Hydrochemistry of the Geothermal in Gonghe Basin, Northeastern Tibetan Plateau: Implications for Hydro-Circulation and the Geothermal System

Abstract

The existence of high-temperature geothermal anomalies in the Gonghe Basin on the northeastern margin of the Tibetan Plateau has highlighted a new perspective on the geothermal system of the Himalayan-Tibetan Plateau orogen. In this study, we collected 32 groups of liquid and gas samples from geothermal water, rivers, and boreholes in the Gonghe basin to analyze hydrochemistry, stable isotopes, and geochronology, which allow us to further reveal the geothermal fluid circulations of geothermal reservoirs. The ion contents of liquids identify two distinguished types of water, namely the Na-SO₄-Cl type primarily from geothermal water and the Na-SO₄-HCO₃ and Na-Ca-CO₃-SO₄ types primarily from cold water. The compositions of the hydrogen and oxygen isotopes of the samples indicate geothermal waters were recharged by atmospheric precipitation and 3000–4600 m high snow mountain meltwater, which may have experienced circulation of 16,300–17,300 years and mixtures of submodern and recent recharge water sources evidenced by isotopes of ³H, ¹³C, and ¹⁴C data. The ³He/⁴He ratios of these geothermal waters varying from 0.03 to 0.84 Ra further highlighted a crustal-dominated heat source in the region. The deep thermal reservoir temperature in the Gonghe Basin at 160 ± 10 °C and the depth of circulation of geothermal water is 2200–2500 m. Based on this evidence, we have established a geothermal fluid circulation model and refined the exchange processes of fluids and geothermal heat, further enriching the details of the geothermal system in Gonghe Basin.

1. Introduction

The Gonghe Basin, located on the northeastern margin of the Tibetan–Himalaya orogen, is the most concentrated area of geothermal activity in China [1]. The heat flow value in the Gonghe Basin is relatively large with the maximum value reaching 135 mW/m² [2]. In 2017, the high-temperature dry-hot rock mass of 236 °C was drilled at a depth of 3705 at the Qiabuqia site in the northeastern Gonghe Basin, further arousing the attention of geothermal communities on the heat source mechanism and geothermal origin [3,4,5].

Over the years, geologists from different fields have conducted research in the Gonghe Basin on the outcropping characteristics of hot springs, types of geothermal resources, types of geothermal reservoirs, and mechanisms of geothermal system genesis and have gained a basic understanding of the geothermal conditions in the Gonghe Basin. Examples include the distribution and outcropping characteristics of hot springs in and around the basin [3] and the two types of geothermal resources in the basin: Fault configurations and sedimentary basins [6]. Yan et al. considered that basin thermal reservoirs can be divided into Quaternary Lower Pleistocene geothermal reservoirs and Neoproterozoic geothermal reservoirs [7]. Meanwhile, there are still controversies about the source of heat in the basin [5,8,9,10,11]. Tang et al. presented a ternary polythermal model for hot dry rocks in the Gonghe Basin by analyzing the tectonic background and heat source mechanism of Gonghe Basin [10]. Lang et al. used the silica-enthalpy model to analyze the cold water in the geothermal fluid with 60–68%, and the geothermal reservoir temperature was approximately 222 °C before being cold-water mixed, and the depth of the thermal cycle was approximately 3200 m in the Guide Basin [12]. Li et al. suggested that the geothermal system of the Zhacang geothermal field was a fractured deep-cycle hydrothermal geothermal system and a strongly tectonically active belt-type dry hot rock geothermal system [13]. Based on the water–thermal–rock coupling model, Jiang et al. analyzed the δD change and suggested it is primarily the consequence of advective and dispersive transport, while the δ¹⁸O change is controlled by water–rock interactions in the Guide Basin geothermal system [14].All of these studies only concentrated on a single hot spring complex for analysis and ignored the whole-basin expressions and the coupling of deep and shallow water cycles.

In this study, the collected water and gas samples for geochemical and isotopic analysis were employed to test the whole-basin geothermal system and cycling processes of the Gonghe Basin. In addition, the geothermal reservoir temperature and circulation depth are evaluated by the geothermometer to build a model map of the Gonghe Basin geothermal system.

2. Geological Setting

2.1. Geology

The Gonghe Basin belongs to one of the sub-basins on the northeastern margin of the Tibetan Plateau with an average elevation of 300–5000 m and shows a diamond shape with a width of 50 km and a length of 200 km. On the structure, the southern and northern margins are under the control of the positive strike-slip faults of the Kunlun and Haiyuan faults, respectively (Figure 1). Both the western and eastern margins are separated by the Ela Shan fault and Dulan-Chaka highland, respectively, as well as by the Riyue Shan fault of the Waligong Tectonic Magmatic Belt [15]. The Yellow River flowing from southwest to northeast forms a deep canyon in the basin [5,15].

Due to the intensive activities of the Neogene piedmont deep fault, the continuous subsiding basin accumulated huge Neogene and Quaternary deposits that only outcrop on the south bank of the Qiabuqia River. In contrast, Middle-Late Triassic igneous bodies and metasedimentary successions surround the basin where continuous uplift accompanies transgression of the East Kunlun and West Qinling faults [5,16,17].

Figure 1. (a) Schematic structural graph of Gonghe Basin and conjunct area, northeastern margin of the Tibetan Plateau (QHNF: Qinghai Nanshan Fault; DMHF: Gonghe Duomaohe Fault); (b) Gonghe Basin, with details shown in Figure 2; (c) terrestrial heat flow (the heat flow data refers to Jiang et al., 2019 [18]). (After [1,5]).

