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Estanho e prata em veios

by A. Panteleyev
British Columbia Geological Survey


Ref: estanho, prata, Sn-Ag, veios, polimetálicos, domo, subvulcânico

Panteleyev, A.(1996): Epithermal Au-Ag: Low Sulphidation, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 45-48.


SYNONYMS: Polymetallic Sn veins, Bolivian polymetallic veins, polymetallic tin-silver deposits, polymetallic xenothermal.

COMMODITIES (BYPRODUCTS): Ag, Sn (Zn, Cu, Au, Pb, Cd, In, Bi, W).

EXAMPLES (British Columbia (MINFILE #) - Canada/International): D zone (104P044, 080,081) and Lang Creek veins (‘Pant’, 104P082), Cassiar district; Cerro Rico de Potosi, Oruro, Chocaya, (Bolivia), Pirquitas (Argentina), Ashio, Akenobe and Ikuno (Japan).


CAPSULE DESCRIPTION: Sulphide and quartz-sulphide veins carrying cassiterite, a wide variety of other base metals and zones with silver minerals. They are associated with epizonal (subvolcanic) quartz-bearing intrusions, or their immediate hostrocks. In some places the ore is in volcanic rocks within dacitic to quartz latitic flow-dome complexes.

TECTONIC SETTING: Continental margin; synorogenic to late orogenic belts with high-level plutonism in intermediate to felsic volcanoplutonic arcs. In British Columbia the only significant Sn-bearing deposits occur with S or A-type granites in eastern tectonic assemblages underlain by continental rocks of North American origin.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: In faults, shears and fractures that cut or are proximal to high-level felsic intrusions and in flow-dome complexes, namely domes and their surrounding tuff rings and explosive breccias.

AGE OF MINERALIZATION: Tertiary in the type area of Bolivia; Cretaceous and Tertiary in Japan; Tertiary and older in British Columbia.

HOST/ASSOCIATED ROCK TYPES: Hostrocks for veins can be of any type and do not appear to be an important control on the occurrence of the deposits; they include sedimentary, volcanic and intrusive rocks and sometimes, metasedimentary rocks at depth. Intrusive rocks with which the mineralization is associated are quartz bearing and peraluminous, but seem to be restricted to intermediate compositions between 60 and 70% SiO2 (dacite to rhyodacite); more felsic rocks are present, but are less common.

DEPOSIT FORM: Veins, commonly with swarms of closely spaced, splaying smaller veins in sheeted zones. Veins vary in width from microveinlets to a few metres, and commonly are less than a metre wide. The ore shoots in veins are commonly 200-300 m along strike and dip but the veins may extend to more than 1000 m in depth and strike length. Vein systems and related stockworks cover areas up to a square kilometre along the tops of conical domes or intrusions 1-2 km wide.

TEXTURE/STRUCTURE: Multistage composite banded veins with abundant ore minerals pass at depth into crystalline quartz veins and upwards into vuggy quartz-bearing veins and stockworks.

ORE MINERALOGY (Principal and subordinate): Pyrite, cassiterite; pyrrhotite, marcasite; sphalerite, galena, chalcopyrite, stannite, arsenopyrite, tetrahedrite, scheelite, wolframite, andorite, jamesonite, boulangerite, ruby silver (pyrargyrite), stibnite, bismuthinite, native bismuth, molybdenite, argentite, gold and complex sulphosalt minerals. These deposits are characterized by their mineralogical complexity. There is no consistency between deposits in vertical or lateral zoning, but individual deposits are markedly spatially and temporally zoned. In some deposits, notably intrusion or dome-hosted examples, core zones are denoted by the high-temperature minerals cassiterite, wolframite, bismuthinite and arsenopyrite. Surrounding ores have varying amounts of stannite and chalcopyrite with, most significantly, sphalerite, galena and various Pb sulphosalt and Ag minerals. Silver in the upper parts of the vein systems occurs in argentite, ruby silver and native silver and at depth is mainly present in tetrahedrite.

GANGUE MINERALOGY (Principal and subordinate): Quartz, sericite, pyrite; tourmaline at depth, kaolinite and chalcedony near surface; rare barite, siderite, calcite, Mn carbonate and fluorite.

ALTERATION MINERALOGY: Quartz-sericite-pyrite is characteristic; elsewhere quartz-sericite- chlorite occurs in envelopes on veins. Near-surface argillic and advanced argillic alteration overprinting is present in some deposits.

WEATHERING: Prominent limonite cappings are derived from the oxidation of pyrite.

