Vertical migration of residual magma in the upper zone of the Bushveld Complex

Summary Migration of residual liquid can potentially affect the textures and mineral compositions in layered intrusions, but is difficult to conclusively demonstrate. In the Upper Zone of the Bushveld Complex a metabasaltic xenolith forms a locally impermeable horizon, which acted as a barrier to vertically migrating residua. Increased Ab content in plagioclase, and K₂O and Zr in whole-rock analyses in the anorthosite directly below the xenolith, compared to the same horizon along strike of the xenolith, demonstrate trapping of residual liquid and/or fluid beneath the xenolith.Comparison of Cu/Ni and Cu/S ratios of the mineralised anorthosite in the normal sequence and below the xenolith suggests that these are primary magmatic sulphides which crystallised within the anorthosite and are not derived by sinking of dense interstitial sulphide liquid originally associated with the overlying magnetite layer or introduced hydrothermally.

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Vertikale Migration von Restmagma in der Upper Zone des Bushveld-Komplexes

Zusammenfassung Die Migration von Restschmelzen kann Texturen und Mineralzusammensetzungen in geschichteten Intrusionen beeinflussen, jedoch ist es schwierig, dies eindeutig nachzuweisen. In der Upper Zone des Bushveld-Komplexes bildet ein metabasaltischer Xenolith einen lokal undurchlässigen Horizont, der für vertikal migrierte Residuen als Barriere fungierte. Erhöhte Ab-Gehalte in Plagioklas, sowie erhöhte K₂O und Zr-Werte in Gesamtgesteins-Analysen in Anorthosit direkt unterhalb des Xenolithen-verglichen mit der Zusammensetzung des gleichen Horizontes in Streichen des Xenolithen-weisen auf Konzentration von Restschmelzen und/oder Fluiden im Liegenden des Xenolithen hin. Der Vergleich von Cu/Ni und Cu/S Verhältnissen der mineralisierten Anorthosite in der normalen Abfolge mit denen unterhalb des Xenolithen zeigt, daß es sich hier um primäre magmatische Sulfide handelt, die innerhalb des Anorthosits kristallisierten. Diese Sulfide lassen sich nicht auf das Absinken von dichter Sulfidschmelze, die ursprünglich mit den überlagernden Magnetit-Bändern in Zusammenhang standen, und auch nicht auf hydrothermale Zufuhr zurückführen.

     

Postcumulus modification of magnetite grains in the upper zone of the Bushveld Complex,South Africa

A detailed electron microprobe study has been made of magnetite grains from magnetitite layer 1 of the upper zone of the Bushveld Complex, in order to establish if there is any evidence for postcumulus processes having affected magnetite compositions. Representative composition profiles obtained from detailed step traverses are presented for touching magnetite grains and where magnetite is in contact with either ilmenite or plagioclase.Magnetite grains in all textural settings are compositionally heterogeneous. Touching magnetite grains display systematic compositional zonations: the rims of grains show a marked decrease in Al and Mg towards the grain boundary; the cores are characterised by a distinctive peak and trough pattern for these elements. These variations can be related to the positions of exsolved phases. Exsolutions in the cores are either pleonaste or an Fe³⁺ bearing AlMg spinel, whereas those at the grain boundaries are generally more Al-rich and include corundum. Similar exsolution related compositional variations for Mg and Al were found in magnetites in contact with ilmenite and plagioclase.A model has been developed which illustrates the possible sequence of postcumulus events during the interval between accumulation of the magnetite and the cessation of subsolidus reactions. The essential feature of the model is the delicate interplay between depletion of the host magnetite by corundum, pleonaste and the Fe³⁺ bearing AlMg spinel exsolutions, and diffusion of Al and Mg into the depleted areas from the surrounding magnetite.Data for Cr contents in magnetites show that although this particular trace element is apparently resistant to redistribution by exsolution processes, it is affected by subsolidus re-equilibration between magnetite and ilmenite, or magnetite and intercumulus liquid.     

