Source and Origin of Subsurface Brine of the Kongquehe Sag Area in Western Lop Nur, China

Abstract

The Kongquehe Sag, located in the western Lop Nur, has abundant pore subsurface brine. In order to study the source and origin, we tested and analyzed the hydrochemical composition and stable isotopes of the subsurface brine. The findings reveal that the brine exhibits a moderate to low degree of mineralization, with values ranging from 50.50 g/L to 91.14 g/L. The stable isotope compositions of unconfined and confined waters are different, with the mean values of δD being −8.00‰ and −51.75‰ and the mean values of δ¹⁸O being 10.08‰ and −6.01‰. These values are indicative of an intense evaporative environment prevalent in the Kongquehe Sag area. Furthermore, the ⁸⁷Sr/⁸⁶Sr ratios vary between 0.710642 and 0.710837, and δ³⁴S values range from 9.2 to 10.7. These data suggest the long-term evolution of sulfur substances, predominantly through dissolution and sedimentation processes, with minimal influence from redox reactions. The data garnered from this research not only offer a novel perspective of the insights gained into the hydrochemical characteristics and the stable isotope signatures of the brines in the Kongquehe Sag area but also enriches the theoretical framework concerning the source and origin of subsurface brines, potentially informing future exploration strategies.

1. Introduction

Brine is rich in various valuable elements, including economically significant ones like K⁺, Li⁺, and B³⁺. These elements have served as vital sources of essential compounds utilized by humans [1,2]. The potassium reserves found in brine are crucial to the global production and supply of potash [3]. In 2020, worldwide potash production capacity reached 62.4 million tons, with K-rich brine deposits accounting for 21.64% of this total [4]. The key production regions include Qarhan Salt Lake, Lop Nur Salt Lake, the Dead Sea, Atacama Salt Lake, and Great Salt Lake [5,6].

Lop Nur is located between the East Tianshan Mountains and the Altun Mountains in China and is the lowest place in the Tarim Basin [7]. As the catchment center, the Lop Nur area accumulated significant amounts of salt brought from the Tarim River basin during geological historic times and is thus thought to be one of the most favorable prospects for potash deposit formation in China [8,9]. Nevertheless, most current research has focused on the eastern part of Lop Nur, and whether the western part has the potential for potassium formation remains unknown. This has recently become an important topic for scholars to study.

Recently, the authors’ team found a number of small salt lakes in the Kongquehe Sag area, mainly composed of Quaternary sediment deposits, distributed in a fusiform ring belt. The outer ring of the salt lake is eolian sand, the secondary outer ring is saline soil, the inner ring is a salt deposit, and the degree of mineralization of surface brine is 320 g/L, and the potassium ion content is 1303 mg/L, reaching the grade of brine potassium salt mining, indicating that the Kongquehe Sag area has good prospects for potassium exploration. However, to date, a comprehensive understanding of the brine in this area is lacking, and the origin of the brine is still unclear.

Isotopes of hydrogen, oxygen, sulfur, and strontium are frequently utilized as tracers to indicate the origin of salt lakes and subsurface brines, as well as water exchange. In addition, calculating and analyzing the hydrochemical composition and characteristic coefficient of subsurface brine can reveal the water chemistry types and evolution of subsurface brine [10,11,12]. Zhao et al. (2024) analyzed the S, Sr, H, and O isotopes of Mahai Salt Lake and its surrounding waters and found that the water sources were river and fault-related waters mixed in a ratio greater than 15:1 [13]. Yang et al. (2024) revealed that Ca-Cl type brines in the Qaidam Basin with high Sr²⁺ concentrations and high ⁸⁷Sr/⁸⁶Sr ratios are supplied by springs or infiltrate along faults [14]. Yang et al. (2023) collected and tested isotopes of surface water and groundwater in five typical watersheds in Qaidam Basin, using hydrochemistry, isotopes, and other methods for cause analyses, understanding the spatial and temporal variations in surface water and groundwater chemistry and isotopes [15].

To reveal the material source and origin of the brine from the Kongquehe Sag area, we analyzed the hydrochemical composition and H–O–S–Sr isotope characteristics of the brine. The obtained results will provide new data for the related exploration of potash salt in the western part of the Lop Nur region.

