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Article

Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential

by
Fan Yu
1,
Qihai Shu
1,2,*,
Xudong Niu
1,
Kai Xing
1,3,
Linlong Li
1,4,
David R. Lentz
3,
Qingwen Zeng
1 and
Wenjie Yang
1
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
2
Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
3
Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
4
School of Earth and Space Sciences, Peking University, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(5), 456; https://doi.org/10.3390/min10050456
Submission received: 11 April 2020 / Revised: 8 May 2020 / Accepted: 15 May 2020 / Published: 18 May 2020
(This article belongs to the Special Issue Magmatic–Hydrothermal Alteration and Mineralizing Processes)
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Abstract

:
The Xianghualing skarn Sn deposit in the southwestern part of the southern Hunan Metallogenic Belt is a large Sn deposit in the Nanling area. In this paper, the garnet has been analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to obtain the concentrations of the major and trace elements. The results reveal that the garnets from the Xianghualing deposit mainly belong to andradite-grossular (grandite) solid solution and are typically richer in Al than in Fe. They show enrichment in heavy rare earth elements (HREEs) and notably lower light rare earth elements (LREEs), and commonly negative Eu anomalies, indicative of a relatively reduced formation environment. The garnets have high Sn concentrations between 2313 ppm and 5766 ppm. It is also evident that there is a positive correlation between Sn and Fe, suggesting that Sn4+ substitutes into the garnets through substituting for Fe3+ in the octahedral position. Combined with previous studies, it can be recognized that the Sn concentrations of garnet in skarn Sn deposits are generally high, whereas the W concentrations are relatively low. This is just the opposite in garnets from skarn W deposits that typically have high W, but low Sn concentrations. In polymetallic skarn deposits with both economic Sn and W, the concentrations of both metals in garnets are relatively high, although varying greatly. Therefore, the Sn and W concentrations in garnets can be used to evaluate a skarn deposit’s potential to produce Sn and (or) W mineralization, which is helpful in exploration.

1. Introduction

The Xianghualing skarn Sn deposit is an important skarn deposit in the center of the Nanling Range, which is situated in the northern part of Linwu County, southern Hunan Province. It contains 0.17 Mt Sn with a grade ranging from 0.93% to 1.39%, and ranks the 10th largest among the Sn skarns in China [1]. The deposit has been widely investigated since the 1980s, which provides systematic understanding of the regional metallogenic background [2], geological characteristics, ore genesis [3,4,5], geochronology, geochemistry of rocks and ores in the mining district [6,7], and crystal chemistry of Sn-bearing minerals [8]. Previously, many attempts have been made to reveal the Sn mineralization potential by geochemical studies on Sn-bearing magmatic minerals (e.g., titanite) in the Xianghualing granite [9,10,11], but to date relatively little attention has been paid to skarn minerals. Garnet is one of the most common minerals in skarn deposits [12,13,14,15] and is also a Sn-rich mineral [16,17]. The compositional variations of garnet can reflect its specific ore-forming environment and processes [18,19,20,21,22]. In addition, the contents of some metal elements (e.g., Sn, W, and Mo) in garnet can also indicate their abundances in the original hydrothermal fluids, which, in turn, can be used in evaluating the potential of economic mineralization of these metals [23,24,25,26,27].
This paper reports the major and trace elements of the garnets from the Xianghualing skarn Sn deposit, which provide insights into the formation conditions of garnets and also the substitution mechanisms of Sn into garnets. Besides, on the basis of a comparison of the garnet compositions from Xianghualing with several other previously studied Sn (–W) deposits, we suggest that the Sn and W concentrations in garnets could be important indicators for Sn and/or W mineralization and therefore can be potentially used in guiding exploration.

2. Geological Setting

2.1. Regional Geology

The studied area in the middle of the Nanling tectono-magmatic belt has a history of multicycle tectonic evolution including several magmatic episodes (Figure 1), and it also contains abundant mineral resources [28]. The region is characterized by exposed shallow-marine clastic sedimentary rocks from Devonian to Triassic, with a small amount of carbonate rocks. Folds and faults are widely developed in this area, resulting in the uplift of basement and the metamorphism of Neoproterozoic to Cambrian (Sinian) sedimentary rocks, the strong folding of Devonian-Permian carbonate rocks, and the general deformation of Upper Permian-Triassic sequences. The pre-Jurassic tectonic-magmatic activities were overprinted by Jurassic-Cretaceous tectonic events, resulting in the formation of NNE-directed rift-related basins and deep faults [29]. NW- and NE-trending faults form the principal geological framework of the region, among which the Chenzhou-Shaoyang Fault and the Chaling-Linwu Fault are the major structures controlling the distribution of granitic intrusions and mineral deposits [30]. The magmatic rocks in the study area were mainly formed in three stages: Silurian, Triassic, and Jurassic, and among them the Jurassic magmatism is the most extensive [31,32,33,34].

