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Article

Thermodynamic Modeling Constrains the Alteration and Mineralization Patterns of the Pulang Porphyry Cu-Au Deposits in Eastern Tibet

by
Shaoying Zhang
1,2,
Wenyan He
2,*,
Huaqing Wang
3 and
Yiwu Xiao
1
1
BGRIMM Technology Group, Beijing 100160, China
2
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
3
China Metallurgical Geology Bureau, Beijing 100025, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(8), 780; https://doi.org/10.3390/min15080780
Submission received: 27 June 2025 / Revised: 23 July 2025 / Accepted: 23 July 2025 / Published: 25 July 2025
(This article belongs to the Special Issue Thermal History and Preservation Mechanisms of Porphyry-Cu Deposits: Evidence from Thermochronology, Mineral and Isotopic Geochemistry)
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Abstract

Thermodynamic simulations of fluid–rock interactions provide valuable insights into mineral deposit formation mechanisms. This study investigates the Pulang porphyry Cu-Au deposit in the Sanjiang Tethys Orogen, employing both Gibbs energy minimization (GEM) and the Law of mass action (LMA) method to understand alteration overprinting and metal precipitation. The modeling results suggest that the ore-forming fluid related to potassic alteration was initially oxidized (ΔFMQ = +3.54~+3.26) with a near-neutral pH (pH = 5.0~7.0). Continued fluid–rock interactions, combined with the input of reduced groundwater, resulted in a decrease in both pH (4.8~6.1) and redox potential (ΔFMQ~+1), leading to the precipitation of propylitic alteration minerals and pyrrhotite. As temperature further decreased, fluids associated with phyllic alteration showed a slight increase in pH (5.8~6.0) and redox potential (ΔFMQ = +2). The intense superposition of propylitic and phyllic alteration on the potassic alteration zone is attributed to the rapid temperature decline in the magmatic–hydrothermal system, triggering fluid collapse and reflux. Mo, mainly transported as HMoO4− and MoO4−2, precipitated in the high-temperature range; Cu, carried primarily by CuCl complexes (CuCl4−3, CuCl2−, CuCl), precipitated over intermediate to high temperatures; and Au, transported as Au-S complexes (Au(HS)2−, AuHS), precipitated from intermediate to low temperatures. This study demonstrates that fluid–rock interactions alone can account for the observed sequence of alteration and mineralization in porphyry systems.

1. Introduction

Porphyry mineralization is closely linked to hydrothermal fluids exsolved from intermediate–felsic magmas, accompanied by intensive hydrothermal alteration processes, indicating that a large amount of fluid is involved in mineralization [1]. Fluid–rock interactions resulting in the migration of dissolved ions can often lead to predictable spatial distributions of hydrothermal alterations and mineralization [2,3,4]. Typically, alteration zones manifest as a distinct pattern, with potassic alteration prevalent in the deeper and central regions. Moving outward and/or upward, this zoning undergoes a transition from chlorite–sericite to sericite alteration and further evolves into (advanced) argillic or propylitic alterations [2,3]. Spatial variations in the types of mineralization, ratios of metals, and ore grades are often linked to specific hydrothermal alteration patterns [2,5,6]. Traditional interpretations of fluid–rock interactions rely on mineral paragenesis and compositional changes. However, overprinting, late-stage alteration, and the poor preservation of fluid characteristics may obscure original fluid signals [7]. Experimental studies face limitations in replicating open systems and selecting appropriate mineral buffers [8]. Additionally, bridging microscale mineralogical features with macroscale hydrothermal processes remains challenging [9].
Recent advances in thermodynamic and reactive transport modeling have enabled the simulation of fluid–rock interactions across a wide range of pressure–temperature conditions and time scales [10]. In addition, thermodynamic simulations offer a distinct advantage by affording unrestricted modeling capabilities for multi-phase, multi-component systems across a wide range of temperatures, pressures, and compositions. Moreover, their temporal scale extends beyond the limitations of field observations and experimental investigations, facilitating a comprehensive understanding of geological processes [11]. Notably, the integration of thermodynamic equilibrium principles in fluid–mineral interactions, coupled with dynamic transport models governing mineral dissolution and precipitation, has played a pivotal role in elucidating and forecasting the genesis of hydrothermal ore deposits within the context of fluid–rock interactions [12]. For instance, thermodynamic simulations of porphyry ore-forming systems have conclusively demonstrated the transient nature of magma–hydrothermal fluid properties and the resulting immiscibility phenomena. Particularly, simulation outcomes within the NaCl-H2O system have confirmed the occurrence of phase separation and the coexistence of high-salinity and vapor-rich fluid inclusions induced by pressure fluctuations [13,14,15]. Subsequently, utilizing experimental gas/liquid partition coefficients for metals and volcanic gas/melt partition coefficients, certain thermodynamic models have explored the capacity for gas transport and metal precipitation [16,17]. They have also examined the influence of atmospheric water mixing on metal precipitation within multi-phase immiscible NaCl-H2O fluids [18]. The studies that use these models underscore that thermodynamic simulations offer novel perspectives and solutions for quantitative research in ore deposits.
At the Pulang porphyry Cu-Au deposit, early models described a classic zonation pattern: a potassic core enveloped by phyllic and propylitic halos [19,20]. However, recent studies reveal extensive overprinting, with phyllic and propylitic alterations occurring throughout both the core and peripheral zones [21]. The pronounced overlay of alteration zones has blurred the correlation between alteration and mineralization at Pulang. As a result, some scholars argue that mineralization should primarily occur within the propylitic alteration zone rather than the typical potassic alteration zone [21,22]. In addition, despite the relatively high redox potential of the ore-bearing porphyries at Pulang, the presence of reduced minerals such as pyrrhotite [23,24], along with CH4- and CO-bearing fluid inclusions [23,25], indicates localized redox reduction.
To better understand these complex alteration patterns and redox features [21,24,25,26], this study integrates mineralogical observations with thermodynamic modeling. We simulate fluid–rock interactions at Pulang to reconstruct alteration sequences; determine controls on Cu, Mo, and Au precipitation; and constrain the physicochemical evolution of ore-forming fluids.

