http://dx.doi.org/10.1016/j.gsf.2016.06.005

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.     

Origin of the UG2 chromitite layer, Bushveld Complex

Chromitite layers are common in large mafic layered intrusions.A widely accepted hypothesis holds that the chromitites formedas a consequence of injection and mixing of a chemically relativelyprimitive magma into a chamber occupied by more evolved magma.This forces supersaturation of the mixture in chromite, whichupon crystallization accumulates on the magma chamber floorto form a nearly monomineralic layer. To evaluate this and othergenetic hypotheses to explain the chromitite layers of the BushveldComplex, we have conducted a detailed study of the silicate-richlayers immediately above and below the UG2 chromitite and anotherchromitite layer lower in the stratigraphic section, at thetop of the Lower Critical Zone. The UG2 chromitite is well knownbecause it is enriched in the platinum-group elements and extendsfor nearly the entire 400 km strike length of the eastern andwestern limbs of the Bushveld Complex. Where we have studiedthe sequence in the central sector of the eastern Bushveld,the UG2 chromitite is embedded in a massive, 25 m thick plagioclasepyroxenite consisting of 60–70 vol. % granular (cumulus)orthopyroxene with interstitial plagioclase, clinopyroxene,and accessory phases. Throughout the entire pyroxenite layerorthopyroxene exhibits no stratigraphic variations in majoror minor elements (Mg-number = 79·3–81·1).However, the 6 m of pyroxenite below the chromitite (footwallpyroxenite) is petrographically distinct from the 17 m of hangingwall pyroxenite. Among the differences are (1) phlogopite, K-feldspar,and quartz are ubiquitous and locally abundant in the footwallpyroxenite but generally absent in the hanging wall pyroxenite,and (2) plagioclase in the footwall pyroxenite is distinctlymore sodic and potassic than that in the hanging wall pyroxenite(An_45–60 vs An_70–75). The Lower Critical Zone chromititeis also hosted by orthopyroxenite, but in this case the rocksabove and below the chromitite are texturally and compositionallyidentical. For the UG2, we interpret the interstitial assemblageof the footwall pyroxenite to represent either interstitialmelt that formed in situ by fractional crystallization or chemicallyevolved melt that infiltrated from below. In either case, themelt was trapped in the footwall pyroxenite because the overlyingUG2 chromitite was less permeable. If this interpretation iscorrect, the footwall and hanging wall pyroxenites were essentiallyidentical when they initially formed. However, all the modelsof chromitite formation that call on mixing of magmas of differentcompositions or on other processes that result in changes inthe chemical or physical conditions attendant on the magma predictthat the rocks immediately above and below the chromitite layersshould be different. This leads us to propose that the Bushveldchromitites formed by injection of new batches of magma witha composition similar to the resident magma but carrying a suspendedload of chromite crystals. The model is supported by the commonobservation of phenocrysts, including those of chromite, inlavas and hypabyssal rocks, and by chromite abundances in lavasand peridotite sills associated with the Bushveld Complex indicatingthat geologically reasonable amounts of magma can account foreven the massive, 70 cm thick UG2 chromitite. The model requiressome crystallization to have occurred in a deeper chamber, forwhich there is ample geochemical evidence. KEY WORDS: Bushveld complex; chromite; crystal-laden magma; crustal contamination; magma mixing; UG2 chromitite     

The significance of magmatic erosion for bifurcation of UG1 chromitite layers in the Bushveld Complex

One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex.     

Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition

The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr₂O₃ content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.     

Evolution of Part of the Lower Critical Zone, Farm Ruighoek, Western Bushveld

This study of a part of the lower Critical Zone, Farm Ruighoek,Western Bushveld, is based mainly on selected drill core samplesfrom two sections approximately 1.2 miles apart. The 1300-ftsequence investigated consists of pyroxenites with two harzburgitebands and sixteen chromitite seams. Results obtained are consistent with the hypothesis that evolutionof the sequence was a cyclic process in which cumulate mineralscrystallized in a zone near the floor of accumulation undergenerally quiescent conditions. Compositional changes of cumulateminerals reflect the influence of a separate intrusion of undifferentiatedparent magma or refusion at depth of crystals formed near thetop of the magma chamber. Interstitial mineral content and plagioclasecomposition reflect changing rates of crystal accumulation.Orthopyroxene grain size and sorting coefficient reflect, inpart, the vertical distance over which crystallization occurred.Textural features and contact relations of chromitite seamsare consistent with the hypothesis that most chromite crystallizedfrom the silicate magma and accumulated during a period of littleor no crystallization of silicate minerals. It is postulatedthat this loose crystal assemblage was enriched by co-accumulationand partial in situ crystallization of chromite-rich immiscibleliquid. Textural, mineralogical, and compositional changes infootwalls and hanging walls of chromitite seams are thoughtto reflect in situ reactions.     

