The Sources of Witwatersrand Gold and Uranium
Authors: Pretorius D. A. 
Published: November 2019

The Witwatersrand basin hosts the most productive gold fields yet mined anywhere in the world. From 1886 to the end of 1987, a total of 41,700 metric tons (1.340 billion oz) of gold and 146,000 metric tons of uranium oxide was recovered from mineralized quartz-pebble conglomerates, quartzites, and thin layers of kerogen deposited between 2750 and 3100 Ma on braid deltas and braid plains marginal to a northeasterly trending shallow-water lake.

There is general agreement on the magnitude and extent of the concentrations of gold and uranium and on the depositional environment of the sediments, but not on the nature and origins of the mineralization. Disagreements revolve about the source, genesis, and mode of emplacement of essentially four components of the ores: gold, uraninite, pyrite, and kerogen. Another 70 ore minerals also contribute to the controversy but rarely advance to the center of the stage. Syngeneticists subscribe to a paleoplacer interpretation of the strata-bound mineralization, their preference being founded primarily on sedimentological evidence. Epige-neticists, arguing from mineralogical and geochemical bases, tend to favor the preferential migration of hydrothermal fluids through the more permeable horizons of coarser elastics. A partial reconciliation has been effected through the modified-placer theory, which envisages the reconstitution of originally detrital heavy minerals by postdepositional hydrothermal activity, possibly associated with up to four episodes of postdepositional metamorphism. No theory has been proposed yet which offers promise of a resolution of the many differences of opinion.

Past approaches to deciphering the history of mineralization in the Witwatersrand basin have concentrated on the host rocks within the depository. Presently, attention is being focused on prospective provenance regions outside the known limits of the depository and on the place of the basin and its successors in the tectonic evolution of the Kaapvaal craton. The long-held concept of Archean greenstone granite terrane’s being the source of detrital gold and uraninite has receded, mainly because of differences in compositions of the ore minerals and because, volumetrically, even the most productive of the world’s greenstone belts falls far short of qualifying as an adequate source of the gold in the Witwatersrand strata, if it be paleoplacer. The most recent basin analysis suggests that, instead of representing a yoked, intracratonic, early Proterozoic depository, the Witwatersrand basin might have developed in a foreland, back-arc environment of late Archean age.

Consideration is being given now to the possibility of the hydrothermal fluids’ having been formed by metamorphic dehydration of thick argillite successions in the lower part of the stratigraphy. Another line of thought is that pyritic, auriferous exhalites formed by shallow-marine hydrothermal discharge in the early stages of the basin’s development and that the endogenous primary mineralization was eroded subsequently, to supply detrital gold to the stratigraphically higher part of the basin fill. Extensive, hydrothermally altered granitic rocks have been found to have been emplaced, during the waning stages of the depository’s history, in a postulated source area close to the basin’s presently known limits, and researchers are exploring the possibility that the intensely mineralized roof zones and cupolas of these rocks were exogenous sources of gold and uranium. One more novel source area, the idea of which is being explored presently, is the magmatic arc which ought to have evolved if the Witwatersrand basin is indeed a product of a foreland, back-arc environment. Along the repeatedly reactivated interface between magmatic arc and back-arc basin, penecontemporaneous granite emplacement, hydrothermal alteration and mineralization, tectonic uplift, deflation and erosion, chemical charging of surface waters, fluvial transportation, sedimentation on braid deltas, introduction of ore minerals, and physical reworking of elastics within the depository possibly followed each other in rapid succession, over and over again, between 2750 and 2850 Ma, when the terminal chapters of basin formation were being recorded. This scenario depicts the Witwatersrand reefs as the consequences of coeval convocations of hydrothermal, precipitation, and placer processes prevailing in the golden age of the late Archean.