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.
