INSIGHT INTO THE ORIGIN OF GOLD AT THE BESSIE G MINE, LA PLATA MINING DISTRICT, SOUTHWESTERN COLORADO

The La Plata Mountains are one of several laccolithic complexes in northwestern New Mexico and southwestern Colorado that formed about 70 Ma during the Laramide orogeny. Base metal and tellurium-rich Au-Ag-Hg-PGE deposits formed in hydrothermal systems during or after the emplacement and crystallization of potassic intrusive rocks. The La Plata mining district records a complex history of mineralization defined by late-stage precious metal deposits that may have formed during regional incipient rifting and mantle magmatism in the Oligocene to Miocene. The district is well known for vein and replacement epithermal Au-Ag deposits in which native gold forms in association with telluride minerals. In previous studies, native gold was mostly considered hypogene though some evidence hinted that it formed either by secondary replacement of telluride minerals or by supergene enrichment.

The Bessie G mine is one of several mines in the La Plata district that produced significant gold and silver. Previous studies at the mine document early assemblages of chalcopyrite-galena-sphalerite-pyrite-tetrahedrite ± arsenopyrite ± cinnabar followed by crystallization of Ag-Au-Hg tellurides and native gold. In this study, petrographic and microprobe analyses on ore samples from the Bessie G are used to test competing ideas on the genesis of late-stage native gold which may provide insight into the progression of telluride and gold mineralization in the district. Preliminary observations document gold in isolated blebs that coat earlier-formed sulfide and telluride minerals in vugs, infilling in microfractures, and rare intergrowths in telluride minerals. Native gold is common in oxidized zones within the ore. These relationships suggest that the gold formed either by late-stage replacement of Au-Ag tellurides or supergene enrichment rather than syngenetic crystallization with the tellurides.