The Porco silver-zinc-tin deposit is located 35 km southwest of the Cerro Rico de Potosí deposit on the southeastern edge of the Los Frailes volcanic field. It was the first silver deposit discovered in Bolivia, with exploitation dating to pre-colonial times. The geology has been described by Sugaki et al. (1983), Cunningham et al. (1993, 1994a, b) and Jiménez et al. (1998).

The deposit is hosted by a north-south-elongated caldera that is 5.0 km x 3.0 km and formed at 12.0 ± 0.4 Ma with the eruption of the crystal-rich dacitic Porco Tuff. Well defined topographic walls of the caldera cut Ordovician phyllites and Cretaceous sandstones. The 12.1 ± 0.4 Ma Apo Porco stock (4,886 masl) occurs on the southern margin of the caldera. Mineralization is associated with the younger 8.6 ± 0.3 Ma Huayna Porco stock (4,528 masl) in the center of the caldera. Radial dykes, alteration and metals are zoned around the stock. To the north, the Porco Tuff is overlain by the ignimbrites of the Los Frailes Formation dated at 6 to 9 Ma.

Mineralization occurs in NNE to NE-trending veins that cut the Porco Tuff about 1 km east of the Huayna Porco stock. The deposit is zoned around the stock with cassiterite proximal to the stock and base metals, mainly sphalerite and galena, further away. The upper parts of the veins are silver-rich with pyrargyrite, acanthite and stephanite. The main structure is the San Antonio vein which strikes N10º - 30ºE and dips between 70º and 85º to the ........

The Porco deposit consists of multiple, relatively thin high-grade veins. The mining methods used vary according to the continuity, dip, and width of these veins. Current mining methods employed include sublevel longhole stoping with backfill, shrinkage stoping, and some cut and fill. Currently the mine is separated into two main areas: Hundimiento and Central zones:

- “Hundimiento” is the more modern section of the mine and is developed mostly with trackless methods using an access ramp to move men and materials between levels. The mineralized zones are predominantly wider and steeper dipping thus, stoping is 85% AVOCA mining and 15% shrinkage. This area is completely isolated from the Central zone and trucks its mineralized material to surface via an access ramp; sublevels are driven on nominal 15 m spacing with sill drifts in vein driven 3.0 m x 3.0 m, an average of 7 stopes produces about 300 t/d of mineralized material (Figure 6-7). All waste rock stays in the mine as backfill; and

- “Central” is 100% conventional shrinkage mining (Figure 6-6). Mineralized material is hauled via rail on each active level to the shaft for hoisting to surface. Levels are spaced at a nominal 45 m and level connections are via manway raises and the main shaft. rifts are 2.2 m x 2.0 m, raises are driven manually, and an average of 17 active stopes produces about 400 t/d of mineralized material. All waste rock stays in the mine as backfill. Currently each mining area prov ........

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