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Petrology and tectonic setting of felsic and mafic volcanic and intrusive rocks in the Finlayson Lake… Piercey, Stephen John 2001

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PETROLOGY A N D TECTONIC SETTING OF FELSIC AND MAFIC V O L C A N I C AND INTRUSIVE ROCKS IN T H E FINLAYSON L A K E VOLCANIC-HOSTED MASSIVE SULPHIDE (VHMS) DISTRICT, Y U K O N , C A N A D A : A RECORD OF MID-PALEOZOIC A R C A N D B A C K - A R C M A G M A T I S M AND M E T A L L O G E N Y by STEPHEN JOHN PIERCEY B.Sc.(Hons.), Memorial University of Newfoundland, 1996 M.Sc., Memorial University of Newfoundland, 1998 A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF T H E REQUIREMENTS FOR T H E D E G R E E OF DOCTOR OF PHILOSOPHY in T H E F A C U L T Y OF G R A D U A T E STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the requjf^d standard T H E UNIVERSITY OF BRITISH COLUMBIA July 2001 © Stephen John Piercey, 2001 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u rposes may be g r a n t e d by t h e head o f my department o r by h i s o r her r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department The U n i v e r s i t y o f B r i t i s h Columbia Vancouver, Canada Abstract The Yukon-Tanana Terrane (YTT) in the Finlayson Lake region (FLR), southeastern Yukon, Canada is host to five volcanic-hosted massive sulphide (VHMS) deposits (total -34 Mt) that have been discovered since the mid-1990's. In this thesis, field, lithogeochemical and Nd isotopic data are presented for felsic and mafic igneous rocks in the F L R to understand the tectonic setting, style of magmatism, and their relationships to V H M S mineralization. All rocks in the FLR were built upon a continental (or continent-derived) substrate of pre-Mississippian (>365 Ma) age. The Fire Lake unit (FLU) reflects Devonian-Mississippian (-365-360 Ma) arc and back-arc magmatism built upon a composite basement of oceanic and continental (or continent-derived) crust above an east-dipping subduction zone. Models proposed herein for the magmatic and tectonic evolution of F L U include: 1) arc magmatism punctuated by back-arc basin generation; 2) ridge propagation into an evolving arc with subsequent evolution to back-arc magmatism; and/or 3) ridge-subduction (slab-window) with eventual back-arc basin magmatism. The Kudz Ze Kayah (KZK) unit overlies the F L U and consists predominantly of crustally derived Devonian-Mississippian (-360-356 Ma) felsic volcanic and high-level subvolcanic rocks and variably carbonaceous sedimentary rocks; the latter are crosscut and overlain by alkalic mafic rocks. The high field strength element (HFSE)-enriched (A-type) felsic rocks and alkalic mafic rocks in the K Z K unit are inferred to represent magmatism within an ensialic back-arc basin upon evolved crust. The Wolverine succession (WS) unconformably overlies the K Z K unit and consists of a lower succession of felsic volcanic and subvolcanic rocks with carbonaceous sedimentary rocks; the upper portion of the succession, above the Wolverine VHMS deposit, consists predominantly of aphyric rhyolitic rocks that are overlain basalt flows. Felsic rocks ofthe WS are broadly similar to those in the K Z K unit and represent ensialic back-arc basin magmatism. However, the succession is younger (-356-346 Ma), and post-dates a period of uplift, deformation, and erosion prior to commencement of back-arc magmatism. Back-arc spreading eventually evolved to true seafloor spreading within the WS. Massive sulphide deposits in the FLR are preferentially associated with rocks indicative of high temperature magmatism (e.g., boninites, A-type felsic rocks) and extensional tectonic activity (e.g., back-arc rifting and spreading). ii Table of Contents Abstract ii Table of Contents iii List of Figures x List of Tables xiv Foreword _ xvi Acknowledgements xviii Chapter 1. Introduction Introduction 1 An Overview of the Classification and Settings of Volcanic-Hosted Massive Sulphide (VHMS) Deposits 2 Objectives 5 Methodology 6 Geological Mapping and Core Logging 6 Lithogeochemistry 6 Nd Isotope Geochemistry 7 Presentation 7 References 9 Chapter 2. Geochemistry and Paleotectonic Setting of Felsic Volcanic Rocks in the Finlayson Lake Volcanic-Hosted Massive Sulphide (VHMS) District, Yukon, Canada Abstract 16 Introduction 18 Regional Setting 19 Geological and Alteration Attributes of Host Rocks 20 Fyre Lake Unit 20 Kudz Ze Kayah Unit 21 Wolverine Succession 22 iii Geochemistry 24 Mobile Element Systematics 25 Immobile Element Systematics 27 Grass Lakes Succession: Fire Lake Unit 27 Grass Lakes Succession: Kudz Ze Kayah Unit 28 Wolverine Succession: Unit 5f/qfp 29 Wolverine Succession: Unit 6 30 Discussion 31 Petrogenesis and Tectonic Setting of Felsic Rocks in the Finlayson Lake District 31 Fire Lake Unit 31 Kudz Ze Kayah Unit 32 Wolverine Succession 35 Modern and Ancient Analogues to the Finlayson Lake District 37 Comparisons to the Geochemistry of Felsic Volcanic Rocks in Archean and Phanerozoic VHMS Environments 40 Conclusions__ 41 References _43 Chapter 3. Boninitic Magmatism in a Continental Margin Setting, Yukon-Tanana Terrane, Southeastern Yukon, Canada Abstract 80 Introduction 81 Geological Setting and Stratigraphy ; 82 Geochemical Characteristics of the Boninitic Rocks 83 Evidence for Boninitic Magmatism Linked to Continental Crust 84 Discussion and Conclusions 85 References 87 iv Chapter 4. Stratigraphic and Regional Implications of Weakly Strained Devono-Mississippian Volcanic Rocks in the Money Creek Thrust Sheet, Yukon-Tanana Terrane, Southeastern Yukon Abstract 99 Introduction 100 Previous Work 101 Stratigraphy ofthe Volcanic Succession 102 Unit MVU, 102 - Unit FMVU 103 Unit MVU2 104 Unit TSU,__ 104 Unit MVU3 105 Intrusive Rocks ; 106 Discussion 107 Definition of the Simpson Range Plutonic Suite 107 Emergent Volcanism in the Undeformed Sequence? 108 Comparisons with Campbell Range Belt 109 Conclusions 110 References 111 Chapter 5. Geological Characteristics of High-Level Subvolcanic Porphyritic Intrusions Associated with the Wolverine Zn-Pb-Cu-Ag-Au Volcanic-Hosted Massive Sulphide Deposit, Finlayson Lake District, Yukon, Canada Abstract 121 a Introduction 122 Regional Setting . 122 Geological Features of Porphyries Associated with the Wolverine V H M S Deposit 124 Wolverine/Lynx Zone 124 Fisher Zone 125 Sable Zone 126 Puck Zone . 126 Discussion : 127 Pre-, Syn-, Post-Mineralization Emplacement of the Porphyries 127 Significance of Porphyries for Localization of Sulphide Mineralization 128 Summary and Conclusions 129 References 13 0 Chapter 6. Geochemistry and Tectonic Significance of Weakly Alkalic Mafic Magmatism in the Yukon-Tanana Terrane, Finlayson Lake Region, Yukon Abstract 142 Introduction 143 Geological and Stratigraphic Setting 144 Lithogeochemistry and Neodymium Isotope Geochemistry 145 Sampling and Analytical Methods 145 Alteration/Metamorphism and Element Mobility 146 Results 147 Discussion 149 Petrogenesis 149 Tectonic Setting 152 Conclusions 154 References 155 Chapter 7. Magmatic Diversity in a Pericratonic Realm: Geochemical and Nd Isotopic Constraints on the Origin of Arc and Non-Arc Magmatism in the Yukon-Tanana Terrane, Yukon, Canada Abstract : 174 Introduction 175 Previous Work 176 Geological Setting of the Fire Lake Unit and YTT in the Finlayson Lake Region 178 vi Lithogeochemistry and Neodymium Isotope Geochemistry 180 Sampling and Analytical Methods 180 Alteration/Metamorphism and Element Mobility 181 Results 181 Back Arc Basin Basalts (BABB) 182 Enriched Mid-Ocean Ridge Basalts (E-MORB) 183 Niobium-Enriched Basalts (NEB-1, NEB-2) 183 High Th Niobium-Enriched Basalts (T-NEB) 184 High Ca Boninites (BON) 185 Island Arc Tholeiites (IAT) 186 LREE-enriched Island Arc Tholeiites (L-IAT) 187 Calc-Alkaline Basalts (CAB) 187 Discussion \ 188 Evidence for Variable Mantle Source Enrichment 188 Evidence for a Subducted Slab Component 191 Evidence for Crustal Contamination 193 Tectonic Setting and Models 196 Implications for Crustal Growth of the Cordillera 202 Conclusions 203 References_ 204 Chapter 8. Neodymium Isotope Geochemistry of Felsic Volcanic and Intrusive Rocks from the YTT in the Finlayson Lake Region Abstract 239 Introduction 240 Geological Setting 241 Geochemistry and Isotope Geochemistry 243 Geochemical Attributes of the Intrusive Rocks 244 vii Nd Isotopic Systematics of Felsic Rocks Discussion 245 246 Role of Crustal Material in Genesis of Felsic Rocks in the Finlayson Lake Region 246 Tectonic Significance 248 Implications for VHMS Mineralization 250 Conclusions 251 References 252 Chapter 9. Summary and Directions for Future Research Summary 276 Directions for Future Research 278 Finlayson Lake Region 279 Yukon-Tanana Terrane 280 Global Questions 281 References__ 283 Appendix 1. Analytical Methods Introduction: Sampling and Crushing Protocol 286 Analytical Methods: Major and Trace Element Geochemical Data 286 Tests for Analytical Precision. 287 Analytical Methods: Neodymium Isotope Geochemical Data 288 References 289 Appendix 2.U-Pb Geochronology of Porphyritic Intrusions in the Kudz Ze Kayah and Wolverine Volcanic-Hosted Massive Sulphide (VHMS) Deposits Methodology 300 Results 300 P98-KZK2 3 00 P98-69A 301 Vl l l Summary 3 01 References 302 ix List of Figures Figure 1.1. Simplifed terane map of the North American Cordilera with location of the Finlayson Lake region 13 Figure 1.2. Idealized cross section through a VHMS deposit 14 Figure 1.3. Schematic representation of a typical seafloor sulphide mound and associated hydrothermal alteration facies 15 Figure 2.1. Location of Finlayson Lake district and VHMS deposits 50 Figure 2.2. Regional geological seting of the Finlayson Lake district 51 Figure 2.3. Schematic stratigraphic relationships of felsic rocks in the Finlayson Lake region 52 Figure 2.4. Photographs of the macroscopic features ofthe felsic rocks in the Finlayson Lake region 53 Figure 2.5. Photomicrographs of mineralogical features of felsic rocks from the Finlayson Lake region 55 Figure 2.6. Major elment plots of felsic rocks: (a) (Na20+K20)-Si02 (Le Maitre plot), (b) Na20+K20-Si02 (TAS plot), (c) A/NK-A/CNK, and (d) AFM plot 57 Figure 2.7. Alteration box plot of felsic rocks from the Finlayson Lake region 58 Figure 2.8. Immobile trace elment discrimination diagrams for felsic rocks from the Finlayson Lake region: (a) Zr/TiOrNb/Y, (b) Nb-Y, (c) Ta-Yb, and (d) Zr-Ga/Al plots 59 Figure 2.9. Primitve mantle-normalized trace elment plots for FLU felsic rocks 60 Figure 2.10. High field strength elment plots of Finlayson Lake region felsic rocks: (a) Zr-Nb, (b) Zr/Ti02-Y/Ti02, (c) Zr/Yb-Sc/Yb, and (d) Zr/Yb-Ti/Yb 61 Figure 2.11. (a) Nh-Y-Ce and (b) Nb-Y-Ga plots for A-type felsic rocks 62 Figure 2.12. Primitve mantle-normalized trace elment plots for KZK unit felsic rocks 63 Figure 2.13. Primitve mantle-normalized trace elment plots of unit 5f7qfp felsic rocks 64 Figure 2.14. Primitve mantle-normalized trace elment plots of unit 6 felsic rocks 65 Figure 2.15. Th/Nbpm-Sc (a) and Th/Nbpm-Ti02 (b) plots of KZK unit felsic rocks 66 x Figure 2.16. Zr/Y-Y and La/Yb n -Yb n plots of Finlayson Lake region felsic rocks and comparisons to Archean (a-d) and Phanerozoic (e-f) VHMS-related felsic rocks 67 Figure 3.1. Simplified geological map of the Yukon-Tanana Terrane in the Finlayson Lake region _90 Figure 3.2. Schematic stratigraphic relationships of the Y T T in the Finlayson Lake region and isotopic features of boninitic and related rocks 91 Figure 3.3. Geochemical plots of Finlayson Lake region boninitic rocks: (a) primitive mantle-normalized plot, (b) La/Sm-Ti0 2 , and (c) Ti/V-Ti/Sc 92 Figure 4.1. Location map of the Finlayson Lake region and the study area 114 Figure 4.2. Geological setting of the undeformed succession in the hanging wall of the Money Creek thrust fault 115 Figure 4.3. Volcanic stratigraphy of the undeformed sequence 116 Figure 4.4. Pillowed flows and volcaniclastic rocks from unit M V U i 117 Figure 4.5. Vesiculated pumiceous basalts (a-b) and hyaloclastitic felsic rocks (b) from unit F M V U , 118 Figure 4.6. Turbiditic sedimentary rocks (a) and flute casts (b) from unit TSUi 119 Figure 4.7. Magma mingling features (a-b) of undeformed sequence mafic and felsic rocks 120 Figure 5.1. Location map of the Finlayson Lake region and the study area 134 Figure 5.2. Regional geological map of the Finlayson Lake region 135 Figure 5.3. Geological map of the Wolverine V H M S deposit area 136 Figure 5.4. Cross-section 16250E of the Wolverine VHMS deposit 137 Figure 5.5. Photos (a-b) of feldspar porphyritic intrusions from the Wolverine/Lynx zone 138 Figure 5.6. Photo of feldspar porphyritic intrusion from the Fisher zone 139 Figure 5.7. Photo of quartz-feldspar porphyritic intrusion from the Sable zone 139 Figure 5.8. Photo of feldspar porphyritic intrusion from the Puck zone 140 Figure 6.1. Location map of the Finlayson Lake region and the study area 160 Figure 6.2. Regional geological map of the Finlayson Lake region_ 161 xi Figure 6.3. Stratigraphy ofthe Finlayson Lake region and unit 4 mafic rocks 162 Figure 6.4. Major and trace element classification plots ofthe unit 4 mafic rocks: (a) TAS plot, (b) Si0 2-(K 20+Na 20) plot, and (c) Zr/Ti0 2 -Nb/Y plot 163 Figure 6.5. FeO*/MgO versus (a) T i0 2 , (b) P 2 0 5 , (c) Cr, and (d) Ni plots ; 164 Figure 6.6. Primitive mantle-normalized trace element plots of (a) group 4a rocks, (b) group 4b rocks, and (c) average group 4a and 4b rocks to modern earth reservoirs 165 Figure 6.7. Chondrite-normalized REE plots of (a) group 4a rocks, and (b) group 4b samples 166 Figure 6.7. Discrimination diagrams for unit 4 samples: (a) Ti-V, (b) Zr/Y-Zr, (c) Zr-Nb-Y, (d) Zr-Ti-Y, and (e) Th-Zr-Nb 167 Figure 6.8. Trace element plots illustrating potential crustal contamination: (a) Th/Nb -S i0 2 , (b) Th/Nb-Ce/Yb, (c) Th/Yb-Nb/Yb, and (d) Nb-U 168 Figure 7.1. Regional geological map of the Finlayson Lake region 212 Figure 7.2. Schematic stratigraphic relationships in the Fire Lake unit in the Finlayson Lake region. 213 Figure 7.3. Discrimination diagrams for Fire Lake unit mafic rocks: (a) Zr/Ti0 2 -Nb/Y, (b) Ti-V, (c) Zr-Nb-Y, and (c) Th-Zr-Nb plots 214 Figure 7.4. Primitive mantle-normalized plots of Fire Lake unit mafic rocks 215 Figure 7.5. Immobile element plots of Fire Lake unit mafic rocks: (a) Al 2 03/Ti0 2 -Nb/Y, (b) Ti/Yb-Nb/Yb, and (c) Zr/Yb-Nb/Yb 217 Figure 7.6. Th/Yb-Nb/Yb plot of Fire Lake unit mafic rocks 218 Figure 7.7. sNd3 5o- l 4 7Sm/ 1 4 4Nd plot of Fire Lake unit mafic rocks 219 Figure 7.8. Simplified geological map of the Y T T in the Finlayson Lake region with distribution of Nd isotopic attributes in the Fire Lake unit 220 Figure 7.9. Simplified geological map of the Y T T in the Finlayson Lake region with distribution of magmatic affinities in the Fire Lake unit 221 Figure 8.1. Regional geological map of the Finlayson Lake region 257 x i i Figure 8.2. Schematic cross-section of the Finlayson Lake region and distribution of granitoid rocks 25 8 Figure 8.3. Discrimination diagrams for Finlayson Lake region felsic rocks: (a) Zr/Ti0 2 -Nb/Y, and (b) A / N K - A / C N K plots 259 Figure 8.4. Discrimination diagrams for Finlayson Lake region felsic rocks: (a) Nb-Y, and (b) Zr-Ga/Al plots 260 Figure 8.5. Key HFSE plots for felsic rocks of the Finlayson Lake region: (a) Zr-Nb, (b) Zr-Ti02, (c) Zr-Sc, and (d) Ti-Sc 261 Figure 8.6. Primitive mantle-normalized trace element plots for felsic rocks from the Finlayson Lake region 262 Figure 8.7. (a) sNd35o-/sm/Nd and (b) sNd 3 5o-' 4 7Sm/ 1 4 4Nd plots for felsic rocks from the Finlayson Lake region 263 Figure 8.8. 8Nd 3 5 0 versus (a) 1 4 7 Sm/ 1 4 4 Nd, (b) La/Yb, (c) Th/Yb, and (d) Zr/Yb for felsic rocks from the Finlayson Lake region in comparison to potential crustal contaminants from the Y T T 264 Figure 8.9. Primitive mantle-normalized trace element plot of basaltic rocks from the top of the Wolverine succession 265 Figure A 1.1. Primitive mantle-normalized plots of the results of replicate analyses of internal standards from this study ; 290 Figure A l .2. Primitive mantle-normalized plots of the average values obtained on replicate analyses of internal standards and comparison to pre-1998 compiled values 291 Figure A2.1. U-Pb condordia diagrams for samples (a) P98-KZK2 and (b) P98-69A 303 xiii List of Tables Table 2.1. Summary of geochemical characteristics of felsic rocks from the Finlayson Lake district. All rocks are separated into stratigraphic position and lithology 68 Table 2.2. Summary of key major and trace element ratios for felsic rocks of the Finlayson Lake district. All rocks are separated into stratigraphic position and lithology 76 Table 3.1. Geochemistry of boninitic rocks from the Finlayson Lake region 93 Table 4.1. Summary of salient stratigraphic, geochemical and temporal differences and similarities of the volcanic rocks of the Money Creek thrust sheet and the Campbell Range belt 121 Table 5.1. Outline ofthe salient geological features of porphyritic intrusions from different zones within the Wolverine VHMS system 141 Table 6.1. Geochemical data for weakly alkaline mafic rocks from unit 4 of the Grass Lakes succession 169 Table 6.2. Neodymium isotope geochemical data for mafic rocks from unit 4 mafic rocks 174 Table 7.1. Average values, 2a errors and data ranges for mafic rock geochemical groups in the Fire Lake unit__ 222 Table 7.2. Average, 2a errors and ranges of key element ratios for mafic geochemical groups in the Fire Lake unit 232 Table 7.3. Neodymium isotope geochemical data for mafic rocks from the Fire Lake unit 237 Table 7.4. Neodymium crustal index (NCI) calculations (DePaolo et al., 1992) for mafic rocks of the Fire Lake unit 238 Table 8.1. Geochemical data for YTT felsic intrusive rocks from the Finlayson Lake region 266 Table 8.2. Key element ratios for the YTT felsic intrusive rocks 272 Table 8.3. Neodymium isotopic geochemical data for YTT felsic rocks in the Finlayson Lake region 274 Table 8.4. Neodymium crustal index (NCI) calculations (DePaolo et al., 1992) for the felsic rocks of the YTT in the Finlayson Lake region 275 xiv Table Al . 1. Precision and accuracy data for in house reference materials analyzed during the course of this study 292 Table A2.1. U-Pb zircon analytical data 304 xv Foreword This thesis represents research with both economic and academic applications produced within a university but in conjunction with researchers in university, government and industry. This thesis is a collection of papers that were produced during the course of this dissertation in conjunction with the aforementioned colleagues and this foreword is presented to acknowledge collaborators contributions in accordance with guidance of The University of British Columbia and the doctoral committee. Chapters 2 through 5 representing published or accepted papers in refereed professional and government journals, and Chapters 6 through 8 representing papers that will be submitted for future publication. Additional field work articles related to the thesis where the thesis author was not the senior author, or the paper was superseded by research in this thesis are presented in Appendix 3. The paper that comprises Chapter 2 has been accepted by Economic Geology and was co-authored with Suzanne Paradis, Donald Murphy and James Mortensen. These co-authors played an editorial role and helped develop and shape the ideas in the paper. The paper was reviewed informally by Alan Galley and Jan Peter (Geological Survey of Canada (GSC)) and by formal journal reviews by Dave Lentz (University of New Brunswick) and an anonymous Economy Geology reviewer. The paper that comprises Chapter 3 has been accepted by Geology and was co-authored by Donald.Murphy, James Mortensen and Suzanne Paradis reflecting their editorial comments and contributions to the development of the ideas in this paper. The paper was informally reviewed by Alan Galley (GSC, Ottawa) and James Hawins (Scripps Institute of Oceanography), and by journal reviewers Rosemary Hickey-Vargas (Florida International University) and Richard Arculus (Australian National University). The paper which constitutes Chapter 4 represents a field work contribution published in Yukon Exploration and Geology and co-authored with Donald Murphy reflect his collaboration and contributions to the field work and editorial comments on the manuscript. The paper was informally reviewed by James Mortensen and formally reviewed by Diane Emond (Yukon Geology Program). Chapter 5 represents a second field work contribution published in Yukon Exploration and Geology and was co-authored by Jan Peter, Geoffrey Bradshaw, Terry Tucker, and Suzanne Paradis. These co-authors xvi contributed to the field work portion ofthe paper and were involved in the development of the ideas in this paper; Dr.'s Peter and Paradis contributed editorially. The paper was informally reviewed by James Mortensen and Alan Galley (GSC, Ottawa), and was formally reviewed by Diane Emond and Julie Hunt (Yukon Geology Program). The papers which comprise Chapters 2 through 5, and in Appendix 3 include: CHAPTER 2 Piercey, S.J., Paradis, S., Murphy, D.C., and Mortensen, J.K., accepted. Geochemistry and Paleotectonic setting of felsic volcanic rocks in the Finlayson Lake volcanic-hosted massive sulphide (VHMS) district, Yukon, Canada. Economic Geology. CHAPTER 3 Piercey, S.J., Murphy, D.C., Mortensen, J.K., and Paradis, S., accepted. Boninitic magmatism in a continental margin setting, Yukon-Tanana Terrane, southeastern Yukon, Canada. Geology. CHAPTER 4 Piercey, S.J., and Murphy, D.C., 2000. Stratigraphy and regional implications of unstrained Devono-Mississippian volcanic rocks in the Money Creek thrust sheet, Yukon-Tanana terrane, Southeastern Yukon. In Yukon Exploration and Geology 1999. Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 67-78. CHAPTER 5 Piercey, S.J., Peter, J.M., Bradshaw, G.D., Tucker, T., and Paradis, S., 2001. Geological attributes of high-level subvolcanic porphyritic intrusions in the Wolverine Zn-Pb-Cu-Ag-Au volcanic-hosted massive sulphide (VHMS) deposit, Finlayson Lake district, Yukon, Canada. In Yukon Exploration and Geology 2000. Exploration and Geological Services Division, Department of Indian and Northern Affairs, p.335-346. xvii Acknowledgements This thesis has benefited from the interaction and assistance of many people. I would like to extend my gratitude to my supervisor Jim Mortensen for guidance during this thesis and for allowing me to follow my own research directions. Jim's open-mindedness and collegiality is the mark of a true mentor; I owe you so much! I have also had the pleasure of working with numerous colleagues during the course of this thesis. Don Murphy is thanked for introducing me to the Finlayson Lake region and for ongoing collaboration on the tectonic and metallogenic history of the Yukon-Tanana Terrane. Suzanne Paradis is thanked for her involvement in most aspects of this thesis, her advice and collaboration with this research, and her patient and meticulous editing of many chapters of this thesis. Jan Peter is thanked for continued collaboration on the geology, alteration, and genesis of deposits of the Finlayson Lake region. Mo Colpron is thanked for being a constant sounding board for many of the ideas in this thesis, and for always providing a place to stay and an enjoyable time while in the Yukon; it's amazing what ideas you come up with late in the evening. Funding for this project was provided by Atna and Expatriate Resources, Yukon Geology Program (Don Murphy), the Geological Survey of Canada through the Ancient Pacific Margin N A T M A P project (Suzanne Paradis) and Finlayson Lake V M S Project (Jan Peter), and by the U B C Geochronology Laboratory and an NSERC operating grant (Jim Mortensen). The author was supported by NSERC PGS-A and PGS-B awards, a Thomas and Margeurite MacKay Scholarship from the University of British Columbia, the Hickok-Radford Fund of the Society of Economic Geologists, and a Geological Society of America Student Research Grant. Other individuals have also made important contributions. Richard Friedman and Janet Gabites are thanked for discussions and interactions during the course of this thesis. Rich is especially thanked for instructing me the finer points of U-Pb geochronology. Kelly Russell and Steve Rowins served as thesis committee members and are thanked for numerous discussions and their comments. I would like to especially thank Kelly for his insistence on data quality and validation, and his viewpoints on the scientific method and scientific philosophy, they will continue to impact my approach to science. Ongoing dialogue with Alan Galley and Dick Tosdal has been beneficial. Discussions with James Hawkins and Tony Crawford on aspects of boninite genesis and magmatism in modern arc and back-arc systems have contributed to the ideas presented in this thesis. Robert Creaser is thanked for undertaking the Nd isotopic analyses and for discussions. Industry colleagues including Rob Carne, Peter Holbek, Paul MacRobbie, Harlan Meade, Terry Tucker, Wayne Roberts and Chris Sebert provided generously their knowledge of the geology and VHMS deposits of the Finlayson Lake region. Paul MacRobbie and Terry Tucker are also thanked for their hospitality and logistical support at the Kudz Ze Kayah and Wolverine camps, respectively, during my research in the Finlayson Lake region. Terry is especially thanked for his continued involvement and interest in the research presented in this thesis. I would like to thank my colleagues and fellow graduate students Scott Heffernan, Steve Quane, and Nancy MacDonald. My office mate Tom Danielson is thanked for numerous discussions, being a constant source of jokes and insults, and for not letting me take myself too seriously. I would especially like to thank my colleagues and friends Lawrence Winter, Steve Israel and Geoff Bradshaw for the numerous discussions, both serious and not so serious, that we've had during my stay at UBC. I thank my parents and sister for their continued financial and emotional support during the course of this thesis. Their constant support during my academic career has been unwavering. Finally, I would like to thank my wife Michelle for her understanding, constant emotional support and companionship. I thank you for still wanting to share your life with me. xviii Chapter 1 Introduction Introduction The Yukon-Tanana Terrane (YTT) represents one ofthe largest and complex terranes in the northern Cordillera with an areal extent the size of California and a multi-stage history that records numerous magmatic, tectonic and metamorphic events throughout the Paleozoic and Mesozoic (Mortensen, 1992 and references therein). This terrane occupies a critical position in the Cordillera located between rocks ofthe North American craton and rocks that have "exotic" origins been attached to the North American craton during Mesozoic accretionary tectonic events (Fig. 1.1; Monger and Nokleberg, 1995). Because of this, an understanding the history ofthe Y T T is required in order to understand the subsequent accretionary history of the potentially more far-travelled, outer terranes along the western margin of North America. The Y T T and possible correlative rocks in Alaska and British Columbia are also metallogenically important because they host significant quantities of base-metals in volcanic-hosted massive sulphide (VHMS) deposits (Hunt, 1998), Au in Cretaceous intrusions (Hart et a l , 2000), and recently discovered emeralds associated with Cretaceous intrusions (Groat et al., 2000). Understanding the petrology and tectonic evolution of the Y T T has important implications for developing genetic and exploration models for these commodities within the Y T T . Of particular importance to this thesis has been the discovery of -34 million tonnes (Mt) of VHMS mineralization in rocks of the Y T T in the Finlayson Lake region since the mid-1990's. These discoveries caused the largest staking rush in the history ofthe Yukon Territory (Hunt, 1998). However, if there are to be subsequent V H M S deposits discovered then understanding the petrology and tectonic setting of both the Y T T and contained V H M S deposits in the Finlayson Lake region is likely to be of paramount importance. In addition to Cordilleran-speciflc questions, the Finlayson Lake district provides an excellent laboratory to understand the relationships between tectonism, magmatism, and hydrothermal system generation within pericratonic or continent margin (bimodal-silicaclastic) V H M S environments. Continent-margin, pericratonic, or bimodal volcanic-silicaclastic (Barrie and Hannington, 1999) VHMS districts are the most prolific environments for VHMS deposits and host by far the most significant 1 accumulations of V H M S mineralization in terms of tonnage and grade. Within these pericratonic environments many of the deposits are of "giant" (>50 Mt) to "super giant" (>100 Mt) status (e.g., Neves Corvo, Spain; Brunswick #12, Canada), and some have exceptionally high precious metal grades (e.g., Wolverine, Finlayson Lake). Deposits in these settings contribute significant quantities of Cu, Pb, Zn, Au, Ag, and Sn to the Canadian and global marketplace. By understanding the setting and relationships of magmatism and tectonics to VHMS hydrothermal system generation insights into the evolution and genesis of V H M S deposits in other pericratonic environments will be enhanced. The Y T T in the Finlayson Lake district of southeastern Yukon provides an excellent laboratory to study the relationships of magmatism and tectonics to VHMS hydrothermal system generation due to the excellent exposure, relatively well constrained stratigraphy and a good knowledge of the relationships of volcanic and plutonic rocks to the V H M S deposits. Similarly, these features provide an opportunity to study the Devonian-Mississippian metallogenic and tectonic history ofthe northern Cordillera. These are the fundamental foci of this thesis. The following sections provide an overview of V H M S deposits and their settings, and outline the objectives and the methodology implemented in the thesis and a summary of the presentation of this thesis. An Overview of the Classification and Settings of Volcanic-Hosted Massive Sulphide (VHMS) Deposits Volcanic-hosted massive sulphide (VHMS) deposits contribute significant quantities of base and precious metals to global marketplace and have been the studied for nearly 50 years. The classification and setting of V H M S deposits have been varied and broad but it's generally agreed that numerous features are common to all VHMS deposits. All form within subaqueous environments either on (exhalative) or slightly below the seafloor (subseafloor replacement); they are associated with subaqueous volcanic and high-level intrusive rocks +/- sedimentary rocks, are often underlain by subvolcanic intrusions, and form in a variety of convergent to divergent tectonic settings (Franklin et al., 1981; Lydon, 1984, 1988; Franklin, 1993, 1995; Ohmoto, 1996). In idealized V H M S deposits the deposits commonly display a distinctive zoning comprising conformable mound-like sulphide lenses with Cu-rich bases grading towards more Zn- and Pb-rich tops (Fig. 1.2; Franklin et al., 1981; Lydon, 1984, 1988; Franklin, 2 1993, 1995; Ohmoto, 1996). The conformable sulphide bodies are commonly underlain by discordant pipe-like stringer or feeder zones consisting of Cu-rich sulphide, chlorite and quartz (Fig. 1.2; op cit). These discordant pipe-like zones are also commonly the areas of the most intense hydrothermal alteration within the V H M S systems (chlorite alteration) and are interpreted to reflect the upflow zones of hydrothermal fluids along synvolcanic faults and other hydrothermal conduits (Figs. 1.2 and 1.3; op cit). In contrast, lateral or semi-conformable fluid flow commonly results in lower grade but greater that background alteration (sericite-carbonate-pyrite, etc.) coupled with metal depletion that is argued to be the zone from which metals that form the deposits were leached (Fig. 1.3; Galley, 1993). Notable, although not always present, in many VHMS deposits and districts are subvolcanic intrusions (SVI) or intrusive complexes (e.g., Flavrian, Noranda; Paradis et al., 1988; Galley, 1996; Large et al., 1996), that underlie the deposits. These intrusive complexes are interpreted to be the heat source that drives hydrothermal circulation in, and possibly contributes metals via magmatic fluids to, VHMS hydrothermal systems (Galley, 1996; Large et al., 1996). Numerous classification schemes have been proposed for V H M S deposits. One of the most common classifications of V H M S deposits is the threefold subdivision that was proposed by Sawkins (1976) which comprised Kuroko-, Cyprus-, and Besshi-style deposits. Kuroko-style deposits are classified as polymetallic (Zn-Pb-Cu±Ag±Au) deposits that are hosted by felsic volcanic rocks, commonly in bimodal volcanic environments interpreted to have formed in arc rifts or back-arc basins; the type deposits occur in the Miocene Kuroko district in Japan (Ohmoto and Skinner, 1983, and references therein). Besshi-style systems are predominantly Cu-rich with lesser Zn, Co and Au and are strata-bound deposits hosted by mafic volcanic rocks with abundant sedimentary rocks. They are interpreted to have formed within forearc or sedimented ridge environments (e.g., Middle Valley; Slack, 1993; Goodfellow et al., 1999; Peter and Scott, 1999). Cyprus-style deposits are predominantly Cu-Zn-rich and named after the classic deposits found in the Cretaceous ophiolitic terranes in Cyprus (Galley and Koski, 1999). Cyprus-style deposits are hosted entirely in mafic volcanic-dominated successions and have a well defined mound-like morphology with a carrot-shaped alteration pipe; most commonly these deposits are found in forearc and back-arc ophiolites (e.g., Newfoundland and Cyprus ophiolites), modern 3 mid-ocean ridges (e.g., T A G and East Pacific Rise), and ancient greenstone belts (e.g., Flin Flon-Snow Lake belt) (Galley and Koski, 1999; Hannington et al., 1995; Herzig and Hannington, 1996). Some of the major problems with this threefold classification of VHMS deposits is that it pigeonholes deposits, and does not always account for tectonic setting in which the deposits have formed. Furthermore, many VHMS deposits have hybrid characteristics that are incompatible with the three end member deposit types. Franklin et al. (1981) and Franklin (1993, 1995) classified V H M S deposits according to their metal contents into those within the Cu-Zn group and Cu-Pb-Zn group. In this classification many of the Cyprus- and Besshi-style systems would be within the Cu-Zn group, whereas the Kuroko deposits would fall within the Cu-Pb-Zn group. Although this classification can result in the equivalence of deposits that are not necessarily the same, it does simplify classification and removes ambiguities provided by the "type" example classification. More recently Barrie and Hannington (1999) have provided a classification scheme that embraces many of the features outlined above but reflects the ambient geological environment in which the VHMS deposits are found in. Their classification is a five fold classification in which deposits are put into five groups depending on the proportions of mafic and felsic volcanic rocks and associated sedimentary rocks, these groups include: 1) mafic-type; 2) bimodal mafic-type; 3) mafic siliciclastic-type; 4) bimodal felsic-type; and 5) bimodal siliciclastic-type. Mafic-type deposits includes deposits hosted by dominantly mafic rocks (>75% mafic rocks), with very rare or absent felsic rocks (<1%) and minor silicaclastic or ultramafic rocks (<10%). This type encompasses ophiolite-hosted systems (e.g., Cyprus), and mafic dominated greenstone-belt associated systems (e.g., Potter mine in the Kidd-Munro assemblage). Bimodal mafic-type deposits occur in sequences where mafic rocks are dominant and comprise >50% of the stratigraphy, felsic rocks are >3%, and sedimentary rocks are subordinate; often there is a 3:1 mafic:felsic ratio. Deposits of this class include many deposits of the Flin Flon-Snow Lake camp, Kidd Creek deposit, and the deposits of the Noranda district. The mafic siliciclastic-type is broadly similar to the Besshi-style systems where there are subequal proportions of mafic volcanic or intrusive rocks and siliciclastic sedimentary rocks (often turbiditic sedimentary rocks); felsic rocks are rare or absent. 4 Examples within this class include: Windy Craggy, B C ; Middle Val ley, Juan de Fuca region; Guyamas Basin, California; and Fyre Lake, Yukon . The bimodal felsic-type is broadly similar to Kuroko-style systems and is defined as deposits in which >50% of the host stratigraphy consists o f felsic volcanic rocks, <15% is silicaclastic sedimentary rocks and mafic volcanic and intrusive rocks account for the remaining rocks. Examples within this class include the Kuroko deposits, some o f the Mt . Read volcanic belt deposits, Tasmania (e.g., Hellyer), and the Buchan's deposit, Newfoundland. The bimodal siliciclastic-type deposits occur districts where volcanic and sedimentary rocks are in subequal proportions and where there is a high proportion of felsic volcanic rocks. These settings host some of the world's largest V H M S systems including the deposits of the Bathurst Camp, N e w Brunswick; Iberian Pyrite Belt, Spain-Portugal; Finlayson Lake district, Yukon; and Mount Windsor Group, Australia. In tandem the combination of the aforementioned classification schemes can provide a descriptive framework for most V H M S deposits. However, some workers have recently observed that many V H M S systems have features that appear to represent hybrids between V H M S deposits and other deposit types. For example, some deposits have features that are intermediate between V H M S and sedimentary exhalative ( S E D E X ) systems (e.g., Brunswick #12; Goodfellow and Peter, 1996); some have features between V H M S and epithermal A u systems (e.g., Eskay Creek; Roth et a l , 1999; Hannington et al., 1999); and some have features hybrid between V H M S and porphyry C u / epithermal A u (e.g., M y L y e l l ; Large et al., 1996; Huston, 2001). The Wolverine deposit discussed in this thesis may represent a V H M S - S E D E X hybrid (Bradshaw et al., 2001). Objectives This study combines geology, geochemistry and isotopic analysis o f felsic and mafic igneous and metaigneous rocks proximal and distal to V H M S mineralization within the Y T T o f the Finlayson Lake region, southeastern Yukon . The broad goals of this thesis are to obtain a better understanding of the petrologic and tectonic evolution of mafic and felsic rocks o f the Y T T in the Finlayson Lake region and their petrological relationships to V H M S mineralization. Encompassed within this broad theme are numerous specific goals of interest. A n improved understanding o f the relative roles of mantle, continental crust, and subducted slab metasomatic agents have played in the evolution of Y T T mafic and 5 felsic rocks in the Finlayson Lake district will be elucidated by using a combined geochemical and Nd isotopic approach to the volcanic and plutonic rocks an. The petrological variations in rocks related and unrelated to V H M S deposit will provide some insight into the thermal and tectonic controls on VHMS system genesis within the Finlayson Lake district. Likewise, the petrotectonic results provide a basis for proposed possible tectonic models for the setting of the Finlayson Lake region by comparison to modern analogues with similar petrotectonic attributes. Understanding these features within the Finlayson Lake district provides insight into the tectonic history of the mid-Paleozoic continental margin of North America and helps elucidate and provide insight into similar settings and V H M S environments worldwide. Methodology Geological Mapping and Core Logging Much of the information presented in this thesis is built on the geological framework provided by field relationships that were determined during 1:500000 scale geological mapping and diamond drill core investigations of deposits in the Finlayson Lake region. Most of the field portions of this thesis were carried out with colleagues in the Yukon Geology Program (Murphy and Piercey, 1999a,b,c, 2000; Piercey and Murphy, 2000; Chapter 4), industry and Geological Survey of Canada (Piercey et al., 2001; Chapter 5). Lithogeochemistry Lithogeochemical studies were employed to understand the petrology and tectonic affinity of volcanic and plutonic rocks in the Finlayson Lake region and to compare to other available data from both the Y T T and similar modern and ancient tectonic environments. This research is the primary and most significant contribution of the results of this thesis. Major, trace and rare earth elements (REE) were determined by X-ray fluorescence (XRF), inductively coupled plasma emission spectrometry (ICP-ES), and inductively coupled plasma mass spectrometry (ICP-MS) at the Geological Survey of Canada, Ottawa, Canada. Details ofthe methodology, precision and accuracy are presented in Appendix 1 and at http://132.156.95.172/chemistry. 