129I and 36Cl concentrations in waters of the eastern Clear Lake area, California: Residence times and source ages of hydrothermal fluids

Abstract

The Clear Lake area of northern California is the location of hot spring activity, some of which is associated with the formation of Au and Hg deposits. We measured 129I I and 36Cl Cl ratios in nine warm springs (formation springs), six cold springs with elevated levels of Cl and I (mineral springs), and five springs of recent meteoric origin. Dating of the I in the formation waters indicated that sources of I in these waters are formations with minimum ages between 60 and 80 Ma. This age range is in good agreement with the ages of the Franciscan Complex and the Great Valley Sequence, the dominant formations in this area. Because the mineral waters are essentially formation waters diluted with meteoric water, I in these waters is of the same origin. Residence times of the waters were calculated based on the build-up of 129I and 36Cl as a consequence of the presence of U and Th in the crust. The residence time of the formation waters in the Great Valley Sequence, the location for most of these springs, was probably not longer than 84,000 yr. The concentrations found for 36Cl and 129I in the mineral waters indicate that these waters have residence times of similar magnitude in formations such as the Franciscan Complex or the Clear Lake Volcanics, which have slightly higher levels of U and Th than the Great Valley Sequence.