Judith A. Kinnaird

Chromitite formation—a key to understanding processes of platinum enrichment

Abstract The mafic layered suite of the 2050 m.y. old Bushveld Complex hosts a number of substantial platinum-group element (PGE)-bearing chromitite layers, including the UG2, within the Critical Zone, together with thin chromitite stringers of the platinum-bearing Merensky Reef. Until 1982 only the Merensky Reef was mined for platinum, although it has long been known that chromitites also host PGE-bearing minerals (PGM). Three groups of chromitites occur: (i) a Lower Group of up to seven major layers hosted in feldspathic pyroxenite; (ii) a Middle Group with four layers hosted by feldspathic pyroxenite or norite; and (iii) an Upper Group, usually of two chromitite packages, hosted in pyroxenite, norite or anorthosite. There is a systematic chemical variation from the bottom to the top chromitite layer in terms of Cr : Fe ratio and the abundance and proportion of PGE. Detailed studies of 87Sr/86Sr isotope variations undertaken on interstitial plagioclase from chromitites and different silicate host rocks show that the magma from which the chromitites formed (interstitial plagioclase Sri <0.7099) usually differed radically from the resident liquid from which the immediate footwall rocks crystallized (Sri, ca 0.7060-0.7064). These high Sr isotope ratios can only have been produced by sudden and extensive contamination by an extremely radiogenic component. The only viable source for this component in the chamber is the felsitic roof rocks or a granophyric roof-rock melt. It is suggested that such contamination occurred when a new magma influx penetrated the residual liquid and interacted with the overlying roof rock as well as mixing with the resident liquid. It is envisaged in this model that chromite cascaded to the floor together with a small amount of magma adherent to the chromite or entrained within the slurry to produce interstitial silicates with enriched isotopic ratios. The close correspondence of chromitite and PGE enrichment strongly suggests that the contamination process that resulted in chromite formation also triggered precipitation of the PGE. The base of each major chromitite layer marks the point where there was a substantial injection of new magma into the chamber, which resulted in erosion of the cumulate pile, interaction with roof rocks and inflation of the chamber. Thus, the major PGM and chromitite ore deposits of the Bushveld Complex are unconformity-related and are associated with mixing of new magma, coupled to simultaneous contamination by granophyric roof-rock melt. The chromitites represent, therefore, the products of a roof contamination and magma mixing process.

Chromite composition and PGE content of Bushveld chromitites: Part 1 – the Lower and Middle Groups

Abstract This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe2+) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe2+ for Mg and Al between spinel and liquid affecting the Mg-Fe2+ spinel-liquid Kd2. Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al2O3 as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the LG-5 (0·9 to 10) and all overlying chromitites (also documented by Scoon and Teigler). The Pt/Ru and Pd/Ru ratios are very variable, but the Ru/Ir, Ru/Rh and Ru/Os ratios of all chromitites are relatively constant, indicating that Pt and Pd respond to different concentration mechanisms to the other PGE. Rh, Ru, Ir and Os were likely concentrated by chromite itself, probably as grains of laurite and alloys incorporated in growing chromite crystals, but the bulk of the Pt, Pd along with lesser proportions of the other PGE were concentrated by sulphide liquid. Most chromitites now have very low contents of S, but mineragraphic and chemical data support the suggestion of Naldrett and Lehmann that vacancies in chromite forming above 900°C were filled by Fe2+ derived from the destruction of interstitial sulphide liquid. Data on En composition through the Bushveld CriticalZone, indicate that the LG-1 to LG-4 chromitites formed at a stage when influxes of magma into the chamber were rapid and primitive, and overrode the effect of fractional crystallization, whereas above this, fractionation mostly overrode influxes of new magma. Irvines model of mixing of resident magma with influxes of more primitive magma is invoked as the origin of the chromitite horizons. It is shown, using the equation for sulphur solubility and the programme MELTS, that influxes and mixing of fresh primitive magma from depth with that in the chamber (i.e. as envisaged for the LG-1 to LG-4) would not have caused sulphide immiscibility along with chromitite crystallisation, but that influxes and mixing of slower-ascending magma, that fractionated en route, could give rise to sulphide liquid segregating along with the chromitite (i.e. the scenario for the LG-5 and overlying chromitites). The modelling also shows that the more fractionated the magma in the chamber becomes, the more sulphide will form, accounting for the overall upward increase in Pt and Pd above the LG-5.