6 Nd Isotope Geochemistry Neodymium isotope geochemistry was employed as a tracer of the relative roles of mantle (juvenile) and continental crustal (evolved) components in the mafic and felsic igneous rocks in the study area. It was also used as another dataset to support interpretations about the roles of these contributions elucidated by trace element geochemical methods. All Nd isotopic determinations were done at the University of Alberta Trace Isotope Facility by Robert Creaser following the methods of Creaser et al. (1997). Further details are presented in Appendix 1. Presentation This thesis constitutes a collection of research manuscripts that are at various stages of publication. All papers relate to different field, geochemical and isotopic characteristics of specific rock units and suites in the study area. Some of the chapters have already been peer reviewed by international technical journals (Chapters 2 and 3) or professional government journals (Chapters 4 and 5), other manuscripts have been internally reviewed and are to be submitted for future publication in international journals (Chapters 6 to 8). For ease to the reader and for publication each chapter is considered a separate entity. Efforts have been made to minimize repetition; however, some overlap between chapters is unavoidable. Outlined below are the subjects of each chapter of this thesis. The contributions of each author to these publications has been previously discussed in the Foreword. Chapter 2 represents the results of a regional geological and geochemical study of V H M S -associated felsic volcanic and high level intrusive rocks in the Finlayson Lake district. This paper documents how there are specific felsic volcanic associations with mineralization and that there most VHMS-associated rocks are associated with high temperature, high field strength element (HFSE)-enriched felsic magmatism, whereas VHMS-barren rocks appear to be associated with HFSE-depleted felsic magmatism. This paper also illustrates that felsic volcanic rocks within the Finlayson Lake district record various episodes of Devonian-Mississippian ensialic arc and back-arc basin magmatism along the western edge North America, with VHMS events occurring during ensialic back-arc basin spreading episodes. This paper also illustrates that the Finlayson Lake district has strong similarities to other 7 pericratonic V H M S districts such as the Bathurst Mining Camp (BMC), Canada, the Mount Windsor Subprovince, Australia, and possibly the modern day Okinawa Trough, Japan. Chapter 3 represents a combined field and geochemical study of continent-margin related boninitic magmatism in the Y T T . Boninitic magmatism is a very unique style of magmatism reflecting high temperature arc magmatism from extremely depleted mantle sources. In the modern and ancient record boninitic magmatism (sensu stricto) is associated with specific settings including greenstone belts, ophiolites, and modern day forearc regions. Common to all of these regions is that these rocks occur in areas of intraoceanic magmatic activity. The Y T T boninites of this paper are unique in that they are the only case of boninites in the world that can be geologically and geochemically tied to continental crust. Chapters 4 and 5 represent field studies of different volcanic and intrusive rocks in the Y T T of the Finlayson Lake region. Chapter 4 is a field study of relatively unstrained volcanic and plutonic rocks within the Money Creek region of the Yukon and their implications for regional stratigraphic and tectonic nomenclature of the region. Chapter 5 represents a field study of the geology and hydrothermal alteration attributes of high-level porphyritic subvolcanic intrusions in the Wolverine V H M S deposit. This study also examines the possible relationships of the intrusions to mineralization within the Wolverine deposit and provides a framework for further geochemical and isotopic study of these intrusions. Chapter 6 is a field, geochemical and Nd isotopic study of weakly alkaline mafic volcanic and high-level intrusive rocks that crosscut and are spatially associated with the K Z K and GP4F VHMS deposits. This paper describes how field, geochemical and Nd isotopic data are used to understand the petrogenesis of these alkalic rocks and their tectonic setting. The results of the study show that these alkalic rocks occur in two suites, one from an enriched asthenospheric source, and the other coming from the same source but with superimposed crustal contamination. Both suites record alkaline magmatism that formed during Devonian-Mississippian ensialic back-arc spreading and are broadly coeval with V H M S system generation. Chapter 7 is a combined field, geochemical and Nd isotopic study of Devonian-Mississippian mafic rocks within the Fyre Lake unit in the lowermost part of the stratigraphy of the Finlayson Lake region. This research illustrates the complex petrological aspects of arc and arc-rift magmatism within 8 the Y T T in this region and the extremely heterogeneous mantle domains recorded by this unit and the varied contributions to the petrology o f these rocks from mantle, subducted slab and evolved continental crust. These petrological characteristics reflect a period o f arc volcanism that was interrupted by arc-rifting and the commencement of back-arc spreading; possible models include simple arc-rifting, propagation of a spreading ridge into an arc environment, and/or ridge subduction and slab-window formation. Chapter 8 presents a reconnaissance N d isotopic study of felsic volcanic and intrusive rocks in the Y T T o f the Finlayson Lake region. This study showed that most felsic rocks o f the Finlayson Lake district contain a significant component o f evolved Proterozoic continental crust. Notable are felsic rocks in the Fire Lake unit that have signatures suggesting derivation from less evolved crustal material and possible derivation from mafic sources. Chapter 9 provides a summary of the major findings of this study and outlines the outstanding problems and potential future research directions. References Barrie, C.T. , and Hannington, M . D . , 1999. Classification of volcanic-associated massive sulfide deposits. In Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Edited by C T . Barrie and M . D . Hannington; Reviews in Economic Geology, v.8, p. 1-11. Creaser, R . A . , Erdmer, P., Stevens, R . A . , and Grant, S.L., 1997. Tectonic affinity o f Nisutl in and A n v i l assemblage strata from the Teslin Tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence. Tectonics, v. 16, p.107-121. Franklin, J . M . , 1993. Volcanic-associated massive sulphide deposits. In Mineral Deposit Modeling. Edited by R . V . Kirkham, W . D . Sinclair, R.I. Thorpe, and J . M . Duke; Geological Association of Canada Special Paper 40, p. 315-334. Franklin, J . M . , 1995. Volcanic-associated massive sulphide base metals. In Geology o f Canadian Mineral Deposit Types. Edited by O.R. Eckstrand, W . D . Sinclair, and R.I., Thorpe; Geological Survey o f Canada (also D N A G ) , #8, p. 158-183. (also Geological Society o f America, The Geology o f North America, v.P-1). Franklin, J . M . , Lydon, J .W., and Sangster, D.F . , 1981. Volcanic-assoicated massive sulfide deposits. Economic Geology 75 t h Anniversary Volume, p.485-627. Galley, A . G . , 1993. Characteristics of semi-conformable alteration zones associated with volcanogenic massive sulphide districts. Journal of Geochemical Exploration, v.48, p. 175-200. 9 Galley, A . G , 1996. Geochemical characteristics of subvolcanic intrusions associated with Precambrian massive sulphide deposits. In Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration. Edited by D.A. Wyman; Geological Association of Canada, Short Course Notes Volume 12. Pages 239-278. Galley, A G . , and Koski, R.A., 1999. Setting and characteristics of ophiolite-hosted volcanogenic massive sulphide deposits. In Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Edited by C T . Barrie and M.D. Hannington; Reviews in Economic Geology, v.8, p.221-246. Goodfellow, W.D., and Peter, J.M., 1996. Sulphur isotope composition of the Brunswick No. 12 massive sulphide deposit, Bathurst Mining Camp, New Brunswick: implications for ambient environment, sulphur source and ore genesis. Canadian Journal of Earth Sciences, v.33: 231-251. Goodfellow, W.D., Zierenberg, R.A., and ODP Leg 169 Shipboard Scientific Party, 1999. Genesis of massive sulfide deposits at sediment-covered spreading centers. In Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Edited by C T . Barrie and M.D. Hannington; Reviews in Economic Geology, v.8, p.297-324. Groat, L .A. , Ercit, T.S., Marshall, D.D., Gault, R.A., Wise, M.A. , Wengzynowski, W., and Eaton, W.D., 2000. Canadian emeralds: The Crown showing, southeastern Yukon. Mineralogical Association of Canada Newsletter, #63, p. 1,12-13. Hannington, M.D., Jonasson, I.R., Herzig, P.M, and Petersen, S., 1998. Physical and chemical processes of seafloor mineralization at mid-ocean ridges. In Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions. Edited by S.E. Humphris, R A . Zierenberg, L.S. Mullineaux, R.S. Thomson; American Geophysical Union Monograph 91, p. 115-157. Hannington, M.D., Poulsen, K.H. , Thompson, J.F.H., and Sillitoe, R.H., 1999. Volcanogenic gold in the massive sulfide environment. In Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Edited by C T . Barrie and M.D. Hannington; Reviews in Economic Geology, v.8, p.325-356. Hart, C.J.R., Baker, T., and Burke, M . , 2000. New exploration concepts for country-rock-hosted, intrusion-related gold systems: Tintina Gold Belt in Yukon. In The Tintina Gold Belt: Concepts, Exploration and Discoveries, British Columbia and Yukon Chamber of Mines Special Volume 2, p.145-171. Herzig, P., and Hannington, M.D., 1996. Polymetallic massive sulfides at the modern seafloor: A review. Ore Geology Reviews, v. 10, p.95-115. Hunt, J.A., 1998. Recent discoveries of volcanic-associated massive sulfide deposits in the Yukon. Canadian Institute of Mining and Metallurgy Bulletin, v. 90, p.56-65. Huston, D.L., 2001. Geochemical dispersion about the Western Tharsis Cu-Au deposit, Mt. Lyell, Tasmania. Journal of Geochemical Exploration, v.72, p.23-46. Large, R.R., Doyle, M . , Raymond, O., Cooke, D., Jones, A., Heasman, L. , 1996. Evaluation of the role of Cambrian granites in the genesis of world class V H M S deposits in Tasmania. Ore Geology Reviews, v. 10, p.215-230. Lydon, J.W., 1984. Volcanogenic massive sulphide deposits Part 1: Descriptive model. In Ore Deposit Models. Geoscience Canada, v.l 1, p. 195-202. 10 Lydon, J.W., 1988. Volcanogenic massive sulphide deposits Part 2: Genetic models. In Ore Deposit Models. Edited by R.G. Roberts and P.A., Sheahan; Geoscience Canada Reprint Series #3, p. 155-181. Monger, J.W.H., and Nokleberg, W.J., 1995. Evolution of the North American Cordillera: generation, fragmentation, displacement and accretion of successive North American plate-margin arcs. In Geology and Ore Deposits ofthe American Cordillera. Edited by A.R. Coyner and P.L. Fahey; Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April 1995, p. 1133-1152. Murphy, D.C., and Piercey, S.J., 1999a. Finlayson project: Geological evolution of Yukon-Tanana Terrane and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon. In Yukon Exploration and Geology; Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 47-62. Murphy, D.C., and Piercey, S.J., 1999b. Geological map of Wolverine Lake area (105G/8), Pelly mountains, southeastern Yukon: Exploration and Geological Sciences Division, Department of Indian and Northern Affairs Canada, Open File 19993 (1:50,000 scale). Murphy, D.C., and Piercey, S.J., 1999c. Geological map of Finlayson Lake area, southeast quarter (105G/7, 8 and parts of 1,2 and 9), southeastern Yukon: Exploration and Geological Sciences Division, Department of Indian and Northern Affairs Canada, Open File 1999-4 (1:100000 scale). Murphy, D.C., and Piercey, S.J., 2000. Syn-mineralization faults and their re-activation, Finlayson Lake massive sulfide belt, Yukon-Tanana terrane, southeastern Yukon. In Yukon Exploration and Geology 1999, Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 55-66. Ohmoto, H. , 1996. Formation of volcanogenic massive sulfide deposits: The Kuroko perspective. Ore Geology Reviews, v.10, p.135-177. Ohmoto, H. , and Skinner, B.J., (editors) 1983. The Kuroko and related volcanogenic massive sulfide deposits. Economic Geology Monograph #5, 604 p. Paradis, S., Ludden, J., and Gelinas, L . , 1988. Evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archean Blake River Group, Abitibi, Quebec. Canadian Journal of Earth Sciences, v.25, p. 134-144. Piercey, S.J., and Murphy, D.C., 2000. Stratigraphy and regional implications of unstrained Devono-Mississippian volcanic rocks in the Money Creek thrust sheet, Yukon-Tanana terrane, Southeastern Yukon. In Yukon Exploration and Geology 1999, Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 67-78. Piercey, S.J., Peter, J.M., Bradshaw, G.D., Tucker, T., and Paradis, S., 2001. Geological attributes of high-level subvolcanic porphyritic intrusion in the Wolverine Zn-Pb-Cu volcanic-hosted massive sulphide (VHMS) deposit, Finlayson Lake district, Yukon, Canada. In Yukon Exploration and Geology 2000, Exploration and Geological Services Division, Department of Indian and Northern Affairs, p.335-346. Roth, T., Thompson, J.F.H., and Barrett, T.J., 1999. The precious metal-rich Eskay Creek deposit, northwestern British Columbia. In Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Edited by C T . Barrie and M.D. Hannington; Reviews in Economic Geology, v.8, p.357-373. 11 Sawkins, F.J., 1976. Massive sulphide deposits in relation to geotectonics. Geological Association of Canada, Special Paper 14, p.221-240. Slack, J.F., 1993. Descriptive and grade-tonnage models for Besshi-type massive sulphide deposits. In Mineral Deposit Modeling. Edited by R.V. Kirkham, W.D. Sinclair, R.I Thorpe, and J.M. Duke; Geological Association of Canada Special Paper 40, p. 343-371. 12 Figure 1.1. Simplified terrane map ofthe North American Cordillera outlining the terranes of North American margin (NAM) affinity relative to those of the Yukon Tanana Terrane (YTT), Slide Mountain Terrane (SMT) and the accreted terranes. Note the location of the YTT between the NAM and the accreted terranes. After M.Colpron (unpublished compilation). 13 "Exhalite" or "Turfite" horizon SiO, +/- py +/- hem Stratification M A S S I V E S U L P H I D E L E N S Sharp hangingwall contact S T O C K W O R K Z O N E cpy+/-py+/-po sulphide mineralization chloritic hydrothermal alteration py-sp+/-ga sulphide mineralization sericitic-chloritic hydrothermal alteration Figure 1.2. Idealized section through a VHMS deposit with a concordant and metal zoned sulphide body underlain by a discordant Cu- and chlorite-rich alteration pipe (modified after Lydon, 1984). Mineral abbreviations are: hem = hematite, cpy = chalcopyrite, py = pyrite, po = pyrrhotite, sp = sphalerite, gn = galena, ba = barite. 14 V--. 100 M / BLACK SMOKER COMPLEX DEBRIS APRON & METALLIFEROUS SEDIMENT ^ S U L F I D E TALUS! DISCONFORMABLE ALTERATION S 0 4 -> H2S METAL LEACHING IENTRAINMENT OF SEAWATER HIGH PERMEABILITY LOW PERMEABILITY 400°C HIGH TEMPERATURE REACTION ZONE Figure 1.3. Schematic representation of typical seafloor sulphide mound with associated hydrothermal alteration facies. Arrows reflect the flow of hydrothermal fluid in different parts of the system. Mineral abbreviations as in Figure 1.2. Modified after Hannington et al. (1995). 15 Chapter 2 Geochemistry and Paleotectonic Setting of Felsic Volcanic Rocks in the Finlayson Lake Volcanic-Hosted Massive Sulphide (VHMS) District, Yukon, Canada Abstract The Finlayson Lake volcanic-hosted massive sulphide (VHMS) district represents one of Canada's most recent VHMS discovery regions with -34 Mt massive sulphide mineralization found since the mid 1990's. Felsic volcanic rocks are associated with three units: the Fire Lake unit (FLU), the Kudz Ze Kayah (KZK) unit, and the Wolverine succession. Significant accumulations of polymetallic felsic volcanic-hosted massive sulphide (VHMS) deposits (Kudz Ze Kayah, GP4F and Wolverine) have only been discovered in the K Z K unit and Wolverine succession. In the hanging wall of the Money Creek thrust (MCT) felsic volcanic and high-level intrusive rocks in the F L U have calc-alkalic and tholeiitic affinities with low HFSE contents and intermediate Zr/Sc and Zr/Ti0 2 ratios. These rocks are interlayered with mafic rocks with arc geochemical signatures, are to date devoid of significant VHMS mineralization, and represent bimodal magmatism within an evolving Devonian-Mississippian continental arc system. The K Z K unit in the footwall of the M C T stratigraphically overlies the F L U and consists of felsic volcanic rocks with high HFSE-contents, within-plate (A-type) signatures, and high Zr/Sc and Zr/Ti0 2 ratios. The K Z K unit felsic rocks are crosscut and overlain by alkalic mafic rocks, are associated with abundant carbonaceous sedimentary rocks, and represent magmatism within a Devonian-Mississippian ensialic back-arc rift/basin environment. The Wolverine succession unconformably overlies the K Z K unit. Felsic rocks below to the Wolverine deposit have similar attributes as the K Z K unit with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc and Zr/Ti0 2 ratios. In contrast, aphyric rhyolite flows in the hanging wall of the deposit have much lower HFSE, have the lowest Zr/Sc and Zr/Ti0 2 ratios in the district. All the felsic rocks ofthe Wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by basaltic rocks with mid-ocean ridge basalt (MORB) chemistry. The Wolverine succession is interpreted to have formed within an Early Mississippian ensialic back-arc basin environment that eventually evolved to sea floor spreading. The variation in the HFSE budgets ofthe felsic rocks of the F L D likely reflects variations in the source and/or temperature of crustal melting. In particular felsic rocks of the F L U have higher Nb/Ta and 16 lower Ti/Sc ratios than other volcanic rocks in the district suggesting possible derivation from mafic crustal sources and/or lower crustal fusion temperatures. The K Z K unit and footwall rocks to the Wolverine deposit are inferred to have formed from high temperature partial melting of continental crust; the hanging wall aphyric rhyolites from the Wolverine succession appear to be lower temperature continental crustal melts. Polymetallic felsic volcanic-associated V H M S deposits within the F L D are preferentially associated with HFSE-enriched felsic rocks with high Zr/Sc and Zr/Ti0 2 ratios. The HFSE and rare-earth element (REE) systematics of VHMS-associated felsic rocks of the F L D are different than prospective felsic rocks from Archean VHMS environments in the Superior Province, and are displaced towards higher Zr/Y and La/Yb n . Their HFSE and REE systematics are similar to many Phanerozoic VHMS environments, in particular those partially to fully underlain by continental crust (e.g., Que River, Mount Windsor, Kuroko, Tobique). The differences in the HFSE and R E E systematics between felsic rocks of the F L D and Archean environments likely reflects the differences in the substrates from which the felsic rocks were derived (e.g., mafic versus felsic). In contrast, the similarities with many Phanerozoic VHMS environments reflect their common association with evolved continental crust, either by derivation from or contamination by a continental substrate. 17 Introduction The Finlayson Lake district (FLD) ofthe Yukon-Tanana Terrane (YTT), central Yukon Territory, is host to some of Canada's most recent volcanic-hosted massive sulphide (VHMS) discoveries such as the Kudz Ze Kayah (KZK), GP4F, Wolverine, Fyre Lake, and Ice deposits (Fig. 2.1). Discovered VHMS deposits in the F L D have a total tonnage of -34 Mt with 21 Mt in the felsic volcanic- and sediment-hosted K Z K , GP4F and Wolverine deposits; the other 13 Mt are in the Cu-Co-Au Fyre Lake deposit and Cu-rich Ice deposit. These VHMS discoveries started one ofthe largest staking rushes in the history of the Yukon Territory (Hunt, 1998), which in turn has spurred an integrated program of regional geological mapping (Murphy, 1998; Murphy and Piercey, 1999, 2000), metallogenic and lithogeochemical studies (Piercey et al., 1999; Bradshaw et al., 2001) ofthe FLD. Volcanic-hosted massive sulphide deposits of the F L D occur within a variably deformed and metamorphosed sequence of mid- to Late-Paleozoic volcanic, plutonic and sedimentary rocks (e.g., Mortensen, 1992a; Murphy and Piercey, 2000). This paper presents a regional geochemical dataset for felsic volcanic rocks from the F L D including those that do not host significant (to date) felsic volcanic-associated V H M S mineralization (Fire Lake unit) and those spatially associated with the Wolverine, GP4F and K Z K deposits. Petrochemical studies of volcanic rocks in other V H M S districts (e.g., Lesher et al., 1986; Barrie et al., 1993; Stoltz, 1995; Barrett and MacLean, 1999; Lentz, 1998, 1999) have shown that volcanic rock geochemical signatures can provide insight into the paleotectonic setting and metallogenic evolution of host-rocks to V H M S districts. Furthermore, these geochemical studies have shown that volcanism associated with V H M S mineralization may provide a fingerprint of the ambient thermal and tectonic controls that govern the genesis of VHMS hydrothermal systems. By association volcanic lithogeochemical signatures may be a guide to the exploration and discovery of new VHMS mineralization. The objectives of this study are to: 1) document the geochemical signatures of felsic volcanic rocks in the FLD; 2) elucidate the paleotectonic and metallogenic settings of the F L D and to compare the district to possible modern and ancient analogues; and 3) investigate the petrologic relationships between felsic volcanic petrogenesis and V H M S hydrothermal systems. 18 Regional Setting The Y T T in the F L D is composed of foliated and lineated greenschist to lower amphibolite grade metasedimentary, metavolcanic and metaplutonic rocks (e.g., Tempelman-Kluit, 1979; Mortensen and Jilson, 1985). Although the region has been strongly deformed and metamorphosed, regional mapping has identified a stratigraphically intact sequence consisting of three mid- to late-Paleozoic unconformity bound successions: the Grass Lakes, Wolverine and Campbell Range successions (Murphy, 1998; Murphy and Piercey, 1999, 2000; Murphy, 2001). The Grass Lakes succession consists of unit 1, the Fire Lake unit (FLU), and the K Z K unit (Figs. 2.2 and 2.3). The lowermost part of the Grass Lakes succession consists of pre-Early Mississippian (pre-365 Ma) quartz-rich, non-carbonaceous metaclastic rocks of unit 1, which are overlain by the -365-360 Ma (Mortensen, 1992a,b and unpublished data) mafic dominated arc- and back-arc related F L U (Mortensen, 1992a,b; Grant, 1997; Murphy and Piercey, 1999,2000; Piercey et al., 1999). Boninitic rocks of the F L U host the -8.5 Mt Besshi-style Fyre Lake Cu-Co-Au V H M S deposit (Figs. 2.2 and 2.3; Murphy, 1998; Murphy and Piercey, 2000). Stratigraphically overlying the F L U is the felsic volcanic- and sedimentary rock-dominated K Z K unit (Murphy, 1998). This unit consists predominantly of Devonian-Mississippian (-360-356 Ma; Mortensen, 1983, 1992a) felsic volcanic and variably carbonaceous sedimentary rocks in the lower parts of the unit (unit 3 of Murphy, 1998). The top of the Grass Lakes succession consists predominantly of alkalic basalts and carbonaceous sedimentary rocks (unit 4 of Murphy, 1998). Coeval with the K Z K unit are the Devonian-Mississippian (360±1 Ma; Mortensen, 1992a) K-feldspar porphyritic to megacrystic granitoids of the Grass Lakes suite (GLS) of intrusions, which are inferred to be the subvolcanic intrusive complex to the K Z K unit V H M S mineralization. The V H M S deposits within the K Z K unit include the Kudz Ze Kayah Zn-Pb-Cu deposit (13 Mt, 5.5% Zn, 1.3% Pb, 1% Cu, 125 g/t Ag, 1.2 g/t Au; Shultze, 1996) and the GP4F Zn-Pb deposit (1.0 Mt, 6.4% Zn, 1.55% Pb, 0.1% Cu, 90 g/t Ag, 1.76 g/t Au,; Figs. 2.2 and 2.3); both deposits are hosted by felsic volcanic and sedimentary rocks. Unconformably overlying the Grass Lakes succession is the Wolverine succession. The Wolverine succession consists predominantly of Early Mississippian (-356-346 Ma; Mortensen, 1992a; 19 Appendix 2) felsic volcanic and carbonaceous sedimentary rocks (Murphy and Piercey, 1999, 2000), which host the Wolverine V H M S deposit (6.2 Mt @ 12.96% Zn, 1.53% Pb, 1.41% Cu, 359.1 g/t Ag, 1.81 g/t Au; Tucker et al., 1997; Bradshaw et al., 2001; Figs. 2.1 and 2.2). The succession contains, from bottom to top, a lower conglomerate unit (unit 51), a lower felsic volcanic dominated unit (unit 5f/qfp), a regional carbonaceous argillite unit (unit 5 cp), the immediate footwall volcanic and subvolcanic rocks to the Wolverine deposit (unit 6-fw), and a hanging wall consisting of aphyric rhyolitic rocks, carbonaceous sedimentary rocks, which near their top contain basalt flows (unit 6-hw) (Murphy and Piercey, 1999, 2000; Bradshaw et al., 2001). The Wolverine deposit occurs at the contact between footwall felsic volcaniclastic rocks (unit 6-hw) and either hanging wall carbonaceous argillite or exhalative rocks (Bradshaw et al., 2001). The Wolverine succession is unconformably overlain by the upper Paleozoic (Pennsylvanian-Permian; Harms in Plint and Gordon, 1997) mafic volcanic and clastic sedimentary rock dominated Campbell Range succession (Murphy and Piercey, 1999; Murphy, 2001). Deformation has influenced rocks of the region but these events largely post-date the formation of the stratigraphy in the Finlayson Lake region. The Money Creek thrust (MCT) has displaced rocks from the Fire Lake unit a minimum of 30 km towards the east-northeast to their present position in the late Paleozoic (Murphy and Piercey, 2000). The region has also been subject to Cretaceous ductile deformation due to low displacement southwest vergent folding and thrusting (Murphy, 1998). A Mississippian event of uncertain kinematics has affected the Grass Lakes succession; however, this is addressed in the context ofthe evolution of the belt in the discussion. Geological and Alteration Attributes of Host Rocks Fire Lake Unit Felsic rocks comprise less than 5% of the F L U in the M C T sheet (Piercey and Murphy, 2000). Samples from the M C T consist of km-scale rhyolite/dacite flows interlayered with calc-alkaline arc and island-arc tholeiitic (Grant, 1997; Grant et al., 1996) pillowed, massive and vesiculated mafic lava flows and volcaniclastic rocks (Piercey and Murphy, 2000). The felsic volcanic and high level subvolcanic rocks in the M C T are fairly well preserved, weakly strained and have minimal alteration (Fig. 2.4a). Most of the felsic rocks are reddish-pink to greenish-white subaerial to shallow subaqueous rhyolites and 20 dacites that are variably K-feldspar porphyritic containing mm-scale subhedral to euhedral feldspar grains (Fig. 2.4a). They commonly constitute rhyolitic lava flows that are in places associated with blocky rhyolitic hyaloclastite (Piercey and Murphy, 2000). Within the sheet, the felsic rocks are associated with only minor clastic sedimentary rocks (Piercey and Murphy, 2000), unlike rocks of the K Z K unit and Wolverine succession (Murphy and Piercey, 1999). High-level quartz- and quartz-feldspar subvolcanic rhyolitic intrusive rocks are spatially associated with, and are inferred to be the feeders of the rhyolitic volcanic rocks (Piercey and Murphy, 2000). These intrusive rocks are pink-white to white-grey and are commonly K-feldspar porphyritic within a medium-grained, and less commonly fine-grained, matrix. Mortensen (1992b) obtained a 360.5±1 Ma age on a felsic porphyry from the M C T , and Piercey and Murphy (2000) documented magma mingling relationships between these intrusions and a calc-alkaline mafic feeder dyke to the basalts constraining the age of concomitant mafic and felsic volcanism within the M C T . The felsic rocks of the F L U are pristine, have no associated V H M S mineralization and relatively minor alteration. On the outcrop and hand specimen scale, the rocks have minor sericite alteration that is likely due to seawater-rock weathering. Petrographically, most contain K-feldspar phenocrysts (Fig. 2.5a), and locally quartz phenocrysts within a siliceous matrix. K-feldspar grains are euhedral to subhedral and mm-scale and in places form glomerophenocrystic aggregates (Fig. 2.5a). Most grains are texturally well preserved with minor sericite patches on their surfaces; in one sample they are totally replaced by red oxides and clay minerals. Quartz grains are typically teardrop-shaped grains or rounded within a siliceous matrix. The matrix is commonly partially replaced by Fe-rich clays and minor sericite but primary textures such as spherulitic quartz are locally preserved; pyrite is rare but present in some samples. Amygdales are typically infilled with quartz, carbonate, chlorite, and/or oxide minerals. Millimeter-scale quartz veinlets are present in more strained samples. Kudz Ze Kayah Unit Felsic rocks iri the K Z K unit range from in situ coherent rocks (rhyolitic flows and/or intrusions?) and volcaniclastic and epiclastic sedimentary rocks. Although the K Z K unit is generally schistose and foliated, more massive coherent rocks preserve some textural characteristics on the outcrop and hand 21 specimen scale (Fig.4b-d). Nevertheless, the original morphology of the coherent rocks is largely obscured by deformation and interpreting whether they are extrusive or intrusive is difficult. Rocks interpreted as volcaniclastic are common within the K Z K unit and range from fine-grained (tuffaceous) to coarse-grained with mm- to cm-scale feldspar crystals (Fig. 2.4d); coarse volcaniclastic rocks are very prevalent within and host the GP4F deposit. The felsic rocks of the K Z K unit are associated with abundant fine-grained carbonaceous sedimentary rocks and lesser siliciclastic rocks which contrasts with the F L U which contains very little carbonaceous material. Crosscutting and overlying the felsic rocks of the K Z K unit are alkalic mafic rocks which are interpreted to be of non-arc origin (Chapter 6). Alteration within the K Z K unit is sporadic. Most rocks have matrix recrystallization due to deformation and replacement of feldspar by sericite. However, although deformed many samples have pristine feldspars and deformation and metamorphism does not appear to have significantly altered the primary mineralogy. Proximal to the K Z K and GP4F VHMS deposits there is an increase in the intensity of alteration and the rocks have sericite, sericite-chlorite and silica alteration. Petrographically, most felsic rocks are variably deformed and have a matrix of recrystallized polycrystalline quartz (Fig. 2.5b) with foliae defined by muscovite (sericite) ± green to brown biotite ± oxides ± chlorite. Rarely, carbonate is intergrown with matrix quartz. Rarely, quartz phenocrysts and crystals are preserved; however, in most cases they are boundinaged into the foliation, internally recrystallized and wrapped by muscovite (± biotite ± oxide) foliae. K-feldspar phenocrysts are common and have variable states of preservation (Fig. 2.5b). Most exhibit partial to full replacement by sericite, and some contain quartz granules replacing the feldspars. Many exhibit elongation to the fabric akin to quartz crystals (Fig. 2.5b). The elongate feldspar grains are often segmented by polycrystalline veinlets of quartz ± sericite ± chlorite (Fig. 2.5b). Crosscutting many ofthe samples are small mm-scale veinlets of quartz and sericite. Wolverine Succession The Wolverine succession is similar to the K Z K unit in being a felsic-volcanic and subvolcanic and sedimentary rock dominated succession; however, it is younger and has more abundant sedimentary rocks. The succession consists of a lower quartz-feldspar conglomerate unit that is overlain by felsic 22 volcanic-dominated unit (unit 5f/qfp), which is in turn overlain by a regional carbonaceous phyllite unit (unit 5cp; see Murphy and Piercey, 1999). The latter carbonaceous sedimentary rocks form the deeper footwall to the Wolverine V H M S deposit; the immediate footwall to the deposit consists of a felsic tuffaceous unit with high level felsic intrusions (unit 6-fw; Bradshaw et a l , 2001; Piercey et al., 2001); the hanging wall consists of aphyric rhyolite flows interlayered with carbonaceous sedimentary rocks (unit 6-hw), which are capped by basalt flows (Bradshaw et al., 2001). Although the Wolverine succession is less deformed than the K Z K unit (Murphy and Piercey, 1999), it is deformed and the original morphology and textures of the volcanic and intrusive rocks are commonly obscured. Nevertheless, in some areas macroscopic textural features are preserved, particularly in drill core where many textures have not been obscured by weathering. The lowermost felsic unit within the Wolverine succession (unit 5f/qfp) consists predominantly of variably strained quartz- and feldspar-porphyritic felsic rocks (Figs. 2.4e and 2.5c), interpreted to be of volcaniclastic origin (tuffs and hyaloclastitic rocks). These volcaniclastic rocks have abundant quartz and K-feldspar crystals that are mm- to cm-scale and heterogeneously distributed within a siliceous to micaceous matrix (Fig. 2.4e). In unit 5f/qfp there are also coherent rocks that are interpreted to be flows or high-level intrusions but often their margins are deformed and flow or intrusion relationships are obscured. Typically the volcaniclastic rocks are more strained that the more coherent rocks. The coherent rocks typically have quartz-phenocrysts (or filled amygdales?) and mm-scale euhedral to weakly strained feldspars. Notably, the rocks of unit 5f/qfp have much higher quartz crystal/phenocryst than those of the K Z K unit (Murphy and Piercey, 1999). Most felsic rocks of unit 5f/qfp have minor to moderately developed sericite alteration with some exhibiting silica alteration; rarely do they exhibit chlorite or carbonate alteration. The footwall to the Wolverine deposit consists of felsic volcaniclastic and high-level intrusive rocks of unit 6-fw (Fig. 2.4f-g; Bradshaw et al., 2001; Piercey et al., 2001). Fine- to coarse-grained felsic volcaniclastic rocks form the immediate footwall to the deposit (Fig. 2.4g). Locally, these volcaniclastic rocks have cm-scale euhedral K-feldspar grains and blue, elliptical quartz-eyes which are elongate in the plane of the foliation (Fig. 2.5e). Elsewhere, they consist of very fine mm-scale feldspar grains and 23 micaceous material (Fig. 2.5e). Also present in the footwall to the deposit are sill-like feldspar- and quartz-feldspar porphyritic intrusive rocks which are relatively undeformed and have very well preserved euhedral mm- to cm-scale K-feldspar phenocrysts with or without quartz phenocrysts within a siliceous matrix (Figs. 2.4f and 2.5d). The footwall rocks to the Wolverine deposit are variably altered. The tuffaceous rocks in the immediate footwall to the deposit are relatively fresh distal from mineralization; however, proximal to the Cu-rich zones of the deposit they have abundant sericite- to chlorite-rich assemblages with or without carbonate alteration (Bradshaw et al., 2001). Felsic intrusions in the footwall of the deposit are less altered but do have a patchy distribution of quartz+sericite + pyrite ± sphalerite ± chlorite ± carbonate veinlets, patchy secondary K-feldspar replacement of primary K-feldspar, and weak silica alteration (Piercey et al., 2001). Felsic rocks above the Wolverine deposit are very distinctive and consist of highly siliceous aphyric rhyolite flows (Figs. 2.4h and 2.5f) and rhyolite breccias, minor fine-grained tuffaceous rocks, interlayered with carbonaceous sedimentary rocks, iron formation and carbonate exhalite (Bradshaw et al., 2001). The aphyric rhyolites are strongly silicified and have a siliceous matrix with partings of green waxy sericite and in places sericite-chlorite. Many of the aphyric rhyolitic rocks near the iron formation and carbonate exhalite are characterized by magnetite-pyrite-carbonate alteration in addition to silica and sericite. Geochemistry Least altered felsic volcanic rocks of rhyolitic to dacitic composition from the F L D were analyzed at Geological Survey of Canada and details ofthe analytical methods are presented in Appendix A. The complete dataset for this paper has been placed in digital form in the CD at the end of this thesis (Table RI). The averages, ranges, and 2a errors for the different suites and lithologies are presented in Table 2.1. Major and trace element ratios are presented in Table 2.2. A set of least-altered samples were collected from the FLD; however, many of the samples do exhibit the effects of alteration and metamorphism; particularly in samples that were collected proximal to the V H M S deposits. Given these constraints we have broken our presentation of the geochemical data into two sections including the: 1) mobile and 2) immobile element systematics. Furthermore, in Tables 24 2.1 and 2.2, for ease of presentation, the data are subdivided based on their stratigraphic position (e.g., F L U , K Z K unit, and Wolverine succession), and geological attributes (e.g., rhyolite flows, subvolcanic intrusives, and volcanosedimentary rocks). Mobile Element Systematics Numerous workers have shown that feldspar and glass destruction, and replacement of primary phases by secondary alteration phases are the most common reactions during hydrothermal alteration of felsic rocks in the V H M S environment (e.g., Munha et al., 1980; Saeki and Date, 1980; Hajash and Chandler, 1981; Lentz, 1999; Large et al., in press). Feldspar destruction reactions result in the loss of alkalis (particularly Na and Ca) during the formation of sericite (Ishikawa et al., 1976; Spitz and Darling, 1978; Munha et al., 1980; Saeki and Date, 1980); whereas, replacement of feldspars and sericite by fixation of Mg (±Fe) from solution to form chlorite leads to gains in Mg (±Fe) in the rocks (Saeki and Date, 1980; Hajash and Chandler, 1981; Date et al., 1983; Lentz, 1999). These results lead us to assume that the alkalis, Si0 2 , and Fe-Mg have been mobile in the rocks of the FLD. Other major elements such as T i 0 2 and A1 2 0 3 are considered immobile (Whitford et al., 1989; Barrett and MacLean, 1999) except under extreme conditions (Hynes, 1980; Finlow-Bates and Stumpfl, 1981). However, given the level of preservation of samples in this study A1 2 0 3 and T i 0 2 are assumed to be immobile. The low field strength elements (LFSE; Ba, Rb, Cs, Sr) are considered mobile during hydrothermal alteration (e.g., MacLean, 1990; Lentz, 1999) and are assumed to be mobile in the rocks of the FLD. The rare earth elements (REE) can be mobile during intense hydrothermal alteration (e.g., Campbell et al., 1984; Valsami and Cann, 1992), but under low-grade alteration (sericite) conditions they remain immobile (Whitford et al., 1988), and in this study we assume they were immobile. The exception, however, is Eu which can be very mobile in the hydrothermal environment (Sverjensky, 1984; Whitford et al., 1988; Wood and Williams-Jones, 1994). The high field strength elements (HFSE) appear to be immobile in nearly all circumstances (e.g., Whitford et al., 1989; MacLean, 1990; Barrett and MacLean, 1999; Lentz, 1999) with minor exceptions (e.g., Finlow-Bates and Stumpfl, 1981). The coherent HFSE behavior in the rocks of the FLD suggests they remained immobile during alteration and metamorphism. By virtue of their presence as 25 essential metallic constituents in V H M S deposits, the metals Cu, Pb, Zn, Ag, Sn, As, and Tl are considered mobile. On Figure 2.6a, the samples are plotted on a total-alkalis versus silica (TAS) plot (Le Bas et al., 1986) which shows that most samples retain a rhyolitic affinity but have significant scatter due to alkali mobility (Fig. 2.6a). The highly silificied nature ofthe aphyric rhyolitic results in many samples plotting outside the bounds of this diagram. A similar alkali and silica scattering is also observed on TAS plot of Irvine and Baragar (1971) (Fig. 2.6b). On the Shand's Index (Fig. 2.6c; Maniar and Piccoli, 1989), most samples exhibit variable A / C N K and A / N K ratios (Fig. 2.6; Table 2.2) which suggest alkali mobility during feldspar destruction; it is likely that most of the rocks had primary A / C N K values close to 1 (cf. Lentz, 1999). Alkali mobility is also reflected in the high Al 2 0 3 /Na 2 0 ratios (Spitz and Darling, 1978), Hashimoto alteration index values (Ishikawa et al., 1976; Date et al., 1983), and sericite index values (Saeki and Date, 1980) (Table 2.2). On an A F M plot (Fig. 2.6d; Irvine and Baragar, 1971) most felsic rocks from the F L D appear to follow calc-alkaline trends with some exceptions (Fig. 2.6d). The F L U tholeiitic rhyolites/dacites plot on the tholeiitic-calc-alkaline boundary (Fig. 2.6d). The Wolverine deposit aphyric rhyolites trend towards the FeO* apex of the diagram, reflecting the presence of pyrite and/or magnetite (Fig. 2.6d). The geochemical data for felsic rocks of F L D are illustrated on an alteration box plot in Figure 7 (Large et al., in press). This diagram relates whole rock geochemistry to potential alteration minerals present in the samples. The diagram is constructed with two alteration indices, the Hashimoto alteration index (AI; Ishikawa et al., 1976; see Table 2.2) to account for feldspar and glass breakdown to sericite and chlorite, and a chlorite-carbonate-pyrite-index (CCPI; see Table 2.2). The diagram also provides a box for felsic rocks that are typically unaltered, and fields for diagenetic and hydrothermal alteration reactions (Fig. 2.7; Large et al., in press). On this diagram, the calc-alkalic rocks of the F L U (see below) plot predominantly within the least altered box. The tholeiitic rhyolitic to dacitic rocks of the F L U lie above the least altered box (Fig. 2.7). This likely reflects their more intermediate compositions that typically have higher least altered CCPI values (Large et al., in press). Numerous rocks from the K Z K unit and the Wolverine succession lie within the least altered box; however, numerous samples trend 26 towards the sericite line (Fig. 2.7). Notable is that most of the hanging wall and footwall rocks to the Wolverine deposit have high AI and CCPI values (Table 7.2). Immobile Element Systematics Throughout the F L D it is clear that many of the major elements, and likely the LFSE, were mobile during alteration and metamorphism (see above). In this section, the immobile element geochemical features of the rocks from the F L D are presented in Figures 7.8 to 7.14 and Tables 7.1 and 7.2. Grass Lakes Succession: Fire Lake Unit Felsic samples from the F L U are those that are found primarily in the M C T sheet and are subdivided into calc-alkaline rhyolite-dacite flows and subvolcanic intrusions and tholeiitic rhyolites. Calc-alkalic rhyolite-dacite flows from the F L U have moderate Zr/Ti0 2 and Nb/Y values that suggest a subalkaline affinity (Fig. 2.8a). The HFSE contents (Nb, Ta, Ga, Zr, Hf, Y) within these rocks are moderate to low and are characteristic of volcanic arc rocks (Fig. 2.8b-c) with I-type affinities (Fig. 2.8d). Primitive mantle-normalized plots are characterized by LPvEE-enrichment, strong negative Nb and Ti anomalies, weakly negative Eu anomalies, low Al , Sc and V, and flat to weakly positive Zr and Hf anomalies (Fig. 2.9a; Table 2.2). The F L U calc-alkalic rhyolites have moderate Zr/Nb and Zr/Y values, similar to published values for calc-alkalic rocks (Leat et al., 1986; Barrett and MacLean, 1999; Fig. 2.10b; Table 2.2). The calc-alkaline rhyolites have moderate Zr/Sc and Zr/Ti0 2 ratios (Fig. 2.10c-d), moderate to low Ti/Sc ratios; chondritic (-36) to super-chondritic (>36) Zr/Hf ratios; and Nb/Ta that are broadly chondritic (-17; Table 2.2). The geochemical attributes of the calc-alkalic QFP intrusions are very similar to the calc-alkalic rhyolite flows with similar volcanic-arc (I-type) affinities (Fig. 2.8; Tables 2.1 and 2.2). The intrusions have similar primitive mantle-normalized patterns to the calc-alkaline flows but with less strongly developed Zr and Hf anomalies (Fig. 2.9; Table 2.2). The intrusions, however, have lower Nb/Ta and Zr/Hf values, and higher Ti/Sc values than the calc-alkalic flows (Table 2.2). Tholeiitic rhyolite flows have distinct geochemical features when compared to the calc-alkalic rocks. Although the flows have Zr/Ti0 2 values similar to the calc-alkalic rocks, they are displaced to 27 lower Nb/Y values (Fig. 2.8a; Table 2.2). In Nb-Y and Ta-Yb space, the tholeiitic flows lie more towards the ocean floor field and are not within the continuous array from the volcanic arc to within-plate fields, reflecting their low Nb and Ta values (Fig. 2.8b-c; Table 2.2.1). The Ga/Al ratios of the tholeiitic rocks straddle the I/S-type to A-type boundary with moderate Zr contents (Fig. 2.8d). The primitive mantle-normalized plot of the tholeiitic flows has a flatter pattern than the calc-alkalic rocks with minor L R E E enrichment; however, like the calc-alkalic suites they have strong negative Nb and Ti anomalies (Nb/Nb* = 0.40-0.47; Ti /Ti* = 0.07-0.15), and depletions in Al , Sc, and V (Fig. 2.9c). Notably Zr and Hf are consistently lower relative to surrounding REE (Zr/Zr* = 0.75-0.90; Hf/Hf* = 0.57-0.92) which is different than the calc-alkalic suites. The tholeiitic rhyolites also have higher Zr/Nb, Nb/Ta and Zr/Hf values, and lower Zr/Y, Zr/Sc, and Ti/Sc values than the calc-alkalic rocks (Fig. 2.10; Table 2.2). Grass Lakes Succession: Kudz Ze Kayah Unit The felsic rocks from the K Z K unit have remarkable similarities in their geochemical characteristics regardless of lithology and as such are treated geochemically as a common entity. When compared to the F L U felsic rocks, the K Z K unit have higher Zr/Ti0 2 ratios, and have Nb/Y ratios that straddle the alkaline-subalkaline boundary (Nb/Y = 0.7; Fig. 2.8a; Table 2.2). The K Z K unit rocks have elevated HFSE (Table 2.2.1) and plot in the fields for within-plate felsic rocks (Fig. 2.8b-c), with predominantly A-type affinities (Fig. 2.8d), and more specifically crustally derived A-type felsic rocks (Fig. 2.11). The primitive mantle-normalized patterns of the K Z K unit rocks are very similar to calc-alkalic rocks ofthe F L U , with L R E E enrichment, negative Nb and Ti anomalies, depletions in Al , Sc and V, weakly negative to strongly positive Zr and Hf anomalies, and strongly negative Eu anomalies (Fig. 2.12). Although having a similar shape to F L U calc-alkalic rocks, the patterns are characterized by higher total REE, HFSE and are shifted upwards relative to those ofthe F L U (Fig. 2.12). The HFSE enrichment in these rocks is illustrated by the moderate covariation of Zr and Nb; with some samples having very high Zr and Nb contents (Zr >500 ppm), typical of peralkalic rocks (Fig. 2.10a; Leat et al., 1986). The K Z K unit have a wide variation in Zr/Y ratios but average values are transitional to calc-alkalic (Fig. 2.10b; Table 2.2; Barrett and MacLean, 1999). Of significant importance is that felsic rocks of the K Z K unit exhibit a strong covariation of Zr with Sc and Ti (Fig. 2. lOc-d), accompanied by very high Zr/Sc and 28 Zr/Ti0 2 ratios (Table 2.2). These high ratios result in distinctive trends relative to the F L U rocks and the hanging wall rhyolites from the Wolverine deposit (Fig. 2.10c-d). The K Z K unit rocks also have high Th contents when compared to the F L U felsic rocks (Table 2.1). The Ti/Sc ratios of the K Z K felsic rocks overlap but have average values much higher than the F L U felsic rocks; Zr/Hf values vary but average values are near chondritic (~36) to weakly super-chondritic (>36; Taylor and McLennan, 1985; Wedepohl, 1995); average Nb/Ta values of the K Z K unit felsic rocks are typical of rocks that have been derived from continental crust (Nb/Ta -11-12 and <17; Table 2.2; Taylor and McLennan, 1985; Wedepohl, 1995; Green, 1995; Barth et al., 2000). Wolverine Succession: Unit 5f/qfp Felsic rocks from unit 5f/qfp from the Wolverine succession have geochemical attributes remarkably similar to those from the K Z K unit. There is considerable geochemical coherency between the different lithologies within unit 5f/qfp. The unit 5f/qfp felsic rocks have high Zr/Ti0 2 values and Nb/Y values that straddle the alkaline-subalkaline boundary (Fig. 2.8a; Table 2.2). Unit 5f/qfp rocks are HFSE-enriched and straddle the field for within-plate to volcanic arc rocks (Fig. 2.8b-c), with features typical of A-type to fractionated I/S-type granitoids (Figs. 2.8d and 2.11). Their primitive mantle-normalized signatures are characterized by LREE-enriched patterns, negative Nb and Ti anomalies, variably negative Eu anomalies, and depletions in Al , Sc, and V (Fig. 2.13). The Zr/Y ratios of the rhyolite flows vary, likely due to Zr compatibility (Watson and Harrison, 1983), but average values are transitional (Fig. 2.10b; Table 2.2; Barrett and MacLean, 1999). Similar to the K Z K unit felsic rocks, the unit 5f/qfp have a moderately strong correlation of Zr with Sc and Ti and correspondingly high Zr/Sc and Zr/Ti0 2 ratios (Fig. 2. lOc-d; Table 2.2). The Ti/Sc values of unit 5f/qfp felsic rocks are high (Table 2.2) and have values similar to those for the upper continental crust (Taylor and McLennan, 1985; Wedepohl, 1995). The Zr/Hf ratios of the rocks are variable but average values are near chondritic (-36); Nb/Ta values are low overlap values for the continental crust (-11-12; Table 2.2; Taylor and McLennan, 1985; Wedepohl, 1995; Green, 1995; Barth et al , 2000). 