The massive mineralizing system centered on Cerro Rico resulted in, by all accounts, the largest single concentration of silver known and produced the textbook Bolivian Tin Belt sub-volcanic dome-hosted silver-tin vein deposit.

Although vein mineralization undoubtedly contributes to the silver content of the pallaco deposits locally, it is the disseminated mineralization within wall rock between and adjacent to the vein system which controls the overall grade of the clasts within the pallaco deposits. The disseminated silver mineralization with the silicified wall rock, particularly the silicified rhyodacite, is preserved in the pallaco clasts. It is this silver mineralization which is primarily recovered by the San Bartolomé Project.

Both wall-rock alteration and vein mineralization show pronounced vertical and horizontal zoning. Alteration grades upward from a high-temperature core of quartztourmaline at depth through quartz-sericite pyrite and quartz-dickite into pervasive silicification. Silicification in the upper levels of the system is so intense that it has leached all the feldspar and ferromagnesium minerals from the host rhyodacite leaving "vugs" (actually molds of the phenocrysts) in their place. These voids were depositional sites for latter disseminated silver and tin mineralization during vein deposition. This “vuggy-silica” alteration (silicified rhyodacite) represents the almost complete replacement of the host rhyodacite by hydrothermal silica (high sulfidation style).

Vein mineralization is zoned vertically from a high-temperature core of quartz-pyritecassiterite-arsenopyrite-bismuthinite at depth through an intermediate zone of quartz-stannite-sphalerite-chalcopyrite-tetrahedrite to an upper zone of ruby silvernative silver-cassiterite-jamesonite-boulangerite. Silver-bearing minerals are zoned from tetrahedrite-andorite at depth upward into argentite-pyrargyrite-native silver in the upper levels of the system.

Vein mineralogical zoning is less pronounced horizontally. Sphalerite, galena and pyrite are reported to increase towards the margins of the intermediate zone. These veins are currently being mined by local cooperatives within the oxide-sulfide transition zone below about the 4400 m elevation. Many are also reported to be rich in silver.

Silver mineralization post-dates tin, and becomes enriched upward in the system. Both the tin and silver disseminated in the quartz-tourmaline and vuggy-silica alteration were deposited contemporaneously with similar vein mineralization. The intensity of the disseminated mineralization increases with increasing proximity to the sheeted-vein zone. Above the 4,400 m elevation, the mineralization is strongly oxidized with less than one percent relic primary sulfide consisting predominantly of pyrite.

Silver occurs as native silver, chlorargyrite, pyargyrite, argentite, argentojarosite, argentianpsilomelane, and silver-bearing iron oxides and hydroxides. On average about 70% of the contained silver is recoverable through cyanide leaching. The other 30% is contained in refractory iron compounds or is silica encapsulated.

Tin occurs as cassiterite in very-fine grained, hypogene intergrowths with quartz and as powdery aggregates likely derived from oxidization of tin sulfide (stannite) and sulfosalts (teallite and franckeite). Quartz is the primary gangue mineral (+90%) along with traces of hematite, magnetite, goethite, zircon and ilmenite. Barite occurs in abundance locally (Huacajchi).

Disseminated silver and tin mineralization in vuggy-silica alteration is the most important ore-source rock within the pallaco/escombreras deposits of the San Bartolomé Project.

The gravel deposits are mined using conventional open-pit methods on 5 m benches using hydraulic excavators mining from the bench crest into 20 t gravel trucks. Trucks are provided and operated by a local contractor; the remaining equipment is owned by Manquiri.

Although a dozer is occasionally used to push ore to the excavator and re-contour pit walls, no dozer-ripping or drilling and blasting is required for mining. No stripping is required, other than soil and minimal internal waste. Mined waste is either used for haul road construction or back filled into the depleted sections of the pit.

Bedrock slope is relatively steep resulting narrow benches which follow the contour of the mountain. The width to length ratio of a typical bench is 3:1 to 4:1. Bench widths very as mining progresses due to changes in pallaco thickness and underlying bedrock slope. Pit development progresses down slope.

The processing plant is located to the south-east of Cerro Rico at an elevation of approximately 4,090 meters. The plant is designed to operate 365 days per year. The mill throughput design is 4,500 metric tons per day of Huacajchi ore and 5,600 metric tons per day for ore sourced from Santa Rita and Diablo; the difference being attributable to different grinding characteristics. The silver is leached using a sodium cyanide (NaCN) solution and the plant is designed to recover approximately 78.0% of the silver from the ore, although metallurgical recovery will vary with different ore types. Thus, the final valuable product from the plant is 99.95% silver, but is called doré.

There are two separate ore types that are handled in the washing and crushing circuit. One is called whole ore and the other is called screen ore. Each ore type has its own crushing process circuit. The whole ore is either direct dumped by truck from the mine into a feed bin or recovered from the stockpile by a front-end loader and delivered to a feed bin.

The whole ore is fed by an apron feeder that delivers the ore to a jaw crusher. The ore is crushed in the jaw crusher to no more than 76 mm. The crushed ore is delivered by conveyor to the crushed ore stockpile. The crushed ore stockpile has a capacity of approximately 5,200 tonnes live storage (i.e., immediately available for use in the downstream processes).

