Decoding the oxygen fugacity of ore-forming fluids from garnet chemistry, the Longgen skarn Pb-Zn deposit, Tibet

Ore Geology Reviews 126 (2020) 103770

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Decoding the oxygen fugacity of ore-forming fluids from garnet chemistry, the Longgen skarn Pb-Zn deposit, Tibet

T

Xiaojia Jianga, Xin Chena, Youye Zhenga,b, Shunbao Gaoc, , Zhaolu Zhangd, Yongchao Zhangc, Shuzhi Zhange ⁎

a

Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China c Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China d Key Laboratory of High-Grade Fe Ores Exploring and Resources Assessing, Shandong Provincial Bureau of Geology ＆ Mineral Resources, and School of Mineral Resources and Environmental Engineering, Shandong University of Technology, Zibo 255049, China e Guilin University of Technology at Nanning, Nanning 532100, China b

ARTICLE INFO

ABSTRACT

Keywords: Garnet Chemical zonation Evolution of fluid Classification of skarn deposit Longgen Pb-Zn deposit

The relationship between the compositional zonation of garnet in skarn deposits and the evolution of physicochemical conditions of ore-forming fluids is not clear and needs further investigation. This study presents the characteristics of trace elements of garnet from the Longgen skarn Pb-Zn deposit, Tibet with an aim to reveal the evolution trend of the physical and chemical conditions (fO2 and pH) of ore-forming fluids and to explore the fingerprint of garnet trace elements in different types of skarn deposits. Based on robust alteration halo, mineral assemblages and petrographic observation, the garnets of the Longgen skarn Pb-Zn deposit are divided into two types: (1) distal exoskarn garnet near limestone with orebearing and fine oscillatory zoning (Adr39.9-75.0; Grt1, Adr represents andradite) and (2) proximal exoskarn garnet (core: Adr32.9-70.7; rim: Adr83.3-99.5) with Pb-Zn-rich mineral and coarse oscillatory zoning (Grt2). According to the total rare earth element (ΣREE) content in the garnet compositional zone, Grt1 is further divided into three component zones: Grt1-1 (heavy rare earth element (HREE)-enriched chondrite-normalized REE distribution patterns and positive Eu-anomaly (δEu > 1, δEu = 2*(Eusample/Euchondrite)/(Smsample/ Smchondrite + Gdsample/Gdchondrite)), Grt1-2 (HREE-enriched REE patterns with negative Eu-anomaly (δEu < 1)), and Grt1-3 (light rare earth element (LREE)-enriched REE patterns with δEu > 1). Similarly, five component zones were identified in Grt2: Grt2-1 (REE-rich REE patterns with δEu < 1), Grt2-2 (LREE-depleted REE patterns with δEu < 1), Grt2-3 (steady REE patterns with δEu < 1), Grt2-4 (LREE-enriched REE patterns with δEu > 1), and Grt2-5 (LREE-enriched REE patterns with δEu < 1) from the core to rim, respectively. The fractionation of LREE with HREE and the behavior of Eu indicate that the physical–chemical conditions during the formation of Grt1-1-Grt1-2 and Grt2-1-Grt2-3 were near neutral (pH = 6–7), and changed to mildly acidic in Grt1-3 and Grt2-4-Grt2-5 (pH < 6–7). In addition, the characteristics of low U (< 1 ppm) and the linear relationship between Sn and Eu indicate that the fO2 of the ore-forming fluid was relatively oxidized in comparison to the garnets found in other skarn deposits, and Grt1-1-Grt1-3 and Grt2-1-Grt2-5 have a gradual increasing oxidative ability. The linear relationship between ΣREE and Y/Ca indicates that Grt1 and Grt2 grew in a near-closed system. The incorporation of REE3+ into the garnet was mainly controlled by a “yttrogarnet” (YAG)-type substitution, Ca site vacancy, fluid chemistry, and physicochemical conditions of precipitation. However, the compositional zone at the transition zone between Al-rich and Fe-rich segments in Grt2-4 may be formed in an open system, as suggested by the exchange of LREE and Cl−. In conclusion, garnet crystal growth was affected by the crystal chemistry and physicochemical conditions of the skarn deposit. This study demonstrates that factor analysis and discrimination diagrams (log Sn vs. log U, log Ce vs. log U, and log (Ce + Hf) vs. log U) are effective methods for distinguishing different types of skarn deposits by their contents of redoxsensitive trace elements in garnet.

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Corresponding author. E-mail address: [email protected] (S. Gao).

https://doi.org/10.1016/j.oregeorev.2020.103770 Received 28 February 2020; Received in revised form 24 August 2020; Accepted 4 September 2020 Available online 11 September 2020 0169-1368/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. a Tectonic framework and location of the Tibetan Plateau (modified from Zhang et al., 2018). b. Distribution of granitoids, tectonic framework, and skarn deposits of the Lhasa terrane (modified after Zhang et al., 2018). Abbreviations: IYZSZ = Indus-Yarlung Zangbo Suture Zone, BNSZ = Bangong-Nujiang Suture Zone, LMF = Luobadui–Milashan Fault, SNMZ = Shiquanhe–Nam Tso Melange Zone, JSSZ = Jinsha Suture Zone, SLS = southern Lhasa subterrane, CLS = central Lhasa subterrane, NLS = northern Lhasa subterrane.

1. Introduction

hydrothermal fluids for the composition of garnet in the evolution of garnet also remains unknown. Many researchers use the major and trace elements of garnet as criteria to distinguish the types of skarn deposits. Relevant studies mainly included major elements (e.g., see the trigonometric discriminant diagram of different component proportions of garnet in Meinert, 1992), metallic ore-forming elements such as Cu in skarn Cudominated deposits (Karimzadeh Somarin, 2004), Sn in skarn W-Sn deposits (Zhou et al., 2017; Park et al., 2017b), and trace elements (Eu, V, Ce, U, and W; Tian et al.,2019). However, these standards suffer from several inconsistencies, such as significant overlapping and/or incomplete classification. Since these inconsistencies have not been solved to date, the application of garnet for the classification of skarn deposits is limited. The Longgen skarn Pb-Zn deposit, located in the southern margin of the Gangdese-Nyainqêntanglha plate, is a medium-sized skarn Pb-Zn deposit in the middle and western parts of the Nyainqêntanglha Pb-ZnFe-Cu metallogenic belt (Zhang et al., 2018). As with most other skarns, multiple events have been recorded in a single garnet particle at Longgen and are characteristically composed of regions of different colors (Guo et al., 2019). This study focuses on the determination of major and trace elements in different garnets using electron probe microanalyses (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Based on the characteristics of both the major and trace elements of different zones of single garnet and different skarn deposits, the formation environment and REE substitution mechanism of a single garnet REE zone, the pH and fO2 evolution of fluid in garnet and the relationship between trace elements of garnet and types of skarn mineralization are discussed.

In skarn minerals, trace elements, and rare earth elements (REE) in particular, are widely used to trace the mineralization processes (Peng et al., 2015). Containing important minerals at the early skarn stage, oscillating zones are ubiquitous in garnet; like Digital Video Disks, these have the potential to record the entire process of mineral growth for most stages of mineralization (Jamtveit et al., 1993; Smith et al., 2004; Ismail et al., 2014; Xiao et al., 2018; Chen et al., 2019). Cu-, Fe-, W-, and Au-dominated skarn-type deposits show that the component annulus in garnet, especially REE annulus, records the crystal growth process and the evolving environment; this information shows its replacement mechanism for temperature, oxygen fugacity, pH, and REE (Zhai et al., 2014; Baghban et al., 2015; Xu et al., 2016; Park et al., 2017a; Zhang et al., 2017b; Fei et al., 2019; Tian et al., 2019). How these trace elements are controlled by the physical and chemical conditions of the ore-forming fluid, especially pH and fO2, in the garnet of a Pb-Zn skarn deposit is worth studying. The relationship between the compositional zonation of garnet in skarn deposits and the evolution of the prevailing physicochemical conditions (primarily the fO2 of fluids) of ore-forming fluids remains controversial. (1) The main element Fe3+ is more concentrated in andradite than grossular, indicating higher fluid oxidation of the andradite zone (Jamtveit et al., 1993; Meinert et al., 2005; Xu et al., 2016; Sun et al., 2020a, 2020b). (2) The δEu value of REE is controlled by the ratio of Eu3+/Eu2+ of the hydrothermal fluid, which can indicate the redox state of the fluid (Bau, 1991; Yang et al., 2007; Xu et al., 2016; Ding et al., 2018; Sun et al., 2020a, 2020b). (3) Trace elements U and Sn in different garnet crystals can be used to identify the relative fO2 of hydrothermal fluids during garnet formation (Smith et al., 2004; Zhang et al., 2017a; Tian et al., 2019). However, whether Sn can also identify the relative fO2 of hydrothermal fluids in Sn skarn deposits remains unknown. In addition, the dominant factor of oxygen fugacity of

2. Geological setting The Longgen Pb-Zn deposit is located in the western part of the 2

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central Lhasa subterrane (Zhang et al., 2019). The Lhasa terrane, which is located in the middle of the Himalaya-Tibet orogenic belt, is adjacent to the Qiangtang terrane of the Bangong-Nujiang suture zone to the north, and connected with the Himalayan belt of the Indus-Yarlung Zangbo suture zone to the south (Zhu et al., 2011; Xu et al., 2016; Zhang et al., 2018; Liu et al., 2019; Jiang et al., 2020; Chen et al., 2020; see Fig. 1a). Moreover, the northern Lhasa subterrane and the central Lhasa subterrane are separated by the Shiquanhe-Nam Tso Melange Zone (SNMZ), while both the central Lhasa terrane and the southern Lhasa terrane are separated by the Luobadui-Milashan Fault (LMF) (Fig. 1b; Zhu et al., 2009). This area is characterized by the development of a large number of granitoid rocks and thrust faults. It has experienced a complex tectonic evolution from the Jurassic to the Cretaceous Neo-Tethys ocean subduction, then during the Cenozoic IndiaAsia continental main collision (65–41 Ma), later collision (40–26 Ma), and post-collision (< 25 Ma) stages (Hou et al., 2004, 2011; Zheng et al., 2015). During these periods, many magmatic activities occurred, including during the early to late Cretaceous (130–80 Ma), from the Paleocene to the Eocene (65–41 Ma), and from the Oligocene to the Miocene (33–13 Ma) (Hou et al., 2004; Zheng et al., 2004; Wen et al., 2008; Zhu et al., 2011; Sun et al., 2018, 2020a, 2020b; Fig. 1b). Numerous granitoid rocks and Permo–Carboniferous metasedimentary rocks that have been exposed on the central Lhasa terrane provide excellent conditions for the formation of skarn deposits (Pan et al., 2006; Zhang et al., 2018). The composite NW-trending fault zone with polyphase and genetic characteristics controls the distribution of skarn deposits along the EW-direction (Zheng et al., 2004; Gao et al., 2012). Many medium to large skarn deposits have been studied in this area, including the Chagele Pb-Zn polymetallic deposit, the Longgen Pb-Zn deposit, the Bangbule Fe-Cu polymetallic deposit, and the Mengya'a PbZn deposit. Together with both porphyry deposits and epithermal deposits, these form the giant Nyainqêntanglha Pb-Zn-Ag-Cu-Fe metallogenic belt (Wang et al., 2015; Gao et al., 2012; Tian et al., 2016; Xu et al., 2016; Zhang et al., 2019; Guo et al., 2019).

