The results of PCA are shown in Figure 4A. The cumulative variance contribution rate of the first two principal components (PC1: 58.63, PC2: 23.60) accounted for 82.23%, indicating that the distinguishing characteristics of the chlorite from the various deposits were prominently separated on the PC2 vs PC1 planes (Figure 4B). FeO, SiO₂, and MgO contributed the most to PC1, whereas Al₂O₃, MnO, and SiO₂ were the main contributors to PC2 (Figure 4B). Clearly, chlorite within the ore deposits of various metal types manifested distinct elemental attributes (Figure 4B). The porphyry Cu-polymetallic deposit strongly correlated with Al₂O₃, MgO, and SiO₂, concentrated mainly in the first, second, and fourth quadrants (blue, cyan, green, and light blue dots in Figure 4B). SiO₂ was the primary factor associated with U deposits distributed throughout the second, third, and fourth quadrants (purple dots in Figure 4B). FeO and MnO are key elements linked to the Sn-polymetallic deposits and are primarily found in the second and third quadrants (red dots in Figure 4B). The distribution of porphyry Cu-Au deposits ly overlaps that of orogenic Au deposits (cyan and yellow dots in Figure 4B).

Because of the complex structure of chlorite and other associated minerals and inclusions in the mineral, it was necessary to eliminate the influence of contamination effects on the obtained EPMA data. Contaminated chlorites often exhibit high values of (CaO + Na₂O+ K₂O) >0.5 wt% (Foster, 1962; Inoue et al., 2010); therefore, it is necessary to exclude the data on this feature before discussion. After excluding unqualified data, the contents of the major elements in the 129 chlorite minerals are listed in Table 1. The main oxide contents are as follows: M-type chlorite contains SiO₂ = 26.79–31.00 wt%, Al₂O₃ = 17.70–21.98 wt%, MgO = 14.22–20.83 wt%, Fe₂O₃= 0.29–3.48 wt%, FeO = 14.24–24.09 wt%, MnO = 0.27–2.13 wt%; Comparatively, H-type chlorite has lower SiO₂ = 24.93–30.13 wt%, higher Al₂O₃ = 17.66–21.43 wt%, lower MgO = 7.96–19.17 wt%, same Fe₂O₃= 0.53–3.70 wt%, higher FeO= 17.15–30.35 wt%, MnO = 0.37–2.19 wt%. Both M- and H-type chlorites have low CaO, Na₂O, K₂O, and Cr₂O₃ contents. The atomic number of each element was calculated using WinCcac based on 14 oxygen atoms (Yavuz et al., 2015). The atomic numbers of each element are listed in Table 2.

Data were calculated using WinCcac software based on 14 oxygen atoms (Yavuz et al., 2015), and the calculation formulas of oxygen fugacity and sulphur fugacity were calculated using the six-component solid solution model formula proposed by Walshe (1986) (for detailed calculation steps, please refer to Zhang et al., 2014); the calculation formula of chlorite thermometer (t) proposed by Kranidiotis and Maclean (1987) was used for the calculation of oxygen and sulphur fugacity.

Chlorite can be divided into metasomatic-type (M-type) and hydrothermal vein-type (H-type) chlorite. However, the classification of the chemical composition is complicated owing to the complexity of the crystal structure. Previous studies have used the Si-TFe/(TFe+ Mg) classification method proposed by Hey (1954) to classify the chlorite in the Naruo deposit as mainly a diabase (Yang et al., 2015). However, this classification method failed to reflect the structural information of chlorite and was not genetically significant (Liu et al., 2016). In this study, the R ²⁺-Si (Wiewióra et al., 1990) and (Al+F2A5)-Mg-Fe (Zane et al., 1998) classification methods have become popular and useful in recent years. The R ²⁺-Si classification diagram shows that the M- and H-type of chlorites are located near the clinochlore and chamosite ranges and belong to the trioctahedral–trioctahedral structure (Figure 6A). Through the further division of the (Al+F2A5)–Mg–Fe three-terminal diagram, it can be clearly seen that there are differences in the chemical compositions of the M- and H-type chlorites. The results show that M-type chlorite is dominated by clinochlore, and all points fall into the Mg end-member area, whereas H-type chlorite includes clinochlore and chamosite, and points fall into the Mg and Fe end-member areas (Figure 6B). The formation of magnesium chlorite represents a low-oxygen fugacity and low-pH environment, whereas the formation of iron chlorite represents a relatively reduced environment (Inoue, 1995). Therefore, the M-type chlorite formed in the early stage was mainly formed in an environment of low oxygen fugacity and low pH value, whereas the H-type chlorite in the later stage had a large amount of iron chlorite, representing the reduced environment, indicating a reduced environment favourable for mineralisation.

