Genesis and evolution of deep K-rich confined brine in Mahai Basin, Qinghai-Tibet Plateau

doi:10.21203/rs.3.rs-3807365/v1

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Genesis and evolution of deep K-rich confined brine in Mahai Basin, Qinghai-Tibet Plateau

https://doi.org/10.21203/rs.3.rs-3807365/v1

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Mahai Basin (MH), situated in the northern Qaidam Basin (QB), occurred abundant K-rich brine resources. The study on origin of deep K-rich confined brine and variation in K-Mg elements correspond to evolution in MH light on the significance of the sustainable development and utilization of brine deposits. This study presents multiple isotopes (δ¹⁸O and δD, ⁸⁷Sr/⁸⁶Sr) and hydrochemical characteristics for river waters, surface brines, intercrystalline brines, confined brines, gravel-type brines and anticlinal brines in the MH. The results indicate that: (1) Compared with anticlinal brine, confined brine is characterized by high K, low Ca and Sr contents, and close to the supply path of water chemistry in Yuqia River (from Na-HCO₃-SO₄ field to Cl-SO₄ field in Ca-SO₄-HCO₃ equivalent diagram), deciphering river water is the main water source for confined brine. (2) The δ¹⁸O-δD values of confine brine are spatially various in MH and fall on the right side of the local evaporation line in this study, which implies that confined brine underwent evaporation and concentration successively. (3) The ⁸⁷Sr/⁸⁶Sr ratios (0.7114298–0.7114522) of confined brine is between river water and Lenghu anticlinal water, and both river water and confine brine dominate in low Sr field, further verified that more recharge budget of river water. (4) Combining with the evolution of sedimentary facies (Dezongmahai lake area as an example), K and Mg concentrations in MH increased progressively from west to east and from south to north, demonstrating the enrichment of K and Mg aligns to the evolution of MH and northeastern part of this basin serving as the final depositional center.

Origin

Brine evolution

K-rich confine brine

Mahai Basin

Qinghai-Tibet Plateau

Potassium (K) is key components of agricultural fertilizers that form essential inputs for plant growth and food production, which plays a vital role in ensuring food security (Zheng et al., 2015). More than 90% of the world's potash salt production capacity is used for potash fertilizer production, and other industrial fields account for about 10% (Zheng et al., 2010, 2015; Zhang and Zheng, 2021). Qaidam Basin (QB), located on the northern Qinghai-Tibet Plateau, is the largest Cenozoic continental sedimentary basin. There are huge Pliocene-Quaternary continental evaporites and abundant brine resources in this basin, especially K-bearing brine resources (Zhang, 1987). Over the recent years, geological exploration and scientific studies have reported that the occurrence of deep K-rich brines in the western-central QB, including confined brine, gravel-type brine, anticlinal brine and oilfield brine, which are important candidates of K resource for exploitation in the future (Fan et al., 2007, 2010; Tan et al., 2011; Li et al., 2014a, 2014b; Yue et al., 2019; Zheng et al., 2018; Li et al., 2022; Ren et al., 2023).

Mahai Basin (MH) is situated in the northern QB. It is one of the main potash salt reservoirs in the QB, and the K resource reserves (KCl) exceed 1.293×10⁷ t (Cao and Wu, 2004). There are abundant K-rich brine resources in this basin. According to the buried depth of brine and sedimentary characteristics of strata in MH, the brines were divided into five types. The first type is the surface brine (the buried depth ranged from 0 to 1.5 m) that distributed in the surface salt lakes (e.g., Dezongmahai Lake, Balunmahai Lake and Niulangzhinv Lake). The second type is intercrystalline brine, which occurred in the salt lattice with the depth of 1.6–18.0 m. The third type is confined brine that existed in a closed environment, which is surrounded by two confining clastic beds. The range of burial depth is between 18.5 m and 164 m (Wei et al., 1992). The fourth type is gravel-type brine. It is a new type of brine reservoir that discovered in alluvial fan between north part of MH and south slope of Saishiteng Mountain, which is characteristics by high Na⁺, Cl⁻ and large water volume. Its buried depth generally below 150 m (Yue et al., 2019; Li et al., 2022). The last type is anticlinal brine that stored in Lenhu anticlinal tectonic belt in the south part of MH, which developed deep Ca-Cl water that discharged as springs along the unconformity zone between Paleogene-Neogene and Quaternary strata (Lowenstein et al., 1989; Li et al., 2016; Shi et al., 2016; Fan et al., 2018).

Although these brines above mentioned deposited in different spatial locations in MH, differential brine types present different hydrochemical characteristics and provide fundamental insights into the understanding of deposition and sources of brines and evaporites (Zhang, 1987; Fan et al., 2007; Tan et al., 2011; Li et al., 2022). Confined brines in MH, as a deep brine, have the characteristics of K-rich and abundant brine resources. Meanwhile, its storage environment and hydrochemical characteristics depict the window for understanding the genesis of various brines because confine brines fall in between shallow intercrystalline brine and deep gravel-type brine. Previous significant research efforts have been directed toward mineral sequence of evaporates, hydrochemical characteristics and spatial variations of K and Mg elements of the confined brine in the MH (Wei et al., 1992). The confined brine formed in a reduced environment and the hydrochemical type is Cl-SO₄ types on the basis of its stable H-O-Sr isotopes (Wei et al., 1992). The results show that mineral sequence evolved from sulfate minerals to chloride minerals and even potash salt minerals during Early Pleistocene to Holocene (Fig. 1c) (Wei et al., 1992). Moreover, the K and Mg contents in confined brines present low in west and south and high in east and north of MH (Wei et al., 1992; Ma et al., 2018). However, the origin of confined brine and K-Mg elemental variation with the evolution of lake in the MH are still unclear.

