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Geochemistry and Mineralogy of Eastern Ag-Rich Epithermal Veins in the Midas District, Nevada, USA




Chitwood, Rue

Type of Degree



Geology and Geography


Five Ag-rich epithermal veins were recently discovered in the eastern section of the Midas district in northern Nevada. Initial data collected indicated that these veins had a significantly higher quantity of silver relative to the main production veins at Midas. The focus of this study was on the geochemistry, ore petrography, and geochronology of three of the five eastern veins: Charger Hill, GP, and Ace. Geochemical ICP-MAS results indicate that Ag and Se occur strongly together suggesting that Ag-selenides are the dominant ore minerals while Au does not appear to have a significant correlation with any elements. An overall Ag:Au ratio for the eastern veins is 176:1. Longitudinal sections indicate that there is an elevation control on mineralization. In addition, mineralization preferentially occurs within the Elko Prince Formation and mafics sills. There is also no significant correlation between vein thickness and ore grade. SEM-EDAX results document most Ag-selenide minerals are an aguilarite-naumannite solid solution, and electrum, which is higher in Au than Ag at the atomic level, occurs in lower quantity relative to the main production veins. Overall, the paragenetic sequence suggests that the minerals were co-precipitated and overall mineralogy is similar to that of the Colorado Grande vein with the exception of the abundance of fluorite. The low quantity of Au appears to be the result of hydrothermal fluid transport dynamics, colloidal/nanoparticle transport, and the higher density of Au vs Ag implying that Au particles would not travel very far from the source. Based on the petrographic, SEM-EDAX, and ICP-MAS evidence found in this study, the eastern veins appear to be a distal product of the same large-scale magmatic-hydrothermal event that formed the main productions veins.