University of Nevada, Reno Relationship of base-metal skarn mineralization to Carlin-type gold mineralization at the Archimedes gold deposit, Eureka, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Matthew H. Hastings Dr. Tommy B. Thompson/Thesis Advisor December 2008 1460760 1460760 2009 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by MATTHEW H. HASTINGS entitled Relationship of base-metal skarn mineralization to Carlin-type gold mineralization at the Archimedes gold deposit, Eureka, Nevada be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN GEOLOGY Tommy B. Thompson, Ph.D., Advisor John L. Muntean, Ph.D., Committee Member Victor R. Vasquez, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Associate Dean, Graduate School December, 2008 i Abstract The Ruby Hill Mine is comprised of three distinctly different gold orebodies occurring near the town of Eureka, Nevada. The deposits are hosted in the Ordovician carbonates immediately adjacent to the Graveyard Flats intrusion, a Cretaceous andesite porphyry. To the west, the Bullwhacker Sill outcrops and plunges gently too the east, presumably to merge with the Graveyard Flats at depth. However, the initial discovery of gold mineralization at Ruby Hill Mine was made over 1000 feet above this skarn and approximately a mile to the west. The two orebodies that make up Archimedes (East and West) appear to be predominantly Carlin-type in their occurrence. The West Archimedes deposit is a sub-horizontal cigar-shaped orebody consisting of oxidized jasperoid and decalcified limestone. Oxidized gold mineralization is principally found in siliceous to variably decalcified breccias associated with intersections of structures such as the East Archimedes fault and the Blanchard-Molly fault system (Dilles et al., 1996). The auriferous jasperoid of West Archimedes is continuous with that that forms the wider orebody to the east. Unlike West Archimedes, the deepest portion of East Archimedes is also host to significant base-metal mineralization with local high-grade sulfide-rich gold. It is here that observations were first made relating to the close proximity of disseminated gold mineralization and base-metal skarn (Dilles, 1996). Gold mineralization also exists approximately 1700-2100 ft below the West Archimedes pit as arsenic sulfide and barite- rich hydrothermal breccias. This orebody is known as Ruby Deeps and is interpreted as being contemporaneous with the Archimedes orebodies. It is thought to be the result of the trapping of ore fluids against the hanging wall of the Bullwhacker Sill. Based on several different lines of evidence drawn from the study of the different orebodies, the results suggest that the mineralization at Archimedes is related to a much later Carlin-type event rather that simply being the latest parts of the magmatic- hydrothermal system responsible for the base-metal skarn. Detailed petrographic studies and analysis using scanning electron microscope techniques provide the most compelling evidence. Arsenian pyrite replaces andradite garnets in high-grade intervals in deep East Archimedes and barite is commonly found as a late hydrothermal product. Well- preserved replacement textures and unique paragenetic relationships offer excellent examples of overprinting by the later Carlin-type event. Additionally, the fact that faults that cut the Cretaceous Bullwhacker sill at depth are hosts to Carlin-type gold mineralization in the Archimedes orebody is a compelling line of evidence for post- intrusion structures serving as mineralizing fluid conduits. Apatite fission track analysis appears to be supportive of this conclusion, exhibiting potentially two different populations of fission tracks with cooling periods ranging from about 100 Ma-70 Ma as well as 35 Ma-9 Ma. These ages are consistent with typical Carlin-type ages as well as the intrusion of the Graveyard Flats. (cid:1)18O and (cid:1)13C ratios were calculated from carbonates found both in veinlets around mineralized structures as well as wall rock alteration. (cid:1)18O values show strong depletions in the deeper parts of East Archimedes as well as along fault zones within the West Archimedes pit. The data obtained from petrographic studies supported by the AFTA data appear to conclusively suggest a dual event model for the genesis of the unique Archimedes deposit, with each period of mineralization potentially separated by as much as 80 million years. ii Acknowledgements This thesis would not have been possible without funding from the Ralph J. Roberts Center for Research in Economic Geology (CREG) at the University of Nevada, Reno. An enormous debt of gratitude is owed to Ralph Roberts and the many others who have helped establish this world-class program for higher education specifically related to the science of ore deposit