A REVIEW of the REHAI (HOT SEA) GEOTHERMAL SYSTEM in TENGCHONG COUNTY, YUNNAN PROVINCE, CHINA Liao Geology Department, Peking University, China

Liao

A REVIEW OF THE REHAI (HOT SEA) GEOTHERMAL SYSTEM IN TENGCHONG COUNTY, YUNNAN PROVINCE, CHINA Liao Geology Department, Peking University, China

Key words: volcanism, thermal springs, magnetotellurics, hot water systems From 1970s to some drillholes for exploration of gold and/or uranium found the high geothermal gradient of about 100% in the southern part of the thermal field. Abstract Geology Tengchong is the main volcanogeothermal region on the Chinese mainland where active thermal springs dis- Volcanism began at the present site of Tengchong in the charge fiercely around the margin of the Cenozoic vol- Pliocene or Miocene and reached its peak roughly Lower canic cones. The Rehai (Hot Sea) geothermal field is the Pleistocene but lasted for the whole Pleistocene largest one. In this paper, a review of the results and a period (Liao, The youngest volcanoes are Mt. summary of the findings by the geological, geochemical Maanshan, Mt. Dayingshan and Mt. Heikongshan. and geophysical reconnaissance surveys, carried out in The newest result of the K-Ar analyses were Ka the Rehai field by the scientists from Pelting University and Ka for Mt. Maanshan, and and engineers from Yunnan Geology and Mineral Re- Ka for Mt. Dayingshan and 7 7 Ka and 14 7 Ka for sources Bureau since is presented. The aim of this Mt. e: , 1993). review is to attract scientists' attention and promote The effect of this long period of volcanism is the geologi- the development of the Rehai. cal setting of geothermal activities in Tengchong area (Fig. Introduction The Rehai (Hot Sea) geothermal system The regional trend of the range is tectonically from north is located in the westernmost part of to south, but is superimposed by many circular nan Province near the border between China and Burma. Their formation could be due to the intrusion The arresting feature of the Rehai to the traveller visit- Jurassic to the Tertiary. The Rehai field is located inside ing, or passing through, is the multitude of vapour a circular structure. plumes ascending from the thermal springs. In the Rehai field, the oldest rock belongs to the The earliest scientific record of thermal activity in the camrian Gaoligongshan gneiss, which was in Rehai (Hot Sea) was in the travel dairy of Xu in drillhole in the southern part of the thermal field and ex- 1639. It was used for hot baths and had some small ad- posed in the Reshuitang area. The late Cretaceous ditional use for sulfurite mining since Ming Dynasty. ite is exposed in the Zaotanghe river valley. The gneiss and granite could be the reservoir and were covered by The scientists of Peking University and their cooperators the Miocene sandstone and conglomerate. The strati- described the geology and geochemistry in relation to the graphic sequence of the Rehai field is as follows: hydrothermal activity and made a preliminary assessment The alluvium distributed along the Zaotanghe river of the potential as a source of geothermal energy (US- and the Menqianhe river; GECR, 1974; Tong, et al., During basalt located in the valley of the 1987, the Second Hydrogeology and Engineering tioned rivers, which came from the Mt. gy Team of Yunnan Geology and Mineral Resource Bu- tou volcano; reau did geological, geochemical and geophysical recon- andesite at the top of the Mt. Bangeshan and naissance, accompanied by the drilling of ten shallow Mt. Xidapotou; holes HET of YGMRB, Miocene Molasse stratum, which is mostly

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tered Table 1 Composition of gases and isotope ratio granite in the valley of the Zaotanghe river; of samples in the Rehai field (taken from Wang the gneiss of Gaoligongshan group, which is dis- et , tributed at the border of the field. Nos. 1 3 4 5 6 7 Many faults, including the important Liuhuangtang- 90 96 90 86 80 92 Huangguaqing fault, are probably good channels for the 56. 33 47. 67 61. 35 41.83 38.08 54. 75 discharge of thermal fluids and are important locally in 32.41 42.38 25.08 46.69 55.20 35.26 controlling hydrothermal activities including thermal springs and quartz veins. 0. 34 0.04 0. 1.03 0. 33 0. 33 Ar 0. 63 0. 56 0. 77 0. 56 0.40 0. 61 Thermal Manifestations He ppm 31.5 5.2 99.6 129.4 39.0 41.0 75. 5 220.0 663.0 802.0 23.0 140.0 The thermal activity at the Rehai field can be divided in- 4. 59 4. 0 6. 02 5. 01 5. 46 5. 4 to two parts: the classical Rehai region at the eastern part and the Reshuitang (Hot Water Pool) region at Names of sampling sites: Zaotanghe; 4. Huang 5. Liuhuangtang; 6. Xiaogunguo; 7. Dagunguo. western part. There is a simple road connecting the east part with the west part about km over the mountains.

The surface manifestations of the Rehai field include Subsurface Temperature boiling springs, fumaroles and steaming ground etc. The temperatures of the thermal springs range from 42% to Subsurface temperature were derived from 19 shallow and the fumaroles are around Sulfur is con- holes drilled from 1976 to 1992. Thirteen holes range in stantly crystallized out around the orifies of the fumalores depth from lOOm to The highest bottom-hole by oxideation of S. temperature recorded at a depth of 380m in is 142%. The temperature gradient of the field is from The natural heat flow of about for the whole 5 to The other six shallow holes with area was measured by Peking University. depth from 19 to 26m were drilled into the seasional variation zone.

The temperature data of shallow holes drilled are not suf- ficient to prepare a geothermal-gradient map. However, Geochemistry the geothermal gradinet is very irregularly with depth. This renders difficult the work of map compilation. The The chemistry of thermal water of the Rehai geothermal shallow drilling depth and the rugged topography also field has been discussed by many papers (Liao et , make the drawing of isotherms on one particular level 1981, 1991; Zhang et 1989; Liao, 1994) impracticable, because many of these holes are either too and will not be repeated here. Hot waters from boiling high or too low in opsition to record the temperature. springs are mainly chloride-bicarbonate-sodium type with Contour maps of isothermal surface were not drawn be- TDS of It is found that the field is a hot-water cause data were also insufficent. But three isothermal system, whose subsurface boiling zone is close to the sur- profiles were dwawn when possible as shown in Fig. 2. face. Cation geothermometers indicate reservior temper- Obviously, profiles 1 and 3 are outside the north and ature in the range of 230% to (Liao, south borders of the high-temperature region.

Gas samples were taken at some sites in the Rehai field. Geophysical Data Results of the gas analyses are listed in Table 1.

Four fundamentally different geophysical methods (mag- netometry, DC sounding, microseismlology and magne- As indicated in the legend of Table the analyses are totelluic sounding) were employed in exploration of the characterized by anomalous and hydrogen and an Rehai field. It is very difficulty to carry sound- average ratio of 4. 6 X Obviously, the re- ings due to the regged topograpy. Groundmagnetic mea- cently published gas analyses and anomalously high surement found the rock inside the, thermal field have ratio of the Rehai field confirm the existence of de- been partly or completely de-magnetized by thermal gassing upper-mantle rocks at crustal levels. The result teration. Unfortunately, groundmagnetic measurement is lacking in perfection due to no data. . can not delineate the boundary of the field.

1208 Two separate microearthquake surveys were made in De- supported by State Science Committee of China. cember 1980 and March 1982 lasting 37 and 31 days re- spectively. A total of events with a magnitude be- References tween 0 and 4. 4 were recorded at ten field stations. It was found that there is a thin crustal layer only about D. H. , Liao, Z. J. , Zhao, C. and Wang, X. B. 7km thick beneath the surface of this field. Away from (1994). The inference of magmatic heat source beneath the center , the thickness of this layer gradually increases the Rehai (Hot Sea) field of Tengchong from the result taking the shape of an umbrella, thus providing condi- of magnetotellunic sounding. Chinese Science Bulletin, tions for the formation of a geothermal reservoir or a , 572-577. magma chamber. It also shows that the seismic velocity of the upper crust in the Tengchong is anomlously low Liao, J. (1989). Late Cenozoic volcanism in et , 1986, chong. In: Ceothermics in Tengchong, Tong, W. and Zhang, M. T. , Science Press, Beijing, 19. Magnetotelluric sounding were made at 11 stations along a km profile, which crosses the Rehai filed Liao, Z. J. Reassessment of the equilibrium in an almost E-W direction perpendicular to the regional temperatures of the Rehai (Hot geothermal field, tectonic trend. An ID inversion algorithm produced the Tengchong County, Yunnan Province, China. In depth section shown in Fig. 3. The crustal resistivity Proc. 12th NZ Geothermal Workshop 1990, pp. anomaly between 5 and 25 km depth has been interpret- 115. ed in terms of anomalous hot crustal probably enclose some magma et al. , 1994; Liao, Liao. J. (1993). Study of the magmatic heat source beneath the Rehai (Hot Sea) prospect, Tengchong (West Yunnan). In: Proc. 15th NZ Geothermal Work- Summary shop 1993, pp.

