Geochemistry of Chlorite from the North Zegulang Ore Block of the Jiama Deposit, Tibet: Implications for Fluid Evolution and the Mineralization Center

Abstract

Jiama is a significant porphyry system in Tibet’s Gangdese Metallogenic Belt, characterized by a typical composite system with multicenter mineralization. The North Zegulang Ore Block, a recently identified mineralization center in the Jiama mining area, has remained understudied, particularly regarding its ore-forming fluid evolution. This study integrates microscopic identification, EPMA, and LA-ICP-MS to investigate the mineral chemistry of widely developed chlorite in the North Zegulang Ore Block, aiming to elucidate fluid evolution and its implications for identifying the mineralization center. Chlorite in the North Zegulang Ore Block is genetically classified into retrograde (Chl-1) and hydrothermal (Chl-2) types. Both are Fe-rich varieties, indicating formation under reducing conditions. Element substitution is dominated by Fe²⁺-Mg²⁺ exchange, accompanied by Tschermak and di-trioctahedral substitutions. The chlorite geothermometer yields formation temperatures of 260–400 °C and log fO₂ values of −38 to −26 for Chl-1, while Chl-2 shows a wider temperature range of 220–400 °C and log fO₂ values of −42 to −20, reflecting a medium-temperature, low-oxygen-fugacity environment. Outward from the granodiorite porphyry, fluid temperature decreases and Cu-Mo grades gradually decrease, confirming the intrusion as the mineralization center. With increasing distance from the mineralization center, chlorite Mg/Sr, Ti/Sr, and Ti/Pb ratios progressively decrease, whereas Th/U ratios and Sr, Th, U, and B contents increase. These systematic variations demonstrate that chlorite serves as an effective exploration tool in collisional-type porphyry systems.

1. Introduction

Chlorite is a hydrous, layered iron-magnesium-aluminum silicate mineral that forms in diverse geological settings, including high- to low-grade metamorphism, diagenesis, and hydrothermal alteration [1,2]. It occurs widely in nature and is closely associated with the mineralization of various metal deposits, such as copper, gold, and uranium [3,4,5]. The composition of chlorite varies significantly with its genesis, making its chemical composition a valuable tool for reconstructing the physicochemical conditions of its formation. Consequently, chlorite serves as a mineral probe for critical parameters in ore-forming systems, including temperature, oxygen fugacity, and sulfur fugacity [6,7,8,9]. Systematic changes in its chemical composition and crystal structure with temperature also qualify chlorite as a reliable geothermometer [10,11,12], used to determine formation temperatures and trace the pathways of ore-forming fluids [13,14,15]. Extensive research on its formation conditions, mechanisms, and temperature has yielded significant insights into its role as an indicator of mineralization processes [16,17,18,19]. Recently, the use of trace element compositions of alteration minerals such as chlorite and epidote as geochemical vectors to mineralization centers has gained considerable traction. This approach is rapidly being adopted in the global mining industry due to its cost-effectiveness, efficiency, and minimal environmental impact [20].

Porphyry deposits are a class of magmatic-hydrothermal deposits characterized by close spatial, temporal, and genetic links to shallow- to ultra-shallow-level intermediate to acidic porphyry intrusions. They typically occur within the porphyry bodies themselves and their surrounding contact zones, primarily forming in tectonic settings such as subduction zones and continental collision zones. Although these deposits generally have relatively low grades, they feature uniform mineralization and enormous tonnages (0.1–20 Gt), with ore bodies situated at shallow depths (1–3 km), making them amenable to large-scale open-pit mining [21]. Notably, porphyry deposits are the principal global source for many critical metals, supplying approximately 75% of the world’s copper, 95% of its molybdenum, 80% of its rhenium, 20% of its gold, and nearly all its selenium and tellurium [21,22]. The Jiama copper-polymetallic deposit is a well-studied, typical example of a collisional porphyry system. Previous research has systematically investigated its geology [23], geochronology [24], mineralogy [25,26,27], petrology and geochemistry [28], fluid inclusions [24], isotope geochemistry [29,30], and alteration characteristics [31,32]. In contrast, the North Zegulang Ore Block, a recently discovered mineralization center within the Jiama district, remains relatively understudied. To date, only detailed geochronological and geochemical investigations have been conducted [33,34]; the evolution of its ore-forming fluids remains poorly constrained. This study, therefore, focuses on the widely developed chlorite in the North Zegulang Ore Block. We aim to characterize its mineral chemistry to decipher the evolution of the ore-forming fluids, establish chlorite as a geochemical vector, and guide further prospecting in the area.

