The last eruptive activity of Usu volcano in 1977-78 involved the development of high temperature (550-710 degrees C) fumaroles. The gases emitted were H2O-rich (95-99 mol%) with Cl/S = 0.05 - 0.9, F/Cl = 0.3 - 0.2 and with RH = -2.5 close to the rock buffer (FeO/FeO1.5). Cooling and oxidation of the high temperature gases resulted in the formation of acidic condensates (pH = 1.6) that interacted with the wall rock. Complete leaching of the cations (Ca, Na, Mg, Al and Fe) from the primary minerals and matrix glass occurred leaving in place only silica. These mobilized cations precipitated as secondary minerals from acidic fluids that circulated in microcracks. SEM study shows mineral associations reflecting increasing fluid oxidation: (a) Al fluorides such as ralstonite (NaMgAlF6. H2O), pyrite, and anhydrite/gypsum; (b) an Al hydroxide, hematite, gypsum and amorphous silica or cristobalite; (c) Al sulfates such as hydronium alunite [(H3O)Al-3(SO4)(2)(OH)(6)], alunite [KAl3(SO4)(2)(OH)(6)], amorphous silica, cristobalite, hematite and anhydrite/gypsum; (d) Al sulfates, Al fluorides, amorphous silica, cristobalite, pyrite and anhydrite/gypsum. A Ti oxide, a Fe-Mg sulfate and barite are present in minor amounts. Clay minerals are absent from the observed assemblages. Primary phenocrysts and matrix glass undergo a complete transformation to silica enriched in fluorine (1-7 wt%). This fluorine enrichment in the silicified parts of silicates and in silica incrustations suggests that F may play a role in silica mobilization. Modeling of the cooling of the high-temperature gases was performed with the program GASWORKS. The calculations suggest that 66% of the total sulfur from the gases may be lost by deposition as native sulfur at temperatures below 160 degrees C. Thermochemical modeling of condensate-rock interaction using CHILLER indicates that the cooling of gases was the source of the altering solutions. Oxidation, by atmospheric Oz, of the sulfur-reduced species in the volcanic gas condensates resulted in their extreme acidification. Condensate-rock interactions produce supersaturation with respect to the following mineral assemblages at 95 degrees C: (I) pyrite, native sulfur, cristobalite, barite, anhydrite, diaspore (log mH(2)S(aq) = -2.32); (2) cristobalite, anhydrite, hematite, alunite, diaspore (log mH(2)S(aq) < -8). Gypsum replaces anhydrite at T = 25 degrees C The calculated mineral assemblages are in agreement with those observed in the field. (C) 2000 Elsevier Science B.V. All rights reserved.
