Field relations and petrographic characteristics of chromitite layers (Steelport Main and Leader seams) and the enclosing pyroxenites in a section of the so-called critical zone of the Bushveld complex (South Africa) are reviewed in the light of various modes of origin proposed for the chrome-rich rocks. It is concluded that textural and other features can best be explained by postulating differential mechanical settling of chromite and bronzite crystals controlled by variations in the velocity of currents in the magma chamber, followed by crystallization of interstitial liquid. Some of the chromite evidently crystallized in place; this could be accounted for by resolution of settled crystals.

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