## 5 Discussion
### Geochemical Characteristics of Garnets in Skarn
#### 5.2.3 Fluid Behavior Processes Recorded by Garnet  
Consequently, U is a variable valence element, including U\({}^{4+}\) and U\({}^{6+}\). U\({}^{4+}\) is highly reactive and easily migrates during fluid activity, while U\({}^{6+}\) has low reactivity. Therefore, the U content in garnets may reflect the _f_/O\({}_{2}\) changes during their formation. Andradite usually forms in relatively oxidizing environments, while grossularite form in relatively reducing environments [1,2]. Near the rock body during skarn formation process, the temperature is relatively high and _f_/O\({}_{2}\) is relatively low. U mainly exists in the form of U\({}^{4+}\) and migrates with the fluid towards the outer skarn, where it is enriched. This results in lower U content in the inner skarn when U enters the lattice of grossularite during its crystallization. In contrast, andradite formed in the outer exhibits higher _f_/O\({}_{2}\) characteristics, with U present as U\({}^{6+}\), which subsequently precipitates in large quantities into the lattice of andradite. In the Tongshan skarn profile, we found garnets in the inner exhibit with relatively low U content, while those in the outer are enriched in U, adhering to the same principle (Figure 15a). The factors that cause the Eu anomaly in garnets are complex and diverse, including fluid temperature, pH, _f_/O\({}_{2}\) and water-rock ratio, etc. The Eu element in the fluid mainly exists as Eu\({}^{2+}\) and Eu\({}^{3+}\). Fluid temperature controls the distribution of Eu, and at temperatures above 250 \({}^{\circ}\)C, Eu\({}^{2+}\) is more easily incorporated into garnets than Eu\({}^{3+}\). Therefore, garnets are more enriched in Eu\({}^{2+}\) under such conditions [55]. Research by [PERSON] and [PERSON] (2005) suggests that Eu\({}^{2+}\) combines with Cl\({}^{-}\) to form EuCl\({}_{4}\)\({}^{2-}\) and migrates [56]. Consequently, its behavior diverges from that of other REEs,  
Figure 15: (**a**) Andradite end member (And) vs. U diagram; (**b**) \(\delta\)Eu vs. U diagram. lies, such as the positive Eu anomaly of andradite, and the weak anomaly or negative Eu anomaly of grossularite. These anomalies may indicate that grossularite is developed in a relatively dry and water-poor environment, while the location where andradite forms exhibits a higher water-rock ratio and greater Cl\({}^{-}\) content. The substantial variation in the Y/Ho ratio in andradite further supports this hypothesis (Figure 14). Simulation studies by [PERSON] have found that andradite exhibits a larger unit cell volume than grossularite, resulting in smaller lattice relaxation of Eu\({}^{2+}\) entering andradite compared to that of Eu\({}^{2+}\) entering grossularite [57]. Eu\({}^{2+}\) is more likely to occupy the Ca position in andradite, which aptly explains the weak Eu anomaly of grossularite and the pronounced positive Eu anomaly of andradite. The garnets, both grossularite and andradite, in the Tongshan skarn profile also support the above conclusions. Grossularite forms in a relatively reduced environment, whereas andradite develops in a relatively oxidized environment. It can be inferred that the proportion of Eu\({}^{3+}\) in the system during andradite formation would be higher, potentially leading to a negative Eu anomaly. However, this inference is not consistent with the observed facts. Consequently, /O\({}_{2}\) may not be the primary factor causing Eu anomalies in garnets. Although the Eu anomalies in different types of garnets may not exhibit a significant relationship with /O\({}_{2}\), the variations of Eu anomalies within the same type could still be influenced by changes in /O\({}_{2}\) characteristics. For instance, in the grossularite developed in sample TS-10-6, its Eu anomaly demonstrates a negative correlation with U content (Figure 15a), which may indicate that the variation is caused by changes in /O\({}_{2}\). As the /O\({}_{2}\) increases, more Eu is converted to Eu\({}^{3+}\) and enters the andradite, leading to a decrease in Eu in the garnet and a reduction in positive Eu anomaly.