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Resolving monazite growth mechanisms in orogenic gold settings : a study from the Klondike Gold District,… Stroh, Brodie A. 2019

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Resolving Monazite Growth Mechanisms in Orogenic Gold Settings: AStudy from the Klondike Gold District, Western YukonbyBrodie A. StrohB.Sc., University of Regina, 2016A THESIS SUBMITTED IN PARTIAL FULFILLMENTOF THE REQUIREMENTS FOR THE DEGREE OFMaster of ScienceinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Geological Sciences)The University of British Columbia(Vancouver)October 2019c© Brodie A. Stroh, 2019The following individuals certify that they have read, and recommend to the Faculty of Graduate andPostdoctoral Studies for acceptance, the thesis entitled:Resolving Monazite Growth Mechanisms in Orogenic Gold Settings: A Study from theKlondike Gold District, Western Yukonsubmitted by Brodie A. Stroh in partial fulfillment of the requirements for the degree of Master of Sciencein Geological Sciences.Examining Committee:Dr. Craig J. R. Hart, Geological SciencesSupervisorDr. Murray M. Allan, Teck Resources LimitedSupervisory Committee MemberDr. James K. Mortensen, Geological SciencesSupervisory Committee MemberDr. Lee A. Groat, Geological SciencesUniversity ExaminerAdditional Supervisory Committee Members:Dr. Tyler K. Ambrose, Yukon Geological SurveySupervisory Committee MemberDr. Matthijs A. Smit, Geological SciencesSupervisory Committee MemberiiAbstractOrogenic gold deposits form some of the world’s largest gold sources; however, the timing of goldmineralization is difficult to constrain. The ∼20 Moz of placer gold mined in the Klondike was derivedfrom orogenic, gold-bearing quartz veins where the exact timing of gold mineralization is unknown despiteprevious age data from hydrothermal micas and rutile. U-Th-Pb dating of hydrothermal monazite can be arobust alternative to constrain the timing of orogenic gold deposits and associated geological events that hasnot been attempted in the Klondike until now.In orogenic gold settings or fluid-affected metamorphic terranes in general, monazite can grow bymetamorphic and hydrothermal processes. The objective of this research is to distinguish between thesemonazite growth processes to provide constraints on orogenic gold mineralization in the Klondike. Veinmaterial, altered wall-rock, and unaltered host-rock samples were investigated at the Virgin, Mitchell-Sheba,and Lone Star gold occurrences. Detailed petrographic analyses were integrated with LA-ICP-MS Th-Pbmonazite dates and ThO2 concentrations to identify metamorphic and hydrothermal monazite.The age of metamorphic monazite in the Klondike is between 189 and 151 Ma, with possible discretepulses at approximately 175 and 160 Ma. These ages overlap with existing 40Ar/39Ar mica cooling agesand represent retrograde monazite growth during crustal exhumation. The age of hydrothermal monazite isbetween 178 to 117 Ma, with possible discrete pulses at approximately 169, 148, and 128 Ma. These agesrepresent episodic vein formation and provide approximate constraints on gold mineralization.The methods developed can be used to identify metamorphic and hydrothermal monazite globally toprovide robust time constraints on metamorphism, vein formation, and gold mineralization. Metamorphicmonazite occurs regardless of proximity to veins, is typically adjacent to or intergrown with other meta-morphic minerals, and has variable ThO2 concentrations that depend on the host-rock composition andmetamorphic grade. In contrast, hydrothermal monazite occurs in or adjacent to veins, is typically adjacentto or intergrown with hydrothermal minerals, and has a distinctly low ThO2 concentration < 2.00 wt.%.iiiLay SummaryThe global demand for metals requires efficient exploration practices and well-constrained metallogenicmodels that include the age of specific deposits. The timing of gold deposits are particularly difficult to con-strain, including those that formed in the Klondike region of northwestern Canada. This research tested anew method that uses the mineral monazite as a tool for dating gold deposits in the Klondike. The approxi-mate timing of gold deposits in the Klondike is between 178 and 117 million years ago. This research alsoprovided new methods to identify monazite and its geological significance, which will help future researchthroughout the world to determine the age of other gold deposits and associated geological processes.ivPrefaceSupervisor, Dr. Craig Hart, and committee member, Dr. Murray Allan, are credited for the initialproject design. The author and committee member, Dr. Tyler Ambrose, are responsible for altering thescope of the project. The author is responsible for the identification of the research objectives. Additionalresearch committee members, Dr. James Mortensen and Dr. Matthijs Smit, are credited for providing inputat committee meetings and through independent correspondence.The author is accountable for the following:1) Fieldwork and sample collection with the help of committee member, Dr. Murray Allan, and fieldassistant, Keagan Parry.2) Sample cutting at the Yukon Geological Survey and the University of British Columbia (UBC) withthe help of field assistant, Keagan Parry.3) Sample selection for analyses.4) Petrographic analysis at the Mineral Deposit Research Unit (MDRU).5) Quartz vein cathodoluminescence (CL) at the Electron Microbeam and X-Ray Diffraction Facil-ity (EMXDF) at UBC with the help of research scientists, Edith Czech, Lan Kato, Jenny Lai, andElisabetta Pani.6) Monazite identification by scanning electron microscope (SEM) at EMXDF at UBC with the help ofresearch scientists, Edith Czech, Lan Kato, Jenny Lai, and Elisabetta Pani.7) Monazite element mapping by electron probe micro-analyzer (EPMA) at the Fipke Laboratory forTrace Element Research (FiLTER) at the University of British Columbia Okanagan (UBC-O) withthe help of technician, David Arkinstall.8) Monazite geochronology by laser ablation inductively-coupled plasma mass spectrome-try (LA-ICP-MS) at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) atUBC with the help of research assistant, Vivian Lai, and Ph.D. candidate, Jamie Cutts.9) Monazite trace element analyses by LA-ICP-MS at PCIGR at UBC with the help of research associate,Dr. Marghaleray Amini.v10) Analytical data management, manipulation, interpretation, and figure preparation with Microsoft Ex-cel (Excel), Reflex ioGAS (ioGAS), MathWorks Matlab (Matlab), the Isoplot Excel macro of Ludwig(2003), Adobe Photoshop, and Adobe Illustrator, under the guidance of the supervisory committee.Independent laboratories performed the following:1) Thin section preparation at Vancouver Petrographics Limited in Langley, British Columbia.2) Whole rock lithogeochemistry at Bureau Veritas in Vancouver, British Columbia.viTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiList of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xivAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Klondike Gold District . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Exploration and Mining History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.5 Study Objectives and Scientific Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.6 Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.7 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Regional and Local Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1 Tectonic Evolution and Assembly of the Yukon-Tanana Terrane . . . . . . . . . . . . . . . . 92.2 Metallogenic Evolution of the Yukon-Tanana Terrane . . . . . . . . . . . . . . . . . . . . . 162.3 Klondike Gold District . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.1 Bedrock Assemblages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.2 Metamorphism and Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20vii2.3.3 Orogenic Gold Mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Resolving Monazite Growth Mechanisms in Orogenic Gold Settings: A Study from theKlondike Gold District, Western Yukon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2 Monazite Growth Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.3 Regional and Local Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.4 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4.1 Sample Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4.2 Whole Rock Lithogeochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.4.3 Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.4.4 Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.4.5 Electron Probe Micro-Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.4.6 Laser Ablation Inductively-Coupled Plasma Mass Spectrometry . . . . . . . . . . . 383.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.5.1 Petrography and Vein Textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.5.2 Whole Rock Lithogeochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.5.3 Monazite Petrography and Characterization . . . . . . . . . . . . . . . . . . . . . . 493.5.4 Monazite Element Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.5.5 Monazite Geochronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.5.6 Monazite Trace Element Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.6.1 Vein Formation and Overprinting Deformation . . . . . . . . . . . . . . . . . . . . 563.6.2 Interpretation of Monazite Dates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.6.3 Interpretation of Monazite Trace Element Data . . . . . . . . . . . . . . . . . . . . 603.6.4 Implications for the Klondike Gold District . . . . . . . . . . . . . . . . . . . . . . 623.6.5 Implications for Monazite Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.1 General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2 Exploration Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.3 Recommendations for Resolving Monazite Growth Mechanisms . . . . . . . . . . . . . . . 784.4 Recommendations for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97A Vein Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97A.1 Macroscopic Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98viiiA.1.1 Lenticular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98A.1.2 Sigmoidal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99A.1.3 Tabular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99A.1.4 Irregular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100A.1.5 Stockwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100A.1.6 Breccia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100A.2 Microscopic Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100A.2.1 Blocky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101A.2.2 Elongate Blocky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101A.2.3 Fibrous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101A.2.4 Stretched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101A.2.5 Deformed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102A.3 Growth Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102A.3.1 Syntaxial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103A.3.2 Antitaxial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104A.3.3 Ataxial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105B List of Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107C Petrographic Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116D Cathodoluminescence Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272E Petrography and Paragenesis Summaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291F Whole Rock Lithogeochemical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301G U-Th-Pb Geochronology Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307G.1 U-Th-Pb Decay System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308G.2 Concordia Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311G.3 Laser Ablation Inductively-Coupled Plasma Mass Spectrometry . . . . . . . . . . . . . . . 314H Review of the Mineral Monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316H.1 The Mineral and Geochronometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317H.2 Identifying the Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318H.3 Resetting the U-Th-Pb Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321I Protocol for Monazite Geochronology at the University of British Columbia . . . . . . . . . . 322I.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323I.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323I.2.1 Scientific Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323I.2.2 Purpose of the Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324ixI.3 Monazite Reference Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324I.3.1 Manangotry Monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325I.3.2 Moacyr Monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326I.4 Analytical Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328I.4.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328I.4.2 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329I.4.3 Data Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329I.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331I.5.1 Dating Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331I.5.2 Reference Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331I.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335I.6.1 External Reference Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335I.6.2 Iolite Unknowns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335I.6.3 Available Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336I.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338I.8 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338J Monazite LA-ICP-MS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339K Monazite Analysis Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382L Data Quality (QA/QC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425xList of TablesTable 2.1 Tectonic Assemblages of the Slide Mountain and Yukon-Tanana Terranes . . . . . . . . . 11Table 2.2 Deformation and Metamorphic Events in the Klondike . . . . . . . . . . . . . . . . . . . 22Table 3.1 Indicators of Monazite Geological Environments . . . . . . . . . . . . . . . . . . . . . . 31Table 3.2 Laser Ablation Parameters for Monazite Petrochronology . . . . . . . . . . . . . . . . . 38Table 3.3 Petrography Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Table A.1 Growth Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Table H.1 Indicators of Monazite Geological Environments . . . . . . . . . . . . . . . . . . . . . . 320Table I.1 Summary of Manangotry Ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327Table I.2 Summary of Moacyr Ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328Table I.3 Experimental LA-ICP-MS Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 330Table I.4 Recommended LA-ICP-MS Parameters for Monazite Geochronology at UBC . . . . . . 331xiList of FiguresFigure 1.1 Western Yukon Metallogenic Belts’ Hard Rock and Placer Gold Abundances . . . . . . 5Figure 2.1 Cordilleran Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 2.2 Summary of Yukon-Tanana Terrane Building Events . . . . . . . . . . . . . . . . . . . 15Figure 2.3 Metallogenic Belts of the Northern Yukon-Tanana Terrane . . . . . . . . . . . . . . . . 17Figure 2.4 Klondike Geological Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 2.5 Klondike Deformation and Mineralization Structures . . . . . . . . . . . . . . . . . . . 25Figure 3.1 Monazite Textures in Orogenic Gold Settings . . . . . . . . . . . . . . . . . . . . . . . 30Figure 3.2 Klondike Geological Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Figure 3.3 Yukon-Tanana Terrane Mesozoic Structural Events . . . . . . . . . . . . . . . . . . . . 36Figure 3.4 Common Pb Correction on U-Th-Pb Concordia . . . . . . . . . . . . . . . . . . . . . . 40Figure 3.5 Virgin Petrographic Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Figure 3.6 Mitchell-Sheba Petrographic Observations . . . . . . . . . . . . . . . . . . . . . . . . . 44Figure 3.7 Lone Star Petrographic Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Figure 3.8 Local Crenulation Adjacent to Vein . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Figure 3.9 Klondike Volcanic Rock Classifications . . . . . . . . . . . . . . . . . . . . . . . . . . 48Figure 3.10 Monazite Element Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Figure 3.11 Monazite Geochronology Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Figure 3.12 Monazite Trace Element Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Figure 3.13 Paragenesis of Klondike Gold Occurrences . . . . . . . . . . . . . . . . . . . . . . . . 57Figure 3.14 Monazite Geochronology Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . 59Figure 3.15 Weighted Mean of Klondike Monazite Eu Anomalies . . . . . . . . . . . . . . . . . . . 61Figure 3.16 Monazite Trace Element Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Figure 3.17 Weighted Mean of Klondike Monazite Ages . . . . . . . . . . . . . . . . . . . . . . . . 66Figure 3.18 Klondike Monazite Age Probability Density . . . . . . . . . . . . . . . . . . . . . . . . 67Figure 3.19 Comparison of Klondike Metamorphic and Hydrothermal Monazite and Mica Ages . . . 70Figure 3.20 Monazite-Forming Geological Events . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Figure 3.21 Klondike Whole Rock Th Versus Zr/TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . 72Figure A.1 Macroscopic Vein Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98xiiFigure A.2 Failure Envelope on the Mohr Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 99Figure A.3 Microscopic Vein Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Figure A.4 Tracking Capability of Vein Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Figure A.5 Vein Opening Trajectory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105Figure E.1 Paragenesis of the Virgin Gold Occurrence . . . . . . . . . . . . . . . . . . . . . . . . 296Figure E.2 Paragenesis of the Mitchell-Sheba Gold Occurrence . . . . . . . . . . . . . . . . . . . . 298Figure E.3 Paragenesis of the Lone Star Gold Occurrences . . . . . . . . . . . . . . . . . . . . . . 300Figure G.1 U-Th-Pb Decay Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309Figure G.2 U-Th-Pb Half Lives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310Figure G.3 U-Th-Pb Isochrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311Figure G.4 U-Pb Concordia Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313Figure G.5 LA-ICP-MS Analysis Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315Figure H.1 Monazite Crystal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317Figure H.2 Monazite REE Distribution Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319Figure I.1 U-Pb ID-TIMS Age Results for Manangotry . . . . . . . . . . . . . . . . . . . . . . . 326Figure I.2 Laser Parameter Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332Figure I.3 Monazite Reference Material Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334Figure I.4 Monazite Reference Material Pucks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337Figure L.1 Geochronology QA/QC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432xiiiList of AbbreviationsAll mineral abbreviations after Siivola and Schmid (2007).BSE backscattered electronCL cathodoluminescenceEDS energy-dispersive X-ray spectroscopyEMXDF the Electron Microbeam and X-Ray Diffraction FacilityEPMA electron probe micro-analyzerExcel Microsoft ExcelFiLTER the Fipke Laboratory for Trace Element ResearchHREE heavy rare earth elementsICP-MS inductively-coupled plasma mass spectrometryID-TIMS isotope dilution thermal ionization mass spectrometryioGAS Reflex ioGASLA-ICP-MS laser ablation inductively-coupled plasma mass spectrometryLREE light rare earth elementMatlab MathWorks MatlabMDRU the Mineral Deposit Research UnitNSERC-CRD Natural Sciences and Engineering Research Council of Canada Collaborative ResearchDevelopmentPCIGR the Pacific Centre for Isotopic and Geochemical ResearchQA/QC Quality assurance/quality controlxivREE rare earth elementsSEM scanning electron microscopeUBC the University of British ColumbiaUBC-O the University of British Columbia OkanaganVMS volcanogenic massive sulphideXRF X-ray fluorescencexvAcknowledgmentsI want to express my most sincere gratitude towards all involved in the completion of this project. I amlucky to interact with and be a part of the group of people that comprise MDRU and UBC. First, I am deeplyindebted to Dr. Tyler Ambrose for taking me under his wing and providing endless enthusiasm. Although heonly joined the project halfway through, the spark he brought with him deserves higher recognition. I thankDr. Craig Hart and Dr. Murray Allan for the opportunity to carry out this research and for the opportunitiesthat came with my time at MDRU. These people, along with the other committee members, Dr. JamesMortensen and Dr. Matthijs Smit, are thanked for their insights, discussions, and mentoring.This research is part of the larger MDRU Yukon-Alaska Metallogeny project that was funded bya Natural Sciences and Engineering Research Council of Canada Collaborative Research Development(NSERC-CRD) grant, Goldcorp Incorporated (now Newmont Goldcorp Corporation), and Sumac MinesLimited. Additional funds were provided through awards and scholarships, including the Society of Eco-nomic Geologists Canada Foundation Graduate Student Fellowship, Northwest Territories and NunavutAssociation of Professional Engineers and Geoscientists Finnigan Award for Northern Research, YukonFoundation Bradshaw Memorial Scholarship, and Plains Midstream Canada Higher Education Award. Ac-cess to field sites and drill core was provided by Klondike Gold Corporation, GroundTruth ExplorationIncorporated, and the Yukon Geological Survey. This research and its contributions would not have beenpossible without the financial and field support of these organizations.Lastly, thanks to all the others who have supported me on this journey. Thanks to Keagan Parry for thetremendous help in the field. All the staff who helped me at EMXDF, FiLTER, PCIGR, and MDRU — youare not forgotten. Thanks to current and former students, including but not limited to, Kaleb Boucher, SamCantor, Dr. Jamie Cutts, Dr. Anais Fourny, Melissa Friend, Bianca Iluanella-Phillips, Niki Kovacs, RhyMcMillan, Dr. Fabien Rabayrol, Ryan Shaw, Tom Ver Hoeve, and Andy Wickham, who each helped withuseful discussions, feedback on writing, or, most importantly, venting. Despite never meeting me, Dr. AnneMagali Seydoux-Guillaume (Jean Monnet University, France), Pauline Jeanneret (Uppsala University, Swe-den), and Sudip Shrestha (UBC-O) helped through email while I needed advice from monazite researchers.Finally, thanks to Dr. Mati Raudsepp, who told me I was a fool for working with monazite ... I probablyshould have listened.xviDedicationTo my parents, who provided me with the opportunity for an educa-tion and passed on their hard work ethic for me to get through it.To Taryn Ortman, who moved with me to Vancouver and supportedme the entire time.To my friends and family, who had to put up with me not beingaround or me not being myself when I was around. I’m back.xviiChapter 1Introduction1.1 RationaleOrogenic gold deposits form along convergent plate margins along structures that act as conduits forhydrothermal fluid generated during metamorphism (e.g., Groves et al., 1998; Bierlein and Crowe, 2000;Goldfarb et al., 2001; Groves et al., 2003; Goldfarb et al., 2005; Goldfarb and Groves, 2015). The termorogenic gold deposit was introduced by Groves et al. (1998) to consolidate and replace previously commonterms including Archean, mesothermal, lode, greenstone, shear zone, turbidite-hosted, motherlode, and low-sulphide gold-quartz, which all referred to deposits formed through the same fundamental process. Althoughorogenic gold deposits form some of the world’s largest gold sources (e.g., Ashanti, Ghana, 3169 t at 2–7 g/t;Golden Mile, Australia, 2079 t at 2 g/t; Frimmel, 2008), several components of their generation are highlydebated (e.g., Goldfarb and Groves, 2015). Key questions remain regarding the fluid and gold sources aswell as the controls on gold deposition.In contrast to other gold-bearing deposit styles such as porphyry Cu (Mo-Au) and intrusion-relatedgold, orogenic gold deposits are genetically linked with metamorphism rather than magmatism. In orogenicgold systems, the mineralizing fluid is inferred to be produced by devolatilization reactions during regionalmetamorphism at pressures and temperatures corresponding to the greenschist-amphibolite transition zone(e.g., Henley et al., 1976; Stuwe, 1998; Goldfarb et al., 1991; Pitcairn et al., 2006). This fluid is channelledin structures and driven upwards before precipitating quartz-dominated, gold-bearing veins with adjacentwall-rock alteration, typically in metamorphic rocks at 3 to 15 km depth (Groves et al., 1998; Goldfarb andGroves, 2015).Whereas the hydrothermal fluids are metamorphic in origin, the source of the gold and the sulphur orchlorine needed to transport it are unclear in many cases. Some models suggest that gold was along with thefluids, from the same source (e.g., Powell et al., 1991; Phillips and Powell, 2010), whereas others proposethat gold is scavenged via wall-rock interactions during fluid migration (e.g., Cox, 2005). Gold is transportedin solution as a bisulphide (Au(HS) –2) or chloride (AuCl-) complex, most commonly with