J. Cent. South Univ. (2019) 26: 3488-3501

DOI: https://doi.org/10.1007/s11771-019-4268-3

Muscovite ⁴⁰Ar-³⁹Ar age and its geological significance in Zhuxi W(Cu) deposit, northeastern Jiangxi

OUYANG Yong-peng(欧阳永棚)¹, WEI Jin(魏锦)¹, LU Yi(卢弋)¹, ZHANG Wei(章伟)²,

YAO Zai-yu(尧在雨)¹, RAO Jian-feng(饶建锋)¹, CHEN Guo-hua(陈国华)¹, PAN Xiao-fei(潘小菲)³

1. No. 912 Geological Surveying Team, Jiangxi Bureau of Geology and Mineral Exploration and Development, Yingtan 335001, China;

2. Faculty of Earth Resources and Collaborative Innovation Center for Scarce and Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, China;

3. Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The Zhuxi W(Cu) skarn deposit, the world’s largest tungsten deposit is newly discovered in Jingdezhen city, northeastern Jiangxi province, China. It mainly occurs near the contact zone between the Yanshanian granites and the Late Paleozoic carbonate rocks. Three types of mineralization including skarn type, altered granite type and quartz vein-veinlet type orebodies have been observed. In this study, the ⁴⁰Ar-³⁹Ar age of hydrothermal muscovite coexisting with copper mineralization in the altered granite type orebody formed near the unconformity interface is determined by step-heating technology using CO₂ laser. The plateau age, isochron age, and inverse isochron age of muscovite are (147.39±0.94) Ma, (147.2±1.5) Ma, and (147.1±1.5) Ma, respectively. These ages are almost identical to the ages of ore-related granite and other mineralization types in the Zhuxi W(Cu) deposit, indicating that the Cu mineralizations occurred at the shallow depth and near the unconformity interface are contemporaneous during the Late Jurassic. This further suggested that the acompanied W and Cu mineralization in the Zhuxi W(Cu) deposit which may be controlled by the magma source is enriched in both W and Cu.

Key words: Muscovite; ⁴⁰Ar-³⁹Ar age; altered granite-type orebody; Zhuxi W(Cu) deposit; northeastern Jiangxi

Cite this article as: OUYANG Yong-peng, WEI Jin, LU Yi, ZHANG Wei, YAO Zai-yu, RAO Jian-feng, CHEN Guo-hua, PAN Xiao-fei. Muscovite ⁴⁰Ar-³⁹Ar age and its geological significance in Zhuxi W(Cu) deposit, northeastern Jiangxi [J]. Journal of Central South University, 2019, 26(12): 3488-3501. DOI: https://doi.org/10.1007/s11771- 019-4268-3.

1 Introduction

Zhuxi is a world-class tungsten-copper skarn deposit which is newly discovered in the northeastern region of Qinzhou Bay-Hangzhou Bay orogenic juncture belt. Its total resources are estimated to be 3.44 million tons of (333+334) WO₃ with an average grade of 0.5% and 112700 tons of Cu with average grade of 0.57%. Geological features [1-4], mineralogy of skarn [5, 6], geochronology of granites and mineralization [3, 7-15], geochemical characteristics of rocks and magmatism-hydrothermal processes [3, 11, 16-20] of the Zhuxi deposit have been studied. Beside the skarn and quartz vein-veinlet mineralization [10], altered granite type orebodies also have been observed in the Zhuxi deposit [19]. Copper and tungsten not only have different metal sources: W is enriched in fertile supracrustal material [21-22], while Cu is more likely derived from the mantle derived mafic magma [23]; but also have different geochemical characteristics during magma evolution: W is incompatible during the evolution of the magma at any oxygen fugacity [24] while a high magma oxygen fugacity is prerequisite for the enrichment of Cu during the magma evolution process [25, 26]. Thus, it is rare for the large scale of copper and tungsten mineralizations which occurred synchronously [27]. Although in situ LA-ICP-MS U–Pb dating of hydrothermal titanite suggested that mineralization of copper and tungsten occurred synchronously (about 150 Ma) in the Zhuxi deposit [28], both of these hydrothermal titanite co-crystallized with scheelite sampled at shallow depth and the copper mineralization of the Zhuxi deposit mainly occurred near the unconformity interface of the Neoproterozoic Wannian Group and the Carboniferous dolomite; additionally, the orebodies of the Zhuxi copper deposit which located closely to the Zhuxi W(Cu) deposit also occur near the unconformity interface, suggesting that copper mineralization near the unconformity interface is important in the area. Furthermore, SONG [15] reported two Zircon U–Pb ages of about 130 Ma for the biotite monzogranite near the unconformity interface. Therefore, there is urgent need for us to confirm the age of copper mineralization occurred near the unconformity interface, which is helpful for better understanding the mineralization process occurred in the Zhuxi W(Cu) deposit. In this study, the copper ore has been sampled from the altered granite which is closely to the unconformity interface to solve this scientific issue.

In the altered granite near the unconformity interface, some chalcopyrite, sphalerite, pyrite and scheelite closely grow along with the muscovite [10, 19]. Because muscovite is relatively stable relative to biotite and phlogopite, and has both high K content and good preservation of radioactive Ar [29], it is commonly used for ⁴⁰Ar-³⁹Ar dating [30-34]. Based on the systematic collection of previous data, detailed field observation and petrographic study, the hydrothermal muscovite closely related to copper mineralization occurred near the unconformity interface was seperated for ⁴⁰Ar-³⁹Ar dating in this paper. The analyzed result will provide precise mineralization geochronology data for the Cu mineralzation occurred near the unconformity interface which is helpful to better understand the mineralization process of the Zhuxi deposit.

2 Geological setting and ore deposit geology

2.1 Regional geology

The Qinzhou Bay-Hangzhou Bay orogenic juncture belt is an extremely complex super-deformed tectonic belt formed by collision between the Yangtze Block and the Cathaysia Block during the Neoproterozoic. It is also an important W-Cu-Au polymetallic metallogenic belt in South China [35] (Figure 1). The Zhuxi deposit is located at the eastern end of the Pingle Geotectogeneand northwestern side of the Northeastern Jiangxi Deep-Fault Zone (Figure 2(a)). It belongs to the Taqian-Fuchun Cu-Au polymetallic ore-forming prospecting area (Figure 2(b)). Since the Neoproterozoic, the area has experienced various stages of extension, compression, shear tectonic deformation, magmatism, metamorphism and sedimentation [1]. The geological conditions are favorable to form a series of W, Cu, Mo and Au polymetallic ore deposits [36-41].

The strata in the Taqian-Fuchun area have a typical binary texture, in which the basement is composed of Neoproterozoic Wannian Group which is mainly composed of a series of deep-sea basin turbidite sedimentary with small amount of submarine volcanic eruptions (Figure 2(b)). The Neoproterozoic rocks turned into green schist facies under later regional metamorphism. The cover consists of the Carboniferous to Triassic continental and marine transitional facies, the shallow sea carbonate platform carbonate and coal-bearing clastic rocks (Figure 2(b)).

Multiple thrusts occurred from northwest to southeast in the ore district [42], and the Neoproterozoic epimetamorphic rocks were thrusting over the Carboniferous-Triassic strata (Figures 2(b) and 3). The Carboniferous-Triassic strata are mainly composed of monoclinic structures extending to the northeast at 50°-55° and dipping to the northwest. The green epimetamorphic rocks of Wannian Group are dominated by tight folds to develop a grid-like shear fissure (Figure 3). Influenced by the regional northwest to southeast thrust nappe structure, the primary NE-oriented fault is formed, followed by the formation of secondary NNE, NW and near EW faults (Figure 2).

Figure 1 Tectonic sketch map showing spatial distribution of late Mesozoic magmatic rocks in SCC (modified from Refs. [39-41])

Figure 2 Sketch map of regional geology in Taqian-Fuchun ore-concentrated area (1-Quaternary; 2-Cretaceous; 3-Jurassic; 4-Triassic; 5-Permian; 6-Carboniferous; 7-Neoproterozoic; 8-Cretaceous granite fine-grained rock; 9-Cretaceous granodiorite porphyry; 10-Cretaceous gabbro; 11-Cretaceous gemstone; 12-Quartz vein; 13-Diorite; 14-Allgovite; 15-Geological boundary/Angle unconformity boundary; 16-Thrust nappe fault; 17-Measured (Inferred) fault; 18-Location of Zhuxi deposit)

Figure 3 Geological map of Zhuxi W(Cu) polymetallic deposit (1-Quaternary; 2- Upper Triassic Anyuan Formation; 3-Upper Permian Changxing Formation; 4-Upper Permian Leping Formation; 5-Middle Permian Mao Formation; 6-Upper Permian Qixia Formation; 7-Upper section of Upper Carboniferous Huanglong Formation; 8-Lower section of Upper Carboniferous Huanglong Formation; 9-Neoproterozoic Wannian Group; 10-Aplite; 11-Granite porphyry; 12-Diorite; 13-Lamprophyre; 14-Tremolite-Actinolite alteration zone; 15-Green alteration zone; 16-Thrust nappe fault; 17-Measured (Inferred) fault; 18-Drilling, exploration line position and its number)

Magmatic rocks are rare and only sporadic Yanshanian intermediate-acid magmatic rocks including granodiorite, biotite granite, two-mica granite, and granitic porphyry developed in this area. The intermediate-basic dykes including lamprophyre, gabbro, diabase and diabase- porphyrite are locally observed (Figures 2(b) and 3). The magmatic rocks are mainly restricted by NE-trending faults as dykes and stocks at the outcrops (Figures 2(b) and 3). In the deep underground, biotite granite stocks were drilled at Zhuxi and Yuexing deposit area.

2.2 Ore deposit geology

In the mining area, the strata include Neoproterozoic Wannian Group, Upper Carboniferous Huanglong Formation, Middle Permian Qixia and Maokou Formation, Upper Permian Leping and Changxing Formation, Upper Triassic Anyuan Formation and Quaternary (Figure 3). The Neoproterozoic Wannian Group is a set of pelitic-arenaceous epimetamorphic rock in the deep-sea basin. The lithology is dominated by phyllitic sandy slate, phyllite and metamorphic siltstone interlay with tuff. The contents of W, Cu, Pb, Zn and other metal elements in Wanian Group are relatively high, which is several times to several hundred times of Clark’s value [43]. The upper Huanglong Formation is composed of limestone- bearing carbonaceous shale, and the lower section is mainly limestone, dolomitic limestone and dolomite. The Qixia, Maokou, Leping, and Changxing Formations are mainly composed of shallow marine carbonate rocks, which are constructed by interacting with marine clastic rocks and coal. The Anyuan Formation is a set of marine-continental transitional coal-bearing clastic rocks. The ore-forming elements W, Cu and Zn in Huanglong, Qixia and Maokou Formation are several times to several tens of times of the crustal element abundance in eastern China [1-2, 44].

The felsic intrusive bodies in the mining area are mainly biotite granite, fine-grained altered muscovite granite, and granitic porphyry. The rocks are formed in the Yanshanian period, mainly intruded in the Wannian Group and the Carboniferous-Triassic strata [3, 17, 18]. The largest pluton is biotite granite which is intersected at the depth of 1600 m in the drill hole between prospecting line of 54-10. The altered fine-grained muscovite granite and granite porphyry mostly occurred as dykes along the interlaminar fracture zone. The lamprophyre, diorite and diorite porphyrite formed in the Yanshanian period [8] are mainly intruded in Carboniferous-Permian strata. In addition, the granodiorite porphyry near the exploration line 23 formed in Jinning period [9]. The Yanshanian biotite granite and fine-grained altered granite are most likely related to mineralization.

The Carboniferous-Triassic monoclinic structure in the mining area extends to northeast and dips to northwest with the angles of 22°-86°. The faults in the mining area are divided into four groups according to their strikes: NE, near EW, NNE, and NE (Figure 3). The NE-trending faults, F1 and F2, are most important ore-controlling faults (Figure 3). The F1 fault strikes 50°-60° to the northeast, and dips to the northwest with the angles of 60°-80°. The F2 fault generally strikes 45°-60° in the northeast. It is steep in the shallow part and tends to be gentle in the deep part (Figure 4). The rock in the fault is strongly folded and broken, and the detachment-expansion space is the main ore-concentrating site of the Zhuxi deposit, so that this fault zone might be the main conduits for ore-forming fluid migration [2, 44].

2.3 Orebody types

The orebodies in Zhuxi generally extend to NE and dip to NW. Current drilling program has domestrated that the size of these orebodies are 1200 m along its strike and 2000 m deep in the dip direction. According to the spatial distribution, wall rock type and morphology, the orebodies are subdivided into skarn type, altered granite type, and quartz vein-veinlet type. Vertically, the altered granite type orebodies occurred in the deepest biotite granite, the skarn type orebodies in the middle part where carbonate rocks altered into skarn, and the quartz vein-veinlet orebodies in the shallow weak-altered carbonate strata (Figure 4).

Skarn type orebodies are most important in the area. They are layered, lenticular produced above the F2 fault and among the Huanglong Formation carbonate rock. The largest orebody is distributed between prospecting line of 18-66 (Figures 3 and 4), extending 1200 m with relatively large dip angles (29°-77°) along its NE strike and tending to NW at the depth of 140-1918 m. The skarn type orebodies show scattered laminars in the shallow part and then merges into thick orebodies in the lower part. They have an average thickness of 146.66 m with average grade of 0.57% WO₃ and 0.57% Cu. Other vein-like and lenticular skarn orebodies controlled by fissure zones and magmatic rocks are mostly found in the Carboniferous Huanglong Formation, the Permian Maokou and Qixia Formation. The middle and deep parts are tungsten and copper mineralization, and the shallow part is copper-zinc (lead) mineralization.

Quartz vein-veinlet type orebodies mainly distribute between prospecting line of 30-54 (Figures 3 and 4). Most quartz vein-veinlet mainly occurs in the carbonate rocks of Maokou and Qixia Formation, and the epimetamorphic rocks of Wannian Group. The overall trend of the ore belt is NE extending 300 m, and dip to NW at depth of 106-446 m. The thickness of a single tungsten orebody is 1.52-4.84 m (the average of 2.81 m). The WO₃ grade is mainly between 0.125% and 0.329%, with the highest grade of 0.767% WO₃ and the average grade of 0.226% WO₃. The thickness of low grade copper orebody is about 5.34 m and the grade of Cu is 0.25%. Some quartz veins occur in the granites and are narrow with variable thickness and low grade mineralization. This ore belt is totally 600 m along the NE strike and dips to NW at depth of 139-510 m. Some single orebodies could be up to 1.70-6.24 m thick (average 3.60 m). Their WO₃ grades are 0.140%-0.298% with average grade of 0.191%.

Figure 4 Orebody characteristic map of line 54 in Zhuxi tungsten-copper deposit (1-Upper Permian Changxing Formation; 2-Upper Permian Leping Formation; 3-Middle Permian Maokou Formation; 4-Middle Permian Qixia Formation; 5-Upper section of Upper Carboniferous Huanglong Formation; 6-Lower section of Upper Carboniferous Huanglong Formation; 7- Neoproterozoic Wannian Group; 8-Sandstone; 9- Limestone; 10-Muddy limestone; 11-Carbonaceous limestone; 12-Dolomites; 13-Phyllite; 14-Granite; 15- Granite porphyry; 16-Geological boundary; 17-Parallel unconformity boundary; 18-Fault; 19-Tungsten orebody; 20-Tungsten-rich orebody; 21-Tungsten-copper orebody; 22-Copper orebody)

Altered granite type orebodies associated with sericitization and chloritization occur between prospecting line of 18-54 in the shape of veins or lenticulars in the altered granite. The orebodies extend along NE for about 600 m and dip to NW at depth of 225-1080 m. A single orebody could be 1.54-11.46 m thick (average of 4.47 m) with 0.125%-0.252% WO₃ (average grade of 0.164%) and 0.22%-0.53% Cu (average grade of 0.28%).

2.4 Ore characteristics

The metal minerals in the Zhuxi deposit are dominated by scheelite and chalcopyrite, with subordinate sphalerite, pyrite, galena, pyrrhotite, stibnite, molybdenite, bismuthinite, tennatite, and arsenopyrite. Non-metallic minerals mainly include quartz, feldspar, muscovite, sericite, garnet, diopside, wollastonite, tremolite, actinolite, epidote, chlorite, fluorite, followed by serpentine, biotite, tourmaline, talc, kaolin, calcite, with a small amount of apatite, zoisite, prehnite, idocrase, clinohumite, forsterite, realgar, and orpiment.

There are three main types of ore textures (crystalline, metasomatic, and exsolution) [5], and three main types of ore structures (fine-veins veinlets, disseminated and massive-agglomerate).

2.5 Alteration and mineralization stages

Intrusion of granites into different wallrock will cause different alteration and alteration zoning. When the granite intruded into the carbonate rock in the Zhuxi deposit, alterations including K-feldspar, greisen and skarn alteration occur from granite to the carbonate rock. When granite intruded within the Neoproterozoic epimetamorphic rocks, the hornfelsic alteration occurs [19].

OUYANG et al [19] divided the ore-forming processes of the Zhuxi deposit into skarn stage (prograde skarn stage, retrograde skarn stage), quartz-sulfide stage, carbonate stage, and surface oxidation period according to the mineral assemblages and paragenesis. In the prograde skarn stage, mineral assemblage mainly consists of garnet, diopside, wollastonite and scheelite; in the retrograde skarn stage, it mainly consists of tremolite, actinolite, chlorite, epidote and serpentine, which are accompanied by strong disseminated scheelite mineralization and a small amount of pyrrhotite and pyrite. To the quartz-sulfide stage, a large amount of quartz, and fluorite, chlorite, muscovite, calcite formed with large amount of chalcopyrite, sphalerite and pyrite and minor of galena, pyrrhotite, molybdenite and scheelite. The mineral assemblages in the carbonate stage contained calcite and chlorite, accompanied with tungsten-rich orebodies. Besides, a small amount of pyrite and chalcopyrite mineralization can also be locally observed.

3 Sample collection and analytical methods

3.1 Sample collection

The muscovite samples used for ⁴⁰Ar-³⁹Ar dating were collected from the altered granite type ores in the ZK5406 drill hole at depth of 2099 m in the Zhuxi tungsten-copper polymetallic deposit (Figure 4). The muscovite on the hand specimen is white-brown, with a radial and scaly structure, and diameter of 0.1-0.5 cm (Figure 5). The other accompanied gauge minerals in the sample are composed of quartz, sericite and chlorite. Some chalcopyrite, sphalerite, and arsenopyrite partially distributed in the gaps of the muscovite particles, showing a typical cogenetic relationship, indicating that the muscovite formed almost simultaneously with the copper mineralization (Figure 5). Additionally, the primary muscovite is totally absent according to the microscopic observation.

3.2 Analytical method

Based on field and microscopic observation, the selected sample was manually crushed, and the muscovite crystals were carefully selected under a binocular microscope. Pure muscovite crystals (purity>99%) were cleaned in the ultrasonic water, sealed in a quartz bottle and sent into a nuclear reactor for neutron irradiation. The irradiation work was carried out in the “pool pile” at the Ar–Ar laboratory in the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China, using the B4 channel, and the neutron flux density was about 2.60×10¹³ cm^-2 S^-1. The total irradiation time was 1444 min, and the integrated neutron flux was 2.25×10¹⁸ cm^-2. The standard used for monitoring neutron irradiation was ZBH-25 biotite standard with a standard age of (132.7±1.2) Ma, and K content is 7.6%.

Figure 5 Photograph of altered granite type copper orebody in Zhuxi tungsten-copper deposit:(Q-Quartz; Mus-Muscovite; Ser-Sericite; Kfs-K-feldspar; Chl-Chlorite; Cp-Chalcopyrite; Sp-Sphalerite; Apy-Aarsenopyrite)

Then the muscovite separates were step-heated within a graphite furnace, in which sample was heated for 30 min and purified for 30 min at every step. Mass spectrometry was performed on a multi-received rare gas mass spectrometer Helix MC with 20 sets of data collected per peak. All data were subjected to mass discrimination correction, atmospheric argon correction, blank correction, and interference element isotope correction after returning to the time zero value. The interference isotope correction coefficient generated during neutron irradiation is obtained by analyzing the irradiated K₂SO₄ and CaF₂, and its value is:(³⁶Ar/³⁷Ar_o)_Ca=0.0002389, (⁴⁰Ar/³⁹Ar)_K=0.004782, (³⁹Ar/³⁷Ar_o)_Ca=0.000806. ³⁷Ar is corrected for radioactive decay; ⁴⁰K decay constant λ=5.543×10^-10 a^-1 [45]; plateau age and isochronal and inverse isochronal lines are calculated using the ISOPLOT program [46]. The plateau age error is given as 2δ. Detailed experimental procedures can be found in related articles [47, 48].

4 Analytical results

The 14-stage heat release analysis of the muscovite sample ZK5406-2099 in the altered granite of the Zhuxi deposit was carried out in the range of 700-1400 °C (Table 1). The total age of muscovite is 146.7 Ma. In the middle and high temperature heat release stage (840-1160 °C), nine ages are close to each other, and the plateau age is (147.39±0.94) Ma (2δ) (Figure 6(a)), which corresponds to 94.8% of ³⁹Ar release. The corresponding ³⁹Ar/³⁶Ar-⁴⁰Ar/³⁶Ar isochronal age is (147.2±1.5) Ma (MSWD=3.7) and the initial value of ⁴⁰Ar/³⁶Ar is (292.6±8.7) (Figure 6(b)). The ³⁹Ar/⁴⁰Ar-³⁶Ar/⁴⁰Ar reverse isochronal age is (147.1±1.5) Ma (MSWD=18) and the initial value of ⁴⁰Ar/³⁶Ar is (294±14) (Figure 6(c)).

5 Discussion

5.1 Mineralization age of Zhuxi deposit

⁴⁰Ar-³⁹Ar dating of potassium-bearing minerals from ore is an important method to determine the ore-forming age of hydrothermal deposits [49-55]. The total age of muscovite (146.7 Ma), plateau age (147.39±0.94 Ma), isochronal age (147.2±1.5 Ma), and reverse isochronal age (147.1±1.5 Ma) are obtained in this study. A systematic study on the fluid inclusions of the Zhuxi deposit shows that the homogenization temperatures of the primary fluid inclusions in quartz in the altered granite type orebodies range from 125 to 265 °C [19], which is lower than the closed temperature (350-400 °C) of muscovite used for ⁴⁰Ar-³⁹Ar dating [56, 57]. This indicates that the muscovite in the Zhuxi deposit did not capture excess Ar during cooling stage or later alteration. Moreover, the ⁴⁰Ar/³⁹Ar initial values of isochronal and inverse isochronal line are (292.6±8.7) and (294±14), respectively, which are close to the ideal atmospheric argon ratio (295.5) within the error range, further indicating that the muscovite did not capture excess argon. It means that the K and Ar in muscovite have maintained a closed system and been not affected by late hydrothermal events after crystallization, its ⁴⁰Ar-³⁹Ar dating results are reliable, and the plateau age of muscovite ((147.39±0.94) Ma) can represent its crystallization age. The potassium-bearing muscovite used for ⁴⁰Ar-³⁹Ar dating in this work is closely cogenetic with chalcopyrite-sphalerite (Figure 5), indicating that muscovite and chalcopyrite-sphalerite are the products formed during the same mineralization alteration stage. Therefore, all the ⁴⁰Ar-³⁹Ar ages (about 147 Ma) being consistent with each other within the error range represent the formation age of the chalcopyrite and sphalerite, which form in altered granite type orebodies.

