Origin and evolution of Li-rich geothermal waters from the Kawu geothermal system, Himalayas: based on hydrochemistry and H-O, Li isotopes

There are 25 Li-rich geothermal fields (>15 mg/L) are displayed throughout nine geothermal zones in Tibet. The Kawu geothermal system is one of the representative high-enthalpy and Li-rich geothermal systems and is located in the Skaya dome, the Himalayas. Their boiling and hot and cold waters are Cl-HCO₃-Na and HCO₃-Ca types, respectively. The parent geothermal fluids are characterized by high Na, Cl, B, Si, As, Li, Rb⁺, Cs⁺ and F, and the Cl⁻ concentration and enthalpy of the parent geothermal liquid are 515 mg/L and 1,485 J/g, respectively (corresponding to a temperature of 323.5°C). The stable oxygen and hydrogen isotopic compositions of geothermal fluids range from −19.7 to −23.4‰ and −155 to −181.2‰, indicating that the ratio of magmatic water mixed into geothermal fluid is between 11.7% and 19.5%. The lithium content in the geothermal system is high (13.04 to 20.8 ppm), but the δ⁷Li is relatively low (1.06 to 1.72‰). The high Cl, Li and B contents, constant Cl/Li and Cl/B ratios and low δ⁷Li indicate that they mainly originate from magmatic fluid, rather than from water rock reaction with the leucogranite and gneiss. While ascending from the reservoirs to the surface, the parent geothermal fluids gradually mixed with the meteoric waters infiltrated via brittle tensional small-scale faults, in turn forming four cooling processes upwelling patterns, i.e. conductive cooling, adiabatic cooling, mixing with cold water, and the combination of these processes.

西藏境内9个地热带中共分布25个锂含量大于15mg/L的富锂地热田。卡乌地热系统是典型的高焓富锂地热系统之一，坐落于喜马拉雅山脉的斯亚穹窿（Skaya dome）。该系统的沸泉水、热水与冷水分别属于Cl-HCO₃-Na型与HCO₃-Ca型水。其母地热流体以高含量的钠、氯、硼、硅、砷、锂、铷离子（Rb⁺）、铯离子（Cs⁺）与氟为特征，母地热流体的氯离子浓度与焓值分别为515mg/L与1485J/g（对应温度323.5℃）。地热流体的稳定氧、氢同位素组成分别介于-19.7‰至-23.4‰与-155‰至-181.2‰之间，表明混入地热流体的岩浆水占比为11.7%~19.5%。该地热系统中的锂含量较高（13.04~20.8ppm），但锂同位素δ⁷Li值相对偏低（1.06‰~1.72‰）。高氯、高锂与高硼含量，恒定的Cl/Li、Cl/B比值以及偏低的δ⁷Li值，表明其主要源自岩浆流体，而非与淡色花岗岩、片麻岩发生的水-岩反应。母地热流体从储层向上运移至地表的过程中，逐渐与通过脆性张性小型断层渗入的大气降水混合，进而形成四种冷却型上升流模式：传导冷却、绝热冷却、冷水混合，以及上述过程的复合模式。