In this study, major and trace ionic concentrations, and H-O-Sr isotopes of twenty-three confined brines and four intercrystalline brines in the MH were determined. Combining with ionic concentrations and H-O-Sr isotopic compositions of rivers (Yuqia River, Aolao River, Naoer River), intercrystalline brines, confined brine, gravel-type brine, Lenhu (LH) anticlinal brine in the MH and lake water, oilfield brines in the western QB, we tried to delineate the hydrochemical characteristics and genesis of confined brine, the evolution of the MH and distribution of K and Mg elements in the study area, and compared the origin of intercrystalline brine in Kunteyi due to both MH and Kunteyi basins were in the same sedimentary environment at the end of Pliocene. This work is beneficial for sustainable development and utilization of brine resources.

Qaidam Basin (QB), located in the northern Qinghai-Tibet Plateau, is a large inland intermountain basin with an area of 12.1×10⁴ km². It is bordered by Qilian Mountains to the northeast, Altyn Mountains to the northwest and Kunlun Mountains to the south (Wei and Jiang, 1994; Yin et al., 2008; Tan et al., 2011; Heermance et al., 2013; Li et al., 2015; Zhu et al., 2018; Fan et al., 2018; Hu et al., 2018; Li et al., 2020). MH (Fig. 1b), located in the northern QB, is a large integrated basin with coexisting solid evaporite and brine resource (Wei et al., 1992). The MH is surrounded by the Saishiteng Mountain in the north, the Altyn Mountain in the northwest, the LH anticline tectonic belt in the southwest and the Nanbaxian anticline tectonic belt in the south, with an area of 3700 km². Controlled by the surrounding diamond-shaped fault, MH is a diamond-shaped basin spreading in the direction of northwest to southeast. It is a sub-subsidence zone on the northern QB, and sedimentary facies mainly developed as alluvial fans, braided rivers and lakes (Wang et al., 2010).

The climate in the MH is extremely arid with average annual precipitation is only 29.61 mm/y, while average potential evaporation is 3040 mm/y. The arid and cold climate and the closed topography determine the unique geological environment, recharge, runoff and groundwater discharge characteristics of the region. (Li et al., 2009; Hu et al., 2015, 2018). The combination of the peripheral mountain system and anticlinal belt makes the MH form a "high mountain deep basin" sedimentary environment. There are three surface salt lakes, namely Dezongmahai Lake, Balunmahai Lake in the eastern MH and Niulangzhinu Lake in the northwestern MH (Fig. 1b). The larger region of three salt lakes contained intercrystalline brines and evaporitic layers, and is referred to as playa. The deep confined brine in the MH is divided into three layers, and mainly distributed below 18.5 m. The first confined brine layer is located in the upper part of Middle Pleistocene and lower part of Late Pleistocene; the second is situated in the middle and lower part of Middle Pleistocene; the third is located in the upper part of Early Pleistocene (Wei et al., 1992). The gravel-type brine is mainly distributed in the alluvial fan in Saishiteng Mountain front (Yue et al., 2019; Li et al., 2022). Anticlinal brine is mainly distributed in the LH anticlinal tectonic belt in the southern MH (Wei et al., 1992). Three hydrochemical types, namely Cl-SO₄, Ca-Cl and Na-HCO₃-SO₄ types, occurred at MH. Intercrystalline brines (such as surface brine and subsurface brine) are Cl-SO₄ types, while gravel-type brine and anticlinal brine are Ca-Cl types; the surface water system of the MH includes Yuqia River and its tributaries (Aolao River and Naoer River) are Na-HCO₃-SO₄ types (Fig. 3) (Lowenstein et al., 1989; Zhang et al., 1993; Fan et al., 2018). In addition, a variety of salt minerals were deposited in the MH, mainly including halite, gypsum, calcite, potash salt and dolomite.

The Quaternary strata in the MH are widely distributed (Fig. 1d), and the surface exposed strata contain Late Pleistocene, Early-Middle Pleistocene and Holocene materials (Hu et al., 2018). The anticlinal tectonic around the basin is covered by the Upper Youshashan Formation (N₂^y) of the Neogene system, and its lithology is mainly composed of shallow lacustrine gray mudstone, sandy mudstone, and partially gypsum. There are Late Pleistocene alluvial sediments, Neogene Upper Youshashan Formation (N₂^y), Upper Ganchaigou Formation (N₁^g) and Paleogene Lower Ganchaigou Formation (E₃^g) in the front overlying strata of Saishiteng Mountain. The lithology is as follows: Upper Youshashan Formation (N₂^y) is dominated by shallow lacustrine gray mudstone, sandy mudstone and local gypsum. Upper Ganchaigou Formation(N₁^g) is a set of semi-deep lacustrine sedimentary brown-red, gray-purple, gray mudstone, sandy mudstone, sandstone mainly mixed with gray marl, occasionally charcoal mudstone; The Lower Ganchaigou Formation(E₃^g) is deposited in a set of deep lacustrine facies with grey-purple-gray mudstone, brown-gray siltstone, argillaceous siltstone and dark gray marl (Li et al., 2022). In general, the Paleogene and Neogene compose the hills and around the MH.