Correlation between chromite composition and PGE mineralization in the Critical Zone of the western Bushveld Complex

Detailed mineralogical investigations of chromite in the Lower and Critical Zones in the northwestern sector of the Bushveld Complex have revealed significant compositional variations with regard to modal proportions, host-rock lithology, and stratigraphic height. Superimposed on these variations are long-range systematic trends in the composition of chromite in the massive layers. These long-range trends are closely linked with the evolution of the silicate cumulates. The massive chromitite layers are divided into two types. Type 1 comprises the chromitites hosted entirely within ultramafic cumulates, while Type 2 chromitites are within cyclic units in which plagioclase cumulates occur. The types are also distinguishable by their respective contents of platinum-group elements (PGEs) and distribution patterns thereof, viz. the ratios between Ru + Os + Ir and Pt + Pd + Rh, or relative element proportions, both of which display a systematic change with height in accordance with chromite composition. The relation between silicate geochemistry, chromite composition, and PGE tenor, leads to the development of a model explaining the formation of PGE-mineralized, sulphide-poor chromitite layers in the Critical Zone of the Bushveld Complex. Presented at the International Conference for Applied Mineralogy, Pretoria, September 1991     

Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.     

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex,South Africa

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe²⁺) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na₂O (2.4 wt%) and H₂O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ¹⁸O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ¹⁸O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.     

Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.     

Magma Mixing as a Trigger for Sulphide Saturation in the UG2 Chromitite (Bushveld): Evidence from the Silicate and Sulphide Melt Inclusions in Chromite

It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements(PGEs: Os, Ir, Ru, Rh,Pt, Pd) or highly siderophile elements(HSEs: Au, Re, PGE). This paper documents sulphide-silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size.Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron(Fe) and lower nickel(Ni) content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter-cumulus plagioclase(An_(68)) and plagioclase enclosed in chromite(An_(13)), as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation.Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA-ICP-MS(laser ablation-inductively coupled plasma-mass spectrometry), which showed the following.(Ⅰ) Chromite contained no detectable HSE in solid solution.(Ⅱ) HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os,Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te.(Ⅲ)In the sulphide inclusions,(a) pyrrhotite did not contain any significant amount of HSE,(b) chalcopyrite contained only some Rh compared to the other sulphides,(c) pentlandite was the main host for Pd,(d)pyrite contained most of the Ru, Os, Ir and Re,(e) Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and(f) no Au was detected. These results show that the use of ArF excimer laser to produce high-resolution trace element maps provides information that cannot be obtained by conventional(spot) LA-ICP-MS analysis or trace element maps that use relatively large beam diameters.     

Genesis of sulfide-rich chromite ores by the interaction between chromitite and pegmatitic olivine–norite dikes in the Potosí Mine (Moa-Baracoa ophiolitic massif, eastern Cuba)

