Sperrylite

Abstract: The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation, a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast, Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals, including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals. We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite.

Abstract: Platinum-group minerals are present as inclusions in both disseminated and massive chromite but are modally more abundant in massive chromite, in the ultramafic zone of the Stillwater Complex. They are small, generally less than 20 mu in diameter and characterized by euhedral and subhedral habits. Their distribution is random (not crystallographically controlled) within the chromite host. The platinum-group mineral inclusions may occur with silicate and base metal sulfide inclusions in an unfractured chromite host. All identified inclusions in unfractured chromite are laurite, (Ru, Os, Ir)S 2 . The Ru/(Ru + Os + Ir)(= Ruranges from 0.92 to 0.83 with Ru Os substitution more extensive than Ru Ir. Both Pd and Rh abundances in laurite are less than 2 wt percent. No Pt was detected. At present, the chemical data do not indicate a correlation between stratigraphic height and the Ru ratio.Several Pt-, Pd-, and Rh-bearing interstitial minerals and a single unusual inclusion phase have been identified in the A chromite seam at West Fork and the B seam at Mountain View. The interstitial platinum-group minerals are sperrylite and isoferroplatinum, whereas the unusual inclusion is a polyphase intergrowth of Pt-Pd-Ni arsenides with chalcopyrite and pent-landite.Whole-rock platinum-group element analyses of the chromite seams range from a few ppb to nearly 16 ppm with Pd > Pt > Rh > Ru > Ir (Os was not analyzed). This is in opposition to the data for the inclusions which show high Ru, Ir, and Os, low Rh and Pd, and no detectable Pt.Thus, in the Stillwater magma chamber, Ru-, Ir-, and Os-bearing minerals appear to have precipitated early and have been included in chromite. In contrast, Pd, Pt, and Rh apparently remained in solution in the magma and are in phases that precipitated late and are interstitial to chromite.

Abstract: The Lac des Iles Complex hosts Pd deposits with unusually high Pd/Pt and Pd/Ir ratios. These high ratios have been attributed to the introduction of Pd by late magmatic–hydrothermal fluids. The presence of hydrous silicates and low-temperature sulfide mineral assemblages has been advanced as evidence for fluid overprint. This idea has been investigated by documenting the change in silicate, sulfide, and platinum-group mineralogy and texture in samples from the deposit ranging from fresh gabbronorite to extensively altered chlorite–actinolite schist. In addition, whole-rock analysis combined with laser ablation analysis of the sulfides was used to determine which phase controlled each of the PGE. In fresh gabbronorite the sulfide assemblage is igneous (pyrrhotite–pentlandite–chalcopyrite) and Pd is present mainly in pentlandite and kotulskite (PdTe) inclusions associated with the sulfide minerals. All other PGE (except Pt) are present in solid solution in pyrrhotite and pentlandite. Platinum is present as sperrylite (PtAs2) and montcheite (PtTe2) inclusions associated with the sulfide minerals. These observations suggest that the PGE in the fresh gabbronorite were all collected by magmatic sulfide liquid. In the chlorite–actinolite schist the sulfide assemblage is a low-temperature assemblage consisting of chalcopyrite, pyrite ± millerite. A minor amount of palladium is present in millerite, but most is present as bismuth-rich kotulskite and Pd arsenides both of which are associated with hydrous silicate. All other PGE (except Pt) are hosted by pyrite. A minor amount of Pt is present in pyrite, but most is found as sperrylite and montcheite associated with hydrous silicate minerals. The pyrite could have formed by Fe-loss from pyrrhotite (resulting in the pyrite inheriting Os, Ir, Ru, and Rh) and Fe-loss from pentlandite to form millerite (resulting in the millerite inheriting some Pd). The PGE concentrations of the whole rock show there is no direct correlation between the sulfide assemblage, the degree of alteration of the silicate minerals and ore grade. Thus, although fluids have clearly altered the sulfide-, silicate-, and platinum-group mineralogy and their textures, it is not clear that these fluids have changed the PGE content of the rocks.

Abstract: 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.