RE-OS ISOTOPIC SYSTEMATICS IN CHROMITITES FROM THE STILLWATER COMPLEX, MONTANA, USA

New Re-Os isotopic data on chromitites of the Stillwater Complex demon strate isotopic equilibrium between cumulate chromite and whole rock. Initial osmium isotopic ratios for the chromitites, chosen for their f reshness, are consistent with derivation from a mantle-derived magma t hat suffered little or no interaction with the continental crust prior to crystallization. Molybdenite, separated from a sample of the G-chr omitite, yields a Re-Os age of 2740 Ma, indistinguishable from the age of the intrusion. The presence of molybdenite documents rhenium, and probably osmium, mobilization by hydrothermal fluids that permeated th e intrusion shortly after crystallization. Initial osmium isotopic var iability observed in chromitites and other rocks from the Stillwater C omplex could result from interaction with these fluids. In this contex t, there is no compelling reason to call on assimilation of crust by m antle-derived magma to explain the osmium or neodymium isotopic variab ility. Although osmium isotopic systematics have been affected by hydr othermal processes, Re-Os results demonstrate that more than 95% of th e osmium, and by inference other PGEs in the Stillwater Complex, deriv e from the mantle.