Metallogenesis of the Bawang Sn-Zn Polymetallic Deposit, Wuxu Ore Field, Guangxi, South China: U-Pb Dating and C-O-S-Pb Isotopic Constraints

: The Bawang deposit, located in the west of Wuxu ore ﬁeld, southern section of the Nandan-Hechi metallogenic belt, is a medium-sized tin–zinc polymetallic deposit. Its genesis has been a matter of debate because of lacking constraints from isotope geology. In this study, LA-MC-ICP-MS U-Pb dating of cassiterite and C-O-S-Pb isotope analyses of monominerals were used to investigate the mineralization age and source of the ore-forming minerals in the Bawang deposit. LA-ICP-MS U–Pb dating of cassiterite yielded ages of 93.1 ± 4.8 Ma and 85.3 ± 6.3 Ma, indicating that the mineralization occurred in the early Late Cretaceous. The δ 13 C PDB and δ 18 O SMOW values of calcites at the ore stage range between − 0.41‰ and 0.44‰ (average = − 11‰) and between 13.8‰ and 15.40‰ (average = 14.59‰), respectively. This shows that ore ﬂuid sources were a mixture of those derived from magma and stratum. Pyrite and sphalerite have uniform δ 34 S values ( − 4.45‰~ − 2.20‰), indicating that sulfur is also derived from the mixing of magmatic hydrothermal and stratum ﬂuids. The Pb isotopic composition of sulﬁde ( 206 Pb/ 204 Pb = 18.4055~18.7625, 207 Pb/ 204 Pb = 15.6745~15.7209, 208 Pb/ 204 Pb = 38.6232~39.0370) is consistent with the granite of the same age, indicating that ore-forming metals are mainly derived from magmatic hydrothermal solution. The Bawang deposit is a hydrothermal vein-type deposit in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal ﬂuid and carbonate rock. There may be large skarn-type ore bodies in the deep part. The results of this study provide insights into the research and exploration of similar deposits in Nandan-Hechi metallogenic belt and in the Youjiang Basin. Bawang deposit is mainly related to magmatism and comes from the crust–mantle mixed-source area, which is consistent with the Lamo deposit and Tongkeng deposit in Dachang ore ﬁeld. In addition, it is very close to that of the Longxianggai granite in the Dachang ore ﬁeld [80].


Introduction
Chronological information on mineralization is key to understanding the genesis of ore deposits. In recent years, LA-ICP-MS U-Pb dating of cassiterite has been used to study the metallogenic age of tin polymetallic deposits [1][2][3][4][5][6]. Furthermore, C-O-S-Pb isotope analysis has been shown to be an effective method for determining the source of ore fluids and metals [7][8][9][10][11]. In this paper, U-Pb isotope chronology was carried out on the Bawang The Wuxu ore field is located in the south of the Nandan-Hechi mineralization belt. The formations from the Lower Devonian to Middle Triassic are exposed, dominated by Middle and Upper Devonian (Figure 1c). The lithology of Middle Devonian to Middle Permian is carbonate with clasolite and silicolite. The Lower Devonian/Upper Permian to Middle Triassic is dominated by clasolite interspersed with carbonate. The Tangding Formation (D1-2t), comprising mudstone, argillaceous siltstone, and sandy mudstone, is the main host rock of Pb-Zn-Sn-Sb deposits. The Nandan Formation (C2P1n), comprising calcarenite, is a secondary ore-bearing horizon. The marl and bioclastic limestone of the Luofu Formation (D2l), the siliceous shale of the Liujiang Formation (D3l), and the banded limestone of the Wuzhishan Formation (D3w) are host rocks of arsenic-mercury deposits [13]. The Wuxu Anticline, Bayue Syncline, and Jiuxu Anticline are the main fold structures and are all NNW-trending (Figure 1d), belonging to the southern extension of the NWtrending Danchi Anticline. It is believed that deflection of the fold axial trace was the result of superimposition of the late SN-trending fold [17]. There are NNW-, NNE-, NE-, and nearly EW-trending faults, with predominant faults trending NNW and NNE, distributed in the core of the Wuxu Anticline, arranged in parallel groups dipping steeply

Ore Deposit Geology
The Bawang deposit is located in the west of the Wuxu ore field and the west wing of the Wuxu Anticline. The strata exposed include Upper Carboniferous Baping, Dashantang, Upper Carboniferous/Lower Permian Nandan, Middle Carboniferous Sidazhai, Upper Carboniferous Linghao, Lower Triassic Shipao, and Middle Triassic Baifeng formations, and a Quaternary system (Figure 2a). The Baping Formation is composed of thin microcrystalline limestone, argillaceous limestone interlaced with chert limestone, silicolite, mudstone, and siltstone. The Dashantang Formation is thick dolomite with dolomitic limestone. The Nandan Formation is divided into four sections: the first section is chert belt limestone and fine crystal limestone mixed with dolomitic limestone; the second section is aplite and silty limestone; the third section is interbedded with thin-medium siltstone limestone and dolomitic limestone, containing a small amount of chert belts and nodules; and the fourth section is thick calcirudite intercalated with calcisiltite. The Sidazhai Formation is composed of two sections: the lower section is medium-thickness calcarenite and calcisiltite containing chert, sandwiched with bioclastic calcarenite, and the upper section is medium calcisiltite with chert belts and nodules. The third and fourth sections of the Nandan Formation are host rock of the ore deposit. microcrystalline limestone, argillaceous limestone interlaced with chert limestone, silicolite, mudstone, and siltstone. The Dashantang Formation is thick dolomite with dolomitic limestone. The Nandan Formation is divided into four sections: the first section is chert belt limestone and fine crystal limestone mixed with dolomitic limestone; the second section is aplite and silty limestone; the third section is interbedded with thin-medium siltstone limestone and dolomitic limestone, containing a small amount of chert belts and nodules; and the fourth section is thick calcirudite intercalated with calcisiltite. The Sidazhai Formation is composed of two sections: the lower section is medium-thickness calcarenite and calcisiltite containing chert, sandwiched with bioclastic calcarenite, and the upper section is medium calcisiltite with chert belts and nodules. The third and fourth sections of the Nandan Formation are host rock of the ore deposit. The strata in the Bawang deposit are monocline (Figure 2b). The strike of the strata is nearly SN and NNW, the dip direction ranges from 240 • to 290 • , and the dip angle is mostly 35 •~6 0 • . The fault structure is generally developed, trending N-S, E-W, and NW-SE. The F 3 fault is located in the middle of the mining area, in which three tin-zinc ore bodies (II-1, II-2, II-3) occur (Figure 2c). It is a S-N trending tensional normal fault with a length of about 1 km, dip direction of 260 •~2 80 • , and dip angle of 55 • -66 • . The F 5 fault is located in the north-central mining area, hosting I ore body. This fault is a zone formed by a series of overlapping dense fractures (joints) striking NEE-NE, with SSE-SE dipping at high angles and reversing locally. The fault is sinistral strike-slip thrusting in the early stage, becoming left-normal-slip in the late stage.
The I ore body is the main one, extending down to 730 m (Figure 3a), 0.33~25.95 m thick, with an average thickness of 9.19 m. The major metal is zinc, associated with Sn, Ag, In, Cd, Ga, and As. The average grade of Zn is 7.09%, Sn 0.70%, Ag 46.79 g/t, In 0.0361%, Cd 0.073%, Ga 0.0061%, and As 1.34%. The I ore body is strictly controlled by the F 5 fault, formed similar to a tube with west side-pitching. In fact, it is an ore body group composed of several bonanzas divided by non-ore surrounding rock (the fourth section of the Nandan Formation, gravel limestone). Most of the single bonanzas dip steeply to the south, and their attitudes are 172 • -182 • ∠76 • -86 • , while some bonanzas are nearly upright or reverse to the north. The plane shape of the bonanza in each level resembles a dumbbell or lotus root, while some are irregular sacs (Figure 3b,c), with a plane length of 40-88 m. The shallow part of the ore body is oxidized (Figure 4a-c) while the deep part is sulfide (Figure 4d,e); their boundary is at about the 500 m level. Primary sulfides are mostly massive, but rarely have disseminated and breccia structures. The metal minerals include sphalerite, pyrite, arsenopyrite, cassiterite, pyrrhotite, chalcopyrite, and so on (Figure 4f-l). The gangue minerals are calcite and dolomite. The wallrock alteration is weak, including limonitization, carbonation, pyritization, and silicification, with a clear boundary to the ore body. by a series of overlapping dense fractures (joints) striking NEE-NE, with SSE-SE dipping at high angles and reversing locally. The fault is sinistral strike-slip thrusting in the early stage, becoming left-normal-slip in the late stage.
The I ore body is the main one, extending down to 730 m (Figure 3a), 0.33 ~ 25.95 m thick, with an average thickness of 9.19 m. The major metal is zinc, associated with Sn, Ag, In, Cd, Ga, and As. The average grade of Zn is 7.09%, Sn 0.70%, Ag 46.79 g/t, In 0.0361%, Cd 0.073%, Ga 0.0061%, and As 1.34%. The I ore body is strictly controlled by the F5 fault, formed similar to a tube with west side-pitching. In fact, it is an ore body group composed of several bonanzas divided by non-ore surrounding rock (the fourth section of the Nandan Formation, gravel limestone). Most of the single bonanzas dip steeply to the south, and their attitudes are 172°-182°∠76°-86°, while some bonanzas are nearly upright or reverse to the north. The plane shape of the bonanza in each level resembles a dumbbell or lotus root, while some are irregular sacs (Figure 3b,c), with a plane length of 40-88 m. The shallow part of the ore body is oxidized (Figure 4a-c) while the deep part is sulfide (Figure 4d,e); their boundary is at about the 500 m level. Primary sulfides are mostly massive, but rarely have disseminated and breccia structures. The metal minerals include sphalerite, pyrite, arsenopyrite, cassiterite, pyrrhotite, chalcopyrite, and so on (Figure 4fl). The gangue minerals are calcite and dolomite. The wallrock alteration is weak, including limonitization, carbonation, pyritization, and silicification, with a clear boundary to the ore body.   The II ore body is hosted in bed sliding fracture zone F3 and is oxidized. It consists of II-1, II-2, and II-3. Their attitudes are 245°-277°∠55°-66°; their lengths are 50, 240, and 360 m, respectively; and their thicknesses are 1.00-1.91 m. Ore contains 0.26-1.44% Pb, 0.79-10.92% Zn, and 0.01-0.56% Sn. The metal minerals are limonite, sphalerite, cassiterite, galena, and so on, while the gangue minerals are mainly calcite.

Cassiterite U-Pb Isotope Dating
The two samples BW4 and BW5 collected for cassiterite U-Pb isotope dating were, respectively, obtained from levels 370 and 282 of the I ore body (Figure 3a). Heavy minerals were separated by gravity concentration, and cassiterite grains were hand-picked under a bingoscope. Large grains with better transparency and fewer cracks and inclusions were selected as targets. Images were taken under transmitted light, reflected light, and backscattered electron (BSE). In the BSE images, most cassiterites were semi-idiomorphic and tetragonal, but a few were irregular. There was almost no aberration in the BSE images, indicating that the composition was relatively uniform ( Figure 5).

Cassiterite U-Pb Isotope Dating
The two samples BW4 and BW5 collected for cassiterite U-Pb isotope dating were, respectively, obtained from levels 370 and 282 of the I ore body (Figure 3a). Heavy minerals were separated by gravity concentration, and cassiterite grains were hand-picked under a bingoscope. Large grains with better transparency and fewer cracks and inclusions were selected as targets. Images were taken under transmitted light, reflected light, and backscattered electron (BSE). In the BSE images, most cassiterites were semi-idiomorphic and tetragonal, but a few were irregular. There was almost no aberration in the BSE images, indicating that the composition was relatively uniform ( Figure 5). The U-Pb isotopic dating of cassiterite was completed in the Isotope Geochemistry Laboratory at the Tianjin Center of China Geological Survey [41]. The instrument was composed of a Thermo Fisher Neptune multireceiver inductively coupled plasma mass spectrometer and an ESI UP193-FX ArF excimer laser ablation sampling system. The experimental process was as follows: According to the reflected light and backscattered electron images of cassiterite, the areas with few inclusions and cracks were tested. Cassiterite grains were analyzed using an energy density of 15 J/cm 2 , a spot size of 75 μm, and a repetition rate of 20 Hz. Helium gas carrying the ablated sample aerosol was mixed with argon and nitrogen (additional diatomic gas) to enhance sensitivity and was finally flowed into the ICP-MS instrument. Due to the weak ion signal of 204 Pb and the interference of 204 Hg in argon gas, the content of 204 Pb is difficult to accurately determine. Therefore, 207 Pb can be used instead of 204 Pb as the U-Pb isochron for young cassiterite samples (generally less than 400 Ma) with low U content and low 207 Pb content. That is, the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron replaces the traditional 238 U/ 204 Pb-206 Pb/ 204 Pb isochron method for data processing [42]. A standard sample (AY-4, 206 Pb/ 238 U age = 158.2 ± 0.4 Ma) with accurately determined ID-TIMS U-Pb age was used as the external standard [43]. Data calculation and graph drawing of the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron and 207 Pb/ 206 Pb-238 U/ 206 Pb concordance ages (Tera-Wasserburg curve) were performed using Isoplot software [44,45].

C-O-S-Pb Isotopes
The samples used for C-O isotope analysis were ore-stage calcite collected from level 282. S-Pb isotope samples were sphalerite and pyrite from the mineralization period; PD2-1 was from level 454, PD4-1 was from level 370, PD345-1 and PD345-2 were from level 345, and others were from level 282. Firstly, the samples were crushed and sieved, taking the particles between 40 and 60 mesh. Then, repeated selection was carried out until the purity of the single mineral was greater than 99%. At last, the samples were ground to less The U-Pb isotopic dating of cassiterite was completed in the Isotope Geochemistry Laboratory at the Tianjin Center of China Geological Survey [41]. The instrument was composed of a Thermo Fisher Neptune multireceiver inductively coupled plasma mass spectrometer and an ESI UP193-FX ArF excimer laser ablation sampling system. The experimental process was as follows: According to the reflected light and backscattered electron images of cassiterite, the areas with few inclusions and cracks were tested. Cassiterite grains were analyzed using an energy density of 15 J/cm 2 , a spot size of 75 µm, and a repetition rate of 20 Hz. Helium gas carrying the ablated sample aerosol was mixed with argon and nitrogen (additional diatomic gas) to enhance sensitivity and was finally flowed into the ICP-MS instrument. Due to the weak ion signal of 204 Pb and the interference of 204 Hg in argon gas, the content of 204 Pb is difficult to accurately determine. Therefore, 207 Pb can be used instead of 204 Pb as the U-Pb isochron for young cassiterite samples (generally less than 400 Ma) with low U content and low 207 Pb content. That is, the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron replaces the traditional 238 U/ 204 Pb-206 Pb/ 204 Pb isochron method for data processing [42]. A standard sample (AY-4, 206 Pb/ 238 U age = 158.2 ± 0.4 Ma) with accurately determined ID-TIMS U-Pb age was used as the external standard [43]. Data calculation and graph drawing of the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron and 207 Pb/ 206 Pb-238 U/ 206 Pb concordance ages (Tera-Wasserburg curve) were performed using Isoplot software [44,45].

C-O-S-Pb Isotopes
The samples used for C-O isotope analysis were ore-stage calcite collected from level 282. S-Pb isotope samples were sphalerite and pyrite from the mineralization period; PD2-1 was from level 454, PD4-1 was from level 370, PD345-1 and PD345-2 were from level 345, and others were from level 282. Firstly, the samples were crushed and sieved, taking the particles between 40 and 60 mesh. Then, repeated selection was carried out until the purity of the single mineral was greater than 99%. At last, the samples were ground to less than 200 mesh in an agate mortar. C-O-S-Pb isotope tests were completed by Langfang Chengxin Geological Services Co., LTD (Langfang, China).
The calcite samples were ground to 200 mesh and baked in an oven at 105 • C for 2 h to remove adsorbed water. The sample tubes were baked for 30 min in a Gasbench at 70 • C. Then, about 0.2 mg of carbonate was placed into the sample tubes and capped. The air in the sample tube was evacuated by high-purity helium gas. The prepared calcite powder and pure (100%) phosphoric acid were fully reacted for more than 8 h, producing CO 2 gas. The CO 2 gas was carried into a MAT253 mass spectrometer with high-purity helium to analyze the C and O isotope composition. Every five samples were added into a group of standard GBW04416 and GBW04417 to compare. Analytical results are reported in δ notation as per mil (‰) deviation relative to the Vienna Pee Dee Belemnite (VPDB) as δ 13 C V-PDB and δ 18 O V-PDB , and the δ 18 O V-SMOW value was calculated using the equation of Friedman and O'Neil [46], i.e., δ 18 O V-SMOW = 1.03086 × δ 18 O V-PDB + 30.86. The analytical accuracy was better than ±0.1‰ (2σ) for both C and O isotopes.
Sulfide monominerals and cuprous oxide were ground to about 200 mesh and mixed evenly in a certain proportion. Then, they were heated to 980 • C in a vacuum of 2.0 × 10 −2 Pa, with the oxidation reaction generating sulfur dioxide gas. The sulfur dioxide gas was collected by a freezing method under vacuum conditions, and the sulfur isotopic composition was analyzed by Delta V Plus gas isotope mass spectrometry. The measured results were based on CDT, recorded as δ 34 S V-CDT . The analytical accuracy was better than ±0.2‰.
A 0.1-0.2 g powder sample was accurately weighed and dissolved in a low-pressure airtight dissolution tank with mixed acid (HF + HNO 3 + HClO 4 ) for 24 h. After the sample was completely dissolved, it was converted into chloride by adding 6 mol/L hydrochloric acid and then dried. The solution was dissolved with 1 mL of 0.5 mol/L HBr and centrifugation. The liquid was added to an anion exchange column (250 µL AG1-X8, 100-200 mesh). The impurities were washed with 0.5 mol/L HBr, and the lead was resolved with 1 mL of 6 mol/L HCl in a PTFE beaker. An ISOPROBE-T thermal ionization mass spectrometer was used for isotope analysis. The samples were placed on a rhenium band with silica gel phosphate and the lead isotope ratio was measured by the static acceptance method. NBS 981 uncorrected results: 208 Pb/ 206 Pb = 2.164940 ± 15, 207 Pb/ 206 Pb = 0.914338 ± 7, 204 Pb/ 206 Pb = 0.0591107 ± 2; Pb background was less than 100 pg during the whole process.

Cassiterite U-Pb Isotope Dating
The results of LA-ICP-MS U-Pb dating of the cassiterite are shown in Table 1 and   (Figure 6c), the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron age was 82.9 ± 5.3 Ma (n = 23, MSWD = 1.08), and the initial 206 Pb/ 207 Pb ratio was 1.218 ± 0.014 (Figure 6d). The two samples' concordant ages and isochron ages are consistent within the error range, indicating that the metallogenic ages obtained in this study are reliable.

C-O Isotopes
Carbon-oxygen isotope results for the five calcite samples (during the mineralization period) from the Bawang deposit are given in Table 2

S-Pb Isotopes
Sulfur isotope compositions in the Bawang deposit are given in Table 3. The δ 34 S values of the nine pyrite samples ranged from −4.45‰ to −2.20‰, with a range of 2.25‰ and an average of −3.50‰, higher than −5.1‰ [47]. The δ 34 S values of the seven sphalerite samples were concentrated, ranging from −4.25‰ to −4.01‰, with a range of 0.24‰ and

C-O Isotopes
Carbon-oxygen isotope results for the five calcite samples (during the mineralization period) from the Bawang deposit are given in Table 2

S-Pb Isotopes
Sulfur isotope compositions in the Bawang deposit are given in Table 3. The δ 34 S values of the nine pyrite samples ranged from −4.45‰ to −2.20‰, with a range of 2.25‰ and an average of −3.50‰, higher than −5.1‰ [47]. The δ 34 S values of the seven sphalerite samples were concentrated, ranging from −4.25‰ to −4.01‰, with a range of 0.24‰ and an average of −4.17‰, close to −4.03‰ [47]. In general, the δ 34 S values of pyrite and sphalerite at all elevations have little difference, showing a single-peak distribution, and the peak values are in the range of −4.5‰~−4‰, close to the δ 34 S values of mantle sulfur and magmatic sulfur (−3‰~+3‰). The δ 34 S value of sphalerite is lower than that of pyrite, indicating that sulfur isotope balance was reached in the ore-forming fluid. The lead isotope compositions in the Bawang deposit are given in Table 4  There are few studies on metallogenic chronology in the Wuxu ore field; however, the only report which is accessible is that of Wei [47]. He obtained metallogenic ages of 92 ± 11 Ma (n = 18, MSWD = 3.6) for the Bawang deposit and 103 ± 9.3 Ma (n = 15, MSWD = 1.4) for the Jianzhupo deposit by LA-ICP-MS U-Pb dating of cassiterite. The metallogenic ages are consistent with those obtained in this paper within the error range, indicating that the tin-polymetallic mineralization in Wuxu ore field occurred in the Late Cretaceous. Previous studies on the chronology of the Nandan-Hechi metallogenic belt mainly focused on Dachang ore field (Figure 7): Wang et al. [27] obtained the 40 [28] reported that the U-Pb age of cassiterite in multiple deposits of Dachang Ore field ranged from 90.3 ± 1.8 to 95.4 ± 4.9 Ma. Huang et al. [51] obtained a U-Pb date of cassiterite at 92.5 ± 4.4 Ma for the Longtoushan deposit. The results show that the tin-polymetallic mineralization time of Wuxu ore field is nearly the same as that of Dachang ore field, and they are the product of the same mineralization event in the early Late Cretaceous.

Metallogenic Epoch
The 207 Pb/ 206 Pb-238 U/ 206 Pb concordant ages of the two cassiterite samples from the I ore body of the Bawang deposit are 93.1 ± 4.8 Ma and 85.3 ± 6.3 Ma, while the 207 Pb/ 206 Pb-238 U/ 206 Pb isochron ages are 89.3 ± 6.3 Ma and 82.9 ± 5.3 Ma, indicating that the cassiterite is the product of mineralization in the early Late Cretaceous.
There are few studies on metallogenic chronology in the Wuxu ore field; however, the only report which is accessible is that of Wei [47]. He obtained metallogenic ages of 92 ± 11 Ma (n = 18, MSWD = 3.6) for the Bawang deposit and 103 ± 9.3 Ma (n = 15, MSWD = 1.4) for the Jianzhupo deposit by LA-ICP-MS U-Pb dating of cassiterite. The metallogenic ages are consistent with those obtained in this paper within the error range, indicating that the tin-polymetallic mineralization in Wuxu ore field occurred in the Late Cretaceous. Previous studies on the chronology of the Nandan-Hechi metallogenic belt mainly focused on Dachang ore field (Figure 7): Wang et al. [27] obtained the 40 [28] reported that the U-Pb age of cassiterite in multiple deposits of Dachang Ore field ranged from 90.3 ± 1.8 to 95.4 ± 4.9 Ma. Huang et al. [51] obtained a U-Pb date of cassiterite at 92.5 ± 4.4 Ma for the Longtoushan deposit. The results show that the tinpolymetallic mineralization time of Wuxu ore field is nearly the same as that of Dachang ore field, and they are the product of the same mineralization event in the early Late Cretaceous.  [20,27,28,[47][48][49][50][51][52][53]. ?-there is no data.
The tin-polymetallic metallogenic ages are consistent with the high-precision zircon U-Pb ages (89-97 Ma) of granitic magmatic rocks of the Nandan-Hechi metallogenic belt reported in recent years, and there is a large time interval between this and the sedimentary age of ore-bearing surrounding rocks (Upper Carboniferous-Lower Permian). The coincidence of mineralization and diagenetic time provides strong support for the time relationship between tin-polymetallic mineralization and Late Yanshanian granitic intrusion in the Nandan-Hechi metallogenic belt. The spatial relationship shows that the mineralization presents three zones, including zinc-tin, antimony, and mercury-arsenic, from the (concealed) granite to the periphery, indicating the characteristics of magmatichydrothermal deposits. In conclusion, the tin-polymetallic mineralization in the Nandan-Hechi metallogenic belt should be attributed to Late Cretaceous granitic post-magmatic hydrothermal processes rather than Late Paleozoic sedimentary exhalative processes. The boundary between the ore body and surrounding rock is distinct (the boundary is fissures of different sizes) [27], and the shear ore-controlled faults (such as F 5 of the Bawang deposit) further confirm that they are epigenetic deposits.

Ore-Forming Fluid Source
Carbon and oxygen isotopic composition of hydrothermal calcite provides an effective means of tracing the source of ore-forming fluids [7][8][9][10][11][54][55][56][57]. There are four sources of carbon in ore-forming fluids: (1) marine carbonate, for which the δ 13 C PDB values are mostly stable at 0‰; (2) metamorphic carbonate, with δ 13 C PDB values ranging from −12.5‰ to −4.7‰ [58]; (3) magma-mantle source, with δ 13 C PDB values of −5‰ to −2‰ and −9‰ to −3‰, respectively [59]; and (4) organic carbon in various rocks, for which the δ 13 C PDB values are about −25‰ [60]. When there is no graphite intergrowth with calcite in metallogenic hydrothermal veins, the carbon isotopic composition of calcite can be approximated as the total carbon isotopic composition of ore-forming hydrothermal fluids [61]. As there is no graphite related to mineralization in the Bawang deposit, the carbon isotopic composition of the calcite in the ore body can approximately represent the carbon isotopic composition of the ore-forming hydrothermal solution.
The δ 13 C PDB values of calcite in the Bawang deposit were confined within a narrow range (−0.41‰ to 0.44‰) during mineralization. This range is between those of mantlederived carbon (or magmatic carbon) (−3‰ to −9‰) [59] and marine carbonate (−2‰ to +2‰) [62]. Therefore, the carbon in the ore-forming fluid could not come from organic carbon sources, but it could come from granite (deep-source carbon) and marine carbonate. The δ 13 O SMOW value is concentrated within 13.85‰ to 15.40‰, close to the range of igneous rock (5‰~15‰) [58]. Similarly, Shao [63] obtained δ 13 C PDB values of calcite in the Tongkeng deposit ranging from −8.9‰ to −0.4‰, suggesting that the tin, zinc, and copper mineralization is involved with magmatic fluid and surrounding rock.
According to the δ 13 C-δ 18 O diagram [64] (Figure 8a), the calcite in the Bawang deposit falls between granite and marine carbonate and is far away from the sedimentary organic range, indicating that the fluid source is characterized by the mixing of magma and stratum. Meanwhile, δ 13 C and δ 18 O are negatively correlated (Figure 8b), similar to results for the metallogenic calcite of Huize lead-zinc deposit in Yunnan province [65], the latemetallogenic calcite of Jiaodong gold deposit [66], and the early metallogenic calcite of Xianghualing polymetallic deposit in Hunan Province [67]. This negative correlation may be related to CO 2 degassing [68]. CO 2 degassing can increase the pH value of the fluid, leading to reduced solubility of tin in the solution, and it is conducive to cassiterite precipitation and mineralization [69]. CO 2 degassing can better explain the characteristics of weak wallrock alteration in the Bawang deposit.  [64], the data from Liu [70], Tan et al. [71], and Ding et al. [72]) and the negatively correlated of δ 13 C and δ 18 O (b).

Source of Ore-Forming Material
Sulfur isotope detection is a mature method used to trace the source of ore-forming materials, and it is widely used to judge the genesis and sulfur source of ore deposits. In general, when using sulfur isotopes to study provenance, it is important to study the isotopic composition of the total sulfur in ore-forming hydrothermal fluids. Only when the sulfide composition is simple, and sulfosalt minerals are not present, can the isotopic composition of total sulfur in ore-forming hydrothermal fluids be substituted with sulfide δ 34 S values [61,73]. There is no sulfosalt mineral in the Bawang deposit, so, it is feasible to use the δ 34 S value of sulfide to replace the isotopic composition of total sulfur in the ore-forming hydrothermal fluid to study the provenance.
The sulfur isotope variation range of pyrite and sphalerite is narrow and the δ 34 S values are similar (−4.45‰ to −2.20‰) (Figure 9), indicating they have the same sulfur source. According to previous studies, the sulfur isotopic composition of the main deposits in Dachang ore field ranges from -8‰ to 5‰ [24,30,31,39,[74][75][76][77], and that in Wuxu ore field ranges from −5.2‰ to 7.4‰ [47]. The δ 34 S value of pyrite in the strata varies from −37‰ to −4‰ [78], and the δ 34 S value of Longxianggai granite varies from −1.3‰ to −1‰ [79]. However, Cheng [80] obtained δ 34 S values ranging from +4.46‰ to +16.36‰. The δ 34 S value by Cheng is close to that of granite in Figure 9. In the whole Nandan-Hechi metallogenic belt, the δ 34 S value is higher when near the (concealed) granite, indicating that the sulfur is mainly derived from the magma. In the process of ore formation, hydrothermal fluid migrates from the center of granite to the periphery, and material favorable for mineralization extracted from the strata joins continuously.   [64], the data from Liu [70], Tan et al. [71], and Ding et al. [72]) and the negatively correlated of δ 13 C and δ 18 O (b).

Source of Ore-Forming Material
Sulfur isotope detection is a mature method used to trace the source of ore-forming materials, and it is widely used to judge the genesis and sulfur source of ore deposits. In general, when using sulfur isotopes to study provenance, it is important to study the isotopic composition of the total sulfur in ore-forming hydrothermal fluids. Only when the sulfide composition is simple, and sulfosalt minerals are not present, can the isotopic composition of total sulfur in ore-forming hydrothermal fluids be substituted with sulfide δ 34 S values [61,73]. There is no sulfosalt mineral in the Bawang deposit, so, it is feasible to use the δ 34 S value of sulfide to replace the isotopic composition of total sulfur in the ore-forming hydrothermal fluid to study the provenance.
The sulfur isotope variation range of pyrite and sphalerite is narrow and the δ 34 S values are similar (−4.45‰ to −2.20‰) (Figure 9), indicating they have the same sulfur source. According to previous studies, the sulfur isotopic composition of the main deposits in Dachang ore field ranges from −8‰ to 5‰ [24,30,31,39,[74][75][76][77], and that in Wuxu ore field ranges from −5.2‰ to 7.4‰ [47]. The δ 34 S value of pyrite in the strata varies from −37‰ to −4‰ [78], and the δ 34 S value of Longxianggai granite varies from −1.3‰ to −1‰ [79]. However, Cheng [80] obtained δ 34 S values ranging from +4.46‰ to +16.36‰. The δ 34 S value by Cheng is close to that of granite in Figure 9. In the whole Nandan-Hechi metallogenic belt, the δ 34 S value is higher when near the (concealed) granite, indicating that the sulfur is mainly derived from the magma. In the process of ore formation, hydrothermal fluid migrates from the center of granite to the periphery, and material favorable for mineralization extracted from the strata joins continuously.
Due to the large molecular mass of lead and the small relative mass difference between different lead isotope molecules, the isotopic composition of ore-forming elements usually does not change during leaching, transportation, and precipitation. Therefore, the characteristics and relations of the lead isotopic composition can be used to obtain information on the mineral source of the deposit.
to −4‰ [78], and the δ 34 S value of Longxianggai granite varies from −1.3‰ to −1‰ [79]. However, Cheng [80] obtained δ 34 S values ranging from +4.46‰ to +16.36‰. The δ 34 S value by Cheng is close to that of granite in Figure 9. In the whole Nandan-Hechi metallogenic belt, the δ 34 S value is higher when near the (concealed) granite, indicating that the sulfur is mainly derived from the magma. In the process of ore formation, hydrothermal fluid migrates from the center of granite to the periphery, and material favorable for mineralization extracted from the strata joins continuously.  The lead isotopic composition of the Bawang deposit is very stable and the distribution range is small, indicating that the metallogenic material may come from a single source or be dominated by a certain source. It is generally believed that the eigenvalues of the lead isotope source region, especially the µ value, can provide information on the geological process and reflect the source of lead. Lead with high µ values (greater than 9.58) is generally considered to be derived from upper crustal material, with enriched U and Th [60]. The µ values of the Bawang deposit are 9.60-9.67, with an average of 9.66 (greater than 9.58), indicating that the material came from the upper crust.
In the 207 Pb/ 204 Pb-206 Pb/ 204 Pb and 208 Pb/ 204 Pb-206 Pb/ 204 Pb tectonic environment evolution diagrams, all samples fall near the evolution line of the upper crust (Figure 10a) or between the orogenic belt and the lower crust (Figure 10b). In the lead isotope ∆γ-∆β genetic classification diagram (Figure 11), the samples are located in the upper crust and in the subduction zone (magmatism) mixed with upper crust and mantle. This indicates that the lead source of the Bawang deposit is mainly related to magmatism and comes from the crust-mantle mixed-source area, which is consistent with the Lamo deposit and Tongkeng deposit in Dachang ore field. In addition, it is very close to that of the Longxianggai granite in the Dachang ore field [80]. Due to the large molecular mass of lead and the small relative mass difference between different lead isotope molecules, the isotopic composition of ore-forming elements usually does not change during leaching, transportation, and precipitation. Therefore, the characteristics and relations of the lead isotopic composition can be used to obtain information on the mineral source of the deposit.
The lead isotopic composition of the Bawang deposit is very stable and the distribution range is small, indicating that the metallogenic material may come from a single source or be dominated by a certain source. It is generally believed that the eigenvalues of the lead isotope source region, especially the μ value, can provide information on the geological process and reflect the source of lead. Lead with high μ values (greater than 9.58) is generally considered to be derived from upper crustal material, with enriched U and Th [60]. The μ values of the Bawang deposit are 9.60-9.67, with an average of 9.66 (greater than 9.58), indicating that the material came from the upper crust.
In the 207 Pb/ 204 Pb-206 Pb/ 204 Pb and 208 Pb/ 204 Pb-206 Pb/ 204 Pb tectonic environment evolution diagrams, all samples fall near the evolution line of the upper crust (Figure 10a) or between the orogenic belt and the lower crust (Figure 10b). In the lead isotope Δγ-Δβ genetic classification diagram (Figure 11), the samples are located in the upper crust and in the subduction zone (magmatism) mixed with upper crust and mantle. This indicates that the lead source of the Bawang deposit is mainly related to magmatism and comes from the crust-mantle mixed-source area, which is consistent with the Lamo deposit and Tongkeng deposit in Dachang ore field. In addition, it is very close to that of the Longxianggai granite in the Dachang ore field [80].    [81], the data of other deposit come from [17,21,24,30,53,72,[82][83][84][85]). The Pb isotopic composition is consistent with that of the granite of the same age, indicating that ore-forming metals were mainly derived from magmatic hydrothermal solution. (3) From the comprehensive ore deposit geological characteristics and the isotope analysis, the Bawang deposit is a hydrothermal vein-type deposit, located in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal fluid and carbonate rock. It is expected that large skarn-type ore bodies are present in the deep zone.

Conflicts of Interest:
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Conclusions
(1) The 207 Pb/ 206 Pb-238 U/ 206 Pb concordant ages of samples BW4 and BW5 from the I ore body of the Bawang tin-zinc polymetallic deposit are 93.1 ± 4.8 and 85.3 ± 6.3 Ma, respectively, while the 206 Pb/ 207 Pb-238 U/ 207 Pb isochron ages are 89.3 ± 6.3 and 82.9 ± 5.3 Ma, respectively. The cassiterite isochron ages of the two samples are consistent with concordant ages within the error range, indicating that the mineralization occurred in the early Late Cretaceous. (2) The C, O, and S isotopic compositions indicate that the ore-forming fluid of the Bawang deposit originated from the mixing of magmatic hydrothermal fluid and stratum. The Pb isotopic composition is consistent with that of the granite of the same age, indicating that ore-forming metals were mainly derived from magmatic hydrothermal solution. (3) From the comprehensive ore deposit geological characteristics and the isotope analysis, the Bawang deposit is a hydrothermal vein-type deposit, located in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal fluid and carbonate rock. It is expected that large skarn-type ore bodies are present in the deep zone.

Conflicts of Interest:
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.