MOUNT ST AUGUSTINE VOLCANO FUMAROLE WALL ROCK ALTERATION - MINERALOGY, ZONING, COMPOSITION AND NUMERICAL-MODELS OF ITS FORMATION PROCESS

Intensely altered wall rock was collected from high-temperature (640 d egrees C) and low-temperature (375 degrees C) vents at Augustine volca no in July 1989, The high-temperature altered rock exhibits distinct m ineral zoning differentiated by color bands. In order of decreasing te mperature, the color bands and their mineral assemblages are: (a) whit e to grey (tridymite-anhydrite); (b) pink to red (tridymite-hematite-F e hydroxide-molysite (FeCl3) with minor amounts of anhydrite and halit e); and (c) dark green to green (anhydrite-halite-sylvite-tridymite wi th minor amounts of molysite, soda and potash alum, and other sodium a nd potassium sulfates). The alteration products around the low-tempera ture 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 depleti ons in Al in all alteration zones and in iron, alkali and alkaline ear th elements in some of the alteration zones. To help understand the or igins of the mineral assemblages in altered Augustine rocks, we applie d the thermochemical modeling program, GASWORKS, in calculations of: ( a) reaction of the 1987 and 1989 gases with wall rock at 640 and 375 d egrees C; (b) cooling of the 1987 gas from 870 to 100 degrees C with a nd without mineral fractionation; (c) cooling of the 1989 gas from 757 to 100 degrees C with and without mineral fractionation; and (d) mixi ng of the 1987 and 1989 gases with air. The 640 degrees C gas-rock rea ction produces an assemblage consisting of silicates (tridymite, albit e, diopside, sanidine and andalusite), oxides (magnetite and hercynite ) and sulfides (bornite, chalcocite, molybdenite and sphalerite). The 375 degrees 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 co oling calculations produce: (a) anhydrite, halite, sylvite; (b) Cu, Mo , Fe and Zn sulfides; (c) Mg fluoride at high temperature (> 370 degre es C); (d) chlorides, fluorides and sulfates of Mn, Fe, Zn, Cu and Al at intermediate temperature (170-370 degrees C); and (e) hydrated sulf ates, liquid sulfur, crystalline sulfur, hydrated sulfuric acid and wa ter at low temperature (< 170 degrees 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 (lg/ a) ratios > 0.41 (> 628 degrees C). This is followed by precipitation of sulfates of Fe, Cu, Pb, Zn and Al at lg/a ratios between 0.41 and - 0.4 (628-178 degrees C). At a lg/r ratio of less than or equal to -0.4 (178 degrees C), anhydrous sulfates are replaced by their hydrated fo rms and hygroscopic sulfuric acid forms. At these low g/a ratios, hydr ated sulfuric acid becomes the dominant phase in the system. Compariso n of the thermochemical modeling results with the natural samples sugg ests that the alteration assemblages include: (1) minerals that precip itate 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 prod uces the observed mineral zoning. Another implication of the numerical simulation results is that most of the observed incrustation and subl imate minerals apparently formed below 700 degrees C.