The Key Lake uranium deposit occurs within a shear zone that intersects the unconformity between early Proterozoic basement gneiss and overlying middle Proterozoic Athabasca Group sandstone. Uranium is concentrated near the unconformity and is associated with intense illite, chlorite, and kaolinite alteration. The delta D and delta 18 O values of the clay minerals suggest that several different fluids have affected the ores and surrounding host rocks. Alteration of the basement gneisses associated with the uranium mineralization consists mainly of Mg chlorite that equilibrated about 200 degrees C with a fluid having a delta D value near -20 per mil and a delta 18 O value near +5 per mil. An illite-rich sandstone alteration halo, preserved only locally above the deposit, equilibrated about 200 degrees C with a basinal brine having a delta D value near -65 per mil and a delta 18 O value near +5 per mil. The uranium was concentrated near the unconformity by the mixing of these two distinct fluids. The delta 18 O values of coexisting quartz and illite sampled from the shear zone hosting the uranium mineralization yield equilibration temperatures of 190 degrees to 230 degrees C, which is postulated to be the temperature of the ore-forming event.Fracturing in the sandstone above the deposit controlled late, low-temperature (50 degrees C) kaolinitization of preexisting hydrothermal illite and chlorite by meteoric waters having delta D values of -50 + or - 30 per mil and delta 18 O values of -7 + or - 3 per mil. The youngest event to affect the Key Lake deposit is related to the influx of modern meteoric waters that have lowered the delta D values but not greatly affected the delta 18 O values of some of the clay minerals associated with the shear and breccia zones. This latest event has recrystallized much of the hydrothermal Mg chlorite associated with the uranium to produce an Fe-Mg chlorite with delta D values as low as -170 per mil.