29 Wolverine Succession: Unit 6 Felsic rocks of unit 6 rocks comprise the footwall and hanging wall to the Wolverine VHMS deposit. Both the footwall intrusions and tuffaceous rocks have similar geochemical attributes with high Zr/Ti0 2 and moderate Nb/Y values (Fig. 2.8a), with HFSE systematics typical of within-plate volcanic arc rocks (Fig. 2.8b-c) and crustally-derived A-type felsic rocks (Figs. 2.8d and 2.11). Primitive mantle-normalized plots for these rocks are characterized by LREE-enrichment, negative Nb and Ti anomalies, negative but erratic Eu anomalies, and depletions in Al , Sc, and V (Fig. 2.14a-b; Table 2.2). The footwall felsic rocks have high Zr and Nb contents (Table 2.2; Fig. 2.10a), and varying Zr/Y ratios, that are on average transitional to calc-alkalic in nature (Fig. 2.10b; Table 2.2). These felsic rocks also have high Zr/Sc and Zr/Ti0 2 values similar to the unit 5f/qfp and K Z K unit felsic rocks (Fig. 2.10c-d; Table 2.2). The Ti/Sc ratios for the footwall rocks are similar to values for the continental crust; Zr/Hf values vary but are subchondritic (<36) to superchondritic (>36); and Nb/Ta values range from crustal values (-11-12) to nearly chondritic (-17) values (Table 2.2; Taylor and McLennan, 1985; Green, 1995; Wedepohl, 1995; Barth et al , 2000). The hanging wall aphyric rhyolites to the Wolverine deposit have very distinctive geochemical features when compared to most other felsic rocks of the FLD. The Wolverine aphyric rhyolites have distinctly low Zr/Ti0 2 values and subalkalic Nb/Y ratios (Fig. 2.8a; Table 2.2). The HFSE contents ofthe aphyric rhyolites are low and the samples plot within fields for volcanic arc felsic rocks (Fig. 2.8b-c). The moderate Ga/Al ratios in the aphyric rhyolites are due to their low A1 2 0 3 contents; this results in them plotting on the boundary ofthe I/S- to A-type field with moderately low Zr contents (Fig. 2.8d). The primitive mantle-normalized plot of the aphyric rhyolites is flatter than the footwall rocks and is less LREE-enriched, but still exhibits negative Nb and Ti anomalies (Fig. 2.14c). Aluminum and Sc are relatively depleted in these rocks, but notably the V values are elevated relative to Sc which suggests possible magnetite accumulation (Fig. 2.14c). The aphyric rhyolites have intermediate Zr/Nb values and the average Zr/Y values are transitional (Fig. 2. lOa-b; Table 2.2). Zirconium covaries with Ti and Sc but with much lower Zr/Sc and Zr/Ti0 2 values when compared to all other felsic rocks in the F L D (Fig. 2. lOc-d; Table 2.2). The average Ti/Sc values ofthe aphyric rhyolites are very low, average Zr/Hf ratios 30 are slightly super-chondritic (>36), and the average Nb/Ta values are slightly sub-chondritic (<17)(Table 2.2; Taylor and McLennan, 1985; Green, 1995; Wedepohl, 1995; Barth et al., 2000). Discussion Petrogenesis and Tectonic Setting of Felsic Rocks in the Finlayson Lake District Fire Lake Unit The geochemical and geological features of felsic rocks in the F L U are consistent with formation within a -365-360 Ma (Grant, 1997; Mortensen, 1992a,b) "continental" margin arc setting. The HFSE and REE depleted signatures of these rocks, coupled with negative Nb and Ti anomalies on primitive mantle-normalized plots, are similar to felsic rocks formed in arc environments (e.g., Pearce and Peate, 1995). Some workers have shown, however, that Nb and Ti depletions and arc signatures in felsic rocks can originate from remelting of rocks with arc parentage (e.g., Whalen et al., 1998; Morris et al., 2000), and can arise from the fractionation of HFSE-enriched accessory phases (e.g., Green and Pearson, 1987; Ryerson and Watson, 1987; Lentz, 1999), regardless of tectonic setting. Nevertheless, the felsic rocks of the F L U are stratigraphically interlayered and magmatically co-mingle with calc-alkalic and island-arc tholeiitic mafic rocks (Piercey and Murphy, 2000), typical of rocks from arc environments suggesting they represent formation within a Devonian-Mississippian arc system. Grant (1997) suggested that this Devonian-Mississippian arc magmatism in the Y T T was built upon a composite basement. Their evidence was primarily based on the mixture of juvenile and evolved geochemical and isotopic features exhibited by the sedimentary rocks and felsic and mafic rocks of the F L U . Some key HFSE ratios (Ti/Sc and Nb/Ta) support the possibility of a composite basement to this arc. For example, felsic volcanic rocks of the F L U have greater Nb/Ta (15.9-21.7) and lower Ti/Sc (191-347) values than most other rocks of the F L D (Table 2.2). It has been established that mantle and mantle-derived rocks typically have Nb/Ta values -17.5 (Sun and McDonough, 1989; Green, 1995) whereas the continental crust and rocks derived from it have values -11-12 (Taylor and McLennan, 1985; Green, 1995). The high Nb/Ta values (20.0-21.7) for the tholeiitic rhyolitic rocks and the lower but still chondritic Nb/Ta values (15.9-17.4) for the calc-alkaline rhyolitic flows suggest possible derivation from mafic or mantle-like crustal sources. This is also supported by the Ti/Sc ratios of the volcanic rocks. As 31 Sc is more compatible than Ti , mafic or mantle-derived crustal sources will have lower Ti/Sc than more felsic sources. The tholeiitic and calc-alkaline rhyolite flows ofthe F L U have distinctly lower Ti/Sc values than most other felsic rocks in the F L D (Table 2.2) supporting derivation from a possible mafic or mantle-like crustal sources. An equally viable alternative for the Nb/Ta and Ti/Sc behavior is that the F L U felsic volcanic rocks are mafic melts that have been contaminated by continental crust during emplacement. Preliminary Nd isotopic data on the tholeiitic and calc-alkalic rhyolites (sNd 3 5 0 = +0.11 to -4.80; Chapter 8) would support either hypothesis. The QFP intrusions in the F L U have distinctly lower Nb/Ta (12.3-13.4) and higher Ti/Sc (174-329) values than the felsic volcanic rocks with values similar to the continental crust (Taylor and McLennan, 1985; Wedepohl, 1995; Green, 1995; Barth et al , 2000). Grant (1997) documented an sNd 3 5 0 value of-12.8 for a F L U QFP intrusion in the M C T and Mortensen (1992b) documented inherited Proterozoic zircon in a M C T QFP, both of which support their derivation from evolved continental crust. The greater crustal signature in the QFP intrusions relative to the volcanic rocks may be due to a longer crustal residence time for the intrusions, allowing them to interact more extensively with continental crust. In summary, the felsic rocks of the F L U are associated with bimodal volcanism with calc-alkaline and tholeiitic affinities, that are interpreted to have formed within a Devonian-Mississippian arc system built upon a composite basement. Whether or not this arc was developed on the distal edge ofthe North American craton (e.g., Mortensen, 1992a; Creaser et al , 1997) is presently uncertain. Kudz Ze Kayah Unit The felsic rocks of the K Z K unit are very distinctive when compared to those in the F L U , or those in the hanging wall of the Wolverine deposit. Compared to the latter felsic rocks, the K Z K unit and felsic rocks in the footwall of the Wolverine deposit, have higher HFSE and ZREE contents and HFSE-ratios (Tables 2.1 and 2.2) suggesting different petrogenetic origins. The Nb/Ta (averages = 11.6-15.8) and Ti/Sc (averages = 313-345) ratios ofthe K Z K unit are notably lower and higher, respectively, relative to F L U felsic rocks and very similar to values for the continental crust (-11-12; Taylor and McLennan, 1985; Wedepohl, 1995; Green, 1995; Barth et a l , 2000). The close similarity of these ratio to those ofthe 32 continental crust suggests that the K Z K unit felsic rocks were derived from continental crustal melting. This hypothesis is further supported by the ubiquitous occurrence of inherited Proterozoic zircon in these -felsic rocks (Mortensen, 1992a; Appendix 2) and evolved Nd isotopic signatures (eNdt = -7.8 to -8.5; Chapter 8). Isotopic data for coeval plutonic rocks in the Grass Lakes suite granitoids (8 7Sr/8 6Srj = 0.716-0.728; eNd t (average) = -15±1; Mortensen, 1992a), add further support for a crustal partial melt origin for the felsic rocks of the K Z K unit. The origin of the K Z K unit felsic rocks from melting continental (or continent-derive) crust is significant because the nature of this melting event likely controlled the HFSE-REE characteristics of these rocks. In the continental crust; most HFSE and REE in the continental crust reside in HFSE-enriched accessory minerals (Green and Pearson, 1987; Ryerson and Watson, 1987; Bea, 1996a,b). The efficiency of melting and dissolution of these minerals during crustal melting will control the HFSE-REE budget of the felsic magma (e.g., Watson and Harrison, 1983; Bea, 1996a,b). Numerous workers have shown that during crustal melting, the HFSE-REE budget of the melts is strongly controlled by the temperature of crustal fusion (Watson and Harrison, 1983; Whalen et al., 1987; Creaser and White, 1991; Bea, 1996a,b; Watson, 1996). The high HFSE and REE contents of the K Z K unit felsic rocks suggest that the HFSE-REE budget reflects high temperature melting of continental crust and efficient dissolution of HFSE-REE-enriched accessory phases. A high temperature crustal melt origin is partly supported by the zircon saturation temperatures (Watson and Harrison, 1983) of the felsic rocks. Zircon saturation temperatures are the temperatures at which zircon saturates in a melt (Watson and Harrison, 1983). Some workers have interpreted zircon saturation temperatures to approximate the emplacement temperature of felsic magmas (Barrie, 1995), and notably the K Z K unit felsic rocks have high zircon saturation temperatures (T-ZrSataverage > 837°C; Table 2.2) supportive of a high temperature crustal melt origin. The elevated HFSE contents in the K Z K unit felsic rocks are also reflected by high HFSE/compatible element ratios. Specifically, the K Z K unit felsic rocks have distinctive trajectories and high Zr/Sc and Zr/Ti0 2 ratios similar to the felsic rocks from the footwall of the Wolverine deposit, but distinctive from those in the F L U and hanging wall of the Wolverine deposit (Fig. 2.10c-d; Table 2.2). Barrett and MacLean (1999) have suggested that covarying Zr-Ti0 2 arrays can reflect mass changes of a 33 single homogeneous protolith. Although there is likely some component of mass change in the K Z K unit rocks, the common incompatible element denominator (Yb) in these plots (Fig. 2.10c-d), suggest that the covarying Zr-Sc and Zr-Ti0 2 arrays cannot be accounted for solely by mass change. Therefore this requires an alternative interpretation. In the Bathurst mining camp, Lentz (1999) found similar Zr-Sc and Zr-Ti0 2 arrays and high Zr/Sc and Zr/Ti0 2 ratios in many VHMS-related felsic rocks and attributed the variations to different temperatures of crustal fusion. Furthermore, Lentz (1999) and Whalen et al. (1998) found that the Bathurst Mining Camp VHMS-related felsic rocks had elevated HFSE and REE contents similar to the K Z K unit rhyolites and attributed these to high-temperature crustal fusion. These studies and the data presented herein suggest that the HFSE-compatible element systematics in the K Z K unit felsic rocks represent thermal controls on crustal melting rather than mass change variations. The geochemical systematics of the K Z K unit felsic rocks suggest a fundamentally different tectonic setting for the K Z K unit rocks relative to the F L U rocks. Mortensen (1992a) suggested that granitoids of the Grass Lakes suite in the K Z K unit reflected magmatism within a west-facing volcano-plutonic arc complex of Devonian-Mississippian age. This interpretation could be bolstered by the negative Nb and Ti anomalies on primitive mantle-normalized plots (Fig. 2.12) of the K Z K unit felsic rocks of the K Z K unit. However, many workers have shown that these signatures can arise from the remelting or arc crust (Whalen et al , 1998; Morris et al , 2000), or from the fractionation of HFSE- and Ti-enriched phases (Green and Pearson, 1987; Ryerson and Watson, 1987; Lentz, 1999). The possibility of Ti-rich phase crystallization is supported in part by the broadly inverse relationship ofthe size of the negative Nb anomaly (Th/Nbp m) to T i 0 2 and Sc (Fig. 2.15a-b) suggesting that the signature does not necessarily reflect an arc signature. The evidence for a crustal melting history also suggests that the negative Nb signature might be inherited from it's crustal source as the upper continental crust has a negative Nb anomaly on primitive mantle-normalized plots (Taylor and McLennan, 1985). Other lines of evidence also support a back-arc setting. For example, the K Z K unit felsic rocks are intercalated with voluminous carbonaceous sedimentary rocks of basinal character, and are crosscut and overlain by HFSE-enriched weakly alkaline mafic rocks with ocean island basalt (OIB) signatures (Chapter 6). The presence of V H M S mineralization also provides indirect evidence for back-arc or non-arc setting because in the 34 modern environment, most hydrothermal vent sites are located in non-arc settings (e.g., Hannington et al., 1998; Herzig and Hannington, 1996). The geological and geochemical characteristics of the K Z K unit suggest formation within a back-arc basin environment. Mortensen (1992a) suggested that Devonian-Mississippian Y T T arc magmatism formed above an west-facing (east-dipping) subduction zone proximal to, but not necessarily part of North America. The felsic rocks in the K Z K unit likely reflect rifting and subsequent ensialic back-arc basin generation within this arc. The formation of the K Z K unit back-arc environment likely involved the westward migration of the Mortensen's (1992a) arc resulting in back-arc rifting and basin generation. Basalting underplating of the crust by enriched asthenospheric or lithospheric magmas, represented by the crosscutting alkaline magmas of unit 4 (Chapter 6), resulted in high temperature melting of continental crust and the formation of the felsic rocks in the K Z K unit. Devonian-Mississippian arc rifting and the synvolcanic faults associated with it (Murphy and Piercey, 2000) are interpreted to have been the regional-scale controls on the localization and formation of the K Z K and GP4F V H M S deposits (Murphy and Piercey, 2000). Wolverine Succession The felsic volcanic rocks of Wolverine succession show considerable variation. Rocks footwall to the Wolverine deposit have similar HFSE-REE systematics as the K Z K unit rocks, whereas those in the hanging wall have distinctive systematics and lower REE and HFSE contents. Rocks in the footwall of the deposit have high HFSE and REE contents (Table 7.1), Ti/Sc and Nb/Ta ratios similar to values for the continental crust (Table 7.2), inherited Proterozoic zircon (Mortensen, 1992a; Appendix 2), and evolved Nd isotopic signatures (sNdt = -7.8 to -8.2; Chapter 8). These features all indicate an origin from high temperature melting of continental crust, similar to rocks from the K Z K unit. The hanging wall aphyric rhyolites in the Wolverine deposit have lower HFSE and REE contents and distinct trajectories in Zr-Sc and Zr-Ti0 2 space (Fig. 2. lOc-d) compared to all other felsic rocks in the FLD. The covariation between Zr and Sc and T i 0 2 in these rocks may reflect mass changes (Barrett and MacLean, 1999), or differential temperatures of crustal fusion (Lentz, 1999). The silicified nature of the aphyric rhyolites point to a possible role for mass change; however, the ratio plots in Figures lOc-d 35 remove this mass effect by having a common incompatible element denominator (Yb). Therefore, the lower Zr-Sc and Zr-Ti0 2 trajectories and accompanying Zr/Sc and Zr/Ti0 2 ratios must reflect processes other than mass change. The lower HFSE and R E E contents, and Zr/Sc and Zr/Ti0 2 ratios, for the aphyric rhyolites may reflect melting ofthe same crustal source as the footwall rocks, but at lower temperatures (e.g., Watson and Harrison, 1983; Bea, 1996a,b; Watson, 1996). Lentz (1999) proposed a similar explanation for felsic rocks of the Bathurst mining camp. This hypothesis is partly supported by Zr-saturation temperatures (Table 2.2). An alternative hypothesis is that the footwall and hanging wall felsic rocks were derived from different crustal sources (e.g., F L U felsic rocks). Preliminary Nd isotopic data, however, illustrate that there is very little difference between the footwall tuffs (eNd, = -8.2) and the hanging wall aphyric rhyolites (eNdt = -7.1) (Chapter 8) supporting differential temperature melting of a common evolved crustal source. The geological and geochemical features of footwall felsic rocks from the Wolverine deposit are identical to those ofthe K Z K unit and suggest the persistence of a similar ensialic back-arc rift/basin geological environment. However, the two units are separated by an angular unconformity, and temporal constraints suggest that the Wolverine succession is younger than the K Z K unit (Mortensen, 1992a; Appendix 2). Although the lower part ofthe Wolverine succession had a similar setting as the K Z K unit there are fragments of rocks from the K Z K unit in the basal unit of the Wolverine succession (Murphy and Piercey, 1999) requiring a period of uplift, erosion, and disruption of K Z K unit back-arc magmatism before the commencement of Wolverine succession back-arc magmatism. Evidence for localized compression has been documented in the upper parts of the K Z K unit by Murphy (1998). This localized deformation episode was likely responsible for the uplift and disruption of back-arc magmatism, and may have been due to slab trajectory changes (e.g., Uyeda and Kanamori 1979; Hawkins et al , 1984), or localized plate reorganizations that induced a brief compressive episode. The back-arc setting for the Wolverine succession is further supported by basaltic rocks in the hanging wall of the Wolverine deposit, near the top of the Wolverine succession. These basalts stratigraphically overlie the aphyric rhyolitic rocks (Bradshaw et al , 2001) and are characterized by MORB geochemical signatures (Plint and Gordon, 1997; Chapter 8). The occurrence of MORB-type 36 basalts suggests that the Wolverine back-arc basin likely evolved to full seafloor spreading. How wide this basin eventually opened is uncertain since much of this record may have been removed by the unconformable overlap of the Late Paleozoic Campbell Range succession (Murphy, 2001). Modern and Ancient Analogues to the Finlayson Lake District Understanding the setting of ancient V H M S districts are often aided by their comparison to similar ancient districts and modern analogues (e.g., Hannington et al., 1995; Herzig and Hannington, 1996). Felsic-associated V H M S deposits from the FLD have similar features to the Cambro-Ordovician Mount Windsor Subprovince of Australia, the Bathurst district of Canada, and possibly the modern day Okinawa Trough of Japan. The Mount Windsor Subprovince exhibits strong similarities to the FLD. Firstly, both districts are characterized by similar lower stratigraphic assemblages consisting of continent-derived sedimentary material (Puddler Creek Formation vs. unit 1 of the Grass Lake succession) (e.g., Berry et al., 1992; Murphy and Piercey, 1999, 2000). Stoltz (1995) suggested that felsic volcanic rocks in the Mount Windsor district were the products of partial melting of Precambrian crustal basement (eNdt = -4.7 to -12.8) due to slab roll back and generation of an intra-continental back-arc basin. A similar petrogenetic origin and geodynamic setting is proposed for the VHMS-related felsic volcanic rocks of the FLD. However, the relationship of VHMS mineralization to felsic volcanism in the Mount Windsor Subprovince differs from that of the FLD. For example, unlike the K Z K , GP4F and Wolverine deposits, which are hosted by felsic volcanic rocks, the Mount Windsor Group deposits occur either at the contact between the felsic dominated Mount Windsor Formation and the overlying basalt-andesite-dacite-rhyolite-volcaniclastic dominated Trooper Creek Formation (e.g., Thalanga deposit), or are hosted within the Trooper Creek Formation (e.g., Highway-Reward deposit; Berry et al., 1992; Stoltz, 1995; Doyle and Huston, 1999). Furthermore, the felsic volcanic rocks within the Mount Windsor Subprovince do not exhibit as elevated HFSE contents as those of the F L D (see Stoltz, 1995 and Lentz, 1998) suggesting a possible lower temperature of crustal fusion. Regardless of these differences, the overall volcano-sedimentary character of the succession, the nature and interpreted origin of the felsic volcanism, 37 formation upon Precambrian continental (or continent-derived) basement, and the intra-continental back-arc basin/rift environment are analogous to the FLD. The Ordovician Bathurst mining camp in Canada is the best ancient analogue to the F L D in terms of stratigraphy, geochemistry and geodynamic setting. The Bathurst mining camp is characterized by a lower continent-derived sedimentary sequence (Miramichi Group), which is disconformably overlain by the felsic volcanic and sedimentary rocks of the Tetagouche Group (van Staal et al , 1991, 1992). The Tetagouche Group consists of two packages of felsic volcanic and sedimentary rocks (Nipisiguit Falls and Flat Landing Brook formations; van Staal et al , 1991, 1992), that are strikingly similar to the rocks of the F L D , and host the massive sulphide deposits of the Bathurst mining camp. Although the deposits of the Bathurst mining camp are larger and have greater tonnage (McCutcheon, 1992), their volcano-sedimentary setting is similar to the Wolverine, GP4F and K Z K deposits of the F L D (e.g., Shultze, 1996; Bradshaw et a l , 2001). Some VHMS deposits also exhibit strong similarities. For example, the Wolverine deposit bears strong similarities to Brunswick #12 deposit including the abundance of carbonaceous rocks in the host stratigraphy, including those on the contact with the massive sulphides (anoxic basin?; cf. Goodfellow and Peter, 1996), the presence of felsic volcanic and volcaniclastic rocks with similar physical and chemical compositions (Lentz, 1999; this study), and the abundance of iron formation and exhalative sedimentary rocks (Peter and Goodfellow, 1996; Bradshaw et al , 2001). Felsic volcanic rocks from the Bathurst mining camp and the F L D are virtually indistinguishable chemically. The felsic rocks from the Tetagouche Group and the K Z K unit and footwall Wolverine succession are characterized by similar HFSE-REE systematics, and they are interpreted to have similar petrogenetic origins, and to have formed in a similar tectonic setting (e.g., Lentz, 1999; this study). For example, van Staal et al. (1991) suggested that much ofthe felsic volcanic rocks of the Tetagouche Group arose from melting of crust due to asthenospheric underplating during continental arc rifting. Lentz (1999) suggested a two-stage crustal melting model within a continental arc-rift to ensialic back-arc environment. Whalen et al. (1998) suggested that the felsic metavolcanic and metaplutonic rocks ofthe Tetagouche Group were formed in an environment where arc activity was shut off due to arc-rifting or ensialic back-arc basin generation. Overall, the stratigraphy, deposit characteristics, felsic volcanic 38 geochemistry and tectonic setting of the Bathurst mining camp is similar to the F L D and provide one of the best analogues. Modern analogues of felsic volcanic- and volcano-sedimentary-hosted V H M S districts are rare (e.g., Halbach et al., 1989, 1993). The Okinawa Trough (OT) has been cited as a potential analogue to both the Bathurst district (Whalen et al., 1998; Lentz, 1996) and the Mount Windsor Subprovince (Stoltz, 1995). The Okinawa Trough is an intra-continental back-arc basin southwest of Japan that hosts Kuroko-style V H M S deposits (JADE field) within a bimodal assemblage of basalt-andesite/dacite-rhyolite, and sedimentary rocks (op cit). The setting and stratigraphy may be analogous to the FLD, and the metal inventory of the sulphide mineralization within the JADE field has somewhat similar mineralogy and local Au and Ag enrichment (Halbach et al., 1989, 1993) akin to the deposits of the F L D (e.g., Shultze, 1996; Tucker et al., 1997; Bradshaw et al., 2001). Similarly, E-MORB and OIB mafic magmatism within the Okinawa Trough (Ishizuka et al., 1990; Chen et al., 1995; Shinjo, 1999; Shinjo et al., 1999) is similar to the mafic magmatism spatially associated with the felsic V H M S deposits in the F L D (Chapters 6 and 8). The are numerous differences between the Okinawa Trough and the FLD, however. First, the geochemical attributes of the felsic rocks of the OT are quite different than those from the FLD. For example, Ishizuka et al. (1990) suggested that the felsic volcanic rocks of the OT were derived from island arc type source material with HFSE depletion unlike most VHMS-associated felsic rocks of the F L D (KZK unit and footwall Wolverine succession). Similarly, Shinjo and Kato (2000) have shown that OT felsic rocks have lower HFSE and juvenile initial 8 7 Sr/ 8 6 Sr and 1 4 3 Nd/ 1 4 4 Nd isotopic ratios (Shinjo and Kato, 2000). These workers have also suggested derivation of the Okinawa Trough felsic rocks from fractional crystallization of basaltic material or A F C processes (Shinjo and Kato, 2000), rather than derivation from crustal melting of evolved older crust, as proposed for the VHMS-associated felsic rocks of the FLD. There are also important differences in the geology of the OT. For example, the abundance of sedimentary rocks associated with the F L D is not present in the OT (e.g., Halbach et al., 1993). Similarly, the abundance of carbonaceous rocks, and possibly anoxic bottom waters during formation of some of the V H M S deposits of the F L D (e.g., Goodfellow and Peter, 1996; Bradshaw et al., 2001) are not 39 present in the deposits ofthe OT (e.g., Halbach et al , 1993). Given these differences between the FLD and Okinawa Trough there may not be an exact modern analogue to the FLD. Nevertheless, the OT provides the best modern geodynamic analogue to the FLD. Comparisons to the Geochemistry of Felsic Volcanic Rocks in Archean and Phanerozoic VHMS Environments Numerous workers have used felsic volcanic geochemistry in an attempt to decipher prospective versus non-prospective volcanic environments for VHMS deposits (e.g., Lesher et al , 1986; Barrie et al , 1993; Lentz, 1998). These prospectivity indexes were based on immobile HFSE-REE ratios and in particular the Zr/Y and La/Yb n ratios of felsic rocks to delineate ore-bearing versus barren felsic volcanic rocks (op cit). Lesher et al. (1986) noted that most VHMS deposits in the Abitibi greenstone belt were associated with tholeiitic felsic rocks with intermediate to high HFSE and REE, low Zr/Y and La/Yb n ratios (Fig. 2.16a,b). They further suggested that these ratios are characteristic of felsic rocks generated at high crustal levels that are associated with subvolcanic intrusive systems. The latter were interpreted as the source of heat for hydrothermal circulation (Lesher et al , 1986). Barrie et al. (1993) extended the study of Lesher et al. (1986) to the bulk ofthe Abitibi Subprovince and found similar results (Fig. 2.16c,d). Lentz (1998) showed that the Phanerozoic VHMS-associated felsic rocks have broadly similar HFSE-REE systematics as Archean examples (Fig. 2.16e,f). Felsic rocks from the F L D are compared to the fields for these different Archean and Phanerozoic prospectivity indexes (Fig. 16). If one considers the felsic rocks ofthe K Z K unit and the rocks footwall to the Wolverine deposit as being the VHMS-hosting felsic rocks (prospective) in the F L D then these rocks partly overlap the FII and FHIa fields of Lesher et al. (1986) and the Selbaie, Noranda and Misema fields of Barrie et al. (1993) (Fig. 2.16a,c). However, there is significant scatter in the Zr/Y-Y systematics of the F L D felsic rocks that reflect partial compatibility or melt kinetic/thermal control on the HFSE distribution (Watson and Harrison, 1983). The La/Yb n systematics of the F L D rocks are less scattered and partly overlap the FII and Selbaie fields (Fig. 2.16b,d). However, for the most part they lie within fields for less productive or barren sequences trending towards higher La/Yb n values (Fig. 2.16b,d). When compared to Phanerozoic indexes, a similar scattering of Zr/Y occurs with most of the 40 rocks overlapping many productive fields (Fig. 2.16e). In La/Yb n -Yb n space, the F L D rocks overlap predominantly with the fields for Que River, Kuroko, Tobique and Mount Windsor (Fig. 2.16f). The difference in the geochemical behaviour of the F L D felsic rocks when compared to the Archean VHMS-hosting rocks, and to many Phanerozoic rocks (Fig. 2.16) likely reflects different crustal substrates from which the felsic rocks were derived. For example, a feature common to the FLD, Que River, Kuroko, Tobique and Mount Windsor deposits/districts is that they are partially to completely underlain by evolved continental crust (Ohmoto and Skinner, 1983; Corbett, 1992; Stoltz, 1995; Whalen et al., 1998; Lentz, 1998,1999). Partial melting or contamination by continental crust during the genesis of felsic rocks would lead to LREE-enrichment due to the LREE-enriched nature of the continental crustal reservoir (Taylor and McLennan, 1985; Wedepohl, 1995). In contrast, the felsic rocks hosting V H M S deposits in Archean greenstone belts are interpreted to have formed from remelting of a mafic substrate in Iceland-type rift environments (Barrie et al., 1993; Prior et al., 1999). Remelting of a mafic (MORB-like) substrate would result in LREE-depleted, tholeiitic felsic rocks common to those hosting the Archean V H M S deposits and districts (Lesher et al., 1986; Barrie et al., 1993; Prior et al., 1999). These contrasting results illustrate that indicies for certain V H M S districts are not necessarily universally applicable to other districts and an understanding of the petrogenetic, tectonic and crustal controls on felsic volcanic genesis are more important than geochemical discrimination fields. Conclusions Felsic volcanic rocks from the F L D are associated with three units: the Fire Lake unit (FLU), the Kudz Ze Kayah (KZK) unit, and the Wolverine succession. Of these units only the Kudz Ze Kayah unit and Wolverine succession are hosted with polymetallic felsic-associated V H M S deposits (Kudz Ze Kayah, GP4F and Wolverine). Geochemical data from these rocks illustrate that felsic rocks from these units have formed under variable petrogenetic conditions within an evolving Devonian-Mississippian arc-back-arc system. Felsic volcanic and high-level intrusive rocks in the F L U have calc-alkalic and tholeiitic affinities with low HFSE contents and intermediate Zr/Sc and Zr/Ti0 2 ratios. These rocks are interlayered with mafic rocks with arc geochemical signatures, are largely VHMS-barren, and represent bimodal magmatism within an evolving Devonian-Mississippian continental arc system. The K Z K unit 41 stratigraphically overlies the F L U and consists of felsic volcanic rocks with high HFSE-contents, within-plate (A-type) signatures, and high Zr/Sc and Zr/Ti0 2 ratios. Felsic rocks of the K Z K unit are crosscut and overlain by alkalic mafic rocks, are associated with abundant carbonaceous sedimentary rocks. Felsic magmatism in the K Z K unit is interpreted to have formed within a Devonian-Mississippian ensialic back-arc rift/basin environment. The Wolverine succession is younger and unconformably overlies the K Z K unit. Felsic rocks footwall to the Wolverine deposit have similar attributes as rocks of the K Z K unit with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc and Zr/Ti0 2 ratios. In contrast, aphyric rhyolite flows in the hanging wall of the deposit have much lower HFSE, have the lowest Zr/Sc and Zr/Ti0 2 ratios in the district. All the felsic rocks of the Wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by basaltic rocks with mid-ocean ridge basalt (MORB) chemistry. The Wolverine succession is interpreted to have formed within an Early Mississippian ensialic back-arc basin environment that eventually evolved to a sea floor spreading environment. The variation in the HFSE budgets of the felsic rocks of the F L D likely reflects variations in the source and/or temperature of crustal melting. In particular, the F L U felsic rocks appear to have higher Nb/Ta and lower Ti/Sc ratios than other volcanic rocks in the district suggesting possible derivation from mafic crustal sources and/or lower crustal fusion temperatures. The K Z K unit and footwall rocks to the Wolverine deposit are interpreted to have formed from high temperature partial melting of continental crust; the hanging wall aphyric rhyolites from the Wolverine succession appear to be generated from lower temperature continental crustal melts. Polymetallic felsic volcanic-associated VHMS deposits within the F L D are preferentially associated with HFSE-enriched felsic rocks with high Zr/Sc and Zr/Ti0 2 ratios. The HFSE and rare-earth element (REE) systematics of VHMS-associated felsic rocks ofthe F L D are different than prospective felsic rocks from Archean V H M S environments in the Superior Province, and are displaced towards higher Zr/Y and La/Yb n . They have similar HFSE and REE systematics, however, to many Phanerozoic V H M S environments, in particular those partially to fully underlain by continental crust (e.g., Que River, Mount Windsor, Kuroko, Tobique). 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Geochemical variations in Middle Ordovician volcanic rocks of the northern Miramichi Highlands and their tectonic significance: Canadian Journal of Earth Sciences, v.28, p. 1031-1049. Watson, E.B, 1996. Dissolution, growth and survival of zircons during crustal fusion: kinetic principles, geological models and implications for isotopic inheritance: Transations ofthe Royal Society of Edinburgh: Earth Sciences, 87, p.43-56. 48 Watson, E.B., and Harrison, T .M. , 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types: Earth and Planetary Science Letters, v.64, p.295-304. Wedephol, K .H. , 1995. The composition of the continental crust: Geochemica et Cosmochimica Acta Cosmochimica Acta, v.59, p. 1217-1232. Whalen, J.B., Currie, K .L . , and Chappell, B.W., 1987. A-type granites: geochemical characteristics, discrimination and petrogenesis: Contributions to Mineralogy and Petrology, v.95, p. 420-436. Whalen, J.B., Rogers, N., van Staal, C.R., Longstaffe, F.J., Jenner, G.A., and Winchester, J.A., 1998. Geochemical and isotopic (Nd, O) data from Ordovician felsic plutonic and volcanic rocks of the Miramichi Highlands: petrogenetic and metallogenic implications for the Bathurst Mining Camp: Canadian Journal of Earth Sciences, v.35, p.237-252. Wheeler, J.O., and McFeely, P., 1991. Tectonic Assemblage Map of the Canadian Cordillera and Adjacent Parts of the United States of America: Geological Survey of Canada, Map 1712A, Scale 1:2000 000. Whitford, D.J., Korsch, M.J., Porritt, P.M., and Craven, S.J., 1988. Rare-earth element mobility around the volcanogenic polymetalic massive sulphide deposit at Que River, Tasmania, Australia: Chemical Geology, v.68, p.105-119. Whitford, D.J., McPherson, W.P.A., and Wallace, D.B., 1989. Geochemistry of the host rocks of the volcanogenic massive sulphide deposit at Que River, Tasmania: Economic Geology, v. 84, p.1-21. Winchester, J.A., and Floyd, P. A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, v.20, p.325-343. Wood, S.A., and Williams-Jones, A . E . , 1994. The aqueous geochemistry of rare-earth elements and yttrium. Part 4. Monazite solubility and R E E mobility in exhalative massive sulphide-depositing environments: Chemical Geology, v.l 15, p.135-162. 49 Figure 2.1. Location of the F L D and VHMS deposits with respect to the Yukon-Tanana Terrane, Yukon, Canada (modified from Wheeler and McFeely, 1991 and Hunt, 1998). 5 0 13T45 ' 1 3 H 5 ' 13 TOO' 61°30' 61 "15' Kilometres 61°00' Q Quaternary sediments Pennsylvanian-Permian Campbell Range Succession Chert, chert-pebble conglomerate, sandstone Mafic volcanic, volcaniclastic SSSK and intrusive rocks I | Diamlctlte, mafic tuff, ollsostromal 1****1 carbonate, chert, sandstone Mississippian Wolverine Succession Unit 6: Felsic volcanic rocks, r-e-formatlon. mixed tuffs Unit 5cp: Carbonaceous arglllite and phyllite Unit 5f/qfp: Quartz-bearing feist volcanlcs and Ngh-level intrusive rocks Mississippian Unit 51: Quartz-feldspar4-shale L: .i chip conglomerate Unconformity Devonian (mostly) Grass Lakes Succession Kudz Ze Kayah Unit (Unit 4): Carbonaceous KSxSS phyllite, rift-related mafic dykes, quartzite Kudz Ze Kayah Unit (Unit 3): Felsic volcanic h <- and shallow level Intrusive rocks, —^ carbonaceous phyllite, turblditic sedimentary rocks Fire Lake Unit (Unit 2): Mafk: volcanic cxxx] and intrusive rocks, carbonaceous phyllite, <^>?