Gold and Uranium in Quartz-Pebble Conglomerates
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.
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