Strongly negative δ34S values associated with secondary pyrite above and below the J-M Reef, Stillwater Complex, Montana

Previous sulfur isotope studies of the Stillwater Complex, and in particular the J-M Reef, have noted the generally restricted range of δ34S values in the range of 0 ± 2.5‰ (e.g., Zientek and Ripley, 1990; Ripley et al., 2017). Detailed studies of high-S zones in both the hanging wall of the Reef Package, which contains the J-M Reef, and the footwall within the Reef Package but below the mineralized J-M-Reef, indicate the presence of significant quantities of secondary pyrite. The pyrite-rich intervals are characterized by δ34S values that strongly differ from those of the J-M Reef, in the range of 0.2 to −17.4‰. The pyrite-rich rocks above and below the J-M Reef are both serpentinized lherzolites. Although the J-M Reef is locally strongly altered to clinozoisite-epidote-rich assemblages, only the sepentinized lherzolites show anomalously low δ34S values. The highly negative δ34S values of the pyrite-rich assemblages result from oxidation processes and S isotope fractionation between reduced and oxidized S species. One possible process involves the production of a mixed sulfate-sulfide fluid with pyrite produced from H2S with low δ34S values. A second alternative is that infiltration of an oxidized fluid and interaction with sulfide-bearing rocks of the Complex produced a sulfate-rich fluid that then underwent partial reduction to sulfide via reaction with reduced Fe-bearing minerals. Strongly negative δ34S values would characterize secondary pyrite produced as a result of either process due to the large S isotope fractionation between oxidized and reduced S species at hydrothermal temperatures of ~400 °C or below. The varying degrees of alteration and restricted pyrite-rich intervals with highly negative S isotope signatures suggest that fluid infiltration was not pervasive; secondary pyrite that was produced as a result of redox-driven reactions is preserved only in olivine-rich lithologies characterized by a high degree of serpentinization.