Metal Sources of World-Class Polymetallic W – Sn Skarns in the Nanling Range , South China : Granites versus Sedimentary Rocks ?

1 Department of Resources Science and Engineering, Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China; wcjiang@cug.edu.cn (W.J.); jhwu@cug.edu.cn (J.W.) 2 Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China 3 School of Earth and Planetary Science, John de Laeter Centre, TIGeR, Curtin University, Bentley, WA 6845, Australia; noreen.evans@curtin.edu.au 4 CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; jingyacao@126.com * Correspondence: lihuan@csu.edu.cn


Introduction
Most skarn deposits are formed by the interaction between shallow magmatic systems and carbonate rocks [1][2][3][4].Since the 1990s, the trace element (and particularly rare earth element; REE) characteristics of skarn-associated plutons and their constituent minerals have been the focus of much research given that these signatures reveal much about deposit genesis [5][6][7][8][9][10].Major and accessory mineral phases in skarns (such as garnet and scheelite) inherit their REE patterns from magmatic fluids and they control the REE signatures of the host skarn [11].However, Giuliani et al. [12] propose that strong REE variations in skarns, characterized by a pronounced negative Eu anomaly and low La/Yb ratios, result from the interaction between scheelite and the surrounding metamorphic rocks.In addition, REE signatures are affected by the distance of the skarn from the related intrusive body, with REE concentrating in skarns during prograde alteration and migrating outward of the skarns during retrograde alteration [13].Furthermore, Alirezaei et al. [8] note that skarns vary in texture from massive to banded and that textural contrast is reflected in their whole rock composition: Fe, Si and S are significantly enriched, and Na, large ion lithophile elements (LILE) and light REE (LREE) are strongly depleted in the massive skarns while Na, K, Si, and S are slightly enriched in the banded skarns.Siesgesmund et al. [14] recently note that LILE (K, Rb, Ba, Sr) and REE are moderately depleted and high field strength elements (HFSE) are slightly enriched in Cu-Au skarns, relative to the intrusive rocks.Correlations between major and trace elements in skarns are weak or absent, but skarn REE patterns can be used as a prospectivity indicator in a metallogenic system [15].However, it is still confused whether the chemical composition of ore-related skarns is correlated with the size of relevant deposit and the primary metallogenic elements enriched in skarns.Determining whether the composition of skarns can be used to indicate the metal source of skarn-type deposits requires further research.
The Nanling Range (Figure 1a), located in the central part of South China, represents the largest W-Sn ore province in the world.More than 30 Mesozoic granite-related skarn-type polymetallic deposits are distributed across this region, represented by the large Shizhuyuan, Xianghualing, Huangshaping, Xintianling, Furong, Baoshan and Yaogangxian deposits (Figure 1b) [16][17][18][19][20].These deposits mainly comprise nonferrous, rare and base metal elements, such as W, Sn, Mo, Bi, Li, Be, Nb, Ta, Cu, Au, Ag, Pb, Zn, U and REE [20][21][22][23][24][25][26][27].Given the economic significance of these deposits, a great deal of research has focused on skarn minerals, including the study of mineralogy, geochemistry, geochronology, formation temperature and pressure, and fluid and ore associations [28][29][30][31][32][33][34][35][36][37].It is traditionally believed that extensive Mesozoic magmatism is one of the primary factors responsible for the formation of coeval polymetallic deposits [9, 16,34,[38][39][40], and that ore-forming elements are mainly derived from parent granitic plutons with minor input from the surrounding strata [11,28,33,34,[41][42][43].However, the strata contribution to the ore element budget may previously have been underestimated.Chen et al. [44] propose that ore-forming metals such as W and Sn are extracted from country rocks by high temperature circulating water and metamorphic fluids released from deep crustal rocks.In addition, systematic and comprehensive studies comparing the behavior of major and trace elements in different types of skarn-related bodies (granites and host strata) at the time of ore formation are lacking.Understanding the behavior of elements during metasomatism, in both the pluton and wallrock, would elucidate metal sources and enhance current exploration models.
In this paper, the whole rock major and trace element geochemistry of three typical Nanling Range skarn-type deposits with different primary metal enhancements are studied (Huangshaping W-Mo-Sn, Shizhuyuan W-Sn-Mo-Bi and Xianghualing Sn deposits).By comparing the chemical composition of these skarns, the related granites and the sedimentary host rocks, this study aims to elucidate the controls on the nature of the metals and deposit size and providing new insights into the genesis of large-scale polymetallic skarn-type deposits in the Nanling Range.
propose that ore-forming metals such as W and Sn are extracted from country rocks by high temperature circulating water and metamorphic fluids released from deep crustal rocks.In addition, systematic and comprehensive studies comparing the behavior of major and trace elements in different types of skarn-related bodies (granites and host strata) at the time of ore formation are lacking.Understanding the behavior of elements during metasomatism, in both the pluton and wallrock, would elucidate metal sources and enhance current exploration models.
The Nanling Range is in the center of the South China Block (Figure 1a).Geographically, it contains most of the bordering areas of the Hunan, Jiangxi, Fujian, Guangdong and Guangxi provinces.Mesozoic (especially Jurassic) granitoids, characterized by calc-alkaline A-type granites, are distributed throughout the area [39,52,53].The sedimentary rocks within the Nanling Range can be divided into three groups: (1) Late Proterozoic basement composed of thick-bedded (metamorphic) detrital sedimentary rocks that are overlain by (2) Devonian-Triassic carbonate rocks and marls intercalated with clastic rock and then by (3) Jurassic-Cretaceous detrital and volcanic rocks deposited in rifted basins [17,54].The Nanling Range has been affected by multiple episodes of tectonic activity since the Phanerozoic [55,56].The structural framework of the Nanling Range was primarily controlled by both the Tethyan tectonic domain and the Indosinian orogenesis pre-Jurassic, but later overprinted by Pacific plate tectonism and intracontinental deep structures [57,58].A three-stage Mesozoic tectonic evolution is proposed by Shu et al. [59] based on sedimentary basin analysis: (1) Late Triassic-Early Jurassic post-orogenic stage; (2) Middle Jurassic tectonic rifting stage and (3) Cretaceous intracontinental extension and faulted-depression stage.

