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Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin- Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation

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Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin- Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation UNLV Theses, Dissertations, Professional Papers, and Capstones 12-1-2012 Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin- Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation Lindsey R. Clark University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geology Commons, and the Mineral Physics Commons Repository Citation Clark, Lindsey R., "Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin-Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation" (2012). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1716. http://dx.doi.org/10.34917/4332697 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. ORE AND GANGUE MINERAL PARAGENESIS OF THE CORTEZ HILLS CARLIN- TYPE GOLD DEPOSIT, NEVADA: EVIDENCE FOR COINCIDENT HIGH-GRADE GOLD DEPOSITION AND COLLAPSE BRECCIATION by Lindsey R. Clark Bachelor of Science in Geoscience University of Nevada, Las Vegas 2009 A thesis submitted in partial fulfillment of the requirements for the Master of Science in Geoscience Department of Geoscience College of Sciences The Graduate College University of Nevada, Las Vegas December 2012 Copyright by Lindsey R. Clark, 2013 All Rights Reserved THE GRADUATE COLLEGE We recommend the thesis prepared under our supervision by Lindsey Clark entitled Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin-Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation be accepted in partial fulfillment of the requirements for the degree of Master of Science in Geoscience Department of Geoscience Jean S. Cline, Ph.D., Committee Co-Chair Adam C. Simon, Ph.D., Committee Co-Chair John L. Muntean, Ph.D., Committee Member Barbara Luke, Ph.D., Graduate College Representative Tom Piechota, Ph.D., Interim Vice President for Research & Dean of the Graduate College December 2012 ii ABSTRACT Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin-Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation by Lindsey R. Clark Dr. Jean S. Cline, Examination Committee Co-chair Professor University of Nevada, Las Vegas Dr. Adam C. Simon, Examination Committee Co-chair Associate Professor University of Michigan The Cortez Hills Carlin-type gold deposit (CTGD), located on the Battle Mountain-Eureka trend of Northern NV, is hosted in a conical shaped polylithic breccia whose central axis strikes parallel to the imbricate Voodoo fault system. Gold grades at the center of the Cortez Hills Breccia Zone (CHBZ) are locally in excess of an ounce per ton. Gold mineralization within the refractory ore at Cortez Hills shares many characteristics with other well studied CTGDs. However, new observations during this study have recognized 1) fragmented realgar that are rimmed by gold-bearing iron-sulfide minerals in four samples, 2) a Hg- and Tl-rich late-ore stage characterized by rare minerals, and 3) textural relationships show that ore-stage mineralizing fluids enhanced porosity through decarbonatization, resulting in contemporaneous brecciation and Au mineralization. The primary objectives of this thesis project were to produce a paragenesis and characterize mineralization at the Cortez Hills deposit with an emphasis on the CHBZ in order to test the hypothesis that the CHBZ contains typical Carlin-type mineralization and iii alteration. The data collected indicate that the CHBZ refractory ore exhibits the following paragenetic relationships. 1) Diagenetic pyrite was deposited in passive margin carbonate sediments. 2) A pre-ore suite of minerals apparently associated with low-grade contact metamorphism includes sphalerite, chalcopyrite, and tremolite. 3) Ore stage alteration minerals include illite, ore-stage jasperoid, and Au- and trace element-rich iron sulfides. 4) Hg- and Tl-rich minerals including aktashite (Cu6Hg3As4S12) and christite (TlHgAsS3) precipitated during a late-ore stage directly following the precipitation of Au-bearing iron sulfides. 5) Late- to post-ore stage minerals include realgar and calcite that are associated with cooling and collapse of the hydrothermal system. As a secondary goal, data were collected to test the hypothesis that Carlin-type mineralization occurred contemporaneously with brecciation throughout the CHBZ; understanding this relationship is necessary to understand how the CHBZ formed. Transects through host rocks that transition from negligible to high Au concentrations show a succession from primarily recrystallized calcite into rocks that contain Au-bearing iron sulfides, illite, fine grained calcite, and insoluble residual material precipitated in dissolution seams. As Au concentrations continue to increase within the ore body, host rocks that contain dissolution seams transition into heavily brecciated rocks with strongly mineralized clasts and a lightly mineralized matrix. Late-ore stage realgar precipitated in open space throughout the breccia, which is evident because it commonly conforms to euhedral crystal faces and cements zones of intense fracturing and areas of high porosity. This study concludes that the dissolution seams reveal Carlin ore fluid pathways through the carbonate host rocks, and extensive fluid-rock interaction along these seams ultimately resulted in brecciation. iv Late-ore-stage realgar precipitation in open space throughout the breccia signifies that brecciation terminated with cessation of the ore fluid incursion throughout the majority of the CHBZ. However, an area within the CHBZ contains fragmented realgar clasts with Au-bearing iron-sulfide rims, indicating localized mineralization both post- and pre-brecciation. The consistent association between fragmentation and mineralization of realgar clasts indicates that the same process that fragmented the realgar also allowed the ore fluid to be reintroduced to previously mineralized rocks. Reactivation along a fault may have fragmented the realgar and reduced pressure sufficiently to allow the Au-bearing fluid to encounter an area that was previously cooler and collapsing, thus mineralizing the same area twice. v ACKNOWLEDGEMENTS First and foremost, I would like to thank Dr. Jean Cline and Dr. Adam Simon, my committee co-chairs, for always making themselves available to answer questions, discuss my ideas, advise me, and provide direction in every aspect. Thank you to Dr. John Muntean for being an actively involved third committee member and providing significant insight into my research and to Dr. Barbara Luke, the graduate college representative. I am also grateful to Barrick Gold Corporation for funding this project, and specifically the Cortez Hills GeoServices group. Special thanks to Page Anderson for all of the time dedicated to assisting me in so many aspects of this project, including sample collection, compiling background information, and suggestions on the presentation of data. Thank you to Roger Bond for handling the financial logistics, allowing Page Anderson to dedicate so much of her time, and for making this project possible. Also, thank you to Tom Whittle, Kerry Hart, and Roger Bond for originally getting this project started. I would also like to acknowledge the Society of Economic Geologists (SEG), Northern California Geological Society, UNLV Graduate and Professional Student Association (GPSA), and the UNLV Geosciences Department for the fellowships, grants, and scholarships that I was awarded throughout my graduate studies. Additionally, special thanks to everyone who helped me collect data and the hours of discussion that came along with it, including Alan Koenig at the USGS Denver LA- ICP-MS lab, Christopher Adcock and Minghua Ren at the UNLV EMiL lab, and Oliver Tschauner and John Howard at the UNLV XRD lab. vi TABLE OF CONTENTS ABSTRACT ..................................................................................................................... iii ACKNOWLEDGEMENTS.............................................................................................. vi LIST OF TABLES ........................................................................................................... ix LIST OF FIGURES .......................................................................................................... .x CHAPTER 1 INTRODUCTION ....................................................................................1 CHAPTER 2 BACKGROUND ..................................................................................... 4 Carlin-type Gold Deposit Genetic Models and Geologic History .......................... 4 Cortez Hills Breccia Zone ....................................................................................... 7 CHAPTER 3 METHODS ............................................................................................
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