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Igneous Geology of the Carlin Trend, Nevada: the Importance of Eocene Magmatism in Gold Mineralization

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Igneous Geology of the Carlin Trend, Nevada: the Importance of Eocene Magmatism in Gold Mineralization University of Nevada, Reno Igneous Geology of the Carlin Trend, Nevada: The Importance of Eocene Magmatism in Gold Mineralization A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology by Michael Walter Ressel, Jr. Dr. Christopher D. Henry/Dissertation Advisor May, 2005 UMI Number: 3210296 UMI Microform 3210296 Copyright 2006 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, MI 48106-1346 Copyright ©, 2005 by Michael Walter Ressel, Jr. All Rights Reserved i ABSTRACT Igneous rocks of five ages are present in the Carlin trend, Nevada, and include: 1) Paleozoic basalt of the Roberts Mountains allochthon, 2) the Jurassic (~158 Ma) Goldstrike intrusive complex, which includes the Goldstrike diorite laccolith and abundant dikes and sills, 3) a Cretaceous (112 Ma) granite stock, 4) lavas and intrusions of the Emigrant Pass volcanic field and widespread epizonal plugs and dikes of Eocene (~40-36 Ma) age that range from rhyolite through basalt, and 5) Miocene (15 Ma) rhyolite lava and tuff. Jurassic and Eocene igneous rocks are by far the most important volumetrically and are spatially associated with nearly all ore deposits of the Carlin trend. This study focuses on the field relations, isotopic dating, and geochemistry of Eocene dikes that intrude sedimentary rocks in many deposits of the Carlin trend, because they are the youngest pre-mineral rocks and have simpler alteration histories than other host rocks. In the Beast, Genesis, Deep Star, Betze-Post, Rodeo-Goldbug, Meikle-Griffin, and Dee-Storm deposits, Eocene dikes are altered, commonly mineralized, and locally constitute ore. Gold-bearing dikes and sedimentary rocks have similar ore mineralogy, including arsenian pyrite, marcasite, and arsenopyrite, with late barite and stibnite. At Beast, as much as half the ore is hosted in a 37.3 Ma rhyolite dike. Post-gold alunite is ~18.6 Ma. At Meikle and Griffin, porphyritic dacite dikes yield concordant U/Pb zircon 40 39 and Ar/ Ar biotite emplacement ages of ~39.2 Ma, and illite from the same QSP- 40 39 altered dacite, with as much 9 ppm Au, yields similar, although imprecise Ar/ Ar ages. Thus, gold mineralization at these deposits closely followed emplacement of Eocene dikes. Eocene dikes and volcanic rocks approximately coeval with ore deposition provide limits on the depth of formation of Carlin trend gold deposits and on the degree of tilting and extension since about 40 to 36 Ma. Altered and mineralized Eocene dikes at ii the Beast, Genesis, Deep Star, Betze-Post, Rodeo-Goldbug, Meikle-Griffin, and Dee deposits were shallowly emplaced as indicated by their strongly porphyritic and commonly glassy, lithophysal, spherulitic, and felsitic textures. An estimated ≤2-km depth of dike emplacement is based on post-Paleozoic offset on faults between the northern Carlin trend and basal lavas of the Emigrant Pass volcanic field. This estimate is significantly less than the ~4-km or more from fluid inclusion studies. A shallow origin is consistent with steep dips of dikes and the subhorizontal volcanic rocks of the northern Emigrant Pass volcanic field, which suggest only modest tilting and extension since ~40 Ma. A demonstrated spatial and temporal link between Eocene magmatism and gold mineralization is suggestive of a genetic link. Abundant Eocene silicic dikes of the northern Carlin trend are apophyses of much larger intrusions at depth. As many as six Eocene plutons are inferred for the Carlin trend based on the age, distribution, and composition of dike sets, and of associated aeromagnetic properties. Plutons are aligned approximately north to south, with generally older plutons and dikes to the north and younger ones to the south, similar to regional trends. Based on geologic relations and published fission-track data, Eocene plutons are also more deeply buried to the north. Carlin-type gold deposits in northeastern Nevada have been variously interpreted as partly syngenetic with Paleozoic carbonate rocks, products of Mesozoic contraction and metamorphism with or without significant magmatism, and of Tertiary age and related or not to magmatism, metamorphism, and/or large-scale extension. A recently established Eocene age for major gold introduction narrows the possibilities, and two principal models have emerged: one involving Eocene magmatism as the heat source to drive shallow hydrothermal circulation and the other advocating deeply sourced metamorphic fluids released into the upper crust during regional extension. Critical to the iii latter argument is the temporal association of extension to gold mineralization, which as yet, is not demonstrated. We argue that Eocene magmatism in the form of large underlying plutons, was the major recognized