the HS- sulphide1ligand (Seward, 1973). Thus, gold in solution is mainly dependent on the activity of reduced sulphur, whichis typically generated at the pyrite-pyrrhotite transition during prograde metamorphism and can be incor-porated into the hydrothermal system at the fluid source or during migration (Toulmin and Barton, 1963;Ferry, 1981; Tomkins, 2010). Gold deposition commonly occurs during sulphidation reactions that result inthe precipitation of pyrite, where the HS- ligand is removed, leading to co-precipitation of gold with pyrite(Williams-Jones et al., 2009). Sulphidation reactions may result from fluid chemistry destabilization fromchanges in pressure, temperature, oxygen fugacity (f O2), pH, and rheological properties of the surroundingrock, which promotes a decrease in HS-, f O2, temperature, and an increase in pH (Williams-Jones et al.,2009).Constraining the timing of orogenic gold formation is challenging because datable minerals in theore system may have poorly established paragenetic relationships with gold, may be too small or scarceto analyze, or may be reset by later thermal events or hydrothermal fluids. The timing of orogenic goldformation can be constrained by 40Ar/39Ar dating of muscovite, sericite, and biotite (e.g., Bierlein et al.,2001; Mortensen et al., 2010), 187Re/187Os dating of arsenopyrite and molybdenite (e.g., Arne et al., 2001;Selby et al., 2002), and U-Pb dating of zircon, titanite, and rutile (e.g., Kerrich, 1993; Lin and Corfu, 2002).Estimating gold formation has traditionally relied on 40Ar/39Ar dating of white micas because they are atypical hydrothermal phase within veins, whereas other datable minerals are less consistently available.However, these micas have a low closure temperature (e.g., ∼380◦C for muscovite; Reiners and Brandon,2006) and are susceptible to resetting by later thermal events and fluids. Moreover, these dates can overlapwith and be difficult to resolve from regional metamorphic cooling dates.U-Th-Pb dating of monazite or xenotime offers an alternative to determine the formation age of orogenicgold deposits (e.g., Brown et al., 2002; Vielreicher et al., 2003; Salier et al., 2005; Sarma et al., 2008;Taylor et al., 2015). Monazite is less susceptible than mica to resetting by a thermal event due to its highclosure temperature of >800◦C (Cherniak et al., 2004; Gardes et al., 2006). Interactions with hydrothermalfluids cooler than 800◦C can modify pre-existing monazite grains and lead to reset ages, which can beutilized to date hydrothermal activity (e.g., Williams et al., 2011). Monazite petrochronology, which isthe interpretation of isotope dates in light of complementary element information (Kylander-Clark et al.,2013), provides the opportunity to understand relationships between monazite growth and well-constrainedgeochronological data. Although monazite petrochronology is most common in metamorphic studies (e.g.,Pyle and Spear, 1999), it has been used to make inferences about monazite formation in ore deposits as well.For example, Schandl and Gorton (2004) used ThO2 concentrations to distinguish between hydrothermaland igneous monazite, and Taylor et al. (2015) used Eu anomalies to identify hydrothermal monazite in anorogenic gold deposit.Linking monazite growth or modification to orogenic gold formation requires establishing a parageneticrelationship with gold, particularly because monazite may grow by multiple processes. For example, in oro-genic gold settings monazite can grow by 1) metamorphic processes, during both prograde and retrogrademetamorphism, and 2) hydrothermal processes, precipitating from fluids in and adjacent to veins. Distin-guishing between these growth processes is vital to determine the geological significance of geochronolog-2ical data.The Klondike gold district (also known as the Klondike; Section 1.2) in the northwestern CanadianCordillera hosts both orogenic gold occurrences and placer gold deposits. An estimated 20 million ouncesof placer gold were recovered from gold-bearing creeks in the Klondike from an area of∼1,200 km2 (Burkeet al., 2005). Coarse, visible gold is a typical feature of many orogenic quartz veins in the bedrock surround-ing the gold-bearing creeks, which suggests the veins are the source of placer gold throughout the district.This link between placer and orogenic gold is confirmed by microchemical and grain shape analysis studiesof gold grains (e.g., Knight et al., 1999a,b; Mortensen et al., 2004, 2006a; Crawford, 2007; Crawford et al.,2007; Chapman et al., 2010a,b, 2011).A regionally widespread, orogenic gold mineralization event in the Late Jurassic is recognized acrossthe Klondike, Sixtymile and White Gold metallogenic belts of western Yukon (Allan et al., 2013; Bailey,2013). The genesis and detailed timing of emplacement and mineralization of the orogenic, gold-bearingquartz veins in the Klondike are poorly constrained. These veins also host micas, but they are difficult to linkto mineralization unambiguously. The veins have been previously dated by 40K/40Ar and 40Ar/39Ar dating ofhydrothermal micas at 183 to 134 Ma (Hunt and Roddick, 1992; Breitsprecher and Mortensen, 2004; Joyceet al., 2015), although it is unclear if these dates were thermally reset during regional cooling. Mortensen(in review) also reported a U-Pb age of 155.6 ± 7.6 Ma from hydrothermal rutile from a single vein. Keyquestions remain and include the geological significance of these dates as representing vein formation orgold mineralization and their relationships to metamorphism and regional exhumation. Thus, the Klondikeis suitable for a monazite petrochronological study to provide geological significance to currently ambiguousdates.This thesis provides constraints on the timing of quartz vein emplacement and gold mineralization in theKlondike and offers insight into monazite growth in orogenic gold settings through petrographic analysis andmonazite petrochronology. Controls on vein and alteration paragenesis in the Klondike are first establishedwith petrographic analysis, quartz vein cathodoluminescence (CL), and whole rock lithogeochemistry. Bycombining petrographic analysis, geochronology, and ThO2 concentration data, the mechanism of monazitegrowth is interpreted in the context of metamorphic and hydrothermal processes. Once monazite growthmechanisms are resolved, the geochronological data become geologically meaningful, and ages of meta-morphic and hydrothermal activity in the Klondike are constrained. The methods established here resolvemonazite growth mechanisms and can be applied not only to monazite growth in orogenic gold settings butalso to growth in metamorphic terranes globally.1.2 Klondike Gold DistrictThe Klondike gold district is located in western Yukon about 400 km northwest of the city of Whitehorseand immediately southeast of Dawson City. The district covers approximately 1,200 km2 and is roughly de-fined by the limits of placer-producing creeks and bedrock gold occurrences. The region was unglaciatedduring the Pleistocene (Duk-Rodkin, 1996), leading to the present-day terrane of rolling hills covered by3thick permafrost soils and extensive vegetation cover. These surficial conditions have been favourable forplacer gold deposition since the Pliocene (Lowey, 2004). The Klondike is underlain by the northern portionof the Yukon-Tanana terrane, a metamorphosed, pericratonic block that was accreted onto the North Ameri-can margin during the Mesozoic assembly of the Canadian Cordillera (Colpron et al., 2006a,b, 2007; Nelsonet al., 2013). As described in Section 2.3, Klondike district rocks were variably derived from felsic to maficvolcanic packages, felsic intrusive suites, and clastic sedimentary units before undergoing greenschist faciesmetamorphism and compressional deformation beginning in the Late Permian (Mortensen, 1990, 1996). Theunits were later stacked into a series of thrust slices during compressional deformation in the Late Triassicto Early Jurassic before exhumation during the Jurassic and extensional deformation in the Late Cretaceous(Mortensen, 1990, 1996; MacKenzie et al., 2008c). Orogenic gold mineralization was deposited in the LateJurassic, although the detailed timing of vein emplacement and mineralization is not well constrained (Allanet al., 2013).There are two distinct generations of quartz veins in the Klondike: early, barren veins and late, gold-bearing veins (MacKenzie et al., 2008c). Barren quartz veins, or segregation veins, formed during metamor-phism, are parallel with the metamorphic fabric, and are not considered to be part of the gold mineralizingevent. The gold-bearing veins that formed during the mineralizing event are largely extensional and formeddiscordant to the metamorphic fabric. These veins are a focus of this thesis and represent the veins referredto throughout the text. Hydrothermally altered and non-hydrothermally altered rocks (or fresh metamorphicrocks) from vein-forming fluids are respectively referred to as altered and unaltered rocks throughout thethesis.1.3 Exploration and Mining HistoryPlacer gold production has continued from its initial discovery in 1896 to the present day, with 47,000ounces recovered in 2017 (Bond and van Loon, 2018). Despite the multi-millions of ounces of gold recov-ered from placer production, only about 1,240 ounces have been mined from bedrock (Figure 1.1; Allanet al., 2013). Prospecting of placer gold deposits to their upper limits led to the discovery of many gold-bearing quartz vein systems throughout much of the Klondike. Limited underground exploration was com-pleted at the Lone Star, Violet, Virgin, Mitchell, Dome Lode/Hunker Dome, and Aime/Payne occurrences(Yukon-MINFILE 115O 072; 115O 073; 116B 007; 115O 068; 115O 067; 115O 061). Mining at Lone Star(also known as the Boulder Lode Mine) took place from 1912 to 1914, producing nearly all hard rock goldever recovered in the Klondike (Yukon-MINFILE 115O 072). Small-scale operations were also carried outat the Violet and Virgin occurrences from 1913 to 1914 (Yukon-MINFILE 115O 073; 116B 007). Sporadichard rock exploration continued, and efforts since 2010 have been focused on the Lone Star and Dome Lodeareas (Allan et al., 2013). Despite over a century of hard rock exploration, no economic hard rock golddiscoveries have been made to define a modern resource. A wide variety of mineralization styles have beenapplied to target the source of the placer gold, including intrusion-related, epithermal, syngenetic, and oro-genic veins. Mortensen et al. (1992), Rushton et al. (1993), and MacKenzie et al. (2008c) have demonstratedthat gold-bearing quartz veins in the Klondike are unrelated to magmatic events and are orogenic in origin.41001,00010,000100,0001 Moz1010010,000 100,00 1 MozApprox. placer gold production(oz).xorppAdlogkcordebcirotsih)zo(ecruoserDawson RangeWhite GoldKlondikeSixtymileFortymile10 100Figure 1.1: Hard rock gold resource for each metallogenic belt of Western Yukon (locations of beltspresented in Figure 2.3) plotted against total historical placer gold production (logarithmic scale).Ellipses indicate approximate uncertainties in resource figures. Negligible hard rock productionrelative to placer production is emphasized in the Klondike (from Allan et al., 2013).1.4 Previous StudiesThe first geological map of the Klondike was made by McConnell (1905) following the initial 1896 goldrush. Bostock (1942), Green (1972), Tempelman-Kluit (1974), and Debicki (1984, 1985) later remappedparts of the region. The most recent map was produced by Mortensen (1996) with updated compilations byMacKenzie et al. (2007) and Mortensen et al. (2019), although more detailed property-scale maps can begathered from assessment reports. Metcalfe (1981) and Mortensen (1990, 1996) built much of the geologicalframework of the Klondike schist and other units throughout the district. Structural studies by MacKenzieet al. (2007, 2008a) uncovered deformational features and associated thrust-bound packages throughoutthe Klondike. Beranek and Mortensen (2011) constrained the timing of initial ductile deformation andassociated metamorphism across the Yukon-Tanana terrane to the Late Permian.Knight et al. (1994) were the first to chemically investigate gold grains, identifying chemical signaturesthat linked placer to lode gold. Since then, there have been microchemical analyses on placer and lode gold(e.g., Knight et al., 1999b; Mortensen et al., 2004, 2006a; Chapman et al., 2010a,b, 2011) and grain shapeevolution studies (e.g., Knight et al., 1999a; Crawford, 2007; Crawford et al., 2007) that have confirmed theplacer to lode gold link and uncovered the local sources of most placer-producing creeks.5Preliminary observations of the gold-bearing quartz veins provided initial control on the vein miner-alogy and paragenesis (Hoymann and Friedrich, 1990; Mortensen et al., 1992). Rushton et al. (1993) in-vestigated vein-forming fluid chemistry, and MacKenzie et al. (2008c) established structural controls on theveins. Later vein studies analyzed petrography and geochemistry (Liverton and Mann, 2011), microstruc-tures and trace element signatures (Wolff, 2012), carbonate in and outside of the veins (Allan et al., 2014),and ductile deformation overprinting veins (Parry, 2018). Most recent studies in the Klondike have focusedon the Lone Star area. MacKenzie et al. (2008b) identified a disseminated-gold-bearing felsic schist unitadjacent to veins in the Lone Star area. Grimshaw (2018) investigated the disseminated-gold-bearing unitand paragenetic evolution of the veins. Mortensen (in review) suggested this unit as a local source of goldfor the veins and provided an absolute age for nearby veining. Lastly, Mortensen et al. (2019) establishedchemostratigraphic constraints of all felsic schist units in the Lone Star area.1.5 Study Objectives and Scientific MotivationThe goal of this thesis is to define the temporal framework for orogenic quartz vein formation and goldmineralization in the Klondike gold district. Three objectives are identified to achieve this goal:1) Establish the gold-bearing vein and hydrothermal alteration evolution of the Klondike.This objective addresses the questions: 1a) What is the vein and alteration paragenesis in theKlondike? 1b) Did the veins form in a single-stage filling event or by multiple crack-seal events? 1c)Has ductile deformation overprinted the veins?Vein mineralogy and paragenesis have previously been investigated for individual occurrences (e.g.,Hoymann and Friedrich (1990) and Rushton et al. (1993) for the Hunker Dome area; Grimshaw(2018) for the Lone Star area), but few studies have investigated the detailed petrography and veintextures of Klondike veins as a whole. MacKenzie et al. (2008c) propose the quartz veins formedin a single-stage filling event that was purely brittle. However, Wolff (2012) revealed that severalvein samples display evidence of multiple crack-seal events during vein formation, Grimshaw (2018)discovered multiple generations of vein quartz in the Lone Star area, and Parry (2018) demonstratedthat quartz veins in the Lone Star area are locally overprinted by ductile deformation. In Chapter 3,I test these findings with a comprehensive paragenetic study that investigates several occurrencesacross the Klondike. Similarities and differences are highlighted between three different occurrences,including pre-mineralization and alteration phases in the adjacent wall-rock. Particular attention ispaid to quartz textures and deformational features to elucidate the mode of vein formation and timingrelative to deformation.2) Constrain the mechanisms that control monazite growth in the Klondike.This objective addresses the questions: 2a) How can metamorphic and hydrothermal monazite growthbe distinguished? 2b) What is the origin of monazite in the Klondike?In orogenic gold geological settings, monazite can grow by metamorphic reactions involving thebreakdown of other minerals on a local scale, or it can precipitate from externally-derived hydrother-6mal fluids in and adjacent to veins (Section 3.2). Understanding the monazite growth mechanism isessential to provide geological context to geochronological studies for which determination of dif-ferent growth processes is required to constrain the timing of mineralization against metamorphism.Petrography is vital for this determination; however, the origin of monazite grains may not alwaysbe clear. In Chapter 3, petrographic observations, monazite dates, and trace element geochemistryare integrated to resolve the mechanisms for monazite growth. The petrochronology presented notonly provides geological significance to monazite dates in the Klondike but also has implications formonazite growth in orogenic gold settings and metamorphic terranes globally.3) Define the temporal framework for metamorphism, vein formation, and gold mineralization inthe Klondike.This objective addresses the questions: 3a) Can metamorphic monazite reveal new details about theprograde and retrograde metamorphism in the Klondike? 3b) Did all orogenic gold occurrences inthe Klondike form during the same metallogenic event? 3c) At what time scale do vein formation andmineralization occur (i.e., are they single, episodic, or protracted events)?As introduced in Section 1.1, determining the absolute age of mineralization in the Klondike hasbeen challenging and ambiguous in the past. Interpreting past results as direct ages of vein formationor gold mineralization is problematic due to uncertainty in whether the dates represent hydrother-mal activity or post-mineralization, regional cooling since dates obtained using the 40K/40Ar and40Ar/39Ar methods are susceptible to resetting. The 155.6 ± 7.6 Ma U-Pb hydrothermal rutile agefrom Mortensen (in review) suggests that vein formation may have occurred as an isolated event.Allan et al. (2013) suggest this single age can be applied across all occurrences in the Klondike andthat vein formation and mineralization is represented in a single, albeit possibly protracted event.In Chapter 3, I test the hypothesis that vein formation and gold mineralization in the Klondike is155.6 ± 7.6 Ma and determine whether they are single, episodic, or protracted events using monazitepetrochronology.1.6 StrategyThe following strategy was implemented to achieve the objectives outlined in Section 1.5:1) Fieldwork. Sampling at each locality involved collecting unaltered and altered host-rock, as wellas vein material. Collecting each type of material was essential to determine the vein and alterationparagenesis of the Klondike as well as compare monazite petrography and geochemistry.2) Petrography and quartz vein cathodoluminescence. The first objective was achieved by detailedthin section and quartz vein cathodoluminescence analysis to establish the vein and alteration evolu-tion in the Klondike. These investigations also provided petrographic context for monazite.3) Monazite identification and petrography. Monazite and surrounding minerals were identified withthe scanning electron microscope (SEM). Monazite grains and textures were studied in backscattered7electron (BSE) mode and later imaged in reflected light4) Monazite element mapping. Element maps were made by the electron probe micro-analyzer(EPMA) to detect zoning within monazite grains that guided laser ablation spots for steps 5 and 6.5) Monazite geochronology. Geochronology by laser ablation inductively-coupled plasma mass spec-trometry (LA-ICP-MS) determined the date and ThO2 concentration of monazite grains.6) Monazite trace element analyses. Trace elements gathered by LA-ICP-MS added elemental infor-mation to compliment monazite dates. Information from steps 3 to 6 are integrated to achieve thesecond and third objectives.The above workflow was designed and executed as part of this study because it is the first of itskind to investigate monazite at the University of British Columbia (UBC). With support from the Elec-tron Microbeam and X-Ray Diffraction Facility, the Pacific Centre for Isotopic and Geochemical Research(PCIGR), the Mineral Deposit Research Unit (MDRU), and the Fipke Laboratory for Trace Element Re-search (FiLTER) at the University of British Columbia Okanagan (UBC-O), UBC now has the capability toidentify, X-ray map, and perform petrochronology on monazite. Section 3.4 discusses the specific methodsused, and Appendix I presents the initial research and analytical protocol for monazite geochronology.1.7 Thesis StructureThree chapters compose the remainder of this thesis. Chapter 2 describes the regional geology, begin-ning with the evolution of the Yukon Tanana terrane before describing the geology and mineralization styleof the Klondike. Chapter 3 presents petrographic and monazite petrochronology investigations. Results areput into the geological framework of the Klondike and Yukon-Tanana terrane, while the broader lessonslearned about monazite growth are related to a global context. Chapter 3 is intended to be published as astand-alone document and thus includes minor repetition of other chapters. Finally, Chapter 4 summarizesthe conclusions of the thesis with implications for exploration and suggestions for future research.8Chapter 2Regional and Local Geology2.1 Tectonic Evolution and Assembly of the Yukon-Tanana TerraneThe Klondike gold district is underlain by the northern portion of the Yukon-Tanana terrane: a meta-morphic block that was accreted onto the margin of North America during the Mesozoic assembly of theCanadian Cordillera (Figure 2.1; Nelson et al., 2013). The Yukon-Tanana terrane has a long-lasting tectonichistory with building events spanning from the Devonian to Paleogene times (Allan et al., 2013). The ter-rane is composed of the Snowcap, Finlayson, Klinkit, and Klondike assemblages (Table 2.1; Colpron et al.,2006b), along with several other post-tectonic magmatic suites and sedimentary formations. Based on pre-vious work (e.g., Colpron et al., 2006b; Nelson et al., 2013, and references therein), the tectonic evolutionand assembly of the Yukon-Tanana terrane can be divided into five basic stages (Figure 2.2):1) Passive margin sedimentation along the North American continent beginning as early as the LateNeoproterozoic and lasting into the Middle Devonian, with continental arc magmatism beginning inthe Middle Devonian, both forming the initial stages of the Yukon Tanana terrane arc;2) Rifting of the Yukon Tanana terrane arc from the North American continent, opening of the SlideMountain Ocean, and ongoing island arc magmatism in the Late Devonian to Early Permian;3) Closure of the Slide Mountain Ocean, continental arc magmatism, and collision of the Yukon-Tananaterrane arc with the North American continent in the Middle to Late Permian;4) Continental arc plutonism and compressional tectonism, possibly associated with orocline develop-ment in the Late Triassic to Middle Jurassic;5) Post-collisional magmatism, sedimentation, and faulting beginning in the mid-Cretaceous.9PacificOceanArcticOceaneasternlimit ofCordillerandeformation58°N50°N58°N144°W132°W160°WFaultFaultDenaliltlliVarious oceanic terranesArctic Northeast-Pacific RealmSlide Mountain / SeventymileYukon-TananaStikiniaQuesnelliaCache CreekTECTONIC REALMSPeri-Laurentian Realm(Intermontane terranes)Figure 2.4 Location0 200kmSelwynBasinlFinlaysondistrictTintinaUndividedCoastal RealmLaurentian Realm(Ancestral North America)Figure 2.1: Distribution of tectonic realms in the Canadian-Alaskan Cordillera, displaying main terranesubdivisions of the peri-Laurentian realm (after Colpron and Nelson, 2011). Heavy black linesshow major faults. Abbreviations: AK = Alaska; BC = British Columbia; NWT = NorthwestTerritories (modified from Allan et al., 2013).10Table 2.1: Tectonic assemblages of the Slide Mountain and Yukon-Tanana terranes (modified fromColpron et al., 2006b).TectonicAssemblageAge Characteristics Tectonic SettingKlondike Middle–LatePermianFelsic metavolcanic rocks,calc-alkaline; minor maficmetavolcanic rocks; coevalintrusions of the Sulphur CreeksuiteContinental arc;Yukon-Tanana terraneKlinkit MiddleMississippian–EarlyPermianMafic to intermediate calc-alkalinevolcaniclastic and volcanic rocks;minor alkali basalt;limestone/marble; basalconglomerateIsland arc;Yukon-Tanana terraneSlide Mountain EarlyMississippian–MiddlePermianBasalt (N-MORB; rare E-MORB toOIB); chert, argillite; gabbro;serpentiniteMarginal ocean basin(back-arc); SlideMountain terraneFinlayson Late Devonian–MiddleMississippianMafic to felsic metavolcanic rocksof arc and back-arc affinities;carbonaceous pelite, chert, minorquartzite; volcaniclastic rocks;marble; coeval with the Grass Lakesand Simpson Range plutonic suitesContinental arc system(including arc andback-arc);Yukon-Tanana terraneSnowcap Pre-LateDevonianPolydeformed and metamorphosedquartzite, psammite, pelite, andmarble; N-MORB to OIBamphibolite; intruded byDevonian–Mississippian plutons ofthe Grass Lakes and SimpsonRange suitesContinental margin;Basement toYukon-Tanana terraneInitial stages of Yukon-Tanana terrane formation began with rifting of the Rodinian supercontinent inthe Neoproterozoic to form the Laurentian craton (ancestral North America; Nelson et al., 2013). Passivemargin marine sedimentation along the northwestern margin of Laurentia lasted into the Middle Devonianand deposited the oldest unit of the Yukon-Tanana terrane, the Snowcap assemblage (Colpron et al., 2006b;Piercey and Colpron, 2009). In the Late Devonian, east-dipping subduction under the craton led to a sig-nificant magmatic arc system, resulting in both plutonic and volcanic rocks of continental arc affinity thatpersisted into the Early Mississippian (Mortensen, 1992; Colpron et al., 2006b; Piercey et al., 2006). Mostof these rocks were deposited in a submarine environment above the Snowcap assemblage (Colpron et al.,2006b; Piercey et al., 2006). Coeval basin sedimentation produced carbonaceous sedimentary layers (nowtermed Nasina assemblage; Mortensen, 1992; Colpron et al., 2006b) and together, these rocks make up the11Finlayson assemblage.Slab rollback caused extension of the overriding plate by about 360 Ma during ongoing depositionof Finlayson strata (Colpron et al., 2007; Allan et al., 2013). Extension led to a westward rifting of thedeveloping arc and opening of the Slide Mountain Ocean. Spreading of the basin persisted from EarlyMississippian to Middle Permian times (Colpron et al., 2007), in which the rocks comprising the SlideMountain terrane formed through MORB volcanism and sedimentation (Colpron et al., 2006b; Nelson et al.,2013). Roughly coeval with the formation of the Slide Mountain terrane, the Klinkit assemblage volcanicrocks erupted from the Middle Mississippian to the Early Permian in an island arc setting as a result of thecontinued east-dipping subduction (Colpron et al., 2006b). The Klinkit assemblage is volumetrically minorand occurs primarily in the southern portion of Yukon-Tanana terrane where it unconformably overliesFinlayson rocks (Colpron et al., 2006b).The Middle to Late Permian marked a distinct shift in the tectonic style in the Yukon-Tanana terrane.East-dipping subduction along the western margin of the terrane was replaced by west-dipping subduction ofthe Slide Mountain Ocean lithosphere under the eastern margin of the Yukon-Tanana terrane (Nelson et al.,2013). This reversal in arc polarity resulted in the closure of the Slide Mountain Ocean and coeval Klondikeassemblage plutonic and volcanic continental arc magmatism from 269 to 253 Ma (Colpron et al., 2006b;Nelson et al., 2006). Upon closure of the ocean, the Yukon-Tanana terrane collided with and overrode theLaurentian margin (260–252 Ma Klondike orogeny of Beranek and Mortensen, 2011). This major collisionproduced ductile shear fabrics and greenschist to lower amphibolite facies metamorphism throughout theterrane (Berman et al., 2007).Tectonism continued into the Mesozoic with renewed convergence along the Laurentian margin leadingto east-dipping subduction beneath the Yukon-Tanana terrane (Nelson et al., 2006). A new continentalmargin magmatic arc developed upon the terrane emplacing Late Triassic to Early Jurassic intrusive rocks(Nelson et al., 2006). These rocks are mainly found in the southern portion of the terrane and includethe Stikine, Minto, Long Lake, and Bennett/Bryde plutonic suites (Joyce et al., 2016). The 204–195 MaMinto suite and 190–180 Ma Long Lake suite were emplaced at respective depths of 23–26 km and 12–15km (Joyce et al., 2016; Topham et al., 2016). The upward-progressing emplacement depths of these suitesand Early Jurassic titanite, hornblende, and mica cooling ages (e.g., 197–189 Ma; Tafti, 2005; Joyce et al.,2015) suggest substantial exhumation of the Yukon-Tanana terrane began near the end of arc construction.Igneous activity subsided from the end of the Early Jurassic to mid-Cretaceous (179–115 Ma) and regionalmetamorphic cooling ages (185–140 Ma; Dusel-Bacon et al., 1993; Breitsprecher and Mortensen, 2004;Knight et al., 2013; Joyce et al., 2015) indicate ongoing regional uplift and exhumation during this magmaticgap (Allan et al., 2013). Contractional deformation continued through this time (Hansen and Dusel-Bacon,1998) and evidence of these structures have been recognized regionally throughout the Klondike, WhiteGold, and Sixtymile metallogenic belts (MacKenzie et al., 2008c; MacKenzie and Craw, 2010; Bailey,2013).The tectonic configuration of the Yukon-Tanana terrane, from the closure of the Slide Mountain Ocean12in the Late Permian Klondike orogeny, to the collisional deformation during the Middle Jurassic, remainscontroversial. Since the Paleozoic, the terrane underpinned the composite Stikine-Yukon-Tanana-Quesnelterranes (Nelson et al., 2006). To accommodate the current configuration of terranes, Mihalynuk et al.(1994) proposed an oroclinal model. In the oroclinal model, the Cache Creek ocean closed and the exoticCache Creek terrane was consumed between clockwise rotation of the Quesnellia arc and counter-clockwiserotation of the Stikinia arc (Mihalynuk et al., 1994). In this scenario, the Yukon-Tanana terrane comprisesthe hinge of the orocline, and the oroclinal closure is marked by decreased arc-style magmatism, thrustingthat accommodated regional shortening, and exhumation during the Middle Jurassic (Allan et al., 2013).Questions remain regarding the viability of the oroclinal model (e.g., Nelson et al., 2013). A possiblealternative to the model is that Stikinia was displaced southwards to its current position west of the CacheCreek terrane, which would imply the existence of a significant, pre-Middle Jurassic sinistral fault on itseastern margin (Nelson et al., 2013).Renewed northeast-dipping subduction of accreted terranes under the Yukon-Tanana terrane during themid- to Late Cretaceous resulted in a pulse of magmatism that includes the Whitehorse plutonic suite andother scattered felsic intrusions and volcanic rocks (Allan et al., 2013). Slab rollback led to extension andnormal faulting (Berman et al., 2007), which emplaced structurally lower sections of the terrane (lower plate)next to structurally higher ones (upper plate; Allan et al., 2013). Upper plate rocks were metamorphosedto the greenschist facies and exhumed during the Jurassic (Knight et al., 2013; Joyce et al., 2015), whereaslower plate rocks exhibit higher metamorphic grade (amphibolite) and were exhumed during Cretaceoustimes (e.g., Finlayson Lake District; Figure 2.1; Berman et al., 2007; Staples et al., 2013; Staples, 2014).Sedimentation from 112–70 Ma produced the Indian River Formation, which is directly overlain by Car-macks Group volcanic rocks at the end of the Cretaceous (72–67 Ma; Lowey and Hills, 1988; Allan et al.,2013; Mortensen and Dusel-Bacon, 2014). Related minor intrusions were emplaced throughout the LateCretaceous (e.g., Casino plutonic suite of Ryan et al., 2013a,b) along with broadly coeval sinistral obliqueextensional faults (Sanchez et al., 2013).The Cenozoic era marked a shift to a transpressional regime in which dextral strike-slip motion wasaccommodated along major faults such as the Tintina and Denali (Gabrielse et al., 2006; Nelson et al.,2013). A suite of dyke swarms, high-level plugs, and pyroclastic rocks affected a large portion of theYukon-Tanana terrane at this time (e.g., Rhyolite Creek complex and Ruby Range plutonic suite of Israelet al., 2011). These magmas were locally emplaced into zones of extension along the Tintina fault zone andoverlap in age with the onset of strike-slip motion; thus a tectonic link between this pulse of magmatismand early activity on the Tintina fault is formed (Gabrielse et al., 2006; Allan et al., 2013). After 37 Ma,the majority of dextral movement shifted from the Tintina to Denali fault (Nelson et al., 2013). Figure 2.2illustrates a summary of the tectonic evolution of the Yukon-Tanana terrane along with its deformation,metallogenic, and local formation events.13PerEra Epoch Age (Ma)Neo-proterozoicPaleozoicMesozoicCenozoicNeogenePaleogeneCretaceousJurassicTriassicPermianCarboniferousDevonianSilurianOrdovicianCambrianTonianCryogenianEdiacaranMississippianPennsylvanianTerreneuvianSeries 2MiaolingianFurongianLowerMiddleUpperLlandoveryWenlockLudlowPridoliLowerMiddleUpperLowerMiddleUpperLowerMiddleUpperLowerMiddleUpperLowerMiddleUpperLowerUpperPaleoceneEoceneOligoceneCisuralianGuadalupianLopingianMiocenePliocenePleistoceneHoloceneQuaternary1000~720~635541.0~509485.4±1.9470.0±1.4458.4±0.9443.8±1.5433.4±0.8427.4±0.5423.0±2.3419.2±3.2393.3±1.2382.7±1.6358.9±0.4346.7±0.4330.9±0.2323.2±0.4315.2±0.2307.0±0.1298.9±0.2273.0±0.1259.1±0.5251.9247.2~237201.3±0.2174.1±1.0163.5±1.0~145.0100.566.056.033.923.035.3332.580.0117PresentCordilleran Tectonic EventsRegional Deforma-tion EventsRegional TectonicAssemblagesLocal Formations of Western Yukon Mineralization EventsTectonic Cross SectionUpper Plate Lower Plate1 - Allan et al., 20132 - Beranek & Mortensen, 20113 - Berman et al., 20074 - Bond, 20165 - Cohen et al., 20186 - Colpron et al., 2006a7 - Colpron et al., 2006b8 - Dusel-Bacon et al., 20029 - Gabrielse et al., 200610 - Hunt and Roddick, 199211 - Israel et al., 201112 - Joyce et al., 201513 - Knight et al., 201314 - Kovacs, 201815 - Lowey  Hills, 198816 - MacKenzie et al., 20085 1TranspressionExtensionCompression PlacerEpithermalOrogenicCarbonate ReplacementPolymetallic VeinsSkarnPorphyryVMSSEDEXIntrusion BrecciaIntrusion-RelatedExhumationDeformationMetamorphismYukon-Tanana TerraneSlide Mountain TerraneSedimentaryVolcanicPlutonic17 - Mihalynuk et al., 199418 - Mortensen, 199019 - Mortensen, 199220 - Nelson et al., 200621 - Nelson et al., 201322 - Ruks et al., 200623 - Staples et al., 201324 - Staples et al., 20143Late DevonianCarboniferousLate PermianLate Triassic to Late JurassicMiddle to Late CretaceousYukon-Tanana TerraneSlide Mountain TerraneLaurentian Realm Oceanic CrustWaterW Eabcdefghijacdeghijklm nob fabcdeabcdefghijklmnoa bcdef ghijklmn opqrst uv14Figure 2.2 (preceding page): Diagram representing the tectonic formation of the Yukon-Tanana terrane and chronostratigraphy of its regionalassemblages, deformation, and mineralization events. Emphasis is on western Yukon rocks and upper plate deformation. Numberedreferences are shown in the diagram. Event descriptions are as follows:Cordilleran Tectonic Events: a) East-dipping subduction under Yukon-Tanana [21]; b) West-dipping subduction leads to closure of SlideMountain Ocean [21]; c) Collision and overriding of North America by Yukon-Tanana leads to the Klondike orogeny [2, 21]; d) East-dippingsubduction under Yukon-Tanana [1, 21]; e) Orocline leading to the closure of the Cache Creek Ocean and collision of Stikinia and Quesnellia[1, 17, 21]; f) Amalgamation between Arctic and peri-Laurentian terranes, final accretion of peri-Laurentian with North America [21]; g)Rifting of Rodinia to form Laurentia/Ancestral North America [21]; h) East-dipping subduction hinge rollback leads to opening of the SlideMountain Ocean [1, 21]; i) East-dipping subduction with slab rollback under Yukon-Tanana [1, 21]; j) Tintina and Denali faults active [9, 21].Regional Deformation Events: a) low P metamorphism [3]; b) M1 at lower greenschist facies [16]; c) M2 at greenschist to lower amphibolitefacies [1, 3]; d) M3 at greenschist to lower amphibolite facies [1, 3]; e) discrete deformation [3]; f) D1 isoclinal folds [16]; g) D2 ductile foliation[2, 3, 16]; h) D3 ductile cleavage, recumbent folds, and regional thrusts [1, 3, 16]; i) D4 brittle-ductile angular kink folds and fractures [1, 16]; j)D5 brittle normal faults and gouge zones [1, 16]; k) Upper plate exhumation [12, 13]; l) Finlayson Lake district metamorphism [24]; m) AustraliaMountain domain metamorphism [1, 3, 23]; n) Finlayson Lake district exhumation [10, 21, 24]; o) Australia Mountain domain exhumation [3, 23].Regional Tectonic Assemblages: a) Snowcap assemblage [1, 7]; b) Finlayson assemblage [6]; c) Klinkit assemblage [6]; d) Klondikeassemblage [6, 19]; e) Slide Mountain Terrane [1, 6, 20].Local Formations of Western Yukon: a) Mount Burnham orthogneiss [18, 22]; b) Simpson Range plutonic suite [1, 6]; c) Sulphur Creekorthogneiss [2, 6, 20]; d) Klondike metaporphyry [18]; e) Jim Creek pluton and Teacher intrusion [1, 2]; f) Whitehorse plutonic suite includingthe Dawson Range batholith and Coffee Creek granite [1]; g) Ruby Range plutonic suite [1, 11]; h) Reid Lakes plutonic and volcanic complex[13]; i) Klondike assemblage [7]; j) Mount Nansen group [1]; k) Carmacks group [1]; l) Rhyolite Creek complex [1, 11]; m) Nasina assemblage[1, 19]; n) Indian River formation [1, 15]; o) Paradise, White Channel, and modern gravels [4].Mineralization Events: a) Pb-Zn Mickey, Clip, Mort, Holly, Columbia Creek; b) Cu-Au Lucky Joe; c) Cu-Au Pattison; d) Cu-Au-Ag-MoCasino, Nucleus-Revenue, Sonora Gulch, Cash, Tad; e) Cu-Mo-Au Mount Cockfield, Mo-Cu-W Swede, Pluto; f) Pb-Zn-Cu-Ag-Au Baldy,Boundary, Bor; g) Au IND; h) Cu-Au Minto, Carmacks Copper [14]; i) Au Pogo; j) U Patt; k) Mo Rebecca, Lucky Joe; l) Au Klondike,Sixtymile, White Gold; m) Au Moosehorn, Boulevard, Coffee; n) Au Antoniuk; o) Au-Ag Augusta; p) Cu-Pb-Ag West Dawson; q) Au-AgMount Nansen; r) Au Grew Creek; s) Ag-Pb-Zn-Cu-Au Tinta Hill, Ag-Pb-Zn Frog; t) Ag Connaught, Pika, Prospector Mountain; u) AgLittle Whiteman; v) Au Dawson Range, Klondike, Sixtymile, White Gold [4].152.2 Metallogenic Evolution of the Yukon-Tanana TerraneThe metallogenic history of the Yukon-Tanana terrane shows diverse mineralization styles characteristicof the regional tectonic and magmatic regimes through time. Although some may be volumetrically minor,mineralization styles found in the Yukon-Tanana terrane include sedimentary exhalative (SEDEX; Pb-Zn),volcanogenic massive sulphide (VMS; Pb-Zn-Cu±Ag±Au), intrusion-related (Au, U), intrusion breccia(Au), porphyry (Cu-Au±Mo±Ag, Mo-Cu-W), skarn (Au-Ag, Cu-Pb-Ag), orogenic (Au, Mo), polymetallicvein (Ag-Pb-Zn±Cu±Au), epithermal (Au±Ag), carbonate replacement (Ag), and placer (Au; Figure 2.2;Allan et al., 2013). Allan et al. (2013) simplified the complex spatial and temporal relationships of thesemineral occurrences into metallogenic belts (Figure 2.3) and eleven main metallogenic events:1) Late Devonian to Early Mississippian (365–342 Ma) SEDEX (e.g., Mickey, Clip, Mort, Holly,Columbia Creek) and porphyry (e.g., Lucky Joe) mineralization associated with arc magmatism.2) Latest Permian (269–253 Ma) VMS mineralization associated with submarine arc magmatism (e.g.,Baldy, Boundary, Bor, and prospects in the Klondike).3) Late Permian (253–250 Ma) post-collisional intrusion-related gold mineralization related to theKlondike orogeny (e.g., IND).4) Late Triassic (212 Ma; Kovacs, 2018) porphyry copper-gold mineralization produced by arc magma-tism (e.g., Minto and Carmacks Copper).5) Early Jurassic (187 Ma) molybdenum mineralization likely associated with orogeny (e.g., Rebecca,Lucky Joe).6) Middle to Late Jurassic (163–155 Ma) regionally widespread orogenic gold mineralization coevalwith regional exhumation and cooling (e.g., occurrences in the Klondike, White Gold, and Sixtymiledistricts).7) Mid-Cretaceous (115–98 Ma) porphyry (e.g., Pattison), intrusion breccia (e.g., Antonuik), skarn (e.g.,Augusta), epithermal (e.g., Mount Nansen), polymetallic vein (e.g., Tinta Hill, Frog), and intrusion-related (e.g., Pogo) mineralization during arc magmatism.8) Mid-Cretaceous (96–92 Ma) orogenic gold mineralization (e.g., Moosehorn, Boulevard, Coffee).9) Early Late Cretaceous (79–72 Ma) porphyry mineralization related to magmatism and faulting (e.g.,Casino, Nucleus-Revenue, Sonora Gulch, Cash, Tad).10) Late Late Cretaceous (72 -67) porphyry (e.g., Mt. Cockfield, Swede, Pluto), polymetallic vein (e.g.,Connaught, Pika, Prospector Mountain), and carbonate-replacement (e.g., Little Whiteman) mineral-ization related to extensional fault systems.11) Paleocene to Eocene (60–55 Ma) intrusion-related (e.g., Patt), skarn (e.g., West Dawson), and epither-mal (e.g., Grew Creek) mineralization coincident with movement on the Tintina fault.160 50kmYUKONALASKADawson138°W142°W64°N63°NChicken163-155 Ma96-92 MaFORT Y-M I L ESI X T Y-M I L EK L O N D I K EW H I T EG O L DD A W S O N  R A N G Eorogenic gold beltintrusion-related mineral beltTintina faultDenali fault200-179 Ma72-67 Ma115-98 MaWhite GoldCoffeeCasinoMinto79-72 MaNucleus-RevenueLittle WhitemanI Yukon RiverWhite River72-67 MaFigure 2.3: Metallogenic belts of the northern Yukon-Tanana terrane (from Allan et al., 2013).2.3 Klondike Gold District2.3.1 Bedrock AssemblagesThe basement of the Klondike gold district is made up of various rock assemblages that formed be-fore and after mineralization (Figure 2.4). Several pre-mineralization assemblages, including Scottie Creek,Mount Burnham, Simpson Range, and Slide Mountain are beyond the center of the district and do not hostgold mineralization. The Scottie Creek formation occurs at the eastern limit of the Klondike, is North Amer-ican rather than part of the Yukon-Tanana terrane, and consists of quartzose psammite, pelitic schist, andminor marble. The Mount Burnham orthogneiss is granitic and typically contains coarse K-feldspar augen(Mortensen, 1990, 1996; Gordey and Ryan, 2005). Together, the Scottie Creek formation and Mount Burn-ham orthogneiss make up the Australia Mountain domain, which is part of the lower plate and is separatedfrom the rest of the Klondike by an inferred normal fault (Australia Creek fault; Gordey and Ryan, 2005;Staples et al., 2013). The Simpson Range plutonic suite occurs beyond the southwestern part of the districtand consists of foliated to gneissic granitoid rocks, mainly of granodiorite to tonalite composition. AlthoughSimpson Range rocks do not host mineralization in the Klondike, they do host several gold occurrences fur-ther south in the White Gold district. Slide Mountain terrane greenstone and ultramafic rocks may representophiolites that were structurally emplaced during thrust faulting (Mortensen, 1990).17Yukon RiverIndian River138°30'0"W138°30'0"W139°0'0"W139°0'0"W139°30'0"W139°30'0"W64°0'0"N64°0'0"N63°45'0"N63°45'0"NTintina Fault1234 71285910 1113 1415 166Dawson CityVirginLone StarMitchell-Sheba0 5 102.5KilometersBedrock Gold OccurrencePost-MineralizationPre-MineralizationSampled OutcropStudied Bedrock Gold OccurrencePlacer Gold Producing Creek (Quaternary)Normal FaultStrike-Slip FaultThrust FaultCarmacks Group Volcanics (Cretaceous)Finlayson Assemblage Schists (Devonian - Carboniferous)Indian River Clastics (Cretaceous)Jim Creek Pluton (Permian)Klondike AssemblageSchists (Permian)Mount Burnham Orthogneiss (Devonian)Ross Plutonic Suite (Paleogene)Scottie Creek Schists (Devonian)Simpson Range Plutonic Suite (Devonian - Carboniferous)Slide Mountain Ultramafics (Carboniferous - Permian)Snowcap Assemblage Schists (Devonian)Sulphur Creek Orthogneiss (Permian)Whitehorse Plutonic Suite (Cretaceous)Quartz-Augen SchistMafic SchistMassive Greenstone (Devonian - Permian)Metaclastic UnitKlondike Schist UndifferentiatedGranodiorite and Porphyry Intrusions (Cretaceous)Felsic SchistYukon-Tannana TerraneLocalitiesFaultsFigure 2.4: Simplified geological map of the Klondike gold district (modified from Mortensen, 1996;Gordey and Ryan, 2005; Mortensen et al., 2019). Studied localities are stared and labelled onthe map. Other localities are labelled as: 1) Ben Levy; 2) Plinc; 3) Orofino; 4) IND; 5) Dysle;6) Nugget; 7) Violet; 8) Boxcar; 9) MacKay; 10) King Soloman Dome; 11) Dome Lode/HunkerDome; 12) Lloyd; 13) Gold Run; 14) Dominion Creek; 15) Aime/Payne; 16) Lower Dominion.18The Snowcap and Finlayson assemblages do not host gold mineralization in the Klondike but they di-rectly underlie the Klondike assemblage, occur along the outskirts of the district, and host significant goldmineralization in the White Gold district (e.g., several prospects in the Golden Saddle camp; Bailey, 2013).The Snowcap assemblage represents the basement of the Yukon-Tanana terrane and dominantly consists ofpsammitic schist, quartzite, carbonaceous schist, and calc-silicate rocks that were metamorphosed to amphi-bolite facies (Colpron et al., 2006a). Unlike the overlying assemblages, Snowcap rocks display garnet-grademetamorphism, have a more complex deformational history, and have been intruded by numerous plutonicsuites (Colpron et al., 2006a). The age of the Snowcap assemblage is constrained to pre-Late Devonianbecause it is unconformably overlain by Late Devonian rock units (Colpron et al., 2006a). The Finlaysonassemblage structurally overlies the Snowcap and is constrained to the Late Devonian to Middle Missis-sippian (365–342 Ma), which is roughly coeval with the Simpson Range and Mount Burnham intrusionsuites (Mortensen, 1990; Colpron et al., 2006b; Ruks et al., 2006). The Finlayson assemblage can broadlybe subdivided into two main categories. The first is represented by metamorphosed felsic to mafic arc andback-arc material that records the terrane’s transition from a continental arc to a back-arc system (Colpronet al., 2006b). The second category is dominantly sedimentary sequences of the Nasina assemblage, whichwere deposited in a back-arc basin environment and now exist dominantly as quartzite and carbonaceousschists (Mortensen, 1992; Colpron et al., 2006b).Permian rocks, including the Klondike assemblage, Sulphur Creek orthogneiss, and Jim Creek pluton,underlie the majority of the Klondike. The Klondike assemblage (commonly known as the Klondikeschist; 269–253 Ma; Colpron et al., 2006b) occurs throughout much of the district and hosts the majorityof gold occurrences. Mortensen et al. (2019) divide the Klondike schist into five categories: 1) a felsicpackage, 2) a quartz-augen schist, 3) a mafic package, 4) a metaclastic unit, and 5) an undifferentiatedpackage. The felsic package comprises quartz-muscovite-feldspar schist, which is interpreted to bederived from a felsic volcanic or volcaniclastic rock, most likely a submarine tuff (Mortensen, 1990).The quartz-augen schist contains quartz±feldspar augen, indicating it is likely derived from a felsicquartz±feldspar phyric porphyry (Mortensen et al., 2019). The mafic package comprised of feldspar-chlorite-quartz±epidote±amphibole±carbonate schist is likely derived from intermediate or mafic volcanicor volcaniclastic rocks (Mortensen, 1990). The metaclastic unit occurs mainly on the east side of theKlondike and is derived from quartz-rich clastic sedimentary rocks. Lastly, the undifferentiated packageincludes intimately inter-layered components of the felsic, mafic, and metaclastic units. Although mostrocks in the Klondike assemblage are metavolcanic, both the felsic and mafic packages have metaplutonicequivalents that occur throughout the district (e.g., felsic metaporphyry and metagabbro of Mortensen,1990). The Sulphur Creek orthogneiss overlaps in time with the Klondike assemblage and may represent,along with the quartz-augen schist, the intrusive equivalent of the felsic package (Mortensen, 1990; Colpronet al., 2006b; Nelson et al., 2006). The Jim Creek pluton (253–250 Ma) is the youngest Permian rock unitin the Klondike and is an undeformed biotite±garnet quartz monzonite that intruded Nasina metaclasticrocks (Beranek and Mortensen, 2011).Several post-mineralization units occur in the vicinity of the Klondike, including the Whitehorse plu-19tonic suite, Indian River Formation, Carmacks Group, and Ross plutonic suite. The Whitehorse plutonicsuite is volumetrically minor to the area and is locally exposed off the southwest corner of the Klondike,away from significant bedrock and placer gold mineralization. However, elsewhere in western Yukon, thissuite includes the Dawson Range batholith, Coffee Creek granite, and Moosehorn Range granitoids, whichare economically relevant (e.g., various intrusion-related mineralization styles in the Cretaceous; Allan et al.,2013). The Indian River Formation is mainly exposed in, and south of the Indian River valley and is com-posed of interbedded sandstone, shale, conglomerate, and minor coal that formed in an alluvial to shallowmarine setting (Lowey and Hills, 1988; Bond and Chapman, 2007). Fossils, U-Pb zircon ages for inter-layered felsic tuff units, and the overlying Carmacks Group constrain the upper and lower age limits of theIndian River Formation, placing its deposition between 112 and 70 Ma (Lowey, 1984; Mortensen and Dusel-Bacon, 2014). The Carmacks Group, which is mainly south of the Indian River, includes calc-alkaline felsicand intermediate volcanic rocks that were deposited from 70 to 67 Ma (Allan et al., 2013). Lastly, the Rossplutonic suite represents a minor group of dykes and small porphyritic intrusions that were emplaced duringthe Eocene throughout the Klondike and adjoining areas (Mortensen, 1996; Gordey and Ryan, 2005).2.3.2 Metamorphism and DeformationThe Paleozoic stratigraphy of the Klondike was modified by several episodes of syn- to post-accretionary deformation. The resulting structures occur throughout much of the Yukon-Tanana terrane butare best described in the Klondike (MacKenzie et al., 2008c; MacKenzie and Craw, 2010). At least fivediscrete deformation events, referred to D1–D5, affected the Permian Klondike assemblage and produceddistinctive structures and metamorphic mineral assemblages (Figure 2.5a; Table 2.2). An early metamorphicevent occurred that pre-dates the deposition of the Permian Klondike assemblage and is therefore excludedfrom the D1–D5 naming convention. This early metamorphic event (M1 of Berman et al., 2007) occurredfrom 365–350 Ma and records relatively low pressure metamorphism during east-dipping subduction andactive arc magmatism. Deformation coeval with this early metamorphism is recorded in texturally distinctgarnet cores but is not preserved at the outcrop scale.The first deformation/metamorphic event (D1/M1) recorded in the Klondike has been overprinted sev-eral times and is poorly preserved and understood. No direct ages for this event exist, but its presence inthe Klondike assemblage and overprinting by D2 suggest it is broadly constrained to 269 to 260 Ma. D1is rarely preserved in the form of rootless isoclinal folds (F1) oriented subparallel to the later S2 foliation.Any pre-existing S1 foliation has been completely transposed by the penetrative S2 fabric (MacKenzie et al.,2008c; MacKenzie