Sulfur Isotope Geochemistry Indicating the Source of Dissolved Sulfate in Gonghe Geothermal Waters, Northwestern China Procedia Earth and Planetary Science 17 ( 2017 ) 157 – 160 Available online at ww[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 157 – 160 15th Water-Rock Interaction International Symposium, WRI-15 Sulfur isotope geochemistry indicating the source of dissolved sulfate in Gonghe geothermal waters, Northwestern China Mingliang Liua, Qinghai Guoa,1, Canhai Zhangb, Mingcheng Zhub, Jiexiang Lia a State Key Laboratory of Biogeology and Environmental Geology, Geological Survey & School of Environmental Studies, China University of Geosciences, 430074 Wuhan, Hubei, P R China b Huanghe Hydropower Development CO., LTD., State Power Investment Corporation, 810008 Xining, Qinghai, P R China Abstract The Gonghe region is characterized by the most active geothermal manifestation in Qinghai Province, northwestern China The geothermal waters from three hydrothermal areas (Qiabuqia, Guide and Xinghai) of Gonghe have different hydrochemical and isotopic compositions Nevertheless, most geothermal water samples fall into the area of evaporitic gypsum defined by the range of its δ34S-sulfate values and a few samples plot right on the border dividing the gypsum area and the atmospheric precipitation area, indicating that the formation of sulfate in Gonghe geothermal waters is commonly attributed to the dissolution of gypsum and/or anhydrite in host rocks, with a possible minor contribution of atmospheric precipitation Moreover, the sulfate in Guide geothermal waters may originate partly from the oxidation of H2S at near-surface oxidizing environment Sulfur isotopes are proved to be good indicators for identifying the geochemical genesis of geothermal water © 2017 2017The TheAuthors Authors Published by Elsevier Published by Elsevier B.V.B.V This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15 (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Keywords: geothermal water; sulfur isotope; indicator, Gonghe region Introduction Sulfur isotopes are often used as a natural tracer to monitor reaction mechanisms, intrinsic and extrinsic processes and rates of sulfate reduction or sulfide oxidation in groundwaters where S content and form varies along groundwater flowpaths1 The sources of sulfate in thermal waters are highly variable, which could be derived from dissolution of sulfate minerals (e.g., gypsum and anhydrate), oxidation of sulfides (e.g., pyrite) and biological activity2 To univocally constrain the sources of sulfate in geothermal waters, the isotopic composition of sulfate is the ideal solution, except when sulfate reduction is the dominant process3 Sulfate from dissolution of marine evaporite, such as gypsum/anhydrite, has δ34S values ranging from +10‰ to +30‰, while sulfides from igneous * Corresponding author Tel.: +86-13554116793; fax: +86-27-87436235 E-mail address: qhguo2006@gmail.com 1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.039 158 Mingliang Liu et al / Procedia Earth and Planetary Science 17 (2017) 157 – 160 rocks typically have δ34S values from -10‰ to +10‰ with an average of 0‰4 Sulfate derived from oxidation of sedimentary sulfides shows usually negative δ34S values5 Anthropogenic-contaminated rainwater has δ34S values ranging from +4‰ to +6‰, while agricultural fertilizers show δ34S from +0.4‰ to +11.9‰5 In this study, we use S isotopes to elucidate the source of dissolved sulfate and controls on redox processes in the Gonghe geothermal system Geological and hydrogeological setting The Gonghe geothermal region can be divided into three major areas, i.e Qiabuqia, Guide and Xinghai hydrothermal areas The Qiabuqia hydrothermal area is situated within the Gonghe Basin and the basement consists of Indosinian intermediate-acidic intrusive rocks and Triassic clastic and carbonate rocks with depths from 930 to 1440 m6 In Qiabuqia, the geothermal water system is composed of two reservoirs at different depths: a shallow reservoir at depths of 100-200 m and a deeper reservoir at 669-718 m7 The shallow reservoir consists of the Lower Pleistocene fine sandstone and mudstone, while the Neogene sandstone and conglomerate