The Huangshaping W-Mo-Sn Polymetallic Skarn Deposit
The Huangshaping deposit, located southwest of Chenzhou City (Figure 1b), is one of the most important W-Mo-Sn polymetallic skarn deposits in the Hunan province.The outcropping strata in this area are mainly clastic and carbonate rocks, consisting of the Upper Devonian Shetianqiao and Xikuangshan Formations and Lower Carboniferous Ceshui, Menggong'ao, Shidengzi and Zimenqiao Formations, respectively (Figure 2a).Among them, the Shidengzi limestones are the richest host rocks, followed by the Ceshui sandstones and shales and the Zimenqiao dolomite.There are several episodes of magmatic activity in the Huangshaping area between 180 and 150 Ma.Among them, the granite porphyry, the latest and most evolved intrusive phase, is regarded as the most important metal source and host rock of the skarns [60][61][62].
At Huangshaping, the dominant mineralization type hosted by the granite porphyry is W-Mo-Sn skarn-type ore (WO 3 = 0.15 Mt and its grade = 0.2%; Mo = 0.043 Mt and its grade = 0.06%) [24,61,62].The ore is mainly located between granite porphyry and carbonate rocks and along faults (Figures 3a  and 4a,b).The skarn-forming metasomatism can be further subdivided into two stages in terms of the mineral assemblages: the early stage prograde skarn contains garnet, diopside and fluorite while the late stage retrograde skarn is dominated by tremolite and magnetite [35,63].

The Shizhuyuan W-Sn-Mo-Bi Polymetallic Skarn Deposit
The Shizhuyuan deposit is located to the southeast of Chenzhou City (Figure 1b), and is often referred to as the "nonferrous metal museum" due to the variety of mineralizing elements.The Middle-Upper Devonian Qiziqiao and Shetianqiao limestones and dolomitic limestones are the most favorable hosts of metal mineralization at Shizhuyuan (Figure 2b).The formation of the large Shizhuyuan deposit is related to the emplacement of the Qianlishan granite complex, with significant metal reserves of W (0.71 Mt with grade of 0.31%), Sn (0.48 Mt with grade of 0.14%), Mo (0.12 Mt with grade of 0.054%) and Bi (0.27 Mt with grade of 0.11%) (unpublished data from [64]).The Qianlishan complex is characterized by a small outcrop area (9.7 km 2 ) and multistage magmatic-hydrothermal activity from 158 to 151 Ma [44,[65][66][67][68]. Three main episodes of magmatism can be distinguished in chronological order: (1) porphyritic biotite granite; (2) equigranular biotite granite and (3) granite porphyry [39,[67][68][69].Among them, the equigranular biotite granite is believed to have the closest relationship to the skarn-forming metasomatism [28].

The Xianghualing Sn Polymetallic Skarn Deposit
The Xianghualing deposit is located about 60 km from Chenzhou City.The exposed strata in the Xianghualing area comprise Cambrian to Quarternary sequences, with no Ordovician and Silurian strata present.The main ore-hosting layers are Cambrian slates and meta-sandstones, as well as Devonian carbonate rocks of the Qiziqiao and Shetianqiao Formations (Figure 2c).The Sn mineralization of the Xianghualing deposit is hosted by the Laiziling pluton, with albite granite as its major rock type [72].Based on geochronological studies, the crystallization age of the Laiziling granite is about 155-150 Ma [23,33,39,73,74].
Mainly mineralized by Sn, the skarn orebodies in the Xianghualing deposit are mainly stratiform, pipe or vein-like, distributed in the contact zone between albite granite and sedimentary rocks along faults (Figure 3c), or within the formation unconformity.The total metal reserve of Sn is estimated at 0.76 Mt (unpublished mining data from the local geological survey) and skarn-type cassiterite-arsenopyrite-pyrrhotite ore is the most economic mineralization (Figure 4e,f).

The Huangshaping Skarn
The Huangshaping skarn is dark green with bronze or brown irregular mineral lumps on the surface, blastic texture and massive structure, dominated by W-Mo-Sn mineralization (Figure 5a).The skarn can be divided into garnet skarn and pyroxene skarn, which formed in the proximal and distal portions of the granite porphyry, respectively.The garnet is mainly grossularite (Figure 5b) and andradite, whereas the pyroxene is dominated by hedenbergite.The garnet skarn can be classified into two sub-types: (1) granite-related type with rufous color, inhomogeneous granularity and dense massive structure; and (2) exoskarn with coarse-grained texture and dark brown or green color.The latter is associated with abundant hydrous skarn minerals (e.g., actinolite, vesuvianite and hornblende) [35].Besides garnet and pyroxene, other minerals in the Huangshaping skarn include chlorite, calcite, fluorite, feldspar and quartz.In addition, metal-bearing minerals include scheelite (Figure 5c), molybdenite, cassiterite, arsenopyrite, pyrite, chalcopyrite, pyrrhotite, magnetite, galena and sphalerite.

The Shizhuyuan Skarn
The Shizhuyuan skarn is brownish-red to gray-green in color, with granoblastic, granular and crystalloblastic textures and massive, comb and banded structures (Figure 5d).
The dominant minerals of the Shizhuyuan skarns are garnet, followed by pyroxene, wollastonite and vesuvianite.Compositionally, pyroxene mainly comprises hedenbergite and diopside, while the garnet belongs to the grossularite-andradite series.According to the mineral paragenesis, garnets from the Shizhuyuan skarn can be classified into early and late stages.The early garnet is characterized by euhedral granular texture whereas the late-stage garnet is a veinlet-type with bright red color and larger single crystal [75].

