δ18O analyses of granitic and volcanic rocks reveal the largest hydrothermal system ever documented around a single granite pluton, occupying a 4,500-km2 area in central Idaho. The remains of this meteoric-hydrothermal system are principally preserved within a sharply bounded, 15-km-wide, 70-km-diameter annulus of low δ18O rock (+2.0 to −8.8‰) termed the Casto Ring Zone. The zone centered on a less depleted (+4.5) core zone consisting of granitic rocks of the Casto pluton. This 700-km2 Eocene subvolcanic batholith has intruded, domed, and hydrothermally metamorphosed a thick sequence of Challis Volcanics, the stratigraphically low rocks in the 2,000-km2 Van Horn Peak and the 1,000-km2 Thunder Mountain cauldron complexes being most strongly altered. Less extreme 18O depletions occur in the youngest major ash-flow sheets of these complexes, indicating a vertical 18O gradient. Water/rock ratios of geothermal systems are surprisingly insensitive to the circulation scale.