PRECIOUS METAL MINERALIZATION AT FLORIDA MOUNTAIN, SILVER CITY DISTRICT, IDAHO

Florida Mountain is part of an east-west trending zone of epithermal precious metal mineralization located in the Silver City District of southwestern Idaho. Deposits in the district are hosted in Miocene bimodal basalts and rhyolites, which are associated with the initial emergence of the Yellowstone hotspot, and in an underlying Late Cretaceous granitoid correlative with the Idaho Batholith. Ores are classified as low-sulfidation type with mineralization at Florida Mountain occurring in veins primarily confined to the deeper granitoid. Petrographic analysis of mineralized quartz veins collected from mine dumps on Florida Mountain indicates ore mineralogy dominated by silver minerals, particularly naumannite (Ag₂Se). Base metal sulfides of galena, chalcopyrite, and sphalerite are also present, with galena hosting many of the silver minerals and electrum occurring within chalcopyrite. Accessory pyrite also exists but in very low abundances. Gangue mineralogy consists mainly of quartz with adularia. Quartz was deposited both pre- and post-mineralization, as well as simultaneously with the major ore-forming event. Preliminary microthermometry of primary fluid inclusions in quartz from mineralized veins reveals homogenization temperatures between 260º and 310ºC. Additional microthermometry and chemical data are forthcoming to further characterize the physicochemical conditions of the mineralizing system. Laser 40Ar/39Ar ages of single sanidine phenocrysts from a sample of the rhyolite range from 16.1 to 15.5 Ma, and a similar range of age was determined for adularia from two samples of vein material. The results suggest that eruption of the rhyolite and formations of the veins were broadly contemporaneous, at about ca. 16 Ma, and were followed by conditions of hydrothermal activity and elevated heat flow until ca. 15.5 Ma.