Journal of Earth Science  2019, Vol. 30 Issue (1): 52-69   PDF    
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Piaoac Granites Related W-Sn Mineralization, Northern Vietnam: Evidences from Geochemistry, Zircon Geochronology and Hf Isotopes
Tuan Anh Nguyen1,2, Xiaoyong Yang1, Hien Vu Thi3, Lei Liu1, Insung Lee4    
1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;
2. Institute of Geological Sciences, Vietnam Academy of Science and Technology(VAST), Hanoi, Vietnam;
3. Department of Geology, Hanoi University of Mining and Geology, Hanoi, Vietnam;
4. School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea
ABSTRACT: Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic compositions of the W-Sn-bearing granites from the Piaoac District have been investigated in detail. LA-ICP-MS U-Pb dating of zircon grains from these granites yielded ages of 82.5±2.3 and 82±1.8 Ma, representing an episode of Late Cretaceous magmatic event. These granites are characterized by high peraluminous and have typical S-type geochemical signatures with high SiO2 (72.37 wt.%-73.07 wt.%), high A/CNK values (1.61-1.65) and Al2O3 (14.4 wt.%-15 wt.%). They are enriched in Rb, U, K, Th, Ta and Pb and display pronounced negative Ba, Sr, Nb, Ti and Eu (Eu/Eu*=0.19-0.24) anomalies. The high degree of fractional crystallization is characterized by low Rb, Sr, Ba and Eu concentrations with high ratios of La/Sm and Eu/Eu*. Zircon grains show εHf(t) values from -9.69 to -0.9 and the corresponding TDM2 range from 1.2 to 1.7 Ga, indicating that these granites could be derived from the Proterozoic basement rocks with minor input from mantle-derived magmas. The calculation of Fe3+ and Fe2+ of biotites indicates a low oxygen fugacity condition (log fO2 ranging from 10-17 to 10-18 bars, below MH), which is favorable for the W-Sn mineralization. Tungsten and tin have been enriched in granitic magmas through fractionation, and low oxygen fugacity conditions have promoted the accumulation and transportation of W-Sn in the hydrothermal fluids, leading to deposition of mineral phases. The geochemical data suggest that Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.
KEY WORDS: geochemistry    zircon U-Pb age    Hf isotope    Piaoac granite    W-Sn mineralization    Northern Vietnam    

0 INTRODUCTION

Late Cretaceous granites of South China Block (SCB) play a major role in the evolution of Eurasia continental crust and in the concentration of economic quantities of important materials especially W-Sn polymetallic deposits such as Gejiu, Bozhushan and Dulong (Liu et al., 2018; Wang et al., 2018; Cheng et al., 2016, 2013, 2012; Xu et al., 2015). In addition, several W-Sn deposits have been discovered in northeastern Vietnam, which are likely formed in similar tectonic setting in relation to Late Cretaceous Sn-polymetallic deposits in SCB (Zhao et al., 2018; Cheng et al., 2016; Romer and Kroner, 2016; Morley, 2012; Sanematsu and Ishihara, 2011). The tungsten-tin deposits in North Vietnam are mainly related with the Piaoac granite complex. The Piaoac complex (Izokh et al., 1965) includes some small tungsten-tin bearing intrusions of constituted by two mica granite, such as Piaoac, Thienke and Dalien massifs (Tri and Khuc, 2011), which have been determined Cretaceous (Chen et al., 2014; Roger et al., 2012; Wang et al., 2011; Anh et al., 2010). Recently, the Dalien deposit has been reported that ore reserves are approximately 87 900 000 tons with 0.19 wt.% WO3, 7.95 wt.% CaF2, 0.18 wt.% Cu, 0.09 wt.% Bi and 0.19 g/t Au (Richards et al., 2003). In addition, Thenke massif is related to tungsten-tin deposit, which has also been exploited.

The Piaoac massif is situated in Cao Bang Province of northern Vietnam (NV), hosted by the Devonian–Triassic sedimentary and volcanic rocks of the Miale and Songhien groups. The pluton is enriched in rare metal such as tungsten, tin, molybdenum and are termed as the W-Sn bearing granite with a close relatinship of large euhedral crystals (e.g., cassiterite, wolframite, molybdenite, fluorite) (Vladimirov et al., 2012; Anh et al., 2010; Ben et al., 1993). However, there are only a few studies on tin mineralization associated with granitic intrusions, which have documented the geochronology, and tectonic setting of these granites. Several important issues regarding the ore genesis, such as emplacement time of the granitic intrusion and tungsten mineralization, the source of ore-forming fluids, the genetic relationship between the granite and tungsten-tin mineralization, are still controversial.

In this paper, we present new geological, geochemical and Hf isotopic data, in order to shed new light on the origin of the tungsten-tin mineralization and related granites in the Cao Bang area, northern Vietnam. And through combining with published of Late Cretaceous magmatic data from southwestern China Block, we suggest tectonic setting related magmatism and mineralization in the area.

