The origin of the gold mineralization in the Witwaterstrand basin of South Africa-the largest known Sold province-has been controversial for decades, with arguments favouring detrital(1,2) (placer), modified placer(3,4) and hydrothermal(5,6) origins. Here we present the results of an extensive geological study of Witwatersrand rocks which show that the gold (and associated uranium) mineralization is hydrothermal in origin and post-dates a regional high-temperature alteration event. Alteration processes identified on a small scale can be mapped out regionally as roughly strata-bound zones of acid metasomatism extending far into the basin : the fluid flow responsible for this alteration was concentrated in small-scale structures localized along lithological boundaries. We find that the gold precipitated as a consequence of interactions of the fluid with shale-derived hydrocarbons present within the basin.