A chill sequence at the base of the Lower Zone of the western Bushveld Complex at Union Section, South Africa, contains aphanitic Mg-rich basaltic andesite and spinifex-textured komatiite. The basaltic andesite has an average composition of 15.2 % MgO, 52.8 % SiO₂, 1205 ppm Cr, and 361 ppm Ni, whereas the komatiite has 18.7 % MgO, 1515 ppm Cr, and 410 ppm Ni. Both rock types have very low concentrations of immobile incompatible elements (0.14–0.72 ppm Nb, 7–31 ppm Zr, 0.34–0.69 ppm Th, 0.23–0.27 wt% TiO₂), but high PGE contents (19–23 ppb Pt, 15–16 ppb Pd) and Pt/Pd ratios (Pt/Pd 1.4). Strontium and S isotopes show enriched signatures relative to most other Lower Zone rocks. The rocks could represent a ~20 % partial melt of subcontinental lithospheric mantle. This would match the PGE content of the rocks. However, this model is inconsistent with the high SiO₂, Fe, and Na₂O contents and, in particular, the low K₂O, Zr, Hf, Nb, Ta, Th, LREE, Rb, and Ba contents of the rocks. Alternatively, the chills could represent a komatiitic magma derived from the asthenosphere that underwent assimilation of the quartzitic floor accompanied by crystallization of olivine and chromite. This model is consistent with the lithophile elements and the elevated Sr and S isotopic signatures of the rocks. However, in order to account for the high Pt and Pd contents of the magma, the mantle must have been twice as rich in PGE as the current estimate for PUM, possibly due to a component of incompletely equilibrated late veneer.

The cumulates of the Bushveld Complex, which form the largest layered intrusion on Earth, are known to have crystallized from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy, to constrain the petrogenesis of the complex. The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallized from multiple, isotopically distinct influxes of melt percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of a distinct magma, which picked up previously formed plagioclase crystals. Sr isotope compositions across the layered sequence seem to be largely decoupled from differentiation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2 km thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of differentiation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance, not only for the petrogenesis but also for the emplacement of the layered intrusion, because devolatilization is directly linked to space creation owing to volume loss, thus producing a lopolith.

The formation of anorthosites in layered intrusions has remained one of petrology's most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a “pothole” in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates

The Paleoproterozoic Monchegorsk Complex, located in the Russian part of the Fennoscandian Shield, constitutes one of the largest mafic-ultramafic layered intrusions in Europe. The complex hosts extensive contact-style platinum group element-Ni-Cu sulfide mineralization along its margin, irrespective of the host lithology, which ranges from peridotite to pyroxenite and gabbronorite. The mineralized intervals reach up to 3 ppm Pt + Pd and attain a thickness of up to 50 m in the central portions of the intrusion, thinning toward the periphery. Our study shows that the key process controlling the size and grade of a contact-style deposit in the Mon-chegorsk Complex was the efficiency of sulfide collection in distinct zones of the intrusion. Strongly mineralized basal contacts are always associated with intense brecciation and the presence of large amounts of felsic pegmatite, suggesting a multistage emplacement of the mafic-ultramafic succession. Thermal modeling demonstrates that multiple episodes of magma influx are required to allow for significant partial melting of the basement. Moreover, the interaction between magma and basement led to the local addition of water and, potentially, carbon dioxide to the magma, resulting in local small-scale dissolution of cumulus phases and a reduction in viscosity of the interstitial melt. This increased the porosity of the mush in the vicinity of the lower intrusion contact, which promoted preferential sulfide liquid accumulation at the base, while the local decrease in magma viscosity facilitated gravitational settling of sulfide droplets. These factors led to an efficient collection of sulfide liquid, especially in the center of the complex, where permeability was maintained the longest due to slower cooling relative to more peripheral parts.

