Formation and evolution of multistage magmatic-hydrothermal fluids at the Yulong porphyry Cu-Mo deposit, eastern Tibet: Insights from LA-ICP-MS analysis of fluid inclusions

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Formation and evolution of multistage magmatic-hydrothermal fluids at the Yulong porphyry Cu-Mo deposit, eastern Tibet: Insights from LA-ICP-MS analysis of fluid inclusions Jia Chang a, Jian-Wei Li a,⇑, Andreas Aude´tat b a

State Key Laboratory of Geological Processes and Mineral Resources and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China b Bavarian Geoinstitute, University of Bayreuth, 95440 Bayreuth, Germany Received 8 October 2017; accepted in revised form 5 April 2018; available online 14 April 2018

Abstract The giant Eocene Yulong porphyry Cu-Mo deposit in eastern Tibet, China was produced by multistage veining, Cu-Mo sulfide deposition, and hydrothermal alteration associated with pulsed magmatic intrusions (i.e., early, transitional, and late stages) lasting for a prolonged period of time. In this study, fluid inclusion microthermometry and laser ablation-ICP-MS microanalysis are combined with textural observations on magmatic apatite and various hydrothermal quartz vein assemblages to constrain the formation and evolution of the multistage magmatic-hydrothermal fluids producing the Yulong porphyry Cu-Mo deposit. Fluid inclusions hosted in quartz veins of the early and transitional stages have very similar compositions, indicating that the initial single-phase intermediate-density (ID) ore-forming fluids of these two mineralizing stages were exsolved from similarly evolved magma reservoirs in the underlying magma chamber. During their ascent, decompression and cooling, the single-phase ID input fluids (
9 wt% NaClequiv. with
1000 ppm Cu and
20 ppm Mo) entered the two-phase field and condensed into a small amount of metal-rich brines (
42 wt% NaClequiv. with
9300 ppm Cu and
330 ppm Mo) coexisting with a large amount of vapors (vapor/brine mass ratio of
4). The condensation of brines and their accumulation at shallow level could be an efficient mechanism to concentrate and then precipitate Cu ± Mo in a small rock volume as represented by the mineralized porphyries. The sequential deposition of Mo and Cu from the condensed brine phase of the early stage caused the local Cu-Mo decoupling in shallow parts of the deposit, whereas the deeply located Mo-rich mineralization of the transitional stage was probably caused by the early precipitation of molybdenite from the single-phase ID fluids before phase separation. It is concluded that the formation of a late Mo-rich mineralization (similar to the transitional-stage mineralization at Yulong) in many other porphyry Cu deposits is due to the subsolidus hydrothermal processes rather than the progressive increase of the Mo/Cu ratio in residual parental melts as magma crystallization proceeds. The high Cs + Rb ± B concentrations of melt and fluid inclusions in sulfur-rich magmatic apatite (0.3–1.4 wt% SO3) suggest a pegmatitic environment for the exsolution of the fluids. The similar element/K ratios of liquid-rich inclusions in

⇑ Corresponding author.

E-mail address: [email protected] (J.-W. Li). https://doi.org/10.1016/j.gca.2018.04.009 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved.

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the late-stage pyrite-quartz veins to the fluid inclusions in the sulfur-rich apatite suggest that the late-stage fluids were also likely derived from highly evolved melt fractions in felsic magmas during the solidification of the magma chamber. Ó 2018 Elsevier Ltd. All rights reserved. Keywords: Porphyry Cu; Fluid inclusions; LA-ICP-MS microanalysis; Cu-Mo decoupling; Sulfur-rich apatite

1. INTRODUCTION Porphyry Cu(-Mo-Au) deposits above upper crustal magma chambers represent focused zones of magma intrusion, heat transfer, fluid flow, hydrothermal alteration, and mineral precipitation of great economic significance (Dilles et al., 2000; Seedorff et al., 2005; Sillitoe, 2010). Processes in the magma chamber are decisive for the generation of ore metal-charged magmatic-hydrothermal fluids (Halter et al., 2005; Nadeau et al., 2010; Aude´tat and Simon, 2012; Wilkinson, 2013). Focusing of large volumes of ascending magmatic derived fluids and efficient physico-chemical mechanisms for metal precipitation in quartz vein stockworks with hydrothermal alteration are prime factors determining the metal enrichment (Hedenquist and Richards, 1998; Aude´tat et al., 2008; Kouzmanov and Pokrovski, 2012). Although the magmatic-hydrothermal process associated with porphyry Cu(-Mo-Au) mineralization has been extensively studied, the following questions remain not well understood: (1) the nature and long-term evolution of source magmas; (2) the main carriers of Cu and Mo and factors governing Cu-Mo decoupling during hydrothermal process; (3) the origin of the common post-ore pyrite + quartz veins and associated sericitic alteration. Basaltic to andesitic magmas are inferred to be the main fluid and metal sources for porphyry Cu(-Mo-Au) deposits. Some studies on porphyry Cu systems with preserved mafic to felsic rocks showed that the metal- and sulfur-rich mafic magmas were typically sulfideundersaturated, whereas andesitic magmas that formed upon magma mixing commonly contained abundant sulfides with the metals mainly inherited from the mafic magmas (Keith et al., 1997; Halter et al., 2005; Aude´tat and Simon, 2012). The similar metal ratios in magmatic sulfides, ore-forming fluids, and ores indicate that these sulfides released their metal load to the ore-forming fluids upon bulk dissolution of the sulfides during magma degassing (Halter et al., 2002a; Stavast et al., 2006; Nadeau et al., 2010). Alternatively, other authors suggested that fluid exsolution from crystallizing basaltic to andesitic magmas without sulfide saturation is more than adequate to form large porphyry Cu(-Mo-Au) deposits (Cline and Bodnar, 1991; Stern and Skewes, 2005; Richards, 2015; Grondahl and Zajacz, 2017). On the other hand, highly evolved melt inclusions coexisting with fluid inclusions in quartz phenocrysts and hightemperature quartz veins have been reported in several porphyry Cu deposits (e.g., Harris et al., 2003; Stefanova et al., 2014; Rottier et al., 2016). Such fluid– melt inclusion populations are argued to represent the transient exsolution of ore-forming fluids (Harris et al.,

2003; Rottier et al., 2016), which is recently realized to be problematic as the high Cu concentration in the melt inclusions is an artefact from experimental re-melting (Rottier et al., 2017). These coexisting, highly evolved melt and fluid inclusions are likely only reflective of extreme and localized fractionation (Halter et al., 2005; Stefanova et al., 2014). Porphyry Cu mineralization is generally produced by a two-phase fluid, comprising a small fraction of hypersaline brine and a much larger mass of low-density vapor either directly exsolved from the melt (Shinohara, 1994) or, more likely, generated when a single-phase fluid decompresses, cools, and intersects its solvus (Burnham, 1979; Cline and Bodnar, 1991). Whether Cu and Mo in porphyry-type ore deposits were deposited dominantly from brines or vapors is hotly debated (Roedder, 1971; Heinrich et al., 1999; Williams-Jones and Heinrich, 2005; Seo et al., 2012; Hurtig and Williams-Jones, 2015). Recent experimental studies suggest that the high Cu concentrations in vapordominated fluid inclusions in quartz are caused by postentrapment diffusion, and thus favored that Cu is mostly transported by hypersaline brines (Lerchbaumer and Aude´tat, 2012). The evidence concerning the behavior of Mo in the two fluid phases is less clear (Wilkinson et al., 2008; Seo et al., 2012; Hurtig and Williams-Jones, 2015), despite recent experimental studies suggest that Mo strongly partitions into brines upon phase separation (Kokh et al., 2016; Zajacz et al., 2017). The temporal and spatial decoupling of Cu and Mo has been documented in many porphyry Cu(-Mo-Au) systems (Sillitoe, 2010 and reference therein). Nevertheless, factors that led to the decoupling remain elusive (Seo et al., 2012; Spencer et al., 2015). In this paper, we present an integrated textural and fluid inclusion study of the Yulong porphyry Cu-Mo deposit, using optical microscope- and scanning electron microscope-cathodoluminescence (Optical- and SEM-CL) imaging of magmatic apatite and hydrothermal quartz veins, electron microprobe (EMP) analysis of magmatic apatite, and petrography, microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of fluid inclusions hosted in magmatic apatite and quartz veins. The results are used to constrain the formation and evolution of the multistage magmatic-hydrothermal fluids of the deposit. This study highlights the relatively constant initial ore-forming fluids produced by an opensystem upper crustal magma chamber in porphyry Cu deposits, the significance of hypersaline brines in concentrating and precipitating Cu ± Mo, and the production of the late-stage infertile fluids from highly evolved felsic melts.

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Fig. 1. Regional and local geology of the Yulong porphyry Cu-Mo deposit in eastern Tibet. (A) Simplified geologic map of the northern Changdu-Simao continental block showing distribution of the volcanic rocks, porphyry intrusions associated with porphyry Cu ± Mo deposits (in red) and weakly mineralized porphyry intrusions (in black) (modified from Hou et al., 2003). The tectonic location of the area is shown in the insert. (B) Geological map of the Yulong deposit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2. GEOLOGIC BACKGROUND The Eocene Yulong porphyry Cu-Mo deposit, the largest one of five similar deposits in the Changdu-Simao continental block in eastern Tibet Plateau, China (Hou et al., 2003), has proven reserves of
6.5 Mt Cu @ 0.62% and
0.4 Mt Mo @ 0.042%. These deposits occur in a belt about 15–30 km wide and
300 km long and show a close spatial and temporal association with the Eocene porphyry intrusions and potassium-rich basaltic to rhyolitic volcanic rocks (Fig. 1A). The north-northwest trending, regional strike-slip faults that accommodated the compressive strains produced by the Asian-Indian continental collision controlled the emplacement and distribution of Eocene volcanic rocks and porphyry intrusions, as well as the associated porphyry Cu(-Mo) deposits (Hou et al., 2003). The Yulong deposit is centered on a composite porphyry stock that intrudes a sequence of Triassic marine clastic and carbonate rocks along the north- to northwest-trending Hengxingcuo anticline (Fig. 1B). The mineralized stock is dominated by a monzonitic granite porphyry (MGP) that was intruded by volumetrically minor K-feldspar granite porphyry (KGP) and then quartz albite porphyry (QAP) (see Fig. 2). These three porphyry intrusions contain similar phenocryst assemblages (K-feldspar, plagioclase, quartz, biotite and/or hornblende) with variable abundance and crystal size that are dispersed in an aplitic groundmass

(quartz, K-feldspar, plagioclase, and/or biotite). The phenocrysts account for 40–50 vol%, 20–30 vol%, and 15–20 vol% of the MGP, KGP and QAP, respectively (Chang et al., 2017). The sequence of vein formation and hydrothermal alteration has been grouped into early, transitional and late stages following the intrusion of MGP, KGP and QAP, respectively as summarized in Table 1. It was estimated that 80% Cu and Mo of the deposit precipitated in the early stage, with the remainder Mo-rich mineralization formed during the transitional stage (Chang et al., 2017). No economic Cu-Mo mineralization has been documented in the late stage. The early-stage mineralization and associated alteration are predominantly hosted by MGP. Intense potassic alteration in the barren core is characterized by texturedestructive replacement of MGP by fine-grained K-feldspar-quartz ± biotite, with or without abundant barren A1E quartz veins (locally >30 vol%; Fig. 3A). Sparse ME and EBE veins also occur in the barren core (Fig. 3B). The A2E and A3E veins are the main Cu and Mo carriers, which are mainly distributed in the strong to moderate potassic alteration zones adjacent to the barren core. Molybdenite of the early stage occurs primarily as disseminations in the quartz-dominated A2E veins, whereas chalcopyrite is mostly present as fracture infillings in A2E veins and paragenetically later A3E veins that are dominated by chalcopyrite ± pyrite (Fig. 3C and D). The

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Fig. 2. Geologic cross-sections through the Yulong porphyry Cu-Mo deposit (see Fig. 1B for locations of the sections; Chang et al., 2017). (A) NNW-SSE cross-section C-C0 parallel to the elongation of the Yulong stock. (B) ENE-WSW cross-section 9-90 perpendicular to the elongated Yulong stock. Notice the distribution of K-feldspar granite porphyry, quartz albite porphyry and BT veins revealed by the drilling holes. Sampling locations for LA-ICP-MS microanalysis of fluid inclusions in quartz veins are indicated.

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Table 1 Explanations of porphyry sequences, vein types, and associated fluid inclusion types. Stage

Porphyry/vein type

Brief description1

Fluid inclusion type2

Early stage (E)

MGP ME vein EBE vein A1E vein A2E vein A3E vein

monzonitic granite porphyry qtz-mgt vein qtz-bio vein qtz ± ksp vein qtz-mo ± cp ± py vein cp ± py ± qtz vein

ID (apatite hosted)

KGP EBT vein A1T vein BT vein A3T vein

K-feldspar granite porphyry qtz-bio vein qtz ± ksp vein symmetrical qtz-mo ± cp ± py vein cp ± py ± qtz vein

ID (apatite hosted)

QAP DL veins

quartz albite porphyry py-qtz vein with sericitic alteration halos

ID (apatite hosted) L-rich ± V

Transitional stage (T)

Late stage (L)

B+V ID (±B ± V) B+V

ID + B + V ID + B + V

1 See more details in Tables 1 and 2 of Chang et al. (2017). Abbreviations: bio = biotite; cp = chalcopyrite; ksp = K-feldspar; qtz = quartz; mgt = magnetite; mo = molybdentie; py = pyrite. 2 The dominant fluid inclusion types in different samples. ID = intermediate-density fluid inclusions; B = brine inclusions; V = vapor inclusions; L-rich = liquid-rich inclusions. See text for definition.

transitional stage postdates the emplacement of KGP dikes. Barren A1T quartz veins, being cut by BT veins, are commonly observed in KGP (Fig. 3E). The mineralization of this stage is largely represented by BT veins that have abundant molybdenite in the marginal zones and open voids in the central part, with no visible alteration halos (Fig. 3F). Some BT veins in KGP differ from BT veins in MGP and hornfels in that they are morphologically irregular (Fig. 3E), likely reflecting their formation before the complete solidification of the KGP dikes. Minor amounts of chalcopyrite ± pyrite are also developed as A3T veins in the transitional stage. The late stage postdates the emplacement of QAP dikes and is characterized by pyrite ± quartz (DL) veins with sericitic alteration halos (Fig. 3G). The DL veins are distributed throughout the deposit, either cutting or displacing all the other vein types. The porphyry-style ore zone in the Yulong stock and the surrounding hornfels has an asymmetric, inverted cupshape in three dimensions, with the highest ore grades in the southeastern part of the deposit extending down to 4000 m a.s.l. (Fig. 2). The grades of Cu and Mo mineralization correlate closely above
4300 m a.s.l., although local decoupling of Cu- and Mo-rich zones is observed. In contrast, Mo mineralization becomes more important in the deeper parts of the deposit (<4300 m a.s.l.). Drill core logging results suggest that the distribution of porphyry-style Cu and Mo grades are primarily controlled by the vein types and abundances, though supergene oxidation, leaching and secondary enrichment have complicated the distribution of Cu grade above
4400 m a.s.l. (Chang et al., 2017). The A2E and A3E veins control the distribution of Cu and Mo grade in the shallow parts (>4300 m a.s.l.) of the deposit, where local Cu and Mo separation is strongly correlated with the decoupled distribution of molybdenite-rich A2E and Cu-Fe sulfide-dominated A3E veins (e.g., north part of section C-C0 , west part of section 9-90 in Fig. 2). The deep-seated, Mo-rich zone, however, is largely controlled by BT veins (Fig. 2).

3. SAMPLES AND METHODS Fluid inclusions were studied in vein quartz and in magmatic apatite on the basis of elaborate petrographic work, in the latter mineral, a few melt inclusions were analyzed as well. Magmatic apatite was selected from a dozen of MGP, KGP and QAP samples to prepare thin sections (
30 lm) and epoxy disks containing a collection of apatite grains. The textures of apatite were examined using a combination of transmitted-light microscopy, optical-CL imaging and SEM characterization prior to EMP analysis. Apatite grains containing sufficiently large fluid and melt inclusions (>15 lm) were targeted for subsequent microthermometry and LA-ICP-MS analysis. Over 300 samples with different type of quartz veins were collected from drill cores to prepare doubly polished thick sections (100–200 lm). All these quartz vein samples were petrographically examined to reveal their paragenetic assemblages and characteristics of fluid inclusions. Thirty thick sections were studied further using SEM-CL imaging to document the relationship between sulfides and various quartz generations in individual veins. Another thirty thick sections were selected based on the type and size of fluid inclusions for microthermometry and LA-ICP-MS analysis, including six A1E veins, three EBE veins, five A2E veins, six A1T veins, six BT veins and two DT veins, which are located along the two cross-sections in Fig. 2. 3.1. Optical-CL and SEM-CL microscope imaging Optical-CL photomicrography of apatite was carried out using an Olympus Vanox microscope attached with a Relion III CL system. The low-vacuum Relion system was set to an electron source operated at 15–20 kV and about 500–800 mA. SEM-CL imaging of carbon-coated quartz vein samples was performed using a Quanta 450 FEG-SEM with a MonoCL4+ detector hosted in the State Key Laboratory of Geological Processes and Mineral

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Fig. 3. Photographs of hand specimens showing various vein types. (A) Quartz block from a several-meter-thick A1E vein. (B) An EBE vein with late overprinted chalcopyrite + pyrite in strongly potassic altered MGP. (C) and (D) Quartz-dominated A2E veins and sulfidesdominated A3E veins in MGP. In panel C the upper A2E vein truncates an A3E veinlet and is cut by another A3E veinlet. (E) A morphologically irregular BT vein cutting an A1T vein in KGP. (F) A typical symmetrical BT vein with central voids in MGP. (G) A DL vein in intensely sericitic altered MGP. Abbreviations: bio = biotite; cp = chalcopyrite; ksp = K-feldspar; mo = molybdenite; py = pyrite; qtz = quartz; tn = tennatite.

Resources (GPMR), China University of Geosciences (Wuhan), with a working distance of
14 mm and an accelerating voltage of 15 kV. 3.2. Electron microprobe analysis EMP analysis of apatite was performed using a JXA8230 Superprobe equipped with wavelength-dispersive spectrometers (WDS) at the Center of Material Research and Analysis, Wuhan University of Technology. The instrument was set to an accelerating voltage of 15 kV and a beam current of 30 nA. A defocused beam of
5 lm was used, with counting times in seconds on a peak/ background of 10/5 for P, Ca, and Mn; 20/10 for F, Na, Mg, Cl, and Fe; 30/15 for Si, Sr, Y, La, and Ce; and

60/30 for S. The standards used for calibrating elements include chrome-diopside (Si), fluorapatite (Ca, P, F), hematite (Fe), celestine (Sr), rhodonite (Mn), albite (Na), olivine (Mg), yttrium aluminum garnet (Y), barite (S), bababudanite (Cl), cerium pentaphosphate (Ce) and lanthanum pentaphosphate (La). F, Cl, Na, and S were measured first. Analytical uncertainties are <0.5% for Ca, <1% for P, <3% for F, <10% for Ce, Cl, and S, <30% for Na and Si, <50% for La and Sr, and <50–100% for Fe, Mn, Mg, and Y, with the latter elements being close to the detection limit. Oxygen was calculated from cation stoichiometry, and the apatite chemical formula calculation was based on 8 cations. An inclusion-rich apatite was selected for EMPWDS mapping to reveal the spatial distribution of S, Ce, Na, F, Cl, Si, Ca, and P. The mapping was conducted at

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an accelerating voltage of 15 kV and a beam current of 30 nA, with a point dwell time of 40 ms. 3.3. Microthermometric measurement Microthermometric measurement of fluid inclusions was carried out at GPMR of China University of Geosciences (Wuhan) using a Linkam THMSG-600 heating–cooling stage that was calibrated to an uncertainty of ±0.2 °C in the range of 56.6 to 0.0 °C. At temperatures above 100 °C the precision is ±2 °C. Salinities were derived from final melting temperature of CO2-clathrate using the equations of Diamond (1992), final melting temperature of ice or dissolution temperature of halite using the equations of Bodnar and Vityk (1994). 3.4. LA-ICP-MS analysis LA-ICP-MS analysis of fluid and melt inclusions was performed at the Bavarian Geoinstitute, Germany using a 193-nm ArF Excimer Laser (GeolasPro from Coherent/ Lambda Physik) combined with a Perkin Elmer Elan DRC-e quadruple mass spectrometer. The sample chamber was flushed with He gas at a rate of 0.4 l/min, to which H2 gas was added on the way to the ICP-MS at a rate of 5 ml/ min (Guillong and Heinrich, 2007). Individual fluid inclusions in apatite and quartz were drilled out as entities. Contributions from co-ablated host were subtracted numerically by assuming that either phosphorus (apatite) or silicon (quartz) was present only in the host. External standardization was based on NIST SRM 610 glass using the values in Jochum et al. (2011), except for sulfur and chlorine which were standardized on a well-characterized piece of natural afghanite (Seo et al., 2011). Elemental concentration ratios measured in the fluid inclusions were converted to absolute values by using Na as the internal standard. We used Na from the microthermometrydetermined NaClequiv. value and applied the empirical correction procedure of Heinrich et al. (2003) to account for the presence of other major cations. Crystallized melt inclusions in apatite were also analyzed by drilling out entirely from the host without prior homogenization. Original melt composition was calculated based on the procedure described in Halter et al. (2002b). External standardization was again based on NIST SRM 610 glass. Internal standardization of melt inclusions was based on SiO2 vs. Al2O3 trends displayed by whole-rock data in Appendix Table A1. Melt inclusions were quantified by normalizing the sum of all major element oxides to 100 wt%. Overall uncertainties in the calculated element concentrations of melt inclusions are considered to be better than 15% (Pettke, 2006). 4. RESULTS 4.1. Petrography and composition of magmatic apatite Apatite mostly occurs as euhedral and subhedral crystals both in phenocrysts (e.g., hornblende and biotite) and groundmass of all the porphyries (Figs. 4 and A1), despite

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the range of grain size is variable in different porphyries (50–200 lm in the MGP, 150–300 lm in the KGP and 70– 150 lm in the QAP). It is notable that apatite is commonly characterized by a perceived core containing numerous fluid ± melt inclusions and a perceived rim essentially free of inclusions (Fig. 4B), with several grains having more complex or reversal textures. The inclusion-rich apatite accounts for 27–30% of all apatite in each porphyry; the distribution of the inclusion-rich apatite, however, show no systematic difference to other (normal) magmatic apatite. A total of 206 EMP spot analyses were made on 96 apatite grains, and the results are tabulated in Table A2 and are illustrated in Fig. 5. The F measurements of most apatite (up to 6.6 wt% F) exceed the maximum F content for ideal, end-member fluorapatite (e.g., 3.76 wt% F; Goldoff et al., 2012). These high F values are analytical artifacts associated with F migration induced by electron beam parallel to the c-axis of apatite (Goldoff et al., 2012). Some high Cl values are also likely problematic for the same reason, but the data for other elements are reliable. Apatite in different porphyries shows similar range of composition, except for the higher Mg in apatite from QAP dikes. The overall SO3 concentrations in apatite range from 0.00 to 1.37 wt% (Fig. 5A). The inclusion-rich core of the apatite grains exclusively contains higher SO3 than the rim of the same grains (Fig. 4B). The concentrations of Na2O, SiO2, Ce2O3 and La2O3 in the inclusion-rich core are also consistently higher, but CaO and P2O5 values are generally lower. EMP-WDS mapping results of an inclusion-rich apatite grain also reveal zoning in S contents, with the high S concentration of the inclusion-rich core corresponding well to the elevated Na, Ce, and Si (Fig. A2). There is no apparent compositional difference between the inclusion-poor rim of inclusion-rich apatite and normal magmatic apatite. The occurrence of magmatic anhydrite as phenocrysts (Fig. 4A) or as inclusions in apatite indicates oxygen fugacities higher than 2.2 log units above the fayalite-magnetitequartz buffer for the magmas (Parat et al., 2011). This consideration suggests that sulfur incorporates mainly as sulfate (S6+) into apatite by replacing P5+ via coupled substitutions (Konecke et al., 2017; Kim et al., 2017). Several exchange reactions governing sulfur concentrations in apatite have been proposed (Liu and Comodi, 1993; Rouse and Dunn, 1982; Tepper and Kuehner, 1999; Parat et al., 2011), such as: P5þ +Ca2þ $ S6þ +Naþ

ð1Þ

2P5þ $ S6þ +Si4þ

ð2Þ

2P5þ +2Ca2þ $ S6þ +Si4þ +Naþ +REE3þ

ð3Þ

with the Ca-site accommodating large cations (e.g., Na+, REE3+) and the P-site accommodating smaller highly charged cations (e.g., S6+, Si4+). Exchange reactions involving REE3+ (Eq. (3)) produce a good correlation between these elements for all apatite and the data plot nicely on the 1:1 trend line (Fig. 5B), whereas the simple substitutions described by Eqs. (1) and (2) produce poor correlations (Fig. 5C and D). This observation implies that the incorporation of sulfur in apatite is controlled predominantly by the coupled substitution mechanism as described in Eq. (3).

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Fig. 4. Back-scattered electron (BSE) images showing textures of magmatic apatite in porphyry intrusions. (A) Apatite closely associated with magmatic anhydrite phenocrysts in MGP. The groundmass is composed of K-feldspar, quartz, and plagioclase. The insert is an inclusion-rich, zoned apatite grain under Optical-CL. (B) An inclusion-rich apatite from KGP. The numbers adjacent to white spots are SO3 concentrations (wt%) revealed by EMP analysis. The numerous small holes in the sulfur-rich core are mostly exposed fluid inclusions. Abbreviations: anh = anhydrite; apa = apatite; bio = biotite; ksp = K-feldspar; plag = plagioclase; qtz = quartz.

4.2. Quartz vein petrography and SEM-CL images Quartz in A2E veins at Yulong is generally anhedral and interlocked (Fig. 6A and B), but local subhedral to euhedral grains also document open-void crystallization. Similar to observations made on quartz-sulfide vein stockworks in other porphyry Cu deposits (Redmond et al., 2004; Rusk, 2012), the A2E veins record two main quartz generations of distinct textures (Fig. 6D), i.e., a predominant and bright-luminescent quartz with white to light gray color (CL-bright) and a secondary, volumetrically smaller and dark-luminescent quartz with dark gray to black color (CL-dark). The CL-bright quartz has weakly oscillatory zoning to homogeneous texture with minor K-feldspar, biotite, and anhydrite inclusions, whereas the CL-dark quartz occurs as a microfracture network along CL-bright quartz

grain boundaries, filling spaces that resulted from fracturing and partial dissolution of CL-bright quartz. The CLdark quartz is also locally in contact with hydrothermal K-feldspar and biotite. Molybdenite in A2E veins is either included in the CL-bright quartz grains or present along the grain boundaries (Fig. 6B and C), indicating that molybdenite precipitated simultaneously with the CLbright quartz. However, chalcopyrite and pyrite in A2E veins are typically in contact with or surrounded by the CL-dark quartz, or filled the open space confined by subhedral to euhedral CL-bright quartz crystals. Therefore, CuFe sulfides are interpreted to precipitate after molybdenite and the bulk CL-bright quartz in A2E veins. Pargenetically earlier ME, EBE and A1E veins are largely composed of homogeneous or oscillatory zoned CL-bright quartz, except for the EBE veins in the shallow ore zone that are

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Fig. 5. Composition of magmatic apatite from various porphyry intrusions at Yulong. (A) SO3 versus La2O3 + Ce2O3 contents. (B) S6+ + Ce3+ + La3+ versus Si4+ + Na+ in atoms per formula unit (apfu). Note the good linear correlation relationship. (C) Na+ vs. S6+ in apfu. (D) Si4+ vs. S6+ in apfu. Linear correlation coefficients (R2) for all analyses in (B)-(D) are 0.93, 0.55 and 0.67, respectively.

overprinted by chalcopyrite ± pyrite together with CL-dark quartz. Minor amounts of subhedral to euhedral quartz in A3E veins generally show oscillatory zonation with gray to dark luminescence, features comparable to the CL-dark quartz in EBE and A2E veins. The A1T and A3T veins show similar luminescent characteristics to A1E and A3E veins, but the CL-bright quartz in BT veins has more contrasting oscillatory growth zones compared to A2E veins (Fig. 6E). Molybdenite flakes occur along the margins of BT veins and cut the oscillatory growth zones of the CL-bright quartz, indicating early precipitation of molybdenite during the vein formation. Minor amounts of chalcopyrite ± pyrite in BT veins precipitated later in the central voids confined by the symmetrical quartz and molybdenite. K-feldspar, anhydrite, and ankerite are occasionally observed as inclusions in the CL-bright quartz. The pyrite-dominated DL veins contain only minor amount of quartz, which occurs as elongated, euhedral crystals aligning along the vein walls. In SEM-CL images, the quartz in the DL veins is generally dull luminescent with sector zones that are overgrown by very bright-luminescent and oscillatory zoned quartz (Fig. 6F), which are in contact with minor tennantite and enargite.

4.3. Petrography of fluid and melt inclusions 4.3.1. Fluid and melt inclusions in magmatic apatite Fluid inclusions in the inclusion-rich apatite from all porphyry intrusions are dominated by intermediatedensity (ID) inclusions (Fig. 7). The ID inclusions have a bubble accounting for 30–60 vol%. Some isolated ID inclusions contain unknown crystal phases that may account for >50 vol% of the inclusions, which are likely resulted from heterogeneous entrapment and thus will not be considered further. Given that the numerous ID inclusions are commonly confined in the core of inclusion-rich apatite and locally with unusually large size (Fig. 7), they are considered to have a primary origin (Roedder, 1984). Melt inclusions and similar mineral inclusions (such as titanite, zircon, K-feldspar, biotite, quartz, magnetite and anhydrite) are present in both inclusion-rich apatite and other normal magmatic apatite. 4.3.2. Fluid inclusions in quartz veins Fluid inclusions in quartz veins are classified according to phase types and ratios at room temperature (Fig. 8). Fluid inclusions with a halite daughter mineral, a saline

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Fig. 6. Photomicrograph (A), BSE image (B) and SEM-CL images (C-F) illustrating textures of representative A2E, BT and DL veins. (A) Part of an A2E vein under transmission light. Note the distribution of molybdenite and chalcopyrite. (B) Mosaic texture of quartz in molybdeniterich zone of the A2E vein in (A). Molybdenite ± anhydrite either exists in quartz grains or along grain boundaries. (C) Coexisting molybdenite and CL-bright quartz in the A2E vein in (A). (D) CL-bright quartz (qtz1) overgrown by CL-dark quartz (qtz2) in the A2E vein in (A). (E) Coexisting molybdenite and oscillatory zoned, CL-bright quartz. (F) Brecciated A2E vein (qtzA) cemented by a DL vein (qtzB). The qtzB is characterized by an abrupt change from dull-luminescent quartz to very bright-luminescent quartz. Abbreviations: anh = anhydrite; bio = biotite; cp = chalcopyrite; ksp = K-feldspar; mo = molybdenite; qtz = quartz; tn = tennantite.

aqueous liquid, and a relatively small vapor bubble are termed as brine inclusions (B type; Fig. 8C–E). Some brine inclusions contain an additional sylvite daughter crystal, plus tiny transparent and opaque minerals (Fig. 8C). The other fluid inclusions primarily consisting of an aqueous

liquid phase and a vapor bubble are subdivided into (1) intermediate-density inclusions containing 30–60 vol% bubble (ID type; Fig. 8A), (2) vapor inclusions containing a bubble accounting for >60 vol% of the inclusions (V type; Fig. 8B and F), and (3) liquid-rich inclusions containing a

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smaller bubble with a volume of 10–30% (L-rich type; Fig. 8G). The ID inclusions typically contain a small opaque and/or a transparent daughter crystal(s) (Fig. 8A). A condensed liquid-CO2 phase was occasionally observed in ID and vapor inclusions (Fig. 8B). Microthermometric measurements and LA-ICP-MS analysis of fluid inclusions in quartz veins focused on fluid inclusion assemblages (FIAs), which were strictly defined as fluid inclusion vacuoles along the same growth zone in a single crystal or along a single healed fracture that formed at about the same time or during the same set of P-T conditions (Goldstein and Reynolds, 1994). Fluid inclusions distributed along growth zones in euhedral quartz are extremely rare and commonly too small to be measured. Fluid inclusions that are suitable for analysis occur either as pseudosecondary/secondary trails or as clusters (the latter cannot be unequivocally equivalents to FIAs). In this study, when fluid inclusions from individual clusters have similar phase ratios, they are considered as analogues to FIAs. The ID, brine and vapor inclusions occur predominantly in the early- and transitional-stage veins (Tables 1 and 2). The brine and vapor inclusions are commonly associated with each other, and occasionally coexisting on boiling FIA trails (Fig. 8D). The liquid-rich inclusions are mainly hosted in the late-stage DL veins, where they locally coexist with vapor inclusions (namely boiling FIAs). The random distribution and superimposition of numerous fluid inclusions in quartz veins made it very difficult to precisely determine their temporal relationship. Therefore, some fluid inclusions analyzed in certain quartz veins were very likely trapped after the vein formation. Nevertheless, the distribution pattern of fluid inclusions in various quartz veins throughout the deposit are still useful to evaluate the fluid evolution processes that have led to hydrothermal alteration and Cu-Mo mineralization. The relative abundance of each fluid inclusion types in various veins was visually estimated based on 22 thick sections that contain abundant well-preserved fluid inclusions (Table 2). Fifteen early-stage veins from the two deepest drill cores (i.e., ZK1007 and ZK1303) were used to document the spatial distribution of different fluid inclusion types (Fig. 9). Brine inclusions generally exceed 50% of all fluid inclusions in the EBE and A2E veins from the Cu-Mo ore zone,

Fig. 7. Photomicrograph illustrating abundant fluid inclusions (FI) in the core of an inclusion-rich apatite grain.

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whereas the abundance of vapor inclusions is generally less than 30%. In contrast, ID inclusions are most abundant in A1E veins in the deep barren zone (46–66%). Liquid-rich inclusions are less abundant (1–8%) and commonly occur as secondary trails in the early-stage veins. The A1T and BT veins have similar abundance of different fluid inclusion types comparing to the early-stage veins in the shallow ore zone, except for one BT vein that contains more ID inclusions. Liquid-rich inclusions dominate in DL veins (>90%). 4.4. Microthermometry 4.4.1. Fluid inclusions in magmatic apatite Only five ID inclusions in the core of the inclusion-rich apatite from a KGP sample were measured for ice melting temperature, as most large fluid inclusions were destroyed during LA-ICP-MS analysis before microthermometric measurements. The results correspond to calculated apparent salinities of 8.4–13.6 wt% NaClequiv with an average of 10.2 wt% NaClequiv. 4.4.2. Fluid inclusions in quartz veins Microthermometric results of FIAs (including fluid inclusions with similar phase ratios in clusters) in vein quartz are summarized in Table A3, and illustrated in Fig. 10. For ID inclusion assemblages without a visible liquid-CO2 phase at room temperature, CO2 clathrate was detected rarely by microthermometry (the detection limit of CO2 is 1–2 mol%; Rosso and Bodnar, 1995). These FIAs are therefore assumed to contain no significant amounts of CO2, and thus their salinities were calculated from ice melting temperatures. The salinities of a few ID FIAs that contain a condensed liquid-CO2 phase were calculated by clathrate melting temperatures. ID fluid inclusions in individual assemblages are relatively consistent in final ice/clathrate melting temperatures, whereas the total homogenization modes and temperatures of them are variable (Table A3). The early- and transitional-stage ID FIAs have similar ranges of ice/clathrate melting temperature translating into similar salinity ranges of
5–15 wt% NaClequiv. (Fig. 10A). Brine FIAs in EBE, A1E and A2E veins have salinities of 35.3–49.5 wt% NaClequiv. calculated from the final dissolution of halite at 260–419 °C, and have bubble disappearance temperatures ranging from 238 to 409 °C (Fig. 10). Similarly, brine FIAs in A1T and BT veins have salinities of 32.2–45.5 wt% NaClequiv. derived from the final halite dissolution temperatures between 206 and 381 °C, and have bubble disappearance temperatures of 263–409 °C. The brine inclusions of both stages mostly reached total homogenization by halite dissolution (Fig. 10B). Microthermometric measurements on vapor inclusions were very difficult because of the poor visibility of phase boundaries. A relatively high-density vapor inclusion from an A2E vein has a salinity of 5.0 wt% NaClequiv. (based on ice melting temperature) and homogenized into vapor at 509 °C. The salinity of this high-density vapor inclusion is likely elevated by co-entrapment of some brine phase. Liquid-rich FIAs in A1E, A2E and DL veins have consistent microthermometric behavior within individual

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Fig. 8. Photomicrographs showing different types of fluid inclusions in quartz veins. (A) Two intermediate-density fluid inclusions from a FIA trail in a deep-seated A1E vein. Note the small opaque and transparent daughter crystals in the inclusions. (B) Vapor inclusion in an A2E vein with a small proportion of liquid-CO2 phase. (C) A polyphase brine inclusion in an A2E vein with a halite, a sylvite, an unknown transparent crystal, and an unknown opaque crystal. (D) Coexisting brine and vapor inclusions on a boiling trail in a shallow A1E vein. (E) A cluster of brine inclusions in an A1T vein. (F) Vapor inclusions on a trail in an A2E vein. (G) Liquid-rich inclusions on a trail in a DL vein.

Typical LA-ICP-MS signals of a quartz-hosted brine inclusion are shown in Fig. 11. Element ratios relative to K of all the analyzed fluid inclusions from magmatic apatite and various quartz veins are shown in Fig. A3 (selectively shown in Fig. 12). We compare fluid inclusion data as element ratios relative to K considering that this element is unlikely to diffusion in or out a fluid inclusion considering its large ionic radius (Zajacz et al., 2009). To do this, the uncertainties caused by salinity correction for absolute concentrations can be eliminated and the compositions of different types of fluid inclusions are comparable. Absolute element concentrations of fluid and melt inclusions are tabulated in Tables 3, 4, A1, and A4 and selectively illustrated in Figs. 13 and 14. These results are described below.

KGP have pure rhyolitic compositions with 77–78 wt% SiO2 and 4–5 wt% K2O, and have concentrations of Cu (
8 ppm) and Mo (<2 ppm) at or below the detection limits (Table A1). These melt inclusions are enriched surprisingly in incompatible elements of Cs (
3000 ppm) and Rb (
1000 ppm) compared to the whole-rock data (Table A1), indicating an extremely evolved melt (Aude´tat and Pettke, 2003). The average salinity of the five ID inclusions measured by microthermometry was adopted as internal standard to quantify the 32 LA-ICP-MS analyses (Table 3). The element/K ratios of ID inclusions hosted in apatite from the three stages of porphyry intrusions are essentially constant (Fig. A3). The ID inclusions in apatite, compared to ID inclusions in vein quartz, have generally higher Cs/K and As/K ratios, and significantly lower Cu/K and Mo/K ratios, though a few apatite-hosted ID inclusions from sample MGP50 are relatively low in Cs/K ratio (Fig. 12). Interestingly, the element/K ratios of ID inclusions in apatite is similar to that of liquid-rich inclusions in the late-stage DL veins.

4.5.1. Fluid and melt inclusions in apatite Most melt inclusions in apatite co-trapped at least one mineral inclusion (e.g., zircon and titanite), rendering it very difficult to precisely constrain the compositions of the melts. Only two melt inclusions analyzed in apatite from

4.5.2. Fluid inclusions in quartz veins Internal standardization for most brine inclusions was achieved by averaging the salinity of fluid inclusions that survived destructive LA-ICP-MS elemental analysis. This approach is associated with an uncertainty of about 5%,

assemblages (Fig. 10A). They have apparent salinities between 3.0 and 7.3 wt% NaClequiv according to their final ice melting temperatures, and homogenized into the liquid phase at temperatures of 167–372 °C. 4.5. Composition of fluid and melt inclusions

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Table 2 A summary of Cu and Mo contents and relative abundance of various fluid inclusions in quartz veins of different stages. Sample name

Early-stage veins 1303-772 1303-788 1303-820 1007-535 1007-699 1007-207 1303-614 1303-598 1303-373 1303-222 1303-133 1303-113 1303-51 1303-14 1007-47

Vein type

Cu (wt%)

Mo (wt%)

Fluid inclusion type (%) ID

V

B

L-rich

A1E vein A1E vein A1E vein A1E vein A1E vein EBE vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein

0.03 0.05 0.14 0.04 0.03 0.10 0.32 0.37 0.80 1.13 2.01 2.42 2.07 0.40 0.75

0.01 0.01 0.01 <0.01 <0.01 <0.01 0.01 0.03 0.04 0.20 0.10 0.19 0.32 0.32 0.01

0.63 0.66 0.55 0.46 0.48 0.15 0.20 0.10 0.12 0.14 0.11 0.10 0.07 0.04 0.09

0.10 0.15 0.09 0.37 0.28 0.20 0.24 0.18 0.21 0.28 0.32 0.25 0.20 0.16 0.19

0.18 0.13 0.31 0.15 0.19 0.62 0.54 0.70 0.63 0.55 0.52 0.61 0.71 0.78 0.71

0.08 0.06 0.05 0.02 0.04 0.04 0.02 0.01 0.04 0.03 0.04 0.03 0.03 0.02 0.01

Transitional-stage veins 0908-312 A1T vein 0908-459 A1T vein 0904-479 BT vein 0912-302 BT vein 1103-497 BT vein

0.21 0.10 0.26 0.17 0.29

0.02 0.04 0.18 0.10 0.02

0.13 0.14 0.44 0.15 0.14

0.34 0.34 0.43 0.39 0.24

0.49 0.48 0.06 0.42 0.59

0.04 0.04 0.07 0.03 0.03

Late-stage veins 1205-342 1007-372

0.40 0.11

0.03 <0.01

-

0.05 0.10

0.00 0.01

0.95 0.90

DL vein DL vein

which, together with uncertainty of 5–10% associated with the external standards, results in a total uncertainty of 10–15%. The salinity of
20% brine inclusions was only roughly estimated (no microthermometric measurement on the same FIAs), introducing a total uncertainty of up to 40% if the entire salinity variation of brine inclusions is considered. For ID, vapor and liquid-rich inclusions, microthermometry and LA-ICP-MS analysis were not performed on the same assemblages due to organizational reasons. Therefore, we adopted the average salinity of all measured assemblages of a given fluid inclusion type as internal standard. Due to the relatively large range of measured fluid salinity, this quantification is associated with an uncertainty of about a factor of two. Microanalysis by LA-ICP-MS was carried out on 120 individual fluid inclusions in various quartz veins. Fluid inclusions from the same assemblages are remarkably reproducible in composition (Table A4). Generally, all inclusions contain Na, K, and Fe as major cations, and Ca, Mn, S, Cu, Pb, and Zn as minor components, with traces of Rb, Cs, Mo, W, As, Bi, and Ag (Table 4). The concentration ratios of Fe/K, Mn/K, Pb/K, Zn/K, Rb/K in all fluid inclusions in quartz veins are essentially constant, whereas element/K ratios of the other elements are variable, especially for Cu/K and Mo/K ratios that vary over two orders of magnitude (Fig. A3). The variation of Cu/K and Mo/K ratios is particularly prominent in brine inclusions. ID and vapor inclusions have higher Cu/K,

S/K, and As/K ratios than brine inclusions. Liquid-rich inclusions in DL veins are generally higher in Cs/K and As/ K ratios and lower in Cu/K and Mo/K ratios compared to the other types of quartz-hosted fluid inclusions (Fig. 12). An average salinity of 9.1 wt% NaClequiv. was adopted as internal standard for the calibration of ID inclusions. The early- and transitional-stage ID inclusions are very similar in the abundance of all the analyzed elements. They have broadly constant concentrations of Na, K, Mn, Fe, Zn, Pb, Rb, and Cs, but variable concentrations of the other elements (e.g., 0.2–2.0 wt% S, 0.01–1.7 wt% Cu and 1–87 ppm Mo). The early- and transitional-stage brine inclusions are also very similar in the analyzed elements. They both contain fairly reproducible concentrations of Na, K, Fe, Mn, Pb, Zn, Cs, Rb, Ag, and Bi, but the values are highly variable for the other elements (e.g., 0.4–2.7 wt% S, 0.005–2.7 wt% Cu, 3–480 ppm Mo). Cu and Mo concentrations in the early-stage brine inclusions show a remarkable evolution trend through different quartz vein types (Fig. 14): the barren A1E and EBE veins contain a group of brine inclusions with high Mo (320–480 ppm) and Cu (0.7–1.2 wt%) concentrations, whereas such brine inclusions are absent in mineralized A2E veins (quartz + molybdenite ± chalcopyrite veins). A similar trend is also observed in the transitional-stage brine inclusions. An average salinity of 5.1 wt% NaClequiv. was used as internal standard to quantify the liquid-rich inclusions.

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et al., 2002; Van Hoose et al., 2013). However, sulfur content in apatite crystallized from ‘‘normal” rhyolitic melts under sulfate-saturated condition is unlikely to exceed
0.7 wt% SO3 based on results of previous experimental studies and analyses of natural samples (see figure 16 in Parat et al. (2011); also see discussion in Van Hoose et al. (2013)). This inconsistency requires an alternative explanation for the origin of sulfur-rich apatite. An important observation in this study is that the S-rich domain (0.3– 1.4 wt% SO3) of apatite is coupled with the fluid ± melt inclusions-rich core of the same apatite grains, and that the melt- and fluid inclusions display an extremely evolved nature marked by unusually high Cs concentrations (Table A1; Fig. 12). It is notable that rhyolitic melts with several thousands ppm Cs (plus
800 ppm boron) are very rare and usually confined to pegmatitic environment (Aude´tat and Pettke, 2003). Similar scenario applies to the high Cs-content of the fluid inclusions. Therefore, it is unlikely that these sulfur-rich cores of magmatic apatite have formed from ‘‘normal” melts. A more possible scenario is that these sulfur-rich cores crystallized from pegmatitic melts, which is expected in the process of solidification of felsic magma reservoirs in the underlying magma chamber. The porphyry intrusions at Yulong with various whole-rock compositions (62–72 wt% SiO2) contain abundant inclusion-rich apatite (
27–30% of all apatite) that occurs commonly as inclusions in phenocrysts (e.g., hornblende and biotite). These intrusions, therefore, either assimilated or mixed with some highly evolved melt fractions, from which the sulfur- and fluid-inclusion-rich cores were inherited. 5.2. Post-entrapment modification of fluid inclusions in quartz veins

Fig. 9. Schematic illustration of vertical distribution of various fluid inclusion types in the early-stage veins.

They are relatively constant in Na, K, As, Cu, Zn, Pb, Rb, and Cs, with the remaining elements being scattered or below the limit of detection. 5. DISCUSSION 5.1. Origin of sulfur-rich magmatic apatite Apatite in magmatic rocks may serve as an indicator for sulfur contents of their parental magmas, but the elusive origin of the sulfur-rich apatite in felsic magmas has hampered its use as a monitor for magmatic sulfur evolution (Streck and Dilles, 1998; Parat et al., 2011; Van Hoose et al., 2013). Magmatic apatite in the porphyry intrusions (62 wt% SiO2 of whole rock; 77–78 wt% SiO2 of melt inclusions in apatite) at Yulong shows a large variation in SO3 contents from 0 to 1.4 wt%. Similar variations (0–2 wt% SO3) have also been documented for apatite from oxidized, felsic magmas related to porphyry Cu systems and arc volcanos elsewhere (Streck and Dilles, 1998; Parat

5.2.1. Interpretation of microthermometric results Post-entrapment modification of fluid inclusions is common in porphyry systems but has not been sufficiently considered in the interpretation of fluid inclusion data. Such modification may result in changes in bulk fluid inclusion density (e.g., stretching or shrinkage; Roedder, 1984) and gain or loss of materials (e.g., H2 and H2O) that may change both fluid inclusion composition and bulk density (Aude´tat and Gu¨nther, 1999). In this study, over 300 doubly polished sections were examined, with only several dozen samples being suitable for microthermometry and LA-ICP-MS analysis. In addition, FIAs in the early- and transitional-stage quartz veins with obvious features of post-entrapment modification such as deformation or decrepitating halos were largely avoided, but the following lines of evidence suggest that the best-preserved ID and brine FIAs still commonly suffered from post-entrapment modification: (1) some unambiguous boiling assemblages homogenized by final dissolution of halite at much higher temperature than that of bubble disappearance; (2) individual ID or brine fluid inclusions in some FIAs show different homogenization behaviors and/or large ranges of homogenization temperature; and (3) two thirds of brine inclusion assemblages homogenized to liquid by halite dissolution with a minimum entrapment pressure estimated at up to

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Fig. 10. Summary plots of microthermometric results of fluid inclusion assemblages. (A) Homogenization temperature (Thom tot) versus salinity of fluid inclusions. (B) Halite melting temperature (Tm halite) versus bubble disappearing temperature (Th liquid-vapor) of brine inclusion assemblages. Isobars based on equation of Becker et al. (2008) are shown to constrain the homogenization pressure of brine inclusions that homogenize by the final melting of halites.


3 kbar (Fig. 10B; Becker et al., 2008), whereas the real fluid entrapment pressure was likely <1 kbar (see section 5.4). However, a few brine inclusion assemblages that homogenized to liquid by halite dissolution still likely formed either by further cooling of brine fluids after phase separation or by heterogeneous entrapment of liquid + halite at halite saturated conditions (Becker et al., 2008). The relatively consistent salinity of most ID and brine FIAs indicate that changes in bulk density and/or slight gain or loss of material have little influence on salinity, whereas the homogenization temperatures of most ID and brine inclusions are considered unreliable. The liquid-rich inclusions in DL veins show consistent microthermometric behavior and thus are unlikely to have suffered from strong postentrapmental modification. Fig. 11. LA-ICP-MS signals of a brine inclusion hosted in quartz (sample 0905-233B, analysis 15Oc28g05) showing the chosen integral interval for the fluid inclusion. To reduce the risk of uncontrolled decrepitation and associated loss of daughter minerals, the quartz was ablated with a small beam diameter, which then was increased in several steps. The inclusion contained 11 wt% Na, 1.0 wt% S, 8700 ppm Cu, 480 ppm Mo, 224 ppm Cs and 17 ppm Ag.

5.2.2. Elemental diffusion and representativeness of fluid inclusions Rapid post-entrapment diffusion of Cu, Ag, Li, and Na in and out of fluid inclusions via cation exchange through quartz has been well documented experimentally (e.g., Zajacz et al., 2009; Li et al., 2009; Lerchbaumer and

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samples indicate that S-rich vapor inclusions in quartz experienced a dramatic increase in Cu, whereas brine inclusions remained largely unmodified (Lerchbaumer and Aude´tat, 2012; Seo and Heinrich, 2013). The variably elevated Cu/K ratio in ID and vapor inclusions at Yulong is most likely due to post-entrapment diffusion of Cu. This view is confirmed by the fact that the ID and vapor inclusions with higher S/K ratio also contain higher Cu/K ratio, but there is no such correlation between S/K and Cu/K ratios in brine inclusions (Fig. A4). ID inclusions are also variable in some other components (e.g., Mo/K and Cs/K ratios in Fig. 12B and C). Such variations are likely caused by the inhomogeneity of initial input fluids, the precipitation of certain elements from fluids before phase separation and/or the misclassification of some vapor or liquid-rich inclusions. The early-stage ID inclusions in deep A1E veins and the two transitionalstage ID inclusions in quartz phenocrysts of KGP are considered as representative of the least differentiated input ID fluids (Fig. 13A). The early- and transitional-stage brine inclusions in EBE and A1E&T veins with high Cu and Mo concentrations likely represent the least differentiated brines (Fig. 13B), as these veins are pre-mineralization and only occasionally overprinted by some late chalcopyrite ± pyrite. 5.3. Evolution of source magmas and initial ore-forming fluids

Fig. 12. Cs/K versus Cu/K, Mo/K, and As/K ratios of all the analyzed fluid inclusions in quartz veins and apatite. Mo/K ratios of intermediate-density inclusions in apatite and liquid-rich inclusions in DL veins are generally below the detection limit.

Aude´tat, 2012). Diffusion of those elements is consistent with the fact that univalent ions with a small radium diffuse rapidly through channels of quartz structure. A recent experimental study and a comparative study of vapor and brine inclusions from coexisting quartz and topaz in natural

The Yulong deposit is a long-lived magmatichydrothermal system fed by an underlying upper crustal magma chamber (Chang et al., 2017). Geologic evidence and molybdenite Re-Os ages on A2E veins revealed at least two pulses of mineralization during the early stage in the time interval of 42.28 ± 0.17–41.46 ± 0.17 Ma (Chang et al., 2017). The transitional-stage mineralization, which is mainly represented by BT veins, formed rapidly from 40.98 ± 0.23 to 40.92 ± 0.17 Ma. Two significantly younger, noneconomic mineralization events (39.56 ± 0.17 and 37.15 ± 0.15 Ma) postdated the intrusion of QAP dikes. It is thus concluded that the Yulong deposit was produced from multiple magmatic-hydrothermal pulses spanning a time period of 5.13 ± 0.23 m.y., but the bulk Cu-Mo mineralization likely formed in a shorter but still long interval of 1.36 ± 0.24 m.y. in at least three major pulses. The prolonged duration for the bulk ore formation at Yulong potentially indicates significant compositional variation of the magmatic hydrothermal fluids as a result of extended magma fractionation (Cline and Bodnar, 1991). For example, the Questa porphyry Mo deposit formed over a period of
0.25 m.y. (Rosera et al., 2013). The second pulse of intermediate-density ore-forming fluids at Questa contained
20 times higher Cs concentration than the first one, observations interpreted to reflect major crystal fractionation of the source magma (Klemm et al., 2008). However, initial single-phase ID ore-forming fluids of the early and transitional stage at Yulong (see section 5.4) were likely very similar, because the least differentiated ID and brine inclusions from the two stages are essentially the same in

Table 3 Results of LA-ICP-MS analysis of fluid inclusions in magmatic apatite. Sample name

1

Na wt%

Cl wt%

K wt%

Mn wt%

Fe wt%

Cu ppm

Zn ppm

As ppm

Rb ppm

Mo ppm

Ag ppm

Cs ppm

W ppm

Pb ppm

Bi ppm

10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2 10.2

2.9 2.7 3.0 3.2 3.5 3.5 3.2 3.5 3.1 3.1 2.8 2.8 3.1 2.8 2.8 3.0 2.8 3.7 3.0 3.4 3.2 3.7 3.6 3.9 3.2 2.6 3.5 3.2 3.0 3.0 2.8 1.4

8.6 12 10 21 11 14 13 6.5 11 11 11 8.8 11 13 13 11 12 9.0 10 10 8.8 12 13 9.1 8.8 8.1 11 8.7 9.5 8.8 7.8 4.2

1.6 1.8 1.3 1.3 1.2 1.2 2.2 0.9 1.2 1.2 1.4 1.6 1.4 1.1 1.5 1.2 1.4 0.9 1.3 1.4 1.6 1.8 1.7 1.3 1.1 1.1 1.2 1.3 1.2 1.3 1.0 0.7

0.37 0.75 0.47 0.21 0.00 0.00 <0.50 0.00 0.15 0.40 0.34 0.13 0.30 0.40 0.51 0.49 0.53 0.00 0.39 0.25 0.21 0.17 0.24 0.19 0.17 0.26 0.38 0.26 0.37 0.33 0.28 0.12

0.75 0.74 0.63 0.66 0.14 0.23 0.45 0.27 0.86 0.61 1.1 1.3 0.65 1.3 0.96 0.80 0.92 <0.01 0.80 0.57 0.49 0.49 0.29 0.37 0.88 2.2 1.1 0.97 0.96 0.78 1.7 5.2

290 79 74 44 44 49 55 22 43 27 49 150 82 110 110 150 110 91 88 54 28 51 53 28 210 66 74 47 57 97 56 23

2700 2100 1300 1600 500 1100 110 530 1200 2800 1700 1900 1300 1800 1600 1100 1600 370 1200 860 1100 690 1100 710 1100 1000 2000 1400 2000 310 2500 570

210 150 240 130 <6.0 270 390 <4.0 170 65 87 490 <4.0 490 520 440 280 510 150 620 500 550 540 320 370 270 710 310 720 270 460 370

270 210 170 180 240 200 490 150 240 260 260 330 260 220 300 280 310 270 250 370 370 290 260 190 200 220 280 240 250 270 230 150

<3.3 <1.0 <1.4 22 <1.7 2.3 <290 <1.2 <0.6 <0.5 1.1 <1.3 <5.8 1.1 <0.6 <0.6 <0.9 <1.1 <1.4 <0.9 <0.3 <0.7 <0.9 <1.0 <0.1 <0.3 <1.3 <0.5 <0.9 <0.2 <0.4 <0.1

6.9 6.0 6.1 3.8 2.7 4.1 4.9 1.5 2.3 3.1 3.2 6.6 13 6.0 4.9 4.6 5.3 2.8 4.0 4.7 3.5 5.1 4.9 3.7 4.5 3.5 5.4 3.9 5.0 4.0 2.7 1.5

990 1100 670 310 380 650 380 170 670 650 460 1100 1100 1200 1400 1100 1200 460 1000 1300 1200 780 990 820 730 790 1800 1100 1100 1100 850 410

<4.5 <0.8 <2.3 <1.5 <3.2 31 <1.0 5.0 2.2 3.0 3.2 15 4.5 <0.7 <0.8 <1.4 3.6 <0.9 <1.6 <1.7 1.5 3.2 1.8 3.0 4.1 630 <2.6 2.3 2.3 2.2 88 220

1500 1900 1800 1600 1800 1100 290 1300 1600 2400 1500 2000 1500 1500 1800 1400 1800 520 1300 1900 1800 1400 1500 1300 1600 1300 1800 1600 1900 1600 1300 760

13 22 24 5.2 11 7.4 3.4 7.1 15 16 35 23 18 18 32 28 25 1.7 21 7.6 6.7 5.0 6.5 12 15 11 18 20 15 8.5 18 3.7

J. Chang et al. / Geochimica et Cosmochimica Acta 232 (2018) 181–205

MGP998 apa16 25um ID MGP998 apa1 20um ID MGP998 apa4 17um ID MGP50 apa1 22um ID MGP50 apa12 20um ID MGP50 apa13 27um ID MGP50 apa10 30um ID MGP50 apa6 20um ID MGP50 apa9 20um ID MGP50 apa15 20um ID MGP50 apa31 22um ID MGP985 apa1 20um ID MGP985 apa25 20um ID MGP985 apa33 20um ID MGP985 apa31 27um ID MGP985 apa29 16um ID MGP985 apa28 19um ID1 MGP985 apa28 45um ID2 MGP985 apa23 25um ID QAP320 apa18 22um ID QAP320 apa17 33um ID QAP320 apa14 24um ID QAP320 apa13 24um ID QAP320 apa9 17um ID KGP438 apa1 40um ID KGP438 apa3 27um ID KGP438 apa12 20um ID1 KGP438 apa12 28um ID2 KGP438 apa15 33um ID KGP438 apa22 22um ID KGP438 apa25 28um ID KGP438 apa34 27um ID

Salinity NaClequiv.1

Average salinity of measured fluid inclusions used for LA-ICP-MS data quantification.

