Geochemical characteristics of absorbed gases in fault gouge from the Daliushu Dam area, NW China

Abstract

A total of eleven fault gouge samples were collected from the Daliushu Dam area in the Zhongwei-Tongxin Fault Zone (ZTFZ) and analyzed for the absorbed gas geochemistry. The concentration of absorbed gas was between 0.04 and 1.65 cm3STP/g, and the chemicals were determined as mainly CO2, N2, H2, Ar, and a few hydrocarbon gases. The ratios of N2 and Ar suggest the presence of water in the fault zone, leading to water-rock interaction and lower N2/Ar ratios. CO2 and H2 showed some signatures implying abiogenic origin, with relatively high δ13CCO2 (-1.7‰ to 0.95‰) and H2 being positively correlated to CO2 (r = 0.83). We speculate that the CO2 and H2 are correlated to lithic origins in the fault zone, carbonate and silicate, respectively. On the other hand, CH4 did show a biogenic signature, with low δ13CCO2 (-44.2‰ to -45.6‰). The variation trends among the absorbed gases in the fault profile show that the gas concentration is mainly related to the fault zone structure, petrology of the fault material, and porosity. The total amounts of absorbed gases including H2, CO2, and Ar observed were higher at site F201 than those at site F3, and since the F201 site is known to be much more active than F3, we propose that CO2, H2, and Ar could be useful indicators of fault activity.