Copper Isotopes and Constraints on the Ore Genesis Process of Cu-Co Ore Deposits at the Idaho Cobalt Belt, USA

Abstract

Quantifying and identifying the introduction of metal in ore deposits that have experienced multiple overprinting hydrothermal events remains an elusive yet essential goal in metallogenic studies. Here, we constrain the origin of Co in the Idaho Cobalt Belt (ICB) that experienced two distinct metal-rich events that introduced Co and Cu. We performed a detailed petrographic study of sulfide ore at Iron Creek in the ICB, in concert with the quantification of trace metal element concentrations and copper isotope values to identify the introduction of Co in the system. The pyrite displays various degrees of alteration, with the highest Co concentrations (up to 6 wt.%) in less-altered pyrite grains (e.g., sharp edges, absence of altered boundaries and fissures) and highest δ65Cu isotope value. The most-altered pyrite grains (e.g., corroded grains, round and altered boundaries) have lower Co contents and lower δ65Cu isotope values that match the copper isotope values of the chalcopyrite. The least-altered pyrite shows a narrow δ65Cu range between −0.39‰ to −0.58‰. In contrast, the most-altered pyrite grains are isotopically depleted, showing a δ65Cu range from −1.35‰ to −0.90‰. Chalcopyrite shows a δ65Cu range between −1.07‰ and −0.77‰. We interpret, from the Cu isotope compositions and Co concentrations in pyrite, that the Co was originally introduced into the siliciclastic host rock package in a Mesoproterozoic SEDEX environment. The heavier Cu was then preferentially leached in a second event, resulting in isotopically lighter Cu in the altered pyrite. Remobilization of the SEDEX cobalt was likely associated with CO2-rich metamorphic fluids present in the region during the Mesoproterozoic East Kootenay orogeny, the late Mesoproterozoic Grenville orogeny, and the Late Jurassic to Late Cretaceous Cordilleran orogeny.