Petrogenesis of the Lower Zone Olivine-Rich Cumulates Beneath the Platreef and Their Correlation with Recognized Occurrences in the Bushveld Complex

Previously unknown Lower zone ultramafic sequences are described beneath the Platreef in the northern limb of the Bushveld Complex (South Africa). The Lower zone is separated from the overlying mineralized Platreef sequence by intervals of country rocks, which are represented by granofels (agmatites) and shales and include thin sills of fine-grained Marginal zone norite and pyroxenite. The chemical compositions of olivine and orthopyroxene in the Lower zone of the northern limb are more primitive than those established for the western and eastern limb and indicate that the Bushveld parental magma should be more Mg rich than previously suggested and therefore may have been able to dissolve more Cr than indicated by known Marginal chilled rock compositions. Olivine and orthopyroxene compositions show multiple reversals within the Lower zone sections which are accompanied by coherent reversals in the Cu/Pd ratio. The style of reversals is consistent with a gradual mixing of fresh magma with more evolved resident liquid in the chamber. Olivine and orthopyroxene contain higher Ni in sulfide-poor Lower zone sequences and are Ni depleted in mineralized rocks, whereas the degree of sulfide mineralization correlates with the S content in the country rock. Assimilation of sedimentary sulfur resulted in sulfide saturation and formation of Ni-depleted silicates in ultramafic rocks. The extremely high Ni content in olivine (up to 0.66 wt % NiO at Mg # = 88) from S-poor ultramafic rocks is attributed to Ni enrichment in residual melt after massive crystallization of Ni-poor orthopyroxene adcumulates. Trace element compositions of the most primitive cumulates and minerals confirm that the enrichment of primary melt of the Bushveld Complex in some incompatible elements is an initial feature of its anomalous mantle source.