Chemical and Isotopic Characteristics of Hot Spring Water and Genesis Analysis Under the Constraint of Nappe Structure in the Midu Area, Dali Prefecture, Yunnan, China

The Midu Basin has a complex geological structure, active neotectonic movement, strong magmatic activity, and abundant geothermal resources, but the current research degree is low. Five representative hot springs in the Midu area were studied; the hydrochemistry and hydrogen and oxygen isotope characteristics of water samples were analyzed; and the recharge elevation, geothermal reservoir temperature, and circulation depth of hot springs were calculated. This study points out that the formation of hot springs is all under the joint action of the Red River Fault and the Xiangyun nappe structure based on hydrochemical characteristics, calculation results, and tectonic location features of hot springs. However, our study further reveals that their development in distinct segments of the nappe structure results in varying surrounding lithologies, which directly govern the intensity and pathways of water–rock interactions. Based on predominant cations in geothermal water, the five representative hot springs can be divided into two types: Na + type and Ca 2+ type. A genetic model of these hot springs is proposed for the first time, whereby they are heated by deep and large faults and recharged by faults and folds, elucidating the lithology–tectonics–fluid linkage in the nappe‐related geothermal system of the region. The research results of this paper provide key data and theoretical support for geothermal research in the Midu area and also provide important guiding significance for geothermal research under the background of the same structure.