## 6 Discussion
### Trace Element Signatures of Ore-Forming Processes  
Sulfide minerals often undergo a \"cleansing\" process during metamorphic growth or recrystallization [3]. This process leads to a significant decrease in the content of trace elements within the recrystallized minerals. The decreasing trend is attributed to the coarsening of pyrite grains and the loss of inclusions during recrystallization, a phenomenon well-documented in deposits subjected to metamorphism and deformation [64, 65].  
Most rare earth elements, Cu and Zn in the studied pyrite of Type-2 massive ores are depleted (Figure 4A), while there are no significant changes in the \(\delta^{34}\)S values (Figure 8A), suggesting that the inclusions in Type-2 massive ores are released during recrystallization. These elements dissolve in the fluid, resulting in an enrichment of trace elements within the fluid phase. Meanwhile, the pyrite of the Type-3 massive ores exhibits sufficient enrichment of Cu, Zn, and REEs with negative \(\delta^{34}\)S values. Consequently, the Type-2 massive ores were possibly derived from the recrystallization of Type-1 massive ores, and the trace elements released during the formation of Type-2 massive ores may play an important role in the metal element re-enrichment in Type-3 massive ores.  
The Co/Ni ratio in pyrite is an important geochemical indicator that provides insights into the ore-forming environment and metal sources in hydrothermal deposits, particularly  
Figure 8: (**A**) Sulfur isotopic compositions of sulfides from the Hongtoushan deposit; (**B**) Co-Ni ratio relationship diagram.  
volcanogenic massive sulfide (VMS) deposits [66; 67; 68]. In sedimentary and diagenetic environments, pyrite typically has a low Co/Ni ratio (<1), as Ni is more readily incorporated into the pyrite structure compared to Co under these conditions [67; 68]. However, the pyrite formed in high-temperature hydrothermal systems associated with VMS deposits tends to have higher Co/Ni ratios (>1), due to the preferential incorporation of Co into the pyrite lattice at elevated temperatures [69; 70]. Furthermore, the Co/Ni ratio in the pyrite can reflect the source of metals in the hydrothermal system. Pyrite with high Co/Ni ratios (>10) is often associated with a magmatic-hydrothermal origin, as magmatic fluids are typically enriched in Co relative to Ni. In contrast, pyrite with lower Co/Ni ratios (1-10) may indicate a mixed source of metals, with contributions from both magmatic fluids and leached host rocks [66; 71].  
The average Co/Ni ratios of pyrite in Type-1 and Type-2 massive ores are 49 and 53, respectively, which are significantly higher than the average Co/Ni ratio of 0.96 in Type-3 massive ores (Figure 8B). The high Co/Ni ratios in Type-1 and Type-2 massive ores suggest a dominant magmatic-hydrothermal input during the early stages of mineralization, while the low Co/Ni ratio in Type-3 massive ores points to a later stage of mineralization characterized by increased fluid-rock interactions [66; 72; 73; 74], and also by a possible mixing of magmatic and circulating seawater-derived fluids, according to the sulfur isotope characters discussed above.