2. Materials and Methods

2.1. Study Area

The Kongquehe Sag area (87°10′10.98″ E, 40°47′31.90″ N) is located in the west of Lop Nur (Figure 1b). The regions feature a warm, temperate, continental, arid climate; rainfall is scarce, evaporation is strong, sunshine is abundant, and heat is abundant [16]. The annual precipitation fluctuates within the range of 20–30 mm, and the annual potential evaporation reaches 2000–3000 mm. The average annual temperature is 11.5 °C; the lowest temperature recorded is observed in December, whereas the highest temperature is observed in July. The annual sunshine duration is 2800–3100 h, and the percentage of sunshine is 58–75%.

In terms of its geotectonic position, the Kongquehe Sag belongs to a secondary structural unit in the north depression of the Tarim Basin [17]. There are two groups of faults, NW-directed and NE-directed, as well as several secondary sags. The strata in this area are mainly Quaternary eolian deposits, scouring proluvial deposits, and chemical deposits. Chemical deposits are mainly distributed in small dry salt lakes on both sides of the Tarim River, and the surface is covered by a large area of salt shells and salt flats (Figure 1c,d). The main components are halite and glauberite. According to the lithological changes in strata revealed by the borehole, the surface is fine-grained sand; the middle part is medium-fine-grained sand, clay and silty soil interbedded layer, and silty soil and fine-grained sand interbedded layer in the lower part (Figure 2).

The subsurface brine mainly exists in the medium-fine-grained sand and coarse-grained sand layers of the Upper Pleistocene of the Quaternary system. It is pore brine with buried water depths of 2.20–7.12 m and unit water influx of 0.42–0.95 L/s·m and is moderately rich in water (0.1–1.0 L/s·m is medium water yield property). The thickness of the aquifer is between 15 and 38 m, and the water permeability coefficient K = 4–8 m/d, permeability is good (1–10 m/d is medium permeability). The water-resisting layer is a Quaternary lacustrine clay and silty clay layer with a thickness ranging from 1.0 to 20.0 m, generally within 3.0–7.0 m. The top and bottom layers are good water barrier layers with horizontal occurrence, which is conducive to the preservation of brine.

2.2. Sampling and Analysis

In this study, a total of 10 confined brine samples were collected from boreholes in 2023, as well as 8 unconfined brine samples in the Kongquehe Sag area (Figure 1a). Before brine collection, the 10 L polyethylene sampling barrel prepared in advance was cleaned three times with brine in the drilling hole, and the deep brine was extracted from the drilling hole to test its temperature and pH on-site [18]. To prevent evaporation of the collected samples, they were packed directly into the sampling drum and wrapped in plastic wrap, and then returned to the laboratory for testing as soon as possible at low temperatures, covered to avoid sunlight during transportation [19]. The hydrochemical composition, along with H–O–S–Sr isotope analyses, was conducted at the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences.

The tested items included pH value, K⁺, Ca²⁺, Na⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, Br⁻, degree of mineralization, and other components. Among them, the contents of Mg²⁺ and Ca²⁺ were determined by complexometric titration, and the test error was less than 0.2%. The content of Cl⁻ was determined by the silver method, and the test error was less than 0.2%. The content of HCO₃⁻ was determined by hydrochloric acid titration, and the test error was less than 0.2%. The content of K⁺ was determined by the sodium tetraphenylborate method, and the test error was less than 0.5%. The content of SO₄²⁻ was determined by the barium sulfate method, and the test error was less than 0.5%. The Na⁺ content was calculated by difference subtraction, and the error was less than 2.0%. The content of Br⁻ was determined by the spectrophotometric method, and the test error was less than 1.0%. The pH value was determined by the glass electrode method, and the test error was less than 1.0%. The degree of mineralization was determined by the gravimetric method, and the test error was less than 1.0%. The density was determined by the hydrometer method, and the test error was less than 1.0%.

The H and O isotopes were measured using a liquid water isotope laser analyzer, and the calculated results were expressed as the kilowatt difference δ of Vienna Standard Mean Ocean Water (VSMOW). The test accuracy reached 0.5‰ and 0.1‰, respectively. S isotopes were determined using a Thermoquest Finnigan Delta PlusXL (Thermo Fisher Scientific, Waltham, MA, USA) isotope ratio mass spectrometer. V-CDT was used as the standard for S isotopes. The analysis error was controlled within ±0.2‰. Sr isotopes were tested using a Nu Plasma II (Nu Instruments, Wrexham, UK) multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and the NIST SRM 987 Sr isotope reference material was used as the standard sample. The test accuracy was 0.000001.

3. Results

3.1. Brine Water Chemical Characteristics

3.1.1. Types of Water Chemistry

The analysis and detection results are shown in

Table 1. The analysis results of hydrochemistry in brine.

Types	Number	Mineralization	K+	Ca2+	Na+	Mg2+	Cl−	SO42−	HCO3−	Br−	Sr2+	Li+	pH	Density
		g/L								mg/L				g/cm3
Confined brines	ZK05	75.83	0.17	1.21	24.95	1.74	40.45	7.19	0.11	10.31	37.68	0.38	8.15	1.13
	ZK07	50.50	0.11	1.09	15.05	1.36	30.31	2.45	0.12	8.26	45.13	0.42	8.06	1.05
	ZK08	58.41	0.18	1.22	18.78	1.38	30.72	6.01	0.12	5.57	34.56	0.27	8.12	1.08
	ZK10	60.48	0.09	1.27	19.30	1.51	31.47	6.67	0.16	7.89	28.98	0.33	8.21	1.09
	ZK11	50.98	0.06	0.58	17.03	0.75	26.63	3.81	0.10	9.98	44.57	0.51	8.17	1.06
	ZK12	57.86	0.10	1.18	18.79	1.30	30.67	5.60	0.21	13.24	39.68	0.45	8.16	1.08
	ZK13	91.14	0.02	2.11	29.79	1.30	53.62	4.23	0.06	5.88	33.59	0.25	7.31	1.15
	ZK14	52.75	0.11	1.28	16.58	1.36	27.78	5.45	0.17	11.98	31.89	0.47	8.17	1.07
	ZK19	71.25	0.18	1.02	23.75	1.52	38.23	6.28	0.25	14.87	39.21	0.36	8.27	1.11
	ZK22	72.15	0.16	1.18	22.12	1.79	38.29	6.74	0.19	13.65	29.23	0.49	8.14	1.08
Unconfined brines	UB01	68.87	0.04	1.67	24.02	0.84	39.37	2.75	0.11	5.57	33.90	0.16	8.22	1.13
	UB02	63.38	0.13	1.33	20.63	1.84	33.46	6.48	0.16	2.86	29.20	0.49	8.15	1.09
	UB03	67.59	0.27	1.02	21.35	2.02	34.60	8.13	0.41	6.32	27.89	0.32	8.31	1.12
	UB04	70.23	0.12	1.21	22.78	1.46	38.98	5.76	0.18	6.88	32.45	0.41	8.22	1.13
	UB05	65.89	0.03	1.76	21.34	1.49	36.24	3.45	0.14	6.45	25.67	0.37	8.28	1.09
	UB06	66.91	0.16	1.45	20.28	1.92	37.72	4.56	0.32	5.34	28.98	0.29	8.15	1.09
	UB07	79.87	0.15	1.41	24.78	1.59	45.12	6.45	0.18	6.09	26.71	0.32	8.21	1.14
	UB08	71.56	0.14	1.22	22.31	1.69	39.08	5.79	0.22	4.57	27.98	0.35	8.19	1.13

. The pH values of confined brines in the study area ranged from 7.31 to 8.27, with an average of 8.08, and unconfined brines ranged from 8.15 to 8.31, with an average of 8.24. The density of confined brines ranged from 1.05 to 1.15 g/cm³, with an average of 1.09 g/cm³, and unconfined brines ranged from 1.09 to 1.14 g/cm³, with an average of 1.12. The mineralization of confined brines was between 50.50 and 91.14 g/L, with an average of 64.14 g/L, and unconfined brines was between 63.38 and 79.87 g/L, with an average of 69.29 g/L. According to the Kurnakov–Valyashko classification [20], the main type of brine in the study area is the magnesium sulfate subtype.