The Saishiteng Mountain system is a part of the northern QB margin tectonic belt, and the main strata in the area are the Dekeng Daban Group in the Paleoproterozoic and the Paleozoic Tanjianshan Group in the Lower Paleozoic, spread NW or NNW (Zhou et al., 2022; Zhuang et al., 2023). The Dakeng Dabanan Group in the region is a set of surface-crust rock assemblages with volcanic-clastic series and middle and advanced metamorphism (Lu et al., 2002; Wang et al., 2022); the Tanjian Mountain Group is a set of early Paleozoic shallow metamorphic Marine volcano-sedimentary formations (Zhuang et al., 2023). There are frequent magmatic activities in the Saishiteng Mountain, and the basic-neutral-acidic intrusive rocks in the area are all occurring, among which the intermediate-acidic intrusive rocks are mainly composed of granodiorite and monoorthitic granite, which are distributed in the Ordovician, Silurian, Carboniferous, Permian and Triassic eras (Zhou et al., 2022). In addition, with the development of igneous rocks, the mountain also developed Jurassic clastic rocks with carbonate rocks, Triassic, Carboniferous, Silurian and other clastic rocks and carbonate rocks (Fig. 1d).

3.1 Sample collection

In this study, a total of twenty-seven brine samples were collected, including twenty-three confined brines and four intercrystalline brines. The confined brine samples were collected from five drills, MH-01 (n = 1), MH-05 (n = 8), MH-07(n = 5), MH-08 (n = 4) and MH-015 (n = 5). Intercrystalline brines were collected from Balunmahai Lake and Niulangzhinu Lake, adjacent to the LH anticlinal tectonic zone. Samples of confined brine were collected at a drilling depth of 20 meters at intervals, while those of intercrystalline brines were collected in the brine distribution area (Fig. 1b). Both brines were filled with brine in 500 mL acid-washed low-density polyethylene (LDPE) bottles. Before filling the brine, the sampling bottles need to be cleaned 3 times with pure water or environment water. Filtration was performed through a 0.45 µm Luerloch syringe filter (polypropylene film) within 48 h after collection (Li et al., 2020).

In the process of sampling confined brine, the depth of drill varies with the sampling depth. The maximum sampling depth of drill MH-05, MH-07, MH-08, MH-01 and MH-015 is 141.2 m, 88 m, 79 m, 26 m and 110 m, respectively, and the brine burial depth is also different. The surface brine burial depth of drill MH-05, MH-07, MH-08, MH-01 and is 1.2 m, 8 m, 19 m and 16 m and 1.6 m, respectively.

3.2 Analytical methods

In this study, 100 mL intercrystalline brine and confined brine were used to analyze the ionic concentrations and isotopic composition. Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, B³⁺, Li⁺ and Sr²⁺ concentrations of intercrystalline brines and confined brines in MH were analyzed in Qinghai Institute of Salt Lake Research, Chinese Academy of Sciences. All analyses were performed within one week of sampling. The concentration of Cl⁻ in brine was determined by silver nitrate point titration. The concentration of SO₄²⁻ in brine was determined by barium sulfate precipitation and gravimetry. The concentration of HCO₃⁻ in brine was analyzed by hydrochloric titration. The concentration of K⁺ in brine was determined by the gravimetric method of precipitated potassium tetraphenylborate [KB(C₆H₅)₄]. The concentrations of Ca²⁺ and Mg²⁺ in brine were determined by EDTA titration. The concentration of Na⁺ in brine is calculated by charge balance [(N_CO3²⁻+N_HCO³⁻+N_SO4²⁻+N_Cl⁻) − (N_K⁺+N_Ca²⁺+N_Mg²⁺)] (N stands for ion equivalent). The analysis accuracy of major cations and anions is better than ± 2%. The concentration of Sr²⁺ in river water, salt water and spring water was determined by atomic absorption spectrometry (GBC-908) with an error of ± 5%. The concentrations of Li⁺ and B³⁺ were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES, ICAP6500 DUO, Thermo Scientific, USA). The analytical accuracy of these elements exceeds ± 5%.

3.2.2 H-O isotopes and ⁸⁷Sr/⁸⁶Sr analyses

The analysis of hydrogen (H) and oxygen (O) isotopes of eight pieces of confined brine was completed at the Open Laboratory of Isotope Geochemistry of the China Geological Survey. The H-O isotope was determined by H₂ and CO₂ on the MAT253 isotope ratio mass spectrometer (IRMS, Thermo Fisher Scientific, USA), and by zinc reduction and CO₂-H₂O equilibrium. The Vienna Standard Mean Ocean water was used as the standard material for H-O isotope analysis, and the analytical accuracy was 1‰ and 0.1‰, respectively.

Strontium (Sr) isotope ratio of eight confined brines were performed on the MAT261 thermoelectric ionization mass spectrometer at the Open Laboratory of Isotope Geochemistry of the China Geological Survey with an accuracy of ± 1.0×10^− 6 (2σSE). All mass fractionation corrections for Sr isotope ratios are based on ⁸⁶Sr/⁸⁸Sr = 0.1194. The analytical error for each sample is within the ± 8.0×10^− 5 (2σSE) range. Sample processing and detection methods refer to Fan et al., 2018; Li et al., 2020; Song et al., 2023; Li et al., 2023.

