Most of the economic gold and uranium placers are developed on low angle disconformities in the Central Rand Group and concentrations of gold and uranium are usually at their optimum on unconformity surfaces. Examples include the Kimberley Reef and South Reef of the East Rand, the Main Reef Leader of the Central Rand, the Carbon Leader of the Carletonville goldfield, the Vaal Reef of the Klerksdorp goldfield and the Basal/Steyn placers of the Welkom goldfield. The individual goldfields represent fluvial fans which are composed of a large number of tectonogenetic sedimentary packages separated by unconformities. The tectonic responses between cycles of sedimentation produced unconformities and tectonically controlled cyclic sedimentation is one of the key factors culminating in the preparation and deposition of auriferous placers within the Witwatersrand succession. Unconformities, which represent breaks in sedimentation, result in the preconditioning of palaeosurfaces and redistribution of sediments and heavy minerals on them. Winnowing of sands produced heavy mineral residual accumulations on erosion surfaces which were generally preserved by small-pebble lags or algal mats. Reworking of units truncated by the unconformities provided additional gold, uranium and heavy minerals to unconformity surfaces.

La recherche de nouveaux procédés propres et durables pour extraire l’uranium des minerais en alternative à l’extraction liquide-liquide conduit à s’intéresser aux procédés d’extraction en milieu CO2 supercritique. L’objectif de ce travail est donc d’étudier la faisabilité de l’extraction de l’uranium des minerais en milieu CO2 supercritique, à l’aide de molécules extractantes adaptées. Dans un premier temps, des mesures de solubilité des ligands sélectionnés pour cette étude préliminaire, trioctylamine et PC88A (acide 2-éthylhexyl 2-éthylhexyl phosphonique), et des complexes métalliques que ces molécules forment avec le molybdène utilisé comme simulant inactif de l’uranium et l’uranium, ont été effectuées en milieu CO2 supercritique. L’étude du procédé d’extraction dynamique en milieu CO2 supercritique a ensuite été réalisée en inactif, d’abord sur une solution aqueuse sulfurique de molybdène puis sur des poudres d’oxydes de molybdène. Les étapes de solubilisation du complexe ligand-Mo en CO2 supercritique et d’attaque du solide par le système extractant (ligand/acide/oxydant) sont les étapes limitantes à contrôler. Le choix d’un système extractant adapté devient alors primordial. En tenant compte des résultats obtenus en inactif (Mo), des essais d’extraction sur minerai d’uranium à partir de PC88A en milieu CO2 supercritique ont finalement été réalisés en présence d’acide sulfurique et d’oxyde de manganèse. Le procédé a ainsi pu être validé, plus de 60 % de l’uranium du minerai a été collecté en sortie de montage. Toutefois, l’effet de certains paramètres opératoires reste à étudier ainsi que la synthèse/sélection de nouvelles molécules extractantes ciblées. / The research of clean and sustainable new processes to extract uranium from ores as an alternative to solvent extraction leads one to consider extraction processes using supercritical carbon dioxide. The aim of this work is to study the extraction feasibility of uranium ores by supercritical CO2, using suitable ligands. First, solubility measurements of selected ligands, trioctylamine and PC88A (2-ethylhexyl 2-ethylhexylphosphonic acid), and metal complexes formed between these ligands and molybdenum (uranium surrogate) or uranium, are performed in supercritical CO2. Supercritical CO2 extractions are then carried out on sulfuric aqueous solutions containing molybdenum and on molybdenum oxides powder. Solubilization of the extractant system (ligand/acid/oxidant) and solid leaching are the key steps which need to be controlled. Well-suited extractant system selection is therefore fundamental. The hypothetical mechanisms, describing supercritical carbon dioxide extraction, seem to be quite different from those usually observed in solvent extraction, especially in terms of selectivity and formed complex structure. Based on the results obtained with molybdenum, extraction trials on uranium ores were then conducted using PC88A in supercritical carbon dioxide with sulfuric acid and manganese oxide. More than 60% of uranium was recovered, which confirmed this new process feasibility. However, the influence of some operating parameters and the synthesis/selection of new suitable ligands remain to be further studied.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Bibliography: leaves 230-233. / Ph.D.

