川西理塘毛垭温泉群的成因及深部地热过程

GENESIS AND DEEP GEOTHERMAL PROCESS OF MAOYA HOT SPRINGS IN LITANG,WESTERN SICHUAN

毛垭温泉目前的成因模式及深部地热过程的研究程度较低。文中以理塘毛垭温泉群和周边的冒火温泉为研究对象,对其进行水化学组分和氢、氧同位素分析。研究结果表明,毛垭温泉群和冒火温泉的水化类型均为Na-HCO_(3)型。温泉水在深循环过程中,深部水、岩、气的相互作用使得热储层中的长石发生水解,是形成Na-HCO_(3)型地热水的主要原因。氢、氧同位素的测量结果表明温泉水均起源于大气降水。毛垭温泉群与冒火温泉相比,具有更高浓度的离子组分,且表现出轻微的氧同位素漂移现象,表明毛垭温泉的循环深度更深,经历了更强烈的水-岩作用,此外,Cl^(-)的深部来源比例更高。通过SiO_(2)温标和硅焓混合模型估算得到毛垭温泉群的浅部热储温度为75~103℃,深部热储温度为235℃,冷水混合比例为87%~94%。基于深部热储温度计算得到毛垭温泉的最终循环深度接近5km。深部地热水受静水压力和水热对流作用,沿理塘断裂带向上运移,在此过程中,受构造裂隙的影响,地热水与冷水发生第1次混合,混合水的温度约为100℃。地热水循环至近地表时,与盆地内的冷水进行第2次混合,最终出露地表,形成中低温温泉群。

Maoya hot spring,as a famous earthquake monitoring site,is seldomly studied in terms of its genesis and deep geothermal process.In this paper,we investigated the chemical and isotopic composition of thermal water in Maoya and Maohuo in Litang to elucidate the hydrochemical characteristics and genesis of the geothermal waters.The study results show that Maoya hot springs and Maohuo hot spring are of the Na-HCO_(3) type as a result of dissolution processes involving feldspars from the reservoir rocks due to the water-CO_(2)-rock interaction during the deep circulation of the geothermal waters.According to the diagram of Cl^(-)and Na+concentrations of the geothermal water samples,Cl^(-)in Maoya hot spring originates from the mixing of granodiorite and basalt aqueous solutions in the process of water rock interaction,while Cl^(-)in Maohuo hot spring mainly originates from granodiorite aqueous solutions.The stable isotopeδD andδ18 O composition of geothermal waters indicates that they are recharged by meteoric precipitation.The Maoya hot springs have the characteristics of higher concentration of ion components and slightly oxygen drifting compared with the Maohuo hot spring,indicating that they have a deeper circulation depth and experience a stronger water-rock interaction.In addition,the ratio of Cl^(-)that comes from deep source in Maoya hot springs is higher than that in Maohuo hot spring.The high temperature geothermal water formed by deep circulation of meteoric water is mixed by the shallow cold water during the ascending process.We employed SiO_(2) geothermometer and Si-enthalpy model to estimate the temperature of shallow reservoir after mixing with cold water and the temperature of deep reservoir and the mixing ratio of cold water,respectively.The results suggest that the temperature of shallow reservoir in Maoya thermal field is in the range of 75~103℃and the temperature of deep reservoir in Maoya thermal field is about 235℃and the mixing ratio of cold water ranges from 87%to 94%.Based on the temperature of deep reservoir,we calculated the depth of the geothermal cycle in Maoya area,which is close to 5km.The heat source triggering the formation of this geothermal system mainly originates from mantle and partial melting body of the crust.In addition,Cenozoic granitoid magmatic residual heat and upper crust radioactive heat can also provide additional heat sources.During the process of surface cold water circulation from shallow to deep,on the one hand,it forms deep geothermal water through normal geothermal gradients,and on the other hand,the mantle fluid upwelling below the Litang Basin and partial melting in the middle crust further heat the groundwater to form a high-temperature deep reservoir.The deep geothermal water is transported to the surface along the Litang Fault under the effect of hydrostatic pressure and hydrothermal convection.During ascending process,the first mixing of groundwater with superficial cold water occurred due to the presence of structural cracks in the crust,and the temperature of the mixing water is about 100℃.When the geothermal water migrates to the near surface,it mixes with the pore water and bedrock fissure water in the basin for the second time,and the mixing proportion of cold water increases(about 90%).Finally,it emerges to the surface,forming a group of medium-low temperature hot springs.