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GEOLOGY AND HISTORY OF THE SAN JOSE MINE

 

This document is a partial translation of a 33-page document titled "Informacion Tecnica Empresa Minera San Jose" apparently prepared by Comibol in the early 1990ís, probably by Ing. Jose del Solar. A full copy can be found in KCA file 2883.0030.

In the Central Andes near the eastern margin of the altiplano, on the west side of the city of Oruro, lies the polymetallic deposit of San Jose, in a series of volcanic hills which are elevated 200 to 300 meters above the nearby level of the altiplano which is 3700 m (12,100 ft) above mean sea level.

The San Jose deposit was known and worked for silver by the Incas. In 1595 as a consequence of the discovery of this deposit by the Spanish, the city of Oruro was founded. From that time until the first part of the 19th century the Spanish exploited the rich veins of San Jose producing important quantities of silver until the war of independence (in the mid 1800ís) resulted in a severe decline of production. In 1865 serious production was re-initiated as a result of the high price of silver and the ability to begin exploitation of tin. Both the San Jose sector and the Itos sector were mined at that time by Compania Minera de Oruro with Chilean and French capital.

In 1906 a mill was installed at Machacamarca (30 kg south of Oruro) to treat the San Jose ores with a median annual production of 1500 tonnes fine tin and 2200 kg fine silver.

In 1948 Banco Minero de Bolivia was in control and with great success exploited the famous glory hole BAMIN with grades of 5.5% tin and 400-500 grams/tonne silver.

Following nationalization of the Mines in October 1952, Empresa Minera San Jose was formed, a subsidiary of the newly created COMIBOL. It continued to produce silver and tin. [A flotation plant with a capacity of 420 tonnes/day was constructed in 1953 and continued to run until 1991. It generated the Itos tailings (1,800,000 tonnes) [11,400 days = 158 tonnes/day].]

Finally, mining activities were stopped in August of 1992 because of the difficulty of marketing the concentrates. The production of the San Jose/Itos complex into concentrates for sale from 1980 to 1990 was 2820 tonnes tin, 268 tonnes silver, 12,500 tonnes lead, and 5137 tonnes antimony.

Structural Geology

The igneous complex which contains the San Jose deposit is emplaced in a sincline of silurian rocks striking N10-30 E.

The San Jose deposit is a typical xenothermal polymetallic deposit, characterized by complex mineralization, predominantly of silver and tin. Veins are the result of fracture filling with bonanza stopes containing base metals and tin (Cu, Pb, Zn, Ag, Sn and W).

The mineralized zone is situated in the rhyolite porphyry in the central part of the igneous complex, in an area 2000 x 1200 meters. It is surrounded by unmineralized monzonite. The majority of the mineralization is found on the contact between the rhyolite and shales and on fractures. Veins which penetrate the shales branch out and disseminate.

The shales are of the Uncia formation. In the area of the mine shale outcrops are minor, but at depth they contact the igneous complex quite extensively. On the shale-rhyolite contact they are brecciated and at times mineralized, while when they contact the quartz monzonite they are only fractured. In the deeper levels the shales are intensely silicified and tourmalinized, giving the appearance of a hornfels.

The mineralization system filling the fracture systems near the surface and down to the 200 meter level is simple. On the 300 meter level the principle veins have few branches and reach their maximum strength. On lower levels the strength of the veins is decreased. At the margins of the vein mineralization exists a mineralization of replacement, impregnation and dissemination, forming porphyritic bodies of tin of great volume, as occurs between Itos and La Colorada. In this [open-pittable?] zone it is estimated that there are four million tonnes of tin porphyry containing cassiterite with an average grade of 0.6% tin.

One characteristic of the deposit is the coexistence of high and low temperature minerals. Apparently there were two stages of hydrothermal activity, one short and high temperature defined by cassiterite, pyrite, quartz and arsenopyrite; and the other later which deposited mostly silver bearing sulfosalts such as jamesonite, freibergite, andorite, boulangerite, zinckenite and franckeite accompanied by galena, chalcopyrite and pyrite.

The minerals are apparently simply zoned. A silver-tin zone occurs between the 90 and 180 meter levels. A silver-lead (sulfosalts) zone occurs below the 220 m level.

The veins have been grouped, considering their structural aspects into three principle systems: North-South (9 named veins including the three largest veins); East-West (10 named veins); and North- Northeast (6 named veins).

Based on a study for Comibol by Pincock, Allen & Holt, the mine contains mineable reserves (delineated in veins which are accessible and at least one meter in width) of 495,000 tonnes with a grade of 480 grams/tonne (15.5 ounces per ton).   These reserves lie between the 360 meter level and the 500 meter level.

The results of diamond drilling directed to exploration at depth confirm the continuation of mineralization with high values of silver down to the 600 meter level. The structural and mineralogical characteristics of the veins developed on the 460 level do not indicate a termination of the veins with depth.

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