HYDROTHERMAL ALTERATION OF ANCIENT MT. TEHAMA: VARIATIONS FROM SULPHUR WORKS TO BOILING SPRINGS LAKE, IN THE MODERN LASSEN VOLCANIC CENTER

The Lassen Volcanic Center (LVC) is located at the southern end of the Cascade Volcanic Arc in Northern California. The goal of this work is to investigate variation of the hydrothermal alteration across the LVC. To do so, we examine hydrothermal alteration at Sulphur Works (SW), in what was the core of ancient Mt. Tehama, and 12 km east, at Boiling Springs Lake (BSL), on the former flank of the volcano. SW has high temperature acidic fumaroles and mud pots (T= 84-91 °C and pH 2.2-3.5). BSL is a ~13,700 m² high-temperature lake with adjacent mud pots and fumaroles (T= 51-52 °C and pH 2.5-2.9). The hydrothermal features at BSL are primarily on the southern shore, at the mouth of a seasonal creek. BSL mud pot samples are composed of montmorillonite, dickite, and kaolinite and lake shoreline samples contain primarily kaolinite and dickite, and less than 2.5% quartz, all of which is consistent with argillic alteration and clay formation at 120-200 °C (Reyes, 1990). In contrast, SW samples from fumarole areas and mud pots, have high proportions of high temperature silica phases (cristobalite and tridymite) and pyrophyllite, which indicate advanced argillic alteration. The formation of pyrophyllite from kaolinite occurs in silica-rich environments at 225-300 °C (John et al, 2008). Thus, there is a lateral alteration trend in Mt. Tehama from advanced argillic alteration at the core (SW) to argillic alteration in the outer flanks (BSL). This trend is consistent with alteration at Mt. Rainier and Mt. Adams where advanced argillic alteration is concentrated in the vent area, transitioning laterally and vertically to argillic alteration (John et al., 2008; Finn et al., 2007). The deep erosion of Mt. Tehama allows our next investigation of vertical alteration trends, with a vertical transect from SW (core) to Pilot Pinnacle, a high elevation remnant flank of the edifice.