Using Sulfide Mineral Chemistry to Understand PGE Mineralization Processes in the Platreef, Bushveld Igneous Complex, South Africa

Abstract The Platreef in the Northern limb of the Bushveld Igneous Complex, South Africa, is a complex lithological unit containing significant PGE-Ni-Cu mineralization. Because the Platreef is characterized by intense magma–country rock interaction, there is no consensus regarding (a) whether crustal assimilation during emplacement triggered the mineralization processes and (b) its correlation in terms of mineralization processes with the PGE-rich Merensky Reef (Western and Eastern limbs of the Bushveld Igneous Complex). Understanding the mineralization processes (processes related to ore genesis) in the Platreef could help with the exploration of new deposits elsewhere. To answer these questions, we hypothesized that major and trace element contents in major sulfide minerals (pyrrhotite, Po; pentlandite, Pn; chalcopyrite, Ccp) from contaminated and little to non-contaminated intervals could be used to track different origins. Three suites of samples were used: (1) atypical samples intersected during deep drilling, showing high PGE grades with minimal evidence of country rock assimilation, followed by increasing assimilation with depth; (2) representative samples of the main orebody (the “Flatreef”); and (3) reference samples from the Merensky Reef. The data were acquired using electron-probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The results validate the hypothesis that sulfide mineral chemistry from contaminated and little to non-contaminated intervals can be used to track different geochemical processes. For instance, Se contents in sulfide minerals associated with mineralization are higher than in those associated with crustal assimilation and lower PGE grades. In contrast, Co contents in Po and Pn increase with crustal contamination. Thus, in the study area, Se/Co in Po and Pn are excellent indicators of the degree of country rock assimilation. Secondly, other trace elements (such as Ru in Po, and Pd in Pn) are significantly higher in intervals with high PGE grades and match the characteristics of reference samples from the Merensky Reef. These results, combined with recently available S and Sr isotopic data, constitute substantial evidence that PGE-Ni-Cu mineralization in the Flatreef and the Merensky Reef were likely caused by the same processes and from the same magmas. Assimilation of local country rocks during emplacement of BIC magmas in the Northern limb was therefore not the trigger of sulfide saturation.