滇东老厂地区地热水成因：来自水化学和碳氢氧硫同位素的约束

Genesis of geothermal water in the Laochang area, eastern Yunnan Province: Constraints from hydrochemistry and C-H-O-S isotopes

关键词:  老厂地区  弥勒-师宗断裂带  地热水  水化学  碳氢氧硫同位素

Key words:  Laochang area  Mile-Shizong Fault Zone  Geothermal water  Hydrochemistry  C-H-O-S isotope

摘要:

滇东弥勒-师宗断裂带地热资源丰富，但是由于研究程度较低，成因机制不明，制约了区内地热资源的可持续开发利用。本文以弥勒-师宗断裂带北段老厂地区天然温泉水和地热钻孔水为研究对象，综合应用野外调查、水文地球化学和环境同位素方法，对区内地热水的地球化学特征和成因机制进行了研究。结果显示，区内地热水pH值介于7.30～8.12之间，TDS在224～382 mg/L之间，属于弱碱性淡水。地热水水化学类型为HCO₃·SO₄-Ca型和HCO₃·SO₄-Ca·Na型，且含有较高含量的Fe、As、Sb等微量组分，不宜饮用。地热水中HCO₃^?的δ¹³C值为?3.31‰～?7.79‰，计算得出参与水岩作用的CO₂的δ¹³C值为?9.50‰～?15.68‰，具有明显的沉积有机质来源特征。离子比值分析及硫同位素特征表明碳酸盐岩矿物和石膏的溶解是区内地热水主要离子来源的控制因素，此外赋存于浅部断裂带内的硫化物矿体氧化以及阳离子交换作用对地热水水化学组分产生了一定的影响。氢氧同位素特征及¹⁴C测年结果表明区内地热水的补给来源为晚更新世时期温度较低的大气降水，补给高程为1984.9～2283.9 m，补给区位于研究区周边的山区。硅焓方程计算的冷水混合比例为71.9%～82.4%，综合硅焓方程计算的热储温度和校正后的石英地热温标计算的热储温度，认为区内地热水的热储温度为87.5～135.7℃，地热水循环深度为1538.0～2502.0 m。研究结果有助于提升滇东弥勒-师宗低温热水带地热水成因研究水平，为区内地热资源的合理开发及保护提供理论支撑。

Abstract:

The Mile-Shizong Fault Zone in Eastern Yunnan is rich in geothermal resources. However, due to the less research and unclear genetic mechanism, the sustainable development and utilization of geothermal resources in the region has been restricted. Taking the hot spring water and geothermal well water in Laochang area in the north section of the Mile-Shizong Fault Zone as the research object, this paper comprehensively applies the integration of field investigation, hydrogeochemistry and environmental isotope to study the characteristics and genetic mechanism of geothermal water in the study area. The pH values of the geothermal water are between 7.30～8.12 and TDS values are between 224～382 mg/L, which belongs to weakly alkaline fresh water. The hydrochemical types of geothermal water are HCO₃·SO₄－Ca type and HCO₃·SO₄－Ca·Na type. The geothermal water contains high contents of trace components such as Fe, As and Sb，and thus are not suitable for drinking. The δ¹³C value of HCO₃^? is between ?3.31‰ and ?7.79‰, and the calculated δ¹³C value of CO₂ involved in water-rock reaction is between ?9.50‰ and ?15.68‰, which is mainly from sedimentary organic matter. Ion ratio analysis and sulfur isotope characteristics show that the dissolution of carbonate minerals and gypsum is the main factor controlling ion concentrations of geothermal water. In addition, the oxidation and cation exchange of sulfide ore bodies in the fault zone have a certain impact on the composition of geothermal water. Hydrogen and oxygen isotope characteristic and ¹⁴C dating results show that the recharge origin of the geothermal water is the atmospheric precipitation with low temperature in the late Pleistocene, and the supply elevation is between 1984.9 m and 2283.9 m. The recharge area is located in the mountainous area with the elevation of 1984.9～2283.9 m around the study area. The mixing ratio of cold water calculated by silicon enthalpy equation is between 71.9% and 82.4%. The heat reservoir temperature calculated by silicon enthalpy equation and corrected quartz geothermal temperature scale is between 87.5℃ and 135.7℃. The circulation depth of geothermal water is between 1538.0 m and 2502.0 m. The research results are helpful to improve the research level of geothermal water genesis in the Mile-Shizong low-temperature hot water zone in Eastern Yunnan, and provide theoretical support for the rational development and protection of geothermal resources in the area.