Bowlesite is a new mineral discovered in the Merensky Reef of the Rustenburg Platinum Mine, Bushveld complex, South Africa. Bowlesite forms tiny grains (maximum dimension 20 μm). It is associated with sulfides including chalcopyrite, pyrrhotite and pentlandite, in contact with silicates including plagioclase, pyroxene- and minor serpentine-subgroup and amphibole-supergroup minerals. Bowlesite is brittle and has a metallic lustre. In plane-polarised light, bowlesite has a light bluish grey colour. It shows weak bireflectance, no pleochroism and has weak anisotropism. Internal reflections were not observed. Reflectance values of bowlesite in air (R1, R2 in %) are: 50.3–51.4 at 470 nm, 48.5–48.9 at 546 nm, 47.9–48.6 at 589 nm and 47.8–48.7 at 650 nm. Ten spot analyses of bowlesite give the average composition: Pt 56.85, Pd 0.02, Sn 34.03 and S 9.15, total 100.05 wt.%, corresponding to the empirical formula (Pt1.001Pd0.001)Σ1.002Sn0.997S1.001, based on 3 atoms per formula unit. The simplified formula is PtSnS. Due to the small size of bowlesite, the crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic PtSnS. The calculated density is 10.06 g⋅cm–3. The mineral is orthorhombic, space group: Pca21 (#29) with a = 6.11511(10), b = 6.12383(10), c = 6.09667(11) Å, V = 228.31(1) Å3 and Z = 4. Bowlesite is isotypic with cobaltite, CoAsS. The origin of bowlesite is probably related to low-T exsolution of Pt–Sn phases from high-T sulfides crystallised from the sulfide melt. The mineral honours Dr. John Bowles (Manchester University, UK) for his contributions to ore mineralogy and mineral deposits related to mafic–ultramafic rocks.

Petrographic and compositional variations in the Lower Main Zone (LMZ) of the western Bushveld Complex indicate changing regimes of magma replenishment. The lowermost unit of the LMZ, designated N-I, is an enigmatic sequence of leuconoritic cumulates, characterized primarily by up-sequence increases in both orthopyroxene Mg# and whole-rock Sr isotope initial ratio. The Sr isotope profile of N-I is ascribed to injection and progressive integration of small influxes of fresh magma with high (Main Zone-type) Sr isotope initial ratios. The basal Fe-enrichment in N-I, on the other hand, is ascribed to a separate, later mechanism involving the downward migration of late-stage Fe-rich liquids. The overlying two units, N-II and G-I, delineated chiefly in terms of basal Mg-enrichment of orthopyroxene, are ascribed to injections of fresh magma into the chamber. Poikilitic orthopyroxene grains in the basal parts of both N-II and G-I suggest entrainment and partial resorption of plagioclase grains from the semi-crystalline resident material into which the fresh magma was intruded.

Platinum group mineral (PGM) assemblages in the Platreef at Sandsloot, northern Bushveld Complex, in a variety of lithologies reveal a complex multi-stage mineralization history. During crystallization of the Platreef pyroxenites, platinum group elements (PGE) and base-metal sulphides (BMS) were distributed thoughout the interstitial liquid forming a telluride-dominant assemblage devoid of PGE sulphides. Redistribution of PGE into the metamorphic footwall by hydrothermal fluids has formed arsenide-, alloy- and antimonide-dominant assemblages, indicating a significant volatile influence during crystallization. Serpentinization of the footwall has produced an antimonide-dominant PGM assemblage. Parts of the igneous reef were subjected to alteration by a late-stage, Fe-rich fluid, producing ultramafic zones where the telluride-dominant assemblage has been recrystallized to an alloy-dominant one, particularly rich in Pt-Fe and Pd-Pb alloys. A thin, small-volume zone of PGE-BMS mineralization along the base of the hangingwall contains a primary PGM assemblage that is locally altered to one dominated by Pt/Pd germanides. This is thought to have formed when the new pulse of Main Zone magma entered the chamber, and scavenged PGE from the underlying Platreef pyroxenites. That each major rock type at Sandsloot contains a distinctive PGM assemblage reflects the importance of syn- and post-emplacement fluid and magmatic processes on the development of Platreef mineralization.

