Genesis analysis of high-temperature geothermal systems in the Western Yunnan: geochemical evidence from geothermal fluids

The formation of high-temperature geothermal systems is widely regarded as inextricably linked to magmatic or volcanic activity, given that both magmatism and volcanism are indicative of extreme high temperatures. However, we cannot rule out the possibility of existence of high-temperature geothermal system in non-magmatic or near-volcanic areas, which is one of the key focuses of research on geothermal system genesis. Taking geothermal fields in western Yunnan as the research objects, we found that the geothermal fluids in Tengchong Rehai (RH), Ruidian (RD), and Mangshi (MS) are rich in Cl and Li. Analysis of the ³He/⁴He isotope ratios in geothermal gases and δ¹³C-CO₂ revealed that mantle-derived helium accounts for a large proportion of the gases in the RH hot springs (over 20%), while it is in the range of 5%–20% in RD and MS, and less than 3% in Jinghong Basin. The mantle-derived CO₂ is over 80% in RH, 24% in RD, and 18% in MS. It is found that the geothermal fields in Tengchong (RH and RD) and Ruili (RL) are affected by magma-derived volatile components, whereas thos in Yunxian (YX), Jinghong (JH), Lincang (LC), MS, and Lianghe (LH) show no such influence. Based on Na-K-Mg cation geothermometry, the reservoir temperatures were accurately determined to be 260 ℃ for RH in Tengchong, approximately 220 ℃ for RD and RL, 200 ℃ for YX, and 180 ℃ for JH. A comprehensive geochemical analysis of geothermal fluids in western Yunnan not only indicates the origin of the regional geothermal system, but also confirms the presence of non-magmatic high-temperature geothermal systems in YX and JH, positioning these areas as promising targets for deep geothermal exploration. Further, these findings indicate that the genesis of the geothermal systems in this region exhibits spatial heterogeneity and differences in heat source characteristics, which are controlled by tectonic differences.