Geochronometric and lead isotope data on samples from the Wallace 1 degree by 2 degrees Quadrangle, Montana and Idaho

Most of the bedrock in the Wallace quadrangle belongs to the Belt Supergroup, a thick (about 18,000 m) sequence of generally fine-grained clastic and carbonate rocks of Middle Proterozoic age.  Regional metamorphism prior to Cambrian time prograded the Belt rocks to greenschist facies, and some metal-bearing veins were emplaced in fractures.  The Belt rocks were intruded in Late Proterozoic time by basic dikes and sills.