4.4.3. Illite Crystallinity SWIR-IC Value

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 conducted by predecessors. Compared with X-ray diffraction illite crystallinity (XRD-IC), 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 formation temperature [21,37,43].

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 indicated by the IC values. *Minerals* **2022**, *12*, 1272 21 of 27

**Figure 17.** No. 88 Exploration Line diagram of two-dimensional spatial change in short-wave infrared illite crystallinity. **Figure 17.** No. 88 Exploration Line diagram of two-dimensional spatial change in short-wave infrared illite crystallinity.

#### **5. Discussion 5. Discussion**

#### *5.1. Metallogenic Center Indication 5.1. Metallogenic Center Indication*

The purpose of auxiliary exploration can be achieved by tracking the migration of ore-forming fluid and change in fluid composition through alteration zoning [21]. By using the SWIR-spectrum test, the alteration type in the rock core can be intuitively interpreted at the micron scale, and the boundary of the alteration zone can be refined through The purpose of auxiliary exploration can be achieved by tracking the migration of ore-forming fluid and change in fluid composition through alteration zoning [21]. By using the SWIR-spectrum test, the alteration type in the rock core can be intuitively interpreted at the micron scale, and the boundary of the alteration zone can be refined through the

the spectral characteristic parameters and mineral abundance changes [21]. Based on spectral (hyperspectral) imaging, determining the best stratigraphic combination and miner-

At the deposit scale, the common spatial zonation pattern in the Jiaojia gold deposit advances toward the hydrothermal center. According to the SWIR results, by establishing 3D alteration-zoning models (Figures 18 and 19), the spatial distribution of mineral assemblages can be determined [46,47]. The model shows the location and geometry of the five alteration zones in the area, clearly reveals their spatial distributions, and confirms the 2D alteration-zoning model (Figure 18). The alteration zoning in this area is obvious, and the distribution is relatively stable. The phengite–carbonate alteration zone intersects with the ore body (Figure 19), which agrees well with, and can be a good indication of, the spatial distribution of the mineralization center. The alteration-zone minerals in the surrounding rocks are controlled by the original rock composition and structural zoning. The pyrite–sericite–quartz alteration zone in the footwall of the fault is close to the main fault plane. The pyrite–sericite–quartz alteration is strong, and the main alteration-zone minerals are developed in the center of the ore body. There were large quantities of phengite and carbonate minerals, followed by montmorillonite and a small quantity of kaolinite. Sericite and carbonate rocks are mainly developed in the monzogranite, in the

spectral characteristic parameters and mineral abundance changes [21]. Based on spectral (hyperspectral) imaging, determining the best stratigraphic combination and mineralogical alteration type related to mineralization is carried out to determine the mining probability with similar stratigraphic and structural conditions [44,45].

At the deposit scale, the common spatial zonation pattern in the Jiaojia gold deposit advances toward the hydrothermal center. According to the SWIR results, by establishing 3D alteration-zoning models (Figures 18 and 19), the spatial distribution of mineral assemblages can be determined [46,47]. The model shows the location and geometry of the five alteration zones in the area, clearly reveals their spatial distributions, and confirms the 2D alteration-zoning model (Figure 18). The alteration zoning in this area is obvious, and the distribution is relatively stable. The phengite–carbonate alteration zone intersects with the ore body (Figure 19), which agrees well with, and can be a good indication of, the spatial distribution of the mineralization center. The alteration-zone minerals in the surrounding rocks are controlled by the original rock composition and structural zoning. The pyrite–sericite–quartz alteration zone in the footwall of the fault is close to the main fault plane. The pyrite–sericite–quartz alteration is strong, and the main alteration-zone minerals are developed in the center of the ore body. There were large quantities of phengite and carbonate minerals, followed by montmorillonite and a small quantity of kaolinite. Sericite and carbonate rocks are mainly developed in the monzogranite, in the quartz–carbonate stage, and are basically not mineralized. A small quantity of chlorite is developed in the deep granite. In addition to sericite, kaolinite has the most obvious spectral characteristics. The sericite–quartz granitized cataclastic rock on the hanging wall of the main fault surface is rich in kaolinite-wx, indicating an acidic and water-rich environment. Kaolinite was basically undeveloped in the mineralized section. The petrological and SWIR spectral characteristics show that the distribution of these minerals is related to mineralization temperature changes. To sum up, the presence of alteration-zone minerals such as phengite, kaolinite, and chlorite can be used as a direct indicator of the deep ore body of the Zhaoxian gold deposit.

In addition to refining the distribution of layered silicate mineralogy, the Al–OH datasets can be used to track the fluid path that forms the clay alteration and mineralization center [21]. Temperature and pH are the most important factors affecting the hydrothermal system [40]. Taking muscovite and phengite as examples, the increase in the pH value helps to replace Alvi with Fe2 + or Mg2 +, so that the Si/Alvi ratio increases and Pos2200 moves toward the long-wave direction. Therefore, muscovite is formed at a lower pH, whereas phengite is formed at a relatively high pH. According to previous studies, the crystallinity of layered silicate minerals can be used to infer the relative temperature change [9,29]. The greater the crystallinity, the higher the formation temperature [40]. Based on the results of the above analysis of the Al–OH spectral characteristics of sericite in the 1D and 2D space (Section 4.4), the Pos2200 and SWIR-IC values of Al–OH are strongly correlated (Figure 20) and closely associated with mineralization. As the spectral test data for the drill holes are relatively continuous, the 3D modeling for these two parameters can be established by referring to the method of grade modeling in Section 4.1.2. Figures 21 and 22 are constructed by ordinary kriging interpolation in SKUA-GOCAD; the ranges of Pos2200 and SWIR-IC interpolation are constrained in the 3D space by the tectonic alteration zone, clearly reflecting that both Pos2200 and SWIR-IC have high values within the tectonic alteration zone and coincide with the ore body. The range of high values for Pos2200 is wider than the range of high values for SWIR-IC. Therefore, Pos2200 and illite crystallinity can be used as vectors and exploration tools to delineate hydrothermal centers.

*Minerals* **2022**, *12*, 1272 22 of 27

body of the Zhaoxian gold deposit.

body of the Zhaoxian gold deposit.

**Figure 18.** Three-dimensional modeling of alteration zones in Zhaoxian gold deposit. **Figure 18.** Three-dimensional modeling of alteration zones in Zhaoxian gold deposit. **Figure 18.** Three-dimensional modeling of alteration zones in Zhaoxian gold deposit.

quartz–carbonate stage, and are basically not mineralized. A small quantity of chlorite is developed in the deep granite. In addition to sericite, kaolinite has the most obvious spectral characteristics. The sericite–quartz granitized cataclastic rock on the hanging wall of the main fault surface is rich in kaolinite-wx, indicating an acidic and water-rich environment. Kaolinite was basically undeveloped in the mineralized section. The petrological and SWIR spectral characteristics show that the distribution of these minerals is related to mineralization temperature changes. To sum up, the presence of alteration-zone minerals such as phengite, kaolinite, and chlorite can be used as a direct indicator of the deep ore

quartz–carbonate stage, and are basically not mineralized. A small quantity of chlorite is developed in the deep granite. In addition to sericite, kaolinite has the most obvious spectral characteristics. The sericite–quartz granitized cataclastic rock on the hanging wall of the main fault surface is rich in kaolinite-wx, indicating an acidic and water-rich environment. Kaolinite was basically undeveloped in the mineralized section. The petrological and SWIR spectral characteristics show that the distribution of these minerals is related to mineralization temperature changes. To sum up, the presence of alteration-zone minerals such as phengite, kaolinite, and chlorite can be used as a direct indicator of the deep ore

**Figure 19.** Relationship between alteration zone and mineralization. **Figure 19.** Relationship between alteration zone and mineralization. ters.

**Figure 20.** Relationship between characteristic parameters of muscovite (2200D represents relative content of muscovite). **Figure 20.** Relationship between characteristic parameters of muscovite (2200D represents relative content of muscovite).

**Figure 21.** Changes in Pos2200 values in alteration zone.

ters.

content of muscovite).

space (Section 4.4), the Pos2200 and SWIR-IC values of Al–OH are strongly correlated (Figure 20) and closely associated with mineralization. As the spectral test data for the drill holes are relatively continuous, the 3D modeling for these two parameters can be established by referring to the method of grade modeling in Section 4.1.2. Figures 21 and 22 are constructed by ordinary kriging interpolation in SKUA-GOCAD; the ranges of Pos2200 and SWIR-IC interpolation are constrained in the 3D space by the tectonic alteration zone, clearly reflecting that both Pos2200 and SWIR-IC have high values within the tectonic alteration zone and coincide with the ore body. The range of high values for Pos2200 is wider than the range of high values for SWIR-IC. Therefore, Pos2200 and illite crystallinity can be used as vectors and exploration tools to delineate hydrothermal cen-

**Figure 20.** Relationship between characteristic parameters of muscovite (2200D represents relative

**Figure 22.** Changes in short-wave infrared illite crystallinity values in alteration zone. **Figure 22.** Changes in short-wave infrared illite crystallinity values in alteration zone.