geology. Specific thanks are due to Shelley Harvey, the woman who simultaneously does many things at once to keep CREG moving forward and the students moving period. Thanks are also due to Dr. Tommy B. Thompson, my advisor, who educated me in almost everything I know about ore deposits as well as many other geology-related subjects. Without his input and constructive criticism, this thesis would not be possible. I also would like to thank Dr. John L. Muntean, for serving on my committee and sharing his time, comments, and ideas that contributed significantly to improve the final product. Thanks also go out to Dr. Victor Vasquez, who also served on my committee and offered a unique perspective and intuitive comments that helped improve both my ability to explain my work and most importantly to communicate the most important parts of this study to people who had never seen nor worked at Archimedes. A huge amount of gratitude is owed to Barrick Gold Company, most notably for donating generously to CREG every year and for continuing to support students through their interest in CREG projects. I would like to thank personnel at Barrick’s Ruby Hill Mine for putting up with me rifling through their core, using their office space, and “wandering aimlessly” around their open pit. Their dedication to their jobs, the mine, and most of all safety was an excellent first example for me of how a world-class miner and its employees should function. Specifically, I deeply appreciate the time and patience given by Kevin Russell. He is truly the end-of-the-line authority on Archimedes and his guidance was instrumental in focusing the development of this thesis. Also, I would like to thank James Berry for his help in providing expertise in pit-mapping and, more importantly, a fresh and inquisitive mind that forced me to know what I was talking about to defend against his thoughtful and candid questions. Also deserving thanks are Barrick Exploration geologists Karl Marlowe, Dave Arbonies, and Paul Dobak. These gentlemen were more than generous when it came to logistics and sharing of data and the amount of work that was able to be done because of them smoothing the way ahead cannot be measured. I would like to thank my friends and coworkers in CREG for their assistance, criticism (both constructive and destructive), and levity. Notably, Wes Sherlock gave much of his time to provide assistance with mineral identification via the SEM as well as reflected light microscopy. Rick Trotman is a good friend who is not afraid to challenge me at every turn and force me to explain my reasoning for things before accepting them to be true. Keith Campbell and Robert Wood are some of the most knowledgeable geologists I’ve ever met and their patience and willingness to answer my many questions was hugely beneficial. Finally, I would like to thank my beautiful wife Theresa, for her understanding, compassion, and motivation. She supported me while in school and forced me to take breaks when I needed to and go back to work when I had to. I would never have been able to finish this thesis without her standing beside me and pushing me forward. iii Table of Contents 1. Introduction 3. Methods 6. Eureka District History 10. Tectonic History 13. Stratigraphy 22. Igneous Rocks 35 Structure 40. Mineralization and Alteration 41 – Jasperoid Hosted Gold Mineralization 47 – Base Metal Skarn Mineralization 50 – Carlin-style Overprinted Skarn Mineralization 57 – Sulfidic Carlin-type Mineralization 64. Age Dating 65. Apatite Fission-Track Dating 73. Stable Isotope Analysis 81. Discussion 93. Conclusions 95. Works Cited 102. Appendices iv List of Tables Table 1. – Summary of apatite fission-track data Pg. 70 Table 2 – (cid:1)34S Isotope values Pg. 80 Table 3 – Comparison of Au-skarn types Pg. 85 Table 4 – Eureka district whole-rock geochemistry Pg. 102 Table 5 - O & C Stable Isotope values Pg. 105 Table 6 – Locations and descriptions for AFTA samples Pg. 108 v List of Figures Figure 1. - Location map Pg. 2 Figure 2. – Schematic 3D cross section Pg. 3 Figure 3. – Water door photo from FAD underground Pg. 7 Figure 4. – Tectonic map showing orogenic rocks Pg. 11 Figure 5. – Modified stratigraphic column for N. Eureka district Pg. 15 Figure 6. – Windfall Formation photomicrographs Pg. 17 Figure 7. – Goodwin Limestone photomicrographs Pg. 20 Figure 8. – Ninemile Formation photomicrographs Pg. 21 Figure 9. – Generalized district geologic map Pg. 23 Figure 10. – Propylitized andesite porphyry photographs Pg. 26 Figure 11. – TAS-diagram for igneous rocks in N. Eureka District Pg. 27 Figure 12. – Graveyard Flats photomicrographs Pg. 29 Figure 13. – Chondrite-normalized REE plot for igneous rocks Pg. 33 Figure 14. – Photographs of 426 fault Pg. 34 Figure 15. – Panorama of E. Arch pit with superimposed structure Pg. 37 