Hydrochemical parameters
The precipitation of travertine is controlled by many factors, which can be grouped into 4 primary fields: (1) hydrochemistry, (2) hydrodynamics, (3) microbiology and (4) depositional environment. Hydrochemical conditions are the most important factors affecting the deposition of travertine (Zhou et al., 2017). The hydrochemical factors of the hot springs mainly includes the concentrations of the major ions and CO2, the partial pressure of CO2 (Table 1), \(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3and\(\ \mathrm{\gamma}\)Mg/\(\mathrm{\gamma}\)Ca (Table 2), the saturation index with respect to anhydrite, aragonite, calcite, dolomite, gypsum and halite (Table 3) and the concentration of CaCO30 (Table 4).

Hydrochemical constituents

Pentecost (1999) proposed that the concentrations of Ca and HCO3 are the most important factors that affect the travertine deposition, since the travertine mainly consist of calcite (CaCO3), and almost all of the travertine deposits contain more than 90% of calcium carbonate. Na concentrations of the Heinitang hot springs range from 165 to 185 mg/L, and for the concentrations of Ca are in the range between 85 and 115 mg/L (Table 1). The concentrations of K and Mg are relatively low, ranging from 23 to 24 mg/L and from 8 to 26 mg/L, respectively. Compared with water samples S2* and S4*, the concentrations of Na and K in the water samples S2 and S4 are almost constant, Ca concentrations increase and Mg contents in the water samples S2 and S4 decrease (Fig. 5a). HCO3 is dominant in the anions of the water samples, and its concentrations are high and range from 780 to 800 mg/L (Fig. 5b). The concentrations of SO4 and Cl are relatively low (Fig. 5b). The concentrations of SO4 are lower than 1 mg/L, and the concentrations of Cl range from 50 to 70 mg/L in the water samples. The concentrations of HCO3, SO4 and Cl are almost unchanged, indicating that there is no significant evaporation from the hot water or other sources of water supply. In addition, the concentrations of F in the water samples are relatively high (3.8-5 mg/L) (Fig. 5c), which are much higher than the World Health Organization (WHO) maximum guideline value (1.5 mg/L). The F concentration in natural waters depends on such factors as temperature, pH, presence or absence of complexing or precipitating ions and colloids, solubility of fluorine-bearing minerals, anion exchange capacity of aquifer materials (OH for F), the size and type of geological formations traversed by water, and the amount of time that water is in contact with a particular formation (Apambire et al., 1997). Minerals which have the greatest effect on the hydrogeochemistry of fluoride are fluorite, apatite, micas, amphiboles, certain clays and villiaumite. Fluorite is the main mineral controlling aqueous fluoride geochemistry in most environments. However, there are some notable exceptions in sedimentary basin environments (Chae et al., 2006).
Fig. 5. (a) Cations, (b) anions, (c) F concentrations, (d) TDS, (e) CO2 concentration and PCO2 of the Heinitang hot water samples, (f) the relationship between pH and PCO2.
As a measure of water salinity, the TDS may be affected by different lithologies and geochemical conditions (Pasvanoğlu, 2013). Relatively high TDS values of the hot springs in western Yunnan are common. The TDS values are above 750 mg/L in the Heinitang hot springs (Fig. 5d), indicating that the hot spring water undergoes a deep circulation and longer residence time.
In general, the precipitation of travertine on the earth’s surface is described by the overall reaction (Dandurand et al., 1982):
Ca2++2HCO3- → CaCO3(s) + H2O+CO2 (g) (2)
In this reaction, CaCO3 does not deposit immediately when CO2 outgases or is consumed. Therefore, the solution loses CO2 according to the following reaction:
H++HCO3- → H2CO3→ H2O+CO2 (g) (3)
Consequently, the CaCO3 becomes supersaturated gradually and precipitates:
Ca2++CO32- → CaCO3 (s) (4)
This shows that the rate of CO2-outgassing can affect the deposition of travertine. Outgassing of CO2 occurs during surface flow as the travertine-depositing hot springs reach equilibrium with the atmosphere, which contributes to the formation of travertine (Lorah and Herman, 1988; Liu et al., 1995). Jones et al. (2005) suggested that the precipitation of travertine was driven by rapid CO2 degassing of CO2-rich spring waters along the flow path of hot springs. The concentrations and the partial pressure of CO2 are high and show high variability in the Heinitang hot springs. The values of CO2 concentrations and partial pressure range from 20 to 80 mg/L and from 0.04 to 0.5 atm, respectively (Fig. 5e). The calculated PCO2 values are significantly higher than that of the atmosphere (0.04% atm). The release of CO2 is not only controlled by PCO2, but also related to the flowing path, the flowing rate and the location of spring. As mentioned above, the deposition of travertine from hot spring vents S2, S3 and S4 stopped during a period of time, this can be caused by the decrease in discharge of the spring’s vents. After the artificial construction of the pools, the spring waters flow along the wall of the pool, increasing the area in contact with the air and accelerating the releasing rate of CO2, which results in the deposition of travertine. This indicates that the high values of CO2, PCO2 and the conditions of CO2-outgassing are necessary factors for the precipitation of travertine from the hot springs.
pH of water is a common chemical parameter affecting the deposition of travertine, along with several other factors, such as the hydrochemical compositions of the water, temperature and PCO2 (Zhou et al., 2014). Shen et al. (1993) reported that pH values and PCO2 are not linearly related, but have a logarithmic relationship under the standard condition and equilibrium with respect to calcite in the solution. The observed values of pH of the water samples range from 6 to 8 and plot on the scatter diagram (correlation index R2 = 0.9766) (Fig. 5f). pH values can reflect the values of PCO2 to some degree.
Table 2 The ratios of Ca and HCO3, Mg and Ca, and the hydrochemical type of the Heinitang hot spring “*” represents the water samples which were collected in 2013.