Casto Ring Zone: A 4,500-km2 fossil hydrothermal system in the Challis Volcanic Field, central Idaho

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.