ALTERATION ASSEMBLAGE OF THE HYDROTHERMAL SYSTEM AT SULPHUR WORK, LASSEN VOLCANIC CENTER

The Lassen Volcanic Center (LVC), located at the southern end of the Cascade Volcanic Arc in Northern California, includes the hydrothermally active area of Sulphur Works (SW) which is three miles to the southwest of Lassen Peak, in what would have been the core area of ancient Mount Tehama. The exposed hydrothermal area at SW includes fumaroles, mud pots and boiling pools with water temperatures that range from 84^o C-91^o C and pH that varies from 2.2-3.5. SW is situated between West Sulphur Creek to the west (pH =6.35) and Tophet Springs to the east (pH =4.7). To investigate the alteration assemblage of the hydrothermal system at SW, we completed preliminary sampling of a small area (150m x 50m). Mineral assemblages include silica polymorphs (quartz, tridymite, and cristobalite), kaolinite, montmorillonite, jarosite and one sample (Big Boiler mud pot) has pyrophyllite. Silica phases indicate decreasing temperature from west (cristobalite) to east (tridymite) across Sulphur Works, across which proportions of clay minerals increase. Clay mineral abundances indicate argillic alteration, and only the Big Boiler site has phyllic alteration. These samples are broadly consistent with previous work at SW that reports similar clay minerals and jarosite, but no pyrophyllite (McHenry et al., 2014). The hydrothermal systems at Mount Rainier and ancient Maidu Volcano (adjacent to LVC) have similar alteration assemblages plus opal and topaz (John et al., 2005 and 2008). Based on clay assemblages we expect to be able to constrain alteration temperatures through the LVC system; previous research indicates temperatures of formation of clay minerals may range from 175^oC-275^oC (John et al., 2008). At Maidu Volcano, mineral assemblages are used to reconstruct alteration across a vertical section of the volcano, which is the next goal for our work at LVC. Additional sampling and analysis will be used to determine mineral trends across a broader transect of LVC, from Sulphur Works to Pilot Pinnacle.