LASER PROBE 40AR/39AR AND CONVENTIONAL K/AR DATING OF ILLITES ASSOCIATED WITH THE MCCLEAN UNCONFORMITY-RELATED URANIUM DEPOSITS, NORTH SASKATCHEWAN, CANADA

Ten Laser probe **4**0Ar/**3**9Ar dates, two of which are step-heat runs showing good plateaus, and 10 conventional K-Ar dates for this material show a distribution with a sharp beginning at approximately 1320 Ma, a marked peak in the 1250 minus 1200 Ma class interval, and a tail to dates as young as 1002 plus or minus 33 (1 sigma ) Ma. These determinations are in complete agreement with direct (U-Pb and Sm-Nd) dates on uraninite. Uranium mineralization and associated hydrothermal alteration started approximately 100 minus 150 Ma after Athabasca sedimentation. Initiation dates for uranium mineralization might reflect a widespread faulting event that affected the eastern part of the basin. The Athabasca Basin in the McClean area has not been disturbed by temperatures greater than approximately 140 degree C for 1. 1 minus 1. 0 Ga. It is suggested that mineralization ceased when fracture permeability had been sealed by crystallization of secondary minerals. Secondary illites interstitial to quartz grains from the HLMl stratigraphic borehole give **4**0Ar/**3**9Ar ages of 1459 plus or minus 4, 1341 plus or minus 4, and 1113 plus or minus 11 Ma, indicating that formation of diagenetic sheet silicates predated uranium mineralization. Recrystallization or formation of sheet silicates in relict sedimentary layers and in subunconformity altered basement referred to as 'regolith' started at approximately the same time. Additional study results are discussed.