constitute the deep reservoir In contrast, the reservoirs below Guide and Xinghai, which are located in the mountain-front fracture zones, consist of the Indosinian-Yanshanian granite In general, the cap-layers of the Qiabuqia hydrothermal area are Quaternary alluvium and proluvium Faults are well developed in the Gonghe geothermal region and act as the channels for the convective circulation of geothermal waters Sampling and analysis Eleven geothermal water samples were collected in March 2014, and the sampling locations are shown in Fig Among the samples, were collected from the Qiabuqia hydrothermal area, from the Guide hydrothermal area and from the Xinghai hydrothermal area At each sampling site, bottles of water samples were collected and reagentgrade HNO3 was added to one of them to bring the pH below for cation analysis Unstable hydrochemical parameters, including temperature and pH, were measured using hand-held meters that were calibrated prior to sampling The cation concentrations were determined by ICP-OES (Thermo Fisher ICAP-6300), and the anion concentrations by ion chromatography (Dionex ICS-1100) within two weeks after sampling For sulfur isotopic analysis, the precipitates obtained in the field were recovered by centrifugation, washed and dried beforehand Prior to further treatment, CdS was converted to BaSO by the semi-melting method with Na2CO3-ZnO at 850℃ The conversion of BaSO4 to SO2 was completed via high-temperature combustion along with SiO2 and V2O5 at 980℃ Sulfur isotope ratios were determined by use of a Finnigan MAT-251 mass spectrometer and expressed in per mil(‰) with conventional delta-notation (δ) as well The δ34S values of sulfate in water samples are reported relative to the Canyon Diablo Troilite standard (δ34S-CDT=0‰) Quality control on sulfur isotope measurements was carried out based on international standard NBS 127 and lab standard LTB-2 whose measured δ34S values in this study are 20.22‰ and 1.84‰ respectively Duplicate preparations and analyses of δ34S agreed within±0.2 ‰.The sampling sites, in-situ parameters, hydrochemistry and isotopic chemistry of all water samples are listed in Table Fig Simplified geological map and sampling locations of the Gonghe geothermal region 159 Mingliang Liu et al / Procedia Earth and Planetary Science 17 (2017) 157 – 160 Table1 Characteristics of the water samples from Gonghe geothermal region Ca2+ Mg2+ Sampling No Sampling location pH temperature(℃) Na2+ Clmg/L HCO3- SO42- δ34S-SO42- GH-01 Qiabuqia hydrothermal area 67 7.8 45.7 1.8 576.5 678.4 603.8 135.6 10.6 GH-02 Qiabuqia hydrothermal area 39 8.1 19.7 7.8 481.2 611.5 323.8 140.8 12.6 GH-03 Qiabuqia hydrothermal area 39 8.2 10.2 3.8 482.4 489.2 404.6 205.3 12.4 GH-04 Qiabuqia hydrothermal area 26 7.8 47.9 13.9 495.8 771.7 165.6 262.5 10.1 GH-05 Qiabuqia hydrothermal area 41 7.7 48.3 11.5 509.7 812.7 130.5 270.4 9.8 GH-06 Qiabuqia hydrothermal area 27 8.2 43.8 17.6 234.3 264.3 157.4 186 7.7 GD-01 Guide hydrothermal area 84 8.1 35.9 1.1 484.6 686.5 74.1 216.2 7.0 GD-02 Guide hydrothermal area 86 8.1 31.2 0.9 425.8 715.3 76.5 221.3 7.2 GD-03 Guide hydrothermal area 80 8.5 58.3 1.4 350.5 272.0 35.7 466.1 8.5 XH-01 Xinghai hydrothermal area 60 7.9 33 4.2 255.2 132.7 148.7 319.1 20.8 XH-02 Xinghai hydrothermal area 47 8.4 28 2.1 272.5 170.9 151.9 397 23.1 Results and discussion 4.1 Hydrochemistry and isotopic characteristics In the Gonghe geothermal region, the geothermal waters are from three units: Qiabuqia, Guide and Xinghai According to Table 1, the average sampling temperature in the Qiabuqia hydrothermal area is 40 ℃, much lower than that in Guide and Xinghai All the samples collected in the Gonghe geothermal region show similar pH values, ranging from 7.7 to 8.4 Most samples contain Cl-, SO42- and HCO3- as predominant anions (Fig 2-b) and Na+ as predominant cation (Fig.2-a) Sulfate is the predominant sulfur species in Gonghe geothermal waters The measured δ34S of dissolved sulfate were between 7.7‰ and +12.6‰ in Qiabuqia hydrothermal area, 7.0‰ and 8.5‰ in Guide hydrothermal area, 20.8‰ and 23.1‰ in Xinghai hydrothermal area respectively Due to the different reservoir lithology and temperature, the hydrochemistry and