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

Abstract

The rocks of the district are all Tertiary or Quaternary. 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 were intruded by dacite porphyry and andesite porphyry stocks and both are overlain by recent dacite porphyry lava flows. The intrusive and sedimentary rock near the intrusive contacts are hydrothermally altered. Mineral deposits of the district are associated with the intrusiv rocks and are of four distinct types: 1)" Sulfide veins containing lead, zinc, and silver associated with dacite porphyry intrusives (Colon and Tesorera Mines). U. S. GOVERNMENT PRINTING OFFICE 1959 0 -511171 1 867.100 2) Oxide veins containing iron and silver associated with andesite porphyry intrusives (Toldos Mine). 3) Partial sulfide replacement of intrusive brecFia containing silver, lead, and zinc (Animas Mine). 4) Disseminated lead-zinc-silver minerals in altered dacite porphyry intrusives and in adjacent altered sedimentary rocks (Hedionda Mine). Disseminated silver minerals may possibly 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.