VERTICAL AND LATERAL VARIATION OF PLATINUM, PALLADIUM, AND RHODIUM IN STILLWATER COMPLEX, MONTANA

Individual samples from the Stillwater Complex, Montana, show a wide variation in content and relative proportions of platinum, palladium, and rhodium, and these metals are preferentially concentrated in chromitite zones. Values range from the limits of determination up to 8 ppm Pt, 11 ppm Pd, and 1.7 ppm Rh. Stibiopalladinite, sperrylite, cooperite, platinum-iron alloy, and laurite, the only platinum mineral phases identified, occur in the chromitite as discrete grains with an average diameter of 5 microns. Chromium shows the strongest correlation, vanadium a moderate correlation, and Ni, Co, and Cu a low correlation with the concentration of the three platinum metals. The three platinum-group metals are most abundant where the oxidation ratio, Fe +3/(Fe+2 + Fe+3), of chromite is the lowest. If chromite is a good petrogenetic indicator of conditions in the magma from which it crystallized and if the three platinum metals are precipitated with the chromite, low oxidation conditions in the magma also favored the precipitation of the three platinum metals from the magma. © 1972 Society of Economic Geologists, Inc.