Rhenium-Osmium and Samarium-Neodymium Isotopic Systematics of the Stillwater Complex

Isotopic data for the Stillwater Complex, Montana, which formed about 2700 Ma (million years ago), were obtained to evaluate the role of magma mixing in the formation of strategic platinum-group element (PGE) ore deposits. Neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas. Ultramafic affinity (U-type) magmas had initial $\varepsilon _{\text{Nd}}$ of -0.8 to -3.2 and a chondritic initial $^{187}$Os/$^{186}$Os ratio of $\sim $0.88, whereas anorthositic affinity (A-type) magmas had $\varepsilon _{\text{Nd}}$ of -0.7 to +1.7 and an initial $^{187}$Os/$^{186}$Os ratio of $\sim $1.13. These data suggest that U-type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas A-type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust. The Nd and Os isotopic data also suggest that Os, and probably the other PGEs in ore horizons such as the J-M Reef, was derived from A-type magmas. The Nd and Os isotopic heterogeneity observed in rocks below the J-M Reef also suggests that A-type magmas were injected into the Stillwater U-type magma chamber at several stages during the development of the Ultramafic series.