No description available.

M.Sc. (Geology) / Within the Black Reef Formation, which forms the basal unit of the Transvaal Supergroup, an auriferous and uraniferous reef occurs. This is known as the Buckshot Pyrite Leader reef. This reef is a conglomerate-hosted gold and uranium deposit, which is similar to the Witwatersrand reefs as they both contain significant amounts of pyrite and have similar depositional environments. This study seeks to identify the geometallurgical characteristics of the gold uranium and thorium that are hosted in the Black Reef at the Modder East operation in Springs. In terms of methodology, a detailed petrographic study was completed on the samples taken from underground and the surface run of mine samples. Mineralogy was conducted on these samples using SEM-based automated technology, namely the FEI Mineral liberation analyser (MLA). Using this, I could further characterise the gold and uranium bearing phases found across the Black Reef. Additionally, mill testing, grading analysis, major and trace element chemistry, density separation, gold and uranium dissolution as well as flotation testing was conducted on the ore in order to determine its geometallurgical characteristics. Through the combination of these methods, this study aims to evaluate the Au, U and Th within the Black Reef with regards to: the metallurgy; the extraction process in relation to the mineralogy of the samples; and the possible implications that these factors could have on the overall recovery of the economic minerals.

No description available.

M.Sc. (Geology) / The Witwatersrand Supergroup is host to a number of auriferous and uraniferous conglomeratic reefs, which have been extensively exploited along the Witwatersrand Basin margins. The current study investigates the Vaal Reef, in the Klerksdorp gold field with particular focus on conducting a geometallurgical characterization of the ore which may ultimately enhance the recovery of gold and uranium and our understanding of how the ore responds to processing. Six samples were collected from AngloGold Ashanti’s Moab Khotsong mine and prepared for a chemical and mineralogical deportment study. These samples were milled and crushed down to 80% passing -75μm and processed for head chemistry assays, grading analysis as well as heavy liquid separation analysis as part of the chemical deportment. The samples were also submitted for gold cyanide, acid uranium and diagnostic leach tests.....

The Koongarra uranium deposit is hosted by quartz-chlorite schists. A conceptual model for the hydrogeology of the deposit is proposed on the basis of lithologic criteria and limited hydraulic testing. Water-level and aquifer-test data are presented that indicate the deposit lies within a partially confined, heterogeneous, anisotropic fractured-rock aquifer. The aquifer is dynamic with annual, diurnal, and semidiurnal water-level fluctuations. The results of aquifer tests indicate a high degree of connectivity in the aquifer. Fracture-dominated flow is observed in some tests, but the overall aquifer response appears to be that of an equivalent porous medium. A homogeneous, anisotropic model is used to estimate the transmissivity tensor for subregions of the aquifer. Anisotropy is well-developed with north- to east-northeast-oriented principal transmissivities. Northeast directions represent large-scale drawdown patterns and are subparallel to bedrock structure and the Koongarra fault. Northerly directions are localized and may reflect a less extensive fracture fabric or a flexure in the bedrock foliation.

