The Petrogenesis of the Lower Critical Zone-Upper Critical Zone Boundary and Middle Group Chromitites, Eastern Bushveld Complex: Emplacement of pyroxenitic sills and chromite slurries into a primary noritic-anorthositic stratigraphy

The Rustenburg Layered Suite is economically the most vital component of the Bushveld Complex, South Africa, and contains numerous, laterally persistent layers of chromitite. Here we investigate the petrogenesis of the Middle Group chromitites from three sections in the Eastern Limb, two located in the northern sector and one from the southern sector. Regional differences in the igneous stratigraphy are explained by each sector having developed as a separate intrusive compartment. The chromitites straddle the MG3 anorthosite, a regional marker that demarcates the boundary between the Lower Critical Zone (LCZ) and the Upper Critical Zone (UCZ). At Jagdlust and Maandagshoek (northern sector), the chromitites are thin and poorly defined and occur in a succession of seven lithological units with a composite thickness of 220 m. The pyroxenitic E (upper) unit (LCZ) contains the MG1 – MG2 chromitites. The F through L units (UCZ), which consist of intercalated layers of pyroxenite, norite, and anorthosite, contain the MG3 – MG5 chromitites, together with the subordinate Upper-Middle Group layers. At Tweefontein (southern sector), the study interval is restricted to four units with a composite thickness of <100 m and yet the chromitites are represented by multiple layers with thicknesses of up to 2 m. This region contains a major resource of chromium ore, together with byproduct platinum-group elements (PGE). The chromitites are exposed in a succession of open pits covering a strike length of approximately 40 km. Descriptions of the igneous stratigraphy are supported by electron microprobe analyses for the orthopyroxene and plagioclase in the host silicates. There are only minor compositional geochemical differences between the pyroxenites and norites, and upward fractionation trends are insignificant. The Sr initial ratio based on plagioclase separates from the silicates is tightly constrained (0.70605 ± 0.00039). The absence of a regular cyclicity, as well as the sharply defined nature of internal contacts, is reconciled with a hypothesis based on successions of intrusive events. The magma was intruded into a crystalline igneous stratigraphy. Intrusion rates associated with emplacement of the thin magma sheets were insufficient to enable a magma chamber to develop. The lithological units developed from two parental magma-types derived from a deep crustal staging chamber. They do not represent magmatic (fractionation) cycles, as is commonly thought. The norites are related to the batch crystallisation of a relatively felsic basaltic magma, the anorthosites having formed in situ, from low degrees of partial melting and metasomatism. The pyroxenites accumulated from a relatively mafic basaltic magma, injected as sills which exploited competency contrasts between the substantially, crystalline layers of norite and anorthosite. The different thermal history of the magma sheets and sills is consistent with the relatively fine-grained nature of the norites as compared with the coarse-grained pyroxenites. Detailed field relationships, including the non-sequential nature of parts of the stratigraphy, are reconciled with each layer of chromitite having accumulated from a chromite slurry. This concept addresses the chromium budget issue by explaining how anomalous concentrations of chromite can form, including the Middle Group layers at Tweefontein which have a composite thickness of 10 to 12 m. The chromite slurries consisted of xenocrysts entrained in the staging chamber, further concentrated in conduits, and transported laterally by the pyroxenitic sills. The sills functioned as loci for repeated magma recharge and contained multiple slurries. The chromite slurries were sufficiently mobile as to have intruded and partially disaggregated the primary stratigraphy, including the MG3 anorthosite. In some instances, the anorthosite is sandwiched between the multiple MG2 and MG3 chromitites, and in other cases the anorthosite hosts individual layers of chromitite. The principal accessory phase in the MG1 and MG2 chromitites is orthopyroxene, consistent with the relationship between the chromite slurries and the mafic basaltic magma. The abundance of plagioclase in the MG3 chromitite reflects the dynamic nature of the intrusive process, the chromite slurry having liberated grains of plagioclase from the anorthosite. The composition of the chromite and the PGE mineralisation contained in the chromitites was established at depth. We find no evidence of magma mixing processes or of the cotectic crystallisation of chromite and plagioclase in the petrogenesis of the Middle Group chromitites.