2. Geological Setting

The Gangdese Porphyry Copper Belt in Tibet is situated within the Tethyan-Himalayan metallogenic domain, one of the three major global porphyry copper belts. Its formation and evolution were controlled by the prolonged influence of the Paleo-Tethyan and Neo-Tethyan tectonic systems, a process that involved complex geological, tectonic, and magmatic activities. This established it as a renowned polymetallic metallogenic belt both in China and worldwide [35,36]. In contrast to typical large to super-large porphyry deposits formed in global subduction settings, most porphyry deposits in the Gangdese belt were generated during the collisional to post-collisional stage [37,38,39]. Throughout its geological evolution, the Gangdese belt has accumulated abundant metallic and non-metallic mineral resources, primarily including Cu, Fe, Mo, Pb, Zn, W, Au, and Ag. Five significant polymetallic clusters have been identified (Figure 1): the Xiongcun Cu-Au cluster, Chongmuda-Chengba Mo-Cu-W cluster, Jiama-Qulong-Bangpu Cu-Mo polymetallic cluster, Tinggong-Chongjiang Cu cluster, and Yaguila-Mengya’a Pb-Zn-Mo polymetallic cluster. Research indicates that the metallogenic element assemblages in these clusters exhibit remarkable north–south (N-S) spatial zonation. From south to north, the trend is Cu-Au → Mo-W (Cu) → Cu-Mo-Pb-Zn (Au-Ag) → Mo (Cu) → Pb-Zn-Mo-W-Fe → Pb-Zn (Ag). This zonation also reflects an age distribution, with older mineralization on the northern and southern sides and relatively younger mineralization in the central part [23]. The Jiama copper-polymetallic deposit is a world-class, collision-type porphyry-skarn system. It is located in the eastern segment of the Gangdese belt, on the southern margin of the Lhasa Terrane, and north of the Yarlung-Zangbo Suture Zone [35].

3. Deposit Geology

The Jiama mining area comprises three main ore blocks: the Main, South Pit, and North Zegulang Ore Blocks (Figure 2) [34,41]. The stratigraphy comprises Jurassic to Cretaceous marine sedimentary rocks deposited in a passive continental margin setting. The main units, from oldest to youngest, are the Upper Jurassic Duodigou Formation limestone and marble (J₃d), the Lower Cretaceous Linbuzong Formation slate and sandy slate (K₁l), and localized Quaternary deposits (Q) in valleys (Figure 2). Structurally, the deposit is controlled by the Jiama-Kajunguo thrust system and the Copper Mountain gliding nappe structures [42]. Magmatic rocks are rarely exposed at the surface; intermediate to acidic intrusions are predominantly concealed. These intrusions include quartz diorite porphyry, granodiorite porphyry, monzogranite porphyry, granite porphyry, and aplite dikes [40].

The Jiama mining area exhibits a typical composite mineralizing system characterized by multicenter mineralization (Figure 3). This system comprises five distinct orebody types: the copper-polymetallic Skarn Orebody, the copper-molybdenum Hornfels Orebody, the molybdenum-copper Porphyry Orebody, independent gold orebodies in peripheral structural alteration zones, and a Manto-type orebody hosted in marble within the Nankeng Ore Block [32,43]. The copper-polymetallic Skarn Orebody forms thick slabs within the Linbuzong Formation (sandy slate, hornfels) and the Duodigou Formation (limestone, marble), and at contacts between marble and porphyritic intrusions, displaying Cu, Mo, Pb, Zn, Au, and Ag mineralization. The copper-molybdenum Hornfels Orebody forms pipe-like masses within silicified hornfels enveloping the porphyries and is dominated by Cu and Mo mineralization. The molybdenum-copper Porphyry Orebody is hosted within the porphyritic intrusions, presenting as vein and disseminated types with primarily Mo and Cu mineralization. Independent gold orebodies occur as veins in peripheral or distal fracture zones, characterized by Au and Ag mineralization [30].

The mining area exhibits a zonal distribution of typical porphyry-style hydrothermal alteration, in addition to early-stage hornfelsization, marbleization, and skarn contact metasomatism. The main alteration types include potassic, phyllic, sericite-chlorite, and propylitic alterations, with localized argillic alteration and tourmalinization (Figure 4) [41,44].

4. Sampling and Analytical Methods

4.1. Sampling

This study focuses on chlorite samples collected from six drill holes (ZK0028, ZK035, ZK036, ZK039, ZK040, and ZK046) along exploration line 0 in the North Zegulang Ore Block of the Jiama copper-polymetallic deposit. Representative samples were selected across vertical profiles from shallow to deep intervals, with precise sampling locations indicated in Figure 4.

Two main genetic types of chlorite are identified in the North Zegulang Ore Block: retrograde and hydrothermal. Metasomatic chlorite occurs primarily in hornfels and porphyry, displaying disseminated, planar, and irregular textures. It is associated with a chlorite-biotite-magnetite assemblage. In hand specimens, it is dark green and commonly replaces biotite or amphibole, with minor coexistence of chalcopyrite and pyrite. Under the microscope, it forms anhedral, irregular, fan-shaped, or vermicular aggregates in green, grass-green, or dark green colors (Figure 5). In contrast, hydrothermal chlorite is mainly hosted in hornfels and spatially associated with quartz-sulfide veins and disseminations containing chalcopyrite and pyrite. Hand specimens are light green and show a strong correlation with mineralization. Microscopically, it exhibits bladed, spherulitic, and radial habits, appearing dark green or grayish-green with corresponding interference colors. Distinct anomalous indigo blue and purple interference colors are also observed (Figure 6). Based on their genesis, the chlorites are classified into two groups: Chl-1 (retrograde) and Chl-2 (hydrothermal).

4.2. Analytical Methods

Electron probe microanalysis (EPMA) of chlorite was conducted at the Key Laboratory of Metallogeny and Mineral Assessment, Ministry of Natural Resources; Institute of Mineral Resources, Chinese Academy of Geological Sciences. Analysis was performed using a JEOL JXA-8230 instrument operating at 15 kV and 10 nA, with a 5 μm spot size. Standards were calibrated against reference materials, with analytical accuracy for major elements better than 0.01%. Subsequently, selected chlorite samples underwent in situ trace element analysis at the Institute of Geochemistry, Chinese Academy of Sciences.

The analysis was performed in the State Key Laboratory of Ore Deposit Geochemistry at the same institute, employing laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The analysis employed a GeoLasPro laser ablation system coupled with an Agilent 7700x ICP-MS, using helium as the carrier gas. The laser spot diameter was 50 μm, with an energy density of 6.06 J/cm² and a frequency of 6 Hz. Analyzed elements included MgO, Al₂O₃, SiO₂, MnO, FeO, Li, B, Na, K, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, As, Sr, Y, Zr, Ag, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Pb, Bi, Th and U. Data calibration utilized USGS reference materials (BIR-1G, BHVO-2G, BCR-2G) and NIST SRM 610 and 612. For quality control, fused glass standards (QCCGSG-1, CGSG-2) were analyzed as unknowns. Data processing was performed using ICPMSDataCal10.2 software.

5. Results

5.1. Major Elements

This study performed electron probe microanalysis (EPMA) on 138 chlorite grains from 39 samples, with results presented in Table S1. The structural formulas of chlorite were calculated on the basis of 28 oxygen atoms, and the Fe³⁺/Fe²⁺ ratio was determined using the charge-balance method of Zheng (1983) [45]. To ensure data quality, analyses yielding a total mass fraction of (Na₂O + K₂O + CaO) > 0.5% were identified as potentially contaminated and excluded from the dataset, following established criteria [46,47].

The EPMA results reveal that chlorite from the North Zegulang Ore Block exhibits the following major element concentration ranges: SiO₂ (23.63–32.37 wt.%, avg. 27.40), Al₂O₃ (16.4–24.88 wt.%, avg. 21.95), FeO (11–35.57 wt.%, avg. 19.55), and MgO (4.83–23.83 wt.%, avg. 17.98). Several classification schemes have been proposed for chlorite [48,49]. Based on the widely adopted Fe-Si classification diagram [48], chlorite in the study area is predominantly classified as Aphrosiderite and Sheridanite (Figure 7), indicating an Fe-rich variety.

