Characterisation of the ultramafic and carbonatite components of the Schiel Alkaline Complex in the Limpopo Province of South Africa

Owing to the poor documentation of the phoscorite-carbonatite association present in the Schiel Complex and the associated economic potential of other known phoscorite-bearing complexes, the Schiel Complex is widely thought to have similar economic potential. This complex is often compared to the lucrative Phalaborwa Complex, as it is thought to have crystallised from a common parental melt, with a similar age of emplacement. This study aims to provide clarity on the physical and chemical characterisation of the various rock types present in the Schiel Complex, with this study being the first petrological investigation based on fresh in-situ samples gathered from 3 borehole cores which were drilled by FOSKOR in the 1960s. The sampled sections of the ultramafics from the Schiel Complex are comprised of end-member rock compositions of either magmatic phoscorites or pyroxenites or metasomatic glimmerites, where gradational contacts between these various end-members produce rock varieties that contain characteristics of one or more end-member types. Carbonatite rocks are present as medium-grained, coarse-grained and banded calcio-carbonatite varieties where the carbonatite rocks are proposed as being the metasomatic medium for glimmerite production. Contrary to previous research, the structure of the ultramafic and carbonatite bodies are present as vein and veinlet structures which seem to originate from a single pipe-like body, from which these rock types intruded into the surrounding syenitic country-rock. Metasomatic alteration of the ultramafic sections of the Schiel Complex also show that the carbonatite rocks must have intruded after some ultramafic magmatism. The presence of the same minerals, with similar chemistries, in both the ultramafic and carbonatite rocks as well as similar REE chondrite-normalised plots show that the various rock types may have originated from a common parental magma, where the accumulation and crystallisation of minerals is the most likely factor in producing the various Schiel Complex rock varieties, causing silicate minerals to be present in the carbonate fraction of the magma, and carbonate minerals in the silicate fraction of the magma. Apatite is the expected rare earth element (REE) mineralising mineral in phoscorites, but is shown to be depleted in REE content in the Schiel Complex due to metasomatic fluid infiltration causing the scavenging and dissipation of REEs. These rocks have also crystallised containing no significant copper-bearing mineralisation, contrary to that which is seen in the Phalaborwa Complex. A comparison of mica minerals between the Schiel Complex rocks and the Phalaborwa Complex rocks show that the two complexes have undergone unique emplacement processes and should not be considered as sister complexes. Efforts to date the glimmerite and carbonatite rocks based on zircon grain U/Pb geochronology proved unsuccessful in constraining the current ages of emplacement provided by previous researchers, but rock relationships show that the current accepted sequence of events cannot be correct, providing scope for further research. This study provides an update on the chemical and physical characteristics, based on the only available sample suite of the ultramafic and carbonatite components, of the Schiel Complex, increasing the depth of documentation of these rare rock types and aiding in refuting some conclusions on the genesis, emplacement and evolution of the Schiel Complex proposed by previous research.Thesis (MSc) -- Faculty of Science, Geology, 202