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Ore Petrography, Geochemistry, and Genesis of Epithermal Silver-Gold Veins

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Ore Petrography, Geochemistry, and Genesis of Epithermal Silver-Gold Veins on Florida Mountain, Silver City District, Idaho by Michael Stuart Mason A thesis submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Master of Science Auburn, Alabama August 1st, 2015 Keywords: Epithermal, Silver-Gold, mid-Miocene, Silver City district, Florida Mountain, Owyhee County Copyright 2015 by Michael Stuart Mason Approved by James A. Saunders, Chair, Professor of Geosciences Willis E. Hames, Professor of Geosciences Robert B. Cook, Professor Emeritus of Geosciences Abstract Geochemical and petrographic studies were conducted on the Black Jack-Trade Dollar vein, a high-grade, low-sulfidation epithermal Ag-Au deposit hosted in mid- Miocene volcanic rocks of the Owyhee Mountains in southwestern Idaho. Petrographic studies of high-grade ore samples were conducted in both reflected and transmitted light. These petrographic studies revealed ore textures that indicate boiling and colloidal transport as important ore deposition mechanisms. Geochemical analysis indicated high levels of Au, Ag, Se, S, and Cu in the ores. Further analysis of the ores with an electron microprobe showed the presence of a compositional continuum of silver sulfo-selenides ranging from naumannite (Ag2Se) to acanthite (Ag2S). Additionally, the microprobe analyses revealed an unidentified Ag-Au-Se-S phase with an estimated formula of 65 Ag3AuSeS. Stable isotope studies of ore-stage chalcopyrite grains showed that δ Cu measurements range from -2 to 1‰ and δ34S measurements range from -1 to 1‰. Both Cu and S isotopes indicate a magmatic source for the metals. Textural, geochemical, and isotopic evidence suggests that the high-grade ores were formed when Au- and Ag-rich hydrothermal fluids were evolved from a magmatic source. These hydrothermal fluids apparently deposited their metals in the shallow vein through a combination of colloidal transport, cooling, and boiling. ii Acknowledgments I would like to thank my advisor Dr. James Saunders for his support and guidance. This thesis would never have been written without many long discussions and much patience on his part. I am grateful to my office mate, Erin Summerlin, for her help in both the field and in the office. Her talents at finding ore samples, keeping me on task, and making me laugh far exceeded my own abilities. I would like to thank the faculty, staff, and students of the Department of Geology for their support throughout my time at Auburn University. Discussions with many of my fellow graduate students, especially Josh Poole and Chris Smith, helped focus and strengthen this thesis, and to them I am grateful. I would especially like to thank my committee members, Dr. Robert Cook and Dr. Bill Hames, for their corrections, suggestions, and help in assembling this thesis. I would like to thank Dr. Ryan Mathur and Dr. George Kamenov for their help in interpretating my copper and lead isotope data. I would also like to thank Dr. Doug Crowe and Chris Fleisher at the University of Georgia for their help in performing sulfur isotope analyses and electron microprobe analyses on my samples. I would like to thank the National Science Foundation for providing support for this project through NSF grant #1004381370012000. I would like to thank Mary O’Malley for her hospitality and vast knowledge of local geography and mining history during my stay in Silver City. Finally, I would like to thank my family. I am forever indebted to them for the love, phone calls, cards, and care packages they sent to me during my time at Auburn University. iii Style manual or journal used: Economic Geology Computer software used: Microsoft Word, Microsoft Excel, Microsoft Powerpoint, Microsoft Paint, Geochemist Workbench iv Table of Contents Abstract ........................................................................................................................................... ii Acknowledgments.......................................................................................................................... iii Table of Contents ............................................................................................................................ v List of Tables ................................................................................................................................. ix List of Figures ................................................................................................................................. x 1. INTRODUCTION .................................................................................................................... 1 Epithermal Gold-Silver Deposits ........................................................................ 1 History of Epithermal Deposit Models ............................................................... 2 Problems with the Current Epithermal Deposit Model ....................................... 6 Objective of Study .............................................................................................. 7 2. PREVIOUS WORKS AND GEOLOGIC SETTING ............................................................... 9 Northern Great Basin Au-Ag Deposits ............................................................... 9 Silver City Area ................................................................................................ 16 Geologic Setting................................................................................................ 24 3. METHODOLOGY ................................................................................................................... 28 Field Methods ................................................................................................... 28 Sample Preparation ........................................................................................... 30 Ore Petrography ................................................................................................ 31 Electron Microprobe Analysis .......................................................................... 32 Geochemical Analysis ...................................................................................... 32 Isotope Analysis ................................................................................................ 33 4. RESULTS ............................................................................................................................... 36 Geochemical Analysis ...................................................................................... 36 Reflected Light Microscopy ............................................................................. 44 Fractal Dendrites .......................................................................................... 44 Quartz Euhedra ............................................................................................. 47 “Vanishing” Silver Phase ............................................................................. 49 Intergrown Silver Phases .............................................................................. 50 Mineralized Veinlets/Fracture Zone Fill/Disseminated Naumannite ........... 52 Relict Grains ................................................................................................. 53 Isolated Aggregates of Sulfides and Selenides .............................................. 55 Paragenesis of the Black Jack-Trade Dollar Vein ....................................... 56 Transmitted Light Microscopy ......................................................................... 59 Massive Quartz and Adularia ....................................................................... 59 Comb Quartz ................................................................................................. 61 Crustiform/Cockade Quartz .......................................................................... 61 Plumose Quartz ............................................................................................. 63 Quartz Pseudomorph After Calcite ............................................................... 64 vi Chalcedony ................................................................................................... 66 Mosaic/Jigsaw/Microcrystalline Quartz ....................................................... 67 Electron Microprobe Analysis .......................................................................... 69 Isotopic Analysis ............................................................................................... 76 Cu Isotopes.................................................................................................... 77 S Isotopes ...................................................................................................... 78 Pb Isotopes .................................................................................................... 80 5. DISCUSSION ......................................................................................................................... 82 Ore Textures...................................................................................................... 82 Geochemistry .................................................................................................... 83 Isotopes ............................................................................................................. 87 Cu isotopes ...................................................................................................
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