LA-ICP-MS Analysis of Minerals from the Shizhuyuan W-Polymetallic Deposit, South China: Implications for Mineralization of Pb, W, Mo and Bi

: The South China Block (SCB) is a globally important metallogenic district containing numerous W-Sn deposits. Extensive studies of W-polymetallic deposits in this region have greatly improved our understanding of the petrogenesis, geochronology and metallogenesis of these systems. However, studies on the mobilization of ore-forming elements between mineralization- and alteration-related minerals using in situ analyses are rare. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we analyzed W, Pb, Mo and Bi concentrations in silicate minerals and scheelite from granites and skarns associated with the Shizhuyuan W-polymetallic deposit in the Nanling Range in the SCB. Data show that muscovitized biotite in granites contains high W contents. Pb mainly occurs in K-feldspar and plagioclase in granites and epidote and scheelite in skarns. Bi mainly occurs in epidote in skarns. Scheelite in skarns contains high W and Mo contents. Pb isomorphously substitutes K or Ca in silicate minerals and scheelite. W isomorphously substitutes Ti in biotite. Mo isomorphously substitutes W in scheelite and occurs as W-bearing submicroscopic inclusions in minerals with low contents. Bi isomorphously substitutes Pb when the Pb content is relatively high and occurs as Bi-bearing micro or submicroscopic inclusions in minerals when the Pb content is low. Biotite and feldspar are altered in a magmatic-hydrothermal process, W enriched in biotite, Pb enriched in feldspar and the W-, Pb-, Mo- and Bi-bearing submicroscopic inclusions are excluded from minerals and released to the magmatic-hydrothermal ﬂuids. Large amounts of W are precipitated in scheelite when the ﬂuids come in contact with carbonate rocks to form skarn, while a few contents of Pb, Mo and Bi are distributed in skarn minerals. Thus, large amounts of Pb, Mo, Bi and residual W remain in the ﬂuids, which results in the formation of a W-Sn-Mo-Bi massive skarn ore.


Introduction
The South China Block (SCB), with most of the deposits formed during the Mesozoic era, has been recognized as one of the most important W-Sn metallogenic districts in the world [1]. It was formed by the amalgamation of the Yangtze Block and Cathaysia Block during the Neoproterozoic era ( Figure 1a) and hosts numerous W-Sn deposits, which are mostly concentrated in the Nanling Range located in the central part of the SCB [2][3][4]. The Nanling Range is well known worldwide for its large-scale Mesozoic magmatism and genetically related W-Sn polymetallic deposits, which account for more than 50% of all Chinese W-Sn resources [5][6][7]. Some of the large, world-class deposits in the Nanling Range include the Shizhuyuan W-polymetallic deposit, Xintianling W-Mo deposit, Furong Sn deposit, Hongqiling Sn-W-Pb-Zn deposit and Xianghualing Sn-Pb-Zn deposit [8][9][10][11][12][13], among which the Shizhuyuan W-polymetallic deposit is one of the largest polymetallic deposits in the world with giant W-Sn-Mo-Bi sources [7,14,15]. Extensive studies of these polymetallic deposits over the past few decades have greatly improved our understanding of their petrogenesis, geochronology, geochemistry and metallogenesis [3,4,[16][17][18][19][20][21]. Although we have previously discussed the distribution and mobilization of Sn in the Shizhuyuan W-polymetallic deposit [13], studies on other trace element occurrences in minerals and mobilization between minerals and their alteration products are still insufficient and could improve and deepen the understanding of W mineralization of giant ore accumulations.  [16] and Jiang et al. [22]).
The development of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allows in situ micro analysis of a large number of trace elements with relatively low detection limits (~1 ppb for a large number of trace elements [23,24]). It has been widely used to determine elemental and isotopic compositions of fluid inclusions and minerals in order to constrain magmatic and hydrothermal processes [20,[25][26][27]. Using in situ LA-ICP-MS, Song et al. [28] studied the behavior of trace elements in scheelite and constrained ore genesis in porphyry-skarn W-Mo systems. Choi et al. [29] examined the REE patterns of scheelite, which showed the relationship between the evolution of ore-forming fluids and the formation of multistage scheelites from the W-skarn deposit. Chen et al. [30] studied the occurrences and contents in magnetite and pyrite from the Hetaoping Fe-Zn-Pb skarn deposit and suggested that the ore-forming fluid in Hetaoping was of magmatic origin.
In this paper, we present in situ LA-ICP-MS element concentration data of major silicate minerals and scheelite in granites and skarns from the Shizhuyuan W-polymetallic deposit. The variation of Pb, W, Mo and Bi contents in different granite-and skarn-hosted minerals are presented and their relative distribution are discussed to highlight the mobilization of these elements during skarn formation and mineralization. We use these data to constrain the behavior of Pb, W, Mo and Bi during mineralization processes in order to improve our understanding of the distribution, migration and mineralization of these trace elements to form the giant deposit.

