Platinum-group elements in sulfide minerals and the whole rocks of the J-M Reef (Stillwater Complex): Implication for the formation of the reef

Abstract

The concentration of platinum-group elements (PGE), Co, Ni, Cu, Re, Au and Ag was determined in base metal sulfides (BMS) minerals and whole rocks of the J-M Reef (footwall, reef and hanging wall) of the Stillwater Complex (U.S.A.). The aims of the study were to establish: (i) whether the BMS minerals (pyrrhotite, pentlandite, and chalcopyrite) are the principal host of these elements; (ii) whether these elements preferentially partition into a specific BMS mineral. The results of this study allowed us to consider and evaluate: the possible parental magma composition, the role of sulfide liquid and the role of alteration in the formation of the J-M Reef. Only a minor quantity of PGE (∼ 10 to 140 ppb) were found in the footwall and hanging wall samples of the J-M Reef, which contain only small amounts of sulfur (∼ 140 to 400 ppm). In contrast, the reef samples enriched in sulfides contain higher values of PGE (49 to 419 ppm). The S, Ni, Cu, and PGE contents follow the same trends indicating that BMS minerals are the principal phases controlling these elements. Palladium and Pt are the more abundant PGE (up to 244 ppm for Pd and 166 ppm for Pt) with an average Pd/Pt ratio of 3.3. Similarly, the Pd/Ir ratio and the mantle normalized PGE patterns indicate that Pd is more strongly enriched in the reef relative to surrounding rocks than the other PGE. Pentlandite is the BMS mineral that contains the largest weight fraction of PGE. Palladium represents ∼ 95% of the PGE found in solution in BMS minerals and is mainly partitioned in pentlandite. In contrast, Pt is almost exclusively found as platinum-group minerals and do not partition in any BMS minerals. The other PGE are largely found in the BMS minerals, mainly pentlandite. The variation of S/Se ratios and the presence of secondary magnetite in some samples indicate that these samples may have lost 20 to 50% of their original S. The highest Pd (and by analogy PGE contents) are found in samples containing secondary magnetite, but these are not necessarily those which have experienced the highest S removal. This indicates that the processes which caused the removal of S and the alteration to magnetite are probably distinct. Our modeling results suggest that PGE (and to a greater extent Pd) enrichment may be summarized into two different steps. First, an immiscible sulfide liquid interacted with a large volume of magma with PGE composition close to high-Mg basalt and collected the PGE. The sulfide liquid percolated down through the crystal mush to collect at a level where the porosity did not permit any further migration. During cooling, magma chamber instabilities triggered the partial desulfurization of the sulfides. Finally, in some parts of reef, a fluid deposited Pd (possibly removed from the footwall) and altered the BMS minerals to magnetite. During this step, Pd was precipitated as an alloy in the most Pd-enriched samples and Pd possibly diffused into pentlandite forming a high-Pd bearing pentlandite. The different contributions of each of the above processes may explain the variability of the Pd grade observed along the J-M Reef.