been studied in the Longgen deposit. The weighted average ages of 206 Pb/208U are (62.9 ± 0.8 Ma) and (64.3 ± 0.7 Ma), indicating that 206 Pb/208U is the product of the early stage of the syn-collision of the Indo-Asian plate (Duan et al., 2014; Zhang et al., 2019). It has been speculated that the granite porphyry formed in the continental marginal arc (Zhang et al., 2019). This is consistent with the Rb-Sr isochronous age of sphalerite (59.1 Ma, unpublished data), and represents the age when the deposit formed (Zhang et al., 2018). Combined with C–H-O isotopes and fluid inclusion microthermometry, the ore-forming fluid was transformed from a magmatic-hydrothermal fluid of high temperature (340–480 °C) and medium salinity (7.9–15.0 wt% NaCl eq.) at the skarn stage to mixed fluid of magmatic water and atmospheric water with medium temperature (160–270 °C) and low salinity (0.9–10.6 wt% NaCl eq.) at the quartz sulfide stage (Zhang et al., 2018). In addition, S-Pb isotopes show that ore-forming materials were mainly derived from granite porphyry. Therefore, it can be concluded that fluid cooling caused by fluid mixing was the most critical mechanism for the formation of the lead–zinc deposits (Zhang et al., 2018). 4. Sampling and methodology Figs. 2 and 3 show the sample locations of skarn samples. Detailed field investigations indicated that the color and grain size of garnets vary significantly with distance from the granite porphyry and carbonate rocks. Hence, garnet samples were divided into two types in the skarn. Garnet 1 (Grt1, 0104-B01 and PM02-B01) coexists with diopside and wollastonite occurs within either marble or at the contact zone with limestone, thus representing distal exoskarn (Fig. 2). The massive garnet 2 (Grt2, PM02-B05 and ZK0004-B14), which coexists with diopside and ore minerals in the Pb-Zn-bearing garnet-diopside skarn, is representative of a proximal exoskarn (Figs. 2–4). Detailed rock and mineral identifications were carried out under an optical microscope. Representative minerals (i.e., garnet) were selected for electron probe microanalyses (EPMA). Back-scattered electron (BSE) images and garnet components were obtained by JSM-IT300 EPMA equipped with an Inca X–Act energy–dispersive spectrometer (EDS). The specific working conditions used an accelerating voltage and beam current of 20 kV and 20nA, respectively, and the spot diameter of the focused electron beam was 5 μm. All test data were corrected for ZAF using the composition of mineral standards provided by the SPI (USA). The detection limits of all relevant elements were 0.01 wt%. In-situ microanalysis (LA-ICP-MS_spot) was carried out on an Agilent 7900 ICP-MS by the Wuhan Sample Solution Analysis Technology Co., Ltd., with a 193 nm ArF excimer Resonetics Resolution GeoLas HD laser ablation system. The frequency and spot size of the laser were 5 Hz and 32 μm, respectively. The trace element content of garnet was calibrated according to different reference materials (NIST610, BHVO-2G, BCR-2G, and BIR-1G) and the Si content of garnet at the corresponding point was used as internal standard (Liu et al., 2008). The analysis time for a single-point test was about 80 s, during which the sample was continuously ablated for 50 s, which was followed by purging for 20 s and cleaning of the sampling system. Time drift correction and quantitative calibration of trace element analyses were performed with ICPMSDataCal10.9 software (Liu et al., 2008). The detection limits of most trace elements ranged between 0.01 and 0.5 ppm. This method is hereinafter referred to as LA-ICP-MS_spot. LA-ICP-MS_Mapping of garnet was carried out in a LA-ICP-MS Laboratory at the Department of Earth Sciences, University of New Brunswick, Canada (McFarlane and Luo, 2012). The LA-ICPMS system used a resonance M−50 193 nm excimer laser system, connected to an Agilent 7700x Quadrupole Inductively Coupled Plasma Mass Spectrometer, equipped with dual external rotary pumps. Garnet was ablated at a scanning speed of 3 μm/s, a diameter of 5 μm, and a frequency of 10 Hz. The laser flux was controlled at ~ 1.5 J/cm2 (Franchini et al., 2015). At the end of the ablation sequence, the laser log file and the ICP-MS intensity data file were synchronized using Iolite software

3. Geology of the deposit The Longgen medium skarn lead–zinc deposit has proven total reserves of 0.13 Mt (lead + zinc metal), consisting of 3.21% lead and 2.43% zinc (Zhang et al., 2018). This deposit consists of 23 orebodies in vein, lenticular, and stratiform, which are mainly located in the contact zone between Permian limestone and Paleocene granite-porphyry (Figs. 2 and 3). The wall rock is composed of intermediate-acidic magmatic rocks (e.g., tuff lava and granite porphyry) and sedimentary rocks (e.g., limestone and quartz sandstone). The carbonate interlayer and weak metamorphic sedimentary rocks may provide suitable storage sites for skarn lead–zinc deposits. Near EW-trending fault structures provide transport channels for Pb-Zn mineralization and also cause large-scale hydrothermal alteration. The skarn system can be divided into an early prograde stage and a late retrograde stage. During the prograde stage, the emplacement of granite porphyry resulted in the metasomatism or contact metamorphism of carbonate and fine sandstone to an anhydrous calcium silicate skarn (garnet-wollastonitediopside). During the retrograde stage, a large number of hydrous silicate minerals, such as actinolite, epidote, chlorite, and a small amount of magnetite, formed (Zhang et al., 2018). In general, garnet skarn and epidote skarn are the most common rock alterations. However, the skarns from all prograde stages are interpenetrated or metasomatized by veins and veinlets of water-bearing minerals (e.g., chlorite, tremolite, epidote, and actinolite), which often coexist with metal sulfides during the quartz sulfide stage. During the early quartz sulfide stage, chalcopyrite, pyrite, and pyrrhotite were the main metal minerals that crystallized; however, during the late quartz sulfide stage, metal minerals are composed of galena, sphalerite, and pyrite. This period was also a rich lead-zinc ore formation period. The LA-ICP-MS zircon U-Pb geochronology and element geochemistry of its granite porphyry have 3

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Fig. 2. Geological map of the Longgen skarn Pb-Zn deposit (modified after Zhang et al., 2018).

(Paton et al., 2011). Trace element mapping data was processed by XMapTools software data processing software based on MATLAB (Lanari et al., 2014). This method is hereinafter referred to as LA-ICPMS_mapping. The high spatial resolution of element measurement with LA-ICP-MS_mapping not only divides the different distribution patterns of REE but also provides a powerful tool for the identification of the relationship between major and trace elements. In geochemical explorations, factor analysis (FA) not only reduces the dimension of multivariate variable groups but also classifies the factors by using the correlation between variables (Chen et al., 2018). FA constructs variable classification criteria by judging the relative size of the load of each variable to each common factor (Chen et al., 2016). If each variable in the factor load matrix matches the load of the common factor, it is necessary to rotate the load matrix so that each

variable has a large load on only one common factor; this further clarifies the variable classification. In the rotation method of the load matrix, the Direct Oblimin can be used when the effect of the Varimax is poor. In addition, to improve the interpretability of the data, principal components and eigenvalues > 1 were used to extract valid variables. FA was performed using SPSS®25 Statistics software. 5. Results 5.1. Garnet textures and zoning Garnet is the most abundant skarn mineral and is widely developed in both proximal exoskarns and distal exoskarns in the Longen skarn deposit (Zhang et al., 2018). Grt1 represents a distal exoskarn, which

Fig. 3. No.00 (a) and No.03 (b) cross-sections of the Longgen Pb–Zn deposit. See Fig. 2 for locations (modified after Zhang et al., 2018; Guo et al., 2019). 4

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Fig. 4. Geological section of the skarn zone of the Longgen skarn Pb-Zn deposit. See Fig. 2 for specific locations.

coexists with epidote, calcite, wollastonite, and minor galena (Fig. 5ac). Its color is pale brown to brown, medium-grained (0.1 to 0.8 cm), is subhedral granular and anisotropic with low birefringence and sector twinning (Zhang et al., 2018), and shows fine oscillatory zoning (Fig. 5f and g). Grt2, which coexists with diopside and ore minerals, is representative of proximal exoskarns (Fig. 5d and e). Its color is reddishbrown to brown, coarse-grained (0.3 to 1.2 cm), visible fluid inclusions (Zhang et al., 2018), euhedral to subhedral granular and anisotropic, and shows clear oscillatory zoning (Fig. 5h and i). In BSE images, most Grt1 grains are displayed in light gray, with varied sizes, and with narrow black-and-white zonings and irregular boundaries (Fig. 6a-c). Grt2 is mostly euhedral and has shock-induced textures (Fig. 6d-f; Gaspar et al., 2008). Furthermore, the color from the core to the rim of this garnet changes from dark gray to light gray (Fig. 6e).

narrow compositional ranges of Adr39.9-75Grs25.0-60.1Pra0.1-0.6 and Adr9.0-37.9Grs62.1-90.9Pra0.1-0.5 (Fig. 7). Moreover, Grt1 has no obvious bright and dark zones in BSE images (Fig. 8a), and the major components show irregular fluctuations in a single garnet (Fig. 8b). In the Grt2, the BSE image clearly shows bright and dark zones (Fig. 8c). The compositions of the bright zones are 32.61–35.58 wt% SiO2, 0.1–3.45 wt% Al2O3, 34.65–35.93 wt% CaO, and 24.28–28.58 wt % FeO*. The Al enrichment is similar to Grt1, with the molecular formula of Adr32.9-70.7Grs29.3-67.1Pra0.0-0.9 (Figs. 7 and 8). In the dark zone, the compositions of SiO2, Al2O3, CaO, and FeO* are 28.99–38.02 wt%, 6.22–14.6 wt%, 29.08–37.55 wt%, and 10.1–24.82 wt%, respectively. Compared with Grt1 and the Al-rich zones in the Grt2, it is Fe-rich, and its molecular formula is Adr83.3-99.5Grs0.5-16.7Pra0.0-1.0. The cross-section line shows that Grt2 presents a clear compositional zonation in a single garnet. From the core to the rim, the andradite composition increases, and the grossular composition decreases (Fig. 8d). Overall, all garnets have a wide compositional range from Adr9.037.9Grs62.1-90.9Pra0.1-0.5 to almost pure andradite (Adr83.3-99.5Grs0.516.7Pra0.0-1.0) (Figs. 7 and 8), which is consistent with the distribution of end-members of garnet in the world skarn-type Zn deposit (Fig. 7).

5.2. Major element compositions The major element compositions of the three generations of garnets in the Longgen skarn Pb-Zn deposit show that the end-member of all components is < 2%, except for grossular and andradite garnet. This indicates that the garnets can be regarded as a grossular-andradite solid solution (Fig. 7). In the Grt1, the concentrations of SiO2, Al2O3, CaO, and FeO* are 35.87–37.92 wt%, 5.09–18.91 wt%, 34.21–38.33 wt%, and 2.63–21.49 wt%, respectively (Table 1). This falls within two relatively

5.3. Trace elements and mapping The most representative samples (i.e., 0104B01 and PM02B05) were selected from Grt1 and Grt2 according to the EPMA dataset. These are closely related to Pb-Zn mineralization. Hence, studying Grt1(0104B01) 5

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Fig. 5. Thin-section photographs and microphotographs of the Longgen skarn Pb-Zn deposit. (a-c) Distal exoskarn garnet (Grt1) coexists with epidote, wollastonite, calcite, galena, and sphalerite. (d-e) Proximal exoskarn garnet (Grt2) coexists with lead-zinc-rich ore and diopside. (f-g) Fine oscillatory zoning in Grt1 (Adr9.0-75.0; Grt1) and is anisotropic with low birefringence and sector twinning. (h-i) Grt2 shows obvious oscillatory zoning (core: Adr32.9-70.7; rim: Adr83.3-99.5) euhedral to subhedral granular and anisotropic.

Fe3+ measured by EPMA. It is also consistent with the relationship between grossularite and andradite (Figs. 8 and 9a, Table 1). These results show that all garnets are depleted of high field strength elements (HFSE, e.g., Ti, Zr, Nb, and Th) and large ion lithophile elements (LILE, e.g., Ba and Rb) relative to the primitive mantle (Fig. 9b-g; Table 2; Sun

and Grt2(PM02B05) can better reflect the chemical properties of the ore-bearing fluids. The results of LA-ICP-MS_spot of Grt1 and Grt2 are listed in Table 2. The negative correlation between FeO* and Al2O3 measured by the LAICP-MS method is consistent with the relationship between Al3+ and

Fig. 6. BSE images of garnet from the Longgen skarn Pb-Zn deposit. (a-c) Grt1 grains are light gray, size-varied, and show black-and-white narrow zonings and irregular boundaries. (d) Shock-induced textures of Grt2. (e-f) Grt2 grains are euhedral and the color of the core to the rim of this garnet changes from dark gray to light gray. 6

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positive loads, while La and Ce appear as negative loads. The positive and negative scores of FS1 of the three types of deposits are relatively symmetrical, while the FS3 of Cu-related skarn deposits are positive, the FS3 of W-related skarn deposits are either positive or negative, and the Zn-related skarn deposits of FS3 are negative. In Fig. 12b, FL2 of LREE (La-Eu) presents a positive load, and FL2 of HREE (Gd-Lu) presents a negative load. U, Hf, and Zr all have positive loads in FL3. The score distribution characteristics of these three types of deposits are similar to those shown in Fig. 12a.

Fig. 7. Triangle discriminant graph of garnet in the Longgen skarn Pb-Zn deposit (the Zn skarn fields originate from Meinert, 1992). Abbreviations: Adr, andradite; Alm, almandine; Grs, grossular; Sps, spessartine.

6. Discussion 6.1. Mechanisms of garnet substitution

and McDonough, 1989). However, Sr is enriched in most of Grt1, while it is depleted in Grt2 relative to the primitive mantle (Fig. 9h). Concentrations of Ti, W, and Y increase from Grt2 Al-rich zones to Grt2 Ferich zones and decrease from Grt1 Al-rich zones to Grt1 Al-poor zones in the mapping (Fig. 10). However, ore-forming elements (e.g., Zn) developed in the form of veins within the garnet and did not show zonal distribution. They are mainly distributed in Grt1 Fe-rich zones and Grt2 Al-poor zones (Fig. 10). In general, from the core to rim, the content of REE in Grt1 and Grt2 gradually declined. Moreover, changes in the main elements (e.g., Al and Fe) affect the overall trends in REE but do not precisely indicate REE zoning (Fig. 10). According to the total content of REE in the LAICP-MS_Mapping, Grt1 and Grt2 are divided into three and five stages, respectively. Grt1 presents three different chondrite-normalized REE fractionation patterns, which are Grt1-1 (ΣREE = 10.51–54.39 ppm, mean = 33.77 ppm, LREE/HREE = 0.19–2.37, smaller positive Euanomaly (δEu = 1.45 average)), Grt1-2 (ΣREE = 38.48–147.66 ppm, mean = 70.39 ppm, LREE/HREE = 0.11–3.61, pronounced negative Eu-anomaly (δEu = 0.54 average)), and Grt1-3 (ΣREE = 5.63–40.44 ppm, mean = 15.51 ppm, LREE/ HREE = 5.27–101.68, and pronounced positive Eu-anomaly (δEu = 5.50 average)) (Fig. 11a and b). Because of the existence of double crystals in Grt1, it is difficult to distinguish the sequence of formation of the three stages. In contrast, Grt2 presents five different and orderly chondrite-normalized REE fractionation patterns, which are Grt2-1 (ΣREE = 80–631 ppm, mean = 326.1 ppm, LREE/ HREE = 0.29–2.88, with pronounced negative Eu-anomaly (δEu = 0.34 average)), Grt2-2 (ΣREE = 33.84–308.72 ppm, mean = 156.82 ppm, LREE/HREE = 0.02–0.67, with pronounced negative Eu-anomaly (δEu = 0.43 average)), Grt2-3 (ΣREE = 24.61–29.10 ppm, mean = 26.73 ppm, LREE/ HREE = 2.28–5.76 with smaller negative Eu-anomaly (δEu = 0.68 average)), Grt2-4 (ΣREE = 39.4–69.14 ppm, mean = 52.53 ppm, LREE/HREE = 39.78–58.92 with small, positive Eu-anomalies (δEu = 2.51 average)), and Grt2-5 (ΣREE = 2.97–14.26 ppm, mean = 8.22 ppm, LREE/HREE = 14.04–43.41 with small, negative Eu-anomalies (δEu = 0.51 average)) (Fig. 11c and d).

Compositional distinctions among most named garnets are centered around the substitution of different elements into the dodecahedral 'X' site, the octahedral 'Y' site, and the tetrahedral 'Z' positions within a generalized crystal-chemical formula X3Y2Z3O12 (Xu et al., 2016). Trace elements in garnet, especially REE, can be used to trace water–rock interactions and fluid properties (Vander Auwera and Andre, 1991; Tian et al., 2019). This is due to the similar ionic potentials (=ionic charge (Z)/ionic radius (R), 2 < Z/R < 4; Railsback, 2003) of REE, which makes it easier for REE to accumulate ions with similar geochemical properties in fluids and solutions (Goldschmidt, 1937). These ions can also be separated from each other based on specific geological environments (Smith et al., 2004). Based on the influence of both the ionic radius and the garnet crystal radius on the structure of garnet (Shannon, 1976), the possible substitution site of REE is the X site (e.g., Ca2+) of a dodecahedron (Carlson, 2012). Because the ionic radius of Eu2+ (1.12 Å) is similar to that of Ca2+ (0.99 Å) and because they have the same ion charge, Eu2+ is more likely to replace Ca2+ at the X site (Gaspar et al., 2008). However, the substitution of REE3+ and Eu2+ in garnet differs; therefore, coupling substitution or vacancy generation are needed to compensate for this charge imbalance (Gaspar et al., 2008; Park et al., 2017a). In general, the following five substitution mechanisms are proposed to explain the incorporation of REE in garnet. (1) Monovalent cations (for example, Na+) replace the X site and form a Na+-REE3+ coupling substitution (Enami et al., 1995; Sepidbar et al., 2017). (2) Incorporation of a divalent cation (e.g., Mg2+ or Fe2+) into the Y site (menzerite-(Y) substitution (Grew et al., 2010; Tian et al., 2019). (3) Yttrogarnet (YAG) type substitution, where trivalent cations (such as Fe3+ or Al3+) are substituted to the Z (Si) site to achieve charge balance (Ding et al., 2018; Fei et al., 2019). (4) The compensation can be changed through the Ca2+ vacancy at the dodecahedron site (Ismail et al., 2014). (5) Substituting F− or OH− into the Si-O tetrahedron (Gross et al., 2008; Fei et al., 2019). Because of the variety of garnet in the Longgen deposit and the complex substitution mechanism of REE in garnet, more than one REE substitution mechanism from garnet may exist in the Longgen deposit. The relationship between REE and major trace elements in Grt1 and Grt2 (Fig. 13a-d) indicates the substitution mechanism of REE to garnet. The content of Na2O in garnet is very low (< 300 ppm) and does not change with the content of REE; therefore, it is difficult to achieve charge balance and substitution of monovalent cations for the X-site (Table 1; Park et al., 2017a). Menzerite-(Y) substitution of divalent cations into the Y site may not be the substitution mechanism of REE to garnet (Grew et al., 2010; Carlson, 2012; Tian et al., 2019). Although the correlation between REE3+ and Mg is negative, this contradicts the positive correlation between both in the menzerite-(Y) substitution (Fig. 13a). The YAG-type substitution mechanism involves the substitution of trivalent cations to tetrahedral positions as a form of charge compensation (Xu et al., 2016). This feature is reflected in the negative correlation between the total ferrum and the REE3+ contents, and also indicates a YAG-type substitution in Grt1-1-Grt1-2, Grt2-1-Grt2-3, and Grt2-5 (Gaspar et al., 2008; Park et al., 2017a; Xiao et al., 2018). However, Grt1-3 does not show a significant correlation between

5.4. Factor analysis of trace elements To use as much garnet trace element data and deposits for the classification as possible, REE, Y, U, Zr, and Hf were selected (18 elements, Supplementary 1) as the basic data for FA. Their deposit types were simplified to three main types (Cu-related, Zn-related, and W-related; Fig. 12). The FA biplot in Fig. 12 is a combination of factor loadings (FL) and factor scores (FS). FL refer to the distribution of variables relative to factor one (F1)/F2 and F3, and FS refer to the distribution of samples. In Fig. 12a, except for La and Ce, FL1 of the other REE elements are obviously positive loads, and FL1 does not contain an obvious negative load element. In addition, in FL3, U, Hf, and Zr elements appear as 7

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X. Jiang, et al.

Table 1 EPMA analysis results (wt%) of garnet from Longgen skarn Pb-Zn deposit. Sample

LG-0104-B01

Type point

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

1 9

1 10

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

CaO MgO TiO2 SiO2 Al2O3 FeO* MnO Cr2O3 Cl F K2O P2O5 Na2O SrO BaO TOTAL Ca Mg Ti Si Al Fe3+ Fe2+ Mn Cr K P Na Sr Ba Adr Gro Pra

34.21 0.10 0.00 36.45 5.09 21.49 0.14 0.01 0.00 0.00 0.00 0.03 0.00 0.15 0.00 97.66 3.01 0.01 0.00 2.99 0.49 1.48 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 75.0 25.0 0.3

34.81 0.01 0.00 36.61 5.56 20.01 0.14 0.00 0.00 0.00 0.00 0.04 0.03 0.13 0.00 97.34 3.06 0.00 0.00 3.00 0.54 1.37 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 71.9 28.1 0.3

35.69 0.00 0.02 37.40 7.50 17.97 0.10 0.00 0.00 0.00 0.00 0.03 0.01 0.13 0.00 98.84 3.07 0.00 0.00 3.00 0.71 1.21 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 63.0 37.0 0.2

35.69 0.02 0.02 36.86 7.89 17.63 0.07 0.00 0.00 0.00 0.00 0.02 0.01 0.21 0.00 98.43 3.08 0.00 0.00 2.97 0.75 1.19 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 61.3 38.7 0.2

35.23 0.00 0.00 36.30 9.67 16.34 0.25 0.02 0.01 0.00 0.00 0.02 0.02 0.16 0.00 98.01 3.04 0.00 0.00 2.92 0.92 1.10 0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.00 54.5 45.5 0.5

35.30 0.01 0.00 35.87 8.79 16.91 0.19 0.00 0.00 0.00 0.00 0.00 0.04 0.16 0.00 97.27 3.07 0.00 0.00 2.91 0.84 1.15 0.00 0.01 0.00 0.00 0.00 0.01 0.01 0.00 57.7 42.3 0.4

36.19 0.00 0.00 36.99 12.32 12.55 0.17 0.02 0.00 0.00 0.00 0.01 0.00 0.14 0.04 98.44 3.07 0.00 0.00 2.93 1.15 0.83 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 41.9 58.0 0.4

36.02 0.01 0.00 36.95 8.97 16.08 0.05 0.00 0.01 0.00 0.01 0.03 0.01 0.15 0.00 98.28 3.10 0.00 0.00 2.96 0.85 1.08 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 56.0 44.0 0.1

36.01 0.03 0.02 36.26 9.29 15.82 0.06 0.00 0.00 0.00 0.00 0.03 0.03 0.19 0.03 97.78 3.11 0.00 0.00 2.92 0.88 1.07 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 54.7 45.3 0.1

