Determinations of δ¹⁸O values from 100 outcrops of Belt Supergroup (Wallace Fm.) metasedimentary rocks in the Idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange. Low grade argillites at large distances (60 km) from the Idaho batholith have high δ¹⁸O values +15, compatible with their probable primary values. Pelitic rocks with anomalously low δ¹⁸O values of + 8.7 to + 12.7‰ occur in the following zones: (1) in a 5000 km² zone of schist and gneiss peripheral to the Idaho batholith, generally coincident with high-grade (sillimanite-bearing) assemblages; (2) in high-grade metasedimentary roof pendants within the Idaho batholith; (3) peripheral to small Cretaceous stocks; and (4) within and near the scapolite-bearing zone south-west of St. Regis. On δ¹⁸O−δ¹⁸O plots, data from coexisting minerals define trends with unit slopes, indicating that the reductions in ¹⁸O occurred under high-grade metamorphic conditions. This metamorphism culminated in the emplacement of the Idaho batholith, probably as a consequence of profound crustal thickening associated with the Cretaceous accretion of the Wallowa-Seven Devils arc terranes with North America. The huge low-¹⁸O region is bounded by a “steep” δ¹⁸O gradient (0.1–0.5‰/km) that occurs in low-grade rocks along and near the Lewis and Clark Line, well below the biotite isograd. This boundary zone may be analogous to, but is not nearly as sharp as, those of meteoric-hydrothermal systems in many regions. The important ore deposits of the Coeur d'Alene district are located in this peripheral zone, suggesting that the metamorphic-hydrothermal system may have been intimately involved in their formation. In addition, the metamorphic-hydrotermal system in Idaho is larger, deeper and higher in temperature than typical meteoric-hydrothermal systems, and it involved fluids with much higher δ¹⁸O values that were probably dominantly derived from formation waters. Accordingly, this system produced rocks with δ¹⁸O values similar to those of the Idaho batholith, and mineral assemblages that approach isotopic equilibrium under high temperature conditions.