Large variations in the isotopic composition of Pb volatilized at different temperatures from rocks occur, and are linearly related. The line so obtained may be treated as a mineral isochron and interpreted in the same way as for the equivalent Rb/Sr case. For the western granodiorite at Yellowknife, we obtain a whole rock Pb/Pb age of 2675 m.y. and a series of mineral ages ranging from this upper limit down to about 2500 m.y. The whole rock age is in good agreement with other work and the range of mineral isochron ages is consistent with a cooling interval of about 150 m.y. as deduced by others. A comparison with other recent age determinations from the same area indicates that almost all the significant igneous activity and the emplacement of the ore minerals occurred in a very short time interval around 2650 m.y. ago.

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