Abstract

Analyses of U/Pb ratios in 30 microsamples of pitchblende and coffinite from the Midwest uranium deposit in northern Saskatchewan, as well as two altered zircons from host rocks, indicate an age of mineralization for primary pitchblende of 1328 ± 17 Ma. The primary material was remobilized at 1110 ± 28 Ma, forming the second generation of pitchblende and coffinite. First-generation pitchblende appears to have lost Pb either by diffusion or by a series of episodic losses at 300–100 Ma, whereas the secondary material proved more susceptible to Pb loss in the recent past.There is evidence of migration of pitchblende downwards into the basement rocks under the deposit at least as recently as 700 Ma ago, but we have been unable to date with certainty any late stages in the paragenetic sequence because of sampling problems associated with the small size of material clearly identifiable as stage 3 or 4 of the paragenetic sequence of Wray et al.

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