Petrogenesis of Discordant Ultramafic Bodies in the Lower Critical Zone at the Winterveld Chromite Mine, Eastern Bushveld Complex, South Africa: Late-Magmatic Intrusions and Dismembering of the LG6 Chromitite

Abstract

Abstract Discordant ultramafic bodies that transgress the Rustenburg Layered Suite are an integral part of the Bushveld Complex. Here we describe clusters of subvertical, ultramafic pipes from the Winterveld chromite mine, in the eastern Bushveld. The main orebody at the mine is the meter wide LG6 chromitite, a prominent marker layer in the Lower Critical zone. The incidence of discordant bodies in the Lower Critical zone is relatively low, and many of the features described here have not previously been reported. Discordant bodies are abundant in the areas next to the mine, and include the platiniferous dunite pipes, which were mined prior to exploitation of the Merensky reef. Most of the discordant bodies, however, are unmineralized and impact negatively on mining of the reef-style deposits. The primary igneous layering adjacent to the Winterveld pipes is downwarped and dismembered, and xenoliths of the LG6 chromitite have been displaced downward. Of interest is the occurrence of lenticular-shaped discordant bodies that have intruded into the center of the LG6 chromitite and have inflated the width of the seam but not replaced the Cr spinel. The discordant bodies are made up of two principal assemblages, dunites (nonpegmatitic) and feldspathic orthopyroxenite pegmatites, which occur either separately or as heterogeneous bodies. Both assemblages have magnesian compositions (Mg number: 87–85) and contain disseminated Cr spinel. They are an entirely separate phenomenon from the iron-rich ultramafic pegmatites; this category of discordant bodies has not been reported from the mine. The dunites at Winterveld are related to the magnesian dunites that form large pipe-like bodies elsewhere in the eastern Bushveld, e.g., the primary component of the platiniferous dunites, including Onverwacht, which is located on the mine leasehold. The feldspathic orthopyroxenite pegmatites, on the other hand, may be compared with the pegmatitic components of the Merensky and UG2 reefs. The discordant bodies are late-magmatic intrusives that crystallized in dilational openings. Two lineages of parental magmas are identified. The magnesian dunites are related to a process of flowage differentiation in which peridotitic magma was injected through subvertical conduits. Forsteritic olivine and Cr spinel were the only liquidus phases. An elevated temperature was maintained by forced upward convection, and the differentiated residue is assumed to have been ejected upward. Similarities with peridotite sheets in the floor rocks, as well as in the layered rocks, including the Lower Critical zone, are significant in this regard. The feldspathic orthopyroxenite pegmatites crystallized from a pyroxenitic magma (B1 type), i.e., the same lineage from which the Lower Critical zone accumulated. The coarse-grained texture of the pegmatites (in comparison to the wall rocks) is ascribed to the intrinsically high heat budget of a layered intrusion subjected to late magmatic processes. The pyroxenitic pegmatites contain partially assimilated xenoliths of the dunite, an indication of the relative timing of the two assemblages. Clinopyroxenite pegmatites are a subordinate assemblage found in conjunction with the dunites and can be ascribed to reaction with either the wall rocks or the later intrusion of pyroxenitic magmas. The occurrence of clinopyroxene-amphibole–bearing pegmatites in association with calc-silicate xenoliths (derived from the floor rocks) adds further complexity to the study. Each category of discordant ultramafic bodies is likely to have a different petrogenesis. The orthomagmatic nature of the magnesian dunites and pyroxenitic pegmatites at Winterveld should not be conflated with the iron-rich ultramafic pegmatites, which are related to late-stage or residual melts derived from within the Rustenburg Layered Suite.