Hydrochemical Characteristics and Sources of Lithium in Carbonate-Type Salt Lake in Tibet

Abstract

With the development of green energy, the demand for lithium resources has increased sharply, and salt lakes are an important source of lithium. In China, the Qinghai–Tibet Plateau has substantial lithium resources, and the Bangor Co Salt Lake is a typical Li-rich carbonate salt lake in northern Tibet. Research into the lithium source of the lake is of great significance for future sustainable industrial development. This article selects the Bangor Co Salt Lake recharge water system (river and cold spring water) and brine samples as the research objects, conducts hydrochemical composition and isotope testing of the water body, and determines the anions, cations, and B isotopes of the samples. This article uses the Piper three-line diagram, Gibbs diagram, and ion ratio relationship to study the hydrochemical characteristics and major ion sources of recharge water systems and salt lakes. The results indicate that the hydrochemical type has transitioned from the strong carbonate type to the moderate carbonate type from the recharge area to the lake area. The major source of ions in lakes is the weathering products of carbonate rocks, followed by evaporite and silicate solutes. The enrichment of lithium in salt lakes is mainly related to the contribution of rivers, followed by geothermal-related cold springs, and early sedimentary carbonate minerals may also make potential contributions. These findings provide a scientific basis for the mechanism of lithium enrichment, as well as for the further development and evaluation of lithium resources.

1. Introduction

Li is known as the “white oil” of the 21st century [1,2,3]. As a raw material, it plays an important role in the field of creating green and sustainable new energy and nuclear fusion. Therefore, lithium resources have been listed as key strategic resources by all countries in the world. Currently, lithium deposits can be categorized into three main types: salt lake brine lithium deposits, pegmatite lithium deposits, and sedimentary lithium deposits [4,5,6]. Salt lake brine lithium deposits have become the main focus of mining companies around the world by virtue of the deposits’ rich resources (accounting for 78% of the world’s reserves), the comprehensive utilization of multiple resources and low development costs [7,8].

The world’s lithium brine resources are mainly distributed on three plateaus: the Andean plateau in western South America, the western plateau in North America, and the Qinghai–Tibet Plateau in China. Among them, the Qinghai–Tibet Plateau has a large number of Li-rich salt lakes, particularly rich in lithium resources. It is an important salt-lake-type lithium metallogenic area, ranking third in the world [9,10]. The exploration of lithium resources on the Tibetan Plateau has yielded approximately 57.55 million tons (lithium carbonate equivalent), equivalent to 34% of the world’s brine-type lithium reserves. The lithium-rich salt lakes on the Tibetan Plateau are mainly concentrated in the Qaidam Basin and northern Tibet [11]. As reported by Zhang et al. [12], the Qinghai–Tibet Plateau is rich in lithium chloride resources, estimated to exceed 24.7 million tons. However, over approximately 80 years, the Qaidam Basin salt lake has gradually developed from potassium resources to the comprehensive development of potassium, lithium, and boron resources, while in Tibet, except for the Zabuye Salt Lake, the resources of the salt lakes have not been developed on a large scale. The research extent of the Li-rich salt lake in Qaidam Basin is much greater than that in Tibet [7,8,13,14,15,16,17,18,19,20]. Nevertheless, Tibet’s salt lakes are rich in lithium resources and a potential focus for future mineral resource development. Bangor Co Salt Lake contains a large amount of lithium and boron elements, with a lithium chloride reserve of 1.04 million tons, with a carbonate hydrochemical type. It is one of the largest lithium and boron mines in Tibet. However, there is currently limited research work on the Bangor Co Salt Lake, mainly focusing on the “boron” aspect, and there are some shortcomings in the studies relating to the origin of lithium in the lake. This seriously hinders the future exploitation and utilization of the lake.

This study involved the implementation of sampling and hydrochemical composition analysis on the brine and peri-lake recharge system of Bangor Co Salt Lake, supplemented with boron isotope data. The research objective is to reveal the evolution of salt lakes and the sources of salt-forming elements. The findings help in understanding the mechanism of lithium enrichment in salt lakes, and offer valuable insights for the future development and assessment of lithium resources.

2. Geological Background

Bangor Co Salt Lake is situated within the faulted basin of the southern portion of the middle Bangor Lake–Nujiang tectonic Zone (BNSZ), one of the four suture zones found on the Tibetan Plateau [21]. It is located within the Lhasa terrane and is positioned to the east of Siling Co, the second largest saltwater lake in China [22]. Bangor Co, in Naqu, is 75 km from Bangor County, conveniently located near major roads with direct access to the Qinghai–Tibet and Sichuan–Tibet highways, as well as the Qinghai–Tibet Railway [23].

Bangor Co’s elevation varies from 4520 to 4525 m, covering around 140 km³. The area experiences a cold, arid climate with an average temperature between −1 and 2℃. Rainfall is mostly in July and August, staying under 150 mm. Annual evaporation ranges from 2200 to 2500 mm. Notably, Bangor Co is a distinct lagoon separate from Siling Co [24]. Bangor Co has three lakes, Bangor Co I, Bangor Co II, and the now-dry Bangor Co III, formed via hydromagnesite deposition from west to east (Figure 1). The local terrain is shaped by tectonic forces and exogenic geological processes, categorized into medium-high mountains, low hills, alluvial and lacustrine plains, lakesides, and modern salt lakes [25,26]. The exposed strata around the lake area include limestone and marlstone from the Mesozoic Jurassic and Cretaceous, as well as sandstone and conglomerate from the Cenozoic Tertiary. They form multiple terraces. Additionally, Tertiary and Quaternary sediments and various salt minerals can be observed in the basin. The Tibet Plateau uplift caused differential uplift in the Bangor area, separating Bangor Co from Siling Co in the late Pleistocene. This formed the current faulted basin with tectonic systems north and south of the lake. Based on the topography and chronology, we can discern the fluctuations in the Bangor Co lake surface. It is characterized by a high water level during the last deglaciation, but since 4.2 ka BP, the lake surface of Bangor Co has gradually receded. Recent research indicates that the lake surface of Bangor Co began to rise steadily again from 1976 to 2020 [24,26,27,28,29]. The Graza River serves as the primary source of recharge for Bangor Co. It originates from Langqing Mountain to the south of Bangor Co. With a length of approximately 180 km, it ultimately flows into Bangor Co III Lake [23]. Both the northern and southern ends of Bangor Co Salt Lake feature several small cold springs. They are additional recharge sources for the salt lake.

3. Materials and Methods

In the winter of 2015, a total of 19 samples were collected from the Bangor Co area during a sampling expedition. These samples included 9 brine samples from Bangor Co Salt Lake, 1 water sample from Siling Co, 2 samples from cold springs near the northern part of the salt lake, 4 samples from cold springs in the southern part of the salt lake, as well as 4 river samples from the Graza River. The geographical distributions of these samples are shown in Figure 1, samples of the physical and chemical properties and sample types are shown in

Table 1. Field sample collection description, and measurement of temperature (T) and density, for the rivers, springs, and surface brine.

No.	T (°C)	pH	Density (g/cm3)	River Discharge (m3/s)	Description
BG-01	7.0	8.4	1.00	1.23	Graza river
BG-02	9.0	8.4	1.00	0.20	Tributaries of the Graza river
BG-03	15.0	8.9	1.00	1.40	Graza river
BG-04	13.0	8.0	1.00	small	Southern spring of Bangor Co
BG-05	15.0	8.0	1.00	small	Southern spring of Bangor Co
BG-06	8.0	8.5	1.00	small	Northern spring of Bangor Co
BG-07	16.5	8.1	1.00	small	Southern spring of Bangor Co
BG-08	5.0	7.9	0.97	small	Southern spring of Bangor Co
BG-09	3.0	8.0	1.00	small	Northern spring of Bangor Co
BG-10	14.0	9.2	1.08	-	Bangor Co II Lake brine
BG-11	15.0	9.0	1.09	-	Bangor Co II Lake brine
BG-12	13.0	9.0	1.08	-	Bangor Co II Lake brine
BG-13	14.0	9.0	1.14	-	Bangor Co III Lake brine
BG-14	13.0	9.2	1.12	-	Bangor Co III Lake brine
BG-15	16.0	9.1	1.10	-	Bangor Co III Lake brine
BG-16	13.0	9.1	1.12	-	Bangor Co III Lake brine
BG-17	14.0	9.0	1.11	-	Bangor Co III Lake brine
BG-18	14.0	9.1	1.12	-	Bangor Co III Lake brine
SLC-1	12.0	8.6	1.00	-	Siling Co Lake brine

Note: dash—no data. Source: Li et al. [21].

