4.4.2. Al–OH Feature Depth 2200D

The quantity of infrared radiation absorbed is a function of the quantity of absorbing material in the sample. However, the absorption intensity is also affected to varying degrees by the physical conditions of the sample, such as the particle size and orientation of the absorbing minerals [15]. As a first approximation, the intensity of the Al–OH band was taken as an indication of the relative abundance of muscovite by assuming similar sample conditions for all pulverized core samples [31]. The intensity of the Al–OH band is the relative absorption depth of layered silicate minerals such as muscovite, kaolinite, and montmorillonite around 2200 nm, which is a fourth-order polynomial fitted between 2180 nm and 2220 nm of the Hull quotient. The derivative is followed by an extraction of relative depth, which indicates muscovite abundance. According to the contour profile, the 2200D in the No. 88 Exploration Line profile has a slight change in the vertical direction, and the correlation with the mineralization is not high, but the relative content of muscovite gradually increases from 88ZK01 to 88ZK05 in the horizontal direction (Figure 16).

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

**Figure 15.** No. 88 Exploration Line diagram of two-dimensional spatial variation of characteristic absorption peak position of muscovite Al–OH Pos2200. **Figure 15.** No. 88 Exploration Line diagram of two-dimensional spatial variation of characteristic absorption peak position of muscovite Al–OH Pos2200. *Minerals* **2022**, *12*, 1272 20 of 27

The illite crystallinity (SWIR-IC) value refers to the ratio of the absorption depth of sericite at 2200 nm to the absorption depth at 1900 nm. Numerous experiments have been

The SWIR-IC values in the study area varied from 0.2 to 3.8 with a majority concentrated between 0.5 and 1.5 (Figure 17). The crystallinities of the muscovite-group minerals in the drill holes were extracted and combined with the spatial locations of the drill holes to make a 2D contour map. As shown in Figure 16, the SWIR-IC values are relatively high near the ore body in the vertical direction, gradually decrease from the center of the mineralization to the sides, and are lower in the deeper areas of the mineralization. In the horizontal direction (Figure 17), the distribution of high IC values is generally consistent with the mineralization strike, thus confirming that the center of mineralization can be

the two have a good negative correlation [21]. Therefore, SWIR-IC, such as XRD-IC, can indicate the crystallinity of sericite minerals. The characteristic absorption peak at 1900 nm may indicate the water absorption of sericite. When the fluid temperature is relatively high, the muscovite minerals contain less crystalline water and have a high SWIR-IC value, so the degree of change in crystallinity can be used to reflect variations in mineral

conducted by predecessors. Compared with X-ray diffraction illite crystallinity (XRD-IC), 206

4.4.3. Illite Crystallinity SWIR-IC Value

formation temperature [21,37,43].

indicated by the IC values.