1 GEOLOGICAL SETTING 1.1 Regional Geology

The geological evolution of Asia is well acknowledged beginning in the Precambrian and continues today. Asia is composed of a series of continental blocks includes large continents such as Siberia, northern China, southern China, Indochina, India, and several microcontinents: Lhasa, Qiangtang, Qaidam, etc. (e.g., Metcalfe, 2011, 2006; Golonka et al., 2006) (Fig. 1). Vietnam is located in the southeastern Asia, constructed by Indochina Block in the south-west and North Vietnam-South China Block (NV-SCB) in north-east (Faure et al., 2014; Zhang et al., 2013; Hoa et al., 2008). The SCB includes the Yangtze Block in the northwest and Cathaysia Block in the southeast (Fig. 1a). The collision between SCB and the Indochina Block occurred during the Middle Paleozoic time, and formed the Ailao Shan and Song Ma sutures (e.g., Thanh et al., 2014, 2011; Tri and Khuc, 2011; Wang et al., 2000; Tri, 1979).

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Figure 1. (a) Simplified geological map of eastern Eurasia, showing major tectonic units (modified from Roger et al., 2000). (b) Schematic geological map of the southwest part of South China Block and Northeast Vietnam (modified from Zhao et al., 2018; Cheng et al., 2016, 2013; Mao et al., 2013). (c) Geological map of the Piaoac pluton (after Dung et al., 2013). The Devonian Miale Formation is distributed in western region, separated by submeridian faults. The Miale Formation is covered untratigraphy by tuffaceous conglomerate, gritstone, conglomerate, and rhyolite of the Early Triassic Song Hien Formation. The NW-SE and subparallel trending form in the same period with Piaoac granite. They are not only magma conduits but separate substantially Devonian carbonate sequences with the granite intrusion. The NE-SW fault occurred latest in the region, cutting almost all geological units

The study area is located in the NV-SCB (Fig. 1a), the striking Red River fault is in the southern boundary of the study area. This fault is the major structure of NV extending several hundreds of kilometers from Tethys to Tonkin (Tapponnier et al., 1990). The striking was formed during Cenozoic extrusion of Sundaland due to Indian collision events (Phan et al., 2012; Leloup et al., 1995). To the north, the study area concerns Guangxi and Yunnan provinces of China, where the stratigraphy is known as Youjiang Basin (Nanpanjiang Basin) (Cheng et al., 2013; Mao et al., 2013; Cheng and Mao, 2010; Galfetti et al., 2008). In ascending stratigraphic order, five lithological units have been recognized (Cheng et al., 2016; Qiu et al., 2016; Chen et al., 2014): Proterozoic strata are distributed in the northwestern part of the region, including metamorphic rocks and minor unmetamorphosed sediments, which is namely the Kangdian massif; Lower Paleozoic rocks consist of Cambrian to Silurian terrigenous-carbonate and clastic sedimentary rocks (Lepvrier et al., 2011); Upper Paleozoic rocks are consisting of limestone, siliceous limestone, and terrigenous rocks; Triassic rocks consist of conglomerates, sandstones, tuffaceous sandstones, siltstones, shales and rhyolite; and Cenozoic sedimentary deposits occur in the northwestern part and downstream Red River (Fig. 1b). Granitic plutons occur rarely in study area, intruded into the sedimentary succession. Paleozoic granites have been known as Dulong-Song- Chay massif, including granitic and orthogneissic series: migmatites, porphyroid granites and augen-gneisses, extending towards the North Vietnam. Permian to Triassic plutons include in the Phia Bioc pluton, Pia Ya pluton and Co Linh pluton, which are exposed in Vietnam. Cretaceous igneous rocks are widespread in the region, including leucogranite of Piaoac pluton in Cao Bang Province; muscovite granite, muscovite-biotite granite of Thienke and Dalien pluton in Thai Nguyen Province, in NV; porphyritic granites, equigranular granite in the Gejiu, Bozhushan and Dulong District, in SCB. These granites are known to be related with Sn-W polymetallic deposits (Zhao et al., 2018; Xu et al., 2015; Cheng et al., 2013; Vladimirov et al., 2012; Wang et al., 2011).

1.2 Ore Deposit Geology of Piaoac District

Occurrences of Piaoac W-Sn ore deposits have been developed in the Piaoac Mountain, Cao Bang Province, mainly around the small Late Cretaceous granite outcrop. The Piaoac granitic massif has an ellipse shape ~20 km2 in total area, consisting mainly of porphyritic muscovite-biotite leucogranite and coarse muscovite granite (Fig. 1c). The western pluton is Devonian Miale Formation mainly with clastic rocks, clay shale, sandstone, limestone and minor calcareous shale. The total thickness of Miale strata in the region is approximately 450–500 m. The eastern Piaoac massif is volcanogenic and terrigenous sedimentary rocks, which makes up the Early Triassic Song Hien Formation including tuffaceous conglomerate, gritstone, conglomerate, rhyolite with about in 700–850 m thickness. The fault structures comprise three groups, i.e., (1) NW-SE faults, which separate substantially Devonian carbonate sequences from the granite intrusion; (2) NE-SW faults, which are in the outer contact zone between granite and Triassic (Vladimirov et al., 2012; Dovjikov et al., 1965); (3) nearly EW-trending faults show Triassic sequences unconformably with Devonian carbonate sequences.

Petrographical study shows that the granites contain plagioclase (20%–25%), alkali feldspar (25%–30%), quartz (30%–35%), biotite (1%–5%), muscovite (5%–8%), with porphyritic texture (Fig. 2). The Piaoac granites are associated with greisen alteration, pegmatite veins and sulfide mineralization. The quartz-muscovite greisenization is distributed in roof of pluton, having an approximate orientation subparallel orientation 70 –80 SW dip. The greisen alteration has common minerals, i.e., cassiterite, wolframite, topaz, rutile and tourmaline as known Santa Aleksandra Sn-W; Lung Muoi Sn-W deposits (Fig. 3). The Cao Nhon F-Be deposit related pegmatite veins occurs in the northwest granite massif. Sulfide mineralization is developed along fault in Tai Soong, which contains chalcopyrite, molybdenite and sphalerite. The mineral paragenesis in the Piaoac ore field is well described by Hien (2012) (Fig. 4).

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Figure 2. Outcrops (a), (b) and micrographs of characteristic textures and mineral assemblage of the rhyolite (c) and Piaoac massif (d), (e), (f), (g), (h). Bt. Biotite; Kfs. potassium feldspar; Mus. muscovite; Pl. plagioclase; Q. quartz; Cl. chlorite
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Figure 3. Back scattered electron images of tungsten-tin ores in the Piaoac pluton. (a), (b), (c) and (g) mineralization in greisen alteration; (d), (e), (f) and (h) mineralization in quartz vein. Fk. Feldspar kali; Py. pyrite; Wof. wolframite; Ccp. chalcopyrite; Mo. molybdenite; Cst. cassiterite; Sph. sphalerite
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Figure 4. Paragenetic sequences for the Piaoac tungsten-tin ore field (modified from Hien et al., 2012)
2 ANALYTICAL METHODS 2.1 Zircon U-Pb Dating

Individual zircon grains for U-Pb isotope analysis were separated from samples POVN-17-05-2 and POVN-17-02 using conventional magnetic and heavy liquid separation methods followed by hand-picking under a binocular microscope. The zircon grains were separated and mounted in epoxy resin, and then polished to section the crystals in about half their thickness. Cathodoluminescence (CL) images of the zircons were taken using electron microprobe JEOL JXA-8100 at CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China (USTC), Hefei.

The laser ablation ICP-MS (LA-ICP-MS) zircon U-Pb analyses were carried out at the same lab. The GeoLas 200M laser-ablation system equipped with a 193 nm ArFexcimer laser was used in connection with ELAN6100 DRC ICP-MS. All measurements were performed using zircon 91500 as the external standard with a recommended 206Pb/238U age of 1 065.4± 0.6 Ma (Wiedenbeck et al., 1995). ICPMSDataCal software was used for quantitative calibration zircon U-Pb dating (Liu Y S et al., 2010, 2008). Concordia diagrams and weighted mean calculations were made using Isoplot/Exver 3.0 (Ludwig, 2003).

2.2 Major and Trace Element Analysis

Major elements and trace elements (including rare earth elements) were analyzed in the ALS Mineral Lab in Guangzhou. After fresh samples were crushed and made to the powdered with grain size less than 200 meshes, a calcined or ignited sample (0.9 g) was added with lithium borate flux (~9.0 g, 50% Li2B4O7-LiBO2), mixed well and fused in an auto fluxer between 1 050 and 1 100 ℃ and then cooled to form a flat molten glass disk. This disk was then analyzed by ME-XRF-06 for major element analyses. Trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS) of solutions on an Elan DRC-II instrument (element, finnigan MAT). Whole-rock powders (50 mg) were dissolved in closed beakers, after 2-day closed beaker digestion using a mixture of HF and HNO3 acids in Teflon screw-cap bombs. Accuracy and precision of the data are better than 5% for trace elements and replicate analyses of international standard reference material (SRM) (Liu et al., 1996).

2.3 Mineral Chemistry Analysis

All elemental analyses of biotite and muscovite were obtained from polished thin sections using a wavelength dispersive JEPL JXA-8800R electron microprobe analyses (EMPA) at the CAS Key Laboratory of Crust-Mantle Materials and Environments, the University of Science and Technology of China (USTC), Hefei. The analysis was taken with an accelerating voltage of 15 kV, a low beam current (10 nA), and a defocused beam (1 μm). The calibration was based on a suite of mineral standards and oxide standards from the American Standard Committee.

2.4 In situ Zircon Hf Isotope Analysis

After zircon U-Pb isotope measurement, the in situ analyses of Lu-Hf isotopes were conducted on a Nu Plasma HR multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), equipped with a GeoLas2005 193 nm excimer ArF laser-ablation system, at the State Key Laboratory of Continental Dynamics in Northwest University, Xi'an, China.

The diameter of the beam was set at 44 μm, and the repetition rate is set at 10Hz with a laser power of 100 mJ. The 176Lu was calibrated using the 175Lu value, while 176Yb/172Yb ratio of 0.588 7 and mean βYb value obtained on the same spot were employed for the interference correction of 176Yb on 176Hf. Zircon standard GJ-1, MON-1 and 91500 were measured with the unknowns in order to evaluate the accuracy of the analytical data. A decay constant for 176Lu of 1.865×10-11 a-1 (Scherer et al., 2001), and the present-day chondritic ratios of 176Hf/177Hf= 0.282 772 and 176Lu/177Hf=0.033 2 (Blichert-Toft and Albarède, 1997) were adopted to calculate εHf(t) values. 176Hf/177Hf of 0.283 25 (Nowell et al., 1998) and 176Lu/177Hf of 0.038 4 (Griffin et al., 2000) were used to calculate single-stage Hf model ages (TDM1). The 'crustal' model age (TDM2) was calculated assuming that the parental magma was produced from average continental crust (176Lu/177Hf=0.015) that originally was derived from the depleted mantle (Griffin et al., 2002). Details of the analytical technique were published by Wu et al. (2006).

3 RESULTS 3.1 Zircon U-Pb Geochronology

Zircon grains selected from samples POVN-17-05-2 and POVN-17-02 are mostly euhedral to subhedral, colorless and prismatic, 50–180 μm in length, and have length/width ratios between 1 : 1 and 3 : 1. Cathodoluminescence images show that representative zircons grains contain unzoned or weakly zoned interior rimmed by clear oscillatory zoning belts which indicates a magmatic origin. The Th/U ratios of all samples range from 0.07 to 3.16, with most of them >0.1, indicating the zircons are magmatic origin (Hoskin and Black, 2000).

The U-Pb results are presented in Table S1. The weighted mean ages for POVN17-05-2 and POVN17-02 samples are 82.5±2.3 Ma (MSWD=4.6, n=16) and 82±1.8 Ma (MSWD=2.0, n=12), respectively (Fig. 5).

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Figure 5. U-Pb Concordia and weighted mean ages for the two samples from the Piaoac granites. (a) Sample POVN17-05-2; (b) sample POVN17-02. Small rectangles indicate the data used to calculate the weighted average ages.
3.2 Major and Trace Elements

Results of major and trace elements for the Piaoac magmatic rocks are listed in Table S2. The Piaoac samples show high contents of SiO2 (72.37 wt.%–73.07 wt.%), high total alkalis (Na2O+K2O) (8.11 wt.%–8.74 wt.%), and Al2O3 (14.4 wt.%–15.01 wt.%). These granites have A/CNK from 1.61 to 1.65, data are plotted in granite classifications (Fig. 6a), showing in peraluminous features (Fig. 6b). The Piaoac samples display high K-calc alkaline series (Fig. 6c). On the Harker diagrams (Fig. 7), the contents of Al2O3, Na2O, MgO, TiO2, CaO and K2O show negative correlations with SiO2, whereas P2O5 has no correlation, implying fractional crystallization during the magma evolution.

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Figure 6. Classification diagram of south China block and northeast Vietnam intrusive rocks. (a) (K2O+Na2O) vs. SiO2 (Middlemost, 1994); (b) A/NK vs. A/CNK (Maniar and Piccoli, 1989); (c) K2O-SiO2 diagram (after Peccerillo and Taylor, 1976). WVI. Literature data of Piaoac rocks from Wang et al. (2011), Vladimirov et al. (2012); Dulong. literature data of Dulong rocks from Xu et al. (2015)
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Figure 7. Harker diagrams of the Piaoac granites

The granites have similar chondrite-normalized REE patterns with Upper continental crust, displaying a fractionated REE patterns ((La/Yb)N=16.6–20.3; (Gd/Yb)N=3.16–4.0) with negative Eu anomalies (Eu*=0.19–0.24) (Fig. 8a). On the primitive mantle-normalized trace-element diagram (Fig. 8b), the Piaoac granites show pronounced positive Rb, U, Ta, Pb, P, Gd anomalies and prominent negative Ba, Nb, La, Ce, Zr, Eu and Ti anomalies.

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Figure 8. Diagrams of REEs and trace elements of the Piaoac granites. (a) Chondrite normalized REEs; (b) primitive mantle normalized trace elements distribution patterns. Chondrite and primitive mantle-normalized data are taken from Sun and McDonough (1989). WVI. literature data of Piaoac rocks from Wang et al. (2011), Vladimirov et al. (2012). Dulong. literature data of Dulong rocks from Xu et al. (2015)
3.3 Mineral Chemistry

Major and minor elements of the biotite and muscovite from the granites are given in Tables S3 and S4. Muscovites are common in all thin sections of granite with medium to large grains, having compositions of 0.19 wt.%–0.58 wt.% TiO2, 0.73 wt.%–1.4 wt.% MgO and 0.06 wt.%–0.89 wt.% Na2O. Muscovites chemistry almost plots in the primary muscovite field, with two analyses falling in secondary field (Miller et al., 1981) (Fig. 9a). Biotite in the Piaoac granites shows high contents of Al2O3 (19.4 wt.%–20.2 wt.%), FeO (total) (21.3 wt.%–22.7 wt.%) and F (1.44 wt.%–2.73 wt.%), low MgO (2.65 wt.%–4.16 wt.%) and MnO (0.28 wt.%–0.63 wt.%). All biotite of the granites falls in the primary biotite field of the 10×TiO2-(FeO+MnO)-MgO diagram (Fig. 9b) (Nachit et al., 2005). In the trioctahedral diagram (Fig. 9c) (Foster, 1960), biotites plot in the siderophyllite field, and muscovites fall in the normal muscovite field.

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Figure 9. Classification mica diagram of the Piaoac granites. (a) Ternary Mg-Ti-Na diagram for muscovite with primary and secondary fields from Miller et al. (1981); (b) 10×TiO2-(FeO+MnO)-MgO diagram of biotites (after Nachit et al., 2005); (c) the Mg-(AlVI+Fe3++Ti)-(Fe2++Mn) ternary diagram (after Foster, 1960)
3.4 Zircon Hf Isotope

Results of Lu-Hf isotopic analyses of samples POVN17-5-2 and POVN17-02 are given in Table S5. The 176Hf/177Hf isotopic ratios of 12 zircon grains from the POVN17-05-2 range between 0.282 449 and 0.282 698, corresponding to εHf(t) values of -9.7– -0.9 and crustal model ages (TDM2) range from 1 208 to 1 767 Ma, when calculated back to 82 Ma. Five zircons from sample POVN17-02 were calculated back to crystallization age of 82 Ma, having initial Hf ratios of 0.28 251–0.282 698, corresponding to εHf(t) values of -7.5– -0.9 and TDM2 ages of 1 209–1 630 Ma, respectively.

4 DISCUSSION 4.1 Petrogenesis of the Piaoac Granites 4.1.1 Genetic type

Granites are classified into four main types: I, S, M and A types according to their geochemical signatures and tectonic settings. S- and I-type granites were proposed by Chappell and White (1974) on studies of the granites from the Lachlan Fold Belt (Australia). The main signatures of S-type granites are an alumina saturation index (A/CNK) >1.1, having low sodium (Na2O < 3.2 wt.% with about 5 wt.% K2O and Na2O < 2.2 wt.% with about 2 wt.% K2O). I-type granites are characterized by (A/CNK) < 1.1, having higher Na2O contents (normally > 3.2 wt.%). A-type granites were firstly mentioned by Loiselle and Wones (1979), these rocks based on anorogenic setting have high alkalis (Na2O+K2O), FeO/MgO, TiO2/MgO ratios, and low CaO, Al2O3 contents (Whalen et al., 1987). The M-type granites are referred to those derived from the mantle, either directly by partial melting of subducted oceanic crust (White, 1979) or by crystal fractionation of basaltic magmas (Whalen, 1985).

Geochemistry of Piaoac granites shows characteristic moderate Na2O (3.36 wt.%–3.66 wt.%), high K2O (4.69 wt.%– 5.10 wt.%), high SiO2 contents, indicating a strongly peraluminous feature (Fig. 6b). In addition, the diagram of P2O5 vs. SiO2 was used to classify the granite type (Chappell, 1999), where the P2O5 contents of I-type granites decrease with increasing SiO2, while they show positive or unchangeable correlation in S-type granites. Since there is no obvious decrease of P2O5 when SiO2 increases in this study (Fig. 7d), suggesting that the Piaoac granite is S-type. These S-type granites are also recognized by the Rb/Sr ratios (> 0.9) (Wang et al., 1993), and our values for Piaoac granites have ratios of 16–16.8. Furthermore, zircon saturation temperatures (TZr) of Piaoac granites, which were proposed by Watson and Harrison (1983), show initial magma temperatures at the source range from 718–736 ℃ (Table S2), suggesting low magmatic temperatures.

In addition, the crystallization of the magma depends on chemical and physical factors, which can be represented by chemical compositions of biotite (Abdel-Rahman, 1994; Munoz, 1992). Biotites of Piaoac granites are of primary magmatic origin (Fig. 9b), so that their chemical compositions may reflect magmatic melt and material sources (Batchelor, 2003; Shabani et al., 2003; Abdel-Rahman, 1994). In equilibrium, biotites in alkaline anorogenic suites are mostly iron-rich, whereas biotites in peraluminous (including S-type) suites are siderophyllitic (Abdel-Rahman, 1994). All of the biotites plot on the peraluminous suites also are siderophyllitic (Figs. 10a, 10b). The FeO/(FeO+MgO) vs. MgO diagram shows the Piaoac granites are derived from crust source (Fig. 10c). Another one, the crystallization temperatures of Piaoac granite calculated through empirical formula of biotite provided by Henry et al. (2005) range from 595 to 651 ℃, and their temperatures are consistent with the result inferred from Ti-Mg/(Mg+Fe) diagram (Fig. 10d), suggesting that the intrusive magma formed at low temperature.

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Figure 10. Chemical compositional diagrams of biotite from the Piaoac granites. (a) Al-Mg diagram after Stussi and Cuney (1996); (b) ternary MgO-FeO-Al2O3 diagram (after Abdel-Rahman, 1994); (c) FeOT/FeOT+MgO vs. MgO diagram (after Zhou, 1986); (d) diagram of Ti-Mg/(Mg+Fe) (after Henry et al., 2005); (e) T vs. log fO2 (after Wones and Eugster, 1965); (f) diagram of Fe3+-Fe2+-Mg2+

In other aspect, the peraluminous granitoids (S-type granite) can be distinguished from mineral contents, textures, rock associations, and mode of emplacement (Barbarin, 1996). Therefore, peraluminous granitoids have been classified into two main types, i.e., muscovite-bearing granitoids (MPGs), and biotite-rich cordierite-bearing granitoids (CPGs). Barbarin (1996) suggested that MPGs are poor in biotites and contain large primary muscovites, while CPGs contain a larger amount of biotites and cordierites. The evolution is caused to affect their mineral contents; MPGs are mainly possessive related 'wet' anatexis of crustal rocks and crystal fractionation of the magmas, while CPGs formed through 'dry' anatexis of crustal rocks enhanced by underplating or injection of hot mantle- derived magmas. Petrography of Piaoac granites shows mainly primary muscovites and few biotites without cordierite. In addition, the granites formed at relatively low magmatic temperatures (TZr=718–736 ℃ and Tbi=595–651 ℃) (Table S2, S4); geochemical data suggest that intrusive magma was derived from crust. Consequently, it is inferred that these granites belong to the MPG-type by Barbarin (1996).

4.1.2 Magma source

Compositions of granites can reflect their source nature, geochemical characteristics, differences derived by different source rocks. The Piaoac granites are peraluminous granites signatures (Fig. 6b). The previous studies suggest that peraluminous granites were derived from partial melting of metasedimentary rocks and mafic source rocks or amphibolites (Chappell et al., 2012; Clemens, 2003; Sylvester, 1998; Ellis and Thompson, 1986). However, the granites produced from melting of meta-igneous rocks are characterized by often high contents in CaO, MgO, FeO, Na2O and Sr (Beard and Lofgren, 1991) showing weakly peraluminous or metaluminous signatures. The Piaoac granites are strongly peraluminous (ASI= 1.61–1.65), K-rich with K2O/Na2O > 1, poor CaO (< 0.64 wt.%) and MgO (< 0.20 wt.%), low Sr (36.8 ppm–40.2 ppm). Therefore, we suggest that the Piaoac magmatic melt was produced from partial melting of metasedimentary protoliths in the crust.

Studies of experimental petrology suggest that peraluminous melts can be depended on changes in the concentrations of CaO and Na2O, which reflects different sources (Sylvester, 1998; Chappell and White, 1992). The granites in Piaoac have low CaO/Na2O ratios (< 0.3) and high SiO2 contents (> 72 wt.%), indicating that granite melts could be produced from pelite rocks (Sylvester, 1998) (Fig. 11a). Furthermore, samples of Piaoac granites are mainly plotted into clay-rich sources field (Fig. 11b), implying these granites were generated from plagioclase-poor (< 5%) pelitic source. In different source rocks diagram (Altherr et al., 2000) (Fig. 11c), Piaoac samples show partial melts generated by partial melting of metapelite source. Results of zircon Hf isotopes have negative εHf(t) values (-9.69 to -0.90) and "crustal" mode ages range from 1.2 to 1.7 Ga (Table S5) (Fig. 12a), indicating that parental magma was produced from Paleo-Mesoproterozoic rock. In addition, previous studies of Nd isotopic data (εNd(t)= -10.4 to -11) enveloped of the Proterozoic crust in South China Block are also consistent with our interpretation (Chen et al., 2014; Wang et al., 2011; Anh et al., 2010).

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Figure 11. Source discrimination diagrams for Piaoac granites. (a) and (b) after Sylvester (1998); (c) after Altherr et al. (2000)
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Figure 12. Diagrams of magma evolution. (a) εHf(t) (from in situ analysis of zircon Hf isotopic composition); (b) εNd(t) (from bulk rock Nd isotopic composition) of the Piaoac granites and Dulong granites, literature data of Piaoac rocks from Chen et al. (2014), Wang et al. (2011) and Anh et al. (2010). Literature data of Dulong rocks from Xu et al. (2015)
4.2 Links between Magmatism and W-Sn Mineralization

The Piaoac granite massif and related ore deposits have been studied at the end of 20th century through forecasting prospecting works on a scale 1 : 500 000 (Dovjikov et al., 1965; Izokh, 1965). Some studies and field observations indicate granites and tungsten ore bodies associated with each other (Dung et al., 2013; Vladimirov et al., 2012; Ben et al., 1993). Ballouard et al. (2016) suggested Nb/Ta ratio (< ~5) is a good marker to distinguish between mineralization and barren granites. The Piaoac samples fall in Sn-W-(U) field in Nb/Ta vs. Zr/Hf diagram (Fig. 13), suggesting W-Sn mineralization occurred during the transition from the magmatic to the hydrothermal stage. The Piaoac granites have widespread characteristics of tin-tungsten related granite in southwestern China, especially granites at the Dulong (Zhao et al., 2018; Xu et al., 2015). These granites generally display high SiO2, K2O, and strong peraluminous features (Fig. 6). In addition, they are similar in trace and rare earth element patterns, such as positive Rb, Ta, Th, Pb, U and negative of Ba, Nb, La, Ti and Eu (Fig. 8), and derived from Proterozoic crust (Fig. 12). The Piaoac granites have unusually higher tin and tungsten contents (34 ppm–38 ppm Sn and 13 ppm–25 ppm W) than average upper crust (5.5 ppm Sn, 2.0 ppm W) and lower crust (1.5 ppm Sn, 0.6 ppm W) (Taylor and Mclenan, 1985). Hence, the extraordinary W-Sn in metapelitc rock may interpret the tungsten-tin enrichment in the Piaoac magmas as Breiter (2012) claimed.

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Figure 13. Nb/Ta vs. Zr/Hf diagram differentiating barren and ore-bearing peraluminous granites. The barren granites plot in the field defined by 26 < Zr/Hf < 46 and 5 < Nb/Ta < 16, whereas ore bearing granites have comparable Zr/Hf ratios between 0 and 46 and Nb/Ta ratios < 5 (Ballouard et al., 2016). The Piaoac granites plot in Sn-W-(U) field, indicating granites are associated with tungsten-tin mineralization in area

Partial melting can move tungsten-tin from the protolith to the melts (Romer and Kroner, 2016), fractional crystallization is another major mechanism to make W-Sn enrichments (Gomes and Neiva, 2002). Piaoac granite is not generated directly by partial melting of metapelitic rock, since their geochemical data suggest these rocks belong to fractional crystallization mechanism during magmatic evolution (Fig. 14). The Piaoac samples show Ba, Sr and Eu negative anomalies, implying fractional crystallization of plagioclase and K-feldspar (Figs. 8a, 8b). Diagrams of Eu/Eu* vs. Ba and Ba vs. Rb (Figs. 15b, 15c) imply that plagioclase fractionation played an important role in the formation the Piaoac granites. In a diagram of (La/Yb)N vs. La (Fig. 15a), these samples had been controlled by fractionation of apatite. In Harker diagrams, the negative correlations Fe2O3T, MgO and TiO2 (Figs. 7e, 7g, 7f) illustrate that the fractionation of Fe-Ti oxides (spinel, ilmenite and titanite) occurred during crystallization (Blevin and Chappell, 1995, 1992).

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Figure 14. Discrimination diagrams of the Piaoac granites. (a) Zr/Nb vs. Zr; (b) La/Sm vs. La; (c) (K2O+Na2O)/CaO vs. (Zr+Nb+Ce+Y) (Whalen et al. 1987), showing the fractional crystallization of the Piaoac granite
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Figure 15. Trace element diagrams for the evolution of the Piaoac granites. (a) (La/Yb)N vs. La diagram, partition coefficients are from Fujimaki (1986) for apatite, Mahood and Hildreth (1983) for zircon and allanite, and Yurimoto et al. (1990) for monazite; (b) Ba vs. Rb, partition coefficients of Rb and Ba are from Philpotts and Schnetzler (1970); (c) Ba vs. Eu/Eu* (after Eby, 1990). Pl. Plagioclase; Kf. K-feldspar; AF. alkali feldspar; Bt. biotite; Aln. allanite; Mnz. monazite; Ap. apatite; Zrn. zircon

During crystallization stages, Sn has two valences include Sn4+ and Sn2+, W also has two valences include W6+ and W4+. The oxygen fugacity plays an important role in controlling valence states of tin-tungsten (Cao et al., 2018; Zhao et al., 2005; Cobbing et al., 1986). The Sn4+ and W6+ are easily to substitute for Ti or Fe in early stage crystalline minerals (e.g., ilmenite, biotite, magnetite and rutile) under oxidized condition (Rice et al., 1998; Lehmann, 1990). However, Sn2+ and W4+ cannot enter the lattice of early forming minerals in reduced magma (Linnen et al., 1996, 1995). In addition, Wones and Eugster (1965) suggested that the oxygen fugacity could be calculated through biotite+sanidine+magnetite. The oxygen fugacity is estimated to be 10-17 to 10-18 bars (Fig. 10e), indicating a low oxidation condition during magma crystallization. The oxygen fugacities of Piaoac granite estimated from the biotites are all below the Fe3O4-Fe2O3 (MH) buffer (Fig. 10f) suggesting reducing condition during magmatic evolution. Lehmann (1990) suggested that low fO2 (ilmenite-series granites with lower Fe3+/Fe2+ ratios) seems to favour the formation of tungsten-tin ore systems. Therefore, Piaoac granitic melts underwent fractional crystallization mechanism, which removed and made W-Sn enrichments in the magma melts to form the tungsten-tin bearing granites. The reducing conditions (fO2 below MH) would efficiently release Sn from the residual granite melts into a hydrothermal fluid, and leading to deposition Sn-rich mineral phases.

