The Genesis of Cs-Rich Geyserite in the Chabu Geothermal Field in Southern Tibet

Abstract

Recent studies have shown that the geothermal systems in Tibet are rich in rare metal elements such as lithium (Li), boron (B), rubidium (Rb), and cesium (Cs). However, the understanding of the origin of Cs-rich geyserite formed by hot springs remains unclear. In this study, a detailed petrological, elemental geochemical, and strontium–neodymium (Sr–Nd) isotopic investigation on Cs-rich geyserite in the Chabu region revealed that opal was the main mineral component of Chabu geyserite; here, some samples were rich in terrigenous clastic material, and well-developed diatom fossils were also present. Chabu geyserite had high contents of SiO₂ (78.95%–94.72%) and Al₂O₃ (3.02%–8.14%) and low contents of Fe₂O₃ (0.21%–1.94%), TiO₂ (0.01%–0.20%), MnO (0.01%–0.15%); additionally, the Fe/Ti ratio, the Al/(Al + Fe) ratio, and the Al/(Al + Fe + Mn) ratio showed large variations. These results indicated different degrees of participation by the terrigenous materials, hydrothermal deposition, and biogenic processes. Chabu geyserite was depleted in transition metal elements (e.g., Sc, V, and Cr) and high field strength elements (e.g., Nb, Zr, and Hf), relatively enriched in large-ion lithophile elements (e.g., Li, Rb, Sr, and Ba), and strongly enriched in Cs, (by up to 100 times the Cs content in the upper crust); in addition, it had low V/Y (1.30–2.00) and U/Th ratios. Chabu geyserite exhibited a right-dipping rare earth element (REE) distribution pattern and had significant negative Eu anomalies (0.26–0.72) and no or weak positive Ce anomalies (0.97–1.36). These results further indicated the influence of terrigenous clastic materials and nonhydrothermal sedimentation factors. The Sr–Nd isotopic composition of Chabu geyserite was significantly different from that of the mantle, with relatively high ⁸⁷Sr/⁸⁶Sr ratios (0.7070–0.7076) and low ¹⁴³Nd/¹⁴⁴Nd ratios (0.512223–0.512314). These ratios were similar to those of the crust. Combined with previous studies, the results from this study indicated that Chabu geyserite was a Cs-rich geyserite and was formed in an intracontinental post-collisional orogenic environment, mainly from crustal material, with the participation of biological and hydrothermal processes.

1. Introduction

Due to the collision between the Indian plate and the Eurasian plate, a wide range of tectonic–magmatic events developed in the Qinghai–Tibet Plateau orogenic belt [1,2]. These events provided excellent conditions for geothermal activity and caused hot springs to form in the Qinghai–Tibet Plateau. This area currently has the most hot springs in China [3,4]. These hot springs not only provide abundant geothermal resources but are also enriched in rare metal elements such as Li, B, Rb, and Cs [5,6,7]. Thus, these hot springs have attracted research attention in relation to geothermal activities and mineral resources [8,9]. Cesium and its derivatives are extensively utilized in catalysts and high-tech applications. One notable use of cesium is in the creation of the world’s most precise atomic clocks, which provide accurate timing for satellite systems. Cesium bromide finds applications in the manufacture of X-ray fluorescent screens and spectrometer prisms, while cesium chloride is employed in producing radioactive isotopes for nuclear medicine, agricultural insecticides, and specialty glasses. With the continuous advancement of science and technology, the global demand for cesium is expected to grow significantly [10]. Previous research has explored the availability and extraction of cesium from cesium-rich siliceous sinter. For example, Zheng et al. (1995) [11] investigated cesium extraction from cesium-rich siliceous sinter deposits in Dagajia, Gulu, and Semi, achieving a cesium recovery rate exceeding 70%, which highlights its promising utilizability. The geothermal area of the Tethys Himalaya belt has developed unique Cs-rich geyserite, which is formed by high-temperature geothermal water activity. This Cs-rich geyserite has been rarely reported in other areas. The Cs content can reach thousands of times that of the continental crust. Some hot springs are known to have a Cs content that is industrial grade. These hot springs have potential for industrial mining. For instance, studies on the cesium-rich geyserite in Dagajia have shown that the Cs resource reaches 14,459.42 tons, classifying it as a super-large-scale cesium deposit [11]. Thus, the hot springs and sinters of the Qinghai–Tibet Plateau are potential resources for Cs rare metals [11,12].