Geological Setting
The Nanling Range (approximately 110-116 • E, 24-27 • N) is located in the central SCB and covers a surface area of ca. 170,000 km 2 [7,31]. It consists of strongly folded and metamorphosed Neoproterozoic-Ordovician flysch and volcanic basement overlain by Late Devonian to Early Triassic sedimentary rocks [2]. Mesozoic granitoid intrusions are well-developed throughout this region ( Figure 1). The granitoid plutons formed by multiple cycles of tectono-magmatism are most directly associated with W-Sn-rare metal mineralization [2,32]. The Nanling Range is an important W-Sn polymetallic metallogenic province with skarn-type W-Sn polymetallic deposits dominant in the west and quartz vein-type W deposits predominant in the east [3,6,7,31].

Petrography
Granite 1 (fine-grained porphyritic biotite granite) is massive with phenocrysts of quartz, feldspar and biotite (Figure 4a). Granite 2 (medium-grained biotite K-feldspar granite) is mainly composed of K-feldspar, plagioclase, biotite, quartz and a few melanocratic opaque minerals (Figure 4b). Granite 3 (fine-grained biotite K-feldspar granite) dominantly consists of biotite, K-feldspar, plagioclase, quartz and a few melanocratic opaque minerals (Figure 4c). Granite 4 (granitic porphyry) is massive with a porphyritic texture, with phenocrysts primarily of quartz, feldspar and small amounts of biotite ( Figure 4d). Granite samples were altered to different degrees, with some K-feldspar being altered into clay minerals, some plagioclase altered to sericite and an aggregate of clay minerals and biotite generally altered to chlorite and muscovite ( Figure 4).  Table S1. Skarns are dominated by a garnet skarn, fluorite-garnet skarn and epidote-fluorite-garnet skarn in the Shizhuyuan deposit. The texture of the skarns varies from lepidoblastic to granoblastic with massive or mesh-vein structures. The garnet skarn consists of garnet, chlorite, epidote, calcite, fluorite and a few opaque minerals ( Figure 4e). The primary minerals of the fluorite-garnet skarn include garnet, fluorite, chlorite, epidote, calcite, opaque minerals and scarce scheelite (Figure 4f,g), while those of the epidote-fluorite-chlorite skarn are epidote, chlorite, fluorite, calcite and opaque minerals (Figure 4h). The garnet was partially altered to chlorite in some thin sections of the three skarns ( Figure 4e).

