## 5 Discussion
### Gold Precipitation  
In the process of gold mineralization within hydrothermal fluids, gold primarily migrated in the form of HS\({}^{-}\) complexes in the Jiaodong gold deposits [59]. Changes in the physicochemical conditions of the fluid can lead to a decrease in the solubility of gold complexes, resulting in gold precipitation [60]. In the Jiaojia gold deposit, pyrites have an average Au content of 0.03 ppm in Stage I, 0.15 ppm in Stage II, and 0.27 ppm in Stage III. Stages II and III were the main mineralization stages.  
Figure 8: Diagram of Co vs. Ni of the pyrite of different generations in the Jiaojia gold deposit.  
During mineralization, the tectonic stress field in the Jiaodong area transitioned from being dominated by compressional stress to being primarily characterized by tensile stress, leading to decompression of the ore-forming fluids [3]. In this environment, the oxygen fugacity of the ore-forming fluids increases, resulting in immiscibility [61]. The change in pressure is another main mechanism of fluid immiscibility in the mid-crust [62]. [PERSON] et al. (2001) have proved that if the rock fractures change due to pressure variations, it may lead to the immiscibility of originally miscible fluids in different spatial environments or promote the mixing of fluids from different sources in orogenic gold deposits [63]. As established above, an increase in the Co content (\(\sim\)216.57 ppm) of pyrite in mineralization Stage III indicates that fluid immiscibility occurred as temperatures decreased in the ore-forming fluids. The immiscibility of ore-forming fluids can lead to the escape of a significant amount of H\({}_{2}\)S gas, destabilization and breakdown of gold-sulfur-hydrogen complexes, and gold precipitation [64; 65; 66], which causes the chemical equilibrium in Equation (1) to shift leftward, leading to Au precipitation.  
\[\text{Au}+\text{H}_{2}\text{S}+\text{HS}\rightleftharpoons[\text{Au(H}_{2})]^{-}+0.5 \text{H}_{2} \tag{1}\]  
Arsenic can replace S in the pyrite lattice, causing lattice distortion or an increase in the cell parameters, thus providing space for the enrichment of gold [67]. During the mineralization stages, As substitution for S\({}_{2}\)\({}^{-}\) in pyrite distorts the pyrite lattice, facilitating the entry of Au\({}^{+}\) into the pyrite lattice [68]. There was a positive correlation in the Jiaojia pyrite between Au and As in all mineralization stages (Figure 9c). Ore bodies close to ore-controlling faults contain more Au resources. The slant depth of the ore bodies is greater than the length along the strike direction. Ore bodies thicken, and gold grades decrease from shallow to deep areas [69].  
Figure 9: Correlation diagram between Au and trace elements of the pyrite of different generations in the Jiaojia gold deposit. Plotting of Au versus Bi (**a**), Au versus Co (**b**), Au versus As (**c**), Au versus Ag (**d**).  
In summary, the precipitation mechanisms of gold at different mineralization stages in the Jiaojia gold deposit are distinct. Gold precipitation resulted from fluid immiscibility caused by pressure and temperature decreases and increases in the oxygen fugacity of the ore-forming fluid.