Chlorite formation is accompanied by three cation replacement modes: Fe²⁺=Mg, Tschermak replacement, and dioctahedral-trioctahedral replacement (Bourdelle et al., 2013). Determining the type of ion replacement reaction involved in chlorite formation in the Naruo deposit is useful for understanding the ion replacement rules and reflecting the physical and chemical circumstances of chlorite production. The diagram of the correlation among ions (clusters) of chlorite in the Naruo deposit shows that, overall, there is no 1:1 linear relationship between the Al^IV and Al^VI contents (Figure 7A), indicating that the ion replacement method of chlorite is not dominated by calcium–magnesium amphibole replacement but by other complex replacement methods (Xie et al., 1997).

The fundamental change in the alteration from clay and mica to chlorite is substituting aluminium for silicon (Hillier, 1993). The strong negative correlation between Al and Si (Figure 7B) verified this dominant alteration process. Al^IV and Fe²⁺/(Fe²⁺+ Mg²⁺) had a mild positive phase association (Figure 5C), demonstrating that when Al^IV and Si were substituted in the tetrahedral position, the absorption of Fe²⁺ and Mg²⁺ occurred in the octahedral position, causing Fe²⁺ to increase and Mg²⁺ to decrease. This assertion was supported by the negative relationship between Fe²⁺ and Mg²⁺ in Figure 5D. As Fe²⁺ replaces Mg²⁺, more Al^IV is replaced by Si due to the modification of the chlorite structure (Xie et al., 1997). Consequently, the substitution of Fe²⁺ with Mg²⁺ promotes the development of chlorite alteration (Zhang et al., 2014). The higher Fe²⁺ content in the H-type chlorite (Figures 7B–D) suggests a more advanced state of chloritization alteration. The principal cations and Mg were shown to have a good linear connection after only one stage of chloritization alteration (Xie et al., 1997). However, the nonlinear relationship between Si, Al^IV, and Mg²⁺ (Figures 7E,F) in the chlorite in this study demonstrates that chloritization in the Naruo deposit was caused by a multitude of hydrothermal events rather than by a single metamorphism.

Geological thermometers are among the most frequently used tools for chlorite minerals in deposit research. The applicable conditions, advantages, and disadvantages of various chlorite geothermometers and their development histories have been comprehensively summarised and introduced by previous researchers (Yavuz et al., 2015; Liu et al., 2016). Because substitution between Al, Fe, and Mg (Figure 7B) occurred during the formation of chlorite in the Naruo deposit, an empirical geothermometer involving Al^IV and Fe²⁺/(Fe²⁺+Mg²⁺) content proposed by Kranidiotis and Maclean (1987) was chosen.

t ° C = 212 × Al IV + 0.35 × Fe Fe + Mg + 18 (1)

The calculated results (Table 2) show that the chlorite formation temperature of the Nara deposit is between 255°C and 342°C (Figure 6A) with an average value of 286°C. The above temperature falls within the 200°C–350°C range for fluid inclusion homogenisation during the chloritization stage (Sun et al., 2015). Given the intimate association between chlorite and the development of chalcopyrite, the geological temperature of chlorite (238°C–342°C) suggested that the Naruo deposit’s Cu mineralisation occurred primarily in a medium-temperature environment. Among them, M-type chlorite is formed at 238°C–310°C (average value of 277°C), which is generally lower than the formation temperature of H-type chlorite at 255°C–342°C (average value of 301°C) (Figure 8A). Chloritization alteration generally occurs during the cooling process after the emplacement of high-temperature magmatic melts (Yang et al., 2005; Yang et al., 2008; Hou et al., 2009; Sillitoe et al., 2010). The phenomenon of increasing H-type chlorite formation temperature in the Naruo deposit implies the presence of multistage magmatic-hydrothermal activity, which agrees with the conclusion that chloritization of the Naruo deposit does not occur in a single metamorphism.

Since Walshe (1986) proposed a six-component solid-solution model of chlorite, it has become possible to calculate the oxygen and sulphur fugacities of chlorite, and this method has been widely used (Xiao et al., 1993; Zheng et al., 1997; Zhang et al., 2014; Zhang et al., 2020). In this work, it was discovered that the Naruo deposit has a mineral combination of chlorite+quartz+Al₂SiO₅ (aluminosilicate mineral) ± oxide ± sulphide; therefore, the oxygen and sulphur fugacity of chlorite may be calculated using the six-component solid solution model (Walshe, 1986). Please refer to Walshe (1986) for the calculation method. The calculation method involving the equilibrium constant adopts the formula modified by Zhang et al. (2014). log ⁡ K 1 = 21.77 × e − 0.003 t (2) log ⁡ K 2 = 0.1368 t − 0.0002 t 2 − 82.615 (3)

