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Oruro - Socavon (La Salvadora), San Jose

Bolivia

Main commodities: Sn Ag Pb
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The Oruro Sn-Ag-Pb mine complex included the San Jose and the Socavón, previously the La Salvadora mines which are distributed along the Cerro Oruro, located within the town of Oruro on the edge of the Altiplano in Bolivia, ~190 km SSE of La Paz (#Location: 17° 57' 38"S, 67° 7' 49"W).

The Oruro Mine or Empresa Minera San Jose was owned and operated by the state owned COMIBOL. The main mines were closed in 1992. The Oruro mines were originally worked by the Incas and reopened by the Spaniards in 1595. In the vicinity of 8000 tonnes of silver are estimated to have been produced from these mines.

For details of the tectonic, regional geological and metallogenic setting see the Central Andes and Bolivian Orocline, and the Andean Tin Belt records.

The sequence in the Oruro area is as follows, from the base:

Silurian
Cancaniri Formation - Greywackes and glacio-marine sediments;
Huanuni Formation - Phyllites;
Llallagua Formation - Quartzites and lesser shale to siltstones;
Siluro - Devonian
Oruro or Uncia Formation - Black shales, slate and argillite with lesser quartzite;
Devonian
Catavi Formation - Fine to medium grained fossiliferous sandstone with shale intercalations. Absent in mine area;
Miocene
San Pedro Stock - Intrusive rhyolitic porphyry. Unaltered and unmineralised;
Older Volcanics ("quartz-monzonite porphyry") - Rhyolitic to dacitic tuffs, breccias to agglomerate, and minor lavas. In places they overlie the stocks, in others they are intruded;
San Jose Explosion Breccia - Explosion breccia, largely associated with the contacts between the intrusive stocks and the Oruro Formation;
Pliocene? to Pleistocene
Younger Volcanics - rhyolitic to rhyodacitic lavas, tuffs and breccias. Widespread to the west of Oruro;
Pleistocene - Older alluvium and lake deposits;
Recent - Alluvial cover.

The San Jose Mine is located near the centre of the Cerro Oruro, a 6 km long and 1 to 3 km wide range of hills rising 300 m above the Altiplano, partly surrounded to the east, south and SW by the town of Oruro.

The Cerro Oruro embraces two rhyolitic stock systems and associated volcanic rocks. Palaeozoic sedimentary rocks which have been intruded and broken up by the extrusive-intrusive activity are restricted to the southern part of the range. The two stock systems comprise the larger northern San Pedro stock which is some 2 x 1.8 km at the surface, and the San Jose-Itos stocks. Neither the San Jose or Itos stocks outcrop extensively as they are largely masked by both the younger and older volcanics. However, near the surface, they occur as two separate bodies. The San Jose stock is 1700 x 1000 m, while Itos is 1800 x 500 m. They are separated by a zone occupied by large blocks of disoriented Oruro Formation argillites. These blocks are up to 200 m or more in diameter. To the south, this band of sediments, although strongly chevron folded, is less intensely deformed and appears to have represented a positive feature during the emplacement of the San Jose and Itos stocks and accompanying extrusives.

At depth the San Jose and Itos stocks converge and coalesce forming a series of dykes. This system is known to a depth of 700 m where it has decreased to three dykes, the largest being 200 m wide. The San Pedro Stock forms a prominent hill on the northern end of the Cerro Oruro while the San Jose-Itos stocks occur towards the centre of the range. The interval between these stock systems, and to the south is occupied principally by the Older Volcanics. In general the latter comprise banded dacitic tuffs which have sparsely distributed angular fragments of tuffaceous material up to 2 cm in diameter. The banding is due to the alignment of the 2 to 3 mm diameter feldspar phenocrysts in the tuffs. In some places, the layering in the tuffs dips into the contact with the San Jose Stock which cuts across the banding. In these cases the contact is marked by a well developed explosive breccia zone, with a variety of rounded to subangular exotic fragments set in a matrix similar to the Older Volcanics. Elsewhere the Older Volcanics appear to rest on the stock suggesting that they in part post date it.

The rhyolitic porphyry of the San Jose Stock is, in general, a more massive and siliceous rock than the Older Volcanics. It comprises subrounded quartz and feldspar phenocrysts from 2 to 3 mm, and occasionally up to 1 cm in diameter set in a fine grey matrix which has a conchoidal fracture. It carries subangular fragments of siltstone and porphyry from 1 to 2 mm across in places.

The San Jose breccia is the name given to a series of explosive (or hydrothermal) breccia zones within the San Jose and Itos stocks. These zones are usually found on the contact of the stocks and the Oruro Formation but at times also cut the stocks. The breccia occurs as pipes, lenses, dykes and irregular bodies, and comprises a coarse fragmental, composed of subangular to subrounded fragments from 1 mm to 1 m in diameter, of light grey altered intrusive porphyry and dark argillite in a black matrix of pulverised argillite.

An extensive zone of alteration is evident in the Cerro Oruro. This alteration is centred on the San Jose and Itos Stocks and affects the two intrusive bodies and the surrounding Older Volcanics. The San Pedro Stock and the Younger Volcanics have not been altered. This alteration zone covers an area which is some 2 km or so in diameter and takes the form of sericitisation and kaolinisation and lesser chloritisation, with a central zone of silicification. Alteration is not visually readily differentiated from the effects of weathering at first sight. The intrusives and Older Volcanics, although altered, still retain their original texture with white kaolinised feldspar set in a grey sericitised matrix. The Devonian sedimentary rocks have not been affected by the alteration where observed in the underground workings, although the altered intrusive and volcanic rocks are apparently strongly pyritic. At the surface, they exhibit a pale yellow-green to red tinge characteristic of oxidised disseminated pyrite.

Unlike the quartz-sulphide cassiterite veins of Potosi, those worked at Oruro are sulphide veins. Fine sulphide has been deposited in planar zones of brecciation. The Grande vein, sighted during a visit in 1997, and which is apparently characteristic of the Oruro type veining, occurs as a zone which averages 1 to 3 m in thickness but is as wide as 10 m in places. It comprises a network of sulphide veinlets ranging from thin stringers 1 mm thick, to pyrite veins 20 to 30 mm thick, deposited between and around the breccia fragments. Breccia fragments range from a few to 10 mm in diameter. This vein has diffuse boundaries, the intensity of sulphide development being proportional to the degree of brecciation. The breccia is largely composed of fragments of the enclosing rock, although occasionally exotic fragments are more abundant than in the surrounding rock.

The altered intrusives and extrusives are said to average 0.2% Sn throughout as fine fracture coatings. The unaltered San Pedro Stock does not carry these high values, nor does the Oruro Formation even in the vicinity of the breccia fill veins.

The 100 m wide interval between the Grande and Moropoto veins at the surface is said to assay 0.6% Sn. The vertical and lateral extent of this high grade mineralisation is not known, although it is believed the grade falls off with depth. The main sulphide present is pyrite with much less arsenopyrite, pyrrhotite, sphalerite, chalcopyrite and galena. The main tin mineral is cassiterite with only traces of stannite.

The head grade being mined in 1997 was 300 g/t Ag, 0.30% Sn and 3% Pb.

The most recent source geological information used to prepare this summary was dated: 1977.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Oruro

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