Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex

Chromitites from layered mafic intrusions are of great economic importance, yet the origin of these deposits remains enigmatic. We describe multiphase silicate inclusions trapped within chromite grains from the G chromitite seam of the Stillwater Complex, Montana, United States. These inclusions are interpreted to represent melt trapped during chromite growth and hence provide information on chromitite formation. Most reheated inclusions have variable quench textures and chemical compositions that are consistent with variable degrees of mixing between a high-Mg basaltic parental magma and a Na-rich trondhjemitic melt. The trondhjemite is suggested to derive from partial melting of mafic or metasedimentary country rocks. Based on these inclusions, we outline a model for chromitite formation involving ponding of a new pulse of primitive magma at the roof of the Stillwater magma chamber, followed by localized partial melting and assimilation of the country rock. The newly formed hybrid melts become oversaturated in chromite, leading to extensive chromite crystallization. Chromitite horizons are proposed to form from dense chromite-rich plumes that periodically sink down from the roof zone to settle out as layers at the basal cumulate mush zone. Numerous radiogenic isotope studies, together with the widespread occurrence of similar multiphase inclusions in chromite from other cumulate complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in many layered intrusions.