Platinum-group and trace elements in Cu-sulfides from the Loolekop pipe, Phalaborwa: implications for ore-forming processes

Abstract

Although Cu-sulfide mineralization within the Loolekop pipe of the Phalaborwa Igneous Complex has been extensively studied, certain aspects of the sulfide metallogenesis remain unclear. This research aims to constrain the relative timing and processes involved in Cu-sulfide mineralization within the Loolekop pipe through textural analysis and trace element composition of the three different types of sulfide assemblages in this intrusion. The first primary magmatic mineralization phase contains dispersed bornite with chalcopyrite exsolution and local traces of chalcocite and covellite related to fluid alteration, whereas the second primary phase is characterized by veins of chalcopyrite with cubanite exsolution. The third mineralization phase relates to leaching of primary sulfide assemblages by oxidizing fluids, resulting in desulfurization and the formation of valleriite [(Fe2+,Cu)4(Mg,Al)3S4(OH,O)6] and magnetite. Whole-rock ICP-MS analyses of platinum-group elements (PGE) indicate a mantle origin of the primary Cu-sulfides (bornite-chalcopyrite assemblages). In situ trace element analyses of sulfide assemblages also show evidence of external material input, most likely of crustal origin, during magma and sulfide liquid ascent, before decoupling of the sulfide liquid and the melt, although the exact source is unclear. The enrichment of Pd-group PGE (PPGE: Pd, Pt) over Ir-group PGE (IPGE: Os, Ir, Ru) in Cu-sulfide phases suggests the presence of an intermediate solid solution (iss)-monosulfide solid solution (mss) system. It is proposed that a Cu- and PPGE-enriched sulfide liquid fractionated from an IPGE-enriched mss, followed by the formation of an iss, and finally the extraction of a PPGE-rich Cu-liquid from the iss forming the bornite-chalcopyrite assemblages. Sulfide textures and the presence of chalcopyrite veins suggest that the iss was leached by high-temperature hydrothermal fluids at depth that remobilized and precipitated Cu along the fractures within the intrusion. Subsequently, all the primary magmatic phases were affected by late-stage fluids, which triggered their alteration into valleriite, also precipitating along fractures within the Loolekop pipe.