process that affected the Carlin trend during gold mineralization. These plutons supplied the heat that drove discrete hydrothermal systems. iv ACKNOWLEDGEMENTS Don Noble introduced me to igneous rocks and ore deposits of Nevada by, as he would say, “brute force”. For his part, there is never a lack of curiosity, dedication, or enthusiasm for geology. I am thankful for the support I received from the Ralph J. Roberts Center for Research in Economic Geology (CREG) for most of my studies and for the discipline demanded by this program. CREG, under the direction of Tommy Thompson, has fueled interest in studies of Carlin-type deposits in Nevada and provided a much needed forum for collaboration and exchange of ideas. Barrick Goldstrike Mines, Newmont Mining Corporation, the U.S. Geological Survey and namely, Steve Peters and Alan Wallace, the Society of Economic Geologists, the Geological Society of America, and Lisa Shevenell of the Nevada Bureau of Mines and Geology (NBMG) provided additional funds during those many pinches. The mines of the Carlin trend are fascinating and extremely hectic places. This work would not have been possible without the support of Barrick and Newmont and in particular, the many geologists from these companies who helped out at every stage, even when gold prices languished. They shared their knowledge without hesitation, were eager to hear of my progress, and rarely passed up a chance to “talk rock” at the outcrop. And like Ralph J. Roberts, these geologists shared with me their passion for gold. Dave Haney gave me a place to live during several field seasons, and what a place it is against the beautiful backdrop of the Ruby Mountains in Lamoille. The Mackay School of Mines provided me with many of the tools I needed to learn: good and knowledgeable faculty and staff, excellent analytical facilities and library, and a bunch of fellow graduate students and friends in the same boat as me. I am fortunate to have collaborated with many from Mackay and elsewhere. I especially thank Chris Henry for his interest and commitment to our work in northeastern Nevada and his v constant egging for resolution. Quite simply, were it not for Chris’ support, this study would not have been completed. I could count on Jim Wise, my good friend, geologist, and officemate, for many lively discussions. Jim never refused to read a manuscript, listen to a practice talk, or flee the office on a moment’s whim. I appreciate the long and sometimes heated discussions on Carlin-type deposits with Marcus Johnston and Greg Ferdock. My committee, including Greg Arehart, Dhanesh Chandra, Kent Ervin, Chris Henry, and Alan Wallace, supported my efforts, was fair, and patient. My “adjunct” committee, consisting of Jim Carr, Li Hsu, and Larry Larson, took the time to write and grade some of my comprehensive exams. Many long days and nights were spent in labs at NBMG doing horrible things to rocks and drinking coffee from beakers. Mario Desilets, Dave Davis, Paul Lechler, and Bret Pecoraro always kept their lab doors open for me. Likewise, I thank John McCormack for providing skillful help with the SEM and electron probe. In addition, I received lots of assistance in telling time via isotope geology. Chris Henry (NBMG), Matt Heizler and Bill McIntosh from the New Mexico Bureau of Mines and Mineral Resources, Jim Mortensen from the University of British Columbia, and Terry Spell from UNLV provided expert analyses of often difficult samples and helped me to interpret the results. In the end, Newmont with the help of Marnie Muirhead, John Jory, and Leroy Schutz, gave me time away from work to focus on the project’s final stage. Very importantly, friends and family have always been supportive in spite of not always understanding my motivations. My wife, Maggie, and our kids, Anna, Chris, and Peter, are a constant source of happiness for me. Their patience and support have been truly amazing. Thank you all, Mike Ressel - April 8th, 2005 vi PREFACE This dissertation contains three stand-alone manuscripts presented in three chapters. Two of the manuscripts have been published in peer-reviewed publications; the third is in review as of April 2005. The papers are arranged in draft form, with text in front and figures, tables, and appendices following text for each paper. At the start of each paper is an introductory section that includes a detailed review of the pertinent literature on the geology of the Carlin trend and Carlin-type deposits. Appendices at the end of each paper describe the analytical methods used and contain supplemental data. This research evolved from a regional synthesis of Eocene magmatism in the northern Great Basin to more focused study of the relationships of magmatism to ore deposition in the Carlin trend gold belt of northeastern Nevada. In the end, however, the detailed work on the trend led to new insights on the regional scale. Thus, this research has gone “full-circle” over the course of several years.
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