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 Hydrothermal and placer origins for Witwatersrand gold have been debated ever since the discovery in 1886 of the Central Rand goldfield in South Africa. The hydrothermal model is supported by recent findings of a complex postdepositional history for the Witwatersrand Supergroup, including extensive deformation, greenschist facies metamorphism with widespread alteration and associated gold mobility. Critical to the debate is the association of half the Witwatersrand gold with migrated hydrocarbons that were not in their present position at the time of sedimentation. Gold is mined from planar reefs that are centimeters to meters thick and of several hundred square kilometers in area. The mineralogy of the reefs is unusual in having negligible iron oxides, hydrocarbons, and abundant round pyrite. The host rocks to gold include conglomerates and sandstones, with no single depositional environment or inferred depositional process that correlates with high gold-grade areas across the basin. In contrast, the chemical association of pyrite and/or migrated hydrocarbons is ubiquitous in the orebodies. Regional metamorphism in all goldfields generated assemblages including pyrophyllite, chloritoid, chlorite, muscovite, and pyrite, with more restricted kyanite, biotite, kaolinite, and pyrrhotite. Peak temperatures of 300° to 400°C have been inferred with isograds semiparallel to stratigraphy. A period of hydrothermal alteration near the peak of metamorphism has overprinted much of the Central Rand Group in the goldfields, extending 300 km around the basin margin. This alteration has involved loss of Si, Fe, Mg, and Ca, with addition of K and Rb. Geometrically, this hydrothermal alteration coincides vertically and laterally with the distribution of economic gold and with areas of widespread sulfide distribution in all sedimentary rock types. The hydrothermal alteration is distinguishable from weathering by its geometry and the addition of K, Rb, and sulfur. The hydrothermal model invokes uranium introduction in meteoric waters along the uplifted basin margin. During burial diagenesis, thermal maturation of organic material in Witwatersrand shales generated hydrocarbons that were carried by the migrating fluids and precipitated near unconformities, commonly in association with preexisting uranium minerals. Gold-bearing H 2 O-CO 2 -H 2 S fluids at 300° to 400°C were introduced to the Central Rand Group along major basin-bounding thrust faults and were channeled between the overlying Klipriviersberg lavas and the underlying marine shales of the West Rand Group. Fluid flow was controlled by bedding subparallel fracture networks and the sedimentary architecture of the basin that favored flow along lithologically complex reef packages on unconformity surfaces. The hydrothermal model predicts the distribution of gold in the Witwatersrand reefs through sulfidation of detrital iron-rich heavy minerals and precipitation with the migrated hydrocarbons. Hydrothermal gold mineralization is inferred to predate Platberg extensional faulting that displaces the orebodies. The hydrothermal replacement model implies significant potential for exploration in younger sedimentary basins with similar tectonic and thermal histories. Basin architecture, structure, alteration, and suitable chemical traps are important exploration criteria.

Abstract Although the gold mining industry in the Witwatersrand has passed its maturity, the Witwatersrand basin remains by far the most important known gold-producing province. Most of the gold occurs together with rounded pyrite and uraninite, and in places with bituminous carbon, along degradation surfaces in coarsegrained siliciclastic rocks of the 2.97 to 2.91 Ga West Rand Group, and more importantly, in similar rocks of the 2.89 to 2.71 Ga Central Rand Group. Significant correlation between gold and detrital zircon distribution, as well as sedimentary lithofacies, preservation of detrital micronuggets in places, and absolute age data provide evidence of a detrital origin of the gold, which was derived from the weathered products of 3.1 to 2.9 Ga greenstone belts to the north and west, and transported and concentrated by fluvial selective entrainment processes. Eolian deflation led to further upgrading of the placer deposits. Microstructural, mineral chemical, and isotopic evidence indicate a detrital origin also for rounded compact pyrite and uraninite particles, from which reducing conditions are inferred for the Archean meteoric environment.These conditions prevailed until at least 2.64 Ga, when the last Witwatersrand-type placer deposits were laid down at the base of the Transvaal basin. A series of tectonothermal overprints are recognized in the Witwatersrand strata. These began with a 2.83 Ga deformation event near the western margin of the Witwatersrand basin in response to accretion of an oceanic arc to the western margin of the Kaapvaal craton. Compression followed by continental rifting led to the deposition of the Ventersdorp Supergroup at 2.70 to 2.67 Ga as a consequence of collision between the Kalahari and Zimbabwe cratons and subsequent orogenic collapse of the Limpopo belt. Thermal subsidence and associated deposition of the 2.64 to 2.43 Ga Chuniespoort Group led to low-grade burial metamorphism of the Witwatersrand strata, to basin dewatering, oil migration, and partial gold mobilization within the Witwatersrand basin fill. Burial to similar depths was achieved again during the deposition of the 2.35(?) to 2.25 Ga Pretoria Group after a major erosive phase. Inferred magmatic underplating and intrusion of large amounts of mafic to ultramafic melts into the lower to middle crust during the emplacement of the 2.06 to 2.05 Ga Bushveld Igneous Complex led to a thermal metamorphic and possibly also metasomatic overprint of the lower parts of the Witwatersrand sequence, but it had little effect on the distribution of the gold. Further local gold mobilization was triggered again by the 2.02 Ga Vredefort meteorite impact that created a secondary permeability. Later overprints during orogenic events along the margins of the Kaapvaal craton, i.e. the 2.0 to 1.8 Kheis, the 1.2 to 1.0 Namaqua, and the 0.6 to 0.5 Ga Pan-African orogenies had little or no significant effect on the distribution of the Witwatersrand gold.