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Timing and tectonic setting of volcanogenic massive sulphide deposits in British Columbia: constraints.. Childe, Fiona Christina 1997

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TIMING AND TECTONIC SETTING OF VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS IN BRITISH COLUMBIA: CONSTRAINTS FROM U-Pb GEOCHRONOLOGY, RADIOGENIC ISOTOPES, AND GEOCHEMISTRY by FIONA CHRISTINA CHILDE B.Sc, McGill University, 1989 M.Sc., McGill University, 1992 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming tp the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1997 © Fiona Christina Childe, 1997 In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Earth and Ocean Sciences The University of British Columbia Vancouver, Canada April 28, 1997 ABSTRACT Volcanogenic massive sulphide (VMS) deposits occur within several of the allochthonous terranes of the North American Cordillera in British Columbia. In this study, the age of mineralization was determined for a number of these deposits. The radiogenic isotopic and geochemical signatures of host rocks and the mineralization were used to constrain the tectonic settings in which these deposits formed. The radiogenic isotopic signatures and geochemical affinities of three of these deposits in Stikinia, the Middle Jurassic Au-Ag-rich Eskay Creek deposit, the Late Triassic Cu-rich Granduc deposit, and the Late Mississippian polymetallic Tulsequah Chief deposit, assist in defining the evolution of this complex terrane. Rhyolite, which hosts and underlies mineralization at Eskay Creek has a primitive radiogenic isotopic and chemical character which is distinct from that of rhyolites of the same age within the region. Similarly, basalt which structurally underlies mineralization at Granduc is isotopically and chemically primitive relative to basalt which occurs within the same group regionally, and indicates formation within a back arc basin or immature island arc. The Tulsequah Chief deposit in northern Stikinia exhibits isotopic and geochemical features indicative of a more evolved island arc setting, which may reflect a variation in the basement to this terrane. The Cu-Zn-rich Kutcho Creek VMS deposit is hosted within felsic-dominated strata of the Kutcho Assemblage. Permo-Triassic to earliest Triassic ages are established for volcanic rocks which host mineralization. These ages, along with primitive radiogenic isotopic and geochemical signatures, indicate that these rocks and their contained mineralization formed in a primitive arc or fore-arc setting built on oceanic basement. Uranium-Pb dating and isotopic and geochemical studies of volcanic and intrusive rocks in fault-bounded slices further south in the Cordillera indicate that this Permo-Triassic magmatic event was widespread, and possibly define a new terrane. Age, isotopic and geochemical information was also obtained for igneous rocks within pendants and belts enclosed within the Coast Plutonic Complex which host VMS mineralization. New data for intrusive rocks from the Scotia-Quaal Belt and Anyox pendant are permissive of formation of these units within Stikinia. ii TABLE OF CONTENTS ABSTRACT " TABLE OF CONTENTS m LIST OF TABLESLIST OF FIGURES xi LIST OF PLA TES FOREWORD . xvii ACKNOWLEDGMENTS CHAPTER 1  1 INTRODUCTIONVMS deposits -3 Objectives 4 Methods —^ Presentation ^ References 8 CHAPTER 2 11 U-PB GEOCHRONOLOGY AND ND AND PB ISOTOPIC CHARACTERISTICS OF THE AU-AG-RICH ESKAY CREEK VMS DEPOSIT, BRITISH COLUMBIA 11 Abstract 1 2 Introduction -1 2 Regional Geology 3 Geology of the Eskay Creek area -15 Mineralization ^ U-Pb Geochronology . 20 Geochemistry  ^5 Nd Isotopic Data 8 iii Pblsotopic Data 30 Discussion 4 Conclusions  8 References 42 CHAPTER 3 SO U-PB GEOCHRONOLOGY, ND AND PB ISOTOPIC SYSTEMATICA AND GEOCHEMISTRY OF THE GRANDUC VMS DEPOSIT, NORTHWESTERN BRITISH COLUMBIA 50 Introduction 1 Regional Geology : 2 Geology of Granduc Mountain 54 Deformation 58 U-Pb GeochronologyLithogeochemistry 62 Nd Isotope Data _71 Pb Isotope Data 3 Discussion : 75 Conclusions 80 References  5 CHAPTER 4 92 U-PB GEOCHRONOLOGY, GEOCHEMISTRY, AND RADIOGENIC ISOTOPIC CHARACTERISTICS OF THE KUTCHO ASSEMBLAGE, HOST TO THE KUTCHO CREEK DEPOSIT, AND RELATED TECTONIC SLIVERS WITHIN THE CANADIAN CORDILLERA 92 PART 1: 93 Geological Setting, U-Pb Geochronology, and Radiogenic Isotopic Characteristics of the Permo-Triassic Kutcho Assemblage, North-Central British Columbia 93 Abstract 9Introduction 4 Geology of the King Salmon Allochthon 95 iv Kuteho Assemblage 95 Sinwa and Inklin Formations . 99 Kutcho Creek deposit 'Previous Correlations and Rb-Sr Geochronology 1°° U-Pb Geochronology of the Kutcho Assemblage, 101 Zircon geochemistry l^4 Lithogeochemistry 1 ^ Pb isotope data —* i3 Discussion ——* * ^ Conclusions -—* ^ References *2^ PART 2: ' 126 U-Pb Geochronology, Geochemistry and Nd Isotopic Systematics of the Sitlika Assemblage, Central British Columbia 126 Introduction . 12Previous work = ^6 Geology -U-Pb geochronology —131 Geochemistry . * 3 4 Nd Isotopic Systematics . —138 Discussion 13 9 References _140 PART 3: 143 Evidence for Early Triassic Felsic Magmatism in the Ashcroft (921) Map Area, British Columbia 14Introduction 3 Nicola Group *44 Kutcho Assemblage i4Geology : 145 U-Pb geochronology ^7 Geochemistry 149 Discussion and Tectonic Implications 152 VMS Potential 15References 4 PART 4: 158 Tectonic and metallogenic implications 15Abstract 15 8 Introduction 5 8 Geology of the Kutcho Assemblage (KA) 159 Geology of the Sitlika Assemblage (SA) 161 Geochemistry 162 Nd Isotope Data 4 Discussion 165 Conclusions _167 References 172 CHAPTER 5 176 U-PB GEOCHRONOLOGY AND RADIOGENIC ISOTOPIC SYSTEMATICS OF THE POLYMETALLIC TULSEQUAH CHIEF AND BIG BULL VMS DEPOSITS, NORTHWESTERN BRITISH COLUMBIA 176 Introduction 177 Regional GeologyGeology of the Tulsequah Chief and Big Bull deposits 180 U-Pb Geochronology 182 Lithogeochemistry 7 Nd Isotopic data 189 Pb Isotopic Data 190 Discussion 3 References 198 vi CHAPTER 6 202 U-PB GEOCHRONOLOGY AND RADIOGENIC ISOTOPIC SYSTEMATICS OF ROOF PENDANTS IN THE COAST PLUTONIC COMPLEX WHICH HOST VMS MINERALIZATION 202 Part I: 203 U-Pb Geochronology and Pb Isotopic Systematics of the Anyox pendant, West-Central British Columbia 20Introduction 3 Geology and mineral deposits of the Anyox pendant 206 Age assignments and terrane correlations for the Anyox pendant 207 U-Pb Geochronology 209 Lithogeochemistry 214 Pb Isotopic Data 6 Discussion 219 Conclusions 221 Part 2: 222 U-Pb Geochronology and Pb Isotopic Systematics of the Ecstall prospect, Scotia-Quaal Belt, West-Central British Columbia 222 Geology of the Scotia-Quaal BeltTerrane affiliation of the Ecstall deposit 223 U-Pb Geochronology 225 Pb Isotopic Data 6 Discussion and Conclusions 227 Part 3: 229 Tectonic and Metallogenic Implications of Paleozoic U-Pb dates from the Anyox Pendant and Scotia-Quaal Belt 22Introduction 9 Pb Isotopic Data 230 Discussion 1 References 232 vii CHAPTER 7 239 AGE AND RADIOGENIC ISOTOPIC CHARACTERISTICS OF VMS MINERALIZATION IN ALLOCHTHONOUS TERRANES OF THE NORTH AMERICAN CORDILLERA IN BRITISH COLUMBIA 239 Introduction 23Regional Tectonic Setting 240 Classification 242 Mafic volcanic & sediment-related deposits 24Felsic volcanic-related deposits 247 Discussion of Nd and Pb isotopic systematics 249 Discussion 252 ReferencesAPPENDIX 1 258 PROCEDURES FOR ISOTOPIC ANALYSES 258 U-Pb Procedures 25Pb Procedures 9 References 264 APPENDDX 2 265 ANALYTICAL PRECISION 265 Lithogeochemical DataPb Isotopic Data 268 References 270 APPENDDX 3 2 71 GEOLOGICAL FIELDWORK PUBLICATIONS 271 viii LIST OF TABLES Table 2. 1 U-Pb zircon analytical data for Eskay and east limb rhyolites. 21 Table 2. 2..Major, trace and rare earth element, and Nd isotopic data for samples from the Eskay anticline. 6 Table 2. 3 Common lead data for sulfides and sulfosalts from the Eskay anticline. 32 Table 3. 1 U-Pb analytical data for samples from Granduc Mountain. 60 Table 3. 2 Major and trace element data for samples from Granduc Mountain. 63 Table 3. 3 Rare earth element data for samples from Granduc Mountain. 67 Table 3. 4 Neodymium isotopic data. 72 Table 3. 5 Lead isotopic data for samples from the Granduc deposit. 73 Table 4 1 U-Pb zircon analytical data for samples from the Kutcho Assemblage. 102 Table 4 2 Major, trace, and rare earth element data for samples from the Kutcho Assemblage. 106 Table 4 3 Neodymium isotopic data for samples from the Kutcho Assemblage. • 113 Table 4 4 Lead isotopic data for samples from the Kutcho Assemblage. 114 Table 4 5 Major and trace element data for 14 additional samples of gabbro which intrude the Kutcho Assemblage. __H9 Table 4 6 U-Pb zircon analytical data for samples from the Sitlika assemblage. 132 Table 4 7 Major and trace element data for samples from the Sitlika assemblage. 135 Table 4 8 Rare earth element data for samples from the Sitlika assemblage. 137 Table 4 9 Nd isotopic data for samples from the Sitlika assemblage. 139 Table 4 10 U-Pb zircon analytical data for a sample from the Ashcroft map area. 148 Table 4 11 Major and trace element data for samples from the Ashcroft map area. 150 Table 4 12 Rare earth element data for samples from the Ashcroft map area. 150 Table 4 13 U-Pb zircon ages and initial 8Nd for the Kutcho Assemblage, Sitlika Assemblage, and felsic volcanic and intrusive rocks from the Venables Valley - Red Hill area. 163 Table 4 14 Ranges of selected major and trace element concentrations for rocks of the KA, SA and W-RH. 16Table 5. 1 U-Pb analytical data for samples from the tulsequah river and glacier area. 184 ix Table 5. 2 Major and trace element chemistry for rocks from the Tulsequah River and Glacier area. 188 Table 5. 3 Nd isotopic data for rocks from the Tulsequah Chief deposit. 189 Table 5. 4 Pb isotopic data for sulphides from the Tulsequah Chief and Big Bull deposits. 191 Table 6. 1 U-Pb analytical data for samples from the Anyox pendant and Scotia-Quaal Belt. 210 Table 6. 2 Major and trace element data for samples from the Anyox pendant. 215 Table 6. 3 Lead isotopic data for samples from the Anyox pendant, Scotia-Quaal Belt and Forrest Kerr pluton. 217 Table 7. 1 Primary commodities, terrane affiliation, host lithologies, age and eNd (initial) values for VMS deposits in allochthonous terranes of the North American Cordillera of British Columbia and southern Alaska. : 243 Table 8. 1 Analyses of in-house standards. 266 Table 8. 2 Reproducibility test for sample Kle._ 268 x LIST OF FIGURES Figure 1. 1 Past and presently producing VMS deposits and developed prospects in British Columbia and southeastern Alaska, North American Cordillera. Figure 2. 1 Location map of the Eskay Creek deposit, Iskut River area, northwestern British Columbia showing the major stratigraphic units, mining camps, deposits and occurrences. 14 Figure 2. 2 Geology of the Prout Plateau. 17 Figure 2. 3 Schematic stratigraphic sections for the east and west limbs of the Eskay anticline. 18 Figure 2. 4 Projection to surface of mineralized zones at the Eskay Creek deposit. 20 Figure 2. 5 U-Pb zircon concordia diagrams for Salmon River Formation rhyolites from the Eskay anticline. 2 Figure 2. 6 Trace element plots for Eskay and east limb rhyolites, a) Zr vs. TiC\ b) Zr vs. Y. 27 Figure 2. 7 Chondorite-normalized REE diagrams showing Eskay and east limb rhyolites and hangingwall basalt. 29 Figure 2. 8 Initial BNd values and associated errors for volcanic rocks from the Eskay anticline. 31 Figure 2. 9 207Pb/206Pb vs. 208Pb/206Pb diagram for sulfide and sulfosalt samples from the Eskay anticline. 40 Figure 3. 1 Location map of the Granduc. 51 Figure 3. 2 Generalized geology of Granduc Mountain. 55 Figure 3. 3 Plan view of the 3100 (950 m) level of the Main Zone of the Granduc mine. 57 Figure 3. 4 U-Pb concordia diagrams a) Footwall Series quartz diorite; b) Footwall Series basalt; c)Mine Series basaltic andesite; and d) felsic lapilli tuff on the southeast flank of Granduc Mountain. 61 Figure 3. 5 Plot of Y versus Zr for a) rocks from Granduc Mountain; and b) regional samples from the Stuhini Group. 69 Figure 3. 6 Plots of a) Ti02 versus Zr; b) AI2O3 versus Zr; and c) Cr203 versus Zr for mafic volcanic and volcaniclastic rocks and diorite to quartz diorite sills and dykes from Granduc Mountain. 70 xi Figure 3. 7 REE plot for basalt and quartz diorite from the Granduc Footwall Series. 71 Figure 3. 8 Plot of BN<J (initial) versus age for basalt and quartz diorite from the Granduc Footwall Series, plotted with Late Triassic rocks of the Stuhini Group and Stikine Plutonic Suite. 72 Figure 3. 9 Plot of 207Pb/206Pb versus 208Pb/206Pb for sulphides from B and F orebodies of the Granduc deposit, veins which cross cut the Footwall Series of the deposit, and the Rock and Roll prospect. 74 Figure 4. 1 Regional setting of the King Salmon. 9Figure 4. 2 Geology of the Kutcho Assemblage. 6 Figure 4. 3 Schematic stratigraphic section for the northern sequence of the Kutcho Assemblage. 97 Figure 4. 4 U-Pb concordia plots for volcanic and intusive rocks of the Kutcho Assemblage. _103 Figure 4. 5 Plot of Y versus Zr for volcanicand intrusive rocks of the Kutcho Assemblage. 109 Figure 4. 6 REE plots for volcanic and intrusive rocks of the Kutcho Assemblage compared with alkaline mafic rocks in Quesnellia. 110 Figure 4. 7 Plot of K20 versus P205 for rocks of the Kutcho Assemblage. 111 Figure 4. 8 eNd(initial) versus age plot of rhyolite from the Kutcho Assemblage, and trondhjemite, quartz-plagioclase porphyry, and gabbro intrusions. 113 Figure 4. 9 207Pb/206Pb versus 208Pb/206Pb plot of sulphides from the Kutcho Creek deposit. Shown for comparison are fields for the Tulsequah Chief, Granduc and Eskay Creek deposits. 114 Figure 4. 10 Location and Generalized geology of the Sitlika Assemblage. 126 Figure 4. 11 Generalized geology of the Kenny Creek - Mount Olsen area. 128 Figure 4. 12 U-Pb concordia diagrams for a) Diver Lake tonalite; and b) Mount Bodine rhyolite. 133 Figure 4. 13 a) Si02 vs. K20 diagram for unaltered rocks of the Sitlika Assemblage, and b) Zr vs. Y diagram for all samples from the Sitlika Assemblage. 136 Figure 4. 14 Chondorite-normalized rare earth element diagram for volcanic and intrusive rocks of the Sitlika Assemblage. 138 Figure 4. 15 Generalized geology of the Lillooet - Ashcroft area. 143 Figure 4. 16 Geology of the Spences Bridge - Cache Creek area. 146 Figure 4. 17 U-Pb concordia diagram for tonalite sample 96A-7. 149 xii Figure 4. 18 a) TiC^ versus Si02 diagram b) Zr versus Y diagram (Barrett and MacLean, 1994) for samples of volcanic and intrusive rocks from the Ashcroft area. 151 Figure 4. 19 Rare earth element diagram for one tonalite and two rhyolite crystal tuffs. 151 Figure 4. 20 Terrane map of the Canadian Cordillera, a) geology of the Kutcho Assemblage and adjacent rocks; b) geology of the Sitlika Assemblage and adjacent rocks; and c) geology of the Venables Valley - Red Hills area. 160 Figure 4. 21 Plot of Zr versus Y for volcanic rocks of the KA, SA, and W-RH, and associated felsic plutonic rocks. _162 Figure 4. 22 Chondorite-normalized REE diagrams for a) rhyolite of the KA, and quartz-plagioclase porphyry and trondhjemite which intrude the KA; b) rhyolite of the SA, and tonalite which intrudes the S A; c) rhyolite tuffs of the W-RH, and tonalite which intrudes the rhyolite tuffs; and d) Permian to Triassic felsic volcanic and plutonic rocks of the Blue Mountains terrane, northwest United States, and felsic volcanic rocks of the Jean Charcot Trough, Vanuatu arc, Southwest Pacific. 164 Figure 5. 1 Simplified tectonostratigraphic map showing the distribution of the Paleozoic Stikine Assemblage. 179 Figure 5. 2 Generalized geology of the Tulsequah Glacier area. 181 Figure 5. 3 U-Pb concordia diagrams for volcanic and intrusive rocks from the Tulsequah River area. 18 5 Figure 5. 4 Y versus Zr diagram for U-Pb samples from the Tulsequah River area. 187 Figure 5. 5 8Nd(initial) versus age for volcanic rocks from the Stikine Assemblage. 190 Figure 5. 6 Plots of a) 207Pb/206Pb versus 208Pb/206Pb, and b) 207Pb/204Pb versus 206Pb/204Pb for sulphides from the Tulsequah Chief and Big Bull deposits. 192 Figure 6. 1 Generalized geology of the Canadian Cordillera between 53° to 58°N, and 128° to 132°W. 204 Figure 6. 2 Generalized geology of Anyox pendant, west-central British Columbia. 205 Figure 6. 3 U-Pb concordia diagrams for a) detrital zircon from turbiditic sedimentary rocks on Granby Peninsula; b) an unfoliated mafic dyke which cross cuts mafic volcanic rocks which host the Hidden Creek VMS deposit; and c) a quartz diorite sill from Mount Clashmore. 212 Figure 6. 4 Plot of Y versus Zr for rocks from the Anyox pendant. 214 Figure 6. 5 Plot of 207Pb/206Pb versus 208Pb/206Pb for galena from the Anyox pendant. 218 xiii Figure 6. 6 Simplified geological map of the Scotia-Quaal belt. 224 Figure 6. 7 U-Pb concordia diagram for a quartz diorite sill from the Ecstall prospect, Scotia-Quaal belt. 225 Figure 6. 8 Plot of 207Pb/206Pb versus 208Pb/206Pb for sulphides from the Ecstall prospect, Scotia-Quaal belt. 6 Figure 7. 1 Volcanogenic massive sulphide deposits and developed prospects in British Columbia and southeastern Alaska. 241 Figure 7. 2 a) 208Pb/204Pb versus 206Pb/204Pb, and b) 207Pb/204Pb versus 206Pb/204Pb diagrams showing fields for Tertiary and Jurassic mineralization in the Stewart Mining Camp, the Eskay Creek, Granduc, Anyox, Windy Craggy, Tulsequah Chief, and Big Bull VMS deposits, the Forrest Kerr pluton, and Devono-Mississippian VMS deposits in the Yukon Tanana terrane. 245 Figure 7. 3 a) eNd(initial) versus age, b) 206Pb/204Pb versus age, and c) eNd(initial) versus 206Pb/204Pb for mineralization and host rocks of the Kutcho Creek, Eskay Creek, Tulsequah Chief, Granduc, Anyox, and Windy Craggy deposits. 246 Figure 8. 1 Pb-Pb diagrams for reproducibility test of sample Kle, showing 2o (95% confidence) error ellipses. 270 xiv LIST OF PLATES Plate 2. 1 Gossanous felsite bluff, south of the Eskay Creek deposit. 11 Plate 2. 2 Black matrix breccia, showing a brecciated, flow-banded rhyolite fragment in a black silicified matrix. 40 Plate 2. 3 Pillow basalts in the hangingwall to mineralization at Eskay Creek. 40 Plate 2. 4 Flow-banded east limb rhyolite with aphanitic matrix and quartz and feldspar phenocrysts.Plate 2. 5 2IB zone argillite-hosted clastic sulfide and sulfosalt mineralization. 40 Plate 2. 6 SEM backscatter image of feldspar inclusions in zircon, Eskay rhyolite 40 Plate 3. 1 Snow capped peak and north face of Granduc Mountain. 5Plate 3. 2 Snow capped north face of Granduc Mountain. 81 Plate 3. 3 Granduc Glory Hole. 8Plate 3. 4 massive basalt from the Granduc Footwall Series with weak pyrite veining. 81 Plate 3. 5 Moderately to strongly sheared sill cross cutting the Granduc Footwall Series. 81 Plate 3. 6 Strongly deformed limestone, Granduc Mine Series. 8Plate 3. 7 Prismatic zircon, sample GD-GC-01, quartz diorite sill. _83 Plate 3. 8 Equant zircon, sample GD-GC-04, Footwall Series basalt. 8Plate 3. 9 Ovoid zircon, sample GD-GC-08, Mine Series basaltic andesite. 83 Plate 3.10 Felsic volcanic fragmental rock, southeast flank of Granduc Mountain. 83 Plate 3.11 Semi-massive sulphides, Granduc B orebody. 8Plate 3. 12 Microcline-calcite-galena-sphalerite veins, Granduc. 83 Plate 4. 1 East face of Sumac Ridge, Kutcho Assemblage. 92 Plate 4. 2 Quartz-plagioclase granophyric intergrowth in rhyolite, northern sequence. 168 Plate 4. 3 Quartz grain with undulatory extinction, northern sequence. 168 Plate 4. 4 Sericite-pyrite alteration in quartz-plagioclase porphyritic rhyolite, Kutcho Creek. 168 Plate 4. 5 Basaltic and rhyoliticfragments in rhyolite mass flow, Kutcho Creek. 168 Plate 4. 6 Plagioclase-augite porphyritic gabbro. : 168 Plate 4. 7 Rhyolite mass flow from the Kutcho Assemblage. 16Plate 4. 8 Quartz-plagioclase glomerocryst. 170 xv Plate 4. 9 Massive sulphide from the Kutcho lens, Kutcho Adit. 170 Plate 4. 10 Photomicrograph of quartz-plagioclase glomerocryst in a quartzo-feldspathic groundmass, Mount Bodine rhyolite. 17Plate 4. 11 Photomicrograph of intergrown quartz and plagioclase grains, tonalite. 170 Plate 4. 12 Photomicrograph of rhyolite crystal tuff showing a broken quartz grain. 170 Plate 4. 13 Photomicrograph of tonalite showing equigranular texture. 170 Plate 5. 1 Looking north to the Tulsequah River and Glacier, from Mount Eaton. 176 Plate 5. 2 Felsic fragmental unit, Tulsequah Chief mine series. 19Plate 5. 3 Silicified felsic fragmental unit, Tulsequah Chief mine series. 196 Plate 5. 4 Baritic pyrite-rich sulphides, Tulsequah Chief Mine Series, I Zone. 196 Plate 5. 5 Deformed dacitic tuffaceous rock, Big Bull deposit. 19Plate 5. 6 Flattened felsic fragmental unit, sample TC-GC-05, Mount Stapler. 196 Plate 5. 7 Deformed andesite tuffaceous rock, sample TC-GC-11, Mount Sittikanay. 196 Plate 5. 8 Deformed heterolithic fragmental unit, sample TC-GC-12, Mount Sittikanay. 196 Plate 6. 1 Abandoned power plant at Anyox. 202 Plate 6. 2 Turbiditic sedimentary rocks on Granby Peninsula, fining upwards from fine sandstone bases to inter-turbidite mudstone tops. 237 Plate 6. 3 Pale green to white quartz diorite sill cross-cut by dark green gabbro dykes, Mount Clashmore. 23Plate 6. 4 Pale green to white weathering, finely laminated, siliceous sedimentary rocks of possible partial exhalitive origin, directly above #1 pit, Hidden Creek Mine. 237 Plate 6. 5 Turbiditic sedimentary rocks on Granby Peninsula cross-cut by base ±precious metal-rich quartz veins. 23xvi FOREWORD This thesis presents geological research with academic and economic applications, produced within a university, in collaboration with both industry and government. This study has benefited from the expertise of many scientists. To recognize the specific input of individuals who contributed to the publications that were produced during the course of this dissertation, and in accordance with guidelines of The University of British Columbia and the doctoral committee, this thesis is presented as a series of research papers. Five papers, which constitute chapters two and four have been submitted for publication in refereed professional and government journals, the names of co-authors, and their specific contributions to these manuscripts are outlined below. Chapters three, five, six and seven represent future contributions to referred publications. Finally, four British Columbia Geological Fieldwork papers which were produced during the course of this thesis are presented in Appendix 3. The paper which comprises chapter two has been published in Economic Geology as a sole authored contribution. Critical reviews by Drs. J.F.H. Thompson and J.K. Mortensen, thesis directors, and Directors of the Mineral Deposit Research Unit (MDRU), and Geochronology Laboratory in the Department of Earth and Ocean Sciences, respectively, Dr. T.J. Barrett, MDRU Project Coordinator, and Drs. T. Barrie and F. Corfu, journal reviewers, greatly improved the manuscript. Chapter four is composed of four individual papers. The first three parts detail the geology, age, radiogenic isotopic systematics and geochemistry of three fault bounded regions within the Cordillera; the final part represents a tectonic synthesis for these regions, based on the data presented in the first three sections. The first part is co-authored by Dr. J.F.H. Thompson, and reflects his contribution to the interpretation of data, assistance in the field and editorial supervision. Part two is co-authored by Mr. P. Schiarizza of the British Columbia Geological Survey, who is currently conducting a mapping project of the region discussed in this section, and contributed his knowledge of the local geology. Part three is co-authored by Dr. J.F.H. Thompson, as well as Drs. R.M. Friedman and J.K. Mortensen, both of the Geochronology Laboratory and MDRU, in the Department of Earth and Ocean Sciences, and reflect their xvii contributions in the field and laboratory, as well as with editorial assistance. Part four, the tectonic synthesis of the chapter is co-authored by all of those people who contributed to parts one through three, as well as K. Bellefontaine of the British Columbia Geological Survey, and J.M. Marr of Agate Bay Resources Ltd. The latter two people contributed ideas which assisted in the initiation of this aspect of my research. I am grateful to all of my co-authors for their assistance. All of the research and ideas not specifically mentioned above was performed by Fiona Childe, in accordance with the guidelines of The University of British Columbia. The papers which comprise chapters two and four are as follows: CHAPTER 2 Childe, F.C. 1996. U-Pb Geochronology and Nd and Pb Isotopic Characteristics of the Au-Ag-Rich Eskay Creek VMS Deposit. Economic Geology, v. 91, pp. 1209-1224. CHAPTER 4 Childe, F.C, and Thompson, J.F.H. accepted. Geological Setting, U-Pb Geochronology, and Radiogenic Isotopic Systematics of the Permo-Triassic Kutcho Assemblage, North-Central British Columbia. Canadian Journal of Earth Sciences. Childe, F.C, Schiarizza, P. 1997. U-Pb Geochronology, Geochemistry and Nd Isotopic Systematics of the Sitlika Assemblage, Central British Columbia. In Geological Fieldwork 1996, Edited by D.V. Lefebure, W.J. McMillan, and J.G. McArthur, British Columbia Ministry of Employment and Investment, Paper 1997-1, p. 69-78. Childe, F.C, Friedman, R.M., Mortensen, J.K., and Thompson, J.F.H. 1997. Evidence for Early Triassic Felsic Magmatism in the Ashcroft Map Area, British Columbia. In Geological Fieldwork 1996, Edited by D.V. Lefebure, W.J. McMillan, and J.G. xviii McArthur, British Columbia Ministry of Employment and Investment, Paper 1997-1, p. 117-124. Childe, F.C, Thompson, J.F.H., Mortensen, J.K., Friedman, R.M., Schiarizza, P., Bellefontaine, K., and Marr, J.M. submitted. Primitive Permo-Triassic Volcanism in the Canadian Cordillera: Tectonic and Metallogenic Implications. Economic Geology. APPENDIX 3 Childe, F., and Mihalynuk, M.G. 1995. U-Pb geochronology of the Mount Stapler Quartz Monzonite: Evidence for Early Jurassic magmatism in the Tulsequah Glacier area, Northwest British Columbia (104K/13). In Geological Fieldwork 1994, Edited by B. Grant and G.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1995-1, pp. 521-527. Sherlock, R.L., Childe, F., Barrett, T.J., Mortensen, J.K., Lewis, P.D., Chandler, T., McGuigan, P., Dawson, G.L., and Allen, R. 1994. Geological investigations of the Tulsequah Chief massive sulphide deposit, Northwestern British Columbia (104K/T2). In Geological Fieldwork 1993, Edited by B. Grant and G.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1995-1, pp. 373-379. Sherlock, R.L., Barrett, T.J., Roth, T., Childe, F., Thompson, J.F.H., Kuran, D., Marsden, H., and Allen, R. 1994. Geological investigations of the 2 IB deposit, Eskay Creek, Northwestern British Columbia (104B/9W). In Geological Fieldwork 1993, Edited by B. Grant and G.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1995-1, pp. 357-372. Mihalynuk, M.G., McMillan, W.J., Mortensen, J.K., Childe, F.C, and Orchard, M.J. 1996. Age of host strata versus mineralization at Erickson-Ashby: A skarn deposit. In Geological Fieldwork 1994, Edited by B. Grant and G.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1995-1, pp. 521-527. xix ACKNOWLEDGMENTS This thesis has benefited from the assistance and expertise of many people. First and foremost, I wish to thank my doctoral supervisors John Thompson and Jim Mortensen for their guidance in the field and laboratory, as well as the countless hours they spent editing this thesis and the publications that were produced during the course of it. I also wish to thank the staff of the Geochronology Laboratory for their assistance, in particular my office mate Richard Friedman. Mary Lou Bevier is thanked for allowing access to unpublished data and procedures. The assistance of MDRU staff Sonya Tietjen and Arne Toma is greatly appreciated. This work benefited greatly from input by members of the British Columbia Geological Survey Branch, in particular Mitch Mihalynuk, Dani Alldrick, Bill McMillan, Paul Schiarizza, and Jim Logan, whose contributions ranged from stimulating geological discussions to digging samples out of the basement for me to analyze. This thesis was funded through the Mineral Deposit Research Unit Volcanogenic Massive Sulphide Deposits of the Cordillera Project, funded by the Natural Science and Engineering Research Council of Canada (NSERC), the Science Council of British Columbia and eleven member mining companies, namely Garanges Inc., Homestake Canada Inc., Inco Ltd., Inmet Mining Corp. (formerly Metall Inc. and Minova Inc.), Kennecott Canada Inc., Lac Minerals, Placer Dome Ltd., Teck Corp., TVI Pacific Inc., Westmin Resources Ltd., and WMC International Ltd. The author was supported in part by a NSERC doctoral scholarship. Ron Britten, Henry Marsden, and Andrew Kaip (all formerly of Homestake Canada Inc.), and Dave Kuran and Tina Roth (Homestake Canada Inc.) are thanked for sharing their wealth of knowledge on the Eskay Creek deposit, and for allowing the author access to company reports, maps, drill logs and core from both the Eskay Creek and Kutcho Creek deposits. Terry Chandler (Redfern Resources Ltd.) is thanked for allowing the author access to similar material for the Tulsequah Chief and Big Bull deposits. Paul McGuigan (Cambria Resources Ltd.) is thanked for sharing his knowledge of the Granduc deposit. Atna Resources Ltd. is thanked for allowing the author access to samples from the Ecstall property. Determining ages of volcanic rocks by U-Pb methods is a challenging and labour intensive endeavor. A significant number of U-Pb age determinations are presented in this thesis. The average weight of these samples was 20-30 kg, they were always the most massive, hardest rock, and the best samples always seemed to be found on the highest mountain tops, lowest valleys or furthest distance from camp. The author wishes to very gratefully acknowledge the assistance of the following people, who assisted collecting these samples and hauling them back to camps across northern and central British Columbia: Tim Barrett, Bob Beck, Dennis Bohme, Ron Britten, Kate Bull, Chris Downie, Andrew Kaip, Jeff Lewis, Peter Lewis, Julia Matsubara, Tanya Mauthner, Rob Macdonald, Paul McGuigan, Mitch Mihalynuk, Jim Mortensen, Matt Phillips, Tina Roth, Paul Schiarizza, Ross Sherlock, John Thompson, and Sarah Vance. And last, but never least, I wish to express my sincere gratitude to Andrew Kaip. Thank you for your unwavering support through these past years. xx CHAPTER 1 INTRODUCTION The North American Cordillera of British Columbia, the Yukon Territory, and Alaska is host to a number of volcanogenic massive sulphide (VMS) deposits (Fig. 1.1). In British Columbia, VMS deposits have both current and historical economic significance, ranging from the past producing Britannia deposit, which in the early part of the twentieth century was the largest single producer of copper in the British Empire, to the currently mined, unusually Au-Ag-rich Eskay Creek deposit, which in 1997 is one of the highest grade gold and silver deposits in North America (Armstrong 1990; Prime Resources Annual Report 1996). Rocks within the Canadian portion of the North American Cordillera can be divided into five morphogeological belts, from east to west these are the Foreland, Omineca, Intermontane, Coast, and Insular Belts; each belt is composed of a collage of tectonostratigraphic terranes of different affinities, bounded by faults (Gabrielse et al. 1991). In the current study, VMS mineralization which formed in the Intermontane Belt, as well as in pendants and belts which now lie within the Coast Belt are examined. These include the Eskay Creek, Granduc, Tulsequah Chief, Big Bull, and Anyox deposits, and the Kutcho Creek and Ecstall prospects. The Intermontane Belt is comprised of the Stikine, Quesnel, Cache Creek and Slide Mountain terranes, the former two consisting of Paleozoic and Mesozoic island arc assemblages, and the latter two of Paleozoic to Mesozoic oceanic assemblages (Gabrielse et al. 1991). These, and other allochthonous terranes of the Cordillera are believed to have accreted on to the margin of Ancestral North America in Mesozoic time (Monger et al. 1982). The Insular Belt is also host to several significant VMS deposits, including the giant Windy Craggy deposit. A recent doctoral^ thesis by Peter (1992) focused on the geology, geochemistry and alteration of the Windy Craggy deposit. The Eskay Creek, Granduc, Tulsequah Chief, and Big Bull deposits formed within Stikinia. Kutcho Creek lies within a fault-bounded allochthon of unknown terrane affinity to the east of Stikinia, and Anyox and Ecstall are hosted within pendants and belts which now lie within 1 Windy Craggy Greens^) h m Niblack-X_ J-Anyox Ecstall •Tulsequah Chief / Big Bull Kutcho Creek Eskay Creek -Granduc Goldstream Chu Chua Tectonostratigraphic terranes past producer current producer developed prospect Figure 1. 1 Past and presently producing VMS deposits and developed prospects in British Columbia and southeastern Alaska, North American Cordillera. 2 metamorphic and plutonic rocks of the Coast Plutonic Complex, in the Coast Belt (Fig. 1.1). In this study the mineralization and host rocks of these deposits and prospects are examined at both the deposit and regional scale. The regional geology and tectonic setting of each deposit is discussed in the chapter pertaining to that deposit. The following section presents an overview of the principal characteristics of VMS deposits and recognized VMS deposit types. VMS deposits Volcanogenic massive sulphide deposits encompass a wide range of styles and compositions of mineralization. However, a number common features to these deposits can be discerned. These include formation in subaqueous environments at or near the seafloor, an association with volcanic +/- sedimentary rocks, and formation near convergent and divergent plate margins (Sawkins 1990; Franklin 1996). Characteristics which may be observed within well preserved VMS deposits include the presence of concordant lenses of massive sulphides overlying discordant stockwork and/or alteration zones, well developed footwall alteration zones, with little alteration extending into the hangingwall to mineralization, a metal zonation from Cu-rich bases to Pb- and Zn-rich tops within lenses in which Fe-rich sulphides (principally pyrite) dominate, exhalitive silica or sulphate horizons which may extend for a greater lateral extent than massive sulphide mineralization, and synvolcanic faults which may have served as conduits for metal-bearing hydrothermal fluids (Sawkins 1976; Hoy 1991; Franklin 1996). For a detailed review of the morphology and general characteristics of VMS deposits the reader is referred to Sawkins (1976), Hoy (1991), and Franklin (1996). A threefold classification scheme for massive sulphide deposits associated with volcanic rocks was first proposed by Sawkins (1976). Sawkins (1976) suggested that VMS deposits could be divided into three broad deposit types, based on the tectonic setting and the host lithologies to mineralization; these are Kuroko-, Besshi-, and Cyprus-type deposits. Kuroko-type deposits are characterized by the presence of polymetallic (Zn-Pb-Cu±Au+Ag) sulphide lenses, hosted within the felsic portion of compositionally bimodal, calc-alkaline volcanic sequences. These deposits are thought to form within island arc and arc-related rift settings. Besshi-type deposits are characterized by the presence of Cu±Zn sulphide lenses, hosted within clastic sedimentary rocks 3 and mafic volcanic rocks of calc-alkaline affinity; Besshi-type deposits are considered to form within marginal and back arc settings, commonly in proximity to sources of terrigenous sediments . The third category, Cyprus-type deposits consist of Cu±Zn sulphide lenses, hosted within basaltic volcanic rocks of tholeiitic magmatic affinity. These deposits form within mid-ocean ridge settings. Worldwide, VMS deposits are commonly concentrated within regions, such as the Hokuroko district in Japan, and the Bathurst and Noranda Camps in Eastern Canada (Ohmoto and Skinner 1983; Lydon 1988; Franklin 1996). Generally, VMS deposits within these regions are concentrated along particular horizons, perhaps in response to a change in the tectonic environment (Ohmoto and Skinner 1983, and references therein; Urabe 1987). Identification of these favourable horizons at an existing deposit, based on age, or lithological, isotopic, or geochemical characteristics is a significant exploration parameter in the search for additional VMS mineralization. On a regional scale, studying the characteristics of VMS mineralization and their host rocks can assist in determining the tectonic setting in which the deposits formed, and therefore yield additional information on the tectonic history of their host terranes. Objectives This thesis focuses on determination of the age, degree of evolution of radiogenic isotopes, and lithogeochemical affinity of several VMS deposits and prospects which formed in allochthonous island arc terranes of the Canadian Cordillera. As described above, VMS mineralization examined in this study is either hosted within Stikinia (Eskay Creek, Granduc, Tulsequah Chief, and Big Bull), or lies within pendants, metamorphic belts, or fault-bounded blocks of uncertain terrane affinity, which may have formed as part of Stikinia or possibly adjacent terranes (Anyox, Ecstall, and Kutcho Creek). The age, radiogenic isotopic signatures, and lithogeochemical affinity of those deposits in the former group can be contrasted with other rocks within the same group or assemblage, to further define the tectonic history of the terrane, whereas in the latter group these properties can be used to suggest whether these deposits and their host rocks could have formed as part of Stikinia. 4 The principal objectives of the current study were to better constrain the age and tectonic setting of a number of VMS deposits and prospects which formed in allochthonous terranes of the Canadian Cordillera in British Columbia. These objectives were achieved through geological mapping and drill core logging, as well as isotopic dating of igneous rocks using U-Pb methods, lithogeochemical and Nd isotopic analysis of igneous rocks, and Pb isotopic analysis of sulphides and igneous rocks. In addition to the deposit specific studies, the regional context of the Kutcho Assemblage, which hosts the Kutcho Creek deposit, was examined. Using the methods outlined above, rocks of the Sitlika Assemblage and Venables Valley areas were examined to determine if they could represent temporal and tectonic equivalents of the Kutcho Assemblage. Methods U-Pb geochronology Age determinations were achieved through U-Pb isotopic dating of zircon (ZrSi04). These analyses were carried out by the author at the Geochronology Laboratory of the University of British Columbia; analytical procedures are outlined Appendix 1. At some deposits U-Pb age determinations provide a refinement of existing radiometric or biochronological constraints, whereas at others they provided new and unexpected ages for VMS mineralization in the North American Cordillera. Pb isotopic data The Pb isotopic signature of mineralization reflects the degree of evolution of Pb, and by analogy other metals occurring in mineralization (Gulson 1986). Therefore the Pb isotopic signatures of sulphides and sulphosalts from deposits and prospects examined in this study were determined to assess the degree of evolution of the metallogenic sources to mineralization. This portion of the study built on the framework established by Alldrick (1991), who determined that mineralization of Early Jurassic and Tertiary age, in the Stewart Mining Camp of the Stikine terrane had distinct Pb isotopic compositions. Analysis of sulphides, sulphosalts, and feldspars 5 were carried out by the author at the Geochronology Laboratory of the University of British Columbia; analytical procedures are outlined Appendix 1. Nd isotopic data The Nd isotopic signatures of igneous rocks associated with several of the VMS deposits studied here were determined to establish the degree of contamination by older, evolved crustal components. Neodymium isotopic characterization of igneous rocks from each of the groups and assemblages which comprise Stikinia was interpreted by Samson et al. (1989) to indicate that these rocks have an island arc affinity, and are the products of mantle-derived continental crust. Data presented in this study are contrasted with the results of Samson et al. (1989) and other relevant studies. Rocks to be analyzed were selected by the author, and analyzed by Reg Theriault, at the Geochronology Laboratory at the Geological Survey of Canada in Ottawa. Analytical procedures follow those outlined in Theriault (1990). Lithogeochemistry Major, trace and rare earth element compositions were determined for volcanic and plutonic rocks at both the deposit and regional scale. The purpose of this portion of the study was to determine the chemical composition and magmatic affinity of these rocks, and compare this data with regional data sets, and detailed data acquired at some of the deposits by other researchers in the MDRU VMS Project. Whole rocks samples were analyzed for major and trace element compositions by X-ray fluorescence at McGill University in Montreal, Quebec, using glass beads for major elements and pressed pellets for trace elements; a subset of samples was analyzed by instrumental neutron activation analysis for rare earth element compositions at Activation Laboratories in Ancaster, Ontario. Presentation As outlined in the foreword, this thesis is presented as a series of discrete chapters, each focusing on a particular region, mineral deposit or assemblage. Five papers, based on two chapters have been accepted or submitted for publication in refereed professional and government journals. The benefits of clarity and continuity for the reader, and the facilitation of publication of 6 portions of this thesis warrant the style adopted here. Efforts have been made to minimize repetition of background material, methodology and references. However, a certain amount of repetition is unavoidable. The subject of each chapter is outlined below. Chapter two discusses the geology, U-Pb geochronology and radiogenic isotopic systematics of the Eskay Creek deposit, a Au- and Ag-rich VMS deposit in the Iskut River area of Northwestern British Columbia (Fig. 1.1). The isotopic age of rocks which comprise the immediate footwall to mineralization is established, and the chemistry and Nd isotopic signature of this unit is contrasted with those of rocks of identical age which are not known to be associated with mineralization. Based on the Pb isotopic signature of mineralization at the deposit, potential metallogenic sources to mineralization are discussed. In addition, possible modern-day analogies to the Eskay Creek deposit are discussed. The third chapter focuses on the Cu-rich Granduc VMS deposit, in the Stewart Mining Camp of Northwestern British Columbia (Fig. 1.1). In this chapter the U-Pb age of magmatic rocks and the Pb isotopic signature of mineralization are used to establish a Late Triassic age for massive sulphide mineralization in this strongly deformed deposit. Determination of this age allows for a regional comparison between the geochemical and Nd isotopic signatures of rocks which host mineralization at Granduc and unmineralized rocks of the Upper Triassic Stuhini Group, as well as a deposit comparison with Windy Craggy, a VMS deposit of comparable age and mineralogy to Granduc, occurring in the Alexander terrane. In addition, it is suggested that the Pb isotopic signature of mineralization determined in this study can now be used in a comparative way to determine if other mineralization in the region is of comparable age to Granduc. Chapter four is presented in four sections. In the first part of the chapter the age, radiogenic isotopic characteristics, and geochemistry of the fault-bounded Kutcho Assemblage in North Central British Columbia is documented (Fig. 1.1). The Kutcho Assemblage is host to the Cu-Zn-rich Kutcho Creek VMS deposit. Data presented here indicate that the Kutcho Assemblage has an uncommon age and primitive character, both of which are distinct from other bimodal island arc sequences in the Cordillera. It is suggested that these rocks are unlike other 7 previously well-documented strata in the Cordillera and as such may represent a formerly unrecognized Cordilleran terrane. In the following two parts of the chapter, fault-bounded rocks of similar lithology, age, and isotopic and geochemical characteristics as rocks of the Kutcho Assemblage are documented. In the final part of the chapter, data for the three regions are compared, and the hypothesis that the rocks within the these assemblages formed in the same time period and tectonic environment is put forward. Chapter five focuses on the age and isotopic characteristics of the Tulsequah Chief and Big Bull polymetallic VMS deposits, in Northwestern British Columbia, as well as volcanic strata of similar age in the region (Fig. 1.1). The isotopic signatures of mineralization and host rocks are contrasted with those of rocks of comparable age which occur further south within Stikinia. The data presented here have implications for both the nature of the basement to this terrane and VMS exploration in the region. Chapter six is written in three parts, and discusses the age and Pb isotopic systematics of pendants or belts encompassed by the Coast Plutonic Complex. Part one discusses the Anyox pendant, which hosts the Anyox deposit, and part two discusses the Scotia-Quaal Belt, which hosts the Ecstall prospect (Fig. 1.1). Part three discusses the possibility of a tectonic link between these pendants and the Stikine terrane, based in part on new U-Pb age data presented in the previous parts of this chapter. Chapter seven classifies VMS deposits in allochthonous terranes of the North American Cordillera in British Columbia based on host lithology, as was first proposed by Thompson et al. (1994). The currently interpreted tectonic setting, age and terrane affinity of several of these deposits are discussed, and this information is contrasted with the radiogenic isotopic signatures of mineralization and host rocks at each deposit. References Alldrick, D.J. 1991. Geology and ore deposits of the Stewart Mining Camp. Ph.D. thesis, The University of British Columbia, 417 p. 8 Armstrong, J.E. 1990. Vancouver Geology. Edited by C. Roots and C. Staargaard, Geological Association of Canada, Vancouver, 128 p. Franklin, J.M. 1996. Volcanic-associated massive sulphide base metals. In Geology of Canadian Mineral Deposit Types, Edited by O.R Eckstrand, W.D. Sinclair, and R.I. Thorpe, Geological Survey of Canada, Geology of Canada no. 8, pp. 158-183. Gabrielse, H., Monger, J.W.H., Wheeler, J.O., and Yorath, C.J. 1991. Morphological belts, tectonic assemblages and terranes. In Upper Devonian to Middle Jurassic Assemblages, Part A of Chapter 1 of Geology of the Cordilleran Orogen in Canada, Edited by H. Gabrielse, and C.J. Yorath, Geological Survey of Canada, Geology of Canada, no. 4, pp. 15-28. Gulson, B.L. 1986. Lead isotopic in mineral exploration. Developments in Economic Geology, 23, Elsevier, 244p. Hoy, T. 1991. Volcanogenic massive sulphide deposits in British Columbia. In Ore deposits, tectonics and metallogeny in the Canadian Cordillera, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, 276 pages. Lydon, J.M. 1988. Volcanogenic massive sulphide deposits, Part 1: A descriptive model. In Ore Deposit Models, Edited by R.G. Roberts and P. A. Sheahan, Geoscience Canada Reprint Series 3, pp. 145-154. Monger, J.W.H., Price, R.A., and Templeman-Kluit, D.J., 1982. Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, 10: 70-75. Ohmoto, H., and Skinner, B.J. 1983. The Kuroko and related volcanogenic massive sulphide deposits: Introduction and summary of new findings. Economic Geology Monograph 5: The Kuroko and Related Volcanogenic Massive Sulphide Deposits, pp. 1-8. 9 Peter, J.M. 1992. Comparative geochemical studies of the Upper Triassic Windy Craggy and modern Guaymus Basin deposits: A contribution to the understanding of massive sulfide formation in volcano-sedimentary environments. Ph.D. thesis, University of Toronto, Toronto. Samson, S.D., McClelland, W.C., Patchett, P.J., Gehrels, G.E., and Anderson, R.G. 1989. Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera: Nature, 337: 705-709. Sawkins, F.J. 1976. Massive sulphide deposits in relation to geotectonics. Geological Association of Canada, Special Paper 14: 221-240. Sawkins, F.J. 1990. Metal deposits in relation to plate tectonics. Second Edition, Springer-Verlag, 461 pages. Theriault, R., J. 1990. Methods for Rb-Sr and Sm-Nd isotopic analyses at the geochronology laboratory, Geological Survey of Canada, in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 89-2, p. 3-6. Urabe, T. 1987. Kuroko deposit modeling based on magmatic hydrothermal theory. Mining Geology, 37: 159-176. 10 CHAPTER 2: U-Pb GEOCHRONOLOGY AND Nd AND Pb ISOTOPIC CHARACTERISTICS OF THE Au-Ag-RICH ESKAY CREEK VMS DEPOSIT, BRITISH COLUMBIA Abstract The Eskay Creek deposit is a Au-Ag-rich volcanogenic massive sulfide deposit located within the Iskut River area of northwestern British Columbia. At Eskay Creek beds of Au-Ag-rich clastic sulfides and sulfosalts hosted within the 'contact' argillite are underlain by the variably altered Eskay rhyolite, which hosts stockwork mineralization, and overlain by the barren hangingwall basalt. The Eskay rhyolite has yielded a Middle Jurassic U-Pb zircon age of 175 +/-2 Ma. Interaction textures between the Eskay rhyolite and 'contact' argillite, which imply emplacement of the rhyolite prior to lithification of the argillite, along with probable local derivation of bedded sulfide clasts indicate a Middle Jurassic age for mineralization, some 7-16 Ma younger than intrusion-related Lower Jurassic mineralization within the region. The data presented in this study demonstrate that there is a second, and significant period of Jurassic mineralization within the Iskut River area. Initial eNd values of +5.5 to +6.9 for the Eskay rhyolite and hangingwall basalt lie within the field for uncontaminated island-arc volcanic rocks and are consistent with formation in a subduction-related environment. Ratios of immobile elements and REE patterns for the Eskay rhyolite suggest a tholeiitic magmatic affinity, whereas unmineralized rhyolite of the same age (174 +2/-1 Ma; U-Pb zircon) is characterized by a slightly less primitive Nd isotopic signature (+4.3) and a transitional to calc-alkaline affinity. Sulfides and sulfosalts from clastic and rhyolite-hosted mineralization in and along strike from the deposit exhibit a homogeneous Pb isotopic signature. The Pb isotopic composition of Eskay Creek sulfides and sulfosalts is inconsistent with leaching of lead from Paleozoic to Mesozoic arc assemblages which may occur at depth below the Eskay Creek deposit, but is consistent with derivation from a Lower or Middle Jurassic source. Introduction The Eskay Creek deposit is located in the Iskut River area (NTS 104B) of northwestern British Columbia. Within this region, Paleozoic to Mesozoic island arc assemblages of the 12 allochthonous Stikine terrane are overlain by marine sedimentary rocks of the Middle to Upper Jurassic Bowser Lake Group and intruded by plutons of the Tertiary Coast Plutonic Complex. A wide range of styles of base and precious metal-rich deposits occur in the Mesozoic arc complexes and Tertiary plutons of the Iskut River area. These include copper-rich volcanogenic massive sulfide (VMS), porphyry copper-gold, gold±silver vein, silver-zinc-lead vein, and gold-silver skarn deposits (Macdonald et al., 1996). The Eskay Creek deposit (Fig. 2.1) is a Au-Ag-rich VMS deposit hosted within a bimodal volcanic sequence, and is distinct in both style and mineralogy from any other known deposits in the Iskut River area. Mineralization consists of stockwork zones hosted within rhyolite, overlain by argillite and siltstone which host clastic sulfide and sulfosalt mineralization, mainly in the 21B zone (Blackwell, 1989; Britton et al., 1989; Roth, 1995). The rhyolite and sedimentary units are in turn overlain by barren pillowed to massive basalt. The 21B zone contains 1.08 million tonnes grading 65.5 grams per tonne gold, 2930 grams per tonne silver, 5.6% zinc and 0.77% copper (Homestake Canada Inc. Feasibility Report August, 1993); mining of the 2 IB zone began in January, 1995. This paper summarizes the results of a multi-isotope study which (1) precisely determines the age of the Eskay Creek deposit, using U-Pb zircon geochronology; (2) uses the Nd and trace element geochemical signatures of the Eskay rhyolite and hangingwall basalt to better characterize the tectonic environment in which the deposit and host magmas formed; (3) compares the chemical and isotopic characteristics of the Eskay rhyolite to unmineralized rhyolite of the same age within the Eskay anticline; and (4) characterizes the Pb isotopic signature of the different styles of mineralization in, and along strike from, the deposit. Regional Geology The Eskay Creek deposit is located within Stikinia, an allochthonous terrane of the Canadian Cordillera. Anderson (1989) divided the Stikine terrane in the Iskut River area into four unconformity-bounded, tectonostratigraphic elements: (1) Paleozoic island-arc rocks of the Stikine Assemblage; (2) Mesozoic island-arc assemblages composed of the Upper Triassic Stuhini Group and the Lower to Middle Jurassic Hazelton Group; (3) Middle to Upper Jurassic sedimentary rocks of the Bowser Lake Group, an overlap assemblage occurring on Stikinia and 13 the adjacent Cache Creek terrane; and (4) Tertiary igneous and metamorphic rocks of the Coast Plutonic Complex (Fig. 2.1). Figure 2. 1 Location map of the Eskay Creek deposit, Iskut River area, northwestern British Columbia showing the major stratigraphic units, mining camps, deposits and occurrences (after Macdonald et al., 1996). The Iskut River area is host to various styles of mineralization. In addition to Eskay Creek, copper-rich VMS deposits (e.g. Granduc; Childe et al., 1994), porphyry deposits (e.g. Kerr and Sulphurets; Macdonald et al., 1996; Kirkham and Margolis, 1995) and Au+Ag-rich vein deposits (e.g. Snip and Brucejack Lake; Macdonald et al., 1996; Davies et al., 1994) are of economic interest. Known ages of mineralization range from Late Triassic to Eocene, with the majority of mineralization having formed in the Early Jurassic (Childe et al., 1994; Kirkham and Margolis, 1995; Macdonald et al., 1996). The Lower to Middle Jurassic Hazelton Group, which hosts the Eskay Creek deposit, comprises subaqueous to locally subaerial, mafic to felsic volcanic and volcaniclastic rocks and their intrusive equivalents, and conglomeratic to argillaceous, and calcareous sedimentary rocks. Rocks of the Hazelton Group have been regionally metamorphosed to lower greenschist to sub-greenschist facies. Abrupt facies changes, a complex structural history, and limited age 14 determinations have hindered the understanding of the stratigraphic history of the Hazelton Group. Stratigraphic sections based on regional mapping by Grove (1986), Anderson and Thorkelson (1990), and Alldrick (1991) divide the Hazelton Group into four formations; from oldest to youngest these are the Unuk River, Betty Creek, Mt. Dilworth and Salmon River Formations. However, recent biochronological (Nadaradju, 1993) and geochronological (Macdonald et al., 1996; this study) information has provided new constraints that require modifications of the existing stratigraphic divisions. Lewis and Anderson (pers. comms., 1995) divided the Hazelton Group into three formations, which are used in this paper. The basal Jack Formation, first defined by Henderson et al. (1992), is composed of pebble to boulder conglomerate and fossiliferous calcareous siltstone, contains Hettangian to Sinemurian fossils, and lies unconformably on rocks of the Upper Triassic Stuhini Group. The second formation is the Betty Creek Formation, composed of andesitic to rhyolitic volcanic and epiclastic rocks, as well as argillite, sandstone, conglomerate and calcareous sedimentary rocks which may be as old as Sinemurian, and in at least one locality as young as Upper Aalenian. The third formation is the Salmon River Formation, and is composed primarily of bimodal volcanic and tuffaceous rocks, as well as locally calcareous argillite to siltstone of Aalenian to Lower Bajocian age. Rocks of the Salmon River Formation record the final stages of arc volcanism in the Stikine terrane, and are overlain by marine sedimentary rocks of the Bowser Lake Group. Plutons coeval with both the Betty Creek and Salmon River Formations are present in the Iskut River area. Those related to Early Jurassic magmatism, including the 197 to 184 Ma Texas Creek Plutonic and Premier Porphyry Suites (Alldrick et al., 1986; Macdonald et al., 1992; Lewis and Mortensen, unpublished data), are associated with significant base and precious metal mineralization (for example Anderson, 1989; Alldrick, 1991; Macdonald et al., 1996). Plutons related to the Middle Jurassic magmatic event, including 181 to 176 Ma quartz diorite to monzodiorite intrusions (M.L. Bevier and R.G. Anderson, pers comm., 1994; J.K. Mortensen, pers. comm. 1995), contain no known mineralization. Geology of the Eskay Creek area Hazelton Group rocks in the Eskay Creek area are folded around the gently northeasterly-plunging Eskay anticline. Strata in the hinge region, in the vicinity of the deposit, are 15 dismembered by several north-northeast trending faults (Fig. 2.2). The core of the Eskay anticline is occupied by sedimentary rocks of the Jack Formation. Overlying strata of the Betty Creek Formation include undated andesite breccias overlain by Upper Pliensbachian sedimentary rocks (Nadaradju, 1993) (Figs. 2.2 and 2.3). The basal footwall volcanic unit of the Salmon River Formation consists of intermediate composition volcanic tuff and breccia, and aphanitic amygdaloidal andesite flows or sills (Roth, 1993). Overlying the footwall volcanic unit, within the Salmon River Formation, are at least two chemically distinct rhyolites, shown on Figure 2.3 as the Eskay rhyolite and the east limb rhyolite (Rye, 1992). The Eskay rhyolite occurs along the west limb and through the fold closure of the Eskay anticline, hosts stringer-style discordant mineralization and is the immediate footwall to stratiform mineralization. West Limb The Eskay rhyolite is characterized by aphanitic massive to flow-banded and auto-brecciated flows and associated tuffs, interpreted by Bartsch (1993) on the basis of facies variations to represent a linear flow dome complex. In proximity to stockwork mineralization, the rhyolite shows moderate to extreme alteration ranging from quartz-sericite-pyrite to chlorite-rich assemblages. A series of north-northeast trending rhyolite masses, termed felsite, which are slightly discordant to stratigraphy, occur along the length of the west limb (Fig. 2.2). Felsite is chemically indistinguishable from the Eskay rhyolite (Rye, 1992; Bartsch, 1993; Roth, 1993) and is interpreted to represent sub-volcanic portions of the Eskay rhyolite (Bartsch, 1993; Edmunds et al., 1994). Along most of the west limb, the Eskay rhyolite grades into a black matrix breccia, which in turn grades into carbonaceous black siltstone and argillite, termed the contact argillite. Black matrix breccia is composed of matrix-supported angular to sub-rounded, shattered, glassy and flow-banded rhyolitic fragments in a black siliceous matrix (Plate 2.2). This texture is interpreted to have formed as a result of intrusion of hot rhyolite into unlithified argillaceous sedimentary rocks, implying that emplacement of the Eskay rhyolite occurred prior to lithification of the contact argillite. Argillite is overlain by pillowed to massive basalt flows and sills which form the hangingwall to mineralization (Figures 2.2 and 2.3 and Plate 2.3). Peperitic textures at the contact between basalt and argillite indicate emplacement of the hangingwall basalt prior to lithification of the contact argillite. The field relationships therefore imply that the Eskay rhyolite 16 and hangingwall basalt were emplaced within a relatively short time span, during accumulation, and prior to lithification of the contact argillite. LEGEND Ashman Formation (Lower Sajocian to Lower Cdlomn) [v] marine sediments Salmon River Formation (Aalenian to Lower Bajocian) F~-1 argillile & turbiditic ^ sediments J J m, contact argillite (bedded argiite & siltstone) \ | rhyolite r^-n footwall volcanic unit |.",| (mixedvolcaniclastks & andesitic Horn or silk) Betty Geek Formation (Upper Pknsbachian) p"\] andesite breccia overlain by L; - oiy/tos, samkrone & limestone Jack Formation (kipattHettangian to bmemurian) • orgilloceous to conglomeratic sedimentary rocks Intrusive Hocks | felsite Eskay Porphyry ^ (184+5/-lka, recalculated from Macdonald etal., 1992) • mineralized zone „ ^ fault * U-Pb sample location y" creek/river fft trend & plunge 45y strike & dip N Figure 2. 2 Geology of the Prout Plateau, showing locations of selected mineralized zones projected to surface (after Edmunds and Kuran, 1991; Bartsch, 1993 and P. Lewis, pers comm., 1996). 17 West Limb East Limb marine sediments Lower Bajodtm to Lower Caiman (Nadaraju, 1993) argillite & turbiditk sediments hangingwall basalt, massive to pillowed flows and sills contact argillite, host to clastk mineralization kleimn to possibly Early Bajocian (Nadaraju, 1993) Eskay rhyolite aphanilk, with black matrix breccia, locally flow banded. U5+/-2Mo (this study) Felsite dykes &/or sills Footwall volcanic unit mixed volcanklastics & andesitic flows or silk Upper Pliesnbachian (Nadaraju, 1993) andesite breccia, overlain-by argillite, sandstone & limestone. V V V.V V 1 arenite, argillite siltsione & volcanic conglomerate. Eskay porphyry 184+5/-lMa (recalculated from Macdonald etal., 1992) WAN; s; 5< fc" S< I A < W- A * -I i, -1 -7 i- J -I ^ ^ 3 * 5" |L. A < U- A * U-marine sediments 3 ca =3 9 =3 fc £ Atone Upper Aahnian 0. Palfy, pers. comm., 1995) East limb rhyolite, quartz + feldspar porphyritic, with minor black matrix breccia, flow banded. 174+2/-lMa (this study) i Footwall volcanic unit mixed volcanidastks & andesitic flows or silk arenite, argillite siltstone & volcank conglomerate. DO 3 Q =3". O =3 3 C2 Figure 2. 3 Schematic stratigraphic sections for the east and west limbs of the Eskay anticline, F = fossil locality, Z = U-Pb zircon sample locality. 18 East Limb The east limb rhyolite has a black aphanitic matrix, well preserved flow-banding and sparse 1-2 mm quartz and potassium feldspar phenocrysts (Plate 2.4). The presence of black matrix breccia along the lower margin of this rhyolite indicates emplacement into unlithified sedimentary rocks. Rhyolite is overlain by argillite which is interpreted to be the stratigraphic equivalent of the contact argillite. Mafic rocks analogous to the hangingwall basalt have not been observed on the east limb. Eskay Porphyry The Eskay porphyry (Figs. 2.2 and 2.3), located within the core of the Eskay anticline, is an orthoclase porphyritic to megacrystic monzodioritic intrusion within sedimentary rocks of the Jack Formation and andesitic volcanic rocks of the Betty Creek Formation. The Eskay porphyry has been dated by U-Pb zircon geochronology at 184 +5/-1 Ma (recalculated from Macdonald et al., 1992) and is contemporaneous with intrusion-related Lower Jurassic mineralization in the Iskut River and Stewart areas. Mineralization Mineralization at Eskay Creek can be broadly divided into two main styles, stratiform and discordant. Stratiform mineralization occurs as beds of clastic sulfides and sulfosalts within the contact argillite in the 21 A, B, and C, hangingwall, and northeast extension zones. Discordant, rhyolite-hosted vein mineralization, which may in part represent feeder zones to clastic mineralization, occurs below the stratiform mineralization in the 109, Pumphouse and Pathfinder zones, and along strike at the Emma and Mackay Adits (Fig. 2.2). At present, the 21B zone is the focus of mining at Eskay Creek. It is an elongate zone, approximately 900 meters long, 60 to 200 meters wide and 1 to 15 meters thick, composed of well-bedded fragmental sulfides, sulfosalts, argillite, altered rhyolite and barite hosted within the contact argillite (Britton et al., 1989; Roth, 1995) (Plate 2.5). The main ore minerals are sphalerite and tetrahedrite, with lesser friebergite, galena, pyrite, boulangerite, and other sulfosalts 19 (Roth, 1995). Well-preserved features such as graded sulfide beds with sharply defined bases, soft-sediment deformation of the sulfide beds and channel structures attest to the clastic nature of the ore within a high-energy, channel-like environment (Roth, 1995). The proximity of stockwork zones to clastic mineralization and the presence of altered rhyolite fragments interbedded with sulphide fragments suggest local derivation of clastic sulfides and sulfosalts (Macdonald et al., 1996). The distribution and mineralogy of the mineralized zones are outlined in Figure 2.4; detailed descriptions are provided by Britton et al. (1989), Roth (1993), Edmunds et al. (1994), and Roth (1995). LEGEND Stratiform mineralization hosted in contact argillite. 21A lone: stibnite-realganinnabar lens 21 B& Northeast Extension lones: clastic sphalerite-tetrahedriteijalena-pyrite beds 21C2one: " Xvl barrteiich beds Hanging wall (HW) lone: massive p0eiphalerit^golena chalcopyrite Discordant mineralization hosted in footwall rhyolite. 21A Zone: ii.'.;.; disseminated stibniteiealgar-arsenopyritHetmhedrite-pyrite Pumphousefathhnder Zones: veins of sphaleritefyrite^aleno-tetrahedrite ga 109 Zone: crustiform quartz-sphalerite^alena-pyritethalcopyritetarbon veins N Northeast Extension Zone -HWZone a Pathfinder Zone 21A Zone, Pumphouse lone / /f 200 100 0 100 200 / / meters Figure 2. 4 Projection to surface of mineralized zones at die Eskay Creek deposit (courtesy of Homestake Canada Inc.). U-Pb Geochronology Zircon was recovered from both the Eskay and east limb rhyolites; heavy mineral extraction procedures and U-Pb zircon analytical procedures follow those of Mortensen et al. (1995). Isotopic ratios were measured using a modified single collector VG-54R thermal 20 ionization mass spectrometer equipped with a Daly photomultiplier. Uranium and Pb analytical blanks were in the range of 1-3 and 8-15 picograms, respectively. Concordia intercept ages and associated errors were calculated using a modified York-II regression model (York, 1969), and ages were calculated using the decay constants recommended by Steiger and Jager (1977). Age assignments follow the time scale of Harland et al. (1990). Analytical results are given in Table 2.1 and Figure 2.5. Table 2. 1 U-Pb zircon analytical data for Eskay and east limb rhyolites. Fraction1 Wt. U PbJ 2»Pb3 Pb4 ""Pb5 Isotopic ralios(±\a,%f Isotopic dalcs(Ma,±2cQ° mg ppm ppm wPb P« % M6Pb/"'U ^"Pb/^U *"Pb/»*Pb ^U/^Pb mto/aiV ao7Pb/*'6Pb Esh/V rhyolite (EC-GC-11) A,f,M5,eq 0.045 2858 71 975 216 8.5 0.02525±0.13 0.!726 ±0.32 0.04957±0.22 160.8±0.4 161.7±0.9 174.8±10.3 D,f,M5,eq 0.034 307! 77 4388 39 7.9 0.02573 ±0.11 0.1757±0.22 0.04954±O.I3 163.8±0.3 164.4±0.7 173.5±6.1 E,f,M5,eq 0.052 3198 84 2317 118 8.2 0.02663 ±0.11 0.1821 ±0.14 0.04959 ±0.07 169.4±0.4 169.8±0.4 175.8±3.1 F,f.M5,eq 0.034 3608 77 4388 39 7.9 0.02554 ±0.I0 0.1745 ±0.27 0.04955 ±0.18 163.8±0.3 163.3±0.8 173.5±6.1 C,m,M5,eq 0.010 6790 173 2450 45 8.1 0.02589 ±0.14 0.1770±0.27 0.04959 ±0.18 164.8±0.4 165.5±0.8 176.0±8.5 East limb rhyolite fEC-GC-03) Morphology 1: subhedral. eouant to prismatic A,m,Nl,eq 0.062 444 112 1446 33 I1J 0.02743 ±0.10 0.1885 ±0.25 0.04984 ±0.17 174.5±0.3 175.4±0.8 187.8±8.0 B,f,Nl,eq 0.115 426 12 3390 25 11.0 0.02729 ±0.10 0.1869 ±0.22 0.04968 ±0.15 173.6 ±0.3 174.0±0.7 180.2±6.8 C,m,Ml,cq 0.115 434 12 4312 20 11.3 0.02728 ±0.10 0.1864±0.21 0.04954 ±0.I2 173.5±0.4 173.5±0.6 173.6±5.6 D,f,M2,eq 0.104 405 11 4143 18 11.0 0.02749 ±0.10 0.1881 ±0.20 0.04963 ±0.19 I74.8±0.3 175.0±0.7 177.9±5.5 L.f.Nl.eq 0.123 313 9 5693 12 11.5 0.02736 ±0.11 0.1881 ±0.20 0.04987±0.12 174.0±0.4 175.0±0.6 188.7±5.7 Morphology 2: euhedral, needles I,f,M5,e* 0.134 1725 46 2947 118 18.4 0.02384 ±0.11 0.1628 ±0.13 0.04952 ±0.07 151.9±0.3 153.2±0.4 172.7±3.0 J,f,M5,e» 0.080 5710 155 2520 281 18.2 0.02461 ±0.13 0.1683 ±0.16 0.04989 ±0.07 156.7±0.4 157.9±0.5 175.8±3.4 K,f,M5,c 0.025 2448 67 1172 82 18.4 0.02477 ±0.08 0.1693 ±0.17 0.04956 ±0.13 157.8±0.2 158.8±0.5 174.5±6.0 'All fractions air abraded unless marked by an *; Grain size, intermediate dimension: m= < 134|xm and >74um, f= <74(im; Magnetic codes: Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8A; Front slope for all fractions=20°; Grain character codes: e=elongate, eq=equant to prismatic. 2Radiogenic Pb 'Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ±20% (Daly collector) 4TotaI common Pb in analysis based on blank isotopic composition 5Radiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers model Pb composition at the ^"Pb/^Pb date of fraction, or age of sample) 21 Figure 2. 5 U-Pb zircon concordia diagrams for Salmon River Formation rhyolites from the Eskay anticline a) Eskay rhyolite (EC-GC-11), showing weighted mean regression line for all fractions; b) East limb rhyolite, showing weighted mean regression line for morphology 2, euhedral needle-shaped zircon. 22 Eskay rhyolite A small quantity of prismatic zircons, with relatively high magnetic susceptibility, and typical length to width ratios of 3:1, were recovered from the Eskay rhyolite (Fig. 2.2). Zircons were pale to dark brown in color, contained numerous colorless to opaque inclusions, and had poor clarity and cloudy patches. SEM analysis revealed that pyrite and potassium feldspar were present as inclusions within some of the grains (Plate 2.6). These two minerals can contain up to several hundred parts per million lead within their crystal lattice, and therefore their presence can significantly contribute to the common lead concentration in a zircon analysis. All of the zircon recovered from this rock was picked, and divided into 5 fractions. Despite strong abrasion of the zircons, all analyses are discordant. Zircon from this rock contain high concentrations of uranium (2858-6790 ppm). Cloudy portions within the zircons, combined with high uranium concentrations, suggest that sections of some grains may have become metamict, and therefore susceptible to lead-loss. It is therefore likely that high uranium concentrations and the possible resulting metamictization of these zircon contributed to their discordance. The 207Pb/206Pb ages of zircon fractions from this rock range from 173.5 to 176.0 Ma. A best fit line through all fractions yields an upper intercept age of 177 +33/-5 Ma and a lower intercept age of 36 +101/-125 Ma. A 207Pb/206Pb weighted mean age for all fractions yields an age of 175 +1-2 Ma. The lack of any visible cores using either binocular microscope or SEM backscatter imaging, combined with a restricted range of Pb/ Pb ages, suggests that these zircon do not contain an inherited component. Therefore the weighted mean age of 175 +1-2 Ma is considered to be the best estimate of the age of crystallization of this rock. East limb rhyolite Two populations of zircon were recovered from a sample of the east limb rhyolite (Fig. 2.2 and Plate 2.4). The first population was relatively non-magnetic and consisted of colorless, subhedral, slightly resorbed equant to prismatic grains, 50 to 100 microns in width, with length to width ratios of 1:1 to 2.5:1. Zircons from this population were characterized by colorless rod-and bubble-shaped inclusions, moderate clarity and occasional turbid cores. Zircons from the second population consisted of relatively high magnetic susceptibility, elongate, colorless, doubly-23 terminated needles, typically missing one or both terminations, probably broken during the mineral separation process. These zircons had widths of 25 to 50 microns, length to width ratios of 5:1 to 10:1, were very clear, and commonly contained funnel-shaped tubes aligned parallel to the c-axis of the grains. These tubes were usually filled with a dark colored, opaque substance. As the zircon was of excellent quality, the opaque material is a likely source of the high magnetic susceptibility of this zircon population. SEM analysis of the opaque material revealed quartz, a phylosillicate (muscovite?), a heavy rare earth yttrium phosphate (xenotime?), and possibly volcanic glass. The presence of rock-forming minerals, in particular micas, is consistent with the magnetic susceptibility of these grains being higher than is characteristic for pure zircon (Hutchison, 1974). Five fractions of the equant to prismatic zircon (A, B, C, D, and L) were analyzed, following strong abrasion. The Pb/ Pb ages of these five fractions range from 173.6 to 188.7 Ma; fraction C is concordant at 173.6 Ma. Turbid cores, observed within some of the zircons in this population not selected for analysis, along with older Pb/ Pb ages than fraction C indicate that the non-concordant fractions contain an inherited component; possible sources to this inherited component include the underlying Paleozoic to Mesozoic arc assemblages. An interpreted age of 174 +6/-1 Ma is obtained from this population of zircon based on the 207pb/206pb age aft(j assodated 206pb/238TJ 207pb/206pb Qf cf)ncor(jant fraction C. Three fractions of zircon with the needle-like morphology (I, J, and K) were also analyzed. Due to the high length to width ratio of the needles, the abrasion process is generally not effective. Fractions I and J were not abraded and fraction K was lightly abraded prior to analysis. All three fractions are discordant; this feature is attributed to lead-loss, which may have been enhanced by the high uranium concentrations and high surface to volume ratio of these grains. The Pb/ Pb ages for the three fractions range from 172.7 to 175.8 Ma, with a weighted mean of 174 +/-2 Ma. The restricted range of 207Pb/206Pb ages and the needle morphology suggest that there is no inherited component within this zircon population, and that the weighted mean age is a valid estimate of the age of crystallization of this population of zircon. The interpreted age of this rhyolite is 174 +2/-1 Ma, reflecting the 207Pb/206Pb age of concordant 24 fraction C, and errors calculated from the minimum Pb/ U age of this fraction and the maximum weighted mean age of zircon with the needle morphology. Geochemistry Major and Trace Elements Major, trace and rare-earth element analyses were conducted for typical samples of the Eskay and east limb rhyolites and the hangingwall basalt (Table 2.2). In the immediate vicinity of the Eskay Creek deposit, the Eskay rhyolite is highly altered to quartz-sericite-pyrite and chlorite; the intensity of alteration decreases with distance away from zones of mineralization (Bartsch, 1993). Although major element concentrations are strongly affected in altered rocks associated with VMS deposits, ratios of immobile elements such as Ti, Al, Zr and Y remain essentially unchanged (for example: MacLean and Kranidotis, 1987; Barrett and MacLean, 1994). The samples analyzed in this study, therefore, are compared in terms of immobile element ratios with more extensive geochemical databases for the Eskay and east limb rhyolites (Bartsch, 1993; Barrett and Sherlock, 1996) (Fig. 2.6). In a plot of Zr vs. Ti02, the Eskay and east limb rhyolites have distinctly different trends, with the Eskay rhyolite having significantly lower Ti02 concentrations and Ti02/Zr ratios (Fig. 2.6a). The wide range of Zr and Ti02 concentrations observed within the Eskay rhyolite appears to be the result of alteration. Mass change effects have produced a dispersion of points towards and away from the origin along the Eskay rhyolite trend (Barrett and Sherlock, 1996). Silicification produces mass gain, which dilutes Zr and Ti02 concentrations, whereas sericitization and chloritization produce mass loss, which residually concentrates Zr and Ti02 (Barrett and MacLean, 1994). A Zr vs. Y diagram is used to characterize the magmatic affinity of the Eskay and east limb rhyolites. Using the divisions suggested by Barrett and MacLean (1994), the Eskay rhyolite, with Zr/Y ratios of 2 to 4.5, is mainly of tholeiitic magmatic affinity, whereas rhyolites on the east limb, with Zr/Y ratios of 4.5 to 8, are of transitional to mildly calc-alkaline affinity (Fig. 2.6b). Regionally, rhyolite from the Salmon River Formation in the Iskut River area ranges from a calc-25 Table 2. 2. Major, trace and rare earth element, and Nd isotopic data for samples from the Eskay anticline. Location: surface surface: EC-94-07 C91-707,181m U-58,15.2m CA90-527, 54m detection U-Pb Sample Number: EC-GC-03 EC-GC-11 limit Lithology: east limb east limb Eskay Eskay hangingwall rhyolite rhyolite rhyolite rhyolite basalt x-rav flouresence SiCh (wt.%) 69.89 77.32 76.38 80.81 46.70 0.006 AI2O3 15.74 12.15 13.30 10.77 17.61 0.0120 TiOj 0.27 0.21 0.07 0.07 1.94 0.0035 Fe203 1.06 1.10 1.31 1.28 9.63 0.003 MnO 0.02 0.02 0.01 0.01 0.14 0.003 MgO 0.19 bdl 0.93 0.83 13.06 0.0095 CaO 0.39 bdl. 0.09 0.01 0.78 0.0015 Na20 4.08 3.46 0.00 0.09 2.47 0.0075 K20 6.84 4.75 6.16 3.35 0.82 0.0025 P2O5 0.02 0.03 0.01 0.04 0.34 0.0035 BaO 0.25 bdl 0.05 0.12 0.13 0.0017 LOI 1.00 0.71 2.20 2.84 6.92 Cu (ppm) 55 9 20 43 75 2 Zn 32 56 55 16 129 2 Co 28 34 39 28 47 10 Ni 12 bdl 0 2 66 3 Cr 157 bdl 5 bdl 397 15 V 19 bdl 7 18 360 10 Zr 325 321 162 . 152 92 1 Y 69 46 92 61 30 1 Rb 100 76 188 109 12 1 Sr 59 47 32 9 62 1 neutron activation La (ppm) 36.3 18.1 4.9 0.1 Ce 71 56 13 1 Nd 28 34 9 1 Sm 6.71 8.58 3.12 0.01 Eu 1.13 bdl 0.89 0.05 Tb 1.5 1.8 0.7 0.1 Yb 7.26 7.41 3.18 0.05 Lu 0.89 1.01 0.43 0.01 Th 7.5 13.1 0.3 0.1 U 2.6 8.9 bdl 0.1 Au bdl 263 9 2 Ag bdl 4 bdl 2 As 1 121 bdl 1 Co 22 31 89 0.1 Cs 1.1 2.4 2.7 0.2 Hf 8.2 5.4 1.7 0.2 Sb 1.4 52.6 4.5 0.1 Sc 1.9 1.1 45.8 0.1 Ta 1.5 2.6 bdl 0.3 mass spectrometry 8.60 3.62 Sm (ppm) 6.28 Nd 27.26 30.39 10.88 ,47Sm/,44Nd 0.1394 0.1712 0.2011 M3Nd/144Nd (meas.) 0.512791+4X10-" 0.512888+llxlO-* 0.512997+7X10-6 £Nd(174Ma) +4.3 +5.5 +6.9 TDM (Ma) 542 - 634 bdl = below detection limit. 26 alkaline to tholeiitic affinity (A. Kaip, pers comm., 1995). The Eskay porphyry, with Zr/Y ratios of 6 to 13, has a transitional to calc-alkaline affinity (Bartsch, 1993). • east limb rhyolites (Bartsch (1993) and this study) D Eskay rhyolite and felsite (Barrett and Sherlock (in press) and this study) 160T Zr/Y=2 Zr/Y=4.5 transitional Zr/Y=7 calc-alkalinel 100 200 Zr (ppm) 300 400 Figure 2. 6 Trace element plots for Eskay and east limb rhyolites, a) Zr vs. Ti02, b) Zr vs. Y. 27 Rare Earth Elements Rare earth element (REE) concentrations are presented in Table 2.2 and compared with the range of REE concentrations determined for these lithologies and the Eskay porphyry by Bartsch (1993) (Figs. 2.7a, b and c). The Eskay rhyolite exhibits a slight enrichment in the light REE (LREE) but a near-flat pattern for the heavy REE (HREE), and a negative europium anomaly; rhyolite from the east limb sampled in this study exhibits a similar REE pattern (Fig. 2.7a). These relatively flat REE patterns suggest that both rhyolites are the products of primitive tholeiitic magmas, either as a result of partial melting of sialic crust or fractional crystallization of basaltic magma. An inherited component to the zircon in the East limb rhyolite indicates that this rhyolite contains some component of partial melting. Although characterized by similar major and trace element chemistry, rhyolite from the southern portion of the east limb sampled in this study has a markedly more tholeiitic REE pattern than the samples reported by Bartsch (1993), suggesting the presence of a second, chemically distinct rhyolite, along the east limb of the anticline (Fig. 2.7b). A steeper REE pattern for the Eskay porphyry indicates a more evolved magmatic affinity than the Eskay and east limb rhyolites (Fig. 2.7c). Samples of the hangingwall basalt analyzed in this study and others (Bartsch, 1993; Barrett and Sherlock, 1996) yield Zr/Y ratios between 2 and 4, consistent with a tholeitic magmatic affinity. High MgO, Cr and Ni contents indicate derivation from relatively unfractionated mantle melts (Barrett and Sherlock, 1996). However, the essentially flat REE pattern for the hangingwall basalt displays enrichment in LREE relative to N-MORB, and is comparable to that of T-MORB (Sun and McDonough, 1989) (Fig. 2.7c). These data suggest minor enrichment of the basalts relative to N-MORB. Nd Isotopic Data The Nd isotopic ratios of the Eskay and east limb rhyolites and the hangingwall basalt were determined to further constrain the characteristics of the magmas and their potential source regions. Isotopic analyses were conducted by R. Theriault at the Geochronology Laboratory of the Geological Survey of Canada. Analytical procedures are described by Theriault (1990). Abundances of Sm and Nd determined by isotope dilution have an uncertainty of 1% or less. 28 8 a 3 <?9 T3 ft 6*1 » a. s o 1000 P 100 10 a o EC-GC-03, east limb rhyolite • U-58,15.2 m, Eskay rhyolite shaded area represents field for rhyolites from the west limb reported by Bartsch (1993) norm:chon 11 , , , L 1000 100 10 —r T.— l t c o EC-GC-03, east limb rhyolite • U-58,15.2 m, Eskay rhyolite shaded area represents field for the Eskay porphyry reported by Bartsch (1993) La Ce Pr Nd Sm Eu(Gd)Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu(Gd)Tb Dy Ho Er Tm Yb Lu to o S3 O* 1000 100 10 -norm:chon o EC-GC-03, east, limb rhyolite • U-58.15.2 m, Eskay rhyolite shaded area represents field for rhyolites from the east limb reported by Bartsch (1993) La Ce Pr Nd Sm Eu(Gd)Tb Dy Ho Er Tm Yb Lu 1000 100 -10 -i i r-d o EC-GC-03, east limb rhyolite, • U-58,15.2 m, Eskay rhyolite • CA90-527,54m, hangingwall basalt shaded area represents field for hangingwall basalts reported by Bartsch (1993) La Ce Pr Nd Sm Eu(Gd)Tb Dy Ho Er Tm Yb Lu Uncertainty for calculated em values is ±0.5 end units. Initial SNd values for the Eskay and east limb rhyolites are +5.5 and +4.3, respectively, whereas the hangingwall basalt has a value of +6.9 (Table 2.2). The range of initial SNd values determined for MORB is about +8 to +12 (DePaolo, 1988). Lavas produced within modern island arcs formed on oceanic crust, with no evidence for continental contamination such as the Isu, New Britain and Mariana arcs, yield initial SNd values of +2 to +10 (DePaolo and Wasserburg, 1977; Nohda and Wasserburg, 1981; Hawkesworth et al., 1993). The shift to slightly more evolved (lower) 6Nd values for island-arc magmas relative to MORB appears to be the result of contamination from components derived from the downgoing slab and/or the volcano-sedimentary pile, thus reflecting the subduction process (DePaolo, 1988). In contrast, lavas produced within continental margin volcanic arcs have lower initial end values, reflecting contamination from more evolved continental basement (Faure, 1986 and references therein). Initial 6Nd values of +4.3 to +6.9 for Salmon River Formation rhyolite and basalt in the Eskay anticline indicate derivation from juvenile source regions, and are consistent with formation in a subduction-related environment (Fig. 2.8). A slightly more evolved signature for the East limb rhyolite, relative to the Eskay rhyolite may be reflecting a greater component of contamination by subduction-related sediments or partial melting of sialic crustal than in the Eskay rhyolite. Initial 6Nd values determined in this study fall within the range of known values for Lower Jurassic felsic volcanic rocks of the Hazelton Group and Lower to Middle Jurassic felsic to intermediate intrusive rocks (Samson et al., 1989; Jackson, 1990; M. L. Bevier and R. G. Anderson, pers. comms., 1994) (Fig. 2.8); there is no comparable published database for mafic rocks or Middle Jurassic felsic volcanic rocks of the Hazelton Group. Pb Isotopic Data Lead isotopic compositions were determined for 21 sulfide and sulfosalt samples from the 2 IB, HW, 109, NEX, and Pathfinder zones, the Emma and Mackay Adits, and stockwork mineralization within the footwall volcanic unit, from directly below the deposit. The Eskay 30 rhyolite is too pervasively altered to yield reliable whole rock lead isotopic data. Minerals analyzed in this study included galena, boulangerite, sphalerite, chalcopyrite and pyrite (Table 3). Analytical procedures for sulfide and sulfosalt trace lead analyses are given in Childe (this volume). The principal objective of this portion of the study was to determine if the various zones of stratiform and discordant mineralization at the deposit and along strike from it are characterized by specific Pb isotopic signatures (Figs. 2.2 and 2.4). 12 10 8 6 ZMd 4 2 0 400 300 200 100 0 Time (Mo) Figure 2. 8 Initial em values and associated errors for volcanic rocks from the Eskay anticline. Shown for reference are fields for Paleozoic to Mesozoic arc assemblages of the Stikine terrane, uncontaminated island arc volcanic rocks (uIAV)5,6-?, and mid-ocean ridge basalts (N-MORB)8, evolution curves for uIAV and N-MORB are shown in dashed and solid lines, respectively. HG = felsic volcanic, and mafic to felsic intrusive rocks of the Hazelton Group 12'3, SG = mafic volcanic and intrusive rocks of the Stuhini Group SA = mafic to felsic volcanic rocks of the Stikine Assemblage 1,4 (' Samson et al. (1989),2 Jackson (1990),3 Bevier and Anderson, (pers. comms., 1994),4 Childe (this volume),5 Nodah and Wasserburg (1981), 6 DePaolo and Wasserburg (1977),7 Hawkesworth et al., (1993), 8 Jahn et al. (1980)). In Figure 2.9 the lead isotopic signature of Eskay Creek mineralization is plotted relative to fields defined by Alldrick et al. (1987) for Lower Jurassic and Tertiary mineralization in the Stewart area. These fields represent data for sulfides (mainly galena) associated with a variety of — - mm IF IMS uM_ I = Eskay Creek ] 31 Table 2. 3 Common lead data for sulfides and sulfosalts from the Eskay anticline. Sample1 DriUHoie Zone Mineral 2 "•Pb/^Pb "•Pb/^Pb "WPb "•Pb/^Pb Number Location (error)*-4 (error)'-4 (error)5-4 (error)3-4 (error)5-4 Ela surface Emma Adit gl 18.818 15.601 38.380 0.82905 2.0396 (0.003) (0.003) (0.008) (0.004) (0.009) E2a surface Emma Adit gl 18.824 15.611 38.396 0.82934 2.0398 (0.002) (0.002) (0.006) (0.004) (0.008) E2b surface Emma Adit py 18.834 15.618 38.409 0.82923 2.0394 (0.012) (0.009) (0.024) (0.009) (0.011) E3a surface Mackay Adit gi 18.840 15.629 38.436 0.82958 2.0402 (0.011) (0.009) (0.024) (0.012) (.0.019) E4a CA-215, 160.2m Pathfinder si 18.876 15.651 38.495 0.82919 2.0394 (0.015) (0.012) (0.032) (0.010) (0.013) E4b CA-215, 160.2m Pathfinder cp 18.819 15.606 38.383 0.82927 2.0396 (0.003) (0.003) (0.008) (0.005) (0.008) E5a CA-215, 156.6m Pathfinder gi 18.823 15.606 38.366 0.82911 2.0383 (0.002) (0.002) (0.007) (0.004) (0.008) E6a U-04, 45.8m 109 gi 18.779 15.565 38.264 0.82884 2.0376 (0.009) (0.008) (0.020) (0.012) (0.014) E6b U-04, 45.8m 109 gi 18.799 15.588 38.319 0.82920 2.0383 (0.030) (0.025) (0.061) (0.008) (0.014) Eoc U-04, 45.8m 109 sph 18.790 15.575 38.298 0.82891 2.0382 (0.009) (0.008) (0.020) (0.006) (0.011) E7a U-06, 7.5m 109 gl 18.811 15.596 38.354 0.82911 2.0390 (0.005) (0.004) (0.011) (0.005) (0.008) E7b U-06, 7.5m 109 gl 18.809 15.597 38.345 0.82923 2.0387 (0.041) (0.034) (0.084) (0.014) (0.018) E8a CA90-422, 52.1m HW gl 18.831 15.614 38.381 0.82919 2.0381 (0.012) (0.010) (0.024) (0.007) (0.011) ESb CA90-422, 52.1m HW py 18.821 15.602 38.346 0.82895 2.0374 (0.008) (0.006) (0.020) (0.022) (0.021) E9b U-56, 32.0m 21B sph 18.816 15.601 38.356 0.82913 2.0385 (0.026) (0.022) (0.055) (0.017) (0.012) E9c U-56, 32.0m 21B py 18.817 15.598 38.351 0.82895 2.0381 (0.005) (0.004) (0.013) (0.010) (0.014) E9d U-56, 32.0m 21B boul 18.816 15.606 38.368 0.82939 2.0391 (0.003) (0.002) (0.008) (0.009) (0.012) Ella surface FW Volcanic gl 18.833 15.623 38.430 0.82952 2.0405 (0.004) (0.003) (0.009) (0.004) (0.009) Ellb surface FW Volcanic gl 18.822 15.608 38.381 0.82922 2.0391 (0.004) (0.003) (0.010) (0.005) (0.009) E12a surface Mackay Adit gl 18.826 15.608 38.382 0.82908 2.0388 (0.003) (0.003) (0.009) (0.005) (0.009) E13a NEX95-5, 71m NEX gl 18.833 15.621 38.424 0.82943 2.0402 (0.018) (0.016) (0.018) (0.009) (0.005) 1 upper case letter and number refers to sample number, lower case letter refers to fraction number. 2 mineral abreviations: gl=galena, py=pyrite, cp=chalcopyrite, sph=sphalerite, boul=boulangerite. 3 errors are quoted at the 2§ (95% confidence) level. 4 values are corrected for instrument fractionation by normalization based on replicate analyses of the NBS-981 standard. 