Cs-rich geyserite in Tibet is concentrated mainly in the southern part of Tibet. For a long time, Cs-rich geyserite has been discovered in areas such as Dagejia, Semi, Gulu, Buxionglanggu, and Gudui; however, relatively few in-depth studies have been performed [11]. Zhao et al. (2007, 2009, 2010) [13,14,15] performed detailed petrological, geochemical, and chronological studies on the Cs-rich geyserite in Dagejia and Gulu, but different views on the genesis of Cs-rich geyserite remain. During the second comprehensive scientific expedition to the Qinghai–Tibet Plateau, we found extensive development of geyserite in Chabu village, Xietongmen, and the Cs content was up to hundreds or even thousands of times the average content of the Earth’s crust. Therefore, a detailed genesis study of Cs-rich geyserite in Chabu can provide a reference for Cs rare metal mineralization and further exploration and utilization in Tibet.

In this study, the focus is on Cs-rich geyserite in Chabu, southern Tibet. Based on detailed field investigations, petrological, elemental geochemical, and isotopic geochemical studies were systematically conducted, and the data were compared with other Cs-rich geyserite in the Tethys Himalayan Belt. The formation mechanism and tectonic setting of the Cs-rich geyserite in the Chabu geothermal field were determined, and the understanding of the genesis of the Cs-rich geyserite on the Qinghai–Tibet Plateau was further enhanced.

2. Regional Geological Background

The collision between the Indian plate and the Eurasian plate formed the Qinghai–Tibet Plateau orogenic belt; this belt is the highest and largest in the world [16]. From north to south, it includes the Qiangtang terrane, the Bangong Lake–Nujiang suture zone, the Lhasa terrane, and the India–Yarlung Zangbo suture zone, and the Himalayan terrane [1]. The Qiangtang terrane is predominantly characterized by the development of Late Paleozoic–Mesozoic strata, Triassic–Cretaceous granites, and Cenozoic volcanic rocks. Additionally, blueschists and eclogites are present in the central part of the terrane [17,18]. The Lhasa terrane is distinguished by the extensive development of Mesozoic–Cenozoic igneous rocks, including the renowned Gangdese granites and Linzizong volcanic rocks. The oldest basement exposed within this terrane is considered to be the Mesoproterozoic–Cambrian, represented by the Nianqingtanggula Group and the Amdo gneiss, while Late Paleozoic-Mesozoic strata are locally developed [19,20]. The Tethyan Himalaya mainly consists of low-grade metamorphic strata ranging from the Cambrian to the Eocene, with relatively limited exposure of Cretaceous–Early Tertiary strata. Metamorphic domes, such as Renbu and Kangmar, are distributed within this terrane, containing Ordovician and Cenozoic granites [21,22]. The direction of the main tectonic line of each terrane is generally east–west, and the India–Yarlung Zangbo suture zone and the Bangong Lake–Nujiang suture zone control the distribution of Cs-rich geyserite [12]. The southernmost terrane of the three terranes is the Himalayan terrane; this terrane has highly significant structures in the east–west direction, closely spaced folds and thrust nappes in the north–south direction, and many extensional and transtensional structures. The Lhasa (Gangdise belt) and Qiangtang terranes extend from south to north and have many prominent conjugate shear faults in the northeast and northwest directions, with numerous rhombic blocks [23]. After the Miocene, the Tibetan Plateau experienced large-scale uplift, followed by extension in the east–west direction [24]; here, a number of nearly north–south trending normal fault systems were formed and cut through the India–Yarlung Zangbo and Bangong Lake–Nujiang suture zones [25,26,27]. These nearly north–south trending normal fault systems caused the formation of rift valleys or graben basins of considerable scale in some areas [28,29,30], and these valleys or basins were spaced approximately 140 km apart from left to right in the plane (Figure 1). Therefore, based on the above geological background, the modern geothermal water activity and distribution are concentrated in the nearly north–south trending rift valleys or graben basins [31]. Four large thermal water zones exist in the Tethys Himalaya from west to east. These are the Tangra Yumco–Gucuo thermal water zone, the Shenzha–Xietongmen thermal water zone, the Yadong–Gulu thermal water zone, and the Sangri–Cuona thermal water belt [32]. The Chabu geothermal field is located south of the Shenzha–Xietongmen thermal water belt and north of the Yarlung Zangbo deep fault.

