Summary:
The San Cristobal volcanic-intrusive center is unconformable on steeply dipping Eocene-Oligocene Potoco Formation red bed sediments, and lies on the NNE-trending San Cristobal Fault, a major, east-verging thrust. The center a volcano about 12 km in diameter with a low relief of up to 400 m, with flow lobes extending outwards, and a summit depression rimmed by domes.
The central depression is about 4 km in diameter and is filled by up to 300 m thickness of volcaniclasticlacustrine sediments.
The mineralized volcaniclastic-lacustrine sediments fill in a central depression, which is either a graben or a caldera, above welded and non welded lithic-crystal tuffs. The central depression is surrounded by early domes, and the sediments are cut by late rhyodacite domes.
Mineralization is epigenetic and post-dates the late rhyodacite domes. Mineralization is preferentially developed in steep fractures in the peripheral domes, and as disseminations and replacement in the lower parts of the volcaniclastic-lacustrine sediments.
Mineralization in the domes/stocks occurs in irregular open space fractures which may form narrow hydrothermal breccias where there are multiple, intersecting fractures. Mineralization is open space fracture- and breccia-fill and is sulfide-rich, coarse grained and often euhedral, comprising pyrite, Fe-rich sphalerite and galena. The galena is argentiferous and is accompanied by acanthine and native silver. This main stage of simple sulfides was followed by a later stage of Ag-, Pb- and Cu- antimony sulfosalts including jamesonite, famatinite, stromeyerite, pyrargyrite, polybasite, freibergite and boulangerite. Late stage pale brown, botryoidal smithsonite coats sphalerite in vugs. The amount of gangue minerals is low and comprises coarse euhedral barite with a bladed texture, less common prismatic quartz in vugs, and fine grained colloform quartz veins with hematite.
San Cristobal is distinct in having massive sulfides with little gangue, a common feature of epithermal deposits in Bolivia. The gangue is late stage and comprises mostly barite, indicating high sulfate activity, and minor quartz. Late stage smithsonite indicates late CO2-rich fluids.