32 styles of mineralization from more than 50 locations within the Iskut River and Stewart areas; also included within these fields are lead isotopic analyses of potassium feldspar from dated intrusive rocks (Alldrick et al., 1987; Godwin et al., 1988; J.K. Mortensen, unpublished data). These data show an evolution of lead to more radiogenic values from the Lower Jurassic to the Tertiary in this portion of the Stikine terrane. Eskay Creek lead analyzed in this study has a restricted range of values and partially overlaps the most radiogenic portion of the Lower Jurassic field. Sulfides and sulfosalts from the different zones and styles of mineralization in and along strike from the Eskay Creek deposit exhibit a remarkably homogeneous lead isotopic signature (Fig. 2.9, insert). This signature is found over a broad area extending from the Eskay Creek deposit, 4.5 km south to the Mackay adit. 2.06 2.05 _Q 2.04 n CL. 2.03 OO o CSI 2.02 2.01 - 2.036 2.042 2.039 F N M E FEPE T B ft ~9 9B ? 9 H / avefoge t—^ error Jurassic Cluster 0.8287 0.8292 0.8297 Tertiary Cluster average error direction of error due to fractionation _L 0.810 0.814 0.818 0.822 0.826 207r-u^ /206r 0.830 0.832 'Pb/^Pb Figure 2. 9 207Pb/206Pb vs. 208Pb/206Pb diagram for sulfide and sulfosalt samples from the Eskay anticline. Shown for reference are Lower Jurassic and Tertiary fields for the Iskut River and Stewart areas of the Stikine terrane (Alldrick et al., 1987; Godwin et al., 1988; J.K. Mortensen, unpublished data), Upper Crustal and Orogene Growth Curves (Zartman and Haines, 1988), and feldspar from Upper Triassic (T) and Devonian QD) intrusive rocks in the Stikine terrane, calculated to 175 Ma, assuming u.=9.74 and K=3.78 0VI.L. Bevier and R.G. Anderson, pers. comms., 1994; Childe, this volume). (B = 21 B zone, E = Emma Adit, F = footwall volcanic unit, H = hangingwall zone, M = Mackay Adit, N = northeast extension zone, P = pathfinder zone, 9 = 109 zone). Note that more radiogenic samples plot closer to the origin on this diagram. 33 Discussion Regional Implications The Stikine terrane is a complex volcanic arc characterized by a wide range of styles and ages of mineralization. Within the Iskut River area, mineralization related to the Hazelton Group consists primarily of Lower Jurassic epithermal and mesothermal gold+silver deposits and porphyry copper-gold deposits. The Middle Jurassic Eskay Creek deposit formed from a sea floor exhalative system which produced well-bedded clastic gold- and silver-rich sulfides and sulfosalts within argillite and in proximity to rhyolite-hosted stockwork zones. This degree of precious metal enrichment within a volcanogenic massive sulfide system is not only unique within the Stikine terrane, but also within the known ancient geologic record. Prior to this study, the isotopic and biochronological age constraints on rocks within the Eskay anticline consisted of a U-Pb zircon age of 184 +5/-1 Ma for the Eskay porphyry (recalculated from Macdonald et al., 1992), Upper Pliensbachian ammonites, bivalves and corals from sedimentary rocks in the Betty Creek Formation, and Aalenian to possibly Early Bajocian radiolaria from the contact argillite in the immediate vicinity of the 21B zone (Nadaraju, 1993). The currently accepted lower stage boundary for the Aalenian (Harland et al., 1990) overlaps the isotopic age of the Eskay porphyry. Based on these data mineralization at Eskay Creek could be related to the Eskay porphyry, a felsic intrusive body coeval with the Texas Creek Plutonic and Premier Porphyry Suites, known mineralizing intrusive suites in the Iskut River and Stewart areas (Alldrick et al., 1986; Macdonald et al., 1996). However, the age of 175 +1-2 Ma determined for the Eskay rhyolite in this study demonstrates that mineralization at Eskay Creek formed in the Middle Jurassic, and is 7-16 Ma younger than the Eskay porphyry. Based on the ages of known mineral deposits in the Iskut River area, the period of time near the Lower Jurassic - Middle Jurassic boundary appears to mark a change in the metallogenic style of the Hazelton Group, from deeper, intrusion-related mineralization (approximately 197 to 184 Ma) to sea floor volcanogenic mineralization (approximately 177 to 173 Ma). 34 Chemical and isotopic characteristics of host rocks On the basis of neodymium isotopic studies of intrusive and volcanic rocks, Samson et al. (1989) suggested that rocks of the Hazelton Group are the products of juvenile, mantle-derived material which formed in a volcanic arc setting, probably in an intra-oceanic environment. Recent studies of modern arc environments suggest that extension, rather than compression may be the dominant tectonic regime at consuming plate margins (Smellie, 1995). Within the immediate Eskay Creek area, the interpretation of rhyolitic magmas erupting along linear fissure vent zones, control on the facies distribution of volcanic and sedimentary rocks by regional and local fault blocks, and the transition to deep marine sedimentary sequences following the cessation of volcanism have been suggested to indicate formation in either a back arc or intra-arc basin environment (Bartsch, 1993). The trace and REE chemistry, and Nd signature of the Eskay rhyolite and hangingwall basalt are consistent with formation in an island arc environment. The hangingwall basalt is characterized by a tholeiitic magmatic affinity. Both REE and Nd isotopic data indicate enrichment of the basalt relative to N-MORB, consistent with minor contamination from an E-MORB-type source. REE patterns and Zr/Y ratios indicate a tholeiitic magmatic affinity for the Eskay rhyolite, whereas coeval rhyolites on the east limb have transitional to calc-alkaline affinities. Regionally other rhyolites of the Salmon River Formation have more calc-alkaline affinities (A. Kaip, pers. comm., 1995). Rhyolites at Eskay Creek may be the products of fractional crystallization of mafic magma, partial melting of sialic crust, or some combination of these two sources. Barrett and Sherlock (1996) have noted that Nb/Zr ratios are inconsistent with derivation of the Eskay rhyolite entirely as a product of fractional crystallization of the hangingwall basalt, indicating that this unit may have some component of partial melt. Zircon inheritance in rhyolite from the east limb implies some component of crustal contamination within this unit. A more evolved magmatic affinity and Nd isotopic signature for the east limb rhyolite would suggest that this rhyolite either assimilated a greater component of partial melt, or slightly less juvenile sialic crust, than the Eskay rhyolite. 35 Metal source Presently there is no clear consensus on the principal source of metals within ancient and modern submarine VMS deposits. The metals within these deposits may be leached from underlying volcanic and sedimentary strata, derived from magmatic sources, or some combination of these two processes (for example Doe and Zartman, 1979; Lydon, 1988; Stanton, 1990; Large, 1992, de Ronde, 1995; Huston et al., 1995). A review by Lydon (1988) concludes that a leaching model for the derivation of metals in VMS deposits is supported by experimental and computer-based studies, observation of sea floor basalts, and similarities of base metal ratios of mineralization with those of the dominant host lithologies. Stable and radiogenic isotopic studies have been cited as evidence for derivation of metals from both the leaching of strata below the ore deposit (for example Thorpe, et al., 1981; Fehn et al., 1983; Lydon, 1988) and , to a lesser extent, magmatic fluids (de Ronde, 1995 and references therein). Stanton (1990) argued for significant magmatic input for metals and fluids to VMS deposits, citing as evidence: (1) the concentration of mineralized zones at specific horizons, as opposed to throughout a volcanic cycle; (2) the apparent discrepancy between the generally smaller size of basalt-hosted VMS deposits relative to larger andesite- to rhyolite-hosted VMS deposits (given the abundance of sea floor basalts); and (3) variations in metal ratios in VMS orebodies relative to potential source rocks. Differences in the Se/S ratios of pyrite in Cu-rich and Cu-poor mineralization are consistent with a component of the sulfur in VMS deposits being derived from a magmatic hydrothermal sources (Huston et al., 1995). Lead isotopic data, in combination with other information, may be used to constrain the source of lead, and by analogy other metals in VMS deposits (Gulson, 1986). In describing a model for the generation of ore fluids by leaching of metals from oceanic crust, Lydon (1988) suggested that the ore components to the fluids are not derived from the immediate footwall to mineralization, but rather from depths of 0.5 to 3 km below the paleo-sea floor, well below the present level of exposure in the Eskay anticline. Based on known stratigraphy in the Iskut River area, the basement to the Eskay Creek deposit at these depths may be composed of volcanic and sedimentary rocks of the Upper Triassic Stuhini Group and Paleozoic Stikine Assemblage, and their intrusive equivalents. Lead isotopic analysis of potassium feldspar from Devono-Mississippian and Upper Triassic intrusive rocks from the Iskut River area, when calculated 36 forward to the Middle Jurassic, show systematically less radiogenic lead isotopic compositions than Eskay Creek sulfides (M. L. Bevier and R. G. Anderson, pers. comms., 1994; Childe, this volume) (Fig. 2.9). These data indicate that leaching of lead from such sources would not result in the isotopic signature documented for Eskay Creek sulfides. Eskay Creek sulfides partially overlap the cluster for Lower Jurassic intrusions and mineralization in the Iskut River area and partially lie in a region that is more primitive, indicating derivation from either a Lower or Middle Jurassic source. Lead leached from Lower Jurassic intrusions of the Texas Creek Plutonic and Premier Porphyry Suites, such as the Eskay porphyry could contribute lead consistent with the observed isotopic signature of Eskay Creek sulfides. However, for intrusions of this age to be a principal source of metals within the deposit they would need to have been significantly more extensive at depth than their exposure in the Eskay anticline may suggest. Alternatively, lead derived from a Middle Jurassic magmatic source could also produce the lead isotopic signature of Eskay Creek sulfides. A potential source of Middle Jurassic magmatic fluids is the Eskay rhyolite. Without a better knowledge of the basement to the Eskay Creek deposit at the depths at which leaching is believed to take place, and a clearer understanding of the sources of metals to VMS deposits, it is not possible to determine which of these latter two potential reservoirs is the principal source of lead and other metals in the Eskay Creek deposit. Modern Analogues Studies of modern sea floor hydrothermal vent fields document the development of sulfide mounds and chimneys overlying stockwork zones hosted within altered volcanic rocks (for example Hannington et al., 1986; Herzig et al., 1993; and Gemmell, 1995). Mineralization at Eskay Creek may have formed in a similar manner, with degradation of accumulations of seafloor sulfides producing the graded sulfide and sulfosalt beds preserved within the deposit. Recently discovered exhalative, gold-rich polymetallic sulfide mineralization within back-arc basins of the western Pacific, such as the Okinawa and Mariana Troughs, and the Lau, Manus, and North Fiji Basins, may be representative of the tectonic settings in which some ancient VMS deposits formed (Herzig et al., 1993; Gemmell, 1995). Herzig et al. (1993) have documented the presence of gold-rich (up to 29 ppm) sulfide chimneys, at water depths of 1600 to 2000 meters, hosted within a sequence of basalt to 37 rhyodacite on the Valu Fa Ridge in the southern Lau Basin. This region represents an active back arc spreading center built on remnant arc crust behind the Tonga-Kermadec subduction zone. The gross sulfide mineralogy, presence of iron-poor sphalerite and native gold in chimney samples, and a tholeiitic magmatic affinity of basalt and dacite from the Valu Fa Ridge have striking similarities to the Eskay Creek deposit (Sunkel, 1990; Herzig et al., 1993). Basalt and basaltic andesite from the Lau Basin show a wide range of 6N<I values (+2.6 to +9.2), which have been interpreted to reflect multiple subduction components within a complex tectonic setting; SNd values for basalts from the Valu Fa Ridge (+7.7 to +8.1) are some of the most primitive within the basin and are comparable to the hangingwall basalt at Eskay Creek (Volpe et al., 1988; Looke et al., 1990). The PACMANUS deposit, located in the eastern Manus Basin, lies within a back arc basin built on remnant arc crust behind the New Britain arc (Gemmell, 1995). Within this deposit sulfides rich in gold and silver (averaging 15 and 230 ppm, respectively) are hosted by a volcanic sequence composed of andesite, dacite and rhyodacite between 1650 and 1680 meters water depth on the flanks of a dacite dome (Binns and Scott, 1993; Binns et al., 1995). Two samples of dacite (fresh and altered) dredged from the dacite dome exhibit a tholeiitic to transitional magmatic affinity based on Zr/Y ratios, and tholeiitic REE patterns intermediate in slope between those of the Eskay rhyolite and hangingwall basalt (Binns and Scott, 1993). The mineralogy, gold concentrations, and spatial association with primitive felsic volcanic rocks of the PACMANUS and Valu Fa Ridge deposits, which are floored by older back-arc crust with no component of continental basement, have similarities to the Eskay Creek deposit and may represent modern equivalents. Conclusions Rhyolite which hosts stockwork mineralization and underlies clastic sulfide-sulfosalt mineralization at the Eskay Creek Au-Ag-rich VMS deposit erupted at 175 +1-2 Ma. This age is 7-16 Ma younger than mineralization associated with the Lower Jurassic Texas Creek Plutonic and Premier Porphyry Suites. A tholeiitic magmatic affinity and relatively primitive Nd signature for volcanic rocks within the footwall and hangingwall to mineralization at Eskay Creek suggest 38 that the deposit formed in an island-arc setting. The Eskay rhyolite, which hosts stockwork mineralization and underlies stratiform mineralization, is characterized by a more primitive magmatic affinity than contemporaneous rhyolites within the Iskut River area. Sulfides and sulfosalts from argillite-hosted stratiform mineralization and rhyolite-hosted discordant mineralization in and along strike from the Eskay Creek deposit are characterized by a homogeneous lead isotopic signature. This signature is consistent with derivation of lead, and presumably other metals, from either a Lower or Middle Jurassic source. 39 Plate 2. 2 Black matrix breccia, showing a brecciated, flow-banded rhyolite fragment in a black silicified matrix. Plate 2. 3 Pillow basalts in the hangingwall to mineralization at Eskay Creek (hammer for scale). Plate 2. 4 Flow-banded east limb rhyolite with aphanitic matrix and quartz and feldspar phenocrysts. Plate 2. 5 2 IB zone argillite-hosted clastic sulfide and sulfosalt mineralizaUon (hammer for scale). Plate 2. 6 SEM backscatter image of feldspar inclusions in zircon, Eskay rhyolite (EC-GC-11) (scale bar 10 microns). 40 ^zircon / feldspar 41 References Alldrick, D.J., 1991, Geology and ore deposits of the Stewart Mining Camp, British Columbia: Unpublished Ph.D. thesis, Vancouver, Canada, The University of British Columbia, 347 p. Alldrick, D.J., Mortensen, J.K., and Armstrong, R.L., 1986, Uranium-Lead age determinations in the Stewart area (104B/1), in Grant, B., and Newell, J.M., eds., Geological Fieldwork 1985, British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1986-1, p. 217-218. 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Theriault, R., J., 1990, Methods for Rb-Sr and Sm-Nd isotopic analyses at the geochronology laboratory, Geological Survey of Canada, in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 89-2, p. 3-6. 48 Thorpe, R. I., Franklin, J. M., and Sangster, D. F., 1981, Evolution of lead in massive sulfide ores of Bathurst District, New Brunswick, Canada: Institute of Mining and Metallurgy Transactions, Section B, p. B55-B56. Volpe, A.M., Macdougall, J.D., and Hawkins, J.W., 1988, Lau Basin basalts (lbb): trace element and Sr-Nd isotopic evidence for heterogeneity in backarc basin mantle: Earth and Planetary Science Letters, v.90, p. 174-186. York, D. 1969, Least-squares fit of a straight line with correlated errors: Earth and Planetary Science Letters, v. 5, p. 320-324. Zartman, R.E., and Haines, S.M., 1988, The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs - a case for bi-directional transport: Geochemica and Cosmochemica Acta, v.52, p. 1327-1339. 49 CHAPTER 3: U-PB GEOCHRONOLOGY, ND AND PB ISOTOPIC SYSTEMATICS, AND GEOCHEMISTRY OF THE UPPER TRIASSIC GRANDUC VMS DEPOSIT, NORTHWESTERN BRITISH COLUMBIA 50 Introduction The Granduc deposit is located 40 km northwest of Stewart, British Columbia, within the rugged Boundary Ranges of the Coast Mountains (Fig. 3.1 and Plate 3.2). Copper mineralization was first discovered and staked in the Granduc area in 1931. These claims lapsed and it was not until 1951, when the area was re-examined, that the level of the glaciers flanking Granduc Mountain had dropped sufficiently for the Granduc main ore zone to become exposed on surface (Dudas and Grove 1970). An underground mine was in production at Granduc from 1971 until 1978, and again from 1980 until 1984, when low metal prices forced final closure of the mine. Total production was 15.2 million tonnes grading 1.3% Cu, with a total of 124,000 kg Ag and 2,000 kg Au recovered (B.C. MINFILE 104B-021). Current published reserves indicate 9.9 million tonnes grading 1.79% copper, with minor gold and silver (Melville et al. 1992). Figure 3. 1 Location map of the Granduc deposit, Stewart Mining Camp, northwestern British Columbia. The Granduc deposit was initially classified as a mesothermal replacement type deposit (Davidson 1960). However, more recently Granduc has been interpreted to have a syngenetic, 51 volcanic-associated origin. The deposit has been classified as a cupriferous iron formation (Kirkham 1979), a concordant massive sulphide deposit (Grove 1986) and most recently a Besshi-type volcanogenic massive sulphide (VMS) deposit (Hoy 1991). Re-classification was based on the recognition that copper-rich massive sulphide-oxide mineralization occurred within a deformed, but broadly conformable zone of sedimentary and volcanic rocks structurally overlying a thick sequence of mafic volcanic flow rocks and associated pyroclastic rocks (Grove 1986). Prior to this study, the age of mineralization at Granduc was poorly constrained. It was not possible to correlate rocks which host the deposit with rocks off of Granduc Mountain, and sedimentary rocks in the area of mineralization lack preserved fossils. Previously the deposit has been assigned alternatively to the Upper Triassic Stuhini Group (Norman and McCue 1966; Hoy 1991), and to the Lower to Middle Jurassic Hazelton Group (Dudas 1973; Grove 1986; Anderson and Thorkelson 1990; Lewis 1992) on the basis of regional correlations and lithological similarities. Grove (1986) assigned a Lower Jurassic age to the Granduc stratigraphy based in part on the presence of Pliensbachian fossils contained within a volcanic-sedimentary sequence from an unspecified locality near Granduc Mountain. In this study, volcanic and intrusive rocks from Granduc Mountain were dated by U-Pb methods, and analyzed for their Nd isotopic signatures and major, trace and rare earth element compositions. The Pb isotopic compositions of mineralization from the Granduc deposit were also determined. New U-Pb age determinations on volcanic and intrusive rocks in the ore zone and footwall of the deposit, and the Pb isotopic composition of stratabound mineralization provide constraints on the age of VMS mineralization at the Granduc deposit, whereas the Nd isotopic signatures and lithogeochemical analyses constrain the tectonic environment in which the deposit formed. Regional Geology The Granduc Deposit occurs within the Stikine island arc terrane (hereafter referred to as Stikinia) along the western margin of the Intermontane Belt and is included within the Stewart Mining Camp in the southeast corner of the Iskut River map area (NTS 104 B) (Fig. 3.1). Regional mapping within the Iskut River map area has identified three Paleozoic to Mesozoic 52 volcanic-plutonic-sedimentary island arc assemblages. From oldest to youngest these are the Paleozoic Stikine Assemblage (Monger 1977), the Upper Triassic Stuhini Group (Souther 1971; Monger 1980) and the Lower to Middle Jurassic Hazelton Group (Leach 1909; Marsden and Thorkelson 1992). The Stikine Terrane is unconformably overlain by the Middle to Upper Jurassic Bowser Lake Group, an assemblage which overlaps the adjacent Cache Creek terrane (Monger and Berg 1984), and is intruded along its western margin by rocks of the Cretaceous to Tertiary Coast Plutonic Complex. The distribution of these units within the Iskut River map area is outlined by Anderson (1989). The Paleozoic Stikine Assemblage is the oldest succession of rocks exposed within Stikinia, and consists of Devonian to Permian volcanic and sedimentary island arc successions (Monger 1977). Stikine Assemblage rocks have not been identified as far south as Granduc Mountain in the Iskut River map area. Rocks of the Stikine Assemblage are unconformably overlain by the Upper Triassic Stuhini Group (Kerr 1948). The Stuhini Group is characterized by thick sequences of Carnian to Norian augite-phyric mafic volcanic pillow lavas, flows and volcaniclastic rocks intercalated with sedimentary sequences that include limestone reef sequences, greywacke, shale, chert and volcanic-derived sedimentary rocks. Local occurrences of bimodal volcanic rocks correlated with the Stuhini Group have been mapped in the northwest portion of the Iskut River map area (Anderson, 1989). Stuhini Group volcanism is contemporaneous with emplacement of the Stikine Plutonic Suite which has a compositional range of gabbro to diorite to monzonite (Woodsworth et al. 1992) . The volumetrically most significant member of the Stikine Plutonic Suite is the Hotailuh Batholith, a 1100 km2 composite intrusion located on the eastern margin of the Stikine Terrane, 200 km northeast of the Granduc deposit (Anderson and Bevier 1992). Mafic volcanic and sedimentary sequences of the Stuhini Group are unconformably overlain by mafic to felsic volcanic and related sedimentary rocks of the Lower to Middle Jurassic Hazelton Group, and cotemporal granodioritic to monzodioritic composition intrusions (Marsden and Thorkelson 1992; Macdonald et al 1996). The Hazelton Group is overlain by marine sedimentary rocks of the Bowser Lake Group. The onset of Bowser sedimentation marks a 53 change in the tectonic regime from an island arc setting producing rhyolitic to basaltic composition volcanic rocks, to the development of a passive basinal sequence characterized by deposition of shale, siltstone and conglomerate. Prospecting in the Stewart Mining Camp has been active since the early 1900s and has led to the discovery of a large number of mineral deposits, including gold-silver-copper-zinc-lead Lower Jurassic mineralization at the Scottie Gold, Big Missouri and Premier-Silbak deposits (Alldrick 1991; Brown 1987); as well as Eocene silver-zinc-lead±molybdenum mineralization in the Prosperity/Porter Idaho camp (Alldrick 1991). Directly to the north of the Stewart Mining Camp is the Iskut River area, which contains a mesothermal gold deposit at Snip (Rhys 1993), complex intrusion-related epithermal gold-silver+base mineralization at Sulphurets (Margolis 1993), and a Au-Ag-rich VMS deposit at Eskay Creek, all of which formed in the Early to Middle Jurassic (Margolis 1993; Macdonald et al. 1996; Childe this volume) (Fig. 3.1). The presence of numerous deposits of widely varying genesis related to Jurassic magrhatism has made Hazelton Group stratigraphy and cotemporal plutons a prime focus for mineral exploration. The present study investigates the possibility that economic mineral occurrences also formed in this region in Late Triassic time. Geology of Granduc Mountain Rocks on Granduc Mountain have been mapped by Davidson (1960), Klepacki and Read (1981), and McGuigan et al. (1992). These rocks can be divided into four main units; from east to west these are the Footwall, Mine, Lower Hangingwall and Upper Hangingwall Series (Fig. 3.2). The Mine Series and Lower Hangingwall Series are separated by the Granduc Fault; the Lower and Upper Hangingwall Series are separated by the Western Fault (Fig. 3.2 and Plate 3.3). The majority of the mined ore at Granduc came from the Main Zone of the deposit, with minor amounts from the North Zone (Grove 1986) (Fig. 3.2). Mineralization within the Main Zone of the Mine Series occurred within six zones, termed the A to F orebodies (Grove 1986) (Fig. 3.3). 54 NORTHLEDUC JURASSIC HAZELTON GROUP |v v ] mafic to fdsic lv v I tuffaceous rocks JURASSIC OR TRIASSIC HANGING WALL SERIES UPPER argiuiic, wackc, tuffaceous rocks & limestone |p3 LOWER distal turbiditic & pelagic sedimentary rocks, mafic volcanic rocks and minor carbonate UPPER TRIASSIC STUHINI GROUP MINE SERIES •semi-massive sulphides hosted in argillite, chert, & magnetite iron formation, with minor limestone and mafic volcanic rocks FOOTWALL SERIES massive basalt to basaltic andesite flow & tuffaceous rocks INTRUSIVE ROCKS — / Granduc Mountain Intrusions I ^ (diorite to quartz-dioritc) FAULT (defined) FAU LT (approximate) SURFACE DRILL HOLE SURFACE SAMPLE MINE WORKINGS CONTOUR INTERVAL: 1000 ft. GD-GC-081 Figure 3. 2 Generalized geology of Granduc Mountain (after Davidson 1960; Klepacki and Read 1981; McGuigan etal. 1992). 55 Footwall Series The Footwall Series consists of mafic volcanic flow rocks of basaltic to andesitic composition with sparse quartz + epidote amygdules and associated pyroclastic deposits. These rocks are intruded by coarse-grained, variably deformed sills of dioritic to quartz dioritic composition. Regional metamorphism reaches greenschist facies, with the development of metamorphic biotite and hornblende (Grove 1986). The Footwall Series are well exposed on the steep northern face of Granduc Mountain where they have been mapped by Davidson (1960); exposure across the top and the more gently sloping south face of the mountain is poor. However, a number of stratigraphic drill holes collared in the Hangingwall Series and drilled through the North Zone into the Footwall Series were archived and available for examination as part of this study. Rocks from the Footwall Series examined in this study do not show evidence of strong hydrothermal alteration and contain only minor pyrite+/-pyrrhotite+/-chalcopyrite veinlets, which are interpreted here to represent stockwork mineralization (Plate 3.4). Deformation through the Footwall Series varies from strong to weak. Localized zones of brecciation within thick sequences of massive fine-grained basaltic composition rock are interpreted to represent primary flow-tops. The variability of strain is particularly evident within thick intersections (up to 110m apparent thickness) of the diorite sill, which shows textural variations ranging from undeformed equigranular zones to moderately sheared fabrics to sharply defined 0.5 to 2 cm thick mylonitic zones (Plate 3.5). In general, the degree of deformation increases uphole, towards the Mine Series and the Granduc and Western Faults. Intrusive contacts between the sills and volcanic rocks are typically sheared, diffuse, and chloritic over several 10s of centimeters. These contact zones also contain minor magnetite and up to a few percent disseminated sulphide, primarily pyrite. Mafic volcanic rocks of the Footwall Series are faulted along their southeastern margin against mafic to felsic tuffaceous and fragmental rocks (Fig. 3.2). Mine Series The Granduc Mine Series consists of semi-massive cupriferous iron sulphide lenses interbedded with argillite, chert, magnetite iron formation, limestone, tuffaceous rocks, and mafic 56 flows or sills within a cataclasite zone (Plate 3.6). The orebodies, with a maximum width of 60 m, strike approximately north-south and dip 60° to the west (Figs. 3.2 and 3.3) (Norman and McCue 1966). Sulphide mineralization consists of pyrite, pyrrhotite, chalcopyrite, sphalerite and galena, in order of decreasing abundance. Trace arsenopyrite, bornite and cobalite have also been reported from within the ore zones (Dudas and Grove 1970; Grove 1986). Primary sulphide-argillite bedding, indicative of subaqueous deposition, has been locally preserved despite large-scale folding and attenuation of the ore lenses (Kirkham 1979). However, the majority of the massive sulphide within the Granduc deposit has been remobilized and recrystallized (Kirkham 1979; Grove 1986). The presence of mafic rocks within the Mine Series suggests that mafic magmatism was occurring during sulphide accumulation. Figure 3. 3 Plan view of the 3100 (950 m) level of the Main Zone of the Granduc mine, showing relative positions of the A, B, C, and F orebodies (after Grove, 1986). Hangingwall Series The Lower Hangingwall Series, separated from the Mine Series by the Granduc Fault, has been coined the 'gash-banded tuff sequence (McGuigan et al. 1992). It is composed primarily of distal turbiditic rocks, pelagic sedimentary rocks and possible fine-grained tuffaceous rocks, together with minor carbonate which often infills brittle fractures in these rocks. Fieldwork conducted as part of this study revealed the presence of local augite-phyric and plagioclase-57 augite-phyric mafic volcanic flows or sills within this package (Fig. 3.2; samples GD-94-03, -04). The Upper Hangingwall Series, to the west of the Western Fault, is composed of thinly bedded brown to green siltstone and argillite, tuffaceous rocks of andesite composition and minor chert and limestone. The Hangingwall Series does not appear to be related to the Mine and Footwall Series. Deformation Two phases of regional deformation have been recorded in the Stewart area, the first occurring in the mid-Cretaceous (thermal peak =110 +/-5 Ma) and the second in the Eocene (thermal peak = 54.8-44.8 Ma) (Alldrick 1991). Complex multi-stage deformation within the Iskut River map area has tectonically juxtaposed lithologically similar volcano-sedimentary sequences of the Triassic Stuhini Group and the Jurassic Hazelton Group, making the stratigraphic relations difficult to determine through field relationships alone. Deformation on Granduc Mountain has been interpreted by Klepacki and Read (1981) to be the result of up to four phases of folding. However, a more recent interpretation by Lewis (1994) attributes deformation to a single event which is correlated with deformation along the Unuk River/Harrymel fault zone (South Unuk cataclasite zone in the terminology of Grove 1986). This fault zone is a north-south trending subvertical shear zone with sinistral displacement, characterized by variable strain gradients over a 60 km mappable extent, from the Iskut River in the north to the Granduc and Western Faults on Granduc Mountain in the south (Lewis 1994). South of Granduc Mountain the fault zone is abruptly truncated by intrusion of granodioritic composition rocks of the Coast Plutonic Complex. U-Pb Geochronology The U-Pb results reported in this paper are based on analysis of zircons recovered from 20-30 kilogram samples collected from drill core or outcrop; sample descriptions are given below. Heavy mineral extraction procedures and U-Pb analytical procedures follow those of Mortensen et al. (1995). All zircon fractions were abraded prior to analysis (Krogh 1982). Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer 58 equipped with a Daly photomultiplier. Procedural blanks were 9-20 picograms for Pb and 1-2 picograms for U. Concordia intercept ages and associated errors were calculated using a modified York-II regression model (York 1969), and the algorithm of Ludwig (1980); ages were calculated using the decay constants recommended by Steiger and Jager (1977). Age assignments follow the time scale of Harland et al. (1990). Analytical results are given in Table 3.1 and shown graphically in Figure 3.4. Footwall Series quartz diorite sill Sample GD-GC-01 was collected near the base of a 110 meter (drilling thickness) intersection of medium- to coarse-grained quartz dioritic composition sill which intrudes the Footwall Series in the North Zone of the Granduc deposit (158-2a, 630.9-641.0 m). Abundant prismatic to equant (l:w = 3:1 to 1:1) zircon with excellent clarity and minor bubble- and rod-shaped colourless inclusions were recovered (Plate 3.7). Six fractions were analyzed; fraction E was concordant at 222.2 Ma, whereas all other fractions were slightly discordant, probably as a OA7 Ofift result of post-crystallization lead-loss (Fig. 3.4a). The Pb/ Pb ages of the discordant fractions ranged from 221.1 to 227.5 Ma. Because of the restricted range of 207Pb/206Pb ages and the lack of visible cores within zircon from this rock, the weighted mean Pb/ Pb age of 223 +1-3 Ma is considered to be the best estimate of the age of crystallization of this unit. Footwall Series basalt flow Sample GD-GC-04 was collected from the Footwall Series within the North Zone of the deposit (Hole 158-2a, 695.8-709.6 m). The sample consisted of a single dark green chlorite-altered basaltic composition flow, with poorly preserved metamorphic hornblende porphyroblasts, sparse quartz + epidote-filled vesicles and 2-3 % disseminated pyrite. Heavy mineral separates yielded a small quantity of equant multi-faceted zircon with few inclusions and high clarity (Plate 3.8). All zircons recovered from the rock were analyzed, and fractions were slightly discordant, probably as a result of post-crystallization lead-loss; the 207Pb/206Pb ages ranged from 221.4 to 224.3 Ma (Fig. 3.4b). Because of the restricted range of Pb/ Pb ages and the lack of visible OAT OArfC cores within zircon from this rock, the weighted mean Pb/ Pb age of 223 +/-5 Ma is considered to be the best estimate of the age of this unit. 59 Table 3. 1 U-Pb analytical data for samples from Granduc Mountain. Fraction1 Wt U Pb2 nig ppm ppm jwpb3 Pb4 ""Pb5 Isotopic ratiosfilcJ.tt)6 Isotopic dates(Ma,±2o)< *"Pb Pg % JMpk/USU Wpb/nsj, ""Pb/^Pb M(iPb/J3!U ^Pb/BSU Knpb/206pb Footwall Series quartz diorite sill CGD-GC-01: DDH 158-2a. 630.9-641.0 m) Ajn.Nl,p 0.030 888 30.0 1618 24 7.0 0.03475±0.14 0.24231*0.30 0.05057*0.22 220.2*0.6 220.3*1.2 221.3*9.9 amNl4> 0.083 1798 60.9 20460 15 8.2 0.03434*0.09 0.2396*0.10 0.05061*0.03 217.7*0.4 218.1*0.4 222.9jtl.4 C,m,Nl,p 0J50 852 28.8 11660 54 9.0 0.03403*0.11 0.2379*0.19 0.05071*0.10 215.7*0.5 216.7*0.8 227.5*4.6 D,cNl.p 0.155 422 14.2 2201 64 8.2 0.03413*0.12 0.2384*0.25 0.05066*0.17 216.3*0.5 217.1*1.0 225.3*7.7 E>,Nl,p 0.077 367 12.7 3989 14 9.3 0.03488*0.11 0.2433*0.22 0.05059*0.13 221.0*0.5 221.1*0.9 222.2*6.2 F,m,MI,p 0.170 652 22.3 7208 32 9.5 0.03434*0.16 0.2394*0.23 0.05057*0.12 217.7*0.7 218.0*0.9 221.1*5.3 Footwall Series basalt flow (GD-GC-04: DDH 158-2a, 695.8-709.6 m) AXNl.eq 0.015 322 10.6 293 34 5.5 0.03460*0.19 0.2414*0.84 0.05061*0.72 219.3*0.8 219.6*3.3 223.0*33.1 BXNl.eq 0.032 485 16.8 564 55 10.6 0.03419*0.14 0.2387*0.44 0.05064*0.34 216.7*0.6 217.4*1.7 224.3*15.7 DXNl.cq 0.014 1036 36.5 2230 16 11.4 0.03450*0.12 0.2409*0.29 0.05057*0.22 218.9*0.5 219.1*1.1 221.4*10.2 E£Nl,eq 0.020 1583 55.6 2144 32 11.7 0.03432*0.10 0.2395*0.24 0.05061*0.15 217.5*0.4 218.0*0.9 223.0*7.1 Mine Series basaltic andesite (GD-GC -08: surface) Ajn.Nlji 0.128 127 4.3 1763 20 6.6 0.03515*0.11 0.2452*0.29 0.05059*0.22 222.7*0.5 222.7*1.2 222.4*10.1 B,m,Nl.s 0.077 139 4.7 1245 19 6.8 0.03503*0.13 0.2444*0.34 0.05061*0.28 221.9*0.6 222.0*1.4 223.1*12.9 CXNU 0.031 131 4.5 561 16 7.7 0.03506*0.13 0.2446*0.57 0.05059*0.50 222.2*0.6 222.2*2.3 222.3*23.1 Felsic lapilli tuffCGD-GC-02 surface) A,c,Nljnf 0.127 163 4.6 1828 19 7.2 0.02921*0.11 0.2005*0.27 0.04980*0.20 185.6*0.4 185.6*0.9 185.4*9.1 BroNljuf 0.143 483 13.7 -6863 18 8.6 0.02864*0.11 0.1974*0.13 0.05001*0.06 182.0*0.4 183.0*0.4 195.3*2.7 C£Nl,p 0.081 193 5.4 2629 10 9.4 0.02828*0.10 0.1942*0.24 0.04981*0.16 179.8*0.3 180.2*0.8 186.3*7.5 'All fractions are air abraded; Grain size, smallest dimension: c= +134nm, m=-134nm+74|j.m, f=-74nm; Magnetic codes Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8 A; Front slope for all fractions=20c; Grain character codes: eq=equant, p=prismatic, mf=mulu-faceted, s=subhedral ^diogenic Pb 'Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ±20% (Daly collector) 4Total common Pb in analysis based on blank isotopic composition sRadiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers model Pb composition at the 207Pb/206Pb date of fraction, or age of sample) 60 (a) 0.034 Footwall Series basalt GD-GC-04 210 0.22 0.26 0.22 207 235 Pb/ U 0.24 207 235 Pb/ U (c) Mine Series basaltic andesite GD-GC-08 CM 0.034 0.032 0.030 felsic lapilli tuff SE flank of Granduc Mtn. GD-GC-02 223 +/-S Ma 0.26 185+/-4Ma 0.22 0.26 0.18 0.20 0.21 207 235 207 235 Pb/ U Pb/ U Figure 3.4 U-Pb concordia diagrams for a) Granduc Footwall Series quartz diorite; b) Granduc Footwall Series basalt; c) Granduc Mine Series basaltic andesite; and d) felsic lapilli tuff on the southeast flank of Granduc Mountain. Mine Series basaltic andesite An intensely sheared rock of basaltic andesitic composition from the Granduc Mine Series was collected on surface (sample GD-GC-08) from within the Glory Hole area (Fig. 3.2). Primary textures were not sufficiently preserved to determine if this unit represents a flow, tuff or sill. A small quantity of ovoid zircons (1: w = 3:1 to 2:1) with resorption pits and no remnant crystal faces were recovered (Plate 3.9). All the zircon from this rock were analyzed in three fractions, all three fractions were concordant, with 207Pb/206Pb ages of 222.3 to 223.1 Ma (Fig. 3.4c). An age of 222 +/-1 Ma, encompassing the ^^b/^'U ages and associated errors of the three concordant fractions is considered the best estimate of the age of this unit. Dacite fragmental, southeast of Granduc Mountain Felsic tuffaceous and fragmental rocks occur on the southeast flank of Granduc Mountain, as well as north and south of the Leduc Glaciers (Fig. 3.2 and Plate 3.10). On the southeast flank of Granduc Mountain these rocks, which are in fault contact with mafic volcanic rocks of the Footwall Series, consist of mafic to felsic debris flows and felsic lapilli to ash tuffs. Two samples of dacitic composition volcaniclastic rocks were collected as part of this study (GD-GC-02 and -03); sample GD-GC-02, from a zone of deformed, variably altered felsic lapilli tuff was dated by U-Pb methods. Three fractions were analyzed, one fraction was concordant at 185.4 Ma, whereas the other two were slightly discordant, with 207Pb/206Pb ages of 186.3 and 195.3 Ma (Fig. 3.4d). An age of 185 +/-4 Ma, based on the weighted mean Pb/ Pb age and error of all fractions is considered the best estimate of the age of this unit. Lithogeochemistry A suite of samples from drill core and surface were analyzed by X-ray fluorescence at McGill University using glass beads for major elements and pressed pellets for trace elements (Table 3.2). A subset of these samples were analyzed for rare earth element (REE) concentrations by instrumental neutron activation at Actlabs (Table 3.3). The purpose of this portion of the study was to determine the compositional range and tectonic affinity of volcanic and intrusive rocks on Granduc Mountain. 62 Table 3.2 Major and trace element chemistry for the Granduc deposit. Sample LITHOLOGY Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total % % % % % % % % % % GD-GC-01 quartz-diorite sill/dyke 57.78 0.76 16.69 5.03 0.20 1.98 4.14 5.88 2.88 0.34 4.13 99.92 GD-GC-02 dacitic tuff 63.81 0.41 13.91 4.31 0.12 1.45 4.24 4.25 2.29 0.17 4.80 99.87 GD-GC-03 dacitic tuff 64.80 0.38 16.68 3.26 0.09 0.37 5.16 3.95 1.48 0.13 3.34 99.74 GD-GC-04 basalt 48.82 0.64 11.32 10.75 0.19 13.06 10.43 1.38 1.37 0.33 2.15 100.63 GD-GC-05 feldspar porphyry 72.14 0.45 8.06 4.84 0.08 1.87 4.46 1.21 1.90 0.20 4.17 99.73 GD-GC-06 basalt 45.50 0.59 10.94 11.53 0.24 15.81 11.21 1.04 1.86 0.16 2.16 101.27 GD-GC-07 tuffaceous chert 89.55 0.19 3.54 2.86 0.00 0.28 0.00 0.08 1.68 0.03 1.50 99.71 GD-GC-08 basaltic-andesite 52.40 0.84 17.55 7.70 0.15 3.00 7.05 3.36 1.97 0.38 6.17 100.57 GD-94-2 basalt 47.66 0.59 12.56 14.78 0.17 9.31 7.65 1.97 3.30 0.20 1.87 100.06 GD-94-3 plag. porphyritic basalt 49.43 1.15 17.36 10.34 0.17 3.26 8.89 4.10 0.69 0.23 4.83 100.45 GD-94-4 plag+aug-porphjiitic basalt 47.69 1.03 17.28 9.77 0.17 6.49 9.66 3.50 0.78 0.26 3.65 100.28 146-3, 5.5 m basalt 47.02 0.86 14.30 12.30 0.18 9.58 8.62 1.85 1.97 0.29 3.35 100.47 146-3, 56.7 m basalt 44.20 0.62 11.22 9.99 0.19 9.67 9.68 0.27 4.97 - 0.20 8.56 99.77 146-3, 102.4 m basalt 47.49 0.83 14.09 12.66 0.16 6.22 6.07 1.28 5.92 0.23 4.89 100.08 146-3, 145.1 m altered zone 54.67 1.52 21.92 3.35 0.17 0.41 5.47 2.85 6.56 0.03 2.38 99.67 146-3, 203.3 m quartz-diorite sill/dyke 58.99 0.70 18.04 2.93 0.12 1.62 4.86 6.51 2.19 0.41 3.08 99.51 146-3, 275.2 m basalt 47.00 0.71 13.13 9.43 0.29 9.44 9.01 2.53 2.83 0.28 4.90 99.75 146-3, 303.3 m chert>' tuff 50.62 0.91 15.38 10.95 0.15 7.07 8.15 3.43 2.06 0.27 1.22 100.34 158-1, 587.1 m feldspathic tuff 48.14 0.92 17.66 8.60 0.16 4.17 7.42 3.58 1.69 0.25 7.87 100.54 158-1, 634.0 m cherty tuff 88.00 0.24 4.40 1.61 0.02 1.50 i 1.21 0.89 0.96 0.09 1.29 100.27 158-1, 654.4 m basalt 48.50 0.74 11.66 11.54 0.21 11.65 ' 11.74 1.67 0.85 0.27 2.13 101.16 158-1, 665.4 m basalt 43.08 0.71 13.40 11.73 0.22 7.70 9.47 2.19 3.30 0.23 6.90 99.32 158-1, 670.3 m diorite sill/dyke 50.99 0.68 14.86 3.05 0.20 2.71 11.65 4.37 2.30 0.27 8.54 99.73 158-1, 816.9 m basalt 47.66 0.71 12.77 11.10 0.22 13.06 7.88 1.87 3.34 0.23 1.63 100.67 158-1, 846.1 m basaltic andesite 52.45 0.98 16.28 11.60 0.10 4.03 6.95 5.21 0.84 0.30 1.38 100.25 158-1, 686.1 m basalt 50.48 0.77 14.05 9.77 0.16 9.60 8.68 3.27 1.20 0.23 2.00 100.40 158-1, 907.7 m basaltic andesite 52.73 0.71 13.44 8.60 0.20 9.96 8.02 3.22 1.85 0.29 1.04 100.23 147-lb, 252.7 m basalt 48.54 2.78 13.44 15.52 0.28 5.70 7.66 3.32 1.58 0.42 1.38 100.77 147-lb, 259.7 m feldspathic tuff? 50.03 0.88 19.59 9.06 0.14 3.28 8.49 4.23 1.42 0.49 2.40 100.13 147-lb, 272.8 m cherty tuff 82.31 0.32 6.35 4.00 0.03 1.37 0.85 0.55 3.21 0.11 0.90 100.63 147-lb, 304.8 m basalt 47.00 0.75 12.63 12.86 0.15 11.26 8.11 1.50 3.67 0.27 2.28 100.73 147-lb, 351.1 m diorite sill/dyke 50.15 0.85 15.97 6.33 0.14 6.36 11.39 3.54 1.14 0.32 3.74 100.02 147-lb, 373.4 m quartz-diorite sill/dyke 58.46 0.83 18.89 3.64 0.07 1.74 4.95 7.34 1.36 0.08 2.33 99.74 Sample LITHOLOGY Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total % % % % % % % % % % 147-lb, 415.1 m black mudstone 47.13 0.84 13.59 14.41 0.16 7.74 3.97 0.20 6.68 0.30 4.34 99.59 147-lb, 473.1 m diorite sill/dyke 50.38 1.02 16.77 2.90 0.19 2.82 9.37 5.76 2.51 0.21 7.33 99.34 147-lb, 485.9 m sheared sill/dyke margin? 37.80 0.59 11.63 29.89 0.10 4.65 4.87 1.64 3.96 0.27 3.45 99.91 147-lb, 517.3 m basalt 46.14 0.79 15.27 9.46 0.14 6.89 8.71 2.64 2.73 0.24 7.41 100.54 102-77, 42.7 m basalt 44.22 2.03 9.41 23.39 0.38 5.45 10.33 1.50 1.17 0.49 0.81 99.51 102-77, 58.2 m feldspathic tuff? 46.90 0.81 18.02 10.67 0.19 3.23 11.12 1.92 2.48 0.35 4.20 100.06 147-lb, 332.2 m gash-banded andesite? 43.01 0.62 10.70 12.75 0.34 10.11 9.16 0.00 3.1 0.23 9.4 99.42 102-77, 127.7 m gash-banded andesite? 50.38 0.92 19.00 10.58 0.17 2.95 7.67 2.31 3.36 0.41 1.49 99.24 119-66, 4.0 m andesite 56.09 0.71 17.87 6.48 0.13 2.02 3.25 4.38 4.75 0.27 2.42 98.37 119-66, 23.6 diorite sill/dyke ? 50.52 0.76 17.74 5.00 0.14 3.35 6.97 5.20 2.4 0.31 5.29 97.68 119-66, 34.7 m basalt 47.82 0.60 10.52 9.79 0.23 12.33 10.32 1.14 2.96 0.19 4.11 100.01 119-66, 54.9 m cherty tuff? 80.57 0.43 7.65 1.61 0.06 0.98 1.53 1.33 2.12 0.1 2.09 98.47 102-79, 2.4 m basalt? 