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Article

The Fluid Evolution in the Skarn Stages of the Baoshan Skarn Cu-Polymetallic Deposit, South China

by
Ping Zheng
1,
Ke Chen
2,3,*,
Jun-Ke Zhang
2,3,
Zhong-Fa Liu
2,3,
Yong-Shun Li
2,3 and
Ming-Peng He
2,3
1
Department of Natural Resources, Hunan Vocational College of Engineering, Changsha 410151, China
2
Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Central South University, Ministry of Education, Changsha 410083, China
3
School of Geosciences and Info-Physics, Central South University, Changsha 410083, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 907; https://doi.org/10.3390/min14090907
Submission received: 4 July 2024 / Revised: 24 August 2024 / Accepted: 4 September 2024 / Published: 5 September 2024
(This article belongs to the Special Issue Geology, Geochemistry, Genesis, Modeling, Structure and Exploration of Copper Polymetallic Deposits)
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Abstract

:
Baoshan is a world-class skarn Cu-polymetallic deposit located at the junction of the Nanling and Qin-Hang metallogenic belts in China. While there has been extensive research on the mineralogy and geochemistry of skarn deposits, studies on the fluid characteristics and evolutionary history from the early to late skarn stages in such deposits are still limited. In this study, we analyzed garnet and pyroxene from the early skarn stage and scheelite from the late skarn stage of the Baoshan deposit. We distinguished two generations of garnet (Grt1 and Grt2), one generation of pyroxene, and three generations of scheelite (Sch I, Sch II, and Sch III) on the basis of mineral assemblages and microscopic characteristics. Grt1 appears coarse-grained, and Grt2 cuts through Grt1 as veinlets. In Grt1, the andradite end-member increases from the core to the rim, while the grossular portion decreases (Ad35–36Gr59–61Sp3–4 to Ad59–61Gr36–37Sp2–3), and in Grt2, the andradite end-member significantly increases (Ad41–73Gr25–55Sp2–3). Grt1 and Grt2 have similar trace element compositions, with enrichment in Zr and depletion in Nb and Hf, depletion in LREE, enrichment in HREE, and weak negative Eu anomalies. Pyroxene coexists with Grt1 and is similarly cut by Grt2, with its composition mainly being diopside (Di82–99Hd0.6–15Jo0–3.2). Sch I and Sch II appear as anhedral to subhedral grains, while Sch III is predominantly found in veinlets. In Sch I and Sch II, most REEs enter the scheelite lattice via the Na-REE coupled substitution mechanism, with a smaller portion substituting Ca vacancies. In Sch III, the substitution mechanism involving Ca site vacancies may dominate. During the early skarn stage, the oxygen fugacity of the fluid gradually decreased from Grt1 and pyroxene to Grt2. In the late skarn stage, fluid oxygen fugacity remains stable from Sch I and Sch II to Sch III in shallow parts but significantly decreases in deeper parts. The garnet and pyroxene from the Baoshan deposit align with typical skarn Cu deposit compositions, while scheelite in the late skarn stage shows Sch I, Sch II, and shallow Sch III as skarn-type and deep Sch III as vein-type scheelite. Early skarn stage fluids were weakly acidic. Sch I, Sch II, and Sch III originated from fluids related to the Baoshan granite porphyry, with Sch III also showing evidence of water–rock interaction. This study reconstructed the fluid evolution history from the early to late skarn stages at the Baoshan deposit, providing insights into the ore-forming processes of other skarn deposits.

1. Introduction

Scheelite (CaWO4), one of the primary host minerals of tungsten (W), has a trace element composition that can effectively reflect hydrothermal conditions and the evolution process [1,2,3,4,5]. Meanwhile, scheelite is also a key mineral for fluid inclusion studies, providing direct evidence of hydrothermal fluids by indicating their temperature, salinity, pressure, and composition [6,7,8]. Moreover, isotope studies (e.g., Sr, Sm-Nd, and U-Pb) of scheelite are valuable for tracing the origin of metallogenic materials and determining the mineralization timing [9,10,11,12,13,14]. Previous research suggests that the REE distribution in scheelite can be used to distinguish different types of deposits, indicating a genetic relationship between the trace element composition of scheelite and geological events [15,16].
Garnet is widely developed in pegmatite, granite, metamorphic rock, and skarn [17,18,19], and most garnets belong to a solid solution series of two or more end-member components [20,21]. In the skarn system, the geochemical composition of garnet is often used to reflect the physicochemical conditions of hydrothermal fluids or to reconstruct the fluid evolution history [22,23,24]. Pyroxene, as a common mineral in skarn-type deposits, is often used to determine deposit types and fluid characteristics [21].
The southern Hunan Province boasts exceptional mineralization conditions and is rich in metal resources such as W, Sn, Cu, Pb, and Zn. It is renowned as a major hub for non-ferrous metals in China. The region’s prominent ore deposits include Sn polymetallic deposits like Xianghualing, Furong, and Hehuaping; W polymetallic deposits such as Shizhuyuan, Xintianling, Yaogangxian, and Weijia; and Cu-Pb-Zn polymetallic deposits including Tongshanling, Baoshan, Huangshaping, and Shuikoushan. Additionally, the Dafang Au deposit is notable. These deposits are closely associated with the Yanshanian magmatic hydrothermal activity in the area. As a representative Cu-polymetallic deposit in the region, extensive research has been conducted on the diagenetic and metallogenic ages, magma source areas and processes, and evolution of ore-forming fluids in the Baoshan deposit [25,26,27,28,29,30,31,32,33]. The fluid evolution history of skarn deposits is crucial for understanding the sources of metals and the mineralization processes in such deposits. Currently, few studies have analyzed the comprehensive fluid evolution history from the early to late skarn stages in skarn deposits. The Baoshan deposit, as a typical skarn deposit, provides a window into studying fluid evolution in skarn systems. Moreover, the widespread presence of garnet, pyroxene, and scheelite in the skarn stages suggests that geochemical studies of these minerals could provide insights into the fluid evolution during the formation of the skarn mineralization system.
In this study, on the basis of detailed geological surveys and microscopic observations, we systematically analyze the major and trace element geochemical characteristics of garnet and pyroxene in the early skarn stage and scheelite in the late skarn stage of the Baoshan Cu-polymetallic deposit using cathodoluminescence (CL), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques to determine the substitution mechanism of rare earth elements in scheelite and the characteristics and evolution history of fluids, especially the transition of fluids from the early to late skarn stage. This helps us understand the role of fluid characteristics and evolution in the mineralization of skarn deposits.

