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Geochemical and Geochronological Constraints on Mineralization

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GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS ON MINERALIZATION WITHIN THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS, BATTLE MOUNTAIN-EUREKA TREND, NEVADA by CHRISTOPHER RONALD KELSON (Under the Direction of Douglas E. Crowe) ABSTRACT The Hilltop, Lewis, and Bullion mining districts (northern Shoshone Range, Nevada) are part of the Battle Mountain-Eureka trend and contain both vein- and porphyry-type deposits. New geochronology data from igneous rocks, porphyry-style Cu-Mo mineralization, and vein-hosted minerals elucidate the relationship between magmatic activity, hydrothermal fluid flow, and mineralization. Mostly felsic intrusive rocks were emplaced throughout the area between 39.3 ± 0.4 and 38.1 ± 0.4 Ma and weak Cu + Mo porphyry-style mineralization is associated with some of the intrusions. Ages of igneous rocks are nearly coincident with molybdenite ages, supporting a relation between pluton emplacement and porphyry Cu-Mo mineralization. Ages of quartz vein-hosted gold (35.9 ± 0.1 Ma, Hilltop deposit) and base-metal minerals (38.3 ± 0.07 Ma, Gray Eagle mine), established via ages of associated gangue clay minerals, are younger than nearby intrusive igneous rocks and may suggest the vein mineralization formed during prolonged hydrothermal activity related to igneous rock emplacement. Quartz vein-hosted sulfide minerals from the northern Shoshone Range are isotopically 34 similar (δ SCDT range from -6 to +9 per mil) to sulfide minerals from other Cu-Mo porphyry deposits and Cordilleran vein-type deposits, supporting a mostly magmatic sulfur source. Carbon isotope data from vein gangue carbonate minerals also support a magmatic origin for ore-forming fluids with variable contributions from host rock organic matter or carbonate rocks 13 (δ CPDB range from -0.2 to –11.6 per mil); carbonate oxygen was derived mainly from 18 magmatic fluids (δ OVSMOW range from –1.3 to +14.4 per mil). Primary fluid inclusion data (salinity range from 0 to 6.4 equiv. wt. % NaCl; Th range from 109-425°C) and measured 18 δ OVSMOW data (-0.97 to +17.3‰) suggest the ore-bearing vein quartz formed from variable 18 amounts of meteoric and magmatic components (calculated δ OVSMOW -16.2 to +13.3‰). Depositional temperatures of base metal minerals, calculated using sulfide sulfur isotope geothermometry, range from 249-502°C and agree with vein quartz primary fluid inclusion Th values. Geochronology, stable isotope, and geothermometry data show that vein- and porphyry- type mineralization is genetically related to Eocene magmatism and that some vein mineralization temperatures exceeded relatively low-temperature epithermal conditions and were more closely related to higher temperature porphyry-style processes. INDEX WORDS: Base Metals, Battle Mountain-Eureka Trend, Bullion District, Carbon, Carbonate, Copper, Cu, Eocene Magmatism, Epithermal, Fluid Inclusions, Geochronology, Geothermometry, Gold, Hilltop District, Hydrothermal, Lewis District, Mineralization, Mo, Molybdenite, Nevada, Oxygen, Porphyry, Quartz, Shoshone Range, Silver, Stable Isotopes, Sulfide, Sulfur, Vein GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS ON MINERALIZATION WITHIN THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS, BATTLE MOUNTAIN-EUREKA TREND, NEVADA by CHRISTOPHER RONALD KELSON B.S., University of Utah, 1994 M.S., Brigham Young University, 1999 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2006 © 2006 Christopher Ronald Kelson All Rights Reserved GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS ON MINERALIZATION WITHIN THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS, BATTLE MOUNTAIN-EUREKA TREND, NEVADA by CHRISTOPHER RONALD KELSON Major Professor: Douglas E. Crowe Committee: Robert B. Hawman Michael F. Roden Paul A. Schroeder Sandra J. Wyld Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia May 2006 DEDICATION This dissertation, representing five years’ worth of being away from home and family while either working in the field or spending long days, weekends, and odd hours in the laboratory and office, is dedicated to my wife Christa. Without her love, support, and patience, I would not have made it through this. iv ACKNOWLEDGEMENTS This research would not have been possible without the generous support of the Cortez Joint Venture, and very special thanks to Mr. Robert C. Hays, Jr., Technical Services Superintendent, Cortez Joint Venture. This research was also funded by the Society of Economic Geologists (Hugh E. McKinstry Grant), the Geological Society of America (Grant No. 7180-02), and the Department of Geology, University of Georgia. Permission of the Cortez Joint Venture to publish this investigation is gratefully acknowledged. Thanks to: Dr. Kenneth A. Foland (OSU), Dr. Matthew T. Heizler (NMT), and Mr. Thomas D. Ullrich (UBC) for their assistance and insight with the 40Ar/39Ar data. Dr. Chris Romanek (SREL) and Mr. Tom Maddux (UGA) assisted with the organic carbon isotope data. Dr. Zachary D. Sharp (UNM) provided the silicate oxygen isotope analyses. Richard Markey (AIRIE, Colorado State University) provided the Re-Os analyses. Ms. Julia Cox and Mr. Chris Fleisher (UGA) assisted with stable isotope and electron microprobe analyses, respectively. I would especially like to thank the members of my dissertation committee for many informative discussions regarding every facet of this study, and for their review of this dissertation. v TABLE OF CONTENTS Page ACKNOWLEDGEMENTS.............................................................................................................v LIST OF TABLES....................................................................................................................... viii LIST OF FIGURES .........................................................................................................................x CHAPTER 1 INTRODUCTION .........................................................................................................1 REFERENCES..........................................................................................................9 2 GEOCHRONOLOGY AND GEOCHEMISTRY OF THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS AND SURROUNDING AREA, BATTLE MOUNTAIN-EUREKA TREND, NEVADA ........................................................11 Abstract ...................................................................................................................12 Purpose of Study .....................................................................................................13 General Geology of the northern Shoshone Range .................................................14 Description of Mines in this Study..........................................................................14 Analytical Methods .................................................................................................20 Results .....................................................................................................................24 Discussion ...............................................................................................................37 Conclusion...............................................................................................................48 Acknowledgements .................................................................................................49 References ...............................................................................................................51 vi 3 GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS ON MINERALIZATION WITHIN THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS, BATTLE MOUNTAIN-EUREKA TREND, NEVADA ..54 Abstract ...................................................................................................................55 Introduction .............................................................................................................57 Regional Geologic Setting.......................................................................................62 Northern Shoshone Range and District Geology ....................................................65 Battle Mountain – Eureka trend ..............................................................................67 Northern Shoshone Range Mineral Deposits ..........................................................68 Methods...................................................................................................................81 Intrusive Igneous Rocks ..........................................................................................87 Geochronology ........................................................................................................98 Relationship between Magmatism and Mineralization.........................................105 Discussion: Northern Shoshone Range Vein-Hosted Mineralization ...................116 Summary and Conclusion .....................................................................................124 Acknowledgements ...............................................................................................125 References .............................................................................................................126 4 CONCLUSION..........................................................................................................137 APPENDICES .............................................................................................................................139
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