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Cobre sedimentar

by David V. Lefebure and Dani J. Alldrick
British Columbia Geological Survey


Ref: cobre, Cu, sedimentar, Cu, Ag (Co, Pb, Zn, PGE, Au, U, Va) arenito, calcocita, calcopirita, carbonato, evaporito

Lefebure, D.V.and Alldrick, D.J. (1996): Sediment-hosted Cu+/-Ag+/-Co, 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 13-16.


SYNONYMS: Sediment-hosted stratiform copper, shale-hosted copper, Kupferschiefer-type, redbed Cu, Cu-shale, sandstone Cu.

COMMODITIES (BYPRODUCTS): Cu, Ag (Co, Pb, Zn, rarely PGE, Au, U, Va).

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Roo (082GSW020), Commerce (082GSE065), Chal 4 (092K068); Redstone (Northwest Territories, Canada), Kennicott (Alaska, USA), Spar Lake (Troy), Rock Creek and Montanore (Montana, USA), White Pine (Michigan, USA), Creta (Oklahoma, USA), Corocoro (Bolivia), Mansfield-Sangerhausen and Spremberg, Kupferschiefer district (Germany), Konrad and Lubin (Poland), Dzherkazgan (Kazakhstan), Copper Claim (Australia), Kamoto and Shaba, Zambia-Zaire copperbelt (Zaire).


CAPSULE DESCRIPTION: Stratabound disseminations of native copper, chalcocite, bornite and chalcopyrite in a variety of continental sedimentary rocks including black shale, sandstone and limestone. These sequences are typically underlain by, or interbedded with, redbed sandstones with evaporite sequences. Sulphides are typically hosted by grey, green or white strata.

TECTONIC SETTINGS: Predominantly rift environments located in both intracontinental and continental-margin settings; they can also occur in continental-arc and back- arc settings.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: The characteristic presence of redbed and evaporite sequences points to deposition of sediments in a hot, arid to semi-arid paleoclimate near the paleoequator. The host rocks are produced in a variety of local anoxic depositional environments, including deltaic sediments, Sabkha-type lagoonal carbonate basins or high intertidal mudflats, and shallow “coal basins”.

AGE OF MINERALIZATION: Proterozoic or younger; Middle Proterozoic, Permian and early Mesozoic most favourable ages.

HOST/ASSOCIATED ROCK TYPES: Most deposits are hosted by pale gray to black shale, but some are found in sandstone, siltstone, limestone, silty dolomite, laminated carbonate units (sabkha origin) and quartzites. Favourable horizons contain reactive organic matter or sulphur. Algal mats, mudcracks and scour-and-fill structures indicative of shallow-water deposition are common. Local channel- conglomerate beds sometimes contain wood fragments. The associated sequence includes redbed sediments, evaporites and sometimes volcanics. In many cases the rift-related layered rocks rest unconformably on older basement rocks.

DEPOSIT FORM: Orebodies are generally conformable with the bedding, although in detail ore may transgress bedding at low angles and is typically more transgressive near the margins of the deposit. Mineralized horizons are from tens of centimetres to several metres thick (rarely more than 5 m); they are often contained within broader zones of anomalous copper values. Tabular ore zones extend laterally for kilometres to tens of kilometres. Less commonly the deposits are elongate lobes. Some deposits have a C-shaped, “roll front” configuration in cross-section. Common lateral and/or vertical zoning is from hematite (barren) > chalcocite > bornite > chalcopyrite > pyrite, or from a chalcociteñbornite core grading to chalcopyrite with peripheral galena and sphalerite.

TEXTURE/STRUCTURE: Sulphides are fine grained and occur as disseminations, concentrated along bedding, particularly the coarser grained fractions, or as intergranular cement. Sharp-walled cracks or veinlets (< 1 cm thick, < than a metre in length) of chalcopyrite, bornite, chalcocite, galena, sphalerite or barite with calcite occur in some deposits, but are not an important component of the ore. Pyrite can be framboidal or colloform. Cu minerals often replace pyrite grains, framboids and nodules; less commonly they form pseudomorphs of sulphate nodules or blade-shaped gypsum/anhydrite grains. They also cluster around carbonaceous clots or fragments.

