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The Platreef, at the base of the northern limb of the Bushveld Complex in South Africa, hosts platinum-group element (PGE) mineralization in association with base-metal sulphides (BMS) and platinum-group minerals (PGM). However, whilst a magmatic origin of the stratiform mineralization of the upper Platreef has been widely confirmed, the processes responsible for the PGE and BMS mineralization and metasomatism of the host rocks in the Platreef are still under discussion. In order to contribute to the present discussion, we present an integrated petrographical, mineral-chemical, whole-rock trace- and major-element, sulphur- and neodymium-isotope, study of Platreef footwall clinopyroxenite drill core samples from Overysel, which is located in the northern sector of the northern Bushveld limb. A metasomatic transformation of magmatic pyroxenite units to non-magmatic clinopyroxenite is in accordance with the petrography and whole-rock chemical analysis. The whole-rock data display lower SiO2, FeO, Na2O and Cr (&lt, 1700 ppm), and higher CaO, concentrations in the here-studied footwall Platreef clinopyroxenite samples than primary magmatic Platreef pyroxenite and norite. The presence of capped globular sulphides in some samples, which display differentiation into pyrrhotite and pentlandite in the lower, and chalcopyrite in the upper part, is attributed to the fractional crystallization of a sulphide liquid, and a downward transport of the blebs. In situ sulphur (V-CDT) isotope BMS data show isotopic signatures (&delta, 34S = 0.9 to 3.1 &permil, &Delta, 33S = 0.09 to 0.32&permil, ) close to or within the pristine magmatic range. Elevated (non-zero) &Delta, 33S values are common for Bushveld magmas, indicating contamination by older, presumably crustal sulphur in an early stage chamber, whereas magmatic &delta, 34S values suggest the absence of local crustal contamination during emplacement. This is in accordance with the &epsilon, Nd (2.06 Ga) (chondritic uniform reservoir (CHUR)) values, of &minus, 6.16 to &minus, 6.94, which are similar to those of the magmatic pyroxenite and norite of the Main Zone and the Platreef in the northern sector of the northern Bushveld limb. Base-metal sulphide textures and S&ndash, Se-ratios give evidence for a secondary S-loss during late- to post-magmatic hydrothermal alteration. The textural evidence, as well as the bulk S/Se ratios and sulphide S isotopes studies, suggest that the mineralization in both the less and the pervasively hydrothermally altered clinopyroxenite samples of Overysel are of magmatic origin. This is further supported by the PPGE (Rh, Pt, Pd) concentrations in the BMS and mass-balance calculations, in both of which large proportions of the whole-rock Pd and Rh are hosted by pentlandite, whereas Pt and the IPGE (Os, Ir, Ru) were interpreted to mainly occur in discrete PGM. However, the presence of pentlandite with variable PGE concentrations on the thin section scale may be related to variations in the S content, already at S-saturation during magmatic formation, and/or post-solidification mobilization and redistribution.

The Mogalakwena platinum mine is Anglo American Platinum's flagship opencast operation and has been the focus of numerous optimization studies. The initial outcomes of a geometallurgical programme were reported on in 2013. This paper further describes the development of a geochemical proxy to identify different ore types at the operation and link these with various metallurgical parameters, with the aim of optimizing the value chain. At Mogalakwena, all blast-hole samples are submitted for chemical analysis by XRF prior to blasting, thus allowing the ore to be stockpiled and treated according to grade. This paper demonstrates that it would be possible to use the chemical data to stockpile and process ores according to ore type (rock type and grade), since preliminary results indicate that the metallurgical characteristics (e.g.hardness, flotation characteristics, rheology, etc.) are linked to rock composition, which defines rock types. This investigation formed part of the geometallurgical programme at Mogalakwena. The final objective of the programme focuses on risk mitigation with improved production forecasts and full implementation of the principles of geometallurgy, resulting in the optimization of metallurgical treatment and adding value to the bottom line.

