.1 0.01 1000  10,000  Gold Skarns Oxidised Reduced NP = Nickel Plate BH = Buckhorn  100,000 1,000,000  Contained Au (Kg) Figure 5.7: Plot of Au/Ag vs Contained Au (Kg) for a selection of gold skarns. See text for further discussion.  5.7	 Discussion and Conclusions Geochemical analysis of skarn alteration at Buckhorn supports many of the conclusions drawn from the detailed petrography. The correlation between gold and bismuth (r=0.92), which was demonstrated with both the correlation matrix and Au-Bi plot, matches with the petrography that described the intergrowth of gold and bismuth minerals (Chapter 4). The correlation between gold and 157  silver (r=0.56) fits with the presence of electrum detected in a previous microprobe study (Gaspar, 2005). The lack of a strong correlation between gold or bismuth and the major and trace elements fits with the petrographic analysis presented in Chapter 4, which showed that gold mineralization postdates the majority of silicate skarn alteration. This implies that the host rock chemistry and therefore the mineralogy and the type of skarn alteration does not impact the location or grade of gold mineralization. Similarly, the lack of correlation between gold and the sulfide forming elements suggests that the sulfide mineralogy does not impact the location or grade of gold mineralization. This conclusion fits with the petrography, which demonstrated that the majority of sulfide alteration predates and is unrelated to gold mineralization (Chapter 4). Notable exceptions to this conclusion are the examples of gold-bearing chalcopyrite; however, these form a minor portion of the total sulfide alteration and therefore do not have a significant impact on the correlation between gold and copper. The negative correlation between gold and aluminum suggests that aluminum poor protoliths are more prospective for gold mineralization, as aluminum is presumed to be less mobile during skarn alteration than the other major elements based on its correlation with the HFSE (Table 5.2); hence, the aluminum content of the skarn alteration reflects the aluminum content of the protolith. The negative correlation between gold and aluminum is also consistent with variation in amphibole chemistry presented in Chapter 4. The relationships between gold and other elements described in this chapter confirm those previously documented at Buckhorn (Hickey, 1990, 1992) and are similar to those at other gold skarn deposits such as Nickel Plate (Ray et al., 1996) and Junction Reefs (Gray et al., 1995). The geochemical analysis also matches the petrographic observation that gold and base metal mineralization at Buckhorn occurred in two distinct events. The base metal event is characterized by high base metal content and low Au/Ag ratios (<4). It may contain a minor amount of gold mineralization, as indicated by EDS elemental maps of gold bearing chalcopyrite (Figure 4.3). Base metal mineralization can be petrographically shown to be part of the retrograde alteration that predates the gold mineralization. The gold mineralizing event has low base metal contents and high Au/Ag ratios (>4). The bulk of gold mineralization at Buckhorn was deposited during this event, and the gold occurs solely in the fracture, intercrystalline and intergrowth settings outlined in Chapter 4. Petrography in Chapter 4 showed that both the base metal and gold mineralizing events occurred during retrograde skarn alteration, and that in some locations the gold mineralizing event overprint the base metal 158  event. However, the presence of base metal rich samples without gold shows that the two mineralizing events were not necessarily spatially coincident. Both mineralizing events occur in the SWOZ and Gold Bowl and are not dependent on the skarn mineralogy (Table 5.1 and Figure 5.4). The geochemical analyses also lead to conclusions regarding the character of the host rocks and the skarn alteration categories. It showed that the host rocks have a different major and trace element signature from the skarn alterations, but there is significant overlap between the two and the host rocks cannot be distinguished from the skarn alteration on the basis of the major and trace elements. Similarly, the overlap in the geochemical signature of the three skarn alteration assemblages is too great to conclusively distinguish them. However they can be easily distinguished based on their mineralogy (Chapter 4). Geochemical analysis of HFSE in the skarn alteration at Buckhorn was able to help determine the protolith of skarn alteration in the SWOZ, a task that is not possible petrographically. The protoliths are distinguished based on their place in the stratigraphy and HFSE content. The HFSE are immobile during skarn alteration and the BMV has notably higher HFSE content than the carbonate member of the BMS, hence, skarn alteration with a BMV protolith has higher HFSE. By determining the protolith this work was able to show that the carbonate member of the BMS host a higher proportion of the skarn alteration and gold mineralization than the other protoliths (Figure 5.5). Determining the protolith also showed that garnet skarn preferentially forms from a metavolcanic protolith and that magnetite skarn forms preferentially from a carbonate protolith The multi-element geochemical analysis of skarn alteration also used to compare Buckhorn to other skarns. This comparison showed that there are differences between some skarn deposits types in terms of Au/Cu ratios, but that there is also significant overlap between other types of skarns. The analysis also showed that, although, the reduced and oxidized gold skarns have different sulfide assemblages there is no difference between them in terms of their Au/Cu or Au/Ag ratios.  159  Chapter 6.	 Summary The Buckhorn gold skarn is hosted in Permian Anarchist Group metasedimentary rocks disconformably overlain by Jurassic Elise Formation metavolcanic rocks. The Anarchist Group was deposited in a back-arc setting and is locally comprised of a lower member of fine-grained siliciclastic rocks and a conformable upper member of carbonate rocks. The Jurassic Elise Formation metavolcanic rocks were deposited sub-aerially in a continental arc setting and are locally made up of basaltic flows and autobreccias. This stratigraphy is intruded by two suites of granitoids. The Middle Jurassic postaccretionary Buckhorn Intrusive Suite (BIS) is genetically related to the skarn alteration and gold mineralization and comprised of a granodiorite stock (170.4 ± 1.7 Ma), marginal diorite (168.9 ± 0.9 Ma), and several generations of dikes: mafic diorite dikes, Early Diorite dikes (168.2 ± 0.7 Ma), Granodiorite dikes (167.5 ± 0.7 Ma), and Quartz Porphyry dikes (163.6 ± 0.8 Ma). The Eocene Roosevelt Intrusive Suite (50.5 ± 2.9 Ma) is comprised of a small (~0.03 km2) granodiorite stock that post-dates skarn alteration and gold mineralization. Skarn alteration at Buckhorn is divided into three categories based on the prograde mineralogy: pyroxene skarn, garnet skarn, and magnetite skarn. Pyroxene skarn is further subdivided based on the retrograde mineralogy: amphibole-pyroxene skarn, magnetite-pyroxene skarn, and epidote-pyroxene skarn. The location of skarn alteration and gold mineralization at Buckhorn is spatially controlled by shear zones and Granodiorite dikes that acted as conduits for hydrothermal fluids. The skarn alteration is zoned from dominantly magnetite and garnet skarn in the proximal Magnetic Mine, to equal portions of magnetite, garnet, and pyroxene skarn in the Gold Bowl, and pyroxene skarn dominated in the distal Southwest Ore-Zone (SWOZ). The SWOZ contains the majority of the gold mineralization and consists of a 1-15 metre thick sub-horizontal layer of massive calcic, Fe-rich, reduced skarn alteration. The SWOZ occurs along a low-angle shear zone at the contact between the carbonate metasedimentary rocks and overlying metavolcanic rocks. Skarn alteration also occurs in millimetre scale veins that cut the skarn and hornfels alteration and are interpreted and the youngest and/or most distal expression of skarn alteration. Based on the mineralogy the hydrothermal fluids that caused prograde alteration were between 430-500° C, fO2=-25 to -20, fS2=-8 to -4.5, and near neutral to slightly acidic pH. Fluids responsible for retrograde alteration were cooler (300-430° C), more reduced (fO2<-26), and had lower sulfur fugacity (fS2=-15 to -6) than fluids responsible for prograde alteration. The physicochemical conditions for prograde and retrograde alteration at Buckhorn are more oxidised and sulfur rich than those 160  proposed for skarn alteration at the Nickel Plate gold skarn in the nearby Hedley district (90 km to the north-west) (Ettlinger et al., 1992). Gold mineralization at Buckhorn occurs in fractures in and intercrystalline space between prograde and retrograde minerals as well as intergrown with retrograde minerals. The gold mineralization is intimately associated and intergrown with bismuth. The intergrowth textures of gold-bismuth and gold-retrograde minerals suggest that gold was scavenged by a bismuth melt from a hydrothermal fluid (Tooth et al., 2011). Based on the conditions of retrograde alteration gold was transported as bisulfide complexes in the fluid. Bismuth scavenging preferentially occurs in reduced, acidic, and low sulfur conditions (Tooth et al., 2008); like those that occurred during retrograde alteration and gold mineralization at Buckhorn. Before this study the mechanism for gold precipitation at Buckhorn was unknown. This work shows that gold precipitation was induced two physicochemical changes. The most prevalent was cooling that results in gold intergrown with bismuth and rimmed by bismuthinite (Tooth et al., 2011), a texture that is abundant in the intercrystalline and fracture hosted settings of gold mineralization. Gold precipitation caused by local retrograde oxidation reactions formed a lesser portion of the gold mineralization. This mechanism forms gold-bismuth mineralization that is intergrown with retrograde minerals (Tooth et al., 2011), the diagnostic texture of the intergrowth style of gold mineralization. The mechanism for gold precipitation in gold skarns has been disputed in the past (Meinert, 2000), but this research at Buckhorn suggests that it may be a combination of cooling and local retrograde oxidation reactions. Re-Os geochronology of molybdenite bearing skarn constrains age of skarn alteration and gold mineralization at Buckhorn to a 4.7 million year window in the Middle Jurassic between 167.5 Ma and 162.8 Ma, and confirms its association with the BIS.  6.1	 Exploration Implications There are significant differences between Buckhorn and Nickel Plate, which has served as the de facto exploration model for gold skarns (Meinert et al., 2005), and an exploration program focused on finding reduced gold