Heat source-related mechanisms and genetic modes for the formation of hot dry rocks in the Reshuiwei geothermal field, Hunan Province

Hot springs with temperatures exceeding 90℃ indicate that the Reshuiwei geothermal field in Hunan Province enjoys sound geothermal geological conditions, making this geothermal field a potential favorable area for hot dry rock (HDR) exploration. To carry out a reasonable evaluation of the geothermal reservoir and prospects for exploration and exploitation of HDRs in the Reshuiwei geothermal field in the Rucheng area, this study comprehensively analyzed the deep geological structures, geochemical characteristics, geophysical characteristics including gravity, magnetic, electrical, and seismic properties, and geothermal field characteristics of the geothermal field. Furthermore, it revealed the deep thermal structures of the geothermal field from deep to shallow and explored the heat source-related mechanisms and geodynamic process for the formation of HDRs in the geothermal field. Primary results are as follows. (1) The Reshuiwei geothermal field exhibits deep reservoir temperatures ranging from 79.4 to 143.9℃, as estimated using a silica geothermometer. (2) The Zhongpeng and Yuwang granite plutons near the Reshuiwei area display average heat production rates between 7.07 and 8.44 μW/m³, which are significantly higher than the average heat production rates of the crust in the major geological units in Chinese continent. (3) The gravity and magnetic characteristics reveal relatively thin lithosphere in the Reshuiwei area. Furthermore, the interpretations of the magnetotelluric sounding and seismic wave velocities reveal that the crust hosts geobodies with high conductivity and low velocity, which coincide with deep-seated fault zones. This indicates that these faults might serve as pathways for the upward intrusion of deep thermal materials. Based on these results, this study summarized the genesis mode of HDRs in the Reshuiwei geothermal field. Specifically, the subduction and retreat of the Pacific Plate led to intensive thermal disturbances to the plate front, further resulting in the uplift of the asthenosphere and the intrusion of mantle-derived thermal materials. As a result, a relatively highly located mantle-derived heat source was formed. Granite plutons with high heat production rates and uranium deposits featuring radioactive heat production form a favorable crustal heat source. Furthermore, deep-seated faults serve as pathways for the upward intrusion of deep thermal materials while supplying heat sources for the formation of shallow HDRs. The favorable thermal source conditions and the upward intrusion of deep thermal materials along deep-seated faults jointly lead to the formation of a favorable HDR target area represented by plutons around Reshuiwei in southeastern Hunan. As inferred from the comprehensively determined geothermal gradients, the concealed and tight granite plutons with scarce fractures and burial depths ranging from 4000-6000 m in the Reshuiwei geothermal field exhibit temperatures reaching 176.8-256.6°C, serving as favorable HDRs to be explored and exploited in the near future.