5.2. Trace Elements

The trace element compositions of Chl-2 analyzed by LA-ICP-MS are presented in Table S2. The results show wide concentration ranges for several elements, including Ti (165.28–844.31 ppm, avg. 418.18), Sr (0.064–34.13 ppm, avg. 3.85), Pb (0.026–10.40 ppm, avg. 1.81), Th (0–4.27 ppm, avg. 1.10), U (0.001–0.515 ppm, avg. 0.182), and B (0.034–11.209 ppm, avg. 2.157). These elements were selected to characterize the trace element geochemistry of chlorite. Box plots (Figure 8) reveal that Ni has the highest concentrations (249.87–676.13 ppm). Besides Ni, the chlorite is characterized by relatively high concentrations of Zn, V, Cr, and Ti; lower concentrations of Sr, Pb, Th, U, and B; and moderately low concentrations of Co, Na, and Li.

6. Discussion

6.1. Substitution Mechanism of Elements in Chlorite

The composition of chlorite in magmatic-hydrothermal deposits is primarily controlled by the physicochemical conditions of the fluids [50]. Based on their genesis, chlorites in this study are classified into two types: retrograde chlorite (Chl-1) and hydrothermal chlorite (Chl-2). The elemental composition of retrograde chlorite is influenced by both the altering hydrothermal fluids and the original rock mineralogy, whereas hydrothermal chlorite composition is predominantly controlled by the altering fluids. The classification diagram (Figure 7) shows that the chlorites in the study area are primarily Fe-rich species, corrensite and pennantite, suggesting formation in a reducing environment [12]. Based on Figure 9e,f, the Fe/(Fe + Mg) ratio in chlorite shows a positive correlation with Al^IV but a negative correlation with Si. This relationship indicates that an Fe-rich environment is associated with low silica activity, suggesting chlorite formation occurred in an Fe-rich, silica-poor system.

Due to their compositional complexity and susceptibility to external physicochemical conditions, chlorites often undergo various ionic substitutions. Four major cationic substitution mechanisms typically operate during chlorite formation [51,52]: (1) Fe-Mg substitution: a continuous (Fe²⁺)^VI ↔ (Mg²⁺)^VI exchange; (2) Tschermak substitution: Al^IV + Al^VI ↔ (Si⁴⁺)^IV + (Mg²⁺, Fe²⁺)^VI [11]; (3) Di-trioctahedral substitution: 3(Mg²⁺, Fe²⁺)^VI ↔ ^VI + 2(Al³⁺, Fe³⁺)^VI [6]; (4) Trivalent cation substitution: (Al³⁺)^VI ↔ (Fe³⁺)^VI and (Si⁴⁺)^VI + (Al³⁺)^VI + ^VI ↔ (Al³⁺)^IV + 2(Fe³⁺)^VI. Additionally, processes such as hydroxyl substitution and dehydrogenation may occur [50]. In the chlorite structure, the tetrahedral sites are occupied mainly by Si and Al, while the octahedral sites host Mg, Fe, and Al^VI. Chlorites from the North Zegulang Ore Block show a strong negative linear correlation between Mg and Fe (R² = 0.89; Figure 9a), indicating the Fe-Mg substitution is the dominant mechanism. Furthermore, a negative correlation exists between (Si + Fe + Mg) and Al (R² = 0.49; Figure 9b), suggesting the presence of the Tschermak substitution. A positive correlation between Al^IV and Al^VI (R² = 0.13; Figure 9c), with Al^IV generally lower than Al^VI, and negative correlations of Fe and Mg with Al (R² = 0.043; Figure 9d), imply the di-trioctahedral substitution is also active.

In summary, substitutions in the octahedral sites of chlorites from the North Zegulang Ore Block of the Jiama deposit are dominated by Fe-Mg exchange, with limited substitution between Mg/Fe and Al^VI. The primary substitution mechanism involves Fe²⁺-Mg²⁺ exchange, accompanied by the Tschermak and di-trioctahedral substitutions.

