A preliminary study on the chemical and isotopic characteristics of geothermal waters and gases in northern Thailand.

Abstract

Hydrogen and oxygen isotopic compositions of hot spring waters and meteoric waters from San Kamphaeng, Fang, and their vicinity in northern Thailand were measured. Although most of the data fall on the global meteoric water line δD=8δ18O+10, some meteoric waters exhibit high δD and δ18O values which plot on a different line defined by δD=8δ18O-5. The isotopic compositions of hot spring waters vary in the range defined by the above meteoric water lines with the δD variation between -50 and -62‰. No large oxygen isotopic shift was observed for all geothermal waters collected in this survey, indicating that the geothermal waters originate from the local meteoric waters.A distinct difference was observed in hydrogen concentrations of soil gases between San Kamphaeng and Fang geothermal areas. In Fang the high H2 concentration areas are distributed around the hot spring zone and a closed fluorite mine which is a product of past hydrothermal activity. No such areas were found in the San Kamphaeng area, suggesting that geothermal activity may be higher in Fang than in San Kamphaeng.The 3He/4He ratios of helium in hot spring gas samples from the Fang area are higher than those from the San Kamphaeng area, although these values are lower than 3×10-7. The δ13C values of CO2 in these hot spring gas samples (-7 to -11‰) are close to those for volcanic CO2. This suggests that the volcanic CO2 is supplied from the deep seated geothermal system and a possibility that mantle He is diluted by a large amount of crustal He originated from abundant radioactive elements in granitic rocks widely distributed in northern Thailand.