Geoscience Horizons

SEG Field Trip

Hydrothermal Alteration in Ancient and Modern Volcanoes, Mount Rainier Area: Applications to Ore Genesis and Volcanic Hazards
Thurs., Nov. 6. Sponsored by Society of Economic Geologists.
David John, Roger Ashley, James Vallance, and James Rytuba, U.S. Geological Survey; Grant Newport, Weyerhauser Corp. Maximum: 35. Cost: Before October 3, $95 SEG members, $125 nonmembers, $45 students; after October 3, $125 SEG members, $155 nonmembers, $55 students. Includes transportation, lunch, refreshments, and a guidebook. Preregistration required - download registration form and pay by credit card, or send a check, payable to Society of Economic Geologists, 7811 Shaffer Parkway, Littleton, CO 80127; (720) 981-7882, fax 720-981-7874, e-mail:, Web site:
Hydrothermal alteration is widespread in Tertiary and Quaternary igneous rocks of the Cascades arcs of Washington. Most alteration in the Tertiary Western Cascades arc resulted from hydrothermal systems related to small plutons, some of which formed porphyry copper and related deposits. Hydrothermal alteration of several Quaternary stratovolcanoes resulted in severely weakened volcanic edifices that were susceptible to failure and catastrophic landslides. Most notable is the 5600 yr B.P. clay-rich Osceola Mudflow that traveled 120 km down-valley from Mount Rainier to Puget Sound, covering about 200 km2. In the morning, we will examine high-level alteration related to a large, early Miocene magmatic-hydrothermal system exposed near Enumclaw, Washington, where advanced argillic alteration is being quarried for silica. The afternoon will be spent examining the Osceola Mudflow and other Holocene lahars from Mount Rainier that contain abundant clasts and matrix material of hydrothermally altered Quaternary rocks from Mount Rainier.

Subaru - Title Sponsor of the 2003 GSA Annual Meeting

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