.

The samples were diluted to determine the concentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, Br⁻, B₂O₃⁻, and Li⁺. Ion analysis tests were carried out at the State Key Laboratory of Loess and Quaternary Geology (SKLLQG). Anion analysis was conducted using ion chromatography (ICS) with RSD (relative standard deviation) <1%. Cation analysis was tested using inductively coupled plasma optical emission spectroscopy (ICP-OES) with an RSD <1%. Trace elements in the water were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Nexion 300D; Perkin Elmer, Waltham, MA, USA). All concentration data are listed in

Table 2. Water chemical and B isotope data of the rivers, spring, and lake brine in Bangor Co area.

No.	Na+	K+	Ca2+	Mg2+	Cl−	CO32−	HCO3−	SO42−	Li	B	TDS	δ11B
	mg/L											(Mean ± 2SD ‰)
BG-01	34.68	5.33	33.55	19.62	5.97	-	214	35.25	0.06	0.81	354	4.62 ± 0.25
BG-02	40.30	4.94	35.65	23.25	9.35	-	234	49.10	0.09	0.85	401	−0.52 ± 0.52
BG-03	62.95	10.00	36.68	30.18	18.33	19.51	253	68.23	0.13	1.14	505	2.93 ± 0.18
BG-04	1128	175	6.45	93.00	841	390	967	1133	3.05	20.7	4803	−1.26 ± 0.17
BG-05	27.40	7.33	56.95	20.00	28.31	-	194	70.70	0.05	0.75	439	−3.30 ± 0.17
BG-06	86.93	22.42	46.30	49.58	36.52	-	556	308	0.51	5.05	1122	7.66 ± 0.19
BG-07	59.18	14.84	77.40	68.55	39.84	-	629	28.78	0.26	1.04	925	−5.96 ± 0.32
BG-08	23.22	5.96	37.93	41.13	4.91	-	343	20.30	0.08	0.73	487	−7.35 ± 0.40
BG-09	407	45.43	12.15	65.93	255	39.51	496	442	0.93	5.31	1789	−0.04 ± 0.19
BG-10	56,290	6941	25.60	431	20,017	5756	6794	66,540	125	818	165,554	1.07 ± 0.19
BG-11	58,880	6961	30.20	440	20,735	6231	6943	69,660	126	824	172,662	1.29 ± 0.15
BG-12	58,990	7137	49.30	454	20,678	6097	6931	68,530	141	823	171,658	1.45 ± 0.25
BG-13	68,050	8501	32.50	397	24,778	8231	9113	77,570	163	1028	200,148	3.59 ± 0.26
BG-14	75,080	9698	22.20	536	27,495	8414	9063	84,330	175	1052	218,204	1.48 ± 0.26
BG-15	95,140	10,730	37.30	528	28,438	9719	8840	123,600	198	1113	280,817	0.59 ± 0.17
BG-16	69,030	8152	373	465	22,259	6829	7638	87,390	141	876	205,099	0.56 ± 0.32
BG-17	73,430	9370	26.40	523	27,270	8389	9820	83,760	181	1049	216,167	0.54 ± 0.37
BG-18	78,420	7949	27.10	459	22,169	6841	7675	108,100	147	884	234,635	0.38 ± 0.18
SLC-1	2681	317	5.45	157	1931	510.9	-	3168	8.61	57.79	8967	0.10 ± 0.22

Note: dash—no data.

.

4. Results

4.1. Hydrochemical Characterization of Bangor Co Brine

The chemical properties of the water in Bangor Co were comprehensively examined and are documented in Table 1. The data from Table 1 reveal that the Bangor Co lake brine was alkaline with a hydrogen ion concentration (pH) value range from 9.0 to 9.2. The density of the brine samples varied between 1.08 and 1.14. Furthermore, the pH value of Siling Co lake water was found to be relatively low, measuring at 8.6, with a corresponding relatively low density of 1 g/m³. The percentage contribution of the major ions was calculated by converting the unit mg/L of concentration to meq/L, and finally, the average was taken as the final result. From the average concentration of the major ions, the dominant order of cations was Na⁺ > K⁺ > Mg²⁺ > Ca²⁺ in the Bangor Co brine. The dominant order of anions was SO₄²⁻ > Cl⁻ > HCO₃⁻ > CO₃²⁻. The information from Table 2 led to the conclusion that the dominant cation in the brine samples of Bangor Co Salt Lake was Na⁺, accounting for 92.19% of the cations, and the anions were dominated by SO₄²⁻, which accounted for 62.62% of the anions. In particular, the content of Ca²⁺ in BG-16 samples was significantly higher than that in the other samples, and the content of SO₄²⁻ was much higher than that of Cl⁻. This may be due to the input of the recharge water. The total dissolved cation and anion (TDS) levels in the Bangor Co lake brine was found to be significantly elevated, ranging from 166 to 281 g/L, with a mean value of 207 g/L. The highest concentration was observed at sampling site BG-15 in the southern region of Bangor Co III lake. The lowest concentration was recorded at sampling site BG-10 located on the west side of Bangor Co III Lake near a cold spring. Notably, Siling Co lake water and Bangor Co lake brine shared similar main ion characteristics. However, the Siling Co ion content was relatively lower, resulting in a TDS level that was two orders of magnitude lower. Consequently, Siling Co lake can be classified as a saltwater lake. Furthermore, the lake brine exhibited significant enrichment of lithium and boron, akin to the volcano–geothermal salt lakes on the Qinghai–Tibet Plateau [30]. Specifically, the average concentrations of lithium and boron in the Bangor Co lake brine were 155 mg/L and 207 mg/L, respectively.

4.2. Hydrochemical Characteristics of the Recharge Water System

The ion content of the recharge water system and the lake brine were different; not only was the content low, but also, the proportion of ions was different. The pH of the recharge water system was around 8 and was weakly alkaline. Most of the samples in the recharge system had a density of 1 g/m³, but one particular sample (BG-08) had a density of 0.97 g/m³. This density of less than 1 g/m³ was possibly caused by multiple factors, including the low temperature, pressure, etc. This sample originates from overflow water from BG-04, and when it flows out of the surface, it may cause some changes in the physical conditions. It can also be seen in Table 2 that the TDS was low; this might have been caused by one of the factors having a density of less than 1 g/m³. Additionally, Table 2 reveals that the major cation and anion content sequences in the recharge system were Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and HCO₃⁻ > SO₄²⁻ > Cl⁻ > CO₃²⁻. In the cationic system, the proportions of Na⁺, Mg²⁺, and Ca²⁺ were similar, at 37.87%, 33.48%, and 24.83%, respectively. The major anion was HCO₃⁻, accounting for 62.02%. The TDS was low and varied from 0.35 to 4.8 g/L, from a fresh water to salt water level. The TDS of BG-04, BG-06, BG-07, and BG-09 exhibited significantly elevated levels compared to the remaining samples.

4.3. Boron Isotopic Composition in Bangor Co Water System

The B isotopes in the Bangor Co water system were analyzed at the Institute of Earth Environment, Chinese Academy of Sciences (IEECAS). According to the research conducted by Li et al. [21], the δ¹¹B values of the river samples exhibited significant fluctuations ranging from −0.52‰ to 4.62‰. The δ¹¹B values of cold spring samples demonstrated exaggerated fluctuations (−7.35‰ to 7.66‰). On the other hand, the lake brine samples demonstrated stable δ¹¹B values ranging from 0.38 ‰ to 3.59‰.

5. Discussion

5.1. Evolution of Hydrochemistry in Bangor Co Area

Based on the hydrochemical chemistry classification developed by Kurnakov–Valyashko [31], the samples obtained from the area primarily exhibit characteristics of the carbonate type (as shown in

Table 3. Chemical classification of salt lakes.

