Rocky Mountain (66th Annual) and Cordilleran (110th Annual) Joint Meeting (19–21 May 2014)

Paper No. 8
Presentation Time: 3:20 PM


BOOKSTROM, Arthur Albin1, BOX, Stephen E.2, COSSETTE, Pamela M.1, GILLERMAN, Virginia S.3, KING, George R.4 and ZIRAKPARVAR, N. Alex5, (1)U.S. Geological Survey, Spokane Office, 904 W. Riverside Ave, Room 202, Spokane, WA 99201, (2)U.S. Geological Survey, 904 W. Riverside Ave, Room 202, Spokane, WA 99201, (3)Idaho Geological Survey, Boise Office, 322 E. Front St., Ste. 242, Boise, ID 83702, (4)Formation Capital US, Salmon, Idaho, Salmon, ID 83467, (5)American Museum of Natural History, Department of Earth and Planetary Science, Central Park West at 79th St, New York, NY 10024,

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.

  • AB_RMGSA14_Blkbrd_tsm_pptv3.pptx (10.8 MB)