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
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Date
2014-12
Authors
Beukes, Jarlen Jocelyn
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Publisher
University of the Free State
Abstract
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.
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Dissertation (M.Sc. (Geology))--University of the Free State, 2014, Mineralogical chemistry, Igneous rocks, Pyroxene, Bushveld Complex (South Africa), Geochemistry -- South Africa -- Mpumalanga