now, all the investigations at the Rehai undertaken Liao, J. The Rehai (Hot Sea) geothermal have in scale. The basic during system in Tengchong County, Yunnan Province, Chi- reconnaissance of a geothermal field is to determine its na. In: Terrestrial Heat Folw and Geothermal Energy in energy potential. To do this it is necessary to assess: Asia, M. Yamano and M. L. Gupta, , Oxford the size of the field, the shape of the field, IBH Publishing PVT. LTD. , 349-364. structure of the field and the capacity of the field to produce energy. Obviously, these problems at the Rehai J. , Zhang, Z.F. and Guo, G.Y. (1981). Ge- field are not yet resolved satisfactorily. But the author ology and Geochemistry of Rehai (Hot geotherma affirms the following statement to be true the spec- field in Tengchong, Yunnan Province, China. In: Proc. tacular thermal springs and fumaroles at the Rehai are 3rd NZ Geothermal Workshop, pp. 19-23. products of a history of intensive volcanism, mainly of the Pleistocene cone; the Rehai field is a hot-water system with a magmatic heat source. Its reservoir tem- J. , Shen, M. and G. Y. (1991). perature can exceed 230%. The reservoir rock body is Characteristics of the geothermal reservoir in the Rehai granite or gneiss, of which the second permeability is Hot Sea field in Tengchong County, Yunnan still uncertain; the geothermal at the Rehai Province, Acta Geologica Sinica, 4 pp. field should be one among the richest in China. Resource 320. as sessment for purpose of electric generation remains to be solved due to lack of adequate drillholes. C. , Tong, W. , Zhang, L. M. and Zhang, Z. X. (1986). Microseismic observation in Acknowl Tengchong volcanogeothermal region. Acta , 547-556. I very much thand my colleagues: Tong Wei, Zhu iang, Guo Guoying, Shen Liu Shibin, Zhao C. , Zhang, L. M. , X. , Zhang, B. S. Fengsan and Zhang Zhifie, for working together in the and Tong, W. Field observation of the seismici- field. I also thand Mr. Jia Xizhi, General engineer of the ty in Tengchong volcanogeothermal region. In 2nd Hydrogeology and Engineering Geology Team of Ceothermics in Tengchong, Tong, W. and Zhang, M. Yunnan, for allowing me use of its data. This study is T. , Science Press, Beijing, pp. 189-209.

1209 Nakai, S. ,Xu, S. , Wakita, H. , Fujjii, N. , Nagao, USGTGR United Survey Group of Tengchong , Orinhashi, Y. , Wang, X. , Chen, J. F. and Geothermal Resources). A Survey Report of Liao, Z. J. (1993) K-Ar ages of young vocanic rocks the Geothermal Resources in Tengchong , Yunnan from Tengchong area, western Yunnan, China. Bulletin Province, China. Special Report of Peking University. Volcanologic Society of Japan, 38 pp. 171. Wang, X. B., Xu, Chen, J. F., Sun, M. Xue, X. F. and Wang, W. Y. (1993). Composition of 2nd Hydrogeology and Engineering Geology Team of gases and isotopic ratios of helium from thermal springs Yunnan, (1987). Reconnaissance Report of the Rehai of Tengchong volcanic region. Chinese Science Bulletin, (Hot geothermal field of Tengchong of Yunnan. 814-817. Internal Report of Yunnan Geology and Mineral Re- source Bureau. Zhang, Z. F. , Liu S. B. and Zhao, F. S. (1989). Geo- chemistry of hydrothermal fluids in Tengchong. In : and Zhang, M. T. (1989) Geother- Geothermics in Tengchong, Tong, W. and Zhang, M. mics in Tengchong, Science Press, Beijing, T. , Science Press, Beijing, 81-101.

Ruidian

Tengcnong

Fig. 1 Location of the Rehai field and sites of drillholes and MT sounding.

Quaternary alluvium U. Pleistocene andesite-basalt M. Carboniferous sandstone-bearing gravel M. Pleistocene basalt PC, Gaoligongshan group of Precambrian L. Pleistocene andesite Cretaceous granite

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Fig. Isothermal Profiles, Locations of drillhole on Fig. Temperature data are taken from 2nd HEGT of Yunnan, 1987).

Fig. Apparent resistivity pseudosection in (taken from Bai dt , 1994)

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