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Article

Critical Minerals in Tibetan Geothermal Systems: Their Distribution, Flux, Reserves, and Resource Effects

by
Di Wang
1,
Fei Xue
1,*,
Lijian Ren
2,3,*,
Xin Li
1,
Songtao Wang
4 and
Xie Qibei Er
5
1
School of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China
2
School of Civil Engineering and Transportation, Hohai University, Nanjing 210024, China
3
School of Civil Engineering, Inner Mongolia University of Technology, Hohhot 010051, China
4
Inner Mongolia Autonomous Region Water Conservancy Development Center, Hohhot 026299, China
5
School of Architecture, Inner Mongolia University of Technology, Hohhot 010051, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(1), 93; https://doi.org/10.3390/min15010093
Submission received: 12 November 2024 / Revised: 23 December 2024 / Accepted: 13 January 2025 / Published: 20 January 2025
(This article belongs to the Special Issue Critical Metal Minerals, 2nd Edition)
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Abstract

:
Critical mineral resources (CMRs) are essential for emerging high-tech industries and are geopolitically significant, prompting countries to pursue resource exploration and development. Tibetan geothermal systems, recognized for their CMR potential, have not yet been systematically evaluated. This study presents a comprehensive investigation of the spatial distributions, resource flux, reserves, and resource effects of CMRs, integrating and analyzing hydrochemical and discharge flow rate data. Geochemical findings reveal significant enrichment of lithium (Li), rubidium (Rb), cesium (Cs), and boron (B) in the spring waters and sediments, primarily located along the Yarlung Zangbo suture and north–south rift zones. Resource flux estimates include approximately 246 tons of Li, 54 tons of Rb, 233 tons of Cs, and 2747 tons of B per year, underscoring the mineral potential of the geothermal spring waters. Additionally, over 40,000 tons of Cs reserves are preserved in siliceous sinters in Tagejia, Gulu, and Semi. The Tibetan geothermal systems thus demonstrate considerable potential for CMRs, especially Cs, through stable discharge and widespread distribution, also serving as indicators for endogenous mineral exploration and providing potential sources for lithium in exogenous salt lakes. This study evaluates the CMR potential of the Tibetan geothermal systems, advancing CMR exploration while contributing to the future security of CMR supplies.

1. Introduction

Critical minerals resources (CMRs) play a significant and irreplaceable role in emerging high-tech industries, including new energy, materials, communications, aerospace, and national defense [1,2,3]. However, CMRs are relatively scarce and unevenly distributed in nature, leading to high supply chain risks [4,5,6]. With rising populations, industrial development, and increasing geopolitical tensions, critical metals are gaining strategic significance [7,8,9]. In response, countries and organizations, including China, the United States (U.S.), and the European Union (EU), are enhancing resource exploration and development while proposing their critical metal lists. The types and quantities of key metals vary based on governmental strategies, predominantly featuring rare metals, rare earth elements, and precious metals [3,10].
Various critical minerals occur naturally, with lithium primarily found in pegmatites, brines, and clay sediments [11,12]. Recently, geothermal brines worldwide have shown promising lithium enrichment, indicating a high potential for industrial extraction and recycling [13,14,15]. The U.S. is investing in geothermal water recycling projects, such as in the Salton Sea region [16,17,18]. In contrast, most geothermal spring waters do not exhibit significant enrichment of critical minerals. However, geochemical studies have demonstrated that spring water and hydrothermal geyserite in the Tibetan Plateau are uniquely enriched in lithium, rubidium, cesium, and boron, drawing considerable attention [19,20,21,22,23,24]. Our team has conducted a comprehensive investigation into the sources, enrichment mechanisms, and resource effects of rare metal elements in Tibetan geothermal springs [15]. According to spatial distribution, isotopic, and elemental geochemistry studies, the CMRs enriched in the Tibetan geothermal springs primarily originate from magmatic-hydrothermal fluids generated by partial melting of the subducted Indian plate, serving as both the heat and material source for geothermal events [15]. The enriched CMRs originate from underground sources, offering valuable insights into deep metallogenic processes and guiding the exploration of hard-rock rare metal deposits. Additionally, drainage recharge from geothermal springs, enriched with unique components, serves as a significant contributor to salt lake formation [15,21,22]. Thus, Tibetan geothermal systems hold triple resource potential: intrinsic resources contained in geothermal systems, indicators for hard-rock rare metal deposits, and sources for salt lake deposits.
However, compared with the relatively abundant studies of the relationship between geothermal systems with deep fluids and salt lakes, the systematic study of the intrinsic resources contained in geothermal systems is still lacking, particularly regarding the annual flux of CMRs from the spring waters and the quantity of CMRs stored in the geothermal sediments. Although previous analyses [22,25,26] have explored CMR fluxes in Tibet, the data remain incomplete. Therefore, this study aims to evaluate the resource potential of Tibetan geothermal systems by investigating the spatial distributions, resource flux, reserves, and effects of CMRs through the integration of hydrochemical data and discharge flow rates, thereby advancing the exploration and development of critical mineral resources and supporting the transition to a green energy era.

