Sm-Nd Dating and In-Situ LA-ICP-MS Trace Element Analyses of Scheelite from the Longshan Sb-Au Deposit , Xiangzhong Metallogenic Province , South China

Longshan is an important Sb-Au ore deposit (3.7 Mt @4.5 wt. % Sb and 4.6 g/t Au) in the Xiangzhong metallogenic province (XZMP), South China. In the present work, trace element composition, Sm-Nd isotope dating, and Sr isotope of scheelite from the Longshan Sb-Au deposit are used to constrain the genesis of the deposit. Based on mineral assemblages and geological characteristics, two types of scheelites can be distinguished (Sch1 and Sch2). Sch1 is granular and cemented by stibnite, while Sch2 is commonly present in stibnite, pyrite, calcite, and quartz veins, indicating that Sch2 is later than Sch1. The Sm-Nd isochron age defined by Sch1 is 210 ± 2 Ma (MSWD = 1.0, n = 4). This age is interpreted as the age of Sb-Au mineralization and overlaps with the 201–228 Ma granitic rocks in the XZMP. Sch1 exhibits high ΣREE + Y contents (43.5 to 104 ppm), low Sr values (2687 to 6318 ppm, average of 4018 ppm), and a narrow range of 87Sr/86Sr values (0.7209 to 0.7210, average of 0.7209). In contrast, the elevated Sr abundance (4525 to 11,040 ppm, average of 6874 ppm) and wide 87Sr/86Sr ratios (0.7209 to 0.7228, average of 0.7214) in Sch2 were possibly caused by fluid-rock interaction mixing with Sr-enriched basement rocks. Sulfides have a narrow range of δ34S values of −1.8‰ to 3.2‰, with an average value of 1.1‰ (n = 7). Geochronological, geochemical and isotopic data suggest that the Longshan Sb-Au deposit is possible genetically related to the Late Triassic granitic intrusion in the XZMP.


Introduction
Scheelite (CaWO 4 ) is a common accessory mineral that occurs in many gold deposits hosted by metamorphic rocks.However, the origin of these gold deposits continues to be debated, including a diversity of proposed genetic models (magmatic versus metamorphic) [1][2][3][4][5][6].The Sm-Nd isotopes of scheelite can potentially be a powerful geochronometer for dating gold deposits [7][8][9][10][11][12][13]. Due to their similar electron configurations and ionic radii, elements such as rare earth elements (REEs), Sr and Y can substitute for Ca 2+ in the scheelite structure [14,15].Hence, scheelite contains abundant trace elements and REEs and has a high Sm/Nd ratio [7,[16][17][18].The REE composition of scheelite can provide information about the source and mineralization conditions of ore-forming fluids [4,15,[19][20][21] as well as genetic types of deposits [22][23][24].Scheelite lattice can accommodate limited Sr but rejects Rb [25,26], and the radiogenic 87 Sr from 87 Rb has a negligible effect on Sr isotope composition.So, the scheelite Sr isotopic system can be used to trace the source of ore-forming fluids [3,7,12,27].The Xiangzhong metallogenic province (XZMP), located at the eastern margin of the Yangtze Block, South China (Figure 1b), is one of the most important Sb-Au metallogenic domains in China [28,29].Various interpretations have been proposed regarding the genesis of these deposits, including sedimentary exhalative processes based on the fact that chondrite-normalized REE patterns of the banded ores in Woxi W-Sb-Au deposit are similar with other SEDEX-type polymetallic ore deposits [30], orogenic-like processes since the characteristics of ore-forming fluids are in agreement with the definition of orogenic gold deposits [31], and intrusion-related deposit systems on account of sulfur and ore-forming fluids in these deposits were magmatic in origin [28].This debate is mainly due to the lack of reliable mineralization ages in these Sb-Au deposits and to the absence of intrusion within the mining district [28].
Minerals 2019, 9, x FOR PEER REVIEW 2 of 20 21] as well as genetic types of deposits [22][23][24].Scheelite lattice can accommodate limited Sr but rejects Rb [25,26], and the radiogenic 87 Sr from 87 Rb has a negligible effect on Sr isotope composition.So, the scheelite Sr isotopic system can be used to trace the source of ore-forming fluids [3,7,12,27] The Xiangzhong metallogenic province (XZMP), located at the eastern margin of the Yangtze Block, South China (Figure 1b), is one of the most important Sb-Au metallogenic domains in China [28,29].Various interpretations have been proposed regarding the genesis of these deposits, including sedimentary exhalative processes based on the fact that chondrite-normalized REE patterns of the banded ores in Woxi W-Sb-Au deposit are similar with other SEDEX-type polymetallic ore deposits [30], orogenic-like processes since the characteristics of ore-forming fluids are in agreement with the definition of orogenic gold deposits [31], and intrusion-related deposit systems on account of sulfur and ore-forming fluids in these deposits were magmatic in origin [28].This debate is mainly due to the lack of reliable mineralization ages in these Sb-Au deposits and to the absence of intrusion within the mining district [28].Longshan is an important Sb-Au deposit hosted by Neoproterozoic slates in the XZMP.Only two studies have constrained the chronology of the Longshan Sb-Au deposit.Quartz fluid inclusion Rb-Sr and pyrite Re-Os studies yielded isochron ages of 175 ± 27 Ma [34] and 195 ± 36 Ma [35], respectively, with large errors.The genesis of the Longshan Sb-Au deposit remains uncertain, particularly as the precise age and origin of the mineralization are not clear.
In this study, the in-situ trace element composition, Sm-Nd isotope dating, Sr isotopes of scheelite, and S isotopes of sulfides were carried out to constrain (1) the age of the Sb-Au mineralization and (2) the possible genetic relationship between the Sb-Au deposit and Late Triassic granitic intrusion in the XZMP.

