GEOLOGIC MAP OF THE BLACKBIRD CO-CU MINE AREA, EAST-CENTRAL IDAHO — ORE TYPES, METASEDIMENTARY STRATA, FOLDS, DIKES, AND METAMORPHIC OVERPRINT

A geologic map of the Blackbird Co-Cu mine area in the Salmon River Mountains, west of Salmon, Idaho was compiled at 1:12,000-scale from 12 overlapping geologic source maps. This map preserves and integrates data from unpublished maps with information from published maps and recent field observations. The map serves as a foundation for ongoing studies of the Blackbird Co-Cu deposit, which is hosted in siliciclastic metasedimentary strata of the Lemhi Group, in the Lemhi sub-basin of the Mesoproterozoic Belt-Purcell basin.

The Blackbird mine area contains multiple semi-concordant cobaltite-biotite ore zones (± later chalcopyrite) in interlayered biotitite and siltite of the banded siltite unit of the Lemhi Group. Cobaltite-biotite lodes and their host strata are folded into large, west-vergent, NNE-plunging F₂ chevron folds and crumpled zones of smaller-scale F₂ folds.

From SE to NW, the discordant Haynes-Stellite tourmaline-matrix breccia was mined for cobaltite. The semi-concordant Merle cobaltite-biotite ore zone is on the east limb of a minor anticline, east of the hinge zone of the large-scale Idaho F₂ syncline. Cobaltite lodes in S. Idaho ore zone are parallel to strain-slip cleavage and transposed metamorphic layering in the tight hinge zone of the north-plunging, chevron-style Idaho syncline. The N. Idaho, Chicago, Brown Bear, Blacktail and Ram ore zones are semi-concordant with the NE-dipping limb of the Idaho syncline. A garnet zone overprints cobaltite-biotite ore.

The Sunshine and E.Sunshine ore zones are west of the Blacktail pit and west of the Calera F₂ anticline. These ore zones are sub-parallel to opposite limbs of the Sunshine F₂ syncline, which is nearly isoclinal, and is overturned to the WSW, such that its limbs dip steeply ENE. Transposed interlayers of silicified siltite and chloritized biotitite host cobaltite in the Sunshine ore zones. Chloritic quartzite in the core of the Sunshine syncline is interpreted as hydrothermally altered rock of a biotitic metasandstone unit that overlies banded siltite.

Mafic dikes cut cobaltite-biotite ore but are themselves cut by discordant polymetallic veins of the Uncle Sam ore zone. Discordant polymetallic breccias of the Dandy ore zone also cut, and locally contain cobaltite-biotite clasts. Chalcopyrite overprints cobaltite in composite Co-Cu ore.