The geochemistry of thermal waters combined with mapping and geophysical studies have been used to define boundaries of the geothermal steam field in The Geysers-Gear Lake area, California. Chemical geothermometry and drill hole data imply that a 200 °C hot-water geothermal system underlies much of the 2.0 to 0.01-m.y.-old Clear Lake Volcanics. Chloride concentrations and electrical resistivity define the concealed extent of Great Valley sedimentary rocks underlying the young volcanic cover. We suggest that the 80 or more vents in the volcanic field act as permeable conduits for water recharge and thus prevent creation of a widespread vapor-dominated system northeast of the Collayomi fault zone.

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