Geologic History of the Blackbird Co-Cu District in the Lemhi Subbasin of the Belt-Purcell Basin
8. OLD G The Geological Society of America Special Paper 522 OPEN ACCESS Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin Arthur A. Bookstrom Stephen E. Box Pamela M. Cossette Thomas P. Frost U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA Virginia S. Gillerman Idaho Geological Survey, 322 E. Front Street, Boise, Idaho 83702, USA George R. King Formation Capital Corporation, 812 Shoup Street, Salmon, Idaho 83467, USA N. Alex Zirakparvar American Museum of Natural History, New York, New York 10024, USA ABSTRACT 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 north- west-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 rela- tionships and isotopic age determinations indicates that most cobalt mineralization occurred in Mesoproterozoic time, whereas most copper mineralization occurred in Cretaceous time.
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