36.61 0.02 0.00 37.58 13.08 12.22 0.12 0.00 0.00 0.00 0.00 0.04 0.02 0.14 0.00 99.82 3.06 0.00 0.00 2.93 1.20 0.80 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 39.9 60.1 0.3

35.23 0.00 0.00 36.30 9.67 16.34 0.25 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.00 97.82 3.04 0.00 0.00 2.92 0.92 1.10 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 54.5 45.4 0.6

35.30 0.01 0.00 35.87 8.79 16.91 0.19 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.00 97.10 3.07 0.00 0.00 2.92 0.84 1.15 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 57.7 42.3 0.4

36.61 0.02 0.00 37.58 13.08 12.22 0.12 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 99.64 3.06 0.00 0.00 2.93 1.20 0.80 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 39.9 60.1 0.3

36.19 0.00 0.00 36.99 12.32 12.55 0.17 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 98.25 3.07 0.00 0.00 2.93 1.15 0.83 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 41.9 58.0 0.4

36.02 0.01 0.00 36.95 8.97 16.08 0.05 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 98.09 3.10 0.00 0.00 2.97 0.85 1.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 56.0 44.0 0.1

34.81 0.01 0.00 36.61 5.56 20.01 0.14 0.00 0.00 0.00 0.01 0.00 0.03 0.00 0.00 97.17 3.06 0.00 0.00 3.01 0.54 1.37 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 71.9 28.1 0.3

35.69 0.00 0.02 37.40 7.50 17.97 0.10 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 98.69 3.07 0.00 0.00 3.00 0.71 1.21 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 63.0 37.0 0.2

35.69 0.02 0.02 36.86 7.89 17.63 0.07 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 98.19 3.08 0.00 0.00 2.97 0.75 1.19 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 61.3 38.7 0.2

Source Sample Type point

LG-PM02-B01 1 1 9 10

LG-ZK0004-B02 1 1 1 2

1 1

1 2

1 3

1 4

1 5

1 1

1 2

1 3

LG-PM02-B05-2 2 2 1 2

2 3

2 4

2 5

2 6

CaO MgO TiO2 SiO2 Al2O3 FeO* MnO Cr2O3 Cl F K2O P2O5 Na2O SrO BaO TOTAL Ca Mg Ti Si Al Fe3+ Fe2+ Mn Cr K P Na Sr Ba Adr Gro Pra

34.21 0.10 0.00 36.45 5.09 21.49 0.14 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 97.48 3.01 0.01 0.00 3.00 0.49 1.48 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 75.0 25.0 0.3

36.66 1.71 2.28 36.47 15.71 3.82 0.12 0.02 0.22 0.92 0.00 0.04 0.19 0.28 0.04 98.46 3.07 0.20 0.13 2.85 1.45 0.25 0.00 0.01 0.00 0.00 0.00 0.03 0.01 0.00 14.7 85.2 0.2

37.64 0.04 0.06 37.92 16.37 8.24 0.22 0.00 0.00 0.08 0.00 0.00 0.00 0.00 0.00 100.57 3.08 0.00 0.00 2.90 1.47 0.53 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 26.3 73.7 0.5

37.38 0.01 0.02 37.60 14.01 11.39 0.10 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.52 3.09 0.00 0.00 2.90 1.27 0.73 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 36.6 63.4 0.2

38.33 0.01 0.00 37.48 18.83 6.19 0.18 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 101.03 3.10 0.00 0.00 2.83 1.67 0.39 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 18.9 81.1 0.4

36.66 1.71 2.28 36.47 15.71 3.82 0.12 0.02 0.00 0.92 0.00 0.00 0.19 0.00 0.00 97.88 3.07 0.20 0.13 2.85 1.45 0.25 0.00 0.01 0.00 0.00 0.00 0.03 0.00 0.00 14.7 85.2 0.2

38.02 1.08 0.55 36.78 18.91 2.63 0.07 0.04 0.00 0.22 0.00 0.00 0.03 0.00 0.00 98.33 3.13 0.12 0.03 2.82 1.71 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9.0 90.9 0.1

37.17 0.02 0.10 37.27 14.17 10.41 0.16 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 99.30 3.10 0.00 0.01 2.90 1.30 0.68 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 34.3 65.7 0.3

37.48 0.02 0.05 37.54 13.45 11.57 0.13 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 100.26 3.10 0.00 0.00 2.90 1.23 0.75 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 37.9 62.1 0.3

38.27 0.00 0.00 37.27 17.04 7.30 0.19 0.02 0.00 0.00 0.00 0.00 0.03 0.00 0.00 100.12 3.13 0.00 0.00 2.85 1.53 0.47 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 23.3 76.6 0.4

36.88 0.00 0.02 36.97 10.00 16.22 0.16 0.00 0.00 0.00 0.00 0.03 0.00 0.16 0.02 100.46 3.10 0.00 0.00 2.90 0.92 1.06 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 53.5 46.5 0.3

37.16 0.00 0.00 37.50 11.68 13.96 0.07 0.00 0.00 0.00 0.00 0.01 0.00 0.12 0.00 100.51 3.09 0.00 0.00 2.92 1.07 0.91 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 45.9 54.1 0.2

35.81 0.01 0.76 36.62 7.91 18.83 0.14 0.01 0.00 0.00 0.00 0.02 0.00 0.47 0.00 100.58 3.04 0.00 0.04 2.90 0.74 1.25 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.00 62.8 37.2 0.3

36.21 0.00 0.00 35.88 8.27 18.63 0.06 0.00 0.00 0.00 0.00 0.03 0.03 0.15 0.00 99.26 3.10 0.00 0.00 2.86 0.78 1.24 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 61.5 38.5 0.1

35.00 0.00 0.00 35.76 1.43 26.81 0.15 0.00 0.00 0.00 0.00 0.06 0.01 0.17 0.00 99.38 3.07 0.00 0.00 2.93 0.14 1.84 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 93.0 7.0 0.3

36.01 0.03 0.02 36.26 9.29 15.82 0.06 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 97.53 3.11 0.00 0.00 2.92 0.88 1.07 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 54.7 45.3 0.1

LG-PM02-B01

38.02 1.08 0.55 36.78 18.91 2.63 0.07 0.04 0.01 0.22 0.00 0.03 0.03 0.16 0.02 98.56 3.12 0.12 0.03 2.82 1.71 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 9.0 90.9 0.1

①

35.07 0.01 0.02 36.45 6.28 21.35 0.28 0.02 0.00 0.00 0.00 0.01 0.03 0.14 0.04 99.68 3.01 0.00 0.00 2.92 0.59 1.43 0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.00 70.7 29.3 0.6

(continued on next page) 8

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X. Jiang, et al.

Table 1 (continued) Sample

LG-0104-B01

Type point

1 1

Source Sample Type point

1 2

LG-PM02-B01 1 3

1 4

1 5

1 6

1 7

1 8

1 9

1 10

1 1

① LG-PM02-B05-2 2 2 1 2

2 3

2 4

2 5

2 6

2 7

2 8

2 9

2 10

CaO MgO TiO2 SiO2 Al2O3 FeO* MnO Cr2O3 Cl F K2O P2O5 Na2O SrO BaO TOTAL Ca Mg Ti Si Al Fe3+ Fe2+ Mn Cr K P Na Sr Ba Adr Gro Pra

35.11 0.00 0.03 34.55 0.27 28.13 0.04 0.00 0.00 0.00 0.00 0.04 0.02 0.13 0.02 98.34 3.13 0.00 0.00 2.87 0.03 1.96 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 98.7 1.3 0.1

35.40 0.00 0.00 34.80 0.14 28.25 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.14 0.01 98.77 3.14 0.00 0.00 2.88 0.01 1.96 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 99.3 0.7 0.0

35.30 0.00 0.01 34.36 0.24 28.25 0.03 0.02 0.01 0.00 0.01 0.03 0.04 0.13 0.00 98.43 3.14 0.00 0.00 2.85 0.02 1.96 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 98.7 1.2 0.1

34.80 0.02 0.00 35.04 0.10 28.43 0.00 0.00 0.00 0.00 0.00 0.03 0.01 0.13 0.00 98.55 3.10 0.00 0.00 2.91 0.01 1.97 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 99.5 0.5 0.0

36.25 0.00 0.00 36.05 6.78 20.74 0.01 0.02 0.00 0.39 0.00 0.02 0.00 0.12 0.00 100.38 3.10 0.00 0.00 2.87 0.64 1.38 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 68.4 31.5 0.0

34.87 0.01 0.00 34.78 1.53 26.55 0.03 0.00 0.00 0.00 0.01 0.03 0.05 0.12 0.06 98.04 3.10 0.00 0.00 2.89 0.15 1.84 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 92.5 7.5 0.1

34.65 0.00 0.00 35.38 0.45 28.35 0.04 0.01 0.00 0.60 0.00 0.04 0.00 0.15 0.00 99.67 3.06 0.00 0.00 2.92 0.04 1.96 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 97.8 2.2 0.1

34.68 0.00 0.03 35.21 0.26 28.58 0.03 0.00 0.01 0.54 0.00 0.04 0.01 0.19 0.04 99.61 3.07 0.00 0.00 2.91 0.03 1.98 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 98.7 1.3 0.1

36.81 0.00 0.08 36.15 9.86 16.26 0.08 0.01 0.00 0.00 0.00 0.02 0.01 0.19 0.00 99.48 3.12 0.00 0.00 2.86 0.92 1.08 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 53.9 46.1 0.2

Source Sample Type point

① LG-PM02-b05-3 2 2 5 6

2 7(rim)

LG-ZK0004-B14 2 2 1(core) 2

2 3

2 4

2 5(rim)

CaO MgO TiO2 SiO2 Al2O3 FeO* MnO Cr2O3 Cl F K2O P2O5 Na2O SrO BaO TOTAL Ca Mg Ti Si Al Fe3+ Fe2+ Mn Cr K P Na

36.79 0.01 0.00 36.64 9.42 16.67 0.12 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.02 100.26 3.11 0.00 0.00 2.89 0.88 1.10 0.00 0.01 0.00 0.00 0.00 0.00

35.64 0.00 0.00 35.79 2.67 24.91 0.14 0.01 0.00 0.00 0.00 0.02 0.00 0.19 0.04 99.94 3.11 0.00 0.00 2.91 0.26 1.70 0.00 0.01 0.00 0.00 0.00 0.00

37.30 0.00 0.00 36.33 12.18 13.09 0.29 0.00 0.00 0.50 0.00 0.04 0.02 0.16 0.02 100.41 3.13 0.00 0.00 2.85 1.13 0.86 0.00 0.02 0.00 0.00 0.00 0.00

37.51 0.01 0.01 37.40 13.63 11.30 0.44 0.00 0.00 0.00 0.01 0.03 0.00 0.17 0.00 100.95 3.10 0.00 0.00 2.89 1.24 0.73 0.00 0.03 0.00 0.00 0.00 0.00

36.77 0.01 0.00 36.34 8.33 17.54 0.14 0.03 0.01 0.41 0.00 0.04 0.01 0.18 0.00 100.31 3.14 0.00 0.00 2.89 0.78 1.17 0.00 0.01 0.00 0.00 0.00 0.00

34.88 0.00 0.00 35.20 1.99 25.99 0.43 0.02 0.00 0.00 0.00 0.04 0.00 0.17 0.00 99.18 3.08 0.00 0.00 2.90 0.19 1.79 0.00 0.03 0.00 0.00 0.00 0.00

34.84 0.00 0.00 34.88 0.40 27.91 0.06 0.00 0.00 0.64 0.00 0.03 0.00 0.14 0.00 98.90 3.10 0.00 0.00 2.90 0.04 1.94 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 98.0 2.0 0.1

35.05 0.01 0.00 35.37 2.58 25.13 0.20 0.03 0.00 0.00 0.00 0.05 0.00 0.16 0.00 99.03 3.09 0.00 0.00 2.91 0.25 1.73 0.00 0.01 0.00 0.00 0.00 0.00

35.37 0.00 0.00 36.65 6.62 20.51 0.29 0.00 0.01 0.00 0.00 0.03 0.00 0.16 0.00 100.08 3.04 0.00 0.00 2.94 0.62 1.37 0.00 0.02 0.00 0.00 0.00 0.00

1 3

1 4

1 5

LG-PM02-b05-3 2 2 1 2

2 3

2 4

34.83 0.02 0.00 34.57 1.20 27.01 0.09 0.01 0.00 0.00 0.00 0.00 0.01 0.13 0.00 99.64 3.11 0.00 0.00 2.88 0.12 1.88 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 94.1 5.9 0.2

36.63 0.00 0.00 35.36 9.86 16.89 0.08 0.00 0.00 0.00 0.00 0.02 0.00 0.14 0.00 99.47 3.12 0.00 0.00 2.81 0.92 1.12 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 54.9 45.1 0.2

29.08 0.00 0.02 28.99 12.87 24.82 0.14 0.15 0.00 0.00 0.07 0.02 0.02 0.16 0.02 96.92 2.58 0.00 0.00 2.40 1.26 1.72 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.00 57.6 42.1 0.4

LG-PM02-B05-2 2 2 2 1 2 3

2 4

35.21 0.00 0.01 35.34 1.27 26.57 0.07 0.02 0.01 0.41 0.00 0.04 0.00 0.08 0.01 99.02 3.11 0.00 0.00 2.92 0.12 1.83 0.00 0.00 0.00 0.00 0.00 0.00

35.18 0.00 0.00 35.36 2.18 25.99 0.09 0.01 0.00 0.00 0.00 0.02 0.00 0.15 0.00 98.98 3.09 0.00 0.00 2.90 0.21 1.78 0.00 0.01 0.00 0.00 0.00 0.00

35.19 0.00 0.00 35.87 3.05 24.86 0.10 0.00 0.00 0.00 0.00 0.05 0.01 0.13 0.01 99.28 3.07 0.00 0.00 2.92 0.29 1.69 0.00 0.01 0.00 0.00 0.00 0.00

35.13 0.00 0.00 35.37 0.79 27.88 0.09 0.00 0.01 0.00 0.00 0.03 0.01 0.16 0.00 99.46 3.09 0.00 0.00 2.90 0.08 1.91 0.00 0.01 0.00 0.00 0.00 0.00

1 2

1 6

1 7

1 8

LG-PM02-b05-3 2 2 1(core) 2

2 3

2 4

36.63 0.00 0.00 35.26 9.56 17.21 0.08 0.00 0.00 0.00 0.00 0.03 0.00 0.18 0.00 99.41 3.13 0.00 0.00 2.81 0.90 1.15 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 56.1 43.9 0.2

36.59 0.01 0.01 38.02 14.60 10.10 0.17 0.01 0.02 0.00 0.03 0.02 0.02 0.15 0.05 100.20 3.04 0.00 0.00 2.95 1.33 0.65 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 32.9 67.1 0.4

37.55 0.00 0.00 34.98 13.22 11.79 0.06 0.06 0.00 1.32 0.00 0.00 0.02 0.12 0.00 99.50 3.19 0.00 0.00 2.77 1.24 0.78 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 38.7 61.1 0.1

36.80 0.01 0.00 37.28 10.55 15.17 0.15 0.00 0.01 0.00 0.00 0.02 0.03 0.10 0.00 100.60 3.09 0.00 0.00 2.92 0.97 0.99 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 50.5 49.5 0.3

36.83 0.01 0.01 36.63 9.60 15.99 0.16 0.00 0.01 0.00 0.00 0.02 0.02 0.15 0.00 101.30 3.12 0.00 0.00 2.90 0.90 1.06 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 54.2 45.8 0.3

2 5

2 6

2 7

2 8

2 9

2 10

34.66 0.00 0.00 35.00 0.89 27.41 0.08 0.00 0.00 0.63 0.00 0.03 0.00 0.16 0.00 98.87 3.08 0.00 0.00 2.91 0.09 1.90 0.00 0.01 0.00 0.00 0.00 0.00

34.81 0.01 0.00 35.56 2.76 25.15 0.08 0.02 0.00 0.00 0.01 0.01 0.01 0.19 0.00 98.59 3.06 0.00 0.00 2.92 0.27 1.73 0.00 0.01 0.00 0.00 0.00 0.00

35.34 0.00 0.02 35.33 2.28 25.48 0.07 0.00 0.00 0.32 0.00 0.03 0.00 0.15 0.00 99.00 3.11 0.00 0.00 2.90 0.22 1.75 0.00 0.00 0.00 0.00 0.00 0.00

36.27 0.00 0.00 35.55 6.22 21.04 0.03 0.01 0.00 0.49 0.00 0.02 0.00 0.19 0.00 99.82 3.12 0.00 0.00 2.86 0.59 1.42 0.00 0.00 0.00 0.00 0.00 0.00

35.93 0.00 0.00 33.85 3.45 24.28 0.01 0.01 0.00 0.00 0.01 0.02 0.00 0.14 0.00 97.69 3.18 0.00 0.00 2.80 0.34 1.68 0.00 0.00 0.00 0.00 0.00 0.00

35.44 0.00 0.00 32.61 0.24 28.00 0.02 0.01 0.00 0.00 0.00 0.03 0.00 0.14 0.00 96.49 3.22 0.00 0.00 2.76 0.02 1.98 0.00 0.00 0.00 0.00 0.00 0.00

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Table 1 (continued) Sample

LG-0104-B01

LG-PM02-B01

Type point

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

1 9

1 10

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

Sr Ba Adr Gro Pra Source

0.01 0.00 55.7 44.3 0.3

0.01 0.00 87.3 12.6 0.5

0.01 0.00 86.8 13.1 0.3

0.01 0.00 43.3 56.7 0.6 ①

0.01 0.00 68.7 31.3 0.6

0.01 0.00 37.0 63.0 0.9

0.01 0.00 59.9 40.0 0.3

0.01 0.00 90.2 9.7 1.0

0.00 0.00 93.6 6.3 0.2

0.01 0.00 85.3 14.7 0.2

0.01 0.00 96.2 3.8 0.2

0.01 0.00 89.4 10.6 0.2

0.01 0.00 95.6 4.4 0.2

0.01 0.00 86.6 13.4 0.2

0.01 0.00 88.8 11.2 0.1

0.01 0.00 70.6 29.4 0.1

0.01 0.00 83.3 16.7 0.0

0.01 0.00 98.8 1.2 0.0

Notes: ①:Zhang et al., 2018; Guo et al., 2019; garnets (structural formula based on 12O); Pra, Pyralspite (sum of Almandine, Pyrope,Spessartine); Gro, Grossular; And, andradite; 0.00, below the detection

Fig. 8. BSE images and relative proportions of Adr and Grs profiles of the Grt1 and Grt2 at the Longgen skarn Pb-Zn deposit. Note: yellow dots indicate EPMA locations, white numbers indicate spot numbers. Abbreviations: Adr, andradite; Grs, grossular. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

trivalent cations and ΣREE3+, thus indicating that there is no YAG-type substitution in Grt1-3 (Fig. 13b). Moreover, F in EPMA data of garnet is almost below the detection limit, thus making the coupling substitution of anions and REE unlikely (Table 1). Therefore, the incorporation of REE3+ into Grt1-3 may follow the mechanism of Ca2+ vacancy. The geochemical behaviors of yttrium (Y) and REE3+ are very similar. If the incorporation of REE3+ and Y into garnet is wholly controlled via coupling substitution, a steady correlation should be found between REE3+ and Y/Ca (Park et al., 2017a). However, no REE3+ vs. Ca correlation was observed in this study (Fig. 13c). Although REE3+ vs. Y shows a slight correlation, there is a significant deviation in Grt2-4 (Fig. 13d), which may be caused by infiltration metasomatism or a

change in the chemical properties of fluids, but not by YAG substitution as previously reported (Park et al., 2017b). Therefore, in Longgen, apart from YAG-type coupling substitution and Ca2+ vacancies, fluid physical and chemical conditions (e.g., pH and fO2) may also have affected the incorporation of REE into garnet (Fu et al., 2018; Tian et al., 2019). 6.2. Physicochemical conditions of the hydrothermal fluid 6.2.1. pH The relationship between pH, anionic ligands, and REE in fluids has been widely discussed and involves important factors that control REE fractionation (e.g., Bau, 1991; Tian et al., 2019). Michard (1989) 10

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Table 2 Representative LA-ICP-MS analyses of garnets from Longgen skarn Pb-Zn deposit. Stage

Grt 1–1

Grt 1–2

SiO2 Al2O3 FeO* Na Mg Al Ca Sc Ti V Cr Mn Co Cu Zn Ga Rb Sr Y Zr Nb Sn Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Pb Th U ΣREE LREE HREE LREE/HREE LaN/YbN δEu δCe

35.3 9.00 18.3 284.47 24.02 4.76 25.98 1.36 83.62 26.18 3.10 0.18 1.87 5.15 10.05 10.02 < LOD 2283.72 21.43 0.09 0.14 3.36 0.05 0.94 0.60 2.10 0.46 3.87 3.10 1.69 5.09 0.58 2.83 0.56 0.96 0.13 0.68 0.13 < LOD < LOD 29.92 1.89 0.01 0.14 22.77 11.81 10.96 1.08 0.64 1.29 0.94

35.2 7.81 19.2 269.38 23.32 4.13 26.45 0.36 89.43 26.57 5.33 0.18 2.48 2.93 13.59 8.03 < LOD 40.65 5.12 < LOD 0.18 2.77 < LOD 0.57 0.25 0.40 0.24 2.66 2.34 1.50 1.73 0.24 0.77 0.14 0.11 0.04 0.08 0.01 < LOD < LOD 50.80 1.14 0.11 0.04 10.51 7.39 3.12 2.37 2.15 2.19 0.37

Stage

Grt 1–3

Grt 2–1

SiO2 Al2O3 FeO Na Mg Al Ca Sc Ti V Cr Mn Co Cu Zn Ga Rb Sr Y Zr Nb

35.5 9.82 16.6 59.00 19.28 5.20 26.78 0.16 9.17 19.09 < LOD 0.14 0.64 1.26 3.44 9.10 0.00 6.51 0.17 < LOD < LOD

36.1 8.54 18.7 69.94 22.30 4.52 25.58 17.89 86.89 22.08 9.10 0.12 1.99 15.18 7.25 12.79 0.43 18.07 479.11 < LOD 0.16

35.4 10.4 17.2 78.70 10.15 5.49 25.45 8.18 98.69 26.57 2.54 0.18 1.12 2.26 2.66 8.18 < LOD 5193.41 96.90 0.29 0.19 2.57 0.03 0.89 0.92 1.85 0.40 3.70 3.31 2.08 9.65 1.63 11.33 2.64 6.91 1.02 7.62 1.34 0.05 0.03 11.06 2.45 < LOD 0.08 54.39 12.25 42.14 0.29 0.09 1.04 0.75

35.3 9.25 17.7 187.17 223.66 4.90 26.33 7.93 106.79 27.77 2.35 0.20 3.39 55.61 14.12 7.47 0.10 132.62 69.31 < LOD 0.23 3.49 0.07 2.49 0.75 0.46 0.10 1.29 1.95 1.54 4.63 1.12 7.45 1.71 5.16 0.72 5.18 1.07 < LOD 0.03 17.01 2.71 0.01 0.05 33.13 6.09 27.04 0.23 0.10 1.51 0.37

35.2 9.68 17.7 105.40 15.03 5.13 25.94 7.34 117.28 27.12 13.41 0.18 1.37 15.10 5.13 8.63 0.02 2777.83 106.41 0.08 0.24 2.30 0.03 0.57 0.95 1.24 0.22 1.72 1.97 1.72 7.35 1.80 11.75 2.74 7.82 0.91 6.78 1.09 < LOD 0.06 13.92 4.05 0.02 0.07 48.07 7.81 40.26 0.19 0.10 1.22 0.64

35.6 9.41 16.4 43.22 13.78 4.98 27.11 2.27 30.74 24.20 0.68 0.17 1.43 0.77 4.09 7.99 < LOD 9.81 27.87 1.93 0.05 1.52 0.04 0.29 0.26 1.48 0.59 8.15 6.15 1.32 9.08 1.22 5.57 0.93 2.15 0.13 1.21 0.26 0.04 < LOD 20.95 0.69 0.02 0.01 38.48 17.94 20.54 0.87 0.15 0.54 0.66

Grt 1–3 35.6 9.76 16.1 197.75 10.28 5.16 26.95 6.02 80.45 28.08 2.89 0.20 1.04 3.17 4.62 6.94 0.16 52.24 126.55 0.39 0.22 1.78 0.04 0.60 0.22 0.20 0.03 1.06 2.91 1.32 12.49 2.61 17.42 3.68 8.13 0.98 6.81 1.14 < LOD 0.04 11.66 1.17 0.01 0.04 59.00 5.73 53.26 0.11 0.02 0.57 0.54

35.6 9.61 16.4 204.38 17.16 5.09 26.83 3.85 37.59 25.54 2.84 0.18 1.73 9.33 3.08 7.81 0.37 265.86 58.24 < LOD 0.04 2.19 < LOD 1.36 20.82 50.92 6.76 28.07 7.76 1.27 12.19 1.96 9.27 1.73 3.70 0.46 2.40 0.35 0.02 < LOD 15.23 1.52 0.02 0.04 147.66 115.60 32.05 3.61 6.22 0.40 1.05

35.8 9.51 16.6 80.50 13.16 5.04 26.63 6.00 87.76 26.82 3.23 0.20 1.16 1.54 3.07 7.94 0.04 240.58 101.88 < LOD 0.18 2.31 0.00 0.15 0.24 0.23 0.08 1.63 2.30 1.43 11.60 2.21 12.83 2.72 6.37 0.77 5.54 0.94 < LOD < LOD 13.97 0.37 < LOD 0.04 48.89 5.90 42.99 0.14 0.03 0.69 0.41

35.6 9.44 16.9 34.51 14.91 5.00 26.68 2.55 26.82 24.80 0.51 0.16 1.48 0.10 2.03 8.23 < LOD 46.42 41.00 0.10 0.06 2.58 0.02 0.22 1.61 6.14 1.76 14.14 6.86 1.41 10.65 1.42 7.60 1.26 2.38 0.26 2.12 0.35 < LOD < LOD 19.49 0.46 < LOD 0.02 57.95 31.91 26.04 1.23 0.54 0.50 0.79

35.6 9.73 16.0 147.48 21.55 5.15 27.16 0.51 6.64 21.06 2.34 0.15 1.16 11.07 3.96 9.55 0.13 7.46 2.21 0.10 0.01 1.46 < LOD 0.60 0.53 2.44 0.70 4.39 0.87 0.63 0.87 0.09 0.16 0.05 0.16 0.04 0.24 0.03 < LOD < LOD 21.88 1.39 < LOD 0.01 11.19 9.56 1.63 5.87 1.61 2.19 0.83

Grt 2–2 37.7 16.2 15.0 48.41 5.98 8.58 21.36 5.15 61.44 26.65 1.41 0.06 0.78 1.43 5.41 48.94 0.07 4608.58 274.34 < LOD 0.06

35.9 9.85 16.9 49.18 12.90 5.21 26.23 0.82 51.39 20.86 2.33 0.10 1.71 2.58 8.18 9.86 < LOD 33.27 44.00 < LOD 0.08

35.9 13.5 13.1 97.58 14.99 7.16 26.31 5.81 28.09 50.11 14.91 0.05 0.42 38.72 4.41 11.87 0.54 7.97 805.22 0.14 0.97

35.7 9.77 16.4 264.84 17.33 5.17 26.78 0.21 10.68 20.30 3.49 0.14 0.79 3.17 5.33 9.04 0.32 8.76 0.33 < LOD < LOD 2.59 0.02 0.83 0.59 2.39 0.53 2.77 0.46 0.47 0.18 0.02 < LOD 0.01 < LOD 0.01 0.03 < LOD < LOD < LOD 22.47 1.11 0.01 0.01 7.46 7.21 0.26 27.94 14.52 4.18 0.97

35.8 10.1 16.2 284.86 22.75 5.34 26.55 0.35 4.03 18.01 3.78 0.16 1.10 14.37 9.09 9.30 0.41 51.75 0.19 < LOD < LOD 2.59 < LOD 0.38 10.63 20.28 2.26 6.21 0.27 0.39 0.27 0.01 < LOD 0.01 0.04 < LOD 0.06 < LOD < LOD < LOD 18.03 1.33 < LOD 0.02 40.44 40.04 0.39 101.68 128.66 4.36 0.97

35.5 9.67 16.7 71.86 16.28 5.12 26.80 0.31 14.92 23.01 < LOD 0.15 1.41 1.75 2.03 8.05 < LOD 6.41 2.23 < LOD < LOD 2.19 0.02 0.15 0.30 2.42 0.71 5.86 0.76 0.75 1.24 0.12 0.46 0.06 0.07 0.01 0.09 < LOD 0.04 < LOD 16.42 0.70 < LOD 0.04 12.84 10.79 2.05 5.27 2.53 2.35 0.90

36.4 9.10 18.0 38.38 15.78 4.82 25.56 0.59 13.10 16.88 2.37 0.10 1.38 0.83 0.83 10.38 < LOD 4.99 8.54 0.12 0.04

37.2 8.69 17.3 43.25 12.68 4.60 25.81 0.85 6.47 17.11 < LOD 0.12 1.87 1.13 1.99 10.27 < LOD 6.47 4.73 0.13 0.09

Grt 2–3 36.2 10.5 16.3 112.53 11.12 5.58 25.90 2.51 109.72 18.27 8.04 0.08 1.11 13.00 9.26 9.06 0.51 3.98 599.32 0.13 0.11

36.9 11.1 15.2 73.08 5.97 5.90 25.74 5.44 77.06 20.36 3.97 0.07 0.87 4.16 5.88 8.67 < LOD 7.76 578.61 0.13 0.13

36.8 10.8 16.1 38.22 8.77 5.70 25.46 2.99 105.11 19.21 0.75 0.07 1.48 188.66 2.21 10.33 < LOD 9.41 424.25 < LOD 0.11

35.7 9.42 17.3 56.35 14.29 4.98 26.38 0.43 62.87 19.12 6.68 0.08 1.95 1.21 9.90 11.36 < LOD 19.25 48.42 2.04 0.11

35.7 9.61 17.2 51.12 16.30 5.09 26.28 0.21 51.29 23.59 2.81 0.10 1.90 3.69 5.40 11.00 0.53 12.10 24.02 4.87 0.03

36.3 9.91 16.9 43.25 8.95 5.25 25.90 0.92 32.19 17.22 5.05 0.10 1.38 2.38 4.64 10.54 0.35 13.01 54.44 2.03 < LOD

36.5 9.88 16.8 120.60 13.84 5.23 25.85 0.55 17.99 17.48 260.75 0.09 0.89 4.47 5.94 11.58 0.12 5.89 9.49 0.25 0.03

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Table 2 (continued) Stage

Grt 1–1

Grt 1–2

Sn Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Pb Th U ΣREE LREE HREE LREE/HREE LaN/YbN δEu δCe

2.43 0.02 0.22 0.37 2.14 0.50 2.04 0.05 0.40 0.14 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD 17.74 0.39 0.02 < LOD 5.63 5.49 0.14 40.58 – 14.41 1.02

4.19 < LOD 0.65 6.09 12.97 2.31 19.89 16.06 2.82 44.63 8.56 58.58 14.41 40.03 4.81 30.97 4.52 < LOD 0.01 59.03 2.33 < LOD 0.03 266.65 60.15 206.51 0.29 0.14 0.30 0.85

Stage

Grt 2–3

Grt 2–4

SiO2 Al2O3 FeO Na Mg Al Ca Sc Ti V Cr Mn Co Cu Zn Ga Rb Sr Y Zr Nb Sn Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Pb Th

39.0 8.81 16.3 29.26 13.31 4.66 25.26 0.26 10.11 15.72 1.67 0.10 1.09 1.85 1.75 9.86 < LOD 6.79 6.03 0.24 0.04 2.08 < LOD 0.19 1.15 2.94 1.25 12.15 3.50 0.57 2.97 0.34 1.00 0.17 0.55 0.04 0.84 0.21 0.03 < LOD 16.88 1.44 < LOD

36.1 4.07 23.7 28.92 21.34 2.15 25.17 0.73 1.57 15.16 0.39 0.15 1.72 0.71 2.71 9.61 < LOD 108.88 0.94 0.43 < LOD 0.77 < LOD 0.32 10.08 27.87 3.45 11.46 0.80 0.76 0.50 0.02 0.09 0.05 0.07 < LOD 0.29 < LOD < LOD 0.02 34.42 0.93 0.00

3.32 0.01 0.50 71.23 176.04 28.08 148.00 40.39 4.72 55.60 9.75 49.35 9.91 23.71 2.10 11.02 1.42 < LOD 0.01 21.58 6.14 0.01 0.01 631.32 468.47 162.85 2.88 4.64 0.30 0.96

2.81 0.03 0.60 6.85 14.86 2.55 18.49 8.70 1.34 11.37 1.49 7.77 1.55 2.87 0.39 1.83 0.26 < LOD < LOD 94.26 1.62 < LOD < LOD 80.33 52.79 27.54 1.92 2.68 0.41 0.87

1.74 0.10 0.44 1.54 1.01 0.37 2.76 4.02 0.81 14.50 7.04 74.66 26.17 85.44 12.07 71.32 7.03 0.03 0.01 18.70 3.01 0.01 0.01 308.72 10.51 298.22 0.04 0.02 0.29 0.32

4.38 0.10 0.31 1.31 0.69 0.47 8.82 16.74 2.80 49.56 10.97 73.61 16.85 43.55 5.60 26.90 3.63 0.03 0.03 23.73 4.17 0.01 0.04 261.51 30.83 230.68 0.13 0.03 0.28 0.22

Grt 1–3 2.01 0.15 1.26 1.47 0.71 0.17 2.71 4.86 1.34 26.94 7.60 56.80 15.56 46.65 5.80 35.86 4.40 < LOD 0.03 35.76 2.54 0.02 0.01 210.87 11.26 199.61 0.06 0.03 0.28 0.29

2.07 0.05 0.50 1.19 3.07 0.54 5.51 9.12 2.51 30.22 6.91 44.70 10.63 30.14 3.72 22.53 3.04 < LOD 0.01 41.26 0.91 0.03 0.03 173.84 21.94 151.90 0.14 0.04 0.42 0.94

2.78 0.10 0.45 0.92 1.67 0.64 9.29 6.56 2.13 10.17 1.37 6.87 1.40 3.71 0.36 2.09 0.20 0.06 < LOD 163.99 1.58 < LOD < LOD 47.37 21.21 26.16 0.81 0.32 0.80 0.51

5.77 < LOD 0.82 0.77 1.25 0.64 9.10 6.04 1.45 7.16 0.79 3.53 0.68 1.30 0.18 0.82 0.13 < LOD < LOD 125.70 2.11 < LOD 0.04 33.84 19.25 14.59 1.32 0.67 0.67 0.41

1.82 0.07 0.42 0.89 1.91 0.86 12.76 9.34 1.15 14.32 2.05 10.20 1.79 3.81 0.55 1.66 0.28 0.03 < LOD 48.03 1.93 < LOD 0.04 61.57 26.92 34.66 0.78 0.39 0.30 0.49

2.90 0.02 0.69 1.42 2.02 0.81 8.65 3.34 0.86 3.53 0.43 1.50 0.33 0.86 0.19 0.47 0.19 < LOD 0.03 20.56 4.87 < LOD 0.02 24.61 17.11 7.50 2.28 2.15 0.76 0.45

36.0 3.02 24.0 26.49 45.37 1.60 25.75 0.87 3.60 17.91 0.42 0.16 1.88 1.14 3.81 8.73 < LOD 13.88 0.33 < LOD 0.04 0.49 0.06 0.59 3.51 4.85 0.52 1.63 0.10 0.01 0.05 0.01 0.06 0.01 0.07 0.03 0.10 0.02 0.03 < LOD 31.20 1.13 0.01

36.1 2.98 24.1 47.35 47.32 1.58 25.61 0.60 3.07 17.15 2.37 0.16 1.82 1.29 2.99 8.49 < LOD 13.92 0.37 < LOD 0.03 2.08 < LOD 0.08 4.77 6.55 0.58 1.52 0.31 0.01 0.26 0.02 0.03 0.01 0.02 < LOD 0.14 0.03 < LOD < LOD 39.53 1.52 < LOD

36.6 3.88 22.7 54.75 60.17 2.05 25.74 0.91 < LOD 6.01 3.76 0.12 2.17 1.27 3.59 11.65 0.30 6.38 < LOD < LOD < LOD 1.68 < LOD < LOD 1.51 0.97 0.05 < LOD 0.20 0.04 0.10 < LOD 0.03 0.01 0.02 0.01 < LOD 0.02 < LOD < LOD 20.78 1.60 0.01

36.4 4.23 22.0 34.27 60.27 2.24 26.14 0.15 0.34 4.72 5.11 0.11 1.36 1.56 2.14 9.98 0.07 5.86 0.06 < LOD < LOD 0.67 < LOD 0.18 2.10 1.58 0.06 0.16 < LOD 0.03 0.16 < LOD 0.06 < LOD < LOD < LOD < LOD < LOD 0.03 0.02 22.99 1.44 < LOD

36.4 3.73 22.6 27.22 68.27 1.97 26.09 0.56 < LOD 6.00 3.32 0.12 2.05 1.16 3.55 11.62 < LOD 7.88 < LOD < LOD < LOD 1.55 0.07 0.62 1.97 1.26 0.06 0.05 < LOD 0.03 0.05 < LOD < LOD 0.01 < LOD 0.01 < LOD 0.01 < LOD < LOD 29.17 1.78 < LOD

2.40 < LOD 0.28 0.53 2.31 1.23 10.79 3.17 0.75 2.99 0.43 1.61 0.31 0.69 0.05 0.48 0.20 < LOD 0.03 14.55 0.97 < LOD 0.04 25.54 18.78 6.76 2.78 0.78 0.74 0.50

2.25 0.06 0.20 1.00 5.03 1.72 13.03 3.39 0.62 1.93 0.17 0.97 0.17 0.34 0.01 0.63 0.09 < LOD < LOD 18.31 0.66 0.01 0.01 29.10 24.79 4.31 5.76 1.15 0.68 0.74

Grt 2–5 35.8 3.17 24.7 23.86 31.57 1.68 25.32 0.57 3.16 15.07 1.65 0.15 2.22 0.93 0.96 8.84 < LOD 14.10 0.78 0.44 0.03 1.65 0.05 0.09 11.45 20.41 1.80 4.69 0.16 0.08 0.11 0.07 0.30 0.03 0.07 0.01 0.17 0.03 0.03 < LOD 43.84 0.68 < LOD

36.2 3.85 23.5 36.80 24.81 2.04 25.45 0.71 1.19 14.83 0.66 0.15 1.65 1.17 3.96 9.58 0.03 15.05 0.96 < LOD 0.01 1.03 0.00 0.17 13.48 30.21 3.34 7.98 0.89 0.72 0.47 0.02 0.06 0.05 0.04 0.02 0.24 0.05 < LOD < LOD 40.02 1.56 < LOD

36.4 3.80 23.2 24.25 28.19 2.01 25.59 0.30 1.81 16.87 1.53 0.16 1.82 0.83 2.03 11.00 < LOD 10.23 0.43 0.43 0.00 0.76 < LOD 0.34 9.81 21.76 2.19 6.11 0.31 0.13 0.26 0.05 < LOD 0.03 0.04 < LOD 0.37 0.06 0.03 < LOD 30.58 0.59 0.04

36.4 5.17 21.5 68.90 25.13 2.74 25.79 0.71 2.72 15.41 5.77 0.14 2.13 0.90 2.01 10.12 0.18 12.16 0.98 0.11 0.01 2.45 < LOD 0.08 8.97 31.38 5.16 19.78 1.84 0.32 0.69 0.11 0.27 0.02 0.05 0.01 0.45 0.09 0.03 < LOD 39.14 1.60 0.01

36.0 3.13 24.3 56.96 36.64 1.66 25.50 1.32 3.81 15.99 2.23 0.16 2.15 1.07 2.37 10.57 < LOD 12.46 0.50 < LOD 0.03 0.69 0.02 0.18 8.25 < LOD 1.10 2.98 0.42 0.05 0.11 0.01 0.15 0.02 0.11 0.04 0.24 0.04 < LOD < LOD 41.06 1.07 < LOD

(continued on next page) 12

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Table 2 (continued) Stage

Grt 1–1

U ΣREE LREE HREE LREE/HREE LaN/YbN δEu δCe

0.02 27.68 21.55 6.13 3.52 0.98 0.52 0.53

Grt 1–2 < LOD 55.44 54.42 1.02 53.31 24.83 3.41 1.16

0.02 39.40 38.60 0.80 48.28 47.77 1.65 0.99

< LOD 57.58 56.62 0.96 58.92 40.44 3.07 1.07

< LOD 41.12 40.31 0.80 50.22 19.20 1.40 1.10

< LOD 69.14 67.45 1.70 39.78 14.30 0.71 1.11

Grt 1–3 0.03 13.53 12.80 0.73 17.55 24.20 0.54 0.87

0.01 10.97 10.63 0.34 30.91 25.32 0.50 0.78

0.01 14.26 13.75 0.51 26.99 25.14 0.16 0.82

0.04 2.97 2.77 0.20 14.04 – 0.83 0.46

< LOD 4.16 3.94 0.22 17.66 – – 0.54

< LOD 3.45 3.37 0.08 43.41 – – 0.46

Note: < LOD = below detection limit; –: no results; The unit of oxide is wt.%, and the unit of element is ppm. REE normalized to chondrite (Sun and McDonough, 1989).

demonstrated a negative correlation between pH and total REE in the fluid, while Bau (1991) reported that pH exerts an important effect on REE fractionation. Under weakly acidic conditions (pH < 6–7), the chondrite-normalized REE distribution model of hydrothermal fluids is characterized by an enrichment of LREE, a depletion of HREE, and a positive Eu-anomaly (δEu > 1; Bau, 1991). However, HREE enrichment and LREE depletion with no or negative Eu anomalies (δEu < 1) in the nearly neutral fluids (pH = 6–7) have been reported (Xu et al., 2016; Tian et al., 2019). The behaviors of REE are also controlled by the Cl− in the fluids, which can carry Eu as an EuCl42− complex and cause δEu > 1 (Zhai et al., 2014; Tian et al., 2019). δEu < 1 can be interpreted by a lack of Cl−, which can act as a ligand for Eu transportation (Gaspar et al., 2008). At the Longgen deposit, Grt1-1 and Grt1-2 are strongly enriched with HREE, depleted of LREE, and show minor δEu < 1 and infrequent δEu > 1, indicating that the hydrothermal fluid may be nearly neutral (Park et al., 2017a; Tian et al., 2019). Grt1-3 is enriched in LREE, depleted in HREE, and pronounced in δEu > 1; thus, the hydrothermal fluid is mildly/weakly acidic (Fu et al., 2018; Tian et al., 2019). However, Grt2-1, Grt2-2, and Grt2-3 indicate that they may have crystallized in a near-neutral environment, and their pH gradually decreases (Xiao et al., 2018; Tian et al., 2019). Grt2-4 and Grt2-5 not only imply that they were crystallized from mildly/weakly acidic fluids, but they largely differ in the content of Cl− (Gaspar et al., 2008). Hence, this may have been caused by environmental mutation. The relationship between Y-ΣREE (Fig. 13d) can also be seen from the change of Al and Fe content (Fig. 9a). Compared with Grt2-5, Grt2-4 enriches LREE, which may be due to the systematic opening of Grt2-4, which results in an exchange of substances and the entry of LREE (Tian et al., 2019).

environment (Fig. 13e and f). 6.2.3. Temperature and fluid/rock interaction Temperature also plays an important role in determining the trace elements of garnet (Sun et al., 2020a, 2020b). However, the homogenization temperatures of garnet fluid inclusions in the Longgen deposit were mainly identified as 440–480 °C, indicating small temperature fluctuation (Zhang et al., 2018). This suggests that the temperature has relatively little effect on the garnet composition in the Longgen deposit. The distribution coefficient of high field intensity elements (Nb, Ta, Zr, and Hf) in the hydrothermal fluid decreases with increasing fluid/ rock ratio (Gaspar et al., 2008). These coefficients reflect the original and developed fluid or the influence of an external fluid (Jamtveit et al., 1993). Furthermore, the change of ∑REE is associated with the entry of externally buffered fluid into the metallogenic system (Smith et al., 2004). However, the contents of Nb, Ta, Zr, and Hf in Longgen garnet are mostly below the detection limit, which may indicate a high fluid/ rock ratio. Therefore, it remains unknown if the garnets had a high fluid/rock ratio during the growth process. 6.3. Evolutions of fluid in garnet REE zoning Trace element analysis can be used to identify the kinetics of crystal growth during fluid evolution recorded by the garnets (Park et al., 2017a). Grt2-1-Grt2-3 and Grt1-1-Grt1-2 have a relatively Al-rich composition and a slightly HREE-enriched pattern (Fig. 10a, b, e, f, and 11) combined with a positive correlation between Y and ΣREE. This indicates that these minerals are formed by diffusion metasomatism and fluid equilibrium crystallization in a closed system (Jamtveit and Hervig, 1994). In advection metasomatism, garnet forms by rapid growth under high fluid/rock ratios, affected by external fluids (Ortoleva et al., 1987). Grt1-3, Grt2-4, and Grt2-5 do not show such a correlation between Y and ΣREE. This result can be explained by the effect of advection substitution. Under non-equilibrium conditions, the rapid growth of minerals is controlled by surface adsorption (Gaspar et al., 2008). Therefore, in this case, the chondrite-normalized REE curve of garnet is LREE-enriched and HREE-depleted (Reed et al., 2000; Park et al., 2017a). The compositional characteristics of Grt2-4 present an example of the identifiable characteristics of garnet produced by advection metasomatism. The relative enrichment of LREE and the corresponding depletion of HREE patterns in Grt1-3, Grt2-4, and Grt2-5 are apparent in Figs. 10, 11, and 14. Different Eu levels confirmed that the fluid composition differed for each stage (Fig. 14). Van Westernen et al. (2000) showed that Eu2+ requires lower relaxation energy than REE3+ to enter the Ca site. In addition, Eu2+ is dominant in skarn systems above 250 °C (Bau, 1991), and the stability of soluble Eu2+ can be enhanced by chlorine (Mayanovic et al., 2007). The Eu in the Grt2-4, Grt1-1, and Grt1-3 was positively fractionated with regard to other REEs, thus indicating a Clrich system (Fig. 14). This is consistent with the Cl-rich phenomenon in hydrothermal fluids, as indicated by the daughter crystal minerals in

6.2.2. fO2 The behavior of redox-sensitive elements (e.g., Eu, Sn, and U) in oreforming fluids is strongly influenced by the fO2, which can thus be used as an indicator of the redox state of hydrothermal fluids (Smith et al., 2004; Tian et al., 2019). Under identical conditions, the closer the ionic radii are, the more likely ionic substitution will occur. Hence, U4+ (0.97 Å) is more likely to substitute Ca2+ (0.99 Å) in garnet than U6+ (0.70 Å; Smith et al., 2004; Xu et al., 2016). The properties of other redox elements are similar to those of U, in which Eu2+ (1.12 Å) and Sn2+ (1.12 Å) easily replace Ca2+. Therefore, their connection (e.g., U, Sn, and Eu) in Longgen Grt1 and Grt2 indicates the fO2 of the hydrothermal fluids in the skarn mineralization (Tian et al., 2019). As mentioned above, in the Longgen skarn Pb-Zn deposit, both Grt1 and Grt2 have lower U contents (< 1 ppm) than other skarn deposits (e.g., Fe, Cu, Mo, and W; Table 2), indicating that it formed under relatively high fO2 (Tian et al., 2019). In Grt1-1-Grt1-3 and Grt2-1-Grt25, the contents of Eu and Sn decreased with the continued evolution of the stages (Fig. 13e and f). This may indicate that with the crystallization of garnet, the environment changed from low oxygen fugacity to high oxygen fugacity, while the overall oxygen fugacity remained high. From core to rim in both Grt1 and Grt2, the linear trend of Eu and Sn may indicate that the garnets grew in a dynamic gradual redox 13

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Fig. 9. Binary plots of the trace elements from the garnet in the Longgen skarn Pb-Zn deposit. The black dotted line represents the value of primitive mantle after Sun and McDonough (1989). 14

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Fig. 10. LA-ICP-MS_mapping of rare earth elements (REE), cerium (Ce), samarium (Sm)/europium (Eu), ytterbium (Y), holmium (Ho), aluminum (Al), iron (Fe), titanium (Ti), zinc (Zn), and tungsten (W) in different types of garnets (Grt1 and Grt2) from the Longgen skarn deposit.

part of the fluid inclusions (Zhang et al., 2018). In summary, Fig. 14 presents a model diagram of the two main types of ore-related garnets in the Longgen skarn deposit. Grt2 shows an obvious crystal growth sequence from the core to rim. Grt2-1 is a closed system with a relatively high ΣREE, near-neutral pH, and relatively low fO2. The residual structure of Grt2-1 formed because of the rapid growth of the crystal and is recorded by REE. With the continuous growth of the crystal, both LREE and ΣREE in the REE distribution

pattern decrease. In Grt2-4, because of the opening of the system, likely initiated by an additional pulse of metalliferous fluid (Li et al., 2018), the pH gradually decreased to weak acidity, fO2 increased, and LREE and Cl− could enter. In Grt2-5, REE were depleted, and both the crystal growth rate and ability to control REE decreased (Fig. 14). Based on the distribution of mineral morphology and REE, it can be speculated that Grt1 was two crystal that grew together. Grt1-1 and Grt1-2 crystallized in closed systems with relatively high ΣREE, near-neutral pH, and 15

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Fig. 11. Chondrite-normalized REE patterns for the single garnet grains Grt1 and Grt2 as identified by LA-ICP-MS_mapping (a, c) and LA-ICP-MS_spot (b, d) in the Longgen deposit. Chondrite values originate from Sun and McDonough (1989).

relatively low fO2. With the fluid evolution, ΣREE decreased in Grt1-3, and HREE decreased significantly; the pH value was also nearly acidic, and the fO2 increased. During the evolution of this garnet, there may have been two increases in the Cl− content in the solution, in Grt1-1 and Grt1-3 (Fig. 14). Both garnet types contain Pb-Zn-bearing hydrothermal fluids in the late stage, which are filled along crystal cracks (Fig. 9-Zn). The equilibrium and non-equilibrium states in the skarn system are reflected in the garnet composition zones, which depend on the spatial location, fluid composition, and physicochemical conditions.

chemical composition of garnet has become a potentially powerful tool with which to identify different skarn mineralization types. Meinert (1992) used in-situ EPMA analysis and proposed a triangle diagram of the proportion of the four end-element components of hydrothermal garnet and plotted a classification chart of skarn deposits. However, these charts overlap in different skarn deposits, such as Fe, Cu, Zn, and Au. Tian et al. (2019) proposed a classification scheme of garnet trace elements and skarn deposits by using in-situ LA-ICP-MS_spot analysis. However, only W, W-Mo, W-Sn, and Cu deposits can be distinguished by this discriminant diagram, and no scheme can be obtained for other deposits (such as Pb-Zn and Fe). To obtain this, FA has been utilized to identify and explain maximal variational trace elements among different skarn deposits (Fig. 12). This study showed that these elements (e.g., U, Hf, Zr, La, and Ce) could

6.4. Fingerprinting skarn deposit types With the development of mineral in-situ analysis technology, the

Fig. 12. FS3 vs FS1 (a) and FS3 vs FS2 (b) biplots for factor analysis in garnets from three major types of skarn deposits. Data listed can be found in Supplementary 1. Abbreviations: FS1: scores of factor one, FS2: scores of factor two, FS3: scores of factor three, FL1: loadings of factor one, FL2: loadings of factor two, FL3: loadings of factor three. 16

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Fig. 13. Plots of total REE3+ with Mg, FeO*, Ca, and Y, as well as changing trends of Eu and Sn with the stages of the garnets (Grt1 and Grt2) in Longgen. (a-d) The substitution mechanism of REE into garnet and the opening and closing of the system. (e-f) The variations of oxygen fugacity.

17

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Fig. 14. Crystal growth and REE model evolution of Grt1 (right) and Grt2 (left) in garnets in Longgen. The black hollow arrow represents the garnet evolution from the core to the rim (Grt1-1-Grt1-3 and Grt2-1-Grt2-5). The black dashed arrow represents the increasing oxygen fugacity trend. The broken red line represents the chondrite-normalized REE distribution pattern of the corresponding stage, and the most obvious position of fluctuation is the Eu element. The solid black arrows represent the direction in and out of the ions. The background color approximates the total amount of REE, which decreases from top to bottom. Abbreviations: LR: light rare earth elements. Abbreviations: Cl−: chloride ion, fO2: oxygen fugacity. Values of chondrite originate from Sun and McDonough (1989). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

problems of most skarn deposits. logCe-logU can distinguish skarn Cu, Pb-Zn, W, and W-Sn deposits (Fig. 15c), while log(Ce + Hf)-logU can distinguish Cu, Cu-Mo, Fe-Zn, Pb-Zn, and W skarn deposits (Fig. 15d). Ce, U, and Sn elements in garnet may be more sensitive to changes in the redox environment of the ore-forming fluid than other redox elements. More interestingly, except for the Pb-Zn skarn deposit, other skarn deposits have a linear relationship (Fig. 15c and d). This may be because U4+ (0.97 Å) can replace Ca2+ (0.99 Å) easier than U6+ (0.7 Å) (Smith et al., 2004). Moreover, Ce3+ (ionic potential = 2.70) compared with Ce4+ (ionic potential = 3.96) can easier interact with Ca2+ (ionic potential = 2.02) (Goldschmidt, 1937; Railsback, 2003). In addition, U4+ (0.97 Å) and Hf4+ (0.81 Å) also have similar geochemical properties. Therefore, in the case of redox environment changes, these redox-sensitive elements will also change, and the elements that coexist with them will vary accordingly. This will result in a distinct linear relationship between different types of deposits (Fig. 15c and d). This study indicates that garnet has different characteristics in different types of skarn deposits (Cu, Cu-Mo, Fe-Zn, Pb-Zn, W, W-Mo, and W-Sn), which can be distinguished with the diagrams mentioned above.

better distinguish between Cu-, W-, and Zn-related skarn deposits. However, to further distinguish the skarn-type deposits, FA appears to be inadequate. Redox-sensitive elements are also an effective tool with which skarn deposits can be distinguished. Hence, this study employed widely-accepted redox-sensitive elements (e.g., W, Sn U, and Ce; Tian et al., 2019) and FA classified elements (e.g., U, Ce, Hf, and Zr) to fingerprint the skarn deposit types. According to the results of FA, U was used as the abscissa to classify the genesis of the deposit. The discriminant diagram and data of skarn deposit types are shown in Fig. 15 and Supplementary 2. With increasing types of deposits and research data, the logW-logU and logSn-logU diagrams (Fig. 15a and b) are increasingly unable to meet the requirements of fingerprinting the skarn deposit types. In logW-logU, different types of skarn deposits overlap (Fig. 15a), while in the logSn-logU diagram, in addition to the large overlap between W-Sn and W, the diagram can still be used to distinguish Cu, W, W-Mo, and Pb-Zn (Fig. 15b). The logCe-logU and log(Ce + Hf)-logU diagrams (Fig. 15c and d) proposed in this paper can solve the classification 18

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Fig. 15. Plots of logW vs logU (a, Tian et al., 2019), logSn vs logU (b, Tian et al., 2019), logCe vs logU (c), and log(Ce + Hf) vs logU (d) in garnets of different skarn deposits. Data originate from Supplementary 2. Abbreviation: log: Logarithm with a base of 10.

Furthermore, the trace elements of garnet, especially valence metal elements, can indicate the formation environment of garnet and can be used to guide the exploration of skarn deposits.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

7. Conclusions 1. In-situ LA-ICP-MS_mapping analysis of single grain garnet shows that the core is rich in REE, relatively rich in HREE and/or LREE. The negative Eu anomaly gradually evolved towards the edge, which is poor in REE, relatively rich in LREE, and has positive/negative Eu anomaly. At the core-rim transition zone, LREE were suddenly enriched, indicating pulsation of material exchange in the ore-forming fluid. 2. The transition from near-neutral pH and oxidative conditions to weak acidity and elevated oxidative conditions (from garnet cores to rims) in the skarn Pb-Zn deposit suggests that the change of physicochemical conditions of fluids may significantly impact the incorporation of REE into the garnet crystal. This is also affected by coupling substitution mechanisms such as YAG-type substitution and Ca-site vacancy. 3. Redox sensitive elements (Ce, U, Sn, and Eu) and elements (Hf and Zr) with strong affinity not only indicate the growth of garnet crystals and evolution of composition zoning, but can also be used as an important indicator to distinguish different skarn deposits. This indicates that the metallogenesis of skarn is mainly restricted by the prevailing redox conditions.

Acknowledgments This work was supported by the Fundamental Research Funds for China Geological Survey (DD20190159–33) and the Central Universities, China University of Geosciences (Wuhan) (No. 2019132). We thank the editor and reviewers who helped to improve this manuscript. We also sincerely thank Dr. Xu, jing, Dr. Jiang, Junsheng, Dr. Liu, Jun, Mr. Cheng, Zihao, and Ms. Guo, Xinran for their help and support in the experiments. Funding information Fundamental Research Funds for China Geological Survey (DD20190159–33), and the Central Universities, China University of Geosciences (Wuhan) (No.2019132). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2020.103770. 19

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