中国典型高温热田热水的锶同位素研究

滇藏地热带的热水大多是大气降水成因，大气降水沿断裂带向下渗透，随地温的增加而逐步升温，并不断地与深部岩石发生水岩相互作用，从造岩矿物中淋滤出锶组分。通过测定热水中的锶同位素组成，可以帮助确定热水的深部滞留环境，研究深、浅层热水的内在联系，区分出不同的水热循环系统。西藏羊八井热田深部由多期次花岗岩组成，岩性差别不大，而锶同位素差异比较明显。热田的深、浅层热水有着相似的^87Sr/^86Sr值，并且只与上新世花岗岩的锶同位素组成相接近，反映了深部热储的岩性特征。在热水从升流部位向排泄区运移时，浅层冷水、始新世火山岩和第四系砂砾岩对热水的锶同位素改造作用较小。云南腾冲热海热田是典型的有幔源岩浆囊作为热源的高温热田，热田由东、西两区组成，其气体和热水在化学组成上有一定的差异，^87Sr/^86Sr值也有明显的不同。因此，可以确定东、西两区的水热系统虽具有共同的热源，但两者之间的水力联系比较微弱，而且热水中的锶组分与热田内大面积出露的火山岩没有关联。

Strontium isotope data for thermal waters in selected high-temperature geothermal fields, China.

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.