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Jones, Rhian Eleri
Languages: English
Types: Doctoral thesis
Subjects: QE
The Platreef, located in the northern limb of the Bushveld Complex in South Africa, is a world-class Ni-Cu-PGE deposit. The complexity of the deposit has meant that despite the numerous studies, developing an accepted genetic model to account for the variations observed has been difficult. While some authors have suggested that it is part of the Upper Critical Zone, correlating it to the Merensky Reef, others have suggested that the Platreef is unrelated to mineralisation found elsewhere in the Bushveld Complex. The model tested is the multiple staging chamber model developed by McDonald and Holwell, that proposes that the parental magma was upgraded in PGE (plus Ni and Cu) prior to emplacement.\ud Key to testing this model has been the analysis of immiscible sulphide inclusions trapped within chromite grains, believed to represent the early parental magma. Analysis has shown that they contain high PGE tenors, significant semi-metal (Bi, Te and As) content and the low S/Se ratios of the inclusions suggest a mantle source. Interaction of the sulphide liquid with multiple batches of magma in the staging chamber is proposed to have enabled enrichment to occur prior to emplacement through a process known as multi-stage dissolution upgrading.\ud The analysis of chromite grains from the three study farms has shown that the variation in chromite composition is dependent on host lithology and the location of the sample along strike of the Platreef. Some correlation can be made with chromites from the UG2 but Platreef chromites cannot be directly correlated to those from the Merensky Reef.\ud Investigation of PGE concentrations within the BMS from Zwartfontein has shown a strong association between PGE and BMS and that the distribution of PGE is consistent with fractional crystallisation of a sulphide liquid.\ud The PGM study has shown that variation along strike and down dip of the Platreef is not strictly controlled by footwall lithologies as previously proposed. Variation is suggested to be the result of differing temperatures and ƒO2 conditions due to the proximity around proposed feeder zones.\ud In order to further test the staging chamber model, S isotope analysis should be carried out on the sulphide inclusions to ascertain if a magmatic signature is present. In addition, further support to the model may be achieved by examining other Lower Zone bodies for chalcophile element depletion.

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