Formation model of Fengshun hydrothermal geothermal system in Cathaysian orogenic system

Taking the Fengshun hydrothermal geothermal field in the Cathaysian orogenic system as an example, this study incorporates orogenic belt background elements into geothermal system and geothermal prediction research, and establishes a genetic model for the Fengshun geothermal field. The results show that the Fengshun area is characterized by well-developed Yanshanian granites with high radiogenic heat production rates, which serve as an effective heat source for the geothermal field. The area receives abundant rainfall, and the well-developed surface runoff, together with a topographic relief of nearly 1 km, provides ample water recharge. A system of steeply dipping conjugate shear faults forms the pathways for the long-distance migration of geothermal fluids. Mesozoic fractured granite bodies along the fault zones and Jurassic clastic rocks constitute the main geothermal reservoirs, whereas Triassic-Jurassic mudstones, shales, tuffs, and Quaternary clay layers act as effective regional caprocks. These features collectively create favorable conditions for a hydrothermal geothermal system in terms of heat source, water supply, reservoir, conduits and caprock. Along the Fenshui fault toward the southeast direction, surface runoff and groundwater flow occur simultaneously, creating favorable conditions for groundwater recharge, long-distance water circulation, and sustained heating of the groundwater. Downstream, the Lianhuashan fault zone and the continuously distributed granite bodies jointly form an effective barrier to groundwater flow, and the combination of the water-bearing reservoir and the obstructing structures constitutes a “ponding structure”. Deep-seated granites generating heat, deep groundwater undergoing long-distance circulation, shear faults conducting both water and heat, fractures and sand bodies providing storage space, fine-grained sedimentary caprocks offering thermal insulation, and lateral confinement by faults and granite bodies—all these elements together constitute the Fengshun hydrothermal geothermal enrichment model. The well-matched combination of heat source and water supply, fault conduits and water circulation, and reservoir and structural barrier is the key factor enabling the Fengshun geothermal field to be exploited continuously at stable temperature and high flow rate. This model explains the genetic mechanism of medium-high temperature geothermal anomalies in an orogenic setting and offers a reference for predicting and exploring geothermal resources in similar geological settings.