*> lesser felsic volcanic rocks Unit 1: Quartz-( + blofite)-rch metaclastic rocks calc-slllcates, rare felsic horizons Intrusive Rocks Cretaceous ^A^*j Peraluminousgranitoids Mississippian j- ^ "*| Grass totes Suite: Peraluminous granitoids r» » - | Simpson Range Pbtonic Suite: Metalumlnous L * . xl granitoids | ~ i JT1 Simpson Flange PUtonb Suite: Sheared metalumlnous granitoids •Mlssissipptan? serpentinized harzburgltes and ultramafic rocks (intrusbns?) Other _ _ _ _ _ Faults, displacement uncertain •H^r-***- Money Creek Thrust Fault VHMS Deposit Figure 2.2. Regional geological setting of the FLD after Murphy and Piercey (1999, 2000). 51 ^<Jr\i-<3pa < ^ • = < ^ N _ <=> 345-350Ma '^"AT^^'. uc n S in 1 co J 9 I W O 1 v e r i n el ~" ~~ =3 ne T f CO • ^..".'."V v v v v v v v Y-^v1: . . . _. _. CO to IO CO pr-VV V V V V V V V V V V V V V V V V V V v v v v v v v v v v v v v v v v v v v v v v -5f/q Wolv 1 3 Wolv Figure 2.3. Schematic stratigraphic relationship of felsic rocks in the hanging wall of the Money Creek thrust in relation to the footwall which forms the bulk of the Finlayson Lake district (a). Schematic stratigraphic sections for felsic rocks ofthe Fire Lake unit (FLU) in the hanging wall ofthe Money Creek thrust fault (b), and felsic rocks of the Kudz Ze Kayah (KZK) unit and Wolverine succession (c). Figures modified from Murphy and Piercey (1999, 2000) and Piercey and Murphy (2000). 52 Figure 2.4. Macroscopic features of felsic rocks from the various stratigraphic levels of the FLD. A. K-feldspar porphyritic rhyolitic flow from the F L U with a massive matrix and small mm-scale K-feldspar phenocrysts scale. Scale at the base of slab is in centimeters. More photos of the rocks in the F L U can be found in Piercey and Murphy (2000); B. Weakly to moderate chlorite-sericite altered K-feldspar porphyritic high-level intrusive rocks from the Kudz Ze Kayah unit containing well preserved cm-scale K-feldspar phenocrysts. Scale at the base ofthe slab is in centimeters; C. Moderately altered aphyric rhyolite flow/intrusion from the Kudz Ze Kayah unit with a network of mm-scale veinlets of sericite+/-quartz. Slab is 20 cm wide; D. Coarse grained felsic volcaniclastic (tuffaceous) rock from the Kudz Ze Kayah unit containing K-feldspar phenocrysts in a siliceous matrix. The dark streaks within this sample are likely preserved sedimentary fragments. Note how the fabric is stronger developed in this sample and how the feldspars are elongate into an elliptical shape. Scale at the base ofthe slab is in centimeters; E. Coarse grained felsic volcaniclastic rock from the unit 5f/qfp of the Wolverine succession containing large cm-scale K-feldspar grains (white) and slightly smaller elliptical black to blue glassy quartz grains (darker grey to black) set within a fine grained matrix. The matrix to this sample is weakly altered to sericite and lesser chlorite. Scale on the side ofthe slab is in centimeters; F. Feldspar porphyritic intrusion with cm-scale feldspars set in a siliceous matrix from the unit 6 in the immediate footwall to the Wolverine VHMS deposit with very weak silica-sericite-pyrite alteration. Scale on the side of the slab is in centimeters; H. Felsic volcaniclastic rock with fine quartz and feldspar crystals with minor sericite-silica-carbonate alteration. This rock in places has coarse cm-scale feldspar crystals and forms the immediate footwall to the Wolverine VHMS deposit (Bradshaw et a l , 2001). Scale at the base of the slab is in centimeters. I. Massive aphyric rhyolite from unit 6 of the Wolverine succession that forms part of the hanging wall to the Wolverine VHMS deposit. This rhyolite is very silicified and has sericite-pyrite veinlets (medium gray) and carbonate veinlets (bright white, left side of the photo). These rocks are associated with iron formations and carbonate exhalative rocks and as such are typically elevated in base metals and silica relative to other rhyolites in the district. Scale at the base of the slab is in centimeters. 53 5 4 Figure 2.5. Photomicrographs of F L D felsic rocks. A. Glomerporphyritic texture of euhedral K-feldspar that is partially replaced by finer sericite within a matrix of quartz-clay minerals and sericite within unit 2 calc-alkalic rhyolites (field of view (FOV) = 3.9mm; cross polarized light (XPL)); B. Relatively unaltered K-feldspar from a K Z K unit FPI. The feldspar grain is deformed and shows brittle fracturing with partial infilling by clays and quartz, and surrounded by a fabric of recrystallized quartz and muscovite which wrap around the feldspar (FOV = 5.1mm; XPL); C. Quartz crystals within unit 5f/qfp fine grained felsic volcaniclastic rock (tuff). The quartz crystals within this rock are boudined, partially cracked, and partially wrapped by a clay-quartz-sericite matrix. Note on the bottom left of the larger grain is a smaller grain with a possible resorbed edge (FOV = 5.1mm; XPL); D. Well preserved euhedral feldspar grain within a matrix of sericite and quartz from unit 6 FPI (FOV = 3.9mm; XPL); E . Boudined quartz grain with possible resorbed edges partially wrapped by muscovite and within a recrystallized quartz-sericite matrix from unit 6 footwall tuff from the Wolverine deposit (FOV = 5.1mm; XPL); F. Rare quartz phenocryst that is boudined into the fabric of a unit 6 rhyolite from the hanging wall of the Wolverine deposit. Matrix contains largely recrystallized quartz, biotite, oxides (magnetite) and sericite (not visible) (FOV = 3.9mm; plane-polarized light (PPL)). 55 5 6 i 1 1 r O O O C D r r C M O O O t D - t C S I O 0^+0 BN 1-2 ' C N 60.S O PH S > o B.ff cfl 2 _ S to _ ° A _ .y _ jrj , 3 • _ ^ —i «S M ' PH t-1 UH I M H tit ••H _> S3 -N IT) U-l .fl .-fl .fl . O CD) + O'BN (»+eN)/IV VO 5 7 100 90 80 70 CCPI 60 50 40 30 20 10 0 epidote calcite dolomite ankerite chlorite pvgte Alteration Field • + i i Diagenetic Field • i v • ++ + + + + _u + ^ + + • 4.. + io least altered felsics: Albiti sericite K-feldspar 0 10 20 30 40 50 60 70 80 90 100 Al Figure 2.7. An alteration box plot of Large et al. (in press) with vectors for various alteration minerals and alteration versus diagenetic trajectories, further details provided in the text. CCPI = chlorite-carbonate-pyrite index, AI = Ishikawa alteration index. Symbols as in Figure 2.6. 5 8 N I 1 1 M I N I 1 1—I M I N I 1 1 1 I ! I 1 I I 1 0 0 0 J3 z .01 10 1 0 0 1 0 :(b) within plate A-type syncollisional S-type volcanic arc P i-type 1 0 0 0 Nb/Y 1 0 0 10 -(c) I I I I I I | I I I [ M i l l I I I I T T T T T syn-collisional S volcanic arc , . . . . l-typ« -mlf Vtf H<r ocean ridge #+4-1+ H+ + r OR-type + -+H++ + + M-type _ J i ; I | i I I l l I i | i i i i u l 1000 1 0 1 0 0 N 100 Yb 104Ga/AI Figure 2.8. A. Modified Zr/Ti0 2-Nb/Y plot (Pearce, 1996) of Winchester and Floyd (1977); B. Nb-Y and C. Ta-Yb plots of Pearce et al. (1984); D. Ga/Al-Zr plot of Whalen et al. (1987). Symbols as in Figure 2.6. Symbols as in Figure 2.6. 59 1000 .01 1000 I (a) ~ l I I I i i I l I I I I I I I 3 J I I L J I I L J I I I I I I I Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc .01 1000 «, 1 0 0 .»—» c CO _ 10 > _ E CL 1 o o :(b) i i i r n — i — i — i — i — i — i — i — i — i — i — i — i — r J i i i i i i i i i i i i _ j i i i L Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr. Eu Gd Dy Er Lu Sc i i i i i i i — i — i — i — i — i — i — i — i — i — i — i — i — r =(c) 01 i i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc Figure 2.9. Primitive mantle-normalized trace element plots for the F L U felsic rocks. A. calc-alkalic rhyolite-dacite flows; B. high level calc-alkalic quartz-feldspar porphyritic intrusions; and C. tholeiitic rhyolite-dacite flows. Primitive mantle values from Sun and McDonough (1989). 60 100 200 300 400 500 600 700 Zr 300 Y /T i0 2 600 200 150 __ _100 N h(c) 50 F ~~1 I f o A ft A • J . + _ L 4 6 Sc/Yb 1 0 200 150 JO _100 N 5 0 ( d ) 1 A A • n4*f ^1 • f-t * + ; ++++ + _ J I I L 500 1000 Ti/Yb 1500 2000 Figure 2.10. High field strength element features of the FLD felsic rocks. A. Zr-Nb plot of Leat et al. (1986); B. Zr/Ti0 2-Y/Ti0 2 plot for deciphering the tholeiitic versus calc-alkaline affinities of the rhyolites from Lentz (1998, 1999); C. Zr/Yb-Sc/Yb, D. Zr/Yb-Ti/Yb. These plots are all normalized to Yb to remove the effects of mass changes due to alteration and metamorphism. Symbols as in Figure 2.6. 61 Figure 2.11. A. Nb-Y-Ce and B. Nb-Y-3*Ga plots of Eby (1992) to illustrate the crustal A-type nature of the felsic rocks of the KZK unit and footwall Wolverine succession. Symbols as in Figure 2.6. 62 QI I I I I I I I I I I I I I I I I I I I I I L Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc Figure 2.12. Primitive mantle-normalized trace element plots for the Kudz Ze Kayah unit felsic rocks. A. aphyric rhyolite flows/intrusions; B. feldspar (+/-quartz)-porphyritic rhyolite flows/ intrusions; and C. fine grained volcaniclastic/tuffaceous rocks. Primitive mantle values from Sun and McDonough (1989). 63 V _ J I I I I I I I I I I I I I I I I I I I L_ Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc n*| i i i i i ' i i i i i i i i i • i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc 01 I i i i i i i i i i i i i i i i i i i i i i l Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc Figure 2.13. Primitive mantle-normalized trace element plots for the unit 5f/qfp felsic rocks of the Wolverine succesion. A. rhyolite flows; B. quartz-feldspar porphyritic high-level intrusions; and C. fine grained volcaniclastic/tuffaceous rocks. Primitive mantle values from Sun and McDonough (1989). 64 QI i i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc 01 i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc 01 I i i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al V Nb Ce Nd Zr Eu Gd Dy Er Lu Sc Figure 2.14. Primitive mantle-normalized trace element plots for the unit 6 felsic rocks of the Wolverine succesion. A. quartz-feldspar porphyritic intrusions; B. footwall felsic volcaniclastic rocks (tuffs); and C. hangingwall aphyric rhyolite flows. Primitive mantle values from Sun and McDonough (1989). 65 20 15 ? 10 (a) A _ A_ A 0 • A O A Q Q A A • 20 (b) A 15 - A 1 ; ^ (Th/Nb; o : A ^ D 5 A o A 0.0 0.2 0.4 TiQ, 0.6 0.8 10 20 30 Sc Figure 2.15. Primitive mantle normalized Th/Nb ratios (Th/Nb p m ) versus T i 0 2 (a) and Sc (b) for the K Z K unit felsic rocks. The inverse relationship between the size o f the negative N b anomaly and T i 0 2 and Sc suggest that the anomaly may be in part reflective of Ti-Sc phase crystallization (e.g., magnetite, ilmenite) as these minerals have high partition coefficients for the H F S E (Green and Pearson, 1987; Ryerson and Watson, 1987). Symbols as in Figure 2.6. 66 Figure 2.16. Zr/Y-Y and La/Yb„-Yb„ plots for outlining ore-bearing versus barren Archean and Phanerozoic felsic volcanic rocks. Groupings in (a) and (b) are Archean Superior Province felsic rocks from Lesher et al. (1986). FI, FII, FHIa and FHIb are different felsic volcanic groupings with the FHIa and FHIb most prospective, FII moderately prospective and FI least prospective. Groupings in (c) and (d) are Archean Superior Province felsic volcanic groupings from Barrie et al. (1993). Grey shaded fields are VHMS-bearing sequences. Groupings in (e) and (f) are for Phanerozoic VHMS environments from Lentz (1998). All of these groupings contain VHMS mineralization. Symbols as in Figure 6. 67 Table 2.1. Summary of geochemical characteristics of felsic rocks from the Finlayson Lake district. FLU-2f-QFP-I (ca) FLU-2f-RHY/DAC (ca) FLU-2f-DAC/RHY (t) Mean 2CT Range Mean 2a Range Mean 2a Range n=3 n=4 n=3 SiC-2 76.2 4.1 73.5-79.3 73.6 1.4 72.2-75.1 70.8 3.8 67.8-73.1 Ti0 2 0.19 0.04 0.17-0.22 0.29 0.12 0.22-0.44 0.42 0.26 0.30-0.63 A I 2 O 3 12.2 1.0 11.4-12.8 13.1 1.4 11.4-14.2 12.4 0.6 11.9-12.7 Fe 20 3T 1.90 1.10 1.00-2.40 3.08 0.90 2.50-4.20 5.37 3.10 2.9-7.10 Fe 20 3 0.70 1.20 0.10-1.30 1.38 0.64 0.90-2.10 3.67 2.37 2.60-5.60 FeO 0.90 0.20 0.80-1.00 1.50 0.97 0.40-2.40 1.53 1.93 0.3-3.0 FeO* 1.71 0.99 0.90-2.16 2.77 0.81 2.25-3.78 4.83 2.79 2.61-6.39 MnO 0.02 0.02 0.01-0.03 0.05 0.01 0.03-0.06 0.12 0.08 0.06-0.17 MgO 0.59 0.16 0.49-0.71 1.25 0.31 0.98-1.61 1.09 0.37 0.79-1.26 CaO 0.84 0.61 0.54-1.34 0.69 0.37 0.41-1.08 1.71 0.93 1.13-2.42 Na20 4.20 1.90 3.10-5.70 3.30 0.42 2.90-3.70 2.13 2.97 O.50-4.20 K 2 0 2.70 2.90 0.46^.49 2.80 0.62 2.03-3.23 1.40 1.06 0.85-2.25 H 2 0 0.87 0.36 0.60-1.10 1.80 0.49 1.30-2.20 3.07 1.55 2.2-4.3 C 0 2 0.37 0.54 0.10-0.80 0.58 0.32 0.30-0.90 1.83 1.42 0.80-2.80 P2O5 0.05 0.02 0.04-0.06 0.07 0.03 0.05-0.10 0.09 0.07 0.06-0.15 Total 100.09 0.32 99.88-100.33 100.54 0.86 100.05-101.64 100.39 1.18 99.43-100.89 Cr (ppm) 28 8 24-34 40 20 25-63 25 7 20-29 Ni <10 - <10 14 7 10-18 <10 - <10 Co <5 - <5 6 - <5-6 <5 - <5 Sc 4.6 2.2 3.1-6.2 7.3 0.3 6.9-7.6 14.0 3.7 12.0-17.0 V 13 3 11-15 39 - <5-39 25 33 8-41 Cu 15 8 11-19 <10 - <10 <10 - <10 Pb 10 8 6-17 6 6 3-18 1 1 1-2 Zn 30 22 19-48 40 8 30-43 76 32 52-97 Bi <0.2 - <0.2 <0.2 - <0.2 <0.2 - <0.2 Cd 0.3 - 0.3-0.3 <0.2 - <0.2 <0.2 - <0.2 In 0.17 - <0.05-0.17 0.08 0.03 0.06-0.11 0.11 0.05 0.07-0.13 Sn 2.3 1.0 1.6-3.0 2.0 0.6 1.4-2.7 1.7 0.9 0.9-2.1 Mo 0.6 0.3 0.4-0.7 1.1 0.5 0.6-1.3 0.3 0.1 0.3-0.4 As NA - - NA - - NA - -Sb 0.7 0.3 0.5-0.8 0.3 0.1 0.2-0.3 <0.2 - <0.2 Ag 0.1 - <0.1-0.1 0.1 - <0.1-0.1 <0.1 - <0.1 Rb 96.6 107.3 9.7-150.0 60.3 20.3 41-78 38.0 21.4 26.0-55.0 Cs 1.34 1.54 0.11-2.20 3.89 3.02 0.85-7.20 3.13 1.64 1.90-4.20 Ba 880 790 290-1400 968 409 470-1300 253 85 190-310 Sr 133 1 64 89-180 140 43 100-190 81 87 33-150 Ti 0.37 0.44 0.06-0.68 0.28 0.16 0.17-0.48 0.19 0.11 0.12-0.28 Ga 12.0 2.4 10.0-13.0 15.3 1.7 13.0-16.0 18.3 2.2 17.0-20.0 Ta 0.7 0.1 0.7-0.8 0.9 0.1 0.8-1.1 0.5 0.1 0.4-0.5 Nb 9.5 1.8 8.5-11.0 15.5 2.9 13.0-19.0 9.9 1.5 8.9-11.0 Hf 3.6 0.1 3.5-3.7 5.8 2.2 4.7-8.6 4.2 1.7 3.5-5.6 Zr 116.7 16.3 110-130 227.5 79.1 190-330 180.0 28.3 160-200 Y 18.7 7.8 13.0-24.0 23.3 4.0 18.0-25.0 50.3 7.8 44.0-54.0 Th 12.7 1.6 12.0-14.0 11.5 2.7 10.0-15.0 2.8 1.2 2.2-3.8 U 2.3 0.8 1.7-2.8 2.2 0.1 2.1-2.2 0.6 0.5 0.3-1.0 68 Table 2.1, (continued). FLU-2f-QFP-I (ca) FLU-2f-RHY/DAC (ca) FLU-2f-DAC/RHY (t) Mean 2a Range Mean 2cr Range Mean 2a Range n=3 n=4 n=3 La 30.0 6.2 25.0-33.0 37.0 2.1 35.0-39.0 21.0 1.4 20.0-22.0 Ce 50.3 5.0 47.0-54.0 71.5 4.9 67.0-76.0 47.7 4.3 45.0-51.0 Pr 5.7 0.9 5.3-6.4 7.8 0.6 7.2-8.4 6.5 0.2 6.3-6.6 Nd 18.7 4.3 16.0-22.0 28.3 1.1 27.0-29.0 29.0 0.0 29.0-29.0 Sm 3.2 1.3 2.4-4.2 5.3 0.4 4.8-5.7 7.8 0.5 7.4-8.0 Eu 0.6 0.2 0.5-0.8 1.0 0.2 0.9-1.2 1.9 0.1 1.9-2.0 Gd 3.1 1.6 2.1-4.3 4.7 0.6 4.1-5.1 8.4 0.4 8.1-8.7 Tb 0.5 0.3 0.3-0.7 0.7 0.1 0.6-0.8 1.4 0.1 1.3-1.4 Dy 2.9 1.3 1.9-3.8 3.8 0.4 3.3-4.1 8.4 1.3 7.5-9.3 Ho 0.6 0.3 0.4-0.8 0.8 0.1 0.6-0.8 1.8 0.4 1.5-2.0 Er 1.7 0.8 1.1-2.2 2.1 0.3 1.7-2.3 4.7 0.8 4.1-5.3 Tm 0.3 0.1 0.2-0.3 0.4 0.1 0.3-0.4 0.8 0.2 0.7-0.9 Yb 1.9 0.7 1.4-2.3 2.6 0.5 1.9-2.9 5.3 1.2 4.4-6.1 Lu 0.3 0.1 0.2-0.4 0.4 0.1 0.3-0.5 0.8 0.2 0.6-0.9 Notes: Abbreviations - FLU-2f prefix = Fyre Lake unit felsic rocks, QFP-I (ca)-calc-alkaline quartz-feldspar porphyritic intrusion; RHY/DAC (ca) - rhyolite-dacite (calc-alkaline); RHY/DAC (t) - rhyolite-dacite (tholeiitic); KZK-3 prefix = Kudz Ze Kayah Unit; RHY - rhyolite; F+/-Q-PI - feldspar +/- quartz porphyritic intrusion; FT (f+/-q) - felsic tuff (feldspar +/- quartz); WV-5f/qfp prefix - Wolverine Succession unit 5f/qfp; RHY (+/-qfp) = rhyolite (+/- quartz and feldspar porphyritic); QFP-I - quartz-feldspar porphyritic intrusion; FT (qfp) - felsic tuff (quartz-feldspar porphyritic); WV-6 prefix = Wolverine Succession unit 6; APHRY - aphyric rhyolite. All totals are calculated on a volatile-bearing basis. 69 Table 2.1. (continued). KZK-3-RHY KZK-3-F+/-Q-PI KZK-3-FT (f+/-q) Mean 2a Range Mean 2a Range Mean 2a Range n=13 n=5 n=9 Si0 2 74.8 1.3 71.8-80.6 72.8 3.9 67.2-77.3 72.9 1.9 67-9-76.1 TiO. 0.24 0.07 0.05-0.43 0.39 0.18 0.17-0.65 0.36 0.11 0.22-0.69 AI2O3 12.3 0.6 10.1-14.3 13.6 1.5 12.2-15.9 13.5 0.7 12.1-15.3 Fe 20 3T 1.87 0.70 0.2-3.9 2.54 0.87 1.7-3.9 2.59 0.56 1.2-3.6 Fe 20 3 0.60 0.31 0-1.40 1.12 0.74 0.3-2.2 1.08 0.38 0.3-1.9 FeO 1.00 0.34 0.2-2.0 1.26 0.18 1.1-1.50 1.35 0.32 0.8-2.0 FeO* 1.69 0.62 0.2-3.5 2.27 0.78 1.47-3.48 2.33 0.50 1.07-3.21 MnO 0.03 0.02 0-0.12 0.02 0.02 0.01-0.05 0.03 0.01 0.01-0.04 MgO 0.58 0.19 0.17-1.19 0.52 0.24 0.2-0.79 0.79 0.23 0.3-1.29 CaO 0.65 0.23 0-1.44 0.33 0.21 0.18-0.70 1.06 0.57 0.22-3.01 NazO 2.25 1.21 0.2-6.0 1.54 1.53 0.3-4.2 3.50 1.21 0.40-5.2 K 2 0 5.94 1.79 1.1-9.97 7.02 3.03 1.92-9.54 3.69 1.22 1.73-6.70 H 2 0 0.75 0.12 0.4-1.1 1.12 0.46 0.7-1.9 1.12 0.28 0.5-1.8 co 2 0.34 0.17 0.1-1.1 0.16 0.09 0.10-0.30 0.47 0.33 0.1-1.5 P2O5 0.05 0.02 0.01-0.17 0.13 0.05 0.09-0.21 0.09 0.05 0.03-0.23 99.67- 98.73-Total 99.80 0.52 97.2-100.8 100.18 0.39 100.71 100.06 0.48 100.79 Cr (ppm) 25 13 12-81 32 33 10-70 26 11 11-50 Ni <10 <10 <10 <10 - <10 16 5 11-21 Co 25 7 13^5 20 9 5-30 24 3 18-28 Sc 4.4 1.1 2.7-8.3 8.3 4.6 3.4-15.0 6.8 1.8 4.1-12.0 V 11 3 5-17 14 8 8-26 16 7 5-36 Cu 13 2 10-16 <10 - <10 47 26 15-86 Pb 11 6 1-42 29 29 25-45 7 4 2-17 Zn 31 15 6-91 34 8 25-45 250 415 5-1580 Bi 0.6 0.3 0.2-1.3 0.7 - <0.2-0.7 0.3 0.1 0.2-0.4 Cd 0.4 0.1 0.2-0.5 <0.2 - <0.2 7.9 - <0.2-7.9 In 0.08 0.02 0.06-0.14 0.06 - <0.06-0.06 0.10 0.03 0.06-0.17 Sn 7.3 1.4 4.0-11.0 4.3 0.5 4.0-5.2 7.1 2.2 2.0-11 Mo 1.0 0.6 0.2-3.5 1.4 0.9 0.6-2.4 1.3 0.3 0.9-1.9 As 1.2 0.6 0.0-2.4 1.1 1.0 0-1.8 1.6 0.8 0.4-2.7 Sb <0.2 - <0.2 <0.2 - <0.2 <0.2 - <0.2 Ag 0.5 0.2 0.2-1.0 0.4 0.4 0.1-0.8 0.7 0.1 0.5-0.8 Rb 134.6 41.7 27-250 147.2 59.0 66-210 96.3 37.5 38-200 Cs 1.00 0.30 0.47-2.30 1.92 1.02 0.88-3.10 6.47 8.19 0.25-37.00 Ba 779 214 290-1500 1229 781 207-1057 1071 430 310-2276 Sr 48 19 9-120 48 27 20-89 73 40 18-207 Tl 0.61 0.24 0.1-1.3 0.67 0.25 0.25-0.87 0.42 0.22 0.11-1.10 Ga 18.2 2.3 12-25 20.6 1.8 19.0-23.0 20.8 2.4 15.0-24.0 Ta 2.5 0.6 1.4-4.7 2.8 0.6 1.7-3.2 2.7 0.6 1.5-3.8 Nb 26.1 6.7 9.8-54.0 30.4 7.9 22.0-42.0 31.2 6.7 11.0-41.0 Hf 7.6 2.4 3.0-17.0 9.5 4.4 3.8-13.0 10.0 2.6 4.4-15.0 Zr 299.4 109.8 72-590 412.8 214.4 133-590 392.4 124.0 178-660 Y 45.5 10.6 22-93 37.6 6.6 31.0-48 49.8 9.5 19.0-68.0 Th 26.3 3.4 14-33 32.4 10.9 19.0-45.0 28.0 6.1 17.0-44.0 (J 4.7 0.6 2.4-6.2 4.3 1.1 3.3-6.1 5.0 1.1 2.5-7.4 70 Table 2. . (continued). KZK-3-RHY KZK-3-F+/-Q-PI KZK-3-FT (f+/-q) Mean 2rj Range Mean 2o Range Mean 2CT Range n=13 n=5 n=9 La 56.6 9.6 29-91 75.4 37.8 35.0-124 68.6 21.6 44.0-136.0 Ce 113.2 20.0 53-190 162.8 67.0 74.0-220 134.2 41.6 85.0-260 Pr 12.8 2.5 6.2-21.0 17.7 7.7 8.6-24.0 15.0 5.0 9.7-29.0 Nd 47.1 9.4 25.0-83.0 65.0 28.9 30.0-88.0 55.6 16.5 35.0-100.0 Sm 8.8 1.7 4.8-16.0 11.3 4.2 6.3-15.0 10.3 3.0 5.1-17.0 Eu 0.9 0.3 0.3-1.9 1.3 0.8 0.4-2.2 1.2 0.4 0.3-2.1 Gd 7.9 1.7 4.5-16.0 8.8 2.9 5.2-12.0 8.9 2.0 4.2-13.0 Tb 1.3 0.3 0.7-2.7 1.3 0.3 1.0-1.6 1.4 0.3 0.6-2.0 Dy 7.7 2.0 3.7-17.0 7.3 1.2 5.9-8.6 8.3 1.6 3.2-11.0 Ho 1.5 0.4 0.8-3.5 1.4 0.2 1.2-1.6 1.7 0.3 0.6-2.2 Er 4.2 1.1 2.1-9.7 3.7 0.4 3.2-4.2 4.5 0.8 1.8-6.0 Tm 0.7 0.2 0.3-1.6 0.6 0.0 0.5-0.6 0.7 0.1 0.3-0.9 Yb 4.5 1.3 2.4-11.0 3.6 0.5 3.1-4.3 4.7 0.9 2.0-6.0 Lu 0.6 0.2 0.4-1.6 0.6 0.1 0.5-0.7 0.7 0.1 0.3-0.9 Notes: Abbreviations - FLU-2f prefix = Fyre Lake unit felsic rocks, QFP-1 (ca)-calc-alkaline quartz-feldspar porphyritic intrusion; RHY/DAC (ca) - rhyolite-dacite (calc-alkaline); RHY/DAC (t) - rhyolite-dacite (tholeiitic); KZK-3 prefix = Kudz Ze Kayah Unit; RHY - rhyolite; F+/-Q-PI - feldspar +/- quartz porphyritic intrusion; FT (f+/-q) - felsic tuff (feldspar +/- quartz); WV-5f/qfp prefix - Wolverine Succession unit 5f7qfp; RHY (+/-qfp) = rhyolite (+/- quartz and feldspar porphyritic); QFP-I - quartz-feldspar porphyritic intrusion; FT (qfp) - felsic tuff (quartz-feldspar porphyritic); WV-6 prefix = Wolverine Succession unit 6; APHRY - aphyric rhyolite. 71 Table 2. (continued). WV-5f7qfp-RHY (+/-qfp) WV-5f/qfp-QFP-I WV-5f/qfp-FT (qfp) Mean 2a Range Mean 2a Range Mean 2a Range n=5 n=2 n=7 Si0 2 76.6 3.4 71.3-79.9 74.4 0.4 74.2-74.5 74.5 2.4 69.1-77.9 Ti0 2 0.23 0.07 0.15-0.33 0.28 0.03 0.27-0.29 0.32 0.18 0.14-0.76 Al.Oj 12.8 2.7 10.6-16.9 13.6 0.8 13.3-13.9 12.6 0.6 11.7-13.6 Fe.OjT 0.82 0.41 0.1-1.1 1.85 1.27 1.40-2.30 2.34 1.56 0.80-6.30 Fe 20 3 0.35 0.49 0-0.70 0.90 0.28 0.80-1.00 1.50 1.04 0.1-3.6 FeO 0.25 0.07 0.20-0.30 0.80 0.85 0.50-1.10 0.98 0.60 0.3-2.40 FeO* 0.78 0.28 0.30-0.99 1.61 1.10 1.22-2.00 2.10 1.40 0.72-5.64 MnO 0.00 0.00 <0.01-0.01 0.06 0.07 0.03-0.08 0.02 0.02 <0.01-0.07 MgO 0.47 0.43 0.04-1.05 0.67 0.49 0.49-0.84 0.70 0.15 0.42-0.97 CaO 0.07 0.06 <0.10-0.12 0.41 0.23 0.33-0.49 0.38 0.27 0.11-1.01 Na20 0.56 0.51 0.20-1.40 0.20 0.00 0.20-0.20 1.59 1.48 0.10-4.40 K 2 0 7.04 0.82 6.08-8.29 7.15 0.55 6.95-7.34 5.76 1.80 2.65-8.55 H 2 0 1.12 0.90 0.20-2.30 1.55 0.14 1.50-1.60 1.24 0.37 0.80-2.00 C0 2 0.10 0.00 0.10-0.10 0.30 0.28 0.20-0.40 0.24 0.18 0.10-0.60 P2O5 0.06 0.04 0.02-0.12 0.15 0.00 0.15-0.15 0.10 0.04 0.06-0.20 99.76-Total 99.93 0.13 100.12 100.56 0.21 100.5-100.6 99.80 0.48 99.1-100.7 Cr (ppm) 51 60 16-120 12 1 11-12 55 32 16-92 Ni <10 - <10 <10 - <10 <10 - <10 Co 28 11 20-35 21 10 17-24 19 5 11-25 Sc 3.5 1.3 1.9-5.3 4.3 0.7 4.0-4.5 5.9 3.3 3.7-15.0 V 15 2 13-16 8 1 7-8 12 8 5-25 Cu <10 - <10 <10 - <10 <10 - <10 Pb 14 10 2-27 18 13 13-22 9 5 2-18 Zn 24 6 19-28 30 6 28-32 31 23 7-71 Bi 0.2 - O.2-0.2 <0.2 - <0.2 0.3 - O.2-0.3 Cd <0.2 - <0.2 0.7 0.4 0.5-0.8 <0.2 - 0.2 In 0.07 0.01 0.06-0.07 0.09 0.00 <0.05-0.09 0.08 0.02 0.05-0.10 Sn 5.8 1.7 3.5-7.7 6.5 1.6 5.9-7.0 6.4 1.7 2.8-8.6 Mo 1.8 1.3 0.3-3.5 0.4 0.3 0.3-0.5 1.0 0.4 0.4-1.6 As NA - - NA - - NA - -Sb 4.1 4.0 0.4-7.6 0.4 0.0 0.4-0.4 1.0 0.7 0.4-1.6 Ag 1.4 0.5 1.0-1.7 0.5 0.1 0.4-0.5 0.3 - O.1-0.3 Rb 214.0 57.3 140-290 190.0 28.3 180-200 161.3 35.9 99-220 Cs 2.64 0.84 1.7-3.9 2.10 0.57 1.9-2.3 4.35 4.49 0.31-16.0 Ba 1482 988 470-2800 1700 566 1500-1900 944 525 480-2100 Sr 35 5 28-41 97 10 93-100 39 18 15-72 Tl 2.10 1.42 0.69-4.10 0.84 0.17 0.78-0.90 0.65 0.23 0.38-1.10 Ga 18.6 5.5 12.0-26.0 19.0 2.8 18.0-22.0 19.1 2.4 16.0-24.0 Ta 2.1 0.7 1.5-2.9 2.6 1.4 2.1-3.1 2.2 0.4 1.4-2.7 Nb 20.4 2.6 17.0-23.0 20.0 5.7 18.0-22.0 24.7 5.0 13.0-32.0 Hf 5.2 0.5 4.7-5.7 5.1 0.7 4.8-5.3 6.0 1.5 4.4-9.7 Zr 176.0 18.2 160-200 175.0 14.1 170-180 228.1 89.0 130-449 Y 28.4 8.9 19.0-40.0 35.0 5.7 33-37 43.4 13.4 14.0-65.0 Th 18.0 5.2 14.0-27.0 17.5 4.2 16-19 22.0 2.5 18.0-25.0 U 3.7 1.0 2.5-5.1 2.6 0.1 2.5-2.6 4.1 0.7 2.8-4.9 72 Table 2 .1 . c^ontinued). WV-5f/qfp-RHY (+/-qfp) WV-5f/qfp-QFP-I WV-5f7qfp-FT (qfp) Mean 2c Range Mean 2CT Range Mean 2o- Range n=5 n=2 n=7 La 36.2 9.0 23.0-48.0 46.0 11.3 42.0-50.0 44.4 8.9 31.0-59.0 Ce 76.4 17.5 52.0-100.0 97.5 35.4 85.0-110.0 89.6 19.2 54.0-120.0 Pr 8.3 2.6 5.0-12.0 10.1 2.5 9.2-11.0 10.1 2.3 5.4-14.0 Nd 28.8 8.3 18.0-41.0 39.0 11.3 35.0-43.0 37.0 9.6 20.0-56.0 Sm 5.4 2.0 3.5-8.2 6.8 1.3 6.3-7.2 7.4 2.1 2.9-11.0 Eu 0.6 0.5 0.2-1.3 1.3 0.3 1.2-1.4 0.7 0.5 0.2-2.2 Gd 4.5 1.8 3.0-7.2 6.2 1.4 5.7-6.7 6.7 1.9 2.6-9.5 Tb 0.7 0.3 0.5-1.1 1.0 0.1 0.9-1.0 1.2 0.4 0.4-1.6 Dy 4.4 1.7 2.9-6.8 5.6 0.6 5.4-5.8 6.9 2.1 2.0-9.6 Ho 0.9. 0.3 0.6-1.4 1.2 0.1 1.1-1.2 1.4 0.4 0.5-2.1 Er 2.5 0.8 1.7-3.6 3.2 0.4 3.0-3.3 3.9 1.2 1.4-5.8 Tm 0.4 0.1 0.3-0.6 0.5 0.0 0.5-0.5 0.6 0.2 0.2-0.9 Yb 2.8 0.6 2.1-3.4 3.3 0.4 3.1-3.4 3.9 1.1 1.8-5.9 Lu 0.4 0.1 0.3-0.5 0.5 0.1 0.5-0.5 0.6 0.1 0.3-0.8 Notes: Abbreviations - FLU-2f prefix = Fyre Lake unit felsic rocks, QFP-I (ca)-calc-alkaline quartz-feldspar porphyritic intrusion; RHY/DAC (ca) - rhyolite-dacite (calc-alkaline); RHY/DAC (t) - rhyolite-dacite (tholeiitic); KZK-3 prefix = Kudz Ze Kayah Unit; RHY - rhyolite; F+/-Q-PI - feldspar +/- quartz porphyritic intrusion; FT (f+/-q) - felsic tuff (feldspar +/- quartz); WV-5f/qfp prefix - Wolverine Succession unit 5f/qfp; RHY (+/-qfp) = rhyolite (+/- quartz and feldspar porphyritic); QFP-I - quartz-feldspar porphyritic intrusion; FT (qfp) - felsic tuff (quartz-feldspar porphyritic); WV-6 prefix = Wolverine Succession unit 6; APHRY - aphyric rhyolite. 73 Table 2.1. (continued). WV-6-FW-QFP-I WV-6-FW-FT (qfp) WV-6-HW-APRHY n=l Mean 2a Range Mean 2a Range (P98-69*) n=6 n=40 Si0 2 75.7 71.4 2.5 69.2-76.4 85.9 1.7 73.0-93.7 Ti0 2 0.41 0.32 0.08 0.19-0.44 0.25 0.04 0.09-0.61 AI2O3 13.9 13.6 1.2 11.5-15.3 5.0 0.7 2.1-11.9 Fe 20 3T 0.20 2.82 0.94 1.80-4.10 2.09 0.39 0.90-7.9 Fe 20 3 0.00 - - - - - -FeO 0.20 NA - - NA - -FeO* 0.20 2.53 0.84 1.62-3.69 1.88 0.35 0.81-7.11 MnO 0.00 0.02 0.03 <0.01-0.07 0.08 0.09 O.01-1.82 MgO 0.09 0.95 0.73 0.22-2.47 0.77 0.13 0.20-2.19 CaO 0.11 0.99 1.00 0.02-2.98 0.49 0.23 <0.10-3.02 Na20 1.30 0.55 0.52 0.10-1.40 0.15 0.08 <0.50-1.50 K 2 0 7.12 5.20 1.12 3.85-7.41 1.22 0.19 0.14-2.53 H 2 0 0.80 1.90 0.13 1.80-2.00 1.11 0.18 0.20-3.1 C0 2 0.10 0.95 0.97 0.20-2.40 0.75 0.32 0.10-4.20 P2O5 0.05 0.11 0.04 0.03-0.15 0.04 0.01 0.01-0.08 Total 99.78 97.51 1.50 94.83-99.22 97.83 1.25 80.25-99.92 Cr (ppm) 91 22 4 16-28 62 6 36-122 Ni <10 <10 - <10 55 14 18-228 Co <5 <5 - <5 13 5 5-58 Sc 5.9 5.9 2.3 3.3-10.0 9.0 1.2 3.4-22.0 V 9 22 7 15-37 94 14 18-209 Cu <10 73 78 11-135 71 12 16-172 Pb 24 25 8 13-35 6 2 1-23 Zn <5 402 742 20-2090 102 58 6-1160 Bi <0.2 1.5 0.9 0.8-2.2 0.3 0.0 0.2-0.4 Cd <0.2 6.6 10.4 0.3-24.0 0.4 0.0 0.3-0.4 In 0.07 0.10 0.03 0.07-0.14 0.07 0.00 0.05-0.09 Sn 3.8 22.9 29.1 4.0-86.0 1.6 0.8 0.6-17.0 Mo 1.3 2.5 1.0 1.2-4.3 1.8 1.4 0.2-25.0 As NA NA - - NA - -Sb <0.2 3.0 1.9 1.1-6.7 1.0 0.7 0.2-12.0 Ag <0.1 0.4 0.2 0.1-0.6 0.2 0.0 0.1-0.3 Rb 150.0 150.0 21.2 110-180 61.1 11.6 7.3-150.0 Cs 2.00 5.62 4.63 2.0-15.0 4.73 1.23 0.33-16.00 Ba 1400 3550 2873 1300-10000 7183 1849 221-33506 Sr 45 69 50 4-170 109 31 5-440 TI 0.85 6.53 8.21 1.0-25.0 0.76 0.19 0.13-2.60 Ga 21.0 21.7 4.0 16.0-28.0 9.0 1.3 3.9-22.0 Ta 2.0 1.9 0.4 1.5-2.6 0.4 0.1 0.1-1.3 Nb 34.0 26.5 8.2 18.0-39.0 5.6 0.9 2.4-18.0 Hf 11.0 8.1 2.8 5.4-12.0 1.9 0.3 0.7-4.4 Zr 500.0 298.3 131.3 190-500 77.1 10.9 28-160 Y 30.0 45.5 11.7 27.0-60.0 14.0 2.1 5.0-28.0 Th 31.0 26.2 8.1 17.0-36.0 4.0 0.5 1.7-8.2 U 2.3 6.8 2.7 3.9-11.0 1.2 0.2 0.5-3.0 74 Table 2.1. (continued). VW-6-FW-QFP-I WV-6-FW-FT (qfp) WV-6-HW-APRHY n=l Mean 2a Range Mean 2a Range (P98-69*) n=6 n=40 La 110.0 66.0 23.3 33.0-101.0 13.7 1.7 5.9-26.0 Ce 200.0 139.5 52.5 67.0-220.0 29.6 4.0 13.0-64.0 Pr 23.0 15.9 6.3 7.5-26.0 3.5 0.4 1.6-6.7 Nd 79.0 56.8 22.9 27.0-94.0 13.3 1.7 6.1-25.0 Sm 12.0 10.5 4.0 5.5-17.0 2.8 0.3 1.3-5.3 Eu 0.9 1.6 1.1 0.2-3.7 0.4 0.1 <LD-0.8 Gd 8.1 8.5 2.9 5.3-13.0 2.5 0.3 1.0-4.6 Tb 1.0 1.3 0.4 0.9-1.9 0.4 0.1 0.2-0.7 Dy 4.8 7.5 1.9 4.9-10.0 2.4 0.3 0.9-4.4 Ho 0.9 1.5 0.4 1.0-2.1 0.5 0.1 0.2-1.0 Er 2.6 4.0 1.0 2.4-5.0 1.4 0.2 0.5-2.6 Tm 0.4 0.6 0.1 0.4-0.8 0.2 0.0 0.1-0.5 Yb 2.9 3.9 1.0 2.3-4.9 1.6 0.2 0.6-3.3 Lu 0.4 0.6 0.1 0.4-0.8 0.3 0.0 0.1-0.6 Notes: Abbreviations - FLU-2f prefix = Fyre Lake unit felsic rocks, QFP-I (ca)-calc-alkaline quartz-feldspar porphyritic intrusion; RHY/DAC (ca) - rhyolite-dacite (calc-alkaline); RHY/DAC (t) - rhyolite-dacite (tholeiitic); KZK-3 prefix = Kudz Ze Kayah Unit; RHY - rhyolite; F+/-Q-PI - feldspar +/- quartz porphyritic intrusion; FT (f+/-q) - felsic tuff (feldspar+/- quartz); WV-5f/qfp prefix - Wolverine Succession unit 5f/qfp; RHY (+/-qfp) = rhyolite (+/- quartz and feldspar porphyritic); QFP-I - quartz-feldspar porphyritic intrusion; FT (qfp) - felsic tuff (quartz-feldspar porphyritic); WV-6 prefix = Wolverine Succession unit 6; APHRY - aphyric rhyolite. 75 Table 2.2. Summary of key major and trace element ratios for felsic rocks of the Finlayson Lake district. FLU-2f-QFP-I (ca) FLU-2f-RHY/DAC (ca) FLU-2f-DAC/RHY (t) Mean 2a Range Mean 2a Min Mean 2a Range n=3 n=4 n=3 FeMg/NaK1 0.36 0.13 0.25-0.43 0.72 0.18 0.56-0.87 2.34 1.81 1.55-3.82 A/CNK 2 1.1 0.1 1.0-1.1 1.4 0.2 1.1-1.5 1.6 0.8 1.2-2.3 A/NK 2 1.2 0.1 1.2-1.3 1.6 0.2 1.4-1.7 4.8 7.3 1.6-10.8 Al 20 3/Na 20 3.1 1.5 2.0-4.1 4.0 0.5 3.6-4.6 4.2 2.4 3.0-5.4 HASHI-AI3 38 29 14-53 50 2 48-52 41 18 26-49 SER-I4 37 38 7-59 46 8 36-53 56 59 16-100 CCPI5 25 7 19-29 40 6 34-45 66 15 58-78 Na/K6 4.76 9.35 0.69-12.39 1.23 0.41 0.90-1.72 1.97 3.70 0-4.94 Sc/Nb 0.49 0.24 0.36-0.68 0.48 0.09 0.39-0.58 1.44 0.60 1.09-1.91 Ti/Sc 265 115 174-329 235 87 191-347 175 58 150-222 Zr/Y 6.8 4.0 4.6-10.0 10.4 6.1 7.6-18.3 3.6 1.1 3.0-4.5 Zr/Sc 27.4 12.7 17.7-35.5 31.1 9.5 26.4-43.4 13.3 4.7 9.4-15.4 Zr/Nb 12.3 0.8 11.8-12.9 15.3 7.8 10.5-25.4 18.2 2.9 16.4-20.4 Zr/Ti0 2 616.4 40.2 591-647 802.2 60.2 750-864 486.7 285.0 254-606 Zr/Hf 32.5 5.7 29.7-37.1 39.4 1.1 38.4-40.4 45.0 17.7 32.1-57.1 Nb/Y . 0.6 0.4 0.4-0.8 0.7 0.1 0.6-0.8 0.2 0.0 0.2-0.2 Nb/Ta7 12.9 0.8 12.3-13.4 17.0 0.9 15.9-17.4 20.8 1.2 20-21.7 104*Ga/Al 1.9 0.3 1.7-2.0 2.2 0.1 2.1-2.3 2.8 0.3 2.5-3.0 Th/Nb 1.3 0.1 1.3-1.4 0.8 0.3 0.5-1.2 0.3 0.1 0.2-0.3 La/Yb„8 11.8 6.3 8.5-16.9 10.7 2.4 8.7-13.6 2.9 0.6 2.6-3.4 Ce/Yb„8 7.1 3.5 5.6-10.0 7.5 2.3 6.0-10.4 2.4 0.5 2.2-2.8 Eu/Eu*' 0.63 0.08 0.57-0.67 0.64 0.04 0.61-0.68 0.73 0.05 0.70-0.78 Nb/Nb*9 0.14 0.01 0.13-0.15 0.24 0.07 0.16-0.32 0.44 0.05 0.40-0.47 Zr/Zr*' 1.03 0.42 0.76-1.35 1.23 0.42 0.99-1.76 0.82 0.11 0.75-0.90 Hf/Hf*9 1.13 0.27 0.93-1.31 1.14 0.41 0.89-1.67 0.70 0.28 0.57-0.92 Ti/Ti* 9 0.09 0.04 0.06-0.12 0.08 0.03 0.06-0.12 0.10 0.06 0.07-0.15 T-ZrSat'° 766 5 763-770 842 19 826-865 833 54 796-872 Notes - Abbreviations as in Table 1; 1- FeO*+MgO/Na20+K20 (Lentz, 1999); 2-A/CNK and A/NK molar Al 20 3/CaO+Na 20+K 20 and Al 20 3/Na 20+K 20 (Maniar and Piccoli, 1989);3- HASHI-AI = Hashimoto index = 100*[(MgO+K2O) / (MgO+K20+Na20+CaO)] (Ishikawa et al., 1976); 4-SER-I = sericite index = K20/(K20+Na20) (Sakai and Date, 1983); 5-CCP1 = chlorite-carbonate-pyrite-index = 100*[(MgO+FeO*) / (MgO+FeO*+K20+Na20)] (Large et al., in press); 6-Na/K = Na 20/K 20; 7-Nb/Ta - note only samples not crushed in tungsten carbide included in calculation; 8 - normalized to chondritic values; 9 - Eu/Eu* = Eu p m/(Gd p m*Sm p m) 0 5, Nb/Nb* = 0.5*Nbpn/(Thpm+Lapm), Zr/Zr* = 0.5*Zrpm/(Gdpm+Smpm), Hf/Hf* = 0.5*Hfpm/(Gdpm+Smpm), Ti/Ti* = 0.5*Tipm/(Gdpm+Smpm), pm = primitive mantle normalized; 10 - T-ZrSat = zircon saturation temperature (Watson and Harrison, 1983). 76 Table 2.2. (continued) K Z K - 3 - R H Y KZK-3-F+/-Q-PI K Z K - 3 - F T (f+/-q) Mean 2a Range Mean 2a Range Mean 2a Range n=13 n=5 n=9 FeMg/NaK 1 0.32 0.11 0.05-0.61 0.36 0.06 0.28-0.45 0.47 0.09 0.32-0.67 A / C N K 2 1.1 0.1 0.9-1.5 1.3 0.1 1.2-1.4 1.2 0.1 1.0-1.4 A / N K 2 1.2 0.1 1.0-1.7 1.3 0.1 1.2-1.5 1.4 0.1 1.2-1.7 A l 2 0 3 / N a 2 0 17.1 11.0 2.2-60.0 18.8 19.7 2.9-53.0 7.3 6.5 2.6-30.3 HASHI -AI 3 68 15 21-98 77 26 32-94 49 14 32-86 SER-I 4 69 17 16-98 78 27 31-97 51 17 28-84 CCPI 5 22 6 5-36 24 3 20-29 30 4 23-38 Na/K 6 0.99 0.91 0.02-5.36 0.53 0.93 0.03-2.19 1.30 0.63 0.06-2.6 Sc/Nb 0.19 0.06 0.06-0.48 0.26 0.12 .15-.44 0.25 0.12 0.15-0.70 Ti/Sc 345 78 64-537 313 104 164-443 322 46 171-392 Zr/Y 6.4 1.9 3.3-14.5 11.2 6.0 3.4-17.7 8.0 2.2 4.1-13.5 Zr/Sc 68.5 23.9 15.3-190.3 50.2 12.7 37.4-67.0 59.2 13.4 23.1-84.0 Zr/Nb 10.9 2.3 7.1-19.6 13.0 5.3 43617.0 12.5 2.2 8.0-16.2 Zr /T i0 2 1204.5 228.9 630-2185 1014.7 265.8 782-1368 1090.2 182.5 809-1593 Zr/Hf 37.0 4.8 24-52 41.7 4.8 35-47 38.4 3.2 32.1-44.0 Nb/Y 0.6 0.1 0.4-0.8 0.8 0.2 0.6-1.1 0.6 0.1 0.5-0.8 Nb/Ta 7 11.6 - - 14.7 - - 15.8 2.5 14.7-17.8 10 4*Ga/Al 2.8 0.3 1.9-3.9 2.9 0.1 2.7-3.0 2.9 0.4 2.1-3.5 Th/Nb 1.2 0.3 0.5-1.9 1.1 0.2 0.9-1.3 1.0 0.2 0.6-1.5 La/Yb„ 8 9.7 1.5 5.9-16.7 14.7 6.7 7.4-23.4 11.1 4.1 6.5-23.8 Ce/Yb„8 7.0 1.0 4.5-12.0 11.7 5.2 5.6-18.4 7.8 2.8 4.8-16.4 Eu/Eu* 9 0.31 0.08 0.11-0.53 0.36 0.12 0.23-0.50 0.38 0.12 0.12-0.73 Nb/Nb* 9 0.19 0.04 0.11-0.33 0.18 0.03 0.15-0.22 0.21 0.04 0.11-0.30 Zr/Zr* 9 0.93 0.26 0.44-1.81 0.95 0.18 0.65-1.08 1.08 0.20 0.82-1.76 Hf/Hf* 9 0.87 0.15 0.54-1.38 0.82 0.08 0.68-0.88 1.03 0.20 0.76-1.65 T i /T i * 9 0.04 0.01 0.02-0.09 0.05 0.01 0.04-0.06 0.05 0.01 0.04-0.07 T-ZrSat 1 0 837 32 756-918 886 66 789-941 875 35 798-960 Notes - Abbreviations as in Table 1; 1- FeO*+MgO/Na 2 0+K 2 0 (Lentz, 1999); 2 - A / C N K and A / N K molar A l 2 0 3 /CaO+Na 2 0+K 2 0 and A l 2 0 3 / N a 2 0 + K 2 0 (Maniar and Piccoli, 1989);3- HASHI-AI = Hashimoto index = 100*[(MgO+K 2O) / (MgO+K 20+Na 20+CaO)] (Ishikawa et al., 1976); 4-SER-I = sericite index = K 2 0 / (K 2 0+Na 2 0 ) (Sakai and Date, 1983); 5-CCPI = chlorite-carbonate-pyrite-index = 100*[(MgO+FeO*) / (MgO+FeO*+K 20+Na 20)] (Large et al., in press); 6-Na/K = N a 2 0 / K 2 0 ; 7-Nb/Ta - note only samples not crushed in tungsten carbide included in calculation; 8 - normalized to chondritic values; 9 - Eu/Eu* = Eu p m / (Gd p m *Sm p m ) 0 ' 5 , Nb/Nb* = 0.5*Nb p m / (Th p m+La p m), Zr/Zr* = 0.5*Zr p m / (Gd p m+Sm p m), Hf/Hf* = 0.5*Hf p m / (Gd p m+Sm p m), T i / T i * = 0.5*Ti p m / (Gd p m+Sm p m), pm = primitive mantle normalized; 10 - T-ZrSat = zircon saturation temperature (Watson and Harrison, 1983). 77 Table 2.2. (continued). WV-5f/qfp-RHY (+/-qfp) WV-5f/qfp-QFP-I WV-5f/qfp-FT (qfp) Mean 2a Range Mean 2a Range Mean 2a Range n=5 n=2 n=7 FeMg/NaK' 0.18 0.11 0.02-0.29 0.34 0.13 0.30-0.39 0.44 0.26 0.15-0.99 A/CNK2 1.5 0.5 1.0-2.2 1.5 0.1 1.5-1.6 1.4 0.3 1.1-2.1 A/NK2 1.6 0.6 1.0-2.3 1.7 0.0 1.7-1.7 1.5 0.3 1.3-2.3 Al203/Na20 43.5 33.7 7.6-84.5 68.0 4.2 66.5-69.5 38.5 38.1 2.9-136 HASHI-AI3 93 6 82-96 93 3 92-94 77 19 38-95 SER-14 93 6 84-97 97 0 97-97 78 21 38-98 CCPI5 14 8 4-21 24 6 21-26 27 11 12-47 Na/K6 0.08 0.07 0.03-0.2 0.03 0.00 0.03-0.03 0.45 0.52 0.02-1.66 Sc/Nb 0.17 0.05 0.10-0.23 0.21 0.02 0.2-0.22 0.25 0.12 0.14-0.47 Ti/Sc 411 62 323-473 396 26 386-405 324 75 200-467 Zr/Y 6.6 1.8 4.8-8.5 5.0 0.4 4.9-5.2 6.2 3.0 2.2-12.9 Zr/Sc 56.2 19.0 35.8-84.2 41.3 3.5 40.0-42.5 42.5 11.3 29.9-62.0 Zr/Nb 8.7 0.8 8.0-10.0 8.8 1.8 8.2-9.4 9.6 3.3 4.8-14.0 Zr/Ti02 816.4 220.9 576-1067 625.2 12.6 621-630 819.0 196.2 391-1033 Zr/Hf 34.0 2.0 31.4-36.2 34.7 2.1 34.0-35.4 36.6 5.0 29.5-46.3 Nb/Y 0.8 0.2 0.6-1.0 0.6 0.1 0.5-0.6 0.6 0.1 0.4-0.9 Nb/Ta7 12.9 0.9 11.9-13.5 - - - 13.7 0.7 12.6-14.2 I04*Ga/Al 2.7 0.3 2.1-2.9 2.6 0.2 2.6-2.7 2.9 0.3 2.3-3.3 Th/Nb 0.9 0.3 0.6-1.4 0.9 0.0 0.9-0.9 1.0 0.2 0.6-1.5 La/Yb„8 9.5 2.5 6.9-13.3 10.1 1.2 9.7-10.6 8.8 2.3 4.6-12.4 Ce/Ybn8 7.3 2.0 5.4-10.2 7.7 1.8 7.1-8.4 6.3 1.5 3.5-7.8 Eu/Eu*9 0.34 0.15 0.16-0.52 0.62 0.01 0.61-0.62 0.36 0.24 0.12-0.89 Nb/Nb*9 0.22 0.05 0.15-0.30 0.21 0.01 0.2-0.21 0.22 0.05 0.14-0.30 Zr/Zr*9 1.01 0.29 0.69-1.35 0.71 0.12 0.67-0.76 0.95 0.28 0.57-1.51 Hf/Hf*9 1.07 0.30 0.79-1.46 0.74 0.09 0.71-0.77 0.93 0.23 0.66-1.52 Ti/Ti*9 0.06 0.01 0.05-0.07 0.06 0.01 0.06-0.06 0.07 0.05 0.03-0.20 T-ZrSat10 829 28 796-861 833 10 829-836 838 23 798-883 Notes - Abbreviations as in Table 1; 1- FeO*+MgO/Na20+K20 (Lentz, 1999); 2-A/CNK and A/NK molar Al203/CaO+Na20+K20 and Al 20 3/Na 2OK 20 (Maniarand Piccoli, 1989);3- HASHI-AI = Hashimoto index = 100*[(MgO+K2O) / (MgO+K20+Na20+CaO)] (Ishikawa et al., 1976); 4-SER-I = sericite index = K20/(K20+Na20) (Sakai and Date, 1983); 5-CCPI = chlorite-carbonate-pyrite-index = 100*[(MgO+FeO*) / (MgO+FeO*+K20+Na20)] (Urge et al., in press); 6-Na/K = Na20/K20; 7-Nb/Ta - note only samples not crushed in tungsten carbide included in calculation; 8 - normalized to chondritic values; 9 - Eu/Eu* = Eupm/(Gdpm*Smpm)0-5, Nb/Nb* = 0.5*Nbpm/(Thpm+Lapm), Zr/Zr* = 0.5*Zrpm/(Gdpm+Smpm), Hf/Hf* = 0.5*Hfpm/(Gdpm+Smpm), Ti/Ti* = 0.5*Tipm/(Gdpm+Smpm), pm = primitive mantle normalized; 10 - T-ZrSat = zircon saturation temperature (Watson and Harrison, 1983). 78 Table 2.2. (continued). WV-6-FW-QFP-I WV-6-FW-FT (qfp) W V - 6 - H W - A P R H Y n=l Mean 2a Range Mean 2a Range (P98-69*) n=6 n=40 FeMg/NaK' 0.03 0.71 0.29 0.42-1.17 3.27 1.93 0.85-36.88 A / C N K 2 1.4 1.8 0.6 1.3-3.0 2.5 0.4 0.4-5.1 A / N K 2 1.4 2.2 0.5 1.4-3.1 3.8 0.7 1.8-14.6 A l 2 0 3 / N a 2 0 10.7 60.7 46.5 10.0-153 40.9 7.1 NA-92.0 HASHI-AI 3 84 81 11 63-98 78 5 22-100 SER-I 4 85 92 6 80-98 88 4 42-100 CCPI 5 3 38 10 28-52 65 3 44-97 Na/K 6 0.18 0.09 0.08 0.02-0.24 0.16 0.08 0-1.38 Sc/Nb 0.17 0.22 0.03 0.17-0.25 1.73 0.16 0.51-3.46 Ti/Sc 417 349 110 248-527 164 20 0-370 Zr/Y 16.7 6.5 1.8 3.4-8.7 5.8 0.5 2.4-10.0 Zr/Sc 84.7 50.6 7.2 40.4-57.6 8.8 1.1 3.5-27.7 Zr/Nb 14.7 11.0 2.2 7.0-13.2 14.2 1.0 5.6-22.8 Zr /T i0 2 1219.5 925.9 233.7 559-1205 307.3 21.5 181-591 Zr/Hf 45.5 35.9 4.1 28.4-41.7 41.1 1.4 35.7-61.9 Nb/Y 1.1 0.6 0.1 0.5-0.7 0.4 0.0 0.2-0.9 Nb/Ta 7 17.0 13.6 2.5 10.4-17.1 15.2 0.6 8.5-18.5 10 4*Ga/Al 2.9 3.0 0.3 2.6-3.5 3.4 0.2 2.1-4.9 Th/Nb 0.9 1.0 0.1 0.9-1.3 0.7 0.0 0.4-1.0 L a / Y b n 8 27.2 12.4 3.5 7.0-17.2 6.5 0.6 3.3-12.0 Ce/Yb„ 8 17.9 9.4 2.6 5.1-12.4 5.1 0.4 1.9-8.6 Eu/Eu* 9 0.28 0.55 0.49 0.1-1.58 0.40 0.06 0.09-0.67 Nb/Nb* 9 0.18 0.18 0.01 0.16-0.20 0.23 0.01 0.17-0.42 Zr/Zr* 9 1.04 0.84 0.26 0.58-1.38 0.86 0.07 0.45-1.40 Hf/Hf* 9 0.83 0.84 0.22 0.69-1.34 0.76 0.06 0.37-1.17 T i / T i * 9 0.04 0.03 0.02 0-0.06 0.14 0.01 0-0.21 T-ZrSat 1 0 933 880 36 823-926 776 18 571-863 Notes - Abbreviations as in Table 1; 1- FeO*+MgO/Na 2 0+K 2 0 (Lentz, 1999); 2 - A / C N K and A / N K molar AI 2 0 3 /CaO+Na 2 0+K 2 0 and Al 2 0 3 /Na20+K 2 0 (Maniar and Piccoli, 1989);3- HASHI-AI = Hashimoto index = 100*[(MgO+K 2O) / (MgO+K 20+Na 20+CaO)] (Ishikawa et al., 1976); 4-SER-I = sericite index = K 2 0 / ( K 2 0 + N a 2 0 ) (Sakai and Date, 1983); 5-CCPI = chlorite-carbonate-pyrite-index = 100*[(MgO+FeO*) / (MgO+FeO*+K 20+Na 20)] (Large et al., in press); 6-Na/K = N a 2 0 / K 2 0 ; 7-Nb/Ta - note only samples not crushed in tungsten carbide included in calculation; 8 - normalized to chondritic values; 9 - Eu/Eu* = Eu p m / (Gd p m *Sm p n , ) 0 ' 5 , Nb/Nb* = 0.5*Nb p m/(Th p m+La p m), Zr/Zr* = 0.5*Zr p m/(Gd p m+Sm p m), Hf/Hf* = 0.5*Hf p „,/(Gd p m +Sm p m ), T i /T i* = 0.5*Ti p m/(Gd p m+Sm p m), pm = primitive mantle normalized; 10 - T-ZrSat = zircon saturation temperature (Watson and Harrison, 1983). 79 Chapter 3 Boninitic Magmatism in a Continental Margin Setting, Yukon-Tanana Terrane, Southeastern Yukon, Canada Abstract Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana Terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate Si0 2 contents, high Mg#'s (MgO/(MgO+FeO*), Al 2 0 3 /Ti02, and Zr(Hf)/ middle rare earth element (MREE) ratios; low T i 0 2 , REE, and high field strength element contents; and U-shaped primitive mantle-normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc - back-arc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting to explain the large volume of temporally equivalent felsic volcanic and intrusive rocks. 80 Introduction Boninitic magmatism represents an enigmatic style of magmatism that is interpreted to reflect melting of highly refractory mantle source regions that have been variably metasomatized by subducted slab fluids (e.g., Crawford et al , 1989; Pearce et al., 1992). These primitive arc rocks have distinctive geochemical characteristics, including (1) high MgO, Mg#'s, compatible element contents (Ni, Cr, Co) and A l 2 0 3 / T i 0 2 ratios, (2) low T i 0 2 , (3) intermediate (andesitic) Si0 2 contents (>53wt%), (4) U-shaped rare-earth element (REE) patterns, (5) extreme high field strength element (HFSE) depletions, and (6) commonly, but not always, Zr and Hf enrichments relative to the middle R E E (Crawford et al., 1989; Pearce etal., 1992). These compositionally distinct rocks have been reported in a variety of settings, including modern forearc regions of intra-oceanic island arcs (Falloon and Crawford, 1991; Pearce et al., 1992), Phanerozoic and Proterozoic supra-subduction zone (SSZ) ophiolites (Rogers et al., 1989; Meffre et al., 1996; Wyman, 1999), and Archean greenstone belts (Kerrich et al., 1998). A common feature of all these boninite suites is their occurrence in SSZ (intraoceanic) settings far removed from continental crust. Although boninite-like rocks have been documented in continental margin environments (e.g., bajaiites, sanukitoids; Rogers and Saunders, 1989), to our knowledge boninites (sensu stricto) have not previously been documented within a continental or epicontinental setting (e.g., continental arc and/or back arc). In the Finlayson Lake region in southeastern Yukon, Canada, boninitic (sensu stricto) volcanic rocks are spatially, temporally, and stratigraphically linked to continental arc and back-arc magmatism in a region partially underlain by continental crust or continent-derived rocks. In this paper we document boninitic magmatism within such an unusual setting. 