承德北部茅荆坝地热田地热流体的水化学和同位素特征及其成因

承德北部茅荆坝地区地表出露的地热水温度高达98.7 °C，赋存于裂隙较发育的侏罗世中粗粒二长花岗岩热储，${mathrm{SO}}_4^{2-} $含量较高，但关于该区地热流体的补给来源、循环演化过程与成因机制研究尚少。为了认识该基岩山区地热系统的成因以合理开发利用地热资源，在区域地热地质调查的基础上，测试分析了不同水体的水化学组分、地下水年龄（3H和14C）、硫酸盐硫、氧同位素（δ34S-${mathrm{SO}}_4^{2-} $和δ18O-${mathrm{SO}}_4^{2-} $）、碳酸盐碳同位素（δ13C-$ {mathrm{HCO}}_3^-$）、锶同位素（87Sr、86Sr）等特征值。结果表明：（1）茅荆坝地区地热水化学类型以${mathrm{SO}}_4 $—$ {mathrm{Na}}$型为主，硅酸盐矿物的溶解及阳离子交换作用促进了地热水中${mathrm{Na}}^+ $、${mathrm{K}}^+ $和SiO2的富集，水中${mathrm{SO}}_4^{2-} $并非来源于硫酸盐岩矿物溶解，推测为H2S气体从深部还原环境上升到浅部氧化后生成${mathrm{SO}}_4^{2-} $，也可能来源于高温地热水与硫反应形成的硫酸盐；（2）地热水n(87Sr)/n(86Sr)均值为0.7092，与海相碳酸盐岩比值接近，揭示热储深部可能存在海相碳酸盐岩储层；（3）地热水属于古地下水，14C校正年龄为11.9～14.9 ka，循环更新能力差，由周边山区的大气降水补给，补给高程在1532～1632 m；（4）地热系统深部热储温度为142～144 °C，高温中心位于热田北部。研究结果对冀北山地地热资源的可持续开发利用具有重要意义。

Hydrochemical and isotopic characteristics and genesis of geothermal fluids in the Maojingba geothermal field,northern Chengde city

The temperature of geothermal water exposed on the surface in the Maojingba area, northern Chengde city, can reach up to 98.7 °C, mostly occurring in the Jurassic coarse-grained diorite with well-developed fractures and high ${mathrm{SO}}_4^{2-} $ content. However, few studies on the recharge, evolution, and genetic mechanism of the geothermal fluids in this area. In order to understand the genesis of the geothermal system in the bedrock mountain area and evaluate its development and utilization potential, this study, based on the regional geothermal geological survey, tested and analyzed the hydrochemical composition of different water bodies, groundwater dating isotopes (3H and 14C), sulfur and oxygen isotopes in sulfate (δ34S-${mathrm{SO}}_4^{2-} $ and δ18O-${mathrm{SO}}_4^{2-} $), carbon isotopes (δ13C-${mathrm{HCO}}_3^- $) and strontium isotope (87Sr and 86Sr) to explore the fluid genesis of the Maojingba geothermal system. The results show that: (1) the chemical type of geothermal water in Maojingba area is mainly $ {mathrm{SO}}_4-$Na type. The dissolution of silicate mineral and cation exchange promote the enrichment of Na+, K+and SiO2 in the geothermal water. The source of ${mathrm{SO}}_4^{2-} $ in water is speculated to be H2S oxidation from deep reduction environment or the reaction between high-temperature geothermal water and sulfur, rather than the dissolution of sulfate minerals. (2) The average ratio of n( 87Sr)/n(86Sr) in the geothermal water is 0.7092, close to the ratio of marine carbonate rock, indicating that marine carbonate rock may occur in the deep geothermal reservoir. (3) The geothermal water in the study area is old groundwater with poor circulation and renewal ability, based the groundwater age of 11.9−14.9 ka from 14C. The recharge of the geothermal water is from precipitation in the surrounding mountains, with elevation of1532−1632 m. (4) The deep geothermal reservoir temperature of the geothermal system is 142−144 °C, and the highest temperature is located in the northern part of the geothermal field. The research results are of great significance for the sustainable development and utilization of geothermal resources in the mountainous areas of northern Hebei Province.