Multiple sulfur isotopes reveal a magmatic origin for the Platreef platinum group element deposit, Bushveld Complex, South Africa

The Platreef ore horizon of the Bushveld Complex, South Africa, is the third largest platinum group element ore deposit in the world, but the origin of its ore remains enigmatic. A complex contact relationship between the igneous and footwall rocks of the Bushveld Complex, coupled with evidence for widespread late-stage hydrothermal processing, obscures the original mineralization history of the Platreef. We constrain the parental magmatic origin of the Platreef by exploiting multiple sulfur isotope contrasts across Bushveld Complex contact zones in order to see through the effects of postmineralization hydrothermal activity. We report S isotope measurements made on samples collected along two profi les through the Platreef into underlying metapelitic and metacarbonate footwall rocks. In both profi les, igneous rocks far from the contact have low Δ 33 S values (average Δ 33 S = 0.15‰), whereas metasedimentary rocks far from the contact have high Δ 33 S values (Δ 33 S to 5.04‰) with a smoothly varying profi le between the two end members. The midpoint in both isotope profi les is displaced into the footwall, defi ning a classic advective-dispersive tracer geometry. This geometry is not present in the associated δ 34 S values. The displacement of the Δ 33 S front suggests fl uid transport and advection of S into the country rocks; this was accompanied by back diffusion of the S isotope tracer into the Platreef. The Platreef magma was apparently S saturated prior to emplacement and, counterintuitively, lost S during the formation of the present Platreef ore horizon.

Mineral supply for sustainable development requires resource governance

Successful delivery of the United Nations sustainable development goals and implementation of the Paris Agreement requires technologies that utilize a wide range of minerals in vast quantities. Metal recycling and technological change will contribute to sustaining supply, but mining must continue and grow for the foreseeable future to ensure that such minerals remain available to industry. New links are needed between existing institutional frameworks to oversee responsible sourcing of minerals, trajectories for mineral exploration, environmental practices, and consumer awareness of the effects of consumption. Here we present, through analysis of a comprehensive set of data and demand forecasts, an interdisciplinary perspective on how best to ensure ecologically viable continuity of global mineral supply over the coming decades.

Complex multistage genesis for the Ni–Cu–PGE mineralisation in the southern region of the Platreef, Bushveld Complex, South Africa

Abstract In the southern portion of the Platreef, Ni–Cu–Pt-group element (PGE) mineralisation results from a complex interplay of pre-, syn- and post-magmatic processes. Sulphide minerals throughout the successio comprise < 1% to > 25% (mode) and occasionally up to 45% over short intersections of core. Generally, these occur as centimetre to millimetre-sized fractionated blebs and interstitial grains of pentlandite, pyrrhotite and chalcopyrite. Where present, zones of massive sulphides rich in chalcopyrite are found close to the footwall contact. More compositionally complex sulphides are associated with felsic melt phases that pervasively infiltrated the package soon after its partial or complete crystallisation. There is a close correlation between Cu and Ni concentrations but a poor correlation between PGE and base metal contents. Thus high Cu+Ni values do not necessarily indicate high PGE grade, although the highest PGE grades are located within sulphide-rich zones towards the floor. Platinum-group minerals (PGMs) are dominated by Pd-bearing tellurides, antimonides, bismuthides, bismutho-antimonides and complex bismuthotellurides. Pt-bearing phases mostly occur as Pt-arsenides and antimonides. PGMs are found in rims around orthopyroxenes, as discrete grains within secondary silicates and as grains adjacent to, or along the margins of, composite sulphides. Rarely are they found as inclusions within sulphide minerals. The present-day distribution of sulphide minerals and PGEs results from a continuum of processes, beginning with scavenging of PGEs by immiscible sulphide droplets. Subsequently, these were modified by the incorporation of meta-sedimentary-derived S, As, Te, Bi and Sb from the devolatilisation of floor rocks and xenoliths. Felsic melts redistributed some of the sulphides and PGMs as they percolated through the Platreef. Finally, late-stage metasomatic fluids altered the primary silicates and led to further remobilisation of the PGEs to form PGMs disseminated within tremolite, talc and serpentine. This final stage may account for the observed decoupling of Cu and Ni from Pt and Pd.

