Application of 3D seismic analysis techniques to evaluate ore resources on Kloof, South Deeps and Driefontein gold mines, Witwatersrand Basin, South Africa.

Abstract

This thesis presents the application of the state-of-the-art processing, interpretation and modeling to the 3D reflection seismic data that were acquired between 1988 and 2003 across the West Rand and West Wits line goldfields of the Archean Witwatersrand Basin. The re-processing of the old 3D seismic data using new imaging techniques, such as 3D Kirchhoff prestack time migration (KPSTM), has led to better imaging of the ore body, structures (faults and dikes), and steeply dipping stratigraphy. Detailed interpretations of the highly auriferous Ventersdorp Contact Reef (VCR) using advanced complex seismic trace and horizon - based seismic attributes, have led to discoveries of ore blocks (~ 1 km long and ~ 250 m wide) that are bound by multi-fault segments of the first-order scale Bank and West Rand faults, leading to an increase of the resource portfolio and potentially, the quantity of the reserves. In particular, the edge detection attributes have resolved faults with throws as small as 10 m and complex structural architectures such as intersecting and cross-cutting faults, and fault bifurcations which are difficult to detect using conventional techniques (e.g., amplitude, dip and azimuth). Potential conduits, such as faults and dikes for migration of water and methane into underground workings were also mapped using edge detection attributes. These results have the potential to play into safe mine planning. The interpretation of the merged 3D seismic datasets, integrated with underground maps, boreholes, absolute and relative geochronological data, has added to our understanding of the gross structural architecture and Neoarchaean tectonic evolution of the goldfields. On a first-order scale (400 m - 2.5 km) the data resolved: (1) the northerly-trending disharmonic Libanon Anticline with a wavelength of 8 km and amplitude of 2 km, which was formed during deformation in the Umzawami Event (ca 2.73 Ga); and (2) the north-northeast trending, west-dipping (65°–70°W) listric West Rand and Bank faults, which were formed during a major extensional event, herein termed the Hlukana- Platberg Event (2.70-2.64). On a second-order scale (25 m - 400 m) the datasets resolved: (1) the Tandeka and Jabulani thrusts 1.5 km below the West Rand and Bank faults in the depth interval of 6-8 km; and (2) a series of drag synclines and rollover anticlines in the immediate footwall and hangingwall of the West Rand and Bank faults. Further to this, the seismic sections across the goldfields provided evidence that the first- and second-order scale faults, thrust and folds were dissected, eroded and overlain by the Transvaal Supergroup above an angular unconformity. The oldest approximate age for the Transvaal Supergroup is given as 2.58 Ga, thus constraining the age of the faults, thrust and folds to the Neoarchaean or pre- 2.58 Ga