Hydrogeochemical Characteristics and Geothermometry of Hot Springs in the Mongolian Altai Region, Mongolia

Accepted Manuscript Geochemistry: Exploration, Environment, Analysis

Hydrogeochemical characteristics and geothermometry of hot springs in the Mongolian Altai region, Mongolia

Bolormaa Chimeddorj, Dolgormaa Munkhbat, Battushig Altanbaatar, Oyuntsetseg Dolgorjav & Bolormaa Oyuntsetseg

DOI: https://doi.org/10.1144/geochem2021-016

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Received 24 February 2021 Revised 29 June 2021 Accepted 3 July 2021

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Hydrogeochemical characteristics and geothermometry of hot

springs in the Mongolian Altai region, Mongolia

Bolormaa Chimeddorja,b, Dolgormaa Munkhbatc, Battushig Altanbaatard, Oyuntsetseg Dolgorjave,

Bolormaa Oyuntsetsega,*

aDepartment of Chemistry, School of Arts and Sciences, National University of Mongolia, Ulaanbaatar

14201, Mongolia

bDepartment of Chemical–Biology, School of Natural Science and Technology, Khovd University,

Peace Avenue 164300 Khovd Province, Mongolia

cDepartment of Chemical and Biological Engineering, School of Engineering and Applied Sciences,

National University of Mongolia, Ulaanbaatar 14201, Mongolia

dMineral Resources and Petroleum Authority of Mongolia, Ulaanbaatar, Mongolia

eInstitute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia MANUSCRIPT *Corresponding Author, ORCID: 0000-0001-7257-3439 (E-mail: [email protected])

Abstract: This study determines the properties of hot spring waters and associated rocks, calculates

reservoir temperatures and depths in the Mongolian Altai region, and constructs a conceptual model

for geothermal water based on these results. The hot springs consist of HCO3-Na, SO4-Na, and HCO3-

SO4-Na mixed type waters. The waters exhibit alkaline pH levels and temperatures in the range of

21.3–35°C. X-ray diffraction analyses of outcrop rocks reveal silicate and carbonate-type minerals such asACCEPTED quartz, albite, orthoclase, dolomite, mica, and actinolite, while correlation analysis indicates that the chemical composition of the hot spring water is directly related to rock mineral composition.

Dissolution of albite, orthoclase, and dolomite minerals has played an important role in the chemical

composition of the waters. Reservoir water circulation depths were 2615–3410 m according to

quartz and chalcedony geothermometry. The results indicate that the spring water in the Mongolian

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Altai region comprises a low mineral content with alkaline pH levels and the reservoir temperature

can reach up to 106°C. We also propose a conceptual model for geothermal water in the Chikhertei

hot spring. The geothermal water in the Mongolian Altai region exhibits a potential for use in heating

systems.

Keywords: Hot spring, hydrogeochemistry, Mongolian Altai, geothermal energy, water–rock

interaction

Hot springs have been widely used for decades in recreational and medical applications

because they impart useful health benefits (Kaven et al., 2020). Nowadays, hot springs are being

utilized by heating and power generation systems because the geothermal energy of hot springs is

renewable (Lund, 2018, Paul et al., 2018). Presently, about 82 countries produce 164,635 GWh/year

of energy for direct use from geothermal energy resources (Lund, et al., 2011), and this energy is

used in agriculture, minor industrial processes, various heating systems, greenhouses, and farms

(Guðnason et al., 2015). Some countries such as Costa Rica, El Salvador, Kenya, and New Zealand produce more than 10% of their total electricity MANUSCRIPTsupply using geothermal resources (Van Nguyen et al., 2015).

In Mongolia, there is an estimated 250 hot and cold springs, among which 43 are hot springs

(Oyuntsetseg et al., 2015). Numerous authors have classified spring water temperatures to exhibit

different ranges of values (Zunic et all., 2019, Durowoju et al., 2015, Thermal Laboratory et al.,

2018). We have used the following range, which has been used repeatedly in the literature: Cold

(<20ºC), Warm (20-35 ºC), Hot (35-45ºC), and Very hot (>45ºC). Generally, Mongolian springs occur in theACCEPTED following five regions: Mongolian Altai, Khuvsgul, Dornod-Mongol, Khangai, and Khentii (Lkhagvadorj and Tseesuren, 2005). Most of the hot springs (32 hot springs) are distributed in the

Khangai Mountain region, where the hot springs are used for medical treatment and spa resorts.

This area is in the central part of Mongolia, which is located 400 km away from the capital city. For

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 the past 10 years, other hot springs such as the Tsenkher, Khujirt, and Shivert have been used for

heating systems with constant flow (Bignall et al., 2005; Tseesuren, 2001).

Hot spring research in the Khangai Mountain region has been more intense than that in the

other four regions (Ganbat and Demberel, 2010; Gendenjamts, 2003; Oyuntsetseg et al., 2015). The

Mongolian Altai region predominantly has cold springs; however, it also has four hot springs

including Chikhert, Gants Mod, Aksu, and Indert. Previous studies in the Mongolian Altai region were

focused on hydrochemical analyses of the hot springs (Yousif and El Aassar, 2018); however, a

geothermal study in this area has not been reported yet (Tugjamba, 2021).