3. Geothermal Field Geology

The Chabu geothermal field is located in Chabu Township, Xietongmen County, Tibet Autonomous Region. The strata of the Cretaceous Chumulong Formation, the Takena Formation, and the Shexing Formation, as well as the Cenozoic magmatic rocks, are exposed in the surrounding areas. Quartz sandstone, siltstone, silty mudstone, and limestone of the Chumulong, Takena, and the Shexing Formations are distributed in the southern, eastern, and northeastern parts of the geothermal field. The Dianzhong Formation (E₁d) which mainly consists of the volcanic breccia and tuff of is distributed in the southern parts of the geothermal field, respectively, and the Palaeogene biotite alkali feldspar granite (E₃χργ), intruding the Dianzhong Formation, is distributed in the southeastern parts of the geothermal field (Figure 2). The Neogene porphyritic (N₁ηγπ) and medium grained (N₁ηγ) biotite monzogranite is widely distributed throughout the geothermal field, and they intruded the Palaeogene biotite alkali feldspar granite (E₃χργ) and were covered with the Quaternary sediment and siliceous sinter (Figure 2). Three groups of faults have developed within the geothermal field in the northeast direction, near north–south direction, and northwest direction, and the main geysers and geyserite in Chabu are located at the intersection of these three groups of faults (Figure 2). The temperature of the geothermal springs in the Chabu geothermal field ranges between 57.3–86.5 °C [34]. The geysers are distributed on both banks of Cebuxiongqu, while the main intermittent geyser, where sinters are widely developed, is located on the east bank. On the west bank, only a few sinter deposits and nearly no active geysers are present.

Geyserite is highly developed on the east bank of Cebuxiongqu, and is predominantly grey–yellow and grey–green (Figure 3a,b). The common structures include thin layers, compact blocks, porous textures, botryoidal stxructures, and crustiform structures (Figure 3c–f), as well as amorphous, cryptocrystalline, clastic, and biogenic structures (Figure 4a–d). In this study, two geyserite profiles, CB01 and CB11, were systematically examined. The geyserite samples of the two profiles all contained clastic materials of different sizes, including quartz, feldspar crystal fragments, and granite breccia. Notably, the CB11-1 sample contained numerous gravel-sized rock fragments cemented by geyserite, and the largest fragment was nearly 60 mm in length (Figure 4a). In addition to these minerals and rock fragments, the main mineral component of geyserite was opal (Figure 4c), and the diatoms were well developed (Figure 4d).

4. Analytical Methods

In this experiment, whole-rock geochemical analysis was carried out on the Chabu geyserite samples, and the experiments were performed by ALS Testing (Guangzhou) Co., Ltd. (Guangzhou, China). The major elements were analyzed using X-ray fluorescence spectrometry using a low-sulfur, low-fluorine major element analysis method for rocks. The detection precision (relative deviation, RD) was controlled below 5%, and the detection accuracy (relative error, RE) was maintained above 95%. The instrument was a PANalytical PW5400X fluorescence spectrometer (Malvern Panalytical, Malvern, UK). The testing method was as follows: the sample was oven-dried at 105 °C, then accurately weighed to the required mass (0.66 g, powder passing through a 200-mesh sieve), and placed in a platinum crucible. Next, a mixture of lithium tetraborate–lithium metaborate–lithium nitrate flux was added; afterwards, the sample was thoroughly mixed, and then melted at 1050 °C in a high-precision fusion machine. The melt was poured into a platinum mold and cooled to form a fused bead. This bead was checked to confirm its acceptable quality. Trace element contents were determined using a four-acid digestion method with inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS). The detection precision (relative deviation, RD) was maintained below 10%, and the detection accuracy (relative error, RE) was above 90%. The testing method was as follows: (1) 0.25 g of the 200-mesh sample was successively added to GR-grade perchloric acid and nitric acid, and the mixture was heated for 5 min for digestion and then cooled for 5 min; (2) hydrofluoric acid was added to the mixture for further digestion, and the sample was then cooled for 5 min; afterwards, hydrochloric acid was added to the sample for further digestion; and (3) the mixture was diluted to volume with dilute hydrochloric acid. The digested sample was analyzed using the Agilent 5110 ICP–MS. If the concentration of Bi, Hg, Mo, Ag, or W was high, the sample was diluted appropriately and then analyzed using the Agilent 7900 ICP–MS (Agilent Technologies, Santa Clara, CA, USA). The rare earth elements (REEs) were measured using a fusion method with ICP–MS, with the detection precision (relative deviation, RD) maintained below 10% and the detection accuracy (relative error, RE) above 90%. The testing method was as follows: lithium borate (LiBO₂/Li₂B₄O₇) flux was added to the 200-mesh sample; afterwards, the sample was thoroughly mixed and melted in a furnace at 1025 °C for 30 min. After the melt cooled, it was digested and diluted to volume with nitric acid, hydrochloric acid, and hydrofluoric acid, and then analyzed using the Agilent 5110 ICP–MS (Agilent Technologies, Santa Clara, CA, USA).

The Sr–Nd isotopic analysis of Chabu geyserite was completed by Wuhan SampleSolution Analytical Technology Co., Ltd. (Wuhan, China). The analytical instrument used was the multi-collector–ICP–MS (MC–ICP–MS, Neptune Plus, Thermo Fisher Scientific, Am Kalkberg, Germany). For Sr isotope sample testing, the precision was in the range of 0.01‰–0.03‰ (2 relative standard error (RSE)), with an accuracy better than 0.03‰; for Nd isotope sample testing, the precision was in the range of 0.01‰–0.05‰ (2RSE), with an accuracy better than 0.05‰.

5. Analysis Results

5.1. Elemental Geochemistry

Table S1 lists the major element results for the Cs-rich Chabu geyserite samples. The SiO₂ content of the Cs-rich Chabu geyserite (78.95%–94.72%, average of 91.14%) was generally lower than that of the marine siliceous rocks (91%–98.8%) and terrestrial siliceous rocks (96.41%–97.29%) [35]. The brecciated geyserite samples from the two profiles, such as CB01-1, CB01-2, CB01-3, and CB11-1, had relatively low SiO₂ content (78.95%–89.45%) but relatively high Al₂O₃ content (3.02%–8.14%, average of 4.96%). As a result, the brecciated geyserite had a relatively low Si/Al ratio (9.05–27.03, average of 18.15). These results indicated the influence of terrestrial detrital material. The remaining samples exhibited significantly low Al₂O₃ content (0.31%–1.16%, average of 0.65%), and relatively high SiO₂ content (78.95%–94.72%, average of 91.14%) and Si/Al ratio (76.35–292.06, average of 152.40). These contents were similar to those of relatively pure siliceous rock. The contents of Fe₂O₃, MgO, CaO, Na₂O, and K₂O were generally low and ranged from 0.21%–1.94% (average of 0.74%), 0.01%–0.38% (average of 0.12%), 0.11%–1.18% (average of 0.46%), 0.05%–1.91% (average of 0.40%), and 0.09%–2.52% (average of 0.50%), respectively. The contents of TiO₂, MnO, and P₂O₅ were all very low and ranged from 0.01%–0.20% (average of 0.09%), 0.01%–0.15% (average of 0.05%), and 0.02%–0.03% (average of 0.03%), respectively; additionally, some samples contained TiO₂, MnO, and P₂O₅, but their contents were below the detection limit (Table S1).

Table S2 provides the trace element results for the Chabu geyserite samples. The trace element contents of brecciated geyserite (Sample CB01-1, CB01-2, CB01-3, and CB11-1) were generally higher than those of the other geyserites, owing to the influence of terrigenous detrital materials. Based on the upper crust-normalized trace element spider diagram (Figure 5a), the contents of the transition metals (such as Sc, V, and Cr) and the high-field-strength elements (Nb, Zr, and Hf) in most geyserites were significantly lower than those in the upper crust; however, the large-ion lithophile elements (such as Li, Rb, Sr, and Ba) were relatively enriched with average Li:Rb:Cs ratio of 1:2.5:44.7. Moreover, the Cs content was strongly enriched and reached up to 100 times the Cs content in the upper crust (4.9 ppm) [36]. The V/Y ratio was low and ranged from 0.98 to 3.33, with an average of 1.89; the U/Th ratio was also low and ranged from 0.18 to 0.56, with an average of 0.28.

The results for REEs in the Chabu geyserite samples are listed in Table S3. The total content of REEs (ΣREE) was generally low; with the exception of sample CB11-1 (160.22 ppm), the ΣREEs of the other geyserites ranged from 7.51 ppm to 80.31 ppm. The ΣREEs of brecciated geyserite (Sample CB01-1, CB01-2, CB01-3, and CB11-1) were generally higher than those of the other geyserites as result of the effect of terrigenous detrital materials. These values were lower than the REE content in North American shale and consistent with the low REE content in siliceous rock [37,38]. In addition, the REE content in brecciated geyserite (58.12–160.22 ppm) was generally higher than those of the other geyserites (7.51–17.08 ppm). Based on the North American shale-normalized REE distribution pattern diagram (Figure 5b), Chabu geyserite exhibited a right-skewed distribution pattern with relative enrichment of light REEs (LREE) and relative depletion of heavy REEs (HREE); here, the LREE/HREE ratio ranged from 2.51 to 3.71, with an average of 3.08. Most samples had an evident negative Eu anomaly (Eu/Eu* = 0.26–0.72), with an average of 0.46; the Ce anomaly (Ce/Ce* = 0.97–1.36) was not pronounced, with an average of 1.06. The (La/Ce)_N ratio (0.91–1.27) was low, with an average of 1.19.

Figure 5

Spidergrams of the Chabu geyserite (a) and REE patterns (b). The value of upper crust shale from Rudnick and Gao [36]. The value of North American from McDonough and Sun [39].

[Figure omitted. See PDF]

5.2. Sr–Nd Isotope Geochemistry

Table S4 provides the results from the Sr–Nd isotopic analysis. The following data were observed: the Sr content in the three samples from Chabu geyserite was in the range of 123–365 ppm, with an average of 139.83 ppm; the ⁸⁷Sr/⁸⁶Sr ratio was in the range of 0.7070–0.7076, with an average of 0.7073, and the corresponding ε (Sr) was in the range of 34.40–42.82 ppm, with an average of 38.61 ppm; the Nd content was in the range of 1.3–3 ppm, with an average of 2.13 ppm; and the ¹⁴³Nd/¹⁴⁴Nd ratio was in the range of 0.512223–0.512314, with an average of 0.512256, and the corresponding ε (Nd) was in the range of −8.10–−6.32, with an average of −7.45.

6. Discussion

6.1. Formation Mechanism of Cs-Rich Geyserite

The geochemical composition of siliceous rock is an important indicator for determining its origin [35,40,41,42,43,44]. For example, the Al₂O₃ and TiO₂ contents can reflect the influence of terrigenous clastic materials [45]. In the brecciated samples (CB01-1, CB01-2, CB01-3, CB11-1) from the Chabu geyserite samples, the Al₂O₃ and TiO₂ contents were significantly higher than those of the other geyserite samples, and the SiO₂ content was significantly lower than those of other geyserite samples. In addition, the Al₂O₃ content had a clear positive correlation with TiO₂ content (Figure 6a) and an evident negative correlation with the SiO₂ content (Figure 6b). These results indicated varying degrees of influence caused by the addition of different terrigenous clastic materials. These results were consistent with the characteristics of brecciated rock fragments and mineral debris, such as quartz and feldspar, in different degrees of development, as observed in the field and using microscopy (Figure 4b). The enrichment of Fe and Mn often reflects the influence of geothermal water activity; thus, ratios of Fe/Ti, Al/(Al + Fe), and Al/(Al + Fe + Mn) are commonly used to determine the origin of the siliceous rocks. For example, Fe/Ti > 30 and Al/(Al + Fe) < 0.35 indicate a hydrothermal origin, Fe/Ti < 30 and Al/(AI + Fe) > 0.5 indicate a non-hydrothermal origin, and Al/(Al + Fe + Mn) > 0.6 often indicates a siliceous rock of biogenic or other non-hydrothermal origin [46,47,48,49,50]. The Fe/Ti ratio and Al/(Al + Fe) ratio of the Chabu geyserite samples were widely varied in the ranges of 7.5–55.71 and 0.28–0.76, respectively. These ranges were distributed in both the hydrothermal and non-hydrothermal genesis zones in the Fe/Ti-Al/(Al + Fe) diagram (Figure 7). The Al/(Al + Fe + Mn) ratio also varied within a range of 0.12–0.73. From the Al-Fe-Mn discrimination diagram (Figure 8), the Chabu geyserite was predominantly distributed in the biogenic or other non-hydrothermal zones, with a small portion in the hydrothermal zone. These results indicated that the formation of Chabu geyserite resulted from a combination of biogenic and hydrothermal genesis and were consistent with the aforementioned characteristics of the well-developed diatoms in the geyserite samples (Figure 4d).

