Abstract The northern limb of the Bushveld Complex of South Africa contains a diverse array of Cr, Ni-Cu-platinum group element (PGE), Fe-V mineralization in mafic-ultramafic rocks and Sn mineralization hosted in granites. The limb has historically been underexplored compared to other parts of the Bushveld Complex and currently represents one of the world’s most interesting exploration frontiers. Successful low-cost open-pit mining of the thick Platreef Ni-Cu-PGE deposit, coupled with rising costs and limited scope for mechanization associated with narrow reef-type deposits in the eastern and western Bushveld, have driven efforts to locate similarly wide magmatic sulfide orebodies at surface or at reasonably shallow depths elsewhere in the northern limb, including recent discoveries of the Flatreef- and Main zone-hosted PGE deposits in the troctolite unit, at Aurora, and in the lower (F) and upper (T) mineralized zones at Waterberg. The Flatreef is hosted within a more consistent series of stratigraphic units than the more varied Platreef located updip, and while it shows similarities in terms of rock types and some geochemical features with the upper Critical zone of the eastern and western Bushveld, strict time equivalence remains to be proven. The various styles of Main zone-hosted PGE mineralization, on the other hand, have no known equivalents in the other limbs of the Bushveld Complex and seem to represent processes and events confined to the northern limb. Potential links based on similar rock types and metal budgets between Aurora and the Waterberg T zone and between the troctolite unit and the Waterberg F zone are attractive but must remain speculative until it becomes clearer whether the northernmost compartment that contains the Waterberg mineralization is linked to the remainder of the northern limb. If both the Flatreef and the Waterberg deposits enter production as planned over the coming decade, they will have dramatic effects on the South African platinum industry and dramatically increase the amount of Pd relative to Pt produced by South Africa due to the Pd-rich nature of all of the northern limb PGE orebodies.

Abstract Sulfate assimilation by mafic to ultramafic melt is thought to be an important process in the genesis of magmatic PGE-Ni-Cu deposits. We consider petrological indicators and possible mechanisms of anhydrite assimilation by ultramafic melts of the northern limb of the Bushveld Complex. On farm Turfspruit, an anhydrite-bearing sedimentary raft of the Duitschland Formation separates the Platreef from underlying Lower zone peridotites. The proportion of anhydrite across the raft increases from negligible in corundum-sillimanite-magnetite hornfels at the base to 95 to 100% in anhydrite marble at the top. Underlying Lower zone peridotites lack anhydrite, whereas overlying Platreef pyroxenites contain both widespread interstitial to euhedral anhydrite as well as spherical to irregularly shaped anhydrite inclusions in association with olivine chadacrysts inside oikocrystic orthopyroxene. Olivine chadacryst compositions (Mg# 79–81 and 0.33–0.46 wt % NiO) support their pristine liquidus origin, although an association of Al-enriched orthopyroxene and interstitial anorthite indicates exchange reactions involving anhydrite and aluminosilicates from hornfels. Plagioclase from the anhydrite-contaminated rocks has an Sr isotope initial ratio (Sr i ) of 0.7047 to 0.7063, similar to the compositions of Bushveld early primitive magmas, in agreement with a relatively nonradiogenic signature of the anhydrite-bearing contaminant with Sr i of 0.7057 to 0.7094. The range of Sr i of plagioclase from the underlying Lower zone peridotites (0.7040–0.7067) and from the Turfspruit platinum reefs just below the Main zone contact (0.7068–0.7084) supports their correlation and synchronous emplacement with the Lower zone and the top of the Upper Critical zone in the western and eastern limbs of the Bushveld. The δ 34 S values of anhydrite (12.2–14.5‰) and a coexisting pyrrhotite-millerite-chalcopyrite sulfide assemblage (6.2–7.8‰) in a hornfelsed raft and overlying pyroxenites are interpreted to have resulted from open-system isotopic exchange, indicating closure temperatures of 750° to 820°C. The assimilation of sedimentary anhydrite is interpreted to be an important component of contact-style mineralization of the Platreef at Turfspruit that took place through the erosion and disintegration of footwall rocks by dynamic pulses of hot magmas. Chemical dissolution, thermal decomposition, and melting of sulfate-bearing rafts or xenoliths are viable assimilation processes that result in the saturation of silicate melt with sulfate, exsolution of immiscible sulfate melts, crystallization of cumulus and interstitial anhydrite, and precipitation of contact-style sulfide mineralization at the base of the intrusion. Reef-style mineralization at the top of the Platreef shows contrastingly negligible compositional and isotopic evidence of sulfate assimilation.