MAGMA CONTAMINATION BY COUNTRY ROCKS DURING FORMATION OF THE NORTHERN LOBE OF THE BUSHVELD COMPLEX, SOUTH AFRICA: EVIDENCE FROM CHROMITE CHEMISTRY

The Bushveld Complex hosts some of the world’s largest repositories of chromium and platinum group elements. We undertook a petrographic and mineral composition study of disseminated chromites which occur within the northern lobe of the intrusion to place some constraints on their origin. Samples from the Platreef in the northern lobe’s central and southern sector investigated possess very fine-grained chromites hosted within silicates. The chromium number, Cr# (100•Cr/(Cr+Al)), varies widely from low values of ~18-32 hosted in altered olivines to high values of ~55-82 hosted in orthopyroxenes and plagioclase. All chromites, irrespective of their host silicate types, show slight decreases in Cr#s from the core to the rim of chromites. These chromites are also characterized by highly variable magnesium numbers, Mg#s (100•Mg/(Mg² + Fe²⁺)), which range from 37-51 for chromites hosted in olivines to 1-13 for chromites hosted in pyroxenes and olivines, respectively. Chromites under study are consistent with trend A defined by LG-1 to LG-4 chromites but extend to much lower Cr#s and higher Mg#s. The chemistry of our chromites suggests the involvement of both relatively high degrees of contamination in their formation and re-equilibration. Plots of oxides such as aluminum, titanium, and magnesium, for example, are consistent with contamination of the Platreef by floor rocks (dolomites and argillaceous shales) to the intrusion.