Sulfur isotope studies of the Stillwater Complex and associated rocks, Montana

The stillwater Complex in south-central Montana is a late Archean layered, ultramafic to mafic intrusion emplaced into middle to late Archean metagraywacke, metashale, and iron-formation. Sulfide minerals are concentrated near the base of the intrusion in some chromitite layers, podiform pegmatoids, discontinuous discordant pods or pipes, and numerous thin, stratiform layers. This investigation focuses on the isotopic composition of sulfur in metamorphosed sedimentary rocks. Stillwater-associated sills and dikes, sulfide accumulations near the base of the complex, and platinum-group element-enriched sulfide mineralization that constitutes the J-M reef and the Picket Pin deposit. The ??34S values for 233 samples analyzed in this study have a mean of 0.4 per mil, a standard deviation of 1.7, and a maximum and minimum of -3.8 and 7.8 per mil, respectively. Despite the very narrow range of values, analysis of variance calculations confirms that the data comprise three statistically distinct groups. -from Authors