Genesis of thermal groundwaters from Siping'an district, China

Abstract

Thermal groundwaters (40–52°C, pH = 7.4–7.8, Eh = 210–245 mV) from Siping'an district, Shanxi Province, northwestern China, are hydrogeochemically unique. Their occurrence is controlled by faulted structures in Precambrian host rocks. Their hydrochemical type (5 springs and 2 wells) is mainly Cl SO 4 Na, with TDS values around 1.0 g/l. Some minor elements such as Si, Br, Sr, and Li, as well as neutral and acid bituminous substances are so enriched that the thermal waters can also be regarded as mineral waters. Their origin is meteoric, as indicated by 3 lines of geochemical evidence: (1) their δD and δ 18O compositions are very close to the Craig meteoric line; (2) their dissolved gas compositions are N 2-dominated, with less O 2 and CO 2; and (3) the 3He/ 4He ratios are low (0.028). Geochemical processes responsible for the genesis of the hydrochemical features of the waters include dissolution, mixing, and oxidation. The most important water-rock interaction is dissolution or hydrolysis of alumino-silicate minerals in the magmatic and metamorphic host rocks, since the waters are still undersaturated with respect to albite, anorthite, K-spar, and chlorite, as shown by saturation indices. The tritium contents of some thermal waters (46–53 TU), higher than the tritium concentration of local meteoric water, result from the mixing of thermal waters with cold, shallow-lying groundwaters that are from the 1960s. The predominant species of Fe in the thermal waters is Fe(OH) 3, as a result of oxidation processes under aerobic conditions of the aquifers.