Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization

Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyri te, and pentlandite) from a suite of samples from the platiniferous J-M Ree f of the Stillwater Complex were measured to elucidate the temporal and gen etic relationship between sulfides and host silicate minerals. Results indi cate that sulfides and coexisting plagioclases are generally not in isotopi c equilibrium, that both sulfides and feldspars record highly radiogenic in itial ratios at 2.7 Ga, and that a component of "post-emplacement" radiogen ic Pb has mixed with common Pb in the sulfides. A model involving introduct ion of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived duri ng an extensive hydrothermal episode. associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radi ogenic Pb may have been derived from primary minerals altered during the lo w-grade metamorphism cannot be discounted. The amount of radiogenic lead ad ded is variable and in some cases negligible. There is a good correlation b etween the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no al teration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary miner als (serpentine, talc, actinolite, chlorite and zoisite) associated with gr eenschist facies metamorphism. Some of the radiogenic Pb in the sulfides ca ll be removed by progressive stepwise leaching. However, in most samples re crystallization of sulfides during metamorphism has mixed common Pb and rad iogenic Pb throughout the crystal structure such that, in these samples, st epwise leaching does not recover initial Pb isotopic ratios. Plagioclases a re much more resistant to low temperature recrystallization and in almost a ll cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their uti lity in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which wer e important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone.