Geological, fluid inclusion and isotopic studies of the Yinshan Cu–Au–Pb–Zn–Ag deposit, South China: Implications for ore genesis and exploration

Journal of Asian Earth Sciences 74 (2013) 343–360

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Geological, fluid inclusion and isotopic studies of the Yinshan Cu–Au–Pb–Zn–Ag deposit, South China: Implications for ore genesis and exploration Guo-Guang Wang, Pei Ni ⇑, Ru-Cheng Wang, Kui-Dong Zhao, Hui Chen, Jun-Ying Ding, Chao Zhao, Yi-Tao Cai State Key Laboratory for Mineral Deposits Research, Istitute of Geo-Fluids, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China

a r t i c l e

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Article history: Available online 6 December 2012 Keywords: Yinshan Cu–Au–Pb–Zn–Ag deposit Fluid inclusions H–O isotopes S–Pb isotopes South China

a b s t r a c t The Yinshan Cu–Au–Pb–Zn–Ag deposit is located in Dexing, South China. Ore bodies are primarily hosted in low-grade phyllite of the Neoproterozoic Shuangqiaoshan Group along EW- and NNW-striking fault zones. Pb–Zn–Ag mineralization is dictated by Jurassic rhyolitic quartz porphyries (ca. 172 Ma), whereas Cu–Au mineralization is associated with Jurassic dacite porphyries (ca. 170 Ma). The main ore minerals are pyrite, chalcopyrite, galena, sphalerite, tetrahedrite–tennatite, gold, silver, and silver sulphosalt, and the principal gangue minerals are quartz, sericite, calcite, and chlorite. Two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing (type III) fluid inclusions can be observed in the hydrothermal quartz-sulfides veins. Type I inclusions are widespread and have homogenization temperatures of 187–303 °C and salinities of 4.2–9.5 wt.% NaCl equivalent in the Pb–Zn–Ag mineralization, and homogenization temperatures of 196–362 °C and salinities of 3.5–9.9 wt.% NaCl equivalent in the Cu–Au mineralization. The pervasive occurrence of type I fluid inclusions with low-moderate temperatures and salinities implies that the mineralizing fluids formed in epithermal environments. The type II and coexisting type III inclusions, from deeper levels below the Cu–Au ore bodies, share similar homogenization temperatures of 317–448 °C and contrasting salinities of 0.2–4.2 and 30.9–36.8 wt.% NaCl equivalent, respectively, which indicates that boiling processes occurred. The sulfur isotopic compositions of sulfides (d34S = 1.7‰ to +3.2‰) suggest a homogeneous magmatic sulfur source. The lead isotopes of sulfides (206Pb/204Pb = 18.01–18.07; 207Pb/204Pb = 15.55–15.57; and 208Pb/204Pb = 38.03–38.12) are consistent with those of volcanic–subvolcanic rocks (206Pb/204Pb = 18.03–18.10; 207Pb/204Pb = 15.56–15.57; and 208 Pb/204Pb = 38.02–38.21), indicating a magmatic origin for lead in the ore. The oxygen and hydrogen isotope compositions (d18O = +7.8‰ to +10.5‰, dD = 66‰ to 42‰) of inclusion water in quartz imply that ore-forming fluids were mainly derived from magmatic sources. The local boiling process beneath the epithermal Cu–Au ore-forming system indicates the possibility that porphyry-style ore bodies may exist at even deeper zones. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Epithermal deposits, first proposed by Lindgren (1933), are important sources of gold, silver and other base metals (Simmons et al., 2005). In the past few years, meteoric water was thought to be crucial for the ore formation of epithermal deposits due to the isotopic features of a significant portion of deeply circulated meteoric water (O’Neil and Silberman, 1974; Simmons et al., 2005; Taylor, 1997). However, the interpretation of stable H and O isotopes data from epithermal ore deposits has recently shifted from the emphasis on the role of meteoric water (e.g., O’Neil and ⇑ Corresponding author. Tel.: +86 25 83597124; fax: +86 25 83592393. E-mail address: [email protected] (P. Ni). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.038

Silberman, 1974; Taylor, 1997) to the recognition that magmatic fluids play key roles as metal contributors to epithermal deposits (Giggenbach, 1992; Kouzmanov et al., 2003; Vennemann et al., 1993). Fluid inclusions studies and thermodynamic modeling indicate that low-medium salinity fluids in epithermal environments can be directly generated in a wide range of cooling paths from a single-phase magmatic fluid (Hedenquist et al., 1998; Heinrich, 2005; Heinrich et al., 2004; Muntean and Einaudi, 2001). More importantly, these low-medium salinity fluids with significant magmatic water components commonly occur in the epithermal systems that show intimate temporal and spatial association with porphyry deposits (Hedenquist et al., 1998; Heinrich, 2005; Muntean and Einaudi, 2001). Previous studies on the epithermal-porphyry system demonstrate that low-moderate temperature and

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salinity fluid inclusions are abundant in the shallow epithermal environment, and boiling fluids inclusions occur in the deeper conditions associated with porphyry-style mineralization (Hedenquist et al., 1998; Muntean and Einaudi, 2001). In this paper, a polymetallic deposit with low-moderate temperature magmatic ore fluids in the shallow epithermal environment and boiling inclusion assemblages at deeper levels below Cu–Au ore bodies will be shown. The Yinshan Cu–Au–Pb–Zn–Ag deposit is located in the famous Dexing copper–gold polymetallic district in South China (Fig. 1). This polymetallic district consists of the Middle Jurassic (ca. 170 Ma) Yinshan Cu–Au–Pb–Zn–Ag deposit (950,000 tons of Cu and 107 tons of Au) (JGEB, 1996; Yinshan Lead-Zinc Mine, 1993; Wang et al., 2011b), the Middle Jurassic (ca. 170 Ma) Dexing porphyry Cu–Au–Mo deposits (6 Mt of Cu and 138 tons of Au) (Wang et al., 2006; Zhu et al., 1983), and the Neoproterozoic (ca. 866 Ma) Jinshan gold deposit (180 tons of Au) (Li et al., 2010; Wei, 1996). The mining history at Yinshan can be traced back to the sixth century during the Sui Dynasty. The modern Yinshan mine began operation in 1958. By the end of 1990, the proved mineral reserves of the ores were up to 670,000 tons Cu, 82 tons Au, 430,000 tons Pb, 520,000 tons zinc, and 3112 tons Ag, with grades of 0.52% Cu and 0.61 g/t Au, 1.25% Pb, 1.02% Zn and 11.5 g/t Ag (Yinshan Lead-Zinc Mine, 1993). Recent deep drilling added new proved mineral reserves of 280,000 tons Cu, 25 tons Au, 120,000 tons Pb–Zn, and 333 tons Ag, with grades of 0.6% Cu, 0.57 Au g/t, 4.5% Pb–Zn, and 7.6 g/t Ag (Xu et al., 2007). Previous studies of the area focus on the deposit geology (Hua, 1987; Ye, 1983), the petrogenesis of volcanic–subvolcanic rocks (JGEB, 1996; Mao et al., 2011; Wang et al., 2006), the relationship between volcanic–subvolcanic activities and polymetallic mineralization (Chen and Du, 1988; Du and Chen, 1987; Hao, 1988; Hua, 1987; JGEB, 1996; Li et al., 2007; Ye, 1983), the mineralogy of ore (Hao, 1988), the ages of alteration minerals (Li et al., 2005), and the comparison with the adjacent Dexing porphyry deposits (Chen and Du, 1988; JGEB, 1996; Zhang et al., 1997). From these studies, it can be concluded that polymetallic mineralization was

controlled by Jurassic (176–166 Ma) volcanic–subvolcanic activities (Hua, 1987; JGEB, 1996; Li et al., 2007; Wang et al., 2012; Ye, 1983). Nevertheless, the characteristics of ore-forming fluids are still poorly constrained because only a few studies have focused on the fluid inclusions of the shallow part ( 150 to 60 m) (Zhang et al., 2007). The recent advancement of deep drilling in the Yinshan deposit has given us a unique opportunity to obtain samples from new depths (up to 1174 m). In addition to providing surface and underground samples, this paper presents detailed field observations, fluid inclusions and H–O–S–Pb isotope data to determine the characteristics, sources and evolution of the ore-forming fluids from the deep to the shallow levels. Moreover, these data have implications for the genetic types of ore deposits and may serve as a useful guide for exploring porphyry-style Cu–Au ore bodies in an even deeper zone. 2. Tectonic settings and regional geology The South China Block consists of the Yangtze Block in the north and the Cathaysia Block in the south. The NEE–SSW Jiangnan orogen probably represents the Neoproterozoic subduction and convergence between the Yangtze and Cathaysia Blocks (Chen et al., 1991; Li et al., 2009; Wang et al., 2008, 2007; Zhou et al., 2009). During the amalgamation of these two blocks, the Neoproterozoic Pingshui and Tieshajie VMS-type copper deposits and Jinshan orogenic gold deposits were developed along the Jiangnan orogen (Chen et al., 2011, 2009; Zhang et al., 2009a). It suggests that such copper–gold rich melts generating from the subducted oceanic crust could form Neoproterozoic juvenile continental crust, which likely is the magmatic and metal sources for the Jurassic Yinshan and Dexing deposits (Wang et al., 2012). It is well known that large-scale Mesozoic magmatism and mineralization is developed in the South China (Hua et al., 2005; Li, 2000; Mao et al., 2008; Zhou, 2007; Zhou et al., 2006). Several tectonic models have been proposed to explain the Mesozoic evolution of South China (He et al., 2010; Li and Li, 2007; Ling et al.,

Fig. 1. (A) The location of the study area in China and (B) a sketch map of the geology and the distribution of ore deposits in Dexing, Northeast Jiangxi Province, South China (modified from Zhu et al.(1983) and Mao et al. (2011)).

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2009; Sun et al., 2007b; Wang et al., 2011a, 2012, 2006; Zhou et al., 2006). For the geodynamic setting of Jurassic Yinshan and Dexing deposits, the models can be roughly classified into three groups: (1) a localized extensional environment in response to far-field stress at plate margins resulting from the initial paleoPacific plate northwestward subduction (He et al., 2010; Wang et al., 2012; Zhou et al., 2006); (2) southwestward subduction of the Pacific plate and the corresponding slab rollback (Sun et al., 2007b; Wang et al., 2011a); and (3) an extensive intra-plate extensional setting corresponding to the flat-slab subduction of the Pacific plate (Li and Li, 2007; Wang et al., 2006). However, lack of arc-related magmatism in SE China during Mesozoic does not supported the second model. Moreover, obvious Triassic E– W tectonic elements in South China (Zhang et al., 2009b) is inconsistent with the existence of westward subduction of the Pacific plate initiated at 250 Ma as discussed in the third model. As a consequence, the last two models still need further studies to test. The Yinshan and Dexing deposits likely were formed in a localized intra-continental extensional setting via partial melting of copper–gold rich Neoproterozoic subduction-related juvenile crust (Wang et al., 2012). In the Dexing region, the Neoproterozoic Shuangqiaoshan Group is covered by the Middle Jurassic Ehuling Formation that outcrops in the NE-trending Kongjia-Yinshan continental volcanic basin (Fig. 1). The Neoproterozoic basement strata of the Shuangqiaoshan Group are rich in volcanic materials composed of shallow sea sedimentary argillo-arenaceous clastic rocks and volcanic clastic rocks intercalated with lava. Under low-grade regional greenschist facies metamorphism, these clastic rocks formed a set of epimetamorphic rocks with a thickness of 2678–5472 m (Wang et al., 2008). The zircon U–Pb data of the volcanic rocks yield a depositional age of ca. 860 Ma (Wang et al., 2008). However, Li et al. (2009) argue that these samples have a younger age of ca. 770 Ma. The ages of 860 Ma and 770 Ma both lead to the conclusion that the Shuangqiaoshan Group was deposited during the Neoproterozoic period. The Jurassic volcanic rocks are over 200– 1000 m in thickness and consist mainly of phreato-magmatic breccia, agglomerate, lavas, and tuff breccia of dacitic or rhyolitic compositions (JGEB, 1996). The Northeast Jiangxi Fault is the dominant linear tectonic feature along the Dexing region (Fig. 1), extending continuously for at least 200 km from Dongxiang in Jiangxi Province northeastward to Shexian in Anhui Province. It dips steeply toward the northwest with a width of 10–20 km. The NE-trending Le’anjiang Fault is in the northwest, and the Sizhoumiao anticlinorium is at the center of this region (Fig. 1). The Neoproterozoic Northeast Jiangxi ophiolites in the Dexing district occur discretely along the Northeast Jiangxi Fault. The main rock types include serpentinized peridotite, cumulate gabbro, dolerite, diorite, anorthosite, plagioclase granite, spilite, keratophyre, basalt and andesite (Li et al., 1997). The formation age of the Northeast Jiangxi ophiolites is approximately 1.0–0.96 Ga, obtained by Sm–Nd isochron and zircon U–Pb data, respectively (Chen et al., 1991; Li et al., 1997). Northeast Jiangxi ophiolites were likely formed in the back-arc basin to the south of the Yangtze Block during the Neoproterozoic period (Li et al., 1997, 2007b). The Middle Jurassic porphyries/subvolcanic rocks are present in the Yinshan and Dexing deposits. The subvolcanic rocks at Yinshan comprise rhyolitic quartz porphyries, dacite porphyries and andesite porphyries. In contrast, the porphyries at Dexing primarily consist of granodiorite porphyries with minor quartz monzodiorite porphyries. The dating of a suite of rock samples collected from rhyolitic quartz porphyries to andesite porphyries in the Yinshan deposit yields a LA-ICP-MS U–Pb zircon age range of ca. 176– 166 Ma (Wang et al., 2012). The Dexing porphyries are emplaced

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simultaneously in the Middle Jurassic (ca. 171 Ma) (Wang et al., 2006; Zhou et al., 2012).

