4.4.1. Al–OH Feature Wavelength Pos2200

The substitution of octahedral Al by other cations such as Fe and Mg is the main reason for the change in muscovite composition. Therefore, the wavelength of the Al–OH band can be used to spectroscopically quantify the octahedral cation composition (Alvi) of muscovite, reflect compositional changes, and infer fluid characteristics [31,40,42]. Sericite exhibits strong absorption near 2200 nm, which is called Pos2200. When Tschermak substitution occurs, the composition of cations in the octahedron changes; usually, Pos2200 > 2205 nm is Al-poor muscovite with high Fe and Mg content [41]. Furthermore, because kaolinite and montmorillonite also have absorption peaks near 2200 nm, when the muscovite-group minerals are mixed with these minerals, the absorption peak position of Al–OH will slightly change.

The sericite minerals identified by SWIR in the eight drill holes in the Zhaoxian mining area mainly include muscovite, paragonite, phengite, and illite. The minimum absorption peak position (Pos2200) of Al–OH is concentrated around 2200 nm, approximately at 2185–2220 nm with a normal distribution, indicating that the dolomite minerals in the study area are distinct (Figure 13). It can be seen from Figure 10 that Pos2200 > 2205 near the ore body is Al-poor muscovite. In the deep part of the mineralization, the Pos2200 value decreases, which reflects the significant correlation between mineralization, depth, and Al–OH wavelength. For the convenience of distinction and description, this study used Pos2205 as the division between muscovite and phengite. *Minerals* **2022**, *12*, 1272 18 of 27

**Figure 13.** Relative frequencies of Al–OH Pos2200 in white mica. **Figure 13.** Relative frequencies of Al–OH Pos2200 in white mica.

The Pos2200 values of the four selected drill holes passing through the main ore bodies are all in a normal distribution, which roughly shows that the Pos2200 wavelength of the shallow sericite is relatively short, generally less than 2204, indicating Al-rich muscovite. With the increase in depth, the Pos2200 value of sericite gradually increased to ap-The Pos2200 values of the four selected drill holes passing through the main ore bodies are all in a normal distribution, which roughly shows that the Pos2200 wavelength of the shallow sericite is relatively short, generally less than 2204, indicating Al-rich muscovite. With the increase in depth, the Pos2200 value of sericite gradually increased to approxi-

proximately 2208. According to Figure 13, the Pos2200 values of the selected drill holes passing through the main ore body at 800–1500 m depths have almost a normal distribu-

The study of the offset of Pos2200 reflects that the gold mineralization corresponds to high values of Pos2200. This phenomenon is more obvious in the contour profile, and the high value area of Pos2200 is basically consistent with the mineralization trend (Figure 15).

mately 2208. According to Figure 13, the Pos2200 values of the selected drill holes passing through the main ore body at 800–1500 m depths have almost a normal distribution; as the depth increases, the Pos2200 values also increase (Figure 14). Conversely, because of the depth of 88ZK05, the Pos2200 value actually decreases in the deepest part. The study of the offset of Pos2200 reflects that the gold mineralization corresponds to high values of Pos2200. This phenomenon is more obvious in the contour profile, and the high value area of Pos2200 is basically consistent with the mineralization trend (Figure 15). proximately 2208. According to Figure 13, the Pos2200 values of the selected drill holes passing through the main ore body at 800–1500 m depths have almost a normal distribution; as the depth increases, the Pos2200 values also increase (Figure 14). Conversely, because of the depth of 88ZK05, the Pos2200 value actually decreases in the deepest part. The study of the offset of Pos2200 reflects that the gold mineralization corresponds to high values of Pos2200. This phenomenon is more obvious in the contour profile, and the high value area of Pos2200 is basically consistent with the mineralization trend (Figure 15).

The Pos2200 values of the four selected drill holes passing through the main ore bodies are all in a normal distribution, which roughly shows that the Pos2200 wavelength of the shallow sericite is relatively short, generally less than 2204, indicating Al-rich muscovite. With the increase in depth, the Pos2200 value of sericite gradually increased to ap-

*Minerals* **2022**, *12*, 1272 18 of 27

**Figure 13.** Relative frequencies of Al–OH Pos2200 in white mica.

**Figure 14.** Wavelength relationship histogram of sericite Al–OH Pos2200 and depth.
