Chromite and Graphite in the Platiniferous Dunite Pipes of the Eastern Bushveld

Abstract

Merensky Reef and UG-2 horizon are the economically most important repositories of platinum group metals (PGM) in the Bushveld Complex. The platiniferous dunite pipes of the Eastern Bushveld, although not producing at present, are of considerable interest for our understanding of transport and deposition of PGM. Platinum mineralogy in the pipes and in the Reef differs distinctly; an attempt has now been made at identifying possible compositional differences in associated chro-mian spinels.

Two types of spinels occur in the pipes: homogeneous spinels plot in the high Fe/high Cr portion of the Mg × 100/Mg + Fe² versus Cr × 100/Cr + Al section of the spinel compositional prism. They thus occupy positions well outside the fields of cumulus Bushveld, Muskox and Stillwater spinels. In addition, there occur inhomogeneous spinels with embayment and resorption textures, and antipathetic Fe/Al variations. Clusters of graphite flakes are widespread in some dunites and may have had a catalytic effect on the distribution of PGM; they are attributed to the activity of late magmatic phases enriched in H₂O and CO₂.