The hydrochemical properties of hot springs are related to the interactions of water with the

surrounding rock minerals. Therefore, mineral interactions, mineral components, and geothermal

calculations can be used to assess the feasibility of using a hot spring as an energy resource. The

purpose of this study was to determine the properties of the hot spring waters and associated rocks

and calculate the reservoir temperatures and depths in the Mongolian Altai region; based on these results, we constructed a conceptual model for theMANUSCRIPT geothermal waters. 1. Study area

1.1. Topography and climate

Figure 1 shows a map of Mongolia. Mongolia is a landlocked country between Russia and China

and its territory covers an area of 1564116 square kilometers. The capital city is Ulaanbaatar, located

about 1700 km away from the study area.

The Mongolian Altai range exhibits a length of 900 km and is located in the northwest of the countryACCEPTED along the border of Mongolia and China (Fig. 1). The Mongolian Altai range extends through the landscapes and provinces of Bayan-olgii, Khovd, Uvs, and Govi-Altai in Mongolia. The total area

of the Mongolian Altai region is approximately 104478.5 km2 (Blomdin et al., 2016). At the top of the

Mongolian Altai range is the Khuiten Mountain, which exhibits a height of 4374 m above sea level.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 The Mongolian Altai Mountains have sharp peaks, steep slopes, and sharp fault scarps owing to

a strong tectonic uplift. In this region, many deep crustal faults are located in the sub-meridian and

sub-latitudinal directions. Additionally, this area is a seismically active zone (Lkhagvasuren et al.,

2017).

This continental area is located in a harsh climate zone owing to the northern latitudes and high

elevations above sea level. The annual average precipitation reaches 50 to 350 mm in the Mongolian

Altai Mountains (Lkhagvasuren et al., 2017). Hot springs in the Mongolian Altai region are located

1690–2780 m above sea level (Blomdin et al., 2016; Lehmkuhl et al., 2016).

The distribution of Mongolian hot springs by temperature difference (warm, moderate, and

high) is shown in Figure 1.

1.2. Geological setting

Sedimentary rocks, volcanogenic sediments, and granitic intrusions are widespread in the

Mongolian Altai region. In the Paleozoic zone, middle Cambrian–Lower Ordovician sandstone- siltstone was commonly deposited in this area. Gneiss,MANUSCRIPT crystalline shale, granite, syenite, gabbro- syenite, grano-syenite, diorite, and granodiorite are the other rocks present in this region.

Furthermore, this region contains valleys, ridges, and mountainous areas with saline soils in

meadow marshes, saline mountain tundra, and alpine meadow soils in high mountain areas, which

were formed by erosion and weathering processes (Blomdin et al., 2016; Byamba et al., 1999). The

Mongolian Altai terrain is characterized by overlapping depressions with Middle Ordovician–Lower

Silurian volcanogenic-terrestrial compositions, overlapping depressions of the Deluun-Sagsai and YamaatACCEPTED rivers, and Devonian volcanic sediment. It contains a variety of components, including molasse, siliceous sub-granite, and the Devonian Altai granite complex (Tomurtogoo, 2008, 2012).

The Tawan Bogd terrain range covers the high mountains in the western part of the Mongolian

Altai range and includes a 4000–6000 m thick unit of late Cambrian–Lower Paleozoic sandstone and

siltstone. The lithology is composed of granite, leucogranite, and granite porphyry that are 385–349

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Ma and granite porphyritic biotite, medium granite, and albite granite that are 456 Ma (Bulgatov et

al., 2014; Byamba et al., 1999). The geological setting of the Mongolian Altai region is shown in

Figure 2.

1.3. Hydrogeology of the study area

The Mongolian Altai terrane is considered to be the remains of oceanic island arcs or a Lake Zone arc

system (Cai et al., 2015 Xiao et al., 2018). The Mongolian Altai range contains various lakes, glaciers,

and hot springs. The majority of the water resources in the Mongolian Altai region are located in the

Khar-Us and Khovd river basins, which cover an area of 85104.8 km2 over five provinces in Western

Mongolia. The deepest lake, Khoton, is about 26–58 m deep, and the biggest lake, Achit, has an area

of approximately 296.8 km square. In total, 96% of the rivers and lakes are considered to be fed by

glaciers, with the longest glacier in the Mongolian Altai range (Potanine) being 20 km long. The

Khovd River is the largest river in the outflow basin of the Mongolian Altai range, which is situated in

Central Asia. The age of the hydrogeology of the Mongolian Altai region is Quaternary and the aquifer contains alluvial, deluvial, and proluvial moraineMANUSCRIPT sediments. The sediments of the Quaternary period represent the layer that is closest to the earth's surface; these were formed

10,000 to 80,000 years ago. The Mongolian Altai region exhibits the following spring water types:

Na-HCO3, Na-SO4, and Na-SO4-HCO3 (Lkhagvasuren et al., 2017).

2. Materials and methods

Hot spring water and rock samples were collected from locations represented by blue dots in

Figure 2. Twelve water samples were collected from four hot spring sources: Aksu (AC), Indert (IN),

Gants ACCEPTEDMod (GM), and Chikhertei (CH). These hot springs are located in Altai region, Bayan-Olgii

Province, Mongolia. Outcrop rock samples were collected from the outflow part of each of the four

hot springs. As shown in Figure 2, rock and hot spring water samples were obtained from the same

four locations. The bedrock of these hot springs was not exposed.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Each water sample was collected and stored in a polyethylene plastic bottle with 2 mL nitric acid

for stabilization. Physicochemical parameters, such as temperature, pH, total dissolved solid (TDS)

levels, and oxidation reduction potential (ORP), were measured in situ using a Hanna HI-98 (USA)

instrument. Bicarbonate ion levels were determined in situ using the 0.01 N hydrochloric acid

titration method with methyl orange endpoint. The cations and anions in the hot spring water

samples, including Ca2+, Mg2+, Na+, and K+, were detected using inductively coupled plasma (ICP)-

optical emission spectrometry (Shimadzu, Japan). All tests were performed in duplicate and

measured in line with ICP standards. Quality assurance and control of the measurements were

performed using a certified reference material (SPS–SW2) for the determination of trace elements.

Sulfate ion and SiO2 levels were determined by spectrophotometry using the alkalinity standard and

ammonium molybdate methods, respectively, with the 4500-SiO-2.C.3g standard silica sample (UV-

M51, Brazil). Chloride ion concentration was analyzed using the AgNO3 volumetric method. The ionic

charge balance error of the analyses was less than ±5%.

Rock samples were air dried at room temperature for 48–72 h after collection. The dried

samples were sieved using a 0.074 millimeter meshMANUSCRIPT and used for elemental and mineral composition

analyses. The mineral composition of the rock samples was investigated by X-ray diffraction (XRD)

(PANalytical X'Pert PRO, Netherlands) using the Co-Kα line (λ = 1.79 Å) powder method with a 2θ

diffraction angle range of 2–50⁰. Laboratory duplicates and internal standards (Si, hkl113) were used

to ensure the precision and accuracy of the XRD analyses. For the XRF measurements, a fused borate

disk was used for calibration and a reference material (Cu-base-FP) was analyzed to monitor

equipment performance and accuracy.

TheACCEPTED results of the hydrochemical analyses are presented using a Piper diagram, Figure 3 and

chemical geothermometry calculations were performed using Aquachem (PHREEQC Ver 2.0). A Piper

diagram describes the chemical composition of groundwater and its different classes (Joshi and

Bhandari, 2013). The geological map and geothermometry model were prepared using ArcGIS and

CorelDraw. The equations provided in Table 1 were used for geothermometry calculations.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 The depth of geothermal waters was calculated using the following equation (Arnorsson et al.,

1983; Besser et al., 2018; Shestakova et al., 2018):

D = (T−T0)/G

where D is the geothermal water circulation depth in m, T is the reservoir temperature in °C, T° is the

annual average temperature in °C, and G is the regional geothermal gradient in °C/km.

3. Results and discussion

3.1. Hydrochemical characteristics

Physicochemical parameters and some ion concentrations are shown in Table 2. The accuracy of

the analyses was checked through the Standard Method 1030E. The ionic error balance (%), was

calculated using cation-anion balance which were expressed as milliequivalents per liter. As shown

exceed 5% (Friedman L.C., 1982). The results show that the hot spring temperatures ranged

between 21.3°C and 35 °C, and the pH levels were high, between 8.32 and 9.26, indicating alkaline conditions. This temperature range of the hot springsMANUSCRIPT in the Mongolian Altai region corresponds to the warm springs (20–35°C) classification (Zunic et all., 2019; Thermal Laboratory et al., 2018).

Alkaline hot springs might be suitable for heating systems because of the low corrosiveness of water

in this pH range (Bleam, 2016).

The ORP values are important for determining the state of the elements and the transfer of

elements to other forms of groundwater. The ORP values of the GM, CH, and AC hot spring samples

ranged between 28.8 and 59.1 mV, whereas those of the IN samples were between 124 and 127 mV. The ORPACCEPTED may depend on the mineral salts, as well as on the TDS in the hot spring and the amount of dissolved oxygen. The SiO2 content (45.3–75.6 mg/L) can be explained by the weathering of siliceous

rocks in the hot spring waters, which caused the transfer of silica from the rocks to the water. The

fluoride concentrations of the hot spring samples ranged from 1.27 to 9.86 mg/L. Hot springs

contain elements such as aluminum, boron, silicon, and iron, which can form stable complexes with

fluorine, thereby increasing the solubility of fluorite and the number of fluoride ions in the water.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 But the solubility of fluorite also depends on hot spring temperature (Edmunds et al., 2013).

Therefore, another reason for the high fluoride content can be related to the host rocks. Some

igneous and sedimentary rocks contain fluoride naturally, which leads to increased fluoride.

The dissolved oxygen (DO) in the hot springs amounted to 0.01 to 0.61 mg/L; these low levels

illustrate that DO decreases as the temperature increases. The TDS values ranged from 127 to 725

mg/L. Interestingly, the TDS values of the IN1 and IN2 samples ranged between 698 and 725 mg/L,

which were higher than those of the other samples. The IN hot spring is located at a lower

elevation area between the mountains. According to these results, we observed that the TDS

values increased with a decrease in elevation of the hot springs in the Mongolian Altai region. TDS

increase also can be related to evaporation at low altitudes (Oyem, H., 2014).

Moreover, the pH and TDS results lend support to the conclusion that the hot springs of the

Mongolian Altai region are advantageous for heating systems because of the low-corrosion

properties and low amounts of mineral accumulation.

We have illustrated the hydrochemical results for the Mongolian Altai hot springs using a Piper

+ − diagram (Fig. 3). According to the Piper diagram, NaMANUSCRIPT and HCO3 predominated in the CH1, CH2, and

+ 2− CH3 samples, which can be characterized as HCO3-Na type waters. The ions Na and SO4

predominated in the GM1, IN1, and IN2 samples, which can be characterized as SO4-Na type waters.

+ − In the АС1, AC2, AC3, CH4, and GM2 samples, which are HCO3-SO4-Na type waters, Na , HCO3 , and

2− SO4 predominated.

3.1.2. Major geochemical processes TheACCEPTED major ion concentrations and their correlations were analyzed to clarify ion relationships, and the results are shown in Table 3. The f(x) coefficient of correlation analysis was equal to 0.96.

Sodium ions predominated in all hot spring samples. As shown in Table 3, Na/SO4, Ca/SO4, Ca/K and

Na/CI ions had positive correlations (r = 0.99, r = 0.98, r = 0.98, and r = 0.98, respectively). With

respect to silica, HCO3 exhibited the highest positive correlation (r = 0.90). This correlation may be

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 related to the dissolution of subsurface rock and metamorphic rock containing albite and silicate

minerals (Fan et al., 2019). According to the correlation results, gypsum and halite minerals may be

dissolved in the hot spring waters.

Saturation Indices (SI) can aid the interpretation of water-rock reactions in hot spring

environments. A positive SI value shows oversaturation while a negative value indicates

undersaturation (Bouragba et al. 2011). The temperature of hot spring water, major ions, pH, and

microelements were used as input data in SI calculation. Table 4 shows the saturation indices of hot

spring samples. SI values for alunite, anhydrite, gypsum, and halite indicate undersaturation whereas

the other minerals were oversaturated.

3.1.3. Gibbs diagram

The Gibbs diagram was used to investigate the hydrochemical components of ground water and

identify the interactions of water and rock (Fig. 4). The element transfers from rock played an

important role in the compositions of the hot springs (Nagaraju et al., 2016; Nwankwoala et al., 2015). Therefore, we investigated the rock sampleMANUSCRIPT compositions further as discussed below.

3.2. Rock characterization

3.2.1. Elemental composition of the rock samples

Major and minor element compositions of the rock samples determined by XRF are shown in

Table 5. The major element oxides were in the following order: SiO2 > Al2O3 > Na2O > K2O > Fe2O3 >

CaO > MgO > TiO2 > P2O5 > MnO. Based on these results, we concluded that the hot spring reservoirs

included metamorphic layers of granite-gneiss (Joshi and Bhandari, 2013). Elemental analyses of the rock samplesACCEPTED showed the presence of the following 30 elements: Ba, Bi, Ce, Co, Cr, Cs, Cu, Ga, Ge, Hf, La, Mo, Nb, Ni, Pb, Pr, Rb, Sb, Se, Sm, Sn, Sr, Ta, Th, U, V, W, Y, Zn, and Zr. Minor element contents in

the rock samples were in the order of Ba > Sr > Rb > Zr > Sb > Cs > Cr > Se > Zn > V. High contents of

barium and strontium were directly related to the decomposition of dolomite and calcite minerals.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Normative mineral composition (norm) was calculated using the bulk chemical analysis of the

rock samples from the four hot springs. Major and minor element compositions of rock samples

were used as input data for this calculation. Silicate minerals (quartz, plagioclase, orthoclase and

hypersthene), oxide minerals (corundum, ilmenite, magnetite), and phosphate minerals (apatite)

were present as normative minerals (Table 6).

3.2.2. Mineralogical characterization of rock samples

XRD analyses of the rock samples detected silicate and carbonate type minerals such as quartz,

albite, orthoclase, dolomite, mica, and actinolite. Albite and mica are typically found in the

metamorphic rock layer (Bleam, 2016; Joshi and Bhandari, 2013). The results of the XRD analysis are

presented in Fig. 5.

