Unconformity-type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks. Mineralization may extend up to 400 m into the underlying basement rocks. Whereas sandstone-hosted unconformity-type deposits have been well studied, deposits hosted primarily in the basement have not. This study examines the deposits at Rabbit Lake, Dawn Lake, and McArthur River, in the Athabasca basin of Canada, which are hosted by the metamorphic Archean and Early Paleoproterozoic rocks forming the basement to younger Late Paleoproterozoic sandstones. Alteration is similar in the three deposits and is characterized by three distinct paragenetic stages: (1) preore alteration involving illitization of plagioclase and amphibole, followed by chloritization of biotite and illite, which formed at ca. 230°C; (2) ore-stage alteration, characterized by uraninite and coarse-grained illite, which formed at ca. 240°C; (3) postore alteration comprising spherulitic dravite, vein chlorite, quartz, calcite, and Fe, Cu, Co, and Pb sulfides, which formed at ca. 135°C. Fluid circulation associated with emplacement of later Mackenzie dikes initiated partial recrystallization of uraninite. A later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca. 50°C.Stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids (δD = -43 to -21‰ . , δ 18 O = 3-8‰) were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the Athabasca Formation and transported it into the basement via infiltration along fracture zones associated with reverse faults. Graphitic units in the basement and preore alteration served as both physical (fractured zones) and chemical (reductants) traps for the uranium mineralization. The basinal fluids were responsible for the preore illite-chlorite, synore uraninite-illite, and the early postore alteration events; this differs from many other sandstone-hosted deposits, where both oxidized basinal and reduced basement-derived fluids were responsible for uranium precipitation.

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Uraninite grains from two sites of unconformity-type uranium mineralization, the high-grade basement-hosted McArthur River deposit and the sediment-hosted Virgin River prospect in the Paleoproterozoic Athabasca Basin in northern Saskatchewan, Canada, were investigated with a scanning electron microscope and analyzed with an electron microprobe. Electron back-scatter images, chemical compositions and calculated chemical U/Pb ages from the uraninite indicate substitution of radiogenic Pb by Ca, Si, and Fe from later fluid-circulation events after initial formation of the uraninite at ca. 1600 Ma. Conversion of uraninite to coffinite,with SiO 2 up to 10 wt%, seems to have occurred at ca. 700 Ma. The intensity and the character of the substitutions depend on both the amount and the composition of the alteration fluids, which may be influenced by the characteristics of the host rocks. SommaiREDes grains d'uraninite provenant de deux zones de minéralisation en uranium de type discordance, le gisement à teneur élevée de McArthur River, situé dans le socle, et la minéralisation de Virgin River, située dans les grès, dans le bassin Paléoprotérozoïque d'Athabasca, Saskatchewan, Canada, ont été étudiés par microscope électronique à balayage et par microsonde électronique. L'imagerie en électrons rétrodiffusés, la composition chimique, et les ages chimiques U-Pb calculés pour l'uraninite indiquent un remplacement du plomb radiogénique par Ca, Si et Fe, au cours d'événements de circulation de fluide après la minéralisation initiale, survenue à environ 1600 Ma. La conversion de l'uraninite à la coffinite, avec près de 10% poids de SiO 2 , est survenue vers 700 Ma. L'intensité et le caractère des substitutions dépendent de la quantité et la composition chimique de la phase fluide, qui pourrait avoir été influencée par les caractéristiques des roches encaissantes.

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