Genesis of Two Different Types of Gold Mineralization in the Linglong Gold Field, China: Constrains from Geology, Fluid Inclusio

Ore Geology Reviews 65 (2015) 643–658

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Ore Geology Reviews

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Genesis of two different types of gold mineralization in the Linglong gold ﬁeld, China: Constrains from geology, ﬂuid inclusions and stable isotope

Bo-Jie Wen a,Hong-RuiFana,⁎,M.Santoshb, Fang-Fang Hu a,FrancoPirajnoc, Kui-Feng Yang a a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China c Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia article info abstract

Article history: The Dongfeng and Linglong gold deposits are located in the northwest Jiaodong Peninsula, North China Craton. Received 21 December 2013 The deposits are mainly hosted in the Mesozoic granitoids and structurally controlled by the Zhaoyuan–Pingdu Received in revised form 26 March 2014 fault zone. Gold mineralization at Dongfeng occurs as disseminated ores and sulfide stockworks, typically Accepted 26 March 2014 enveloped by broad alteration selvages. In contrast, mineralization at Linglong is characterized by massive Available online 13 April 2014 auriferous quartz veins with narrow alteration halos. Three stages of mineralization were identified in both deposits, with the early stage represented by quartz ± pyrite, the middle stage by gold + quartz + pyrite or Keywords: fi Fluid inclusion gold + quartz + base metal sul des, and the late stage by quartz + carbonate ± pyrite, respectively. Four Water–rock interaction types of fluid inclusions were distinguished based on petrography, microthermometry, and laser Raman Phase separation spectroscopy, including (1) pure CO2 fluid inclusions (type I), (2) H2O–CO2–NaCl fluid inclusions (type II),

Dongfeng gold deposit (3) H2O–NaCl fluid inclusions (type III), and (4) daughter mineral-bearing or multiphase fluid inclusions (type IV). Linglong gold deposit In the Dongfeng gold deposit, the early- and middle-stage quartz mainly contains primary type II fluid inclusions Northwest Jiaodong that completely homogenized at temperatures of 276–341 °C with salinities of 2.8–11.7 wt.% NaCl equivalent, Eastern China and temperatures of 248–310 °C with salinities of 3.3–10.8 wt.% NaCl equivalent, respectively. A few primary type I fluid inclusions could be observed in the early-stage quartz. In contrast, the late-stage quartz contains only the type III fluid inclusions with homogenization temperatures of 117–219 °C, and salinities of 0.5–8.5 wt.% NaCl equivalent. The estimated pressures for the middle-stage fluids are 226–338 MPa, suggesting that gold mineralization mainly occurred at paleodepths of deeper than 8.4–12.5 km. The mineralization resulted

from extensive water–rock interaction between the H2O–CO2–NaCl fluids and wallrocks in the first-order fault. In the Linglong gold deposit, the early-stage quartz mainly contains primary type II fluid inclusions and a few type I fluid inclusions, of which type II fluid inclusions have salinities of 3.3–7.5 wt.% NaCl equivalent and homogenization temperatures of 271–374 °C. The middle-stage quartz mainly contains all four types of fluid inclusions, among which the type II fluid inclusions yield homogenization temperatures of 251–287 °C and salinities of 5.5–10.3 wt.% NaCl equivalent, while the type III fluid inclusions have homogenization temperatures of 244–291 °C and salinities of 4.1–13.3 wt.% NaCl equivalent. Fluid inclusions in the late-stage quartz are type III fluid inclusions with low salinities of 0.3–8.2 wt.% NaCl equivalent and low homogenization temperatures of 103–215 °C. The trapping pressure estimated for the middle-stage fluids is 228–326 MPa, suggesting that the gold mineralization mainly occurred at paleodepths of about 8.4–12.1 km. Precipitation of gold is possibly a con-

sequence of phase separation or boiling of the H2O–CO2–NaCl fluids in response to pressure and temperature fluctuations in the open space of the secondary faults. The δ34S values of pyrite are similar for the Dongfeng and Linglong deposits and show a range of 5.8 to 7.0‰ and 5.9 to 7.4‰, respectively. Oxygen and hydrogen stable isotopic analyses for quartz yielded the following results: δ18O=−3.8 to +6.4‰ and δD=−90.5 to −82.7‰ for the Dongfeng deposit, and δ18O = 0.0 to +8.9‰ and δD=−77.4 to −63.7‰ for the Linglong deposit. Stable isotope data show that the ore-forming fluids of the two gold deposits are of magmatic origin, with gradual incorporation of shallower meteoric water during/after mineralization. © 2014 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: +86 10 82998218; fax: +86 10 62010846. China is the largest gold-producer in the world. Its gold production is E-mail address: [email protected] (H.-R. Fan). increasing rapidly and had reached to 428.163 metric tons in 2013

(China Gold Association, http://www.cngold.org.cn/newsinfo.aspx? Gold Group Co., Ltd, unpublished data). The annual gold production in ID=996). The Jiaodong gold province located in the Jiaodong Peninsula the Dongfeng and Linglong deposits has climbed to ~3.6 metric tons in of eastern China (Fig. 1) is the most important gold-producing district 2013 (Shandong Gold Group Co., Ltd, personal communication). and is the host for several world-class gold deposits (N100 t gold) in The Jiaodong gold province hosts dozens of gold deposits. Genetic the country (Fan et al., 2003; Hu et al., 2013; Qiu et al., 2002; Zhou differences between the Linglong-type deposits and the Jiaojia-type and Lü, 2000). The region occupies less than 0.2% of China's land area, deposits have not been investigated, though most of them have been but yields about a quarter of the country's gold production. Gold extensively described. This paper attempts to evaluate the contrast be- deposits in the Peninsula are mainly distributed along three gold belts tween the Dongfeng and Linglong gold mineralization in the Linglong from west to east, i.e., the Zhaoyuan–Laizhou, Penglai–Qixia and gold field from field observations, ore geology, fluid inclusion and stable Muping–Rushan belts (Fan et al., 2003; Hu et al., 2006). These gold isotope analysis in order to reveal the nature and evolution of the ore- deposits are controlled by NE- or NNE-trending faults and hosted in forming fluid system, and to probe the ore genesis in both types of the Precambrian high-grade metamorphic basement rocks as well as deposits. the Mesozoic granitoids. They were divided into two types according to ore occurrence, referred to as “Linglong-type” and “Jiaojia-type” (Goldfarb and Santosh, 2014; Qiu et al., 1988). The Linglong-type lode 2. Regional geology gold mineralization is characterized by massive auriferous quartz veins with narrow alteration halos and usually occurs in subsidiary The Jiaodong gold province is located along the southeastern margin second- or third-order faults. The Jiaojia-type disseminated and of the North China Craton (NCC) and at the western margin of the Pacific stockwork gold mineralization is usually surrounded by broad alter- Plate. It is bounded by the NE- to NNE-trending Tan–Lu fault zone to the ation zones and generally develops along major first-order regional west and by the Su–Lu ultrahigh pressure metamorphic belt to the faults. The lode gold deposits usually have smaller reserves and higher south (Fig. 1). Exposed rocks in the area comprise metamorphosed grades, whereas the disseminated and stockwork gold deposits have Precambrian basement sequences and a series of Mesozoic intrusive larger reserves and lower grades. and volcanic rocks (Zhou and Lü, 2000). The Precambrian sequences in- The Zhaoyuan–Laizhou gold belt, located in the northwest Jiaodong, clude the Archean Jiaodong Group and the Proterozoic Jingshan and shows the highest concentration of gold deposits, with over 80% of the Fenzishan Groups (Guo et al., 2005; Yang et al., 2012). These groups Jiaodong gold concentrated within an area of ~3500 km2 (Zhou and consist of mafic to felsic volcanic and sedimentary rocks metamor- Lü, 2000). The Linglong gold field in this belt is the typical example of phosed to amphibolite and granulite facies. The Mesozoic volcanic “Linglong-type” lode gold mineralization, which together with some rocks, namely Qingshan Formation, are mainly distributed in the Jiaolai other deposits such as the Taishang gold deposit, account for an overall Basin, and were formed at 108–110 Ma (Qiu et al., 2001a). The Qingshan gold reserve of more than 1000 metric tons. Recently, a giant and new Formation comprises two units, with the lower assemblage composed “Jiaojia-type” gold mineralization (Dongfeng gold deposit) was discov- of trachybasalt, latite, and trachyte, overlain by an upper assemblage ered. It has been confirmed that the gold reserve at Dongfeng is dominated by rhyolite flows and pyroclastic rocks (Li et al., 2006; Qiu 158.475 metric tons, with average grade of 2.75 × 10−6 (Shandong et al., 2001a).

Fig. 1. Simpliﬁed geological map of the Jiaodong Peninsula showing location of the major gold deposits (modiﬁed after Fan et al., 2003). The size of the symbols of the gold deposits indicates the gold reserves: large symbol means Au N 50 t, small symbol means Au b 50 t. The Dongfeng and Linglong deposits occur at the northwestern part of the gold province. B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 645

Mesozoic granitoid rocks are widespread in the Jiaodong gold prov- trending faults, which cut through the Mesozoic granitoids. There are ince. These rocks can be subdivided into three major groups according three ore-controlling fault zones, including the Sanshandao–Cangshang, to their formation ages: late Triassic granitoids, late Jurassic granitoids, Jiaojia–Xincheng, and Zhaoyuan–Pingdu fault zones from west to east. and early Cretaceous granitoids. The late Triassic granitoids such as More than half of the gold reserves in this area are controlled by the the Jiazishan, Chashan and Xingjia plutons mainly intruded in the Zhaoyuan–Pingdu fault zone (Deng et al., 2011). The Linglong-type southeastern fringe of Jiaodong from 225 to 205 Ma (zircon U–Pb lode gold mineralization and the Jiaojia-type disseminated and method; Chen et al., 2003; Guo et al., 2005; Yang et al., 2005). These gra- stockwork gold mineralization show coexistence in the Northwest nitic rocks, which show typical mantle-derived features (Gao et al., Jiaodong. Sericite/muscovite 40Ar/39Ar and single grain pyrite Rb–Sr 2004; Guo et al., 2005), were generated following the collision between dating have been carried out to determine the ages of the gold deposits the NCC and the Yangtze Craton during the middle-late Triassic in a in this area, which are between 123 and 114 Ma (Li et al., 2003, 2008; post-collisional setting (Tan et al., 2012; Xu et al., 2006; Yang et al., Yang and Zhou, 2001; Zhang et al., 2003). 2007). These intrusions are dominated by quartz syenite, pyroxene sy- The Linglong gold field is situated to the east of the Zhaoyuan– enite and alkaline gabbro (Tan et al., 2012). The late Jurassic granitoids, Laizhou gold belt and in the northern tip of the Zhaoyuan–Pingdu dated from 160 to 150 Ma by single zircon U–Pb method (Guo et al., fault zone. Mineralization in this field is dominated by auriferous quartz 2005; Miao et al., 1997; Wang et al., 1998; Yang et al., 2012), are repre- veins, with minor amount of disseminated sulfide replacements, and/or sented by the Linglong and Luanjiahe suites in the western Jiaodong, stockworks. The gold field is bordered to the southeast by the Potouqing and the Kunyushan, Queshan, Wendeng, and Duogushan suites in the fault, the northern segment of the Zhaoyuan–Pingdu fault (Fig. 2), eastern Jiaodong. They consist of medium-grained metaluminous to which is the major first-order structure controlling the Jiaojia-type min- slightly peraluminous biotite granite, granodiorite and monzonite eralization in this gold field (Qiu et al., 2002). The Potouqing fault trends (Tan et al., 2012), and were likely derived from the partial melting of a N60–70°E and dips S30–40°E. Granitic cataclasite, tectonic breccias and thickened Archean lower crust (Yang et al., 2012). The early Cretaceous a small amount of mylonite occur around the fault. In addition, hydro- granitoids, emplaced from 130 to 105 Ma (zircon U–Pb method; Miao thermal alteration and mineralization along the fault are well devel- et al., 1997; Guo et al., 2005; Zhang and Zhang, 2007; Goss et al., oped. The Linglong fault, cutting through the central part of the 2010; Yang et al., 2012), include Guojialing, Aishan, Nantianmen, and Linglong gold field, underwent multistage complex tectonic movements Beifengding suites in the western Jiaodong, and Sanfoshan, Weideshan, since its formation. The fault trends N25–30°E, and dips N65–85°W and Haiyang, Yuangezhuang, Yashan, and Laoshan suites in the eastern SE. Granitic cataclasite, hydrothermal alteration and very weak mineral- Jiaodong. They consist of granodiorite, porphyritic granite, and ization occur along the fault. Second-order faults in the gold field are monzonitic granite, and show a mixed source of crustal and mantle generally 100–5800 m in strike length and 1–10 m in width. These faults components (Guo et al., 2013; Liu et al., 1997; Song and Yan, 2000; consistently trend NNE to NEE, dip 50–75°NW and SE, and are the major Yang et al., 2012, 2013; Zhang et al., 2006). structures controlling the occurrence of felsic to mafic dikes, and Mafic to felsic dikes are common within the gold districts. They con- auriferous Linglong-type quartz veins (Qiu et al., 2002). Granitoids are sist of dolerite, lamprophyre, diorite (porphyry), granodiorite, granite well developed in the field, including Linglong gneissic biotite granite (porphyry) and syenite, thus spanning the range from medium/high-K and Luanjiahe medium-coarse grained granite with local exposures of calc-alkaline to shoshonitic rocks (Cai et al., 2013; Guo et al., 2004; the Guojialing granodiorite. The gold deposits are usually hosted within Tan et al., 2007, 2012; Yang et al., 2004). These rocks were mostly the Linglong granite, but deep within the Jiuqu area, the ores occur in emplaced at ca. 122–114 Ma and a few at 110–102 Ma (Qiu et al., both the Linglong granite and the Guojialing porphyritic granodiorite 2001b; Tan et al., 2008; Yang and Zhou, 2001; Zhang et al., 2002; Zhu (Chen et al., 1993, 2004; Li et al., 2004; Lu et al., 1999; Mao et al., and Zhang, 1998). The former has been correlated to magma genesis 2005; Yang et al., 2013). Intermediate and mafic dykes, consisting of during Cretaceous lithospheric thinning and asthenospheric upwelling diorite, dioritic porphyrite, and lamprophyre, are widely developed (Cai et al., 2013; Tan et al., 2008). within the gold field. They are spatially associated with the gold Two main stages of deformation have been identified in Jiaodong mineralization. during the late Mesozoic. The first stage involved northwest–southeast oblique compression, presumably related to the subduction of the Izanagi–Pacific plate, which produced prominent NNE- to NE-trending 3. Ore geology brittle–ductile shear zones with sinistral oblique reverse movements. This was followed by reactivation with development of brittle 3.1. Geology of the Dongfeng gold deposit structures and half-graben basins. These structures were accompanied by hydrothermal alteration and gold mineralization (Fan et al., 2003; The Dongfeng gold deposit is situated in the northwestern segment Hu et al., 1998; Li et al., 2013; Wang et al., 1998; Zhai et al., 2002). of the Jiaodong gold province, about 20 km north of Zhaoyuan City, and Northwest Jiaodong, where the Zhaoyuan–Laizhou gold belt is locat- forms part of the Linglong gold field (Figs. 2 and 3). Seven gold ore bod- ed, is a key part of the Jiaodong gold province (Fig. 1). Exposed rocks in ies have been identified in the Dongfeng gold deposit, which are jointly this area include metamorphosed Precambrian sequences and Mesozoic controlled by the NNE-trending Potouqing fault and the NE-trending intrusions (Wang et al., 1998; Zhou and Lü, 2000). The Precambrian se- Zhaoyuan–Pingdu fault. Ore bodies occur in the footwall of the main quence is mainly composed of basement rocks of the Late Archean fault plane. 1711 and 171sub-1 are the major ore bodies. As the largest Jiaodong Group with an age range of 2707–2726 Ma (Jahn et al., ore body, 1711 is layer-like and occurs at depths of 120 to 1700 m. The 2008). Plutonic rocks have been traditionally divided into two suites, ore body generally strikes NE 60° and dips SE 36.5° to 43.5°, with a the Linglong suite and the Guojialing suite. The former suite consists length of 2500 m and thicknesses of 0.27 to 26.06 m. Au grades in the of medium-grained metaluminous to slightly peraluminous biotite 1711 ore body range from 1.00 to 26.34 g/t, with an average value of granites, and the later one is composed of porphyritic hornblende– 2.71 g/t. 171sub-1 ore body is located under 1711 ore body. Their occur- biotite granodiorites (Deng et al., 2011; Fan et al., 2003; Yang and rences are similar. 171sub-1 ore body is also layer-like, with depths of Zhou, 2001). The emplacement ages of the two suites of granitoids are 370 to 1270 m and thicknesses of 0.50 to 18.46 m. Its Au grades range 160–156 Ma and 130–126 Ma, respectively (Miao et al., 1997; Qiu from 1.00 to 17.35 g/t, with an average value of 2.97 g/t. Gold ore bodies, et al., 2002; Wang et al., 1998; Yang et al., 2012, 2014). hosted in the late Jurassic granites, occur in large and thick fracture Gold deposits of the Northwest Jiaodong are mainly hosted in the alteration zones. Phyllic granite and phyllic cataclasite constitute the Mesozoic granitoids or along the contacts between the granitoids and roof and floor of the ore bodies. Outside the alteration zone, the hanging metamorphic rocks. They are usually controlled by the NE- and NNE- wall is composed of the Luanjiahe medium-coarse grained monzonite 646 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

Fig. 2. Simplified regional geological map of the Linglong gold field. granite, whereas the Linglong gneissic medium-fine grained biotite deposit (Figs. 2 and 3). The ore bodies are mainly hosted by the Linglong granite constitutes the footwall. granite, but in the Jiuqu area, these are hosted in both the Linglong gran- Mineralization appears associated with pyrite–sericite–silica altered ite and the Guojialing porphyritic granodiorites (Chen et al., 1993, 2004; rocks or fine pyrite veins (Figs. 4Cand5B). The major ore minerals in- Li et al., 2004; Lu et al., 1999; Yang et al., 2013, 2014; Zhang, 2002). In- clude native gold, electrum, and pyrite, whereas the subordinates are termediate and mafic dykes are widely developed within the gold de- chalcopyrite, galena, and sphalerite. The main gangue minerals consist posit. The mineralization occurs typically in the form of auriferous of quartz, plagioclase and sericite, with minor amounts of chlorite and quartz veins with lesser disseminated sulfide replacements and/or calcite (Fig. 6). Native gold grains occur mainly in fissures of pyrite stockworks. The Linglong gold deposit is structurally controlled by and quartz or as inclusions in pyrite crystals and gangue minerals both the NEE- to NNE-trending Potouqing fault zone and the NNE- (Fig. 5F). trending Linglong fault zone. The two major fault zones and their branches control hundreds of auriferous quartz veins, which display a 3.2. Geology of the Linglong gold deposit general NNE–NE trend, with local abrupt changes in the attitude of the fractures (Yang et al., 2014). The Linglong gold deposit is located in the western part of the north- The Linglong gold deposit occurs in the Dongshan and Xishan min- ern tip of the Zhaoyuan–Pingdu fault zone, west of the Dongfeng gold ing areas. More than two hundreds veins are exposed at the surface,

Fig. 3. Geological proﬁle crossing the ore bodies of the Dongfeng and Linglong deposits. B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 647

Fig. 4. Photographs showing the ore geology of the Dongfeng and Linglong gold deposits. (A) K-feldspathization and silicification. (B) Early-stage quartz vein (Q1) in associated with K-feldspathization. (C) Disseminated and stockwork mineralization, i.e., pyrite + sericite + silica alteration rocks containing the middle-stage quartz (Q2). (D) Lode mineralization, containing the middle-stage quartz (Q2), surrounded by silicification and sericitization. (E) Middle-stage quartz-sulfide vein (Q2) in the K-feldspathization and sericitization. (F) Late-stage quartz-carbonate vein (Q3). among which approximately 30 veins are economically viable, such as from ore bodies to wallrocks in the two gold deposits as: pyrite + Veins 9, 10, 36, 47, 51, 55, 56, 58, 98, 108, and 175. Overall, these veins sericite + silica → silica + sericite → K-feldspar → fresh granite. strike NE 35° to 70° and dip NW. However, near the Potouqing fault, Although the two gold deposits have similar alteration zonings, intensi- the veins have SE dips at shallow levels and NW at depth. The major ty of the hydrothermal alteration at the Linglong deposit is much weak- veins run for a few thousands of meters, with the longest reaching er than that at the Dongfeng deposit. Locally in the Linglong deposit, the more than 5500 m. These veins vary in width from a few meters to width of the alteration zone is less than 1 m or the alteration zone is tens of meters. even lacking in some cases. In contrast, alteration zones are well devel- Auriferous quartz veins (Figs. 4D, E and 5A) usually have variable oped at the Dongfeng deposit with widths varying from several meters grades from a few grams to a dozen or so per ton, with the highest to tens of meters, sometimes up to a few hundreds of meters. reaching up to hundreds of grams per ton. Native gold, electrum and py- The ore-forming stages are also similar between the Dongfeng and rite are the major ore minerals with minor of chalcopyrite, galena, and Linglong gold deposits. Based on mineral paragenesis and crosscutting sphalerite (Fig. 5C and D). Minor magnetite, hematite, pyrrhotite, and relationships, four hydrothermal stages can be distinguished. Stage 1 arsenopyrite are found locally. The main gangue minerals comprise is characterized by the assemblage of quartz ± pyrite (Fig. 4B).Itisde- quartz, sericite, feldspar, calcite, and chlorite (Figs. 5H and 6). Native fined by milky white quartz veins or pyrite-quartz veins containing few gold grains occur mainly in fissures of pyrite and quartz or as inclusions coarse euhedral and subhedral pyrite. K-feldspathization, silicification in pyrite crystals and gangue minerals, similar to that in the Dongfeng and sericitization is often developed. In this stage, gold is scarcely gold deposit (Fig. 5EandG). precipitated. Stage 2 is characterized by the assemblage of gold + quartz + pyrite (Fig. 4D). Generally, this stage is displayed by white- 3.3. Hydrothermal alteration and mineralizing stages gray quartz vein networks containing abundant pyrite, with minor chalcopyrite, galena, sphalerite, in the lode gold mineralization. Corre- Hydrothermal alteration is widespread, including potassic, sericitic, spondingly, disseminated sulfides in the pyrite + sericite + silica alter- pyritic, silicic alterations as well as chloritization and carbonatization ation rocks are the most important form of ores. In both cases, (Fig. 4A–F). These alterations are characterized by distinct zonings pyrite occurs as coarse euhedral cubes and subhedral aggregates. The 648 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

Fig. 5. Photomicrographs under reflected light showing important mineral assemblages at Dongfeng and Linglong deposits. (A) Lode ore. (B) Disseminated and stockwork ore. (C) Isolated electrum, native gold and pyrite in quartz. (D) Coexistence of quartz, pyrite, galena, chalcopyrite, and sphalerite. (E) Fractures in early precipitated pyrite that are filled with gold. (F) Native gold inclusion in pyrite and quartz. (G) Native gold in pyrite and its fractures. (H) Directional distribution of quartz in pyrite. Qz: quartz, Au: native gold, Py: pyrite, Gn: galena, Sph: sphalerite, Cp: chalcopyrite, El: electrum. structurally-controlled deformation and brecciation suggest deuteric summary, stage 1 is the early stage of mineralization, stages 2 and 3 con- mechanical stress. Stage 3 is characterized by the assemblage of stitute the middle stage when the major enrichment of gold occurred gold + quartz + base metal sulfide (Fig. 4E). In this stage, large and the practically barren stage 4 marks the late stage of mineralization amounts of sulfide minerals precipitated, including pyrite, galena, sphal- (Fig. 6). erite, chalcopyrite and minor pyrrhotite. Quartz is usually dark-gray. Py- rite occurs as fine-grained subhedral and anhedral aggregates. The other 4. Fluid inclusions sulfide minerals show fine-grained anhedral aggregates. Stage 4 is characterized by the assemblage of quartz + carbonate ±pyrite (Fig. 4F). 4.1. Sample descriptions and analytical methods White quartz and milky carbonate often occur together. The carbonates consist of calcite and minor ankerite. Pyrite occurs sporadically in minor Samples for fluid inclusion study were collected from the two amount. There is almost no gold mineralization in this stage. In deposits. Twenty two (early, 9; middle, 8; late, 5) and twenty (early, B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 649

Fig. 6. Paragenetic sequences of the Dongfeng and Linglong gold deposits.

6; middle, 10; late, 4) doubly polished thin sections (about wavelength of 532 nm and a source power of 44 mW was used in detec- 0.20–0.30 mm thick) were prepared from quartz samples associated tion. The spectral range falls between 100 and 4000 cm−1 for the anal- with different stages in the Dongfeng and Linglong deposits, respective- ysis of CO2,N2,CH4, and so on in the vapor phase. ly. Fluid inclusion petrography involved careful observation of the shapes, characteristics of spatial distribution, genetic and composition fl types, and vapor/liquid ratios. Samples with abundant and representa- 4.2. Petrography and types of uid inclusions tive fluid inclusions were selected for microthermometric measure- fl ments and laser Raman spectroscopy analyses. Four different compositional types of uid inclusions are distin- – – – Microthermometric measurements on the fluid inclusions were guished: pure CO2 (type I), H2O CO2 NaCl (type II), H2O NaCl (type fl carried out using a Linkam THMS 600 programmable heating–freezing III), and daughter mineral-bearing or multiphase (type IV) uid inclu- stage combined with a Zeiss microscope at the Institute of Geology sions, based on the combination of petrography at room temperature, and Geophysics, Chinese Academy of Sciences (IGGCAS). The stage phase transitions observed during heating and cooling, and laser was calibrated using synthetic fluid inclusions supplied by FLUID INC Raman spectroscopy (Fig. 7). through calibration against the triple-point of pure CO2 (−56.6 °C), the freezing point of water (0.0 °C) and the critical point of water 4.2.1. Type I inclusions (374.1 °C). Most measurements were carried out at a heating rate of Type I inclusions consist of almost pure carbonic fluid lacking any 0.2 to 0.4 °C/min. Carbonic phase melting (Tm-CO ) and clathrate melting 2 visible H2O at room temperatures, including monophase CO2 (vapor (Tm-clath) were determined by temperature cycling (Diamond, 2001; or liquid), and two-phase CO2 (VCO +LCO )(Fig. 7A and B). They are – 2 2 Fan et al., 2003; Roedder, 1984). 0.1 0.2 °C/min, the heating rate for usually dark with oval to negative crystal morphologies. These inclu- measurements, was adopted near phase transformations. The precision sions, ranging from 6 to 13 μm in size, have been mostly found of measurements was ±0.2 °C at temperatures below 30 °C and ±2 °C coexisting with type II inclusions in the early-stage quartz of the two de- at temperatures above 30 °C. posits, and with other three types of inclusions in the middle-stage Five types of temperature observations were made in this study in- quartz of the Linglong deposit. The typically isolated and scattered cluding the melting temperature of CO2 (Tm-CO ), final melting temper- 2 nature of these inclusions indicates that they are primary inclusions. atures of ice (Tm-ice), final melting temperatures of clathrate (Tm-clath), the homogenization temperatures of the CO2 (Th-CO2) and the total homogenization temperatures (Th-tot). Using Tm-ice and Tm-clath, salinities 4.2.2. Type II inclusions of the H2O–NaCl (Bodnar, 1993)andH2O–CO2–NaCl (Collins, 1979) Type II inclusions are composed of H2OandCO2 phases with fluid systems can be calculated. The density of the CO2 can be well re- 20–70 vol.% carbonic phase (Fig. 7C, D and H). They can be further fl stricted through Th-CO2.Th-tot can re ect the temperature of different divided into two subtypes, containing two-phase (VCO2 +LH2O)and fl stages of ore-forming uid to some extent. Mole fractions of composi- three-phase (VCO2 +LCO2 +LH2O)inclusionsatroomtemperaturewith tions, density of carbonic liquid and bulk fluid, and bulk molar volume varying sizes between 5 and 23 μm. As the predominant type of fluid in- of fluid inclusions were calculated by the Flincor computer software clusions, they are abundant in quartz formed in the early- and middle- (Brown and Lamb, 1989). stages. They generally occur in isolation or in cluster. Sometimes they Laser Raman spectroscopic analysis of the fluid inclusions was car- appear as trails along healed fractures which do not cut across the ried out on the LabRam HR800 Raman microspectrometer (produced crystal boundaries of quartz. These features suggest that they are prima- by French HORIBA Scientific) at the IGGCAS. An argon ion laser with a ry or pseudosecondary. 650 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

Fig. 7. Photomicrographs of typical fluid inclusions in the Dongfeng and Linglong deposits. (A) One phase type I fluid inclusion. (B) Two phases type I fluid inclusion. (C) Three phases type II fluid inclusion. (D) Two phases type II fluid inclusion. (E) Type III fluid inclusion. (F) Type IV fluid inclusion. (G) Type III fluid inclusions on a cluster distribution. (H) Type II fluid inclusions on a cluster distribution. (I) Boiling fluid inclusions association. (J–K) A comparison of the characteristics of fluid inclusions from the Dongfeng gold deposit. (L–M) A comparison of the characteristics of fluid inclusions from the Linglong gold deposit.

