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Includes 16 papers presented at an international colloquium., Includes 16 papers presented at an international colloquium.

The uranium deposits are located in the vicinity of an unconformity between Archaean and Aphebian basement and Helikian Athabasca sedimentary cover and occur in either or in both. The metallogenic role of the unconformity and of the weathering and alteration associated with the palaeosurface are still disputed. Petrographic, mineralogical and isotope studies are used to elucidate these. The basement has been subject to retrograde greenschist facies metamorphism. Below the Athabasca sandstone the basement shows assemblages resulting from two alteration phases, representing a weathering zone later impregnated by formation waters from the Athabasca sandstones. Athabasca sandstone diagenesis is characterised by various neoformed minerals. The alteration halo around the uraniferous deposit consists of ferrimuscovite, aluminous illite and alumino-magnesian chlorite whilst the gangue is characterised by abundant magnesian chlorite. It is interpreted as deriving from diagenetic-hydrothermal fluids of the Athabasca sandstone. The halo around the deposits give K/Ar dates of about 1 250 Ma and oxygen and hydrogen isotope patterns similar to those of Athabasca sandstone formation waters. In mineralised facies, anomalously low K/Ar ages are found; these and the anomalous hydrogen isotope values are interpreted as indicating retrograde hydrogen isotope exchange in clays with post-Cretaceous meteoric waters under the influence of a radiation field. This phenomenon is an important complement to clay mineral studies in uranium prospecting, and a combination of K/Ar data and clay water content data may also be useful., The uranium deposits are located in the vicinity of an unconformity between Archaean and Aphebian basement and Helikian Athabasca sedimentary cover and occur in either or in both. The metallogenic role of the unconformity and of the weathering and alteration associated with the palaeosurface are still disputed. Petrographic, mineralogical and isotope studies are used to elucidate these. The basement has been subject to retrograde greenschist facies metamorphism. Below the Athabasca sandstone the basement shows assemblages resulting from two alteration phases, representing a weathering zone later impregnated by formation waters from the Athabasca sandstones. Athabasca sandstone diagenesis is characterised by various neoformed minerals. The alteration halo around the uraniferous deposit consists of ferrimuscovite, aluminous illite and alumino-magnesian chlorite whilst the gangue is characterised by abundant magnesian chlorite. It is interpreted as deriving from diagenetic-hydrothermal fluids of the Athabasca sandstone. The halo around the deposits give K/Ar dates of about 1 250 Ma and oxygen and hydrogen isotope patterns similar to those of Athabasca sandstone formation waters. In mineralised facies, anomalously low K/Ar ages are found; these and the anomalous hydrogen isotope values are interpreted as indicating retrograde hydrogen isotope exchange in clays with post-Cretaceous meteoric waters under the influence of a radiation field. This phenomenon is an important complement to clay mineral studies in uranium prospecting, and a combination of K/Ar data and clay water content data may also be useful.

The uranium at Ecarpiere (France) was recovered from ground ore by a classical acid hydrometallurgical process; the tailings were stored about 30 years ago. The investigation combined uranium series disequilibrium studies on the solids; a petrological, mineralogical and geochemical study of the tailing solid samples; geochemistry of the porewater and mineralogical and geochemical investigation of the particles left on the filters; and leaching tests on the tailings with demineralised water and simulated rain water. The main conclusions are that significant diagenesis has occurred over the last 30 years and that the leaching behaviour of radium and other metals is different in fresh and stored tailings and in tailings from different sites., The uranium at Ecarpiere (France) was recovered from ground ore by a classical acid hydrometallurgical process; the tailings were stored about 30 years ago. The investigation combined uranium series disequilibrium studies on the solids; a petrological, mineralogical and geochemical study of the tailing solid samples; geochemistry of the porewater and mineralogical and geochemical investigation of the particles left on the filters; and leaching tests on the tailings with demineralised water and simulated rain water. The main conclusions are that significant diagenesis has occurred over the last 30 years and that the leaching behaviour of radium and other metals is different in fresh and stored tailings and in tailings from different sites.