81 Geological Setting and Stratigraphy The Yukon-Tanana terrane (YTT) in the Finlayson Lake region of southeastern Yukon (Fig. 3.1) consists of variably deformed and metamorphosed volcanic and sedimentary rocks of pre-Late Devonian to Permian age (Murphy, 1998; Murphy and Piercey, 1999, 2000). Although these rocks are locally highly deformed an original stratigraphic succession has been determined from regional mapping. The Y T T in the Finlayson Lake region has been subdivided into three unconformity-bounded stratigraphic successions: the Grass Lakes succession (upper Devonian to lowest Mississippian), Wolverine Lake succession (Early Mississippian) and Campbell Range succession (Pennsylvanian-Permian) (Fig. 3.1; Murphy and Piercey, 2000). Boninitic rocks that are the subject of this paper form part ofthe Fire Lake unit (FLU) of the Grass Lake succession. The lowermost stratigraphic unit within the Grass Lakes succession consists of a pre-Devonian (>365 Ma) assemblage of quartz-rich, non-carbonaceous siliciclastic, with lesser carbonate, metasedimentary rocks (Fig. 3.1). Overlying this unit is the F L U ; the contact between the two units is marked by interlayering of psammite and biotite-chlorite schist identical to the overlying mafic schist of the F L U , implying a transitional contact. Alternatively, interlayered mafic schist may be transposed dikes feeding the F L U . In either case, however, the F L U is tied stratigraphically to unit 1. Above this contact, the F L U consists of variably metamorphosed and deformed -365-360 m.y. old (Mortensen, 1992a; Grant, 1997) mafic-dominated succession of volcanic rocks with geochemical signatures that vary from boninitic through island-arc tholeiitic, calc-alkaline and non-arc (Figs. 3.1 and 3.2; Grant, 1997; Piercey et al., 1999). The F L U passes conformably upward into the -360 m.y. old felsic volcanic- and sedimentary rock-dominated Kudz Ze Kayah unit, which is in turn unconformably overlain by the Mississippian felsic volcanic- and sedimentary rock-dominated Wolverine succession and the mafic-dominated late Paleozoic Campbell Range succession (Murphy and Piercey, 2000). The arc-back-arc systems of the Yukon-Tanana terrane are thought to have been formed on crust of continental or transitional composition. This conclusion is based on the ubiquitous presence of inherited zircons in felsic volcanic and intrusive rocks and their radiogenic isotopic compositions 82 (Mortensen, 1992a, 1992b, 1994; Grant, 1997). The quartz-rich and noncarbonaceous nature of the basal clastic unit of the Grass Lakes succession also implies proximity to a continental mass. Geochemical Characteristics of the Boninitic Rocks Geochemical data for the boninitic rocks of the F L U are presented in Figures 3.3 and 3.4 and Table 3.1. These rocks have variable Si0 2 (48.9-59.2 wt%), MgO (8.8-16.8 wt%), and Mg#'s (52.5-65.71) reflecting element mobility during seawater alteration and/or metamorphism. Nevertheless, the average Si0 2 (52.9 wt%), MgO (13.3 wt%), and Mg# (60.4) of these rocks are consistent with Crawford et al.'s (1989) boninite definition (Si0 2 > 53 wt%; Mg# > 60). The high MgO contents are coupled with high Ni (80-507 ppm), Co (26-80 ppm), and Cr (282-1590 ppm) contents. In contrast, they have low T i 0 2 content (0.1-0.3 wt%), moderate A1 2 0 3 content (7.7-14.6 wt%) and high A l 2 0 3 / T i 0 2 ratios (45.1-83.3). The CaO/Al 2 0 3 ratio of the samples from the F L U range from 0.3 to 1.2 reflecting likely CaO mobility due to seawater alteration and/or regional metamorphism; however, the average CaO/Al 2 0 3 ratio of the samples (0.7) is similar to Crawford et al.'s (1989) high-Ca boninite (HCB) designation (CaO/Al 2 0 3 > 0.8). Furthermore, the presence of preserved plagioclase phenocryst pseudomorphs in many samples and moderate Sc contents (30-55 ppm) is typical of HCB (Crawford et al., 1989; Meffre et al., 1996). The F L U samples have Zr/Y = 0.9 to 3.0, and Zr/Hf = 26.2-38.1. On primitive mantle-normalized plots (Fig. 3.3a) the F L U boninites have distinctive U-shaped trace element patterns with strong medium R E E depletion relative to the heavy R E E ([Gd/Yb]n = 0.3-0.8), with minor light REE depletion to slight enrichment ([La/Sm]n= 0.7-2.6) typical of boninites (e.g., Hickey and Frey, 1982; Pearce et al., 1992). The extreme HFSE depletion of the F L U boninites (Fig. 3.3a) is illustrated by their very low Nb (0.4-0.9 ppm), Zr (5.4-20.0 ppm), and Hf (0.2-0.5 ppm) contents. However, in spite of their overall depletion, Zr and Hf are slightly enriched relative to the middle R E E (Zr/Zr* = 1.2-2.6; Hf/Hf* = 1.3-2.8; Zr/Sm = 26.2-58.6; Fig. 3.3a), typical of boninites (Fig. 3.3a; Hickey and Frey, 1982; Pearce et al., 1992). The REE-HFSE and transitional element (TE) systematics of boninitic rocks are distinct from those of most ocean floor basalts (Fig. 3.3b-c). The boninites generally have low T i 0 2 contents and overlap fields for modern boninites, for Cambrian low-Ti tholeiites (LOTI) from Tasmania, and they are 83 similar to the north Tonga Ridge HCB but are displaced to lower La/Sm values (Fig. 3.3). The low Ti02 values coupled with the elevated Sc and V contents result in low Ti/Sc (17.5-50.3) and Ti/V values (3.6-9.6), typical of boninites (Hickey and Frey, 1982), and overlapping the fields for modern boninites, the Tongan HCB, and HCB from the Troodos ophiolite (Fig. 3.3). The geochemical features outlined above and comparisons to modern and ancient boninites clearly illustrate that volcanic from the FLU are boninitic in composition, and, more specifically, similar to HCB. Evidence for Boninitic Magmatism Linked to Continental Crust Numerous lines of field, geochemical, and isotopic evidence suggest that the basement beneath the FLU was at least in part continental. Isotopic evidence from igneous rocks throughout the Finlayson Lake area clearly points to a basement of continental crust or derivation from continental crust (sedimentary). Most Late Devonian to mid-Mississippian felsic igneous rocks in the Finlayson Lake area are isotopically evolved and have abundant inherited Proterozoic zircon (Mortensen, 1992a, 1992b, 1994; Grant, 1997; summarized in Fig. 3.2). These include felsic volcanic rocks in the Money Creek thrust sheet that are coeval with the FLU; younger felsic volcanic rocks of the Kudz Ze Kayah unit of Grass Lakes succession and felsic volcanic and subvolcanic rocks of Wolverine succession; and younger intrusions of the Grass Lakes and Simpson Range plutonic suites, both of which intrude the FLU (Fig. 3.2). The isotopic character and peraluminous nature of the Grass Lakes suite in particular suggest that it is the product of partial melting of continental crustal basement (Mortensen, 1992a). Second, mafic rocks of the FLU have eNdt values that range from +3.8 to 1^.8 compatible with variable influence from juvenile mantle and evolved (TDM = 0.78-2.00 Ga) crustal material (Fig. 3.2; Grant, 1997). Third, Pb-isotopic data on sulfides from felsic volcanic-associated syngenetic volcanic-hosted massive sulfide (VHMS) mineralization (Mortensen, 1994) have evolved crustal Pb-isotope signatures that lie on the "shale curve", a strongly radiogenic Pb-isotopic growth curve (u. = 12.16) defined for miogeoclinal rocks of the Cordillera (Godwin and Sinclair, 1982; Fig. 3.2). The FLU is locally stratigraphically sandwiched by quartz-rich clastic rocks of pericratonic nature and provenance. It everywhere overlies with transitional contact a unit of quartz-rich clastic rocks, pelite, 84 and marble with negative eNdt (t = 350 Ma; -1.7 to -11.4) values and Proterozoic depleted mantle model ages ( T D M = 1.38-1.98 Ga) (Fig. 3.2). At the Fyre Lake massive sulfide deposit, boninitic volcanic rocks are overlain by massive sulfide and up to 700 m of quartz-rich turbiditic sedimentary rocks (Foreman, 1998). Within this sedimentary package, Foreman (1998) has documented mature sandstone layers up to 10 m thick, with lesser volcanic-derived sandstone, chert, and limestone. It is uncertain whether these are first-cycle quartz-rich material from a cratonal basement or a recycled component from sedimentary and igneous rocks derived from a continental block. Nonetheless, the abundance of mature quartzose clastic material in rocks proximal to the boninites imply a nearby continental source. Although the bulk of the evidence presented above points to a dominantly continental character of the Yukon-Tanana terrane lithosphere, we cannot rule out that the lithosphere immediately underlying the Fire Lake boninites was of oceanic character. The current location of the Yukon-Tanana terrane, sandwiched between rocks of the ancestral North American continental margin and dominantly allochthonous terranes of intra-oceanic origin, leaves open the possibility that Yukon-Tanana terrane is underlain by lithosphere transitional between continental and oceanic, i.e., thinned continental lithosphere with rifted domains within which oceanic crust may have formed. Discussion and Conclusions The generation of boninitic magmatism requires unique thermal and petrological conditions (shallow melting, elevated geothermal gradient, subducted slab flux; Crawford et al., 1989; Pearce et al., 1992). These conditions have led to a variety of models for their setting and genesis, including (1) arc-initiation and catastrophic melting (Stern and Bloomer, 1992; Pearce et al., 1992), (2) ridge subduction (Crawford et al., 1989), (3) back-arc basin initiation (Coish et al., 1982), (4) plume-island arc interaction (Kerrich et al., 1998), and (5) ridge propagation into a forearc region (Falloon and Crawford, 1991; Monzier et al., 1993; Meffre et al , 1996). A common feature to all of these models and settings, however, is that they all form in SSZ or SSZ-like environments (e.g., greenstone belts), with the imposition of an external heat source on a variably metasomatized, depleted mantle wedge to generate the boninites. 85 The geochemical attributes of the F L U boninites also require satisfaction of these thermal and petrological source characteristics. Furthermore, their similarity to modern and ancient HCBs provides additional constraints on their petrotectonic origin. The HCB subclass of boninites is found in many ancient (e.g., Troodos, Koh) and modern (e.g., Tonga, North Fiji Basin) SSZ settings. In these regions, HCB generation is commonly interpreted to have formed near areas of back-arc spreading (e.g., Falloon and Crawford, 1991; Monzier et al., 1993; Meffre et al , 1996; Hawkins and Castillo, 1999), and in the case of the Tonga and North Fiji Basin HCB are associated with the propagation of spreading ridges into forearc regions (Falloon and Crawford, 1991; Monzier et al , 1993). We propose that the formation of boninitic rocks within the Finlayson Lake region and their association with continental crust may be due to the propagation of a spreading ridge into an arc built on composite basement of continental and oceanic affinity. In this model, the normal island arc and calc-alkaline arc volcanism (Grant, 1997; Piercey et al., 1999) would be disrupted due to the propagating ridge. Boninite genesis would be due to the influx of H 2 0 from the subducted slab and heat associated with ridge-related asthenospheric uprise inducing melting of the refractory mantle wedge (e.g., Falloon and Crawford, 1991; Monzier et al., 1993; Meffre et al., 1996); the subsequent decompression of the asthenosphere forming non-arc (MORJ3 and OIB) volcanic rocks (Piercey et al., 1999). In the region overlain by continental crust, the upwelling asthenospheric mantle and/or melts derived from it were likely important in inducing crustal melting, leading to the formation of the temporally equivalent felsic magmas evident in the Grass Lakes succession (Piercey et al., 2000). These petrological models can explain the thermal and petrological requirements of the Fire Lake boninites; however, the Fire Lake boninites require satisfaction of an additional constraint, escaping crustal contamination. We suggest that rapid arc rifting associated with ridge propagation resulted in extreme extension and attenuation of the crust (e.g., Stern and Bloomer, 1992). Rapid arc extension resulted in lithosphere-penetrating synvolcanic faults that would have quickly removed any crustal material that could have contaminated the boninites, and would have provided conduits that would permit rapid ascent of the boninites to the surface. Field evidence for rapid extension, arc rifting, and synvolcanic faulting are supported by an increase in the intensity of mafic-ultramafic intrusions proximal 86 to the boninitic rocks, and the thick clastic infill sequence overlying the Fyre Lake V H M S deposit (Figs. 3.1 and 3.2; Foreman 1998; Murphy and Piercey, 2000). We do not necessarily advocate that spreading-ridge propagation is the sole model for boninite genesis within the Finlayson Lake region; other models may be plausible. Additional chemostratigraphy and temporal constraints are required to evaluate this and other possible models (e.g., arc initiation, back-arc initiation, slab window). What is significant, however, is that numerous lines of field, geochemical, and isotopic evidence can link boninitic magmatism (sensu stricto) to continent margin arc and back-arc magmatism, and extremely unusual setting for boninitic magmatism. References Brown, A V , and Jenner, G . A , 1989, Geological setting, petrology and chemistry of Cambrian boninite and low-Ti tholeiite lavas of western Tasmania, in Crawford, A . J , ed, Boninites and related rocks: London, Unwin-Hyman, p. 232-263. Cameron, W . E , 1985, Petrology and origin of primitive lavas from the Troodos ophiolite, Cyprus, Contributions to Mineralogy and Petrology, v. 89, p. 239-255. Coish, R A , Hickey, R , and Frey, F . A , 1982, Rare earth element geochemistry ofthe Berts Cove ophiolite, Newfoundland: Complexities in ophiolite formation, Geochimica et Cosmochimica Acta, v. 46, p. 2117-2134. Crawford, A . J , Falloon, T . J , and Green, D . H , 1989, Classification, petrogenesis, and tectonic setting of boninites, in Crawford, A . J , ed, Boninites and related rocks: London, Unwin Hyman, p. 1-49. Falloon, T . J , and Crawford, A . J , 1991, The petrogenesis of high-calcium boninite lavas dredged from the northern Tonga , Earth and Planetary Science Letters, v. 102, p. 375-394. Foreman, I, 1998, The Fyre Lake project, 1997: Geology and mineralization of the Kona massive sulfide deposit, in Yukon exploration and geology 1997, Department of Indian and Northern Affairs Canada, Exploration and Geological Services Division, p. 105-113. Godwin, C.I , and Sinclair, A . J , 1982, Average lead isotope growth curve for shale-hosted zinc-lead deposits, Canadian Cordillera, Economic Geology, v. 77, p. 675-690. Grant, S .L , 1997, Geochemical, radiogenic tracer isotopic, and U-Pb geochronological studies of Yukon-Tanana Terrane rocks from the Money Klippe, southeastern Yukon, Canada, [M.Sc. thesis], Edmonton, Canada, University of Alberta, 177 p. Hawkins, J .W, and Castillo, P , 1999, Early history ofthe Izu-Bonin-Mariana arc system: Evidence from the Belau and the Palau Trench, The Island Arc, v. 7, p. 559-78. 87 Hickey, R . L , and Frey, F . A , 1982, Geochemical characteristics of boninite series volcanics: Implications for their source, Geochimica et Cosmochimica Acta, v. 46, p. 2099-2115. Kerrich, R , Wyman, D , Fan, J , and Bleeker, W , 1998, Boninite-low Ti-tholeiite associations from the 2.7 Ga Abitibi greenstone belt, Earth and Planetary Science Letters, v. 164, p. 303-316. Meffre, S, Aitchison, J . C , and Crawford, A . J , 1996, Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia, Tectonics, v. 15, p. 67-83. Monzier, M , Danyushevsky, L . V , Crawford, A . J , Bellon, H , and Cotten, J , 1993, High-Mg andesites from the southern termination of the New Hebrides island arc (S W Pacific), Journal of Volcanology and Geothermal Research, v. 57, p. 193-217. Mortensen, J . K , 1992a, Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana Terrane, Yukon and Alaska, Tectonics, v. 11, p. 836-853. Mortensen, J . K , 1992b, New U-Pb ages for the Slide Mountain Terrane in southeastern Yukon Territory, in Radiogenic age and isotopic studies: Report 5: Geological Survey of Canada Paper 91-2, p. 167-173. Mortensen, J . K , 1994, Nd, Sr and Pb isotopic constraints on mechanisms of crustal growth in the Yukon-Tanana Terrane in Yukon Territory, Canada, and Eastern Alaska, U.S.A., in Abstracts of the Eighth International Conference on Geochronology, Cosmochronology and Isotope Geology (ICOG-8), p. 227. Murphy, D . C , 1998, Stratigraphic framework for syngenetic mineral occurrences, Yukon-Tanana Terrane south of Finlayson Lake: A progress report, in Yukon exploration and geology 1997, Department of Indian and Northern Affairs Canada, Exploration and Geological Services, p. 51-58. Murphy, D . C , and Piercey, S.J, 1999, Finlayson project: Geological evolution of Yukon-Tanana Terrane and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon, in Yukon Exploration and Geology; Department of Indian and Northern Affairs Canada, Exploration and Geological Services Division,, p. 47-62. Murphy, D . C , and Piercey, S.J, 2000. Syn-mineralization faults and their re-activation, Finlayson Lake massive sulfide belt, Yukon-Tanana terrane, southeastern Yukon, in Yukon exploration and geology 1999, Department of Indian and Northern Affairs Canada, Exploration and Geological Services Division, p. 55-66. Pearce, J . A , van der Laan, S.R, Arculus, R .J , Murton, B . J , Ishii, T , Peate, D . W , and Parkinson, I.J, 1992, Boninite and harzburgite from Leg 125 (Bonin-Mariana forearc): A case study of magma genesis during the initial stages of subduction, in Fryer, P , et a l , eds. Proceedings of the Ocean Drilling Program, Scientific Results, Sites 778-786, Bonin/Mariana Region: College Station, Texas, p. 623-659. 88 Piercey, S.J., Murphy, D.C., Mortensen, J.K., and Paradis, S., 2000, Arc-rifting and ensialic back-arc basin magmatism in the northern Canadian Cordillera: Evidence from the Yukon-Tanana Terrane, Finlayson Lake region, Yukon, in Slave-Northern Cordilleran Lithospheric Experiment (SNORCLE) - Lithoprobe Report 72, Lithoprobe Secretariat, Vancouver, BC, Canada, p. 129-138. Rogers, N.W., and Saunders, A.D. , 1989, Magnesian andesites from Mexico, Chile and the Aleutian Islands: Implications for magmatism associated with ridge-trench collision, in Crawford, A.J., ed., Boninites and related rocks: London, Unwin-Hyman, p. 416-445. Rogers, N.W., MacLeod, C.J., and Murton, B.J., 1989, Petrogenesis of boninitic lavas from the Limassol Forest Complex, Cyprus, in Crawford, A.J. , ed., Boninites and related rocks: London, Unwin-Hyman, p. 288-313, Stern, R.J., and Bloomer, S.H., 1992, Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California Arcs, Geological Society of America Bulletin, v. 104, p. 1621-1636. Sun, S.-s., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in Saunders, A.D. , and Norry, M.J., eds., Magmatism in the ocean basins, Geological Society of London Special Publication 42, p. 313-345. Wyman, D.A., 1999, Paleoproterozoic boninites in an ophiolite-like setting, Trans-Hudson orogen, Canada, Geology, v. 27, p. 455-458. 89 _ \ Quaternary deposits Intrusions j p l Cretaceous intrusions (~] Simpson Range plutonic suite PH Grass Lakes suite intrusions Layered Rocks Wolverine Lake succession Grass Lakes Succession ggg Kudz Ze Kayah unit and unit 4 (23 Fire Lake unit • Mafic + ultramafic intrusions O Unit 1 Faults, displacement uncertain - * — ' — i - Money Creek thrust fault Figure 3.1. Simplified geologic map of the Yukon-Tanana terrane in the Finlayson Lake region, Yukon, Canada. Stars north ofthe Fyre Lake deposit are boninite locations, all other boninitic rocks come from drill core from the Fyre Lake volcanic-hosted massive sulfide deposit. Modified after Murphy and Piercey (2000). 9 0 PH Q- 2 g ta £ co CO ro CD 00 to C O cks cr i 1 Ro o o a> t o c p 03 O II s CD PN to + + + -i Lake Deposi | Fyre jVHMS V \ V « rj> _A O U r T f r = n N 1 (D CO ^ ->-* ' _ .9. II 9 P II <a o c o <P _ o § •a & 1 2 3 Z CL u 00 00 GO c o 'S.-4 3 o cu -3 & es Jo J- "fl 1 - 1 3 5 « U T3 C \ M -fl ^ r* 60 .s +H Q S3 U L D co •cos c J 3 CD ^H D 9 5 ^ s—' "fc! = e-1 s P f t ' S - H 43 2 3 CO 1—'CO AS o CJ ;>> o 1= 0 a o © © o t N o 'J •~ o £ "fl 3 s O IB T3 o E u es"3 3 1 0 •v £• "o es 2* .tS •7! fj\ e w ^ H a i n C J fS T3 3 si C o fl M U co CO r j 3 o Is S J o 5. -a 30 3 O M • c © o o GO CO o o fl «. CO 3 O to CO CM o £ a c3 o eS * HH 3 60 ' £ ,3 co •a 3 U 60 U H J O ui CJ CJ ed B o -3 U 00 f N cn o — 3 SO tZ 'I S *H § A s l H tJ (5 ° A Z. co CS g .9- co o « 1^  C es O _g u u C CJ C o c c3 o o ^ 3 p g ra ra 1 o '5. o 91 0 •) I i i i i i i i i i i i i i i i i i i i i i Th La Pr Sm Hf Ti Tb Y Yb Al Sc Nb Ce Nd Zr Eu Gd Dy Er Lu V 0.0 0.5 1.0 1.5 2.0 2.5 Ti0 2 0 50 100 150 Ti/Sc Figure 3. A: Primitive mantle-normalized trace element plot for the FLU boninitic rocks. B: Modified after Wyman (1999), Brown and Jenner (1989), and Meffre et al. (1996), N-MORB normal mid-ocean ridge basalt. C: From Hickey and Frey (1982) with additional data from Troodos, Tonga and Limassol Forest; SHMB siliceous high magnesian basalt. Data from Troodos, Limassol Forest complex, and Tonga from Cameron (1985), Rogers et al. (1989), and Falloon and Crawford (1991), respectively. Triangles are boninitic samples from the Finlayson Lake region. 92 Table 3.1. Geochemistry of boninitic rocks from the Finlayson Lake region. Sample Name P99-FL-8 P99-FL-15 P99-FL-18F P99-FL-20D P99-FL-20E P99-FL-20F P99-FL-20G Drill Hole 99-39 96-53 97-98 96-36 96-16 96-3 96-20 Depth 57.97 65.44 227.28 100.72 7.19 55.23 246.27 Easting 419217 411803 - 419259 419096 419017 419114 Northing 6789015 6789121 - 6798025 6789180 6789205 6789086 Rock Type' MG MG MG MG MG MG MG Si0 2 53.40 53.20 54.90 55.10 52.70 51.60 50.30 Ti0 2 0.12 0.29 0.20 0.12 0.14 0.20 0.15 A1203 10.00 13.10 10.70 7.70 10.20 9.70 10.20 Fe 20 3T 14.80 9.00 9.20 9.70 9.50 10.00 10.00 Fe 20 3 2.50 2.30 2.20 2.60 2.10 3.00 3.30 FeO 11.00 6.00 6.30 6.40 6.70 6.30 -6.00 FeO*2 13.25 8.07 8.28 8.74 8.59 9.00 8.97 MnO 0.22 0.17 0.23 0.27 0.16 0.19 0.18 MgO 14.68 11.01 11.49 16.75 ' 15.54 15.88 15.08 CaO 3.11 8.12 8.56 7.61 7.98 8.03 11.46 Na 20 0.30 3.30 3.50 0.60 1.10 2.00 1.70 K 2 0 0.20 0.76 0.60 0.18 0.25 0.98 0.20 P2O5 0.02 0.04 0.02 0.03 0.01 0.02 0.02 H 2OT 5.70 2.40 2.10 4.00 3.90 3.00 2.30 C0 2 T 0.40 0.20 0.50 0.30 0.30 0.10 0.10 Mg#3 52.56 57.71 58.12 65.71 64.40 63.83 62.70 Total 101.43 100.72 101.07 101.39 100.92 100.81 100.81 Cr (ppm) 810 318 830 910 1200 1200 1300 Ni 247 174 346 507 344 387 361 Co 80 42 46 57 53 55 52 Sc 39 43 40 41 39 43 45 V 182 181 181 175 216 188 226 Cu <10 <10 <10 <10 22 <10 <10 Pb <1 6 3 <1 2 2 3 Zn 163 74 73 168 67 83 65 Rb 3.6 30.0 17.0 1.6 4.6 22.0 2.1 Cs 1.30 4.30 4.10 0.41 1.40 12.00 0.43 Ba 240 1500 940 170 500 890 180 Sr 17 170 110 30 74 73 110 Ga 9.0 10.0 9.2 8.5 9.3 9.4 10.0 Ta 0.07 0.06 0.05 <0.05 O.05 <0.05 O.05 Nb 0.53 0.89 0.52 0.53 0.39 0.59 0.70 Hf 0.23 0.52 0.37 0.26 0.19 0.42 0.29 Zr 6.7 16.0 11.0 7.5 5.4 11.0 8.2 Y 4.6 11.0 8.2 3.4 5.8 8.2 7.6 Th 0.11 0.14 0.09 0.06 0.07 0.10 0.09 U 0.11 0.14 0.16 0.10 0.08 0.18 0.37 93 Table 3.1. (continued). Sample Name P99-FL-8 P99-FL-15 P99-FL-18F P99-FL-20D P99-FL-20E P99-FL-20F P99-FL-20G La 0.40 0.80 0.40 0.30 0.30 0.50 0.40 Ce 1.00 1.80 1.10 0.80 0.70 1.20 0.90 Pr 0.13 0.29 0.16 0.12 0.09 0.19 0.12 Nd 0.60 1.50 1.00 0.60 0.40 1.00 0.60 Sm 0.17 0.59 0.39 0.17 0.20 0.42 0.25 Eu 0.09 0.14 0.11 0.11 0.07 0.11 0.13 Gd 0.43 1.00 0.76 0.26 0.41 0.72 0.55 Tb 0.08 0.20 0.15 0.06 0.09 0.15 0.13 Dy 0.62 1.40 1.20 0.46 0.77 1.10 0.96 Ho 0.15 0.36 0.27 0.12 0.18 0.27 0.25 Er 0.46 1.10 0.83 0.39 0.56 0.78 0.75 Tm 0.08 0.19 0.15 0.07 0.11 0.14 0.13 Yb 0.58 1.30 1.10 0.53 0.71 1.00 1.00 Lu 0.11 0.23 0.19 0.09 0.13 0.17 0.17 94 Table 3.1. (continued). Sample Name 97DM-58/59 97DM-62 P99-FL-21 P99-FL-22C P99-FL-22A P99-FL-21A P99-FL-21F Drill Hole - - FL96-67 FL96-59 FL96-59 FL96-65 FL96-28 Depth - - 64.2 124.1 25.11 381 41.8 Easting 413874 415308 418877 418740 418740 419579 419176 Northing 6796389 6794113 6789081 6789196 6789196 6788650 6789105 Rock Type1 P-MG CS MG MG MG-A MG MG Si0 2 52.00 59.20 51.20 54.30 54.70 52.50 51.00 Ti0 2 0.21 0.17 0.23 0.17 0.18 0.18 0.19 A1203 11.90 11.30 11.50 9.70 12.30 10.80 10.10 Fe 20 3T 9.80 6.50 9.40 8.00 7.50 9.60 9.70 Fe 20 3 2.80 1.50 - - - - -FeO 6.30 4.50 - - - - -FeO*2 8.82 5.85 8.46 7.20 6.75 8.64 8.73 MnO 0.15 0.10 0.33 0.23 0.16 0.21 0.19 MgO 13.13 8.88 13.41 13.56 11.86 13.31 16.56 CaO 9.01 6.61 11.35 8.72 7.54 6.95 7.10 Na 20 2.50 2.00 1.20 1.60 2.50 2.80 1.70 K 2 0 0.12 0.13 0.30 1.06 0.94 1.43 0.42 P2O5 0.02 0.04 0.01 0.01 0.01 0.01 0.02 H 2OT 2.70 3.60 2.40 2.60 2.80 2.80 4.00 C0 2 T 0.20 2.70 0.20 0.60 0.40 0.20 0.10 Mg#3 59.82 60.29 61.32 65.32 63.73 60.64 65.49 Total 100.89 100.63 101.53 100.55 100.89 100.79 101.08 Cr (ppm) 760 520 756 672 582 1150 1090 Ni 184 155 291 200 166 340 364 Co 36 34 58 39 36 74 52 Sc 44 30 46 39 38 42 43 V 201 136 227 171 147 201 176 Cu 28 111 124 <10 <10 25 <10 Pb 2 1 4 4 8 6 1 Zn 53 53 268 75 69 346 103 Rb 2.2 5.6 7.7 47.0 41.0 57.0 7.0 Cs 0.75 10.00 3.00 17.00 8.70 16.00 1.20 Ba 110 130 350 2300 2700 1100 800 Sr 74 71 150 92 140 120 84 Ga 10.0 7.8 10.0 8.0 8.3 10.0 9.4 Ta <0.05 0.05 0.05 0.05 0.07 <0.05 0.05 Nb 0.52 0.68 0.56 0.67 0.72 0.45 0.59 Hf 0.48 0.34 0.49 0.43 0.42 0.35 0.54 Zr 15.0 11.0 17.0 14.0 16.0 12.0 20.0 Y 7.5 5.9 5.6 6.2 5.9 4.2 6.8 Th 0.09 0.12 0.10 0.12 0.16 0.08 0.09 U 0.06 0.12 0.13 0.12 0.11 0.12 0.42 95 Table 3.1. (continued). Sample Name 97DM-58/59 97DM-62 P99-FL-21 P99-FL-22C P99-FL-22A P99-FL-21A P99-FL-21F La 0.50 1.00 0.40 0.40 0.60 0.40 0.50 Ce 1.40 1.90 0.60 1.00 1.60 0.70 1.00 Pr 0.22 0.24 0.12 0.13 0.18 0.12 0.17 Nd 1.00 1.10 . 0.70 0.60 0.80 0.60 0.90 Sm 0.44 0.39 0.29 0.25 0.29 0.22 0.40 Eu 0.16 0.10 0.14 O.02 <0.02 <0.02 0.08 Gd 0.70 0.65 0.56 0.48 0.48 0.41 0.66 Tb 0.14 0.12 0.12 0.11 0.11 0.09 0.14 Dy 1.10 0.92 0.90 0.87 0.83 0.67 1.00 Ho 0.25 0.20 0.20 0.22 0.21 0.16 0.25 Er 0.73 0.62 0.61 0.75 0.65 0.48 0.77 Tm 0.12 0.10 0.10 0.13 0.11 0.08 0.13 Yb 0.91 0.70 0.76 0.94 0.84 0.60 0.92 Lu 0.15 0.13 0.12 0.16 0.13 0.11 0.15 9 6 Table 3.1. (continued). Sample Name P99-FL-21I P99-FL-21E P99-FL-22B P99-FL-25 P99-FL-24A P99-FL-21D Drill Hole FL96-28 FL96-28 FL96-59 96FL-43 97FL-109 FL96-28 Depth 75.7 32.8 52.5 145.5 348.7 31.3 Easting 419176 419176 418740 419285 - 419176 Northing 6789105 6789105 6789196 6788984 - 6789105 Rock Type1 MG MG MG MG MG MG Si0 2 48.90 52.50 51.50 50.50 57.00 50.70 Ti0 2 0.26 0.24 0.18 0.12 0.19 0.26 A1203 13.40 14.30 11.00 9.70 12.40 14.60 Fe 20 3T 11.40 10.70 9.50 9.90 8.70 10.50 Fe 20 3 - - - - - -FeO - - - - - -FeO*2 10.26 9.63 8.55 8.91 7.83 9.45 MnO 0.17 0.18 0.17 0.18 0.20 0.21 MgO 13.81 10.84 14.43 15.26 8.84 11.93 CaO 4.23 4.26 8.37 11.13 7.47 4.81 Na 20 2.70 3.30 1.30 1.60 3.70 3.40 K 2 0 2.20 0.71 0.53 0.32 0.49 0.76 p2o5 0.06 0.02 0.01 0.02 0.02 0.02 H 2OT 3.80 3.80 3.50 2.30 1.80 4.00 C0 2 T 0.10 0.40 0.60 0.10 0.30 0.40 Mg#3 57.38 52.96 62.80 63.14 53.03 55.81 Total 101.03 101.25 101.09 101.13 101.11 101.59 Cr (ppm) 1480 501 1200 1590 282 529 Ni 351 202 320 430 80 227 Co 51 41 49 50 26 44 Sc 31 55 43 41 42 52 V 220 254 211 198 240 244 Cu <10 <10 101 <10 19 <10 Pb 4 2 3 3 3 3 Zn 219 112 126 71 86 112 Rb 49.0 13.0 22.0 4.9 16.0 11.0 Cs 23.00 3.10 5.50 3.10 4.50 2.60 Ba 2900 1400 760 320 400 1700 Sr 60 160 70 79 87 160 Ga 11.0 12.0 9.3 10.0 11.0 12.0 Ta 0.06 0.06 <0.05 0.05 <0.05 <0.05 Nb 0.44 0.58 0.57 0.50 0.61 0.59 Hf 0.39 0.54 0.33 0.22 0.29 0.36 Zr 13.0 19.0 9.6 7.2 8.5 12.0 Y 5.8 10.0 6.8 7.0 8.8 8.2 Th 0.11 0.10 0.09 0.09 0.07 0.10 U 0.29 0.18 0.07 0.43 0.17 0.09 97 Table 3,1. (continued). Sample Name P99-FL-21I P99-FL-21E P99-FL-22B P99-FL-25 P99-FL-24A P99-FL-21D La 1.00 0.50 0.30 0.40 0.40 0.50 Ce 1.30 1.20 0.80 0.90 0.80 1.00 Pr 0.15 0.19 0.11 0.11 0.11 0.16 Nd 0.70 1.00 0.50 0.50 0.50 0.80 Sm 0.25 0.48 0.28 0.18 0.28 0.40 Eu <0.02 0.06 0.05 0.07 0.08 <0.02 Gd 0.58 0.94 0.57 0.40 0.61 0.78 Tb 0.13 0.20 0.12 0.09 0.14 0.18 Dy 0.90 1.60 0.98 0.78 1.10 1.30 Ho 0.22 0.37 0.23 0.19 0.29 0.29 Er 0.65 1.20 0.74 0.69 0.96 0.91 Tm 0.10 0.19 0.13 0.13 0.15 0.14 Yb 0.79 1.40 0.99 1.00 1.20 1.00 Lu 0.14 0.21 0.17 0.17 0.21 0.14 Notes: Samples with drill hole and depths are from the Fyre Lake volcanic-hosted massive sulphide deposit. Those without drill hole numbers are regional samples and locations are given as universal transverse mercator (UTM) coordinates (Zone 9V). All samples were analyzed at the Geological Survey of Canada, Ottawa, Canada. Most major elements were analyzed by XRF, FeO by wet chemical methods, and C 0 2 and H 2 0 by infared spectroscopy, trace elements were analyzed by ICP-ES and ICP-MS. Analytical precision calculated from repeat analyses of internal standards of similar matrix and composition (basaltic) is given as percent relative standard deviation (%RSD = 100*standard deviation/mean), and yielded values of: 0.43-6.52% for the major elements, 0.8-8.8% for the transition elements (TE=V, Ni, Cr, Co), 2.21-4.55% for the high field strength elements (HFSE=Nb, Zr, Hf, Y, Sc, Ga) (with the exception of Ta for which is was 17.55%), 2.62-9.35% for the low field strength elements (LFSE= Ba, Cs, Rb, Sr, Th, U) with the exception of Ba (15.75), 2.16-6.47% for the rare-earth elements (REE=La-Lu), and 1.12-9.17% for Cu, Pb, and Zn. All totals are calculated on a volatile-bearing basis. 1 - MG - massive greenstone; CS - chlorite schist, P-MG - plagioclase bearing metagabbro, MG-A - massive greenstone with amygdules 2- FeO* = FeO+0.89981*Fe2O3 3 - Mg# = MgO / (MgO +FeO*) 98 Chapter 4 Stratigraphic and Regional Implications of Weakly Strained Devono-Mississippian Volcanic Rocks in the Money Creek Thrust Sheet, Yukon-Tanana Terrane, Southeastern Yukon Abstract Relatively unstrained Devonian-Mississippian volcanic and volcano-sedimentary rocks have been documented in the Money Creek thrust sheet in Finlayson Lake map area. The succession comprises a five unit volcanic stratigraphy containing subaerial and subaqueous mafic and felsic volcanic and volcaniclastic rocks and associated sedimentary rocks that are underlain, and locally crosscut by, sub-volcanic mafic intrusions and quartz porphyritic granite. Magma-mingling relationships between mafic dykes and quartz-porphyritic granite suggest that mafic and felsic volcanism was broadly coeval. A published 360.5 +/ 1 Ma U-Pb date on a quartz porphyritic granitic intrusion establishes the age of volcanism. Biotite-hornblende granitic rocks of the Simpson Range Plutonic Suite (SRPS) intrude and metamorphose the volcanic sequence and related sub-volcanic intrusive rocks, and coupled with previously published U-Pb dates (345-350 Ma), this relationship implies that the SRPS is a distinctly younger pulse of magmatism. Mafic and ultramafic rocks of the Money Creek Thrust sheet have previously been correlated with the Pennsylvanian-Permian Campbell Range Belt and together both have been considered part of the Anvil Allochthon or Slide Mountain Terrane. Field characteristics, age, and geochemistry show that neither correlation is valid. 99 Introduction The Yukon-Tanana Terrane (YTT) over much of the Yukon Territory is characterized by the presence of abundant greenstone, chloritic schist, gabbro, diabase and ultramafic rocks (e.g., Tempelman-Kluit, 1979; Mortensen, 1992a,b; and others). The origin and significance of the greenstones are ambiguous and a source of controversy (e.g., Tempelman-Kluit, 1979; Mortensen and Jilson, 1985). In most previous work, all mafic rocks have been placed under the same header, be that Anvil Allochthon (Tempelman-Kluit, 1979; Erdmer, 1981, 1985), Slide Mountain Terrane (Mortensen and Jilson, 1985; Mortensen, 1992a,b), or Anvil Assemblage (Wheeler and McFeely, 1991). However, recent mapping in different parts of the Y T T has shown that there are different greenstone units within the Y T T (e.g., Stevens et al., 1995; Murphy and Timmerman, 1997; Murphy, 1998; Murphy and Piercey, 1999; Piercey et al., 1999a,b). Others have questioned the validity of correlating the Slide Mountain Terrane with the Anvil Assemblage based on geochemical and isotopic grounds (Creaser et al., 1997; Grant, 1997; Grant et al., 1996). Recent geochemical and isotopic data suggest that there are differences in the chemical and isotopic compositions of the greenstones (e.g., Piercey et al., 1999a,b; Creaser et al., 1997; Grant et al., 1996; Grant, 1997); however, only a few workers have outlined field criteria to distinguish between the different greenstone units (e.g., Murphy and Timmerman, 1997; Murphy, 1998; Murphy and Piercey, 1999a, b, c). Most of the ambiguities in the distinction between the greenstone units arise because of the highly variable degree of strain recorded within them. In the Finlayson Lake region, Murphy and Timmerman (1997), Murphy (1998), and Murphy and Piercey (1999a, b, c) show that the Y T T contains at least three distinct mafic horizons at different stratigraphic levels. Similarly, other workers have shown the different mafic units have distinctive geochemical characteristics. For instance, the lowermost mafic unit (unit 2) exhibits primitive arc (Piercey et al., 1999a,b; Sebert and Hunt, 1999) through calc-alkalic geochemistry (Grant et al., 1996; Grant, 1997; Piercey et al., 1999a,b); the middle mafic unit (unit 4) has rift-like geochemistry (Chapter 6); and the upper mafic units of the Campbell Range belt exhibit rift alkaline through E-MORB to N-MORB signatures (Plint and Gordon, 1997; Piercey et al., 1999a, b; Piercey et al., 1999). 100 In this paper we present results from mapping of a previously undocumented section of Devono-Mississippian basalt and rhyolite and spatially associated intrusive rocks in the hanging wall ofthe Money Creek thrust (Murphy and Piercey, 2000; Figs. 4.1 and 4.2). These rocks are now considered unique in that they are essentially unstrained and relatively undeformed, providing a window into the early Mississippian tectonic setting that has not been complicated by strain and metamorphism. Our goals for this paper are as follows: 1) to describe the stratigraphy of the undeformed volcanic sequence; 2) to describe the spatially associated intrusive rocks and clarify the definition of the Simpson Range plutonic suite (SRPS); 3) to provide insights into the nature and setting of volcanism recorded in the undeformed volcanic sequence; and 4) to compare and contrast the style of volcanism with that ofthe younger, and also relatively well-preserved rocks of the Campbell Range belt with which they have been correlated and lumped together into Slide Mountain Terrane. Previous Work Tempelman-Kluit (1977, 1979) briefly described the volcanic rocks as an undeformed Cretaceous volcanic 'plug' comprising subequal amounts of porphyritic, pyritic quartz-rhyolite and hornblende andesite that were inferred to "invade" the Money Klippe. Erdmer (1981) also interpreted the volcanic assemblage to be a Cretaceous 'plug' that "invaded" the Money Klippe. He stated that the margins of the plug overlie the cataclastic rocks, but the contact was not readily visible and did not exhibit significant thermal alteration. He also said that the present outcropping of the plug likely represented the erosion level of a subaerial volcanic edifice. Mortensen (1983) questioned the Cretaceous age for the volcanic 'plug' based on a ca. 345-380 Ma U-Pb age from near concordant zircon fractions from a quartz-porphyry body that intruded the mafic rocks ofthe plug. Although imprecise, these ages disproved a Cretaceous age and Mortensen (op cit) included the plug in the Anvil Assemblage. Similarly, the latter age designations led Erdmer (1985) to include the volcanic plug in the Anvil Allochthon. Mortensen and Jilson (1985) and Mortensen (1992b) suggested that the plug intruded the Anvil Assemblage and was transported with it during subsequent, post-Mississippian thrusting. Mortensen (1992b) refined the age of the quartz-porphyry body within the plug producing a discordant crystallization age of 360.5 ±1.9 Ma. 101 Grant (1997) undertook a geochemical, isotopic and U-Pb geochronological study of SRPS and Y T T metasedimentary rocks, and his study partly encompassed the undeformed volcanic sequence. He mapped a small portion of the sequence and documented the occurrence of rhyolites, flow-banded rhyolites, quartz-potassium feldspar rhyolitic porphyry, and black to green plagioclase- and augite-bearing mafic rocks. He suggested that the composite plug was part of the SRPS and was the structural top of the Money Klippe. U-Pb dating of the quartz-feldspar porphyry (356.5 +3.4/-4.0 Ma) affirmed Mortensen's (1992b) age; geochemical and isotopic data suggested that both the felsic and mafic members of the plugs reflected magmatism within a continental arc with variable crustal thickness. Hunt and Murphy (1998) mapped a portion of the composite plug as part of a study of the geology near the Fyre Lake Cu-Co-Au volcanogenic massive sulphide (VMS) deposit (Yukon MINFILE, 1997, 105G 034). Their work documented mafic volcanic breccias and augite-porphyritic volcanic rocks lying as roof pendants within quartz-porphyry. Furthermore, they suggested that the mafic rocks were not part of the SRPS, but were part of Yukon-Tanana Terrane, correlating with the mafic metavolcanic unit that hosts the Fyre Lake deposit. Additional regional mapping in the vicinity of the porphyry and outside this region are also presented in Murphy and Piercey (2000). Stratigraphy of the Volcanic Succession The unstrained volcanic succession consists of subaqueous and subaerial mafic and felsic volcanic rocks and associated marine sedimentary rocks. Most rocks are pristinely preserved within the sequence. However, near the margins of the sequence along the Money Creek thrust (cf. Murphy and Piercey, 2000), or near younger SRPS intrusive rocks, the volcanic rocks have been altered and deformed (see Intrusive Rocks; Fig. 4.2). Unit MVUi In the lower unit (MVUi), mafic volcanic rocks are subaqueous in nature and range from pillowed to massive flows with abundant volcaniclastic rocks. Pillowed flows typically contain green to black pillows that are well preserved with very little flattening, and range in size from 10-15 cm up to 1.5 m in diameter (Fig. 4.4a). Typically all pillows have -1-2 cm glassy rinds that grade into inter-pillow hyaloclastite breccia that contains 1-2 cm angular fragments of commonly glassy pillowed material, and 102 are associated with red (hematitic) to purple inter-pillow chert. Pillow lavas are variably vesicular and range from non-vesicular up to 10% vesiculation. Minor vesiculation is common in all pillow lavas. Vesicles are often infilled with quartz and/or carbonate material. The relationship between massive and pillowed flows within M V U i is uncertain; however, it is assumed based on the stratigraphic continuity of the unit that the relationships are conformable. Massive flows (?) have similar colouration as the pillowed flows, but their extent is uncertain. Mafic volcanic rocks of unit M V U i are somewhat distinctive from the other mafic units; they contain abundant spherulites up to 13 mm in diameter. The spherulites occur as rounded blebs with radial devitrification structures. One to three mm-sized euhedral augite (?) or hornblende phenocrysts are common within the volcanic rocks of unit M V U i . Volcaniclastic rocks of unit M V U i consist primarily of pillowed breccias and re-sedimented (?) pillowed fragments (Fig. 4.4b). Most volcaniclastic breccias contain 0.5-3.5 cm-sized angular fragments of variably vesicular (+amygdaloidal) pillow material that are locally bleached to a white colouration (Fig. 4.4b). The angular nature of the clasts suggests deposition proximal to the parent mafic flows, possibly due to auto-brecciation of the parent flows. Volcaniclastic fragments of similar size but with more rounded character may have been re-sedimented by bottom currents; however, this interpretation needs to be tested by more detailed mapping. No tuffaceous rocks or other rocks indicative of explosive volcanism were documented or observed by the authors. Unit FMVUi Both felsic and mafic rocks occur in unit F M V U i and are in part sub-aerial. The top of unit M V U i exhibits a ~25 m transition zone of interlayered (?) reddish, highly vesiculated mafic lavas (FMVUi), and green, weakly to non-vesiculated mafic lava. Mafic volcanic rocks from unit F M V U i are typically reddish to maroon in colour and locally have a white bleached appearance. Typically, the mafic rocks in this unit exhibit a greater degree of vesiculation than in unit M V U i ranging from 5-30%, with most -10-15% (Fig. 4.5a). Vesicles are commonly relatively small, leading to the surfaces of the basalt flows having a very pumiceous nature. In other places the vesicles are up to 5 mm in diameter; most larger vesicles are 2-3 mm in size (Figs. 4.5a and b). In many outcrops, the vesicles exhibit a trachytic alignment that gives a 103 crude indication of flow banding; however, this feature is not ubiquitous. Commonly, the larger vesicles are infilled by carbonate and/or quartz. Fragmentation and brecciation of the mafic lavas is not as common as in unit M V U i ; however, this may be a function of the limited exposure of unit F M V U i . Breccias that have been observed have fragments that are angular and typically range in size from 1-5 cm. Locally these breccias have a matrix of rusty carbonate that may represent a post- depositional (post-volcanic) infilling and breakup by later carbonate-rich fluids, possibly from the SRPS. Felsic volcanic rocks are interlayered with the mafic volcanic flows, but are less abundant than the mafic volcanic rocks (Fig. 4.5c). The felsic volcanic rocks are typically massive, reddish pink to pink, non-vesicular rhyolite to rhyodacite (Fig. 4.5c). They are of limited areal extent (Fig. 4.3) and are commonly associated with rhyolitic hyaloclastite breccias with large 20-30 cm blocky fragments (Fig. 4.5d). UnitMVU2 The contact between unit F M V U i and the mafic volcanic rocks of unit MVU2 is gradational and characterized by an increase in green mafic volcanic rocks, not that dissimilar from those of unit M V U i . Mafic volcanic rocks from unit M V U 2 are typically pillowed to massive, dark black to greenish with 0-5% vesiculation. Inter-pillow hyaloclastite breccias are common, as are minor cherts; however, on the whole, volcaniclastic rocks are minor in comparison to unit M V U i . Unit TSU, Although relatively thin (-5-50 m), turbiditic clastic rocks of unit TSU] form a distinctive marker horizon within the undeformed volcanic sequence. The thickest section of unit TSUi consists of minor interlayered coarse, green greywacke, coarse white greywacke, and finely laminated finer clastic rocks. Laterally away from this location where the unit is thinner, finely laminated clastic rocks predominate (Figs. 4.2, 4.3 and 4.6). Green greywacke is volumetrically minor and consist of green medium-grained sand with fragments of -0.5-1-cm-sized volcanogenic (?) material and mafic minerals. Green greywacke was observed in only one location forming 2-5-m-thick beds, typically overlain by very fine- to fine-grained, dark green to grey, finely laminated cherty siltstones (Fig. 4.6a). These generally occur in 0.5-1 104 m-thick beds with abundant mm-scale internal laminae interlayered with beds of white greywacke (Figs. 4.6a and b). Laterally away from the interlayered coarse clastic rocks, finely laminated turbiditic rocks predominate, and in one location they are very fine-grained to glassy. These glassy units may be turbiditically re-sedimented volcanogenic ash. In the thickest portion of this section, the white greywacke unit is composed of coarse white sand with reddish pink and green fragments (Fig. 4.6b). The sands typically form 0.5-2-m-thick beds which are commonly interlayered with ~30-cm-thick fine cherty layers as described above. The reddish pink clasts are typically rounded and 2-3 mm in diameter with some resemblance to the underlying rhyolitic rocks of unit F M V U i . Similarly, smaller green fragments have features akin to the underlying mafic volcanic rocks; however, some larger fragments appear to be akin to the finely laminated chert layers. The white sand layers of this unit provide the best evidence for the upright-facing direction of the sequence. The lower parts of white sandy beds consist of abundant clasts ofthe underlying rocks, as described above. Well defined 2-3 cm lode and flute casts ornament the bases of beds (Fig. 4.6b), providing unambiguous stratigraphic-top indicators. The sandy material within the TSUi unit decreases laterally from point X to nil to the northeast and southwest (Fig. 4.2) and is replaced by very siliceous, in places glassy, fine-grained