VERTICAL VARIATION OF HYDROTHERMAL ALTERATION FROM THE CORE TO UPPER FLANK OF ANCIENT MT. TEHAMA, LASSEN VOLCANIC CENTER

Ancient Mt. Tehama is an extinct partially eroded stratovolcano in the Lassen Volcanic Center (LVC), located at the southernmost end of the Cascade Volcanic Arc in Northern California. Within the eroded edifice is an active hydrothermal system that includes fumaroles, mud pots (84-91 °C, pH=2.2-3.5), steam vents, and acidic creeks (pH=4.7-6.35). The goal of this work is to investigate the variation of hydrothermal alteration along a vertical transect from the exposed core at Sulphur Works (SW) to Pilot Pinnacle, a high elevation remnant of Mt. Tehama. Previous studies at Mt. Rainier and Mt. Adams used alteration mineral assemblages of debris flows and geophysical surveys to reconstruct the hydrothermal system and found a general trend of increasing temperature and alteration from argillic to advanced argillic nearest feeder dikes and conduits (John et al. 2008; Finn et al., 2007). At LVC, the SW samples have advanced argillic alteration (up to 300 °C) and Boiling Springs Lake, 12 km to the east of SW, has lower temperature argillic alteration (up to 200 °C), consistent with large scale gradation of decreasing temperature from core to areas outside the remnant edifice. However, the alteration mineral assemblages of the vertical transect from Sulphur Works to Pilot Pinnacle indicate that alteration zones are more complex than progressive gradational zoning. Pyrophyllite, an indicator mineral of advanced argillic alteration (200-300 °C), is present at lower elevations nearest Sulphur Works as expected, and is also sporadically present in some higher elevation samples closer to Pilot Pinnacle. Additional work will incorporate fossil vs. modern hydrothermal system designations identified by John et al. (2006) and the chemistry of hydrothermal fluids to better characterize the distribution and gradients of alteration assemblages.