THE GENESIS OF METAMORPHOSED PALEOROTEROZOIC MASSIVE SULFIDE OCCURRENCES IN CENTRAL COLORADO: GEOLOGICAL, MINERALOGICAL AND SULFUR ISOTOPE CONSTRAINTS

The Paleoproterozoic era (2.5-1.6 Ga) constitutes a period in Earth’s history when some of the world’s largest massive sulfide deposits formed. Those in central-south Colorado were metamorphosed to the middle-upper amphibolite facies and are generally small Cu-Zn±Pb±Au±Ag occurrences (Sedalia: 1.25 Mt @ 3.3% Cu, 5.6% Zn, 23 g/t Ag, and 0.3 g/t Au; Dawson: 0.41 Mt @ 9.2 g/t Au). They formed in a volcanic-arc setting on the margin of the Yavapai crustal province. Uranium-Pb ages on spatially related granitoids range from ~1.7 to ~1.3 Ga, while Pb isotope studies on galena from massive sulfides suggest mineralization formed around 1.7 Ga. Comparison of Rb vs Yb+Ta and Rb vs Y+Nb for granitoids spatially related to the Dawson deposit (south of Cañon City) show volcanic-arc signatures that exhibit LREE enrichment and HREE depletion with negative Eu anomalies. Deposits in the Dawson-Green Mountain trend (DGMT) and the Gunnison belt are composed of Cu-Zn-Au-(Pb-Ag) mineralization that were overprinted by epithermal 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 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. Metamorphosed altered rocks intimately associated with mineralization primarily consist of various amphiboles (gedrite, tremolite, hornblende), gahnite, biotite, garnet, cordierite, and carbonate. The Zn/Cd ratios of sphalerite (44 to 307) in DGMT deposits 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 = -3.3 to +6.5‰; n = 86) suggest sulfur was derived from magmatic sources. The preferred genetic model is that the deposits are VMS deposits with transitional affinities towards volcanic-sediment hosted massive sulfide (VSHMS) deposits.