DISTRIBUTION OF SULFIDE MINERALS IN METASEDIMENTARY DEPOSITS OF THE APPLE CREEK FORMATION, SALMON RIVER MOUNTAINS, IDAHO

The Idaho Cobalt Belt (ICB) is located in east-central Idaho and is a geologically complex and economically significant area within the Salmon River Mountains. Metallic deposits of the ICB are rich in cobalt, copper, and gold. The belt extends 60 km and trends northwest-southeast. Ore occurs as sulfide deposits in the Mesoproterozoic Apple Creek Formation, a 1410 ± 10 Ma old turbidite sequence of metasedimentary siltite. Individual units include the fine siltite unit, diamictite unit, coarse siltite unit, and banded siltite unit. Mineralization primarily occurs in the banded siltite unit in sulfide rich lenses but may also occur in discordant and massive sulfide breccias. Thirty hand samples were collected from the coarse and banded siltite units throughout the ICB and distribution and characterization of sulfides within these units are the focus of this study. Comparison of composition was determined via petrographic thin sections of each sample and SEM analysis of the sulfide minerals. Not surprisingly, the distribution of sulfides (chalcopyrite, pyrite, cobaltite) varies spatially within and adjacent to the ICB. Most often sulfides occur disseminated within the siltites and decrease closer to the Idaho Batholith. Southern samples exhibit concentration of sulfides parallel to foliation in the siltites. Additionally, sulfides in samples closest to the Blackbird Mine District are larger, hexagonal, and cross-cut quartz veins within the siltites. Southern and central samples contain mica-poor quartz veins roughly orthogonal to the orientation of micas within the siltite. In northern samples, closer to the batholith, the presence of chlorite and biotite increases in the quartz veins, and the orientation of the veins is no longer orthogonal.