The Sunshine mine, near Kellogg ID, is a mesothermal Ag-Pb vein deposit in the Coeur d’Alene mining district. Proterozoic siliciclastic rocks of the Ravalli Group, Belt Supergroup, host the ore bodies. The recently discovered West Chance ore body has been under development for the past five years. This tabular ore body strikes west and dips steeply to the south, has 300m (~1000 ft) strike length and extends approximately 914m (~3000 ft) down dip. Ore is located where the WNW-striking Chance fault changes to a predominately west-striking structure. This study consists of a fluid inclusion and structural analysis of the West Chance ore body to determine pressure, temperature, and composition parameters of mineralization, and to evaluate possible structural controls on ore deposition.
Fluid inclusion analysis of over 60 primary inclusions from quartz veins within the West Chance ore body show the dominant type of fluid inclusion to be liquid-rich inclusions composed of H2O-NaCl. Homogenization temperatures, T(h), range from 190.6°C to 325.8°C, with a mean value of 270.9°C. There is no systematic variation of T(h) with respect to depth and no evidence of boiling. Salinities increase with depth through the vein system; thus range from 0.5 wt.% NaCl in the upper portions to 12.2 wt.% NaCl lower in the ore body. Increasing salinity with depth suggests that ore deposition in the West Chance is a result of mixing of two fluids of similar temperature but varying salinities or that boiling occurred higher in the system yielding the observed salinity contrast. A constructed P-V-T diagram suggests that the West Chance ore body formed at pressures in the range of 0.5-2 kbars, corresponding to depths of 1.75-7 km for fluid pressure under lithostatic load.
Structural analysis consisted of mapping, petrofabric analysis, and compilation of existing data and showed that the mineralized fault system consists of an anastomosing series of smaller faults that strike WSW to WNW. Megascopic features and petrographic textures indicate that the fault has been reactivated over time. Ore shoots plunge steeply to the west with a 60-70° rake. Previous work on structural control of ore fluids has suggested mineralization took place during left-lateral or reverse motion. The present study of the West Chance ore body shows there is evidence for both types of motion and supports left-lateral, reverse, and right-lateral as a sequence of fault motions, with right lateral motion occurring after ore deposition. However, there are no constraints on the absolute timing of faulting
