Abstract The Witwatersrand Basin is an Archaean basin situated on the Kaapvaal Craton of Southern Africa. The results presented here focus on the structural geometry and development of the basin. Detailed structural sections across the western and northwestern parts of the basin are presented, using seismic data integrated with borehole, mine and outcrop information. The structural development of the basin can be expressed in simple terms, and in a manner that is standard practice within the oil industry. There are several clearly identified stages in basin evolution. The basin was initiated as a rift during Dominion Group times ( c. 3074 Ma), with post-rift thermal subsidence during the early part of West Rand Group times. Thermal subsidence would have been completed by late West Rand Group times ( c. 2900 Ma). Minor volcanic interludes within the West Rand Group sequence may testify to the existence of phases of extension during West Rand Group times. Onset of compression and thrusting outside the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during late West Rand Group times ( c. 2950 Ma). Foreland basin development culminated in Central Rand Group times, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during Klipriviersberg Group times (2714 Ma) by the accumulation of basic volcanic rocks. Further thrusting occurred at the end of Klipriviersberg Group times. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2709 Ma), a major rift event across much of the area. Thermal subsidence related to this rift phase occurred during late Platberg Group times. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, and it is difficult to elucidate the earlier compressional thrust-fold structures. Careful integration of seismic, borehole, mine and outcrop data allows structures from the thrusting event be recognized and more complete sections drawn. These present a larger-scale view of the structure of the basin than has previously been obtained.

Abstract This paper examines progressive evolution of fault architectures through late orogenic compression- to post-orogenic extensional deformation in the Witwatersrand Basin, South Africa. The results indicate that rapid extrusion of mafic lavas of the lower Klipriviersberg Group formed a rigid ‘lid’ over the thrust front, changing its mechanical character and thereby driving a change of structural style from fold growth to passive roof duplex. Flexural tightening of folds in the core of the triangle zones at this time may have helped provide the dynamic permeability for distributed ingress of hydrothermal fluids and consequent gold mineralization. Shortly afterwards, the kinematic environment changed to become extensional. However, this study shows sharp lateral partitioning of the duration of kinematic style and structural amplification, such that thrusting and extension coexisted along strike in the upper Klipriviersberg Group. Thus the switch from thrusting to extension was progressive within the region, but locally very rapid. As the local kinematic environment became extensional, the fault system evolved progressively, with the early stages of kinematic changes being dominated by a process of reactivation by architectural scavenging, in which new extensional structures developed by selectively reusing and incorporating geometrical segments of earlier formed thrust and normal faults. Three basic stages can be identified in this evolution: broad extension above underlying detachments, involving reactivation of lateral structures; a period of intensive reactivation and kinematic reworking incorporating frontal structures; and an abandonment stage when the detailed influence of the earlier architecture diminished and the fault system developed larger through-going normal faults. The interaction of the newly developing fault system with the pre-existing architecture constitutes pre-programming of the final geometry, in which individual large faults are composed of a reticulated network of new and inherited segments. The observations are consistent with fault scale being a key control on the fault reactivation involved. This study has involved full integration of a dataset comprising 2D and 3D seismic reflection data, geological mine plans, logging of over 120 km of drill core and underground mapping in deep mine workings that pass 3 km into the seismic volume at 2–3 km depth.

The Ventersdorp Contact Reef, one of the major gold-bearing conglomerate horizons in the Witwatersrand Basin, occurs as a distinct horizon between the overlying Klipriviersberg Group lavas (2714 Ma) and the underlying Central Rand Group rocks (<2894 to >2714 Ma). The Ventersdorp Contact Reef has been metamorphosed and hydrothermally altered under greenschist facies conditions (290–350 °C and 0.2–0.3 GPa). A study of S, O, and H isotopes was carried out to constrain the sources of hydrothermal fluids and gold mineralization in the Ventersdorp Contact Reef. A narrow range of δ 34 S values (−1.5‰ to +1.8‰) for authigenic pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena from the Ventersdorp Contact Reef and its hanging-wall and footwall lithologies suggests a reconstitution of detrital sulfides during fluid circulation at peak metamorphic conditions. The δ 18 O values of authigenic quartz and calcite range from +8.9‰ to +11.3‰, and the δ 18 O and δD values of muscovite separates are between +7.2‰ and +8.4‰ and −62‰ and −31‰ respectively. The estimated δ 18 O (+4.8‰ to +6.1‰) and δD (−27‰ to −39‰) values of the fluids in equilibrium with these minerals under the relevant P-T conditions suggest a metamorphic origin for the fluid. It is concluded that the metamorphic fluids involved were probably derived from the Basin itself, and allogenic sulfides in the Ventersdorp Contact Reef were reconstituted during fluid circulation at peak metamorphic conditions. Accordingly, gold appears to have been locally remobilized and possibly derived from pre-existing placer concentrations.