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.