laminated turbiditic sedimentary rocks. Unit MVU3 The contact between unit TSUi and M V U 3 is abrupt, and marked by an abundance of mafic and lesser felsic volcanic rocks. Immediately above the TSU1-MVU3 contact is a variably vesiculated and plagioclase-phyric, green, pillowed basalt sequence that is interlayered with ca. 5-m-thick flows (?) of plagioclase-phyric, red to white rhyolite and rhyodacite. Felsic volcanic rocks are only observed in this location. The remainder of the unit consists predominantly of pillowed and massive flows akin to the underlying mafic units. Pillowed lavas are variably vesicular and have vesicles infilled with carbonate and/or quartz. Pillows range from 30-60 cm in diameter and commonly have reddish to purple inter-pillow chert. Pillows and massive flows range in colour from black to purple and in places greenish. The areal extent of the massive flows is uncertain. Plagioclase phenocrysts are present in some samples; rarely 105 hornblende (pyroxene?) phenocrysts are present. Sedimentary and volcaniclastic rocks are rare in unit MVU3. Locally near the top of this unit are discontinuous marble horizons of detrital origin. Intrusive Rocks Intrusive rocks are spatially associated with the undeformed volcanic sequence; however, not all of them are temporally or genetically related to the sequence. Intrusive rocks in this region include: 1) mafic/ultramafic intrusive phases; 2) quartz-porphyritic intrusions; and 3) intrusions of the Simpson Range plutonic suite (SRPS). Our data show that the first two types are coeval and comagmatic with the undeformed volcanic sequence and the SRPS post-dates the volcanic rocks. Gabbroic and ultramafic rocks are common within the undeformed sequence, particularly along the Money Creek thrust (see Murphy and Piercey, 1999b,c). Near the thrust, the gabbroic rocks are variably strained; however, original gabbroic textures are recognizable. Spatially associated with the gabbroic rocks are serpentinized harzburgitic ultramafic rocks that appear to underlie the gabbros and may represent either residues from gabbroic melt extraction or cumulations of ultramafic minerals below a gabbroic magma chamber. The authors infer that the mafic and ultramafic intrusions are comagmatic with the basalts based on their spatial association, similar U-Pb age constraints and similarity in the petrology of the volcanic and gabbroic rocks (Grant, 1997). The gabbroic and ultramafic rocks may thus be the intrusive roots ofthe basalts in the undeformed volcanic sequence. Quartz-porphyritic (QP) intrusive rocks look superficially similar to the SRPS rocks, but typically lack hornblende, biotite and are older than the SRPS (-357-361 Ma; Mortensen, 1992a,b; Grant, 1997; Grant et al, 1996). The QP are generally medium-grained, less commonly fine-grained, and are typically pink to pink-white to white-grey. The QP typically contain 2-3 mm quartz phenocrysts dispersed in a pink to white, fine- to medium-grained groundmass. At location Y the quartz-porphyritic rocks are intruded by, yet mingle with, mafic dykes that are interpreted to represent feeders to the overlying basaltic units (Fig. 4.7). The mafic dykes that intrude the QP are medium- to fine-grained with abundant 2-3 mm euhedral augite and plagioclase phenocrysts, and are widely spaced with south southeast trends. Dyke margins are variably straight, but do not exhibit well-defined chill margins typical of warm dykes intruding cold wall rocks (Fig. 4.7). In places along the 106 dykes they exhibit tentacle-like terminations into the surrounding QP (Fig. 4.7b), whereas in other places the dyke walls are bulbous and grade into a flame-like termination into the QP (Fig. 4.7a). Common are ball structures of mafic material disassociated from the dyke margins and floating within the QP (Fig. 4.7a). These magma-mingling relationships with the quartz-porphyritic rocks suggest that they are coeval and that the quartz-porphyritic intrusions are the sub-volcanic feeders to rhyolite and rhyodacite interstratified with basalt in parts of the volcanic sequence. The -357-361 Ma ages on the quartz porphyry would therefore constrain the age of part of the volcanic succession. The SRPS consists of numerous granitoid types including hornblende-granodiorite, biotite-monzogranite, and K-feldspar granite (cf. Mortensen, 1983; Mortensen, 1992a,b; Grant, 1997). The relationships between the different granitoid phases were not discerned during our field study. The SRPS clearly intrudes the mafic rocks of the undeformed sequence, as evidenced by distinctive patchy purple-yellow epidote-rich alteration of the mafic rocks along the contact, dykes of hornblende-bearing granitoids cutting the sequence, and pendants and xenoliths of mafic volcanic rock within the granitoids. U-Pb geochronological constraints suggest that the Simpson Range plutonic suite intruded between 345 and 350 Ma (Mortensen, 1992b; personal communication, 1997; Grant, 1997), well after the extrusion of the volcanic sequence at ca. 357-361 Ma. Discussion The new data and conclusions from our study of the undeformed volcanic rocks of the Money Creek Thrust sheet have broader implications. This discussion will be divided into 3 parts dealing with 1) definition of the Simpson Range plutonic suite; 2) emergent volcanism in the undeformed sequence; and 3) criteria to distinguish the succession in the Money Creek thrust sheet from the Campbell Range belt with which it has been correlated. Definition of the Simpson Range Plutonic Suite As defined by Mortensen (1983), the SRPS consists of hornblende-biotite granodiorite and quartz diorite; that is, granitoids of metaluminous affinity that differed from suites of peraluminous affinity in YTT such as the Mink Creek or Houle River orthogneiss bodies. Erdmer (1985), Grant et al. (1996) and Grant (1997) subsequently included the mafic and felsic rocks of the volcanic 'plug' and related gabbroic 107 intrusions in the SRPS. The geological relationships described in this report, as well as published and unpublished U-Pb geochronological data show that the volcanic succession is older than and crosscut by intrusions of the SRPS; hence, they should not be included in the SRPS. Emergent Volcanism in the Undeformed Sequence? Given the state of strain that most greenstones within the YTT exhibit, it is typically impossible to determine the ambient volcanic environment in most areas of the YTT. The lack of strain in volcanic rocks in this part of the Money Creek thrust sheet gives us the opportunity to determine the local environment of deposition. The change from predominantly subaqueous pillowed flows and related volcaniclastic rocks in unit M V U i to mixed subaerial and subaqueous volcanism in unit F M V U i is inferred to represent the emergent growth of a volcanic edifice from subaqueous to subaerial conditions. Unit M V U i basalts are unequivocally subaqueous exhibiting features such as pillowed flows, pillowed breccias, and inter-pillow chert and hyaloclastite breccias. Near the top of M V U i , however, the dominantly subaqueous basalts change to reddish lava flows alternating with black to green subaqueous flows, implying a transition to more oxidizing conditions indicative of a subaerial environment. Furthermore, in the overlying F M V U i the presence of trachytic textures, abundant vesiculation, scoriaceous textures, and predominant reddish colouration are features typical of volcanism in a subaerial environment. We suggest that based on this transition from subaqueous to subaerial conditions there was growth of a volcanic edifice from below to above the ambient sea level, akin to many volcanic archipelagos present in the modern oceans. It appears from the volcanic stratigraphy that the phase of emergent volcanism in FMVUt was replaced by subaqueous turbiditic sedimentation and mixed mafic and felsic magmatism ( T S U i , MVU2, MVU3). It is possible that this return to subaqueous activity may represent either an inundation of the volcanic edifice by rising sea levels, or more likely rifting or intra-volcano subsidence (caldera?) with subsequent subaqueous activity. Given the presence of turbiditic sedimentation in association with subaqueous lavas, the latter situation is most probable. 108 Comparison with Campbell Range Belt The correlation between the mafic and ultramafic rocks of the Money Creek thrust sheet and similar rocks in the Campbell Range belt (CRB) is rendered obsolete by the geological relationships presented in this study, and published and unpublished U-Pb geochronological data. The volcanic and sub-volcanic rocks of the Money Creek thrust sheet are Devono-Mississippian and those of the CRB are Pennsylvanian to Permian (Harms, in Plint and Gordon, 1997). In this section we compare and contrast the volcanic rocks of the Money Creek thrust sheet and the Campbell Range belt to provide criteria with which to distinguish them (Table 4.1). Mafic volcanic rocks are common to both sequences; they differ primarily in the nature of the other rock types. For example, subaqueous pillow lavas and massive flows are common to both sequences, as are pillowed breccias and vent-proximal volcaniclastic material (e.g., Murphy and Piercey, 1999a; Plint and Gordon, 1997). However, rhyolitic and rhyodacitic rocks are a common feature of the rocks ofthe Money Creek thrust sheet and are notably absent in the CRB rocks. Turbiditic sedimentary rocks such as TSUi are present, although not necessarily abundant, in the Money Creek thrust sheet. In contrast, sedimentary rocks including chert, chert-pebble conglomerate, cherty argillite, siltstones, sandstones and olistostromal carbonate blocks are the common sedimentary rocks of the CRB (Plint and Gordon, 1997; Murphy and Piercey, 1999a, b, c; 2000). Although the sedimentary rocks of the CRB may have had similar origins as TSUi (i.e., mass flows), they do not exhibit the turbiditic layering and characteristics common of those in the Money Creek thrust sheet (op cit). There is also a significantly greater abundance of chert and chert-rich material in the CRB than in the Money Creek thrust sheet, suggestive of a quiescent environment with significant gaps in volcanism that allowed the accumulation of chemical sediments. The sequence in the Money Creek thrust sheet also contains abundant evidence for subaerial volcanism that has yet to be described or observed in the CRB (op cit). In addition to the field-based differences, the sequences differ geochemically. Grant (1997) showed that the mafic and felsic rocks from the Money Creek thrust sheet have geochemical and isotopic signatures typical of calc-alkalic continental arc magmatism. In contrast, the CRB is characterized by 109 non-arc signatures consisting of various basalt types ranging from rift through normal mid-ocean ridge basalt (N-MORB) composition (Plint and Gordon, 1997; Piercey et al, 1999a,b). To summarize, the undeformed Money Creek package can be separated from the Campbell Range belt (and possibly similar rocks of the Slide Mountain Terrane?) lithologically, geochemically (arc versus non-arc) and geochronologically (Mid- versus Late-Paleozoic). By using this combined approach, greenstones from other portions ofthe YTT and Slide Mountain Terrane may be effectively discriminated and separated. Conclusions 1) Undeformed volcanic rocks ofthe Money Creek thrust sheet provide a window into the Mid-Paleozoic volcanic history of this portion of the YTT. This undeformed sequence consists of a five-component volcano-sedimentary stratigraphy with associated sub-volcanic intrusions that record volcanism in a shallow water subaqueous to subaerial setting (archipelago-like environment). 2) Magma-mingling relationships between augite-plagioclase-porphyritic mafic dykes, interpreted as feeders to the basalts in the volcanic sequence, and -357-361 Ma quartz-porphyritic granitoids, inferred to feed the felsic volcanic rocks, imply coeval Devono-Mississippian mafic and felsic volcanism. This relationship provides a key temporal pin on primitive through mature arc activity (e.g., Grant, 1997; Piercey et al, 1999a,b) in Yukon-Tanana Terrane. 3) Many workers have suggested that the mafic and felsic volcanic rocks, as well as quartz-porphyritic granitoids and gabbroic-ultramafic intrusions were part ofthe SRPS (e.g., Erdmer, 1981, 1985; Grant et al, 1996; Grant, 1997). We exclude these units from the SRPS based on our field observations and U-Pb geochronological data and restrict the SRPS to metaluminous granitoids of -345-350 Ma age. 4) Field, geochemical and geochronological criteria can be used to distinguish the Devono-Mississippian mafic volcanic rocks from Pennsylvanian-Permian rocks of the Campbell Range belt. Greenstones within the Yukon-Tanana Terrane are diverse in geological character and paleotectonic environments of formation. It is only with keen field observations and geological mapping augmented with relevant laboratory data (i.e, geochemistry, geochronology) can one obtain a clearer picture ofthe 110 nature and origin of mafic to felsic volcanism within the Y T T and other terranes ofthe northern Cordillera. References Creaser, R . A , Erdmer, P , Stevens, R . A , and Grant, S .L , 1997a. Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence. Tectonics, v.6, p. 107-121. Erdmer, P , 1981. Comparative studies of cataclastic allochthonous rocks in McQuesten, Laberge and Finlayson map areas. In Yukon Geology and Exploration 1979-80. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 60-64. Erdmer, P . E , 1985. An examination of cataclastic fabrics and structures of parts of Nisutlin, Anvil and Simpson allochthons, central Yukon: Test ofthe arc-continent collision model. Journal of Structural Geology, v.7, p.57-72. Grant, S .L , 1997. Geochemical, radiogenic tracer isotopic, and U-Pb geochronological studies of Yukon-Tanana Terrane rocks from the Money Klippe, southeastern Yukon, Canada. Unpublished M.Sc. thesis, University of Alberta, 177 p. Grant, S .L , Creaser, R . A , and Erdmer, P , 1996. Isotopic, geochemical and kinematic studies of the Yukon-Tanana Terrane in the Money Klippe, SE Yukon. Lithoprobe SNORCLE Report #50, p.58-60. Hunt, J . A , 1998. Recent discoveries of volcanic-associated massive sulfide deposits in the Yukon. Canadian Institute of Mining and Metallurgy, Bulletin, v.90, p.56-65. Hunt, J . A , and Murphy, D . C , 1998. A note on preliminary bedrock mapping in the Fire Lake area. In Yukon Exploration and Geology 1997. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p.59-68. Mortensen, J . K , 1983. Age and evolution of the Yukon-Tanana Terrane, southeastern Yukon Territory. Unpublished Ph.D thesis, University of California, Santa Barbara, 155 p. Mortensen, J . K , 1992a. Pre-Mid-Mesozoic tectonic evolution ofthe Yukon-Tanana Terrane, Yukon and Alaska. Tectonics, v.l 1, p.836-853. Mortensen, J . K , 1992b. New U-Pb ages for the Slide Mountain Terrane in southeastern Yukon Territory. In: Radiogenic Age and Isotopic Studies: Report 5, Geological Survey of Canada, Paper 912, p. 167-173. Mortensen, J . K , and Jilson, G . A , 1985. Evolution ofthe Yukon-Tanana Terrane: Evidence from southeastern Yukon Territory. Geology, v.13, p.806-810. Murphy, D . C , 1998. Stratigraphic framework for syngenetic mineral occurrences, Yukon-Tanana Terrane south of Finlayson Lake: A progress report. In Yukon Exploration and Geology 1997. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 51-58. I l l Murphy, D.C., 1999.Yukon-Tanana Terrane and its relationship to Slide Mountain 'Terrane', Finlayson Lake area, southeastern Yukon. Slave-Northern Cordillera Lithospheric Experiment (SNORCLE), Lithoprobe Report 69, p. 138-141. Murphy, D.C., and Timmerman, J.T., 1997. Preliminary geology of the northeast third of the Grass Lakes area, Pelly Mountains, southeastern Yukon. In Yukon Exploration and Geology 1996, Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p.29-32. Murphy, D.C., and Piercey, S.J., 1999a. Finlayson project: Geological evolution of Yukon-Tanana Terrane and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon. In Yukon Exploration and Geology 1998. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p.47-62. Murphy, D.C., and Piercey, S.J., 1999b. Geological map of Wolverine Lake area (105G/8), Pelly Mountains, southeastern Yukon. Exploration and Geological Sciences Division, Yukon, Indian and Northern Affairs Canada, Open File 1999-3, 1:50 000 scale. Murphy, D.C., and Piercey, S.J., 1999c. Geological map of Finlayson Lake area, southeast quarter (105G/7, 8 and parts of 1, 2 and 9), southeastern Yukon. Exploration and Geological Sciences Division, Yukon, Indian and Northern Affairs Canada, Open File 1999-4, 1:100 000 scale. Murphy, D.C. and Piercey, S.J., 2001. Syn-mineralization faults and their re-activation, Finlayson Lake massive sulphide belt, Yukon-Tanana Terrane, southeastern Yukon. In Yukon Exploration and Geology 1999. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada p.55-66. Piercey, S.J., Hunt, J.A., and Murphy, D . C , 1999a. Lithogeochemistry of meta-volcanic rocks from Yukon-Tanana Terrane, Finlayson Lake region, Yukon: Preliminary results. In Yukon Exploration and Geology 1998. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs, p. 125-138. Piercey, S.J., Murphy, D . C , Mortensen, J.K., and Hunt, J.A., 1999b. Geochemistry of metavolcanic rocks from the Yukon-Tanana Terrane and Campbell Range belt, Finlayson Lake region, southeastern Yukon Territory: Preliminary eesults. Slave-Northern Cordillera Lithospheric Experiment (SNORCLE), Lithoprobe Report 69, p.312. Plint, H.E., and Gordon, T .M. , 1997. The Slide Mountain Terrane and the structural evolution of the Finlayson Lake fault zone, southeastern Yukon. Canadian Journal of Earth Sciences, v.34, p.105-126. Stevens, R.A., Erdmer, P., Creaser, R.A., and Grant, S.L., 1996. Mississippian assembly of the Nisutlin Assemblage: Evidence from primary contact relationships and Mississippian magmatism in the Teslin tectonic zone, part of the Yukon-Tanana Terrane of south-central Yukon. Canadian Journal of Earth Sciences, v.33, p. 103-116. Tempelman-Kluit, D.J., 1977. Geology of Quiet Lake (105F) and Finlayson Lake (105G) map areas, Yukon Territory. Geological Survey of Canada, Open File 486. Tempelman-Kluit, D.J., 1979. Transported cataclasite, ophiolite and granodiorite in Yukon: Evidence for arc-continent collision. Geological Survey of Canada Paper 79-14, 27 p. 112 Wheeler, J.O., and McFeely, P., 1991. Tectonic Assemblage Map of the Canadian Cordillera and Adjacent Parts of the United States of America. Geological Survey of Canada, Map 1712A, 1: 2 000 000 scale. 113 k m Figure 4.1. Geological setting and distribution of Yukon-Tanana terrane in the Yukon and the location of the study area in relation to the Finlayson Lake region. Map modified from Hunt (1998) and Wheeler and McFeely (1991). 114 Intrusions Layered Rocks x xi X H Sheared SRPS strongly foliated granite (Grass Lakes suite) weakly foliate biotite-muscovite granite biotite-nornblende granite and quartz monzonite (Simpson Range Plutonic Suite (SRPS)) L" ~. —1 mafic schist A A A A A A A A A A A A carbonaceous phyllite quartz-(f eld spar) porphyritic granite (high level) serpentinized ultramafic rocks 2 3 Kudz Ze Kayah unit - felsic A volcanic and volcaniclastic _a, J rocks carbonaceous phyllite and grey quartzite carbonaceous phyllite and quartzite, minor chert and marble massive reddish pink to pink non-vesicular rhyolite to rhyodacite Fire Lake unit - mafic schist pillowed and massive lava flows in Money Creek thrust marble and calc-schist (unit 1) quartz-rich metaclastic rocks (unit 1) Figure 4.2. Geological setting of the undeformed succession in the hanging wall of the Money Creek thrust sheet. Map modified from Murphy and Piercey (1999b). Further details on the regional setting can be obtained from Murphy and Piercey (199a,b,c). Grid around map is in UTM coordinates (NAD 27, Zone 9V), X and Y are locations discussed in the text. 115 fl S V / f l ^ V 3 %V " fl %V " %V A A %V A i [ 7 ! ^ ffV^ J 7 J ^ J V B ^ flV^ fl ^ ^1 345-350 Ma fl <#> »^ fl f ^  ^  ^  = % ^ >#> = <x <%> * Figure 4.3. Volcanic stratigraphy of the undeformed volcanic sequence and relationships to intrusive rocks in the region. Age quotations on this figure are from Mortensen (1992a, b), Grant et al. (1996) and Grant (1997). 116 Figure 4.4. (a) Typical large-sized pillowed mafic lava flows from unit MVU,. These pillowed flows are not dissimilar to those found in units MVU, and MVU 3 . (b) Typical pillowed breccias and volcaniclastic rocks of MVUI. Directly above the pen is a dark fragment of pillowed material that is similar to the dark finer grained mafic matrix; to the left of the pen are bleached augite-bearing pillowed fragments. 117 Figure 4.5. In (a) are highly vesiculated (pumiceous) subaerial basaltic lavas from unit FMVU,. Some of the vesicles in the basaltic rocks become quite large; in (b) this single vesicle is nearly 2 cm in diameter. Subaerial to subaqueous felsic flows in unit FMVU, commonly exhibit a rubbly in situ brecciation along their margins as show in (c). 118 Figure 4.6. Typical finely laminated, locally glassy, siliceous sedimentary rocks common in unit TSU, (a). These laminated sedimentary rocks are interlayered with white sandy layers at location X in Figure 2. Directly atop of the pencil in (b) is a typical flute cast within a white greywacke common along their basal surfaces in unit TSU,. 119 Figure 4.7. Mafic dykes intruding and mingling with a -361 Ma (Mortensen, 1992b) quartz-porphyritic granite intrusion. In (a) the dyke margins are characterized by flame-like terminations into the granite host. Commonly, the dykes have ball structures of mafic material disassociated from the dyke margins within the granite host. Tentacle-like intrusive contacts and very diffuse margins of the mafic dykes with the granite host are also common (b). These features suggest that the granite host was still warm and not solidified during the intrusion of the dykes implying that they are nearly coeval. 120 Table 4.1. Summary of salient stratigraphic, geochemical and temporal differences and similarities of the volcanic rocks of the Money Creek thrust sheet and the Campbell Range belt. Money Creek Thrust Sheet Campbell Range Belt Mafic volcanism1 Pillowed to massive flows and volcaniclastic rocks; subaqueous and subaerial Pillowed to massive flows and volcaniclastic rocks; all subaqeous Felsic volcanism1 Rhyolite-rhyodacite; subaqeous and subaerial Felsic volcanism absent Sedimentary rocks' Turbiditic sedimentary rocks; minor detrital carbonate rocks; minor volcanic greywackes; rare chert Dark argillite and sandstone; diamictite; volcanic greywacke; abundant chert and chert- pebble conglomerate; carbonate (olistostromal) Age2 Devonian-Mississippian Pennsylvanian-Permian Geochemistry3 Arc: calc-alkaline, island-arc tholeiitic Non-Arc: N-MORB, E-MORB, OIB Tectonic setting4 Continental arc Marginal basin with terrigenous input Notes: 1- CRB data from Murphy and Piercey (1999a,b,c) and Plint and Gordon (1997); 2- Age data from Mortensen (1983, 1992b), Grant et al. (1996), and Grant (1997); 3-Geochemical attributes from the undeformed sequence from Grant (1997), CRB from Plint and Gordon (1997) and Piercey et al. (1999a,b), N-MORB = normal mid-ocean ridge basalt, E-MORB = enriched mid-ocean ridge basalt, OIB = ocean island basalt; 4- Interpretations based on inferences from this paper and Grant (1997), Grant et al. (1996), and Plint and Gordon (1997). 121 Chapter 5 Geological Characteristics of High-Level Subvolcanic Porphyritic Intrusions Associated with the Wolverine Zn-Pb-Cu-Ag-Au Volcanic-Hosted Massive Sulphide Deposit, Finlayson Lake District, Yukon, Canada Abstract During the 2000 field season, a project was initiated to study the geology, geochemistry and alteration characteristics of high-level subvolcanic porphyritic intrusions associated with the Wolverine volcanic-hosted massive sulphide (VHMS) deposit in the Finlayson Lake district, Yukon. Subvolcanic porphyritic intrusions within the Wolverine deposit are located at approximately 10-20 m beneath exhalative sulphide bodies or iron-formation in four zones (Wolverine/Lynx, Fisher, Sable and Puck). Most intrusions are K-feldspar porphyritic (Fisher and Wolverine/Lynx Zones); however, a few are quartz and K-feldspar porphyritic (Puck and Sable zones). Feldspar porphyritic intrusions consist of euhedral to subhedral grains of K-feldspar in a grey fine-grained matrix. Quartz-feldspar porphyritic intrusions contain slightly smaller feldspar crystals and blue to black glassy quartz eyes set in a fine-grained matrix. Most of the intrusions have non-peperitic upper margins with carbonaceous argillite (Wolverine/Lynx, Fisher, Puck). Some ofthe quartz-feldspar porphyritic intrusions are in contact with fine-grained volcaniclastic rocks along their upper margins (Sable); both types of intrusions have lower contacts with fine-grained volcaniclastic sedimentary rocks. These intrusions are, for the most part, unaltered and have only minor sericite-silica±chlorite±pyrite alteration and small mm- to cm-scale veinlets of quartz-sericite±chlorite±pyrite±sphalerite. This suggests a pre- to syn-mineralization timing for the emplacement of the intrusions. The contribution of these intrusions to the heat and metal budget of the Wolverine deposit is the focus of ongoing research. 121a Introduction Numerous workers have suggested that subvolcanic intrusions play a critical role in the generation and maintenance of hydrothermal systems responsible for the formation of volcanic-hosted massive sulphide (VHMS) deposits (Campbell et al, 1981; Cathles, 1981, 1983; Lesher et al, 1986; Galley, 1996; Large et al, 1996; Barrie et al, 1999). Most workers suggest that these high-level subvolcanic intrusions provide the heat for generating and maintaining hydrothermal systems (Campbell et al, 1981; Galley, 1996; Barrie et al, 1999), and some workers also suggest that these intrusions may contribute metals to the hydrothermal system (Large et al, 1996). Within the Wolverine VHMS deposit, high-level subvolcanic porphyritic intrusions are spatially associated with massive sulphide mineralization in many areas. Herein we provide field and petrographic observations of these high-level subvolcanic porphyritic intrusions. This paper is a companion to a more extensive study of the genesis and setting of the Wolverine VHMS deposit (Bradshaw et al, 2001) and on chemical sedimentary (exhalative) rocks associated with this deposit (Peter et al, in prep). Regional Setting The Finlayson Lake VHMS district (Figs. 5.1 and 5.2) is hosted by variably deformed and metamorphosed (greenschist to lower amphibolite facies) Devonian-Mississippian volcanic, intrusive and sedimentary rocks (Tempelman-Kluit, 1979; Mortensen and Jilson, 1985; Mortensen, 1992; Murphy and Timmerman, 1997; Murphy, 1998; Murphy and Piercey, 1999a,b,c, 2000; Piercey and Murphy, 2000; Murphy, 2001). Murphy and co-workers (pp. cit.) have broken the volcanic, intrusive, and sedimentary rocks of the district into three stratigraphic successions (Grass Lakes, Wolverine Lake, and Campbell Range successions), which have been formed on a substrate of non-carbonaceous, biotite-rich clastic metasedimentary rocks and associated metacarbonates and marble (unit 1; Fig. 5.2). The base of the Grass Lakes succession consists of unit 1 (metaclastic rocks) which is overlain by rocks of the Fyre Lake unit (unit 2), which consists of a Devonian-Mississippian (-365-360 Ma) package of mafic volcanic and intrusive rocks, with lesser felsic volcanic and sedimentary rocks (Fig. 5-2). These rocks are interpreted to have represented arc magmatic activity that progressed to back-arc basin 122 formation (Grant, 1997; Sebert and Hunt, 1999; Piercey et al., 1999, 2000a). Boninitic volcanic rocks within this package are host to the Besshi-style Cu-Co-Au V H M S deposits (Foreman, 1998; Sebert and Hunt, 1999). The Kudz Ze Kayah unit (unit 3) stratigraphically overlies the Fyre Lake unit and is a -360 Ma felsic-volcanic and sedimentary rock dominated unit with lesser mafic volcanic and intrusive rocks, felsic intrusive rocks (Grass Lakes suite intrusions), and sedimentary rocks (Fig. 5.2). These rocks are interpreted to have formed in an Okinawa Trough-style (Piercey et al., 2000a) ensialic back-arc basin and host the felsic-associated Zn-Pb-Cu Kudz Ze Kayah and Zn-Pb GP4F V H M S deposits (Fig. 5.2). The Wolverine succession unconformably overlies the Grass Lakes succession and consists of a lower package (unit 51) of quartz-feldspar pebble conglomerate, overlain by -345 Ma (Piercey, unpublished data) rhyolitic flows, volcaniclastic rocks and quartz-feldspar bearing volcaniclastic and epiclastic sedimentary rocks and high-level feldspar! quartz porphyritic rhyolitic intrusive rocks (unit 5f/qfp; Fig. 5.2). This felsic-dominated package is overlain by unit 5cp, carbonaceous argillite and quartzite of regional extent, which forms part of the deeper footwall of the Wolverine deposit (Tucker et al., 1997; Bradshaw et al., 2001; Fig. 5.2). The Wolverine deposit is hosted by unit 6, which consists of rhyolitic volcaniclastic and epiclastic rocks, high-level rhyolitic intrusive rocks and flows, variably carbonaceous sedimentary rocks, and is capped by interlayered felsic and mafic volcaniclastic and basaltic lava flows (Bradshaw et al., 2001; Figs. 5.2-5.4). Geochemical compositions of the felsic volcanic rocks in the Wolverine succession suggest that the rocks formed within an ensialic back-arc basin setting, akin to that of the Grass Lakes succession (Piercey et al., 2000a,b). Capping the entire Finlayson Lake district, and unconformably overlying the rocks of the Wolverine Lake succession are Pennsylvanian-Permian (T. Harms in Plint and Gordon, 1997) rocks of the Campbell Range succession (Murphy, 2001; Fig. 5.2). This succession consists of largely clastic sedimentary rocks, pillowed to massive mafic lava flows, high-level diabase intrusive rocks, leucogabbroic intrusive rocks of MORB chemistry and chert-rich sedimentary rocks, that host numerous Cyprus-style V H M S occurrences (Mann and Mortensen, 2000; Baknes in Hunt, 1997; Pigage, 1997). 123 Plint and Gordon (1997) and Murphy and Piercey (1999a,b,c) give more detailed descriptions of the geology of the Campbell Range succession. Geological Features of Porphyries Associated with the Wolverine VHMS Deposit Within the Wolverine VHMS deposit area, most porphyritic intrusions are K-feldspar porphyritic and only a few are quartz-feldspar porphyritic. In all cases the intrusions are spatially associated with, and underlie, massive sulphide or iron formation (Wolverine/Lynx, Fisher, Sable and Puck zones; Fig. 5.3). Notably, to date significant massive sulphide accumulations have been only delineated in the Wolverine/Lynx zone; however, all zones contain iron formation and carbonate exhalite suggesting that they are part of the same hydrothermal system. The zones discussed in this paper are treated as representing parts of the same system and are distinguished partly on the geographic distribution of diamond drill holes outlined during exploration and delineation of the Wolverine V H M S system. Outlined below are the geological attributes of porphyritic intrusions within the four zones of the Wolverine V H M S system (Fig. 5.3). Salient geological features of the intrusions are given in Table 5.1 and their locations within the Wolverine VHMS deposit area are shown in Figure 5.3. Wolverine/Lynx Zone Porphyritic intrusions associated with the Wolverine/Lynx zones and the Wolverine deposit are sill-like feldspar porphyritic intrusions (FPI) (Fig. 5.4) with euhedral to subhedral 0.1-1 cm K-feldspar set in a grey, siliceous fine-grained matrix; the phenocrysts comprise -20% of the rock on average (Fig. 5.5). The upper contacts of intrusions are against black carbonaceous argillite, and for the most part are sheared due to Cretaceous deformation (Murphy, 1998). Where the contacts are not sheared, the intrusions do not exhibit peperitic margins indicative of minimal interaction with wet, unconsolidated sedimentary rocks. The lower margins of the intrusions are against fine-grained, variably crystal-rich, felsic volcaniclastic rocks (tuffs). Most of the intrusions are sill-like in morphology (Fig. 5.4) range in true thickness from -1 to 14 m with an average thickness of -7.5 m. These intrusions are located on average 20 m below the massive sulphide mineralization in the Wolverine/Lynx zones (cf. Bradshaw et al , 2001). The feldspar porphyritic intrusions of the Wolverine/Lynx zone are unaltered to weakly altered. The most common type of alteration within the FPI consists of a patchy distribution of partial to complete 124 replacement of K-feldspar by grey to black secondary K-feldspar (as identified by X-ray diffraction at the Geological Survey of Canada; Fig. 5.5b). Veinlet-style alteration is less common than secondary K-feldspar and consists of millimetre-scale veinlets of weak fine-grained sericite and quartz throughout (Fig. 5.5a). Pyrite-quartz and pyrite-quartz-sericite veinlets commonly crosscut many of the samples; in a few samples chlorite-pyrite-quartz veinlets are present (e.g., Fig. 5.5a). The intensity of veinlet-style alteration commonly decreases with distance from the upper and lower contacts. Fisher Zone The porphyritic intrusions of the Fisher zone have strong petrographic similarities and stratigraphic position to those from the Wolverine/Lynx zone. The intrusions are mostly feldspar porphyritic intrusions with some portions having minor quartz (QFP). Euhedral to weakly subhedral 0.1-1 cm K-feldspar phenocrysts make up -20% of the rock within a grey fine-grained matrix (Fig. 5.6). Some QFP have 1-2 mm, quartz-filled amygdules. In one drill hole the edges of the FPI have rhyolite extrusive characteristics with autobrecciated rhyolite fragments and a less feldspar-phyric margin that grades inward into a more feldspar-rich interior, suggesting rapid quenching of the margin and slower cooling of the interior. As with intrusions associated with the Wolverine/Lynx zone, the Fisher zone intrusions appear to be at the same stratigraphic level having upper contacts with black argillite that lack peperitic textures, and the lower contacts are against fine-grained felsic volcaniclastic rocks (tuffs?). The average thickness of the intrusion varies from 1 m to -50 m; however, there may have been considerable structural thickening within the nose of a Cretaceous fold (cf. Murphy, 1998); primary interpreted thicknesses are estimated to vary from -1 to 14 m. The feldspar porphyritic intrusions from the Fisher zone are not associated with significant amounts of massive sulphide mineralization; however, they lie at a similar stratigraphic level, and are overlain by minor disseminated sulphide and iron-formation (cf. Peter et al., in prep) inferring a correlation with the Wolverine/Lynx zone intrusions. The Fisher zone intrusions are relatively unaltered and contain minor veinlet-style alteration. This veinlet-style alteration consists of 1-7 mm wide veins of pyrite-sericite-quartzichlorite and is very similar to that in the Wolverine/Lynx zone, including a similar spatial distribution in some diamond drill holes (Fig. 5.6). In addition, partial to complete replacement and patchy overprinting of feldspars by 125 black, secondary K-feldspar is also the prevalent type of alteration within the intrusions of the Fisher zone. Sable Zone The Sable zone intrusions differ significantly from the Wolverine/Lynx and Fisher zone intrusions. Al l ofthe Sable zone intrusions are quartz-feldspar-porphyritic intrusions (QFP) with quartz and K-feldspar phenocrysts comprising -20-30% ofthe rock within a grey to weak grey-green, fine-grained matrix (Fig. 5.7). Quartz phenocrysts are dark grey to black, glassy, eye-shaped, and 0.1-0.5 cm in diameter (Fig. 5.7). Potassic feldspar phenocrysts are less abundant than quartz phenocrysts and comprise -5-10% of the rock as euhedral grains 0.1-0.5 cm in diameter (Fig. 5.7). The QFP have non-peperitic upper and lower contacts with fine-grained felsic volcaniclastic rocks. Intrusions within the Sable zone are 2-16 m in thickness and lie below minor amounts (-30 cm) of massive sulphide, which in turn are overlain by exhalative iron formation similar to the other zones (cf. Peter et al , in prep). However, given their distinctive petrographic characteristics and stratigraphic position these intrusions are not likely correlative to those in the Wolverine/Lynx and Fisher zones. Only veinlet-style alteration is observed in the Sable zone QFP, with 1-4 mm wide veinlets of sericite-pyrite, pyrite-quartz±fine-grained sphalerite, and pyrite-sphalerite. In some samples sulphide veinlets constitute 5-10% of the rock. Puck Zone Both quartz-feldspar- and feldspar-porphyritic intrusions are present in the Puck zone. Feldspar porphyritic intrusions are present in 2 drill holes and are similar to those in the Wolverine/Lynx and Fisher zones. These intrusions are 0.5-3.5 m thick and consist of euhedral, 0.1-2 cm diameter K-feldspar phenocrysts within a grey, fine-grained matrix (Fig. 5.8). The Puck feldspar porphyritic intrusions have non-pepperitic upper contacts with black argillite and lower contacts against fine-grained felsic volcaniclastic rocks, akin to feldspar porphyritic intrusions in other zones. A 25 m thick QFP occurs in one drill hole and consists of 3-4 mm elliptical-shaped blue quartz and euhedral K-feldspar in a siliceous grey matrix. Upper and lower contacts ofthe QFP are similar to those ofthe feldspar porphyritic intrusions. Both QFP and feldspar porphyritic intrusions are stratigraphically overlain by minor massive 126 sulphide and iron formation (cf. Peter et al., in prep) inferring a possible correlation to intrusions within the Wolverine/Lynx and Fisher zones. Similar to intrusions in other zones the alteration intensity is weak. Feldspar porphyritic intrusions have some fine sericite-pyrite veins and notably the K-feldspar phenocrysts are partially to completely replaced by patchy secondary K-feldspar (Fig. 5.8). The Q F P have minor disseminated pyrite throughout and some o f the feldspars are incipiently replaced by pyrite. Both intrusion types have low-level alteration and mineralization akin to that in other zones. Discussion The spatial proximity o f high-level subvolcanic intrusions to massive sulphides and iron formation within the Wolverine V H M S deposit points to a passive (to possibly active) role of these intrusions in the genesis and localization o f the Wolverine V H M S deposit. The following discussion w i l l briefly address two questions, namely: 1) Are the porphyritic intrusions pre-, syn- or post-mineralization?; and 2) What is the significance of the porphyritic intrusions to the localization o f sulphide mineralization? Pre-, Syn- or Post-Mineralization Emplacement of the Porphyries The close spatial association of the porphyritic intrusions to massive sulphides and iron formations within the Wolverine deposits raises the question of whether the intrusions are o f syn-, post-or pre-hydrothermal origin. Some possible evidence for a post-mineralization origin for the porphyritic intrusions is their non-peperitic upper margins and lack of evidence for interaction with wet unconsolidated material (e.g., modification of bedding, etc.). This would suggest emplacement into solidified material and a l ikely non-syn-volcanic/syn-sedimentary and post-hydrothermal origin. It is notable, however, that many o f the contacts between the intrusions and overlying argillites are sheared due to Cretaceous deformation and much o f the evidence for intrusion-sediment interaction may have been removed and obscured. Further evidence against a post-hydrothermal origin is quite strong. Firstly, the intrusive rocks within the deposit only occur stratigraphically below the massive sulphide or iron formations. Although this does not necessarily prove that they are not post-mineralization, the presence of interpreted VHMS-re la ted alteration of the intrusions (e.g., sericite-chlorite-quartz veinlets), and 127 crosscutting sulphide-bearing veinlets in the intrusions suggest that they were not emplaced after hydrothermal activity. Determining the syn- or pre-mineralization timing of intrusion is less clear. A pre-mineralization origin is compatible with the presence of hydrothermal alteration and crosscutting sulphide veining. For example, the intrusions are emplaced prior to generation of the V H M S system and then subsequently altered as hydrothermal activity commenced to generate the Wolverine deposit and related iron formations. Similarly, geological evidence for a syn-mineralization history (syn-sedimentary/syn-volcanic in origin) for the intrusions (e.g., peperites, contorted bedding) is lacking and at present most evidence points to a pre-mineralization origin. Nevertheless, the spatial association of these intrusions to VHMS mineralization is significant (see below) and further facies mapping of the intrusion-sediment contacts are required to test the syn- versus pre-mineralization origin of these intrusions. Significance of Porphyries for Localization of Sulphide Mineralization The presence of porphyritic intrusions proximal to (-10-20 m below) the Wolverine VHMS deposit and iron-formation/exhalites is likely significant from a regional perspective. There are high-level intrusions outside of the Wolverine succession (e.g., Murphy and Piercey, 1999a); however, they are not as volumetrically condensed and are more regionally dispersed when compared to those proximal to the Wolverine V H M S system. Given the presence of these coherent magmatic rocks (i.e, flows and intrusions, not volcaniclastic or epiclastic) close to exhalative activity, and relatively minor abundance elsewhere in the Wolverine succession, suggest that unique conditions prevailed proximal to the Wolverine deposit area that controlled the emplacement of these rocks. The authors suggest that their presence in the Wolverine deposit was likely controlled by synvolcanic structures (growth faults?). Furthermore, we suggest that the structures that controlled the intrusion emplacement were also responsible for controlling the hydrothermal fluids that formed the Wolverine deposit and related exhalites. It is also notable that coherent aphyric rhyolitic flows (Fig. 5.4) that overlie the Wolverine massive sulphides are also restricted to the area proximal to the Wolverine deposit and have not been recognized, thus far, elsewhere in the district. It is possible that coherent facies (i.e, not volcaniclastic or epiclastic) felsic volcanic rocks may reflect proximity to volcanic vents and through-going structural 128 conduits that allowed their emplacement. We acknowledge, however, that we have not yet documented facies changes, thickness variations or syn-volcanic diking to support the proximity of the intrusions to syn-volcanic faults and further mapping of thickness variations, and volcanic facies associated with the intrusions and coherent facies rocks is required to test this syn-volcanic-emplacement hypothesis. The proximity of these intrusions to massive sulphide mineralization also suggests that they were possible heat and/or metal sources. The authors are presently unable to address whether these intrusions were the sources of metals without further data; however, some insight can be provided into the possibility of these intrusions as heat sources for hydrothermal circulation that formed the Wolverine deposit. When compared to many subvolcanic intrusive rocks in world-class V H M S districts (e.g., Flavrian-Noranda; Biedelman Bay-Mattabi; Cambrian granites- Mount Read; Galley, 1996; Large et al., 1996), the intrusions that constitute the footwall of the Wolverine system are volumetrically minor (i.e., 10's of m 2 rather than 10's of km2). Therefore, we suggest that these intrusions alone are of insufficient size (see Figs. 5.4 and 5.7) to be the sole thermal control on the generation and maintenance of the hydrothermal system responsible for the Wolverine deposit (e.g., Cathles, 1981, 1983; Barrie et al., 1999). What the intrusions likely reflect, however, are leaks from a larger underlying intrusive system that has yet to be identified that may have driven hydrothermal circulation. It is possible that a subvolcanic intrusive system akin to the Grass Lakes suite intrusions in the Kudz Ze Kayah unit may underlie the Wolverine succession. However, the Wolverine succession has not been uplifted significantly, whereas the Kudz Ze Kayah unit and Grass Lakes succession have experienced uplift (Murphy and Piercey, 1999a). The lack of uplift may have prevented the surface expression of a deeper intrusive system to the Wolverine succession. Summary and Conclusions Initial results from our study of high-level subvolcanic porphyritic intrusions within the Wolverine V H M S deposit show that footwall intrusions are largely K-feldspar porphyritic with lesser quartz- and K-feldspar-porphyritic intrusions. Most of the intrusions have non-peperitic upper margins against carbonaceous argillite (Wolverine/Lynx, Fisher, Puck); however, some of the QFP intrusions have upper margins against fine-grained volcaniclastic rocks (Sable); both have lower margins against fine-129 grained volcaniclastic sedimentary rocks. These intrusions are located at variable distances (-10-20 m) below exhalative sulphide bodies or iron formation activity. For the most part, the intrusions are unaltered to weakly altered having sericite-quartz±chlorite+pyrite alteration and small mm- to cm-scale veinlets of quartz-sericite±chlorite±pyrite±sphalerite. The presence of alteration and mineralization within the intrusions suggests a pre- to syn-mineralization timing for their emplacement. The large number of coherent intrusive rocks proximal to the Wolverine deposit, and their relative scarcity elsewhere in the district suggests that the emplacement of these intrusions may have been controlled by synvolcanic faults, which themselves controlled hydrothermal fluid flow. Although abundant, the volume ofthe intrusions is quite small (10's of m2), and the authors suggest that the size of these intrusions was insufficient to have generated the hydrothermal fluid flux required to form the Wolverine deposit. However, the intrusions may represent leaks (or apophyses?) from a larger intrusive system at depth that may have controlled the hydrothermal budget ofthe Wolverine deposit. The authors are presently uncertain of the role the intrusions have played in the metal budget of the Wolverine deposit, and this is the focus of ongoing research. References Barrie, C T , Cathles, L . M , Erendi, A , Schwaiger, H , Murray, C , 1999. Heat and fluid flow in volcanic-associated massive sulfide-forming hydrothermal systems. Reviews in Economic Geology, v.8, p.201-219. Bradshaw, G D , Tucker, T . L , Peter, J . M , Paradis, S, and Rowins, S . M , 2001. Geology ofthe Wolverine volcanic-hosted massive sulphide deposit, Finlayson Lake district, Yukon Territory, Canada. In Yukon Exploration and Geology 2000, Exploration and Geological Services Division, Indian and Northern Affairs Canada, p.269-287. Campbell, L H , Franklin, J . M , Gorton, M . P , Hart, T . R , and Scott, S.D, 1981. The role of subvolcanic sills in the generation of massive sulfide deposits. Economic Geology, v.76, p.2248-2253. Cathles, L . M , 1981. Fluid flow and genesis of hydrothermal ore deposits. Economic Geology 75th Anniversary Volume, p. 424-457. Cathles, L . M , 1983. An analysis ofthe hydrothermal system responsible for massive sulfide deposition in the Hokuroko basin of Japan. Economic Geology Monograph 5, p.439-487. Foreman, I, 1998. The Fyre Lake project, 1997: Geology and mineralization of the Kona massive sulfide deposit. In Yukon Exploration and Geology 1997, Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 105-113. 