Geology and mineral deposits of the San Cristobal district, Villa Martin province, Potosi, Bolivia

Abstract

The San Cristobal silver district near Uyuni, Bolivia, was jointly studied in detail during the months of August to November 1966 by members of the Servicio Geologico de Bolivia and the U.S. Geological Survey, under the auspices of the Agency for International Development, U.S. Department of State. The rocks of the district are Cenozoic. The oldest rocks are the red beds of the Potoco Formation, which are unconformably overlain by agglomerate, conglomerate, sandstone, and tuff of the flat-lying Quehua Formation. Both formations are intruded by dacite porphyry and andesite porphyry stocks, and both are overlain by recent dacite porphyry lava flows. The intrusive and sedimentary rocks near the intrusive contacts are hydrothermally altered. Mineral deposits of the district ai*e associated with the intrusive rocks and are of four distinct types: 1. Sulfide veins containing lead, zinc, and silver associated with dacite porphyry intrusives (Colon and Tesorera mines). 2. Oxide veins containing iron and silver associated with andesite porphyry intrusives (Toldos mine). 3. Partial sulfide replacement of intrusive breccia containing silver, lead, and zinc (Animas mine). 4. Disseminated lead-zinc-silver minerals in altered dacite porphyi*y intrusives and in adjacent altered sedimentary rocks (Hedionda mine). Disseminated silver minerals may provide a basis for the development of open-pit silver mines. This possibility should be further investigated, beginning with geochemical surveys and hydrothermal alteration studies.