Structural controls on Witwatersrand gold mineralisation

Structural controls on the distribution of gold in the Witwatersrand Basin can be seen at scales ranging from that of thin sections up to regional seismic data sets. At the thin section scale, gold occurs largely in fractures with hydrocarbons. These fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basinʹs development. The distribution of thrust–fracture networks (and therefore gold) is controlled by the mechanical attributes of the host stratigraphy. Thrust displacements are generally very low, with structures exploiting depositional contacts and mesoscale sedimentary bedforms. Many of the small faults are isolated from each other with bimodal vergence between larger-scale thrusts of the same age. Early-formed faults and fractures become folded and faulted by subsequent propagation of larger-scale thrusts. These characteristics are consistent with formation of the mineralised thrust–fracture networks in the frontal, low-displacement parts of a thrust system. In the Carletonville goldfield, the prospectivity of the Ventersdorp Contact Reef (VCR) has been evaluated by using underground observations to pinpoint the structural habitat of gold, and therefore characterise the size of prospective zones around structures intersecting the VCR, mapped within 3D seismic data. In the Welkom goldfield, the controls on gold distribution at the Tshepong mine have been characterised from 3D seismic data, drilling and underground observations, and used to develop an ore-body model that aids reserve estimation and production planning.