The occurrences of ultramafic pegmatites at Dwarsrivier Mine, Lower Critical Zone, Eastern Bushveld Complex, South Africa

Abstract

This study presents a detailed petrological and geochemical investigation of the concordant ultramafic pegmatites of the Lower Critical Zone at Dwarsrivier Mine, in the eastern Bushveld Complex. The pegmatites are coarse-grained and of variable size and mineralogy. They occur within the LG-6 chromitite layer as well as in the silicate rocks of the Rustenburg Layered Suite. Field observations into their occurrence, and their association with the LG-6 chromitite layer, suggest that the pegmatites intrude, deform and disrupt the LG-6 chromitite. The mineralogy of the pegmatites is similar as the surrounding pyroxenite rocks, although they are texturally different from the surrounding pyroxenite host rocks. They comprise variable amounts of clinopyroxene, orthopyroxene, olivine, plagioclase, biotite and accessory amphiboles. Based on dominant mineralogy, the pegmatites can be grouped into clinopyroxene-, orthopyroxene- and olivine-rich pegmatites. The major and trace element composition of the pegmatites is different from the surrounding pyroxenite host rocks. The whole rock geochemistry of the pegmatites indicate that the pegmatites have a slightly higher concentration of incompatible trace elements compared to the pyroxenite host rocks, although compositional overlaps exist. Whole rock Mg# and trace element ratios suggests that the pegmatites are the products of less fractionated liquid(s) than their surrounding pyroxenite rocks. When the whole rock geochemical data of the pegmatites are compared to other similar concordant pegmatites in the eastern Bushveld Complex, as well as the discordant pegmatites from the western Bushveld Complex, it is established that the pegmatites at Dwarsrivier mine are not genetically related to the discordant, intrusive iron-rich pegmatites. They are also different from the concordant pegmatites found elsewhere in the eastern Bushveld Complex. The pegmatites are interpreted as the products of a volatile-rich melt which promoted the recrystallization or sub-solidus enlargement of the pyroxenes. The occurrence of cumulus olivine at the stratigraphic position of the Lower Critical Zone is exceptional. Olivine crystalized as a result of continuous phase shifts within the pegmatite melt. Some of the pegmatites are serpentinised by late hydrothermal fluids. The olivine-rich pegmatites display the most prominent serpentinisation, and are composed of a mineral assemblage dominated by serpentine and magnetite.