Cause of granite in Lancang County and its geothermal implications

Geothermal energy is a clean, abundant, and dependable energy source. Because the formation of a geothermal field is often closely linked to the distribution and development of granite, it is crucial to understand the regularity and formation mechanism of granite geothermal resources to advance the field of geothermal energy. Lancang County is located on the southern edge of the Lincang granite, and contains many hot springs. In this paper, we synthesize rock geochemistry, zircon U–Pb chronology, and other methods to interpret the genesis of granites in Lancang County and their geothermal formation patterns. The study area mainly comprises a Middle Triassic S-type granite with high contents of heat-producing elements. Zircon U–Pb dating results show that the intrusion time of the Lancang granite was 239 Ma. The granite is rich in light rare earth elements, depleted in heavy rare earth elements, and characterized by a negative Eu anomaly. The ɛ_Hf values range from −20.3 to 13.6, indicating the presence of mantle material in the source area. Igneous rocks in Lancang County likely formed in the background of an active continental margin associated with the subduction of the Tethys Ocean to the east, which formed an intra-land orogeny. High contents of radioactive thermogenic elements in the granite, fracture development, and additional heat sources from metamorphic rocks combined to generate high-temperature geothermal resources in the study area. The hydrothermal geothermal model of the granite has the following characteristics: a near heat source, large drop, water storage in fractures, simultaneous geothermal influence in deep and shallow layers, and hot spring and geothermal distribution along deep and large faults, etc. The geothermal reservoirs in the study area are classified into four types: internal fissures in granite, granite paleo-weathering crust, tectonic nappe fractures, and sedimentary deposits associated with granite cooling collapse. These types are of great significance to the understanding of the formation of hydrothermal geothermal heat in this granite area.