The Potosí Mine is located in the Moa-Baracoa massif in the easternmost part of the Cuban Ophiolitic Belt. Chromite mineralization occurs within the mantle-crust transition zone. Two events of magma intrusion overprint the chromitite bodies: one gave rise to the crystallization of pegmatitic olivine-norite dikes, and the other produced pegmatitic gabbro dikes. Sulfide-poor chromite ores, brecciated chromite ores, and sulfide-rich chromite ores can be distinguished in the different chromitite bodies. Sulfide-poor ores represent more than 80 vol% of the chromitites. This type occurs far from the zones intruded by pegmatitic gabbro dikes and shows petrographic and chemical features similar to other chromitite bodies described in the Moa-Baracoa massif. Brecciated chromite ores occur within pegmatitic gabbro dikes. In this type, chromite crystals occur included within chromian diopside and plagioclase. These silicates often contain droplet-like sulfide aggregates. Sulfide-rich ores are spatially associated to the contacts between sulfide-poor chromite and pegmatitic olivine-norite dikes. These ores mainly consist of recrystallized (coarse) chromite with interstitial pyrrhotite, pentlandite, cubanite, and chalcopyrite. Chromite from sulfide-rich ores exhibits TiO₂, FeO, V₂O₃, MnO, and especially, Fe₂O₃ contents, higher than those of chromite from brecciated ores and much higher than those of chromite from sulfide-poor ores. The sulfide-rich ores are PGE-rich (up to 1,113 ppb of total PGE), and show nearly flat chondrite-normalized PGE patterns, slightly above 0.1 times chondritic values. Mineralogical and chemical data indicate that the chromite ores of the Potosí Mine were modified by the intrusions of olivine-norite and gabbro dikes. The interaction between pre-existing sulfide-poor chromite ores and the intruding volatile-rich silicate melts produced strong brecciation, partial dissolution, and recrystallization (coarsening) of chromite. The sulfide assemblage formed by fractionation of the immiscible sulfide melt segregated from the volatile-rich silicate melt that generated the pegmatitic olivine-norite. The segregation of the sulfide melt can be interpreted as the consequence of chemical interaction between intruding melts and the host chromite. The variable extent of this interaction produced chromite ores with variable sulfide ratios. The magmatic nature of the sulfide mineralization is supported by sulfur isotope data, which range from -0.4 to +0.9‰. Sulfide melt collected incompatible PGE (Rh, Pt, Pd) to produce the typical flat chondrite-normalized pattern of sulfide-rich chromite ores.     

New insights into precious metal enrichment on the Isle of Rum,Scotland

The Rum Layered Suite (NW Scotland) is generally regarded as one of a handful of classic examples of open‐system layered mafic‐ultramafic intrusions, or ‘fossilized’ basaltic magma chambers, world‐wide. The eastern portion of the Rum intrusion is constructed of sixteen repeated, coupled, peridotite–troctolite units. Each major cyclic unit has been linked to a major magma replenishment event, with repeated settling out of ‘crops’ of olivine and plagioclase crystals to form the cumulate rocks. However, there are variations in the lithological succession that complicate this oversimplified model, including the presence of chromitite (>60 vol. percent Cr‐spinel) seams. The ～2 mm thick chromitite seams host significant platinum‐group element (PGE) enrichment (e.g. ～2 ppm Pt) and likely formed in situ, i.e. at the crystal mush–magma interface. Given that the bulk of the world's exploited PGE come from a layered intrusion that bears remarkable structural and lithological similarities to Rum, the Bushveld Complex (South Africa), comparisons between these intrusions raise intriguing implications for precious metal mineralization in layered intrusions.     

Platinum-group element distribution in base-metal sulfides of the UG2 chromitite,Bushveld Complex,South Africa—a reconnaissance study

Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole-rock analysis, ore microscopy, SEM/Mineral Liberation Analysis (MLA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The top and base of the UG2 main seam have the highest bulk-rock Pd and Pt concentrations. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite, and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains. In situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated platinum-group element (PGE) contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350–1,000 ppm Pd, 200 ppm Rh, 130–175 ppm Ru, 20 ppm Os, and 150 ppm Ir, and is the principal host of Pd and Rh in the studied ores of the UG2. Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Pentlandite consistently hosts elevated contents of the whole-rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %), and Ru (1–39 %). Platinum-group mineral (PGM) investigations support these mass balance results; most of the PGM are Pt-dominant such as braggite/cooperite and Pt-Fe alloys or laurite (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare. Both PGE concentrations and their distribution in base-metal sulfides (BMS) in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE.     

Structural features of ophiolitic chromitites in the Zambales Range,Luzon, Philippines

The chromitite-bearing peridotites of the Zambales mafic-ultramafic complex form the lowermost level of the Zambales ophiolite, which exposes a complete ophiolitic sequence. The chromitites occur close to the peridotite/gabbro transition zone.The chromite orebodies are structurally classified into three major types: (1) concordant tabular deposits, (2) strings of pods and (3) pocketlike deposits.Concordant tabular deposits show a gradational transition from chromitite to host rock (modal grading) and are characterized by the parallelism of ore and host-rock structures. Primary magmatic features like inch-scale layering, size grading, glomeroporphyric chromite aggregates, skeletal chromite growth and adcumulus growth (cumulus textures) are common.The concordant chromite bodies are often tectonically disrupted and boudined forming strings of pods or fault-controlled pocketlike deposits. With increasing tectonization chromite shows pull-apart textures and lineations (plastic deformation), shearing, prismatic jointing, brecciation and mylonitization (brittle deformation). Recrystallization of cataclastic chromite occurs on a microscopic scale.Plastic deformation is caused by mantle flow and/or the volume increase of the peridotites during serpentinization. The influence of mantle flow is indicated by the orientation of the pod strings and lineations in chromitite perpendicular to the ridge axis. Brittle deformation of chromite (cataclasis) and disruption by faults is related to the emplacement of the ophiolite.     

