Idaho Cobalt Belt - Blackbird, Blacktail, Salmon Canyon, Chicago, Merle, Brown Bear, Ram

Idaho, USA

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The Blackbird cobalt deposit is near the centre of the 60 km long, NW-SE oriented Idaho Cobalt Belt in central northeastern Idaho, within 50 km of, and parallel to, the border with Montana to the NE. The Blackbird mine is located 40 km west of the town of Salmon, ~175 km SW of Butte, Montana, and 230 km NE of Boise, Idaho, in western USA (#Location: 45°: 7' 35"N, 114° 21' 0"W). Other deposits in the belt include Blacktail, Brown Bear, Chicago, Dandy, Salmon Canyon, Iron Creek, Sunshine, Merle, Black Pine, Haynes-Stellite and Idaho.

The mineral deposits of the Idaho Cobalt Belt lie within the broad, ~100 km wide, NE-SW oriented Great Falls tectonic zone, and are hosted within three NW-SE trending thrust fault-defined rock packages in the footwall of the regional-scale, Late Cretaceous Iron Lake Fault (Lund and Tysdal, 2007). These thrust slices are located within the southwestern section of the Mesoproterozoic Belt-Purcell basin, which is dominated by S-tectonites, in which metamorphic and planar fabrics were imparted during the Cretaceous (Sevier) Orogeny.

The three thrust slices comprise, from east to west, the lower (Haynes-Stellite), middle (Blackbird) and upper (Indian Creek) plates, composed respectively of i). middle greenschist facies biotite-phyllite and metasandstone; ii). pervasively foliated biotite-phyllite to schist at middle to upper greenschist facies; iii). distinctly layered, tightly folded, garnet-biotite gneiss and schist at upper greenschist to lower amphibolite facies grade. These rocks are interpreted to represent metamorphosed equivalents of the Yellowjacket, Apple Creek and Gunsight formations of the Ravalli, Piegan (or Middle Belt carbonate) and Missoula Groups respectively, of the Belt Supergroup.

Within the main Belt basin, the Belt-Purcell Supergroup consists of i). lower Belt formations, a southwest-derived deep-water turbidite wedge with locally abundant mafic sills, ii). the Ravalli Group, a largely sub-aerial, southwest-derived fluvial-deltaic complex, iii). the Piegan Group (Middle Belt carbonate), a cyclic carbonate and siliciclastic sequence of either lacustrine or marine setting, capped by a sequence of mafic to felsic lavas, and iv). the Missoula Group, a south- and east-derived fluvial succession (Winston and Link, 1993; Link et al., 2007; Winston, 2007). Most of the exposed stratigraphic section of the Belt-Purcell Supergroup was deposited between 1470 and 1400 Ma (Anderson and Davis, 1995; Evans et al., 2000). Intrusive rocks of Mesoproterozoic (porphyritic biotite granite and granodiorite), Neoproterozoic to Late Cambrian (including Cambrian syenite-diorite suites), Cretaceous (the extensive S-type Idaho Batholith granites) and Eocene age, outcrop within the district and surrounding region (Evans and Zartman, 1990; Tysdal et al., 2003; Lund et al., 2010).

Stratabound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation within the linear Idaho Cobalt Belt. The Apple Creek Formation generally comprises a thick sequence of metamorphosed argillite, siltite and quartzite with intercalations of biotite-rich layers. A large body of Mesoproterozoic granite, augen gneiss and mafic intrusive rocks (the 1380 to 1370 Ma Big Deer Creek pluton) occurs within 5 km to the north and east of the Blackbird Mine area.

The Co-Cu mineralisation of the Blackbird Mine is predominantly hosted within the banded siltite unit of the Apple Creek Formation, locally extending into the underlying coarse siltite unit of the same formation and the overlying feldspathic quartzites of the basal Gunsight Formation (Lund and Tysdal, 2007). A gold-bearing layer occurs stratigraphically above the Co-Cu deposits, associated with metachert that has been interpreted to be siliceous tuff. Gold is also found within the Co-Cu ores of the Blackbird district (Nash et al., 1987).

Mineralisation, is overall strata-bound and includes both "stratiform" and vein-type orebodies. The major ore minerals are cobaltite and chalcopyrite, with minor pyrite, arsenopyrite, safflorite and pyrrhotite. Quartz, biotite, chlorite, muscovite, garnet, tourmaline, apatite and siderite are gangue minerals in the Blackbird deposit (Slack, 2007). There is a close association between cobalt-copper mineralisation and the occurrence of biotite-rich rocks, with the biotite-rich rocks interpreted to represent mafic tuffs by some authors (e.g., Nash and Hahn, 1987). High concentrations of REEs and yttrium are also recodrded from the Idaho Cobalt Belt (Gillerman et al., 2000; Slack, 2006, 2007).

Both stratabound and vein-type mineralisation occurs at the Salmon Canyon prospect, hosted within upper amphibolite facies schists and gneisses of the Apple Creek Formation. Cobaltite mostly occurs as porphyroblasts within the coarse-grained stratbound ores in association with arsenopyrite. Chalcopyrite, pyrite, pyrrhotite and minor sphalerite are also present in the ores, which have been metamorphosed and remobilised (Nold, 1990). At the Black Pine prospect, which is located between the Iron Creek prospect and Blackbird deposit, mineralisation comprises bedded chalcopyrite with a variable cobaltiferous arsenopyrite content and minor cobaltite (Bending and Scales, 2001). There are two mineralised zones at the Iron Creek prospect, i). a lower magnetite-rich, and ii). an upper sulphide-rich (cobaltiferous pyrite and chalcopyrite) zone. Two generations of pyrite have been recognised: the fine-grained stratiform and the coarse-grained epigenetic varieties. Most of the cobalt is contained within the epigenetic variety of pyrite (Nash, 1989a).