4.1 Elemental concentrations of brine in MH

The ionic concentrations, sample types, water types and pH values of intercrystalline brines and confined brines in the MH are shown in Table 1. The ionic concentrations of Yuqia River water, intercrystalline brine, gravel-type brine and LH anticlinal brine reported by previous studies in the MH are also listed in Table S1-3. Significant differences in ionic concentrations can be observed in the different water bodies. The confined brine, gravel-type, and intercrystalline brine are characterized by higher Na⁺, K⁺, and Cl⁻ contents than other ions (Fig. 2). The intercrystalline brine (surface lake brine) reported by Wei et al. (1992) is dominated by higher Mg²⁺ and Ca²⁺, while K⁺ is relatively depleted (Fig. 2). However, the anticlinal brine in the LH is enriched in Cl⁻ and relatively depleted in K⁺ (Fig. 2, Table S2) (Li et al., 2016; Shi et al., 2016). In addition, the Yuqia River water in the MH is enriched in Ca²⁺ and HCO⁻ and depleted in K⁺ (Fig. 2; Table S1). The confined brine and gravel-type brine in the MH have high K concentrations, low boron (B) and lithium (Li) concentrations, compared with other brines (e.g., intercrystalline brine, and anticlinal brine) (Fig. 4). While Yuqia River water exhibits a characterized by higher B contents, low Li and K contents. Furthermore, the anticlinal brine in LH presents a relatively high Li background (Fig. 4). The Sr²⁺ concentrations of confined brine and intercrystalline brine (in this study) in the MH are18.08 and 296.63 mg/L, respectively (Table 1).

Table 1

Major and trace concentrations and isotopic compositions of intercrystalline brine and confined brine in Mahai Basin. Note: from number 1 to number 23 stands for confined brine; from number 24 to number 27 stands for inter-brine; the major ionic (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻ ) and ‘TDS’ concentration unit is g/L; the trace ionic (LI⁺, B³⁺, Sr²⁺) concentration unit is mg/L. -: untested data.
Number	Sample code	pH	TDS	K⁺	Na⁺	Ca²⁺	Mg²⁺	Cl⁻	SO₄²⁻	HCO₃⁻	Li⁺	B³⁺	Sr²⁺	⁸⁷Sr/⁸⁶Sr	Error (2σ)	δ¹⁸O	δD	Water type
1	MH05-1.2	6.54	305.40	5.82	100.50	0.85	6.37	184.86	6.86	0.14	14.79	38.43	24.35	0.711432	0.000009	1.50	-21.50	Cl-SO₄
2	MH05-21.2	6.56	304.53	5.82	99.78	0.85	6.31	184.86	6.85	0.16	9.58	36.10	24.41	-	-	-	-	Cl-SO₄
3	MH05-41.2	6.60	305.33	5.72	99.13	0.84	6.30	186.40	6.82	0.12	9.44	39.15	24.30	-	-	-	-	Cl-SO₄
4	MH05-61.2	6.63	306.80	5.71	99.78	0.90	6.51	186.79	6.93	0.14	9.68	35.34	24.72	0.711499	0.000007	2.10	-26.1	Cl-SO₄
5	MH05-81.2	6.75	303.75	5.75	98.95	0.86	6.44	184.86	6.84	0.12	9.33	34.56	24.32	-	-	-	-	Cl-SO₄
6	MH05-101.2	6.68	303.43	5.68	98.68	0.85	6.41	184.86	6.83	0.11	9.19	35.16	24.19	-	-	-	-	Cl-SO₄
7	MH05-121.2	6.56	302.57	5.68	97.96	0.87	6.64	184.86	6.84	0.13	9.46	34.65	24.65	0.71143	0.000004	1.50	-27.4	Cl-SO₄
8	MH05-141.2	5.87	301.96	5.27	95.47	0.87	7.44	186.40	7.02	0.11	10.46	35.68	25.05	-	-	-	-	Cl-SO₄
9	MH07-8	5.81	308.10	4.65	93.16	0.34	8.13	176.20	19.55	0.25	9.81	32.51	10.88	-	-	-	-	Cl-SO₄
10	MH07-28	5.97	304.97	10.47	93.18	0.34	7.95	173.69	19.10	0.25	9.59	31.71	10.53	0.711471	0.000007	2.30	-24.20	Cl-SO₄
11	MH07-48	6.14	315.16	10.44	95.16	0.36	8.13	181.01	19.47	0.26	9.97	41.58	11.16	-	-	-	-	Cl-SO₄
12	MH07-68	6.14	311.65	10.77	95.67	0.35	7.98	177.16	19.67	0.26	9.33	35.75	10.83	0.711472	0.000007	1.90	-21.00	Cl-SO₄
13	MH07-88	6.17	323.46	10.56	98.99	0.32	7.82	180.05	25.08	0.23	9.52	40.92	10.68	-	-	-	-	Cl-SO₄
14	MH08-19	6.02	300.16	10.97	87.01	0.60	12.55	181.01	10.80	0.28	14.77	47.74	14.85	0.711522	0.000006	1.60	-17.80	Cl-SO₄
15	MH08-39	5.99	304.25	7.91	87.58	0.61	12.63	184.86	10.65	0.25	14.39	49.31	14.93	-	-	-	-	Cl-SO₄
16	MH08-59	6.06	304.72	7.67	88.17	0.61	12.39	184.86	10.73	0.23	14.67	41.38	15.17	0.711486	0.000007	2.40	-22.7	Cl-SO₄
17	MH08-79	6.03	294.87	7.73	86.75	0.72	10.67	179.08	10.16	0.21	12.21	37.65	17.13	-	-	-	-	Cl-SO₄
18	MH01-16	6.14	309.93	7.27	98.66	0.46	7.99	182.94	14.38	0.15	11.54	49.90	13.42	0.711298	0.000005	2.00	-19.5	Cl-SO₄
19	MH015-3		198.96	5.35	46.91	0.73	17.61	119.12	12.12	0.12	19.15	0.80	-	-	-	-	-	Cl-SO₄
20	MH015-20		320.68	2.41	101.90	0.71	11.84	192.69	8.05	0.04	9.10	0.80	-	-	-	-	-	Cl-SO₄
21	MH015-50		323.23	5.46	102.67	0.77	12.52	193.57	8.02	0.02	10.87	0.80	-	-	-	-	-	Cl-SO₄
22	MH015-80		319.94	5.66	101.78	0.76	12.11	191.81	7.92	0.04	6.30	0.80	-	-	-	-	-	Cl-SO₄
23	MH015-110		320.31	5.53	101.76	0.80	12.56	191.81	7.80	0.05	6.13	0.80	-	-	-	-	-	Cl-SO₄
24	NL-01	6.47	316.97	4.33	72.62	14.27	21.18	202.74	0.49	0.19	36.86	127.24	709.06	0.711662	0.000006	-1.1	-23.8	Ca-Cl
25	NL-04	6.55	313.91	3.47	78.17	12.85	18.45	199.80	0.52	0.17	11.94	54.38	304.40	0.711696	0.000008	-2.5	-27.4	Ca-Cl
26	BLMH-14	6.83	306.96	2.15	102.04	3.65	8.34	188.42	2.03	0.14	6.47	34.04	73.88	0.711693	0.000008	-2	-30.2	Ca-Cl
27	BLMH-15	6.76	309.81	2.47	98.84	4.80	10.22	171.95	1.34	0.06	5.28	8.48	99.19	0.711714	0.000009	-1.6	-29.6	Ca-Cl

The confined brine and intercrystalline brine (surface lake brine) from Wei et al. (1992) belong to the Cl-SO₄ types (Fig. 3). And the pH values of confined brine are ranging from 5.81 to 6.75 (Table 1). The intercrystalline brine from this study and anticlinal brine belong to Ca-Cl types (Fig. 3), and the pH values of intercrystalline brine range from 6.47 to 6.83 (Table 1). While the Yuqia River water is Na-HCO₃-SO₄ (Fig. 3).

4.2 H-O-Sr isotopic compositions of the brine in the MH

The δD and δ¹⁸O values of the confined brine and Ca-Cl type intcrystalline brine are shown in Table 1 and Fig. 5. Previously reported the δD and δ¹⁸O values of the Yuqia River water, snowmelt water, Cl-SO₄ types intercrystalline brine (surface lake brine and subsurface brine), confined brine (from Wei et al. (1992)), gravel-type brine in the western QB, freshwater lake water for the MH area, anticlinal brine in the LH and intercrystalline brine in Kunteyi are also listed in Table S4 and Fig. 5. The isotopic compositions in the MH are heterogenous among different brines. The δD and δ¹⁸O values confined brine in the MH range from − 17.80‰ to -27.40‰ for δD, and from 1.50‰ to 2.40‰ for δ¹⁸O (see Fig. 5, Table 1). While the δD and δ¹⁸O values of confined brine from Wei et al. (1992) are ranging from − 75.47‰ to -79.42‰ for δD, and from − 10.48‰ to -11.32‰ for δ¹⁸O. The H-O isotopic compositions of above two confined brines are distributed in two regions as shown in Fig. 5. The δD and δ¹⁸O values of intercrystalline brine in Kunteyi are similar with confined brine (in this study) for distributing area, which range from − 6.10‰ to -81.00‰ for δD, and from + 2.10‰ to -8.20‰ for δ¹⁸O (Fig. 5 and Table S4) (Li et al., 2019). The H-O isotopes of Yuqia River water, snowmelt water and groundwater from Zhu et al. (2016) are similar, and are distributed on both sides of the Global Meteoric Water Line (GMWL, δD = 8 × δ¹⁸O+10) (Craig, 1961). The H-O isotopes of anticlinal tectonic brine in LH is mainly distributed in the right side of the evaporation line (LEL, δD = 5 × δ¹⁸O-25) (Li et al., 2022), with an average of -39.03‰ and 2.03‰. The gravel-type brine in the western QB has higher δ¹⁸O values, and lower δD values, which suggest that the origin of gravel-type brine may be related to water-rock reactions (Li et al., 2022) (Fig. 5, Table S4).

The ⁸⁷Sr/⁸⁶Sr ratios of confined brine and intercrystalline brine from this study in the MH is shown in Fig. 6 and Table 1. The ratios of ⁸⁷Sr/⁸⁶Sr of confined brine is 0.711298 to 0.711522, with an average of 0.71145 (n = 8), which is slightly higher than that of anticlinal brine (0.71135, n = 1) (Cao and Wu., 2004; Shi et al., 2016), slightly lower than that of Yuqia River (0.71183, n = 1) (see Fig. 6 and Table S5) (Liu et al., 2022; Liu et al., 2023) and significantly lower than those of the ⁸⁷Sr/⁸⁶Sr ratios in hot spring and seepage in Da Qaidam, spring in Eboliang ((Liu et al., 2022; Liu et al., 2023). In addition, the ⁸⁷Sr/⁸⁶Sr values of intercrystalline brine (in this study) near Dezongmahai Lake and Balunmahai Lake range from 0.711662 to 0.711714 (n = 4). The ⁸⁷Sr/⁸⁶Sr ratios of gravel-type brine and oilfield water in the western QB range from 0.711794 to 0.711979 (n = 4) and from 0.711163 to 0.711937 (n = 8), respectively (Tan et al., 2011; Li et al., 2022) (see Fig. 6 and Table S5).

5.1 Genesis of the confined brine in Mahai Basin

5.1.1 Elemental constraint

Spatially, the confined brine in the MH is referred to as “underground bridge” because it fell in between shallow intercrystalline brine and deep gravel-type brine. The understanding on genesis of confined brine is crucial to interpret the formation and evolution of new gravel-type brines in this region. The K⁺ concentrations of confined brine in the MH is the highest, with an average of 7.41 g/L, among distinct type brines (including surface lake brine, subsurface brine, gravel-type brine and anticlinal brine). The anticlinal brine in LH anticlinal tectonic belt has a significant low K⁺ and relatively high Sr²⁺, Ca²⁺ concentrations, while the confined brine has a characterized by high K⁺ and relatively low Sr²⁺ concentrations, indicating that the source of solutes in anticlinal brine and confined brine is obviously diverse (Table S2-5). Both as deep brines, the K⁺ concentrations of the gravel-type brine are slightly lower than that of confined brine, with an average of 2.45 g/L. Yue et al. (2019) and Li et al. (2022) concluded that the formation of gravel-type brine is related to the leaching of salt layers, and therefore presents high Na⁺ and Cl⁻ concentrations than those in the confined brine (Table 1 and Table S3), this characteristics demonstrates that the formation of confined brine is independent of the leaching of evaporites in the study area.

In the Ca-SO₄-HCO₃ diagram, the intcrystalline brines (reported by Wei et al. (1992 and this study) are Ca-Cl type brine that plots near high Ca end member, parallel with the gravel-type brine and anticlinal brine (Fig. 3, Table S2-3) (Wei et al., 1992; Lowenstein et al., 1989; Zhang et al., 1993; Fan et al., 2018; Yue et al., 2019; Li et al., 2022). On the contrary, confined brine in this study plots near high Mg end member, and are Cl-SO₄ type brine (Fig. 3; Table 1). The Yuqia River water falls into the Na-HCO₃-SO₄ field. The route of Yuqia River water crosses from Na-HCO₃-SO₄ field to Cl-SO₄ field, and is closed to confined brine in the Ca-SO₄-HCO₃ diagram, illustrating Yuqia River water is main recharge source for the confined brine in the MH (Fig. 3).

In addition, the Li-B-K equivalent diagram indicates the confined brine in MH is relatively enriched in K and depleted in B (Fig. 4), which is different with anticlinal brine and shallow intercrystalline brine. Similarly, the confined brine has characteristics of low Sr²⁺ and Ca²⁺ (Table 1). These comparisons demonstrate that the river water provided abundant water volume for confine brine and enriched solutes of brines by evaporation and concentration in the basin.

5.1.2 H-O isotopic and ⁸⁷Sr/⁸⁶Sr ratio constraint

Hydrogen (H) -oxygen (O) isotopes can indicate the origin and evolution of water bodies (Fan et al., 2010; Clark and Fritz, 2013; Li et al., 2022; Song et al., 2023). The δ¹⁸O and δD values of the confined brine in the MH are distributed in different regions as shown in Fig. 5a. For example, the δ¹⁸O (-4.8‰ to -8.2‰) and δD values (-4.8‰ to -8.2‰) of confined brine in MH from Wei et al. (1992) are distributed on both sides of the GWML (δD = 8 × δ¹⁸O+10) (Craig, 1961), while the δ¹⁸O (1.5‰ to 2.4‰) and δD values (-17.8‰ to -27.4‰) from this study are plot near the local evaporation line (LEL) (δD = 5 × δ¹⁸O-25, Li et al., 2022). Therefore, the origin of confined brine in the MH needs to be discussed based on the spatial distribution.