Structures at the base of the Upper Group 2 chromitite layer, Bushveld Complex, South Africa, on Karee Mine (Lonmin Platinum)

Exposures in a now-infilled pit mined for platiniferous UG2 chromitite in the Bushveld Complex, South Africa, are described. The layer of chromitite is underlain by anorthosite, providing a dramatic colour contrast. The interface between these two rock types shows evidence of various scales of irregularities. In plan view, small circular depressions, less than 3 cm across and 5 mm deep, occupy about 20% of the surface. Between them, the contact is planar. The anorthosite, immediately underlying the chromitite, has a planar fabric visible in thin sections that is not disturbed beneath these small depressions. Another set of depressions occurs, about 40 cm in diameter and with variable depth (< 40 cm). Again they are approximately circular. Larger structures, called potholes, reach several metres. No regular distribution pattern is apparent in any of these structures.

Several possible processes are reviewed for the origin of these irregularities, especially the small-scale structures, but none explains all the features noted. These processes include remelting, diapirism, impact-generated dimpling, gas escape, and interference rippling. We present a photographic record of these structures, but present no definitive model for their interpretation.     

Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex

A petrogenetic model for the Merensky Reef in the Rustenburg section of the Bushveld Complex has been developed based on detailed field and petrographic observations and electron microprobe data. The model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage. The model is devised to account for several key observations: (1) Although the dominant rock type in the Rusterburg sections is pegmatoidal feldspathic pyroxenite, there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite, and small amounts of olivine are present in nearly all pegmatoids. (2) The pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter-thick layer of anorthosite. The thicknesses of the two layers exhibit a well-defined, positive correlation. (3) Inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid. The leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef. (4) Chromite in seams adjacent to plagioclase-rich rocks is characterized by higher Mg/Mg+Fe and Al/R3 and lower Cr/R3 than that in seams adjacent to pyroxene-rich rocks. Similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ. The chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates. It is proposed that the present reef was originally a melt-rich horizon in norite immediately overlain by relatively crystallized pyroxenite. Magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef. The melt-rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water-undersaturated. Hydration of the intercumulus melt was accompanied by melting, and the hydration/melting front migrated downward into the footwall and upward into the hanging wall. In the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase. With continued hydration chromite was stabilized as melt alumina content increased. The regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front. The stratigraphic sequence downward through the base of the reef or pegmatoid (melt)-chromitite-anorthosite-norite represents the sequence of stable mineral assemblages across the hydration/melting front. The sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures. With cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite-olivine-pyroxene-plagioclase, with peritectic loss of some olivine. Calculations of mass balance indicate that a significant proportion of the melt was lost from the melt-rich horizon. Variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the Merensky Reef resulted because the processes of hydration, melting and melt loss operated to varying extents.     

Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis

In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides, sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2. This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously, they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide, Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and may possibly have been derived from the UG2 chromitite itself.     

The distribution of chromium among orthopyroxene,spinel and silicate liquid at atmospheric pressure

