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Andean Tin Belt |
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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 Andean tin-tungsten-silver belt comprises an arcuate interval of some 2 000 km, extending from the Peruvian border through south western Bolivia into northern Argentina. Within this, there is a broad metal zoning from north to south of tungsten to tin-tungsten to tin, with variable quantities of silver. This belt embraces the legendary tin and silver mines of Potosi and Oruro.
In the interval from La Paz to the Peruvian border tungsten is the major component of the tin-tungsten deposits. Between La Paz and Oruro both tin and tungsten deposits are worked. Oruro is a tin-silver mine, while further south at Huanuni and Llallagua only tin is extracted. The Cerro Rico de Potosi produces tin and silver. Further south again, Chorolque produces only tin while Pirquitas in Argentina is a tin-silver mine.
Tin mineralisation occurs in a variety of modes and host rocks. These include:
Porphyry tin deposits - The most significant by far in terms of production, are extensive disseminated and associated vein deposits accompanying intensely altered Miocene and Pliocene dacitic and rhyolitic sub-volcanic stocks. These are the porphyry tin deposits. The largest tested reserves of this type are at Llallagua (Catavi), where a bulk mining reserve, amenable to open cut extraction, of 80 million tonnes of 0.3% Sn has been outlined. Underground block caving has exploited 0.5% Sn mineralisation of this type within the deposit. Others include Cerro Rico de Potosi and Chorolque.
These deposits are, in most cases, characterised by the presence of circular to oval shaped, pervasively altered, rhyolitic to dacitic sub-volcanic stocks which decrease in diameter with depth in most cases, to a few relatively thin dykes. These stocks usually carry around 0.15 to 0.3% Sn throughout as microscopic cassiterite disseminations, cassiterite bearing fracture coatings or sulphide veinlet networks. The stocks embrace smaller but still substantial volumes of from 0.5% to 1% Sn, often associated with sheeted sulphide, or quartz-sulphide vein systems. Some deposits have accompanying tin bearing quartz veins cutting both the stocks and surrounding country rocks. Explosive hydrothermal breccias are often found in association with the mineralised stocks, particularly on their margins (see the Llallagua description).
Deposits of this type range from those indicated above, which are associated with sub-volcanic stocks, to purely volcanic complexes at which no intrusives other than dykes are recognisable. Examples of the latter are Chocaya, Tatasi and Colquechaca. These have only minor tin with more Zn and Ag, and comprise rhyolitic to dacitic lavas, lava domes, pyroclastics and volcanic breccias resting unconformably on older sedimentary rocks. These systems usually cover tens of km2, having a central extensive zone of hydrothermal alteration several kilometres across which embrace mineralised veins. At Chocaya and Tatasi the major veins extend downwards into the underlying sediments.
Alteration types within the stocks of the volcanic complexes include quartz-tourmaline, sericite-tourmaline and sericite-clay, with carbonate in some instances. At some centres there is a peripheral chlorite zone. Disseminated cassiterite and sulphide mineralisation generally accompanies the sericite alteration assemblages. At Chorolque the l km diameter stock is strongly brecciated (explosive and hydrothermal breccia) and intensely altered to a quartz-tourmaline rock. This is surrounded by a halo of sericite-tourmaline alteration which is within the intruded Tertiary volcanics and Palaeozoic sediments. At a distance of 600 m outwards from the stock margin there is a weak chloritic zone. The central quartz tourmaline core carries no disseminated or stockwork tin. It does however, embraces all of the high-grade quartz cassiterite veins. The surrounding 400 m wide sericite-tourmaline zone has strong veinlet and sulphide mineralisation (from 0.04 to 0.1% Sn over a 200 m width) in both the Tertiary volcanics and Palaeozoic sediments. This example is described here as it exhibits some variations on the pattern seen in the deposits inspected, where sericite-clay and sericite-tourmaline alteration predominate. Alteration within the volcanic complexes where there are no outcropping stocks is usually sericite-clay with very little disseminated tin.
Stratabound sulphide deposits - Stratabound/lithology controlled sulphide rich deposits and non sulphide vein and/or disseminated mineralisation. Both of these comprise mineralisation restricted to particular lithologies or stratigraphic units. They occur in the same relatively narrow arcuate belt as the porphyry deposits and are hosted by both Siluro-Devonian and Cretaceous sediments. The stratabound sulphide rich deposits include deposits such as Colavi in Bolivia and Pirquitas in Jujuy, northern Argentina.
Colavi comprises three siltstone units carrying up to 20% pyrite and from 0.8% to 1.8% Sn as 5 to 15 µm cassiterite over thicknesses averaging from 0.5 to 2 m and strike lengths of several hundred metres (or more). Mineralised horizons can be traced for strike lengths of several km's. These are found within Cretaceous sandstone-siltstone units intruded in part by Tertiary acid sills.
At Pirquitas, a stratigraphic thickness of up to 100 m of black pyritic shale and quartzite carrying 0.18 to 0.25% Sn, as 50 µm cassiterite and lesser sulphides, and 75 g/t Ag, in a veinlet system. See the la more detailed description in the Pirquitas record.
The non sulphide occurrences range from quartzites with 0.5% to 1% Sn over thicknesses of 2 m or so as "invisible" cassiterite, to groups of individual veins of high grade, coarse cassiterite within coarse dogs-tooth open space filling quartz veins restricted to particular lithologies within an alternating quartzite-shale unit. An example of the latter is Huanuni, historically one of Bolivia's largest tin producers.
Vein Deposits within sedimentary rocks - Three deposits of this type were visited, namely La Chojlla, Huanuni and Kellguani. These deposits display variations in age, type of host rock and form of the quartz-veins. What they do have in common is that they comprise cassiterite bearing quartz veins cutting quartzite-shale-siltstone or greywacke-shale sequences.
For details consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1979.
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.
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Selected References:
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Lehmann B, Ishihara S, Michel H, Miller J, Rapela C, Sanchez A, Tistl M, Winkelmann L 1990 - The Bolivian Tin province and regional Tin distribution in the Central Andes: a reassessment: in Econ. Geol. v85 pp 1044-1058
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Muller B, Frischknecht R, Seward T, Heinrich C, Camargo Gallegos W 2001 - A fluid inclusion reconnaissance study of the Huanuni tin deposit (Bolivia), using LA-ICP-MS micro-analysis: in Mineralium Deposita v36 pp 680-688
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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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