Mineralogical and geochemical variations in the UG2 reef at Booysendal and Zondereinde mines, with implications for beneficiation of PGM
Thesis (MSc)--Stellenbosch University, 2016.
ENGLISH ABSTRACT: The layered intrusion of the Bushveld Complex, South Africa hosts the world’s largest concentration of platinum group elements (PGE), which are principally mined from three mineralised horizons namely, the Merensky reef, the Upper Group Two (UG2) reef and the Platreef. The PGE contents of these horizons are conventionally beneficiated via comminution, froth flotation and smelting techniques. The mineralogical and geochemical characteristics of the platiniferous reefs, and any variations thereof, are known to be intimately connected with the performance of the above-mentioned techniques. In particular, the chromite-rich UG2 reef presents a variety of complications, for example the ore/gangue relationships, mineral chemistry and textural characteristics, which can impact upon its beneficiation potential. This study was primarily aimed at evaluating and constraining the mineralogical and geochemical characteristics of two UG2 reef mining cuts from Booysendal mine (eastern Bushveld Complex) and Zondereinde mine (western Bushveld Complex). The results of comprehensive petrographic (2-D and 3-D), compositional and geochemical investigations were then placed within the context of the milling and flotation process in order to comment on the impact that any variability might have during the beneficiation of the PGE contents. In addition to these aims, the validity of results obtained from 3-D microfocus X-Ray Computed Tomography (XCT) were assessed within the context of this study. In this study it was found that the main ore zone from each UG2 reef sample is characterised by cumulate chromite grains with variable characteristics depending on the grain size, composition, degree of compaction and grain shape. The Zondereinde UG2 reef in particular was interpreted as having experienced a significant degree of compaction due to the lack of intercumulate silicate phases within the chromitite units. All mineral phases within both UG2 reef sample sets exhibit variable alteration features which results in the replacement of primary silicates by hydrous silicate minerals. PGE grade is commonly distributed with a top- and bottom-loaded profile in the main chromitite layers. Some PGE exist as platinum group minerals (PGM) with average grain sizes of less than 3 μm, associated predominantly with nickel and copper sulphide minerals. The Booysendal UG2 reef is dominated by a PGE-sulphide assemblage whereas Zondereinde is dominated by PGE-alloys. The mineralogical and geochemical characteristics further described in this study can be utilised to refine the milling regime and flotation parameters in order to maximise plant efficiencies. It is suggested that the Booysendal UG2 reef’s lesser degree of compaction within the chromitite horizons and predominance of PGE-sulphide mineral compositions may yield better liberation and faster recoveries of PGM contents when compared to the Zondereinde UG2 reef. In the case of the Zondereinde UG2 reef, the homogeneity of chromite textures might serve to simplify the refinement of milling regimes so as to not over- or under-grind the reef contents. This, coupled with a strong association of PGM with comparably higher sulphide mineral proportions will benefit the beneficiation process. The interpretation of results obtained from 3-D XCT proved the technique to be a powerful tool in terms of the broad-scale characterisation of chromite textures and PGM distribution. The technique however suffers from resolution limitations when attempting to accurately discern individual PGM grains. This is interpreted to be an artefact of the typically small grain size of PGM from the UG2 reef.
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