A series of experiments has been carried out in which a synthetic silicate melt, of composition equivalent to a “U-type” Bushveld Complex parent liquid, was equilibrated with bronzitic orthopyroxene and chromite spinel between 1334 and 1151°C over a range of oxygen fugacities between the nickel-nickel oxide and iron-wüstite buffers. The partition coefficient for Cr between bronzite and melt increases with falling temperature along a given oxygen buffer, and decreases with falling oxygen fugacity at a given temperature, showing an overall range from 1.1 to 11.7. The Cr content of the melt in equilibrium with spinel (Cr solubility) decreases with falling temperature and increases with lower oxygen fugacity. This variation may be quantified in terms of temperature-dependent solubility of Cr³⁺ combined with a changing ratio of Cr³⁺ to Cr²⁺ in the melt. Iso-oxidation curves for Cr are approximately parallel to Fe-Si-O buffer curves and to curves for equal Fe³⁺/Fe²⁺ determined by Hill and Roeder (1974), and agree within experimental error with the results of Schreiber and Haskin (1976). When the changing oxidation state of Cr is allowed for, the orthopyroxene partition coefficient may be expressed as the sum of the temperature-dependent coefficient for Cr³⁺ and a partition coefficient of about 1 for Cr²⁺.The experimental results are used to construct a series of model curves for liquid and bronzite compositional variations during fractional crystallization of a Bushveld parent liquid. Trends for Cr variation are shown to depend critically upon oxygen fugacity, and on whether the liquid is saturated with chromite. The position of the peritectic between chromite and orthopyroxene is shown to be very sensitive to oxygen fugacity within one and a half log units of the QFM buffer. This observation may explain the contrasting distribution of chromite seams in the Bushveld Complex, where chromite occurs within bronzitite-norite sequences, and in the Stillwater Complex in which chromite is restricted to olivine cumulate layers.     

Platinum group element distribution in the lower and middle group chromitites in the western Bushveld Complex

Summary All analysed massive chromitite layers of the Critical Zone of the Bushveld Complex are enriched in PGE's over their silicate host rocks. The concentration factor has been found to increase with stratigraphic height. The PGE-distribution of the Lower Group and Middle Group chromitites shows a systematic relationship to the chromite mineralogy of the chromitites. The LG1- to LG4-chromitite layers are characterized by the dominance of the Ru-group elements (Ru, Os, Ir). The LG5- to LG7-chromitite layers contain almost equal amounts of the two PGE-groups and in the MG-chromitites the elements of the Pt-group (Pt, Pd, Rh) are the most abundant. The chromite mineralogy subdivides the chromitites in a similar way.

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PGE-Verteilung in den Lower und Middle Group Chromititen des westlichen Bushveld Complexes

Zusammenfassung Alle untersuchten massiven Chromitite der Critical Zone des Bushveld Complexes sind im Hangenden ihrer silikatischen Nebengesteine an PGE's angereichert. Es stellte sich heraus, dass der Konzentrationsfaktor innerhalb der stratigraphischen Abfolge zum Hangenden hin zunimmt.Die PGE Verteilung in den Lower und Middle Group Chromititen ändert sich systematisch mit der Mineralogie der Chromite in den Chromititen. Die LG 1 bis LG 4 Chromititlagen sind durch ein Vorherrschen der Elemente der Ru-Gruppe (Ru, Os, Ir) gekennzeichnet.Die LG 5 bis LG 7 Chromititlagen enthalten beinahe die gleichen Gehalte an Elementen beider PGE-Gruppen. In den MG-Chromititen sind die Elemente der Pt Gruppe (Pt, Pd, Rh) am weitesten verbreitet. Mit Hilfe der Mineralogie der Chromite können die Chromitite auf ähnliche Weise untergliedert werden.

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With 11 Figures     

Surface chemical mechanisms of inadvertent recovery of chromite in UG2 ore flotation: Residual layer identification using statistical ToF-SIMS analysis

In the UG2 ore (Bushveld Complex, South Africa) flotation, normally more than 3% of the gangue minerals, principally chromite with talc and pyroxene, report to the concentrate diluting the PGM recovery and contributing to subsequent processing costs. Previous studies have identified residual talc-like layers on orthopyroxene surfaces in Merensky ore flotation contributing to inadvertent flotation of relatively large particles (20–150 µm) of this mineral. Chromite (75–150 µm) from flotation of UG2 ore has been similarly examined. Statistical comparison of ToF-SIMS analysis of particles from concentrate and tails reveals no significant difference in Cu, Pb, Ni and collector (IBX and DTP) signals between these streams but surface exposure of Mg and Si is favoured in the concentrate. The flotation rate of coarse chromite correlates with the exposures of magnesium and silicon in patches on the chromite surface; higher exposures give earlier flotation. Conversely, there is a negative correlation with signals corresponding to the chromite surface, i.e. Cr, Fe, Al. Flotation of chromite without collector has confirmed this statistical discrimination. Hydrophobic talc-like residual layers, similar to those found on orthopyroxene surfaces, probably from partial alteration, explain this flotation mechanism.