Figure 16. – Schematic 3D cross section with drillholes superimposed Pg. 41 Figure 17. – W. Archimedes jasperoid core photograph Pg. 42 Figure 18. – Oxidized E. Archimedes photomicrographs Pg. 44 Figure 19. – SEM backscatter image of Fe-sulfates Pg. 46 Figure 20. – SEM backscatter image of Cu-Co-Sn mineral Pg. 46 Figure 21. – E. Archimedes skarn core photograph Pg. 47 Figure 22. – Skarn mineralogy photomicrographs Pg. 49 Figure 23. – Photomicrographs of overprinted skarn minerals Pg. 52 Figure 24. – SEM backscatter image of overprinted garnets Pg. 53 Figure 25. – SEM backscatter image of overprinted high-grade Au zone Pg. 55 Figure 26. – SEM backscatter image of paragenetic relationships in Au zone Pg. 56 Figure 27. – Ruby Deeps core photograph Pg. 57 Figure 28. – Ruby Deeps orebody photomicrographs Pg. 59 Figure 29. – SEM backscatter image of arsenian rims from Ruby Deeps Pg. 60 Figure 30. – SEM backscatter image of zoned arsenian marcasite overgrowths Pg. 61 Figure 31. – Paragenetic diagram for Archimedes system Pg. 63 Figure 32. – AFTA sample locations Pg. 65 Figure 33. – Diagram illustrating effect of Cl on apatite annealing temperature Pg. 66 Figure 34. – Photomicrograph of apatite with fission tracks and inclusions Pg. 67 Figure 35. – Constraints on cooling periods from AFTA data Pg. 68 Figure 36. – Thermal history reconstruction for composite AFTA data Pg. 71 Figure 37. – Average sample parameters for AFTA analysis Pg. 72 Figure 38. – Carbon vs. Oxygen stable isotope ratios from Archimedes Pg. 74 Figure 39. – Isotopic exchange ratios for vein vs. wallrock carbonates Pg. 75 Figure 40. – Ranged (cid:1)13C isotopic values from Archimedes pit Pg. 77 Figure 41. – Ranged (cid:1)18O isotopic values from Archimedes pit Pg. 78 1 Introduction The Archimedes system is located approximately one mile west of the town of Eureka, in east-central Nevada (see Figure 1). Archimedes is an interpreted Carlin-type system comprised of three orebodies: West Archimedes, East Archimedes, and Ruby Deeps. It is currently operated by Barrick Gold Co., and retains the name Ruby Hill Mine. Unlike the lead-zinc-silver/gold mineralization mined from Ruby Hill in the late 1800’s and early 1900’s, the economic gold mineralization at Archimedes is conspicuously devoid of lead or zinc. The Ruby Hill Mine is an oxide operation, involving the crushing and cyanide heap-leaching of the ore. The system was originally separated into two different orebodies (East and West Archimedes) to ease tensions with locals concerning water rights. West Archimedes was mined first, as it was above the water table. As the price of gold rose, East Archimedes became more economically viable and is currently producing. West Archimedes can best be described as an oxidized carbonate-hosted, structurally and lithologically controlled gold deposit. The result is a 780 meter-long cigar shaped orebody plunging approximately S60E at -10o. East Archimedes is the second part of the system, and appears to be a more steeply dipping root to the West Archimedes orebody. East Archimedes is a partially oxidized, steeply plunging, auriferous breccia body. Perhaps the most interesting thing about this otherwise obviously Carlin-type system (as well as the subject of this project) is the proximity of East Archimedes to the Graveyard Flats andesite porphyry and associated base-metal skarn mineralization. The schematic cross section in Figure 2 defines the inferred relationships of the gold orebodies to intrusive rocks as well as a base metal skarn. The Ruby Deeps orebody is 2 hosted in the Cambrian Windfall Formation at depths in excess of 1700-2000 ft from the original surface. This orebody occurs in the footwall of a shallowly dipping andesite porphyry sill. This sill is interpreted to have originated as an offshoot of the Graveyard Flats intrusive that was likely emplaced along a preexisting thrust fault. Ruby Deeps is both mineralogically and morphologically different from the Archimedes system, and is characterized by barite/arsenic sulfide hydrothermal breccias with disseminated auriferous arsenian pyrite. The purpose of this project is to explain and differentiate between different types of gold mineralization seen within the Archimedes and Ruby Deeps deposits. The controversy that has arisen from the proximity of these different orebodies and the base-metal skarn at depth has been drawn from the similarities of distal carbonate- replacement deposits and Carlin- type systems. The purpose of the research presented in this study is to evaluate and observe the relationship between the skarn mineralization at depth and the Archimedes and Ruby Deeps orebodies. Conclusions drawn from Figure 1. – Location map for Archimedes showing proximity to prolific Battle Mountain-Eureka trend of ore deposits this research will hopefully shed