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Géochimie des éléments de terres rares et du plomb dans les oxydes d'uranium naturels

Abstract : Upon their crystallization natural uranium oxides incorporate variable quantities of elements, which depend on their ionic radius, the physical-chemical conditions (temperature, pressure, pH, redox conditions, and nature of ligands in the mineralizing fluids), the composition of the rocks with which the mineralizing fluid has been equilibrated, and post-depositional reequilibration due to later fluid circulations. REE represent a particularly interesting set of elements because their ionic radii are close to that of U4+ in eight-fold coordination and most of them are not sensitive to changes in the redox conditions. Hence, they are much less mobile than radiogenic Pb which has a much larger ionic radius than U4+ and thus may better preserve their primary distribution within uranium oxides. As a consequence, REE distributions in uranium oxides from different types of uranium deposits appear to be characterized by specific signatures. An in situ analysis methodology of the rare earth elements in uranium oxides on micro-domains by ionic microprobe (CAMECA IMS-3f) has been developed. Analyses performed on uranium oxides selected among the main deposits of the world show that every uranium deposit type presents specific REE patterns. According to the type of deposit, REE fractionation is mainly controlled by one of these following parameters: temperature, nature of ligands in solution, source or host rock composition, and uraninite crystal-chemical parameters. Hydrothermal alteration affects the light REE abundances and fractionations in uranium oxides, with the occasional appearance of W tetrad effects, while intermediate and heavy REE remain unaffected. A thermodynamic approach has been used to determine the role of some parameters on the incorporation of REE in uranium oxides: although high temperatures increase REE complexation in the mineralizing fluids, where they would preferentially form complexes with chloride, hydroxide and to a lesser extent fluoride ions. Saturation index calculations for hydrothermal fluids with respect to REE sesquioxides indicate that temperatures = 300°C are required for the incorporation of significant amounts of REE in uranium oxides
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Jessica Bonhoure. Géochimie des éléments de terres rares et du plomb dans les oxydes d'uranium naturels. Autre. Institut National Polytechnique de Lorraine, 2007. Français. ⟨NNT : 2007INPL082N⟩. ⟨tel-01752855⟩

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