The Xianghualing Skarn
The gray to dark green Xianghualing skarn has a granular to crystalloblastic texture and a massive structure (Figure 5g).The skarn has a variety of mineral assemblages: (1) skarn developed near the metasomatic contact zone is dominanted by diopside, wollastonite, vesuvianite and zoisite (Figure 5h) with minor garnet; (2) skarn controlled by fractures or structures is mainly composed of diopside, actinolite, tremolite (Figure 5i) and vesuvianite; and (3) skarn located at the unconformity between Devonian and Cambrian strata mainly consists of actinolite, associated with silicification [33].Ore minerals in the skarn are dominated by cassiterite, followed by pyrite, pyrrhotite, arsenopyrite, stannite, galena, sphalerite, magnetite and chalcopyrite.
After sampling, each sample was cut for thin sections and the remainder was crushed to >200 mesh size using an iron pestle and mortar.From the Huangshaping skarn, major element analysis was conducted at Kyushu University (Fukuoka, Japan), using X-ray fluorescence spectrometry (XRF, Rigaku RIX 3100, Tokyo, Japan), and the trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer Elan 9000, Perkin, Waltham, MA, USA) at the ALS Laboratory in Vancouver, Canada.The XRF (Panalytical, Almelo, The Netherlands) and ICP-MS (Perkin Elmer Elan 9000, Perkin, Waltham, MA, USA) analysis of the whole-rock compositions for the Xianghualing and Shizhuyuan skarns was carried out at the ALS Laboratory in Guangzhou, China.For analysis of major elements, samples were decomposed by lithium borate fusion, and the major elements were analyzed by XRF with a determination of loss-on-ignition at 1000 • C. The "total" is a combination of all data.The analytical process for trace element analysis is as follows: A prepared sample was added to a lithium metaborate/lithium tetraborate flux, mixed well and fused at 1025 • C. The melt was cooled and dissolved in a nitric, hydrochloric and hydrofluoric acid mixture and diluted for ICP-MS to determine trace element content.Standard sample JG-2 was used to detect the reliability of analytical results with errors for major elements and most trace elements of about 1-5% and 5-10%, respectively.

Results
Major element compositions of the skarns from the Huangshaping, Shizhuyuan and Xianghualing deposits are presented in Table S1, and trace element compositions are given in Tables S2-S4.Previously published geochemical data on these skarns are discussed.The loss-on-ignition (LOI) for most samples was <10 wt %.The skarns from the three deposits are characterized by variable chemical compositions, notably for SiO 2 , Al 2 O 3 , total Fe, MgO and CaO.The oxides vary between deposits too, with the Huangshaping skarn displaying the widest ranges of SiO 2 (28.77-87.18%),total Fe (0.98-46.25%),MgO (0.09-20.07%),CaO (1.33-37.78%),Na 2 O (0.03-3.95%) and K 2 O (0.12-6.88%), the Shizhuyuan skarn showing the most variable MnO contents (0.12-2.46%), the Xianghualing skarn yielding the widest range of Al 2 O 3 (2.89-21.06%),TiO 2 (<0.01-1.11%)and P 2 O 5 (0.01-0.20%), respectively (Figure 6a-i).When comparing the compositional range of skarns with those of skarn-related granites and host sedimentary strata, the Huangshaping skarn is characterized by a relatively wide elemental distribution, with only a few samples plotting around or within the granite range, while other samples plot close to the range defined by the host strata.By contrast, the Shizhuyuan skarn has a relatively narrow range of major elements that corresponds most closely to sedimentary strata.In addition, the SiO 2 contents of skarn samples from different deposits show distinct linear trends with other oxides, such as Al 2 O 3 , total Fe, MnO and CaO (Figure 6).

Results
Major element compositions of the skarns from the Huangshaping, Shizhuyuan and Xianghualing deposits are presented in Table S1, and trace element compositions are given in Tables S2-S4.Previously published geochemical data on these skarns are discussed.The loss-on-ignition (LOI) for most samples was <10 wt %.The skarns from the three deposits are characterized by variable chemical compositions, notably for SiO2, Al2O3, total Fe, MgO and CaO.The oxides vary between deposits too, with the Huangshaping skarn displaying the widest ranges of SiO2 (28.77-87.18%),total Fe (0.98-46.25%),MgO (0.09-20.07%),CaO (1.33-37.78%),Na2O (0.03-3.95%) and K2O (0.12-6.88%), the Shizhuyuan skarn showing the most variable MnO contents (0.12-2.46%), the Xianghualing skarn yielding the widest range of Al2O3 (2.89-21.06%),TiO2 (<0.01-1.11%)and P2O5 (0.01-0.20%), respectively (Figure 6a-i).When comparing the compositional range of skarns with those of skarn-related granites and host sedimentary strata, the Huangshaping skarn is characterized by a relatively wide elemental distribution, with only a few samples plotting around or within the granite range, while other samples plot close to the range defined by the host strata.By contrast, the Shizhuyuan skarn has a relatively narrow range of major elements that corresponds most closely to sedimentary strata.In addition, the SiO2 contents of skarn samples from different deposits show distinct linear trends with other oxides, such as Al2O3, total Fe, MnO and CaO (Figure 6).Likewise, the trace element compositions of all skarn samples are also highly variable (Figure 7a-i).The Xianghualing skarn has the highest enrichments of LILE such as Ba (10.1-1045 ppm), Rb (139.5-2440ppm) and Cs (14.8 to 610 ppm), followed by the Huangshaping and Shizhuyuan skarns.In addition, Sr is extremely enriched in the Shizhuyuan skarn (20.4-680 ppm), followed by the Xianghualing (20.1-154 ppm) and Huangshaping (6.8-99.6 ppm) skarns.Regarding the HFSE, the Huangshaping skarn contains the most variable contents of Nb (0.30-88.2 ppm), Hf (0.2-28.3 ppm) and Th (0.82-48 ppm), respectively.The Xianghualing skarn has the highest Zr (up to 534 ppm) and U (up to 90.3 ppm) concentrations, followed by the Shizhuyuan (up to 365 and 36.7 ppm, respectively) and Huangshaping skarns (up to 130 and 32 ppm, respectively).On primitive mantle-normalized spider diagrams (Figure 8a-c), the Huangshaping skarn samples are strongly depleted in Ti, but enriched in Hf, while the Shizhuyuan skarn samples show moderate values for most elements.The weakest Ba anomaly and most significant U anomalies are recorded in the Xianghualing skarn.By comparing skarn compositions with the associated granite and sedimentary strata, it can be concluded that the Huangshaping skarn is most closely related to the granite, whereas the Shizhuyuan skarn seems to be more similar to regional strata in terms of its trace element composition.Likewise, the trace element compositions of all skarn samples are also highly variable (Figure 7a-i).The Xianghualing skarn has the highest enrichments of LILE such as Ba (10.1-1045 ppm), Rb (139.5-2440ppm) and Cs (14.8 to 610 ppm), followed by the Huangshaping and Shizhuyuan skarns.In addition, Sr is extremely enriched in the Shizhuyuan skarn (20.4-680 ppm), followed by the Xianghualing (20.1-154 ppm) and Huangshaping (6.8-99.6 ppm) skarns.Regarding the HFSE, the Huangshaping skarn contains the most variable contents of Nb (0.30-88.2 ppm), Hf (0.2-28.3 ppm) and Th (0.82-48 ppm), respectively.The Xianghualing skarn has the highest Zr (up to 534 ppm) and U (up to 90.3 ppm) concentrations, followed by the Shizhuyuan (up to 365 and 36.7 ppm, respectively) and Huangshaping skarns (up to 130 and 32 ppm, respectively).On primitive mantle-normalized spider diagrams (Figure 8a-c), the Huangshaping skarn samples are strongly depleted in Ti, but enriched in Hf, while the Shizhuyuan skarn samples show moderate values for most elements.The weakest Ba anomaly and most significant U anomalies are recorded in the Xianghualing skarn.By comparing skarn compositions with the associated granite and sedimentary strata, it can be concluded that the Huangshaping skarn is most closely related to the granite, whereas the Shizhuyuan skarn seems to be more similar to regional strata in terms of its trace element composition.[25,31].Strata composition of the Shidengzi Formation Carboniferous limestone are reference from [31], Devonian mudstone and shale composition are reference from [77].Granite composition of the Huangshaping granite porphyry are reference from [24], Shizhuyuan equigranular biotite granite are reference from [44,68], Xianghualing albite granite are reference from [76].Normalized values for primitive mantle are from [78].
The rare earth elements (REE) of three skarn samples are characterized by variable total REE contents and fractionation between LREE and HREE (Figure 9a-c).The Xianghualing skarn samples have the most variable and highest total REE contents (up to 693.5 ppm), with high La (1.8-104 ppm), Ce (3.5-245 ppm) (Figure 7g-i) and Yb (0.13-60.2 ppm).The average total REE content of the Huangshaping and Shizhuyuan skarns is lower.In addition, the fractionation between LREE and HREE in the Shizhuyuan skarn (highest average (La/Yb)N = 7.7) is stronger than that at the Huangshaping and Xianghualing skarns (Figure 9).The negative Eu anomaly in the Xianghualing skarn samples is the weakest and shows a similar REE pattern to its hosted granite.By contrast, the Huangshaping skarn samples have the largest Eu anomalies, which are also similar to its parent granite (Figure 9a).

Magma Differentiation and Metasomatism
The skarns at the Huangshaping, Shizhuyuan and Xianghualing deposits are located at the contact zones between granitic intrusions and strata (mainly carbonate rocks) (Figures 2-4).This reflects the undisputed role of granites in the formation of skarns.It is important to examine the differences in magma composition of the skarn-related granites at each deposit and to assess the possible impact on the respective metal contents.
The rare earth elements (REE) of three skarn samples are characterized by variable total REE contents and fractionation between LREE and HREE (Figure 9a-c).The Xianghualing skarn samples have the most variable and highest total REE contents (up to 693.5 ppm), with high La (1.8-104 ppm), Ce (3.5-245 ppm) (Figure 7g-i) and Yb (0.13-60.2 ppm).The average total REE content of the Huangshaping and Shizhuyuan skarns is lower.In addition, the fractionation between LREE and HREE in the Shizhuyuan skarn (highest average (La/Yb) N = 7.7) is stronger than that at the Huangshaping and Xianghualing skarns (Figure 9).The negative Eu anomaly in the Xianghualing skarn samples is the weakest and shows a similar REE pattern to its hosted granite.By contrast, the Huangshaping skarn samples have the largest Eu anomalies, which are also similar to its parent granite (Figure 9a).[25,31].Strata composition of the Shidengzi Formation Carboniferous limestone are reference from [31], Devonian mudstone and shale composition are reference from [77].Granite composition of the Huangshaping granite porphyry are reference from [24], Shizhuyuan equigranular biotite granite are reference from [44,68], Xianghualing albite granite are reference from [76].Normalized values for primitive mantle are from [78].
The rare earth elements (REE) of three skarn samples are characterized by variable total REE contents and fractionation between LREE and HREE (Figure 9a-c).The Xianghualing skarn samples have the most variable and highest total REE contents (up to 693.5 ppm), with high La (1.8-104 ppm), Ce (3.5-245 ppm) (Figure 7g-i) and Yb (0.13-60.2 ppm).The average total REE content of the Huangshaping and Shizhuyuan skarns is lower.In addition, the fractionation between LREE and HREE in the Shizhuyuan skarn (highest average (La/Yb)N = 7.7) is stronger than that at the Huangshaping and Xianghualing skarns (Figure 9).The negative Eu anomaly in the Xianghualing skarn samples is the weakest and shows a similar REE pattern to its hosted granite.By contrast, the Huangshaping skarn samples have the largest Eu anomalies, which are also similar to its parent granite (Figure 9a).

Magma Differentiation and Metasomatism
The skarns at the Huangshaping, Shizhuyuan and Xianghualing deposits are located at the contact zones between granitic intrusions and strata (mainly carbonate rocks) (Figures 2-4).This reflects the undisputed role of granites in the formation of skarns.It is important to examine the differences in magma composition of the skarn-related granites at each deposit and to assess the possible impact on the respective metal contents.

Magma Differentiation and Metasomatism
The skarns at the Huangshaping, Shizhuyuan and Xianghualing deposits are located at the contact zones between granitic intrusions and strata (mainly carbonate rocks) (Figures 2-4).This reflects the undisputed role of granites in the formation of skarns.It is important to examine the differences in magma composition of the skarn-related granites at each deposit and to assess the possible impact on the respective metal contents.
In addition, the granites are believed to have contributed a large proportion of the volatile components (especially fluorine, [24,25,36,42,44,62,68,72]) and ore-forming elements [16,40,47,52].Although the parental magmas of the three granitic plutons are highly fractionated, the degree of differentiation, which can be inferred from whole-rock element compositions, could vary between the plutons.In general, the Rb content will increase with increasing degree of differentiation, whereas Ba and Sr are usually enriched in the early stage during magmatic evolution without an obvious variation of K.This means that K/Rb and Rb/Sr ratios can be used to evaluate the degree of fractionation of the magmas [24,79,80].The Xianghualing granite has the lowest K/Rb ratio (mean = 22), but the highest Rb/Sr ratio (mean = 146.9),indicating that it is the most differentiated (Table S5, [81]).Moreover, the Laiziling pluton (i.e., the Xianghualing granite) can be divided into several magmatic-hydrothermal zones from the base to the top [73], also suggesting strong crystal fractionation.On the other hand, previous studies reveal that the Huangshaping granite porphyry is coarse-grained and porphyritic in texture, and crystallized over an extended period (~10 Myr; [82]) and at a high crystallization temperatures (~915 • C; [34]).This contrasts with the relatively low crystallization temperatures (680-725 • C) for both the Shizhuyuan and Xianghualing granite which have a hypidiomorphic granular texture [33,68].In summary, the Xianghualing granite appears to exhibit the strongest degree of magma differentiation, followed by the Huangshaping granite and finally, the Shizhuyuan granite.
By comparing the compositions of the skarns with the composition of the associated granites and sedimentary strata, it is evident that the Huangshaping skarn, which can be classified as a siliceous skarn, has the most variable elemental composition, and one that shows affinities to both the granite and strata compositions (Figures 6 and 7), probably indicating varied protoliths of the skarn components.The average content of Si in the Huangshaping skarn is much higher than those in other skarns.This may imply that Si was sourced from the highly evolved granite in this deposit.Due to the very low abundances of both Si and Al in the host strata (Table S6), the migration or exchange of these two elements between two end members (granites and strata) is probably mainly controlled by fluids derived from the granites.Both the Shizhuyuan and Xianghualing skarns exhibit positive correlations between Al, Mn and Ca oxides and SiO 2 , whereas the Huangshaping skarns show two opposite trends (see arrows in Figure 6a,d,e), indicating contributions from both the granites and strata.The negative correlation between SiO 2 and CaO in the Shizhuyuan and Xianghualing skarns can be attributed to their protolith properties (carbonate rocks).Highly differentiated magma-related fluids may provide strong exchange capacity and thus mobilize these major elements.
The average Al 2 O 3 content in the skarns show a decreasing trend from Xianghualing (average = 13.62%) to Shizhuyuan (average = 8.80%) and then to Huangshaping (average = 4.54%), which is inconsistent with other oxides.This may indicate that the Xianghualing skarn inherited most of its metal elements from the granite.By contrast, this effect is less pronounced at the Huangshaping and Shizhuyuan skarns, respectively.It is also reasonable to speculate that the activity of Al was suppressed by the gradual enrichment of Fe during metasomatism (Table S5).This is evidenced by the gradation of garnet types from grossular to almandine with increasing metasomatism, and the dominant pyroxene composition being hedenbergite [35].
Trace element contents may also reflect variations in the granite composition of the studied skarns.Some trace elements, such as Ba, Rb, Cr, Cs, V, Ga and most REEs, are enriched in skarns and in the associated granites, especially in the Xianghualing samples (Table S6).This implies the dominant control of granites on these elements during metasomatism.Although the average contents of other trace elements (such as Nb, Y, Zr, Th and U) in skarns are lower than those of granites, their maximum values are comparable to as those in granites.This further suggests that granites influenced the enrichment of those trace elements in the skarns.Moreover, consistent linear trends for these skarns can be observed on the Zr vs. Hf, Ba vs. Sr, Zr vs. Ti and U vs. Th diagrams (Figure 10), probably indicating that these elements may actived in pairs during metasomatism, aided by the intrusion of granites.In addition, the two distinct and contrasting trends of the Huangshaping skarn on the Zr vs. Ti diagram (green arrows, Figure 10c) indicates at least two types of skarns in this area.
the REE signatures of the skarns.By contrast, the REE pattern of the Huangshaping skarn can be classified into two types: (1) one shows a steep REE pattern and significant negative Eu anomaly, similar to that of the parent granite; and (2) the other shows a shallower REE pattern and a moderate Eu anomaly, reflecting control by both granite and strata.The REE contents of most skarns moderately decrease along with increasing SiO2, except for some Huangshaping samples which show the opposite trend for La and Lu (Figure 7h,i).All of this could imply varied protoliths of the Huangshaping skarn.In summary, the Xianghualing skarn composition is most strongly influenced by the parent granite composition, by contrast, this effect is less pronounced at the Shizhuyuan and Huangshaping skarns, respectively.Major elements such as Si and Al, and most trace elements such as Ba, Rb, Cs, Cr, V, Ga, Nb, Zr, U and Th in the skarns are dominantly controlled by the composition of the related granites.

Strata Contribution to Skarns
The sedimentary strata also has an impact on the composition of skarns.The mean total FeO content of the skarns is 16.26%, 10.86% and 10.02% at Huangshaping, Shizhuyuan and Xianghualing, respectively (Table S5).These Fe contents can be can be primarily ascribed to stratigraphic fluids released from host sediments or regional strata, due to the very low contents of Fe in the three related granites.High contents of CaO (43.62%) and MgO (20.07%) in the skarns may stem from the carbonate rocks at each deposit, and the similarity of MnO and TiO 2 contents in skarns and strata indicate that such components are dominantly inherited from the local strata (Figure 6d,f).
The Sr contents are relatively high in both the sedimentary strata and skarns (average = 143.3ppm) at Shizhuyuan (Table S6), indicating the dominant role of strata in Sr enrichment.The ∑REE content varies widely within individual skarns, especially at Xianghualing where the ∑REE content ranges from 8.0 to 693.5 ppm, also suggesting the combined control of both granite and strata on its composition at this site.The ratios of (LREE/HREE) T (6.1 to 8.4) and (La/Yb) N (3.8 to 7.7) in skarns are also in accordance with the average ratios of the strata.Moreover, the steepness of the REE pattern from LREE to HREE at both Shizhuyuan and Xianghualing is more similar to the parent strata than the corresponding granite on chondrite-normalized REE diagrams (Figure 9), suggesting that the host strata influenced the REE signatures of the skarns.By contrast, the REE pattern of the Huangshaping skarn can be classified into two types: (1) one shows a steep REE pattern and significant negative Eu anomaly, similar to that of the parent granite; and (2) the other shows a shallower REE pattern and a moderate Eu anomaly, reflecting control by both granite and strata.The REE contents of most skarns moderately decrease along with increasing SiO 2 , except for some Huangshaping samples which show the opposite trend for La and Lu (Figure 7h,i).All of this could imply varied protoliths of the Huangshaping skarn.
In summary, the REE signatures of the Shizhuyuan and Xianghualing skarns are more strongly influenced by the sedimentary strata, while the REE signatures at the Huangshaping skarn suggest influence of both strata and granite.Major elements such as Fe, Mn, Mg and Ca in the skarns are dominated by the host sedimentary strata, and Sr and REE patterns are mainly inherited from the surrounding sedimentary rocks.

Source of Metals and Scales of Skarn Mineralization
As described below, numerous previous studies have focused on the temperature, pressure, crystallization depth and ore-forming relationships in the skarns at the Huangshaping, Shizhuyuan and Xianghualing deposits.For the Huangshaping skarn, two crystallization stages (>500 • C and 250-476 • C, respectively) are distinguished [32].Homogenization temperatures of fluid inclusions in the Shizhuyuan skarn minerals range from 350 • C to 535 • C [28].Based on the analysis of hessonite inclusions, the formation temperature of the Xianghualing skarn is estimated to be 430-570 • C [29].Thus, the diagenetic pressures are estimated to be 0.2-0.85kbar at Huangshaping [32,83], 1.8-3.2kbar at Shizhuyuan [30] and 0.2-1.0kbar at Xianghualing [29], corresponding to the formation depth of skarns and related mineralization at 0.7-3.0km for Huangshaping, 5.4-12 km for Shizhuyuan, and 0.7-3.5 km for Xianghualing.In summary, the Shizhuyuan skarns formed under the highest temperatures and pressures and the deepest formation depth, consistent with most W skarns (formation depth of 5-20 km; [84].Consequently, it can be inferred that the precipitation of multiple metals (especially W) in the Shizhuyuan area occurred in a deep, high-pressure setting, in which magma differentiation was largely suppressed.By contrast, local strata composition strongly influenced skarn formation and mineralization.
Skarn mineralization in the Nanling Range can be divided into three types in terms of dominant mineralization element: (1) the Huangshaping W-Mo-Sn; (2) Shizhuyuan W-Sn-Mo-Bi; and (3) the Xianghualing Sn.The W skarns are closely related to calc-alkaline plutons and can be further divided into reduced and oxidized types [4].The reduced type W skarns are composed of hedenbergite, grandite garnet and low-molybdenum scheelite in the early stages.During the later stages, garnet is subcalcic, and dominated by spessartine and almandine.Moreover, enrichment of W is likely to be favored by reduced environments and under more reduced conditions, there is strong affinity between W and Sn [85].The Sn skarn is closely related to high-silica magma under reducing conditions [4].For the W-Mo-Cu assemblage, a reducing environment with high fluorine is favored.Therefore, it can be concluded that reducing environments were dominated in the skarn mineralization in the Nanling Range, with an increasing reduction states from the Xianghualing Sn to the Huangshaping W-Mo-Sn and then to the Shizhuyuan W-Sn-Mo-Bi deposit.The abundant Bi contents in the Shizhuyuan skarn may be attributed to the strong influence of the sedimentary wall rocks at Shizhuyuan.
As discussed above, the Xianghualing and Huangshaping skarns have a greater affinity to their strongly differentiated parent granites, compared to the Shizhuyuan skarn which has strong affinities to the host sediments.The Xianghualing granite is the most evolved, enriching in most trace elements (e.g., Ba, Rb, Cr, Cs, V, Ga and most REEs), and its related skarn is also the richest.However, Sn is the only ore-forming element at Xianghualing and metal reserves are relatively low compared to the W-Mo-Sn skarn at Huangshaping.Compared to the Xianghualing and Huangshaping skarns, the Shizhuyuan skarn is characterized by the largest ore-forming scale, most varied mineral species and smallest pluton size (~10 km 2 ).It is interesting that such a small pluton contributed to the formation of a giant polymetallic deposit.There are many similarities between these three deposits, such as well-developed structures and multiphase magmatic-hydrothermal activity.We suggest that strongly evolved granites (e.g., the Xianghualing granite) may be less favorable for the development of mineralization, hence, resulting in a relatively small size of deposits.By contrast, less differentiated, smaller plutons formed at greater depths (e.g., the Shizhuyuan granite) may favor the formation of mineralized skarns where substantial amount of ore-forming metals stems from host sedimentary strata.The sedimentary rocks at Shizhuyuan have high concentrations of metals (average W contents of 5-6 ppm and average Sn contents of 2-8 [86], and the hot environment resulting from deep magmatic activity probably creating suitable conditions for widespread fluid convection in the sedimentary strata as well as strong water-rock interaction [70].Although the abundance of ore metals in the strata could be lower than in the granite, fluid circulation through a large volume of strata may concentrate these elements more effectively than a limited volume of granite.The Shizhuyuan deposit was less influenced by granite-derived fluids due to the relatively low degree of differentiation of the magma; however, continuous magmatic activity and an extended hot environment likely accounted for the prolonged circulation of stratigraphic fluids.These fluids carried ore-forming elements through fractures widely developed in the deposit area, constantly assimilating ore-forming material from surrounding strata, and eventually forming the giant Shizhuyuan skarn deposits (Figure 11).By contrast, the Xianghualing granite experienced the strongest degree of magmatic differentiation, resulting in relatively low abundance and a single element (Sn) skarn mineralization with a minor contribution from the strata.The metasomatism and mineralization at Xianghualing has more affinities with local magmatism.At Huangshaping deposit, both granite and strata may equally have contributed in forming the W-Mo-Sn mineralization (Figure 11).sedimentary strata as well as strong water-rock interaction [70].Although the abundance of ore metals in the strata could be lower than in the granite, fluid circulation through a large volume of strata may concentrate these elements more effectively than a limited volume of granite.The Shizhuyuan deposit was less influenced by granite-derived fluids due to the relatively low degree of differentiation of the magma; however, continuous magmatic activity and an extended hot environment likely accounted for the prolonged circulation of stratigraphic fluids.These fluids carried ore-forming elements through fractures widely developed in the deposit area, constantly assimilating ore-forming material from surrounding strata, and eventually forming the giant Shizhuyuan skarn deposits (Figure 11).By contrast, the Xianghualing granite experienced the strongest degree of magmatic differentiation, resulting in relatively low abundance and a single element (Sn) skarn mineralization with a minor contribution from the strata.The metasomatism and mineralization at Xianghualing has more affinities with local magmatism.At Huangshaping deposit, both granite and strata may equally have contributed in forming the W-Mo-Sn mineralization (Figure 11).

Conclusions
(1) Concentrations of Si, Al and most trace elements (e.g., Ba, Rb, Cs, Cr, V, Ga, Nb, Zr, U and Th) in ore-related skarns controlled by related granites but FeO, CaO, MgO, MnO, TiO 2 , Sr and REE signatures in most skarns are significantly controlled by the composition of the host sediments.The composition of strata plays a crucial role in the enrichment of polymetallic elements, scale and metallogenic ore species of the W-Sn skarn mineralization in South China.
(2) The formation of the Shizhuyuan W-Sn-Mo-Bi polymetallic skarn deposit is strongly controlled by strata during metasomatism.The Xianghualing Sn skarn is controlled by the granite composition.Granite and strata may have contributed almost equally to the Huangshaping W-Mo-Sn deposit, however.
Author Contributions: H.L., W.J. and J.W. conceived and designed the experiments; H.L., W.J., J.W. and J.C. took part in the field investigation; W.J., H.L. and N.J.E.interpreted the data and took part in the discussion; W.J. wrote the original draft; H.L. and N.J.E.reviewed and edited the paper.
Funding: This work was funded by the National Natural Science Foundation of China (Grant Nos.41502067).

Figure 1 .
Figure 1.Regional geological map.(a) Simplified geological sketch map showing the distribution of Jurassic granitoids in South China (after [24]); (b) Geological sketch map of W-Sn polymetallic deposits in the central Nanling Range, southern Hunan (modified from [22]).

Figure 1 .
Figure 1.Regional geological map.(a) Simplified geological sketch map showing the distribution of Jurassic granitoids in South China (after [24]); (b) Geological sketch map of W-Sn polymetallic deposits in the central Nanling Range, southern Hunan (modified from [22]).

Figure 6 .
Figure 6.Correlation plots of skarn oxide compositions, compared to those of granites and strata of the Huangshaping, Shizhuyuan and Xianghualing deposits.(a) SiO2 vs. Al2O3 diagram; (b) SiO2 vs. FeO T (Fe2O3 T ) diagram; (c) SiO2 vs. MgO diagram; (d) SiO2 vs. MnO diagram; (e) SiO2 vs. CaO diagram; (f) SiO2 vs. TiO2 diagram; (g) SiO2 vs. Na2O diagram; (h) SiO2 vs. K2O diagram and (i) SiO2 vs. P2O5 diagram.Colored arrows show correlation trends for skarns (color coded to match data points).The orange box shows the range of composition of granites from all three deposits.The gray box shows the composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.No strata analysis was available at the Shizhuyuan and Xianghualing deposits.Data for the granites are reference from[24,44,68,76].Data for the strata are from[31,77].

Figure 6 .
Figure 6.Correlation plots of skarn oxide compositions, compared to those of granites and strata of the Huangshaping, Shizhuyuan and Xianghualing deposits.(a) SiO 2 vs. Al 2 O 3 diagram; (b) SiO 2 vs. FeO T (Fe 2 O 3 T ) diagram; (c) SiO 2 vs. MgO diagram; (d) SiO 2 vs. MnO diagram; (e) SiO 2 vs. CaO diagram; (f) SiO 2 vs. TiO 2 diagram; (g) SiO 2 vs. Na 2 O diagram; (h) SiO 2 vs. K 2 O diagram and (i) SiO 2 vs. P 2 O 5 diagram.Colored arrows show correlation trends for skarns (color coded to match data points).The orange box shows the range of composition of granites from all three deposits.The gray box shows the composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.No strata analysis was available at the Shizhuyuan and Xianghualing deposits.Data for the granites are reference from[24,44,68,76].Data for the strata are from[31,77].

Figure 7 .
Figure 7. Correlation plots of trace elements vs. SiO2 in skarns.Colored arrows show correlation trends for skarns (color coded to match data points).(a) SiO2 vs. Ba diagram; (b) SiO2 vs.Rb diagram; (c) SiO2 vs. Sr diagram; (d) SiO2 vs. Hf diagram; (e) SiO2 vs. Th diagram; (f) SiO2 vs. Zr diagram; (g) SiO2 vs. REE diagram; (h) SiO2 vs. La diagram and (i) SiO2 vs. Lu diagram.The orange box shows the range of composition of granites from all three deposits.The box shows the composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.No strata analysis was available at the Shizhuyuan and Xianghualing deposits.Data sources for the granites and strata are the same as in Figure 6.

Figure 7 .
Figure 7. Correlation plots of trace elements vs. SiO 2 in skarns.Colored arrows show correlation trends for skarns (color coded to match data points).(a) SiO 2 vs. Ba diagram; (b) SiO 2 vs.Rb diagram; (c) SiO 2 vs. Sr diagram; (d) SiO 2 vs. Hf diagram; (e) SiO 2 vs. Th diagram; (f) SiO 2 vs. Zr diagram; (g) SiO 2 vs. REE diagram; (h) SiO 2 vs. La diagram and (i) SiO 2 vs. Lu diagram.The orange box shows the range of composition of granites from all three deposits.The box shows the composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.No strata analysis was available at the Shizhuyuan and Xianghualing deposits.Data sources for the granites and strata are the same as in Figure 6.

Figure 9 .
Figure 9. Chondrite-normalized REE patterns for granites, strata and skarns of the (a) Huangshaping, (b) Shizhuyuan and (c) Xianghualing deposits.Data sources are the same as in Figure 8. Normalized values for chondrite are from [78].

Figure 9 .
Figure 9. Chondrite-normalized REE patterns for granites, strata and skarns of the (a) Huangshaping, (b) Shizhuyuan and (c) Xianghualing deposits.Data sources are the same as in Figure 8. Normalized values for chondrite are from [78].

Figure 9 .
Figure 9. Chondrite-normalized REE patterns for granites, strata and skarns of the (a) Huangshaping, (b) Shizhuyuan and (c) Xianghualing deposits.Data sources are the same as in Figure 8. Normalized values for chondrite are from [78].

Figure 10 .
Figure 10.Correlation of trace elements in skarns, illustrating the geochemical behavior of typical trace elements during metasomatism.(a) Zr vs. Hf diagram; (b) Ba vs. Sr diagram; (c) Zr vs. Ti diagram and (d) Th vs. U diagram.The colored arrows are provided to highlight correlation trends (black arrow shows overall trend for all skarns, with the green arrow for Huangshaping and blue arrow for Shizhuyuan).The orange box shows the range of composition of all granites from the three deposits and the gray box shows the range of composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.Data sources are the same as in Figure 8.

Figure 10 .
Figure 10.Correlation of trace elements in skarns, illustrating the geochemical behavior of typical trace elements during metasomatism.(a) Zr vs. Hf diagram; (b) Ba vs. Sr diagram; (c) Zr vs. Ti diagram and (d) Th vs. U diagram.The colored arrows are provided to highlight correlation trends (black arrow shows overall trend for all skarns, with the green arrow for Huangshaping and blue arrow for Shizhuyuan).The orange box shows the range of composition of all granites from the three deposits and the gray box shows the range of composition of the Carboniferous limestone at Huangshaping and Devonian limestone at other deposits in the Nanling range.Data sources are the same as in Figure 8.

Figure 11 .
Figure 11.A genetic model for W-Sn polymetallic skarn deposits in the Nanling Range, South China, showing relative material contributions from granite and strata, respectively.

1 )
Concentrations of Si, Al and most trace elements (e.g., Ba, Rb, Cs, Cr, V, Ga, Nb, Zr, U and Th) in ore-related skarns controlled by related granites but FeO, CaO, MgO, MnO, TiO2, Sr and REE signatures in most skarns are significantly controlled by the composition of the host sediments.The composition of strata plays a crucial role in the enrichment of polymetallic elements, scale and metallogenic ore species of the W-Sn skarn mineralization in South China.2) The formation of the Shizhuyuan W-Sn-Mo-Bi polymetallic skarn deposit is strongly controlled by strata during metasomatism.The Xianghualing Sn skarn is controlled by the granite composition.Granite and strata may have contributed almost equally to the Huangshaping W-Mo-Sn deposit, however.

Figure 11 .
Figure 11.A genetic model for W-Sn polymetallic skarn deposits in the Nanling Range, South China, showing relative material contributions from granite and strata, respectively.