Fuchsloch, Warrick C2019-05-222019-05-222018Fuchsloch, Warrick Clinton (2018) Pegmatites of the Cape Cross-Uis pegmatite belt, Namibia: structural, mineralogical, geochemical and mineral chemical characterisation with implications for petrogenesis and mineralisation,University of the Witwatersrand, Johannesburg,https://hdl.handle.net/10539/27136https://hdl.handle.net/10539/27136A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg, 2018The Neoproterozoic Pan-African Damara Orogen in Namibia is host to a variety of mineral resources, most occurring within the metasedimentary and igneous lithologies of the inland branch of the orogen, known as the Damara Belt. Examples of economic deposits include base metals, semi-precious stones and rare-metal pegmatites hosting minerals such as cassiterite, columbitegroup minerals, petalite, spodumene, lepidolite and U-bearing phases. The Cape Cross-Uis pegmatite belt is one of several pegmatite belts within the Damara Belt, located in the Northern tectonostratigraphic Zone. The LCT-type pegmatites of the belt were divided into three different types based on characteristics, mineralogy and whole-rock geochemistry. The types from most abundant to least are: 1) metasediment-hosted, Nb-Ta-Snbearing, unzoned pegmatites, 2) granite-hosted, garnet-tourmaline-bearing, crudely zoned pegmatites and 3) metasediment-hosted, Li-bearing, complexly zoned pegmatites. To the southeast of Uis, the Nainais pegmatites occur within the Nainais-Kahero pegmatite belt and were studied for comparison to the Cape Cross-Uis pegmatites. They are similar in almost all aspects to the NbTa-Sn-bearing type pegmatites and were grouped together accordingly. Field characteristics and structural analyses of the pegmatites indicate an overall northeast trend of pegmatites in line with the belt-wide northeast regional lineation. Furthermore, the pegmatites intrude various Damaran structures, however, they are not co-genetic with these structures and cross-cutting relationships and a lack of micro- or macroscopic deformational features within pegmatites indicate an exclusively post-orogenic emplacement age. Regional mapping of the pegmatite types also indicates that there is no apparent mineralogical or whole-rock geochemical spatial relationship of pegmatites with resident granites. Whole-rock geochemistry of fine-grained border zones within pegmatites reveals that fractionation was the dominant process by which incompatible elements within the pegmatitic melt were enriched. Assimilation was discounted as a potential process in which elements such as Sn were incorporated into the pegmatitic magma as assimilation indicators such as MgO+FeO-CaO do not correlate with any trace elements. In addition, there is a mass balance problem since biotite schist country rocks on average show 70 ppm Sn and pegmatites may reach up to 1 wt% Sn in late-stage alteration zones such as greisens. Furthermore, the dominant morphologies of greisens indicate that late-stage, Cl-complexed, Sn- and possibly Ta-Nb-enriched fluids were trapped by larger feldspar crystals and therefore could not have remobilised Sn from country rocks in a leaching, circulating, hydrothermal system. Whole-rock geochemistry also shows that pegmatites and granites either follow completely different fractionation trends or overlap in fractionation trends, suggesting a decoupling of elements and preclusion from being coeval. Rare-earth elements confirm that a granite-pegmatite cogenetic relationship is unlikely since REE values of pegmatites (0.1-5 chondrite normalised) are two orders of magnitude lower than granites (10-100 chondrite normalised) where one would expect concentrations of incompatible elements in pegmatites to be far greater than in granites if a granite-parent, pegmatite-daughter type relationship is implied. In addition, a lack of pegmatite spatial association with granites supports this hypothesis. Fractionation trends and minor element substitution trends in CGM suggest that mineral competition for elements within the melt produced Fe-rich CGM phases over a wide range of T/(Ta+Nb) values (fractionation indicator in CGM; 0.03-0.96) which supports an LCT, Petalitesubtype pegmatite signature. Furthermore, the CGM mineral chemical data and textures of zonation patterns in CGM may indicate that Ta, Nb and Sn were complexed and subsequently enriched in a late-stage exsolved pegmatitic aqueous fluid, which led to economic mineralisation of cassiterite, CGM and tapiolite in greisenised zones within pegmatites. Additional evidence is shown by the elevated values of Ta, Nb and Sn in host rock metasomatic contact zones which are abundant in tourmaline. Tourmaline mineral chemistry reveals that assimilation played a negligible role in the element diversity of the pegmatites, supporting whole-rock geochemical data. Tourmaline compositions indicate that the pegmatite magmas went through a phase of mafic depletion and Fe-enrichment in the early stages of crystallisation. During an intermediate stage of magma evolution, early CGM stabilised and competed with tourmaline for Fe in the magma, subsequently decreasing Fe contents in tourmaline and increasing Al, Li and Mn (elbaitic compositions). Highly mafic tourmaline compositions from pegmatites are rare and most likely indicate a crustal anatexis petrogenetic model for the pegmatites of the Cape Cross-Uis pegmatite belt, confirming whole-rock geochemical and field observationOnline resource (xxiv, 287 leaves)enPegmatitesMineralogyThesis