## 5 Discussion  
### Indicators of Hydrothermal Centers  
The formation temperature of the minerals is directly correlated with the absorption peak wavelength of Fe-OH in chlorite minerals (pos2260) and the crystallinity index (CSM) [6; 7; 8; 13; 14; 15; 16; 17; 18; 19; 20]. CSM scalar values greater than 1 are indicative of chlorite with a high metamorphic grade, while values less than 1 are generally associated with chlorite produced by lower temperature alteration events; the higher the CSM value, the higher the alteration temperature [1; 2; 3; 8; 13; 14; 15; 16; 17; 18; 19; 20]. The compositions of many minerals that contain Fe-OH have slightly changed. This indicates fluid chemicals are involved during the alteration. These compositional changes cause the wavelength position of the Fe-OH absorption feature to shift at around 2260 nm. It indicates the geochemical conditions during the alteration [6; 7; 8]. In most cases, the absorption trough for Fe-rich chlorite occurs between 2260 and 2265 nm, while the absorption peak for Mg-rich chlorite lies between 2255 and 2260 nm [11; 22]. Fe-chlorite is cooled prior to Mg-chlorite, followed by Mg-Fe chlorite, Fe-Mg chlorite, and finally Mg-chlorite [3; 5]. The formation temperature of the minerals is closely linked to the absorption peak wavelength of Al-OH in muscovite minerals (Pos2200) and the crystallinity index (ISM) [19; 20]. ISM scalar values greater than 1 are indicative of illite with a high metamorphic grade, while values less than 1 are generally associated with illite produced by lower temperature alteration events; the higher the ISM value, the higher the alteration temperature [1; 2; 3; 8; 13; 14; 15; 16; 17; 18; 19; 20]. The chemical composition of the fluids changed during alteration, leading to wavelength shifts in the absorption features. For adsorbed water, it was at approximately 1,900 nm, and for Al-OH, it was at around 2200 nm. As the alteration became stronger, the adsorbed water gradually diminished. Moreover, both the absorption peak wavelength of the Al-OH feature within the range of 2160 nm to 2240 nm and the crystallinity index increased [6; 7; 8; 13; 14; 15; 16; 17; 18; 19; 20]. Therefore, the wavelength position can indicate the geochemical conditions during the alteration. This study mapped out the mineral alteration in the middle section (200 m) of the borehole drills and tunnels (Figure 9) and integrated the results with the compilation work. The illitization (of the muscovite group) Al-OH absorption peak wavelength within the 200 m middle section spreads across the area between the 1' and 17' lines, with a predominance at <2206 nm. This distribution shows a good correlation with the No. 2 ore body. The crystallinity of illite, with values > 1.1, forms a distinct belt extending from north to west, with two anomalous centers concentrated at the 1' and 5'-9' lines. In the case of chloritization (of the chlorite group), the Fe-OH absorption peak wavelength shows a sheet-like anomaly at >2254 nm along the 0'-7' lines. This continuous anomaly extends up to 500 m long and 20 m wide further northwest, which is similar to the direction of the ore body. This study has also located a northwestward weak anomaly (circumscribed by values > 2251) in the deeper section along the 15' line. There is an anomalous zone along the 9' and 17' lines with crystallinity of chlorite larger than 3.0. Additionally, combined with the engineering practice of the middle section at 0 m, chloritization (of the chlorite group) shows a sheet-like anomaly at the 1' line, with the Fe-OH absorption peak wavelength exceeding 2254 nm. This anomaly extends approximately 50 m long and 20 m wide to the northwest, which is similar to the direction of the ore body. It continues to stretch in the southeastern or southwestern direction.