Tungsten from typical magmatic hydrothermal systems in China and its environmental transport

Science of the Total Environment 657 (2019) 1523–1534

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Tungsten from typical magmatic hydrothermal systems in China and its environmental transport Qinghai Guo ⁎, Yumei Li, Li Luo State Key Laboratory of Biogeology and Environmental Geology, School of Environmental Studies, China University of Geosciences, 430074 Wuhan, Hubei, PR China

H I G H L I G H T S • Very high W concentrations are observed in some geothermal waters from YST. • Vastly different W/Mo ratios of the YST waters are ascribed to their various geneses. • Fe-bearing minerals in hot spring sediments are the major sink for geothermal W. • Low pH is favorable for the adsorption of geothermal W onto Fe-bearing minerals. • Immobilization of geothermal W was little affected by its polymerization/ thiolation.

a r t i c l e

i n f o

Article history: Received 6 September 2018 Received in revised form 27 November 2018 Accepted 10 December 2018 Available online 12 December 2018 Editor: Xinbin Feng Keywords: Geothermal tungsten Geochemical origin Environmental transport The YST Geothermal Province China

⁎ Corresponding author. E-mail address: [email protected] (Q. Guo).

https://doi.org/10.1016/j.scitotenv.2018.12.146 0048-9697/© 2018 Elsevier B.V. All rights reserved.

G R A P H I C A L

A B S T R A C T

pyrite

tungsten-rich hot spring

goethite outflow channel

a b s t r a c t Tungsten is of extraordinarily high concentrations in the geothermal waters discharging from several representative Tibetan magmatic hydrothermal systems (up to 1103 μg/L), which are also characterized by exceptionally high W/ Mo molar ratios (up to 1182). The geochemical origins of the tungsten in these geothermal waters were investigated, with a comparison to those from Rehai, the sole magmatic hydrothermal system in Yunnan, which is another major part of the Yunnan-Sichuan-Tibet Geothermal Province of China. The results show that the lithology of reservoir host rocks is the primary factor controlling the tungsten concentrations of the geothermal waters, although the contribution of magmatic fluid input cannot be ruled out. In this study, the geothermal waters are generally rich in sulfide, and therefore the molybdenum in the reservoir fluids has been substantially precipitated as the form of molybdenite; in contrast, the reservoir fluids are well undersaturated with respect to tungstenite which is much more soluble than molybdenite. Thus the neutral/alkaline hot springs, i.e. the evolved reservoir fluids, have high W/Mo molar ratios as well. In the hot spring sediments, the distribution pattern of tungsten is quite different. The concentrations of tungsten are the highest in the sediments with high iron concentrations collected from the acid hot spring vents and outflow channels. The adsorption of aqueous tungsten onto iron-bearing minerals, like goethite or pyrite, is favorable at acid pH values and thereby responsible for the very high tungsten concentrations of these acid hot spring sediments. The proportions of thiotungstates in total tungsten are quite low for all the hot springs, as indicated by thermodynamic calculations, suggesting that thiolation of tungstate has little impacts on the environmental transport and fate of geothermal tungsten in the investigated hydrothermal areas. This is the first study to report the tungsten geochemistry of hot springs in mainland China. © 2018 Elsevier B.V. All rights reserved.

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1. Introduction The important role of tungsten in biology has been proposed for a relatively long period (da Silva and Williams, 1991; Johnson et al., 1996; Kletzin and Adams, 1996; Bevers et al., 2009; Majumdar and Sarkar, 2011), while only recently its toxicity received increasing attentions. An investigation on the toxicological profile of tungsten, especially its relations to the leukemia cluster events occurring in Nevada, USA, was initiated by the United States Centers for Disease Control and Prevention (CDC) in the early 2000s (USCDC, 2003), after which more and more studies suggested that exposure to elevated tungsten concentrations may cause various pathological changes or diseases, like rhabdomyosarcoma, lung inflammation, DNA damage in bone marrow, etc. (Kalinich et al., 2005; Roedel et al., 2012; Kelly et al., 2013; George et al., 2015), although the general knowledge frame necessary for a full understanding of the toxicology of tungsten remains somewhat sketchy and fragmentary until now (Schell, 2005; Sheppard et al., 2006; Sheppard et al., 2007; Schell and Pardus, 2008). The background concentrations of tungsten in natural waters, one of the most important environmental media, are generally very low. The concentration of tungsten in precipitation was reported to be 0.76 ng/L (Kist, 1994), and river water contains tungsten with concentrations mostly below 0.1 μg/L (Krauskopf, 1974; Van der Sloot et al., 1985). Reported average tungsten concentrations for seawater vary greatly, from only 0.2 ng/L (Kunzendorf and Glasby, 1992) to 10 ng/L (Sohrin et al., 1987; Sohrin et al., 1999). However, under the impacts of natural processes (e.g. volcanic activities, weathering or leaching of tungsten-rich rocks, etc.) or anthropogenic activities (e.g. exploitation and smelting of tungsten ores, use of tungsten as a replacement for lead ammunition, discharge of certain industrial and domestic wastewaters, etc.), the aqueous tungsten concentrations in the environments might be much higher. As one of the most important sources of tungsten in environment, geothermal waters were paid increasing attention in recent years. In contrast to other types of natural waters, geothermal waters are characterized by higher tungsten concentrations. A study on tungsten-rich hydrothermal vent fluids from two known locations in the North Pacific Ocean and the East China Sea showed tungsten concentrations as high as 2.8 and 22.6 μg/L, respectively (Kishida et al., 2004). Recently published tungsten concentrations of the geothermal waters discharging from the basaltic areas in Iceland can be up to 77.9 μg/L (Stefánsson and Arnórsson, 2005). The highest concentration of tungsten in the hot springs in the South Nahanni area of Canada is 224.5 μg/L (Hall et al., 1988). The discharge of geothermal waters (as the form of either hot spring or geothermal well discharge) commonly causes the elevated tungsten concentrations of other natural waters. A good example is the Gibbon River in the Yellowstone National Park (USA), where the tungsten concentration increased downstream from 0.28 μg/L (background value) to a value up to 6.4 μg/L with a factor of over 20, due to the substantial geothermal input of tungsten with high concentrations up to 43.5 μg/L (McCleskey et al., 2010). Likewise, the tungsten concentrations as high as 100 ng/L in the North Atlantic Ocean (Merian et al., 2004) were attributed primarily to the input of tungsten from the hydrothermal vents in the ocean floors (Koutsospyros et al., 2006), although a certain contribution from the intensive industrial activities in the areas around the North Atlantic may not be completely ruled out. As a transition metal in Group VIb of the Periodic Table of Elements, tungsten occurs predominantly in natural waters as tungstate, a hexavalent oxyanion (Johannesson et al., 2000; Arnórsson and Óskarsson, 2007). In addition to tungstate, it can also occur in water in other forms, including polytungstates and thiotungstates. With low concentrations and under neutral/alkaline conditions, aqueous tungsten exists mainly in the form of monomeric tungstate; however, at high tungsten concentrations (≥0.01 mol/L) and under acid conditions (pH b 5), polytungstates are very likely to form (Wesolowski et al., 1984; Wood, 1992; Rodríguez-Fortea et al., 2008). In reducing, sulfide-rich aqueous environments, the oxygen in tungstate is prone to be

