Ore-Forming Fluids Characteristics and Metallogenesis of the Anjing Hitam Pb-Zn Deposit in Northern Sumatra, Indonesia

0. INTRODUCTION

Sumatra is located at the confluence of the Tethyan metallogenic belt and the circum-Pacific metallogenic belt (Pei et al., 2013; Hutchison and Taylor, 1978), and has experienced multiple phases of tectono-magmatic activities (Liu Y et al., 2015; Gao et al., 2013; Liu J L et al., 2010; Liu Y C et al., 2008). This complex tectonic history has led to the formation of many high-quality precious/base-metal deposits, notably the Geuneu Au-Ag deposit, Lebong Tandai Au-Ag deposit, Timbulan Cu deposit, Danau Ranau Kelayang Cu-Mo deposit, Nam Satu and Sungeilsahen Sn deposits (Deng et al., 2017, 2015; Wang C M et al., 2017, 2016), which constitute one of the most important Au-Ag-Cu-Sn mineral provinces in Indonesia (Crow and Barder, 2005; Crow and van Leeuwen, 2005). In recent years, several medium to large Pb-Zn deposits have been discovered in the Sidikalang area of northern Sumatra (Gao et al., 2013; Silic and Seed, 2001). However, the source, nature and evolution of its ore-forming fluids have not been well constrained, which limits not only the mineral prospecting in the area, but also our understanding in the regional metallogenesis.

The Anjing Hitam deposit in the Sidikalang area is the largest Pb-Zn deposit in Sumatra (Fig. 1), containing over 10 million tons (Mt) of proven lead and zinc resources (Silic and Seed, 2001). The stratiform orebodies are hosted in the middle member of the Carboniferous–Permian Kluet Formation of the Tapanuli Group, representing a typical sediment-hosted base metal sulfide deposit in the region.

Figure  1.  Regional tectonic setting of Sumatra (modified after Genrich et al., 2000).

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This paper aims to investigate the Anjing Hitam ore- forming fluid properties and sources via detailed fluid inclusion and carbon-oxygen isotope analyses, and to discuss the deposit type and possible metallogenic mechanism of the deposit.

1. BACKGROUND GEOLOGY

1.1 Regional Geology

The Anjing Hitam deposit is located in northern Sumatra, which lies in the southern part of the Sibumasu terrane (Barber and Crow, 2003; Metcalfe, 2002). The Sibumasu terrane extends from Sumatra, via Peninsular Malaysia, central-eastern Thailand, eastern Myanmar to SW Yunnan, and was likely part of the Gondwana supercontinental margin connected to northwestern Australia (Wu and Suppe, 2018; Hu et al., 2017; Jamaludin et al., 2017; Burrett et al., 2014; Barber and Crow, 2003). During the Permian, Sibumasu was likely cracked off from Gondwana, drifted across the Paleotethys, and accreted onto the southeastern Eurasia margin in the Mesozoic (Ueno, 2003; Metcalfe, 1996; Şengör, 1987, 1979; Stöcklin, 1974). This formed the flysch sequences and the overlying marine sedimentary rocks (Metcalfe, 2002), followed by the formation of the Woyla suture zone in Sumatra after the accretion. In the Paleocene, the oblique subduction of the Indian Plate beneath Sibumasu might have occurred (Hou et al., 2006a, b ), forming regional magmatism and dextral shearing.

Major rocks exposed at Sidikalang include the Carboniferous–Permian Tapanuli Group, which comprises the Alas Formation (mainly carbonate rocks) in the lower part and the Kluet Formation (mainly clastic rocks) in the upper part (Cameron et al., 1982). These Paleozoic rocks are deformed and weakly contact metamorphosed by intrusions. The Paleozoic sequences are almost entirely covered by the Pleistocene tuffs centered on the Toba crater in Sumatra (Reynolds, 2004).

1.2 Ore Geology

The major structure at Anjing Hitam is the NW-trending Sopokomil dome (Fig. 2), which is about 5 km long and 2 km wide. Apart from the Sopokomil dome, a contemporaneous fault (Balga fault) is identified as the boundary fault of Anjing Hitam deposit. Multistage deformation structures of the mine are well developed. Although small-scale isoclinal folds are extensively developed, mineralization is essentially stratiform.

Figure  2.  Regional geological map of the Anjing Hitam area (modified after Reynolds, 2004).

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The Anjing Hitam Pb-Zn orebody is mainly hosted in the Kluet Formation (Figs. 3–4), which comprises three members: (from top to bottom) the Jehe Member largely comprises massive thick-bedded pale to dark grey dolostones, commonly extensively brecciated and veined in multiple events. The strongly deformed Julu Member comprises carbonaceous (dolomitic) shale and siltstone. The Julu Member (main ore host) sandstone contains stratiform, lenticular and massive Pb-Zn sulfide orebodies, and commonly contains also overprinting Pb-Zn vein mineralization.

Figure  3.  Sketch map of the Anjing Hitam Pb-Zn deposit (modified after Reynolds, 2004).

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Figure  4.  Cross-section diagram of the Anjing Hitam Pb-Zn deposit (modified after Reynolds, 2004).

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Drill-hole data reveal that the alterations at Anjing Hitam include carbonization, silicification and sericitization (Fig. 5). All lithologies within the Kluet Formation are carbonate- bearing to some extent, and all are apparently dolomitised. Dolomitization is extensively developed in the marl of the Julu Member. Pyrite-calcite-dolomite assemblage has a spotty texture, particularly in the footwall of the Julu Member carbonaceous shale, which is closely spatially related to the stratiform Zn-Pb mineralisation. Silicification is less extensive and is associated mainly with the overprinting vein-type Zn-Pb mineralization.

Figure  5.  Photographs showing the geological characteristics of Anjing Hitam lead-zinc deposit.

(a) Banded orebody; (b) massive orebody; (c) stratiform orebody; (d) sulfide and its paragenetic gangue minerals bended synchronously with fold; (e) quartz-carbonate veins cut the fold; (f) late hydrothermal superimposition and modification; (g) silicification, calcitization, sericitization; (h) skeleton crystal structure of pyrite; (i) the automorphic, hypidiomorphic and xenomorphic granular of the main ore textures. Qtz. Quartz; Cal. calcite; Ser. sericite; Gn. galena; Sp. sphalerite; Py. pyrite.

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Based on the detailed field geological and petrographic observations, the Anjing Hitam Pb-Zn mineralization is divided into two stages: sedimentary and hydrothermal mineralization.

Sedimentary mineralization (stage Ⅰ) is the main ore-forming stage. It occurs as multiple stratiform orebodies within the Julu Member, particularly in the thinly bedded carbonaceous siltstones but also in the coarser wacked interbeds. The Pb-Zn mineralization zone at Anjing Hitam is up to 800 m long and 100–300 m wide (Fig. 3; Reynolds, 2004), and up to 20 m thick (Figs. 5a–5d). The orebodies are generally stratiform, stratoid or lenticular, and extended along dip and strike with the strata. The drill hole SOP60D intersects 17.67 m of 10.6% Zn and 5.0% Pb. The ores are mainly massive, banded with local brecciation. The ore texture is dominated by subhedral-xenomorphic granular and metasomatic texture. Major metallic minerals include galena, sphalerite and pyrite, whilst gangue minerals are dominated by quartz and carbonates. Wall rock alterations include mainly weak-moderate silicification, carbonization and sericitization (Fig. 5g). The stratiform orebodies are bended by the late folding (Fig. 5d), and crosscut by irregular veins and stockworks of remobilised sulphide.