The Paleoproterozoic Monchegorsk Complex in northwest Russia represents one of the largest layered intrusions in Europe and hosts several examples of broadly stratiform platinum group element (PGE) mineralization at different stratigraphic levels of the intrusion that have been suggested to represent reef-style mineralization. The Sopcha reef occurs in the ultramafic lower portion of the complex and constitutes an up to 6-m-thick succession of layered, mineralized dunite, harzburgite, and olivine-orthopyroxenite, with peak grades of 3.4 ppm Pt + Pd and 1.1 wt % Ni. Another PGE occurrence is hosted by the leucogabbronoritic to anorthositic Vuruchuaivench intrusion, which represents part of the mafic upper portion of the Monchegorsk Complex. The disseminated sulfide mineralization reaches up to 7.3 ppm Pt + Pd and is concentrated in several lenticular bodies over a strike length of ~5 km, rather than in a laterally continuous reef as previously suggested. Moreover, our work identified a previously unreported minor enrichment in precious metals of up to 0.2 ppm Pt + Pd in leucogabbroic rocks of the Monchetundra intrusion, which represents the uppermost portion of the Monchegorsk Complex and belongs to the more than 60-km-long mafic Main Ridge. Detailed lithophile and chalcophile element data, coupled with mineral chemistry, indicate that the PGE mineralization at Sopcha and Vuruchuaivench does not represent classic reef-style mineralization, which is commonly narrow and marked by a sharp increase in Cu/Pd ratios, reflecting the in situ sulfide saturation within a large magma chamber. Instead, it is more likely that the Sopcha reef was emplaced as a crustally contaminated and sulfide-saturated, olivine-rich crystal mush that was sourced from a deeper chamber. The Sopcha mineralization is characterized by Pd/Pt > 5 and Pd/Ir > 55, similar to contact-style mineralization elsewhere in the complex, possibly suggesting a common origin of the sulfides. The mineralized Vuruchuaivench rocks have similar Pd/Pt but much higher Pd/Ir ratios of up to 600, whereas the unmineralized host rocks, below as well as above the mineralization, have Pd/Ir ratios <100 and Pd/Pt ratios <2. These data indicate that the PGE-rich sulfides did not segregate in situ from the same magma that crystallized the host gabbronorites and anorthosites at Vuruchuaivench. Considering R factor and sulfide fractionation modeling results, we suggest that the mineralized Vuruchuaivench rocks represent a sill-like intrusion of gabbroic crystal mushes, which have entrained fractionated sulfide liquid that is related to an earlier sulfide saturation event. In contrast, the mineralized leucogabbroic rocks from the Monchetundra intrusion are characterized by a sharp increase in Cu/Pd ratios, which is consistent with a classic PGE reef model, in which sulfide saturation was triggered in situ by extensive fractionation and possibly affected the entire magma chamber. Furthermore, the Pd/Ir and Pd/Pt ratios of the mineralized horizon are distinctly lower at <66 and <1 in comparison to all other types of mineralization in the Monchegorsk Complex. The potential of this mineralization style elsewhere in the Main Ridge remains to be evaluated.

The Lower Zone of the Bushveld Complex comprises an up to 2-km-thick package of different ultramafic rock types with an approx. 90-cm-thick, sulphide-bearing noritic interval that occurs in the western and eastern limbs. The distribution and geometry of the zone are highly variable across the Complex, showing pronounced, yet laterally discontinuous layering on different scales. Together with the ubiquitous lack of large-scale fractionation in the Mg# of orthopyroxene, variable Sr isotope compositions and erratic Pt/Pd ratios, these observations strongly suggest an emplacement of the Lower Zone as a sill complex, as these contrasting geochemical characteristics are difficult to account for in a large Bushveld magma chamber, as previously suggested. It is more likely that these sills were episodically fed from a sub-Bushveld staging chamber, and variably contaminated, while passing through the crust before their final emplacement in the Lower Zone. Detailed mineralogical and microtextural work based on high-resolution elemental mapping of a set of samples, covering the entire Lower Zone stratigraphy of the western Bushveld shows that the variations in the late crystallising interstitial mineral mode are different from what would be expect, if all phases crystallised from a fixed initial mass of interstitial liquid. The interstitial mineral mode, represented by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.