Sampling and Analytical Methods
Thirteen representative granite (granites 1-4) and skarn (garnet skarn, fluorite-garnet skarn and epidote-fluorite-chlorite skarn) samples were collected at the 490 m levels of Shizhuyuan in the Shizhuyuan ore district. One hundred and fifty-three spots of silicate minerals, alteration minerals and scheelite were selected from 13 sample thin sections for analysis. Element concentrations were determined by LA-ICP-MS using a GeoLas 193-nm Laser Ablation system coupled with an Agilent 7500a ICP-MS instrument at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument, as well as data reduction, have been described by Liu et al. [23]. Each analysis was performed using a laser spot size of 44 µm with a pulse rate of 6 Hz and an energy of 60 MJ and started with a gas blank of approximately 20-30 s followed by data acquisition of 50 s from the sample. The element contents were calibrated against multiple reference materials (NIST 610, BCR-2G, BIR-1G and BHVO-2G) as external standards. In particular, 29 Si was used as an internal standard for K-feldspar, plagioclase, biotite, muscovite, garnet, epidote and chlorite, while 42 Ca was used as an internal standard for scheelite [23]. Major element concentrations of chloritized biotite, epidote and chloritized garnet were corrected with EMPA (Electron microprobe analysis) data (see electronic Appendix A in Yuan et al. [13]), which were obtained from polished thin sections using a JEOL JXA-8100M electron microprobe (EMP) at the Analytical Laboratory of Beijing Research Institute of Uranium Geology (BRIUG), Beijing, China. The operating conditions of the EMP included an accelerating voltage of 15 kV and a probe current of 20 nA focused to a spot of 1 µm and a ZAF (atomic number effect, absorption effect and fluorescence effect) correction procedure for data reduction. The offline reduction of data obtained by LA-ICP-MS was performed with the program ICPMSDataCal (V9.0, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China) obtained from Liu et al. [23], including an integration selection of analyte signals and background, time-drift correction and quantitative calibration [23,38]. The LA-ICP-MS analytical results of major elements and W, Mo, Bi and Pb for minerals are summarized in Table S1 and the results of major elements and W, Mo, Bi and Pb for international reference materials are summarized in Table S2 (data of major elements are cited from Yuan et al. [13]). The measurement accuracy of element concentrations was better than 5% for most elements, based on repeated analyses of the reference materials. Table S3 shows the summary statistics of the major elements and Pb, W, Mo and Bi contents for K-feldspar, plagioclase, altered biotite, garnet, chlorite, epidote and scheelite from different rock samples.

Major Elements
Primary K-feldspar is mainly orthoclase (Or 81-98 ) with Ab 2-19 (albite) and lacking a significant An component (anorthite). Primary plagioclase in granites is enriched in Na 2 O (Ab 95-99 ) with low K 2 O (Or 0-3 ) and CaO (An 0-3 ) (Table S1 [13]). Biotite is generally altered to muscovite and chlorite to different degrees in granites. Among K-feldspar, plagioclase and altered biotite in granites, K-feldspar contains higher K 2 O contents, while altered biotite contains higher FeO and TiO 2 contents. Muscovitized biotite contains relatively lower FeO, K 2 O and TiO 2 compared with chloritized biotite (Table S3).

Pb, W, Mo and Bi
The analyses regarding Pb, W, Mo and Bi contents of main silicate minerals and scheelite are listed in Table S1 and shown in Figure 5. Table S3 contains the summary statistics of Pb, W, Mo and Bi contents for the analyzed minerals. In granites, K-feldspar contains the highest Pb contents (91.9 ppm on average), whereas muscovitized biotite contains relatively high W contents (43.7 ppm on average) compared with other minerals. Mo and Bi contents are low in feldspar (<2.23 ppm and <0.22 ppm, respectively) and muscovitized biotite (<1.71 ppm and <0.24 ppm, respectively) ( Figure 5, Table S3).
In skarns, epidote and scheelite have relatively high Pb (43.6 ppm and 21.2 ppm on average, respectively) and Bi contents (16.3 ppm and 1.57 ppm on average, respectively). Scheelite contains higher W and Mo contents (624,452 ppm and 838 ppm on average, respectively) compared with silicate minerals ( Figure 5, Table S3).