48.20 0.81 15.87 14.61 0.21 5.69 3.03 2.72 4.32 0.3 3.66 99.42 102-79, 12.2 m argillaceous chert 90.51 0.08 1.73 2.85 0.04 0.88 2.00 0.00 0.25 0.31 1.22 99.87 102-79, 87.8 m basaltic andesite 53.79 0.76 14.46 12.58 0.22 5.44 6.72 2.77 2.67 0.35 0.92 100.68 102-79, 111.9 m andesite 63.25 0.61 13.22 9.12 0.09 0.39 2.34 4.20 4.18 0.03 1.7 99.13 158-2a, 422.7 m lapilli tuff 50.15 1.25 17.26 12.77 0.19 3.75 7.47 4.04 0.74 0.26 2.34 100.22 158-2a, 512.1 m banded chert \v/ sulphides 83.99 0.28 4.83 4.72 0.06 1.21 1.55 1.01 1.3 0.06 2.09 101.1 158-2a, 523.0 m basalt 50.85 1.06 17.41 11.14 0.16 5.14 7.07 4.24 1.97 0.34 1.13 100.51 158-2a, 586.7 m quartz diorite sill/dyke 57.74 0.66 17.27 5.01 0.15 1.60 3.39 5.35 4.74 0.36 2.64 98.91 158-2a, 737.0 m basalt 47.63 0.63 10.54 12.02 0.18 14.09 8.80 1.61 1.17 0.15 3.29 100.11 158-2a, 784.8 m basalt 46.21 0.63 11.63 10.70 0.24 9.89 16.35 0.45 1.08 0.26 2.56 100 158-2a, 816.5 m basalt 48.27 0.68 12.17 10.12 0.21 13.22 10.26 1.25 2.19 0.2 1.52 100.09 153-1, 250.2 m basalt 50.44 0.93 17.98 11.15 0.18 3.54 9.08 3.70 0.62 0.17 1.94 99.73 153-1, 420.0 m chlorite schist 57.74 0.95 21.62 1.57 0.08 0.27 3.75 6.61 3.37 0.02 3.12 99.1 153-1, 444.4 m quartz diorite sill/dyke 57.94 0.63 17.46 4.86 0.11 1.38 3.94 6.07 4.04 0.35 3.02 99.8 153-1, 521.8 m basalt 47.50 0.83 14.30 12.80 0.18 9.66 7.50 1.85 3.25 0.27 1.76 99.9 153-1, 478.2 m diorite sill/dyke 51.81 0.90 16.90 4.40 0.07 6.63 8.72 4.34 2.13 0.74 3.97 100.61 153-1, 588.6 m basalt 49.68 0.68 12.05 8.61 0.16 11.70 9.80 2.66 1.06 0.17 3.18 99.75 153-1, 64.9 m basalt 48.77 0.72 12.73 10.88 0.18 11.67 10.01 1.37 1.68 0.21 2.34 100.56 Sample BaO Co Cr203 V Cu Ni Zn Ga Nb Rb Sr Pb Zr Y Zr/Y ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm GD-GC-01 344 28 int 106 261 10 211 16 10 135 159 9 146 25 5.8 GD-GC-02 829 18 50 71 76 14 45 10 7 54 210 5 99 15 6.6 GD-GC-03 654 19 47 45 96 20 44 16 7 35 395 4 98 10 9.8 GD-GC-04 279 71 814 213 115 226 57 11 2 51 174 10 50 14 3.6 GD-GC-05 2817 66 268 133 88 50 52 8 5 57 176 8 78 19 4.1 GD-GC-06 198 65 1176 204 108 414 169 10 2 87 128 11 48 11 4.4 GD-GC-07 4618 44 58 60 85 0 188 8 8 18 39 1102 46 36 1.3 GD-GC-08 711 20 0 197 140 4 106 18 3 63 284 3 65 18 3.6 GD-94-2 675 39 961 269 92 199 129 13 1 126 149 6 33 17 2.0 GD-94-3 386 25 33 312 53 11 101 19 2 11 432 2 100 24 4.3 GD-94-4 395 26 207 269 37 88 92 17 1 13 489 2 64 16 4.0 146-3, 5.5 m 0.03 35 561 305 34 129 87 14 4 73 268 8 61 16 3.8 146-3, 56.7 m 0.02 18 694 249 460 112 181 10 3 326 90 10 44 10 4.4 146-3, 102.4 m 0.03 60 369 314 1013 130 120 15 4 403 102 9 59 12 4.9 146-3, 145.1 m 0.23 8 381 42 480 11 92 11 10 236 304 22 69 7 9.9 146-3, 203.3 m 0.04 16 2 90 27 6 71 19 11 87 416 7 73 29 2.5 146-3, 275.2 m 0.03 62 572 266 413 181 168 13 4 178 233 10 51 13 3.9 146-3, 303.3 m 0.03 30 348 271 147 98 86 16 5 103 198 9 70 18 3.9 158-1, 587.1 m 0.03 26 21 231 93 25 89 17 5 68 262 6 60 15 4.0 158-1, 634.0 m 0.03 38 67 33 71 20 53 4 6 32 49 0 89 14 6.4 158-1, 654.4 m 0.02 50 847 261 290 175 90 12 4 23 189 7 51 14 3.6 158-1, 665.4 m 0.07 45 559 275 1886 182 175 14 5 137 182 18 54 15 3.6 158-1, 670.3 m 0.06 8 97 145 68 22 69 13 6 78 264 7 66 19 3.5 158-1, 816.9 m 0.03 42 740 259 144 230 183 12 4 163 168 ~7 50 14 3.6 158-1, 846.1 m 0.02 54 87 330 499 30 41 16 5 27 275 9 71 20 3.6 158-1, 686.1 m 0.02 38 740 294 342 116 42 14 4 40 176 9 57 17 3.4 158-1, 907.7 m 0.04 20 616 232 95 139 97 12 5 64 415 8 65 17 3.8 147-lb, 252.7 0.06 36 91 400 182 74 58 21 14 49 240 9 514 41 12.5 147-lb, 259.7 0.06 16 48 251 192 20 31 17 4 32 565 10 66 17 3.9 147-lb, 272.8 0.52 37 78 76 774 42 17 5 6 53 84 1 71 12 5.9 147-lb, 304.8 0.05 17 902 249 353 184 265 15 5 131 99 17 60 11 5.5 147-lb, 351.1 0.02 10 149 235 46 40 70 15 4 35 371 9 60 17 3.5 147-lb, 373.4 0.02 5 87 132 64 9 31 12 10 50 308 3 77 30 2.6 Sample BaO Co Cr203 V Cu Ni Zn Ga Nb Rb Sr Pb Zr Y Zr/Y ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm 147-lb, 415.1 0.12 80 340 293 113 76 104 14 5 250 59 10 59 13 4.5 147-lb, 473.1 0.03 16 70 122 106 53 95 12 6 129 227 5 73 11 6.6 147-lb, 485.9 0.02 54 278 481 9284 120 102 21 5 277 144 15 54 7 7.7 147-lb, 517.3 0.02 14 327 269 117 87 81 15 3 150 228 69 54 14 3.9 102-77, 42.7 m 0.04 20 41 375 1889 48 404 25 15 31 105 38 158 49 3.2 102-77, 58.2 m 0.1 20 85 282 211 37 85 19 4 68 541 21 58 15 3.9 147-lb, 332.2 361 23 1023 293 388 157 222 14 7 133 49 2 34 16 2.1 102-77, 127.7 1509 13 20 295 81 18 57 19 4 87 400 17 58 22 2.6 119-66, 4.0 m 1548 17 4 143 65 14 44 18 7 126 261 11 93 16 5.8 119-66, 23.6 614 31 7 174 557 52 86 17 4 99 308 15 64 19 3.4 119-66, 34.7 m 205 24 880 250 105 168 168 14 5 188 108 4 36 17 2.1 119-66, 54.9 m 2383 136 29 143 20 14 24 8 9 39 96 7 103 28 3.7 102-79, 2.4 m 1483 120 182 264 248 47 186 16 6 146 171 10 61 23 2.7 102-79, 12.2 m 206 57 22 190 157 73 33 7 8 2 40 13 31 16 2.0 102-79, 87.8 m 911 34 111 244 322 25 199 17 6 85 228 15 66 22 3.1 102-79, 111.9 437 14 3 22 43 8 63 33 53 59 84 19 859 79 10.8 158-2a, 422.7 361 26 48 328 121 23 102 20 5 20 287 9 100 29 3.4 158-2a, 512.1 871 98 19 156 452 70 32 9 9 22 50 14 62 19 3.4 158-2a, 523.0 1143 26 124 324 141 35 46 17 6 63 240 8 73 25 2.9 158-2a, 586.7 1048 16 14 102 183 8 138 18 10 133 355 22 215 28 7.7 158-2a, 737.0 251 46 941 261 101 223 56 12 6 38 139 3 33 20 1.7 158-2a, 784.8 368 46 728 260 34 162 106 12 6 22 223 15 40 21 1.9 158-2a, 816.5 311 57 878 259 79 290 120 13 6 73 204 7 41 22 1.9 153-1, 250.2 m 350 37 24 295 124 9 67 18 4 10 364 7 71 25 2.9 153-1, 420.0 m 529 12 132 60 30 5 25 15 10 90 394 15 69 11 6.2 153-1,444.4 m 817 10 13 113 83 11 170 20 12 114 290 10 176 31 5.6 153-1, 521.8 m 693 5 487 308 46 80 121 16 5 131 256 9 48 20 2.4 153-1,478.2 m 213 19 272 182 31 139 27 16 5 109 365 4 57 15 3.9 153-1, 588.6 m 147 26 851 240 137 139 58 11 6 38 219 3 35 21 1.7 153-1, 64.9 m 394 33 649 273 57 177 171 13 5 57 263 12 40 21 1.9 Table 3.3 Rare earth element chemistry for the Granduc deposit. Sample Hf Sc Ta Th U W La Ce Nd Sm Eu Tb Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm GD-GC-01 4.4 9.2 1.2 3.0 2.3 54 13.9 33 17 4.04 0.89 0.7 2.78 0.36 GD-GC-04 0.9 41.5 <d/l 1.1 1.0 33 8.7 18 10 2.56 0.86 0.4 1.44 0.20 ON Major and Trace Elements Igneous rocks sampled for geochemical analysis from the Granduc Mine and Footwall Series can be divided into two main groups on the basis of lithology: mafic volcanic and volcaniclastic rocks, and mafic to intermediate sills and dykes. Mafic volcanic and volcaniclastic rocks range in composition from Mg-rich basalt to basalt and basaltic andesite, with Si02=43-53 %, TiO2=0.6-2.8 %, Al203=9.4-19.6 %, MgO=3.0-15.8 %, and Cr203=33-1176 ppm. These rocks have Zr/Y ratios of 1.7 to 5.5, which are consistent with tholeiitic to transitional magmatic affinities, with the exception of two samples with extremely high concentrations of Zr (514 and 859 ppm), and correspondingly high Zr/Y ratios (12.5 and 10.8, respectively) (Table 3.2 and Fig. 3.5a). The Zr/Y ratios determined for these rocks in part overlap the more primitive end of the data cluster determined for mafic rocks the Stuhini Group in the Telegraph Creek map area, north of Granduc (Fig. 3.5b). A trend from low to high Zr, Ti02, and AI2O3, and high to low Cr203, can be established for mafic volcanic and volcaniclastic rocks from Granduc Mountain (Figs. 3.6a, b, and c). This variation in composition corresponds to the geographic location of samples, with the most primitive compositions (lowest Zr, Ti02, and Al203, highest Cr203) occurring at the greatest depths in drill core (all drill holes sampled were drilled from west to east) and at the easternmost surface exposures of the Footwall Series, a few meters west of the shear zone that separates Footwall Series rocks from volcaniclastic rocks of Jurassic age discussed above (Fig. 3.2). This variation in the chemical composition of the mafic volcanic rocks may represent the fractionation of a mafic magma from more primitive to more evolved compositions, and if so would suggest the VMS mineralization within the Mine Series may be overlying a chemically evolving volcanic pile. Variably deformed sills and dykes which intrude mafic volcanic and volcaniclastic rocks of the Footwall Series range in composition from diorite to quartz diorite, and are compositionally similar to the basalt to basaltic andesite composition flows and tuffaceous rocks they intrude (SiO2=50-59 %, TiO2=0.6-1.0 %, Al203=14.9-18.9 %, MgO=1.4-6.6 %, and Cr203=7-278 ppm). Intrusive equivalents of the most primitive Mg-rich basaltic rocks were not represented in the drill core sampled in this study. Ratios of Zr/Y of 2.5 to 7.7 for the sills and dykes are consistent with predominantly tholeiitic to transitional magmatic affinities (Fig. 3.5a). Diorite to quartz diorite 68 sills and dykes lie along the trends established for the mafic volcanic rocks, with the exception of three rocks, which have markedly higher Zr concentrations (146-215 ppm), and slightly elevated Y concentrations and Zr/Y ratios relative to other samples of the sills analyzed in this study (Fig. 3.6a, b, and c). These high Zr concentrations may represent a high Zr mineral, such as zircon, becoming concentrated by chemical or physical mechanisms within some parts of these sills. 40 10 1 1 1 i 1 i 1 •! 1 1 _ (») / Zi/Y = 2 -Granduc Mountain » mafic volcanic & votcanictastic rocks • diorite to quartz-diorite sills & dykes / Zr/Y = 4.5 A dacitic tuffaceous rocks Tholeiitic two tdditiotul mific volcanic rocks at • / / O Zr-514, Y-41 and Zr-859. Y=79 /< • 4» &> Transitional a • -"Zr/Y = 7 - / y h i i i Calc-atkaline - / • i A 1 1 1 0 20 40 60 80 100 120 140 160 180 200 220 Zr (ppm) Figure 3. 5 Plot of Y versus Zr for a) rocks from Granduc Mountain; and b) regional samples from the Stuhini Group (data from Logan et al., 1989; Brown and Gunning 1989; Brown et al. 1992; Kaip, 1997) (limits after Barrett and MacLean 1994). 69 2.5 (a) 2 -0s *S 1.5 <S i O H 0, possible fractionation trend-primitive basalts. • mafic volcanic & volcaniclastic rocks • diorite to quartz-diorite sills & dykes ve ^ ^ • 50 100 150 Zr (ppm) 200 250 25 ^ 20 " ox ^15-r m io O (b) 4 If primitive • *' • basalts D X o • \ possible fractionation trend i i i • mafic volcanic & volcaniclastic rocks o diorite to quartz-diorite sills & dykes 1200 50 100 150 Zr (ppm) 200 250 primitive basalts • mafic volcanic & volcaniclastic rocks D diorite to quartz-diorite sills & dykes possible fractionation trend n 50 100 150 Zr (ppm) 200 250 Figure 3. 6 Plots of a) Ti02 versus Zr; b) A1203 versus Zr; and c) Cr203 versus Zr for mafic volcanic and volcaniclastic rocks and diorite to quartz diorite sills and dykes from Granduc Mountain. 70 Rare Earth Elements Basalt and quartz diorite from the Footwall Series of the Granduc deposit have similar REE patterns, characterized by depletion in heavy-REE relative to light-REE, and Ce„/Ybn ratios of 3.2 and 3.8, respectively (Fig. 3.7). The sample of quartz diorite has a minor negative Eu anomaly, which is consistent with plagioclase fractionation. This sample also corresponds to one of the samples noted above to have an elevated Zr concentration. The similarity between the pattern for this rock and the basalt, combined with the higher overall REE concentrations of the sill sample is consistent with concentration of zircon or some other Zr- and REE-rich mineral within portions of the sill. The REE patterns of these rocks are slightly enriched relative to basalts of N- and E-MORB character, and indicate derivation from somewhat more evolved sources than N- and E-MORB lavas (Sun and McDonough 1989). Figure 3. 7 REE plot for basalt and quartz diorite from the Granduc Footwall Series. Nd Isotope Data The samples of basaltic volcanic rock from the Footwall Series and quartz dioritic intrusion, both of which were dated in this study, were analyzed for their Nd isotopic compositions to further constrain potential magmatic source regions for these rocks. Neodymium isotopic analyses were conducted by R. Theriault at the Geochronology Laboratory of the Geological Survey of Canada; analytical procedures are described by Theriault (1990). Analytical uncertainty is +0.5 eNd units; abundances of Sm and Nd were determined by isotope dilution and have an uncertainty of 1% or less. Analytical results are presented in Table 3.4 and are shown graphically in Figure 3.8. 100 Norm: chon • GD-GC-Ol quarte-diorite o OD-GC-04 basalt La Ce Pr Nd Sm Eu Od Tb Dy Ho Er Tm Yb Lu 71 Basalt from the Granduc Footwall Series and quartz diorite intrusive into it have initial 8Nd values of +6.1 and +6.8, respectively. These values, which are within error of each other, indicate that these rocks were derived from primitive magmatic sources with no detectable component of old, evolved sialic basement. For comparison, initial em values determined in this study are plotted with those of regional samples of mafic volcanic and volcaniclastic rocks of the Stuhini Group (n = 4, initial SNd = +1-3 to +7.7) and mafic intrusions of the Stikine Plutonic Suite (n = 3, initial sNd = +4.0 to +7.7) (Fig. 3.8) (Samson et al. 1989; Jackson 1990); values determined in this study overlap the most primitive end of the data array. Table 3. 4 Neodymium isotopic data. Sample Sm Nd meas. ""Nd/'^Nd age1 (ppm) (ppm) (error x 10"6, 2CT) (present day) (Ma) (initial) GRANDUC GD-GC-01 4.43 18.48 0.1448 0.512908 (7) +5.3 223 +6.8 Footwall Series quartz diorite sill GD-GC-04 3.36 13.46 0.1511 0.512884(6) +4.8 223 +6.1 Footwall Series basalt flow used for the calculation of eNd (initial). 2error = +0.5 em units. 10 3 6 ;§ d4 + • this study • Jackson (1990) A Samson et al. (1989) 8 A B 200 210 220 230 Age (Ma) Figure 3. 8 Plot of sNd (initial) versus age for basalt and quartz diorite from the Granduc Footwall Series, plotted with Late Triassic rocks of the Stuhini Group and Stikine Plutonic Suite (Samson et al. 1989; Jackson 1990). 72 Pb Isotope Data Lead isotopic compositions were determined for sulphides from the B and F ore zones in the Granduc deposit, as well as for galena and potassium feldspar from Pb-Zn-Ag veins which cross cut the Footwall Series (Fig. 3.3). Sample preparation and Pb isotopic analyses were carried out at the Geochronology Laboratory at the University of British Columbia. Analytical procedures are outlined in Appendix 1; analytical results are given in Table 3.5 and shown graphically in Figure 3.9. Table 3. 5 Lead isotopic data for samples from the Granduc deposit. SAMPLE' DRILL HOLE SAMPLE MIN1 "'Pb/^Pb W7Pb/M4Pb M*Pb/M4Pb ""Pb/^Pb "•Pb/^Pb NUMBER LOCATION DESCRIPTION (% error)54 (% error)3-4 (% error)5-4 (% error)5-4 (% error)5-4 Gla 102-77, 112ft. B orebody gl 18.62 15.56 38.19 0.8358 2.051 (0.027) (0.027) (0.030) (0.008) (0.009) Gib 102-77, 112ft. B orebody gl 18.64 15.59 38.27 0.8362 2.053 (0.022) (0.023) (0.026) (0.006) (0.008) G2a 102-77,356ft. B orebody gl 18.65 15.59 38.27 0.8359 2.052 (0.015) (0.017) (0.021) (0.006) (0.009) G2b 102-77, 356ft. B orebody gl 18.65 15.58 38.27 0.8355 2.052 (0.019) (0.021) (0.024) (0.005) (0.010) G2c 102-77, 356ft. B orebody py 18.65 15.58 38.24 0.8355 2.051 (0.044) (0.038) (0.056) (0.025) (0.032) G3a 158-1, 3135ft. vein gi 19.18 15.64 38.70 0.8151 2.018 (0.019) (0.021) (0.024) (0.006) (0.009) G3b 158-1,3135ft. vein gi 19.18 15.63 38.71 0.8151 2.018 (0.012) (0.014) (0.019) (0.007) (0.010) G3c 158-1, 3135ft. vein kf 19.19 15.63 38.71 0.8146 2.017 (0.011) (0.013) (0.018) (0.006) (0.009) G3d 158-1, 3135ft. vein kf 19.18 15.62 38.66 0.8144 2.015 (0.026) (0.027) (0.030) (0.006) (0.008) G9a 119-57, 97ft. F orebody gl 18.64 15.58 38.25 0.8357 2.052 (0.010) (0.013) (0.017) (0.005) (0.008) G9b 119-57, 97ft. Forebody gl . 18.63 15.57 38.24 0.8359 2.052 (0.014) (0.016) (0.019) (0.005) (0.008) 1 upper case letter and number refers to sample number, lower case letter refers to fraction number. 2 mineral abreviations: gl=galena, py=pyrite, kf= potassium feldspar. 3 errors are quoted at the 2a (95% confidence) level. 4 values are corrected for instrument fractionation by normalization based on replicate analyses of the NBS-981 standard. 73 Alldrick (1991) has characterized the Pb isotopic signature of Jurassic and Tertiary mineralization in the Stewart Mining Camp. Lead isotopic values group in two distinct clusters, the isotopically more primitive cluster correspond to Jurassic gold-silver-copper-zinc-lead mineralization, and the isotopically more evolved cluster to Tertiary silver-zinc-lead+molybdenum mineralization associated with Hyder Plutonism. Values determined in the present study are plotted relative to Alldrick's (1991) Jurassic and Tertiary clusters (Fig. 3.9). X5 o CN x5 PH oo o CN 2.06 2.04 2.02 2.001 0.810 Tertiary Cluster Granduc Pb-Zn-Ag veins J_ _1_ 0.820 207 Jurassic Cluster 0.830 Pb/206Pb Granduc deposit (B&F orebodies) Rock & Roll prospect 1 . I 0.840 Figure 3. 9 Plot of 207Pb/206Pb versus 20SPb/206Pb for sulphides from B and F orebodies of the Granduc deposit, veins which cross cut the Footwall Series of the deposit, and the Rock and Roll prospect (Dean and Carr, 1991). Plotted for comparison are clusters defined for Jurassic and Tertiary mineralization in the Stewart Mining Camp (Alldrick, 1991). Syngenetic Cu-rich VMS mineralization Mineralization within the main orebodies at Granduc consists primarily of laminated and variably deformed massive to semi-massive pyrite-pyrrhotite-chalcopyrite in an argillaceous and variably magnetite-rich matrix (Plate 3.11). In the archived core examined, galena occurs as thin (l-3mm) quartz-calcite-galena+pyrite veinlets cross-cutting the banded copper-rich mineralization. On first inspection, the intimate relationship between the galena-rich veinlets and the banded mineralization within these highly deformed rocks appeared to reflect a remobilization of the galena. However, the possibility that the galena was part of a younger mineralizing event could not be dismissed. Therefore, laminated pyrite, as well as galena was analyzed for its Pb 74 isotopic composition. Analysis of six galena samples from the B and F orebodies in the Main Zone of the Granduc deposit yielded ratios of 207Pb/206Pb = 0.8355-0.8362 and 208pb/206Pb = 2.051-2.052 (Fig. 3.9 and Table 3.5). Lead values determined for pyrite from the B orebody plots within the range of galena values (207Pb/206Pb = 0.8355 and 208Pb/206Pb = 2.051), suggesting that Pb within the two minerals was derived from the same metallogenic source. Thus the galena analyzed in this study appears to represent a minor, yet primary component to Granduc mineralization which was preferentially remobilized during deformation. The values determined for galena and pyrite from the Granduc deposit are less radiogenic than Jurassic mineralization in the Stewart area, and therefore suggest a pre-Jurassic age for this style of mineralization, consistent with the age of the host rocks. Pb-Zn-Ag Veins A limited number of undeformed, coarse-grained microcline-calcite-sulphide veins (10-15 cm apparent thickness) cross cut Footwall Series in the North Zone of the Granduc deposit; insufficient core is preserved from the mined-out zones to determine if these veins cross cut the orebodies in significant quantities. Sulphides in these veins consist of pyrite, galena, and sphalerite; original assays of bulk rocks containing these veins show enrichments of lead, zinc and silver (Plate 3.12). Galena and microcline from one of these veins yielded Pb isotopic ratios of 207pD/206ph = 0.8144-0.8151 and 208Ph/206Ph = 2.015-2.018 (Fig. 3.9 and Table 3.5). These values are significantly more radiogenic than the Pb isotopic signature determined for syngenetic mineralization in the Granduc deposit and plot within the cluster defined by Alldrick (1991) for mineralization related to Tertiary intrusions in the Stewart Mining Camp. Discussion Ages of magmatic rocks on Granduc Mountain In the current study mafic volcanic rocks from the Footwall and Mine Series of the Granduc deposit were dated by U-Pb methods at 223 +/-5 and 222 +/-1 Ma, respectively, and are therefore assigned to the Upper Triassic Stuhini Group. These results establish that magmatic 75 rocks in the Mine and Footwall Series are the same age, a association that could not be confirmed by field relationships alone, due to the high degree of deformation on Granduc Mountain. A quartz diorite body which intrudes the Footwall Series was dated by U-Pb methods at 223 +/-3 Ma; this unit is therefore assigned to the Stikine Plutonic Suite. The age determined for this unit is within error of those determined for mafic volcanic rocks in this study and establishes that plutonism was contemporaneous with volcanism in the Footwall and Mine Series of the Granduc deposit. The age of this intrusion is also identical to an age of approximately 223 Ma determined for the Bucke Glacier Stock, 6 -8 km north of Granduc Mountain (Lewis 1994), and indicates that Late Triassic plutonism in the area was not restricted to Granduc Mountain. On the southeast flank of Granduc Mountain mafic volcanic rocks of the Footwall Series are in fault contact with deformed mafic to felsic tuffaceous and fragmental rocks (Fig. 3.2). A tuffaceous rock of dacitic composition was dated by U-Pb methods at 185 +/-4 Ma, and these rocks are therefore assigned to the Lower Jurassic Hazelton Group. An Early Jurassic age for volcanic rocks in the southeast flank of Granduc Mountain implies that these rocks are unrelated to volcanic and intrusive rocks of the Granduc Mine and Footwall Series. Age of Cu-rich VMS mineralization As a result of the high degree of deformation, determining the age of the magmatic rocks is not sufficient to establish an age for VMS mineralization at Granduc. Therefore the Pb isotopic signature of mineralization at Granduc was determined as part of this study. The Pb isotopic signatures of Jurassic and Tertiary age mineralization in the Stewart Mining Camp has been documented by Alldrick (1991); the data clusters can be used in a comparative way to establish an age for other mineralization in the region. In this study, Cu-rich VMS mineralization at Granduc was determined to have a Pb isotopic signature that is distinctly more primitive than both Tertiary and Jurassic mineralization in the region; these data imply a pre-Jurassic age for VMS mineralization at Granduc (Fig. 3.9). Together, the Pb isotopic data and U-Pb age determinations for igneous rocks in the Mine and Footwall Series suggest a Late Triassic age for Cu-rich VMS mineralization at Granduc, consistent with a syngenetic interpretation of mineralization (Grove 1986; Hoy 1991). 76 An interpreted Late Triassic age for Granduc VMS mineralization indicates that in addition to the Jurassic and Tertiary metallogenic epochs in the region, there was also a Late Triassic metallogenic event. The Pb isotopic signature of this mineralization is distinct, and more primitive than signatures of Jurassic and Tertiary mineralization in the region. It should therefore be possible to use sulphide Pb isotopic signatures from other syngenetic mineral occurrences to determine if they are related to this Late Triassic mineralizing event. The Rock and Roll prospect, located 7 km west of the Snip deposit, is interpreted to be a VMS occurrence of probable Late Triassic age (B.C. MTNFUE # 104B 377). The prospect, with indicated reserves of 0.58 million tonnes grading 3.1 % Zn, 0.8 % Pb, 0.6 % Cu, 2.8 g/t Au and 336 g/t Ag, consists of sediment-hosted base metal sulphides which occur in proximity to mafic volcanic and volcaniclastic rocks, interpreted to be part of the Upper Triassic Stuhini Group (B.C. MINFILE). Galena from the Rock and Roll prospect has a Pb isotopic signature that is very close to that determined in this study for mineralization at Granduc (207Pb/206Pb = 0.8363 and 208Pb/206Pb = 2.050) (Dean and Carr 1991) (Fig. 3.9). The similarity in the Pb isotopic signatures for mineralization from the two areas suggest that a Late Triassic age designation for mineralization at the Rock and Roll prospect is valid. Age ofPb-Zn-Ag veins Galena and microcline from undeformed, coarse-grained Pb-Zn-Ag veins which cross cut the Footwall Series have a Pb isotopic signature which is significantly more evolved than that of the Cu-rich VMS mineralization discussed above. These veins have a Pb isotopic composition which plots within the cluster defined for Tertiary mineralization in the region (Fig. 3.9) (Alldrick 1991). This Tertiary mineralization is characterized by Ag-rich galena-sphalerite veins and includes the Prosperity, Porter-Idaho and Indian deposits (Alldrick 1991). Based on the Pb isotopic signature of these veins and the similarity in the styles of mineralization, these veins are interpreted to be of Tertiary age. Although Pb and Zn were not present in recoverable amounts at the Granduc deposit, Ag was recovered. It is possible that Tertiary Pb-Zn-Ag veins may have locally contributed to the silver content of the deposit. 77 Lithogeochemistry and Nd isotopic signatures The Granduc Footwall, Mine and locally Lower Hangingwall Series contain mafic volcanic and volcaniclastic rocks of high-Mg basaltic to basaltic andesitic composition. The most primitive compositions occur at the southeast extreme of the Footwall Series and at the greatest depths in drill core; the most evolved compositions occur in the Mine Series and near the top of the drill holes, if this trend in the geochemical data is reflecting a fraction trend from less to more evolved affinities, it would suggest that this sequence was younging from east to west, towards the Mine Series. The Footwall Series is intruded by dioritic to quartz dioritic composition sills and dykes of comparable age and chemical compositions similar to those of the basaltic to andesitic volcanic and volcaniclastic rocks which they intrude, and these rocks have tholeiitic to transitional magmatic affinities. Two samples, consisting of basalt from the Footwall Series and quartz diorite which intrudes it have similar REE patterns and initial ENd values which are identical within error (+6.1 and +6.8, respectively) (Figs. 3.7 and 3.8). The light-REE enriched and heavy-REE depleted REE patterns suggest derivation from magmatic sources which are slightly enriched relative to MORB, and are consistent with formation in an oceanic back arc basin or an early, tholeiitic stage of island arc formation, and inconsistent with formation in either a mid-ocean ridge or continental margin setting. The primitive Nd isotopic signatures indicate that these rocks are the products of juvenile magmatic sources and have not interacted with significant amounts of old, evolved sialic crust. The similarities in age, Nd isotopic signature, and major, trace and rare earth element chemistry indicate that mafic volcanic and volcaniclastic rocks of the Granduc Mine and Footwall Series are probably comagmatic with the diorite to quartz diorite sills and dykes which intrude the Footwall Series. Regionally, volcanic rocks of the Upper Triassic Stuhini Group have tholeiitic to calc-alkaline magmatic affinities and initial 6Nd values of+1.3 to +7.7 (Figs. 3.5b and 3.8). Upper Triassic volcanic and intrusive rocks from Granduc lie at the more primitive end of the regional data set, both in terms of Zr/Y ratios and Nd isotopic signatures. Based on the available Nd data and the ages assigned to the rocks analyzed, these data suggest that Late Triassic mafic rocks of the Stuhini Group and Stikine Plutonic Suite become more evolved (i.e., tend towards lower 78 initial SN<I values) with time, which is consistent with the broad-scale maturation of an island arc (c.f. Donnelley 1968; Baker 1968; Gill 1970). These data indicate that to this point, economic VMS mineralization has been found in the oldest, and most chemically and isotopically primitive part of the Stuhini Group. Comparison with the Late Triassic Windy Craggy deposit The Alexander terrane is host to the world class mafic volcanic and sediment-hosted Early Norian Windy Craggy deposit, with reserves of 300 Mt of 1.4 % copper (Orchard 1986; Hoy 1991; B.C. MTNF1LE #114P-002). The Granduc and Windy Craggy deposits, which formed more or less contemporaneously in different island arc terranes, both comprise copper-rich mineralization hosted within variably calcareous and argillaceous sedimentary and mafic submarine volcanic rocks which are intruded by diorite sills. Both deposits have similar ore mineralogy, including locally high concentrations of magnetite within the ore zones. However, the Windy Craggy deposit is significantly larger than the Granduc deposit. The two deposits differ in other respects as well; lithogeochemical analyses of mafic rocks from Windy Craggy have been interpreted by Maclntyre (1986) to have a calc-alkaline to alkaline affinity, with Zr/Y ratios mainly of 6 to 18 and REE patterns which show a strong LREE-enrichment relative to MORB. The range of Pb isotopic compositions determined for Windy Craggy mineralization is 207pb/206Ph = 0.8332-0.8341, and 208pD/206pD = 2.037-2.040 (Peter, 1992); these values are markedly more radiogenic than those of syngenetic mineralization from the Granduc deposit. Windy Craggy has been interpreted to have formed within subaqueous rift valleys associated with spreading centers as part of a transform fault system adjacent to a continental mass, analogous to the present day Guaymas Basin (Maclntyre 1986). In comparison, mafic rocks from Granduc are mainly tholeiitic, with a lesser degree of LREE-enrichment, and the associated mineralization has a less evolved Pb isotopic signature. These comparisons suggest that VMS mineralization in the Granduc deposit formed in a more oceanic, or primitive tectonic setting than contemporaneous mineralization at Windy Craggy. A model for the formation of the Granduc deposit 79 VMS mineralization at Granduc is interpreted to have accumulated within a subaqueous sediment-dominated sequence overlying a mafic volcanic pile. Contemporaneous, and probably comagmatic mafic intrusive rocks within the Footwall Series may be related to more extensive intrusions at depth, such as are seen to the north at the Bucke Glacier Stock, and may have acted as a heat source to drive a hydrothermal system. Kirkham (1979) has suggested that the bedded oxide-sulphide metalliferous sediments produced in the Red Sea brine deeps might be a possible modern day analogue to the Granduc deposit, and adopted Sato's (1972) model invoking expulsion of high density Type I brines from source vents. If these vents were located in areas of high relief on the paleo-seafloor, brines could have flowed downslope and pooled within local depressions, to produce distal, layered sulphide-oxide-silicate accumulations. Accumulation of sulphides by this mechanism is consistent with the apparent lack of hydrothermal alteration, and paucity of stringer mineralization within the volcanic pile underlying the Granduc ore lenses. This allows for the preservation of multiple ore lenses, such as those seen at Granduc, either by multiple, probably linear vents or within multiple localized depressions in which sulphides accumulated. The high degree of deformation precludes determining unequivocally if VMS mineralization at Granduc formed by the mechanism described above. However, this model is consistent with the observations made and data presented in this study. Conclusions Mafic volcanic rocks of the Granduc Footwall and Mine Series, and similar composition sills and dykes which intrude the Footwall Series, are of Late Triassic age and therefore are assigned to the Stuhini Group and Stikine Plutonic Suite, respectively. These volcanic and intrusive rocks are characterized by predominantly tholeiitic to transitional magmatic affinities, primitive Nd isotopic signatures and REE patterns which are slightly enriched relative to N- and E-MORB; the chemical signatures of these rocks suggest formation within an oceanic back arc basin or early stages of island arc magmatism. The similarity in age and isotopic and chemical signatures of the volcanic and intrusive rocks suggest that they are comagmatic. The age of igneous host rocks, along with a Pb isotopic signature of syngenetic mineralization which is less radiogenic than Jurassic and Tertiary mineralization in the region, supports a Late Triassic age for VMS mineralization at Granduc. 80 Plate 3. 2 Snow capped north face of Granduc Mountain. Plate 3. 3 Granduc Glory Hole photo showing surface traces of the Granduc and Western faults and sample locations, looking northeast. Plate 3. 4 massive basalt from the Granduc Footwall Series with weak pyrite veining. Plate 3. 5 Moderately to strongly sheared sill cross cutting the Granduc Footwall Series. Plate 3. 6 Strongly deformed limestone, Granduc Mine Series (hammer for scale). 81 Plate 3. 7 Prismatic zircon, sample GD-GC-01, quartz diorite sill (grain length = lOOu.) Plate 3. 8 Equant zircon, sample GD-GC-04, Footwall Series basalt (grain length = 50u) Plate 3. 9 Ovoid zircon, sample GD-GC-08, Mine Series basaltic andesite (grain length = IOOLI) Plate 3. 10 Felsic volcanic fragmental rock, southeast flank of Granduc Mountain, close to locality of sample GD-GC-02 (hammer for scale). Plate 3.11 Semi-massive sulphides (pyrite, chalcopyrite, and pyrrhotite) within an argillaceous matrix, Granduc B orebody. Plate 3. 12 Microcline-calcite-galena-sphalerite veins, which cross cut mafic volcanic rocks of the Granduc Footwall Series. 83 Plate 3.7 Plate 3.8 References Anderson, R.G. 1989. 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In Magmatism in the ocean basins, Edited by A.D. Saunders and M.J. Norry. Geological Society Special Publication 42: 313-345. 90 Theriault, R., J. 1990. Methods for Rb-Sr and Sm-Nd isotopic analyses at the geochronology laboratory, Geological Survey of Canada. In Radiogenic Age and Isotopic Studies, Report 2, Geological Survey of Canada, Paper 89-2, pp. 3-6. Woodsworth, G.J., Anderson, R.G., Armstrong, R.L., Struik, L.C, and van der Heyden, P. 1992. Plutonic Regimes. Chapter 15 In Geology of the Cordilleran Orogen in Canada, Geological Survey of Canada. Edited by H. Gabrielse and C.J. Yorath, p. 491-531. York, D. 1969. Least-squares fit of a straight line with correlated errors. Earth and Planetary Science Letters, 5: 320-324. 91 CHAPTER 4: U-PB GEOCHRONOLOGY, GEOCHEMISTRY, AND RADIOGENIC ISOTOPIC CHARACTERISTICS OF THE KUTCHO ASSEMBLAGE, HOST TO THE KUTCHO CREEK DEPOSIT, AND RELATED TECTONIC SLIVERS WITHIN THE CANADIAN CORDILLERA Plate 4.1. East face of Sumac Ridge, Kutcho Assemblage. 92 PART 1: Geological Setting, U-Pb Geochronology, and Radiogenic Isotopic Characteristics of the Permo-Triassic Kutcho Assemblage, North-Central British Columbia Abstract The Kutcho Assemblage is a latest Permian to Early Triassic volcano-sedimentary sequence within the fault-bounded King Salmon Allochthon. Volcanic and volcaniclastic rocks consist of massive to pillowed flows and tuffs of basaltic to basaltic andesitic composition, as well as flows, mass flows, and pyroclastic flows of rhyodacitic to rhyolitic composition. The volcanic sequence is intruded by gabbro, diorite, trondhjemite and quartz-plagioclase porphyry. Volcanic and intrusive rocks have tholeiitic magmatic affinities, with the exception of the gabbro sills and dykes, which are chemically similar to alkaline arc magmas. Cu-Zn volcanogenic massive sulphide mineralization at the Kutcho Creek deposit is hosted by rhyolite mass flows near the top of the Kutcho Assemblage. Rhyolite mass flows from the hangingwall and footwall to mineralization have U-Pb ages of 242 +/-1 Ma and 246 +11-S Ma, respectively, whereas a quartz-plagioclase porphyritic intrusion to the south of the mineralization has a U-Pb age of 244 +1-6 Ma. The age of the Kutcho Assemblage, and the primitive Pb isotopic signature of its contained massive sulphide mineralization (207Pb/206Pb = 0.83988-0.84112 and 208Pb/206Pb = 2.0517-2.0556) are inconsistent with formation as part of the adjacent Stikine or Quesnel terranes. Primitive Nd isotopic signatures (initial sm = +7.5 to +7.8), and trace and rare earth element chemistry indicate that volcanic rocks of the Kutcho Assemblage, related intrusions and VMS mineralization formed in an intraoceanic island arc environment, probably directly on oceanic crustal basement. Gabbro sills and dykes, which are interpreted to be slightly younger than volcanic rocks of the Kutcho Assemblage, appear to have formed in response to a change in the tectonic regime, perhaps as a result of a collisional event. 93 Introduction The Kutcho Assemblage, previously termed the Kutcho sequence and Kutcho Formation, occurs within the King Salmon Allochthon in north-central British Columbia. It is host to the Kutcho Creek Cu-Zn volcanogenic massive sulphide (VMS) deposit (Monger and Thorstad 1978; Bridge et al. 1986; Thorstad and Gabrielse 1986). The King Salmon Allochthon lies in thrust and fault contact with the island arc Stikine and Quesnel terranes, and the oceanic Cache Creek terrane (hereafter called Stikinia, Quesnellia and Cache Creek, respectively) (Fig. 4.1). The King Salmon Allochthon has been tectonically emplaced onto Cache Creek, which is a Mississippian to Jurassic oceanic melange composed of mafic to ultramafic volcanic and intrusive rocks, and thick sequences of shallow water carbonates, ribbon chert and argillaceous sediments (Monger et al. 1991). Stikinia and Quesnellia, which lie in fault contact with the King Salmon Allochthon, are Paleozoic and Mesozoic island-arc assemblages, consisting of mafic to felsic arc-related volcanic rocks and associated plutonic and sedimentary rocks (Monger et al. 1991). Stikinia is host to several significant VMS deposits, including the Tulsequah Chief, Granduc, and Eskay Creek deposits; VMS deposits of comparable size and grade are not known in Cache Creek or Quesnellia (Hoy 1986). Figure 4. 1 Regional setting of the King Salmon Allochthon (modified from Thorstad and Gabrielse, 1986). 94 In this study, magmatic rocks of the Kutcho Assemblage were dated by U-Pb methods. Samples were also analyzed for their Nd isotopic signatures and major, trace and rare earth element compositions. Lead isotopic compositions were determined for their contained sulphides. The ages determined in this study are of regional significance, as they define the time period in which the Kutcho Assemblage formed, and they bracket the age of VMS mineralization at Kutcho Creek. Radiogenic isotopic compositions, and lithogeochemical and REE signatures determined in this study and parallel studies (Barrett et al. 1996) provide new information on the tectonic setting and terrane affiliation of the Kutcho Assemblage, and the environment of formation of the Kutcho Creek deposit. New constraints on the timing and setting of VMS mineralization at Kutcho Creek has implications for mineral exploration. Geology of the King Salmon Allochthon The King Salmon Allochthon is composed of three lithologically distinct packages of rocks. From oldest to youngest these are: 1) volcanic rocks of the Kutcho Assemblage; 2) limestone and marble which have been correlated with the Upper Triassic Sinwa Formation (Monger and Thorstad 1978); and 3) argillaceous sediments and siltstones which have been correlated with the Inklin Formation of the Lower Jurassic Laberge Group (Gabrielse 1962) (Fig. 4.1). The Kutcho Assemblage is intruded by gabbro, diorite, trondhjemite and quartz-plagioclase porphyry. Previous studies on the Kutcho Assemblage include those by Gabrielse (1962) and (1978), Pearson and Panteleyev (1975), Panteleyev and Pearson (1976), Monger and Thorstad (1978), Thorstad (1983), and Thorstad and Gabrielse (1986). Extensive drilling, mapping and geophysical studies were conducted by Esso Minerals Canada and Sumitomo Metal Mining Co. Ltd. Results of this work are in part summarized by Bridge et al. (1986). Kutcho Assemblage Rocks of the Kutcho Assemblage can be broadly divided into southern and northern sequences (Fig. 4.2). The northern sequence, which hosts the Kutcho Creek deposit, consists of 95 northward younging rhyolitic mass flows and tuffs, overlain by fine-grained volcanic and sedimentary rocks, which are intruded by gabbro sills and dykes (Fig. 4.3). The southern sequence is compositionally bimodal and consists of basaltic and rhyolitic flows and tuffaceous sedimentary rocks, with minor argillite, cross-cut by mafic and felsic intrusive rocks. As a result of limited exposure and small scale folding and faulting within the central part of the Kutcho Assemblage, the relationship between the northern and southern sequences is unclear. The southern sequence may represent either a lower portion of the assemblage, or a fine-grained distal equivalent to the coarse, proximal mass flows of the northern sequence (P. Lewis, pers. comm. 1995). I I argillite II 1 il limestone I ° o I volcanic-derived conglomerate Kutcho Assemblage Northern sequence I • • I tuff-argil lite f< 's | quartz-feldspar porphyritic rhyolite h~ -H rhyolite lapilli to crystal tuff I" v v vl feldspar-quartz porphyritic rhyolite LEGEND Southern sequence I ' I felsic tuffs, flows, & derived sediments Kv-^v^l mafic volcanic rocks & tuffs I ! argillite Intrusive rocks 1= A gabbro IT^/il quartz-plagioclase porphyry \~ / / trondhjemite t^^'l diorite geologic contact fault syncline anticline trace of ore horizon ore zones (projected to surface) adit Figure 4. 2 Geology of die Kutcho Assemblage (modified from Holbek et al., 1991). 96 a © © a 0. TO argillite & siltstone intruded • by gabbro sills plagioclase/augite-porphy ritic. gabbro sills quartz - plagioclase porphyritic rhyolite with rare mafic and pummaceous fragments I242+/-1 Mai Kutcho Creek deposit massive sulphide +/- dolomite lenses sericite schist - felsic tuff/ sediment with coarse dolomite/pyrite alteration plagioclase-quartz porphyritic rhyolite |246+7/-SMa1 Figure 4. 3 Schematic stratigraphic section for the northern sequence of the Kutcho Assemblage. Within the northern sequence, the oldest exposed unit consists of coarse-grained plagioclase-quartz porphyritic rhyolite fragmental rocks which are interpreted to have formed from mass flows and pyroclastic flows. Plagioclase is the dominant phenocryst, with lesser quartz and granophyric intergrowths of plagioclase and quartz (Plate 4.2). This unit is overlain by rhyolite lapilli and crystal tuffs, which form the immediate footwall to massive sulphide mineralization at Kutcho Creek. The hangingwall to mineralization consists of coarse-grained quartz-plagioclase porphyritic rhyolite fragmental rocks of probable mass flow origin; quartz phenocrysts are dominant and are characterized by numerous embayments and undulatory extinction (Plate 4.3). Rhyolite within the footwall and immediate hangingwall to mineralization has experienced intense sericite-pyrite-carbonate alteration which obscures primary textures (Plate 4.4). Where preserved, angular to rounded fragments up to 25 cm across occur in a crystal-rich matrix. Interbedded within the rhyolite mass flows in the hangingwall are rare mafic and pumiceous clasts (Plate 4.5). These clasts are flattened parallel to the regional east-west foliation which affects all rocks in the Kutcho Assemblage to varying degrees, and appear to occur along specific stratigraphic horizons. 97 The hangingwall rhyolite is overlain by bedded felsic tuffaceous rocks which grade upward into argillite. Coarse-grained plagioclase-augite porphyritic gabbro sills and dykes intrude the upper portion of the volcano-sedimentary sequence, at and above the rhyolite-sediment contact (Plate 4.6). Rare narrow porphyritic gabbroic dykes also occur lower in the sequence of rhyolite mass flows; these are interpreted to be feeders to the extensive sills emplaced higher in the stratigraphy. Interaction zones between the gabbro and rhyolite, and the local presence of peperitic textures at the contact between gabbro and argillite suggest that gabbro sills and dykes were emplaced prior to lithification of the upper tuffaceous rhyolitic units and the overlying argillite. Minor alteration of the lower portions of the gabbroic bodies suggest emplacement during the waning stages of hydrothermal activity. Within the southern sequence, basalt occurs as massive to pillowed flows and tuffs, and rhyolite forms fine-grained quartz-plagioclase porphyritic flows, tuffs and coarse fragmental rocks. Thin beds of argillite occur as rare interbeds in the tuffaceous rocks; the presence of argillite and pillowed basalt indicate deposition of the Kutcho Assemblage in a subaqueous environment. Trondhjemite, quartz-plagioclase porphyry, and diorite intrude the southern sequence (Fig. 4.2). The mineralogy and texture of felsic units which intrude the southern sequence are very similar to those of the rhyolite in the northern sequence. These units consist predominantly of quartz and plagioclase, with no potassium feldspar. In both the rhyolitic volcanic rocks and porphyry, quartz and plagioclase occur as individual phenocrysts or as glomerocrystic and granophyric intergrowths (Plates 4.2 and 4.8). Unlike the other felsic units, the trondhjemite intrusion has a medium-grained equigranular texture. However, quartz-plagioclase intergrowths similar to those observed in porphyritic units also occur in the trondhjemite. Deformation in the Kutcho Assemblage is interpreted to have occurred in Jurassic time, contemporaneously with emplacement of the King Salmon Allochthon onto Cache Creek. Deformation resulted in a penetrative axial planar foliation and the formation of upright to overturned southwest vergent asymmetric to near isoclinal folds (Thorstad and Gabrielse 1986). 