The 3033 Ma Stella layered intrusion of South Africa consists largely of magnetite gabbros and gabbros that are hosted by greenstones of the Kraaipan belt. The intrusion contains a 100-m-thick, platinum group element (PGE)-enriched interval that includes a number of laterally continuous PGE reefs constituting the oldest mineralization of this type known on Earth. The richest of the reefs is hosted by magnetitite and contains 1015 ppm Pt + Pd over 1 m, representing by far the highest PGE grades known up to this time in magnetitite-hosted Pt-Pd reefs. The PGEs are interpreted to have been concentrated by sulfide melt, after S saturation had been reached in the advanced stages of magmatic differentiation, in response to magnetite crystallization. Reaction between sulfide melt and oxides led to late magmatic S loss, causing a paucity of sulfides in most of the PGE mineralized interval. As a result, the reefs cannot be distinguished macroscopically from their unmineralized host rocks, and we suggest that similar mineralization may have been overlooked in the upper parts of other tholeiitic intrusions elsewhere.
Maier, W., Barnes, S-J., Gartz, V., & Andrews, G. (2003). Pt-Pd reefs in magnetitites of the Stella layered intrusion, South Africa: A world of new exploration opportunities for platinum group elements. Geology, 31(10), 885-888. https://doi.org/10.1130/G19746.1