Constraints on the Petrogenesis and Metallogenic Setting of Lamprophyres in the World-Class Zhuxi W–Cu Skarn Deposit, South China

: Whole-rock and apatite geochemical analyses and zircon U–Pb dating were carried out on the lamprophyres in the world-class Zhuxi W–Cu skarn deposit in northern Jiangxi, South China, in order to understand their origin of mantle sources and their relationship with the deposit, as well as metallogenic setting. The results show the lamprophyres were formed at ca. 157 Ma, just before the granite magmatism and mineralization of the Zhuxi deposit. These lamprophyres have from 58.98–60.76 wt% SiO 2 , 2.52–4.96 wt% K 2 O, 5.92–6.41 wt% Fe 2 O 3t , 3.75–4.19 wt% MgO, and 3.61–5.06 wt% CaO, and enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE), and depletion of high-ﬁeld-strength elements (HFSE). Apatites in the lamprophyres are enriched in LREE and LILE, Sr, S, and Cl, and have 87 Sr / 86 Sr ratios ranging from 0.7076 to 0.7078. The conclusions demonstrate that the lithospheric mantle under the Zhuxi deposit was metasomatized during Neoproterozoic subduction. Late Jurassic crustal extension caused upwelling of the asthenospheric mantle and consecutively melted the enriched lithospheric mantle and then crustal basement, corresponding to the formation of lamprophyres and mineralization-related granites in the Zhuxi deposit, respectively.


Introduction
The Yangtze and Cathaysia blocks were collided by a north-directed subduction during the Neoproterozoic (Jiangnan orogeny) [1] and together they form the South China Block. Multiple episodes of Mesozoic magmatism occurred in the South China Block, including voluminous granites associated with W deposits and subordinate bimodal volcanic rocks, mafic-ultramafic intrusions, and extensive swarms of mafic dikes [2][3][4]. Although tectonic models proposed to interpret the Mesozoic magmatism in the South China Block are controversial, it is generally accepted that the South China Block was in an extensional setting due to lithospheric thinning or back-arc extension [5][6][7][8]. Previous studies of mafic rocks show that the lithospheric mantle below the Yangtze and Cathaysia blocks is different [9]. Along the eastern boundary of the Yangtze Block, Late Jurassic high-Mg andesites in Youjiang basin have similar compositions to those of the Neoproterozoic high-Mg rocks and were derived from partial melting of the enriched lithospheric mantle metasomatized by the Neoproterozoic subduction [10]. constrained by muscovite 40 Ar- 39 Ar, molybdenite Re-Os, and U-Pb titanite [20,21]. The Zhuxi granites are highly siliceous (>70 wt%) and peraluminous, and show typical S-type granite characteristics. [14,19]. Major and trace element, as well as Sr-Nd-Hf isotopic compositions of Zhuxi granites indicate they are derived from partial melting of the Neoproterozoic Shuangqiaoshan Group in this region [14,19]. Figure 1. Regional geological map of the Jiangnan porphyry-skarn W belt (JNB) (modified from [13]).
Lamprophyre dikes intruded in the Carboniferous-Permian strata are 50-500 m long and 1-10 m wide. They were intersected by drilling along exploration lines 7, 62, and 78 and show 4-15 m thickness. No direct contact between these lamprophyres and granites was found. However, presence of pervasive sulfide mineralization along fractures in some of these lamprophyres indicates the lamprophyre formed earlier than the Zhuxi granites and ore deposit. The Zhuxi lamprophyre is characterized by a porphyritic texture in which phenocrysts (30 vol.%) of amphibole and biotite are enclosed in groundmass (70 vol.%) of plagioclase, accessory apatite, titanite, and zircon. Amphibole is subhedral to anhedral. Some grains were partially replaced by biotite. Biotite is euhedral to subhedral with 10-50 μm width and 50-200 μm length and was locally altered into chlorite. Minor quartz is also found in the groundmass. Alteration minerals in the lamprophyres include chlorite, sericite, and calcite ( Figure 4).  [14]). Figure 2. Geological map of the Zhuxi W-Cu deposit (after [14]).
Intrusions in the Zhuxi deposit include biotite granite, muscovite granite, granitic porphyry, and lamprophyre. The granitic porphyry crosscuts the muscovite granite. Both occur as dikes at depths of 600-2400 m below the surface and intrude the biotite granite pluton ( Figure 3). The Zhuxi granites generally consist of quartz, alkaline feldspar, and minor plagioclase, biotite, and muscovite. Accessory minerals in them include zircon, apatite, and Ti/Fe oxides. Veinlet-disseminated scheelite and chalcopyrite mineralization in them is accompanied by pervasive chlorite and sericite alteration. These granites formed at ca. 153 Ma [14,19], which is contemporaneous with mineralization ages constrained by muscovite 40 Ar- 39 Ar, molybdenite Re-Os, and U-Pb titanite [20,21]. The Zhuxi granites are highly siliceous (>70 wt%) and peraluminous, and show typical S-type granite characteristics. [14,19]. Major and trace element, as well as Sr-Nd-Hf isotopic compositions of Zhuxi granites indicate they are derived from partial melting of the Neoproterozoic Shuangqiaoshan Group in this region [14,19].
Lamprophyre dikes intruded in the Carboniferous-Permian strata are 50-500 m long and 1-10 m wide. They were intersected by drilling along exploration lines 7, 62, and 78 and show 4-15 m thickness. No direct contact between these lamprophyres and granites was found. However, presence of pervasive sulfide mineralization along fractures in some of these lamprophyres indicates the lamprophyre formed earlier than the Zhuxi granites and ore deposit. The Zhuxi lamprophyre is characterized by a porphyritic texture in which phenocrysts (30 vol.%) of amphibole and biotite are enclosed in groundmass (70 vol.%) of plagioclase, accessory apatite, titanite, and zircon. Amphibole is subhedral to anhedral. Some grains were partially replaced by biotite. Biotite is euhedral to subhedral with 10-50 µm width and 50-200 µm length and was locally altered into chlorite. Minor quartz is also found in the groundmass. Alteration minerals in the lamprophyres include chlorite, sericite, and calcite ( Figure 4).