ORE CONTROLS: Sets of closely spaced veins, commonly in sheeted zones, fractures and joints within and surrounding plutons are related to the emplacement and cooling of the host intrusions. The open space filling and shear-replacement veins are associated with stockworks, breccia veins and breccia pipes. A few deposits occur in faults, shears, fold axes and cleavage or fracture zones related to regional tectonism. Some early wallrock replacement along narrow fissures is generally followed and dominated by open- space filling in many deposits.

GENETIC MODEL: Dacitic magma and the metal-bearing hydrothermal solutions represent the uppermost products of large magmatic/hydrothermal systems. The Sn is probably a remobilized component of sialic rocks derived from recycled continental crust.

ASSOCIATED DEPOSIT TYPES: Polymetallic veins Ag-Pb-Zn ; epithermal Au-Ag: low sulphidation , mantos , porphyry Sn , placers . This deposit type grades with depth into Sn veins and greissens (I13) associated with mesozonal granitic intrusions into sediments. Cassiterite in colluvium can be recovered by placer mining. Mexican-type rhyolite Sn or “wood tin” deposits represent a separate class of deposit (Reed et al., 1986).

COMMENTS: Many Sn-bearing base metal vein systems are known to occur in eastern British Columbia, but there is poor documentation of whether the Sn is present as cassiterite or stannite. The former can be efficiently recovered by simple metallurgy, the latter cannot.



OTHER EXPLORATION GUIDES: The vein systems may display impressive vertical and horizontal continuity with marked metal zoning. Bolivian polymetallic vein deposits have formed at depths of 0.5 to 2 km below the paleosurface. Deeper veins of mainly massive sulphide minerals contain Sn, W and Bi; the shallower veins with quartz-barite and chalcedony-barite carry Ag and rarely Au. Metal zoning from depth to surface and from centres outward shows: Sn + W, Cu + Zn, Pb + Zn, Pb + Ag and Ag ± Au; commonly there is considerable ‘telescoping’ of zones. Oxidized zones may have secondary Ag minerals, such as Ag chlorides.


TYPICAL GRADE AND TONNAGE: Considerable variation in metal contents of ores is evident between deposits. Potentially bulk-mineable bedrock deposits contain in the order of 0.2% Sn with 70-179 g/t Ag (Cerro Rico, Potosi, Bolivia).

ECONOMIC LIMITATIONS: These veins tend to be narrow.

IMPORTANCE: These veins are an important source of cassiterite for economic placer deposits around the world and the lodes have been mined in South America. They are currently attractive only when they carry appreciable Ag. In some deposits Au content is economically significant and Au-rich zones might have been overlooked during past work. Future Sn production from these veins will probably be as a byproduct commodity, and only if cassiterite is the main Sn mineral.


Cunningham, C.G., McNee, J., Pinto Vasquez, J. and Ericksen, G.E. (1991): A Model of Volcanic Dome-hosted Precious Metal Deposits in Bolivia; Economic Geology, Volume 86, pages 415-421.

Ericksen, G.E. and Cunningham, C.G. (1993): Epithermal Precious-metal Deposits Hosted by the Neogene and Quaternary Volcanic Complex in the Central Andes; in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., Editors, Geological Association of Canada, Special Volume 40, pages 419-431.

Grant, J.N., Halls, C., Avila, W., and Avila, G. (1977): Igneous Systems and the Evolution of Hydrothermal Systems in some Sub-volcanic Tin Deposits of Bolivia; in Volcanic Process in Orogenesis, Geological Society of London, Special Paper Publication 7, Pages 117-126.

Ludington, S.D., Orris, G.J., Cox, D.P., Long, K.R. and Asher-Bolinder, S. (1992): Mineral Deposit Models; in Geology and Mineral Resources of the Altiplano and Cordillera Occidental, Bolivia, U.S. Geological Survey, Bulletin 1975, pages 63-89.

Nakamura, T. and Hunahashi, M. (1970): Ore Veins of Neogene Volcanic Affinity in Japan; in Volcanism and Ore Genesis, Tatsumi, T., Editor, University of Tokyo Press, pages 215-230.

Reed, B.L., Duffield, W., Ludington, S.D., Maxwell, C.H. and Richter, D.H. (1986): Descriptive Model of Rhyolite-hosted Sn; in Mineral Deposit Models, U.S. Geological Survey, Bulletin 1693, pages 168-171.

Togashi, Y. (1986): Descriptive Model of Sn-Polymetallic Veins; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, page 109.

DEPÓSITOS - 25/04/2004 18:36:00

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