The compositions of coexisting and individual cooperite (ideally PtS) and braggite (ideally (Pt,Pd)S) grains from the Merensky Reef of the Bushveld Complex, as well as cooperite, braggite and vysotskite (ideally PdS) grains from the UG-2 of the Bushveld Complex were investigated. There is a clearly defined miscibility gap between cooperite and braggite, but no evident gap between braggite and vysotskite. Partition coefficients between cooperite and braggite are determined on coexisting phases. The KDbraggite/cooperite in atomic ratios are estimated to be 0.54 for Pt, 15.81 for Pd and 5.93 for Ni. For Rh and Co the KDbraggite/cooperite are estimated to be > 1.40 and > 1.46 respectively. No systematic behaviour is detected for Fe and Cu. Coupled substitutions of Pd + Ni for Pt in cooperite and braggite/vysotskite are indicated. Within the cooperite of the Merensky Reef, the Pd:Ni ratio is approximately 9:11. The substitution trend in braggite, which extends to vysotskite in the UG-2, is dependent on the base-metal sulphide (BMS) association. If pentlandite is the dominant Ni-bearing BMS, the Pd:Ni ratio is about 7:3 in the Merensky Reef and in the UG-2. Millerite as the dominant Ni-bearing BMS in the UG-2 changes this ratio to 3:1. It is concluded that the Ni-content in braggite/vysotskite from BMS assemblages does not depend on the NiS activity, but rather on temperature of formation.

The occurrence of spheroidal, ovoid or deeply embayed plagioclase inclusions within cumulus bronzite and olivine grains of both pyroxenites and norites here implies the existence of plagioclase feldspar in melts prior to the separation of ultramafic cumulates from these melts. Variations of Sr-isotope initial ratio are shown to exist within the feldspar populations of such rocks, even on a grain-to-grain scale. Five separate stratigraphic intervals within the Lower and Critical Zones exhibit this texture, and in each case these are intervals wherein normal fractionation trends (decline in both whole-rock and orthopyroxene Mg/(Mg+Fe2+); decline in Cr contents of orthopyroxene) became reversed. The data support a model for evolution of the texture during major episodes of influx of hot, primitive magmatic liquid into the chamber. Progressive mixing of the new liquid with resident liquids (bearing feldspar on the liquidus) effected progressive changes in the composition of the hybrid liquids produced, and resorption and imposition of reversed zoning upon the inclusions.

Spinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite.

The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.

The UG1 Footwall unit is a layered pyroxenite-norite-leuconorite-anorthosite sequence between the Middle Group 4 and Upper Group 1 chromitites of the Upper Critical Zone, and is c. 300 m thick at Rustenburg Platinum Mines, Union Section, where it shows an oscillatory fluctuation in whole-rock Mg/(Mg + Fe), Cr/Co, Ni/V and Fe/Ti ratios with stratigraphic height. This permits subdivision into 8 sub-cycles which match a subdivision based on cyclical variations in orthopyroxene and feldspar compositions. Constituent pyroxene grains of pyroxenites, norites and leuconorites alike contain rounded and embayed plagioclase inclusions in abundance. Sr-isotope disequilibrium prevails in some samples between the orthopyroxene and feldspar populations. Chemical and isotopic data support a model of pulsatory injection of limited volumes of a more primitive, mafic liquid into a resident column of depleted residua, from which sodic labradorite and Mg-poor bronzite were crystallizing. The depleted liquid is equated with the supernatant liquid residuum of buried cumulates (Sric. 0.7054) and the primitive liquid with magma parental to the UG1-UG2 lineage (Sri ⩾ 0.7068). The increase in leucocratic character of the 300 m column, with height, is attributed to the rising of low-density liquids enriched in the components of feldspar during separation of the pyroxenites. Deposition of the UG1 chromitite layers is attributed to mixing of a major influx of primitive liquid with a feldspathic residuum at the top of the UG1 Footwall unit. There is no evidence to indicate the participation of a discrete A-type liquid (Irvine and Sharpe, 1982) in this process.

Hybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.

In the Brits area of the Bushveld Complex, the stratigraphically lowermost magnetitite layer of the Upper Zone is underlain by an anorthosite containing discrete, euhedral magnetite crystals, measuring 0.1–0.3 mm in diameter, which form thin layers (1 mm-1 cm thick) and laminae (commonly one grain in thickness). Discordant relationships between laminae and layers indicate a consistent younging direction upwards towards the contact with the overlying magnetitite. Our interpretation is that these crystals are cumulus and have escaped modification by postcumulus overgrowth. They also record, in places, the morphology of the floor onto which they accumulated. Their preservation is largely attributed to adcumulus growth of plagioclase before, during and after, accumulation, but may also indicate that there was only a limited supply of magnetite to the floor, thus precluding grain contact and prohibiting grain enlargement. The results of a study of compositional variations across individual plagioclase grains suggest that the presence of magnetite within the crystallising mush hindered the development of an ideal plagioclase adcumulate.

