The Key Lake deposit is one of several large, high-grade, unconformity-related uranium deposits located at the eastern margin of the Athabasca Basin in northern Saskatchewan, Canada. The deposit consists of the Gaertner orebody, now mined out, and the Deilmann orebody, which is presently being mined. In the past, radiometric dating efforts yielded an age of oldest ore-forming event of 1250 ± 34 Ma at the Gaertner orebody and 1350 ± 4 Ma at the Deilmann orebody. This unlikely age difference called for further investigation. Innovative preparation techniques were used to separate the paragenetically oldest U mineral, an anisotropic uraninite. Ore microscopy and U/Pb isotopic data show that the oldest event of uranium emplacement occurred simultaneously at the two orebodies, at 1421 ± 49 Ma. The primary ore-forming phase was followed by younger generations of U mineralization and periods of remobilization. Sm/Nd data of Key Lake uraninite form an isochron corresponding to an age of 1215 Ma. This is interpreted as the age of a uranium remobilization or a new mineralizing event. The lead found in the Athabasca Group above the Deilmann deposit and in galena appears to be a mixture of a common lead and radiogenic lead mobilized from the orebody over a time span of at least 1000 Ma.