Occurrence of Pb, W, Mo and Bi
Granites are composed mainly of K-feldspar, plagioclase, altered biotite and quartz, while skarns consist mainly of garnet, chlorite, epidote and scheelite.
K-feldspar and plagioclase contain relatively high Pb contents (9.70-144 ppm) and low W (<0.53 ppm), Mo (<2.23 ppm) and Bi (<0.22 ppm) contents ( Figure 5). The pure mineral would display a flat time-resolved depth profile, while the mineral inclusions usually show some peaks in time-resolved depth profiles [25,39,40]. Time-resolved depth profiles for Pb are rarely flat and covariant with Na, K and Al (major elements for feldspar), which indicates that Pb enters the crystal lattice of feldspar by isomorphic substitution (Figure 6a,b). Feldspar is a tectosilicate mineral with a general chemical formula M[(Al,Si) 4 O 8 ], where M represents ions such as Na + , Ca 2+ , K + and other trace elements (Sr 2+ , Pb 2+ ) with a low valence state, large radius and high coordination [41]. Pb has a similar radius to K (Pb 2+ (1.20 Å), K + (1.33 Å)) [42]. Thus, Pb could replace K outside the silicon-oxygen framework with the substitution reaction Pb 2+ + Al 3+ → K + + Si 4+ . Some studies have shown that Bi could enter the lattice by isomorphously substituting Pb in some minerals such as galena [43][44][45]. Since Pb 2+ and Bi 3+ have the similar radius and electronic configuration (Pb 2+ (1.20 Å), Bi 3+ (1.20 Å)) [42], Bi is speculated to isomorphously substitute Pb in K-feldspar and plagioclase with the substitution reaction Bi 3+ + Al 3+ → Pb 2+ + Si 4+ . The concentrations of W and Mo are low (Figure 6a,b) and have no obvious correlations with other elements, implying that the analyzed W and Mo were present in the form of Wand Mo-bearing submicroscopic (invisible) inclusions in the feldspar [46].
Epidote contains relatively high Pb and Bi and low W and Mo contents compared with garnet and chlorite in skarns (Figures 5 and 6d). Epidote is a nesosilicate mineral with the chemical formula Ca 2 Fe 3+ Al 2 [SiO 4 ][Si 2 O 7 ]O(OH). It is suggested that octahedral-coordinated Pb could partially substitute for Ca due to the similar ionic radius and spectra covariation (Figure 6d). Epidote contains low Bi contents due to the substitution between Pb 2+ and Bi 3+ (Figure 6d). W and Mo occur as W-and Mo-bearing submicroscopic inclusions residing in epidote. The concentrations of Pb, W, Mo and Bi are relatively low in chlorite in skarns ( Figure 5). Chlorite is a phyllosilicate mineral and its ideal structure is characterized by TOT sheets and regularly alternating octahedral sheets [52]. The general chemical formula of chlorite is Y x [T 4 O 10 ](OH) 8 , in which the letter Y represents the octahedral-coordinated cations, such as Al 3+ , Mg 2+ , Fe 2+ and Fe 3+ and the letter T represents the tetrahedral-coordinated cations, such as Si 4+ and Al 3+ [53,54]. Time-resolved depth profiles for Pb and Bi were rather smooth and consistent, demonstrating the occurrences as isomorphisms in chlorite (Figure 6e). The major elements Fe, Mg and Al in chlorite have large differences in ionic radius in relation to Pb (Fe 2+ (0.76 Å), Mg 2+ (0.60 Å), Al 3+ (0.50 Å), Pb 2+ (1.20 Å)) [42], therefore Pb generally does not isomorphously substitute with them in chlorite. The flat spectra of Pb and the covariation with K indicate that Pb occurred as an isomorphous substitution of K (Figure 6e). The Pb content was rather low due to the low K 2 O concentration (0.0071-0.075 wt %). Chlorite contains few Bi contents due to the isomorphism between Pb 2+ and Bi 3+ (Figure 6e).
Scheelite has remarkably high Mo contents (Figure 5c). Scheelite is a calcium tungstate mineral in the tetragonal crystal system with the chemical formula CaWO 4 . Scheelite has a simple crystal structure with irregular dodecahedral [CaO 8 ] 14− (which can accommodate alkaline earths and alkalis such as Ca 2+ and Na + ) and tetrahedral [WO 4 ] 2− sites (which can accommodate highly charged, smaller cations such as W 6+ and Mo 6+ ) [55]. Furthermore, W and Mo had a common evolutionary trend during the magmatic evolution due to the similar chemical properties regarding ionic radius, ionic valence state and electronegativity [56][57][58]. Scheelite contains few Pb contents due to the substitution between Pb 2+ and Ca 2+ (Figure 6f). Some spectra of Bi in scheelite are rather flat, while some spectra show peaks, which demonstrates that Bi occurs both as Bi-bearing micro inclusions and isomorphous substitution in scheelite through the equation as follows: Ca 2+ + W 6+ → Bi 3+ + V 5+ [59] (Figure 6f).

Alteration of K-feldspar
Parts of K-feldspar in granites are cloudy and dirty on the surface and locally have transitioned into clay minerals (Figure 4a). As K-feldspar is generally altered to kaolinite during the process of hydrothermal alteration [60,61], the clay minerals in this study are all treated as kaolinites. The alteration reaction is as follows [60]: Major element contents of kaolinized K-feldspar and K-feldspar show little variation, with the exception of Al (Table S3; 18.7-20.4 wt % for K-feldspar and 14.6-47.0 wt % for kaolinized K-feldspar, respectively). Pb content decreased slightly during the alteration, while Bi content was almost unchanged (Figure 7a,d). Both K-feldspar and kaolinized K-feldspar are depleted in W and Mo (Figure 7b,c). Due to the similar ionic radius, Pb occurs in the lattice of K-feldspar and kaolinized K-feldspar as an isomorphous substitution for K (Figure 6g). Kaolinite is an abundant micro and submicrometer-sized sheet silicate mineral [Al 4 Si 4 O 10 (OH) 8 ] built up of alternating tetrahedral (T) and octahedral (O) sheets with no interlayer space [62]. When K-feldspar is altered to kaolinite, [AlO 4 ] is replaced by [SiO 4 ] and K + is replaced by Al 3+ on the octahedral site and K content decreases overall [61,63], therefore Pb is also excluded from the mineral lattice due to isomorphous substitution. Thus, the more intensely K-feldspar alters to kaolinite, the more K and Pb are released into the magmatic-hydrothermal fluids. As Bi isomorphously substitutes Pb in K-feldspar, Bi is also excluded from the feldspar mineral lattice and may enter fluids when K-feldspar alters to kaolinite. W, Mo and the residual Bi may occur as W-, Mo-and Bi-bearing submicroscopic inclusions residing in kaolinized K-feldspar considering their jagged spectra (Figure 6g).

Alteration of Plagioclase
Parts of plagioclase in granites are altered to sericite and clay minerals (Figure 4c,d). Sericite is a fine-grained flaky aggregate of muscovite. The sericitization of plagioclase is as follows [64]: The K 2 O, Al 2 O 3 and FeO contents are higher in sericitic plagioclase than in fresh plagioclase (Table S3). During the sericitization of plagioclase, Pb contents decrease, while W, Mo and Bi contents remain relatively low and almost unchanged (Figure 7). Sericitic plagioclase contains low Pb and Bi contents due to the substitution between K + , Pb 2+ and Bi 3+ , respectively (Figure 6h).
Parts of plagioclase are cloudy on the surface and transitioned into kaolinite through the reaction [ 4 (kaolinite) + 4SiO 2 + 2Na + The differences regarding Pb, W, Mo and Bi between kaolinized plagioclase and plagioclase are negligible (Figure 7). Moreover, Pb could enter the lattice of kaolinized plagioclase as an isomorphous substitution of K, while W, Mo and Bi may occur as W-, Mo-and Bi-bearing submicroscopic inclusions residing in the minerals (Figure 6i).

