Abstract
A geologic map and stratigraphic column of the Nietverdiend basin are presented, based on recent mapping and two boreholes drilled in the area. The floor of this part of the Bushveld Complex is undulous with two approximately north-south-trending structures composed of metasedimentary rocks flanking the lower and critical zones; the floor is confined to a north-south-aligned, kidney-shaped basin. The marginal zone is composed of gabbroic and noritic rocks showing erratic variability in mafic mineral composition and reversed zoning in the plagioclases. The lower and critical zones consist of bronzitites alternating with olivine- and/or chromite-bearing rock types. Throughout these two zones the compositions of the silicates remain approximately constant. The chemistry of the LG-1 to LG-4 chromitite layers was investigated in detail and the nature of the cation substitutions assessed. There is an increase in Cr 2 O 3 abundance from layer LG-1 to LG-4 with Al 2 O 3 and/or Fe 2 O 3 being substituted in the spinel structure. Principal component analysis is used to group and differentiate among the four chromitite layers. A dissimilarity between the chemistry of the chromitite layers from the southern and northern sectors of the basin is illustrated. Extensive assimilation of floor rocks by the Bushveld magma, especially during the earlier part of the intrusion is postulated.
