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University of Nevada, Reno Igneous and Hydrothermal Geology of The University of Nevada, Reno Igneous and Hydrothermal Geology of the Central Cherry Creek Range, White Pine County, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by David J. Freedman Dr. Michael W. Ressel, Thesis Advisor May, 2018 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by DAVID JOSEPH FREEDMAN entitled Igneous and Hydrothermal Geology of the Central Cherry Creek Range, White Pine County, Nevada be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Michael W. Ressel, PhD, Advisor John L. Muntean, PhD, Committee Member Douglas P. Boyle, PhD, Graduate School Representative David W. Zeh, PhD, Dean, Graduate School May, 2018 i Abstract The central Cherry Creek Range exposes a nearly intact, 8-km thick crustal section of Precambrian through Eocene rocks in a west-dipping homocline. Similar tilts between Eocene volcanic rocks and underlying Paleozoic carbonates demonstrate that tilting and exhumation largely occurred during post-Eocene extensional faulting, thus allowing for relatively simple paleo-depth determinations of Eocene intrusions and mineral deposits. The study area is cored by the Cherry Creek quartz monzonite pluton (35 km2 exposure; ~132 km2 coincident magnetic anomaly), which was emplaced into Precambrian and Cambrian meta-sedimentary strata between 37.9-36.2 Ma and is exposed along the east side of the range. The pluton and overlying Paleozoic strata are cut by abundant 35.9-35.1 Ma porphyritic silicic dikes. A range of polymetallic mineralization styles are hosted by the intrusive rocks along two deeply-penetrating, high-angle faults and their intersections with favorable Paleozoic units. Geochemical, geochronological, petrographic, and field mapping show progression among hydrothermal ore deposits from the mesozonal intrusion-proximal through the epizonal intrusion-distal environments. Data indicate that both the intrusions and hydrothermal deposits at Cherry Creek are exclusively Eocene, and were developed prior to the westward tilt that affects rocks above and below the Eocene unconformity. The deepest-formed and highest- temperature deposits are intrusion-related Sn-W-(Mo) greisen and Au-rich polymetallic vein mineralization that occurred at paleodepths of 7-8 km and developed during the emplacement of the Cherry Creek pluton based on 187Re/187Os and 238U/206Pb ages. Carbonate replacement deposits containing Ag-Pb-Zn-(W) occurred at intermediate depths of ~4-5 km during the Eocene, whereas distal-disseminated Au-Ag and Carlin-Type Au deposits (CTDs) formed slightly later than high-temperature deposits in shallower environments, from 1-3 km below the Eocene ii unconformity. The close timing and spatial associations suggest a link between all deposits and the development of the Cherry Creek intrusive complex. These findings imply that Eocene CTDs relate to large deep-seated plutons and/or batholiths, not to shallow porphyry-type plutons, and that shallow CTDs are potentially zoned with deep Sn-W-Mo greisen and high-temperature Au-rich vein and replacement deposits. Late Eocene magmatic-hydrothermal events at Cherry Creek regionally coincided with a major metallogenic period in the Cordillera that resulted in the formation of economically important CTDs. iii Acknowledgements This thesis would not have been possible without the help of many people over the last three years. First, thanks to David Tretbar and Summit Mining International for going above and beyond to fund the project, provide support in Cherry Creek during the 5 months of field work, and for his continued interest in the project. Thanks to my thesis committee, especially to my adviser Dr. Mike Ressel for his patience and enthusiasm during the writing process. Thanks to Dr. John Muntean for his tireless hard work towards the success of the CREG program and its students. Thanks to Dr. Doug Boyle who served as outside committee member. Thanks to Will Ostrenga and Jamie Robinson for their great company and geologic conversation in the field. Thanks to Dr. Paul O’Sullivan, Dr. Rob Creaser, and Kathleen Zanetti for their assistance with the geochronology presented in this thesis. Thanks to my classmates in CREG, especially to Micah Claypoole, Rob Selwood, and Patrick Quillen who saw the whole thing. iv Table of Contents INTRODUCTION ......................................................................................................................... 1 Geologic History of the Great Basin ................................................................................................................ 4 Local Geology .................................................................................................................................................. 9 METHODOLOGY ...................................................................................................................... 19 Field Mapping and Petrology ........................................................................................................................ 19 Geochemistry ................................................................................................................................................ 20 Petrography ................................................................................................................................................... 22 Geochronology .............................................................................................................................................. 23 238U/206Pb Geochronology ......................................................................................................................... 23 40Ar/39Ar Geochronology ........................................................................................................................... 23 187Re/187Os Geochronology ....................................................................................................................... 26 IGNEOUS GEOLOGY ................................................................................................................ 29 Porphyritic Quartz Monzonite of the Cherry Creek Pluton ........................................................................... 29 Tonalite of the Silver Canyon Stock ............................................................................................................... 36 Porphyritic Rhyolite and Dacite Dikes ........................................................................................................... 39 Granite Aplite and Aplitic Quartz Monzonite ................................................................................................ 45 Porphyritic Dacite and Andesite Dikes .......................................................................................................... 48 Diorite and Diabase Dikes ............................................................................................................................. 50 v HYDROTHERMAL GEOLOGY ................................................................................................... 57 Contact-Metamorphic and Skarn Rocks ........................................................................................................ 60 Hydrothermal Deposits in the Cherry Creek Pluton ...................................................................................... 63 Quartz-Muscovite Greisen and Phyllic Alteration in the Salvi Fault Zone ................................................ 63 Phyllic Alteration and Quartz-Mineralized Faults at Cherry Hill and Bull Hill ........................................... 71 Phyllic Alteration and Pluton-Hosted Sulfide Mineralization at the Maryanne Deposit .......................... 74 Sericite-Carbonate-Carbonate Alteration in El Rey Canyon ...................................................................... 83 Sodic-Calcic Alteration in Quartz Monzonite ............................................................................................ 85 Hydrothermal Deposits in Quartzite and Phyllite ......................................................................................... 88 McCoy Creek Polymetallic Veins ............................................................................................................... 88 Exchequer Polymetallic Veins ................................................................................................................. 100 Hydrothermal Deposits in Carbonate and Shale ......................................................................................... 107 Polymetallic Carbonate and Carbonate Breccia Replacement Deposits ................................................. 107 Flint Canyon Au-Ag-Bearing Jasperoids .................................................................................................. 120 DISCUSSION...........................................................................................................................127 Intrusive History of Cherry Creek ................................................................................................................ 127 Significance of Cherry Creek Magmatism in Eocene Nevada ...................................................................... 129 Hydrothermal History of Cherry Creek ........................................................................................................ 130 Hydrothermal Zonation in Cherry Creek ....................................................................................................
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