and Craw, 2010). Thus, D1 is commonly grouped with D2 as a composite D1-2 event.The second deformation/metamorphic event (D2/M2) was temporally coincident with the initial ac-cretion of the Yukon-Tanana terrane onto the margin of ancestral North America at 260 to 252.5 Ma(Klondike orogeny of Beranek and Mortensen, 2011). Upper greenschist facies metamorphism occurredin the Klondike assemblage and greenschist to locally garnet grade amphibolite facies metamorphism (at 9kbar and 600◦C; M2 of Berman et al., 2007) occurred in the structurally underlying assemblages at this time.S2 is a sub-horizontal to shallowly dipping, penetrative schistosity defined by micas and is the dominant de-20formation fabric throughout the Klondike and much of the Yukon-Tanana terrane. Metamorphic segregationand barren quartz veins on the mm to cm scale developed as part of the event are now parallel to the S2foliation (MacKenzie et al., 2008c).The penetrative S2 foliation was locally overprinted by D3 deformation structures created during theprogressive collision of the Yukon-Tanana terrane with the North American margin in the Early Jurassic(Figure 2.5b; MacKenzie et al., 2008c). These structures include shallow to moderately southwest dippingductile crenulation cleavage (S3), open to tight, northeast verging recumbent folds (F3), and regional-scalethrust faults. Deformation was heterogeneously distributed, with crenulation and folding more commonlydeveloped in micaceous rocks and in proximity to thrust faults. The thrust faults imbricated the Paleozoicstratigraphy and emplaced tectonic slices of Slide Mountain assemblage rocks along thrust surfaces withinthe Klondike (Mortensen, 1990, 1996). D3 occurred with a third regional metamorphic event (M3), whichis dated by U-Pb analysis of monazite at 195-187 Ma (Berman et al., 2007) in Finlayson assemblage rocks.This metamorphism was mainly greenschist, locally garnet grade amphibolite facies (up to 7.8 kbar and595◦C), but in the overlying Klondike assemblage, it was restricted to lower greenschist facies metamor-phism (Berman et al., 2007). Although metamorphic events continued to occur at deeper structural levels(e.g., Staples et al., 2013; Staples, 2014), this was the last recorded significant metamorphic event that af-fected the upper plate rocks of the Klondike.Exhumation and minor brittle-ductile deformation (D4) took place during a gap in magmatic activitybetween 179 and 115 Ma (MacKenzie et al., 2008c). D4 deformation manifests as angular kink folds (F4),fold axial surface fractures (S4), high angle reverse faults, shears, gouge zones, and commonly mineral-ized quartz veins which occur in steeply dipping orthogonal sets trending west to northeast (Figure 2.5c;MacKenzie et al., 2008c). F4 kink folds contain early angular folding and commonly a cross-cutting brit-tle fracture along the axial plane (S4). This close spatial association between angular folding and brittlefracturing is indicative of a progressive transition from the brittle-ductile transition zone to the brittle defor-mation regime that occurred during fold development (Bailey, 2013). D3 and D4 structures both representrelatively shallow depth compressional deformation and may reflect a continuum of deformation which pro-gresses from ductile to brittle deformation that may be coincident with regional uplift and exhumation ofthe Klondike (MacKenzie et al., 2008c; Upton and Craw, 2014; Mortensen, in review).The final deformation event observed in the Klondike (D5) is characterized by steeply dipping, west tonortheast-trending normal faults that are interpreted to be related to mid- to Late Cretaceous regional exten-sion (MacKenzie et al., 2008c; Sanchez et al., 2013). The exact timing of this event is poorly understood.However, the structures are broadly coeval with mid- to Late Cretaceous magmatism, suggesting that theywere active during or after this time (Allan et al., 2013; Sanchez et al., 2013). The normal faults are typi-cally marked by gouge zones which are hydrothermally altered, pyritized, and silicified. They may also haveoccurred along reactivated D4 structures, leading to their similar orientations (MacKenzie et al., 2008c).21Table 2.2: Summary of deformation and metamorphic events in the Klondike gold district (modified from MacKenzie et al., 2008c).Event DeformationStageDeformationFeatureDeformationOrientationMetamorphic Facies AssociatedMetalsAge (Ma)D5 Normal faulting Normal faults,gouge zonesSteeply dipping,west to northeasttrending- Pyrite 100–66D4 Fault-fold Kink folds (F4), foldaxial planarfractures (S4),high-angle reversefaults, shears, gougezonesSteeply dipping twoorthogonal setstrending west tonortheast- Au, pyrite 179–155D3/M3 Thrust stacking Crenulationcleavage (S3) andrecumbent folds(F3)Shallowlysouthwest dippingLower greenschist - 195–187D2/M2 PenetrativefoliationFoliation (S2),isoclinal folds (F2)Variable Upper greenschist - 260–252.5D1/M1 First foliation Foliation (S1),segregationsVariable Unconstrained - 269–260EarlyMetamorphismDiscretedeformation- - Variable, low P - 365–350222.3.3 Orogenic Gold MineralizationA regionally widespread, orogenic gold mineralization event occurred in the Late Jurassic (163–155Ma) in the Klondike, Sixtymile, and White Gold districts (Allan et al., 2013; Bailey, 2013; Mortensen, inreview). This event overlaps in time with regional exhumation (Knight et al., 2013; Joyce et al., 2015)during the final amalgamation of the peri-Laurentian terranes with ancestral North America (Figure 2.2;Nelson et al., 2013). Mineralization occurred during a northern Cordillera-wide magmatic gap in the area(no magmatism from 179 to 115 Ma; Allan et al., 2013), and thus is unrelated to regional magmatic activity.Mineralized features occur as simple extensional veins, veinlet arrays, brittle-ductile shears, stylolites, axialplanes of kink folds (F4), and breccias (Allan et al., 2013). These mineralized features are typically a func-tion of the host lithology. For example, breccias are common in highly competent rocks, such as quartziteand felsic metaplutonic rocks (e.g., Golden Saddle in the White Gold district; Bailey, 2013), whereas mica-ceous rocks typically exhibit simple vein arrays (e.g., Klondike occurrences; Allan et al., 2013). Regardlessof the style of mineralization, gold concentration always occurs in discordant D4 structures (Figure 2.5c)that cut pre-existing D1-3 fabrics at a high angle and are later overprinted by D5 normal faulting (MacKenzieet al., 2008c).In the Klondike, mineralization mostly forms steeply to moderately dipping quartz-sulphide±carbonateveins that cut the penetrative S2 foliation (Mortensen et al., 1992; Rushton et al., 1993). The veins are tabularto irregular, typically occur in anastomosing swarms with northeast or northwest trends, and can locally beup to three meters wide (MacKenzie et al., 2008c). Texturally, they appear massive and only rarely displaysyntaxial growth textures typical of multiple crack-seal events (Allan et al., 2013; Mortensen, in review, seeAppendix A for more detailed descriptions of vein growth textures). Angular wall-rock fragments are alsolocally present within vein material. The veins consist almost entirely of milky white quartz (∼99%) withminor pyrite, and lesser carbonate, feldspar, muscovite, baryte, scheelite, chalcopyrite, galena, sphalerite,rutile, and sulfosalts (Hoymann and Friedrich, 1990). Gold in veins is typically within pyrite along thevein margins and locally occurs as free grains (Figure 2.5d; Rushton et al., 1993; Mortensen, in review).Fluid inclusions in vein quartz have low salinity (<6 wt. % NaCl equivalent), variable CO2 content, andwere trapped at temperatures estimated from 200 to 350 ◦C and pressures of 300 to 2300 kbar (Rushtonet al., 1993). This considerable variation in trapping conditions is interpreted by Rushton et al. (1993) toreflect rapid exhumation during vein formation. All veins in the Klondike are broadly similar in terms ofmineralogy, texture, and fluid chemistry. It is therefore assumed that they represent a single, albeit possiblyprotracted syn to post D4 orogenic gold event (Rushton et al., 1993; Allan et al., 2013).A variety of alteration styles are typical for the orogenic gold mineralizing event, including K-feldspar,sericite-illite, silica, kaolinite ± dickite, pyrite, carbonate, graphite, and no visible alteration (Allan et al.,2013). Multiple types of alteration may even be superimposed within a single structure. Regardless of miner-alization or alteration style, pyrite is the most common sulphide and typically occurs along vein margins andas a disseminated alteration in the wall-rock (Allan et al., 2013). Hydrothermal alteration in the Klondikeis relatively minor and is rarely greater than a few meters beyond veins. It exists as pyrite-sericite±Fe-carbonate (e.g., mafic-hosted Mitchell-Sheba occurrences), kaolinite-silica (quartz-K-feldspar; e.g., quartz-23augen schist-hosted Virgin occurrence), and little to no silica (quartz)-pyrite (e.g., felsic-hosted Lone Staroccurrence). In some instances, gold may accompany hydrothermal pyrite and be hosted within the alteredwall-rock (e.g., Liverton and Mann, 2011). Subtle differences in alteration styles likely reflect host litholo-gies as the mineralizing fluids react differently based on the host mineral assemblage. Fluid desulphidationleading to gold deposition was mainly driven by fluid/rock reaction in the mafic rocks and by decompressionand cooling in the felsic rocks where alteration is less prominent and there is little evidence for fluid/rockinteraction (Mortensen, in review).24S3S4D5S2 / S1S1-2S1-2S3S3 S1-2S1-2S4S4S4VeinA)B) C)D)Figure 2.5: Deformation and mineralization structures in the Klondike. A) Schematic illustration ofthe general relationships between different deformational structures in the Klondike and elsewherein the Yukon-Tanana terrane (from Bailey, 2013). B) S1-2 transposition fabric overprinted by S3spaced crenulation. C) S1-2 transposition fabric overprinted by S4 kink fold and mineralized vein.D) Gold along a discordant vein margin adjacent to pyritic wall-rock.25Chapter 3Resolving Monazite Growth Mechanisms inOrogenic Gold Settings: A Study from theKlondike Gold District, Western Yukon3.1 IntroductionOrogenic gold deposits form along convergent plate margins along structures that act as conduits forhydrothermal fluid generated during metamorphism (e.g., Groves et al., 1998; Bierlein and Crowe, 2000;Goldfarb et al., 2001; Groves et al., 2003; Goldfarb et al., 2005; Goldfarb and Groves, 2015). The termorogenic gold deposit was introduced by Groves et al. (1998) to consolidate and replace previously commonterms including Archean, mesothermal, lode, greenstone, shear zone, turbidite-hosted, motherlode, and low-sulphide gold-quartz, which all referred to deposits formed through the same fundamental process. Althoughorogenic gold deposits form some of the world’s largest gold sources (e.g., Ashanti, Ghana, 3169 t at 2–7 g/t;Golden Mile, Australia, 2079 t at 2 g/t; Frimmel, 2008), several components of their generation are highlydebated (e.g., Goldfarb and Groves, 2015). Key questions remain regarding the fluid and gold sources aswell as the controls on gold deposition.Constraining the timing of orogenic gold formation is challenging because datable minerals in theore system may have poorly established paragenetic relationships with gold, may be too small or scarceto analyze, or may be reset by later thermal events or hydrothermal fluids. The timing of orogenic goldformation can be constrained by 40Ar/39Ar dating of muscovite, sericite, and biotite (e.g., Bierlein et al.,2001; Mortensen et al., 2010), 187Re/187Os dating of arsenopyrite and molybdenite (e.g., Arne et al., 2001;Selby et al., 2002), and U-Pb dating of zircon, titanite, and rutile (e.g., Kerrich, 1993; Lin and Corfu, 2002).Estimating gold formation has traditionally relied on 40Ar/39Ar dating of white micas because they are atypical hydrothermal phase within veins, whereas other datable minerals are less consistently available.However, these micas have a low closure temperature (e.g., ∼380◦C for muscovite; Reiners and Brandon,262006) and are susceptible to resetting by later thermal events and fluids. Moreover, these dates can overlapwith and be difficult to resolve from regional metamorphic cooling dates.U-Th-Pb dating of monazite or xenotime offers an alternative to determine the formation age of orogenicgold deposits (e.g., Brown et al., 2002; Vielreicher et al., 2003; Salier et al., 2005; Sarma et al., 2008;Taylor et al., 2015). Monazite is less susceptible than mica to resetting by a thermal event due to its highclosure temperature of >800◦C (Cherniak et al., 2004; Gardes et al., 2006). Interactions with hydrothermalfluids cooler than 800◦C can modify pre-existing monazite grains and lead to reset ages, which can beutilized to date hydrothermal activity (e.g., Williams et al., 2011). Monazite petrochronology, which isthe interpretation of isotope dates in light of complementary element information (Kylander-Clark et al.,2013), provides the opportunity to understand relationships between monazite growth and well-constrainedgeochronological data. Although monazite petrochronology is most common in metamorphic studies (e.g.,Pyle and Spear, 1999), it has been used to make inferences about monazite formation in ore deposits as well.For example, Schandl and Gorton (2004) used ThO2 concentrations to distinguish between hydrothermaland igneous monazite, and Taylor et al. (2015) used Eu anomalies to identify hydrothermal monazite in anorogenic gold deposit.Linking monazite growth or modification to orogenic gold formation requires establishing a parageneticrelationship with gold, particularly because monazite may grow by multiple processes. For example, in oro-genic gold settings monazite can grow by 1) metamorphic processes, during both prograde and retrogrademetamorphism, and 2) hydrothermal processes, precipitating from fluids in and adjacent to veins. Distin-guishing between these growth processes is vital to determine the geological significance of geochronolog-ical data.The Klondike gold district (also known as the Klondike; Section 1.2) in the northwestern CanadianCordillera hosts both orogenic gold occurrences and placer gold deposits. An estimated 20 million ouncesof placer gold were recovered from gold-bearing creeks in the Klondike from an area of∼1,200 km2 (Burkeet al., 2005). Coarse, visible gold is a typical feature of several orogenic quartz veins in the bedrock sur-rounding the gold-bearing creeks, which suggests the veins are the source of placer gold throughout thedistrict. This link between placer and orogenic gold is confirmed by microchemical and grain shape analysisstudies of gold grains (e.g., Knight et al., 1999a,b; Mortensen et al., 2004, 2006a; Crawford, 2007; Crawfordet al., 2007; Chapman et al., 2010a,b, 2011).A regionally widespread, orogenic gold mineralization event in the Late Jurassic is recognized acrossthe Klondike, Sixtymile and White Gold metallogenic belts of western Yukon (Allan et al., 2013; Bailey,2013). The genesis and detailed timing of emplacement and mineralization of the orogenic, gold-bearingquartz veins in the Klondike are poorly constrained. These veins also host micas, but they are difficult to linkto mineralization unambiguously. The veins have been previously dated by 40K/40Ar and 40Ar/39Ar dating ofhydrothermal micas at 183 to 134 Ma (Hunt and Roddick, 1992; Breitsprecher and Mortensen, 2004; Joyceet al., 2015), although it is unclear if these dates were thermally reset during regional cooling. Mortensen(in review) also reported a U-Pb age of 155.6 ± 7.6 Ma from hydrothermal rutile from a single vein. Key27questions remain and include the geological significance of these dates as representing vein formation orgold mineralization and their relationships to metamorphism and regional exhumation. Thus, the Klondikeis suitable for a monazite petrochronological study to provide geological significance to currently ambiguousdates.This study provides constraints on the timing of quartz vein emplacement and gold mineralization in theKlondike and offers insight into monazite growth in orogenic gold settings through petrographic analysis andmonazite petrochronology. Controls on vein and alteration paragenesis in the Klondike are first establishedwith petrographic analysis, quartz vein CL, and whole rock lithogeochemistry. By combining petrographicanalysis, geochronology, and ThO2 concentration data, the mechanism of monazite growth is interpreted inthe context of metamorphic and hydrothermal processes. Once monazite growth mechanisms are resolved,the geochronological data become geologically meaningful, and ages of metamorphic and hydrothermalactivity in the Klondike are constrained. The methods established here resolve monazite growth mechanismsand can be applied not only to monazite growth in orogenic gold settings but also to growth in metamorphicterranes globally.3.2 Monazite Growth MechanismsMonazite (LREE(PO4)) is a common accessory mineral in a wide variety of igneous, sedimentary, meta-morphic, and hydrothermal rocks (see Appendix H for a literature review summary of monazite). Becauseof its widespread occurrence, resistance to weathering, and high closure temperature of >800◦C (Cherniaket al., 2004; Gardes et al., 2006), monazite is commonly present as detrital or inherited grains from formerrocks and protoliths, which may lead to inherited dates. New monazite growth in orogenic gold deposit ge-ological settings and in metamorphic terranes in general can form by two main mechanisms: metamorphicprocesses and hydrothermal precipitation. Both mechanisms require a fluid phase to form monazite and mayoverlap in time and space, sometimes occurring in the same sample. Defining metamorphic, hydrothermal,and mixed growth processes becomes essential to distinguish geological environments and provide meaningto dates obtained from monazite. Figure 3.1 illustrates the expected textures of metamorphic and hydrother-mal monazite, whereas Table 3.1 highlights the common petrographic and geochemical indicators that havebeen used to distinguish monazite from different geological environments.Metamorphic monazite grows by the breakdown of other adjacent and nearby minerals within the samerock. It commonly forms in metamorphic rocks at amphibolite facies conditions, but may also grow undergreenschist facies pressures and temperatures (Rasmussen et al., 2007; Williams et al., 2007; Allaz et al.,2013; Grand’Homme, 2016). Though heat and pressure are the major driving factors in metamorphism,metamorphic fluids are present as an intergranular fluid phase during metamorphism (Feenstra and Franz,2015). The intergranular fluid helps facilitate new mineral growth by the breakdown of adjacent mineralsthrough a local dissolution-precipitation process (Etheridge, 1983). In the case of monazite, it typicallyforms by the breakdown of light rare earth element (LREE)- or P-bearing minerals including allanite, apatite,and pre-existing monazite (e.g., Overstreet, 1967; Bollinger and Janots, 2006; Finger and Krenn, 2007;Janots et al., 2008; Johnson et al., 2015). Because fluids play a role in metamorphic reactions, completely28dry metamorphism is rare, and the reactions typically involve some level of metasomatism (Putnis andAustrheim, 2010). Fluids involved in metamorphic monazite growth are interpreted to be locally generated,have not travelled a significant distance (i.e., have not left the rock from which they were derived), and arein thermodynamic equilibrium with the host-rock. Thus, metamorphic monazite may grow at any locationin a metamorphic rock (Figure 3.1).In contrast, hydrothermal monazite precipitates from dissolved components in vein-forming hydrother-mal fluid, both in and adjacent to quartz veins. The hydrothermal fluids involved are externally derived,having travelled through interconnected open spaces (i.e., flowed from m to km scale along permeable struc-tures), and may be out of thermodynamic equilibrium with the host-rock. Thus, hydrothermal monazite isexpected to form in and adjacent to veins, not in the unaltered host-rock (Figure 3.1).Metamorphic and hydrothermal growth mechanisms represent two end members of a spectrum wheremonazite can also grow by a mixture of local metamorphic and external hydrothermal fluids. For example,there may be some input of hydrothermal fluids to metamorphic monazite during its growth near a vein.Therefore, it grows through the local breakdown of another mineral but has mixed contributions from bothlocally and externally derived fluids. Mixed monazite growth results in grains that may be petrographicallyambiguous and pose a challenge to identifying its growth mechanism. Consequently, specific petrologicaland geochemical indicators such as petrographic associations, element zonation, ThO2 concentration, andrare earth elements (REE) geochemistry can be invoked to aid in this identification (Table 3.1).2910 metersNHost-Rockwith fabricQuartz VeinAlteration Zonewith fractures30 µmqtzmsmnzA 30 µmqtzmsmnzB30 µmqtzmsmnzCfracture30 µmqtzpymnzD fracturert qtz30 µmmspymnzEBAECDAdjacent to ms-qtz(unknown)Aligned in schistosity(metamorphic)Adjacent to fracture(hydrothermal?)Hosted in vein material(hydrothermal)Adjacent to py(hydrothermal?)Figure 3.1: Schematic diagram illustrating different petrographic contexts of monazite in orogenic goldsettings. The block diagram shows a quartz vein and alteration zone cutting through unalteredmetamorphic rock. A–E represent expected monazite textures at each location. Next to eachimage are the petrographic associations in bold and interpretation in brackets. Only B and D canbe unambiguously interpreted because of their locations in the unaltered host-rock and vein.30Table 3.1: Indicators used to distinguish between different monazite geological environments. Igneousmonazite is included because remnant igneous grains may be present in metamorphic terranes andorogenic gold deposits. The strength of each indicator to distinguish geological environmentsdecreases towards the bottom.Indicator Igneous Metamorphic Hydrothermal Reference(s)PetrographicAssociationsrIntergrown withigneous mineralsand textures (e.g.,qtz, fsp, bt)rIntergrown withmetamorphicminerals andtextures (e.g., bt)rIntergrown withhydrothermalminerals andtextures (e.g.,sulphides, oxides,micas, alb, qtz)r(Schandl andGorton, 2004)rThis studyThO2Concentrationr3 to 13 wt.%rLower incarbonatitesrVariablerIncreases withincreasing grader<2 wt.