98 Sinwa and Inklin Formations Tuff and argillite at the top of the northern sequence of the Kutcho Assemblage are capped by coarse conglomeratic sedimentary rocks, composed primarily of clasts of the underlying mafic and felsic volcanic rocks, as well as rare limestone clasts (Fig. 4.2). Conglomerate is interbedded with, and overlain by lenses of foliated limestone and finely crystalline marble. Limestone interbedded with the conglomerate was sampled for micropaleontological analysis in an attempt to constrain the age of sedimentation, but microfossils were not recovered. Regionally, limestone which overlies the Kutcho Assemblage contains a sparse fauna of crinoids, corals, bryoza and pelecypods of probable Triassic age (Panteleyev 1977; Monger and Thorstad 1978). Upper Triassic index fossils reported by Panteleyev (1977) and Thorstad and Gabrielse (1986) from west of the Kutcho Assemblage include the bivalves Lima sp. (?), a probable Palaeocardita, (Minetrigonia, sp.) as well as a Mesozoic scleractinian coral; these rocks have been correlated with the Upper Triassic Sinwa Formation (Monger and Thorstad 1978) (Fig. 4.2). Overlying the limestone, and locally directly overlying the Kutcho Assemblage, are thick sequences of argillite and siltstone (Monger and Thorstad 1978). These sedimentary rocks, which have not yielded fossils in the Cry Lake map area, have been correlated with the Inklin Formation of the Lower Jurassic Laberge Group, on the basis of lithology and regional relationships (Monger and Thorstad 1978; Gabrielse 1962) (Fig. 4.2). Kutcho Creek deposit Mineralization at the Kutcho Creek deposit consists of three massive sulphide lenses interpreted to lie at the same stratigraphic level; from east to west these are the Kutcho, Sumac West and Esso West zones (Bridge et al. 1986). The sulphide lenses are conformable to bedding, with a general strike of 280° and dips of 45° to 50° north (Bridge et al. 1986) (Fig. 4.2). The largest and closest to surface of the three is the Kutcho lens, with a length of 1400 meters and a maximum thickness of 27 meters, containing probable reserves of 14 Mt, grading 1.8% copper, 99 3.0% zinc, 28 g/t Ag and 0.34 g/t Au (Holbek et al. 1991). The smaller, and deeper Sumac West and Esso West lenses have reserves of 5.3 Mt of 1.1% copper, 1.6% zinc, 14 g/t Ag and 0.10 g/t Au, and 3.3 Mt of 3.4% copper, 5.6% zinc, 63 g/t Ag and 0.56 g/t Au, respectively (Holbek et al. 1991). The lower margins of the sulphide lenses are gradational into pyrite-sericite-carbonate-altered lapilli to crystal tuffs (Bridge et al. 1986). Within the altered footwall, pyrite occurs as fine- to coarse-grained (1 mm to 1.5 cm) cubes; sericite is typically wispy and outlines the foliation in the rocks. Carbonate alteration consists of coarse-grained (0.5 to 1.5 cm) dolomite rhombs and fine-grained ankerite. The upper contacts of the lenses are typically sharp, and the immediate hangingwall to mineralization consists of coarse-grained quartz-plagioclase porphyritic rhyolite mass flows; localized alteration extends a short distance upwards into the hangingwall (Plate 4.4). The main sulphide minerals within the deposit are pyrite, sphalerite, chalcopyrite, and bornite, with minor chalcocite (Plate 4.9). Galena has been reported from within the sulphide lenses (Bridge et al. 1986), but we observed it only within cross cutting fractures during this study. Within the exploration adit a laminated dolomitic rock occurs as a discrete zone within massive sulphide of the Kutcho lens. This unit contains fine-grained disseminated sulphides, which in part define the laminations, and is brecciated and infilled with an assemblage of quartz-calcite-pyrite-bornite-covellite. The laminated dolomite facies is interpreted to be exhalitive in origin. The Kutcho Creek deposit is jointly owned by Homestake Canada Ltd. and Sumac Mines Ltd. (controlled by Sumitomo Metal Mining Co. Ltd.). The Kutcho lens has been considered for open pit mining due to its proximity to surface, the general topography of the area and continuity of the orebody (Bridge et al 1986). However, location and lack of road access has made development of the Kutcho Creek deposit non-economic to this time. Previous Correlations and Rb-Sr Geochronology The Kutcho Assemblage was originally correlated with the Lower Permian Asitka Group of the Stikine terrane on the basis of lithological similarities and a Lower Permian Rb-Sr whole rock age of 275 +25 Ma (Panteleyev and Pearson 1976; Monger 1977). However, the presence 100 of gradational contacts with overlying limestones and a second Rb-Sr whole rock age of 210 +10 Ma led Monger and Thorstad (1978) and Thorstad and Gabrielse (1986) to suggest that the Kutcho Assemblage was Upper Triassic in age. Thorstad and Gabrielse (1986) correlated the Kutcho Assemblage with Upper Triassic arc sequences, most notably the Takla Group, and concluded that the Kutcho Assemblage could have formed as an island arc built directly on Cache Creek. An Upper Triassic age designation for the Kutcho Assemblage implied that mineralization at Kutcho Creek formed in the same time interval as several other significant VMS deposits in the North American Cordillera, including the Windy Craggy (Orchard 1986), Greens Creek (Newberry et al. 1990) and Granduc (Childe et al. 1994) deposits. The Upper Triassic age assignment and correlations with VMS deposits elsewhere in the Cordillera is inconsistent with U-Pb zircon geochronology presented in this paper. U-Pb Geochronology of the Kutcho Assemblage Three units were dated using U-Pb zircon methods in the Geochronology Laboratory at the University of British Columbia. Samples included quartz-plagioclase porphyritic rhyolite from the immediate hangingwall to the Kutcho Creek deposit (KC-GC-01) and plagioclase-quartz porphyritic rhyolite from the footwall of the deposit (KC-GC-04), both from within the northern sequence, and quartz-plagioclase porphyry which intrudes the southern sequence (KC-GC-03) (Figs. 4.2 and 4.3). Multiple samples of the trondhjemite and hangingwall gabbro were also collected for U-Pb dating, but failed to yield zircon or other dateable minerals. Heavy mineral extraction procedures and U-Pb zircon analytical procedures follow those of Mortensen et al. (1995). Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Uranium and Pb analytical blanks were in the range of 1-2 and 6-15 picograms, respectively. Concordia intercept ages and associated errors were calculated using a modified York-II regression model (York 1969), and the algorithm of Ludwig (1980); ages were calculated using the decay constants recommended by Steiger and Jager (1977). Age assignments follow the time scale of Harland et al. (1990), except for the age of the Permo-Triassic boundary, which follows the designation of Renne et al. (1995). Analytical results are given in Table 4.1 and shown graphically in Figure 4.4. 101 Fraction1 Wt U Pb1 ""Ph3 Pb4 "Pb5 Isotopic ratiosQUa.*/.)' Isotopic datcs(Ma,±2o-ys mg ppm ppm wPb Pg (%) **PbP*V «W»U »W*Pb ^Ph/"^ *"PbP*U »"Pb/**Pb hangingwall quartz-plagioclase porphyritic rhyolite (KC-GC-01) Cjn,Nljx,t 0.096 137 5 2242 14 6.2 0.03791*0.10 0.2668±0.26 0.05104*0.18 239.9*0.5 240.1*1.1 242.8*8.5 >UMl4> 0.096 157 6 1656 21 7.0 00.03738±0.15 0.2627*0.28 0.05097*0.18 236.6*0.7 236.8*1.2 239.6*8.4 Mjn.Nl.p 0.717 138 5 3290 72 6.6 0.03755*0.45 0.2648*0.48 0.05114*0.13 237.7*2.1 238.5*2.1 247.0*6.0 N,m.Nl.p 0.280 140 5 3611 27 6.7 0.03822*0.15 0.2689*0.24 0.05102*0.13 241.8*0.7 241.8*1.0 241.8*6.0 0,m,NI,p 0.101 145 5 2916 12 6.8 0.03818*0.31 0.2686*0-39 0.05102*0.20 241.5*1.5 241.5*1.7 241.8*9.2 0.072 151 5 1774 15 6.5 0.03737±0.45 0.2633*0.52 0.05111*0.21 236.5*2.1 237.3*2.2 245.7*9.8 footwall plaeioclase-auartz porDhvritic rhyolite (KC-GC-04) A^n,N2.p 0.165 80 3 651 46 6.4 0.03690*0.16 0.2595*0.55 0.05101*0.47 233.6*0.7 234.3*2.3 241.1*21.7 D.m,N2,p 0.598 79 3 718 155 6.5 0.03704*0.12 0.2611*0.40 0.05112*0.32 234.5*0.5 235.6*1.7 246.3*14.7 Fyn,M2,p 0.424 74 3 656 Ul 8.7 0.03722*0.12 0.2624*0.566 0.05113*0.49 235.6*0.5 236.6*2.3 246.8*22.6 F,m,N2,p 0.630 78 3 989 123 6.8 0.03842*0.35 0.2704*0.54 0.05116*0.33 243.0*1.7 243.0*2.3 242.7*16.8 Gjn,N2,p 0.170 73 3 691 43 6.7 0.03747±0.13 0.2646*0.38 0.05122*0.33 237.2*0.6 238.4*1.6 250.6*13.0 H£N2,p 0.036 90 3 731 10 6.8 0.03624*0.17 0.2552*0.56 0.05108*0.48 229.5*0.7 230.8*2.3 244.6*21.9 plagioclase-auartz porphyry (KC-GC-031 D.m,Ml,p 0.192 103 4 649 71 8.2 0.03687±0.18 0.2601*0.44 0.05116*0.33 233.4*0.8 234.7*1.8 247.9*15.2 EXNI4) 0.368 101 4 1164 74 7.8 0.03641±0.10 0.2562*0.27 0.05102*0.19 230.6*0.5 231.6*1.1 241.6*8.6 F.m,Nl,p 0.303 76 3 1080 51 7.4 0.03728*0.11 0.2626*0.29 0.05110*0.21 235.9*0.5 236.8*1.2 245.2*9.5 •All fractions are air abraded; Grain size, smallest dimension: m=-134um+74nm, f=-74|*m; Magnetic codes: Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8A; Front slope for all fractions=20°; Grain character codes: p=prismatic, l=tips 2Radiogenic Pb 3Measured ratio corrected for spike and Pb fractionation of0.0043/amu ±20% (Daly collector) 4Total common Pb in analysis based on blank isotopic composition 'Radiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers model Pb composition at the ^b/^Pb date of fraction, or age of sample) Table 4 1 U-Pb zircon analytical data for samples from the Kutcho Assemblage. Zircons from the three units were visually and chemically similar. Zircons were small and prismatic (length.width = 2:1 to 3:1), with few inclusions, good clarity, and no visible cores, but numerous fractures and generally rough surfaces. Only the best quality grains, free of opaque inclusions were selected, and all zircon fractions were abraded prior to analysis (Krogh 1982). 102 0.040 0.036 0.25 0.27 207Pb/235U 0.27 ^Pb/^U 0.040 (c) o. CM 8 .038 0.036 0.25 207Pb/235U 0.27 Figure 4. 4 U-Pb concordia plots of a) hangingwall quartz-plagioclase porphyritic rhyolite (KC-GC-01); b) footwall plagioclase-quartz porphyritic rhyolite (KC-GC-04); and c) quartz - plagioclase porphyry (KC-GC-03). 103 Hangingwall quartz-plagioclase porphyritic rhyolite (KC-GC-OJ) Analysis of six fractions of zircon from this rock yielded zu'PbruoPb ages of 240 to 247 Ma (Table 4.1, Fig. 4.4a). Fractions O and N were overlapping and concordant; all other fractions were slightly discordant, probably as a result of post-crystallization Pb-loss. An Earliest Triassic age of 242 +/-1 Ma was calculated for this unit based on the ^Pb/^U ages and errors of the two concordant fractions. Footwall plagioclase-quartz porphyritic rhyolite (KC-GC-04) Analysis of six fractions of zircon from this rock yielded zl"Pbr"°Pb ages of 241 to 251 Ma (Table 4.1, Fig. 4.4b). Fraction F was concordant, whereas all other fractions were slightly discordant, probably as a result of post-crystallization Pb-loss. The weighted mean of the 207Pb/206Pb ages of all six fractions from this unit is 246 +/-7 Ma. A weighted mean age is considered valid for this rock due to the restricted range of 207Pb/206Pb ages and the lack of visible 207 206 cores within the zircon. An Earliest Triassic age of 246 +7/-5 Ma, based on the Pb/ Pb weighted mean age and upper intercept error of all fractions and the upper error limit for the 206Pb/238U age of concordant fraction F, is considered to be the best estimate of the age of this unit. Quartz-plagioclase porphyry (KC-GC-03) Analysis of three fractions of zircon from this rock yielded 207Pb/206Pb ages of 242 to 248 Ma (Table 4.1, Fig. 4.4c). All fractions were slightly discordant, probably as a result of post-crystallization Pb-loss. Because of the restricted range of 207Pb/206Pb ages and the lack of visible cores within zircon from this rock, the weighted mean Pb/ Pb age of all fractions (244 +1-6 Ma) is considered to be the best estimate of the age of this unit. Zircon geochemistry Zircon from all three units are characterized by low U concentrations; this feature is consistent with the low concentrations of high field strength and rare earth elements in rocks of 104 the Kutcho Assemblage (Tables 4.1 and 4.2). In addition, there is a strong positive correlation between the weight of individual zircon fractions and the total common Pb in each analysis (Table 4.1), indicating that the source of the common Pb may have been in the zircon crystals themselves, perhaps as microinclusions of colourless minerals with high common Pb concentrations, such as feldspar or apatite. The combination of low U concentrations and increasing common Pb with sample size contributed to relatively low ratios of radiogenic to common Pb (206Pb/204Pb) in zircon from the Kutcho Assemblage. Lithogeochemistry Major, trace, and rare earth (REE) element concentrations were determined for volcanic rocks of the Kutcho Assemblage and intrusions that cross cut it (Table 4.2). The chondorite-normalized Ce/Yb ratio (Ce„/Ybn) is used to describe the slope of the REE patterns, and therefore the degree of evolution of the rocks. Rocks of the Kutcho Assemblage are compositionally bimodal, comprising basalt to basaltic andesite and their intrusive equivalents (Si02 = 45 to 55%), and rhyodacite to rhyolite and their intrusive equivalents (Si02 = 65 to 80%) (Table 4.2). On the basis of chemical composition these rocks can be divided into four main groups: 1) felsic volcanic and intrusive rocks; 2) mafic volcanic rocks and related intrusions; 3) gabbro sills and dykes; and 4) compositionally distinctive mafic, pumiceous, and dacitic fragments and basalt. Felsic volcanic and intrusive rocks Rhyodacite and rhyolite are compositionally indistinguishable from trondjhemite and quartz-plagioclase porphyry (Table 4.2). These rocks are characterized by low concentrations of K and high field strength elements (HFSE) (i.e. Th, U, Zr, Hf), high concentrations of Na, and Zr/Y ratios of 2.2-4.0. (Table 4.2 and Fig. 4.5). Rhyolite, trondhjemite, and quartz-plagioclase porphyry have near-flat REE patterns (CtJYbn= 0.7-1.0), low overall REE concentrations, and negative Eu anomalies, the latter being consistent with plagioclase fractionation (Fig. 4.6a). Both the REE signatures and Zr/Y ratios of these rocks indicate a tholeiitic magmatic affinity. 105 Table 4.2 Major, trace, and rare earth element data for samples from the Kutcho Assemblage. Sample Number Lithology Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total % % % % % % % % % % KC-GC-01 HW qtz + plag porph. rhyolite 77.40 0.27 12.11 2.16 0.02 0.42 1.88 4.45 0.89 0.05 0.88 100.60 KC-GC-02 quartz + plag porphyry 71.25 0.51 13.51 4.46 0.08 1.20 2.34 4.80 0.25 0.09 1.89 100.40 KC-GC-03 trondhjemite 77.22 0.25 12.54 2.48 0.08 0.16 0.06 5.54 0.95 0.04 1.34 100.70 KC-GC-04 FW plag + qtz. porph. rhyolite 80.22 0.13 11.73 1.06 0.04 0.57 0.04 5.95 0.06 0.02 0.79 100.60 KC-GC-05 gabbro sill 47.57 0.80 11.76 12.79 0.22 8.02 11.55 2.76 2.59 0.77 1.94 100.90 KC-GC-06 gabbro sill 53.36 0.61 18.37 8.09 0.17 2.88 5.62 3.77 4.15 0.60 2.47 100.28 KC-GC-07 trondhjemite 74.16 0.50 13.55 3.37 0.05 0.98 0.26 6.05 0.14 0.09 1.22 100.39 KC-GC-08 gabbro sill 51.68 0.61 18.19 8.27 0.20 3.17 6.80 3.31 3.58 0.60 3.62 100.13 JTK-1 felsic fragment 77.67 0.21 10.17 3.26 0.06 0.41 3.70 3.00 0.66 0.04 1.44 100.66 JTK-2 mafic fragment 62.13 0.55 15.50 4.35 0.10 2.36 3.84 3.92 2.53 0.07 4.96 100.38 JTK-4 quartz + plag porphyry 72.86 0.54 13.43 3.68 <dl 0.81 1.73 5.67 0.29 0.12 1.27 100.47 JTK-6 chlorite schist 48.00 0.59 16.70 9.28 0.16 8.24 9.25 3.89 0.10 0.05 4.22 100.55 JTK-8 biotite - chlorite schist 63.78 0.63 15.62 6.66 0.14 2.89 1.22 5.56 1.38 0.08 2.54 100.54 JTK-11 rhyolite 76.47 0.40 12.51 2.62 0.13 0.19 0.16 5.78 0.08 0.07 1.41 99.83 JTK-13 basalt 47.90 1.11 16.40 11.05 0:21 7.92 9.58 2.88 0.03 0.12 3.51 100.79 JTK-18 basalt 50.28 0.81 16.59 9.77 0.14 8.79 7.24 3.40 0.12 0.09 3.40 100.72 JTK-19 quartz + plag porphyry 76.91 0.25 12.43 2.30 0.03 0.33 0.12 6.12 0.92 0.04 0.78 100.26 JTK-20 rhyolite 76.28 0.38 12.21 3.15 0.19 1.03 0.12 5.73 0.10 0.07 1.16 100.50 JTK-22 quartz + plag porphyry 66.08 0.57 14.64 6.39 0.16 3.01 1.79 5.50 0.16 0.08 2.62 101.03 JTK-27 plag. porphyritic fragment 64.77 0.44 14.62 6.48 0.09 3.62 4.45 2.53 0.93 0.10 2.61 100.70 JTK-28 pumiceous fragment 48.88 0.58 17.36 8.63 0.31 2.91 8.29 1.18 4.18 0.08 7.67 100.18 JTK-29 mafic fragment 44.36 0.51 15.96 6.48 0.28 1.95 13.25 2.47 3.36 0.07 11.05 99.84 JTK-30 gabbro dyke/sill 51.48 0.71 16.47 9.15 0.26 6.25 5.70 3.00 1.19 0.89 5.61 100.82 JTK-31 mafic fragment 41.20 0.94 14.34 7.41 0.32 8.62 8.46 3.69 0.10 0.06 14.19 99.49 JTK-34 vesicular basaltic flow 46.76 0.64 16.49 9.73 0.15 7.57 9.17 2.77 0.02 0.05 7.11 100.54 JTK-39 diorite 48.52 1.31 15.52 11.90 0.18 7.49 9.00 3.47 0.12 0.13 2.92 100.67 JTK-45 tuffaceous sediment 49.06 1.87 15.07 12.78 0.19 5.93 9.36 3.08 0.07 0.16 2.99 100.64 JTK-48 tuffaceous sediment ? 49.11 0.67 14.33 8.63 0.19 6.80 13.82 2.04 0.05 0.07 5.06 100.88 90-Kll-44.2m rhyolite 78.13 0.12 10.60 2.78 0.03 0.18 0.63 5.97 0.07 0.02 1.46 100.04 90-Kll-46.6m vesicular basaltic flow 55.23 2.01 14.28 11.36 0.18 5.02 3.69 6.13 0.04 0.28 2.19 100.48 90-Kll-132.6m fine tuffaceous sediment ? 45.59 1.31 15.91 9.51 0.12 6.63 11.56 3.76 0.03 0.11 5.00 99.64 90-Kll-214.9m fine tuffaceous sediment 61.46 0.46 17.67 4.87 0.08 2.16 3.21 7.17 0.31 0.11 1.91 99.44 90-Kll-443.2m basalt 47.44 1.13 16.20 10.04 0.16 7.92 11.25 2.86 0.05 0.10 3.01 100.28 detection limit 60 ppm 35 ppm 120 ppm 30 ppm 30 ppm 95 ppm 15 ppm 75 ppm 25 ppm 35 ppm Table 4.2 (con't). Sample Number BaO Co 0203 V Cu Ni Zn Ga Nb Rb Sr Pb Th u Y Zr ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm Ppm KC-GC-01 126 37 <dl 37 45 <dl 63 12.0 4.0 12.0 71.0 <dl <dl 2.0 39.0 125.0 KC-GC-02 57 25 16 58 166 16 50 15.0 4.0 2.0 102.0 <dl <dl 2.0 43.0 110.0 KC-GC-03 276 45 2 21 14 <dl 90 15.0 5.0 9.0 39.0 <dl <dl 3.0 55.0 175.0 KC-GC-04 13 42 <dl 10 24 <dl 55 12.0 5.0 1.0 21.0 <dl <dl 2.0 52.0 138.0 KC-GC-05 532 39 249 350 153 40 114 16.0 1.0 61.0 441.0 4.0 1.0 2.0 17.0 55.0 KC-GC-06 1125 27 15 162 292 5 168 18.9 3.0 73.8 708.5 4.7 2.2 <dl 22.3 87.9 KC-GC-07 60 34 <dl 37 13 <dl 66 13.5 5.0 <dl 37.3 <dl <dl <dl 36.5 132.4 KC-GC-08 495 27 22 170 238 5 161 17.6 4.4 63.9 267.6 8.2 1.3 <dl 22.6 86.7 JTK-1 322 36 11 36 9 3 43 12.0 4.0 8.0 73.0 <dl <dl 2.0 47.0 103.0 JTK-2 290 6 22 61 9 8 110 15.0 2.0 32.0 50.0 <dl <dl 6.0 73.0 44.0 JTK-4 34 17 <dl 44 12 <dl 98 13.0 4.0 4.0 104.0 <dl <dl 1.0 46.0 100.0 JTK-6 39 36 232 272 73 52 83 15.0 1.0 1.0 226.0 <dl <dl 4.0 14.0 24.0 JTK-8 234 10 <dl 155 28 <dl 110 17.0 3.0 11.0 39.0 <dl <dl 6.0 25.0 78.0 JTK-11 <dl 25 <dl 14 23 <dl 89 14.0 5.0 1.0 49.0 <dl <dl 2.0 42.0 121.0 JTK-13 24 34 254 331 5 43 100 16.0 2.0 2.0 156.0 3.0 <dl 9.0 21.0 45.0 JTK-18 23 31 407 274 43 80 109 15.0 2.0 2.0 106.0 <dl <dl 8.0 18.0 39.0 JTK-19 300 25 <dl 19 2 <dl 61 14.0 6.0 9.0 36.0 <dl <dl 3.0 44.0 175.0 JTK-20 3 18 13 22 85 1 669 14.0 5.0 2.0 34.0 <dl <dl 3.0 32.0 119.0 JTK-22 7 25 <dl 104 10 <dl 94 15.0 4.0 2.0 67.0 <dl <dl 5.0 23.0 79.0 JTK-27 183 38 74 154 58 8 91 12.1 3.6 13.9 121.3 <dl <dl <dl 29.1 80.8 JTK-28 553 36 27 272 85 14 165 23.2 3.0 60.0 102.5 7.2 <dl <dl 25.9 15.7 JTK-29 571 23 43 264 103 7 125 19.4 2.6 55.8 122.7 <dl <dl <dl 28.6 16.9 JTK-30 129 21 178 199 336 34 278 16.3 5.1 11.9 239.6 12.7 <dl <dl 18.2 63.6 JTK-31 88 39 351, 172 519 69 358 15.3 3.6 <dl 51.5 1.8 <dl <dl 31.6 49.3 JTK-34 70 31 288 244 58 49 91 13.2 2.8 <dl 260.2 <dl <dl <dl 10.6 16.9 JTK-39 148 43 244 384 78 33 104 16.5 3.1 <dl 225.8 <dl <dl <dl 21.3 49.1 JTK-45 148 33 148 359 37 21 112 17.9 3.3 <dl 103.2 <dl <dl <dl 27.1 72.1 JTK-48 69 32 552 249 51 83 90 14.5 2.9 <dl 222.8 1.0 <dl <dl 12.8 28.0 90-Kll-44.2m <dl 31 39 0.0 441 <dl 123 14.7 5.0 <dl 27.5 <dl <dl <dl 96.4 224.8 90-Kll-46.6m 139 34 60 263 64 16 134 18.0 3.2 <dl 59.4 <dl <dl <dl 54.4 185.1 90-Kll-132.6m 95 47 376 281 108 92 115 16.3 3.4 <dl 116.4 <dl <dl <dl 28.7 76.1 90-Kll-214.9m 67 18 19 68 68 6 79 16.3 5.1 1.8 116.9 <dl <dl <dl 8.0 50.8 90-Kll-443.2m 108 34 522 285 49 93 97 14.8 3.5 <dl 98.4 <dl <dl <dl 20.2 52.2 detection limit 17 ppm 10 ppm 15 ppm 10 ppm 2 ppm 3 ppm 2 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm Table 4.2 (con't). Sample Number Hf Sc Ta Th U W La Ce Nd Sm Eu Tb Yb Lu Ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm KC-GC-01 3.8 8.9 1.1 0.9 0.5 262 5.1 17 12 3.15 0.56 0.8 4.28 0.62 KC-GC-02 2.9 14.0 0.8 0.8 0.6 181 5.3 15 12 3.71 1.03 1.0 5.18 0.73 KC-GC-03 4.6 6.8 0.6 1.0 0.7 168 9.5 29 19 4.33 0.73 1.0 5.83 0.84 KC-GC-04 2.9 10.8 1.1 0.4 0.5 260 3.6 13 10 3.50 0.67 1.2 6.49 0.93 KC-GC-05 1.0 37.9 0.3 1.5 0.8 73 8.2 20 12 2.85 0.92 0.5 1.39 0.20 JTK-18 0.8 36.9 <dl 0.2 0.3 14 1.8 5 4 1.44 0.53 0.5 1.68 0.25 JTK-34 1.5 27.1 <dl <dl <dl 32 4.5 13 10 3.36 1.19 0.9 3.12 0.43 detection limit 0.2 ppm 0.1 ppm 0.3 ppm 0.1 ppm 0.1 PPm 1 ppm 0.1 ppm 1 ppm 1 ppm 0.01 ppm 0.05 ppm 0. 1 ppm 0.05 ppm 0.01 ppm o oo Mafic volcanic rocks and related intrusions Basalt to basaltic andesite and diorite have low concentrations of both HFSE and large ion lithofile elements (LLLE) (i.e. K, Ba, Rb, Sr, Pb) and Zr/Y ratios of 2.1-3.4 (Table 4.2 and Fig. 4.5). A sample of basalt (JTK-18) has low overall concentrations of REE and a slightly light REE depleted pattern (Cer/Ybn^ 0.8) (Fig. 4.6b). Similar to felsic rocks of the Kutcho Assemblage, mafic volcanic and related intrusive rocks have Zr/Y ratios and a REE pattern indicative of a tholeiitic magmatic affinity. 100 80 S o. 60 • 1 1 gabbro o mafic fragment • diorite # dacitic fragment a trondhjemite * pumiceous fragment • mafic volcanic • tuffaceous sediment X felsic volcanic • chlorite schist or intrusive A biotite-chlorite schist a _ tholeiitic Zr/Y = 4.5 transitional Zr/Y = 7 calc-alkaline 100 150 Zr (ppm) Figure 4. 5 Plot of Y versus Zr (fields from Barrett and MacLean, 1994). 250 Gabbro sills and dykes The chemical signature of the gabbro is distinct from that of mafic volcanic rocks of the Kutcho Assemblage. Gabbro has low concentrations of HFSE and Zr/Y ratios of 2.6-3.9, but high concentrations of P, Cu and LILE (Table 4.2 and Fig. 4.5). On a plot of P205 vs. K20 the fields for gabbro and basalt are distinct from each other, with gabbro having high P2O5 and variable K20, whereas basalt has both low P2Os and K20 (Fig. 4.7). 109 100 (a) 10 1 100 (b) Norm:chon BKC-GC-01 rhyolite " AKC-GC-02 qtz-plag porphyry xKC-GC-03 trondhjemite •KC-GC-04 rhyolite La Ce Nd Sm Eu Tb Yb Lu 10 : 1 100 (a) Norm: chon + JTK-18 basalt •JTK-34 basalt La Ce Nd Sm Eu Tb Yb Lu 10 r 1 Norm:chon A KC-GC-05 gabbro field for alkaline intrusions from Mt Polley f|] field for alkaline intrusions from MtMilligan f ' field for alkaline intrusions from the Iron Mask BatholiUi La Ce Nd Sm Eu Tb Yb Lu Figure 4. 6 REE plots of a) felsic volcanic and intrusive rocks; b) mafic volcanic rocks; and c) gabbro, fields of data from alkaline mafic rocks in Quesnellia at Mt. Polley (Fraser et al. 1995), Mt. Milligan (Barrie 1993), and the Iron Mask Batholith (Snyder and Russell 1995) are shown for comparison. 110 Gabbro is depleted in heavy-REE relative to light-REE, with a near-constant slope that decreases from La to Lu (CeJYbn= 3.6) (Fig. 4.6c). This pattern is significantly more evolved than those of volcanic rocks of the Kutcho Assemblage and is comparable to patterns determined for alkaline arc magmas which intrude the northern Cordillera (Fig. 4.6c). On a plot of alkali vs. silica the gabbroic sills and dykes have an alkaline affinity; in comparison, volcanic rocks of the Kutcho Assemblage and related felsic intrusions have a predominantly subalkaline affinity (Fig. 4.8). • gabbro • basalt O diorite Omafic fragment s pumiceous fragment # dacitic fragment 0.4 0.6 0.8 P2o5(wt%) 1.0 1.2 Figure 4. 7 Plot of K20 versus P2Os which differentiates tholeiitic basalt in the Kutcho Assemblage from gabbroic rocks which intrude the upper part of the assemblage and clasts in felsic fragmental units. Compositionally distinctive mafic, dacitic, and pumiceous fragments and basalt A distinct group of mafic to intermediate composition volcanic rocks is recognized on the basis of extremely low Zr/Y ratios (0.6-1.6) (Table 4.2 and Fig. 4.5). This group consist of dacitic (JTK-2), mafic (JTK-29, -31) and pumiceous (JTK-28) fragments interbedded with rhyolite mass flows of the northern sequence, and a vesicular basaltic flow (JTK-34) (Plate 4.5). The basalt from this group has a near-flat REE pattern (Cer/Yb„= 1.1), with slightly higher light REE concentrations than the other sample of basalt analyzed in this study (Fig. 4.6b). Sample JTK-28, -29, and -31 have high concentrations of alkali earths, whereas the other two have low concentrations (Fig. 4.9). The rocks from this group with the lowest Zr/Y ratios (0.6; JTK-2, -28, -29) have high concentrations of K and other LILE, the range of which overlap those of the gabbro, but low P concentrations, comparable to those of the basalts (Table 4.2 and Fig. 4.7); the rocks with slightly higher Zr/Y (1.6; JTK-31, -34) are more similar to basalt of the Kutcho Assemblage. Ill It is unclear from these data whether these mafic, dacitic, and pumiceous fragments and basalt represent the most primitive end member of mafic volcanic rocks in the Kutcho Assemblage, or if they are a second, extremely primitive alkaline to subalkaline basalt type characterized by low Zr/Y ratios but variable major element chemistry. Nd isotope data The Nd isotopic signatures of hangingwall rhyolite and gabbro from the northern sequence, and trondhjemite and quartz-plagioclase porphyry from the southern sequence were determined to further constrain potential magmatic source regions. Neodymium isotopic analyses were conducted by R. Theriault at the Geochronology Laboratory of the Geological Survey of Canada. Analytical procedures are described by Theriault (1990). Analytical uncertainty is ±0.5 6N<I unit; abundances of Sm and Nd were determined by isotope dilution and have an uncertainty of 1% or less. High positive initial 6Nd values of +7.5 to +9.1 for the five units analyzed attest to the primitive nature of volcanic and related intrusive rocks in the Kutcho Assemblage (Table 4.3). In Figure 4.8 these values are compared with those of island arc and oceanic terranes which now lie in proximity to the Kutcho Assemblage. The Nd isotopic signature of felsic rocks of the Kutcho Assemblage are comparable to those of mafic rocks of the Nicola Group (+6.8 to +7.8) and Fennel Formation (+7.7 to +10.2), the most primitive mafic rocks of the Stuhini Group (+1.3 to +7.7), and mafic to intermediate composition alkaline arc magmas which intrude Quesnellia and northern Stikinia (+2.7 to +7.9) (Samson et al. 1989; Jackson 1990; Smith and Lambert 1995; Cassidy et al. 1996; Childe this volume) (Fig. 4.9). Rhyolite from the Sitlika Assemblage, in central British Columbia has a comparable Nd signature to those determined in this study, whereas Paleozoic and Jurassic rocks from Stikinia have more evolved signatures (Samson et al. 1989; Childe 1997; Childe and Schiarizza 1997; Childe this volume) (Fig. 4.8). 112 Table 4 3 Neodymium isotopic data for samples from die Kutcho Assemblage. Sample Sm Nd U,Sm/mNd meas. MJNd/mNd _ 2 age1 (ppm) (ppm) (error x 10"6,2a) (present day) (Ma) (initial) KC-GC-01 3.69 11.84 0.1884 0.513024(5) +7.5 245 +7.8 HW quartz porphyritic mass flow KC-GC-02 3.88 11.93 0.1966 0.513029 (5) +7.6 245 +7.6 trondhjemite KC-GC-03 5.22 19.88 0.1586 0.512969(5) +6.6 245 +7.8 quartz + plagioclase porphyritic felsic intrusive KC-GC-05 3.77 14.97 0.1521 0.512942(9) +6.1 245 +7.5 HWgabbro JTK-34 3.58 10.51 0.2058 0.513117(12) +9.4 245 +9.1 basaltic flow 'used for the calculation of sNd (initial). 2error = ±0.5 eNd units. 0 1 1 1 1 1 1 0 100 200 300 400 500 Age (Ma) Figure 4. 8 eNd(initial) versus age plot of rhyolite from the Kutcho Assemblage, and trondhjemite, quartz-plagioclase porphyry, and gabbro intrusions. Shown for comparison are fields for various volcanic assemblages in the Canadian Cordillera (Stikine Assemblage and Stuhini Groups: Samson et al., 1989; Jackson, 1990; Childe, this volume; Hazelton Group: Samson et al., 1989, Bevier and Anderson, pers. comm., 1995; Childe, 1997; Sitlika Assemblage, Childe and Schiarizza, 1997; Bonaparte subterrane: Jackson, 1990; Smith and Lambert, 1995; Fennell Formation: Smith and Lambert, 1995). Pb isotope data Lead isotopic compositions were determined for copper- and iron-rich sulphides from the Kutcho, Esso West and Sumac West lenses. Analytical procedures are outlined in Childe (this volume), and analytical results are given in Table 4.4 and Figure 4.9. 113 Table 4 4 Lead isotopic data for samples from the Kutcho Assemblage. SAMPLE SAMPLE MTN? 204Pb/2MPb 207Pb/2<MPb 208Pb/2MPb 20,Pb/206Pb 208Pb/204Pb NUMBER LOCATION ('/.error)2-3 (% error)23 ('/.error)2-3 ('/.error)2-3 ('/.error)2-3 Kla Kutcho lens bn 18.514 15.568 38.039 0.84089 2.0546 (0.037) (0.037) (0.038) (0.004) (0.005) Klc Kutcho lens bn 18.508 15.561 38.044 0.84080 2.0556 (0.037) (0.031) (0.041) (0.020) (0.019) Kid Kutcho lens 18.471 15.520 37.895 0.84022 2.0517 (0.018) (0.017) (0.020) (0.007) (0.007) K2a Sumac West lens py 18.584 15.614 38.128 0.84000 2.0520 (0.086) (0.079) (0.089) (0.034) (0.024) K3a Esso West lens py+cp 18.434 15.507 37.872 0.84123 2.0545 (0.107) (0.107) (0.108) (0.014) (0.012) K3b Esso West lens py 18.494 15.555 37.986 0.84112 2.0540 (0.324) (0.324) (0.325) (0.020) (0.019) K4b Esso West lens py 18.377 15.451 37.774 0.84075 2.0538 (0.345) (0.344) (0.346) (0.025) (0.023) K6b Sumac West lens bn 18.510 15.546 37.990 0.83988 2.0524 (0.032) (0.029) (0.033) (0.012) (0.010) 1 mineral abbreviations: py=pyrite, bn=bornite, cp=chalcopyrite. 2 errors are quoted at the 2a (95% confidence) level. 3 values are corrected for instrument fractionation by normalization based on replicate analyses of the NRS-Q81 stanrlam 2.07 2.06 PH O JO 2.05 00 © 2.04 2.03 more radiogenic T Kutcho Creek deposit < (Early Triassic) Tulsequah Chief deposit (Late Mississippian) Granduc deposit (Late Traissic) Eskay Creek deposit (Middle Jurassic) • Kutcho lens • Esso West lens o Sumac West lens (J average 2a error direction of error due to fractionation 0.83 0.84 207pb/206pb Figure 4. 9 207Pb/206Pb versus 208Pb/206Pb plot of sulphides from the Kutcho Creek deposit. Shown for comparison are fields for the Tulsequah Chief, Granduc and Eskay Creek deposits (Childe, 1997; Childe et al., 1994; Childe, 1994). 114 Sulphides from the Kutcho Creek deposit have a restricted range of Pb isotopic compositions, with no distinction between values for the three sulphide lenses. For comparison, the Pb isotopic signature of Kutcho Creek mineralization is plotted relative to Early Mississippian to Middle Jurassic VMS deposits in Stikinia (Fig. 4.9) (Childe 1997; Childe this volume). The Pb isotopic signature of Permo-Triassic Kutcho Creek mineralization is distinctly more primitive than Mississippian to Jurassic VMS mineralization in Stikinia. Discussion Age of volcanic and intrusive rocks U-Pb dating indicates that rhyolite from the top of the northern sequence has an Early Triassic age of 242 +/-1 Ma, whereas rhyolite from the base of the northern sequence and porphyry which intrudes the southern sequence have ages of 246+7/-5 and 244 +1-6 Ma, respectively, which overlap the Permo-Triassic boundary. The Early Triassic age of 242 +/-1 Ma from the northern sequence constrains the minimum age of volcanism in the Kutcho Assemblage (Fig. 4.3). Within terranes of island arc affinity in the Cordillera, such as Stikinia and Quesnellia, the Late Permian to Early Triassic typically corresponds to a hiatus in volcanism, marked by regional unconformities (Gabrielse and Yorath 1991). The discrepancy between the age of the Kutcho Assemblage and island arc assemblages of Stikinia and Quesnellia suggests that the Kutcho Assemblage did not form as part of one of these terranes (Childe et al. submitted). The absolute age of the gabbro, which is chemically distinct from rocks of the majority of the Kutcho Assemblage remains unconstrained. However, rhyolite which is cross cut by the gabbro is dated at 242 +/-1 Ma and therefore provides a maximum age for the gabbro. In addition, field observations indicate that the gabbro was emplaced prior to both lithification of the stratified rocks that overlie the Kutcho Assemblage and cessation of the hydrothermal system associated with VMS mineralization. Consequently, the gabbro sills and dykes are interpreted to be only slightly younger than the felsic volcanic rocks which they intrude. 115 Lithogeochemistry and Nd isotopic signatures Basalt of the Kutcho Assemblage, and diorite which intrudes it have low-K tholeiitic compositions; a sub-group of basaltic to dacitic fragments and basalt is characterized by lower Zr/Y ratios and variable K concentrations. Rhyodacite and rhyolite of the Kutcho Assemblage, and trondhjemite and quartz-plagioclase porphyry which intrude it are low-K, high-Na tholeiites. The Zr/Y ratios and REE patterns of the felsic volcanic rocks are indistinguishable from those of the felsic intrusive rocks, and indicate derivation from tholeiitic magmatic sources. The Nd isotopic signatures of rhyolite, trondhjemite and quartz-plagioclase porphyry indicate derivation from primitive sources, with little or no interaction with old, evolved sialic crustal components. The similarity in age, mineralogy, Nd isotopic signature, and trace, major and rare earth element chemistry suggests that volcanic rocks of the Kutcho Assemblage and felsic intrusions which cross cut it are cogenetic. Gabbro has a high initial 8Nd, comparable to values determined for felsic volcanic rocks of the Kutcho Assemblage. However, the major and REE chemistry of these high-K alkaline gabbros is more evolved than that of mafic volcanic and related intrusions of the Kutcho Assemblage, and are chemically and isotopically similar to Triassic to Jurassic alkaline (shoshonitic) arc magmas which intrude Quesnellia and northern Stikinia (Barrie 1993; Mortimer 1987; Lang et al. 1995; Mortensen et al. 1995). Kutcho Creek deposit The Kutcho Creek VMS deposit is hosted by rhyolite mass flows in the northern sequence of the Kutcho Assemblage. The coarse, angular nature of rhyolite clasts, the lack of sorting and the presence of pumiceous fragments within the northern sequence indicates that VMS mineralization formed proximal to a felsic volcanic source. The Kutcho Creek deposit has previously been described as having aspects of both a Besshi-type deposit (Pearson and Panteleyev 1975), based primarily on the recognition of concordant, bedded cupriferous iron sulphide lenses, and a Kuroko-type deposit (Thorstad and Gabrielse 1986), based on the association of massive sulphide mineralization with thick sequences of what were interpreted to be predominantly calc-alkaline felsic volcaniclastic rocks, in a bimodal volcanic sequence. However, 116 the Cu-Zn-rich and Pb-poor metal suite, the lack of sulphate facies, and host rocks consisting of low-K tholeiitic rhyolite are fundamentally different from either Besshi- or Kuroko-type deposits. The U-Pb zircon ages of 242 +/-1 Ma and 246+7/-5 Ma determined for rhyolites from the northern sequence bracket the age of VMS mineralization at Kutcho Creek (Fig. 4.3). The Pb isotopic signature of Kutcho Creek mineralization is more primitive than signatures of Paleozoic to Jurassic VMS deposits in Stikinia, implying that Pb, and by implication other metals in the Kutcho Creek deposit were derived from more primitive sources than metals in VMS deposits in Stikinia. A latest Permian to Early Triassic age implies that the Kutcho Creek deposit did not form contemporaneously with Upper Triassic VMS deposits elsewhere in the Cordillera, and the isotopic data indicates that the Kutcho Assemblage and its contained mineralization is unlikely to have formed as part of Stikinia. Tectonic setting The primitive trace and rare earth element chemistry, and Nd isotopic signature of volcanic rocks from the Kutcho Assemblage suggests formation in an intraoceanic island arc built directly on oceanic crustal basement (Childe et al. submitted). Compositionally bimodal volcanic rocks, which are chemically and mineralogically similar to the Kutcho Assemblage have been documented by Vallier (1995) in the Blue Mountains Region of the northwestern United States. Vallier (1995) suggests that these rocks may have formed in the fore-arc portion of an intraoceanic island arc; Barrett et al. (1996) and Childe et al. (submitted) suggest that the Kutcho Assemblage may have formed in a similar tectonic environment. Cu-Zn VMS mineralization formed proximal to a felsic volcanic center near the end of this volcanic episode. Following rhyolitic volcanism, and during the waning stages of hydrothermal activity, gabbro dykes and sills intruded the upper part of the volcanic sequence and overlying volcano-sedimentary rocks. The gabbroic rocks have the chemical signature of alkaline arc magmas, in contrast with the low-K tholeiitic signature of the underlying felsic and mafic rocks. Alkaline arc magmas commonly form in complex tectonic settings related to subduction or post-subduction processes such as collision (eg Box and Flowers 1989; Barrie 1993; Lang et al. 1995; Cassidy et al. 1996). The Tabar-Feni chain of volcanic islands consists of alkaline arc magmas 117 compositionally similar to those which intrude the Kutcho Assemblage (Wallace et al. 1983; Kennedy et al. 1990; Mclnnes and Cameron 1994). The Tabar-Feni chain occurs in a fore-arc position and is interpreted to have formed following collision of the Ontong-Java oceanic plateau with the subduction zone and consequent cessation of subduction (Mclnnes and Cameron 1994). Although a collisional origin for the gabbroic rocks which intrude the Kutcho Assemblage cannot be demonstrated, a fore-arc setting similar to Tabar-Feni appears likely. Regardless, the chemistry of the gabbroic rocks which intrude the upper part of the Kutcho Assemblage strongly suggests that a tectonic change occurred immediately after formation of massive sulphide mineralization and the deposition of extensive rhyolitic mass flow deposits. Conclusions The Kutcho Assemblage consists of compositionally bimodal tholeiitic volcanic rocks of latest Permian to Early Triassic age. The volcanic sequence is intruded by compositionally similar diorite, trondhjemite and quartz-plagioclase porphyry, as well as alkaline gabbro. The age and Nd isotopic signature of volcanism, and the Pb isotopic signature of VMS mineralization indicate that the Kutcho Assemblage did not form as part of Stikinia or Quesnellia. The Kutcho Assemblage and contained mineralization is interpreted to have formed as part of an intraoceanic island arc. Gabbro which intrudes it may have formed in response to a change in the tectonic environment. 118 Table 4.5 Major and trace element data for 14 additonal samples of gabbro which intrude the Kutcho Assemblage. Sample Number Lithology Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total % % % % % % % % % % 90-K27-116m gabbro 49.06 0.71 13.27 11.48 0.20 7.25 10.25 2.70 2.96 0.84 1.62 100.50 E-53, 58.5m gabbro 47.12 0.71 1347 936 0.19 639 733 3.26 4X11 038 6.12 99.72 E-53, lOCrn gabbro 4838 0.62 16J.7 8.72 0.15 4.92 734 336 135 031 8.26 100.72 E-53,118m gabbro 44.26 0.66 14J.8 932 0.18 63 9.63 2.69 0.24 033 1148 10032 KT50, 60m gabbro 47.99 033 1238 12.94 0.21 8.14 1032 0.79 339 033 1.78 10033 86, 53m gabbro 5133 0.60 1749 833 0.15 3.15 7.16 3.06 4.05 031 3.29 10034 KT30, 44.5m gabbro 50.59 0.76 14.62 11.19 0.19 6.22 7.88 3.10 3.46 0.97 1.37 100.56 KT30,64m gabbro 47.23 0.81 15.51 12.20 0.19 6.79 6.87 2.71 4.38 1.02 2.57 100.50 KT30,101.5m gabbro 50.03 0.71 .14.45 10.30 0.19 5.68 8.45 2.63 3.74 0.90 3.06 100.33 KT30,150m gabbro 50.50 0.73 14.09 11.01 0.19 6.26 7.95 3.18 3.14 0.89 1.95 100.10 KT30,180m gabbro 49.50 0.75 13.80 11.57 0.20 7.02 8.17 2.53 3.75 0.94 2.07 100.51 KT30,201.5m gabbro 45.97 0.68 12.01 10.70 0.21 6.57 10.02 1.90 1.15 0.82 10.17 100.40 KT30,224m gabbro 46.02 0.63 15.08 9.36 0.16 5.81 6.35 4.54 0.95 0.80 10.61 100.45 detection limit 60 ppm 35 ppm 120 ppm 30 ppm 30 ppm 95 ppm 15 ppm 75 ppm 25 ppm 35 ppm Sample Number BaO Co Cr203 V Cu Ni Zn Ga Nb Rb Sr Pb Th U Y Zr ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm 90-K27-116m 459 37 364 280 247 62 151 15.2 3.4 64.8 274.4 5.3 2.9 1.1 17.3 47.0 E-53, 583m 258 36 281 267 646 51 215 153 3.1 1023 2192. 6.2 <dl <dl 17.0 54.7 E-53, lOCrn 126 33 163 195 606 35 211 16.2 33 213 2583 73 <dl <dl 16.9 553 E-53,118m 33 34 253 241 663 56 219 15 J. 33 4.2 335.7 15.2 <dl <dl 143 52.7 KT50, 60m 893 33 298 360 434 48 181 153 1.7 71-4 667.9 6.9 0.9 1.1 17 X) 43.7 86, 53m 1010 23 57 181 654 12 216 16 J. 23 753 9863 7.9 <dl <dl 143 543 KT30, 44.5m 575 38 258 275 637 46 219 15.3 1.8 71.2 714.2 9.6 <dl 1.1 17.5 56.7 KT30,64m 636 36 258 292 648 51 237 16.9 2.1 92.3 756.7 9.7 1.1 1.0 18.9 60.3 KT30,101.5m 624 32 240 254 489 44 185 14.9 2.8 78.2 342.9 8.5 <dl 1.5 17.4 56.4 KT30,150m 616 41 258 262 656 51 218 14.9 2.6 65.5 499.0 7.5 <dl 0.5 17.0 57.3 KT30,180m 658 36 297 274 542 54 209 15.4 3.0 76.1 339.5 8.5 <dl 2.3 18.9 56.6 KT30,201.5m 397 29 347 231 591 181 212 13.6 3.2 16.0 164.1 4.3 <dl 0.2 16.2 54.2 KT30,224m 269 30 165 199 452 32 191 15.4 4.3 13.2 124.7 2.3 <dl <dl 15.0 56.4 detection limit 17 ppm 10 ppm 15 ppm 10 ppm 2 ppm 3 ppm 2 ppm 1 ppm 1 ppm 1 ppm 1 ppm I ppm 1 ppm 1 ppm 1 ppm 1 ppm References Barrett, T.J., and MacLean, W.H. 1994. Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstones and younger volcanic rocks. In Alteration and Alteration Processes associated with Ore-forming Systems: Geological Association of Canada, Short Course Notes. Edited by DR. Lentz, 11: 433-467. Barrett, T.J., Thompson, J.F.H., and Sherlock, R.L. 1996. Geology and Geochemistry of the Kutcho Creek VMS deposit, Northern British Columbia. Exploration and Mining Journal. Barrie, CT. 1993. Petrochemistry of shoshonitic rocks associated with porphyry copper-gold deposits of central Quesnellia, British Columbia, Canada. Journal of Geochemical Exploration, 48, 225-258. Box, S.E., and Flower, M.F.J. 1989. Introduction to special section on alkaline arc magmas. Journal of Geophysical Research, 94: 4467-4468. Bridge, D.A., Marr, J.M., Hashimoto, K., Obara, M., and Suzuki., R. 1986. Geology of the Kutcho Creek volcanogenic massive sulphide deposits, northern British Columbia. 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A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8: 523-548. Jackson, J.L. 1990. Tectonic analysis of the Nisling, Northern Stikine and Northern Cache Creek terranes, Yukon and British Columbia. Ph.D. thesis, The University of Arizona, Tucson, Arizona, 200 pages. Kennedy, A.K., Hart, S.R., and Frey, F.A. 1990. Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea. Journal of Geophysical Research, 95: 6929-6942. Krogh, T.E. 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochemica and Cosmochemica Acta, 46: 637-649. Lang, J.R., Lueck, B.A., Mortensen, J.K., Russell, J.K., Stanley, C.R., and Thonjpson, J.F.H. 1995. Triassic-Jurassic silica-undersaturated and silica-saturated alkalic intrusions in the Cordillera of British Columbia: Implications for arc magmatism. Geology, 23: 451-454. Ludwig, K.R. 1980. Calculation of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, 46: 212-220. 