Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

Ore Geology Reviews 114 (2019) 103141

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Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

T

Xinghai Langa,b, , Xuhui Wanga, , Yulin Denga, Juxing Tangc, Fuwei Xiea, Zongyao Yangd, Qing Yina, Kai Jiange ⁎

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a

College of Earth Science and MNR Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation, Chengdu University of Technology, Chengdu 610059, China State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China c Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China d Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, China e Tibet Tianyuan Mineral Exploration Co. Ltd., Shigatse 857000, China b

ARTICLE INFO

ABSTRACT

Keywords: Porphyry deposit SEM-CL Fluid inclusion CO2-rich Xiongcun

The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CLdark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, > 70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18Ofluid and δDfluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were lowsalinity, CO2-rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial oreforming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the oreforming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO2-poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation.

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Corresponding authors at: College of Earth Science, Chengdu University of Technology, No. 1, East 3rd Road, Erxianqiao, Chengdu 610059, China. E-mail addresses: [email protected] (X. Lang), [email protected] (X. Wang).

https://doi.org/10.1016/j.oregeorev.2019.103141 Received 16 July 2018; Received in revised form 8 September 2019; Accepted 26 September 2019 Available online 26 September 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

relations of veins, such as the reopening of older veins by later veins or the infilling of dissolution vugs in early quartz by later quartz, based on hand-sample observations and transmitted light microscopy (Rusk and Reed, 2002; Redmond et al., 2004; Müller et al., 2010; Rusk, 2012; Maydagán et al., 2015). Non-detection of these superpositions complicates the understanding of the origin and evolution of fluids and of ore

Porphyry Cu (Mo–Au) deposits are characterized by the superposition of multiple mineralization, alteration, and vein-forming events (Hedenquist and Lowenstern, 1994; Gustafson and Hunt, 1975; Sillitoe, 2010), but it is not possible to identify some of the superposition

Fig. 1. (a) Simplified map showing the location of the Himalayan–Tibetan orogeny; (b) simplified regional geological map of the Himalayan–Tibetan orogeny showing the location of GPCB, modified from Zhu et al. (2011); (c) geological map of GPCB, modified from Yang et al. (2009); (d) geological map of the Xiongcun district, modified from Tang et al. (2015); (e) geological map of the No. 2 porphyry Cu–Au deposit, modified from Tang et al. (2012). 2

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precipitation mechanisms. In recent years, studies of hydrothermal quartz using scanning electron microscope-cathodoluminescence (SEMCL) have shown it to be a powerful tool for distinguishing multiple generations of quartz precipitated at different times in an individual vein (Rusk and Reed, 2002; Redmond et al., 2004; Landtwing et al., 2005; Klemm et al., 2007, 2008; Pudack et al., 2009; Müller et al., 2010; Maydagán et al., 2015; Ni et al., 2017; Mao et al., 2017). Early studies of fluid inclusions in porphyry Cu deposits recognized that two coexisting fluid phases, high-salinity brine and low-salinity vapor, are extremely ubiquitous (Roedder, 1971, 1992; Denis et al., 1980; Bodnar and Vityk, 1994). However, in most deposits, it is unclear whether these fluids directly exsolved from a pluton underlying the mineralized stocks or whether they were generated by fluid immiscibility in a low-salinity single-phase fluid (Roedder, 1971; Bodnar and Vityk, 1994; Clinem and Bodnar, 1994; Campos et al., 2002, 2006; Audetat et al., 2008; Rusk et al., 2008). The spatial and temporal evolution of fluids in two world-class porphyry Cu systems has recently been reconstructed in detail (Redmond et al., 2004; Landtwing et al., 2005, 2010; Rusk et al., 2004, 2008). These studies indicated that the initial ore-forming fluids were represented by low-salinity single-phase fluids trapped in fluid inclusions in early barren quartz veins in the deep parts of a deposit. Upon depressurization and cooling, the low-salinity single-phase fluids underwent phase separation into high-salinity brine and low-salinity vapor. Therefore, the deep early barren quartz veins in porphyry Cu deposits may record the initial ore-forming fluids directly exsolved from the parent magma. The Gangdese porphyry copper belt (GPCB) in China is marked by abundant Cenozoic collision-related Cu–Mo deposits (Fig. 1c; Hou et al., 2015). However, the Xiongcun Cu–Au district in the belt is unique for its Jurassic age and genetic relation to the subduction of the Neo-Tethys (Tafti et al., 2009; Lang et al., 2014a). Three porphyry Cu–Au deposits (No. 1, No. 2, and No. 3) have been discovered in the Xiongcun district (Fig. 1d). In terms of ore-forming fluids, only Xu et al. (2009) investigated the hydrothermal evolution of the No. 1 deposit and the hydrothermal evolution and ore precipitation mechanisms of the No. 2 deposit have not been considered at all by previous studies. Importantly, some early barren quartz veins have been identified in the deep parts of the No. 2 deposit. These veins provide an opportunity to determine the nature of the initial ore-forming fluids in porphyry Cu deposits. In this contribution, we systematically present SEM-CL textures, microthermometric data, stable isotope compositions (H, O, He, Ar), and Raman data of the No. 2 deposit. A combination of these data yields new insights into the nature of the initial ore-forming fluids, the fluid evolution processes, and the ore precipitation mechanisms in this deposit.

Dong et al., 2010) has been found in its eastern segment, suggesting that a Precambrian crystalline basement is present locally. 2.2. GPCB GPCB is approximately 50 km wide and 400 km long, and situated at the southern margin of the Lhasa terrane (Fig. 1c; Hou et al., 2009). Significant Miocene collision-related porphyry Cu–Mo deposits have been discovered in GPCB, such as the Qulong, Jiama, Bangpu, Bairong, Tinggong, and Chongjiang deposits (Qu et al., 2001; Hou et al., 2003; Rui et al., 2003). These deposits are usually associated with Miocene intrusions with an adakite-like affinity (Hou et al., 2004; Hou and Cook, 2009). Mesozoic porphyry Cu–Au deposits are also found in this belt (e.g., in Xiongcun district; Tafti et al., 2009, 2014; Lang et al., 2014a). They are associated with Jurassic porphyries exhibiting geochemical characteristics indicative of an arc affinity, suggesting an origin related to Mesozoic Neo-Tethys subduction (Lang et al., 2014a; Tang et al., 2015). 2.3. Deposit geology The Xiongcun district, located in the western segment of GPCB (Fig. 1c), comprises three deposits, No. 1, No. 2, and No. 3 (Fig. 1d). Constraints from Re–Os isotopic dating of molybdenite indicate the occurrence of two individual mineralization events in the district, which are represented by the No. 1 (161.5 ± 2.7 Ma) and No. 2 deposits (172.6 ± 2.1 Ma) (Lang et al., 2014a). The No. 3 deposit is a newly discovered deposit. The strata exposed in the district belong to the Lower–Middle Jurassic Xiongcun Formation (Fig. 1d), which comprises volcanic breccia, tuff, lava, conglomerate, sandstone, siltstone, argillite, slate, and a lesser amount of limestone (Lang et al., 2019). The intrusive rocks in the district are of Jurassic and Eocene ages (Fig. 1d). The Jurassic intrusions include Early Jurassic quartz diorite porphyry (181–175 Ma; Lang et al., 2014a), Early–Middle Jurassic quartz diorite porphyry (174 Ma; Lang et al., 2014b), and Middle Jurassic quartz diorite porphyry (167–161 Ma; Lang et al., 2014a) and diabase dikes (165 Ma; Lang et al., 2018). The Eocene intrusions include biotite granodiorite, quartz diorite, granitic aplite dikes (Tang et al., 2012), and lamprophyre dikes (47 Ma; Lang et al., 2017). The main structures in the district are faults F1 and F2 (Fig. 1d), which occur along the hanging wall and footwall of the No. 1 deposit, respectively. Steep postmineralization structures are also present, trending NE, N, EW, and NW (Fig. 1d). The No. 2 deposit is mainly hosted in the Early Jurassic quartz diorite porphyry and surrounding contemporary tuff (Fig. 1e, 2; Tang et al., 2012, 2015; Lang et al., 2014a). It contains 1.34 Mt Cu, 76.34 t Au, and 193.78 t Ag, with average grades of 0.35%, 0.22 g/t, and 1.30 g/t, respectively. The tuff is bedded and has a fine-grained texture (Fig. 3a). It hosts only a small part of Cu–Au mineralization, which dominantly occurs in the Early Jurassic quartz diorite porphyry (Fig. 1e, 2). It is not possible to identify the texture and composition of the protolith of the most intensely altered Early Jurassic quartz diorite porphyry (Fig. 3b), but plagioclase and hornblende phenocrysts can be observed in the more weakly altered porphyry (Fig. 3c). A NW-trending intrusion (Early–Middle Jurassic quartz diorite porphyry) cuts the No. 2 deposit (Fig. 1e). This intrusion is distinguished from the Early Jurassic quartz diorite porphyry by having up to 15% round-to-subhedral quartz phenocrysts (Fig. 3d). It contains only minor, low-grade Cu–Au mineralization. Mineralization in the No. 2 deposit is dominated by hypogene sulfides with variable magnetite (Fig. 4). The sulfide minerals comprise pyrite, chalcopyrite, and minor molybdenite, as well as traces of sphalerite and galena. Surficial oxidation is weak and has only resulted in a partial replacement of hypogene chalcopyrite and pyrite with chalcocite and minor covellite (Fig. 4i).

2. Geological background 2.1. Tectonic framework The Tibetan plateau was produced by the Indo–Asian continental collision since the Paleocene (Klootwijk et al., 1992; DeCelles et al., 2004). It can be divided from south to north into the Himalayas, the Lhasa terrane, and the Qiangtang terrane along the Yarlung–Zangbo Suture Zone (YZSZ) and Banggong–Nujiang Suture Zone (BNSZ) (Fig. 1b; Yin and Harrison, 2000). The Lhasa terrane can be further divided into northern, central, and southern subterranes by the Shiquan River–Nam Tso Mélange Zone (SNMZ) and Luobadui–Milashan Fault (LMF) (Fig. 1b; Zhu et al., 2011). The discontinuously exposed Mesozoic ophiolitic melanges along SNMZ probably represent relics of an Early Cretaceous oceanic basin (Zhang et al., 2004; Zhu et al., 2011). An eclogite- and blueschist-facies Triassic metamorphic belt along the eastern LMF has been interpreted to be a Paleo-Tethyan suture zone that separates the central from the southern Lhasa subterrane (Liu et al., 2010). The southern Lhasa subterrane is characterized by a juvenile crust (Hou et al., 2015), but the Early Paleozoic granite (ca. 496 Ma; 3

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3. Alteration zones and quartz-dominated vein types

V1 veins are rare and observed only in deep drill holes (Fig. 2). No obvious crosscutting relations are observed between V1 veins and other veins. They have irregular, wavy margins (Fig. 5g), and their thicknesses generally range from < 1 cm to 3 cm. They are dominated by equigranular quartz grains and are devoid of sulfides. V2 veins are the most abundant vein type in this deposit, especially in the potassic alteration zone in the central orebody (Fig. 2), and host a bulk of Cu–Au mineralization. V2 veins have irregular, wavy margins similar to V1 veins (Fig. 5h), and their thicknesses range from ~ 0.5 cm to 3 cm. They are usually cut by later veins with straight walls, such as V3 veins (Fig. 5i, j). The V2 veins are quartz-dominated, with pyrite and chalcopyrite as the dominant sulfides and with variable amounts of localized magnetite. Pyrite and chalcopyrite crystals (0.1–1 mm) generally occur along the centerlines of V2 veins (Fig. 5h). Minor anhydrite and biotite are also observed in V2 veins. V3 veins are rarer than V2 veins and generally occur in the central orebody (Fig. 2). They range from a few millimeters to 3 cm in thickness and tend to have straight walls (Fig. 5i, j). The crosscutting relations clearly indicate that they postdate V2 veins and predate V4 veins (Fig. 5i–l). Molybdenite crystals in the veins are generally thin and platy, and pyrite and chalcopyrite are occasionally observed. V4 veins are the youngest generation of veins observed in the deposit. Vein truncation relations distinctly indicate that they postdate epidote veins (EV) (Fig. 5l), which crosscut V3 veins (Fig. 5k), generally range in thickness from ~ 1 mm to 2 cm, and are composed of epidote with minor calcite and sericite. V4 veins range from ~ 0.5 cm to several centimeters

Field investigation and petrographic and mineralogical observations have enabled the delineation of various alteration zones in the No. 2 deposit, where the hydrothermal alteration is potassic, sodic-calcic, chloritesericite, phyllic, and propylitic (Fig. 2). (1) Potassic alteration is dominated by biotite and variable concentrations of magnetite (the magnetite content increases from shallow to deep levels in the deposit) (Fig. 5a, b), with a locally minor K-feldspar. It contains high-grade Cu–Au mineralization. (2) Sodic-calcic alteration is characterized by albite and actinolite and minor magnetite, chlorite, and tourmaline. This alteration is weakly mineralized and overprints the pre-existing mineralized potassic alteration (Fig. 5c). (3) Chlorite-sericite alteration contains chlorite, sericite, and minor clay and epidote (Fig. 5d). Because it overprints the older, well-mineralized potassic alteration, it also contains high concentrations of copper and gold. (4) Phyllic alteration is composed mainly of quartz, sericite, and pyrite, and minor chlorite and epidote (Fig. 5e) and contains low-grade Cu–Au mineralization. (5) Propylitic alteration comprises chlorite, epidote, and carbonate, and minor sericite and pyrite (Fig. 5f). This alteration is barren of Cu–Au mineralization. Four stages of quartz-dominated veins are clearly identified in the No. 2 deposit on the basis of their crosscutting relations and mineralogy. From early to late, these are early barren quartz veins (V1), quartz–pyrite–chalcopyrite ± magnetite veins (V2), quartz–molybdenite ± pyrite ± chalcopyrite veins (V3), and late quartz veins (V4). Their characteristics are described as follows.

Fig. 2. North–south geologic cross-section (A–B is shown in Fig. 1e) of the No. 2 deposit in the Xiongcun district, modified from Yin et al. (2017). Drill holes 7223, 7229, and 7238 have not defined the footwall of the orebody, which is also open down-dip to the north. Circles, triangles, and squares represent locations of samples used for fluid inclusion petrographic, H–O, and He–Ar isotope analyses. 4

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Fig. 3. Photographs showing rock types from the No. 2 deposit in the Xiongcun district. (a) Bedded tuff; (b) Early Jurassic quartz diorite porphyry, with intense potassic alteration obscuring the protolith texture and composition; (c) Early Jurassic quartz diorite porphyry with weak hydrothermal alteration and observable plagioclase and hornblende phenocrysts; (d) Early–Middle Jurassic quartz diorite porphyry in which abundant subhedral quartz phenocrysts can be observed. Abbreviations: Qtz = quartz, Bi = biotite, Pl = plagioclase, Hbl = hornblende.

Fig. 4. Ore mineral assemblages from the No. 2 deposit in the Xiongcun district. (a) Magnetite is replaced by pyrite and chalcopyrite in the potassic alteration zone; (b) chalcopyrite coexisting with pyrite in quartz–sulfide veins; (c) chalcopyrite in quartz–sulfide veins; (d) magnetite coexisting with pyrite and chalcopyrite in quartz–sulfide veins; (e) chalcopyrite coexisting with pyrite in the chlorite-sericite alteration zone; (f) molybdenite coexisting with minor chalcopyrite; (g) coexisting pyrite, chalcopyrite, and sphalerite; (h) irregular anhedral galena in the gangue minerals; (i) chalcopyrite replaced by covellite in the near-surface part of the deposit. Abbreviations: Mag = magnetite, Ccp = chalcopyrite, Py = pyrite, Mol = molybdenite, Sp = sphalerite, Gn = galena, Cv = covellite.

in thickness and have straight walls (Fig. 5l). They are dominated by quartz (over 95 vol%), and minor calcite and epidote are also observed within them. Occasional pyrite is the only sulfide observed in V4 veins.

Approximately 60 samples were collected for petrographic observation of fluid inclusions, of which 17 representative samples (Table 1) were examined for inclusion abundance and further analyzed by microthermometry, SEM-CL analysis, and laser Raman spectroscopy. In addition, 11 samples were collected for oxygen and hydrogen isotope analyses, and 5 samples were collected for helium and argon isotope analyses. In this contribution, the sample-naming principle is as follows: using the example of sample 7238–726, the sample was collected from drill hole No. 7238 and from a depth of 726 m below the surface.

4. Sampling and analytical methods 4.1. Sampling All samples used in this study are from quartz-dominated veins and collected from six drill holes along the cross-section A–B (Fig. 2). 5

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Fig. 5. Photographs showing various alteration and vein types from the No. 2 deposit in the Xiongcun district. (a) Potassic alteration is dominated by biotite at shallow levels in the deposit; (b) potassic alteration is dominated by biotite and magnetite at deep levels of the deposit; (c) sodic-calcic alteration overprints older potassic alteration; (d) chlorite-sericite alteration within which chlorite and sericite can both be observed; (e) phyllic alteration composed mainly of quartz, sericite, and pyrite, and minor chlorite and epidote; (f) propylitic alteration comprises chlorite, epidote, and carbonate, and minor sericite and pyrite; (g) V1 veins are dominated by equigranular quartz grains and are devoid of sulfides; (h) V2 veins contain pyrite and chalcopyrite crystals that generally occur along the centerline; (i–j) V3 veins crosscutting V2 veins; (k) EV veins crosscutting V3 veins; (l) V4 veins crosscutting EV veins. Abbreviations: Bi = biotite, Qtz = quartz, Ab = albite, Act = actinolite, Chl = chlorite, Ser = sericite, Ccp = chalcopyrite, Py = pyrite, Ep = epidote, Mol = Molybdenite.

4.2. SEM-CL

recorded as grayscale images, and therefore, the luminescence intensity can only qualitatively be referred to as CL-bright, CL-gray, and CL-dark.

SEM-CL petrography was conducted at Nanjing Hongchuang Exploration Technology Service Co. Ltd. to characterize the quartz generations and their textural correlations, determine the sulfide distribution, and identify the fluid inclusion assemblages. SEM-CL images were obtained using a TESCAN MIRA3 instrument equipped with a motor-driven elliptical mirror that focused the CL signal onto a standard photomultiplier detector. SEM-CL images were taken at an acceleration voltage of 15 kV and a beam current density of 10–15 nA/mm using an EDAX Phoenix digital acquisition system. CL signals were

4.3. Petrography and microthermometry The petrographic studies and inclusion analyses strictly concentrated on the fluid inclusion assemblages (Goldstein and Reynolds, 1994). A vast majority of the inclusions analyzed in this study occurred in groups or clusters of inclusions with similar liquid-to-vapor ratios in a single grain or in several adjacent grains of quartz. Where groups or clusters of inclusions exhibited similar heating and freezing behaviors, 6

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Average Cu and Au grade for the 2 m interval in which the sample lies; 2 Relative abundance of fluid inclusions in V2 veins refers to the relative abundance of fluid inclusions in Q2–3 quartz of V2 veins, because the relative abundance of fluid inclusions in Q1 quartz of V2 veins is similar to V1 veins. Mineral abbreviations are as follows: Qtz = quartz, Ccp = chalcopyrite, Py = pyrite, Anh = anhydrite; Mag = magnetite, Cal = calcite, Mol = molybdenite, Ab = albite, Act = actinolite, Bt = biotite, Kfs = K-feldspar, Ser = sericite, Chl = chlorite, Ep = epidote.

Anh, Py, Ccp Mol, Py, Ccp Mol, Py, Ccp, Chl Py Cal Cal Anh, Py, Ccp Mol Anh, Py, Ccp Anh, Py, Ccp, Mag Ep, Chl, Cal Anh, Py, Ccp Anh, Py, Ccp Py, Cal

it can be inferred that they trapped similar fluids under similar conditions. Where possible, inclusion types were linked to specific quartz generations based on the transmitted light and SEM-CL observations. Fluid inclusion microthermometry was conducted using a Linkam THMSG600 Heating–Freezing stage combined with a Nikon microscope at the MNR Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation at the Chengdu University of Technology. The thermocouples were calibrated by measuring the melting temperatures of pure water (0.0 °C) and pure CO2 (−56.6 °C) using synthetic fluid inclusions. The measurement accuracy of phase transitions in fluid inclusions was ± 0.2 °C for temperatures below 0 °C, ± 0.5 °C between 0 °C and 100 °C, and ± 2 °C between 100 °C and 600 °C. Ice melting temperatures were determined using a heating rate of no more than 0.1 °C/s. Homogenization temperatures (Th) were assessed at a heating rate of ≤1 °C/s. Testing of the homogenization of halitebearing inclusions was conducted on only one assemblage of halitebearing inclusions per sample chip to avoid analyzing inclusions that had become stretched due to previous heating. The salinities of aqueous inclusions were calculated using the final melting temperatures of ice (Bodnar, 1993). The salinities of CO2-bearing fluid inclusions were calculated using the melting temperatures of clathrate (Collins, 1979). The salinities of halite- or sylvite-bearing fluid inclusion were calculated using the dissolution temperatures of daughter minerals (Hall et al., 1988). 4.4. Laser Raman spectroscopy Laser Raman spectroscopy analyses were performed at the State Key Laboratory for Mineral Deposit Research, Nanjing University. The vapor and liquid compositions and daughter phases in single-fluid inclusions were analyzed with a Renishaw RM2000 Raman microprobe using an Ar-ion laser with a surface power of 5 mW to excite the radiation (514.5 nm). The area of the charge-coupled device detector was 20 μm2, and the spectral scanning range was set between 1000 and 4000 cm−1 for vapor and liquid compositions and 100 cm−1 and 1600 cm−1 for daughter phases, with an accumulation time of 30 s for each scan and a spectral resolution of ± 2 cm−1. 4.5. He and Ar isotope analysis Five samples containing pyrite and quartz gathered from veins were selected for helium and argon isotope analyses, respectively. He and Ar isotopic analysis was conducted using the one-step crushing technique at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology (BRIUG). The crusher was made of 316 l stainless steel, and the noble gas isotope analyses were performed with a Noblesse mass spectrometer in the static mode. The details of the crushing and analytical procedures have been provided by He et al. (2011). Helium blanks were negligible (3He blank < 3 × 10−17 cm3 STP) and Ar blanks were small, about 0.1% relative to the signals. The air standard, which has a 3He/4He ratio of 1.39 × 10−6 (Ra) and a 40 Ar/36Ar ratio of 298.5, was measured every 2 weeks. The uncertainty of the average 3He/4He ratio in calibration measurements was better than 5%, so a 5% error was assigned to the calculated 3He/4He ratio and included in the unknown sample correction. The results for the samples were normalized to the value of the air standard and corrected for system blanks. 4.6. H and O isotope analysis Quartz grains were extracted from crushed and washed fragments of the samples and purified by hand-picking under a binocular microscope. The oxygen and hydrogen isotopic compositions of the quartz were determined using a Finnigan-MAT 253 mass spectrometer at the Analytical Laboratory, BRIUG. The analytical method has been described by Zhu et al. (2010). Oxygen was liberated from quartz through

1

2 3 2 7 49 52 100 99 100 10 49 27 17 99 11 21 100 0 0 0 9 0 0 0 0 0 13 0 7 5 0 6 3 0 0 0 0 8 45 40 0 1 0 9 46 42 41 1 37 39 0 1 2 0 45 3 2 0 0 0 34 0 11 21 0 32 23 0 0 0 0 19 1 3 0 0 0 27 3 5 12 0 9 7 0 0.39 0.34 0.59 0.84 0.28 0.48 0.23 0.32 0.32 0.83 0.28 0.18 0.49 0.03 0.59 0.55 0.21 10 15 22 15 12 22 15 10 15 8 6 20 22 45 10 11 19 726 605.5 586 600 470.1 362.1 164 319 101 596 462.2 363 229 465 254 235 105 7238-726 7238-605.5 7238-586 7238-600 7238-470.1 7238-362.1 7238-164 7231-319 7231-101 7229-596 7229-462.2 7229-363 7229-229 7226-465 7223-254 7223-235 7253-105

V1/potassic V1/potassic V1/potassic V2/potassic V3/chlorite-sericite V3/chlorite-sericite V4/potassic V4/sodic-calcic V4/potassic V2/potassic V3/potassic V2/chlorite-sericite V2/potassic V4/sodic-calcic V2/chlorite-sericite V2/chlorite-sericite V4/sodic-calcic

Qtz Qtz, Qtz Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz,

Ccp

Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz, Qtz,

Bt, Kfs, Mag Bt, Kfs, Mag Bt, Kfs, Mag Bt, Kfs, Mag Chl, Ep, Ser Chl, Ep, Ser Mag, Bt, Kfs Ab, Act, Mag, Ser Mag, Bt, Kfs Bt, Kfs, Mag Bt, Kfs, Mag Chl, Ep, Ser Mag, Bt, Kfs Ab, Act, Mag, Ser Chl, Ep, Ser Chl, Ep, Ser Ab, Act, Mag, Ser

0.16 0.18 0.35 0.57 0.12 0.42 0.12 0.16 0.14 0.65 0.18 0.09 0.41 0.02 0.26 0.59 0.11

97 95 98 12 2 3 0 0 0 7 2 8 4 0 5 7 0

B80 B40H B40

Fluid inclusion types (%) Au grade1 (ppm) Cu grade1 (wt%) Alteration minerals Vein minerals Vein width (mm) Vein/Alteration Depth below surface (m) Sample no.

Table 1 Deposit-scale distribution of fluid inclusion types showing the relative abundance of fluid inclusions in 17 selected samples from the No. 2 deposit in the Xiongcun district.

B15H

B15H+

B15

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reaction with BrF5 (Clayton and Mayeda, 1963) and converted to CO2 on a platinum-coated carbon rod. The hydrogen isotope ratios of bulk fluid inclusions in quartz were measured by mechanical crushing of about 5 g of quartz grains to 1–5 mm in size according to Simon’s (2001) method·H2O was collected from quartz decrepitation under a vacuum at 550 °C, and then reduced by Zn at 400 °C to obtain H2. All obtained data were normalized to V-SMOW standards with analytical precisions of more than ± 0.2‰ and ± 1‰ for δ18Oquartz and δDfluid, respectively.

et al., 2004; Oyu Tolgoi, Mongolia: Müller et al., 2010; Altar, Argentina: Maydagán et al., 2015). Where Cu–Fe sulfides (pyrite and chalcopyrite) are present in V2 veins, they are intergrown with the later generation of CL-gray Q2 quartz grains (Fig. 7a, b), suggesting that Cu–Fe sulfides precipitate contemporaneously with Q2 quartz. The next quartz generation, Q3, is volumetrically minor and observed in microfractures crosscutting the earlier quartz generations. Q3 quartz has low CL intensity and is systematically intergrown with Cu–Fe sulfides (Fig. 7c). The Q2 and Q3 quartz generations are both intergrown with Cu–Fe sulfides in V2 veins and have the same inclusion types (see below); therefore, they are no longer distinguished in the following and are collectively referred to as Q2–3 quartz. V3 veins also contain three different generations of quartz. The characteristics of the earliest, roughly equigranular CL-bright quartz (Q4) are consistent with those of Q1 quartz observed in V1 and V2 veins except that the grains of Q4 quartz have larger diameters (Fig. 8b). Q5 quartz, which crystallize after Q4 quartz, exhibits subhedral comb-like textures with well-developed oscillatory growth zoning (Fig. 8b). Q4 and Q5 quartz have similar CL intensities, and their growth sequence indicates that they are formed during successive hydrothermal episodes. In addition, CL-dark quartz (Q6) can be observed in the cores of V3 veins. These grains also exhibit oscillatory growth zoning (Fig. 8b). Molybdenite grains can be intergrown with any of the quartz generations in V3 veins (Fig. 8b), suggesting that molybdenite precipitation accompanies the formation of V3 veins. Characteristically, V4 veins have significantly lower CL intensity than V1, V2, and V3 veins. Two distinct quartz generations can be observed in V4 veins in SEM-CL images. Q7 quartz forms euhedral to subhedral crystals, which exhibit strong oscillatory zoning and a CLgray color (Fig. 8c). Microscopic fractures are well-developed in V4 veins, and filled by later Q8 quartz characterized by a significantly lower CL intensity (Fig. 8c).

5. Results 5.1. Quartz generations More than 150 SEM-CL images are collected from the 17 quartzdominated veins (V1, V2, V3, and V4) mentioned above, to clarify the sequence of quartz crystallization. Eight quartz generations (Q1–Q8) are identified within these veins through examination of SEM-CL textures. V1 veins are dominated by mosaics of roughly equigranular CLbright quartz (Q1, Fig. 6), usually with a homogeneous texture, i.e., lacking growth zonation or other internal variations in CL intensity. The CL intensity of these quartz grains is higher than that of quartz in any of the other vein types. We do not observe any instance of Q1 quartz crosscutting the earlier quartz, indicating that it is the earliest quartz generation. Microfractures filled with later CL-gray quartz (Q2) are occasionally seen in V1 veins. These anastomosing microfractures are typically < 20 μm in thickness (Fig. 6). V2 veins contain three distinct generations of quartz (Q1, Q2, and Q3; Fig. 7). The earliest, roughly equigranular CL-bright Q1 quartz shows characteristics consistent with the Q1 quartz observed in V1 veins and accounts for > 70 vol% of the V2 veins. In some V2 veins, however, slightly lower CL intensity quartz (Q2) occurs at many CLbright quartz (Q1) grain boundaries (Fig. 7c), which is not observed in V1 veins. In addition, abundant patches of irregular CL-gray Q2 quartz are widely distributed in CL-bright Q1 quartz (Fig. 7a, b); these have larger diameters than the Q1 quartz grains. The occurrence of CL-gray Q2 quartz as Q1 quartz overgrowths or as discontinuous patches in Q1 quartz indicates that the CL-bright Q1 quartz forms first and is later dissolved by quartz-undersaturated fluid, leaving a high-porosity quartz framework, which is later infilled by CL-gray Q2 quartz. Similar SEMCL textures have been observed in other porphyry Cu deposits (Butte, Montana: Rusk and Reed, 2002; Bingham Canyon, Utah: Redmond

5.2. Fluid inclusion types On the basis of the phases present at room temperature (25 °C–32 °C), six types of fluid inclusions are distinguished in the different sets of quartz-dominated veins using the terminology by Rusk et al. (2008): namely B40, B40H, B80, B15H, B15H+, and B15. In this classification system, the letter “B” means that a vapor bubble is present, and the number indicates the average volume percent occupied by the vapor bubble in inclusions of that type. The letter “H” refers to the presence of halite as a transparent daughter mineral, and “H+”

Fig. 6. SEM-CL image of quartz from V1 veins. 8

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Fig. 7. SEM-CL images of quartz from V2 veins.

indicates the presence of other transparent daughter mineral (sylvite) besides halite. The petrographic features of each type of inclusion are as follows. B40 inclusions contain liquid and a bright vapor bubble that occupies 30–60 vol% of the inclusion and lack halite daughter minerals at room temperature (Fig. 9b, c). They range in size from < 10 μm to 20 μm and typically take the shape of negative quartz crystals or rounded rectangles (Fig. 9b). Approximately 15% of B40 inclusions contain an opaque daughter mineral, most commonly a mineral that is triangular in cross-section and identified by laser Raman spectrometry as chalcopyrite. Most B40 inclusions are grouped in clusters in individual quartz grains, and only rarely identified as primary along euhedral growth zones of quartz or as secondary or pseudosecondary along the healed fractures. B40H inclusions contain liquid plus dark vapor bubbles (30–60 vol %) plus halite at room temperature (Fig. 9e, f). They are 8–20 μm in diameter, and their shapes vary from rounded to elongate and are irregular (Fig. 9e, f). Approximately 50% of B40H inclusions contain an opaque daughter mineral, which is most likely to be hematite (red, platy) or chalcopyrite (opaque, triangular) (Fig. 9e, f). Like B40 inclusions, B40H inclusions are clustered and variably scattered and occur

less commonly as secondary or pseudosecondary trails. B80 inclusions contain vapor and liquid phases at room temperature. They have bright vapor bubbles (> 70 vol%), show rounded or negative crystal shapes, and generally vary in size from 5 to 20 μm (Fig. 9g, h, o). Approximately 30% of B80 inclusions contain an opaque daughter mineral; laser Raman spectrometry confirms that these are chalcopyrite (Fig. 10d). B80 inclusions are typically densely scattered or clustered within quartz grains. At room temperature, B15H inclusions contain liquid plus 5–25 vol % vapor plus halite, which is the only transparent daughter mineral (Fig. 9i, j). They range in size from 5 to 30 μm, and their shapes are irregular and vary from rounded to elongate (Fig. 9i, j). In addition, red hematite can be observed in B15H inclusions (Fig. 9j). These inclusions are, in general, variably scattered in quartz grains. B15H+ inclusions are similar to B15H fluid inclusions but contain other transparent daughter minerals besides halite. B15H+ inclusions can have up to four daughter minerals and can be identified as halite, sylvite, chalcopyrite, and hematite based on the crystal habit and color (Fig. 9k). They are between 10 and 25 μm in diameter and have irregular, regular, or negative crystal shapes. B15 inclusions contain liquid and 5–25 vol% vapor at room 9

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Fig. 8. (a) Photograph showing a V3 vein crosscutting a V2 vein; (b) SEM-CL image showing a V2 vein cut by a V3 vein; (c) SEM-CL image of quartz from V4 veins.

temperature. They are 5–30 μm in diameter, and their shapes vary among equant, rounded, elongate, and irregular (Fig. 9l, q). No daughter mineral is observed in B15 inclusions. The inclusions are scattered as primary individuals or distributed along microfractures as trails of secondary inclusions.

quartz generations. B40H and B80 inclusions are mainly observed in Q2–3 quartz in V2 veins. In deep V2 veins (about 550 m below the present surface), B40H inclusions represent 15% to 25% of the inclusion population in Q2–3 quartz (Fig. 11). At 200 m below the present surface, this number decreases to 10%. B80 inclusions are also abundant in Q2–3 quartz in V2 veins, especially in deep V2 veins, where they represent approximately 40% of the inclusion population in that quartz type. This number decreases to 25% by 200 m below the present surface (Fig. 11). In addition, immiscible inclusion assemblages dominated by B80 and B40 inclusions can be identified in Q2–3 quartz in V2 veins (Fig. 9m). B15H+ inclusions are only present in Q2–3 quartz in V2 veins. They account for 10% of the inclusion population in Q2–3 quartz in deep V2 veins and 5% in shallow V2 veins (Fig. 11). Fluid inclusion assemblages comprising B80, B15H+, and B40H inclusions are widely identified in Q2–3 quartz in V2 veins (Fig. 9n). B15H inclusions are abundant in V2 veins from shallow depths (about 220 m below the present surface) and in V3 veins (Fig. 11). In deep V2 veins, fewer B15H inclusions are present than B40H inclusions; they represent 10% of the inclusion population in Q2–3 quartz, where

5.3. Deposit-scale distribution of fluid inclusions The distribution of the different inclusion types on the deposit-scale is studied to delineate the temporal and spatial relations among inclusions, veins, and ore zones. The relative abundance of each inclusion type is visually estimated in representative samples from different depths of the deposit (Table 1). A general pattern of fluid inclusion distribution relative to ore zones and vein types can be inferred from Table 1 and is illustrated in Fig. 11. B40 inclusions are abundant in V1 and V2 veins from the deepest samples (about 550–730 m below the present surface) (Fig. 11). They constitute more than 95% of all inclusions in CL-bright Q1 quartz in V1 and V2 veins. A few B40 fluid inclusions can also be observed in Q2–3 quartz in V2 veins, but no B40 fluid inclusions are observed in other 10

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Fig. 9. (a) SEM-CL image showing the relation between Cu–Fe sulfides and quartz in V2 veins; (b) a B40 inclusion containing 60 vol% vapor and a triangular opaque daughter mineral, likely chalcopyrite; (c) a B40 inclusion containing 35 vol% vapor; (d) a B40 inclusion containing 50 vol% liquid CO2 + vapor CO2; (e–f) B40H inclusions containing 35–60 vol% vapor, an opaque daughter mineral, and a transparent halite daughter; (g) a B80 inclusion containing 80 vol% vapor; (h) a B80 inclusion containing 95 vol% vapor and a triangular opaque daughter mineral; (i) a B15H inclusion containing 10 vol% vapor and a transparent halite daughter; (j) a B15H inclusion containing 20 vol% vapor, an opaque daughter mineral, and a transparent halite daughter; (k) a B15H+ inclusion containing 10 vol% vapor plus three transparent daughter minerals; (l) B15 inclusions containing 10 vol% vapor; (m) B80 and B40 inclusion assemblages; (n–o) B80, B40H, and B15H+ inclusion assemblages; (p) SEM-CL image showing primary CL bands in quartz in V3 veins; (q) B15 and B15H inclusion assemblages in a V3 vein.

the halite-bearing inclusions are dominated by B40H inclusions. In shallow V2 veins, however, B15H inclusions are the main type of halite daughter mineral-bearing inclusion, accounting for 35% of the inclusion population in Q2–3 quartz. B15H inclusions are more common in V3 veins, accounting for 45% of the inclusion population. No B15H inclusions are observed in V1 and V4 veins (Table 1). B15 inclusions occur in all vein types from all depths in the deposit but do not dominate in V1 and V2. They are most abundant in V3 and V4 veins (Fig. 9q). In V4 veins, B15 inclusions are the only inclusion type identified and occur both as primary inclusions in Q7 quartz and secondary inclusions in Q8 quartz. In V3 veins, they account for more

than 50% of the inclusion population, but this number decreases to less than 20% in V2 veins and < 5% in V1 veins (Table 1). 5.4. Microthermometry of fluid inclusions 5.4.1. V1 veins B40 inclusions are the only type in V1 veins used for microthermometric study. All B40 inclusions form clathrate and “double bubbles” (liquid CO2 + vapor CO2) upon cooling (Fig. 9d, Supplementary Fig. 1). These inclusions yield solid CO2 melting temperatures ranging from −57.1 °C to −56.6 °C, indicating that CO2 is the 11

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Fig. 10. Raman spectra of fluid inclusions of the No. 2 deposit in the Xiongcun district. Abbreviations: Qtz = quartz, Ccp = chalcopyrite.

dominant gas, which is also confirmed by the Raman spectrum (Fig. 10a). The clathrate melting temperatures range from 2.7 °C to 8.9 °C, corresponding to salinities of 2.2–12.3 wt% NaCl equiv (Fig. 12b). All B40 inclusions homogenize to liquid or vapor between 278 °C and 389 °C (peaking at 310 °C–330 °C) (Fig. 12a), with densities ranging from 0.60 to 0.93 g/cm3 (Table 2).

at 337 °C–444 °C, with a peak at 380 °C–400 °C (Fig. 12c). However, the melting temperatures of ice or clathrate in B80 inclusions are difficult to detect because of the large bubble size, and values are only obtained for a few inclusions. The formation of clathrate upon cooling in some B80 inclusions indicates the presence of CO2, and in fact, laser Raman analysis indicates the presence of CO2 in most B80 inclusions analyzed (Fig. 10b). Thus, the vapor bubbles in B80 inclusions are dominated by CO2 rather than H2O. The salinities of B80 inclusions are estimated to be 1.8–3.9 wt% NaCl equiv (Fig. 12d), with densities ranging from 0.36 to 0.54 g/cm3 (Table 2). For the B15H inclusions, most of which are homogenized by liquid–vapor (L–V) homogenization after halite dissolution (Fig. 13), L–V homogenization occurs between 256 °C and 348 °C (peaking at 270 °C–290 °C) and halite dissolution temperatures range from 117 °C to 292 °C, reflecting salinities of 28.5–37.5 wt% NaCl equiv (Fig. 12c, d) with densities ranging from 0.98 to 1.27 g/cm3 (Table 2). For B15H+ inclusions, which homogenize through halite dissolution above the temperature of L–V homogenization, L–V homogenization occurs between 239 °C and 413 °C (peaking at 330 °C–360 °C) and the halite and sylvite dissolution temperatures are 383 °C–528 °C and 127 °C–215 °C, respectively. Halite dissolution occurs at temperatures up to 212 °C higher than bubble homogenization in some other groups of inclusions. These dissolution temperatures correspond to a salinity of 56.9–69.7 wt % NaCl equiv. Approximately 50% of the B40H inclusions homogenize by halite dissolution above the temperature of L–V homogenization, whereas the rest homogenize by L–V homogenization after halite dissolution (Fig. 13). L–V homogenization occurs between 307 °C and 514 °C (peaking at 380 °C–410 °C) and halite dissolution temperatures range from 238 °C to 511 °C, reflecting salinities of 34.0–52.4 wt% NaCl equiv (Fig. 12c, d) with densities ranging from 0.97 to 1.21 g/cm3 (Table 2).

5.4.2. V2 veins All fluid inclusion types (B40, B80, B15H, B15H+, B40H, and B15) in V2 veins are used for microthermometric study. The microthermometric results for B40 inclusions in V2 veins are similar to those for B40 fluid inclusions in V1 veins. The solid CO2 melting temperatures of B40 inclusions in Q1 and Q2–3 quartz in V2 veins range from −57.0 °C to −56.6 °C and −57.1 °C to −56.7 °C, respectively, indicating that CO2 is the dominant gas in the vapor bubbles. The clathrate melting temperatures range from 2.3 °C to 9.0 °C and 6.2 °C to 8.5 °C, respectively, corresponding to respective salinities of 2.0–12.8 and 3.0–7.0 wt% NaCl equiv (Fig. 12d). The B40 inclusions in Q1 and Q2–3 quartz in V2 veins have Th values ranging from 283 °C to 354 °C (concentrating at 310 °C–330 °C) and 273 °C to 332 °C (concentrating at 300 °C–310 °C) (Fig. 12c) and densities ranging from 0.59 to 0.95 g/cm3 and 0.60 to 0.83 g/cm3, respectively (Table 2). Microthermometric results are also obtained for two immiscible inclusion assemblages (IFA-1 and IFA-2) trapped in Q2–3 quartz and dominated by B40 and B80 inclusions (Table 2). They display melting of solid CO2 at temperatures between −56.9 °C and −56.6 °C. Clathrate melting temperatures range from 5.8 °C to 8.3 °C, corresponding to salinities of 3.3–7.7 wt% NaCl equiv (Fig. 12d). They homogenized to liquid or vapor between 323 °C and 348 °C (peaking at 325 °C–340 °C) (Fig. 12c), with densities ranging from 0.58 to 0.84 g/cm3 (Table 2). The B80 inclusions in Q2–3 quartz in V2 veins homogenize to vapor 12

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Fig. 11. Deposit-scale distribution of fluid inclusions of the No. 2 deposit in the Xiongcun district. The lower limit of brine inclusions is indicated by the thick black line. Below the line, B15H, B15H+, B40H, and B80 fluid inclusions are absent, and B40 fluid inclusions dominate all samples. The distribution of fluid inclusions is represented by schematic diagrams of the fluid inclusion types.

No CO2 is detected in B40H inclusions (Fig. 10e), which intergrow closely with B15H + and B80 inclusions and have similar homogenization temperatures (Figs. 12c and 13), indicating that they result from entrapment of heterogeneous fluids after the development of fluid immiscibility. Thus, they cannot provide a meaningful estimate for P–T conditions of inclusion trapping (Audétat and Günther, 1999). The B15 inclusions homogenize to liquid at temperatures between 226 °C and 393 °C, with a peak at 260 °C–290 °C (Fig. 12c). Their ice melting temperatures, which are between −17.0 °C and −5.9 °C, reflect salinities of 9.1–20.2 wt% NaCl equiv (Table 2; Fig. 12d) with densities ranging from 0.79 to 1.01 g/cm3 (Table 2). No CO2 is detected in B15 and B15H inclusions.

(Table 2). No CO2 is detected in B15 and B15H inclusions (Fig. 10c). 5.4.4. V4 veins B15 inclusions are the only type recognized in V4 veins and homogenized to liquid at temperatures between 166 °C and 229 °C, with a peak at 170 °C–200 °C (Fig. 12g). Their ice melting temperatures are between −13.2 °C and −3.1 °C, reflecting salinities of 5.1–17.1 wt% NaCl equiv (Fig. 12h) with densities ranging from 0.92 g/cm3 to 1.01 g/ cm3 (Table 2). No CO2 is detected in B15 inclusions (Fig. 10f). Microthermometry is not performed on the secondary B15 inclusions in Q8 quartz. 5.5. Pressure estimation

5.4.3. V3 veins Microthermometric analysis for V3 veins is conducted on B15H and B15 inclusions. Almost all B15H inclusions homogenize by L–V homogenization after halite dissolution (Fig. 13). L–V homogenization occurs between 171 °C and 320 °C (peaking at 240 °C–260 °C) and halite dissolution temperatures range from 124 °C to 236 °C, reflecting salinities of 28.7–33.9 wt% NaCl equiv (Fig. 12e, f) with densities ranging from 1.05 to 1.23 g/cm3 (Table 2). The B15 inclusions homogenize to liquid at temperatures between 173 °C and 317 °C, with a peak at 210 °C–230 °C (Fig. 12e). Their ice melting temperatures are between −21.1 °C and −3.4 °C, reflecting salinities of 5.6–23.1 wt% NaCl equiv (Fig. 12f) with densities ranging from 0.81 g/cm3 to 1.06 g/cm3

We estimate the trapping pressure of CO2-bearing fluid inclusions on the basis of their Th values, homogenization behaviors, and proportion of CO2 phase within them by using the Flincor program (Brown, 1989) and the formula by Bowers and Helgeson (1983) for the H2O–CO2–NaCl system. The estimated trapping pressures of inclusions range from 90 MPa to 180 MPa (average = 140 MPa) in V1 veins, 85 MPa to 160 MPa (average = 130 MPa) in Q1 quartz in V2 veins, and 50 MPa to 110 MPa (average = 80 MPa) in Q2–3 quartz in V2 veins. No pressure estimation is obtained for V3 and V4 veins due to the shortage of CO2-bearing fluid inclusions within them. Accurate estimates of the trapping pressure are required to 13

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Fig. 12. Histograms of Th and salinity for all inclusion types of the No. 2 deposit in the Xiongcun district.

determine the exact trapping temperature and whether fluid inclusions are trapped during phase separation (Roedder and Bodnar, 1980; Brown and Hagemann, 1995). The B40 and B80 inclusions from Q2–3 quartz in V2 veins with variable CO2 content occur as primary clusters (Fig. 9m), and thus, are interpreted as having formed through contemporaneous trapping. They exhibit consistent Th values (Table 2; Fig. 12c), indicating that phase separation probably occurs in a singlephase fluid. Furthermore, the B15H, B40H, and B15H+ inclusions are closely spatially associated with the B80 inclusions in Q2–3 quartz in

V2 veins (Fig. 9n), further supporting the view that fluid immiscibility occurs in Q2–3 quartz in V2 veins. Thus, the Th values of immiscible inclusion assemblages IFA-1 and IFA-2 (Table 2) are inferred to be close to the trapping temperature, and we can estimate their exact trapping pressures. However, only the minimum trapping pressure for the V1 veins and Q1 quartz in V2 veins can be estimated, due to a lack of evidence for fluid boiling or fluid immiscibility. The estimated trapping pressures for V1 veins (90–180 MPa) and Q1 quartz in V2 veins (85–160 MPa) are thus the minimum trapping pressures, while the 14

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0.60–0.93 0.59–0.95 0.60–0.83 0.58–0.84 0.63–0.84 0.79–1.01 0.98–1.27 – 0.97–1.21 0.36–0.54 0.81–1.06 1.05–1.23 0.92–1.01 2.2–12.3 2.0–12.8 3.0–7.0 4.1–7.0 3.3–7.7 9.1–20.2 28.5–37.5 56.9–69.7 34.0–52.4 1.8–3.9 5.6–23.1 28.7–33.9 5.1–17.1 124–236 (NaCl)

The measured He and Ar isotopic compositions of fluid inclusions in four pyrite samples from V2 veins and one quartz sample from a V4 vein are listed in Table 3. The 4He and 40Ar concentrations of the four pyrite samples are 7.27–31.40 × 10−8 cm3 STP g−1 and 7.92–22.07 × 10−8 cm3 STP g−1, respectively. The 3He/4He values vary from 0.29 Ra to 0.84 Ra, with an average of 0.60 Ra, and the 40Ar/36Ar values range from 964.1 to 1567.8, with an average value of 1272.7. The concentrations of 4He and 40 Ar in the quartz sample are 5.51 × 10−8 cm3 STP g−1 and 7.69 × 10−8 cm3 STP g−1, respectively, and its 3He/4He and 40Ar/36Ar values are 0.08 Ra and 451.3, respectively. The proportions of mantle 4He can be calculated according to Hemantle (wt%) = 100 × (R–Rc)/(Rm–Rc), where Rm = 6, Rc = 0.01, and the R parameters represent the 3He/4He ratios of the fluids in the mantle, crust, and sample (Anderson, 2000). The calculated results of the percentage of Hemantle for the four pyrite samples from V2 veins and the one quartz sample from a V4 vein are in the range of 4.64%–13.90% (average = 9.92%) and 1.19%, respectively. 5.7. H and O isotopes The oxygen and hydrogen isotope results are listed in Table 4. From V1 to V4 veins, the measured δDfluid values are −80.3‰, −86.4‰ to −81.1‰, −88.4‰ to −84.0‰, and −92.8‰ to −90.2‰, and the measured δ18Oquartz values are 10.9‰, 10.2‰ to 11.7‰, 8.7‰ to 12.6‰, and 9.2‰ to 11.0‰, respectively. The δ18Ofluid values are calculated using the quartz–water equilibrium function (Clayton et al., 1972), 1000lnαquartz-water = 3.38 × 106 × T−2 − 3.40, using the peak temperatures of fluid inclusions in quartz. The calculated δ18Ofluid values from V1 to V4 veins are 5.0‰, 4.3‰ to 5.8‰, −1.0‰ to 2.9‰, and −3.2‰ to −1.4‰, respectively. 6. Discussion 6.1. Source of the ore-forming fluids The isotopic composition of noble gases can be used as an ideal tracer for the crustal and mantle contributions to ore-forming processes

7

6

5

4

3

Temperature of solid CO2 melting. Temperature of ice melting. Temperature of clathrate melting. Homogenization temperature of CO2 phase. Temperature of homogenization to liquid (L) or vapor (V). Temperature of halite or sylvite melting. Immiscible inclusion assemblages dominated by B40 and B80 fluid inclusions. 2

V4

Q7

– Q4–Q6 V3

1

−13.2 to −3.1

27.9–30.9

−57.1 −57.0 −57.1 −56.9 −56.9

B40 B40 B40 IFA-17 IFA-27 B15 B15H B15H+ B40H B80 B15 B15H B15 V1 V2

Q1 Q1 Q2–3

to to to to to

Tm, CO2 (°C)1

−56.6 −56.6 −56.7 −56.6 −56.6

−21.1 to −3.4

−17.0 to −5.9

8.0–9.1

(310–330) (310–330) (300–310) (325–340) (325–340) (260–290) (270–290) (330–360) (380–410) (380–400) (210–230) (240–260) (170–200) 278–389 283–354 273–332 323–341 324–348 226–393 256–348 239–413 307–514 337–444 173–317 171–320 166–229 20.5–31.1 28.9–30.9 28.6–30.7 30.1–30.8 28.9–30.8 2.7–8.9 2.3–9.0 6.2–8.5 6.2–7.9 5.8–8.3

Th, L-V (°C)5 (peak range) Th, CO2 (°C)4

5.6. He and Ar isotopes

Tm, ice (°C)2

Tm, clath (°C)3

Fig. 13. Halite melting temperature versus Th (liquid–vapor) plot for B40H, B15H, and B15H+ inclusions.

estimated trapping pressures for Q2–3 quartz in V2 veins (50–110 MPa) are the exact trapping pressures.

Fluid inclusion type Quartz Generations Vein type

Table 2 Microthermometric results of the different inclusion types hosted in the different vein types from the No. 2 deposit in the Xiongcun district.

Tm, halite or sylvite (°C)6

117–292 (NaCl) 383–528 (NaCl) 127–215 (KCl) 238–511 (NaCl)

Density (g/cm3) Salinity (wt % NaCl equiv)

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Note: 40Ar* is radiogenic 40Ar, given all of the Ar come from the fluid inclusions, which can be expressed as 40Ar* = 40Arsample − 295.5 × 36Arsample; F4He values demonstrate enrichment of 4He in the fluid relative to air, which can be illustrated as F4He = (4He/36Ar)sample/(4He/36Ar)air; 4Hemantle values represent weight percent of mantle helium compared with crustal helium, which could be calculated as Hemantle (wt%) = 100 × (R – Rc)/(Rm – Rc), where Rm = 6 Ra, Rc = 0.01 Ra and R represent the 3He/4He ratios of the fluids in the mantle, crust, and the sample (Anderson, 2000).

11.28 4.64 13.90 9.85 1.19 12,691 8691 5288 3405 1954 0.69 0.29 0.84 0.60 0.08 0.53 0.88 0.97 1.19 0.48 1410.2 1567.8 1148.6 964.1 451.3 16.66 6.43 9.35 15.31 2.65 0.0150 0.0051 0.0110 0.0229 0.0170 31.40 7.27 9.59 12.90 5.51 Pyrite from V2 vein Pyrite from V2 vein Pyrite from V2 vein Pyrite from V2 vein Quartz from V4 vein 7229–229.3 7231–396.1 7229–321 7229–324.9 7223–246.1

21.08 7.92 12.59 22.07 7.69

Ar (×10−8 cm3 STP/g) 36

Ar (×10−8 cm3 STP/g) He (×10−8 cm3 STP/g)

40 4

Sample description Samples

Table 3 He and Ar isotope compositions and ratios of fluid inclusions from the No. 2 deposit in the Xiongcun district.

40

Ar*(×10−8 cm3 STP/g)

40

Ar/36Ar

40

Ar*/4He

3

He/4He (Ra)

F4He

4

Hemantle (wt%)

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(Turner et al., 1993; Stuart et al., 1995). Noble gases trapped in fluid inclusions have three potential sources: air-saturated water (ASW), mantle-derived fluids, and crust-derived fluids (Turner et al., 1993; Stuart et al., 1995; Burnard et al., 1999). The He and Ar isotopic compositions of ASW (meteoric or marine) are similar to those of the atmosphere, with 3He/4He = 1 Ra and 40Ar/36Ar = 295.5 (Turner et al., 1993, Stuart et al., 1995; Burnard et al., 1999). Atmospherically derived He is unlikely to affect He abundance and isotopic compositions of most crustal fluids owing to the insolubility of He in water and the low He concentration in the atmosphere (Marty et al., 1989; Stuart et al., 1994). Therefore, He in the ore-forming fluids of the deposit can have only two possible sources: the mantle and the crust. The 3He/4He ratios (0.08–0.84 Ra; Table 3) of the ore-forming fluids of the No. 2 deposit in the Xiongcun district are higher than those of He produced in the continental crust (0.01–0.05 Ra; Turner et al., 1993) but lower than those of the mantle (6–9 Ra; Dunai and Baur, 1995; Gautheron and Moreira, 2002), demonstrating that they contain both mantle- and crust-derived He. In the 3He versus 4He and 3He/4He versus 40Ar/36Ar diagrams (Fig. 14), all data points are distributed between the mantle and crustal He. The characteristics of the He isotopic composition detailed above indicate a mixed fluid source of crust and mantle origin. However, the estimated percentage of Hemantle (average = 8.17%; Table 3) of the No. 2 deposit is relatively low, suggesting that the oreforming fluids are predominantly crustal with trace amounts of mantle fluids. On the other hand, in terms of Ar isotopes, the measured 40 Ar/36Ar value of the V4 vein quartz sample is 451.3 (Table 3), which is lower than those of the V2 vein pyrite samples (964.1–1567.8; Table 3) and close to the atmospheric 40Ar/36Ar value (40Ar/36Ar = 295.5; Turner et al., 1993, Stuart et al., 1995; Burnard et al., 1999). In addition, on the 3He/4He versus 40Ar/36Ar diagram (Fig. 14b), the V4 vein quartz sample lies near the trend line of modified air-saturated water (Stuart et al., 1995), indicating that some meteoric water is mixed into the ore-forming fluid in the late stage. The V1 veins from the No. 2 deposit show relatively high δ18Ofluid (5.0‰) and δDfluid (−80.3‰) values (Table 4), consistent with those for early veins reported in other porphyry systems (e.g., Ulrich et al., 2001; Cooke et al., 2011). The V2 veins also have relatively high δ18Ofluid (4.3‰ to 5.8‰) and δDfluid (−86.4‰ to −81.1‰) values, similar to those of the V1 veins (Table 4). In the δDfluid versus δ18Ofluid plot (Fig. 15), samples from V1 and V2 veins plot in or adjacent to the box for primary magmatic water, indicating that the early-stage oreforming fluids were magmatic in origin. The four samples from V3 veins show lower δ18Ofluid (−1.0‰ to 2.9‰) and δDfluid (−88.4‰ to −84.0‰) values than V1 and V2 veins and plot in the region between the primary magmatic water box and the meteoric water line, suggesting that the ore-forming fluids that formed V3 veins were a magmatic–meteoric mixture. The δ18Ofluid (−3.2‰ to −1.4‰) and δDfluid (−92.8‰ to −90.2‰) isotopic compositions of the fluids that formed the V4 veins are extremely depleted, strongly implying the involvement of meteoric water in late-stage fluids. The H–O isotopic values gradually decrease from V1 to V4 veins (Fig. 15), indicating that early-stage oreforming fluids had a dominantly magmatic signature and that the oreforming fluids were diluted by meteoric water during the late stage. In summary, the He–Ar and H–O isotope data indicate that the oreforming fluids were initially magmatic in origin (crust-derived) and evolve to being meteoric water-dominated in the late stage. 6.2. Initial ore-forming fluids Low-salinity (~2–12 wt% NaCl equiv) CO2-rich B40 inclusions occur primarily in V1 veins from the deepest samples (550–730 m below the present surface). In these deep V1 veins, B40 inclusions account for more than 95% of the inclusions present, and no halitebearing inclusions are identified. The relatively high oxygen and hydrogen isotopic values and presence of CO2-rich inclusions in quartz from V1 veins suggest that the B40 inclusions trap the magmatic fluids 16

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Table 4 O and H isotope compositions from the No. 2 deposit in the Xiongcun district. Samples

Sample description

Th (°C)

δDfluid (‰)

δ18OQuartz (‰)

δ18Ofluid (‰)

7231-396 7229-229.2 7229-321 7229-324.9 7223-246 7231-175 7238-595.5 7229-182.5 7223-221.3 7229-483.5 7223-93.5

Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz

330 330 330 330 330 235 235 235 235 190 190

−80.3 −86.4 −82.4 −86.1 −81.1 −84.0 −88.4 −84.4 −86.3 −92.8 −90.2

10.9 11.7 10.9 10.2 11.6 8.7 9.7 11.5 12.6 9.2 11.0

5.0 5.8 5.0 4.3 5.7 −1.0 0.0 1.8 2.9 −3.2 −1.4

from from from from from from from from from from from

V1 V2 V2 V2 V2 V3 V3 V3 V3 V4 V4

vein vein vein vein vein vein vein vein vein vein vein

responsible for the formation of V1 veins. In addition, we identify a transition zone (thick black line) in cross-section A–B (Fig. 11), below which B40 inclusions dominate and no B15H, B15H+, and B40H inclusions are present and above which B80, B15H, B15H+, and B40H inclusions dominate (Fig. 11). Based on the distribution of these inclusions, we conclude that B40 inclusions in deep V1 veins trap a lowsalinity, CO2-rich single-phase fluid of magmatic origin that represents the initial fluids of the magmatic-hydrothermal systems that formed the No. 2 deposit. The initial low-salinity, CO2-rich single-phase fluids indicate that the No. 2 deposit formed deeper than that of typical porphyry system (1–5 km; Cooke et al., 2005; Sillitoe, 2010). First, CO2 has a lower solubility in magma than does water and will exsolve at a higher pressure (Holloway, 1976; Giggenbach, 1997; Lowenstern, 2001), so the fluids exsolved from deeper magma tend to have higher CO2/H2O ratios than those exsolved in shallow environments. In other words, the emplacement depth of porphyry is a factor controlling the content of CO2 in a magmatic-hydrothermal system, and CO2-rich porphyry systems generally develop at greater depths than the 1–5 km depths inferred for most porphyry systems. Several CO2-rich porphyry systems developed at unusual depths have been identified elsewhere, for example, the emplacement depth of the Butte deposit is constrained to 6–9 km (Rusk et al., 2008), the Dabaoshan porphyry Mo deposit formed at depths of 6–7 km (Mao et al., 2017), and the estimated formation depth of porphyry deposits in the East Qinling Mo belt, including Yuchiling, Jinduicheng, Nannihu, and Shangfanggou, is up to 8 km (Li et al., 2009, 2012; Yang et al., 2009a–c). Second, similar low-salinity single-phase fluids have been recognized and described in deep veins from a few porphyry systems, such as Alta Stock, Utah (John, 1989); Bingham Canyon, Utah (Redmond et al., 2004; Landtwing et al., 2010); Questa, New Mexico (Klemm et al., 2008); Morococha district, Peru (Catchpole

Fig. 14. (a) 3He versus 4He and (b) 3He/4He (Ra) versus district.

40

Fig. 15. Hydrogen and oxygen isotopic compositions of fluids in quartz from the No. 2 deposit in the Xiongcun district. Boxes indicating the isotopic values for primary magmatic and metamorphic water are from Taylor (1997).

et al., 2015); and Altar, Argentina (Maydagán et al., 2015). The estimated pressures of 150–270 MPa for the low-salinity single-phase fluids in these porphyry systems correspond to a depth of 5–9 km at lithostatic pressures (Rusk et al., 2008; Catchpole et al., 2015; Maydagán et al., 2015; Mao et al., 2017). Given the above, the presence of low-salinity, CO2-rich single-phase fluids in porphyry systems indicates an unusually great depth of ore deposit formation. Given that our calculated minimum trapping pressures for inclusions in V1 veins range from 90 MPa to 180 MPa (average = 140 MPa), it can be concluded that the

Ar/36Ar plots of inclusion-trapped fluids in pyrite and quartz from the No. 2 deposit in the Xiongcun

17

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fluids may have exsolved from the magma at a depth of ≥ 5.2 km, indicating that the formation depth of the No. 2 deposit is greater than the typical 1–5 km depth of porphyry systems (Cooke et al., 2005; Sillitoe, 2010). This may be the reason why the Xiongcun district is the only preserved Jurassic porphyry system discovered in GPCB, which has undergone large-scale uplift and denudation since the Cretaceous.

undergo separation into hypersaline liquid (B40H, B15H, and B15H+ inclusions, with maximum fluid salinities of up to ~70 wt% NaCl equiv) and vapor phases (B80 inclusions) at temperatures of 325 °C to 340 °C and pressures of ~50–110 MPa (Fig. 16b). The fluids then continue to evolve to form V3 veins. The types of inclusions in V3 veins include B15 and B15H inclusions. These fluid inclusions are trapped under hydrostatic conditions accompanied by the involvement of meteoric water (Fig. 16c). They are characterized by low homogenization temperatures (< 320 °C) and being CO2-poor and intermediate- or low-salinity (5.6–33.9 wt% NaCl equiv). No CO2 is detected in these inclusions, indicating that the previous fluid immiscibility causes the ore-forming fluids to lose most of their CO2. Finally, only B15 inclusions are observed in V4 veins. They have lower homogenization temperatures (< 230 °C) and salinities (< 17.1 wt% NaCl equiv) than V3 veins, indicating that the fluids are diluted by the injection of considerable meteoric water into the hydrothermal system in the late stage (Fig. 16d). In summary, the temperature and CO2 content of the oreforming fluids decrease from the early to the late stage of the hydrothermal process. The salinity varies over a comparatively large range in

6.3. Fluid evolution The hydrothermal evolution at the No. 2 deposit in the Xiongcun district is recorded by fluid inclusions trapped in multiple quartz generations and vein types. The B40 inclusions in Q1 quartz in the V1 and V2 veins trap initial ore-forming fluids that are directly exsolved from the parent magma (Fig. 16a). They are trapped under temperatures of more than 330 °C and under high-pressure conditions (greater than 130 MPa) and are characteristically CO2-rich and low-salinity (~2–12 wt% NaCl equiv) (Fig. 16a). Subsequently, during the deposition of later Q2–3 quartz in V2 veins, fluid immiscibility develops, accompanied by pressure fluctuations. Early low-salinity, CO2-rich fluids

Fig. 16. Schematic diagrams showing the evolution of the magmatic-hydrothermal system of the No. 2 deposit in the Xiongcun district. (a) As crystallization progressed in the magma chamber beneath the mineralization zone, low-salinity, CO2-rich single-phase fluids exsolved at high temperatures and lithostatic pressure. Under these conditions, B40 fluid inclusions were trapped in V1 veins accompanied by potassic alteration. (b) As the low-salinity, CO2-rich single-phase fluids ascended, they underwent phase separation into hypersaline liquid and vapor phases during the deposition of later Q2–3 quartz in V2 veins. In addition, fluid immiscibility led to CO2 escape and chalcopyrite precipitation. (c) V3 veins formed at hydrostatic pressure accompanied by the involvement of meteoric water, and fluid-cooling due to the mixing of magmatic fluids with meteoric water probably induced molybdenite precipitation. (d) V4 veins formed at lower temperatures and pressures, accompanied by the involvement of large quantities of meteoric water. 18

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V2 veins due to fluid immiscibility in the initial low-salinity fluids, and then, gradually decreases due to the input of meteoric water into the hydrothermal system. Similar hydrothermal evolution processes can be observed in other porphyry systems (Yang et al., 2012, 2013; Zhong et al., 2014).

late quartz veins (V4). SEM-CL images reveal that these veins are composed of eight generations of quartz. b) The initial ore-forming fluids in this deposit were low-salinity (~2–12 wt% NaCl equiv), CO2-rich, single-phase fluids. This finding, combined with the results of pressure estimation, indicates that the formation depth of the No. 2 deposit is deeper than that in a typical porphyry system. c) The ore-forming fluids were initially magmatic in origin and moderate-temperature, CO2-rich, and low-salinity. Subsequently, immiscibility developed in the initial ore-forming fluids, generating a hypersaline liquid and low-salinity vapor phases, as well as leading to the separation of a CO2 phase from the fluids. Due to subsequent injection of meteoric water into the hydrothermal system, the oreforming fluids gradually evolved into meteoric water-dominated, low temperature, low-salinity, and CO2-poor fluids. d) Fluid immiscibility accompanied by pressure fluctuations and phase separation were the prime factors controlling chalcopyrite precipitation, whereas fluid-cooling due to the mixing of magmatic fluids with meteoric water probably induced molybdenite precipitation.

6.4. Ore precipitation mechanisms In the No. 2 deposit, chalcopyrite mainly occurs in V2 veins, but molybdenite is mainly present in V3 veins. The two vein types show various fluid inclusions, H–O isotopic compositions, and SEM-CL textures, indicating that the deposition of chalcopyrite and molybdenite is controlled by different mechanisms. SEM-CL images reveal extensive evidence in V2 veins for retrograde dissolution of early Q1 quartz leaving a high-porosity quartz framework (pore space) and microfractures, which are later infilled with Q2–3 quartz (Fig. 7a, b). Experimental results show that retrograde dissolution of quartz is more likely to occur in response to pressure fluctuations than to temperature fluctuations (Kennedy, 1950; Fournier, 1999). In addition, pressure fluctuations are likely to occur in magmatic-hydrothermal systems as a hydrothermal fluid breaches the boundary between ductile rock at lithostatic pressure (Rusk and Reed, 2002). Indeed, the fluid inclusions in V2 veins record large pressure fluctuations (> 130 MPa to 50 MPa). We, therefore, propose that chalcopyrite precipitation in V2 veins may be related to pressure fluctuation. Furthermore, pressure fluctuations often cause fluid immiscibility or boiling (Roedder, 1971, 1992). In V2 veins, chalcopyrite is in contact with the late generation of Q2–3 quartz (Fig. 7), which contains high-salinity (B15H, B15H+, and B40H) and low-salinity (B80) fluid inclusion assemblages, which are interpreted to reflect fluid immiscibility. Moreover, the correspondence of the high-grade ore zone with the occurrence of phase separation indicates a strong relation between fluid immiscibility and chalcopyrite precipitation (Fig. 11). Therefore, fluid immiscibility processes accompanied by pressure fluctuations and phase separation in the hydrothermal fluids may be the key factor that induces chalcopyrite precipitation. The temperature interval of chalcopyrite precipitation is approximately equal to the precipitation temperature of Q2–3 quartz in V2 veins (325 °C–340 °C). In contrast to V2 veins, SEM-CL images show that the V3 veins are dominated by quartz displaying growth zonation (Fig. 8b), and the dissolution of quartz and microfracture development have never been observed in V3 veins. In addition, no immiscible fluid inclusion assemblages are observed in V3 veins. These findings indicate that fluid immiscibility is not the dominant factor driving molybdenite deposition. The fluid inclusions in V3 veins exhibit lower Th values (< 320 °C) and salinities (5.6–33.9 wt% NaCl equiv) than do those in V2 veins, suggesting that molybdenite and quartz precipitation in V3 veins have been triggered by a decrease in temperature and salinity. One possible explanation for such a decrease is the mixing of minor amounts of meteoric water into the fluid. Quartz in V3 veins is characterized by subhedral comb-like textures with well-developed oscillatory growth zoning (Fig. 8b), implying that there is open space for crystallization under hydrostatic conditions. Under such conditions, it is possible for early high-temperature magmatic fluids to mix with low-temperature, low-salinity meteoric water. Indeed, the H–O isotopic characteristics of V3 veins confirm that mixing of this type is responsible for the development of V3 veins (Fig. 15). Thus, we consider that the mixing of magmatic fluids with meteoric water may be the primary factor causing molybdenite precipitation.

Acknowledgments This research was jointly supported by the Deep Resources Exploration and Mining, the National Key R&D Program of China (grant numbers: 2018YFC0604105), the National Natural Science Foundation of China (grant numbers: 41972084), the Opening Foundation of State Key Laboratory for Mineral Deposits Research (grant numbers: 2017LAMD-K04), the Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University (grant numbers: 18LCD04), the Opening Foundation of State Key Laboratory of Ore Deposit Geochemistry (grant numbers: 201503), and the China Geological Survey Programs (grant numbers: DD20160346). We are grateful to Prof. Yanjing Chen and anonymous reviewers for their constructive comments, which have considerably improved the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2019.103141. References Anderson, D.L., 2000. The statistics and distribution of helium in the mantle. Int. Geol. Rev. 42, 289–311. Audétat, A., Günther, D., 1999. Mobilization and H2O-loss from fluid inclusions in natural quartz crystals. Contrib. Miner. Petrol. 137, 1–14. Audetat, A., Pettke, T., Heinrich, C.A., Bodnar, R.J., 2008. The composition of magmatichydrothermal fluids in barren and mineralized intrusions. Econ. Geol. 103 (5), 877–908. Bodnar, R.J., 1993. Revised equation and table for determining the freezing point depression of H2O-NaCl solutions. Geochim. Cosmochim. Acta 57 (3), 683–684. Bodnar, J.R., Vityk, M.O., 1994. Interpretation of microthermometric data for H2O-NaCl fluid inclusions. In: De Vivo, B., Frezzotti, M.L. (Eds.), Fluid Inclusions in Minerals: Methods and Applications. Virginia Technical Institute, Blacksburg, VA, pp. 117–130. Bowers, T.S., Helgeson, H.C., 1983. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H2O-CO2-NaCl on phase relations in geologic systems: equation of state for H2O-CO2-NaCl fluids at high pressures and temperatures. Geochim. Cosmochim. Acta 47 (7), 1247–1275. Brown, E., 1989. FLINCOR: a microcomputer program for the reduction and investigation of fluid-inclusion data. Am. Mineral. 74, 1390–1393. Brown, P.E., Hagemann, S.G., 1995. MacFlincor and its application to fluid in Archean lode-gold deposits. Geochim. Cosmochim. Acta 59, 3943–3952. Burnard, P.G., Hu, R.Z., Turner, G., Bi, X.W., 1999. Mantle, crustal and atmospheric noble gases in Ailaoshan gold deposit, Yunnan province. China. Geochim. Cosmochim. Acta 63, 1595–1604. Campos, E., Touret, J.L.R., Nikogosian, I., Delgado, J., 2002. Overheated, Cu-bearing magmas in the Zaldívar porphyry-Cu deposit, northern Chile. Geodynamic consequences. Tectonophysics 345, 229–251. Campos, E., Touret, J.L.R., Nikogosian, I., 2006. Magmatic fluid inclusions from the Zaldívar deposit, northern Chile: the role of early metalbearing fluids in a porphyry

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