Alteration of Biotite
Granite-hosted biotite is generally altered to muscovite and chlorite to varying degrees (Figure 4a,b).
The W contents decrease and the Pb, Mo and Bi contents are almost unchanged during the alteration of biotite to muscovite (Figure 7). Muscovite is a TOT-type phyllosilicate with the general formula KAl 2 [AlSi 3 O 10 ][OH] 2 . Ti is a common trace element in both biotite and muscovite [48]. In this study, muscovitized biotite and muscovite contained TiO 2 ranging from 0.084-1.16 wt % and 0.017-9.11 wt %, respectively. Furthermore, W was speculated to reside in the lattice as an isomorphous substitution of Ti 4+ at the octahedral site, based on their similar time-resolved LA-ICP-MS spectra (Figure 6j,k) and similar crystal chemical properties, with the substitution equation Fe 2+ + W 6+ → 2Ti 4+ [50,66]. During the alteration process of biotite to muscovite, Al 2 O 3 content remarkably increases, while FeO, MnO, CaO, MgO, TiO 2 and W contents decrease (Table S3, Figure 7b). As a result of this alteration, Al 3+ likely enters octahedral-coordinate positions to replace Fe, Mg and Ti in muscovite. As a result of the isomorphous substitution of W and Ti, W may also be replaced by Al and thus may be leached by magmatic-hydrothermal fluids as supported by the lower W contents of hydrothermal muscovite. Moreover, Pb has no obvious correlations with other elements in muscovite and muscovite-altered biotite. The time-resolved LA-ICP-MS spectra of Pb for muscovitized biotite and muscovite suggest that Pb occurs as an isomorphous substitution of K in the mineral lattice (Figure 6j,k).
Compared with muscovitized biotite, chloritized biotite has variable contents of TiO 2 , Pb, W, Mo and Bi ( Figure 7). As mentioned above, chlorite is a TOT-type phyllosilicate mineral. In this case, W was also speculated to reside in the lattice as an isomorphous substitution of Ti 4+ at the octahedral site (Figure 6l). The spectra of Pb in chloritized biotite are covariant with K, which indicates that Pb occurs as an isomorphous substitution of K, with Pb content being rather low due to low K concentration. Chloritized biotite contains relatively low Bi contents as Bi 3+ isomorphously substitutes Pb 2+ . Finally, Mo occurs as Mo-bearing submicroscopic inclusions residing in the muscovite, muscovitized biotite and chloritized biotite (Figures 6j-l and 7d).

Alteration of Garnet
Garnet is partially altered to chlorite in skarns (Figure 4e). When garnet is altered to chlorite, Al, Fe, Mn and Mg contents markedly increase, while the Ca and Ti contents decrease (Table S3). In addition, Pb, W, Mo and Bi have no obvious correlations with other elements in chloritized garnet and vary widely, not obviously changing during the alteration (Figure 7).

