A Preliminary Analysis Of The Formation Of The Jifei Thermal Spring And Travertine, Yunnan, China

The Jifei thermal spring (24°44′N, 99°28′E) locates at Changning County, West Yunnan Province where geothermal resources are abundant. The water temperature varies from 35℃to 81℃; its total flux is about 10 L/s and chemical type of water is HCO3-Na?Ca type. The spring is famous for its huge travertine, which covers about 4000 m2 and has as much as 18 travertine cones with very different shapes and sizes.Local hydrological conditions, hydrochemistry compositions, and isotope compositions are analyzed to establish a conceptual model of occurrence of the Jifei thermal spring. The geothermal reservoir is recharged from precipitation at high altitude (about 1888~1956 m) in the southwest mountainous areas. The precipitation begins to infiltrate through large fault fracture zones by water pressure derived and the water temperature increases with increasing circulation depth. The water temperature is up to about 120℃at 2014~2080 m depth. Then the geothermal groundwater begins to ascend through available channels (fault intersection). When the geothermal groundwater ascends to or close to surface, it mixes with local shallow groundwater.Subsequently, the mixed water occurs in the form of spring groups with temperature from 35℃to 81℃in the Jifei canyon. All the process takes about 35 a~50 a, which is long enough for the geothermal water to catch abundant of Ca2+ and HCO3-, both of which are formed by water-rock interaction and CO2, which comes from mantle, carbonate rocks metamorphism and so on. That is why Jifei thermal spring has hydrochemical condition for travertine deposition.Based on the locations, causes and shapes of travertine cones, the Jifei travertine cones are divided into 4 kinds. As the strength of hydrothermal activities are different in various formation stages of travertine, the formation process of travertine is divided into 3 stages generally: initial stage, growth stage and death stage. By the calcium ion equilibrium and considering the deposition rate of other travertine, this study estimates that the Jifei travertine depositions date to the Holocene.Wenquanxiang (WQX) thermal spring locates at Changning County too. It's geological setting and climate are similar with Jifei thermal spring. However, no travertine is found in WQX geothermal area. Through comparing Jifei thermal spring with WQX thermal spring, this study finds that both high contents of HCO3- and CO2 in water are the main condition for the travertine deposition. 7 famous domestic and foreign travertine springs are selected to analyze the similarity of hydrochemistry data in travertine areas. The necessary hydrochemistry conditions of travertine deposition are summarized as follows: the water temperature not higher than 90℃; HCO3 type water, which means HCO3- is the main anion. HCO3- or Ca2+ account for not less than 30% equivalent of the anions and cations, respectively; spring water has amount of deep source CO2 and significantly high PCO2 value, which is higher than that of atmosphere.