Multiple,isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

The Bushveld Complex and other layered intrusions show significant initial isotopic heterogeneity, both between and within co-existing cumulate minerals. Various processes have been proposed to account for this, including (i) intrusion of variably contaminated crystal mushes from deeper staging chambers, (ii) blending of semi-consolidated crystal mushes as a result of subsidence during cooling, (iii) variable infiltration of contaminants into a partially solidified crystal mush, (iv) density-driven mixing of minerals from isotopically distinct magma pulses, (v) contamination of crystals at the roof of the intrusion and mechanical incorporation of such contaminated crystals into the lower crystallisation front as a result of gravitational instability at the upper crystallisation front, and (vi) late-stage metasomatic processes. In order to assess the likely process(es) responsible for initial isotopic heterogeneities within the Bushveld Complex, we analysed core and rim domains of 12 plagioclase crystals from the Main and Upper zones of the Bushveld Complex for their Sr-isotopic compositions. The data show the presence of multiple, isotopically heterogeneous populations of plagioclase occurring within the same rocks. The data presented here are best explained through the intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton.     

The palaeomagnetism of the main zone of the eastern Bushveld Complex

Palaeomagnetic data were acquired from eighteen sampling sites situated in the main zone of the eastern Bushveld Complex, Transvaal, South Africa. Specimens were subjected to alternating field and thermal demagnetization. Two mean magnetization directions, which are approximately antipodal, were found. One direction represents subzone B of the main zone in the eastern Bushveld Complex and yields a palaeomagnetic pole at . The second direction represents subzone C of the main zone in the eastern Bushveld Complex with virtual geomagnetic pole at . The positions of these poles on the apparent polar wander path (APW) for Africa indicate that the critical zone had acquired its remanent magnetization before the main zone. Fold tests prove that the main zone in the eastern Bushveld Complex had acquired its remanent magnetization with the igneous layering in a horizontal position.     

Sulphur saturation in the lower and critical zones of the Eastern Bushveld Complex

A suite of ultramafic and mafic rocks from the lower, critical and lower portion of the main zones of the Bushveld Complex has been analysed for Th, Cs, Zr, Ni, Cr and Au by INAA and XRF spectrometry. The incompatible elements Th, Cs, and Zr correlate positively, and show a gradual upward increase in abundance. Assuming constant average proportion of intercumulus material, this upward increase implies that the zones of the Complex studied represent crystallization of a single magma type some 3600 m thick. Pyroxenites dominate the lower portion of the section studied and their Ni content shows an initial rapid decrease from 850 ppm in the lowermost rocks, to around 500 ppm, with considerable scatter. This distribution is most likely to have resulted from bottom crystallization with superimposed convective overturn near the transient floor of the chamber. Gold abundances are generally higher in chromitites, and correlate positively with Ni, indicating the presence of significant amounts of cumulus immiscible sulphide. In the silicate rocks, Au does not correlate with any of the analysed elements, and it is concluded that Au was trapped in small quantities of immiscible sulphide which precipitated continuously during crystallization. There is an upward increase in the amount of cumulus immiscible sulphide, indicating a progressive increase in sulphur solubility in the magma.     

The use of plagioclase composition as an indicator of magmatic processes in the Upper Zone of the Bushveld Complex

Summary Analytical data on the composition of plagioclase from the lower part of the Upper Zone in the eastern Bushveld Complex is presented. Detailed electron microprobe investigations failed to establish any cyclic variation through that sequence but revealed similar variations in An content, potassium and iron concentrations below and above magnetite layers. These findings can be attributed to the heterogeneous nature of the plagioclase both within individual grains and within a given sample, which would mask any possible trends of cryptic variation. The Sr concentration and Sr/Al₂O₃ ratio of plagioclase, determined by XRF on plagioclase separates, however change slightly at the level of the Main Magnetite Layer, which can possibly be related to the breakdown of density stratified liquid layers within the resident magma. Analyses of plagioclase separates are thus considered to be more suitable to indicate magmatic processes than plagioclase compositions determined by electron microprobe.

