CHEMICAL MODELING OF VOLCANIC GAS INTERACTIONS WITH GROUND WATER AND ANDESITE: INSIGHTS INTO HYDROTHERMAL ALTERATION AT MOUNT RAINIER, WASHINGTON

Mineralogic, petrographic, and isotopic studies of hydrothermally altered Quaternary volcanic rocks at Mount Rainier, have identified several distinctive mineral assemblages formed by volcanic gas interactions with edifice ground waters and rocks. We are using chemical reaction-path modeling to simulate the geochemical effects of volcanic gas condensation into ground water followed by reaction of the gas-water mixture with andesite rock. We use as input for the modeling a range of published gas compositions from active volcanoes and analyses of fresh Rainier andesite. The minerals predicted to form vary as a function of gas composition (especially SO₂, H₂, H₂S, and H₂O proportions), gas/water/rock proportions, temperature, presence or absence of atmospheric oxygen, and rock composition. Zoned acid-sulfateÞargillicÞpropylitic alteration sequences are generally predicted to form by titration of gases with high SO₂/H₂ (5-10) and SO₂/H₂S (>1) into ground waters and the progressive reaction of the resulting highly acidic fluids with andesite; a typical predicted mineral precipitation sequence is native sulfur (formed only by gas condensation)ÞsilicaÞsilica-alunite-kaolinite-anhydrite±hematite±pyriteÞsmectite-pyriteÞalbite-carbonate-chlorite±zeolite. Smectite-pyrite grading into carbonate-albite-chlorite±zeolite is predicted to form by progressive reaction of andesite with condensates of gases with low SO₂/H₂S (<1) and SO₂/H₂ (<~5). Formation of alteration jarosite apparently requires gas condensation into ground waters under atmospheric oxygen pressures, and is enhanced by replacement of Na- and K-rich minerals. These results are generally consistent with field, mineralogic, petrographic, isotopic, and other lines of evidence: alunite-rich acid-sulfate assemblages formed by condensation of SO₂ into ground waters; smectite-pyrite assemblages formed on the fringes of the acid sulfate alteration or where H₂S-rich gases condensed into abundant ground-waters (perhaps beneath glaciers); and, jarosite-rich assemblages formed where gases vented directly into the atmosphere. Improved understanding of alteration processes and distribution on active volcanoes can be used in hazards assessments and in the interpretation of volcano-related ore genesis.