The effects of carbonate contamination on Ni-Cu-PGE deposit genesis in the Platreef, northern Bushveld Complex: A case study using Niggli numbers

Abstract

Crustal contamination in Ni-Cu-PGE deposit genesis is generally regarded as an essential, or at least highly beneficial, process in triggering sulfide saturation, either through the addition of external sulfur or as a mechanism for increasing oxygen fugacity through the incorporation of volatile-rich lithologies. The Platreef, northern limb of the Bushveld Complex, South Africa, forms one of the world's largest resources of platinum-group elements (PGE), with additional Ni-Cu-Co mineralisation, and represents a unique deposit, intersecting numerous footwall lithologies from the underlying Transvaal Supergroup. In this study, Niggli Numbers, a geochemical tool used to classify rocks on the molecular proportions of their major element geochemistry, are used to examine the degree and styles of contamination in the Platreef at three locations: Tweefontein, Overysel, and a newly drilled deeper section at Sandsloot. The potential impact of these varying contamination styles on PGE-Ni-Cu mineralisation is then discussed. At Tweefontein and Overysel, the highest PGE-Ni-Cu grades are found in largely uncontaminated pyroxenites, exemplified by Niggli c values <20. Where carbonate contamination is strong, and Niggli c exceeds 30, it is not associated with elevated grades. In contrast, at Sandsloot elevated PGE and Ni grades are strongly associated with carbonate contamination, with Niggli c commonly exceeding 20, and Niggli mg exceeding 0.8, indicating dolomitic contamination, in the highest-grade horizons. Although other pre-emplacement models may yet explain the elevated grades observed at Sandsloot, there remains a clear correlation between interactions with the Malmani dolomite and elevated Ni-Cu-PGE contents which warrants further investigation.