6.2. Formation Temperature and Oxygen Fugacity of Chlorite

Chlorite, forming under medium- to low-temperature conditions, exhibits a composition highly sensitive to its environment, which establishes it as a reliable geothermometer for reconstructing formation temperatures [10,47,53]. In this study, the chlorite basal spacing geothermometer [54,55] was applied. The calculation formula is as follows:

d001 (0.1 nm) = 14.339 − 0.1155 Al^IV − 0.0201Fe²⁺

(1)

d001 (0.1 nm) = 14.339 − 0.001T (°C)

(2)

The two formulas above yield formation temperatures ranging from 220 to 400 °C (average 350 °C) (Table S3). This temperature range corresponds to a medium-temperature hydrothermal alteration environment, and its wide spread suggests multiple hydrothermal events in the mining area. Chl-1 yields temperatures of 260–400 °C (avg. 348 °C), while Chl-2 records a broader range of 220–400 °C (avg. 350 °C). Previous studies have reported positive correlations between temperature and Al^IV content and negative correlations with octahedral vacancies and Si within the 100–350 °C interval [11,51]. In this study, formation temperature shows good linear correlations with both Al^IV and Si (Figure 10), which is consistent with previous findings and supports the applicability of this geothermometer [56,57].

Fluid inclusion data from the Jiama deposit indicate copper mineralization occurred at 235–451 °C [58]. The chlorite temperatures obtained here, particularly the concentrated peak between 280 °C and 420 °C (Table S3), align well with this copper precipitation range. This agreement suggests that most chlorite formed during the main mineralization stage, with a minor portion forming post-mineralization, which is consistent with its modes of occurrence: within hornfels and porphyry, and closely associated with chalcopyrite and pyrite in hornfels. As the ore-forming fluids evolved from acidic to neutral or weakly alkaline and cooled through water-rock interaction, physicochemical changes likely triggered simultaneous chlorite formation and metal sulfide precipitation [13]. Therefore, chlorite development is closely linked to hydrothermal mineralization and may serve as an indicator of ore fluid precipitation, offering potential exploration significance.

Walshe (1986), Bryndzia (1987), Scott (1987) established methods to estimate oxygen fugacity, sulfur fugacity, temperature, and pressure from chlorite composition [59,60]. These approaches were later successfully applied by Xiao et al. (1993) and Zheng et al. (1997) to determine the oxygen and sulfur fugacity of chlorite in gold deposits, yielding reliable results [61,62]. In this study, Walshe’s oxygen fugacity calculation method was adopted, yielding log fO₂ values ranging from −42 to −20 (avg. −31) for chlorites from the North Zegulang Ore Block (Table S3), indicating a low-oxygen-fugacity environment. Specifically, Chl-1 yields log fO₂ values of −38 to −26 (avg. −31), while Chl-2 shows a broader range from −42 to −20 (avg. −31). The wider variations in both temperature and oxygen fugacity recorded by Chl-2 may be attributed to the open and dynamic nature of the hydrothermal system, which facilitated the mixing of fluids from different sources and with varying properties. In contrast, the retrograde system likely represents a relatively closed and stable local reaction, resulting in the narrower ranges preserved in Chl-1.

6.3. Coupling of Temperature with Mineralization: Implications for Fluid Evolution

In porphyry mineralization systems, fluid temperature and oxygen fugacity are critical factors controlling metal precipitation [63]. This study projects the average temperature of chlorite samples onto a cross-section, integrating geological data with Cu and Mo grade intensities (Figure 11) to investigate the coupling between chlorite temperature and mineralization in the North Zegulang Ore Block. As shown in Figure 11c, crystallization temperatures decrease gradually outward from the deeply emplaced granodiorite porphyry (330–350 °C) to below 330 °C, with a corresponding weakening of Cu-Mo mineralization, identifying the intrusion as the mineralization center. A notable temperature anomaly (>350 °C) around the middle of drill hole ZK036 and its vicinity is attributed to post-mineralization emplacement of diorite porphyry. A slight temperature increase in the shallow part of ZK028 is likely influenced by shallow intrusions from the Main Ore Block.

Based on the basic geological information, mineralization intensity, and fluid temperatures estimated from chlorite geochemistry along section line 0, it is clearly revealed that Cu and Mo mineralization in the North Zegulang Ore Block shows a close genetic relationship with the emplacement of granodiorite porphyry. Metal precipitation predominantly occurred at temperatures between 330 °C and 380 °C (Figure 11). This mineralization was subsequently overprinted by magmatic intrusions and ore-forming fluids from the Main Ore Block, leading to the development of thick, high-grade Cu-Mo skarn mineralization between the two domains. Subsequent to metal precipitation, the emplacement of diorite porphyry partially overprinted the chlorite alteration, thereby affecting the temperature signatures reconstructed from its composition.

