STRUCTURE, TECTONICS, AND METAMORPHISM OF THE BLACKBIRD CO-CU DEPOSIT AND ITS METASEDIMENTARY HOST ROCKS

The Blackbird Co-Cu deposit is in gray siliciclastic metasedimentary strata of the Lemhi Group in the rift-style Lemhi sub-basin of the Mesoproterozoic Belt-Purcell basin. The Blackbird deposit contains at least 11 early semi-concordant cobaltite-biotite ore zones and many late, mostly discordant polymetallic breccias and veins. Age dates, fluid inclusions and stable-isotope ratios indicate that both ore types are of metamorphic-hydrothermal origin.

Cobaltite surrounds xenotime cores (dated 1.37 Ga) in cobaltite-biotite ore. Xenotime cores are coeval with nearby metagabbro and A-type monzogranite plutons, which cut F₁ folds that formed during the E. Kootenay orogeny. Cobaltite-biotite ore and its host rocks (banded biotitite phyllite and silicified siltite) are folded into west-vergent, north-plunging F₂ folds and crumpled zones, in which spindle-shaped bodies of high-grade ore parallel F₂ fold axes. Later cobaltite occupies strain-slip cleavage of F₂ folds. Mafic-alkalic dikes of within-plate geochemical character cut cobaltite-biotite lodes and F₂ folds, and one F₂ fold is cut by the syenitic Deep Creek pluton (~500 Ma). Thus, the ages of cobaltite-biotite lodes and F₂folds are bracketed by 1.37 and 0.5 Ga. Cobaltite-biotite mineralization probably occurred during the Grenville orogeny (~ 1.2 to 1.0 Ga). Xenotime inner rims in cobaltite-biotite ore yield ages of ~1.3 to 1.1 Ga; Lead isotopic ratios in country rocks indicate metamorphic homogenization at ~1.2 Ga; and north Idaho garnets are dated ~1.2 to 1.0 Ga.

Post-cobaltite-biotite garnets yield Lu-Hf ages from ~151 to ~93 Ma. The garnet zone grades downward to garnet-free rocks. This metamorphic-zone boundary implies heating from above and from NW to SE, which is consistent with NE trending F₃ fold axes. F₄crenulations, rolled garnets, ball texture in ore, and post-garnet thrust faults indicate late SW-NE compression.

Cretaceous polymetallic breccias and veins cut and overprint cobaltite-biotite ore and mafic dikes. The polymetallic assemblage includes quartz ± siderite ± chalcopyrite ± pyrite ± pyrrhotite ± marcasite ± magnetite ± cobaltian pyrite ± cobaltite-glaucodot-arsenopyrite ± monazite (~110-92 Ma) ± xenotime outer rims (102-93 Ma). Most late breccias and veins, and Tertiary normal faults follow axial-plane cleavage of F₂ folds.