Sulfur Isotopes as a Record of Complex Sulfide Saturation Histories in Conduit-Type Ni-Cu-PGE Deposits: An Example from the Eastern Gabbro, Coldwell Complex, Canada

Abstract Copper-platinum-group element (PGE) mineralization in the Eastern Gabbro Suite is restricted to the Marathon Series. The mineralization occurs in the Marathon deposit and the Sally deposit, as well as Four Dams, Boyer, and Redstone occurrences, which are to the north of Marathon (i.e., the northern deposits). The northern deposits must have formed by distinct processes because they are characterized by Cu/Pd and S/Se ratios that are higher and lower than mantle values, respectively. Base-metal sulfides in the northern deposits exhibit a range of δ34S (–0.7 to 3.78‰) and Δ33S (–0.44 to 0.34‰) values. The nonmantle values imply that externally derived S was added to the northern deposit magmas, with the sources of S likely having been Archean in age. The variability toward mantle values likely resulted from destruction of the contaminant signatures by interaction of the sulfide liquids with multiple uncontaminated pulses of magma. This interpretation can also explain the systematically low S/Se ratios of the northern deposits because, unlike S isotopes that equilibrate to mantle values at high R factors, S/Se ratios equilibrate to values lower than mantle at equivalent R factors. Taken together, it is evident that, compared to Marathon, the northern deposits experienced more complex sulfide saturation histories, which were characterized by multiple sulfide segregation events—some of which depleted the melts in PGE—and derivation of S from Archean reservoirs containing elemental S and S produced by sulfate reduction.