Concentration of PGE in the Earth's Crust with Special Reference to the Bushveld Complex

Abstract The Earths mantle is the principal reservoir from which platinum-group element (PGE) concentrations in the crust are derived. The transfer of the PGE is accomplished by two main methods, first the development of mantle partial melts and their intrusion into the crust, and second the emplacement of mantle slabs in the subduction/collision zones. The first mechanism is far more important than the second. Once in the crust, a number of mechanisms serve to concentrate the PGE sufficiently so that they can be exploited economically as the principal product, rather than as a by-product. These include (i) the development of an Ni-Cu sulfide liquid in a mafic intrusion, the concentration of this liquid, followed by cooling and fractional crystallization that results in a residual sulfide liquid highly enriched in Cu, Pt, and Pd; (ii) the formation of layers of very high-PGE tenor sulfides at specific horizons within a layered intrusion, either with or without associated chromitite; (iii) the emplacement of magma carrying PGE-rich sulfide along the margins of layered intrusions; (iv) the delayed separation of immiscible sulfides until the late stages of the differentiation of a layered intrusion; (v) chromite crystallization without the development of sulfide immiscibility; (vi) hydrothermal redistribution and concentration of PGE from zones of low grade disseminated sulfide; (vii) secondary concentration of PGE along with chromite during recrystallization of Ural-Alaskan intrusions and the subsequent development of placer deposits during the weathering of these bodies; and (viii) the concentration of Pt during the formation of black shale deposits. The Bushveld Igneous Complex of South Africa hosts 75% of the worlds resources of Pt, 54% of Pd resources, and 82% of Rh resources, and contains examples of mineralization formed by processes (ii), (iii), (iv), (v), and (vi) listed above. Of these, process (ii) accounts for 90% of the current economic reserves and resources, and type (iii) for 9%. The Merensky Reef (32% of total resources) is a PGE-enriched horizon that contains 1–3 thin seams of chromite, and an average of 1–3 wt% sulfide, across the mining width. The sulfides are thought to have been the principal collectors for the PGE. The Reef resulted from two or more influxes of hot, sulfide-bearing, mafic magma that gave rise to the horizon. The thickness of the ultramafic cumulates (mainly orthopyroxenite, but including peridotite in some areas) resulting from these influxes varies from 50 cm to several meters, although mining is usually focused on a zone that is rarely greater than 1 m in thickness. The genesis of the Reef is still debated, some arguing that the PGE have been concentrated from below by ascending hydrothermal fluids, and others arguing that they have been carried from above by sulfides settling from the magma, giving rise to the Merensky pyroxenites. What is clear is that the pyroxenite, norite, and anorthosite overlying the Reef are composed of minerals derived from two magma types, one rich in MgO (∼12 wt%) and Cr and poor in Al2O3 (∼12 wt%) and the other with the composition of a typical tholeiite. The UG-2 chromitite accounts for 58% of the economic resources, and comprises of a chromitite seam 60 cm–1 m thick (sometimes divided by an internal parting of pyroxenite) and 1–3 overlying thinner seams of chromite. The sulfide content of UG-2 is significantly lower than that in the Merensky Reef, ranging from 0.5 to 1.5 wt%, although the sulfides are thought to have played a role in the concentration of at least some of the PGE. There are up to 13 chromitite horizons below that of the UG-2, and all contain PGE, although the total PGE contents and the (Pt + Pd)/(Ru + Ir + Os) ratios are much lower than those of UG-2. High 87Sr/86Sr ratios found within the pyroxenite “partings” within UG-2 suggest that mixing with melted roof rocks may have played a role in causing both chromitite and sulfide to form. The Platreef is the main example of type (iii) mineralization and currently accounts for 9% of the total resources, although active exploration is occurring along this zone and this proportion will probably rise in the future. The Reef is much thicker than the Merensky Reef and UG-2, and is currently mined open-cast over a thickness of about 50 m. The Platreef is zoned, ranging from an upper, orthopyroxene cumulate to a lower zone of pyroxenite, feldspathic pyroxenite, and norite that has interacted strongly with shale, iron formation, and dolomite sediments forming the immediate footwall. In this article, it is suggested that the Platreef is the consequence of several surges of magma that were responsible for different units, including the UG-2 and Merensky Reef, within the main chamber of the Bushveld. These magmas were displaced and exited up the walls of the chamber in response to new influxes of magma entering the main chamber. Cylindrical, zoned pipes of ultramafic rock containing very high Pt grades cut cumulates in the lower part of the Bushveld Complex, and were thought to be the consequence of hydrothermal remobilization. None are currently in production, and they constitute a historic PGE resource that never contributed significantly to the overall resources of the complex.