Whole-Rock Major and Trace Elements Analyses
The whole-rock geochemical analyses were carried out in the State Key Laboratory for Mineral Deposits Research, Nanjing University (Najing, China) by Thermo Scientific ARL 9900 X-ray fluorescence (XRF) and the analytical precisions are better than 1%. For trace element analyses, about 50 mg sample powder was dissolved in high-pressure Teflon bombs using an HF + HNO 3 mixture and measured by a Finnigan Element II inductively coupled plasma mass spectrometer (ICP-MS). Rh was used as an internal standard to monitor signal drift. The analytical precisions were estimated to be better than 10% for all trace elements.

Major Elements, Trace Elements, and Sr Isotopes in Apatite
Major element analyses of apatite were performed in the Testing Center of the China Metallurgical Geological Bureau, Shandong, China, using a JEOL JXA-8230 electron microprobe analyzer (EMPA) equipped with four wavelength dispersive spectrometers. The operating conditions were 15 kV voltage, 20 nA current, and 5 µm beam size. The measurement times were 10 s for Ca and P, and 20 s for Na, Mg, Si, Fe, Mn, Sr, F, and Cl. The standards used were apatite (Ca, P), jadeite (Na, Si), diopside (Mg), olivine (Fe), rhodonite (Mn), celestite (Sr), phlogopite (F) and tugtupite (Cl). Precision for EMPA analysis was calculated from counting statistics, and was generally better than 1% for measurements >10 wt%, and better than 5% for contents >0.5 wt%.
In situ trace element analyses of apatite used a RESOlution S-155 laser ablation system coupled to a Thermo iCAP Qc inductively coupled plasma mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. The NIST 612 and 610 glass standards and the USGS reference glasses (BIR-1G, BCR-2G, and BHVO-2G) were repeatedly analyzed between every four apatite samples. Both standards and samples were ablated using a 33 µm spot size, 10 Hz repetition rate, and corresponding energy density of~3 J/cm 2 . The Ca measured by EPMA was used as the internal standard, whereas the USGS reference glasses noted were used for external calibrations. Data reduction was offline after analysis and was conducted by using the ICPMSDataCal software (version 11, the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China ) [22].
The in situ Sr isotope ratios were determined with a NWR193 laser ablation system coupled to a Nu Plasma II MC-ICP-MS instrument at the State Key Laboratory of Geological Processes and Mineral Resources (Wuhan, China). The measurements involve correction of critical spectral interferences that include Kr, Rb, and doubly charged rare earth elements. A modern-day coral (Qingdao), well characterized for its 87 Sr/ 86 Sr isotopic composition by isotope dilution thermal ionisation mass spectrometry (ID-TIMS), was used as an external standard. The analysis conditions were 55 µm spot size, 10 Hz repetition rate, and~11 J/cm 2 energy density. The average 87 Sr/ 86 Sr ratio obtained for the coral standard was 0.70926 ± 0.00002, which is within analytical error of TIMS values of 0.70925 ± 0.00002 [23].

Zircon U-Pb Age
Zircon grains from the Zhuxi lamprophyre are generally subhedral to euhedral, dominantly short-prismatic with a length of 50 to 100 µm. Cathodoluminescence images show that zircon typically has a dark core with a bright rim. Twelve zircon rims have 8.82-156 ppm Th, 374-1459 ppm U, with the Th/U ratios of 0.01 to 0.28. They yield a concordia age of 157.9 ± 1.5 Ma (MSWD = 0.93) and a weight mean 206 Pb/ 238 U age of 157.9 ± 1.7 Ma (MSWD = 1.9). These lamprophyre ages are slightly older than the emplacement ages of the mineralization-related granites (148-153 Ma) in the Zhuxi deposit [14,19] ( Figure 5; Table 1). In situ trace element analyses of apatite used a RESOlution S-155 laser ablation system coupled to a Thermo iCAP Qc inductively coupled plasma mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. The NIST 612 and 610 glass standards and the USGS reference glasses (BIR-1G, BCR-2G, and BHVO-2G) were repeatedly analyzed between every four apatite samples. Both standards and samples were ablated using a 33 μm spot size, 10 Hz repetition rate, and corresponding energy density of ~3 J/cm 2 . The Ca measured by EPMA was used as the internal standard, whereas the USGS reference glasses noted were used for external calibrations. Data reduction was offline after analysis and was conducted by using the ICPMSDataCal software (version 11, the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China ) [22].
The in situ Sr isotope ratios were determined with a NWR193 laser ablation system coupled to a Nu Plasma II MC-ICP-MS instrument at the State Key Laboratory of Geological Processes and Mineral Resources (Wuhan, China). The measurements involve correction of critical spectral interferences that include Kr, Rb, and doubly charged rare earth elements. A modern-day coral (Qingdao), well characterized for its 87 Sr/ 86 Sr isotopic composition by isotope dilution thermal ionisation mass spectrometry (ID-TIMS), was used as an external standard. The analysis conditions were 55 μm spot size, 10 Hz repetition rate, and ~11 J/cm 2 energy density. The average 87 Sr/ 86 Sr ratio obtained for the coral standard was 0.70926 ± 0.00002, which is within analytical error of TIMS values of 0.70925 ± 0.00002 [23].

Zircon U-Pb Age
Zircon grains from the Zhuxi lamprophyre are generally subhedral to euhedral, dominantly short-prismatic with a length of 50 to 100 μm. Cathodoluminescence images show that zircon typically has a dark core with a bright rim. Twelve zircon rims have 8.82-156 ppm Th, 374-1459 ppm U, with the Th/U ratios of 0.01 to 0.28. They yield a concordia age of 157.9 ± 1.5 Ma (MSWD = 0.93) and a weight mean 206 Pb/ 238 U age of 157.9 ± 1.7 Ma (MSWD = 1.9). These lamprophyre ages are slightly older than the emplacement ages of the mineralization-related granites (148-153 Ma) in the Zhuxi deposit [14,19] (Figure 5; Table 1).  Table 1).

Whole-Rock Major and Trace Elements
Lamprophyres are rich in volatiles and their loss on ignition (LOI) is usually high (many published data above 15 wt%; e.g., [24]). The LOI values of Zhuxi lamprophyres are very low, ranging from 2.98 to 4.37 wt%, indicating they are relatively fresh. In order to better describe the geochemical characteristics of Zhuxi lamprophyres, we collected from literature the data of mafic rocks that are temporally and spatially close to the Zhuxi lamprophyres, including the Wushan lamprophyres [25] Table 1).

Strontium Isotopes of Apatite
Ten apatite grains were selected for Sr isotope analysis and the results indicate a relatively homogeneous 87 Sr/ 86 Sr ratio from 0.7076 to 0.7078 ( Figure 9; Table 3). Considering the extremely low Rb/Sr ratios in these apatites, the measured 87 Sr/ 86 Sr represent the initial 87 Sr/ 86 Sr of the magma.  Table 3).  [32] for the apatites in Zhuxi lamprophyres (see Table 3).

Strontium Isotopes of Apatite
Ten apatite grains were selected for Sr isotope analysis and the results indicate a relatively homogeneous 87 Sr/ 86 Sr ratio from 0.7076 to 0.7078 ( Figure 9; Table 3). Considering the extremely low Rb/Sr ratios in these apatites, the measured 87 Sr/ 86 Sr represent the initial 87 Sr/ 86 Sr of the magma.  [32] for the apatites in Zhuxi lamprophyres (see Table 3).

Strontium Isotopes of Apatite
Ten apatite grains were selected for Sr isotope analysis and the results indicate a relatively homogeneous 87 Sr/ 86 Sr ratio from 0.7076 to 0.7078 ( Figure 9; Table 3). Considering the extremely low Rb/Sr ratios in these apatites, the measured 87 Sr/ 86 Sr represent the initial 87 Sr/ 86 Sr of the magma.  Table 3).  Table 3). Table 3. Major elements (in wt%) and trace elements (in ppm), and Sr isotope compositions of apatites from the Zhuxi lamprophyres.

Source Mineralogy
Relatively homogeneous whole-rock geochemistry, weak negative Eu anomalies, as well as the homogeneous apatite composition (sensitive to crustal contamination; e.g., [34]) indicate assimilation of silicic upper crustal material is not significant in the Zhuxi lamprophyres. High K 2 O and large-ion lithophile elements (LILE) in the Zhuxi lamprophyres require phlogopite or amphibole in the source region [35], since Rb and Ba are compatible in phlogopite [36], while Rb, Sr, and Ba are moderately compatible in amphibole. Melts derived from partial melting of phlogopite-bearing rocks typically have significantly higher Rb/Sr (>0.1) and lower Ba/Rb values (<20) [37] (Figure 10a), whereas melts from amphibole-bearing sources have extremely high Ba/Rb values (>20) [37]. The Zhuxi lamprophyres with low Ba/Rb ratios (<20), but high Rb/Sr ratios (up to 0.43) indicate phlogopite in their source. Deggen et al. [38] modeled the garnet, garnet-spinel, and spinel stability field during partial melting of lherzolite. Based on the K/Yb/1000 vs. Dy/Yb diagram (Figure 10b), unlike Quzhou and Longyou mafic-ultramafic intrusions plotting in the garnet stability field, the Zhuxi and Wushan lamprophyres plot in the garnet-spinel transition field, indicating garnet and spinel are both present in the source region of Zhuxi lamprophyre, corresponding to a depth of 75-85 km [39][40][41].

Mantle Metasomatism by Subducted Components
The trace element compositions (Figure 7; Figure 11) of Zhuxi lamprophyres show they are very different from the ocean-island basalts (OIB) and mid-ocean-ridge basalts (MORB), but are broadly similar to average continental crust [29] and global subducting sediments-II [28]. Different La/Yb ratios between them may be attributed to different degrees of partial melting. The enrichment of large-ion lithophile elements (LILE) and depletion of high-field-strength elements (HFSE) in primitive mantle normalized diagrams are generally considered as fingerprints of subduction processes (e.g., [42,43]), during which the HFSEs are stored in minerals, including rutile and ilmenite in the subducted slab, whereas the LILEs easily transfer into the fluids [44,45]. In the Th/Yb versus Nb/Yb diagram (Figure 11b), the diagonal mantle array is defined by the averages of N-MORB, E-MORB, and OIB, and the Zhuxi and Wushan lamprophyres containing subduction components are displaced toward higher Th/Yb values. The Nb/U ratios of Zhuxi lamprophyres range from 4.86 to 5.86, much lower than those in the MORB and OIB (47 ± 7; [46,47]), and the lower crust (Nb/U ≈ 25; [48]), but are similar to the pelagic sediments [49] (Figure 11c). Thus, the enriched lithospheric mantle of the Zhuxi lamprophyres is possibly modified by subducted sediments through melts or fluids. Yang and Jiang [50] proposed various origins for lamprophyres from the Jiurui district of Middle-Lower Yangtze River Belt, and suggested Nb/Ta ratios in all major silicate Earth reservoirs are Figure 10. (a) Rb/Sr vs. Ba/Rb diagram (see e.g., [37]); (b) plot of K/Yb/1000 vs. Dy/Yb for the Zhuxi lamprophyres (see Table 2), Wushan lamporphyres [25], and Quzhou and Longyou ultramafic intrusions [4]. Various lherzolite melting curves are from [38].

Mantle Metasomatism by Subducted Components
The trace element compositions (Figure 7; Figure 11) of Zhuxi lamprophyres show they are very different from the ocean-island basalts (OIB) and mid-ocean-ridge basalts (MORB), but are broadly similar to average continental crust [29] and global subducting sediments-II [28]. Different La/Yb ratios between them may be attributed to different degrees of partial melting. The enrichment of large-ion lithophile elements (LILE) and depletion of high-field-strength elements (HFSE) in primitive mantle normalized diagrams are generally considered as fingerprints of subduction processes (e.g., [42,43]), during which the HFSEs are stored in minerals, including rutile and ilmenite in the subducted slab, whereas the LILEs easily transfer into the fluids [44,45]. In the Th/Yb versus Nb/Yb diagram (Figure 11b), the diagonal mantle array is defined by the averages of N-MORB, E-MORB, and OIB, and the Zhuxi and Wushan lamprophyres containing subduction components are displaced toward higher Th/Yb values. The Nb/U ratios of Zhuxi lamprophyres range from 4.86 to 5.86, much lower than those in the MORB and OIB (47 ± 7; [46,47]), and the lower crust (Nb/U ≈ 25; [48]), but are similar to the pelagic sediments [49] (Figure 11c). Thus, the enriched lithospheric mantle of the Zhuxi lamprophyres is possibly modified by subducted sediments through melts or fluids. Yang and Jiang [50] proposed various origins for lamprophyres from the Jiurui district of Middle-Lower Yangtze River Belt, and suggested Nb/Ta ratios in all major silicate Earth reservoirs are subchondritic (chondritic Nb/Ta = 19.9; [51]) and the superchondritic Nb/Ta ratios occur in subduction-related melts rather than subduction-related fluids [52,53]. The Nb/Ta ratios of Zhuxi lamprophyres are lower than 18, indicating their mantle metasomatism is likely related to the subduction-related fluids. This conclusion is also supported by their (Ta/La) N [51]) and the superchondritic Nb/Ta ratios occur in subduction-related melts rather than subduction-related fluids [52,53]. The Nb/Ta ratios of Zhuxi lamprophyres are lower than 18, indicating their mantle metasomatism is likely related to the subduction-related fluids. This conclusion is also supported by their (Ta/La)N and (Hf/Sm)N ratios (Figure 11d)   Table 2).

Significance of Apatite Geochemistry
As discussed above, the Zhuxi lamprophyres could be a result of partial melting of the enriched lithospheric mantle that was metasomatized by the fluids released from subduction-related and pelagic-sediment-like materials. Subduction typically incorporates mobile elements into the lithospheric mantle [57,58]. This could explain the enrichment of LILE (Figure 8b) and Sr (ca. 5000-6000 ppm; Table 3) in the apatites of the Zhuxi lamprophyres. Reported Sr concentrations in basalt apatites are from 621-1066 ppm, and those in the granite apatites are much lower [59].
Volatile element (F, Cl, and S) content in mantle apatites are related to various subduction component sources. High F contents in basaltic magmas are derived from partial melting of F-rich minerals (F-phlogopite, fluoroapatite, and F-rich aragonite) in the source region. Cl and H2O are efficiently extracted from the slab, whereas F is largely returned to the deep mantle [60]. Chlorine contents in mantle-derived apatites are very heterogeneous. Metasomatic apatites are quite Cl-rich, whereas primitive apatites contain very little Cl [61]. Mantle-derived melt inclusions and volcanic glasses show the Cl concentration lower than 0.1 wt% [62,63]. Based on the Cl partition coefficient between apatite and basaltic melts of ~0.8 [64], the calculated Cl in mantle apatite is lower than 0.08 wt%. The relatively high Cl contents (0.37-0.57 wt%) of apatite in the Zhuxi lamprophyres are Nb diagram (data for each source member is from [46,49]); (d) (Ta/La) N vs. (Hf/Sm) N diagram ( [56]), for the Zhuxi lamprophyres (see Table 2).

Significance of Apatite Geochemistry
As discussed above, the Zhuxi lamprophyres could be a result of partial melting of the enriched lithospheric mantle that was metasomatized by the fluids released from subduction-related and pelagic-sediment-like materials. Subduction typically incorporates mobile elements into the lithospheric mantle [57,58]. This could explain the enrichment of LILE (Figure 8b) and Sr (ca. 5000-6000 ppm; Table 3) in the apatites of the Zhuxi lamprophyres. Reported Sr concentrations in basalt apatites are from 621-1066 ppm, and those in the granite apatites are much lower [59].
Volatile element (F, Cl, and S) content in mantle apatites are related to various subduction component sources. High F contents in basaltic magmas are derived from partial melting of F-rich minerals (F-phlogopite, fluoroapatite, and F-rich aragonite) in the source region. Cl and H 2 O are efficiently extracted from the slab, whereas F is largely returned to the deep mantle [60]. Chlorine contents in mantle-derived apatites are very heterogeneous. Metasomatic apatites are quite Cl-rich, whereas primitive apatites contain very little Cl [61]. Mantle-derived melt inclusions and volcanic glasses show the Cl concentration lower than 0.1 wt% [62,63]. Based on the Cl partition coefficient between apatite and basaltic melts of~0.8 [64], the calculated Cl in mantle apatite is lower than 0.08 wt%. The relatively high Cl contents (0.37-0.57 wt%) of apatite in the Zhuxi lamprophyres are attributed to the addition of Cl-rich brines released by the subduction slab into their source region [62,65,66]. Sulfur solubility in melts can be enhanced by increasing f O 2 , temperature, and pressure. Basaltic melts typically have higher S than felsic melts [67,68]. The apatites in the Zhuxi lamprophyres belongs to high-S apatite, based on the classification of [32], and the S most likely originated from pelagic-sediment-like materials as discussed above, since in the subduction setting the pelagic clays and cherts have the highest S (1000s of ppm), whereas volcaniclastics have very low S contents (100s ppm; [69]).

Timing of Metasomatic Enrichment and Implication for Metallogenic Setting
After a tectonic transition from the Tethys regime (continental collision between the South China and Indonesian blocks) to the Pacific regime (Paleo-Pacific plate subduction) during ca. 200-180 Ma in south China [70], voluminous Jurassic to Cretaceous intrusions and subordinate volcanic rocks formed in proposed tectonic settings, including subduction of the Paleo-Pacific plate [5], lower-crustal delamination [7], intraplate lithospheric rifting [6], and a mantle plume [71]. The most popular concept is that the South China Block was under an extensional setting since the Early Jurassic ( [8] and the references therein). The Zhuxi lamprophyres are coeval with the Taoxikeng lamprophyres in south Jiangxi [72] and formed at ca. 158 Ma. During this period, the voluminous W-Sn granites formed in the Nanling Range in south Jiangxi due to lithospheric thinning-extension and upwelling of asthenospheric mantle [2,3]. A contemporaneous (153-163 Ma) NNE-trending A-type granite belt in southeast Hunan and north Guangxi was suggested to have developed in an intra-arc rift or back-arc setting as a consequence of Paleo-Pacific plate slab roll-back [73]. This progressive slab rollback resulted in a coastward migration of magmatism accompanied by regional extension [74]. The Zhuxi lamprophyres, as well as the recently reported high-Mg andesites (ca. 159 Ma) at the north and south end of Jiangnan Orogen [10], support the conclusion that during the Late Jurassic to Early Cretaceous the Jiangnan Orogen was in an intra-arc rift or back-arc extension setting [75].
The Zhuxi lamprophyres were derived from the enriched mantle modified by fluid-related subduction metasomatism. Although some tectonic models suggested that the fluids were derived from the subducted Paleo-Pacific plate (e.g., [8]), it is questionable if the Paleo-Pacific plate subducted that far into the interior of the South China Block in Late Jurassic. [10] advocated that the enriched mantle below the Yangtze Block was related to an ancient metasomatism caused by amalgamation between the Yangtze Block and the Cathaysia Block (Jiangnan Orogen). This conclusion is also supported by [15] who proposed that the intrusion related to the giant Dexing porphyry copper deposit is a product of partial melting of the early Neoproterozoic relict island arcs that are Cu enriched. The Yangtze Block has a distinct enriched mantle from that of the Cathaysia Block [9]. Along the eastern border of the Yangtze Block (Jiangnan Orogen), the Late Jurassic Yangtun and Liuliang andesites as well as the Early Neoproterozoic subduction-related bonitite-series rocks [76], and high-Mg basalts [77,78] share a similar metasomatic source attributed to the Early Neoproterozoic subduction [10]. The Zhuxi lamprophyres have similar major element, trace element, and initial 87 Sr/ 86 Sr (0.7076-0.7078) compositions, indicating they are most likely derived from partial melting of the same lithospheric mantle metasomatized by fluids released from the Neoproterozoic-subducted sediments during the Jiangnan orogeny.
Although tungsten deposits are generally associated with granitoid magmas, mafic intrusions may also play a vital role in the formation of ore systems. For example, in the world-class Cantung W-Cu deposit where lamprophyres also occur, the intrusion of a hot mafic magma into a larger and cooler felsic magma chamber results in enriching the felsic melt in volatiles and metals [79]. Lamprophyres in the Zhuxi deposit formed earlier than the granites related to W mineralization; they could have substantially contributed volatiles to prolonged generation of granitic partial melts in the lower crust (i.e., [80]) and the large amount of CH 4 in fluid inclusions of the Zhuxi deposit reported by [81] could be a result of mantle outgassing [82]. This conclusion is supported by [83], who speculated that aqueous fluids at Maikhura could be entirely or in part supplied from crystallizing granitoid magma, whereas high-carbonic fluids are from a mafic magma source situated at a greater depth.

Conclusions
The enriched lithospheric mantle below the Zhuxi W-Cu skarn deposit was metasomatized by fluids (high Cl and oxidized S) exsolved from subduction sediments during the Neoproterozoic collision between Yangtze and Cathaysia blocks (Jiangnan orogeny). Late Jurassic crustal extension in this region caused upwelling of the asthenospheric mantle accompanied by heat advection. This process resulted in partial melting of the enriched lithospheric mantle and crustal basement that formed first the lamprophyres (ca. 157 Ma) and then the mineralization-related granites (148-153 Ma) in the Zhuxi deposit. The heat offered by mafic magmas may also enhance felsic magma buoyancy and possibly modify the thermal environment into which the felsic magmas are emplaced, which facilitates more efficient fractionation of the felsic magma.