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Cerro Rico de Potosi |
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Bolivia |
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Sn Ag
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Super Porphyry Cu and Au
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The Cerro Rico de Potosi tin/silver ore deposit was discovered in 1545, and is located immediately to the south of the town of Potosi, ~425 km SE of La Paz, just to the east of the Altiplano of Bolivia (#Location: 19° 37' 8"S, 65° 44' 57"W).
Since 1545 it has been exploited primarily for silver, although tin has become the more important ore metal over the last century. Total silver production to 1977, was estimated at 30 000 tonnes and by 1996 had reached 60 000 tonnes. The upper portion of the mineralised hill was being worked by 300 small cooperatives operating around 575 'mines' when visited in 1977. The lower part of the orebody was then being mined by the state owned COMIBOL. Production during 1976 was 2 273 tonnes of tin, 5.3 tonnes of silver and 10 tonnes of lead.
For details of the tectonic, regional geological and metallogenic setting see the Central Andes and Bolivian Orocline, and the Andean Tin Belt records.
Geological Setting
The geological setting and sequence in the area is as follows, from the base:
Ordovician or Silurian, >1500 m thick - Shale with lesser sandstone which grades to quartzite.
Early Tertiary to Cretaceous
• Puca Formation - which is absent in the Potosi area, but where present in the district comprises a basal cross bedded sandstone that is 700 m thick, overlain by100 m of upper calcareous sandstone marl and limestone of late Cretaceous age. These are overlain by 1800 m of alternating marl, mudstone and sandstone, possibly of Early Tertiary age.
Middle Tertiary
• Agua Dulce Formation, 450 m thick - Basal unit comprising red shale, red and buff sandstone and fine conglomerate. Overlain by andesitic flows with lesser tuffs, glassy flows and coarse breccia. This formation is absent around the Cerro Rico Stock.
• Canteria Formation, >350 m thick - coarse agglomerates overlain by breccia tuffs and thin lavas. Rhyolitic to dacitic and garnet bearing in part. Some tuffs have a characteristic purple colour.
• Cerro Rico Formation, which is composed of:
- Pailavira Conglomerate, 10 to 17 m thick - Pebbles of shale, quartzite and igneous rocks from the Agua Dulce and Canteria Formations set in a fine (bedded) matrix.
- Venus Breccia, 0 to 100 m thick - Angular fragments of igneous and sedimentary rocks in a tuffaceous matrix.
- Caracoles Tuff, >300 m thick - Well bedded ash with intercalated breccia, fine conglomerate and sandstone.
Miocene
• Kari Kari Diorite - A garnet bearing diorite porphyry. This intrusive hosts minor tin occurrences.
Pliocene
• Cerro Rico Stock - Feldspar quartz porphyry. A sub-volcanic stock. The host of much of the mineralisation.
Pleistocene ?
• Huakachi Rhyolite, >260 m thick - Rhyolitic tuff, breccia and minor flows. Tuffs carry pebbles of pyritised porphyry similar to that of the Cerro Rico Stock. These volcanics are not in contact with the Cerro Rico Stock.
Mine Geology
The Potosi Mine is developed largely within the 13.8±0.2 Ma Cerro Rico Stock and surrounding intruded rocks. The latter comprise Ordovician shales in the lower parts of the mine and the flat lying Pailavira Conglomerate, Venus Breccia and Caracoles tuff at higher levels. Emplacement of the stock was controlled by either on a ring fault of the resurgent lower Miocene Karikari caldera or by a tensile connecting fault between the NNW striking series of en echelon faults that were generated by dextral movement along the nearby major Carma fault.
The Cerro Rico Stock has the form of an inverted cone with oval shaped dimensions of 1700 x 1200 m at the surface. The stock stands out as a prominent sharp conical hill rising some 700 m above the surrounding countryside. Approximately 600 m below the base of the hill the stock has diminished to two small dykes less than 100 m in width and less than 1 km in length.