Figure 1. (a) Schematic structural graph of Gonghe Basin and conjunct area, northeastern margin of the Tibetan Plateau (QHNF: Qinghai Nanshan Fault; DMHF: Gonghe Duomaohe Fault); (b) Gonghe Basin, with details shown in Figure 2; (c) terrestrial heat flow (the heat flow data refers to Jiang et al., 2019 [18]). (After [1,5]).

2.2. Geothermal and Geological Conditions

The terrestrial heat flows on the northeastern margin of the Qinghai-Tibet Plateau are obviously greater than the regional crust average (65 mW/m²) [19]. According to the statistical data of geothermal anomalies in the northeastern Tibetan Plateau area (92°–108° E, 33°−41° N; 37 hot springs (groups); 63 geothermal wells), nine sites show high-temperature geothermal anomalies (>90 °C), which are primarily located in the Gonghe Basin and its surrounding areas [5]. There are more than 80 hot springs (40–100 °C) reported in the Gonghe Basin and its surrounding areas, and 17 geothermal wells have been drilled [1,4,5,11] (Figure 2). Geothermal anomaly hot springs are primarily distributed along the Elashan Fault zone and the Riyueshan Fault zone and a few of them are in the blind thrust interior of the basin. Most geothermal wells in the shallow Neogene and Paleogene clastic rocks encountered hydrothermal geothermal reservoirs, and the deep Triassic granites encountered hot dry rocks [7,10,20]. The basin has hydrothermal-type geothermal resources and hot dry rocks [10]. The deep part is dominated by hot dry rock with no or little fluid content and high temperatures (such as the Qiabuqia hot dry rock). In addition, the Gonghe Basin has a high earth heat flow value of up to 134 mW/m². According to temperature-logging data, the average geothermal gradient of basal granite was 6.7–6.8 °C/100 m, and the gradient value in the fault zone can reach up to 14 °C/100 m (at a depth of 2214–2248 m) [21].

Figure 2. The distribution of spring groups and geothermal drilling wells in the Gonghe Basin and the sampling area, with kriging grid method as the temperature background (spring point and temperature are shown in Table 1) [1].

Figure 2. The distribution of spring groups and geothermal drilling wells in the Gonghe Basin and the sampling area, with kriging grid method as the temperature background (spring point and temperature are shown in Table 1) [1].

Table 1. Main hot spring distribution locations and temperature statistics (see the above picture for the serial number plane location and the data [1]).

Code	Spring Name	Location		Temperature (°C)
		Longitude	Latitude
1	Bayingou	98.931969	36.438813	43
2	Wahong	98.821297	36.305936	46
3	Qinggenhe	99.124605	35.070330	68
4	Sangchi 1	99.198644	36.051766	62
5	Sangchi 2	99.233341	36.016961	30
6	Wenquan	99.430527	35.405138	61
7	Ayihai	100.701491	36.159602	32
8	Lagan	100.558969	35.766052	38
9	Qunaihai	101.040638	36.141444	96
10	Zhacang	102.634475	35.968611	93
11	Xinjie	101.389222	35.958500	64
12	Reshui	101.591113	36.401655	39
13	Xiema	101.678497	36.200894	32
14	Lancai	101.804258	35.587875	69
15	Qukuhu	101.971769	35.377741	49

Table 1. Main hot spring distribution locations and temperature statistics (see the above picture for the serial number plane location and the data [1]).

Code	Spring Name	Location		Temperature (°C)
		Longitude	Latitude
1	Bayingou	98.931969	36.438813	43
2	Wahong	98.821297	36.305936	46
3	Qinggenhe	99.124605	35.070330	68
4	Sangchi 1	99.198644	36.051766	62
5	Sangchi 2	99.233341	36.016961	30
6	Wenquan	99.430527	35.405138	61
7	Ayihai	100.701491	36.159602	32
8	Lagan	100.558969	35.766052	38
9	Qunaihai	101.040638	36.141444	96
10	Zhacang	102.634475	35.968611	93
11	Xinjie	101.389222	35.958500	64
12	Reshui	101.591113	36.401655	39
13	Xiema	101.678497	36.200894	32
14	Lancai	101.804258	35.587875	69
15	Qukuhu	101.971769	35.377741	49

3. Sampling and Analysis

3.1. Sampling

To understand the geothermal system and cycling process of the Gonghe Basin, 16 sets of water samples and 6 sets of gas samples were collected in the Gonghe Basin between August 2019 and August 2020 (sampling location is shown in Figure 2). The samples were selected from 3 geothermal fields of Qunaihai, Zhacang, and Xinjie distributed along the Riyueshan Fault in the eastern Gonghe Basin for water chemistry and isotopic analysis. Field test indicators such as pH (measurement error is ±0.05) were tested with a multifunctional portable tester (Hach hQ40d). The samples were sealed in 5 L high-density polyethylene bottles. Ten water samples from previous publications were also compiled.

3.2. Methods

We carried out detailed major ionic components and isotope analysis on geothermal water and gas collected from the Gonghe Basin. We also studied the geothermal reservoir temperature and circulation depth. The samples were tested for major ions, trace elements, isotopes, and gases.

The primary tested ions were K⁺, Na⁺, Ca²⁺, Mg²⁺, SO₄⁻, Cl⁻, F⁻, and NO₃⁻, where major cations were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) with a precision higher than 0.5%, and anions were assayed with a DIONEX-500 Ion Chromatograph with a 0.05 mg/L detection limit.

