Geological Setting, Petrology, and Geochemistry of Granitic Pegmatites and Leucogranites Hosting U-Th-REE Mineralization at Fraser Lakes Zone B, Wollaston Domain, Northern Saskatchewan, Canada

Exploration & Mining Geology, Vol. 21, No. 1, 2013

CHRISTINE L. MCKECHNIE, IRVINE R. ANNESLEY , AND KEVIN M. ANSDELL

The U-Th-REE mineralization at Fraser Lakes Zone B is hosted by granitic pegmatites and leucogranites, which lie along the deformed contact between Paleoproterozoic metasedimentary gneiss of the Wollaston Group and Archean orthogneiss, approximately 25 km from the southeastern edge of the Athabasca Basin. The pegmatites/leucogranites are subcordant to concordant with the regional foliation and are concentrated within a northeast-plunging antiformal fold nose, the study area, which lies west of Fraser Lakes. The mineralized pegmatites/leucogranites in the western part of the study area (Group A) are typically enriched in U (± Th) with Th/U ~1, and contain uraninite, thorite, zircon, and allanite. Those intruding the central part of the study area (Group B) are generally enriched in Th and LREE, with Th/U >2, and contain monazite, thorite (commonly U-enriched), and zircon. The pegmatites and leucogranites in Group A tend to be slightly enriched in SiO2 and depleted in TiO2 relative to those in Group B and are interpreted to represent more evolved crustal melts. Both groups are peraluminous and show varied chemistry. Partial melting (~850ºC and 9 kbar) of a metasedimentary rock-dominated source, entrainment of accessory minerals as xenocrysts, and assimilation–fractional crystallization processes combined to enrich the granitic pegmatites/leucogranites in U, Th, and REE elements. Transfer of melt from the source region to the crystallization sites was assisted by deformation within and along major structural zones, such as the folded Archean–Paleoproterozoic discontinuity. The character of mineralization and structural control in the study area is reminiscent of the alaskite-hosted deposits at Rössing, Namibia.

Keywords: U-rich pegmatites, Th-REE-rich pegmatites, Fraser Lakes Zone B, Wollaston Domain, Athabasca Basin, U metallogeny