Base-metal sulphides (BMS) are important host minerals for platinum-group elements (PGE) in orthomagmatic Ni-Cu-PGE deposits. The PGE distribution and concentration in the BMS provide important genetic, exploration and mineral processing information. Therefore, detailed mineralogical work and an in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analysis of trace elements were conducted on the BMS of mineralised samples from the Overysel and Sandsloot localities of the Mogalakwena Platinum Mine. These localities access the Platreef of the northern Bushveld Complex. Platinum-group element enrichment trends in the BMS are similar in the Overysel and Sandsloot samples, although the overall PGE concentrations in the Overysel BMS are lower than those in the Sandsloot BMS. A general decoupling of the PGE from the BMS due to fluid activity, as advocated by other authors for the Sandsloot reef lithologies, was not observed in the samples studied. A major portion of the PGE mineralisation is associated with BMS as is indicated by the ‘moderate’ (Overysel r = +0.62) to ‘very good’ (Sandsloot r = +0.79) PPGE-S bulk-rock correlation, whereas an excellent PPGE-Ni correlation (especially at Overysel) indicates that pentlandite is a major PGE-carrier. These results are corroborated by the trace element contents of the BMS and by mass balance calculations. For instance, the LA-ICP-MS BMS study revealed average concentrations of 65 to 165 ppm Pd and 0.17 to 13 ppm Rh at Overysel, and 156 to 412 ppm Pd and 18 to 57 ppm Rh at Sandsloot in pentlandite of both the mineralized pyroxenites. In contrast, chalcopyrite and pyrrhotite are almost devoid of PGE. Platinum and the IPGE (Ir, Os, Ru) are very low in the BMS and, therefore, are thought to be hosted in discrete PGM grains. This conclusion is supported by the mass balance calculations for Pt and by the actual presence of PGM grains in both sample suites. The presence of discrete PGM grains explains the calculated gap between the whole-rock PPGE concentrations and the PPGE concentration in the mineral phases as revealed by the mass balance calculations. The PPGE concentrations in the BMS and the complementary mass balance calculations are in general agreement with those of the Merensky Reef and the UG-2, in both of which large proportions of the whole-rock Pd and Rh are hosted by pentlandite, whereas Pt and the IPGE mainly occur in discrete PGM rather than being hosted by BMS. However, comparison of our results with those of the Merensky Reef and the UG-2 indicates that the average maximum Pd and Rh concentrations in pentlandite are distinctly lower in the Platreef.

Automated mineralogy methods and tools, such as the Mineral Liberation Analyser (MLA) and the QEMSCAN, are now widely used for ore characterization, process design and process optimization. Several case studies published recently demonstrate that large gains can be obtained through grinding and flotation optimization guided by automated mineralogy data. However, since automated mineralogy can only provide the information pointing to where the process gains can be made, it does not directly impact the production gain. Thus the question is often asked: how to value the contribution of automated mineralogy to process improvement at a particular plant. This appears to be a difficult question to answer. On close examination however, it is found that this is essentially a question of the value of information and this is reasonably well documented in various other industries. Hubbard, 2010, in chapter 7 “Measuring the Value of Information”, dealt with exactly this type of problem. The value of information is the reduced risk of an investment and opportunity loss. The methods Hubbard developed can be applied to estimate the value of automated mineralogy, as well as metallurgical test work, both producing information that reduces the risk of investment.This paper first introduces Hubbard’s theory on the value of information and how to measure it. It then applies his methods to estimate the value of automated mineralogy, using Anglo Platinum’s fine grinding project as an example. In the end, a general model is developed to allow the simulation of the value of automated mineralogy in different mining operations constrained by different parameters.

The Merensky Reef of the Bushveld Complex of South Africa is marked by prominent lateral and vertical variations in its geology, platinum group element grade distribution and platinum group mineralogy. At Bafokeng Rasimone Platinum Mine on the western limb of the complex eleven distinct Merensky Reef facies have been identified. The reef facies show different mineral processing behaviour. Detailed geometallurgical characterisation of three reef facies (FW 1A contact, FW 3 pothole and pothole edge reef facies) at Bafokeng Rasimone Platinum Mine has been carried out in an attempt to understand differences in flotation performance. Results illustrate that the FW 1A contact facies has the best Pt and Pd grades and recoveries in the flotation concentrates and the pothole edge facies has the worst. The differences are related not only to mineralogical and textural characteristics of the platinum group minerals and base metal sulphides in the different facies, but also pertain to the geological position and the mineralogy of the host rocks that are introduced as dilution to achieve a realistic mining cut.