A comparison of R.P.M. Union and Amandelbult Sections reveals close geochemical and stratigraphic correlations, but the sequence at the latter is more complex. Mafic members at Union Section are consistently more magnesian than their equivalents at Amandelbult Section and (where available data allow comparisons) than at Rustenburg Section. This, taken together with regional patterns of progressive attenuation and elimination of leucocratic rocks beneath harzburgitic layers, identifies Union Section as proximally located with reference to an irruptive centre from which primitive liquids were injected along the floor/supernatant liquid interface. Successive injections led to thermal erosion of the floor, causing ‘dimpling’ and ‘potholing’ on a local scale, and elimination of noritic-anorthositic layers on a regional scale. Such erosion of the floors beneath new inputs of magmatic liquid is likely to be initiated by only partial remelting, mainly of lower-temperature phases (sodic rims to zoned plagioclase grains, and Fe-enriched pyroxenes) within intercumulus space. It is argued that resultant contamination of the primitive liquids may have led to direct superposition of anorthosites (accompanied by rare troctolite) upon harzburgite, without intervening pyroxenite members, as in the Pseudoreef Multicyclic Unit.

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.

Apatite in most igneous intrusions has a high Cl/F ratio. However, chlor-apatite has been reported in the lower portions of the Bushveld and Stillwater Complexes. This has been used as evidence supporting the early separation of a Cl-rich discrete hydrous fluid in these intrusions. Mineralogical evidence is presented here to demonstrate that the Bushveld Complex, at least, formed from a nearly anhydrous magma, and did not release a hydrous fluid before apatite began to crystallize. It is suggested that apatite in the earliest cumulates equilibrated with trapped interstitial liquid, which converted it from the typical F-rich composition of cumulus apatite to a Cl-rich composition. This is an analogous process to that in which cumulus mafic minerals may become more Fe-rich on cooling and reaction with interstitial liquid.

A detailed geochemical study is presented of the uppermost Critical Zone, especially of the footwall and hanging wall to the Merensky Reef, at Impala Platinum Mines in the Bushveld Complex. The approximately 100 m-thick sequence below the Merensky Reef consists of 13 distinct layers which have sharp boundaries. They are adcumulates with varying proportions of cumulus plagioclase, orthopyroxene and chromite.

Experimental studies on the composition of coexisting orthopyroxene liquid indicate that the magma which produced this sequence contained between 4 and 6% MgO. The magma from which the Merensky Reef formed was more evolved than the footwall magma.

Significant variations exist for both the En content of orthopyroxene and mg# number of whole-rock analyses in short vertical sections. Pyroxenite and norite always have higher values than anorthosite. Extremely sharp breaks in these values correlate with changes in modal proportions, and argue against both significant fractionation within the studied interval, and infiltration metasomatism. Quantitative modelling shows that the entire footwall section could have contained pyroxene with a uniform primary composition of En82, and that all the variation now observed reflects the effect of reaction with trapped magma.

Two independent methods for determining the proportion of trapped liquid are presented, based on mg# number and incompatible element abundances. Both yield a uniform proportion in all samples of approximately 10%. Immiscible sulphide liquid from the Merensky Reef can be shown to have infiltrated downwards for <5 m, despite its high density contrast with silicate magma, very low viscosity and low crystallization temperature. Residual silicate magma would have had even more restricted mobility. The migration of residual liquid or fluid through pothole structures in the floor of the Merensky Reef is not supported by the present data.

Ratios between elements Mg, Fe, Co, Cr, Ni, V, and Sc are consistently different in mafic rocks of the upper critical zone, and those above the Bastard unit. Within the 300 m section above the Merensky Reef, 87Sr/86Sr ratios increase from c.0.7063 to c.0.7087, irrespective of rock type. Decoupling of Mg/(Mg + Fe2+) ratios and the Ca contents of plagioclase, and wide variations in the proportions of anorthosite within the Bastard, Merensky, and Merensky Footwall units, are inconsistent with anorthosite formation by simple fractional crystallization of magma batches of limited volume. Conversely, significant differences in Sr-isotope ratios show that these anorthosites could not have shared a common parental liquid. These data are used to develop a model whereby (a) the 300 m column above the critical zone represents the mixing of liquids of isotopically and geochemically discrete upper critical and main zone lineages, (b) mafic layers of the Bastard, Merensky, and Merensky Footwall units crystallized from discrete injections of primitive, mafic liquid while (c) the leucocratic upper parts of these units crystallized during progressive hybridization of liquid residua, which remained after significant separation of mafic phases, with a supernatant column representing the liquid residua of earlier cycles, and (d) the buoyancy of plagioclase, and enlargement of the primary phase volume of plagioclase consequent upon an increase in An/Ab ratio of hybrid liquids, were significant factors in the generation of anorthositic layers.

