HYPOGENE HYDROTHERMAL ALTERATION OF QUATERNARY VOLCANIC ROCKS FROM MOUNT RAINIER, WASHINGTON

Hydrothermal alteration decreases rock strength and stability of stratovolcanoes. To better understand the origin of debris flows derived from collapse of hydrothermally altered rock, studies of the distribution, origin, and effects of hydrothermal alteration on Mount Rainier are underway. Geologic, mineral, and chemical analyses of Holocene debris flows show that a large volume of the volcano was variably altered. Three major alteration assemblages are present: (1) smectite-group clay minerals + pyrite; (2) kaolinite + opal/chalcedony ± alunite + pyrite/marcasite; and (3) alunite + jarosite + opal. Where unaffected by post-flow weathering, the clay-rich matrix of the Osceola Mudflow, the largest Holocene debris flow from Mount Rainier, contains abundant hydrothermal pyrite.

Assemblage 1 is most common. Clay minerals replace mafic phenocrysts and groundmass and fill vesicles. Plagioclase phenocrysts commonly are fresh or weakly altered. XRD and SEM-EDA studies suggest a variety of smectite minerals. Pyrite is present as disseminated crystals, vesicle linings, and narrow vein fillings. Anhydrite/gypsum and barite are present locally.

Less common assemblage 2 shows complete replacement of primary igneous materials and includes clasts of multiple-generation hydrothermal(?) breccias cemented by opal, alunite, kaolinite, and pyrite. Alunite mostly forms fine-grained (<5 µm) lath-like crystals locally intergrown with kaolinite. Coarser-grained alunite (up to 30 µm) locally fills vugs and contains P-rich domains. Pyrite/marcasite content reaches 20%. Vug-filling anhydrite/gypsum, sulfur, cinnabar, barite, and fluorite also occur. Assemblage 3 is present in brecciated clasts in the Paradise Lahar on the south side of Mount Rainier. It comprises clasts of opalized andesite (± pyrite) cemented by multiple generations of compositionally zoned natroalunite and natrojarosite (molar K/(K+Na) < 0.0 to 0.3). Vug-filling sulfur is present locally.

The alteration assemblages reflect a complex interplay among volcanism, igneous intrusions, magmatic degassing, atmospheric oxygen, meteoric water, and a fluctuating ground water table on the volcano. Ongoing detailed geochemical, stable isotopic, and geochronologic studies will further elucidate these processes.