6.4. Chlorite Trace Elements as Vectors to Mineralization

Numerous studies have demonstrated that trace elements in chlorite can serve as effective vectors in subduction-related porphyry deposits [64,65,66]. Analysis of chlorite for elements including K, Li, Mg, Ca, Sr, Ba, Ti, V, Mn, Co, Ni, Zn, and Pb [14] shows that Ti, V, and Mg decrease exponentially with distance from the deposit center, whereas other elements increase. Ratios including Ti/Pb, Mg/Sr, Ni/V, and Ti/Sr also vary systematically, decreasing away from the mineralization center. Although the correlations of Zn, Mn, Sc, Sr, and Li with temperature vary among deposits, studies confirm that Ti and V contents decrease with distance from the hydrothermal center [17,67].

In this study, trace elements and their ratios in chlorite from the North Zegulang Ore Block of the Jiama deposit were analyzed. The results indicate that with increasing distance, the Mg/Sr, Ti/Sr, and Ti/Pb ratios systematically decrease, whereas the Th/U ratio increases (Figure 12a,b). Similarly, Sr, U, B, and Th contents generally increase with distance (Figure 12e–h). These patterns confirm that chlorite is an effective exploration tool for collisional-type porphyry systems.

The vertical distribution of chlorite trace elements over a 1400 m interval is shown in Figure 13. The Mg/Sr, Ti/Sr, and Ti/Pb ratios are enriched toward the mineralization center, whereas the Th/U ratio, along with Sr, U, B, and Th contents, decreases. Although local variations occur due to minor apophyses (Figure 13) or shallow intrusions from the Main Ore Block (Figure 12), the overall trends remain clear. Additionally, elemental anomalies at sampling points 035–489.5, 039–158.7, and 035–36 (the numbers indicate the depth below surface in meters) in Figure 12e–h suggest the presence of an unidentified minor apophysis between these points (Figure 14).

Wilkinson et al. (2015) proposed using chlorite trace elements to estimate the location of mineralization centers in porphyry deposits and developed a corresponding formula [14]. Cooke et al. (2020) successfully applied this formula in a blind test at the Resolution porphyry Cu-Mo deposit [68]. The distance between a chlorite sample and the mineralized center is calculated as:

$$x={\displaystyle \frac{ln(R/a)}{b}}$$

where x is the distance in meters, R is the element ratio, and a and b are exponential fit parameters (from [14]).

The systematic variation of element concentrations with proximity to the mineralization center in the North Zegulang Ore Block, as documented above, provides the basis for this calculation. To validate its applicability to collisional-type porphyry deposits, we applied the formula to samples along Exploration Line 0. For this study, R represents the Ti/Sr ratio, with a = 3 × 10⁶ and b = −0.0088. The calculated distances are plotted for verification (Figure 15). Figure 15 shows that the calculated center coincides with the known, drilled mineralization center, demonstrating the formula’s applicability to collisional-type porphyry deposits such as Jiama. This confirms chlorite as an effective exploration tool for collisional-type porphyry systems.

7. Conclusions

(1) Based on genetic type, chlorites from the North Zegulang Ore Block are classified into two categories: retrograde chlorite (Chl-1), which occurs mainly in hornfels and porphyry and displays a weak correlation with mineralization; and hydrothermal chlorite (Chl-2), which is predominantly hosted in hornfels and exhibits a strong genetic link to mineralization. Both types are Fe-rich varieties, with Fe²⁺-Mg³⁺ substitution as the dominant mechanism, accompanied by Tschermak and di-trioctahedral substitutions.

(2) Chl-1 formed at temperatures of 260–400 °C with log fO₂ values of −38 to −26. Chl-2 exhibits a wider formation temperature range of 220–400 °C and log fO₂ values of −42 to −20, indicating a medium-temperature, low-oxygen-fugacity environment. Chlorite-associated Cu-Mo mineralization primarily occurred under medium-temperature (330–380 °C) and low log fO₂ (−38 to −28) conditions.

(3) Outward from the granodiorite porphyry, fluid temperature gradually decreases, accompanied by a systematic decrease in Cu and Mo grades. This trend confirms the granodiorite porphyry as the mineralization center of the North Zegulang Ore Block. With increasing distance from the mineralization center, the Mg/Sr, Ti/Sr, and Ti/Pb ratios in chlorite progressively decrease, whereas the contents of Sr, Th, U, and B increase. These systematic variations demonstrate that chlorite serves as an effective exploration tool in collisional-type porphyry systems.