The oxygen fugacity of chlorite ranges between - 49.6 and −32.7, with an average of −37.3. The M-type chlorite (−41.9–32.7) and H-type chlorite (−39.1–33.3) in the Naruo deposit have similar lgfO₂ value ranges (Figure 6B). The sulphur fugacity lgfS₂ value of chlorite is between −14.8 and −10.1. The two types of chlorites have similar sulphur fugacity values: lgfS₂ (M-type) = −14.8–10.1, lgfS₂ (H-type) = −13.5–10.3 (Figure 8C). The precipitation of Cu sulphide indicates the progressive transition of the ore-bearing hydrothermal fluid environment from oxidation to reduction (Sillitoe et al., 2010; Li et al., 2006; Hou et al., 2020), resulting in lower oxygen and sulphur fugacity in the environment. However, the oxygen and sulphur fugacities of the late H-type chlorite did not vary appreciably (Figures 8B,C), confirming that several magmatic-hydrothermal processes influenced the mineralisation of the Naruo deposit.

The chemical elements of chlorite in porphyry Cu deposits exhibit specific spatial changes (Wilkinson et al., 2015; Cooke et al., 2020; Fan et al., 2021). For example, the chemical composition of chlorite in Indonesia’s Batu Hijau porphyry Cu deposit shows systematic changes from the hydrothermal mineralisation centre to the edge of the deposit, especially within a radius of 2.5 km from the mineralisation centre (Wilkinson et al., 2015). This has also been verified in a resolution porphyry deposit in the United States, and the hydrothermal mineralisation centre was successfully found in the blind area (Cooke et al., 2020). Consequently, the alteration of chlorite chemical components is becoming increasingly popular in the prospecting and investigation of porphyry deposits, although Xiao et al., 2020a, b) proposed that using elements such as Fe, Mg, Co, and Ni as geochemical vectors might not be appropriate. Previous studies have revealed a significant positive correlation between the chlorite Al^Ⅳ, Fe²⁺/(Fe²⁺+Mg²⁺), and Cr/Ti ratios within the porphyry body and the ore grades of Cu and Au, serving as indicative markers for locating enriched ore bodies within porphyry-type deposits (Yang et al., 2015).

To mitigate the influence of multiphase hydrothermal activities on chlorite composition and considering the heightened sensitivity and mobility of H-type chlorite, we chose the elevation of M-type chlorite samples within the magmatic metallogenic system (ZK0001, ZK0701, and ZK0801) as the abscissa. This selection enabled us to observe the chemical indicators of chlorite in ore prospecting, particularly considering the pronounced sensitivity of H-type chlorite to external environmental factors. According to the Cu grade and AA’ profile (Figure 2), an altitude of 4784 m (red dotted line) is the most Cu-rich mineralisation centre (Figure 9). As shown in Figure 7, some chlorite chemical components follow specific patterns as they progress from the shallow mineralisation centre to the deep intrusions. The contents of Si, Fe, and the ratio of Fe/(Fe+Mg) in the main elements of chlorite gradually decrease but increase with depth. In contrast, the Al, Mg, and Mn content gradually increased and then decreased with depth. In particular, Si and Al showed different trends from the other elements at an altitude of 4,300 m (Figures 9A,B).

Xiao et al., 2020a, b) proposed that in the process of chloritization of the mafic minerals in the Xiaokelehe and Atlas porphyry Cu deposits, the contents of the major elements Fe, Mg, and Mn are greatly affected by the composition and type of the precursor minerals, whereas the contents of elements such as Al and Ti can be used as a reflection of the hydrothermal characteristics. Therefore, we believe that the Al and Si elements of the Naruo metasomatic-type chlorite may have originated primarily from hydrothermal fluid, making them more sensitive and reflecting more information about hydrothermal characteristics. In contrast, Fe, Mg, Mn, and other elements were significantly influenced by the type and composition of the precursor minerals and reflected less on the hydrothermal composition; therefore, they only exhibited a single change trend (Figures 9C–F). As a result, we believe there might be another hydrothermal mineralisation centre deep in the Naruo porphyry system, but it is still necessary to strengthen the mineralogy and trace element research of chlorite to determine the location of another mineralisation centre and properly direct upcoming prospecting work.

In addition, the continuous precipitation of metal sulphides also reduces oxygen and sulphur fugacity as the ore-forming system cools, atmospheric water is added, and other factors (Richards et al., 1993; 1995). In contrast, the temperature, oxygen fugacity, and sulphur fugacity of the late H-type chlorite are significantly higher than those of the early M-type chlorite (Figure 10), indicating the addition of magmatic-hydrothermal fluid at a later stage. Among the contemporaneous chlorites, the oxygen and sulphur fugacity values of the proximal chlorite near the mineralisation centre were higher than those of the distal chlorite, which is compatible with the properties of the fluid in the magma dissolving centre. The chlorite temperature, oxygen fugacity, and sulphur fugacity values demonstrated a pattern of decreasing and then increasing from the shallow mineralisation centre to the deep intrusion, suggesting that the second stage of hydrothermal mineralisation may have occurred from the deep.