3.3. Water–rock interactions

Results of the correlation analysis of elemental compositions of hot spring water and rock are

presented in Table 7. The correlation analysis for water–rock interaction was calculated using

centered log-ratio (CLR) transformation using R softwareMANUSCRIPT by R Core Team. The sum of all

components was equal to 100% and the f(X) value of correlation analysis was 0.91. As shown in

Table 7, the following components showed strong positive correlation with each other: Na(w) and

f(SiO2(r)); CI(w) and f(Na2O(r)); Ca(w) and f(CaO)(r) whereas the following components showed

strong negative correlation: Na(w) and f(Al2O3(r)) and CaO(r), MnO(r). The positive correlations

indicate the interaction between hot spring water and reservoir rock. 3.3.1. ACCEPTEDGeochemical models of minerals Rock transformation processes occur over thousands of years and are affected by natural

factors. We used the following models to explain the effect of rock weathering processes on the

composition of hot springs in the Mongolian Altai region. Water of these hot springs predominantly

contain HCO3-Na, SO4-Na, and HCO3-SO4-Na ions, and rocks contain albite, orthoclase, dolomite,

calcite, mica, and other minerals.

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Geochemical model of the HCO3-Na type hot spring: Hot spring water + albite, orthoclase, and

dolomite → HCO3-Na + (SiO2 + AI2O3 + K2O + Na2O)mud

+ − 2KAISi3O8 + 2H2CO3 + 9H2O ↔ 2K + 2HCO3 + 4H4SiO4 + AI2Si2O5(OH)4 (1)

+ − 2NaAISi3O8 + 2H2CO3 + 9H2O ↔ 2Na + 2HCO3 + 4H4SiO4 + AI2Si2O5(OH)4 (2)

Equations 1 and 2 show the dissolution of albite and orthoclase to form KHCO3, NaHCO3, and

kaolinite (Oyuntsetseg, 2014; Yuan et al., 2017). Moreover, kaolinite can react with hot spring water

to form orthosilicic acid and gibbsite.

The interaction of dolomite and calcite minerals with water is shown in Equation 3 and 4,

respectively.

2+ 2+ − MgCa(CO3)2 + 2CO2 + 2H2O ↔ Ca + Mg + 4HCO3 (3)

2 - CaCO3( ) CO2 H2O→Ca 2HCO3 (4)

According to Equation 3 and 4, an increase in the calcium and magnesium ion concentrations

may lead to cation exchange reactions between sodium and calcium or magnesium. By contrast, minerals containing Na+ ions such as albite, can interactMANUSCRIPT with water and exchange dissolved Ca2+, which may cause an increase in Na+ ions in the water. The cation exchange reaction is shown in

Equation 5 (Kononov, 1983; Yuan et al., 2017; and Zhang, B. 2020).

Ca2+ + 2Na-mineral → 2Na+ + Ca-mineral (5)

Geochemical model of SO4-Na type hot spring: HCO3-Na type water shown in model 1 forms SO4-Na

type water and calcite compounds by interacting with gypsum. Equations 6 and 7 show the

formation of SO4-Na type water, which occurs in response to the reactions between HCO3-Na type water ACCEPTEDand gypsum. 2NaНСО3 + СаSO4*2Н2O → Са(НСО3)2 + Na2SO4 2Н2O (6)

2+ 2− CaSO4 * 2H2O → Ca + SO4 + 2H2O (7)

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 According to Equation 8, hydrogen sulfide in groundwater can react with organic carbon and

oxygen, and this may lead to the formation of sulfate ions.

+ 2− H2S + 2O2 ↔ 2H + SO4 (8)

2− − Hot springs predominately contain SO4 , whereas cold springs predominately contain HCO3 .

While a temperature increase can lead to a decrease in the dissolution of carbonate ions, a

significant rise in temperature can lead to an increase in the dissolution of sulfate ions (Joshi and

Bhandari, 2013; Kononov, 1983).

3.4. Geothermal reservoir

3.4.1. Chemical geothermometers

Various types of geothermometers can be used to determine reservoir temperature, including

silicate mineral (quartz and chalcedony) and cation (Na-K; Na-K-Ca) thermometers (Klein, 2007 and

Peiffer et al., 2014). These methods are based on the interactions between water and rock

(Dolgorjav, 2009). We used the methods of Fournier (1977) and Arnorsson (1983) for the geothermometer calculations, and the results areMANUSCRIPT shown in Table 8. Quartz and chalcedony geothermometers are based on the solubilities of different forms of silica

in the rock (Fournier, 1989). Temperature estimated by these geothermometers can range from 20

to 250°C, because of different silica phases and solubility characterization (Reed et al., 2007 and

Fournier, 1992).

As shown in Table 8, the quartz and chalcedony geothermometry values were ranged between

82.4 and 106.4°C. But the reservoir temperature might be underestimated due to partial equilibration.ACCEPTED Moreover, silica geothermometers generally give lower temperature estimation than other cation geothermometers. Hot spring circulation depth is estimated by SiO2 geothermometers

(quartz and chalcedony). The circulation depth ranged from 2615 m to 3410 m (Table 8).

