EVALUATION OF ORE MINERALOGY ASSOCIATED WITH PRECIOUS METAL MINERALIZATION AT FLORIDA MOUNTAIN, SILVER CITY DISTRICT, IDAHO

Epithermal precious metal mineralization at Florida Mountain is part of an east-west trending zone of mineralization within the Silver City Mining District, Idaho. Deposits in the district are hosted in Miocene bimodal volcanic rocks associated with the initial emergence of the Yellowstone hotspot and in an underlying Late Cretaceous granitoid correlative with the Idaho Batholith. Low-sulfidation style mineralization at Florida Mountain occurs primarily in veins within the Cretaceous granitoid. Petrographic analysis of mineralized quartz veins collected from mine dumps on Florida Mountain indicates ore mineralogy consisting of a variety of silver minerals along with base metal sulfides. Analysis of the composition of the silver minerals via scanning electron microscopy with an electron-dispersive X-ray system shows multiple silver minerals including naumannite (Ag₂Se), aguilarite (Ag₄SeS), acanthite (Ag₂S), proustite (Ag₃AsS₃), and chlorargyrite (AgCl). Base metal sulfides of galena, chalcopyrite, and sphalerite are also present, with aguilarite/acanthite intergrown with galena and chalcopyrite. Aguilarite was also observed to replace galena in some samples. Pyrite occurs in very low abundances. Ore mineralogy suggests that base metal sulfides formed prior to and contemporaneous with the main phase of silver mineralization. The occurrence of chlorargyrite indicates that some of the mineralization was subjected to secondary oxidation. Gangue minerals of quartz, adularia, and calcite were deposited before, during, and after the main phase of mineralization. Microthermometry of primary fluid inclusions in quartz, calcite, and adularia from mineralized veins reveals homogenization temperatures between 260º and 310ºC with an average salinity of 0.44 wt % of NaCl equivalent. 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.