3.1.2. Ion Content

The ion content analysis results indicate that Na⁺ and Cl⁻ were absolutely dominant among the macroions. From the perspective of cation composition, ρ(Na⁺) > ρ(Mg²⁺) > ρ(Ca²⁺) > ρ(K⁺), the mass concentration of Na⁺ + K⁺ was between 15.16 and 29.81 g/L, and the mass concentrations of Mg²⁺ and Ca²⁺ were small. The main anion composition was ρ(Cl analysis results) > ρ(SO₄²⁻) > ρ(HCO₃⁻), the mass concentration of Cl⁻ was between 26.63 and 53.62 g/L, and the mass concentration of Cl⁻ was more than one order of magnitude different from that of HCO₃⁻. The order of trace element ion contents in the brine was Sr²⁺ > Br⁻ > Li⁺. The content of Sr²⁺ was 25.67–45.13 mg/L, with an average of 33.18 mg/L; the content of Br⁻ was 2.86–14.87 mg/L, with an average of 8.09 mg/L; and the content of Li⁺ was 0.16–0.51 mg/L, with an average of 0.37 mg/L.

3.1.3. Hydro-Geochemical Coefficients

Previous studies have shown that the source and evolution of subsurface brine can be revealed by calculating the hydrogeochemical coefficients of the brine [21]. For any ion x, ρx represents the mass concentration, and nx represents the molarity. The hydrogeochemical coefficients in the study area were calculated according to the test results of sample analysis. As shown in

Table 2. Hydrogeochemical coefficients of brine sample.

Types	Number	nNa+/nCl−	ρBr−*103/ρCl−	nSO42−*50/nCl−	nCa2+/nMg2+	ρK+*103/ρCl−
Confined brines	ZK05	0.95	0.25	6.56	0.42	4.16
	ZK07	0.85	0.3	3.31	0.49	3.94
	ZK08	0.94	0.18	7.22	0.54	3.41
	ZK10	0.95	0.25	7.82	0.51	2.9
	ZK11	0.99	0.37	5.28	0.47	2.32
	ZK12	0.94	0.43	6.74	0.55	3.22
	ZK13	0.88	0.47	2.6	1.28	0.68
	ZK14	0.92	0.43	7.24	0.57	4.11
	ZK19	0.96	0.39	6.07	0.41	4.81
	ZK22	0.92	0.45	8.21	0.4	5.17
Unconfined brines	UB01	0.94	0.36	5.87	0.31	4.12
	UB02	0.96	0.28	7.93	0.65	3.14
	UB03	0.83	0.31	7.91	0.38	4.98
	UB04	0.95	0.35	5.98	0.39	4.78
	UB05	0.98	0.34	7.98	0.48	3.09
	UB06	0.93	0.32	5.66	0.29	3.97
	UB07	0.96	0.33	7.45	0.48	4.32
	UB08	0.91	0.21	6.43	0.39	4.01

, the sodium/chloride coefficient ranged from 0.83 to 0.99, with an average value of 0.93. The bromine/chlorine coefficient ranged from 0.18 to 0.47, with an average value of 0.33. The desulfurization coefficient ranged from 2.60 to 8.21, with an average value of 6.46. The coefficients of calcium/magnesium ranged from 0.29 to 1.28, with an average of 0.50. The potassium/chloride coefficient ranged from 0.68 to 5.17, with an average value of 3.73.

3.2. Characteristics of Isotopic Composition of Brine

3.2.1. H–O Isotope Composition Characteristics

Hydrogen and oxygen isotopic compositions are strong evidence for the analysis of water-recharged sources and have been widely used in groundwater and brines in various geological periods [22]. In research, water movement can be traced on a regional scale.

Table 3. H–O–S–Sr isotope composition in different water bodies.

