Local-to micro-scale structural controls on shear-zone-hosted gold mineralisation, Ngubevu area, Natal Thrust Front, Tugela Terrane, South Africa.

Abstract

The research described in this dissertation focused on shear-zone-hosted precious and base metal mineralisation in the Ngubevu area, in the Natal Thrust Front of the Tugela Terrane in the Natal Metamorphic Province of South Africa. This province forms part of a 400-km-wide metamorphic belt referred to as the Namaqua-Natal Metamorphic Province, which stretches from the Northern Cape on the west coast to KwaZulu-Natal on the east coast of the country. Unlike the western Namaqualand Metamorphic Province, which is well-explored and well-exploited for several mineral deposits, the Natal Metamorphic Province is relatively poorly explored. One reason for this is that the nature of structural controls on mineralisation in the Natal Metamorphic Province is poorly understood. In the research described in this dissertation, known occurrences of shear-zonehosted precious and base metal deposits in the Golden Eagle and Champion Mines in the Ngubevu area, which have been described in the literature, were studied in the micro-scale, and the findings obtained were integrated with existing knowledge of regional- to local-scale deformation in the Tugela Terrane, in order to deduce the structural controls of such mineralisation in the area. The micro-scale data, studied by spatial analytical methods (i.e., point pattern analysis, Fry analysis, fractal analysis and shape analysis) show that the mineralisation in the area operated in at least two spatial scales, within each thin section sample, of at most 0.005 μm and at least 0.002 μm and that, within each thin section sample, the occurrence of shear-zone-hosted precious and base metal deposits is non-random as they exhibit clustered and/or regular spatial distributions. Microfractures in the area generally show major northerly trends in their alignments. Likewise, microscale mineral distributions show NNE–SSW and NNW–SSE trends in their orientations. These findings suggest that N–NE-directed compression controlled the occurrence and distribution of shear-zone-hosted precious and base metal mineralisation in the Ngubevu area. This N–NEdirected compression and thrusting during deformation (D1) controlled the circulation and concentration of mineralising hydrothermal fluids in the Natal Thrust Front. These hydrothermal fluids formed quartz veins along nearly E–W-trending foliations resulting in mineralisation in the clustered and regular spatial patterns observed in the micro-scale.