skarns should not discount rocks because they do not fit the Nickel Plate model. Some of the exploration implications that come out of the better understanding of Buckhorn and the variation within reduced gold skarns include:  161  •	 Prior to this study the age of skarn alteration and gold mineralization was unclear. However local mapping, U-Pb geochronology of the host and intrusive rocks, and Re-Os of molybdenite in skarn alteration showed that the skarn alteration and gold mineralization was genetically related to the post-accretionary Middle Jurassic BIS. This means that postaccretionary plutons are also prospective for the development of gold skarns, and not only syn-accretionary plutons, such as those responsible for gold skarn formation at Nickel Plate. •	 Skarn alteration with carbonate metasedimentary protolith should be targeted because at Buckhorn skarn alteration with a such a protolith hosts proportions more gold mineralization than skarn from other protoliths. This contrasts other gold skarns where mineralization is predominately hosted in siliciclastic and volcaniclastic protoliths (Meinert, 2000). This study showed that the content of certain immobile HFSE can be used to distinguish the protoliths of skarn alteration. •	 Syn-mineralization shear zones and dikes acted as fluid conduits and controlled the location of skarn alteration and gold mineralization at Buckhorn, and should therefore be targets for exploration. The orientation of some of the dikes at Buckhorn may have been influenced by syn-sedimentary faults in the metasedimentary host rocks. Therefore, when exploring for reduced gold skarns the orientation of any structures should be determined because the orientation of skarn alteration can be controlled by both pre- and syn-mineralization features. •	 Scapolite and arsenopyrite are vectors for gold mineralization at Nickel Plate, but they are not necessarily indicative of gold mineralization in all reduced skarns, as scapolite is not present and arsenopyrite is rare and neither is associated with gold mineralization at Buckhorn. •	 Magnetite and chalcopyrite are nearly absent from Nickel Plate, but can be present in significant amounts in other reduced gold skarns such as at Buckhorn. Therefore their presences should not be used to reject an exploration target. •	 At Buckhorn gold precipitated at the end of retrograde alteration, and was induced by cooling and local retrograde oxidation reactions. This change in physical conditions is expressed as a decrease in aluminum and iron content in amphibole, the development of epidote, pyrite, and hydrated Fe-silicate minerals. The latter mineralogical changes can be detected during core logging and indicate that the physicochemical conditions for gold precipitation occurred. 162  •	 Millimetre to centimetre scale skarn veins are the youngest and/or most distal expression of skarn alteration at Buckhorn. Their identification in otherwise non skarn-altered rocks suggests that an area is prospective for gold skarn mineralization. •	 Economic gold skarns can occur within the same skarn system as uneconomic skarn alteration (ie. The SWOZ vs. The Magnetic Mine). It is therefore important to understand the characteristics of gold skarn alteration and properly characterize any known uneconomic skarn alteration to determine if it is part of a larger gold skarn system. This could be done using the following tools: ºº Property scale mapping and thin section petrography could be done to determine the character and distribution the skarn alteration assemblages, host rocks, and local intrusions. At Buckhorn such mapping would show that the most distal exposed skarn alteration is made up of roughly equal portions of garnet and pyroxene (ie. The Gold Bowl), which suggests that a more distal and more prospective pyroxene dominated zone may exist (ie. The SWOZ). ºº Whole rock multi-element geochemical analysis could characterize the mineralization and determine which elements correlate with gold. Based on the work at Buckhorn and other deposits reduced gold skarns are characterized by a gold - bismuth ± tellurium ± arsenic ± cobalt signature and have a high (>1) Au (ppm)/Cu (%) ratio. ºº Several techniques could be used to determine the composition of the skarn minerals, with iron and calcium rich pyroxene, garnet and amphibole alteration being indicative of reduced gold skarns. Microprobe analysis could be used, but it is likely prohibitively costly and time consuming for mineral exploration. Elemental analysis by EDS is more cost effective, but less precise that microprobe analysis, but it would still provide sufficient information for mineral exploration. X-Ray diffraction could also be used and in some cases the optical properties of the minerals may also be useful for distinguishing the mineral species.  6.2	 Future Research During the course of this study several questions arose that have been left unanswered, but would be worthwhile investigating with future research. 163  The spatial relationship of the different settings of gold mineralization is currently unknown, and it is unknown if their distribution is impacted by larger structures and/or other features. Determining the distribution of the different settings of gold mineralization could help identify fluid pathways and/or other controls on mineralization. This could potentially be used to fine tune the milling process to account for the different types of gold mineralization and improve the efficiency of gold recovery. The age of the mafic diorite is yet to be determined, it cross-cuts the Buckhorn Granodiorite and is therefore younger than 170.4 Ma (Figure 3.10), but its age has not been further constrained because it was not found in contact with any other intrusions and did not contain any zircons for U-Pb geochronology. Ar-Ar geochronology could be used to further constrain the age of the mafic diorite, which would help describe the genesis of the BIS. The current constraints on the age of skarn alteration and gold mineralization suggest that it took place over an extended period of time, between 167.5 and 162.8 Ma. Dating the prograde garnet by Lu-Hf geochronology and retrograde amphibole by Ar-Ar geochronology could possibly further constrain the age of skarn alteration and gold mineralization. This could lead to a better understanding of timing and duration of the skarn alteration system at Buckhorn, and help develop a better deposit model for reduced gold skarns.  164  Chapter 7.	 Conclusions Work in the six preceding chapters leads to conclusions regarding the Buckhorn host rocks, skarn alteration, gold mineralization, and the classification as a reduced gold skarn.  7.1	 Host Rocks •	 The metasedimentary host rocks are correlated with the Permian Anarchist Group and are equivalent to other rocks in the southern Canadian cordillera including the Harper Ranch Group, Attwood Group, and the Mount Roberts Formation. This correlation is based on the lithological similarities and the Permian fossil ages previously determined. •	 The metavolcanic host rocks unconformably overly the metasedimentary host rocks and are correlated with the Jurassic Elise formation of the Nelson-Rossland area. The Elise formation is age equivalent to other early Mesozoic arc related volcanic rocks in the southern Canadian cordillera including the Nicola Group in the Hedley area and the Brooklyn Formation in the Greenwood area. The correlation of the metavolcanic rocks at Buckhorn with the Jurassic Elise formation is based on the similarity in lithologies and the Early Jurassic (192.4 ± 1.0 Ma) age determined for a monzodiorite dike comagmatic with the metavolcanic sequence. •	 The Buckhorn Intrusive Suite (BIS) is correlated with a number of age equivalent intrusive suites including the Cahill Creek Pluton (168.9 ±9 Ma) in the Hedley district; The Nelson (167 Ma), Trail (169±3 Ma) and Bonnington (167 Ma) Plutons and the Rossland Monzonite (167.5±0.5 Ma) in the Nelson-Rossland area. The BIS is also shown to be more complex than previously known: ºº Different generations of granodiorite that are texturally and mineralogically similar are distinguished on the basis of age (BS048 170.4 ± 1.1 Ma, BS074 167.8 ± 1.5 Ma, BS057 166.0 ± 1.0 Ma). ºº The Buckhorn Diorite (BS060 169.0 ± 0.9 Ma) is now shown to be a marginal phase of the Buckhorn Granodiorite based on the gradational contact between the two and the similarity in age.  165  ºº The Early Diorite dikes (BS075 169.4 ± 1.3 Ma, BS046 168.2 ± 0.7 Ma) are now distinguished from the Buckhorn Diorite on the basis of texture, but were shown to be the same age. ºº The Granodiorite dikes are shown to host both the two low angle and steeply dipping foliations, this means that the foliations occurred after the emplacement of the dikes in the Middle Jurassic (BS059 167.4 ± 0.8). This contrasts earlier work that proposed that only the steeply dipping foliation post-dated the igneous rocks and affected the skarn alteration. •	 The Early Eocene age and complex melting and emplacement history of the Roosevelt Granodiorite is confirmed on the basis of the diverse inherited zircon population discovered in the geochronological sample.  7.2	 Skarn Alteration •	 Skarn alteration is associated with the low angle shear zones and Granodiorite dikes, both of which acted as fluid pathways and control the location and intensity of skarn alteration. •	 Skarn alteration overprints metavolcanic, carbonate, and to a minor extent clastic metasedimentary rocks. The first two host approximately equal portions of skarn alteration and gold mineralization. The protolith of skarn alteration was determined by using the HFSE content as a proxy for the protolith. This analysis also shows that garnet skarn is more prevalent in a metavolcanic protolith, magnetite in a carbonate protolith, whereas pyroxene skarn forms in all protoliths. •	 Skarn alteration at Buckhorn is now split into three major alteration assemblages based on the prograde mineralogy: pyroxene skarn, garnet skarn, and magnetite skarn. Pyroxene skarn is further subdivided based on the specific prograde and retrograde alteration assemblage: amphibole-pyroxene skarn, magnetite-pyroxene skarn, and epidote-pyroxene skarn. •	 The skarn alteration is zoned from garnet and magnetite dominated in the Magnetic Mine to pyroxene dominated in the SWOZ. This zonation suggests that skarn forming fluids were sourced from the Buckhorn Granodiorite and Buckhorn Diorite near the Magnetic Mine and travelled to the south forming the Magnetic Mine, the Gold Bowl and ultimately the SWOZ, cooling and become more reduced along the way. 166  •	 A similar zonation can be seen from east to west in the SWOZ, implying that the fluids that caused skarn alteration on the east side of the SWOZ were more oxidised that expected based on the larger scale zonation. This secondary zonation is likely due to the presences of the Footwall Mylonite that channelled skarn forming fluids and allowed them to travel a greater distance with less interaction with the host rocks. •	 The physicochemical conditions of prograde and retrograde skarn alteration are constrained based on the mineralogy. Prograde alteration occurred between 430-500° C, fO2=-25 to -20, fS2=-8 to -4.5, and near neutral to slightly acidic pH. Retrograde alteration occurred at cooler (300-430° C), more reduced (fO2<-26), and lower sulfur fugacity (fS2=-15 to -6) conditions than prograde alteration. •	 In addition to forming at the metre scale, skarn alteration is also documented to form in veins at the centimetre scale. Based on the cross-cutting relationships the skarn veins are the youngest and/or most distal stage of skarn alteration and gold mineralization at Buckhorn. •	 Much of the skarn alteration is affected by deformation that manifests as brittle fractures in the prograde minerals and as low angle foliations in the retrograde minerals. Based on the similarities in appearance and the comparable age constraints, the deformation in the skarn is equivalent foliations in the host rocks.  7.3	 Gold Mineralization •	 Gold mineralization occurs in five distinct settings at the Buckhorn gold skarn: (1) in intercrystalline space between prograde and retrograde minerals, (2) in fractures in prograde minerals, (3) intergrown with retrograde minerals, (4) in skarn veins through skarn and non skarn-altered rocks, and (5) in chalcopyrite mineralization. •	 Gold hosted in chalcopyrite formed first, followed by the fracture and intercrystalline gold, then the gold intergrown with retrograde minerals, and finally the skarn vein hosted gold. More than one setting of gold mineralization may occur in a single sample. •	 In all settings gold mineralization is accompanied by bismuth mineralization in the form of native bismuth, bismuthinite, and other bismuth minerals.  167  •	 Gold mineralization is part of retrograde alteration and postdates the majority of the calcsilicate skarn alteration, and all of the foliations. •	 Gold has a positive correlation with bismuth (r=0.92) and silver (r=0.56), and negative correlation with aluminum (r=-0.50), but has no significant correlation with any other major, trace or rare earth elements. The lack of correlation matches with the petrography and geochemical analysis that shows that gold mineralization is spatially, texturally, and geochemically distinct from the prograde and base metal sulfide alteration. •	 Gold mineralization is spatially associated with more magnesium rich and aluminum poor amphiboles, which suggests that they formed under cooler and/or more oxidised conditions, both of which occurred towards the end of retrograde alteration at Buckhorn. •	 Gold was transported as bisulfide complexes in the hydrothermal fluid between 241-300°C, fO2=-42 to -36, and pH=6 to 8, before being scavenged by a bismuth melt. Precipitation of the gold-bismuth melt was induced by cooling and retrograde oxidation reactions. Precipitation caused by cooling is responsible for the fracture and intercrystalline hosted gold. Gold intergrown with retrograde minerals resulted from gold precipitation caused by that retrograde reaction. •	 The maximum age of gold mineralization is constrained by the Granodiorite dikes to 167.5 ± 0.8 Ma and the minimum age is constrained by the molybdenite that postdates the retrograde alteration at 162.8 ± 0.7 Ma.  7.4	 Comparison to Other Gold Skarns •	 Buckhorn is classified as a reduced gold skarn based on the skarn mineralogy (pyroxene > garnet + magnetite, pyrrhotite > pyrite + chalcopyrite), mineral compositions (pyroxene = hedenbergite-augite, amphibole = ferrohornblende-ferroactinolite), and gold mineralization elemental assemblage (gold-bismuth). •	 Other characteristics of the skarn alteration at Buckhorn are similar to so-called oxidised gold skarns (high magnetite content, minor pervasive chalcopyrite), suggesting that there is there is a continuum between reduced and oxidised gold skarns  168  •	 The oxygen and sulfur fugacity conditions for prograde and retrograde skarn alteration at Buckhorn also show that it formed at more oxidised conditions than Nickel Plate, and is further evidence that there may be a continuum between reduced and oxidised gold skarns. •	 When compared to the Nickel Plate deposit, one of the largest and best studied reduced gold skarns, there are significant differences between the two deposits. ºº Buckhorn does not contain scapolite, which is spatially and temporally associated with gold mineralization at Nickel Plate where it indicates that gold was transported as chloride complexes. 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Wotruba, P. R., Benson, R. G., and Schmidt, K. W., 1986, Battle Mountain describes the geology of its Fortitude gold-silver deposit at Copper Canyon: Mining Engineering, v. 38, p. 495-499.  179  Appendix A.	 Microprobe Analysis Table A.1: Microprobe analysis of pyroxene Label  090-1-10  090-1-11  090-1-2  090-1-6  090-1-7  090-2-2  090-2-4  Mineral  Px  Px  Px  Px  Px  Px  Px  Au (ppm)  0.2  0.2  0.2  0.2  0.2  0.2  0.2  SiO2  48.769  49.145  49.476  49.075  49.252  49.439  49.086  TiO2  0.006  0.000  0.000  0.061  0.000  0.046  0.017  Al2O3  0.287  0.423  0.146  1.543  0.091  0.534  0.415  FeO  23.244  22.162  22.283  29.033  21.833  21.208  22.544  MnO  0.465  0.526  0.378  0.355  0.406  0.388  0.406  MgO  4.098  4.315  4.314  5.071  4.495  4.862  4.109  CaO  21.493  23.005  22.736  11.438  22.808  22.668  22.635  Na2O  0.074  0.056  0.193  0.166  0.196  0.246  0.188  NiO  0.015  0.000  0.000  0.000  0.000  0.055  0.023  Cr2O3  0.000  0.014  0.010  0.000  0.000  0.000  0.012  Total  98.451  99.645  99.535  96.742  99.080  99.447  99.434  Si  1.986  1.969  1.983  2.052  1.980  1.973  1.972  Al (T)  0.014  0.020  0.007  0.000  0.004  0.025  0.020  Fe3+ (T)  0.000  0.011  0.010  0.000  0.015  0.002  0.008  Sum T  2.000  2.000  2.000  2.052  2.000  2.000  2.000  Al (M1)  0.000  0.000  0.000  0.076  0.000  0.000  0.000  Fe3+ (M1)  0.020  0.035  0.031  0.000  0.035  0.043  0.041  Ti  0.000  0.000  0.000  0.002  0.000  0.001  0.001  Cr  0.000  0.000  0.000  0.000  0.000  0.000  0.000  Mg  0.249  0.258  0.258  0.316  0.269  0.289  0.246  Fe2+  0.771  0.697  0.706  1.015  0.684  0.663  0.708  Mn  0.016  0.018  0.013  0.013  0.014  0.013  0.014  Ni  0.001  0.000  0.000  0.000  0.000  0.002  0.001  Sum M1  1.057  1.008  1.008  1.422  1.002  1.012  1.011  Ca  0.938  0.988  0.977  0.512  0.983  0.969  0.974  Na  0.006  0.004  0.015  0.013  0.015  0.019  0.015  Sum M2  0.943  0.992  0.992  0.526  0.998  0.988  0.989  Mg#  0.244  0.270  0.267  0.237  0.283  0.304  0.258  Name  Hed  Hed  Hed  aug  hed  hed  hed  180  Table A.1 Continued Label  091-2-2  091-2-5  092-1-1  092-1-2  092-1-3  092-1-5  092-1-6  Mineral  Px  Px  Px  Px  Px  Px  Px  Au (ppm)  0.5  0.5  46.2  46.2  46.2  46.2  46.2  SiO2  37.291  46.175  50.275  49.336  49.687  49.622  49.954  TiO2  0.126  0.169  0.015  0.000  0.019  0.000  0.000  Al2O3  11.295  5.215  0.030  0.245  0.425  0.205  0.145  FeO  15.623  28.513  20.856  24.337  22.037  22.919  21.227  MnO  0.585  0.402  0.667  0.265  0.477  0.519  0.525  MgO  0.039  4.347  4.930  2.702  4.098  3.882  4.571  CaO  32.953  11.375  23.185  22.893  22.555  23.297  23.016  Na2O  0.000  0.554  0.013  0.113  0.193  0.046  0.171  NiO  0.000  0.000  0.009  0.000  0.044  0.000  0.000  Cr2O3  0.006  0.010  0.010  0.000  0.022  0.020  0.000  Total  97.919  96.758  99.991  99.891  99.558  100.510  99.609  Si  1.497  1.916  2.000  1.994  1.993  1.978  1.996  Al (T)  0.503  0.084  0.000  0.006  0.007  0.010  0.004  Fe3+ (T)  0.000  0.000  0.000  0.000  0.000  0.012  0.000  Sum T  2.000  2.000  2.000  2.000  2.000  2.000  2.000  Al (M1)  0.032  0.171  0.001  0.005  0.013  0.000  0.002  Fe3+ (M1)  0.463  0.000  0.000  0.010  0.008  0.024  0.015  Ti  0.004  0.005  0.000  0.000  0.001  0.000  0.000  0.000  0.000  0.000  0.001  0.001  0.000  Cr Mg  0.002  0.269  0.292  0.163  0.245  0.231  0.272  Fe2+  0.061  0.990  0.694  0.813  0.731  0.728  0.694  Mn  0.020  0.014  0.022  0.009  0.016  0.018  0.018  0.000  0.000  0.000  0.001  0.000  0.000  Ni Sum M1  1.450  1.011  1.000  1.016  1.001  1.002  Ca  0.582  0.506  0.988  0.991  0.969  0.995  0.985  Na  0.045  0.001  0.009  0.015  0.004  0.013  Sum M2  0.000  0.550  0.989  1.000  0.984  0.999  0.998  Mg#  0.037  0.214  0.296  0.167  0.251  0.241  0.282  Name  alum ferrian alum aug subilicic essenite  hed  hed  hed  hed  hed  181  Table A.1 Continued Label  092-1-7  092-2-2  092-2-3  092-2-4  092-2-5  092-2-6  092-2-7  Mineral  Px  Px  Px  Px  Px  Px  Px  Au (ppm)  46.2  46.2  46.2  46.2  46.2  46.2  46.2  SiO2  49.490  49.850  49.229  49.809  49.156  49.640  50.972  TiO2  0.000  0.003  0.013  0.003  0.025  0.000  0.022  Al2O3  0.143  0.241  0.266  0.300  0.192  0.187  0.615  FeO  22.858  22.522  23.323  21.836  23.784  23.633  27.130  MnO  0.359  0.570  0.486  0.374  0.566  0.553  0.326  MgO  4.141  4.942  3.858  4.178  3.113  3.390  6.159  CaO  22.772  20.315  23.018  22.925  22.794  23.160  12.875  Na2O  0.225  0.126  0.160  0.091  0.129  0.078  0.066  NiO  0.000  0.000  0.010  0.009  0.005  0.017  0.000  Cr2O3  0.030  0.000  0.032  0.000  0.050  0.002  0.010  Total  100.017  98.569  100.395  99.525  99.813  100.658  98.175  Si  1.977  2.018  1.965  1.998  1.984  1.983  2.085  Al (T)  0.007  0.000  0.013  0.002  0.009  0.009  0.000  Fe3+ (T)  0.016  0.000  0.023  0.000  0.007  0.008  0.000  Sum T  2.000  2.018  2.000  2.000  2.000  2.000  2.085  Al (M1)  0.000  0.011  0.000  0.012  0.000  0.000  0.030  Fe3+ (M1)  0.039  0.000  0.046  0.000  0.023  0.023  0.000  Ti  0.000  0.000  0.000  0.000  0.001  0.000  0.001  Cr  0.001  0.000  0.001  0.000  0.002  0.000  0.000  Mg  0.247  0.298  0.230  0.250  0.187  0.202  0.376  Fe2+  0.709  0.762  0.710  0.733  0.772  0.759  0.928  Mn  0.012  0.020  0.016  0.013  0.019  0.019  0.011  Ni  0.000  0.000  0.000  0.000  0.000  0.001  0.000  Sum M1  1.008  1.092  1.003  1.008  1.004  1.003  1.346  Ca  0.975  0.881  0.984  0.985  0.985  0.991  0.564  Na  0.017  0.010  0.012  0.007  0.010  0.006  0.005  Sum M2  0.992  0.891  0.997  0.992  0.996  0.997  0.569  Mg#  0.258  0.281  0.244  0.254  0.195  0.210  0.288  Name  hed  aug  hed  hed  hed  hed  aug  182  Table A.1 Continued Label  092-2-8  092-3-1  092-3-2  092-3-3  092-3-4  093-1-3  093-2-2  Mineral  Px  Px  Px  Px  Px  Px  Px  Au (ppm)  46.2  46.2  46.2  46.2  46.2  51.1  51.1  SiO2  50.330  49.408  49.819  49.418  49.616  50.255  49.891  TiO2  0.000  0.013  0.050  0.000  0.005  0.000  0.003  Al2O3  0.375  0.198  1.470  0.357  0.308  0.466  0.870  FeO  23.613  23.062  28.498  22.706  22.756  22.265  18.370  MnO  0.530  0.634  0.332  0.398  0.482  0.387  0.244  MgO  4.959  3.755  5.628  3.917  3.887  6.081  6.811  CaO  19.253  23.292  