Petrographic Analysis of Bonanza Epithermal Vein Textures at Buckskin National and Fire Creek Deposits, Northern Nevada
Type of DegreeMaster's Thesis
Geology and Geography
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The Buckskin National and Fire Creek epithermal deposits, which are located in the Northern Great Basin in northern Nevada, exhibit a variety of gangue-mineral vein textures. Gangue minerals, which are valueless minerals, commonly include quartz, chalcedony, adularia, and calcite, and occur with ore-minerals in the veins. In this study, petrographic analysis was conducted of gangue textures and associated ore minerals in two deposits in both transmitted and reflected light. The petrographic details were then compared to Sleeper and Midas deposits, which are well-known epithermal ores in northern Nevada. In addition, X-ray powder diffraction (XRD) and hot-cathode CL data augmented petrographic interpretations. Silica vein textures found in the Buckskin National and Fire Creek deposits generally exhibit jigsaw quartz textures in colloform-bands, replace calcite, and also define enigmatic fibrous-acicular structures. This fibrous-acicular texture appear to be composed of pseudomorphs after unknown mineral. Other vein textures that also found in two deposits include comb, flamboyant, plumose, and groups of pseudomorphs after bladed calcite such as lattice-bladed, parallel-bladed, and pseudoacicular. XRD analysis indicates that the Buckskin National veins commonly contain quartz, kaolinite, and minor amount of adularia, whereas the Fire Creek samples consist of quartz, later calcite, and adularia. The CL microscope provides important additional information on original silica textures and as silica phases emit distinct CL colors that indicate hydrothermal origins of quartz in the veins of this study. The CL technique also sheds light on textures of colloidal precursors that have already (re)crystallized to quartz and chalcedony in some colloform bands. The Buckskin National deposit and the Fire Creek deposit in northern Nevada contain similar silica textures to other epithermal deposits and can exhibit specific characteristics that are controlled by the complexity of hydrothermal events, ore-fluid evolution, and water-rock chemical reactions with wall rocks of varying composition. However, results from this study have implications for interpreting vein textures in other epithermal deposits around the world including the Sleeper, Midas, and Mule Canyon deposits in the US, Koryu and Hishikari deposits in Japan, and the epithermal deposits in Queensland, Australia.