Types	Number	δD	δ18O	δ34S	87Sr/86Sr
		‰
Confined brines	ZK05	−54.13	−5.52	10.40	0.710750
	ZK07	−55.67	−5.80	10.70	0.710642
	ZK08	−51.01	−4.99	10.10	0.710768
	ZK10	−47.94	−6.70	10.20	0.710773
	ZK11	−47.39	−5.85	9.60	0.710791
	ZK12	−57.60	−7.64	10.30	0.710757
	ZK13	−54.89	−5.78	9.70	0.710732
	ZK14	−48.64	−6.25	9.20	0.710820
	ZK19	−49.55	−6.32	9.80	0.710805
	ZK22	−50.73	−5.28	10.30	0.710837
Unconfined brines	UB01	−5.06	8.78	10.20	0.710796
	UB02	−9.41	3.08	9.90	0.710812
	UB03	−12.48	8.15	10.20	0.710793
	UB04	−5.54	11.91	10.40	0.710821
	UB05	−11.94	8.71	9.80	0.710779
	UB06	−6.70	12.89	10.40	0.710783
	UB07	−3.34	12.56	10.50	0.710809
	UB08	−9.58	14.56	10.30	0.710818

shows that the δD values of the confined brines in the study area ranged from −57.60‰ to −47.39‰, with an average of −51.76‰, and the δ¹⁸O values ranged from −7.64‰ to −4.99‰, with an average of −6.01‰. The δD values of the unconfined brines ranged from −12.48‰ to −3.34‰, with an average of −8.00‰, and the δ¹⁸O values ranged from 3.08‰ to 14.56‰, with an average of 10.08‰. Among them, δD and δ¹⁸O of unconfined brines > δD and δ¹⁸O of confined brines.

3.2.2. Characteristics of S Isotope Composition

The difference in the δ³⁴S values of sulfate in natural water reflects the characteristics of sulfur isotope fractionation [23]. The source of sulfur in water can be determined by comparison. Table 3 shows that the δ³⁴S values of the confined brines in the study area ranged from 9.20 to 10.70, with an average of 10.03. The δ³⁴S values of unconfined brines varied from 9.80 to 10.50, with an average of 10.21. Among them, δ³⁴S of unconfined brines > δ³⁴S of confined brines, but the differences were not large.

3.2.3. Characteristics of Sr Isotope Composition

Unlike hydrogen and oxygen isotopes, the strontium isotopic composition of groundwater is similar to the isotopic composition of the bedrock with which it interacted and is almost unaffected by fractionation or mineral precipitation. Therefore, strontium isotopes are often used as indicators of sources of solutes in natural waters, water–rock interactions, and water mixing [24]. Table 3 shows that the ⁸⁷Sr/⁸⁶Sr values of confined brines in the study area ranged from 0.710642 to 0.710837, with an average value of 0.710767. The ⁸⁷Sr/⁸⁶Sr values of unconfined brines ranged from 0.710779 to 0.710821, with an average of 0.710801. Among them, ⁸⁷Sr/⁸⁶Sr of unconfined brines > ⁸⁷Sr/⁸⁶Sr of confined brines.

4. Discussions

4.1. Source of Brine Supply

In the Piper diagram (Figure 3), the cations in the subsurface brine are concentrated in the Na⁺ + K⁺ end element; the contents of these two cations accounted for more than 90% of the total cations, and the contents of Mg²⁺ and Ca²⁺ were very low. The main anions were Cl⁻ + SO₄²⁻, which accounted for more than 90% of the total anions. Of these, Cl⁻ was the main anion, with a small amount of SO₄²⁻; HCO₃⁻ and CO₃²⁻ contents were very low, reflecting that these brine samples were rarely affected by atmospheric water mixing. The macroelement contents are almost the same between the confined brines and unconfined brines. Because the study area is located in the depression of the northern Tarim Basin, the ice-snow melting water, rainfall infiltration, and groundwater recharge of the northern mountain system continue to gather in this area from the surrounding areas. After dissolving soluble salt ions from the rocks, the water migrates to low-lying areas [25]. After long-term accumulation, coupled with evaporation and concentration under arid climate conditions, brine with a high degree of mineralization is ultimately formed [26].

To investigate the origin of subsurface brine in the Kongquehe Sag area, the ion characteristic coefficients of the five brines were analyzed [27]. The average sodium/chloride coefficient of brine in the study area was 0.93, which is higher than that of normal seawater (0.87) and close to 1. This can be attributed to the dissolution of salt and indicates non-marine brine. The bromine/chlorine coefficient of the brine in the study area was an average of 0.35. That of normal seawater is 3.4, that of sedimentary metamorphic brine is >3.4, and that of halite-dissolution brine was 0.083–0.83. Thus, the subsurface brine in the study area is of the formation mechanism of the dissolution and filtration of salts. The desulfurization coefficient had an average value of 6.11. Generally, when this value is closer to 0, it indicates a better sealing property. The coefficient of deep subsurface brine in the study area was small (2.60), indicating a better-sealing property, whereas that of the shallow brine was worse. The calcium/magnesium coefficient had an average value of 0.56. Generally, a longer sealing time represents a better-sealing property and a higher degree of metamorphism. The coefficient of the deep subsurface brine in the study area was larger (1.28), indicating that the sealing property was relatively good and the metamorphism degree was relatively high. The potassium/chloride coefficient had an average value of 3.47. In general, when the potassium/chloride coefficient is greater than 75, the brine is rich in K, indicating that the deposition period of the study area has not reached the stage of K salt precipitation [28]. In summary, the subsurface brine in the Kongquehe Sag area may have come from early terrestrial saline lake deposits; the deep brine system was relatively closed during the formation process; the degree of metamorphism was higher; and there were sources of halite-dissolution brine.

4.2. Cause Analysis

The H–O isotope data of the subsurface brine samples from the study area were projected into the H–O relationship diagram, as shown in Figure 4. They all deviated from the GMWL and were located at the lower right of the GMWL, roughly divided into confined brines and unconfined brines, and the δD and δ¹⁸O of the unconfined brines were significantly more enriched than those of the confined brines. This phenomenon is more common in dry climates or closed basins [29]. Because the climate of the Tarim Basin, where the Kongquehe Sag area is located, is extremely arid, the annual evaporation is much larger than the precipitation, and the subsurface brine is mainly replenished by surface water. The unconfined brines are shallowly buried, and the dynamic fractionation of H–O isotopes in the water body continues during the evaporation process, which makes the isotopes more enriched. In addition, in the process of lateral infiltration and recharge, unconfined brines may dissolve and filter the salt substances in the formation and achieve equilibrium with oxygen-containing minerals by isotope exchange reaction, which also leads to the fractionation of H–O isotopes in the water body [30]. Therefore, the subsurface brine in the Kongquehe Sag area is likely formed by the combined action of evaporation and salt leaching.

The factors of sulfur isotope fractionation mainly include the mixing of different sulfur isotope sources and biochemical action [31]. A large number of studies have shown that in the low-temperature environment of the crust surface, the reduction reaction of oxy-type sulfide caused by microbial activity plays an important role in the fractionation of sulfur isotopes, but the physical fractionation of the sulfur isotope composition of sulfate in inorganic chemical reactions is not obvious [32]. The S isotopic composition of dissolved sulfate in natural water is basically the same as that of the original sulfate, which is crystallized by evaporation and concentration and dissolved into water via sulfate mineral dissolution. Compared with the δ³⁴S value (9.9) of solid sulfate minerals in this area [33], the δ³⁴S value (10.11) of subsurface brine is very similar (Figure 5), which reflects the long-term evolution of sulfur substances, mainly through dissolution and sedimentation, whereas the redox process under the influence of microorganisms is not obvious. In addition, because of the wide distribution for the evaporites of Mesozoic–Cenozoic in the Tarim Basin, the gypsum salt layers and glauberite-halite layer are widely found in the Kuqa Basin, south of the Tianshan Mountains in the upper reaches of the Tarim River basin, and the δ³⁴S values range from 7.17‰ to 13.15‰, with an average of 10.19‰ [34]. It is concluded that the sulfates of the subsurface brine in the Kongquehe Sag area were may mainly derived from the weathering products of the Mesozoic–Cenozoic gypsum calcium glauber layer in the Tarim Basin.

