2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): ZIMBELMAN, D.R., G.O.Logic, PO Box 1878, White Salmon, WA 98672, hazard@gorge.net; BREIT, G.N., USGS, Box 25046, MS 973, Denver, CO 80225; and RYE, R.O., USGS, Box 25046, MS 963, Denver, CO 80225
Keywords: sulfate, volcano, alteration
Active andesitic stratovolcanoes contain a diverse suite of sulfates, often accompanied by native sulfur and generally lack sulfides. The sulfates are a product of reactions between volcanic gases and acid hydrothermal solutions with the volcanic rock. At most volcanoes the sulfates are intimately intergrown with clays and vary on a micron scale in crystal form, anion and cation chemistry, and isotopic composition. These characteristics result from variable exposure to acid fluids and changes in fluid composition. Sulfate alteration is an indicator of structural weakening of a volcanic edifice; many active volcanoes have weakened to the point of collapse, with associated loss in human life and property. Citlaltepetl volcano, Mexico, has experienced three distinct growth stages separated by two large edifice collapse events. The current summit contains historic flows, active fumaroles, and altered rock with alunite, sodium alum, minamiite, gypsum, anhydrite, and alunogen/metaalunogen accompanied by native sulfur, kaolin, and various forms of silica. Rocks from an intermediate stage represent deeper alteration zones and contain alunite, minamiite, letovicite, sodium alum, and phillipsite, accompanied by kaolin and various forms of silica. The oldest parts of the edifice are more deeply incised and alteration consists of minor occurrences of minamiite and alunite accompanied by various forms of silica, halloysite, and iron-oxides and hydroxides. Active stratovolcanoes in the Cascade Range contain a range of alteration minerals, including sulfates. For example, Mount Shasta contains altered zones similar to those at Citlaltepetl’s summit, consisting of kaolin, gypsum/anhydrite, alunite, and cristobalite. In contrast, Mount Adams, Hood, and Rainier contain more smectite than kaolin, jarosite, and widespread occurrences of various soluble sulfate minerals. In active andesitic environments, alteration mineral assemblages, crystal forms, chemical zonings, and isotopic characteristics suggest deposition in rapidly changing conditions, including swift cooling of hydrothermal systems by predominantly meteoric waters, and in association with fluids of very low pH and high fO2.
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