Strontium isotope variations in the Flatreef on Macalacaskop, northern limb, Bushveld Complex: implications for the source of platinum-group elements in the Merensky Reef

The Platreef of the Bushveld Complex is one of three key stratigraphic units, along with the Merensky and UG-2 reefs, in which platinum-group elements are concentrated. The correlation between the Platreef and the stratigraphy from the western and eastern limbs of the BC has been challenging due to the heterogeneous and variable nature of the Platreef along strike. However, the discovery of the Flatreef, interpreted as the down dip, sub-horizontal extension of the Platreef, on the farms Turfspruit and Macalacaskop, opened new avenues for enquiry that allow the study of a magmatic stratigraphy less affected by footwall interaction. In this study, we report on the Sr isotope composition of plagioclase through an ~ 280-m-thick intersection of the Flatreef and its footwalls and hanging walls as intersected by borehole UMT-393 drilled on the farm Macalacaskop. Comparison of the Sr-isotopic composition and the anorthite content of plagioclase across this intersection with data from the Merensky and Bastard cyclic units elsewhere in the Bushveld Complex supports the contention that the Flatreef is a correlative of the Upper Critical Zone–Main Zone transition, including the Merensky and Bastard cyclic units. The available data suggest that the sulfides of the Flatreef and its correlatives in the remainder of the Bushveld Complex originally formed in a deeper staging chamber. During continued magma ascent some of the sulfides were entrained and deposited in the Flatreef.