Characteristic Coefficient		Kc (%)	Kn1	Kn2	Kn3	Kn4
Carbonate type	Strong	>29	≫1	≫1	≫1	≫1
	Moderate	8–29	≫1	≫1	≫1	≫1
	Weak	0.1–8	≫1	≫1	≫1	≫1
Sulfate type	Na sulfate	0–0.1	≥1	≥1	≫1	>/<
	Mg sulfate	–	≤1	≤1	≫1	>/<1
Chloride type		–	≪1	≪1	≤1	<1

$\mathrm{Kc}=\frac{{\mathrm{NaHCO}}_3+{\mathrm{Na}}_2{\mathrm{CO}}_3}{\mathrm{total} \mathrm{salt}}\times 100\%$ (calculated with mg/L or wt\%) ${\mathrm{Kn}}_1=\frac{{\mathrm{CO}}_3+{\mathrm{HCO}}_3}{\mathrm{Ca}+\mathrm{Mg}}$; ${\mathrm{Kn}}_2=\frac{{\mathrm{CO}}_3+{\mathrm{HCO}}_3+{\mathrm{SO}}_4}{\mathrm{Ca}+\mathrm{Mg}}$; ${\mathrm{Kn}}_3=\frac{{\mathrm{SO}}_4}{\mathrm{Ca}}$ and ${\mathrm{Kn}}_4=\frac{{\mathrm{CO}}_3+{\mathrm{HCO}}_3}{\mathrm{Ca}}$ (calculated with mg/L or wt\%).

and

Table 4. Characteristic coefficient of each sample in Bangor Co area.

Sample	Characteristic Coefficient
	Kn1	Kn2	Kn3	Kn4	Kc (%)
BG-01	1.06	1.28	2.53	2.09	86.75%
BG-02	1.03	1.30	2.72	2.15	83.17%
BG-03	1.06	1.13	3.19	2.96	25.53%
BG-04	0.70	1.03	1.63	1.12	67.45%
BG-05	3.57	6.50	161.43	88.8	78.50%
BG-06	1.41	2.41	6.71	3.93	38.64%
BG-07	1.08	1.14	2.82	2.66	32.14%
BG-08	1.11	1.43	3.40	2.62	42.68%
BG-09	1.55	3.06	30.72	15.54	68.25%
BG-10	8.15	45.42	1319.91	236.9	12.05%
BG-11	8.41	46.40	1174.02	212.93	14.87%
BG-12	7.87	43.32	707.73	128.54	12.81%
BG-13	10.22	66.37	1632.1	251.41	15.49%
BG-14	9.38	47.80	1969.29	386.52	13.46%
BG-15	12.22	58.83	1255.26	260.77	15.99%
BG-16	8.93	65.78	1923.19	261.14	14.24%
BG-17	9.81	48.68	1655.78	333.81	14.16%
BG-18	6.15	37.87	116.54	18.92	14.25%
SLC-01	1.26	6.16	304.98	62.56	8.88%

). However, BG-04 deviates from this trend with a Kn₁ value of less than 1, indicating its classification as a sulfate type. Considering the location of BG-04 and the surrounding geological formations, this special type may be attributed to the nearby sinter. Therefore, it may cause changes in the chemical composition of cold spring water. Additionally, by calculating the Kc value of the samples, it was determined that two river samples can be classified as strong carbonate types, and one can be classified as a moderate carbonate type. All cold springs can be classified as belonging to the strong carbonate type. For lake brine samples, they can be classified as moderate carbonate types. Observing the direction of water flow, it becomes evident that the hydrochemical type gradually transitions from strong carbonate to moderate carbonate as one moves closer to the lake brine. The hydrochemical types can also be differentiated based on the collection from the Bangor Co I Lake and Bangor Co II Lake brine samples. Notably, the Kc value of Bangor Co II Lake brine was found to be lower than that of Bangor Co I Lake brine. This indicates a greater presence of a certain component in Bangor Co II Lake brine. The evaporation and concentration of ore-forming materials carried via the recharge water system causes a significant mineralization process in salt lakes. The determination of water chemistry in a natural water body can be achieved by analyzing the relative content of major anions and cations. To comprehensively understand the evolution of the water body’s components, a Piper trilinear diagram can be employed as a visual tool for hydrochemical characterization and systematic discussion [32].