Na-K-Mg ternary diagram: A ternary diagram (Na-K-Mg) was used to estimate the reservoir

temperature and equilibrium state of the water and rock interactions (Fan et al., 2019; Sebesan et al.,

2015) (Fig. 6). As shown in Fig. 6, hot springs in the Mongolian Altai region included a partially

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 equilibrated water zone. Generally, water-rock interactions are divided into three zones: mature

water, partially equilibrated water, and immature water. As shown in Fig. 6, the hot springs in the

Mongolian Altai region include a “partially-equilibrated water” zone on the Gi enbach plot. This

indicates that the water-rock interactions occurred partially, not fully. On the other hand, mixing

with rainwater, cold spring, or groundwater in the aquifer may prevent the achievement of complete

equilibrium.

3.4.2. Geothermal water circulation depth

We calculated the reservoir depth of the hot springs by measuring reservoir temperatures and

estimating local geothermal gradients (Shestakova et al., 2018). These are shown in Table 9. The

geothermal gradient of the Mongolian Altai region was calculated as 20–30 °С/km (Lkhagvadorj and

Tseesuren, 2005).

According to Mongolian statistics on climate, the annual average temperature of the research

area is between −2 °C and −4 °C. Based on this information, we calculated the depth of water circulation as 2615 m to 3410 m. MANUSCRIPT 3.4.3. Conceptual model for the geothermal water of the CH hot spring reservoir

We propose a conceptual model of geothermal water for the CH hot spring reservoir in the

Mongolian Altai region, which is based on its geology, the chemical composition of hot springs, and

reservoir temperature and depth. The conceptual model of the CH hot spring is shown in Fig. 7.

According to the model, precipitation (rain, snow, etc.) from the atmosphere reaches the hot

spring system through tectonic faults and other possible geological features at a depth of 2550 m a.s.l., whereACCEPTEDin the water interacts with aluminosilicate minerals (Bulgatov et al., 2014; Byamba et al., 1999). The dissolution of albite, dolomite, and gypsum minerals generates HCO3-Na and SO4-Na

types of water in this area. Moreover, rock weathering and precipitation play an important role in

the CH hot spring system. According to our calculation, the reservoir temperature of this area is

approximately 96.2 °С at a depth of 3070 m below surface, which corresponds to a geothermal

source with a low temperature (Nickolson, 1993).

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 4. Conclusions

In this study, we investigated the chemical composition, mineral components of rocks,

geothermal temperature, and reservoir depth of hot springs in the Mongolian Altai region.

The chemical compositions of the hot springs revealed that the springs contain HCO3-Na, SO4-Na,

and HCO3-SO4-Na types of water with low TDS values in the alkaline pH range, and their

temperatures ranged as 21.3–35 °C. According to the Na-K-Mg ternary diagram, hot springs in the

Mongolian Altai region have a partially equilibrated state of water–rock interactions. The results

suggest that water-rock interactions and the geographical features of the reservoir are responsible

for the main hydrochemical properties.

A preliminary study of the reservoir temperature ran ed from 82.4°С to 106.4°С. This result may

be estimated lower because of the partial equilibrium. The reservoir depth of the hot springs ranged

from 2615 m to 3410 m according to the quartz and chalcedony geothermometers. We constructed

a conceptual model for the geothermal water in the Mongolian Altai region, which can be used in other areas with similar geological settings. MANUSCRIPT Hot spring water in the Mongolian Altai region has the potential to be used in heating systems.

Corrosion and mineral accumulation in heating systems based on these hot springs would be low,

based on their alkaline properties and low TDS concentration. This type of hot spring resource could

thus be directly used for heating households, greenhouses, and swimming pools.

A limitation of this study is the lack of isotopic research on the hot springs and water resource

calculations. Further research, including studying the isotopic characteristics of the thermal waters

in the Mongolian Altai region, will be carried out in the future. Water source investigations are

importantACCEPTED for utilizing thermal groundwater in heating systems in remote areas of Western

Mongolia.

Acknowledgments: This work was supported by the post-graduate fund of Khovd University [grant

number A/51]. We would like to acknowledge the support provided by the Laboratory of

Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Environmental Analysis, National University of Mongolia and Khovd University, Mongolia. We

appreciate Dr. Eric Stemmler for helping with R software, CLR transformation.

Author contributions: Bolormaa Oyuntsetseg conceived and designed the study; Bolormaa

Chimeddorj collected and analyzed the samples; Bolormaa Oyuntsetseg supervised the project;

Bolormaa Chimeddorj, Dolgormaa Munkhbat, Battushig Altanbaatard and Oyuntsetseg Dolgorjav

analyzed the data; and Dolgormaa Munkhbat, Bolormaa Chimeddorj, and Bolormaa Oyuntsetseg

took the lead in writing the manuscript. All authors read, provided critical feedback, and approved

the submitted paper.

