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1 NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.

2 Geothermal Resources Council TRANSACTIONS, Vol. 14, Part II, August 1990 GEOLOGY, GEOCHEMISTRY AND ISOTOPIC COMPOSITION OF HIGH- TEMPERATURE GEOTHERMAL SYSTEMS IN WEST YUNNM, CHINA Shen Minzi and Liao Zhijie Department of Geology, Peking University, Beijing, China ABSTRACT There are 34 high-temperature hydrothermal systems with reservoir temperatures of greater than 150-C in west Yunnan province of south west China. The Hot Sea geothermal field in Tengchong county is the most important scientific research and development field in the province. The geological settings of (1) a collision zone between the Eurasian and Indian plates for west Y unnan, and (2) a convergence zone for the Tengchong volcano-geothermal area are favorable for the development of a heat source and for the ascent of geothermal waters to the surface. Most high-temperature geothermal waters are of the HC4-Na type but some C1-HCq-Na and HC4-C1- Na types occur in the Hot Sea geothermal field and Langpu Hot Pool. The carbon-13 and sulfur-34 suggest that some mantle-derived carbonaceous and sulfur materials might have arisen to shallow depth and mixed with the hydrothermal waters. The deuterium and oxygen-18 study of spring waters of high-temperature geothermal systems confirmed they are originally meteoric. The oxygen and hydrogen isotope compositions as well as the tritium contents have been used to clarify the subsurface processes of the Hot Sea geothermal system. The "O(H20-S0,) geothermometer temperature of the Hot Sea geothermal system is in the narrow range of G near those obtained by the chemical geothermometers. The oxygen isotope equilibration of sulfate-water in the system is kept quite well. INTRODUCTION The west Yunnan active hydrothermal areas extend from latitude 23.45" to 28-45" and from longitude 97.30'E to 'E. There are more than 660 thermal springs and 34 of them are high-temperature hydrothermal systems with reservoir temperatures more than 150'C by chemical geothermometers. The regional geology and geochemistry investigation was one aspect of a comprehensive scientific expedition to the Hengduan Range of the Qinghai-Xizang (Tibet) Plateau. The most active hydrothermal systems on mainland China are present along the western foot of the Gaoligong Mountah of the Hengduan Range in Tengchong volcano-geothermal zone. There are 58 hydrothermal aread'] with various manifestations, such as thermal springs, bailing springs, steaming ground, fumaroles, and sinter deposits; distributed in one county. Geology, geochemistry and isotope studies of these systems have been carried out by various means in the 1980s. The study area w as essentially in its natural state and field work was limited to surface only. The recent investigation would be equivalent to the development or pre-exploration prospecting phase. This paper describes the high-temperature hydrothermal systems of west Yunnan and discusses those in Tengchong, especially the Hot Sea geothermal field in detail. GEOLOGICAL PATTERNS OF WEST YUNNAN West Yunnan is close to the collision zone between the Eurasian and Indian plates. The regional geological structure is quite complex as a result of several orogenic events. Systems of anticlinoriums and synclinoriums run almost parallel to each otherr2] (Figure 1). The anticlinoriums consist of metamorphic complexes, namely from west to east the Tengchong- Gaoligongshan belt, the Changning-Lancang belt and the Ailaoshan belt. Between the metamorphic belts there are the Paleozoic Baoshan basin and the Mesozoic Simao-Lanping basin. Some famous large faults, such as the Nu-g fault, the Lancangjiang fault and the Red River fault, constitute the boundaries between the uplifts and troughs. In the metamorphic belts the Early Paleozoic gneiss is overlain by granite and volcanic rocks. Among them the largest one is the Lincang granite batholith in the Changning-Lancang belt. The Baoshan basin comprises mainly Paleozoic marine deposits. The Simao-Lanping basin was filled with west Yunnan red strata of continental facies. The granite with different K-Ar ages ( Ma for the Lincang granite)[*] could supply heat for geothermal activities. The anticline structure is favorable for concentrating the heat, and the intensive Mesozoic Yanshan movement and Neotedonic activities offered a lot of channels for ascending geothermal fluids to reach the surface. Most of the thermal springs issue near the tenso-sheer faults, which are related to the major faults or to fault intersections. Tengchong and its neighboring areas are located at the convergence zone between two continental plates. The high-temperature Himalayan metamorphism, intrusions of complex granitic magma, late Cenozoic volcanism, and frequent seismic events are characteristic here. The active boiling springs 975

Shen and Liao occur around the margins of the volcanic flows, Some magma pockets could still remain and the follow-up magmatism may exist within the shallow crust, which might serve as the local

