Abstract

Platinum-group metals show a wide variation in content and relative proportions in the rocks of the Basal, Ultramafic, Banded, and Upper zones of the stratiform mafic and ultramafic Stillwater Complex, southwestern Montana. Reported values range from the limits of determination to approximately 8 ppm Pt, 11 ppm Pd, 1.7 ppm Rh, 0.5 ppm Ir, 1.0 ppm Ru, and 0.1 ppm Au as analyzed by fire assay-spectrochemical and neutron activation techniques. The highest concentrations occur within particular horizons of the Basal zone; in the chromitites of the Peridotite member of the Ultramafic zone, in pegmatitic bronzitites in the Bronzitite member of the Ultramafic zone, in a specific horizon in the Banded zone, and in disseminated sulfide horizons in the Upper zone. These occurrences represent manifold enrichment over the estimated levels of about 12 ppb Pt, 55 ppb Pd, and <5 ppb Rh in the parent basaltic magma(s) of the complex. The presence of platinum-iron alloy, isoferroplatinum, platinian rhodium, rhodian platinum, palladian gold, cooperite, laurite, braggite, vysotskite, moncheite, kotulskite, merenskyite, sperrylite, and stibiopalladinite has been confirmed by various investigators. The variations of Pt, Pd, and Rh content with stratigraphic position in the Basal zone and in individual cumulate units of the Peridotite member show repetitive, cyclic patterns similar to the patterns exhibited by the grain size, modal proportions, whole-rock compositions, and compositions of individual silicate minerals. Correlation between chemical and physical properties of the cumulates and the platinum-group metal distributions suggests that the distribution patterns may be ascribed to oscillation of physical conditions or composition of the parent basaltic magma, or to oscillation of both.

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