Au cours de ces dernières décennies, des études ont étés menées pour identifier plusieurs traceurs des matériaux du cycle du combustible nucléaire, dans le cadre de la lutte contre la prolifération nucléaire. Ces matériaux sont généralement collectés lors d’inspections dans des installations nucléaires, ou saisis lors de contrôles de trafics illicites. Les informations fournies par ces traceurs sont parcellaires et ne permettent pas de déterminer avec exactitude la provenance et l’historique industriel de ces matériaux.Le but de ce travail de thèse est de démontrer le potentiel de l’utilisation des isotopes du molybdène pour le traçage des matériaux du cycle du combustible nucléaire. Le choix s’est porté sur le molybdène car en raison de la similarité de leurs propriétés chimiques, le molybdène et l’uranium sont étroitement associés dans les minerais d’uranium et tout au long de la chaîne de purification de l’uranium. L’étude s’est focalisée sur une partie de l’amont du cycle du combustible, depuis l’extraction des minerais d’uranium jusqu’à la production des concentrés miniers d’uranium : divers procédés physiques et chimiques sont appliqués, à la fois pour purifier l’uranium et abaisser la concentration en molybdène.Au cours de cette étude, une nouvelle méthode de séparation du molybdène a été développée pour caractériser sa composition isotopique dans des minerais, minéraux et concentrés miniers d’uranium. La variabilité des compositions isotopiques du molybdène dans un gisement d’uranium est principalement due aux mécanismes d’adsorption et/ou de précipitation du molybdène. Les gisements magmatiques et sédimentaires ont des compositions isotopiques différentes, ce qui permet ainsi leurs distinctions. Les concentrés miniers d’uranium produits à partir de ces deux types de gisements ont des compositions isotopiques similaires aux minerais. Ces résultats soulignent ainsi le potentiel des isotopes du molybdène comme traceur des origines des concentrés miniers d’uranium. Cependant, un fractionnement des isotopes du molybdène a été établi lors de la production des concentrés miniers d’uranium pour deux usines au Niger. Les procédés de purification de l’uranium tels que la lixiviation, l’extraction par solvant et la précipitation ont été reproduits en laboratoire sur des échantillons réels pour expliquer le fractionnement isotopique du molybdène lors de la production des concentrés miniers. Au cours de ces procédés, le fractionnement peut être positif (lixiviation), négatif (extraction par solvant, précipitation à l’eau oxygénée) ou nul (précipitation à l’ammoniaque). Dans le cas des échantillons du Niger, la somme de ces procédés est négative, dans le sens des données expérimentales que nous avons obtenues, démontrant ainsi également le potentiel de l’utilisation des isotopes du molybdène comme traceur des procédés de transformations des matériaux du cycle du combustible nucléaire. / Nuclear forensics aims at determining the age, provenance as well as industrial or storage history of uranium ores and uranium ore concentrates that are part of the nuclear fuel cycle. Several potential tracers have already been identified for this purpose. However, these tracers are not providing always unambiguous information. This study is focused on establishing Mo isotopes as a new tracer of uranium ore provenance and of ore processing for its application in nuclear forensics. Molybdenum and uranium share a number of common geochemical properties. In the nuclear fuel cycle, molybdenum is an impurity that is difficult to separate during uranium extraction and purification processes, while its concentration is required to be lower than some specification limits. We focused this study on the first part of the nuclear fuel cycle, from the uranium ores extraction to the production of uranium ore concentrates.We developed an enhanced separation method for Mo from a uranium-rich matrix (uranium ores, uranium minerals, uranium ore concentrates) in order to analyze the mass fractionation induced by processes typical of the nuclear fuel cycle. Molybdenum isotopic compositions in uranium ores depend of adsorption and precipitation processes. The δ98Mo values of sedimentary uranium ores is shifted to negative values relative to magmatic ores. This provides a means of distinguishing these types of uranium ores. Uranium ores concentrates produced from both uranium ore natures (magmatic and sedimentary) have Mo isotope compositions similar to the uranium ores. These results suggest that molybdenum isotopes have a strong potential of as a tracer for identifying the origin of the uranium ore concentrates. However, Mo isotopes fractionations were established during the production of uranium ore concentrates in the both Niger mills. We reproduced in laboratory the lixiviation, solvent extraction and precipitation processes to explain these observations. The Mo isotopes fractionation is positive for the lixiviation process, negative for the solvent extraction and precipitation with hydrogen peroxide, and null for ammonia precipitation. In the case of the Niger samples, the sum of these processes is negative and agrees with our experimental data. Mo isotopes have a strong potential as a tracer for identifying the origin and transformation of uranium in the nuclear fuel cycle, in the framework of nuclear forensics.