THE TIMING, CHARACTER, AND SIGNIFICANCE OF LATE STAGE CARBONATE, SULFATE AND HYDROLYTIC ALTERATION ASSEMBLAGES IN THE CRIPPLE CREEK GOLD TELLURIDE DISTRICT, COLORADO

The Cripple Creek district has produced ~23 Moz of gold (+ 7 Moz of undeveloped resources) from an intensely altered Oligocene alkaline diatreme complex. Au-Te mineralization and large volumes of attendant low-T K-metasomatism (adularia-pyrite) were developed after a complex magmatic and hydrothermal history. The youngest hydrothermal features, however, are dominated by carbonate and sulfate-rich mineral assemblages; these clearly cut Au mineralization and are commonly associated with hydrolytic alteration assemblages that include sericite, well-crystallized kaolinite and rare dickite. Some remarkable features include massive zones of hydrothermal carbonate (dominantly calcite, but including Fe, Mg and Mn-rich types) and sulfate (anhydrite), which occur as irregular masses, plugs, and breccia pipes 10's to 100's of meters in diameter. Accessory minerals accompanying these assemblages include Mo and Pb sulfides, fluorite, rhodochrosite, REE phases, and rare creedite. Zones of hydrolytic alteration can extend for 100's of meters in and around these features, and near-surface dissolution of sulfates and carbonate have resulted in collapse features and "rubble breccias".

Mechanisms for the generation of the high-level zones of hydrolytic alteration include gas condensation at the water table, oxidation of H₂S (and possibly SO₂) in ascending fluids to form sulfuric acids in the near surface environment, supergene oxidation of sulfides, or some combination of these processes. Existing isotope data and other geochemical evidence do not unequivocally distinguish these mechanisms. It is clear, however, that large volumes of SO₄ and carbonate-rich fluids were introduced at high levels in the late (waning?) stages of hydrothermal activity. These fluids also appear to have introduced significant Mo, Pb and possibly other metals, and their attendant styles of Mo-rich and hydrolytic alteration contrast with the more alkaline (adularia-pyrite + Au-Te bearing) assemblages that characterize the main phase of mineralization in the district. The late stage SO₄ and CO₃ assemblages at Cripple Creek may bear similarity with zones of SO₄-rich alteration described in other alkaline epithermal systems such as Lihir, and the zones of massive carbonate seen in alkaline systems in Montana and the Black Hills of WY-SD.