ORE MINERALOGY (Principal and subordinate): Chalcocite, bornite and chalcopyrite; native copper in some deposits. Pyrite is abundant in rocks outside the ore zones. Enargite, digenite, djurleite, sphalerite, galena, tennantite, native silver with minor Co-pyrite and Ge minerals. In many deposits carrollite (CuCo2S4) is a rare mineral, however, it is common in the Central African Copperbelt.

GANGUE MINERALOGY (Principal and subordinate): Not well documented; in several deposits carbonate, quartz and feldspar formed synchronously with the ore minerals and exhibit zonal patterns that are sympathetic with the ore minerals. They infill, replace or overgrow detrital or earlier authigenic phases.

ALTERATION MINERALOGY: Lateral or underlying reduced zones of green, white or grey colour in redbed successions. In the Montana deposits these zones contain chlorite, magnetite and/or pyrite. Barren, hematite-rich, red zones grade into ore in the Kupferschiefer. Kupferschiefer ore hosts also show elevated vitrinite reflectances compared to equivalent stratigraphic units.

WEATHERING: Surface exposures may be totally leached or have malachite and azurite staining. Near surface secondary chalcocite enrichment is common.

ORE CONTROLS: Most sediment-hosted Cu deposits are associated with the sag phase of continental rifts characterized by deposition of shallow-water sediments represented by redbed sequences and evaporites. These formed in hot, arid to semi-arid paleoclimates which normally occur within 20-30øof the paleoequator. Hostrocks are typically black, grey or green reduced sediments with disseminated pyrite or organics. The main control on fluid flow from the source to redoxcline is primary permeability within specific rock units, commonly coarse-grained sandstones. In some districts deposits are located within coarser grained sediments on the flanks of basement highs. Growth faults provide local controls in some deposits (e.g., Spar Lake).

ASSOCIATED DEPOSIT TYPES: Sandstone U (D05), volcanic redbed Cu , Kipushi Cu-Pb-Zn  evaporite halite, sylvite, gypsum and anhydrite; natural gas in Poland.

GENETIC MODELS: Traditionally these deposits have been regarded as syngenetic, analogous to sedex deposits or late hydrothermal epigenetic deposits. Currently most researchers emphasize a two-stage diagenetic model. Carbonaceous shales, sandstones and limestones deposited in reducing, shallow subaqeuous environments undergo diagenesis which converts the sulphur in these sediments to pyrite. At a later stage during diagenesis, saline low-temperature brines carrying copper from a distant source follow permeable units, such as oxidized redbed sandstones, until they encounter a reducing unit. At this point a redoxcline is established with a cuperiferous zone extending “downstream” until it gradually fades into the unmineralized, often pyritic, reducing unit. The source of the metals is unresolved, with possible choices including underlying volcanic rocks, labile sediments, basement rocks or intrusions.

COMMENTS: Sediment-hosted Cu includes Sabkha Cu deposits which are hosted by thin- bedded carbonate-evaporite-redbed ‘sabkha’ sequences.


GEOCHEMICAL SIGNATURE: Elevated values of Cu, Ag, Pb, Zn and Cd are found in hostrocks, sometimes with weaker Hg, Mo, V, U, Co and Ge anomalies. Dark streaks and specks in suitable rocks should be analysed as they may be sulphides, such as chalcocite.

GEOPHYSICAL SIGNATURE: Weak radioactivity in some deposits.

OTHER EXPLORATION GUIDES: Deposits often occur near the transition from redbeds to other units which is marked by the distinctive change in colour from red or purple to grey, green or black. The basal reduced unit within the stratigraphy overlying the redbeds will most often carry the highest grade mineralization.