Water isotopes (δ¹⁸O and δD) were analyzed by a laser absorption water isotope analyzer spectrometer, with analytical precisions of ±0.1‰ (1σ) for δ¹⁸O and ±0.5‰ (1σ) for δD in the Water Isotope. The results are expressed as δD and δ¹⁸O (δ = (R_sample/R_standard − 1) × 1000) relative to the Vienna Standard Mean Ocean Water (VSMOW) standard.

The isotope ¹³C_DIC and radiocarbon (¹⁴C_DIC) activity concentrations were analyzed at the Radiocarbon Laboratory of Beta Analytics (Miami, Florida, USA) with an analytical precision of ±0.3‰(1σ) for δ¹³C_DIC and ±0.001–0.004 pMC (1σ) for ¹⁴C. The ¹⁴C_DIC concentrations were expressed as pMC (percent Modern Carbon) using NIST oxalic acid, using SRM 4990C as the standard of reference. The δ¹³C_DIC values were relative to the Pee Dee Belemnite (PDB) standard. Helium and Neon isotopic abundances and the values of ³He/⁴He and He/Ne were measured with a MAP 215 noble gas mass spectrometer.

4. Results

4.1. Water Chemistry

The results of major ion concentrations are shown in

Table 2. Physical and chemical compositions of geothermal and river waters in Gonghe Basin.

Sample ID	Longitude	Latitude	SO42− (mg/L)	Cl− (mg/L)	NO3− (mg/L)	F− (mg/L)	HCO3− (mg/L)	CO32− (mg/L)	Ca2+ (mg/L)	Mg2+ (mg/L)	Na+ (mg/L)	K+ (mg/L)	TDS (mg/L)	PH
S1	101.3020	35.9683	567	313	0.005	4.19	15.40	24.20	55.80	0.17	429	17.50	1488	8.97
S2	101.3010	35.9677	541	307	0.005	4.47	24.60	18.10	53.80	0.23	448	17.90	1474	8.87
S3	101.3010	35.9683	546	329	0.005	4.49	55.30	6.04	58.00	0.42	429	18.70	1470	8.72
S4	101.3020	35.9681	539	326	0.005	4.48	49.10	9.06	59.90	0.54	438	18.70	1454	8.64
S5	101.3890	35.6253	115	19.30	0.60	4.54	280	6.04	37.90	6.05	130	5.96	478	8.46
S6	101.3900	35.6253	234	17.20	1.26	8.77	169	9.06	11.80	0.53	169	7.35	524	8.57
H1	101.3890	35.6253	41.70	3.55	1.59	0.55	280	3.02	64.90	11.80	26.20	2.39	302	8.58
H2	101.3840	36.0403	70.90	9.13	1.84	0.33	249	3.02	61.30	23.30	28.10	2.58	330	8.45
H3	101.2780	36.0992	59.40	12.20	2.20	0.17	221	3.02	49.20	16.90	30.00	3.18	288	8.52
H4	101.2690	36.0692	51.20	12.20	2.16	0.16	218	3.02	47.00	17.60	30.00	3.38	290	8.48
H5	101.2790	36.0622	53.60	12.20	0.78	0.21	237	6.04	44.50	23.90	24.30	2.19	300	8.48
H6	101.3910	36.0394	46.00	10.10	0.80	0.21	227	6.04	42.80	23.30	20.00	2.19	274	8.46

. Both geothermal water and river water are alkaline, but the pH range of geothermal water of 8.46–8.97 is slightly different from surface water of 8.45–8.58, and the TDS of geothermal water ranging from 478 to 1488 mg/L is also different from surface water ranging from 274 to 330 mg/L. The Piper diagram (Figure 3) shows the differences in the water chemistry types of geothermal water and river water. The geothermal water hydrochemical types are Na-SO₄-Cl, Na-Ca-HCO₃-SO₄, and Na-SO₄-HCO₃ (Figure 3). The hydrochemical types of river water contain Ca-Na-HCO₃, Ca-Mg-HCO₃-SO₄, Ca-Mg-Na-CO₃-SO₄, and Ca-Mg-HCO₃.

4.2. Fluids and Gas Isotopes

4.2.1. Isotope Compositions of Fluids Samples

The water samples from the Gonghe Basin show that deuterium and oxygen-stable isotopes can effectively determine the source of geothermal water (

Table 3. Isotope ratios of geothermal and cold waters in Gonghe Basin.

Sample ID	Longitude	Latitude	Sampling Elevation (m)	δ18O (‰)	δD (‰)	δ13C (‰)	3H (TU)	87Sr/86Sr	Reference
S1	101.301660	35.968333	2511	−11.1	−84.3	−13.80	<3	0.714475
S2	101.301380	35.967777	2511	−11.2	−85.7	−15.20	<3	0.714370
S3	101.301110	35.968333	2509	−11.1	−84.4	−6.00	<3	0.714410
S4	101.301940	35.968055	2508	−11.1	−84.6	−10.40	<3	0.714328
S5	101.389440	35.625277	3240	−10.4	−71.2	−12.10	9.6	0.712943
S6	101.389720	35.625277	3242	−12.4	−89.9	−9.10	<3	0.714421
S7	101.036430	36.140809	2557	−10.4	−83.3	−5.40	—	0.712577
S8	101.038070	36.141305	2577	−10.1	−81.5	−5.60	—	0.712627
S9	100.690410	36.207874	2644	−9.6	−66.7	−10.50	—	0.711735
S10	100.683490	36.203893	2655	−9.4	−65.4	—	—	0.711801
G18	—	—	2700	−10.6	−74.6	—	—	—	[29]
G20	—	—	2886	−8.7	−77.8	—	—	—	[29]
GH18	—	—	2796	−8.9	−78.0	—	—	—	[30]
GH48	—	—	2660	−8.8	−64.0	—	—	—	[30]
G29	—	—	—	−10.4	−86.4	—	—	—	[31]
GD-06	—	—	—	−9.6	−68.0	—	—	—	[32]
GD-07	—	—	—	−10.0	−69.0	—	—	—	[32]
GD-01	—	—	—	−9.0	−64.0	—	—	—	[32]
GD-04	—	—	—	−8.7	−65.0	—	—	—	[32]
GD-05	—	—	—	−7.9	−59.0	—	—	—	[32]

) [22,23]. The different isotopic compositions of deuterium and oxygen in different groundwater cycles are caused by the lower water vapor pressure, and the δD and δ¹⁸O are enriched in the liquid phase and are depleted in the gas phase. While the δD value primarily depends on the recharge temperature and altitude and is slightly influenced by mixing, the δ¹⁸O value is primarily dependent on the degree of exchange in the water–rock interaction and the water–rock ratios. For the geothermal water in this study, the δ¹⁸O, δD, and δ¹³C values range from −12.45‰ to −9.45‰, from −89.89‰ to −65.46‰, and from −15.20‰ to −5.40‰, respectively. In addition, we also collected data on δ¹⁸O and δD from previous articles. The tritium values of underground water samples in the study area, except sample S5, are <3. Sr isotopes are extremely important tracers to effectively limit mineral weathering in geothermal water [24]. In addition, Sr isotopes have negligible biological and geological fractionation [25]. The abundance of ⁸⁷Sr relative to the abundance of stable ⁸⁶Sr is expressed as the ratio ⁸⁷Sr/⁸⁶Sr. ⁸⁷Sr/⁸⁶Sr is extensively used in the mixing processes of groundwater, water–rock interactions, the flow paths of groundwater, and so on [25,26,27,28]. The range of the ⁸⁷Sr/⁸⁶Sr ratio is 0.711–0.714.

4.2.2. Isotope Compositions of Gas Samples

Helium isotopes are widely used as tracers of volatilities from the crust and mantle in various tectonic environments [33,34]. The source of ³He is the mantle, and ⁴He is primarily derived from the radioactive decay of U and Th in the crust [35]. Accordingly, the ³He/⁴He ratio can be used to investigate the contribution rate of crust–mantle material sources in geothermal systems [35,36]. The results of gas isotopes are shown in

Table 4. Isotope ratios of gas in Gonghe Basin.

Sample ID	Longitude	Latitude	R/Ra	3He/4He	4He/20Ne
G1	101.301666	35.968333	0.05	7.26 × 10−8	24.0
G2	101.301388	35.967777	0.49	6.87 × 10−7	1.3
G3	101.301111	35.968333	0.04	5.87 × 10−8	48.0
G4	101.301944	35.968055	0.07	1.01 × 10−7	6.2
G5	101.389444	35.625277	0.84	1.18 × 10−6	1.0
G6	101.389722	35.625277	0.03	4.53 × 10−8	58.0

.

The ³He/⁴He ratio is presented as R/Ra, where R is the ³He/⁴He ratio in the measured sample and Ra is the ³He/⁴He ratio with a constant of 1.384 × 10⁻⁶ in the atmosphere [37]. The ³He/⁴He ratios in our samples are between 0.03 and 0.84 and the ⁴He/²⁰Ne ratios range from 1 to 58. The latter is much higher than air (0.318) and air-saturated water (0.26 at 10 °C), indicating that the isotopic component of helium is excluded from the atmosphere [38,39].

5. Discussion

5.1. Origin and Recharge Source of Geothermal Fluids

5.1.1. Deuterium and Oxygen Stable Isotopes Characteristics

The deuterium and oxygen isotope results of geothermal water in the study area were compared with the local meteoric water line (LMWL) of δD (‰) = 7.27δ¹⁸O + 3.37 [40]. As shown in Figure 4, the collected samples are distributed along the local atmospheric precipitation line similarly to the previous samples. This indicates the recharge source may be alpine snowmelt water or high-altitude atmospheric precipitation. With respect to the LMWL, the positive shift of the oxygen isotope from the geothermal water is interpreted as an exchange of ¹⁸O between water and enriched heavy oxygenic rock under high temperatures (>150 °C).

5.1.2. Recharge Elevation

The δD and δ¹⁸O values of meteoric precipitations have an elevation effect [41] with a negative correlation between the isotope value and altitude. Therefore, the average elevation of the groundwater recharge zone in the aquifer can be calculated. Since the values of δD and δ¹⁸O of atmospheric precipitation near the groundwater samples sites are known, the recharge elevation can be obtained according to the following equation:

$$H=\frac{\delta S-\delta d}{K}+h$$

(1)

where H is the recharge elevation (m), S is the isotopic composition of the water sample (‰), d is the isotopic composition of atmospheric precipitation near the sampling site (‰), K is the isotopic elevation gradient (−n‰/100 m), and h is the elevation of the sampling site (m).

Generally, the isotopic elevation gradient (K) varies from −0.15‰/100 m to −0.5‰/100 m for δ¹⁸O and from −1.2‰ to −4‰ for δD [42]. Li et al. obtained an elevation gradient of δD in the eastern Tibetan Plateau of −2.6‰/100 m, and an elevation gradient of δ¹⁸O of −0.4‰/100 m [43]. We collected local isotopic data of atmospheric precipitation with δd values of −7.82‰ and −51.3‰ [44]. We then calculated the corresponding recharge elevation of geothermal water (

Table 5. The recharge elevation of water.

Sample ID	Sites	Elevation (m)	δ18O	δD	Recharge Elevation (m)	Recharge Elevation (m)	Average (m)
S1	Zhacang	2511	−11.08	−84.34	—	3782	3782
S2		2511	−11.26	−85.67	—	3833	3833
S3		2509	−11.12	−84.36	—	3781	3781
S4		2508	−11.12	−84.61	—	3789	3789
S5	Xinjie	3240	−10.39	−71.21	3883	4006	3944
S6		3242	−12.45	−89.89	4400	4726	4563
S7	Qunaihai	2557	−10.40	−83.31	—	3788	3788
S8		2577	−10.06	−81.50	—	3739	3739
S9	Qiabuqia	2644.	−9.60	−66.70	3097	3236	3166
S10		2655	−9.50	−65.50	3063	3200	3131
G18		2700	−10.60	−74.60	3395	3596	3496
G20		2886	−8.70	−77.80	—	3905	3905
GH18		2796	−8.90	−78.00	—	3823	3823
GH48		2660	−8.80	−64.00	2905	3148	3027

). The S1, S2, S3, S4, S7, and S8 samples occurred during oxygen transfer, resulting in differences in recharge elevations calculated using δ¹⁸O values, so only stable hydrogen isotopes were used to calculate recharge elevations for these samples. The calculated recharge elevations are 3000–4600 m. Therefore, the sources of groundwater recharge are likely atmospheric precipitation from the surrounding hills and meltwater from the western snow-capped mountains.

The O isotope exchange between water and rock can lead to a water body relatively rich in ¹⁸O, and was firstly proposed to result from an excess deuterium parameter d = δD − 8δ¹⁸O [45]. With greater water–rock interactions, when the exchange of O isotopes between water and rocks is greater, the d value of groundwater is smaller. In addition, deuterium excess (d) is an effective parameter to express the degree of deviation of groundwater isotopes relative to local atmospheric precipitation.

Table 6. Deuterium excess parameters of geothermal water.

Sample ID	δ18O (‰)	δD (‰)	d
S1	−11.08	−84.34	4.30
S2	−11.26	−85.67	4.41
S3	−11.12	−84.36	4.60
S4	−11.12	−84.61	4.35
S5	−10.39	−71.21	11.91
S6	−12.45	−89.89	9.71
S7	−10.40	−83.31	−0.11
S8	−10.06	−81.50	−1.02
S9	−9.63	−66.69	10.35
S10	−9.45	−65.46	10.14

shows the deuterium excess values of underground geothermal water are between −1.02‰ and 11.91‰, indicating a normal atmospheric precipitation supply. Specifically, the d values of the S1, S2, S3, S4, S7, and S8 samples are significantly smaller than that of other geothermal water samples, suggesting a strong water–rock interaction to result in oxygen shift and slow the groundwater runoff rate. The retention time in the aquifer is relatively long, and the renewal alternating ability is relatively weak.

5.2. Hydrogeochemical Processes

5.2.1. General Hydrogeochemistry

The principal component contents of the water samples of various types are presented in the Schoeller diagram (Figure 5) to compare the different characteristics of hydrochemistry types between geothermal water and cold water in different areas. There is a similar trend of ion variation in different regions and types of groundwater. It can be seen from Figure 5 that the main performance of river water is Ca²⁺ > Mg²⁺ > Na⁺ + K⁺ and HCO₃⁻ > SO₄²⁻> Cl⁻, and geothermal water is Na⁺ + K⁺ > Ca²⁺ > Mg²⁺, SO₄²⁻ > Cl⁻ > HCO₃⁻ and SO₄²⁻ > HCO₃⁻ > Cl⁻. It is indicated that they have experienced similar recharge, runoff, and circulation processes.

5.2.2. Hydrochemical Facies Variation

Generally, Cl⁻ is present in almost all groundwater and primarily comes from salt rock dissolutions [46,47]. The strong electrolytes of chloride salt determine its large solubility. Even at high temperatures, it is rare for the water–rock interaction to affect the existence of Cl⁻. Thus, Cl⁻ is often used to trace the sources of geothermal water and other substances with good correlations in the system [46,48].

The relationships between the concentration of primary ions and Cl⁻ in the water samples are shown in Figure 6. The higher Na⁺/Cl⁻ and K⁺/Cl⁻ of the geothermal water ratios than that of cold water indicate the former participates in longer and deeper regional flow paths. The relatively high Ca²⁺/Cl⁻ ratio in river water may be due to the dissolution of calcite or dolomite from the Neogene sediments, whereas the lower Ca²⁺/Cl⁻ ratio in geothermal water may reflect calcite precipitation along the way. Higher HCO³⁻/Cl⁻ ratios in river water reflect a short flow path and rapid water circulation, whereas the lower HCO³⁻/Cl⁻ ratios in geothermal water indicate that geothermal water flowed through a longer flow path underground. The strong relationship between HCO³⁻ and Cl⁻ and SO₄²⁻ and Cl⁻ suggests that geothermal water is diluted by cold water.

The ratio coefficients between different chemical compositions in groundwater can be used to research certain hydrogeochemical processes. By analyzing the characteristic coefficients of geothermal water, the geological environment in which the geothermal fluid is present can be determined, and the hydrochemical characteristic coefficients of geothermal water in the research area are shown in

Table 7. Geothermal water characteristic coefficients.

