Applied Geochemistry, VoL 7, pp. 111-121, 1992 0883-2927/92 $5.00 + .00 Printed in Great Britain (~ 1992 Pergamon Press Ltd Uranium mobility in the sediment-hosted uranium deposit of Coutras, France J. D. MEUNIER CREGU and GS CNRS-CREGU, BP 23, 54501 Vandoeuvre les Nancy Cedex, France J. BRUHLET COGEMA, 2 rue Paul Dautier, BP 4, 78141 V61izy-Villacouhlay Cedex, France and M, PAGEL CREGU and GS CNRS-CREGU, BP 23, 54501 Vandoeuvre les Nancy Cedex, France (Received 6 December 1989; accepted in revised form 30 September 1991) 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 23°Th]234U in rocks, which 234 238 range from 0.55 to 12, demonstrate recent U accumulation and leaching. The U/ U 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, C1, HCO3, PO4, Ra, Ba, Mn, higher 234U/238UAR (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 234, 234 both relative excesses and deficits of U are observed. Preferential mobility of U 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. INTRODUCTION THE GEOCHEMICAL behavior of U in low-temperature environments is essentially controlled by redox pro- cesses, adsorption and the presence of ligands (LANG- MUIR, 1978; NGUYEN-TRUNC,1985). Uranium occurs as uranyl (UO 2+) complexes such as uranyl- carbonates or uranyl-phosphates at near-neutral to basic conditions. Uranium (IV) minerals, such as pitchblende or coffinite, are not stable in oxidizing environments, so that many U deposits located in near-surface sedimentary rocks are destroyed by weathering processes. Activity measurements on members of the U decay series frequently show disequilibrium parent- daughter relations, which provide a good framework for studying geochemical processes because the radioactive daughters show different geochemical behavior and have different half-lives. The 23°Th, which is part of the 238U decay chain, is highly insoluble in aqueous media and tends to be rapidly adsorbed (BACON and ANDERSON, 1982). Recent leaching or mineralization of U can then be demon- strated by analyzing the 23°Th/238U activity ratio in rock samples (ZIELINSKI et al., 1986). The 226Ra, radioactive daughter of 23°Th, is mobile in reducing media in the absence of SO4 ions, whereas U is insoluble. Radium has often been used in hydro- chemistry as a potential tracer for U ore deposits, but Ra may also be easily adsorbed to create local false anomalies in 222Rn (LEVINSONand COETZEE, 1978). The 238U and its grand-daughter 234U are fre- quently out of equilibrium despite similar chemical properties. Mechanisms of 234U/238U fractionation have been reviewed by OSMONDand COWART (1976) and recently by IVANOVICH and HARMON(1982). Two mechanisms are proposed that might favor preferen- tial mobilization of daughter 234U" (1) transfer of 234U to solution by alpha recoil of 234Th at the solid/liquid boundary (KIGOSHI, 1971; FLEISHERand RAABE, 1978); (2) increased vulnerability to solution after recoil displacement due to unstable location or oxidation from U 4+ to U6+ (RosHOLT et al., 1963; KOLODNY and KAPLAN, 1970; CHERDYNTSEV, 1971; FLEISHER and RAABE, 1978; FLEISHER, 1982; PETIT et al., 1985). Generally, oxidizing waters exhibit high U contents and 234U/238U activity ratios (4/8AR) near 1 whereas reducing groundwaters have low U contents and high 4/8AR. The 4/8AR are different in ground- water at different positions relative to ore bodies 111