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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.
IDENTIFICATION
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).
GEOLOGICAL
CHARACTERISTICS
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.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Ag, Cu, Zn,
Pb, Sn, W, As, Bi.
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.
ECONOMIC FACTORS
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.
REFERENCES
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
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