Rhenium-osmium systematics and major and trace element chemistry of cobaltite (CoAsS): Evidence for Late Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization in the Idaho Cobalt Belt, Belt-Purcell Basin, USA

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  • We report the first study of the Re-Os systematics of cobaltite (CoAsS) using disseminated andmassively mineralized samples from two breccia-type and two stratabound deposits from the Co-Cu-Au Idaho Cobalt Belt, Lehmi Sub-basin to the Belt-Purcell Basin, Idaho, USA. Using a 185Re+190Os spike solution, magnetic and non-magnetic fractions of cobaltite mineral separates give reproducible Re-Os analytical data for aliquot sizes of 150 to 200 mg. Cobaltite has highly radiogenic 187Os/188Os (17‒45) ratios and high 187Re/188Os (600‒1800) ratios but low Re and total Os contents of ca. 0.4‒4 ppb and 14‒64ppt, respectively. With proportions of radiogenic 187Os between 30 and 74%, cobaltite from the Idaho Cobalt Belt is amenable to Re-Os age determination using the isochron regression approach. The Re-Os data for disseminated cobaltite mineralization in a quartz-tourmaline breccia from the Haynes-Stellite deposit yield a Model 1 isochron age of 1349 ± 76 Ma (2σ, n = 4, mean squared weighted deviation MSWD = 2.1, initial 187Os/188Os ratio of 4.7 ± 2.2). This middle Mesoproterozoic age information was preserved despite a possible phase of metamorphism or a pulse of metamorphic- hydrothermal remobilization of pre-existing colbatite mineralization along fold cleavages during the Grenvillian orogeny. This phase of remobilization was tentatively identified through a Model 3 isochron age of 1132 ± 240 Ma (2σ, n = 7, MSWD = 9.3, initial 187Os/188Os ratio of 9.0 ± 2.9) in the quartz- tourmaline breccia from the Idaho Zone. Although all Mesoproterozoic cobaltite mineralization was subject to lower-upper greenschist to lower amphibolites in garnet zone metamorphism from Late Jurassic to Late Cretaceous during the Cordilleran orogeny in the Blackbird district, the fine- to coarse-grained massive cobaltite mineralization from the Chicago Zone is the only studied deposit that has severely disturbed Re-Os systematics with evidence for a linear trend of mixing with (metamorphic?) fluids in the 187Os/188Os vs. 1/192Os space. Given the position of the Chicago Zone in the Blackbird metamorphic domain characterized by middle to upper greenschist metamorphism, and, the thermal conditions of Cretaceous metamorphism between 400 and 520ºC in the overlying garnet-bearing Indian Creek metamorphic domain, a maximum closing temperature of 400ºC for the Re-Os isotopic system in cobaltite is proposed. In addition, our study suggests that small, high-grade, structurally-controlled satellite deposits, which are located ca. 15 km to the SE of the Blackbird district (e.g., Black Pine), are product of Cretaceous remobilization of Mesoproterozoic cobaltite deposits in the Blackbird district. The new Re-Os ages and the extremely high initial 187Os/188Os ratios are in favor of a magmato-hydrothermal genetic model for a multi-stage REE-Y-Co-Cu-Au mineralization between ca. 1370 Ma and 1349 Ma related to the emplacement of the Big Deer Creek granite at ca. 1377 Ma. Following deposition of paragenetically early xenotime, gadolinite and Be-minerals, a mixture of Mesoproterozoic evaporitic brines and magmatic fluids derived metals and reduced sulfur from mafic and oceanic island-arc Archean to Paleoproterozoic rocks from the Laurentian basement of the Great Falls Tectonic Zone. Cobaltite mineralization (and the closure of the Re-Os isotope system in this mineral) occurred upon cooling of these fluids at an inferred temperature of 300ºC or below.

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    Attribution-NonCommercial 4.0 International