2. Geological Setting

The Pulang porphyry Cu-Au deposit is situated in the southern segment of the Yidun arc, part of the Sanjiang Tethys Orogen in eastern Tibet (Figure 1). The Yidun Terrane is situated between the Songpan-Garze Fold Belt to its west and the Qiangtang Terrane to its east, bounded by the eastern Garze-Litang Suture Zone and the western Jinshajiang Suture Zone and the Yangtze Craton to the southeast (Figure 1a,b). The N-S-trending Fault then cuts the Yidun Terrane into distinct sections, the Zhongza block to the west and the Yidun arc to the east, where the Pulang porphyry Cu-Au deposit is located [27]. The Yidun arc mainly comprises Late Triassic arc-like magmatic rocks (238–210 Ma; [28]) and coeval volcaniclastic rocks, which intrude or conformably overlie Middle–Upper Triassic fine-grained clastic, bioclastic, and mudstone rocks, whereas the Zhongza block mainly consists of deformed or metamorphosed Paleozoic rocks [29].
Within the southern Yidun arc (namely the Zhongdian arc; Figure 1c), the exposed Upper Triassic strata, the Qugasi and Tumugou formations, comprise clastic and volcanic rocks with carbonate, sandstone, and slate interlayers [30]. Late Triassic magmatic rocks, however, are mainly quartz dioritic, monzonitic, and granodioritic porphyry intrusions and mostly intruded into the above Triassic strata [31,32]. These porphyries exhibited consistent initial 87Sr/86Sr ratios of 0.7058~0.7077 and εNd values of −1.88~−1.93, with elevated Sr/Y ratios and highly fractionated REE patterns, resembling adakitic signatures [33], and therefore these porphyries are believed to have originated from the partial melting of the asthenospheric mantle wedge that had undergone metasomatism [27,33,34,35]. Nevertheless, recent studies also suggest the potential for rejuvenating the Neoproterozoic arc root considering tectonic reconstruction models of the Paleo-Tethys Ocean [36]. While having limited surface exposure, these porphyries contain numerous porphyry deposits, including the Pulang (4.18 Mt Cu with average grades of 0.50~0.53%), Xuejiping (0.28 Mt with 0.53%), Lannitang (0.18 Mt with 0.50%), and other deposits (Figure 1c). Although these deposits consistently share similar strata and porphyry intrusions [22,24,37], they exhibit distinct variations in hydrothermal alteration zonations, particularly in the Pulang deposits [21,38].
A sequence of sinistral strike-slip faults oriented in the NW-SE direction forms within the Yidun arc (Figure 1b,c; [27,39]). Mesozoic intrusions exhibit an elongated NW-SE orientation, running parallel to the nearby fault systems. These intrusions often display a transition from ductile to brittle deformation along their edges while remaining relatively less deformed at their cores, where the magmatic lineations and flow structures are generally subparallel to each other [40].

2.1. Ore Geology

The Pulang porphyry Cu-Au deposit, located at 28°02′19″ N, 99°59′23″ E in the Zhongdian arc, is one of the most important porphyry copper systems (Figure 2). It contains 447 million tonnes (Mt) of premining reserves averaging 0.52% Cu and 0.18 g/tonne (g/t) Au. It has been mined since 2017 with a designed annual production of ~40 kt Cu [21]. The Pulang deposit spans 36.4 km2 and is divided into three distinct mineralized zones. The largest, South Pulang, holds about 96% of the district’s total reserves, while the smaller East and North Pulang parts contain scattered Cu-Au-bearing veins.
Two main lithologic units are present at Pulang: sedimentary rocks and porphyry intrusions from the Late Triassic period. Sedimentary rocks, accounting for approximately 70% of the surface outcrop across the ore district, belong to the Tumugou Formation, which consists of two members: T3t1 and T3t2. The T3t1 member, predominantly distributed in the eastern region, is composed of black carbonaceous slate, andesite, and metasandstone, with a total thickness exceeding 2700 m. The T3t2 member, mainly found in the western area, consists of black carbonaceous slate containing euhedral pyrite, carbonaceous phyllite, metasandstone, and layered limestone, with a cumulative thickness greater than 680 m. The porphyry intrusions, covering an outcrop area of approximately 8.9 km2, comprise four major types arranged chronologically from oldest to youngest: pre-mineralization fine-grained quartz diorite porphyry (FQD) and coarse-grained quartz diorite porphyry (CQD), inter-mineralization quartz monzonite porphyry (QMP), and post-mineralization diorite porphyry (DP) and granite porphyry (GP). Although these intrusions share similar mineral assemblages, their mineralogical compositions differ significantly. Specifically, FQD, CQD, and DP are characterized by a higher content of mafic minerals such as amphibole and biotite, whereas the QMP and GP are rich in feldspar and quartz [41].
Hydrothermal alteration at Pulang is classified into potassic, propylitic, and phyllic types (Figure 3). Potassic alteration predominantly occurs within and adjacent to the causative quartz monzonite porphyry (QMP) and is mainly composed of hydrothermal K-feldspar, biotite, and quartz (Figure 3a). Chalcopyrite is commonly present in disseminated or sparsely disseminated forms within the potassic zone (Figure 3a). Anhydrite, part of the potassic alteration paragenesis, primarily occurs as an alteration mineral rather than as vein infill (Figure 3b), being associated with K-feldspar and biotite during early potassic alteration stages, indicative of the high-temperature magmatic–hydrothermal environment. Propylitic alteration, characterized by epidote, chlorite, actinolite, and quartz (Figure 3c), typically manifests as vein-like structures or overprints the early potassic zone (Figure 3d). Chalcopyrite is also frequently observed in localized clusters or fine veinlets within propylitic veins. Phyllic alteration is structurally controlled along fracture zones and is locally enriched in disseminated pyrite, with minor chalcopyrite and molybdenite (Figure 3e).

2.2. Typical Drill Hole Logging

Due to overlapping alteration zones and difficulties in accurately assessing their presence and intensity, shortwave infrared spectroscopy (SWIR) was employed to map alteration and mineralization in typical drill holes (e.g., ZK0403) at the Pulang porphyry deposit (Figure 4a). Alteration intensity was estimated by integrating conventional drill core logging with methods similar to those described by [42]. Macroscopic observations of mineral assemblages (e.g., biotite, chlorite, epidote, sericite) were recorded from drill core ZK0403 and correlated with spectral absorption features identified by SWIR analysis. The results show that deep sections of ZK0403 exhibit strong biotite alteration, whereas shallow sections are dominated by advanced K-feldspar alteration (Figure 4b). Intermediate and shallow zones are characterized by well-developed chlorite + epidote + actinolite, indicative of propylitic alteration. Additionally, the chlorite assemblages are mostly not found in conjunction with biotite, with only residual biotite or amphibole alteration features preserved (Figure 4b). The widespread propylitic alteration in these zones suggests that biotite alteration was originally intense but subsequently overprinted by late-stage, low-temperature fluid alteration, resulting in a transformation into chlorite, epidote, and actinolite. Although hydrothermal K-feldspar is pervasive throughout the drill hole, its intensity is weaker than that of biotite alteration. In shallow sections, the replacement of biotite by chlorite leaves remnants of hydrothermal K-feldspar phenocrysts, imparting a characteristic pinkish-red hue to the rock surface (Figure 4b). Near fractures and breccia zones, plagioclase is frequently altered by late-stage, low-temperature acidic fluids, producing abundant sericite, pyrite, and quartz assemblages characteristic of phyllic alteration (Figure 4b). Phyllic alteration is concentrated in zones of pronounced rock fragmentation, contrasting with the pervasive distribution of propylitic alteration throughout the drill core. Additionally, argillic alteration is more prominent in intermediate and shallow sections, often imparting a rough, clay-like texture to feldspar minerals.
The SWIR results indicate that mafic minerals are predominantly Mg-biotite (Figure 4c), with a small amount of biotite also present, consistent with the previous EPMA data on biotite [24]. Mg-biotite is characterized by a higher magnesium content, whereas regular biotite contains more iron. SWIR analysis shows that Mg-biotite typically exhibits absorption peaks in the 2300~2350 nm range, while biotite has a stronger absorption peak around 2200 nm [24,26]. A small amount of amphibole is visible in the deeper sections, but biotite and Mg-biotite are well-developed throughout the drill hole, especially in the shallow sections (Figure 4c). Due to its susceptibility to hydrothermal alteration, biotite often transforms into chlorite, with Fe-Mg chlorite being commonly observed. Phyllic alteration, represented by sericite and phengite, is closely associated with fracture zones. Argillic alteration is particularly prominent in the ZK0403 drill hole, overprinting earlier potassic and propylitic alteration zones. This alteration is especially pronounced in the 0~20 m interval. Clay minerals are predominantly illite, with kaolinite being rare.
Chalcopyrite is the dominant copper sulfide mineral at Pulang, with a more pronounced development in the shallow sections, as evidenced by widespread copper mineralization throughout the ZK0403 drill hole (Cu > 0.3 wt%; Figure 4d,e). Molybdenite, a minor component of the Pulang ores, is more commonly found in the deeper sections and exhibits a spatial distribution distinct from that of chalcopyrite (Figure 4d,e). Native gold primarily occurs as inclusions or veins within chalcopyrite [36], and the strong correlation between Au and Cu grades suggests that most of the gold precipitated alongside chalcopyrite [21]. Additionally, pyrrhotite, which is indicative of a reduced environment, is predominantly found in the central part of the drill hole, where propylitic alteration is well-developed (Figure 4d).

