The genesis of metamorphosed Proterozoic massive sulfide occurrences in central Colorado: Geological, mineralogical and sulfur isotope constraints
Berke, Edward Higgins
Spry, Paul G
Johnson, Benjamin W
Is Version Of
Geological and Atmospheric Sciences
Paleoproterozoic massive Cu-Zn±Pb±Au±Ag sulfide deposits metamorphosed to the middle-upper amphibolite facies in central-south Colorado formed in a volcanic-arc setting on the edge of the Yavapai crustal province. Previously published U-Pb ages on spatially-related granitoids range from ~1.9 to ~1.1 Ga, while Pb isotope studies on galena from massive sulfides suggests mineralization formed at around 1.8-1.7 Ga. Some deposits in the Dawson-Green Mountain trend (DGMT) and in the Gunnison belt are composed of Cu-Zn-Au-(Pb-Ag) mineralization that were overprinted by later Au-(Ag-Cu-Bi-Se-Te) mineralization. Sulfide mineralization is spatially related to bimodal, mafic-felsic volcanic rocks (gabbro, amphibolite, rhyolite, and dacite) and granitoids but it occurs mostly in biotite-garnet-quartz±sillimanite±cordierite schists and gneisses, spatially related to nodular sillimanite rocks, and in some locations, exhalative rocks (iron formations, gahnite-rich rocks, quartz-garnetite). The major metallic minerals of the massive sulfides include chalcopyrite, sphalerite, pyrite, pyrrhotite, and magnetite, with minor galena and gahnite. Altered rocks intimately associated with mineralization primarily consist of various amphiboles (gedrite, tremolite, and hornblende), gahnite, biotite, garnet, cordierite, and rare högbomite. The Zn/Cd ratios of sphalerite (44 to 307) in deposits in the DGMT fall within the range of global volcanogenic massive sulfide (VMS) deposits but overlap with sphalerite from sedimentary exhalative (Sedex) deposits. Sulfur isotope values of sulfides (δ34S = -4.4 to +6) suggest sulfur was largely derived from magmatic sources and that variation in isotopic values resulting from thermochemical sulfate reduction is due to small differences in physicochemical conditions. The preferred genetic model is for the deposits to be bimodal-mafic (Gunnison) to mafic-siliciclastic VMS deposits.