Organic Carbon-rich Sulfide-rich Metasediments from the Fraser Lakes Zone B Uranium-thorium-rare earth element (U-Th-REE) Deposit, northern Saskatchewan (Canada): Trace-metal Concentration Data, Paleoredox Environment, and Implications for the Metal Source of U/C-type Uranium Deposits

English Abstract:

The U-Th-REE mineralization at Fraser Lakes Zone B is hosted by ca. 1800 Ma granitic pegmatite/leucogranite sheets intruding the tectonic decollement between basal Paleoproterozoic Wollaston Group pelitic (+/-graphitic) gneisses and underlying Archean granitoid orthogneisses. These pelitic gneisses are inferred to be metamorphosed black shales deposited in a back-arc continental margin marine environment. The "Rossing-type" Zone B deposit and its host rocks are located ~20-25 km southeast of the Athabasca Basin (eastern Wollaston Domain, northern Saskatchewan), and provide evidence for being both potential economic ore and a source rock for unconformity-type (U/C-type) uranium deposits. Although U/C-type uranium deposits have been studied extensively, questions still remain, including the origin of their transition metals (e.g. Cr, Co, Cu, Mo, Ni, V, and Zn). Using trace-metal geochemical data from the metamorphosed sulfide-rich black shales at Fraser Lakes, we present new insights into Paleoproterozoic shale formation and their metal potential for U/C-type deposits.

The graphite-sulfide-bearing pelitic gneisses are highly variable in composition, with C and S ranging from 0.5 to 7.5 and 0.2 to 16.0 wt%, respectively, with varied metal concentrations. However, the highest Ni (2600 ppm), Cu (1500 ppm), Mo (1740 ppm), Zn (943 ppm), and Co (137 ppm) are hosted in the most S-rich rocks; whereas the highest Cr (259 ppm) and V (815 ppm) occur in the most Fe-rich rocks. Uranium (up to 100 ppm) and Th (up to 557 ppm) are enriched relative to the North American Shale Composite (NASC) with values of 2.66 ppm U and 12.30 ppm Th. The abundances of Co, Cr, La, Mo, Ni, Th, U, V, and Zn are comparable to, but much higher than, those of post-Archean shale.

After assessing the degree of element mobility, we conclude the wide range of chemical variation in the fresh graphitic pelitic gneisses reflects original sedimentary facies variations of metal content and constrains original paleoredox conditions. This study documents that pre-Athabasca metal enrichments in the basal Wollaston Group represent the most likely metal source for U/C-type deposits.