Fluid inclusion and isotope geochemistry of the Qian'echong giant porphyry Mo deposit, Dabie Shan, China: A case of NaCl-poor, CO2-rich fluid systems

Journal of Geochemical Exploration 124 (2013) 1–13

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Fluid inclusion and isotope geochemistry of the Qian'echong giant porphyry Mo deposit, Dabie Shan, China: A case of NaCl-poor, CO2-rich fluid systems Yong-Fei Yang a, Yan-Jing Chen a, b,⁎, Nuo Li a, Mei Mi b, You-Ling Xu c, Fa-Ling Li c, Shou-Quan Wan c a b c

Key Laboratory of Orogen and Crust Evolution, Peking University, Beijing 100871, China Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, CAS, Guangzhou 510640, China Geological Team 3, Henan Bureau of Land and Resources, Zhengzhou 464000, China

a r t i c l e

i n f o

Article history: Received 20 August 2011 Accepted 22 June 2012 Available online 27 July 2012 Keywords: Fluid inclusion Qian'echong porphyry Mo deposit Dabie Shan Continental collision NaCl-poor and CO2-rich fluid

a b s t r a c t The Qian'echong Mo deposit in Guangshan county, Henan Province, China, is a giant porphyry Mo deposit formed in Early Cretaceous in the Dabie Shan. Mo mineralization is associated with the Qian'echong granite porphyry, mainly presenting as numerous veinlets in the altered wallrocks, with potassic, phyllic, argillic and propylitic alteration developed. The hydrothermal ore-forming process can be divided into four stages: quartz+ potassic feldspar+ magnetite stage 1, quartz + molybdenite stage 2, quartz+ carbonate+ polymetal sulfide stage 3 and quartz+ carbonate+ fluorite stage 4. Fluid inclusions (FIs) can be distinguished between pure carbonic, carbonic, aqueous and solid-bearing types, but only the stage 1 quartz contains all the four types of FIs. The stage 2 quartz has three of the four types of FIs, with exception of PC-type. The stage 3 minerals are developed with the aqueous FIs with or without daughter minerals, but short of any kinds of CO2-bearing FIs. In stage 4 minerals, only the W-type FIs can be observed. The FIs in minerals of stages 1, 2 and 3 are mainly homogenized at temperatures of 260−400 °C, 200−340 °C and 160−300 °C, with salinities of 2.00−11.58 wt.% NaCl.eqv, 1.06− 10.98 wt.% NaCl.eqv and 0.53−9.47 wt.% NaCl.eqv, respectively. The estimated minimum trapping pressures are up to 100 MPa in stage 1 and to 62 MPa in stage 2, respectively, corresponding to an initial mineralization depth of no less than 4 km. The quartz separates from veinlets yield δ18O values of 7.1−10.2‰, corresponding to δ18OH2O values of −1.4−5.7‰, and the δDH2O values of fluid inclusions of −55−−72‰, suggesting that the ore-fluids evolved from magmatic to meteoric in sources. Therefore, the initial fluids forming the Qian'echong deposit, compared to those forming the porphyry systems in volcanic arcs, are characterized by relatively high temperature, high salinity, high fO2, CO2-rich, and NaCl-poor, considering that no halite has been observed in S-type FIs. We suggest that the “CO2-rich fluid” is a distinctive feature of porphyry systems developed in continental collision regime. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Porphyry deposits are significant repositories of copper, gold, and molybdenum, which are characterized by low-grade, great-reserves, and vein stockworks and disseminated mineralization. They are most commonly discovered in continental and oceanic magmatic arcs, such as the famous Pacific Rim. Recently, lots of studies of mineralizations in Chinese intercontinental settings, such as the Lower Yangtze porphyry–skarn ore belt (Chen et al., 2007a; Xie et al., 2009; Yang and Lee, 2011; Yang et al., 2011), the NE Qinling porphyry Mo belt (Chen et al., 2000; Li et al., 2007a; Mao et al., 2008; Zhang et al., 2011), and the Gangdese porphyry copper belt (Zheng et al., 2002), suggest that the intercontinental settings are also favorable for developing porphyry systems. Consequently, Chen and his coauthors (e.g. Chen, 1998; Chen ⁎ Corresponding author at: Key Laboratory of Orogen and Crust Evolution, Peking University, Beijing 100871, China. E-mail address: [email protected] (Y.-J. Chen). 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2012.06.019

and Fu, 1992; Chen et al., 2000, 2004, 2005, 2007a, 2009) proposed a tectonic model for collisional orogeny, metallogeny and fluid flow (the CMF model), which was introduced in b Hydrothermal Processes and Mineral Systems> (Pirajno, 2009). The Qinling–Dabie Shan is a typical collisional orogenic belt suturing the Yangtze and Sino-Korea continents. The Qinling orogen contains six giant Mo deposits (each with Mo reserve of >0.5 Mt in metal) and tens of small (b 0.01 Mt), medium (0.01−0.1 Mt) and large (0.1−0.5 Mt) deposits, with total reserve of ~6 Mt Mo metal (Chen et al., 2009), which are mainly associated with Mesozoic porphyry or porphyry– skarn systems that are proven to have formed in post-subduction collision regime (Chen and Li, 2009; Chen et al., 2000, 2009; Li et al., 2007a, 2011a, 2011b; Mao et al., 2008, 2010). As the eastern extension of the Qinling Orogen, the Dabie Shan is famous for the development of ultra-high pressure eclogites (Ames et al., 1993; Chavagnac and Jahn, 1996; Wang et al., 1989; Zheng et al., 2006; and references therein), and universally considered as an ore-barren area due to strong weathering denudation. However, in recent geological investigation,

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the Tangjiaping large porphyry Mo deposit, Qian'echong giant porphyry Mo deposit and Shapinggou giant porphyry Mo deposit have been discovered in Dabie Shan, ending the “barren Dabie Shan” history. The Qian'echong Mo deposit was firstly discovered in 2006 by Geological Survey Team 3 of Henan Bureau of Land and Resources, with a proven reserve of 0.6 million tons of Mo metal with an average grade of 0.081%. As a new discovered deposit, it is not mined yet and the scientific research is weak. This contribution reports the results of fluid inclusion studies of the Qian'echong giant Mo deposit, and accordingly, discusses several problems related to geological characteristics of the porphyry systems developed in post-subduction collision regime. 2. Regional geology The Dabie Shan lies at the eastern end of the E–W-trending Central China orogenic belt suturing the Sino-Korea and Yangtze Cratons, bounded by the Xiangfan−Guangji fault to the south and Luanchuan− Gushi fault to the north (Fig. 1). The NW-trending Guishan–Meishan fault is the final suture zone between the Sino-Korea and Yangtze ancient continents, equivalent to the Shang-Dan fault in Qinling area (Chen and Fu, 1992; Hu et al., 1988). To the north of Guishan−Meishan fault, the Qinling Group and the Erlangping Group composed the Caledonian

metamorphic accretion belt of the Sino-Korea continent, locally overlain by Late Paleozoic strata. To the south of Guishan−Meishan fault, the Xinyang Group, the Xiaojiamiao Formation and the Dabie metamorphic complex occur southwardly, with the Tongbai−Shangcheng and Xiaotian−Mozitan faults as their boundaries, respectively. The Xinyang Group is a Hercynian–Indosinian (Devonian to Early Triassic) accretionary complex, containing ophiolitic slices and Precambrian fragments. The Xiaojiamiao Formation, constituting the Caledonian accretionary complex, is a low-grade metamorphosed Neoproterozoic–Ordovician clastic–argillic–carbonate sequence. The Dabie metamorphic complex comprises of high-grade metamorphosed plutons and supracrustal rocks either of Archean or Paleoproterozoic ages. All the above NW-trending strata and faults are crosscut by NE- to NNE-striking faults developed since Jurassic. Intrusions in the Dabie Shan are mainly the Jinningian (ca 1.0 Ga) and Yanshanian granitoids (Fig. 1). The Jinningian granitoids mainly intruded into the Dabie metamorphic complex, occurring in the southeast of the Dabie Shan. The Yanshanian granitoids widely occur along the NW- and NNE-trending faults or at their intersection, consisting of granitic batholiths and small stocks. The small Yanshanian granitic stocks, such as Tianmugou, Xiaofan, Mushan, Dayinjian, Tangjiaping and Shapinggou, are associated with porphyry Mo deposits, constituting the Dabie porphyry Mo belt (Fig. 1).

Fig. 1. The geological map of the Dabie orogen, showing the locations of porphyry Mo deposits. Modified after Chen and Wang (2011).

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3. Local and ore geology The Qian'echong Mo deposit is located in northern Dabie Shan, to the south of the Guishan−Meishan fault (Fig. 1). The strata at the deposit include the Xiaojiamiao Formation and the Nanwan Formation of the Xinyang Group, with Tongbai−Shangcheng fault as their boundary (Fig. 2). The Xiaojiamiao Formation consists of muscovite–albite schists, muscovite–quartz schists, two-mica oligoclase schists and lenticular marble intercalations. Lithologies of the Nanwan Formation are muscovite–quartz schists, two-mica quartz schists, epidote–biotite quartz schists and biotite–plagioclase schists. Structures are dominated by the NW-trending faults that are subsidiary to the Tongbai– Shangcheng fault and crosscut by the N–S-trending faults. Three types of dykes outcropped at the Qian'echong deposit (Fig. 2). The diorite dyke swarms are NW-trending, composed of plagioclase, hornblende, K-feldspar, and quartz, with accessory minerals of apatite and magnetite. The N–S trending quartz albitophyre dykes crosscut the diorite porphyry dykes, comprising of quartz, plagioclase, K-feldspar and minor muscovite. The latest granite porphyry dykes

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occur along the NW-trending faults and is considered to connect with the concealed granite. The ore-associated Qian'echong concealed granite intruded into the Nanwan Formation (Fig. 3). Lithologies of the granite change from biotite monzogranite at depth, through to biotite-bearing granite porphyry at the top, and grading into granite porphyry dykes towards the outside, and all of them have similar mineralogical compositions. The biotite-bearing granite porphyry stock and granite porphyry dykes contain 40−50% phenocrysts, which are composed of plagioclase, K-feldspar, quartz and minor biotite. The matrix comprises of fine-grained K-feldspar, plagioclase, quartz and biotite, with accessory minerals of aspidelite, zircon, apatite, magnetite and ilmenite. The LA-ICP-MS zircon U−Pb dating on the granite porphyry dykes got a weighted average age of 128.8±2.6 Ma (Yang et al., 2010), indicating that the porphyry was formed at Early Cretaceous. The biotite monzogranite, biotite-bearing granite porphyry and granite porphyry dykes are all characteristics of high-Si, high-K, alkali-rich and negative εNd(t) values of −18.01 to −21.37, with Nd model ages of 2.3−2.4 Ga, suggesting that the magma was derived from the subducted Dabie metamorphic complex (Yang et al., 2010).

Fig. 2. Simplified geological map of the Qian'echong Mo deposit. Modified after Geological Survey Team 3 of Henan Bureau of Land and Resources (2009).

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Fig. 3. Sketch geological profile for prospecting line No. 0 of the Qian'echong deposit, showing the orebodies hosted by wallrocks. Modified after Geological Survey Team 3 of Henan Bureau of Land and Resources (2009).

Three orebodies (M1, M2, M3) mainly occur in the outer contact zones of the Qian'echong granite porphyry (Figs. 3 and 4), containing 0.6 million tons of metal Mo, with an average grade of 0.081%. The main orebody (M3) is about 1500 m long, 400−1000 m wide, and lenticular in shape (Fig. 4). The ores are major of variously altered schists, with minor altered porphyry rocks. Prime ore minerals include molybdenite, magnetite, pyrite, chalcopyrite, galena and sphalerite. The main gangue minerals are quartz, feldspar, epidote, biotite, sericite, fluorite and calcite. Structures of ores include veinlets (Fig. 5A, B, D, E), disseminations (Fig. 5C), stockworks and breccias (Fig. 5F). Various ore textures were observed, such as flaky (Fig. 5K), replacement (Fig. 5J), idiomorphic to hypidiomorphic grain and phenocryst. The alterations at the deposit include: (1) potassic alteration, with biotite and feldspar as predominant hydrothermal minerals; (2) silicification, in the forms of quartz-(sulfide) stockworks or veinlets; (3) sericitization, typified by transformation of feldspar and biotite to sericite, with disseminated pyrite (Fig. 5H) and quartz–sericite veinlets; (4) argillization, typified by transformation of feldspar to kaolinite, usually controlled by fractures; (5) propylitization, with epidote, chlorite and calcite as predominant hydrothermal minerals (Fig. 5I); (6) carbonation, mainly typified by carbonate veinlets; and (7) fluoritization, presented as disseminated purple grains or veinlets. The hydrothermal alteration and mineralization can be roughly divided into four stages. Stage 1 is characterized by the assemblage of K-feldspar+ quartz+ epidote + magnetite + pyrite. Magnetite is mostly disseminated, coexisting with K-feldspar and epidote (Fig. 5G); pyrite occurs as idiomorphic to hypidiomorphic cubes or replaces magnetite (Fig. 5J); and little molybdenite scatters in the porphyry. Coeval alteration includes potassic feldspathization, silicification and propylitization. Stage 2 is conducive to Mo mineralization, characterized by the assemblage of quartz+molybdenite+pyrite. Molybdenite is flaky (Fig. 5K), disseminated in veins or being skinny along veins or cracks. The stage 3 is characterized by the quartz+calcite+polymetal sulfides assemblage, with sphalerite and galena occurring as xenomorphic grain in veins and coexisting with pyrite and calcite (Fig. 5L). Silicification and phyllic

alteration are most conspicuous in stages 2 and 3. The stage 4 is characterized by quartz–carbonate, carbonate or/and carbonate–fluorite veinlets, with no or little (if there is) sulfide, which crosscut the earlier veins, stockworks and altered porphyry blocks. The carbonation and argillization developed in stage 4. The Re–Os isotope dating on molybdenite separated from the ores got a weighted average age of 127.82 ± 0.87 Ma (Yang et al., 2010), slightly younger than the porphyry complex aged 128.8 Ma, indicating that the porphyry Mo system formed in the Early Cretaceous extension of the Dabie Shan. 4. Fluid inclusions 4.1. Samples and analytical methods Samples were collected from drill cores of ZK005 and ZK703, including various kinds of veinlets formed in different stages, and then doubly polished into thin sections (b 0.30 mm thick). Microthermometric measurements were performed using the Linkam THMSG600 heatingfreezing stage and employing standard procedures, in the Fluid Inclusion Laboratory of the Beijing Institute of Geology for Mineral Resources. Stage calibration was carried out at −56.6 °C, −10.7 °C and 0.0 °C using synthetic FIs supplied by FLUID INC. The measurement precisions are estimated at ±0.2 °C for b30 °C, ±1 °C for the interval of 30− 300 °C, and ±2 °C for >300 °C, respectively. Melting temperatures of solid CO2 (Tm,CO2), freezing point of NaCl–H2O inclusions (Tm,ice), final melting temperatures of clathrate (Tm,cla), homogenization temperatures of CO2 phase (Th,CO2), dissolution temperatures of daughter minerals (Tm,d) and total homogenization temperatures of FIs (Th) were measured. Heating rate was 1−5 °C/min during the initial stages of each heating run and reduced to 0.3−1 °C/min close to the phase change points. Compositions of individual FIs, including vapor, liquid and daughter minerals, were identified using Laser Raman spectroscopy in Laboratory of Orogen and Crust Evolution, Peking University. An argon laser with a wave length of 514.5 nm was used as laser source at

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power of 1000 mW. Integration time was 10 s, with ten accumulations for each spectral line. The spectral resolution is ±2 cm −1 with a beam size of 2 μm. Instrumental setting was kept constant during all analyses. Salinities of carbonic (CO2–H2O) and aqueous (NaCl–H2O) inclusions were calculated using the final melting temperatures of CO2clathrate (Collins, 1979) and ice points (Bodnar, 1993), respectively. Because daughter minerals in fluid inclusions do not melt in the heating process, the salinities of fluid phases were calculated using the final melting temperatures of CO2-clathrate (Collins, 1979) and ice points (Bodnar, 1993), which do not include the contribution of daughter minerals.

H2O (Fig. 7D), but the vapor phase of the W-type FIs in stage-1 quartz contains a small amount of CO2 (Fig. 7C). The vapor and liquid phases of the SW-subtype FIs are dominated by H2O; but in the SC-subtype FIs, the vapor is dominated by CO2, and the liquid phases contain H2O and CO2. The spectra of opaque daughter minerals show the peak characteristic of chalcopyrite (290 cm −1; Fig. 7E). One kind of transparent daughter minerals shows two peaks at 298 cm −1 and 1090 cm −1 (Fig. 7F) which are taken as identifying characteristics of calcite. The other kind of transparent daughter minerals shows non-identifying peaks in Laser Raman spectra.

4.2. Types and populations of fluid inclusions

The microthermometric data of FIs are summarized in Table 2 and Fig. 8, which clearly show the linkage between mineral assemblages, physico-chemical conditions and metallogenic stages. The stage 1 quartz veinlets contain lots of W-, C-, and S-types and minor PC-type FIs. In C-type FIs, the CO2 phase accounts for 40−80% in volume. The melting temperatures of solid CO2 (Tm,CO2) range from − 56.8 to − 58.6 °C, slightly below the triple-phase point (− 56.6 °C) of CO2, suggesting minor amounts of dissolved components in the carbonic phase (Lu et al., 2004). The clathrate melting (Tm,cla) occurs in the interval of 5.1− 9.0° C, corresponding salinities of 2.00 − 8.82 wt.% NaCl.eqv; and the carbonic phase homogenized to vapor at temperatures from 16.7 to 31.1 °C (Th,CO2). The C-type FIs are totally homogenized to liquid or vapor at temperatures ranging from 295 to 396 °C (Th), concentrating 320− 380 °C, but some C-type FIs decrepitated at temperatures of 266 to 425 °C before homogenization. Their densities range from 0.45 to 0.84 g/cm 3. The W-type FIs yield ice-melting temperatures (Tm,ice) from − 7.8 to − 1.3 °C, with salinities ranging from 2.24 to 11.46 wt.% NaCl.eqv. They are homogenized to liquid at temperatures of 257 − 400 °C, with densities of 0.60 −0.84 g/cm 3. The S-type FIs contain chalcopyrite and some transparent daughter minerals, which do not melt in the heating process, with vapor H2O or CO2 phase accounting for 10 − 40% in volume. Their fluid phases are homogenized to liquid at temperatures of 211 to 422 °C, yielding salinities of 2.07 − 11.58 wt.% NaCl.eqv (Fig. 9). The FIs in stage 2 minerals are W-, C- and S-types. The C-type FIs contain 20 − 50% CO2 in volume at room temperature and yield melting temperatures of solid CO2 mainly ranging from − 57.0 to − 59.0 °C, suggesting a small quantity of other gasses dissolved in the carbonic phase. Clathrate melting temperatures range 6.6−8.9 °C, corresponding to salinities of 2.20−6.37 wt.% NaCl.eqv. The total homogenization temperatures range 211−348 °C, peaking at 300 °C, with densities of 0.53−0.83 g/cm 3. The W-type FIs contain vapor phase of 10−40% in volume. Their freezing points vary from −0.6 to −7.4 °C, corresponding to salinities of 1.06−10.98 wt.% NaCl.eqv, and densities of 0.62−0.97 g/cm3. They are mainly homogenized to liquid at temperatures between 146 and 370 °C, suggesting that some secondary FIs are incorporated. In the S-type FIs, chalcopyrite, calcite and unidentified transparent daughter minerals can be observed, which do not melt in heating runs, while the rest of the phases of the inclusions are homogenized to liquid at temperatures of 160 to 331 °C, corresponding to salinities of 1.22−10.61 wt.% NaCl.eqv. FIs in the stage 3 quartz and calcite are major of W-type, and minor of S-type. The W-type FIs yield ice-melting temperature of − 6.2 to −0.3 °C, corresponding to salinities of 0.53−9.47 wt.% NaCl.eqv. Their homogenization temperatures widely vary from 137 to 300 °C, with densities ranging from 0.76 to 0.97 g/cm3. The fluid phases in a few SW-subtype FIs that contain calcite and opaque daughter minerals are homogenized to liquid at temperatures of 143−299 °C, yielding salinities of 2.34−8.28 wt.% NaCl.eqv. Only W-type FIs can be observed in stage 4 minerals (calcite, fluorite and quartz), but they are too small to conduct microthermometric study.

Four fluid inclusion types are identified based on their phases at room temperature (21 °C) and subzero temperatures, phase transitions during heating and cooling runs (−196 −+ 600 °C), and Laser Raman spectroscopy (Chen et al., 2007b; Fan et al., 2011; Lu et al., 2004). They are aqueous, carbonic, pure carbonic and solid-bearing, (Fig. 6). The pure carbonic (PC) type FIs appear as one (vapor CO2) or two phases (liquid CO2 + vapor CO2) at room temperature (Fig. 6A), commonly coexisting with carbonic type FIs. They are irregular, ellipsoidal and negative quartz crystal in shapes, with sizes ranging from 10 μm to 20 μm. The carbonic (C) type FIs are two-phase (vapor CO2 + liquid H2O) or three-phase (vapor CO2 + liquid CO2 + liquid H2O) CO2–H2O system (Fig. 6B, C), with CO2 phase accounting for 20–80% in volume. They appear irregular, ellipsoid and negative quartz crystal in shape, 5 to 20 μm in size, isolated or as clusters in occurrence. The aqueous (W) type FIs are two-phase (liquid and vapor water) NaCl–H2O systems (Fig. 6D, E, F), with bubbles usually accounting for 5–60% in volume. They are irregular, ellipsoid, strip and negative crystal in shape, with sizes ranging from 5 to 30 μm. The solid-bearing (S) type refers daughter mineral-bearing FIs or multiphase FIs, consisting of one or more daughter minerals and fluid with one to three phases. They are ellipsoid, negative crystal and irregular in shape, and 5 to 25 μm in size. As identified by Laser Raman spectroscopy, the most common opaque daughter mineral is chalcopyrite. Transparent daughter minerals include calcite and unidentified one. The S-type FIs can be divided into two subtypes, i.e. SC-subtype with carbonic fluid phase (Fig. 6G) and SW-subtype with aqueous fluid phase (Fig. 6H, I). The relative abundances of fluid inclusions were investigated for each type of FIs in samples of different stages (Table 1). The stage 1 quartz contains all the four types of FIs, but the PC-type FIs only occur in the minerals from the deep portion of the ore-system; while stage 4 minerals only contain W-type FIs. The stage 2 quartz contains three of the four type FIs, with exception of the PC-type. In stage 3 minerals the C- and PC-types FIs have not been observed, but the W- and S-type FIs are abundant. The populations of C-type FIs decrease both from stage 1 to stage 2 and from deep to shallow, and on the contrary, the W-type FIs increase from stage 1 to stage 4. The abundance of S-type FIs also decreases from stage 1 to stage 3, but slightly increases from deep to shallow. All the variations above indicate that the fluids changed from carbonic to aqueous along with the temporal evolution and upward migration. 4.3. Laser Raman spectroscopy analysis The spectra of the PC-type FIs only have obvious peaks of CO2 (1285 cm −1 and 1388 cm −1, Fig. 7A), indicating that they are totally composed by CO2. The vapor phases of the C-type FIs are dominated by CO2 (Fig. 7B), and the liquid phases contain H2O and liquid CO2. The vapor and liquid phases of the W-type FIs are dominated by

4.4. Microthermometry

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Fig. 5. Photographs showing ore petrography of the Qian'echong Mo deposit. (A) the stage 1 potassic feldspar–epidote veinlet; (B) the stage 1 potassic feldspar–quartz–epidote– magnetite veinlet; (C) the stage 1 quartz–pyrite veinlet; (D) the stage 2 quartz–molybdenite veinlet; (E) the stage 1 potassic feldspar–quartz veinlet cut by the stage 3 quartz– calcite–molybdenite veinlet; (F) the stage 4 calcite veinlet, showing that the calcite cemented breccias of wallrocks; (G) the stage 1 magnetite coexisting with potassic feldspar, quartz and epidote; (H) the beresitization, sericite coexisting with quartz and pyrite; (I) the propylitic alteration, epidote coexisting with calcite and chlorite; (J) the stage 1 magnetite replaced by pyrite; (K) the stage 2 foliaceous molybdenite; (L) the stage 3 pyrite coexisting with sphalerite and galenite. Abbreviations: Cc: calcite; Ep: epidote; Gn: galenite; Kfs: potassic feldspar; Mt: magnetite; Mo: molybdenite; Py: pyrite; Qtz: quartz; Sp: sphalerite; Srt: sericite.

4.5. Minimum trapping pressure of FIs and mineralization depth According to homogenization temperatures, homogenization behaviors and proportion of the CO2 phase in the C-type FIs, and the total homogenization temperatures of the C-type FIs, the minimum trapping pressures of FIs were estimated using the Flincor program (Brown, 1989) and the formula of Bowers and Helgeson (1983) for

the H2O–CO2–NaCl system. The homogenization temperatures of the C-type FIs used here are all above 300 °C. The minimum trapping pressures of FIs were estimated to be 15−100 MPa in stage 1 and 5 −62 MPa in stage 2, respectively, given their averages of homogenization temperatures being 358 ± 20 °C and 315 ± 15 °C, respectively. This estimation shows that the trapping pressure of FIs gradually decreased from stage 1 to stage 2, which is similar to many magmatic

Fig. 4. Sketch geological profiles for prospecting lines Nos. 0, 7, 8, 15 and 16 of the Qian'echong Mo deposit, showing the occurrence of orebodies. Modified after Geological Survey Team 3 of Henan Bureau of Land and Resources (2009).

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Fig. 6. Microphotographs of fluid inclusions in the Qian'echong Mo deposit. (A) the PC-type fluid inclusion; (B) H2O-rich C-type fluid inclusion; (C) CO2-rich C-type fluid inclusion; (D) liquid rich W-type fluid inclusion; (E) the pseudosecondary W-type fluid inclusions occurring as a cluster; (F) the W-type fluid inclusion developed in the calcite; (G) the SC-subtype inclusion containing daughter chalcopyrite; (H) the SW-type inclusion containing daughter chalcopyrite; and (I) the SW-type inclusion containing daughter chalcopyrite and an unknown transparent mineral. Abbreviation: VCO2: CO2 vapor; LCO2: CO2 liquid; VH2O: H2O vapor; LH2O: H2O liquid; Cp: chalcopyrite; Tr: unidentified transparent mineral.

hydrothermal systems in East Qinling, such as the Shangfanggou porphyry Mo–Fe deposit (Yang et al., 2009), Yuchiling porphyry Mo deposit (Li et al., 2009) and the Qiyugou breccia-pipe gold deposit (Fan et al., 2011; Li et al., 2012), and to the Luoboling porphyry Cu– Mo deposit (Zhong et al., 2011) and Wuziqilong porphyry Cu deposit (Chen et al., 2011 ) in the Zijinshan ore field, Fujian province, SE China. The pressure decrease indicates that most of the magmatic hydrothermal systems were formed in a rapid crust uplifting setting caused by synorogenic crustal thickening or post-orogenic delamination of lithospheric root. The porphyry-type mineral system is characteristic of pulsating hydraulic broken-and-healing caused by fluid boiling and precipitation. The pressure of fluid system frequently alternates between superlithostatic and hydrostatic. Therefore, the lowest trapping pressure of FIs represents the hydrostatic system, while the highest pressure reflects the lithostatic or super-lithostatic system. According to the minimum pressure estimated for the Qian'echong deposit, the stage 1 mineralization at Qian'echong likely occurred at depth of 2−4 km, while the mineralization of stage 2 occurred at depth of 0.5−2.5 km, given that the density of rocks covering the Qian'echong porphyry is 2.5 g/cm3. Hence it can be concluded that the initial Mo-mineralization at Qian'echong mainly occurred at depth of no less than 4 km. This depth is coincident with the estimated mineralization depths (1−5 km) of the porphyry deposits in the world (Pirajno, 2009). 5. Hydrogen and oxygen isotopic compositions The hydrogen and oxygen isotopes were analyzed in the Stable Isotope Laboratory of Mineral Resources Institute, Chinese Academy of

Geological Sciences, using the Finnigan MAT253 mass spectrometer. 13 quartz samples from the ZK005 and ZK703 drills were analyzed. Oxygen was liberated from quartz by reaction with BrF5 (Clayton and Mayeda, 1963) and converted to CO2 on a platinum-coated carbon rod for oxygen isotope analysis. The water of the fluid inclusions in quartz was released by heating the samples to above 500 °C by means of an induction furnace, and then reacted with zinc powder at 410 °C to generate hydrogen (Friedman, 1953) for isotope analysis. The results were reported in per mil relative to V-SMOW standards, with precisions of ±2‰ for δD and ±0.2‰ for δ18O. The δ18OH2O values of ore-forming fluids from quartz were calculated using the equation 1000lnαquartz-H2O =3.38×106 T−2 −3.40 (Clayton et al., 1972). The δ 18OQ values range from 7.1 to 10.2‰, from which the δ 18OH2O values are calculated between − 1.4 and 5.7‰ (Table 3). The δDH2O ratios are between − 55 and − 72‰. In the δD vs. δ 18O plot (Fig. 10), 4 samples from stage 1 plot in and adjacent to the box of primary magmatic water, indicating that the initial ore-forming fluids were magmatic in origin. The temperatures used here in calculation are homogenization temperatures that are the minimum trapping temperature of FIs and are lower than the temperatures of the ore-fluids, which resulted that the calculated δ 18OH2O values are no higher than the actual δ 18OH2O values. 9 samples from stages 2 and 3 are plotted between the magmatic water box and the meteoric water line, suggesting that the fluids possibly mixed with meteoric water. The input of the meteoric water could reduce the δ18O values. The characteristics of the hydrogen and oxygen isotopic compositions of ore-fluids at Qian'echong deposit are similar to those of the Dayinjian porphyry Mo deposit in Dabie Shan (Li et al., 2010).

Y.-F. Yang et al. / Journal of Geochemical Exploration 124 (2013) 1–13 Table 1 Distribution of fluid inclusion types in samples from the different stages of the Qian'echong Mo deposit. Stage Drill hole

Analysis number

Depth (m)

Mineral assemblage of veins

FIs types (%)

1

45 53 66 72 75 76 23

877.5 746 438 245 164 102 583

5 5

09 01

179 24

34 49 55 56 61 62

1080 843 703 676 589 523

65 67 70 71 39 26 21 17 15 14 08 60 63

442 430 269 259 799 641 488 376 347 304 103 597 501

07 06

95 76

Qtz, Py, Cpy Qtz, Kfs, Py Qtz, Kfs, Ep Qtz, Kfs, Py, Mt, Mo Qtz, Kfs, Py Qtz, Kfs, Py, Mt Qtz, Kfs, Ms, Fl, Py, Mo Qtz, Kfs, Ep, Mt Qtz, Kfs, Chl, Py, Mt, Cpy Qtz, Srt, Py Qtz, Srt, Mo Qtz, Mo, Py, Cpy Qtz, Mo Qtz, Mo Qtz, Chl, Kfs, Srt, Py, Mo, Mt Qtz, Py, Mo Qtz, Kfs, Mo Qtz, Mo Qtz, Py, Mo Qtz, Mo Qtz, Py, Cpy, Mo Qtz, Ep, Mo Qtz, Ep, Mo, Kfs Qtz, Ep, Mo Qtz, Ep, Py, Cpy, Mo Qtz, Chl, Mo Qtz, Cc, Srt, Mo Qtz, Cc, Py, Cpy, Sp, Gn Qtz, Cc, Mo Qtz, Cc, Py

005

703

2

005

703

3

005

703

PC C

W

S

40 40 40 50 55 20 50 10 50 10 60 35 50

15 5 45 30 40 30 15

20 40 25 60

40 15

25 25 20 20 10 20

5 10 15 35 35

5

75 70 70 65 55 45

70 80 85 15 50 10 75 85 80 80 90 80 90 95 100

25 20 15 35 15 15 20 20 10 20 10 5

100 95 5

Abbreviations: Cc: calcite; Chl: chlorite; Cpy: chalcopyrite; Ep: epidote; Fl: fluorite; Kfs: potassic feldspar; Mt: magnetite; Mo: molybdenite; Py: pyrite; Qtz: quartz; Srt: sericite

6. Discussion 6.1. Initial NaCl-poor ore-forming fluids The halite-bearing FIs are generally observed in early-stage hydrothermal minerals of the porphyry deposits in the world (Chen et al., 2007b). At the Qian'echong deposit, however, it is very unique that no halite-bearing inclusion has been identified in hydrothermal minerals, but on the contrary, the W-, C- and chalcopyrite-bearing S-types of FIs have been frequently observed. This implies that the NaCl activity in the initial ore-forming fluid was relatively low. The exsolution pressure of Cl from magma is lower than CO2 and H2O (Giggenbach, 1997). In the upward intruding process of magma, Cl would be immiscible in and exsolved from the CO2bearing magma after CO2 and H2O, and then dissolved into the fluids mainly composed of CO2 and H2O. Therefore, the CO2-bearing magma will firstly generate the low-salinity CO2-rich fluid at depth, and secondly the high-salinity fluid at shallow (Shinohara and Kazahaya, 1995). Baker (2002) also found that the initial magmatic–hydrothermal fluids of the intrusion-related gold deposits in deep environment are NaCl-poor and CO2-rich. This is why no halite-bearing fluid inclusion has been observed at the Qian'echong deposit in this study. In the porphyry Cu–Mo deposit at Butte, Montana, the initial ore-forming fluid is proven NaCl-poor and CO2-bearing, forming at high pressures from 200 to 250 MPa (Rusk et al., 2008). The same NaCl-poor, CO2-bearing initial ore-forming fluids have also been recognized and described in deep veins from the Bingham porphyry deposit, Utah (Redmond et al., 2004), and the Questa porphyry Mo deposit, New

9

Mexico (Klemm et al., 2008). In this study, the minimum trapping pressures of the stage 1 FIs at Qian'echong were estimated up to 100 MPa, indicating that the pressures of initial magmatic fluids might be higher than 100 MPa, corresponding to a deeper initial mineralization environment. Hence the initial ore-forming fluids were NaCl-poor, which is supported by shortage of halite-bearing inclusion. 6.2. Origin of the CO2-rich FIs Mainly based on the study of the Circum Pacific metallogenic belt, it is generally believed that the magmatic–hydrothermal systems are characteristic of containing aqueous FIs with or without daughter minerals, and occasionally if there is, minor CO2–H2O FIs (Bodnar, 1995; Cline and Bodnar, 1991; Klemm et al., 2007, 2008; Lu et al., 2004; Masterman et al., 2005), but without PC-type and SC-subtype FIs. However, abundant C- and PC-types and SC-subtype of FIs have been recently reported in the magmatic–hydrothermal systems formed in intracontinental tectonic settings in China, particularly the continental collision regimes, for examples, the Wunugetushan porphyry Cu–Mo deposit, Inner Mongolia (Li et al., 2007b); the Yuchiling porphyry Mo deposit, Henan (Li et al., 2009); the Tangjiaping porphyry Mo deposit, Henan (Chen and Wang, 2011); the Qiyugou breccia-related gold deposit, Henan (Fan et al., 2011); and the Sandaozhuang skarn Mo–W deposit, Henan (Shi et al., 2009). The Qian'echong Mo deposit and its associated granitic porphyry were formed at the beginning of the extension of the Dabie collisional orogen. Its ore-fluid system was also rich in CO2 and evidently resulted in the development of FIs of PC- and C-types and SC-subtype. Chen et al. (2007b, 2008, 2009) and Chen and Li (2009) proposed that the ore-fluids of porphyry systems formed in volcanic arcs were mainly derived from metamorphic dehydration of subducted oceanic slab that could be roughly regarded as “altered oceanic basalt” (NaCl-brine or seawater-bearing oceanic crust), and thereby, rich in H2O, Na and Cl, but poor in CO2 (or carbonate), K and F; on the other hand, the porphyry systems formed in continental collision regime originated from the thickened lower continental crust or lithospheric mantle that are relatively poor in H2O and NaCl, and have higher CO2/H2O, K/Na and F/Cl ratios relative to oceanic slabs. Therefore, the CO2-rich ore-fluids, indicated by the C- and PC-types and SC-subtype of FIs, can be taken as a diagnostic marker of the porphyry systems formed in intra-continental settings, considering that the porphyry systems formed in volcanic arcs are short of these CO2-rich FIs. 6.3. Evolution of fluid system and mineralization process Fluid inclusion is the “fossil” of ancient fluid-systems. Among the FIs trapped in minerals of multistage hydrothermal system, only the primary FIs in the earliest-stage minerals can represent the nature and genesis of the original fluids (Chen et al., 2007b). The spatial and temporal relationship between orebodies and porphyry at Qian'echong, the characteristics of the hydrogen and oxygen isotopic compositions, and the occurrence of the PC-, C- and S-types of FIs in quartz formed in stage 1, suggest that the Qian'echong Mo deposit was formed by an initially high-salinity carbonic-rich fluid system originating from intracontinental magmatism (Chen and Li, 2009; Chen and Wang, 2011). The Qian'echong porphyry is high-Si, high-K and alkali-rich, with high content of Mo (Geological Survey Team 3 of Henan Bureau of Land and Resources, 2009). Fluid exsolved from this magma is characterized by CO2-rich, K-rich, high temperature, high salinity and probably Mo-rich. The fluid filtered through and reacted with the cooling porphyry rocks and hosting schists, causing K-silicate alterations (muscovite, biotite and K-feldspar). In stage 1, the coexistence of PC-, C-, W- and S-types FIs and the divergent homogenization ways of C-type FIs, possibly resulted from the earlier fluid boiling or phase separation. The fluids were relatively oxidizing, CO2-rich and S2− poor, and therefore,

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Y.-F. Yang et al. / Journal of Geochemical Exploration 124 (2013) 1–13

Fig. 7. The LRM spectra of fluid inclusions. (A) CO2-spectrum of the PC-type fluid inclusion; (B) CO2-spectrum of the vapor in C-type fluid inclusion; (C) H2O- and CO2-spectra of the vapor in stage 1 W-type fluid inclusion; (D) H2O-spectrum of the vapor in stages 2 and 3 W-type fluid inclusion; (E) daughter chalcopyrite in the S-type fluid inclusion; (F) daughter calcite in the S-type fluid inclusion.

unfavorable for sulfide deposition and Mo mineralization, which is supported by the precipitation of abundant magnetite. Because of the consumption of alkali ions and OH − in stage 1 alteration, the stage 2 fluids became more acidic, with increasing H+ activity, leading to more escape of CO2 (2H++CO32−→H2O +CO2↑). The fO2 of

the ore-fluids magnetite and In stage 2, the become lower less carbonic,

decreased due to the precipitation of abundant escape of CO2, resulting increase in S 2 − activity. homogenization temperatures and salinities of FIs compared to those in stage 1; the fluids become which is indicated by the decrease of volume

Table 2 Microthermometric data of fluid inclusions of the Qian'echong Mo deposit. Stage

Host

Type

N

Tm,CO2 (°C)

1

Quartz

C W SC SW C W SC SW W SW W

46 152 10 74 41 454 10 70 110 11 35

−56.8−−58.6

2

3

Quartz

Quartz Calcite

Tm,ice (°C)

Tm,cla (°C)

Tm,d (°C)

Th (°C)

Salinity (wt.% NaCl)

Density (g/cm3)

5.1−9.0

266−425

295−396 (L,V) 257−400 (L) *337−422 (L) *211−391 (L) 211−348 (L) 146−370 (L) *263−301(L) *160−331 (L) 137−297 (L) *143−299 (L) 188−300 (L)

2.0−8.8 2.2−11.5 2.8−5.3 2.1−11.6 2.2−6.4 1.1−11.0 5.3−6.5 1.2−10.6 0.5−9.5 2.3−8.2 0.9−5.9

0.45−0.84 0.60−0.84

−7.8−−1.3 −57.0−−58.6

7.2−8.6 −7.9−−1.2

−57.0−−59.0

6.6−8.9 −7.4−−0.6

−57.1−−58.0

6.5−7.2 −7.1−−0.7 −6.2−−0.3 −5.3−−1.3 −3.6−−0.5

262−348

0.53−0.83 0.62−0.97

0.76−0.97 0.76−0.90

Notations: N, number of fluid inclusions analyzed; L and V mean that the FIs homogenized to liquid (L) or vapor (V); and the Th with * stands for the homogenization temperature of all the fluid phases in the S-type FIs.

Y.-F. Yang et al. / Journal of Geochemical Exploration 124 (2013) 1–13

11

Fig. 8. Histograms of salinities and homogenization temperatures of fluid inclusions in minerals of different stages.

proportions of CO2 phase in C-type FIs, the decrease of frequency percentage of C-type FIs, and shortage of the PC-type inclusion. These changes of ore-fluids facilitated the deposition of sulfides

including molybdenite, together with phyllic alteration, forming the stage 2 quartz-molybdenite stockworks. Further cooling of the ore-fluid system and inputting of meteoric water caused the development of stage 3 quartz–carbonate–sulfide Table 3 The oxygen and hydrogen isotopic ratios of Qian'echong Mo deposit.

Fig. 9. Representative isochores for C-type fluid inclusions of stages 1 and 2 and their average homogenization temperatures (vertical line) with 1σ standard deviation (vertical dashed and dot-dashed lines). The isochores are calculated using the Flincor program (Brown, 1989) and the formula of Bowers and Helgeson (1983). The red and green boxes indicate the P–T windows of stages 1 and 2, calculated using the 1σ temperature range. The homogenization temperatures of the C-type FIs used here are all above 300 °C.

δ18OH2O (‰)

δDH2O (‰)

9

4.2

−62

329 310 337

8.5 9.3 9

2.6 2.8 3.3

−57 −59 −61

2 1 1 2 1 3 2 2

302 380 370 300 400 260 309 310

8.9 10.2 9.8 9.2 9.7 7.1 7.6 7.5

2.1 5.7 5.0 2.3 5.6 −1.4 1.0 1.0

−58 −57 −72 −62 −66 −60 −59 −55

2

320

7.6

1.4

−61

Sample no.

Depth (m)

Mineralogy of veins

Stage Th (°C)

005-72

245

1

367

005-71 005-65 005-62

259 442 523

2 2 2

005-56 005-45 005-38 005-34 005-26 703-7 703-8 703-17

676 877 1018 1080 1278 95 103 376

703-39

799

Qtz–Kfs–Py– Mo–Mt Qtz–Py–Mo Qtz–Py–Mo Qtz–Chl–Kfs– Py–Mo Qtz–Mo Qtz–Py Qtz–Py Qtz–Py–Srt Qtz–Py Qtz–Mo–Cc Qtz–Chl–Mo Qtz–Mo–Ep– Kfs Qtz–Mo

δ18Oquartz (‰)

Abbreviations: Cc: calcite; Chl: chlorite; Ep: epidote; Kfs: potassic feldspar; Mt: magnetite; Mo: molybdenite; Py: pyrite; Qtz: quartz; Srt: sericite

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Y.-F. Yang et al. / Journal of Geochemical Exploration 124 (2013) 1–13

a key to increase the activities of NaCl and S2− in the fluids, to reduce the oxidizing of the fluids, and to result in precipitation of sulfides in hydrothermal minerals. Acknowledgments This study was financially supported by the National 973 Research Program (No. 2006CB403508) and the National Natural Science Foundation of China (Nos. 40730421 and 40425006). The field investigation was supported by the Geological Team 3 of Henan Bureau of Land and Resources. Fluid inclusion study was helped by Professor Li-juan Wang. References

Fig. 10. The δD~δ18O systematics for the Qian'echong Mo deposit. (Base map after Taylor, 1974).

veinlets. In stage 3, the fluids got more dilute and CO2-poor, and therefore, none of the C- and PC-types and SC-subtype FIs could be observed. The majority of FIs in stage 3 minerals are W-type, with minor S-type. The stage 4 presents as sulfide-barren carbonate ± fluorite ± quartz veinlets that only contain small-sized W-type FIs. This stage represented the waning hydrothermal system or the termination of ore-forming process, and resulted from CO2-poor, low-temperature and dilute hydrothermal fluids sourced from meteoric water. 7. Concluding remarks (1) The Qian'echong Mo deposit is a giant porphyry ore system in the Dabie collision orogen. Its mineralization process includes four stages, characterized by K-feldspar–quartz–epidote–magnetite veinlets stage 1, quartz–molybdenite stockworks stage 2, quartz– sulfide–carbonate stockworks stage 3, and carbonate±fluorite± quartz veinlets stage 4, respectively. Wallrock alterations in stage 1 are mainly K-silication, silicification and propylitization; in stages 2 and 3 are sericitization and silicification alteration; and in stage 4 are carbonation and argillization. (2) The aqueous, solid-bearing aqueous, pure carbonic, carbonic and solid-bearing carbonic FIs are presented in the hydrothermal minerals at the Qian'echong deposit. The former two types of FIs are characteristic of all the magmatic hydrothermal mineral systems; the latter three types (or subtype) of FIs are distinctive characteristics of the magmatic-hydrothermal mineral systems formed in continental collision regime, including porphyry deposits. (3) As indicated by the occurrence, microthermometric data and estimated minimum trapping pressures of fluid inclusions, as well as the H–O isotope systematics, the ore-forming fluid-system evolved from high-temperature, high-salinity, high-pressure and CO2-rich, to low-temperature, low-pressure, low-salinity and CO2-poor, through continuous fluid boiling and inflow of meteoric water. The fluid boiling characterized by CO2 escape and mixing with meteoric water were two key factors resulting in deposition of sulfides or ore-metals. (4) Easy exsolution of CO2 from the CO2-rich magma and the high content of CO2 in initial fluids result in low NaCl activity in ore-forming fluids, and thereby, causing shortage of halite-bearing inclusion to be a unique feature of hydrothermal minerals. CO2-escape may be

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