Abstract
Recent mobility of U was investigated in the Coutras U deposit (Gironde, France) located in Lutetian sediments near the surface. The Coutras deposit is a belt of tabular ore bodies separated into two areas by the Dronne River: the Jean Vincent Area, close to the surface, and the le Fieu area, located at a maximum depth of 100 m. The immobility of Th is supported by whole-rock Th contents which do not vary significantly with U content. The present hydrological regime in the Jean Vincent area is characterized by oxidizing waters flowing toward the Dronne River. Activity ratios (AR) of 230Th/ 234U in rocks, which range from 0.55 to 12, demonstrate recent U accumulation and leaching. The 234U/ 238U AR range from 0.94 to 1.52 in the rocks and from 0.94 to 1.61 in the waters. Recent accumulation of U is favored along redox barriers where Mo, V, Se and As are also mobilized. Waters in the Le Fieu area are nearly stagnant; they are reduced and exhibit a different chemical composition than waters in the Jean Vincent area, with higher contents of Ca, Mg, Na, K, Cl, HCO 3, PO 4, Ra, Ba, Mn, higher 234U/ 238U AR (1.33–4.63) and lower U content. The Le Fieu rock samples do not show any isotopic evidence of recent leaching of U, but both relative excesses and deficits of 234U are observed. Preferential mobility of 234U in the reduced Le Fieu area is interpreted according to the recoil model. The results place new constraints on the model of Cowart and Osmond and should be useful in prospecting for tabular U deposits in reduced environments.
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