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

R.E. Criss·E. B. Ekren·R.F. Hardyman
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Abstract

δ 18 O analyses of granitic and volcanic rocks reveal the largest hydrothermal system ever documented around a single granite pluton, occupying a 4,500-km 2 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 δ 18 O 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-km 2 Eocene subvolcanic batholith has intruded, domed, and hydrothermally metamorphosed a thick sequence of Challis Volcanics, the stratigraphically low rocks in the 2,000-km 2 Van Horn Peak and the 1,000-km 2 Thunder Mountain cauldron complexes being most strongly altered. Less extreme 18 O depletions occur in the youngest major ash-flow sheets of these complexes, indicating a vertical 18 O gradient. Water/rock ratios of geothermal systems are surprisingly insensitive to the circulation scale.

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R.E. Criss, E. B. Ekren, R.F. Hardyman. 1984. Casto Ring Zone: A 4,500-km2 fossil hydrothermal system in the Challis Volcanic Field, central Idaho. https://doi.org/10.1130/0091-7613(1984)12%3C331%3Acrzakf%3E2.0.co%3B2

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