The Geology and Geochemistry of the Millennium Uranium Deposit, Athabasca Basin, Saskatchewan, Canada

Renewed interest in the development and expansion of nuclear energy has lead to increased uranium exploration, in order to meet the demand for new uranium resources. The Athabasca Basin in northern Saskatchewan, Canada, hosts the world's largest and highest grade unconformity-associated uranium deposits. Uranium mineralization is hosted at the Athabasca Group unconformity or within the crystalline basement rocks. The unconformity hosted deposits have been studied extensively for more than 30 years and multiple models have been developed. In comparison to the unconformity hosted deposits, the basement-hosted deposits have not been well documented. The basement hosted Millennium deposit was discovered in 2000, and is located 35 km north of the Key Lake mine. The orebody is situated between a deformed graphitic pelite known as the "Marker Unit" (a reverse fault) and the "Mother" fault located foot-wall to the main orebody. Movement along these two sub-parallel reverse faults resulted in the development of a dilational structure that increased the permeability of the basement rocks within the hanging-wall block.;The chemical Pb and isotopic 207Pb/206Pb 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 groups reflect lead loss associated with post depositional fluid events. However, unlike other uranium deposits from the Athabasca basin, disseminated uraninite (style 4) have 207 Pb/206Pb ages from 1770-1650 Ma. These ages are older than the depositional age for the Athabasca sediments (∼1710 Ma) and similar to U-Pb dates obtained from the Beaverlodge vein-type uranium deposits. This suggests the basement rocks, in addition to the overlying Athabasca Group rocks, are a possible source of uranium mineralization at the Millennium deposit. The 207Pb/206Pb ages obtained for galena associated with style 4 uraninite is ∼1400 Ma and reflects the time when the disseminated uraninite was reset during the primary mineralization event.;Within, the Millennium deposit uranium mineralization occurs in four distinct mineralization styles which include: style 1- massive replacement, style 2- fracture filling veins, style 3- "mini" roll-fronts, and style 4- disseminated mineralization. Massive replacement uraninite is associated with chlorite whereas fracture filled uraninite is associated with euhedral quartz-carbonate veins. The disseminated grains are altered uraninite with galena, are associated with hematite and clay minerals, and are assumed to be primary grains that were altered by a sulfur rich fluid. In drill core, massive replacement and fracture filling veins are the most prevalent mineralization styles. While, "mini" roll-fronts, and disseminated mineralization are only minor constituent of the uranium mineralization. Mineralization consists mainly of pitchblende and uraninite with lesser amounts of coffinite. Fluid inclusion studies on euhedral quartz-uraninite veins identified two fluids. The first fluid is moderately saline with 10-15wt% NaCl. The second fluid is highly saline with 22-24wfllo NaCl which is within the range of fluid salinities for basinal brines within the Athabasca Basin. Heating results were unsuccessful due to decrepitation of the fluid inclusions. Uraninites have low delta 18O values -35‰ to -15‰, and similar to those measured for unconformity hosted uranium deposits in the Athabasca Basin. Metamorphic and secondary euhedral quartz grains associated with vein-type uraninite have delta 18O values of 11.1‰ to 15.1‰ and 16.2‰ to 19.1‰ respectively. The low delta18O values for uraninite and the high delta18O values for quartz suggest that the original isotopic composition of both minerals have been overprinted by recent low temperature meteoric waters.