2.2. Geology of the Deposit

The Xianghualing deposit is located in the northern part of the southern Hunan Province, which is near the center of the Nanling Range [29]. It is one of the largest Sn (–W) deposits in the southwestern part of the South Hunan Metallogenic Belt, and tectonically it lies in the intersection area between the NE-trending, deep-seated Chenzhou-Linwu fault zone and the S-trending fault zone. The study area appears as a NE-trending tectono-magmatic dome, which is due to Jurassic magmatic diapirism under a regional extensional setting [6]. The Cambrian slate and meta-sandstone are the main components of the core of the Xianghualing deposit (Figure 2), and the flanks are mainly composed of the Devonian clastic rocks, including the Middle Devonian Tiaomajian Formation conglomerates, sandstones, and shales, the Middle Devonian Qiziqiao Formation limestone and dolomites, and the Upper Devonian Shetianqiao Formation dolomitic limestones and sandstones. In addition, Carboniferous carbonates and Jurassic clastic rocks are also present locally [35]. The Cambrian and Middle Devonian strata are considered to be the major host rocks in the Xianghualing Sn deposit [29,36]. The Laiziling and Jianfengling stocks are the two largest granitic stocks in this area, which intrude into Cambrian meta-sandstone and Devonian carbonate of the Tongtianmiao dome, respectively. The Laiziling intrusion can be divided into four vertical zones from the base to the top: albite-microcline granite zone, albite granite zone, greisen zone, and pegmatite zone [30,32]. The U–Pb zircon age determined from the Laiziling Granite is 155 ± 2 Ma [37].
The Xianghualing deposit is a stratabound skarn Sn deposit [5], and the orebodies are mainly stratiform, pipe- and vein-like in shape [38,39,40]. Field investigations have revealed three major stages of skarn alteration and metal mineralization in the Xianghualing deposit: (1) A prograde pre-mineralization skarn stage (Figure 3a), followed by (2) a dominant Sn-mineralization stage with significant retrograde alteration, and finally (3) a subordinate Pb–Zn sulfide stage. Typical minerals of the prograde stage consist of garnet, pyroxene and olivine. The Sn mineralization stage is mainly composed of cassiterite, with retrograde minerals including actinolite, tremolite, chlorite, vesuvianite and quartz. Retrograde alteration often overprinted prograde garnet-bearing assemblages. The Pb–Zn sulfide stage is composed of Pb–Zn sulfides, with minor pyrite, pyrrhotite and arsenopyrite (Figure 3b), as well as coexistent phlogopite, chlorite, fluorite, quartz and calcite (Figure 3). The ore mineralization is generally zoned in the altered country rocks surrounding the plutons: Sn mineralization occurs in skarns proximal to the granites and Pb–Zn sulfides in more distal areas away from the granites [30]. The cassiterite U-Pb dating and muscovite 40Ar–39Ar dating for the Xianghualing deposit gave two ages of 157 ± 6 Ma and 154.0 ± 1.1 Ma [35,38], respectively; these results are consistent with the zircon U–Pb dating results of the Laiziling intrusion, implying that the magmatism and mineralization could be genetically related and then quickly cooled below the muscovite Ar retention blocking temperature (<350 °C).

3. Sampling and Analytical Methods

The samples analyzed in this study were all collected from the Xianghualing mining area (Figure 2). Generally, the garnet could be divided into two types. The first type garnet is euhedral and larger in size (>3 mm) (Figure 3c–e); the second type is anhedral and much smaller (<100 μm) (Figure 3f), and is clearly overprinted by later retrograde alteration. Based on microscopic observation, neither garnet type has obvious growth zoning. In this study, three garnet samples (XHL-1 and XHL-2 of the first type, and XHL-3 of the second type) have been selected for laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) element analysis.
The analysis was performed at the Ore Deposit and Exploration Centre (ODEC), Hefei University of Technology, Hefei, China, using an Agilent 7900 ICP-MS coupled with a 193 nm ArF excimer Resonetics RESOlution laser-ablation system, named PhotonMachines Analyte HE. Helium (0.90 L/min) and argon (0.87 L/min) were used as the carrier gas and make-up gas, respectively. The two gases were mixed through a T-joint before entering ICP. The laser was operated at 8 Hz, with a laser fluency of 2 J/cm2, and a spot size of 30 μm. Each analysis included 20 s of background acquisition (gas blank), 40 s of data acquisition, and again 20 s of background signal, and therefore the total time for every spot analysis was 80 s [41]. The external standards were BCR-2G, NIST610, and NIST612, analyzed once every 10 analyses in order to correct the time-dependent drift of sensitivity and mass discrimination. The laser for LA-ICP-MS mapping was operated at 10 Hz, with a sample movement speed of 40 μm/s. The mapping was conducted in line scanning style, with each line being parallel and the size consistent with the laser ablation spot [41]. Standards were analyzed in duplicate at the start and end of each mapping run, which included reference glasses NIST610, NIST612 and garnet standard MON-GT. The total analysis time of mapping was approximately 2 h for each sample [42]. Elements analyzed in this study include Mg, Al, Si, Ca, Ti, Mn, Fe, Cu, Zn, Y, Mo, Sn, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, W, and Pb. The off-line data processing was performed using the ICPMSDataCal program [43]. Trace element compositions were calibrated against multiple-references materials without applying internal standardization. The sum of all element concentrations, expressed as oxides according to their oxidation states, is normalized to be 100 wt % [43].

4. Results

4.1. Major Element Geochemistry

The major element compositions of the garnets from the Xianghualing skarn Sn deposit are summarized in Table 1, and a full list of the data are given in Table S1 in Supplementary Materials. The three analyzed garnet samples share similar compositions (Table 1). The results indicate that the main components of the Xianghualing garnets are SiO2, CaO, Al2O3, and Fe2O3T, and the average concentrations are 38.31 wt % (range 37.19–39.34 wt %) for SiO2, 31.82 wt % (30.73–32.98 wt %) for Al2O3, 15.39 wt % (14.04–16.85 wt %) for Al2O3, and 11.70 wt % (9.43–13.30 wt %) for Fe2O3, respectively, with lower amounts of MnO (average 1.43 wt %), TiO2 (average 0.30 wt %), and MgO (average 0.12 wt %).
The data show that the garnets from the Xianghualing deposit generally form from a grossular-andradite solid solution. They are Al-rich and dominated by grossular (60.59–70.75 mol %) and less andradite (18.91–29.76 mol %), with minor amounts of spessartine, pyrope, and almandine accounting for less than 5.5 mol % of the total (Table 1; Figure 4). In general, the contents of grossular and andradite are negatively correlated, but there is no obvious compositional zoning from the core to the rim of an individual garnet grain, although in some grains there are narrow rims surrounding the cores based on the mapping results (Figure 5). The Xianghualing garnets are similar to other skarn Sn deposits worldwide in composition [44] as shown in Figure 4.

4.2. Trace Element Geochemistry

The concentrations of trace elements and rare earth elements of the garnets from the Xianghualing skarn Sn deposit are summarized in Table 2 and listed in Table S1. The three garnet samples show little compositional variations. All garnets contain very low concentrations of Cu (<0.31 ppm), Mo (<0.46 ppm), and Pb (<5.50 ppm), but have higher and variable Sn concentrations from 2314 to 5766 ppm (4295 ppm on average). The concentrations of W vary from 0.22 to 30.0 ppm, Zn from 7.95 to 12.6 ppm, and Pb from 0 to 0.55 ppm.
The rare earth elements (REEs) concentrations of the Xianghualing garnets are all very low. Among them, the average Nd, Sm, Gd, Dy, Er, and Yb contents are generally higher than 1 ppm, whereas the concentrations of all the remaining REE elements (including La, Ce, Pr, Eu, Tb, Ho, Tm, and Lu) are lower than 1 ppm. The ∑REE abundances in garnets range from 6.97 to 30.2 ppm, with an average of 14.7 ppm. Both LREE/HREE and (La/Yb)N ratios are generally lower than 1, ranging from 0.01 to 0.75 and from 0 to 0.04, respectively (Table 2). The garnets display obvious negative Eu anomalies (Eu/Eu* of 0.15–0.79), except for one spot that has a Eu/Eu* of 2.37. Similarly, Ce also shows negative anomalies (Ce/Ce* = 0–0.71).

