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Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin

By
Arthur A. Bookstrom
Arthur A. Bookstrom
U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA
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Stephen E. Box
Stephen E. Box
U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA
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Pamela M. Cossette
Pamela M. Cossette
U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA
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Thomas P. Frost
Thomas P. Frost
U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA
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Virginia S. Gillerman
Virginia S. Gillerman
Idaho Geological Survey, 322 E. Front Street, Boise, Idaho 83702, USA
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George R. King
George R. King
Formation Capital Corporation, 812 Shoup Street, Salmon, Idaho 83467, USA
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N. Alex Zirakparvar
N. Alex Zirakparvar
American Museum of Natural History, New York, New York 10024, USA
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Published:
September 01, 2016

The Blackbird cobalt-copper (Co-Cu) district in the Salmon River Mountains of east-central Idaho occupies the central part of the Idaho cobalt belt—a northwest-elongate, 55-km-long belt of Co-Cu occurrences, hosted in grayish siliciclastic metasedimentary strata of the Lemhi subbasin (of the Mesoproterozoic Belt-Purcell Basin). The Blackbird district contains at least eight stratabound ore zones and many discordant lodes, mostly in the upper part of the banded siltite unit of the Apple Creek Formation of Yellow Lake, which generally consists of interbedded siltite and argillite. In the Blackbird mine area, argillite beds in six stratigraphic intervals are altered to biotitite containing over 75 vol% of greenish hydrothermal biotite, which is preferentially mineralized.

Past production and currently estimated resources of the Blackbird district total ~17 Mt of ore, averaging 0.74% Co, 1.4% Cu, and 1.0 ppm Au (not including downdip projections of ore zones that are open downward). A compilation of relative-age relationships and isotopic age determinations indicates that most cobalt mineralization occurred in Mesoproterozoic time, whereas most copper mineralization occurred in Cretaceous time.

Mesoproterozoic cobaltite mineralization accompanied and followed dynamothermal metamorphism and bimodal plutonism during the Middle Mesoproterozoic East Kootenay orogeny (ca. 1379–1325 Ma), and also accompanied Grenvilleage (Late Mesoproterozoic) thermal metamorphism (ca. 1200–1000 Ma). Stratabound cobaltite-biotite ore zones typically contain cobaltite1 in a matrix of biotitite ± tourmaline ± minor xenotime (ca. 1370–1320 Ma) ± minor chalcopyrite ± sparse allanite ± sparse microscopic native gold in cobaltite. Such cobaltite-biotite lodes are locally folded into tight F2 folds with axial-planar S2 cleavage and schistosity. Discordant replacement-style lodes of cobaltite2-biotite ore ± xenotime2 (ca. 1320–1270 Ma) commonly follow S2 fractures and fabrics. Discordant quartz-biotite and quartz-tourmaline breccias, and veins contain cobaltite3 ± xenotime3 (ca. 1058–990 Ma).

Mesoproterozoic cobaltite deposition was followed by: (1) within-plate plutonism (530–485 Ma) and emplacement of mafic dikes (which cut cobaltite lodes but are cut by quartz-Fe-Cu-sulfide veins); (2) garnet-grade metamorphism (ca. 151–93 Ma); (3) Fe-Cu-sulfide mineralization (ca. 110–92 Ma); and (4) minor quartz ± Au-Ag ± Bi mineralization (ca. 92–83 Ma).

Cretaceous Fe-Cu-sulfide vein, breccia, and replacement-style deposits contain various combinations of chalcopyrite ± pyrrhotite ± pyrite ± cobaltian arsenopyrite (not cobaltite) ± arsenopyrite ± quartz ± siderite ± monazite (ca. 144–88 Ma but mostly 110–92 Ma) ± xenotime (104–93 Ma). Highly radiogenic Pb (in these sulfides) and Sr (in siderite) indicate that these elements resided in Mesoproterozoic source rocks until they were mobilized after ca. 100 Ma. Fe-Cu-sulfide veins, breccias, and replacement deposits appear relatively undeformed and generally lack metamorphic fabrics.

Composite Co-Cu-Au ore contains early cobaltite-biotite lodes, cut by Fe-Cu-sulfide veins and breccias, or overprinted by Fe-Cu-sulfide replacement-style deposits, and locally cut by quartz veinlets ± Au-Ag ± Bi minerals.

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GSA Special Papers

Belt Basin: Window to Mesoproterozoic Earth

John S. MacLean
John S. MacLean
Department of Physical Science, Southern Utah University, 351 W. University Boulevard, Cedar City, Utah 84720, USA
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James W. Sears
James W. Sears
Department of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, Montana 59812-1296, USA
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Geological Society of America
Volume
522
ISBN print:
9780813725222
Publication date:
September 01, 2016

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