Regional Geology
South China consists of the Yangtze Block in the northwest and the Cathaysia Block in the southeast (Figure 1a, [36,37]).The XZMP, located at the eastern margin of the Yangtze Block [28], includes the Xuefengshan Range in the west and the Shaoyang Basin in the east (Figure 1b).Strata exposed in the XZMP consist mainly of Neoproterozoic metamorphic basement and Paleozoic to Cenozoic sedimentary rocks (Figure 1b).The Neoproterozoic rocks are part of the crystalline basement of South China and consist of slate, conglomerate, siltstone, pelite, chert, and shale.These basement rocks compose the majority of the wall rock for the Sb-Au and Au-Sb deposits [28,38].
In the XZMP, intrusive rocks are mostly observed to outcrop at the margins of the Shaoyang Basin and are dominated by Late Triassic granites and subordinate Devonian granitoids (Figure 1b, [33,[39][40][41][42]. Numerous zircon SHRIMP and LA-ICP-MS U-Pb ages demonstrate that extensive Late Triassic granites were emplaced between 228 and 201 Ma in the XZMP [29].Magmatic rocks are lacking in most of the Sb-Au deposit areas, although small felsic dikes are observed in some deposits.For example, mineralized veins cutting granite porphyries and quartz porphyries are observed in the Fuzhuxi and Liaojiaping Sb-Au deposits, and in the Banxi Sb deposit (Figure 1b, [28]).Moreover, Bouguer gravity and aeromagnetic anomalies suggest that hidden granitic intrusions may be present beneath the Longshan Sb-Au deposit [43].
More than 170 Sb-Au deposits have been discovered in the XZMP, hosted by Neoproterozoic clastic rocks and Devonian to Carboniferous carbonate rocks [44,45].The Sb polymetallic deposits can be classified into two types, the Woxi-type and the Xikuangshan-type, based on ore-forming elements and host rocks [46].The Woxi-type deposits contain a W-Sb-Au metal association and, in some cases, Sb-W mineralization, and is represented by the Zhazixi deposit [12,[47][48][49][50].These deposits are hosted by Proterozoic low-grade metamorphosed clastic rocks.The Xikuangshan-type deposits in the Shaoyang Basin are hosted by Devonian-Carboniferous limestone-sandstones and characterized by Sb mineralization [51].

