RE-OS AND SM-ND ISOTOPE GEOCHEMISTRY OF THE STILLWATER COMPLEX, MONTANA - IMPLICATIONS FOR THE PETROGENESIS OF THE J-M REEF

Re-Os and Sm-Nd isotopic data have been obtained for mafic and ultrama fic cumulates from the 2700-Ma Stillwater Complex and associated fine- grained sills and dykes, so as to better constrain the geochemical cha racteristics of Stillwater parental magmas and to trace the source(s) of the precious metals that have been concentrated in the J-M Reef, th e major platinum-group element mineral deposit in the complex. Initial Os isotopic compositions (Os-187/Os-188) for chromitites from the Ult ramafic series range from a radiogenic isotopic composition of 0.1321 (gamma(Os) = +21) for the platinum group element (PGE)-enriched B chro mitite seam from the West Fork area to a near-chondritic isotopic comp osition of 0.1069-0.1135 (gamma(Os) = -2 to +4.1) for the PGE-poor G a nd H chromitite seams, respectively, near the middle of the Ultramafic series. Osmium isotopic data for the POE-rich B chromitite seam are g enerally isochronous with whole-rock and mineral data for the J-M Reef (gamma(Os) = +12 to +34). Re-Os isotopic data therefore document a co ntrast between POE-poor cumulates from the Ultramafic series and POE-e nriched cumulates from both the Ultramafic series and the J-M Reef, su ggesting that Os and probably the other PGE were derived from at least two isotopically distinct sources. Moreover, these Re-Os isotopic cha racteristics correlate with petrogenetic subdivisions of the Stillwate r 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 m ajor element, trace element, and precious metal abundances and isotopi c compositions. Although crustally contaminated komatiites can mimic t he Os and Nd isotopic characteristics of the U-type magma, the combina tion of low initial Os isotopic values (gamma(Os) similar to 0) with l ow initial Nd isotopic values (epsilon(Nd) less than or equal to -1), high Pb-207/Pb-204 for a given Pb-206/Pb-204 (Wooden et al., 1991), an d high (Ce/Yb)(n) ratios in U-type cumulates and fine-grained sills an d dykes is more consistent with the involvement of a Re-poor, but trac e-element-enriched portion of the subcontinental lithospheric mantle i n the petrogenesis of Stillwater U-type magmas. However, the radiogeni c 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) gamma(Os) and epsilon(Nd) val ues suggests that mixing of geochemically distinct magmas may have bee n an important process throughout the history of the Stillwater magma chamber. Magma mixing may then explain not only the PGE-enriched J-M R eef but also the anomalous enrichment of the PGE in the B chromitite s eam from the West Fork area and the variable gamma(Os) values observed in other chromitite seams of the Ultramafic series. The intimate asso ciation of these magma types, derived from or modified in the Archaean continental lithosphere, may then be crucial to the formation of magm atic PGE mineral deposits.