Abstract The sediment-hosted gold–bearing cobalt–copper Blackbird deposits of east–central Idaho, U.S.A., are unique deposits hosted in enigmatic and poorly known Mesoproterozoic strata. New regional mapping studies elucidate the geographic distribution of units of the very thick Mesoproterozoic succession and the geometry of major structures. Blackbird deposits are hosted predominantly by the banded siltite unit of the Mesoproterozoic Apple Creek Formation; a few mineralized zones are in the underlying coarse siltite unit of the Apple Creek and the overlying basal Gunsight Formation. The depositional mechanism for hosting strata was predominantly turbidity– current flow. Blackbird deposits were long thought to be hosted in the Yellowjacket Formation, which was mapped as the dominant formation throughout central Idaho but, as originally defined, formed in a relatively shallow–water marine environment. Our conclusions about the host strata, in conjunction with previous interpretations that Blackbird deposits formed in a rift setting, resolves previous contradictions between models for environments of sediment deposition and origin of metals accumulation. Along the west side of the Blackbird district, the Apple Creek and Yellowjacket formations are separated by the northwest–trending Late Cretaceous Iron Lake fault. The Yellowjacket Formation is confined to the Iron Lake thrust plate west of the district and does not contain sediment–hosted deposits. In contrast, the Blackbird deposits lie in the complexly deformed upper part of the underlying Poison Creek plate, specifically at the northeast–trending hinge zone of the Blackbird Mountain oblique ramp. Near the ramp, both hanging–wall and footwall rocks are strongly deformed, resulting in major overturned folds in the hanging wall and imbricate thrust plates in the footwall. The upper imbricate in the Poison Creek plate contains chloritoid–garnet schists that were long recognized as being above the garnet isograd and having a different mineralization style. The intermediate imbricate forms the lower to middle greenschist Blackbird subplate containing bedding–parallel and structurally remobilized cobalt–copper deposits. The structurally lowest imbricate forms the Haynes– Stellite subplate and includes the youngest rocks (lower Gunsight Formation) and mineralized tourmaline breccias. Subsequent to deformation related to the Iron Lake thrust fault, several normal faults reactivated the Iron Lake thrust fault, the Blackbird Mountain ramp, and the buried Great Falls tectonic zone in the basement. This complex deformation resulted in juxtaposition of metamorphic facies and stratigraphic units from different structural levels as well as from different parts of the stratigraphic section. Present exposure of strata of the Apple Creek sedimentary basin, in which mineralized strata formed, is controlled by northwest– trending thrust faults and normal faults. Our study results in limiting the permissive stratigraphy that hosts the deposits in a meaningful way for future identification of related deposits. It also shows that the previous outline of an Idaho cobalt belt did not represent geometry of the rift basin per se, but instead is the expression of structural control of exposure of the hosting Apple Creek Formation, primarily those strata that lie in the Poison Creek plate.

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 cobaltite 1 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 F 2 folds with axial-planar S 2 cleavage and schistosity. Discordant replacement-style lodes of cobaltite 2 -biotite ore ± xenotime 2 (ca. 1320–1270 Ma) commonly follow S 2 fractures and fabrics. Discordant quartz-biotite and quartz-tourmaline breccias, and veins contain cobaltite 3 ± xenotime 3 (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.