R. K. W. Merkle

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.

40Ar/39Ar age constraints on ore deposition and cooling of the Bushveld Complex, South Africa

The UG-2 chromitite layer in the upper critical zone of the Rustenburg Layered Suite is one of the prominent platinum-group element enrichment zones of the c. 2.06 Ga Bushveld Complex (South Africa). Eleven biotite crystals were selected from a production concentrate from the Impala Platinum mine and degassed incrementally with a CO2 laser. The apparent age spectra display widely varying characteristics, ranging from essentially 100% concordant to strongly discordant (in some cases undulatory) as a result of various degrees of chloritization of the grains. The least discordant spectra (seven grains, 90–100% 39Ar released) yield integrated and plateau ages ranging from 2037 to 2049 Ma (relative to the Fish Canyon sanidine standard at 28.02 Ma) and share a weighted mean age of 2042.4 ± 3.2 Ma (2σ, excluding systematic errors). This age is slightly younger than existing constraints from U–Pb dating (c. 2055–2061 Ma), which seem to indicate a cooling rate of around 20 °C/Ma. However, the recently inferred c. 1% bias between currently used calibrations of the 40Ar/39Ar and U–Pb systems implies a very rapid cooling rate for the Bushveld Complex between 700 and <500 °C. The possibility of subtle bias as a result of recoil artefacts must be considered. In all cases, the heterogeneous age spectra yielded older apparent plateau ages, revealing the likelihood for erroneous conclusions to be drawn from less complete data sets.

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

Abstract 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 Fe 3+ 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 Fe 3+ 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.

Native osmium from the Guli Massif, Northern Siberia, Russia

SummaryNative osmium from two different placer occurrences (i.e., Ingaringda and Burlakovsky) within the Guli Massif, Maimecha-Kotui Province, was evaluated for mineral compositions that show systematic differences between the localities. Grains of native osmium show increases in iridium towards the rim, and nuggets consisting of aggregates define trends of preferential substitution of osmium by iridium. A statistically reliable difference in Os/Ir ratios between the two studied placers can be demonstrated. From textural and compositional characteristics it is concluded that the native osmium of the Burlakovsky placer formed at temperatures higher than can be assumed to be reasonable for crustal conditions. The difference in the Os/Ir ratios between the two placers can be attributed to fractional crystallization of native osmium either in the mantle source, which was tapped in different events to form the Guli Massif, or to decreasing Os/Ir values during crystallization under crustal conditions after intrusion of the silicate melt(s).

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.

The Effects of Metasomatising Fluids on the PGE-content of the UG-1 Chromitite Layer

Research in recent years has indicated the importance of late magmatic and hydrothermal processes for the formation and modification of platiniferous rocks in the Bushveld Complex. However, very little is known about the exact nature of these processes and their effects (i.e., mobilisation or addition of PGE) on the I platinum-group minerals or on whole rock PGE patterns.

Distinguishing between legally and illegally produced gold in South Africa

The identification of gold‐bearing material is essential for combating the theft of gold in South Africa. Material seized in police operations is generally a mixture of gold from different mines, and as such cannot be traced back to a single location. ICP‐OES analysis of material dissolved by acid dissolution provided a database of gold compositions comprising gold from South African mines, illegal gold stolen from the mines, and commercial gold alloys and jewelery. Discrimination between legal and illegal gold was possible due to the presence of Pb, As, Sb, Sn, Se, and Te in the stolen material, elements which are not present in legally produced gold. The presence of these elements is a quick and simple way to distinguish between gold alloys based on refined gold, such as in commercially manufactured jewelery, and gold alloys containing a proportion of unrefined and therefore illegally obtained gold.

Hydrothermal hogbomite associated with vanadiferous-titaniferous (V-Ti) bearing magnetite bands in Bhakatarhalli chromite mine, Nuggihalli greenstone belt, western Dharwar Craton, Karnataka, India

Hogbomite, a rare exotic mineral, is found to be associated with the vanadiferous–titaniferous (V-Ti) bearing magnetite bands at Bhakatarhalli, Nuggihalli greenstone belt, western Dharwar Craton, India. We report on a second occurrence of hogbomite from the Dharwar craton in Karnataka, which is the sixth documented occurrence of this mineral from India. We evaluate the chemical characteristics of hogbomite and associated Fe-Ti-minerals in an attempt to identify its formation as a primary hydrothermal mineral in a metamorphosed magnetite layer. We report here the presence of hogbomite as a complex oxide of Fe, Mg, Al and Ti with accessory of Zn, V and Sn. Petrographic studies suggest the (V–Ti) bearing magnetite (Mt) contain spinel, hogbomite, chlorite, martite, ilmenite (Il) and minor amounts of diaspore. The hogbomite displays euhedral to subhedral textures, and is up to 250 μm along the grain boundaries of magnetite and ilmenite. In the samples studied, hogbomite is prismatic, irregular and elongated in shape. The genesis of hogbomite in veins between magnetite and ilmenite implies its precipitation from fluids without involving complicated reactions. Several models were proposed for the formation of hogbomite; however, the subject is still debatable.

The crystal chemistry and Raman spectroscopy of kimberlitic (G-9) garnets

Abstract The Raman spectra of G-9 garnets from kimberlites and their relationship to compositional changes, and therefore changes in their cell edges, are evaluated. Increasing CaO and Cr2O3 cause the cell constant to become larger and positions of Raman bands to shift. G-9 garnets have characteristic Raman spectra but the application of Raman spectroscopy as sole identification tool, without the need for micro-analytical techniques, requires further research.