Lead isotopes are used to distinguish between Precambrian mineralization and Mesozoic or Cenozoic mineralization in an area of Belt Supergroup rocks in northwestern Montana and northern Idaho. The Precambrian lead is characterized by a comparatively uniform isotopic composition (Pb 206 /Pb 204 = 16.15-16.73; Pb 207 /Pb 204 = 15.37-15.45; Pb 208 /Pb 204 = 35.88-36.46) and model ages of 1.5-1.2 b.y. We interpret it to be the product of approximate single-stage development, although the model ages need not be exact geochronologic ages. The possibility that some vein-type deposits formed at a later time by remobilization of this lead already in Belt rocks is also discussed. The Mesozoic or Cenozoic lead, which has undergone at least a two-stage development, is generally more radiogenic and ranges widely in isotopic composition.Deposits associated with the Osburn fault system, including those of the Coeur d'Alene mining district, and with a second belt of mineralization which extends southward from near Bonners Ferry, Idaho, and joins the Osburn fault system near Thompson Falls, Montana, contain the Precambrian type of lead. Deposits adjacent to the Kootenay Arc mobile belt in Idaho and occurrences throughout southern Lincoln and Sanders Counties, Montana, appear to be largely Mesozoic or Cenozoic. Small, isolated deposits of Precambrian and Mesozoic or Cenozoic ages have been identified in the weakly mineralized northeastern part of the study area. The ability to distinguish, by their lead isotopes, two generations of ore widely spaced in time but overlapping geographically promises to be a valuable aid in unraveling the structural history and in carrying out future prospecting within this area.

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