Abstract
The Heinitang hot spring is located in the western Yunnan of China and lies in the Tibet-Yunnan geothermal zone and the Tengchong Block. The hot water is of HCO3-Ca•Na type and the F concentration is relatively high (ranging from 3.8 to 5 mg/L). Fossil travertine mounds and cones and new travertine are found at the spring vents. Hydrochemical analyses indicate that the primary factor affecting the deposition of travertine is the concentration of ion pair CaCO30 in the Heinitang hot springs. The Heinitang hot spring shows a positive correlation between the concentration of CaCO30 and the saturation index with respect to aragonite, calcite and dolomite. They are not linearly related, but have an exponential relationship. When the aragonite, calcite and dolomite are in the state of dissolved equilibrium, the corresponding concentrations of CaCO30 are 0.01 mmol/L, 0.008 mmol/L and 0.012 mmol/L, respectively, indicating that the precipitation of travertine can occur when the concentration of CaCO30 is higher than 0.008 mmol/L. In addition, the concentrations of CaCO30in the Heinitang hot springs decrease gradually with the deposition of travertine. The precipitation of travertine cannot occur when the concentration of CaCO30 decreases to lower than 0.008 mmol/L.
Keywords: hot springs; travertine; hydrochemistry; CaCO30; Yunnan;

Introduction

The terms “travertine” and “tufa”, in its broadest sense, refer to non-marine carbonate precipitates near springs, rivers, lakes and caves, which consist of calcite or aragonite (Viles and Goudie, 1990; Pentecost, 2005). Tufa is the precipitation of calcium carbonate under cool water (near ambient temperature) and travertines are calcareous deposits that precipitate where thermal groundwaters emerge at springs (Ford and Pedley, 1996; Li and Robert, 1998; Fedoseev et al., 2017). Tufa or travertine can provide valuable information to study the paleo-climate and paleo-environment (Chafetz et al., 1991; Fouke et al., 2000) and have implications for hydrogeological evolution history and present or past fluid dynamic regimes (Andreo et al., 1999; Naroa et al., 2018). Travertine may reflect changes in temperature and rainfall by changing isotopic compositions, minerals, the rate of growth and texture. In addition, travertine have various applications in construction, agriculture, decorative arts, etc. (Erdoğan, 2011).
Travertine deposits form by important precipitation of carbonates and are distributed over the world (Pentecost, 1995a). However, the precipitation of travertine only occurs near a small portion of hot springs (Zhou et al., 2017). Travertine deposits are formed when thermal groundwater, rich in Ca, HCO3 and CO2, flows to the surface and outgases CO2 to the atmosphere. Re-equilibration of CO2 in groundwater can result in saturation of CaCO3 and travertine deposits downstream from the hot spring (Pentecost, 1999; Acikel and Ekmekci, 2016). Therefore, it is significant to study the hydrochemical characteristics of hot springs to better understand the factors affecting the precipitation of travertines. Interest in the travertine-depositing hot springs has increased and the studies of hydrochemical characteristics of hot springs have attracted attention since the 19th century. In the travertine-depositing hot springs, the concentrations of major and trace elements were determined by numerous researchers. The characteristics of hydrochemical compositions in travertine-depositing hot springs in Italy are high concentrations of CO2, Ca, Mg and SO4 (Pentecost, 1995b). The Pamukkale hot springs in Turkey contain high concentrations of HCO3, Ca and SO4 in the ranges from 1100 to 1300 mg/L, from 400 to 500 mg/L and from 600 to 750 mg/L, respectively (Yesertener and Elhatip, 1997). Kawai et al. (2009) proposed that the precipitation rate of CaCO3 is mainly controlled by Ca and alkalinity, which is relatively low when Ca concentrations are less than 65 mg/L. However, the precipitations of travertine from the Jifei hot spring in western Yunnan of China took place on a cliff face when the concentration of Ca is 58.1 mg/L (Liu et al., 2012; Jones and Peng, 2016). The high concentrations of Ca and Mg of hot springs can arise apparently saturation with respect to calcite and aragonite when the thermal groundwater rises to the surface with the CO2-outgassing. In addition, other hydrochemical factors proposed as influential in the precipitation of travertine from hot springs include the saturation index with respect to carbonate (Dandurand et al., 1982), hydrochemical type (Dilsiz et al., 2004), temperature and pH (Veysey et al., 2008) and the relationship between major elements (Kele et al., 2008). The hydrochemical type and saturation index with respect to calcite are common indications in the analyses of the travertine-depositing hot springs. The Pamukkale travertine-depositing hot springs exhibited a HCO3•SO4-Ca type and positive values of saturation index calculated with Phreeqc-2 (Dilsiz, 2006). Fouke (2011) established a quantitative correlation among the temperature, pH and flux and classically-defined travertine depositional facies at the Mammoth hot springs in the Yellowstone National Park. Kele et al. (2011) suggested that the Jandarma hot spring in Turkey show a positive correlation between the equivalent concentrations of Ca and HCO3 and the values of\(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3 range from 1.5 to 1.8. Wang et al. (2015) noted that the concentrations of Ca and HCO3 decrease gradually along the path of travertine deposition at the Lianchangping hot springs in western Yunnan of China and the values of\(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3 are almost constant (nearly 1). Frery et al. (2017) noted that the precipitation of travertine takes place near the outlet of the hot springs that represent the chemical characteristics of high PCO2 values and saturation index (SI) values with respect to carbonate minerals. However, there is limited data on the influence of CaCO30 ions pairs on travertine deposition from hot springs.
It seems that further research is needed to interpret the hydrochemical factors affecting the deposition of travertine. So far, substantial papers to clarify this matter have been published. In the present paper, we examine 8 water samples collected at the Heinitang hot springs in western Yunnan of China in 2013 and 2018 during field investigations where travertines are depositing from the hot springs. This paper places its emphasis on a new hydrochemical factor affecting the deposition of travertine from the Heinitang hot springs by analyzing the concentrations of CaCO30 and establish the relationship between the concentrations of CaCO30 and the saturation index with respect to aragonite, calcite and dolomite that other researchers have not done.