2. Geological Setting

2.1. Regional Geology

The southern Hunan ore concentration area, part of the South China metallogenic domain, is situated within the late Proterozoic collisional contact zone between the Yangtze and Cathaysia blocks (Figure 1) [34]. It is also located at the intersection of the east–west-trending Nanling W-Sn polymetallic metallogenic belt and the northeast-trending Qin-Hang Cu-Pb-Zn polymetallic metallogenic belt in South China, giving it a unique tectonic and dynamic background. Since the Paleozoic era, the southern Hunan Province has experienced complex tectonic and magmatic activities. The Caledonian orogeny caused the metamorphism of Cambrian–Sinian sedimentary rocks and the uplift of basement folds. Devonian–Permian carbonate rocks underwent significant folding. Jurassic–Cretaceous tectonic activity resulted in the formation of northeast-trending rift-related basins and major faults [35].
The strata in the southern Hunan Province are relatively complete. Except for the Ordovician and Silurian periods, all strata from the Sinian to the Quaternary are exposed (Figure 2). Among these strata, the Paleozoic carbonate rocks are the most significant ore-bearing layers in the area, predominantly controlling the mineralization pattern, which is primarily characterized by skarn-type mineralization [37,38]. Influenced by the collision and orogeny of the Tethys Indosinian block, the North China block, and the South China block, a series of NNE-trending thrust nappe structures formed during the Indosinian period, establishing the main tectonic framework of this area [39,40,41]. Under the influence of Paleo-Pacific subduction and north–south landmass extrusion, a series of NE-trending thrust nappe structures developed during the Yanshanian period [42,43]. During the extensional setting of the Middle–Late Jurassic, intense magmatic activity led to the formation of extensive post-collision granites, which were accompanied by significant mineralization [44,45,46,47]. Since the convergence of the Neoproterozoic Yangtze Block and Cathaysia Block, this area has experienced several stages of tectonic–magmatic activity. The Yanshanian stage was the most intense and is most closely associated with mineralization, primarily controlled by NE-trending structures [37]. The genetic types of W-Sn deposits in this area are complex, encompassing porphyry, greisen, altered granite, skarn, quartz vein, and cassiterite sulfide types. The Cu-Pb-Zn polymetallic deposits are predominantly skarn-type and hydrothermal vein-type.

2.2. Deposit Geology

The Baoshan deposit is located at the intersection of the Qin-Hang and Nanling ore belts, with estimated total resources of 0.24 Mt Cu at a grade of 1.28%, 0.47 Mt Pb at 3.82%, 0.51 Mt Zn at 4.34%, and 0.012 Mt Mo [25]. The exposed lithology associated with the Baoshan deposit consists predominantly of neritic marine sediments. These include the Upper Devonian Xikuangshan Formation, the Lower Carboniferous Menggong’ao, Shidengzi, Ceshui, and Zimenqiao Formations, as well as the Middle-Upper Carboniferous Hutian Group (Figure 3). The area is characterized by NE-trending thrust nappe faults and closed inverted folds, which developed in response to the subduction of the Paleo-Pacific plate. Additionally, these structures have been intersected by three NW-trending strike-slip faults at the Baoshan deposit. Intense magmatism is observed at the Baoshan deposit, which includes granite porphyry (180.5 ± 1.6 Ma) [35], granodioritic cryptoexplosive breccias (157 ± 1.8 Ma) [37], and lamprophyres (156 ± 2 Ma) [48]. Additionally, granodiorite porphyry formed between 162 and 155 Ma [31,35,37,38,43,49]. This timing aligns with the mineralization age indicated by molybdenite Re-Os dating at 160 ± 2 Ma [49] and grossular-rich garnet U-Pb dating at 162.6 ± 2.9 Ma [43]. As a representative Cu-polymetallic deposit, the Cu skarn mineralization is hosted within the limestone of the Shidengzi Formation and is controlled by the NE-trending Baoling inversion anticline in the central mining district. In contrast, the Pb-Zn hydrothermal vein-type mineralization primarily occurs in the dolomite of the Zimenqiao Formation along faults, predominantly located in the eastern, western, and northern mining districts [50]. The Baoshan deposit exhibits intense hydrothermal alteration, mainly including potassic alteration, silicification, sericitization, skarnization, carbonatization, and fluoritization, among which skarnization, silicification, fluorization, and carbonatization are most closely related to mineralization (Figure 4). The minerals in these hydrothermal alteration zones are quite complex, including garnet, diopside, epidote, chlorite, actinolite, quartz, fluorite, and sericite. Potassic alteration, epidotization, sericitization, and silicification are commonly found in the ore-forming granodiorite porphyry (Figure 4a); skarnization and silicification are closely associated with Cu and Mo mineralization (Figure 4b).

