Isotopic analyses of lead extracted from a variety of minerals from Blind River, Ontario, are reported. The detrital minerals monazite and zircon both give lead‐ratio ages of 2500 million years. The uraninite ore gives a lead‐ratio age of 1700 m y. Other isotopic evidence is quoted to suggest that the age of the sediment in which the uranium is found may also be approximately 1700 m y, or older. The lead found in pyrite, pyrrhotite, sericite, and feldspar has anomalous isotopic ratios which can be explained by the hypothesis that they received additions of radiogenic lead from the uraninites (presumed to be 1700 m y old) 1200 to 1300 m y ago. In any case the age of these minerals, in the sense of time of last chemical alteration, is not greater than 1450±150 m y. All our measurements can be interpreted without assuming a major period of mineralization more recent than 1000 m y ago, although we are unable to rule out such a possibility from our evidence.
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