Abstract Thermal/mineral waters of the Clear Lake region, California are among the most challenging geothermal fluids in the world to study because they display enormous chemical and isotopic diversity and do not geochemically resemble fluids in typical, high-temperature (≥200°C) geothermal systems (Goff et al., 1993a, 1993b). The Clear Lake region contains no boiling hot springs, hot fumaroles, or springs actively depositing sinter, features commonly linked with high-temperature reservoirs. Regionally, the fluids display tremendous variations in chemical and isotopic composition, caused more by variations in bedrock composition than by subjacent magmatic heat sources (Goff et al., 1977; Thompson et al., 1981a; 1992; Donnelly-Nolan et al., 1993). The distribution of fluids is roughly coincident with the late Tertiary and Quaternary Clear Lake volcanic field (2.1 Ma to 10 ka; Donnelly-Nolan et al., 1981; Hearn et al., 1981). The region lies northeast of The Geysers steam field, the largest geothermal field in the world, yet drilling of approximately 25 exploration wells has not found a commercially exploitable geothermal system. Because conditions in most of these wells are very hot (≥200°C at 2000 m) but relatively impermeable, the Clear Lake region is rated as one of the best hot dry rock geothermal prospects in the United States (Goff and Decker, 1983). Gas geochemistry is becoming more widely used for geothermal prospecting, especially in areas where spring chemistry is ambiguous or where spring waters are not directly derived from deep reservoirs (Goff et al., 1985; 1991; Janik et al, 1991; 1992). Geothermal gases commonly originate from