% r(Schandl andGorton, 2004)r(Williams et al.,2007)r(Cuney and Kyser,2015)r(Taylor, 2015)rThis studyREEGeochemistryrVariable tosignificantnegative EuanomalyrNegative Euanomaly (smallerafter plagioclasebreakdown)rVariable to nonegative Euanomalyr(Schandl andGorton, 2004)r(Williams et al.,2007)r(Taylor, 2015)Other ElementConcentrationsrHigh U rU and, to alesser extent, Thand Y increasewith graderLow Th/U r(Taylor, 2015)r(Grand’Homme,2016)ElementZonationrConcentricrSector rPatchyrMottledrIntergrowth-likerConcentricrStrongest ThzonationrNonerMottledrPatchyr(Zhu and O’Nions,1999)r(Spear and Pyle,2002)r(Schandl andGorton, 2004)r(Williams et al.,2007)r(Didier, 2013)GrainDistributionrSparse andhomogeneousrSparse andhomogeneousrCommonlyoccurs in clustersr(Schandl andGorton, 2004)r(Williams et al.,2007)rThis studyGrain Size rUnknown r<10 to >200µmrIncreases withgraderSmall(commonly <20µm)r(Schandl andGorton, 2004)r(Rasmussen et al.,2006)313.3 Regional and Local GeologyThe Klondike gold district is underlain by the northern portion of the Yukon-Tanana terrane: a metamor-phic block that was accreted onto the margin of North America during the Mesozoic assembly of the Cana-dian Cordillera (Nelson et al., 2013). The Yukon-Tanana terrane is composed of the Snowcap, Finlayson,Klinkit, and Klondike assemblages (Colpron et al., 2006b), along with several post-tectonic magmatic suitesand sedimentary formations. Based on previous work (e.g., Colpron et al., 2006b; Nelson et al., 2013, andreferences therein), the tectonic evolution and assembly of the Yukon-Tanana terrane can be divided intofive basic stages:1) Passive margin sedimentation along the North American continent beginning as early as the LateNeoproterozoic and lasting into the Middle Devonian, with continental arc magmatism beginning inthe Middle Devonian, both forming the initial stages of the Yukon Tanana terrane arc;2) Rifting of the Yukon Tanana terrane arc from the North American continent, opening of the SlideMountain Ocean, and ongoing island arc magmatism in the Late Devonian to Early Permian;3) Closure of the Slide Mountain Ocean, continental arc magmatism, and collision of the Yukon-Tananaterrane arc with the North American continent in the Middle to Late Permian;4) Continental arc plutonism and compressional tectonism, possibly associated with orocline develop-ment in the Late Triassic to Middle Jurassic;5) Post-collisional magmatism, sedimentation, and faulting beginning in the mid-Cretaceous.The Paleozoic stratigraphy of the Yukon-Tanana terrane was modified by several episodes of syn- topost-accretionary deformation. At least five discrete events can be distinguished, referred to as D1-5, whichresulted in the structural and metamorphic mineral assemblages that are best observed in the Klondike (asoutlined in MacKenzie et al., 2008c). D1 represents a poorly-preserved deformation/metamorphic eventthat has been overprinted by D2. D1 and D2 are coincident with the initial accretion of the Yukon-Tananaterrane onto the margin of North America (Klondike orogeny of Beranek and Mortensen, 2011) and D2 isstructurally represented as a penetrative foliation (S2). D2 is synchronous with upper greenschist metamor-phism in the Klondike and locally garnet grade amphibolite metamorphism in the underlying assemblagesfrom 260 to 252.5 Ma (M2 of Berman et al., 2007). S2 was locally overprinted by shallowly southwest dip-ping D3 structures created during ongoing collision that include crenulation cleavage (S3), recumbent folds(F3), and regional-scale thrust faults (MacKenzie et al., 2008a). The thrust faults imbricated the Paleozoicstratigraphy and emplaced tectonic slices of Slide Mountain assemblage rocks along thrust surfaces withinthe Klondike (MacKenzie et al., 2008a; MacKenzie and Craw, 2010, 2012). Accompanying metamorphism(M3 of Berman et al., 2007) produced lower greenschist conditions in the Klondike and was the last knownmetamorphism in the district at 195 to 187 Ma. D4 deformation was temporally coincident with regional ex-humation and a magmatic gap from 179 to 115 Ma. It typically manifests as steeply dipping, brittle-ductilekink folds and typically mineralized quartz veins (MacKenzie et al., 2008c; Allan et al., 2013). Lastly, D5 ischaracterized by steeply dipping normal faults that are interpreted to be related to mid- to Late Cretaceous32regional extension (MacKenzie et al., 2008c; Sanchez et al., 2013). Normal faulting in the mid- to LateCretaceous emplaced structurally lower sections of the Yukon-Tanana terrane (lower plate; e.g., AustraliaMountain Domain and Finlayson Lake District) next to structurally upper plates including the Klondike(Allan et al., 2013; Sanchez et al., 2013). The lithological assemblages of the lower plate have a distinctivemetamorphic and deformation history (Staples et al., 2013; Staples, 2014) from the rocks of the upper plate.The basement of the Klondike gold district is made up of several rock assemblages that formed be-fore and after mineralization (Figure 3.2). The Klondike assemblage (commonly known as the Klondikeschist; 269–253 Ma; Colpron et al., 2006b) occurs throughout much of the district and is divided into fivecategories by Mortensen et al. (2019): 1) a felsic package, 2) a quartz-augen schist, 3) a mafic package,4) a metaclastic unit, and 5) an undifferentiated package. The felsic package comprises quartz-muscovite-feldspar schist which is interpreted to be derived from a felsic volcanic or volcaniclastic rock, most likely asubmarine tuff (Mortensen, 1990). The quartz-augen schist contains quartz±feldspar augen, indicating it islikely derived from a felsic quartz±feldspar phyric porphyry (Mortensen et al., 2019). The mafic packagecomprised of feldspar-chlorite-quartz±epidote±amphibole±carbonate schist is likely derived from inter-mediate or mafic volcanic or volcaniclastic rocks (Mortensen, 1990). The metaclastic unit occurs mainly onthe east side of the Klondike and is derived from quartz-rich clastic sedimentary rocks. Lastly, the undif-ferentiated package includes intimately inter-layered components of the felsic, mafic, and metaclastic units.Although most rocks in the Klondike assemblage are metavolcanic, both the felsic and mafic packages havemetaplutonic equivalents that occur throughout the district (e.g., felsic metaporphyry and metagabbro ofMortensen, 1990).Orogenic gold mineralization is restricted to the metamorphic rocks of the Klondike assemblage. Min-eralization occurs within steeply to moderately dipping quartz-sulphide±carbonate veins that are discordantto the penetrative foliation (S2; Mortensen et al., 1992; Rushton et al., 1993). The veins are tabular to ir-regular, typically occur in anastomosing swarms with northeast or northwest trends, and can locally be upto three meters wide (MacKenzie et al., 2008c). Texturally, they appear massive and only rarely displaysyntaxial growth textures typical of multiple crack-seal events (Allan et al., 2013; Mortensen, in review).The veins consist almost entirely of milky white quartz (∼98%) with minor pyrite and lesser carbonate,feldspar, muscovite, baryte, scheelite, chalcopyrite, galena, sphalerite, rutile, and sulfosalts (Hoymann andFriedrich, 1990). Gold in veins typically occurs with pyrite along the vein margins and locally as free grains(Rushton et al., 1993; Mortensen, in review). Hydrothermal alteration is relatively minor and exists inthe forms of pyrite-sericite±Fe-carbonate (e.g., mafic-hosted Mitchell-Sheba occurrences), kaolinite-silica(quartz-K-feldspar; e.g., metaporphyry-hosted Virgin occurrence), and little to no silica (quartz)-pyrite (e.g.,felsic-hosted Lone Star occurrence). Fluid inclusions within vein quartz have low salinity (<6 wt. % NaClequivalent), variable CO2 content, and were trapped at temperatures estimated from 200 to 350◦C and pres-sures of 300 to 2300 kbar (Rushton et al., 1993). This considerable variation in trapping conditions isinterpreted by Rushton et al. (1993) to reflect rapid exhumation during vein formation. All veins in theKlondike are broadly similar in terms of mineralogy, texture, and fluid chemistry. It is therefore assumedthat they represent a single, albeit possibly protracted syn to post D4 orogenic gold event (Rushton et al.,331993; Allan et al., 2013). Figure 3.3 illustrates the timing of mineralization relative to metamorphic anddeformation events as based on available time constraints in the literature.This study investigates five bedrock gold occurrences: Virgin, Mitchell, Sheba, Lone Star, and IND. Allbut IND are hosted in the Klondike assemblage and are interpreted to be examples of orogenic gold occur-rences. The Virgin gold occurrence (also known as Virgin; Yukon-MINFILE 116B 007) is hosted in a quartz-augen quartz-albite-muscovite±K-feldspar schist that is mapped as the quartz-augen sub-unit (Mortensen,1996; Mortensen et al., 2019). Quartz±pyrite veins with a kaolinite-quartz-K-feldspar-pyrite alteration halooccur at Virgin. The Mitchell and Sheba gold occurrences (also known as Mitchell-Sheba; Yukon-MINFILE115O 068) occur approximately 850 m apart along the same north-south trending vein system. The host-rock at Mitchell-Sheba is an albite-chlorite-epidote-muscovite±carbonate±quartz±amphibole schist that ismapped as the mafic schist sub-unit (Mortensen, 1996; Mortensen et al., 2019). Quartz±pyrite±galenaveins occur at Mitchell-Sheba with a muscovite-carbonate-pyrite alteration halo, which is itself locallygold-bearing (Mortensen et al., 1992; Rushton et al., 1993; Chapman et al., 2010b). The Lone Star goldoccurrence (also known as Lone Star; Yukon-MINFILE 115O 072) is hosted in a quartz-muscovite-albite-chlorite schist that is mapped as the felsic schist sub-unit (Mortensen, 1996; Mortensen et al., 2019).Quartz±pyrite±galena veins occur at Lone Star with a locally developed quartz-pyrite alteration halo. TheLone Star and Virgin occurrences are the only localities in the Klondike that have been mined for bedrockgold, albeit with negligible production (Allan et al., 2013). The IND gold occurrence (also known as IND;Allan et al., 2013) is hosted in a meta-muscovite-garnet-biotite syenogranite with a locally developed weakschistosity that is mapped as the Jim Creek pluton (Mortensen, 1996; Mortensen et al., 2019). Mineraliza-tion at IND occurs along the contact between the Jim Creek pluton and the Nasina assemblage, which isa carbonaceous quartz-K-feldspar-biotite±muscovite schist. IND is interpreted as an intrusion-related goldsystem (Allan et al., 2013). New igneous monazite ages obtained during this study for the Jim Creek plutonat IND are similar to igneous zircon ages published by Beranek and Mortensen (2011) of 252.5 Ma for theJim Creek pluton (see appendices J and K for analysis information). Thus, the IND occurrence serves as aproof-of-concept for monazite geochronology and is not discussed further.34Yukon RiverIndian River138°30'0"W138°30'0"W139°0'0"W139°0'0"W139°30'0"W64°0'0"N64°0'0"N63°45'0"NTintina Fault1234 71285910 1113 1415 166Dawson CityVirginLone StarMitchell-Sheba0 5 102.5KilometersAlaskaYukon NWTBCBedrock Gold OccurrencePost-MineralizationPre-MineralizationSampled OutcropStudied Bedrock Gold OccurrencePlacer Gold Producing Creek (Quaternary)Normal FaultStrike-Slip FaultThrust FaultCarmacks Group Volcanics (Cretaceous)Finlayson Assemblage Schists (Devonian - Carboniferous)Indian River Clastics (Cretaceous)Jim Creek Pluton (Permian)Klondike AssemblageSchists (Permian)Mount Burnham Orthogneiss (Devonian)Ross Plutonic Suite (Paleogene)Scottie Creek Schists (Devonian)Slide Mountain Ultramafics (Carboniferous - Permian)Snowcap Assemblage Schists (Devonian)Sulphur Creek Orthogneiss (Permian)Quartz-Augen SchistMafic SchistMassive Greenstone (Devonian - Permian)Metaclastic UnitKlondike Schist UndifferentiatedGranodiorite and Porphyry Intrusions (Cretaceous)Felsic SchistYukon-Tannana TerraneLocalitiesFaultsFigure 3.2: Simplified geological map of the Klondike gold district (modified from Mortensen, 1996;Gordey and Ryan, 2005; Mortensen et al., 2019). Inset: Yukon-Tannana terrane is yellow; White-horse is the large black square; Dawson City is the small black square; map location is the redbox. Studied localities are stared and labelled on the map. Other localities are labelled as: 1) BenLevy; 2) Plinc; 3) Orofino; 4) IND; 5) Dysle; 6) Nugget; 7) Violet; 8) Boxcar; 9) MacKay; 10)King Soloman Dome; 11) Dome Lode/Hunker Dome; 12) Lloyd; 13) Gold Run; 14) DominionCreek; 15) Aime/Payne; 16) Lower Dominion.35Mesozoic CretaceousJurassicTriassicLowerMiddleUpperLowerMiddleUpperLowerUpper251.9247.2~237201.3±0.2174.1±1.0163.5±1.0~145.0100.566.0PerEra Epoch Age (Ma) Upper Plate Lower PlateM3M4D3D4D5[2,3,8][1,3] [1,3,8][1,8][6,7]Australia Moun-tain Domain [1,3,10]Australia Moun-tain Domain [3,10][1,8]Finlayson Lake District [5,9,11]Finlayson Lake District [11][1][1,3]M2 D2Metamorphism Deformation Exhumation Mineralization[4]Figure 3.3: A summary of the Yukon-Tanana terrane Mesozoic structural events that surround orogenicgold mineralization in the Klondike. Numbered metamorphic events after Berman et al. (2007).Numbered deformation events after MacKenzie et al. (2008c). Emphasis is on upper plate defor-mation events. Numbered citations: 1) Allan et al. (2013); 2) Beranek and Mortensen (2011); 3)Berman et al. (2007); 4) Cohen et al. (2018); 5) Hunt and Roddick (1992); 6) Joyce et al. (2015);7) Knight et al. (2013); 8) MacKenzie et al. (2008c); 9) Nelson et al. (2013); 10) Staples et al.(2013); 11) Staples (2014).3.4 Methods3.4.1 Sample StrategySampling in the Klondike included seventeen localities (Virgin, Ben Levy, Orofino, IND, Dysle,Nugget, Violet, Lone Star (Boulder Lode), Mitchell, Sheba, MacKay, King Soloman Dome, Dome Lode(Hunker Dome), Lloyd, Dominion Creek, Aime (Payne), and Lower Dominion), almost all of which areknown gold occurrences. The Sixtymile and White Gold districts were also sampled for monazite dating.Eleven localities were selected for petrographic analysis (Aime, Dysle, IND, Lone Star, MacKay, Mitchell,Nugget, Orofino, Sheba, Violet, and Virgin) and five were selected for monazite petrochronology (IND,Lone Star, Mitchell, Sheba, Virgin). The selected localities were designed to investigate gold occurrencesthroughout the Klondike that are hosted in different lithologies.Unaltered, altered, and vein samples were collected at each locality and consist of float, outcrop, ordrill core material where possible. Detailed outcrop sketches, maps, or core logs were completed to providegeological context. Systematic sampling began with fresh, unaltered host-rock that was commonly collected5–20 m away from veins. Sampling continued towards the veins with lightly, moderately, and heavily alteredsamples. Lastly, vein material with adjacent wall-rock was sampled. Uncertainties related to sampling36include sampling float material with limited bedrock information at some localities and bias introduced bysampling a subset of potentially diverse vein types. Samples were oriented perpendicular to the penetrativefabric and prepared into 30 µm thick, polished thin sections by Vancouver Petrographics Limited in Langley.The thin sections were used for all analyses described (except for whole rock lithogeochemistry) to preservethe textures in situ. Metamorphic rocks are named with porphyroblastic minerals listed first, followed bymajor rock-forming minerals (≥5%) in decreasing abundance. Veins are named in the same format, withspecific terminology discussed in Appendix A.3.4.2 Whole Rock LithogeochemistryCorresponding unaltered and altered rock sample pairs were sent to Bureau Veritas Mineral Laboratoriesin Vancouver for preparation and whole rock lithogeochemical analysis. The rocks were crushed to< 2 mmand pulverized to < 75 µm before undergoing a Li borate fusion digestion. X-ray fluorescence (XRF) wasused to analyze major elements (reported as oxides), and inductively-coupled plasma mass spectrometry(ICP-MS) was used to analyze lithophile and rare earth elements. A 4-acid digestion with ICP-MS finish wasused to analyze other trace elements. Quality assurance/quality control (QA/QC) (presented in Appendix L)was conducted in Microsoft Excel (Excel), and Reflex ioGAS (ioGAS) was used for data manipulation andplotting.3.4.3 PetrographyPetrographic analysis was carried out at MDRU, at UBC. A Nikon Eclipse E600 POL polarizingmicroscope fitted with a Canon Rebel EOS T21 optical camera was used in transmitted and reflected lightto identify mineralogy, textures, and establish a paragenetic sequence.3.4.4 Scanning Electron MicroscopeSEM analysis was done at the Electron Microbeam and X-Ray Diffraction Facility (EMXDF) at UBC.Thin sections were carbon coated with an Edwards Auto 306 carbon coater before copper tape was placed onthe back of the slides to better fixate the electron beam during analysis. A Philips XL-30 SEM with a BrukerQuantax 200 energy-dispersion X-ray microanalysis system, XFlash 6010 SDD detector, and Robinson CLdetector was used to investigate micro-textures and identify unknown minerals. Identification of differentquartz vein generations by CL contributes to an understanding of the paragenetic sequence. Potential mon-azite grains identified with BSE imaging were confirmed using energy-dispersive X-ray spectroscopy (EDS)semi-quantitative analysis and images were taken of each grain at various scales.3.4.5 Electron Probe Micro-AnalyzerChemical zonations of representative monazite grains from each locality were investigated by elementalX-ray mapping at FiLTER at UBC-O. A Cameca SXFive Field Emission EPMA at 15 Kv and 200 nA with afixed beam, 100 ms dwell time, and 0.5 to 0.1 µm step size produced Si, Ca, Y, Th, and U maps of individualmonazite grains.373.4.6 Laser Ablation Inductively-Coupled Plasma Mass SpectrometryMonazite petrochronology by LA-ICP-MS was conducted for all isotope analyses at PCIGR at UBC.Ablation spots were placed on grain surfaces with no apparent preference to internal mineral structure,ensuring sufficient distance from grain edges and neighbouring ablation sites. Spots were placed insidechemical zonations where X-ray maps identified large enough zones (>10 µm). Two separate analyticalprocedures, with different laser ablation systems, ablation spot locations, and mass spectrometers, were usedto first collect U-Th-Pb isotopes and second trace element isotopes. Table 3.2 summarizes the different laserablation parameters used for each procedure. These parameters give the highest reproducibility and optimalprecision, whereas other tested parameters are presented in Appendix I. The standard-sample bracketinganalytical technique was applied by using interpolation between the bracketing reference materials to correctfor instrumental drift. Data were reduced with the Iolite 3.4 extension for Igor Pro (Paton et al., 2011). Iolitewas used to select integrations from the raw data, subtract baselines, compute exponential functions tocorrect for downhole fractionation, and calculate common Pb corrections. Reduced data were monitoredfor QA/QC (presented in Appendix L) and investigated with Excel, the Isoplot Excel macro of Ludwig(2003), and MathWorks Matlab (Matlab). Analyses were considered unsuccessful and disregarded based oninclusions, missed or partly missed spots, instrument error, and age uncertainty >8% (>10% was used atMitchell-Sheba as monazite grains there are smaller and have higher uncertainty).Table 3.2: Laser ablation parameters for monazite petrochronology.Laser AblationParametersU-Th-Pb Analysis Trace Element AnalysisInstrument ESI-New Wave Research NWR193UC Resonetics RESOlution M-50LRWavelength 193 nm 193 nmSpot Diameter 10 µm 19 µmEnergy Density 3 J/cm2 2 J/cm2Repetition Rate 8 Hz 8 HzAblation Time 25 s 40 sGas Blank Time 30 s 20 sThe U-Th-Pb analyses of monazite were performed using an ArF excimer laser ablation system (193nm; ESI-New Wave Research NWR193UC) coupled to a high-resolution ICP-MS (ThermoFinnigan Ele-ment2). All grains were ablated using a fluence of 3.00 J/cm2, ablation frequency of 8 Hz, and a spot sizeof 10 µm. The grains were analyzed for m/z (mass/charge) corresponding to isotopes of Pb (204Pb, 206Pb,207Pb, and 208Pb), U (235U and 238U), and 232Th, as well as 202Hg to monitor for background common Pb.External standardization included bracketing groups of ten unknown monazite analyses with two analyses ofthe Manangotry monazite (see Section I.3.1; 555 ± 1 Ma; Paquette et al., 1994; Horstwood et al., 2003; Pa-quette and Tiepolo, 2007). The Moacyr monazite (see Section I.3.2; 504.3 ± 0.2 Ma; Seydoux-Guillaumeet al., 2002a,b; Gasquet et al., 2010) was used as a secondary monitor to ensure the accuracy of the ageanalyses. Data reduction in Iolite was performed using the VisualAge data reduction scheme (Petrus and38Kamber, 2012). After common Pb contamination was confirmed by plotting Wetherill (Wetherill, 1956) andTera-Wasserburg (Tera and Wasserburg, 1972) concordia diagrams, a common Pb correction was performedusing the method of Andersen (2002). Low 204Pb counts led to this method being used over a conventional204Pb correction. The 208Pb/232Th dates are ultimately used over the U-Pb dates because:1) There is significant uncertainty in the 207Pb/235U ages due to low U contents (and thus negligible207Pb) in young monazite (Grand’Homme et al., 2016a). Varying amounts of initial Pb suggest muchof the 207Pb may originate from initial Pb rather than radiogenic 207Pb and cause discordance.2) Excess 206Pb from the decay of 230Th (an intermediate decay product of 238U) can result in dise-quilibrium of the U decay series in young monazite, causing inaccurate (too old) 206Pb/238U datesand reverse discordant concordia plots (Scharer, 1984). Figure 3.4 demonstrates how excess 206Pb isidentified after common Pb correction.3) The Th/U ratio is typically high and almost always >1 in monazite (Van Emden et al., 1997;Grand’Homme, 2016). Because 232Th is abundant and has a natural isotope abundance of almost100% (Rosman and Taylor, 1998), 208Pb originating from initial Pb is negligible compared toradiogenic 208Pb (Bosse et al., 2009).4) Data collected from this study indicate the U concentration is relatively low (∼125 ppm average) andU-Pb dates typically have poor accuracy and precision even after common Pb correction (uncertaintycommonly >20%). The 208Pb/232Th dates are the most reproducible, contain low uncertainty (most<8%), and are concordant on the U-Th-Pb concordia diagram (some discordance indicates excess206Pb; Figure 3.4).The trace element analyses of monazite were performed using an ArF excimer laser ablation system(193 nm; Resonetics RESOlution M-50-LR, ASI Australia) coupled to a quadrupole ICP-MS (Agilent7700x). All grains were ablated using a fluence of 2.00 J/cm2, ablation frequency of 8 Hz, and a spotsize of 19 µm. The grains were analyzed for m/z corresponding to thirty-three isotopes that include thefull suite of rare earth elements. External standardization was done by bracketing groups of five unknownmonazite analyses with one analysis of the BCR2G glass reference material (Jochum et al., 2005), whereasNIST610 and NIST612 glasses were used as secondary monitors. Because these reference materials arenot appropriately matrix-matched to monazite, no internal standardization was used, and data from thisprocedure is semi-quantitative.398 8.5 9 9.505101520170 Ma 180 Ma208Pb/232Th (x10-3)206 Pb/238 U (x10-2)190 MaFigure 3.4: U-Th-Pb concordia plot demonstrating the effects of common Pb correction and identify-ing excess 206Pb, which is from the decay of 230Th. The thick black line and white circles are theconcordia. Arrows indicate how the same data point moves after common Pb correction, correct-ing for common Pb in both the 206Pb/238U and 208Pb/232Th systems. Grey: concordant analysiscontaining no common Pb that did not move after correction. Orange: discordant analysis con-taining little common Pb that became concordant after correction. Blue: discordant analysis con-taining common Pb that became concordant within error, possibly containing excess 206Pb. Red:discordant analysis containing abundant common Pb that remained discordant after correction,indicating excess 206Pb.403.5 Results3.5.1 Petrography and Vein TexturesKlondike Gold DistrictThe Klondike schist comprises quartz-muscovite-feldspar schist in the felsic sub-units (Figure 3.5aand Figure 3.7a) and albite-chlorite-epidote-muscovite±carbonate±quartz±amphibole schist in the maficsub-unit (Figure 3.6a). The rocks exhibit a penetrative schistosity defined by the alignment of micas andalternating mica- and quartz-feldspar-rich layers (Figure 3.5b). An overprinting crenulation cleavage islocally observed where micas are abundant (Figure 3.5b). The protoliths of the Klondike schist range fromfelsic to mafic volcanic, felsic intrusive, and clastic sedimentary rocks (Mortensen et al., 2019). The rangein protoliths led to the different sub-units outlined in Section 3.3 that are defined by subtle mineralogicaland textural differences. Gold occurrences described in this study occur in all sub-units of the Klondikeschist (Figure 3.2). The host-rock mineralogy and textures are summarized for the Virgin, Mitchell-Sheba,and Lone Star occurrences in Table 3.3 and explained below.Gold-bearing quartz veins in the Klondike schist consist almost entirely of quartz (>98%) with minorpyrite (<2%) and trace galena (Table 3.3). Trace vein phases vary by occurrence but may contain carbonate,muscovite, chlorite, baryte, rutile, and chalcopyrite. Gold typically occurs with pyrite along vein margins(Figure 2.5d). The alteration is typically quartz-muscovite-pyrite, with accessory phases including chlorite,carbonate, monazite, rutile, gold, baryte, galena, and chalcopyrite. There are minor to significant variationsof this alteration assemblage that depend on the local host-rocks (Table 3.3) and are explained below.Gold-bearing quartz vein orientations range from sub-concordant to perpendicular to the wall-rockschistosity. Two different vein textures are observed petrographically (Figure 3.5d): 1) elongate, euhedralquartz that grew syntaxially from the wall-rock; and 2) blocky quartz that comprises the majority of veinmaterial (see Appendix A for a summary of vein growth textures). These earlier quartz textures were laterdeformed and partially recrystallized (Figure 3.7b). Post-vein deformation features range from subtle kinksof wall-rock fabric to incipiently boudinaged veins (e.g., Figure 3.8), to boudinaged veins with extensivelysheared alteration halos. Microstructures such as undulose extinction, deformation bands, grain boundarymigration, and sub-grain rotation in vein-forming quartz also indicate varying degrees of post-vein deforma-tion. Vein quartz has dark, nearly homogeneous CL responses that yield little additional textural information(see Appendix D for all CL images). Brittle fractures, locally filled with carbonate and sparsely galena,cross-cut the veins but only rarely propagate into the wall-rock (Figure 3.6b). Vugs indicate weathering outof carbonate and pyrite in the veins and adjacent wall-rock (Figure 3.5f).41Table 3.3: Summary of petrographic investigations. Minerals are divided into rock-forming minerals, accessory minerals, and sulphides andgold. Minerals for each category are listed from most abundant (left) to least abundant (right). Mineral abundances are shown for theunaltered host-rock, altered host-rock, and vein material of the Virgin, Mitchell-Sheba, and Lone Star gold occurrences. * = trace; ** =trace–<2%; *** = ≥2%–<5%; X = ≥5%–<10%; XX = ≥10%–<30%; XXX = ≥30%–<50%; XXXX = ≥50%. A similar mineralogytable for all samples is in Appendix E.Rock Type Qtz Ab Ms Chl Cb Ep Kfs Am Bt Kln Brt Ser Rt Ap Mag Mnz Zrn Xtm Ttn Ilm Py Gn Ccp Sp Au ApyHost-Rock XXX XX XX *** X *** ** * * *Altered Host-Rock XXX * X XXXX ** ** * *** *Vein XXXX ** * ** *Host-Rock *** XXX X XX *** XX ** *** ** ** ** ** * *Altered Host-Rock XXXX X XXX ** * * * *** ** ** * * *Vein XXXX ** * ** * ** *Host-Rock XXX XX XX X X ** ** * * * **Altered Host-Rock XXXX ** X ** ** * *** * * * *Vein XXXX ** ** ** * *VirginMitchell-ShebaLone StarRock-Forming Minerals Accessory Minerals Sulphides and Gold421000 µm2000 µm200 µm60 µmA) B)D)E)C)F)BS17-VG7 BS17-VG4BS17-VG6BS17-VG5BS17-VG6BS17-VG1qtz-alb-ms-KspS2S3 S3S3qtz2qtz3qtz1kln-qtz4-Ksp2-ms2 alterationmnz2 qtz1klnrthost-rockS2S3 S3S3Figure 3.5: Hand sample, microscope, and BSE images displaying petrographic relationships of theVirgin gold occurrence. A, B, and E are from the host-rock, whereas C, D, and F are from veinmaterial. Mineral generation numbers are linked to interpretations made in Section 3.6 (e.g.,qtz2 vs. qtz3). A: Unaltered quartz±albite-augen quartz-albite-muscovite±K-feldspar