Multiple Sulfur Isotope Studies of the Stillwater Complex, Montana

English Abstract:

Models for the genesis of the J-M Reef of the Stillwater Complex include magma mixing and the attainment of high R-factors to produce PGE-rich immiscible sulfide droplets (e.g., Campbell et al., 1983), leaching of trace sulfides in the Ultramafic Series and concentration at the level of the Reef by magmatic fluids (e.g., Boudreau and McCallum, 1992; Boudreau and Meurer, 1999), and the emplacement of PGE-rich magmas that had been upgraded via sulfide dissolution at depth (e.g., Keays and Lightfoot, 2013). In order to better evaluate the extent to which crustal sulfur was key for PGE enrichment we have initiated a multiple sulfur isotope study of the Complex. The work compliments the early S isotope studies of the Complex by Zientek and Ripley (1990), but the utilization of both ³³S and ³⁴S permits the detection of deviation from the terrestrial fractionation line that may be due to assimilation of S from Archean country rocks and was previously undetectable.

 We have measured Δ³³S values for samples of country rocks and sulfides within country rocks, as well as rocks of the Reef Package in the area of the Stillwater Mine. Δ³³S values for both massive sulfides in country rocks and pyroxene-hornfels below the Basal Series range from 0 to 0.25 ‰. δ³⁴S values generally range from -1.3 to +1.3 ‰, with one massive sulfide sample having a value of 3.9 ‰ Samples of sulfides from the J-M Reef have δ³⁴S values from -1 to +1.5 ‰. J-M Reef samples have Δ³³S values from 0.005 to 0.05 ‰. The Δ³³S values of sulfides so far measured from the J-M Reef are not considered to be anomalous, however the range of sulfur isotopic values found in the metasedimentary country rocks does not preclude a contribution of country rock sulfur to the magmas that produced the Stillwater Complex. The relative uniformity of Δ³³S values in the igneous rocks of the Reef is consistent with no country rock contribution of sulfur, and exclusive incorporation of mantle S or, alternatively, contribution of country rock S characterized by similar ∆³³S values. A multi-component mixing process involving sulfur from various country rocks and that of mantle derivation remains feasible. Samples of Archean gneisses and other intrusive rock types are being analyzed to further constrain the involvement of country rock S in the formation of the J-M Reef.