Strontium isotope data for thermal waters in selected high-temperature geothermal fields, China.
Strontium isotope data for thermal waters in selected high-temperature geothermal fields, China.
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作者：
Dor Ji
云南-西藏地热带的地热水主要来源于大气降水。在重力的作用下，大气降水沿着破碎带向下移动，通过吸收围岩的热量来增加温度，并从造岩矿物中溶解锶组分。因此，热水的87Sr/86Sr值能够保存水-岩作用强烈的流体滞留证据是可行的。研究热水和围岩的~(87)Sr/~(86)Sr值，可以解释浅层热水与深层热水的关系，并能识别该区某些储层是否存在水力连通。花岗岩中的~(87)Sr/~(86)Sr值与西藏羊八井地热田北部不同。而浅层和深部热水稳定，87Sr/86Sr值基本相似，变化范围为0。7122比0。7126。热水87Sr/86Sr略高于冷水，与上新世花岗岩一致，与中新世花岗质糜棱岩不同。这表明上新世花岗岩中的热水主要是聚集和储存的。当浅层热水从NW向SE流动时，富CO2水蒸气逸出，导致碳酸盐岩过饱和，如CaCO3、MgCO3和SrCO3。除围岩溶解作用外，碳酸盐沉淀作用将是主要作用。第四系冲积层对浅层热水的初始87Sr/86Sr值没有明显影响。云南省腾冲县热海地热田是我国大陆特有的与地幔岩浆房有关的高温地热系统。东区热水具有高SiO_2浓度、低Na/K值、87Sr/86Sr值在0。7164比0。7187。而在西部，热水中SiO_2含量低，Na/K值相对较高，87Sr/86Sr值在0。7266比0。7302.油田西部高黎贡山组砂质板岩和片岩中发现了120(C)的储集层。此外，东西部热液区位于不同的北S走向断裂上。根据这些热水的地球化学特征，保证两个热液系统具有非常弱的水力联系是合理的。此外，热水中的锶组分与该地区的大型火山岩无关。这些成果将对该区地热勘探和资源评价具有重要的指导意义。
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 87Sr/86Sr values of thermal waters can preserve evidence of fluid residence where water-rock interaction occurs intensively. Studies on the 87Sr/86Sr 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 87Sr/86Sr 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 87Sr/86 Sr, varying from 0. 7122 to 0. 7126. The thermal waters are slightly higher than cold groundwater in 87Sr/ 86Sr, accordant to Pliocene epoch granite and different from Miocene epoch granitic mylonite in 87Sr/86Sr. 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 87Sr/86 Sr 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 87Sr/86Sr 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 87Sr/86Sr 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.