Gold mineralisation in the Pietersburg Greenstone Belt, Eersteling Gold Mine

Abstract

The Eersterling Gold Mine was South Africa’s first hard rock commercial gold mine, and it is situated in the Pietersburg Greenstone Belt (PGB). The gold mineralisation in this Archaean greenstone belt is sediment and shear zone hosted. Borehole samples were taken from multiple gold reefs; Pienaar, Girlie – Pienaar, Maltz and Zandrivier – Arsenopyrite. The material was described, documented, and analysed to determine the occurrences of gold mineralisation and associated sulphides. The results collected from the various methodologies illustrated that the different reefs experienced multiple phases of deformation and mineralisation which was associated with a hydrothermal system – that produced quartz and carbonate veins. The different sulphides found within these reefs include pyrite, chalcopyrite, galena, sphalerite, pyrrhotite, and arsenopyrite. These sulphides occur in massive, net textured and disseminated forms and indicate phases of remobilisation. Visible gold was noted using petrological techniques from Z01 samples taken from the Zandrivier Reef. Elemental maps from TESCAN Scanning Electron Microscope (SEM) mineralogical analysis of the Pienaar and Girlie – Pienaar Reefs indicate enrichment of invisible gold within pyrite, pyrrhotite and chalcopyrite. However, this may be an analytical artifact because the traces of gold are far too uniform to be real. This observation requires further analysis which is beyond the scope of this research project. In contrast, gold in the Zandrivier Reef mainly occurs as inclusions on the margins of these sulphides. The Zandrivier Reef also contains insignificant traces of native gold in arsenopyrite. The traces of gold from these reefs are also associated with high traces of bismuth, which is a common gold pathfinder element. From gold fire assay results provided by the SGS laboratory, the Pienaar and Zandrivier Reefs have the highest gold grades at an average of 4 to 7 ppm Au. However, the limited petrographic results collected in this research report do not necessarily support this result. Therefore, it is recommended that more research should be undertaken to determine the enrichment of gold within these sulphides.