3 Shen and Liao occur around the margins of the volcanic flows, Some magma pockets could still remain and the follow-up magmatism may exist within the shallow crust, which might serve as the local powerful heat source to the active high-temperature hydrothermal systems. The Hot Sea geothermal field could be present in a circular structure, originated from a probable shallow-buried cupola[21. The high-temperature hydrothermal systems are mainly distributed in the Tengchong-Gaoligongshan belt and the Changning-Lancang belt. The wall rocks of most reservoirs are composed of crystalline rocks, such as granite, basalts, migmatites or gneisses and occasionally are of limestones. have low concentrations, with HC4 of ppm, Cl of ppm and TDS of < 1 g/l. The high temperature geothermal waters either in Tengchong or in other places have uniform SiO, content, in the range of ppm. It is noted that in the Hot Sea geothermal field the waters from wells 22-3 and 22-6 are stored in open ponds with intensive evaporation. The concentrations of constituents in these waters might be increased. Therefore, the Cl-HCO,-Na type water of with C1 of 372 ppm, higher than others, is suggested to represent the deep water, ascending to the surface with its original composition. This C1-type water more or less is similar to those of some well-known global geothermal fields and could have more potential for geothermal energy. The thermal waters of HCO, -Cl type with lower temperature might be formed by the mixing of deep thermal water with shallow groundwater around the periphery of the system. Figure 1. Sketch map showing the tectonic regions and the distribution of high-temperature hydrothermal areas in west Yunnan. ENVIRONMENTAL ISOTOPES OF HIGH-TEMPERATURE GEOTHERMAL WATERS Carbon-13 and Sulfur-34 The carbon isotope study of thermal waters has been used to identify the sources of carbonate materials. The distribution of 6% in most geothermal waters is in the range of t9 to -l%o, the values of geothermal waters in The Geysers, Steamboat and Yellowstone Park (Figure 2), and it is heavier than that of ground water, which usually has values of - 25 to -12%0, depending on the amount of dissolved carbonate materials. It was inferred there should be a deep carbonate material to serve as another source for the geothermal waters. Based on the 6I3C values and mixing model of b13c-l/c the geothermal waters have some carbonaceous materials with 6I3C values of -6.4 to -2.6%0 (Figure 3) that have emerged from the mantle. " I h I. Tengchong-Gaoligongshan belt 11. Nujiang fault belt 111. Boashan basin IV. Changning-Lancang metamorphic belt V. Simao-Lanping basin VI. Ailoashan metamorphic belt GEOCHEMISTRY OF HIGH-TEMPERATURE GEOTHERMAL WATERS Chemistry and isotopic compositions of hightemperature geothermal waters are shown in Table 1. The first four geothermal systems are located in Tengchong county and the others are scattered in different places (Figure 1). The chemical types of waters are rather simple. Most of the hydrothermal waters are HCQ-Na, but the Hot Sea and Langpu are Cl-HC0,-Na and HC0,-Cl-Na types. It is clear that only Hot Sea, Langpu and Ruidian thermal waters contain high contents of HC4 (500-1,000 ppm), CL ( ppm) and TDS (1-2 g/l). Others Figure 2. Frequency of distribution of carbon 13 of geothermal waters in west Yunnan. After mass and isatope balance calculations it was shown that about 1,300 ppm and 1,100 ppm of mantle carbonaceous material has entered the Langpu and Hot Sea geothermal waters,. The sulfur isotope study was conducted in Hot Sea geothermal field with samples of native sulfur, artrfinal... precipitation of BaSO, and CdS, some marcasite and alunogen. Most of the &34S fell into 0 976

4 &4%0. It is coincident with the idea deduced from carbon-13, that some mantle-derived materials may have arisen to shallow depth then mixed with the thermal waters. Because of no isotopic fractionation among the sulfur-bearing materials with different valences it could be supposed the temperature at shallow depth might be so high as to be unbearable for the sulfur bacteria[']. Deuterium and Oxygen-18 Deuterium and oxygen-18 analyses have been performed for most geothermal systems in Tengchong county. It has been confirmed that all geothermal waters including the high-temperature ones are originally meteoric. According to the mechanism of steam separation and isotopic fractionation['] the isotopic compositions of deep fluid with 276'C temperature from the Hot Sea geothermal system have been calculated[51 (point A at Figures 4 and 5). Using the altitude effect of deuterium, the recharge area of Hot Sea could be 800 m higher than the geothermal field and probably is situated in the east granite mountain areas. The oxygen isotope shifts of Ruidian and Hot Sea geothermal fields are %0 and %0, respectively. The various values of 6D, 6''O and C1 of thermal springs in Hot Sea have proven that there are subsurface processes of boiling and dilution under the geothermal field (Figures 4 and 5). Tritium The tritium content ((1 to 16 TU) of hot springs in the Hot Sea geothermal system confirms that mixing of ascending hut water occurs somewhere at depth..the springs formed by subsurface boiling have a tritium content of less than 5 TU. A tritium content of Shen and Liao -12 'r l. :E: '' I? iic Figure 3. Plot of 6 C and l/ct.10 for geothermal waters of west Yunnan TU is found in springs formed by mixing and subsurface bailing, and TU is found in springs formed by subsurface boiling-mixing. Isotopic Geothermoh The present study is the first time the sulfatewater geothermometer was applied to a hightemperature geothermal system of China. The ''0 values of dissolved sulfate are given in Table 1 for Table 1. The chemical and isotopic compositions of high-temperature geothermal systems in west Yunnan Locality Hot Sea Langpu Ruidian a Panzhihua 95 Bongbeng 95 Lanniba 93 Longwozhai 97 Quanqiaohe 96 Xingf u 95 Malutianta 96 Lingcang 64 Ebrybie 63 Laymadong 84 Eryuan 65 Niu jie d a Cl Cl HCO~-C~-N~ HC03-Cl-Na HCOj-Cl-Na HC03-Cl-Na C~-HLCO~-N~ HC03-Cl-Na IICO3-Nd HCO3-Na HC03--Na HCO3-Na tlco+ja ~03-Na HCO3-Na HCOj-Na I IC03-S04-Na IiC03-Na HC03-NCi C ' l ~ w *Xia Juanjuan, 1984, The isotopic geothermometer study of sulfate-water systems. Masters Thesis. Peking University (in Chinese) 977

