Titre du document / Document title

Mount St. Augustine volcano fumarole wall rock alteration: mineralogy, zoning, composition and numerical models of its formation process

Auteur(s) / Author(s)

GETAHUN A. ⁽¹⁾ ; REED M. H. ⁽¹⁾ ; SYMONDS R. ⁽²⁾ ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

⁽¹⁾ Department of Geological Sciences, University of Oregon, Eugene, ETATS-UNIS
⁽²⁾ U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, ETATS-UNIS

Résumé / Abstract

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. Intensely altered wall rock was collected from high-temperature (640°C) and low-temperature (375°C) vents at Augustine volcano in July 1989. The high-temperature altered rock exhibits distinct mineral zoning differentiated by color bands. In order of decreasing temperature, the color bands and their mineral assemblages are: (a) white to grey (tridymite−anhydrite); (b) pink to red (tridymite−hematite−Fe hydroxide−molysite (FeCl₃) with minor amounts of anhydrite and halite); and (c) dark green to green (anhydrite−halite−sylvite−tridymite with minor amounts of molysite, soda and potash alum, and other sodium and potassium sulfates). The alteration products around the low-temperature vents are dominantly cristobalite and amorphous silica with minor potash and soda alum, aphthitalite, alunogen and anhydrite. Compared to fresh 1986 Augustine lava, the altered rocks exhibit enrichments in silica, base metals, halogens and sulfur and show very strong depletions in Al in all alteration zones and in iron, alkali and alkaline earth elements in some of the alteration zones. To help understand the origins of the mineral assemblages in altered Augustine rocks, we applied the thermochemical modeling program, GASWORKS, in calculations of: (a) reaction of the 1987 and 1989 gases with wall rock at 640 and 375°C; (b) cooling of the 1987 gas from 870 to 100°C with and without mineral fractionation; (c) cooling of the 1989 gas from 757 to 100°C with and without mineral fractionation; and (d) mixing of the 1987 and 1989 gases with air. The 640°C gas−rock reaction produces an assemblage consisting of silicates (tridymite, albite, diopside, sanidine and andalusite), oxides (magnetite and hercynite) and sulfides (bornite, chalcocite, molybdenite and sphalerite). The 375°C gas−rock reaction produces dominantly silicates (quartz, albite, andalusite, microcline, cordierite, anorthite and tremolite) and subordinate amounts of sulfides (pyrite, chalcocite and wurtzite), oxides (magnetite), sulfates (anhydrite) and halides (halite)

Revue / Journal Title

Journal of volcanology and geothermal research   ISSN 0377-0273   CODEN JVGRDQ 

Source / Source

1996, vol. 71, n^o2-4, pp. 73-107

Langue / Language

Anglais

Editeur / Publisher

Elsevier, Amsterdam, PAYS-BAS  (1976) (Revue)

Mots-clés anglais / English Keywords

fumaroles ; Augustine ; Alaska ; hydrothermal alteration ; temperature ; zoning ; paragenesis ; volcanism ; staining ; tridymite ; anhydrite ; hematite ; molysite ; alunite ; sulfates ; aphthitalite ; alunogen ; silica ; halides ; iron oxides ; magnetite ; sulfides ; mineral assemblages ; digital simulation ; cooling ; fluid-rock interaction ; gaseous phase ; phase equilibria ; sublimates ; duricrust ; United States ; North America ;

Mots-clés français / French Keywords

Fumerolle ; Volcan Augustine ; Alaska ; Altération hydrothermale ; Température ; Zonalité ; Paragenèse ; Manifestation volcanique ; Coloration ; Tridymite ; Anhydrite ; Hematite ; Molysite ; Alunite ; Sulfate ; Aphthitalite ; Alunogen ; Silice ; Halogénure ; Oxyde fer ; Magnetite ; Sulfure ; Association minérale ; Simulation numérique ; Refroidissement ; Mont St. Augustine ; Modèle thermochimique ; Terre rare ; Microscopie électronique ; Donnée MEB ; Analyse chimique ; Interaction fluide roche ; Phase gazeuse ; Diagramme équilibre ; Sublimat ; Incrustation ; Etats Unis ; Amérique du Nord ;

Mots-clés espagnols / Spanish Keywords

Fumarola ; Volcán Augustine ; Alaska ; Alteración hidrotermal ; Temperatura ; Zonalidad ; Paragénesis ; Manifestación volcánica ; Coloración ; Tridimita ; Anhidrita ; Hematites ; Milisita ; Alunita ; Sulfato ; Aphthitalita ; Alunógeno ; Sílice ; Haluro ; Óxido de hierro ; Magnetita ; Sulfuro ; Asociación mineral ; Simulación numérica ; Enfriamiento ; Interacción flúido roca ; Fase gaseosa ; Diagrama equilibrio ; Sublimado ; Incrustación ; Estados Unidos ; America del norte ;

Localisation / Location

INIST-CNRS, Cote INIST : 17236, 35400004415445.0002