122 Mclnnes, B.I.A., and Cameron, EM. 1994. Carbonated, alkaline hybridizing melts from a sub-arc environment: Mantle wedge samples from the Tabar-Lihir-Tanga-Feni arc, Papua New Guinea. Earth and Planetary Sciences, 122, 125-141. Monger, J.W.H. 1977. Upper Paleozoic Rocks of the Western Canadian Cordillera and their bearing on Cordilleran Evolution. Canadian Journal of Earth Sciences, 14: 1832-1859. Monger, J.W.H., and Thorstad, L. 1978. Lower Mesozoic stratigraphy, Cry Lake (1041) and Spatsizi (104H) map areas, British Columbia. In Current Research, Part A, Geological Survey of Canada, Paper 78-1 A, pp. 21-24. Monger, J.W.H, Wheeler, J.O., Tipper, H.W., Gabrielse, H., Harms, T., Struik, L.C., Campbell, R.B., Dodds, C.J., Gehrels, GE, and O'Brien, J. 1991. Part B. Cordilleran terranes. In Upper Devonian to Middle Jurassic assemblages, Chapter 8 of Geology of the Cordilleran Orogen in Canada. Edited by H. Gabrielse and C.J. Yorath. Geological Survey of Canada, 4: 281-327. Mortensen, J.K., Ghosh, D.K., and Ferri, P. 1995. U-Pb geochronology of intrusive rocks associated with Cu-Au deposits in the Canadian Cordillera. In Porphyry Deposits in the Northwestern Cordillera of North America. Edited by T.G. Schroter. G-2: 491-531. Mortimer, N. 1987. Late Triassic, arc-related, potassic igneous rocks in the North American Cordillera. Geology, 14: 1035-1038. Newberry, R.J., Brew, D.A., and Crafford, T.C. 1990. Genesis of the Greens Creek (GC) volcanogenic massive sulphide (VMS) deposit, SE. Alaska: A geochemical study. Geological Association of Canada Annual Meeting Program with Abstracts, pp. A-96. Orchard, M.J. 1986. Conodonts from western Canadian chert: Their nature, distribution and stratigraphic application. In Investigative Techniques and Applications, Ellis Horwood Ltd., Chichester, pp. 94-119. 123 Panteleyev, A. 1977. Kutcho Creek Map-Area (104I/1W). In Geological Fieldwork. Edited by N.C. Carter and R.J, Moir. B.C. Ministry of Mines and Petroleum Resources, p. 43. Panteleyev, A., and Pearson., D.E. 1976. Kutcho Creek Map-Area (104I/TW). In Geological Fieldwork, B.C. Ministry of Mines and Petroleum Resources, pp. 74-76. Pearson., D.E., and Panteleyev, A. 1975. Cupiferous Iron Sulphide Deposits, Kutcho Creek Map-Area (104I/1W); in Geological Fieldwork, B.C. Ministry of Mines and Petroleum Resources, pp. 86-92. Renne, P.R. 1995. Synchrony and Causal Relations Between Permian-Triassic Boundary Crises and Siberian Flood Volcanism. Science, 269: 1413-1416. Samson, S.D., McClelland, W.C., Patchett, P.J., Gehrels, G.E., and Anderson, R.G. 1989. Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera: Nature, 337: 705-709. Smith, A.D., and Lambert, R.S. 1995. Nd, Sr, and Pb isotopic evidence for contrasting origins of ;ate Paleozoic volcanic rocks from the Slide Mountain and Cache Creek terranes, south-central British Columbia. Canadian Journal of Earth Sciences, 32: 447-459. Snyder, L. and Russell, J.K. 1995. Petrogenetic relationships and assimilation processes in the alkalic Iron Mask batholith, south-central British Columbia. In Porphyry Deposits in the Northwestern Cordillera of North America. Edited by T.G. Schroter, G-2: 593-608. Steiger, R.H., and Jager, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 14: 359-362. 124 Thompson, J.F.H., Mortensen, J.K., and Lang, J.R. 1995. Magma suites and metallogeny -examples from the Canadian Cordillera. PACRTM '95 Abstracts, pp. 569-574. Theriault, R., J. 1990. Methods for Rb-Sr and Sm-Nd isotopic analyses at the geochronology laboratory, Geological Survey of Canada. In Radiogenic Age and Isotopic Studies, Report 2, Geological Survey of Canada, Paper 89-2, pp. 3-6. Thorstad, L.E. 1983. The Upper Triassic "Kutcho Formation" Cassiar Mountains, North-Central British Columbia. M.Sc. thesis, University of British Columbia, Vancouver, British Columbia, 271 pages. Thorstad, L., and Gabrielse, H. 1986. The Upper Triassic Kutcho Formation Cassiar Mountains, North-Central British Columbia. Geological Survey of Canada Paper 86-16, 53 pages. Vallier, T.L. 1995. Petrology of pre-Tertiary igneous rocks in the Blue Mountains region of Oregon, Idaho, and Washington: Implications for the geologic evolution of a complex island arc. In Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Petrology and tectonic evolution of pre-Tertiary rocks of the Blue Mountains Region. Edited by T.L. Vallier, and H.C. Brooks. U.S. Geological Survey Professional Paper 1438, pp. 125-209. Wallace, D.A., Johnson, R.W., Chappell, B.W., Arculus, R.J., Perfit, MR., and Crick, I.H. 1983. Cainozoic volcanism of the Tabar, Lihir, Tanga, and Feni Islands, Papua New Guinea: Geology, whole-rock analyses, and rock-forming mineral compositions. Bureau of Mineral Resources, Geology, and Geophysics, Australian Department of Resources and Energy, Report 243, 62 pages. York, D. 1969. Least-squares fit of a straight line with correlated errors. Earth and Planetary Science Letters, 5: 320-324. 125 PART 2: U-Pb Geochronology, Geochemistry and Nd Isotopic Systematics of the Sitlika Assemblage, Central British Columbia Introduction Rocks of the Sitlika assemblage occur within the eastern Sitlika Range and adjacent parts of the Hogem Batholith, east of Takla Lake in central British Columbia (Figs. 4.10 and 4.11). The Sitlika assemblage is currently the focus of a two year 1:50,000 scale mapping project headed by one of the authors of this report (Schiarizza and Payie, 1997; Schiarizza et al., 1997). This report presents U-Pb zircon geochronology for felsic volcanic and intrusive rocks, major and trace element analyses for the principal igneous lithologies of the Sitlika assemblage and Nd isotopic and rare earth analyses for rocks dated in this study. 125°45' 56° 5 km 55° 15 LEGEND Lower to Middle Jurassic E^l Hazelton Group LAAJ tuff, volcanic breccia Psimian to Lower Triassic SMka Assemblage (rekitrve ages unknown) [Hill phyUlte, argillite |.vi| rhyolite. basalt, tonalite greywacke. sittstone Upper Paleozoic K^n Cache Creek Group limestone, phytlite, chert, greenstone IN1KUSIVE ROCKS Mesozoic or Tertiary (?) syenite to granodiortte Jurassic |y$J Hogem Batholith Rermo-Trlasslc \^/\ serpentlnrre, harzburgtte rtver fault Figure 4. 10 Location and Generalized geology of the Sitlika Assemblage, northcentral British Columbia (after Paterson 1974). Previous work Volcanic and sedimentary rocks, directly east of the Takla Fault, were originally correlated with the Cache Creek Group (Armstrong, 1949). Further mapping by Paterson (1974) identified the presence of three principal lithologies within and south of the Sitlika Range: argillite, volcanic 126 rock and greywacke. Based on the occurrence of felsic volcanic and volcaniclastic rocks within this sequence, Paterson (1974) concluded that these rocks were not part of the Cache Creek Group, and hence informally named them the Sitlika assemblage. On the basis of similarities in lithologies and structural style Monger et al. (1978) suggested that the Sitlika assemblage may represent an offset portion of the Kutcho Assemblage, a fault-bounded volcano-sedimentary sequence which lies some 300 km north of the Sitlika assemblage. The Kutcho Assemblage is host to the Kutcho Creek volcanogenic massive sulphide deposit, with reserves of 17 Mt, grading 1.6% Cu and 2.3% Zn, 29 g/t Ag and 0.3 g/t Au (Bridge et al., 1986); the identification of displaced slivers of the Kutcho Assemblage in the Cordillera has implications for base metal exploration. Recent studies have documented the precise age and geochemical characteristics of the Kutcho Assemblage and provide a basis for comparison between the Kutcho and Sitlika Assemblages (Childe and Thompson, 1995; Thompson et al, 1995; Childe and Thompson, submitted). One of the most distinctive characteristic of the Kutcho Assemblage is the Permo-Triassic to earliest Triassic age of magmatism (Childe and Thompson, submitted). This time period is typically characterized by a regional unconformity in terranes of island-arc affinity in the Cordillera (Gabrielse and Yorath, 1991). Geology The Sitlika assemblage comprises greenschist facies metavolcanic and metasedimentary rocks in the central part of the Intermontane Belt. They are in fault contact with the Stuart Lake Belt of the Cache Creek terrane to the east, and juxtaposed against unmetamorphosed volcanic and sedimentary rocks of the Stikine terrane to the west, across the Late Cretaceous or Early Tertiary Takla fault. In the Kenny Creek - Mount Olson area, the Sitlika Assemblage is subdivided into three units, corresponding to the divisions originally defined by Paterson (1974) (Fig. 4.11). The volcanic unit comprises mafic to felsic flow and fragmental rocks, along with comagmatic intrusions. Mafic rocks are dominant, and include thoroughly reconstituted actinolite-epidote-chlorite schists, as well as more massive greenstone with variable preservation of vesicles, plagioclase phenocrysts and pillow structures. The subordinate felsic volcanic rocks include quartz-sericite schists, with or without relict quartz and feldspar phenocrysts, as well as massive feldspar porphyry and quartz-feldspar porphyry. Light grey felsic volcanic rocks also 127 Figure 4. 11 Generalized geology of the Kenny Creek - Mount Olsen area (after Schiarizza and Payie 1997), showing geochemistry and geochronology sample locations. 128 Upper Cretaceous Sustut Group •:- •.- - j Tango Creek Formation: polymictic conglomerate; sandstone, ; ;"• shale Jurassic or Cretaceous (?) * * * Medium to coarse grained biotite granodiorite STIKINE TERRANE Lower to Middle Jurassic Hazelton Group Medium to dark green, brownish-weathered andesite, basalt ry^ysj and associated breccias and tuffs; commonly feldspar or P \ v \ \ N| feldspar-pyroxene-phyric; lesser amounts of volcanic conglomerate, sandstone and siltstone Late Triassic to Early Jurassic (?) v • J Topley Intrusionsf?): Red to pink, fine to medium-srained •• ~ ~ 1 granite; lesser amounts of feldspar porphyry SITLIKA ASSEMBLAGE Middle to Upper Jurassic (?) Western clastic unit: dark grey phyllite and slate; foliated chert-pebble conglomerate and chert-grain sandstone; lesser amounts of foliated limestone and grey phyllite containing flattened sedimentary and volcanic-lithic granules Triassic (?) Eastern clastic unit: variably foliated siltstone, sandstone and conglomerate containing felsic volcanic and plutonic clasts; medium to dark grey slate and phyllite; locally includes foliated limestone, limestone conglomerate and green chloritic phyllite Early Triassic j Light grey, medium to coarse-grained tonalite; medium green, I medium-grained tonalite to quartz diorite Late Permian or Early Triassic Medium grained epidote-chlorite-feldspar schist to semischist; sericite-chlorite-feldspar schist; weakly foliated chloritized hornblende diorite Permian to Early Triassic Volcanic unit: medium to dark green chlorite schist, fragmental chlorite schist and pillowed metabasalt; chlorite-sericite schist containing felsic metavolcanic fragments; lesser amounts of quartz-sericite schist, quartz-feldspar porphyry, metasandstone and metachert CACHE CREEK TERRANE Pennsylvanian to Triassic Cache Creek Group Sedimentary unit: light to medium grey quartz phyllite, platy quartzite and metachert; lesser amounts of recrystallized limestone, dark grey phyllite, massive to pillowed greenstone, fragmental greenstone and chlorite schist; minor amounts of metasandstone Mafic unit: Medium to dark green, massive to pillowed ,JA"J greenstone, fragmental greenstone and chlorite schist; "A minor amounts of metagabbro, amphibolite, serpentinite, listwanite, slate, ribbon chert and metasandstone Ultramafic unit: serpentinite, serpentinized ultramafite and serpentine-magnesite-talc schist; serpentinite melange containing knockers of greenstone, diabase, amphibolite, chert and limestone; locally includes mariposite-quartz-magnesite-altered rock and nephrite Legend to accompany Figure 4.11. 129 constitute the dominant clast type in fragmental sericite-chlorite schists that are common within the unit. These fragmental rocks generally interfinger with pillowed mafic volcanic rocks, and may represent mass flow deposits derived from adjacent felsic volcanic buildups. Mafic to intermediate composition intrusive rocks within the volcanic unit include fine- to medium-grained feldspar-chlorite schist and semischist, derived from sills, dykes and small plugs of diabase, gabbro, and diorite. Felsic intrusive rocks include widespread dykes and sills of variably foliated quartz-feldspar porphyry, as well as a small multiphase tonalite stock that intrudes pillowed volcanic rocks and fragmental schist west of Diver Lake (Fig. 4.11). Clastic sedimentary rocks that crop out mainly east of the volcanic unit correspond to Paterson's (1974) greywacke division. These rocks rest stratigraphically above the volcanic unit in sections exposed north of Beaverpond Creek and west of Mount Bodine; the contact is abrupt but apparently conformable. The basal part of the eastern clastic unit comprises green chloritic phyllite containing lenses of recrystallized limestone and dolostone, as well as conglomerate and coarse sandstone. The conglomerates contain mainly felsic volcanic clasts, with some felsic plutonic clasts, limestone clasts and mafic volcanic clasts. The volcanic and plutonic clasts are lithologically similar to rocks found within the Sitlika volcanic unit. Higher stratigraphic levels consist mainly of dark green slate intercalated with thin to thick, massive to graded beds of volcanic-lithic sandstone and siltstone. The eastern clastic unit is not dated, but is presumed to be Early Triassic and/or younger as it conformably overlies the volcanic unit. Clastic metasedimentary rocks that crop out west of the Sitlika volcanic unit are equivalent to Paterson's (1974) argillite division, and consist of dark grey phyllite, chert-pebble conglomerate, chert-quartz sandstone, and limestone. These rocks are not well exposed, but apparently form a narrow continuous belt that occurs east of the Takla fault over the full length of the map area (Fig. 4.11). The contact with the adjacent Sitlika volcanic unit is not well exposed, but is inferred to be a fault. The western clastic unit is not dated, but is tentatively correlated with the Middle to Upper Jurassic Ashman Formation at the base of the Bowser Lake Group (Tipper and Richards, 1976). This correlation is based on their general lithologic similarity, and in particular on the predominance of chert clasts in the coarser clastic intervals, which apparently does not occur in older rocks found in this part of the central Intermontane Belt. 130 All three units of the Sitlika assemblage are characterized by a single penetrative cleavage or schistosity defined by the preferred orientation of metamorphic minerals and variably flattened clastic grains or volcanic fragments. This metamorphic foliation is axial planar to folds that are most commonly observed in the eastern clastic unit. The folds are upright, with axes that plunge north to northwest or south to southeast. North of Beaverpond Creek, the volcanic unit and lower portion of the overlying eastern clastic unit comprise a moderately east-dipping homocline cut by a steep, east-dipping cleavage. Farther east, the eastern clastic unit is repeated across several upright folds. The wide outcrop expanse of the eastern clastic unit thins dramatically to the south, apparently due to truncation along the fault system that marks the Sitlika - Cache Creek contact. The volcanic belt is correspondingly wider in the south, in part due to internal folding, as indicated by a faulted anticline and adjacent syncline that repeat the volcanic unit and overlying eastern clastic unit south and west of Mount Bodine. The Sitlika assemblage is bounded to the east by a unit of serpentinite melange that is included in the Cache Creek Group. Metasedimentary and metavolcanic rocks comprising the bulk of the Cache Creek Group farther east rest structurally above the serpentinite melange unit across an east-dipping thrust fault (Paterson, 1974). Limited structural data suggests that the Sitlika - Cache Creek contact (specifically, the contact between the Sitlika assemblage eastern clastic unit and the serpentinite melange) is a steeply dipping dextral strike-slip fault that postdates the contractional deformation within the Cache Creek Group (Schiarizza and Payie, 1997). U-Pb geochronology Two samples were collected for U-Pb zircon geochronology. These consisted of a tonalite from the Diver Lake area, with abundant 3-6 mm glassy quartz phenocrysts set in a fine grained crystalline groundmass (PSC95-16-4), and a rhyolite from the Mount Bodine area, with 1-2 mm quartz and plagioclase phenocrysts (SA-GC-01) (Plates 4.10 and 4.11). Sample preparation and U-Pb analyses were carried out at the Geochronology Laboratory of the University of British Columbia. The samples were processed and zircon was separated using conventional crushing, grinding, Wilfley table and heavy liquid techniques. All fractions 131 Fraction1 Wt U Pb2 f^Pb1 Pb 4 ""Pb5 Isotopic raliosQ 1 a.%) 6 Isotopic dates(Ma,±2CT)li mn ppm ppm wPb Pg '/. ^Pb^U ^Pb/"^ »W*Pb "W^ll  mPbP*U "W*Pb Mount Bodine rhvolite SA-GC-01 A,m,Ml,p 0.120 77 3 1730 14 9.7 0.04082±0.13 0.2892±0.29 0.05138±0.23 257.9±0.7 257.9*1.3 257.8*10.5 BJCMIJJ 0.122 60 2 604 29 8.9 0.03680*0.13 0.2603±0.43 0.05129±OJ4 233.0±0.6 234.9±1.8 253.9*15.7 CCMlj 0.102 33 2 917 14 8.1 0.03782±0.12 0.2671*0.35 0.05123*0.28 239.3*0.6 240.4*1.5 251.0*12.7 «M2,p 0.052 67 2 579 15 8.5 0.03714*0.14 0.2632±0.48 0.05140±0.40 235.1±0.7 237.2±2.0 258.7*18.2 Diver Lake tonalite PSC95-16-4 A,c,Nl,p 0.099 194 7 2469 19 7.8 0.03801*0.22 0.2682±0.31 0.05112*0.19 240.5±1.0 241.2*1.3 248.8±8.3 B,c,N2Ip 0.035 174 7 1254 12 8.6 0.03807*0.18 0.2689*0.40 0.05122*0.32 240.9*0.8 241.8*1.7 250.9*14.6 Cjn,Nlj> 0.140 244 9 4202 19 7.7 0.03771 ±0.11 0.2652*0.21 0.05100*0.13 238.6±0.8 238.8±0.9 240.7±5.9 D,c,Ml,p 0.285 230 37 6801 23 8.1 0.03816*0.12 0.2689±0.21 0.05111*0.11 241.4*0.6 241.8±0.9 245.7*5.0 E,c.Nl.p 0.196 199 32 3925 24 7.5 0.03816±0.I0 0.2686±0.21 0.05105±0.13 241.4±0.5 241.6±0.9 243.0±6.2 'All fractions are air abraded; Grain size, smallest dimension: c= +134|xm, m=-134nm+74um, f=-74|ini; Magnetic codes: Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8A; Front slope for all fractions=20°; Grain character codes: p=prismatic. 2Radiogenic Pb 'Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ±20% (Daly collector) *Toial common Pb in analysis based on blank isotopic composition 'Radiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers model Pb composition at the 207PbPo6Pb date of fraction, or age of sample). Table 4 6 U-Pb zircon analytical data for samples from the Sitlika assemblage. were air abraded prior to analysis, to reduce the effects of surface-correlated lead loss (Krogh, 1982). Zircon grains were selected based on criteria such as magnetic susceptibility, clarity, morphology and size. Procedures for dissolution of zircon and extraction and purification of uranium and lead follow those of Parrish et al. (1987). Uranium and lead were loaded onto single, degassed refined rhenium filaments using the silica gel and phosphoric acid emitter technique. Procedural blanks were 9 and 6 picograms for lead and uranium, respectively. Errors assigned to individual analyses were calculated using the numerical error propagation method of Roddick (1987) and all errors are quoted at the 2a level. Ages were calculated using the decay constants recommended by Steiger and Jager (1977). Common lead corrections were made using the two-stage growth model of Stacey and Kramers (1975). Discordia lines were regressed using a modified York-II model (York, 1969; Parrish et al., 1987). Uranium-lead analytical results are presented in Table 4.6. 132 a 0.040 0.039 § 0.038 _Q CM 0.037 L 0.036 0.25 pbrou 0.28 0.041 a? 0.0391 Sitlika Assemblage Mt. Bodine rhyolite SA-GC-01 258 +10/-1 Ma (based on concordant fraction A) CO CM n CL 0.037 0.035 _i i i i i i_ 0.25 0.26 0.27 0.28 207pb/235(J 0.29 0.30 Figure 4. 12 U-Pb concordia diagrams for a) Diver Lake tonalite (PSC95-16-4); and b) Mount Bodine rhyolite (SA-GC-01). 133 Diver Lake tonalite The Diver Lake tonalite (PSC95-16-4) contained abundant coarse-grained, prismatic zircon with few inclusions and good clarity. Analysis of five fractions yielded Pb/ Pb ages of 241 to 251 Ma. Fraction E was concordant, with a ^Vb/^U age of 241.4 Ma, while fraction D slightly overlapped concordia, with a Pb/ U age of 241.4 Ma (Fig. 4.12a). An Early Triassic age of 241 +/-1 Ma, which is based on the ^Pb/238!! age and associated errors of fractions E and D, is considered to be the best estimate for the age of this rock. Mount Bodine rhyolite The Mount Bodine rhyolite (SA-GC-01) contained a small quantity of fine-grained prismatic zircon with few inclusions and good clarity. Zircon from this rock was characterized by extremely low U concentrations (53 to 77 ppm), which is in part reflected in low Pb/ Pb ratios (Table 4.6). All of the zircon recovered from this rock was divided into four fractions, analyses yielded 207Pb/206Pb ages of 251 to 259 Ma. Fraction A was concordant, with a 206Pb/238U age of 257.9 Ma (Fig. 4.12b). A Permian age of 258 +10/-1 Ma, based on the 206Pb/238U age and 206pb/238u and 207pb/206pb errQrs 0f fi.act;on A, is considered to be the best estimate of the age of this rock. Geochemistry Major and Trace Elements A suite of thirteen igneous rock samples from the Sitlika assemblage were analyzed for major and trace element abundances. Based on these analyses, the Sitlika assemblage has a roughly bimodal distribution of compositions, containing basalt (47-50% Si02) and dacite to rhyolite, and their intrusive equivalents (62-85% Si02) (Table 4.7). The Mount Bodine rhyolite (SA-GC-01) has a high Si02 concentration (85%), combined with relatively low concentrations of other major elements (AI2O3, Fe203, CaO, Na20) which indicates silicification of this rock, and is 134 I: 8 3 B & 8 3 & s 5? 3 ST 00 fl> Table 4.7 Major and trace element data for rocks from the Sitlika assemblage. sample lithology Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total BaO number % % % % % % % % % % ppm SA-GC-01 rhyolite 85.25 0.16 8.40 1.58 0.02 0.00 0.15 4.53 0.23 0.02 0.25 100.59 <d/l 95-SA-07 dacite 67.86 0.70 11.37 5.93 0.26 2.46 0.44 3.04 0.55 0.19 4.35 100.31 103 PSC95-16-1-1 tonalite 74.47 0.30 13.45 2.55 0.05 0.65 1.95 5.09 0.69 0.06 0.99 100.28 181 PSC95-16-1-2 QFP 66.42 0.65 16.01 4.72 0.11 1.35 3.66 5.56 0.30 0.18 1.34 100.33 170 PSC95-16-2 QFP to tonalite 65.18 0.74 16.12 5.17 0.10 1.56 3.72 5.34 0.34 0.18 1.82 100.30 116 PSC95-16-4 tonalite 74.43 0.31 13.92 2.09 0.03 0.71 2.45 5.16 0.62 0.06 0.87 100.69 258 PSC95-16-9-3 pillowed metabasalt 49.00 2.17 16.40 12.62 0.19 4.79 5.08 5.38 0.04 0.24 3.85 99.85 191 PSC95-17-2 biot-chl. schist 48.53 1.81 14.64 15.65 0.24 5.56 6.37 3.79 0.45 0.11 3.42 100.67 257 PSC95-17-7-2 chlorite schist 62.09 1.38 15.02 8.18 0.17 2.53 2.68 7.23 0.18 0.20 0.98 100.68 132 PSC95-17-11 chlorite schist 46.98 1.53 16.15 12.36 0.19 5.04 9.05 4.24 0.07 0.28 4.70 100.67 135 PSC95-18-6-1 pillowed metabasalt 49.89 1.42 15.64 13.91 0.22 4.92 7.70 4.38 0.12 0.14 2.56 100.98 138 PSC95-22-2 chl-ser-qtz schist 74.14 0.12 14.68 1.29 0.04 0.35 0.31 7.05 1.28 0.02 0.82 100.13 125 PSC95-22-3 chlorite schist 43.91 1.35 17.13 11.78 0.18 6.84 7.89 4.20 0.22 0.18 6.68 100.46 119 Detection Limits (ppm): 60 35 120 30 30 95 15 75 25 35 17 sample Co Cr203 Cu Ni V Zn Ga Nb Pb Rb Sr Th U Y Zr number ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm SA-GC-01 47 <d/l 7 <d/l <d/l 68 9.8 5.3 <d/l <d/l 23.1 <d/l <d/l 40.0 180.3 95-SA-07 18 <d/l 83 <d/l 83 1219 19.0 3.3 160.9 4.6 35.6 <d/l <d/l 50.2 166.0 PSC95-16-1-1 54 <d/l 5 <d/l 25 57 13.9 4.1 <d/l 8.8 132.5 <d/l 4.3 27.4 139.3 PSC95-16-1-2 36 <d/l 4 <d/l 56 60 16.6 3.9 <d/l 2.7 200.2 <d/l 4.5 40.4 131.0 PSC95-16-2 14 <d/l 3 <d/l 68 59 15.0 4.1 <d/l 3.9 217.0 <d/l 4.4 42.7 117.8 PSC95-16-4 60 <d/l 4 <d/l 27 43 13.7 3.9 <d/l 5.4 110.9 <d/l 4.2 29.4 127.3 PSC95-16-9-3 44 66 46 11 402 136 20.1 3.6 2.3 <d/l 106.4 4.2 6.7 43.6 121.1 PSC95-17-2 45 35 32 <d/l 490 117 18.1 3.4 2.1 5.7 118.9 5.1 7.1 30.6 46.5 PSC95-17-7-2 19 <d/l 12 <d/l 164 Ul 19.5 4.0 1.0 1.0 73.9 1.3 5.8 53.2 268.1 PSC95-17-11 37 217 27 45 253 118 18.0 2.8 1.8 <d/l 133.2 4.0 6.6 37.6 109.9 PSC95-18-6-1 37 <d/l 53 3 391 126 19.2 3.0 1.6 <d/l 125.0 4.7 7.0 37.4 106.6 PSC95-22-2 30 <d/l 23 9 <d/l 60 17.2 8.2 <d/l 8.5 25.6 <d/l 4.0 85.5 233.7 PSC95-22-3 47 271 89 62 288 124 14.9 3.8 1.9 2.6 73.5 3.0 6.6 28.0 74.6 Detection 10 15 2 3 10 2 1 1 1 1 1 1 1 1 1 Limits (ppm): consistent with field observations. In a plot of Si02 vs. K20 (Pecerrillo and Taylor, 1976), unaltered rocks from the Sitlika assemblage lie within the field for low-K magmas (Fig. 4.13a). A plot of Zr vs. Y indicates that intrusive and volcanic rocks of the Sitlika assemblage have a predominantly tholeiitic magmatic affinity, with Zr/Y ratios of 1.5 to 5.0 (Fig. 4.13b and Table 4.7). Zr(ppm) Figure 4. 13 a) Si02 vs. K20 diagram for unaltered rocks of the Sitlika Assemblage (fields from Peccerillo and Taylor, 1976); b) Zr vs. Y diagram for all samples from the Sitlika Assemblage (fields from Barrett and MacLean, 1994). Rare Earth Elements Rare earth element (REE) concentrations were determined for samples of rhyolite, tonalite, and basalt from the Sitlika Assemblage (Table 4.9 and Figs. 4.14a and b). All three rocks are characterized by low REE abundances and near-flat REE patterns. A small negative europium 136 Table 4.8 Rare earth element data for samples from the Sitlika assemblage. sample Au Ag As Br Cs Hf Hg Ir Sb Sc Se Ta W La Ce Nd Sm Eu Tb Yb Lu number ppb ppm ppm ppm ppm ppm ppm ppb ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm SA-GC-01 <d/l <d/l <d/l <d/l <d/l <d/l <d/l <d/l 0.1 6.4 <o71 1.4 251 6.6 23 16 4.4 0.8 1.0 4.4 0.6 PSC95-16-4 <d/l «M <d/l <d/l <d/l 4.5 <o71 <d/l 0.6 6.1 0.8 1.6 302 5.8 <d/l 16 2.3 0.9 0.6 3.1 0.5 PSC95-18-6-1 <d/l «M 2.0 <d/l <d/l 2.9 <d/l <d/l 0.6 39.1 <d/l <d/l 15 4.3 <o71 16 3.6 1.4 0.9 3.6 0.5 Detection 2 2 1 0.5 0.2 0.2 1 1 0.1 0.1 0.5 0.3 1 0.1 1 1 0.01 0.05 0.1 0.05 0.01 Limits: ppb ppm ppm ppm ppm ppm ppm ppm ppm •o 3 ppm ppm ppm ppm ppm ppm ppm ppm ppm •a 3 ppm anomaly for the rhyolite is consistent with fractionation of plagioclase in this unit. With the exception of the europium anomaly, the patterns for the rhyolite and tonalite are extremely similar (Fig. 4.14a). The low overall REE concentrations and near-flat REE patterns for rocks of the Sitlika Assemblage suggest derivation from primitive magmatic sources. 100 10 Norm:chon ' SA-GC-01 rhyolite T PSC95-16-4 tonalite shaded area represents field for felsic volcanic and intrusive rocks of the Kutcho Formation (Childe and Thompson, in prep.) LaCe Nd Sm Eu Tb Yb Lu 100 : b 10 r 1 Norm:chon ° PSC95-18-6-1 basalt shaded area represents field for mafic fragmental rocks of the Kutcho Formation (Barrett et al., in press) LaCe NdSmEu Tb Yb Lu Figure 4. 14 Chondorite-normalized rare earth element diagram for a) rhyolite (SA-GC-01) and tonalite (PSC95-16-4), showing the field for felsic volcanic and intrusive rocks of the Kutcho Assemblage (Childe and Thompson, submitted), and b) basalt (PSC95-18-6-1) from the Sidika Assemblage, showing the field for mafic volcanic rocks of the Kutcho Assemblage (Childe and Thompson, submitted). Nd Isotopic Systematics The Nd isotopic ratio of rhyolite and tonalite dated in this study were determined to further constrain the degree of evolution of this magma. Isotopic analysis of the rhyolite was conducted by R. Theriault at the Geochronology Laboratory of the Geological Survey of Canada; 138 analysis of the tonalite was conducted at Memorial University. Analytical procedures are described by Theriault (1990). Abundances of Sm and Nd determined by isotope dilution have an uncertainty of 1% or less. Uncertainty for calculated 8Nd values is ±0.5 ewa units. Neodymium analytical results are presented in Table 4.9. An initial ENd value of +8.2 for the Mount Bodine rhyolite is one of the highest values reported for a felsic rock within the Cordillera and indicates derivation of this magma from primitive, unenriched magmatic sources, with no evidence for contamination by old, isotopically evolved sialic crust. An initial ENd value of +2.7 for tonalite that intrudes the Sitlika assemblage indicates that this unit is also derived from primitive magmatic sources. Table 4 9 Nd isotonic data for samples from the SiUika assemblage. Sample Sm (ppm) Nd (ppm) meas. ,<5Nd/'**Nd (error x 10"4, 2a) (present day) age1 (Ma) (initial) Mount Bodine rhyolite SA-GC-01 4.77 16.12 0.1789 0.513029 (6) + 7.6 258 + 8.2 Diver Lake tonalite PSC95-16-4 2.61 9.99 0.16158 0.512723 (19) + 1.7 241 +2.7 'used for the calculation of eNd (initial). 2error = +.0.5 eNd units, discussion Ages of 258 +10/-1 Ma and 241 +/-1 Ma, determined for rhyolite and tonalite, respectively, indicate that magmatic activity in the Sitlika assemblage was occurring in the Permo-Triassic, and in part overlapped in time with magmatism in the Kutcho Assemblage. Major and trace element chemistry shows that the Sitlika assemblage is composed of low-K intrusive and bimodal volcanic rocks with a tholeiitic magmatic affinity. Rare earth element chemistry and Nd isotopic systematics indicate derivation from primitive magmas, uncontaminated by old, evolved crust. Rocks of the Sitlika assemblage formed in the same time period as the Kutcho Assemblage, and the principal lithologies are indistinguishable from those of the Kutcho Assemblage. As such, this region represents a viable exploration target for Kutcho Creek-equivalent VMS mineralization in the Cordillera. 139 References Armstrong, J.E. (1949): Fort St. James Map Area, Cassiar and Coast Districts, British Columbia; Geological Survey of Canada, Memoir 252. Barrett, T.J., and MacLean, W.H. (1994): Chemostratigraphy and Hydrothermal Alteration in Exploration for VHMS Deposits in Greenstones and Younger Volcanic Rocks, in Alteration and Alteration Processes Associated with Ore-forming Systems, Lentz, D.R., Editor, Mineral Deposits Division, Geological Association of Canada, Short Course Notes Volume 11, pages 433-467. Bridge, D.A., Marr, J.M., Hasimoto, K., Obara, M., and Suzuki, R. (1986): Geology of the Kutcho Creek Volcanogenic Massive Sulphide Deposit, Northern British Columbia; in Mineral Deposits of the Northern Cordillera, Canadian Institute of Mining and Metallurgy Special Volume 37, Morin, J.A., Editor, pages 115-128. Childe, F.C, and Thompson, J.F.H. (submitted): Geological Setting, U-Pb Geochronology and Radiogenic Isotopic Characteristics of the Permo-Triassic Kutcho Assemblage, Northcentral British Columbia. Childe, F.C, and Thompson, J.F.H. (1995): U-Pb Age Constraints and Pb Isotopic Signature of the Kutcho VMS Deposit: Implications for the Terrane Affiliation of the Kutcho Formation, North Central British Columbia; Geological Association of Canada -Mineralogical Association of Canada Program with Abstracts, page A 16. Gabrielse, H., and Yorath, C.J. (1991): Tectonic synthesis, Chapter 18, in Geology of the Canadian Orogen in Canada, Gabrielse, H., and Yorath, C, Editors, Geological Survey of Canada, Geology of Canada, Number 4, pages 677-705. 140 Krogh, T. (1982): Improved Accuracy of U-Pb Zircon Ages by the Creation of More Concordant Systems Using Air Abrasion Technique; Geochemica et Cosmochemica Acta, Volume 46, pages 637-649. Monger, J.W.H., Richards, T.A., and Paterson, I. A. (1978): The Hinterland Belt of the Canadian Cordillera: New Data from Northern and Central British Columbia; Canadian Journal of Earth Sciences, Volume 15, pages 823-830. Paterson, I. A. (1974): Geology of the Cache Creek Group and Mesozoic Rocks at the Northern End of the Stuart Lake Belt, Central British Columbia; Geological Survey of Canada Special Paper 74-1, Part B, pages 31-41. Parrish, R.R., Roddick, J. C, Loveridge, W. D. and Sullivan, R. W. (1987): Uranium-Lead Analytical Techniques at the Geochronological Laboratory, Geological Survey of Canada; in Radiogenic Age and Isotopic Studies, Report 1, Geological Survey of Canada, Paper 87-2, pages 3-7. Peccerillo, A., and Taylor, S.R. (1976): Geochemistry of Eocene Calc-alkaline Volcanic Rocks from the Kastamonu Area, Northern Turkey; Contributions to Mineralogy and Petrology, Volume 68, pages 61-81. Roddick, J.C. (1987): Generalized Numerical Error Analysis with Applications to Geochronology and Thermodynamics; Geochimica et Cosmochimica Acta, Volume 51, pages 2129-2135. Schiarizza, P., and Payie, G. (1997): Geology of the Sitlika Assemblage in the Kenny Creek -Mount Olson Area (93n/12, 13), Central British Columbia; In Geological Fieldwork 1996, Grant, B., Editor, British Columbia Ministry Of Employment And Investment, Paper 1997-1, 79-100. Schiarizza, P., Payie, G., Holunga, S., and Wright, D. (1997): Geology, Mineral Occurrences and Geochemistry of the Kenny Creek - Mount Olson Area (93n/12, 13), British 141 Columbia; In Geological Fieldwork 1996, Grant, B., Editor, British Columbia Ministry of Employment and Investment, Open File 1997-2. Stacey, S.J. and Kramers, J.D. (1975): Approximation of Terrestrial Lead Isotope Evolution by a Two-stage Model; Earth and Planetary Science Letters, Volume 26, pages 207-221. Steiger, R. H. and Jager, E. (1977): Subcornmission on Geochronology: Convention on the Use of Decay Constants in Geo- and Cosmochronology; Earth and Planetary Science Letters, Volume 36, pages 359-362. Thompson, J.F.H., Barrett, T.J., Sherlock, R.L., and Holbec, P. (1995): The Kutcho VMS deposit, British Columbia: A Felsic Volcanic-hosted Deposit in a Tholeiitic Bimodal Sequence, Geological Association of Canada - Mineralogical Association of Canada Program with Abstracts, page A 104. Tipper, H.W., and Richards, T.A. (1976): Jurassic Stratigraphy and History of North-Central British Columbia; Geological Survey of Canada, Bulletin 270, 73 pages. Theriault, R.J. (1990): Methods for Rb-Sr and Sm-Nd Isotopic Analyses at the Geochronology Laboratory, Geological Survey of Canada, in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 89-2, pages 3-6. York, D. (1969): Least-squares Fitting of a Straight Line with Correlated Errors; Earth and Planetary Science Letters, Volume 5, pages 320-324. 142 PART 3: Evidence for Early Triassic Felsic Magmatism in the Ashcroft (921) Map Area, British Columbia Introduction This report presents U-Pb geochronology, and major, trace and rare earth element data for a package of felsic to intermediate composition volcanic and intrusive rocks. These rocks, previously correlated with the Nicola Group, occur within the western portion of the Ashcroft (921) map area in southwestern British Columbia (Fig. 4.15). TERTIARY lA A \ Kamloops Gp. CRETACEOUS iv v i undivided 1 1 volcanic rocks | Kinasvale-Spences ' 1 Bridge Gp. JURASSIC Ashcroft Formation PERMIAN TO JURASSIC \7£' I Mount lytton Complex PALEOZOIC TO JURASSIC |-_~| Cache Creek Gp. sedimentary rocks TRIASSIC H Pavillion Beds 'Nicola Group" PERMIAN Cache Creek Gp.-Marble Canyon Fm. PENNSYLVANIAN TO TRIASSIC I y I Cache Creek Gp. melange i I area of • 1 following figure ,au» — river SPENCES BRIDGE: Figure 4. 15 Generalized geology between Lillooet and Ashcroft, the outlined area is detailed in Figure 2 (FR-Y = Fraser River - Yalakom); (modified from Monger et al., 1991). 143 Rocks examined in the current study include silicic ash and crystal tuffs, and an intrusion of dioritic to tonalitic composition. These units are lithologically similar to volcanic rocks of the Kutcho Assemblage and contemporaneous plutonic bodies which intrude the Kutcho Assemblage, 950 km to the north (Marr, pers. comm., 1995; Childe and Thompson, 1995b). The objectives of this study were to obtain precise age and geochemical data for these rocks, to determine if they could represent an offset portion of the Kutcho Assemblage. Nicola Group The Nicola Group is a Late Triassic to Early Jurassic island arc assemblage within the Quesnel terrane. It is comprised of submarine to subaerial, predominantly mafic volcanic and volcaniclastic rocks, their intrusive equivalents and associated clastic and chemical sedimentary rocks (Preto, 1977; Monger etal, 1991). The Nicola Group has been broadly divided into western, central and eastern belts on the basis of lithology and lithogeochemistry (Mortimer, 1986; Monger et al, 1991). Variation from calc-alkaline to shoshinitic compositions from west to east has been interpreted to reflect eastward dipping subduction in the Nicola arc (Mortimer, 1986). Mafic and lesser felsic volcanic and intrusive rocks previously assigned to the western belt of the Nicola Group have been mapped in the Ashcroft (921) and Hope (92H) map areas by Preto (1977), Grette (1978), Travers (1978), Shannon (1982), and Monger and McMillan (1984). Kutcho Assemblage The Kutcho Assemblage, in north-central British Columbia, forms the lowermost unit in the fault-bounded King Salmon Allochthon. It is composed of tholeiitic bimodal volcanic rocks and sedimentary rocks and is intruded by contemporaneous, probably comagmatic plutonic rocks (Gabrielse, 1979; Thorstad and Gabrielse, 1986; Childe and Thompson, 1995a and b, submitted). Magmatism occurred in the Latest Permian to Early Triassic (Childe and Thompson, 1995a and b). The Kutcho Assemblage is host to the Kutcho Creek volcanogenic massive sulphide deposit, with reserves of 17 Mt, grading 1.6% Cu, 2.3% Zn, 29 g/t Ag and 0.3 g/t Au (Bridge et al, 1986). 144 Geology The current study examines felsic volcanic and intrusive rocks formerly assigned to the Nicola Group between the Martell and Bonaparte Faults, in the Venables Valley and Red Hill areas (Fig. 4.16). Within the Venables Valley area, Grette (1978) divided rocks into three main units. From inferred oldest to youngest these units are: (1) mafic to felsic volcanic rocks, related intrusive rocks, and volcanic derived sedimentary rocks; (2) thick, massive to bedded limestone; and (3) argillite and thin bedded limestone, with minor volcanic rocks. To the north, in the Red Hill area, Ladd (1981) proposed four subdivisions for volcanic rocks, thought to be interbedded, and hence of contemporaneous age, which were assigned to the Nicola Group. These are (1) felsic crystal tuffs characterized by large quartz grains; (2) chlorite-rich mafic schist, with relict phenocrysts; (3) silicified greenstone; and (4) altered massive chloritic basalt. On Red Hill, Ladd (1981) mapped felsic tuffs cross cut by a series of fine- to coarse grained granodioritic to tonalitic plutons. Three kilometers southwest of Red Hill, Ladd (1981) observed trondhjemite grading into rhyolite tuffs, and suggested that the intrusion was hypabyssal, with the volcanic and intrusive units being emplaced in the same magmatic event. Felsic volcanic rocks from the Venables Valley and Red Hill consist of massive to bedded crystal to ash tuffs. Crystal tuffs are characterized by 2 to 5 mm diameter glassy quartz and/or plagioclase phenocrysts within a fine-grained quartzo-feldspathic matrix. Quartz and plagioclase commonly occur as broken grains, reflecting the pyroclastic nature of these rocks (Plate 4.12). The crystal tuffs contain scattered flow-banded, aphanitic clasts and sporadic layers of wispy chlorite, which may be remnants of flattened pumice fragments. Ash tuffs are characterized by a fine-grained quartzo-feldspathic matrix with rare 1 to 2 mm diameter quartz and plagioclase crystals. The diorite to tonalite body that intrudes the volcanic rocks on Red Hill has a medium-grained granitic texture, and contains varying proportions of plagioclase, hornblende and quartz, with minor secondary calcite and epidote (Plate 4.13). 145 LEGEND Quaternary | | alluvium Eocene |^ | Kamloops Group Cretaceous Intrusive Rocks Late Triassic to Jurassic V v^==j Guichon Creek Batholhh -4 granodioritx & qtz monzonite P" ^ | granodiorite Early Triassic l"^] Spences Bridge Group diorite to tonalite Jurassic p—r-i f x x trondjhemite Ashcroft Formation Upper Triassic (?) Hm&J Nicola Group Lower Triassic v v \rf felsic, with minor v v vl mafic volcanic rocks Permian to Triassic Cache Creek Group ' fault •yk" U-Pb geochronology or geochemistry sample location @) fossil locality: F1 late Camian to middle Norian (Travers, 1978; Monger and McMillan, 1984) F2 probable Late Triafnc (Orette, 1978) mineral occurrence road 0 km Figure 4. 16 Geology of the Spences Bridge - Cache Creek area (modified from Ladd, 1981; Monger and McMillan, 1984). 146 Biochronological constraints from south, east and north of the study area consist of probable Late Triassic conodonts from limestone (Grette, 1978), Late Triassic ammonites and pelecypods from argillite (Monger and McMillan, 1984), and middle Norian ammonites from sediments overlain by mafic volcanic rocks of the Nicola Group (Travers, 1978) (Fig. 4.16). However, these age determinations do not assist in constraining the age of felsic magmatism, as the dated sedimentary rocks are not found in conformable contact with the felsic rocks. Grette (1978) obtained an Early Jurassic Rb-Sr whole rock isochron age of 196 +/-15 Ma, with a Sr, of 0.7043 (+0.0002) from felsic volcanic and intrusive rocks within the study area; Grette attributed the elevated Sr, to contamination by seawater Sr. The isochron is highly dependent on a sample of altered dacite. Hydrothermal alteration has the potential to perturb Rb-Sr systematics (Shirey, 1991). Regardless of the mechanism of isotopic disturbance, the validity of this age determination is suspect. U-Pb geochronology One sample of tonalite (96A-7) and three samples of crystal tuff (96A-1, 96-A3, and CC-GC-01) were collected and processed for U-Pb zircon analysis, as described below. Of the four samples, only the tonalite contained sufficient zircon for U-Pb analysis. The low Zr concentrations in the crystal tuffs (65-91 ppm), along with the relatively fine grain size may be responsible for the scarcity of recoverable zircon in the tuffs. Sample preparation and U-Pb analyses were carried out at the Geochronology Laboratory of the University of British Columbia. The samples were processed and zircon was separated using conventional crushing, grinding, Wilfley table and heavy liquid techniques. All fractions were air abraded prior to analysis, to reduce the effects of surface-correlated lead loss (Krogh, 1982). Zircon grains were selected based on criteria such as magnetic susceptibility, clarity, morphology and size. Procedures for dissolution of zircon and extraction and purification of uranium and lead follow those of Parrish et al. (1987). Uranium and lead were loaded onto single, degassed refined rhenium filaments using the silica gel and phosphoric acid emitter technique. Procedural blanks were 9 and 6 picograms for lead and uranium, respectively. Errors assigned to individual analyses were calculated using the numerical error propagation method of Roddick 147 (1987) and all errors are quoted at the 2a level. Ages were calculated using the decay constants recommended by Steiger and Jager (1977). Age designations were based on the time scale of Harland et al. (1990) and the revised age of the Permo-Triassic boundary by Renne et al. (1995) Common lead corrections were made using the two-stage growth model of Stacey and Kramers (1975). Discordia lines were regressed using a modified York-II model (York, 1969; Parrish et al., 1987). Uranium-lead analytical results are presented in Table 4.10. Table 4 10 TJ-Pb zircon analytical data for a sample from the Ashcroft map area. Fraction1 Wt U Pb2 "W Pb4 JOSPb5 Isotopic ratiosfolo,'/.)6 Isotopic dalcs(Ma.±2o)< mg ppm ppm wPb Pg % "^Pb^U ^PbA'Hj ""Pb/^Pb **pb/238u ^Pb/"^ ""Pb/^Pb tonalile 96A-7 A, c,Nl,t 0.455 183 7 7528 B, c,Nl,t 0.204 139 5 8223 C. c,Nl,t 0.226 135 5 7877 D. m,Nl,t 0.265 169 6 8496 'All fractions are air abraded; Grain size, smallest dimension: c= +134u.m, m=-134ixm+74u.m Magnetic codes: Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8 A; Front slope for all fractions=20c Grain character codes: t=tabular ^diogenic Pb 3Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ±20% (Daly collector) •Total common Pb in analysis based on blank isotopic composition 5Radiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers (1975) model Pb composition at the ^"Pb/^Pb age of fraction, or age of sample) 26 8.3 0.03694±0.09 0.2601*0.09 0.05107±0.06 233.8±0.4 234.8*0.4 244.0±2.7 8 7.3 0.03712±0.13 0.2610*0.20 0.05100±O.U 235.0±0.6 235.5±0.8 240.9±5.2 9 7.6 0.03723±0.10 0.2615*0.19 0.05095*0.10 235.7±0.5 235.9*0.8 238.4*4.8 12 7.8 0.03666*0.14 0.2578*0.15 0.05101*0.05 232.1*0.7 232.9*0^6 241.2*2.5 Red Hill tonalite The tonalite (96A-7) contained abundant coarse-grained tabular to prismatic zircon with few colourless baguette-shaped inclusions and good clarity. Analysis of four fractions of zircon yielded 207Pb/206Pb ages of 238 to 244 Ma (Fig. 4.17 and Table 4.10). The error ellipse of fraction C slightly overlaps the concordia curve, with a 206Pb/238U age of 236 Ma, which provides a minimum age for this rock. However, an Early Triassic age of 242 +1-2 Ma, based on the weighted mean 207Pb/206Pb age of all four fractions, is considered the best estimate of the age of this rock. 148 § 0.036 0.038 242 +/-2 Ma (based on weighted mean age of all four 0.035 0.25 0.26 207 m. 0.27 Figure 4. 