11.860  23.195  22.984  19.402  23.239  Na2O  0.147  0.117  0.173  0.121  0.069  0.109  0.173  NiO  0.004  0.031  0.014  0.000  0.017  0.000  0.005  Cr2O3  0.000  0.024  0.052  0.014  0.004  0.006  0.000  Total  99.210  100.532  97.896  100.125  100.128  98.971  99.607  Si  2.028  1.970  2.050  1.975  1.985  2.014  1.959  Al (T)  0.000  0.009  0.000  0.017  0.015  0.000  0.040  Fe3+ (T)  0.000  0.020  0.000  0.008  0.001  0.000  0.001  Sum T  2.028  2.000  2.050  2.000  2.000  2.014  2.000  Al (M1)  0.018  0.000  0.071  0.000  0.000  0.022  0.000  Fe3+ (M1)  0.000  0.037  0.000  0.034  0.020  0.000  0.054  Ti  0.000  0.000  0.002  0.000  0.000  0.000  0.000  Cr  0.000  0.001  0.002  0.000  0.000  0.000  0.000  Mg  0.298  0.223  0.345  0.233  0.232  0.363  0.399  Fe2+  0.796  0.712  0.981  0.716  0.740  0.746  0.548  Mn  0.018  0.021  0.012  0.013  0.016  0.013  0.008  Ni  0.000  0.001  0.000  0.000  0.001  0.000  0.000  Sum M1  1.130  0.996  1.413  0.998  1.010  1.145  1.009  Ca  0.831  0.995  0.523  0.993  0.985  0.833  0.978  Na  0.011  0.009  0.014  0.009  0.005  0.008  0.013  Sum M2  0.843  1.004  0.537  1.002  0.990  0.842  0.991  Mg#  0.272  0.239  0.260  0.246  0.238  0.327  0.421  Name  aug  Wol  aug  hed  hed  aug  hed  183  Table A.1 Continued Label  093-2-7  093-3-1  094-1-2  094-1-8  094-2-3  094-2-6  094-3-6  Mineral  Px  Px  Px  Px  Px  Px  Px  Au (ppm)  51.1  51.1  51.1  51.1  51.1  51.1  51.1  SiO2  48.145  49.710  49.449  49.101  47.656  48.509  48.539  TiO2  0.020  0.007  0.059  0.023  0.017  0.000  0.026  Al2O3  0.348  0.280  0.357  0.295  0.565  0.256  1.780  FeO  23.579  21.792  22.415  23.790  26.317  26.505  22.540  MnO  0.360  0.324  0.603  0.477  0.410  0.352  0.247  MgO  3.431  4.759  4.451  3.655  2.242  2.135  3.796  CaO  22.610  23.119  22.125  22.415  21.294  21.871  21.391  Na2O  0.103  0.059  0.075  0.125  0.302  0.106  0.773  NiO  0.013  0.022  0.000  0.006  0.001  0.000  0.000  Cr2O3  0.004  0.032  0.032  0.000  0.000  0.000  0.014  Total  98.612  100.103  99.565  99.887  98.805  99.734  99.106  Si  1.962  1.977  1.984  1.974  1.956  1.978  1.945  Al (T)  0.017  0.013  0.016  0.014  0.027  0.012  0.055  Fe3+ (T)  0.021  0.010  0.000  0.012  0.016  0.010  0.000  Sum T  2.000  2.000  2.000  2.000  2.000  2.000  2.000  Al (M1)  0.000  0.000  0.001  0.000  0.000  0.000  0.029  Fe3+ (M1)  0.045  0.026  0.016  0.034  0.067  0.030  0.083  Ti  0.001  0.000  0.002  0.001  0.001  0.000  0.001  Cr  0.000  0.001  0.001  0.000  0.000  0.000  0.000  Mg  0.208  0.282  0.266  0.219  0.137  0.130  0.227  Fe2+  0.738  0.689  0.737  0.754  0.821  0.864  0.672  Mn  0.012  0.011  0.020  0.016  0.014  0.012  0.008  Ni  0.000  0.001  0.000  0.000  0.000  0.000  0.000  Sum M1  1.005  1.010  1.043  1.025  1.039  1.036  1.021  Ca  0.987  0.985  0.951  0.966  0.937  0.955  0.919  Na  0.008  0.005  0.006  0.010  0.024  0.008  0.060  Sum M2  0.995  0.990  0.957  0.975  0.961  0.964  0.979  Mg#  0.220  0.290  0.266  0.225  0.143  0.131  0.252  Name  hed  hed  hed  hed  hed  hed  hed  184  Table A.1 Continued Label  094-3-8  Mineral  Px  Au (ppm)  51.1  SiO2  49.152  TiO2  0.000  Al2O3  0.973  FeO  23.370  MnO  0.375  MgO  3.518  CaO  21.762  Na2O  0.718  NiO  0.000  Cr2O3  0.000  Total  99.867  Si  1.964  Al (T)  0.036  Fe3+ (T)  0.000  Sum T  2.000  Al (M1)  0.010  Fe3+ (M1)  0.082  Ti  0.000  Cr  0.000  Mg  0.210  Fe2+  0.699  Mn  0.013  Ni  0.000  Sum M1  1.013  Ca  0.932  Na  0.056  Sum M2  0.987  Mg#  0.231  Name  hed  185  Table A.2: Microprobe analysis of amphibole Label  090-1-5  091-1-1  091-1-3  091-1-4  091-2-1  091-2-3  091-2-4  Mineral  Amph  Amph  Amph  Amph  Amph  Amph  Amph  Au (ppm)  0.2  0.5  0.5  0.5  0.5  0.5  0.5  SiO2  41.356  44.394  44.239  46.513  43.502  45.991  44.692  TiO2  0.029  0.079  0.104  0.074  0.215  0.038  0.328  Al2O3  9.451  6.001  5.037  3.894  7.107  4.717  5.986  Cr2O3  0.000  0.012  0.000  0.035  0.067  0.019  0.017  FeO  28.936  29.316  28.536  28.475  29.424  28.781  30.302  MnO  0.276  0.276  0.340  0.357  0.445  0.370  0.448  MgO  2.818  4.444  4.867  4.823  3.514  4.377  3.189  CaO  10.789  11.349  11.317  11.514  11.427  11.485  11.420  Na2O  0.632  0.570  0.614  0.523  0.724  0.525  0.573  K2O  2.071  0.859  0.512  0.340  0.440  0.420  0.564  F  0.000  0.151  0.122  0.130  0.000  0.094  0.000  Cl  0.147  0.218  0.225  0.157  0.184  0.223  0.312  H2O  1.853  1.631  1.652  1.712  1.816  1.684  1.688  Sum  98.357  99.301  97.566  98.548  98.865  98.722  99.519  Ideal site assignments Si  6.658  6.983  7.059  7.344  6.885  7.262  7.061  Al (IV)  1.342  1.017  0.941  0.656  1.115  0.738  0.939  Sum T  8.000  8.000  8.000  8.000  8.000  8.000  8.000  Al (VI)  0.451  0.095  0.006  0.068  0.211  0.139  0.175  Ti  0.004  0.009  0.013  0.009  0.026  0.004  0.039  Fe 3+  0.409  0.649  0.685  0.393  0.599  0.403  0.466  Cr  0.000  0.001  0.000  0.004  0.008  0.002  0.002  Mg  0.676  1.042  1.158  1.135  0.829  1.030  0.751  Fe 2+  3.460  3.203  3.123  3.366  3.296  3.397  3.538  Mn  0.000  0.000  0.016  0.023  0.031  0.023  0.029  Sum C  5.000  5.000  5.000  5.000  5.000  5.000  5.000  Fe 2+  0.027  0.004  0.000  0.000  0.000  0.000  0.000  Mn  0.038  0.037  0.030  0.024  0.029  0.027  0.031  Ca  1.861  1.913  1.935  1.948  1.938  1.943  1.933  Na  0.074  0.047  0.035  0.028  0.033  0.030  0.036  Sum B  2.000  2.000  2.000  2.000  2.000  2.000  2.000  Na  0.123  0.127  0.155  0.132  0.189  0.130  0.140  K  0.425  0.172  0.104  0.069  0.089  0.085  0.114  Sum A  0.548  0.299  0.259  0.201  0.278  0.215  0.254  F  0.000  0.008  0.006  0.007  0.000  0.005  0.000  Cl  0.004  0.006  0.006  0.004  0.005  0.006  0.009  OH  1.996  1.986  1.987  1.989  1.995  1.989  1.991  Sum W  2.000  2.000  2.000  2.000  2.000  2.000  2.000  186  Table A.2 Continued Label  091-2-6  093-2-5  094-2-2  094-2-4  094-3-4  094-3-9  Mineral  Amph  Amph  Amph  Amph  Amph  Amph  Au (ppm)  0.5  51.1  51.1  51.1  51.1  51.1  SiO2  43.602  50.866  47.743  48.296  46.644  46.522  TiO2  0.176  0.008  0.009  0.000  0.000  0.000  Al2O3  6.861  1.343  4.659  4.121  5.101  5.056  Cr2O3  0.000  0.000  0.041  0.000  0.000  0.000  FeO  29.272  25.404  24.115  24.055  25.146  24.517  MnO  0.327  0.383  0.318  0.233  0.345  0.303  MgO  3.677  7.526  7.901  8.544  7.339  7.843  CaO  11.418  11.858  11.568  11.322  11.606  11.224  Na2O  0.791  0.175  0.775  0.595  0.834  1.017  K2O  0.566  0.080  0.279  0.205  0.369  0.324  F  0.000  0.067  0.008  0.074  0.044  0.192  Cl  0.208  0.014  0.170  0.085  0.178  0.115  H2O  1.792  1.919  1.822  1.841  1.777  1.693  Sum  98.691  99.643  99.408  99.371  99.384  98.804  Ideal site assignments Si  6.922  7.776  7.288  7.325  7.164  7.159  Al (IV)  1.078  0.224  0.712  0.675  0.836  0.841  Sum T  8.000  8.000  8.000  8.000  8.000  8.000  Al (VI)  0.205  0.018  0.126  0.062  0.087  0.076  Ti  0.021  0.001  0.001  0.000  0.000  0.000  Fe 3+  0.535  0.188  0.410  0.569  0.525  0.557  Cr  0.000  0.000  0.005  0.000  0.000  0.000  Mg  0.870  1.715  1.798  1.932  1.680  1.799  Fe 2+  3.352  3.060  2.660  2.437  2.705  2.568  Mn  0.017  0.018  0.000  0.000  0.003  0.000  Sum C  5.000  5.000  5.000  5.000  5.000  5.000  Fe 2+  0.000  0.000  0.009  0.045  0.000  0.030  Mn  0.027  0.032  0.041  0.030  0.042  0.039  Ca  1.942  1.942  1.892  1.840  1.910  1.851  Na  0.031  0.026  0.058  0.085  0.048  0.080  Sum B  2.000  2.000  2.000  2.000  2.000  2.000  Na  0.213  0.026  0.172  0.090  0.200  0.224  K  0.115  0.016  0.054  0.040  0.072  0.064  Sum A  0.327  0.042  0.226  0.129  0.272  0.287  F  0.000  0.004  0.000  0.004  0.002  0.010  Cl  0.006  0.000  0.005  0.002  0.005  0.003  OH  1.994  1.996  1.995  1.994  1.993  1.987  Sum W  2.000  2.000  2.000  2.000  2.000  2.000  187  Appendix B.	 Four Acid Digestion ICP-MS Geochemical Analysis Table B.1: Major element and mineralizing element geochemical analysis of Buckhorn skarn alteration and host rocks Table B.1 Hole ID  Sample ID  From  To  Lithology  188  Au ppm  Ag ppm  Bi ppm  Fe %  Ca %  P%  Mg %  Ti %  Al %  Na %  K%  S%  D07-369  BX02864  399  402  Hornfels BMV  1.3  2.516  516.42  7.51  10.27  0.103  3.01  0.334  7.01  1.154  1.56  0.76  D07-369  BX02865  402  404.4  Hornfels BMV  0.05  1.157  5.36  6.66  9.52  0.093  3.13  0.362  8.38  1.3  1.64  0.6  D07-369  BX02866  D07-369  BX02867  404.4  406.4  Skarn  Amp-Px  0.5  12.981  86.32  14.82  13.82  0.042  1.54  0.187  3.41  0.034  0.01  2.13  406.4  407.3  Skarn  Amp-Px  0.1  5.083  12.36  9.68  17.4  0.041  0.97  0.187  3.38  0.187  1.12  1.07  D07-369 D07-369  BX02868  407.3  408.4  Skarn  Gar  0.4  14.779  33.51  11.34  17.78  0.029  1.02  0.125  2.76  0.039  0.15  0.66  BX02869  408.4  411  Skarn  Gar  0.7  15.186  45.05  10.76  18.15  0.037  0.97  0.174  3.13  0.051  0.62  0.66  D07-369  BX02870  411  412.4  Skarn  Amp-Px  0.05  0.484  2.22  12.25  13.64  0.053  1.38  0.393  5.92  0.374  0.51  0.24  D07-369  BX02871  412.4  413.7  Skarn  Amp-Px  0.05  0.184  1.63  10.08  10.53  0.056  1.42  0.406  5.98  1.962  1.03  0.1  D07-369  BX02872  413.7  415.9  Skarn  Ep-Px  0.05  0.289  3.09  12.3  14.94  0.048  1.21  0.29  4.89  0.066  0.26  0.43  D07-369  BX02873  415.9  418.1  Skarn  Gar  0.05  1.013  1.12  9.81  9.3  0.059  1.62  0.336  5.8  1.014  2.7  0.36  D07-369  BX02874  418.1  420.2  Skarn  Ep-Px  0.05  0.585  1.18  9.01  9.97  0.057  1.59  0.373  6.09  0.607  2.77  0.34  D07-369  BX02875  420.2  421.8  Skarn  Amp-Px  0.3  66.454  203.92  13.15  12.99  0.049  1.48  0.308  3.48  0.04  0.16  1.7  D07-369  BX02876  421.8  422.9  Skarn  Amp-Px  2  5.952  63.92  8.61  16.86  0.039  1.28  0.265  3.57  0.042  0.89  0.82  D07-369  BX02877  422.9  424.6  Skarn  Amp-Px  9.1  6.017  175.95  10.02  17.73  0.027  0.91  0.187  3.56  0.055  1.43  1.68  D07-369  BX02878  424.6  426.5  Skarn  Gar  0.4  6.218  27.23  9.47  16.61  0.039  0.94  0.267  4.28  0.057  1.36  0.44  D07-369  BX02879  426.5  428.6  Skarn  Amp-Px  10.7  11.638  143.97  11.18  16.66  0.037  1.15  0.186  3.07  0.067  1  1.09  D07-369  BX02882  430.4  432.4  Skarn  Amp-Px  15.3  23.543  1049.84  12.38  16.76  0.038  1.49  0.118  2.02  0.075  0.9  0.93  D07-369  BX02883  432.4  434.4  Skarn  D07-369  BX02884  434.4  436.7  Skarn  D07-369  BX02885  436.7  438.8  Skarn  D07-369  BX02886  438.8  440.6  Marble BMS  D07-369  BX02887  440.6  443.2  Skarn  D07-369  BX02888  443.2  445.4  Marble BMS  D07-369  BX02889  445.4  447.9  Skarn  Sample  BS083 BS082  Skarn Class  Amp-Px  35  28.03  1938.04  13.67  16.22  0.03  1.46  0.162  2.39  0.069  0.19  0.96  Amp-Px  22.8  17.405  1633.41  14.42  16.28  0.028  1.52  0.105  1.35  0.059  0.06  1.23  Amp-Px  1.5  5.339  274.62  11.02  16.87  0.02  2.16  0.087  1.99  0.072  0.51  0.78  Amp-Px  3.6  5.987  855.62  7.59  19.88  0.016  1.73  0.06  1.14  0.08  0.43  0.66  Amp-Px  2.4  17.437  425.27  10.42  14.87  0.008  2.39  0.21  4.31  0.133  0.58  0.2  Amp-Px  0.05  0.989  12.92  4.05  19.96  0.012  4.65  