According to Figure 2, the water composition of Graza is identified as HCO₃⁻-Ca²⁺-Na⁺-Mg²⁺. The cold spring water exhibits similarities to river water. However, the southern cold spring (BG-04) and the northern beaded spring (BG-09) are of Na⁺-SO₄²⁻ type. This distinction may be attributed to the high levels of evaporation and concentration near the lake. The lake brine is classified as Na⁺-SO₄²⁻ type. Along the path from the recharge area to the salt lake terminal, the cations can be seen transitioning from the middle of the Piper trilinear diagram to Na⁺+K⁺. The anions evolve from (CO₃²⁻+HCO₃⁻)-SO₄²⁻ to Cl⁻-SO₄²⁻, and they demonstrate a greater inclination towards SO₄²⁻. The cations in the brine of Bangor Co Salt Lake exhibit a bias towards the (Na⁺+K⁺) end-member, and the anions display a bias towards the Cl⁻+SO₄²⁻ end-member. The water chemistry is classified as carbonate type, with lower concentrations of CO₃²⁻ and HCO₃⁻. This phenomenon indicates evaporation and concentration leading to the precipitation of carbonate minerals. This observation aligns with the known distribution patterns of salt minerals in the field [25]. Additionally, the cation triangulation diagram reveals that the evaporite weathering zone is situated closer to the Na⁺ and K⁺ side, and the carbonate weathering zone is closer to the Ca²⁺-Mg²⁺ side. In the anion diagram, the evaporite weathering zone is located proximate to the SO₄²⁻-Cl⁻ side, and the carbonate weathering zone is situated proximate to the HCO₃⁻+CO₃²⁻ side [33]. Consequently, it can be deduced that the predominant characteristic of river water and cold spring water is carbonate weathering.

Moreover, as the spring water samples approach the salt lake, the hydrochemical types exhibit a greater resemblance to the salt lake brine. This similarity is also reflected in the increasing ion content along the flow direction of the samples, because the proximity to the salt lake leads to a gradual reduction in water flow and an increase in evaporation and concentration.

5.2. Main Source of Ions in the Recharge System

The effectiveness of the Gibbs model in analyzing the impact of precipitation, rock weathering, and evaporation crystallization on natural water bodies has been acknowledged [34]. Therefore, this study employs the Gibbs model to delve deeper into the primary factors influencing the evolution of water bodies in the Bangor Co area. As depicted in Figure 3a, most of the river and spring water is concentrated in the upper and middle parts. This indicates that rock weathering and evaporative crystallization are controlling them. Additionally, two springs are observed to fall to the right of the dashed line in the figure, probably due to the dissolution of sodic minerals through leaching from the water. Figure 3b demonstrates that the majority of the recharge water system is situated above the dotted line. This suggests an abundance of HCO₃⁻ in the water body within this region. This enrichment can be attributed to the infiltration of hydromagnesite during weathering processes, and an enhancement in the HCO₃⁻ content. Additionally, the Bangor Co lake brine is positioned at the highest point and surpasses the TDS = 100,000 threshold of seawater. This indicates a significant influence of evaporation crystallization on the lake brine and a stronger evaporation effect compared to seawater.

The chemical composition of natural water bodies comes from a variety of sources, including rock weathering, atmospheric precipitation, and mixing with fluids. Therefore, it is feasible to infer potential sources in natural water by examining elemental proportions and correlations [35,36,37]. Consequently, the utilization of ion ratios can be employed to ascertain the primary source of major ions in the recharge water system.

The majority of Na⁺, K⁺, and Cl⁻ present in the water column can be attributed to the process of weathering and the dissolution of evaporite and silicate rocks [38]. The observation that river and cold spring water deviate from the 1:1 line (Figure 4a) implies that the sources of Na⁺ and K⁺ extend beyond evaporite dissolution, and potentially involve the dissolution of silicate minerals.

The (Cl⁻+SO₄²⁻)/HCO₃⁻ ratio can distinguish between carbonate rock weathering and evaporative rock weathering. According to Figure 4b, it can be seen that the river water samples and most cold spring samples are above the 1:1 line, and two cold spring samples are below the 1:1 line (BG-06 and BG-08). This suggests that the river water ions are affected by the weathering of carbonate rocks, and the cold spring experiences carbonate rock weathering and evaporite weathering. There are many salt minerals deposited near the salt lake. The samples of BG-04 and BG-09 were located near the salt lake, so mineral re-dissolution may have occurred.

