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Clay minerals associated with the unconformity-type uranium deposits of the East Alligator Rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the Kombolgie unconformity. Alteration develops a similar pattern at regional and smaller scales, with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity (e.g., faults, fractures, breccias). These discontinuities were mechanically active during the hydrothermal activity. The hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the Kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks. The time-space alteration sequence established in this study can be summarized as follows: (1) early crystallization of illite ± illite-smectite (I-S) mixed layer minerals + LREE-Sr–bearing aluminum phosphate-sulfates (APS) in sandstones above the unconformity, illite ± I-S mixed layer minerals + sudoite + APS on both sides of the unconformity and in fault gouges, and illite ± I-S mixed layer minerals + chlorite ± apatite deeper in the basement; and (2) late precipitation of chlorite in the open fracture network mainly centered below the unconformity. The main stage of uranium deposition was associated with chloritization in the basement rocks. Smaller particle size, lath-shaped or flaky morphology, common interstratification with expandable layers, lower crystallinity, and Mg-rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and/or chlorite. Hydrothermal illite can be distinguished from diagenetic and metamorphic illite-phengite by its poor crystallinity along the c-axis, interstratification with minor amounts of smectite layers, and a 1M polytype in which trans-vacant octahedral sites predominate (1 Mt). Chemically, the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher Mg content. The hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size, spherulitic habit, poor crystallinity along the c-axis, and common association with corrensite and/or chlorite-corrensite mixed layers. Most of the hydrothermal chlorite is Mg rich and has a structural formula close to that of clinochlore (XFe

The Bernardan uraniferous district is located in the Western Marche Complex (French Massif Central) and is represented by disseminated uranium deposits. The U orebodies occur as episyenitic pipes: vuggy granites that resulted from quartz leaching. Observations of deeper structures confirm the widespread occurrence of coffinite and the absence of visible primary U minerals. Barren episyenites also exist and may be either connected or unconnected with uraniferous zones. This study describes the alteration history of the episyenitic pipes and discusses its effects on the present U distribution. The physicochemical conditions of alteration events are supported by detailed alteration petrography, fluid inclusion studies, stable isotope data, and rare earth element contents of secondary minerals including carbonates, fluorite, and clay minerals of the illite-smectite conversion series. Isotopic ages have been collected using K-Ar radiochronology on alkali feldspars and illite-smectite mixed layers. From the petrography of episyenites, three successives crystallization stages are evident: 1. Geodic crystallization of secondary K feldspars + or - quartz + dolomite +- fluorite occurred in all types of episyenitic pipes. This stage followed the formation of episyenites (308-315 Ma). Secondary minerals began to form from saline and 18 O-rich fluids at high temperature, up to 360 degrees C for dolomite, in closed systems, as indicated by the absence of significant fracture network. 87 Sr/ 86 Sr ratios in carbonates are compatible with the values of alumino-potassic granitic bodies from the Western Marche Complex. This event is interpreted as the result of progressive cooling of residual fluids derived from fractional crystallization of the leucogranites of the Bernardan deposits. 2. The second stage occurred in the Dogger epoch (170-140 Ma). It consisted of argillization of some episyenitic pipes in response to infiltration of 18 O-rich fluids that originated in sedimentary basins. This alteration stage occurred at 100 degrees C. The close association of U silicates with clay minerals suggests that U was transported by these fluids. However, its source may not lie in the sedimentary basins. 3. The distribution of mineralization has been modified by present-day supergene mobilization which has led to oxidation and the formation of high-grade ore near the top of the deposit and in horizons adjacent to major faults or fractures subject to infiltration.