0.108  2.2  0.045  0.81  0.28  Amp-Px  24.1  15.078  4000.1  8.97  19.43  0.012  2.17  0.072  1.12  0.046  0.09  0.65  Table B.1  189  Hole ID  Sample ID  From  To  Lithology  Sample  Skarn Class  Au ppm  Ag ppm  Bi ppm  Fe %  Ca %  P%  Mg %  Ti %  Al %  Na %  K%  S%  D07-369  BX02890  447.9  450.2  Skarn  BS084  Amp-Px  62  32.792  4000.1  9.71  27.69  0.008  2.1  0.13  1.15  0.03  0.01  1.03  D07-369  BX02892  452.7  456.1  Marble BMS  Amp-Px  2.2  1.337  1320.03  3.24  24.63  0.008  1.07  0.163  2.98  0.45  0.3  0.38  D07-369  BX02893  456.1  458.8  Marble BMS  Amp-Px  0.5  9.44  265.43  2.68  22.65  0.007  1.27  0.177  3.49  0.793  0.84  0.32  D07-369  BX02894  458.8  460.4  Skarn  Ep-Px  1.3  11.612  587.14  12.62  14.95  0.024  2.08  0.181  5.22  0.024  0.01  0.37  D07-369  BX02895  460.4  462.5  QP Dike  0.05  0.248  7.71  4.2  8.3  0.009  1.75  0.382  8.44  3.096  2.86  0.07  D07-369  BX02896  462.5  465  Hornfels BMS  0.05  0.175  2.24  3.39  2.3  0.007  2.02  0.312  9.74  1.869  3.75  0.2  D07-369  BX02897  465  468.6  Hornfels BMS  0.05  0.127  1.5  3.88  1.73  0.005  2.11  0.371  9.68  1.601  3.36  0.26  D08-410  BX04561  368.8  372.7  Hornfels  0.05  0.615  3.16  7.08  8.9  0.116  3.63  0.515  9.02  1.259  2.45  0.22  D08-410  BX04562  372.7  374.3  Skarn  Amp-Px  0.05  0.27  0.99  6.7  17.35  0.067  2.52  0.21  5.21  0.228  0.91  0.21  D08-410  BX04563  374.3  375.4  Skarn  Amp-Px  0.05  0.309  2.18  10.44  8.98  0.109  2.56  0.431  8.84  1.726  0.43  0.2  D08-410  BX04564  375.4  376.4  Skarn  Amp-Px  0.1  0.568  4.92  15.99  14.51  0.047  2.26  0.299  3.82  0.009  1.01  4.34  D08-410  BX04565  376.4  379  Skarn  Amp-Px  0.05  0.955  5.44  21.45  11.27  0.04  1.51  0.248  3.35  0.122  0.35  4.13  D08-410  BX04566  379  381.8  Skarn  Amp-Px  0.05  1.301  5.23  23.99  12.05  0.04  1.15  0.246  3.1  0.184  0.52  3.63  D08-410  BX04567  381.8  383.6  Skarn  Amp-Px  0.2  1.26  4.11  21.15  12.14  0.044  1.28  0.207  2.88  0.135  1.3  4.27  D08-410  BX04568  383.6  385.9  Skarn  Gar  0.05  1.239  3.54  20.25  13.61  0.037  1.28  0.216  3.11  0.045  0.13  3.41  D08-410  BX04569  385.9  387.3  Skarn  Gar  0.05  1.026  5.39  20.84  15.71  0.033  0.72  0.094  2.99  0.027  0.05  3.47  D08-410  BX04570  387.3  389.1  Skarn  Gar  0.5  0.911  5.6  18.88  15.17  0.032  0.98  0.086  3.16  0.059  0.09  2.8  D08-410  BX04571  389.1  389.9  Skarn  Gar  0.2  1.516  5.49  21.89  14.04  0.04  1.06  0.077  2.7  0.038  0.04  3.7  D08-410  BX04572  389.9  391  Skarn  Amp-Px  0.2  1.002  23.09  13.34  15.56  0.017  1.42  0.054  2.69  0.008  0.02  4.51  D08-410  BX04573  391  392.8  Skarn  Amp-Px  0.8  1.683  40.91  21.71  15.25  0.02  1.26  0.03  2.25  0.044  0.03  4.94  D08-410  BX04574  392.8  395.3  Skarn  Amp-Px  46.2  3.563  1743.94  21.44  14.71  0.024  1.65  0.024  1.06  0.073  0.02  3.62  D08-410  BX04575  395.3  397.9  Skarn  Amp-Px  8.5  1.805  401.14  19.72  15.5  0.016  1.6  0.029  1.03  0.067  0.01  3.41  D08-410  BX04576  397.9  399.8  Skarn  Amp-Px  21.9  1.734  1108.77  15.36  18.5  0.012  1.73  0.021  1.2  0.052  0.02  0.64  D08-410  BX04578  399.8  401.4  Skarn  Amp-Px  42.9  5.418  3062.05  24.53  12.32  0.021  2.13  0.019  0.73  0.063  0.03  3.6  D08-410  BX04579  401.4  403  Skarn  Mag  22.5  2.409  2353.82  40.55  10.47  0.01  1.57  0.015  0.38  0.064  0.05  2.2  D08-410  BX04580  403  405.6  Skarn  Amp-Px  18.5  3.881  2604.79  19.97  11.1  0.023  2.68  0.03  0.78  0.08  0.06  3.19  BS090  BS091  BS092  Table B.1 Hole ID  Sample ID  From  To  Lithology  Sample  190  Skarn Class  Au ppm  Ag ppm  Bi ppm  Fe %  Ca %  P%  Mg %  Ti %  Al %  Na %  K%  S%  D08-410  BX04581  405.6  407.5  Skarn  BS093 BS094  Amp-Px  51.1  15.705  4000.1  21.99  16.19  0.02  2.42  0.027  1.63  0.064  0.08  3.26  D08-410  BX04582  407.5  409  Skarn  Amp-Px  9.7  1.368  1086.84  15.53  10.51  0.083  3.37  0.268  4.55  0.09  0.39  1.79  D08-410  BX04583  409  411.5  Skarn  Amp-Px  4.8  1.036  1607.57  13.08  9.65  0.122  5.12  0.43  7  0.012  0.09  0.09  D08-410  BX04584  411.5  415  Marble BMS  0.5  0.165  138.61  2.37  25.07  0.007  4.78  0.026  0.75  0.014  0.09  0.02  D08-410  BX04585  415  416.3  Marble BMS  0.4  0.465  363.53  2.74  29.74  0.006  2.34  0.011  0.42  0.006  0.03  0.02  D08-410  BX04586  416.3  417.6  Hornfels BMS  0.1  0.116  34.37  0.74  2.88  0.001  0.14  0.003  0.16  0.007  0.01  0.02  D08-413  BX04635  278  280.8  Hornfels BMS  0.05  0.18  1.91  4.75  8.09  0.098  3.69  0.398  9.11  1.734  2.15  0.42  D08-413  BX04636  280.8  283.3  Hornfels  Ep-Px  0.05  0.134  1.5  5.42  9.49  0.077  3.47  0.303  7.35  1.386  1.34  0.19  D08-413  BX04638  283.3  284.6  Skarn  Gar  0.05  0.51  3.82  9.19  11.29  0.096  2.75  0.403  8.11  1.44  0.51  0.66  D08-413  BX04639  284.6  286.2  Skarn  Gar  0.05  0.714  3.38  17.02  17.57  0.04  1.01  0.319  4.18  0.086  0.15  2.26  D08-413  BX04640  286.2  287.6  Skarn  Gar  0.05  0.951  10.93  20.7  14.89  0.032  1.09  0.247  3.52  0.131  0.22  2.56  D08-413  BX04641  287.6  289.1  Skarn  Amp-Px  0.05  0.726  6.26  17.55  17.38  0.036  1.02  0.235  3.6  0.084  0.12  2.29  D08-413  BX04642  289.1  290.4  Skarn  Amp-Px  0.05  0.847  5.57  21.09  14.82  0.043  1.32  0.203  2.62  0.116  0.15  2.67  D08-413  BX04643  290.4  291.7  Skarn  BS085  Amp-Px  0.05  0.929  6.3  26.11  12.36  0.036  1.24  0.197  2.32  0.115  0.21  4.45  D08-413  BX04644  291.7  294.2  Skarn  BS016  Amp-Px  31.1  3.666  65.7  21.34  13.13  0.032  1.35  0.125  1.77  0.113  0.15  3.15  D08-413  BX04645  294.2  296.8  Skarn  BS086  Amp-Px  92.4  8.018  666.7  16.83  16.67  0.018  2.15  0.078  1.57  0.074  0.05  1.68  D08-413  BX04646  296.8  298.7  Skarn  Amp-Px  14.3  1.77  325.18  18.86  14.86  0.036  1.66  0.105  1.93  0.095  0.1  2.26  D08-413  BX04647  298.7  300.8  Skarn  Amp-Px  68.2  6.069  1207.42  18.91  16.07  0.033  2.05  0.051  0.6  0.084  0.03  2.67  D08-413  BX04648  300.8  303.3  Skarn  Amp-Px  117.5  15.651  4000.1  20.51  14.99  0.019  2.77  0.028  0.24  0.068  0.01  2.46  D08-413  BX04649  303.3  306  Skarn  Mag-Px  7.8  3.21  4000.1  40.18  9.65  0.009  1.38  0.01  0.15  0.032  0.01  1.93  D08-413  BX04650  306  308  Skarn  BS017  Mag  3.6  1.277  2448.35  52.05  6.91  0.011  0.74  0.013  0.24  0.045  0.03  2.02  D08-413  BX04651  309  311.5  Skarn  BS088  Amp-Px  3.6  0.85  567.62  16.25  18.23  0.009  3.27  0.014  0.23  0.057  0.03  0.15  D08-413  BX04652  311.5  313.6  Skarn  Amp-Px  1.2  0.131  66.49  9.75  11.18  0.004  6.85  0.012  0.28  0.057  0.03  0.05  D08-413  BX04653  313.6  316.1  Hornfels  Amp-Px  0.05  0.066  31.08  5.74  13.11  0.007  9.88  0.028  0.43  0.049  0.05  0.02  D08-413  BX04654  316.1  317.6  Hornfels  Amp-Px  0.05  0.076  4.62  2.75  17.28  0.005  7.74  0.057  0.97  0.024  0.05  0.02  D08-413  BX04656  317.6  319  Hornfels  Amp-Px  2.3  0.36  60.6  7.76  15.53  0.006  7.22  0.022  0.26  0.032  0.06  0.7  BS087  Table B.1  191  Hole ID  Sample ID  From  To  Lithology  Skarn Class  Au ppm  Ag ppm  Bi ppm  Fe %  Ca %  P%  Mg %  Ti %  Al %  Na %  K%  S%  D08-413  BX04657  319  321.6  Hornfels  Amp-Px  5.3  0.764  834.57  5.6  30.35  0.005  1.31  0.008  0.14  0.013  0.01  1.6  D08-443  BX11281  87  88.8  D08-443  BX11282  88.8  90.8  Hornfels  0.05  0.149  1.31  4.85  7.82  0.119  1.47  0.642  8.62  3.097  1.86  0.27  Hornfels  0.05  0.204  1.61  4.41  5.38  0.076  1.43  0.626  8.55  3.55  3.07  0.33  D08-443  BX11283  90.8  93.4  Skarn  Gar  43.6  4.933  3207.88  10.12  19.4  0.066  1.48  0.484  5.68  0.13  0.02  0.09  D08-443  BX11284  93.4  95.9  Skarn  D08-443  BX11285  95.9  98.1  Skarn  BS078  Gar  53.7  6.602  2425.66  10.49  22.63  0.06  1.03  0.379  5.48  0.021  0.01  0.13  BS079  Gar  24.8  4.873  1583.99  13.48  17.89  0.072  1.62  0.216  2.98  0.036  0.01  0.36  D08-443  BX11287  101  103.3  Skarn  BS077 BS089  Gar  0.1  2.98  25.46  12.82  18.84  0.051  0.86  0.171  3.68  0.03  0.07  0.4  D08-443  BX11288  103.3  104.9  Skarn  Gar  0.05  43.23  157.23  11.23  15.3  0.05  1.31  0.193  4.86  0.028  0.08  0.16  D08-443  BX11289  104.9  106  Skarn  Ep-Px  0.05  0.736  7.18  7.96  15.17  0.038  0.87  0.314  8.1  0.022  0.04  0.02  D08-443  BX11290  D08-443  BX11291  106  108.6  Skarn  Amp-Px  0.05  0.599  5.11  5.62  11.56  0.039  1.04  0.266  5.64  0.148  0.72  0.02  108.6  110.1  Skarn  Ep-Px  0.05  0.052  2.51  5.07  10.35  0.063  1.1  0.319  4.76  0.532  1.73  0.02  D09-526  BX19604  518  522  BMS  D09-536  BX21686  1164  1167.7  QP Dike  0.05  0.141  0.99  3.04  6.22  0.065  1.69  0.626  7.81  2.604  1.8  0.49  0.05  0.073  0.38  1.55  2.6  0.073  0.7  0.236  6.53  3.744  2.93  0.09  D09-536  BX21687  1167.7  1170.5  QP Dike  0.05  1.342  0.5  2.05  3.46  0.092  0.66  0.286  6.55  3.296  2.76  0.15  D09-536  BX21688  1170.5  1174.5  Skarn  D09-536  BX21689  1174.5  1177.5  Skarn  Ep-Px  0.05  13.507  5.33  9.75  13.38  0.093  1.05  0.281  8.6  0.1  0.69  1.13  Ep-Px  0.05  0.984  2.61  10.6  16.43  0.046  2.55  0.142  5.43  0.04  0.01  1.35  D09-536  BX21690  1177.5  1180  Skarn  Gar  0.05  0.911  0.56  10.22  19.1  0.003  3.22  0.08  3.35  0.032  0.02  0.45  D09-536  BX21691  1180  1183.5  Skarn  D09-536  BX21692  1183.5  1187.5  Skarn  Mag-px  0.05  0.566  6.28  32.56  10.08  0.005  3.87  0.035  0.41  0.029  0.01  0.73  Mag-Px  0.05  3.143  1.51  10.44  16.24  0.006  5.4  0.034  1.03  0.032  0.01  1.41  D09-536  BX21693  1187.5  1191.7  Skarn  D09-536  BX21694  1191.7  1195.3  Skarn  Gar  0.05  4.436  3.41  20.26  14.02  0.011  4.89  0.034  0.8  0.031  0.01  2.09  Gar  0.05  0.376  2.38  15.67  20.18  0.01  1.67  0.146  4.47  0.021  0.01  0.51  D09-536  BX21696  1195.3  1199.3  Skarn  Gar  0.7  0.344  3.83  10.27  22.66  0.026  1.16  0.181  5.24  0.016  0.01  0.02  D09-536  BX21697  1199.3  1203.3  D09-536  BX21698  1203.3  1207.3  Skarn  Gar  0.05  0.548  1.89  23.43  18.36  0.031  1.22  0.144  3.97  0.025  0.01  1.7  Skarn  Mag-Px  0.05  0.693  3.43  23.18  15.1  0.013  2.59  0.071  2.82  0.05  0.05  3.03  D09-536  BX21700  1207.3  1211.3  Skarn  Gar  0.05  0.819  1.63  18.42  16.14  0.023  2.99  0.09  2.58  0.042  0.03  1.97  D09-536  BX21701  D09-536  BX21702  1211.3  1215  Skarn  Gar  0.05  0.054  0.79  11.25  22.31  0.032  0.57  0.18  5.22  0.011  0.01  0.02  1215  1218.9  Skarn  Gar  0.05  1.083  1.06  13.13  23.51  0.015  0.36  0.114  4.3  0.006  0.01  0.02  D09-536  BX21703  1218.9  1222  Skarn  Gar  0.1  1.097  0.54  10.72  23.54  0.024  0.58  0.184  5.54  0.01  0.01  0.02  D09-536  BX21704  1222  1224.9  Skarn  Gar  0.05  1.475  1.49  8.85  18.76  0.022  2.17  0.147  5.05  0.127  0.47  0.06  Sample  BS039  BS081  BS080  Table B.1 Hole ID  Sample ID  From  To  Lithology  Skarn Class  Au ppm  Ag ppm  Bi ppm  Fe %  Ca %  P%  Mg %  Ti %  Al %  Na %  K%  S%  D09-536  BX21705  1224.9  1228.9  QP Dike  Amp-Px  0.05  0.073  0.19  1.83  3.69  0.093  0.93  0.31  6.85  4.077  2.93  0.02  D09-536  BX21706  1228.9  1232.7  Quartz Porphyry Dike  0.05  0.128  0.15  1.63  2.74  