Origin of secondary sulfate minerals on active andesitic stratovolcanoes

Abstract

Sulfate minerals in altered rocks on the upper flanks and summits of active andesitic stratovolcanoes result from multiple processes. The origin of these sulfates at five active volcanoes, Citlaltépetl (Mexico), and Mount Adams, Hood, Rainier, and Shasta (Cascade Range, USA), was investigated using field observations, petrography, mineralogy, chemical modeling, and stable-isotope data. The four general groups of sulfate minerals identified are: (1) alunite group, (2) jarosite group, (3) readily soluble Fe-and Al-hydroxysulfates, and (4) simple alkaline-earth sulfates such as anhydrite, gypsum, and barite. Generalized assemblages of spatially associated secondary minerals were recognized: (1) alunite+silicaFpyriteFkaoliniteFgypsumFsulfur, (2) jarosite+alunite+silica; (3) jarosite+smectite+silicaFpyrite, (4) Fe-and Al-hydroxysulfates+silica, and (5) simple sulfates+silicaFAl-hydroxysulfatesFalunite.Isotopic data verify that all sulfate and sulfide minerals and their associated alteration assemblages result largely from the introduction of sulfur-bearing magmatic gases into meteoric water in the upper levels of the volcanoes. The sulfur and oxygen isotopic data for all minerals indicate the general mixing of aqueous sulfate derived from deep (largely disproportionation of SO 2 in magmatic vapor) and shallow (oxidation of pyrite or H 2 S) sources. The hydrogen and oxygen isotopic data of alunite indicate the mixing of magmatic and meteoric fluids. Some alunite-group minerals, along with kaolinite, formed from sulfuric acid created by the disproportionation of SO 2 in a condensing magmatic vapor. Such alunite, observed only in those volcanoes whose interiors are exposed by erosion or edifice collapse, may have d 34 S values that reflect equilibrium (350F50 8C) between aqueous sulfate and H 2 S. Alunite with d34 S values indicating disequilibrium between parent aqueous sulfate and H 2 S may form from aqueous sulfate created in higher level low-temperature environments in which SO 2 is scrubbed out by groundwater or where H 2 S is oxidized. Jarosite-group minerals associated with smectite in only slightly altered volcanic rock are formed largely from aqueous sulfate derived from supergene oxidation of hydrothermal pyrite above the water table. Soluble Al-and Fehydroxysulfates form in low-pH surface environments, especially around fumaroles, and from the oxidation of hydrothermal pyrite. Anhydrite/gypsum, often associated with native sulfur and occasionally with small amounts of barite, also commonly form around fumaroles. Some occurrences of anhydrite/gypsum may be secondary, derived from the dissolution and reprecipitation of soluble sulfate. Edifice collapse may also reveal deep veins of anhydrite/gypsumFbarite that formed from the mixing of saline fluids with magmatic sulfate and dilute meteoric water. Alteration along structures associated with both 0009-2541/$ -see front matter D

Hydrothermal alteration, fumarolic deposits and fluids from Lastarria Volcanic Complex: A multidisciplinary study

“…Barite is a typical mineral found in fumarolic deposits as incrustations and sublimates (e.g., Africano and Bernard, 2000;Distler et al, 2002;Zimbelman et al, 2005;Zaitsev and Keller, 2006), and its origin could be related to reaction of wall rock with fumarolic gases in a higher oxidized environment, probably related to surface conditions, and at very low pH (Africano and Bernard, 2000;Zaitsev and Keller, 2006). On the other hand, despite that bloedite is a typical mineral from marine and lacustrine salt deposits, normally associated with halite and polyhalite, and formed under surface conditions in arid environments (e.g., Braitsch, 1971), has been also reported in volcanic caves as stalactites (Benedetto et al, 1998;Porter, 2000;White, 2010), probably related to the remobilization of sulphur deposits by meteoric water (Heliker et al, 2003).…”

“…Subsequent alteration of andesite by acid sulphuric action produces the assemblage alunite +kaolinite+silica+pyrite (Zimbelman et al, 2005). In the case of H 2 S, a typical compound of the hydrothermal fluids, under oxidizing conditions may form sulphuric acid by the next reaction:…”

“…where the alunite can be formed from the sulphuric acid, similarly then SO 2 disproportion (Zimbelman et al, 2005). Low-temperature meteoric water or high-temperature magmatic fluids can produce this oxidation process, which is a source of sulphate for supergene acid-sulphate alteration (Zimbelman et al, 2005).…”

“…Low-temperature meteoric water or high-temperature magmatic fluids can produce this oxidation process, which is a source of sulphate for supergene acid-sulphate alteration (Zimbelman et al, 2005). In the case of supergene alunite formation, alunite occurs at or below the water table (Rye et al, 2000).…”

An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards

“…Within such systems, dome rocks can be altered to clay minerals, significantly weakening portions of an edifice, reducing slope stability and ultimately resulting in failure (Lopez and Williams 1993;Boudon et al 1998;Reid et al 2001;Crowley et al 2003;Zimbelman et al 2005;Opfergelt et al 2006;John et al 2008). At the Santiaguito lava dome complex of Santa María volcano in Guatemala, there is both an evident hydrothermal system and a regional climate characterized by intense bouts of precipitation during the wet season.…”