Abstract: The Tibet Plateau is rich in geothermal resources. Among them, in Chaka and Quzhuomu areas of Shannan, under similar background of geohistorical evolution, geothermal springs with significantly different water chemical compositions were formed, and two different types of springs, silica and travertine, were deposited on a large scale. In this paper, the causal mechanisms of the differences in hydrochemistry between the two types of geothermal systems are discussed through the comparative analysis of the parameters of the two types of geothermal springs, normal and trace chemical compositions, and the stable isotopes of H and O. The results show that Quzhuomu geothermal springs are recharged by meteoric water, and the components source is mainly water-rock interaction, and the water chemical type is mainly HCO3-Ca-Na type. Due to the long hydrochemical evolution process of Chaka and the mixing of a higher proportion of magma residual fluid, the water chemical type evolves into Cl-Na type. The estimation results of various geothermal thermometer show that the thermal reservoir temperature of Quzhuomu is between 98.8-181.3 ℃ , and the proportion of cold water mixing before excretion is 43.4%, while the thermal reservoir temperature of Chaka (183.2-204.3 ℃ ) is significantly higher than that of Quzhuomu. Compared with Quzhuomu, the longer water cycle evolution process of Chaka geothermal springs, the mixing of a higher proportion of magma residual fluid and the higher thermal reservoir temperature, are the main cause mechanism that controls the large-scale deposition of silica during the discharge process of geothermal springs and the abnormal enrichment of some typical incompatible elements in the water body.

Key words: geothermal springs, travertine, silica sinter, hydrochemistry evolution, genesis mechanisms