Two Sources of Uranium at the Millennium Uranium Deposit, Athabasca Basin, Saskatchewan, Canada

Summary Renewed interest in nuclear energy has lead to increased uranium exploration to meet the demand for new uranium resources. The Athabasca Basin in northern Saskatchewan, Canada, hosts the world’s highest grade unconformity-related uranium deposits. Uranium mineralization is hosted within the Athabasca Group above the unconformity and the crystalline basement rocks. The Athabasca Group-hosted deposits have been studied extensively for more than 30 years and multiple depositional models have been developed. The Millennium uranium deposit, located 35 km north of the Key Lake mine, occurs between two subparallel reverse faults. The zone between these two faults represents a dilational structure that increased the permeability of the basement rocks. Uranium mineralization occurs in a variety of styles including (1) massive replacement, (2) fracture filling veins, (3) fine-grain aggregates associated with “mini” roll fronts, and (4) disseminated grains. Massive replacement uraninite is associated with chlorite whereas fracture filled uraninite is associated with euhedral quartz-carbonate veins. These two styles of uranium mineralization constitute the majority of the ore whereas fine-grain aggregate and disseminated uraninites are minor components of the ore. The disseminated grains are altered uraninite with galena, are associated with hematite and clay minerals, and are thought to be primary grains that were altered by a sulfur rich fluid. Massive and vein-type uraninites have low δ 18 O values from -35‰ to -15‰, which are typical of oxygen isotopic values of uraninite from other uranium deposits in the Athabasca basin. Relict metamorphic and secondary euhedral quartz grains associated with vein-type uraninite have δ 18 O values of 11.1‰ to 15.1‰ and 16.2‰ to 19.1‰ respectively. The low δ 18 O values for uraninite and the high δ 18 O values for quartz suggest that the original isotopic composition of both minerals have been modified by recent low-temperature meteoric waters. The chemical Pb and isotopic 207 Pb/ 206 Pb ages of the massive (style 1), vein-type (style 2), and fine-aggregate (style 3) uraninite cluster at 1400-1200 and 1100-900 Ma. The ~1400 Ma ages coincide with the primary mineralization event for many of the uranium deposits (1550-1400 Ma) within the Athabasca Basin. The younger age group reflects lead loss associated with post depositional fluid events. However, unlike other uranium deposits from the Athabasca basin, disseminated uraninite (style 4) have 207 Pb/ 206 Pb ages from 1770-1650 Ma. These ages are older than the depositional age for the Athabasca sediments (~1710 Ma) and are similar to the ages from the Beaverlodge vein-type uranium deposits. These ages suggest that the basement rocks that host the Millennium deposit contained disseminated uraninite similar to fine-grain uraninite from the metasediments from the Karpinka Lake uranium prospect (50 km southwest of the Key Lake), suggesting that the basement rocks, in addition to the Athabasca Group rocks, can be a possible source of uranium. The 207 Pb/ 206 Pb age of the galena associated with style 4 uraninite is ~1400 Ma and reflects the time when the disseminated uraninite was reset during the primary mineralization event.