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Plagioklaszusammensetzung als Indikator für magmatische Prozesse in der Upper Zone des Bushveld Komplexes

Zusammenfassung Analytische Daten von Plagioklasen aus dem unteren Teil der Upper Zone im östlichen Bushveld Komplex werden präsentiert. Detaillierte Untersuchungen mittels Elektronen-strahl-Mikrosonde ergaben keine Hinweise auf eine zyklische Variation in dieser Abfolge, zeigten aber eine dänliche Variation des An-Gehaltes, Bowie der Kalium- und Eisengehalte im Liegenden und Hangenden von Magnetitlagen. Dies läßt sich mit der heterogenen Natur der Plagioklase, sowohl in Einzelkörnern, als auch innerhalb einer Probe erklären, die jeden möglichen verborgenen Variationstrend verdecken würden. Der mittels XRF Analytik an separierten Plagioklasen bestimmte Gehalt an Sr und das Sr/Al₂O₃ Verhältnis dndern sich allerdings geringfügig im Bereich des Main Magnetite Layer. Dies wird möglicherweise mit derv Zusammenbruch von dichtegeschichteten Schmelzlagen im Magma in Beziehung gebracht. Die Analyse von Plagioklaskonzentraten scheint daher geeigneter zu sein magmatische Prozesse anzuzeigen als Mikrosondenuntersuchungen.

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Argon diffusion in plagioclase and implications for thermochronometry: A case study from the Bushveld Complex, South Africa

Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K-Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision ⁴⁰Ar/³⁹Ar data and quantifies Ar diffusion kinetics of low-K minerals. We present results of five diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed diffusion kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155-178 kJ/mol and ln(D₀/a²) varies between 3.5 and 6.5. These diffusion parameters correspond to closure temperatures of 225-300 °C, for a 10 °C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 °C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150-200 °C over ∼600 Ma.     

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.     

Discordant ultramafic pegmatoidal pipes in the Bushveld Complex

Discordant ultramafic pipes cut most of the layered sequence of the Bushveld Complex. We have studied one pipe in detail, the Tweefontein pipe, which cuts the Critical Zone, eastern Bushveld Complex, because it is well-exposed in a new road cutting. Field relations suggest that these pipes were emplaced while the layered rocks were extremely hot and incapable of brittle failure. The existence of displaced chromitite and anorthosite fragments in this discordant body is suggestive of an intrusive magmatic, rather than metasomatic, mode of emplacement. Initial Sr isotopic ratios of plagioclase from the pipe are in the range 0.7073 to 0.7079, which contrast with typical ratios of 0.7055 to 0.7065 for the Critical Zone, and >0.708 for Main Zone. These data preclude an origin for the pipe as residual magmas from the adjacent layered rocks. The compositions of, and extensive exsolution in, pyroxenes in the pipe indicate temperatures of formation comparable to those of the layered sequence itself, and that they underwent slow cooling comparable to the surrounding layered rocks, such that they both have similar closure temperatures. Preferential replacement of leuconoritic layers suggests a temperature of emplacement in excess of the plagioclase–pyroxene cotectic temperature. The per mil δ¹⁸O difference between plagioclase and pyroxene (Δ_plag–px) for samples from within the pipes ranges from 0.4 to 1.0, and averages 0.7 (for nine pairs), compared to Δ_plag–px of 0.4 to 0.6 for host rocks, again consistent with magmatic temperatures of formation. Oxygen isotope ratios for plagioclase and pyroxene in the pipes and layered host rocks are comparable, and preclude a significant fluid contribution from metamorphosed sediments in the floor of the Bushveld Complex in the formation of the primary mineralogy. The presence of hornblende, and occasional higher Δ_plag–px values than in the normal layered sequence rocks suggest lower temperature equilibration in the pipe, probably in the presence of a fluid. Higher absolute δ¹⁸O values for both minerals in a few of the pipe and host samples suggest reaction with a later fluid. These discordant ultramafic pipes are considered to form by emplacement of magma batches, which are Sr-isotopically distinct from those which produced the adjacent layered rocks of the Bushveld Complex, but were nevertheless extremely closely related in time to the main intrusive events. Dissolution of host rocks, rather than purely mechanical dilation, provided the space for pipe emplacement. However, the pipe may have acted ultimately as a channelway for low-temperature hydrothermal fluids related to later faulting in the immediate vicinity. Received: 10 October 1998 / Accepted: 22 May 2000     

