SULFUR AND HELIUM ISOTOPES IN THE COBALT+COPPER± GOLD DEPOSITS OF THE IDAHO COBALT BELT

Co+Cu±Au mineralization of the Idaho Co belt occurs in the Apple Creek Fm, a Mesoproterozoic siltite-argillite sequence that may correlate with rocks of the Belt Supergroup. At Blackbird, the midpoint of the belt and the major cluster of mines and prospects, the ore is spatially associated with mafic dikes and strata (termed biotitite). Proterozoic metaplutonic rocks and Phanerozoic plutonic and volcanic rocks also outcrop nearby. Because the Idaho Co belt is the most important identified domestic Co resource, an understanding of its genesis is critical for assessment of Co potential elsewhere.

We present new measurements of S isotopes in sulfides and He isotopes in quartz. At Blackbird where mineralization is in the upper Apple Ck, delta 34-S is quite uniform (cobaltite: 8.0±0.4‰ (n=18), cpy: 7.1±1.0‰ (n=11), py: 7.8±0.1‰ (n=2)). Sulfur is lighter at Black Pine ~15 km SE where mineralization is in the middle Apple Ck (cobaltite: 6.4‰ (n=1), cpy: 5.0‰ (n=1), py 4.0‰ (n=1)), and at Iron Creek ~15 km farther SE where mineralization is in the lower Apple Ck (cpy: 5.8‰ (n=1), py: 4.6±1.0‰ (n=3)). The cobaltite-cpy fractionation is fairly consistent (0.7±0.4‰, n=6) which may reflect equilibration during original deposition or in some subsequent metamorphic/recrystallization event, although mineral textures suggest that cpy postdated cobaltite. The observed isotopic range, 3.4-8.9‰, is more restricted than most stratiform Cu or sedex Pb-Zn deposits, including Belt-Purcell Basin examples at Spar Lake, Sheep Creek, and Sullivan. The mean value, 7.2‰, is high with respect to most, though not all, volcanogenic massive sulfide deposits. On the other hand, the isotopic variation with stratigraphic position suggests that S may have been acquired from, or influenced by, local sediments or sedimentary rocks. Acquisition of ore constituents from Apple Ck sediments has also been suggested based on Pb isotope data for Blackbird (Panneer et al., 2004). In contrast, He isotope measurements gave R/Ra values of 0.02 and 8, the larger value suggesting a mantle or magmatic component in ore-associated volatiles. Thus, synsedimentary mafic volcanism or post-sedimentary bimodal granite-gabbro plutonism may also have been important for the ore-forming hydrothermal system, possibly by supplying heat.