Characteristics,extent and origin of hydrothermal alteration at Mount Rainier Volcano,Cascades Arc,USA: Implications for debris-flow hazards and mineral deposits |
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Authors: | David A. John Thomas W. Sisson George N. Breit Robert O. Rye James W. Vallance |
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Affiliation: | 1. U.S. Geological Survey, MS-901, 345 Middlefield Rd., Menlo Park, CA 94025, USA;2. U.S. Geological Survey, MS-910, 345 Middlefield Rd., Menlo Park, CA 94025, USA;3. U.S. Geological Survey, MS-964, Federal Center, Denver, CO 80225, USA;4. U.S. Geological Survey, MS-963, Federal Center, Denver, CO 80225, USA;5. U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683, USA |
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Abstract: | Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km3 Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures. |
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Keywords: | hydrothermal alteration volcano hazards debris flows O, H, S stable isotopes, magmatic-hydrothermal system |
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