Chemical Variation in Coexisting Chromite and Olivine in Chromitite Zones of the Stillwater Complex

Abstract The chromitite zones in the Stillwater Complex consist of alternating chromite-rich and olivine-rich stratiform layers. On the basis of previous work, it was concluded that where olivine and chromite occur together as cumulus minerals, they formed as simultaneous crystallization products, in equilibrium with each other and the Stillwater magma. This hypothesis was tested in the present study by determining the proportions and compositions of 35 coexisting olivine and chromite pairs encountered in channel samples of the G and H chromitite zones in the Mountain View area. It was found that the cation fractions of Mg and Fe+2 in the chromite and olivine, and the cation fractions of Cr, Al, and Fe+3 in the chromite, are clearly related to the proportions of the two minerals in the 35 layers. The variations in these cation fractions were compared with theoretical variations expected on the basis of free energies of formation of end-member olivines and spinels. It was found that the observed and expected variations were of the same general magnitude and direction, and that the compositions of coexisting olivine and chromite in G- and H-zone layers can be accounted for by means of a simple thermodynamic model. The regularity in concomitant chemical and volumetric variation of chromite and olivine is such that the grade of the chromite in any layer in the G and H zones can be estimated given only the local stratigraphic section and the proportions of chromite and olivine in the layers. Work is continuing on the possibility of using olivine-chromite pairs as relative geothermometers.