The magma source(s) and Ni-PGE potential of the Ventersdorp large igneous province

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This thesis investigates the magmas source(s) of the Ventersdorp Supergroup on the Kaapvaal Craton in South Africa. The investigation is addressed in three interlinked geochemical studies using new radiogenic Sr-Nd-Hf isotopes, stable in-situ oxygen isotopes, multiple sulphur isotopes, and platinum group elements (PGE), in combination with new trace element analysis and new in-situ major element analysis. The Ventersdorp Supergroup is a Neoarchean sequence comprising volcanic and sedimentary rocks that have been metamorphosed to greenschist facies. Little is known and agreed about the source(s) of the magmas and the petrogenesis of the Ventersdorp Supergroup. The basal group, the Klipriviersberg Group, comprises flood basalts with komatiites of the Westonaria Formation at its base. The whole-rock Sr-Nd-Hf isotopes of the Westonaria komatiites and the in-situ oxygen isotope compositions of fresh clinopyroxene in the komatiites are investigated. Komatiites are high-magnesian magmas that are mainly restricted to the Archean, reflecting the hotter mantle of the Archean that was dominated by mantle plumes. The komatiite investigation shows that the Westonaria komatiites were sourced from a depleted sublithospheric mantle and that the magmas experienced insignificant crustal contamination. This is supported by the mantle compositions of the freshly preserved clinopyroxene grains. The radiogenic isotope compositions of the flood basalts of the Klipriviersberg Group, together with the felsic lavas of the Platberg Group (medial group) and the lavas of the Pniel Group (uppermost group) show that the Ventersdorp Supergroup comprises three large igneous provinces (LIPs) that include the Klipriviersberg LIP, Platberg silicic LIP, and the Allanridge LIP. This isotope study contributes to the current redefinition of the Ventersdorp Supergroup from being one magmatic event to three magmatic events over 70 million years. The study also emphasises the effects of alteration and metamorphism on the Rb-Sr isotope system and the significance of the robust Lu-Hf isotope system. The Ni-PGE potential of the Ventersdorp Supergroup is also investigated by analysis of the PGE contents and multiple sulphur isotope composition of the Klipriviersberg LIP maficultramafic lavas. The Klipriviersberg LIP has low PGE contents and has mass-independent iii sulphur isotope signatures that are readily explained by the sulphide minerals that are associated with secondary features such as veins and amygdales. The investigation shows that the depleted sublithospheric mantle source of the Klipriviersberg LIP was poor in PGE. This study demonstrates the importance and value of multi-sourced data when dealing with altered geological samples
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2023
Magmas source, Geochemical, Radiogenic isotope