schist withoverprinting ductile deformation fabrics (PPL; S2 – schistosity, S3 – crenulation); B: overprintingductile deformation fabrics (hand sample); C: Alteration showing monazite 2, rutile, and kaolinite(BSE); D: Syntaxial quartz 2 growth from the wall-rock and blocky quartz 3 (XPL); E: Pervasivekaolinite-quartz 4-K-feldspar 2-muscovite 2 alteration of the wall-rock (XPL); F: Vugs along avein boundary indicating weathered out pyrite and carbonate (hand sample).432000 µm2000 µm200 µm1000 µmA)C)E)D)F)B)BS17-MT5KSD-13-02_57.8m KSD-13-01_326.1mcb3S2ab-chl1-ep-ms1-amqtz2qtz3BS17-MT3chl2KSD-13-02_67.2mqtz1pycb2KSD-13-01_213.0m1000 µmqtz3host-rock1000 µmpymt qtz3abms2cpFigure 3.6: Microscope images displaying petrographic relationships of the Mitchell-Sheba gold oc-currence. A and D are from the host-rock, whereas B, C, E, and F are from vein material. Mineralgeneration numbers are linked to interpretations made in Section 3.6 (e.g., qtz2 vs. qtz3). A: Unal-tered albite-chlorite-epidote-muscovite±carbonate±quartz±amphibole schist (PPL); B: Carbon-ate 3 infilling late brittle fractures within a quartz 3 vein (XPL); C: Stretched quartz 2 indicatingataxial vein growth and later blocky quartz 3 (XPL); D: Pervasive muscovite 2 and carbonate 2alteration (XPL); E: Pyrite replacing magnetite (RL); F: Chalcopyrite infilling brittle fractures inpyrite (RL).44500 µm500 µm 2000 µm2000 µm1000 µmC) D)E)A)F)LS16-70_33.1mms1cppy1py2S2S3S3py1qtz1cb1qtz4 alterationqtz1-ms1-alb-chl1qtz3gnpy2LS16-70_33.1mLS16-69_89.0mLS16-70_33.1mLS16-69_89.0m2000 µmB)qtz5qtz3LS16-70_99.4mFigure 3.7: Microscope images displaying petrographic relationships of the Lone Star gold occurrence.A, C, D, and E are from the host-rock, whereas B and F are from vein material. Mineral generationnumbers are linked to interpretations made in Section 3.6 (e.g., qtz3 vs. qtz5). A: Unalteredquartz-muscovite-albite-chlorite schist (PPL; S2 – schistosity); B: Blocky vein quartz 3 showingductile deformation in the form of undulose extinction and subgrain rotation (XPL). Subgrainsrotate to recrystallize and form quartz 5; C: Chlorite porphyroblast replaced by carbonate 1 (XPL);D: Pyrite 1 aligned with the schistosity (RL; S2 – schistosity, S3 – crenulation); E: Pervasivequartz 4-pyrite 2 alteration (XPL); F: Galena and chalcopyrite infilling brittle fractures in pyriteindicating they are late in the hydrothermal sequence (RL).45�1�1Figure 3.8: Crenulation locally well-developed in a mica-rich alteration halo adjacent to an incipientlyboudinaged vein. The principle stress direction is indicated by the crenulation and vein deforma-tion.Virgin Gold OccurrenceQuartz veins at Virgin are hosted in quartz±albite-augen quartz-albite-muscovite±K-feldspar schist(Figure 3.5a). Mortensen (1996) and Mortensen et al. (2019) included these rocks in the quartz-augenschist sub-unit of the Klondike schist (Figure 3.2), which is interpreted to be a hypabyssal equivalent of themetamorphosed felsic volcanic rocks. Quartz±albite-augen comprise 5–10% of the rock and are surroundedby a finer-grained groundmass of quartz (40%), albite (20%), and muscovite (20%; Table 3.3). Minor phases(< 5%) include K-feldspar, kaolinite, and carbonate, whereas trace phases (<< 1%) include monazite,zircon, and xenotime. The veins at Virgin consist almost entirely of quartz with minor to trace pyrite,muscovite, galena, and baryte (Table 3.3). Wall-rock alteration adjacent to the veins is dominated by veryfine-grained kaolinite, quartz, and K-feldspar with other minor to trace phases, including pyrite, rutile,baryte, monazite, and muscovite (Figure 3.5e).Mitchell-Sheba Gold OccurrenceQuartz veins at Mitchell-Sheba are hosted in fine-grained, albite-chlorite-epidote-muscovite±carbonate±quartz±amphibole schist (Figure 3.6a). Mortensen (1996) and Mortensen et al. (2019) included theserocks in the mafic schist sub-unit of the Klondike schist (Figure 3.2). Albite is abundant in the rock (up to50%), with lesser chlorite (∼25%), epidote (∼10%), and muscovite (∼5%; Table 3.3). Minor phases includecarbonate, quartz, pale-green amphibole, and magnetite, whereas trace phases include biotite, K-feldspar,apatite, titanite, and zircon. The veins consist almost entirely of quartz, with minor to trace pyrite, mus-covite, carbonate, chlorite, rutile, and galena (Table 3.3). Syntaxial quartz growth is not apparent in veinsat Mitchell-Sheba. Instead, quartz grains are elongated perpendicular to the vein and do not show growthcompetition, consistent with ataxial growth (Figure 3.6c). Alteration adjacent to the veins is muscovite-carbonate dominated, with chlorite, pyrite, and Fe alteration of pre-existing chlorite, muscovite, and am-phibole (Figure 3.6d). Pyrite occurs as rims that have partially replaced magnetite (Figure 3.6e), and alsooccurs as finely disseminated grains throughout altered wall-rock. Trace alteration phases include rutile,galena, chalcopyrite, monazite, arsenopyrite, sphalerite, xenotime, and ilmenite. Rutile, arsenopyrite, and46galena occur as inclusions in pyrite, whereas chalcopyrite and galena occur within fractures in pyrite andquartz (Figure 3.6f).Lone Star Gold OccurrenceQuartz veins at Lone Star are hosted in quartz-muscovite-albite-chlorite schist (Figure 3.7a). Mortensen(1996) and Mortensen et al. (2019) included these rocks in the felsic schist sub-unit of the Klondike schist(Figure 3.2). Quartz dominates the host-rock (∼50%), and lesser amounts of muscovite (∼25%), albite(∼15%), and chlorite (∼10%) make up the remaining bulk (Table 3.3). Chlorite is dominantly aligned inthe schistosity but locally forms porphyroblasts that have been partially to entirely replaced by carbonate(Figure 3.7c). Unlike the rest of the Klondike, pyrite occurs in relatively high abundance (∼1%) and isaligned in the schistosity and crenulation (Figure 3.7d). Trace host-rock phases include baryte, apatite,monazite, and rutile. The veins at Lone Star consist almost entirely of quartz with minor to trace pyrite,carbonate, baryte, galena, and chalcopyrite (Table 3.3). Syntaxial quartz vein growth is absent at Lone Starbut may have been overprinted by later ductile deformation (Figure 3.7b). Wall-rock alteration from theveins is relatively weak and consists mainly of quartz and pyrite (Figure 3.7e). Minor to trace alterationphases include carbonate, chlorite, baryte, rutile, monazite, galena, chalcopyrite, and sphalerite. Galena andchalcopyrite occur within fractures in pyrite and quartz (Figure 3.7f). Sub-concordant veins at Lone Stargenerally have the same vein and alteration assemblage as discordant veins, but locally have more carbonate,chlorite along vein margins, and weaker alteration.3.5.2 Whole Rock LithogeochemistryResults from the whole rock lithogeochemistry analyses demonstrate that the host-rock at each goldoccurrence derived from chemically different protoliths (Figure 3.9). No distinct differences exist betweenunaltered and altered rocks of the same lithology, which confirms the reliability of the Winchester and Floyd(1977) diagram, which utilizes relatively immobile elements (Ti, Zr, Nb) to look beyond metamorphismand alteration to record the primary volcanic protolith compositions. Analyses from Virgin samples clusterin the rhyolite field, which agrees with the felsic quartz-albite-muscovite±K-feldspar assemblage deter-mined in Section 3.5.1. Analyses from Mitchell-Sheba samples plot in the andesite/basalt field, consistentwith the mafic albite-chlorite-epidote-muscovite±carbonate±quartz±amphibole host. Analyses from LoneStar samples plot across the boundary of the rhyodacite/dacite and andesite fields, which overlaps with thewidely exposed QMS sub-unit in the area (outlined in Mortensen et al., 2019) and agrees with the felsicto intermediate quartz-muscovite-albite-chlorite host. As noted by Mortensen et al. (2019), much of thehost-rock at Lone Star is rhyolitic; however, some of the rocks have whole rock compositions that trend tomore intermediate compositions. These relatively more intermediate compositions overlap with the new,mainly dacitic analyses and, despite local heterogeneity, the host-rock at Lone Star is herein described asintermediate to distinguish it from the relatively more felsic rocks at Virgin and the relatively more maficrocks at Mitchell-Sheba.Unaltered and altered rock sample pairs were investigated to determine elements that were added/sub-47tracted during alteration to compare with petrographic observations and provide insight into the alterationassemblage. Rocks altered by the vein-forming fluids contain marginally higher concentrations of Ce, whichis a major element in monazite, than their unaltered pairs (e.g., ∼32 ppm Ce in a moderately altered sampleand ∼16 ppm Ce in an unaltered sample from Mitchell-Sheba; see Appendix F for all whole rock lithogeo-chemical data). No other elements, including other major elements of monazite such as P and Th, exhibit aconsistent variation between unaltered and altered samples, but certain elements are found to vary in agree-ment with the mineralogy summarized in Table 3.3. For example, altered rocks have elevated concentrationsof Si and Al at Virgin, consistent with higher abundances of quartz and kaolinite. At Lone Star, unaltered andaltered rocks are indistinguishable from the geochemical data, consistent with the generally weak alterationhalo. This result from Lone Star is also consistent with most orogenic gold deposits as the hydrothermal flu-ids are not significantly out of thermodynamic equilibrium with the wall-rocks (Goldfarb and Groves, 2015).Local lithological heterogeneity may complicate these interpretations of unaltered and altered material.PhonoliteTrachy AndesiteAndesiteBasaniteNephelliniteAlkali BasaltAndesite/BasaltSub-Alkaline BasaltRhyodacite/DaciteRhyoliteComendite/PantelleriteTrachyteDome Lode (n=2)Lower Dominion (n=6)MacKay (n=2)Mitchell-Sheba (n=2)Lone Star (n=3)Virgin (n=3)UnalteredAlteredNb/YZr/TiO210.010.10.1 0.2 1 2 10Figure 3.9: Volcanic rock classification diagram based on immobile trace elements (after Winchesterand Floyd, 1977). Whole rock lithogeochemistry samples from the Klondike are plotted andcoloured based on locality. Grey-scale localities were not investigated by monazite petrochronol-ogy. Lone Star samples are from Nugget but represent the same host-rock at Lone Star, 1.8 kmaway. Mitchell-Sheba samples are from Mitchell but represent the same host-rock at Sheba, 850maway. Dome Lode: mafic protolith; Lower Dominion: mafic and intermediate protoliths; MacKay:felsic protolith; Mitchell-Sheba: mafic protolith; Nugget/Lone Star: intermediate protolith; Vir-gin: felsic protolith.483.5.3 Monazite Petrography and CharacterizationMonazite is a typical trace phase in the unaltered rocks investigated from Virgin and Lone Star. Theseanhedral grains range in shape from equant to elongate, and average ∼25 µm in length with sparse grainslarger than ∼60 µm. They are typically adjacent to muscovite and quartz and are locally aligned with theschistosity. They may also be adjacent to pyrite in unaltered rocks at Lone Star. No monazite was observedin the unaltered rocks at Mitchell-Sheba.Monazite was rarely identified in vein material, but is typical in the adjacent, altered wall-rock at Virgin,Lone Star, and Mitchell-Sheba (Figure 3.5c). The size and shape of monazite grains in altered rock issimilar to those hosted in unaltered material, though the average size of grains at Mitchell-Sheba is ∼15µm. Texturally, grains in altered rock are also adjacent to muscovite and quartz and are locally alignedwith the schistosity, which makes distinguishing them from grains in unaltered rocks a challenge. Monazitein altered rock, however, is locally adjacent to fractures, pyrite, or other hydrothermal minerals such askaolinite at Virgin.Several petrographic, U-Th-Pb, and trace element parameters were considered to determine relation-ships between the parameters and monazite isotope data from the LA-ICP-MS procedures. The petrographicparameters that display significant relationships with monazite data are incorporated into several figures inSection 3.5 and Section 3.6. These petrographic parameters include: 1) sample alteration intensity, whichrepresents the type of sample from which a monazite grain is derived and is divided into unaltered, lightlyaltered, and heavily altered intensities, as well as disseminated gold zone and vein/wall-rock material; and2) grain petrographic association, which is the petrographic context of a monazite grain with respect toadjacent minerals and textures. Petrographic associations are divided into four categories (Figure 3.1): 1)adjacent to muscovite-quartz, which is the typical petrographic setting of monazite grains with ambiguousorigins; 2) aligned in schistosity, which indicates the monazite grain may have grown during metamorphismor hydrothermally precipitated along the planar features; 3) adjacent to fracture, which may indicate themonazite grain precipitated hydrothermally along the fracture; and 4) adjacent to pyrite, which may indicatethe monazite grain precipitated hydrothermally because pyrite is typically a hydrothermal mineral in theKlondike (aside from the pre-existing pyrite phase at Lone Star). Other petrographic parameters include:1) sample number, which is the sample from which a monazite grain is derived; 2) grain form, which isa subjective parameter of monazite grain form from anhedral to euhedral; 3) grain form factor, which is acalculated proxy for grain irregularity (4pi(area)/√perimeter; values closer to 1 are more equant); 4) grainarea, which is the 2-dimensional area of a monazite grain; 5) grain aspect ratio, which is a calculated valueof the largest diameter and the smallest orthogonal diameter of a monazite grain (ma joraxis/minoraxis);6) grain compactness, which is a calculated value of the area and largest diameter of a monazite grain(√4area/pi/ma joraxis); and 7) grain roundness, which is also a calculated value of the area and largestdiameter of a monazite grain (4area/pi√ma joraxis).Parameters from the monazite U-Th-Pb isotope analyses include: 1) 208Pb/232Th date; 2) ThO2 con-centration, which was previously used by Schandl and Gorton (2004) to distinguish between igneous and49hydrothermal monazite; and 3) U/Th ratio, which was previously used by Janots et al. (2012) to distinguishmonazite from different host-rocks. Lastly, parameters from the monazite trace element isotope analysesinclude: 1) Eu anomaly (Eu/Eu*), which is used to indicate monazite growth coeval with plagioclase andapatite; 2) Gd/Yb ratio, which is used to indicate monazite growth coeval with garnet and apatite; and 3) Caabundance, which is used to distinguish Ca-rich monazite varieties (brabantite/cheralite).3.5.4 Monazite Element MappingEPMA element maps of Th, Y, U, Ca, and Si in monazite reveal subtle to clear zonation patterns thatare dominantly patchy or mottled (Figure 3.10; see Appendix K for all element maps). The zonation style isconsistent across localities, sample alteration intensities, and monazite grain petrographic associations. Thezoned domains are typically smaller than 10 µm. Th is the most prominently zoned element, with domainsof relatively high and low concentration in most grains. Y zonation is typically more subdued but generallyfollows the same pattern as Th. Y maps also show instances where xenotime is grown adjacent to monazite(e.g., see LS16-69 88.8m - M5 in Appendix K). U is typically homogeneous and displays no zonation. Fewmonazite grains are entirely free of zonation, and even fewer grains display a core and rim with a high Thand Y core (e.g., see LS16-69 27.4m - M6 in Appendix K). Ca and Si maps were used to check for apatiteand silicate micro-inclusions.50LS16-60_17.5m - M10BSE 30 µmThKSD-13-02_125.3m - M7BSE 10 µmThMA11-VG2 - M16BSE 10 µmThFigure 3.10: Representative monazite BSE and element maps. Th maps are displayed because theyshow the most visible zonation. Top: Monazite MA11-VG2-M16 from a vein/wall-rock sam-ple at Virgin displays mottled zonation. The grain is surrounded by vein quartz and interpretedas hydrothermal. Middle: Monazite KSD-13-02 125.3m-M7 from a heavily altered sample atMitchell-Sheba displays mottled zonation. The grain is adjacent to quartz and is interpreted ashydrothermal. Bottom: Monazite LS16-69 17.5m-M10 from a vein/wall-rock sample at LoneStar displays patchy/mottled zonation. The grain is aligned in the schistosity and may have beenreset by hydrothermal fluids.513.5.5 Monazite GeochronologyOverall, 272 geochronology spot analyses yielded 208Pb/232Th dates from 189±14 to 116±4 Ma andThO2 concentrations from 7.38±0.13 to 0.13±0.01 wt.% (Figure 3.11). Where compositional domains weresufficiently large, LA-ICP-MS spots were targeted to sample the individual domains. Different spot analysesof zoned grains yielded dates that are within uncertainty of one another, with monazite LS16-69 89.4m-M11being the only exception (see Appendix J for all monazite dates). Variations in date and ThO2 concentrationsexist between occurrences and grain petrographic associations, which are summarized below. AppendixJ reports all monazite geochronology data, including quantitative concentrations, common Pb-correctedisotope ratios, and calculated dates. Figure 3.11 summarizes the data by occurrence and displays ThO2concentration plotted against date and U-Th-Pb concordia diagrams. The sample alteration intensity andpetrographic associations are respectively indicated in Figure 3.11 by marker shape and colour. There areno relationships between ThO2 concentrations and date with respect to other parameters including samplenumber, grain form, grain form factor, grain area, grain aspect ratio, grain compactness, grain roundness,analysis U/Th ratio, and trace element values from Section 3.5.6 including Eu anomaly, Gd/Yb ratio, andCa abundance.Virgin Gold OccurrenceMost of the 52 analyses from the Virgin gold occurrence samples are concordant on the U-Th-Pbconcordia; however, some indicate excess 206Pb by their discordance after common Pb correction (Fig-ure 3.11). The analyses yielded dates from 185±14 to 117±4 Ma and ThO2 concentrations from 7.38±0.13to 0.32±0.02 wt.% (Figure 3.11). These results plot in two distinct populations: 1) all points with ThO2>2.00 wt.% (n = 16) are relatively older and generally correspond to unaltered samples and grains alignedin the schistosity; and 2) although some overlap exists, most points with ThO2 <2.00 wt.% (n = 32) arerelatively younger and are generally from altered samples and grains adjacent to pyrite. Analyses youngerthan 166 Ma shift from variable ThO2 concentrations to <2.00 wt.%. Relatively older analyses are gener-ally from more euhedral grains, whereas relatively younger analyses (especially younger than 166 Ma) arefrom more anhedral grains. Monazites that are aligned in the schistosity (n = 2) gave dates from 176±4to 173±3 Ma and ThO2 concentrations from 4.64±0.13 to 3.96±0.11 wt.%, though only two such pointsexist. Monazites that are adjacent to pyrite (n = 12) gave dates from 178±6 to 122±7 Ma and lower ThO2concentrations from 1.80±0.09 to 0.32±0.02 wt.%.Mitchell-Sheba Gold OccurrenceAll 23 of the analyses from the Mitchell-Sheba gold occurrence samples are concordant on the U-Th-Pbconcordia (Figure 3.11). The analyses yielded dates from 175±13 to 126±8 Ma and ThO2 concentrationsfrom 1.57±0.15 to 0.16±0.01 wt.% (Figure 3.11). Monazites that are adjacent to pyrite (n = 6) gave datesfrom 171±14 to 140±13 Ma and low ThO2 concentrations from 1.57±0.15 to 0.16±0.01 wt.%. Theseanalyses overlap with the second population at Virgin, in that they are <2.00 wt.% ThO2 and are fromaltered samples and grains adjacent to pyrite. As monazite does not occur in unaltered rocks at Mitchell-Sheba, the analyses do not overlap with the first population at Virgin (i.e., ThO2 >2.00 wt.% that correspond52120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)Virginn = 526 7 8 9 10-5051152025206 Pb/238 U (x10-2)208Pb/232Th (x10-3)Virgin120 Ma 140 Ma 160 Ma 180 Ma120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)n = 23Mitchell-Sheba6 7 8 9 10-50510152025206 Pb/238 U (x10-2)208Pb/232Th (x10-3)Mitchell-Sheba120 Ma 140 Ma 160 Ma 180 Ma120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)Lone Starn = 1976 7 8 9 10-50510152025206 Pb/238 U (x10-2)208Pb/232Th (x10-3)Lone Star120 Ma 140 Ma 160 Ma 180 MaAdjacent to fractureAdjacent to pyAdjacent to ms-qtzMonazite Grain Petrographic Association ColourAligned in schistositySample Alteration Intensity MarkerUnalteredLightly alteredHeavily alteredDisseminated Au zoneVein/wall-rock53with unaltered samples and grains aligned in the schistosity).Lone Star Gold OccurrenceMost of the 197 analyses from the Lone Star gold occurrence samples are concordant on the U-Th-Pbconcordia; however, some indicate excess 206Pb by their discordance after the common Pb correction (Fig-ure 3.11). The analyses yielded dates from 189±14 to 116±4 Ma and ThO2 concentrations from 5.33±0.16to 0.13±0.01 wt.% (Figure 3.11). These analyses do not define distinct relationships between date, ThO2concentration, and sample alteration intensity. Like Virgin, the grains that are aligned in the schistosityare generally older whereas the grains adjacent to pyrite are younger, though much overlap exists. UnlikeVirgin, there is no relationship between the date and the form of the grains. Monazites that are aligned inthe schistosity (n = 74) gave dates from 189±8 to 144±8 Ma and ThO2 concentrations from 4.31±0.50 to0.26±0.01 wt.%. Monazites that are adjacent to pyrite (n = 50) gave dates from 178±8 to 116±4 Ma andThO2 concentrations from 3.97±0.18 to 0.14±0.01 wt.%.3.5.6 Monazite Trace Element AnalysesOverall, 90 trace element spot analyses yield a Gd/Yb range from 21 to 315 and Eu/Eu* from 0.31 to1.09 (Figure 3.12). There is a general trend of monazite with high Gd/Yb and low Eu/Eu* to low Gd/Yband high Eu/Eu*. Figure 3.12 summarizes these values by occurrence and displays Gd/Yb vs. Eu/Eu*. Asin Figure 3.11, the marker shape and colour of Figure 3.12 corresponds to the sample alteration intensityand the monazite grain petrographic association, respectively. There are no observed relationships of Gd/Yband Eu/Eu* with other parameters including sample number, grain form, grain form factor, grain area, grainaspect ratio, grain compactness, grain roundness, and geochronology values from Section 3.5.5 including208Pb/232Th date and ThO2 concentration. Monazite trace element data, including the Gd/Yb ratio and Euanomaly (Eu/Eu*), is presented in Appendix J.There is little variation in Gd/Yb and Eu/Eu* between occurrences and monazite grain petrographicassociation. The 20 analyses from Virgin samples yield a Gd/Yb range from 118 to 291 and Eu/Eu* from0.37 to 0.72 (Figure 3.12). Grains with a higher Eu/Eu* correspond to relatively older dates, though thereare few grains with both U-Th-Pb and trace element analyses at Virgin to show this. No analyses weresuccessful in grains aligned with the schistosity. One analysis from a grain adjacent to pyrite yielded aGd/Yb of 278 and Eu/Eu* of 0.39. Trace element analyses were not conducted at Mitchell-Sheba as grainswere too small for the required 19 µm spot size. The 70 analyses at Lone Star yield a Gd/Yb range from 21Figure 3.11 (preceding page): Monazite geochronology results. Each data point represents oneLA-ICP-MS analysis. Thin grey lines represent uncertainty in 2σ . Where no uncertainty lineis visible, the uncertainty is less than the size of the marker. Multiple samples are shown on thesame diagram, and thus the marker shape is dictated by the sample alteration intensity. Markercolour is based on the monazite grain petrographic association. Left: ThO2 vs. date plots. Right:U-Th-Pb concordia diagrams. Top: Results from Virgin. Middle: Results from Mitchell-Sheba.Bottom: Results from Lone Star.540.4 0.6 0.8 1Eu/Eu*050100150200250300Gd/YbVirginn = 200.4 0.6 0.8 1Eu/Eu*050100150200250300Gd/YbLone Starn = 70Adjacent to fractureAdjacent to pyAdjacent to ms-qtzMonazite Grain Petrographic Association ColourAligned in schistositySample Alteration Intensity MarkerUnalteredLightly alteredHeavily alteredDisseminated Au zoneVein/wall-rockFigure 3.12: Monazite trace element results. Ratios are normalized to the primitive mantle using thevalues from Hofmann (1988). Each data point represents one LA-ICP-MS analysis. Multiplesamples are shown on the same diagram, and thus the marker shape is dictated by the samplealteration intensity. Marker colour is based on the monazite grain petrographic association. Left:Results from Virgin. Right: Results from Lone Star.to 315 and Eu/Eu* from 0.31 to 1.09 (Figure 3.12). No relationship was found between Eu/Eu* and date.Monazite at Lone Star aligned in the schistosity (n = 16) gave Gd/Yb from 21 to 227 and Eu/Eu* from 0.37to 1.09, whereas monazite adjacent to pyrite (n = 15) gave Gd/Yb from 43 to 154 and Eu/Eu* from 0.38 to0.99.3.6 DiscussionDating minerals that have poorly understood relationships with gold have hindered previous attemptsto confidently determine the timing of orogenic gold formation in the Klondike. Because monazite does notoccur directly in the quartz veins and intergrown with gold, the same problem arises for the Th-Pb monazitedating attempted herein. However, dates from monazite in the alteration halo of gold-bearing veins can beused to indicate the timing of hydrothermal events during which the veins and gold formed. To distinguishbetween monazite formed by metamorphic or hydrothermal processes and to provide geological significanceto the dates, petrographic analysis, monazite geochronology, and monazite