GEOCHRONOLOGIC EVIDENCE FOR THE TIMING OF PRECIOUS METALS MINERALIZATION IN THE BESSIE G MINE, LA PLATA MOUNTAINS, COLORADO

Precious metals mineralization in the La Plata Mountains is spatially associated with Laramide intrusive rocks emplaced between 75 and 65 Ma. A long-standing assumption was that mineralization was related to Laramide magmatic-hydrothermal processes, but more recently it was proposed that mineralization was linked to regional Oligocene magmatic activity. A lack of geochronologic control on the timing of mineralization made it difficult to test these competing ideas.

Thermochronologic analyses reveal a complex history for the epithermal vein system in the Bessie G mine of the La Plata mining district. ⁴⁰Ar/³⁹Ar analyses on roscoelite from veins define an early stage of hydrothermal activity marked by cooling below temperatures of 300-400° C at 57.46 ± 0.11 Ma. These data suggest that some of the mineralization was related to the cooling of fluids released by Laramide intrusive rocks. Fission-track analyses on apatite from Laramide intrusive host rocks define a later stage of thermal activity defined by cooling below 130° C at 27.23 ± 0.14 Ma. The cooling ages for roscoelite and apatite thus reveal multiple hydrothermal events at different times and perhaps different peak thermal conditions.

Our data are consistent with rapid heating and cooling in the Bessie G vein system at ~27 Ma which was synchronous with Oligocene magmatic activity throughout the western San Juan Mountains. Late-stage coloradoite + native gold overprint earlier Au-Ag telluride minerals in the Bessie G deposit. The ~57 Ma magmatic-hydrothermal event was related to Au-Ag telluride mineralization which was superceded by the hydrothermal activity at ~27 Ma that likely produced the coloradoite + native gold. Paragenetic and fluid inclusion data from previous studies support this interpretation. These findings offer constraint on the timing of ore deposition and reveal the complex nature of hydrothermal activity and mineralization in the La Plata Mountains.