213 Proc. 8th NZ Ceothctmal Workshop 1986 RESEARCH FOR HIGH TEMPERATURE HYDROTHERMAL SYSTEMS IN WEST YUNNAN PROVINCE (P R CHINA) LIAO ZHIJIE, TONG WEI, LIU SHIBIN, ZHAO FENGSAN Department of Geology, Beijing University, Beijing, P.R. China ABSTRACT The Tengchong-Gaoligongshan belt, which is part of the Mesozoic Yanshan Lhasa-Tengchong Fold Zone, consists of Early Paleozoic gneiss, Carboniferous There are more than 660 thermal springs in West plates, Mesozoic-Palaeogene granites and Pliocene- Yunnan Province, 30 of which are high-temperature Pleistocene volcanic rocks. The last metamorphic hydrothermal systems with reservoir temperature of period occurred about 23-50 Ma ago. Along the east above 150OC. fringe of this belt, the Gaoligongshan metamorphic gneiss and migmatite is separated from the Baoshan All thermal springs in West Yunnan are controlled by Paleozoic Basin by the Nujian fault. Among 193 tectonics, most of them are distributed along thermal areas in this belt nine high-temperature anticlinoriums, consisti,ng of metamorphic rocks and hydrothermal systems are located in Tengchong and its granites. In this paper, the relationship between surroundings; these amount to about 5 per cent of the thermal areas and tectonics, correlation of thermal total thermal areas in this belt. springs and tectonics, correlation of thermal springs between West Yunnan and North Thailand, and the The Baoshan Basin has complete Paleozoic marine importance of geothermal prospects in West Yunnan deposits with common and short disconformity. The will be discussed. latest marine deposit is M. Jurassic. There are about 60 thermal areas with low temperature in this belt. INTRODUCTION The Changning-Lancang Metamorphic belt consists of The term "high-temperature hydrothermal system" is Late Proterozoic gneiss consisting of an eugeosycline used here to describe a system where the reservoir sediment-volcanic complex on the east and Late temperature is more than 150OC. There are about 30 Proterozoic miogeosyncline deposits on the west; high-temperature hydrothermal systems in West Yunnan between these there are U. Paleozoic limestones and Province. Except for the Hot Sea geothermal field in basalts. The largest batholith in this area is the the Tengchong Cenozoic Volcanic Region, issuing chlo- Lincang granite with an K-Ar age of 244-96 Ma. The ride sodium waters, most thermal springs discharge last metamorphic stage was 210-280 Ma ago. In this almost neutral sodium bicarbonate waters, which are belt there are 167 thermal areas, 17 of which belong similar to those in NortheriThailand. to high temperature hydrothermal systems, making up about 10% of the total of thermal springs. The study of high-temperature hydrothermal areas in West Yunnan Province is an interesting topic and is The Simao-Lanping Trough was formed in the Middle important not only for basic problems in geology, but Triassic and filled with West Yunnan Red Strata, also for geothermal developments in the region. including sediments from M. Triassic to Eocene. In this belt (600 km long) there are 82 thermal areas with lower temperature. The distribution of thermal areas under the control of geological tectonics The easternmost Ailaoshan Metamorphic belt consists of three deep fractures. The axis of the Ailaoshan The province of West Yunnan (China P.R.) is close to is a mylonite belt which separates the intense the collision zone between Eurasian and Indian metamorphic gneiss, migmatites and migma-granites on Plates. Its geological structure is very complex and the east from weakly metamorphic Proterozoic slabs on is the result of several orogenic events. A system the west. The eastern foot of the Ailoashan Ranges of anticlinoriums and synclinoriums run almost is bounded by the Red River fault, showing dextral parallel to each other (see Fig.1). The anticlin- slip, along which many Neogene basins occur. The oriums consist of metamorphic complexes, namely the western fault is upthrusted from east to west, along Tengchong Gaoligongshan belt, the Changning-Lancang which there are many ultrabasic rocks (age 910-1300 belt and the Ailaoshan belt (from west to east). Ma). The last metamorphic stage of Ailaoshan gneiss Between the metamorphic belts there are the Paleozoic occurred 11.5-44.4 Ma ago. In the Ailaoshan belt, Baoshan Basin, which is the northern end of the Shan there are 25 thermal areas, 2 of which belong to Plateau of Burma, and the Mesozoic Simao-Lanping high-temperature hydrothermal systems. Basin, which extends into Laos and east Thailand. Many famous large faults constitute the boundary On the northwestern end of the Red River faults, between the uplifts and troughs, i.e. the Nujiang there is the Jingsajiang eugeosyncline. Diancangshan Fault, the Lancanjiang Fault and the Red River Fault. gneiss forms high mountains. The 57 thermal areas The Yangzi Platform is on the east of the Red River discharge from faults in Paleozoic limestones and Fault. Permian basalts. There are three high temperature hydrothermal systems in this area. 214 LIAO et al. Fig. 1 Sketch map showing the tectonic regions and the distribution of high- temperature hydrothermal areas in West YuLlnan . 