3. Sampling and Analytical Methods

3.1. Sampling

On the basis of detailed geological surveys and systematic observations of various samples from the mining area, this study selects representative samples from each mineralization stage of the Baoshan deposit, specifically garnet and pyroxene from the early skarn stage and scheelite from the late skarn stage. The samples were systematically polished into thin sections. Combined with the results of microscopic petrographic identification, representative samples were selected for major and trace element analysis. Two representative samples from the skarn zone were taken from the 50 m and −190 m levels of Line 169 (S-50-169-1 and S-190-169-1) for garnet major and trace element analysis and pyroxene major element analysis (Figure 3). Three representative scheelite-bearing samples were collected from the Baoshan deposit for scheelite trace element analysis. Two samples are from the 50 m and −190 m levels of Line 169 (S-50-169-2 and S-190-169-2), which is the most significant exploration profile of the Baoshan deposit (Figure 5). The other one is collected from the 50 m level of Line 153 (S-50-153-2) for comparison. Standard polished thin sections (~30 µm) of each sample were prepared for EPMA and LA-ICP-MS analysis. Then, 100 µm to 500 µm size of scheelite grains were separated from the ores for cathodoluminescence imaging.

3.2. Analytical Methods

EPMA analysis, including quantitative energy dispersive spectroscopy (EDS), in situ major element analysis, and back-scattered electron (BSE) observations, were performed at the School of Geosciences and Info-Physics (Central South University) on a Shimadzu EPMA-1720. The analytical parameters include a 15 kV accelerating voltage, 2.0 × 10−8 A probe current, 5 µm spot size, and 0.01% detection limit.
The cathodoluminescence imaging of the garnet and scheelite was conducted via a CAMECA cathodoluminoscope at 10 kV and 20 nA at the Guangzhou Touyan Analytical Technology Co., Ltd., China. Then, trace elements of scheelite were measured by use of an NWR 193 nm laser ablation system at the same place. The ion-signal intensities were collected by an iCAP RQ inductively coupled plasma mass instrument. The sealed ablation cell where the samples were placed was constantly flushed with pure Helium gas (~1.5 L/min). The laser beam size, energy, and repetition rate were 30 µm, 3 J/cm2, and 6 Hz, respectively. Each analysis comprised a 30 s measurement of background acquisition, 40 s analysis for the sample, and 20 s washout time. The following isotopes were monitored: 23Na2O, 71Ga, 85Rb, 88Sr, 89Y, 91Zr, 93Nb, 97Mo, 135Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 151Eu, 157Gd, 159Tb, 161Dy, 165Ho, 166Er, 169Tm, 172Yb, 175Lu, 181Ta, 208Pb, 232Th, and 238U. The standard sample was Glass NIST SRM610 with Ca as the internal standard, and SRM612 as the system monitoring sample [52]. The ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals and trace element analysis [53].

4. Results

4.1. Petrography and Cathodoluminescence (CL)

The detailed mineral paragenesis is presented in Figure 6. Garnet and diopside are typical minerals of the early skarn stage of the Baoshan deposit. On the basis of field geological intercalation relationships and micro-petrographic characteristics, the garnet in the Baoshan deposit is divided into two generations (Figure 7a). The early generation garnet (Grt1) is brown-red, with a coarse-grained granular structure (Figure 7b). It exhibits mostly total extinction under crossed polarizers with weak heterogeneity. It is often replaced by minerals such as diopside, epidote, actinolite, quartz, calcite, and late sulfides. The late generation garnet (Grt2) appears as dark brown veinlets, cutting through and replacing the early garnet and pyroxene (Figure 7a,c). Under the microscope, it appears yellow-brown, with a subhedral–anhedral structure and total extinction.
In the Baoshan deposit, the diopside does not show obvious multi-generation characteristics in the field. It appears gray-green, often replacing Grt1 in a bay-shaped manner and then being cut by Grt2 (Figure 7d). Under the microscope, it is colorless to yellow-green, with obvious pleochroism and a subhedral short-columnar to granular structure. It is often replaced by late-stage carbonate minerals.
Scheelite appears in subhedral to anhedral grains (20–400 µm) and is predominantly found in skarn and sulfide ores, exhibiting a disseminated appearance. It commonly coexists with garnet, epidote, chlorite, pyrite, and magnetite (Figure 8a–c). No obvious zonation in structure or composition is visible under transmitted or reflected light (Figure 8d–f). On the basis of mineral cutting relationships, it can be inferred that scheelite crystallized after garnet (Figure 8a) but before the formation of retrograde skarn minerals (e.g., epidote and chlorite; Figure 8d), as well as magnetite and sulfides (Figure 8e,f).
However, the cathodoluminescence (CL) images of scheelite shown in Figure 9 reveal a different story. Three generations of scheelite are identified from the Baoshan deposit. Scheelite I (Sch I) appears as euhedral to subhedral grains with oscillating bands and a relatively CL-dark appearance (Figure 9a–d). Scheelite II (Sch II) generally has a homogeneous CL intensity and replaces Sch I (Figure 9a–c). Sch II can be easily distinguished from Sch I by the absence of oscillating bands and its relatively CL-bright in appearance (Figure 9b). Unlike Sch I and Sch II, Scheelite III (Sch III) predominantly exhibits a veinlet appearance with the darkest color in the CL images and inhomogeneous CL intensity (Figure 9c,d). Representative scheelite grains and the locations of LA-ICP-MS measurements in three samples are presented (Figure 9e–g).