Grain-size distribution and rheology of the upper mantle

The physical processes that govern the grain size of rocks in the upper mantle are examined. The analysis is based on the experimental data on creep, recrystallization, and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere. The grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate. Above this depth, the microstructures record the thermomechanical history of rocks rather than the in situ stress.In the case of the oceanic lithosphere where the thermomechanical history is best known, the following features of grain-size distribution are found. At the uppermost mantle, where the amount of grain growth is limited, the grain size is determined by the initial value and the growth rate, and, where the effect of grain growth dominates, it increases with depth. When the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform, the grain size is then determined by the applied stress. This grain size is, however, frozen, when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization. Thus, at the intermediate depth region, the grain size records the fossil (frozen) stress at which the microstructures of rock have been frozen. Since the frozen stress increases with age, the grain size in this depth interval decreases with depth. Finally, the grain size below this level reflects the in situ stress, and increases with depth, its extent being dependent on the nature of return flow in the deep mantle.Thus the grain size versus depth relation may show a sigmoid curve. The qualitative features of this curve may be similar also in the case of the continental lithosphere, if a similar thermal event (i.e., the intrusion of hot material and subsequent cooling) occurs. The results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules (Avé Lallemant et al., 1980). The present model suggests that the depth of minimum grain size (65 and 150 km at the continental rift zone and the shield region respectively) corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels (~ 1 MPa) and in the geological time scale. This result leads to a new definition of the thickness of lithosphere in terms of its rheological properties. This thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness. The model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy.     

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years, yet its origin remains unresolved.In the present study, we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy.Our results indicate that the Merensky Reef formed through a range of diverse processes.Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth.The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow.Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation.The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts.Olivine-hosted melt inclusions containing Cl-rich apatite, sodic plagioclase, and phlogopite suggest the presence of highly evolved, volatile-rich melts.Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase, possibly triggered by flux of heat, acidic fluids, or hydrous melt.Together, these data suggest that the reef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes.     

Coexisting Ca-poor pyroxenes in the Main Zone of the Bushveld Complex

Electron microprobe analyses of Ca-poor pyroxenes in gabbroic rocks of the Main Zone of the Bushveld Complex reveal that inverted pigeonites have lower Mg/Fe ratios than coexisting hypersthenes. Textural relationships, however, indicate that the two Ca-poor pyroxenes did not crystallize simultaneously from the magma. Early pigeonite reacted with the magma to form hypersthene and the difference in the Mg/Fe ratio of these two pyroxenes reflects the difference of this ratio between early pigeonite and the magma at the time of reaction. Some of the grains of early pigeonite, now inverted to hypersthene, evidently escaped this reaction with the magma. Bulk compositions of pyroxenes intermediate between that of pigeonite and hypersthene are postulated on the grounds of varying amounts of exsolved augite in the hypersthene which has originated from pigeonite by reaction with magma.     

PGE mineralization in the western sector of the eastern Bushveld Complex

Summary Unusual facies of the Merensky Reef, the UG-2 and the UG-1 chromitite layers are developed in the western sector of the eastern Bushveld Complex. Within the basal pyroxenite of the Merensky unit, mineralization can be developed at up to four levels. Some of these contain significant mineralization with an increase in the Pt/Pd ratio upward in the succession.The UG-2 chromitite layer consists of a lower, sulphide-rich layer and an upper, sulphide-poor layer. Although these two layers are separated by a pyroxenite parting in places, both contain high platinum-group element (PGE) values. Textural features such as inclusions of base metal sulphides in chromite grains, and the moulding of sintered chromite grains around sulphides, indicates that immiscible sulphide liquid separated prior to or simultaneously with chromite crystallization. The presence of platinum minerals within the sulphides of the inclusions and enclosed in all the base metal sulphides interstitial to chromite, indicates that the PGE were extracted from the magma by the sulphide liquid.Textural and compositional evidence suggests that the sulphide enrichment in the UG-1 chromitite layer is also of magmatic origin, but that these sulphides underwent remobilization at high temperatures.Magma mixing processes are considered to have produced the chromitite layers. The high sulphide content associated with the chromitite layers in the upper critical zone in this sector is ascribed to favourable compositions and proportions of the magmas involved in the mixing process.

