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BLACKBIRD SEDIMENT-HOSTED Cu-Co / E15

Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas

IDENTIFICATION

SYNONYM:

Sediment-hosted Cu-Co deposit.

COMMODITIES (BYPRODUCTS):

Cu, Co, (Au, Bi, Ni, Ag; possibly Pb, Zn).

EXAMPLES

(British Columbia - Canada/International): Canadian examples are not known; Blackbird, Bonanza Copper and Tinker's Pride (Idaho, USA), possibly Sheep Creek deposits (Montana, USA).

GEOLOGICAL CHARACTERISTICSregrese arriba
CAPSULE DESCRIPTION:

Pyrite and minor pyrrhotite, cobaltite, chalcopyrite, arsenopyrite and magnetite occur as disseminations, small veins and tabular to pod-like lenses in sedimentary rocks. Chloritic alteration and tourmaline breccias are locally associated with mineralization

TECTONIC SETTINGS:

Near continental margins or in intracratonic basins. Within the Belt-Purcell basin, which may have formed in a large inland sea, extensional tectonics are suggested by possible turbidite deposition, growth faulting, gabbroic sills and (?)tuff deposition. Alternative setting is marine, in an incipient or failed rift along a continental margin..

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:

These deposits are not well understood. Possible turbidite deposition in marine or inland sea, associated with basaltic pyroclastic volcanics or mafic synsedimentary gabbroic sills; alternatively, tidal flat environment.

AGE OF MINERALIZATION:

Can be of any age. The Blackbird deposits at the type locality are assumed to be approximately 1460 Ma, the age of the hostrocks.

HOST / ASSOCIATED ROCK TYPES:

Fine-grained metasedimentary rocks; thin-bedded siltstone, fine-grained quartzite, black argillite and calcareous siltstone; garnet schist, phyllite, quartz-mica schist. In the Blackbird district synaeresis cracks (subaqueous shrinkage cracks) occur within immediate hostrocks, sedimentary structures indicative of shallow water, and locally subaerial exposure in overlying rocks, suggest shallow water environment. Numerous biotite-rich beds within the host succession may be mafic tuff units (or diorite sills ?). Sheep Creek deposits are within correlative Newland Formation dolomitized shales and conglomerates.

DEPOSIT FORM:

Irregular, tabular to pod-like deposits, from approximately 2 to 10 m thick.

TEXTURES/STRUCTURE:

Fine to fairly coarse grained, massive to disseminated sulphides; pyrite locally has colloform textures. Locally sheared; vein sulphides in some deposits; quartz-tourmaline breccia pipes (?).

ORE MINERALOGY (Principal and subordinate):

Cobaltite, chalcopyrite, pyrite, pyrrhotite, gold and silver in breccia pipes; arsenopyrite, magnetite, cobaltian pyrite. Sheep Creek: pyrite, marcasite, chalcopyrite, tennantite plus cobalt minerals; covellite, bornite in barite.

GANGUE MINERALOGY:

quartz, biotite, barite; tourmaline, hornblende, chlorite, muscovite, ankerite, dolomite, siderite, calcite and apatite.

ALTERATION MINERALOGY:

Silicification and intense chloritization; locally quartz- tourmaline breccias.

WEATHERING:

Supergene enrichment with ludlamite and vivianite; erythrite (cobalt bloom); intense gossans at surface.

ORE CONTROLS:

Regional controls include synsedimentary extensional fault structures, basin margin and growth faults. Local controls include association with mafic tuffs and stacked deposits at several stratigraphic intervals separated by barren rock.

GENETIC MODEL:

Based on stratabound nature of deposits and similarity with unmetamorphosed Sheep Creek deposits, the Blackbird lenses are interpreted to be either syngenetic or diagenetic.

ASSOCIATED DEPOSIT TYPES:

Possibly Besshi volcanogenic massive sulphide deposits (G04), Fe formations (F10), base metal veins, tourmaline breccias.

COMMENTS:

Sheep Creek deposits are a relatively new exploration target in Belt rocks in Montana. They are in equivalent, lower metamorphic grade hostrocks to those of the Blackbird deposits, and have similar mineralogy and trace metal geochemistry. Lower Purcell Supergroup rocks and other structurally controlled sedimentary basins associated with variable mafic magmatism are prospective hosts in Canada.

EXPLORATION GUIDES regrese arriba
GEOCHEMICAL SIGNATURE:

Enriched in Fe, As, B, Co. Cu, Au, Ag and Mn; may be depleted in Ca and Na. Sheep Creek also contains high Ba.

GEOPHYSICAL SIGNATURE:

Airborne and ground geophysical surveys, such as electromagnetics or magnetics should detect deposits that have massive sulphide zones, especially if these are steeply dipping. However, the presence of graphite-rich zones in the host sediments can complicate the interpretation of EM conductors. Also, if the deposits are flat lying and comprised of fine laminae distributed over a significant stratigraphic interval, the geophysical response is usually too weak to be definitive. Induced polarization can detect flat-lying deposits, especially if disseminated feeder zones are present.

OTHER EXPLORATION GUIDES:

Proximity to mafic tuffs or possibly early gabbroic sills, rapid sedimentary facies changes indicative of growth faults; regional pyrite development; may grade laterally to pyritic zones with anomalous Pb-Zn.

ECONOMIC FACTORS regrese arriba
GRADE AND TONNAGE:

The Blackbird district deposits range from less than 100 000 t to 1.3 Mt containing 0.4 - 0.6 % Co and 1.3% Cu. Two zones of the Sheep Creek deposits contain respectively 4.5 Mt of 2.5% Cu and 0.12% Co, and 1.8 Mt with 6% Cu. Variable gold, up to 20 g/t in Blackbird lenses.

ECONOMIC LIMITATIONS:

Generally lower copper grades favour open pit mining; Au and Ag are important byproducts.

IMPORTANCE:

Small past producers of copper, cobalt and gold in Idaho.

REFERENCES regrese arriba
  • ACKNOWLEDGMENT: This deposit profile draws heavily from the USGS descriptive deposit model of Blackbird Co-Cu by Robert Earhart.

  • Anderson, A.L. (1947): Cobalt Mineralization in the Blackbird District, Lemhi County, Idaho; Economic Geology, Volume 42, pages 22-46.

  • Earhart, R.L. (1986): Descriptive Model of Blackbird Co-Cu; in Mineral Deposit Models, Cox, D.P and Singer, D.A., Editors, US Geological Survey, Bulletin 1693, page 142.

  • Himes, M.D. and Petersen, E.U. (1990): Geological and Mineralogical Characteristics of the Sheep Creek Copper-Cobalt Sediment-hosted Stratabound Sulfide Deposit, Meagher County, Montana; in Gold '90 Symposium, Salt Lake City, Utah, Chapter 52, Society of Economic Geologists, pages 533-546.

  • Hughes, G.J. (1982): Basinal Setting of the Idaho Cobalt Belt, Blackbird Mining District, Lemhi County, Idaho; in the Genesis of Rocky Mountain Ore Deposits; Changes with Time and Tectonics, Denver Region Geologists Society, pages 21-27.


Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas
Business contact: Commentaries, proposals or details for negotiation: Mr. Jorge Perazzo
51 1 2638212 (spanish please) 1—602-499 2708 (USA)
jperazzo@miningperu.com
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