LATE MESOPROTEROZOIC SEDIMENT-HOSTED SULFIDE MINERALIZATION IN THE BLACKBIRD DISTRICT, IDAHO COBALT BELT, USA ‒ EVIDENCE FROM COBALTITE RE-OS SYSTEMATICS

The Blackbird district within the Idaho Cobalt Belt contains the largest reserves of cobalt in the United States. Polymetallic sulfide deposits with cobaltite are hosted by Mesoproterozoic metasedimentary rocks with maximum and minimum ages of 1409 ± 10 Ma and 1370 ± 4 Ma, respectively [1]. The age of cobaltite mineralization remains controversial: (i) multiple growth of xenotime and cobaltite with original growth in the Mesoproterozoic (ca. 1370 to ca. 1050 Ma) and subsequent late Cretaceous (ca. 110 to ca. 92 Ma) metamorphic overprint and remobilization [1, 2], (ii) sulfide mineralization linked to late Cretaceous regional thrusting [1, 3].

Samples were collected within the Blackbird mining district [4]: (i) layers of massive coarse-grained to fine-grained brownish pink cobaltite ± xenotime at the Chicago Zone, Blackbird mine, (ii) layers of disseminated brownish pink cobaltite in a biotite, quartz, K-feldspar rock at the Idaho Zone, Blackbird mine, (iii) tourmalinized breccia of quartz, biotite, K-feldspar, xenotime cemented by disseminated brownish pink cobaltite at the Haynes-Stellite deposit, and (iv) coarse-grained disseminated shiny grey cobaltite in a quartz-chlorite rock, Black Pine deposit. Standard mineral separation procedures produced magnetic (M) and non-magnetic (NM) cobaltite fractions from the 70-200 and +70 mesh size fractions by using successive 0.3, 0.6 and 0.9 amp currents.

Re-Os data of these mineral fractions show a large range of ¹⁸⁷Re/¹⁸⁸Os (39 to 18,466) and ¹⁸⁷Os/¹⁸⁸Os (0.69 to 360.5) ratios. Re and Os abundances range from 157 to 39,235 ppt and 10 to 491 ppt, respectively. The aliquots do not define an all-sample isochron in ¹⁸⁷Os/¹⁸⁸Os vs. ¹⁸⁷Re/¹⁸⁸Os space. However, the significant variability of ¹⁸⁷Os/¹⁸⁸Os and correlation with ¹⁸⁷Re/¹⁸⁸Os of the samples from Haynes-Stellite, the Idaho Zone and the non-magnetic fraction of the Chicago Zone may indicate the preservation of primary Re-Os isotope systematics in fine-grained disseminated brownish pink cobaltite. These aliquots define a Model 3 isochron (n = 7) with an age of ca. 1150 Ma. The other data points (coarse-grained cobaltite from the Chicago Zone and the Black Pine deposit) suggest disturbance of the primary Re-Os isotopic system by younger dynamothermal metamorphic event(s) (late Grenvillian? Cretaceous?).