Trace elements can also reflect the formation process of siliceous rocks. For example, hydrothermal sedimentation often has enriched uranium (U) and chromium (Cr) and a high U/Th ratio, whereas terrigenous clastic materials have high thorium (Th) and zirconium (Zr) contents [52,53,54,55,56]. For example, the hot brine areas in the Red Sea and the hydrothermal sedimentary areas in the East Pacific Rise have relatively high U contents and U/Th ratios [57]; however, Chabu geyserite had relatively low U contents and U/Th ratios, with significant differences from the modern hydrothermal sedimentary areas in the lg(U)-lg(Th) diagram, and its low U/Th ratios were similar to those of Dagejia and Gulu geyserites (Figure 9). In the Cr–Zr diagram (Figure 10), the brecciated geyserite samples (CB01-1, CB01-2, CB01-3, and CB11-1) show high Zr contents compared to Dagejia and Gulu geyserites due to the impact of terrigenous detrital materials. The other samples have low Cr and Zr contents and are similar to Dagejia and Gulu geyserites, while the Cr content was significantly lower, indicating that the impact of geothermal water deposition and terrigenous clastic material input is not significant. In addition, siliceous rocks formed by hydrothermal deposition typically exhibit distinct positive europium (Eu) anomalies and negative cerium (Ce) anomalies [58,59,60,61], whereas Chabu geyserite showed evident negative Eu anomalies, and its Eu/Eu* ratios were in the range of 0.26–0.72 (average of 0.46); at the same time, no or weak Ce anomalies were present and had Eu/Eu* ratios in the range of 0.97–1.36 (mean of 0.46). These results further highlighted that hydrothermal genesis was not the primary cause of the formation of Chabu geyserite.

Sr–Nd isotopic composition can be used to determine the material source of rocks. For example, mantle source areas often have low ⁸⁷Sr/⁸⁶Sr ratios and high ¹⁴³Nd/¹⁴⁴Nd ratios; in contrast, crustal source areas mostly have high ⁸⁷Sr/⁸⁶Sr ratios and low ¹⁴³Nd/¹⁴⁴Nd ratios [62]. The ⁸⁷Sr/⁸⁶Sr ratio of the Chabu geyserite samples were in the range of 0.7070–0.7076. These values were significantly higher than that of the mantle (0.7028), lower than that of the upper crust (0.714), and close to that of the lower crust (0.7060) [63]. In the ε (Sr) –ε (Nd) diagram (Figure 11), the Sr–Nd isotopic composition of Chabu geyserite was relatively depleted compared with that of Dagejia and Gulu geyserites and closer to that of the crust. All samples plot in the region of magmatic rocks in the Gangdese belt, indicating that the water-rock interaction played a role in the formation of Chabu geyserite, which is consistent with the development of terrigenous clastic materials in Chabu geyserite.

In summary, Chabu geyserite is a Cs-rich geyserite formed mainly by crustal material, with the combined participation of biological and hydrothermal processes. Recent research has also shown that the geothermal systems in Tibet are often enriched with rare metal elements such as Li, Rb, and Cs. These systems provide the material base for the formation of rare metal-rich geyserite [6,64]. As the geothermal fluid rises to the surface, the geothermal fluid rapidly evaporates due to the decrease in temperature and pressure; thus, this process leads to silica supersaturation and the formation of colloidal opal, where Cs replaces H+ and becomes enriched in the colloidal opal [11,23,64,65,66].

