Unconventional olivine-rich cumulates, magma dynamics and development of platinum-group element mineralisation in the Main Zone of the northern Bushveld Complex

Kennedy, Bianca 2018. Unconventional olivine-rich cumulates, magma dynamics and development of platinum-group element mineralisation in the Main Zone of the northern Bushveld Complex. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The Troctolite Unit (TU) is a distinctive PGE-rich olivine horizon in the upper Main Zone (MZ) of the northern Bushveld Complex. The TU is unique to the central sector of the northern limb and is yet to be noted elsewhere in the Bushveld. This study is the first of its kind on the TU and describes the lithology, petrology, mineralogy, texture relationships, geochemistry and PGE enrichment and mineralisation of this poorly known part of the northern Bushveld. Samples were sourced from the VSF2 and BV1 boreholes and MZ samples were obtained from outcrop in the same area as the VSF2 borehole. Samples were analysed using various techniques to characterise the TU from a chemical to a textural scale and so gain a better understanding as to how the unit formed. This study showed that the TU is much thicker (> 250 m) than previously thought, can be subdivided into 4 subunits and lacks ‘conventional’ orthomagmatic indicators such as well defined ‘cyclic units’ of harzburgite-troctolite-anorthosite. There are no thermal or intrusive contacts with the surrounding MZ that might indicate a younger intrusion and transitions with the MZ appear to be gradual. The ultramafic portions of the lower TU are unusually poor in Cr and the TU as a whole lacks any chromite. There are a number of unusual mineral associations and textures throughout the TU that distinguishes it from the MZ above and below, including: 1) interstitial olivine (with single and polyphase- and/or fluid inclusions), 2) rounded and zoned plagioclase with embayed contacts, 3) a variety of textures (mottles, replacement and reaction textures) and, 4) an abundance of hydrous minerals (amphibole, biotite, apatite) throughout the TU. Fluid inclusions are widespread in the TU and found in olivine, plagioclase and quartz. There are increases in An# and Mg# ratios between the MZ and the TU but these remain nearly constant through the TU and do not significant fractionate between ultramafic and felsic units. Despite the high An# and Mg# values the TU has Cr/MgO ratios consistently <80, akin to MZ (<60) rather than Critical Zone (CZ) values (>80). Incompatible trace element ratios likewise show little differences from MZ values. Trace element studies show that TU olivine is enriched in Ni and moderately enriched in Cr relative for their Fo#. Parts of the TU contain significant PGE enrichment (3PGE>1 ppm) that is generally associated with less mafic lithologies (anorthosite>troctolite>harzburgite>pyroxenite) and concentrated at the changeover between leucocratic-mafic lithologies. The PGE in the TU show stronger fractionation of PPGE/IPGE than found in the CZ or Platreef. Fractionation of Pt and Pd is apparent between depleted (Pt>Pd) and enriched samples (Pd>Pt). PGE enrichment decouples with Cr enrichment and tends to occur stratigraphically before elevated Cr grades. PGE enrichment shows no trend or association with Cr, Cu or Ni. Au concentrations in the TU are surprisingly low. Platinum-group minerals (PGM) are Pd>Pt dominant, with limited Rh phases. The type of PGM, size (<100 nm to 100 µm), morphology and association varies in each high grade zone and there is no obvious trend or association with lithology, BMS, mineralogy, texture or grade. The majority of PGM are mineralised in the BMS-silicate alteration zones with limited PGM enclosed in BMS. Some of the BMS have significant Pd concentrations. These have been shown to be hydrothermal in origin based on lack of IPGE and very high S/Se ratios. BMS can be split into at least two assemblages based on the bimodal relationship of S/Se and IPGE; magmatic-hydrothermal and hydrothermal. Results indicate that the TU could not have formed from new magma input. Rather it is suggested that the TU formed syn-MZ formation by flux melting of MZ proto-cumulates. MZ cumulates at this point are neither consolidated nor a mush and is proposed to have some kind of layering or stratification. The source of these fluids are the interstitial fluids of the underlying MZ cumulates as well as fluids being driven off sedimentary xenoliths percolating upwards. The introduction of fluids changes the localised activity of the system expanding the stability field of olivine and facilitating unconventional mineral changes and phase associations. The fluxing fluids are the most obvious source for the extreme and localised PGE enrichment. The fluids scavenged PGE, S and other metals, from MZ proto-cumulates and possibly the Platreef and so formed hydrothermal BMS, oxidised the magmatic BMS present in the MZ and leached the BMS to form PGM. The bimodal source of PGE i.e. magmatic and later hydrothermal is evident in the non-distinct PGE fractionation patterns, range in PGE concentration and ratios of BMS and low temperature PGM and BMS.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Uncontrolled Keywords:	Platinum-group elements, Bushveld Complex, northern limb, Troctolite Unit, BMS, PGM, PGE, flux melting, fluids
Funders:	Cardiff University School of Earth and Ocean Sciences, Cardiff University College ofPhysical Sciences and Engineering
Date of First Compliant Deposit:	13 June 2019
Last Modified:	19 Apr 2023 08:25
URI:	https://orca.cardiff.ac.uk/id/eprint/123415

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