Abstract

The Cr abundances in magnetite separates from the Main Magnetite layer, 1 magnetite layers 1 and 2, and the intervening anorthositic horizons have been determined from two widely spaced localities in the eastern Bushveld Complex. Chromium abundance is greatest at the base of the Main Magnetite layer (around 7,000 ppm) and decreases rapidly with height through this layer and the overlying anorthositic horizon. The base of layer 1 is associated with a high Cr content (about 1,500 ppm), which is followed by a steady upward decrease. The base of layer 2 is also characterized by a high Cr abundance (about 900 ppm) and detailed studies indicate a slight rise in Cr near the base of this layer.Chromium abundance profiles are examined in the light of several models which have been proposed for the origin of mineralogical layering. The general features of the abundance profiles are compatible with a variety of models; however, it appears that the detailed features of these profiles are best accounted for by a model involving bottom growth, with a slow and variable rate of supply of Cr to the crystallization zone.

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