Re and Os distribution and Os isotope composition of the Platreef at the Sandsloot–Mogolakwena mine, Bushveld complex, South Africa

Rhenium and Os concentrations and Os isotopic compositions were determined for a suite of previously characterized samples from a traverse across the Platreef at the Sandsloot–Mogolakwena mine of the Northern Limb of the Bushveld complex. The samples comprise five pyroxenites and five “parapyroxenites,” contact rocks representing variable mixtures of calc–silicate country rocks and magmatic rocks. The isotopic data from the pyroxenites define a Re–Os regression line with an age (2011 ± 51 Ma) that agrees within uncertainty with the accepted age for the Bushveld complex, whereas the parapyroxenite data scatter about this trend. The initial 187Os/188Os ratios of the pyroxenites (0.1820–0.1848) are similar to, but slightly more radiogenic than, previously published values for the Merensky Reef. Our preferred interpretation is that this section of the Platreef formed from a well mixed, crustally contaminated magma, genetically related to that which gave rise to the Merensky Reef. The Platreef parental magma may also have experienced minor additional contamination by wall rock interaction during emplacement.

Research highlights
► New osmium isotopic results from the Platreef, Bushveld Complex.
► Evidence for genetic link between the Platreef and the Merensky Reef.
► Extensive crustal contamination at depth, possibly from black shale assimilation.
► Minor additional contamination from wallrocks during emplacement.