successively substituted by sulfur to form thiotungstates (Mohajerin et al., 2014a). The speciation of tungsten in water has non-negligible effects on its toxicity and geochemical behaviors. Monomeric tungstate is reported to be not as toxic and mobile as polymeric tungstates in aquatic environment (Strigul et al., 2009; Strigul et al., 2010; Sun and Bostick, 2015). Nevertheless, aqueous monomeric tungstate still behaves conservatively in alkaline environments, and is apt to be adsorbed onto iron and manganese oxides/hydroxides under neutral/acid conditions (Gustafsson, 2003; Koschinsky and Hein, 2003; Seiler et al., 2005; Johannesson and Tang, 2009; Kashiwabara et al., 2013). In general, high pH and low redox potential are favorable for the enrichment of tungsten in water (Cutler, 2011; Johannesson et al., 2013). In recent years, many experimental studies were carried out to investigate the transformation and transport of various tungsten species at the interfaces between water and minerals, including pyrite (Cui and Johannesson, 2017), goethite (Xu et al., 2009; Davantès and Lefèvre, 2015; Cui and Johannesson, 2017), hematite (Davantès and Lefèvre, 2015; Rakshit et al., 2017), ferrihydrite (Gustafsson, 2003; Kashiwabara et al., 2013), boehmite (Hur and Reeder, 2016), and clay minerals (Sen Tuna and Braida, 2014). The geochemistry of tungsten in natural environments was also studied, like surface water systems (Johannesson et al., 2000; Johannesson and Tang, 2009; Mohajerin et al., 2016), nonthermal groundwater systems (Cutler, 2011; Johannesson et al., 2013; Mohajerin et al., 2014b), and geothermal water systems (Kishida et al., 2004; Stefánsson and Arnórsson, 2005; Arnórsson and Óskarsson, 2007). Nevertheless, the geochemical data for tungsten in natural water systems are still limited (compared to those for molybdenum), especially in high-temperature hydrothermal systems. The systematic studies on the geochemical geneses and the environmental fates of geothermal tungsten are sparse. Therefore, the aims of this study are to present the distribution of tungsten in the hot springs and sediments in several representative hydrothermal areas in China (including Daggyai, Semi and Gudui in Tibet and Rehai in Yunnan), to identify the geochemical origins of tungsten in these geothermal systems, and to investigate the critical environmental geochemical processes controlling the fate of geothermal tungsten. This work represents the first study of tungsten in the hightemperature geothermal waters of mainland China. 2. Site description The hydrothermal areas investigated in this study, including Daggyai, Semi, Gudui, and Rehai, are all situated in the YunnanSichuan-Tibet Geothermal Province (abbreviated as the YST Geothermal Province; Fig. 1) and representative for the geothermal systems there. They have all been proven or speculated to be magma-heated systems (see the Supplementary data and the references therein). The YST Geothermal Province is the sole high-temperature geothermal region in mainland China, and belongs to the Mediterranean-Himalayas geothermal belt globally (Tong and Zhang, 1994; Liao and Zhao, 1999; Tong et al., 2000). The detailed site descriptions of Daggyai, Semi, Gudui, and Rehai are presented in the Supplementary data. 3. Materials and methods 3.1. Sample collection and analysis In this study, 63 geothermal water samples were collected from 4 selected hydrothermal areas (41 from Rehai, 12 from Daggyai, 4 from Semi, and 6 from Gudui; see Fig. 1 for the specific sampling locations). Prior to sample collection, the high-density polyethylene bottles for sampling were soaked in 5% HCl and then rinsed with deionized water 3 times in the laboratory, and the bottles were also rinsed with filtered geothermal water immediately before sampling. Water temperature, pH, and conductivity were determined with hand-held meters calibrated before sampling. All other samples, except for those for alkalinity analysis, were filtered through 0.2 μm pore-size cellulose-acetate

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(a)

Manidang River

(b) 1. Holocene sediment; 2. Miocene granite; 3. Jurassic adamellite; 4. Triassic slate; . Sampling location 1. Upper Triassic slate; 2. Lower Jurassic slate; 3. Holocene alluvium; 4. Silica sinter; 5. Travertine; 6. Diabase; 7. Fault breccia; 8. Fault; 9. Sampling location; 10. River

(c) (d)

1. Quaternary sediment; 2. Tertiary sandstone and conglomerate; 3. Cretaceous sandstone and mudstone; 4. Cretaceous granite; 5. Jurassic sandstone; 6. Sampling location; 7. Fault; 8. River

1. Middle Pleistocene basalt; 2. Lower Pleistocene andesite; 3. Miocene sandstone; 4. Cretaceous granite; 5. Proterozoic metamorphic rocks; 6. Sampling location; 7. Fault; 8. River

Fig. 1. Simplified geological maps of Gudui (a), Semi (b), Daggyai (c), and Rehai (d) as well as sampling locations.

membranes on site. Sulfide and total alkalinity were measured in the field using a HACH colorimeter (methylene blue method) and the Gran titration method, respectively. For the analysis of total metal concentrations, reagent-grade 14 M HNO3 was added to one sample split collected at each site to decrease the pH to b1, while no chemical agents

were added to sample splits for the analysis of major anions. In the laboratory, concentrations of SO42−, Cl−, and F− were determined by ion chromatography (IC), Na, K, Mg, Ca, B, and Si by inductively coupled plasma optical emission spectrometry (ICP-OES), and Al, Fe, W, and Mo by inductively coupled plasma mass spectrometry (ICP-MS). It is

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worth noting that the analysis of W was made using unacidified sample splits as well to avoid potential precipitation of tungsten oxide. The onsite parameters as well as the tungsten and molybdenum concentrations of the water samples are presented in Table 1, and the concentrations of other hydrochemical constituents in Table S1.

Table 1 Sampling sites, in-situ parameters, and tungsten and molybdenum concentrations of the water samples. T in °C, EC in μs/cm, tungsten and molybdenum in μg/L, and sulfide in mg/L. n.d.: not detected. Sample no.

Sampling area

DGJ00 DGJ01 DGJ04 DGJ05 DGJ06 DGJ07 DGJ08 DGJ09 DGJ10 DGJ11 DGJ12 DGJ13 SM01 SM02 SM03 SM04 GD01 GD02 GD02-1 GD02-2 GD02-3 GD03 LGG-HT LGG-LT DRTY01 DRTY02 DRTY03 DRTY04 DRTY08 DRTY10 DRTY11 WGQ DGG-AS ZZQ ZZQD1 ZZQD2 ZZQD3 ZZQD4 ZZQD5 TQL TQL#1 YJQ-L YJQ-R GMQ HTJ-L HTJ-R DGG SCQ SRBZ SRBZ#1 SRBZ#2 XKT-L XKT-R ZTH ZTH#1 HMZP-L HMZP-M HMZP-R HMZP-RD1 HMZP-RD2 HMZP-RD3 HMZP-RD4 HMZ

Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Semi Semi Semi Semi Gudui Gudui Gudui Gudui Gudui Gudui Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai

Sampling T 79.3 79.5 80.1 78.9 74.1 82.1 75.5 41.5 81.9 79.9 77.8 80.2 85.9 79.5 73.0 84.9 41.7 70.9 61.8 53.9 45.7 78.4 66.1 62.1 41.6 65.4 68.7 68.7 87.5 52.0 74.0 38.4 49.3 89.0 73.0 46.5 40.5 39.4 38.3 82.3 81.8 91.1 81.1 90.5 92.4 85.4 84.1 46.0 77.7 86.6 91.2 75.8 95.0 91.6 82.3 64.2 57.5 91.8 84.0 70.1 62.6 58.9 82.5

pH

EC

W

Mo

W/Mo molar ratio

Sulfide

8.59 8.24 7.40 8.25 6.96 6.97 6.92 6.90 7.35 6.99 7.00 6.00 7.15 7.17 3.39 8.46 7.56 7.47 7.72 7.76 7.97 8.37 2.73 2.66 1.85 1.85 1.93 1.93 2.57 1.38 1.90 5.36 2.08 2.81 3.46 3.62 3.30 3.21 3.29 7.96 9.00 8.88 9.32 8.12 7.40 6.88 7.45 7.60 7.90 8.80 8.90 7.46 7.53 7.13 8.20 7.14 7.40 9.96 8.37 8.72 8.90 8.99 7.70

1873 1910 1912 2004 1916 1872 1994 1678 1914 1966 1805 1502 2307 2655 2443 4184 2672 3735 3632 3660 3576 3840 1168 1209 2373 1629 1480 1484 579.4 8549 485.4 382.8 4442 600.3 556.6 569.2 540.9 553.4 567.6 3576 3640 3705 3599 3707 3401 2935 4288 1854 2516 2455 2480 2208 2378 2352 2433 2039 1955 2363 2200 2250 2281 2298 2814

899.9 868.5 879.2 950.1 897.6 890.4 923.1 919.9 925.6 961.2 849.3 795.7 298.4 334.0 18.8 1103 440.1 289.1 286.3 283.6 283.7 636.8 0.19 0.20 0.21 0.08 0.13 0.36 1.62 0.14 0.05 0.25 0.51 12.3 12.0 10.72 7.50 6.91 5.65 78.6 63.0 77.4 69.8 73.0 64.2 49.8 87.3 11.5 47.7 40.8 38.4 40.2 43.2 41.4 35.5 17.6 42.8 52.7 51.2 50.9 50.0 50.2 39.9

32.2 30.2 27.4 32.0 36.1 41.5 37.4 26.1 33.2 29.3 28.6 33.3 3.48 3.99 0.43 0.48 2.21 0.60 1.31 0.50 0.44 0.89 n.d. 0.28 0.30 0.31 0.26 0.25 0.20 0.48 0.31 n.d. 0.29 0.31 0.39 0.23 0.43 0.35 0.25 0.26 n.d. 0.43 0.33 0.49 0.31 0.38 0.32 0.58 0.26 n.d. n.d. n.d. 0.29 0.37 n.d. 0.27 0.35 0.25 0.33 0.23 0.28 0.31 0.33

14.5 14.9 16.7 15.4 12.9 11.2 12.8 18.3 14.5 17.0 15.4 12.4 44.5 43.5 22.8 1182 103 251 114 294 333 371 – 0.37 0.37 0.13 0.25 0.74 4.24 0.15 0.09 – 0.91 20.8 16.0 23.7 9.01 10.35 11.60 159 – 94.2 109 76.8 106.3 67.7 142 10.4 96.0 – – – 77.5 57.9 – 33.5 63.8 109.1 80.4 116.2 93.9 83.9 62.9

0.15 0.09 0.07 0.13 0.15 0.11 0.09 0.20 0.49 0.26 0.15 0.12 0.06 0.20 0.02 2.95 n.d. 0.29 0.05 0.02 0.01 0.18 0.02 0.02 0.01 n.d. 0.01 0.02 n.d. n.d. 0.08 0.75 0.07 0.04 n.d. n.d. n.d. n.d. n.d. 2.60 1.40 5.90 3.80 5.20 4.20 2.10 0.24 0.04 0.20 0.65 0.74 0.17 0.71 0.60 0.41 0.13 0.15 0.43 0.15 0.01 n.d. n.d. 0.31