Deposits within the Blackbird district are intrafolial concordant layers and lenses in schist and gneiss and discordant lenses in metamorphosed siliciclastic rocks of the Apple Creek Formation. Several of these sulphide deposits (e.g., Dandy) are discordant to metamorphic layers, forming vein-type structures (Lund et al., 2011). Elsewhere within the district, (e.g., Haynes-Stellite deposit), discordant tourmalinised breccia bodies contain abundant cobaltite and xenotime (Lund et al., 2011; Slack, 2012). The sulphide-rich deposits, concordant to metamorphic layering, vary from <1 to 4 m thick and mainly comprise cobaltite and chalcopyrite in a gangue of Fe-rich biotite with lesser quartz, chlorite, garnet, chloritoid, tourmaline and/or muscovite (Anderson, 1947; Purdue, 1975; Modreski, 1985; Nash and Hahn, 1989). The Sunshine prospect differs in having a quartz-rich gangue with minor chlorite (Eiseman, 1988). Rare magnetite is generally a minor constituent, but is locally abundant in several small deposits and prospects (Slack, 2012). Other ore minerals include pyrite, pyrrhotite, arsenopyrite, glaucodot, safflorite, bismuthinite, native Bi, Bi tellurides and gold. Very high Bi contents of >1 wt.% are locally present, including 9.2 wt.% in a Cu-rich sample from the Ram deposit. Sparse uraninite, sphalerite, stannite and an unidentified Bi selenide are also present (Slack, 2012), while some of the sulphide deposits, e.g., Sunshine, Merle, Brown Bear and Ram, contain minor to abundant allanite, apatite, xenotime, and/or monazite (Slack, 2006).

Aleinikoff et al. (2012) present data they interpret to indicate i). the host sedimentary rocks to the mineralisation were deposted between 1409±10 and ~1370 Ma, ii). granitoid plutons in the district were emplaced between 1383±4 and 1359±7 Ma and may be related to the first phase of the REE, Co-Cu and Au mineralisation, iii). Cretaceous xenotime, monazite and associated cobaltite formed in multiple phases that are interpreted as remobilised and reprecipitated Mesoproterozoic sulphide ore during Middle to Late Cretaceous metamorphic events.

Production in the Blackbird district commenced in 1917, after the initial discovery of sulphide mineralisationin 1893, with minor Co being removed from the Haynes-Stellite Mine between 1917 and 1920 (Bennett, 1977). Major production began in 1949 and continued intermittently until 1960 at the Blackbird (Uncle Sam) underground mine and the Blacktail open pit, together producing ~5 Mt @ 1.5 wt.% Cu, 0.6 wt.% Co (Nash and Hahn, 1989; Bending and Scales, 2001).

Bennett (1977) used Hanna Mining Co.data to calculate unmined reserves for the Blacktail, Brown Bear, Chicago, Dandy and Idaho deposits at 5.9 Mt @ 1.31 wt.% Cu, 0.55 wt.% Co.

Formation Metals Inc. (2009; 2012) reported a total resource for four deposits of:
    11.7 Mt @ 0.7 wt.% Co, 1.1 wt.% Cu, including:
    The Ram deposit, which has,
        proven, probable and diluted reserves, (0.2% Co cut-off), of 2.64 Mt @ 0.56 wt.% Co, 0.60 wt.% Cu, 0.4 g/t Au, plus,
        inferred resource of 1.12 Mt @ 0.59 wt.% Co, 0.79 wt.% Cu, 0.53 g/t Au.

The Black Pine deposit (~15 km SE of the district) has a small sulphide resource of 1.0 Mt @ 4.5 wt.% Cu, 0.08 wt.% Co, 1.03 g/t Au (Johnson et al., 1998).

Slack et al., 2012) indicates that past production + reserves and resources in 14 deposits of the Blackbird district, excluding Black Pine, total 16.8 Mt @ 0.74 wt.% Co, 1.37 wt.% Cu, 1.04 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Aleinikoff J N, Slack J F, Lund K, Evans K V, Fanning C M, Mazdab F K, Wooden J L and Pillers R M,  2012 - Constraints on the Timing of Co-Cu ± Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks : in    Econ. Geol.   v.107 pp. 1143-1175
Johnson C A, Bookstrom A A and Slack J F,  2012 - Sulfur, Carbon, Hydrogen, and Oxygen Isotope Geochemistry of the Idaho Cobalt Belt: in    Econ. Geol.   v.107 pp. 1207-1221
Landis G P and Hofstra A H,  2012 - Ore Genesis Constraints on the Idaho Cobalt Belt from Fluid Inclusion Gas, Noble Gas Isotope, and Ion Ratio Analyses : in    Econ. Geol.   v.107 pp. 1189-1205
Lund K, Tysdal R G, Evans K V, Kunk M J and Pillers R M,  2011 - Structural Controls and Evolution of Gold-, Silver-, and REE-Bearing Copper-Cobalt Ore Deposits, Blackbird District, East-Central Idaho: Epigenetic Origins : in    Econ. Geol.   v.106 pp. 585-618

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