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 st ........

The San Cristobal Mine is the largest open-pit mining operation in Bolivian history.

The mining process begins with the drilling of “blast holes” in a grid of 7 by 10 meters, depending on the exact type of rock being extracted from a given location.

This activity is conducted using modern drilling equipment, capable of penetrating the rock and performing at speeds of approximately 1 meter per minute.

MSC uses a system of “silent blasts” to break off rocks in the target area of the open pit, which means a minimum of explosives is used to break the rock without propelling it over a long distance.

The explosives used in this procedure are known as ANFO, a mixture of ammonium nitrate and either diesel fuel or used oil. Blasts are generally performed at noon and follow a strict security protocol that involves isolating the area by closing access roads for a radius of 500 meters in all directions. After the blast, the explosion of all active charges is carefully verified to confirm that no non-blasted charges remain.

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