The composition of magmas proposed as parental to the layered suite of the Bushveld Complex, and some models for the manner of their emplacement, are reviewed briefly. Included are some contributions published in South Africa, with which overseas readers might be less familiar. Emphasis is given to the broader features of the cumulates, and the contradictions raised by whole-rock compositional, Sr-isotopic, and trace-element data that cloud their correlation with proposed parental magmas. It is concluded that the Lower, Critical and Main Zones are the derivatives of only two primary magmatic lineages, while a third was added to residual liquids from which the layered rocks above the Pyroxenite Marker were formed. Excessive amounts of olivine and chromium in the cumulates of the Lower and Lower Critical Zones in the northern sector of the Western Limb can seemingly not be accounted for by the composition and volume of the putative magmas. This is attributed to (1) this sector being a proximal facies located close to the original feeder, and/or (2) crystal-charged magma batches, expelled from a lower magma chamber, being periodically injected into and dispersed within the liquids already in place in the Bushveld chamber. Thus, ongoing changes in the bulk composition of the liquids within the chamber would not be reflected in the rinds of earlier, chilled-facies rocks. The expulsion of significant volumes of liquid residua from the chamber during cumulate deposition is not ruled out.

Mafic rocks of the Bushveld Complex at the southeastern end of the western limb, intersected in bore core from the Cullinan Diamond Mine, are described. A 260 m thick ultramafic body of orthopyroxene and chromite cumulate rocks, with mg# – 100*Mg/(Mg+Fe) – values from 77 to 84 and 0.25 to 0.5% Cr2O3 in the pyroxene, is considered to have affinity to the Critical Zone. Such an interpretation considerably extends the eastern limit of Critical Zone rocks of the western limb of the Bushveld Complex. The whole-rock composition of the lower, chilled basal contact of this body has 10% MgO and 500 ppm Cr, and is comparable to magmas considered parental to the Bushveld Complex. Due to intrusion of a younger sill, the upper contact is not preserved in the bore core. The cumulate rocks have higher interstitial component, inferred from incompatible trace element abundances (Zr, Ti and K), than normal Critical Zone rocks, interpreted to be a result of more rapid cooling due to proximity to the basal contact. The near-constancy of mg# in the pyroxene in the entire succession suggests that large volumes of magma flowed through this conduit, with only the liquidus phases of orthopyroxene and chromite being precipitated.

Five generations of sills, intruded into the underlying metasedimentary rocks, are identified. The oldest is tholeiitic, and was metamorphosed prior to the emplacement of the Bushveld Complex. The second equates to the magma proposed as being parental to the Bushveld Complex (2060 Ma). The third represents the products of differentiation of that magma. The fourth is syenitic, and related to the Pienaars River Alkaline Complex (1430–1300 Ma). The fifth is tholeiitic (1150 Ma), and cuts the Cullinan kimberlite.

The Upper Critical Zone of the Bushveld Complex consists of several cyclic units, each ranging from ultramafic to leucocratic. One view is that they were initiated by addition of, and crystallization from, relatively magnesian magma. Experimental studies on plausible parental magma compositions to this sequence show that plagioclase joins orthopyroxene in the crystallization sequence once the Mg# of the orthopyroxene has decreased below 83. Some norites from the Upper Critical Zone contain orthopyroxene with this Mg#, supporting the validity of the experimental studies. There are no orthopyroxene compositions with higher Mg# than 83 in the entire Upper Critical Zone in either the eastern or western limbs of the Bushveld Complex. This observation suggests that all these rocks formed from plagioclase-saturated magmas. If this interpretation is correct, pyroxenites in this succession are not the result of crystallization from a magnesian magma. Instead, they result from the mechanical separation of plagioclase and orthopyroxene, probably due to crystal sorting during settling, from a magma lying at the cotectic for these two minerals. Rocks exist in the Upper Critical Zone that contain non-cotectic proportions of cumulus plagioclase and orthopyroxene, again supporting models of crystal sorting during settling. In such a model, anorthosites result from the delayed accumulation of plagioclase relative to pyroxene, and not to formation from a magma saturated only in plagioclase.