2.3. Paragenesis of Ore and Alteration Minerals

Based on drill hole alteration mapping, potassic alteration can be divided into an early stage dominated by K-feldspar alteration and a later stage characterized by biotite alteration. During the K-feldspar alteration stage, the primary minerals include K-feldspar, quartz, and anhydrite, with occasional sulfide occurrences. In contrast, the biotite alteration stage, marked by hydrothermal biotite + quartz veins, also features fine-grained chalcopyrite, bornite, and minor molybdenite, with bornite partly replacing chalcopyrite (Figure 5a). Mineralization during the propylitic alteration stage is more prevalent and represented by minerals such as chlorite, epidote, and actinolite. Chlorite often forms along mineral fractures, exhibiting disseminated or vein-like patterns, and coexists with sulfides like chalcopyrite and molybdenite (Figure 5b). Actinolite, on the other hand, tends to be more euhedral and may host molybdenite and other sulfides along its fractures (Figure 5c). Phyllic alteration, marked by the transformation of plagioclase into sericite or illite, typically overlays the potassic and propylitic alteration zones, indicating a relatively later stage of formation (Figure 5d). Except for pyrite, other metal sulfides are infrequent in these zones.
By integrating drill core logging with microscopic observations of mineral assemblage relationships, the paragenesis sequence of mineralization at Pulang was established (Figure 6). K-feldspar formation precedes biotite crystallization and is locally accompanied by anhydrite and magnetite. Sulfides such as chalcopyrite, bornite, and molybdenite are commonly observed within biotite-hosted veins. During the propylitic stage, actinolite, chlorite, and epidote are primarily formed through the metasomatic replacement of amphibole, biotite, and anorthite, respectively. Chalcopyrite and molybdenite are frequently associated with these propylitic mineral assemblages. Notably, pyrrhotite is exclusively found in association with chlorite, indicating a close genetic link to the propylitic alteration stage. Phyllic alteration, characterized by assemblages of sericite + quartz + pyrite, typically overprints slightly earlier chlorite + quartz + chalcopyrite veins, suggesting a later stage of formation relative to the propylitic alteration. In the shallowest levels of the deposit, kaolinite and illite are disseminated and overlie the potassic, propylitic, and phyllic alteration zones.

2.4. Fluid Inclusion Petrography

Fluid inclusion assemblages (FIAs) from the propylitic alteration stage were identified through petrographic observations and laser Raman spectroscopy (532 nm). These veins typically exhibit a harbor-like shape, with chlorite and chalcopyrite occurring along the edges or fractures (Figure 7a). Chlorite often forms a curved or worm-like pattern embedded within quartz, while chalcopyrite fills the gaps between chlorite and quartz, creating a harbor-like structure (Figure 7b). The FIAs are predominantly located along healed fractures within chlorite-surrounding quartz, suggesting a close association with the propylitic alteration (Figure 7c). These inclusions are primarily vapor-rich, a feature rarely observed in other alteration stages (Figure 7c). The inclusions are mainly distributed along healed fractures within quartz and chlorite and are spatially associated with chalcopyrite and pyrrhotite. These petrographic characteristics imply that the inclusions were likely trapped during or shortly after mineralization, rather than being purely secondary. Hence, we interpret these FIAs as being closely related to the ore-forming propylitic fluids. The Raman spectroscopy results indicate that most of these vapor-rich FIAs are composed of methane (Figure 7d). This observation, along with the presence of pyrrhotite during the propylitic alteration stage (Figure 4d), suggests a decrease in redox potential during this alteration stage.

3. Methods and Data Sources

The Raman spectra of fluid inclusions were collected in the range of 100~4000 cm−1 using a Horiba LabRAM HR Evolution Raman microscope (HORIBA Scientific, Paris, France) at the School of Gemology, China University of Geosciences (Beijing). The system was equipped with a Peltier-cooled CCD detector, edge filters, and a 100 mW, 532 nm Nd-YAG laser. A 100×, 50×, and 10× objective lens with a polaroid was used for laser focusing and sample observation. The spatial resolution was <1 μm, and spectral resolution was ~1 cm−1. Spectra were acquired at 50 mW laser power with an acquisition time of 3 s and two accumulations.
Thermodynamic modeling was carried out using the GEM-Selektor (GEMS) code package (3.9.6) based on the Gibbs energy minimization (GEM) method [43,44] and using the Reaktoro code package (2.13.0) based on the Law of mass action (LMA) method [45,46]. The initial step involved employing the LMA and GEM methods to reconstruct the original composition of the ore fluid, which encompassed factors like major elemental compositions, pH, and redox potentials (ƒO2), as well as the concentrations of sulfur, chlorine, copper, molybdenum, and gold. Subsequently, reactive transport simulations using the GEM2MT module implemented in GEMS were constructed to elucidate the influence of fluid–rock interactions on hydrothermal alteration patterns and ore metal precipitation at the Pulang deposit. Finally, a pH-logƒO2 phase diagram of altered minerals was constructed using the mineral equilibrium interactions and the ThermoFun (0.4.5) module in Reaktoro to test the above forward simulation results.
The thermodynamic dataset for minerals, gases, and aqueous species used in the calculations was taken from the MINES 2023 database [47]. All activity models and equations of state were calculated using the TSolMod library class [48] implemented in GEMS. The activity model used for aqueous species was the extended Debye–Hückel equation with NaCl as the background electrolyte [49] and extended term parameters calibrated for the systems NaCl, KCl, NaOH, and KOH [50,51]. The thermodynamic properties of aqueous species were corrected for pressure and temperature using the revised Helgeson–Kirkam–Flowers (HKF) equation of state [49,52,53]. The detailed mineralogical analysis conducted on the Pulang porphyry Cu-Au deposit provided essential data on alteration zones, mineral assemblages, and fluid inclusion characteristics. These observations served as input for the thermodynamic models, which simulate the fluid–rock interactions and constrain ore-forming processes.

4. Thermodynamic Modeling of Fluid–Rock Interactions at Pulang

4.1. Effects of S and Cl Volatiles on Metal Solubility and Metal Ratios

To constrain the variability in volatile components in the simulations, vapor-like and intermediate-density fluid inclusions from typical porphyry deposits were collected (Supplementary Table S1). The S (SO2 and H2S), Cl (HCl and NaCl), and CO2 contents reported for vapor-like and intermediate-density fluid inclusions from porphyry deposits are similar, namely 0.1~2.0 wt% S, 0.6~9.6 wt% Cl, and 1.1~5.3 wt% CO2. Since sulfur plays a critical role in controlling metal solubilities, titration models were designed by progressively increasing the S content from 0.01 to 10.0 wt% at constant NaCl concentrations of 1, 5, and 10 wt%. The CO2 content was held constant at 2 wt%, the median value for vapor-like and intermediate-density porphyry fluids. Redox potential is also a key consideration.
Although there is no direct method for measuring redox-sensitive ions in fluid inclusions, mineral assemblages indicative of high redox potentials, such as magnetite, anhydrite, and rutile in the potassic alteration zone of the Pulang deposit, suggest that the initial buffer mineral assemblage for the model can be set as magnetite + anhydrite + rutile, reflecting the initial fluid environment. Temperature and pressure were set at 600 °C and 1500 bar to simulate the conditions of high-temperature fluids exsolved from magma.
The simulated results show that the mineral assemblages of magnetite + anhydrite + rutile are highly sensitive to S concentration, and therefore the modeled fluid S concentration holding such mineral assemblages may represent those present in porphyry deposit environments (Supplementary Table S2). When Cl concentrations are set at 1 wt%, 5 wt%, and 10 wt%, the corresponding S concentration ranges required to stabilize the magnetite + anhydrite + rutile assemblage are 0.51~5.61 wt%, 0.41~5.51 wt%, and 0.81~4.41 wt%, respectively (Figure 8). In contrast, the stability of the buffer assemblage is less influenced by Cl concentration, particularly when S concentration falls within the typical range observed in porphyry systems. As S concentration increases under sulfur-saturated conditions, the buffer assemblage transitions to pyrrhotite + anhydrite + rutile, with pyrrhotite content increasing in response to higher NaCl concentrations.
Copper in solution primarily migrates as CuCl (CuCl4−3, CuCl2−, CuCl), with a minor contribution from CuS (CuHS, Cu(HS)2−; Supplementary Table S2). Therefore, Cu solubility decreases with increasing S concentration under relatively low-NaCl conditions (1–5 wt%). However, at higher salinity (10 wt% NaCl), Cu solubility stabilizes at approximately 20,000 ppm. Gold mainly migrates as AuS (Au(HS)2−, AuHS) and AuCl (AuCl3−2, AuCl2−, AuCl). The modeling results show that the solubility of both the AuS and AuCl complexes increases with rising S and Cl concentrations. For instance, at 5 wt% NaCl, as the sulfur concentration increases from 0.1 wt% to 10 wt%, the solubility of AuS increases from 0.01 ppm to 0.67 ppm, while that of AuCl increases from 0.04 ppm to 0.86 ppm. Molybdenum is primarily transported as molybdate species (HMoO4− and MoO4−2). The model results indicate that Mo solubility decreases exponentially with increasing S concentration, from 10,000 ppm down to 1 ppm. Moreover, Mo solubility is only weakly influenced by Cl concentration: even as NaCl content increases from 1 wt% to 10 wt%, Mo solubility rises only modestly from 18 ppm to 60 ppm.
To evaluate the accuracy of thermodynamic simulations, the modeled solubilities of Cu, Mo, and Au were compared with the metal concentrations reported in fluid inclusions from porphyry deposits (Figure 8). At a NaCl concentration of 5 wt%, the simulated Cu solubility ranges from 665 to 5800 ppm. When constrained by typical porphyry S concentrations, the modeled Cu solubility falls between 760 and 1765 ppm—lower than the range observed in fluid inclusions (10~33,000 ppm). At higher salinity (10 wt% NaCl), the simulated Cu solubility can reach up to 30,000 ppm. In contrast, the simulated Au solubility (0.04–3.12 ppm) closely matches the Au content in vapor-like and intermediate-density fluid inclusions (0.05–3.34 ppm). Molybdenum is primarily transported as HMoO4− and MoO42−, and its solubility is negatively correlated with S concentration, consistent with the experimental findings [54]. The simulated Mo solubility constrained by the magnetite + anhydrite + rutile buffer assemblage ranges from 45 to 12,238 ppm, which significantly exceeds the values typically measured in porphyry fluid inclusions (1.2~290 ppm, median ~50 ppm). The differential effects of S on Cu, Mo, and Au solubilities lead to considerable variation in the Cu/Au and Cu/Mo ratios. For example, at typical porphyry conditions, 2.7 wt% S and 5 wt% NaCl, the simulated Cu/Au ratio (4764) and Cu/Mo ratio (8) closely approximate those observed in the Bingham porphyry deposit, with Cu/Au ~4300 and Cu/Mo ~6 [55]. In contrast, increasing Cl concentration significantly enhances Cu solubility, thereby elevating both the Cu/Au and Cu/Mo ratios.

4.2. Reconstructing the Physical and Chemical Conditions of the Initial Fluid

Fluid inclusion studies have constrained the key physicochemical properties of ore-forming fluids at Pulang, including homogenization temperatures (140~395 °C), salinities (4.6~24.7 wt% NaCl), and major components (H2O, CO2, CH4, CO) [25]. However, some key parameters, such as pH values, redox potentials, and volatile contents, are challenging to obtain, particularly in which sulfur (S) and chlorine (Cl) serve as some of the most crucial ligands for metal migration and exert a pivotal influence over the levels of dissolved Cu and Au in the fluid [13,50,56]. The solubility of metal complexes in solutions gradually decreases due to changes in temperature/pressure and fluid–rock interactions, leading to the subsequent precipitation of metals [57,58]. Therefore, to obtain the physical and chemical parameters of the initial ore-bearing fluid, the above mineral precipitation process is reversibly treated; that is, the combined LMA and GEM methods are used to construct the phase equilibrium process between the volatile-bearing fluid, alteration minerals, and metals. Regarding the thermobarometry of hydrothermal biotite and fluid inclusion analysis [25,59], the temperature and pressure conditions are 450 °C and 1000 bar, respectively. The presence of magnetite + hematite + anhydrite in the initial alteration stage of porphyry deposits indicates a relatively high redox potential, such as ΔFMQ = +2 [60,61]. At 450 °C, the system redox potential obtained is logƒO2 = −23.82. In addition, due to the complex composition and valence state of volatile S in fluid, to simulate the feasibility of using the LMA and GEM methods to simulate the concentration of S in solution, S in solution is simplified into two end members, H2S and SO2, and other compounds in the S valence state can be reached from the above end members; that is, H2Saq + 3O2aq = 2SO2aq + 2H2O. In this paper, by using the thermodynamic database of MINES 2023 and the ThermoFun module in Reaktoro, the equilibrium constant of the above-mentioned mineral interaction equation is calculated as log K = 72.5548, and the calculated ratio of SO2/H2S concentration in the fluid is 3.527. Considering the S concentration of fluid inclusions in typical porphyry deposits (Supplementary Table S1), the initial fluid is set to contain 3.5 wt% SO2 and 1 wt% H2S. According to the simulation results in Section 4.1, the NaCl and HCl concentrations are set at 8 wt% and 1 wt%. The CO2 content was fixed at 2 wt% in this study based on previous work and observed fluid inclusion compositions, which suggested that CO2 was a dominant component within this concentration range (Supplementary Table S1). In addition, CO2 has been shown to play a significant role in the ore-forming fluids of some porphyry deposits [55,62]. Its presence can inhibit the stability of major ore-bearing complexes (e.g., CuHSaq, CuCl2−, Au(HS)2−, FeCl42−) and promote the precipitation of Cu, Au, and Fe [63]. A lower CO2 content would likely lead to changes in fluid density, which could impact the calculated fluid pressures and homogenization temperatures [64]. Conversely, a higher CO2 content (up to 5 wt%) may influence the solubility of other gas species, potentially altering the homogenization temperatures [65]. However, based on the stability of the fluid inclusion phases and the consistency of CO2 content in porphyry deposits, we believe that 2 wt% is a reasonable and representative value for the fluid conditions at the Pulang deposit. As a result, a corresponding three-terminal phase equilibrium simulation of volatile-bearing fluid, potassic alteration assemblages, and metal sulfide was constructed: (1) 1 kg aqueous solution containing 8 wt% NaCl, 2 wt% CO2, 1 wt% HCl, 3.5 wt% SO2, and 1 wt% H2S; (2) mineral assemblages representing potassic alteration, including K-feldspar (KAlSi3O8), albite (NaAlSi3O8), biotite (K(Mg,Fe)3AlSi3O10(OH)2), quartz (SiO2), magnetite (Fe3O4), anhydrite (CaSO4), and rutile (TiO2), with excess amounts ensured; and (3) sulfide assemblages representing metal sources, including chalcopyrite (CuFeS2), molybdenite (MoS2), and gold (Au), ensuring an excess of these minerals.
The simulation results show that the pH and redox potential (ΔFMQ) of the equilibrated fluid are 4.70 and +2.92. Furthermore, when the solution reaches metal saturation, the dissolved concentrations are as follows: Cu at 1137.17 ppm, Mo at 1.25 ppm, and Au at 1.16 ppm. These values fall within the typical compositional range of metal elements in ore-forming fluids of porphyry deposits (Supplementary Table S1). This observation suggests that the calculated fluid composition can reasonably approximate the composition of initial ore-forming fluids in a porphyry system.

4.3. One-Dimensional Reactive Transport Simulations

To model hydrothermal alteration overprinting and its correlation with mineralization at Pulang, GEMS was employed to establish a one-dimensional reactive transport model. The model used the initial ore-forming fluid composition obtained in Section 4.2 as the starting point, and the QMP was selected as the host rock (Rock 1; Supplementary Table S3), and then 2 wt% H2O and 2 wt% CH4 were added to represent the initial rock composition with reduced components (Rock 2). The model encompasses 25 rock units, through which the ore-bearing fluid traverses from bottom to top (Figure 9a). As the fluid progresses and cools, minerals continuously precipitate from the fluid due to decreasing temperatures and ongoing water–rock interactions. The specific model settings are as follows:
  • Each rock unit contains 100 g of Rock 1, but rock units 11 to 15 consist of 100 g of Rock 2. The initial ore-bearing fluid initiates interactions from unit 1 and reacts sequentially with subsequent rock units, simulating the mixing process between magmatic–hydrothermal fluids and meteoric water that has acquired reduced components (e.g., from wall rock interactions).
  • The initial ore-bearing fluid is set to 1000 g, which is in equilibrium with mineral assemblages representing the potassic alteration stage, as shown in Section 4.2.
  • The simulation temperature decreases stepwise by 10 °C per unit as the fluid reaches equilibrium with each rock unit, ranging from 450 °C to 200 °C. This temperature range effectively covers the temperature range of the alteration and mineralization stages.
  • Since pressure variations have a minimal impact on metal solubility in porphyry ore-forming systems, this study’s simulation does not incorporate pressure changes. The simulation pressure is based on the results from magmatic biotite and fluid inclusion analysis and is set to a constant value of 1000 bar.
Minerals 15 00780 g009
Figure 9. Thermodynamic simulation of fluid–rock interaction processes at Pulang porphyry Cu-Au deposit. (a) Schematic diagram of fluid–rock interaction processes. (b) Changes in fluid pH value. (c) Changes in fluid redox potential (ΔFMQ) values. FMQ buffer is extrapolated beyond its calibrated range and serves only as reference scale for redox potential. (d) Alteration mineral associations of potassic alteration. (e) Alteration mineral assemblages of propylitic alteration. (f) Alteration mineral association of phyllic alteration. (g) Metal precipitation in fluid–rock interactions. (h) Changes in concentration of dissolved metal ions.
Figure 9. Thermodynamic simulation of fluid–rock interaction processes at Pulang porphyry Cu-Au deposit. (a) Schematic diagram of fluid–rock interaction processes. (b) Changes in fluid pH value. (c) Changes in fluid redox potential (ΔFMQ) values. FMQ buffer is extrapolated beyond its calibrated range and serves only as reference scale for redox potential. (d) Alteration mineral associations of potassic alteration. (e) Alteration mineral assemblages of propylitic alteration. (f) Alteration mineral association of phyllic alteration. (g) Metal precipitation in fluid–rock interactions. (h) Changes in concentration of dissolved metal ions.
Minerals 15 00780 g009
While the FMQ buffer assemblage is not thermodynamically stable below ~400 °C [66], it is used here solely as a comparative reference to express redox potentials in terms of log fO2 (e.g., ΔFMQ values). We acknowledge this extrapolation and do not imply the physical presence of the FMQ assemblage at these lower temperatures. As the initial ore-bearing fluid flows through the QMP, within the temperature range of 450 °C to 400 °C, due to the high fluid/rock ratios in the initial stage, the pH (4.73~4.62) and ΔFMQ (+3.54~+3.26) value of fluid are mostly controlled by the fluid itself, and low pH and high ΔFMQ values are shown. Mineral assemblages representing potassic alteration are precipitated continuously, such as K-feldspar, biotite, anhydrite, magnetite, and hematite. Along with the precipitation of these minerals and decreasing fluid/rock ratios, the pH of the fluid increases from its initial value near 4.7 to around 5.2, while the fluid redox potential gradually decreases (Figure 9), with the ΔFMQ value decreasing from +3.5 to around +2.8 (Figure 9). As the temperature continues to decrease, the typical propylitic alteration mineral assemblages, mainly clinochlore and daphnite, begin to precipitate. In addition, influenced by the presence of CH4 mixtures, a significant precipitation of pyrrhotite and calcite begins within this interval. The pH of the fluid increases from 5.2 to 6.1, while the ΔFMQ value decreases sharply from +2.8 to +1.9. As the temperature decreases further, sericite begins to precipitate gradually. This is accompanied by a slight increase in fluid pH from 5.8 to approximately 6.0, while the ΔFMQ value slightly rises.
Although the temperature reduction reduces the concentration of the Cu, Au, and Mo complexes, the response of the stability of each metal complex to temperature varies significantly. With carriers such as HMoO4− and MoO4−2, Mo predominantly precipitates within the high-temperature range of 450~370 °C, and solubility drops from 1.25 ppm to 1.57 × 10−3 ppm. The precipitation of the Cu element, with CuCl and a small amount of CuS as the carriers, is also controlled by temperature. In the range of 450~370 °C, the solubility of Cu decreases from 794.12 ppm to 6.25 ppm, and it continues to slightly decrease to ~1 ppm in the propylitic alteration stage. The solubility of the Au element with AuS and AuCl as carriers also decreases with the drop in temperature but is less than that of Cu and Mo. This results in the precipitation of Au being obvious in the low-temperature range.

4.4. pH-logƒO2 Phase Diagram

To validate the evolutionary trajectory of ore-forming fluids derived from GEM simulations, a corresponding pH-logƒO2 phase diagram based on the paragenesis of alteration minerals and the LMA method was constructed. Based on the results of the mineral thermobarometry and fluid inclusions, the temperature and pressure of potassic, propylitic, and phyllic alteration are limited to 400 °C, 300 °C, and 300 °C and 1000 bar, respectively. The ion concentrations under various temperature and pressure conditions were revised according to the fluid ion concentration in the GEM model. During the transformation of biotite into chlorite, the GEM model indicates fluid concentrations of Mg2+ and K+ ions at 0.00023 and 0.1090 mol/kg, respectively. As K-feldspar transforms into muscovite, the GEM model reveals fluid concentrations of Mg2+ and K+ ions at 0.00015 and 0.032 mol/kg, respectively. To ensure thermodynamic model consistency, this study also utilizes the MINES 2023 database [47] and imports it into the Reaktoro software package [45,46] for database management. Subsequently, the ThermoFun module is employed to calculate the equilibrium constants (log K values) for the mineral interactions (Supplementary Table S5), and the pH-ƒO2 phase diagram is built on these log K values.
Limited by the coexistence of K-feldspar, biotite, anhydrite, magnetite, and hematite in the potassic alteration zone, the fluid pH (~7) and redox potential (ΔFMQ = +3) are representative of strong oxidation and neutral fluid (Circle 1 in Figure 10a). When the temperature drops to about 300 °C, the fluid pH value limited by propylitic alteration minerals, such as the chloritization of biotite and deposition of pyrrhotite and calcite, is about 4.83 (Circle 2 in Figure 10b). Affected by the deposition of hematite and magnetite in the early potassic alteration stage, as well as the replenishment of reduced substances, the fluid redox potential sharply decreases to around ΔFMQ = 0. In addition, the stable phase region of chalcopyrite + bornite is greatly expanded as temperature decreases, so the appearance of biotite + chalcopyrite + bornite veins in the potassic alteration stage should be caused by temperature reductions rather than redox potential changes, which is consistent with previous experiments [67]. As the temperature decreases further, the fluid pH value fluctuates to the stable domain of sericite (pH = ~4.97; Circle 3 in Figure 10b), and the fluid redox potential increases slightly (ΔFMQ = 1), due to the decrease in reducing S concentration caused by pyrite precipitation.

5. Discussion

5.1. Causes of Hydrothermal Alteration Overprinting

The grade map results from ZK0403 indicate that the drill hole is predominantly copper-mineralized (Cu > 0.3 wt%), with particularly strong mineralization in the 0~200 m depth, where Cu grades exceed 1.0 wt% (Figure 4e). Traditional logging results indicate that the drill hole exhibits potassic alteration in the shallow sections and propylitic alteration in the deep sections (Figure 4b). Consequently, Cu mineralization is closely linked to potassic alteration. Moreover, the SWIR results reveal that biotite alteration is well-developed throughout the drill hole, with biotite in the shallow sections being partly altered to chlorite, displaying strong propylitic alteration characteristics. Therefore, Cu mineralization at Pulang is most probably associated with the biotite alteration stage. However, despite the strong biotite alteration in deep sections, its Cu grades (0.3 wt% < Cu < 0.4 wt%) are lower than those within the shallow propylitic alteration zone (Cu > 0.6 wt%). Chlorite is primarily derived from the alteration in biotite (Figure 4), and it is observed that chalcopyrite and chlorite coexist in a harbor-like pattern (Figure 5b). The enrichment of copper during the propylitic alteration stage may be attributed to the changes in fluid properties, which promoted the transformation of biotite into chlorite, as well as the dissolution and subsequent re-precipitation of minor copper minerals that had formed during the potassic alteration stage.
Notably, both GEM and LMA modeling reflect similar changes in fluid pH and redox potential at the Pulang porphyry deposit: the initial fluid associated with potassic alteration has a nearly neutral pH (5.0~7.0) and high redox potential (ΔFMQ = +3.54~+3.26), while the fluid pH (4.8~6.1) and redox potential (ΔFMQ = +1) related to propylitic alteration both decrease, and the fluid pH (5.8~6.0) and redox potential (ΔFMQ = +2) related to phyllic alteration stabilize and increase slightly. The simulations confirm the governed role of fluid–rock interactions in fluid pH and redox potential, and the fluctuations in fluid pH are influenced both by the fluid–rock ratios and the presence of alteration minerals. This suggests that a single magmatic fluid can generate all the observed alteration and mineralization features in a porphyry system solely through temperature reduction. The widespread occurrence of vein crosscutting relationships in porphyry deposits, where quartz + chalcopyrite veins with K-feldspar or biotite alteration halos are crosscut by chlorite + actinolite + chalcopyrite veins, which are in turn crosscut by low-temperature sericite + pyrite + quartz veins [68], may be the result of the rapid cooling of a single magmatic fluid, rather than indicating the presence of different fluid components from the porphyry intrusions at distinct stages. In the case of the Pulang deposit, the intense propylitic alteration overprinting the potassic alteration is more likely a consequence of “thermal collapse” and the reflux of the hydrothermal system, triggered by the rapid temperature decrease within the magmatic system, which overlays the high-temperature alteration zone.
However, certain reduction-related features at Pulang, such as the presence of pyrrhotite and CH4-bearing fluid inclusions, are likely attributed to the influx of reduced substances. This interpretation is supported by mineral paragenesis data (Figure 4), redox potential measurements of chlorite [59], and the process of pyrrhotite formation resulting from interactions between ore-bearing fluids and the CH4-bearing QMP (Figure 9). These scenarios collectively emphasize the critical role of reduced groundwater (i.e., meteoric water modified through interaction with reduced wall rock components) in generating the deposit’s reduction-related characteristics. This hypothesis is further corroborated by isotopic evidence from multiple C-H-O studies on quartz, sericite, and calcite. Du et al. (2017) [69] analyzed oxygen isotopes in early-stage quartz associated with potassic alteration, revealing δ18O values ranging from 7.6‰ to 11.4%, indicative of a magmatic origin. In contrast, later-stage quartz formed during more complex fluid interactions displayed lower δ18O values (−1.8‰ to 6.7‰), suggesting mixing between magmatic fluids and meteoric water that had potentially acquired reduced components via wall rock interactions. Similarly, Yang et al. (2021) [25] examined sericite and documented a temporal shift in both hydrogen and oxygen isotopic signatures from predominantly magmatic to more mixed signatures, reflecting increasing meteoric water influx. C-H-O isotopes in calcite showed δ13C values consistent with a magmatic carbon source, while δ18O values displayed a decreasing trend linked to enhanced fluid–rock interactions [70]. Hu et al. (2024) [71] further reported oxygen isotopic fractionation in quartz, highlighting the role of fluid–rock interactions during mineralization. In addition, sulfur isotopes in pyrrhotite exhibited δ34S values indicative of a mixed sulfur source involving both magmatic and sedimentary wall rock contributions [23], a conclusion also supported by fluid inclusion data. Collectively, these isotopic signatures point to a hybrid fluid source involving the mixing of magmatic–hydrothermal fluids with reduced substances derived from sedimentary wall rocks, which ultimately drove the redox evolution observed in the Pulang deposit.

5.2. Comparison of Simulated Metal Solubilities and Metal Concentrations in Fluid Inclusion

As a common mineral assemblage in the deep zones of porphyry deposits, the buffer minerals represented by magnetite + anhydrite + rutile are stable within a limited sulfur concentration range of 0.5~5.0 wt%, with the upper limit of modeled sulfur concentrations exceeding those found in fluid inclusions (0.1~2.0 wt%) in porphyry deposits. Although sulfides such as pyrite and pyrrhotite are rare in the early-formed minerals of porphyry deposits, the presence of sulfide solid solutions in zircons [72], despite the absence of sulfides in typical mineral assemblages, suggests a complex geological history in which sulfur concentrations in the source fluids were likely higher. The sulfur concentrations observed in fluid inclusions are probably the result of multiple phases of boiling and sulfide precipitation. Therefore, the modeled sulfur concentrations may better represent the initial fluid conditions.
The modeled Cu solubility (760~1765 ppm) is relatively lower than the observed values (10~33,000 ppm). Compared with other sulfides, chalcopyrite crystals are common in the fluid inclusions [73], and the Cu concentration of some fluid inclusions is as high as 10 wt% [74], which indicates that the Cu in the fluid inclusions may diffuse through the lattice of host minerals [72,74], and significant Cu secondary enrichment occurs, resulting in a high degree of oversaturation in the fluid inclusions. This is consistent with the extremely high Cu solubility (~30,000) seen when the NaCl concentration is 10 wt%, which is similar to the Cu content of fluid inclusions that have undergone Cu secondary enrichment [74,75]. Therefore, the simulated Cu solubility is more likely to represent a lower limit for Cu content in fluid inclusions. Au solubility is both controlled by AuCl and AuS complexes and increases slightly with increasing S and Cl concentrations, and the simulated Au solubility is basically within the Au content range of vapor-like and intermediate-density fluid inclusions from porphyry deposits. The abnormal solubility of Mo in the S unsaturated state may be due to the lack of S, which leads to the failure to generate Mo stable compounds, so it can only exist in the form of HMoO4− and MoO4−2 [76]. Therefore, only when the concentration of S reaches or exceeds the typical porphyry range can the Mo solubility value in the source region be represented. The simulation results indicate that the continuous supplementation of S in the solution leads to a gradual reduction in the redox potential and pH of the system, which is the main reason for the formation of molybdenite resulting from the decrease in the solubility of HMoO4− and MoO4−2.

5.3. Mechanisms of Metal Precipitation

The GEM modeling results indicate that the solubilities of the Cu, Au, and Mo complexes are primarily controlled by temperature. Mo predominantly precipitates at high temperatures, Cu at intermediate-to-high temperatures, and Au at intermediate-to-low temperatures. This temperature-dependent behavior aligns well with the typical distribution of mineralization in porphyry deposits: Mo mineralization occurs near the porphyry intrusion, Cu is found in veined or disseminated patterns overlying the Mo mineralization, and Au typically forms shallow, low-temperature stockwork mineralization [77].
In summary, fluids exsolved from the porphyry intrusion exhibit elevated redox potential (ΔFMQ = +3.54~+3.26; Figure 11) and a nearly neutral pH value (5.0~7.0), reflecting the high redox characteristics observed in the mineral oxygen fugacity of magmatic amphibole and biotite. These fluids interact with the quartz monzonite porphyry (QMP), forming potassic mineral assemblages, including hydrothermal K-feldspar, biotite, magnetite, and anhydrite, alongside the predominant precipitation of molybdenite and chalcopyrite. As temperatures decrease and reduced groundwater mixes with the ore-forming fluid, the reaction with the QMP leads to the development of a propylitic assemblage dominated by chlorite, calcite, and pyrrhotite, indicative of a reduced environment. During this stage, fluid pH drops (4.8–6.1), and redox potential decreases (ΔFMQ = +1), with a limited precipitation of molybdenite, partial chalcopyrite, and small amounts of gold. As fluid temperature continues to decrease, and the sulfide-promoted disproportionation of SO2 occurs (4SO2 + 4H2O = 3H2SO4 + H2S), the fluid pH stabilizes around the sericite field (pH = 5.8–6.0), while redox potential (ΔFMQ = +2) slightly increases, influenced by pyrite precipitation. During this phase, gold and minor chalcopyrite precipitate at lower temperatures.

6. Conclusions

(1)
The initial fluid associated with potassic alteration exhibits a nearly neutral pH (5.0–7.0) and a high redox potential (ΔFMQ = +3.54 to +3.26). In contrast, fluids linked to propylitic alteration show a decrease in both pH (4.8~6.1) and redox potential (ΔFMQ = +1). Fluids related to phyllic alteration stabilize at a slightly higher pH (5.8~6.0) and redox potential (ΔFMQ = +2).
(2)
The precipitation of Mo, Cu, and Au is temperature-dependent. Mo primarily precipitates between 450 °C and 370 °C, while Cu solubility decreases from 794.12 ppm to ~1 ppm within this range. Although Au solubility also decreases, it does so to a lesser extent, leading to notable Au precipitation at lower temperatures.
(3)
The reduced characteristics of the Pulang deposit, including pyrrhotite and CH4-bearing fluid inclusions, are primarily a result of the mixing of magmatic–hydrothermal fluids with reduced meteoric water. Fluid–rock interactions further modify the fluid’s properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15080780/s1, Table S1: Composition information of Vapor-rich and Intermediate-density fluid inclusions in typical porphyry deposits; Table S2: Metal solubility under sulfur-titration model of different NaCl concentrations; Table S3: Composition of initial fluid and rock end members in fluid-rock interaction processes; Table S4: Modeled results of one-dimensional reactive transport model of the Pulang porphyry Cu-Au deposits; Table S5: Mineral reaction formula and corresponding equilibrium constant (log K).