3. Deposit geology The wall rocks consist of the Neoproterozoic Shuangqiaoshan Group and Middle Jurassic volcanic–subvolcanic rocks of the Ehuling Formation (Figs. 2 and 3). The low-grade metamorphic rocks of the Shuangqiaoshan Group exposed in the most parts of this deposit include phyllite and tuffaceous phyllite with a thickness of 2500 m (Ye, 1983). The Middle Jurassic volcanic–subvolcanic rocks in the Yinshan deposit are divided into three cycles based on stratigraphic and cutting relations. The first cycle is characterized by eruptions of rhyolitic lava (0.8 km2) with a thickness of ca. 450 m in the Xishan district, accompanied by intrusive stocks of rhyolitic quartz porphyries (0.02–0.04 km2) in the Jiuqu and Beishan districts. The second cycle is represented in the formation of approximately 700 m thick dacitic volcanic rocks at the Xishan district and nearly EW-trending dacite porphyries (0.09 km2) in the Jiuqu district. The last cycle is represented as small intrusive stocks of andesitic porphyries (<0.01 km2) in the former two cycles of volcanic rocks in the Xishan area (JGEB, 1996) (Fig. 2). The zircon U–Pb ages of quartz porphyries and dacite porphyries (ca. 175–170 Ma, Wang et al., 2012) are consistent with the muscovite 40Ar–39Ar ages (ca. 175 Ma) (Li et al., 2007a), indicating that the mineralization was coeval with the volcanic–subvolcanic events. The major structures developed in this deposit are the Yinshan anticline and faults. The axis of the Yinshan anticline is oriented at NE 45–50° and plunges in the NE direction (Fig. 2). Both limbs are composed of Shuangqiaoshan Group low-grade metamorphic rocks. The overlying Ehuling Formation volcanic rocks are not folded, indicating that this folding event took place before the Middle Jurassic period. The Yinshan anticline, as well as the regional-scale Sizhoumiao synclinorium, was likely formed during the Neoproterozoic period resulting from the assemblages of the Yangtze and Cathaysia blocks (Li et al., 2010; Liu, 2005). Accompanying the formation of Yinshan anticline, a series of faults were generated. These major NE-oriented faults were developed well along the axial plane of the Yinshan anticline, and second-order EW- and NWW-oriented faults were formed in the two limbs. During the Middle Jurassic, the reactivation of pre-existing faults in the Dexing area and Yinshan deposit most likely occurred (Wang et al., 2012, 2006). The subvolcanic rocks associated with mineralization were emplaced at the intersections of the major NE-trending fault and the second-order EW- and NWW-trending faults. Polymetallic ore bodies formed primarily along the second-order EW- and NWW-trending faults (Mao et al., 2011; Ye, 1983). The Yinshan deposit can be divided into six ore sections: the Nanshan, Yinshan East, Yinshan West, Jiuqu, Jiulongshangtian (simplified as Jiulong), and Beishan districts from the south to the north (Fig. 2). Cu–Au ore bodies are distributed in the Jiuqu, Yinshan West, and Yinshan East districts, whereas Pb–Zn–Ag ore bodies are in the Beishan, Jiulong, Nanshan and Yinshan East districts. The trends of the ore bodies are E–W striking in the NW limb of the Yinshan anticline and N–S striking in the SE limb. Most of the ore bodies dip steeply and extend >1050 m vertically along faults. The lengths of individual ore bodies vary primarily between 300 and 600 m, with a maximum of 1050 m; the thicknesses vary from 1 to 15 m (Fig. 3). Based on the results of previous field investigations, petrographic observations and mineral assemblages, two types of mineralization have been identified: Pb–Zn–Ag mineralization, associated with the first cycle of rhyolitic quartz porphyries, and Cu–Au mineralization, associated with the second cycle of dacite porphyries (Hua, 1987; Li et al., 2005; Zhang et al., 1997). All of

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Fig. 2. Geological map of the Yinshan Cu–Au–Pb–Zn–Ag polymetallic deposit (modified from Wang et al. (2012)).

the ore bodies clearly exhibit fault-controlled features (Fig. 4A–D). Sericitic alteration and carbonatization are generally associated with Pb–Zn–Ag mineralization, and phyllic alteration is commonly observed along the Cu–Au ore bodies (Fig. 4M and O). The broadscale pattern of alteration surrounding the two types of mineralization is propylitic alteration (Fig. 4N and P). The quartz-sulfide veins formed in the Pb–Zn–Ag mineralization consist of ore minerals, including galena, sphalerite, pyrite, native silver and Pb–Ag–

Sb-sulfosalts (Fig. 4E and F, and I and J), and gangue minerals, such as quartz, calcite, sericite and chlorite (Fig. 4M and N). The Cu-Au mineralization is dominated by quartz-sulfide veins (Fig. 4G and H, and K and L), comprised of ore minerals such as chalcopyrite, pyrite, tetrahedrite, and native gold and gangue minerals such as sericite, quartz, chlorite and minor calcite (Fig. 4O and P). The ore mineral assemblages in the Cu–Au ore zones vary with depth. Tetrahedrite–tennatite association is abundant in the shallow

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Fig. 3. Cross sections through the majority of ore bodies in the Yinshan deposit, showing the locality of samples and the according types of fluid inclusions (modified from JGEB (1996); the location is indicated in Fig. 2).

environment, whereas chalcopyrite–pyrite assemblage is predominant in deeper levels.

4. Analytical methods Doubly polished thin sections were prepared from samples of Yinshan deposit. Various inclusions in quartz and sphalerite were selected for microthermometry based on their petrographic characters. The microthermometric measurements were carried out with a Linkam THMS600 heating-freezing stage with a temperature range of 195 °C to +600 °C. The stage was calibrated by measuring the melting points of pure water inclusions (0 °C), pure CO2 inclusions (398 °C), and potassium bichromate (398 °C). The accuracy of the measured temperatures was approximately ±0.2 °C during cooling and ±2 °C between 100 and 600 °C. The salinities of NaCl–H2O inclusions were calculated using the final melting temperatures of the ice (Bodnar, 1993). Salinities of halite-bearing fluid inclusions were calculated using the dissolution temperatures of daughter minerals (Hall et al., 1988). The compositions of single fluid inclusions were identified using a Renishaw RM2000 Raman microprobe with an Ar ion laser with a surface power of 5 mW to excite the radiation (514.5 nm); the detector change-coupled device (CCD) area was 20, and the scanning range of spectra was set between 1000 and 4000 cm 1 with an accumulation time of 30 s for each scan. All the fluid inclusion studies were carried out in the State Key Laboratory for Mineral Deposit Research, Nanjing University.

The sulfur isotope measurements of sulfide were performed on powders made from ore samples from the Yinshan deposit. Sulfide was extracted, and the sulfur isotope composition was determined based on the method of Robinson and Kusakabe (1975). It was then analyzed with a MAT251EM mass spectrometer at the Stable Isotope Laboratory of Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS). The sulfur isotope ratios were reported as d34S relative to the Caòon Diablo Troilite (CDT); the analytical reproducibility was ±0.2‰. The oxygen isotope compositions of quartz from Pb–Zn–Ag and Cu–Au mineralization were determined. Quartz samples were hand-picked and/or separated using a magnetic separator. Oxygen was liberated from quartz by reaction with BrF5 (Clayton and Mayeda, 1963) and converted to CO2 on a platinum-coated carbon rod. The d18O determinations were made using a MAT-252 mass spectrometer at the State Key Laboratory for Mineral Deposit Research, Nanjing University. Reproducibility for isotopically homogeneous pure quartz was about ±0.1‰ (1r). The hydrogen isotopic compositions of the fluid inclusions were analyzed for the same quartz samples measured for oxygen isotopes. Samples were first degassed of labile volatiles by being heated under a vacuum at 150 °C for 3 h. Water was released by heating the samples to approximately 500 °C by means of an induction furnace. Water was converted to hydrogen by passage over heated zinc powder at 410 °C (Friedman, 1953), and the hydrogen was analyzed with a MAT-252 mass spectrometer. The analysis of standard water samples suggested a precision for dD of ±3‰ (1r). The hydrogen isotopic analyses were performed at

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Fig. 4. Photographs of representative samples from the Yinshan deposit. (A) Pb–Zn–Ag veins controlled by faults; (B) Pb–Zn–Ag veins associated with quartz; (C and D) Cu–Au veins controlled by faults; (E and F) polished surface of Pb–Zn–Ag ore with quartz; (G) polished surface of Cu–Au ore with chalcopyrite and pyrite; (H) polished surface of Cu– Au ore containing tetrahedrite, chalcopyrite and pyrite; (I) quartz coexisting with sphalerite and galena; (J) concentric structure of sphalerite; (K) quartz coexisting with pyrite and chalcopyrite; (L) quartz coexisting with chalcopyrite, tetrahedrite and pyrite; (M) calcite, quartz and sericite with Pb–Zn–Ag mineralization; (N) chlorite partially occur in the Pb–Zn–Ag mineralization zone; (O) quartz, sericite and pyrite with Cu–Au mineralization; (P) chlorite commonly occur in the Cu–Au mineralization zone. Ccp chalcopyrite, Gn galena, Py pyrite, Q quartz, Sph sphalerite, Ttr Tetrahedrite, Cc calcite, Chl chlorite, Ser sericite.

the Stable Isotope Laboratory of Institute of Mineral Resources, CAGS. For Pb isotopic determination, fresh plagioclase of Yinshan samples was analyzed for common Pb. Samples were dissolved in concentrated HF, and Pb was purified by the cation-exchange technique described by He et al. (2005). Isotopic ratios were measured with a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) at the Institute of Geology, CAGS. Thallium was added as an internal standard to determine mass fractionation. Long-term analysis of the NBS981 standard yielded 206 Pb/204Pb = 16.940 ± 0.010 (±2r), 207Pb/204Pb = 15.498 ± 0.009, and 208Pb/204Pb = 36.716 ± 0.023.

