Geochemical characteristics and genesis mechanisms of Kawu geothermal water in Tibet
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摘要:
卡吾是藏南地区典型的高温水热系统，具有较大的开采潜力，而现阶段对其成因机制还远未充分认识，限制了地热资源的进一步开发利用。为了进一步探究卡吾地热区地热成因及热源，基于卡吾地热区地热水和浅层冷水的水文地球化学和氢氧同位素特征，评估了地热系统的热储温度，探讨了地热水形成过程中的水文地球化学过程（水-岩反应、冷水混合、水-汽分离等），识别了地热系统的深部热源，进而揭示了地热系统的成因机制。结果表明：地热水水化学类型主要为HCO3-Cl-Na 型，Na-K温标计算的深层统一热储温度为280℃，K-Mg温标和石英温标计算的浅层热储温度约175℃，冷水混合比例为50%～76%，还原的深部热储氘氧同位素范围分别为−207.20‰～−185.25‰，−22.26‰～−17.74‰。基于以上认识，提出了卡吾地热系统的成因模式：卡吾为一具有岩浆热源的的地热系统，深部统一的母地热流体沿区域内不同断裂向上运移并经过不同水文地球化学过程形成4个不同分布的浅部热储，最终出露形成卡吾地热水。研究成果为卡吾地热区地热资源的合理开发和高效利用提供了重要的指导作用，并可为藏南地区同类型地热系统成因机制的研究提供借鉴思路。
Abstract:Objective The Kawu area is a typical high-temperature hydrothermal system in southern Tibet, with high potential for exploitation. However, the current understanding of its genesis mechanisms is still insufficient, which limits the further development and utilization of geothermal resources. This study aims to investigate the genesis and thermal sources of the Kawu geothermal region.
Methods Hydrogeochemical characteristics and hydrogen-oxygen isotope data of geothermal and shallow cold waters were analyzed to assess the thermal reservoir temperatures of the geothermal system. The hydrogeochemical processes involved in the formation of geothermal waters, including water-rock interactions, cold water mixing, and water-steam separation, were explored. Additionally, the deep thermal sources of the geothermal system were identified, shedding light on the genesis mechanisms of the system.
Results The results indicated that the geothermal waters primarily exhibited a neutral to weakly alkaline HCO3-Cl-Na chemical type. The Na-K geothermometer estimated the uniform deep thermal reservoir temperature at 280℃, while the K-Mg and quartz geothermometers estimated the shallow thermal reservoir temperature at approximately 175℃. The cold-water mixing ratio ranged from 50% to 76%. The deuterium and oxygen isotopic values of the reduced deep thermal reservoir ranged from −207.20‰ to −185.25‰ and −22.26‰ to −17.74‰, respectively. Based on these findings, a conceptual model for the Kawu geothermal system is proposed. It is suggested that Kawu area is a geothermal system with a magmatic heat source, where a uniform deep geothermal fluid rises along various regional fractures and undergoes different hydrogeochemical processes, resulting in the formation of four distinct shallow thermal reservoirs with varying distributions. This ultimately leads to the emergence of Kawu geothermal water.
Conclusion The study provides important guidance for the rational development and efficient utilization of geothermal resources in the Kawu geothermal region and offers valuable insights for studying the genesis mechanisms of similar geothermal systems in southern Tibet.
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图 1 西藏卡吾地质简图及采样点分布图（a）和我国藏南热泉点、构造分布图（b）
Figure 1. Simplified geological map of Kawu and sampling point distribution (a), and distribution of hot springs and tectonics in southern Tibet, China (b)
图 2 卡吾地区地表水、地热水Piper三线图
Figure 2. Piper diagram of surface water and geothermal water in Kawu area
图 3 卡吾地区地表水、地热水特征离子含量关系图
Figure 3. Major ions in surface water and geothermal water in Kawu area
图 4 卡吾地区地热水样品的Na-K-Mg三角图
Figure 4. Na-K-Mg ternary diagram of geothermal water samples in Kawu area
图 5 卡吾地区地热水样品Na/K活度比与石英温度关系图(a)和lg(SiO2)-lg(K2/Mg)图(b)
Figure 5. Na/K activity ratios versus Quartz temperatures diagram (a), and lg(SiO2)-lg(K2/Mg) diagram (b) of geothermal water samples in Kawu area
图 6 卡吾地区部分地热水样点硅−焓方程法图示
Figure 6. Schematic of the Silicon-Enthalpy method for selected geothermal water samples in Kawu area
图 8 卡吾地区地热水氢氧同位素偏移过程示意图
Figure 8. Schematic diagram of hydrogen and oxygen isotope deviations in geothermal water in Kawu area
图 9 卡吾地热系统成因模式概念图（R1～R4为浅部热储编号）
Figure 9. Conceptual diagram of genesis model for the Kawu geothermal system
表 1 水-汽分离前地热水的热焓（H0）及蒸汽损失比例（fv）计算结果
Table 1. Calculation of geothermal water enthalpy (H0) and steam loss ratio (fv) before vapor separation
编号 KW01 KW02 KW03 KW04 KW05 KW06 KW08 KW09 KW10 KW11 H0/(kJ·kg−1) 438.55 515.39 478.63 494.33 654.31 505.63 392.39 492.23 513.84 359.68 fv/% 3.60 6.95 5.34 6.03 13.00 6.52 1.59 5.94 6.88 0.16 
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表 2 卡吾地区地热水氢氧同位素值偏移还原计算
Table 2. Calculation of geothermal water hydrogen and oxygen isotope deviations in Kawu area
编号 实测数据 水汽分离前 冷水混合前 δD/‰ δ18O/‰ δD/‰ δ18O/‰ δD/‰ δ18O/‰ KW01 −159.9 −18.8 −161.14 −19.00 −185.25 −18.77 KW02 −161.7 −19.17 −164.09 −19.57 −186.58 −20.31 KW03 −161 −19.91 −162.84 −20.21 −186.11 −22.26 KW04 −163 −19.56 −165.07 −19.90 −191.33 −21.31 KW05 −164.4 −19.92 −168.86 −20.66 −187.43 −22.21 KW06 −162.7 −19.29 −164.94 −19.66 −189.87 −20.60 KW08 −159.9 −18.65 −160.45 −18.74 −187.88 −17.74 KW09 −161.7 −19.32 −163.74 −19.66 −187.63 −20.63 KW10 −162 −19.19 −164.36 −19.58 −187.30 −20.35 KW11 −163.9 −19.78 −163.96 −19.79 −207.20 −21.94
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