0: Tengchong-Gaoligongshan anticlinorium; P: Baoshan synclinorium; Q: Changning-Lancang anticlinorium; R: Lanping-Simao synclinorium; S: Ailaoshan anticiinorium; T: Jinshajiang folded belt; U: Yangzi platform; V: South China fold belt. Fig. 2 Hyd rochemic.al charact erist icsof thermal waters from high- temperature hydrothermal systems in West Yuan 011-19; .I11-1117; 01111-1112; \ AIVl-IVS; AVl-VJ. 215 LIAO et al. The above-mentioned last five zones are called the The data for thermal springs in Burma are old and Sangiang (three river) Indosinican Fold Belt, where lack information about chemistry and temperature Mesozoic Yanshiln Movement and Neotectonic activities (Chhibber, 1934). Most springs are distributed along were very intense. the Shan Scarp from Mandalay to Tenasserim and occur in the Precambian Chaumg Magyi Supergroup and Mong Northern Yunnan, which is located east of the Long Schists (Hutchison, 1982). Jinshajiang Fault and the Red River Fault, belongs to the Yangzi Platform. There are 77 thermal spring The thermal springs in Burma are the continuation of systems in the area; three high-temperature hydro- the Tengchong-Gaoligongshan geothermal belt. The thermal systems are located along North-South thermal belt in Burma appears to be very narrow. trending tensional faults. The Fan Si Pan Massif of North Vietnam, which consists of gneiss-granites, appears to be the The high temperature hydrothermal systems continuation of the Ailaoshan Metamorphic belt; it has many low temperature springs (Waring, 1965). About 70% of all thermal areas in West Yunnan were visited and sampled for partial chemical analysis. A total of 40 thermal spring systems of North The results of the analyses are not listed in this Thailand have been studied (Ramingwong et al., 1978) paper since they take up too much space. But we and have been mapped by the Geothermal Exploration would like to discuss the high temperature hydro- Project, Department of Mineral Resources of Thailand. thermal systems, which contain some information about the deeper crust. These systems with reservoir This region has many granites of different ages and temperature of more than 150°C can be grouped using lies within the Chiang Mai-Tak gneiss belt the Si02 and Na-K-Ca geothermometers. (Hutchinson, 1982). It is possibly the continuation of Changning-Lancang belt of West Yunnan. About 40% The general characteristics of the 34 high tempera- of hot springs in North Thailand discharge hot water ture systems are presented in Table 1, from which the near or at boiling temperature. The chemistry of the following can be inferred: sodium-bicar6onate waters in N Thailand is remarkably similar to that of the thermal waters issuing from Firstly. most of the high-temperature hydrothermal most of thermal systems in West Yunnan. The systems are located in the above-mentioned anti- reservoir temperature, as indicated by the Na-K-Ca clinorium-like metamorphic belts. As stated above, geothermometers, falls for most springs within the there are about nine high temperature thermal systems rather narrow range of 140° to 180° (Hochstein and in the Tengchong-Gaoligongshan belt, 17 ~ in the Caldwell, 1985). Changning-Lancang belt and two in the Ailaoshan belt. The other six systems are also located in the fault The contributing factors which produce high temp- zones. The most reservoirs are crystalline rocks erature spring systems both in West Yunnan and such as granites, basalts, migmatites or gneiss and. Southeast Asia need further discussion. on a few occasions, are limestones. The occurrence of hot spring systems in N Thailand Second.ly, the chemical types of water found in has been associated with granites and their high high-temperature hydrothermal systems can be heat generating capacity. "Preliminary field surveys classified into following two (Fig.2): indicate that most of the hot springs are regionally related to the granitic rock outcrops" (Ramingwong et Alkali chloride waters: The hot springs containing al., 1978). Analyses by the University of Chiang Mai alkali chloride are found only in two areas: the Hot (Ramingwong et al., 1980) have shown that the heat Sea Field and the Langpu Hot Pool of Tengchong generating capacity of N. Thailand granites is County, a Pliocene-Pleistocene andesitic and basaltic anomalously high with the average value of 9 f 3 x area. The dissolved minerals in this water are 10-6 W/m3. The high-generating capacity of massive mainly sodium chloride but it contains high granites in North Thailand causes anomalously heat concentrations of silica, fluoride, arsenic, lithium, flows and the enhanced heat flow within the granites rubidium and cesium. The pH ranges from slightly is the most likely heat source for these acid to slightly alkaline (pH 5-9). The TDS are more high-temperature hydrothermal system (Hochstein and than lg/l. Caldwell, 1985).