R.A. Creaser Arthur A. Bookstrom N.J. Saintilan 2017 <p id="sp0010">We report the first study of the Re-Os systematics of cobaltite (CoAsS) using disseminated grains and massive sulfides from samples of two breccia-type and two stratabound deposits in the Co-Cu-Au Idaho cobalt belt (ICB), Lemhi subbasin to the Belt-Purcell Basin, Idaho, USA. Using a<span>&nbsp;</span>¹⁸⁵Re&nbsp;+&nbsp;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&nbsp;mg. Cobaltite from the ICB has highly radiogenic<span>&nbsp;</span>¹⁸⁷Os/¹⁸⁸Os ratios (17–45) and high<span>&nbsp;</span>¹⁸⁷Re/¹⁸⁸Os ratios (600–1800) but low Re and total Os contents (ca. 0.4–4&nbsp;ppb and 14–64 ppt, respectively). Containing 30 to 74% radiogenic<span>&nbsp;</span>¹⁸⁷Os, cobaltite from the ICB is amenable to Re-Os age determination using the isochron regression approach.</p><p id="sp0015">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&nbsp;±&nbsp;76&nbsp;Ma (2σ,<span>&nbsp;</span><i>n</i>&nbsp;=&nbsp;4, mean squared weighted deviation MSWD&nbsp;=&nbsp;2.1, initial<span>&nbsp;</span>¹⁸⁷Os/¹⁸⁸Os ratio&nbsp;=&nbsp;4.7&nbsp;±&nbsp;2.2). This middle Mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic-hydrothermal remobilization of pre-existing cobaltite that formed along fold cleavages during the ca. 1190–1006&nbsp;Ma Grenvillian orogeny. This phase of remobilization is tentatively identified by a Model 3 isochron age of 1132&nbsp;±&nbsp;240&nbsp;Ma (2σ,<span>&nbsp;</span><i>n</i>&nbsp;=&nbsp;7, MSWD&nbsp;=&nbsp;9.3, initial<span>&nbsp;</span>¹⁸⁷Os/¹⁸⁸Os ratio of 9.0&nbsp;±&nbsp;2.9) for cobaltite in the quartz-tourmaline breccia from the Idaho zone in the Blackbird mine.</p><p id="sp0020">All Mesoproterozoic cobaltite mineralization in the district was affected by greenschist- to lower amphibolite-facies (garnet zone) metamorphism during the Late Jurassic to Late Cretaceous Cordilleran orogeny. However, the fine- to coarse-grained massive cobaltite mineralization from the shear zone-hosted Chicago zone, Blackbird mine, is the only studied deposit that has severely disturbed Re-Os systematics with evidence for a linear trend of mixing with (metamorphic?) fluids.</p><p id="sp0025">The new Re-Os ages and extremely high initial<span>&nbsp;</span>¹⁸⁷Os/¹⁸⁸Os ratios of cobaltite reported here favor a magmatic-hydrothermal genetic model for a multi-stage REE-Y-Co-Cu-Au mineralization occurring at ca. 1370 to 1349&nbsp;Ma, and related to the emplacement of the Big Deer Creek granite pluton at ca. 1377&nbsp;Ma. In our model, deposition of paragenetically early xenotime and gadolinite was followed by an influx of Mesoproterozoic evaporitic brines and magmatic-hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island-arc Archean to Paleoproterozoic rocks in the Laurentian basement. Cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300&nbsp;°C or below.</p> application/pdf 10.1016/j.oregeorev.2017.02.032 en Elsevier Re-Os systematics and geochemistry of cobaltite (CoAsS) in the Idaho cobalt belt, Belt-Purcell Basin, USA: Evidence for middle Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization article