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 goldfield, China: Constrains from geology, fluid inclusions and stable isotope

Bo-Jie Wen a, Hong-Rui Fan a,⁎, M. Santosh b, Fang-Fang Hu a, Franco Pirajno c, Kui-Feng Yang a

a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinab School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, Chinac Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia

⁎ Corresponding author. Tel.: +86 10 82998218; fax: +E-mail address: fanhr@mail.iggcas.ac.cn (H.-R. Fan).

http://dx.doi.org/10.1016/j.oregeorev.2014.03.0180169-1368/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 December 2013Received in revised form 26 March 2014Accepted 26 March 2014Available online 13 April 2014

Keywords:Fluid inclusionWater–rock interactionPhase separationDongfeng gold depositLinglong gold depositNorthwest JiaodongEastern China

The Dongfeng and Linglong gold deposits are located in the northwest Jiaodong Peninsula, North China Craton.The deposits are mainly hosted in the Mesozoic granitoids and structurally controlled by the Zhaoyuan–Pingdufault zone. Gold mineralization at Dongfeng occurs as disseminated ores and sulfide stockworks, typicallyenveloped by broad alteration selvages. In contrast, mineralization at Linglong is characterized by massiveauriferous quartz veins with narrow alteration halos. Three stages of mineralization were identified in bothdeposits, with the early stage represented by quartz ± pyrite, the middle stage by gold + quartz + pyrite orgold + quartz + base metal sulfides, and the late stage by quartz + carbonate ± pyrite, respectively. Fourtypes of fluid inclusions were distinguished based on petrography, microthermometry, and laser Ramanspectroscopy, including (1) pure CO2 fluid inclusions (type I), (2) H2O–CO2–NaCl fluid inclusions (type II),(3) H2O–NaCl fluid inclusions (type III), and (4) daughtermineral-bearing ormultiphase fluid inclusions (type IV).In the Dongfeng gold deposit, the early- andmiddle-stage quartz mainly contains primary type II fluid inclusionsthat completely homogenized at temperatures of 276–341 °C with salinities of 2.8–11.7 wt.% NaCl equivalent,and temperatures of 248–310 °C with salinities of 3.3–10.8 wt.% NaCl equivalent, respectively. A few primarytype I fluid inclusions could be observed in the early-stage quartz. In contrast, the late-stage quartz containsonly the type III fluid inclusions with homogenization temperatures of 117–219 °C, and salinities of0.5–8.5 wt.% NaCl equivalent. The estimated pressures for the middle-stage fluids are 226–338 MPa, suggestingthat goldmineralizationmainly occurred at paleodepths of deeper than 8.4–12.5 km. Themineralization resultedfrom 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 quartzmainly contains primary type IIfluid inclusions and a few typeI fluid inclusions, of which type II fluid inclusions have salinities of 3.3–7.5 wt.% NaCl equivalent and homogeni-zation 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 of5.5–10.3 wt.% NaCl equivalent, while the type III fluid inclusions have homogenization temperatures of244–291 °C and salinities of 4.1–13.3 wt.% NaCl equivalent. Fluid inclusions in the late-stage quartz are type IIIfluid inclusions with low salinities of 0.3–8.2 wt.% NaCl equivalent and low homogenization temperatures of103–215 °C. The trapping pressure estimated for the middle-stage fluids is 228–326 MPa, suggesting that thegoldmineralization 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 temperaturefluctuations 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‰ and5.9 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‰ andδD = −77.4 to −63.7‰ for the Linglong deposit. Stable isotope data show that the ore-forming fluids of thetwo gold deposits are of magmatic origin, with gradual incorporation of shallower meteoric water during/aftermineralization.

© 2014 Elsevier B.V. All rights reserved.

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1. Introduction

China is the largest gold-producer in theworld. Its gold production isincreasing rapidly and had reached to 428.163 metric tons in 2013

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(China Gold Association, http://www.cngold.org.cn/newsinfo.aspx?ID=996). The Jiaodong gold province located in the Jiaodong Peninsulaof eastern China (Fig. 1) is the most important gold-producing districtand is the host for several world-class gold deposits (N100 t gold) inthe country (Fan et al., 2003; Hu et al., 2013; Qiu et al., 2002; Zhouand Lü, 2000). The region occupies less than 0.2% of China's land area,but yields about a quarter of the country's gold production. Golddeposits in the Peninsula are mainly distributed along three gold beltsfrom west to east, i.e., the Zhaoyuan–Laizhou, Penglai–Qixia andMuping–Rushan belts (Fan et al., 2003; Hu et al., 2006). These golddeposits are controlled by NE- or NNE-trending faults and hosted inthe Precambrian high-grade metamorphic basement rocks as well asthe Mesozoic granitoids. They were divided into two types accordingto ore occurrence, referred to as “Linglong-type” and “Jiaojia-type”(Goldfarb and Santosh, 2014; Qiu et al., 1988). The Linglong-type lodegold mineralization is characterized by massive auriferous quartzveins with narrow alteration halos and usually occurs in subsidiarysecond- or third-order faults. The Jiaojia-type disseminated andstockwork gold mineralization is usually surrounded by broad alter-ation zones and generally develops along major first-order regionalfaults. The lode gold deposits usually have smaller reserves and highergrades, whereas the disseminated and stockwork gold deposits havelarger reserves and lower grades.

The Zhaoyuan–Laizhou gold belt, located in the northwest Jiaodong,shows the highest concentration of gold deposits, with over 80% of theJiaodong gold concentrated within an area of ~3500 km2 (Zhou andLü, 2000). The Linglong gold field in this belt is the typical example of“Linglong-type” lode gold mineralization, which together with someother deposits such as the Taishang gold deposit, account for an overallgold reserve of more than 1000 metric tons. Recently, a giant and new“Jiaojia-type” gold mineralization (Dongfeng gold deposit) was discov-ered. It has been confirmed that the gold reserve at Dongfeng is158.475 metric tons, with average grade of 2.75 × 10−6 (Shandong

Fig. 1. Simplified geological map of the Jiaodong Peninsula showing location of the major goindicates the gold reserves: large symbol means Au N 50 t, small symbol means Au b 50 t. The

Gold Group Co., Ltd, unpublished data). The annual gold production inthe Dongfeng and Linglong deposits has climbed to ~3.6 metric tons in2013 (Shandong Gold Group Co., Ltd, personal communication).

The Jiaodong gold province hosts dozens of gold deposits. Geneticdifferences between the Linglong-type deposits and the Jiaojia-typedeposits have not been investigated, though most of them have beenextensively described. This paper attempts to evaluate the contrast be-tween the Dongfeng and Linglong gold mineralization in the Linglonggold field from field observations, ore geology, fluid inclusion and stableisotope analysis in order to reveal the nature and evolution of the ore-forming fluid system, and to probe the ore genesis in both types ofdeposits.

2. Regional geology

The Jiaodong gold province is located along the southeasternmarginof theNorth China Craton (NCC) and at thewesternmargin of the PacificPlate. It is bounded by the NE- to NNE-trending Tan–Lu fault zone to thewest and by the Su–Lu ultrahigh pressure metamorphic belt to thesouth (Fig. 1). Exposed rocks in the area comprise metamorphosedPrecambrian basement sequences and a series of Mesozoic intrusiveand volcanic rocks (Zhou and Lü, 2000). The Precambrian sequences in-clude the Archean Jiaodong Group and the Proterozoic Jingshan andFenzishan Groups (Guo et al., 2005; Yang et al., 2012). These groupsconsist of mafic to felsic volcanic and sedimentary rocks metamor-phosed to amphibolite and granulite facies. The Mesozoic volcanicrocks, namely Qingshan Formation, are mainly distributed in the JiaolaiBasin, andwere formed at 108–110Ma (Qiu et al., 2001a). TheQingshanFormation comprises two units, with the lower assemblage composedof trachybasalt, latite, and trachyte, overlain by an upper assemblagedominated by rhyolite flows and pyroclastic rocks (Li et al., 2006; Qiuet al., 2001a).

ld deposits (modified after Fan et al., 2003). The size of the symbols of the gold depositsDongfeng and Linglong deposits occur at the northwestern part of the gold province.

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Mesozoic granitoid rocks are widespread in the Jiaodong gold prov-ince. These rocks can be subdivided into three major groups accordingto their formation ages: late Triassic granitoids, late Jurassic granitoids,and early Cretaceous granitoids. The late Triassic granitoids such asthe Jiazishan, Chashan and Xingjia plutons mainly intruded in thesoutheastern fringe of Jiaodong from 225 to 205 Ma (zircon U–Pbmethod; Chen et al., 2003; Guo et al., 2005; Yang et al., 2005). These gra-nitic rocks, which show typical mantle-derived features (Gao et al.,2004; Guo et al., 2005), were generated following the collision betweenthe NCC and the Yangtze Craton during the middle-late Triassic in apost-collisional setting (Tan et al., 2012; Xu et al., 2006; Yang et al.,2007). These intrusions are dominated by quartz syenite, pyroxene sy-enite and alkaline gabbro (Tan et al., 2012). The late Jurassic granitoids,dated from 160 to 150 Ma by single zircon U–Pb method (Guo et al.,2005; Miao et al., 1997; Wang et al., 1998; Yang et al., 2012), are repre-sented by the Linglong and Luanjiahe suites in the western Jiaodong,and the Kunyushan, Queshan, Wendeng, and Duogushan suites in theeastern Jiaodong. They consist of medium-grained metaluminous toslightly peraluminous biotite granite, granodiorite and monzonite(Tan et al., 2012), and were likely derived from the partial melting of athickened Archean lower crust (Yang et al., 2012). The early Cretaceousgranitoids, emplaced from 130 to 105 Ma (zircon U–Pb method; Miaoet al., 1997; Guo et al., 2005; Zhang and Zhang, 2007; Goss et al.,2010; Yang et al., 2012), include Guojialing, Aishan, Nantianmen, andBeifengding suites in the western Jiaodong, and Sanfoshan, Weideshan,Haiyang, Yuangezhuang, Yashan, and Laoshan suites in the easternJiaodong. They consist of granodiorite, porphyritic granite, andmonzonitic granite, and show a mixed source of crustal and mantlecomponents (Guo et al., 2013; Liu et al., 1997; Song and Yan, 2000;Yang et al., 2012, 2013; Zhang et al., 2006).

Mafic to felsic dikes are commonwithin the gold districts. They con-sist of dolerite, lamprophyre, diorite (porphyry), granodiorite, granite(porphyry) and syenite, thus spanning the range from medium/high-Kcalc-alkaline to shoshonitic rocks (Cai et al., 2013; Guo et al., 2004;Tan et al., 2007, 2012; Yang et al., 2004). These rocks were mostlyemplaced at ca. 122–114 Ma and a few at 110–102 Ma (Qiu et al.,2001b; Tan et al., 2008; Yang and Zhou, 2001; Zhang et al., 2002; Zhuand Zhang, 1998). The former has been correlated to magma genesisduring Cretaceous lithospheric thinning and asthenospheric upwelling(Cai et al., 2013; Tan et al., 2008).

Two main stages of deformation have been identified in Jiaodongduring the late Mesozoic. The first stage involved northwest–southeastoblique compression, presumably related to the subduction of theIzanagi–Pacific plate, which produced prominent NNE- to NE-trendingbrittle–ductile shear zones with sinistral oblique reverse movements.This was followed by reactivation with development of brittlestructures and half-graben basins. These structures were accompaniedby hydrothermal alteration and gold mineralization (Fan et al., 2003;Hu et al., 1998; Li et al., 2013; Wang et al., 1998; Zhai et al., 2002).

Northwest Jiaodong, where the Zhaoyuan–Laizhou gold belt is locat-ed, is a key part of the Jiaodong gold province (Fig. 1). Exposed rocks inthis area includemetamorphosed Precambrian sequences andMesozoicintrusions (Wang et al., 1998; Zhou and Lü, 2000). The Precambrian se-quence is mainly composed of basement rocks of the Late ArcheanJiaodong Group with an age range of 2707–2726 Ma (Jahn et al.,2008). Plutonic rocks have been traditionally divided into two suites,the Linglong suite and the Guojialing suite. The former suite consistsof medium-grained metaluminous to slightly peraluminous biotitegranites, and the later one is composed of porphyritic hornblende–biotite granodiorites (Deng et al., 2011; Fan et al., 2003; Yang andZhou, 2001). The emplacement ages of the two suites of granitoids are160–156 Ma and 130–126 Ma, respectively (Miao et al., 1997; Qiuet al., 2002; Wang et al., 1998; Yang et al., 2012, 2014).

Gold deposits of the Northwest Jiaodong are mainly hosted in theMesozoic granitoids or along the contacts between the granitoids andmetamorphic rocks. They are usually controlled by the NE- and NNE-

trending faults, which cut through the Mesozoic granitoids. There arethree ore-controlling fault zones, including the Sanshandao–Cangshang,Jiaojia–Xincheng, and Zhaoyuan–Pingdu fault zones from west to east.More than half of the gold reserves in this area are controlled by theZhaoyuan–Pingdu fault zone (Deng et al., 2011). The Linglong-typelode gold mineralization and the Jiaojia-type disseminated andstockwork gold mineralization show coexistence in the NorthwestJiaodong. Sericite/muscovite 40Ar/39Ar and single grain pyrite Rb–Srdating have been carried out to determine the ages of the gold depositsin this area, which are between 123 and 114 Ma (Li et al., 2003, 2008;Yang and Zhou, 2001; Zhang et al., 2003).

The Linglong gold field is situated to the east of the Zhaoyuan–Laizhou gold belt and in the northern tip of the Zhaoyuan–Pingdufault zone. Mineralization in this field is dominated by auriferous quartzveins, with minor amount of disseminated sulfide replacements, and/orstockworks. The gold field is bordered to the southeast by the Potouqingfault, the northern segment of the Zhaoyuan–Pingdu fault (Fig. 2),which is themajor first-order structure controlling the Jiaojia-typemin-eralization in this gold field (Qiu et al., 2002). The Potouqing fault trendsN60–70°E and dips S30–40°E. Granitic cataclasite, tectonic breccias anda small amount of mylonite occur around the fault. In addition, hydro-thermal alteration and mineralization along the fault are well devel-oped. The Linglong fault, cutting through the central part of theLinglong goldfield, underwentmultistage complex tectonicmovementssince its formation. The fault trends N25–30°E, and dips N65–85°W andSE. Granitic cataclasite, hydrothermal alteration and veryweakmineral-ization occur along the fault. Second-order faults in the gold field aregenerally 100–5800m in strike length and 1–10m inwidth. These faultsconsistently trendNNE toNEE, dip 50–75°NWand SE, and are themajorstructures controlling the occurrence of felsic to mafic dikes, andauriferous Linglong-type quartz veins (Qiu et al., 2002). Granitoids arewell developed in the field, including Linglong gneissic biotite graniteand Luanjiahe medium-coarse grained granite with local exposures ofthe Guojialing granodiorite. The gold deposits are usually hostedwithinthe Linglong granite, but deep within the Jiuqu area, the ores occur inboth the Linglong granite and the Guojialing porphyritic granodiorite(Chen et al., 1993, 2004; Li et al., 2004; Lu et al., 1999; Mao et al.,2005; Yang et al., 2013). Intermediate and mafic dykes, consisting ofdiorite, dioritic porphyrite, and lamprophyre, are widely developedwithin the gold field. They are spatially associated with the goldmineralization.

3. Ore geology

3.1. Geology of the Dongfeng gold deposit

The Dongfeng gold deposit is situated in the northwestern segmentof the Jiaodong gold province, about 20 km north of Zhaoyuan City, andforms part of the Linglong gold field (Figs. 2 and 3). Seven gold ore bod-ies have been identified in the Dongfeng gold deposit, which are jointlycontrolled by the NNE-trending Potouqing fault and the NE-trendingZhaoyuan–Pingdu fault. Ore bodies occur in the footwall of the mainfault plane. 1711 and 171sub-1 are the major ore bodies. As the largestore body, 1711 is layer-like and occurs at depths of 120 to 1700 m. Theore body generally strikes NE 60° and dips SE 36.5° to 43.5°, with alength of 2500 m and thicknesses of 0.27 to 26.06 m. Au grades in the1711 ore body range from 1.00 to 26.34 g/t, with an average value of2.71 g/t. 171sub-1 ore body is located under 1711 ore body. Their occur-rences are similar. 171sub-1 ore body is also layer-like, with depths of370 to 1270 m and thicknesses of 0.50 to 18.46 m. Its Au grades rangefrom1.00 to 17.35 g/t, with an average value of 2.97 g/t. Gold ore bodies,hosted in the late Jurassic granites, occur in large and thick fracturealteration zones. Phyllic granite and phyllic cataclasite constitute theroof and floor of the ore bodies. Outside the alteration zone, the hangingwall is composed of the Luanjiahe medium-coarse grained monzonite

Fig. 2. Simplified regional geological map of the Linglong gold field.

646 B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

granite, whereas the Linglong gneissic medium-fine grained biotitegranite constitutes the footwall.

Mineralization appears associated with pyrite–sericite–silica alteredrocks or fine pyrite veins (Figs. 4C and 5B). The major ore minerals in-clude native gold, electrum, and pyrite, whereas the subordinates arechalcopyrite, galena, and sphalerite. The main gangue minerals consistof quartz, plagioclase and sericite, with minor amounts of chlorite andcalcite (Fig. 6). Native gold grains occur mainly in fissures of pyriteand quartz or as inclusions in pyrite crystals and gangue minerals(Fig. 5F).

3.2. Geology of the Linglong gold deposit

The Linglong gold deposit is located in thewestern part of the north-ern tip of the Zhaoyuan–Pingdu fault zone, west of the Dongfeng gold

Fig. 3. Geological profile crossing the ore bodie

deposit (Figs. 2 and 3). The ore bodies aremainly hosted by the Linglonggranite, but in the Jiuqu area, these are hosted in both the Linglong gran-ite and theGuojialing porphyritic granodiorites (Chen et al., 1993, 2004;Li et al., 2004; Lu et al., 1999; Yang et al., 2013, 2014; Zhang, 2002). In-termediate and mafic dykes are widely developed within the gold de-posit. The mineralization occurs typically in the form of auriferousquartz veins with lesser disseminated sulfide replacements and/orstockworks. The Linglong gold deposit is structurally controlled byboth the NEE- to NNE-trending Potouqing fault zone and the NNE-trending Linglong fault zone. The two major fault zones and theirbranches control hundreds of auriferous quartz veins, which display ageneral NNE–NE trend, with local abrupt changes in the attitude of thefractures (Yang et al., 2014).

The Linglong gold deposit occurs in the Dongshan and Xishan min-ing areas. More than two hundreds veins are exposed at the surface,

s of the Dongfeng and Linglong deposits.

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 withK-feldspathization. (C) Disseminated and stockworkmineralization, i.e., pyrite + sericite + silica alteration rocks containing themiddle-stage quartz (Q2). (D) Lodemineralization, con-taining 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-stagequartz-carbonate vein (Q3).

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among which approximately 30 veins are economically viable, such asVeins 9, 10, 36, 47, 51, 55, 56, 58, 98, 108, and 175. Overall, these veinsstrike NE 35° to 70° and dip NW. However, near the Potouqing fault,the veins have SE dips at shallow levels and NW at depth. The majorveins run for a few thousands of meters, with the longest reachingmore than 5500 m. These veins vary in width from a few meters totens of meters.

Auriferous quartz veins (Figs. 4D, E and 5A) usually have variablegrades from a few grams to a dozen or so per ton, with the highestreaching up to hundreds of grams per ton. Native gold, electrum and py-rite are the major ore minerals with minor of chalcopyrite, galena, andsphalerite (Fig. 5C and D). Minor magnetite, hematite, pyrrhotite, andarsenopyrite are found locally. The main gangue minerals comprisequartz, sericite, feldspar, calcite, and chlorite (Figs. 5H and 6). Nativegold grains occurmainly in fissures of pyrite and quartz or as inclusionsin pyrite crystals and gangue minerals, similar to that in the Dongfenggold deposit (Fig. 5E and G).

3.3. Hydrothermal alteration and mineralizing stages

Hydrothermal alteration is widespread, including potassic, sericitic,pyritic, silicic alterations as well as chloritization and carbonatization(Fig. 4A–F). These alterations are characterized by distinct zonings

from ore bodies to wallrocks in the two gold deposits as: pyrite +sericite + silica → silica + sericite → K-feldspar → fresh granite.Although the two gold deposits have similar alteration zonings, intensi-ty of the hydrothermal alteration at the Linglong deposit is muchweak-er than that at the Dongfeng deposit. Locally in the Linglong deposit, thewidth of the alteration zone is less than 1 m or the alteration zone iseven lacking in some cases. In contrast, alteration zones are well devel-oped at the Dongfeng deposit with widths varying from several metersto tens of meters, sometimes up to a few hundreds of meters.

The ore-forming stages are also similar between the Dongfeng andLinglong gold deposits. Based on mineral paragenesis and crosscuttingrelationships, four hydrothermal stages can be distinguished. Stage 1is characterized by the assemblage of quartz ± pyrite (Fig. 4B). It is de-fined bymilkywhite quartz veins or pyrite-quartz veins containing fewcoarse euhedral and subhedral pyrite. K-feldspathization, silicificationand sericitization is often developed. In this stage, gold is scarcelyprecipitated. Stage 2 is characterized by the assemblage of gold +quartz + pyrite (Fig. 4D). Generally, this stage is displayed by white-gray quartz vein networks containing abundant pyrite, with minorchalcopyrite, galena, sphalerite, in the lode gold mineralization. Corre-spondingly, disseminated sulfides in the pyrite + sericite + silica alter-ation rocks are the most important form of ores. In both cases,pyrite occurs as coarse euhedral cubes and subhedral aggregates. The

Fig. 5. Photomicrographs under reflected light showing importantmineral assemblages at Dongfeng and Linglong deposits. (A) Lode ore. (B) Disseminated and stockwork ore. (C) Isolatedelectrum, native gold and pyrite in quartz. (D) Coexistence of quartz, pyrite, galena, chalcopyrite, and sphalerite. (E) Fractures in early precipitatedpyrite that arefilledwith gold. (F)Nativegold 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.

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structurally-controlled deformation and brecciation suggest deutericmechanical stress. Stage 3 is characterized by the assemblage ofgold + quartz + base metal sulfide (Fig. 4E). In this stage, largeamounts of sulfideminerals precipitated, including pyrite, galena, sphal-erite, chalcopyrite andminor pyrrhotite. Quartz is usually dark-gray. Py-rite occurs asfine-grained subhedral and anhedral aggregates. The othersulfideminerals show fine-grained anhedral aggregates. Stage 4 is char-acterized by the assemblage of quartz + carbonate ±pyrite (Fig. 4F).White quartz and milky carbonate often occur together. The carbonatesconsist of calcite andminor ankerite. Pyrite occurs sporadically in minoramount. There is almost no gold mineralization in this stage. In

summary, stage 1 is the early stage ofmineralization, stages 2 and3 con-stitute the middle stage when the major enrichment of gold occurredand the practically barren stage 4 marks the late stage of mineralization(Fig. 6).

4. Fluid inclusions

4.1. Sample descriptions and analytical methods

Samples for fluid inclusion study were collected from the twodeposits. Twenty two (early, 9; middle, 8; late, 5) and twenty (early,

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

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6; middle, 10; late, 4) doubly polished thin sections (about0.20–0.30 mm thick) were prepared from quartz samples associatedwith different stages in the Dongfeng and Linglong deposits, respective-ly. Fluid inclusion petrography involved careful observation of theshapes, characteristics of spatial distribution, genetic and compositiontypes, and vapor/liquid ratios. Samples with abundant and representa-tive fluid inclusions were selected for microthermometric measure-ments and laser Raman spectroscopy analyses.

Microthermometric measurements on the fluid inclusions werecarried out using a Linkam THMS 600 programmable heating–freezingstage combined with a Zeiss microscope at the Institute of Geologyand Geophysics, Chinese Academy of Sciences (IGGCAS). The stagewas calibrated using synthetic fluid inclusions supplied by FLUID INCthrough 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(374.1 °C). Most measurements were carried out at a heating rate of0.2 to 0.4 °C/min. Carbonic phasemelting (Tm-CO2

) and clathratemelting(Tm-clath) were determined by temperature cycling (Diamond, 2001;Fan et al., 2003; Roedder, 1984). 0.1–0.2 °C/min, the heating rate formeasurements, was adopted near phase transformations. The precisionof measurements was ±0.2 °C at temperatures below 30 °C and ±2 °Cat temperatures above 30 °C.

Five types of temperature observations were made in this study in-cluding the melting temperature of CO2 (Tm-CO2

), final melting temper-atures of ice (Tm-ice), final melting temperatures of clathrate (Tm-clath),the homogenization temperatures of the CO2 (Th-CO2

) and the total ho-mogenization temperatures (Th-tot). Using Tm-ice and Tm-clath, salinitiesof the H2O–NaCl (Bodnar, 1993) and H2O–CO2–NaCl (Collins, 1979)fluid systems can be calculated. The density of the CO2 can be well re-stricted through Th-CO2

. Th-tot can reflect the temperature of differentstages of ore-forming fluid to some extent. Mole fractions of composi-tions, density of carbonic liquid and bulk fluid, and bulk molar volumeof fluid inclusions were calculated by the Flincor computer software(Brown and Lamb, 1989).

Laser Raman spectroscopic analysis of the fluid inclusions was car-ried out on the LabRam HR800 Raman microspectrometer (producedby French HORIBA Scientific) at the IGGCAS. An argon ion laser with a

wavelength of 532 nmand a source power of 44mWwas used in detec-tion. The spectral range falls between 100 and 4000 cm−1 for the anal-ysis of CO2, N2, CH4, and so on in the vapor phase.

4.2. Petrography and types of fluid inclusions

Four different compositional types of fluid inclusions are distin-guished: pure CO2 (type I), H2O–CO2–NaCl (type II), H2O–NaCl (typeIII), and daughter mineral-bearing or multiphase (type IV) fluid inclu-sions, based on the combination of petrography at room temperature,phase transitions observed during heating and cooling, and laserRaman spectroscopy (Fig. 7).

4.2.1. Type I inclusionsType I inclusions consist of almost pure carbonic fluid lacking any

visible H2O at room temperatures, including monophase CO2 (vaporor liquid), and two-phase CO2 (VCO2

+ LCO2) (Fig. 7A and B). They are

usually dark with oval to negative crystal morphologies. These inclu-sions, ranging from 6 to 13 μm in size, have been mostly foundcoexistingwith type II inclusions in the early-stage quartz of the twode-posits, and with other three types of inclusions in the middle-stagequartz of the Linglong deposit. The typically isolated and scatterednature of these inclusions indicates that they are primary inclusions.

4.2.2. Type II inclusionsType II inclusions are composed of H2O and CO2 phases with

20–70 vol.% carbonic phase (Fig. 7C, D and H). They can be furtherdivided into two subtypes, containing two-phase (VCO2

+ LH2O) andthree-phase (VCO2

+ LCO2+ LH2O) inclusions at room temperature with

varying sizes between 5 and 23 μm. As the predominant type of fluid in-clusions, they are abundant in quartz formed in the early- and middle-stages. They generally occur in isolation or in cluster. Sometimes theyappear as trails along healed fractures which do not cut across thecrystal boundaries of quartz. These features suggest that they are prima-ry or pseudosecondary.

Fig. 7. Photomicrographs of typical fluid inclusions in theDongfeng and Linglong deposits. (A) One phase type I fluid inclusion. (B) Two phases type I fluid inclusion. (C) Three phases typeII 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 inclu-sions 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 ofthe characteristics of fluid inclusions from the Linglong gold deposit.

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4.2.3. Type III inclusionsType III inclusions are one-phase (LH2O) or two-phase (VH2O + LH2O)

liquid-rich aqueous inclusions (Fig. 7E andG). The two-phase inclusionsare more common with vapor volume occupying 2–40% of the totalcavity volume. These inclusions, varying in size from 5 to 14 μm, havea variety of shapes ranging from irregular to elliptical and negativeshapes. They are commonly present in quartz of all stages, particularlyin the late-stage quartz crystals. In general, the primary type III inclu-sions occur as isolated singles or group in late-stage quartz. Secondarytype III inclusions cutting across the crystal boundaries of quartz canbe observed as arrays or trails along healed fractures in early- and

middle-stage quartz. It's noteworthy that primary type III inclusionsare also well developed in the middle-stage quartz of Linglong deposit.Trace content of CO2 can still be identified in the vapor bubbles by laserRaman spectroscopy (Fig. 9e), although no visible CO2 phase appearsduring heating or cooling runs.

4.2.4. Type IV inclusionsType IV inclusions are scarce and are usually composed of aque-

ous liquid, a vapor bubble, and a calcite crystal at room temperature(Figs. 7F, 9g and h). They are irregular or circular in shape with7–12 μm in size and are only observed in the middle-stage quartz

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crystals. They usually occur as isolated individuals coexisting withtype I, type II and type III inclusions (Fig. 7I).

4.3. Microthermometry and laser Raman spectroscopy

4.3.1. Dongfeng gold deposit

4.3.1.1. Early stage. Type II inclusions are dominant in the early-stagequartz, coupled with some type I inclusions. For type II inclusions, themelting temperatures of solid CO2 (Tm-CO2

) range from −56.9 °C to−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.The melting temperatures of clathrates (Tm-clath) were observed be-tween 3.1 °C and 8.4 °C, corresponding to salinities between 2.8 and11.7 wt.% NaCl equivalent (Fig. 8 and Table 1). The carbonic phase(Th-CO2

) was partially homogenized to liquid at temperatures rangingfrom 25.6 °C to 30.9 °C. Total homogenization (Th-tot) of the carbonicand aqueous phases (L + V to L, few L + V to V) was observed at tem-peratures ranging from 276 °C to 341 °C. However, some inclusionswith greater vapor/liquid ratios decrepitated between 310 °C and330 °C prior to final homogenization. The calculated CO2 densitiesrange from 0.25 to 0.63 g/cm3 with XCO2

from 0.02 to 0.18 and bulkdensities from 0.57 to 0.97 g/cm3. For type I inclusions, final meltingto liquid was observed during heating, with Tm-CO2

ranging from−57.0 °C to −56.6 °C. Partial homogenization (Th-CO2

) of CO2 (L + Vto L) occurs between 28.9 °C and 30.9 °C, corresponding to densitiesof 0.53 to 0.63 g/cm3.

4.3.1.2. Middle stage. Type II fluid inclusions are the most abundantinclusions in the middle-stage quartz. Tm-CO2

ranges from −56.9 °Cto −56.6 °C and is generally near the pure CO2 melting point(−56.6 °C), indicating that the dominant composition is CO2. Melting

Fig. 8. Histograms of homogenization temperatures (Th-tot)

of the CO2 clathrate (Tm-clath) in the presence of CO2 liquid occurs be-tween 3.8 °C and 8.3 °C, corresponding to the fluid salinities of 3.3 to10.8 wt.% NaCl equivalent. CO2 generally homogenized to the liquidphase and Th-CO2

range from 13.6 °C to 30.9 °C. The densities of theCO2 phase are calculated to be between 0.30 and 0.83 g/cm3 with XCO2

varying from 0.01 to 0.19. Densities of the bulk inclusions range from0.76 to 1.05 g/cm3. Most of the type II fluid inclusions homogenized inthe range of 248 °C to 310 °C (L + V to L, few L + V to V or the criticalstate) (Fig. 8 and Table 1), excepting some inclusions decrepitating attemperatures from 253 °C to 254 °C before total homogenization.

4.3.1.3. Late stage. Type III aqueous inclusions from the late-stage quartzyield final ice melting temperatures (Tm-ice) of−5.5 °C to−0.3 °C, cor-responding to salinities varying from 0.5 to 8.5 wt.% NaCl equivalent.The temperatures of homogenization to liquid phase are between117 °C and 219 °C (Fig. 8 and Table 1). Densities of the bulk inclusionsrange from 0.86 to 1.00 g/cm3.

4.3.1.4. Laser Raman spectroscopy. Laser Raman spectroscopy shows thatCO2 and H2O are the main volatiles in the measured fluid inclusionsfrom the early- and middle-stage quartz (Fig. 9a–c). No CH4, N2 orother gas phaseswere detected in thesefluid inclusions. This is in accor-dance with the microthermometric results that the melting tempera-tures of solid CO2 are near −56.6 °C, the triple point of pure CO2. Inthe late-stage quartz, fluid inclusions mainly consist of H2O, in theabsence of any other major volatile phase (Fig. 9f).

4.3.2. Linglong gold deposit

4.3.2.1. Early stage. Type II inclusions and a few type I inclusions exist inthe early-stage quartz. For type II inclusions, melting of the solid CO2

(Tm-CO2) occurred between−56.9 °C and−56.6 °C. These temperatures

. (A) Dongfeng gold deposit. (B) Linglong gold deposit.

Table 1Microthermometric 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.%NaCl equiv.

CO2 density(g/cm3)

Bulk density(g/cm3)

Dongfeng golddeposit

Early I 10 −57.0 to −56.6 28.9–30.9 0.53–0.63II 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 0.76–1.05Late III 22 −5.5 to −0.3 117–219 0.5–8.5 0.86–1.00

Linglong golddeposit

Early I 8 −57.2 to −56.6 26.6–30.7 0.55–0.69II 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 0.53–0.75II 17 −58.9 to −56.6 4.1–7.1 12.5–28.4 251–287 5.5–10.3 0.65–0.84 0.82–1.01III 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

Note: N, numbers of measured fluid inclusion; Tm-CO2, final melting temperature of solid CO2; Tm-clath, final 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.

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are equal to or slightly lower than themelting temperature of pure solidCO2 (−56.6 °C) and thus indicates that the gas phase consists predom-inantly of CO2. Melting of the CO2 clathrate (Tm-clath) was observedbetween 5.9 °C and 8.3 °C, corresponding to the salinities rangingfrom 3.3 to 7.5 wt.% NaCl equivalent. The homogenization of the CO2

(Th-CO2) into liquid occurred between 26.5 °C and 29.0 °C. Total homog-

enization (Th-tot), mostly into liquid phase, was recorded between271 °C and 374 °C (Fig. 8 and Table 1). Decrepitation temperatures ofsome inclusions with greater vapor/liquid ratios range from 250 °Cto 272 °C. CO2 densities from type II inclusions are from 0.63to 0.69 g/cm3 with XCO2

from 0.01 to 0.16 and bulk densities from 0.87to 1.03 g/cm3. For type I inclusions, Tm-CO2

ranges from −57.2 °Cto−56.6 °C. The homogenization of the CO2 (Th-CO2

) into liquidwas ob-served between 26.6 °C and 30.7 °C, corresponding to densities of 0.55to 0.69 g/cm3.

4.3.2.2. Middle stage. All the four types of fluid inclusions are developedin the middle-stage quartz. For type I inclusions, final melting to liquidwas observed during heating, with Tm-CO2

ranging from −57.2 °C to−56.6 °C. Partial homogenization (Th-CO2

) of CO2 (L+V to L) occurs be-tween 22.2 °C and 30.9 °C, corresponding to densities of 0.53 to0.75 g/cm3.

Tm-CO2of type II fluid inclusions ranges from −58.9 °C to −56.6 °C

and is generally near or below the pure CO2 melting point (−56.6 °C),indicating there are other gas components, such as CH4 and N2

(Roedder, 1984), in the gas phase. Melting of the CO2 clathrate (Tm-clath)in the presence of CO2 liquid occurs between 4.1 °C and 7.1 °C, corre-sponding to the fluid salinities of 5.5 to 10.3 wt.% NaCl equivalent. CO2

generally homogenized to the liquid phase and Th-CO2ranges from

12.5 °C to 28.4 °C. The densities of the CO2 phase are calculated to bebetween 0.65 and 0.84 g/cm3 with XCO2

varying from 0.05 to 0.43.Densities of the bulk inclusions range from 0.82 to 1.01 g/cm3. Most ofthe type II fluid inclusions homogenized in the range of 251 °C to287 °C (some L + V to L, others L + V to V or the critical state) (Fig. 8and Table 1) with the exception of some inclusions decrepitating attemperatures from 240 °C to 270 °C before total homogenization.

Type III aqueous inclusions from the middle-stage quartz yield finalice melting temperatures (Tm-ice) of−9.4 °C to−2.5 °C, correspondingto salinities varying from 4.1 to 13.3 wt.% NaCl equivalent. Densities ofthe bulk inclusions range from 0.82 to 0.90 g/cm3. The temperaturesof homogenization to the liquid phase are between 244 °C and 291 °C(Fig. 8 and Table 1).

Vapor bubbles of the type IV inclusions disappeared firstly duringheating,whereas the daughterminerals did not dissolve even if temper-ature was up to 500 °C.

4.3.2.3. Late stage. Type III aqueous inclusions from the late-stage quartzyield final icemelting temperatures (Tm-ice) of−5.3 °C to−0.2 °C, cor-responding to salinities varying from 0.3 to 8.2 wt.% NaCl equivalent.

The temperatures of homogenization to the liquid phase are between103 °C and 215 °C (Fig. 8 and Table 1). Densities of the bulk inclusionsrange from 0.88 to 0.98 g/cm3.

4.3.2.4. Laser Raman spectroscopy. The data from laser Raman spectros-copy show that CO2 and H2O are the main volatiles in the measuredfluid inclusions from the early- and middle-stage quartz (Fig. 9a–cand e). Minor quantity of CH4 was detected in some of the middle-stage type II fluid inclusions (Fig. 9d). This is in accordance with themicrothermometric results that melting temperatures of solid CO2 insome inclusions are below−56.6 °C. Daughter minerals are almost allcalcite in the type IV inclusions (Fig. 9g and h). In the late-stage quartz,fluid inclusions mainly consist of H2O (Fig. 9f). Other gas phase wasbarely found.

5. Stable isotopes studies

Quartz and pyrite grains were handpicked from the 40–60 meshcrushings under a binocular (purity N 99%). Analyses of hydrogen, oxy-gen and sulfur isotopic compositions were performed at the AnalyticalLaboratory of the Beijing Research Institute of Uranium Geology.

Hydrogen isotope analyses of the inclusionfluidswere performed onthe ten quartz vein samples, which are from different ore-formingstages of the two gold deposits.Water was released by heating the sam-ples to approximately 500 °C in an induction furnace. Sampleswere firstdegassed of labile volatiles by heating to 180–200 °C until the vacuum isless than 10−1 Pa. Water was converted to hydrogen by passage overheated zinc powder at 400 °C and the hydrogen was analyzed with aMAT-253 mass spectrometer. Analyses of standard water samplessuggest a precision for δD of ±2‰.

Oxygen isotope analyses were performed on ten quartz vein sam-ples, which are used for hydrogen isotope analyses. The pure mineralswere crushed into 200 mesh and the crushings reacted with BrF5 at500–600 °C for 14 h, generating O2 which subsequently reacted withgraphite to produce CO2 at 700 °C with platinum catalyst. The CO2 wasthen measured byMAT-253mass spectrometer for oxygen isotope. Re-producibility for isotopically homogeneous pure quartz is about±0.2‰.

Ten pyrite samples from ores of the two gold deposits were put touse for sulfur isotope analyses. The pyrite grains were mixed with cu-prous oxide and crushed into 200 mesh powder. SO2 was producedthrough the reaction of pyrite and cuprous oxide at 980 °C under a vac-uum pressure of 2 × 10−2 Pa. The SO2 was then measured by MAT-251mass spectrometer for sulfur isotope. All the analytical uncertaintieswere better than ±0.2‰.

The stable isotope data obtained in this and previous studies (Houet al., 2006) are shown in Table 2. δD of the inclusion fluids in quartzfrom the Dongfeng gold deposit vary from −90.5‰ to −82.7‰, withan average value of −86.6‰. δD of the inclusion fluids in quartz fromthe Linglong gold deposit vary from −77.4‰ to −63.7‰, with an

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 fluidinclusion. (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 fluidinclusion, containing water only. (g–h) Type IV fluid inclusions with calcite as a daughter mineral.

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average value of−69.1‰. Oxygen isotopic compositions of hydrother-mal waters in equilibriumwith quartz were calculated using an extrap-olation of the fractionation formula from Clayton et al. (1972). Thecalculations of the fractionation factors were made using the meanvalue of the homogenization temperatures of fluid inclusions from thesame ore-forming stage quartz samples. The calculated oxygen isotopecomposition of the fluid from the Dongfeng gold deposit is character-ized by δ18O of −3.8‰ to +6.4‰, with an average value of 0.0‰. Sim-ilarly, the fluid from the Linglong gold deposit is characterized by δ18Oof 0.0‰ to +8.9‰, with an average value of +4.9‰. In a plot of δD vs.δ18O, ten quartz samples plot are adjacent to the primary magmatic

water field (Fig. 10). The δ34SV-CDT values of pyrite range from +5.8‰to +7.0‰ and from +5.9‰ to +7.4‰ in the Dongfeng and Linglonggold deposits, respectively.

6. Discussion

6.1. Fluid evolution in the two gold deposits

Fluid inclusion studies and laser Raman spectroscopy suggest thatthe ore-forming fluid in the two gold deposits have similar chemicaland physical properties. The early-stage quartz contains the type I and

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

Name Sample Mineral Stage δ18Oqz Th (°C) δ18Ofluid δD δ34S Data sources

Dongfeng gold deposit PZ46812-1 Quartz Early 6.3 308 −0.8 −82.7 This paperPZ4966-1 Quartz Middle 6.4 282 −1.7 −85.7PZ41207-2 Quartz Middle 14.5 282 6.4 −90.5PZ46812-3 Quartz Late 11.6 157 −3.8 −87.5

Linglong gold deposit 10X74 Quartz Early 12.5 294 4.9 −67.1 This paper10X78 Quartz Early 11.8 294 4.2 −63.710LL04 Quartz Early 12.9 294 5.3 −70.110X79 Quartz Middle 14.4 274 6.0 −69.9LL-Q-06 Quartz Middle 17.3 274 8.9 −77.410X80 Quartz Late 14.7 165 0.0 −66.5

Dongfeng gold deposit 10LL18 Pyrite Middle 6.7 This paper10LL20 Pyrite Middle 6.510LL22 Pyrite Middle 6.810LL23 Pyrite Middle 5.810LL24 Pyrite Middle 7.0

Linglong gold deposit 10LL03 Pyrite Early 6.6 This paperJQ-Q-04 Pyrite Middle 6.5LL-Q-02 Pyrite Middle 6.2LL-Q-06 Pyrite Middle 5.910X80 Pyrite Late 7.4

Linglong gold deposit LL-108-1 Pyrite Middle 7.7 Hou et al. (2006)LL-108-2 Pyrite Middle 8.3LL-108-5 Pyrite Middle 7.6LL-108-6 Pyrite Middle 8.5LL-108-7 Pyrite Middle 7.5LL-53-4 Pyrite Middle 7.0LL-48-1 Pyrite Middle 8.6LL-48-4 Pyrite Middle 7.2LL-50-3 Pyrite Middle 6.4LL-50-5 Pyrite Middle 7.3LL-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 7.0LL-171-6 Pyrite Middle 8.3LL-171-7 Pyrite Middle 7.8LL-171-10 Pyrite Middle 7.9

Fig. 10. δD and δ18O characteristics of the ore-forming fluids at the Dongfeng and Linglonggold deposits. (A) Dongfeng gold deposit. (B) Linglong gold deposit.

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type II inclusions, whereas the late-stage quartz contains only the typeIII inclusions. In the early stage, ore-forming fluid belongs to H2O–CO2–NaCl system,which is characterized bymedium-high temperature,enrichment of CO2, and medium-low salinity (Fig. 11 and Table 1), incontrast to typical high temperature and high salinity magmatic fluids.These features, combined with the analytical results of hydrogen andoxygen isotopes (Fig. 10), indicate that the hyperthermal, volatiles-abundant and Au-rich primary ore-forming fluid probably mixed withmeteoric water infiltrating downward along fractures, when it movedupward through the fractures, altering the primary characteristics ofthe ore-forming fluid. The fluid evolved into H2O–CO2–NaCl systemwith medium-low temperature, less CO2, and variable salinity in themiddle stage. During the mineralization, more meteoric water was in-volved (Fig. 10). Finally, the ore-forming fluid, in the late stage, turnedinto 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 similardistribution in the δD-δ18O isotopic diagram (Fig. 10), suggesting similarsources of ore-formingfluids, for the data from the two deposits plot be-tween the magmatic field (or the metamorphic field) and the globalmeteoric water line (Fig. 10). Since the Mesozoic age of mineralizationis about 2 billion years younger than the timing of metamorphism inthe basement rocks, the ore-forming fluids could not have been derivedfrom themetamorphicfluids. Furthermore, the goldmineralization ages(123–114Ma) are younger than the ages of the regionalMesozoic gran-ites such as the Linglong (160–156 Ma) and Guojialing (130–126 Ma)granitoids, thus precluding the possibility of magmatic sources for theore-forming fluids. Recent studies suggest the role of mantle-derived

Fig. 11. Temperature vs. salinity plot of the fluid inclusions, showing fluid evolution at theDongfeng and Linglong deposits. (A) Dongfeng gold deposit. (B) Linglong gold deposit.

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fluids in the metallogenic process (Deng et al., 2003; Liu et al., 2002,2003; Mao et al., 2002). A number of mafic dikes, whose formationages are close to the gold mineralization ages, are widely distributedaround the Jiaodong gold province. Some researchers considered thatthe ore-forming fluids had a magmatic source, derived throughdegassing of mantle-derived magmas in the shallow part of crust (Fanet al., 2003, 2005).

H2S is an important medium for the migration and precipitation ofAu. The S of H2S is bound within sulfide, especially pyrite, which oftenaccompanies native gold. Therefore, the source of Au can be traced

Fig. 12. A comparison of sulfur isotopic compositions ofAfter Hou et al. (2006).

from the sulfur isotopic compositions of the sulfide minerals. The twogold deposits have consistent δ34S values (Table 2), which were obtain-ed from pyrite in equilibrium with the mineralization of the two de-posits. Hou et al. (2006) analyzed 16 pyrite samples from the Linglonggold field, and reported δ34S values varying from 6.4‰ to 8.6‰ withan average value of 7.6‰ (Table 2), which are in accordance with thisstudy. At the same time, Hou et al. (2006) carried out reported contrast-ing of δ34S values among metamorphic rocks of the Achaean JiaodongGroup, Mesozoic basic-intermediate dikes, Mesozoic granites, andgold ores (Fig. 12). The results show that the δ34S values of these geo-logic units are comparable, especially, the δ34S values of Mesozoicmantle-derived basic-intermediate dikes deviate from the mantlevalues (δ34S = ~0‰). Mao et al. (2008) suggested that the similar sul-fur isotopic compositions of the Mesozoic rocks implied homogeniza-tion of the sulfur isotopic system through crust–mantle interaction. Inother words, during the Mesozoic mineralization events, the ore-forming fluids were sourced from a common fluid reservoir probablylinked to processes of crust–mantle interaction.

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

HS− and Cl−, which can form stable complexes with gold ions, arethe most important ligands in hydrothermal solutions. Initially gold isdissolved 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 andSeward, 2004; Williams-Jones et al., 2009; Zotov et al., 1991). Takinginto consideration that gold is usually accompanied with sulfide inthese deposits, especially pyrite, we infer that gold bisulfide was themost possible species transporting gold. The abundance of type I andtype II inclusions in the early stage of the two gold deposits suggeststhat the initial ore-forming fluids were enriched in CO2. CO2 can bufferthe pH of the solution (Phillips and Evans, 2004), which provides favor-able conditions for gold bisulfide migration.

Although there are similarities in fluid sources between Dongfengand Linglong gold deposits, the ore forming mechanisms appear tobe different. Our petrographic studies, Raman spectroscopy andmicrothermometry on fluid inclusions show that the type II inclusionsand two phase type III inclusions not only coexist in the middle-stagequartz of the Linglong gold deposit (Fig. 7L andM), but also have consis-tent homogenization temperatures. In addition, the type III inclusions inthe middle-stage quartz usually contain trace amounts of CO2 (Fig. 9E),although no CO2 phase was observed during the heating–cooling runs.Furthermore, four different types of fluid inclusions coexist in somedomains (Fig. 7I). Type II and type III inclusions have similar range of

sulfide ores and rocks from the Jiaodong gold fields.

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homogenization temperatures; homogenization temperatures of typeIV inclusions were not observed. These features imply that phase sepa-ration or boilingmight have occurred in themiddle stage at the Linglonggold deposit. Accompanied with a significant drop in temperature andpressure of the fluid, the boiling resulted in CO2 escape and consump-tion of H+ through the reaction: H+ + HCO3

− = H2O + CO2.This process might have led to the decomposition of gold bisulfide[Au(HS)0, Au(HS)2−)], as the activity of H+ is the key factor to maintainHS− and gold bisulfide [Au(HS)0, Au(HS)2−)] stable in the fluids (Chenet al., 2007). Phase separation can release H2S from the liquid into thevapor phase, which also decreases the stability of Au–S complexes(Cox et al., 1995; Jia et al., 2000; Naden and Shepherd, 1989; Zhanget al., 2012). Subsequently, Au precipitated from the ore-forming fluid.In contrast, type II inclusions appear alone in the middle stage of theDongfeng gold deposit (Fig. 7J and K). This suggests that large-scalephase separation or boiling did not occur in the main mineralizationperiod. Hydrothermal alteration is widely developed in the Dongfenggold deposit, muchmore intense than that at the Linglong gold deposit.This may imply that intense water–rock interaction occurred betweenore-forming fluid and wall rocks at the Dongfeng gold deposit. Thisprocess dramatically changed the physical and chemical conditions ofore-forming fluid in themainmineralization period, and finally resultedin the precipitation and mineralization of gold. In the process of water–rock interaction, H2Swas expended to generate pyritewith iron derivedfrom the wall rocks, and pH of the fluid ascended by release of CO2.These factorsmade gold bisulfide unstable and to eventually precipitatemetallic gold.

6.4. Pressure and depth of gold deposition

Using the Flincor computer software with the equations of Brownand Lamb (1989) for the H2O–CO2–NaCl system, pressures in therange of 226–338 MPa were obtained in the middle stage of theDongfeng gold deposit, and 228–326MPa for the Linglong gold deposit.If these pressures are lithostatic, given 2.7 g/cm3 as the density of uppercrust rocks, the corresponding depth of mineralization is in the range of8.4–12.5 kmand 8.4–12.1 km, respectively. The trapping pressure in theDongfeng gold deposit should be higher than that we estimated,because the trapping temperature is above the homogenization tem-perature. In contrast, the trapping pressure in the Linglong gold depositshould be equal to the estimated values, because of fluid boiling. There-fore, the depth range computed for the Dongfeng gold deposit is only aminimum estimate, and the actual depths must be higher, and themin-eralization occurred at a deeper domain than that of the Linglong golddeposit. This result is in accordance with the fact that the ore bodiesexploited for gold in the Dongfeng gold deposit are at greater depthsthan those in the Linglong gold deposit.

