CONTRASTING STYLES OF GOLD-SILVER MINERALIZATION IN THE LA PLATA MINING DISTRICT, SOUTHWESTERN COLORADO: THE PORPHYRY TO EPITHERMAL TRANSITION

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 caused domal uplift of the region and contact metamorphosed the surrounding country rocks. Later Cretaceous syenite and monzonite porphyry intrusions crosscut the diorite-monzonite porphyry. These younger more alkalic stocks appear to have initiated the ore-forming processes in the district. In particular, the Allard syenite hosts porphyry copper mineralization containing Au-Ag-Te-PGEs. Reconnaissance copper isotope data from the Allard tunnel, which accesses the lower exposure of the porphyry system, gives a δ⁶⁵Cu value of 0.965, which lies within the range of typical porphyry coppers in the Western U.S. Mineralization in the district includes disseminated chalcopyrite and pyrite with Pt and Pd, contact-metamorphic Au-bearing sulfides, Au-bearing pyrite, mixed sulfides with Ag and free Au, chalcocite veins, ruby silver veins, and telluride deposits (Eckel, 1949).

Crosscutting and offsetting veins of chalcopyrite and pyrite (Allard), massive chalcopyrite, azurite, and malachite (Copper Hill), and spotty telluride clusters (at Cumberland and Mayday) are visible in collected samples. The Allard stock hosts two types of mineralization: porphyry copper (at Allard tunnel) and copper-rich deep hydrothermal veins (at Copper Hill Glory Hole), both of which are anomalously enriched in PGEs. The district also contains two geochemically similar Au-Ag-Te deposits: mineralized Manto-like limestone replacement at Mayday and Idaho mines and low sulfidation epithermal veins at Cumberland, Bessie G, and other deposits in the district. Due to the >1km of topographic relief, the transition between porphyry and epithermal styles of mineralization is uniquely exposed and able to be sampled in the La Plata district. Detailed mineralogy as well as trace element and copper/sulfur isotope geochemistry are currently ongoing.