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Volume 41 Issue 4
Aug.  2020
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Runsheng Chen, Lüyun Zhu, Shao-Yong Jiang, Ying Ma, Qinghai Hu. Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction. Journal of Earth Science, 2020, 31(4): 635-652. doi: 10.1007/s12583-020-1283-0
Citation: Runsheng Chen, Lüyun Zhu, Shao-Yong Jiang, Ying Ma, Qinghai Hu. Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction. Journal of Earth Science, 2020, 31(4): 635-652. doi: 10.1007/s12583-020-1283-0

Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction

doi: 10.1007/s12583-020-1283-0
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  • Unlike classic skarn-type scheelite deposits directly acquiring sufficient Ca2+ from surrounding limestones, all of the scheelite orebodies of the Shangfang tungsten (W) deposit occur mainly in amphibolite, and this provides a new perspective on the mineralization mechanism of W deposits. The ability of hydrothermal scheelite (CaWO4) to bind REE3+ in their Ca2+ crystal lattices makes it a useful mineral for tracing fluid-rock interactions in hydrothermal mineralization systems. In this study, the REE compositions of scheelite and some silicate minerals were measured systematically in-situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to assess the extent of fluid-rock interactions for the Late Mesozoic quartz-vein-type Shangfang W deposits. According to the variations in CaO and REE among scheelite and silicate minerals, the amphibole and actinolite in amphibolite may be able to release large amounts of Ca2+ and REE3+ into the ore-forming fluids during chlorite alteration, which is critical for scheelite precipitation. Furthermore, an improved batch crystallization model was adopted for simulating the process of scheelite precipitation and fluid evolution. The results of both the in-situ measurements and model calculations demonstrate that the precipitation of early-stage scheelite with medium rare-earth elements (MREE)-rich and [Eu/Eu*]N < 1. The early-stage scheelite would consume more MREE than LREE and HREE of fluid, which will gradually produce residual fluids with strong MREE-depletion and[Eu/Eu*]N>1. Even though the partition coefficient of REE is constant, the later-stage scheelite will also inherit a certain degree of MREE-depletion and [Eu/Eu*]N future from the residual fluids. As a common mineral, sheelite forms in various types of hydrothermal ore deposits (e.g., tungsten and gold deposits). Hence, the improved batch crystallization model is also possible for obtaining detailed information regarding fluid evolution for other types of hydrothermal deposits. The results from model calculations also illustrate that the Eu anomalies of scheelite are not an effective index correlated to oxygen fugacity of fluids but rather are dominantly controlled by the continuous precipitation of scheelite.
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    Zhou, X. M., Li, W. X., 2000. Origin of Late Mesozoic Igneous Rocks in Southeastern China:Implications for Lithosphere Subduction and Underplating of Mafic Magmas. Tectonophysics, 326(3/4):269-287. https://doi.org/10.1016/s0040-1951(00)00120-7 doi:  10.1016/s0040-1951(00)00120-7
    Zhou, X. M., Sun, T., Shen, W. Z., et al., 2006. Petrogenesis of Mesozoic Granitoids and Volcanic Rocks in South China:A Response to Tectonic Evolution. Episodes, 29(1):26-33. https://doi.org/10.18814/epiiugs/2006/v29i1/004 doi:  10.18814/epiiugs/2006/v29i1/004
    Zhu, L. Y., Liu, Y. S., Hu, Z. C., et al., 2013. Simultaneous Determination of Major and Trace Elements in Fused Volcanic Rock Powders Using a Hermetic Vessel Heater and LA-ICP-MS. Geostandards and Geoanalytical Research, 37(2):207-229. https://doi.org/10.1111/j.1751-908x.2012.00181.x doi:  10.1111/j.1751-908x.2012.00181.x
    Zhu, L. Y., Jiang, S. Y., Chen, R. S., et al., 2019. Origin of the Shangfang Tungsten Deposit in the Fujian Province of Southeast China:Evidence from Scheelite Sm-Nd Geochronology, H-O Isotopes and Fluid Inclusions Studies. Minerals, 9(11):713. https://doi.org/10.3390/min9110713 doi:  10.3390/min9110713
    Zhu, L. Y., Zhang, G. L., Liu, Y. S., et al., 2019. Improved in-situ Determination of Sr Isotope Ratio in Silicate Samples Using LA-MC-ICP-MS and Its Wider Application for Fused Rock Powder. Journal of Earth Science, 31(4):262-270. https://doi.org/10.1007/s12583-019-1214-0 doi:  10.1007/s12583-019-1214-0
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Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction

doi: 10.1007/s12583-020-1283-0

Abstract: Unlike classic skarn-type scheelite deposits directly acquiring sufficient Ca2+ from surrounding limestones, all of the scheelite orebodies of the Shangfang tungsten (W) deposit occur mainly in amphibolite, and this provides a new perspective on the mineralization mechanism of W deposits. The ability of hydrothermal scheelite (CaWO4) to bind REE3+ in their Ca2+ crystal lattices makes it a useful mineral for tracing fluid-rock interactions in hydrothermal mineralization systems. In this study, the REE compositions of scheelite and some silicate minerals were measured systematically in-situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to assess the extent of fluid-rock interactions for the Late Mesozoic quartz-vein-type Shangfang W deposits. According to the variations in CaO and REE among scheelite and silicate minerals, the amphibole and actinolite in amphibolite may be able to release large amounts of Ca2+ and REE3+ into the ore-forming fluids during chlorite alteration, which is critical for scheelite precipitation. Furthermore, an improved batch crystallization model was adopted for simulating the process of scheelite precipitation and fluid evolution. The results of both the in-situ measurements and model calculations demonstrate that the precipitation of early-stage scheelite with medium rare-earth elements (MREE)-rich and [Eu/Eu*]N < 1. The early-stage scheelite would consume more MREE than LREE and HREE of fluid, which will gradually produce residual fluids with strong MREE-depletion and[Eu/Eu*]N>1. Even though the partition coefficient of REE is constant, the later-stage scheelite will also inherit a certain degree of MREE-depletion and [Eu/Eu*]N future from the residual fluids. As a common mineral, sheelite forms in various types of hydrothermal ore deposits (e.g., tungsten and gold deposits). Hence, the improved batch crystallization model is also possible for obtaining detailed information regarding fluid evolution for other types of hydrothermal deposits. The results from model calculations also illustrate that the Eu anomalies of scheelite are not an effective index correlated to oxygen fugacity of fluids but rather are dominantly controlled by the continuous precipitation of scheelite.

Runsheng Chen, Lüyun Zhu, Shao-Yong Jiang, Ying Ma, Qinghai Hu. Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction. Journal of Earth Science, 2020, 31(4): 635-652. doi: 10.1007/s12583-020-1283-0
Citation: Runsheng Chen, Lüyun Zhu, Shao-Yong Jiang, Ying Ma, Qinghai Hu. Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction. Journal of Earth Science, 2020, 31(4): 635-652. doi: 10.1007/s12583-020-1283-0
  • Tungsten is a rare metal but is critical for some military and industrial products, such as filaments, electrodes, projectiles, catalysts, and radiation shielding. According to the statistics of the U.S. Geological Survey, China is the world's leading producer of tungsten accounting for ~82% of world production, most of which is found in South China (Schulz et al., 2018). At present, it is widely accepted that skarnization is the most critical mechanism for scheelite precipitation (Song et al., 2018; Zhou et al., 2018; Guo S et al., 2016). Commonly, scheelite precipitates at the contact zone between granitic intrusions and carbonate sedimentary rocks, such as limestones and dolostones (Song et al., 2014; Singoyi and Zaw, 2001; Larsen, 1991). For skarn-type scheelite deposits, sedimentary carbonate rocks and granitic intrusions are two crucial factors for scheelite precipitation. However, in some newly discovered world-class scheelite deposits in South China (e.g., Dahutang, Shimenshi), scheelite occurs mostly as veinlets or as disseminations in granite and granite-granodiorite porphyries (Peng et al., 2018; Zhou et al., 2018; Sun and Chen, 2017; Mao et al., 2013), challenging the previous assumption that Ca-rich sedimentary wall rocks are essential for the formation of large scheelite deposits. Therefore, a series of petrological, geochemical, and geochronological studies were carried out on these veinlet-disseminated scheelite deposits in recent years to understand the origin of these scheelites and their mechanisms of formation (Sun et al., 2019; Chen C et al., 2018; Chen S A et al., 2018; Liang et al., 2018; Peng et al., 2018; Song et al., 2018; Mao J W et al., 2017; Wang et al., 2017; Huang and Jiang, 2014; Mao Z H et al., 2013). For instance, Peng et al. (2018) suggested the Ca component of scheelite was most likely released from the breakdown of plagioclase during the process of greisenization and alkali-feldspar metasomatism in the Dahutang W deposit, and this mineralization mechanism was quite different from classic skarn-type scheelite deposits. In other words, hydrothermal alteration during fluid-rock interactions also would be a critical factor for scheelite mineralization. The Shangfang W deposit is a large-scale tungsten deposit (66 500 t WO3 @ 0.23%) located in Southeast China. Unlike classic skarn scheelite deposits and the newly discovered Dahutang scheelite deposits, the scheelite ore bodies of the Shangfang W deposit are mainly hosted in amphibolite, which provides a new perspective for understanding different mineralization mechanisms of scheelite.

    As a common mineral, scheelite also forms in various types of hydrothermal ore deposits, and incorporates abundant REE via substitution for Ca2+ in its crystal lattice (Ghaderi et al., 1999; Raimbault et al., 1993). To maintain electrostatic neutrality in the crystal structure, several important substitution mechanisms have been proposed by Nassau and Loiacono (1963), Burt (1989) and Ghaderi et al. (1999): (1) 2Ca2+= REE3++Na+; (2) Ca2++W6+=REE3++Nb5+; (3) 3Ca2+=2REE3++ Ca, where Ca is a Ca-site vacancy. Whichever mechanism is adapted for substitution, the REE pattern of scheelite correlates with that of the ore-forming fluids (Li et al., 2018; Zhang et al., 2018; Sun and Chen, 2017; Zhao et al., 2017; Guo Z J et al., 2016). Different scheelites or even different parts of one crystal may record just the REE pattern of fluids of different stages, and a need remains for an efficient and high-resolution analytical method to obtain reliable results (Brugger et al., 2000a). When the in-situ trace elemental composition analytical technique for scheelite was available (Sylvester and Ghaderi, 1997), the economic geologist immediately tried to use it for tracing the origin and evolution of the ore-forming fluids (Brugger et al., 2000a; Ghaderi et al., 1999). Obviously, the REE pattern of scheelite (especially for the quartz-vein type) not only met the research requirements for analysing the fluid evolution for W deposits but also was applied to research mineralization conditions for other types of hydrothermal ore deposits (especially for Archean orogenic gold (Au) deposits) (Sciuba et al., 2019; Souza Neto et al., 2008; Brugger et al., 2000a; Cole et al., 2000; Ghaderi et al., 1999). For instance, Brugger et al. (2000a) investigated trace elements of scheelite from the Mt. Charlotte and Drysdale Au deposits, western Australia, and demonstrated that the REE pattern of scheelite is sensitive to the dynamics of hydrothermal deposits.

    In this paper, based on careful investigation of the geology and petrography of the Shangfang W deposit, we present in-situ LA-ICP-MS analytical results of scheelite and have modelled the REE variations between scheelite and fluids. The origin and evolution of ore-forming fluids are the essential cornerstones for understanding the mechanisms of mineralization. According to analytical data and the results of the model calculations, we reveal the evolution of hydrothermal fluids and the mineralization processes of the Shangfang W deposit and suggest a new genetic model for scheelite deposits, which improves our understanding of the mineralization mechanisms of scheelite. Furthermore, an improved model is set up to trace the fluid evolution and the precipitation process of scheelite in quartz veins for some hydrothermal deposits.

  • The South China Block (SCB), located at the eastern margin of the Eurasian Plate, is surrounded by the Qinling-Dabie- Sulu orogenic belt to the north, the Tibetan Block to the west, and the Indochina Block to the south (Fig. 1). It was formed by the amalgamation of the Yangtze Block to the northwest and the Cathaysia Block to the southeast during the Neoproterozoic along the Jiangshan-Shaoxing fault (Charvet, 2013; Zhao et al., 2011). The Yangtze and Cathaysia blocks have distinctive Precambrian basements and different tectonic evolution histories (Yu et al., 2012; Zhao et al., 2011; Li et al., 2009a; Ling et al., 2003). The Cathaysia Block is composed predominantly of Neoproterozoic basement rocks with small amounts of Mesoproterozoic and Paleoproterozoic rocks, which are younger than the basement of the Yangtze Block consisting of Proterozoic rocks with rare Archean outcrops (Yu et al., 2012, 2005a; Wang et al., 2008; Chen et al., 1991).

    Figure 1.  Simplified geological map of South China and the distribution of some granite-related W-Sn deposits (modified after Zhou et al., 2006).

    The SCB contains one of the most extensive areas of Mesozoic magmatic activity in the world, which is characterized by voluminous granitic intrusive-volcanic rocks and related hydrothermal deposits (Wang et al., 2016; Mao et al., 2006; Zhou and Li, 2000; Gilder et al., 1996). These granitoids and volcanic rocks were formed in two periods: the Early Mesozoic Indosinian Period (Triassic) and the Late Mesozoic Yanshanian Period (Jurassic and Cretaceous) (Zhou et al., 2006). The Indosinian granites in the SCB are predominantly composed of S-type granites with minor calc-alkaline Ⅰ-type granites, without coeval volcanic rocks (Wang G G et al., 2015; Li and Li, 2007; Wang Y J et al., 2007; Zhou et al., 2006; Sun et al., 2005; Zhou and Li, 2000). In contrast, the Yanshanian granites are characterized by predominant Ⅰ-type granites with fewer S- and A-type granites and are associated with coeval volcanic rocks. The Early Yanshanian (Jurassic) granites are mainly distributed in the interior of the SCB, with minor coeval volcanic rocks, whereas the Late Yanshanian (Cretaceous) granites are concentrated in the coastal region of the SCB and are associated with voluminous felsic volcanic rocks (Liu et al., 2015; Li et al., 2009b; Hua et al., 2005), and it is proposed that the Mesozoic magmatism in the SCB experienced an oceanward-younger migration trend, especially in the Late Mesozoic (Li et al., 2014; Zhou et al., 2006). It has been a widely accepted view that there were three episodes of Mesozoic large- scale tungsten mineralization in South China from multiple lithospheric extensions in South China, including the spans from 170−150, 140−126, and 110−80 Ma (Hua et al., 2005, Mao et al., 2006). As shown in Fig. 1, there are many large-scale W deposits formed in the interior of South China as two large-scale metallogenic provinces, named the Nanling region and the Jiangnan Orogen (Peng et al., 2018; Zhao et al., 2017; Huang and Jiang, 2014; Mao Z H et al., 2013; Mao J W et al., 2006). In general, the mineralization episodes of tungsten are roughly coincident with those of other metals (e.g., Sn, Fe, Cu, Pb-Zn, Ag and Au), which together show three peaks respectively in the Late Triassic, Mid–Late Jurassic, and the Cretaceous (Ma et al., 2019; Xiong et al., 2017; Hu and Zhou, 2012; Mao et al., 2011).

  • The Shangfang W deposit (26°58′45″N to 27°02′45″N, 118°31′00″E to 118°35′15″E) is located approximately 25 km east of Jian'ou City and covers an area of ~39 km2 (Fig. 2). The strata exposed in the ore district mainly belong to the Paleoproterozoic Dajinshan Formation, composed of biotite leptynite, biotite-plagioclase leptynite, quartz-mica schist, and amphibolite (Figs. 2, 3, 4). The Paleoproterozoic strata have been intruded by several phases of granitic rocks, including biotite syenogranite and granite porphyry dykes (Figs. 2, 3). The biotite syenogranite has a genetic relationship with tungsten mineralization and associated hydrothermal alterations, although only a few granite bodies are in contact with the ore bodies (Zhu et al., 2019; Chen et al., 2013). The biotite syenogranite occurs both at the surface and various underground levels. These rocks (Figs. 4e4f) consist of K-feldspar (30%−40%), plagioclase (20%−30%), quartz (20%−30%), and minor biotite (~5%) with a zircon U-Pb age of 158.8±1.6 Ma (Chen et al., 2013). The intrusions have high SiO2 (71.11 wt.%−75.39 wt.%) content and alkalis (Na2O+K2O=8.19 wt.%−8.92 wt.%), with A/CNK and Mg# values of 1.01 to 1.11, and 23 to 33, respectively and, as such, belong to the high-K calc-alkaline series. They are enriched in light rare earth elements (LREE) and light-ion lithophile elements (LILE) but are depleted in the heavy rare earth elements (HREE) and the high field strength elements (HFSE). The biotite syenogranites have high initial 87Sr/86Sr ratios (0.711 7 to 0.717 1), and low values of εNd(t) (-13.55 to -9.35). Taken together, it is suggested that the Shangfang biotite syenogranites were produced mainly by partial melting of the lower crust with only small contributions from the mantle components (Chen et al., 2013). The Shangfang deposit is characterized by NE- and NW-trending faults, and the NE-trending faults controlled the emplacement of the granite porphyry dykes, due to a large-scale movement of East Asian lithospheric plate (Wang et al., 2018).

    Figure 2.  Geologic sketch map of the Shangfang W deposit.

    Figure 3.  Geologic cross section along prospecting lines No. 0 (a) and No. 2 (b) of the Shangfang W deposit.

    Figure 4.  Photographs and photomicrographs showing petrology features of the relative rock in Shangfang tungsten deposit. (a) Amphibolite; (b) photomicrograph of amphibolite under cross-polarized light; (c) biotite-plagioclase leptynite; (d) photomicrograph of biotite-plagioclase leptynite under cross-polarized light; (e) biotite syenogranite; (f) photomicrograph of biotite syenogranite under cross-polarized light. Amp. Amphibole; Pl. plagioclase; Qz. quartz; Ser. sericite; Spn. sphene; Kfs. K-feldspar.

    The W mineralization at Shangfang, including veins and dissemination ores, is hosted by the altered amphibolites of the Paleoproterozoic Dajinshan Formation. The Shangfang W deposit consists mainly of nine ore bodies and contains an estimated resource of 66 500 t of WO3 at an average grade of 0.23% (Zhu et al., 2019; Chen et al., 2013). Each ore body is either lens- or tubular-shaped (Fig. 3). The mineralization of the Shangfang deposit is aligned along with a NE-SW trend, with a maximum length of 800 m, and a thickness of up to 30 m. The W and Mo mineralization in the Shangfang deposit displays zoning which is characterized by upper scheelite ore bodies and lower molybdenite ore bodies (Fig. 3). Among the 20 exploration lines, only several ones found Mo orebody in a small amount. In general, the amount of W ore is far more than that of Mo ore that only occurs as the molybdenite along some quartz veinlet.

    The ore minerals within various veins consist mainly of scheelite, pyrrhotite, pyrite, chalcopyrite and molybdenite, with minor galena and sphalerite, and these minerals are generally enriched in the quartz-scheelite veins (Figs. 5, 6). The scheelite is generally disseminated in the quartz veins as tetragonal dipyramidal crystals with a grain size range from 0.1 to 30 mm. The optical microscope cathodoluminescence (OM-CL) images of scheelite in the quartz veins show clear growth zoning including a dark-blue core and a brighter-blue rim (Fig. 6c), possibly suggesting that they were formed in equilibrium-controlled growth. Major gangue minerals are quartz, chlorite and actinolite for quartz-vein-type ores (Figs. 5, 6).

    Figure 5.  Photographs and photomicrographs showing features of the ore in Shangfang tungsten deposit. (a) Photographs under ultraviolet light showing scheelite occurring as veinlets in quartz veins or as disseminations in altered amphibolite; (b) quartz-scheelite vein; (c) quartz-scheelite-pyrrhotite vein; (d) quartz-molybdenite-pyrrhotite vein, with molybdenite mainly present in the contact zone between amphibolite and quartz veins; (e) subhedral pyrrhotite intergrowths with molybdenite and scheelite in quartz veins; (f) sulfides from a quartz-polymetallic vein, including pyrrhotite, sphalerite, and chalcopyrite. Sch. Scheelite; Qz. quartz; Po. pyrrhotite; Mol. molybdenite; Sp. sphalerite; Ccp. chalcopyrite.

    Figure 6.  Photographs of ore mineralization and hydrothermal alteration features in the Shangfang W deposit. (a) The quartz veins containing scheelite and hydrothermal alteration on the amphibolite; (b) photographs under ultraviolet light showing scheelite occurring as veinlets in quartz veins or as disseminations in alteration zone; (c) optical microscope cathodoluminescence images of scheelite in the quartz veins showing the equilibrium-controlled growth zoning; (d) chlorite replacing amphibole in amphibolite; (e), (f) chlorite replacing amphibole and intergrowths with scheelite, quartz and pyrrhotite. Po. Pyrrhotite; Sch. Scheelite; Chl. chlorite; Amp. amphibole; Qz. quartz; Act. actinolite.

    The emplacement of the Shangfang biotite syenogranites resulted in thermal metamorphism of the amphibolites of the Paleoproterozoic Dajinshan Formation in a zone several hundred metres wide (Figs. 2, 3). The key components of the alteration assemblages are quartz, chlorite and sericite, with minor epidote. The silicic alteration occurred as a medium- to fine-grained quartz coexisting with sericite in the altered biotite syenogranites or quartz-scheelite-sulfide veins/veinlets in the altered amphibolite (Fig. 6a) and is the most widespread alteration type in this deposit. Sericite commonly replaces plagioclase in biotite syenogranites. As shown in Fig. 6a, the quartz veins cut the amphibolite and produce clear alteration zones containing massive sulphide and scheelite. The disseminated scheelite in the alternation zones could be interpreted as syngenetic hydrothermal mineralization, possibly related to the chemical element exchange between the hydrothermal quartz vein and amphibolite (Fig. 6b). According to the petrological context, there are large numbers of chlorite formed by replacement of amphibole or actinolite in the amphibolites (Figs. 6d6f). Hence, chloritization appears to be closely related to scheelite precipitation. Furthermore, carbonatization postponed the formation of the scheelite and molybdenite and represents the latest stage of hydrothermal activity.

  • Both the major element and rare earth element (REE) compositions of scheelite and silicate minerals were obtained directly from ~300 μm thick sections. An Agilent 7500a ICP- MS instrument in combination with an excimer 193 nm laser ablation system (GeoLas 2005) was used for in-situ analysis at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan.

    All measurements were performed in a time-resolved analysis mode. Helium was used as the carrier gas and was merged with argon (make-up gas) via a T-connector before entering the ICP. Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50 s for data acquisition from ablation of the sample. Between analysis of reference materials and samples, the precipitate aerosols in the ablation cell and inlet lines were cleaned by ablating a washing sample (Zhu et al., 2019). More details regarding the instrumental parameters and operational details can be found in Zhu et al. (2013). The element compositions were calibrated against multiple reference materials (BCR-2G, BIR-1G, and BHVO-2G), and a summed metal oxide normalization was applied (Liu et al., 2008). The preferred values of the element compositions for reference materials were taken from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). The summed results of the metal oxides for plagioclase, actinolite, amphibole, and chlorite were normalized to 100%, 97%, 97%, and 86%, respectively. The data reduction software, ICPMSDataCal, was used for the offline selection and integration of the background and analytical signals, time-drift corrections and quantitative calibrations (Liu et al., 2008).

    The REE compositions of the amphibolite and biotite syenogranite powders were measured by solution ICP-MS (Agilent 7700). The samples were digested by HF+HNO3 in steel-jacketed Teflon digestion vessels. The detailed sample digestion procedure for ICP-MS analysis and the analytical precision and accuracy of the method for trace element determination have been described by Liu et al. (2008).

  • The major element and REE compositions in six quartz-vein-type scheelite samples were measured simultaneously by LA-ICP-MS, and the analytical results for these are summarized in Table 1. All scheelite samples consist of CaO (17.6 wt.% to 23.0 wt.%), WO3 (75.3 wt.% to 81.9 wt.%) and MoO2 (0.08 wt.% to 0.34 wt.%). There are minor Na2O, MgO, Al2O3, SiO2, P2O5, MnO, and FeO fractions in the scheelite. The chondrite-normalized REE distribution patterns of the scheelites show both negative and positive Eu anomalies ([Eu/Eu*]N= 2×[Eu]N/([Sm]N+[Gd]N)). Furthermore, the ΣMREE contents (Sm to Ho, except Eu) for all scheelites were also calculated and the detection limits are listed in Table S1. The variations in [La/Yb]N of scheelite ranges from 0.67 to 4.6. However, most are approximately 1.2−2.5, indicating that fractionation between LREE and HREE is not very substantial. As the possible reactants and the products of the fluid-amphibolite alteration, the amphibole, actinolite, plagioclase, and chlorite were analyzed by LA-ICP-MS to obtain their major element and REE compositions. The analytical results of these silicate minerals are summarized in Table 2. Furthermore, the REE compositions of amphibolite and biotite syenogranite were also measured by ICP-MS and are listed in Table 3.

    Sample SFWZK001-2 SFWZK001-4 SFWZK204-4 SFWZK803-1 SFZK1003-2 SFWZK803-1 SFWZK803-6 SFZK1003-2
    Elements Unit 01 02 03 04 05 01 02 03 04 05 06 07 08 09 10 11 12 01 02 01 02 03 04 05 06 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 07 08 09 10 01 02 03 04 05 01 02 03 04
    CaO wt.% 20.9 20.7 20.7 20.4 20.4 17.8 17.7 17.6 17.6 17.6 18 17.9 17.6 17.8 17.8 17.7 17.7 22.8 22.7 22 22 21.9 21.9 22 22.8 19.1 19.1 19.8 19.3 19.3 19.2 19.1 19.1 19 19.1 19.1 19.2 19.3 19.3 20.1 22.1 21.8 22 22.4 23 22.9 22.9 22.7 22.4 19.1 19 19.1 19
    WO3 wt.% 78.7 78.9 78.8 79.2 79.2 81.8 81.9 81.9 81.7 81.9 81.5 81.4 80.5 81.6 81.6 81.8 81.9 76.7 76.7 77.1 77.3 77.4 77.5 77.4 75.3 80.3 80.2 79.5 80.2 80.5 80.5 80.5 80.5 80.5 80.4 79.9 80.2 80.4 78.5 79.5 77.2 77.4 77.2 77 75.9 76.3 76.3 76.6 76.7 80.3 80.3 80.1 80.4
    Mo ppm 1 002 1 265 1 229 854 562 1 327 1 480 1 442 1 303 1 261 1 547 1 965 1 830 1 750 1 869 1 493 1 474 699 651 1 725 1 745 2 125 2 016 2 073 1 632 1 067 1 025 1 129 668 718 676 612 588 707 690 928 972 473 1 344 1 279 1 963 1 844 1 862 1 915 1 892 2 269 2 180 2 565 2 101 1 034 1 022 968 959
    La ppm 123 165 272.5 93.7 125 124 202 215 185 211 134 197 175 184 148 217 140 95.5 81.9 310 252 210 173 188 127 147 189 92 177 62.9 38.4 67.9 70.8 89.7 145 44.9 62.9 47.3 151 28.3 205 240 227 167 262 219 210 169 204 142 171 130 187
    Ce ppm 345 478 745 294 388 316 520 580 604 662 350 557 461 487 475 544 406 376 322 1 019 744 655 539 547 377 593 702 339 516 194 152 253 252 311 445 185 272 173 651 124 685 833 768 495 888 720 687 468 670 476 646 522 679
    Pr ppm 42.8 58.4 88.3 38.7 58 33 56.2 67.1 80.8 84.2 48.2 78.2 51 63.4 66.9 55.4 44.7 68.3 58 135 91.4 81 70.1 67.3 49.7 96.9 106 54.9 65.1 27.3 26.5 42 39.8 47 58.9 35.2 50.5 27.6 123 25.2 92 117 108 62.9 133 101 96.5 57.6 95.3 63.4 102 86.7 99.2
    Nd ppm 148 183 260 125 263 81.2 166 218 250 259 227 323 139 259 276 155 130 354 297 449 290 233 215 197 157 415 431 257 251 112 131 199 188 205 217 198 266 132 678 145 298 402 374 193 543 403 381 190 393 215 426 385 382
    Sm ppm 40.8 42.6 54.6 31.7 98.4 13.4 33.6 47.4 49.6 52 75.2 98.3 28.6 76.9 80.8 29.8 25.1 123 97.1 93.9 61.9 42.1 40.4 37.6 34.7 116 123 99.7 81.5 32.2 56.9 84.1 73.5 75.1 62.9 86.5 104 57.8 268 61.1 59.7 89.6 79.6 38.6 145 107 104 48.5 113 48.4 120 116 107
    Eu ppm 21.3 27.1 37.1 21 20.7 14 21.2 23.7 31.5 28.5 10 14.6 23.7 13.7 14.6 24.3 17.6 11.7 10.9 33 17.4 20.4 16.6 16.7 15.1 18.1 18.8 10.8 18.5 7.09 6.67 8.99 8.7 10.1 18.5 6.85 10.8 6.57 14.2 6.78 26.1 27.5 22.4 16.8 20.1 17.5 18.6 19.5 18.5 15.3 17.8 14.2 18.9
    Gd ppm 43.5 39.2 44.9 29.4 135 9.62 29.4 42 37.1 41.5 90.7 108 23.9 78.6 88.5 22.8 19.8 139 111 76 50.3 30.4 27.8 27.2 26.1 120 131 137 107 45.5 97.5 130 113 106 79.1 139 159 93.4 342 83.8 40.5 74.7 56.2 27.5 129 112 109 51.3 123 48.7 132 131 117
    Tb ppm 7.8 7.92 9.31 5.98 25.4 1.91 5 7.75 7.63 8.37 14.7 19.1 5 13.2 15.1 3.88 3.75 23.1 18.5 16.6 10.6 6.88 6.3 6.15 5.5 23.3 24.9 27.1 20.8 7.41 17.5 24.4 20.8 21.3 17.3 26.5 30.5 17.4 61.6 16.2 9.28 15.6 13 6.41 24.9 21 21.2 9.67 24.2 10.4 26.5 25 23
    Dy ppm 53.2 56 73 43.6 167 15.4 34 54.3 60.3 61 85.1 119 36.6 79.5 93.2 25.4 25.3 137 106 119 73 50.7 47.2 45 37.9 158 167 174 138 44.3 101 147 132 133 116 168 198 103 374 102 66.8 110 93.6 45.6 162 137 135 62.3 158 74.5 182 161 150
    Ho ppm 10.8 11.5 15.46 9.22 32.4 3.11 6.82 10.9 12.7 12.8 15.5 22 7.63 14.3 16.9 4.89 5.43 24.7 19.4 25.1 14.6 10.3 9.75 9.58 7.8 29.1 29.3 30.8 24.5 7.45 16.6 25 22.4 23.7 22 30.7 36.7 17.5 65.2 18.2 13.5 22.9 19.5 9.61 30.9 26.6 26.7 12 31.2 14.4 33.5 28.7 27.2
    Er ppm 33.3 39.8 54.9 30.7 88.8 12.9 23.5 35.8 47.6 46.1 38.4 61.7 27.2 39.9 47.6 17.2 17.7 64.5 50.8 85.1 49.1 39 36.4 34.1 26.7 90.8 90.5 89.2 70.4 21.6 41.5 63 60.9 65.1 72.4 81.4 102 45.3 174 50.1 46.2 74.6 66.6 34.4 92.1 78.6 79.3 38 90.7 49.4 103 88.2 84.2
    Tm ppm 4.9 6.38 9.45 4.95 10.4 2.91 4.56 6.08 8.85 8.88 4.48 8.51 5.29 5.36 6.29 3.45 3.41 8.07 6.56 15.1 8.74 7.74 7.01 6.8 4.86 12.8 12.3 10.7 9.03 2.41 4.35 7.07 6.62 7.68 9.58 8.77 11.8 4.79 19.1 5.35 8.17 13.1 12 6.19 13.8 11.6 12.2 6.84 13.3 7.94 14.1 11.4 11.2
    Yb ppm 37.8 48.2 74.9 36.4 58.1 24.6 39.5 46.2 67.8 70.3 26.6 55.2 43 37.3 41.6 33.8 29.9 45.6 36.1 107 60.5 60 53.7 51.1 38 84.5 80.8 65.6 56.9 14.9 23.1 38 37 42.9 62.2 47.9 65.2 26.3 107 28.1 56.9 88.2 86.5 46.6 88.6 75.2 80.8 51.4 81.1 58.5 92.9 74.4 75
    Lu ppm 5.9 7.67 11.4 5.56 7.86 3.89 6.13 7.22 10.2 10.6 3.81 7.66 6.94 5.52 6.12 5.75 4.84 5.41 4.33 14.7 8.4 8.12 7.77 7.17 5.17 10.4 10.2 8.03 7.26 2.16 2.74 4.37 4.45 5.16 7.75 5.87 7.9 3.06 12.5 3.34 7.13 11.7 11.9 6.48 11.4 9.67 10.8 7.46 10.9 7.8 12 9.55 9.68
    ΣMREE ppm 156 157 197 120 458 43 109 162 167 176 281 366 102 263 295 87 79 447 353 330 210 140 131 126 112 445 475 469 373 137 290 411 362 359 297 450 529 289 1 111 281 190 313 262 128 492 403 396 184 449 196 494 461 425
    [Eu/Eu*]N 1.54 1.99 2.22 2.07 0.55 3.58 2.02 1.59 2.15 1.81 0.37 0.43 2.7 0.54 0.53 2.74 2.33 0.27 0.32 1.16 0.92 1.66 1.43 1.52 1.47 0.47 0.45 0.28 0.6 0.57 0.27 0.26 0.29 0.35 0.8 0.19 0.26 0.27 0.14 0.29 1.53 1 0.97 1.5 0.44 0.49 0.53 1.19 0.48 0.95 0.43 0.35 0.51
    [La/Yb]N 2.34 2.45 2.61 1.85 1.54 3.62 3.68 3.33 1.95 2.15 3.6 2.56 2.91 3.54 2.54 4.6 3.36 1.5 1.63 2.08 2.98 2.51 2.31 2.64 2.41 1.25 1.68 1 2.24 3.03 1.19 1.28 1.37 1.5 1.67 0.67 0.69 1.29 1.02 0.72 2.59 1.96 1.88 2.57 2.12 2.09 1.86 2.35 1.8 1.74 1.32 1.26 1.79
    Serial numbers of tested numbers are named as Sch- + number, e.g., Sch-01, Sch-02.

    Table 1.  Results of major element and REE compositions in scheelite from the Shangfang W deposit

    Sample SFZK603-4 SFWZK803-6 SFWZK803-1 SFWZK603-3
    Elements Unit Amp-1 Amp-2 Amp-3 Amp-4 Amp-5 Plg-1 Plg-2 Plg-3 Plg-4 Chl-1 Chl-2 Act-1 Act-2 Act-3 Act-4 Act-5 Act-6 Act-7 Act-8 Act-9 Act-10
    Na2O wt.% 0.4 0.24 0.38 0.34 0.26 5.23 5.57 5.5 5.22 0.012 0.015 0.73 0.72 0.76 0.45 0.72 0.68 0.8 0.73 0.61 0.72
    MgO wt.% 16.5 17 15.1 15.9 16.6 0.2 0.196 0.24 0.23 20.2 20.1 15.4 15 14.8 16.8 15.4 15.3 14.5 14.8 15.5 15.8
    Al2O3 wt.% 4.02 3.36 3.51 4.24 6.98 27.5 26.1 26.7 24.9 16.2 16.3 4.19 4.34 4.74 2.45 4.46 4.07 4.81 4.47 3.92 4.43
    SiO2 wt.% 51.3 51.2 51.7 50.3 46.9 57.1 58.8 58.1 60.3 30.5 30.3 52.1 51.8 51.4 54 51.6 52 50.9 51.6 52.2 51.4
    K2O wt.% 0.18 0.093 0.16 0.14 0.1 3.33 4.3 3.71 5.79 0.12 0.11 0.4 0.43 0.48 0.25 0.44 0.43 0.48 0.44 0.37 0.42
    CaO wt.% 12.6 11.7 12 11.7 9.4 6.26 4.6 5.29 3.12 0.22 0.28 13.3 13.2 13.1 13.7 13.3 13.3 13.2 13.1 13.3 13.3
    MnO wt.% 1.52 1.85 2 1.84 1.82 0.01 0.01 0.01 0.01 0.73 0.74 1 1.1 1.1 0.9 1 1 1.1 1 1 0.9
    FeO wt.% 10.2 11.4 11.9 12.3 14.6 0.19 0.2 0.22 0.22 17.8 17.9 12.8 13.2 13.4 11.3 12.9 13.1 14 13.6 12.9 12.8
    La ppm 0.15 0.27 0.32 0.22 0.11 0.08 0.17 0.05 0.12 0.059 0.021 2.57 2.53 2.35 1.14 2.06 2.47 2.9 2.45 1.82 2.01
    Ce ppm 1.25 2.8 3.11 1.84 1.69 0.12 0.24 0.12 0.2 0.11 0.08 9.02 9.47 9.89 4.1 7.73 8.99 11.5 9.32 7.29 7.33
    Pr ppm 0.34 0.72 0.72 0.49 0.39 0.011 0.018 0.012 0.018 0.017 0.003 3 1.34 1.48 1.5 0.57 1.2 1.21 1.78 1.29 0.98 0.96
    Nd ppm 2.94 5.25 6.36 3.57 3.74 0.034 0.095 0.034 0.078 0.065 0.003 4 5.73 6.45 6.03 2.72 4.34 5.12 8.21 5.99 3.92 4.03
    Sm ppm 2.76 3.05 4.1 2.11 2.29 0.004 0.03 0.007 0.007 0.11 0.13 2.08 2.6 2.27 0.8 1.24 1.72 3.24 2.2 1.36 1.18
    Eu ppm 0.29 0.26 0.49 0.3 0.27 0.138 0.135 0.078 0.048 0.0039 0.06 0.56 0.6 0.61 0.25 0.52 0.45 0.95 0.54 0.38 0.53
    Gd ppm 2.86 2.68 3.69 3.48 2.74 0.007 0.016 0.016 0.02 0.44 - 2.32 3.12 3.43 1.07 2.29 1.57 3.62 2.62 1.26 1.05
    Tb ppm 0.28 0.32 0.55 0.4 0.3 0.002 0.002 0.001 0 0.049 0.04 0.53 0.64 0.6 0.24 0.51 0.33 0.89 0.56 0.38 0.26
    Dy ppm 1.22 1.49 3.12 2.58 1.93 0.014 0.013 0.007 0.004 0.28 0.36 3.56 4.94 4.46 1.83 3.16 2.34 7.49 4.08 2.11 1.96
    Ho ppm 0.16 0.22 0.53 0.57 0.36 0.004 0.004 0.002 0.001 0.083 0.074 0.76 0.96 0.9 0.3 0.67 0.52 1.57 0.85 0.52 0.4
    Er ppm 0.21 0.61 1.54 1.28 1.2 0.004 0.007 0 0.006 0.27 0.3 2.04 3.09 3.08 1.04 2.06 1.55 5.09 2.78 1.73 1.11
    Tm ppm 0.05 0.08 0.19 0.18 0.14 - 0.002 0.001 0.001 0.035 0.015 0.33 0.53 0.53 0.18 0.38 0.26 0.93 0.48 0.3 0.22
    Yb ppm 0.45 0.44 1.31 1.13 0.89 - 0.003 0.005 0.006 0.11 0.21 2.42 4.21 4.02 1.08 2.4 1.99 7.14 3.49 2.26 1.64
    Lu ppm 0.039 0.12 0.16 0.14 - 0.12 0.002 0 0.002 0.043 0.025 0.42 0.67 0.56 0.22 0.41 0.31 1.05 0.54 0.35 0.26

    Table 2.  Results of major element and REE compositions in silicate minerals (chlorite, amphibole, actinolite and plagioclase)

    Rock Syenogranite Amphibolite
    Elements Unit SFZK001 SFZK001 SFZK001 SFZK401 SFZK101 SFZK802 SFD1107 SFD1102 SFD1104 SFD1103 SFZK802 SFZK1601
    La ppm 32.4 27.7 87.7 64.1 66.5 67.7 10.3 8.14 6.77 7.89 10.6 7.51
    Ce ppm 26.7 19.3 60.5 44.4 47.3 46.2 10.9 6.24 5.39 5.13 11.2 6.82
    Pr ppm 18.3 14.9 43.1 32.3 33.7 34.2 8.95 6.85 6.41 6.78 9.93 7.03
    Nd ppm 13.9 11.8 31.1 23.8 25.1 24.9 8.63 6.99 6.74 7.10 10.0 7.42
    Sm ppm 8.29 7.16 14.98 13.02 13.83 13.06 7.43 6.69 6.53 6.69 8.76 6.91
    Eu ppm 3.15 3.57 7.02 5.77 5.54 6.07 5.00 5.42 5.36 5.54 7.20 6.49
    Gd ppm 5.20 4.51 8.71 7.97 8.39 7.67 6.11 6.02 5.92 6.01 7.32 6.19
    Tb ppm 4.72 4.26 7.13 7.22 7.87 6.20 6.94 7.04 6.85 7.13 8.15 7.04
    Dy ppm 3.98 3.65 5.36 5.96 6.45 4.55 7.04 7.16 7.01 7.26 8.06 7.30
    Ho ppm 3.90 3.41 4.70 5.24 5.91 3.66 7.01 7.32 7.07 7.26 8.05 7.44
    Er ppm 3.60 3.25 4.29 4.71 5.23 3.27 6.40 6.56 6.35 6.56 7.10 6.75
    Tm ppm 4.32 3.92 4.46 5.00 5.41 3.24 6.89 7.16 7.03 7.30 7.57 7.30
    Yb ppm 4.56 4.06 4.16 4.48 5.09 3.18 6.33 6.61 6.45 6.67 6.86 6.80
    Lu ppm 5.68 4.86 4.59 4.73 5.27 3.38 6.76 7.03 6.89 7.30 7.16 7.30

    Table 3.  Results of REE compositions in syenogranite and amphibolite

  • In this study, all scheelite crystals were investigated under the OM-CL and compositions of the measured REE were obtained using LA-ICP-MS. After the chondrite normalization, the in-situ analytical results are displayed in Fig. 7, which illustrates distinct variations in REE patterns among different parts of the scheelite crystals. The difference in REE patterns focuses mainly on the [Eu/Eu*]N values and ΣMREE contents. As shown in Fig. 8, there is a continuously increasing trend of [Eu/Eu*]N with reductions in ΣMREE contents. In general, when ΣMREE contents are higher than 200 ppm, the REE patterns of most scheelites show negative Eu anomalies ([Eu/Eu*]N < 1). While the ΣMREE contents drop gradually from ~600 ppm to ~200 ppm, the [Eu/Eu*]N values slowly climb and approach 1. If the ΣMREE content is lower than 200 ppm, the [Eu/Eu*]N increases sharply although the decline in ΣMREE contents is limited. Compared with the rim, the core of scheelite grain usually contains lower [Eu/Eu*]N but higher ΣMREE contents. As the distribution pattern of REE is a hump containing an Eu gap (Brugger et al., 2000b), the continuous precipitation of scheelite possibly consumed the MREE and sequentially boosted the [Eu/Eu*]N value in theory. These geochemical variations of scheelite could be interpreted as the equilibrium-controlled growth, in combination with the growth zoning of scheelite crystal.

    Figure 7.  Chondrite-normalized REE distribution patterns for scheelite samples from the Shangfang W deposit.

    Figure 8.  The variation trends of ΣMREE contents and [Eu/Eu*]N values of the in-situ analytical results of scheelite. Microscopic images of scheelite under optical microscope cathodoluminescence and laser ablation pits.

  • In the Shangfang W deposit, most of the ore bodies are hosted in amphibolite. As the major roles of fluid-rock interaction, the REE compositions of the amphibolites and the biotite syenogranites were measured by ICP-MS and are displayed in Fig. 9a after chondrite normalization. The amphibolite is characterized by nearly flat chondrite-normalized REE patterns with slightly negative Eu anomalies, while the biotite syenogranite shows LREE-enriched patterns and negative Eu anomalies. Obvious macroscopic chlorite alteration demonstrates that the silicate minerals in amphibolite were altered during fluid-rock interactions. To reveal the element transportation among different minerals during chlorite alteration, the major silicate minerals (e.g., amphibole, actinolite, plagioclase, and chlorite), which coexist with scheelite, were measured by LA-ICP-MS to obtain in-situ REE compositions. As shown in Fig. 9b, the REE contents of amphibole and actinolite are higher by approximately one to two orders of magnitude, in contrast to those of plagioclase and chlorite. Furthermore, comparing the abundance of CaO and ∑REE among the amphibole, actinolite, plagioclase, and chlorite (Fig. 10), it appears that amphibole (~9.4%−12.6%) and actinolite (~13.1%−13.7%) possibly release large quantities of Ca2+ and REE3+ into the fluids during chlorite alteration. According to previous research, the zircon U-Th-Pb age of the syenogranite (158.8±1.6 Ma) coincides with the mineralization age of the W deposit derived from the molybdenite Re-Os isochron age of 158.1±5.4 Ma (Chen et al., 2013) and the scheelite Sm-Nd isochron age of 157.9±6.7 Ma (Zhu et al., 2019), which means that the fluid may have been derived from the syenogranite and then interacted with the amphibolite. Therefore, the composition of the initial ore-forming fluid correlates with both the silicate minerals in the amphibolite and which the fluids from the syenogranite, both of which contributed their REE to scheelite.

    Figure 9.  (a) Chondrite-normalized REE patterns for silicate minerals (amphibole, plagioclase, chlorite) in the Shangfang W deposit; (b) chondrite- normalized REE patterns for ore-related amphibolite and syenogranite from drill cores in the Shangfang W deposit (all results were normalized against chondrite values from Sun and Mcdonough (1989)).

    Figure 10.  Variations in CaO and ∑REE among amphibole, actinolite, plagioclase, and chlorite.

  • In general, the theoretical composition of the initial fluid and the partition coefficients are crucial for modeling REE variations during mineralization (Li et al., 2018; Zhang et al., 2018; Zhao et al., 2018). According to previous research on the Shangfang W deposit, it is acceptable to assume the REE compositions of the magma-derived fluid are similar to that of the syenogranite, based on drill core data. Because both amphibole and actinolite may contribute their REE to the initial fluid to different degrees, it is necessary to assume four different initial fluids for our model calculation, namely, C01, C02, C03, and C04 that respectively consist of 80 wt.% REE of amphibole+20 wt.% REE of magma-derived fluid, 80 wt.% REE of actinolite+ 20 wt.% REE of magma-derived fluid, 20 wt.% REE of amphibole+80 wt.% REE of magma-derived fluid and 20 wt.% REE of actinolite+80 wt.% REE of magma-derived fluid. The average values of the in-situ analytical results were adopted to represent the REE compositions of amphibole and actinolite. The calculated REE patterns of C01, C02, C03, and C04 are displayed in Fig. 11. As the variation in mixture ratio is not beyond the defined proportion (20%−80%), the REE compositions of the initial fluid should be within the area confined by C01, C02, C03, and C04. The partition coefficients of REE between scheelite and fluid adopted the values that were calculated theoretically from the ionic radius of REE by Brugger et al. (2000b). Because the ionic radius of Eu2+ is far different from that of REE3+, the partition coefficients for Eu are lower than that of its adjacent elements. In theory, the redox condition will control the valence state of Eu, which may produce the variation of partition coefficient of Eu. During the interaction between amphibolite and syenogranite-derived fluids, the Fe2+ and Fe3+ binding in the minerals of mafic rock (especially amphibole) would be the buffer of oxygen fugacity. Therefore, it is reasonable to accept that Eu2+ and Eu3+ may coexist in the ore-forming fluids. In order to better assess the influence of continuous precipitation of scheelite on Eu anomaly of fluids, we assume the oxygen fugacity is stable and the partition coefficients of Eu are constant. If the amounts of Eu2+ are equal to those of Eu3+, the bulk partition coefficient of Eu can be calculated according to the respective partition coefficient and proportion of Eu2+ and Eu3+, which was suggested by Li et al. (2018). More details about the parameters for the model calculation are listed in the supplementary (Table S2).

    Figure 11.  Chondrite-normalized REE patterns for assuming initial ore-forming fluid after rock-fluid interaction in the Shangfang W deposit (normalization values are from Sun and Mcdonough (1989). Note: C01=80 wt.% REE (amphibole)+20 wt.% REE (magma-derived fluid), C02=80 wt.% REE(actinolite)+20 wt.% REE (magma-derived fluid), C03=20 wt.% REE (amphibole)+80 wt.% REE (magma-derived fluid), C04=20 wt.% REE (actinolite)+80 wt.% REE (magma-derived fluid).

    Then, an improved batch crystallization model is modified and adapted to calculate the REE compositional variations of scheelite and fluids for the Shangfang W deposit. In contrast to other fluid-derived REE-free minerals (e.g., quartz, sulfide), scheelite is the dominant REE-bearing mineral for the ore-forming fluid system of the Shangfang W deposit. Therefore, we assume in this model that each batch consists of scheelite precipitation (i.e., step Ⅰ) and precipitation of REE- free minerals (i.e., step Ⅱ) (Fig. 12). As the partition coefficients between scheelite and fluid remain constant, the calculation of the REE compositions of scheelite and fluid during step Ⅰ is carried out according to Eq. (1). In step Ⅱ, the precipitation of REE-free minerals does not consume REE but consumes other components of the fluid, which increases the relative concentrations of REE in the fluid. Then, the REE compositions of fluids can be calculated based on the following Eq. (2).

    Figure 12.  The computational flow chart of an improved batch crystallization model for scheelite in quartz veins.

    where n is the repetition of the batch; F is the residual mass fraction of fluid; R is the concentration ratio of fluid by precipitation of quartz and sulfide; $ {D}_{i} $ values are the partition coefficients of element i between scheelite and fluid; $ {C}_{o}^{i} $, $ {C}_{l}^{i} $ and $ {C}_{s}^{i} $ are the concentrations of element i in the original fluid, residual fluid and precipitated scheelite, respectively; $ {C}_{o}^{i}1 $, $ {C}_{o}^{i}2 $, $ {C}_{o}^{i}3 $ and $ {C}_{o}^{i}4 $ are the concentrations of element i in the different assumed initial ore-forming fluids, respectively. R is the mass fraction of REE-free minerals (e.g., quartz and sulfide), the precipitation of which will increase the REE concentration of the residual fluids in each batch. All the calculated results of model are listed in the supplementary (Tables S3, S4).

    Furthermore, the degree of fluid evolution correlates to the variable n, which is the repetition of the batch for this model. Then, the relationship between variable n and the mass fraction of residual fluid of each batch after precipitation of scheelite and other REE-free minerals is expressed by the following Eq. (3).

    where αn and βn are the mass fraction of the residual fluid after the precipitation process of scheelite and REE-free minerals for the n-th batch, respectively.

    If we assume the total repetitions for the batch are 20, the accumulated mass fractions of the precipitated scheelite and the REE-free minerals can be calculated from the following Eq. (4).

    where ΔMSch and ΔMREE-free are the accumulated mass fractions of scheelite and REE-free minerals during the whole process, respectively. According to this model, the values of ΔMSch and ΔMREE-free attain of ~2.64% and ~90.13%, the summation of which (~92.67%) indicates that more than 90% of fluid has been consumed. In other words, the fluid has evolved to the later stage when the repetition of the batch is 20. The proportion of scheelite (~2.64%) is less than that of REE-free minerals (~90.13%), which agrees with the actual composition of the quartz-vein-type ores. Therefore, the assumed values of F, R, and n are appropriate for modelling REE variations of fluid and scheelite during mineralization. It appears that this model might apply to cases other than scheelite deposits, and quite possibly to other types of hydrothermal deposits coexisting with scheelite in quartz veins.

    Both the variations in REE compositions for scheelite and the images from optical microscope cathodoluminescence (OM-CL) have demonstrated the ΣMREE contents and the [Eu/Eu*]N of scheelite between the early and later stages are distinctly different. The early-stage scheelite is in character of ΣMREE > 350 ppm and [Eu/Eu*]N < 0.6. Meanwhile, later stage scheelite displays distinct character of ΣMREE < 200 ppm and [Eu/Eu*]N > 1.2. Then, the repetitions of the batch were set respectively to n=1−5 and n=16−20 for modelling the REE variations of different stage scheelites (Fig. 13).

    Figure 13.  Model calculations and measured REE compositions of scheelite during the batch fractionation of scheelite at the early stage (a) and at the later stage (b). Chondritic values are from Sun and Mcdonough (1989).

    Regardless of which type of stage is being discussed, all results of the model calculations show excellent agreement with the in-situ analytical results, both for REE patterns and for contents. Furthermore, the REE compositions of fluid in equilibrium with scheelite were also calculated and are summarized in Fig. 14. The results of both the in-situ measurements and the model calculations show similar trends for the fluid. It is evidence that the precipitation of early-stage scheelite (MREE- enriched and [Eu/Eu*]N > 1) consumes the MREE compositions of the initial fluid. Subsequently, the residual fluid precipitates later-stage scheelite with future of MREE-depleted and [Eu/Eu*]N > 1. According to the new model, the calculated trends (from n=1 to n=20) of [Eu/Eu*]N over ΣMREE contents by the initial fluids of C01, C02, C03, and C04 agree with the analytical results (Fig. 15), which further demonstrates that our model is reliable for understanding the process of scheelite precipitation in the Shangfang W deposit. The calculated results demonstrate that scheelite displays distinct variations in Eu anomalies as does the continuous precipitation of scheelite, although the oxygen fugacity in the model is constant. Even for the same period of mineralization, it still possibly forms scheelite with various REE patterns and different Eu anomalies merely by the different extents of interaction between the same initial fluid and the silicate minerals in the rock.

    Figure 14.  REE compositions of the ore-forming fluid equilibrating with scheelite at the early stage (a) and the later stage (b). Chondritic values are from Sun and Mcdonough (1989).

    Figure 15.  The trend of [Eu/Eu*] over ΣMREE concertation from model calculations (from n=1 to n=20) and in-situ analytical results.

  • The Late Mesozoic granitic magmatism was most intense in the interior of the SCB with a peak age at 165–155 Ma (Li et al., 2009a; Zhou et al., 2006). This episode of igneous activity is also predominantly distributed throughout the Nanling Range, from Northeast Guangxi Province (e.g., the Miaoershan granite), through Hunan and Jiangxi provinces, and eastward to southern Fujian Province (e.g., the Wuping, and Gutian granites) (Liu et al., 2015; Xie et al., 2008; Wang L J et al., 2007; Zhou et al., 2006; Yu et al., 2005b). According to the molybdenite Re-Os isochron age of 158.1±5.4 Ma (Chen et al., 2013) and the scheelite Sm-Nd isochron age of 157.9±6.7 Ma (Zhu et al., 2019) for the Shangfang W deposit, it is reasonable to accept that the mineralization backgrounds of the Shangfang tungsten deposit are similar to those of the tungsten deposits in the Nanling region. By heating of mantle-derived magmas, the Shangfang biotite syenogranites possibly melted from the basement rocks of the Cathaysia Block, which is composed predominantly of Neoproterozoic basement rock with small quantities of Mesoproterozoic and Paleoproterozoic rocks (Chen et al., 2013). As shown in Fig. 16, the initial ore-forming fluids were magma-derived and had similar REE compositions to those of the biotite syenogranites. The fluids caused extensive chlorite alteration of amphibolite, during which amphibole and actinolite are replaced by chlorite. And Ca and REE in these minerals were released into fluids. The various REE patterns and different Eu anomalies recorded the process of continuous precipitation of scheelite for interaction between fluid and rock. Therefore, the chlorite alteration of amphibole and actinolite is the critical ore-forming mechanism responsible for the Shangfang quartz-vein-type W deposits.

    Figure 16.  An idealized model of the mineralization process in the Shangfang W deposit.

  • (1) The precipitation of scheelite (CaWO3) in Shangfang is correlative to the fluid-amphibolite alteration. Unlike classic skarn scheelite deposits, this type of deposit does not directly acquire sufficient Ca from the surrounding limestone, but rather from amphibolite by chlorite alteration, which is a new critical ore-forming mechanism responsible for the quartz-vein-type W deposits.

    (2) As a common mineral, scheelite forms in various types of hydrothermal deposits and incorporates abundant REE via substitution for Ca2+ in its crystal lattice. The REE pattern of scheelite is an effective index to trace the REE evolution of the fluids during mineralization. For the process of mineralization in the Shangfang W deposit, the early-stage scheelite would consume more MREE than LREE and HREE of fluid, which will gradually produce residual fluids with strong MREE-depletion and [Eu/Eu*]N > 1. Even the partition coefficient of REE is constant, the later-stage scheelite will also inherit a certain degree of MREE-depletion and [Eu/Eu*]N future from the residual fluids.

    (3) It is possible to form scheelite with various REE patterns and different Eu anomalies solely by continuous precipitation of scheelite and by different extents of interaction between fluid and rock during the same period of mineralization. Hence, the Eu anomalies in scheelite are not an effective index to distinguish the oxygen fugacity of the fluid.

  • Special thanks should go to three anonymous reviewers who have given detailed suggestions and comments, which improved the quality of this manuscript. We highly appreciate the constructive comments of Prof. Jianwei Li and Prof. Kuidong Zhao regarding an earlier version of this manuscript. We are also grateful for the skilled assistance of Prof. Xinfu Zhao during operation of the OM-CL. This study was financially supported by the National Science Foundation of China (No. 41803012) and the China Postdoctoral Science Foundation (No. 2017M622546). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1283-0.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1283-0.

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