Re--Os and Sm--Nd Isotope Geochemistry of the Stillwater Complex, Montana: Implications for the Petrogenesis of the J-M Reef

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Abstract
Re—Os and Sm—Nd isotopic data have been obtained for mafic and ultramafic cumulates from the 2700-Ma Stillwater Complex and associated fine-grained sills and dykes, so as to better constrain the geochemical characteristics of Stillwater parental magmas and to trace the source(s) of the precious metals that have been concentrated in the J-M Reef, the major platinum-group element mineral deposit in the complex. Initial Os isotopic compositions (187Os/188Os) for chromitites from the Ultramafic series range from a radiogenic isotopic composition of 0.1321 (γOs = +21) for the platinum group element (PGE)-enriched B chromitite seam from the West Fork area to a near-chondritic isotopic composition of 0.1069–0.1135 (γOs=−2 to +4.1) for the PGE-poor G and H chromitite seams, respectively, near the middle of the Ultramafic series. Osmium isotopic data for the PGE-rich B chromitite seam are generally isochronous with whole-rock and mineral data for the J-M Reef (γOs = + 12 to + 34). Re—Os isotopic data therefore document a contrast between PGE-poor cumulates from the Ultramafic series and PGE-enriched cumulates from both the Ultramafic series and the J-M Reef, suggesting that Os and probably the other PGE were derived from at least two isotopically distinct sources. Moreover, these Re-Os isotopic characteristics correlate with petrogenetic subdivisions of the Stillwater Complex based on field mapping, petrology, REE geochemistry, and Sm—Nd isotope geochemistry. The data are best explained by mixing of two magma types, referred to as U-type and A-type magmas, with differing major element, trace element, and precious metal abundances and isotopic compositions. Although crustally contaminated komatiites can mimic the Os and Nd isotopic characteristics of the U-type magma, the combination of low initial Os isotopic values (γOs∼0) with low initial Nd isotopic values (εNd≦−1), high 207Pb/204Pb for a given 206Pb/204Pb (Wooden et al., 1991), and high (Ce/Yb)n ratios in U-type cumulates and fine-grained sills and dykes is more consistent with the involvement of a Re-poor, but trace-element-enriched portion of the subcontinental lithospheric mantle in the petrogenesis of Stillwater U-type magmas. However, the radiogenic initial Os isotopic compositions of the J-M Reef and other portions of the intrusion with elevated PGE concentrations suggest that A-type parental magmas incorporated Os from radiogenic early Archaean crust. The relatively large range in (Ce/Yb)n, γOs, and εNd values suggests that mixing of geochemically distinct magmas may have been an important process throughout the history of the Stillwater magma chamber. Magma mixing may then explain not only the PGE-enriched J-M Reef but also the anomalous enrichment of the PGE in the B chromitite seam from the West Fork area and the variable values observed in other chromitite seams of the Ultramafic series. The intimate association of these magma types, derived from or modified in the Archaean continental lithosphere, may then be crucial to the formation of magmatic PGE mineral deposits.

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