The silicate mineralogy of the MG4 chromitite package in the eastern part of the Bushveld complex, South Africa

Abstract

Stratiform chromitite layers are peculiar to large layered mafic intrusions. The origin of these chromitite layers has been widely debated. Some petrologists suggested that the layers formed as a result of the mixing of two compositionally different magmas whereas others suggest that the chromitite layers formed from changes in pressure. The former hypothesis is widely accepted, and states that chromitite forms when a more evolved magma is injected into the chamber occupied by a more primitive one. To evaluate this hypothesis, a study has been conducted on the silicate textures and major element geochemistry of the silicate-rich layers above and below the MG4 chromitite package in the Critical Zone of the Rustenburg Layered Suite, part of the Bushvel Complex in South Africa.The MG4 chromitite package consists of several chromitite seamsseparated by silicate layers. Orthopyroxene and plagioclase (interstitial plagioclase) are observed in large amounts throughout the silicate layer, with less abundant clinopyroxene and some trace amounts of biotite. Throughout the silicate-rich layers above and below the MG4 chromitite layers (MG4 pyroxenite), the orthopyroxene exhibits no major compositional variation in major elements (Mg#= 1.15-1.25). This is also observed in the clinopyroxene composition throughout the study area. However, plagioclase, which dominates the lower part of the stratigraphy, varies in composition with a decrease in the calcium content (Ca= 0.8-0.5) and a simultaneous increase in the sodium content (Na=0.2-0.5). These similarities between the rocks above and below the MG4 chromitite layers suggest that the chromitite layer originated from a single magma or a mixture of two magmas with similar composition. This model is supported by the observed thin sections where orthopyroxene occurs as euhedral grains throughout the section especially above the 63.13m depth lying above the plagioclase –rich layer. Trace element analysis further suggest that the magma that crystallized the plagioclase-rich lower part mixed with the influx of new magma rich in Mg to crystallize the rocks of the upper sequence dominated by orthopyroxene, clinopyroxene and Na-rich plagioclase.