trace element analyses werecombined. The petrographic analysis not only provides a detailed context for the monazite grains, butalso allowed for paragenetic, textural, and microstructural summaries of the veins in Section 3.6.1. InSection 3.6.2, petrography and monazite dates are integrated to create a set of assumptions that are usedto apply geological significance to the geochronological analyses. In Section 3.6.3, monazite trace element55analyses are interpreted to provide insight into the metamorphic and hydrothermal conditions under whichthe grains formed. This information is integrated to make inferences about the Klondike in Section 3.6.4and monazite growth in Section 3.6.5.3.6.1 Vein Formation and Overprinting DeformationQuartz veins in the Klondike were previously interpreted to form in two distinct generations: early,barren segregation veins and late, gold-bearing discordant veins that were purely brittle (MacKenzie et al.,2008c). This study investigated the gold-bearing veins, and new observations provide a paragenetic sum-mary in Figure 3.13, evidence of protracted vein formation in the brittle-ductile transition zone, and evidenceof ongoing ductile deformation after vein emplacement.Quartz vein textures indicate at least two stages of formation. The first is indicated by epitaxial quartzgrowth near vein boundaries (Qtz2; Figure 3.5d and Figure 3.6c). Vein boundary relationships (e.g., Qtz2grains approximately equal in width to Qtz1 grains and muscovite continuing into the vein as inclusiontrails) suggest the wall-rock acted as a crystallographic template for vein materials. At Virgin, Qtz2 areelongate-blocky, subhedral grains that display growth competition and are interpreted to have grown syn-taxially from the wall-rock during at least one crack-seal event (Figure 3.5d). At Mitchell-Sheba, Qtz2 arestretched grains without growth competition, suggesting ataxial growth during multiple crack-seal events(Figure 3.6c). Though subtle differences exist because of the vein opening width, both the syntaxial andataxial quartz show epitaxial growth from the wall-rock and are interpreted as the same initial generation ofepitaxial vein quartz (Qtz2). This generation is absent at Lone Star, possibly because the vein opening waswider and blocky quartz precipitated, or because it has been overprinted by recrystallization. The secondstage of vein formation is indicated by blocky quartz grains that compose most of the veins (Qtz3; Fig-ure 3.5d and Figure 3.6c). The blocky texture suggests at least one additional crack-seal event, where thedilation rate exceeded crystal growth, allowing quartz growth into free space (Hilgers et al., 2001). AlthoughKlondike veins were previously thought to have formed in a single-stage (MacKenzie et al., 2008c), veintextures here display evidence of multiple crack-seal events, in agreement with Wolff (2012). The earlyvein formation as narrow epitaxial growths before wider open space filling suggests deformation conditionsbecoming increasingly brittle over time.Varying amounts of overprinting deformation presented in Section 3.5.1 suggests the veins formedduring a sequence of progressive brittle-ductile deformation, rather than as late, purely brittle features. Veinorientations ranging from sub-concordant to discordant to the wall-rock schistosity as well as macro- andmicro-structural evidence suggest at least some level of ductile deformational overprint and local dynamicrecrystallization on all veins (e.g., Figure 3.5d and Figure 3.6c). This ductile deformation represents theprogressive D3/D4 events because D3 crenulation and D4 kink banding are locally emphasized adjacentto veins where pervasive mica alteration has occurred (Figure 3.8). Late in the deformation sequence,carbonate veinlets filled brittle fractures in quartz veins (Figure 3.6b) but rarely propagated into the host-rock, indicating that competent quartz behaved brittlely whereas micaceous wall-rock accommodated strainin a ductile manner.56Ductile SchistosityDuctile Crenulation\KinksBrittle Kinks\FaultsQuartzAlbiteK-FeldsparCarbonateMuscoviteRutileMonazitePyriteGoldGalenaTimeDeformationMineral PhasesPre-Vein Emplacement Vein Emplacement Post-Vein EmplacementBaryteChloriteChalcopyriteQtz1Cb1Qtz2 Qtz3Qtz4Qtz5Cb3Cb2Ms1 Ms2Chl1 Chl2Mnz1Mnz2Figure 3.13: Simplified paragenesis chart for the Klondike gold occurrences based on petrographicand CL analysis of 37 polished thin sections from the Virgin, Mitchell-Sheba, and Lone Star goldoccurrences. Red represents phase formation, whereas blue indicates breakdown by oxidationor weathering. Chl1: metamorphic/recrystallized; Chl2: host alteration; Ms1: metamorphic/re-crystallized; Ms2: host alteration; Qtz1: metamorphic/recrystallized; Qtz2: syntaxial vein filling;Qtz3: blocky vein filling; Qtz4: host alteration; Qtz5: dynamic recrystallization of pre-existingvein quartz; Cb1: metamorphic/recrystallized; Cb2: infrequent vein filling/host alteration; Cb3:fracture filling; Mnz1: metamorphic/recrystallized; Mnz2: host alteration. Metamorphic mineralsare solid then dashed because most of the mineral growth is interpreted to occur during schistos-ity formation, but some may be related to the crenulation forming event. Metamorphic carbonate(Cb1) is not dashed because it locally replaces pre-existing, metamorphic chlorite (Chl1; Fig-ure 3.7c). Baryte, galena, and chalcopyrite begin dashed because they can be included in quartzand pyrite but are typically late in brittle fractures. Brittle fracturing likely continued during ex-humation and weathering and is dashed.Vein quartz investigation by CL indicates dark, nearly homogeneous responses that yielded little ad-ditional textural information beyond what was gathered by the polarizing microscope (see Appendix D forall CL images). Oscillatory zonation is locally visible in CL in Qtz2 but is typically absent between thedark, homogeneous features of Qtz3 and recrystallized Qtz5. These dark and homogeneous CL features are57interpreted as due to quartz deformation and recrystallization. According to Rusk (2012), homogeneous/-mottled CL textures are typical of orogenic quartz veins. Moreover, quartz recrystallization is reflected byhomogeneous/mottled CL textures and low contrast (Rusk, 2012). It is difficult to infer whether the quartzinitially precipitated with little or no CL texture or whether a previously existing CL texture was overprintedduring ductile deformation and dynamic recrystallization.The exact timing of gold precipitation relative to vein formation and deformation remains ambiguous asveins overprinted by various amounts of deformation are locally gold-bearing. Mortensen (in review) notesthat gold commonly occurs in pyritic selvages along vein margins and suggests that a substantial portionof gold in the Klondike was introduced during early vein formation stages. Conversely, Grimshaw (2018)notes a late, brittle deformation stage of gold precipitation at Lone Star, suggesting that gold was locallyintroduced late in the deformation sequence. This evidence suggests that the timing of gold precipitationmay span the timing of vein formation or vary locally by occurrence.3.6.2 Interpretation of Monazite DatesMonazite dates from 189±14 to 116±4 Ma represent a wide range that are complemented with petro-graphic analysis and additional elemental information to resolve geologically meaningful events. As pre-sented in Section 3.5.5, two distinct ThO2 vs. date populations emerge from the analyses when all availableinformation is integrated: 1) analyses >2.00 wt.% ThO2 are relatively older and generally correspond tounaltered samples and grains aligned in the schistosity; and 2) analyses <2.00 wt.% ThO2 are relativelyyounger and generally correspond with altered samples and grains adjacent to pyrite. These populations areevident at the Virgin and Mitchell-Sheba occurrences but are absent at Lone Star.Based on regional geological information in Section 3.3, monazite growth processes outlined in Sec-tion 3.2 and petrographic observations from Section 3.5.1, a set of assumptions are made regarding theinterpretation of monazite age analyses. These include: 1) the analyzed monazite are not igneous becausethe dates are significantly younger than the Klondike assemblage protolith ages of 269 to 253 Ma (Colpronet al., 2006b) and mostly overlap with a regional magmatic gap from 179 to 115 Ma (Allan et al., 2013); 2)monazite in unaltered samples of felsic host rocks are metamorphic because they are assumed to be beyondthe influence of hydrothermal alteration; 3) all monazite at Mitchell-Sheba are assumed to be hydrother-mal because monazite does not occur in the unaltered mafic lithology; 4) monazite that occurs adjacent topyrite at Virgin and Mitchell-Sheba are assumed to be hydrothermal because the pyrite is hydrothermal andoverprints the schistosity (see Section 3.5.1); and 5) monazite aligned in the schistosity is not necessarilymetamorphic because hydrothermal monazite may preferentially form along these planar features.New geochemical data discriminate bases for four additional assumptions. First, hydrothermal monaziteis assumed to have<2.00 wt.% ThO2 concentration because none of the grains at Virgin and Mitchell-Shebathat are adjacent to pyrite have >2.00 wt.% (Figure 3.11). Therefore, although monazite from unalteredsamples has variable ThO2 concentrations that may be <2.00 wt.%, all monazite >2.00 wt.% at Virgin areassumed to be metamorphic. Second, all analyses at Virgin below the youngest date of 161 Ma or lowestThO2 concentration of 1.32 wt.% from unaltered samples (Figure 3.11) are assumed to be hydrothermal.58The results affected by this assumption are all within the range of analyses interpreted as hydrothermal byprevious assumptions. Third, analyses from grains adjacent to muscovite-quartz or fractures lack relation-ships with sample alteration intensity, ThO2 concentration, or age (Figure 3.11). Therefore, these monazitesmay have grown by a mixed process, have an unknown provenance, and are not ascribed a metamorphicor hydrothermal origin unless a previous assumption constrained them. Lastly, only monazite from unal-tered samples at Lone Star are interpreted because results from altered samples may have been partially tocompletely reset (further discussed in Section 3.6.4). By applying the assumptions formulated above, Fig-ure 3.14 highlights results for grains constrained to have metamorphic or hydrothermal origin. Appendix Jincludes the interpretations for each analysis.120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)Virginn = 52120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)n = 23Mitchell-Sheba120 140 160 180 20002468208Pb/232Th Date (Ma)ThO2 (wt.%)Lone Starn = 197Adjacent to fractureAdjacent to pyAdjacent to ms-qtzMonazite Grain Petrographic Association ColourAligned in schistositySample Alteration Intensity MarkerUnalteredLightly alteredHeavily alteredDisseminated Au zoneVein/wall-rockMetamorphic HydrothermalMonazite Growth ProcessFigure 3.14: ThO2 vs. date diagrams from Figure 3.11 with metamorphic and hydrothermal growthprocess interpretations. Thin grey lines represent uncertainty in 2σ . Where no uncertainty lineis visible, the uncertainty is less than the size of the marker. Each data point represents oneLA-ICP-MS analysis. Multiple samples are shown on the same diagram, and thus the markershape is dictated by the sample alteration intensity. Marker colour is based on the monazite grainpetrographic association.59Distinct ThO2 vs. date monazite populations are demonstrated in Figure 3.14 that are assumed to repre-sent different monazite growth mechanisms (i.e., metamorphic vs. hydrothermal). To affirm this assumption,two tests were completed to rule out the populations instead being caused by the Th concentration of thewhole rock or different monazite varieties. First, to test whether the distinct monazite populations are con-trolled by the Th concentration of the whole rock, monazite ThO2 concentrations were normalized to wholerock Th (ppm) for two samples from Virgin. One sample is unaltered and hosts monazite with relativelyold dates and high ThO2 concentration, whereas the other sample is heavily altered and hosts monazitewith relatively young dates and low ThO2 (i.e., the first and second populations outlined in Section 3.5.5,respectively). The same two populations emerged after normalization, and thus, they are not an expressionof the Th concentration of the host-rock. Second, to test whether the distinct monazite populations are re-lated to different monazite varieties, relative Ca abundances were assessed to check for the Ca-rich monaziteend-member: brabantite/cheralite. The Ca investigation determined that most analyses have similar Ca con-centrations, which suggests that the distinct monazite populations also do not result from different monazitevarieties. Moreover, Ce values are always at least double that of La, Nd, and Sm, which suggests these mon-azites are all the monazite-Ce variety. The results of these tests confirm that different growth mechanismsresulted in the distinct monazite populations.3.6.3 Interpretation of Monazite Trace Element DataFew trace element values and ratios present discernible relationships between locality, sample alter-ation intensity, or monazite grain petrographic association (Figure 3.12). The lack of relationships can beexplained by the reactions that control trace elements in monazite occurring on a local grain scale ratherthan a whole rock equilibrium scale, which is evidenced by the broad range in the monazite Gd/Yb andEu/Eu* values (Gd/Yb from 21 to 315 and Eu/Eu* from 0.31 to 1.09). The broad ranges suggest that min-erals adjacent to the analyzed monazite controlled the amount of REE that became incorporated into it. Forexample, plagioclase feldspar is well known to sequester Eu2+ (Weill and Drake, 1973), and monazite with alow Eu/Eu* likely grew coeval with and adjacent to plagioclase whereas monazite with a high Eu/Eu* likelygrew away from plagioclase.Hydrothermal monazite is expected to have little to no Eu anomaly because hydrothermal systemsdo not involve plagioclase precipitation (Table 3.1; Taylor, 2015). Although the trace element data lacksrelationships between Eu/Eu* and sample alteration intensity or monazite grain petrographic association,REE geochemistry can help to distinguish between metamorphic and hydrothermal monazite. Figure 3.15presents the Eu/Eu* values of all Klondike analyses, and a clear separation exists at ∼0.80. Below thisseparation are analyses with strongly negative Eu anomalies, many of which are from grains aligned in theschistosity, consistent with metamorphic growth. Those below ∼0.80 Eu/Eu* that are from grains adjacentto pyrite may be related to the first generation of pyrite at Lone Star and can be interpreted as metamorphicas well. Values that are above ∼0.80 Eu/Eu* have weakly negative or lack Eu anomalies, many of whichare from grains adjacent to pyrite that can be interpreted as hydrothermal. Those above ∼0.80 Eu/Eu* thatare from grains aligned in the schistosity have either grown metamorphically without adjacent plagioclaseor precipitated hydrothermally along the fabric plane. Therefore, analyses below the ∼0.80 Eu/Eu* sepa-60ration are interpreted as metamorphic, whereas analyses above it are mainly interpreted as hydrothermal.Although still useable, the REE geochemistry is less reliable as a monazite indicator in cases like this whereno clearly defined vein-hosted monazite exists. Where both trace element and geochronology data werecollected from the same monazite grain, the Eu/Eu* and geochronology interpretations for growth mecha-nism typically agree with each other, but the geochronology interpretation is always used because it relieson more parameters and has greater control.0.30.40.50.60.70.80.911.1Eu/Eu*0 10 20 30 40 50 60 70 80 90Analysis NumberKlondikeMetamorphicHydrothermalAdjacent to fractureAdjacent to pyAdjacent to ms-qtzMonazite Grain Petrographic Association ColourAligned in schistositySample Alteration Intensity MarkerUnalteredLightly alteredHeavily alteredDisseminated Au zoneVein/wall-rockFigure 3.15: Summary of all Klondike monazite Eu anomaly values from monazite (normalized to theprimitive mantle using values from Hofmann, 1988) with growth mechanism interpretations. Eachdata point represents one LA-ICP-MS analysis. Multiple samples are shown on the same diagram,and thus the marker shape is dictated by the sample alteration intensity. Marker colour is based onthe monazite grain petrographic association.The Gd/Yb ratio indicates preferential incorporation of heavy rare earth elements (HREE). In moststudies with monazite trace elements, HREE are investigated as a proxy for garnet activity as it incorporatesor releases HREE upon growth or breakdown (e.g., Pyle and Spear, 2003). Garnet is absent in the meta-morphic rocks of the Klondike, so we can assume the Gd/Yb ratio is controlled by apatite activity as it alsopreferentially incorporates HREE during growth (Zirner et al., 2015, and references therein). Thus, monazitedata in Figure 3.12 with relatively high Gd/Yb ratios (>150) may have grown coevally with and adjacentto apatite whereas monazite with relatively low Gd/Yb ratios (<150) likely grew away from apatite. Theoccurrence of apatite adjacent to monazite is consistent with this interpretation. Apatite further complicatesmonazite trace element interpretations because, like plagioclase, it too sequesters Eu2+ (e.g., Hiroshi, 1970;Zirner et al., 2015). Because of this, monazite analyses with 1) high Gd/Yb and low Eu/Eu* (<0.7) likely61resulted from grains that grew coeval with and adjacent to apatite and plagioclase; 2) low Gd/Yb and lowEu/Eu* likely resulted from grains that grew coeval with and adjacent to only plagioclase; 3) low Gd/Yband high Eu/Eu* (>0.7) likely precipitated from hydrothermal fluids without apatite and plagioclase; and4) high Gd/Yb and high Eu/Eu* do not exist and indicate no monazite grew without apatite, plagioclase, orhydrothermal fluids (Figure 3.16).0.4 0.6 0.8 1Eu/Eu*050100150200250300Gd/Yb 12 34Figure 3.16: Monazite trace element data of Figure 3.12 from Virgin and Lone Star are overlain withfour interpretive quadrants that correspond to the four points in the text. These quadrants describethe minerals that monazite may have grew coeval with and adjacent to based on the trace elementdata interpretation. 1) high Gd/Yb and low Eu/Eu* likely resulted from grains that grew coevalwith and adjacent to apatite and plagioclase; 2) low Gd/Yb and low Eu/Eu* likely resulted fromgrains that grew coeval with and adjacent to only plagioclase; 3) low Gd/Yb and high Eu/Eu* likelyprecipitated from hydrothermal fluids without apatite and plagioclase; and 4) high Gd/Yb and highEu/Eu* do not exist and indicate no monazite grew without apatite, plagioclase, or hydrothermalfluids. The exact locations of the quadrants are arbitrary.3.6.4 Implications for the Klondike Gold DistrictEvidence for Reset Dates at the Lone Star Gold OccurrenceThe distinct metamorphic and hydrothermal monazite populations at Virgin and Mitchell-Sheba are ab-sent at Lone Star (Figure 3.14), suggesting different geological controls on monazite origin. At Virgin, anal-yses with young dates and low ThO2 concentrations are assumed to be hydrothermal because they directly62overlap with analyses interpreted as hydrothermal based on petrographic associations adjacent to pyrite. AtLone Star, however, none of the analyses are interpreted as hydrothermal because the pyrite there may haveexisted before hydrothermal activity. Analyses at Lone Star with relatively young dates (<151 Ma) havehighly variable ThO2 concentrations that can be relatively high (>2.00 wt.%). These young analyses withhigh ThO2 concentrations suggest that there may have been partial to complete resetting of metamorphicdates by subsequent hydrothermal fluids such that the monazites maintain the ThO2 concentration of thepre-existing, metamorphic monazite. Moreover, because the ThO2 concentrations from unaltered samplesat Lone Star are relatively low (0.13 to 2.26 wt.%), it is unknown if relatively old dates with high ThO2are metamorphic or partially reset dates, and if analyses with relatively young dates and low ThO2 arehydrothermal or partially reset dates. This evidence at Lone Star for partial to complete resetting of meta-morphic monazite dates in the alteration halo of quartz veins suggests that only monazite from unalteredsamples can be used in age interpretations.Resetting dates at Lone Star may happen because of the scale of chemical equilibrium between the flu-ids and wall-rock may be smaller than at Virgin (i.e., grain scale vs. rock scale), which may be due to lowerpermeability. Because the fluid is not pervasive through the entire rock, local dissolution-reprecipitationreactions may have inherited the ThO2 concentration of the former monazite but reset its date. Fluid immo-bility of Th and mobility of Pb under metamorphic conditions (Schandl and Gorton, 2004; Cuney and Kyser,2015) result in partially reset monazite that appears to form by a mixed metamorphic-hydrothermal process.At Virgin, any dissolution-reprecipitation reactions may have occurred with a broader chemical equilibrium,which resulted in Th also partitioning into the fluid to achieve complete chemical resetting with lower ThO2and younger dates.Meaningful information can be inferred from reset monazite dates at Lone Star. The analyses withrelatively young dates and high ThO2 may represent one of three options: 1) unusually young metamorphicmonazite; 2) hydrothermal monazite with unusually high ThO2 concentration; or 3) partially to completelyreset metamorphic monazite. The first option is unlikely because the interpreted metamorphic monazitefrom Virgin and Lone Star are relatively old (>151 Ma). The second option is also unlikely because theinterpreted hydrothermal monazite at Virgin and Mitchell-Sheba have relatively low ThO2 concentrations(<2.00 wt.%). Therefore, the most plausible explanation is that the Lone Star analyses with relatively youngdates and high ThO2 represent metamorphic monazite that have been partially to completely reset. Monazitewith relatively old dates and high ThO2 are interpreted to have at least initially been metamorphic becauseno interpreted hydrothermal monazite exists with high ThO2 concentration. Although it is unknown whenexactly hydrothermal activity began at Lone Star, we can assume that the ages obtained from Virgin andMitchell-Sheba apply here because of the similar ages of possibly reset dates. The youngest date of 116±4Ma constrains the youngest possible age of hydrothermal activity at Lone Star.The Disseminated Gold Unit at the Lone Star Gold OccurrenceThe disseminated gold-bearing unit in the Lone Star area represents a more widespread and economi-cally viable exploration target than the gold-bearing quartz veins that are of limited areal extent (Mortensen63et al., 2019). The gold in this unit was initially proposed to be syngenetic in origin, possibly deriving fromvolcanogenic massive sulphide (VMS) occurrences in the area (Mortensen et al., 2006b; MacKenzie et al.,2008b). Grimshaw (2018) suggested the source of disseminated gold is instead a distal alteration from thequartz veins, linking the two to the same mineralizing event. However, Mortensen (in review) disputesan epigenetic origin and argues for the original interpretation of Mortensen et al. (2006b) that the gold isgenetically linked to VMS occurrences and is syngenetic. One sample from the disseminated gold unitwas investigated during this study, which yielded an age range from 149±4 to 116±4 Ma and ThO2 from4.42±0.28 to 0.62±0.03 wt.