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 stockpil ........