Geochemical evidence for multiphase emplacement in the southern Platreef

Abstract The Platreef is predominantly a pyroxenitic PGE–Cu–Ni-bearing package which lies at the base of the northern limb of the Bushveld Complex. Geochemical data from two cores from the southern sector of the Platreef on the farm Turfspruit suggest that the Platreef here is a complex intrusive body comprising three or four feldspathic pyroxenite sills each with a distinctive chemistry. Each package is characterised by difference in Mg#, SiO2:Al2O3 and CaO:Al2O3 ratios, trace element abundances and Pt:Pd and Ni:Cu ratios. Compositional breaks in major and trace element chemistry between the different pyroxenite packages are supported by changes in mineral chemistry. The more primitive feldspathic pyroxenite with the highest Cr content and Mg# occurs towards the top of the succession, whereas pyroxenites lower in the succession show varying degrees of interaction with floor rocks and hornfels xenoliths resulting in inhomogeneous textures, variability in composition even on a metre scale, increased incompatible elements and migration of sulphur from shale to magma. The earliest intrusive pulse that contributed to the Platreef package appears to have reacted with the country rocks and cooled rapidly to form a grey micronoritic layer, equated with the Marginal Zone of the eastern and western limbs of the Bushveld Complex. Thick fine-to-medium grained peridotites ascribed to the Lower Zone are equated with the pyroxenites, harzburgites and serpentinites of the Grasvally area south of Mokopane. These olivine-bearing layers have subsequently been intensely serpentinised. As further pulses of magma followed, it is envisaged that metasedimentary material became detached from the floor and later intrusive pulses, which gave rise to feldspathic pyroxenites, flowed under or over these layers. The result is a succession of pyroxenite sills that are separated from each other by interlayers of serpentinite, clinopyroxenite, pegmatitic norite or cordierite spinel hornfels. The Main Zone differs mineralogically and geochemically from the Platreef. It hosts both xenoliths of hornfels and Platreef pyroxenite, especially towards the base, indicating that the Main Zone intruded post-Platreef. Thus, it is implied that shales or dolomites were originally the roof rocks to the Platreef that became incorporated into the Main Zone magma during emplacement. Within the Platreef package, the number of pyroxenitic layers may vary but preliminary attempts have been made to correlate three pyroxenites in one core with four pyroxenites in a second core from boreholes 2 km apart. The most striking characteristics are that the highest Pt+Pd concentration and lowest Pt:Pd ratio occur in the basal pyroxenite in association with sulphides, with an upward increase in Pt:Pd ratio and Ni:Cu so that the top pyroxenite has a Pt:Pd ratio of 2 and Ni:Cu of > 3 whilst the bottom pyroxenite has a ratio of Pt:Pd ratio of 1 and a Ni:Cu of < 2. These increased ratios upward are not a simple fractionating sequence from the bottom to the top of the Platreef otherwise the top pyroxenite would be the most evolved, which is not the case as it has the most primitive Mg# and highest Cr content. The Platreef is, therefore, a complex zone of sill inter-fingered lithologies reflecting a multiphase emplacement.

A new look at sulphide mineralisation of the northern limb, Bushveld Complex: a stable isotope study

Abstract The Platreef is the main platinum-group element (PGE)-bearing horizon in the northern limb of the Bushveld Complex, South Africa. It is considered to be richer in sulphides than other similar horizons within the Bushveld Complex, in particular the Merensky Reef. Previous work has indicated that assimilation of dolomite may be a mechanism to add sulphur to the magma from which the Platreef formed. Sulphur isotope data presented in this study indicates an additional sulphur source contributing to the Platreef. In the southern Platreef, the Duitschland Formation of the Transvaal Supergroup forms part of the direct footwall. Within this sequence are pyrite-rich shales, which we suggest contributed to the sulphur budget of the Platreef. Local variations in sulphur isotopes, as well as a decrease in crustal sulphur in the Platreef further away from the footwall, also indicate local rather than regional contamination processes. Platreef samples have δ18O values that are higher than expected in a mantle-derived magma, but there is no apparent systematic variation in δ18O with distance from the footwall. This may possibly indicate contamination of the Bushveld magma pre-intrusion, probably in a staging chamber. However, when compared to data from the central sector of the Platreef itself, analysis from this study have lower δ18O values indicating changes in the degree of contamination with varying footwall lithology. Also, differences between plagioclase and pyroxene δ18O values indicate exchange with fluids. It is possible that late-stage deuteric fluids may have caused this exchange.

