Abstract: Hydrothermal uranium veins, associated with the Hercynian leucogranites show important variations in their mineralogical, chemical and structural features in relation to the host rock lithology. These are described with particular reference to the Chardon deposit, Vendée where the veins cut granite, basic lithologies, and shales. The following features are described: Changes in the thickness of veins near to contact zones, particularly those between granites and basic lithologies, lamprophyres, and shales. Changes in the gangue mineral assemblage with the preferential development of carbonate in veins cutting basic lithologies, and of silica in veins which cut granite. Paragenetic zoning in the veins in passing from granites to their metamorphic aureoles.
Comparisons between deposits of Vendée, Limousin, and Erzgebirge allow the following generalizations to be made: Open faults and subsequent mineralization are concentrated at boundaries between competent and more plastic lithologies. Mineralizing fluids cause wall-rock alteration characterized by the removal of Si from granite and of Ca, Mg, Fe from metamorphic and basic rocks. The chemical and structural characteristics of wallrocks are important controls on the mineralization but in acid lithologies the main controls on the pitchblende vein formation are the structural characteristics of the wallrock.