Funding

This work was supported by the post-graduate fund of Khovd University [grant number A/51].

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 1. Equations used in the geothermometer calculations.

Geothermometer Equation (T in °C) Reference

Fournier (1977) Quartz

Fournier (1977) Quartza

Arnorsson (1983) Chalcedony

Fournier (1977) Chalcedony

(a) - Adiabatic

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 2. Physical and chemical parameters of hot springs in the Mongolian Altai area ORP, TDS DO Na K Ca Mg HCO3 CO3 CI SO4 SiO2 F ID T, °C pH IEB, % mV mg/L GM1 30.4 9.21 40.0 127 0.07 45.4 1.23 2.02 0.22 41.6 23.5 17.8 8.91 46.7 9.86 0.121 GM2 24.5 9.18 59.1 145 0.03 45.4 1.23 2.21 0.22 47.3 22.4 13.7 10.7 48.2 7.31 0.054 CH1 22.0 9.22 43.3 221 0.53 68.6 1.84 5.33 0.42 72.6 15.1 7.70 68.3 64.5 2.97 0.133 CH2 23.8 9.26 51.9 230 0.01 71.8 1.83 2.50 0.24 74.6 15.2 7.31 66.1 61.1 1.27 0.291 CH3 21.3 9.11 50.6 259 0.30 71.1 1.75 2.00 0.49 68.7 17.8 9.62 61.5 64.2 3.16 0.041 CH4 24.7 9.14 44.7 225 0.40 73.6 1.86 3.22 0.41 78.0 15.9 8.11 67.4 60.9 1.99 0.016 CH5 24.7 9.13 52.6 225 0.40 71.4 1.89 2.81 0.54 79.4 12.5 7.14 67.9 60.4 4.12 0.045 AC1 31.0 9.00 28.8 187 0.31 38.4 1.22 2.80 0.10 50.5 18.6 3.41 14.5 51.6 4.82 0.066 AC2 35.0 8.60 30.6 188 0.28 32.9 1.02 3.20 0.16 48.7 14.6 2.62 12.9 49.5 4.05 0.148 AC3 32.0 8.65 35.8 174 0.44 34.5 1.20 2.80 0.10 48.1 15.7 2.30 14.5 45.3 2.82 0.014 IN1 25.0 8.40 124 725 0.45 196 12.5 24.2 1.22 110 8.65 47.7 322 75.6 6.2 0.080 IN2 28.0 8.32 127 698 0.61 186 15.2 22.2 1.22 91.5 6.12 47.7 319 69.2 2.54 0.022

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 3. Correlations of major ions (meq/L) in hot spring samples.

Na K Ca Mg HCO3 SO4 CI Si Na 1.00 K 0.96 1.00 Ca 0.96 0.98 1.00 Mg 0.93 0.90 0.89 1.00

HCO3 0.76 0.56 0.59 0.67 1.00

SO4 0.99 0.97 0.98 0.92 0.70 1.00 CI 0.98 0.95 0.94 0.94 0.59 0.93 1.00 Si 0.85 0.71 0.72 0.75 0.90 0.83 0.65 1.00 f(X)= 0.96

Table 4. Saturation indices for hot springs of Mongolian Altai region Samples GM1 GM2 CH1 CH2 CH3 CH4 CH5 AC1 AC2 AC3 IN1 IN2 Albite 7.59 8.13 8.31 8.11 8.07 7.99 7.81 6.65 6.56 6.07 6.81 6.56 Alunite -6.55 -4.77 -2.29 -3.22 -2.71 -3.14 -3.61 -7.51 -5.96 -7.95 -2.19 -2.64 Anhydrite -1.9 -1.8 -0.84 -1.12 -1.29 -1.06 -1.09 -1.47 -1.26 -1.32 0.21 0.21 Calcite 2.34 2.37 2.62 2.39 2.18 2.4 2.32 2.41 2.28 2.23 2.6 2.44 Fluorite 2.56 2.45 1.93 0.88 1.59 1.31 1.9 2.15 2.17 1.81 2.98 2.14 Gibbsite 1.23 1.65 1.98 1.75 1.75 1.66 1.49 0.56 0.79 0.08 0.71 0.51 Gypsum -1.66 -1.49 -0.51 -0.08 -0.94 -0.75 -0.79 -1.23 -1.07 -1.09 0.51 0.48 Halite -4.94 -5.04 -5.18 -5.19 -5.06 -5.14 -5.21 -5.73 -5.89 -5.92 -4.05 -4.08 K-feldspar 8.54 9.14 9.32 9.1 9.09 8.97 8.8 7.66 7.5 7.11 8.11 7.93 K-mica 16.6 18.0 18.8 18.2 18.1 17.9 17.4 14.4 14.7 12.9 15.1 14.5 Kaolinite 7.97 8.99 9.47 8.97 9.07 8.82 8.49 6.63 7.11 5.83 7.02 6.6 Quartz 2.33 2.43 2.34 2.32 2.38 MANUSCRIPT2.34 2.34 2.33 2.34 2.42 2.38 2.38