The main sources of strontium in seawater are shell sources (mean value 0.7119) and mantle sources (mean value 0.7035), and the strontium isotope composition of seawater is a comprehensive reflection of the mixture of these two sources. Since the beginning of the Mesozoic era, the average value of ⁸⁷Sr/⁸⁶Sr in seawater has shown an overall increasing trend, reaching a maximum value of 0.7093 at present [35]. The ⁸⁷Sr/⁸⁶Sr of the brine (mean value 0.7107) in the study area is significantly lower than that of crustal source strontium of the continental surface weathering system (Figure 6), and the ⁸⁷Sr/⁸⁶Sr isotopic characteristics are close to but slightly higher than those of seawater. It has been reported that the Ordovician carbonate ⁸⁷Sr/⁸⁶Sr in the Tazhong area of the Tarim Basin ranges from 0.70644 to 0.70938 (similar to Ordovician seawater, 0.7078–0.7090). Silurian formation water ⁸⁷Sr/⁸⁶Sr is between 0.71057 and 0.71258. The ⁸⁷Sr/⁸⁶Sr of Carboniferous formation water is between 0.71100 and 0.71900 [36]. This ratio in the Paleogene marine rocks of the Kuqa Depression ranges from 0.708718 to 0.709326, and that of the Miocene Marine rocks with sea–land mixing ranges from 0.709909 to 0.710868 [37]. Comparison with ⁸⁷Sr/⁸⁶Sr of existing brines in the Tarim Basin reveals that the characteristics of subsurface brine ⁸⁷Sr/⁸⁶Sr in the study area are most similar to those of Miocene salt rocks, which indicates that Miocene salt may be the source salt of this set of brines.

4.3. Genetic Analysis of Brine

Generally, the coupling of “extreme components” of the three elements of provenance, structure, and climate is the key to the formation of brine [38]. Through the past combined with this study, the drainage system of the Kongquehe Sag area is well developed. Since the Middle Pleistocene, the Tarim River has been flowing through the area intermittently. All kinds of salt-bearing rocks in the South Tianshan Mountains have been weathered and denuded by nature for a long time, and most of the broken materials are transported and deposited in the Kongquehe Sag by the action of running water, providing this area with rich salt provenance for a long time [39].

In the late Pliocene, under the continuous subduction of the Indian plate to the north, the Cenozoic strata in the Tarim Basin began to shrink, resulting in the X-type conjugate faults in the NW-directed and NE-directed and forming migration channels or water-bearing spaces for groundwater in the Kongquehe Sag area [40,41]. Under the circumstances of high pressure and a closed reduction environment, groundwater continuously migrated and circulated in formation pores, fault fractures, and other parts, and a water–rock reaction occurred with the surrounding rock. It produces material exchange and enrichment to form high-salinity brine.

Since the Quaternary period, the long-term climate environment in this area was dominated by extreme drought (Figure 7), which promoted the rapid and intense evaporation of surface water. Surface evaporation causes a large amount of the original surface water to concentrate and crystallize into salt, laying the foundation for the enrichment of the brine [42,43].

5. Conclusions

The subsurface brine of the Kongquehe Sag area mainly exists in the medium-fine-grained sand and coarse-grained sand layers of the Upper Pleistocene of the Quaternary system. The hydrochemical type is of the magnesium sulfate subtype, and the mineralization of the subsurface brines exhibits a moderate to low degree, with values ranging from 50.50 g/L to 91.14 g/L. The stable isotope compositions of unconfined and confined waters are different, with the mean values of δD being −8.00‰ and −51.75‰, and the mean value of δ¹⁸O being 10.08‰ and −6.01‰. These values are indicative of an intense evaporative environment prevalent in the Kongquehe Sag area. Furthermore, the ⁸⁷Sr/⁸⁶Sr ratios vary between 0.710642 and 0.710837, and δ³⁴S values range from 9.2 to 10.7. These data suggest the long-term evolution of sulfur substances, predominantly through dissolution and sedimentation processes, with minimal influence from redox reactions. The obtained results will provide new data for the theory of the source and origin of subsurface brine and related exploration of potash salt in the Lop Nur region.