Studies of Geothermal Background and Isotopic Geochemistry of Thermal Water in Jiangxi Province
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CNIC-01104 CN9700766 GIEC-0002 CHINA NUCLEAR SCIENCE AND TECHNOLOGY REPORT STUDIES OF GEOTHERMAL BACKGROUND AND ISOTOPIC GEOCHEMISTRY OF THERMAL WATER IN JIANGXI PROVINCE m m China Nuclear Information Centre Atomic Energy Press 1982^ ZHOU Wenbins Associate professor» deputy president of East China Geological Institute. Graduated from Geological Department of Eest China Geological Institute in 1982, majoring in hydrogeology and engineering geology. CNIC-001104 GIEC-0002 Studies of Geothermal Background and Isotopic Geochemistry of Thermal Waters in Jiangxi Province ZHOU Wenbin SUN Zhanxue LI Xueli SHI Weijun (East China Geological Institute, Linchuan, Jiangxi) ABSTRACT The terrestrial heat flow measurement, isotope and geochemical techniques have been systematically applied to the geothermal systems in Jiangxi Province. Re- sults show that the thermal waters in the study area all belong to the low-medium temperature convective geothermal system, which essentially differs from high tem- perature geothermal systems with deep magmatic heat sources. It has been proven that the isotope and geochemical techniques are very useful and effective in geother- mal exploration. INTRODUCTION Thermal waters are widely distributed in Jiangxi Province and their genesis and origin have been a subject of dispute for a long time. In 19891 we were support- ed by the China National Fund of Natural Sciences to carry out a project to ap- proach the geothermal background of Jiangxi Province and its relations with hy- drothermal uranium mineralization. In 1991, the International Atomic Energy A- gency (IAEA) decided to fund the project of research contract entitled " An Oxy- gen and Hydrogen Isotope Study of Thermal Springs in Jiangxi Province" as a part of the " Regional Coordinated Project on Isotope and Geochemistry in Geothermal Exploration Areas in Africa, Asia and the Pacific and the Middle East". According to the Research Contract (No. 6447/RO/RB and No. 6447/Rl/RB) signed be- tween the IAEA and the East China Geological Institute, the programme of the work included: (1) Monitoring of hot springs and ground water in Jiangxi Province for isotopic and geo chemical analyes; (2) Elaboration of the data to evaluate the formation conditions of hot springs. This paper will demonstrate the major results of the above-mentioned programs. 1 GEOLOGICAL SETTING Jiangxi Province is located in the southeastern coastal area of China. Roughly divided by the Zhe-Gan Railway, Jiangxi lies on two major tectonic units, the north belonging to the southeastern margin of Yangtze Metaplatfrom, and the center and south being a part of the southern China Fold System. Jiangxi Province has a multilayered and heterogeneous deep structures, and ;the tectonic landforms has a mirror symmetry with the fluctuation of the Moho. The deep structure can be divided into three types (see Fig. 1) , in which (1) the lifted zone of the upper mantle, (2) the sunken zone of the upper mantle, and (3) the transitional zone of the deep structure. Faults are well developed in Jiangxi and can be classified into four groups: the nearly E W group (mainly in the Yangtze Metaplatform), the NE-NEE group (mainly in the center and south), the NNE-nearly SN group (mainly in the center and south) and the NW group (mainly in the center-south and the north east) in which the nearly NE-NNE group was mostly developed and followed by the nearly EW-nearly SN group. It had been proved that there are 10 pieces of deep faults and 8 pieces of great faults in Jiangxi Province (Fig. 1). The deep faults began to devel- 3 op in the later stage of the basement folding and fell into the present pattern in the Indo-Chinese epoch and Yanshan epoch. Expended and deepened in multiple move- ment, these faults controlled the volcanic and igneous activities, the formation of red basins • diagenesises»mineralizations, hot spring distribution and regional geothermal regime. -^_ Jiujiang , Lushan \ Xiushui A5 Nancliang Yiyang • 6 A12 Linchuan 9 • 14 6^ Maanping Lean Ji an III XIS Xingguo u - * m i Chong>;ii , • 'Ganzhou A17 I - II Xunwu Ill LongnanI Fig. 1 Schematic map showing the tectonic units, heat flow points, hot springs and uranium mineralization distribution 1. heat flow measurement points j 2. hot springsi 3. U mineralization points» 4. lifted area of the upper mantle j 5. sunken area of the upper mantle» 6. structural transitional zone. 2 GEOTHERMAL BACKGROUND 2.1 Terrestrial heat flow Terrestrial heat flow is key to learn the geothermal regime of the concerned area. By defintion, terrestrial heat flow is given as follows* 4 (1) where, q—terrestrial heat flow, mW/m2; K—heat conductivity of media, W/m °C j T—temperature, °C ; Z—depth, m, T~—geothermal gradient *C /km. From 1987 through 1990, East China Geological Institute, co-operated with Institute of Geology, Academia Sinica, measured 16 terrestrial heat flow values for Jiangxi province for the first time. Temperature measurement has been made for 26 bore-holes with depth from 110 to 1040 m, and 172 pieces of rock samples have been collected to determine their heat conductivities. According to Table 1, the terrestrial heat flow values for Jiangxi Province vary from 57. 8 to 82.1 mW/m2,averaging 70. 5 mW/m2,accord with the regional heat flow for the southeastern coastal area of China. Fig. 1 also shows the heat flow anomalies in Jiangxi,such as Dexing and Rongnan. In summary,the amonalies have the following characteristics: (1) Along the transitional zone of deep structure, that is, the positions between the lifted zone and sunken zone of upper mantle where grear changes take place, the heat flows increase apparently (or anomalies) \ (2) The heat flows become considerably higher (or anomal) along the active deep faults; ( 3) The heat flow anomalies are in accordance with presently active hy- drothermal activities and Mesozoic-Cenozoic volcanic-igneous activities in distribu- tion. (4) The heat flows increase greatly in uranium-riched rocks and uranium de- posits. 2. 2 Affecting Factors 2. 2. 1 Geological structure Researches reveal that heat flow values are different in different tectonic units. In old and stable platform areas, heat flow values are lower and more homogenous than those in young orogenic belts and rift zones. Generally speaking, the younger the tectonic units and the stronger activities of structures are, the higher the heat flow values. The distribution of heat flow in Jiangxi province reflects such laws. Owing to that northern Jiangxi belongs to Yangtze Metaplatform which is a stable massif while central and southern Jizngxi is a part of South China Fold System which is an active land mass, heat flow values in the former are lower than those in the latter. 5 Table 1 Data of measured and corrected heat flow values in Jiangxi Province Gradient Conductivity Heat flow No. Loaction Longitude Latitude Depth Rock G±SD K±SD Measured Corrected W/ (mlC) mW/m* 1 Ruichang 115*38' 29*40' 390~500 sandstone 19. 27±1.05 3. 07±0.07 59.2 2 Nanchang 115*52' 28*23' 230~750 mudstone 23. 70±0. 58 2. 38±O. 08 56-4 750-1040 sandstone 17. 98±0. 43 3.48±0.16 62-5 58. 6° 3 Xiushui 114*22' 29*01' 40~260 mudstone 21.00±l. 12 2. 75±O. 01 57-8 4 Dexing 117*41' 28*55' 760~940 phyllite 20. 94±0. 67 4. 40drl. 18 92.1 72. 2b) 5 Leping 117*01' 28*55' 645~765 sandstone 25. 00±0. 00 3. 30±0. 44 82-5 6 Yiyang 117*26' 28*21' 220—310 tufistone 27. 40±1.00 2. 90±0.11 79-5 meta 7 Jinxian 116*13' 28*16' 90- 170 -sandstone 23. 15±2. 84 3. 04db0. 21 70.4 sand- 8 Fengcheng 115*49' 28*15' 140--280 mudstone 20. 33±0. 66 3. 28±0. 64 66-7 390--470 limestone 21.95±0. 92 3. ll±0. 23 68-2 67.3° mud- 9 Xinyu 114*59' 27*55' 382--502 sandstone 28. 67±1.46 2. 76±0. 07 79.2 sand 40—•210 10 Pingxiang 113*48' 27*32' stone 17. 43±0. 46 3.85 67. 1 tuff, 200~320 11 Pingxiang 113*48' 27*32' sandstone 22. 39±0. 70 3. 98dbl. 16 89-1 72.6 12 Yongping 117*43' 28*14' 20~110 migmatite 20. 30±2. 20 3-26±O. 29 66.2 porphyroc 20. 17±0. 30 3. 25±O. 20 65-6 13 Lean 115*58' 27*45' 1OO~26O -lastic lava 280—370 dacite 22.03±1.65 3. 23±13 71-2 67.6'a) porphyroc- 14 Lean 115*58' 27*45' 32. 19±1.40 3. 14±0. 15 101.0 79. 9b><0 440—940 lastic lava porphyroc- 15 Lean 115*58' 27*45' 320—430 25. 85±1.70 3. 02± 78. 0 62. lastic lava 100—300 porphyroc- a) 16 Lean 115*58' 27*45' lastic lava 26. 84±1.14 3. 02± 81.0 74.6 sandstone, 17 Ganzhou 114*51' 25*45' 100—300 shale 33. 63±2. 26 2.18±0.14 73.3 62-1° 18 Chongyi 114*21' 25*33' 70—260 granite 26. 03±0. 71 3. 34±0. 06 86-9 75. 6b) 19 Longnan 114*46' 24*52' 300—390 sandstone 17. 92±1.83 3. 75±O. 68 67-2 a, b, c, d ) separately stand for average heat flow of different measure section for a bore-hole, heat reflect correction, groundwa- ter disturbance correction and radiogenic heat correction. It can be seen from Fig. 1 that the distribution of surface heat flow anomalies are controlled by deep seated structural transitional zones and deep-great faults (or fault basins).