BESSHI MASSIVE SULPHIDE Zn-Cu-Pb / G04
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Cortesia del Gobierno de BRITISH COLUMBIA.
Ministerio de Enegia y Minas |
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SYNONYM:Besshi type, Kieslager.
COMMODITIES (BYPRODUCTS):Cu, Zn, Pb, Ag, (Au, Co, Sn, Mo, Cd).
EXAMPLES ((British Columbia - Canada/International): Goldstream (082M141), Standard (082M090), Montgomery (082M085), True Blue (082F002), Granduc (?) (104B021), Windy Craggy (?) (114P020); Greens Creek (Alaska, USA), Besshi (Japan).
GEOLOGICAL CHARACTERISTICS
CAPSULE DESCRIPTION:Deposits typically comprise thin sheets of massive to well layered pyrrhotite, chalcopyrite, sphalerite, pyrite and minor galena within interlayered, terrigenous clastic rocks and calcalkaline basaltic to andesitic tuffs and flows.
TECTONIC SETTINGS:Oceanic extensional environments, such as back-arc basins, oceanic ridges close to continental margins, or rift basins in the early stages of continental separation.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:Terrigenous clastic rocks associated with marine volcanic rocks and sometimes carbonate rocks; these may overlie platformal carbonate or clastic rocks.
AGE OF MINERALIZATION:Any age. In British Columbia, most deposits are Cambrian, Late Triassic and less commonly Mississippian-Permian in age.
HOST / ASSOCIATED ROCK TYPES:Clastic sediments and marine volcanic rocks; basaltic tuffs and flows, shale and siltstone, commonly calcareous; less commonly chert and Fe formations. Possibly ultramafics and metagabbro in sequence.
DEPOSIT FORM:Typically a concordant sheet of massive sulphides up to a few metres thick and up to kilometres in strike length and down dip; can be stacked lenses.
TEXTURES/STRUCTURE:Massive to well-layered, fine to medium-grained sulphides; gneissic sulphide textures common in metamorphosed and deformed deposits; durchbewegung textures; associated stringer ore is uncommon. Crosscutting pyrite, chalcopyrite and/or sphalerite veins with chlorite, quartz and carbonate are common.
ORE MINERALOGY (Principal and subordinate):Pyrite, pyrrhotite, chalcopyrite, sphalerite, cobaltite, magnetite, galena, bornite, tetrahedrite, cubanite, stannite, molybdenite, arsenopyrite, marcasite.
GANGUE MINERALOGY (Principal and subordinate):Quartz, calcite, ankerite, siderite, albite, tourmaline, graphite, biotite.
ALTERATION MINERALOGY:Similar to gangue mineralogy - quartz, chlorite, calcite, siderite, ankerite, pyrite, sericite, graphite.
ORE CONTROLS:Difficult to recognize; early (syndepositional) faults and mafic volcanic centres.
GENETIC MODEL:Seafloor deposition of sulphide mounds in back-arc basins, or several other tectonic settings, contemporaneous with volcanism.
ASSOCIATED DEPOSIT TYPES:Cu, Zn veins.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE:Cu, Zn, Ag, Co/Ni>1; Mn halos, Mg enrichment.
GEOPHYSICAL SIGNATURE:Sulphide lenses usually show either an electromagnetic or induced polarization signature depending on the style of mineralization and presence of conductive sulphides.
OTHER EXPLORATION GUIDES:Mafic volcanic rocks (tholeiitic, less commonly alkalic) associated with clastic rocks; Mn-rich garnets in metamorphosed exhalative horizons, possible structures, such as faults; possible association with ultramafic rocks.
ECONOMIC FACTORS
GRADE AND TONNAGE:Highly variable in size. B.C. deposits range in size from less than 1 Mt to more than 113 Mt. For example, Goldstream has a total resource (reserves and production) of 1.8 Mt containing 4.81 % Cu, 3.08 % Zn and 20.6 g/t Ag and Windy Craggy has reserves in excess of 113.0 Mt containing 1.9 % Cu, 3.9 g/t Ag and 0.08% Co. The type-locality Besshi deposits average 0.22 Mt, containing 1.5% Cu, 2-9 g/t Ag, and 0.4-2% Zn (Cox and Singer, 1986).
IMPORTANCE:Significant sources of Cu, Zn and Ag that can be found in sedimentary sequences that have not been thoroughly explored for this type of target.
REFERENCES
Cox, D.P. and Singer, D.A., Editors (1986): Mineral Deposit Models; U.S. Geological Survey, Bulletin 1693, 379 pages. Höy, T. (1991): Volcanogenic Massive Sulphide Deposits in British Columbia; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, McMillan, W.J., Coordinator, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991- 4, pages 89-123. Franklin, J.M., Lydon, J.W. and Sangster, D.M. (1981): Volcanic-associated Massive Sulfide Deposits; Economic Geology, 75th Anniversary Volume, pages 485-627. Hutchinson, R.W. (1980): Massive Base Metal Sulphide Deposits as Guides to Tectonic Evolution; in The Continental Crust and its Mineral Deposits, Strangway, D.W., Editor, Geological Association of Canada, Special Paper 20, pages 659-684. Fox, J.S. (1984): Besshi-type Volcanogenic Sulphide Deposits - a Review; Canadian Institute of Mining and Metallurgy, Bulletin, Volume 77, pages 57-68. Slack, J.F. (in press): Descriptive and Grade-Tonnage Models for Besshi-type Massive Sulphide Deposits; Geological Association of Canada, Special Paper.
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Cortesia del Gobierno de BRITISH COLUMBIA.
Ministerio de Enegia y Minas |
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Geological Targets