4.2. Major and Trace Element Geochemistry of Garnet

The major element analysis results of garnet (Supplementary Materials S1) show that in Grt1, the SiO2 content ranges from 35.62 to 40.43 wt. %, with an average of 38.08 wt. %; TiO2 content ranges from 0.17 to 0.41 wt. %, with an average of 0.33 wt. %; Al2O3 content ranges from 6.70 to 11.60 wt. %, with an average of 9.232 wt. %; FeOT content ranges from 12.76 to 20.97 wt. %, with an average of 16.48 wt. %; MnO content ranges from 0.74 to 1.57 wt. %, with an average of 1.10 wt. %; MgO content ranges from 0.07 to 0.28 wt. %, with an average of 0.13 wt. %; and CaO content ranges from 30.52 to 32.49 wt. %, with an average of 31.55 wt. %. In Grt1, the andradite end-member proportion increases from the core to the rim, while the grossular end-member proportion gradually decreases (Ad35–36Gr59–61Sp3–4–Ad59–61Gr36–37Sp2–3) (Figure 10a). In Grt2, the SiO2 content ranges from 40.49 to 42.89 wt. %, with an average of 41.99 wt. %; TiO2 content ranges from 0.05 to 0.51 wt. %, with an average of 0.24 wt. %; Al2O3 content ranges from 3.97 to 9.14 wt. %, with an average of 5.52 wt. %; FeOT content ranges from 14.12 to 23.36 wt. %, with an average of 19.74 wt. %; MnO content ranges from 0.53 to 0.98 wt. %, with an average of 0.70 wt. %; MgO content ranges from 0.06 to 0.18 wt. %, with an average of 0.11 wt. %; and CaO content ranges from 28.27 to 30.24 wt. %, with an average of 29.10 wt. %. Compared to Grt1, the proportion of andradite end-member in Grt2 significantly increases (Ad41–73Gr25–55Sp2–3) (Figure 10a).
The trace element analysis of garnet (Supplementary Materials S2) shows that Grt1 and Grt2 have similar trace element compositions, characterized by enrichment in Zr and depletion in Nb and Hf. The Zr, Nb, and Hf contents in Grt1 and Grt2 are 113.7–274.8 ppm and 59–213 ppm, 1.9–6.6 ppm and 7.6–15.6 ppm, and 2.5–6.7 ppm and 1.2–6.4 ppm, respectively. The ∑REE contents are 27.4–69.8 ppm and 30.8–59.8 ppm, both showing enrichment in LREE and depletion in HREE (LREE/HREE = 1.6–5.4 and 2.1–5.3) (Figure 11a,b), with weak negative Eu anomalies.

4.3. Major Element Geochemistry of Diopside

The major element analysis results of pyroxene from the Baoshan deposit (Supplementary Materials S3) show that the SiO2 content ranges from 53.60 to 56.14 wt. %, with an average of 54.75 wt. %; Al2O3 content ranges from 0.12 to 1.72 wt. %, with an average of 0.57 wt. %; FeO content ranges from 0.19 to 4.77 wt. %, with an average of 2.71 wt. %; MgO content ranges from 15.00 to 18.33 wt. %, with an average of 16.46 wt. %; and CaO content ranges from 23.67 to 24.53 wt. %, with an average of 24.20 wt. %. The composition of pyroxene in Baoshan is mainly diopside (Di82–99Hd0.6–15Jo0–3.2) (Figure 10b).

4.4. Trace Element Geochemistry of Scheelite

The trace element compositions of the three generations of scheelite from the representative samples are summarized in Supplementary Materials S4. Mo, Sr, and Y are highlighted in Figure 12 due to the significant variance in their concentrations among the different scheelite types.
The trace element analyses included 21 spots for Sch I (6 from S-190-169-2, 8 from S-50-169-2, and 7 from S-50-153-2), 21 spots for Sch II (7 from S-190-169-2, 7 from S-50-169-2, and 7 from S-50-153-2), and 13 spots for Sch III (7 from S-190-169-2 and 6 from S-50-169-2), resulting in a total of 55 corresponding LA-ICP-MS analyses.
Overall, the trace element composition in scheelite is generally consistent across the different exploration lines (Lines 169 and 153). Sr shows a fluctuating evolution from Sch I to Sch II to Sch III, whereas other elements exhibit either increasing (Y and Ba) or decreasing (Nb and Mo) trends. The highest concentrations of Sr and Y and the lowest concentrations of Nb and Mo appear in Sch III. In general, the trace element concentration of scheelite does not vary significantly with depth, except for Mo, which breaks this pattern in Sch III.

5. Discussion

5.1. REE Pattern and Substitution Mechanism

Three primary substitution mechanisms for the incorporation of REE(+Y) into the crystal lattice of scheelite have been identified [56]: (1) 2Ca2+ = REE3+ + Na+; (2) Ca2+ + W6+ = REE3+ + Nb5+; and (3) 3Ca2+ = 2REE3+ + ☐Ca (Ca site vacancy). At the Baoshan deposit, two primary REE patterns in scheelite have been observed: right-dip and hump-shaped (Figure 13). Sch I and Sch II exhibit high LREE/HREE ratios and a slight negative Eu anomaly (Figure 13a,b,d). In contrast, Sch III is characterized by high total REE concentrations, with the hump center between Sm and Gd, and either a positive or negative Eu anomaly (Figure 13c). These patterns suggest the presence of two distinct REE substitution mechanisms.
Most spots of Sch I and Sch II fall near the 1:1 correlation line, with Na increasing alongside ∑REE + Y-Eu (Figure 14a). This suggests a Na-REE coupled substitution mechanism. However, a few spots deviate from this line, indicating that REEs might enter the scheelite lattice via the replacement of Ca site vacancies. The insufficient concentration of Na (average 13.5 ppm) and the weak correlation between Na and ∑REE + Y-Eu (Figure 14) [2] suggest that the Ca site vacancy substitution mechanism may be dominant. Sch III falls far below the 1:1 correlation line, further indicating the Ca site vacancy mechanism (Figure 14a). Figure 14b also supports this, showing low Nb and high ∑REE + Y-Eu concentrations across the three generations of scheelite. In this case, REEs of any size can enter the scheelite lattice in pairs without preference. Given the weak influence of other factors on the REE pattern [56], the REE pattern in scheelite from the Baoshan deposit likely reflects the composition of the ore-forming fluid during its precipitation [58].

5.2. Redox State of Ore-Forming Fluids

The composition of pyroxene in Baoshan is predominantly diopside, indicating that the mineralizing hydrothermal fluids during the early skarn stage were generally under high oxygen fugacity conditions [59]. Additionally, changes in the U content in garnet can indicate variations in the oxygen fugacity of the mineralizing fluids, as low oxygen fugacity conditions tend to increase the U content in garnet [60,61]. The U content in Grt1 shows an increasing trend from the core to the rim; in Grt2, the U content is significantly higher than in Grt1 (Figure 15). This indicates that the oxygen fugacity of the mineralizing fluids during the early skarn stage gradually decreased.
Compared to the Eu3+ or the other REE3+, Eu2+ can be incorporated into the scheelite directly due to having the same charge as Ca2+. The EuN vs. δEu diagrams can well distinguish the ionic valences of Eu in scheelite (Figure 16a) [56]. Eu3+ and Eu2+ can be identified by the data array with a slope of one or not, respectively. Sch I is characterized by moderate EuN* and weak Eu anomalies with all points falling on the 1:1 correlation line, indicating that Eu3+ is dominant and Sch I precipitated from an oxidized hydrothermal fluid. The feature of Sch II is not similar to Sch I with a relatively lower EuN/δEu ratio and it diverges from the slope of one, which suggests a relative Eu2+-dominant fluid and weak oxidized conditions. Sch I and Sch II from Line 153 exhibit similar trends with relatively lower EuN/δEu ratios, implying that they precipitated in a more reduced environment (Figure 16a). Compared to Sch I and Sch II, Sch III features significant high EuN* characteristics with a strong correlation between EuN and δEu, which indicates that Eu3+ is dominant in the hydrothermal fluid. However, there was significantly negative Eu anomalies of Sch III in the deeper parts (Figure 16c), indicating Sch III was probably inherited from the hydrothermal fluid. No evidence indicates the existence of the pH transformation or the precipitation of the Eu-rich minerals, which are considered as the other factors controlling the Eu [58], due to the similar Eu anomalies during precipitation of the Sch I and Sch II and weak negative Eu anomalies of garnet at the Baoshan deposit (Figure 16c).
As mentioned above, as a redox-sensitive element, Mo is commonly used to identify redox conditions [2] and is favored to enter scheelite as Mo6+ under oxidizing conditions or precipitated as molybdenite under reducing conditions. The gradual decrease in concentration of Mo during the formation of Sch I and Sch II, implying a progressive decrease in oxidation conditions (Figure 16d), is consistent with the result of Eu. However, the sharp decrease in Mo concentration in the deeper Sch III indicates a significant reduction in oxygen fugacity at depth (Figure 16d).

5.3. Sources and Evolution of Ore-Forming Fluids

In skarn deposits, the end-member compositions of garnet and pyroxene can be used to identify the types of ore-forming metals [21]. The garnet and pyroxene from the Baoshan Cu-polymetallic deposit mostly fall within the range typical for skarn Cu deposits (Figure 10a,b). Their compositions differ significantly from those found in typical skarn Sn, Mo, and Zn deposits (Figure 10a,b), suggesting that there may not be an overprint of a Sn polymetallic mineralization system in the Baoshan deposit. This is consistent with the macroscopic mineralogical characteristics, which lack cassiterite and stannite. In the late skarn stage, the first two generations of the scheelite (Sch I and Sch II) and shallow Sch III were identified to be skarn-type scheelite (Figure 17a,b), whereas deep Sch III was suggested to be a vein-type scheelite (Figure 17a,b).
The REE distribution pattern of garnet can effectively reflect the pH conditions of the mineralizing fluid. Garnet formed under near-neutral conditions typically exhibits an REE distribution pattern characterized by enrichment in HREE, depletion in LREE, and a negative or absent Eu anomaly. Garnet formed under weakly acidic conditions has an REE distribution pattern more controlled by Cl− ions in the fluid, with Eu present as Eu Cl 4 2 − , enhancing the stability of Eu2+ in the fluid and leading to a positive Eu anomaly in garnet [17,63]. The REE distribution patterns of the two generations of garnet in the Baoshan deposit are similar, showing relative enrichment of LREE, depletion of HREE, and weak negative or absent Eu anomalies (Figure 11a,b). This indicates that during the early skarn stage, when garnet formed, the mineralizing hydrothermal fluid was weakly acidic in pH.
The narrower Y/Ho ratios of Sch I, Sch II, and Sch III indicate that they may precipitate in a single fluid (Figure 16b). As mentioned above, the scheelite from the Baoshan deposit may preserve the REE pattern of the ore-forming fluid owing to its substitution mechanisms. The REE pattern of granodiorite porphyry is similar to those of Sch I and Sch II, exhibiting in a right-dip appearance, which indicates that Sch I and Sch II may be crystallized from the magmatic hydrothermal fluid associated with the granodiorite porphyry (Figure 13c) [64]. On the contrary, the hump-shaped REE pattern in Sch III and its independence from the Na-REE coupled substitution, as well as the congruent variation of LREE between Sch III and garnet (Figure 11a,b and Figure 12c), suggest that remobilization of REE from the garnet may be inherited by Sch III. High concentration of Sr in scheelite can be attributed to fluid–rock interactions [65,66] and the mixing with meteoric water [67]. Sch III should have been expected to obtain more Sr and a wide range of variation in Eu during the fluid–rock reactions with the limestone of the Shidengzi Formation due to its high Sr and low Eu characteristics (Figure 16c) [57,66]. The relatively stable Y/Ho ratio from Sch I to Sch III indicates that the increase in Sr content is not due to the addition of meteoric water. The increase in Sr content in Sch III may be related to water–rock interactions.

6. Conclusions

(1)
In the early skarn stage of the Baoshan deposit, two generations of garnet (Grt1 and Grt2) were identified, with Grt1 having a coarse granular texture and distinct zonation, and Grt2 cutting across Grt1 as veinlets. Additionally, pyroxene in the early skarn stage coexists with Grt1 and is similarly cut by Grt2. In the late skarn stage, three generations of scheelite (Sch I, Sch II, and Sch III) were identified. Sch I and Sch II appear as anhedral to subhedral grains, while Sch III is predominantly found in veinlets.
(2)
In Sch I and Sch II, most of the REEs enter the scheelite lattice via the Na-REE coupled substitution mechanism, with a smaller portion entering by substituting Ca vacancies. In Sch III, the substitution mechanism involving Ca site vacancies may dominate.
(3)
During the early skarn stage, the oxygen fugacity of the fluid gradually decreased from Grt1 and pyroxene to Grt2. During the late skarn stage, the fluid oxygen fugacity shows little change from Sch I and Sch II to Sch III in the shallow part, while it significantly decreases from Sch I and Sch II to Sch III in the deeper part.
(4)
The garnet and pyroxene from the Baoshan deposit align with typical skarn Cu deposit compositions, while scheelite in the late skarn stage shows Sch I, Sch II, and shallow Sch III as skarn-type and deep Sch III as vein-type scheelite. In the early skarn stage, the pH of the ore-forming hydrothermal fluid was weakly acidic. Sch I, Sch II, and Sch III originated from the magmatic hydrothermal fluids related to the Baoshan granite porphyry. The formation of Sch III also involved water–rock interactions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14090907/s1. Table S1: The major element characteristics of garnet in Baoshan Cu-polymetallic deposit; Table S2: The trace element characteristics of garnet in Baoshan Cu-polymetallic deposit; Table S3: The major element characteristics of pyroxene in Baoshan Cu-polymetallic deposit; Table S4: LA-ICP-MS Data of Scheelite from the Baoshan Deposit.

Author Contributions

Conceptualization, P.Z. and K.C.; methodology, J.-K.Z.; software, J.-K.Z.; validation, Z.-F.L. and K.C.; formal analysis, M.-P.H.; investigation, J.-K.Z.; resources, Z.-F.L.; data curation, Y.-S.L.; writing—original draft preparation, P.Z.; writing—review and editing, K.C. and J.-K.Z.; visualization, Z.-F.L.; supervision, Y.-S.L.; project administration, K.C.; funding acquisition, P.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Laboratory Project of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education (Project number: 2023YSJS02); Hunan Provincial Department of Education Scientific Research Project (Project number: 22C0830); "Chu Yi" High level Teacher Team Construction Project of 2022 Vocational Education in Hunan Province.

Data Availability Statement

Data are contained within the Supplementary Materials and article.