Deposit Geology
The Longshan Sb-Au deposit is located in the central part of the so-called "Longshan dome" (Figure 2).The dome is made up of Neoproterozoic slates.Cambrian to Ordovician shales are distributed around the dome in a ring belt, and Devonian to Carboniferous siltstone and mudstone are distributed along the east and west sides of the Longshan dome (Figure 2).Late Triassic granitic dikes are developed around the eastern part of the dome (Figure 2), and the zircon U-Pb ages (220-217 Ma, [52]) are consistent with those of the Late Triassic granites formed in the XZMP [29].The Longshan Sb-Au deposit (3.7 Mt @4.5 wt.% Sb and 4.6 g/t Au, [53]) contains two mining zones: Longshan and Xiejiashan (Figure 2).These include 27 NE-striking, NWW-striking and NNEstriking vein groups.The No.1 and No.2 vein groups (Figure 3a), which are the largest ore bodies, occur as a steeply NWW-striking and NE-dipping (65-80°) body within metamorphic slates.It is more than 1400 m long and 0.2 to 1.3 m wide and continues for 1050 m down-dip, with an average grade of 0.77 to 6.79 wt.% Sb and 0.3 to 4.6 g/t Au.Diorite dikes occurred on the northwest side of the Longshan mining zone (Figure 3a), however, we were unable to collect the sample due to a lack of outcrop.The Longshan deposit is underlain by rocks of the Neoproterozoic Jiangkou Formation containing a series of low-grade metamorphic slates.The Jiangkou Formation can be further divided into four members from bottom to top (Figure 3).The first and second members consist of pebbly sandy slate and tuffaceous pebbly sandy slate, and the orebodies mainly occur in the two members.The third and fourth members consist of dark-gray and gray-green pebbly sandy slate (Figure 3).The Longshan Sb-Au deposit (3.7 Mt @4.5 wt.% Sb and 4.6 g/t Au, [53]) contains two mining zones: Longshan and Xiejiashan (Figure 2).These include 27 NE-striking, NWW-striking and NNE-striking vein groups.The No.1 and No.2 vein groups (Figure 3a), which are the largest ore bodies, occur as a steeply NWW-striking and NE-dipping (65-80 • ) body within metamorphic slates.It is more than 1400 m long and 0.2 to 1.3 m wide and continues for 1050 m down-dip, with an average grade of 0.77 to 6.79 wt.% Sb and 0.3 to 4.6 g/t Au.The Longshan Sb-Au deposit (3.7 Mt @4.5 wt.% Sb and 4.6 g/t Au, [53]) contains two mining zones: Longshan and Xiejiashan (Figure 2).These include 27 NE-striking, NWW-striking and NNEstriking vein groups.The No.1 and No.2 vein groups (Figure 3a), which are the largest ore bodies, occur as a steeply NWW-striking and NE-dipping (65-80°) body within metamorphic slates.It is more than 1400 m long and 0.2 to 1.3 m wide and continues for 1050 m down-dip, with an average grade of 0.77 to 6.79 wt.% Sb and 0.3 to 4.6 g/t Au.Diorite dikes occurred on the northwest side of the Longshan mining zone (Figure 3a), however, we were unable to collect the sample due to a lack of outcrop.The Longshan deposit is underlain by rocks of the Neoproterozoic Jiangkou Formation containing a series of low-grade metamorphic slates.The Jiangkou Formation can be further divided into four members from bottom to top (Figure 3).The first and second members consist of pebbly sandy slate and tuffaceous pebbly sandy slate, and the orebodies mainly occur in the two members.The third and fourth members consist of dark-gray and gray-green pebbly sandy slate (Figure 3).Diorite dikes occurred on the northwest side of the Longshan mining zone (Figure 3a), however, we were unable to collect the sample due to a lack of outcrop.The Longshan deposit is underlain by rocks of the Neoproterozoic Jiangkou Formation containing a series of low-grade metamorphic slates.The Jiangkou Formation can be further divided into four members from bottom to top (Figure 3).The first and second members consist of pebbly sandy slate and tuffaceous pebbly sandy slate, and the orebodies mainly occur in the two members.The third and fourth members consist of dark-gray and gray-green pebbly sandy slate (Figure 3).Wallrock alteration in the Longshan deposit is pervasive and is best developed adjacent to quartz veins.The scale and intensity of alteration show a correlation with the width of the quartz veins.Alteration styles include sulfidation, silicification, sericitization, and carbonatization (Figure 4).Sulfidation is caused by the presence of arsenopyrite and pyrite.Wallrock alteration in the Longshan deposit is pervasive and is best developed adjacent to quartz veins.The scale and intensity of alteration show a correlation with the width of the quartz veins.Alteration styles include sulfidation, silicification, sericitization, and carbonatization (Figure 4).Sulfidation is caused by the presence of arsenopyrite and pyrite.Based on field and microscopic observations, we recognized a mineral paragenesis, including four distinct stages accompanied by varying degrees of alteration (Figure 5).The pre-mineralization Stage-1 is represented by deformed segregation quartz veins that are several to tens of cm in thickness (Figure 4a).Stage-2 is marked by NE-striking and NW-dipping quartz-sulfide veins with widths of 1 to 20 cm.The quartz-sulfide veins cut the Stage-1 quartz veins, with distinct boundaries (Figure 4b).Based on field and microscopic observations, we recognized a mineral paragenesis, including four distinct stages accompanied by varying degrees of alteration (Figure 5).The pre-mineralization Stage-1 is represented by deformed segregation quartz veins that are several to tens of cm in thickness (Figure 4a).Stage-2 is marked by NE-striking and NW-dipping quartz-sulfide veins with widths of 1 to 20 cm.The quartz-sulfide veins cut the Stage-1 quartz veins, with distinct boundaries (Figure 4b).The veins mainly comprise galena, quartz, stibnite, and pyrite (Figures 4c and 6a,b).Minor chalcopyrite associated with sphalerite and pyrrhotite occur in the ores (Figure 7a).The wallrock alteration including sulfidation and silicification is weak in this stage.Stage-3 represents the main stibnite and gold mineralization stage, which is marked by NWW-striking and NE-dipping quartz-sulfide veins with widths of 0.2 to 1.4 m (Figure 4d-f).High-grade stibnite ore (Figure 6d) is infilled with native gold (Figure 7f).Pyrite infilled with native gold (Figure 7e) is associated with stibnite (Figure 6e) and jamesonite (Figure 7g).Arsenopyrite developed proximally to the stibnite vein (Figure 4g).Stage-4 is marked by NNE-striking and NW-dipping quartz-sulfide veins, from 0.1 to 0.68 m in width (Figure 4i-l), locally cutting the Stage-3 quartz veins (Figure 4h).Coarse-grained disseminated arsenopyrite and pyrite (Figure 4j,k and Figure 6f) are developed in the wallrock during Stage-4; meanwhile, abundant sericitization (Figure 4k) occurs within the discolored wallrock.Arsenopyrite replaced by chalcostibite (Figure 7h) or native antimony (Figure 7i) is observed.The local development of an assemblage of calcite and stibnite (Figure 4l) occurs in the quartz vein.
The veins mainly comprise galena, quartz, stibnite, and pyrite (Figures 4c and 6a,b).Minor chalcopyrite associated with sphalerite and pyrrhotite occur in the ores (Figure 7a).The wallrock alteration including sulfidation and silicification is weak in this stage.Stage-3 represents the main stibnite and gold mineralization stage, which is marked by NWW-striking and NE-dipping quartzsulfide veins with widths of 0.2 to 1.4 m (Figure 4d-f).High-grade stibnite ore (Figure 6d) is infilled with native gold (Figure 7f).Pyrite infilled with native gold (Figure 7e) is associated with stibnite (Figure 6e) and jamesonite (Figure 7g).Arsenopyrite developed proximally to the stibnite vein (Figure 4g).Stage-4 is marked by NNE-striking and NW-dipping quartz-sulfide veins, from 0.1 to 0.68 m in width (Figure 4i-l), locally cutting the Stage-3 quartz veins (Figure 4h).Coarse-grained disseminated arsenopyrite and pyrite (Figures 4j,k and 6f) are developed in the wallrock during Stage-4; meanwhile, abundant sericitization (Figure 4k) occurs within the discolored wallrock.Arsenopyrite replaced by chalcostibite (Figure 7h) or native antimony (Figure 7i) is observed.The local development of an assemblage of calcite and stibnite (Figure 4l) occurs in the quartz vein.The scheelite is well formed in Stage-3 and can be divided into two types.Coarse-grained Sch1 is coexisted with stibnite and quartz (Figures 6c and 7b).Sch2 is commonly present in pyrite, calcite and quartz veins (Figure 4f) and is associated with pyrite (Figure 7c) and calcite (Figure 7d).The different mineral assemblages indicate that Sch2 is later than Sch1.

Sample Descriptions
More than 120 hand-specimen samples, representative of all stages and mineralization styles, were collected from Longshan deposit.Eighteen scheelite and sulfide samples (Table S1) collected from underground adits at the +298 m to +720 m levels, were investigated in this study.The scheelite grains were separated for dating using conventional heavy liquid and magnetic techniques.Standard petrographic thin-sections were prepared from scheelite and sulfide samples for optical microscopy to identify mineral paragenetic relationships.Backscattered electron (BSE), scanning electron microscopy (SEM) and cathodoluminescence (CL) images (Figure 8) were obtained using a Hitachi SU-70 analytical field emission scanning electron microscope (Hitachi, Ltd., Chiyoda, Japan) at the University of Tasmania (UTAS), Australia.The scheelite is well formed in Stage-3 and can be divided into two types.Coarse-grained Sch1 is coexisted with stibnite and quartz (Figures 6c and 7b).Sch2 is commonly present in pyrite, calcite and quartz veins (Figure 4f) and is associated with pyrite (Figure 7c) and calcite (Figure 7d).The different mineral assemblages indicate that Sch2 is later than Sch1.

Sample Descriptions
More than 120 hand-specimen samples, representative of all stages and mineralization styles, were collected from Longshan deposit.Eighteen scheelite and sulfide samples (Table S1) collected from underground adits at the +298 m to +720 m levels, were investigated in this study.The scheelite grains were separated for dating using conventional heavy liquid and magnetic techniques.Standard petrographic thin-sections were prepared from scheelite and sulfide samples for optical microscopy to identify mineral paragenetic relationships.Backscattered electron (BSE), scanning electron microscopy (SEM) and cathodoluminescence (CL) images (Figure 8) were obtained using a Hitachi SU-70 analytical field emission scanning electron microscope (Hitachi, Ltd., Chiyoda, Japan) at the University of Tasmania (UTAS), Australia.

Trace Element
Each sample was prepared as a 1-inch-diameter polished mount.Quantitative LA-ICP-MS trace element analyses of scheelite from the Longshan Sb-Au deposit was analyzed using a Resonetics Resolution laser ablation system equipped with a Coherent COMPex Pro 110 ArF Excimer laser (Coherent, Inc., Santa Clara, CA, USA) operating at a 193-nm wavelength with a 20-ns pulse width.

Trace Element
Each sample was prepared as a 1-inch-diameter polished mount.Quantitative LA-ICP-MS trace element analyses of scheelite from the Longshan Sb-Au deposit was analyzed using a Resonetics Resolution laser ablation system equipped with a Coherent COMPex Pro 110 ArF Excimer laser (Coherent, Inc., Santa Clara, CA, USA) operating at a 193-nm wavelength with a 20-ns pulse width.The laser is coupled to an Agilent 7700 quadrupole inductively-coupled plasma mass spectrometer (ICP-MS) housed at CODES Analytical Laboratories, UTAS.All scheelite samples were ablated on 51 µm diameter spots using the laser at 10 Hz and an energy of density of 2.7 J/cm 2 .Each analysis was pre-ablated with five laser pulses to remove the surface contamination.Once the pre-ablation had washed out (20 s), a blank gas was analyzed for 30 s followed by 60 s of sample ablation for scheelite.All data reduction calculations and error propagations were undertaken within the software LADR designed at CODES analytical laboratories, UTAS.Data reduction was undertaken using stoichiometric values of Ca as the internal standard for scheelite.Glass standard NIST612 was measured using a 74 µm spot size (10 Hz, 3.5 J/cm 2 ) as the primary calibration standard.Since the low content of rare earths in NIST612 in comparing with the studied mineral, we chose different laser sizes for the analysis of standards and scheelite.GSD-1G and BCR-2 (51 µm, 10 Hz, 3 J/cm 2 ) were used as the secondary standards.Standard standard and unknowns analyses were used to compare values for each element and to correct for differences in spot size between the primary.Measured isotopes and detection limits are given in Table S2.

Sm-Nd and Sr Isotope
Based on field investigations and microscopic observations, scheelite chips were cut from hand specimens and lightly crushed to a size of 40 to 60 mesh.Scheelite was prepared using standard heavy liquid methods, with final purification achieved by handpicking under a binocular microscope with the aid of a UV lamp.The cleaned fractions, with a purity of 99%, were powdered to ~74 µm in an agate mortar.Approximately 50 mg of scheelite samples was dissolved in Teflon bombs using a mixture of acid solution (2 mL 22 M HF + 1 mL 15 M HNO 3 + 0.5 mL concentrated HClO 4 ).Two separate sample aliquots were dissolved: one for the determination of present-day 87 Sr/ 86 Sr and l43 Nd/ 144 Nd ratios and the other for spiking with 149 Sm-146 Nd spike and determination of Sm and Nd concentrations.The bombs were placed in an oven at 190 • C for two days.After drying on a hot plate at 150 • C, 3 mL 6 M HCl was added and evaporated to dryness again.Finally, the samples were dissolved with 1 mL 2.5 M HCl.The solution was centrifuged, and the supernatant liquid was loaded onto the preconditioned resin column with 2 mL of AG50W × 12 (200-400 mesh) for the separation of Sr and LREE from the sample matrix.Sm and Nd were separated by HEHEHP chromatographic columns.Isotope analyses were performed by thermal ionization mass spectrometry (TIMS) using a Triton instrument from Thermo Fisher Scientific at the Tianjin Institute of Geology and Mineral Resources.Sr isotopic ratios were normalized to an 86 Sr/ 88 Sr ratio of 0.1194, and Nd isotope ratios were normalized to a 146 Nd/ 144 Nd ratio of 0.7219.The reproducibility of the isotopic ratios was approximately 0.003% at the 2σ level, and the precisions for the Nd and Sm concentrations at the 2σ level were approximately ± 0.5% of the quoted values.Concentrations for the rock standard USGS BCR-2 determined during this study were 6.58 ppm Sm and 28.92 ppm Nd.BCR-2 was used to evaluate the separation and purification process of Nd and yielded 143 Nd/ 144 Nd=0.512636 ± 0.000006, which was consistent with the reference value of 0.512634 ± 0.000012 [54].The NBS987 standard yielded an 87 Sr/ 86 Sr value of 0.710245 ± 0.000025 (2σ, n = 10); and the JMC Nd standard provided a l43 Nd/ 144 Nd value of 0.511132 ± 0.000020 (2σ, n = 10).Blanks during this study were 30 pg for Sm, 54 pg for Nd, and 0.56 ng for Sr and Rb.The Sm-Nd isochron age was calculated using the ISOPLOT plotting program [55].In this study, the decay constant λ used for 147 Sm was 6.54 × 10 −12 a −1 , and the 147 Sm/ 144 Nd and 143 Nd/ 144 Nd values of the chondritic uniform reservoir (CHUR) used for the εNd(t) calculations were 0.1967 and 0.512638 [56], respectively.

Sulfur Isotopes
Arsenopyrite and pyrite grains were carefully handpicked under a binocular microscope after the samples had been crushed and sieved to a size of 60 to 80 mesh with a purity of 99%.The sulfur isotope analyses of seven sulfide samples were performed on the SO 2 gas extracted from pyrite using conventional methods.Sulfide was mixed with Cu 2 O in a 1:10 proportion and heated under vacuum (2.0 × 10 −2 Pa) at 980 • C to release SO 2 , after which the sulfur isotope ratios were measured using a MAT-251 mass spectrometer.The analytical precision was better than ± 0.2‰, relative to the Vienna Canon Diablo Troilite (V-CDT) sulfide.