2. Classification Criteria for Critical Mineral Resources

Various countries and organizations are establishing their criteria for critical minerals, as illustrated in Figure 1. The United States Geological Survey (USGS) defines critical minerals as non-fuel minerals essential to economic or national security, with supply chains vulnerable to disruption [27]. The absence of these minerals could compromise the national economy and security [28]. Accordingly, the USGS has compiled a list of 50 critical minerals, distinguishing rare earth elements and platinum group elements as separate entries [27]. Similarly, the European Union (EU) has identified 30 critical minerals vital for industrial and economic development, which are also at high risk of supply disruption [29]. The Australian Government recognizes 31 critical minerals, factoring in relationships with partner countries additionally [30]. Japan has developed a comprehensive evaluation method that includes five risk categories including supply, price, demand, recycling restrictions, and potential, resulting in the identification of 34 critical resource commodities [31,32]. In 2016, the Chinese Government proposed a critical minerals list featuring 24 commodities categorized into four groups based on their strategic importance: bulk mineral resources, scarce mineral resources, high-performance material resources, and clean energy mineral resources [10].
Different countries select minerals for inclusion in their lists according to specific national contexts and objectives. Notably, China uniquely classifies bulk mineral resources (such as oil, gas, coal, iron, and copper) as critical minerals (Figure 1), reflecting its focus on industrialization, infrastructure development, urbanization, and a vast manufacturing sector as a developing country [3]. Overall, critical metal lists worldwide predominantly feature rare metals like Li, Be, Rb, Cs, Nb, Ta, Zr, Hf, and W, along with rare earth elements and scarce metals such as Ga, Ge, Se, Cd, In, Te, and Re (Figure 1). Additionally, graphite and fluorite are recognized as critical minerals across various countries and organizations, underscoring the competitive landscape in the new materials and energy sectors [33,34,35,36].
Extensive studies have confirmed the unique enrichment of elements such as Li, Rb, Cs, and B in Tibetan geothermal systems, including both spring waters and geyserite [19,21,24,25,26,37,38,39,40]. Based on these findings, Tibetan geothermal systems exhibit CMR potential. These systems not only support geothermal energy development [41] but also draw growing interest in Earth science, particularly in studying CMR formation mechanisms [15,24], highlighting the interplay of water, heat, and minerals [22].

3. Geological Background and Occurrence of the Tibetan Geothermal Systems

The formation of the Tibetan Plateau is closely tied to the tectonic evolution of the Tethys Ocean, driven by the sequential accretion of distinct terranes to the southern margin of the Asian continent over various geological periods. From north to south, these terranes comprise the Qiangtang, Lhasa, and Himalayan terranes [42]. The boundaries between these terranes are delineated by several major suture zones, which, in the Tibetan region, are arranged from north to south as the Jinshajiang Suture, the Bangong-Nujiang Suture, and the Yarlung Zangbo Suture (Figure 2). The eventual closure of the Neo-Tethys Ocean initiated the collision between the Indian subcontinent and the Asian landmass in the early Paleogene, triggering the uplift of the Himalayas and the significant rise of the Gangdese Mountains, ultimately giving rise to the present-day Tibetan Plateau [42].
Geothermal activity is widespread throughout the Qinghai–Tibet Plateau geothermal belt, situated in the eastern part of the Mediterranean-Himalayan geotropics [43,44]. As documented in the reports [45,46], a total of 677 geothermal springs have been found in Tibet exhibiting various types such as boiling springs, hot springs, fumaroles, geysers, and explosive springs. The basic information, water chemical composition, trace elements, flow rate data, and estimated annual resource flux of the Tibetan geothermal springs are listed in Supplementary Materials Table S1 [45,46]. As exhibited in Figure 2, their spatial distribution is uneven, with geothermal events primarily concentrated within the Lhasa and Himalayan terranes, south of the Bangong-Nujiang suture zone, while the Qiangtang terrane to the north displays weaker activity. Numerous relics of past hydrothermal activity, including large-scale sinter islands in Zhabuye salt lake [47], indicate that intense geothermal processes occurred during past geological periods. Additionally, eastern and central Tibet experience more frequent and vigorous geothermal activity compared to the western region.
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Figure 2. Schematic map of tectonics, distribution of geothermal springs, and division of geothermal zones in Tibet (data from [46]). The yellow lines delineate the geothermal activity zones in Tibet, classified based on the density, temperature, type, and development of geothermal springs, as outlined in previous research [15]. Five geothermal activity zones are identified: I, II, III, IV, and V, representing the low-temperature zone in northern Tibet, medium- to high-temperature zone in central Tibet, high-temperature zone in southern Tibet, medium- to high-temperature zone in western Tibet, and low- to medium-temperature zone in eastern Tibet, respectively. Abbreviations: BNS: Bangonghu-Nujiang Suture; JSS: Jinshajiang Suture; YZS: Yarlung Zangbo Suture; TKR: Tangra Yum Co-Kong Co rifts; CAR: Cuona Co-Aguo Co rifts; WCR: Woka-Cuona rifts; XDR: Xainza-Dinggye rifts; YGR: Yadong-Gulu rifts.
Figure 2. Schematic map of tectonics, distribution of geothermal springs, and division of geothermal zones in Tibet (data from [46]). The yellow lines delineate the geothermal activity zones in Tibet, classified based on the density, temperature, type, and development of geothermal springs, as outlined in previous research [15]. Five geothermal activity zones are identified: I, II, III, IV, and V, representing the low-temperature zone in northern Tibet, medium- to high-temperature zone in central Tibet, high-temperature zone in southern Tibet, medium- to high-temperature zone in western Tibet, and low- to medium-temperature zone in eastern Tibet, respectively. Abbreviations: BNS: Bangonghu-Nujiang Suture; JSS: Jinshajiang Suture; YZS: Yarlung Zangbo Suture; TKR: Tangra Yum Co-Kong Co rifts; CAR: Cuona Co-Aguo Co rifts; WCR: Woka-Cuona rifts; XDR: Xainza-Dinggye rifts; YGR: Yadong-Gulu rifts.
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In the aspect of temperature, high-temperature geothermal systems are predominantly found in southern Tibet, where most springs exceed 75 °C. These springs are densely located along the Yarlung Zangbo River and its banks, hosting significant geothermal systems such as Tagejia, Semi, Gudui, and Yangbajing. In contrast, eastern Tibet features low-temperature geothermal events, with spring water temperatures below 50 °C, despite intense geothermal activity (Figure 2). Consequently, the spatial distribution of geothermal springs in Tibet is characterized by a pattern of stronger activity in the south, weaker in the north, more abundant in the east, and sparser in the west (Figure 2). Based on the distribution pattern, Tibet can be divided into five geothermal activity zones (Figure 2, [15]): the low-temperature geothermal activity zone in northern Tibet (zone I), the medium- to high-temperature geothermal activity zone in central Tibet (zone II), the high-temperature geothermal activity zone in southern Tibet (zone III), the medium- to high-temperature geothermal activity zone in western Tibet (zone IV), and the low- to medium-temperature geothermal activity zone in eastern Tibet (zone V).

4. Geochemical Features and Spatial Distribution of Tibetan CMR-Rich Geothermal Systems

This study analyzes the hydrogeochemical characteristics of the Tibetan geothermal springs using data from the book “Thermal Springs in Tibet” [46], and the data are listed in Supplementary Materials Table S1. The geothermal springs in Tibet exhibit relatively low total dissolved solids (TDS), with an average of 0.92 g/L [46]. Only three springs exceed 25 g/L, reaching brine levels, primarily influenced by intense evaporation-concentration processes [24]. According to the Piper diagram (Figure 3), Tibetan geothermal spring waters display diverse chemical compositions, with cation concentrations in the order of Na+ > Ca2+ > K+ > Mg2+ and anion concentrations of HCO3− > Cl− > SO42−. The dominant water chemistry types are Cl-Na and Cl·HCO3-Na, similar to other global geothermal fields (Figure 4). However, geochemical studies further reveal that Tibetan geothermal spring waters are uniquely enriched in critical minerals such as Li, Rb, Cs, and B, distinguishing them from other geothermal systems worldwide (Figure 4). To evaluate the potential of CMRs in geothermal systems, the cut-off and industrial grades for these minerals in China are summarized in Table 1.

4.1. Geochemical Features of Critical Mineral Resources in Tibetan Geothermal Spring Water

4.1.1. Lithium

The average Li content in the Earth’s crust is approximately 20 ppm [58], classifying it as a rare metal element. Li is often referred to as “white oil” due to its critical role in modern high-tech industries, including ceramics, electronics, metallurgy, medical applications, and optics [11]. As the world transitions to a green energy era [59], the growing demand for electric vehicles, renewable energy sources, and energy storage solutions has highlighted lithium’s significance, with about 65% of total lithium demand attributed to rechargeable batteries [60]. China’s push for electric vehicles as part of its national strategic industry has rapidly increased its lithium battery demand, making it the world’s largest consumer of lithium resources [61]. The importance of lithium continues to rise amidst a changing international landscape and intensifying competition for energy resources [3,62,63]. Currently, brine and hard-rock deposits account for approximately 90% of global lithium reserves [11,64,65], but with the advancement of lithium extraction technology from brine [66], low-grade lithium-rich hot spring water has broad prospects for development and utilization [13,67].
In Tibet, the average lithium concentration in spring waters is 5.48 mg/L, with 30 springs exceeding 10 mg/L and 8 geothermal springs reaching the cut-off grade of 25 mg/L (Table 1). The Zhumaisha geothermal spring records the highest concentration at 65.87 mg/L [15]. In contrast, geothermal springs in Yellowstone National Park typically have lithium concentrations under 10 mg/L [53], and those in the volcanic geothermal fields of Iceland and Japan generally fall below 1 mg/L [68,69]. Similarly, geothermal regions in China exhibit similar temperature and TDS values to those in Tibet, but with significantly lower lithium concentrations, typically below 1 mg/L [49,50,51]. Overall, Tibetan geothermal spring waters demonstrate the highest degree of lithium enrichment globally [14,15,22,24].

4.1.2. Rubidium and Cesium

Rb and Cs are alkali rare metals having remarkably similar physical and chemical properties [70]. They are indispensable in traditional industries such as electronics, catalysis, specialty glass, biochemistry, and medicine, as well as in emerging sectors including new energy, materials, aerospace, national defense, and information technology [71,72,73]. Rb and Cs are primarily found in nature alongside rare metals or salt minerals like Li, Be, and Nb, with independent deposits being quite rare [74]. Their resources are mainly concentrated in granite [75] and pegmatite [76,77], yet total global reserves of Rb and Cs are both below 200,000 tons, projected to be depleted within a few years [78]. To ensure a stable resource supply, there is increasing interest in the salt lakes [79] and geothermal systems in Tibet, particularly given their widespread distribution and cost-effective extraction methods [79], with Tibetan geothermal systems showing significant resource potential [15,25,47].
The average crustal contents of Rb and Cs are approximately 90 ppm and 3 ppm, respectively [48]. In contrast, geothermal spring waters exhibit average concentrations of 0.75 mg/L for Rb and 3.58 mg/L for Cs, surpassing by several orders of magnitude those found in natural waters, such as the upper Ganga River (3.83–5.14 μg/L for Rb and 0.18–0.22 μg/L for Cs) [80] and the Rhône River (1.29 μg/L for Rb and 0.02 μg/L for Cs) [81]. Across the Tibetan region, 141 geothermal springs have Cs concentrations that meet the cut-off grade (0.5 mg/L), with 12 springs reaching industrial-grade levels. Although Rb enrichment is relatively low, 9 springs exceed the cut-off grade. Notably, the Cs concentration in the Semi spring has been recorded as high as 76.37 mg/L [25], and our previous study identified the Zhumaisha spring with Rb and Cs concentrations of 6.83 mg/L and 57.99 mg/L, respectively [15]. Compared to other geothermal fields, Tibetan geothermal spring waters exhibit significantly higher concentrations, particularly with Cs enrichment being unique globally (Figure 4). Unlike typical geothermal or river waters, where Rb concentrations surpass Cs, with Cs/Rb ratios less than 1 [80,81,82], the Tibetan geothermal springs display higher Cs concentrations than Rb, resulting in Cs/Rb ratios greater than 1 (Figure 5a). Moreover, there is a trend indicating that higher Cs concentrations correspond to greater Cs/Rb ratios, similar to those in fluid inclusions from lithium-rich pegmatite deposits (3–15) [83], suggesting that the rare metal elements in Tibetan geothermal spring water may originate from deep magmatic-hydrothermal fluids [15,22].

4.1.3. Boron

Boron (B) is a non-metallic critical mineral characterized by unique physical and chemical properties, including low weight, high hardness, flame retardance, heat resistance, abrasion resistance, and non-magnetic characteristics [84]. It is widely utilized in traditional industries such as metallurgy, ceramics, glass, and enamel [85], as well as in high-tech applications including permanent magnets, superconductors, whiskers, and medical technologies [86,87,88,89].
Globally, boron mineral resources can be categorized into six types based on their genetic origin: volcanic-sedimentary, sedimentary-metamorphic, modern salt lake, subsurface brine, marine evaporite, and skarn types [84]. The Tibetan geothermal springs exhibit remarkable boron enrichment, with an average concentration of 31.57 mg/L (Figure 4). Notably, there are 25 spring areas where boron concentrations exceed 100 mg/L [46], significantly higher than the boron concentration found in natural seawater (4.5–5.0 mg/L) [90]. The Zhumaisha and Semi geothermal springs have exceptionally high concentrations of 506.75 mg/L and 626.60 mg/L, respectively [22]. While the Mediterranean-Himalayan geothermal belt includes high-boron geothermal fields, such as the Kizildere geothermal area in Turkey [68] and the Mount Taftan geothermal field in Iran [91], their boron concentrations are considerably lower than that found in the Tibetan geothermal springs (Figure 4). Given the high B concentrations in these springs, they represent a potential resource for the Chinese government, particularly as China’s boron mineral resources are largely imported from countries such as Turkey, the U.S., and Chile [78].

4.1.4. Other Critical Mineral Resources

The Tibetan geothermal system is characterized by elevated levels of heavy metals, including As and Sb, which are recognized as CMRs in several countries (Figure 1). High-As geothermal systems are primarily located in southern Tibet [39], with the Moluojiang geyser exhibiting an arsenic concentration of 126.00 mg/L [46], significantly higher than those found in other high-As geothermal systems worldwide, such as the Los Humeros geothermal system in Mexico [69] and the El Tatio geothermal field in Chile [92]. Additionally, the Semi spring records the highest antimony content in Tibet, reaching 2.90 mg/L [46]. The environmental and resource implications of these harmful elements discharged by geothermal springs are not well understood, underscoring the urgent need for further research into their distribution patterns, sources, accumulation processes, and potential for comprehensive utilization in geothermal systems [39].

4.2. Cesium-Bearing Geyserite Deposits

In addition to the dissolved CMRs present in spring water, the Tibetan geothermal systems also host a unique type of solid Cs deposit. The siliceous sinters found in Tagejia, Semi, and Gulu exhibit exceptionally high Cs content, with an average concentration of 515 ppm and a maximum Cs2O grade of 1.25% [25]. This concentration is several times greater than the minimum industrial grade for solid cesium deposits, establishing these as hydrothermal-origin Cs mineral resources [25,47,93]. Comprehensive studies have examined their geochemical characteristics, isotopic compositions, geochronology, origin, and reserves [25,47,93,94,95]. Additionally, our team has identified new occurrences of Cs-rich siliceous sinters in Kawu, Chatuogang, and Langjiu (unpublished data).

4.3. Spatial Distribution of CMR-Rich Geothermal Systems

Based on our previous research [15] and on data from the literature, we present figures illustrating the spatial distribution of Li, Rb, Cs, and B concentrations in Tibetan geothermal spring water (Figure 6 and Figure 7). The results indicate a strong interrelation among the concentrations of these elements. Notably, certain geothermal systems, such as the Zhumaisha and Semi springs, demonstrate simultaneous enrichment of critical minerals, including high concentrations of Li, Rb, Cs, and B. Such phenomena are rare globally (Figure 6 and Figure 7). These enriched geothermal springs are primarily located along the Yarlung Zangbo suture zone and the north–south trending rift zones, particularly in areas where these zones intersect. Prominent geothermal systems, including the Semi, Tagejia, Kawu, Chatuogang, Gulu, and Gudui, are situated in these highly enriched regions. The representative rift zones extend from east to west and include the Woka-Cuona, Yadong-Gulu, Xainza-Dinggye, Tangra Yum Co-Kong Co, and Cuona Co-Aguo Co [15]. Among them, the Yadong-Gulu rift zone is the largest, spanning 500 km [96], and is noted for its intense geothermal activity, featuring fields such as Yangbajing, Yangyi, and Gulu. In contrast, the Bangong-Nujiang suture zone and the northern Qiangtang Terrane exhibit weaker geothermal activity and contain fewer geothermal springs having high concentrations of CMRs, as exemplified by the Xiakangjian spring [97].

5. Resource Flux and Reserves of Critical Metals in Tibetan Geothermal Systems

The preceding discussion has demonstrated the mineral potential of the Tibetan geothermal systems, focusing on two key aspects: the liquid resource flux derived from spring water discharge and the solid reserves stored in geothermal sinters.

5.1. Resource Flux of Critical Mineral Resources in Geothermal Spring Water

High-temperature geothermal waters in the southern Tibetan Plateau demonstrate notably low Mg/Li ratios, making them suitable for industrial exploitation (Figure 5b). Additionally, the Li/TDS ratios in these waters significantly exceed those found in productive salt lake deposits (Figure 5b), such as the Zhabuye salt lake and the Uyuni salt flat [26]. This results in a remarkably high concentration of lithium, highlighting its value for extraction. Long-term observations of the geothermal springs in the Gudui geothermal field indicate that the lithium and boron concentrations, along with the discharge flow rates, have remained relatively stable over the past 50 years [45,46,98,99]. While previous studies have analyzed the resource flux of CMRs in Tibet [22,25,26], their datasets are incomplete. In this study, we utilize a comprehensive dataset from the literature [46], which includes detailed discharge flow rates and geochemical data (Supplementary Materials Table S1). Accordingly, the estimated annual resource flux of the geothermal systems is also included.
As illustrated by the summary Table 2 and statistical donut diagram (Figure 8), 2 geothermal springs (Figure 6a) have achieved dissolved lithium concentrations that meet industrial standards (≥50 mg/L), collectively yielding a total resource flux of approximately 2 tons per year. Additionally, 7 springs exceed the cut-off grade of 25 mg/L for lithium brine deposits, contributing an estimated total resource flux of around 27 tons annually. Moreover, 30 geothermal springs exhibit lithium concentrations greater than 10 mg/L, collectively discharging approximately 127 tons of dissolved lithium each year. A total of 122 geothermal springs feature lithium concentrations exceeding 1 mg/L, with a combined resource flux of approximately 237 tons per year. Overall, this dataset includes 218 geothermal springs suitable for lithium resource estimation, with a total lithium resource flux of around 246 tons annually.
In contrast, no geothermal spring meets the industrial grade for rubidium brine deposits. However, 7 springs have rubidium concentrations that surpass the cut-off grade of 3 mg/L, contributing roughly 4 tons of rubidium annually (Figure 7a). A total of 29 geothermal springs display rubidium contents greater than 1 mg/L, yielding a total resource flux of about 22 tons each year. This dataset encompasses 193 geothermal springs available for rubidium resource estimation, with a total rubidium resource flux of approximately 54 tons annually. For cesium, 6 springs reach industrial grade (≥20 mg/L, Figure 7b), producing an estimated resource flux of about 23 tons each year. Notably, 120 geothermal springs have cesium concentrations exceeding the cut-off grade of 0.5 mg/L, providing a total resource flux of approximately 202 tons per year. In total, 185 geothermal springs are included in this dataset for cesium resource estimation, yielding an overall cesium resource flux of around 233 tons annually.
For boron, 5 geothermal springs achieve dissolved boron concentrations at industrial grade (≥315 mg/L, Figure 6b), resulting in a total resource flux of around 125 tons per year. Additionally, 15 springs exceed the cut-off grade of 125 mg/L for boron brine deposits, yielding a combined resource flux of approximately 360 tons annually. Furthermore, 39 geothermal springs display boron concentrations greater than 50 mg/L, collectively discharging about 1536 tons of dissolved boron each year. A total of 107 geothermal springs have boron concentrations exceeding 10 mg/L, contributing a total resource flux of approximately 2446 tons annually. In summary, this dataset includes 262 geothermal springs available for boron resource estimation, with an overall boron resource flux estimated at around 2747 tons per year.
The calculated amounts of resource flux in this study are underestimated because some discharge flow data in the dataset are missing and inaccurate. Advanced methods for measuring flow rate are applied in our group’s fieldwork, and the complete and precise dataset is being constructed and will be presented in future research. Additionally, this study only estimates the resource flux of surface springs, without accounting for the broader potential of geothermal waters, which remains underexplored. Some preliminary studies have concentrated on the resource potential of geothermal waters throughout the world [13,14]. Therefore, although the current resource flux appears low, further data collection and in-depth research are expected to reveal greater resource potential within geothermal systems.

5.2. Reserves of Cesium in Geyserite Deposits

We have compiled reserve data for Cs-bearing geyserite deposits in Tibet from the existing literature [25]. Three geothermal systems are identified as rich in cesium siliceous sinters. The Tagejia spring features the largest exposure of siliceous sinters, covering approximately 419,500 m2, which can be divided into nine distinct segments. The average thickness of these geyserites ranges from 2 to 15 m, with cesium grades varying between 0.08% and 0.26%. The total estimated reserve of cesium in the Tagejia geyserite deposits is approximately 14,459 tons. In the Gulu geothermal field, the exposure area of cesium-rich siliceous sinters is about 101,300 m2, maintaining a consistent thickness of 5 m. The cesium content in these geyserites varies from 0.05% to 0.10%, surpassing the industrial grade for hard rock-type cesium deposits, resulting in a resource reserve of 969 tons. The Semi geothermal spring, located along the steep banks of the Yarlung Zangbo River [94,95], has a limited exposure area of only about 1020 m2. Despite the cesium content reaching 1.30% and the thickness of the sinter bodies measuring 8.3 m, the total resource reserve of cesium is only about 25 tons. Consequently, based on previous investigations, the total cesium reserves within the siliceous sinters of the Tibetan geothermal systems are estimated to be approximately 15,454 tons.
In addition to the previously discussed geyserite deposits, several Tibetan geothermal systems exhibit significant potential for further exploration. Notably, the Gudui geothermal field, which includes the Buxionglanggu, Sagalangga, Riruo, Chaka, and Baburisu springs, is estimated to contain substantial reserves [99]. Our group is conducting detailed fieldwork and geochemical analyses to quantify cesium reserves in siliceous sinters using advanced technologies such as high-resolution remote sensing image interpretation, high-resolution drone-based 3D oblique photography, and high-density geochemical surveys. Preliminary findings indicate that cesium reserves in the Gudui geothermal field exceed 10,000 tons (unpublished data). Additionally, the Chabu and Chatuogang geothermal systems exhibit high cesium concentrations in their geyserite deposits, with predicted cesium reserves also exceeding 10,000 tons (unpublished data), underscoring the promising mineralization potential of CMRs in these areas.
As a result, abundant dissolved critical minerals are discharged with the flowing of geothermal spring water, and large amounts of solid reserves are preserved in geothermal sediments, highlighting the mineral potential of the Tibetan geothermal systems.

6. Resource Effects of Tibetan Geothermal Springs

Currently, efforts are underway to utilize high-salinity deep geothermal brines having lithium concentrations exceeding 150 mg/L, which are considered economically viable for lithium extraction [13]. Notable examples include the Salton Sea in the United States [100,101,102] and the Campania geothermal area in Italy [13]. In contrast, Tibetan geothermal spring water exhibits lower lithium concentrations, which currently hinders economic extraction. However, advancements in extraction technologies for lithium, rubidium, cesium, and boron from brines [56,79,103,104,105] may facilitate the ecological recovery of CMRs from these geothermal spring waters in the future. When combined with deep geothermal brines, the geothermal spring water will become a stable and cost-effective source of supply of CMRs [14].
Given the formation processes of geothermal systems, the anomalously enriched critical minerals must originate from depth. Two primary mechanisms account for the sources of critical minerals in the Tibetan geothermal systems: water–rock interactions, which leach unique elements from surrounding rocks [60,106,107], and deep fluids that supply ore-forming materials to the geothermal water [15,20,23,99,108]. Regardless of the source, these mechanisms indicate the presence of geological bodies enriched with strategic metal elements at depth, which has significant implications for mineral exploration, particularly in the search for hard rock-type deposits. Notably, many Tibetan geothermal springs are spatially associated with the recently identified potential rare metal metallogenic belt in the Himalayas [109,110,111,112]. Our research group is actively investigating their spatial, temporal, and genetic correlations.
Based on the spatial distribution (Figure 6 and Figure 7) and resource flux (Figure 8) analyses, it is evident that nearly all enriched geothermal systems are concentrated along the Yarlung Zangbo suture zone and the north–south trending rift zones. In contrast, northern Tibet exhibits weaker geothermal activity and enrichment, but there are various lithium-rich salt lake deposits such as Zhabuye, Laguo Co, and Mami Co [113,114]. Furthermore, numerous relics of hydrothermal activity, specifically paleo-sinter deposits, are found surrounding these salt lakes [47,97]. This close spatial association suggests that ancient geothermal events may have contributed ore-forming materials to the development of these salt lake deposits. Within Zhabuye salt lake, a variety of hydrothermal-origin travertine islands exist, exhibiting an average lithium content of 76.8 ppm, with a maximum of 320 ppm. This indicates that the geothermal water responsible for forming these travertines likely had elevated lithium concentrations, thus contributing significantly to the lithium resources available in Zhabuye salt lake [47,115].
In addition to the historical geothermal activity, recent studies have increasingly highlighted the crucial role of geothermal spring discharge in forming modern salt lakes [47,97,114,115,116,117,118,119,120,121,122,123]. Comprehensive investigations in the Nalenggele River Basin and its terminal salt lakes in the Qaidam Basin [120], as well as the Xiakangjian Geothermal Spring–Suomei Zangbo River–Laguo Co Salt Lake metallogenic system in northern Tibet [97], revealed that geothermal spring water accounts for 84.9%–93.4% and approximately 90% of the lithium resources in the respective tail salt lakes. Thus, the geothermal springs in northern Tibet serve as a significant source of lithium for salt lake deposits, highlighting another dimension of the resource potential of the Tibetan geothermal springs. In contrast, CMRs sourced from geothermal systems in southern Tibet can be retained within the salt lake basins. However, southern Tibetan geothermal systems such as the Semi, Tagejia, Kawu, and Zhumaisha primarily discharge CMRs into the Yarlung Zangbo, leading to resource loss and impacting the geochemistry of rivers and even oceans [40].
In conclusion, the resource effects of the Tibetan geothermal systems are reflected in three aspects. First, these geothermal springs are widely distributed, numerous, and have exhibited stable discharge over several decades, showing potential for CMR development and utilization. Moreover, fertile geothermal water serves as rare fluid “probes” that can indicate the current activity status of the continental crust, providing valuable insights for endogenous mineral exploration. Finally, the discharge from these springs is thought to be a potential source for lithium deposits found in certain exogenous salt lakes. The Tibetan geothermal springs thus play a pivotal role in the deep-to-shallow migration and accumulation of critical minerals, facilitating the transition between endogenous and exogenous mineral formation, as well as contributing to the source-to-sink processes. Further detailed investigations are warranted to explore these dynamics more thoroughly.

7. Conclusions

(1)
Critical minerals such as lithium, rubidium, cesium, and boron display abnormal enrichment in the Tibetan geothermal systems including spring water and sediments, and they are majorly concentrated along the Yarlung Zangbo suture zone and the N–S trending rift zones.
(2)
The analysis of available geothermal springs in the dataset indicates a total lithium resource flux of ~246 t/a (over cut-off grade for 27 t/a), alongside a rubidium flux of ~54 t/a (over cut-off grade for 4 t/a), a cesium flux of approximately 233 t/a (over cut-off grade for 202 t/a), and a substantial boron flux estimated at around 2747 t/a (over cut-off grade for 360 t/a), highlighting the significant mineral potential of Tibetan geothermal spring water.
(3)
Abundant cesium reserves are preserved in the geothermal sediments of Tibetan geothermal systems such as in Tagejia, Gulu, Semi, Gudui, Chatuogang, and Chabu, with total cesium reserves exceeding 40,000 t in forms of siliceous sinters.
(4)
The Tibetan geothermal systems host potential for CMRs through stable discharge and widespread distribution, serve as valuable indicators of endogenous mineral exploration, and act as potential sources for lithium deposits in exogenous salt lakes, thereby facilitating the deep-to-shallow and source-to-sink migration and accumulation of critical minerals.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15010093/s1, Table S1. Basic information, water chemical composition, trace elements, flow rate data, and estimated annual resource flux of the Tibetan geothermal springs. Data are summarized and calculated based on [45,46].