The various types of uranium deposit in the Carswell structure are related to a series of metallogenic events from Aphebian to Hercynian times. The behaviour of the metallic elements is discussed for the various stages: lower Proterozoic sedimentation and volcanism, Hudsonian metamorphism and anatexis, late Hudsonian retrograde metamorphism, post Hudsonian basement weathering, middle Proterozoic Athabasca sediment formation and diagenesis, middle Proterozoic unconformity-related deposit formation, middle and upper Proterozoic disturbances, Ordovician formation of the Carswell structure, and remobilisation of the deposits since 380 Ma. The proposed model for the formation of the unconformity-related deposits involves leaching of the basement (rich in U, Au, Ni) by brines issuing from the sedimentary cover. During the Hudsonian anatexis, Mo and U are expressed as molybdenite and uraninite respectively. Retrograde metamorphism and weathering give rise to significant adsorbed uranium. During a tectonic event, hydrothermal oxidising and acid fluids move through the fractured weathered basement solubilising the metallic elements as chlorides. In graphitic metasediments, graphite and sulphides react with the brines to form methane and hydrogen sulphide. These reducing fluids react with metal-rich fluids above the graphitic sediments to form zones of mineralisation., The various types of uranium deposit in the Carswell structure are related to a series of metallogenic events from Aphebian to Hercynian times. The behaviour of the metallic elements is discussed for the various stages: lower Proterozoic sedimentation and volcanism, Hudsonian metamorphism and anatexis, late Hudsonian retrograde metamorphism, post Hudsonian basement weathering, middle Proterozoic Athabasca sediment formation and diagenesis, middle Proterozoic unconformity-related deposit formation, middle and upper Proterozoic disturbances, Ordovician formation of the Carswell structure, and remobilisation of the deposits since 380 Ma. The proposed model for the formation of the unconformity-related deposits involves leaching of the basement (rich in U, Au, Ni) by brines issuing from the sedimentary cover. During the Hudsonian anatexis, Mo and U are expressed as molybdenite and uraninite respectively. Retrograde metamorphism and weathering give rise to significant adsorbed uranium. During a tectonic event, hydrothermal oxidising and acid fluids move through the fractured weathered basement solubilising the metallic elements as chlorides. In graphitic metasediments, graphite and sulphides react with the brines to form methane and hydrogen sulphide. These reducing fluids react with metal-rich fluids above the graphitic sediments to form zones of mineralisation.

The lateritisation released the uranium from especially resistant mineral structure which resulted in a preconcentration zone where a large proportion of uranium is adsorbed and therefore is easily leached by an acidic brine. Some uranium deposits are located on a basement high where relatively fresh rocks were in contact with the Athabasca cover, a situation which favoured the interaction of sedimentary brine with the sulphur- and graphite-rich sediments., The lateritisation released the uranium from especially resistant mineral structure which resulted in a preconcentration zone where a large proportion of uranium is adsorbed and therefore is easily leached by an acidic brine. Some uranium deposits are located on a basement high where relatively fresh rocks were in contact with the Athabasca cover, a situation which favoured the interaction of sedimentary brine with the sulphur- and graphite-rich sediments.

Recent mobility of U was investigated in the Coutras U deposit (Gironde, France) located in Lutetian sediments near the surface. The 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 characterised by oxidising waters flowing toward the Dronne River. Activity ratios (AR) of 230Th/234U in rocks, which range from 0.55 to 12, demonstrate recent U accumulations 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 favoured along redox barriers where Mo, V, Se and As are also mobilised. Waters in the Le Fieu area are nearly stagnant; they are reduced and exhibit a different chemical composition from waters in the Jean Vincent area, with higher contents of Ca, Mg, Na, K, Cl, HCO3, PO4, Ra, Ba, Mn, higher 234U/238U AR (1.33-4.63) and lower U contents. 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., Recent mobility of U was investigated in the Coutras U deposit (Gironde, France) located in Lutetian sediments near the surface. The 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 characterised by oxidising waters flowing toward the Dronne River. Activity ratios (AR) of 230Th/234U in rocks, which range from 0.55 to 12, demonstrate recent U accumulations 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 favoured along redox barriers where Mo, V, Se and As are also mobilised. Waters in the Le Fieu area are nearly stagnant; they are reduced and exhibit a different chemical composition from waters in the Jean Vincent area, with higher contents of Ca, Mg, Na, K, Cl, HCO3, PO4, Ra, Ba, Mn, higher 234U/238U AR (1.33-4.63) and lower U contents. 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.

In Europe, many uranium deposits are either of Permian age or are located in or close to Permian sedimentary rocks. From the point of view of uranium metallogeny, the Permian period was important because the sediments were deposited under contrasting redox conditions, the intense surface weathering was favourable to uranium leaching, volcanic emissions were significant (illustrated in particular by the intercalation of cinerites in the sediments) and extensional tectonism facilitated fluid circulation in the sedimentary basins and in the basement. While the granite-associated uranium deposits have been dated by the U-Pb isotopic method as Permian, no uranium deposits located in Permian sedimentary rocks are known to be of Permian age. However, during the Permian, significant metal preconcentration took place. Four models are discussed: formation of uranium deposits during the Permian, uranium preconcentration during the Permian, formation of uranium deposits in the Permian host rocks and spatial association of uranium deposits with Permian formations., In Europe, many uranium deposits are either of Permian age or are located in or close to Permian sedimentary rocks. From the point of view of uranium metallogeny, the Permian period was important because the sediments were deposited under contrasting redox conditions, the intense surface weathering was favourable to uranium leaching, volcanic emissions were significant (illustrated in particular by the intercalation of cinerites in the sediments) and extensional tectonism facilitated fluid circulation in the sedimentary basins and in the basement. While the granite-associated uranium deposits have been dated by the U-Pb isotopic method as Permian, no uranium deposits located in Permian sedimentary rocks are known to be of Permian age. However, during the Permian, significant metal preconcentration took place. Four models are discussed: formation of uranium deposits during the Permian, uranium preconcentration during the Permian, formation of uranium deposits in the Permian host rocks and spatial association of uranium deposits with Permian formations.

There is a good correlation between uranium, total sulphur and organic carbon. Uranium is mainly fixed in organics as organo-uranyl compounds. Uranium minerals detected include coffinite, pitchblende and secondary hexavalent U minerals. Accumulation of uranium was favoured by a permeable, organic-rich and slightly acid environment. Adsorption processes probably enhanced ore formation. The lack of oxidised tongues may be the result of oxygen-depleted uranium-bearing solutions when they reached the organic-rich zone. The Coutras uranium deposit is then a good example of a tabular-type deposit which was deposited during early diagenesis., There is a good correlation between uranium, total sulphur and organic carbon. Uranium is mainly fixed in organics as organo-uranyl compounds. Uranium minerals detected include coffinite, pitchblende and secondary hexavalent U minerals. Accumulation of uranium was favoured by a permeable, organic-rich and slightly acid environment. Adsorption processes probably enhanced ore formation. The lack of oxidised tongues may be the result of oxygen-depleted uranium-bearing solutions when they reached the organic-rich zone. The Coutras uranium deposit is then a good example of a tabular-type deposit which was deposited during early diagenesis.

The temperature range is large. High-temperature fluids carry significant amounts of uranium but most uranium deposits appear to have been formed at temperatures of between 90 and 250 degrees C. Chemical compositions of the fluids are quite variable. Very often the fluids are CO2-rich but this is not invariably the case. The most consistent and important condition for uranium transport seems to be a high level of oxidation. Relatively saline brines appear to have played an important role in the genesis of some of the largest deposits., The temperature range is large. High-temperature fluids carry significant amounts of uranium but most uranium deposits appear to have been formed at temperatures of between 90 and 250 degrees C. Chemical compositions of the fluids are quite variable. Very often the fluids are CO2-rich but this is not invariably the case. The most consistent and important condition for uranium transport seems to be a high level of oxidation. Relatively saline brines appear to have played an important role in the genesis of some of the largest deposits.

The uraniferous mineralisation in the Brousse-Broquies Basin, spatially associated with the unconformity between a metamorphic basement and a Stephano-Permian nonmetamorphic cover, is unique among the uranium deposits of Western Europe. It is essentially in the form of the tetragonal uranium oxide alpha U3O7. Chemical analysis shows that Pb may have been separated out. Depositional conditions have been deduced from mineral assemblages in the sequence of alterations and paragenesis. It implies a slightly alkaline pH. The temperature of formation, deduced from fluid-inclusion study, is 140 (plus or minus 25) degrees C and the fluid is a brine rich in NaCl and containing divalent cations., The uraniferous mineralisation in the Brousse-Broquies Basin, spatially associated with the unconformity between a metamorphic basement and a Stephano-Permian nonmetamorphic cover, is unique among the uranium deposits of Western Europe. It is essentially in the form of the tetragonal uranium oxide alpha U3O7. Chemical analysis shows that Pb may have been separated out. Depositional conditions have been deduced from mineral assemblages in the sequence of alterations and paragenesis. It implies a slightly alkaline pH. The temperature of formation, deduced from fluid-inclusion study, is 140 (plus or minus 25) degrees C and the fluid is a brine rich in NaCl and containing divalent cations.