4.2.3. Type III inclusions middle-stage quartz. It's noteworthy that primary type III inclusions are also well developed in the middle-stage quartz of Linglong deposit. Type III inclusions are one-phase (LH2O) or two-phase (VH2O +LH2O) liquid-rich aqueous inclusions (Fig. 7E and G). The two-phase inclusions Trace content of CO2 can still be identiﬁed in the vapor bubbles by laser are more common with vapor volume occupying 2–40% of the total Raman spectroscopy (Fig. 9e), although no visible CO2 phase appears cavity volume. These inclusions, varying in size from 5 to 14 μm, have during heating or cooling runs. a variety of shapes ranging from irregular to elliptical and negative shapes. They are commonly present in quartz of all stages, particularly 4.2.4. Type IV inclusions in the late-stage quartz crystals. In general, the primary type III inclu- Type IV inclusions are scarce and are usually composed of aque- sions occur as isolated singles or group in late-stage quartz. Secondary ous liquid, a vapor bubble, and a calcite crystal at room temperature type III inclusions cutting across the crystal boundaries of quartz can (Figs. 7F, 9g and h). They are irregular or circular in shape with be observed as arrays or trails along healed fractures in early- and 7–12 μm in size and are only observed in the middle-stage quartz B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 651

crystals. They usually occur as isolated individuals coexisting with of the CO2 clathrate (Tm-clath) in the presence of CO2 liquid occurs be- type I, type II and type III inclusions (Fig. 7I). tween 3.8 °C and 8.3 °C, corresponding to the ﬂuid salinities of 3.3 to

10.8 wt.% NaCl equivalent. CO2 generally homogenized to the liquid

4.3. Microthermometry and laser Raman spectroscopy phase and Th-CO2 range from 13.6 °C to 30.9 °C. The densities of the 3 CO2 phase are calculated to be between 0.30 and 0.83 g/cm with XCO2 4.3.1. Dongfeng gold deposit varying from 0.01 to 0.19. Densities of the bulk inclusions range from 0.76 to 1.05 g/cm3. Most of the type II ﬂuid inclusions homogenized in 4.3.1.1. Early stage. Type II inclusions are dominant in the early-stage the range of 248 °C to 310 °C (L + V to L, few L + V to V or the critical quartz, coupled with some type I inclusions. For type II inclusions, the state) (Fig. 8 and Table 1), excepting some inclusions decrepitating at − melting temperatures of solid CO2 (Tm-CO2) range from 56.9 °C to temperatures from 253 °C to 254 °C before total homogenization. −56.6 °C, equal to or slightly lower than the triple point of pure CO2 (−56.6 °C), indicating that the gas phase is mainly composed of CO2. 4.3.1.3. Late stage. Type III aqueous inclusions from the late-stage quartz The melting temperatures of clathrates (Tm-clath)wereobservedbe- yield ﬁnal ice melting temperatures (Tm-ice)of−5.5 °C to −0.3 °C, cor- tween 3.1 °C and 8.4 °C, corresponding to salinities between 2.8 and responding to salinities varying from 0.5 to 8.5 wt.% NaCl equivalent. 11.7 wt.% NaCl equivalent (Fig. 8 and Table 1). The carbonic phase The temperatures of homogenization to liquid phase are between

(Th-CO2) was partially homogenized to liquid at temperatures ranging 117 °C and 219 °C (Fig. 8 and Table 1). Densities of the bulk inclusions 3 from 25.6 °C to 30.9 °C. Total homogenization (Th-tot) of the carbonic range from 0.86 to 1.00 g/cm . and aqueous phases (L + V to L, few L + V to V) was observed at temperatures ranging from 276 °C to 341 °C. However, some inclusions 4.3.1.4. Laser Raman spectroscopy. Laser Raman spectroscopy shows that with greater vapor/liquid ratios decrepitated between 310 °C and CO2 and H2O are the main volatiles in the measured fluid inclusions 330 °C prior to final homogenization. The calculated CO2 densities from the early- and middle-stage quartz (Fig. 9a–c). No CH4,N2 or 3 fl range from 0.25 to 0.63 g/cm with XCO2 from 0.02 to 0.18 and bulk other gas phases were detected in these uid inclusions. This is in accor- densities from 0.57 to 0.97 g/cm3. For type I inclusions, final melting dance with the microthermometric results that the melting tempera- − to liquid was observed during heating, with Tm-CO2 ranging from tures of solid CO2 are near 56.6 °C, the triple point of pure CO2.In − − fl 57.0 °C to 56.6 °C. Partial homogenization (Th-CO2)ofCO2 (L + V the late-stage quartz, uid inclusions mainly consist of H2O, in the to L) occurs between 28.9 °C and 30.9 °C, corresponding to densities absence of any other major volatile phase (Fig. 9f). of 0.53 to 0.63 g/cm3. 4.3.2. Linglong gold deposit 4.3.1.2. Middle stage. Type II fluid inclusions are the most abundant − inclusions in the middle-stage quartz. Tm-CO2 ranges from 56.9 °C 4.3.2.1. Early stage. Type II inclusions and a few type I inclusions exist in to −56.6 °C and is generally near the pure CO2 melting point the early-stage quartz. For type II inclusions, melting of the solid CO2 − − − ( 56.6 °C), indicating that the dominant composition is CO2. Melting (Tm-CO2) occurred between 56.9 °C and 56.6 °C. These temperatures

Fig. 8. Histograms of homogenization temperatures (Th-tot). (A) Dongfeng gold deposit. (B) Linglong gold deposit. 652 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

Table 1 Microthermometric data of ﬂuid inclusions at the Dongfeng and Linglong gold deposits.

Name Stage Type N Tm-CO2/°C Tm-clath/°C Th-CO2/°C Tm-ice/°C Th-tot/°C Salinity/wt.% CO2 density Bulk density NaCl equiv. (g/cm3) (g/cm3)

Dongfeng gold Early I 10 −57.0 to −56.6 28.9–30.9 0.53–0.63 deposit II 30 −56.9 to −56.6 3.1–8.4 25.6–30.9 276–341 2.8–11.7 0.25–0.63 0.57–0.97 Middle II 31 −56.9 to −56.6 3.8–8.3 13.6–30.9 248–310 3.3–10.8 0.30–0.83 0.76–1.05 Late III 22 −5.5 to −0.3 117–219 0.5–8.5 0.86–1.00 Linglong gold Early I 8 −57.2 to −56.6 26.6–30.7 0.55–0.69 deposit II 27 −56.9 to −56.6 5.9–8.3 26.5–29.0 271–374 3.3–7.5 0.63–0.69 0.87–1.03 Middle I 11 −57.2 to −56.6 22.2–30.9 0.53–0.75 II 17 −58.9 to −56.6 4.1–7.1 12.5–28.4 251–287 5.5–10.3 0.65–0.84 0.82–1.01 III 8 −9.4 to −2.5 244–291 4.1–13.3 0.82–0.90 Late III 28 −5.3 to −0.2 103–215 0.3–8.2 0.88–0.98 fl fi fi Note: N, numbers of measured uid inclusion; Tm-CO2, nal melting temperature of solid CO2;Tm-clath, nal melting temperature of the clathrate phase; Th-CO2, temperature of CO2 (L + V) to CO2 (L) or CO2 (V); Tm-ice, final melting temperature of water ice; Th-tot, temperature of total homogenization of the inclusions; wt.% NaCl equiv., weight percent NaCl equivalent.

are equal to or slightly lower than the melting temperature of pure solid The temperatures of homogenization to the liquid phase are between

CO2 (−56.6 °C) and thus indicates that the gas phase consists predom- 103 °C and 215 °C (Fig. 8 and Table 1). Densities of the bulk inclusions 3 inantly of CO2. Melting of the CO2 clathrate (Tm-clath)wasobserved range from 0.88 to 0.98 g/cm . between 5.9 °C and 8.3 °C, corresponding to the salinities ranging from 3.3 to 7.5 wt.% NaCl equivalent. The homogenization of the CO2 4.3.2.4. Laser Raman spectroscopy. The data from laser Raman spectros-

(Th-CO2) into liquid occurred between 26.5 °C and 29.0 °C. Total homog- copy show that CO2 and H2O are the main volatiles in the measured enization (Th-tot), mostly into liquid phase, was recorded between fluid inclusions from the early- and middle-stage quartz (Fig. 9a–c 271 °C and 374 °C (Fig. 8 and Table 1). Decrepitation temperatures of and e). Minor quantity of CH4 was detected in some of the middle- some inclusions with greater vapor/liquid ratios range from 250 °C stage type II fluid inclusions (Fig. 9d). This is in accordance with the to 272 °C. CO2 densities from type II inclusions are from 0.63 microthermometric results that melting temperatures of solid CO2 in 3 − to 0.69 g/cm with XCO2 from 0.01 to 0.16 and bulk densities from 0.87 some inclusions are below 56.6 °C. Daughter minerals are almost all 3 − to 1.03 g/cm . For type I inclusions, Tm-CO2 ranges from 57.2 °C calcite in the type IV inclusions (Fig. 9g and h). In the late-stage quartz, − fl to 56.6 °C. The homogenization of the CO2 (Th-CO2) into liquid was ob- uid inclusions mainly consist of H2O(Fig. 9f). Other gas phase was served between 26.6 °C and 30.7 °C, corresponding to densities of 0.55 barely found. to 0.69 g/cm3. 5. Stable isotopes studies 4.3.2.2. Middle stage. All the four types of fluid inclusions are developed in the middle-stage quartz. For type I inclusions, final melting to liquid Quartz and pyrite grains were handpicked from the 40–60 mesh − N was observed during heating, with Tm-CO2 ranging from 57.2 °C to crushings under a binocular (purity 99%). Analyses of hydrogen, oxy- − 56.6 °C. Partial homogenization (Th-CO2)ofCO2 (L + V to L) occurs be- gen and sulfur isotopic compositions were performed at the Analytical tween 22.2 °C and 30.9 °C, corresponding to densities of 0.53 to Laboratory of the Beijing Research Institute of Uranium Geology. 0.75 g/cm3. Hydrogen isotope analyses of the inclusion fluids were performed on fl − − Tm-CO2 of type II uid inclusions ranges from 58.9 °C to 56.6 °C the ten quartz vein samples, which are from different ore-forming and is generally near or below the pure CO2 melting point (−56.6 °C), stages of the two gold deposits. Water was released by heating the sam- indicating there are other gas components, such as CH4 and N2 ples to approximately 500 °C in an induction furnace. Samples were first (Roedder, 1984), in the gas phase. Melting of the CO2 clathrate (Tm-clath) degassed of labile volatiles by heating to 180–200 °C until the vacuum is −1 in the presence of CO2 liquid occurs between 4.1 °C and 7.1 °C, corre- less than 10 Pa. Water was converted to hydrogen by passage over sponding to the fluid salinities of 5.5 to 10.3 wt.% NaCl equivalent. CO2 heated zinc powder at 400 °C and the hydrogen was analyzed with a generally homogenized to the liquid phase and Th-CO2 ranges from MAT-253 mass spectrometer. Analyses of standard water samples 12.5 °C to 28.4 °C. The densities of the CO2 phase are calculated to be suggest a precision for δDof±2‰. 3 between 0.65 and 0.84 g/cm with XCO2 varying from 0.05 to 0.43. Oxygen isotope analyses were performed on ten quartz vein sam- Densities of the bulk inclusions range from 0.82 to 1.01 g/cm3. Most of ples, which are used for hydrogen isotope analyses. The pure minerals the type II fluid inclusions homogenized in the range of 251 °C to were crushed into 200 mesh and the crushings reacted with BrF5 at 287 °C (some L + V to L, others L + V to V or the critical state) (Fig. 8 500–600 °C for 14 h, generating O2 which subsequently reacted with and Table 1) with the exception of some inclusions decrepitating at graphite to produce CO2 at 700 °C with platinum catalyst. The CO2 was temperatures from 240 °C to 270 °C before total homogenization. then measured by MAT-253 mass spectrometer for oxygen isotope. Re- Type III aqueous inclusions from the middle-stage quartz yield final producibility for isotopically homogeneous pure quartz is about ±0.2‰. ice melting temperatures (Tm-ice)of−9.4 °C to −2.5 °C, corresponding Ten pyrite samples from ores of the two gold deposits were put to to salinities varying from 4.1 to 13.3 wt.% NaCl equivalent. Densities of use for sulfur isotope analyses. The pyrite grains were mixed with cu- 3 the bulk inclusions range from 0.82 to 0.90 g/cm . The temperatures prous oxide and crushed into 200 mesh powder. SO2 was produced of homogenization to the liquid phase are between 244 °C and 291 °C through the reaction of pyrite and cuprous oxide at 980 °C under a vac- −2 (Fig. 8 and Table 1). uum pressure of 2 × 10 Pa. The SO2 wasthenmeasuredbyMAT-251 Vapor bubbles of the type IV inclusions disappeared firstly during mass spectrometer for sulfur isotope. All the analytical uncertainties heating, whereas the daughter minerals did not dissolve even if temper- were better than ±0.2‰. ature was up to 500 °C. The stable isotope data obtained in this and previous studies (Hou et al., 2006) are shown in Table 2. δD of the inclusion fluids in quartz 4.3.2.3. Late stage. Type III aqueous inclusions from the late-stage quartz from the Dongfeng gold deposit vary from −90.5‰ to −82.7‰, with yield final ice melting temperatures (Tm-ice)of−5.3 °C to −0.2 °C, cor- an average value of −86.6‰. δD of the inclusion fluids in quartz from responding to salinities varying from 0.3 to 8.2 wt.% NaCl equivalent. the Linglong gold deposit vary from −77.4‰ to −63.7‰, with an B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 653

Fig. 9. Representative Raman spectra of vapor bubbles of fluid inclusions in quartz. (a) Two phase type I fluid inclusion. (b) Three phase type II fluid inclusion. (c) Two phase type II fluid inclusion. (d) Spectrum for three phase type II fluid inclusion, showing a small amount of CH4. (e) Vapor bubble in type III fluid inclusion, containing trace content of CO2. (f) Type III fluid inclusion, containing water only. (g–h) Type IV fluid inclusions with calcite as a daughter mineral.

34 average value of −69.1‰. Oxygen isotopic compositions of hydrother- water field (Fig. 10). The δ SV-CDT values of pyrite range from +5.8‰ mal waters in equilibrium with quartz were calculated using an extrap- to +7.0‰ and from +5.9‰ to +7.4‰ in the Dongfeng and Linglong olation of the fractionation formula from Clayton et al. (1972).The gold deposits, respectively. calculations of the fractionation factors were made using the mean value of the homogenization temperatures of fluid inclusions from the 6. Discussion same ore-forming stage quartz samples. The calculated oxygen isotope composition of the fluid from the Dongfeng gold deposit is character- 6.1. Fluid evolution in the two gold deposits ized by δ18Oof−3.8‰ to +6.4‰,withanaveragevalueof0.0‰.Sim- ilarly, the fluid from the Linglong gold deposit is characterized by δ18O Fluid inclusion studies and laser Raman spectroscopy suggest that of 0.0‰ to +8.9‰, with an average value of +4.9‰.InaplotofδDvs. the ore-forming fluid in the two gold deposits have similar chemical δ18O, ten quartz samples plot are adjacent to the primary magmatic and physical properties. The early-stage quartz contains the type I and 654 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

Table 2 Stable isotope data (reported as per mil values) for minerals from the Linglong gold ﬁeld.

18 18 34 Name Sample Mineral Stage δ Oqz Th (°C) δ Oﬂuid δD δ S Data sources Dongfeng gold deposit PZ46812-1 Quartz Early 6.3 308 −0.8 −82.7 This paper PZ4966-1 Quartz Middle 6.4 282 −1.7 −85.7 PZ41207-2 Quartz Middle 14.5 282 6.4 −90.5 PZ46812-3 Quartz Late 11.6 157 −3.8 −87.5 Linglong gold deposit 10X74 Quartz Early 12.5 294 4.9 −67.1 This paper 10X78 Quartz Early 11.8 294 4.2 −63.7 10LL04 Quartz Early 12.9 294 5.3 −70.1 10X79 Quartz Middle 14.4 274 6.0 −69.9 LL-Q-06 Quartz Middle 17.3 274 8.9 −77.4 10X80 Quartz Late 14.7 165 0.0 −66.5 Dongfeng gold deposit 10LL18 Pyrite Middle 6.7 This paper 10LL20 Pyrite Middle 6.5 10LL22 Pyrite Middle 6.8 10LL23 Pyrite Middle 5.8 10LL24 Pyrite Middle 7.0 Linglong gold deposit 10LL03 Pyrite Early 6.6 This paper JQ-Q-04 Pyrite Middle 6.5 LL-Q-02 Pyrite Middle 6.2 LL-Q-06 Pyrite Middle 5.9 10X80 Pyrite Late 7.4 Linglong gold deposit LL-108-1 Pyrite Middle 7.7 Hou et al. (2006) LL-108-2 Pyrite Middle 8.3 LL-108-5 Pyrite Middle 7.6 LL-108-6 Pyrite Middle 8.5 LL-108-7 Pyrite Middle 7.5 LL-53-4 Pyrite Middle 7.0 LL-48-1 Pyrite Middle 8.6 LL-48-4 Pyrite Middle 7.2 LL-50-3 Pyrite Middle 6.4 LL-50-5 Pyrite Middle 7.3 LL-47-1 Pyrite Middle 7.4 Lingnan gold deposit LL-171-2 Pyrite Middle 7.8 Hou et al. (2006) LL-171-3 Pyrite Middle 7.0 LL-171-6 Pyrite Middle 8.3 LL-171-7 Pyrite Middle 7.8 LL-171-10 Pyrite Middle 7.9

type II inclusions, whereas the late-stage quartz contains only the type

III inclusions. In the early stage, ore-forming fluid belongs to H2O– CO2–NaCl system, which is characterized by medium-high temperature, enrichment of CO2, and medium-low salinity (Fig. 11 and Table 1), in contrast to typical high temperature and high salinity magmatic fluids. These features, combined with the analytical results of hydrogen and oxygen isotopes (Fig. 10), indicate that the hyperthermal, volatiles- abundant and Au-rich primary ore-forming fluid probably mixed with meteoric water infiltrating downward along fractures, when it moved upward through the fractures, altering the primary characteristics of

the ore-forming fluid. The fluid evolved into H2O–CO2–NaCl system with medium-low temperature, less CO2, and variable salinity in the middle stage. During the mineralization, more meteoric water was involved (Fig. 10). Finally, the ore-forming fluid, in the late stage, turned

into H2O–NaCl system with low temperature, low salinity and no CO2 (Fig. 11 and Table 1).

6.2. Source of ore-forming materials and ﬂuids

The hydrogen and oxygen isotopes of the two deposits show similar distribution in the δD-δ18O isotopic diagram (Fig. 10), suggesting similar sources of ore-forming fluids, for the data from the two deposits plot between the magmatic field (or the metamorphic field) and the global meteoric water line (Fig. 10). Since the Mesozoic age of mineralization is about 2 billion years younger than the timing of metamorphism in the basement rocks, the ore-forming fluids could not have been derived from the metamorphic fluids. Furthermore, the gold mineralization ages (123–114 Ma) are younger than the ages of the regional Mesozoic granites such as the Linglong (160–156 Ma) and Guojialing (130–126 Ma) Fig. 10. δDandδ18O characteristics of the ore-forming fluids at the Dongfeng and Linglong granitoids, thus precluding the possibility of magmatic sources for the gold deposits. (A) Dongfeng gold deposit. (B) Linglong gold deposit. ore-forming fluids. Recent studies suggest the role of mantle-derived B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 655

from the sulfur isotopic compositions of the sulfide minerals. The two gold deposits have consistent δ34Svalues(Table 2), which were obtained from pyrite in equilibrium with the mineralization of the two deposits. Hou et al. (2006) analyzed 16 pyrite samples from the Linglong gold field, and reported δ34S values varying from 6.4‰ to 8.6‰ with an average value of 7.6‰ (Table 2), which are in accordance with this study. At the same time, Hou et al. (2006) carried out reported contrast- ing of δ34S values among metamorphic rocks of the Achaean Jiaodong Group, Mesozoic basic-intermediate dikes, Mesozoic granites, and gold ores (Fig. 12). The results show that the δ34S values of these geo- logic units are comparable, especially, the δ34SvaluesofMesozoic mantle-derived basic-intermediate dikes deviate from the mantle values (δ34S=~0‰). Mao et al. (2008) suggested that the similar sulfur isotopic compositions of the Mesozoic rocks implied homogenization of the sulfur isotopic system through crust–mantle interaction. In other words, during the Mesozoic mineralization events, the ore- forming fluids were sourced from a common fluid reservoir probably linked to processes of crust–mantle interaction.

6.3. Gold transport and deposition and a comparison of ore forming mechanism between the two gold deposits

HS− and Cl−, which can form stable complexes with gold ions, are the most important ligands in hydrothermal solutions. Initially gold is dissolved and transported in the form of gold bisulfide [Au(HS)0, − − Au(HS)2 )] and gold chloride [AuCl2 ](Benning and Seward, 1996; Hayashi and Ohmoto, 1991; Seward, 1973, 1990; Stefansson and Seward, 2004; Williams-Jones et al., 2009; Zotov et al., 1991). Taking into consideration that gold is usually accompanied with sulfide in these deposits, especially pyrite, we infer that gold bisulfide was the most possible species transporting gold. The abundance of type I and type II inclusions in the early stage of the two gold deposits suggests

that the initial ore-forming fluids were enriched in CO2.CO2 can buffer Fig. 11. Temperature vs. salinity plot of the fluid inclusions, showing fluid evolution at the the pH of the solution (Phillips and Evans, 2004), which provides favor- Dongfeng and Linglong deposits. (A) Dongfeng gold deposit. (B) Linglong gold deposit. able conditions for gold bisulfide migration. Although there are similarities in fluid sources between Dongfeng fluids in the metallogenic process (Deng et al., 2003; Liu et al., 2002, and Linglong gold deposits, the ore forming mechanisms appear to 2003; Mao et al., 2002). A number of mafic dikes, whose formation be different. Our petrographic studies, Raman spectroscopy and ages are close to the gold mineralization ages, are widely distributed microthermometry on fluid inclusions show that the type II inclusions around the Jiaodong gold province. Some researchers considered that and two phase type III inclusions not only coexist in the middle-stage the ore-forming fluids had a magmatic source, derived through quartz of the Linglong gold deposit (Fig. 7L and M), but also have consis- degassing of mantle-derived magmas in the shallow part of crust (Fan tent homogenization temperatures. In addition, the type III inclusions in et al., 2003, 2005). the middle-stage quartz usually contain trace amounts of CO2 (Fig. 9E), H2S is an important medium for the migration and precipitation of although no CO2 phase was observed during the heating–cooling runs. Au.TheSofH2S is bound within sulfide, especially pyrite, which often Furthermore, four different types of fluid inclusions coexist in some accompanies native gold. Therefore, the source of Au can be traced domains (Fig. 7I). Type II and type III inclusions have similar range of