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 5. Major and minor element compositions of rock samples. Sample Major elements, %

ID SiO2 TiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO P2O5 CH 75.5 0.213 12.5 2.34 1.15 0.290 3.05 3.92 0.033 0.067 GM 75.3 0.176 12.7 2.54 0.74 0.411 4.50 2.57 0.036 0.034 IN 70.2 0.352 13.8 3.04 2.27 0.712 3.42 3.75 0.062 0.119 AC 69.3 0.355 13.6 4.32 2.69 1.23 2.78 2.41 0.077 0.086 Minor elements, mg/kg Ba Cs Rb Sb Sr Zn Zr V Se Cr CH 386 48.0 190 40 176 18.0 64.0 25.1 30.0 43.0 GM 289 39.0 80.2 40 116 48.2 104 16.2 30.0 39.0 IN 936 42.0 132 40 425 32.3 164 35.0 53.0 39.0 AC 235 30.0 199 40 70.2 18.1 36.0 15.3 30.0 33.0

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 6. Calculated Normative Minerals Volume Norm, % Normative Minerals CH GM IN AC Quartz 39.41 36.37 29.67 30.16 Plagioclase 31.85 42.95 41.01 38.26 Orthoclase 24.53 16.14 23.93 23.94 Corundum 0.89 0.83 0.19 0.04 Hypersthene 2.85 3.36 4.45 6.86 Ilmenite 0.24 0.19 0.39 0.39 Magnetite 0.09 0.10 0.12 0.17 Apatite 0.14 0.06 0.23 0.18

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 7. Correlation analysis results for CLR transformed sample compositions

CO3(w) Ca(w) Cl(w) HCO3(w) K(w) Mg(w) Na(w) SO4(w) Al2O3(r) CaO(r) Fe2O3(r) K2O(r) MgO(r) MnO(r) Na2O(r) P2O5(r) SiO2(r) TiO2(r)

CO 3(w) Ca(w) -0.36 Cl(w) -0.21 -0.60

HCO3(w) 0.92 -0.12 -0.57 K(w) -0.91 0.52 0.28 -0.90 Mg(w) 0.01 -0.89 0.89 -0.31 -0.09 Na(w) 0.46 -0.98 0.41 0.28 -0.67 0.77

SO4(w) -0.89 0.38 -0.15 -0.67 0.66 -0.21 -0.39

Al2O3(r) -0.45 0.92 -0.25 -0.35 0.72 -0.65 -0.98 0.30 CaO(r) -0.41 1.00 -0.61 -0.16 0.55 -0.88 -0.97 0.46 0.90

Fe2O3(r) 0.09 0.87 -0.56 0.22 0.21 -0.84 -0.84 -0.12 0.83 0.83

K2O(r) -0.76 -0.05 0.04 -0.60 0.41 0.14 0.05 0.90 -0.14 0.04 -0.53 MgO(r) -0.06 0.88 -0.40 0.04 0.39 -0.75 -0.90 -0.06 0.91 0.84 0.98 -0.48 MnO(r) -0.24 0.95 -0.43 -0.10 0.52 -0.79 -0.97 0.15 0.97 0.92 0.95 -0.30 0.98

Na2O(r) 0.10 -0.70 0.95 -0.29 0.02 0.90 0.53 -0.44 -0.36 -0.73 -0.51 -0.22 -0.40 -0.48 P2O5(r) -0.72 0.87 -0.47 -0.44 0.70 -0.70 -0.85 0.79 0.77 MANUSCRIPT0.90 0.51 0.46 0.55 0.70 -0.70 SiO2(r) 0.41 -0.96 0.36 0.27 -0.66 0.74 1.00 -0.31 -0.99 -0.95 -0.87 0.13 -0.93 -0.98 0.47 -0.80

TiO2(r) -0.50 0.99 -0.48 -0.28 0.65 -0.81 -0.99 0.48 0.95 0.99 0.81 0.05 0.85 0.94 -0.62 0.91 -0.97 (w) – water, (r) – rock, f(x) = 0.91

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Downloaded from http://pubs.geoscienceworld.org/geea/article-pdf/doi/10.1144/geochem2021-016/5360166/geochem2021-016.pdf by guest on 30 September 2021 Table 8. Reservoir temperature (T, °C). Sample Average Quartz Quartzad Chalcedony Chalcedony ID (1) (2) (3) (4) CH 96.2 111 110 82.2 81.6 GM 82.4 96.8 98.2 68.2 66.5 IN 106.4 121 119 92.6 92.8 AC 84.4 98.8 100 70.1 68.6 (1, 2, 4) Fournier (1977); (3) Arnorsson (1983); ad – adiabatic.

Table 9. Temperatures and depths of the hot springs. Reservoir Water circulation Sample ID temperature (°С) Depth (m) CH 96.2 3070 GM 82.4 2615 IN 106 3410 AC 84.4 2680

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Fig. 1. Hydrogeological regions and locations of hot springs in Mongolia. ACCEPTED