5. Fluid inclusions results 5.1. Fluid inclusion types and occurrence The specimens were collected from open pits, underground tunnels of different levels at +96 m, +48 m, 150 m, 195 m, 240 m and 330 m, as well as from two deep drill holes from 287 to

1174 m in the Cu–Au zones and from 236 to 872 m in the Pb–Zn–Ag zones, respectively (Fig. 3). Thirty-five samples were selected for further fluid inclusion studies. Fluid inclusions that occurred as individual inclusions and random groups in intragranular quartz crystals were interpreted as primary in origin, whereas those aligned along micro-fractures in transgranular trails were designated secondary (Lu et al., 2004; Roedder, 1984). According to the nature of phase relationships at room temperature and phase transitions during heating and cooling, three types of fluid inclusions were recognized in the studied quartz and sphalerite samples using the nomenclature of Shepherd et al. (1985): two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing fluid inclusions (type III). Two-phase liquid-rich fluid inclusions (type I) consisted of a vapor bubble and a liquid phase. The vapor phase normally occupies between 5 and 15 vol.%. The inclusions are of regular and irregular shapes and 4–15 lm in size. Type I inclusions were dominant in the Pb–Zn–Ag and Cu–Au ore zones (Figs. 3 and 5A–C). Two-phase vapor-rich fluid inclusions (type II) were characterized by a dark vapor bubble generally exceeding 50% of the inclusion volume. The inclusions were 2–20 lm in size and shaped in

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Fig. 5. Microphotographs showing different types of fluid inclusions observed in the Yinshan polymetallic deposit. (A) type I inclusions in quartz of Pb–Zn–Ag mineralization; (B) type I inclusions in sphalerite of Pb–Zn–Ag mineralization; (C and D) type I inclusions in quartz of Cu–Au mineralization; (E and F) type II and coexisting type III inclusions in quartz of Cu–Au mineralization. L liquid phase, V vapor phase, S solid phase, Op opaque minerals.

elongated and negative-crystal forms. They occurred in clusters, with type III fluid inclusions in regions ( 984 m) deeper than the Cu–Au ore bodies (Figs. 3 and 5D and E). Halite-bearing fluid inclusions (type III) contain three phases: a vapor bubble, a brine liquid, and a cubic solid identified as halite. The inclusions vary from 4 to 13 lm in size and have elongated to rounded forms. These types of inclusions were interpreted to be of primary origin and were commonly associated with type II fluid inclusions in regions ( 984 m) deeper than the Cu–Au ore bodies (Figs. 3 and 5D and E). In summary, there were numerous type I inclusions in the Pb– Zn–Ag mineralization zones. In contrast, three types of inclusions were observed in the Cu–Au ore zones. Type I inclusions were common in the quartz veins of the Cu–Au ore bodies from the depth to the surface. Type II and type III inclusions were only observable at the deeper parts ( 984 m) below the Cu–Au ore bodies (Fig. 3). 5.2. Microthermometric data The fluid inclusion microthermometric results are given in Table 1, Figs. 6 and 7. Type I primary inclusions in quartz from the deep levels ( 875 m) and in quartz and sphalerite from the shallow depths ( 150 to 240 m) of the Pb–Zn–Ag ore zones were

studied (Fig. 3). At the deep levels ( 875 m) of the Pb–Zn–Ag ore zones, type I fluid inclusions from quartz yielded ice-melting temperatures from 6.2 to 2.5 °C, with salinities ranging from 4.2 to 9.5 wt.% NaCl equivalent. Type I inclusions were homogenized to liquids at temperatures of 240–303 °C (Fig. 6C and D). At the shallow levels ( 150 to 240 m), the homogenization temperatures (Th) of the inclusions in quartz grains ranged between 187 and 278 °C. The salinities varied between 4.3 and 8.7 wt.% NaCl equivalent (Fig. 6A and B). Some sphalerite at shallow levels ( 150 to 240 m) allowed the observation of growth lines and primary type I fluid inclusions under transmitted light. Ice melting was measured at temperatures from 5.8 to 4.3 °C, with salinity values of 6.9–8.9 wt.% NaCl equivalent and vapor bubble disappearance at temperatures between 192 and 262 °C (Fig. 6A and B). Type I primary inclusions from quartz grains at deep ( 909 to 847 m), intermediate ( 359 to 330 m) and shallow levels ( 240 to +96 m) of the Cu–Au ore zones were studied. At deep levels ( 240 to +96 m), the final melting temperatures of ices ranged between 6.5 and 2.4 °C, corresponding to salinities of 4.0– 9.9 wt.% NaCl equivalent. Homogenization of inclusions took place at temperatures between 256 and 362 °C (Fig. 7E and F). At intermediate levels ( 359 to 330 m), type-I inclusions were homogenized to liquids at temperatures from 217 to 314 °C. Their freezing

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Table 1 Microthermometric results of fluid inclusions from the Yinshan Cu–Au–Pb–Zn–Ag deposit. Mineralization types

Depth (m)

Mineral assemblages

Host mineral

Inclusion type

Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au

Shallow levels ( 150 to 240) Shallow levels ( 150 to 240) Deep levels ( 875) Shallow levels (+96 to 195) Middle levels ( 359 to 330) Deep levels ( 909 to 847) Deepest level ( 984) Deepest level ( 984)

Q + Sph + Gn + Py + Cc Q + Sph + Gn + Py + Cc Q + Sph + Gn + Py + Cc Q + Tnt + Ttr + Ccp + Py Q + Tnt + Ttr + Ccp + Py Q + Ccp + Py Q + Ccp + Py Q + Ccp + Py

Q Sph Q Q Q Q Q Q

I I I I I I II III

Tm (°C)

5.6 5.8 6.2 6.1 5.4 6.5 2.5

to to to to to to to

TmH (oC) 2.6 4.3 2.5 2.1 2.5 2.4 0.1 196 to 426

Salinity (NaCl eq. wt.%)

Th (°C)

Number of measurements

4.3–8.7 6.9–8.9 4.2–9.5 3.5–9.3 4.2–8.4 4.0–9.9 0.2–4.2 31.0–36.9

187–278 192–262 240–303 196–306 217–314 256–362 349–448 317–438

69 10 21 87 31 47 21 16

Th: Homogenization temperature, Tm: final melting temperature of ice, ThHv: dissolution temperature of halite, Sph: sphalerite, Q: quartz, Ccp: chalcopyrite, Py: pyrite, Gn: galena, Tnt: tennatite, Ttr: tetrahedrite, Cc: calcite.

points ranged from 5.4 to 2.5 °C, corresponding to salinities from 4.2 to 8.4 wt.% NaCl equivalent (Fig. 7C and D). At shallow levels ( 909 to 847 m), type-I inclusions showed final ice melting temperatures from 6.1 to 2.4 °C, with salinities values of 3.5– 9.3 wt.% NaCl equivalent. They were homogenized to liquids at temperatures from 196 to 306 °C (Fig. 7A and B). Ice from type II fluid inclusions at levels beneath the Cu–Au ore bodies melted between 2.5 and 0.1 °C, corresponding to salinities of 0.2–4.2 wt.% NaCl equivalent. Total homogenization of typeII fluid inclusions to the vapor phase occurred between 349 and 448 °C (Fig. 7G and H). Total homogenization of type III fluid inclusions at levels deeper than the Cu–Au ore bodies occurred to liquid between 317 and 438 °C. Halite dissolution temperatures ranged from 196 to 426 °C, corresponding to salinities from 30.9 to 36.9 wt.% NaCl equivalent (Fig. 7G and H).

had d34S values from 1.7‰ to +3.2‰; one pyrite sample of rhyolitic quartz porphyry had d34S values from +1.6; eight sulfide samples from Cu–Au mineralization had d34S values from 0.2‰ to +1.6‰; and two pyrite samples of dacite porphyry had d34S values from 0.2‰ to +1.4‰. The lead isotope compositions of sulfide minerals from the Yinshan deposit had 206Pb/204Pb ratios ranging from 18.01 to 18.07, 207 Pb/204Pb from 15.55 to 15.57, and 208Pb/204Pb from 38.03 to 38.12. The feldspars from the volcanic–subvolcanic rocks had 206 Pb/204Pb ratios ranging from 18.03 to 18.10, 207Pb/204Pb from 15.56 to 15.57, and 208Pb/204Pb from 38.02 to 38.21 (Table 4 and Fig. 11).

5.3. Laser Raman spectroscopy analysis

The data of dDH2O ( 66‰ to 42‰) and d18OH2O (+7.8‰ to +10.5‰) of fluid inclusions at Yinshan were plotted within the magmatic water box, indicating a primary magmatic origin for the mineralizing process (Table 2 and Fig. 9). Although voluminous literature in the past few years demonstrates that meteoric waters might play a major role in epithermal-porphyry systems (Taylor, 1997; Simmons et al., 2005 and the references therein), increasing evidence indicates a primary magmatic origin for epithermal deposits (e.g., Giggenbach, 1992; Kouzmanov et al., 2003; Vennemann et al., 1993). These H O isotopes data from the Yinshan deposit are consistent with many epithermal deposits (Giggenbach, 1992; Kouzmanov et al., 2003; Vennemann et al., 1993), which show that magmatic fluids play a key role as metal contributors. It should be noted that the fluids containing significant or even exclusively magmatic water components in the epithermal deposits generally show intimate spatial association with porphyry-style deposits (Heinrich, 2005). The sulfur isotopes ( 1.7‰ to +3.2‰) from sulfides from the Yinshan deposit indicate a homogeneous magmatic source for the polymetallic mineralization (Ohmoto, 1979; Ohmoto and Goldhaber, 1997) (Fig. 10). Moreover, these data are similar to those obtained for the adjacent Dexing porphyry deposit ( 4‰ to +3.1‰) (Zhu et al., 1983) and worldwide porphyry to epithermal deposits (Hedenquist and Lowenstern, 1994; Ohmoto and Goldhaber, 1997) but are lower than those of the regional Shuangqiaoshan Group strata (+3.1 to +6.0‰) (Wei, 1996), implying a magmatic origin from igneous rocks. The lead isotope compositions of sulfide minerals and feldspars from volcanic–subvolcanic rocks show similar ranges, with 206Pb/204Pb ratios of 18.01–18.10, 207Pb/204Pb ratios of 15.55–15.57 and 208Pb/204Pb ratios of 38.02–38.21. The lead isotope data were plotted on the 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs.

Representative inclusions were measured using Laser Raman micro-spectroscopy to constrain their gas compositions. In the samples from Pb–Zn–Ag veins, the results showed that the vapor phases of the type I inclusions were dominated by H2O (Fig. 8A) or by minor amounts of CH4, N2, and CO2 (Fig. 8B). In the samples of Cu–Au veins, the data indicated that the vapor phases of the type-I inclusions were characterized by H2O (Fig. 8C), with minor amounts of CO2, N2, and CH4 (Fig. 8D). At the 984 m level, below the Cu–Au ore bodies, type-II inclusions from the boiling inclusions assemblage contained a small amount of CO2 (Fig. 8E), but type-III halite-bearing inclusions were composed of pure H2O (Fig. 8F). The Laser Raman spectroscopy analysis of fluid inclusions showed that NaCl–H2O is the dominant composition in these two types of mineralizing fluids at the Yinshan deposit, with variable volatile compositions. 6. O–D–S–Pb isotopes The total range of d18O values for quartz veins from the Cu–Au and the Pb–Zn–Ag mineralization at Yinshan was very narrow, between 16.10‰ and 18.75‰ (Table 2). The calculated oxygen isotopic compositions of the ore-forming fluids from quartzes ranged from 7.8‰ to 10.5‰, using the equation of Clayton et al. (1972) and the corresponding fluid inclusion homogenization temperatures of the sample. The dD values of the extracted waters for quartz samples ranged from 66‰ to 42‰ (Table 2 and Fig. 9). Sulfur isotopic analysis was performed on pyrite, sphalerite, and galena samples from the Pb–Zn–Ag ore bodies and on pyrite and chalcopyrite samples from the Cu–Au ore bodies (Table 3, and Fig. 10). Thirteen sulfide samples from Pb–Zn–Ag mineralization

7. Discussion 7.1. Source of ore-forming fluids and materials

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351

Fig. 6. Histograms of homogenization temperatures (Th) and salinities for type-I fluid inclusions in the Pb–Zn–Ag mineralization of the Yinshan deposit. Symbols: red back slash-type I inclusions from quartz; blue slash-type I inclusions from sphalerite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 206

Pb/204Pb diagrams modified by Zartman and Doe (1981) (Table 4, Fig. 11). Most of the ore samples fall inside the volcanic–subvolcanic rocks envelope, indicating that Yinshan magmatic rocks provide lead for the ores (Fig. 11). The ores have Pb isotope ratios higher than the Neoproterozoic Shuangqiaoshan Group (Zeng, 2002), which indicates that this regional basement strata are not the primary lead sources (Fig. 11). Consequently, lead isotopic ratios from the Yinshan deposit support the existence of a magmatic source rather than a strata source for the sulfide ores of this deposit. 7.2. Characteristics and evolution paths of ore-forming fluids

From the deep to the shallow zones, the widespread type I inclusions in the Pb–Zn–Ag ore zones showed a gradual decrease in temperatures with homogenization temperature (Th) ranges from 240 to 303 °C and from 187 to 262 °C (Fig. 12) but showed constant salinity ranges from 4.2 to 9.5 wt.% and from 4.3 to 8.9 wt.% NaCl equivalent (Fig. 13A). The type I inclusions in the Cu–Au ore zones also showed the same temperature-decreasing trend from the depth to the surface, with Th ranges of 256– 362 °C, 217–314 °C and 196–306 °C (Fig. 12), but constant salinity values of 4.0–9.9 wt.%, 4.2–8.4 wt.% and 3.5–9.3 wt.% NaCl equivalent, respectively (Fig. 13B). As shown in the inset of Fig. 13 (modified after Shepherd et al. (1985)), the clear temperaturedecreasing trends with constant salinities from the greatest depth upward in every type of mineralization indicate that simple cooling, rather than the mixing or boiling process, is the main factor controlling fluid evolution.

As presented, at greater depths than the Cu–Au ore bodies ( 984 m), type II and coexisting type III inclusions possess an intimate spatial relationship (Fig. 5E and F). These two types of inclusions display similar homogenization temperatures (317–448 °C) and contrasting salinities of 0.2–4.2 wt.% and 30.9–36.9 wt.% NaCl equivalent (Fig. 7G and H). The mode of homogenization of halite-bearing inclusions is vapor disappearance. Consequently, boiling events can be discriminated on the basis of similar temperatures and variable salinities, the intimate spatial relation between type II and type III inclusions, and the mode of homogenization of type III inclusions. These coexisting fluid inclusions are interpreted as primary fluids that boil at high temperatures (317–448 °C), similar to porphyry copper deposits worldwide (Hedenquist and Lowenstern, 1994; Heinrich, 2005). This boiling process only occurs partially at the much deeper levels ( 984 m); therefore, it is not considered a main factor controlling exposed ore bodies at the Yinshan deposit. The bulk salinity of the magmatic fluids is generally believed to be between 5 and 10 wt.% NaCl equivalent (Burnham, 1979; Hedenquist and Lowenstern, 1994). Modeling under relatively high pressure condition (12 kbar) also demonstrates that the salinity of the evolved magmatic fluids is low to moderate (Cline and Bodnar, 1991; Sun et al., 2007a). In addition, fluid inclusion observations from the deeper parts of some porphyry copper deposits demonstrate that magmatic fluids may initially be a single-phase fluid of intermediate density and salinity of less than 12 wt.% NaCl equivalent (e.g., Bingham, Utah: Redmond et al., 2004; Butte, Montana: Rusk et al., 2004). Depending on the trap-

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Fig. 7. Histograms of homogenization temperatures (Th) and salinities for fluid inclusions in the Cu–Au mineralization of the Yinshan deposit. Symbols: red back slash-type I inclusions, deep sky blue crossing line-type II inclusions, slateblue long string-type III inclusions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ping pressure, these low-medium salinity initial magmatic fluids can evolve in two distinct ways (Hedenquist et al., 1998; Heinrich, 2005; Muntean and Einaudi, 2001). Commonly in porphyry deposits, low-medium-salinity supercritical fluids encounter a two-phase field and undergo boiling processes, thus generating two fluids: a high salinity liquid coexisting with a vapor (Audetat et al., 1998; Burnham, 1979; Hedenquist and Lowenstern, 1994; Henley and McNabb, 1978; Yao et al., 2012; Yu et al., 2012).

Alternatively, the ascending supercritical fluids may evolve into a low-to-moderate temperature aqueous liquid without ever entering a two-phase field (Heinrich, 2005; Heinrich et al., 2004). Given abundant low-medium salinity type I inclusions demonstrating significant magmatic features, these ore fluids in the Yinshan deposit can be directly evolved from initial magmatic fluids without the boiling process by following the latter model.

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Fig. 8. Representative Raman spectra of fluid inclusions in the Yinshan deposit. (A) H2O-spectrum of vapor in type I inclusions of Pb–Zn–Ag mineralization; (B) H2O-, CO2-, CH4-, and N2-spectra of vapor in type I inclusions of Pb–Zn–Ag mineralization; (C) H2O- and CO2-spectra of vapor in type I inclusions of Cu–Au mineralization; (D) H2O-, CO2-, CH4-, and N2-spectra of vapor in type I inclusions of Cu–Au mineralization; (E) CO2-spectrum of vapor in type II inclusions coexisting with type III inclusions of Cu–Au mineralization; (F) H2O-spectrum of vapor in type III inclusions coexisting with type II inclusions of Cu–Au mineralization.

7.3. Possible metals transport and deposition mechanism It is well known that metals transport in most hydrothermal solutions mainly as aqueous complexes (Barnes, 1979), therefore, determining the nature and stability of aqueous metal complexes is essential to understand the transport and deposition of metals. Previous experimental studies have shown that chloride complexes of Pb2+, Zn2+ and Ag+ play important roles in the transport of metals in hydrothermal ore solutions (Ruaya and Seward, 1986; Seward and Barnes, 1997; Seward, 1976). Copper chloride

complexes are likely to be the most important aqueous species of copper in epithermal-porphyry environments (Landtwing et al., 2005; Muntean and Einaudi, 2001; Pudack et al., 2009; Roedder, 1971; Ulrich et al., 2002; Yao et al., 2012). Many experimental studies have also been conducted to testify that copper chloride complexes are predominate species in solutions at elevated temperatures (Crerar and Barnes, 1976; Hemley et al., 1992; Liu and McPhail, 2005; Seyfried and Ding, 1993; Xiao et al., 1998). Gold is likely transported as a bisulfide or as another sulfur complex at temperatures up to 350 °C (Seward, 1973; Stefánsson and Sew-

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Table 2 Oxygen and hydrogen isotopic data for quartz from the Yinshan mine. Sample number

Samples location (depth: m)

d18Omineral ‰

Th (°C)

d18Ofluid ‰

YS030 YS024 YS025 Dongqu-01 Dongqu-16 YS239 5–10 orebody YS575 YS630 Beishan03 YS196 YS210 YS697 YS680

Shallow levels of Cu–Au district ( 240 to +96) Shallow levels of Cu–Au district ( 240 to +96) Shallow levels of Cu–Au district ( 240 to +96) Shallow levels of Cu–Au district ( 240 to +96) Shallow levels of Cu–Au district ( 240 to +96) Middle levels of Cu–Au district ( 359 to 330) Middle levels of Cu–Au district ( 359 to 330) Middle levels of Cu–Au district ( 359 to 330) Deep levels of Cu–Au district ( 909 to 847) Shallow levels of Pb–Zn–Ag district ( 240 to 150) Shallow levels of Pb–Zn–Ag district ( 240 to 150) Shallow levels of Pb–Zn–Ag district ( 240 to 150) Shallow levels of Pb–Zn–Ag district ( 240 to 150) Deep levels of Pb–Zn–Ag district ( 875)

18.59 18.75 17.99 18.13 18.61 17.99 16.94 16.60 16.10 17.76 17.08 18.57 17.27 18.35

233 233 233 254 254 265 265 265 334 233 244 235 244 267

8.8 8.9 8.2 9.4 9.8 9.7 8.7 8.3 10.3 8.0 7.8 8.9 8.0 10.2

dDmineral ‰ 47 47 66 52 59 62 62 51 64 52 49 52 47 42

Fig. 10. Histograms of d34S values of sulfide minerals from different types of mineralization and intrusions in the Yinshan deposit. Ccp chalcopyrite, Py pyrite, and Sph sphalerite.

Fig. 9. dD and d18O values of the ore-forming fluids in the Yinshan deposit. Also shown are the isotopic fields for common magmatic waters (Taylor, 1997).

Table 3 Sulfur isotopic composition of sulfide samples from the Pb–Zn–Ag and the Cu–Au mineralization. d34SV-CDT‰

Mineralization

Orebody

Sample no.