Sample ID	Sites	γNa+/γCl−	100 × γSO42−/γCl−	γCl−/(γHCO3− + CO32−)
S1	Zhacang	1.37	181.15	7.90
S2		1.46	176.22	7.19
S3		1.30	165.96	5.36
S4		1.34	165.34	5.61
S5	Xinjie	6.74	595.85	0.07
S6		9.83	1360.47	0.10

.

The alteration coefficient γNa⁺/γCl⁻ is used to describe the degree of groundwater metamorphism, the hydrogeochemical environment, and the confinement of the geothermal reservoir. The smaller the metamorphic coefficient, the better the aquifer confinement and the greater the extent of metamorphism. The metamorphic coefficient of Zhacang geothermal water ranges from 1.30 to 1.46 and the metamorphic coefficient of Xinjie geothermal water ranges from 6.74 to 9.83. Therefore, the geothermal reservoir of Zhacang has better confinement, a higher degree of metamorphism, and a more reductive environment. The metamorphic coefficients of both geothermal reservoirs are greater than 1, indicating that the geothermal water formation in these two reservoirs is influenced by the infiltration of atmospheric precipitation, and the Xinjie geothermal reservoir is more influenced by infiltrated water. The desulfurization coefficient 100 × γSO₄²⁻/γCl⁻ is used to denote the groundwater desulfurization degree. If the desulfurization coefficient is smaller, it means that the environment in which the geothermal water is contained is better and the reduction is higher. If the desulfurization coefficient is less than 1, it means that the environment where the aquifer is located is completely reduced. The 100 × γSO₄²⁻/γCl⁻ in the Zhacang geothermal reservoir ranges from 165.34 to 181.15 and the 100 × γSO₄^2-/γCl⁻ in the Xinjie geothermal reservoir ranges from 595.85 to 1360.47, which indicate that the geothermal water of both reservoirs is desulfated to some degree, and the geological environment is more reductive. The salinization coefficient γCl⁻/(γHCO₃⁻ + CO₃²⁻) can reflect the salinization degree of groundwater, and the higher the salinization coefficient, the greater the salinization degree of groundwater. The γCl⁻/(γHCO₃⁻ + CO₃²⁻) of Zhacang geothermal water in the study area ranged from 5.36 to 7.90, with a mean value of 6.52; the γCl⁻/(γHCO₃⁻ + CO₃²⁻) of Xinjie geothermal water ranged from 0.07 to 0.10, with a mean value of 0.08. This indicates that the runoff path of Zhacang geothermal water is longer than that of Xinjie geothermal water, the water circulation rate is slower, and the salinization rate is stronger.

5.3. Deep Circulation of Geothermal Fluid

The age of the minerals, the value of initial ⁸⁷Sr/⁸⁶Sr at the time of rock formation, and the Rb/Sr ratio together determine the ratio of ⁸⁷Sr/⁸⁶Sr. The variable amount of radioactive ⁸⁷Sr is produced by the decay of ⁸⁷Rb in rocks [49,50]. Nevertheless, the residence time of groundwater is very short compared to the half-life of ⁸⁷Rb, so the radioactivity of ⁸⁷Sr can be neglected during groundwater movement. Due to its large mass number, the isotope mass fractionation of strontium isotopes during mineral dissolution and precipitation is also negligible. Thus, isotopes of strontium in water have the advantage of retaining the isotopic properties from rocks of their origin, such as silicates and carbonates. The values of ⁸⁷Sr/⁸⁶Sr in the Gonghe Basin are comparatively uniform, in the range of 0.711 to 0.714, which are more positive than that of the homogenous mantle (0.702–0.705) and marine carbonate (0.707–0.709) [51,52]. The geothermal water flows through the aquifer of the Triassic clastic rock and Neogene sandstone and mudstone in the Gonghe Basin, which significantly contribute to increased ⁸⁷Sr/⁸⁶Sr values in the geothermal waters.

Radioisotopes are primarily used to determine the age of groundwater in hydrogeology, which refers to the time that water is retained in an aquifer [53,54]. It reflects the closed conditions of the groundwater occurrence environment and its relationship with other water environments, so the determination of groundwater age is helpful to determine the recharge cycle of groundwater resources [55].

This study applies the ¹⁴C method to determine the specific age of geothermal water collected from the Gonghe Basin. The upper limit of ¹⁴C dating is 50,000–60,000 years, with a maximum of 70,000 years [56]. According to the different hydrogeochemical conditions of the groundwater system, the Pearson model was used to correct the age [57].

Due to the variety of carbon sources in groundwater, there are still difficulties in obtaining the correct ages using ¹⁴C. Therefore, ³H has been chosen to assist in determining the age of groundwater. Only six sets of water samples have been tested for tritium (Table 2), and the age of geothermal water is estimated using qualitative methods as listed in

Table 8. An indicative standard of T to judge the age of the groundwater.

Areas	3H	Age
continental areas	<0.8	Submodern groundwater, replenished before 1952
	0.8–5	Mixed water of submodern and recent replenishment
	5–15	Modern water (5–10 yr)
	15–30	Groundwater containing some nuclear tritium
	30–50	Contains significant amounts of water replenished in the 1960s or 1970s
	>50	Mainly from the 1960s

[30]. According to statistics, most of the geothermal water in the study area has a tritium value of less than 3 and is only one point greater than 5, indicating that the recharge source of geothermal water is essentially a mixture of submodern and recent recharge water. This is consistent with the above analysis of the retention time of geothermal in the aquifer, and the corrected age was approximately 16,300–17,300 years (

Table 9. Results of geothermal water age calculation.

