ABSTRACT: Understanding of very thick Late Devonian shelf strata in Idaho is hindered by formation terminologies. Interpreted genetically, and in combination with lower accommodation settings in Montana, strata reveal craton-to-basin geometries and analogues similar to other western Laurentian basins. The Jefferson Formation Birdbear Member and Three Forks Formation in Montana are correlated to the Jefferson Grandview Dolomite in Idaho using regional sequence stratigraphic surfaces. A new stratigraphic framework defines three widely deposited latest Frasnian sequences and Early Famennian intrashelf basin paleogeography. Peritidal to marine mixed siliciclastic and carbonate rocks of the Middle Devonian lower Jefferson Formation in Idaho are overlain by the Frasnian Dark Dolomite. These rocks are overlain by similar lithologies, including thick evaporite solution breccias of the latest Frasnian and Early Famennian upper Jefferson Formation. Latest Frasnian sequences are similar to Nisku–Winterburn sequences in western Canada. Overlying Famennian successions are correlatives to the Three Forks Formation Logan Gulch Member in Montana and the Palliser–Wabamun units of Alberta. Biohermal Dark Dolomite in the central Lemhi Range and Borah Peak area of the Lost River Range was deposited west of the Lemhi Arch, with buildups also established on ramps near the shelf break in the Grandview Canyon area (Grandview Reef). During onset of the Antler Orogeny, prior to deposition of the Middle Famennian Three Forks Trident Member and widespread disconformities, a latest Frasnian outer shelf barrier formed above the Grandview Reef. Cyclic, heterolithic, peloidal western Grandview Dolomite facies were deposited and are ~330 m thick, although correlative facies of the Jefferson D4 through D6 members are twice as thick behind the shelf edge in the central Lemhi and Borah Peak area. Lower Grandview Dolomite black subtidal carbonate and Nisku buildups (Gooseberry Reef) formed in three late Frasnian sequences and under a basal Famennian sequence boundary. At this time, the Lemhi Arch foundered, but remained unstable—it was termed the “Beaverhead Mountains uplift.” An intrashelf basin dominated midshelf paleogeography during the Early Famennian, accommodating thick shallow water barrier sandstone, solution-collapse breccia, and restricted marine dolostone and limestone of the upper Grandview Dolomite. Crinoid packstone beds near the top of the Jefferson Formation occur below the Three Forks Trident Member in the Lost River Range. Similar nodular, crinoidal limestone with cephalopods occurs under an unconformity with the Sappington Formation in the Beaverhead Mountains. These rocks were previously called the False Birdbear and were grouped with the Jefferson Formation; however, they comprise their own ~15-m-thick sequence and are unrelated to the rarely fossiliferous and dolomitized upper Grandview Dolomite. Open marine shale–limestone sequences of the 80-m-thick Trident Member were deposited on the Idaho shelf above and below regional surfaces and hiatuses. These rocks were variably accommodated on reactivated paleohighs and in local seaways on the craton margin. An unconformity developed on the outer Idaho shelf in the latest Devonian during deposition of the Sappington Formation on the Lemhi Arch and in the Central Montana Trough. Sappington strata were either not deposited on the western shelf or accumulated under deep water conditions and were eroded during regional Mississippian basin inversion and turbidite deposition.

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.