Vein mineralization (stage Ⅱ) is featured by numerous large-scale polymetallic-sulfide-carbonate-quartz veins that crosscut stage Ⅰ stratiform orebodies. These veins are dominated by calcite, dolomite and quartz, while sphalerite, galena and pyrite are sparsely disseminated in the quartz-calcite veins or stockwork (Figs. 5e–5f).

2. ANALYTICAL METHODS AND RESULTS

2.1 Analytical Methods

Based on field geology and mineralogy, representative calcite samples from the two mineralization stages were selected for the fluid inclusion analysis. Nine polished calcite thin sections (0.06–0.08 mm thick) were prepared for the fluid inclusion study that used heating and freezing measurements. Microthermometric measurements were performed using a Linkam THMS600 heating-freezing stage at the Central South University (Changsha, China). The stage, which was calibrated by using synthetic fluid inclusions, has a maximum temperature limit of 600 ºC and a minimum temperature limit of -196 ºC. The estimated precisions of its measurements are ±0.1 and ±1 ºC for freezing and heating, respectively. Salinities of NaCl-H₂O were estimated using the ideal geometrical mixing equation proposed by Brown and Lamb (1989) using the computer program FLINCOR. Analysis of fluid inclusion composition was performed using an SP3400-1 gas chromatograph and DX-120 ion chromatograph. Detailed operational and analytical processes were as outlined in Wang (1998) and Zhu et al. (2003).

Carbon-oxygen isotope analyses were conducted at the ALS Laboratory Group (Guangzhou), following the analytical procedures similarly described in Li et al. (2013). Using the cavity enhanced absorption spectroscopy with the PDB and SMOW standard, the analysis accuracy was ±0.05‰.

2.2 Fluid Inclusion Petrography

Fluid inclusions in calcite were systematical studied. Calcite in the primary sedimentary period is presenting mostly as belt shape or contaminated products out of sphalerite, galena and pyrite. The orebody in the primary sedimentary period had folding deformation. Calcite in the hydrothermal transformation period is dominantly net-veinlet and veinlet. Sphalerite, galena and pyrite are presenting mostly as belt shape or contaminated products out of calcite.

Fluid inclusion petrography is conducted with the following rules: isolated fluid inclusions and random groups in intragranular calcites crystals were interpreted as primary in origin, whereas those aligned along micro-fractures in transgranular trails were designated as pseudosecondary (Lu et al., 2004; Goldstein, 2003; Roedder, 1984). But secondary inclusions are small and unsuitable for analysis.

According to the phase relationships at room temperature and phase transitions during heating and cooling, the following two types of well-developed primary and pseudosecondary inclusions were observed in these calcite samples: aqueous inclusions and gas-liquid two-phase inclusions. In the sedimentary- stage samples, the inclusions (2–6 μm in diameter) occur in isolation or in cluster and are elliptical, irregular, or elongated in shape. They have varying vapor-liquid ratios of 5% to 20% (Figs. 6a–6b). The hydrothermal-stage inclusions were distributed in clusters, 2–8 μm in diameter, and are irregular or elliptical in shape and have vapor-liquid ratios of 5% to 20% (Figs. 6c–6d).

Figure  6.  Microphotographs of fluid inclusions from the Anjing Hitam lead-zinc deposit.

L. Liquid phase; V. vapor phase.

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2.3 Microthermometric Results

The microthermometric results are summarized in Table 1. The sedimentary stage fluid inclusions were homogenized to liquid at 105 to 199 ºC (mostly 120 to 160 ºC; Fig. 7), averaging at 131 ºC. The inclusion freezing temperatures range from -14.5 to -9.5 ºC, averaging at -13.8 ºC. The calculated salinities are of 9.6 wt.% to 16.6 wt.% NaCleqiv, averaging at 13.2 wt.% NaCleqiv.

Table  1.  Summary of microthermomentric data for fluid inclusions from the Anjing Hitam Pb-Zn deposit

Mineralization stage	Mineral	Freezingtemperature (Tm ice/℃)				Homogenizationtemperature (Th LV/℃)			Salinity (wt.% NaCLeqiv)
		Num.	Range	Average value		Range	Average value		Range	Average value
Primary stage	Calcite	42	-14.5– -9.5	-12.6		105–199	131		9.6–16.6	13.2
Hydrothermal stage	Calcite	46	-20.5– -18.5	-19.7		206–267	234		19.0–22.5	20.4

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Figure  7.  Histograms of homogenization temperature and salinity of fluid inclusion.

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The hydrothermal-stage fluid inclusions were homogenized to liquid at 206 to 267 ºC, averaging at 234 ºC. The freezing temperatures of these inclusions range from -20.5 to -18.5 ºC. The calculated salinities are between 19.0 wt.% and 22.5 wt.% NaCleqiv, averaging at 20.4 wt.% NaCleqiv.

2.4 Liquid-Gas Compositions of Inclusions

Analytical results are listed in Table 2 and summarized below.

Table  2.  Gas and liquid compositions of fluid inclusions

Sample	Mineral	Mineralization stage	Liquid compositions (μg/g)								Ore	Mineralization stage	Gas compositions (μL/g)
			F-	Cl-	NO3-	SO42-	K+	Na+	Mg2+	Ca2+			H2	N2	CO	CH4	CO2	C2H6	H2O
S1	Calcite	Primary sedimentary stage	0.1	15.8	/	6.4	1.6	17.1	/	4.5	Calcite	Primary sedimentary stage	3.4	0.1	/	2.2	294	0.3	1 232
S2	Calcite		/	7.6	/	8.3	1.4	13.2	/	2.6			2.2	0.2	/	4.3	165	0.2	1 862
S3	Calcite		/	9.3	/	5.2	1.1	14.7	0.1	3.7			3.6	0.1	/	5.3	453	0.2	2 376
S6	Calcite	Hydrothermal stage	1.2	10.2	0.2	3.6	0.5	4.7	1.2	8.6	Calcite	Hydrothermal stage	3.5	9.6	0.3	0.4	26.8	/	3 522
S7	Calcite		0.7	13.2	0.1	3.6	0.7	7.8	0.6	4.2			1.3	2.2	0.2	0.1	10.1	/	1 180
S8	Calcite		0.3	11.6	0.2	2.3	1.3	14.1	0.6	4.7			0.9	1.3	0.2	0.1	10.3	/	1 616

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(1) For the sedimentary-stage fluid inclusions, the liquid- phase cations were mostly Na⁺ and Ca²⁺, plus minor K⁺ and Mg²⁺. The anions are dominated by Cl⁻ and minor SO₄²⁻ and F⁻. These ions (in decreasing abundance) are: Na⁺ > Cl⁻ > SO₄²⁻ > Ca²⁺ > K⁺ > Mg²⁺ > F⁻. The gaseous phase components are mainly H₂O and CO₂ with some H₂, N₂, and CH₄, and a small amount of C₂H_6.

(2) For the hydrothermal reformation stage fluid inclusions, the liquid-phase cations are mostly Na⁺ and Ca²⁺, plus minor K⁺ and Mg²⁺. The anions are dominated by Cl⁻, plus some SO₄²⁻, F⁻ and NO₃⁻. These ions (in decreasing abundance) are: Cl⁻ > SO₄²⁻ > Ca²⁺ > Na⁺ > Mg²⁺ > K⁺. The gaseous phase components are mainly H₂O with some CO₂, H₂, N₂, and a small amount of CH₄ and CO.

2.5 Carbon and Oxygen Isotopes

Five sedimentary-stage and four hydrothermal vein-stage fluid inclusion samples from calcite were used for the carbon- oxygen isotope analyses, and the results are shown in Table 3. For the sedimentary-stage fluid inclusions, the δ¹⁸O_SMOW values range from 14.4‰ to 15.3‰ (mean of 16.5‰). The δ¹⁸C_PDB and δ¹⁸O_H2O values range from -2.2‰ to -0.5‰ (mean of -1.6‰) and from 1.1‰ to 4.8‰ (mean of 2.8‰), respectively. For the hydrothermal vein-stage fluid inclusions, the δ¹⁸O_SMOW values range from 14.8‰ to 15.4‰ (mean of 15.0‰). The δ¹⁸C_PDB and δ¹⁸O_H2O values range from -6.7‰ to -4.8‰ (mean of -5.6‰) and from 5.0‰ to 7.6‰ (mean of 6.8‰), respectively.

Table  3.  Carbon and oxygen isotope compositions of Anjing Hitam Pb-Zn desposit

Sample	Mineralization period	Ore	Mineral	δ13CPDB (‰)	δ18OSMOW (‰)	T (℃)	δ18OH2O (‰)
S1	Primary stage	Banded ore	Calcite	-1.8	15.3	130	1.6
S2		Massive ore	Calcite	-2.2	18.5	130	4.8
S3		Massive ore	Calcite	-1.3	17.6	125	3.5
S4		Banded ore	Calcite	-0.5	14.4	135	1.1
S5		Massive ore	Calcite	-2.0	16.6	130	2.9
S6	Hydrothermal stage	Vein ore	Calcite	-6.7	15.4	235	5.0
S7		Vein ore	Calcite	-4.8	15.2	230	7.6
S8		Vein ore	Calcite	-5.9	14.7	235	7.3
S9		Vein ore	Calcite	-5.1	14.8	230	7.2

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3. DISCUSSION

3.1 Ore-Forming Fluid Properties

Stage I (sedimentary) fluid inclusion assemblages of the Anjing Hitam Pb-Zn deposit are relatively simple: the inclusions are mainly composed of gas-liquid two-phase aqueous solution with liquid-rich phase, resembling those from MVT- or SEDEX-type deposits (Li et al., 2018; Zhou et al., 2018; Li and Xi, 2015; Chen et al., 2007). The homogenization temperatures of stage Ⅰ inclusions are 105–199 ºC, characteristic of low-temperature mineralization. The salinity is medium to low (9.6 wt.%–16.6 wt.% NaCleqiv) of typical SEDEX deposits (60–280 ºC and 4 wt.%–23 wt.% NaCleqiv, respectively; Lu and Shan, 2015; Wang et al., 2014; Wilkinson, 2014; Liu et al., 2008; Basuki, 2002; Cooke et al., 2000; Lydon, 1983; Badham and Williams, 1981).

The homogenization temperature and salinity of the stage Ⅱ fluid inclusions are 206–267 ºC and 19.0 wt.%–22.5 wt.% NaCleqiv, respectively, indicating a medium temperature and salinity for the ore fluids. The temperature and salinity are higher than those of stage Ⅰ.

3.2 Fluid Source

The primary inclusions are formed simultaneously with the host minerals during the diagenesis and mineralization, which captures the ore-forming fluids (Wang X et al., 2017; Li et al., 2013; Lu et al., 2004). Therefore, their compositions can reflect the physicochemical conditions in which the minerals were formed. Our analyses of the gas-liquid compositions of the fluid inclusions show that the Anjing Hitam stage Ⅰ ore-forming fluids belong to a CO₂-rich H₂O-NaCl system (Table 2). The gas phase of the inclusions contains CO₂, N₂, CH₄ and C₂H₆, and the H₂O and CO₂ contents are relatively high, resembling those of typical deep-sourced fluids (Lu et al., 2004). The enrichment of Ca²⁺ and CO₂ in the fluid inclusions suggests that the ore-forming fluids may have influenced by the fractional crystallization of deep-lying granitic intrusion (Li et al., 2017). Cations in the inclusions include mainly Na⁺ and Ca²⁺, and the anions include mainly Cl⁻, with Na⁺/K⁺=9.4–13.3 (> 10) and F⁻/Cl⁻ of about 0 (< 1). These characteristics are similar to those of SEDEXT-type deposits (Roedder, 1984), suggesting that the Anjing Hitam stage Ⅰ ore-forming fluids were mostly likely to be derived from the exhalative sedimentary process (Zhang, 1992). Stable isotopes are wide used to identify the ore-forming fluid sources (Zheng, 2001; Ohmoto and Goldhaber, 1997; Taylor, 1997; Rye and Ohmoto, 1974). There are three sources of CO₂ in hydrothermal solutions: magmatic carbon, sedimentary carbon and organic carbon. Our carbon isotope analysis shows that the calcite δ¹³C_PDB values of the stage Ⅰ ores are -2.2‰ to -0.5‰, which fall between the ranges of magmatic carbon (δ¹³C_PDB=0.8‰ to 2.0‰; Kroopnick et al., 1972) and sedimentary carbon (δ¹³C_PDB= -9.0‰ to -3.0‰; Hoefs, 1973). This further suggests that the stage Ⅰ ore-forming fluids were sourced from both deep-sourced magmatic rocks and exhalative sedimentary process. Similarly, oxygen isotope analysis shows that the calcite δ¹⁸O_H2O values of the stage Ⅰ ores are 1.1‰–4.8‰, falling between the seawater (δ¹⁸O= -1.0‰ to 1.0‰; Sheppard et al., 1971) and magmatic water (δ¹⁸O=5.5‰–9.0‰; Sheppard et al., 1971) fields. Though the Late Carboniferous–Permian granic activities weren't found at Anjing Hitam, the Kluet Formation, which is the host rock for the SEDEX orebodies and mainly composed of clastic rock and mafic volcanic rocks, is widely exposed in the region (Cameron et al., 1982). It indicates that the local magmatic activity may have contributed to the SEDEX mineralization.

Gaseous components of the stage Ⅱ fluids include CO₂, N₂, CH₄, CO, and the H₂O and CO₂ contents are high. Cations are also mainly Na⁺ and Ca²⁺, and anions are mainly Cl⁻, with Na⁺/K⁺=9.4–11.1 (> 10) and F⁻/Cl⁻=0–0.01 (< 1), which are similar to those of the stage Ⅰ ore. The calcite δ¹³C_PDB and δ¹⁸O_H2O values of the stage Ⅱ ores are -6.7‰ to -4.8‰ and 14.7‰ to 15.4‰, respectively, which are different from those of the stage Ⅰ ore (especially in δ¹⁸O_H2O values). These characteristics suggest that the stage Ⅱ ore-forming fluids were mainly derived from magmatic process but also inherited some isotope features from the stage Ⅰ SEDEX-mineralization. During the Cenozoic, the Indian Plate moved eastward and subducted beneath Sumatra, forming the three back-arc basins of North Gate, Central Post and South Sumatra (Cao et al., 2005). The subduction was accompanied by intense magmatism (Ulmer and Trommsdorff, 1995), as shown by the extensive Pleistocene tuff deposition around the Toba crater at Sidikalang. The Pleistocene tuff is located to the northern of the Anjing Hitam area (Fig. 2). This intense magmatism may have been responsible for stage Ⅱ hydrothermal vein-type mineralization. Therefore, we propose that the stage Ⅱ ore-forming fluids were mainly derived from magmatic rocks, and contain inheritance from the wall rocks (including stage Ⅰ SEDEX ores).

3.3 Ore Deposit Type

SEDEX deposits are mostly developed on the divergent plate margins and intraplate rift (Sun et al., 2011; Betts et al., 2003; Lydon, 1996), or back-arc rift basins (Betts and Lister, 2002). This is mainly because the upper crustal permeability can be enhanced significantly under extensional environments, which would promote fluid circulation (Russell, 1983). In addition, the syn-sedimentary faults resulted from the disintegration (Miall, 1984) would provide the ore-bearing fluids with migration channels and space for precipitation, which are also key conditions for the SEDEX mineralization (Zhang et al., 2010). For example, the famous Lasbela-Khuzdar SEDEX Pb-Zn belt in Pakistan, which hosts the Surmai, Gunga, Dhungei, Duddar and many other Pb-Zn deposits (Leach et al., 2005; Janković, 2001; Turner, 1992; Sillitoe, 1978), is considered to have resulted from the rifting of the Cimmerian terrane (including Transcaucasia, Central Iran, Southern Afghanistan, Southern Pamirs, Qiangtang, Sibumasu, ect.; Ueno, 2003; Metcalfe, 1997, 1996; Şengör, 1987) from the Gondwana supercontinent. In northern Sumatra, the Kluet Formation clastic rocks are found to have decreasing grain sizes from northeast to southwest, suggesting the formation of a passive continental rift basin to the southwest (Barber and Crow, 2003; Cameron et al., 1980). This tectono-sedimentary setting in in northern Sumatra resembles that of the Lasbela-Khuzdar SEDEX Pb-Zn belt.

SEDEX Pb-Zn deposits commonly contain the following features: (1) metals are mainly Pb, Zn and Ag, and the ore minerals are dominated by sphalerite, galena and pyrite and Zn-rich minerals (Leach et al., 2005; Lydon, 1996); (2) stratiform orebodies hosted by marine sedimentary rocks (Leach et al., 2010, 2005; Han and Sun, 1999) such as fine marine clastic and carbonate rocks. The sedimentary rocks are commonly folded; (3) massive or bedded ore structure; (4) orebodies are controlled by the syn-sedimentary faults; (5) most deposits were formed mostly during the Paleoproterozoic, Cambrian and Early Mesozoic (Leach et al., 2010; Goodfellow and Lydon, 2007; Laznicka, 2006); (6) ore-forming structures are mostly sedimentary and textensional-related, and the orebodies are controlled by rift-controlled the sedimentary basins (Sun et al., 2011; Betts et al., 2003; Lydon, 1996). Many of these features are present at Anjing Hitam, including: (1) ore minerals are mainly sphalerite, galena and pyrite; (2) orebodies are hosed in the Upper Carboniferous–Permian Kluet Formation marine sedimentary rocks; (3) the deposit is bounded by the regional syn- sedimentary Balga fault; (4) during the Late Carboniferous– Permian, Sumatra was likely situated in an extensional, passive continental margin environment (Ueno, 2003; Metcalfe, 1996; Şengör, 1987). Therefore, the Anjing Hitam deposit is best classified to a modified SEDEX type.

4. CONCLUSIONS

(1) The Anjing Hitam Pb-Zn orebodies are mainly stratiform and hosted in the middle part of the Carboniferous– Permian Kluet Formation of the Tapanuli Group. According to the mineral assemblage and paragenesis, the mineralization can be divided into two stages: stage Ⅰ exhalative sedimentary mineralization and stage Ⅱ hydrothermal vein-type mineralization.

(2) The stage Ⅰ ore-forming fluids were likely derived mainly from exhalative sedimentary (SEDEX) processes involving some deep-seated magma sourced fluids. The stage Ⅱ ore-forming fluids were likely magmatic fluids with certain input from the wall rocks and the remobilization of stage Ⅰ SEDEX ores.

(3) The stage Ⅰ SEDEX Pb-Zn mineralization at Anjing Hitam Pb-Zn deposit was formed in a Late Carboniferous to Permian passive continental rift basin, associated with the opening of the Paleotethys.

ACKNOWLEDGMENTS

We gratefully acknowledge the anonymous reviewers for their critical comments and constructive suggestions, which have improved the quality of the paper greatly. This study was financially supported by the National Basic Research Program of China (No. 2014CB440901). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0859-z.