Influence de l'altération météorique sur l'uranium à l'état de traces dans le granite à deux micas de St-Sylvestre

Details Influence de l'altération météorique sur l'uranium à l'état de traces dans le granite à deux micas de St-Sylvestre Influence de l'altération météorique sur l'uranium à l'état de traces dans le granite à deux micas de St-Sylvestre

Jan 1969

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969gecoa..33...39b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 33, Issue 1, pp.39-47

Physics

1

Scientific paper

In the two mica granite of the St-Sylvestre massif, the transition from fresh rock at depth to the zone of surface weathering is marked by the destruction of uraninite which contains about half of the total uranium in the rock. In general, a considerable portion of the uranium (65-70%) is mobilized. In the lower part of the weathered zone, even though the chemical alteration effects on the rock are weak and practically limited to a partial oxidation of ferrous iron, the uranium so released precipitates as autunite along cracks; but for sample sizes in the kilogramme range, the uranium content does not change. In the upper part of the zone of weathering, where the chemical alteration effects are stronger, the autunite itself is dissociated and the uranium is again released. A fraction of the uranium is removed from the surface zone and, as a result, the average content of the surface samples (9.10 ) is appreciably less than the average content of the unaltered rock at depth (14 to 22 ). Some uranium is also redistributed in a disseminated form on the adsorptive surfaces of alteration products, so that, locally, the uranium content changes little, though it sometimes increases. In this type of granite, the distribution of the uranium in near surface rocks is essentially determined by all the processes of the weathering cycle. This results in a geochemical distribution of uranium which is very different from that encountered is unaltered rocks at depth.

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