Geochemical controls on the distribution of PGE mineralisation in skarns formed during emplacement of the Platreef in the Northern limb of the Bushveld Complex, South Africa

Abstract

The skarns in drill core SS330, extracted at the contact zone between the Platreef magmas from the Northern limb of the Bushveld Igneous Complex and the country rock dolomite, are investigated to understand processes that were involved in their genesis and have influenced the distribution of the platinum group element (PGE) mineralisation.The skarn lithologies are diopside-rich rocks (i.e. clinopyroxenite and feldspathic clinopyroxenite), and forsterite-rich rocks (olivine clinopyroxenite and serpentinite). The geochemical data show that the skarns were formed by metasomatic transformations of dolomite; such that Ca was pivotal in the genesis of diopside-rich rocks whereas Mg played a major role in the genesis of forsterite-rich rocks when compared to magmatic rocks from the Platreef. The metasomatic fluids involved in the skarn- and reef-forming processes were Si–Al–Fe-rich with variable amounts of the base metal sulphide (BMS) elements (e.g. Cu, Ni, Zn, Co) and originated from three mafic magmas. It is suggested that the fluids evolved along three crystallisation trends (i.e. upwards, downwards and inwards) leading to SiO2, Na2O, Fe2O3, Cu, Ni, Pt and Pd enrichments and Al2O3 depletion which resulted in the formation of the PGE and PGE-BMS reefs. On these bases, the proposed indices (e.g. SiO2/Al2O3, Na2O/Al2O3, Fe2O3/Al2O3, Si + Na + Fe/Al) can be used as lithogeochemical vectors to locate the position of the PGE reefs downhole in drill core SS330.