PGE enrichment in chromitite layers and the Merensky Reef of the western Bushveld Complex; a Re-Os and Rb-Sr isotope study

Four poikilitic pyroxenites of the Bastard Unit, all being relatively low in osmium concentrations, define a Re-Os isochron age of 2043 ± 11 Ma (with initial ¹⁸⁷Os/¹⁸⁸Os = 0.151), which is in accordance with the normally cited crystallization age of the Bushveld Complex. The isochron fit demonstrates an excellent radiogenic Os isotopic homogeneity of this portion of the complex at the time of crystallization and excludes any later disturbance of the Re-Os isotope system. Initial ¹⁸⁷Os/¹⁸⁸Os ratios of platinum-group element (PGE) enriched sulfides and whole rocks of the directly underlying Merensky Reef are highly radiogenic and variable (with values between 0.168 and 0.181). Separates of chromite grains and interstitial phases of Critical Zone chromitite layers show initial ¹⁸⁷Os/¹⁸⁸Os signatures ranging from the near-chondritic mantle values of lower group chromitites (i.e. ~0.120) to 0.137 for middle group chromitites and to 0.150 for the upper group 2 chromitite layer. Osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold, showing the presence of highly PGE-enriched interstitial sulfides. Equal or very similar initial ¹⁸⁷Os/¹⁸⁸Os signatures of the interstitial minerals and the cumulus chromite grains together with their coupled increasing and decreasing trends in this ratio clearly demonstrate that interstitial sulfides and associated cumulus chromite of the different chromitite layers scavenged their osmium from the same or similar sources. Therefore, a PGE enrichment of the interstitial sulfides by a primary magmatic process, such as precipitation of immiscible sulfide droplets during magma mixing appears more likely than a scenario involving fluid or melt transport of PGE into these layers. Such a primary magmatic PGE enrichment process is also preferred for the formation of the Merensky Reef.