ORIGIN OF PODIFORM CHROMITE DEPOSITS BY MAGMA MINGLING

Abstract

I have oversaturated the immiscible system SiO2Al2O3FeOK2O with FeCr2O4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts. Although both melts are oversaturated with chromite, chromite only nucleates in the fayalite-rich melt where the crystal/melt interfacial energy (γSL) is lowest. In less than three days at 1150°C, all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools, producing nodular cumulate textures as observed in podiform chromite ore. When these results are applied to the lithospheric mantle, podiform chromite deposits may form when droplets of olivine-normative high-pressure, low-viscosity melt mingle with siliceous low-pressure, more viscous melt in shallow magma conduits. It is inferred from the experimental analogue that chromite only nucleates in the melt where γSL is lowest, which in the mantle would be the high-pressure melt. The low-pressure melt only serves as the diffusive chromium reservoir. The richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt.