Crustal contamination and PGE mineralization in the Platreef,Bushveld Complex,South Africa: evidence for multiple contamination events and transport of magmatic sulfides

Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein, where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel, the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks, but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation, the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas with the crustal rocks that underlie the Complex.