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Stockworks com estanho

by W. David Sinclair
Geological Survey of Canada, Ottawa

Ref: pórfiro, estanho, cassiterita, brechas, stockworks, veios

Sinclair, W.D.. (1995): Porphyry Sn, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Energy of Employment and Investment, Open File 1995-20, pages 97-100.


SYNONYM: Subvolcanic Sn


EXAMPLES (British Columbia - Canada/International): Mount Pleasant (New Brunswick, Canada), East Kemptville (Nova Scotia, Canada), Catavi, Chorolque and Cerro Rico stock (Bolivia), Ardlethan and Taronga (Australia), Kingan (Russia), Yinyan (China), Altenberg (Germany).


CAPSULE DESCRIPTION: Fine-grained cassiterite in veinlet and fracture stockwork zones, breccia zones, and disseminated in porphyritic felsic intrusive rocks and associated country rocks.

TECTONIC SETTING: Zones of weak to moderate extension in cratons, particularly post orogenic zones underlain by thick crust, possibly cut by shallow-dipping subduction zones.

GEOLOGICAL SETTING: High-level to subvolcanic felsic intrusive centres in cratons; multiple stages of intrusion may be present.

AGE OF MINERALIZATION: Paleozoic to Tertiary.

HOST/ASSOCIATED ROCK TYPES: Predominantly genetically related intrusive rocks and associated breccias, but may also include related or unrelated sedimentary, volcanic, igneous and metamorphic rocks. Genetically related felsic intrusive rocks are F and/or B enriched and are commonly porphyritic. Tuffs or other extrusive volcanic rocks may be associated with deposits related to subvolcanic intrusions.

DEPOSIT FORM: Deposits vary in shape from inverted cone, to roughly cylindrical, to highly irregular. They are typically large, generally hundreds of metres across and ranging from tens to hundreds of metres in vertical extent.

TEXTURE/STRUCTURE: Ore is predominantly structurally controlled in stockworks of crosscutting fractures and quartz veinlets, or disseminated in hydrothermal breccia zones. Veins, vein sets, replacement zones may also be present.

ORE MINERALOGY (Principal and subordinate): Cassiterite; stannite, chalcopyrite, sphalerite and galena. Complex tin- and silver-bearing sulphosalts occur in late veins and replacement zones.

GANGUE MINERALOGY: Pyrite, arsenopyrite, löllingite, topaz, fluorite, tourmaline, muscovite, zinnwaldite and lepidolite.

ALTERATION MINERALOGY: In the Bolivian porphyry Sn deposits, sericite + pyrite ± tourmaline alteration is pervasive; in some deposits it surrounds a central zone of quartz + tourmaline. Sericitic alteration is typically bordered by weak propylitic alteration. In other deposits (e.g. , Ardlethan, Yinyan), central zones are characterized by greisen alteration consisting of quartz + topaz + sericite; these zones grade outward to quartz + sericite + chlorite alteration.

WEATHERING: Oxidation of pyrite produces limonitic gossans. Deep weathering and erosion can result in residual concentrations of cassiterite in situ or in placer deposits downslope or downstream.

ORE CONTROLS: Ore minerals occur in fracture stockworks, hydrothermal breccias and replacement zones centred on 1-2 km2, genetically related felsic intrusions.

GENETIC MODEL: Magmatic-hydrothermal. Large volumes of magmatic, highly saline aqueous fluids under pressure strip Sn and other ore metals from temporally and genetically related magma. Multiple stages of brecciation related to explosive fluid pressure release from the upper parts of small intrusions result in deposition of ore and gangue minerals in crosscutting fractures, veinlets and breccias in the outer carapace of the intrusions and associated country rocks. Mixing of magmatic with meteoric water during waning stages of the magmatic- hydrothermal system may result in deposition of some Sn and other metals, particularly in late-stage veins.

ASSOCIATED DEPOSIT TYPES: Sn veins (I13), Sn-polymetallic veins (H07).

COMMENTS: Some of the deposits listed (e.g. , Taronga, East Kemptville) are not "subvolcanic" but they are similar to some porphyry Cu deposits with regard to their large size, low grade, relationship to felsic intrusive rocks and dominant structural control (ie., mineralized veins, fractures and breccias).


GEOCHEMICAL SIGNATURE: Sn, Ag, W, Cu, Zn, As, Pb, Rb, Li, F, B may be anomalously high in hostrocks close to mineralized zones and in secondary dispersion halos in overburden. Anomalously high contents of Sn, W, F, Cu, Pb and Zn may occur in stream sediments and Sn, W, F (topaz) and B (tourmaline) may be present in heavy mineral concentrates.

GEOPHYSICAL SIGNATURE: Genetically related intrusions may be magnetic lows (ilmenite- rather than magnetite-dominant); contact aureole may be magnetic high if pyrrhotite or magnetite are present in associated skarn or hornfels zones. Radiometric surveys may be used to outline anomalous U, Th or K in genetically related intrusive rocks or in associated altered and mineralized zones.

OTHER EXPLORATION GUIDES: Sn (-Ag) deposits may be zoned relative to base metals at both regional (district) and local (deposit) scales.


GRADE AND TONNAGE: Tens of millions of tonnes at grades of 0.2 to 0.5% Sn. Mount Pleasant (New Brunswick): 5.1 Mt @ 0.79% Sn; East Kemptville (Nova Scotia): 56 Mt @ 0.165% Sn; Catavi (Bolivia): 80 Mt @ 0.3% Sn; Cerro Rico stock, Bolivia: averages 0.3% Sn; Ardlethan (Australia): 9 Mt @ 0.5% Sn; Taronga (Australia): 46.8 Mt @ 0.145% Sn; Altenberg, (Germany): 60 Mt @ 0.3% Sn; Yinyan (China): "large" (50 - 100 Mt?) @ 0.46% Sn

ECONOMIC LIMITATIONS: Low grades require high volumes of production which may not be justified by demand.

IMPORTANCE: A minor source of tin on a world scale; when it was in production, East Kemptville was the major producer of tin in North America.


Grant, J.N., Halls, C., Avila, W. and Avila, G., (1977): Igneous Systems and 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.

Grant, J.N., Halls, C., Sheppard, S.M.F. and Avila, W., (1980): Evolution of the Porphyry Tin Deposits of Bolivia; in Granitic Magmatism and Related Mineralization, Ishihara, S. and Takenouchi, S., Editors; The Society of Mining Geologists of Japan, Mining Geology Special Issue, No. 8, pages 151-173.

Guan, X., Shou, Y., Xiao, J., Liang, S. and Li, J., (1988): A New Type of Tin Deposit - Yinyan Porphyry Tin Deposit; in Geology of Tin Deposits in Asia and the Pacific, Hutchison, C.S., Editor, Springer-Verlag, Berlin, pages 487-494.

Lin, G., (1988): Geological Characteristics of the Ignimbrite-related Xiling Tin Deposit in Guangdong Province; in Geology of Tin Deposits in Asia and the Pacific, Hutchison, C.S., Editor, Springer-Verlag, Berlin, pages 494-506.

Reed, B.L., (1986): Descriptive Model of Porphyry Sn; in Mineral Deposit Models, Cox, D.P. and Singer, D.A. Editors; U.S. Geological Survey, Bulletin 1693, pages 108.

Sillitoe, R.H., Halls, C. and Grant, J.N., (1975): Porphyry Tin Deposits in Bolivia; Economic Geology, Volume 70, pages 913-927.

Taylor, R.G. and Pollard, P.J., (1986): Recent Advances in Exploration Modelling for Tin Deposits and their application to the Southeastern Asian Environment; GEOSEA V Proceedings, Volume 1, Geological Society of Malaysia, Bulletin 19, pages 327-347.

DEPÓSITOS - 30/04/2004 23:33:00

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