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PGE-Vererzung im westlichen Sektor des östlichen Bushveld-Komplexes

Zusammenfassung Ungewöhnliche Fazies des Merensky-Reefes sowie der UG-2 und der UG-1 Chromitite kommen im westlichen Sektor des östlichen Bushveld Komplexes vor. In den basalen Pyroxeniten der Merensky-Einheit liegt Vererzung in bis zu vier verschiedenen Niveaus vor. Einige von diesen enthalten signifikante Metallgehalte, wobei das Pt/Pd Verhältnis gegen das Hangende hin zunimmt.Der UG-2 Chromitit besteht aus einer unteren, Sulfid-reichen, und einer oberen, Sulfid-armen Lage. Obwohl diese beiden Lagen stellenweise durch eine pyroxenitische Zwischenschicht getrennt sind, enthalten beide hohe Platin-Gruppen-Elementgehalte (PGE). Texturen wie z.B. Einschlüsse von Buntmetallsulfiden in Chromitkörnern, und die Anordnung von gesinterten Chromitkörnern um Sulfide herum weisen darauf hin, daß eine unmischbare Sulfidschmelze vor oder gleichzeitig mit der Chromitkristallisation abgetrennt wurde. Das Vorkommen von Platin-Mineralen in den Sulfiden der Einschlüsse, und in allen Buntmetallsulfiden die zwischen Chromitkörnern vorkommen, zeigen, daß die PGE durch eine Sulfidschmelze aus dem Magma entfernt worden sind.Texturelle und chemische Parameter zeigen, daß die Sulfidanreicherung in den UG-1 Chromititen auch einen magmatischen Ursprung hat, jedoch waren diese Sulfide später von einer Hochtemperatur-Mobilisation betroffen.Die Chromitit-Lagen werden durch Magmen-Mischung, der hohe Sulfid-Gehalt in den Chromitit-Lagen der oberen Kritischen Zone in diesem Sektor durch günstige Zusammensetzungen und Verhältnisse der Magmen, die an diesem Mischungsprozess teilgenommen haben erklärt.

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Mercury distribution amongst co-existing silicates within the Bushveld Complex

Thirty separates of plagioclase, orthopyroxene and clinopyroxene from the lower Main Zone of the Northern Limb of the Bushveld Complex were analysed for their mercury contents using combustion atomic absorption spectroscopy with gold amalgamation pre-concentration. The average mercury contents of plagioclase, orthopyroxene and clinopyroxene were found to be 0.9 ppb, 1.2 ppb and 1.1 ppb, respectively. Mercury within the separates does not vary systematically with any of the major element oxides present in the minerals. Based on a positive 1:1 correlation between mercury in orthopyroxene and clinopyroxene, we estimate D^Opx_Hg ≈ D^Cpx_Hg, and on this basis, can exclude the presence of significant Hg²⁺ within the melts from which these minerals crystallised. The lack of correlation between mercury in plagioclase and that in the mafic silicates may suggest diffusional loss of the element from the former during slow cooling under magmatic conditions and better retention of mercury by the mafic silicates under the same conditions. Alternatively and more likely, this lack of correlation may support earlier arguments based on distinct Sr-isotopic disequilibrium between co-existing plagioclase and mafic silicates, that plagioclase and the mafic silicates in the Northern Limb of the Bushveld Complex may have crystallised from different melts within a variably contaminated, sub-Bushveld staging chamber.     

Iron in plagioclase in the Bushveld and Skaergaard intrusions: implications for iron contents in evolving basic magmas

