Multiple S isotope studies of the Stillwater Complex and country rocks: An assessment of the role of crustal S in the origin of PGE enrichment found in the J-M Reef and related rocks

Abstract

The J-M Reef in the Stillwater Complex of Montana is the highest grade Pd deposit in the world, averaging approximately 14ppm Pd. One suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal S. Because δ34S values alone are often insufficient to document in igneous rocks the assimilation of S from Archean sedimentary rocks, a combined study of both δ33S and δ34S values, and computed Δ33S values, was undertaken. δ34S and Δ33S values of cordierite-bearing hornfels, as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations. One is characterized by anomalous positive Δ33S values from 0 to 0.32‰ and δ34S values from 0.5 to −1.6‰, whereas the other is characterized by anomalous negative Δ33S values from 0 to −0.22‰ and δ34S values from 1.5 to 5.8‰. δ34S and Δ33S values for rocks from the Basal Series of the Complex range from −0.9 to 3.0‰ and 0.01 to 0.18‰, respectively; the anomalous S isotope ratios indicate that for some samples at least 50% of the their S was of crustal origin. In contrast, most of the rocks from the J-M Reef are characterized by δ34S and Δ33S values between −1.3 and 1.8‰ and −0.06 to 0.06‰, respectively. These values deviate only slightly, if at all, from those thought to be representative of mantle S, and suggest that (i) contamination was not an important process for PGE enrichment in the Complex, (ii) the contaminant was also characterized by near 0 Δ33S values, or (iii) S isotopic exchange in the staging chambers was an important process. Upgrading of PGE values via reaction between sulfide in the chamber and uncontaminated mantle-derived magma may also have resulted in the conversion of S isotopic ratios that may have been indicative of crustal S addition, to mantle-like values. Rare δ34S values up to 7.2‰ in the J-M Reef and 8.2‰ in chromitite from the Ultramafic Series provide strong evidence for localized assimilation of crustal S which did not experience isotopic homogenization. The S isotopic ratios of the J-M Reef are non-unique in terms of a specific genesis as they may be interpreted in either a downward, sulfide collection model, or an upward fluid transport model.