Strontium Isotope Data For Thermal Waters In Selected High-Temperature Geothermal Fields, China

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Journal Article: Strontium Isotope Data For Thermal Waters In Selected High-Temperature Geothermal Fields, China

Abstract
Most of geothermal waters are meteoric origin in the Yunnan-Tibet Geothermal Belt. Pulled by gravity, meteoric waters move down along fracture zone, increase temperature by absorbing heat from wallrock and dissolve strontium component from rock-forming minerals. Therefore, it is feasible that the Sr-87/Sr-86 values of thermal waters can preserve evidence of fluid residence where water-rock interaction occurs intensively. Studies on the Sr-87/Sr-86 values of thermal waters and wallrock can explain the relationship between shallow and deep thermal waters and identify whether some reservoirs are hydraulically connected in the region. Values for Sr-87/Sr-86 in granites are different from the northern part of the Yangbajain geothermal field, Tibet. However. the shallow and deep thermal waters are stable and quite similar in Sr-87/Sr-86, varying from 0.7122 to 0.7126. The thermal waters are slightly higher than cold groundwater in Sr-87/Sr-86, accordant to Pliocene epoch granite and different from Miocene epoch granitic mylonite in Sr-87/Sr-86. It implies the thermal waters are mainly assemblage and storage in Pliocene epoch granite. When the shallow thermal water flows from NW to SE, CO2-rich steam escaping results in carbonate to become over-saturation, such as CaCO3, MgCO3 and SrCO3. The carbonate precipitation will be the main process other than wallrock dissolution. Quaternary alluvium does not apparently affect the initial Sr-87/Sr-86 value of thermal waters in the shallow reservoir. The Rehai geothermal field of Tengchong County in Yunnan Province is a unique high-temperature geothermal system related to mantle-derived magma chamber in the mainland. In the eastern part of the field, thermal waters are of high SiO2 concentration, low Na/K ratio and Sr-87/Sr-86 values in the range of 0.7164 to 0.7187. However, in the western part, thermal waters are of low SiO2 concentration, relatively high Na/K ratio and Sr-87/Sr-86 values in the range of 0.7266 to 0.7302. The reservoir of 120(C is found in sandy slate and schist of the Gaoligongshan group in the western part of the field. Further, the eastern and western hydrothermal areas are located in different N-S striking faults. According to geochemical characteristics of these thermal waters, it is rational to assure that both hydrothermal systems have a very weak hydraulic connection. Moreover, strontium component of the thermal waters does not relate to large-scale volcanic rocks in the region. These results will be of great benefit to geothermal exploration and resource assessment in the field.

Authors 
P. Zhao, J. Dor, E. J. Xie and J. Jin








Published Journal 
Acta Petrologica Sinica, 2003





DOI 
Not Provided
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Citation

P. Zhao,J. Dor,E. J. Xie,J. Jin. 2003. Strontium Isotope Data For Thermal Waters In Selected High-Temperature Geothermal Fields, China. Acta Petrologica Sinica. (!) .