Sulfide saturation history of the Stillwater Complex, Montana: chemostratigraphic variation in platinum group elements

A platinum group element (PGE) investigation of a 5.3 km-thick stratigraphic section of the Stillwater Complex, Montana was undertaken to refine and test a geochemical technique to explore for platiniferous horizons in layered mafic/ultramafic complexes. PGE, Au, major, and trace elements were determined in 92 samples from outcrops along traverses in the Chrome Mountain and Contact Mountain areas in the western part of the Stillwater Complex where the J-M reef occurs ∼1,460 m above the floor of the intrusion. A further 29 samples from a drill hole cored in the immediate vicinity of the J-M reef were analyzed to detail compositional variations directly above and below the J-M reef. Below the J-M reef, background concentrations of Pt (10 ppb) and Pd (7 ppb) are features of peridotites with intermediate S concentrations (mostly 100–200 ppm) and rocks from the Bronzitite, Norite I, and Gabbronorite I zones (mostly Pt > Au. Within the middle and upper banded series, S abundances fluctuate considerably, but exhibit an overall upward increase. The behavior of these elements records periodic sulfide saturation during deposition of the Peridotite zone, followed by crystallization under sulfide-undersaturated conditions until saturation is achieved at the base of the J-M reef. Following formation of the reef, sulfide-saturated conditions persisted throughout the deposition of most of the remaining Lower Layered Series. This resulted in a pronounced impoverishment in PGE abundance in the remaining magma, a condition that continued throughout deposition of the remainder of a succession, which is characterized by very low Pt (1.5 ppb) and Pd (0.7 ppb) abundances. Because only unmineralized rock was selected for study in the 5.3 km-thick section, the results provide an unbiased picture of the variation in background PGE levels during crystallization of the Stillwater Complex. In contrast, the variations in the drill core samples through the reef provide a detailed record of ore formation. Plots of Pt, Pd, Pd/S, and Pt + Pd as a function of stratigraphic height in the intrusion show that the location of the J-M reef is defined by an abrupt change in these concentrations and ratios. Although this is the most abr