Abstract

The Fraser Lakes area in the Wollaston Domain, in northern Saskatchewan, Canada, is located 25 km from the southeastern edge of the uranium-rich Athabasca Basin; it hosts a number of U- and Th–REE-bearing granitic pegmatites and leucogranites. At Fraser Lakes Zone B, the pegmatites and leucogranites intrude the deformed contact between Paleoproterozoic metasedimentary gneisses of the Wollaston Group and Archean orthogneisses, and have characteristics typical of the Abyssal pegmatite subclass. For two examples of Group-A U- and Th-enriched pegmatite and leucogranite, and two examples of Group-B Th- and LREE-enriched pegmatite and leucogranite samples that have minimal petrographic evidence of later alteration, we analyzed selected minerals by electron microprobe to provide constraints on the age and temperature of intrusion. The Group-A pegmatites contain uraninite with variable CaO and SiO2, indicative of later recrystallization, uranoan thorite spatially associated with complexly zoned zircon (with some intermediate solid-solution between the two minerals), rare coffinite, and titaniferous magnetite and ilmenite where they intrude the Archean orthogneisses. CHIME dating of the most pristine uraninite yielded ages between 1.85 and 1.80 Ga, consistent with crystallization from the pegmatite-forming melt. The Group-B pegmatites contain monazite-(Ce) with significant Th substitution, uranoan thorite, zircon (also showing extensive solid-solution toward thorite), and rare xenotime and pyrochlore. CHIME dating of monazite from the Group-B pegmatites gave older ages of ca. 2.1 to 2.2 Ga, suggesting that they are xenocrysts from the source region of the melt. Biotite dehydration reactions have led to significant development of leucosome in the host pelitic gneisses, and likely resulted in the formation of granitic melt at depth.

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