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Geology of the Chukar Footwall Mine, Maggie Creek District, Carlin Trend, Nevada

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Geology of the Chukar Footwall Mine, Maggie Creek District, Carlin Trend, Nevada University of Nevada, Reno GEOLOGY OF THE CHUKAR FOOTWALL MINE, MAGGIE CREEK DISTRICT, CARLIN TREND, NEVADA A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology By Juan Antonio Ruiz Parraga Dr. Tommy Thompson, Thesis Advisor May, 2007 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1447807 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 1447807 Copyright 2007 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, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. © Juan Antonio Ruiz Parraga, 2007 All rights Reserved Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE GRADUATE SCHOOL University of Nevada, Reno We recommend that the thesis prepared under our supervision by JUAN ANTONIO RUIZ PARRAGA Entitled Geology Of The Chukar Footwall Mine, Maggie Creek District, Carlin Trend, Nevada be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE my B. Thompson; Ph.D., Advisor Richard A. Sch1 u D., Committee Member Victor Vasquez, Ph.D., Graduate School Representative arsha H. Read, Ph. D., Associate Dean, Graduate School May, 2007 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT The Chukar Footwall mine forms part of the NW striking Carlin trend in northern Nevada, and lies beneath the southwest highwall of the Gold Quarry, a world-class deposit with total 1999 reserves, resources, and mineral inventories in excess of 24M oz gold. The Chukar Footwall orebodies are hosted in the planar to wispy silty limestone and calc-silicates of the Silurian-Devonian Roberts Mountains Formation (SDrm). The Devonian Popovich Formation (Dp), a micritic package locally hosting economic gold mineralization, structurally overlies the SDrm. The Raven dike intrudes the DSrm at several mine levels along northwest- trending structures. The dike is composed of abundant dark green, millimeter sized subhedral-anhedral phenocrysts in a light colored aphanitic groundmass. It is altered to clay and pyrite, but does not appear to be an ore fluid feeder structure. The time of the Raven dike emplacement into NNW-trending structures has been determined by U-Pb zircon geochronology, producing an age of 200.3±5.1 Ma (early Jurassic). Apatite separates from the Raven dike yielded a fission track pooled age of 17.7±3.7 Ma for a thermal event that may have been associated with mineralization at Chukar Footwall; however, initial cooling began at 26.2±5.5 Ma. These rocks were probably deformed during the Antler orogeny, generating the Chukar anticline, a northeast trending open structure with a subhorizontal plunge. High gold grades are commonly situated along the hinge and the southeast limb and in small parasitic folds. A conjugate system of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ii structures is represented by northwest and northeast striking faults, where the former cuts the latter. Kinematic indicators denote predominantly oblique normal slips for both fault sets. The Chukar Footwall mine exhibits the hydrothermal alteration assemblages typical of Carlin-type gold deposits: (a) decalcification, (b) dolomitization, (c) silicification, (d) argillization, and (e) baritization. The uniqueness of this deposit relative to the more typical Carlin-type is the (1) sharp boundaries between fresh and altered rocks, and (2) presence of abundant visible gold. Gold mineralization is spatially related with strong decalcification in the vicinity of intersections of northeasterly structures with the Chukar anticline. Coarse, visible gold occurs in decarbonatized silty limestone along fractures of all orientations as well as along bedding planes. Also, visible gold is present in late barite veinlets, coprecipitating with the latter phase. Stable isotope transects reveal systematic 5180 and 513C shifts in wall rocks where approaching structures and/or changes of intensity of hydrothermal alteration. At a mine scale, these shifts define a generalized trend toward lighter oxygen and heavier carbon values for the wallrocks from the higher to the deeper mine levels, with a negative correlation between oxygen and carbon. Late stage calcite veins 5 13C values lie within the values of the nearby altered limestone, suggesting the carbon could have been derived from the wallrocks. Isotope and fluid inclusion data point to gangue precipitation from interaction between meteoric water and the wallrocks. 634S from sulfides and sulfates are consistent Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. iii with a sulfur derivation from sedimentary sources. A correlation has been found between the presence of visible gold and the isotopic signature of barite. Barite precipitated from meteoric water as it descended into open structures. Microthermometric data from late stage barite±gold veins indicate significant variation in both homogenization temperatures and salinities with depth. Mean salinities and mean homogenization temperatures from the deeper mine levels range from 3.03 to 3.18 wt % NaCI equiv and between 183.2° and 179.7° C. In contrast, lower salinities (-1.16 wt % NaCI equiv) and homogenization temperatures (177.3°C) were recorded in samples from shallower levels. Similarly, recorded data from late calcite yielded very low salinities (up to 0.71 wt % NaCI equiv) and low homogenization temperatures (between 87.6° to 117° C). Neither C 0 2 nor CH4 were detected in calcites. These data suggest the participation of, at least, two contrasting fluids. The Chukar Footwall orebody forms part of the Gold Quarry gold system to which same basic genetic ideas may apply. The metallogenic evolution of the Chukar Footwall deposit began with a significant pre-ore episode of dissolution- collapse breccia between the Roberts Mountains Formation and the Popovich Formation that formed a semi-impermeable cap for later hydrothermal fluids. During ore-stage, ore fluids were channelized along major active NE-striking structures, and probably micron-size Au and base metals precipitation took place as a result of sulfidation and a shift toward higher pH values. During Late Miocene extension, NW-striking faults were open for descending cool, weakly saline meteoric fluids and ascending relatively hotter, more saline hydrothermal Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fluids, possibly producing metal remobilization and visible gold precipitation along structures. It seems that the common link among the Gold Quarry gold systems is its structural relation to the Deep Sulfide Feeder and Chukar Gulch faults, which served as a major conduits for hydrothermal fluids. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS I would like to thank Dr. Tommy Thompson and Dr. Richard Schweickert for being my M.S. thesis advisors, and for their insights on the geological aspects of north-central Nevada. Dr. Thompson gave me the opportunity and the appropriate financial assistance for this research through the Ralph J. Roberts Center for Research in Economic Geology (CREG). Thanks to the Chukar Footwall geologists Joe Sagar, Kevin Creel, and Mike Robinson for their logistical support, patience, and ideas about Chukar Footwall’s geology during underground mapping. My appreciation is extended to Newmont for letting me work at Chukar Footwall, giving me free access to information, and conducting the ICP-MS analyses. I would also like to thank to Dr. John McCormack (UNR) for the SEM analyses and comments, Dr. Ray Donelick and Paul O’Sullivan for performing the critical U/Pb zircon data and apatite fission track dating from the Raven dike. Also thanks to Dianna Neff for sharing the splendid view of Gold Quarry. Finally, discussions with my fellow students Keith Campbell, Craig Mach, and Robert Wood are greatly appreciated, i Salud a todos! Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. v i TABLE OF CONTENTS Pag* Title page Copyright page Signature page Abstract i Acknowledgements v Table o f contents v i List of tables viii List of figures xii Plates x iii 1. INTRODUCTION 1 Purpose and Objectives 5 Methodology 6 Previous Work 7 2. PHANEROZOIC GEOLOGICAL EVOLUTION OF 8 NORTH-CENTRAL NEVADA 3. LITHOSTRATIGRAPHIC SETTING OF THE CHUKAR 24 FOOTWALL DEPOSIT Silurian-Devonian Roberts Mountains Formation 26 Devonian Popovich Formation 3 0 Stylolites 31 Metasomatism: Diopside Hornfels 32 Breccias Bodies: The Contact Zone 33 Intrusive Rocks:
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