U-TH-REE-MINERALIZED GRANITIC PEGMATITES FROM FRASER LAKES ZONE B: FERTILE CRUSTAL MELTS AND POTENTIAL U PROTORE?

The Fraser Lakes Zone B deposit consists of stacked granitic pegmatite/leucogranite sheets with U-Th-REE mineralization that intruded along the deformed contact between Paleoproterozoic metasedimentary gneisses of the Wollaston Group and underlying Archean orthogneisses. The U-Th-REE mineralization is a mixture of primary magmatic and xenocrystic peritectic radiogenic minerals, with minor younger hydrothermal mineralization. The latter is due to later fluid flow events, most likely related to those generating the rich U deposits in the nearby prolific Athabasca Basin. The mineralogy and chemistry, lack of an associated large intrusive body in the area, and concordant nature of the Fraser Lakes Zone B pegmatites suggests they did not form via fractionation of a hypothetical large granitoid body at depth.

Instead, petrological evidence from the pegmatites and their host rocks suggests that biotite-dehydration reactions generated the pegmatitic melts at 6–9 kbar and 800–850ºC during peak thermal metamorphism of the Trans-Hudson Orogeny at ca. 1815 Ma. The lack of connecting pathways to migmatitic leucosomes in the host pelitic gneisses, plus textural and geochemical data, suggests that their source is U-Th-REE-enriched, peraluminous pelitic gneisses at depth, similar in composition to those in the immediate vicinity of the pegmatites, but more garnet-rich.

Melt transfer from the source through the crustal melt transfer zone at Fraser Lakes to the hypothetical final zone of upper crustal emplacement would have been facilitated by major structures in the Fraser Lakes Zone B area; the most important being the highly sheared unconformity between the Wollaston Group metasediments and underlying Archean gneisses, sheared fold limbs, and major antiformal fold noses.

Our model for the formation of these mineralized pegmatites is similar to those for the origin of U-enriched pegmatites and leucogranites in the Pan-African Orogen of Namibia, the Svecofennian Orogen of Scandinavia, and the Grenville Province in eastern Canada. This model is applicable to other pegmatite-hosted U-Th-REE deposits in the Wollaston and Mudjatik domains of northern Saskatchewan, Canada. Also, similar pegmatites are thought to act as primary uranium sources for unconformity-related uranium deposits in the nearby Athabasca Basin.