## 5 Discussion
### Mechanism of the Mineralization-Alteration Zone  
The temperature, sulfur fugacity, and solution pH are the principal controls of sulfide assemblages and the alteration type [2]. And the evolution of ore-forming fluids is not only a crucial component in the formation of various alterations but also a reflection of the ore-forming process [39; 40; 41; 42].  
Fluid inclusions and isotopes studies indicate that the ore-forming fluids and materials of the Beiya gold-polymetallic deposit originate from the magmatic-hydrothermal system related to alkaline porphyryr [23; 24; 25; 26; 43]. In the early stage of the magmatic-hydrothermal system, the ore-forming fluid is at a high temperature (400-560 \({}^{\circ}\)C) and high salinity (38-67 wt.% NaCl equiv.) fluid [24; 26]. This early-stage high temperature and salinity ore-forming fluid, on the one hand, causes osmotic potassium metasomargin in the alkaline porphyryr [44], resulting in the formation of potassium silicate minerals of K-feldspar and hydrothermal biotite, along with enrichments of K\({}_{2}\)O, MgO, and Fe\({}_{2}\)O\({}_{3}\)\({}^{*}\) and losses of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and CaO; the early garnet-containing skarn alteration- mineralization occurs synchronously [45], which is predominantly induced by fluids with high temperature and salinity of 360-575 \({}^{\circ}\)C and 36-69 wt.% NaCl equiv., respectively, as well as a low to moderate salinity of 2-25 wt.% NaCl equiv. conducted in epidotes or garnets within the skarn [26; 46]. On the other hand, the early-stage ore-forming fluid extracts metallogenic elements such as Au, Cu, and Mo from the alkaline porphyryr, generating Cu-Au-Mo mineralization in the potassic zone and initial Au mineralization in the wall-rocks [44; 45].  
With the processes of cooling, boiling, and/or mixing with meteoric waters, the ore-forming fluid at Beiya transforms to a predominant low to moderate temperature (310-440 \({}^{\circ}\)C) and low salinity (1-15 wt.% NaCl equiv.) fluid [26; 46], creating a general phyllic and propylitic alteration, which is similar to sericitic alteration occurring at temperatures between 370 and 450 \({}^{\circ}\)C in the Butte porphyry copper-molybdenum deposit [39]. The early garnet-containing skarn undergoes retrograde alteration as well, producing epidote, chlorite, and quartz through fluid metasomargin [45]. In addition, the diffusion and migration ability of Au and Cu decreases at this stage in the thermodynamics state; when the hydrothermal fluid migrates to the lithologic and structural discontinuity interface, Au, Cu, and other metallogenic elements precipitate in these special locations, mostly in phyllic and contact alteration zones (Figures 4 and 6).  
In the late-stage evolution of ore-forming fluid, cooling and meteoric water mixing are the main processes to form a low temperature (160-340 \({}^{\circ}\)C) and low salinity (1-12 wt.% NaCl equiv.) of the hydrothermal fluid [26; 46], which promotes the formation of argillic alteration and migrates Au, Ag, Pb, and Zn outward to form low- to medium-temperature hydrothermal vein- and distal hydrothermal-type mineralization in the wall-rocks (Figures 4 and 6).