0.072  0.54  0.248  7.25  2.536  3.39  0.02  D10-569  BX24565  201  203.5  Skarn Veins  0.05  0.633  3.73  4.94  5.2  0.078  2.17  0.587  8.1  3.108  2.6  1.32  D10-569  BX24566  203.5  207  Skarn Veins  24.1  9.246  2247.42  4.65  9.35  0.062  1.76  0.545  7.3  2.101  1.3  0.14  D10-569  BX24567  207  211  Skarn Veins  0.05  0.91  19.9  6.34  12.31  0.08  1.84  0.812  7.66  1.095  0.29  0.08  D10-595  BX25014  123  125.4  Skarn Veins  0.05  0.051  1.95  3.11  1.24  0.006  1.72  0.457  7.82  2.201  2.77  0.17  D10-595  BX25015  125.4  128.8  Skarn Veins  4  1.027  1094.66  3.31  7.82  0.024  2.28  0.534  8.25  2.375  1.46  0.08  D10-595  BX25016  128.8  131.7  Skarn Veins  1.4  0.329  519.61  3.35  2.07  0.039  1.97  0.449  7.92  3.596  2.2  0.69  D10-595  BX25017  131.7  134.1  Skarn Veins  3.5  0.526  611.12  3.38  5.92  0.022  1.74  0.498  8.53  2.804  1.79  0.5  D10-595  BX25018  134.1  137.5  Skarn Veins  0.1  0.216  66.25  4.02  3.16  0.021  2.06  0.457  7.37  3.09  1.94  0.68  Sample  BS095 BS096  BS097  BS098  192  Table B.2: Trace element geochemical analysis of Buckhorn skarn alteration and host rocks Table B.2  193  Sample ID  Mn ppm  Cr ppm  Cu ppm  Pb ppm  Zn ppm  As ppm  Mo ppm  W ppm  V ppm  Co ppm  Ni ppm  Rb ppm  Sr ppm  Y ppm  Zr ppm  Nb ppm  Sn ppm  Cs ppm  BX02864  1572  99  449.3  407.15  116.4  11.3  4.07  7.8  214  25.1  24.4  58  348  15.4  25  5.06  1.8  3.2  BX02865  1174  88  300.91  91.96  201.8  7.6  3.05  4.5  226  29.3  22.8  60  390  12.1  20.4  3.04  3.7  7.9  BX02866  3789  51  1402.21  523.89  925.1  11.6  238.42  4.4  91  76.5  28.7  0.6  271  16.4  26.3  1.95  3.4  0.2  BX02867  1372  28  1226.66  156.96  180.3  25.4  3.19  200.1  53  73.4  27.5  39.4  335  12.9  24.9  1.78  5  3.3  BX02868  2129  36  2294.64  466.8  618.6  32.5  2.51  30.1  62  59  15.1  6.9  168  15.9  19.1  1.57  10.2  4  BX02869  2651  43  1550.76  545.46  286.3  21  13.32  24.6  67  41.7  8.9  18.9  122  18  27.6  2.28  10.1  5.5  BX02870  2700  51  30.9  33.21  140.5  22.3  9.43  2.6  126  35.4  9.5  15.3  352  14.5  39.3  4.56  3.2  4.7  BX02871  2560  44  14.27  12.72  108.8  5.2  9.61  2.4  133  15.3  8  26.7  228  15.3  41.7  4.77  1.9  1.9  BX02872  3611  137  14.76  14.01  104.4  11.1  15.49  5.6  110  21.6  12.6  11  249  18.5  36.6  3.95  4.2  9.4  BX02873  2606  46  450.65  12.6  144.2  6.8  2.38  2  126  28.6  15.2  71.6  217  16  32.6  4.23  1.7  3.5  BX02874  2324  41  182.73  9.44  125.1  11.6  5.31  2  133  31.7  19.8  75.2  228  16.7  32.3  4.72  2.4  6.1  BX02875  2935  33  5054.48  700  460.4  6.8  44.23  3.7  117  68  22.9  10  256  20  31.4  3.28  5.2  18.2  BX02876  1677  22  423.18  225.7  149.7  5.8  45.89  1.9  88  30.6  11.8  24.6  265  14.4  34.6  3.25  4.2  4.6  BX02877  1665  30  1194.16  62.71  116.6  2.5  24.18  10  71  42.2  14.2  34  155  17.5  32.3  2.22  8.8  4.2  BX02878  1771  31  1157.83  214.73  152.4  8.1  11.62  12.9  89  32  9.2  41.4  146  18.4  38.7  2.95  9.6  4.5  BX02879  1770  28  2313.42  200.12  274.7  6  45.41  10.8  85  47.6  15.4  24  178  18.9  24.7  1.97  6.6  4.2  BX02882  2031  46  2740.35  544.9  331.3  3.8  56.03  1.1  51  50.9  19.3  23.4  142  9.9  19.6  1.37  3  6  BX02883  2206  54  3075.11  579.57  406.9  4.8  25.11  1.5  56  60.4  19.3  6.4  182  9.6  24.7  2.11  3.8  5.9  BX02884  2000  43  2409.15  316.56  301.8  85.2  8.04  4.6  38  143.2  26.2  3.6  149  9.2  15.8  1.19  2.1  4.1  BX02885  1610  46  1342.34  77.22  194.2  10.2  5.61  0.8  51  45.8  9.6  23.3  195  9.8  13.1  0.94  1.9  7.1  BX02886  1797  32  684.5  99.35  110.6  514  1.34  143.9  31  59  8.7  25.5  256  11.6  13.4  0.69  1.1  5.8  BX02887  1475  70  383.98  685.67  182  15.4  3.53  3.5  79  42.1  9.1  26.2  310  14.9  38.6  2.71  5.7  1.8  BX02888  1225  17  139.37  31.46  66.9  50.4  4.72  1.3  37  16.1  4.2  55.7  303  10.6  17.6  1.34  0.6  8.7  BX02889  1448  39  1516.93  124.05  206.7  67.7  3.69  94.3  39  63.7  9.6  7  234  7.3  13.2  0.93  1.8  2.6  BX02890  1507  42  1759.12  295.02  255.2  65.4  4.68  200.1  55  114.8  17.1  4.9  236  9.6  11.1  0.02  1.8  1.1  BX02892  1061  28  161.64  49.88  23.1  162.3  6.99  32.5  58  149.8  8.5  16.4  519  11.6  25.6  2.01  1.1  2.2  BX02893  1195  29  55.85  357.23  52.8  23.9  3.72  4  69  7.7  13.6  28.5  389  13.2  31.5  2.11  0.7  2.1  BX02894  3229  43  23.71  127.22  202.3  8.3  6  1.7  74  39.8  18  1.6  300  45.1  28  2.2  3.4  0.1  BX02895  1012  43  3.06  10.39  71.5  2.9  1.3  4.8  104  15.8  14.5  88.9  381  29.1  49.4  3.93  1.2  1.9  Table B.2 Sample ID  Mn ppm  Cr ppm  Cu ppm  Pb ppm  Zn ppm  As ppm  Mo ppm  W ppm  V ppm  Co ppm  Ni ppm  Rb ppm  Sr ppm  Y ppm  Zr ppm  Nb ppm  Sn ppm  Cs ppm  BX02896  203  81  24.13  14.79  34.2  45  4.8  6.7  123  32.2  23.3  134.9  212  18.8  6.9  2.96  1.2  8.7  194  BX02897  134  54  50.48  7.7  30.1  28.9  1.07  5.8  117  18.1  20.8  152.1  168  12.8  11  3.7  1  10.2  BX04561  1227  156  112.73  20.91  78.7  17.3  1.9  6.4  294  40.8  23.9  106.3  518  15.4  25.3  2.97  2  12.1  BX04562  1787  69  136.44  2.9  58  56.3  0.64  9.3  153  26  15.3  44  435  12.7  18.7  1.8  3.7  8.2  BX04563  1799  148  180.58  5.05  93.9  45.2  0.82  6.4  198  40.7  18.4  38.1  373  12.1  29.2  2.65  0.8  13.4  BX04564  2442  103  613.47  4.67  92.9  5.7  0.34  1.3  157  94  31.6  85.3  348  15.9  21.9  2.4  4  25.6  BX04565  1981  134  713.01  3.8  69.4  9.2  4.48  10.8  123  125.1  36.6  19.4  159  16.4  30.9  2.23  13.7  13.7  BX04566  2357  185  1088.63  4.18  61.6  28  69.88  24.2  125  152.5  45.8  10.7  123  16.4  32.6  2.24  14.5  7.5  BX04567  2135  139  999.65  2.85  50.2  13.3  158.98  6.4  91  155.8  42.6  30.7  136  12.4  29  1.88  10.3  8.3  BX04568  2832  144  899.59  2.23  38.5  8.4  141.77  11.4  130  122.2  28.1  9.3  77  20.3  35.8  2.85  12.6  13.3  BX04569  2847  324  758.95  3.19  26.3  36.3  92.68  200.1  106  136.8  27.4  5  65  35.1  17.4  1.88  19.9  7.8  BX04570  2981  335  665.31  2.69  39  10.1  2.38  12  113  111.4  19.2  4  71  29.5  17.8  1.8  18.1  4.6  BX04571  2822  252  1122.39  2.08  40.8  9.6  2.44  6.3  99  162.5  28.7  5.3  127  22.9  19.4  1.42  15.7  11.9  BX04572  1906  110  442.58  2.68  23.3  9.6  4.69  5.9  76  62.5  7.8  4.2  412  22.8  13.2  1.36  11.1  6.8  BX04573  2813  134  1027.34  3.1  44.5  11.9  2.81  2.6  57  227.4  28.4  2.8  51  21.7  6.8  1.12  8.8  5.7  BX04574  2918  112  772.73  12.57  67.8  4.3  1.7  1.9  39  160.2  22.8  1.3  32  15.8  4.8  0.78  3.2  2.7  BX04575  2800  125  692.01  8.29  83.6  12.2  0.31  10.3  36  129.6  24.2  1.2  34  19.7  5.2  1  3.9  2.7  BX04576  3133  199  109.25  17.49  78.6  15.2  0.58  46.4  37  44  9.5  2.5  68  25.7  4.9  0.99  9.1  5.9  BX04578  2494  140  369.51  61.69  72.6  5.8  0.6  6.8  47  64.4  12.4  3.7  85  15  3.8  0.73  3.2  3.8  BX04579  1280  115  101.23  43.64  45.4  8.3  0.62  1.1  34  30.8  2.6  3  111  5.8  2  0.54  2.2  3.3  BX04580  1961  77  216.21  97.28  73.9  15.9  1.64  0.9  54  50  7.7  7.4  151  20  5.8  1.01  1.8  9.4  BX04581  2043  78  349.1  419.96  90.5  0.6  0.43  5.4  28  37.3  5.3  7.3  227  28.4  0.1  1.14  4.2  5.4  BX04582  1050  46  368.42  17.44  120  18.5  1.86  2.7  65  53.8  7.2  35.9  243  19.3  21.9  10.76  1.5  16.2  BX04583  1073  149  121.08  76.48  151.6  433.3  2.82  4.4  105  43  3.7  9  209  10.8  32  13.66  4.4  4.1  BX04584  618  61  9.1  11.24  34.6  23.8  0.61  0.3  12  9.8  3.4  6.7  241  7.3  8.2  0.56  0.3  1.6  BX04585  641  58  6.25  22.84  39.4  324.7  2.8  0.4  15  47  7.6  2.3  334  5.3  3.1  0.25  0.3  1  BX04586  156  514  4.53  4.67  5.1  8.3  0.32  0.2  5  2.4  4.6  0.5  34  2.2  0.3  0.2  0.1  0.2  BX04635  1188  211  75.55  4.16  62.9  8.3  0.71  2.1  239  18.2  28.2  94  378  15.4  14.4  2.88  0.6  7.1  BX04636  1479  176  98.66  3.23  78  8.9  1.99  2.2  245  25.3  32.1  63.7  337  11.7  11.1  2.18  0.6  5.7  Table B.2  195  Sample ID  Mn ppm  Cr ppm  Cu ppm  Pb ppm  Zn ppm  As ppm  Mo ppm  W ppm  V ppm  Co ppm  Ni ppm  Rb ppm  Sr ppm  Y ppm  Zr ppm  Nb ppm  Sn ppm  Cs ppm  BX04638  2361  271  364.98  7.55  71.4  17.2  0.97  1.5  241  60.9  27.8  21.9  459  16.5  43.5  3.42  2.1  2  BX04639  5221  225  540.51  4.48  63.1  7.9  72.48  13.5  120  60.8  11.2  3.6  167  19.3  42.1  2.76  7.5  4.4  BX04640  2947  292  662.62  5.12  74.9  24.9  103.35  46.2  117  80.6  14.2  5.1  146  23.6  30.3  2.66  14  5.9  BX04641  3409  187  490.77  3.59  58.3  11.3  170.65  21.5  125  63.8  11.9  3.7  167  19.8  27.2  2.33  9.7  6.1  BX04642  2749  210  689.16  3.07  67.9  15.8  70.62  21.2  96  84.6  13.1  1.8  130  16.7  30.4  2.11  10.8  1.7  BX04643  2511  155  799.6  3.73  69.5  6.2  13.43  12  79  149.4  21.6  1.5  83  14.9  24.9  2.08  9.5  0.5  BX04644  2532  152  1065.53  3.82  67.8  8.2  7.7  2.2  71  97.2  12.7  1.1  88  6.2  14.5  1.37  7.3  0.2  BX04645  2864  168  487.49  16.91  64.5  11.8  0.54  2.5  53  69.4  8.1  0.4  96  12.3  11.8  0.94  4.8  0.2  BX04646  2697  201  555.96  8.3  63.4  7.7  0.37  1.4  51  89.6  12.2  1.2  110  10.9  15.2  1.13  5.4  0.2  BX04647  2967  132  976.21  26.32  58  4.3  0.48  0.9  42  93.6  10.8  0.4  47  8  8.8  0.52  1.9  0.5  BX04648  2920  136  715.81  265.73  71.9  0.5  0.6  1.8  23  99.3  11.1  0.6  40  3.7  4.6  0.33  1.1  0.5  BX04649  1219  141  292.2  106.71  34  645.9  0.31  8.8  22  777.3  8.6  1  61  3.8  0.4  0.17  1  1.2  BX04650  768  120  260.1  16.88  25.4  807.5  0.24  1.9  20  1445.5  13.6  1.6  61  5.8  1.9  0.39  1.6  2.1  BX04651  1975  82  24.32  12.63  48.2  23.1  0.75  3.1  11  30.9  2.5  1.8  158  5.8  5  0.26  1.2  2.6  BX04652  1599  112  14.35  3.02  60.3  35.4  2.46  0.2  5  45.3  2.5  1.7  57  3.2  2.6  0.3  1.5  2  BX04653  1101  41  4.31  2.39  51.8  4.5  0.75  0.3  15  10.7  3.5  2.6  64  4.3  5.6  0.53  1.5  0.9  BX04654  794  104  8.36  7.33  45.4  2.7  2.22  1.1  15  5.5  5.7  3.1  153  5.5  10.3  1.11  0.7  1  BX04656  1041  54  173.78  23.52  59.5  6.5  0.72  0.5  8  23.1  7.8  3.7  165  3  6.2  0.21  0.5  1.4  BX04657  839  47  210.93  18.23  24.7  44.7  0.56  0.6  7  56.9  6.2  0.9  293  4.1  2.9  0.11  0.7  0.7  BX11281  866  297  86.1  9.49  62.4  41.9  2.35  1.6  196  31.9  16  45.9  380  30.5  38.9  3.89  1.1  0.6  BX11282  762  275  123.36  9.32  58.5  65.6  3.76  1.6  167  20.6  13.2  70.6  287  31.5  44.9  4.21  0.9  1.1  BX11283  4530  119  12.64  76.2  126.4  859.1  5.41  1.9  129  847.8  19.7  1.2  231  23  41.9  2.97  3.9  1.3  BX11284  4200  168  77.64  74.22  70.9  1669.2  2.15  2.3  114  1709.5  25.6  1.6  55  27.2  40.6  2.06  4.9  1.7  BX11285  4692  57  9.43  55.43  214.9  2408.9  13.9  4.4  59  3305.4  45.3  2.1  156  19.6  22.2  1.61  2.4  3.4  BX11287  4864  93  18.31  161.38  606.7  93.5  2.6  2.6  43  90.5  8.2  12.3  112  18.1  33.2  2.29  2.2  18.7  BX11288  5311  33  12.37  700  3178.6  71.1  20.86  1.2  47  39.3  15.9  9.2  387  15.5  21.9  2.33  1.5  17.4  BX11289  2738  108  2.7  60.42  144.7  13.2  2.91  1.2  82  13.9  13.7  1.8  644  28.2  