5 Shen and Liao Hot Sea. From the difference in '*O content between sulfate and water, SO, -bo, isotopic equilibration temperatures TS04-%0 were calculated by use of the relationship: SO,-&O = looln S04-&0 3.06Sxl06/9-4.9 " representing the mean of fractionation factors experimentally determined by Lloyd[*] and Mieutani and Rafter'']. The temperatures obtained, 'C, are very uniform, and approximate the mean value of 215'C obtained by chemical geothermometers[81. The low scatter and good correlation with the chemical geothermomebrs suggest that the isotopic composition of sulfate is not a Eected by secondary processes such as steam loss or dilution, or the oxidation of hydrogen sulfide; therefore, it can be assume to reflect a close approach to equilibrium. Because of the slow rate of sulfate- water equilibration and the neutral to slightly alkaline waters of the Hot Sea geothermal system the application of the sulfate-water geothermometer here was verified and gave positive results IC \ 276'C L /A C1 Figure 4. Deuterium-chloride relations for waters of Hot Sea geothermal field. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) and the State Education Committee. Special thanks are given to Tong Wei, Zhang Mhgtao, Liu Shibin, Zhang Zhifei, Zhu Meixiang, Guo Guoying, Zhao Fengsan, Mu Zhichao and Chen Chengye. REPEREICES HI Tong Wei, Zhang Mingtao (eds), 1989, GEOTHERMICS IN TENGCHONG, Science Press, Bdjing. (In Chinese with English introduction and comprehensive summary). Lia0 Zhijie, Tong Wei, Liu Shibin, Zhao Fengsan, 1986, "Research for high temperature hydrothermal systems in west Yunnan province (P.R. China)," Proceedings of New Zealand Geothermal Workshop, 1986, Shen Minzi, Chen Chengye, Zhao Fengsan, 1989, Carbonate chemistry and carbon isatope characteristics in west Y unnan hydrothermal areas (in press). Truesdell, A.H., Nathenson, M., Rye, R.O., 1977, The effect of subsurface boiling and dilution on the isotopic compositions of Yellowstone thermal waters, J. Geophys. Res, 82, Shen Mind, Hou Fagao, Lin Ruifen, Ni Bag, 1988, Application of oxygen and hydrogen isotopes to waters in the Tengchong hydrothermal systems of China, Proceedings of a regional executive management seminar on isotope techniques and workshop on isotope hydrology, Bdjing, Lloyd, R.M., 1968, Oxygen isotope behaviour in the sulfate-water systems, J. Geophys. Res., 73, Mieutani, Y., Rafter, T.A., 1969, Oxygen isotopic composition of sulfates, New Zealand Journal of Science, 12, Zhang Zhifei, Liu Shibin, Zhao Fengsan, 1987, 1989, Geochemistry uf thermal waters in the Tengchong volcanic geothermal area, west Yunnan province, China, Geathermics, 16, b1*oc%.) 18 Figure 5. 6D relations for waters of Hot Sea geothermal field. For Figures 4 and 5 the lines AB and A'B' represent the subsurface boiling, the lines AC, BC and BC' - the mixing of geothermal water with ground water, the line BD - surface evaporation. 1/ Xia Juanjuan, 1984, The isotopic geothermometer study of sulfate-water systems, Masters Thesis, Peking University (in Chinese). 978

NOTICE CONCERNING COPYRIGHT RESTRICTIONS

NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials These materials have been made available for use in research, teaching, and private study, but may not be used for