Trace Element Concentration in Scheelite
A total of 38 LA-ICP-MS REE analyses were carried out on four samples of scheelite from the Longshan Sb-Au deposit, and the results are presented in Tables 1 and 2

Sm-Nd and Sr Isotope Results for the Scheelite
The Sm-Nd concentration and isotope data for scheelites from the Longshan deposit are presented in Table S3.The four Sch1 samples show variations in Sm (3.90 to 20.3 ppm) and Nd (6.24 to 15.4 ppm) concentrations.The 147 Sm/ 144 Nd ratios of Sch1 range from 0.317 to 0.799, and presentday 143 Nd/ 144 Nd ratios range from 0.512459 to 0.513120.The Sch1 samples define an excellent linear array with an MSWD =1.0, yielding an age of 210 ± 2 Ma, with an initial ratio of 0.512025 ± 0.000006 (Figure 10).The Sm (0.683 to 11.9 ppm) and Nd (1.01 to 13 ppm) concentrations of the Sch2 samples are highly variable.The 147 Sm/ 144 Nd ratios of Sch2 range from 0.410 to 0.554, and present-day 143 Nd/ 144 Nd ratios range from 0.512384 to 0.512907.Sch1 and Sch2 have elevated Sr concentrations (2687 to 6318 ppm and 4525 to 11,040 ppm, respectively) and very low Na and Mo concentrations.The Na and Mo concentrations in Sch1 range from below detection limit (bdl) to 1.64 ppm and 0.004 to 0.017 ppm, respectively.The Na and Mo concentrations in Sch2 range from bdl to 13.1 ppm and 0.014 to 0.648 ppm, respectively.The Rb contents of the Sch1 and Sch2 scheelite samples are bdl, a feature that is consistent with the crystal chemistry of scheelite (less than 3 ppm Rb, [26]).

Sm-Nd and Sr Isotope Results for the Scheelite
The Sm-Nd concentration and isotope data for scheelites from the Longshan deposit are presented in Table S3.The four Sch1 samples show variations in Sm (3.90 to 20.3 ppm) and Nd (6.24 to 15.4 ppm) concentrations.The 147 Sm/ 144 Nd ratios of Sch1 range from 0.317 to 0.799, and present-day 143 Nd/ 144 Nd ratios range from 0.512459 to 0.513120.The Sch1 samples define an excellent linear array with an MSWD =1.0, yielding an age of 210 ± 2 Ma, with an initial ratio of 0.512025 ± 0.000006 (Figure 10).The Sm (0.683 to 11.9 ppm) and Nd (1.01 to 13 ppm) concentrations of the Sch2 samples are highly variable.The 147 Sm/ 144 Nd ratios of Sch2 range from 0.410 to 0.554, and present-day 143 Nd/ 144 Nd ratios range from 0.512384 to 0.512907.S3), with εNd(t) ranging from −6.65 to −6.75.While the initial 143 Nd/ 144 Nd values for the Sch2 are variable (0.511632 to 0.512208).This variation is also reflected in the εNd(t) values, which range from −3.12 to −14.4.The 87 Sr/ 86 Sr ratios determined for the Sch1 are rather uniform and range from 0.7209 to 0.7210, with an average value of 0.7209.In contrast, the 87 Sr/ 86 Sr ratios of Sch2 range widely from 0.7209 to 0.7228, with an average value of 0.7214.The 87 Sr/ 86 Sr ratios of Sch2 are higher than that of Sch1 while lower than ( 87 Sr/ 86 Sr)(t) of the basement slates in the Banxi Group (0.7430 to 0.7494, average of 0.7467, [12]).

Timing of Sb-Au Mineralization
A linear array on an isochron diagram may also be the result of mixing between two discrete end-member compositions with different 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios [8].However, the 143 Nd/ 144 Nd ratios plotted versus their corresponding values of 1/Nd do not indicate an inherited collinearity (Figure 10b).Considering that all Sch1 samples were collected from the same ore vein and should be regarded as products of the same hydrothermal event, the scheelite is chemically homogeneous (Figure 8) and the variation range of the calculated εNd(t) for Sch1 is relatively narrow (Table S3).The calculated initial 143 Nd/ 144 Nd values at an age of 210 Ma for Sch1 are the same within error as the initial 143 Nd/ 144 Nd value of the isochron (Figure 10a).These features suggest that all Sch1 samples had the same initial isotope ratios of daughter isotopes, and the scheelite Sm-Nd isochron age can be regarded as a single Sch1 hydrothermal event.The high-grade ore of the Sch1 and stibnite vein (Figure 5e) is 25.2 wt.% Sb, 0.23 g/t Au and 13 wt.% W from Stage-3.The mineral assemblage consists of quartz, scheelite, and stibnite.Thus, we believe that the isochron age of 210 ± 2 Ma (Figure 10a) represents the age of the Sb-Au mineralization in the Longshan deposit.
As noted above, many low-temperature Sb-Au deposits are hosted in the Proterozoic metamorphosed clastic rocks in the XZMP (Figure 1b).The Chanziping and Daping gold deposits (Figure 1b) have Rb-Sr isochron dates for fluid inclusions trapped in quartz yielding 205.6 ± 9.4 Ma and 204.8 ± 6.3 Ma, respectively [58].Scheelite coexisting with stibnite from the Zhazixi Sb-W deposit yielded Sm-Nd isochron ages of 227.3 ± 6.2 Ma [59].Mineralized granite and granite porphyry yielded K-Ar dates of 209 Ma for the Fuzhuxi Sb-Au, 200 Ma for the Liaojiaping Sb-Au, and 194-204 Ma for The initial 143 Nd/ 144 Nd values at an age of 210 Ma for Sch1 are narrow variation (0.512022 to 0.512027, Table S3), with εNd(t) ranging from −6.65 to −6.75.While the initial 143 Nd/ 144 Nd values for the Sch2 are variable (0.511632 to 0.512208).This variation is also reflected in the εNd(t) values, which range from −3.12 to −14.4.The 87 Sr/ 86 Sr ratios determined for the Sch1 are rather uniform and range from 0.7209 to 0.7210, with an average value of 0.7209.In contrast, the 87 Sr/ 86 Sr ratios of Sch2 range widely from 0.7209 to 0.7228, with an average value of 0.7214.The 87 Sr/ 86 Sr ratios of Sch2 are higher than that of Sch1 while lower than ( 87 Sr/ 86 Sr)(t) of the basement slates in the Banxi Group (0.7430 to 0.7494, average of 0.7467, [12]).

Timing of Sb-Au Mineralization
A linear array on an isochron diagram may also be the result of mixing between two discrete end-member compositions with different 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios [8].However, the 143 Nd/ 144 Nd ratios plotted versus their corresponding values of 1/Nd do not indicate an inherited collinearity (Figure 10b).Considering that all Sch1 samples were collected from the same ore vein and should be regarded as products of the same hydrothermal event, the scheelite is chemically homogeneous (Figure 8) and the variation range of the calculated εNd(t) for Sch1 is relatively narrow (Table S3).The calculated initial 143 Nd/ 144 Nd values at an age of 210 Ma for Sch1 are the same within error as the initial 143 Nd/ 144 Nd value of the isochron (Figure 10a).These features suggest that all Sch1 samples had the same initial isotope ratios of daughter isotopes, and the scheelite Sm-Nd isochron age can be regarded as a single Sch1 hydrothermal event.The high-grade ore of the Sch1 and stibnite vein (Figure 5e) is 25.2 wt.% Sb, 0.23 g/t Au and 13 wt.% W from Stage-3.The mineral assemblage consists of quartz, scheelite, and stibnite.Thus, we believe that the isochron age of 210 ± 2 Ma (Figure 10a) represents the age of the Sb-Au mineralization in the Longshan deposit.
As noted above, many low-temperature Sb-Au deposits are hosted in the Proterozoic metamorphosed clastic rocks in the XZMP (Figure 1b).The Chanziping and Daping gold deposits (Figure 1b) have Rb-Sr isochron dates for fluid inclusions trapped in quartz yielding 205.6 ± 9.4 Ma and 204.8 ± 6.3 Ma, respectively [58].Scheelite coexisting with stibnite from the Zhazixi Sb-W deposit yielded Sm-Nd isochron ages of 227.3 ± 6.2 Ma [59].Mineralized granite and granite porphyry yielded Minerals 2019, 9, 87 14 of 20 K-Ar dates of 209 Ma for the Fuzhuxi Sb-Au, 200 Ma for the Liaojiaping Sb-Au, and 194-204 Ma for the Banxi Sb deposits [28].These dates show that there was a widespread multi-commodity W-Sb-Au mineralizing event in the XZMP ca.210 Ma, which was coeval with the Late Triassic granites (201-228 Ma) in the XZMP [29].

Genesis of the Longshan Sb-Au Deposit
Due to their similar electronic configurations, ionic radii, and valence states, Mo enters the scheelite lattice as Mo 6+ by substituting for W 6+ under oxidizing conditions.With changing redox conditions, more Mo 6+ is reduced to Mo 4+ and precipitates as MoS 2 in the fluid under reduced conditions [15,60].As shown in Figure 11a, the Mo concentrations of our samples are much lower than those of the scheelites from skarn-and porphyry-type W deposits and Au-W deposits [1,4,15,16,50,61].Further, the precipitation of Stage-4 native antimony (Figure 7i) in the Longshan Sb-Au deposit reflects ore formed under reduced conditions [62].This interpretation means the ore-forming fluids were relatively reduced.the Banxi Sb deposits [28].These dates show that there was a widespread multi-commodity W-Sb-Au mineralizing event in the XZMP ca.210 Ma, which was coeval with the Late Triassic granites (201-228 Ma) in the XZMP [29].

Genesis of the Longshan Sb-Au Deposit
Due to their similar electronic configurations, ionic radii, and valence states, Mo enters the scheelite lattice as Mo 6+ by substituting for W 6+ under oxidizing conditions.With changing redox conditions, more Mo 6+ is reduced to Mo 4+ and precipitates as MoS2 in the fluid under reduced conditions [15,60].As shown in Figure 11a, the Mo concentrations of our samples are much lower than those of the scheelites from skarn-and porphyry-type W deposits and Au-W deposits [1,4,15,16,50,61].Further, the precipitation of Stage-4 native antimony (Figure 7i) in the Longshan Sb-Au deposit reflects ore formed under reduced conditions [62].This interpretation means the oreforming fluids were relatively reduced.The different hydrothermal fluids that precipitated scheelite can potentially be distinguished in the LREE-MREE-HREE ternary diagram (Figure 12).Geochemical differences between REE for Sch1 and Sch2 indicate genetic similarities to porphyry-and skarn-type W deposits and quartz vein Au-W deposits, respectively [1,4,15,16,50,61,63]. Considering that the Sm-Nd age and narrow 87 Sr/ 86 Sr ratios of Sch1 (Table S3) compare well with those of the Late Triassic granites (Figure 11b, [41]), we The different hydrothermal fluids that precipitated scheelite can potentially be distinguished in the LREE-MREE-HREE ternary diagram (Figure 12).Geochemical differences between REE for Sch1 and Sch2 indicate genetic similarities to porphyry-and skarn-type W deposits and quartz vein Au-W deposits, respectively [1,4,15,16,50,61,63]. Considering that the Sm-Nd age and narrow 87 Sr/ 86 Sr ratios of Sch1 (Table S3) compare well with those of the Late Triassic granites (Figure 11b  The evolution of the ore-forming fluid can be reflected by compositional changes in scheelite [16,18,64,65].In the present study, Sch1 in the Longshan deposit has higher REE and lower Sr concentrations than Sch2 (Table 2, Figure 9), indicating that the early fluids were likely characterized by high REE and low Sr contents.The decrease in REE in Sch2 was likely caused by the precipitation of Sch1 and the resulting decrease in REE in the fluid.However, Sch2 has higher Sr concentrations than Sch1 (Table 2, Figure 9).Strontium is compatible in scheelite [16], meaning that the precipitation of Sch1 would have decreased rather than increased the Sr content in fluid.Differentiation of magma cannot elevate the Sr content of exsolved fluids [66,67].Therefore, an extra source of Sr is required to account for the formation of Sr-enriched Sch2.Sr-enriched scheelite (1455-6810 ppm, [68]) is also analyzed in the Woxi W-Sb-Au deposit (Figure 1b), where high-Sr fluid is thought to have resulted from the hydrothermal leaching of Proterozoic clastic rocks [68].The basement sequences in the Longshan region are mainly part of the Neoproterozoic Banxi and volcanic and pyroclastic rocks are Sr-enriched (302 to 942 ppm, [69]) and possibly provided the extra Lengjiaxi groups that contain pyroclastic and tholeiitic to calc-alkaline volcanic rocks [36,37,69].These mafic-intermediate Sr in the Sch2 through fluid-rock interaction.The extra Sr from the basement strata may also be characterized by the Sr isotopic signatures of Sch2.The higher 87 Sr/ 86 Sr ratios of Sch2 (Table S3, Figure 11b) indicate that the fluids were affected by contamination of radiogenic 87 Sr-enriched strata of the Banxi Group during intense wall rock alteration [12,68].The similar evolving fluids system has been reported from the Woxi W-Sb-Au deposit in the XZMP [12].In addition, native gold filling in the pyrite (Figure 7e) and arsenopyrite hosted in the altered host rock also suggest that the fluid-rock interaction caused Au precipitation [18].
The restricted range of δ 34 S values (−1.8‰ to 3.2‰) (n = 7, Figure 13, Table S4) is similar to that of sulfides in granitic rocks worldwide (δ 34 S = +1.0 ± 6.1‰) [70], suggesting a magmatic source for the sulfur.The concentrations of Au in the Neoproterozoic strata are 3.6 ppb in the Lengjiaxi Group and 3.1 ppb in the Banxi Group.These values are only slightly higher than the concentrations of Au in the The evolution of the ore-forming fluid can be reflected by compositional changes in scheelite [16,18,64,65].In the present study, Sch1 in the Longshan deposit has higher REE and lower Sr concentrations than Sch2 (Table 2, Figure 9), indicating that the early fluids were likely characterized by high REE and low Sr contents.The decrease in REE in Sch2 was likely caused by the precipitation of Sch1 and the resulting decrease in REE in the fluid.However, Sch2 has higher Sr concentrations than Sch1 (Table 2, Figure 9).Strontium is compatible in scheelite [16], meaning that the precipitation of Sch1 would have decreased rather than increased the Sr content in fluid.Differentiation of magma cannot elevate the Sr content of exsolved fluids [66,67].Therefore, an extra source of Sr is required to account for the formation of Sr-enriched Sch2.Sr-enriched scheelite (1455-6810 ppm, [68]) is also analyzed in the Woxi W-Sb-Au deposit (Figure 1b), where high-Sr fluid is thought to have resulted from the hydrothermal leaching of Proterozoic clastic rocks [68].The basement sequences in the Longshan region are mainly part of the Neoproterozoic Banxi and volcanic and pyroclastic rocks are Sr-enriched (302 to 942 ppm, [69]) and possibly provided the extra Lengjiaxi groups that contain pyroclastic and tholeiitic to calc-alkaline volcanic rocks [36,37,69].These mafic-intermediate Sr in the Sch2 through fluid-rock interaction.The extra Sr from the basement strata may also be characterized by the Sr isotopic signatures of Sch2.The higher 87 Sr/ 86 Sr ratios of Sch2 (Table S3, Figure 11b) indicate that the fluids were affected by contamination of radiogenic 87 Sr-enriched strata of the Banxi Group during intense wall rock alteration [12,68].The similar evolving fluids system has been reported from the Woxi W-Sb-Au deposit in the XZMP [12].In addition, native gold filling in the pyrite (Figure 7e) and arsenopyrite hosted in the altered host rock also suggest that the fluid-rock interaction caused Au precipitation [18].The restricted range of δ 34 S values (−1.8‰ to 3.2‰) (n = 7, Figure 13, Table S4) is similar to that of sulfides in granitic rocks worldwide (δ 34 S = +1.0 ± 6.1‰) [70], suggesting a magmatic source for the sulfur.The concentrations of Au in the Neoproterozoic strata are 3.6 ppb in the Lengjiaxi Group and 3.1 ppb in the Banxi Group.These values are only slightly higher than the concentrations of Au in the upper continental crust (1.5 ppb, [71]).The Neoproterozoic strata are unlikely to be the dominant source of Au.In the Longshan deposit, the measured Au concentration in sedimentary pyrite are 1.81-3.46ppm (unpublished data) and no free gold is observed within sedimentary pyrite.The occurrence of native gold and the high concentration of invisible gold in the pyrite (147-350 ppm, unpublished data) in Stage-3 suggest the late input of Au-rich fluid.The granitic dikes developed around the eastern part of the Longshan dome (Figure 2) also have a higher concentration of Sb (27.2-41.9)ppm [52] than that of Sb (0.

Conclusion
(1) The Longshan Sb-Au deposit formed ca.210 ± 2 Ma, which was coeval with the Late Triassic granites in the XZMP (201-228 Ma).The ages and sulfur isotope indicated that Longshan might be an intrusion-related Sb-Au deposit. (

Figure 1 .
Figure 1.(a) Tectonic framework of China including the location of the XZMP.(b) Regional geological map of the XZMP in Central Hunan, South China, showing the distribution of granitic intrusions and important Sb, Au and W deposits (modified after Geological Map of the Hunan Province, the People's Republic of China [32]; Chu et al. [33]).

