G. von Gruenewaldt

Contrasting platinum-group element concentration patterns in cumulates of the Bushveld complex

Chromite-rich lithologies in both the lower and the critical zones of the Bushveld Complex in the Potgietersrus area display flat, chondrite-normalized, platinum-group-element (PGE) concentration patterns, whereas those of associated sulphide-bearing, but chromite-poor rocks are considerably steeper. The low (Pt+Pd)/(Os+Ir+Ru) ratio in the chromite-bearing rocks is maintained irrespective of the amount of sulphide or chromite in the rock. This feature suggests that the partitioning of the individual PGE into PGE-bearing phases during conditions in the magma under which crystallization of chromite in excess of the normal cotectic amounts was favoured differed from conditions under which an immiscible sulphide liquid separated from the same magma in the absence of enhanced chromite crystallization. These changes in the partitioning coefficients of the individual PGE are considered to reflect changes in the solubility of these elements in response to variations in the intensive parameters in the magma necessary to bring about the enhanced crystallization of chromite.

The Formation of Stratiform PGE Deposits in Layered Intrusions

The principle stratiform platinum group element deposits are the Merensky Reef and UG-2 chromitite of the Bushveld and the J-M Reef of the Stillwater Complexes. In the Bushveld Complex, cryptic variation in the MgNo of bronzite and in the An and Sr content of plagioclase within the Merensky cyclic unit, the apparent crossing of the plagioclase cotectic by the magmareponsible for this and other units, and variations in 87Sr/86Sr initial ratio, and, in the Stillwater Complex, the change in the order of liquidus phases, point to the involvement of at least two different magmas in each intrusion.

Ilmenite-Apatite Enrichments in the Upper Zone of the Bushveld Complex: A Major Titanium-Rock Phosphate Resource

Detailed modal analyses of apatite-enriched layers in the upper 1000 m of cumulates of the Upper Zone have shown them to contain appreciable quantities of granular ilmenite. This greatly enhances the economic significance of these layers as a potential resource of combined apatite and ilmenite. The mineralized layers encountered in the very limited number of borehole intersections through the uppermost 1000 m of the Upper Zone suggest a widespread development of two mineralized zones. A lower zone is approximately 30 m thick with a combined apatite and ilmenite content of slightly more than 20 vol%, of which granular ilmenite constitutes between 4 and 6 vol%. The upper mineralized zone is up to 40 m thick, with an apatite plus ilmenite content of between 18 and 20 vol% and a granular ilmenite component of approx-imately 10 vol%. Textural and compositional relations suggest the periodic development of an immiscible Fe-Ti-Ca-O-P liquid during fractional crystallization of the topmost 1000 m of the Upper Zon...

Platinum group element proportions in chromitites of the Bushveld complex: implications for fractionation and magma mixing models

Abstract Chromitite layers in the Critical Zone of the Bushveld complex display a systematic change in the chondrite normalized platinum group element (PGE) concentration patterns. The bell-shaped pattern of the lower layers of the lower group of chromitite layers suggests a very low sulphide content and that ruthenium, osmium and iridium are largely contained as magmatic platinum group minerals (PGM) in the chromite grains. Elevated rhodium concentrations compared to platinum and palladium suggests a possible solid solution component of rhodium in chromite. The upper of the lower group of chromitite layers and the middle group of layers have gentle positive patterns, which suggest the presence of a base metal sulphide component. Pt ( Pt + Pd ) ratios within these layers can be related to the thickness of the chromitite layer and total PGE content. Positive slopes for the upper group of chromitite layers and the Merensky Reef reflects sulphide dominant PGE mineralization of samples from these layers used for this investigation. It is also demonstrated that all chromitite layers of the Bushveld complex obtained their PGE content largely by a combination of enclosure in chromite of early, magmatic PGMs and subsequent sulphide collection from a smaller volume of magma as that from which the chromite crystallized. The results also indicate that the Ir Os ratio seems to be a sensitive petrogenetic indicator. Variations in this ratio over the sequence of chromitite layers suggest that the B 2 B 3 parental melts of the Bushveld had a higher Ir Os ratio than the earlier B1 parental magma.

PGE mineralization in the western sector of the eastern Bushveld Complex

SummaryUnusual 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.ZusammenfassungUngewöhnliche 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.

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

SummaryAnalytical 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.ZusammenfassungAnalytische 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.

An evaluation of proposed platinum group element contents in the parental magmas of the Bushveld complex

Abstract Samples collected by the authors and representing three proposed parental magmas of the Bushveld complex were analyzed for their platinum group element (PGE) contents by three different laboratories. Results differ strongly between laboratories, but imply that the parental magmas may have had flatter chondrite normalized patterns and an overall lower content than previously reported. It seems, however, that the Bushveld magmas were enriched in PGEs relative to average mafic rocks. A clear difference between the three magma types could not be substantiated. At present the PGE content of proposed parental melts of the Bushveld complex must be considered to be insufficiently known to warrant any quantitative models.

Shear aggregation (convective scavenging) and cascade enrichment of PGEs and chromite in mineralized layers of large layered intrusions

SummaryShear driven coagulation/aggregation is a common method to concentrate and separate suspended particulate matter from fluids. Convective boundary layers are of primary importance in securing concentration. The theoretical indication is that ore grade PGEs may be scavenged from a primary melt of Bushveld composition and aggregated in convective boundary layers well within the expected lifetime of a magma chamber. The boundary layer dynamics should also secure the observed peculiarities of Bushveld PGE concentration profiles: peaks at top and bottom of, say, hosting chromitite layers. In the environment of double diffusive convection, precipitates of immiscible sulphide liquids in the cooler upper sections of the magma will be transported downward through undersaturated, hotter layers of melt, to be resorbed and to enrich the lower layers, similar in operation to a chemical fractionation cascade. Further cooling secures 1) ore grade levels of concentrates in zones of higher shear at the bottom and 2) supersaturation, assistingin situ solidification there.ZusammenfassungKoagulation/Aggregation, die durch Scherung verursacht wird, ist eine verbreitete Methode um in Suspension gehaltene Feststoffe aus fluiden Phasen zu konzentrieren und abzutrennen. Konvektive Grenzlagen sind von grundsätzlicher Bedeutung um eine Konzentration zu ermöglichen. Es gibt theoretische Hinweise dafür, daß PGEs in abbauwürdigem Ausmaß aus einer primären Schmelze von Bushveld-Zusammensetzung abgetrennt und in konvektiven Grenzlagen aggregiert werden können; dies kann durchaus in der wahrscheinlichen “Lebenszeit” einer Magmenkammer stattfinden. Die Dynamik der Grenzschichten sollte auch die beobachteten Besonderheiten von PGE Konzentrationsprofilen im Bushveld erklären: Peaks am oberen und unteren Rand von, beispielsweise, Chromititlagen. Im Milieu doppelt diffusiver Konvektion werden Prezipitate von entmischter Sulfidschmelze in den kühleren oberen Bereichen des Magmas nach unten transportiert, um im untersättigten, heisseren unteren Bereich des Magmas resorbiert zu werden und die unteren Lagen anzureichern, heir bestehen Ähnlichkeiten zu einer chemischen Fraktionierungskaskade. Weitere Abkühlung stellt sicher, daß 1. abbauwürdige Gehalte in Zonen von intensiver Scherung am unteren Rand der Magmenkammer auftreten und daß 2. Übersättigung zur Verfestigung in situ beiträgt.