Petrography and Geochemistry of Hypogene Mineralization at the Mike Gold - Copper Deposit, Eureka County, Nevada
AuthorParrish, Benjamin Michael
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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The Mike Carlin-type gold and copper deposit is a exploration property within the central Carlin trend, 9 miles northwest of the town of Carlin in Eureka County, Nevada. The deposit is hosted in Paleozoic lower-plate rocks in the Carlin window, a fenster in the Roberts Mountains allochthon, and is localized along the Good Hope fault and Tusc Corridor in the historic Maggie Creek mining district. Gold and copper mineralization is localized along the intersection of northeast-striking faults that intersect the northwest-striking Good Hope fault, and is stratigraphically controlled. The deposit is characterized by an extensive leached zone of iron and copper oxide minerals where supergene copper enrichment has occurred at paleo-water table levels. Previous work has been done in the form of a master's thesis and Newmont reports to characterize the geology of the Mike deposit as well as mineralogy and distribution of supergene copper. The primary focus of this thesis is characterization of the unoxidized portion of the Mike deposit to determine the primary source of hypogene copper through bulk deposit geochemistry and petrographic analysis of sulfide minerals. This thesis focuses on statistical analysis of geochemical data sets and petrographic analysis of ore and gangue mineralogy and textures. Combined data from multiple Spearman coefficient correlation matrices, correspondence analysis, and principal component analysis confirm a strong association between gold, silver, arsenic, antimony, thallium and lesser association with tellurium. Notable correlations also exist between copper, bismuth, tin, tellurium, and tungsten and moderate correlations between lead, silver, and molybdenum. Geochemical correlation patterns of gold are similar to those of other Carlin-type gold deposits where gold correlates with arsenic, antimony, and thallium, though the Mike deposit gold also shows correlations with silver and tellurium. Copper correlations suggest a magmatic source of hypogene copper based on bismuth, tin, and tungsten associations with copper and indicates that the copper and gold ore events are discrete. Petrographic analysis has identified silicification and carbonatization replacement textures as well as open space quartz vein textures, skarn, and hornfels. Identified gangue minerals include quartz, multiple carbonate phases, sericite, smectite, kaolinite, adularia, wollastonite, chondrodite, magnetite, vesuvianite, epidote, diopside, hematite, and goethite. Sulfide ore-related mineral phases include chalcopyrite, bornite, chalcocite, covellite, digenite, pyrrhotite, bournonite, marcasite, and pyrite. Petrographic observations indicate that copper, zinc, and lead sulfides post-date Carlin-type gold mineralization, which is in the form of arsenic-rich pyrite. δ34 S analyses implies an igneous source for base metal sulfides with some assimilation of sedimentary sulfur, and depositional temperature range of sphalerite and galena between 401° C - 355° C ± 20°. Qualitative examination of base metal sulfide and Carlin-type pyrite rims by SEM-EDS analysis shows minor arsenic enrichment in rims and overgrowths on pyrite where base metals are also present, supporting paragenetic interpretations.