Hydrothermal alteration of monazite in the Precambrian crystalline basement of the Athabasca Basin (Saskatchewan, Canada): implications for the formation of unconformity-related uranium deposits

Microanalytical studies of basement rocks below the Athabasca sandstone basin indicate that monazite is the dominant uranium-bearing mineral in the study area. Drill core samples of hydrothermally altered basement show that monazite is commonly altered to a Th-silicate phase, and uranium has been significantly mobilized. On average, 75% of the uranium bound to monazite is leached out during monazite alteration. In contrast, no substantial mobilization of uranium from detrital minerals (e.g. zircon) has yet been observed in the Athabasca sandstones. It is suggested that hydrothermal alteration of granitic rocks (especially potassic pegmatoids and potassic orthogneisses) of the sub-Athabasca basement, represents the most important uranium source for the unconformity-type deposits.