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

摘要

Five Ag-rich epithermal veins were recently discovered in the eastern section of the Midas district in northern Nevada. Initial geochemical analysis indicated that these veins had significantly higher silver values compared to the historic veins at Midas. The focus of this study was on the geochemistry, ore petrography, and geo-chronology of three of the five eastern veins: Charger Hill, GP, and Ace. Geochemical ICP-MAS results indicate that Ag and Se vary strongly together, suggesting that Agselenides are the dominant ore minerals, while Au does not appear to have a significant correlation with any other elements. An overall Ag/Au ratio for the eastern veins is 176:1, which is significantly more disproportionate to the 12:1 Ag/Au ratio for the historic veins. Longitudinal sections indicate that mineralization is confined to an elevation range of 4300-4900 ft, which indicates mineralization has a strong relationship to temperature/pressure conditions. In addition, mineralization cuts across all rock types and fault structures. There is also no apparent correlation between vein thickness and ore grade. SEM-EDAX results document that most Ag-selenide minerals are an aguilarite-naumannite solid solution, and electrum, which contains more Au than Ag, occurs in lower quantity relative to the historic veins. In general, the paragenetic sequence suggests that these minerals were co-precipitated, and the overall mineralogy is similar to that of the Colorado Grande vein, one of the historic veins, with the exception of the higher abundance of fluorite in the eastern veins. The low quantity of Au in the eastern veins relative to the historic veins 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 petrographic, SEM-EDAX, and ICP-MAS evidence the eastern veins appear to be a distal product of the same large-scale magmatic-hydrothermal event that formed the historic veins at Midas