Minerals

Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag

Beishan Beishan Beishan Beishan Beishan Beishan quartz porphyry Jiulong Jiulong Jiulong Jiulong Jiulong Jiulong Nanshan Nanshan Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu quartz porphyry Jiuqu dacite porphyry Jiulong

YS199 YS199 BS-16 BS-18 BS-19 YS416

Gn Sph Sph Sph Sph Py

1.7 2.0 1.4 1.6 1.9 1.6

JL-11 JL-4 YS403 YS374 YS379 YS430 YS435 YS436 YS025 YS402 YS467 YS474 YS515 YS544 YS774 YS306 YS321 JL-4

Sph Sph Sph Sph Sph Sph Gn Gn Py Py Py Py Py Py Py Py Py Ccp

1.4 1.5 0.7 1.9 3.2 0.4 2.3 2.1 1.4 1.2 0.8 1.2 1.0 1.6 0.3 0.2 1.4 0.2

Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Pb–Zn–Ag Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au Cu–Au

Ccp: chalcopyrite; Gn: galena; Py: pyrite; Sph: sphalerite.

ard, 2004). Heinrich et al. (2004) emphasized that initial sulfur richness was an indispensable chemical requirement for the transport of gold from the magmatic source to the epithermal environment. The stability constants for bisulfide reach maxima at ca. 350 °C due to the increased association of H+ and HS as H2S (Williams-Jones et al., 2009). At higher temperatures (>350 °C) of magmatic-hydrothermal systems, AuCl2 becomes an important agent for transporting gold in solutions (Gammons and WilliamsJones, 1997; Williams-Jones et al., 2009). Hydrothermal ore deposits are formed as a consequence of the instability of metal complexes transported by fluids in the Earth’s crust (Seward and Barnes, 1997). Boiling (Calagari, 2004; Roedder, 1971, 1984; Yao et al., 2012), fluid mixing (O’Neil and Silberman, 1974; Simmons et al., 2005; Taylor, 1997), cooling (Landtwing et al., 2005; Redmond et al., 2004; Ulrich et al., 2002) and fluid– rock interaction (Beane and Titley, 1981; Hezarkhani et al., 1999; Rose, 1970) can all play important roles in the deposition of polymetallic sulfides from ore fluids. The boiling process of magmatic fluids results in the occurrence of coexisting vapor-rich and halite-bearing inclusions (Bodnar and Vityk, 1994; Cline and Bodnar, 1994; Lu et al., 2004; Roedder, 1984; Roedder and Bodnar, 1997). Such a fluid inclusion assemblage is characteristic of numerous porphyry-epithermal systems and is therefore favorable to metal sulfides, gold and silver precipitation during ore-formation processes (Bodnar, 1995; Hedenquist et al., 1998; Roedder, 1971; Roedder and Bodnar, 1997; Sillitoe, 2010; Yao et al., 2012). Nevertheless, two-phase liquid-rich (type I) inclusions are predominant in ore-forming fluids; boiling inclusion assemblages could only be observed beneath Cu–Au ore bodies in the Yinshan deposit (Figs. 3 and 5). Consequently, the boiling process is only of local importance for metal deposition

G.-G. Wang et al. / Journal of Asian Earth Sciences 74 (2013) 343–360 Table 4 Pb isotopic compositions of feldspars from igneous rocks and sulfides in the Yinshan deposit. Sample no.

Location

Minerals

206

Pb/204Pb

YS474 Dongqu-4 YS306 YS774 YS402 YS024 YS025 YS199 BS-19 BS-16 BS-18 YS199 YS374 YS379 2009-11-2 JL-4 YS435 YS436 YS418 YS190 YS242 YS254 YS258 YS260 YS266 YS250 YS251

Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Jiuqu Beishan Beishan Beishan Beishan Beishan Beishan Beishan Jiulong Jiulong Nanshan Nanshan BQP BQP XVR XVR XVR XVR XVR XAP XAP

Py Ccp Py Py Py Py Py Sph Sph Sph Sph Sph Sph Sph Sph Sph Gn Gn Pl Pl Pl Pl Pl Pl Pl Pl Pl

18.06 18.03 18.06 18.01 18.04 18.04 18.05 18.02 18.02 18.03 18.02 18.01 18.02 18.04 18.02 18.03 18.07 18.07 18.07 18.03 18.04 18.08 18.06 18.07 18.10 18.04 18.05

207

Pb/204Pb

15.55 15.56 15.57 15.56 15.56 15.56 15.57 15.56 15.57 15.57 15.57 15.56 15.56 15.56 15.57 15.56 15.57 15.57 15.56 15.56 15.56 15.57 15.57 15.57 15.57 15.56 15.56

208

Pb/204Pb

38.03 38.04 38.12 38.06 38.06 38.08 38.09 38.06 38.07 38.08 38.07 38.04 38.03 38.06 38.07 38.05 38.08 38.09 38.09 38.08 38.10 38.11 38.09 38.09 38.21 38.02 38.06

JQP: Jiuqu rhyolitic quartz porphyry; BQP: Beishan rhyolitic quartz porphyry; XVR: Xishan rhyolitic volcanic rocks; XAP: Xishan andesite porphyry; Ccp: chalcopyrite; Py: pyrite; Sph: sphalerite; Gn: Galena; Pl: plagioclase.

rather than being the main contributor to the formation of the exposed ore bodies. Fluid mixing between magmatic fluids and external meteoric water has long been thought to effectively lead to the deposition of metals from ore solutions (Beane and Titley, 1981; Reynolds and Beane, 1985; Taylor, 1997). Evidence for the involvement of meteoric water has come from H–O stable isotope studies for many porphyry-epithermal deposits (Hayba, 1997; Simmons et al., 2005; Taylor, 1997). The fluid-mixing model requires two essential features described as follows: first, the stable isotope data plots into the region between the magmatic water box and meteoric water line in the H–O diagram, suggesting the mixing process between magmatic and meteoric waters took place; and second, the salinities of ore-forming solutions decrease with temperatures, indicating that low salinity and temperature meteoric waters input (Simmons

Fig. 11. Plots of

206

Pb/204Pb vs.

207

Pb/204Pb and

206

Pb/204Pb vs.

355

et al., 2005; White and Hedenquist, 1995). However, hydrogen and oxygen isotope compositions are predominantly magmatic in origin (Fig. 9), and the salinity values of ore-forming fluids are relatively constant (Fig. 13) in Yinshan, implying the fluid mixing model cannot be used to explain the genesis of ore deposition. Experimental studies show that decreasing temperature significantly influences the dissociation of metal chloride complexes, implying fluid cooling is particularly effective for depositing metals transported in solutions (Liu and McPhail, 2005; Luo and Millero, 2007; Seward, 1984; Seward and Barnes, 1997; Xiao et al., 1998). Recently, studies of some famous deposits, including the Sungun deposit, Iran (Hezarkhani et al., 1999); Bajo de la Alumbrera, Argentina (Ulrich et al., 2002); and Bingham deposit, USA (Redmond et al., 2004), suggest that fluid cooling is the major factor causing sulfide saturation and deposition. Similar to the abovementioned deposits, ore-forming fluids in the Pb–Zn–Ag and Cu– Au mineralizations of the Yinshan deposit also show clear temperature decreasing trends from the deeper zones and upward (Fig. 12). The temperatures of ore-forming fluids decrease from 303 to 187 °C and from 362 to 196 °C in the Pb–Zn–Ag and Cu– Au ore bodies, respectively. According to the experiment results (Liu and McPhail, 2005; Seward and Barnes, 1997), the stability of chloride complexes decreases rapidly as the solutions cool over the temperature range of Yinshan. Consequently, such temperature gradients could efficiently induce the deposition of polymetallic sulfides. In addition to the chloride complex, gold is likewise likely transported as a sulfur complex (Heinrich et al., 2004; Seward and Barnes, 1997; Seward, 1973), and where sulfur is removed from the hydrothermal fluid during polymetallic sulfide precipitation, gold is also precipitated. Therefore, fluid cooling most probably is the main driving force for polymetallic sulfide depositions in Yinshan. The polymetallic veins in the Yinshan deposit are related to phyllic alteration and sericitization (Fig. 4M and O). Phyllic/sericitic alteration is characterized by the replacement of all rock minerals with fine-grained muscovite (sericite) and quartz accompanied by variable amount of pyrite (Gustafson and Hunt, 1975; John et al., 2010). Extensive alteration of feldspars to muscovite and quartz neutralizes the acidity generated by the disproportionation of magmatic sulfur dioxide upon cooling (Giggenbach, 1992). During the neutralization process, increasing pH causes a marked decrease in H+ concentration, which has a significant effect on the deposition of sulfides (John et al., 2010; Liu and McPhail, 2005). The precipitation of sericite, quartz and sulfides in the veins of most porphyryepithermal deposits (John et al., 2010; Kamilli and Ohmoto, 1977; Mao et al., 2011; Sillitoe, 1989) suggests that the near-neutral pH fluids generated during phyllic/sericitic alteration are helpful to

208

Pb/204Pb for sulfide minerals from the Yinshan deposit.

