Evolution of Part of the Lower Critical Zone, Farm Ruighoek, Western Bushveld

Abstract

This study of a part of the lower Critical Zone, Farm Ruighoek, Western Bushveld, is based mainly on selected drill core samples from two sections approximately 1.2 miles apart. The 1300-ft sequence investigated consists of pyroxenites with two harzburgite bands and sixteen chromitite seams. Results obtained are consistent with the hypothesis that evolution of the sequence was a cyclic process in which cumulate minerals crystallized in a zone near the floor of accumulation under generally quiescent conditions. Compositional changes of cumulate minerals reflect the influence of a separate intrusion of undifferentiated parent magma or refusion at depth of crystals formed near the top of the magma chamber. Interstitial mineral content and plagioclase composition reflect changing rates of crystal accumulation. Orthopyroxene grain size and sorting coefficient reflect, in part, the vertical distance over which crystallization occurred. Textural features and contact relations of chromitite seams are consistent with the hypothesis that most chromite crystallized from the silicate magma and accumulated during a period of little or no crystallization of silicate minerals. It is postulated that this loose crystal assemblage was enriched by co-accumulation and partial in situ crystallization of chromite-rich immiscible liquid. Textural, mineralogical, and compositional changes in footwalls and hanging walls of chromitite seams are thought to reflect in situ reactions.