Published online by Cambridge University Press: 09 July 2018
The uranium deposit (1350 Ma) of Cigar Lake (Canada) is surrounded by a late hydrothermal alteration halo (330 Ma) containing Fe-illites and kaolinites. Crystallochemical characterization of the kaolinites has been carried on the microscale using XRD, electron microscopy (SEM and TEM) coupled with EDX spectrometry and EPR. The large, well-crystallized particles show large amounts of Fe (0.9–1.8%) and V (0.3–0.5%). According to EPR measurements performed on both random powders and oriented samples, V occurs as the vanadyl ion VO2+, in substitution within the octahedral sheet of the kaolinite structure in the same way as Fe3+. Kaolinite growth proceeded through the hydrothermal alteration of anterior phyllosilicates devoid of V, induced by fluids which leached V-rich titano-magnetites in the surrounding sandstones.
Le gisement d'uranium (1350 Ma) de Cigar Lake (Canada) présente une auréole d'altération hydrothermale (330 Ma) contenant des illites ferrifères et des kaolinites. Nous avons réalisé une étude cristallochimique des kaolinites à l'échelle de la particule en combinant la diffraction des RX, la microscopie électronique (MEB et MET) couplée à la spectrométrie RX en dispersion d'énergie et la résonance paramagnétique électronique (RPE). Les particules, bien formées et de haute cristallinité, montrent un taux élevé de Fe (0.9–1.8%) mais surtout de V (0.3–0.5%). D'après les données RPE, enregistrées sur des poudres et des échantillons orientés, le V apparait sous forme d'ions vanadyle VO2+, en substitution, comme Fe3+ dans la couche octaèdrique de la kaolinite. Le développement des kaolinites résulte de l'altération hydrothermale de phyllosilicates antérieurs, dépourvus de V, par des fluides enrichis en cet élément lors du lessivage de titanomagnétites dans les grès environnants.
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