Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainlyon geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterallyand upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.

The Upper Critical Zone of the Bushveld Complex consists of several cyclic units, each ranging from ultramafic to leucocratic. One view is that they were initiated by addition of, and crystallization from, relatively magnesian magma. Experimental studies on plausible parental magma compositions to this sequence show that plagioclase joins orthopyroxene in the crystallization sequence once the Mg# of the orthopyroxene has decreased below 83. Some norites from the Upper Critical Zone contain orthopyroxene with this Mg#, supporting the validity of the experimental studies. There are no orthopyroxene compositions with higher Mg# than 83 in the entire Upper Critical Zone in either the eastern or western limbs of the Bushveld Complex. This observation suggests that all these rocks formed from plagioclase-saturated magmas. If this interpretation is correct, pyroxenites in this succession are not the result of crystallization from a magnesian magma. Instead, they result from the mechanical separation of plagioclase and orthopyroxene, probably due to crystal sorting during settling, from a magma lying at the cotectic for these two minerals. Rocks exist in the Upper Critical Zone that contain non-cotectic proportions of cumulus plagioclase and orthopyroxene, again supporting models of crystal sorting during settling. In such a model, anorthosites result from the delayed accumulation of plagioclase relative to pyroxene, and not to formation from a magma saturated only in plagioclase.

When traced from northwest to southeast in the western limb, there is a change in the relative proportions of plagioclase to orthopyroxene in the Upper Group 2 chromitite cyclic unit. In the northwest the unit is dominated by ultramafic rocks. In the southeast the plagioclase to pyroxene ratio exceeds that of the cotectic proportion, relations that may result from the lateral transport of suspended plagioclase grains to the southeast.