5. Discussion

5.1. REE Patterns in Garnet and Their Significance

The REE enrichments in skarn minerals like garnet are controlled by several factors, including the REE concentrations in the source magmas and/or wallrocks (typically carbonate), the REE concentrations in the hydrothermal fluids forming the skarns, and the mobility of REEs (i.e., their ability to be released during hydrothermal alteration) [45,46]. In addition, several physicochemical parameters of the hydrothermal fluids responsible for the skarn formation, including salinity, pH, fO2, XCO2, pressure, and temperature, have also been suggested to be critical in the REE incorporation into garnets [47,48,49]. As shown in Figure 6, the chondrite-normalized REE patterns [50] of the garnets from the Xianghualing skarn Sn deposit indicate that the garnets are generally enriched in heavy REE (HREE) relative to light REE (LREE), with pronounced negative Eu anomalies. This is also similar to the garnets from many other Sn-bearing skarn deposits in China, such as the Shizhuyuan skarn W–Sn–Mo–Bi deposit [51,52] and the Jinchuantang skarn Sn–Bi deposit [53] in Hunan Province, the Gejiu skarn Sn deposit in Yunnan Province [54], and the Huanggangliang skarn Sn–Fe deposit in Inner Mongolia [55]. As documented in previous studies, such REE patterns indicate that the incorporation of the REEs into garnets was controlled by crystal chemistry through isomorphic substitution [19,53], and that the HREEs with smaller ionic radii are more compatible to be accommodated in the octahedral coordination site of garnet lattice than the LREEs with larger ionic radii [56,57]. The REEs in garnets of the Xianghualing deposit display similar patterns and therefore are believed to be controlled by the crystal chemical structure.
The obviously negative Eu anomalies of the garnets from the Xianghualing deposit imply a relatively low oxygen fugacity of the mineralizing fluid. Under reduced conditions, unlike other REEs that only have +3 valence (and +4 valence for Ce), Eu can also occur as Eu2+. The reduced Eu2+ (1.25 Å) can less preferentially substitute for Ca2+ (1.12 Å) than the oxidized cation Eu3+ (1.07 Å) in the octahedral position [58,59]. As a consequence, negative Eu anomalies would be generated in a reduced environment. On the contrary, many Cu-rich skarn deposits formed in oxidized conditions [1,60], and as a result, they show typical positive Eu anomalies (Figure 7). To sum up, the negative Eu anomalies of the Xianghualing garnets indicate a reduced condition of the magmatic systems and their exsolved hydrothermal fluids. This is consistent with many other hydrothermal Sn deposits in China and worldwide, which are commonly related to highly reduced granitic system [1,60,61].