31.2  7.09  1.6  2.3  BX11290  2736  153  1.19  39.63  137.5  9.3  3.55  1.9  75  11.6  20  14.4  328  28.3  32.2  4.63  1.2  0.6  BX11291  2544  221  1.08  5.95  92.1  17.1  0.94  1.5  92  9  25.5  32.5  268  24.4  41  3.26  1.1  0.5  Table B.2  196  Sample ID  Mn ppm  Cr ppm  Cu ppm  Pb ppm  Zn ppm  As ppm  Mo ppm  W ppm  V ppm  Co ppm  Ni ppm  Rb ppm  Sr ppm  Y ppm  Zr ppm  Nb ppm  Sn ppm  Cs ppm  BX19604  408  47  59.45  8.3  37.3  52.7  2.1  2.1  162  16.5  7.2  40  374  24.1  38.9  3.75  0.8  0.7  BX21686  202  33  23.07  7.9  14.8  2.7  1.34  1  53  4.8  3.1  89.1  439  10.3  25  13.64  1.4  3.1  BX21687  294  45  586.57  6.83  22.2  3.5  2.07  1.7  75  4.2  3.9  83.8  366  12.2  34.1  12.11  1.6  3.1  BX21688  866  41  6420.05  11.16  137.9  22.4  12.7  2  72  41.8  31.6  42.1  1046  16.4  29.2  11.57  8.2  2.5  BX21689  1684  43  474.57  5.98  60.1  22.8  1.26  5.4  46  15.3  6.9  0.9  640  8.1  20.7  5.71  5.4  0.3  BX21690  3186  43  431.34  2.62  59.9  8.1  1.16  2.5  46  7.3  3.6  0.6  46  8.4  20.4  2.52  2.8  0.3  BX21691  1371  21  205.77  5.19  70.4  5.1  1.28  2.5  34  28  10.2  0.8  20  1.3  2.5  0.31  0.8  0.6  BX21692  2209  20  1348.73  35.58  141.9  5  3.59  2.5  16  27.3  8.3  0.6  27  2.4  6.8  0.7  1  0.6  BX21693  1869  22  1915.28  10.93  163.2  4.4  17.67  0.9  29  31.4  10.1  0.7  26  1.5  5.9  0.5  0.9  0.8  BX21694  3776  54  138.95  5.89  30.5  5.2  2.25  9.5  74  14.9  4.5  0.6  82  14.1  31.7  4.15  4.4  0.5  BX21696  4265  75  12.74  5.79  23  6  4.88  4.6  67  5.4  2.3  0.6  60  19.8  43.6  5.78  4.6  0.8  BX21697  3532  56  311.32  5.51  32.4  9.7  2.94  9.6  60  11.7  6.2  1  48  15.4  33.5  4.8  4.5  0.9  BX21698  2697  33  494.81  4.84  41.9  6.2  0.87  0.8  47  18.7  7.9  1.1  34  7.7  15.8  2.08  1.8  0.6  BX21700  2684  34  485.02  3.53  47.1  5  1.58  1  46  17.6  6.7  0.8  28  8.7  19.7  2.29  1.4  0.6  BX21701  4100  61  20.25  6.62  10.2  5.9  2.92  10.5  74  3.1  3.8  0.1  51  24.8  43.1  5.13  8.1  0.1  BX21702  4031  82  589.46  3.1  49.9  7.7  3.04  31.4  53  6.8  4  0.05  32  23.8  37.1  4.25  13.3  0.1  BX21703  4701  58  484.13  14.25  36.4  5.6  2.02  3.3  64  6.4  5.8  0.1  33  30.3  60.8  5.59  6.1  0.2  BX21704  3492  51  557.95  18.25  65.2  4  6.19  7.8  59  9.8  6.7  19.2  218  15.7  30.4  5.11  3.7  0.6  BX21705  368  31  4.97  13.35  17.2  3.5  2.56  1.4  78  5.1  3.8  92.2  286  12.7  41.9  14.27  1.5  3.3  BX21706  212  41  12.39  12.12  15.6  3.1  1.41  2.6  53  3.4  2.9  138  262  10.2  24  14.78  1.3  3.5  BX24565  410  60  282.65  12.21  37.1  53  2.95  1.4  177  28.7  15.7  64.3  294  22.6  30.6  3.31  0.6  1.2  BX24566  906  46  84.18  195.05  70.7  683  33.81  1.5  158  575.8  13.7  30.6  350  17.4  44.2  2.98  1.6  0.4  BX24567  1566  95  58.73  43.68  60.1  170.6  49.42  1.6  228  57  28.6  7.2  412  22.2  60.2  5.01  2.6  0.2  BX25014  163  80  24.33  2.94  26.8  78.3  1.46  3.3  137  12.1  14.4  102.7  169  24.1  7.7  4.8  1.1  8.3  BX25015  619  147  17.37  6.22  29.5  122.8  2.96  2.2  174  16.6  54.4  52.9  537  76.7  57.9  3.79  1  4.2  BX25016  206  49  71.68  11.39  25.2  43.7  1.9  1.8  135  16.8  12.6  86.6  275  48.9  31.4  3.52  0.1  11  BX25017  380  74  75.94  7.98  21  19.8  31.94  3.3  123  12.7  17.9  49  442  22.4  44.1  4.09  0.7  1.8  BX25018  261  48  105.26  3.87  27.7  540.1  2.15  2.2  99  33.4  10.1  62.2  284  24.8  17.4  5.18  0.6  10.2  Table B.3: Trace element geochemical analysis of Buckhorn skarn alteration and host rocks (Part 2) Table B.3  Ba ppm  Th ppm  U ppm  Ga ppm  Hf ppm  Cd ppm  Sb ppm  Be ppm  Sc ppm  Li ppm  Ta ppm  BX02864  626  1.8  1.5  13.01  0.81  0.69  10.38  0.5  22  21  0.3  BX02865  1201  1.5  1.3  15.01  0.69  2.51  5.29  1  25  29.2  0.1  BX02866  11  0.6  3.4  12.48  0.79  16.6  4.48  0.5  7.4  9.8  0.1  BX02867  220  0.5  2.8  12.94  0.63  2.06  3.08  0.5  6.7  7  0.05  BX02868  148  0.4  2.7  12.01  0.61  9.06  3.1  0.5  4.8  5  0.05  BX02869  327  0.8  3.1  12.73  0.82  4.05  3.22  0.5  6.3  5  0.1  BX02870  450  1.7  2.3  16.01  1.38  0.19  6  1  14.4  4.8  0.2  BX02871  574  2  1.8  13.52  1.39  0.11  4.06  1  16.9  4.7  0.2  BX02872  171  1.7  2.3  15.99  1.21  0.13  5.15  0.5  11.6  5  0.2  BX02873  1619  2.1  1.2  14.14  1.11  1.04  5.04  0.5  13.7  6.2  0.2  BX02874  1525  2.3  1.3  15.46  1.11  0.5  5.39  0.5  14.9  7.3  0.2  BX02875  185  1.3  7.8  19.72  0.97  9.46  5.19  0.5  12.2  9.7  0.2  BX02876  584  1.2  3.6  15.18  1.07  1.25  4.28  0.5  9.7  11.1  0.2  BX02877  190  0.7  2.3  16.12  0.99  1.78  4.4  0.5  5.7  4.8  0.1  BX02878  663  0.9  2.4  19.43  1.05  2.31  2.22  0.5  8.5  4.5  0.1  BX02879  431  0.5  2.5  13.53  0.72  3.94  4.75  0.5  6.9  5.5  0.05  BX02882  503  0.4  1.8  8.67  0.53  5.65  3.98  0.5  4.2  7.5  0.05  BX02883  163  0.7  1.9  10.43  0.71  6.38  4.6  0.5  5.3  6.9  0.1  BX02884  85  0.3  1.6  6.19  0.44  4.56  5  0.5  4.2  7.4  0.05  BX02885  423  0.3  1.5  8.95  0.35  2.2  3.05  0.5  4  13.8  0.05  BX02886  218  0.3  1.9  5.35  0.38  1.11  4.1  0.5  2.9  10.1  0.05  BX02887  642  1.5  6.2  15.66  1.03  1.33  3.58  0.5  11  12.2  0.1  BX02888  106  0.7  3  5.64  0.54  0.33  1.35  0.5  5.8  19.2  0.05  BX02889  93  0.6  1.8  7.7  0.37  3.44  4.89  0.5  3.8  8.1  0.05  BX02890  0.5  1.5  2.1  6.46  0.4  4  5.25  0.5  4.7  6.2  0.05  BX02892  152  1.6  3.8  6.04  0.82  0.31  6.23  0.5  9  6.8  0.05  BX02893  503  1.5  4.2  6.77  0.83  0.39  1.42  0.5  10  6.4  0.1  BX02894  3  1.3  3.8  22.62  0.88  0.39  3.58  0.5  9.9  4.3  0.1  BX02895  5056  3  0.8  9.96  1.4  0.1  1.4  1  22.7  6.5  0.2  Sample ID  197  Table B.3 Sample ID BX02896  Ba ppm  Th ppm  U ppm  Ga ppm  Hf ppm  Cd ppm  Sb ppm  Be ppm  Sc ppm  Li ppm  Ta ppm  1476  2.4  0.2  17.43  0.25  0.05  1  0.5  25.9  38.5  0.2  198  BX02897  1133  2.2  0.3  18.08  0.29  0.03  0.71  1  24.6  43  0.2  BX04561  1293  1.4  0.9  16.5  0.71  0.24  3.9  0.5  37.1  34.7  0.1  BX04562  390  0.9  0.6  11.72  0.58  0.23  1.13  1  17.2  52.9  0.05  BX04563  70  1.3  1.9  16.95  0.82  0.14  2  0.5  24.8  56  0.1  BX04564  76  1.3  2  15.11  0.59  0.32  1.14  0.5  12.4  26.7  0.1  BX04565  109  0.7  2.7  17.57  0.88  0.15  2.7  0.5  9.6  14.3  0.05  BX04566  86  0.6  3.1  15.32  0.92  0.17  1.95  0.5  9.4  10.6  0.05  BX04567  37  0.7  2.5  13.17  0.85  0.13  2.5  0.5  9.1  11.3  0.05  BX04568  115  0.5  2.3  13.41  1.27  0.22  1.4  0.5  6.8  9.4  0.2  BX04569  53  0.3  4.3  14.23  0.46  0.22  1.05  0.5  2.5  5.5  0.05  BX04570  41  0.4  1.6  14.45  0.45  0.21  2.45  0.5  2.5  6.7  0.05  BX04571  45  0.2  1.1  13.07  0.41  0.25  1.33  0.5  2.8  12.7  0.05  BX04572  59  0.3  0.8  14.7  0.31  0.18  5.08  1  2  66.8  0.05  BX04573  42  0.1  0.6  12.13  0.15  0.33  1.49  0.5  0.3  7.8  0.05  BX04574  22  0.05  0.7  7.5  0.16  0.31  3.04  0.5  1.7  7.4  0.05  BX04575  18  0.05  0.8  7.5  0.11  0.48  2.62  0.5  0.8  6.6  0.05  BX04576  77  0.05  1.4  9.53  0.15  0.16  3.83  0.5  0.7  11.9  0.05  BX04578  50  0.1  1.1  11.57  0.07  0.37  5.88  0.5  1  10.5  0.05  BX04579  25  0.05  0.6  24.37  0.07  0.15  6.06  1  0.4  7.5  0.05  BX04580  105  0.05  1.1  10.67  0.16  0.31  13.57  0.5  1.4  17.4  0.05  BX04581  60  0.05  1.3  15.78  0.01  0.61  17.89  0.5  0.05  10.8  0.05  BX04582  96  8.9  42.3  29.24  0.84  0.42  46.32  0.5  8.9  18.6  0.7  BX04583  34  14.5  17.7  37.19  0.96  0.36  15.99  0.5  8.3  34.2  0.9  BX04584  38  0.3  1.2  1.95  0.22  0.4  2.73  0.5  0.9  11.4  0.05  BX04585  13  0.3  3  1.74  0.09  0.43  4.37  0.5  0.5  8.7  0.05  BX04586  5  0.2  0.2  0.62  0.01  0.09  1.53  0.5  0.5  5.7  0.05  BX04635  767  1.3  0.5  15.58  0.46  0.15  1.45  0.5  26.3  47.5  0.1  BX04636  639  1  0.5  12.47  0.43  0.16  1.67  0.5  23.9  49.7  0.05  Table B.3  Ba ppm  Th ppm  U ppm  Ga ppm  Hf ppm  Cd ppm  Sb ppm  Be ppm  Sc ppm  Li ppm  Ta ppm  BX04638  411  1.7  1.3  15.29  0.95  0.25  5.9  0.5  27.2  18.6  0.2  BX04639  38  0.8  2.1  12.6  1.32  0.25  3.93  0.5  12.8  4.6  0.1  BX04640  60  0.6  3.2  15.46  0.94  0.35  3.96  0.5  10.1  5.9  0.05  BX04641  41  0.6  2.5  15.29  0.86  0.24  5.74  0.5  10.3  4.8  0.1  BX04642  31  0.7  2.4  14.79  0.88  0.2  9.41  0.5  9.3  5.4  0.1  BX04643  30  0.5  2.8  13.03  0.89  0.19  4.74  0.5  7.8  4.5  0.05  BX04644  17  0.3  1.4  13.68  0.49  0.17  4.44  0.5  5.1  3.5  0.05  BX04645  7  0.3  1  9.72  0.36  0.21  8.9  0.5  2.5  3.3  0.05  BX04646  15  0.5  1.2  11.33  0.51  0.14  6.77  0.5  4.7  2.7  0.05  BX04647  5  0.2  0.9  6.02  0.27  0.28  6.32  0.5  2.2  3.8  0.05  BX04648  4  0.1  0.7  4.66  0.18  0.3  6.08  0.5  1.4  5.1  0.05  BX04649  12  0.1  0.2  21.21  0.01  0.21  4.03  0.5  0.4  5  0.05  BX04650  22  0.05  0.4  31.47  0.07  0.16  3.56  1  0.4  3.2  0.05  BX04651  17  0.1  0.4  7.75  0.09  0.25  3.67  1  0.5  5.6  0.05  BX04652  11  0.1  0.6  3.61  0.06  0.22  3.48  0.5  0.2  5.5  0.05  BX04653  4  0.2  2.4  3.4  0.17  0.25  2.53  1  0.6  2.9  0.05  BX04654  8  0.2  3.3  3.05  0.26  0.29  1.54  0.5  1  5.1  0.05  BX04656  20  0.1  1.1  3.76  0.06  0.45  3.98  0.5  0.3  2.7  0.05  BX04657  9  0.05  0.7  1.93  0.06  0.24  6.22  0.5  0.4  2.8  0.05  BX11281  985  1.3  0.7  15.93  1.39  0.16  2.69  0.5  26.7  4.8  0.2  BX11282  1389  1.6  1.1  15.64  1.55  0.11  1.79  0.5  27.4  5.1  0.2  BX11283  23  1.7  3.3  12.2  1.64  0.28  7.68  0.5  22.7  6.5  0.1  BX11284  15  0.8  1.8  11.88  1.36  0.35  3.02  0.5  16.7  6.8  0.1  BX11285  20  0.6  1.7  8.99  0.74  0.2  4.98  0.5  9.9  6.5  0.05  BX11287  42  0.9  1.5  9.16  0.99  7.63  4.74  0.5  7.4  5.7  0.1  BX11288  69  1.2  2.1  12.61  0.99  37.18  9.44  0.5  8.4  7.6  0.1  BX11289  33  1.8  1.5  19.53  1.14  1.31  7.91  0.5  15.8  11.1  0.4  BX11290  374  1.3  1.5  13.88  1.27  0.47  6.98  0.5  13  11.1  0.2  BX11291  811  1.6  1.9  9.47  1.42  0.12  8.3  0.5  14.3  4.8  0.1  Sample ID  199  Table B.3  Ba ppm  Th ppm  U ppm  Ga ppm  Hf ppm  Cd ppm  Sb ppm  Be ppm  Sc ppm  Li ppm  Ta ppm  BX19604  831  1.2  0.4  17.59  1.45  0.22  2.92  0.5  23.1  3.6  0.2  BX21686  841  5.7  2.2  18.82  1.08  0.04  1.26  2  5.7  10.9  0.8  BX21687  788  5.9  2.2  18.49  1.05  0.18  1.06  2  7.2  10.6  0.6  BX21688  205  8.4  13.9  39.76  1  3.38  39.67  0.5  8.9  8.5  0.6  BX21689  4  3.9  8.3  27.08  0.72  0.61  23.14  0.5  4.2  3.6  0.3  BX21690  3  0.2  1  10.52  0.45  0.56  1.95  0.5  1.5  2.1  0.05  BX21691  3  0.05  0.1  8.23  0.03  0.25  1.15  0.5  0.4  1.9  0.05  BX21692  2  0.1  0.2  4.66  0.14  2.12  1.11  1  0.9  2.8  0.05  BX21693  2  0.05  0.2  5.33  0.11  2.94  1.54  0.5  0.8  2.6  0.05  BX21694  3  0.3  2.1  13.13  0.68  0.24  2.4  0.5  2.4  1.9  0.2  BX21696  3  0.5  3.1  15.28  0.98  0.15  1.58  0.5  3.3  2.6  0.3  BX21697  3  0.4  2.3  14.38  0.84  0.18  2.15  0.5  3  2.2  0.2  BX21698  3  0.2  0.6  10.28  0.28  0.1  1.37  0.5  1.6  2.3  0.05  BX21700  2  0.2  0.5  9.09  0.4  0.3  1.4  0.5  1.5  2.1  0.05  BX21701  1  0.4  4.2  14.89  0.97  0.11  1.94  0.5  3.5  1.3  0.2  BX21702  2  0.3  4.4  13.95  0.79  0.99  1.33  0.5  2.2  1.6  