Hydrogeochemistry of high-temperature geothermal systems in China: A review

As an important part of the Mediterranean-Himalayas geothermal belt, southern Tibet and western Yunnan are the regions of China where high-temperature hydrothermal systems are intensively distributed, of which Rehai, Yangbajing and Yangyi have been investigated systematically during the past several decades. Although much work has been undertaken at Rehai, Yangbajing and Yangyi to study the regional geology, hydrogeology, geothermal geology and geophysics, the emphasis of this review is on hydrogeochemical studies carried out in these geothermal fields. Understanding the geochemistry of geothermal fluids and their environmental impact is critical for sustainable exploitation of high-temperature hydrothermal resources in China. For comparison, the hydrogeochemistry of several similar high-temperature hydrothermal systems in other parts of the world are also included in this review. It has been confirmed by studies on Cl⁻ and stable isotope geochemistry that magma degassing makes an important contribution to the geothermal fluids from Rehai, Yangbajing and Yangyi, though meteoric water is still the major source of recharge for these hydrothermal systems. However, the mechanisms of magma heat sources appear to be quite different in the three systems, as recorded by the ³He/⁴He ratios of escaping geothermal gases. A mantle-derived magma intrusion to shallow crust is present below Rehai, although the intruding magma has been heavily hybridized by crustal material. By contrast, the heat sources below Yangbajing and Yangyi are inferred to be remelted continental crust. Besides original sources, the geochemistry of characteristic constituents in the geothermal fluids have also been affected by temperature-dependent fluid–rock interactions, boiling and redox condition changes occurring in the upper part of hydrothermal systems, and mixing with cold near-surface waters. The geothermal fluids from Rehai, Yangbajing and Yangyi contain very high concentrations of some toxic elements. Since local drinking water sources may be mixed with geothermal water, and irrigation with water containing geothermally-derived harmful elements, possibly leading to accumulation in crops consumed by human beings, the natural geothermal spring discharge or anthropogenic geothermal wastewater drainage of these fields poses a threat to the environment and human health. Future research work should focus on estimation of stable O and H isotope compositions of magmatic water related to high-temperature hydrothermal systems in China, which is of significance for the quantitative source study of geothermal fluid recharged by degassed magmatic waters. Attention should also be paid to some constituent species in geothermal fluid of strong environmental significance, such as thioarsenate that is crucial for the fate of As discharged from geothermal springs, especially sulfidic hot springs.