Assessing the extent of local assimilation within the Platreef, Northern Limb of the Bushveld Igneous Complex, using sulfur isotopes and trace element geochemistry

Abstract

The proximity to metasedimentary footwall units in the Northern Limb of the Bushveld Igneous Complex (BIC) has resulted in a complex local contamination of in this maficultramafic intrusive body, including the units containing platinum group element (PGE) mineralization. To assess the extent of incorporation of non-magmatic material and its effects on PGE mineralization, geochemical and isotopic data were collected from drill core UMT094 on the Turfspruit project, where core logging has shown a clear macroscopic division between mineralization and local footwall contamination. The S isotopic data combined with whole rock geochemistry data (including CaO/Al2O3, V/Ti, Ni/Cr, S/Se, LOI) present substantial evidence to assess the range of incorporation of local footwall material. A δ34SVCDT profile shows a steady decrease from the footwall assimilation zone (δ34S = +8 ‰) to near constant δ34S values (δ34S < +4 ‰) below mineralization. Through PGE mineralization, the δ34S data converge to the range that has been documented for the Merensky Reef in the Eastern and Western Limbs of the BIC (δ34S: ~ 0 to +3 ‰). Other geochemical parameters through mineralization, such as S/Se and CaO/Al2O3, also match the ranges documented for the Merensky Reef. In addition, parameters such as whole rock V/Ti are shown to be useful indicators of the type of contaminant (e.g. V/Ti > 2 for intervals assimilating shales and V/Ti < 1 for intervals assimilating carbonates; 1 < V/Ti < 2 for uncontaminated magmatic units). The results indicate that there is negligible local contamination through mineralization and that the primary mechanism of PGE mineralization in the Platreef was no different than the mechanism that generated the Merensky Reef in the Eastern and Western limbs of the BIC.