# Boron Isotope Geochemistry of the Lakkor Co Salt Lake (Tibet) and Its Geological Significance

Authors: B. K. Li; H. D. Cheng; H. Z. Ma
Journal: Geofluids
 (2022)

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## Abstract

The origin of boron in modern salt lakes has long been a controversial subject. This study focuses on the boron geochemistry of Lakkor Co salt lake, located on the Shiquanhe-Lakkor Co-Yongzhu-Jiali Ophiolite Mélange Zone (SYMZ). Lakkor Co is one of the main boron-rich salt lakes in the Tibetan plateau where the boron concentration ranges from 401 to 849 mg L-1. The boron isotope value (δ11B) of the carbonate deposit within the Lakkor Co region is -28.08‰. The boron isotope value of the Somme river decreases from -9.97‰ to -11.15‰ after it flows through the carbonate deposit region, indicating that the carbonate deposit in this region releases boron, which has a more negative isotope value. However, the boron isotope value of Bai Kok river increases from -15.63‰ to -13.76‰, when it flows through the Quaternary clay region in Ganga Co, indicating that the carbonate deposit undergoes a process of boron adsorption. It can be concluded that the Quaternary carbonate deposits may be a source of boron and also serve as a temporary boron reservoir, thus playing an important role in the formation of modern salt lake boron deposits in Tibet.


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## Body

## 1. Introduction

Tibet is a special area having the largest distribution of modern salt lakes in China, with 536 salt lakes of various types larger than 1 km2 [1]. Its boron reserves, with great potential economic value, account for 53% of China’s salt lakes and can be divided into solid and liquid types [1, 2]. Therefore, the source of boron in modern salt lakes is a hot topic of research. The existing studies [3–6] suggest that the boron-rich material in salt lakes mainly comes from the earth’s crust. Specifically, groundwater continuously absorbs heat from the surrounding rocks during deep circulation, interacts with them through the water–rock interaction, and then carries dissolved minerals from the rocks to the surface of the earth’s crust in the form of geothermal water. Another part of the boron component comes from surface run-off recharge from chemical weathering of bedrock in the source area.Boron is a soluble element and has a large fractionation, with boron ratios (δ11B) ranging from –70‰ to +60‰ in nature [7, 8]. The boron isotope fractionation signature can be used to trace the material origin of the boron and can also be used to indicate the migration and enrichment of the boron in saline waters [9–12]. With the development of analytical techniques [13–17], boron isotopes have been applied to the study of salt lake boron sources. Based on the isotopic composition of B and Li in the water source, previous studies [12, 18, 19] concluded that B and Li in the Da Qaidam Lake area have the same material source, both originating from deep water in the perimeter (thermal spring). In 2014, Wei et al. [20] performed boron isotope analyses of different phases (evaporite, carbonate, and silicate clay) in the sediments of the Dongtai Salt Lake in the Qaidam basin, using a split-phase dissolution method. Their results indicated that δ11B values of evaporites and carbonates vary in relation to evaporation processes, while δ11B values of silicate phases are related to variations in weathering intensity within the basin. Recent studies on Damxung Co salt lake in central Tibet found that the main material source of the lake’s boron is Quaternary carbonate deposit deposits [21]. As mentioned above, previous boron isotopic studies of Tibetan lakes are still relatively few, especially the systematic comparative study with the same tectonic setting. Hence, the preboron isotope comparison work would be significant for a better understanding of the material origin of boron and its processes in the source-sink.Taking the typical boron-rich salt lake of Lakkor Co as the research object, this paper analyzes the characteristics of element geochemistry and boron isotope geochemistry of the recharge water and lake surface water. Combined with the previous studies on Damxung Co salt lake and the data on boron content in Lakkor Co petrology, the supply, and mineralization sources of boron in Lakkor Co, the loss and potential sources of boron in the process of boron transportation are further discussed.


## 2. Geological Background

The Tibetan Plateau is a product of the collision of the Indian and Eurasian plates and consists of a collage of four near east-west tectonic blocks [22]. The Songpan-Ganzi, Qiangtang, Lhasa, and Himalayan blocks are located from north to south and are separated by the Jinsha River suture zone (JSSZ), the Bangong Lake-Nujiang Suture Zone (BNSZ), and the Yarlung Tsangpo River suture zone (YZSZ) (Figure 1).Figure 1
Map showing the location and tectonic setting of the study area (modified from Lü et al. [21]).The BNSZ is one of the most important suture zones on the Tibetan Plateau. It divides the suture zone to the Lhasa and Qiangtang plates. BNSZ is mainly composed of ophiolite zones, mixed rock zones, and deep-sea complex marble deposits, and it also contains many salt lakes, such as Bangor Co, Dong Co, and Chagcam Caka [1]. The Shiquanhe-Lakkor Co-Yongzhu-Jiali Ophiolite Mélange Zone (SYMZ) is located on the southern flank of the BNSZ. Jurassic basalts of SYMZ have geochemical properties of MORB-type (midocean ridge basalts) and IAT-type (island arc labradorite basalts) volcanic rocks and belong to intraocean arc type and midocean ridge type ophiolite mélanges [23].There are also numerous boron-rich salt lakes on the SYMZ, such as Mami Co, Lakkor Co, Jibu Caka, and Damxung Co. (Figure1) [1]. There are still some divergent views on the period of formation of the Lakkor Co ophiolite. It is accepted that the zircon U-Pb isotopic ages range from 190 to 124 Ma [24, 25], spanning the Jurassic-Cretaceous period. In addition, there is also some disagreement about the tectonic setting in which the Lakkor Co ophiolite was formed. The Tibetan Bureau of Geology and Mines suggests that the ophiolite is a tectonic overburden of the Dong Co ophiolite. Fan et al. [25] and Wang et al. [23, 26] proposed that it formed in a tectonic setting on a subduction zone, while Zhang et al. [24] suggested that it is a branch of the BMSZ.Lakkor Co, located in the SYMZ, is considered to have formed in the interarc basin [26]. Lakkor Co ophiolite mélange is exposed in the Lakkor Co area, and the lithology is mainly composed of metamorphic peridotite, gabbro, gabbro wall, pillow lava, plagioclase, and siliciclastic rocks (Figure 2). Lakkor Co area has undergone multistage evolution, multistage superimposition, and transformation to form the existing landform. Lakkor Co is located in the Lakkor back-arc basin in the near-trending tectonic fault basin, which is distributed linearly along the Jibucaka basin and belongs to the secondary structural unit of the Anglonggangri-Bango continental margin. There is more development of Quaternary carbonate deposit in the Lakkor Co basin and its river upstream (Figure 2).Figure 2
Geological overview of the Lakkor Co area and sampling location map (modified from Dong et al. [30]).The Lakkor Co salt lake is 30 km to the south of Gaize county [1], with the lake surface water area of 95.67 km2, east-west spread, east-west length of 12-17 km, and north-south width of 5-8 km. Its brine water depth mainly ranges from 1 to 29.2 m, and the total brine water volume is about 1470×106m3. According to a detailed investigation report of the region [27], the Lakkor Co salt lake has potassium resources (KCl) of 744×104t, lithium resources (LiCl) of 247×104t and boron resources (B2O3) of 382×104t. Bai Kok and Somme rivers are the main recharge rivers of the Lakkor Co salt lake (Figure 2). Somme river has an average flow of 2.15 m3/s, and the Bai Kok river has an average flow of 1.00 m3/s. The Bai Kok river flows through two small lakes, Ganga Co and Ziza Co, with an average retention time of 37.97 days for Ganga Co and 86.21 days for Ziza Co [28]. The average annual temperature in the area is below 0°C, with large annual and daily temperature differences. The annual precipitation ranges from 150 to 220 mm, with an average of 160 mm, mainly in the rainy season from July to August. The average annual evapotranspiration is 1765 mm [29]. The upper reaches of the Bai Kok river form two temporary lakes, Ziza Co and Ganga Co. The western part of Lakkor Co is Jibu Caka, which is also a boron-rich salt lake with a slightly higher water level than Lakkor Co. Currently, there is no surface hydraulic connection between them.


