Lead-isotope data from a number of mining districts in the Northern Rocky Mountains in Idaho, Montana, and Washington are examined to help evaluate the lead-isotope evidence that the major Coeur d’Alene lead-zinc deposits were formed in Precambrian time rather than at the time of intrusion of the Idaho batholith. This regional reconnaissance is extended by detailed isotope studies that the Geological Survey of Canada has made on ore deposits in British Columbia, as described by Leech and Wanless in this volume.

The new evidence does not clearly prove or disprove the apparent Precambrian age of Coeur d’Alene ore lead. Such ore lead with model ages of 1.2 to 1.4 billion years has so far been found in abundance in only two districts of the region: the East Kootenay district, British Columbia, and the Coeur d’Alene district, Idaho. Elsewhere in the Northern Rockies ore lead in so-called “Coeur d’Alene-type” ore deposits in Belt rocks is more evolute and ranges widely in isotopic composition.