The radioactive disequilibria (U-Ra) of the uranium from the phosphatic deposit of the Ganntour (Western Morocco)

In the context of the study of uranium distribution in the phosphate deposit of the Ganntour, the state of relative equilibrium uranium-equivalent radium (U/eRa) for 110 phosphatic samples of various localities and facies, was analysed by gamma multichannel spectrometry. The equilibrium (U/eRa = 1) is not always maintained. Disequilibrium is more frequent and is greater in the superficial part of the deposit than at a depth of more than 10 m. In the superficial part the U/eRa ratio ranges from 0.6 to 2.2 (with the exception of a single value as high as 4.5), whereas in the deeper zones U/eRa varies only between 0.8 and 1.3. Given that the radioactive equilibrium of pure uranium along with its daughter products requires only a million years, the observed disequilibria indicate that some radioisotopes (mainly U) have migrated recently or even continue to do so. The superficial part of the deposit contains secondary U enrichments, the importance of which varies from place to place, but values greater than 300 ppm are rarely reached. In this zone, the disequilibrium varies with increasing depth as follows: at first unfavorable to U, it later becomes favorable (U/eRA > 1), passes through a maximum, then decreases and finally tends to disappear. Moreover, the high disequilibria favorable to U correspond to the highest secondary U accumulation. In these conditions the above variation is explained by meteoric leaching of the U in the uppermost part of the profile, followed by its accumulation below the leached zone, Ra remaining stable. In depth, the rare and weak disequilibria suggest the existence of minor U movements. In the dry zone, the disequilibria in favour of U seem to confirm a low build-up of this element migrating mainly from the surface. However, within the water-bearing zone, if U migration is weak, losses and gains would roughly balance each other. Article published in 'Sciences Geologiques - Bulletin', tome 40, no. 4, 1987, pp. 349-362. (author)