Platinum-group elements within the Merensky reef, Western Limb, Bushveld complex: results of a high resolution mineralogical and geochemical study
The formation of the Merensky reef still remains controversial despite of its economic importance and decades of research. Remaining questions like, the variability of the Merensky reef and how this effects the platinum distribution (e.g. whether the grade or distribution of platinum is influenced by the presence or not of pegmatoidal Merensky reef, whether PGE distribution are more associated with sulphides or with chromites) are still unanswered. Data were generated in order to address some of these questions. This study was undertaken in the south-western portion of the Western lobe of the Bushveld Complex on intersections of pegmatoidal and non-pegmatoidal Merensky reef from Impala Platinum Mine. The two non-pegmatoidal reefs correspond to the normal Merensky ‘A’ type reef and the pegmatoidal reef corresponds to the Merensky ‘B’ type reef according to the classification by Leeb-du Toit (1986). The core was analysed in 2 cm intervals. Samples were analysed by optical microscopy. Quantitative analysis was done using scanning electron microproscopy and electron microprobe analysis. Major elements and trace elements were determined by using ICP-MS (inductively coupled plasma mass spectrometry). Platinum-group elements (PGE) were determined by Ni-S fire assay with an ICP-MS finish and sulphur by an Eltra Infrared Analyser. Macroscopic investigation of the drillcores identified an anorthositic footwall with an overlying basal chromitite stringer and a pyroxenite hangingwall for the two non-pegmatoidal reefs. The pegmatoidal reef consists of an anorthositic footwall, a bottom chromitite stringer, a pegmatoidal layer with an overlying top chromitite stringer and a pyroxenite hangingwall. Microscope analysis showed one sulphide inclusion visible in a chromitite grain which displayed a negative crystal shape imposed by the crystal structure of the host chromite. This could indicate the presence of sulphide liquid in the system at a very early stage. This might be an indication of PGE accumulation in a deeper staging chamber. There is a correlation between the bottom chromitite stringer from the pegmatoidal Merensky reef and the single basal chromitite stringer from the non-pegmatoidal reef. There is also a Cr2O3 correlation between the single basal chromitite stringer from the non-pegmatoidal reef and the top chromitite stringer from the pegmatoidal reef. Whole rock geochemistry is strongly governed by the mutual influence and proportion of co-precipitating minerals competing for the same major cations like chromium, iron, aluminium and magnesium. Whole rock Mg# is the lowest in the chromitite layer, which is in contrast with what is seen in the mineral chemistry where Mg# is more primitive in the chromitite layer. This could be due to the subsolidus effect where the orthopyroxene within the chromitite layer is more enriched in Mg, due to the exchange with the chromite The general evolution from bottom to top of the pegmatoidal reef is not so clear, with considerable irregularity. Whole rock PGE content indicated that there is a close relationship between chromium and PGE enrichment, with the highest PGE content associated with the basal chromitite stringer in the case of the non-pegmatoidal reef and with the top chromitite stringer in the case of the pegmatoidal reef. Extremely high Pt/Pd ratios of up to 8.2 and Pt up to 40 ppm in the non-pegmatoidal chromitite stringer is noted but could be an artefact of the small sample sizes used. Whether some of the results found are a local or a general characteristic, can only be determined by analysing more sections. The results of this study indicate that a combination of geochemical processes and multiple replenishments of magma with subsequent processes such as: crystallization of PGE as PGM, (Tredoux et al., 1995), collection of PGE by an immiscible sulphide liquid (Campbell et al., 1983 and Barnes and Maier, 2002b) and perhaps redistribution of PGE by late magmatic/hydrothermal fluids (Boudreau & Meurer, 1999). Trends of a deeper staging chamber as suggested by Hutchinson et al (2015) is supported by several of the observations made in this study and these processes could be responsible for the formation of the pegmatoidal and non-pegmatoidal Merensky reefs.
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