## 7 Geothermal Genesis Model  
Using the structural information obtained from the MT, seismic reflection, and large-scale seismic tomography, combined with the analysis of geology, water chemistry, and helium isotope tracing, we established a complete geothermal system model in the Huailaia area.  
First, under the crystalline basement of the Huailaia Basin, there is a high-conductivity and reflection anomaly, indicating the relatively active structural features below the basement. Combining the results of helium isotope tracing and the lack of an obvious heat source in the Huailaia area, it is thought that the high conductivity and reflection anomaly under the basement is caused by the upwelling of mantle-derived material. The upwelling mantle-derived material carries large amounts of heat and is enriched under the basement. Due to melting, a magmatic heat source formed, which is the main reason for the abundant geothermal resources in the Huailaia area. In contrast, the Xuanhua Basin, adjacent to the Huailaia Basin, is relatively poor in geothermal resources due to its distance from the heat source.  
Secondly, the crystalline basement connects the deep heat source and the groundwater circulation system through the faults (F5, F6, and F7 in Figure 9a) in the basement. Then, the heat is transferred to the surface through the groundwater cycle. The crystalline basement plays an important role in heat storage; it ensures that heat will not be lost quickly and can then be continuously transferred to the surface.  
Third, under the influence of NW extensional tectonic stress, the regional faults (F1, F2, F3, and F4 in Figure 1) create the permeability of the upper crust, which ensures the recharge and circulation of groundwater that allows for the continuous transference of heat energy to the surface.  
Finally, the Tertiary and Quaternary clay and silt layers overlying the bedrock provide the geothermal water with a good aquifer and caprock (Figure 2).  
The geothermal system in the Huailaia area has the typical characteristics of a magma-type geothermal system. From bottom to top, the geothermal structure of the Huailaia area can be divided into a magma-type heat source, a crystalline basement heat storage layer, and caprock. The F1-F7 faults constitute the circulation network of the geothermal system, which connects the deep heat source to the groundwater. A conceptual model of the geothermal system in the Huailaia area was established, as shown in Figure 10.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Hot Spring**} & \multicolumn{2}{c}{**Reference [1]**} & \multicolumn{2}{c}{**Reference [2]**} \\ \cline{2-5}  & **Circulation Depth** & **Thermal Storage** & **Circulation Depth** & **Thermal Storage** \\  & **(m)** & **Temperature (°C)** & **(m)** & **Temperature (°C)** \\ \hline Nuanquan & 1910.5 & \textgreater{}66 & — & — \\ \hline Houhaoyao & 4025.3 & 115 & 4600 & 106 \\ \hline \hline \end{tabular} The “—” indicates that the data are not calculated. Note: the temperature calculation uses the geochemical geothermometer, and the premise is that the reaction between the water–rock system reaches a balance. The Nuanquan hot spring has not reached a balance, and the calculated result is lower than the actual temperature.  
\end{table}
Table 2: The groundwater circulation depth and the thermal storage temperature.