The provenance and evolution of thermal spring waters from the Dogai Coring area of Qiangtang basin in Tibet, China

Abstract

Chemistry of major and minor elements, 87Sr/86Sr, δO, δD, and δ34S of thermal spring water, and 34S of gypsum were measured from Jurassic strata in the Qiangtang basin, China. The aim is to elucidate the provenance and evolution of thermal spring water, further to reveal geological information on the deep basin. Analysis results show that the spring waters are rich in K and Ca but deficient in Mg and Br. Most spring water samples have TDS within the range of 10–100 g/L, with a few containing 1–10 g/L TDS, and can be referred to as saline springs. High consistency of spring water data with seawater curves indicates that Na and Cl in spring water are derived from residual seawater sequestered in strata, or dissolution of self-evaporiting salts in strata. However, the mean Cl/Br value of spring water samples ranges from 1000 to 30,000, further indicating that solutes in spring water are derived from dissolved evaporites (e.g., NaCl) in strata. Sulfur isotopic characteristics indicate that thermal spring waters commonly dissolve Jurassic gypsum in strata and dolomitization of calcite is the mainly reason for Mg loss. The interpretation of Ca enrichment of spring waters of northwest Wan'an Lake (NWA), Yuanquan River (YQR), and east Thermal Spring (ETS) include: dissolution of gypsum and albitization of plagioclase. Spring waters of south Dogai Coring Lake (SDC) are enriched with more Ca than that at the NWA, YQR and east ETS, mainly due to dissolution of gypsum and dolomitization of calcite. The enrichment of K of spring waters at NWA, YQR and ETS can be attributed to albitization and dissolution of K-feldspar in groundwater. Relatively increased K in SDC spring waters are possibly derived from dissolution of potassium minerals (e.g., KCl). The 2nd member of Xiali Formation has 87Sr/86Sr values distributed in a consistent range with those of thermal spring waters in the Dogai Coring area, which is most likely the source horizon of salts (e.g., halite, gypsum, and sylvine). Modern meteoric water enter strata and dissolve ancient evaporites.