Abstract 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.

Abstract The most important source of gold in the world is in conglomerates of lower Proterozoic age, and production of uranium from this type of host rock also has been substantial. The largest known reserves of both these ores are contained in quartz-pebble conglomerates and associated coarse-grained arenites. Pyrite is an important by-product, and platinum group metals, thorium, and silver have also been recovered. Mineralized conglomerates have been discovered in many formations on every continent, but only four regions have sustained persistent mining operations: Witwatersrand in South Africa, Blind River-Elliot Lake in Canada, Tarkwa in Ghana, and Jacobina in Brazil. The most significant of these, by far, is the Witwatersrand Basin, knowledge of which has provided the basis for understanding the processes of ore formation which are common to all the deposits. The lower age limit for the development of mineralized conglomerates is 3,100 m.y. and the upper limit 1,900 m.y. Rudites in typical Archean greenstone assemblages have not been exploited, and upper Proterozoic conglomerates (700 – 1,600 m.y.) are conspicuously devoid of conglomerate-hosted ore deposits. The development of middle Proterozoic iron-formations and red beds (1,600–2,200 m.y.) terminated the metallogenic epochs in which the auriferous and uraniferous conglomerates were formed. The strata that contain the rudites are preserved on the flanks of stable blocks of elevated Archean basement. Greater uplift of the more central parts of such blocks caused the lower Proterozoic formations immediately above the basement to be stripped away, whereas the relatively negative regions surrounding the blocks have preserved the upper Proterozoic and Phanerozoic cover. The original basins containing the conglomerates were large in extent, at least 600 km long × 250 km wide, and were formed in an intracratonic or continental-shelf environment. Up to 15,000 meters of sediments and volcanics were laid down. Fluvial, deltaic, neritic, and shallow marine sediments are characteristic, and deep water turbidites and extensive chemical sediments are absent. The conglomerates are the products of a fluvial system in which low sinuosity, high energy, shallow depth braided streams were operative. Gravels and sands, the latter carrying heavy minerals, accumulated either in paleovalleys on the erosion surface of the Archean basement or on fluvial fans or fan deltas which developed where major river systems debouched into a large lake or inland sea. The structural fabric of the region played a critical role in the style and facies of sedimentation. The geomorphology of the basement reflected a pattern of superimposed interference folding and associated faulting, and the structural domes formed in such a pattern constrained the siting of the fluvial fans. Repeated diapiric-like uplift of the domes and of the sets of anticlines on which they are situated led to tilting and increased gradients of the depositional paleoslope, erosion surfaces on unconformities, and rim faulting around the peripheries of the domes and along the limbs of the anticlines. All of these contributed to the processes of reworking and winnowing of the sands and gravels, which were essential to the concentration of the heavy minerals. Reworking was further enhanced where transgression of the depository waters up the fluvial fans resulted in reconstitution, by wave action, of the sediments on the surface of the fan. Very fine grained gold and uraninite which moved beyond the midfan area, where mineralized braided-stream channel conglomerates are at an optimum, were trapped, in rare instances, by algal mats that grew below wave base in the low-energy fan-base environment. The unconformity is the site of maximum concentration of heavy minerals, whether these be in residual lag gravels, winnowed sands, or algal mats. Most of the conglomerates occur as basal components, or within the lowermost portion, of the stratigraphic succession. Such deposits conform more to the paleovalley fill-type of sedimentation which appears to have a lower economic potential than fluvial fan-type assemblages. Uranium is generally more prevalent than gold in the basal conglomerates. Fans develop higher in the sedimentary pile, and the most significant mineralization has been found in the upper half of a complete stratigraphic succession. Where the later part of the basin history is characterized by a depository shrinking in size and by generally regressive conditions, factors are at an optimum for the progressive reworking of material and the generation of high-grade concentrations of gold and uranium in coarse clastic sediments. Extensive sedimentölogical, mineralogical, and geochemical studies in the past 25 years have produced evidence that very strongly supports a placer origin for the mineralization in the lower Proterozoic conglomerates and sands. Remobilization of gold and uranium and reconstitution of the latter have been effected by diagenesis and metamorphism. The intimate relationship between sedimentary features, many tectonically controlled, and the sites of maximum concentration of heavy minerals points to a syngenetic origin for the mineralization. The source of the detrital minerals is in all cases indicated to be the Archean granite-greenstone basement terrane on which the sedimentary basins rest. The gold and pyrite were derived from volcanogenic mineralization in the greenstones. The uranium was contributed by a paleosol which formed in a regolith over younger, potash- and silica-rich granitoids belonging to the granitic component of the Archean assemblage. The degree of tectonic uplift of the provenance area and the consequent level of erosion determined the relative proportions in the mix of granite and greenstone debris and, therefore, the prevalence of uranium or gold, respectively, in the conglomerates and sands of such lower Proterozoic basins as the Jacobina, Tarkwaian, Huronian, Pongola, Transvaal, and Witwatersrand.