5.2. Substitution of Sn into Garnet

Garnet has a general chemical formula X3Y2Z3O12, where X represents divalent cations in eightfold coordination, Y represents trivalent cations in octahedral coordination, and Z is dominantly Si in tetrahedral coordination [63,64]. According to the results listed in Table 1, the Xianghualing garnet belongs to the series of grossular-andradite solid solution. It can be inferred that the X position of the eightfold coordination is mainly Ca2+, containing a small amount of Fe2+, Mn2+, Mg2+, and other divalent cations (e.g., REEs), and the Y position is mainly occupied by Fe3+ and Al3+.
The incorporation of trace elements into garnet crystals, like the REEs proposed above, is essentially controlled by crystal chemistry [65]. According to the Goldschmidt’s first rule, the ionic radius and charge mismatch between the host cation and its substituent, at a given site, exert the principal control on substitution mechanisms [19,60]. The ionic radius of Sn4+ is 0.69 Å, which is close in size to Fe3+ (0.67 Å) in the octahedral site of garnet [60], making it possible for Sn4+ to substitute Fe3+. In this study, there is a generally positive correlation between the content of Fe and Sn in Xianghualing garnets based on the mapping results (Figure 5). This suggests that Sn4+ is likely to occupy the octahedral sites of garnet through replacement of Fe3+. The possible mechanism for such a substitution (Equation (1)) could be:
3Sn4+ + □ = 4Fe3+
Here □ is a divalent cations-site vacancy in the octahedral site [66].
It should be noted that the above substitution of Sn4+ for Fe3+ requires not only a modification of the crystal structure, but also charge balance. The charge difference between quadrivalent Sn and trivalent Fe makes it necessary to get a compensatory mechanism to maintain charge balance. As a result, two other cosubstitution mechanisms (Equations (2) and (3)) have also been proposed [52,67,68,69]:
Sn4+ + Fe2+ = 2Fe3+
Sn4+ + Mg2+ = 2Fe3+
As shown in Figure 8a, however, the atoms of divalent iron and the content of Sn do not show any positive correlations, indicating that the cosubstitution (Equation (2)) might be weak, although it cannot be fully ruled out. As for the Mg-Sn cosubstitution (Equation (3)), because of the relatively low Mg concentrations of the garnets in Xianghualing (Table 1) and the poor correlations between the Mg2+ and Sn content (Figure 8b), it is also unlikely to be the major diadochic substitution mechanism.
Based on the above observations, it is concluded that the incorporation of Sn into prograde skarn garnets in the Xianghualing deposit are mainly through the substitution 3Sn4+ + □ = 4Fe3+. Interestingly, as shown in the mapping results (Figure 5), the concentrations of Sn and W in the garnets are also positively correlated. This has also been confirmed by the LA-ICP-MS analysis results as shown in Figure 8c. This implies that the mechanism for W substitution into the garnet may be similar to that of Sn.
It should be noted that the ability of Sn substitution into garnet is controlled by the Sn contents in hydrothermal fluids and the partitioning coefficient between fluids and garnet. The partitioning coefficient is further related to the Sn availability in the hydrothermal system; the mineral composition into which Sn substitutes [16,70]; as well as the fluid temperature, pH, salinity, pressure, and the chlorine and possibly fluorine contents [70,71,72]. Based on the microthermometric data for primary fluid inclusions from the different Sn-bearing deposits [73,74,75,76], including Xi’ao skarn Cu–Sn deposit, Huanggang skarn Fe–Sn deposit, Shizhuyuan skarn W–Sn deposit and Xianghualing skarn Sn deposit, it can be concluded that they have similar formation temperatures of garnet (mainly 400–500 °C), salinities (mainly around 10 wt % NaCl equivalent) and pressures (around 500 bars). Hence, it is reasonable to conclude that these parameters of the Sn-bearing hydrothermal fluids have similar effects on the partitioning coefficient of Sn between fluids and garnets. Therefore, the ability of Sn substitution into garnet largely depends on the concentrations of Sn in the fluids.

5.3. Exploration Implications

The garnets of the Xianghualing deposit contain relatively high Sn concentrations (>2000 ppm), especially for the sample XHL-2-2, which reaches up to >5700 ppm (Table 2). Similarly high Sn concentrations in garnets have also been observed in other skarn Sn deposits from previous studies. The garnet from a skarn Sn polymetallic deposit in Finland contains up to 1.44 wt % SnO2 [77]. Also, the content of SnO2 in garnets at Doradilla, New South Wales, Australia is as high as 0.5–3.5 wt % [68]. The studies on several Sn-bearing skarn deposits in South China (e.g., Yaogangxian, Dachang, Xinlu, Debao, Gejiu, and Shizhuyuan) also show that the contents of SnO2 in garnets are generally in the range of 0.02–2 wt %, particularly in Gejiu, where the highest value can be up to 5.14 wt % [51,52]. For garnets from skarn deposits containing both Sn and W, including the Baiganhu skarn W-Sn deposit and the Huangshaping skarn W-Sn deposit, the LA-ICP-MS analysis results show that both are rich in Sn (Figure 9), with a maximum of 4990 ppm and 8364 ppm, respectively [23,65]. Combined with the results of this study, it can be generally concluded that the garnets in Sn-bearing skarn deposits are commonly enriched in Sn. A reasonable explanation given by Zhou et al [23] is that the substitution of Sn4+ for Fe3+ can more readily take place when the hydrothermal fluids responsible for the Sn mineralization contain elevated concentrations of Sn, regardless of other factors, such as redox state, pH, and temperature conditions [23]. As mentioned above, the abundance of Sn in garnet largely depends on the Sn concentrations in the hydrothermal fluids. Accordingly, the Sn content of garnet can be used to evaluate the potential of Sn mineralization in a skarn system.
As for the two skarn deposits containing both economic Sn and W (Baiganhu and Huangshaping), the W concentrations in garnets are also very high, although variable: the range is between <1 to 2092 ppm [23,65]. For the Weondong skarn W deposit (without Sn) from South Korea [78], the garnet W concentrations (5–760 ppm) are as high as in these Sn–W skarn deposits, and significantly higher than the Xianghualing skarn Sn deposit (Figure 9). However, the Sn concentrations (32–228 ppm) in Weondong garnets are lower than these Sn-bearing skarn deposits (Figure 8). These observations indicate that skarn Sn deposits commonly contain Sn-rich, but W-poor garnets, whereas skarn W deposits commonly contain W-rich, but Sn-poor garnets; for skarn deposits containing both Sn and W, the garnets may be enriched in both the two ore-forming elements. This might be applied in skarn occurrences where exploration is ongoing to reveal their Sn and/or W mineralization potential.

6. Conclusions

(1) Garnets in the Xianghualing skarn Sn deposit in South China belong to typical grossular-andradite solid solution. They are characterized by HREE enrichment, LREE depletion and negative Eu anomalies, indicative of a reduced magmatic-hydrothermal environment. The enrichment of Sn in garnet is mainly controlled by the substitution into the Fe3+ octahedral site.
(2) Garnets from skarn Sn deposits like Xianghualing are generally Sn-rich and W-poor, whereas those from skarn W deposits (e.g., Weondong) are W-rich and Sn-poor. Interestingly, the Sn and W concentrations in garnets from skarn Sn-W deposits can both be very high. Such observations are potentially useful in guiding exploration for these skarn occurrences without defined Sn and/or W resources to evaluate their potentials to form economic Sn and (or) W mineralization.

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/10/5/456/s1, Table S1: Summary of the major element compositions (in wt %) and trace and rare earth elements compositions (ppm) of the Xianghualing garnets.

Author Contributions

Q.S. designed the project; F.Y., L.L., Q.Z., and W.Y. collected the samples; F.Y., X.N., and K.X. performed the LA-ICP-MS analyses and reduced the data; F.Y. and Q.S. wrote and organized the paper, with a careful discussion and revision by D.R.L. All authors discussed the results and evaluated the data. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Natural Science Foundation of China (41973043, 41602083), the fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, the MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (MSFGPMR201804), and the 111 Project of the Ministry of Science and Technology (BP0719021). DL was supported by a NSERC Discovery grant.

Acknowledgments

We are grateful to Fangyue Wang for his assistance during the analyses. We also thank the three anonymous reviewers and an academic editor for their constructive comments on the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Regional geology of the central Nanling region and the location of the Xianghualing skarn Sn deposit, southeastern China (modified after [6,30]). (a): Tectonic map of South China, showing the location of southern Hunan Province; (b): Geological map of southern Hunan Province, showing the distribution of intrusive rocks and major W–Sn deposits.
Figure 1. Regional geology of the central Nanling region and the location of the Xianghualing skarn Sn deposit, southeastern China (modified after [6,30]). (a): Tectonic map of South China, showing the location of southern Hunan Province; (b): Geological map of southern Hunan Province, showing the distribution of intrusive rocks and major W–Sn deposits.
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Figure 2. Geological map of the Xianghualing skarn Sn deposit (modified after [6]).
Figure 2. Geological map of the Xianghualing skarn Sn deposit (modified after [6]).
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Figure 3. Photos of hand specimens and thin sections of samples from the Xianghualing skarn Sn deposit. (a) Massive garnet-bearing skarn in reddish brown; (b) massive sulfide ore containing pyrite and pyrrhotite; (c) an euhedral garnet crystal; (d) an euhedral garnet hydrothermally altered by later retrograde minerals; (e) the gaps between garnet grains filled with later calcite; (f) garnet grains postdated by later calcite and cassiterite. Abbreviations: Cal = calcite; Cst = cassiterite; Grt = garnet; Po = Pyrrhotite and Py = Pyrite.
Figure 3. Photos of hand specimens and thin sections of samples from the Xianghualing skarn Sn deposit. (a) Massive garnet-bearing skarn in reddish brown; (b) massive sulfide ore containing pyrite and pyrrhotite; (c) an euhedral garnet crystal; (d) an euhedral garnet hydrothermally altered by later retrograde minerals; (e) the gaps between garnet grains filled with later calcite; (f) garnet grains postdated by later calcite and cassiterite. Abbreviations: Cal = calcite; Cst = cassiterite; Grt = garnet; Po = Pyrrhotite and Py = Pyrite.
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Figure 4. Ternary diagram summarizing garnet compositions in the Xianghualing skarn Sn deposit. Dashed area indicates the garnet composition ranges for skarn Sn deposits worldwide [44]. Abbreviations: Ad = andradite; Gr = grossular; Sp = spessartine; Py = pyrope and Al = almandine.
Figure 4. Ternary diagram summarizing garnet compositions in the Xianghualing skarn Sn deposit. Dashed area indicates the garnet composition ranges for skarn Sn deposits worldwide [44]. Abbreviations: Ad = andradite; Gr = grossular; Sp = spessartine; Py = pyrope and Al = almandine.
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Figure 5. LA-ICP-MS element mapping results of (a) Fe, (b) Al, (c) Sn, and (d) W for a garnet crystal in Xianghualing.
Figure 5. LA-ICP-MS element mapping results of (a) Fe, (b) Al, (c) Sn, and (d) W for a garnet crystal in Xianghualing.
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Figure 6. Chondrite-normalized REE patterns of the garnets from Xianghualing. Dashed area indicates the Chondrite-normalized REE partition curves of the garnets from other Sn-rich deposits including Jinchuantang [53], Shizhuyuan [51] and Huanggangliang [55], which have been shown here for comparison. All data were normalized to the C1 chondrite values [50].
Figure 6. Chondrite-normalized REE patterns of the garnets from Xianghualing. Dashed area indicates the Chondrite-normalized REE partition curves of the garnets from other Sn-rich deposits including Jinchuantang [53], Shizhuyuan [51] and Huanggangliang [55], which have been shown here for comparison. All data were normalized to the C1 chondrite values [50].
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Figure 7. Garnet Eu/Eu* ratios of Xianghualing and several other skarn deposits. Note: The compiled skarn Sn-bearing deposits include Jinchuantang skarn Sn–Bi deposit [53] and Shizhuyuan skarn W–Sn–Mo–Bi deposit [51]. The Cu skarns include Hongniu-Hongshan [19] and Yongping [62] skarn Cu deposits.
Figure 7. Garnet Eu/Eu* ratios of Xianghualing and several other skarn deposits. Note: The compiled skarn Sn-bearing deposits include Jinchuantang skarn Sn–Bi deposit [53] and Shizhuyuan skarn W–Sn–Mo–Bi deposit [51]. The Cu skarns include Hongniu-Hongshan [19] and Yongping [62] skarn Cu deposits.
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Figure 8. Geochemical compositions of garnets from Xianghualing. Plots of (a) Sn (ppm) vs. Fe2+ (apfu), (b) Sn (ppm) vs. Mg2+ (apfu), and (c) Sn (ppm) vs. W (ppm). apfu = atoms per formula unit.
Figure 8. Geochemical compositions of garnets from Xianghualing. Plots of (a) Sn (ppm) vs. Fe2+ (apfu), (b) Sn (ppm) vs. Mg2+ (apfu), and (c) Sn (ppm) vs. W (ppm). apfu = atoms per formula unit.
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Figure 9. Binary plots of Sn vs. W concentrations of garnets from different deposits. Note: skarn W–Sn deposits include Baiganhu [23] and Huangshaping [65]. The data of the Weondong skarn W deposit are from [78].
Figure 9. Binary plots of Sn vs. W concentrations of garnets from different deposits. Note: skarn W–Sn deposits include Baiganhu [23] and Huangshaping [65]. The data of the Weondong skarn W deposit are from [78].
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Table
Table 1. Summary of the major element compositions (in wt %) of the Xianghualing garnet.
Table 1. Summary of the major element compositions (in wt %) of the Xianghualing garnet.
SampleXHL-1 (n = 7)XHL-2 (n = 10)XHL-3 (n = 11)
MaximumMinimumAverageMaximumMinimumAverageMaximumMinimumAverage
MgO0.120.090.110.120.090.110.180.090.14
Al2O315.7314.7715.2316.8515.1116.1716.0614.0414.79
SiO239.3437.8938.5838.2537.1937.8039.1137.9638.61
CaO32.2231.7131.9331.7730.7331.3232.9831.3032.21
TiO20.550.360.490.270.090.200.640.120.27
MnO1.441.281.371.661.401.521.641.261.39
FeO11.9310.6811.4513.3010.9411.8812.909.4311.70
Total99.3598.9899.1499.1698.9198.9999.3798.9599.1
Mg0.010.010.010.010.010.010.020.010.02
Al1.461.361.411.561.401.501.471.301.37
Si3.072.973.033.002.922.973.082.993.03
Ca2.712.672.682.672.582.632.772.642.71
Ti0.030.020.030.020.010.010.040.010.02
Mn0.100.090.090.110.090.100.110.080.09
Fe2+0.300.210.270.270.180.230.330.160.23
Fe3+0.560.390.480.650.480.550.650.40.54
Ad34.7531.4733.236.6330.2832.6437.6628.0834.2
Gr63.9660.7162.2365.1358.6862.667.4257.7860.99
Sp + Py + Al4.684.374.565.114.484.755.294.434.81
Note: All the calculations were based on 12 oxygens. Abbreviations: Ad = andradite; Gr = grossular; Sp = spessartine; Py = pyrope and Al = almandine.
Table
Table 2. Summary of the compositions (in ppm) for trace and rare earth elements of the Xianghualing garnet.
Table 2. Summary of the compositions (in ppm) for trace and rare earth elements of the Xianghualing garnet.
SampleXHL-1 (n = 7)XHL-2 (n = 10)XHL-3 (n = 11)
MaximumMinimumAverageMaximumMinimumAverageMaximumMinimumAverage
Cu0.3100.120.1400.060.2300.07
Zn12.477.9510.3812.569.991112.249.1510.76
Mo0.2500.150.460.010.200.280.080.19
Sn522823143924576637905037570624043856
W28.20.4510.929.991.1518.8923.090.224.89
Pb2.1100.420.7800.115.500.171.68
La0.080.020.040.040.010.020.0400.01
Ce0.450.340.400.550.300.420.4100.14
Pr0.250.180.200.300.150.200.2600.08
Nd3.461.962.693.161.802.613.1201.02
Sm3.002.042.473.701.602.662.8501.11
Eu0.620.260.390.740.340.520.590.020.22
Gd3.882.002.595.761.243.213.930.081.40
Tb0.500.320.421.140.130.570.620.070.29
Dy2.931.782.356.790.463.033.321.021.96
Ho0.590.350.471.370.060.580.750.330.50
Er1.800.981.373.630.031.482.080.821.61
Tm0.340.130.220.500.010.190.410.130.30
Yb2.171.141.783.1001.243.680.672.42
Lu0.320.190.270.3000.130.620.060.41
Y15.6610.5013.3337.241.3315.3517.969.2313.39
ΣREE17.2413.7315.6630.197.1216.8419.286.9711.46
ΣLREE7.754.946.198.084.276.427.210.062.57
ΣHREE10.298.2612.5322.581.9310.4212.076.878.89
ΣLREE/ΣHREE0.750.60.490.362.220.620.60.010.29
δEu0.560.340.460.790.350.582.370.150.61
δCe0.690.430.570.720.510.612.4800.56
LaN/YbN0.030.010.020.0400.010.0300.01

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MDPI and ACS Style

Yu, F.; Shu, Q.; Niu, X.; Xing, K.; Li, L.; Lentz, D.R.; Zeng, Q.; Yang, W. Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential. Minerals 2020, 10, 456. https://doi.org/10.3390/min10050456

AMA Style

Yu F, Shu Q, Niu X, Xing K, Li L, Lentz DR, Zeng Q, Yang W. Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential. Minerals. 2020; 10(5):456. https://doi.org/10.3390/min10050456

Chicago/Turabian Style

Yu, Fan, Qihai Shu, Xudong Niu, Kai Xing, Linlong Li, David R. Lentz, Qingwen Zeng, and Wenjie Yang. 2020. "Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential" Minerals 10, no. 5: 456. https://doi.org/10.3390/min10050456

APA Style

Yu, F., Shu, Q., Niu, X., Xing, K., Li, L., Lentz, D. R., Zeng, Q., & Yang, W. (2020). Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential. Minerals, 10(5), 456. https://doi.org/10.3390/min10050456

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