Water Geochemistry Indicating the Geological Genesis of a Typical Unexplored Geothermal System (shidian) in the Yunnan-Sichuan-Tibet Geothermal Belt (YST), China

Distinguishing between non-magmatic and magmatic hydrothermal systems coexisting in a magmatic geothermal belt is of great significance. Shidian, located in the Yunnan-Sichuan-Tibet geothermal belt (YST), is representative of those hydrothermal areas unexplored or explored to a low degree. The hydrochemical types of the Shidian geothermal waters reflect the lithology of the reservoirs there. Lack of acid sulfate springs and much lower chloride concentrations in the neutral-alkaline springs are the major differences of the Shidian hydrothermal area from the Rehai hydrothermal area. Typically, co-existence of Na-Cl type, Na-HCO3 type, and H-SO42− type hot springs in a geothermal system can be indicative of magmatic heat source, which is not the case in Shidian. The water isotopic compositions of the Shidian geothermal waters are controlled by water-rock interactions and their mixing with shallow groundwaters. In contrast, the enrichment of oxygen-18 and deuterium in the Rehai geothermal waters is the result of magmatic fluid input and adiabatic cooling of parent geothermal fluid (PGF) during its ascent. Moreover, the reservoir temperatures in Shidian calculated using chemical geothermometers range from 70 to 118 °C, much lower than those in Rehai. To sum up, a combination of the hydrochemical, water isotopic, and reservoir temperature data shows little possibility of the existence of a magmatic heat source in Shidian. The ways to identify the heat source of a hydrothermal system used in this study are helpful for further exploration and exploitation of not only the Shidian system but also the other unexplored systems in YST.