Mineralization of Pb, W, Mo and Bi
The migration and mineralization of W are closely related to reducing F-rich environments [22,67]. In addition, Sn, Bi and Mo are the most common and important accessory elements in W-polymetallic deposits [68]. Enrichments of F and Cl in granitic magmas could decline the solidus line and decrease viscosity and density of magma to prolong its fractional crystallization and will highly activate massive incompatible elements (e.g., REE and Pb) and concentrate ore-forming elements (W, Mo and Bi) [22,69,70]. As a consequence, F − and Cl − bearing magmatic-hydrothermal fluids are significantly important in transporting ore-forming elements from granitic magmas during the final stages of crystallization [22,71,72].
The metallogenesis of the Shizhuyuan W-polymetallic deposit is considered to be driven directly by the intrusion of the Qianlishan granitic complex [8,16,19]. Previous studies characterized the Qianlishan granitic complex as being comprised of highly fractionated A-type granite and crystallized from highly reduced magma (source magma), which contains considerable highly volatile components including H 2 O, F, B and Cl contents [16,22,71]. Previous studies indicated that there was a strong F complexation during the magmatic-hydrothermal evolution in the Shizhuyuan deposit [8,22]. Chen et al. [16] reported high F contents (up to 8690 ppm) in Qianlishan granites. Hydrothermal zircons from Qianlishan granites also implied that F-rich magmatic-hydrothermal fluid derived from granites played an important role in W-Sn mineralization of the Shizhuyuan deposit [19,22]. Exsolution of the magmatic-hydrothermal fluid from the source magma and the associated mineralization may be summarized as follows. At the beginning, the source magma ascended below the Shizhuyuan area due to the regional tectonic-magmatic process in the SCB, which was highly rich in F, B and Cl and accumulated W together with other ore-forming elements (e.g., Mo, Bi) [19,32]. The abundances of ore-forming elements may have been further enhanced by fractional crystallization (forming the Qianlishan granites) in reduced systems and thus formed the highly evolved late residual magmatic-hydrothermal fluids [16,19,73]. During the fractional crystallization, parts of Pb and W entered the lattice of feldspar and biotite as isomorphisms, respectively. When biotite and feldspar altered in the later magmatic-hydrothermal process, W enriched in biotite and Pb enriched in feldspar were excluded from the mineral lattice and did not enter the alteration products (e.g., muscovite and kaolinite, Figure 8). Thus, Pb and W were released into the magmatic-hydrothermal fluids. As the W-, Pb-, Mo-and Bi-bearing submicroscopic inclusions residing in the minerals were very tiny, they could easily enter the fluids during the alterations as well. In addition, W and F formed W-F complexes, such as WF 6 and decomposed to H 2 WO 4 according to the reaction WF 6 + 4H 2 O → H 2 WO 4 + 6HF, which could react with Ca 2+ to form scheelite [50]. Moreover, Mo readily formed strong, complexing anions, which were transported in the fluids in the form of oxyacid or oxysalt (such as MoO 3 ·nH 2 O and MoO 2 Cl 2 ) due to the high electrovalence and small ionic radius [74]. In contrast, Pb mainly occurred in the fluids in the form of volatile chemical compounds (such as PbCl 2 ) [70]. In this case, large amounts of W were precipitated in scheelite when fluids came in contact with carbonate rocks to form skarn, while a few contents of Pb, Mo and Bi were distributed in the skarn minerals at the same stage. Thus, large amounts of Pb, Mo, Bi and residual W remained in the ore-forming fluids, resulting in the formation of a W-Sn-Mo-Bi massive skarn ore at a later stage [8,14].

Conclusions
W is mainly distributed in biotite and muscovitized biotite in the granite complex and scheelite in skarns. Pb is mainly distributed in feldspar in granites and epidote in skarns. Scheelite contains considerable amounts of W, Pb, Mo and Bi. Pb mainly occurs as an isomorphous substitution and is associated with K (i.e., in feldspar, micas and chlorite) or Ca (i.e., in scheelite and epidote) in minerals. W occurs as an isomorphous substitution and mainly correlates with Ti in biotite. Mo isomorphously substitutes W in scheelite with high content, while it may occur as Mo-bearing submicroscopic inclusions in silicate minerals with relatively low content. Bi mainly occurs as an isomorphous substitution of Pb when the Pb content is relatively high (such as in K-feldspar). When the Pb content is relatively low, Bi can occur as Bi-bearing micro or submicroscopic inclusions in minerals (such as in garnet).
During the later hydrothermal alteration (e.g., the alterations of biotite and feldspar), large amounts of W enriched in biotite and Pb enriched in feldspar were excluded from the mineral lattice and entered the magmatic-hydrothermal fluids. During the formation of the skarn deposits, a significant amount of W entered scheelite and few contents of Pb, Mo and Bi were precipitated in skarn minerals at the same stage. Thus, large amounts of Pb, Mo, Bi and residual W remained in the ore-forming fluids, resulting in the formation of W-Sn-Mo-Bi massive skarn ore at a later stage.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2075-163X/10/9/748/s1, Table S1: LA-ICP-MS analytical results for silicate minerals and scheelite from the Shizhuyuan W-polymetallic deposit (data of major elements for muscovitized biotite, chloritized biotite, muscovite, K-feldspar, kaolinized K-feldspar, plagioclase, sericitic plagioclase, kaolinized plagioclase, garnet, chloritized garnet, chlorite and epidote are cited from electronic Appendix C in Yuan et al. [13]), Table S2: LA-ICP-MS analytical results for international reference materials (data of major elements are cited from electronic Appendix B in Yuan et al. [13]), Table S3: Summary statistics of major elements, Pb, W, Mo and Bi concentrations for major silicate minerals as well as scheelite and alteration minerals (major elements for silicate minerals are cited from Yuan et al. [13]).