Besides, 38 sediment samples were collected from the hot spring vents and outflow channels where geothermal waters were also sampled. Immediately after sampling, sediments were put into sealed N2filled plastic bags, preserved in anaerobic boxes with an O2 adsorbent (AnaeroPack, Mitsubishi) and an anaerobic indicator (Oxoid, England), and transported to the lab at 4 °C. They were stored at −20 °C in the lab until freeze-dried, disaggregated, and milled for further analyses. A split of each sediment sample was completely dissolved (total digestion) in a Savillex container with ultrapure concentrated HCl, HF and 6 M HClO4 (after Matthews et al. (2004)), and then the W and Fe concentrations of the liquids were determined by ICP-MS for calculating the corresponding concentrations of the sediments, the results being shown in Table 2. Notably, Na2WO4·2H2O was used as a reference material to testify the reliability of the digestion method. Three reference samples were tested and their recovery rates were up to 89%, 91%, and 97%, respectively. All the sediments were also analyzed with a FEI Quanta200 Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray (EDX) analytical capability. Images were taken at 20 kV excitation to highlight compositional variations in the samples. Energy dispersive X-ray spectroscopy (EDX) was performed at 20 kV as well to obtain the elemental composition. Special attention was paid to the samples rich in either W or Fe. 3.2. Geochemical calculation and modeling Tungsten speciation was modeled for all the surface geothermal water samples based on total tungsten and sulfide concentrations using the PHREEQC code in combination with the WATEQ4F database (Parkhurst and Appelo, 1999) updated with thermodynamic data for tungstates from Smith and Martell (2004), for thiotungstates from Mohajerin et al. (2014a), and for polytungstates from Cruywagen and van der Merwe (1987), Smith and Martell (2004), and Rozantsev and Sazonova (2005) (see Table S2). The results are presented in Table S3. With the aid of PHREEQC, the saturation indices of these surface geothermal waters with respect to common tungsten-, molybdenum-, and iron-bearing minerals, such as tungstenite, wolframite, scheelite, molybdenite, goethite, and pyrite, were also calculated. The necessary thermodynamic data for the dissolution reactions of tungstenite and molybdenite are from Stefánsson and Arnórsson (2005), Arnórsson and Ívarsson (1985), and Robie and Hemingway (1995), and those for wolframite and scheelite from Reed and Palandri (2010). Moreover, to evaluate the role of iron oxides in aqueous tungsten adsorption onto hot spring sediments, the surface complexation of tungsten was simulated by assuming hydrous ferric oxide (HFO) as the adsorbing agent in the hot spring sediment samples. The relevant surface complexation reactions as well as their surface complexation constants (Johannesson et al., 2013) are presented in Table S2. To calculate the saturation indices of the reservoir fluids with respect to tungsten- and molybdenum-bearing minerals, the geochemical code SOLVEQ-XPT developed by Mark H. Reed and his colleagues (Reed et al., 2010) was used. During the calculation of saturation indices, the pH values of the reservoir fluids (corresponding to the neutral/alkaline springs) were also estimated using an approach proposed by Reed and Spycher (1984). In brief, the total mole of hydrogen ion species (MH+t) in a water sample, independent of temperature, pH and other chemical properties of the solution, was firstly computed from its hydrochemical analytical results obtained at the lab temperature, and then the computed total mole of hydrogen ion species in the sample, along with the hydrochemical compositions measured in the laboratory, were used to calculate its pH value and target saturation indices at the reservoir temperature. Notably, if the water sample under consideration was from a hot spring with sampling temperature close to local boiling point that should undergo a near-surface boiling process and produce a gas assemblage in spring vent (e.g. the YJQ spring and the GMQ spring in Rehai), the contributions of the flashed gases (including CO2, H2S, H2 and CH4) to MH+t had to be taken into account as well based on their weight

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Table 2 Concentrations of tungsten and iron in the sediments collected in the vents and the outflow channels of the hot springs (in ppm). Sample no.

Area

W

Fe

Sample no.

Area

W

Fe

Sample no.

Area

W

Fe

DRTY01 DRTY02 DRTY03 DRTY04 DRTY08 DRTY10 DRTY11 ZZQ ZZQD1 ZZQD2 ZZQD3 ZZQD4

Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai

7.88 8.06 6.49 7.29 16.0 4.76 7.31 71.0 991 536 218 197

7644 15,030 13,284 4000 736 1747 840 1375 28,902 21,675 8011 4859

TQL#1 YJQ-L YJQ-R GMQ DGG SRBZ#1 XKT-L XKT-R ZTH#1 HMZPL HMZPM DGJ05 DGJ07

Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Rehai Daggyai Daggyai

9.95 18.8 11.0 11.6 0.32 4.50 7.82 11.8 2.57 160 7.25 8.03 10.4

4108 3651 4340 4505 167 1471 3723 2961 9490 61,275 47,335 480 416

DGJ08 DGJ09 DGJ10 DGJ11 DGJ12 DGJ13 GD02 GD02-1 GD02-2 SM01 SM02 SM03 SM04

Daggyai Daggyai Daggyai Daggyai Daggyai Daggyai Gudui Gudui Gudui Semi Semi Semi Semi

9.12 16.6 10.6 12.6 11.2 17.4 11.7 10.3 6.44 62.4 54.6 286 35.0

234 463 422 331 451 840 2772 2172 1152 2728 4826 3249 2678

4. Results

which are the discharge of deep geothermal waters (or evolved deep geothermal waters). This group of springs has variable HCO3/Cl milliequivalent ratios. For those with high HCO3/Cl ratios, their elevated HCO3 concentrations resulted from either the mixing of HCO3-rich shallow waters or the attack of CO2 in ascending deep waters on rocks at near-surface environments, as Giggenbach (1988) proposed. The geological origins of the hot springs are confirmed by the triangular diagram of Na-K-Mg1/2 (Fig. 3) where the acid SO4 springs and those affected by the mixing of acid waters are well within the immature water area with some strongly acid springs very close to the Mg apex, and most neutral/alkaline Cl or HCO3-Cl springs are closely around the full equilibrium line or located in the partially equilibrated or mixed area.

4.1. General hydrochemistry of geothermal waters

4.2. Tungsten and its speciation in geothermal waters

The geothermal waters sampled in this study feature various hydrochemical types and geological geneses. In the SO4/Cl milliequivalent ratio vs. pH diagram (Fig. 2), they are classified into three groups. The first group is characterized by high SO4/Cl ratios and very low pH values, and these springs are typical steam-heated acid waters. The second group has moderate SO4/Cl ratios and a wide range of pH values, suggesting that they were formed by the mixing of “pure” steam-heated waters with neutral/alkaline Cl-rich waters (or HCO3rich waters for the case of the WGQ spring). The hot springs falling into the third group are of low SO4/Cl ratios and high pH values,

The tungsten concentrations of the geothermal waters investigated in this study vary greatly, from 0.05 to 1103 μg/L (Table 1). In general, the Tibetan geothermal waters have much higher tungsten concentrations (ranging from 289.1–1103 μg/L except for an acid spring with tungsten concentration of 18.8 μg/L) than the Rehai geothermal waters (ranging from 0.05–87.3 μg/L). In either Tibet or Rehai, the Cl-rich geothermal waters are far richer in tungsten than those steam-heated SO4 waters or their mixtures with other waters, as shown in Fig. 4-a. Another important feature of the neutral/alkaline Cl or HCO3-Cl waters in Tibet and Rehai is that their tungsten concentrations are

fractions in the gas phase (available in Liao and Zhao (1999) and Zhao et al. (2002)) and the weight ratio of total gas flashed from the geothermal fluid in reservoir to the spring water sample whose composition has been analyzed. Specifically, the weight proportion (β) of the total flashed gases in the reservoir fluid was calculated as below: β¼

H f −H w Hg −H w

where Hw, Hf and Hg are the enthalpy values of the hot spring, its corresponding reservoir fluid, and the flashed gases, respectively.

1000

Na/1000

SO4/Cl milliequivalent ratio

Group 1

100 Group 2

10

Full equilibrium line 160 200

1

120 80

240

Partially equilibrated or mixed

0.1

280

Group 3 320

0.01 0

2

4

6

8

10

pH Fig. 2. Plot of SO4/Cl milliequivalent ratio vs. pH of all the hot springs. Legend of symbols: , neutral/alkaline hot springs in Rehai; , steam-heated acid hot springs in Rehai; , hot springs in Rehai formed via mixing of “pure” steam-heated SO4 waters with neutral/ alkaline Cl-rich waters; , neutral/alkaline hot springs in Tibet; , hot springs in Tibet formed via mixing of “pure” steam-heated SO4 waters with neutral/alkaline Cl-rich waters.

