The Fraser Lakes Zone B U-Th-REE deposit and its host rocks : implications for pegmatite- and leucogranite-hosted U-Th-REE deposits in northern Saskatchewan, Canada
The Fraser Lakes Zone B U-Th-LREE deposit is located approximately 50 km SE of the Key Lake uranium mine in the Wollaston Domain of Northern Saskatchewan, Canada. Here, a number of variably radioactive pegmatites and leucogranites intruded the highly sheared, unconformable contact between Paleoproterozoic Wollaston Group metasedimentary rocks and the underlying Archean orthogneisses, in a NNE-plunging antiformal fold nose. The U-Th-LREE mineralized pegmatites were subdivided into two groups based on their mineralogical and spatial differences. Both groups contain variable amounts of quartz, k-feldspar, plagioclase, biotite, magnetite, and ilmenite. The Group A pegmatites, in the eastern part of the fold nose, contain small amounts of uraninite, uranoan thorite, zircon, and rare coffinite and allanite, and are typically U- ±Th-enriched (Th/U ~1). The Group B pegmatites, in the central part of the fold nose, are Th- and LREE- enriched (Th/U up to ~20) and contain small amounts of monazite, uranoan thorite, and zircon, with rare allanite and pyrochlore-group minerals. The Group A pegmatites are interpreted to represent slightly more evolved crust melts that underwent higher degrees of restite unmixing, whereas the Group B pegmatites are postulated to have resulted from partial melting and incorporation of more restitic and peritectic material. Field relationships and chemical age dating of the pegmatites suggests that they formed between 1.85 to 1.80 Ga, during peak thermal metamorphism and ensuing decompression related to the ca. 1.9 to 1.8 Ga Trans-Hudson Orogen (THO). Examination of the pelitic gneiss host rocks at Fraser Lakes Zone B revealed that the area underwent high temperature (up to ~780°-800° C), low to medium pressure (max pressures of ~7-8 kbar), upper amphibolite to granulite facies metamorphism (accompanied by partial melting), followed by isothermal decompression (down to ~3 kbar) at approximately 1.81-1.80 Ga. After this, the rocks began to cool and retrograde metamorphism took place at amphibolite to greenschist facies. Later faulting and hydrothermal fluid flow served to alter the pegmatites and led to local remobilization of U and other metals in fractures and faults. The pegmatite-forming melts are interpreted to have formed by partial melting (via biotite-dehydration reactions) of U-, Th- and REE-enriched rocks similar to the Wollaston Group at depth. The melts migrated upwards to the emplacement level within and along major structural zones, including sheared fold limbs, metamorphic foliations, and the deformed Archean/Paleoproterozoic contact. The melt was concentrated in antiformal fold noses and other dilational zones, where it crystallized to form the pegmatite and leucogranite bodies. The style of mineralization and the structural control is similar to other pegmatite-hosted uranium deposits, including the Rössing alaskite-hosted deposit in Namibia. The knowledge gained from this study can be applied to exploration for similar deposits in northern Saskatchewan and other areas with similar geology.
abyssal-type granitic pegmatites, U-rich pegmatites, Th-LREE-rich pegmatites, uranium metallogeny, Fraser Lakes Zone B
Master of Science (M.Sc.)