Abstract The Platreef is one of the largest and most valuable Ni-Cu-PGE orebodies on Earth. It is located at the base of the northern limb of the 2.06 Ga Bushveld Complex and stratigraphic relationships with other limbs of the complex and stratiform orebodies such as the Merensky Reef and UG2 chromitite are not clear. The Bushveld Complex intruded along the axis of the >2.9 Ga Thabazimbi-Murchison lineament and this may have acted as a barrier between the northern limb and the rest of the complex for some or all of the intrusion history. Research since the turn of the millenium has demonstrated that the Platreef represents a sill or complex of sills intruded into basement granite-gneiss and sediments of the Transvaal Supergroup. Different sills display variable lithologic units, thicknesses, bulk chemical signatures, and mineralization arising from different inputs of magma and the effects of local wall-rock contamination. Chilling and injection of Main zone gabbronorites took place into already solidified and deformed Platreef, indicating a major break in time between these events. Aspects of mineral chemistry and bulk geochemistry and Nd and Os isotopes in the Platreef overlap completely with the Merensky Reef but not the Upper Critical zone. Conventional and mass independent S isotopes suggest a mantle source of S that was overprinted by addition of local crustal S where the Platreef intruded pyrite-rich shales. Assimilation and introduction of external S is viewed as an ore-modifying process, not as the primary trigger for mineralization. The genesis of the Platreef is more likely to have involved introduction of PGE-rich sulfide droplets with the intruding Platreef magma. These sulfides may have been derived from the same magma(s) that formed the Merensky Reef and which injected up and out along the intrusion walls as the chamber expanded. Alternatively, the sulfides may have formed in pre-Platreef staging chambers where they were upgraded by repeated interactions with batches of Lower zone magma before being expelled as a crystal-liquid-sulfide mush by an early injection of Main zone magma, prior to the formation of the bulk of the Main zone which crystallized above (and partially eroded) the solidified Platreef.

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.

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 The total grams of platinum group elements (PGE) in the Critical zone of the Bushveld when projected to the original horizontal and expressed as g/m 2 is relatively uniform for the Critical zone of the Bushveld Complex as a whole, varying from 157 to 171 g/m 2 in the western Bushveld and from 105 to 116 in the eastern Bushveld. However, in the interval from the top of the UG-2 to the top of the Merensky Reef, the northwestern Bushveld is twice as rich as the southwestern Bushveld, which is again significantly richer than the eastern Bushveld. This latter trend is also paralleled by the PGE "tenor factor" of the sulfides in samples close to the upper chromite seam of the Merensky Reef. These observations support the conclusions of Eales et al. ( 1988 , 1990 ), Scoon and Teigler ( 1994 ), and Maier and Teigler ( 1995 ) that a feeder for the Bushveld existed in the northwest during Merensky Reef time. Variation in the Cu/Pd ratio of sulfide-poor rocks suggests that the more mafic sequences in the interval between the top of the UG-2 and the Merensky Reef in the southwest Bushveld are due to influxes of an unusually PGE rich mafic magma across cumulates crystallizing from a magma with both plagioclase and orthopyroxene on the liquidus. The Merensky and Bastard reefs in this area are the culmination of these mafic influxes and their mixing with resident magma, albeit influxes that in these cases developed immiscible sulfide. The decrease in PGE tenor and increase in Cu/Pd ratio from near the upper chromite seam of the Merensky Reef upward into the overlying Merensky pyroxenite is consistent with the fractional segregation of sulfide from a PGE-enriched magma. A model is proposed for the emplacement of the Merensky Reef whereby influxes of magma spread away from the centers, mixing with resident magma, segregating sulfide, and becoming progressively depleted in PGE as they progressed. In general, the average PGE content of massive chromitites increases upward from the LG-1 to the UG-2/3. The LG-1 to LG-4 chromitites differ from those higher in the stratigraphy in having a much lower (Pt + Pd)/(Ru + Ir + Os) ratio. It has been suggested that the latter developed along with an immiscible magmatic sulfide liquid, while the former lacked this sulfide liquid. All chromitites are characterized by very high PGE/S ratios, and, in the case of those that contained original sulfide liquid sulfide, this is attributed to the original sulfide having been destroyed due to Fe entering the structure of cooling chromite, the f S2 rising, and the sulfur eventually being lost to the environment. Along with the sulfur, Cu and some of the Ni and Pd have also been lost, but not Pt, Rh, Ru, Ir, or Os. Ideas on the formation of Bushveld chromitites fall into two main groupings, those that maintain that they formed in situ ("onstage") and those that maintain that the chromite crystallized elsewhere ("offstage") and was introduced as a slurry. The problem that has puzzled researchers is what causes chromite to separate from a magma alone, without accompanying silicates. Observations reported here have shown that there are similar systematic variations in PGE profiles in the MG-3 and MG-4 chromitites in areas that are separated by 270 to 300 km and a progressive upward increase in V content in the UG-2 at Waterval shaft, all of which could not have resulted from introduction of the chromite in these units as a slurry; such observations argue strongly for an on-stage origin. An investigation has been made of spinel and orthopyroxene in a magma with the average composition of Critical zone magma, as a function of temperature, pressure, Cr 2 O 3 content, and H 2 O content and mixing with both fractionated mafic magma and felsic melt. Chromitites can form in situ if spinel crystallization precedes orthopyroxene on cooling. Accepting the usual maximum f o2 for the Bushveld as <QFM, MELTS modeling shows that this will only be the case if the Cr 2 O 3 content is >0.20 wt percent. Increasing total pressure has no effect on the f O2 at which spinel precedes orthopyroxene, indicating that increase in pressure will not precipitate a chromitite. The application of MELTS to mixtures of primary with 10 and 20 percent fractionated magma, and with admixtures of 20 percent average continental upper crust slightly increases the f O2 at which spinel precedes orthopyroxene, indicating that these mechanisms will not cause spinel to appear on the liquidus before orthopyroxene. It has been found that if the mixed magmas both have olivine (rather than orthopyroxene) along with spinel on the liquidus, a small amount of spinel will crystallize alone before it is joined by olivine; however, the modeling shows that those who have extended this concept to orthopyroxene—rather than olivine-rich magmas—are incorrect. The same is true of adding H 2 O. Calculations show that the limited amount of chromite that could form from a magma containing 0.25 percent Cr 2 O 3 at or below QFM means that a 70-cm-thick massive chromitite would have to seperate from a column of magma ≥1,000 m, supporting Eales's ( 2000 ) conclusion that the chromite in the Critical zone must have come from a volume of magma equal to many times the existing volume of cumulates. The whereabouts of the missing magma (the Critical zone cumulates are, on average, less than 2,000 m thick) is unknown, but it is suggested that part has exited the complex up its walls, leaving the Platreef and Sheba's Ridge deposits as witnesses to its passing—the pudding basin model. While contamination of Critical zone magma with a felsic melt will not promote chromite crystallization, it will also not prevent it at a given f O2 . Contamination with a melt of average upper crustal composition will, however, promote sulfide immiscibility, and it is proposed that this is the reason why some chromitites have developed with immiscible sulfide and are therefore rich in Pt and Pd, and others have not.