Immature water area

K/100 Na-K isotherms K-Mg isotherms

√Mg

Fig. 3. Triangular diagram of Na-K-Mg1/2 for all the hot springs. Legend of symbols is the same as in Fig. 2.

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4.3. Tungsten in hot spring sediments

10000.00

(a)

W (mg/L)

1000.00 100.00 10.00 1.00 0.10 0.01 0.01

0.1

1

10

100

1000

SO4/Cl milliequivalent ratio

10000.00

(b)

W/Mo molar ratio

1000.00 100.00 10.00 1.00 0.10 0.01 0.01

0.1

1

10

100

1000

SO4/Cl milliequivalent ratio

Fig. 4. Plots of W concentration (a) and W/Mo molar ratio (b) vs. SO4/Cl milliequivalent ratio of all the hot springs. Legend of symbols is the same as in Fig. 2.

Although the tungsten concentrations of the Tibetan geothermal waters are generally much higher than the Rehai geothermal waters, those of the hot spring sediments don't show a distinct difference between Tibet and Rehai except for several Rehai samples with extraordinarily high tungsten concentrations (Fig. 5). In both Tibet and Rehai, the sediment samples collected in the tungsten-rich, neutral/alkaline hot springs have relatively low total tungsten concentrations. Instead, some acid hot spring sediments (e.g. the ZZQD1 sample in Rehai (water pH 3.46) and the SM03 sample in Semi (water pH 3.39)) feature much higher total tungsten concentrations (991 and 287 ppm, respectively). 4.4. Variations of tungsten, iron, and sulfide concentrations along the outflow channels of three representative hot springs The concentration variations of tungsten, iron, and sulfide along the outflow channels of three hot springs, ZZQ (pH 2.81), GD02 (pH 7.47), and HMZP-R (pH 9.96), representative of acid, neutral, and alkaline springs in the study areas, respectively, were delineated in Fig. 6. The sulfide concentrations of all three springs decreased rapidly to below the detection limit (0.01 mg/L). The concentrations of tungsten and iron in the source water of the ZZQ spring are 12.3 μg/L and 0.94 mg/L, respectively, and decreased dramatically to 5.65 μg/L and 0.50 mg/L at the site around 10 m downstream of the spring vent. In contrast, although the iron concentrations also showed sharp decrease trends within short distance downstream of both the GD02 and HMZP-R springs, their tungsten concentrations changed little. 5. Discussion 5.1. Source of tungsten as well as tungsten molybdenum relations in geothermal reservoir fluids As the acid sulfate springs occurring in the other high-temperature magmatic hydrothermal areas worldwide, e.g. Wairakei (New 1000

800

Tungsten (ppm)

overwhelmingly higher than molybdenum concentrations with W/Mo molar ratios from 10.4 to 1182 (Table 1; Fig. 4-b), which is distinctly different from other types of natural waters (e.g. river water (Johannesson et al., 2000) and sea water (Firdaus et al., 2008)) as well as the geothermal waters elsewhere (like Iceland (Stefánsson and Arnórsson, 2005; Arnórsson and Óskarsson, 2007)). In contrast, the acid SO4 waters in the study areas have comparable tungsten and molybdenum concatenations, with W/Mo ratios mostly ranging between 0.1 and 0.9 (Table 1; Fig. 4-b). PHREEQC modeling for the aqueous equilibrium speciation distribution of tungsten based on the chemical compositions of the geothermal water samples were performed. The results show that deprotonated tungstate (WO42−) is the predominant species in all the neutral/alkaline hot springs with proportions in total tungsten higher than 99% (Table S3). In contrast, quite a number of acid hot springs have significant proportions of polytungstates in total tungsten, with the highest up to 92.8% (the ZZQ spring; see Table S3). HW6O215− or W10O324− is the major polytungstate for all the hot springs. In addition, protonated tungstate, including HWO4− and H2WO4, also account for much higher proportions in acid springs than in neutral/alkaline springs (Table S3). Sulfide concentrations are up to 2.95 mg/L for the Tibetan geothermal waters and 5.90 mg/L for the Rehai geothermal waters, respectively. However, the modeling results indicate that 48 out of 51 hot springs investigated, in both Tibet and Rehai, contain very low concentrations of thiotungstates with the proportions of thiotungstates in total tungsten b0.1% (Table S3). HTJ-R and HTJ-L (located at Rehai) are two springs with the highest thiotungstates concentrations (0.40 and 0.32 μg/L, respectively) whose proportions in total tungsten are 0.8% and 0.5%, respectively. The other spring with relatively high proportion of thiotungstates (2.6%) is WGQ, which has only 0.01 μg/L thiotungstates, even much lower than HTJ-R and HTJ-L, due to its very low total tungsten concentration (0.25 μg/L).

600

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0 A

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Type Fig. 5. Box and whisker plot of tungsten in acid spring sediment samples in Rehai (group A), neutral/alkaline spring sediment samples in Rehai (group B), acid spring sediment samples in Tibet (group C), and neutral/alkaline spring sediment samples in Tibet (group D). The boxes show the mean value minus standard error, the mean value, and the mean value plus standard error. The smallest and largest values are indicated by the small horizontal bars at the end of the whiskers. ○: outlier values.

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0.8

0.008 3.46

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Fig. 6. Variations of tungsten and iron concentrations along the outflow channels of the hot springs ZZQ (a), GD02 (b), and HMZP-R (c). ○: tungsten concentration; △: iron concentration; the numbers in the figure indicate the pH values of hot spring waters at the sampling sites.

Zealand), El Tatio (Chile), The Geysers (USA) (Giggenbach and Stewart, 1982), Waiotapu (New Zealand) (Giggenbach et al., 1994), and Yellowstone (USA) (Nordstrom et al., 2009), those investigated in this study are also essentially locally-perched non-thermal waters heated by H2S-rich steam separated from reservoir fluid, as suggested by their hydrochemical compositions. Hence, these acid hot springs have totally different chemistry than the reservoir fluids, and they contain low concentrations of tungsten because tungsten is depleted in both the local non-thermal waters and the separated vapor phase. Furthermore, under acid conditions, the concentration of aqueous tungsten is

controlled by the low solubility of tungsten oxide (WO3·H2O). That is, even if there were an excess of tungsten in the acid hot springs investigated in this study, it would be apt to precipitate as the form of tungsten oxide, resulting in their low tungsten concentrations. In contrast, the alkaline hot springs likely form from the depressurization boiling of reservoir fluids, and the neutral springs from the conductive cooling of reservoir fluids or from the mixing of boiled reservoir fluids with small fractions of local non-thermal waters. Therefore, they have similar hydrochemical compositions to reservoir fluids. The high concentrations of tungsten in these neutral/alkaline springs should derive