130 Galley, A . G . , 1996. Geochemical characteristics o f subvolcanic intrusions associated with Precambrian massive sulphide deposits. In Trace Element Geochemistry o f Volcanic Rocks: Applications for Massive Sulphide Exploration. Geological Association of Canada, Short Course Notes Volume 12. Edited by D . A . Wyman. Pages 239-278. Grant, S.L., 1997. Geochemical, radiogenic tracer isotopic, and U-Pb geochronological studies o f Yukon-Tanana Terrane rocks from the Money Kl ippe , southeastern Yukon, Canada: Unpublished M . S c . thesis, University o f Alberta, 177 p. Hunt, J .A. , 1997. Massive sulphide deposits in the Yukon-Tanana Terrane and adjacent terranes. In Yukon Exploration and Geology, 1997, Exploration and Geological Services Divis ion, Indian and Northern Affairs Canada, p. 91-98. Hunt, J .A. , 1998. Recent discoveries o f volcanic-associated massive sulfide deposits in the Yukon: Canadian Institute of Min ing and Metallurgy Bulletin, v.90, p.56-65. Large, R., Doyle, M . , Raymond, O., Cooke, D. , Jones, A . , and Heasman, L . , 1996. Evaluation o f the role o f Cambrian granites in the genesis o f world class V H M S deposits in Tasmania. Ore Geology Reviews, v. 10, p.215-230. Lesher, C M . , Goodwin, A . M . , Campbell, I .H., and Gorton, M . P . , 1986. Trace element geochemistry o f ore-associated and barren felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences, v.23, p.222-237. Mann, R . K . , and Mortensen, J .K. , 2000. Geology, geochemistry and lead isotope analysis o f mineralization o f the Strike prospect, Campbell Range, southeastern Yukon . In Yukon Exploration and Geology 1999. Exploration and Geological Services Divis ion, Department of Indian and Northern Affairs Canada, p.237-245. Mortensen, J .K. , 1992. Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana Terrane, Yukon and Alaska: Tectonics, v . l 1, p.836-853. Mortensen, J .K. , and Jilson, G . A . , 1985. Evolution of the Yukon-Tanana Terrane: Evidence from southeastern Yukon Territory: Geology, v.13, p.806-810. Murphy, D . C , 2001. Yukon-Tanana Terrane in southwestern Frances Lake area, southeastern Yukon. In Yukon Exploration and Geology 2000. Exploration and Geological Services Divis ion, Indian and Northern Affairs Canada, p.217-233. Murphy, D . C , 1998. Stratigraphic framework for syngenetic mineral occurrences, Yukon-Tanana Terrane south of Finlayson Lake: A progress report. In Y u k o n Exploration and Geology 1997. Exploration and Geological Services Divis ion, Yukon , Indian and Northern Affairs Canada, p. 51-58. Murphy, D . C , and Timmerman, J.T., 1997. Preliminary geology of the northeast third of the Grass Lakes area, Pelly Mountains, southeastern Yukon . In Yukon Exploration and Geology 1996. Exploration and Geological Services Divis ion, Yukon , Indian and Northern Affairs, Canada, p.29-32. Murphy, D . C , and Piercey, S.J., 1999a. Finlayson project: Geological evolution of Yukon-Tanana Terrane and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon . In Yukon Exploration and Geology. Exploration and Geological Services Divis ion , Department o f Indian and Northern Affairs, p.47-62. 131 Murphy, D.C, and Piercey, S.J, 1999b. Geological map of Wolverine Lake area (105G/8), Pelly Mountains, southeastern Yukon: Exploration and Geological Sciences Division, Department of Indian and Northern Affairs Canada, Open File 19993 (1:50,000 scale). Murphy, D.C, and Piercey, S.J, 1999c. Geological map of Finlayson Lake area, southeast quarter (105G/7, 8 and parts of 1,2 and 9), southeastern Yukon: Exploration and Geological Sciences Division, Department of Indian and Northern Affairs Canada, Open File 1999-4 (1:100000 scale). Murphy, D.C, and Piercey, S.J, 2000. Syn-mineralization faults and their re-activation, Finlayson Lake massive sulfide belt, Yukon-Tanana Terrane, southeastern Yukon. In Yukon Exploration and Geology 1999. Exploration and Geological Services Division, Department of Indian and Northern Affairs, p.55-66. Peter, J.M, Mihalynuk, M.G, Colpron, M , and Tucker, T.L, in preparation. Hydrothermal exhalative sedimentary rocks of the Finlayson Lake area, Little Salmon area, and Big Salmon Complex, Yukon Tanana Terrane. Pigage, L.C, 1997. Mapping and stratigraphy at Ice. Expatriate Resources Ltd, Internal Company Report, 5p. Piercey, S.J, and Murphy, D.C, 2000. Stratigraphy and regional implications of unstrained Devono-Mississippian volcanic rocks in the Money Creek thrust sheet, Yukon-Tanana Terrane, Southeastern Yukon. In Yukon Exploration and Geology 1999. Exploration and Geological Services Division, Department of Indian and Northern Affairs, p.67-78. Piercey, S.J, Hunt, J.A, and Murphy, D.C, 1999. Lithogeochemistry of meta-volcanic rocks from Yukon-Tanana Terrane, Finlayson Lake region, Yukon: Preliminary results. In Yukon Exploration and Geology 1998, Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 125-138. Piercey, S.J, Murphy, D.C, Mortensen, J.K, and Paradis, S, 2000a. Arc-rifting and ensialic back-arc basin magmatism in the northern Canadian Cordillera: Evidence from the Yukon-Tanana Terrane, Finlayson Lake region, Yukon. Slave-Northern Cordilleran Lithospheric Experiment (SNORCLE) - Lithoprobe Report 72, p. 129-138. Piercey, S.J, Murphy, D.C, and Mortensen, J.K, 2000b. Magmatic diversity in a pericratonic realm: tales from the Yukon-Tanana Terrane in the Finlayson Lake region, southeastern Yukon, Canada: Geological Society of America, Program with Abstracts, v.32, #6, p.A-62. Plint, H.E, and Gordon, T.M, 1997. The Slide Mountain Terrane and the structural evolution ofthe Finlayson Lake Fault Zone, southeastern Yukon. Canadian Journal of Earth Sciences, v.34, p. 105-126. Sebert, C. and Hunt, J.A, 1999. A note on preliminary lithogeochemistry ofthe Fire Lake area. In Yukon Exploration and Geology 1998, Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 139-142. Tempelman-Kluit, D.J, 1979. Transported cataclasite, ophiolite and granodiorite in Yukon: evidence for arc-continent collision. Geological Survey of Canada Paper 79-14, 27 p. Thompson, R.I, Nelson, J.L, Paradis, S, Roots, CF, Murphy, D.C, Gordey, S.P, and Jackson, L.E, 2000. Ancient Pacific Margin NATMAP project, year one. In Current Research 2000-A1, Geological Survey of Canada, p. 1-8. 132 Tucker, T , Turner, A J , Terry, D A , and Bradshaw, G A , 1997. Wolverine massive sulfide project, Yukon. In Yukon Exploration and Geology 1996. Exploration and Geological Services Division, Department of Indian and Northern Affairs, p.53-55. 133 Figure 5.1. Geological setting and distribution ofthe Yukon-Tanana Terrane in the Yukon and the location of the study area with respect to the Finlayson Lake region. Map modified from Hunt (1998). 134 13T45 ' 131°15' 13 TOO' 61°30' Kilometres 61°15' 61 "00' Quaternary sediments Pennsylvanian-Permian Campbell Range Succession Chert, chert-pebble conglomerate, sandstone p & f e ! Mafic volcanic, volcaniclastic and intrusive rocks ; ; ; ; ; ] Dtamictite, mafic tuft, ollsostromal 1* * « f I carbonate, chert, sandstone Mississippian Wolverine Succession I- 1 Unit 6: Felsic voteanlc rocks, L__JJ Fe-formation, mixed tuffs Unit 5cp: Carbonaceous argillite I ! and phyllite Unit 5f/qfp: Quartz-bearing felsic volcanics and high-level intrusive rocks Mississippian Unit 51; Quartz-feldspar + shale V. 'A chip conglomerate - - Unconformity Devonian (mostly) Grass Lakes Succession Unit 4: Carbonaceous phyllite, rift-related mafk: rocks, quartzite Unit 3: Felsic volcanic and shallow level Intrusive rocks, carbonaceous phyllite, turbiditic sedimentary rocks Unit 2: Mafic volcanic and Intrusive rocks, carbonaceous phyllite, lesser felsic volcanic rocks Unit 1: Quartz-( + blotlte)-rich metaclastlc rocks calc-silicates, rare felsic horizons Intrusive Rocks Cretaceous 1~A~A] Peraluminous granitoids I A A I Mississippian j" ^ ~hj Grass totes Suite: Peraluminous granitoHs l—-—"l Simpson Range Plutonic Suite: Metalumlnous I " » " xl granitoids Simpson Range Plutonic Suite: Sheared <• C ^ - l metalumlnous granitoids Mississippian? serpentinized harzburgrtes and ultramaflc rocks (intrusions?) Other 1 f — | VHMS Deposit Faults, disptacement uncertain Money Creek Thrust Fault Figure 5.2. Geological map of the Finlayson Lake region modified after Murphy and Piercey (1999c) with locations of different zones of the Wolverine VHMS deposit and other VHMS deposits of the Finlayson Lake district. 135 136 137 0 1 2 ) 4 5 6 Figure 5.5. (a) Feldspar porphyritic intrusions from the Wolverine/Lynx zone with subhedral K-feldspar grains set in a siliceous matrix. Note the dark grey to black crosscutting quartz-sulphide-sericite veinlets; (b) partial to fully replaced K-feldspar grains with secondary black K-feldspar. 138 Figure 5.6. Unaltered to weakly altered feldspar porphyritic intrusion from the Fisher zone with black to grey quartz-sericite-chlorite-sulphide veinlets cutting across it. Figure 5.7. Sable zone quartz-feldspar porphyritic intrusion with darker grey quartz eyes and lighter grey K-feldspar. Note the smaller size of feldspar grains as compared to the Wolverine/Lynx and Fisher zones. 139 Figure 5.8. Puck zone feldspar porphyritic intrusion with variably broken K-feldspar grains with partial replacement by secondary K-feldspar. 140 Table 5.1. Outline of the salient geological features of porphyritic intrusions from different zones within the Wolverine V H M S system. Descriptions are outlined in zones from northwest to southeast. Zone Fisher Wolverine/Lynx Sable Puck Intrusion Type' FPI FPI QFP FPI+QFP Massive Sulphides? minor abundant minor minor Exhalites yes yes yes yes Upper Contacts argillite and silicified argillite; partly sheared argillite and silicified argillite; partly sheared fine-grained felsic volcaniclastic rocks FPI: siliceous to weakly siliceous argillite; QFP: argillite Lower Contacts medium- to fine-grained felsic volcaniclastic rocks fine-grained felsic volcaniclastic ± argillite fine-grained felsic volcaniclastic rocks fine-grained felsic volcaniclastic rocks Alteration2 veinlets of py-ser-qtz ± chl; secondary black K-feldspar veinlets of py-ser-qtz ± chl; secondary black K-feldspar veinlets of py-ser-qtz-sph veinlets of py-ser; py replacement in K-fsp; secondary black K-feldspar Notes: 1- FPI - feldspar porphyritic intrusion; QFP - quartz-feldspar porphyritic intrusion; 2 - py - pyrite; ser - sericite; qtz - quartz; chl - chlorite; sph - sphalerite; K-fsp - K-feldspar 141 Chapter 6 Geochemistry and Tectonic Significance of Weakly Alkalic Mafic Magmatism in the Yukon-Tanana Terrane, Finlayson Lake Region, Yukon Abstract Devonian-Mississippian, weakly alkalic mafic rocks from the Yukon-Tanana Terrane (YTT) in the Finlayson Lake region, southeastern Yukon, occur as sills, dykes and mafic flows that crosscut felsic volcanic rocks and are interlayered with variably carbonaceous sedimentary rocks.. The mafic rocks have non-arc, alkalic, ocean island basalt (OIB)-like geochemical signatures with moderate Ti0 2 and P 2 0 5 contents, and elevated high field strength element (HFSE) and light rare earth element (LREE) contents. A subset of the dykes (group 4b) has similar geochemical characteristics but with higher Th/Nb, Nb/Nb*, lower Nb/U and higher Zr and L R E E contents. A group 4b sample yielded an 8 N d 3 5 0 value of -2.80 compared to a value of+1.02 in a group 4a sample. The geochemical and isotopic features of the alkalic rocks are consistent with formation from either lithospheric or asthenospheric sources during decompression melting of the mantle. Group 4b suite are equivalent to the group 4a suite but are interpreted to have been crustally contaminated. The alkalic basalts of this study represent magmatism associated with -360 Ma ensialic back-arc rifting and basin generation within the Yukon-Tanana Terrane. It is envisioned that Devonian-Mississippian east-dipping subduction within the Y T T was disrupted in the Finlayson Lake region by subduction hinge roll-back and the onset of back-arc extension. Decompression melting of the mantle associated with back-arc generation resulted in mantle melting and the formation of the alkalic basalts. The spatial association of this mafic magmatism with crustally-derived felsic volcanic rocks and contained volcanic-hosted massive sulphide (VHMS) mineralization suggests that the associated deposits (Kudz Ze Kayah, GP4F) formed within an ensialic back-arc rift/basin environment. 142 Introduction The application of trace element geochemistry to mafic rocks in ancient and modern volcanic arc and back-arc environments has provided significant insight into the petrogenesis of these rocks and their relationships to the tectonic evolution of arc and back-arc regions (e.g., Pearce and Peate, 1995; Stern et al., 1995; Shinjo et al., 1999). In the northern Cordillera the origins of mafic magmatism in the pericratonic Yukon-Tanana Terrane (YTT) has been problematic. Central to the problems has been the stratigraphic grouping of mafic rocks without regard to stratigraphic context, geochemistry or tectonic setting (e.g., Anvil Allocthon; Templeman-Kluit, 1979). There has also been contradictory nomenclature of YTT-associated mafic rocks. For example, some workers have correlated mafic rocks of the Y T T with the Anvil Allochthon or Anvil assemblage considering them allochthonous with respect to Y T T (Tempelman-Kluit, 1979). Other workers have considered some of the mafic rocks stratigraphically part of Y T T , whereas other mafic rocks were considered allocthonous and correlated with the Slide Mountain Terrane (e.g., SMT; Mortensen and Jilson, 1985; Mortensen, 1992). Recent stratigraphic mapping in the Y T T have built on the latter work and have confirmed that mafic rocks are essential constituents of Y T T , are not necessarily equivalent to the SMT, and within the Y T T mafic units are not necessarily equivalent to one other (Stevens et al., 1995; Murphy, 1998; Murphy and Piercey, 1999, 2000; Piercey and Murphy, 2000). Similarly, geochemical and isotopic data have illustrated that there is considerable complexity in the mafic units. For example, using trace element geochemistry and Nd isotope geochemistry Creaser et al. (1997) and Grant (1997) questioned the correlation of Anvil Assemblage greenstones to the SMT showing that the Anvil Assemblage greenstones had arc affinities, whereas the SMT typically have mid-ocean ridge basalt (MORB) chemistry (Nelson and Bradford, 1993; Nelson, 1993; Plint and Gordon, 1997). Similarly, Murphy (1998), Murphy and Piercey (1999, 2000), and Piercey et al. (1999) illustrated that within the Finlayson Lake region there are three mafic units, each with distinct chemical attributes, but internally each exhibits considerable geochemical diversity. In this paper we present a regional geological and geochemical dataset for Devonian-Early Mississippian within-plate mafic magmatic rocks from the Y T T in the Finlayson Lake region in an 143 attempt to documented stratigraphically constrained petro-tectonic relationship for weakly alkalic Y T T -associated mafic rocks. Accordingly, the main objectives of this study are thus: 1) to document the geological and geochemical characteristics of these within-plate magmatic rocks; 2) to provide insights into the origins and petrogenesis of the YTT-associated Mid-Paleozoic within-plate mafic magmatic rocks; and 3) to ascertain the significance of this style of mafic magmatism to the tectonic evolution ofthe Y T T in northern Cordillera. Geologic and Stratigraphic Setting The Y T T in the F L D (Fig. 6.1) is composed of foliated and lineated greenschist to lower amphibolite grade metasedimentary, metavolcanic and metaplutonic rocks (e.g., Tempelman-Kluit, 1979; Mortensen and Jilson, 1985; Fig. 6.2). Although the region has been strongly deformed and metamorphosed regional mapping has identified a stratigraphically intact sequence consisting of three mid- to late-Paleozoic unconformity-bound successions: the Grass Lakes, Wolverine and Campbell Range successions (Fig. 6.2; Murphy, 1998; Murphy and Piercey, 1999, 2000; Murphy, 2001). The alkalic rocks of this study form part ofthe Grass Lakes succession. The Grass Lakes succession consists of unit 1, the Fire Lake unit (FLU), the Kudz Ze Kayah (KZK) unit and unit 4 (Figs. 6.2-6.3). The lowermost part of the Grass Lakes succession consists of pre-Devonian (pre-365 Ma) quartz-rich, non-carbonaceous metaclastic rocks of unit 1, which are overlain by the -365-360 Ma (Mortensen, 1992 and unpublished data) mafic dominated arc- and back-arc related F L U (Mortensen, 1992; Grant, 1997; Murphy and Piercey, 1999, 2000; Piercey et a l , 1999). Stratigraphically overlying the F L U is the felsic volcanic- and sedimentary rock-dominated K Z K unit (Fig. 6.2; Murphy, 1998). This unit consists predominantly of Devonian-Mississippian (-360-356 Ma; Mortensen, 1992) felsic volcanic and variably carbonaceous sedimentary rocks in the lower parts of the unit (unit 3 of Murphy, 1998; Figs. 6.2-6.3). Near the top of the succession the unit passes gradationally into interlayered alkalic basalts and carbonaceous sedimentary rocks of unit 4 that are the subject of this study (Murphy, 1998; Figs. 6.2-6.3). The most common occurrence of the mafic rocks is as meter-scale layers of chloritic schist interlayered with carbonaceous phyllitic rocks, indicating a volcanic (flow?) origin. In some regions volcanic flow textures (massive flows and hyaloclastite) are 144 preserved and the volcanic rocks are associated with carbonaceous quartzite and phyllite. Intrusive forms of the mafic rocks are commonly found proximal to the Kudz Ze Kayah (KZK) volcanic-hosted massive sulphide (VHMS) deposit (Fig. 6.2). In this region, diabase sills are interlayered with carbonaceous phyllite and locally preserve both salt and pepper and cumulate textures. Hydrothermally unaltered mafic dykes near KZK are commonly biotite-rich and cut the footwall rhyolitic rocks to the deposit. The dykes are interpreted to post-date the hydrothermal system that formed the KZK deposit. Similarly, in the GP4F VHMS deposit (Fig. 6.2) numerous mafic dykes cut across the felsic volcaniclastic stratigraphy; some of the mafic rocks may be interlayered flows or volcaniclastic sedimentary material. Biotite-rich (biotitite) dykes are also sporadically present within unit 3 regionally; however, the greatest concentration of mafic magmatism appear to be concentrated in the northern Finlayson Lake region, near the KZK and GP4F deposits (Fig. 6.2). Coeval with and crosscutting the KZK unit are the Devonian-Mississippian (360+1 Ma; Mortensen, 1992) K-feldspar porphyritic to megacrystic granitoids of the Grass Lakes suite (GLS) of intrusions (Figs. 6.2-6.3). These intrusions are interpreted to be the subvolcanic intrusive complex to the KZK unit VHMS mineralization (Chapter 8). The crosscutting relationship ofthe GLS granitoids (Figs. 6.2-6.3) suggest that the alkalic rocks of the study are pre- to syn-360 Ma. Unconformably overlying the Grass Lakes succession is the Early Mississippian Wolverine succession (-356-345 Ma; Mortensen, 1992; Appendix 2; Figs. 6.2-6.3). The Wolverine succession consists predominantly of felsic volcanic and carbonaceous sedimentary rocks, with minor mid-ocean ridge basalt (MORB) mafic flows (Plint and Gordon, 1997; Chapter 8) near the top of the succession (Figs. 6.2-6.3). The Wolverine succession is unconformably overlain by the upper Paleozoic (Pennsylvanian-Permian; Harms in Plint and Gordon, 1997), mafic volcanic and clastic sedimentary rock dominated Campbell Range succession (Murphy and Piercey, 1999; Murphy, 2001; Figs. 6.2-6.3). Lithogeochemistry and Neodymium Isotope Geochemistry Sampling and Analytical Methods Samples of mafic volcanic and subvolcanic rocks in this study unit were collected during regional mapping and were analyzed at the laboratories of the Geological Survey of Canada, in Ottawa, Canada (Table 6.1). Samples were analyzed using fused bead X-ray fluorescence (XRF) for most of the major 145 elements. Water ( H 2 O t ) and C 0 2 T were analyzed by infrared spectroscopy, and FeO was analyzed by modified Wilson titration. Trace elements were analyzed by combined inductively coupled plasma emission spectrometry (ICP-ES) and mass spectrometry (ICP-MS). Analytical precision calculated from repeat analyses of internal basaltic reference materials (Appendix 1) is given as percent relative standard deviation (%RSD = 100*standard deviation/mean), and yielded values of: 0.43-6.52% for the major elements, 0.72-8.80% for the transition elements (V, Ni, Cr, Co), 2.21-5.92% for the HFSE (Nb, Zr, Hf, Y, Sc, Ga), 2.35-6.96% for the low field strength elements (LFSE) Cs, Rb, Th and U but slightly higher for Ba and Sr (1.49-15.75%), 2.15-6.47% for the REE (La-Lu), and 1.12-98.12% for Cu, Pb, and Zn. However, concentrations of these were close to detection limits in the internal reference materials leading to decreased precision and high %RSD values (Appendix 1). Estimated 2a analytical uncertainties for given elements are presented in Appendix 1. Two samples were analyzed for their Nd isotopic compositions at the University of Alberta radiogenic isotope facility. Samples were analyzed by thermal ionization mass spectrometry (TI-MS) using the preparation and error treatment methodology of Creaser et al. (1997) (see also Appendix 1). Although the age of the mafic rocks is -360 Ma, the initial sNd values were calculated at 350 Ma to facilitate comparison to other Y T T Nd isotopic data (Grant, 1997; Creaser et al., 1997). Furthermore, the variations between eNd 3 5 0 and eNd 3 6 0 are insignificant within the timescale of the evolution of the Sm-Nd system and are less than 0.1 epsilon unit (e.g., Hamilton et al., 1983; Goldstein et al., 1984). Results for the Nd isotopic analyses are presented in Table 6.2. Alteration/Metamorphism and Element Mobility Field and petrographic data on sampled mafic rocks indicate that they have been affected by low-grade hydrothermal alteration (background seawater alteration) and/or greenschist facies regional metamorphism. Although primary volcanic/intrusive features are readily observable at both outcrop (e.g., cumulate textures) and thin section scale (e.g., relict plagioclase), the primary mineralogy of the rocks has largely been replaced. In particular, petrographic examination indicates that the matrices of the rocks have been replaced by an assemblage of chlorite, tremolite-actinolite, quartz, muscovite, epidote and carbonate. Precursor plagioclase and mafic phenocrysts have been replaced by carbonate, muscovite and epidote, and chlorite and tremolite-actinolite, respectively. The greenschist facies metamorphic and 146 seafloor conditions that these samples have experienced suggest that most major elements (e.g., Si02, Na 2 0, K 2 0 , CaO) and LFSE (Cs, Rb, Ba, Sr, U) are likely to have been mobile (MacLean, 1990). The potential for element mobility is supported by the metamorphic mineral assemblages present and the high volatile contents of many samples (H 2 0+C0 2 = 2.4-14.6%; Table 6.1). In contrast, the major elements Al 203, T i 0 2 (and possibly P 20 5), the transition elements, HFSE, R E E and Th were likely immobile under the above conditions (e.g., Pearce and Cann, 1973; Wood, 1980; Meschede, 1986; MacLean, 1990; Rollinson, 1993; Jenner, 1996). Furthermore, the major element ratios FeO*/MgO and Mg# (MgO/(FeO*+MgO)) should not have been appreciably changed except at high water to rock ratios (Alt and Emmerman, 1985; Humphries and Thompson, 1978), which appears not to be the case for the samples of this study due to their low grade assemblages. Given the likelihood of element mobility discussions in this paper concentrate primarily on immobile element systematics; however, we will also discuss the major and mobile element systematics as in many cases they mirror the immobile element systematics. Results The unit 4 mafic volcanics are characterized by highly variable Si0 2 contents and total alkali (Na 20+K 20) contents (Table 6.1), likely reflecting alkali mobility. Nevertheless, the samples straddle the alkaline-subalkaline boundary on the total alkalis versus silica plot (Fig. 6.4a). The samples have broadly basaltic to andesitic compositions on LeMaitre et al.'s (1989) plot (Fig. 6.4b). The immobile element-based Zr/Ti0 2 -Nb/Y plot mirrors the mobile element alkali indexes as the rocks have basaltic affinities (Fig. 6.4c) with Zr/Ti0 2 =50.34-92.17, and weakly to moderately alkalic Nb/Y ratios (Nb/Y=0.36-2.96; Table 6.1). The weak to moderate alkalinity of the rocks is also supported by their elevated T i 0 2 (1.53-4.82%) and P 2 0 5 contents (0.16-0.80%)(Table 6.1) which show a positive trend with increasing FeO*/MgO (0.91-3.90, average = 2.10) (Fig. 6.5a,b). In contrast, FeO*/MgO exhibits a negative trend with the compatible elements Ni and Cr (Fig.5c,d). The A l 2 0 3 / T i 0 2 ratios of the mafic rocks (4.20-9.26) (Table 6.1) are lower than primitive mantle (-21) and normal mid-ocean ridge basalts (N-MORB - 11), but range between those of ocean island basalts (OIB - 5) and enriched M O R B (E-MORB - 9.5; Sun and McDonough, 1989) (Table 6.1). 147 The immobile trace element features of the unit 4 mafic rocks are plotted on a series of discrimination diagrams and primitive mantle- and chondrite-normalized plots in Figures 6.6 through 6.8. The mafic rocks can be subdivided on primitive mantle-normalized plots: group 4a with a positive Nb anomaly (Nb/Nb*>l) and group 4b with a weakly negative Nb anomaly (Nb/Nb*<l)(Fig. 6.6). Both groups of samples are characterized by smooth downward sloping profiles that have minor depletions in Al , Sc, and to a lesser extent V (Fig. 6.6a). The group 4a samples are also remarkably similar to basalts with OIB trace element signatures (Fig. 6.6c). The downward sloping profiles in Figure 6.6 are echoed by the R E E profiles (Fig. 6.7) with both groups exhibiting LREE-enrichment (Ce/Ybn = 2.11-16.77) and fractionation of the M R E E from the HREE (Gd/Yb n = 1.40-3.86). On various discrimination diagrams most of the samples have within-plate affinities. The elevated T i 0 2 contents of the unit 4 mafic rocks is reflected by the samples lying on the OIB-MORB boundary on Ti-V plot (Fig. 6.8a). The high Zr contents and Zr/Y ratios of the unit 4 rocks (Table 6.1) result in the samples plotting in or near the field for within-plate basalts, with the exception of two samples which plot in the MORB field (Fig. 6.8b). In Zr-Nb-Y space most group 4a samples have within-plate signatures, whereas group 4b samples lie predominantly in the enriched-MORB (E-MORB) field (Fig. 6.8c). A similar distinction is observed in Zr-Ti-Y space (Fig. 6.8d; Table 6.1). On the Th-Nb-Zr plot the separations of group 4a and 4b is quite systematic with the group 4a samples lying largely in the field for OIB with their positive Nb anomalies (Nb/Nb* = 1.11-1.81) and low Th/Nb (0.07-0.10), whereas the group 4b samples are displaced towards the Th apex, reflecting their negative Nb anomalies (Nb/Nb* =0.67-0.90) and higher Th/Nb ratios (0.14-0.22)(Fig. 6.8e; Table 6.1). The Th/Nb ratios of the group 4b suite are also correlated with Si0 2 and Ce/Yb which is not the case for the group 4a samples (Fig. 6.9a,b). In the Th/Yb-Nb/Yb plot the group 4a and 4b samples lie on different trajectories, with the group 4a samples plotting within the mantle array and the within-plate enrichment vector, whereas the group 4b samples have higher Th/Yb at a given given Nb/Yb value (Fig. 6.9c). Although TJ can be mobile under oxidizing fluid conditions (Brenan et al , 1995), for the most part there is a strong correlation with immobile Nb suggests that U has been immobile and that the Nb-U systematics of the rocks reflect primary values. It is clear on this plot that there is a strong separation in Nb/U ratios 148 between the group 4a samples (Nb/U=34-68) and the group 4b samples (Nb/U= 17-35) (Fig. 6.9d). Both group 4a and 4b samples have similar and largely chondritic (~36) to slightly super-chondritic (>36) Zr/Hf ratios (e.g., Eggins et a l , 1997), ranging from 36.6-46.7 (average = 41.7) and 35.7-57.1 (average = 43.6) (Table 6.1). A sample from the group 4a mafic volcanics yielded sNd 3 5 0 = +1.05 and /sm/Nd = -0.41 and a group 4b sample yielded eNd 3 5 0 = -2.80 and / s m / N d = -0.29. These eNd 3 5 0 values are much lower than those for the depleted mantle (DM) reservoir at 350 Ma which has a value of eNd 3 5 0 = +9.5 and / s m / N d = +0.09 (Goldstein et al , 1984). Discussion Petrogenesis Deciphering the origin of mafic magmatism within this study requires distinguishing between mantle source features versus superimposed features due to fractional crystallization and/or other open system processes (i.e, crustal contamination). The major element index FeO*/MgO and compatible element behaviour provide insight into the role of fractional crystallization. What is notable with the rocks from this study is that they have relatively fractionated FeO*/MgO values and thus are relatively evolved, consistent with ferromagnesian mineral fractionation. For example, the inverse correlation between Ni and Cr and FeO*/MgO are consistent with crystallization of olivine and spinel. Similarly, depletions of A l , Sc and V in both the group 4a and 4b suites on primitive manle-normalized plots further suggest that these rocks have crystallized and fractionated ferromagnesian minerals (olivine, pyroxene, spinel) and feldspars. The latter compatible element geochemical features of these rocks, however, appear to be mainly superimposed post-magma generation effects and provide little information regarding the tectonic setting, mantle source region, or between-suite characteristics of the mafic rocks. In contrast, the immobile, incompatible-element characteristics are largely unaffected by fractional crystallization and thus can provide evidence bearing on the latter attributes of the volcanic rocks and are examined below. The immobile major and incompatible trace element systematics ofthe unit 4 alkalic mafic rocks are distinctive. The low A l 2 0 3 / T i 0 2 ratios coupled with high T i 0 2 and P 2 0 5 are major element features 149 suggesting that these rocks have been derived from an enriched mantle source region, similar to OIB. Similarly, the HFSE and REE characteristics and primitive mantle-normalized patterns of the group 4a rocks (Fig. 6.6) are indistinguishable from rocks from OIB-sources. Rocks with alkalic affinities and OIB signatures are found in a variety of settings continental margin environments including continental rifts (Goodfellow et al., 1995) and continental arc rift environments (van Staal et al., 1991; Shinjo et al., 1999). There is considerable debate as to whether these alkalic rocks in these environments come from asthenospheric or subcontinental lithospheric mantle sources. The data for the alkalic rocks in this study are compatible with either source. For example, numerous workers have shown that magmas derived from enriched asthenospheric mantle can yield rocks with enriched OIB-like trace element signatures (e.g., Sun and McDonough, 1989; van Staal et al., 1991; Shinjo et al., 1999). This is partly supported by the strong similarities between the group 4a rocks and rocks from OIB sources (Fig. 6.6c). Equally viable, however, is a lithospheric source. For example, numerous workers have shown that the subcontinental lithospheric mantle (SCLM) is strongly enriched in incompatible trace elements (Pearce, 1983; McDonough, 1990; Hawkesworth et al., 1990; Menzies, 1990). Furthermore, the low eNd35o value of the group 4a sample, as compared to the D M reservoir at 350 Ma (+9.5), is a feature present in some S C L M -derived magmas (Hawkesworth et al., 1990; Menzies, 1990). Some workers have also suggested that small volumes of alkaline magmatism represent melts of dominantly lithospheric origin (e.g., LaFleche et al., 1998), whereas larger volumes represent asthenospheric-derived melts (e.g., McKenzie and Bickle, 1988). The relatively low volume of magma associated with the unit 4 alkalic magmatism is in part supportive of this. However, the association of these melts with large volumes of felsic volcanic rocks interpreted to have formed from basaltic underplating (Piercey et al., 2000; Chapter 2), suggest that these magmas may have solidified in subvolcanic magma chambers and did not erupt on surface. From the data provided above neither lithosphere nor asthenosphere are unequivocal sources for the unit 4 magmatism; hence, these mafic rocks are compatible with derivation from either lithospheric or asthenospheric sources. The group 4b suite of mafic rocks has similar HFSE-REE characteristics to the group 4a suite and were likely derived from similar sources. However, they have additional geochemical features, including 150 the presence of a weak negative Nb anomaly, which point to a role for additional component in their genesis. Negative Nb anomalies are common features of rocks associated with arc lavas and incipient back-arc basin basalts due to the influx of LFSE into the mantle source regions from the subducted slab (Hawkins, 1995; Pearce and Peate, 1995; Shinjo et al , 1999). However, similar negative Nb anomalies are features common to the bulk and upper continental crust (e.g., Taylor and McLennan, 1985; Wedepohl, 1995), and rocks contaminated by continental crust can also exhibit negative Nb anomalies. These possibilities are examined below. Numerous lines of field and geochemical evidence argue against an arc origin for the negative Nb anomalies in the group 4b suite rocks. Their very high HFSE and R E E contents, and OIB-like signatures are dissimilar from most arc rocks (e.g., Pearce and Peate, 1995). Also, these alkalic rocks are associated with abundant HFSE-enriched (A-type) felsic rocks, and with interpreted extensional synvolcanic faults (see below and Murphy and Piercey, 2000), and VHMS mineralization, interpreted to have formed in an ensialic arc-rift or back-arc basin (Piercey et al , 2000). Given these constraints the negative Nb anomaly on these plots is unlikely to reflect formation of these rocks within an arc environment. Nevertheless, there may be a subducted slab component in the genesis of these rocks. For example, high LFSE/Nb values have been recognized in back-arc lavas in many modern back-arc environments where they have been attributed to fluids or melts from the subducted slab metasomatizing the back-arc mantle (Hawkins, 1995; Shinjo et a l , 1999). If the negative Nb anomalies in the group 4b suite are from a subducted slab component then this slab component must have contained evolved continental crustal material to account for the eNd35o = -2.80 and T D M = 1.64 Ga for the group 4b sample. The Nd isotopic character of the single group 4b sample analyzed, and by association the group 4b suite rocks, strongly indicates that these magmas were contaminated by continental crust. Numerous lines of geochemical evidence further support this. For example, in Th/Yb-Nb/Yb space (Fig. 6.9) the group 4a samples have a typical within-plate enrichment trend with variable amounts of enriched (OIB-like) component in their source. In contrast, group 4b samples lie on a distinct trajectory with higher Th/Yb at a given Nb/Yb as compared with group 4a samples (Fig. 6.9). In this diagram (Fig. 6.9), samples that have been influenced by Th-enrichment, either through subducted slab metasomatism or 151 crustal contamination, exhibit vertical increases with higher Th at a given Nb content than rocks that have not been influenced by Th-enrichment (e.g., Pearce, 1983). Crustal contamination is further supported by the Th/Nb-Si0 2 systematics of the rocks as the group 4b samples show a strong increase in Th/Nb with increasing Si0 2 typical of magmas that have undergone crustal contamination (Fig. 6.9). In contrast, the group 4a samples have a nearly constant, to slightly increasing Th/Nb with Si0 2 , typical of uncontaminated mafic magmas (Fig. 6.9). A similar distribution exists in Th/Nb-Ce/Yb space where group 4a samples show a flat distribution with constant Th/Nb with variable Ce/Yb contents, and the group 4b samples exhibiting a rapid increase in Th/Nb with increasing Ce/Yb (Fig. 6.9). Similarly, the Nb/U systematics of the rocks support crustal contamination as values of group 4a (Nb/Ua Vg = 51.39) samples lie primarily within the array for oceanic basalts (Nb/U = 47±10; Hoffman et a l , 1986), whereas the Nb/U values for the group 4b samples are much lower (Nb/Ua Vg = 21.17) similar to crustal values (Nb/U=12.09-Taylor and McLennan, 1985; Nb/UUppercmst=10.40 N b / U i o w e r c r u s t = 12.15-Wedepohl, 1995)(Fig. 6.9). Finally, group 4b samples contain greater average contents of crustally enriched immobile trace elements including Zr, Hf and the L R E E relative to the group 4a suite (Table 6.1). Taken together both suites of rocks from the K Z K unit are consistent with formation from lithospheric or asthenospheric sources; however, the group 4b suite of mafic rocks have experienced crustal contamination either en-route to the surface or within crustal-level subvolcanic magma chambers. Tectonic Setting The alkalic (OIB-like) signatures in the rocks from the Grass Lakes succession (unit 4) have enriched signatures similar in composition to rocks from continental arc-rift zones (e.g., van Staal et al , 1999; Shinjo et a l , 1999), continental rift zones (e.g., Goodfellow et al , 1995; Spath et al , 2001), oceanic islands (e.g., Floyd, 1989), and large igneous provinces (LIP) such as flood basalt provinces and mafic dyke swarms (e.g., Lassiter and DePaolo, 1997). The relatively small volume of mafic magmatism, association with V H M S deposits and volume of felsic volcanic rocks precludes a LIP or ocean island environment. Within the northern Cordillera alkalic magmatism has been documented in the pericratonic terranes (Dusel-Bacon and Cooper, 1999; Colpron, 2001; this study), and within rocks of the North 152 American Miogeocline in both the Cassiar Terrane (Mortensen, 1982; Mortensen and Godwin, 1983) and Selwyn Basin (Goodfellow et al., 1995). The alkaline rocks from the miogeocline have been interpreted to represent continental rift zones within the N A M , some of which are temporally equivalent to rocks of this study (Mortensen, 1982; Mortensen and Godwin, 1983; Gordey et al., 1987; Goodfellow et al., 1995). However, the alkalic rocks of the K Z K unit have numerous differences to alkalic rocks of the N A M and these argue against a continental rift setting. For example, mafic alkalic rocks from the Selwyn Basin are ultrapotassic with extreme L R E E and HFSE contents (Goodfellow et al., 1995), significantly higher than the alkalic rocks of this study. Secondly, felsic rocks associated with alkalic magmatism in the N A M are strongly peralkalic (Mortensen, 1982; Mortensen and Godwin, 1983) in contrast to the subalkaline to weakly alkaline felsic rocks that are spatially associated with the rocks of this study (Piercey et al., 1999, 2000). Thirdly, some of the alkalic mafic rocks of the Selwyn Basin are spatially associated with sedimentary exhalative (SEDEX) massive sulphide deposits (Goodfellow et al., 1995) whereas those of K Z K unit are associated with VHMS mineralization. Fourthly, alkalic rocks of this study are preceded and possibly concomitant with mafic and felsic rocks that represent arc magmatic activity (Piercey et al., 1999, 2000) a feature absent in all alkalic rocks of the N A M . Finally, rocks of the N A M and the Y T T in the Finlayson Lake region occur in fundamentally different stratigraphic settings and with the exception of alkalic rocks in the Cassiar Terrane, are younger than alkalic rocks in the N A M (Mortensen, 1982; Mortensen and Jilson, 1985; Mortensen, 1992). These features all argue against a continental rift setting for alkalic rocks of unit 4 and against possible correlations to the N A M . The geological and geochemical attributes of alkalic rocks from unit 4 support a continental arc rift to ensialic back-arc rift/basin setting (Piercey et al., 2000). Using various geological, geochemical and isotopic techniques numerous workers have shown that the Devono-Mississippian evolution of the Y T T is the product of continental arc magmatism, possibly built on the distal edge of the North American margin (e.g., Mortensen, 1992 and references therein; Creaser et al., 1997, 1999; Grant, 1997; Piercey et al., 1999a,b, 2000). In the Finlayson Lake region arc volcanic activity is recorded primarily by the rocks of Fire Lake unit and is temporally bracketed between 365-360 Ma (Mortensen, 1992 and unpublished data). At -360 Ma (oldest U-Pb age of A-type Grass Lakes granitoid; Mortensen, 1992; cf. Creaser et al., 153 1999) this arc was rifted and incipient ensialic back-arc basin development occurred (Piercey et al., 2000). Piercey et al. (2000) suggested that this rifting was due to westward migration of an east dipping subduction zone within the Y T T (cf. Mortensen, 1992). The alkalic basalts from this study are likely the manifestations of this arc-rifting event. The likely mechanism by which the alkaline basalts formed involved decompression (e.g., McKenzie and Bickle, 1988) of either enriched lithospheric or asthensopheric mantle associated with rifting resulting in partial melting and melt generation. The spatial association of alkaline basalts with HFSE-enriched felsic rocks suggests the possibility that the alkalic magmas formed during arc rifting may have provided the heat engine to drive crustal partial melting to form associated felsic rocks (e.g., Huppert and Sparks, 1988). The data presented herein suggest that the alkalic rocks from this study represent a Devonian-Mississippian arc-rifting episode within the Y T T . The spatial association of these alkalic rocks to extensional synvolcanic structures and VHMS deposits (GP4F and K Z K ; Murphy and Piercey, 2000) suggests that arc rifting and subsequent extension were important large-scale controls on hydrothermal system generation within the Finlayson Lake district. The association of alkalic mafic rocks with HFSE-enriched felsic rocks (Piercey et al., 2000; Chapter 2) may be a regional scale tectonomagmatic relationship that identifies potential VHMS-bearing stratigraphy within the Y T T and similar continent-margin bimodal-volcanic silicaclastic VHMS environments. Conclusions The conclusions of this study are: 1) Devonian-Mississippian mafic rocks from the Yukon-Tanana Terrane in the Finlayson Lake region are spatially associated with VHMS mineralization and HFSE-enriched felsic volcanic rocks. These alkalic rocks occur as sills and dykes that crosscut felsic volcanic rocks, and mafic flows that are interlayered with variably carbonaceous sedimentary rocks. 