Figure 1 .
Figure 1.(a) Tectonic framework of China including the location of the XZMP.(b) Regional geological map of the XZMP in Central Hunan, South China, showing the distribution of granitic intrusions and important Sb, Au and W deposits (modified after Geological Map of the Hunan Province, the People's Republic of China [32]; Chu et al. [33]).

Figure 3 .
Figure 3. (a) Simplified geological map of the Longshan mining district in Longshan deposit, South China; (b) cross-section map of the Longshan mining district in Longshan deposit, South China (modified after Hunan Bureau of Geology and Mineral Resources [53]).

Figure 3 .
Figure 3. (a) Simplified geological map of the Longshan mining district in Longshan deposit, South China; (b) cross-section map of the Longshan mining district in Longshan deposit, South China (modified after Hunan Bureau of Geology and Mineral Resources [53]).

Figure 3 .
Figure 3. (a) Simplified geological map of the Longshan mining district in Longshan deposit, South China; (b) cross-section map of the Longshan mining district in Longshan deposit, South China (modified after Hunan Bureau of Geology and Mineral Resources [53]).

Figure 5 .
Figure 5. Paragenesis of the mineral assemblages showing the mineralized sequence of the Longshan Sb-Au deposit, South China.Line thickness indicates approximate relative mineral abundance.

Figure 5 .
Figure 5. Paragenesis of the mineral assemblages showing the mineralized sequence of the Longshan Sb-Au deposit, South China.Line thickness indicates approximate relative mineral abundance.

Figure 8 .
Figure 8.The (a) Backscattered electron (BSE) image; (b) Scanning Electron Microscopy (SEM) image; (c) Colorful Cathodoluminescence (CL) image; and (d) Grey Cathodoluminescence (CL) image of the Sch1 from the Longshan Sb-Au deposit indicate that its chemical composition is homogeneous.

Figure 8 .
Figure 8.The (a) Backscattered electron (BSE) image; (b) Scanning Electron Microscopy (SEM) image; (c) Colorful Cathodoluminescence (CL) image; and (d) Grey Cathodoluminescence (CL) image of the Sch1 from the Longshan Sb-Au deposit indicate that its chemical composition is homogeneous.
. Sch1 exhibits high REE and Y contents with ΣREE + Y abundances ranging from 43.5 to 104 ppm.The REE N distribution patterns of Sch1 (Figure 9a,b) are similar, with a positive "hump" in the MREE.However, Sch2 exhibits low REE and Y contents with ΣREE + Y abundances ranging from 0.349 to 25.1 ppm.The REE N distribution patterns of Sch2 (Figure 9c,d) are similar, with depletions in the LREE.

Figure 11 .
Figure 11.(a) Comparison of Mo content (ppm) in scheelite from different type of deposits; (b) Plot of 87 Sr/ 86 Sr ratios for scheelite from the from the Longshan Sb-Au deposit, South China.The ( 87 Sr/ 86 Sr) (t = 210 Ma) ratios for Late Triassic granites are from Gao et al. [41].The ( 87 Sr/ 86 Sr)(t = 210 Ma) ratios for Banxi Group basement strata are from Peng and Frei [12].

Figure 11 .
Figure 11.(a) Comparison of Mo content (ppm) in scheelite from different type of deposits; (b) Plot of 87 Sr/ 86 Sr ratios for scheelite from the from the Longshan Sb-Au deposit, South China.The ( 87 Sr/ 86 Sr) (t = 210 Ma) ratios for Late Triassic granites are from Gao et al. [41].The ( 87 Sr/ 86 Sr)(t = 210 Ma) ratios for Banxi Group basement strata are from Peng and Frei [12].
, [41]), we interpret Sch1 as a possible precipitate from the primitive ore-forming fluids derived from the Late Triassic granitic intrusion in the XZMP.Minerals 2019, 9, x FOR PEER REVIEW 15 of 20 interpret Sch1 as a possible precipitate from the primitive ore-forming fluids derived from the Late Triassic granitic intrusion in the XZMP.
4 ppm) in the upper continental crust [71].Based on the in-situ trace element compositions, Sm-Nd isotope dating of scheelite and the S isotope of sulfides, we infer that Longshan is an intrusion-related Sb-Au deposit associated with the emplacement of the Late Triassic granitic intrusion in the XZMP.Minerals 2019, 9, x FOR PEER REVIEW 16 of 20 upper continental crust (1.5 ppb, [71]).The Neoproterozoic strata are unlikely to be the dominant source of Au.In the Longshan deposit, the measured Au concentration in sedimentary pyrite are 1.81-3.46ppm (unpublished data) and no free gold is observed within sedimentary pyrite.The occurrence of native gold and the high concentration of invisible gold in the pyrite (147-350 ppm, unpublished data) in Stage-3 suggest the late input of Au-rich fluid.The granitic dikes developed around the eastern part of the Longshan dome (Figure 2) also have a higher concentration of Sb (27.2-41.9)ppm [52] than that of Sb (0.4 ppm) in the upper continental crust [71].Based on the in-situ trace element compositions, Sm-Nd isotope dating of scheelite and the S isotope of sulfides, we infer that Longshan is an intrusion-related Sb-Au deposit associated with the emplacement of the Late Triassic granitic intrusion in the XZMP.

Figure 13 .
Figure 13.Sulfur isotope composition from the Longshan Sb-Au deposit, South China (Stratum sulfur isotope compositions data are from Hu et al. [28]).
) The are two types of scheelite that can be distinguished (Sch1 and Sch2) in the Longshan Sb-Au deposit.Sch1 exhibits high ΣREE + Y contents, low Sr values and a narrow range of 87 Sr/ 86 Sr values, while Sch2 shows low ΣREE + Y contents, elevated Sr abundance and wide 87 Sr/ 86 Sr ratios.The basement rocks may provide the extra Sr and radiogenic 87 Sr in Sch2 through fluid-rock interaction.

Table 1 .
REE and trace element compositions (ppm) of scheelite from the Longshan Sb-Au deposit, South China.

Table 2 .
REE and trace element compositions (ppm) of scheelite from the Longshan Sb-Au deposit, South China.