0.2  BX21703  2  0.4  3.4  14.72  1.54  0.84  0.86  0.5  3.7  2.3  0.3  BX21704  200  1.4  2  11.79  0.75  0.91  2.32  0.5  3.1  3.9  0.2  BX21705  816  6.1  2  18.26  1.37  0.03  1.27  2  7.7  8.5  0.7  BX21706  972  5.6  2.2  19.44  1.05  0.07  0.43  2  5.7  12  0.8  BX24565  254  1.2  0.4  15.48  1.03  0.17  2.37  0.5  24.4  6  0.2  BX24566  617  1.4  2.5  15.29  1.44  0.55  5.98  0.5  22.2  4.5  0.1  BX24567  129  1.3  2.4  16.88  1.82  0.16  7.86  0.5  33.7  5.2  0.2  BX25014  699  2  0.5  18.03  0.22  0.02  0.69  1  22.6  50.6  0.2  BX25015  635  1.1  1.9  15.58  1.66  0.06  3.06  1  24.3  9.7  0.2  BX25016  604  1.2  2.3  16.83  0.84  0.05  7.45  1  19.2  38.9  0.2  BX25017  796  1.3  1.4  16.33  1.53  0.06  1.96  1  20.4  10.5  0.2  BX25018  445  1.3  0.6  16.96  0.5  0.06  1.37  1  17.1  29.7  0.3  Sample ID  200  Table B.4: Rare earth element geochemical analysis of Buckhorn skarn alteration and host rocks Table B.4 La ppm  Ce ppm  Pr ppm  Nd ppm  Sm ppm  Eu ppm  Gd ppm  BX02864  8  17.26  2.3  9.1  2.5  0.6  3.2  BX02865  11.6  21.58  2.8  10.4  2.4  0.6  BX02866  8.7  15.79  2.3  10.3  2.4  1.5  BX02867  2.3  4.97  0.8  3.5  1.2  BX02868  1.3  2.73  0.6  2.6  BX02869  6  10.75  1.5  6.4  BX02870  8.4  18.91  2.8  BX02871  7.8  17.3  2.5  BX02872  11.7  22.09  BX02873  6.3  BX02874  7.1  BX02875  Tb ppm  Er ppm  Tm ppm  Yb ppm  Total HREE  Dy ppm  Ho ppm  0.5  3.3  0.6  1.7  0.2  1.7  0.2  11.4  2.6  0.4  2.4  0.5  1.2  0.2  1.3  0.2  8.8  2.8  0.4  2.5  0.4  1.4  0.2  1.4  0.2  9.3  0.5  1.6  0.2  1.7  0.3  1  0.1  1.1  0.2  6.2  1.1  0.8  1.8  0.3  2.2  0.4  1.3  0.2  1.4  0.2  7.8  1.9  1  2.4  0.4  2.3  0.5  1.6  0.2  1.6  0.2  9.2  10.3  2.8  0.7  2.7  0.4  2.7  0.5  1.3  0.2  1.3  0.2  9.3  9.8  2.5  0.6  2.7  0.4  3.1  0.5  1.5  0.2  1.5  0.2  10.1  2.8  10.2  2.5  0.7  2.9  0.4  3.1  0.6  1.8  0.2  1.8  0.3  11.1  14.52  2.1  8.5  2.4  0.4  3  0.4  3  0.5  1.7  0.2  1.8  0.2  10.8  16.99  2.4  10  2.6  0.5  3.2  0.4  3.1  0.6  1.7  0.2  1.9  0.3  11.4  11.1  20.35  2.7  10.6  2.7  1.4  3  0.4  3.2  0.6  1.7  0.2  1.9  0.3  11.3  BX02876  8.8  15.11  2.2  8.4  2  0.7  1.9  0.3  2.4  0.5  1.3  0.2  1.3  0.2  8.1  BX02877  1.9  3.55  0.6  2.9  1.2  0.7  2  0.4  2.3  0.6  1.6  0.2  1.6  0.2  8.9  BX02878  3.6  7.52  1.2  5.5  1.8  0.8  2.4  0.4  2.8  0.5  1.6  0.2  1.7  0.3  9.9  BX02879  2.3  4.84  0.8  3.7  1.5  0.7  2.4  0.4  2.6  0.6  1.6  0.2  1.6  0.2  9.6  BX02882  0.8  1.7  0.3  1.3  0.6  0.2  0.7  0.1  1.1  0.2  0.8  0.05  0.9  0.1  3.95  BX02883  1  2.28  0.4  1.5  0.6  0.3  0.9  0.2  1.2  0.3  0.8  0.05  0.8  0.1  4.35  BX02884  1  2.46  0.4  1.6  0.6  0.3  0.9  0.1  1.1  0.2  0.7  0.1  0.8  0.1  4  BX02885  0.6  1.28  0.3  0.9  0.5  0.3  0.8  0.1  1.2  0.2  0.7  0.1  0.8  0.1  4  BX02886  2.2  2.28  0.3  1.9  0.5  0.5  1.2  0.2  1.7  0.3  0.9  0.1  0.9  0.1  5.4  BX02887  8.9  14.83  2  8.1  1.7  1  2.7  0.3  2  0.4  1.2  0.2  1.3  0.2  8.3  BX02888  4.4  7.17  1.5  4.4  0.9  0.4  1.1  0.2  1.4  0.3  1  0.1  1  0.1  5.2  BX02889  3.9  4.28  0.5  2.9  0.6  0.4  0.8  0.1  0.7  0.2  0.6  0.05  0.6  0.1  3.15  BX02890  0.3  4.13  0.2  1.1  0.6  0.1  1.8  0.1  0.9  0.2  0.8  0.05  0.5  0.1  4.45  BX02892  7.9  13.39  2.2  8.7  1.7  0.5  1.7  0.3  1.7  0.4  1.1  0.1  1  0.1  6.4  BX02893  9.7  17.62  2.7  11.1  2.1  0.6  2.3  0.4  2.4  0.5  1.3  0.2  1.2  0.2  8.5  BX02894  12.5  19.95  2.8  12.6  3  1.2  4.1  0.7  4.8  1.1  3  0.4  2.7  0.4  17.2  Sample ID  Lu ppm  201  Table B.4 La ppm  Ce ppm  Pr ppm  Nd ppm  Ho ppm  BX02895  14  25.54  3.8  17.5  3.8  0.3  7.1  BX02896  14.9  31.73  5  22.6  4.5  1  5.2  0.8  5.3  1  3.1  0.4  2.8  0.4  20.9  0.7  3.9  0.8  1.8  0.2  1.5  0.2  14.3  BX02897  10.5  21.03  3.1  14.2  3.2  0.9  4.2  0.5  3.1  0.5  1.3  0.2  1.1  0.2  11.1  BX04561  7.5  15.3  2  9.9  2.5  BX04562  5.3  9.66  1.4  5.7  1.2  0.8  3.5  0.4  3.2  0.6  1.5  0.2  1.5  0.2  11.1  0.4  2  0.3  2.3  0.5  1.4  0.2  1.2  0.2  8.1  BX04563  14.1  22.21  2.5  9.6  1.9  0.9  2.4  0.3  2.3  0.4  1.1  0.1  1  0.2  7.8  BX04564  23.8  32.63  3.5  BX04565  1.8  3.85  0.7  12.7  2.4  1.1  2.5  0.4  2.6  0.5  1.7  0.2  1.6  0.2  9.7  3.3  1.3  0.6  1.8  0.3  2.5  0.6  1.6  0.2  1.9  0.3  9.2  BX04566  1.5  3.66  0.6  3.6  1  0.7  1.7  0.3  2.5  0.6  1.7  0.2  1.8  0.3  9.1  BX04567  1.6  3.48  BX04568  1.7  2.72  0.6  3  1  0.4  1.4  0.2  2  0.5  1.3  0.2  1.1  0.2  6.9  0.5  1.9  0.9  0.7  2  0.4  3.1  0.7  2.2  0.3  2.1  0.4  11.2  BX04569  1.3  BX04570  1.4  1.97  0.5  2.6  1  1.3  2.4  0.5  3.8  0.9  3.4  0.4  3.2  0.5  15.1  1.64  0.2  1.6  1.1  0.9  2.1  0.4  3.2  0.9  2.7  0.4  2.6  0.4  12.7  BX04571  0.6  0.65  0.4  0.8  0.5  0.6  1.4  0.3  2.5  0.7  2.1  0.3  2.2  0.4  9.9  BX04572  1.9  1.88  0.3  1.1  0.6  0.5  1.6  0.3  2.6  0.6  2.3  0.3  2.3  0.3  10.3  BX04573  0.3  0.24  0.3  0.4  0.4  1.2  0.2  2.2  0.6  2.2  0.3  2.4  0.3  9.4  0.27  0.2  0.2  0.2  0.8  0.1  1.5  0.4  1.8  0.2  1.8  0.3  6.9  BX04574 BX04575  Sm ppm  Eu ppm  Gd ppm  Tb ppm  Er ppm  Tm ppm  Yb ppm  Lu ppm  Total HREE  Dy ppm  Sample ID  0.24  0.2  0.2  0.2  0.8  0.2  2  0.5  2.2  0.2  1.9  0.3  8.1  BX04576  0.2  0.37  0.5  0.3  0.5  1.1  0.2  2.1  0.6  1.8  0.2  1.3  0.2  7.5  BX04578  0.6  0.94  0.7  0.3  0.2  0.6  0.1  1.1  0.3  0.9  0.1  0.7  0.1  3.9  BX04579  0.4  0.78  0.4  0.1  0.3  0.4  0.1  0.3  0.05  0.3  1.45  BX04580  0.5  0.43  0.4  0.2  0.1  0.6  0.1  1.2  0.3  1  0.1  0.8  4.1  0.2  0.5  0.4  1.6  0.1  1.2  0.7  1.6  0.1  1.3  0.2  6.8  BX04582  43.3  67.37  7.9  32.6  5.4  3.1  4.4  0.5  3.2  0.5  1.3  0.1  1  0.1  11.1  BX04583  17  29.14  3.2  12.4  2.1  1.5  2.1  0.3  1.6  0.3  0.9  0.1  0.9  0.2  6.4  BX04584  2.3  2.8  0.5  1.8  0.4  0.5  0.3  0.1  0.3  0.05  0.3  1.55  BX04585  1.8  2.79  0.4  1.4  0.3  0.3  0.4  0.2  0.05  0.3  1.25  BX04586  1.1  2.19  0.3  1.3  0.2  0.2  0.2  0.1  0.05  BX04635  5.4  13.09  1.8  8.3  2.1  1.4  0.2  BX04581  0.2  1.73  202  0.3 0.9  2.7  0.4  2.4  0.5  0.55 1.1  0.2  8.9  Table B.4 La ppm  Ce ppm  Pr ppm  Ho ppm  BX04636  5.3  12.26  1.7  7.5  1.7  0.6  1.6  BX04638  27.1  42.13  4.3  14.9  3  1.2  2.8  0.3  1.7  0.4  1.2  0.1  1.1  0.1  6.5  0.4  2.8  0.5  1.5  0.2  1.4  0.2  9.8  BX04639  3.8  8.65  1.5  8.7  2.4  2.1  2.3  0.4  2.6  0.6  2.1  0.2  1.6  0.3  10.1  BX04640  2.5  5.15  0.9  4.9  1.3  BX04641  2.3  4.57  0.8  3.9  1.2  1  2.1  0.4  2.7  0.7  2.3  0.3  2.4  0.4  11.3  0.9  2  0.3  3.2  0.7  2.5  0.3  2.6  0.4  12  BX04642  1.2  2.79  0.5  2.8  1  0.6  1.6  0.2  2.1  0.5  1.9  0.2  1.8  0.2  8.5  BX04643  0.7  1.97  0.4  BX04644  0.4  1.33  0.2  2.2  0.9  0.5  1.8  0.3  2  0.5  1.8  0.2  1.9  0.2  8.7  1.1  0.4  0.2  0.6  0.1  1  0.2  0.7  0.05  0.7  0.1  3.45  BX04645  0.5  1.09  0.2  0.9  0.3  0.3  0.7  0.1  1.2  0.3  0.9  0.1  1  0.2  4.5  BX04646  0.8  BX04647  0.4  1.88  0.3  1.2  0.3  0.3  0.7  0.2  0.8  0.3  0.8  0.1  0.7  0.2  3.8  0.56  0.1  0.4  0.2  0.1  0.3  0.5  0.2  0.5  0.05  0.6  2.15  BX04648  0.5  BX04649  0.4  1.03  0.4  0.1  0.2  0.4  0.3  0.05  0.3  1.25  0.82  0.3  0.1  0.3  0.3  0.05  0.2  0.95  BX04650  0.2  0.44  0.2  0.2  0.4  0.4  0.05  0.2  1.35  BX04651  0.6  0.65  0.2  0.3  0.2  0.05  0.2  0.95  BX04652  0.2  0.28  0.1  0.05  0.1  0.45  BX04653  0.6  1.06  0.1  0.5  0.2  BX04654  1.2  2.27  0.4  1.8  0.4  BX04656  0.5  0.67  0.1  0.5  BX04657  0.5  0.69  0.1  0.7  0.1  0.1  0.3  BX11281  7.9  23.24  3.4  16.2  4.2  1.1  4.6  0.8  4.9  1  BX11282  7.7  22.59  3.5  16.9  4.3  0.8  5.1  0.9  5.2  BX11283  26.1  41.75  5  18.8  3.3  1.5  4.1  0.6  BX11284  6.1  10.86  1.8  9.2  2.8  1.6  3.7  0.6  BX11285  3.5  7.65  1.3  6.9  2.1  1.2  2.5  BX11287  4.5  7.96  1.2  6  1.6  0.7  BX11288  5  11.23  1.8  9  1.7  0.5  BX11289  5.3  13.64  2.2  11.8  2.9  BX11290  4.5  12.15  2.1  10.5  2.8  0.2  Nd ppm  Sm ppm  Eu ppm  0.1  0.7  0.1  0.2  Gd ppm  Tb ppm  0.1  0.2  Er ppm  Tm ppm  Yb ppm  Lu ppm  Total HREE  Dy ppm  Sample ID  203  0.3  0.2  0.3  0.05  0.2  1.05  0.5  0.4  0.2  0.05  0.3  1.45  0.2  0.2  0.1  0.05  0.1  0.65  0.1  0.05  0.2  2.6  0.4  2.4  0.4  17.1  1  3  0.4  2.9  0.4  18.9  3.7  0.7  2.3  0.3  2.1  0.3  14.1  3.6  0.8  2.2  0.3  2  0.3  13.5  0.4  2.6  0.5  1.4  0.2  1.3  0.2  9.1  2.1  0.4  2.3  0.5  1.5  0.2  1.3  0.2  8.5  2  0.3  2  0.4  1.2  0.2  1.3  0.2  7.6  0.4  4  0.6  4.2  0.9  2.5  0.3  2.2  0.4  15.1  0.3  3.4  0.7  4.1  0.8  2.4  0.4  2.8  0.4  15  0.1  0.2  0.85  Table B.4 La ppm  Ce ppm  Pr ppm  Nd ppm  BX11291  8.4  17.46  2.7  14.1  3  0.5  4  BX19604  7.8  22.14  3.1  16.7  3.9  1.2  4.6  BX21686  17  36.15  4.1  17.2  2.8  0.8  BX21687  18  37.65  4.3  17.4  3.3  BX21688  35.6  64.34  6.9  27.1  4.2  BX21689  18.7  32.25  3.4  13  BX21690  0.2  1.11  0.4  Sample ID  BX21691  0.33  Sm ppm  Eu ppm  Gd ppm  Tb ppm  Ho ppm  0.6  3.9  0.8  2.4  0.3  2.2  0.3  14.5  0.8  4.8  1  2.8  0.4  2.5  0.4  17.3  2.7  0.4  2.3  0.4  1.2  0.2  1.1  0.2  8.5  0.8  3  0.5  2.9  0.5  1.4  0.2  1.4  0.2  10.1  2.6  4  0.6  3.5  0.6  1.7  0.2  1.4  0.2  12.2  2.2  1.4  2  0.3  1.7  0.3  0.7  0.1  0.7  4.7  1.6  0.7  1.6  0.2  1.3  0.2  0.6  0.05  0.5  0.5  0.1  0.05  0.1  0.55  1.6  0.5  0.05  0.1  1.25  0.2  0.2  0.2  0.5  0.4  Er ppm  Yb ppm  Lu ppm  5.8 0.1  4.55  BX21692  0.5  0.62  0.2  BX21693  0.1  0.49  0.1  1  0.3  0.1  0.3  BX21694  0.1  1.09  0.5  5.6  2.5  1  2.8  0.4  2.3  0.4  1.1  0.1  0.9  0.1  8.1  BX21696  0.2  1.58  0.7  8  3  1.3  3.6  0.5  3.2  0.6  1.5  0.2  1.4  0.2  11.2  BX21697  0.3  1.22  0.5  5.5  2.7  1.1  2.8  0.4  2.2  0.4  1.1  0.1  1  0.1  8.1  0.55  0.2  2.8  1.2  0.5  1.6  0.2  1.2  0.2  0.5  0.05  0.5  BX21698  0.2  Tm ppm  Total HREE  Dy ppm  0.2  0.05  0.55  4.25  204  BX21700  0.1  0.52  0.3  2.6  1.1  0.4  1.3  0.2  1.2  0.2  0.6  0.05  0.6  BX21701  0.4  2.15  0.7  8.4  3.4  2.1  4.3  0.6  4.1  0.8  2.1  0.3  1.9  0.3  4.15 14.4  BX21702  0.5  2.69  0.8  8.4  3.2  2.6  4.3  0.6  3.5  0.7  1.9  0.3  1.9  0.3  13.5  BX21703  0.3  1.74  0.6  8.1  3.5  2.1  4.6  0.7  4.6  0.9  2.5  0.3  2.5  0.4  16.5  BX21704  2.3  6.11  1.1  7.6  2.6  1.1  2.6  0.4  2.4  0.4  1.3  0.2  1.2  0.2  8.7  BX21705  17.3  38.29  4.2  18.4  3.1  0.7  3  0.5  2.8  0.6  1.5  0.2  1.5  0.2  10.3  BX21706  15.4  34.27  4  17.5  3.2  0.7  3  0.4  2.5  0.4  1.3  0.2  1.2  0.2  9.2  BX24565  6.4  19.08  2.8  14.3  3.5  0.9  4.2  0.6  4.6  0.8  2.5  0.3  2.2  0.3  15.5  BX24566  6.2  15.67  2.3  11.5  2.6  0.7  3.3  0.6  3.7  0.7  2.2  0.3  1.9  0.2  12.9  BX24567  18.9  31.99  3.9  17.4  3.7  1.2  4  0.6  4.5  0.9  2.7  0.3  2.3  0.3  15.6  BX25014  13.2  28.44  3.6  17.2  3.5  0.8  4.2  0.6  4.9  1  2.6  0.3  1.9  0.2  15.7  BX25015  9.5  22.23  4.3  23.9  6.6  1.3  9.1  1.5  10.6  2.3  7.1  0.8  6  0.8  38.2  BX25016  13  30.42  5.4  29.5  6.6  1.6  9.5  1.4  9.4  1.9  5.6  0.6  4.6  0.7  33.7  BX25017  6.6  19.86  3.4  17.5  4  1.1  4.6  0.7  4.3  1  2.7  0.3  2.1  0.3  16  BX25018  8  22.16  3.3  16.6  4  1.2  4.9  0.8  6  1.2  3.6  0.4  3  0.3  20.2  

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