Trace Elements and Pb-O Isotopes of Scheelite: Metallogenic Implications for the Shimensi W-Polymetallic Deposit in South China

: The world-class Shimensi tungsten (W)-polymetallic deposit is located in Jiangnan Orogen, with an estimated reserve of 742.5 kt WO 3 @ 0.195% W, 403.6 kt Cu and 28 kt Mo. In this paper, the trace elements and Pb-O isotopes of scheelite (the main ore mineral) are presented to study the ore-forming material source and ore-forming ﬂuid evolution. The results show that the REE distribution in scheelite is mainly controlled by the substitution mechanism of 3Ca 2+ = 2REE 3+ + (cid:3) Ca (where (cid:3) Ca is the Ca-site vacancy). Oxygen isotope data indicate that the scheelite mineralization occurred under high-temperature oxygen isotope equilibrium conditions, and that the ore-forming ﬂuid has a magmatic–hydrothermal origin. The variation in scheelite Eu anomalies and the wide range of scheelite Y/Ho ratio indicate that the ore-forming ﬂuid evolves from reducing to oxidizing, and the early-stage and late-stage ore-forming ﬂuid may have been relatively rich in F − and HCO 3 − , respectively. The signiﬁcant Mo decrease in scheelite from the early to late stage that are opposite to the inﬂuence of f O 2 variation may have resulted from the crystallization of molybdenite and Mo-rich scheelite. Lead isotopes of the ore minerals of scheelite, wolframite, molybdenite and chalcopyrite can be divided into three groups, similar to these of feldspars in different granites. Both the Mesozoic porphyritic and ﬁne-grained biotite granites have Pb isotope ratios similar to the ores, which suggests that the former two are the main ore material source.


Introduction
Several giant granite intrusion-related tungsten deposits, including the Dahutang and Zhuxi [1][2][3][4], have been discovered in northern Jiangxi Province (South China). The Shimensi W deposit is an important part of the Dahutang ore field. Previous studies have focused on the deposit geology, ore-forming age, ore-forming fluid and geochemical features of ore-related granites [1,[5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Moreover, Sun and Chen [19] and Chen et al. [20] reported the scheelite elemental data of veinlet-disseminated and quartz-vein mineralization types, respectively. These works play an important role in aiding our understanding of the Shimensi ore-forming genesis. The evolution process of different ore stages and the origin of ore-forming metals remain a controversial topic in the literature. In this study, the trace elements of scheelite and the Pb and O isotope signatures of the minerals in the Shimensi deposit were analyzed, attempting to provide a better understanding of the metallogenic material sources and ore-forming nature of the Shimensi deposit. Map presenting the distribution of granites and volcanic rocks in south China, and also the location of the Jiangnan Orogen and the Shimensi ore deposit (modified after [21]).
Neoproterozoic and Mesozoic granitic rocks are well-developed in the area. The Neoproterozoic biotite granodiorite, distributed approximately E-W-trending, is part of the Jiuling granodiorite batholith, the largest granitoid complex in SE China (outcrop size >2500 km 2 ) [32]. The batholite granodiorite was zircon LA-ICP-MS and SHRIMP U-Pb dated to be ca. 820 Ma [11,14,32,33] and intruded the Shuangqiaoshan Group. It is peraluminous and characterized by high CaO/Na 2 O, and low Rb/Sr and Rb/Ba ratios, and has Sr-Nd isotopes similar to those of the Shuangqiaoshan Group [33]. The Mesozoic granitic rocks, predominately consisting of porphyritic biotite granite, fine-grained biotite granite and granite porphyry, intruded in both the Neoproterozoic biotite granodiorite and Shuangqiaoshan Group, and show a close relationship with the W mineralization [7][8][9][10]34]. The overall NE-ENE-trending faults cut the Neoproterozoic strata and biotite granodiorite, and controlled the distribution of both granite emplacement and W-polymetallic mineralization. A total of 15 W-polymetallic deposits are present in the ore field (size: 20 × 10 km), among which five are medium to (super-)large in size, i.e., the Shimensi, Dongdouya, Dalingshang, Shiweidong and Kunshan ( Figure 2).  [16]).

Deposit Geology
Large-scale Neoproterozoic and Mesozoic magmatism occurred in the Shimensi mining area, with minor Quaternary overburden sediments ( Figure 3). The grayish Neoproterozoic coarse-grained biotite granodiorite covers 80% of the deposit area, and contains 30-35% quartz, 27-35% plagioclase, 10-15% potassic feldspar, 5-10% biotite and accessory (<5%) muscovite and cordierite [33]. The Mesozoic granites include porphyritic biotite granite (ca. 148.3-147.4 Ma), fine-grained biotite granite (ca. 146.1-144.7 Ma) and granite porphyry (ca. 143.1-143.0 Ma) [10,34]. The porphyritic biotite granite, distributed in central Shimensi, intruded in the Neoproterozoic granodiorite batholith in the forms of sills and dykes. The fine-grained biotite granite, distributed in southeastern Shimensi as sills, cut both the porphyritic biotite granite and Neoproterozoic biotite granodiorite. The granite porphyry occurs in central Shimensi as narrow dykes and cut other granitoids. All of these three granites feature high-Si, peraluminous, high-K and calc-alkaline granites, and belong to the S-type granite [22]. The W mineralization mainly occurs in Neoproterozoic biotite granodiorite and Mesozoic porphyritic biotite granite. Considering that the former has a higher CaO content than the latter and there are no carbonate sequences present in the area, the Neoproterozoic biotite granodiorite is suggested to provide Ca to the scheelite (CaWO 4 ) orebodies [22,33]. Scheelite mineralization can also be observed in the fine-grained granite, whereas chalcopyrite mineralization occurs in the granite porphyry [33]. There is also a 0.5-1.5 m thick pegmatite zone with K-feldspar megacrysts that has developed along the contact zone between the Neoproterozoic biotite granodiorite and Mesozoic porphyritic biotite granite. Local structures include ductile shear zones, faults and joints, and trend NNE to ENE and NW, which has been interpreted to have formed by Neoproterozoic and Mesozoic tectonic movements [16]. Feng et al. [1] reported a molybdenite Re-Os age of 143.7 ± 1.2 Ma for the Shimensi mineralization, which is consistent with the scheelite Sm-Nd age (142.4 ± 8.9 Ma) reported by Huang [35], suggesting that the Shimensi W mineralization occurred in the Early Cretaceous and is likely associated with the Mesozoic granites, especially porphyritic biotite and fine-grained granites. Three types of mineralization can be identified at Shimensi, i.e., veinlet-disseminated, hydrothermal crypto-explosive breccia and quartz vein, which commonly coexist spatially and overlap one another (Figures 4 and 5). The veinlet-disseminated orebodies, accounting for over 90% of the W reserve, are distributed in the Neoproterozoic biotite granodiorite and Mesozoic granitic rocks (Figure 5a,b), especially in their exo-contact zone. The ore minerals are mainly scheelite with minor wolframite and chalcopyrite, which intergrow with some sulfides, including molybdenite, pyrrhotite, arsenopyrite, bornite, sphalerite, pyrite, enargite and tennantite. Major non-metallic minerals include quartz, biotite, muscovite, K-feldspar, dolomite and fluorite [22]. Alteration is closely related to wall rock lithology: K-feldspar is present in the contact zone close to the Mesozoic porphyritic biotite granite, whereas biotite is present close to Neoproterozoic biotite granodiorite. Greisenization and chlorite were developed in both the endo-and exo-contacts.  Hydrothermal crypto-explosive breccia orebodies in central Shimensi lie mainly over the Mesozoic porphyritic biotite granite, and partially extend into the Neoproterozoic biotite granodiorite. This type accounts for approximately 5% of the total W reserve. The breccia clasts were derived mainly from the Neoproterozoic granodiorite and less from the Mesozoic porphyritic biotite granite. The felsic matrix comprises mainly quartz, together with scheelite, molybdenite, chalcopyrite, bornite and minor wolframite (Figure 5c). The alteration mainly consists of K-feldspar and greisenization, with minor fluorite and tourmaline.
The quartz-vein orebodies are distributed throughout central Shimensi and cut the orebodies of other types. These veins are typically parallel to each other, mostly 100-200 m long and 20-40 cm wide. The orebodies have higher WO 3 and Cu grades than the other mineralization styles. Ore minerals mainly include wolframite and scheelite, with minor chalcopyrite (Figure 5d). Non-metallic minerals include quartz, muscovite and calcite. K-feldspar and greisen alterations are the main wall-rock alteration styles, and are accompanied by weak albite, chlorite and carbonate alterations.

Analytical Methods
Electron microprobe (EPMA) analysis of the samples was conducted at the Wuhan Center of Geological Survey, China Geological Survey, using a JEOL JXA8100 electron probe microanalyzer (EPMA; JEOL, Japan). The EPMA was operated at 15 kV accelerating voltage with 10 nA beam current, 5 µm beam size and 10-30 s counting time. All data were corrected using the ZAF procedure. The detection limit is 0.01% for most elements, except for Mo and Ti (0.02%).
Scheelite cathodoluminescence (CL) imaging and in situ LA-ICP-MS analysis were conducted at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University. A Gatan Chromal CL cold cathode generator and an FEI Quanta FEG 650 scanning electron microscope (SEM; FEI, Hillsboro, America) were used for the imaging. SEM was operated at~12 keV under a vacuum of~30 Pa. LA-ICP-MS in situ analysis of scheelite trace elements was performed on a 193 nm ArF excimer laser ablation system, connected to an Agilent 7500a ICP-MS (Agilent Technologies, Palo Alto, America) with a transfer tube. Helium was used as the carrier gas, which mixed with argon make-up gas via a T-connector before entering the ICP. The carrier and make-up gas flows were optimized by ablating the NIST SRM 610 standard, in order to obtain the maximum signal intensity while keeping the ThO/Th (0.1-0.3%) ratio low to reduce the oxide and doubly charged ion interference. Each LA-ICP-MS analysis comprised a~20 s background acquisition (gas blank) followed by a 60 s sample data acquisition. Every tenth spot analysis was followed by one NIST SRM 610 standard analysis to correct the time-dependent drift of sensitivity and mass discrimination of the ICP-MS. Reference glasses (GSE, NIST612 and NIST614) were analyzed prior to and following the sample measurements.
The lead and oxygen isotope analyses were performed at the Beijing Research Institute of Uranium Geology. The scheelite O isotope compositions were measured using a MAT-253 mass spectrometer (Finnigan MAT, Bremen, German). The analytical precision for δ 18 O was ±0.5‰. The Pb isotopes of sulfide separates were measured on a GV Isoprobe-T thermal ionization mass spectrometer (TIMS; GV, Manchester, British). The measurement precision for 204 Pb/ 206 Pb was less than 0.05%, with 208 Pb/ 206 Pb generally no more than 0.005% for 1 µg Pb. Detailed analytical procedures are described by Zhu et al. [36] and Yang et al. [37].

Cathodoluminescence Image
The scheelite samples varied from red to yellow to blue (most common) under CL imaging. The scheelite grains with red CL had the highest total REE content (∑REE = avg. 1315.24 ppm, data obtained from LA-ICP-MS), whilst those with blue CL images had the lowest ∑REE (avg. 76.41 ppm, data obtained from LA-ICP-MS). Scheelite samples with yellow CL images presented intermediate ∑REE content. Scheelites with red and yellow CL images presented positive and negative Eu anomalies, respectively, whilst those with blue CL images presented only indistinct Eu anomalies (Figure 5e). However, the Sm/Nd range for an individual sample was narrow (0.14-0.37).

Lead Isotopes
Scheelite: The Pb isotope variation range of most scheelite samples (n = 9) was narrow, with 206
Mode 1 : scheelite has high Na content. Meanwhile, the upward parabolic REE patterns in scheelite imply that MREEs occupy the Ca sites, as the ionic sizes of MREE 3+ (1.06Å) and Ca 2+ (1.12Å) are similar [42]. For Shimensi, the low-salinity ore-forming fluids (0.88-9.47 wt% NaCl eq ; [2]) indicate low Na concentration. The low Na concentration and poor Na vs. REE correlation in all our samples indicate that Mode 1 is unimportant for the REE incorporation.
Mode 2 : the Nb concentration is high. Both HREEs and MREEs are relatively depleted (while LREEs are enriched) as a result of their substitution for Nb 5+ [42,43]. Nb content for the scheelite samples (1.29-219.04 ppm) is much lower than the ∑REE content, and the lack of positive Nb 5+ vs. ∑REE correlation in most samples suggests that Mode 2 is unimportant (Figure 7a).

Sources and Evolution of the Ore-Forming Fluid
The calculated δ 18 O for ore-forming fluid is 7.0-9.6‰ and falls in the magmatic water range [49], which suggests a magmatic-hydrothermal fluid origin. However, the different trace and REE elements between veinlet-disseminated and quartz-vein-hosted scheelite (Figures 6 and 7) may indicate that the ore-forming fluid is not homogenous, which is consistent with the presence of a magmatic hydrothermal fluid as early ore-forming fluid and of a mixing of meteoric water as late ore-forming fluid, as shown by the study of quartz fluid inclusions and H-O isotopes [2].
Europium can substitute for Ca in scheelite both as Eu 3+ and as Eu 2+ . If it enters as Eu 3+ , it behaves similarly to the other REE 3+ elements and would not change the trend of Eu anomaly in the hydrothermal fluids; if it enters as Eu 2+ , ore-fluid-positive Eu anomalies occur when Eu 2+ >> Eu 3+ [19,42]. In Figure 7b, most of the data are presented along a horizontal line, indicating the domination of Eu 2+ in the ore-forming fluid. This is consistent with the mostly positive Eu anomalies of these scheelites ( Figure 6). However, data projections of sample DHT241 show positive Eu N vs. Eu* N correlation, suggesting that Eu is dominated by Eu 3+ . Such a variation characteristically indicates the evolution of ore-forming fluids from reducing to oxidizing from the early to late stage. The significant decrease in scheelite Mo content from the early to late stage (Figure 7c) is in contrast with the influence of f O 2 variation [50][51][52]. Considering that the early-stage scheelite intergrows with molybdenite, an early-stage crystallization of molybdenite and Mo-rich scheelite could be imagined to significantly decrease the ore-fluid Mo content, and thus the Mo content of the late-stage scheelite, even in relatively oxidizing conditions. As REE and Y have similar partition coefficients in fluid and scheelite, the Y/Ho and La/Ho ratios in the minerals crystallized at the same stage should be similar, but show negative correlation in the minerals crystallized at different stages [53]. The different slopes between Y/Ho and La/Ho (Figure 7e) and very wide range of scheelite Y/Ho ratios (Y/Ho = 15.24-114.02; Figure 7f) may also reflect that the scheelite samples were formed at different stages. The difference of complexing F − and HCO 3 − would cause the differentiation between Y and Ho [20,54], although the two elements have the same valence state and similar ionic radius: Y/Ho is >28 when Fis present in the fluid and < 28 when HCO 3 − prevails. Therefore, the Y/Ho difference in the analyzed samples indicates that the early-stage and late-stage ore-forming fluid may have been relatively rich in F − and HCO 3 − , respectively. The F − in the early stage may have been gradually lost because of the fluorite crystallization, and greisenization. Meanwhile, meteoric water infiltration may have also led to the ore-fluid HCO 3 increase.

Provenance of Metals
The Pb isotopic compositions of the major metallic minerals obtained from Shimensi (scheelite, wolframite, chalcopyrite and molybdenite) fall into three groups ( Figure 8): (1) wolframite-chalcopyrite-molybdenite; (2) scheelite-chalcopyrite-molybdenite; and (3) scheelite-chalcopyrite; this can reflect both multiphase mineralization and multiple material sources. Due to the lack or minor presence of radiogenic Pb in the feldspar, the Pb isotope composition in the feldspar samples can approximate that present in the granite [55][56][57]. The Shimensi feldspar samples can also be divided into three Pb isotope compositional groups ( Figure 8): (1) the compositions of the lead isotope in the Neoproterozoic biotite granodiorite, which has a lower radiogenic Pb content similar to the low-grade metamorphosed Shuangqiaoshan Group clastic rocks [58]. This is consistent with Sr-Nd isotopes of Neoproterozoic biotite granodiorite [33], indicating that Neoproterozoic biotite granodiorite is derived from the partial melting of the Shuangqiaoshan Group basement rocks. (2) The Mesozoic porphyritic/fine-grained biotite granites have similar Pb isotope compositions to most of the scheelite samples, which indicates that the main ore stage is closely related to the granites. (3) The granite porphyry has a higher radiogenic Pb content than the porphyritic/fine-grained biotite granites.
As depicted in Figure 8, the Pb isotope compositions of the Shimensi metallic minerals are similar to those of the Early Cretaceous porphyritic/fine-grained biotite granites, suggesting that the latter two may have been the ore-forming material source. However, the lead isotopic compositions of the ore mineral, with higher radioactive lead values, are consistent with the compositions of the granite porphyry, which implies that granite porphyry may also play a role in the deposition of W mineralization.
As a result, we proposed that the veinlet-disseminated, hydrothermal crypto-explosive breccia and quartz-vein type mineralizations of the Shimensi deposit were formed in a uniform ore-forming system, which is related to the Mesozoic granites, especially Early Cretaceous porphyritic/fine-grained biotite granites. The high Ca Neoproterozoic biotite granodiorite is a high-quality surrounding rock, providing Ca to the scheelite. The contact zones between Mesozoic granites and Neoproterozoic biotite granodiorite are an optimal prospecting target for veinlet-disseminated and hydrothermal crypto-explosive breccia mineralization, whereas the fracture structures are good for quartz vein-type mineralization.

Conclusions
(1) The acquired data reveal that 3Ca 2+ = 2REE 3+ + Ca ( Ca = Ca-site vacancy) was the principal substitution mechanism responsible for the REE fixation occurring in the Shimensi scheelite. (2) The Shimensi ore-forming fluid was mainly sourced from magmatic water, which possibly mixed with meteoric water in the late ore stage. The early-stage and late-stage ore-forming fluids may have been relatively rich in F − and HCO 3 − , respectively, and evolved from reducing to oxidizing.
(3) The Pb isotope compositions of the metallic minerals were similar to those of the porphyritic/fine-grained biotite granites. The main-stage ore-forming materials were consequently derived from these Mesozoic biotite granites.

Data Availability Statement:
All data used in this study are freely available.