TYPICAL GRADE AND TONNAGE: Average deposit contains 22 Mt grading 2.1 % Cu and 23 g/t Ag (Mosier et al., 1986). Approximately 20% of these deposits average 0.24 % Co. The Lubin deposit contains 2600 Mt of >2.0% Cu and ~ 30-80 g/t Ag. Spar Lake pre-production reserves were 58 Mt grading 0.76% Cu and 54 g/t Ag. Montanore contains 134.5 Mt grading 0.74% Cu and 60 g/t Ag, while Rock Creek has reserves of 143.7 Mt containing 0.68 % Cu and 51 g/t Ag.

ECONOMIC LIMITATIONS: These relatively thin horizons require higher grades because they are typically mined by underground methods. The polymetallic nature and broad lateral extent of sediment-hosted Cu deposits make them attractive.

IMPORTANCE: These deposits are the second most important source of copper world wide after porphyry Cu deposits. They are an interesting potential exploration target in British Columbia, although there has been no production from sediment-hosted Cu deposits in the province. The stratigraphy that hosts the Spar Lake, Montanore and Rock Creek deposits in Montana extends into British Columbia where it contains numerous small sediment-hosted Cu-Ag deposits.


ACKNOWLEDGEMENTS: Nick Massey contributed to the original draft of the profile.

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Bateman, A.M. and McLaughlin, D.H. (1920): Geology of the Ore Deposits of Kennecott, Alaska; Economic Geology, Volume 15, pages 1-80.

Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C. and Kirkham, R.V., Editors, (1989): Sediment-hosted Stratiform Copper Deposits; Geological Association of Canada, Special Paper 36, 710 pages.

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Ensign, C.O., White, W.S., Wright, J.C., Patrick, J.L., Leone, R.J., Hathaway, D.J., Tramell, J.W., Fritts, J.J. and Wright, T.L. (1968): Copper Deposits of the Nonesuch Shale, White Pine, Michigan; in Ore Deposits of the United States, 1933- 1967; The Graton-Sales Volume, Ridge, J.D., Editor, American Institute of Mining, Metallurgy and Petroleum Engineers, Inc., New York, pages 460-488.

Fleischer, V.D., Garlick, W.G. and Haldane, R. (1976): Geology of the Zambian Copperbelt; in Handbook of Strata-bound and Stratiform Ore Deposits, Wolf, K.H., Editor, Elsevier, Amsterdam, Volume 8, pages 223-352.

Gustafson, L.B. and Williams, N. (1981): Sediment-hosted Stratiform Deposits of Copper, Lead and Zinc; in Economic Geology Seventy-Fifth Anniversary Volume, Skinner, B.J., Editor, Economic Geology Publishing Company, pages 139-178.

Hayes, T.S. and Einaudi, M.T. (1986): Genesis of the Spar Lake Strata-bound Copper- Silver Deposit, Montana: Part I. Controls Inherited from Sedimentation and Preore Diagenesis; Economic Geology, Volume 81, pages 1899-1931.

Kirkham, R.V., (1989): Distribution, Settings, and Genesis of Sediment-hosted Stratiform Copper Deposits, in Sediment-hosted Stratiform Copper Deposits, Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C. and Kirkham, R.V., Editors, Geological Association of Canada, Special Paper 36, pages 3-38.

Kirkham, R.V., Carri‚re, J.J., Laram‚e, R.M. and Garson, D.F. (1994): Global Distribution of Sediment-hosted Stratiform Copper Deposits; Geological Survey of Canada, Open File Map 2915, 1: 35 000 000.

Mosier, D.L., Singer, D.A. and Cox, D.P. (1986): Grade and Tonnage Model of Sediment- hosted Cu; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, pages 206-8.

Renfro, A.R. (1974): Genesis of Evaporite-associated Stratiform Metalliferous Deposits - a Sabkha Process; Economic Geology, Volume 69, pages 33-45.

DEPÓSITOS - 26/04/2004 18:30:00

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