Galena samples representing the vertical and geographical extent of the Coeur d'Alene mining district have been isotopically analyzed for Pb. The isotopic composition is essentially uniform at Pb 2O6 /Pb 2O4 = 16.44, Pb 2O7 /Pb 2O4 = 15.58, Pb 2O8 /Pb 2O4 = 36.52. The common Pb age from this average is about 1,400 million years. This is consistent with other isotope data in the district and may be considered the time of primary mineralization. The original Belt series and its metamorphic equivalents are, therefore, older than 1,400 m.y. Precambrian Pb in veinlets cutting Laramide monzonite dikes and stocks present positive evidence for sulfide remobilization in Tertiary time in the vicinity of these igneous intrusions. Some minor rock Pb was also introduced as galena during this event.

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