17 U-Pb concordia diagram for tonalite sample 96A-7. Geochemistry major and trace elements A suite of one intrusive and eight volcanic rock samples were analyzed for major and trace element abundances, using x-ray fluorescence at McGill University in Montreal, Quebec. Volcanic rocks range in composition from dacite to rhyolite (61-80% Si02, 0.19-0.30% Ti02), whereas the intrusive rock has a tonalitic composition (63% Si02, 0.62% Ti02) (Fig. 4.18a and Table 4.11). All samples have relatively low K20 (0.05-1.70%) and high Na20 (3.18-6.26%) concentrations. Volcanic and intrusive rocks analyzed in this study have Zr/Y ratios of 1.1 to 5.5 (Table 4.11). Using the limits defined by Barrett and MacLean (1994), these rocks have a predominantly tholeiitic magmatic affinity (Fig. 4.18b). rare earth elements Rare earth element (REE) concentrations were determined for samples of two rhyolite crystal tuffs and the tonalite by neutron activation analysis at Actlabs in Ancaster, Ontario. All samples are characterized by near-flat patterns (Fig. 4.19 and Table 4.12). These patterns, 149 Table 4.11. Major and trace element data for samples from the Ashcroft map area. sample lithology Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total BaO number % % % % % % % % % % ppm 96 A-1 qtz+plag crystal tuff 69.76 0.30 12.12 2.77 0.15 1.54 3.79 3.18 1.70 0.06 4.21 99.63 309 96A-2 qtz+plag crystal tuff 79.63 0.22 11.88 0.78 0.02 0.41 0.27 6.26 0.07 0.05 0.53 100.13 28 96A-3 qtz+plag crystal tuff 77.90 0.19 10.00 1.72 0.10 0.70 2.40 4.45 0.22 0.04 2.45 100.19 95 96A-4 plag crystal tuff 60.80 0.60 15.24 8.61 0.16 3.64 1.53 6.00 0.05 0.11 3.43 100.21 64 96A-5 siliceous ash tuff 76.77 0.26 9.78 2.79 0.10 1.22 2.00 4.95 0.20 0.07 2.36 100.52 37 96A-6 qtz+plag crystal tuff 75.58 0.26 12.18 1.96 0.08 0.63 1.27 6.15 0.20 0.06 1.66 100.04 46 96A-7 tonalite 62.54 0.62 15.59 7.11 0.13 2.78 4.83 3.27 1.61 0.09 1.97 100.61 433 CC-GC-01 qtz crystal tuff 76.79 0.24 13.70 0.57 0.01 1.12 0.23 4.46 1.45 0.04 1.30 99.95 251 CCH-SIT qtz+plag crystal tuff 76.51 0.27 13.65 1.23 0.00 0.20 0.27 5.90 0.92 0.03 1.03 100.03 137 Detection Limits (ppm): 60 35 120 30 30 95 15 75 25 35 17 sample Co Cr203 Cu Ni V Zn Ga Nb Pb Rb Sr Th U Y Zr number ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm 96 A-1 23 17 11 7 26 103 13 3.4 1.1 20.7 170.1 <d/l 4.5 35.6 84.0 96A-2 44 <d/l <d/l <d/l <d/l 51 10 3.7 <d/l <d/l 111.1 <d/l 3.6 34.2 108.1 96A-3 44 27 6 11 <d/l 72 10 3.0 <d/l 2.8 163.5 <d/l 3.8 33.7 91.4 96A-4 36 <d/l 6 4 197 104 16 4.3 2.7 <d/l 91.9 <d/l 5.2 25.6 27.8 96A-5 28 16 7 <d/l 49 67 8 3.8 <d/l <d/l 82.8 <d/l 4.1 27.4 60.7 96A-6 36 20 <d/l 7 23 61 10 4.7 <d/l 1.6 69.2 <d/l 3.9 21.5 79.1 96A-7 31 47 68 9 150 97 14 3.1 2.5 36.4 155.6 3.3 5.5 36.8 112.2 CC-GC-01 30 <d/l 4 <d/l 23 45 15 7.6 <d/l 14.5 61.6 <d/l <d/l 11.9 65.0 CCH-SIT 44 <d/l 12 <d/l 17 40 13 5.5 <d/l 7.9 75.4 <d/l <d/l 32.2 88.8 Detection 10 15 2 3 10 2 1 1 1 1 1 1 1 1 1 Limits (ppm) Table 4.12. Rare earth element data for samples from the Ashcroft map area. sample Au Ag As Br Cs Hf Hg Ir Sb Sc Se ' Ta W La Ce Nd Sm Eu Tb Yb Lu number ppb ppm ppm ppm ppm ppm ppm ppb ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm 96A-3 <d/l <d/l <d/l <d/l <d/l 3.3 <d/l <d/l 0.4 10.8 <d/l 0.9 212 5.5 14 9 2.64 0.80 0.7 3.17 0.44 96A-6 <d/l <d/l 1 <d/l <d/l 2.3 <d/l <d/l 0.5 7.9 <d/l 1.0 150 4.4 11 7 1.66 0.51 0.4 1.79 0.25 96A-7 <d/l <d/l 2 <671 1.1 4.5 <d/l <d/l 0.6 23.1 <d/l <d/l 108 6.5 19 11 3.18 0.86 0.8 3.48 0.48 Detection 2 2 1 0.5 0.2 0.2 1 1 0.1 0.1 0.5 0.3 1 0.1 1 1 0.01 0.05 0.1 0.05 0.01 Limits Ppb ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm combined with low REE concentrations indicate derivation of the rhyolite tuffs and tonalite from nrimitive maematic sources. Zr (ppm) Figure 4. 18 a) Ti02 versus Si02 diagram b) Zr versus Y diagram (Barrett and MacLean, 1994) for samples of volcanic and intrusive rocks from the Ashcroft area. 100 10 ; 1 i r—i 1 r——i r —i i i i 1 • - *96A-3 crystal tuff -- • 96A-6 crystal tuff -• 96A-7 tonalite -^8 _ Norm: chon 1 1 1 1 1 1 1 L. i i i i « i La Ce Nd Sm Eu Tb Yb Lu Figure 4. 19 Rare earth element diagram for one tonalite and two rhyolite crystal tuffs. 151 Discussion and Tectonic Implications An Early Triassic age of 242 +/-2 Ma for the tonalite dated in this study is older than rocks of the Nicola Group and contemporaneous plutonism, but is indistinguishable from rocks of the Kutcho Assemblage (McMillan et al., 1982; Monger and McMillan, 1984; Childe and Thompson, 1995a and b). The strong similarity between the REE patterns of the rhyolites and tonalite suggests that the volcanic and intrusive rocks were derived from the same magmatic source, and are probably of similar age. The low-K tholeiitic chemistry of these rocks is comparable to those of the Kutcho Assemblage, and dissimilar from the calc-alkaline to shoshinitic chemistry characteristic of the Nicola Group (Mortimer, 1986; Thompson etal, 1995; Childe and Thompson, submitted). Based on the data presented in this paper, felsic volcanic and intrusive rocks which occur between the Martell and Bonaparte Faults, near Ashcroft, are tentatively correlated with the Permo-Triassic Kutcho Assemblage, rather than the Late Triassic to Early Jurassic Nicola Group. Mafic volcanic rocks assigned to the Nicola Group occur both to the east and west of the Bonaparte Fault (Fig. 4.16). The presence of Late Triassic fossils imply that this correlation is valid for basaltic rocks which occur east of the Bonaparte Fault. However, the age of basaltic rocks that occur west of the Bonaparte Fault, in proximity to, and possibly interbedded with rhyolite tuffs, is not constrained. These basaltic rocks may be contemporaneous with the Early Triassic felsic rocks, rather than the younger Nicola Group lavas. Detailed mapping and geochemistry, accompanied by additional U-Pb geochronology are necessary in order to determine the extent of Early Triassic age rocks with primitive arc affinity in this region. VMS Potential The presence of rocks of Kutcho Assemblage age and affinity within the Ashcroft map area raises the potential for Kutcho Creek-equivalent Cu-Zn volcanogenic massive sulphide mineralization. 152 A number of copper occurrences are known within the Venables Valley - Red Hill area. The Red Hill showing (B.C. MINFJJJE 092INW042) contains chalcopyrite, chalcocite, and secondary Cu minerals that occur within chlorite and sericite altered pyritic greenstone, in proximity to Early Triassic rhyolite tuffs and tonalite. Also on Red Hill are unnamed gossans containing malachite and azurite (Ladd, 1981). Rhyolite tuff-hosted mineralization includes two unnamed copper occurrences, which occur approximately 1 km east of the trondhjemite body, southwest of Red Hill (Ladd, 1981). Based on the correlation of these rocks with the Kutcho Assemblage proposed in this paper, mineralized occurrences within this area should be explored for Cu-Zn VMS potential. 153 References Barrett, T.J., Sherlock, R.L. and Thompson, J.F.H. (1996): Geology and Geochemistry of the Kutcho Creek VMS deposit, northern British Columbia; Exploration and Mining Journal. Barrett, T.J. and MacLean, W.H. (1994): Chemostratigraphy and Hydrothermal Alteration in Exploration for VHMS Deposits in Greenstones and Younger Volcanic Rocks, in Alteration and Alteration Processes associated with Ore-forming Systems, Lentz, D.R., Editor, Geological Association of Canada, Short Course Notes volume 11, pages 433-467. Bridge, D.A., Marr, J.M., Hashimoto, K., Obara, M. and Suzuki., R. (1986): Geology of the Kutcho Creek Volcanogenic Massive Sulphide deposits, Northern British Columbia; in Mineral Deposits of Northern Cordillera, Canadian Institute of Mining Special Volume 37, Morin, J.A. Editor, pages 115-128. Childe, F.C. and Thompson, J.F.H. (1995a): U-Pb Age Constraints and Pb Isotopic Signature of the Kutcho VMS Deposit: Implications for the Terrane Affiliation of the Kutcho Formation, North Central British Columbia; Geological Association of Canada Mineralogical Association of Canada Program with Abstracts, page A 16. Childe, F.C. and Thompson, J.F.H. (1995b): U-Pb Age Constraints and Pb Isotopic Signature of the Kutcho VMS Deposit: Implications for the Terrane Affiliation of the Kutcho Formation, North Central British Columbia; The Gangue, Grant, B. Editor, Issue 49, pages 6-8. Childe, F.C. and Thompson, J.F.H. (submitted): Geological Setting, U-Pb geochronology, and Radiogenic Isotopic Characteristics of the Permo-Triassic Kutcho Assemblage, Northcentral British Columbia. 154 Gabrielse, H. (1979): Geology of the Cry Lake Map Area; Geological Survey of Canada, Open File Report 610. Grette, J. (1978): Cache Creek and Nicola Groups near Ashcroft, B.C.; unpublished M.Sc. thesis, University of British Columbia, 88 pages. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A., G. (1990): A Geologic Time Scale, 1989: 263 pages. Krogh, T. (1982): Improved Accuracy of U-Pb Zircon Ages by the Creation of More Concordant Systems Using Air Abrasion Technique; Geochemica et Cosmochemica Acta, Volume 46, pages 637-649. Ladd, J.H. (1981): A Report on the Geology of the Cache Creek - Nicola Contact Southwest of Ashcroft (921/11); in Geology in British Columbia, B.C. Ministry of Energy, Mines and Petroleum Resources, pages 91-97. McMillan, W.J., Armstrong, R.L. and Harakal, J. (1982): Age of the Coldwater Stock and Nicola Batholith, Near Merritt; in Geological Fieldwork 1981, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1982-1, pages 102-105. Monger, J.W.H. and McMillan, W.J. (1984): Bedrock geology of the Ashcroft (921) Map Area; Geological Survey of Canada, Open File 980. Monger, J.W.H, Wheeler, J.O., Tipper, H.W., Gabrielse, H., Harms, T., Struik, L.C, Campbell, R.B., Dodds, C.J., Gehrels, GE. and O'Brien, J. (1991): Part B, Cordilleran terranes; Upper Devonian to Middle Jurassic assemblages, in Geology of the Cordilleran Orogen in Canada, Gabrielse, H. and Yorath, C.J. Editors; Geological Survey of Canada, pages 281-327. 155 Mortimer, N. (1986): The Nicola Group: Late Triassic and Early Jurassic Subduction-Related Volcanism in British Columbia; Canadian Journal of Earth Sciences, Volume 24, pages 2521-2536. Parrish, R.R., Roddick, J. C, Loveridge, W. D. and Sullivan, R. W. (1987): Uranium-Lead Analytical Techniques at the Geochronological Laboratory, Geological Survey of Canada; in Radiogenic Age and Isotopic Studies, Report 1, Geological Survey of Canada, Paper 87-2, pages 3-7. Preto, V.A. (1977): The Nicola Group: Mesozoic Volcanism Related to Rifting in Southern British Columbia; Geological Association of Canada, Special Paper Number 16, pages 39-57. Renne, P.R. (1995): Synchrony and Causal Relations Between Permian-Triassic Boundary Crises and Siberian Flood Volcanism; Science, Volume 269, pages 1413-1416. Roddick, J, C. (1987): Generalized Numerical Error Analysis with Applications to Geochronology and Thermodynamics; Geochimica et Cosmochimica Acta, Volume 51, pages 2129-2135. Shannon, K. (1982): Cache Creek Group and Contiguous Rocks Near Cache Creek, B.C.; unpublished M.Sc. thesis, University of British Columbia, 72 pages. Shirey, S.B. (1991): The Rb-Sr, Sm-Nd and Re-Os Isotopic Systematics: A Summary and Comparison of Their Applications to the Cosmochronology and Geochronology of Igneous Rocks; Mineralogical Association of Canada Short Course Handbook on Applications of Radiogenic Isotope Systems to Problems in Geology, Heaman, L. and Ludden, J.N. Editors, pages 103-166. Stacey, S. J. and Kramers, J. D. (1975): Approximation of Terrestrial Lead Isotope Evolution by a Two-stage Model; Earth and Planetary Science Letters, Volume 26, pages 207-221. 156 Steiger, R. H. and Jager, E. (1977): Subcommission on Geochronology: Convention on the Use of Decay Constants in Geo- and Cosmochronology; Earth and Planetary Science Letters, Volume 36, pages 359-362. Thompson, J.F.H., Barrett, T.J., Sherlock, R.L. and Holbec, P. (1995): The Kutcho VMS Deposit, British Columbia: A Felsic Volcanic-hosted Deposit in a Tholeiitic Bimodal Sequence, Geological Association of Canada - Mineralogical Association of Canada Program with Abstracts, page A 104. Thorstad, L. and Gabrielse, H. (1986): The Upper Triassic Kutcho Formation, Cassiar Mountains, North-Central British Columbia; Geological Survey of Canada, Paper 86-16, 53 pages. Travers, W.B. (1978): Overturned Nicola and Ashcroft strata and their relation to the Cache Creek Group, Southwestern Intermontane Belt, British Columbia; Canadian Journal of Earth Sciences, Volume 15, pages 99-116. York, D. (1969): Least-squares Fitting of a Straight Line with Correlated Errors; Earth and Planetary Science Letters, Volume 5, pages 320-324. 157 PART 4: Primitive Permo-Triassic volcanism in the Canadian Cordillera: Tectonic and metallogenic implications Abstract Permo-Triassic volcanic rocks of the Kutcho and Sitlika Assemblages, and the Venables Valley - Red Hill area occur as fault-bounded slivers adjacent to the Cache Creek terrane in the Canadian Cordillera. Rocks from the three areas consist of isotopically primitive (initial em = +7.6 to +8.2), compositionally bimodal low-K tholeiitic basalt and rhyolite, with associated intrusions. These arc sequences are distinct in age, chemistry and radiogenic isotopic signature from other allochthonous arc sequences in the Cordillera. Data presented in this paper suggest that these primitive arc assemblages were built directly on oceanic crustal basement, and as such may represent a previously unrecognized intraoceanic arc terrane. Introduction The Kutcho Assemblage (KA), and overlying marine sediments comprise the King Salmon Allochthon, a tectonic wedge which lies in thrust and fault contact with the oceanic Cache Creek terrane and the island-arc Stikine and Quesnel terranes (Fig. 4.20a). The age, terrane affiliation, and tectonic setting of the KA have been the subject of considerable debate (e.g., Monger, 1977; Panteleyev and Pearson, 1976; Thorstad and Gabrielse, 1986). Recent U-Pb zircon geochronology has yielded Latest Permian to Early Triassic ages for volcanic rocks of the KA and plutons which intrude it (Childe and Thompson, 1995; Childe and Thompson, submitted), whereas geochemical (Barrett et al., 1996; Childe and Thompson, submitted) and radiogenic isotope (Childe and Thompson, submitted) data define a distinctive primitive arc chemistry for the KA. These characteristics are inconsistent with formation of the KA within the recognized elements of the adjacent Quesnel or Stikine island arc terranes, as defined by Souther (1991). In this paper the age, geochemistry and Nd isotopic signature of magmatic rocks of the KA are 158 compared with two lithologically similar, fault-bounded sequences which occur in similar tectonic positions within the Canadian Cordillera; these are the Sitlika Assemblage (SA) in the Manson River (NTS 93N) map area, and a sequence of felsic volcanic and intrusive rocks of the Venables Valley - Red Hill area (W-RH) in the Ashcroft (NTS 921) map area (Figs. 4.20b and c). Geology of the Kutcho Assemblage (KA) Childe and Thompson (submitted) divide rocks of the KA into northern and southern sequences; the stratigraphic relationship between the two sequences is unclear. The Kutcho Creek volcanogenic massive sulfide (VMS) deposit, with reserves of 14 Mt grading 1.8% Cu, 3% Zn, 29 g/t Ag and 0.3 g/t Au is hosted within rhyolite near the top of the northern sequence (Fig. 4.20a) (Bridge et al., 1986). The southern sequence contains compositionally bimodal volcanic rocks, consisting of interbedded basalt and rhyodacite to rhyolite, with minor sedimentary intervals. Mafic volcanic rock types consist of pillowed to massive basalt flows and possible basaltic tuffs. Felsic volcanic components consist of aphanitic to fine-grained plagioclase+quartz porphyritic flow and fragmental rocks, mass flows, and crystal to ash tuffs. Sedimentary rocks consist of thin (<0.5 m) beds of argillite. The presence of pillowed lavas and argillite attest to deposition in a subaqueous environment. Facing directions in the southern sequence are rare, but where present indicate younging to the south. The southern sequence is intruded by trondhjemite and quartz-plagioclase porphyry, as well as minor diorite (Fig. 4.20a). Quartz-plagioclase porphyry from the southern sequence has been dated at 244+6 Ma (Table 4.13) (Childe and Thompson, submitted). The northern sequence, which youngs consistently to the north, consists of fragmental coarse-grained plagioclase±quartz porphyritic rhyodacite, overlain by lapilli to crystal tuffs, which in turn are overlain by coarse-grained quartz+plagioclase porphyritic fragmental rocks. Massive sulphide mineralization occurs at the base of the upper unit (Fig. 4.20a). The coarse fragmental rocks, of probable mass flow and pyroclastic origin, are overlain by fine-grained rhyolite tuffs which grade upward into argillite. Plagioclase-augite porphyritic gabbros intrude the upper part of the volcanic section and overlying argillite. Tuff and argillite are overlain by conglomeratic sediments, derived at least in part from the underlying volcanic rocks, and lenses of unfossiliferous 159 Geology of the Kutcho Assemblage LEGEND Middle Triassic to Lower Jurassic (?) T7/\ Inklin & Sinwa w A Formations Upper Permian to lower Triassic Kutcho Assemblage liejwl northern sequence J southern sequence Paleozoic to Mesozoic Cache Creek terrane - j Stikine terrane Quesne] terrane INTRUSIVE ROCKS Upper Permian to Lower Triassic f7j gabbro fTj qtz.-pag. porphyry J7-T j trondhjemite ^m\» Kutcho Creek orebody (projected to surface) Geology of the Sitlika Assemblage 125°45' 156" LEGEND Lower to Middle Jurassic Hazelton Group r *" 1 tuff, volcanic breccia Permian to Lower Triassic own) Sitlika Assemblage (relative ages unki j phyllite, argimte jv v v I rhyolite, basalt, tonalite j' ' greywacke,sirtsrone Upper Paleozoic Cache Creek Group \/*\ limestone, phyllrte. chert, greenstone INTRUSIVE ROCKS Mesozoic or Tertiary (?) | syenite to granodiortte Jurassic £pj Hogem Batholith Permo-Triassic {//^ serpentlntte. harzburglte / river C Geology of the Venables Valley - Red Hills area 50M3' LEGEND Quaternary | aHuvlum Cretaceous ^ Spences Bridge Group Upper Triassic (?) BBj Nicola Group Lower Triassic I; *; ;| felsic, with minor kljU mafic vok:anlc rocks Permian to Triassic [^>^| Coche Creek Group Intrusive Rocks Late Triassic to Jurassic |^ + ^ granodiortte Early Triassic dlortte to tonalite |" , "| trondhjemite fault road river 50°30" 121'20' Figure 4. 20 Terrane map of the Canadian Cordillera, a) geology of the Kutcho Assemblage and adjacent rocks; b) geology of the Sitlika Assemblage and adjacent rocks; and c) geology of the Venables Valley - Red Hills area (modified from Thorstad and Gabrielse, 1986; Paterson, 1974; Monger and McMillan, 1984). limestone, which is correlated on the basis of lithology, with the Upper Triassic Sinwa Formation (Thorstad and Gabrielse, 1986). Plagioclase+quartz porphyritic rhyodacite in the lower part of the northern sequence has been dated at 246 +7/-5 Ma, and quartz+plagioclase porphyritic rhyolite has been dated at 242 +1 Ma (Table 4.13, Childe and Thompson, submitted). The later 160 date constrains the upper age limit for volcanism in the KA, but the base of the KA is not exposed, and therefore the maximum age of volcanism in the KA remains unconstrained. Geology of the Sitlika Assemblage (SA) The SA, which lies between the Takla and Vital faults in central British Columbia, consists of sedimentary and compositionally bimodal volcanic rocks (Paterson, 1974; Schiarizza and Payie, 1997; Childe and Schiarizza, 1997) (Fig.4.20b). Volcanic rocks of the SA consist of quartz and/or plagioclase porphyritic rhyodacite to rhyolite flows and tuffs, and pillowed to massive basalt flows, and are intruded by gabbro, diorite, and tonalite (Schiarizza and Payie, 1997); several volcanic-hosted base metal showings occur within the SA. Quartz porphyritic rhyolite from the SA, and equigranular to quartz+plagioclase porphyritic tonalite which intrudes it have been dated at 258 +10/-1 Ma and 241 +/-1 Ma, respectively (Table 4.13) (Childe and Schiarizza, 1997). Volcanic rocks of the SA are conformably overlain by marine sedimentary rocks, which include siltstone, sandstone, conglomerate, and limestone; this sequence is similar to sedimentary rocks which overlie the KA. Previous workers (Monger et al., 1978; Thorstad and Gabrielse, 1986) have observed that the S A resembles the KA in terms of general composition and structural style; these observations are now reflected in the tectonic assemblage map of the Canadian Cordillera (Wheeler and McFeely, 1991). The present study provides the first geochronological and geochemical data which verifies a correlation between the SA and KA. Geology of Volcanic and Plutonic Rocks in the Venables Valley - Red Hill Area (W-RH) Fault-bounded volcanic and plutonic rocks correlated by Grette (1978) and Ladd (1981) with the western belt of the Upper Triassic Nicola Group occur between the Martell and Bonaparte faults in the W-RH of southern British Columbia (Fig. 4.20c). Felsic volcanic rocks in the W-RH consist of massive to bedded rhyolite ash tuff and quartz and/or plagioclase crystal tuffs; mafic volcanic rocks consist of schistose and altered basalt, which may either be interbedded and coeval with the rhyolite tuffs, or fault-bounded slivers of the Nicola Group or Cache Creek terrane; volcanic-hosted Cu showings occur on Red Hill (Ladd, 1981). Trondhjemite intrusions 161 cross cut and grade into the rhyolite tuffs (Ladd, 1981). A diorite to tonalite body which intrudes the rhyolite tuffs was dated at 242 ±2 Ma, precluding correlation of the rhyolite tuffs with Late Triassic rocks (Childe et al., 1997). Geochemistry Major and Trace Elements The ranges of selected major and trace element concentrations, and Zr/Y ratios for rocks of the KA, SA and W-RH are summarized in Table 2. Volcanic rocks of the KA and S A are compositionally bimodal, with basalt and dacite to rhyolite, whereas those from the W-RH are dacite to rhyolite, but as discussed above may include interbedded basalt. Volcanic rocks from the three areas are geochemically similar and characterized by low K and Zr and high Na contents (Table 4.14). Zircon in the felsic volcanic and intrusive rocks in all areas have distinctly low U contents (Table 4.13). Based on the limits defined by Barrett and MacLean (1994), Zr/Y ratios from the three areas indicate tholeiitic magmatic affinities (Fig. 4.21). Felsic plutonic rocks from each of the three regions are comparable in composition to the felsic volcanic rocks which they intrude. E Q. Q. 125 100 75 50 25 -0 0 KA • SA A W-RH Zr/Y=2.0 50 100 150 200 Zr (ppm) tholeiitic affinity Zr/Y=4.5 —i 250 300 Figure 4. 21 Plot of Zr versus Y for volcanic rocks of the KA, SA, and W-RH, and associated felsic plutonic rocks (data from Childe and Thompson, submitted; Childe and Schiarizza, 1997; Childe et al., 1997). 162 Table 4 13 U-Pb zircon ages and initial eNd for the Kutcho Assemblage (KA), Sitlika Assemblage (SA), and felsic volcanic and intrusive rocks from the Venables Valley - Red Hill area (W-RH); abbreviations: eP = Early Permian IP = Late Permian, eTr = Early Triassic, *age designations are based on the time scale of Harland et al. (1991) and the age of the Permian-Triassic boundary determined by Renne et al., (1995); data sources: KA = Childe and Thompson, (submitted), SA = Childe and Schiarizza, (1997), W-RH = Childe et al., (1997), **error = +0.5 eNd units. Region Lithology U-Pb age • Period U in zircon (ppm) ** 8Nd (initial) KA qtz_-plag. porphyritic rhyolite 242 +/-1 Ma eTr 137-157 +7.8 KA plag.-qtz. porphyritic rhyolite 246 +7/-5 Ma lP-eTr 73-90 -KA qtz.-plag. porphyry 244 +/-6 Ma lP-eTr 76-103 +7.8 KA trondhjemite - - - +7.6 SA tonalite 241 +/-1 Ma eTr 17+244 -SA qtz. porphyritic rhyolite 258 +10/-1 Ma eP-lP 53-77 +8.2 W-RH tonalite 242 +/-2 Ma eTr 135-183 -Table 4 14 Ranges of selected major and trace element concentrations for rocks of the KA, SA and W-RH; data sources KA = Childe and Thompson, (submitted); SA = Childe and Schiarizza, (1997); W-RH = Childe et al., (1997). Region Lithology S1O2 K20 Na20 Zr Zr/Y Cc/Ybn (%) (%) (%) (ppm) KA basalt 41-52 0.02-0.12 2.86-3.69 17-45 1.6-2.6 0.8-1.1 KA rhyodacite to rhyolite 76-80 0.06-0.89 3.00-5.95 103-225 2.2-3.7 0.7-1.0 KA qtz.-plag. porphyry & trondhjemite 66-77 0.14-0.95 4.80-6.12 79-175 2.2-4.0 0.9-1.4 SA basalt 47-50 0.04-0.45 3.79-5.38 47-121 1.5-2.9 -SA dacite to rhyolite 62-85 0.18-1.28 3.0+7.23 166-268 2.7-5.0 1.3 SA tonalite 65-74 0.30-0.69 5.09-5.56 118-139 2.8-5.1 1.4 W-RH dacite to rhyolite 61-80 0.07-1.70 3.18-6.26 28-108 1.1-3.7 1.+1.5 W-RH tonalite 63 1.61 3.27 112 3.0 1.1 Rare Earth Elements Rare Earth Element (REE) plots of felsic volcanic rocks of the KA, SA, and W-RH, and felsic plutonic rocks are presented in Figure 4.22. The slope, and therefore degree of evolution, of REE patterns is described using the chondorite-normalized Ce/Yb ratio (Cen/Ybn), thereby removing the potential problem of La mobility due to alteration or metamorphism, and utilizing the better precision of Yb measurements. REE signatures from felsic rocks in all three areas are similar and are characterized by near-flat REE patterns (Cc/Ybn= 0.9-1.5) and low overall REE concentrations. All rocks, with the exception of tonalite from the SA, have negative europium anomalies, consistent with plagioclase fractionation. Within each area, volcanic and plutonic rocks have comparable REE signatures. Basalt from the KA reported by Childe and Thompson (submitted) have near-flat to slightly LREE depleted (Cen/Ybn = 0.8-1.1) REE patterns, consistent with derivation from an N-MORB-type source (Sun and McDonough, 1989). 163 100 F 10 a : Norm: chon B KC-GC-01 rhyolite a KC-GC-02 qtz-plag porphyry x KC-GC-03 trondhjemite • KC-GC-04 rhyolite 100 10 Norm: chon o96A-3 rhyolite tuff •96A-6 rhyolite tuff •96A-7 tonalite shaded area represents field for felsic volcanic rocks of the Kutcho Assemblage and related intrusions (from Fig. 3a) 100 La Ce Nd Sm Eu Tb Yb Lu La Ce Nd Sm Eu Tb Yb Lu 10 : 100 F Norm: chon * SA-OC-01 rhyolite • PSC95-16-4 tonalite shaded area represents field for felsic volcanic rocks of the Kutcho Assemblage and related intrusions (from Fig 3a) 10 r Norm: chon Jean Charcot Trough dacite Blue Mountains Region felsic tholeiites La Ce Nd Sm Eu Tb Yb Lu La Ce Nd Sm Eu Tb Yb Lu Figure 4. 22 Chondorite-normalized REE diagrams for a) rhyolite of the KA, and quartz-plagioclase porphyry and trondhjemite which intrude the KA (Childe and Thompson, submitted); b) rhyolite of the SA, and tonalite which intrudes the SA (Childe and Schiarizza, 1997); c) rhyolite tuffs of the W-RH, and tonalite which intrudes the rhyolite tuffs (Childe et al., 1997); and d) Permian to Triassic felsic volcanic and plutonic rocks of the Blue Mountains terrane, northwest United States (Vallier, 1995), and felsic volcanic rocks of the Jean Charcot Trough, Vanuatu arc, Southwest Pacific (Nakada et al., 1994). Nd Isotope Data Pdiyolite mass flows of the KA, and trondhjemite which intrudes it, have initial 8Nd values of +7.6 to 7.8, while rhyolite from the SA has an initial value of +8.2 (Childe and Thompson, submitted; Childe and Schairizza, 1997). These high positive values suggest derivation from primitive magmatic sources, with no incorporation of old, evolved sialic components. In comparison, felsic rocks of the Stikine island arc terrane have reported values of +2.7 to 5.8, which, while still primitive, are more evolved than those from the KA and SA (Samson et al., 1989; Bevier and Anderson, pers. comms., 1995; Childe, unpublished data). No comparable database exists for felsic rocks in Quesnellia. 164 Discussion Comparison of the KA, SA and W-RH Rocks of the KA, SA, and W-RH are comprised of Permo-Triassic quartz- and plagioclase-porphyritic felsic volcanic rocks, with related intrusions. Contemporaneous basaltic flow rocks exist in the KA and SA, and may also be present in the W-RH. Silicic rocks from the three areas include low-K, high-Na tholeiitic volcanic rocks and related plutons, with near-flat REE patterns; Nd isotopic signatures of these rocks in the KA and S A are extremely primitive. Basaltic volcanic rocks of the KA and SA are low-K, with a N-MORB REE signature. The similarities in mineralogy, chemistry, radiogenic isotopic signature and age indicate that rocks of the KA, SA, and W-RH could have formed either as a single assemblage, which was subsequently dispersed by faulting, or as separate arc assemblages which formed within the same environment in the same time period. It is not possible to distinguish between these two alternatives on the basis of field and laboratory data, and therefore we favor the second, more conservative interpretation. Tectonic Environment The chemical and isotopic characteristics of the KA, SA and W-RH summarized above are consistent with formation within an intraoceanic island arc environment. Barrett et al. (1996) have suggested that rocks of the KA probably formed within a fore-arc setting above a subduction zone, an interpretation which is consistent with the data presented in this paper. Permo-Triassic rocks with similar chemistry and mineralogy to those of the KA, S A and W-RH occur within the Wallowa and Baker terranes of the Blue Mountains Region (BMR) of Oregon, Idaho, and Washington (Vallier, 1995). These felsic volcanic-dominated sequences are characterized by low-K, high-Na tholeiitic quartz and/or plagioclase porphyritic dacite to rhyolite and trondhjemite, and low-K tholeiitic basalt, with near-flat REE patterns that remain almost constant with increasing Si02 (Ce„/Yb„ = 1.0-1.9, Fig. 4.22d) (Vallier, 1995). These rocks are interpreted by Vallier (1995) to have formed as early, probably fore-arc sequences, within a Permo-Triassic intraoceanic island arc. The recognition of several sequences of Permo-Triassic tholeiitic volcanic rocks throughout the Cordillera, usually occurring as tectonic slivers, suggests 165 ' that this magmatic event may have been widespread. The Kutcho Creek Cu-Zn and Iron Dyke Cu-Au VMS deposits are hosted within tholeiitic rhyolites of the KA and the Wallowa terrane of the BMR, respectively (Bridge et al., 1986; Bussey and LeAnderson, 1994). The presence of significant VMS mineralization in the KA and BMR indicates a potential for comparable deposits within rocks of this age and chemical affinity, such as the SA and W-RH, which are already know to host showings of possible volcanogenic origin, and other not yet recognized correlative tectonic slices. Magmatic Source Chemically similar rocks to the KA, SA and W-RH are found in the northern Jean Charcot back-arc Trough (NJC) of the Vanuatu arc, in the southwest Pacific. Igneous rocks from the NJC include a compositionally bimodal volcanic assemblage, with low-K (<1%) N-MORB-type basalt, and low-K (<1%), high-Na (>6%) dacite that compositionally resembles trondhjemite; basalt has a tholeiitic affinity, and dacite has a tholeiitic to transitional affinity (Nakada et al., 1994). The two rock types have comparable, near-flat REE patterns (basalt: Ce„/Ybn= 0.9-1.6; dacite: Cen/Ybn^ 1.6-1.7) and almost identical ratios of incompatible elements (Fig. 4.22d) (Nakada et al., 1994). Nakada et al. (1994) concluded that production of the dacite by partial melting of crustal material would not produce the geochemical coherence observed between the basalt and dacite; rather the high-Na dacite evolved from a mantle-derived basaltic magma as a product of fractional crystallization. Permian to Early Triassic tholeiitic compositionally bimodal rocks of the BMR, which are discussed above, are interpreted to have formed by the same process, but within a fore-arc setting (Vallier, 1995). Low-K, high-Na felsic lavas of the KA, SA and W-RH, and low-K basalt of the KA and SA exhibit nearly identical geochemical relations as bimodal rocks of the NJC and BMR. We suggest, that similar to the NJC and BMR, felsic rocks of the KA, S A and W-RH formed as the products of fractional crystallization of a mantle derived, low-K tholeiitic basalt. The chemical similarities between bimodal volcanic rocks of the NJC, a young back-arc trough, and the BHR, KA, SA and W-RH, which are interpreted to represent ancient fore-arc assemblages, suggest felsic magmas produced through the fractional crystallization of mafic melts can form in different tectonic settings within the arc environment. 166 Terrane Affiliation of the KA, SA and W-RH The present tectonic position of the KA, SA, and W-RH suggests that these rocks may have formed either as part of Stikinia or Quesnellia, directly on the Cache Creek terrane, or as a discrete, previously unrecognized intraoceanic arc terrane. Rocks of the KA, S A and W-RH formed in the Permian to Early Triassic. Within Stikinia and Quesnellia this period corresponds to a hiatus in volcanism, marked by regional unconformities; therefore correlation of the volcanic sequences from the KA, SA and W-RH with these terranes is unlikely. The primitive arc chemistry and Nd signatures of rocks of the KA, SA and W-RH indicate formation in an intracoeanic island arc, probably built directly on oceanic crustal basement. Thorstad and Gabrielse (1986) suggest that the KA formed on the Cache Creek terrane. However, where visible, the contact between the Cache Creek and KA, SA and W-RH is tectonic. Furthermore, the distinctive Tethyan faunal assemblages that characterize Cache Creek (Gabrielse and Yorath, 1991) have not been identified in the KA. In the French Range (FR), 150 km north of the KA, undated mafic to felsic volcanic rocks and Permian sedimentary rocks, which may be correlative with the KA, directly overlie Cache Creek rocks (Monger, 1975); further work in this area is required to document a potential relationship among the FR, KA, and Cache Creek terrane. Conclusions Low-K tholeiitic felsic ± mafic lavas and associated plutons of the KA, SA and W-RH are primitive arc magmas which have experienced no significant contamination by sialic crust, and probably formed in an intraoceanic island arc setting. These magmatic rocks are distinct in age, chemistry and radiogenic isotopic signature from the Stikine and Quesnel island arc terranes and may have formed on a substrate of oceanic affinity, such as the Cache Creek terrane. 167 Plate 4. 2 Quartz-plagioclase granophyric intergrowth in rhyolite, northern sequence (field of view = 8 mm). Plate 4. 3 Quartz grain with undulatory extinction, northern sequence (field of view = 8 mm). Plate 4. 4 Sericite-pyrite alteration in quartz-plagioclase porphyritic rhyolite in the immediate hangingwall of the Kutcho Creek deposit (hammer for scale). Plate 4. 5 Basaltic (dark) and rhyolitic (light) fragments in rhyolite mass flow from the northern sequence (ruler for scale). Plate 4. 6 Plagioclase-augite porphyritic gabbro (ruler for scale). Plate 4. 7 Rhyolite mass flow from the Kutcho Assemblage. 168 Plate 4.2 Plate 4.3 Plate 4. 8 Quartz-plagioclase glomerocryst. Plate 4. 9 Massive sulphide from the Kutcho lens, Kutcho Adit (mainly pyrite and chalcopyrite), small scale faults oudined by white chalk). Plate 4. 10 Photomicrograph of quartz-plagioclase glomerocryst in a quartzo-feldspathic groundmass, Mount Bodine rhyolite, sample SA-GC-01 (field of view = 5 mm). Plate 4. 11 Photomicrograph of intergrown quartz and plagioclase grains, tonalite, sample PSC95-16-4 (field of view = 5 mm). Plate 4. 12 Photomicrograph of rhyolite crystal tuff (96A-2) showing a broken quartz grain (photograph width 5 mm). Plate 4. 13 Photomicrograph of tonalite (96A-7) showing equigranular texture (photograph width 5 mm), p = plagioclase, q = quartz. 170 Plate 4.8 Plate 4.9 Plate 4.10 Plate 4.11 Plate 4.12 Plate 4.13 171 References Barrett, T.J., and MacLean, W.H., 1994, Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstones and younger volcanic rocks, in Lentz, DR., ed., Alteration and Alteration Processes associated with Ore-forming Systems: Geological Association of Canada, Short Course Notes, v.l 1, p. 433-467. Barrett, T.J., Sherlock, R.L., and Thompson, J.F.H., 1996, Geology and geochemistry of the Kutcho Creek VMS deposit, northern British Columbia: Exploration and Mining Journal. Bridge, D.A, Marr, J.M., Hashimoto, K., Obara, M., and Suzuki., R., 1986, Geology of the Kutcho Creek volcanogenic massive sulphide deposits, northern British Columbia: in Mineral Deposits of Northern Cordillera, Canadian Institute of Mining Special Volume 37, Morin, J.A, ed., p. 115-128. Bussey, S.D., and LeAnderson, P.J., 1994, Geology of the Iron Dyke Mine and surrounding Permian Hunsaker Creek Formation: in Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Stratigraphy, Physiography, and Mineral Resources of the Blue Mountains Region, Vallier, T.L., and Brooks, H.C., eds., U.S. Geological Survey Professional Paper 1439, p.151-181 Childe, F.C, and Thompson, J.F.H., 1995, U-Pb age constraints and Pb isotopic signature of the Kutcho Deposit: Implications for the terrane affiliation of the Kutcho Assemblage, north central British Columbia: Geological Association of Canada - Mineralogical Association of Canada Annual Meeting, Program with Abstracts, p. a 17. Childe, F.C, and Thompson, J.F.H., submitted, Geological setting, age, and radiogenic isotopic characteristics of the Permo-Triassic Kutcho Creek Cu-Zn VMS deposit, northcentral British Columbia: Canadian Journal of Earth Sciences. Childe, F.C, and Schiarizza, P., 1997, U-Pb geochronology, geochemistry and Nd isotopic systematics of the Sitlika Assemblage, northcentral British Columbia: in Geological 172 Fieldwork 1996, Grant, B., ed., B.C. Ministry of Employment and Investment, Paper 1997-1, p. 69-78. Childe, F.C, Friedman, R.M., Mortensen, J.K., and Thompson, J.F.H., 1997, Evidence for Early Triassic felsic magmatism in the Ashcroft (921) map area, British Columbia: in Geological Fieldwork 1996, Grant, B., ed., B.C. Ministry of Employment and Investment, Paper 1997-1, p. 117-124. Gabrielse, H., and Yorath. C.J., 1991, Tectonic synthesis; in Geology of the Cordilleran Orogen in Canada, H. Gabrielse and C.J. Yorath (ed.); Geological Survey of Canada, no. 4, p. 677-705. Grette, J., 1978, Cache Creek and Nicola Groups near Ashcroft, British Columbia: unpublished M.Sc. thesis, University of British Columbia, 88p. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A., G., 1990, A Geologic Time Scale, 1989: 263 pages. Ladd, J.H., 1981, A report on the geology of the Cache Creek - Nicola contact southwest of Ashcroft (921/11): in Geology in British Columbia, B.C. Ministry of Energy, Mines and Petroleum Resources, pages 91-97. Monger, J.W.H., 1975, Upper Paleozoic rocks of the Atlin terrane, northwestern British Columbia and south-central Yukon: Geological Survey of Canada Paper 74-47, 63p. Monger, J.W.H., 1977, Upper Paleozoic rocks of northwestern British Columbia: Report of Activities, Part A, Geological Survey of Canada Paper 77-1 A, p. 255-262. Monger, J.W.H., and McMillan, W.J., 1984, Bedrock geology of Ashcroft (921) map area: Geological Survey of Canada, Open File Map 980. 173 Monger, J.W.H., Richards, T.A., and Paterson, I. A., 1978, The Hinterland Belt of the Canadian Cordillera: new data from northern and central British Columbia, Canadian Journal of Earth Sciences, v. 15, p. 823-830. Nakada, S., Maillet, P., Monjaret, M-C, Funjinawa, A., and Urabe, T., 1994, High-Na dacite from the Jean Charcot Trough (Vanuatu), southwest Pacific: Marine Geology, v. 116, p. 197-213. Panteleyev, A., and Pearson., D.E., 1976, Kutcho Creek Map-Area (104I/1W): in Geological Fieldwork, B.C. Ministry of Mines and Petroleum Resources, p.74-76. Paterson, I. A., 1974, Geology of the Cache Creek Group and Mesozoic rocks at the northern end of the Stuart Lake Belt, central British Columbia; Geological Survey of Canada Special Paper 74-1, Part B, pages 31-41. Renne, P.R., 1995, Synchrony and Causal Relations Between Permian-Triassic Boundary Crises and Siberian Flood Volcanism: Science, Volume 269, pages 1413-1416. Samson, S.D., McClelland, W.C., Patchett, P.J., Gehrels, G.E., and Anderson, R.G, 1989, Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera: Nature, v.337, p. 705-709. Schiarizza, P., and Payie, G., 1997, Geology of the Sitlika Assemblage in the Kenny Creek -Mount Olson area (93N/12, 13), central British Columbia: in Geological Fieldwork 1996, Grant, B., ed., B.C. Ministry of Employment and Investment, Paper 1997-1, p. 79-100. Souther, J.O., 1991, Volcanic Regimes, in Geology of the Canadian Orogen in Canada, Gabrielse, H., and Yorath, C, eds.; Geological Survey of Canada, Geology of Canada, no. 4, p. 457-490. 174 Sun, S. -s., and McDonough, W.F., 1989, Chemical and Isotopic systematics of oceanic basalts: implications for mantle composition and processes: in Magmatism in the Ocean Basins, Saunders, A.D., and Norry, M.J., eds., Geological Society Special Publication Number 42, p. 313-345. Thorstad, L., and Gabrielse, H., 1986, The Upper Triassic Kutcho Formation, Cassiar Mountains, North-Central British Columbia: Geological Survey of Canada Paper 86-16, 53 p. Vallier, T.L., 1995, Petrology of pre-Tertiary igneous rocks in the Blue Mountains region of Oregon, Idaho, and Washington: Implications for the geologic evolution of a complex island arc: in Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Petrology and tectonic evolution of pre-Tertiary rocks of the Blue Mountains Region, Vallier, T.L., and Brooks, H.C., eds., U.S. Geological Survey Professional Paper 1438, p. 125-209. Wheeler, J.O., and McFeely, P., 1991, Tectonic Assemblage map of the Canadian Cordillera and adjacent parts of the United States of America: Geological Survey of Canada, Map 1712A. 175 CHAPTER 5: U-PB GEOCHRONOLOGY AND RADIOGENIC ISOTOPIC SYSTEMATICS OF THE POLYMETALLIC TULSEQUAH CHIEF AND BIG BULL VMS DEPOSITS, NORTHWESTERN BRITISH COLUMBIA Plate 5.1. Looking north to the Tulsequah River and Glacier, from Mount Eaton 176 Introduction The Tulsequah Chief and Big Bull polymetallic volcanogenic massive sulphide (VMS) deposits are located on Mount Eaton, near the confluence of the Tulsequah and Taku Rivers in the Tulsequah map area (104K) of northwestern British Columbia (Figs. 5.1 and 5.2). The two deposits are separated by a distance of about 8.5 km. Massive sulphide mineralization was discovered at Tulsequah Chief in 1924, and at Big Bull in 1929. Mines at Tulsequah Chief and Big Bull were in production from 1951 to 1957, producing 0.58 Mt grading 1.8% Cu, 1.3% Pb, 6.7% Zn, 3.43 g/t Au and 108 g/t Ag at Tulsequah Chief and 0.36 Mt grading 1.2% Cu, 1.9% Pb, 7.3% Zn, 5.14 g/t Au and 154 g/t Ag at Big Bull (Carmichael and Curtis, 1994). The deposits, which are now owned by Redfern Resources Ltd., have geological reserves of 8.7 Mt grading 1.3% Cu, 1.2% Pb, 6.4% Zn, 2.4 g/t Au and 100 g/t Ag at the Tulsequah Chief deposit, and 0.58 Mt grading 1.1% Cu, 1.5% Pb, 5.6% Zn, 3.4 g/t Au and 154 g/t Ag at the Big Bull deposit (Carmichael et al. 1995; The Northern Miner, March 1996). In this study, igneous rocks from the Tulsequah Chief and Big Bull deposit were sampled for analysis by U-Pb methods in an attempt to constrain the age(s) of VMS mineralization. Volcanic rocks from areas north and south of the deposit were also collected for dating, in an attempt correlate these rocks with those which host VMS mineralization at Tulsequah Chief, and therefore identify additional prospective stratigraphy for VMS exploration in the region. Igneous rocks from the Tulsequah Chief deposit were analyzed for their Nd isotopic signatures, and Pb isotopic compositions were determined for sulphides from both the Tulsequah Chief and Big Bull deposits, to determine the degree of evolution of these deposits and their host rocks. Regional Geology Regional mapping in the Tulsequah River and Glacier areas was begun by Kerr (1931), and was continued by Souther (1971), and more recently by Mihalynuk et al. (1994a and b). Within the Tulsequah River and Glacier areas, the Llewellyn Fault separates Mesozoic and Paleozoic rocks of lower metamorphic grade to the east from rocks of higher metamorphic grade 177 to the west (Mihalynuk et al. 1994a and b) (Fig. 5.2). Rocks west of the faults are divided into three suites, the Whitewater and Boundary Ranges metamorphic suites, and the lower grade Mount Stapler Suite (Mihalynuk et al. 1994a; Mihalynuk and Rousse 1988). Mihalynuk et al. (1994a) has suggested that schistose rocks, quartzite, metabasite and ultramafite of the Whitewater Suite may be correlative with parts of the Yukon-Tanana terrane (in the sense of Mortensen 1992) (Fig. 5.1), and has mapped volcanic and sedimentary rocks of the Mount Stapler Suite as being gradational into rocks of the Whitewater Suite. Rocks east of the Llewellyn fault were first correlated with the Upper Triassic Stuhini Group of the Stikine terrane by Souther (1971). However, subsequent biochronological data presented by Nelson and Payne (1984) indicated that rocks from the upper part of Mount Eaton are of late Paleozoic (Middle Pennsylvanian to Early Permian) age, and much of this area was reassigned to the Paleozoic Stikine Assemblage of the Stikine terrane (Fig. 5.1). In the Tulsequah River and Glacier areas, Nelson and Payne (1984) divided volcanic and sedimentary rocks of the Stikine Assemblage into three structural-stratigraphic blocks, separated by known or suspected faults. These are the weakly deformed Mount Eaton block and the more strongly deformed Mount Strong and Sittikanay blocks. The Tulsequah Chief and Big Bull deposit are hosted within arc-related bimodal volcanic rocks of the Mount Eaton block (Fig. 5.2). The Stikine terrane is interpreted to be an allochthonous terrane of island arc affinity, whereas the Yukon-Tanana terrane is interpreted to have a continental arc affinity, possibly with links to ancestral North America (Monger et al. 1972; Samson et al. 1989; Mortensen 1992). However, based on several lines of evidence, Mihalynuk et al. (1994c) have suggested that early Mesozoic and Paleozoic arc assemblages of the Stikine terrane may have been built on the flanks of a continental margin represented by the Yukon-Tanana terrane. The radiogenic isotopic signatures of igneous rocks produced within island and continental arcs are distinctly different, with those from continental arcs being significantly more evolved (i.e., Samson et al. 1989). The radiogenic isotopic signatures determined in the current study are compared with published values for both the Stikine and Yukon-Tanana terranes, in order to test the hypothesis of Mihalynuk et al. (1994c). 