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. Mesoproterozoic cobaltite mineralization accompanied and followed dynamother- mal metamorphism and bimodal plutonism during the Middle Mesoproterozoic East Kootenay orogeny (ca. 1379–1325 Ma), and also accompanied Grenville-age (Late Meso- proterozoic) thermal metamorphism (ca. 1200–1000 Ma). Stratabound cobaltite-biotite Bookstrom, A.A., Box, S.E., Cossette, P.M., Frost, T.P., Gillerman, V.S., King, G.R., and Zirakparvar, N.A., 2016, Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin, in MacLean, J.S., and Sears, J.W., eds., Belt Basin: Window to Mesoproterozoic Earth: Geological Society of America Special Paper 522, p. 185–219, doi:10.1130/2016.2522(08). © 2016 The Authors. Gold Open Access: This chapter is published under the terms of the CC-BY license and is available open access on www.gsapubs.org. 185 Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/980372/spe522-08.pdf by guest on 28 September 2021 186 Bookstrom et al. ore zones typically contain cobaltite1 in a matrix of biotitite ± tourmaline ± minor xeno- time (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-sul- fide veins and breccias, or overprinted by Fe-Cu-sulfide replacement-style deposits, and locally cut by quartz veinlets ± Au-Ag ± Bi minerals. INTRODUCTION west-trending belt of Cu-Co mines, prospects, and geochemi- cal anomalies. The Idaho cobalt belt (ICB) is ~55 km long and Blackbird Cobalt-Copper (Co-Cu) District extends ~27.5 km to the southeast and northwest of its centroid. Broad ridges are deeply weathered to saprolite, and hillsides Location are mostly covered with talus and periglacial colluvium. Bed- The Blackbird Co-Cu district is in the Salmon River Moun- rock outcrops are sparse, except along steep upper slopes and tains of east-central Idaho and is ~35 km west-southwest of the road cuts. town of Salmon, Idaho (Figs. 1A and 1B). The Salmon River As shown in Figure 1B, the geologically defined Blackbird Mountains are interpreted as an uplifted and dissected plateau district is within the legally defined Blackbird Mining District, with broad ridges at nearly concordant elevations, incised by which is wider but shorter than the Idaho cobalt belt, and includes steep-walled tributaries to Salmon River Canyon, which is epithermal gold-silver (Au-Ag) deposits outside of the Idaho ~1.5 km deep. The Blackbird district occupies the central, wid- cobalt belt. The Blackbird mine area is in the west-central part of est, and most-mineralized part of the Idaho cobalt belt, a north- the Blackbird district, where it is elongate northwest. Figure 1. Simplified geologic map of the Salmon River Mountains with a tectonic index map (inset A) showing selected Precambrian features: (A) Tectonic index map (inset) showing the Idaho cobalt belt (ICB) in relation to the Mesoproterozoic Belt-Purcell Basin (Y) and the Lemhi subbasin (after O’Neill et al., 2007; Box et al., 2012; Burmester et al., 2013). Basement domains that surround or extend under different parts of the Belt-Purcell Basin include the Archean Wyoming Province (A), Medicine Hat block (A), and Paleoproterozoic-Neoarchean Clearwater block (Clwtr Block, XA) after Vervoort et al. (2015) and Wang (2015); Paleoproterozoic Selway terrane (X) after Foster et al. (2006). Major faults of the Great Falls tectonic zone (GFTZ [X] and thrust belt [XA]) are from O’Neill (1993) and Sims et al. (2004). The Perry line (PL) is a Belt Basin growth fault (after Winston, 1986). The eastern margin of the East Kootenay orogen (Y) is drawn to include sites where igneous or metamorphic minerals yield isotopic age determinations between ca. 1370 and 1320 Ma. Other features shown include the Great Divide megashear (GDM; after O’Neill et al., 2007), the Sheep Creek Cu-Co deposit (SC; after Graham et al., 2012), the rifted continental margin (RCM; after O’Neill et al., 2007), and accreted oceanic terranes (Pz-Mz—Paleozoic to Mesozoic). (B) Simplified geologic map of the Salmon River Mountains, after Evans and Green (2003), showing selected Co-Cu ore zones and prospects of the Idaho cobalt belt (after Johnson et al., 1998), all of which are in Mesoproterozoic strata of the Lemhi subbasin within the Poison Creek thrust plate. Stippling represents the southern boundary of the regional garnet zone (after Lewis et al., 2012). The Salmon Canyon fault is interpreted here as a thrust fault, which may have been a southeast-vergent thrust during the Nevadan orogeny, but may have been reactivated as a northwest-dipping lateral ramp during northeast-vergent thrusting related to the Sevier orogeny (as modeled by Skipp, 1987). Garnet-bearing rocks south of this fault are likely in its footwall, or in that of the Iron Lake thrust fault, or both. Also shown are boundaries of the legally defined Blackbird mining district (after Ross, 1941), the Blackbird Co-Cu-Au dis- trict (as defined geologically), and the Blackbird mine area, which is within the fault-bounded Blackbird structural block (Bsb) of Vhay (1948). Other abbreviated labels indicate the Salmon River (SR), the Big Deer Creek fault (BDf), and the Poison Creek fault (PCf). Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/980372/spe522-08.pdf by guest on 28 September 2021 Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin 187 Regional Geologic Setting Province and the Medicine Hat block. As shown in Figure 1A, Co-Cu mines and prospects of the ICB and the Blackbird the western Belt-Purcell basin and most of the Lemhi subbasin district are hosted in Mesoproterozoic metasedimentary rocks are within the East Kootenay orogen, which underwent dynamo- of the Lemhi subbasin of the Belt-Purcell Basin (Fig. 1A). The thermal metamorphism and plutonism in Ectasian time. Lemhi subbasin probably is underlain by cratonized oceanic The Belt-Purcell Basin and the Lemhi subbasin also are rocks of the Paleoproterozoic Selway terrane along the Great within the Cordilleran orogen (Fig. 2A), which underwent oro- Falls tectonic zone between Archean terranes of the Wyoming genesis and plutonism from Late Jurassic through Cretaceous to 114°30'W B C ru A B le s a h BRITISH r y ALBERTA w COLUMBIA a G t u e r l c o h ro CANADA E g f 49°N a a e u n lt s t t ic USA Grt RCM z K o RC n Clrwtr o ) e M Belt-Purcell lt BlocBlockk o u t Medicine Ha a e n F (XA)(XA) basin (Y) Block (A)G Selway o n y s a FTZ terrane (X n g y a RC c in M MONTANA x r o er p ro SC iv S IDAHO Helena WASHINGTON g R t e sub-basin o n g Salmon Canyon n H ( o Accreted Y PL Copper ult lm ) thrust belt (XA) n fa Sa terranes Salmon Wyomin anyo Salmon C (Pz-Mz) IC Province SR .