Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus

Chromite is developed as an accessory mineral in a 1,700-m-thick sequence of lower and lower critical zone cumulates in the area south of Potgietersrus and as a major constituent in a number of chromitite layers within this sequence. The lower zone consists of a 1,500-m-thick sequence of ultramafic rocks characterized by varying proportions of olivine, orthopyroxene, and chromite, which allows the pile of cumulates to be subdivided into 37 cyclic units. The critical zone, on the other hand, is only 350 m thick, of which only the lower 150 m contains chromite-bearing lithologies.Electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the Mg/(Mg + Fe (super +2) ) and Cr/(Fe (super +2) + Fe (super +3) ) ratios of chromite and the Mg/(Mg + Fe (super +2) ) ratio of the mafic silicates. The composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks. The analytical data have also shown the TiO ₂ content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma.A detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f (sub O ₂ ) and a rise in the Cr ₂ O ₃ content of the liquid, thereby causing an increase in the Cr ₂ O ₃ content of the chromite within the chromitite layers.Postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering. Sintering took place in the presence of a reactive Mg-rich liquid which resulted in an increase in the Mg/Mg + Fe (super +2) ratio of the chromite. This process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent.