Journal of Earth Science  2019, Vol. 30 Issue (4): 679-688   PDF    
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Genesis of the Binh Do Pb-Zn Deposit in Northern Vietnam: Evidence from H-O-S-Pb Isotope Geochemistry
Chaowen Huang 1, Huan Li 1, Chun-Kit Lai 2,3     
1. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;
2. Faculty of Science, Universiti Brunei Darussalam, Gadong BE1410, Brunei Darussalam;
3. Centre of Excellence in Ore Deposits(CODES), University of Tasmania, Hobart, Tasmania 7001, Australia
ABSTRACT: The Binh Do Pb-Zn deposit in the Phu Luong region (Thai Nguyen Province, northern Vietnam) is located on the southern margin of the South China Block. The fault-controlled Pb-Zn orebodies are mainly hosted in Upper Paleozoic carbonate formations. In order to reveal the mineralization type and metallogenesis of this deposit, multi-isotopic (S, Pb, H and O) analyses on typical ore and gangue minerals were conducted. The average ore sulfide δ34S∑S value is 4.3‰, suggestive of magmatic sulfur. The ore sulfide Pb isotope compositions are homogeneous, with the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 18.501 to 18.673, 15.707 to 15.798, and 38.911 to 39.428, respectively. Lead isotope model ages of the ore sulfides (240-220 Ma) are consistent with the timing of regional Triassic S-type granite emplacement (250-220 Ma), suggesting that the metals may have been granite-derived. The quartz δDV-SMOW (-82.4‰ to -70.5‰) and δ18OH2O (-0.4‰ to +6.4‰) values suggest that the ore-forming fluids were composed of mixed magmatic and meteoric waters. Combined with the geological features of the Pb-Zn deposit in the region, we propose that the Pb-Zn deposits belong to magmatic-hydrothermal type, rather than MVT-type as previously suggested. The Triassic granites may have contributed the ore-forming material and heat that drove the hydrothermal system. The ore-forming fluids may have migrated into interlayer faults and fractures of the carbonate strata, diluted by subsurface meteoric water and deposited successively the vein-type and stratiform-type Pb-Zn ores.
KEY WORDS: Binh Do deposit    northern Vietnam    Pb-Zn belt    Triassic granitoids    isotope    geochemistry    
0 INTRODUCTION

The Northern Vietnam Pb-Zn belt, situated on the southern margin of the South China block, is a key Pb-Zn province in eastern Indochina (Li et al., 2018a; Tan et al., 2018; Anh et al., 2012; Lu et al., 2009; Yan et al., 2006). A number of Pb-Zn deposits, such as Binh Do (Vietnamese: Bình Đô), Bình Chai, Lǔng Hoài, Po Pen, Phia Khao, Bô Luông, Dèo An, Than Tàu, La Poin, Na Son and Cho Dìen have been discovered (Pham-Ngoc et al., 2016; Chen et al., 2014, 2013; Can et al., 2011; Wang et al., 2011; Ishihara et al., 2010; Won-In and Charusiri, 2003), contributing to 80% of the total Pb-Zn reserves (~20 Mt) in Vietnam (Anh et al., 2012). Isotope geochemical studies on these deposits are generally lacking (e.g., Anh et al., 2012; Ishihara et al., 2010; Tran et al., 2008). For example, Pb-S isotope data were only reported from the Cho Dìen deposit (Wang et al., 2015), and no H-O isotope data were reported from the entire region. This hampers our understanding on the ore deposit type(s) and metallogenesis of this Pb-Zn belt, notably on the question whether these Pb-Zn deposits are MVT-type or magmatic- hydrothermal-type (Anh et al., 2012).

In this paper, we present new field geological and multi- isotope (S, Pb, H, O) data from the Binh Do deposit, discuss the ore-forming fluid and material source, and explore any metallogenic links between the regional large-scale Pb-Zn mineralization and Triassic (Indosinian) granitic magmatism.

1 GEOLOGICAL SETTING AND STRATIGRAPHY 1.1 Regional Geology

Northern Vietnam is situated in the southern margin of the South China Block, bordered with the Indochina Block to the southwest along the Ailaoshan-Song Ma suture zone (Fig. 1a; Lai et al., 2014a, b ). Pre-Cenozoic lithostratigraphy in this region mainly comprises Cambrian-Lower Ordovician, Silurian- Permian, Lower-Middle Triassic, Upper Triassic and Upper Mesozoic sequences (Fig. 1b; Xia et al., 2016; Gonez et al., 2012). The Cambrian-Lower Ordovician sequences are dominated by terrigenous carbonate and shallow marine clastic sedimentary rocks mainly exposed in the western and southern parts of northern Vietnam (Lepvrier et al., 2011). The Silurian- Permian sequences are characterized by marine-facies sedimentation. There was likely a brief tectonic uplift episode in the Early-Late Permian, followed by a rapid marine transgression (Cheng et al., 2016; Findlay, 1997). The Lower-Middle Triassic sequences is mainly exposed in the Song Hien zone, dominated by turbidite deposits such as conglomerate, sandstone, shale and chert. In the Late Triassic, the region was uplifted due to the Triassic Indosinian orogeny, and the sedimentary rocks are composed mainly of molasse formation (Metcalfe, 2002). By the Late Mesozoic, graben deposition of terrigenous red sandstone occurred (Cai and Zhang, 2009).

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Figure 1. Regional geological map of the northern Vietnam Pb-Zn belt (modified after Chen et al., 2014).

Northern Vietnam had experienced multi-phase magmatic activities during the Ordovician-Silurian, Permian, Triassic and Cretaceous (Chen et al., 2014; Wang et al., 2011; Liu et al., 2007; Yan et al., 2006, 2003; Gilley et al., 2003; Carter et al., 2001; Roger et al., 2000). The Ordovician-Silurian magmatic rocks are mainly composed of the Song Chay granite complex and the Phan Ngame granite complex. The Song Chay complex is the largest granite complex (outcrop area: 2 500 km2) in Vietnam, consisting mainly of diorite and medium-fine-grained foliated granite (zircon U-Pb ages: 465 to 424 Ma; Carter et al., 2001; Roger et al., 2000). The Permian magmatic rocks are mainly composed of ultramafic rocks, gabbro and granites, which have been interpreted as products of the Emeishan large igneous province (LIP) (Tran et al., 2008; Hanski et al., 2004; Polyakov et al., 1999). Triassic intrusions are widely distributed in the region, and zircon U-Pb dating on the post-collisional granites and granodiorite yielded ca. 252 to 245 Ma (Halpin et al., 2016; Chen et al., 2014). Cretaceous granitoids (ca. 94 to 87 Ma) are mainly distributed in the Tinh Tuc area north of the Bac Kan fault (Roger et al., 2012; Wang et al., 2011).

1.2 Ore Deposit Geology

The Binh Do Pb-Zn deposit is located in the NE-trending Ha Giang-Bac Kan fault zone, which lies in the southeastern part of the northern Vietnam Pb-Zn belt (Fig. 1b). Exposed stratigraphy at the mine includes the Cambrian sequence, Lower Devonian Song Cau Group, and the Mia Le and Khao Loc formations (Fig. 2a). Cambrian rocks are distributed in the southern part of the mine, and consist mainly of sandstone, siltstone, shale and mica schist with limestone interbeds. The Song Cau Group is distributed in the eastern part of the mine, and consists mainly of conglomerate, sandstone, siltstone, shale and limestone. The Mia Le Formation is distributed in the central and western parts of the mine and consists mainly of (argillaceous)- sandstone and (marly)-shale. The Khao Loc Formation (main ore host strata) is mainly composed of marble, chert and limestone. No magmatic rocks are reported at Binh Do.

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Figure 2. Simplified geological map (a) and cross section (b) of the Binh Do Pb-Zn deposit.

The Pb-Zn orebodies at Binh Do are mostly hosted in carbonate sequences and controlled by faults. The orebodies can be divided into stratiform-type (No. Ⅰ and No. Ⅱ) and vein-type (No. Ⅲ and No. Ⅳ), among which the stratiform No. Ⅱ orebody is the largest (Fig. 2a). The stratiform-type orebodies occur along stratigraphic interfaces (Fig. 2b), with a length of 0.8-1 km and a width of 20-50 m. The Pb-Zn ores from the stratiform-type orebodies are mainly banded (Fig. 3a) and massive. The vein-type orebodies develop along NNE-trending faults/fractures (Fig. 2b), having an outcropping length of 0.3-0.5 km and a width of 10-30 m. The ores are veined (Fig. 3b), brecciated (Fig. 3c) or disseminated (Fig. 3d). In addition, the stratiform-type orebodies and the vein-type orebodies are developed separately and no contact relations can be found (Fig. 2b). Thus, there is no way to determine whether they formed simultaneously or successively in the field. Moreover, supergene ores are also developed on/near the ground surface (Fig. 3e). Metallic minerals of the stratiform-type and vein-type orebodies are similar, and include mainly galena (Fig. 3f), sphalerite (Fig. 3g), pyrite (Fig. 3h) and rare pyrrhotite (Fig. 3i). These sulfide minerals intergrow with each other and possess hypidiomorphic and xenomorphic granular textures. Non-metallic minerals include mainly quartz and calcite. Boundaries between orebodies and wall rocks are clear and featured by an alteration halo. Major wall-rock alteration styles include silicification, carbonation and pyritization. Based on the paragenetic sequence of minerals, two stages of mineralization (hydrothermal stage and supergene stage) have been determined (Fig. 4).

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Figure 3. Field occurrences and photomicrographs of typical ores in the Binh Do Pb-Zn deposit. (a) Stratiform-type orebodies; (b) vein-type orebodies; (c) brecciated Pb-Zn ores; (d) disseminated Pb-Zn ores with strong pyritization; (e) secondary oxidization zones; (f) euhedral-subhedral galena intergrows with quartz; (g) euhedral galena and sphalerite intergrows with quartz; (h) disseminated pyrite in quartz; (i) xenomorphic sphalerite intergrows with galena and pyrrhotite. Gn. Galena; Sp. sphalerite; Py. pyrite; Po. pyrrhotite; Cal. calcite; Qz. quartz.
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Figure 4. Paragenetic mineral sequences and ore-forming stages of the Binh Do Pb-Zn deposit.
2 ANALYTICAL METHODS

Samples were collected from underground tunnels in the Binh Do mining area, and we conducted S-Pb isotope analyses on sulfides and H-O isotope analyses on quartz. After the samples were crushed, washed and dried, minerals such as pyrite, galena, sphalerite and quartz were selected under the double eyepiece, with over 98% purity.

Hydrogen-oxygen isotope analyses were conducted at the Analytical Laboratory of BRIUG, using a Finningan MAT-253 mass spectrometer. Oxygen was extracted from quartz using a quantitative reaction with BrF5 in externally heated nickel vessels at ca. 700 ℃ and converted to CO2 on a platinum-coated carbon rod. Analyses of the hydrogen isotopic compositions of fluid inclusions were conducted on the same quartz samples measured for oxygen isotopes. Quartz separates were first degassed of labile volatiles and secondary fluid inclusions by heating under vacuum to 120 ℃ for 3 h. The released water was trapped and converted to hydrogen by passing over heated zinc, and then analyzed with the mass spectrometer. The detailed analytical procedures for H-O isotope analyses were described in Ni et al. (2017) and Li et al. (2013). The isotopic data are reported in per mil relative to the V-SMOW standards for oxygen and hydrogen, with an analytical precision of ±0.2‰ for δ18O, and ±2‰ for δD.

Sulfur and lead isotope analyses were conducted at the same laboratory. Sulfur isotope analyses were conducted using a MAT-251 EM mass spectrometer, with analytical procedures similar to Li et al. (2016). Pure sulfide samples (200-mesh) were combusted under vacuum with CuO in a 1 000 ℃ oven. Liberated SO2 was frozen in a liquid nitrogen trap after cryogenic separation from other gases. All values are reported as per mil (‰) relative to Canyon Diablo Troilite (CDT), with analysis accuracy better than ±0.2‰. The lead isotopic compositions of the sulfide samples were also analyzed on a MAT-261, using the method similarly described in Li et al. (2018b) and Xu et al. (2017). Sulfide samples were dissolved completely in ultrapure acids of HNO3+HCl at 180 ℃. The Pb in the samples was separated and purified using a two-column AG 1-X8 anion resin method. Pb isotopic ratios were corrected to reference values of Pb standard NBS-981 (Todt et al., 1996), with analytical reproducibility of ~0.1% (2σ) for 206Pb/204Pb, 207Pb/204Pb, and ~0.2 % (2σ) for 208Pb/204Pb.

3 RESULTS

The analytical results of the S-Pb isotopes for 12 sulfide samples (4 pyrite, 4 galena and 4 sphalerite) are shown in Table 1. Taken together, there are no big differences in these values between the stratiform-type and vein-type ores. However, small variations can be still observed. Overall, the pyrite yielded the highest δ34S values (+4.9‰ to +6.2‰), followed by galena (δ34S= +1.3‰ to +3.9‰) and sphalerite (δ34S= +2.3‰ to +3.6‰) (Fig. 5a). In addition, a weak decreasing trend in δ34S values for galena and pyrite from stratiform-type ore to vein- type ore can be observed. Lead isotopes for all the sulfides are overall similar, with 206Pb/204Pb=18.501-18.673, 207Pb/204Pb= 15.707-15.798, and 208Pb/204Pb=38.911-39.428. However, there is a slight decreasing trend in the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios for galena (and an increasing trend for sphalerite) from the stratiform-type to vein-type ore. The μ and ω values of all the sulfides range 9.66 to 9.83 and 38.97 to 41.00, respectively, with the Pb model ages ranging ca. 220 to 238 Ma (Table 1).

Table 1 S and Pb isotope composition of ore minerals from Binh Đo deposit
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Figure 5. (a) Histogram of S isotopes and (b) 1 000×ln δ34Ssphalerite-galena vs. δ34S diagram (after Pinckney and Rafter, 1972) for sulfide minerals from the Binh Do Pb-Zn deposit.

The H-O isotopic results of 6 quartz samples are shown in Table 2. The δDV-SMOW values range -82.4‰ to -70.5‰, and the δ18OV-SMOW values range +9.6‰ to +16.4‰ (corresponding δ18OH2O values = -0.4‰ to 6.4‰). Moreover, there is an increasing trend in δ18O values from stratiform-type ore to vein-type ore (Table 2).

Table 2 Hydrogen and oxygen isotopic compositions of the Binh Đo Pb-Zn deposit
4 DISCUSSION 4.1 Sources of S and Pb

Sulfide mineral assemblage is simple at Binh Do, and comprises mainly galena, sphalerite and pyrite. No barite, alunite, gypsum or other sulfate minerals are present, suggesting that the ore-forming fluids occurred mainly in the form of HS- and S2- (Ohmoto, 1972). In addition, the symbiotic relationship and hypidiomorphic-xenomorphic textures of these sulfide minerals indicate that they were precipitated almost simultaneously. Therefore, the sulfur isotopes of sulfides can approximate those of the ore fluids. In addition, the S isotopic ranges of all sulfides are relatively narrow (δ34S= +1.3‰ to +6.2‰) and peaked at ~2 (Fig. 5a), implying that the ore-forming fluid had reached equilibrium in S isotope. This is further evidenced by the enrichment of 34S order (δ34Spyrite > δ34Ssphalerite > δ34Sgalena) which is in accordance with the order of equilibrium of thermodynamic fractionation of sulfur isotopes in mineral phase (Ohmoto, 1986; Rye and Ohmoto, 1974).

Since the sulfur isotope had reached equilibrium, fractionation equations can be used to calculate the temperature of the symbiotic mineral formations (Rye and Ohmoto, 1974). In this study, the temperatures of mineral pairs (pyrite-sphalerite and sphalerite-galena) are calculated at 172.2 to 277.8 ℃ and 157.1 to 272.2 ℃, respectively (mean ~230 ℃). These results show that these minerals were formed under similar temperature and chemical conditions. Therefore, the method from Pinckney and Rafter (1972) can be used to further determine the average isotopic composition of total sulfur (δ34S∑S) in the ore-forming fluids. As shown in Fig. 5b, the average ore fluid δ34S∑S value (+4.3‰) is close to the magmatic sulfur (δ34S= 0±3‰), and thus the sulfur may have been magma-derived.

Compared with the δ34S of different deposit types, the Binh Do data are markedly different from those of typical MVT-type Pb-Zn deposits (e.g., the Xiangxi deposit, δ34S= +11.0‰ to +31.4‰; Zhang et al., 2005) but similar to the granite intrusion-related Cho Dìen Pb-Zn deposit in the northern Vietnam Pb-Zn belt (δ34S= +4.1‰ to +6.8‰; Wang et al., 2015) and in the other Pb-Zn belts (Huang et al., 2019; Liu et al., 2018; Xing et al., 2016; Corsini et al., 1980). Therefore, the sulfur of the Binh Do Pb-Zn ores was likely derived from buried intrusive bodies.

Lead isotope compositions of the Binh Do ore sulfides are featured by their narrow range (< 1%), suggestive of a single or highly similar Pb source. The Th/U ratios of the sulfides range from 3.9 and 4.0, similar to the average Th/U ratios of mainland China (4.20±0.13; Zhu et al., 1998), indicating an affinity of Pb source between the northern Vietnam Pb-Zn belt and the South China Block. The Th/U ratios and ω and μ values are close to those of typical magmatic-hydrothermal Pb-Zn deposits in South China (e.g., the Fulaichang Pb-Zn deposit in Guizhou; Tang et al., 2012). This suggests that the Pb isotopes of the Binh Do Pb-Zn deposit is characterized by a single-stage common Pb composition, and its model age may represent the Pb-Zn ore- forming age. In the Pb isotope discrimination diagram (Figs. 6a, 6b), the Binh Do data all plot near or above the upper crustal curve, indicating an upper crustal source. This is further evidenced by the Δβγ diagram (Fig. 6c), on which all the sample data fall inside the field of supracrustal lead. The Pb isotope model ages of the Binh Do sulfides (240 to 220 Ma) are younger than the Upper Paleozoic wall-rocks, but similar to those of the Early Triassic granitoids in northern Vietnam. In northern Vietnam, the extensive Indosinian (Triassic: ca. 250 to 220 Ma) peraluminous S-type granitic magmatism is widely regarded to have been derived from upper crust remelting (Chen et al., 2014, 2013; Roger et al., 2012). These S-type granites may have contributed (at least part of) the metals to Binh Do, although Triassic intrusions are yet to be found in the mining area at the present exploration depths.

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Figure 6. (a) 207Pb/204Pb vs. 206Pb/204Pb diagram, (b) 208Pb/204Pb vs. 206Pb/204Pb diagram, (c) Δβ vs. Δγ diagram (after Zhu et al., 1998), and (d) δDV-SMOW vs. δ18OH2O diagram of the Binh Do Pb-Zn deposit. In (c): 1. Mantle lead; 2. supracrustal lead; 3a, 3b. mixed supracrustal and mantle lead (magmatism); 4. chemical deposit lead; 5. submarine hydrothermal lead; 6. metamorphic lead (medium-grade metamorphism); 7. metamorphic (lower crust) lead (high-grade metamorphism); 8. orogenic lead; 9. ancient shale (upper crust) lead; 10. retrograde metamorphism lead. Δβ and Δγ are relative deviations between contemporaneous mantle values and 207Pb/204Pb and 208Pb/204Pb ratios, respectively. The lead isotope curves for the mantle, orogeny, upper crust, and lower crust are from Zartman and Doe (1981). The base diagram of H-O isotopes is from Taylor (1974).
4.2 Fluid Characteristics and Ore Genesis

Hydrogen and oxygen isotopes can effectively identify the ore-forming fluid source. The δD values (Table 2) of the Binh Do ore-forming fluids fall inside the magmatic water field (-80‰ to -40‰), indicating that the ore-forming fluids may have been magma-derived. In the δD vs. δ18OH2O diagram (Fig. 6d), the Binh Do data plot between the magmatic water and the meteoric water line, but closer to the former. Integrated with the S-Pb isotope results, it is inferred that the Binh Do ore-forming fluids were likely magmatic-derived, probably mixed with some meteoric water.

Ore-forming temperature is also a good indicator to differentiate different types of Pb-Zn deposits. Basinal brines- derived MVT-type Pb-Zn deposits are commonly characterized by low ore-forming temperatures (90 to 150 ℃) (Leach et al., 2010, 2005; Basuki, 2008, 2002), which are considerably lower than the temperature (mean 230 ℃) calculated from the S isotope data of sulfide mineral pairs at Binh Do. The H-O isotope characteristics of the Binh Do Pb-Zn ores (esp. vein-type) are quite similar to those of the magmatic-hydrothermal type Pb-Zn deposits in Guangxi (South China) (Zhou et al., 2018; Chai et al., 2015), further supporting that the ore-forming fluids were mainly magmatic-hydrothermal with minor meteoric water input.

The Binh Do Pb-Zn deposit is hosted mainly in carbonate rocks, composed of stratiform-type and vein-type orebodies that are controlled by faults and fractured zones, and no magmatic rocks are outcropped. These features are highly similar to the other Pb-Zn deposits in northern Vietnam, such as the Bình Chai, Lǔng Hoài and Po Pen deposits (Anh et al., 2012; Ishihara et al., 2010; Tran et al., 2008). Opinions vary on the metallogenic style of the northern Vietnam Pb-Zn belt: previous local researchers argued that the carbonate-hosted stratiform Pb-Zn orebodies are similar to typical MVT Pb-Zn deposits, whereas Anh et al. (2012) suggested that although no intrusions are exposed in most of the Pb-Zn deposits, the strong coeval magmatism may have been genetically linked with the regional Pb-Zn mineralization. Through systematic and laboratory studies, we found that there are obvious differences in many aspects between the Binh Do Pb-Zn deposit and the typical MVT or SEDEX deposits (Li K et al., 2018; Sun et al., 2017a; Li and Xi, 2015). We propose that concealed intrusions may have provided the fluids, metals and heat for the Binh Do Pb-Zn mineralization. Our field observations reveal a clear mineral zoning pattern at Binh Do, where the lower-temperature assemblage of sphalerite+galena+pyrite passes downward to the higher- temperature assemblage of pyrrhotite+scheelite. This zoning pattern of increasing temperature with depth is atypical in sedimentary-type (MVT or SEDEX) Pb-Zn deposits but common in magmatic-hydrothermal W-Sn-Pb-Zn polymetallic deposits (e.g., Jiang et al., 2018; Li et al., 2018c, d, 2017, 2014a, b ; Wu et al., 2018; Sun et al., 2017b.), thus implying a heat-source (probably intrusion) at depth and suggesting a magmatic-hydrothermal origin for the Binh Do Pb-Zn mineralization.

4.3 Metallogenic Model

It is noticeable that there are some regular variations of S-Pb-H-O isotopes from vein-type ores to stratiform-type ores: δ34S values decrease whereas 18OV-SMOW values increase, and 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios increase in galena whereas decrease in sphalerite (Tables 1 and 2). This may indicate a time sequence of the mineralization in the Binh Do deposit, resulting in the vein-type and stratiform-type ores successively (i.e., vein-type ores might be formed slightly earlier than the stratiform-type ores). In the granite intrusion-related hydrothermal systems, δ34S values normally increase whereas 18OV-SMOW values decrease along with the mineralization processes, whereas Pb/Pb ratios may differ among intergrown minerals (Li et al., 2018b). The higher δ34S values for the sulfides from the stratiform-type ores could be a product of quantitative BSR (bacterial sulfate reduction), but it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite (related with the carbonate rocks) was the source of heavy hydrothermal sulfur (Ma et al., 2004). This may indicate an increased stratigraphic control but decreased magmatic impact with the advance of ore-forming process. The decreased 18OV-SMOW values from vein-type ores to stratiform-type ores also support this inference (Fig. 6d). Overall, the slightly different but overall consistent values of these isotopes in the Binh Do deposit also suggest that the vein-type ores to stratiform- type ores have the similar source of ore-forming materials but precipitated successively in different structures.

Combined the S-Pb-H-O isotopic data with field observations and previous literature, we propose a metallogenic model for the northern Vietnam Pb-Zn belt (Fig. 7): In the Early Triassic, northern Vietnam may have collided with the Indochina Block and underwent the Indosinian orogenic events. Under the strong extrusion environment, partial melting of the thickened crust resulted in large-scale emplacement of post-orogenic S-type granites, providing massive heat and ore-forming materials. Meanwhile, the multi-level and multi-scale faults are reactivated, providing channels for the migration of the magmatic-hydrothermal fluids. The good permeability of the carbonate rocks has given impetus to the migration and precipitation of the hydrothermal fluids. The final deposition of the Pb-Zn materials was caused by the mixing of deep-seated magma-derived fluid with meteoric water in the shallower crust, forming the vein-type and stratiform-type Pb-Zn orebodies successively.

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Figure 7. A genetic model to explain the ore-forming process of the Binh Do Pb-Zn deposit, northern Vietnam Pb-Zn belt.
5 CONCLUSIONS

(1) The Binh Do Pb-Zn stratiform-/vein-type orebodies are hosted in Upper Paleozoic carbonate rocks. The δ34S values range from +1.3% to +6.2% (δ34spyrite > δ34ssphalerite > δ34sgalena), indicating a magmatic sulfur with an equilibrized ore-forming temperature of ~230 ℃.

(2) Lead isotope compositions of the Binh Do ore sulfides are similar, suggesting a homogeneous upper crustal lead source. The one-stage Pb model age is ca. 238 to 220 Ma, broadly coeval with the regional Triassic (Indosinian) S-type granitic magmatism. Isotope evidence and vertical ore mineral zoning pattern suggest the ore-forming fluids, metals and heat may have originated from concealed S-type granites in the area.

(3) The δDV-SMOW and δ18OH2O values of the ore-related quartz range -82.4‰ to -70.5‰ and -0.4‰ to +6.4‰, respectively, indicating that the ore-forming fluids were mainly magmatic-hydrothermal derived with minor meteoric water input.

(4) We suggest that the Binh Do Pb-Zn deposit is best attributed to be magmatic-hydrothermal type, rather than MVT type as previously proposed.

ACKNOWLEDGMENTS

We thank Mr. Tianguo Wang for his help in sample collection. This work was partially financed by the National Natural Science Foundation of China (No. 41502067). The editor and anonymous reviewers are thanked for the insightful comments and suggestions. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0872-2.


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