Chemical and isotopic characteristics and origin of spring waters in the Lanping–Simao Basin, Yunnan, Southwestern China

Abstract

The chemical and isotopic characteristics (oxygen, hydrogen, and strontium) of spring waters and isotopic compositions of helium (He) and neon (Ne) in gases escaping from spring waters in the Lanping–Simao Basin are studied. A total of twenty-one spring water samples (twelve hot springs, four cold springs, and five saline springs) and eleven gas samples were collected from the study area, including one spring and one gas sample from northern Laos. It is found that saline spring waters in the study area are of chloride type, cold spring waters are of carbonate type or sulfate type, and hot spring waters are of various types. High total dissolved solids levels in saline springs are significantly related to Upper Cretaceous–Paleocene salt-bearing strata. On the basis of hydrochemical geothermometry, the reservoir temperatures (Tr) for hot springs, cold springs, and saline springs are 65.5–144.1, 37.8–64.4, and 65.1–109.0°C, respectively, and the circulation depths of saline springs are much larger than those of hot and cold springs. The oxygen and hydrogen isotopic compositions of springs in the Lanping–Simao Basin and northern Laos are primarily controlled by meteoric waters with obvious latitude and altitude effects, and are also influenced by δ18O exchange to some extent. Most Sr2+ in spring waters of the study area is derived from varied sources (carbonate, evaporite, and silicate mineral dissolution), and the Sr isotopic compositions are greatly influenced by volcanic rocks. Wide distribution of crust-derived He in the Lanping–Simao Basin and northern Laos reveal that faults in these areas may not descend to the upper mantle. It is concluded that water circulation in the study area may be limited above the upper mantle, while saline springs may originate from the Upper Cretaceous–Paleocene evaporites. Hydrochemical characteristics demonstrate affinities among the Lanping–Simao Basin, northern Laos, and Yanjing, eastern Tibet, while disaffinities are observed between these areas and Tengchong on the basis of the hydrochemical characteristics and noble gas isotopic compositions.