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” that follow a reticulate of 7 to 10 meters, depending on the exact type of rock that should be extracted in each place.

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.

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.

The capacity of the crusher plant is 2,200 tons per hour.

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.

The construction of the mineral concentration plant, which uses the flotation method, began in 2005. The plant started operating mid-2007. 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.

In order to optimize the work of the plant, an Expert System is used, i.e. a fuzzy logic based software to program operational rules in milling, flotation, thickening and other processes.

The minerals of value present in the ore body are in the form of sulfides and oxides. Only the sulfides can be processed in our 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). Silver sulfide (argentite) is contained in both products. The objective of our facility is to separate the lead and zinc sulfides from each other and from the non-usable remainder material through several phases of crushing, milling, and differential flotation, filtering and drying to obtain two enriched concentrates with a humidity of less than 10% ready for marketing.

To separate the sulfides from the sterile material, it is first necessary to reduce the size of the material to be treated until the sulfide particles are freed and can be separated from the remainder material. This is achieved in the crushing process, followed by the milling process.

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 flotation process begins when the pulp is loaded into special receptacles known as “flotation cells”, adding various chemical reagents and stirring the mixture.

Due to the characteristics of the mineral processed by this plant, the first process is conducted using lead flotation, followed by zinc flotation.

Pressurized air is introduced into the cell to create small bubbles in the pulp, to which adherence to the mineral particles to be selected is promoted as an effect of floating reagents, such as MIBC and F150, and modifying agents such as zinc oxide, sodium cyanide, sodium silicate and copper sulfate. In this way, the mineral is brought to the surface in the form of froth for subsequent withdrawal from the flotation cell. For lead flotation, Aerophine 3418A and Flotec 2200 LF are used. For zinc flotation, Danafloat 468 and Flotec 2041 are used.

These processes are performed in circuits of five serial cells each, with three products resulting from each circuit: “concentrates”, “middlings” and “tailings”. The concentrates go on to the thickening and filtering process. The middlings, from which it has not yet been possible to fully extract the mineral, go on to a re-milling and classification process, at which point the undersized material is sent to the flotation circuit for cleaning and re-cleaning, and the concentrate is finally extracted. The tailings are sent to the tailings thickener and from there to the tailings deposit.

Upon conclusion of the flotation process, the products obtained contain a high percentage of water and proceed to the thickening phase, where the solids (concentrates) are separated from the liquids (process water) in metallic circular tanks called thickeners.

The concentrates (zinc-silver and lead-silver) emerging from the thickeners are sent separately to the filtration area where water is extracted in two independent circuits equipped with horizontal press filters, and where the temperature is increased until the final humidity is under 10%. The product is concentrate ready for loading and railway transportation to the port.

The water recovered in this process is pumped to the flotation area and reused.

The process, from mine to market, takes approximately 40-75 days. From the mine to the plant is a short period of days, while the larger portion of time corresponds to time from the plant to port and then to the smelters.

The entire process is designed to maximize the economic yield of the company, minimize environmental impact, recycle resources wherever available, and apply world-class mining technology to the operation. Minera San Cristobal regularly reviews the process, with routine maintenance and quality control.