The Key Lake orebodies are at the southeastern rim of the Athabasca Basin in middle Proterozoic Athabasca sandstone and immediately underlying Aphebian metasediments of the Wollaston Fold Belt. The two known orebodies are tectonically and lithologically controlled by a northeast-striking fracture zone. Ore minerals consist of uranium oxides and silicates and nickel sulfides and arsenides. The presence of tetragonal U 3 O 7 and bravoite, with maximum formation temperatures of 135 degrees and 137 degrees C, respectively, and intergrowth textures of other ore minerals suggest a low-temperature genesis for this stage. Metallogenic research suggests the deposits evolved in the following manner: (1) Uranium was deposited in Aphebian sediments surrounding Archean cores. (2) Metamorphism during the Hudsonian orogeny may have caused further concentration of uranium. (3) Weathering during the middle Proterozoic mobilized the uranium and nickel. (4) Both elements migrated into tectonic traps where they were concentrated. (5) Subsequently, the basement was covered by more than 1,000 m of the Athabasca Formation. (6) Diagenetic processes mobilized and redeposited (more or less in situ) the uranium, destroying the original radiogenic equilibrium and forming a new "primary" generaton of uranium oxide with a rejuvenated age of about 1,200 m.y. (7) Episodic uplifts resulted in erosion of the overlying Athabasca Formation and consequently changed the static equilibrium, producing new generations of sooty pitchblende and coffinite. Alternately, the weathering cycle (3 and 4) could be disregarded and redeposition of ore attributed solely to diagenetic processes.