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originally from the enhanced leaching of tungsten from reservoir host rocks at elevated temperatures. The four hydrothermal areas in this study are characterized by comparable reservoir temperatures estimated by use of Na-K geothermometer (generally 240–280 °C; see Table S4). Nevertheless, the tungsten concentrations of the neutral/alkaline hot springs in Daggyai, Semi, and Gudui (289.1–1103 μg/L) are much higher than those in Rehai (11.5–87.3 μg/L). Thus, one may consider that the distinct concentration difference of tungsten in hot spring between three Tibetan hydrothermal areas and Rehai is a reflection of the diversity of reservoir rocks as well as their tungsten concentrations. According to the analyses of tungsten in representative international geological reference samples (GeoRem; Govindaraju, 1994; Hu and Gao, 2008), the tungsten concentrations of most common rocks are at the same order of magnitude, such as basalt (0.21–0.9 ppm), diabase (0.31 ppm), andesite (0.47–0.56 ppm), rhyolite (1.5 ppm), shale (0.79–1.4 ppm), sandstone (1.16 ppm), and limestone (0.67 ppm), with two exceptions of dunite (0.024 ppm) and granite (8.4 ppm). Tungsten is more enriched in granite because it behaves incompatibly during magmatic evolution, as confirmed by its positive concentration correlations with typical incompatible elements (like K) in basaltic to silicic volcanic rocks of the tholeiite series in Iceland (Stefánsson and Arnórsson, 2005; Arnórsson and Óskarsson, 2007). Thus, it is plausible that the geothermal waters discharging from the granite-hosted Daggyai reservoirs have higher tungsten concentrations than those from the metamorphic rockshosted Rehai reservoirs. Moreover, the investigated hydrothermal systems have been confirmed or speculated to be heated by magma chambers (see supplementary data and the references therein), and thereby it cannot be ruled out that magmatic fluid input make a contribution to the tungsten in the geothermal waters as well. In fact, tungsten is well known to be rich in magmatic fluid, and is prone to form volatile compounds at magmatic temperatures (Arnórsson and Óskarsson, 2007). The enrichment of tungsten in the minerals occurring around the fumaroles in active volcanoes has also been observed (Bykova et al., 1995; Hamasaki, 2002). Thus, the concentration variation of tungsten in the magmatic fluids released from different magmas is possibly another reason for the difference in the tungsten concentrations of the sampled geothermal waters. However, this speculation requires further validation. Both tungsten and molybdenum are transition metals in Group VIb and have similar chemical properties. They are also marked by comparable crustal abundances with that of tungsten being somewhat lower (in molar unit). As reported elsewhere, the molar ratio of tungsten to molybdenum in the upper continental crust (UCC) is between 0.52 and 0.90 (Taylor and McLennan, 1985; Taylor and McLennan, 1995; Wedepohl, 1995; Gao et al., 1998; Rudnick and Gao, 2003). Likewise, the concentration of tungsten in most common rocks is lower than but still at the same order of magnitude as that of molybdenum (in molar unit; see Table S5) (GeoRem; Govindaraju, 1994; Hu and Gao, 2008). However, as mentioned earlier, the tungsten concentrations of

the neutral/alkaline hot springs collected in this study are much higher than their molybdenum concentrations, with W/Mo molar ratios ranging from 10.4 to 1182. One possible factor responsible for the much higher tungsten concentrations could be the non-stoichimetric rock dissolution occurring in the reservoirs. But preferential leaching of tungsten than molybdenum from host rocks in natural water systems (including geothermal water systems) has been seldom reported. In contrast, substantial removal of molybdate from the geothermal reservoir fluids is more likely to be the primary factor resulting in their extraordinarily high W/Mo molar ratios. Indeed, most geothermal fluids are oversaturated with respect to molybdenite (Fig. 7-a), suggesting that this mineral is expected to be stable and to constrain the mobility of molybdenum in geothermal water. Instead, all the fluids are far from being equilibrated with tungstenite (Fig. 7-b), and therefore the tungsten concentrations seem to be not controlled by tungstenite. Tungsten and chloride (typical conservative constituent) in the geothermal waters collected in each hydrothermal area show significant positive relations, while the concentrations of molybdenum do not display any regular variation with those of chloride (Fig. 8; only the plots for Rehai and Daggyai are presented, and those for Semi and Gudui are not because too few hot spring samples were collected there), indicating as well that tungsten is basically conservative in the geothermal waters but molybdenum is not. Thus, tungsten should be much more mobile than molybdenum in sulfide-rich waters where the concentrations of molybdenum are generally low due to solubility control of molybdenite. Arnórsson and Ívarsson (1985) also reported that hightemperature (N200 °C) and sulfide-rich geothermal waters in Iceland contain molybdenum with very low concentrations and concluded that molybdenum in high-sulfide geothermal waters is prone to precipitate as molybdenite or with other sulfide minerals. In general, the aqueous tungsten concentrations in the studied hydrothermal systems (especially for neutral and alkaline geothermal waters) are much higher than those in common low-temperature groundwater systems, primarily due to the intense interactions between geothermal fluids and reservoir hostrocks at elevated temperatures. Thus, compared with non-thermal aquatic systems, the tungsten released from the geothermal systems involved in this study poses a more severe threat to the local environments. Environmental transport and fate of geothermal tungsten are worth being further investigated in the study areas. 5.2. Environmental transport and fate of geothermal tungsten The tungsten concentrations of the sediments collected at tungstenrich neutral/alkaline hot springs are much lower than those at some relatively tungsten-depleted acid hot springs (e.g. ZZQD1 and SM03). The mechanisms for the accumulation of tungsten in the hot spring sediments, therefore, are critical for a deep understanding of environmental transport and fate of geothermal tungsten in the investigated hydrothermal areas. Just like the reservoir fluids, all the hot springs are also

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Fig. 7. Plots of the saturation indices of reservoir fluids with respect to molybdenite (a) and tungstenite (b) vs. their pH values.

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Fig. 8. Plots of tungsten and molybdenum vs. chloride in the hot springs at Rehai and Daggyai. Tungsten and chloride in the hot springs at both Rehai and Daggyai show significant positive relations (R2 = 0.852 at a significance level of b0.001 for Rehai and R2 = 0.820 at a significance level of b0.001 for Daggyai, respectively).

undersaturated with respect to tungstenite, indicating that the aqueous tungsten is unlikely to precipitate as the form of tungstenite on the surface. More complicatedly, the saturation indices of the hot springs to wolframite and scheelite do not change monotonously with their pH values (Fig. 9). At medium pH values (approximately from 3 to 8.5 for wolframite and 3.5–7 for scheelite, respectively), most hot springs are oversaturated to wolframite and scheelite, while the reverse is true at

lower or higher pH values. However, both wolframite and scheelite were not detected in any sediment samples by a thorough SEM-EDX analysis, demonstrating that even if these minerals were present in some hot spring sediments, they would be of very small quantity and not be the primary tungsten-bearing phases. In fact, if the tungsten in the hot springs had precipitated substantially as wolframite or scheelite, the sediments in the tungsten-rich neutral hot springs, which also have

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Fig. 9. Plots of the saturation indices of hot spring water samples with respect to wolframite (a), scheelite (b), goethite (c), and pyrite (d) vs. their pH values. Legend of symbols is the same as in Fig. 2.

