The production and storage of uranium mine mill tailings have the potential to contaminate local groundwater and surface waters with metals and metalloids. As such, an understanding of the solids reservoirs for potential contaminants in uranium ore blends and leach residues (solid wastes generated by the milling of ore) is required to predict long-term controls on these contaminants in tailings porewaters. This study characterized the distribution of the elements of concern (EOCs; As, Mo, Ni, and Se) in uranium ores and waste rock used to blend the mill feeds in the milling process and leach residues from the Key Lake mining operation, Saskatchewan. This study also evaluated the alteration of the clay minerals in these uranium ores, waste rocks and leach residues. X-ray diffraction, electron micro-probe, and mineral liberation analyses showed that the reservoirs for As, Mo, Ni, and Se (in ores and ore blends) were dominated by sulphides including cobaltite, gersdorffite, molybdenite, pyrite, galena and chalcopyrite, secondary Ni-arsenates (annabergite?), Fe-arsenate (scorodite?) and Ni-Co/Ni-sulfates. The secondary arsenates and sulfates present in special waste were identified as major As, Mo, Ni, and Se bearing minerals and most likely the product of oxidation of arsenide-bearing sulphide minerals within the special waste rock. Analyses also showed that sulphides and arsenates occurred in trace amounts in the ores and special waste rock (0.5 to 1.0 wt %). Data showed that 55 to 90% pyrite, 36 to 51% chalcopyrite, 23 to 37% molybdenite, and 52 to 70% galena remained unleached in the leach residues after milling of the ore blends. The percentages of unleached minerals varied between mill feeds and were dependent on the grain-size distribution and the degree of mineral liberation. Cation exchange capacity (CEC) analysis indicated an increase of the CEC values in the leach residues suggesting possible evolution of 2:1 layers into high-charge layers during the milling.