## 3. Materials and Methods

A total of 21 samples were collected, including 10 Lakkor Co salt lake brine samples, 3 Jibu Caka brine samples near Lakkor Co, 2 water samples from small freshwater lakes within the chain of lakes upstream of Lakkor Co, 1 cold spring sample around the lake, 4 river samples (collected before and after the river flows through a zone of Quaternary carbonate deposit deposits), and 1 carbonate deposit sample. All samples were collected in the winter of 2015 during one sampling expedition (Figure2). The elemental analysis results and locations for all samples are shown in Table 1 and Figure 2.Table 1
Field sample collection, description, and measurement of temperature (T), pH, and density for different sample types.
No.Na+K+Ca2+Mg2+Cl-CO32-HCO3-SO42-BLi+Br-TDSδ11Bmg/Lg/L(mean±2SD‰)BJR-0158.057.5612.6633.4525.2124.88297.0772.813.450.440.050.54−13.76±0.18BJR-02104.508.845.5039.7527.410236.07173.83.710.5000.61−15.63±0.19SMR-0169.847.7926.7226.6716.190303.02121.82.541.080.150.58−11.15±0.21SMR-0281.137.8930.7127.1415.150327.33143.22.691.180.130.64−9.97±0.22LGCS-0115.906.9152.6919.678.780312.9428.541.270.150.130.47-18.29CGC61.678.737.7736.4123.7553.65270.2984.023.520.480.050.56−14.91±0.23ZZC14.904.5542.3817.706.600278.2330.783.960.110.220.40−15.48±0.11JB-0117150.003597.5057.431013.7530379.031507.16011895.00683.58334.00068.14−4.01±0.42JB-0217557.503692.5069.001038.2530930.371460.83012147.50695.57590.75069.73−4.52±0.25JB-0318127.503832.5051.481076.7530967.981480.34013035.00744.77352.00071.32−4.42±0.27LG-0110465.001823.0011.50553.007813.761582.76012585.00517.99186.600.0436.69−6.42±0.27LG-0217030.002881.0019.05887.6013561.012377.80020880.00804.21300.20060.53−6.24±0.29LG-0315960.002673.0025.39826.8012825.182450.97019470.00789.63278.90057.06−6.17±0.21LG-0417040.002901.0049.32892.8013484.402554.62020970.00849.47299.20060.93−5.01±0.31LG-058445.001483.0029.11443.156531.941175.4909865.00401.56144.653.2529.41−6.11±0.27LG-0616890.002844.0016.99881.0013549.352182.70020700.00842.38298.20060.08−6.37±0.19LG-0714720.002634.0028.23807.5013472.352658.26018900.00772.64272.40055.98−6.44±0.19LG-0813950.002454.0024.97755.9013123.822652.17017330.00709.71256.00052.83−6.00±0.31LG-0913750.002433.0023.05751.9013502.282542.42017270.00711.00253.901.2152.82−5.68±0.23LG-1013600.002392.0022.98735.6013590.852575.35016630.00683.13249.40052.00−6.75±0.27LGS-01370 ppm100 ppm−28.08±0.11Brine samples were diluted and the concentrations of Na+, Mg2+, K+, Ca2+, Cl-, and SO42- were determined, and the dilution rate of brine samples was 50 times. All analyses for major ions in this study were performed at the Qinghai Institute of Salt Lakes (ISL), Chinese Academy of Sciences, according to the research procedures of ISL [31].The concentrations of Ca2+, Mg2+, and Cl- were determined using chemical titration. For K+ and SO42-, their concentrations were obtained using the gravimetric analysis. The concentration of Na+ was estimated via subtraction, and the amount of B2O3 and Li was analyzed by plasma spectrometry (ICAP6500DUO, USA). Br was analyzed by the UV-Vis spectrophotometer (TU-1810, China). Analytical uncertainties are less than ±5% for B2O3, Li, and Br, less than ±2% for Na+, and less than ±0.5% for other elements like Ca2+, Mg2+, Cl-, K+, and SO42-.B and Li concentrations of samples were determined using an ICAP 6500 DUO ICP-OES system (Thermo Fisher, America) at Qinghai Institute of Salt Lakes, Chinese Academy of Sciences (ISL). The analytical precision was better than ±2%. All data are listed in Table1.Chromatographic isolation of B was carried out using Amberlite IRA 743 resin (80-100 mesh), packed into a customized PDF microcolumn with an internal diameter of 3.2 mm and a column length of 2 cm. To achieve B purification, the samples were passed through columns, and the total elution volume (about 0.55 ml) was eluted with 3 mL of 0.5 M HNO3. The procedures were according to the literature reports [17, 32]. The concentration of boron for both SRM 951 standard and sample solution was 50 ppb, with a high 11B intensity of ~0.7 V/50 ppb, due to the performance of X-cone.All B isotope measurements were carried out at the State Key Laboratory of Loess and Quaternary Geology (SKLLQG), Institute of Earth Environment, Chinese Academy of Sciences (IEECAS). The measurements were performed using a NEPTUNE Plus MC-ICP-MS system (Thermo Finnigan, America), which enabled a static measurement of m/z 10 and m/z 11 with the low-mass Faraday cup (L4) and the high-mass Faraday cup (H4). The major challenge of boron isotope measurements by MC-ICP-MS is a strong B memory effect, which requires long washout time. He et al. [17] found that an acidic NaF solution can be used to reduce boron signals to blank levels within 4 min. Therefore, an acidic NaF solution was used as a rinse solution.Theδ11B value of the samples can be calculated by the following equation:
(1)δ11B=2×B11/B10nB11/B10NIST951n−1+B11/B10NIST951n+1−1×1000,where NIST951n−1 and NIST951n+1 indicate the standard measured values before and after the sample. A standard sample was tested before testing the sample, and a standard sample was also tested after testing the sample. Two reference samples were the corrected test samples. The 11B in the external precision for δ11B values is 0.2‰ (2SD), estimated by duplicate and replicate analyses of a brine sample and SRM 951, with different concentrations varying from 60 to 800 ppb.


