An interpretation of the alteration assemblages at Sulphur Springs, Valles Caldera, New Mexico

Abstract

Sulphur Springs, located near the western edge of the Valles caldera, north central New Mexico, is the only vapor‐dominated surface manifestation of any significance found in the Valles. The percolating acid sulfate waters and fumarolic gases have leached and oxidized the country rock near the hot springs to a bleached wasteland. The near‐surface mineral assemblages at 90°C are restricted to one and two phases occurring in contact with a variety of solutions and gases. The conditions of local equilibrium at 90°G are approximately halotrichite + alunogen, log fO2 −45, log fS2−12, pH 0.3; szomolnokite + pyrite, log fO2 −46, log fS2−16, pH 0.8; alunite, log fO2−50, log fS2−10, pH 1–2; kaolinite + sulphur, log fO2 −53, log fS2 −10, pH 1–2, log aK+ = 0.0012 m, log aSiO2 = 0.0033 m; and pyrite, log fO2 −60, log fS2 −19, pH 4.3. This study has demonstrated that local equilibrium is closely approached even under the low‐temperature and ‐pressure conditions found near the surface.