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

This study of a part of the lower Critical Zone, Farm Ruighoek,Western Bushveld, is based mainly on selected drill core samplesfrom two sections approximately 1.2 miles apart. The 1300-ftsequence investigated consists of pyroxenites with two harzburgitebands and sixteen chromitite seams. Results obtained are consistent with the hypothesis that evolutionof the sequence was a cyclic process in which cumulate mineralscrystallized in a zone near the floor of accumulation undergenerally quiescent conditions. Compositional changes of cumulateminerals reflect the influence of a separate intrusion of undifferentiatedparent magma or refusion at depth of crystals formed near thetop of the magma chamber. Interstitial mineral content and plagioclasecomposition reflect changing rates of crystal accumulation.Orthopyroxene grain size and sorting coefficient reflect, inpart, the vertical distance over which crystallization occurred.Textural features and contact relations of chromitite seamsare consistent with the hypothesis that most chromite crystallizedfrom the silicate magma and accumulated during a period of littleor no crystallization of silicate minerals. It is postulatedthat this loose crystal assemblage was enriched by co-accumulationand partial in situ crystallization of chromite-rich immiscibleliquid. Textural, mineralogical, and compositional changes infootwalls and hanging walls of chromitite seams are thoughtto reflect in situ reactions.