## 6 Analysis of Deep Geothermal Elements
### The Formation of the Heat Source  
Heat sources can be divided into magmatic and non-magmatic types [7]. From the structure revealed by the magnetotelluric profile, there is a high-conductivity C1 structure under the basement of the Huailai Basin, which appears as a bright spot reflector in the seismic reflection. In the larger-scale seismic tomography (Figure 9b) [38], C1 is connected to a low-velocity P-wave body that extends southwest to the depth of the Ordos block below 400 km, which is interpreted as an upwelling of asthenospheric mantle [39]. Thus, it has the ability to form a heat source.  
Helium isotope, as an inert gas, provides an excellent tracing effect. In geothermal systems, helium isotopes can provide clearer mantle source information. Generally, \({}^{3}\)He isotopes exist in the mantle, \({}^{4}\)He isotopes are mainly generated in the crust, and the helium isotopes in the atmosphere are mainly derived from the degassing of the earth. In the long evolution, the value of \({}^{3}\)He/\({}^{4}\)He forms the three-terminal component of the atmosphere, the crust, and the mantle. The value of \({}^{3}\)He/\({}^{4}\)He in the atmosphere is basically constant. When the gas in the mantle enters the crust and degasses, the value of \({}^{3}\)He/\({}^{4}\)He is reduced. Therefore, the value of \({}^{3}\)He/\({}^{4}\)He in hot spring gas mainly depends on the flux of mantle-derived helium isotope and can reflect the deep material composition of hot spring water [2]. Table 1 shows the contribution rate of the three different sources in the value of \({}^{3}\)He/\({}^{4}\)He. The larger the value, the greater the proportion of helium isotopes from a certain source. The helium isotope mainly comes from crust sources, which is mixed with considerable mantle source helium isotope content. These mantle-derived helium isotopes were interpreted as the result of the melting and degassing of mantle-derived materials [2].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hot Spring** & **Atmosphere Source** & **Crust Source** & **Mantle Source** \\ \hline Nuanquan & 0.1 & 75.6 & 24.3 \\ \hline Houhaoyao & 0.1 & 92.4 & 7.5 \\ \hline \end{tabular}
\end{table}
Table 1: The contribution rate of three different source in the value of \({}^{3}\)He/\({}^{4}\)He.  
In addition, there is no significant increase in the content of radioactive elements in the hot spring water (gas). The main cations are Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\); among them, K\({}^{+}\) mainly comes from the dissolution of silicate and sulfate [1,2]. There are no increases in the content of other radioactive elements (e.g., Th and U) or their decay elements (e.g., Pb and Rn), which shows that the crust is not radioactive, further proving that the heat source is not radioactive.  
Therefore, the formation of abundant geothermal resources requires the contribution of deeper heat sources. The existence of deep mantle-derived materials can reasonably explain the rich geothermal resources in the Huailai area. Mantle material upwelling is generally characterized by high electrical conductivity. The high-conductor C1 we found in the Huailai area supports this inference. Mantle-derived materials upwelled to the Huailai area and accumulated under the crystalline basement. Along with melting, a magma heat source formed, which provided the conditions for the formation of abundant geothermal resources in the Huailai area.