SHERMAN-TYPE PB-ZN-BA MINERALIZATION IN STAR BASIN, WEST FLANK OF THE SAWATCH UPLIFT, GUNNISON COUNTY, COLORADO

Areas of Pb-Zn-Ba mineralization in Star Basin include the Star and New Star Mines and the Hope Claims on the south side of Italian Creek. Star Mine mineralization was localized in a paleokarst feature in the Leadville Fm and planar layering of sulfide minerals and barite indicate deposition against open cavity walls. Mineralization at New Star is probably in the Leadville, but that in the Hope Claims is partly in the Belden Fm. The mineralization has many similarities to Sherman-Type deposits east of the Sawatch Uplift.

δS34 values for 11 sulfides range from +2.3 to +9.8 ‰ (+6.8 ‰ ave) and +9.2 to +20.7 ‰ (+15.2 ‰ ave) for sulfate in 3 barites. The sulfide values are readily explained by reduction of Pennsylvanian marine sulfate (~ +15 ‰ ). Nearby Pennsylvanian evaporites and those in the Eagle basin would have provided a readily available source of dissolved sulfate at any time since the mid-Pennsylvanian. Three galenas from the Star and New Star workings show moderate J-type anomalies (206/204, 18.9-20.5; 207/204, 15.7-16.0; 208/204, 38.8-40.1). They are less radiogenic than some galenas from other Sherman-type deposits, but far more radiogenic than common lead in the 34 Ma North Italian Mountain granite and galenas from the large carbonate-hosted Pb-Zn deposits in Colorado. 87Sr/86Sr ratios of barite (0.7199), 2 dolomites from pyrite-rich rocks (0.7177 and 0.7136), and dolomite from dolomitized Leadville Fm (0.7278) show that Sr in the ore-depositing and dolomitizing fluids was far more radiogenic than original Sr expected in Leadville limestone.

These fluids extracted Pb and Sr from Proterozoic clastic fragments in Pennsylvanian rocks and probably directly from crystalline basement rocks as they were expelled from the Colorado Trough and its marginal basins during the Ancestral Rocky Mountain tectonic episode. The mineralization is probably late Pennsylvanian-early Permian in age and has no genetic connection with the Northern Italian Mountain granite.