## 5 Discussion
### The Definition of the Inner and Outer Boundaries of Skarn  
[PERSON] et al. (2020) utilized the inactive elements present in the whole-rock data for binary diagrams to differentiate between the inner and outer skarn [8]. By plotting Al\({}_{2}\)O\({}_{3}\) against Lu, Dy, Y, and Nd, they were able to clearly distinguish between the two types of skarn. It is found that most of the inner and outer skarn on the Tongshan skarn profile fall within a similar range [8], and only a few data points (such as TS-10-7 and TS-10-2) have poor differentiation effects (Figure 16). The observation of the petrographic characteristics shows that TS-10-7 is mainly composed of garnet and diopside, and later epidote intersects and replaces the garnet. No detailed residual plagioclase or pyroxene was observed, indicating that the sample belongs to the outer skarn. On the Al\({}_{2}\)O\({}_{3}\)-Nd diagram (Figure 16b), TS-10-7 does not fall within the range of either type of skarn, and on other diagrams, TS-10-7 also cannot be well distinguished for its skarn type. The TS-10-2 area is mainly composed of diopside and chalcopyrite, and it also belongs to the obvious category of outer skarn. On the Al\({}_{2}\)O\({}_{3}\)-Y and Al\({}_{2}\)O\({}_{3}\)-Dy diagrams (Figure 16a,c), TS-10-2 is significantly higher than the range of outer skarn, but its chemical composition is obviously different from that of inner skarn. These characteristics indicate that there is still a certain degree of uncertainty in distinguishing between inner and outer skarn based on whole-rock geochemical features. Further research is needed to determine the geochemical discrimination criteria for inner and outer skarn. Skarn minerals, such as garnet, may also provide mineralogical constraints for distinguishing the boundary between inner and outer skarn. The Fe, In, Al, Mn, Ni, Ti, Zr and Nb contents of garnets in inner and outer skarn are significantly different. Garnets in inner skarn exhibit high concentrations of Al, Mn, Ni, Ti, Zr and Nb, whereas those in outer skarn possess lower levels of these elements but substantially higher Fe contents. The contents of these elements show a cliff-like change between the two types of skarn (Figure 17). Moreover, the contents of Ti, Zr and Nb elements in garnets show a negative correlation with the andradite end-member (And) of garnet (Figure 18). These mineralogical and chemical characteristics can be explained as follows: due to the relatively high formation temperature and the low water-rock ratio of inner skarn; immobile HFSE often accumulates in the grossularite formed during the high-temperature stage. In contrast, outer skarn has a high water-rock ratio and strong fluid activity, and immobile HFSE is not easily enriched in them. Therefore, the garnets formed in this area show low and fluctuating contents of these elements. The trace element composition of garnets in the outer skarn sample TS-10-7 markedly differs from that of the inner skarn, exhibiting high Fe and In levels, along with low Al, Mn, Ni, Ti, Zr and Nb contents. These characteristics are similar to those of other garnets in outer skarn, further confirming that the TS-10-7 sample is a typical outer skarn sample. The major and trace element characteristics of skarn minerals partially compensate for the limitations of using a single whole-rock geochemical method. Comprehensive  
Figure 16: Al\({}_{2}\)O\({}_{3}\) vs. Y, Nd, Dy, Lu diagrams. (**a**) Al\({}_{2}\)O\({}_{3}\) vs. Y diagram, (**b**) Al\({}_{2}\)O\({}_{3}\) vs. Nd diagram, (**c**) Al\({}_{2}\)O\({}_{3}\) vs. Dy diagram, (**d**) Al\({}_{2}\)O\({}_{3}\) vs. Lu diagram. Refering intrusion, inner skarn, outer skarn and marble data from appendix tables A2 of [PERSON] et al. (2020) [8]. Limestone data from [PERSON] et al. (2020) and [PERSON] et al. (2015) [8,58]. judgments from multiple methods on samples near the boundary are the most effective way to distinguish between inner and outer skarn. Figure 17: Analysis of major and trace elements present in garnets within the TS-10 profile.