Thermal waters along the Konocti Bay fault zone, Lake County, California: a re-evaluation

Abstract

The Konocti Bay fault zone (KBFZ), initially regarded by some as a promising target for liquid-dominated geothermal systems, has been a disappointment. At least five exploratory wells were drilled in the vicinity of the KBFZ, but none were successful. Although the Na-K-Ca and Na-Li geothermometers indicate that the thermal waters discharging in the vicinity of Howard and Seigler Springs may have equilibrated at temperatures greater than 200°C, the spring temperatures and fluid discharges are low. Most thermal waters along the KBFZ contain >100 mg/l Mg. High concentrations of dissolved magnesium are usually indicative of relatively cool hydrothermal systems. Dissolution of serpentine at shallow depths may contribute dissolved silica and magnesium to rising thermal waters. Most thermal waters are saturated with respect to amorphous silica at the measured spring temperature. Silica geothermometers and mixing models are useless because the dissolved silica concentration is not controlled by the solubility of either quartz or chalcedony. Cation geothermometry indicates the possibility of a high-temperature fluid (> 200°C) only in the vicinity of Howard and Seigler Springs. However, even if the fluid temperature is as high as that indicated by the geothermometers, the permeability may be low. Deuterium and oxygen-18 values of the thermal waters indicate that they recharged locally and became enriched in oxygen-18 by exchange with rock. Diluting meteoric water and the thermal water appear to have the same deuterium value. Lack of tritium in the diluted spring waters suggest that the diluting water is old.