Geology, Geochemistry and Geochronology of the mid-Miocene, Low-Sulfidation Epithermal Gold-Silver Ores on War Eagle Mountain, Silver City District, Idaho

Abstract

Located in southwest Idaho’ s Silver City district, War Eagle Mountain’s epithermal Au-Ag veins are structurally controlled arrays that are hosted by a Cretaceous granitoid. Taking into consideration similar deposits at Silver City hosted by mid-Miocene volcanics at DeLamar and Florida Mountains, this study presents a genetic model for ores in the district. Ore deposition occurred from solutions <250°C and were of low salinities (0.5-1wt % NaCl equivalent). Gold mineralization occurs mainly in the form of electrum of approximately 60% Au. Silver occurs in naumannite, aguilarite, arcanthite and electrum, and at least two previously undocumented Ag-Se-S phases. Pb isotope studies demonstrate that primitive basalts are perhaps the primary source of gold, with more evolved variants only being modified by minor crustal inputs. New 40Ar/39Ar data for tecto-magmatism and mineralization that cluster around (16.7–15.1 Ma) provides definitive evidence of the connection between the initial emergence of the Yellowstone hotspot and epithermal mineralization. This conclusion has implications for understanding (and exploring) similar epithermal systems in the northern Great Basin.