6.5. Deposit genesis

The gold deposits in the Jiaodong province are mostly hosted in theMesozoic granitoids and are structurally controlled by faults and shearzones that cut the Mesozoic granitoids. Previous studies have shownthat the ages of the gold deposits in the Jiaodong gold provincecluster between 123 and 114 Ma as determined by sericite/muscovite40Ar/39Ar and single grain pyrite Rb–Sr dating (Hu et al., 2013; Li et al.,2003, 2006, 2008; Yang and Zhou, 2001; Zhang et al., 2003). The forma-tion ages of the Mesozoic granitoids are 160–156 Ma and 130–126 Ma,respectively, as obtained by zircon U–Pb dating (Guo et al., 2005; Miaoet al., 1997; Qiu et al., 2002; Wang et al., 1998; Yang et al., 2012,2013). The timing of gold mineralization is significantly younger thanthe ages of the regional granitoid magmatism as the Linglong andGuojialing granitoids, indicating that the gold mineralization has nodirect relationship to the granitoid magmatism. Instead, most golddeposits show temporal and spatial association with abundant maficto intermediate dikes that arewidespread in the Jiaodonggold province,

and which have been dated at ca. 122 to 119 Ma and, less commonly, at110 to 102 Ma (Cai et al., 2013; Qiu et al., 2001b; Yang and Zhou, 2001;Zhang et al., 2002; Zhu and Zhang, 1998).

Yang and Zhou (2001) and Li et al. (2008) dated pyrite from theLinglong gold field and reported ages in the range of 122–123 Ma and120.6 ± 0.9 Ma, respectively, consistent with the ages from other golddeposits in the Jiaodong gold province. The Dongfeng and Linglonggold deposits have similar characteristics inmineralogy, lithology, alter-ation patterns, and ore-forming fluids, suggesting that these two golddeposits were formed during the same metallogenic event at about120 Ma. This metallogenic event is widely developed in the Jiaodonggold province. During this period, post-collisional extension occurredin the North China and Yangtze Cratons with transfer of the principalstress-field from north–south to east–west directions, and east–westlithospheric extension caused by subduction of the Paleo-Pacific plate(Fan et al., 2005; Mao et al., 2003a, 2003b, 2004, 2006, 2008). Simulta-neously, lithospheric thinning, whichwas caused by the removal of lith-ospheric mantle and the upwelling of new asthenospheric mantle,induced partial melting and dehydration of the lithospheric mantleand lower crust due to an increase of temperature (Yang et al., 2003).The mantle-derived magma migrated upward to the shallow crust, itmight have degassed considerable volume of fluids. These fluidsmixed with meteoric water to form the ore-forming fluids.

The first-order faults in this area underwent multi-stage reactiva-tion, and the stress types were diverse at different periods. Repeatedstress and tectonic movements caused the rocks around faults tobecome highly cataclastic. Large and small fissures and cavities weredeveloped within the cataclastic rocks, which provided the pathwaysfor ore-forming fluids against the wall rocks, creating favorable condi-tions for fluid permeation and hydrothermal alteration. The first-orderfaults are the main migration pathway of the ore-forming fluids. Hightemperature and strong water–rock interaction occurred along thefirst-order faults, resulting in the formation of “Jiaojia-type” disseminat-ed and stockwork gold mineralization. In contrast, the secondary faultswere less activated and the rocks around these show less degree offracture. They were thus unfavorable water–rock interaction but servedas open conduits for the migration of ore fluids. During the process ofore fluid migration from the first-order faults to the secondary faults,the temperature gradually decreased. Opening of the faults led to suddendecompression and fluid phase separation (boiling). A sharp fall in tem-perature and large-scale exsolution of volatiles also occurred at the sametime. This process brought about the precipitation of the “Linglong-type”lode gold mineralization.

It is worth raising that the above mentioned ore-forming processmight be closely related to seismic activity. The role of seismic pressurefluctuations has been advocated for orogenic mineral systems in theSouth Island of New Zealand (Craw et al., 2013). The movement of orefluids is partly controlled by permeability, and enhanced permeabilityis considered to be proximal to earthquakes, where shear failure is likelyto occur, thereby explaining the common fault- to shear zone-controllingore deposits. Fault jogs and flower structures are particularlyefficient in the channeling of fluids during aftershocks, becausethose structures promote vertical flow, and can tap metal-rich reser-voirs (Craw et al., 2013). A fault-valve will drive fluid-pressure, andenhance permeability and fault ruptures can connect shallow low-pressure reservoirs with deeper high-pressure reservoirs, resultingin strong degassing. Importantly, the presence of breccias and vein-ing imply episodic phases of fluid flow and quite likely with fluidpressures changing from lithostatic to hydrostatic (Sibson, 2001;Sibson et al., 1975). Aftershocks, seismic slips and suction-pumpmechanisms are induced by rapid transfer of fluids in dilationalfault jogs and bends, resulting in the abrupt reduction of fluid pres-sure at structural sites, triggering phase separation and ore precipi-tation throughout the aftershock phases (Sibson, 2001; Sibsonet al., 1975). These phenomena lead to multiple phases of ore miner-al precipitation, accompanied by related alteration minerals.

657B.-J. Wen et al. / Ore Geology Reviews 65 (2015) 643–658

7. Conclusions

(1) In the Dongfeng and Linglong gold deposits, four types of fluidinclusions were observed, including: pure CO2 (type I), H2O–CO2–NaCl(type II), H2O–NaCl (type III), and daughter mineral-bearing or multi-phase (type IV) fluid inclusions. In the early stage of the two deposits,type II fluid inclusions are well developed and accompanied withsome type I fluid inclusions. In the middle stage, quartz from Dongfengcontains only type II fluid inclusions, whereas both of type II and type IIIfluid inclusions occur in quartz from Linglong,with a few type I and typeIVfluid inclusions. In the late-stage quartz of the two deposits, only typeIII fluid inclusions are present.

(2) Deep-seated magmatic water may be the source of the ore-forming fluids. The fluid gradually blended with shallower meteoricwater during mineralization. The ore-forming fluids initially had thecharacteristics of medium-high temperature, were enriched in CO2,and had medium-low salinity. They finally evolved into low tempera-ture, CO2-free, low salinity, and meteoric water-dominated fluids inthe late stage.

(3) The mineralization of the Dongfeng gold deposit resulted fromintense water–rock interaction between the H2O–CO2–NaCl fluids andwallrocks in the first-order fault, whereas precipitation of gold is possi-bly a consequence of phase separation or boiling of the H2O–CO2–NaClfluids in response to pressure and temperature fluctuations in theopen space of the secondary faults within the Linglong gold deposit.

(4) Large-scale gold mineralization in the Jiaodong region occurredduring tectonic regime inversion and lithosphere thinning.

Acknowledgments

Sincere thanks are due to the managements and staffs of theLinglong Gold Mine and the Dongfeng Gold Mine for their help duringfieldworks. Two anonymous referees are thanked for their constructiveand valuable comments which greatly contributed to the improvementof the manuscript. This study was financially supported by the NaturalScience Foundation of China (41173056), geological surveying projectof China Geological Survey (12120114032301), and project of theState Key Laboratory of Lithospheric Evolution (1303).

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