A study of epigenetic mineralisation in the Central Zone of the Damara Orogen, Namibia with special reference to gold, tungsten, tin and rare earth elements

Abstract

Epigenetic, rare earth element, gold, tin and tungsten, mineralisation in the NE-trending, intracratonic branch of the deeply eroded, Late Proterozoic/Early Palaeozoic, Pan-African Damara Orogen is hosted by meta-arkoses, marbles and metaturbidites in the magmatic arc (Central Zone; CZ) of the orogen, a tectonostratigraphic entity that is characterised by multiple deformation, greenschist/amphibolite-facies, low-pressure/high-temperature metamorphism and numerous granitic and pegmatitic intrusions. This thesis integrates regional and detailed geological mapping with petrographic studies, whole rock geochemistry (obtained by X-ray fluorescence spectrometry, atomic absorption spectrometry, fire assay and infra-red spectroscopy), mineral chemistry studies (electron microprobe and X-ray diffraction) and geochronological work (Rb-Sr whole-rock and mineral age determinations and Pb-Pb model ages) to examine epigenetic mineralisation in the Usakos-Karibib-Omaruru district, central Namibia. The investigated area straddles the magnetically defined Omaruru Lineament, which divides the CZ into southern (SCZ) and northern (NCZ) portions. Mapping and lithostratigraphic work support the geophysical evidence for this subdivision: in the SCZ, predominantly continental and shallow marine Damaran metasediments are floored by a 1.7-2.0 Ga granite gneiss basement inlier, that is transected by numerous ENE-trending aurif erous megashears. These structures controlled late Proterozoic mafic dyke emplacement, Damaran rifting and sedimentation, alkaline volcanism and the localisation of hydrothermal fluids in the overlying cover rocks. In the NCZ, where basement inliers are not exposed, the metamorphosed equivalents of Damaran shallow marine carbonates are overlain by deeper water sediments and turbidites. Sedimentation was controlled by rifting, facies belts being oriented parallel to the axis of the orogen. Felsic and mafic volcanic rocks comprise only a fraction (< 5 % ) of the Damara Sequence.