## 5 Discussion
### Geochemical Characteristics of Garnets in Skarn
#### 5.2.1 Mechanism of Rare Earth Element into Garnet
In addition, the content of Y is also lower than that of REE, indicating that the garnets in Tongshan deposits may not follow this substitution mechanism. (3) REE\({}^{3+}\) can couple with trivalent cations, such as Al\({}^{3+}\) and Fe\({}^{3+}\), to enter both X and Z sites, which is represented as \([\text{X}^{2+}]^{\text{VIII}}\)\(+\)\([\text{Si}^{4+}]^{\text{IV}}\)\(\rightarrow\)\([\text{REE}^{3+}]^{\text{VIII}}\)\(+\)\([\text{Y}^{2+}]^{\text{VI}}\). This substitution mechanism is called YAG-type substitution [49]. In samples TS-10-4/5/6/7, there is a positive correlation between REE\({}^{3+}\) and Al\({}^{3+}\) in spessarting garnets, while in sample TS-10-8, there is a positive correlation between REE\({}^{3+}\) and Fe\({}^{3+}\) (Figure 13a,b). This suggests that REEs may enter the garnets in the samples through YAG-type substitution. These characteristics also indicate that Fe\({}^{3+}\)-REE is more likely to enter the crystal of Al-rich grossularite from inner skarn; Al\({}^{3+}\)-REE is more likely to enter the crystal of Fe-rich and/or-dite from outer skarn. The different REE substitution mechanisms in garnets within the inner and outer skarn zones are discovered for the first time. These findings suggest that the incorporation process of REEs into the garnet lattice as trace elements, exhibits a stronger affinity with low-concentration isovalent elements in the system. For instance, in the inner skarn, the Fe\({}^{3+}\) content is significantly lower than that of Al\({}^{3+}\). As a result, trivalent REEs are more likely to combine with the lower concentration of Fe\({}^{3+}\). This phenomenon provides mineralogical constraints that can be utilized to differentiate between the inner and outer skarn zones. (4) REEs can enter the X site of spessarting garnets through a vacancy in the octahedral site, which is represented as \(3[\text{X}^{2+}]^{\text{VIII}}\)\(\rightarrow\)\(2[\text{REE}^{3+}]^{\text{VIII}}\)\(+\)\([\text{Y}^{\text{VII}}]\)[50,51]. From a geochemical perspective, we cannot rule out the possibility of REEs entering the garnets in this way. [PERSON] et al. (2009) found that there was no correlation between REE\({}^{3+}\) and Al in the Navachab gold deposit, but there was a correlation between REE\({}^{3+}\) and Y [52]. They suggested that during the process of REEs entering the garnet lattice, crystal chemistry can only control a small portion of the elements that enter the garnet through substitution mechanisms, and the chemical composition of the fluid plays a major role. In our samples, U, Y and REE also show a positive correlation (Figure 13c,f). The REE in andradradite from outer skarn TS-10-4/5 also showed large fluctuations at different positions in the garnet (Figure 13 horizontal coordinate), while the Y/Ho ratio (Figure 14) indicates the presence of strong water-rock reactions in the meta-igneous rocks. Therefore, in the Tongshanskarn, the entry of trace elements into garnets may not only be controlled by equilibrium crystal chemistry, but the fluid and rock chemistry of water-rock reactions (such as surface adsorption and inclusions) may also play an important role [14].