The stock is principally of a porphyry composed of 40 to 50% phenocrysts of corroded quartz and altered relics of plagioclase, sanidine and biotite in a strongly altered, dense groundmass. The phenocrysts are now present as altered white kaolinitic and sericitised feldspar from several mm's to 1 cm across which crumble at a touch. Lesser quartz crystals of around 2 mm in diameter comprise less than 1% of the rock. The matrix is a fine grained, brown, sericitic material, probably originally dacitic in composition. The porphyry is strongly pyritic, present as disseminations, fracture coatings and a network of veins totalling up to 5% of the rock.
Mineralisation
A predominatly vertical alteration pattern has been imposed on the stock, as the result of a high sulphidation lithocap with vuggy silica and quartz-dickite alteration accompanied by disseminated silver mineralisation near the summite. This zone passes downward into an interval of strong sericite development, before an underlying zone of tourmalinisation in the deepest levels (Dietrich, et al., 2000)
Strongly developed polymetallic veins strike NE-SW and cross cut the stock and enclosing country rocks. These veins comprise pyrite, galena, sphalerite, complex Ag sulphides, wolframite and cassiterite with a gangue of quartz, tourmaline, kaolinite, alunite, sericite and siderite.
In the upper sections of the mine a series of sheeted veins occupy a volume of some 200 x 300 x 300 m in the siliceous core of the stock representing 30 million tonnes of 0.6% Sn. In the lower levels of the mine, in the foot-wall of the "feeder dykes", silicification of the intruded Ordovician shales and Pailavira conglomerate was apparently more intense with denser fracturing and subsequent veining.
An outward zonation of metals has been recognised within and adjacent to the stock, from sulpho-salts in the core to silver and then to zinc. In the zinc zone sphalerite occurs as centimetric veins within the intruded sediments. Similarly there is an upward zonation from tin to bismuth to tungsten. Copper and stannite increase at depths of greater than 750 m below the peak of the hill.
Silver levels fall off deeper in the stock as the tin increases. The rich silver grades mined in the early years of production in the upper parts of the orebody were in the zone of oxidation.
In the upper levels of the mine tin is present largely as cassiterite with minor stannite. The cassiterite is usually associated with quartz. The sulphides present are predominantly pyrite and arsenopyrite with lesser sphalerite, galena and tetrahedrite.
Secondary copper salts forming stalactites are widespread throughout the mine suggesting the presence of significant quantities of copper minerals within the stock.
This summary is drawn from a visit in 1977, literature available at that time and from more recent sources.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2000.
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.
Cerro Rico de Potosi
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Selected References:
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Bartos, P.J., 2000 - The Pallacos of Cerro Rico de Potosi, Bolivia: a new deposit type: in Econ. Geol. v.95, pp. 645-654.
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Cunningham, C.G., Zartman, R.E., McKee, E.H., Rye, R.O., Naeser, C.W., Sanjines, O.V., Ericksen, G.E. and Tavera, F.V., 1996 - The age and thermal history of Cerro Rico de Potosi, Bolivia: in Mineralium Deposita v.31, pp. 374 - 385.
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Dietrich, A., Lehmann, B. and Wallianos, A., 2000 - Bulk rock and melt inclusion geochemistry of Bolivian Tin Porphyry systems: in Econ. Geol. v.95, pp. 313-326.
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Kamenov, G., Macfarlane, A.W. and Riciputi, L., 2002 - Sources of Lead in the San Cristobal, Pulacayo, and Potosi Mining districts, Bolivia, and a re-evaluation of regional ore Lead isotope provinces: in Econ. Geol. v.97, pp. 573-592.
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Rice, C.M., Steele, G.B., Barfod, D.N., Boyce, A.J. and Pringle, M.S., 2005 - Duration of Magmatic, Hydrothermal, and Supergene Activity at Cerro Rico de Potosi, Bolivia: in Econ. Geol. v.100, pp. 1647-1656.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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