δ34S values vary in these three hydrothermal areas Fig Relative Na+, Mg2+ and Ca2+ concentrations (a) and Cl-, SO42- and HCO3- concentrations (b) of all water samples (in equivalents) in the Gonghe geothermal region 4.2 Source of dissolved sulfate The variety of possible complex fractionation mechanisms, non-equilibrium state and uncertainties about the permeability of the geothermal system, make the interpretation of the sources of dissolved sulfates a difficult task Based on the previous data, the calculated reservoir temperatures were between 170 and 230 ℃ in the Gonghe geothermal region where S bacteria critical for S-isotope fractionation could not survive The isotopic composition of sulfur is plotted against the total concentration of sulfate of the geothermal waters in Fig Most samples from the three geothermal areas fall into the evaporitic gypsum and atmospheric precipitation regions, which show that 160 Mingliang Liu et al / Procedia Earth and Planetary Science 17 (2017) 157 – 160 the sources of sulfate for the Gonghe geothermal waters are commonly attributed to the dissolution of gypsum and/or anhydrite in host rock, with a possible contribution of atmospheric precipitation However, the SO4/Ca milligram equivalent ratios of the Gonghe samples are much greater than unity (Fig 4), implying that there are some other geochemical processes responsible for the occurrence of sulfate in the geothermal waters Considering that Nabearing silicates are the primary minerals in the host rocks of all reservoirs investigated in this study, the following processes are very likely to be related to the surplus of SO relative to Ca in the Gonghe geothermal waters: (1) Na-silicates + CO2 + H2O = Na+ + HCO3- + hydrated-silicates (2) CaSO4 + 2HCO3- = CaCO3 + SO42- + CO2 + H2O These reactions indicate that HCO3- generated by the attack of CO2 to Na-silicates can incorporate with Ca2+ from the dissolution of gypsum (or anhydrite) to precipitate calcite, which in turn increases the SO 4/Ca ratio of water Accordingly, the Na-SO4 and Na-SO4·Cl geothermal waters prevail in the study areas Moreover, some geothermal water samples, collected from Guide have quite high H2S concentrations, and therefore part of sulfate in Guide geothermal waters may originate from the H 2S oxidation at near-surface oxidizing environment Although the pyrite is not found in the stratigraphic sequence of Gonghe geothermal area, the possible sulfate source from oxidation of pyrite can not be ruled out Considering the human activities are not frequent in the study area, the anthropogenic influence is neglected Further analyses such as sulphate reduction processes, would be necessary to decipher the origin of sulfate content at Gonghe geothermal area Fig δ34S values vs SO4 content of Gonghe geothermal waters Fig Plot of Ca vs SO4 for the Gonghe geothermal waters Acknowledgements The research work was financially supported by the National Nature Science Foundation of China (No 41572335, 41120124003 and 41521001) and the Science and Technology Item of State Power Investment Corporation (No 2015-138-HHS-KJ-X) References Wynn JG, Sumrall JB, Onac BP Sulfur isotopic composition and the source of dissolved sulfur species in thermo-mineral springs of the Cerna Valley, Romania Chemical Geology 2010;271(1-2): 31-43 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Geology 2011; 38(2): 119-124 (in Chinese with English abstract) Yan WD, Wang YX, Gao XZ, et al Distribution and aggregation mechanism of geothermal energy in Gonghe basin Northwestern Geology 2013; 46(4): 223-230 (in Chinese with English abstract) ... state and uncertainties about the permeability of the geothermal system, make the interpretation of the sources of dissolved sulfates a difficult task Based on the previous data, the calculated... occurrence of sulfate in the geothermal waters Considering that Nabearing silicates are the primary minerals in the host rocks of all reservoirs investigated in this study, the following processes are... developed in the Gonghe geothermal region and act as the channels for the convective circulation of geothermal waters Sampling and analysis Eleven geothermal water samples were collected in March