Constraining the timing of deformation in the southwestern Central Zone of the Damara Belt, Namibia

Abstract Structural investigations and U–Pb sensitive high-resolution ion microprobe (SHRIMP) dating of rocks from the southwestern Central Zone of the Damara Belt, Namibia, reveal that a major SE-verging deformation event (D2) occurred at between 520 and 508 Ma. During D2, SE-verging simple shear and NE–SW pure shear extension in a constrictional stress field produced recumbent, south- to SE-verging, kilometre-scale folds and ductile shear zones, a NE–SW extensional lineation and conjugate shear bands, and was coeval with granitoid emplacement and high-grade metamorphism. The timing of this event is constrained by anatectic leucosomes in D2 shear zones (511±18 Ma) and extensional shear bands (508.4±8.7 Ma) as well as by syntectonic grey granites (520.4±4.2 Ma), and is similar to ages for high-grade metamorphism in the Central Zone. An upright folding event (D3) occurred at c. 508 Ma, resulting in the formation of basement-cored fold interference domes. The timing of deformation and metamorphism at 520–508 Ma in the mid-crustal SW Central Zone contrasts with ages of 560–540 Ma for shallow crustal NW-verging folding and thrusting elsewhere in the Central Zone that was concomitant with voluminous magmatism. This magmatism led to metamorphism and anatexis of the basement and the emplacement of anatectic red granites at 539±17 to 535.6±7.2 Ma, which contain 1013±21 Ma inherited zircons. The Central Zone therefore contains a record of crustal thickening, heating of the mid-crust, exhumation and orogen-parallel extension over the life of an orogen.

Geochemical effects of magma addition: compositional reversals and decoupling of trends in the Main Zone of the western Bushveld Complex

Abstract The Main Zone of the Bushveld Complex, which is ~3-3.5 km thick, comprises a sequence of gabbronorites with minor anorthosites and pyroxenites. The Pyroxenite Marker (PM), a thin orthopyroxenite layer occurring towards the top of the Main Zone in the eastern Bushveld, marks the change from an inverted pigeonite-bearing microgabbronorite below, to a primary orthopyroxene-bearing porphyritic gabbronorite above. In the western Bushveld the PM has only been observed in core material although its surface position can be inferred from the mineralogical and textural changes. Whole-rock geochemistry of surface and core samples from the Brits and Marikana areas, together with mineral compositional data, have been integrated with published analyses to elucidate the magmatic processes that occurred during the addition of new magma into the chamber, just below the level of the PM. Modal and major-element data show that most lithologies lie close to the plagioclase-two pyroxene cotectic. However, there are three units below the PM in which distinct modal layering is developed. Changes in geochemical trends for trace-element abundances in both whole-rock and mineral-separate data occur at ~150 m below the PM, below a layered package, the Hexrivier Unit. However, there is a displacement of at least 40 m between the beginning of the reversals in An content in plagioclase and in Mg# in pyroxene which occur at 124 m and 80 m below the PM respectively. In addition, a gradual change occurs in the mineral parameter Mg# in opx minus An in plagioclase, with the plagioclase becoming more primitive relative to the pyroxene. This decrease and the decoupling of geochemical trends has not been noted in the Main Zone of the Bushveld Complex before. The decoupling between plagioclase and pyroxene compositional reversals is not easily explained by modal effects, an influx of phenocrysts in the new magma, or by infiltration metasomatism. Instead we propose that new magma pulses first entered the chamber some 150 m below the PM and that this magma had a composition that crystallized more primitive plagioclase but similar pyroxene to the magma residing in the chamber. The chamber was intruded by progressively more magma that had a more primitive composition particularly in terms of pyroxene composition. This model can explain the decoupling between plagioclase and pyroxene compositional trends. Continued mixing between resident magma and the new influxes occurred over an interval of ~150-200 m. Above the PM fractional crystallization processes dominated and continued into the Upper Zone.