6.2. Formation Background of Cs-Rich Geyserite

Silica rocks from different formation environments often have distinct geochemical characteristics [50,67,68]. For example, the MnO/TiO₂ ratio and the Al₂O₃/(Al₂O₃ + Fe₂O₃) ratio are often used to reflect the formation environment of siliceous rocks. Mid-ocean ridge environments typically have high MnO/TiO₂ ratios (>3.5) and low Al₂O₃/(Al₂O₃ + Fe₂O₃) ratios (<0.4), whereas ocean basin environments have moderate MnO/TiO₂ ratios (0.5–3.5) and Al₂O₃/(Al₂O₃ + Fe₂O₃) ratios (0.4–0.6). Due to high amounts of terrigenous clastic materials and high TiO₂ and Al₂O₃ contents, continental margin environments generally have low MnO/TiO₂ ratios (<0.5) and high Al₂O₃/(Al₂O₃ + Fe₂O₃) ratios (>0.6) [47,48,69,70,71]. However, both MnO/TiO₂ ratio (0.15–15) and Al₂O₃/(Al₂O₃ + TFe₂O₃) ratio (0.34–0.81) of the Chabu geyserite samples varied in large ranges, and this variation could be related to its multiple formation factors and the influence of terrigenous clastic materials; the determination of its formation environment is challenging.

Trace elements and REEs can also reflect the formation environment of siliceous rocks. For example, ocean basin environments have high V/Y ratios (approximately 5.8), mid-ocean ridge environments have moderate V/Y ratios (approximately 4.3), and continental margin environments have low V/Y ratios (approximately 1.34) [53,72]. The V/Y ratio of Chabu geyserite was mostly concentrated between 1.30 and 2.00, with an average of 1.89. These results indicated that it was closer to a continental margin environment. Indicators, such as the (La/Ce) _N ratio and Ce anomaly, can also be used to identify the formation environment of siliceous rocks; here, samples with high (La/Ce) _N ratios (>3.5) and significant negative Ce anomalies (Ce/Ce* < 0.49) typically form in mid-ocean ridge environments. In contrast, continental margin environments are characterized by low (La/Ce)_N ratios (approximately 1) and less pronounced negative Ce anomalies or even positive Ce anomalies (Ce/Ce* > 0.65), and ocean basin environments have moderate (La/Ce) _N ratios (>3.5) and negative Ce anomalies (Ce/Ce* = 0.50–0.80) [72,73,74]. The (La/Ce) _N ratio of Chabu geyserite was in the range of 0.91–1.27 (average of 1.19); this range was consistent with the characteristics of Dagejia and Gulu geyserites [13,14] and indicated its similarity to siliceous rocks from continental margin environments. In addition, the right-skewed REE distribution pattern, which was characterized by the enrichment of LREE and the depletion of HREE, with a high LREE/HREE ratio (2.51–3.71, average of 3.08), further reflected its similarity to siliceous rocks from continental margin environments [75]. The above characteristics overall indicated that the Cs-rich geyserite in Chabu was formed in an environment relatively rich in terrestrial materials.

Recent research has shown that the collision between the Indian and Eurasian continents started in the early Palaeogene, forming the world’s largest and highest altitude land–land collision orogenic belt on the Qinghai–Tibet Plateau [2,16,76,77,78,79,80]. The recently reported A-type granites from the end of the Palaeogene are considered products of a post-collisional tectonic setting [81]. The subsequent tearing of the Indian lithospheric subduction slab provided favorable conditions for the thermal asthenospheric upwelling; this upwelling subsequently formed a series of north–south rift zones in the Qinghai–Tibet Plateau orogenic belt and led to the extensive development of tectonic–magmatic activity, thus providing the foundation for geothermal activity on the plateau [82,83,84,85,86,87], such as Gudui boiling springs in the Cona–Woka Rift, Gulu boiling springs in the Yangdong–Gulu Rift, and Chabu boiling springs in the Dinggye–Xainza Rift. The formation of these geothermal fields with extensive development of geyserite is controlled by faults, with the most development occurring at the intersection of faults. For example, the Gudui geothermal field is located at the intersection of NS–trending faults and EW–trending faults, the Gulu geothermal field is situated at the intersection of NS–trending and NE–trending faults, and the Chabu geothermal field is found at the intersection of NS–trending, NE–trending, and NW–trending faults [88,89]. Chronological studies of sinters on the Qinghai–Tibet Plateau revealed that these sinters all formed in the late Quaternary Period, starting at approximately 690 ka and continuing to modern age [8,9,11,15,23,90,91]. Therefore, the sinters in the Qinghai–Tibet Plateau, including Chabu geyserite, formed in an intracontinental post-collisional orogenic environment.

