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 i n osmium concentrations, define a Re-Os isochron age of 2043 +/- 11 Ma (wit h initial Os-187/Os-188 = 0.151), which is in accordance with the normally cited crystallization age of the Bushveld Complex. The isochron fit demonst rates an excellent radiogenic Os isotopic homogeneity of this portion of th e complex at the time of crystallization and excludes any later disturbance of the Re-Os isotope system, Initial Os-187/Os-188 ratios of platinum-grou p element (PGE) enriched sulfides and whole rocks of the directly underlyin g Merensky Reef are highly radiogenic and variable (with values between 0.1 68 and 0.181). Separates of chromite grains and interstitial phases of Crit ical Zone chromitite layers show initial Os-187/Os-188 signatures ranging f rom the near-chondritic mantle values of lower group chromitites (i.e. simi lar to 0.120) to 0.137 for middle group chromitites and to 0.150 for the up per group 2 chromitite layer. Osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold, sho wing the presence of highly PGE-enriched interstitial sulfides. Equal or ve ry similar initial Os-187/Os-188 signatures of the interstitial minerals an d the cumulus chromite grains together with their coupled increasing and de creasing trends in this ratio clearly demonstrate that interstitial sulfide s and associated cumulus chromite of the different chromitite layers scaven ged their osmium from the same or similar sources. Therefore, a PGE enrichm ent of the interstitial sulfides by a primary magmatic process, such as pre cipitation 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 fo r the formation of the Merensky Reef. (C) 1999 Elsevier Science B.V. All ri ghts reserved.