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Fig. 12. Longitudinal section through the Yinshan deposit shows data from fluid inclusion studies. The histogram of homogenization temperatures (Th) of fluid inclusions from every level was plotted. The arrow up means the decreasing trend of Th. Here, we use the values that are the peak value of each sample.

metals deposition. Consequently, phyllic/sericitic alteration in Yinshan is also a crucial controlling factor for ore formation. 7.4. Genetic type and implication for exploration Compared with typical epithermal deposits (Carman, 2003; Roedder and Bodnar, 1997; Sillitoe and Hedenquist, 2003; Simmons et al., 2005), most of the abundant type-I fluid inclusions show a low-moderate homogenization temperature range (<300 °C) and salinity values (3.5–9.9 wt.% NaCl equivalent), suggesting that the mineralization at Yinshan occurred chiefly under an epithermal environment. Ore bodies at Yinshan are clearly controlled by subvolcanic rocks and occur in quartz vein-types along steep fault zones, which are similar to most epithermal deposits (Carman, 2003; Christie et al., 2007; Cooke and McPhail, 2001). The ore minerals of chalcopyrite, pyrite, tetrahedrite, tennatite, nature gold and nature silver and the gangue minerals of quartz, calcite and chlorite present at Yinshan (Fig. 4) are generally formed in epithermal regimes (e.g. see texts in Einaudi et al., 2003; Heald et al., 1987; Sillitoe, 1993). Hence, the fluid inclusions data, mineralogy and ore deposit geology indicate that the Yinshan deposit has a close affiliation with epithermal environments. Epithermal deposits can be broadly grouped into high-, intermediate-, and low-sulfidation types based on the sulfidation states of their hypogene sulfide assemblages, sulfide contents and metal concentrations (Einaudi et al., 2003; Hedenquist, 1987; Sillitoe and Hedenquist, 2003). In high-sulfidation (HS) epithermal deposits, the sulfidation state ranges from high for the enargite-bearing assemblages to intermediate for the tennantite tetrahedrite + pyrite assemblages. Intermediate-sulfidation (IS) epithermal deposits share many similar sulfide assemblages with HS deposits, except that the enargite-bearing assemblages are lacking

and the Ag:Au ratios (at least 10:1) are higher. Low sulfidation (LS) deposits contain the low-sulfidation pair of pyrite–arsenopyrite. In addition, LS deposits are sulfide-poor, dominated by gold typically of bonanza grades, and have very low contents of Cu (typically <100–200 ppm) (Einaudi et al., 2003). Lack of high sulfidation enargite minerals indicates the Yinshan deposit is not likely an HS deposit. Moreover, the high sulfide mineral contents in this deposit are not consistent with the sulfide-poor features of LS deposits. Therefore, the Yinshan deposit is similar to the IS epithermal deposit on the basis of the recognition of intermediate-state sulfidation mineral associations containing tennantite, tetrahedrite, pyrite, chalcopyrite (Fig. 4); the abundant base-metal polymetallic veins (Fig. 4); and the high Ag:Au ratios (13.3) (JGEB, 1996). IS and/or HS deposits generally show a close spatial and temporal association with porphyry systems, including the Sam Goosly deposit of Canada (Panteleyev, 1986), the Far Southeast-Lepanto deposits of Philippines (Hedenquist et al., 1998), and several deposits (e.g., Valea Morii and Rosia Poeni) of Romania (Ivascanu et al., 2002). Panteleyev (1986) proposed the term ‘‘transitional deposit’’ to describe the transition between porphyry and epithermal mineralization. Transitional deposits contain an abundance of sulfide minerals such as pyrite, chalcopyrite, and tetrahedrite–tennatite, which are similar to epithermal deposits (Muntean and Einaudi, 2001; Panteleyev, 1986). However, they include sericite (fine-grained muscovite) rather than alunite, indicating higher temperatures and greater formational depths (Hedenquist et al., 1998; Panteleyev, 1986). Moreover, fluid inclusion homogenization temperatures of transitional deposits approach those of porphyry copper deposits (Hedenquist et al., 1998; Panteleyev, 1986; Wojdak and Sinclair, 1984). Wojdak and Sinclair (1984) described the temperatures of mineralization at the Equity Silver transitional deposit as ranging from approximately 200 °C to over 400 °C, and Hedenquist et al. (1998)

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Fig. 13. Salinities vs. Th illustrating the distribution pattern of the data points from the Yinshan deposit. Inset illustrates temperature–salinity trends or fluid evolution paths as a result of different geological processes (modified from Shepherd et al. (1985)). (I) isothermal mixing; (II) boiling process; (III) simple cooling; (IV) mixing of fluids with different homogenization temperatures and salinities; (V) leakage or necking down. Symbols: Fig. 13 A square-type I inclusions in quartz (-240 to -150m), rhomb-type I inclusions in quartz (-875m), hexagon-type I inclusions in sphalerite (-240 to -195m); Fig. 13 B square-type I inclusions (-195 to -96m), circle-type I inclusions (-359 to 330m), triangle-type I inclusions (-909 to -847m), cross type II inclusions (-984m), cross within square-type III inclusions (-984m).

reported the temperature ranges from 200 °C to 350 °C. The Cu–Au ore bodies in the Yinshan deposit show some significant features similar to the transitional deposit. Firstly, sericite, not alunite or adularia, commonly occurs along the polymetallic veins in Yinshan (Fig. 4), indicating deeper conditions and higher temperatures. Secondly, the highest homogenization temperature of type I liquid-rich inclusions is up to 360 °C at the deepest zone of the Cu–Au zones of the Yinshan deposit (Fig. 7E), which is analogous to the transitional deposit (Hedenquist et al., 1998; Panteleyev, 1986) but is higher than the generally epithermal deposits (Simmons et al., 2005). More importantly, the types of fluid inclusions at Yinshan show obvious changes from the liquid-rich two-phase inclusions (type I) to boiling inclusion assemblages (type II and type III) beneath the Cu–Au ore bodies ( 984 m) (Figs. 5E and F and 7G and H), implying that the exposed deepest Cu–Au ore zone may represent the transitional zone from an epithermal to a porphyry system. If this assumption is valid,

the widespread boiling fluid inclusions and unexposed porphyry Cu–Au ore bodies may be observed at greater depths. 8. Conclusions Liquid-rich two-phase (type I) inclusions are dominant in the Yinshan deposit and show gradually decreasing temperatures from the depths upward. These low-intermediate temperatures type I inclusions indicate an epithermal environment. Polymetallic mineralization was controlled by the Middle Jurassic (176–166 Ma) rhyolitic to dacitic subvolcanic porphyries. Isotopic data of ore fluids indicate a predominant magmatic origin, without the incursion of meteoric water. Local boiling events took place below the Cu–Au ore bodies, providing new insights for exploring mineral porphyry ore bodies at even deeper levels.

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