## 4. Results and Discussion

### 4.1. Results

pH values in different rivers and springs range from 7.9 to 8.4 and are generally slightly alkaline (Table2). The brines are slightly alkaline, with pH from 9.0 to 9.2, showing that Na+, Cl-, and SO42- are the dominant ions, and water types are Na-Cl-SO4 or Na-SO4-Cl (Figure 3). By contrast, river water and spring water are of Ca–Mg–HCO3 or Mg–Ca–HCO3 types. The freshwater lakes are also of Ca–Mg–HCO3 or Mg–Ca–HCO3 types.Table 2
Water chemical data of the rivers, spring, lake brine, and solid boron ore in Lakkor Co area.
No.Sample typeT (°C)pHDensityRiver discharge (m3/s)DescriptionBJR-01River98.411.0Bai Kok riverBJR-02River138.011.0Bai Kok riverSMR-01River57.90.972.17Somme riverSMR-02River158.012.13Somme riverLGCS-01Spring128.21SmallSpring waterGGCLake water158.911.0Ganga Co Lake waterZZCLake water78.411.23Ziza Co Lake waterJB-01Lake brine118.61.06—Jibu Caka Lake brineJB-02Lake brine68.61.06—Jibu Caka Lake brineJB-03Lake brine78.71.06—Jibu Caka Lake brineLG-01Lake brine139.11.12—Lakkor Co Lake brineLG-02Lake brine139.21.12—Lakkor Co Lake brineLG-03Lake brine149.01.14—Lakkor Co Lake brineLG-04Lake brine149.01.11—Lakkor Co Lake brineLG-05Lake brine149.11.12—Lakkor Co Lake brineLG-06Lake brine169.11.1—Lakkor Co Lake brineLG-07Lake brine139.01.08—Lakkor Co Lake brineLG-08Lake brine149.21.08—Lakkor Co Lake brineLG-09Lake brine1591.09—Lakkor Co Lake brineLG-10Lake brine128.61—Lakkor Co Lake brineLGS-01Carbonate deposit (mud stone)Figure 3
Piper plot showing the water chemical variability of Lakkor Co area.The boron concentrations in Lakkor Co lake water are 401.56-849.47 mg L-1, and their δ11B values are -6.75 to -5.01‰ (Figure 2). Samples from Jibu Caka lake water (JB-01 to 03) to the west of Lakkor Co have B concentrations of 683.58 to 744.77 mg L-1 with δ11B values of -4.52 to -4.01‰ (Table 1 and Figure 2). The water from the chain of lakes upstream of the Lakkor Co displays the B concentration of 3.52 mg L-1 (sample No. CGC) and 3.96 mg L-1 (sample No. ZZC), and the δ11B values of -14.91 and -15.48‰, respectively (Table 1 and Figure 2). Bai Kok river and Somme river, as the main recharge rivers of the Lakkor Co salt lake, exhibit different boron isotopic signatures. The Bai Kok river to the south of Lakkor Co salt lake has the B concentration of 3.45 mg L-1 and 3.47 mg L-1 and the δ11B values of -15.63 and -13.76‰. The Somme river to the east of Lakkor Co salt lake exhibits the B concentration of 2.54 and 2.69 ppm, and the δ11B values of -11.15 to -9.97‰. Carbonate deposit is mainly composed of hydromagnesite and aragonite, as well as small amounts of muscovite, calcite, quartz, and albite. The δ11B value of the carbonate deposit is -28.08‰.


### 4.2. Mechanisms Controlling Source Water Chemistry of Lakkor Co

The scatter diagram method reported by Gibbs [33] illustrated three important natural mechanisms controlling the major ion chemistry of the water including water–rock interaction, evaporation, and atmospheric precipitation. The TDS concentrations were plotted against the weight ratios of Na+/(Na+ + Ca2+) for cations and the weight ratios of Cl−/(Cl− + HCO3−) for anions (Gibbs, 1970; Figure 4). It was found that majority of the brine samples fell into evaporation dominant area, indicating the important role of evaporation on brine chemistry, and the source water, including spring water, river water, and small lake water, fell out of the water–rock interaction, evaporation, and atmospheric precipitation area, indicating that the source of chemical elements in the water may be related to the early deposited.Figure 4
Gibbs’ diagram of the Lakkor Co region.In the ratio of Na/Cl and (Ca2+ + Mg2+)/(HCO3- + CO3-) in the water of Lakkor Co area. The (Ca2++Mg2+) and (HCO3- + CO3-) molar ratio is distributed around the 1 : 1 line (Figure 5), indicating that the source of Ca2+ + Mg2+ leaching earlier carbonate strata. The region deposited a large number of carbonate minerals [1] (Figure 6). Somme river region has early deposited carbonate stratigraphy, and the carbonate deposition of Gangga Co is being formed. Therefore, we believe that the weathering and formation of carbonates have an important influence on the hydrochemical properties of regional waters.Figure 5
Molar ratio of Na/Cl and (Ca2+ + Mg2+)/(HCO3- + CO3-) in water of the Lakkor Co region.Figure 6
Carbonate deposits of Lakkor Co area on Google Earth.


### 4.3. Distribution Characteristics of [B] andδ11B within the Watershed of Lakkor Co Lake

The Bai Kok river is recharged from the south with boron levels of 3.45 ppm and 3.47 ppm and the Somme river is recharged from the east with boron levels of 2.54 ppm and 2.69 ppm. The two lagoon waters in the Bai Kok river basin have similar boron levels to the Bai Kok river. The boron content of the spring water is comparable to or higher than that of the river water. The higher boron content of the river may be related to the lithology of the outcrop. The boron content of the lake water ranges from 401.56 ppm to 849.47 ppm, due to the recharge and the degree of concentration, as evidenced by the mineralization. The carbonate deposit also exhibits a high boron content of 0.037% and a lithium content of 0.010%. The boron content of the spring water collected in this work is 1.24 ppm. A previous research [31] found that the elemental boron content of the spring water outcropping from Lakkor Co is 3.24 to 21.39 ppm. Overall, the Lakkor Co recharge water bodies all exhibit high boron concentrations.The boron isotopic compositions of the Lakkor Co salt lake recharge and lake waters and the carbonate deposit all show negative values, with the carbonate deposit (δ11B=−28.08) being lower than the spring water (δ11B=−18.29), river water (-9.97 to -15.63), and lake surface water (-5.01 to -6.75). The main reason is likely the adsorption of boron by the clay minerals. In the Lakkor Co salt lake region, clay adsorption of boron is very common. The Bai Kok river upstream of the two small lake basins and Somme river flowing area have carbonate deposit development (Figure 2). In the northwest of Lakkor Co, there is enrich-boron mineral named graphite precipitation due to clay adsorption and capillary evaporation in the surface layer [1] (Figure 2). This is mainly due to the adsorption of clay minerals, the coprecipitation of carbonates, and the deposition of evaporite minerals during the preferential entry of 10B into the solid phase, which makes the boron isotopic composition of the water column positive [13, 34–36].Boron isotope ratios of the Bai Kok river in the south, ranging from−15.63±0.19 to −14.91±0.23 to −13.76±0.18, are lower than those of the Somme river in the east, ranging from −11.15±0.21 to −9.97±0.22. The overflow spring in this area has the lowest B isotope ratio. This is due to the fact that Somme river is mainly developed in the area of the Lakkor Co ophiolite development, while the Bai Kok river weathering rocks are more distant from the BNSZ.Lakkor Co regional water is not only rich in boron elements but also rich in lithium elements. Positive correlations have been determined among Li and B in different water types, suggesting common sources and similar geochemical behavior for Li and B ions (Figure7).Figure 7
Boron-lithium elemental ratio chart.


### 4.4. Calculated Amount of Boron Resources Input to Salt Lakes by Rivers

As noted earlier, the current boron reserves at Lakkor Co are3.82×106 ton. Somme river and Bai Kok river are the main recharge rivers flowing down into Lakkor Co lake. The total amount of B input into the lake by Somme river and Bai Kok river water can be calculated based on water-salt balance, according to the following equation:
(2)ΣW=S×C×10−6,.whereΣW is the annual total amount of B input into Lakkor Co lake by the rivers in tons, S is the flow of river water, and C is the annual concentration of B in the river water. The average flow velocity of the Somme river is about 2.15 m3/s, and the B content is about 2.60 mg L-1. The calculated results show that the annual total B input into Lakkor Co lake by Somme river is about 176 tons. The average flow velocity of the Bai Kok river is about 1.0 m3/s, and the B content is about 3.58 mg L-1. The calculated results show that the annual total B input into Lakkor Co lake by Bai Kok river is about 112 tons. The input from both rivers is therefore roughly estimated to be 288 tons per year. For the overall reserves of boron (382×104 tons), the riverine recharge is considerable.


### 4.5. The Source of Boron in the Lakkor Co Area

As mentioned above, the potential sources of boron in the salt lake are surface weathering, deep hydrous rock action, and Quaternary carbonate deposits. The riverine boron input is estimated to be the main source of boron recharge in the Lakkor Co region.


#### 4.5.1. Carbonate Deposit

There is a large amount of carbonate in the Lakkor Co region. The boron content in carbonate deposit of Gangga Co is 370 ppm. Carbonate deposit (LGS-01) is mainly composed of hydromagnesite and aragonite, as well as small amounts of muscovite, calcite, quartz, and albite. Keren and O’Connor [37] noticed that the effect of excessive boron accumulation by Ca-bearing clay might partly be caused by the reaction of scattered CaCO3 with borate-rich solutions that lead to the precipitation of Ca-bearing borates. Therefore, the high boron content in this carbonate deposit may be caused by the precipitation of small amounts of boron minerals. The effect of regional widespread [1] (Figure 6) carbonate deposition on B migration is significant.The study found that the Bai Kok river changes in boron isotope ratios as it flows through areas of carbonate deposit, which is exposed at Ganga Co, from the inlet to the lake to the outlet of the river. The B isotopic ratio changes from−15.63±0.19 to −14.91±0.23 to −13.76±0.18, while Ganga Co hosts a range of clays which are largely distributed in the lake and along the lake shore. Field investigations show that the average retention time of water bodies in Ganga Co is about 38 days [28]. This indicates that the boron element has sufficient time to be adsorbed by the clay and for fractionation to occur in this lake system. The boron isotopic value for carbonate deposit (LGS-01) at the edge of Gangga Co is -28.08.Previous research shows that the isotope fractionation factor between boron adsorbed on sediments and dissolved boron in seawater is 0.975 atpH=8.2 [35]. The fractionation factor between the coprecipitated boron in carbonate and dissolved boron is 0.974 at pH=8.2 [38, 39]. In this study, the average δ11B value of carbonate deposit is –28.08‰, and the average δ11B value of surface brine is –14.91‰. This translates to a fractionation factor (α=Rcarbonatedeposit/Rbrine) of 0.986, which is in good agreement with the values for stromatolite (0.990) and travertine (0.990). However, the Somme river shows a decrease in B isotope ratios from -9.97 to -11.15, because it flows through the carbonate deposit region which has higher exposure of carbonate deposit than the river. The carbonate deposit acts as a B source in this area.Many scholars have studied the equilibrium fractionation of boron isotope adsorption between illite and seawater [40–42]. Their results showed that 10B is more likely to be adsorbed on the clay surface during the adsorption, and the adsorption process may be partially reversible. This indicates that boron from the lighter carbonate deposits was partially released during the passing of the Somme river water through the Quaternary carbonate deposits, allowing the boron isotopic composition of the Somme river water to be prioritized. Besides being highly mobile, boron can easily accumulate in secondary phases, mainly in clay minerals and in Fe oxides and hydroxides. These newly formed solids can not only reincorporate a large fraction of B inherited from parent minerals but can also incorporate boron from the coexisting solution [43–46]. Damxung Co is another boron-rich salt lake in Tibet, where the carbonate deposits present are the main sources of boron [21].Therefore, it can be concluded that adsorption and desorption processes of boron in Quaternary carbonate deposit were concurrent and had a nonnegligible role in the formation of salt lake boron ore (Figure8).Figure 8
Schematic diagram of the process of boron release from carbonate deposit.


#### 4.5.2. Boron Resources Derived from Underground Rock Weathering and Water–Rock Interaction

By studying the hydrogen and oxygen isotopes and lithium content of spring water and river water, Dong [30] found that the lithium resources of Lakkor Co are derived from two sources, namely bedrock weathering and deep water–rock recharge. The spring water of Lakkor Co comes from meteoric precipitation. The boron contents of Lakkor Co river and the two main rivers are very high, and the spring also shows a high boron content [30]. Therefore, it is inferred that the boron source of Lakkor Co is similar to the lithium source, which has two forms of bedrock weathering and deep water–rock interaction. The boron content data of rocks in the Lakkor co area are shown in Table 1.The geological survey results of Lakkor Co area show that the contents of boron and lithium in the bedrock of this area are relatively high, ranging from 4.35 to 28.2 ppm [47]. The enrichment of boron and lithium in the salt lake of Lakkor Co area is mainly related to the boron- and lithium-rich rocks formed by structural evolution. Since boron can enter the lattice of serpentinites well, serpentinites are the most important host rocks of boron [48]. The large amount of serpentinized ultramafic rocks on the ocean floor is an important reservoir of boron, with boron contents of about 10×10−6 to 91×10−6 (Figure 2) [49, 50]. Due to the tectonic evolution of the SYMZ, boron-rich rocks are exposed at the surface or deposited near the surface, which is the main reason for the formation of more boron-rich salt lakes on the SYMZ.Wang et al. [51] studied the characteristics of high-temperature lithium-rich geothermal water in the southern Qinghai-Tibet Plateau, which is mostly neutral or alkaline NaCl-type water and rich in characteristic elements such as B, Li, Rb, Cs, and F. Tan et al. [52] used hydrogen-oxygen isotope data to explain the circulation of hot water underground. In most of the high-temperature geothermal systems of Tibet, they found rapid circulation of groundwater and upwelling of residual magmatic water. Moreover, in most cases, granite outcrops were observed within a short distance from the hot springs. Dong [30] studied the isotopes in water bodies of the Lakkor Co and found that the water of both springs and river water is from atmospheric precipitation. Therefore, the main source of boron and other substances in the springs here is still the rapid circulation of groundwater.Therefore, it can be concluded that the sources of boron in the Lakkor Co salt lake are related to the tectonic activity caused by the formation of the BNSZ when boron-rich rocks are exposed or enriched near the surface, as well as the deep water–rock interaction, bedrock weathering, and atmospheric precipitation.


### 4.6. A Comparative Study of Boron Isotopes in Boron-Rich Salt Lakes of Lakkor Co and Damxung Co

Lakkor Co and Damxung Co share the same geological setting (Figure1), i.e., the same island arc environment as the SYMZ ophiolite mélange. B is a fluid-active element, preferentially entering fluids with significant isotopic fractionation (up to 40‰). 11B tends to be enriched in fluid phases [53–55], especially in primitive oceanic basalts (δ11B=−12‰to0‰) [56], seawater (δ11B=+39.6‰) [57], and hydrothermal fluids. Thus, the altered oceanic crust is enriched with heavy B isotopes. The δ11B ranges from -4‰ to +25‰, with a mean value of +3.4‰ [58]. The δ11B of serpentinized ultramafic rocks on the ocean floor generally ranges from +7‰ to +20‰ [49, 55]. Some studies have suggested that heavy boron isotopes in volcanic arcs are mainly derived from the combined contribution of subduction erosion and forearc serpentinized mélange [55, 59, 60]. However, recent studies have shown that not all mantle wedges have high δ11B values, and the variation of B isotopes in the mantle is within the range of -15. 3‰ to 9.7‰. This is mainly related to the fact that fluid present in the sediments reduces δ11B values [61]. The boron isotopes of the recharging waters in the SYMZ volcanic arc are negative, which suggests that the boron isotopes of the Linguistic Bedrock are also relatively negative.In addition, it can be seen from the data that the B isotope ratios of the lake water of Damxung Co are significantly lower than those of the recharge waters (Figure9), while the carbonate deposit has lower B isotope ratios. This is consistent with the results of related previous studies and demonstrates that the carbonate deposit is the main boron source for the lake in Damxung Co [21]. However, it can be seen that, although the carbonate deposits in the area exhibit lower B isotope ratios, the B isotope ratios of the recharge waters are similar. Moreover, the B isotopes of the lake water are more positive than both of the above. This result means that clay adsorption occurs during the flow of the river and leads to B fractionation. Moreover, precipitation of borate minerals occurs on the shore of the lake, which is also the reason for the higher boron isotope ratios of the lake water in Lakkor Co than the recharge water.Figure 9
Comparison of boron isotopes between Lakkor Co and Damxung Co (boron isotope data of Damxung Co are taken from Lü et al. [21]).


## 4.1. Results

pH values in different rivers and springs range from 7.9 to 8.4 and are generally slightly alkaline (Table2). The brines are slightly alkaline, with pH from 9.0 to 9.2, showing that Na+, Cl-, and SO42- are the dominant ions, and water types are Na-Cl-SO4 or Na-SO4-Cl (Figure 3). By contrast, river water and spring water are of Ca–Mg–HCO3 or Mg–Ca–HCO3 types. The freshwater lakes are also of Ca–Mg–HCO3 or Mg–Ca–HCO3 types.Table 2
Water chemical data of the rivers, spring, lake brine, and solid boron ore in Lakkor Co area.
No.Sample typeT (°C)pHDensityRiver discharge (m3/s)DescriptionBJR-01River98.411.0Bai Kok riverBJR-02River138.011.0Bai Kok riverSMR-01River57.90.972.17Somme riverSMR-02River158.012.13Somme riverLGCS-01Spring128.21SmallSpring waterGGCLake water158.911.0Ganga Co Lake waterZZCLake water78.411.23Ziza Co Lake waterJB-01Lake brine118.61.06—Jibu Caka Lake brineJB-02Lake brine68.61.06—Jibu Caka Lake brineJB-03Lake brine78.71.06—Jibu Caka Lake brineLG-01Lake brine139.11.12—Lakkor Co Lake brineLG-02Lake brine139.21.12—Lakkor Co Lake brineLG-03Lake brine149.01.14—Lakkor Co Lake brineLG-04Lake brine149.01.11—Lakkor Co Lake brineLG-05Lake brine149.11.12—Lakkor Co Lake brineLG-06Lake brine169.11.1—Lakkor Co Lake brineLG-07Lake brine139.01.08—Lakkor Co Lake brineLG-08Lake brine149.21.08—Lakkor Co Lake brineLG-09Lake brine1591.09—Lakkor Co Lake brineLG-10Lake brine128.61—Lakkor Co Lake brineLGS-01Carbonate deposit (mud stone)Figure 3
Piper plot showing the water chemical variability of Lakkor Co area.The boron concentrations in Lakkor Co lake water are 401.56-849.47 mg L-1, and their δ11B values are -6.75 to -5.01‰ (Figure 2). Samples from Jibu Caka lake water (JB-01 to 03) to the west of Lakkor Co have B concentrations of 683.58 to 744.77 mg L-1 with δ11B values of -4.52 to -4.01‰ (Table 1 and Figure 2). The water from the chain of lakes upstream of the Lakkor Co displays the B concentration of 3.52 mg L-1 (sample No. CGC) and 3.96 mg L-1 (sample No. ZZC), and the δ11B values of -14.91 and -15.48‰, respectively (Table 1 and Figure 2). Bai Kok river and Somme river, as the main recharge rivers of the Lakkor Co salt lake, exhibit different boron isotopic signatures. The Bai Kok river to the south of Lakkor Co salt lake has the B concentration of 3.45 mg L-1 and 3.47 mg L-1 and the δ11B values of -15.63 and -13.76‰. The Somme river to the east of Lakkor Co salt lake exhibits the B concentration of 2.54 and 2.69 ppm, and the δ11B values of -11.15 to -9.97‰. Carbonate deposit is mainly composed of hydromagnesite and aragonite, as well as small amounts of muscovite, calcite, quartz, and albite. The δ11B value of the carbonate deposit is -28.08‰.


## 4.2. Mechanisms Controlling Source Water Chemistry of Lakkor Co

The scatter diagram method reported by Gibbs [33] illustrated three important natural mechanisms controlling the major ion chemistry of the water including water–rock interaction, evaporation, and atmospheric precipitation. The TDS concentrations were plotted against the weight ratios of Na+/(Na+ + Ca2+) for cations and the weight ratios of Cl−/(Cl− + HCO3−) for anions (Gibbs, 1970; Figure 4). It was found that majority of the brine samples fell into evaporation dominant area, indicating the important role of evaporation on brine chemistry, and the source water, including spring water, river water, and small lake water, fell out of the water–rock interaction, evaporation, and atmospheric precipitation area, indicating that the source of chemical elements in the water may be related to the early deposited.Figure 4
Gibbs’ diagram of the Lakkor Co region.In the ratio of Na/Cl and (Ca2+ + Mg2+)/(HCO3- + CO3-) in the water of Lakkor Co area. The (Ca2++Mg2+) and (HCO3- + CO3-) molar ratio is distributed around the 1 : 1 line (Figure 5), indicating that the source of Ca2+ + Mg2+ leaching earlier carbonate strata. The region deposited a large number of carbonate minerals [1] (Figure 6). Somme river region has early deposited carbonate stratigraphy, and the carbonate deposition of Gangga Co is being formed. Therefore, we believe that the weathering and formation of carbonates have an important influence on the hydrochemical properties of regional waters.Figure 5
Molar ratio of Na/Cl and (Ca2+ + Mg2+)/(HCO3- + CO3-) in water of the Lakkor Co region.Figure 6
Carbonate deposits of Lakkor Co area on Google Earth.


## 4.3. Distribution Characteristics of [B] andδ11B within the Watershed of Lakkor Co Lake

The Bai Kok river is recharged from the south with boron levels of 3.45 ppm and 3.47 ppm and the Somme river is recharged from the east with boron levels of 2.54 ppm and 2.69 ppm. The two lagoon waters in the Bai Kok river basin have similar boron levels to the Bai Kok river. The boron content of the spring water is comparable to or higher than that of the river water. The higher boron content of the river may be related to the lithology of the outcrop. The boron content of the lake water ranges from 401.56 ppm to 849.47 ppm, due to the recharge and the degree of concentration, as evidenced by the mineralization. The carbonate deposit also exhibits a high boron content of 0.037% and a lithium content of 0.010%. The boron content of the spring water collected in this work is 1.24 ppm. A previous research [31] found that the elemental boron content of the spring water outcropping from Lakkor Co is 3.24 to 21.39 ppm. Overall, the Lakkor Co recharge water bodies all exhibit high boron concentrations.The boron isotopic compositions of the Lakkor Co salt lake recharge and lake waters and the carbonate deposit all show negative values, with the carbonate deposit (δ11B=−28.08) being lower than the spring water (δ11B=−18.29), river water (-9.97 to -15.63), and lake surface water (-5.01 to -6.75). The main reason is likely the adsorption of boron by the clay minerals. In the Lakkor Co salt lake region, clay adsorption of boron is very common. The Bai Kok river upstream of the two small lake basins and Somme river flowing area have carbonate deposit development (Figure 2). In the northwest of Lakkor Co, there is enrich-boron mineral named graphite precipitation due to clay adsorption and capillary evaporation in the surface layer [1] (Figure 2). This is mainly due to the adsorption of clay minerals, the coprecipitation of carbonates, and the deposition of evaporite minerals during the preferential entry of 10B into the solid phase, which makes the boron isotopic composition of the water column positive [13, 34–36].Boron isotope ratios of the Bai Kok river in the south, ranging from−15.63±0.19 to −14.91±0.23 to −13.76±0.18, are lower than those of the Somme river in the east, ranging from −11.15±0.21 to −9.97±0.22. The overflow spring in this area has the lowest B isotope ratio. This is due to the fact that Somme river is mainly developed in the area of the Lakkor Co ophiolite development, while the Bai Kok river weathering rocks are more distant from the BNSZ.Lakkor Co regional water is not only rich in boron elements but also rich in lithium elements. Positive correlations have been determined among Li and B in different water types, suggesting common sources and similar geochemical behavior for Li and B ions (Figure7).Figure 7
Boron-lithium elemental ratio chart.


## 4.4. Calculated Amount of Boron Resources Input to Salt Lakes by Rivers

As noted earlier, the current boron reserves at Lakkor Co are3.82×106 ton. Somme river and Bai Kok river are the main recharge rivers flowing down into Lakkor Co lake. The total amount of B input into the lake by Somme river and Bai Kok river water can be calculated based on water-salt balance, according to the following equation:
(2)ΣW=S×C×10−6,.whereΣW is the annual total amount of B input into Lakkor Co lake by the rivers in tons, S is the flow of river water, and C is the annual concentration of B in the river water. The average flow velocity of the Somme river is about 2.15 m3/s, and the B content is about 2.60 mg L-1. The calculated results show that the annual total B input into Lakkor Co lake by Somme river is about 176 tons. The average flow velocity of the Bai Kok river is about 1.0 m3/s, and the B content is about 3.58 mg L-1. The calculated results show that the annual total B input into Lakkor Co lake by Bai Kok river is about 112 tons. The input from both rivers is therefore roughly estimated to be 288 tons per year. For the overall reserves of boron (382×104 tons), the riverine recharge is considerable.


## 4.5. The Source of Boron in the Lakkor Co Area

As mentioned above, the potential sources of boron in the salt lake are surface weathering, deep hydrous rock action, and Quaternary carbonate deposits. The riverine boron input is estimated to be the main source of boron recharge in the Lakkor Co region.


### 4.5.1. Carbonate Deposit

There is a large amount of carbonate in the Lakkor Co region. The boron content in carbonate deposit of Gangga Co is 370 ppm. Carbonate deposit (LGS-01) is mainly composed of hydromagnesite and aragonite, as well as small amounts of muscovite, calcite, quartz, and albite. Keren and O’Connor [37] noticed that the effect of excessive boron accumulation by Ca-bearing clay might partly be caused by the reaction of scattered CaCO3 with borate-rich solutions that lead to the precipitation of Ca-bearing borates. Therefore, the high boron content in this carbonate deposit may be caused by the precipitation of small amounts of boron minerals. The effect of regional widespread [1] (Figure 6) carbonate deposition on B migration is significant.The study found that the Bai Kok river changes in boron isotope ratios as it flows through areas of carbonate deposit, which is exposed at Ganga Co, from the inlet to the lake to the outlet of the river. The B isotopic ratio changes from−15.63±0.19 to −14.91±0.23 to −13.76±0.18, while Ganga Co hosts a range of clays which are largely distributed in the lake and along the lake shore. Field investigations show that the average retention time of water bodies in Ganga Co is about 38 days [28]. This indicates that the boron element has sufficient time to be adsorbed by the clay and for fractionation to occur in this lake system. The boron isotopic value for carbonate deposit (LGS-01) at the edge of Gangga Co is -28.08.Previous research shows that the isotope fractionation factor between boron adsorbed on sediments and dissolved boron in seawater is 0.975 atpH=8.2 [35]. The fractionation factor between the coprecipitated boron in carbonate and dissolved boron is 0.974 at pH=8.2 [38, 39]. In this study, the average δ11B value of carbonate deposit is –28.08‰, and the average δ11B value of surface brine is –14.91‰. This translates to a fractionation factor (α=Rcarbonatedeposit/Rbrine) of 0.986, which is in good agreement with the values for stromatolite (0.990) and travertine (0.990). However, the Somme river shows a decrease in B isotope ratios from -9.97 to -11.15, because it flows through the carbonate deposit region which has higher exposure of carbonate deposit than the river. The carbonate deposit acts as a B source in this area.Many scholars have studied the equilibrium fractionation of boron isotope adsorption between illite and seawater [40–42]. Their results showed that 10B is more likely to be adsorbed on the clay surface during the adsorption, and the adsorption process may be partially reversible. This indicates that boron from the lighter carbonate deposits was partially released during the passing of the Somme river water through the Quaternary carbonate deposits, allowing the boron isotopic composition of the Somme river water to be prioritized. Besides being highly mobile, boron can easily accumulate in secondary phases, mainly in clay minerals and in Fe oxides and hydroxides. These newly formed solids can not only reincorporate a large fraction of B inherited from parent minerals but can also incorporate boron from the coexisting solution [43–46]. Damxung Co is another boron-rich salt lake in Tibet, where the carbonate deposits present are the main sources of boron [21].Therefore, it can be concluded that adsorption and desorption processes of boron in Quaternary carbonate deposit were concurrent and had a nonnegligible role in the formation of salt lake boron ore (Figure8).Figure 8
Schematic diagram of the process of boron release from carbonate deposit.


### 4.5.2. Boron Resources Derived from Underground Rock Weathering and Water–Rock Interaction

By studying the hydrogen and oxygen isotopes and lithium content of spring water and river water, Dong [30] found that the lithium resources of Lakkor Co are derived from two sources, namely bedrock weathering and deep water–rock recharge. The spring water of Lakkor Co comes from meteoric precipitation. The boron contents of Lakkor Co river and the two main rivers are very high, and the spring also shows a high boron content [30]. Therefore, it is inferred that the boron source of Lakkor Co is similar to the lithium source, which has two forms of bedrock weathering and deep water–rock interaction. The boron content data of rocks in the Lakkor co area are shown in Table 1.The geological survey results of Lakkor Co area show that the contents of boron and lithium in the bedrock of this area are relatively high, ranging from 4.35 to 28.2 ppm [47]. The enrichment of boron and lithium in the salt lake of Lakkor Co area is mainly related to the boron- and lithium-rich rocks formed by structural evolution. Since boron can enter the lattice of serpentinites well, serpentinites are the most important host rocks of boron [48]. The large amount of serpentinized ultramafic rocks on the ocean floor is an important reservoir of boron, with boron contents of about 10×10−6 to 91×10−6 (Figure 2) [49, 50]. Due to the tectonic evolution of the SYMZ, boron-rich rocks are exposed at the surface or deposited near the surface, which is the main reason for the formation of more boron-rich salt lakes on the SYMZ.Wang et al. [51] studied the characteristics of high-temperature lithium-rich geothermal water in the southern Qinghai-Tibet Plateau, which is mostly neutral or alkaline NaCl-type water and rich in characteristic elements such as B, Li, Rb, Cs, and F. Tan et al. [52] used hydrogen-oxygen isotope data to explain the circulation of hot water underground. In most of the high-temperature geothermal systems of Tibet, they found rapid circulation of groundwater and upwelling of residual magmatic water. Moreover, in most cases, granite outcrops were observed within a short distance from the hot springs. Dong [30] studied the isotopes in water bodies of the Lakkor Co and found that the water of both springs and river water is from atmospheric precipitation. Therefore, the main source of boron and other substances in the springs here is still the rapid circulation of groundwater.Therefore, it can be concluded that the sources of boron in the Lakkor Co salt lake are related to the tectonic activity caused by the formation of the BNSZ when boron-rich rocks are exposed or enriched near the surface, as well as the deep water–rock interaction, bedrock weathering, and atmospheric precipitation.


## 4.5.1. Carbonate Deposit

There is a large amount of carbonate in the Lakkor Co region. The boron content in carbonate deposit of Gangga Co is 370 ppm. Carbonate deposit (LGS-01) is mainly composed of hydromagnesite and aragonite, as well as small amounts of muscovite, calcite, quartz, and albite. Keren and O’Connor [37] noticed that the effect of excessive boron accumulation by Ca-bearing clay might partly be caused by the reaction of scattered CaCO3 with borate-rich solutions that lead to the precipitation of Ca-bearing borates. Therefore, the high boron content in this carbonate deposit may be caused by the precipitation of small amounts of boron minerals. The effect of regional widespread [1] (Figure 6) carbonate deposition on B migration is significant.The study found that the Bai Kok river changes in boron isotope ratios as it flows through areas of carbonate deposit, which is exposed at Ganga Co, from the inlet to the lake to the outlet of the river. The B isotopic ratio changes from−15.63±0.19 to −14.91±0.23 to −13.76±0.18, while Ganga Co hosts a range of clays which are largely distributed in the lake and along the lake shore. Field investigations show that the average retention time of water bodies in Ganga Co is about 38 days [28]. This indicates that the boron element has sufficient time to be adsorbed by the clay and for fractionation to occur in this lake system. The boron isotopic value for carbonate deposit (LGS-01) at the edge of Gangga Co is -28.08.Previous research shows that the isotope fractionation factor between boron adsorbed on sediments and dissolved boron in seawater is 0.975 atpH=8.2 [35]. The fractionation factor between the coprecipitated boron in carbonate and dissolved boron is 0.974 at pH=8.2 [38, 39]. In this study, the average δ11B value of carbonate deposit is –28.08‰, and the average δ11B value of surface brine is –14.91‰. This translates to a fractionation factor (α=Rcarbonatedeposit/Rbrine) of 0.986, which is in good agreement with the values for stromatolite (0.990) and travertine (0.990). However, the Somme river shows a decrease in B isotope ratios from -9.97 to -11.15, because it flows through the carbonate deposit region which has higher exposure of carbonate deposit than the river. The carbonate deposit acts as a B source in this area.Many scholars have studied the equilibrium fractionation of boron isotope adsorption between illite and seawater [40–42]. Their results showed that 10B is more likely to be adsorbed on the clay surface during the adsorption, and the adsorption process may be partially reversible. This indicates that boron from the lighter carbonate deposits was partially released during the passing of the Somme river water through the Quaternary carbonate deposits, allowing the boron isotopic composition of the Somme river water to be prioritized. Besides being highly mobile, boron can easily accumulate in secondary phases, mainly in clay minerals and in Fe oxides and hydroxides. These newly formed solids can not only reincorporate a large fraction of B inherited from parent minerals but can also incorporate boron from the coexisting solution [43–46]. Damxung Co is another boron-rich salt lake in Tibet, where the carbonate deposits present are the main sources of boron [21].Therefore, it can be concluded that adsorption and desorption processes of boron in Quaternary carbonate deposit were concurrent and had a nonnegligible role in the formation of salt lake boron ore (Figure8).Figure 8
Schematic diagram of the process of boron release from carbonate deposit.


## 4.5.2. Boron Resources Derived from Underground Rock Weathering and Water–Rock Interaction

By studying the hydrogen and oxygen isotopes and lithium content of spring water and river water, Dong [30] found that the lithium resources of Lakkor Co are derived from two sources, namely bedrock weathering and deep water–rock recharge. The spring water of Lakkor Co comes from meteoric precipitation. The boron contents of Lakkor Co river and the two main rivers are very high, and the spring also shows a high boron content [30]. Therefore, it is inferred that the boron source of Lakkor Co is similar to the lithium source, which has two forms of bedrock weathering and deep water–rock interaction. The boron content data of rocks in the Lakkor co area are shown in Table 1.The geological survey results of Lakkor Co area show that the contents of boron and lithium in the bedrock of this area are relatively high, ranging from 4.35 to 28.2 ppm [47]. The enrichment of boron and lithium in the salt lake of Lakkor Co area is mainly related to the boron- and lithium-rich rocks formed by structural evolution. Since boron can enter the lattice of serpentinites well, serpentinites are the most important host rocks of boron [48]. The large amount of serpentinized ultramafic rocks on the ocean floor is an important reservoir of boron, with boron contents of about 10×10−6 to 91×10−6 (Figure 2) [49, 50]. Due to the tectonic evolution of the SYMZ, boron-rich rocks are exposed at the surface or deposited near the surface, which is the main reason for the formation of more boron-rich salt lakes on the SYMZ.Wang et al. [51] studied the characteristics of high-temperature lithium-rich geothermal water in the southern Qinghai-Tibet Plateau, which is mostly neutral or alkaline NaCl-type water and rich in characteristic elements such as B, Li, Rb, Cs, and F. Tan et al. [52] used hydrogen-oxygen isotope data to explain the circulation of hot water underground. In most of the high-temperature geothermal systems of Tibet, they found rapid circulation of groundwater and upwelling of residual magmatic water. Moreover, in most cases, granite outcrops were observed within a short distance from the hot springs. Dong [30] studied the isotopes in water bodies of the Lakkor Co and found that the water of both springs and river water is from atmospheric precipitation. Therefore, the main source of boron and other substances in the springs here is still the rapid circulation of groundwater.Therefore, it can be concluded that the sources of boron in the Lakkor Co salt lake are related to the tectonic activity caused by the formation of the BNSZ when boron-rich rocks are exposed or enriched near the surface, as well as the deep water–rock interaction, bedrock weathering, and atmospheric precipitation.


## 4.6. A Comparative Study of Boron Isotopes in Boron-Rich Salt Lakes of Lakkor Co and Damxung Co

Lakkor Co and Damxung Co share the same geological setting (Figure1), i.e., the same island arc environment as the SYMZ ophiolite mélange. B is a fluid-active element, preferentially entering fluids with significant isotopic fractionation (up to 40‰). 11B tends to be enriched in fluid phases [53–55], especially in primitive oceanic basalts (δ11B=−12‰to0‰) [56], seawater (δ11B=+39.6‰) [57], and hydrothermal fluids. Thus, the altered oceanic crust is enriched with heavy B isotopes. The δ11B ranges from -4‰ to +25‰, with a mean value of +3.4‰ [58]. The δ11B of serpentinized ultramafic rocks on the ocean floor generally ranges from +7‰ to +20‰ [49, 55]. Some studies have suggested that heavy boron isotopes in volcanic arcs are mainly derived from the combined contribution of subduction erosion and forearc serpentinized mélange [55, 59, 60]. However, recent studies have shown that not all mantle wedges have high δ11B values, and the variation of B isotopes in the mantle is within the range of -15. 3‰ to 9.7‰. This is mainly related to the fact that fluid present in the sediments reduces δ11B values [61]. The boron isotopes of the recharging waters in the SYMZ volcanic arc are negative, which suggests that the boron isotopes of the Linguistic Bedrock are also relatively negative.In addition, it can be seen from the data that the B isotope ratios of the lake water of Damxung Co are significantly lower than those of the recharge waters (Figure9), while the carbonate deposit has lower B isotope ratios. This is consistent with the results of related previous studies and demonstrates that the carbonate deposit is the main boron source for the lake in Damxung Co [21]. However, it can be seen that, although the carbonate deposits in the area exhibit lower B isotope ratios, the B isotope ratios of the recharge waters are similar. Moreover, the B isotopes of the lake water are more positive than both of the above. This result means that clay adsorption occurs during the flow of the river and leads to B fractionation. Moreover, precipitation of borate minerals occurs on the shore of the lake, which is also the reason for the higher boron isotope ratios of the lake water in Lakkor Co than the recharge water.Figure 9
Comparison of boron isotopes between Lakkor Co and Damxung Co (boron isotope data of Damxung Co are taken from Lü et al. [21]).


## 5. Conclusions

(1)
The formation of the SYMZ island arc caused the boron-rich rocks to be exposed at the surface, and boron element was concentrated in rivers by weathering of boron rocks and deep water–rock interaction. Then, the boron-rich water body was further enriched and concentrated in the salt lake and formed the modern salt lake type boron deposit(2)
The carbonate deposits in the Lakkor Co and Damxung Co regions are different. In the Damxung Co region, the carbonate deposits are mainly a source of boron release and are currently the only major boron source. However, in the Lakkor Co region, there are carbonate deposits with both adsorption and desorption processes of boron. The main source is still the weathering of boron-rich rocks and deep water–rock interaction(3)
The processes of adsorption and desorption of boron in Quaternary carbonate deposit were concurrent and had a nonnegligible role during formation of salt lake boron ore



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*Source: 3724800-2022-06-22.xml*