Table 1 ⁴⁰Ar/³⁹Ar step wise heating dating data of sericites from muscovite of altered granite type orebody from Zhuxi tungsten-copper deposit

Figure 6 ⁴⁰Ar-³⁹Ar spectrum age (a), isochronal age (b) and inverse isochronous age diagram (c) of muscovite in Zhuxi tungsten-copper deposit

Large amount of previous geochronology studies have been performed on the granites and mineralization in the Zhuxi deposit. LI et al [7] obtained two granite porphyry zircon U-Pb ages of (150.6±1.9) Ma and (149.5±1.9) Ma. PAN et al [18] and SONG et al [14] obtained the zircon U-Pb ages of granite porphyry are (150±1) Ma and (148.3±1.4) Ma, respectively. The reported zircon U-Pb ages of the altered muscovite granites are (146.90±0.97) Ma [17], (149.2±1.5) Ma [3] and (149±1) Ma [14]. HE et al [13], SONG et al [14] and PAN et al [18] obtained zircon U-Pb ages of biotite granites are (147.7±2.2) Ma, (149.38±0.86) Ma, and (153.5±1) Ma, respectively. All the results indicate that the magmatic rocks in the Zhuxi mining area are mainly formed at (146.90±0.97) Ma-(153.5±1) Ma. Additionally, LIU et al [58] obtained an scheelite Sm-Nd isochron age of (144±5) Ma; PAN et al [10] obtained two muscovite ⁴⁰Ar-³⁹Ar plateau ages of (150.04±0.94) Ma and (150.24±0.94) Ma for the muscovite–quartz– calcite–scheelite vein which formed at shallow depth (885 m and 1161 m, respectively), and a Re-Os age of (145.1±1.5) Ma for molybdenite coexisting with scheelite; YU [12] obtained a U-Pb age of hydrothermal titanite from ore sample is (153±3) Ma; SONG [15] obtained ages of hydrothermal titanite are (148.1±7.4) Ma, (148.9±1.5) Ma, and (149.9±1.3) Ma for retrograde-altered exoskarn, Cu ore formed at shallow depth, and W ore, respectively. These ages show that the skarn type orebodies were mainly formed at (144±5)-(153±3) Ma.

The hydrothermal alteration ages ((147.39± 0.94) Ma) for the altered granite where Cu orebodies formed near the unconformity interface are not the same as those alteration caused by the early Cretaceous (about 130 Ma) magmatism which is more close to the copper orebodies spatially [15]. Nevertheless, this age is consistent with most of the results obtained by previous studies [3, 10, 14, 15, 18], suggesting that both the Cu orebody at the shallow depth or near the unconformity interface formed synchronously. This indicates that the W and Cu were most likely originated from the same hydrothermal fluids.

5.2 Possible ore-forming process

The Neoproterozoic Wannian Group is composed of pelitic-arenaceous turbidite interbedded with metamorphosed basaltic-andesitic rock and rich in W and Cu. The present δ³⁴S values of chalcopyrite, pyrite, and sphalerite range from 2.2‰ to 4.3‰ [2, 59], which is close to the majority of magmatic hydrothermal deposits (3‰-1‰) [60]. And the Late Paleozoic Carbonate rocks also contain high concentrations of W, Cu and Zn [2, 19]. Moreover, these data are similar to those obtained from the regional Wannian Group (3.1‰-6.0‰) [61-63], the adjacent Dexing deposit (-4‰-3.1‰) [64, 65], and from primitive mantle (nearly zero) [66]. This suggests that sulfur in the Zhuxi deposit was derived from local magmatic and sedimentary sources. Pb isotopic compositions of the sulfides, being plotted between the mineralization-related magmatic (i.e., granite) and the wall rocks (Wannian metasedimentary) probably suggests that metal elements such as Cu, Pb and Zn, due to their close geochemical association, associate with these two sources (granites and Wannian metasedimentary) [59]. The consistent ages of mineralization of altered granite, skarn and quartz vein-veinlet types with granitic rock further presented the relationship between mineralization with the magmatism.

Additionally, other melt sources have been suggested by previous studies. For example, CHEN et al [3] proposed that the Late Paleozoic Carbonate rocks also contain high concentrations of W, Cu, and Zn, which may have provided the metal source for the Zhuxi tungsten-copper polymetallic deposit. Meanwhile, the quartz vein-veinlet orebodies were formed in the fracture zone or interlaminar fracture in the Carboniferous-Permian carbonate strata [19]. These strata also have been considered to possibly provide metal source for the Zhuxi tungsten-copper polymetallic deposit [2]. Furthermore, the ore-related granites were formed by partial melting of the Wannian Group [3, 14, 18], which is riched in both copper (38.1×10^-6 [67]) and tungsten (11.82×10^-6 [68])). Therefore, we favor that both the tungsten and copper of the Zhuxi deposit were sourced from the ore-related granites which were formed by partial melting of the Wannian Group.

When the granitic magma and high- temperature hydrothermal fluids carrying tungsten- copper elements intruded along the NE-trending fault zone, it reacted with the Late Paleozoic carbonate rocks to form skarn, skarnized marble and skarnized dolomitic marble and is accompanied by strong tungsten-copper polymetallic mineralization.

6 Conclusions

1) The plateau age of hydrothermal muscovite in the Cu ore formed near the unconformity interface of the Zhuxi W (Cu) deposit is (147.39±0.94) Ma and the isochronal age is (147.2±1.5) Ma. The inverse isochronal age is (147.1±1.5) Ma, which is almost identical with the granite and metallogenic ages determined by other methods, indicating that both the Cu mineralization occurred at the shallow depth or near the unconformity interface synchronous in the Late Jurassic.

2) The synchronous W and Cu mineralization in the Zhuxi W(Cu) deposit may be dominantly controlled by the magma source which enrich both W and Cu.

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(Edited by FANG Jing-hua)

中文导读

赣东北朱溪钨(铜)矿床白云母⁴⁰Ar–³⁹Ar定年及其地质意义

摘要：景德镇朱溪钨铜多金属矿床是近年在赣东北地区新发现的世界最大钨矿床，该矿床主要产于燕山期酸性岩体与晚古生代碳酸盐岩接触带附近，发育有矽卡岩型、蚀变花岗岩型和石英细脉–网脉型矿体。本文利用激光阶段加热技术对不整合接触界面附近蚀变花岗岩型矿体中与铜关系密切的蚀变矿物白云母进行⁴⁰Ar–³⁹Ar年龄测定，获得矿床成矿年，白云母坪年龄为(147.39±0.94) Ma、等时线年龄为(147.2±1.5) Ma、反等时线年龄为(147.1±1.5) Ma，这与前人用其他方法测定的花岗岩成岩和成矿年龄近乎一致，表明浅部或不整合接触界面附近的铜矿均形成于晚侏罗世，同时进一步表明朱溪钨(铜)矿床的形成可能受富含钨铜的岩浆岩控制。

关键词：白云母；⁴⁰Ar–³⁹Ar年龄；蚀变花岗岩型矿体；朱溪钨(铜)矿床；赣东北

Foundation item: Project(41873059) supported by the National Natural Science Foundation of China; Project(JGMEDB [2017]78) supported by the Jiangxi Geological and Mineral Exploration and Development Bureau Foundation, China; Project(2011BAB04B02) supported by the National Science and Technology Support Plan Project, China; Project(201411035) supported by the Welfare Research Program of Ministry of Land and Resources, China; Project(20150013) supported by Jiangxi Provincial Geological Exploration Fund Management Center, China

Received date: 2019-01-08; Accepted date: 2019-03-11

Corresponding author: PAN Xiao-fei, PhD, Assistant Researcher; Tel: +86-13693394272; E-mail: Pan_smile0551@sina.com; ORCID: 0000-0002-2541-861X