Conflicts of Interest

All authors (Ping Zheng, Ke Chen, Jun-Ke Zhang, Zhong-Fa Liu, Yong-shun Li, and Ming-peng He) declare that they have no financial and personal relationships with other people or organizations that can inappropriately influence their work. There is no professional or other personal interest of any nature or kind in any product, service, or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

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Figure 1. (a) Sketch map of the regional tectonic framework. (b) Distributions of the Jurassic granites and deposits in the South China Block. Modified from Zhou et al. [36].
Figure 1. (a) Sketch map of the regional tectonic framework. (b) Distributions of the Jurassic granites and deposits in the South China Block. Modified from Zhou et al. [36].
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Figure 2. Sketch map of the regional geology showing the tectonic framework and mineralization resources. Modified from Li et al. [43].
Figure 2. Sketch map of the regional geology showing the tectonic framework and mineralization resources. Modified from Li et al. [43].
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Figure 3. Simplified geologic map of the Baoshan Cu-polymetallic deposit. After Zhang et al. [32].
Figure 3. Simplified geologic map of the Baoshan Cu-polymetallic deposit. After Zhang et al. [32].
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Figure 4. Hydrothermal alteration characteristics of the Baoshan deposit. (a) Development of potassic alteration, epidotization, chloritization, and silicification in the ore-forming granodiorite porphyry. (b) Skarnization and silicification are closely associated with Cu mineralization. Ep = epidote; Qtz = quartz; Chl = chlorite; Ccp = chalcopyrite; Grt = garnet; Cb = carbonate minerals.
Figure 4. Hydrothermal alteration characteristics of the Baoshan deposit. (a) Development of potassic alteration, epidotization, chloritization, and silicification in the ore-forming granodiorite porphyry. (b) Skarnization and silicification are closely associated with Cu mineralization. Ep = epidote; Qtz = quartz; Chl = chlorite; Ccp = chalcopyrite; Grt = garnet; Cb = carbonate minerals.
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Figure 5. Cross section along the No. 169 prospecting line from the Baoshan Cu-polymetallic deposit. After Li et al. [51].
Figure 5. Cross section along the No. 169 prospecting line from the Baoshan Cu-polymetallic deposit. After Li et al. [51].
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Figure 6. Mineral paragenesis of Baoshan deposit.
Figure 6. Mineral paragenesis of Baoshan deposit.
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Figure 7. Macroscopic and microscopic features of the two generations of garnet and pyroxene in the Baoshan deposit. (a) Replacement and cross-cutting relationship between diopside and garnet. (b) Grt1 develops a coarse-grained granular structure (−). (c) Grt2 replaces Grt1; transmitted light (−). (d) Diopside replacing Grt1 in a bay-shaped pattern; transmitted light (+). Grt = garnet; Di = diopside; Cb = carbonate minerals; Py = pyrite.
Figure 7. Macroscopic and microscopic features of the two generations of garnet and pyroxene in the Baoshan deposit. (a) Replacement and cross-cutting relationship between diopside and garnet. (b) Grt1 develops a coarse-grained granular structure (−). (c) Grt2 replaces Grt1; transmitted light (−). (d) Diopside replacing Grt1 in a bay-shaped pattern; transmitted light (+). Grt = garnet; Di = diopside; Cb = carbonate minerals; Py = pyrite.
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Figure 8. Three representative samples of scheelite from the Baoshan deposit. (a) Scheelite-bearing quartz veinlet cut the garnet skarn. (b) Scheelite in the skarn-hosted ores coexisting with retrograde minerals (epidote and chlorite). (c) Disseminated scheelite in the sulfide ores. (d) Scheelite has been replaced by epidote and chlorite; reflected light. (e) Scheelite exhibits an island appearance, cut by pyrite and magnetite; reflected light. (f) Coarse-grained scheelite coexists with the latter colloform pyrite and is altered along the fractures. Sch = scheelite; Py = pyrite; Grt = garnet; Qtz = quartz; Cb = carbonate; Ep = epidote; Chl = chlorite; Act = actinolite; Hem = hematite; Mag = magnetite; Ccp = chalcopyrite.
Figure 8. Three representative samples of scheelite from the Baoshan deposit. (a) Scheelite-bearing quartz veinlet cut the garnet skarn. (b) Scheelite in the skarn-hosted ores coexisting with retrograde minerals (epidote and chlorite). (c) Disseminated scheelite in the sulfide ores. (d) Scheelite has been replaced by epidote and chlorite; reflected light. (e) Scheelite exhibits an island appearance, cut by pyrite and magnetite; reflected light. (f) Coarse-grained scheelite coexists with the latter colloform pyrite and is altered along the fractures. Sch = scheelite; Py = pyrite; Grt = garnet; Qtz = quartz; Cb = carbonate; Ep = epidote; Chl = chlorite; Act = actinolite; Hem = hematite; Mag = magnetite; Ccp = chalcopyrite.
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Figure 9. CL images of scheelite from the Baoshan deposit showing three generations and locations for the LA-ICP-MS. (a,b) Sch I and Sch II. (c) Sch I, Sch II, and Sch III. (d) Sch I and Sch III. (e) Scheelite from the −190 m levels of Line 169. (f) Scheelite from the 50 m levels of Line 169. (g) Scheelite from the 50 m levels of Line 153.
Figure 9. CL images of scheelite from the Baoshan deposit showing three generations and locations for the LA-ICP-MS. (a,b) Sch I and Sch II. (c) Sch I, Sch II, and Sch III. (d) Sch I and Sch III. (e) Scheelite from the −190 m levels of Line 169. (f) Scheelite from the 50 m levels of Line 169. (g) Scheelite from the 50 m levels of Line 153.
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Figure 10. Triangular diagrams of garnet (a) and pyroxene (b) end-member compositions in the Baoshan deposit. The Au, Cu, Zn, Sn, W, and Mo skarn deposit fields are from Meinert et al. [54]. Pyr = Pyrope; Spe = Spessartine; Alm = Almandine; Uva = Uvarovite; Gro = Grossular; And = Andradite; Jo = Johannsenite; Di = Diopside; Hd = Hedenbergite.
Figure 10. Triangular diagrams of garnet (a) and pyroxene (b) end-member compositions in the Baoshan deposit. The Au, Cu, Zn, Sn, W, and Mo skarn deposit fields are from Meinert et al. [54]. Pyr = Pyrope; Spe = Spessartine; Alm = Almandine; Uva = Uvarovite; Gro = Grossular; And = Andradite; Jo = Johannsenite; Di = Diopside; Hd = Hedenbergite.
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Minerals 14 00907 g011
Figure 11. REE distribution pattern of Grt1 (a) and Grt2 (b) from the Baoshan deposit, chondrite-normalized according to the literature [55].
Figure 11. REE distribution pattern of Grt1 (a) and Grt2 (b) from the Baoshan deposit, chondrite-normalized according to the literature [55].
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Minerals 14 00907 g012
Figure 12. Plot of Mo (a), Sr (b), and Y (c) contents of scheelite from the Baoshan deposit, grouped into three generations.
Figure 12. Plot of Mo (a), Sr (b), and Y (c) contents of scheelite from the Baoshan deposit, grouped into three generations.
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Minerals 14 00907 g013
Figure 13. Chondrite-normalized REE patterns of Sch I from Line 169 (a), Sch II from Line 169 (b), Sch III from Line 169 (c), and Sch I + Sch II from Line 153 (d) from the Baoshan deposit. The chondrite-normalized REE patterns of granodiorite porphyry, garnet, and limestone of the Shidengzi Formation are also shown for comparison [31,35,37,38,57].
Figure 13. Chondrite-normalized REE patterns of Sch I from Line 169 (a), Sch II from Line 169 (b), Sch III from Line 169 (c), and Sch I + Sch II from Line 153 (d) from the Baoshan deposit. The chondrite-normalized REE patterns of granodiorite porphyry, garnet, and limestone of the Shidengzi Formation are also shown for comparison [31,35,37,38,57].
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Minerals 14 00907 g014
Figure 14. (a) Na and (b) Nb vs. ∑REE + Y-Eu diagram for three generations of scheelite from the Baoshan deposit.
Figure 14. (a) Na and (b) Nb vs. ∑REE + Y-Eu diagram for three generations of scheelite from the Baoshan deposit.
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Figure 15. U vs. X And Grt diagram of garnet from the Baoshan deposit.
Figure 15. U vs. X And Grt diagram of garnet from the Baoshan deposit.
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Minerals 14 00907 g016
Figure 16. (a) EuN vs. δEu diagram, (b) Y/Ho vs. La/Lu diagram, (c) Sr vs. δEu diagram, and (d) Mo vs. δEu diagram for different types of scheelite from the Baoshan deposit. EuN = Eusample/Euchondrite; δEu = EuN/(SmN\NdN)1/2. Chondrite values according to the literature [55].
Figure 16. (a) EuN vs. δEu diagram, (b) Y/Ho vs. La/Lu diagram, (c) Sr vs. δEu diagram, and (d) Mo vs. δEu diagram for different types of scheelite from the Baoshan deposit. EuN = Eusample/Euchondrite; δEu = EuN/(SmN\NdN)1/2. Chondrite values according to the literature [55].
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Minerals 14 00907 g017
Figure 17. (a) Y vs. Ho diagram and (b) LREE-MREE-HREE diagram for scheelite from the Baoshan deposit. After Ding et al. [62].
Figure 17. (a) Y vs. Ho diagram and (b) LREE-MREE-HREE diagram for scheelite from the Baoshan deposit. After Ding et al. [62].
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Zheng, P.; Chen, K.; Zhang, J.-K.; Liu, Z.-F.; Li, Y.-S.; He, M.-P. The Fluid Evolution in the Skarn Stages of the Baoshan Skarn Cu-Polymetallic Deposit, South China. Minerals 2024, 14, 907. https://doi.org/10.3390/min14090907

AMA Style

Zheng P, Chen K, Zhang J-K, Liu Z-F, Li Y-S, He M-P. The Fluid Evolution in the Skarn Stages of the Baoshan Skarn Cu-Polymetallic Deposit, South China. Minerals. 2024; 14(9):907. https://doi.org/10.3390/min14090907

Chicago/Turabian Style

Zheng, Ping, Ke Chen, Jun-Ke Zhang, Zhong-Fa Liu, Yong-Shun Li, and Ming-Peng He. 2024. "The Fluid Evolution in the Skarn Stages of the Baoshan Skarn Cu-Polymetallic Deposit, South China" Minerals 14, no. 9: 907. https://doi.org/10.3390/min14090907

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