The evolved, iron-rich rocks of the tholeiitic Bushveld and Skaergaard intrusions are similar in containing cumulus magnetite, ilmenite, plagioclase, clinopyroxene, apatite and olivine, and also orthopyroxenes/pigeonite in Bushveld. Here, we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses. To aid this analysis a revised mass balance model for the liquid evolution of Skaergaard is presented. For plagioclase in the Upper Zone of Skaergaard it was previously demonstrated that total FeO increases from ~0.25 to ~0.45 wt% with differentiation and correlates inversely with An% [100 × Ca/(Na + Ca)]. The reverse trend is observed in two recently published datasets for Bushveld, showing that total FeO in plagioclase decreases upward through the magnetite-bearing Upper Zone from ~0.30 to ~0.15% and from ~0.40 to ~0.25% in the western and northern limbs, respectively, and correlates positively with An%. The partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid. Although Bushveld formed under slightly more oxidizing conditions than Skaergaard, differences in the partition coefficients cannot explain the two observed trends. We therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different. The inferred liquid composition for Bushveld contained about 15 wt% total FeO at the level of magnetite-in, which is slightly less than the total FeO content of the subsequent cumulates. In contrast, the Skaergaard liquid contained more total FeO than the ensuing cumulates. As a result, in Bushveld residual liquids total FeO decreased after magnetite saturation, whereas in Skaergaard the residual liquids continued to become enriched in iron. This conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks. Despite the mineralogical similarities of evolved iron-rich rocks of Skaergaard and Bushveld, their liquid evolution trends were very different, and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided.     

Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex

We have analysed sulphides, silicates, and chromites of theMerensky Reef for platinum-group elements (PGEs), Re and Auusing laser ablation-inductively coupled plasma mass spectrometryand synthetic pyrrhotite standards annealed with known quantitiesof noble metals. Os, Ir and Ru reside in solid solution in pyrrhotiteand pentlandite, Rh and part of the Reef’s Pd in pentlandite,whereas Pt, Au, Re and some Pd form discrete phases. Olivineand chromite, often suspected to carry Os, Ir and Ru, are PGEfree. All phases analysed contain noble metals as discrete micro-inclusionswith diameters typically <100 nm. Inclusions in sulphidescommonly have the element combinations Os–Ir–Ptand Pt–Pd–Au. Inclusions in olivine and chromiteare dominated by Pt ± Au–Pd. Few inclusion spectracan be related to discrete noble metal phases, and few inclusionshave formed by sub-solidus exsolution. Rather, some PGE inclusions,notably those in olivine and chromite, are early-magmatic nuggetstrapped when their host phases crystallized. We suggest thatthe silicate melt layer that preceded the Merensky Reef wasPGE oversaturated at early cumulus times. Experiments combinedwith available sulphide–silicate partition coefficientssuggest that a silicate melt in equilibrium with a sulphidemelt containing the PGE spectrum of the Merensky ore would indeedbe oversaturated with respect to the least soluble noble metals.Sulphide melt apparently played little role in enriching thenoble metals in the Merensky Reef; rather, its role was to immobilizea pre-existing in situ stratiform PGE anomaly in the liquid-stratifiedmagma chamber. KEY WORDS: Bushveld Complex; Merensky Reef; laser-ablation ICP-MS; platinum-group mineralization     

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.     

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     

Connectivity between the western and eastern limbs of the Bushveld Complex

The mafic layered rocks of the Bushveld Complex are 6–8 km thick and crop out over an area of 65,000 km². Previous interpretations of the Bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies. However, the mafic sequences in these arcuate western and eastern limbs are remarkably similar, with at least six petrologically distinctive layers and sequences being recognisable in both limbs. Such similarity of sequences in two totally discrete bodies 200–300 km apart is petrologically implausible, and it is suggested that they formed within a single lopolithic intrusion.

All previous Bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma. Isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km. With this revised whole crustal model, it becomes possible to construct a gravity model, consistent with observed data, which includes a 6 km-thick sequence of mafic rocks connecting the western and eastern limbs of the Bushveld Complex. The exact depth at which the mafic rocks of the Bushveld Complex lie in the centre of the structure cannot be constrained by the gravity data.

Such a first-order model is an approximation, because there have been subsequent deformation and structural readjustments in the crust, some of them probably related to the emplacement of the Bushveld Complex. Specifically, the observed geometry of the rocks around the Crocodile River, Dennilton, Marble Hall and Malope Domes suggests that major upwarping of the crust occurred on a variety of scales, triggered by emplacement of the Bushveld Complex.     

Plagioclase content of cyclic units in the Bushveld Complex,South Africa

Contributions to Mineralogy and Petrology - The Upper Critical Zone of the Bushveld Complex, South Africa, has been divided into so-called cyclic units. Ideally, they should consist of (from the...