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1200

W (ppm)

900

confirmed by Cui and Johannesson (2017), which was ascribed to the low point of zero charge of pyrite or the fact that the adsorption of tungstate (WO42−) is strongest at pH values near the acid dissociation constants of H2WO4 (pKa1 = 3.6 and pKa2 = 5.8). To further evaluate the contribution of iron oxides in hot spring sediments to the immobilization of geothermal tungsten, the surface complexation of aqueous tungsten onto hydrous ferric oxide (HFO) was modeled at various pH values, initial solution tungsten concentrations, and HFO-water ratios. The results (Fig. 11) confirm that pH is the most important parameter controlling adsorption of tungsten onto hydrous ferric oxide. In general, tungsten can substantially complex with hydrous ferric oxide at relatively low pH values with the fractions of adsorbed tungsten close to 100%, which decrease with further increasing solution pH values. The critical pH value, beyond which the fraction of adsorbed tungsten starts to decline, is dependent on primarily HFOwater ratio and secondly solution tungsten concentration. Specifically, with the increase of HFO-water ratio and the decrease of initial solution tungsten concentration, this critical pH value increases. It is also worth noting that at a low HFO-water ratio (0.1 mmol/L), the complexation of tungsten with HFO is not favored under very acid conditions (pH b 2) as strongly as under weakly acid conditions, especially when the initial solution tungsten concentration is high (5 μmol/L). It seems to be meaningful to explain why the sediments associated with the strongly acid hot springs with pH values lower than 2 in the study areas are not rich in tungsten. Polymerization may have effects on adsorption of tungstate onto iron-bearing minerals as well. Sun and Bostick (2015) claimed that the polymerization of aqueous tungstate likely hinders its immobilization due to the lower affinities of polymerized tungstates for mineral surfaces than monomeric tungstates. Nevertheless, as mentioned

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higher saturation indices to wolframite or scheelite, would have been much more enriched in tungsten compared to those in the tungstendepleted acid springs with pH values around 3, like ZZQD1 and SM03. But the fact is that the latter is characterized by far higher tungsten concentrations than the former (Table 2). In addition, the high concentrations of tungsten in some acid spring sediments may also be induced by precipitation of tungsten oxide in view of its low solubility under acid conditions, but it was not found in any samples by the SEM-EDX analysis as well. A lack of the thermodynamic parameters of tungsten oxide impeded our attempt in calculating the saturation indices of the geothermal water samples with respect to tungsten oxide, which would be helpful for judging if the enrichment of tungsten in those acid spring sediments resulted from the formation of tungsten oxide. The ubiquitous existence of iron-bearing phases in the hot spring sediments is suggested by the analytical results of their iron concentrations (Table 2). Indeed, most hot springs are oversaturated with respect to pyrite, and almost all the neutral/alkaline hot springs are oversaturated to goethite (Fig. 9). The occurrence of framboidal pyrite and goethite was also confirmed by the SEM-EDX analyses of some iron-rich hot spring sediment samples (Fig. S1). Thus, it is reasonable to speculate that as a siderophile element, the tungsten in the hot spring sediments is primarily associated with iron-bearing minerals. The substantial adsorption of aqueous tungsten species (including tungstates, thiotungstates and polytungstates) onto common iron-bearing minerals has been observed in many experimental studies. For instance, Xu et al. (2009) and Kashiwabara et al. (2013) reported strong tungsten adsorption onto goethite and ferrihydrite, respectively; Cui and Johannesson (2017) argued that pyrite can also be an important sink of tungsten in natural waters in view of its high affinity for both tungstate and tetrathiotungstate, although these tungsten species were still less adsorbed onto pyrite than goethite. In this study, the connection of tungsten with iron-bearing minerals is especially clear for those acid hot spring sediments enriched in tungsten, as shown by a statistically significant positive correlation (R2 = 0.675; at a significance level of b0.01) between the tungsten and iron concentrations of the sediment samples collected from the vents of the acid hot springs in Rehai as well as their outflow channels (Fig. 10). However, not all iron-rich sediment samples have relatively high tungsten concentrations, nor do all acid hot spring sediment samples. The typical examples of iron-rich but tungsten-depleted hot spring sediments are the samples HMZPM, DRTY02, and DRTY03, the pH values of the corresponding springs being 7.40, 1.85, and 1.93, respectively. The DRTY02 and DRTY03 springs have very low tungsten concentrations (0.08 and 0.13 μg/L, respectively). It implies that several conditions, including low environmental pH, high concentration of iron in sediment, and not too low concentration of tungsten in hot spring, are all indispensible for the enrichment of tungsten in hot spring sediment, among which low environmental pH seems to be the most critical. Indeed, the stronger adsorption of tungsten onto pyrite and goethite under acid conditions than neutral/alkaline conditions was

Fraction tungsten sorbed

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Fig. 11. Simulative results of surface complexation of aqueous tungsten onto hydrous ferric oxide (HFO) at various pH values (from 1.5 to 10), initial solution tungsten concentrations (0.001 and 5 μmol/L), and HFO-water ratios (0.1, 1.0, and 10 mmol HFO/ L water).

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earlier, the hot spring sediment samples with the highest tungsten concentrations were all collected from the vents or the outflow channels of the acid springs in which tungstate polymerized to relatively high degree, e.g. SM03 and ZZQD1 where the proportions of polytungstates in total tungsten can be up to 83.7% and 76.3%, respectively. So tungstate polymerization should be a secondary factor compared to environmental pH in terms of the impact on tungsten adsorption. Moreover, the thiolation of tungstate in sulfidic water is another process that possibly affects its adsorption onto iron-bearing minerals. However, although tungsten and molybdenum, both as Group VIb transition metals, have similar chemical behaviors in many aspects, tungstate is marked by much lower potential for thiolation than molybdate (Mohajerin et al., 2014a). As expected, the proportions of thiotungstates in total tungsten for the collected hot spring samples are very low (mostly b0.1% and the highest being just 2.6%; see Table S3). Thiotungstates are also more conservative and less particle reactive than tungstate (Cui and Johannesson, 2017) and thiomolybdates (Mohajerin et al., 2016) in sulfidic waterpyrite system. Thus, while it was proposed that the formation of thiomolybdates is critical for the fixation of molybdenum by pyrite (Vorlicek et al., 2004), the role of aqueous tungstate thiolation in its scavenging by the hot spring sediments investigated in this study should be unimportant. The distribution of tungsten in the hot springs and the sediments reveals that tungsten is conservative in the neutral/alkaline waters, but is prone to be captured by the hot spring sediments under acid conditions. Therefore, for the acid springs, geothermal tungsten tends to be immobilized in the sediments or the soils near the spring vents, causing the tungsten contamination within the hydrothermal areas or in their vicinities. In contrast, for the neutral/alkaline springs, the majority of aqueous tungsten is capable of transporting along their outflow channels all the way to the local surface waters. These surface water bodies are typically the local rivers (e.g. the Zaotang River in Rehai, the Changma River in Daggyai, and the Manidang River in Gudui), and receive almost all the hot springs waters. They flow far beyond the boundaries of the hydrothermal areas, and then discharge into the corresponding upper level rivers, posing a threat to the water quality safety there. Thus, well-directed treatment strategies need to be made for different types of hot springs that serve as potential sources of tungsten contamination.