The screen ore is fed by an apron feeder to an ore-sizer which reduces the larger particles to less than 10 cm. The sizer is a dentate roller crusher. The sized material is delivered to a washing system consisting of a rotary drum washer and a triple deck shaker screen. The wash water and fine particles passing through the bottom deck (8 mesh screen) are pumped to a tailings disposal facility. The top deck oversize material normally coarser than 50 mm is fed into a 120 cm short head cone crusher. The crushed product is combined with the middle deck oversize material (>2.4 mm to < 50 mm size) and conveyed to the crushed ore stockpile which provides feed for the grinding circuit.

The crushed ore is reclaimed from the crushed ore stockpile using a front-end loader. The loader feeds a bin that discharges into a conveying system then the ore is fed to a Semi-Autogenous Grinding (SAG) mill. As the ore is discharged from the conveyor into the SAG mill, water and lime slurry (pH modifier) are added. New grinding balls are added to replenish the ball charge of the mill as the grinding balls in the mill are consumed. The slurry discharging from the SAG mill flows onto a trommel screen. The screen removes the oversize material, which is returned to the SAG mill. The crushed ore is ground in the SAG mill to approximately 48 mesh at which size the ore is reduced sufficiently in size to where the particles can be efficiently broken in the downstream ball mill.

Cyclone overflow slurry produced by the grinding circuit is directed over a trash screen and then pumped to the cyanide leach circuit. The slurry is leached in a series of seven agitator tanks using NaCN and plant air.

The leached slurry flows by gravity to the counter current decantation (CCD) circuit that performs a solid/liquid separation. The leached slurry is washed by the CCD process: underflow from each thickening stage is pumped to the next stage downstream while the solution overflowing each thickener is directed to the previous stage upstream. The CCD circuit yields two product streams: a pregnant solution that contains the valuable metals and thickened slurry that contains the washed tailings particles and a minute amount of metal values in solution.

The pregnant solution (first-stage thickener overflow solution) flows by gravity to the pregnant solution tank of the Merrill-Crowe plant for recovery of the precious metal values. The washed, thickened slurry is pumped to the paste thickener for dewatering and eventual disposal.

Pregnant solution is pumped from the pregnant solution tank to a Merrill-Crowe processing circuit. The Merrill-Crowe process, which consists of precipitating silver (and any other precious metals) from the pregnant solution using zinc dust, is commonly used for ores containing large quantities of silver in addition to the gold.

Pregnant solution that is treated using the Merrill-Crowe zinc precipitation process must not contain any particulate matter or any dissolved oxygen. In the first step of the process, pregnant solution passes through pressure-leaf-type clarifier filters to remove any fine particles that are contained in the solution until turbidity less than 100 NTU is achieved. After the solids have been removed, the clarified pregnant solution flows to a deaeration tower. The clarified pregnant solution enters the upper portion of the deaeration tower, which is maintained at a negative pressure by a vacuum pump that continually withdraws air from the deaerator. As the air is withdrawn, dissolved oxygen contained in the pregnant solution vaporizes and is removed. During the next stage of the process, the oxygen-free pregnant solution is pulled from the deaeration tower by a pump, and zinc dust is injected into the suction line of the pump. The silver precipitates as soon as the zinc makes contact with the deaerated pregnant solution.

After zinc addition, the silver precipitate slurry is pumped to plate-and-frame-type precipitate filters that separate the liquid from the solids. The silver precipitate is retained on the filter cloths that cover the filter plates. The precipitate filter filtrate, which is now barren solution, flows to a barren solution tank. Barren solutions normally contain minimum silver values (< 1.0 ppm). The barren solution is pumped from the barren solution tank back to the countercurrent decantation (CCD) circuit to be used as washing solution for the leached slurry, also is used throughout the processing plant as process and gland seal water.

At the end of the filtration cycle, when the precipitate filter is filled with solids, the filter is opened and the filter cake is manually removed and placed into pans for further processing. The pans containing filter cake are loaded into a dryer oven to evaporate all the water content. The filter cake is heated overnight to temperatures ranging from 100 to 110o C to entirely remove present moisture. After the moisture is removed from the filter cake, the cake is ready for smelting. The purpose of smelting is to remove any remaining impurities in the precipitate. These impurities may include copper, excess zinc from the precipitation step, lead, cadmium, and other base metals.

The precipitate is mixed with a combination of fluxes that are used to form a slag containing all the impurities extracted during the smelting process. The fluxprecipitate mixture is placed in a smelting furnace and heated to approximately 1,250ºC to form a molten mixture. A slag forms on the top of the molten mixture and the silver forms a molten bath at the bottom. The slag is removed into slag pots, and the silver is poured into bar molds.

The slag is cooled and later processed to concentrate and recover any silver entrained in the slag. The slag then is added back to the melting furnace for retreatment. The treated slag is periodically returned to the SAG mill. The metal that is formed into bar molds is called doré—a mixture of silver and minor amounts of any remaining impurities. The doré bars are cooled, cleaned, weighed, sampled and stamped for identification, and placed in the vault awaiting shipment. Silver is analyzed to determine the grade before shipping.