The river water and cold spring samples have high Ca²⁺ and Mg²⁺ content, and the (Ca²⁺ + Mg²⁺)/HCO₃⁻ ratio can be applied to allow for an analysis of the source of Ca²⁺ and Mg²⁺. The sample sites fall near the 1:1 line (Figure 4c), indicating similar sources of Ca²⁺, Mg²⁺ and HCO₃⁻ in the water. Consequently, it is inferred that Ca²⁺ and Mg²⁺ in the recharge water system mainly originate from carbonate rocks. From Figure 4d, it can be seen that the sample is below the 1:1 line. This indicates that calcium and magnesium ions migrated from the water. We conclude that this phenomenon may be caused by the deposition of salt minerals. This phenomenon can be explained by the large amount of hydromagnesite deposited near the Bangor Co Salt Lake. Because a large amount of hydromagnesite was deposited on the terraces around the Bangor Co Salt Lake, this phenomenon is very special [39]. Hydromagnesite is rich in a large amount of magnesium.

In summary, carbonate weathering dominates in rivers, followed by silicate and evaporite weathering. Cold springs are also dominated by carbonate weathering, followed by the dissolution of evaporites, with a lower influence of silicate weathering.

5.3. Sources of Lithium in Brine

Based on previous research, the origins of lithium in the salt lake exhibit a diverse range and can be categorized into five distinct types. Firstly, weathering activates the lithium in the surrounding rock of the basin, and then, the lithium enters the salt lake with the fluid [40,41]. Secondly, deep upwelling geothermal fluids engage in water–rock reactions, leaching lithium, potassium, boron, and other elements from the rocks. With intensified fluid circulation, lithium and other elements gradually accumulate [42,43]. Thirdly, the lithium-containing salts that formed in the early stages are dissolved back into the salt lake through the action of the surface water, groundwater, and circulating brine [44,45]. Fourthly, magmatic activity contributes significantly to the lithium resources found in the salt lake [46,47,48,49,50]. Fifthly, the process of atmospheric salt replenishment also serves as a source of lithium in salt lakes [51,52,53]. The abundance of boron (B) and lithium (Li) in most salt lakes on the Tibetan Plateau suggest a potential common material source for these two elements. For example, Du et al. [54] suggested that the higher the content of these two elements in the salt lake, the stronger the correlation between the elements.

The concentration of lithium (Li) in the Bangor Co lake brine varies from 125 to 198 mg/L, with an average value of 155 mg/L. The concentration of boron (B) ranges from 817 to 1113 mg/L. Bangor Co Salt Lake is rich in Li and B, and the two elements have a good positive correlation (R² = 0.98) (Figure 5a). This implies that they have similar geochemistry. According to Li. [55], the δ¹¹B values of lake brine exhibit a wide range from 0.38‰ to 3.59‰, indicating the presence of multiple sources of recharge. The main water system of Bangor Co lake flows through granite rich in tourmaline, mainly composed of crustal materials [56,57]. These monzonitic granites exhibit significant enrichment of lithium, with concentrations ranging from 24.3 × 10⁻⁶ to 104.5 × 10⁻⁶ [57]. It even surpasses the levels found in Earth’s crust (20 × 10⁻⁶ [58]). This finding implies that the surrounding geological structure is an important source of lithium in Bangor Co Salt Lake. It is similar to the element B; given lithium’s propensity for leaching and migration with fluids, it is easily transported. Table 2 demonstrates that the concentration of lithium ions in the rivers of the Bangor Co region is several times higher than the average lithium concentration observed in global rivers (0.002~0.023 mg/L [59]). We consider the Graza River to be one of the sources of lithium in the salt lake, as it has been a source of supply for Bangor Co for a long time.

