PGE mineralization at the Allard stock: Implications for the porphyry to epithermal transition, La Plata Mountains, Colorado

Abstract

Geochemical and petrographic studies on the Allard stock, which hosts a porphyry Cu-Ag-Au-Te-PGE deposit in the La Plata Mountains of southwestern Colorado, suggest mantle sources for both metals and sulfur. Microprobe analysis of ores verifies the PGE host mineral phase(s) as Pd±Pt±Bi-tellurides and Ag±Pd tellurides, and a possible previously unidentified mineral with a suggested formula of PdTe2. The La Plata mining district, named after the laccolithic La Plata mountains, lies ~20 km northwest of Durango, Colorado, at the southwestern-most part of the Colorado Mineral Belt. Intrusion of Cretaceous laccolithic diorite-monzonite porphyries from igneous activity that propagated inward from the flat-slab subuction of the Farallon plate during the Laramide Orogeny (70-80 Ma) caused domal uplift of strata and contact metamorphosed the surrounding country rocks. Younger alkaline stocks appear to have initiated the ore-forming processes in the district by providing fluids and reopening existing faults. One of these alkaline porphyry stocks, The Allard stock (~67 Ma), hosts both massive, stockwork, and disseminated chalcopyrite and pyrite, Ag-Au tellurides, and PGEs. Sulfur isotope values for the Allard stock range from -7.8 to +1‰ δ34S(VCDT), indicating a magmatic source with a minor 32S-enriched component of contamination apparently derived from the country rocks. Copper isotope values range from +0.965 to +2.700‰ δ65Cu, within the range for typical porphyry coppers. Lead isotope data suggests a mantle source for lead from sulfides in the stock mineralization. Due to the ~1 km of topographic relief, the transition between porphyry and epithermal styles of mineralization is well exposed and amenable to sampling in the La Plata district. Mineralization in the district includes porphyry mineralization (the Allard stock) as well as contact-metamorphic Au-bearing sulfides, limestone replacement (skarn) with sulfides and tellurides, Au-bearing pyrite, mixed sulfides with free Au, chalcocite veins, ruby silver veins, and Au-Ag-telluride deposits. Because of their close spatial association, hydrothermal fluids that mineralized the Allard stock porphyry are likely coeval or analogous to fluids that mineralized the Au-Ag-tellurides in the epithermal deposits and perhaps even the skarn deposits. Geochemically, the porphyry and epithermal ores in the La Plata district are extremely similar (Au, Ag, Te). Therefore the migration of fluids from the mantle source, to the porphyry regime, and finally to the epithermal environment seems a likely model for ore deposition in the La Plata district.