The mineralogy and geochemistry of the Rooikoppies iron-rich ultramafic pegmatite body, Karee Mine, Bushveld Complex, South Africa

Abstract

At the Karee Mine of the Lonplats mining company in the Bushveld Complex, South Africa, the Rooikoppies iron-rich ultramafic pegmatite (IRUP), which covers the stratigraphy from below the UG2 chromitite layer up to the Main Zone, replaces cumulus anorthosite and pyroxenite. The Rooikoppies IRUP was studied using transmitted and reflected light microscopy, X-ray fluorescence, X-ray diffraction, and electron microprobe techniques. Two visually different varieties of IRUP were observed: a) a relatively smaller grained greyish variety and b) a relatively coarser grained greenish variety. In drill core, the IRUP body was observed to be in contact with the host cumulate rocks either by means of gradual or sharp contacts. Bulk rock compositions indicate that the Rooikoppies IRUP is enriched in Fe2O3, MgO, and CaO (relative to the cumulate host rocks) while having lower concentrations of Al2O3. Chemical differences between cumulus host rocks and IRUP are accompanied by changes in mineral assemblage and mineral chemistry. Spatially related IRUP samples revealed areas with potentially more pronounced increases in iron and magnesium contents relative to the host cumulate rocks. Element ratios indicate that aluminium acted as an immobile element during the formation of the IRUP body and that the addition of iron, magnesium, and calcium, through the action of hydrothermal fluids, diluted the already existing cumulus feldspar, resulting in the low concentrations of Al2O3 in the IRUP samples. The addition of iron, magnesium, and calcium also resulted in the crystallization of large proportions of clinopyroxene and olivine, and resulted in changes in the mineral assemblage and mineral chemistry, relative to the host cumulate rocks. The Rooikoppies IRUP body can be classified as a silicate rich variety (Viljoen and Scoon, 1985), consisting of clinopyroxene, olivine, plagioclase, secondary magnetite and ilmenite. It is suggested that the formation of the Rooikoppies IRUP is not due to a single event, but rather that the IRUP body formed through multiple replacement events, resulting in a network of chemically different zones within one large IRUP body.