Samples of confined brine from Wei et al. (1992) were collected near Dezongmahai Lake (Fig. 1b). This lake area is the main sink area of Yuqia River water, and there is abundant groundwater recharge (Wei et al., 1992; Zhu et al., 2015). The regression lines of δ¹⁸O-δD values of Yuqia River water, snowmelt water and groundwater in MH are all closed to the GWML(δD = 8 × δ¹⁸O+10) (Craig, 1961), indicating that confine brines reported by Wei et al. (1992) were influenced by the recharge of river water. This conclusion has been verified by plotting δ¹⁸O and δD field, near the GMWL, from underground water that collected in the lower reaches of Yuqia River (Zhu et al., 2015). On the contrary, confine brines in this study were collected in the southwest part of MH and far away the recharge route of river water (Fig. 1b). The positive H-O isotopes of confined brine in this study fall in vicinity of the LEL (δD = 5 × δ¹⁸O-25) (Li et al., 2016; Li et al., 2022; Ren et al., 2023) (Fig. 5a). This heavier characteristics of δD and δ¹⁸O values is consistent with intercrystalline brine or confined brine (the buried depth is between 10 m and 133 m, Table S5) in the Kunteyi (Fig. 5a), which are plotted on the right side of the LEL. These studies show the confined brine in MH experienced strong evaporation and concentration. In Fig. 5b, from the early Pleistocene to the Holocene, the δD and δ¹⁸O values gradually became positive, which further indicates that the confined brine experienced evaporation and concentration of primary river water. Therefore, the genesis of confined brine in MH is may related to evolution of depocenter of lake during the Quaternary.

The ⁸⁷Sr/⁸⁶Sr ratios of different waters plotted into three fields (see a, b, c in Fig. 6). One is a field, presenting high Sr isotopic ratios of hot springs in the Da Qaidam, cold spring in Eboliang anticline and terminal salt lakes in the northern QB. Liu et al. (2022, 2023) concluded in detailed that high radiogenetic Sr setting of springs affected by host rocks in the Qilian Mountain. The other is b field, taking on low Sr isotopic ratios and high Sr contents of deep anticlinal brine and oilfield brines in the western QB. Strong water-rock interaction in the western QB is the fundamental cause for high Sr contents in deep brines (Fan et al., 2007, 2010, 2018; Tan et al., 2011). The last is c field, all data are characteristics by low Sr contents and low Sr isotopic ratios. In this field, river water and confined brine in MH dominated this region. The Sr isotopic ratios of both waters are paralleled each other, although relatively high Sr contents contained in confined brine. Those characteristics reveal that the Yuqia River water is the main recharge source, which is confirmed by evidence lines of element and H-O isotopes.

5.2 Formation and evolution of confined brine

From the above discussion on the general chemical compositions and characteristics of H-O-Sr isotopes in confined brine in the MH, the origin of confined brine mainly originated from recharge of Yuqia River water. Different spatial locations of confined brine in MH should be influenced by different waters, such as river water and anticlinal water. In the north part of MH, river water diluted partly intercrystalline brine and confined brine. In the south part of MH, the water chemistry of intercrystalline brine and confined brine altered by the participation of a small amount of anticlinal brine.

Although the origin of confined brine in MH is accepted based on the above discussions, K and Mg contents in confined brine are varied in the different locations. This variation might be related to the evolution of salt lake. After the end of Pliocene, the MH and Kunteyi separated from a uniform lake and formed a closed basin (Wei et al., 1992). As a secondary basin in QB, MH has experienced the migration of depositional center during the Quaternary (Wei et al., 1992; Ma et al., 2018). From the early Pleistocene to the Holocene, the precipitation of salt minerals was different, evolving from sulfate minerals to chloride minerals and further potash salts (Fig. 1c), which may also be related to sediment center migration, evaporation and concentration (Wei et al., 1992). In the early Pliocene, there were two depositional centers in the MH, located in the northwest (zk5612) and southeast (zk7204) of the basin, which may be related to the northwest strike and southwest dip reverse fault uplift of the MH. In Fig. 7d, the K⁺ and Mg²⁺ concentrations of these two regions have the highest values. Meanwhile, there were scattered salt-lake facies in the near Dezongmahai Lake region (Fig. 7h), indicating this area was one of the depositional centers during in the early Pliocene. From the Middle Pleistocene to the Holocene, the migration of the depositional center in the MH changed from west to east and from south to north, and the northeastern (zk7606) part of the basin is the last depositional center. The migration of sediment center makes the content of K and Mg increase from west to east and from south to north (Fig. 7a, b, c) (Wei et al., 1992; Ma et al., 2018). From the Middle Pleistocene to the Holocene, the salt-lake facies, shallow lake facies and the lake front facies have undergone a depositional change similar to the migration of depositional center, such as from west to east and from south to north (Fig. 7e, f, g) (Ma et al., 2018).

In our collection area of confined brine (MH-01, MH-05, MH-07, MH-08, MH-015 sediment cores), the content changes of K and Mg have remained stable, indicating that the composition of confined brine in this area does not change with the migration of sedimentation center from the Middle Pleistocene to the Holocene. The enrichment of K, Mg and other elements in confined brine is mainly corresponded to evaporation and concentration (Fig. 7a, b, c).

Mahai Basin (MH) contained the second largest K-rich brine deposit in the Qaidam Basin, Qinghai-Tibet Plateau. The deep K-rich confined brine in the MH has the characteristics of high K, low Ca and Sr, which are distinct from the deep anticlinal brine with high Ca and Sr. The H-O isotopes of confined brines from the MH are scattered in the different regions as shown in δD-δ¹⁸O diagram, the confined brine from previous study distributed nearby river water is plotted near the global meteoric water line, while the δD and δ¹⁸O values of confined brine that localized far away river water in this study fall in vicinity of the local evaporation line, indicating the evaporation and concentration successively. In addition, Yuqia River water and confined brine from the MH show the similar ⁸⁷Sr/⁸⁶Sr ratios and both waters located on the low ⁸⁷Sr/⁸⁶Sr ratio and low Sr content field. These comparisons of elemental and H-O-Sr isotopic compositions demonstrate that river water is the main recharge source for confine brine in MH. The increase of K and Mg content in confined brine follows the rule from west to east and from south to north during from the Early Pleistocene to the Holocene. The evolution of basin restricts the enrichment of K and Mg. Combining with the sedimentary facies of MH, the northeastern part of MH is the last depositional center.

