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 bas in indicate that monazite is the dominant uranium-bearing mineral in the st udy area. Drill core samples of hydrothermally altered basement show that m onazite is commonly altered to a Th-silicate phase, and uranium has been si gnificantly mobilized. On average, 75% of the uranium bound to monazite is leached out during monazite alteration. In contrast, no substantial mobiliz ation 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 orthogneisse s) of the sub-Athabasca basement, represents the most important uranium sou rce for the unconformity-type deposits.