FERRIFEROUS AND VANADIFEROUS KAOLINITES FROM THE HYDROTHERMAL ALTERATION HALO OF THE

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 VO 2§ in substitution within the octahedral sheet of the kaolinite structure in the same way as Fe 3§ 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.