Laser probe 40Ar/39Ar and conventional K/Ar dating of illites associated with the McClean Uranium deposits, N. Saskatchewan, Canada

Mineralization at McClean occurs in elongate pods at the unconformity between the Athabasca sandstone sequence and the underlying Archean/Aphebian basement. Laser probe 40Ar/39Ar and conventional K/Ar dates were obtained on illites from the following environments (i) alteration halo around high grade U(-Ni-Co) mineralization, (ii) recrystallized sedimentary layers within the sandstone sequence, (iii) interstitial to quartz grains in sandstone, and (iv) regolith. The alteration halo illites gave ages between 1321+/-44 and 1002+/-33 Ma (n=20) which are in good agreement with published U-Pb dates on other U deposits in the area. One sample was analyzed twice, and gave a significant age difference of 20 Ma. Hence the age range is interpreted as being either a reflection of variable argon retentivity or due to continued hydrothermal activity (i.e. U mineralization) or a combination of both. Dates obtained on (ii), (iii) and (iv) were 1438-1038 (n=8), 1459-1113 (n=3) and 1482-1262 (n=6) Ma respectively. The older ages are in good agreement with a published Rb-Sr whole rock age of 1470 Ma on tuffaceous sediments from the Athabasca group. The spread to younger ages is interpreted as due to resetting by the hot (∼ 2000C) formation water which transported the U, and variable argon retentivity. Diffusion studies on a single sample of illite from the sedimentary sequence indicate a low blocking temperature of about 1400C and imply that there have been very low temperatures in the area for the last 1000 Ma