2) The mafic rocks have non-arc, alkalic, ocean island basalt (OIB)-like geochemical signatures with moderate T i 0 2 and P 2 0 5 contents, and elevated high field strength element (HFSE) and light rare earth element (LREE) contents. A subset of the dykes (group 4b) has similar geochemical characteristics but with higher Th/Nb, Nb/Nb*, lower Nb/U and higher Zr and L R E E contents; a group 4b sample 154 yielded an eNd35o value of-2.80 compared to a value of+1.02 in a group 4a sample. The geochemical and isotopic features of the alkalic rocks are consistent with formation from either lithospheric or asthenospheric sources during decompression melting of the mantle. The group 4b mafic rocks are interpreted to be equivalent to the group 4a suite but have been contaminated by continental crust. 3) The alkalic basalts of this study represent magmatism associated with -360 Ma ensialic back-arc rifting and basin generation within the Yukon-Tanana Terrane. It is envisioned that Devonian-Mississippian east-dipping subduction within the Y T T was disrupted in the Finlayson Lake region by subduction hinge roll-back and the onset of back-arc extension. Decompression melting of the mantle associated with back-arc generation resulted in melting and the formation of the alkalic basalts in this study. The spatial association of this mafic magmatism with volcanic-hosted massive sulphide (VHMS) mineralization suggests that the associated deposits (Kudz Ze Kayah, GP4F) formed within an ensialic back-arc rift/basin environment. References Alt, J.C., and Emmerman, R., 1985. Geochemistry of hydrothermally altered basalts. Deep Sea Drilling Project Hole 504B, Leg 83. Initial Reports of the Deep Sea Drilling Project, v.83, p. 249-262. Brenan, J.M., Shaw, H.F., Ryerson, F.J., and Phinney, D., 1995. Mineral-aqueous fluid partitioning of trace elements at 900°C at 2.0 Gpa: constraints on the trace element chemistry of mantle and deep crustal fluids. Geochimica et Cosmochimica Acta, v.59, p.3331-3350. Colpron, M . , 2001. Geochemical characterization of Carboniferous volcanic successions from Yukon-Tanana Terrane, Glenlyon map area (105L), central Yukon. In Yukon Exploration and Geology 2000. Exploration and Geological Services Division, Indian and Northern Affairs, Yukon, p. 111 -136. Creaser, R.A., Erdmer, P., Stevens, R.A., and Grant, S.L., 1997. Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin Tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence. Tectonics, v. 16, p. 107-121. Creaser, R.A., Goodwin-Bell, J-A.S, and Erdmer, P., 1999. Geochemical and Nd isotopic constraints for the origin of ecologite protoliths, northern Cordillera: implications for the Paleozoic tectonic evolution of the Yukon-Tanana terrane. Canadian Journal of Earth Sciences, v.36, p. 1697-1709. Dusel-Bacon, C.,'and Cooper, K . M . , 1999. 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In Continental Mantle. Edited by M.A. Menzies. Oxford Monographs on Geology and Geophysics, Oxford Press, Oxford, pp. 67-86. Meschede, M , 1986. A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram. Chemical Geology, v.56, p.207-218. Mortensen, J.K, 1982. Geological setting and tectonic significance of Mississippian felsic metavolcanic rocks in the Pelly Mountains, southeastern Yukon Territory. Canadian Journal of Earth Sciences, v.19, p.18-22. Mortensen, J.K, 1992. Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana Terrane, Yukon and Alaska. Tectonics, v.ll , p.836-853. Mortensen, J.K, and Godwin, C.I, 1982. Volcanogenic massive sulfide deposits associated with highly alkaline rift volcanics in the southeastern Yukon Territory. Economic Geology, v.77, p.1225-1230. Mortensen, J.K, and Jilson, G.A, 1985. Evolution of the Yukon-Tanana Terrane: Evidence from southeastern Yukon Territory. Geology, v. 13, p.806-810. Murphy, D.C, 1998. Stratigraphic framework for syngenetic mineral occurrences, Yukon-Tanana Terrane south of Finlayson Lake: A progress report. In Yukon Exploration and Geology 1997, Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 51-58. Murphy, D.C, and Piercey, S.J, 1999. Finlayson project: Geological evolution of Yukon-Tanana Terrane and its relationship to Campbell Range belt, northern Wolverine Lake map area, southeastern Yukon. In Yukon Exploration and Geology; Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 47-62. Murphy, D.C, and Piercey, S.J, 2000. Syn-mineralization faults and their re-activation, Finlayson Lake massive sulfide belt, Yukon-Tanana terrane, southeastern Yukon. In Yukon Exploration and Geology 1999: Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 55-66. Nelson, J.L, 1993. The Sylvester Allochthon: upper Paleozoic marginal-basin and island-arc terranes in northern British Columbia: Canadian Journal of Earth Sciences, v.30, p631-643. 157 Nelson, J . L , and Bradford, J . A , 1993. Geology ofthe Midway-Cassiar area, northern British Columbia. British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, Bulletin 83, 94 p. Pearce, J . A , 1983. Role of sub-continental lithosphere in magma genesis at active continental Margins. In Continental Basalts and Mantle Xenoliths. Edited by C.J. Hawkesworth and M.J. Norry. Shivan, Nantwich, U.K., p.230-249. Pearce, J . A , and Cann, J.R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters, v. 19, p.290-300. Pearce, J . A , and Norry, M . J , 1979. Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks. Contributions to Mineralogy and Petrology, v.69, p.33-47. Pearce, J . A , and Peate, D . W , 1995. Tectonic implications of the composition of volcanic arc magmas. Annual Reviews in Earth and Planetary Science, v.23, p.251-285. Piercey, S.J, Hunt, J . A , and Murphy, D . C , 1999. Lithogeochemistry of meta-volcanic rocks from Yukon-Tanana Terrane, Finlayson Lake region, Yukon: Preliminary results. In Yukon Exploration and Geology 1998, Exploration and Geological Services Division, Department of Indian and Northern Affairs, p. 125-138. Piercey, S.J, Murphy, D . C , Mortensen, J . K , and Paradis, S, 2000. Arc-rifting and ensialic back-arc basin magmatism in the northern Canadian Cordillera: Evidence from the Yukon-Tanana terrane, Finlayson Lake region, Yukon. In Slave-Northern Cordilleran Lithospheric Experiment (SNORCLE) - Lithoprobe Report 72, pp. 129-138. Plint, H . E , and Gordon, T . M , 1997. The Slide Mountain Terrane and the structural evolution of the Finlayson Lake Fault Zone, southeastern Yukon. Canadian Journal of Earth Sciences, v. 34, p.105-126. Rollinson, H . R , 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman Group, U K Ltd , 352 pages. Shervais, J .W, 1982. Ti-V plots and the petrogenesis of modern and ophiolitic lavas: Earth and Planetary Science Letters, v.59, p. 101-118. Shinjo, R , Chung, S-L, Kato, Y , and Kimura, M , 1999. Geochemical and Sr-Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin: Journal of Geophysical Research, v. 104, #B5,p.l0591-10608. Spath, A , LeRoex, A . P , and Opiyo-Akech, N , 2001. Plume-lithosphere interaction and the origin of continental rift-related alkaline volcanism - the Chyulu Hills volcanic province, southern Kenya. Journal of Petrology, v.42, p.765-787. Stern, R . A , Syme, E . C , Bailes, A . H , and Lucas, S.B, Paleoproterozoic (1.90-1.86 Ga) arc volcanism in the Flin Flon Belt, Trans-Hudson Orogen, Canada: Contributions to Mineralogy and Petrology, v. 119, p. 117-141. 158 Stevens, R.A., Erdmer, P., Creaser, R.A., and Grant, S.L., 1996. Mississippian assembly of the Nisutlin assemblage: evidence from primary contact relationships and Mississippian magmatism in the Teslin tectonic zone, part of the Yukon-Tanana Terrane of south-central Yukon. Canadian Journal of Earth Sciences, v.33, p. 1342-1363. Sun, S.-s., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins. Edited by A.D. Saunders and M.J. Norry. Geological Society Special Publication 42, p.313-345. Taylor, S.R., and McLennan, S.M., 1985. The continental crust: its composition and evolution. Blackwell Scientific Publications, Oxford, 312 p. Templeman-Kluit, D.J., 1979. Transported cataclasite, ophiolite and granodiorite in Yukon: evidence for arc-continent collision. Geological Survey of Canada Paper 79-14, 27 p. van Staal, C.R., Winchester, J.A., and Bedard, J.H., 1991. Geochemical variations in Middle Ordovician volcanic rocks of the northern Miramichi Highlands and their tectonic significance. Canadian Journal of Earth Sciences, v.28, p. 1031-1049. Wedepohl, K.H. , 1995. The composition of the continental crust. Geochemica et Cosmochimica Acta Cosmochimica Acta, v.59, p. 1217-1232. Wheeler, J.O., and Mcfeely, P., 1991. Tectonic Assemblage Map of the Canadian Cordillera and Adjacent Parts of the United States of America: Geological Survey of Canada, Map 1712A, Scale 1: 2000 000. Winchester, J.A., and Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology, v.20, p.325-343. Wood, D.A., 1980. The application of the Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province. Earth and Planetary Science Letters, v.50, p. 11-30. 159 Figure 6.1. Location of the F L D and VHMS deposits with respect to the Yukon-Tanana Terrane, Yukon, Canada (modified from Wheeler and McFeely, 1991 and Hunt, 1998). 160 131°45' 13H5' 13 TOO' 61°30' Kilometres 61 "15' 61°00' Q Quaternary sediments Pennsylvanian-Pertnian Campbell Range Succession Chert, c h e r t - p e b b l e c o n g l o m e r a t e , Ma f i c vo lcan ic , vo lcan ic last ic s S S S a n d intrusive rocks ; ; ; ; j Dlamictite, m a f i c tuff, ollsostromal 11 -111 c a r b o n a t e , chert, sandstone Mississippian Wolvenne Succession IT-V."."] Unit 6: Felsic v o l c a n i c rocks, [-----I Fe-formation, m ixed tufts • Unit 5cp : C a r b o n a c e o u s argillite a n d phyllite Unit 5f/qfp: Quartz -bear ing felsic vo lcan ics a n d high- level intrusive rocks Mississippian 1 ~| Unit 51: Quartz - feldspar+shale I .........J c h i p c o n g l o m e r a t e - Unconformity Devonian (mostly) Grass Lakes Succession ppp*Sj Unit 4: C a r b o n a c e o u s phyllite, rift-ESffiS&l related m a f i c rocks, quartzite Unit 3: Felsic v o l c a n i c a n d h shallow level intrusive rocks, ' — ^ c a r b o n a c e o u s phyllite, turbiditic sedimentary rocks _ _ _ _ U n i t 2 : Maf ic v o l c a n i c a n d intrusive B<>oo rocks, c a r b o n a c e o u s phyllite, lesser felsic v o l c a n i c rocks I' • ' . . *| Unit 1: Quartz-(+biotlte)-rich metac las t ic I':' • ••! rocks ca lc -s i l icates , tare felsic horizons Intrusive Rocks Cretaceous Peraluminous granitoids Mississippian ^ ' _J Grass LakBS Suite: Peraluminous granitoids i — ; — x ] Simpson Range Plutonic Suite: Metalumlnous I * " I granitoids v^g Simpson Range Plutonic Suite: Sheared TJ'H m e t a l u m h o u s granitoids Mississippian? serpentinized harzburgltes a n d ultramafic rocks (intrusions?) Other f f~ | VHMS Deposit Faults, d i s p l a c e m e n t uncertain M o n e y Creek Thrust Fault Figure 6.2. Geological map of the Finlayson Lake region modified after Murphy and Piercey (1999c) with locations of different zones of the Wolverine VHMS deposit and other VHMS deposits of the Finlayson Lake district. 161 Figure 6.3. Schematic stratigraphy of the Finlayson Lake district with the stratigraphic location of VHMS deposits. Alkalic mafic rocks from this study occur near the top of the Wolverine succession unit and cross-cut the Kudz Ze Kayah and GP4F deposits. Modified after Murphy and Piercey (1999, 2000). Patterns for rock units as in Figure 6.2. 162 20 18 16 S? 14 i i 2 2* 10 n 8 ra (a) -i 1 1 1 ' 1 1 r Alkaline SubAlkaline O Group 4a ® Group 4a 35 40 45 50 55 60 65 70 75 80 85 SiO, (wt%) 15 5 10 O + ™" i - U1 / \ ® / S1LT\ U • © Pc •B • 01 Pc = picrobasalt B=basalt; 01 = basaltic andesite 02 = andesite, 03 = dacite R = rhyolite, S1 = trachybasalt 52 = basaltic trachyandesite 53 = trachyandesite T = trachyte 35 45 55 65 Si02 (wt%) 75 N .01 .001 (c) -1 1—i—i i i i 11 1 1—i—i i i 111 rn 1—i—i i i i i Phonolite § Andesite/Basalt SubAlkaline Basalt D oBa $ Alk-Bas Bsn/Nph _j i i t i i i .01 10 Nb/Y Figure 6.4. Major and trace element classification diagrams for the unit 4 mafic rocks including: (a) total alkalis versus silica (Irving and Baragar, 1971); (b) SiO^KjO+NajO) plot of LeMaitre (1989), and (c) Zr/Ti02-Nb/Y of Winchester and Floyd (1977). 163 900 800 700 f 600 Q. T500 o 400 300 200 100 0 2 3 FeO*/MgO _(c) • \ D -• \ -spinel K fractionation • \ -• ^ © © • © 2 3 FeO*/MgO 400 300 E a. a. '200 100 1 2 3 FeO*/MgO 2 3 FeO*/MgO Figure 6.5. FeO*/MgO plots against (a) Ti02 and (b) P205 illustrate the incompatible nature of these elements during magma evolution. The inverse relationships between FeO*/MgO and Cr and Ni are consistent with spinel and olivine crystallization. Symbols as in Figure 6.4. 164 •1 I I I I I I I I I I I I I I 1 I I I I I I I Th La Pr Sm Hf Ti Tb Y Yb Al Sc Nb Ce Nd Zr Eu Gd Dy Er Lu V 1000 £ 100 C ra 1 10 n—i—i—i—i—i—i—i—r E(b) Q G G '••is _J I I I I I I I L_ Th La Pr Sm Hf Ti Tb Y Yb Al Sc Nb Ce Nd Zr Eu Gd Dy Er Lu V 1000 £ 100 1 10 "u. Q . O or =(c) ~i—i—i—r • Group 4a-Avg Q Group 4r>Avg (> Global OIB «0> Global N-MORB_ O SCLM-Avg «§» SCLM-Med Th La Pr Sm Hf Ti Tb Y Yb Al Sc Nb Ce Nd Zr Eu Gd Dy Er Lu V Figure 6.6. Primitive mantle-normalized plots of (a) group 4a and (b) group 4b samples. In (c) the average values for the group 4a and 4b samples are compared to values for ocean-island basalts (OIB) and normal mid-ocean ridge basalts (N-MORB) (Sun and McDonough, 1989), and median (SCLM-Med) and average (SCLM-Avg) values for the subcontinental lithospheric mantle (McDonough, 1990). Primitive mantle values from Sun and McDonough (1989). Symbols as in Figure 6.4. 165 1000 o £ 10 ~i 1 1 1 1 1 1 1 1 1 1 1 1 1 r (a) _i i i i j i i i i_ La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu 1000 i 1 1 1 1 1 1 1 r i 1 1 r (b) La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 6.7. Chondrite-normalized plots of (a) group 4a and (b) group 4b samples. Chondrite values from Sun and McDonough (1989). Symbols as in Figure 6.4. 166 Figure 6.8. Discrimination diagrams for the unit 4 mafic rocks including: (a) Ti-V plot of Shervais (1982), B O N = boninite, LOTI = low-Ti tholeiite, IAT = island arc tholeiite, MORB = mid-ocean ridge basalt, B A B B = back-arc basin basalt, OIB = ocean island basalt; (b) Zr/Y-Zr plot of Pearce and Norry (1979); (c) Zr-Nb-Y plot of Meschede (1986), WPA = within-plate alkaline, WPT = within-plate tholeiitic, E-MORB = enriched-MORB, N-MORB = normal-MORB, VAB = volcanic-arc basalts; (d) Zr-Ti-Y plot of Pearce and Cann (1973), WPB = within-plate basalt, OFB = ocean floor basalt, L K T = low K tholeiite, C A B = calc-alkaline basalts; and (e) Th-Zr-Nb plot of Wood (1979). Symbols as in Figure 6.4. 167 40 45 50 Si02 (wt %) 55 60 j i_ _i i i_ 10 20 30 40 50 60 70 Ce/Yb :(c) ~ i 1 1—i—i—i i i I i i 1 i i—n-, - • ' 9 ; / " V Subduction Zone Enrichment Crustal Contamination? 4 Within Plate Enrichment _i i i i i i_ 10 Nb/Yb 100 80 70 60 £ 50 a a 40 30 20 10 (d) ' 6 y • / , =47+/-10 / D / O - qo -' // -1 2 U (ppm) Figure 6.9. Trace element plots that illustrate the differences between the group 4a and 4b samples. The sympathetic relationships of Th/Nb with Si0 2 (a) and Ce/Yb (b) in the group 4b are consistent with them having experienced crustal contamination. In Th/Yb-Nb/Yb space (c) (modified after Pearce (1983) and Stern et al. (1995)) the group 4b samples exhibit a distinct crustally influenced trajectory. The Nb/U ratios of the group 4a samples lie within the field for modern oceanic basalts (Hoffman et al., 1986), whereas the group 4b samples have a value that is lower and consistent with crustal influence (Nb/U ~12; Taylor and McLennan, 1985). Symbols as in Figure 6.4. 168 Table 6.1. Geochemical data for weakly alkaline mafic rocks from unit 4 of the Grass Lakes succession. All samples are in UTM zone 9V. Sample Name P98-8 P98-9b P98-28A P98-35 P98-36 P98-45 P98-48 Rock Type1 CS CS BCS MB MB MB CS Easting 414061 413795 419847 425862 425450 414457 411645 Northing 6815190 6815954 6803506 6817028 6817394 6822258 6811313 Group 4a 4a 4a 4a 4a 4a 4a Si0 2 (wt%)2 47.49 50.11 55.09 46.22 45.86 50.93 47.96 Ti0 2 3.16 3.82 2.20 1.53 1.69 3.36 1.97 A1203 15.57 18.95 16.07 14.19 13.38 15.29 11.10 Fe 20 3T 14.22 14.46 9.18 11.47 10.27 14.34 14.23 Fe 20 3 1.79 1.50 1.46 1.82 6.02 2.00 1.83 FeO 11.20 11.67 6.89 8.63 3.84 11.07 11.21 FeO* 12.81 13.02 8.20 10.27 9.25 12.88 12.86 MnO 0.24 0.14 0.15 0.10 0.11 0.24 0.24 MgO 6.33 5.44 4.10 7.14 7.64 3.34 14.12 CaO 10.30 2.82 6.19 16.16 20.35 7.48 9.57 Na20 2.24 2.89 3.55 2.38 0.62 4.11 0.11 K 2 0 1.24 1.94 3.82 1.64 0.16 1.54 1.62 P2O5 0.45 0.72 0.50 0.18 0.33 0.62 0.27 H 2 0 4.40 4.60 1.50 3.20 1.90 3.00 4.80 C 0 2 6.70 1.50 2.00 9.00 2.30 2.30 2.80 Cr (ppm) 115 28 98 736 690 <10 558 Ni 72 21 40 303 350 <10 345 Co 48 55 35 44 22 33 57 Sc 30 13 26 35 21 19 30 V 288 120 292 177 160 182 209 Cu 92 <10 12 <10 <10 14 <10 Pb 5 3 4 10 5 20 2 Zn 78 100 58 106 55 121 128 Rb 34 55 82 66 6.2 32 57 Cs 0.86 1.60 6.80 7.10 0.41 18.00 4.20 Ba 1036 2300 3038 1283 100 1225 1655 Sr 231 130 323 334 790 464 44 Ga 20 20 18 12 16 24 15 Ta 2.5 4.6 4.1 1.7 2.6 4.1 1.8 Nb 39 74 64 26 55 68 28 Hf 4.8 6.0 3.3 2.2 3.0 6.2 2.9 Zr 212 270 143 88 140 247 111 Y 46 25 24 16 20 50 21 Th 3.1 7.0 5.2 1.9 3.8 5.5 2.2 U 0.78 1.40 1.00 0.38 1.30 1.30 0.54 169 Table 6.1. (continued). Sample Name P98-8 P98-9b P98-28A P98-35 P98-36 P98-45 P98-48 La 31.00 52.00 35.00 14.00 28.00 46.00 20.00 Ce 63.00 110.00 66.00 25.00 60.00 93.00 42.00 Pr 8.40 12.00 7.30 3.20 7.20 12.00 5.50 Nd 34.00 52.00 29.00 13.00 29.00 48.00 22.00 Sm 7.80 9.60 6.10 3.10 5.00 10.00 4.80 Eu 2.70 2.80 1.80 1.00 1.80 3.30 1.50 Gd 8.40 8.10 5.10 3.20 4.70 11.00 4.80 Tb 1.50 1.10 0.79 0.51 0.64 1.60 0.72 Dy 8.60 5.20 4.30 3.00 3.40 8.90 3.90 Ho 1.70 0.87 0.82 0.57 0.65 1.70 0.75 Er 4.00 2.00 2.00 1.40 1.60 4.20 1.80 Tm 0.53 0.27 0.29 0.20 0.23 0.57 0.24 Yb 3.10 1.70 1.80 1.40 1.40 3.80 1.60 Lu 0.41 0.28 0.27 0.20 0.18 0.52 0.23 A l 2 0 3 / T i 0 2 4.93 4.96 7.30 9.26 7.91 4.55 5.63 Zr/Y 4.61 10.80 5.96 5.50 7.00 4.94 5.29 Zr/Nb 5.44 3.65 2.23 3.38 2.55 3.63 3.96 Zr /Ti0 2 75.18 75.63 67.77 65.19 85.89 77.43 60.66 Zr/Hf 44.17 45.00 43.33 40.00 46.67 39.84 38.28 Ti /V 58.70 178.35 43.32 45.72 61.07 105.08 52.49 Th/Nb 0.08 0.09 0.08 0.07 0.07 0.08 0.08 Ce/Yb n 3 5.27 16.77 9.50 4.63 11.11 6.34 6.80 Gd/Yb n 3 2.20 3.86 2.30 1.85 2.72 2.35 2.43 Nb/Y 0.85 2.96 2.67 1.63 2.75 1.36 1.33 Nb/U 50.00 52.86 64.00 68.42 42.31 52.31 51.85 Nb/Nb* 4 1.34 1.31 1.60 1.71 1.81 1.45 1.43 Notes: 1- CS = chlorite schist, BCS = biotite bearing chlorite schist, MB = metabasalt, MCS = muscovite bearing chlorite schist; 2 - recalculated volatile free; 3 - chondrite normalized to values of Sun and McDonough (1989); 4 - Nb/Nb* = (2*Nb/Nbpm) / (Th/Thpm+La/Lapni) where pm=normalized to primitive mantle values of Sun and McDonough (1989). 170 Table 6.1. (continued). Sample Name P98-47 98DM-141 P98-2 P98-3 P98-5a P98-7 P98-43 Rock Type' CS MCS MDS CS CS MGS BMDD Easting 413765 431291 414979 414591 414328 414655 414449 Northing 6811497 6817068 6816617 6817360 6818770 6814941 6814176 Group 4a 4a 4b 4b 4b 4b 4b Si0 2 (wt%)2 50.84 48.65 54.89 53.11 41.03 51.26 52.67 Ti0 2 2.05 2.39 3.00 3.02 4.82 1.52 2.84 A1203 14.29 16.45 14.31 13.47 20.24 14.54 14.14 Fe 20 3T 13.40 13.09 14.63 14.55 16.24 10.07 13.73 Fe 20 3 1.55 1.39 2.78 1.63 2.38 3.76 2.97 FeO 10.63 10.54 10.68 11.62 12.45 5.70 9.73 FeO* 12.03 11.79 13.18 13.09 14.59 9.08 12.41 MnO 0.25 0.15 0.20 0.20 0.22 0.16 0.15 MgO 6.81 9.08 4.52 3.36 6.41 7.32 5.70 CaO 10.93 8.41 5.23 8.56 6.91 12.33 4.61 Na20 2.10 0.23 3.74 4.45 2.60 3.15 0.10 K 2 0 0.28 2.47 0.11 0.07 2.15 0.09 6.69 P2O5 0.25 0.24 0.56 0.52 0.80 0.16 0.39 H 2 0 4.30 4.10 3.90 3.00 5.30 2.30 2.40 c o 2 5.90 10.50 3.10 5.30 2.40 0.10 0.40 Cr (ppm) 233 142 62 32 74 402 80 Ni 54 58 19 <10 37 59 17 Co 41 23 36 37 50 36 45 Sc 37 25 32 33 50 44 34 V 278 235 317 309 410 255 330 Cu <10 19 16 11 40 56 <10 Pb 14 3 2 6 2 7 5 Zn 196 80 147 204 210 66 125 Rb 7.5 47 3.8 1.7 69 1.4 240 Cs 0.66 1.30 0.67 0.60 1.80 0.09 7.70 Ba 281 2300 86 30 1100 22 570 Sr 157 250 122 213 190 304 130 Ga 18 18 22 22 33 17 26 Ta 1.3 1.4 2.2 2.0 2.7 0.9 1.8 Nb 15 21 30 29 50 12 29 Hf 3.2 2.9 6.2 5.9 9.8 2.1 4.2 Zr 117 120 259 244 410 75 240 Y 42 20 50 48 87 29 47 Th 1.5 1.8 6.4 6.4 8.2 1.7 4.7 U 0.44 0.45 1.80 1.40 3.10 0.34 1.70 171 Table 6.1. (continued). Sample Name P98-47 98DM-141 P98-2 P98-3 P98-5a P98-7 P98-43 La 14.00 14.00 34.00 34.00 53.00 12.00 25.00 Ce 31.00 30.00 70.00 72.00 120.00 25.00 54.00 Pr 4.50 3.60 9.50 9.50 15.00 3.60 6.40 Nd 20.00 17.00 39.00 38.00 67.00 16.00 29.00 Sm 5.40 4.30 8.80 8.40 14.00 4.00 7.90 Eu 1.70 1.80 2.50 2.40 4.30 1.40 2.30 Gd 6.40 4.30 9.10 9.00 16.00 4.80 8.50 Tb 1.10 0.65 1.50 1.40 2.50 0.81 1.30 Dy 6.80 3.70 8.50 8.00 14.00 4.80 7.50 Ho 1.40 0.73 1.70 1.60 3.00 1.00 1.60 Er 3.70 1.80 4.30 4.10 7.40 2.60 4.20 Tm 0.54 0.27 0.62 0.58 1.20 0.37 0.63 Yb 3.70 1.70 4.30 4.00 7.00 2.50 4.10 Lu 0.53 0.26 0.59 0.56 1.00 0.34 0.58 Al 2 0 3 /Ti0 2 6.97 6.89 4.77 4.46 4.20 9.60 4.98 Zr/Y 2.79 6.00 5.18 5.08 4.71 2.59 5.11 Zr/Nb 7.80 5.71 8.63 8.41 8.20 6.25 8.28 Zr/Ti0 2 63.24 58.25 92.17 87.77 92.13 50.34 86.64 Zr/Hf 36.56 41.38 41.77 41.36 41.84 35.71 57.14 Ti/V 39.89 52.55 53.14 53.94 65.07 35.03 50.32 Th/Nb 0.10 0.09 0.21 0.22 0.16 0.14 0.16 Ce/Ybn 3 2.17 4.57 4.22 4.66 4.44 2.59 3.41 Gd/Ybn 3 1.40 2.05 1.72 1.82 1.85 1.56 1.68 Nb/Y 0.36 1.05 0.60 0.60 0.57 0.41 0.62 Nb/U 34.09 46.67 16.67 20.71 16.13 35.29 17.06 Nb/Nb*4 1.11 1.42 0.67 0.65 0.81 0.90 0.89 Notes: 1- CS = chlorite schist, BCS = biotite bearing chlorite schist, MB = metabasalt, MCS = muscovite bearing chlorite schist; 2 - recalculated volatile free; 3 - chondrite normalized to values of Sun and McDonough (1989); 4 - Nb/Nb* = (2*Nb/Nbpm) / (Th/Thpm+La/Lapm) where pm=normalized to primitive mantle values of Sun and McDonough (1989). 172 Table 6.2, Neodymium isotope geochemical data for mafic rocks from unit 4 mafic rocks. Sample Suite 143Nd7144Nd' 1 4 7Sm/ 1 4 4Nd eNd35o2 /Sm/Nd 2 eNdo2 T 3 Sm (ppm) Nd (ppm) P99-9b P99-2 Group 4a Group 4b 0.512505 (07) 0.512364 (07) 0.1151 0.1395 +1.05 -2.80 -0.41 -0.29 -2.59 -5.34 1.02 1.64 9.50 8.88 49.90 38.48 Notes: 1 - estimated Nd/ Nd uncertainties, in brackets, at the 2a level, numbers in brackets are errors in the last two decimal places; 2 - calculated using 1 4 3Nd7 1 4 4Nd of CHUR = 0.512638 and 1 4 7 Sm/ 1 4 4 Nd = 0.1966 (Hamilton et a l , 1983); 3 - calculated using the values of 1 4 3 Nd/ 1 4 4 Nd = 0.513163 and 1 4 7 Sm/ 1 4 4 Nd = 0.2137 for the D M reservoir (Goldstein et a l , 1984). 173 Chapter 7 Magmatic Diversity in a Pericratonic Realm: Geochemical and Nd Isotopic Constraints on the Origin of Arc and Non-Arc Magmatism in the Yukon-Tanana Terrane, Yukon, Canada. Abstract The Fire Lake unit (FLU) of the Yukon-Tanana Terrane (YTT) in the Finlayson Lake region, Yukon, Canada, consists of a Devonian-Mississippian (-365-360 Ma) package of mafic and lesser ultramafic volcanic and subvolcanic rocks. In this paper field, geochemical and Nd isotopic attributes of these rocks are evaluated to understand the interrelationships between tectonic and magmatic processes within the YTT and their roles to Cordilleran crustal growth. The rocks of the FLU can be broken into different magmatic suites reflecting variable influence from subducted slab metasomatism, varying mantle enrichment, and contamination by continental crust. The subdivisions include suites of arc parentage from a variably enriched mantle wedge; from most depleted to enriched these are: high Ca boninites (BON); island arc tholeiites (IAT); light rare-earth element (LREE)-enriched island arc tholeiites (L-IAT); and calc-alkaline basalts (CAB). Those with non-arc parentage include, from most depleted to enriched: back-arc basin basalts (BABB); enriched mid-ocean ridge basalts (E-MORB); Nb-enriched basalts with moderate La/Ybn ratios (NEB-1); and Nb-enriched basalts with high La/Ybn ratios (NEB-2). A Nb-enriched basalt suite with subduction-influenced signatures (high Th; T-NEB) is also present in the FLU. Neodymium isotopic data for the mafic rocks of the FLU have a wide range in eNd350 values from +8.5 to -5.0 reflecting contributions from depleted mantle, enriched mantle, and evolved continental crust. The spatial distribution of these different suites implies mid-Paleozoic subduction above an east-dipping subduction zone with a composite basement of both juvenile (oceanic) and evolved (continental or continent-derived) crustal material. Potential models are presented herein to explain the diversity of magmatic rocks in the FLU including: 1) arc magmatism punctuated by arc-rifting and back-arc basin generation; 2) ridge propagation into an evolving arc and subsequent back-arc basin formation; and/or 3) ridge subduction (slab-window) beneath evolving arc and subsequent back-arc basin generation. All models are consistent with current paradigms for the tectonic evolution of the mid-Paleozoic Ancient Pacific margin of the northern Cordillera. The occurrence of abundant juvenile Nd isotopic signatures in the rocks of the FLU suggests that the pericratonic terranes of the Cordillera may have contributed much 174 more juvenile material to Cordilleran crustal growth in the Phanerozoic than has previously been considered. Introduction Convergent margin magmatism is considered to be a significant process in the growth of continental crust and the evolution of the Earth's crust and mantle (Taylor and McLennan, 1985; Armstrong, 1988; Samson et al., 1989, 1990, 1991; Samson and Patchett, 1991; Patchett and Gehrels, 1998; Kerrich et al., 1999; Polat and Kerrich, 2000). Since the Archean subduction zones have been the sites where oceanic crust has been returned to the mantle, new crust has been generated through convergent margin magmatism, and where there has been a fractionation of elements and isotopes between the subducted slab, overlying mantle wedge and crust, and the deep mantle (Taylor and McLennan, 1985; Armstrong, 1988; Hoffman, 1988; Samson et al., 1989; Samson and Patchett, 1991; Pearce and Peate, 1995; Kerrich et al, 1999; Kamber and Collerson, 2000; Polat and Kerrich, 2000). Furthermore, convergent margins are locations where both juvenile and evolved crust have been mechanically added via accretion to the crustal land mass leading to crustal growth and assembly of continents (Armstrong, 1988; Samson et al., 1989; Monger and Nokleberg, 1994; Patchett and Gehrels, 1988). Many workers have considered the North American Cordillera as a model for Phanerozoic crustal growth, exemplifying the roles that magmatism and accretion play in the growth and assembly of continental crust (Armstrong, 1988; Samson and Patchett, 1991; Monger and Nokleberg, 1994; Patchett and Gehrels, 1998). These workers have shown that these accretionary and magmatic processes within the terranes of the Cordillera have resulted in certain terranes reflecting the addition of new juvenile crust (Wrangellia, Alexander, Stikine; Samson et al., 1989, 1990), whereas other terranes consist largely of recycled continental crustal material (Cassiar, North American Margin; e.g., Boghassian et al., 1996; Garzione et al., 1996). Still other terranes appear to be mixtures of both juvenile and evolved material (Yukon-Tanana, Kootenay, Slide Mountain; e.g., Stevens et al., 1996; Creaser et al., 1997). The Yukon-Tanana Terrane (YTT) is a critical terrane within the Cordilleran mosaic because it lies between the outboard terranes of predominantly juvenile character, but is proximal to the North American cratonic 175 margin (NACM). The "pericratonic", or continent-margin, nature or the terrane has resulted in the terrane having mixed signals with a significant evolved component (e.g., Mortensen, 1992a,b; Stevens et al , 1996; Creaser et a l , 1997; Grant, 1997), but with contributions of juvenile material (e.g., Creaser et al , 1997, 1999; Grant, 1997). These features suggest that many ofthe inboard terranes like the Y T T may have contributed much more juvenile material to the Cordilleran collage than has previously been considered. In this paper we provide field, geochemical and Nd-isotopic data for Devonian-Mississippian mafic volcanic and subvolcanic rocks from the Yukon-Tanana Terrane in the Finlayson Lake region, southeastern Yukon. Within this region there is considerable geochemical diversity with many samples exhibiting features common to intraoceanic arc environments, whereas others exhibit the influence of continental crust typical of continental margin arc environments. The geochemical and Nd isotopic data in this paper are presented in an attempt to resolve the relative roles that the mantle, subducted slab and evolved crustal material have played in the generation of these Y T T mafic magmatic rocks. Furthermore, these data will be used to illustrate the relationships that these magmatic rocks have to the tectonic evolution ofthe Y T T and the crustal growth and assembly ofthe North American Cordillera. Previous Work The Yukon-Tanana Terrane (YTT) is the largest and most enigmatic terrane in the northern Cordillera (Mortensen, 1992a) and the setting and origin ofthe terrane has been variously interpreted (e.g., Templeman-Kluit, 1979; Mortensen and Jilson, 1985). Early workers considered the terrane to be entirely allocthonous and a product of arc-continent collision (Templeman-Kluit, 1979; Erdmer, 1985), which some workers still consider as the most viable model for the terrane (Hansen and Dusel-Bacon, 1998). Templeman-Kluit (1979) interpreted the terrane to consist of three unrelated allochthonous including the Nisutlin Allochthon (siliceous cataclasite), Anvil Allochthon (ophiolitic rocks), and Simpson Allochthon (sheared granite). Templeman-Kluit (1979) and subsequent workers (Erdmer, 1985; Hansen and Dusel-Bacon, 1998) suggested that these allochthons were assembled in the Permo-Triassic, obducted onto the North American Margin (NAM) in the Jurassic, and disassembled during Cretaceous 176 extension. A key premise of the aforementioned workers is that stratigraphy could not be determined in the Y T T , and that all rocks of the terrane, including the mafic rocks of this study, were allocthonous. Mortensen and Jilson (1985) and Mortensen (1992a,b) provided an alternative to the latter and suggested that stratigraphy could be defined within the terrane. Mortensen and Jilson (1985) and Mortensen (1992a,b) divided the terrane into three units including a pre-Devonian-Mississippian "lower unit" of dominantly metasedimentary rocks, overlain by the Devonian-Mississippian volcanic and sedimentary rock dominated "middle unit", capped by the Pennsylvanian to Permian "upper unit" of carbonate and quartzite. In addition, Mortensen and co-workers documented two Devonian-Mississippian granitic suites (Grass Lakes, Simpson Range) that intruded parts of the Y T T stratigraphy. Mortensen and co-workers considered some mafic rocks interlayered with felsic rocks to be parts of Y T T but they also considered others allochthonous and correlated them with the ophiolitic Slide Mountain Terrane (SMT). In the view of Mortensen and co-workers (Mortensen and Jilson, 1985; Mortensen, 1992a,b) the Y T T represented an evolving Devonian-Mississippian volcano-plutonic arc developed upon a continental basement above an east-dipping subduction zone. They also suggested that the terrane was deformed between the Permian and Triassic, was thrust imbricated with the SMT between the Late Triassic to mid-Cretaceous, and was joined with the N A M by mid-Cretaceous by displacement along the Finlayson Lake fault zone, an interpreted steep transpressive structure (Mortensen and Jilson, 1985; Mortensen, 1992a,b). More recently, Stevens et al. (1995) showed that stratigraphy could also be documented in the Y T T in the Teslin zone south of the Tintina Fault (Fig. 7.1). They documented that many rocks of the "Nisutlin Allochthon" had primary depositional contacts, and that rocks formerly considered part of the Anvil Allocthon had intrusive contacts with the Nistulin "assemblage". Geochemical and Nd isotopic data within the Teslin zone by Creaser et al. (1997) suggested that sedimentary rocks of the Teslin zone had isotopic attributes consistent with influence from both evolved and juvenile sources, and that mafic rocks of the Anvil "assemblage" had signatures consistent with formation within an arc environment. They also suggested that the Y T T in the Teslin zone was part of a continental arc developed on the distal margin of N A M with both inputs from juvenile and evolved continental material. At the same time, Grant (1997) documented similar features in sedimentary rocks of the Y T T in the Finlayson Lake region. 177 Furthermore, he provided geochemical and isotopic data for granitoid and mafic rocks in the region to illustrate that they were crust-mantle mixtures and formed within a subduction zone environment. The discovery of volcanic-hosted massive sulphide (VHMS) mineralization in the Y T T of the Finlayson Lake region has led to resurgence of research in the Y T T and this new work has generated significant new ideas about the stratigraphy and setting of the terrane. Murphy and co-workers (Murphy, 1998, 2001; Murphy and Piercey, 1999, 2000; Piercey and Murphy, 2000) have confirmed that the Y T T in the Finlayson Lake has a coherent stratigraphy. These workers have shown that most ofthe mafic rocks in the terrane are not allochthonous and not correlative with the SMT. Work outside of the Finlayson Lake region as part ofthe Ancient Pacific Margin N A T M A P project has shown that elsewhere in the Yukon, British Columbia and Alaska the Y T T consists of coherent stratigraphic successions (Nelson et al , 2000; Colpron et al , 2001). Coincident geochronological, lithogeochemical and tracer isotopic studies have illustrated that rocks of the Y T T represent episodic, arc and back-arc magmatism that was built upon a continental substrate with various juvenile and evolved inputs (Piercey et al , 1999, 2000; Piercey and Murphy, 2001; Colpron, 2001; Colpron et al , 2001). Together these studies are beginning to unravel a complex mid- to Late-Paleozoic continental arc and back-arc tectonic and magmatic history for the Y T T (Colpron et a l , 2001), and provide a starting point for this study. Geological Setting of the Fire Lake Unit and YTT in the Finlayson Lake Region The Y T T in the F L D (Fig. 7.1) is composed of foliated and lineated greenschist to lower amphibolite grade metasedimentary, metavolcanic and metaplutonic rocks (e.g., Tempelman-Kluit, 1979; Mortensen and Jilson, 1985; Figs. 7.1-7.2). Although the region has been strongly deformed and metamorphosed, regional mapping has identified a stratigraphically intact sequence consisting of three mid- to late-Paleozoic unconformity bound successions: the Grass Lakes, Wolverine and Campbell Range successions (Figs. 7.1-7.2; Murphy, 1998, 2001; Murphy and Piercey, 1999, 2000). The lowermost unit in the Y T T of the Finlayson Lake region consists of pre-365 Ma (Mortensen and Murphy, unpublished data) quartz-rich, non-carbonaceous, metaclastic rocks of unit 1 of the Grass Lakes succession (Figs. 7.1-7.2). The Fire Lake unit (FLU) was deposited upon this continent (?)-derived substrate. The F L U is a mafic and ultramafic dominated unit that consists of a variety of volcanic, subvolcanic and intrusive rocks 178 that are variably preserved. In many regions the rocks are highly sheared and have been converted to chlorite schists with much of their igneous protolith history obliterated. However, commonly associated with these schists are coherent massive greenstones of interpreted volcanic origin, which display pseudomorphs after igneous phenocrysts (e.g., plagioclase). Many parts of the F L U are very well preserved. For example, the hanging wall of the Money Creek thrust fault contains abundant variably vesiculated pillowed to massive mafic flows with preserved igneous and volcanic textures (Piercey and Murphy, 2000). Furthermore, many of these rocks have well-preserved primary igneous minerals (e.g., hornblende, plagioclase feldspar). The F L U has a variable thickness and distribution. In the eastern portion of the Finlayson Lake region it is quite thin and can be only a few meters thick in some localities (Fig. 7.1). In contrast, near the Fyre Lake deposit, the F L U is up to 800m thick, and is associated with abundant turbiditic, clastic sedimentary rocks (Figs. 7.1-7.2). There is also an increase in the abundance of mafic and ultramafic intrusions near the Fyre Lake deposit (Figs. 7.1-7.2). Rocks of the F L U within the hanging wall of the Money Creek thrust fault are thicker and much better preserved relative to those in most parts of the footwall of the thrust (Fig. 7.1-7.2). Age constraints on the F L U range from -365 Ma (Mortensen and Murphy, unpublished data) to -360 Ma (Mortensen, 1992a,b). Stratigraphically overlying, and likely interlayered with, the F L U is the felsic volcanic and sedimentary rock dominated Devonian-Mississippian (-360-356 Ma) Kudz Ze Kayah (KZK) unit of the Grass Lakes successi