Masters Degrees (Geology)
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Browsing Masters Degrees (Geology) by Subject "Bushveld Complex (South Africa)"
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Item Open Access Geochemical and mineralogical investigation of the Merensky Reef and its noritic hangingwall at Two Rivers Platinum Mine and Eerste Geluk, Eastern Bushveld, with special reference to the PGE distribution and cryptic variation of the mineral chemistry(University of the Free State, 2014-12) Beukes, Jarlen Jocelyn; Gauert, C. D. K.This research study focuses on the unusual occurrence of noritic lenses (termed “brown sugar norite” by mine geologists), within the pyroxenite of the Merensky Reef as well as its hanging wall at Two Rivers Platinum Mine, situated on the southern sector of the eastern limb of the Bushveld Complex. The primary purpose of this study is to determine the origin of these noritic lenses (hereafter referred to as BSN) and their influence on PGE distribution within the Merensky Reef. This study will also attempt to characterise the cumulate rocks associated with the Merensky Reef unit through geochemistry and mineralogy. Furthermore, a comparison with similar rock types of different genetic facies types of the same stratigraphy north of the Steelpoort fault at Eerste Geluk will be performed. The BSN is a fine-grained mela-gabbronorite and only occurs where the upper chromite stringer of the Merensky Reef unit is present. Orthopyroxene is the dominant cumulate phase in both the BSN and pyroxenite of the MR followed by interstitial plagioclase. Clinopyroxene occurs mostly as an exsolved lamellae phase within orthopyroxene and as intermittent rims around orthopyroxene. This can be attributed to exsolution of the Ca end member during decrease in temperature and compositional change of the melt during cooling. Some of the chromite crystals present in the pyroxenite are well-rounded possibly indicating magmatic erosion. Textural features of minerals from the different rock types such as plagioclase inclusions within orthopyroxenes as well as triple junctions of orthopyroxene crystals suggest disequilibrium and recrystallization of mineral phases respectively. The dominant mineral phases control most of the chemical composition of the rocks in accordance with their mineral proportions as they concentrate most of the lithophile elements. The main difference between the Merensky reef at Two Rivers Platinum and the Merensky reef north of the area at the farm Eerste Geluk is the absence of brown sugar norite at the latter. Also, the minerals of the Eerste Geluk Merensky lithologies display a higher degree of alteration or deformation and a higher concentration of hydrous minerals. Eerste Geluk is situated proximal to the Steelpoort fault which suggests that the rocks in the area were affected by faulting and late hydrothermal fluids which resulted in the alteration of minerals. Strontium isotope analyses of five representative samples of the Merensky interval at TRP yielded 87Sr/86Sr ratios typical of Critical Zone magma. Though both the pyroxenite and BSN have 87Sr/86Sr ratios representative of Critical Zone magma, the BSN has a lower ratio relative to pyroxenite. This suggests that it formed from a more primitive magma. Whole rock MgO content is higher in the BSN, ranging between 24-28 wt. % compared to the 21-23 wt. % MgO found within the surrounding Merensky reef pyroxenite. This provides some evidence suggesting that the BSN formed from a more primitive magma. EPMA results show cryptic vertical variation of En content, Al2O3, TiO2 and MnO in orthopyroxene and An content variation in plagioclase. This indicates fractionation and replenishment of magma. Base metal sulphides and associated PGMs occur disseminated throughout the Merensky pyroxenite interval. The PGMs analysed by EPMA are relatively enriched in Pt but are poor in Pd and Rh. These findings are consistent with the ICP-MS study done on the base metal sulphides. Textural features such as zonation of these PGMs suggest the action of late stage magmatic processes. The occurrence of the BSN has not influenced the content of the PGE mineralisation. It contains relatively little if any base metal sulphides and PGMs. It is therefore suggested that the BMS and PGM saturation was not affected during crystallization of BSN. With regards to emplacement, it is suggested that the BSN formed prior to the MR and that a magmatic erosion caused by the injection of the new MR magma may have disturbed the previously formed BSN layer. It thus resulted in isolated lenses of relict and primitive BSN. The BSN is not laterally consistent in the TRP area and may be attributed to this phenomena. The absence of BSN in other mines of the Bushveld may be due to this reason, or the occurrence of the BSN has been overlooked due to its similarities to MR pyroxenite.Item Open Access Platinum-group elements within the Merensky reef, Western Limb, Bushveld complex: results of a high resolution mineralogical and geochemical study(University of the Free State, 2016) Magson, Justine; Tredoux, M.; Roelofse, F.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.