Base-metal sulfides in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite, and pyrite were determined in samples from the mineralized portion of four Merensky Reef intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for trace elements (PGE, Ag, and Au). Whole rock trace element analyses were performed on representative samples to obtain mineralogical balances. In Merensky Reef samples from the western Bushveld, both Pt and Pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns. In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite only contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores. Palladium and Rh concentrations in pentlandite reach up to 700 and 130 ppm, respectively, in the samples from the eastern Bushveld, and up to 1,750 ppm Pd and up to 1,000 ppm Rh in samples from the western Bushveld. Only traces of Pt are present in the base-metal sulfides (BMS). Pyrrhotite contains significant though generally low amounts of Ru, Os, and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries only extremely low PGE concentrations. Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10–40 %) of the Os, Ir, and Ru. Chalcopyrite and pyrrhotite usually contain less than 10 % of the whole rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of discrete platinum-group minerals such as cooperite/braggite, sperrylite, moncheite, and isoferroplatinum. Distribution patterns of whole rock Cu, Ni, and S versus whole rock Pd and Pt show commonly distinct offsets. The general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, is Pd in pentlandite → Pd in whole rock → (Cu, Ni, and S). The relationship is not that straightforward in general; some of the reef sequences studied only partially show similar trends or are more complex. In general, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation.

The Gamsberg zinc deposit in the Northern Cape in South Africa has been the subject of a number of studies by Anglo American over the years. Two distinct ore types have been identified, namely pelitic and garnet-magnetite ore. The ore is characterised by relatively high manganese levels. Manganese reporting to the flotation concentrate is problematic for downstream refining. Anglo Research was asked by the exploration division of Anglo Base to study the relationship between the mineralogy and the floatability of the ore from reef intersections obtained from the eastern ore body. The two ore types were found to have different mineralogical characteristics resulting in different flotation responses both in terms of zinc and manganese recoveries and concentrate grades. The presence of the manganese sulphide, alabandite, was also established, and was found to be extremely detrimental to sphalerite flotation. The reasons for this were explained using the surface analysis facility at Anglo Research.

Anglo American Platinum has invested strongly in its mineralogical capability in recent years, taking advantage of the advances in hardware and software in the last decade. Currently, the Anglo American research facility in Johannesburg has 15 MLA and 5 Qemscan instruments in operation. These machines provide support to Anglo American Platinum Concentrator operations. A comprehensive monthly composite sample analyses programme was initiated in 2004 to support a drive to improve metals’ extractions and to enhance product specification—the feedstock to the downstream smelting stage. Over the last 20 years, the ore profile mined by the group’s mining operations has changed significantly with the proportion of UG2 reef and Platreef growing exponentially—the easier to process Merensky reef proportion has dropped to low levels in the same period. UG2 and Platreef Concentrator extractions are compromised by the more ‘difficult’ mineralogy—finer platinum group minerals (PGMs) and larger proportion of dissociation from sulfides. Guided by process mineralogy a change in processing technology has resulted—driven by the pressure to maintain or improve metal-extraction efficiencies. The recent adoption of stirred milling employing ceramic media has proven to be a ‘step change’ process step to meet this objective. A total of 23 stirred milling units have been installed and commissioned in the period 2003–2009 at the majority of Anglo American Platinum Concentrator plants. The development of a large-scale IsaMill™ unit—10,000 L with 3 MW of installed drive power—was a milestone decision allowing the economic mainstream application of stirred milling to follow shortly afterwards when a suitable ceramic grinding medium was identified. The mill scale-up was a successful partnering initiative with Anglo American Platinum working closely with Xstrata Technology of Australia and the manufacturers Netzsch Feinmahltechnik GmbH. A period of flow-sheet evolution guided by process mineralogy in the last decade has led to a new process flowsheet at the UG2 Concentrators. The impact of this flowsheet has led to a step change in PGM metal recoveries. Since the mainstream regrind circuit additions or ‘MIG’ stirred milling applications have settled in after an early operational ramp-up and learning period, UG2 recoveries at the two main production units of Anglo American Platinum—the Rustenburg and Amandelbult mining sites—have increased by more than 5%. This is the largest step change increase in these operations’ history. Further improvement and flow-sheet modification guided by process mineralogy are confidently expected in the next decade.