Author Contributions

Conceptualization, F.X.; methodology, F.X.; investigation, F.X.; formal analysis, F.X.; funding acquisition, F.X.; writing—original draft preparation, F.X. and D.W.; writing—review and editing, F.X. and L.R.; visualization, F.X. and D.W.; supervision, F.X.; project administration, F.X.; data curation, X.L., S.W. and X.Q.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, Grant No. 42402071, the Natural Science Foundation of Jiangsu Province, Grant No. BK20241537, the Second Tibetan Plateau Scientific Expedition and Research Program (STEP), Grant No. 2022QZKK0202, the Fundamental Research Funds for the Central Universities, Grant No. B230201014, and the Qinghai Provincial Key Laboratory of Geology and Environment of Salt Lakes, Grant No. 2024-KFKT-A07.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Discussions with Hongbing Tan, Hohai University, during the preparation of this manuscript provided significant inspiration for this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 00093 g001
Figure 1. Critical mineral resources lists for Australia [30], China [10], EU [29], USA [27], and Japan [32].
Figure 1. Critical mineral resources lists for Australia [30], China [10], EU [29], USA [27], and Japan [32].
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Figure 3. Piper diagram of geothermal spring water in Tibet (data from [46]).
Figure 3. Piper diagram of geothermal spring water in Tibet (data from [46]).
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Figure 4. Box pattern and line chart of chemical composition and trace element concentrations of typical geothermal springs worldwide. The triangles represent the average concentration of global typical geothermal spring waters. Data sources: Tengchong area of China [48], Southeast Coast of China [49,50], Taiwan Island of China [51], geothermal system in the Andes Mountains [52], geothermal fields of Yellowstone National Park in the U.S. [53], Kizildere geothermal field in Turkey [54], Los Humeros geothermal field in Mexico [55]. Data for Tibetan springs are from [46].
Figure 4. Box pattern and line chart of chemical composition and trace element concentrations of typical geothermal springs worldwide. The triangles represent the average concentration of global typical geothermal spring waters. Data sources: Tengchong area of China [48], Southeast Coast of China [49,50], Taiwan Island of China [51], geothermal system in the Andes Mountains [52], geothermal fields of Yellowstone National Park in the U.S. [53], Kizildere geothermal field in Turkey [54], Los Humeros geothermal field in Mexico [55]. Data for Tibetan springs are from [46].
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Figure 5. Elemental ratios of the Tibetan geothermal springs (data from [46]). (a) The relationship between Cs and Rb concentrations, (b) the relationship between Mg and Li concentrations, and (c) the relationship between TDS and Li concentrations.
Figure 5. Elemental ratios of the Tibetan geothermal springs (data from [46]). (a) The relationship between Cs and Rb concentrations, (b) the relationship between Mg and Li concentrations, and (c) the relationship between TDS and Li concentrations.
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Figure 6. Spatial distribution of critical minerals’ concentrations in the Tibetan geothermal springs (data from [46]). (a) Spatial distribution of Li concentrations, and (b) spatial distribution of B concentrations. Red names shown in the diagrams represent the geothermal springs whose concentrations exceed the cut-off or industrial grades. Abbreviations are the same as in Figure 2.
Figure 6. Spatial distribution of critical minerals’ concentrations in the Tibetan geothermal springs (data from [46]). (a) Spatial distribution of Li concentrations, and (b) spatial distribution of B concentrations. Red names shown in the diagrams represent the geothermal springs whose concentrations exceed the cut-off or industrial grades. Abbreviations are the same as in Figure 2.
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Figure 7. Spatial distribution of critical minerals’ concentrations in the Tibetan geothermal springs (data from [46]). (a) Spatial distribution of Rb concentrations, and (b) spatial distribution of Cs concentrations. Red names shown in the diagrams represent the geothermal springs whose concentrations exceed the cut-off or industrial grades. Abbreviations are the same as in Figure 2.
Figure 7. Spatial distribution of critical minerals’ concentrations in the Tibetan geothermal springs (data from [46]). (a) Spatial distribution of Rb concentrations, and (b) spatial distribution of Cs concentrations. Red names shown in the diagrams represent the geothermal springs whose concentrations exceed the cut-off or industrial grades. Abbreviations are the same as in Figure 2.
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Figure 8. Donut chart of resource flux of Li, Rb, Cs, and B in the Tibetan geothermal springs.
Figure 8. Donut chart of resource flux of Li, Rb, Cs, and B in the Tibetan geothermal springs.
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Table 1. Cut-off and industrial grades of lithium, rubidium, cesium, and boron in brine in China. The lithium and boron standards are based on “People’s Republic of China geological and mineral industry standard: Specifications for salt-lake, salt mineral exploration” [56], and the rubidium and cesium standards are based on “People’s Republic of China geological and mineral industry standard: Specifications for rare metal mineral exploration geological and mineral industry standard” [57].
Table 1. Cut-off and industrial grades of lithium, rubidium, cesium, and boron in brine in China. The lithium and boron standards are based on “People’s Republic of China geological and mineral industry standard: Specifications for salt-lake, salt mineral exploration” [56], and the rubidium and cesium standards are based on “People’s Republic of China geological and mineral industry standard: Specifications for rare metal mineral exploration geological and mineral industry standard” [57].
Critical MineralLithiumRubidiumCesiumBoron
Cut-off grade (mg/L)2530.5125
Industrial grade (mg/L)505020315
Table 2. Summary of estimated total annual resource flux of lithium, rubidium, cesium, and boron in Tibetan geothermal systems. The “n” represents the number of geothermal springs reaching this grade rank.
Table 2. Summary of estimated total annual resource flux of lithium, rubidium, cesium, and boron in Tibetan geothermal systems. The “n” represents the number of geothermal springs reaching this grade rank.
Grade Rank (mg/L)Annual Resource Flux of Li (t/a)Grade Rank (mg/L)Annual Resource Flux of Rb (t/a)Grade Rank (mg/L)Annual Resource Flux of Cs (t/a) Grade Rank (mg/L)Annual Resource Flux of B (t/a)
≥502 (n = 2)≥34 (n = 7)≥2023 (n = 6)≥315125 (n = 5)
25–5025 (n = 5)1–318 (n = 22)0.5–20179 (n = 114)125–315235 (n = 10)
10–25100 (n = 23)0–132 (n = 164)0–0.531 (n = 65)50–1251176 (n = 24)
1–10109 (n = 92) 10–50910 (n = 68)
0–19 (n = 96) 0–10301 (n = 155)
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Wang, D.; Xue, F.; Ren, L.; Li, X.; Wang, S.; Er, X.Q. Critical Minerals in Tibetan Geothermal Systems: Their Distribution, Flux, Reserves, and Resource Effects. Minerals 2025, 15, 93. https://doi.org/10.3390/min15010093

AMA Style

Wang D, Xue F, Ren L, Li X, Wang S, Er XQ. Critical Minerals in Tibetan Geothermal Systems: Their Distribution, Flux, Reserves, and Resource Effects. Minerals. 2025; 15(1):93. https://doi.org/10.3390/min15010093

Chicago/Turabian Style

Wang, Di, Fei Xue, Lijian Ren, Xin Li, Songtao Wang, and Xie Qibei Er. 2025. "Critical Minerals in Tibetan Geothermal Systems: Their Distribution, Flux, Reserves, and Resource Effects" Minerals 15, no. 1: 93. https://doi.org/10.3390/min15010093

APA Style

Wang, D., Xue, F., Ren, L., Li, X., Wang, S., & Er, X. Q. (2025). Critical Minerals in Tibetan Geothermal Systems: Their Distribution, Flux, Reserves, and Resource Effects. Minerals, 15(1), 93. https://doi.org/10.3390/min15010093

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