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.

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.

Crushing
The material removed from the mine is hauled to the crusher in high-tonnage trucks to be fed into the conical or primary crusher.

The objective of this primary crushing phase is to reduce the size of the rocks extracted from the mine to an average diameter of 15 centimeters, which is small enough to be fed into the mills for further processing.

The equipment is complemented by a metal detector equipped with a magnetic separator, a hydraulic fracturing machine to break oversized rocks that cannot enter the mouth of the crusher, and a dust suppression system.

All material crushed in the primary crusher is sent to the processing plant via a 1,700 meter-long conveyor belt, with a transportation capacity of 2,300 tons per hour.

The material is dropped from a height of 43 meters, forming a big cone known as the stockpile, where the crushed material is stored.

The stockpile is covered with a metallic dome to prevent dust contamination to the outside and has water sprayers designed to sprinkle the material so as to diminish inner dust generation as the material is dropped. The stockpiled material is subsequently loaded through three chutes onto another shorter conveyor belt, which feeds the material into the plant’s milling section.

Milling
Lime is added to the dry material taken from the stockpile in order to regulate the pH during the process.

This mixture enters the semi-autogenous grinding (SAG) mill, where the milling process begins. Inside the SAG mill (36 feet in diameter), the dry material is mixed with water during milling. The mixture of ground material and water is known as pulp.

The pulp from the SAG mill travels into a cylindrical sieve known as a trommel. The excess is recirculated again into the SAG mill via a conveyor belt system, and the undersized material is emptied into a box that feeds the ball mills to continue reducing the size for subsequent treatment.

The ball mills, each of which has a diameter of 22 feet, stand parallel to each other, forming two working lines. These operate in a closed circuit with two batteries of hydro cyclones, through which it is possible to divide the pulp into two sizes. The oversized material is sent back to the mills and the undersized material — with a diameter of 0.1 millimeter or less — is sent on to the flotation process.

The concentration of zinc, lead and silver in the mineralized rock extracted from the Jayula and Tesorera pits in the San Cristóbal mine is very low; it is therefore necessary to have the material undergo a process designed to increase the content of these elements and thus obtain products with a commercial value. In this sense, the concentration plant was designed to obtain two enriched concentrates.

The plant has a nominal production capacity of approximately 52,000 tons of dry mineral per day. It operates 24/7/365, except on a few days scheduled for routine maintenance.

The minerals of value present in the ore body are in the form of sulfides and oxides. Only the sulfides can be processed in the flotation plant. Sulfides are minerals, the basic composition of which is sulfur combined with different metals — in this case zinc, lead and silver. Oxides are comprised of oxygen and metals. The plant’s outputs are zinc sulfide (sphalerite) and lead sulfide (galena). ........