4.3 Tectonic Implications in Northern Vietnam and Southwestern China

Our data suggest that the Piaoac granites belong to S-type granite and were emplaced around ca. 82 Ma, The negative εHf(t) values of these zircon (-9.69 to -0.90) indicate minor input from juvenile crust or mantle into the magma. Vladimirov et al. (2012) suggested that the granites in this area were formed through heating and anatexis of continental crust related Pacific "hot ring". In general, S-type granites' formation is related to collisional event, with the emplacement of post-collisional granites occurring in an extensional geodynamic setting (Barbarin, 1999; Sylvester, 1998). In Rb-(Y+Nb) tectonic discrimination diagram (Fig. 16), the Piaoac granite samples mainly fall into the post collision extensional (post-CEG) field, suggesting their post-collision affinity.

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Figure 16. Tectonic discrimination diagrams of the Piaoac granite is from Pearce et al. (1984) and the post-CEG from Förster et al. (1997). VAG. Volcanic-arc granites; WPG. within plate granites; syn-COLG. syn- collisional granites; ORG. ocean-ridge granites; post-CEG. post-collision extensional granites. Symbols are as in Fig. 7

Moreover, W-Sn deposits related the Late Cretaceous igneous rocks are also widespread in southwestern China such as Gẹjiu, Bozhushan and Dulong in the Southeast Yunnan Province (Cheng et al., 2013, 2012; Mao et al., 2013; Cheng and Mao, 2010) (Fig. 1b). Specially, Dulong tin-polymetallic deposit has emplacement ages at 79–92 Ma (Zhao et al., 2018; Sheng et al., 2015; Xu et al., 2015; Feng et al., 2012; Liu et al., 2007). The Dulong granites and Piaoac granite are not only closely spatial and temporal related but also have similar geochemical characteristics (see Figs. 6, 8), implying that they formed in a similar geodynamic setting. Previous studies indicated that the lithospheric extension with the asthenospheric upwelling mantle during Late Cretaceous is the cause to form granites in the southwestern China Block including in Dulong area (Cheng et al., 2013, 2012; Cheng and Mao, 2010; Yan et al., 2005). Therefore, we suggest that the Piaoac intrusion was associated with large-scale crustal extension in South China and Northern Vietnam during Late Cretaceous (Fig. 17).

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Figure 17. Geodynamic setting and mineralization model during the Late Cretaceous in Northeast Vietnam (NV) and the southern part of South China Block (SCB) (modified after Zhao et al., 2018). (a) A model for lithospheric extension (during 80–90 Ma). The NV-SCB underwent lithospheric extension with strong upwelling of the asthenospheric mantle that triggered mafic melt underplating and/or continental lithosphere delamination during Late Cretaceous (Zhao et al., 2018; Cheng et al., 2013, 2012; Cheng and Mao, 2010). The lithospheric extension may be consequence of the late episode of the Paleo-Pacific Plate subduction under the continental margin of the SE Asia (Sun, 2016; Roger et al., 2012). During extension, partial melting of the overlying Meso- to Paleoproterozoic metasedimentary rocks produced felsic magmas such as the Dulong granites in southwestern South China Block with higher magmatic temperatures (TZr < 806 ℃) (Xu et al., 2015), while partial melting of Meso- to Paleoproterozoic metasedimentary with lower magmatic temperatures (TZr < 736 ℃) in Cao Bang District formed the Piaoac granitic magmas. (b) Model of the formation of the Piaoac S-types granites and related Sn-W mineralization in Cao Bang Province, Northeast Vietnam. Partial melting moved W-Sn from the Meso- to Paleoproterozoic metasedimentary rocks to the melts. The Piaoac granites show negative anomalies in Ba, Nb, Sr, Ti, Zr, and positive anomalies in Rb, U, Ta, indicating that granitic melts have undergone highly fractional crystallization of plagioclase, K-feldspar, monazite, apatite and zircon. This fractional crystallization would have led to further enrichment of W-Sn in the magma (Fogliata et al., 2012; Teixeira et al., 2012). During crystallization, Piaoac granite underwent the reduced magmas (below MH), the partitioning of W-Sn into crystallizing phases was low, which would efficiently remove of W-Sn into a hydrothermal fluid, and lead to deposition of W-Sn mineral phases
5 CONCLUSIONS

(1) The Piaoac plutons in northeastern Vietnam were emplaced around 82 Ma of Late Yanshannian Period, showing strong peraluminous feature (ASI > 1.1). Zircon Hf isotopic data indicate that these granites were generated by partial melting of metapelitic rocks from the Proteozoic continental crust.

(2) The reducing condition of Piaoac granites during crystal fractionation of K-feldspar, plagioclase, biotite, and apatite are major causes of transferring W-Sn in hydrothermal fluid.

(3) The Piaoac granites formed in an extensional setting is consistent with the extensional regime proposed for the southwest SCB, indicating a large-scale lithospheric extension event in the NV-SCB during Late Cretaceous.

ACKNOWLEDGMENTS

This study was supported by the National Key R &38; D Program of China (No. 2016YFC0600404) and the National Natural Science Foundation of China (Nos. 41673040 and 41611540339). The authors are grateful to Hou Z H, Deng J H, Gu H L, Qi H S, Ren Y S, and Shu S Y, for assistance in zircon U-Pb dating and Lu-Hf isotope analyses. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0865-6.

Electronic Supplementary Materials

Supplementary materials (Tables S1–S5) are available in the online version of this article at http://dx.doi.org/10.1007/s12583-018-0865-6.


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