Abstract 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.

Abstract The structural history of part of the Purcell Mountains of British Columbia is interpreted in the light of the discovery of the Precambrian age of the Hellroaring Creek stock. This stock, the first Precambrian granitic intrusion recognized in situ in the Canadian Cordillera, cuts Moyie (Purcell) Intrusions and verifies their Precambrian age. The Sullivan ore body, one of the world’s largest deposits of lead and zinc, is younger than a low-grade regional metamorphism of its host metasedimentary rocks but, on the basis of potassium-argon dating of a lamprophyre, is probably no younger than 765 million years. The lead-isotope content of the Sullivan deposit is homogeneous and indicates a single mineralization. Deposits of the East Kootenay district contain two main types of lead; a relatively nonradiogenic variety and a markedly radiogenic type. The irdistribution is unrelated to geography and to major structural trends, out there is an apparent relation between isotopic composition and specific combinations of structure and host rock. Deposits that are structurally conformable with strata of the Aldridge Formation probably belong to the relatively nonradiogenic lead group, although there is no relation between isotopic compositions and occurrences in the Aldridge Formation i n general. Deposits believed to be genetically related to the Moyie Intrusions in which they occur are probably characterized also by lead of the relatively nonradiogenic group, a correlation that is in harmony with their proposed origin. Another correlation is between isotopic composition and size: small deposits, with the significant exception of those probably related to Moyie Intrusions, are likely to be of the markedly radiogenic type, whereas the commercial lead production of the district has come from deposits of the relatively nonradiogenic type. If any single lead was common to a ll the deposits of the dist rict its presence in many of them, especially the smaller ones, has been masked by additions of more radiogenic lead.