%.Although monazite investigation of the disseminated gold unit does not constrain the source of the gold,it provides evidence for hydrothermal fluid activity. Most monazite grains in this unit occur adjacent to pyriteor fractures, and no grains are aligned with the metamorphic fabric. If the disseminated gold unit is trulysyngenetic, monazite dates from this unit should overlap with metamorphic monazite ages from unalteredrocks at Lone Star. However, monazite dates from this unit overlap with interpreted monazite resetting datesat Lone Star and suggest they represent resetting from hydrothermal fluids. These dates provide evidencefor vein-related hydrothermal fluid activity and alteration within the disseminated gold unit. Highly variableThO2 concentrations and Eu/Eu* values are also consistent with hydrothermal resetting of metamorphicmonazite.The above findings do not necessarily constrain the origin of the gold but provide evidence that rockscontaining disseminated gold at least interacted with hydrothermal fluids at the time of vein formation.Therefore, the new monazite data supports the possibility of epigenetic gold but does not exclude syngeneticgold. Combining this with the information from Grimshaw (2018) and Mortensen et al. (2019), the gold ofthe disseminated gold unit may have derived from a syngenetic source but was locally remobilized on aunit-wide scale by late hydrothermal fluids. Because this study focusses on gold-bearing quartz veins andonly one sample of the disseminated gold unit was investigated, more work is recommended to confirmthese findings.Age of Metamorphic and Hydrothermal MonaziteMonazite dates are given geological significance by applying the assumptions formulated in Sec-tion 3.6.2 and avoiding reset dates at Lone Star. Figure 3.17 presents a summary of all monazite dates andgrowth mechanism interpretations from the Virgin, Mitchell-Sheba, and Lone Star gold occurrences. Thewide range of both metamorphic and hydrothermal ages suggests protracted monazite growth throughoutthe Klondike. Uncertainty on all individual ages averages 4%.The range of metamorphic monazite ages spans 39 m.y., from 189 to 151 Ma. This range includes anal-yses from Virgin and Lone Star, which have similarly distributed individual ranges, although the youngestmetamorphic age at Virgin is 161 Ma. The range of hydrothermal monazite ages overlaps with the metamor-phic monazite ages by 28 m.y. and spans 62 m.y., from 178 to 117 Ma. The hydrothermal range includesanalyses from Virgin and Mitchell-Sheba, which have similarly distributed individual ranges. Field evi-dence for protracted vein formation over 62 m.y. are variably deformed veins. Some veins formed early in64the deformation sequence and are more strained, whereas others formed later and are less strained.The protracted metamorphic and hydrothermal age ranges may arise from multiple distinct age popula-tions that overlap and cross-contaminate in time through analytical uncertainties. Probability density plotspresented in Figure 3.18 indicate that two distinct age components contribute to the metamorphic monaziteage range, whereas three components contribute to the hydrothermal range. Assuming two and three agecomponents, approximate weighted average ages without meaningful uncertainties for each of these distinctpopulations were resolved with the unmix ages function of Isoplot (Sambridge and Compston, 1994). Twodistinct metamorphic monazite-forming reactions occur at approximately 175 and 160 Ma. Whereas the firstpeak at 175 Ma is evident at both Virgin and Lone Star, the youngest metamorphic age of 161 Ma at Virginsuggests the second peak at 160 Ma does not occur there. Thus, the first metamorphic peak at 175 Ma likelyapplies to the Klondike as a whole, but it is unknown if the second peak at 160 Ma is restricted to LoneStar or if it is absent only at Virgin. Three distinct hydrothermal monazite-forming reactions occur at ap-proximately 169, 148, and 128 Ma. These hydrothermal pulses suggest that vein formation in the Klondikewas episodic. Each one of these episodes is evident at Virgin, but are more subdued at Mitchell-Sheba.Although the exact timing of vein formation at Lone Star is unknown, these pulses may also apply thereand throughout the Klondike. The same three hydrothermal pulses are not evident in previously published40Ar/39Ar hydrothermal mica ages, possibly due to an inadequate amount of available data. There are sev-eral hydrothermal mica ages between 140 and 150 Ma, however (Figure 3.19), which overlaps with the 148Ma hydrothermal monazite pulse.650 50 100 150 200 250Analysis Number110120130140150160170180190200210208Pb/232Th Date (Ma)Metamorphic189 - 151 MaHydrothermal178 - 117 MaNumber of analyses:Lone Star = 197Virgin = 52Mitchell-Sheba = 23MetamorphicHydrothermalPossibly ResetUnknownFigure 3.17: Summary of all monazite dates in the Klondike and overlain metamorphic and hydrothermal age ranges. Thin grey lines rep-resent uncertainty in 2σ . Each data point represents one LA-ICP-MS analysis. Marker colour is based on monazite growth mechanisminterpretation.66051015110 120 130 140 150 160 170 180 190 200Frequency208Pb/232Th Age148128169Hydrothermaln = 51051015Frequency175160Metamorphicn = 73Figure 3.18: Probability density plots for metamorphic and hydrothermal monazite ages in theKlondike. The histograms display the age data. The curved black lines are the relative proba-bility of ages and display two distinct metamorphic populations and three distinct hydrothermalpopulations. The vertical black lines represent the unmixed approximate weighted mean ages ofthe distinct populations.67Relationship Between Exhumation and Monazite GrowthThe monazite ages presented herein overlap with previously published 40Ar/39Ar mica ages interpretedto date coeval regional cooling, exhumation, and hydrothermal activity in the Klondike (Figure 3.19; Bre-itsprecher and Mortensen, 2004; Joyce et al., 2015, unpublished MDRU data). Prograde metamorphicmonazite may not have formed because no monazite occurs that overlaps with the known prograde meta-morphism periods of 260 to 252.5 Ma (M2) or 195 to 187 Ma (M3; Berman et al., 2007). Instead, themetamorphic monazite overlaps in time with metamorphic cooling mica ages and is therefore interpretedas retrograde monazite growth during rehydration, cooling, and exhumation. Infiltration of metamorphicfluids during retrograde conditions may cause monazite to form by the breakdown of allanite, apatite, orpre-existing monazite (Ayers et al., 2002; Bollinger and Janots, 2006; Finger and Krenn, 2007; Johnsonet al., 2015). Thus, retrograde monazite may form with or without the presence of pre-existing monazitegrains. Hydrothermal monazite ages overlap with hydrothermal mica ages, suggesting that vein formationbegan shortly after exhumation. The hydrothermal ages extend younger than the metamorphic cooling ages,suggesting that vein formation continued after cooling through the closure temperature of mica. Vein forma-tion continuing after mica cooling is further supported by vein-forming fluid inclusion trapping temperaturesof 200 to 350 ◦C (Rushton et al., 1993), which is below the closure temperature for mica (e.g., ∼380◦C formuscovite; Reiners and Brandon, 2006).Comparing the Klondike to Other Phanerozoic Orogenic Gold SystemsThe Klondike gold district shares many characteristics with other Phanerozoic orogenic gold systems(e.g., criterion listed in Bierlein and Crowe, 2000). The Meguma and Cariboo orogenic systems are com-pared to the Klondike below because they form during progressive brittle-ductile deformation during simi-larly protracted periods.The Meguma terrane in Nova Scotia has numerous vein-hosted gold deposits that are interpreted witha multi-stage model in which the mineralizing fluids may be from both magmatic and metamorphic sources(Sangster and Smith, 2007; Chen, 2015). Re-Os dating of arsenopyrite in Meguma deposits indicatesepisodic mineralization from 460 to 380 Ma, which began before Acadian deformation and metamorphismat 410 to 400 Ma (Chen, 2015). The pre-deformation ages and 80 m.y. of episodic mineralization suggestmagmatic fluid contributed to the system and the orogenic deposit model only partly fits these deposits.Unlike Meguma, no evidence exists for a magmatic input of fluids in the Klondike (Allan et al., 2013), butmultiple metamorphic fluid sources may explain 62 m.y. of episodic vein formation from 178 to 117 Ma.Staples et al. (2016) suggest that contemporaneous amphibolite-facies metamorphism and devolatilizationof the underlying Finlayson Lake domain from 169 to 142 Ma (Staples, 2014) may be the source of thevein-forming fluids in the overlying Klondike assemblage. Another source of fluids could be similar meta-morphism of the underlying Australia Mountain domain from 146 to 118 Ma (Staples et al., 2013). Vryet al. (2010) suggest that vein-forming fluids can be generated during exhumation, which implies that thefluids may also have derived from Klondike assemblage rocks during exhumation from ∼185 to ∼140 Ma(Breitsprecher and Mortensen, 2004; Joyce et al., 2015, unpublished MDRU data). Therefore, there are three68possible explanations as a fluid source for the orogenic quartz veins in the Klondike: 1) prograde metamor-phism of rocks in the Finlayson Lake domain; 2) prograde metamorphism of rocks in the Australia Mountaindomain; and 3) exhumation of the Klondike assemblage (Figure 3.20). A combination of them explains the62 m.y. of protracted vein formation in the Klondike, which is substantially less time than mineralization atMeguma. The overlapping timing of these events with the pulses of vein-forming hydrothermal fluids sug-gests that Klondike exhumation may be responsible for the first pulse, Klondike exhumation and FinlaysonLake domain metamorphism for the second pulse, and Australia Mountain domain metamorphism for thethird pulse (Figure 3.20).The Cariboo gold district in east-central British Columbia is an orogenic gold district with protractedvein formation from 155 to 134 Ma during progressive ductile to brittle deformation (Rhys et al., 2009;Mortensen et al., 2011; Allan et al., 2017). Veins that formed early in the deformation sequence are onlyweakly mineralized and are overprinted by ductile deformation, whereas veins that formed later have ore-grade gold values and are only weakly strained to unstrained (Allan et al., 2017). Although the veins formedover 21 m.y., mineralization is interpreted to have lasted 15 m.y., from 149 to 134 Ma (Rhys et al., 2009;Mortensen et al., 2011; Allan et al., 2017). Many similarities exist between the Cariboo and Klondikedistricts, including their overlapping age of formation. Veins in both districts show varying levels of de-formation, which indicates protracted vein formation continued as deformation became increasingly brittle.Although the absolute timing of mineralization is unknown in the Klondike and evidence exists for goldprecipitation at both an early and late stage of vein formation, Grimshaw (2018) demonstrated that gold isintroduced during a late stage of vein filling at Lone Star. Therefore, both the Cariboo and Klondike veinsformed over a protracted period during progressive brittle-ductile deformation in which gold was at least lo-cally added late to the hydrothermal system. The Klondike veins appear to have formed over a substantiallylonger period than at Cariboo, which is attributed to the possibility of multiple sources of fluid (Figure 3.20).69Yukon River174165173170165166160175173143172174171 165175 166158183183180176169 172170173165169173173179177141181138152162148138°30'0"W138°30'0"W139°0'0"W139°0'0"W139°30'0"W64°0'0"N64°0'0"N63°45'0"NTintina FaultDawson CityVirginLone StarMitchell-Sheba0 5 102.5KilometersBedrock Gold OccurrencesVirginMetmorphic mnz: 185 - 161 MaHydrothermal mnz: 178 - 117 MaMitchell-ShebaHydrothermal mnz: 175 - 126 MaLone StarMetmorphic mnz: 189 - 151 MaMica Ages (Ma)MetamorphicHydrothermal0.01  0.05  0.1   0.25  0.5   0.75  0.9   0.95  0.99Probability110120130140150160170180190Age (Ma)Metamorphic MicaHydrothermal MicaMetamorphic MonaziteHydrothermal MonaziteFigure 3.19: Comparison of Klondike metamorphic and hydrothermal monazite and mica ages. Top:Split probability plot summarizing the previously published and new metamorphic and hydrother-mal ages in the Klondike. 208Pb/232Th monazite ages presented herein overlap with previouslypublished 40Ar/39Ar mica ages. No uncertainty in age is shown for simplicity. Mica ages arefrom Breitsprecher and Mortensen (2004); Joyce et al. (2015); and unpublished MDRU data. Allmonazite ages are from this study. Bottom: Geological map of the Klondike summarizing the lo-cations of the ages from the top figure. Monazite age ranges are summarized in the legend. Somemica ages mask others. Legend for geology corresponds to Figure 3.2.7001020304050INCONNUTHRUSTSELWYN BASINSLIDE MOUNTAINCONTINENTALLITHOSPHEREDuctile thrustingAUSTRALIA MOUNTAINFINLAYSONExtensionkmPermo-Triassic and Early Jurassicmetamorphic domainLATE JURASSIC - EARLY CRETACEOUSMetamorphic fluidOrogenic gold mineralization120160100140180200JurassicCretaceousPer Age(Ma)MonaziteFormingEventsGeological EventsPrograde MetamorphismExhumationKlondike Vein Formation Rangeand PulsesKlondike Retrograde MetamorphismRange and PulsesM3KlondikeFinlayson AustraliaFigure 3.20: Geological events that cause retrograde metamorphism and orogenic, gold-bearing quartzvein formation in the Klondike. Top: Possible hydrothermal fluid sources for the Klondike veins(from Staples et al., 2016). Finlayson Lake and Australia Mountain domains are underthrustedto 25 to 30 km depth in the Late Jurassic to Early Cretaceous, causing overlying rocks to be ex-humed with compensating extension above. The Finlayson Lake and Australia Mountain domainsare metamorphosed, devolatilized, and are two possible sources of hydrothermal fluid that lead toorogenic quartz vein formation. Devolatilization and a third possible fluid source also occur in theoverlying Klondike assemblage rocks where vein formation and gold mineralization occur. Bot-tom: Temporal summary of the possible geological events that cause monazite-defined retrogrademetamorphism and vein formation in the Klondike. Prograde metamorphism in the Klondike (M3)is not temporally linked to monazite-forming events. Klondike exhumation is likely responsiblefor both peaks of retrograde metamorphism. Klondike exhumation may be responsible for thefirst pulse of vein-forming hydrothermal fluids, Klondike exhumation and Finlayson Lake domainmetamorphism for the second pulse, and Australia Mountain domain metamorphism for the thirdpulse.713.6.5 Implications for Monazite GrowthHost-Rock Control on ThO2 in Metamorphic MonaziteMetamorphic monazite at Lone Star generally has lower ThO2 concentration than at Virgin becauseof differences in their respective host-rocks. The felsic rock at Virgin is higher in incompatible elements,including Th, which allows for more available Th to partition into monazite. The intermediate rock atLone Star is lower in incompatible elements, including Th, which allows for less available Th to partitioninto monazite. Figure 3.21 presents whole rock lithogeochemical data from the three occurrences and usesZr/TiO2 as a measurement from mafic (low) to felsic (high). The mafic host-rock at Mitchell-Sheba has lowTh and Ce, which corresponds to no metamorphic monazite, whereas the felsic host-rock at Virgin has highTh and Ce, which corresponds to metamorphic monazite with high ThO2 concentration. The intermediatehost-rock at Lone Star has moderate Th and Ce, which corresponds to metamorphic monazite with low ThO2concentration. Moreover, the felsic host-rocks would likely have had abundant primary apatite to serve as aprecursor to metamorphic monazite, whereas apatite would likely have been absent in the mafic host-rocks(Overstreet, 1967). Therefore, not only does the variable ThO2 concentration of metamorphic monaziteincrease with increasing grade (Schandl and Gorton, 2004), but it also depends on the composition of thehost-rock. Under the greenschist metamorphic conditions of the Klondike, monazite develops with 0.13–7.38 wt.% ThO2, but is typically< 5 wt.% (Figure 3.14). Rocks with a mafic protolith produce no monaziteunder these conditions, whereas rocks with an intermediate protolith produce monazite with 0.13–2.26 wt.%ThO2 and rocks with a felsic protolith produce monazite with 1.32–7.38 wt.% ThO2.Zr/TiO2Th (ppm)VirginMitchell-ShebaNugget/Lone Star0 - 18.518.5 - 5555 - 6969 - 76.176.1 - 103.6Locality Ce (ppm)Figure 3.21: Whole rock lithogeochemical data suggesting the host-rock composition controls themetamorphic monazite composition. Zr/TiO2 is treated as a measurement from mafic (low) tofelsic (high). More felsic host-rocks have increasing Th and Ce concentrations, which translatesto metamorphic monazite growth with higher ThO2 concentration. Mafic rocks have no metamor-phic monazite. Thus, the host-rock composition controls the ThO2 concentration in metamorphicmonazite.723.7 ConclusionsResults and interpretations from the above research provide three main contributions including 1) in-sight into the vein and alteration evolution of the Klondike; 2) a protocol to distinguish between metamor-phic and hydrothermal monazite in orogenic gold settings and fluid-affected metamorphic terranes; and 3)a temporal framework to understand retrograde metamorphism and orogenic quartz vein formation in theKlondike. Sample context from detailed petrographic analyses were integrated with LA-ICP-MS Th-Pbmonazite dates and trace element geochemistry to identify the different monazite growth mechanisms andconstrain the timing of vein formation.A summary of the gold-bearing quartz veins and associated alteration in the Klondike is presentedin Figure 3.13. The veins consist mainly of quartz (>98%) with minor pyrite (<2%). Wall-rock alter-ation from the vein-forming hydrothermal fluids varies with the local host-rock composition and includeskaolinite-quartz-K-feldspar-pyrite alteration in the quartz-augen schist sub-unit, muscovite-carbonate-pyritealteration in the mafic schist sub-unit, and quartz-pyrite alteration in the felsic schist sub-unit. The veinsformed in at least two stages over a progression from brittle-ductile-dominated to brittle-dominated defor-mation. Evidence for the first formation stage is the epitaxial quartz growth near vein boundaries, whereasthe second stage is indicated by blocky quartz that occupies most of the veins. Evidence for prolongedvein formation over the brittle-ductile transition includes post-vein deformation features such as subgrainrotation, recrystallization, and various vein orientations relative to the wall-rock fabric.Petrographic analysis and petrochronology are combined to distinguish between metamorphic and hy-drothermal monazite in orogenic gold settings and fluid-affected metamorphic terranes. Metamorphic mon-azite can form anywhere in a metamorphic rock, is typically adjacent to or intergrown with other metamor-phic minerals, and may be aligned with a metamorphic fabric. They have a variable ThO2 concentration(∼0–8 wt.%) that depends on the host-rock composition and metamorphic grade. If grown coeval with andadjacent to plagioclase, metamorphic monazite will have a negative Eu anomaly. In contrast, hydrothermalmonazite can precipitate or modify pre-existing grains in and adjacent to veins, is typically adjacent to orintergrown with other hydrothermal minerals, and may also be aligned in a metamorphic fabric. It has adistinctly low ThO2 concentration <2.00 wt.% and does not have a Eu anomaly. Although useful for deter-mining growth zonation patterns, grain-scale element maps do not help distinguish between metamorphicand hydrothermal monazite because both have similar patchy or mottled Th zonation styles. Monazites thatlack relationships with petrographic association, Th-Pb date, and ThO2 concentration may represent partialto complete resetting of pre-existing monazite by hydrothermal fluids.The data indicate that retrograde metamorphic monazite in the Klondike formed over a protracted pe-riod of 39 m.y., from 189 to 151 Ma. This period overlaps with existing 40Ar/39Ar mica cooling agesand exhumation from 185 to 140 Ma (Breitsprecher and Mortensen, 2004; Joyce et al., 2015, unpublishedMDRU data). Retrograde monazite and crustal exhumation may have occurred in discrete peaks at approxi-mately 175 and 160 Ma, but the youngest peak was not recorded at Virgin. Gold-bearing veins formed overa protracted period of 62 m.y., from 178 to 117 Ma. This prolonged range may arise from three distinct73hydrothermal pulses at approximately 169, 148, and 128 Ma. Those hydrothermal pulses may derive frommultiple geological events including the Klondike exhumation (185–140 Ma; Breitsprecher and Mortensen,2004; Joyce et al., 2015, unpublished MDRU data), Finlayson Lake domain metamorphism (169–142 Ma;Staples, 2014), and Australia Mountain domain metamorphism (146–118 Ma; Staples et al., 2013). Thesevein formation ages constrain the approximate timing of gold mineralization, but the exact timing is un-known because the samples examined have ambiguous relationships between monazite and gold.74Chapter 4Conclusions4.1 General ConclusionsResults and interpretations from the research presented in this thesis provide three main contributions in-cluding 1) insight into the vein and alteration evolution of the Klondike; 2) a protocol to distinguish betweenmetamorphic and hydrothermal monazite in orogenic gold settings and fluid-affected metamorphic terranes;and 3) a temporal framework to understand retrograde metamorphism and orogenic quartz vein formationin the Klondike. Sample context from detailed petrographic analyses were integrated with LA-ICP-MS Th-Pb monazite dates and trace element geochemistry to identify the different monazite growth mechanismsand constrain the timing of vein formation. Conclusions made from the new results and interpretations aresummarized below and are organized by the thesis objectives and sub-questions outlined in Chapter 1.Objective 1 – Establish the gold-bearing vein and hydrothermal alteration evolution of the Klondike1a) What is the vein and alteration paragenesis in the Klondike?A summary of the gold-bearing quartz veins and associated alteration in the Klondike is presentedin Figure 3.13. The veins consist mainly of quartz (>98%), minor pyrite (<2%), and trace phasesincluding carbonate, muscovite, chlorite, baryte, rutile, galena, chalcopyrite, and gold. Wall-rockalteration from the vein-forming hydrothermal fluids varies with the local host-rock composition andincludes kaolinite-quartz-K-feldspar-pyrite alteration in the quartz-augen schist sub-unit, muscovite-carbonate-pyrite alteration in the mafic schist sub-unit, and quartz-pyrite alteration in the felsic schistsub-unit.1b) Did the veins form in a single-stage filling event or by multiple crack-seal events?The veins formed in at least two stages, both of which precipitated the vein and alteration mineralslisted above. Evidence for the first stage is the epitaxial quartz growth near vein boundaries, whichoccurred through multiple crack-seal events in the brittle-ductile realm. The second stage is indicated75by blocky quartz that occupies most of the veins and formed with at least one crack-seal event thatwas dominantly brittle.1c) Has ductile deformation overprinted the veins?The two stages of vein formation occurred over a progression from brittle-ductile-dominated to brittle-dominated deformation and the veins are not all late, purely brittle features as previously interpretedby MacKenzie et al. (2008b). Evidence for prolonged vein formation over the brittle-ductile transitionincludes post-vein deformation features such as subgrain rotation, recrystallization, and various veinorientations relative to the wall-rock fabric.Objective 2 – Constrain the mechanisms that control monazite growth in the Klondike2a) How can metamorphic and hydrothermal monazite growth be distinguished?Petrographic analysis and petrochronology are combined to distinguish between metamorphic andhydrothermal monazite in orogenic gold settings and fluid-affected metamorphic terranes. Metamor-phic monazite can form anywhere in a metamorphic rock, is typically adjacent to or intergrown withother metamorphic minerals, and may be aligned with a metamorphic fabric. They have a variableThO2 concentration (∼0–8 wt.%) that depends on the host-rock composition and metamorphic grade.If grown coeval with and adjacent to plagioclase, metamorphic monazite will have a negative Euanomaly. In contrast, hydrothermal monazite can precipitate or modify pre-existing grains in andadjacent to veins, is typically adjacent to or intergrown with other hydrothermal minerals, and mayalso be aligned in a metamorphic fabric. It has a distinctly low ThO2 concentration <2.00 wt.% anddoes not have a Eu anomaly. Although useful for determining growth zonation patterns, grain-scaleelement maps do not help distinguish between metamorphic and hydrothermal monazite because bothhave similar patchy or mottled Th zonation styles. Monazites that lack relationships with petrographicassociation, Th-Pb date, and ThO2 concentration may represent partial to complete resetting of pre-existing monazite by hydrothermal fluids.2b) What is the origin of monazite in the Klondike?Retrograde metamorphic and hydrothermal monazite occur throughout most of the Klondike host-rocks. The retrograde metamorphic monazite is present in rocks of the quartz-augen schist and felsicschist sub-units but is absent in the mafic schist sub-unit. Rocks of the metaclastic sub-unit were notinvestigated. The hydrothermal monazite occurs adjacent to veins, but is absent in vein material anddoes not have a direct genetic link with gold formation. Monazite that is adjacent to veins at LoneStar may represent hydrothermally reset grains of pre-existing, likely retrograde monazite.Objective 3 – Define the temporal framework for metamorphism, vein formation, and goldmineralization in the Klondike3a) Can metamorphic monazite reveal new details about the prograde and retrograde metamorphism inthe Klondike?76The data indicate that retrograde metamorphic monazite formed over a protracted period of 39 m.y.,from 189 to 151 Ma. This period overlaps with existing 40Ar/39Ar mica cooling ages and exhumationfrom 185 to 140 Ma (Breitsprecher and Mortensen, 2004; Joyce et al., 2015, unpublished MDRUdata). Retrograde monazite and crustal exhumation may have occurred in discrete peaks at approxi-mately 175 and 160 Ma, but the youngest peak was not recorded at Virgin.3b) Did all orogenic gold occurrences in the Klondike form during the same metallogenic event?The orogenic gold occurrences in the Klondike likely formed during the same metallogenic event.Vein formation ages range from 178 to 117 Ma at the Virgin and Mitchell-Sheba occurrences. Theupper limit of vein formation is not constrained at Lone Star, but the 178 to 117 Ma range from otheroccurrences overlaps with reset monazite dates and the youngest possible hydrothermal age of 116Ma at Lone Star.3c) At what time scale do vein formation and mineralization occur (i.e., are they single, episodic, orprotracted events)?The veins formed over a protracted period of 62 m.y., from 178 to 117 Ma. This prolonged rangemay arise from three distinct hydrothermal pulses at approximately 169, 148, and 128 Ma. Thosehydrothermal pulses may derive from multiple geological events including the Klondike exhumation(185–140 Ma; Breitsprecher a