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). The cooling calculations produce: (a) anhydrite, halite, sylvite; (b) Cu, Mo, Fe and Zn sulfides; (c) Mg fluoride at high temperature (> 370 °C); (d) chlorides, fluorides and sulfates of Mn, Fe, Zn, Cu and Al at intermediate temperature (170–370 °C); and (e) hydrated sulfates, liquid sulfur, crystalline sulfur, hydrated sulfuric acid and water at low temperature (< 170°C). The volcanic gas-air mixing calculation produces major amounts of Na and K sulfates, minor amounts of hematite and trace amounts (< 1%) of anhydrite at log gas/air (Ig/a) ratios > 0.41 (> 628°C). This is followed by precipitation of sulfates of Fe, Cu. Pb. Zn and Al at Ig/a ratios between 0.31 and -0.4 (>628-178°C). At a lg/r ratio of ≤ -0.4 (178°C). anhydrous sulfates are replaced by their hydrated forms and hygroscopic sulfuric acid forms. At these low g/a ratios. hydrated sulfuric acid becomes the dominant phase in the system.

Comparison of the thermochemical modeling results with the natural samples suggests that the alteration assemblages include: (1) minerals that precipitate from direct cooling of the volcanic gas; (2) phases that form by volcanic gases mixing with air: and (3) phases that form by volcanic gas-air-rock reaction. A complex interplay of the three processes produces the observed mineral zoning. Another implication of the numerical simulation results is that most of the observed incrustation and sublimate minerals apparently formed below 700°C.