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The Platreef magma event at the world-class Turfspruit Ni-Cu-PGE deposit: implications for mineralisation processes and the Bushveld Complex stratigraphy

Stephenson, Hannah 2018. The Platreef magma event at the world-class Turfspruit Ni-Cu-PGE deposit: implications for mineralisation processes and the Bushveld Complex stratigraphy. PhD Thesis, Cardiff University.
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Abstract

The Platreef Ni-Cu-PGE deposit as identified on the farm Turfspruit, northern limb of the Bushveld Complex, extends westwards and down-dip from the surface exposure. The orebody transitions with increasing depth from highly contaminated ‘Platreef-style’ mineralisation to a dominant magmatic stratigraphy hosting ‘Merensky-style’ mineralisation associated with persistent chromitites in the Flatreef and Deep Platreef. Four drill holes sampling each sector of the orebody have been examined to determine; the origin of the stratigraphy, the controls on the Ni-Cu-PGE mineralisation and the relationship of the Platreef to the wider Bushveld Complex. Orthomagmatic processes, operating at depth in a staging chamber, are proposed to be dominant in generating the high PGE tenors in sulphides (> 600 ppm) and a magmatic platinum-group mineral (PGM) assemblage consisting of; Pt/Pd sulphides, Pt-Fe alloys, Pt-tellurides and laurite. This is supported by magmatic δ34S values (av. +2 ‰) determined in sulphides in unaltered pyroxenites. Two stages of crustal contamination affected the development of the Platreef orebody on Turfspruit: (1) early-stage contamination homogenised in the parental magma at depth in a staging chamber; (2) in situ contamination through interaction with the footwall sediments of the Duitschland Formation (DF). The lithostratigraphy of the DF varies between the carbonate-dominated Upper DF and the shale-dominated Lower DF; which, results in mineralogical and geochemical heterogeneity across the deposit. This variability is especially evident in the δ34S values with contaminated δ34S signatures identified throughout the shallow Platreef (up to +19.6‰) and a systematic increase to heavy isotopic values (> +10 ‰) with depth is recorded in the Deep Platreef; in contrast, sulphides in the Flatreef give mainly magmatic δ34S values. The presence of elevated δ34S values in the Deep Platreef corresponds with a progressive decoupling of PGE grade from sulphides and the introduction of arsenide and antimonide PGMs. The stratigraphic position of the Platreef, directly below the Main Zone, has led previous workers to conclude that it correlates with the Upper Critical Zone. This work, however, suggests that the relationship is more complex. The magmatic lineage of the Platreef across Turfspruit represents a ‘hybrid’ composition with contrasting Cr/MgO and Ce/Sm ratios, which consistently plot within the Critical and Main Zone ranges, respectively. Furthermore, there is no systematic increase in the Pt/Pd ratio with depth; this represents a key test of models proposing their lateral continuity. The northern limb is proposed as a discrete system, part of a larger network of sills and magma chambers, operating independently but connected at depth to the wider Bushveld Complex.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Ocean Sciences
Funders: Ivanplats Pty Ltd
Last Modified: 02 Aug 2019 15:06
URI: http://orca-mwe.cf.ac.uk/id/eprint/124662

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