Author Contributions

Conceptualization, S.Z. and W.H.; Methodology, S.Z.; Software, S.Z.; Validation, S.Z.; Formal analysis, S.Z.; Investigation, S.Z. and W.H.; Resources, W.H.; Writing—original draft, S.Z.; Writing—review & editing, W.H.; Supervision, W.H.; Project administration, W.H., H.W. and Y.X.; Funding acquisition, W.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (42302082, 42372098), and the National Key Research and Development Project of China (2022YFF0800902).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to thank the editor and anonymous reviewers for their constructive comments which significantly improved this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) The tectonic outline of the India and Eurasia collision zone and (b) the Yidun arc (simplified from [21,28]); (c) a simplified geological map of the southern portion of the Yidun arc (modified after [27]).
Figure 1. (a) The tectonic outline of the India and Eurasia collision zone and (b) the Yidun arc (simplified from [21,28]); (c) a simplified geological map of the southern portion of the Yidun arc (modified after [27]).
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Figure 2. A simplified geological map of the Pulang porphyry Cu-Au deposit, showing major lithology units (modified from [21]).
Figure 2. A simplified geological map of the Pulang porphyry Cu-Au deposit, showing major lithology units (modified from [21]).
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Figure 3. Photographs of hydrothermal alteration at Pulang. (a) Chlorite (Chl), epidote (ep), actinolite (act), and chalcopyrite (ccp) veins in the potassic alteration zone. (b) Anhydrite (Anh) in the potassic alteration zone, associated with K-feldspar and quartz. (c) Actinolite, chlorite, and epidote in the propylitic alteration zone. (d) The propylitic vein minerals include chalcopyrite (ccp), pyrite (py), and quartz (qtz) intruding the potassic alteration zone. (e) Phyllic alteration with disseminated pyrite and minor chalcopyrite and molybdenite.
Figure 3. Photographs of hydrothermal alteration at Pulang. (a) Chlorite (Chl), epidote (ep), actinolite (act), and chalcopyrite (ccp) veins in the potassic alteration zone. (b) Anhydrite (Anh) in the potassic alteration zone, associated with K-feldspar and quartz. (c) Actinolite, chlorite, and epidote in the propylitic alteration zone. (d) The propylitic vein minerals include chalcopyrite (ccp), pyrite (py), and quartz (qtz) intruding the potassic alteration zone. (e) Phyllic alteration with disseminated pyrite and minor chalcopyrite and molybdenite.
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Figure 4. Comprehensive interpretation of ZK0403 at Pulang porphyry Cu-Au deposit. (a) Geological log, (b) alteration intensity, (c) shortwave infrared spectroscopy diagram, (d) mineralized intensity of ZK0403, and (e) Cu-Mo grade diagram of ZK0403. Bt—biotite; Chl—chlorite; Ep—epidote; Kfs—K-feldspar; Qtz—quartz; Ser—sericite; Cal—calcite; Dol—dolomite; Kln—kaolinite; Ilt—illite; Phg—phlogopite; Ank—ankerite.
Figure 4. Comprehensive interpretation of ZK0403 at Pulang porphyry Cu-Au deposit. (a) Geological log, (b) alteration intensity, (c) shortwave infrared spectroscopy diagram, (d) mineralized intensity of ZK0403, and (e) Cu-Mo grade diagram of ZK0403. Bt—biotite; Chl—chlorite; Ep—epidote; Kfs—K-feldspar; Qtz—quartz; Ser—sericite; Cal—calcite; Dol—dolomite; Kln—kaolinite; Ilt—illite; Phg—phlogopite; Ank—ankerite.
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Figure 5. Photomicrographs of alteration and sulfide minerals. (a) Quartz + biotite + chalcopyrite + bornite veins. (b) Chlorite coexisting with chalcopyrite. (c) Actinolite hosts molybdenite along its fractures. (d) Phyllic alteration overlays the propylitic alteration zone. Ccp—chalcopyrite; Bn—bornite; Mol—molybdenite; Qtz—quartz; Bt—biotite; chl—chlorite; Act—actinolite; Ser—sericite; Ilt—illite.
Figure 5. Photomicrographs of alteration and sulfide minerals. (a) Quartz + biotite + chalcopyrite + bornite veins. (b) Chlorite coexisting with chalcopyrite. (c) Actinolite hosts molybdenite along its fractures. (d) Phyllic alteration overlays the propylitic alteration zone. Ccp—chalcopyrite; Bn—bornite; Mol—molybdenite; Qtz—quartz; Bt—biotite; chl—chlorite; Act—actinolite; Ser—sericite; Ilt—illite.
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Figure 6. Sequence and paragenesis of alteration and metallic minerals.
Figure 6. Sequence and paragenesis of alteration and metallic minerals.
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Figure 7. Photomicrographs and Raman spectroscopy of vapor-rich fluid inclusions in propylitic alteration zone. (a) Mineral assemblages of propylitic veins. (b) Chlorite and chalcopyrite within quartz veins. (c) Fluid inclusion assemblages in chlorite-surrounded quartz. (d) Raman spectroscopy analysis of fluid inclusion assemblages.
Figure 7. Photomicrographs and Raman spectroscopy of vapor-rich fluid inclusions in propylitic alteration zone. (a) Mineral assemblages of propylitic veins. (b) Chlorite and chalcopyrite within quartz veins. (c) Fluid inclusion assemblages in chlorite-surrounded quartz. (d) Raman spectroscopy analysis of fluid inclusion assemblages.
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Figure 8. Thermodynamic simulation of S titration model. (a) Control effect of sulfur concentration variations on buffer mineral assemblages at low salinity (NaCl = 1 wt%) and (b) its effect on metal solubility, and (c) comparison with metal content in fluid inclusions of porphyry deposits. (d) Control effect of sulfur concentration variations on buffer mineral assemblages at moderate salinity (NaCl = 5 wt%) and (e) its effect on metal solubility, and (f) comparison with metal content in fluid inclusions of porphyry deposits. (g) Control effect of sulfur concentration variations on buffer mineral assemblages at low salinity (NaCl = 10 wt%) and (h) its effect on metal solubility, and (i) comparison with metal content in fluid inclusions of porphyry deposits.
Figure 8. Thermodynamic simulation of S titration model. (a) Control effect of sulfur concentration variations on buffer mineral assemblages at low salinity (NaCl = 1 wt%) and (b) its effect on metal solubility, and (c) comparison with metal content in fluid inclusions of porphyry deposits. (d) Control effect of sulfur concentration variations on buffer mineral assemblages at moderate salinity (NaCl = 5 wt%) and (e) its effect on metal solubility, and (f) comparison with metal content in fluid inclusions of porphyry deposits. (g) Control effect of sulfur concentration variations on buffer mineral assemblages at low salinity (NaCl = 10 wt%) and (h) its effect on metal solubility, and (i) comparison with metal content in fluid inclusions of porphyry deposits.
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Figure 10. pH-logƒO2 phase diagram of alteration minerals at (a) 400 °C and (b) 300 °C. The mineral equilibrium interaction lines are mainly calculated according to the log K values in Supplementary Table S4. Circle 1 represents the pH and logƒO2 values defined by the mineral assemblages of potassic alteration, and Circles 2 and 3 represent propylitic alteration and phyllic alteration, respectively.
Figure 10. pH-logƒO2 phase diagram of alteration minerals at (a) 400 °C and (b) 300 °C. The mineral equilibrium interaction lines are mainly calculated according to the log K values in Supplementary Table S4. Circle 1 represents the pH and logƒO2 values defined by the mineral assemblages of potassic alteration, and Circles 2 and 3 represent propylitic alteration and phyllic alteration, respectively.
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Figure 11. (a,b) Schematic diagram of alteration zones and hydrothermal fluid evolution of Pulang porphyry Cu-Au deposit (modified from [78,79]).
Figure 11. (a,b) Schematic diagram of alteration zones and hydrothermal fluid evolution of Pulang porphyry Cu-Au deposit (modified from [78,79]).
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Zhang, S.; He, W.; Wang, H.; Xiao, Y. Thermodynamic Modeling Constrains the Alteration and Mineralization Patterns of the Pulang Porphyry Cu-Au Deposits in Eastern Tibet. Minerals 2025, 15, 780. https://doi.org/10.3390/min15080780

AMA Style

Zhang S, He W, Wang H, Xiao Y. Thermodynamic Modeling Constrains the Alteration and Mineralization Patterns of the Pulang Porphyry Cu-Au Deposits in Eastern Tibet. Minerals. 2025; 15(8):780. https://doi.org/10.3390/min15080780

Chicago/Turabian Style

Zhang, Shaoying, Wenyan He, Huaqing Wang, and Yiwu Xiao. 2025. "Thermodynamic Modeling Constrains the Alteration and Mineralization Patterns of the Pulang Porphyry Cu-Au Deposits in Eastern Tibet" Minerals 15, no. 8: 780. https://doi.org/10.3390/min15080780

APA Style

Zhang, S., He, W., Wang, H., & Xiao, Y. (2025). Thermodynamic Modeling Constrains the Alteration and Mineralization Patterns of the Pulang Porphyry Cu-Au Deposits in Eastern Tibet. Minerals, 15(8), 780. https://doi.org/10.3390/min15080780

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