When traced from northwest to southeast in the western limb, there is a change in the relative proportions of plagioclase to orthopyroxene in the Upper Group 2 chromitite cyclic unit. In the northwest the unit is dominated by ultramafic rocks. In the southeast the plagioclase to pyroxene ratio exceeds that of the cotectic proportion, relations that may result from the lateral transport of suspended plagioclase grains to the southeast.

Lacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, 1100 m above the Main Zone — Critical Zone contact, a 10 m thick pyroxenitic layer was intersected in an exploration borehole from the southern sector of the western Bushveld.

Within the pyroxenitic layer, mineral chemistry defines a series of five cycles of upward Mg and Cr enrichment in pyroxenes, and Ca enrichment in plagioclase. The mineral chemistry, supported by textural evidence, suggests the influx of successive surges of magma. Sustained streaming of magma gave rise to adcumulate textures in the central portion of each cycle, with orthocumulate textures at bases and tops of cycles representing waxing and waning stages of magma surges.

Compositional variations with respect to minor elements A1, Cr, Ti and Mn, and major elements Fe and Mg, in orthopyroxenes along ca. 160 km of strike of the Lower (LZ), Lower Critical (LCZ) and Upper Critical (UCZ) Zones are reviewed on the basis of 1900 analyses by electron microprobe. AI increases with stratigraphic height and declining Mg/(Mg + Fe2) ratios (hereafter MMF ratios) through the LZ and LCZ, reaching peak values close to the base of the UCZ, where the first cumulus plagioclase appears in the succession. Above this, Al contents decline as MMF ratios decline. Through the same interval, subdued increase in Ti occurs through >1000 m of ultramafic cumulates, but this increase accelerates within the ca. 450 m UCZ sequence. Mn increases linearly with declining MMF ratios through the entire succession, Cr levels are highest in orthopyroxenes of the ultramafic LZ and LCZ, and olivine norites of the UCZ, but decline in more evolved norites and associated anorthosites of the UCZ.

This pattern of cryptic variations, displayed by a thick succession of cumulates, is consistent with the model of Bence and Papike (1972) and Grove and Bence (1977) for basaltic rocks, which links the levels of minor elements in pyroxenes with entry of plagioclase into the paragenesis.

The Waterberg platinum deposit is an extraordinary example of a vein-type hydrothermal quartz-hematite-PGE (platinum-group element) mineralization. This study concentrates on the geochemical character of the ores and the platinum-group mineral (PGM) assemblage by application of reflected-light and scanning electron microscopy followed by electron probe microanalysis.

The PGM-bearing quartz veins show multiple banding indicating numerous pulses of fluid infiltration. Mineralization was introduced contemporaneously with the earliest generation of vein quartz and hematite. High oxygen and low sulfur fugacities of the mineralizing fluids are indicated by hematite as the predominant opaque mineral and the lack of sulfides.

The ‘Waterberg type’ mineralization is characterized by unique metal proportions, namely Pt>Pd>Au, interpreted as a fingerprint to the cradle of the metals, namely rocks and ores of the Bushveld Complex, or reflecting metal fractionation during ascent of an oxidized, evolving fluid. The PGM assemblage signifies three main depositional and alteration events. (1) Deposition of native Pt and Pt–Pd alloys (>90% of the PGM assemblage) and Pd–Sb–As compounds (Pt-rich isomertieite and mertieite II) from hydrothermal fluids. (2) Hydrothermal alteration of Pt by Cu-rich fluids and formation of Pt–Cu alloys and hongshiite [PtCu]. (3) Weathering/oxidation of the ores producing Pd/Pt-oxides/hydroxides.

Platinum-group element transport was probably by chloride complexes in moderately acidic and strongly oxidizing fluids of relatively low salinity, and depositional temperatures were in the range 400–200°C. Alternatively, quartz and ore textures may hint to noble metal transport in a colloidal form and deposition as gels.

The source of the PGE is probably in platiniferous rocks or ores of the Bushveld Complex which were leached by hydrothermal solutions. If so, further Waterberg-type deposits may be present, and a prime target area would be along the corridor of the Thabazimbi-Murchison-Lineament where geothermal springs are presently still active.