The deep high K confined brine is not only present in the MH but also distributed in the kunteyi Basin that serves as the secondary basin of QB. Hence, investigating the origin of deep brine is beneficial for enhancing comprehension of confined brine in the evaporitic basin.

Acknowledgement This work was co-supported by the Key Projects of the National Natural Science Foundation of China (Grant No. U21A2018) and the Natural Science Foundation of Qinghai Provincial (Grant No. 2021-ZJ-932). We thank Qingkuan Li, Haotian Yang and Tianyuan Chen for their help in the sampling and their helpful suggestions in the preparation of manuscript.

Author Contributions FY helped in sample collection, experiment, data analysis, and writing. QF done project design, discussion, and revision. HG helped in discussion and sample collection. SW helped in data collection and sample collection. JL and HB done discussion, revision, and sample collection.

Conflict of interest The authors declare that there are no know financial conflicts of interest or personal relationships that could have influenced the work reported in this paper.

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SupplementaryTable1.docx
Supplementary Table 1 Hydrochemical compositions of snowmelt water, meteoric water and Yuqia water in Mahai Basin. Note: the ‘TDS’ concentration unit is g/L; other ionic concentration unit is mg/L. Above data from Zhu et al. (2015). -: means no data.
SupplementaryTable2.docx
Supplementary Table 2 Hydrochemical compositions of inter-brine and anticlinal brine in Mahai Basin.Note: the major ionic (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, HCO₃^- ) and ‘TDS’ concentration unit is g/L; the trace ionic (LI⁺, B³⁺) concentration unit is mg/L.Inter-brine data from Zhang. (1987); Anticlinal brine data from Li et al. (2016); LH: Lenhu. -: means no data.
SupplementaryTable3.docx
Supplementary Table 3 Hydrochemical compositions of gravel-type brine in Mahai Basin. Note: the major ionic (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, HCO₃^- ) and ‘TDS’ concentration unit is g/L; the trace ionic (LI⁺, B³⁺) concentration unit is mg/L. Gravel-type data from Yue et al. (2019). -: means no data.
SupplementaryTable4.docx
Supplementary Table 4 δ¹⁸O and δD values of distinct water bodies in the western Qaidam Basin. Note: a: from Zhu et al. (2015); b and d: from Zhang, (1987); c ande: from Wei et al. (1992); f: from Li et al. (2022); g: from Fan et al. (2007); h: from Liu et al. (2022); j: from Li et al. (2016); MH: Mahai Basin; LH: Lenhu; Ⅱ～Ⅴ: Lenhu anticlinal tectonic members.
SupplementaryTable5.docx
Supplementary Table 5 ⁸⁷Sr/⁸⁶Sr values and Sr concentration of distinct water bodies in Mahai Basin and its surrounding area. Note: a: from Liu et al. (2022) and Liu et al. (2023); b: from Liu et al. (2022) and Liu et al. (2023); c: from Li et al. (2014); Anticlinal brine: the ⁸⁷Sr/⁸⁶Sr ratio from Cao and Wu. (2004), the Sr content from Shi et al. (2016); d: from Tan et al. (2011). -: means no data.

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Genesis and evolution of deep K-rich confined brine in Mahai Basin, Qinghai-Tibet Plateau

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Abstract

Figures

1 Introduction

2 Geology setting

3 Sample and analytical methods

3.1 Sample collection

3.2 Analytical methods

3.2.2 H-O isotopes and ⁸⁷Sr/⁸⁶Sr analyses

4 Results

4.1 Elemental concentrations of brine in MH

4.2 H-O-Sr isotopic compositions of the brine in the MH

5 Discussion

5.1 Genesis of the confined brine in Mahai Basin

5.1.1 Elemental constraint

5.1.2 H-O isotopic and ⁸⁷Sr/⁸⁶Sr ratio constraint

5.2 Formation and evolution of confined brine

6 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Genesis and evolution of deep K-rich confined brine in Mahai Basin, Qinghai-Tibet Plateau

Status:

Version 1

Abstract

Figures

1 Introduction

2 Geology setting

3 Sample and analytical methods

3.1 Sample collection

3.2 Analytical methods

3.2.2 H-O isotopes and 87Sr/86Sr analyses

4 Results

4.1 Elemental concentrations of brine in MH

4.2 H-O-Sr isotopic compositions of the brine in the MH

5 Discussion

5.1 Genesis of the confined brine in Mahai Basin

5.1.1 Elemental constraint

5.1.2 H-O isotopic and 87Sr/86Sr ratio constraint

5.2 Formation and evolution of confined brine

6 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

3.2.2 H-O isotopes and ⁸⁷Sr/⁸⁶Sr analyses

5.1.2 H-O isotopic and ⁸⁷Sr/⁸⁶Sr ratio constraint