Abstract

The lower zone of the Potgietersrus limb of the Bushveld Complex consists of a succession of ultramafic rocks, at least 1,600 m thick, that can be subdivided into 37 cyclic units. Ni, Cu, and Pt mineralization is developed at two levels in the sequence, namely, a zone of mineralization within the Volspruit pyroxenite subzone and mineralization associated with the chromitite layers of the Drummondlea harzburgite-chromitite subzone. Of the two, the mineralization in the Volspruit subzone is more significant, in that values over the basal 6 m of the mineralized zone are on the order of 0.27 percent Ni, 0.19 percent Cu, 3.09 ppm Pt, and 2.56 ppm Pd. The composition of the coexisting silicates suggests that separation of an immiscible sulfide liquid was brought about by a drop in temperature of the magma, possibly caused by the emplacement of less primitive, cooler magma that mixed with the residual, crystallizing, lower zone magma.In the Drummondlea subzone, sulfide mineralization is developed within the upper parts of the chromitite layers and the overlying olivine-rich rocks. In this case, separation of sulfide liquid evidently took place in response to the crystallization of large quantities of chromite as a result of which the FeO content of the magma was lowered, as well as its sulfur-carrying capacity.

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