Sample ID	14C (pMC)	Uncorrected Age	Corrected Age
S1	43.24	6931	3433
S2	35.06	8665	6097
S3	101.40	Modern	Modern
S4	45.36	6535	217
S5	85.39	1306	Modern
S6	38.39	7915	199
S7	2.30	31,185	17,332
S8	2.73	29,768	16,393
S9	32.68	9246	3026

).

5.4. Heat Source and Reservoir Temperature

Gas sources are usually identified by plotting the R/Ra ratio versus the ⁴He/²⁰Ne ratio [58,59,60]. The three end members of the bottom diagram as mixing lines are the atmosphere, the mantle, and the crust. Their coordinates are (R/Ra = 1, ⁴He/²⁰Ne = 0.318), (R/Ra = 8, ⁴He/²⁰Ne = 1 × 10³), and (R/Ra = 0.02, ⁴He/²⁰Ne = 1 × 10³), respectively [61,62]. The ³He/⁴He ratio is generally denoted as R. The atmospheric ³He/⁴He ratio Ra is 1.384 × 10⁻⁶ [63,64].

The ³He/⁴He ratios of the Gonghe Basin geothermal waters range from 0.03 to 0.84 Ra, indicating that no more than 3% of helium originates from the mantle. The air pollution of most gas samples is approximately 1%, except two samples fall in the range between 20% and 50% (Figure 7). We interpret that the two abnormal data were polluted by air when they were sampled. The remaining four data show that in the geothermal system, the mantle material’s contribution to the gas is very weak. Therefore, based on the tritium contents and Helium compositions, we conclude that the major source of heat for the geothermal system in the Gonghe Basin is primarily the crust.

The geothermometer relies on the relationship between the balance temperature of geothermal water and some minerals to estimate the reservoir temperature [65]. It is based on the equilibrium state of minerals and water within the deep geothermal reservoir, meaning that with the flow of thermal water from deep to shallow areas, the temperature decreases but the chemistry remains relatively stable, so it can be used to estimate the temperature of the geothermal reservoir. The selection of various geothermometers to calculate the reservoir temperature requires balance with the minerals in the geothermal reservoir [66,67]. In this study, nine geothermometers (

Table 10. Different types of chemical geothermometers.

Type	Geothermometer	Code	Expression
silica geothermometer	SiO2	Geothermometer 1	T = 1315/(5.205 − lg (SiO2)) − 273.15
	SiO2	Geothermometer 2	T = 1309/(5.19 − lg (SiO2)) − 273.15
	SiO2	Geothermometer 3	T = −1107/(0.0254 + lg (SiO2)) − 273.15
	Chalcedonite	Geothermometer 4	T = 1032/(4.69 − lg (SiO2)) − 273.15
Cation geothermometer	Na-K	Geothermometer 5	T = 856/(0.857 + lg (Na/K)) − 273.15
	Na-K	Geothermometer 6	T = 1217/(1.483 + lg (Na/K)) − 273.15
	Na-K	Geothermometer 7	T = 933/(0.933 + lg (Na/K)) − 273.15
	Na-K	Geothermometer 8	T = 1390/(1.75 + lg (Na/K)) − 273.15
	K-Mg	Geothermometer 9	T = 4410/(13.95 − lg (K2/Mg)) − 273.15

) are selected for calculating the balance temperature (

Table 11. Reservoir temperature estimation by chemical geothermometry (°C).

Sample ID	Geothermometer 1	Geothermometer 2	Geothermometer 3	Geothermometer 4	Geothermometer 5	Geothermometer 6	Geothermometer 7	Geothermometer 8	Geothermometer 9
S1	163	163	227	139	108	151	129	170	139
S2	163	163	227	139	106	149	127	168	135
S3	169	169	218	146	113	155	134	174	127
S4	171	171	215	149	111	153	132	172	123
S5	121	121	311	93	117	158	138	177	61
S6	144	144	260	118	113	155	133	174	96

). During the upward migration of geothermal water, the content of the chemical components of the thermal fluid will be changed due to boiling and steam escape or dilution and mixing of the geothermal water and river water, which could destroy the original high-temperature equilibrium environment. This reaction leads to an imbalance between the chemical composition of the geothermal temperature and the minerals in the geothermal reservoir. Therefore, before calculating the thermal storage temperature, it is necessary to judge the water–rock mineral balance state of the geothermal water in the ground and analyze the reliability of the use of the geothermal temperature scale.

The Na-K-Mg ternary diagram provides a way to classify geothermal water as fully equilibrated with the rock, partially equilibrated, or immature. This demonstrates whether the solute thermometer is applicable to geothermal water [68,69]. Some water samples of geothermal water from the Gonghe Basin are located near fully equilibrated mineral dissolution lines (Figure 8). Under these conditions, the water–rock reactions reached equilibrium. The temperature of the geothermal reservoir resulting from the Na-K-Mg triangulation is approximately 160 °C. The distribution of water samples along the mixing line suggests a tendency to mix with cold water.

