STYLE, EXTENT, AND ORIGIN OF HYDROTHERMAL ALTERATION AT MOUNT RAINIER VOLCANO, CASCADE ARC, USA

Mount Rainier grew episodically over the past 500 kyr during alternating periods of high and low effusion. Hydrothermal alteration focused in permeable breccia units promoted edifice degradation during times of relative volcanic quiescence. Multiple Holocene debris-flow deposits contain abundant hydrothermal alteration products, notably the 3.8-km³ Osceola Mudflow which formed about 5600 ybp by collapse of the NE side and upper 1000+ m of the edifice and flowed >120 km into Puget Sound. Mineralogy and stable isotope compositions of alteration products are consistent with 3 hydrothermal environments: magmatic-hydrothermal (MH), steam-heated, and fumarolic. The Osceola Mudflow and coeval F tephra contain the highest temperature and most deeply formed hydrothermal minerals. MH alteration assemblages in clasts include vuggy silica, alunite, topaz, pyrophyllite, dickite, and illite (all +quartz, pyrite). Steam-heated opal-alunite-kaolinite and smectite-pyrite alteration are also common components of the Osceola deposit. By contrast, the alteration-rich Paradise lahar formed by collapse of the uppermost S side of the edifice at about the same time, but it lacks MH assemblages. Younger debris-flow deposits on the W side of the volcano (Round Pass and distal Electron Mudflows) contain only smectite-pyrite assemblages, whereas proximal Electron Mudflow and a <100 ybp rock avalanche on Tahoma Glacier contain MH assemblages also exposed in the avalanche headwall of Sunset Amphitheater. These deposits reflect progressive exhumation of deeper alteration products. Mineralogy and stable isotopic composition of the alteration phases, geologic and geophysical relations, and observations from analogue fossil hydrothermal systems, allow inference of alteration geometry on the pre-Osceola-collapse edifice. Relatively narrow zones of MH alteration in the core of the volcano graded to more widely distributed, but weak, smectite-pyrite alteration assemblages farther out on the upper flanks, capped by a steam-heated alteration zone. Fossil smectite-pyrite alteration zones also extended along dikes cutting the W and E sides of the edifice. The Osceola collapse removed most altered rock from the core and east flank of the volcano, but collapse of the uppermost west flank altered rocks presents a continuing hazard.