Once geothermal tungsten enters the surface environment along with the discharge of hot springs, it may undergo diversified environmental geochemical processes. For the neutral/alkaline hot springs, tungsten behaves conservatively and tends to remain in the aqueous phase along their outflow channels, regardless of the iron concentrations of the sediments through which they flow; in contrast, the sharp decrease of aqueous tungsten concatenation was observed in the flow path of a representative acid hot spring in Rehai (the ZZQ spring) where the sediments are extremely high in both tungsten and iron. Nevertheless, the sediments collected from the acid hot springs with very low tungsten concentrations (b1 μg/L) are not tungsten-rich, and the tungsten concentrations of the iron-rich neutral/alkaline hot spring sediments are relatively low as well, indicating that low environmental pH, high concentration of iron in sediment, and not too low concentration of tungsten in hot spring are all necessary for the enrichment of tungsten in hot spring sediment. The polymerization of tungstate, which occurred substantially only in some acid hot springs, and the thiolation of tungstate, which are of low extents for all the springs, don't seem to affect much its adsorption onto hot spring sediments. In view of its different geochemical behaviors in various hot springs, geothermal tungsten, as a toxic element, requires well-directed treatment strategies to prevent it from contaminating the other natural waters downstream of the investigated hydrothermal areas that serve as drinking water sources.

6. Conclusion

Arnórsson, S., Ívarsson, G., 1985. Molybdenum in Icelandic geothermal waters. Contrib. Mineral. Petrol. 90 (2–3), 179–189. Arnórsson, S., Óskarsson, N., 2007. Molybdenum and tungsten in volcanic rocks and in surface and b100 °C ground waters in Iceland. Geochim. Cosmochim. Acta 71 (2), 284–304. Bevers, L.E., Hagedoorn, P.-L., Hagen, W.R., 2009. The bioinorganic chemistry of tungsten. Coord. Chem. Rev. 253 (3), 269–290. Bykova, E.Y., Znamenskii, V., Kovalenker, V., Marsii, I., Baturin, S., 1995. Associations and deposition conditions of molybdenum minerals in exhalation products of the Kudryavyi volcano, Iturup, Kuril Islands. Geol. Ore Deposits 37 (3), 227–235. Cruywagen, J.J., van der Merwe, I.F., 1987. Tungsten (VI) equilibria: a potentiometric and calorimetric investigation. J. Chem. Soc. Dalton Trans. 7, 1701–1705. Cui, M., Johannesson, K.H., 2017. Comparison of tungstate and tetrathiotungstate adsorption onto pyrite. Chem. Geol. 464, 57–68. Cutler, N.K., 2011. The Geochemistry of Groundwater and Sediments Governing Tungsten Concentration in the Basin-fill Aquifers Fallon, Nevada. University of Nevada, Reno. Davantès, A., Lefèvre, G., 2015. In situ characterization of (poly) molybdate and (poly) tungstate ions sorbed onto iron (hydr) oxides by ATR-FTIR spectroscopy. Eur. Phys. J. Spec. Top. 224 (9), 1977–1983. Firdaus, M.L., Norisuye, K., Nakagawa, Y., Nakatsuka, S., Sohrin, Y., 2008. Dissolved and labile particulate Zr, Hf, Nb, Ta, Mo and W in the western North Pacific Ocean. J. Oceanogr. 64 (2), 247–257. Gao, S., Luo, T.-C., Zhang, B.-R., Zhang, H.-F., Han, Y.-W., Zhao, Z.-D., Hu, Y.-K., 1998. Chemical composition of the continental crust as revealed by studies in East China. Geochim. Cosmochim. Acta 62 (11), 1959–1975. GeoRem, d. http://georem.mpch-mainz.gwdg.de. George, I., Hagege, A., Herlin, N., Vrel, D., Rose, J., Sanles, M., Orsiere, T., Uboldi, C., Grisolia, C., Rousseau, B., 2015. Assessment of respiratory toxicity of ITER-like tungsten metal nanoparticles using an in vitro 3D human airway epithelium model. Toxicol. Lett. 238 (2, S), S179. Giggenbach, W.F., 1988. Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta 52 (12), 2749–2765. Giggenbach, W.F., Stewart, M.K., 1982. Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas. Geothermics 11 (2), 71–80.

The neutral/alkaline geothermal waters discharging from Daggyai, Semi, and Gudui, three typical magmatic hydrothermal systems in Tibet, have much higher tungsten concentrations than those from Rehai, the sole magmatic hydrothermal systems in Yunnan, although they are characterized by comparable high W/Mo molar ratios. The acid steam-heated waters or their mixture with ascending chloriderich geothermal waters in these hydrothermal areas, however, are all tungsten-depleted. The distinct differences of tungsten concentration and W/Mo molar ratio among the geothermal waters of various types or from different areas are ascribed to their different geological and geochemical geneses. In general, the concentrations of tungsten in deep geothermal fluids depend on the lithology of reservoir host rocks, and possibly, the input of tungsten-rich magmatic fluids. The W/Mo molar ratios of surface geothermal waters, on the other hand, are also affected by the precipitation of molybdenite from the corresponding reservoir fluids. Moreover, common reservoir processes, like boiling and dilution, have effects on the tungsten concentration and speciation of reservoir fluid as well. Boiling of reservoir fluid is capable of increasing the tungsten concentration in liquid phase. Conversely, dilution of reservoir fluid by cold groundwater water results in the decrease of tungsten concentration in liquid phase, because tungsten is generally much richer in reservoir fluid than in cold groundwater water. Boiling and dilution can also decrease the temperature of reservoir fluid, which in turn changes its tungsten speciation.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos. 41772370, 41572335, and 41521001), the National Key R&D Program of China (No. 2018YFC0604303), and the Fundamental Research Funds for the Central Universities, China University of Geosciences, Wuhan (No. CUGQYZX1717). The very helpful comments of the reviewers are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.12.146. References

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