7. Conclusions

1. Chabu geyserite exhibited various structures, commonly thin layers, compact blocks, porous textures, botryoidal structures, and crustiform structures, as well as amorphous, cryptocrystalline, clastic, and biogenic structures. The main mineral component of geyserite was opal, and diatoms were also highly developed.

2. Chabu geyserite had a high SiO₂ content and low contents of Al₂O₃, TFe₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, MnO, and P₂O₅; the contents of the transition metal elements (e.g., Sc, Cr, and Ni) and high-field-strength elements (e.g., Nb, Zr, and Hf) were significantly lower than those in the upper crust; the large-ion lithophile elements (e.g., Li, Rb, Sr, and Ba) were relatively enriched, and Cs was strongly enriched. A right-skewed distribution pattern with relative enrichment of LREE and relative depletion of HREE was observed, and the samples had a relatively enriched Sr–Nd isotopic composition.

3. Chabu geyserite is a Cs-rich geyserite formed mainly by crustal material, with the combined participation of biological and hydrothermal processes, in an intracontinental post-collisional orogenic environment.

Author Contributions

R.-M.L.: Conceptualization, Investigation, Writing original draft. Y.-B.L.: Investigation, Writing-original draft, Writing-review & editing, Supervision. C.-J.Y.: Data curation, Resources. J.-S.Z.: Investigation, Resources. C.S., L.Y. and C.Y.: Data curation. G.-M.X.: Formal analysis. D.L.: Software. L.-C.Y., Z.-L.Z. and Z.-W.S.: Visualization. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. Diagram of active tectonic system and geothermal distribution in Tibet (after [33]). (1) Boundary faults and fault depression zones primarily characterized by normal faulting; (2) boundary faults primarily characterized by strike-slip motion; (3) suspected active faults; (4) early compressive deep faults; (5) typical Quaternary geothermal spring display areas; (F1) Southern Tibet detachment; (F2) Yarlung Zangbo deep fault; and (F3) Bangong Co–Nujiang Fault zone.

Figure 2. Geological map of the Chabu geothermal field (after [34]).

Figure 3. Chabu Geyserite outcrop and structure. (a,b) Profile outcrop; (c) lamellar structure; (d) massive structure; (e) vesicular structure; (f) crusty structure.

Figure 4. Chabu Geyserite structure. (a) Brecciated vein debris; (b) mineral debris such as quartz and feldspar; (c) gelatinous opal; (d) diatom organism.

Figure 6. Plots of (a) Al2O3 vs. SiO2 and (b) Al2O3 vs. TiO2 for Chabu geyserite.

Figure 7. Diagrams of Al/(Al + Fe + Mn) vs. Fe/Ti for Chabu geyserite (after [51]).

Figure 8. Triangular diagrams of Fe-Mn-Al in Chabu geyserite (after [48]).

Figure 9. Diagrams of Th vs. U for Chabu geyserite (after [14,47]).

Figure 10. Diagrams of Cr vs. Zr for Chabu geyserite (after [14]).

Figure 11. Diagrams of εNd vs. εSr for Chabu geyserite. The mantle, lithosphere mantle, upper crust and lower crust values are from Zhao [63].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15010036/s1, Table S1: Major (wt.%) element test results of the Chabu Geyserite; Table S2: Trace element (ppm) test results of the Chabu Geyserite; Table S3: Rare earth element test results of the Chabu Geyserite; Table S4: Sr-Nd isotope test results of the Chabu Geyserite.

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