178 Stikine Terrane Paleozoic Stikine Assemblage Yukon-Tanana/ Nisling Terrane Coast Belt 68° N— Taku Terrane, Gravina Belt Forrest Kerr pluton _ . map area boundary 132° w 130° W Figure 5. 1 Simplified tectonostratigraphic map showing the distribution of the Paleozoic Stikine Assemblage, the Tulsequah Chief and Big Bull deposits, and the Forrest Kerr pluton (104B = Iskut River map area; 104G = Telegraph Creek map area; 104K = Tulsequah map area) (after McClelland 1992). 179 Rocks of the Stikine Assemblage east of the Llewellyn fault are overlain by volcanic and sedimentary rocks of the Upper Triassic Stuhini Group of the Stikine terrane and sedimentary rocks of the Lower to Middle Jurassic Laberge Group. Paleozoic and Mesozoic rocks are cross cut by Cretaceous to Tertiary intrusions, including dykes of the Eocene Sloko Group (Mihalynuk et al. 1994a). Geology of the Tulsequah Chief and Big Bull deposits For a detailed description of the geology and mineralization of the Tulsequah Chief and Big Bull deposits, the reader is referred to papers by McGuigan et al. (1993), Sherlock et al. (1994), Carmichael and Curtis (1994), Carmichael et al. (1995), Sebert et al. (1995), and Sebert and Barrett (1996). The Tulsequah Chief footwall, mine, and hangingwall series are a northward-younging bimodal volcanic sequence. The footwall series consists of mafic volcanic rocks; these rocks are conformably overlain by felsic tuffaceous rocks and feldspar- and quartz-porphyritic flows and breccias of the mine series, which are in turn overlain by mafic volcanic flows and volcaniclastic sedimentary rocks which comprise the hangingwall series (McGuigan et al. 1993; Sebert and Barrett 1996). Mineralization at Tulsequah Chief occurs within the hydrothermally altered lower portion of the felsic volcanic mine series as massive to semi-massive sulphide lenses, termed the ABi, AB2, F, G, H, and I lenses (McGuigan et al. 1993) (Plates 5.2, 5.3 and 5.4). Mineralization can be divided into three main facies: pyrite, zinc, and copper (McGuigan et al. 1993). The pyrite facies consists of predominantly of massive pyrite, with only minor base metals. The zinc facies comprises semi-massive pale yellow sphalerite, pyrite, galena, chalcopyrite, and tetrahedrite, in a matrix of barite, quartz and sericitically altered felsic volcaniclastic material. The copper facies consists mainly of massive pyrite, with up to several percent disseminated chalcopyrite. Felsic volcanic rocks of the mine series have been dilated by the intrusion of a semi-concordant sill of gabbroic composition. The sill is compositionally and mineralogically similar to the mafic volcanic rocks which comprise the hangingwall series of the deposit and may represent a sub-volcanic equivalent to the mafic flows (Sebert and Barrett 1996; Sherlock et al. 1996). 180 Quaternary S80.000E Unconsolidated outwash and alluvium SLOKO GROUP Eocene fZ3 Early Jurassic Quartz monzonitc Quartz monzonite MOUNT EATON SUITE Permian and Pennsylvania Volcanic and sedimentary rocks Mississippian to Pennsylvania!. Tuffaceous muds tone and greywacke Agglomerate, breccia and minor flows B88iB838a Polymictic volcanic conglomerate and sandstone Massive basslt to andesite tuff Gabbro Rhyolite to andesite Volcanic and sedimentary rocks MOUNT STAPLER SUITE Paleozoic to Mesozoic metamorphosed strata I Quartz monzonitc to diorite Metabasalt Rhyolite flows, breccia and felsic tuff Argillite, siltstone, greywacke, phyllite Figure 5. 2 Generalized geology of the Tulsequah Glacier area, showing locations of U-Pb geochronology samples (after Mihalynuk et al. 1994) 181 At the Big Bull deposit the oldest rocks exposed consist of mafic lapilli and ash tuffs with rare mafic flow rocks. These rocks are overlain by fine-grained felsic crystal to ash tuffs, which are in turn overlain by intermediate composition, fine- to coarse-grained fragmental and tuffaceous rocks with interbedded massive, black manganese oxides and silicates. The sequence is capped by mafic lapilli, ash and crystal tuffs characterized by streaks and patches of black hematite (Carmichael et al. 1995) (Plate 5.5). Massive to semi-massive sulphides consisting of pyrite, galena, sphalerite, chalcopyrite and tetrahedrite in a matrix of barite and sericitized lithic fragments are hosted within hydrothermally altered, fine-grained felsic crystal to ash tuffs of dacitic composition within this sequence. In contrast to the Tulsequah Chief mine series, felsic flow rocks are rare to absent around the Big Bull deposit. In addition, felsic tuffaceous rocks are finer grained than those at the Tulsequah Chief deposit. Carmichael et al. (1995) has suggested that these differences may indicate formation of Big Bull mineralization in a more distal environment than mineralization at Tulsequah Chief. U-Pb Geochronology In the current study samples were collected for analysis by U-Pb methods to address a number of specific problems. One of these was to attempt to verify and refine a preliminary latest Devonian to earliest Mississippian U-Pb zircon date of 351 +15/-6 Ma on a felsic volcaniclastic rock in the upper part of the Tulsequah Chief mine series (Sherlock et al. 1994). Sherlock et al. (1994) report a high degree of inheritance in zircon from the dated sample, a feature which is inconsistent with formation within the basal assemblage of a terrane that has been interpreted to be composed almost completely of juvenile, mantle-derived material (Samson et al. 1989). Samples of the sill of gabbroic composition which intrudes the Tulsequah Chief mine series were also collected, along with volcanic rocks from the Big Bull deposit, but none of these samples yielded suitable material for analysis by U-Pb methods. Volcaniclastic rocks were also collected from the Mount Stapler Suite on Mount Stapler, and the Sittikanay block on Mount Sittikanay, to determine the time of volcanism in these areas; the results are presented below. The U-Pb results reported in this paper are based on analysis of zircons recovered from 20-30 kilogram samples collected from drill core, outcrop or underground workings; sample descriptions are given below. Heavy mineral extraction procedures and U-Pb analytical 182 procedures follow those of Mortensen et al. (1995). All zircon fractions were abraded prior to analysis (Krogh 1982). Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Procedural blanks were 6-20 picograms for Pb and 1-2 picograms for U. Concordia intercept ages and associated errors were calculated using a modified York-II regression model (York 1969), and the algorithm of Ludwig (1980); ages were calculated using the decay constants recommended by Steiger and Jager (1977). Age assignments follow the time scale of Harland et al. (1990). Analytical results are given in Table 5.1 and shown graphically in Figure 5.3. Tulsequah Chief Mine Series Two felsic volcanic rocks of rhyodacitic to rhyolitic composition, from the Tulsequah Chief mine series, were sampled for analysis for by U-Pb methods. Sample TC-GC-02 was a massive quartz+feldspar porphyritic flow of rhyodacitic composition from near the top of the felsic volcanic sequence. Sample TC-GC-04 was a massive to autobrecciated quartz+feldspar porphyritic flow of rhyolitic composition which directly overlay massive sulphide mineralization of the H zone mineralization, and occurred at a lower stratigraphic level than sample TC-GC-02. Both of these samples occur at a lower stratigraphic level than the sample dated by Sherlock et al. (1994). Sample TC-GC-02 contained small equant to prismatic zircons, some of which were characterized by slightly turbid centers. In the hand-picking of zircon grains for analysis, only those with the best clarity and least turbidity were selected. Analysis of four fractions yielded Pb/ Pb ages of 325 to 340 Ma; two fractions, G and H, were overlapping and concordant (Table 5.1, Fig. 5.3a). A Late Mississippian age of 327 +/-1 Ma was calculated as the age of this rock, based on the ^Pb/^U ages and errors of concordant fractions G and H. Fractions A and B are slightly discordant (2 and 6%), probably as a result of post-crystallization lead-loss. Fraction A gives a slightly older Pb/ Pb age than the other fractions, but it is within error of the date determined from the two concordant fractions. Zircons from sample TC-GC-04 were visually similar to those described above from sample TC-GC-02. Analysis of three fractions yielded 207Pb/206Pb ages of 325 to 330 Ma; error 183 ellipses of all three fractions had some overlap with concordia, but the degree of concordance is observed to increase with Pb/ U age (Table 5.1, Fig. 5.3b). A Late Mississippian age of 330 +11/-1 Ma was calculated as the age of this rock based on the ^b/23^ age and 206Pb/^sU and errors of fraction E, the fraction which has the most significant overlap with concordia. Table 5. 1 U-Pb analytical data. Fraction1 Wt U Pb5 **Pb3 Vb4 "W Isotopic ratios(±lg.V.)< Isotopic datcs(Ma,*2<T)li mB ppm ppm *"Pb pg «/. "'Pb^HJ "W^J "Pb^Pb ^Pb/^U ""PbP'U «W*Pb TULSEQUAH CHIEF Mine Series rhyodacite (TC-GC-02, underground via 5400 level portal) A£N2,p 0.052 293 16 976 51 13.6 0.05107*0.11 03751*0.36 0.05327*0.29 321.1*0.7 323.4*2.0 340.2*13.2 B£N2,p,s 0.112 235 12 1606 53 12.9 o!o5071±0.20 0J702±0.35 0.05294*0.24 318.9*1.2 319.8*1.9 326.2*10.7 G£M2.s 0.198 305 16 1019 198 12.2 0.5202*0.12 03795*0.21 0.05292*0.14 326.9*0.8 326.7*1.2 325.2*6.5 U£M2,s 0.054 366 20 1642 40 13.8 0.5207*0.16 0.3807*0.31 0.05302*0.21 327.2*1.0 327.5*1.7 329.7*9.6 Mine Series rhyolite (TC-GC-04, DDH TCU-91-32, 312.4-326 D^Mlj) 0.028 197 10 647 26 12.6 0.04957*0.12 9 m) 0.3616*0.43 0.05290*0.36 311.9*0.7 313.4*2.3 324.6*16.2 E£Nl4> 0.033 241 13 908 29 12.5 0.05246*0.12 03835*0.33 0.05302*0.25 329.6*0.8 329.6*1.9 329.7*113 lLm,M2^> 0.024 318 17 768 33 13.6 0.05160±0.13 0.3771*0.41 0.05300*0.33 3243*0.8 324.9*2.3 329.0*14.9 Mount Stapler rhyolite (TC-GC-05, surface) A£NU 0.189 530 31 612 563 14.2 0.05442*0.11 0.4039*0.30 0.05383*0.24 341.6*0.7 344.5*1.7 364.0*10.5 BXNl^ 0.079 420 24 519 200 13.9 0.05315±0.15 03933*0.37 0.05367*0.28 333.8*0.9 336.8*2.1 357.3*12.8 CXNlj) 0.040 189 11 1385 19 12.8 0.05594±0.U 0.4158*0.27 0.05392*0.19 350.9*0.7 353.1*1.6 367.6*8.3 DXNU 0.032 396 23 773 57 13.7 0.05490*0.13 0.4076*0.35 0.05385*0.25 344.5*0.9 347.1*2.1 364.6*11.4 E£M2,s 0.063 273 16 1005 59 13.7 0.05496*0.11 0.4071*030 0.05372*0.22 344.9*0.7 346.8*1.8 359.5*9.8 GXM2p^ 0.100 150 9 770 68 14.7 0.05465±0.13 0.4038*0.37 0.05358*0.29 343.0*0.8 344.4*2.2 353.5*13.0 Mount Sittikanay volcaniclastic (TC-GC-12, surface) Ajn,Nl,p 0.040 458 22 2595 21 11.9 0.04715*0.11 0.3450*0.22 0.05306*0.14 297.0*0.6 300.9*1.2 331.3*6.3 BXNl^. 0.063 317 16 3822 15 13.3 0.04810*0.10 0.3503*0.20 0.05282*0.12 302.9*0.6 305.0*1.1 321.0*5.2 DXMlji 0.020 229 11 1903 7 7.7 0.04932*0.14 03630*0.29 0.05339*0.21 310.3*0.8 314.5*1.6 345.3*9.4 'All fractions are air abraded; Grain size, smallest dimension: c= +134|im, m=-134uiti+74u,m, f=-74|*m; Magnetic codes Franz magnetic separator sideslope at which grains are nonmagnetic; e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8A; Front slope for all fractions=20°; Grain character codes: p=prismatic, s=subhedral. 2Radiogenic Pb 'Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ±20% (Daly collector) *T6taI common Pb in analysis based on blank isotopic composition 5Radiogenic Pb 'Corrected for blank Pb, U and common Pb (Stacey-Kramers model Pb composition at the 207Pb/20SPb date of fraction, or age of sample). 184 Figure 5. 3U-Pb concordia diagrams for a) rhyodacite from the Tulsequah Chief mine series, b) rhyolite from the Tulsequah Chief mine series, c) rhyolite from Mount Eaton, and d) volcaniclastic from Mount Sittikanay. 185 Mount Stapler A sample of a strongly foliated rhyolite ash to lapilli tuff was sampled from Mount Stapler (TC-GC-05) and yielded a limited quantity of small, prismatic zircons (Plate 5.6). Analysis of six fractions yielded 207pb/206pb ages 0f 35410 353 Ma. All fractions are slightly discordant and this feature is attributed mainly to post-crystallization Pb-loss (Fig. 5.3c). The range of 207pb/206pb ages may also reflect a minor degree of inheritance in some zircon fractions from this rock. A weighted mean 207pb/206pb age was calculated for the fractions which show the least degree of inheritance (Fractions B, E and G), and yielded an Earliest Mississippian date of 357 +6 Ma. Because of the interpreted inheritance, this age is considered to be an estimate of the maximum age of this rock. A second sample, which consisted of an intrusion of quartz monzonitic composition, was collected by the author and M. Mihalynuk from Mount Stapler, west of the Llewlyn fault. Analytical results for this sample, which yielded an Early Jurassic crystallization age, are presented in Appendix 3. Mount Sittikanay volcaniclastic Two volcaniclastic rocks were sampled for U-Pb dating from Mount Sittikanay (TC-GC-11 and -12). The samples consisted of a strongly foliated, fine-grained medium green andesite (TC-GC-11) (Plate 5.7) and a strongly to moderately foliated, heterolithic volcaniclastic debris flow, with a strong to moderate deformational fabric (TC-GC-12) (Plate 5.8). Sample TC-GC-11 failed to yield minerals suitable for dating, but sample TC-GC-12 contained a small amount of zircon. Zircons from sample TC-GC-12 were prismatic (l:w = 2:1 to 3:1) with good clarity and minor colourless bubble and rod-shaped inclusions. All of the zircon from this rock was picked and separated into three fractions. Analysis of the three fractions yielded Pb/ Phages of 321, 331, and 345 Ma. All three fractions were slightly discordant (6-10%), and this feature is attributed to post-crystallization Pb-loss (Table 5.1, Fig. 5.3d). A weighted mean age of 325 +/-186 10 Ma was calculated for the two fractions with the lowest Pb/ Pb ages. As neither point is concordant, this age can be regarded as a maximum age for this rock. The 207Pb/206Pb age of fraction D is not within error of this age; this may result from either inheritance or incorporation of detrital material during deposition of this volcaniclastic rock. Lithogeochemistry Samples collected for analysis by U-Pb methods in the current study were also analyzed for their major and trace element concentrations (Table 5.2). Felsic volcanic rocks from the Tulsequah Chief deposit and Mount Stapler have rhyodacitic to rhyolitic compositions, and Zr/Y ratios of 4.9 to 6.7 indicate transitional magmatic affinities, based on the limits defined by Barrett and MacLean (1994) (Table 5.2 and Fig. 5.4). Felsic volcanic rocks from the Big Bull deposit have dacitic to rhyodacitic compositions, and tholeiitic to transitional magmatic affinities (Zr/Y = 2.9 to 5.7). Volcaniclastic rocks from Mount Sittikanay have basaltic to andesitic average compositions and tholeiitic magmatic affinities (Zr/Y = 3.7 to 4.0). Sills which intrude the Tulsequah Chief mine series have gabbroic compositions and tholeiitic magmatic affinities (Zr/Y = 2.6 to 3.6). Compositions determined here for rocks form the Tulsequah Chief deposit are consistent with results determined by Sebert and Barrett (1996). 60 Zr (ppm) Figure 5. 4 Y versus Zr diagram for U-Pb samples from the Tulsequah Glacier area (data fields after Barrett and MacLean 1994). 187 Table 5.2 Major and trace element data for U-Pb samples from the Tulsequah River and Glacier areas. sample lithology Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 LOI Total BaO number % % % % % % % % % % ppm TC-GC-01 mine series rhyodacite 78.42 0.22 12.23 2.04 0.01 0.01 0.27 4.01 1.62 0.03 0.89 99.75 488 TC-GC-02 mine series rhyodacite 70.95 0.27 14.14 3.52 0.08 2.08 3.20 1.23 2.15 0.03 2.16 99.81 1033 TC-GC-03 gabbro sill 48.74 0.83 18.00 8.76 0.14 7.97 9.69 2.49 1.84 0.15 1.67 100.28 749 TC-GC-04 mine series rhyodacite 74.59 0.26 12.61 2.48 0.05 0.29 1.51 6.28 0.33 0.07 1.41 99.88 1387 TC-GC-05 Mt. Stapler rhyodacite 73.30 0.21 14.72 1.58 0.02 0.43 0.98 3.58 2.71 0.04 2.06 99.63 1515 TC-GC-08 Big Bull dacite 73.69 0.28 13.77 2.82 0.03 0.30 0.96 4.51 2.42 0.06 1.17 100.15 1317 TC-GC-09 gabbro sill 53.08 0.89 19.79 6.31 0.11 6.32 6.11 5.24 1.33 0.16 1.28 100.78 482 TC-GC-10 gabbro sill 48.41 0.90 17.50 9.31 0.17 9.14 10.45 2.90 0.73 0.16 1.02 100.85 618 TC-GC-11 Mt. Sittikanay fragmental 65.43 0.79 13.81 7.73 0.14 3.31 1.82 4.77 0.08 0.18 2.91 101.01 79 TC-GC-12 Mt. Sittikanay fragmental 53.92 0.89 16.07 8.50 0.15 3.99 5.18 5.17 0.37 0.49 5.58 100.40 464 BB-GC-01 Big Bull dacite 68.44 0.56 14.13 5.91 0.13 1.35 2.06 5.25 0.64 0.24 1.28 99.99 480 Detection Limit (ppm): 60.00 35.00 120.00 30.00 30.00 95.00 15.00 75.00 25.00 35.00 17 sample Co Cr203 Cu Ni V Zn Ga Nb Pb Rb Sr U Y Zr Zr/Y number ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm TC-GC-01 48 10 33 8 26 66 n/a n/a n/a 15 50 n/a 20 129 6.5 TC-GC-02 37 54 57 15 34 131 n/a n/a n/a 52 377 n/a 22 148 6.7 TC-GC-03 46 522 48 164 190 52 n/a n/a n/a 42 445 n/a 18 64 3.6 TC-GC-04 35 84 130 17 35 82 n/a n/a n/a 5 150 n/a 28 138 4.9 TC-GC-05 28 98 66 16 35 33 n/a n/a n/a 71 104 n/a 13 71 5.5 TC-GC-08 22 4 40 <d/l 21 64 14 9 2 42 137 3 26 147 5.7 TC-GC-09 32 566 31 176 216 89 18 5 3 24 221 1 22 58 2.6 TC-GC-10 26 487 77 164 244 97 18 1 2 9 774 <d/l 21 70 3.3 TC-GC-11 14 32 86 18 203 118 16 4 1 1 331 <d/l 26 104 4.0 TC-GC-12 19 32 60 4 196 113 16 4 <d/l 8 301 1 28 103 3.7 BB-GC-01 42 12 50 12 51 79 n/a n/a n/a 14 213 n/a 41 118 2.9 Detection 10 2 15 3 10 2 1 1 1 1 1 1 1 1 Limit >d/l = below detection limit n/a = not available Nd Isotopic data Rhyolite from the Tulsequah Chief mine series and the mafic sill which intrudes the mine series were analyzed for their Nd isotopic compositions to determine the degree of evolution of these rocks and therefore better constrain potential magmatic sources. Although suitable material was not found in the mafic sill to permit determination of an age by U-Pb methods, the similarity in chemistry and mineralogy between this intrusive unit and mafic flows in the hangingwall of the deposit noted by Sebert and Barrett (1996) suggests that the sill may be an intrusive equivalent of the hangingwall mafic flows, and therefore was probably emplaced contemporaneously with mafic volcanic flows in the hangingwall to mineralization. The rhyolite flow and gabbro sill had ENd (initial) values of +2.7 and +3.9, respectively (Table 5.2). In Figure 5, (initial) values for these rocks are compared with the regional data set for the Stikine Assemblage, as well as for data from the Yukon-Tanana terrane (Samson et al. 1989; Mortensen 1992; Logan et al. 1993). The data set for the Stikine terrane consists of mafic to felsic volcanic rocks (n= 5, e^d (initial) = 5.1-5.9) from the northern Iskut River and southern Telegraph Creek map areas, 150 to 200 km to the south, comparable data for the Stikine Assemblage in the Tulsequah Chief map area is not available. There is a minor, but detectable difference that exceeds analytical error in the 8Nd (initial) values determined for rocks from the Tulsequah Chief deposit and the regional data set for the Stikine terrane; values determined here are isotopically somewhat less primitive than those of the regional data set. In contrast, igneous rocks from the Yukon-Tanana continental-arc terrane have significantly more evolved 8Nd values than those determined here. The tectonic implications of these results are discussed below. Sample Sm Nd M'Sm/l44Nd meas. ""Nd/'^Nd age1 (ppm) (ppm) (error x 10^,20) (present day) (Ma) (initial) TULSEOUAH CHIEF DEPOSIT TC-GO02 3.29 15.54 0.1280 0.512619(11) -0.4 327 +2.7 mine series rhyolite A S- 93-145 3.02 10.85 0.1681 0.512771 (4) +2.6 327 +3.9 mine series gabbro sill 'used for the calculation of eNd (initial). 2error = +0.5 eNd units. Table 5. 3 Nd isotopic data for rocks from the Tulsequah Chief deposit. 189 250 • • Yukon-Tanana terrane (Mortensen 1992) B • Tulsequah Chief deposit rhyodacite flow and gabbro sill • Stikine Assemblage, Iskut River area (56o00' to 56°30') (Samson et al. 1989; Logan etal. 1993) 300 350 Age (Ma) 400 450 Figure 5. 5 eNd(initial) versus age for volcanic rocks from the Stikine Assemblage (Iskut River area data from Samson et al. 1989; Logan et al. 1993) Pb Isotopic Data Sulphides from the Tulsequah Chief and Big Bull deposits were analyzed for their Pb isotopic compositions to assess the degree of evolution of metallogenic sources to mineralization. Sulphides from the ABi, AB2, H, and I zones of the Tulsequah Chief deposit yielded Pb isotopic ratios of 207Pb/206Pb = 0.8379-0.8387 and 208Pb/206Pb = 2.053-2.056, the Big Bull deposit yielded comparable ratios of 207Pb/206Pb = 0.8375-0.8380 and 208Pb/206Pb = 2.053-2.056 (Table 5.6). In Figure 6 the Pb isotopic signature of sulphides from the Tulsequah Chief deposit are compared with those of Tertiary mineralization in the Stewart Mining Camp, Triassic and Jurassic VMS deposits, and the Late Devonian Forrest Kerr pluton, all of which comprise, or intrude the Stikine terrane in the Iskut River and southern Telegraph Creek map areas, and Devono-Mississippian VMS deposits in the Yukon-Tanana terrane in the Yukon and Alaska. Lead isotopic data from the Upper Triassic Granduc and Middle Jurassic Eskay Creek deposits, as well as Tertiary mineralization define an evolution from less to more evolved radiogenic lead compositions with time from the Mesozoic to the Cenozoic within the Stikine terrane; this evolution is best observed in the increasing 206Pb/204Pb values (Fig. 5.6) (Alldrick 1991; Childe this volume). With the addition of feldspar isotopic compositions from the Forrest Kerr pluton, 190 Table 5. 4 Pb isotopic data for sulphides from the Tulsequah Chief and Big Bull deposits. SAMPLE IUMBER SAMPLE LOCATION MIN.1 !WPb/swPb (% error)" M'Pb/5MPb (% error)" !W,Pb/!MPb (% error)" ""Pb/^Pb (% error)" 20»pb/206p (% error)1 TULSEQUAH CHIEF DEPOSIT Tla I zone gl 18.634 15.637 38.303 0.8387 2.056 (0.011) (0.014) (0.018) (0.005) (0.003) T3a AB2 zone gl 18.654 15.637 38.298 0.8378 2.053 (0.127) (0.127) (0.128) (0.013) (0.009) T4a AB1 zone gl 18.674 15.660 38.352 0.8381 2.054 (0.033) (0.033) (0.036) (0.008) (0.003) T5a H zone gl 18.638 15.633 38.292 0.8383 2.055 (0.013) (0.015) (0.020) (0.007) (0.006) T6a H zone gl 18.631 15.631 38.270 0.8385 2.054 (0.031) (0.032) (0.034) (0.007) (0.006) T7a sx clasts gl 18.651 15.637 38.301 0.8380 2.054 (0.014) (0.016) (0.019) (0.005) (0.003) T8a H zone gl 18.639 15.635 38.291 0.8384 2.054 (0.040) (0.041) (0.043) (0.007) (0.006) TlOa AB1 zone gl 18.642 15.635 38.296 0.8383 2.054 (0.011) (0.014) (0.018) (0.005) (0.002) Tlla AB2 zone gl 18.651 15.651 38.355 0.8387 2.056 (0.017) (0.016) (0.023) (0.011) (0.008) T13a AB1 zone gl 18.630 15.629 38.275 0.8385 2.055 (0.011) (0.014) (0.018) (0.005) (0.003) T14a AB1 zone gl 18.630 15.618 38.242 0.8379 2.053 (0 011) (0.014) (0.018) (0.005) (0.003) T16a H zone gl 18.645 15.642 38.319 0.84385 2.055 (0.043) (0.044) (0.045) (0.006) (0.005) T17a H zone gl 18.637 15.631 38.286 0.8383 2.054 (0.018) (0.019) (0.023) (0.006) (0.004) BIG BULL DEPOSIT Bla Big Bull gl 18.637 15.650 38.37 0.8398 2.059 (0.030) (0.030) (0.033) (0.008) (0.010) Bib Big Bull gl 18.619 15.619 38.261 0.8389 2.055 (0.020) (0.021) (0.025) (0.009) (0.010) B2a Big Bull gl 18.663 15.640 38.354 0.8380 2.055 (0.024) (0.021) (0.030) (0.016) (0.0015) B3c Big Bull gl 18.674 15.656 38.398 0.8384 2.056 (0.016) (0017) (0.022) (0.008) (0.010) B4a Big Bull gl 18.775 15.758 38.697 0.8393 2.061 (0.056) (0.050) (0.063) (0.028) (0.025) B4b Big Bull gl 18.671 15.644 38.368 0.8379 2.055 (0.035) (0.033) (0.046) (0.014) (0.027) B5a Big Bull gl 18.654 15.627 38.309 0.8377 2.054 (0.014) (0.016) (0.020) (0.005) (0.008) B5c Big Bull gl 18.681 15.652 38.381 0.8379 2.055 (0.034) (0.035) (0.037) (0.007) (0.009) B5d Big Bull gl 18.664 15.632 38.314 0.8376 2.053 (0.050) (0.050) (0.052) (0.011) (0.011) 1 mineral abbreviations: gl=galena 2 errors are quoted at the 2a (95% confidence) level. 3 values are corrected for instrument fractionation by normalization based on replicate analyses of the NBS-981 standard. 191 2.10 JO o 2.08 2.06 2.04 -2.02 -2.00 0.81 Devono-Mississippian VMS deposits, Yukon-Tanana terrane (Lange et al. 1993) Late Devonian Forrest Kerr pluton Late Triassic Granduc deposit, Stikine terrane Middle Jurassic Eskay Creek deposit, Stikine terrane Tertiary mineralization Stewart Mining Camp (Alldrick 1990) A Tulsequah Chief deposit © Big Bull deposit •+-0.82 0.83 0.84 207Pb/206Pb 0.85 0.86 15.80 15.70 15.60 15.50 Devono-Mississippian VMS deposits, Yukon-Tanana terrane (Lange etal. 1993) Forrest Kerr pluton \ Late Triassic Granduc deposit, Stikine terrane Tertiary mineralization Stewart Mining Camp (Alldrick 1990) Eskay Creek deposit 18.3 18.5 18.7 18.9 19.1 19.3 206 Pb/204Pb Figure 5. 6 Plots of a) 207Pb/206Pb versus 208Pb/206Pb, and b) 207Pb/204Pb versus 206Pb/204Pb for sulphides from the Tulsequah Chief and Big Bull deposits, references for otiier data fields given in text. 192 this evolution can be extended back to the Late Devonian (Fig. 5.6). In comparison, Devono-Mississippian VMS mineralization in the Yukon-Tanana terrane has a wide range of Pb/ Pb values and significantly higher Pb/ Pb values than mineralization or magmatism of any age in the Stikine terrane. These elevated isotopic compositions in the Yukon-Tanana terrane, relative to younger mineralization and magmatism in the Stikine terrane, appear to reflect a continentally derived, more evolved source to mineralization. Early Mississippian Tulsequah Chief mineralization is more evolved than would be predicted by the evolution of lead in the Iskut River area of the Stikine terrane, with 206Pb/204Pb values comparable to the approximately 100 Ma younger Granduc deposit, and Pb/ Pb values intermediate between those of the island-arc Stikine terrane and continental-arc related mineralization. Discussion In the current study U-Pb dates of 327 +/-1 Ma and 330 +10/-1 Ma were determined from volcanic rocks of rhyodacitic to rhyolitic composition from the Tulsequah Chief mine series; these dates indicate a Late Mississippian age for VMS mineralization at Tulsequah Chief. The date determined for the felsic rock lower in the stratigraphy (330 +10/-1 Ma) is slightly older than the age determined for the rock which occurs at a higher level in the stratigraphy (327 +/-1 Ma), and may reflect an age difference between the upper and lower portions of the volcanic sequence which hosts mineralization. The dates determined for felsic rocks in the Tulsequah Chief mine series are in good agreement with Middle Pennsylvanian to Early Permian dates determined on fossils in limestone units near the peak of Mount Eaton (Nelson and Payne 1984), and suggests that the time span between the eruption of felsic volcanic rock which hosts VMS mineralization at the base of Mount Eaton, and the deposition of limestone and andesite at the top of Mount Eaton may be as little as 10-20 m.y. However, the dates determined here are significantly younger than an Early Mississippian date of 351 +15/-6 Ma determined by Sherlock et al. (1994) on a felsic volcaniclastic rock near the top of the Tulsequah Chief mine series. Zircon from this rock were characterized by significantly older (up to 2040 Ma) Pb/ Pb ages, which was attributed to inherited cores or xenocrystic grains within the zircon population. In contrast, zircons analyzed from the same felsic volcanic sequence at Tulsequah Chief in this study did not exhibit any 193 evidence of inheritance (Figs. 5.3a and b). The question as to the source of the Precambrian zircon component in a volcaniclastic rock of the Stikine Assemblage, which is not present in underlying flow rocks, is an intriguing one. The source of this xenocrystic zircon could potentially be far-traveled continental detritus incorporated into the rock during deposition, or an assimilated component of older crustal material. Although attempts to date felsic volcaniclastic rocks which host mineralization at Big Bull were unsuccessful, the similarity in the Pb isotopic signatures of sulphide from the Big Bull and Tulsequah Chief deposits suggests that the two deposits are of comparable age. Nelson and Payne (1984) noted lithological similarities between volcanic and sedimentary rocks on Mount Eaton and those on Mount Sittikanay. Mihalynuk et al. (1994) suggested that volcanic and sedimentary rocks of the Mount Sittikanay block, about 20 km south of the Tulsequah Chief deposit, are the finer-grained, deformed distal equivalents of rocks of similar lithology which occur on Mount Eaton (Fig. 2). A maximum age of 325 +/-10 Ma determined for a volcaniclastic rock from Mount Sittikanay suggests that this correlation is valid and that the stratigraphic sequence which hosts mineralization at Tulsequah Chief may extend south of the deposit to Mount Sittikanay. An Early Mississippian age of 334 +5/-4 Ma determined by Brown and McLelland (1995) for felsic tuffaceous rocks of the Stikine Assemblage in the Golden Bear area, some 100 km south of Tulsequah Chief, further extends prospective stratigraphy for the exploration of Tulsequah Chief-equivalent VMS mineralization. A maximum age of 357 +/-6 Ma determined for a felsic volcanic rock from the Mount Stapler block indicates that these rocks may be significantly older than those which host VMS mineralization at Tulsequah Chief. This unit lies west of the Llewlyn fault and has been correlated with rocks of the Whitewater Suite, which may be related to the Yukon-Tanana terrane (Mihalynuk et al. 1994a). Correlation of these rocks with Yukon-Tanana rather than Stikinia may account for the significant difference in age between these felsic volcanic rocks and those to the south on Mount Eaton and Mount Sittikanay. Neodymium isotopic analyses of rocks from Mount Stapler may assist in resolving this question. 194 Volcanic rocks which host VMS mineralization at the Tulsequah Chief and Big Bull deposits are interpreted to be part of the most northerly extreme of the Paleozoic Stikine Assemblage, the oldest recognized sequence within Stikinia, a terrane of island arc affinity (Monger 1977; Nelson and Payne 1984) (Fig. 5.1). However, Mihalynuk et al. (1994) have suggested that northern Stikinia may have been built in part on rocks of continental affinity, such as the Yukon-Tanana terrane. The Nd and Pb isotopic signatures from the Tulsequah Chief and Big Bull deposits are slightly more evolved than would be expected, based on data from the southern portions of Stikinia. However, these values are significantly less evolved than signatures from the Yukon Tanana terrane, and do not support these rocks being built directly on bona fide Yukon-Tanana rocks (Figs. 5.4 and 5.5). Available isotopic data are consistent with the basement at Tulsequah Chief and Big Bull being composed of one or more of the following: strongly attenuated continental arc components; continentally derived detritus directly overlying oceanic crust, or older, and more evolved island arc components than are present below rocks in more southerly parts of the Stikine Assemblage. 195 Plate 5. 2 Felsic fragmental unit, Tulsequah Chief mine series. Plate 5. 3 Silicified felsic fragmental unit, Tulsequah Chief mine series. Plate 5. 4 Baritic pyrite-rich sulphides, Tulsequah Chief Mine Series, I Zone (hammer for scale). Plate 5. 5 Deformed dacitic tuffaceous rock, Big Bull deposit (hammer for scale). Plate 5. 6 Flattened felsic fragmental unit, sample TC-GC-05, Mount Stapler (pen for scale). Plate 5. 7 Deformed andesite tuffaceous rock, sample TC-GC-11, Mount Sittikanay. Plate 5. 8 Deformed heterolithic fragmental unit, sample TC-GC-12, Mount Sittikanay (lens cap for scale). 196 References Alldrick, D.J. 1991. Geology and Ore Deposits of the Stewart Mining Camp. Ph.D. thesis, University of British Columbia, Vancouver, 417p. Barrett, T.J. and MacLean, W.H. 1994. Chemostratigraphy and hydrothermal alteration in exploration for VFfMS deposits in greenstones and younger volcanic rocks. In Alteration and Alteration Processes associated with Ore-Forming Systems. Edited by D.R. Lentz, Geological Association of Canada, Short Course Notes, pp. 433-467. Brown, D.A., and McLelland, W.C. 1995. U-Pb dates for northern Stikinia: Tulsequah/Chutine areas (104F, K). Geological Association of Canada-Mineralogical Association of Canada Joint Annual Meeting, Program with Abstracts, Victoria, p. A-10. Carmichael, R.G. and Curtis, K.M. 1994. 1993 Exploration Program: Geology, Geophysics and Diamond Drilling at the Big Bull Mine Area (NTS 104K/12E). Internal Report for Redfern Resources Ltd., 22p. Carmichael, R.G. Sherlock, R.L., and Barrett, T.J. 1995. The geology of the Big Bull polymetallic volcanogenic massive sulphide deposit, northwestern British Columbia (104K/12). In British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork 1994. Edited by B. Grant and J.M. Newell, p. 541-550. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, AG, and Smith, D.G. 1990. A Geological Time Scale, 1989. Cambridge University Press, Cambridge, 263 p. Kerr, FA. 1931. Explorations between the Stikine and Taku Rivers, B.C. Geological Survey of Canada, Summary Report 1930, Part A, pp. 41-55. Krogh, T.E. 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochemica and Cosmochemica Acta, 46: 637-649. 198 Lange, I.M., Nockelburg, W.J., Newkirk, SR., Aleinikoff, J.N., Church, S.E., and Krouse, H.R. 1993. Devonian volcanogenic massive sulphide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska range, Alaska. Economic Geology, 88: 344-376. Logan, J.M., Drobe, R., McClelland, W.C., and Anderson, R.G. 1993. Devonian intraoceanic arc magmatism in northwestern Stikinia. Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Program with Abstracts, Edmonton, p. A-60. Ludwig, K.R. 1980. Calculation of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, 46: 212-220. McClelland, W.C. 1992. Permian and older rocks of the southwestern Iskut River map area, northwestern British Columbia. In Current Research, Part A, Geological Survey of Canada, Paper 92-1A, pp. 303-307. McGuigan, P., Melnyk, W., Dawson., G.L., and Harrison, D. 1993. Tulsequah Chief Mine, Northwestern B.C. 1992 Exploration Program: Diamond Drilling, Geology and Reserve Estimation. Unpublished Report by Cambria Geological Ltd., Vancouver, 3 volumes. Mihalynuk, M.G., and Rousse, J.N. 1988. Preliminary geology of the Tutshi Lake area, northwestern British Columbia (104M/15). In Geological Fieldwork 1987, Edited by B. Grant and J.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1988-1, p. 217-231. Mihalynuk, M.G., Smith, M.T., Hancock, K.D., and Dudka, S. 1994a. Regional and economic geology of the Tulsequah River and Glacier areas (104K/12 &13). In Geological Fieldwork 1993, Edited by B. Grant and J.M. Newell, British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1994-1, p. 171-200. 199 Mihalynuk, M.G., Smith, M.T., Hancock, K.D., and Dudka, S. 1994b. Tulsequah River a map area geology (104K/12). British Columbia Ministry of Energy, Mines and Petroleum Resources, Open File 1994-3. Mihalynuk, M.G., Nelson, J., and Diakow, L. 1994c. Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera. Tectonics, 13: 575-595. Monger, J.W.H. 1977. Upper Paleozoic rocks of the western Canadian Cordillera and their bearing on Cordilleran evolution. Canadian Journal of Earth Sciences, 14: 1832-1859. Monger, J.W.H., Souther, J.G., and Gabrielse, H. 1972. Evolution of the Canadian Cordillera: A plate-tectonic model. American Journal of Science, 272: 577-602. Mortensen, J.K. 1992. Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana terrane, Yukon and Alaska. Tectonics, 11: 836-853. Mortensen, J.K., Ghosh, D.K., and Ferri, P. 1995. U-Pb geochronology of intrusive rocks associated with Cu-Au deposits in the Canadian Cordillera. In Porphyry Deposits in the Northwestern Cordillera of North America. Edited by T.G. Schroter. G-2: 491-531. Nelson, J. and Payne, J.G. 1984. Paleozoic volcanic assemblages and volcanogenic massive sulphide deposits near Tulsequah, British Columbia. Canadian Journal of Earth Sciences, 21: 379-381. Samson, S.D., McClelland, W.C., Patchett, P.J., Gehrels, G.E., and Anderson, R.G. 1989. Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera, Nature, 337: 705-709. Sebert, C, Curtis, KM., Barrett, T.J., and Sherlock, R.L. 1995. Geology of the Tulsequah Chief VMS deposit, Northwestern British Columbia (104K/12). In British Columbia Ministry of 200 Energy, Mines and Petroleum Resources Geological Fieldwork, Edited by B. Grant and J.M. Newell, pp. 529-540. Sebert, C. and Barrett, T.J. 1996. Stratigraphy, alteration and mineralization at the Tulsequah Chief massive sulphide deposit, Northwestern British Columbia. Exploration and Mining Geology. Sherlock, R.L., Childe., F., Barrett., T.J., Mortensen, J.K., Lewis, P.D., Chandler, T., and Allen., R. 1994. Geological Investigations of the Tulsequah Chief Massive Sulphide Deposit, Northwestern British Columbia (104K712). In British Columbia Ministry of Energy, Mines and Petroleum Resources Geological Fieldwork, Edited by B. Grant and J.M. Newell, p.373-379. Souther, J.G. 1971. Geology and mineral deposits of the Tulsequah map-area, British Columbia. Geological Survey of Canada, Memoir 362. Steiger, R.H., and Jager, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 14: 359-362. Theriault, R., J. 1990. Methods for Rb-Sr and Sm-Nd isotopic analyses at the geochronology laboratory, Geological Survey of Canada, in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 89-2, p. 3-6. York, D. 1969. Least-squares fit of a straight line with correlated errors. Earth and Planetary Science Letters, 5: 320-324. 201 CHAPTER 6: U-PB GEOCHRONOLOGY AND RADIOGENIC ISOTOPIC SYSTEMATICS OF ROOF PENDANTS IN THE COAST PLUTONIC COMPLEX WHICH HOST VMS MINERALIZATION Plate 6.1. Abandoned power plant at Anyox. 202 Part I: U-Pb Geochronology and Pb Isotopic Systematics of the Anyox pendant, West-Central British Columbia Introduction The Anyox pendant lies within the Coast Plutonic Complex in the Nass River (103P/5) map area of west-central British Columbia (Figs. 6.1 and 6.2). Mafic volcanic and sedimentary rocks of uncertain, but probable Mesozoic age, on the eastern side of the pendant are host to the Anyox volcanogenic massive sulphide (VMS) deposits. These deposits were mined between 1914 and 1935, and the two largest orebodies, the Hidden Creek and Bonanza deposits produced 21.7 Mt grading 1.5% Cu, 9.25 g/t Ag, and 0.17 g/t Au, and 0.66 Mt grading 2.2% Cu, respectively (Hoy 1991). Both the age and terrane affiliation of the Anyox pendant have been topics of debate (e.g., Alldrick 1986a; Grove 1986; Smith 1993). Prior to this study there were no U-Pb ages and only limited biochronological data for rocks of the Anyox pendant. In the current study volcanic, intrusive, and sedimentary rocks were collected in an attempt to constrain the age of VMS mineralization in the Anyox pendant using U-Pb methods. In addition, sulphides from the Hidden Creek orebody, and base and precious metal-rich quartz veins from Granby Peninsula, in the easternmost portion of the pendant, were analyzed for their Pb isotopic compositions. Detrital zircon grains from turbiditic sedimentary rocks, also on Granby Peninsula, were dated by U-Pb methods to begin to constrain the provenance of this strata, and a diorite sill from Mount Clashmore was dated to begin to establish the age of some of the igneous rocks in the central to western portions of the pendant. 203 129' 128" S S S J J- J- J- J S S S J C t ' ' wm H Legend Tertiary continental arc volcanic rocks Coast Plutonic Suite (undifferentiated) Juro-Cretaceous Bowser Lake Group Early to Middle Jurassic Intrusions Lower to Middle Jurassic Hazelton Group Late Triassic Intrusions Upper Triassic Stuhini Group Paleozoic Stikine Assemblage Devonian Intrusive rocks Scotia-Quaal Belt Metamorphic rocks within the Coast Plutonic Complex Alexander - Wrangel terranes (undifferentiated) Figure 6. 1 Generalized geology of the Canadian Cordillera between 53° to 58°N, and 128° to 132°W (after Wheeler and McFeely, 1991). 204 + + + + + + + + + + + + + + + + + + + Legend Tertiary pp 3 granite, quartz monzonite, I—.—I quartz monzodiorite Jurassic +/- Triassic l.v. •. .1 sandstone, siltstone, turbidite, 1. • .'• • -I in part correlated with the Bowser Lake Group Jurassic or Triassic (?) p<v>i pillowed to massive volcanic ixxSI & tuffaceous rocks, intruded by gabbro, overlain by sedimentary rocks Paleozoic to Mesozoic (?) |7{V7| cataclastic & mylonitic granitoid ll^SMil sheared volcanic and sedimentary rock, |wii8"fi| intruded by gabbro, diorite, & quartz diorite [riV^i] sheared plutonic rocks Lx**H ultramafic rock Anyox VMS deposits ® Hidden Creek (2) Double Ed (3) Bonanza 0 Redwing © Eden fault syncline, anticline U-Pb sample locality Middle Jurassic fossil locality (Goutier et al. 1990; Evinchick and Holm in press) 10 kilometres Figure 6. 2 Generalized geology of Anyox pendant, west-central British Columbia (after Alldrick 1986a; Grove 1986; Evinchick and Holm 1997). 205 Geology and mineral deposits of the Anyox pendant The eastern portion of the Anyox pendant has been mapped, and its contained mineral deposits described by Sharp (1980), Alldrick (1986a and b), Grove (1986), and Macdonald et al. (1996). Geochemical studies of the tholeiitic mafic volcanic rocks that host VMS mineralization were conducted by Smith (1993) and Macdonald et al. (1995 and 1996). Until recently there has been no systematic mapping of the western portion of the pendant. Geological mapping of the entire Anyox pendant, as well as description of mineral deposits within the western portion of the pendant, is the focus of collaborative project by the Geological Survey of Canada and the British Columbia Geological Survey (Alldrick et al. 1996; Evenchick and Holm 1997; Evenchick et al. 1997). The western two thirds of the Anyox pendant is composed of strongly to moderately deformed metavolcanic and metasedimentary rocks that have been intruded by gabbroic to quartz-dioritic composition sill-like bodies (Alldrick et al. 1996; Evinchick and Holm 1997). The Maple Bay quartz-Cu+/-precious metal vein systems occur along b