Lead Isotopes and Mineralization Ages in Belt Supergroup Rocks, Northwestern Montana and Northern Idaho

Lead isotopes are used to distinguish between Precambrian mineralization and Mesozoic or Cenozoic mineralization in an area of Belt Supergroup rocks in north-western Montana and northern Idaho. The Precambrian lead is characterized by a comparatively uniform isotopic composition (Pb2øø/Pb2ø4--16.15-16.73; Pb2ø7/Pb •ø4--- 15.37-15.45; Pb"S/Pb " • = 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 dis-cussed. The Mesozoic or Cenozoic lead, which has undergone at least a two-stage de-vel. opment, is generally more radiogenic and ranges widely in isotopic omposition. 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 southxvard 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 Pre-cambrian 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 prospect-ing within this area