The multi-mineral equilibrium method uses the relationship between the dissolution state of multiple minerals in the geothermal system and the temperature of geothermal water, and when multiple mineral components in the geothermal system reach the dissolution equilibrium at the same time, this temperature is the geothermal reservoir temperature [70]. The solubility state of multiple minerals is drawn as a function of temperature. If some or more minerals are close to equilibrium for a given temperature, the geothermal fluid and the mineral will reach equilibrium, and the equilibrium temperature is the heat storage temperature. The saturation index of each mineral was calculated using PREEQC software. The saturation index (SI) is defined as SI = Log(Q/K), where Q is the anion and cation activity product of a certain mineral in an aqueous solution; K is the mineral equilibrium constant at a certain temperature, which depends on the available thermodynamic database. Since the rocks in the working area of Gonghe Basin are primarily sandstone and granite, minerals such as dolomite, quartz, calcite, K-feldspar, chalcedony, chlorite, and Ca-Montmorillonite were selected for calculation, and the saturation index (SI) and temperature curves were drawn (Figure 9). To remove the influence of CO₂ degassing, the geothermal water is supplemented with an equal amount of CO₂. In samples S3 and S4, 0.01 mol/L of CO₂ was added to the fluid, and 0.05 mol/L of CO₂ was added to S5 and S6 [71,72]. It can be seen from the figure that multiple mineral saturation index curves converge at one point simultaneously, so the geothermal reservoir temperature is estimated to be approximately 160–170 °C.

Considering the result of the Giggenbach diagram, Na-K geothermometer, and multi-mineral balance method with excellent concordance, the geothermal reservoir temperature of geothermal water in the Gonghe Basin can be appropriately estimated at 160 ± 10 °C.

Geothermal water is generally heated up by geothermal sources during deep circulation. For a detailed understanding of geothermal water origin in the Gonghe Basin, the circulation depth of the geothermal water was estimated. The circulation depth can be roughly estimated by the following Formula (2). The calculated circulation depth is approximately 2200–2500 m.

$$H=\frac{\left(T-T_0\right)}{\Delta t}+h$$

(2)

where H is the depth of circulation (m), T₀ is the constant temperature (°C), Δt is the geothermal gradient, and h is the depth of the constant temperature zone.

According to the statistical results of the weather stations in the study area, the annual average temperature in the Gonghe Basin is 3.9 °C. The average geothermal gradient is 6.7–6.8 °C/100 m [21], and we take 6.7 °C/100 m for calculation. The constant temperature zone depth is 20 m based on experience.

For igneous rocks and metamorphic rocks, the aquifers are primarily distributed in strata containing fractures. When fractured zones appear in the strata or fractures develop in the strata, the strata in this section may contain water. Through the analysis of the logging data in the 2280–2400 m interval (Figure 10), we also found that the sound amplitude scanning image of this interval has a relatively obvious strip pattern, and the number of fractures increased sharply, which further indicates that there are cracks in this interval, which is consistent with the calculation results.

5.5. Conceptual Model of the Gonghe Geothermal System

Based on the discussion of the geological environment and geothermal fluid characteristics, a diagram of the genesis model of the geothermal system in the Gonghe Basin is proposed (Figure 11). Snowmelt from the surrounding high mountains or atmospheric precipitation recharges the geothermal water. According to the elevation effect of hydrogen and oxygen isotopes, the recharge elevation is estimated to be 3000–4600 m. The Gonghe basin is high in the west and low in the east. The mountain snowmelt in the west and atmospheric precipitation infiltrate and migrate laterally along the loose sediment and sandstone, and some of them flow into rivers and migrate with surface water, while the other part migrated along the fault to the deep area. Long-term groundwater with an age of approximately 16,300–17,300 years flows at depth and absorbs heat from the surrounding thermal rocks, resulting in a deeply flowing fluid with a temperature of approximately 160 ± 10 °C. The circulation depth is estimated to be approximately 2–3 km. The low thermal conductivity of the overlying Cenozoic clastic sedimentary rocks in the study area can be considered a good cover for geothermal systems. Geothermal fluids are transported upward to the surface through deep faults and fractures under the effect of a pressure difference, forming an abnormal geothermal water region.

6. Conclusions

Taking the hydrochemistry and isotopic composition (δD, δ¹⁸O, δT, δ¹³C, δ¹⁴C, ⁸⁷Sr/⁸⁶Sr, and ³He/⁴He) in the area as the study objects, combined with regional geological information and drilling results, the conclusions are as follows:

The main cations are Na⁺ and Ca²⁺ and the main anions are SO₄²⁻, Cl⁻, and HCO₃⁻ in the geothermal water of the Gonghe Basin, and the hydrochemical chemistry type is primarily Na-SO₄-Cl. When the fluid flowed through the Triassic silicate reservoir, the water–rock reaction occurred with the surrounding rocks. During upward flow, deep geothermal water suffered different degrees of the cooling process, alone or simultaneously with the cold-water mixing process and transfer cooling process.

The results of oxygen and hydrogen isotope tests and the elevation effects of hydrogen and oxygen isotopes indicate that the geothermal water originated from atmospheric precipitation or snowmelt water from the high surrounding mountains, and the geothermal fluid experiences oxygen drift under the high-temperature underground environment. The isotopic ratios of helium range from 0.03 to 0.84 Ra, suggesting a predominantly atmospheric and crustal source. According to ¹⁴C, ¹³C, and the tritium isotope, we determined the corrected age of groundwater as approximately 16,300–17,300 years and the result of mixtures of submodern and recent recharge water sources.

The cationic geothermometer, Na-K-Mg geothermometer, and multi-mineral equilibrium methods were used to calculate the geothermal reservoir temperature in the Gonghe Basin to be 160 ± 10 °C with a circulation depth of approximately 2200–2500 m.