Combined with the geological background of the region, we believe that there are other sources of Li. This phenomenon is consistent with the results revealed by the B isotope in previous discussions. The heavy B isotope values (e.g., BG-06, 7,66‰) in cold spring water indicate that it is also one of the sources of lithium in salt lakes. Notably, BG-04 exhibits a higher lithium concentration (3.05 mg/L) and a negative B isotope value (−1.26‰). It may be closely related to deep hydrothermal fluids with generally lighter B isotope values [60,61]. This suggests that cold spring water may have undergone a geothermal fluid influence during its early stages, resulting in a heightened lithium enrichment. Intense hydrothermal activities, both low-temperature and high-temperature, are prominently observed in the vicinity of the Bangong Co–Nujiang deep fault zone. These hydrothermal fluids exhibit significant enrichment of lithium, boron, rubidium, cesium, and various other elements [30]. These fluids in the upward process undergo water–rock interaction with the surrounding rock, leading to Li-rich minerals. Many references, such as “Tibetan Salt Lakes”, indicate the occurrence of substantial geothermal activity during the initial phases in the Bangor Co area [62]. Extensive occurrences of lithium and boron hot springs have been observed in the southern region of Bangor Co, accompanied by the identification of early sedimentary deposits of travertine during the conducted fieldwork. The peripheral vicinity of the lake is characterized by pronounced and frequent seismic activity. This intense geological activity has facilitated the creation of migration pathways and the formation of lithium-enriched geological formations. Deep geothermal fluids rise towards the surface and near surface areas, and then, mix with shallow groundwater to form springs that flow into the lake. The interaction between water and surrounding rocks during this process leads to the leaching of lithium elements. These distinctive geological structural conditions, combined with salinity-forming elements, play a crucial role in the geochemical processes of boron, lithium, and other elements in the lake. Bangor Co is a representative example of a carbonate-type salt lake and has deposited a large amount of hydromagnesite. Li et al. [9] investigated the lithium content in hydromagnesite samples obtained from the Bangor Co region, resulting in measured values of 100 mg/L and 44 mg/L. It is worth noting that Li and Mg, due to their similar ionic radii, frequently undergo isostructural substitution in minerals [63]. Additionally, Bangor Co has experienced drying up followed by continuous expansion of its lake [27]. Furthermore, Li. [55] propose that the leaching of lithium from early sedimentary carbonate clay minerals may have occurred as a consequence of freshwater infiltration. Therefore, during the initial phase of salt lake development, lithium-rich hydromagnesite and other carbonate deposits underwent adsorption of a specific quantity of lithium. Subsequently, the dissolution and filtration of freshwater facilitated further migration of the previously enriched lithium into the salt lake in the later phase.

In order to provide additional verification regarding the origin of lithium in Bangor Co Salt Lake, we conducted a correlation analysis between the CO₃²⁻+HCO₃⁻ content and lithium concentration in recharge water (Figure 5b). The findings indicate a strong correlation between the recharge water regime and the lithium content (R² = 0.94). This is consistent with the previous discussion on the prevalence of carbonate weathering in the recharge water system. This suggests that the enrichment of lithium in Bangor Co is associated with the weathering of carbonate rocks. Consequently, the main source of lithium in Bangor Co is input from river and cold springs, followed by the contribution of early sedimentary mineral dissolution.

6. Conclusions

Bangor Co Salt Lake is a typical lithium-rich carbonate salt lake in northern Tibet, China. The research on the lithium source of the lake is of great significance for sustainable industrial development in the future. In this study, the following conclusions were obtained through the analysis of major and trace elements and boron isotopes in samples of brine, river water, and cold spring water from Bangor Co:

• The ion content in lake brine is much higher than that in the recharge water system. The recharge water samples in the study area changed from the recharge area to the lake area, and the hydrochemical type changed from the strong carbonate type to the moderate carbonate type. The salt lake was in the stage of carbonate precipitation, changing from the strong carbonate type to the moderate carbonate type. The hydrochemical evolution changed from rock weathering control to evaporation and crystallization control.
• Based on the geological background, anion/cation ratio relationships and boron isotopes in the Bangor Co area’s carbonate weathering products are mainly transported into the salt lake from the replenishment river and cold springs, followed by evaporative and silicate weathering materials.
• Geothermal-related cold springs and early sedimentary rocks and rivers have potential contributions to the lithium sources of salt lakes.