Magmatic and Metamorphic Uraninite Mineralization in the Western Margin of the Trans-Hudson Orogen (Saskatchewan, Canada): A Uranium Source for Unconformity-Related Uranium Deposits?
Résumé
The genetic model for the giant unconformity-related uranium deposits of the Athabasca Basin is still being debated, with one of the main issues being the source of the uranium concentrated by Mesoproterozoic era (ca. 1.60-1.00 Ga) diagenetic-hydrothermal events at the interface between the Athabasca Basin and the underlying Archean/Paleoproterozoic basement rocks. Currently, accessory minerals like monazite, zircon, and/or apatite from the sedimentary basin and basement rocks are proposed as the primary uranium source for these high-grade uranium deposits. Numerous occurrences of U mineralization of Hudsonian age have been documented for decades all around the Athabasca Basin; however, so far these have not been regarded as viable U sources. Here, a systematic and detailed study of two areas of basement rocks near the eastern part of the Athabasca Basin is presented (i.e., the Way Lake property, lying outside the current margin of the basin, and the Moore Lakes property, currently covered by the basin). This study highlights the significant and widespread occurrence of Hudsonian (ca. 1.81-1.76 Ga) uranium oxide (UO2) mineralization in these zones. Two types of mineralization are identified and documented here: magmatic uranium oxides related to granitic pegmatites and leucogranites, which are more common, and high-temperature, vein-hosted uranium oxides, which have the highest grades. The two types were formed during the peak (1.82-1.81 Ga) and/or postthermal peak (1.81-1.72 Ga) events related to the evolution of the Trans-Hudson orogeny. The magmatic uranium oxides formed by partial melting of Wollaston Group metasedimentary rocks. The origin of the vein-type occurrences is unclear, but their high thorium and rare earth element (REE) contents suggest a high-temperature process associated with Ca and/or Na metasomatism. The uranium oxides are associated with other U-, Th-, and REE-bearing accessory minerals like U-rich thorite, thorite, zircon, and/or monazite, adding to the exceptional U contents (100-2,460 ppm) of these UO2-bearing rocks (up to 200 times more primarily enriched in U than other basement or basin rock types). A 3-D model of a 1,300 X 630 x 200-m basement zone from the Way Lake property indicates that uraninite-bearing granitic pegmatites and leucogranites represent 7% of the total volume of crystalline rock. Within this rock volume are approximately 8,121 (assuming a mean U content of 250 ppm) to 16,242 (assuming a mean U content of 500 ppm) tonnes U. The U tonnage of this limited rock volume, contained mainly by the Hudsonian-age UO2, corresponds to between 4% (for McArthur River) and 103% (for Rabbit Lake) of the U tonnage of known unconformity-related U deposits of the basin. Some of the studied rock samples, even macroscopically fresh and located far away from any known unconformity-related U. deposit, present clear evidence of alteration, including clay minerals, aluminophosphate-sulfate (APS) minerals, and UO2 dissolution, indicating the percolation of the brines associated with the formation of unconformity-related uranium deposits when the basin was far more geographically extensive. Due to geologic similarities between the studied zones and the basement domains from the eastern part of the Athabasca Basin, i.e., the Hearne province, it is proposed that these domains hosted widespread Hudsonian-age uranium oxide protores. These protores provided easily leachable uranium for the metal enrichment of basinal brines during their percolation within the basement and the formation of the unconformity-related U deposits. These observations bring new insight to the debate about the genetic model of unconformity-related U deposits, and reinforce the metal source potential of the basement compared to that of the sedimentary basin.