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Geochemistry and Origin of Scheelites from the Xiaoyao Tungsten Skarn Deposit in the Jiangnan Tungsten Belt, SE China

: The type, association, variations, and valence states of several metal elements of scheelite can trace the source and evolution of the ore-forming ﬂuids. There are four types of scheelite from the Xiaoyao deposit: (1) scheelite intergrown with garnet in the proximal zone (Sch1a) and with pyroxene in the distal zone (Sch1b), (2) scheelite replaced Sch1a (Sch2a) and crystallized as rims around Sch1b (Sch2b), (3) quartz vein scheelite with oscillatory zoning (Sch3), and 4) scheelite (Sch4) within micro-fractures of Sch3. Substitutions involving Mo and Cd are of particular relevance, and both elements are redox-sensitive and oxidized Sch1a, Sch2b, Sch3 are Mo and Cd enriched, relatively reduced Sch1b, Sch2a, Sch4 are depleted Mo and Cd. Sch1a, Sch2a, Sch3, and Sch4 are characterized by a typical right-inclined rare earth element (REE) pattern, inherited from ore-related granodiorite and modiﬁed by the precipitation of skarn minerals. Sch1b and Sch2b are characterized by low light rare earth element / heavy rare earth element (LREE / HREE) ratios, inﬂuenced by a shift in fO 2 during ﬂuid-rock alteration. Sch1b, Sch2b and Sch3 have higher Sr contents than those of Sch1a and Sch2a, reveal that host-rock alteration and ﬂuid–rock interaction have elevated Sr contents. The Y / Ho ratios of scheelite gradually increase from skarn to quartz vein stages, due to ﬂuid fractionation caused by ﬂuid–rock interaction. Thus, the variation in REE and trace elements in scheelite in time and space reﬂects a complex magmatic-hydrothermal process involving various ﬂuid–rock interactions and ﬂuid mixing. Four crosscutting

Tungsten orebodies mainly occur within skarns at contact zones between the Kaobeijian granodiorite pluton and their wall rocks ( Figure 2). The largest tungsten orebody (W1) developed along the contact between granodiorite and the limestone of the Lantian Formation ( Figure 3). It has a length of 1300 m and a width of 5-35 m. It strikes in an NE direction and dips toward the north at an angle of 80 • (Figure 3).
The skarn stage is characterized by a large amount of garnet and pyroxene distributed in different zones (Figure 4a,b). For example, in drill core ZK4704, the skarn has a pyroxene-dominated distal zone and a garnet-dominated proximal zone from shallow to deep. The garnets in proximal zones are always reddish to red and fine-to coarse-grained and euhedral to subhedral with oscillatory zonation (Figure 4a, sample 15XY-31). They commonly intergrow with pyroxenes. The pyroxenes are generally pale green in color. They occur as fine-grained (0.2−1.0 mm) subhedral to anhedral minerals. The distal zones are dominated by dark green, coarse-grained pyroxene crystals without garnet (Figure 4b, sample . The pyroxene grains are anhedral to subhedral, varying from 100 to 500 µm in size. The retrograde alteration stage is characterized by the precipitation of hydrous silicate minerals such as epidote, actinolite, and chlorite; and the associated mineralization is scheelite replacing garnet and pyroxene (Figure 4c). The quartz-scheelite-sulfide stage is characterized by a scheelite-molybdenite-pyrrhotite-chalcopyrite-quartz assemblage in scheeliteand sulfide-rich veins cutting the early scheelite-bearing skarn ores (Figure 4d−f). The scheelite in this stage generally occurs as large anhedral grains that intergrow with quartz. The carbonate stage is characterized by a calcite-sphalerite-galena assemblage in sulfide-rich veins that cut through the tungsten ores and wall rock. The skarn stage is characterized by a large amount of garnet and pyroxene distributed in different zones (Figure 4a,b). For example, in drill core ZK4704, the skarn has a pyroxene-dominated distal zone and a garnet-dominated proximal zone from shallow to deep. The garnets in proximal zones are always reddish to red and fine-to coarse-grained and euhedral to subhedral with oscillatory zonation (Figure 4a, sample 15XY-31). They commonly intergrow with pyroxenes. The pyroxenes are generally pale green in color. They occur as fine-grained (0.2-1.0 mm) subhedral to anhedral minerals. The distal zones are dominated by dark green, coarse-grained pyroxene crystals without garnet (Figure 4b, sample 15XY-14). The pyroxene grains are anhedral to subhedral, varying from 100 to 500 µm in size. The retrograde alteration stage is characterized by the precipitation of hydrous silicate minerals such as epidote, actinolite, and chlorite; and the associated mineralization is scheelite replacing garnet and pyroxene (Figure 4c). The quartz-scheelite-sulfide stage is characterized by a scheelite-molybdenite-pyrrhotite-chalcopyrite-quartz assemblage in scheelite-and sulfide-rich veins cutting the early scheelite-bearing skarn ores (Figure 4d-f). The scheelite in this stage generally occurs as large anhedral grains that intergrow with quartz. The carbonate stage is characterized by a calcite-sphalerite-galena assemblage in sulfide-rich veins that cut through the tungsten ores and wall rock.

Sample Descriptions
The samples for electron microprobe and in-situ LA-ICP-MS trace elements analysis were collected from the drill cores taken from the largest W1 orebody (Figure 3). The mineralogical and petrographic features and sampling depth of the samples are summarized in Table 1. Samples were evaluated petrographically employing optical and CL microscopy to characterize the abundance and physical properties of scheelite, including shape, size, color, and luminescence. Selected grains were analyzed by electron probe microanalysis (EPMA) for major element geochemistry and in situ LA-ICP-MS for trace element geochemistry.
Based on CL textures and mineral paragenesis, four types of scheelite have been recognized. Scheelite 1 (Sch1) occurs as disseminated crystal in the skarn (Figure 5a,e), which is further divided based on different pyroxene, garnet and scheelite assemblages. Scheelite 1a (Sch1a) is intergrown with garnet in the proximal zone ( Figure 5a) and Scheelite 1b (Sch1b) is intergrown with pyroxene in the distal zone ( Figure 5e). Sch1a shows yellowish fluorescent in short-waved UV light. It is mainly fine-grained, euhedral to subhedral, with diameters of 0.1 to 0.2 mm (Figure 5a). Sch1a shows homogeneous textures with dark-CL (weak CL response, Figure 5b), reflecting that no dissolution took place during any stage of growth. Sch1b shows blue fluorescent in short-waved UV light and generally occurs as anhedral coarse grains of 0.5-1.0 mm in size (Figure 5e). The homogeneous textures, as shown by the strong and bright CL of Sch1b, may be precipitation features associated with precipitating under stable conditions (Figure 5f). Scheelite 2a (Sch2a) selectively replaced parts of Sch1a that likely represent relict fluid pathways during the alteration stage (Figure 5c,h). Sch2a occurs as irregular, with a light-CL with a grain size of 0.2−0.5 mm (Figure 5c,h). The Sch2a crystal shows a strong correlation with molybdenite ( Figure 5d) and is often associated with epidote, actinolite, chlorite, and quartz ( Figure 5g). Scheelite 2b (Sch2b) is crystallized as rims around the Sch1b with dark-CL (Figure 5f). Sch2b generally occurs as anhedral coarse grain of 1.0−1.5 mm in size ( Figure 5e). CL images of Sch2a and Sch2b from the mineralized skarn reveal later dissolutionreprecipitation reactions (Figure 5c,f,h).

Sample Descriptions
The samples for electron microprobe and in-situ LA-ICP-MS trace elements analysis were collected from the drill cores taken from the largest W1 orebody ( Figure 3). The mineralogical and petrographic features and sampling depth of the samples are summarized in Table 1. Samples were evaluated petrographically employing optical and CL microscopy to characterize the abundance and physical properties of scheelite, including shape, size, color, and luminescence. Selected grains were analyzed by electron probe microanalysis (EPMA) for major element geochemistry and in situ LA-ICP-MS for trace element geochemistry.
Based on CL textures and mineral paragenesis, four types of scheelite have been recognized. Scheelite 1 (Sch1) occurs as disseminated crystal in the skarn (Figure 5a,e), which is further divided based on different pyroxene, garnet and scheelite assemblages. Scheelite 1a (Sch1a) is intergrown with garnet in the proximal zone ( Figure 5a) and Scheelite 1b (Sch1b) is intergrown with pyroxene in the distal zone ( Figure 5e). Sch1a shows yellowish fluorescent in short-waved UV light. It is mainly fine-grained, euhedral to subhedral, with diameters of 0.1 to 0.2 mm (Figure 5a). Sch1a shows homogeneous textures with dark-CL (weak CL response, Figure 5b), reflecting that no dissolution took place during any stage of growth. Sch1b shows blue fluorescent in short-waved UV light and generally occurs as anhedral coarse grains of 0.5-1.0 mm in size (Figure 5e). The homogeneous textures, as shown by the strong and bright CL of Sch1b, may be precipitation features associated with precipitating under stable conditions (Figure 5f). Scheelite 2a (Sch2a) selectively replaced parts of Sch1a that likely represent relict fluid pathways during the alteration stage (Figure 5c,h). Sch2a occurs as irregular, with a light-CL with a grain size of 0.2-0.5 mm (Figure 5c,h). The Sch2a crystal shows a strong correlation with molybdenite ( Figure 5d) and is often associated with epidote, actinolite, chlorite, and quartz ( Figure 5g). Scheelite 2b (Sch2b) is crystallized as rims around the Sch1b with dark-CL (Figure 5f). Sch2b generally occurs as anhedral coarse grain of 1.0-1.    Figure  5j, k). The rim is oscillatory growth zoning and the core is homogeneous, as shown on CL images (Figure 5j, k). Scheelite 4 (Sch4) occurs as rims around Sch3 (Figure 5j) or as micro-fractures within Sch3 with light-CL ( Figure 5k). The occurrence of micro-fractures in Sch3, which are filled with Sch4, indicates recrystallization during late hydrothermal processes. The formation of Sch4 is accompanied by the precipitation of molybdenite and other sulfides (chalcopyrite, pyrrhotite; Figure 5l)

Cathodoluminescence
Cathodoluminescence is an efficient technique for determining the zonation patterns and growth history of scheelite which are otherwise not unobservable using light microscopy. CL imaging of scheelite was obtained using a Gatan MonoCL 4 system attached to the FEI Nova NanoSEM 450 scanning electron microscope (FEI corporation, Hillsboro, OR, USA) at 10 kV, at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing.

Electron Microprobe Analysis
Chemical compositions of scheelite were determined by electron probe microanalyses using a JEOL JXA-8230 Superprobe (JOEL Ltd., Akashima, Tokyo, Japan) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. The operating conditions were an accelerating voltage of 15 kV, a beam current of 10 nA, and a count time per element of 10 s for major elements. The quantitative analyses were performed with carbon-coated thin sections in an evacuated chamber with less than 4.0 ppm mbar. The following natural minerals and synthetic oxides were used for calibration: K-fedspar (K), jadeite (Na), apatite (P), wollastonite (Ca), hematite (Fe), MnO (Mn), scheelite (W), Cr 2 O 3 (Cr), wulfenite (Mo), and rutite (Ti).

In situ LA-ICP-MS Trace Element Analysis
Trace element concentrations were determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a New Wave UP 213 laser ablation system coupled to a Thermo Element II single collector high-resolution magnetic sector (ICP-MS) at the National Research Center for Geoanalysis, Beijing, China. Helium is advantageous as a carrier gas and was thus applied in this study. Single-spot analyses were performed by laser-ablating spot diameters of 40 µm with a repetition rate of 10 Hz at an energy density of 23-25 J/cm 2 . Each LA-ICP-MS analysis incorporated a~20 s background acquisition (gas blank) followed by a 60 s data acquisition from the sample. After every ten spots, analysis was followed by one NIST SRM 610 analysis to correct the time-dependent drift in sensitivity and mass discrimination of the ICP-MS. Reference glasses (GSE, NIST612, NIST614) were analyzed prior to and after the sample measurements. Calcium44 was used as an internal standard and fixed at 19.55 wt. %, based on the average EMPA composition.

Trace Element Geochemistry.
The representative trace element analyses from LA-ICP-MS on garnet, pyroxene and scheelite are listed in Table 3; and the chondrite-normalized REE patterns are shown in Figure 7.
Garnet is characterized by a slight LREE enrichment relative to HREE with low LREE/HREE ratios of 1.4-3.0 (Figure 7a,b), and the total REE contents are 85.9-91.0 ppm, accompanied by Eu/Eu* values   Sch3 has different REE patterns in a single grain and varying total REE contents of 162.9-890.1 ppm (Figure 7e-h). The change from core to rim in a single Sch3 grain shows a systematic change from negative to positive Eu-anomaly (Eu/Eu* = 0.51-4.89) and progressive increase in the LREE/HREE ratios of 2.5-96.2 (Figure 7e-f). Compared with Sch2a, Sch3 have higher Sr contents of 92.0-185.8 ppm and Y contents of 3.0-142.6 ppm.
Sch4 displays the same REE patterns as the core of Sch3 with low LREE/HREE ratios of 8.4-11.9, lower total REE contents of 188.6-249.1 ppm and Eu/Eu* ratios of 0.40-0.73, compared with those of Sch3 (Figure 7g,h). Sch4 has lower Sr contents, 87.6-89.2 ppm, and Y contents, 7.9-16.4 ppm, than those of Sch3. Sch3 has different REE patterns in a single grain and varying total REE contents of 162.9-890.1 ppm (Figure 7e−h). The change from core to rim in a single Sch3 grain shows a systematic change from negative to positive Eu-anomaly (Eu/Eu* = 0.51−4.89) and progressive increase in the LREE/HREE ratios of 2.5−96.2 (Figure 7e−f). Compared with Sch2a, Sch3 have higher Sr contents of 92.0-185.8 ppm and Y contents of 3.0-142.6 ppm.
Sch4 displays the same REE patterns as the core of Sch3 with low LREE/HREE ratios of 8.4−11.9, lower total REE contents of 188.6−249.1 ppm and Eu/Eu* ratios of 0.40−0.73, compared with those of Sch3 (Figure 7g, h). Sch4 has lower Sr contents, 87.6-89.2 ppm, and Y contents, 7.9−16.4 ppm, than those of Sch3. The chondrite values are from [33]. The granodiorite data are from [28].

Oxidation State of Ore-Forming Fluids
Molybdenum cation Mo 6+ enters scheelite by substituting W 6+ under high f O 2 conditions; under low f O 2 and high f S 2 conditions, Mo 6+ is reduced to Mo 4+ and precipitates as molybdenite. Thus, the molybdenum contents within scheelite crystals can be used to indicate the oxidation conditions of ore-forming fluids [34][35][36]. In the proximal zone, the MoO 3 contents of Sch1a are high with 5.77-10.30%, indicating oxidized conditions ( Figure 8A). The redox state of early ore-forming fluids could be inherited from magmas, and Xiaoyao granodiorite is oxidized. The existence of accessory magnetite in the Xiaoyao granodiorite indicates that the magmas had a high oxygen fugacity. Sch1b in the distal zone has lower MoO 3 contents of 0.38-0.42% (Table 2 and Figure 8B). The carbonaceous wall rocks may lessen the effects of such a relatively oxidized magma [17,18]. However, the lack of association between molybdenite and Sch1b under reduced conditions suggests that the ore solution may be already depleted in molybdenum and/or under low f S 2 conditions [34]. The activity of molybdenum decreases from the proximal zone to the distal zone [16]. In addition, Sch2b and Sch3 have higher MoO 3 contents (8.07-10.17% and 4.09-9.78%, respectively), compared with Sch1b and Sch2a (Table 2 and Figure 8E-G). The late oxidized Mo-rich granodiorite dike overprints the sample location of Sch2b (Figure 3), and Sch2b and Sch3 have the same REE patterns with ore-related granodiorite (Figure 7d,f). The elevated Mo content may be provided by the high oxidation state, as well as an overprinting or remobilization of the oxidized Mo-rich magmatic-hydrothermal system [17]. However, Sch2a and Sch4 have low MoO 3 contents (1.71-3.51% and 0.19-0.76%, respectively), accompanied by MoS 2 precipitation ( Figure 8C,H). These variations may be related to low f O 2 and high f S 2 , caused by fluid-rock interaction. Furthermore, the precipitation of Mo-rich scheelite likely resulted in a Mo-depletion in residual fluids. The varying redox state of ore-forming fluids is influenced by the replenishment of oxidized Mo-rich magmatic-hydrothermal fluids and various interactions with carbonaceous wall rocks.  The presented results indicate a positive correlation between Mo and Cd in the scheelite ( Figure 9a); therefore, the Cd content in scheelite increases under oxidized conditions. In contrast, under reduced conditions, the Cd content of scheelite decreases. This is corroborated by the changing valence state of Cd in an evolving ore forming fluid. Cd has two valence states, i.e. +1 and +2. Cd migrates as Cd 1+ and its ionic radius is 1.14 Å; Cd migrates as Cd 2+ and its ionic radius is 0.97 Å [37]. Cd 2+ has the same valence states as Ca 2+ and its ionic radius is close to 0.99 Å of Ca 2+ and preferentially enters scheelite by substituting Ca 2+ in fluids under oxidized conditions. The Cd average content of scheelite in the proximal skarn is 5.60 ppm, while, in the distal skarn, it is 0.15 ppm (Table 3), indicating lower oxidized conditions in the distal zone. These results are consistent with the lower Mo contents of the distal zone. This suggests that, in addition to the Mo content, the Cd content of scheelite can also be used to estimate the redox conditions of ore-forming fluids.  [9], Xitian [38]; the reduced tungsten skarn deposits, including the Zhuxi [15], Xianglushan [14], Kyanigan [39].

Dissolution and Re-Precipitation
The majority of the analyzed skarn-related scheelite has been extensively modified by dissolution and re-precipitation (DRP) processes. The high-Mo primary Sch1a is replaced to various extents by a secondary variety along fractures and/or grain boundaries ( Figure 8A, B). This high-Mo Sch1a may have originally formed at high temperatures and under relatively oxidizing conditions. Furthermore, it was subsequently modified through the decrease in MoO3 contents and molybdenite precipitation, caused by a partial reduction of Mo 6+ to Mo 4+ in response to decreasing fO2. Secondary Sch2a is generally in sharp contact with the primary Sch1a and intergrown with molybdenite ( Figure  8C). Low-Mo Sch1b has been modified by DRP processes (Figure 8D, E) through increases om MoO3 contents, caused by the substitution of W 6+ and Mo 6+ in response to increasing fO2. The replacement of garnet released large amounts of calcium ion into the ore-forming fluid and thus raised the activity of calcium ion in solution. Consequently, this led to a disequilibrium between precipitated scheelite and evolving fluids [16,17]. In addition, the W contents of garnet from the Xiaoyao deposit are  [9], Xitian [38]; the reduced tungsten skarn deposits, including the Zhuxi [15], Xianglushan [14], Kyanigan [39].

Dissolution and Re-Precipitation
The majority of the analyzed skarn-related scheelite has been extensively modified by dissolution and re-precipitation (DRP) processes. The high-Mo primary Sch1a is replaced to various extents by a secondary variety along fractures and/or grain boundaries ( Figure 8A,B). This high-Mo Sch1a may have originally formed at high temperatures and under relatively oxidizing conditions. Furthermore, it was subsequently modified through the decrease in MoO 3 contents and molybdenite precipitation, caused by a partial reduction of Mo 6+ to Mo 4+ in response to decreasing f O 2 . Secondary Sch2a is generally in sharp contact with the primary Sch1a and intergrown with molybdenite ( Figure 8C). Low-Mo Sch1b has been modified by DRP processes (Figure 8D,E) through increases om MoO 3 contents, caused by the substitution of W 6+ and Mo 6+ in response to increasing f O 2 . The replacement of garnet released large amounts of calcium ion into the ore-forming fluid and thus raised the activity of calcium ion in solution. Consequently, this led to a disequilibrium between precipitated scheelite and evolving fluids [16,17]. In addition, the W contents of garnet from the Xiaoyao deposit are 219-384 ppm (Table 3); therefore, garnet breakdown can lead to disequilibrium. Fluid-rock interaction led to the dissolution of the primary scheelite and the formation of secondary varieties [35]. These have been identified as important components in the formation of Xiaoyao tungsten skarn and similar deposits in the same district. Carbonate rocks contributed to an increase in the Ca contents of ore-forming fluids, enhanced WO 4 2− solubility, and consequently led to disequilibrium. In addition, the re-equilibration of scheelite must have resulted in an increase in the extent of grain boundaries. This is favorable for fluid infiltration and further promotes the dissolution and re-precipitation of scheelite.

Recrystallization
Sch3 displays a homogeneous core and growth zoning rim ( Figure 8F,G). In this study, scheelite grains with distinctive texture comprise a Mo-rich core, and a low-Mo rim ( Figure 8F,G). Both generations have significantly different trace element compositions (Figure 7e-h). Both the oxidation state and the types of REE complexing may also be important during the recrystallization of scheelite and may have caused the observed LREE/HREE fractionation. Sch3 grains were modified by DRP processes and were replaced to various extents by Sch4 along fractures together with MoS 2 and other sulfide precipitations (e.g., chalcopyrite and pyrrhotite) ( Figure 8H). Therefore, scheelite from the Xiaoyao tungsten skarn deposit can be concluded to have undergone complex re-equilibration processes, where the precursor scheelite was partially modified or locally completely destroyed. Identification of such multiple re-equilibration processes reflects the dynamic changes in the fluid systems that are responsible for tungsten skarn formation.

Control of Chemical Variation in Scheelite.
Scheelite contains multiple groups of trace elements, such as REEs, LILEs (Sr and Pb), Mo, As, and HFSEs (Y, Nb, and Ta) ( Table 3). Scheelite can incorporate significant amounts of REEs, Sr, Pb, and Y because of substitution for Ca 2+ , while Mo, As, Nb and Ta enter scheelite via substitution for W 6+ [5]. Subtle variations in the trace element geochemistry of scheelite samples from this study were found between sample locations and scheelite types; the most significant include variations in Mo, Sr, Y, LREEs, and HREEs. Different substitution mechanisms lead to different chondrite-normalized REE patterns of scheelites [5]. Three main mechanisms have been identified as responsible for REE substitution [5]: (1) 2Ca 2+ = REE 3+ + Na + ; (2) Ca 2+ + W 6+ = REE 3+ + Nb 5+ ; and (3) 3 Ca 2+ = 2 REE 3+ + Ca, where Ca is a Ca-site vacancy. Regarding mechanism (1), MREEs preferentially substitute into a Ca site because of their similar ionic radii if Na is available to provide the charge balance in scheelite. Mechanism (2) requires that scheelites are Nb 5+ -enriched and have Nb contents close to the ΣREEs, with a positive correlation of Nb 5+ and Nd 3+ [6]. As shown in Figure 7, none of the scheelites from Xiaoyao display MREE-enriched REE patterns, and contain very low Nb contents of 1.33-38.45 ppm, which are much lower than their REE contents of 21-1032 ppm (Table 3). This indicates that mechanism (3) is more applicable than either (1) or (2). Because of the vacancy in mechanism (3), the restrictions of ionic radii were slightly modified, and the scheelite may inherit its REE composition from ore-forming fluids [9]. Sch1a, Sch2a, Sch3, and Sch4 in the proximal zone display LREE-enriched and slightly HREE-depleted patterns (Figure 7). These are similar to the pattern of ore-related granodiorite, indicating that the scheelites inherited REE compositions from magmatic-hydrothermal fluids. However, they have higher LREE/HREE ratios compared with the ore-related granodiorite which may be caused by the precipitation of skarn minerals with low LREE/HREE ratios of 1.4-3.0 (Figure 7b). Many researchers reported that the REE patterns of skarn-type scheelites are enriched in LREEs but depleted in HREEs, which is likely caused by the precipitation of HREE-rich skarn minerals [9]. Prominent examples include the Zhuxi and Jitoushan tungsten skarn deposits in the JNB [9] and the Kyanigan tungsten skarn deposit in the Mogok metamorphic belt, Myanmar [39]. However, the Sch1b and Sch2b of the distal zone with low LREE/HREE ratios of 0.3-2.1 were identified by their distinctive REE fractionation patterns (Figure 7d). Such low LREE/HREE ratios can be interpreted by several explanations. Firstly, mineral/mineral REE partitioning may have occurred. However, because pyroxene intergrowth with scheelite has much lower REE contents of 0.6-13.1 ppm (Table 3), this is insufficient to change the LREE/HREE ratios, and thus REE partitioning cannot be explained via this process. Secondly, if the HREE-enriched patterns were assumed to be inherited from the ore-forming fluids, the ore-forming fluids should be HREE-enriched; in contrast to the LREE-enriched patterns of ore-related granodiorite, the inherited process cannot be explained. Thirdly, a minor distortion in Ca 2+ will result in a shift from LREE enrichment to HREE enrichment, depending on the reduction in the relative size of the crystallographic site from light to heavy REE (i.e., from 1.12 A • to 1.00 A • ) [40]. The extensive replacement of Ca 2+ for Sr 2+ may be linked to site distortions because of differences in their ionic radii (Ca 2+ , 1.12 A • ; Sr 2+ , 1.26 A • ) [40]. Sch1b and Sch2b in the distal zone are characterized by low LREE/HREE ratios and relatively low Sr contents of 76.7-159.6 ppm. Both display a positive correlation between LREE/HREE ratios and Sr contents (Figure 8b), which may be slightly influenced by Ca 2+ site distortions.
Moreover, changes from LREE-to HREE-enrichment correlate with variation in MoO 3 contents in the distal zone ( Figure 9c). This is corroborated by high MoO 3 contents of 8.06-10.17% for Sch2b and low MoO 3 contents of 0.38-0.42% for Sch1b in the distal zone (Table 2 and Figure 6a), suggesting that scheelites evolved from LREE-enrichment to HREE-enrichment because of a shift in f O 2 . This is consistent with the change from HREE-enrichment to LREE-enrichment because of a shift in f O 2 [41].  (Figure 8b), which may be slightly influenced by Ca 2+ site distortions. Moreover, changes from LREE-to HREE-enrichment correlate with variation in MoO3 contents in the distal zone (Figure 9c). This is corroborated by high MoO3 contents of 8.06%−10.17% for Sch2b and low MoO3 contents of 0.38%−0.42% for Sch1b in the distal zone (Table 2 and Figure 6a), suggesting that scheelites evolved from LREE-enrichment to HREE-enrichment because of a shift in fO2. This is consistent with the change from HREE-enrichment to LREE-enrichment because of a shift in fO2 [41].   Figure 11e). Furthermore, the Sr contents of Sch2a, Sch2b, and Sch3 are higher compared with those of Sch1a and Sch1b (Figure 11e). This may be attributed to fluid-rock interactions, which, in general, elevate Sr contents due to Ca-bearing minerals such as carbonates (i.e., as a substitution for Ca 2+ ) during wall-rock alteration by the tungsten-bearing fluid [42,43]. Many  Figure 11e). Furthermore, the Sr contents of Sch2a, Sch2b, and Sch3 are higher compared with those of Sch1a and Sch1b (Figure 11e). This may be attributed to fluid-rock interactions, which, in general, elevate Sr contents due to Ca-bearing minerals such as carbonates (i.e., as a substitution for Ca 2+ ) during wall-rock alteration by the tungsten-bearing fluid [42,43]. Many studies of tungsten deposits reported fluid-rock interactions as important processes during mineralization, especially in skarn deposits [44]. Studies of the Sr content and isotope on scheelite from the Felbertal tungsten deposit, Austria [45], and the Dahutang tungsten deposit [33] indicated that fluid-rock interaction and alterations in host rocks by granite-derived hydrothermal fluids can cause the observed Sr enrichment. The lower Sr concentrations of Sch4 in the quartz vein (Figure 11e) may be due to dilution via mixing of magmatic fluids with externally-derived (meteoric) fluids [40], which is consistent with the lower REE contents. The large variation in Sr contents in scheelite reflect the changing Sr composition of the mineralizing fluids, which is, in turn, influenced by fluid-rock interaction and fluid mixing.
Yttrium concentrations in scheelite also vary substantially (0.5-496.8 ppm; Figure 11a-d). Yttrium is positively correlated with REEs, LREEs, and especially HREEs (Figure 11b-d), which is likely because Y has a similar charge to REE 3+ and a similar ionic radius in eightfold coordination (1.015 Å), near Ho (1.02 Å) [5]. Similar to HREE, Y also displays negative correlations with Mo ( Figure 11a) and is related to the redox state. However, both Sch1a and Sch2a deviate from this correlation. Garnet intergrowth with Sch1a and Sch2a has significant Y contents (i.e., a Y content of 24.45 ppm; Table 3), suggesting that its precipitation may influence Y, which is depleted in Sch1a and Sch2a. Variations in Y concentrations in the Xiaoyao scheelite most likely relate to the redox state or the precipitation of Y-rich hydrothermal minerals. Yttrium concentrations in scheelite also vary substantially (0.5−496.8 ppm; Figure 11a−d). Yttrium is positively correlated with REEs, LREEs, and especially HREEs (Figure 11b−d), which is likely because Y has a similar charge to REE 3+ and a similar ionic radius in eightfold coordination (1.015 Å), near Ho (1.02 Å) [5]. Similar to HREE, Y also displays negative correlations with Mo ( Figure  11a) and is related to the redox state. However, both Sch1a and Sch2a deviate from this correlation. Garnet intergrowth with Sch1a and Sch2a has significant Y contents (i.e., a Y content of 24.45 ppm; Table 3), suggesting that its precipitation may influence Y, which is depleted in Sch1a and Sch2a. Variations in Y concentrations in the Xiaoyao scheelite most likely relate to the redox state or the precipitation of Y-rich hydrothermal minerals. Because of their similar charge and ionic radii, Y-Ho, Zr-Hf, and Nb-Ta pairs display coherent behavior and hold their respective chondritic ratios in mafic to intermediate rocks [46]. In the present study, Y and Ho are positively correlated with Y/Ho of 13.78−43.82, whereas Zr-Hf and Nb-Ta relationships displayed widely scattered patterns with Zr/Hf of 4.63-20.83 and Nb/Ta < 0.05 ( Table  3). The positive correlation between Y and Ho suggests their similar chemical behavior in hydrothermal systems [46]. However, an explanation for the deviation of these ratios from the Because of their similar charge and ionic radii, Y-Ho, Zr-Hf, and Nb-Ta pairs display coherent behavior and hold their respective chondritic ratios in mafic to intermediate rocks [46]. In the present study, Y and Ho are positively correlated with Y/Ho of 13.78-43.82, whereas Zr-Hf and Nb-Ta relationships displayed widely scattered patterns with Zr/Hf of 4.63-20.83 and Nb/Ta < 0.05 (Table 3). The positive correlation between Y and Ho suggests their similar chemical behavior in hydrothermal systems [46]. However, an explanation for the deviation of these ratios from the chondritic values is still under debate. These elements are mainly concentrated in minor/accessory minerals, the crystallization of which can modify the ratio [47]. Trace element fractionation by scheelite in the hydrothermal system leads to the variable non-chondritic Y/Ho and Zr/Hf ratios of scheelite [39]. The Y/Ho ratios of scheelite from Xiaoyao gradually increase (17.86-26.54, 13.70-34.50, and 22.32-43.82) from the skarn stage to the quartz vein stage. This may be due to fractionation caused by fluid-rock interaction and metasomatism [48] or the breakdown of skarn minerals with high Y/Ho (28.94-39.65, Table 3). The Zr/Hf ratios of scheelite increase (4.63-6.92 and 6.23-20.83) from the skarn stage to the retrograde alteration stage, which may be due to fractionation or the breakdown of skarn minerals with high Zr/Hf (average of 19.38, Table 3). The Zr/Hf ratios of scheelite decrease (5.37-10.33) in the quartz-vein stage, which may be due to the precipitation of hydrothermal zircon. This suggestion is supported by the mineral assemblage of hydrothermal zircon, which is intergrown with scheelite in the quartz-vein stage (Figure 5h). Compared with most geological materials (e.g., Nb/Ta of chondrites~17.5; average continental crust~11) [49], scheelites have a very low Nb/Ta ratio (<0.05), reflecting relatively high abundances of Ta. Fluid fractionation leads to an enrichment of Ta relative to Nb [50]. The data in this paper support this model and show that hydrothermal fluids efficiently fractionate the Nb/Ta ratio.
Based on the above textural and geochemical characteristics, the REE and trace element characteristics of scheelites are influenced by substitution mechanisms, fluid-rock interaction, redox state, fluid mixing, fluid fractionation, and mineral/mineral REE partitioning. The texture may reflect multiple fluid pluses with fluctuating chemical properties and depositional conditions and may indicate modification of the pre-existing scheelite by late-stage fluids in the ore-system.

Implication for Ore-Forming Processes
Granodiorite has a zircon U-Pb isotope age of 149.4 ± 1.1 Ma, which is consistent with the molybdenite Re-Os age (148.7 ± 2.5 Ma) at the Xiaoyao deposit, suggesting that it is genetically related to skarn formation and mineralization [28]. Paragenetic studies on textural and physicochemical data show that the Xiaoyao skarn may be classified as an oxidized tungsten deposit that can form in an evolving magmatic-hydrothermal system. Alterations and mineralizations at the Xiaoyao have been associated with calc-alkaline magmatic activity.
The tungsten-rich magmatic fluids migrated and interacted with wall rock, thus forming garnet skarns in the proximal zone and pyroxene skarns in the distal zone ( Figure 3). Andradite is characterized by low LREE/HREE ratios and a negative Eu anomaly. The tungsten was incorporated into garnet, reaching high contents of 219−384 ppm ( Table 3). The precipitation of garnet with low LREE/HREE ratios likely resulted in an enrichment of LREE and a depletion of HREE in the residual fluids. Sch1a in the proximal zone inherited magmatic-hydrothermal fluids with LREE enrichment, HREE depletion, and negative Eu anomaly (Table 3 and Figure 7b). The tungsten-rich magmatic fluids interacted with the carbonaceous wall rock, which may have led to lower f O 2 in the distal Sch1b. The low f O 2 can lead to HREE and Y enrichment in scheelite and an alteration in carbonate wall rocks by granite-derived hydrothermal fluids, which release high Sr contents. Sch1b in the distal zone is characterized by an HREE-enriched pattern with lower MoO 3 contents, as well as higher Sr and Y contents (Table 3 and Figure 7d).
Following intensive fluid-rock interaction, the ore-forming fluids showed lower f O 2 . Replacement of garnet released large amounts of calcium ions into the ore-forming fluid, thus raising the activity of calcium ion in solution, and leading to the dissolution and re-precipitation of Sch1a and Sch1b, as well as the formation of Sch2a and Sch2b [16,17]. The lower f O 2 resulted in a decrease in MoO 3 contents in Sch2a. The HREE-depleted patterns of Sch1a and Sch2a were influenced by the precipitation of garnet with low LREE/HREE ratios (Figure 7b). The fluid-rock interaction and alteration in the host rocks by granite-derived hydrothermal fluids can cause Sr enrichment in Sch2a and Sch2b. Sch2a is characterized by HREE-depletion patterns with lower MoO 3 , and higher Sr contents (Table 3 and Figure 7b). As a result of the overlaying of the late oxidized granodiorite dike to the sample of Sch2b (Figure 3), the ore-forming fluids became higher in f O 2 . Sch2b displayed a similar pattern, with granodiorite with higher MoO 3 and higher Sr contents (Table 3 and Figure 7d).
The subsequently appearing magmatic-hydrothermal fluids formed quartz-scheelite veins, which typically crosscut early tungsten skarn ores (Figure 5a). The ore-forming fluids increased in f O 2 and interacted with the wall rock, thus causing the hydrothermal growth of Sch3. Sch3 exhibits a homogeneous core and growth zoning rim (Figure 5f−h). The homogeneous core of Sch3 displays a similar pattern to that of ore-related granodiorite, which further demonstrates that scheelite is derived from magmatic-hydrothermal fluids. The growth zoning rim displays a strong HREE-depletion pattern, similar to Sch1a and Sch2a, which may be caused by the precipitation of HREE-rich hydrothermal minerals. Continuing fluid-rock interaction can cause Sr enrichment. Sch3 is characterized by LREE-enrichment patterns with higher MoO 3 contents, higher REE, Sr, and Y contents compared with Sch2a (Table 3 and Figure 7f). Mixing with meteoric water resulted in the recrystallization of Sch3 and formation of Sch4. Meteoric waters normally show very low contents of REE and trace elements, which could cause a decrease in REE and trace element contents of ore-forming fluid as a result of dilution. Sch4 has a similar pattern to Sch3, but has lower REE, Mo, Sr, and Y contents (Table 3 and Figure 7h). In addition, the hydrogen-oxygen isotope data from both the Jitoushan and Dongyuan deposits (which are closely situated to the Xiaoyao deposit) further demonstrate that the ore-forming fluids are mainly magmatic with the addition of meteoric water during the late stage [51,52].
In summary, textural and trace element signatures indicate a magmatic-hydrothermal formation model and the mechanism of tungsten ore enrichment. Fluid-rock interaction and fluid mixing may be important enrichment mechanisms that generated the Xiaoyao tungsten deposit.

Conclusions
(1) Scheelite can record the following complex growth histories based on CL images: homogeneous Sch1a and Sch1b, dissolution and re-precipitation Sch2a and Sch2b, oscillatory zoning Sch3, and Sch4 in micro-fractures. These four types of scheelite have different MoO 3 and Cd contents, indicating fluctuating changes in the oxygen fugacity from the skarn stage to the quartz vein stage; (2) Variations in REE patterns in scheelite are influenced by the REE patterns of ore-related granodiorite, the precipitation of hydrothermal minerals with low LREE/HREE ratios, and changing oxygen fugacity during fluid-rock interaction; (3) The increasing Sr contents and Y/Ho ratios of scheelite from the skarn stage to the quartz vein stage indicate continuing fluid-rock interaction. This fluid-rock interaction may be an important enrichment mechanism underlying the generation the Xiaoyao tungsten deposit.