青海共和盆地地热流体地球化学特征及热储水-岩相互作用过程

Geochemical characteristics of geothermal fluids and water-rock interaction in geothermal reservoirs in and around the Gonghe Basin,Qinghai Province

地热流体地球化学组成及其运移规律和成因机制研究对地热资源勘查和开发利用具有重要意义。当前,青海省地热资源开发利用程度低,更缺乏针对地热流体地球化学特征进行深入研究的系统性工作。青海共和盆地是青藏高原北缘的一个断陷盆地,盆地内地热资源丰富。本文以共和盆地及周围部分山区的地热系统为研究对象,基于系统地球化学采样和测试开展了地热流体地球化学组成及热储水-岩相互作用过程分析,认为:从共和下更新统热储、新近系热储到鄂拉山构造岩浆带再到瓦里贡山构造岩浆带,地热水中SiO2含量依次升高,反映热储温度依次升高;上述地热地区热储中原生铝硅酸盐矿物的溶解和蚀变矿物的形成是控制地热水中阳离子含量的最重要的水文地球化学过程,而补给水下渗和地热水径流及升流过程中盐类矿物的溶滤则是水中阴离子(特别是SO^(2-)4和Cl^-)的主要来源。

Investigations of the geochemical compositions of geothermal fluids as well as their movements and geneses are of great significance for the exploration and exploitation of hydrothermal resources.Currently,the exploitation and utilization of geothermal resources in Qinghai Province are not well developed,and more seriously,there is a lack of systematic studies on the geochemical features of the geothermal fluids there.The Gonghe Basin of Qinghai Province,located in the northern margin of the Qinghai-Tibet Plateau,is a rift basin formed in the early Cenozoic where abundant geothermal resources occur.With the geothermal areas in and around the basin as the study areas,we determined the geochemical compositions of the geothermal fluids and investigated the water-rock interactions occurring in the reservoirs based on systematic geochemical sampling and subsequent hydrochemical analyses.The results showed that the SiO2 concentrations of the geothermal waters displayed an increase trend from the Lower Pleistocene and Neogene reservoirs within the Gonghe Basin to the Ela and Waligong tectonic-magmatic belts,implying that the reservoir temperatures increased in the same way.The dissolution of primary aluminosilicate minerals and the formation of hydrothermally altered minerals in the reservoirs are the predominant hydrogeochemical processes controlling the concentrations of major cations in the geothermal waters,while the leaching of salty minerals during the infiltration of the recharging waters and the runoff and upflow of the geothermal waters made a substantial contribution to the occurrence of major anions,especially SO^(2-)4 and Cl^-.