Fig. 12. A comparison of sulfur isotopic compositions of sulfide ores and rocks from the Jiaodong gold fields. After Hou et al. (2006). 656 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 homogenization temperatures; homogenization temperatures of type and which have been dated at ca. 122 to 119 Ma and, less commonly, at IV inclusions were not observed. These features imply that phase sepa- 110 to 102 Ma (Cai et al., 2013; Qiu et al., 2001b; Yang and Zhou, 2001; ration or boiling might have occurred in the middle stage at the Linglong Zhang et al., 2002; Zhu and Zhang, 1998). gold deposit. Accompanied with a significant drop in temperature and Yang and Zhou (2001) and Li et al. (2008) dated pyrite from the pressure of the fluid, the boiling resulted in CO2 escape and consump- Linglong gold field and reported ages in the range of 122–123 Ma and + + − tion of H through the reaction: H +HCO3 =H2O+CO2. 120.6 ± 0.9 Ma, respectively, consistent with the ages from other gold This process might have led to the decomposition of gold bisulfide deposits in the Jiaodong gold province. The Dongfeng and Linglong 0 − + [Au(HS) , Au(HS)2 )], as the activity of H is the key factor to maintain gold deposits have similar characteristics in mineralogy, lithology, alter- − 0 − HS and gold bisulfide [Au(HS) ,Au(HS)2 )] stable in the fluids (Chen ation patterns, and ore-forming fluids, suggesting that these two gold et al., 2007). Phase separation can release H2S from the liquid into the deposits were formed during the same metallogenic event at about vapor phase, which also decreases the stability of Au–S complexes 120 Ma. This metallogenic event is widely developed in the Jiaodong (Cox et al., 1995; Jia et al., 2000; Naden and Shepherd, 1989; Zhang gold province. During this period, post-collisional extension occurred et al., 2012). Subsequently, Au precipitated from the ore-forming fluid. in the North China and Yangtze Cratons with transfer of the principal In contrast, type II inclusions appear alone in the middle stage of the stress-field from north–south to east–west directions, and east–west Dongfeng gold deposit (Fig. 7J and K). This suggests that large-scale lithospheric extension caused by subduction of the Paleo-Pacific plate phase separation or boiling did not occur in the main mineralization (Fan et al., 2005; Mao et al., 2003a, 2003b, 2004, 2006, 2008). Simulta- period. Hydrothermal alteration is widely developed in the Dongfeng neously, lithospheric thinning, which was caused by the removal of lith- gold deposit, much more intense than that at the Linglong gold deposit. ospheric mantle and the upwelling of new asthenospheric mantle, This may imply that intense water–rock interaction occurred between induced partial melting and dehydration of the lithospheric mantle ore-forming fluid and wall rocks at the Dongfeng gold deposit. This and lower crust due to an increase of temperature (Yang et al., 2003). process dramatically changed the physical and chemical conditions of The mantle-derived magma migrated upward to the shallow crust, it ore-forming fluid in the main mineralization period, and finally resulted might have degassed considerable volume of fluids. These fluids in the precipitation and mineralization of gold. In the process of water– mixed with meteoric water to form the ore-forming fluids. rock interaction, H2S was expended to generate pyrite with iron derived The first-order faults in this area underwent multi-stage reactiva- from the wall rocks, and pH of the fluid ascended by release of CO2. tion, and the stress types were diverse at different periods. Repeated These factors made gold bisulfide unstable and to eventually precipitate stress and tectonic movements caused the rocks around faults to metallic gold. become highly cataclastic. Large and small fissures and cavities were developed within the cataclastic rocks, which provided the pathways 6.4. Pressure and depth of gold deposition for ore-forming fluids against the wall rocks, creating favorable conditions for fluid permeation and hydrothermal alteration. The first-order Using the Flincor computer software with the equations of Brown faults are the main migration pathway of the ore-forming fluids. High and Lamb (1989) for the H2O–CO2–NaCl system, pressures in the temperature and strong water–rock interaction occurred along the range of 226–338 MPa were obtained in the middle stage of the first-order faults, resulting in the formation of “Jiaojia-type” disseminat- Dongfeng gold deposit, and 228–326 MPa for the Linglong gold deposit. ed and stockwork gold mineralization. In contrast, the secondary faults If these pressures are lithostatic, given 2.7 g/cm3 as the density of upper were less activated and the rocks around these show less degree of crust rocks, the corresponding depth of mineralization is in the range of fracture. They were thus unfavorable water–rock interaction but served 8.4–12.5 km and 8.4–12.1 km, respectively. The trapping pressure in the as open conduits for the migration of ore fluids. During the process of Dongfeng gold deposit should be higher than that we estimated, ore fluid migration from the first-order faults to the secondary faults, because the trapping temperature is above the homogenization tem- the temperature gradually decreased. Opening of the faults led to sudden perature. In contrast, the trapping pressure in the Linglong gold deposit decompression and fluid phase separation (boiling). A sharp fall in tem- should be equal to the estimated values, because of fluid boiling. There- perature and large-scale exsolution of volatiles also occurred at the same fore, the depth range computed for the Dongfeng gold deposit is only a time. This process brought about the precipitation of the “Linglong-type” minimum estimate, and the actual depths must be higher, and the min- lode gold mineralization. eralization occurred at a deeper domain than that of the Linglong gold It is worth raising that the above mentioned ore-forming process deposit. This result is in accordance with the fact that the ore bodies might be closely related to seismic activity. The role of seismic pressure exploited for gold in the Dongfeng gold deposit are at greater depths fluctuations has been advocated for orogenic mineral systems in the than those in the Linglong gold deposit. South Island of New Zealand (Craw et al., 2013). The movement of ore fluids is partly controlled by permeability, and enhanced permeability 6.5. Deposit genesis is considered to be proximal to earthquakes, where shear failure is likely to occur, thereby explaining the common fault- to shear zone-controlling The gold deposits in the Jiaodong province are mostly hosted in the ore deposits. Fault jogs and flower structures are particularly Mesozoic granitoids and are structurally controlled by faults and shear efficient in the channeling of fluids during aftershocks, because zones that cut the Mesozoic granitoids. Previous studies have shown those structures promote vertical flow, and can tap metal-rich reser- that the ages of the gold deposits in the Jiaodong gold province voirs (Craw et al., 2013). A fault-valve will drive fluid-pressure, and cluster between 123 and 114 Ma as determined by sericite/muscovite enhance permeability and fault ruptures can connect shallow low- 40Ar/39Ar and single grain pyrite Rb–Sr dating (Hu et al., 2013; Li et al., pressure reservoirs with deeper high-pressure reservoirs, resulting 2003, 2006, 2008; Yang and Zhou, 2001; Zhang et al., 2003). The forma- in strong degassing. Importantly, the presence of breccias and vein- tion ages of the Mesozoic granitoids are 160–156 Ma and 130–126 Ma, ing imply episodic phases of fluid flow and quite likely with fluid respectively, as obtained by zircon U–Pb dating (Guo et al., 2005; Miao pressures changing from lithostatic to hydrostatic (Sibson, 2001; et al., 1997; Qiu et al., 2002; Wang et al., 1998; Yang et al., 2012, Sibson et al., 1975). Aftershocks, seismic slips and suction-pump 2013). The timing of gold mineralization is significantly younger than mechanisms are induced by rapid transfer of fluids in dilational the ages of the regional granitoid magmatism as the Linglong and fault jogs and bends, resulting in the abrupt reduction of fluid pres- Guojialing granitoids, indicating that the gold mineralization has no sure at structural sites, triggering phase separation and ore precipi- direct relationship to the granitoid magmatism. Instead, most gold tation throughout the aftershock phases (Sibson, 2001; Sibson deposits show temporal and spatial association with abundant mafic et al., 1975). These phenomena lead to multiple phases of ore miner- to intermediate dikes that are widespread in the Jiaodong gold province, al precipitation, accompanied by related alteration minerals. B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658 657

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