Paragenesis of cobalt and nickel at the Black Butte Copper Project, Meagher County, Montana

The Black Butte Copper Project (formerly known as the Sheep Creek deposit) is a sediment hosted, laterally extensive massive sulfide deposit hosted in the mid-Proterozoic Newland Formation in Central Montana. Formation of the deposit occurred during two stages of mineralization: Stage I occurred during deposition of sediment in the Helena Embayment, and Stage II occurred post-deposition. Stage I mineralization is characterized by a large quantity of porous pyrite hosting significant Co/Ni/As mineralization in the form of both ion substitution within the pyrite chemical lattice and small (< 1 microm) mineral inclusions of Ni-rich alloclasite. Stage II mineralization reworked the existing Co/Ni/As mineralization, removing the metals from the pyrite and reprecipitating them as distinct siegenite ((Co,Ni)3S4) and tennantite (Cu 12As4S13) grains, usually adjacent to one another. Stage II introduced minor, if any, additional Co/Ni/As mineralization into the deposit. Although siegenite is abundant in Co-rich portions of the ore body and is readily identifiable in hand specimen and under the microscope, stoichiometric relationships based on drill-core assay data suggest that Co and Ni were originally introduced into the deposit in the form of alloclasite.;Although many of the characteristics of SEDEX type deposits are present at Black Butte (e.g., low temperature of formation, laterally extensive massive sulfide horizons, interbedded black shales, abundant barite and local phosphate horizons, hosted within a continental rift) the lack of economic Pb and Zn mineralization in the central ore body and the abundance of Cu/Co/Ni is more typical of red-bed copper deposits. The Neihart Quartzite, resting below the base of the Belt Supergroup, is a possible source bed for Cu-rich ore fluids, which, depending on the pH and redox state, may have been rich in Co and Ni as well.