197

Vein type

Sample

salinity NaClequiv.1

Na wt%

198

Table 4 Representative LA-ICP-MS analyses of fluid inclusions in quartz veins. S wt%

K wt%

Ca wt%

Mn wt%

Fe wt%

Cu ppm

Zn ppm

As ppm

Rb ppm

Mo ppm

Ag ppm

Cs ppm

W ppm

Pb ppm

Bi ppm

intermediate-density (ID) inclusions in deep A1E veins 1007-535A ID(?) 14 lm 9.1 2.8 0705-420A ID1 14 lm 9.1 2.7 0705-420B ID1 25 lm 9.1 2.4 0705-420D ID1 17 lm 9.1 2.7 1303-772A ID2 30 lm 9.1 2.8 1303-772B ID 20 lm 9.1 2.4 1007-669 ID2 17 lm 9.1 2.6 0905-577B ID1 17 lm 9.1 2.8 0905-577D ID2 16 lm 9.1 1.8

<1.1 0.51 <0.26 <0.46 0.44 <1.3 <0.13 <0.61 <1.1

<0.49 0.77 0.95 1.2 1.0 0.71 1.2 0.91 <0.60

<10 <4.9 <2.4 <4.4 0.29 <13 <1.2 <7.0 <9.9

0.39 0.14 0.14 0.11 0.13 0.08 0.22 0.19 0.04

1.8 0.80 0.62 0.72 0.51 0.79 1.0 0.68 4.0

2800 3500 2700 2800 2700 5400 1200 1700 1200

1000 600 500 600 520 380 670 670 <220

54 53 55 43 110 200 34 65 <46

260 240 270 260 180 180 230 210 70

87 16 6.6 41 4.5 20 16 <6.7 19

<11 <4.3 <1.8 2.7 3.4 <13 5.2 <5.3 <14

160 170 180 90 410 170 150 130 59

220 110 96 45 150 51 85 76 <19

850 580 560 700 540 400 960 600 <33

30 20 28 8.4 1.8 18 17 10 10

Early-stage A1E vein EBE vein EBE vein EBE vein EBE vein EBE vein EBE vein

brine inclusions in shallow A1E and EBE veins 1007-132 brine1 13 lm 42.8 0905-233A brine 18 lm 44.2 0905-233B brine1 17 lm 43.9 0905-233B brine3 17 lm 44.5 1007-207 brine1 20 lm 40.4 1007-207 brine3 15 lm 41.4 0905-62 brine3 17 lm 41.0

11 11 11 11 12 11 10

1.1 0.86 1.0 0.71 1.1 <0.44 0.78

6.5 7.0 6.8 7.2 4.3 5.5 6.8

<2.6 <4.0 1.7 0.87 <3.2 <3.5 1.3

0.87 0.89 0.73 1.0 0.63 0.64 0.70

5.4 5.7 5.1 6.7 3.9 4.5 5.1

12000 7300 8700 5000 7100 10000 120

4400 4200 3900 5300 2400 2700 4700

31 80 62 79 26 42 50

1100 1200 1400 1500 800 1100 1100

450 200 480 28 350 390 41

24 22 17 16 23 24 19

180 470 220 1200 200 310 550

280 370 390 510 300 360 270

3900 4000 3800 5100 3200 3400 4200

32 53 34 130 23 33 70

Early-stage A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein A2E vein

brine inclusions in A2E veins 0905-108 brine2 12 lm 1303-14A brine2 23 lm 1303-14A brine4 17 lm 1303-14B brine2 17 lm 0705-12.5 brine2 20 lm 0705-12.5 brine3 17 lm 0904-141 brine2 24 lm 0705-233A brine2 17 lm 0705-233B brine2 14 lm 0705-233B brine5 18 lm

11 13 11 10 11 10 11 10 8.3 11

<0.44 1.3 <0.62 <0.33 0.68 <0.32 0.50 0.89 <0.35 <0.17

6.4 7.3 8.3 7.3 7.4 5.5 6.8 7.0 6.0 6.2

<3.4 1.2 1.7 <2.6 <1.2 <2.4 1.2 <2.9 <3.0 1.3

0.87 0.65 0.86 0.79 0.58 0.64 0.86 0.84 0.64 0.78

6.3 5.5 6.9 5.0 3.9 4.6 5.5 6.1 5.5 5.6

6600 13000 10000 55 160 7800 5200 7400 3400 960

4900 4800 6000 5000 3400 3000 4300 4700 3700 4100

58 52 99 44 59 320 100 65 79 85

1100 1000 1300 1100 910 1000 1100 1300 1100 1200

38 86 210 <3.3 6.7 19 25 31 28 64

20 49 34 52 23 21 22 14 13 18

610 360 760 1200 910 280 820 560 380 900

180 230 300 300 490 360 98 340 4.6 440

4400 4600 6500 6200 4400 2700 5100 4200 4000 4100

110 120 77 66 93 55 110 66 58 48

Tansitional-stage intermediate density inclusions in qtz. phen. of KGP and A1T veins 2.4 <0.13 0.94 qtz. phen. 0908-380C ID1 22 lm 9.1 qtz. phen. 0908-380C ID2 20 lm 9.1 2.5 <0.30 0.80

<1.3 <2.8

0.19 0.18

1.1 1.0

900 1200

1000 910

23 22

160 140

22 19

1.7 <3.2

150 130

80 66

800 670

10 9.0

Transitional-stage brine inclusions in A1T veins or qtz. phen. of KGP A1T vein 0908-315A brine1 15 lm 42.4 10 A1T vein 0908-315B brine 20 lm 42.4 11 qtz. phen. 0908-315C brine2 30 lm 39.2 9.4 A1T vein 1103-439A brine1 17 lm 41.2 11 A1T vein 1103-439B brine2 25 lm 41.2 11 A1T vein 0908-312 brine2 15 lm 37.6 10 A1T vein 0908-306 brine 30 lm 42.8 12

<5.1 <2.3 <11 <7.1 <17 <4.6 1.5

0.78 0.68 1.1 0.68 0.66 0.49 0.82

5.5 5.6 8.6 4.1 5.3 3.3 4.7

1800 7800 18000 2400 12000 260 10000

4000 3300 3200 3000 3200 2300 3800

27 84 290 68 120 <18 48

1100 1100 840 990 900 750 820

20 4.8 280 <8.7 77 <5.1 190

19 15 <10 23 <17 14 20

750 420 700 320 520 290 190

380 1100 2200 780 630 310 920

4300 3100 2800 2700 2200 2600 4000

65 66 53 40 52 24 47

43.9 49.5 48.0 41.6 40.6 37.5 42.7 41.6 35.4 42.5

<0.56 0.78 <1.2 <0.71 <1.6 <0.47 1.4

7.1 5.2 3.0 5.3 3.6 4.2 4.6

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Early-stage A1E vein A1E vein A1E vein A1E vein A1E vein A1E vein A1E vein A1E vein A1E vein

<3.2 2.8 22 <7.5 <21 520 450 <36 <9.5 <1.7 <7.7 <18 150 400 330 220 <7.2 2.3 <5.9 <13 <6.4 <2.0 <8.5 <10 24 110 150 45 Salinity used for LA-ICP-MS data quantification; underlined values represent rough estimations (see text for details). 1

<160 480 370 <260 230 78 230 230 <0.13 0.32 0.14 <0.20 <0.01 0.13 0.05 0.01 <7.2 <1.6 <8.8 <13 <0.31 0.54 0.75 <0.56 <0.74 <0.20 <0.81 <1.3 1.7 1.6 1.6 1.7 5.1 5.1 5.1 5.1 Late-stage liquid-rich inclusions in DL veins DL vein 1205-342A aqu1 17 lm 1205-342C aqu 17 lm DL vein DL vein 1007-372A aqu1 25 lm DL vein 1007-372B aqu 20 lm

199

concentrations of all the analyzed elements (Fig. 13). The repeated exsolution of compositionally similar fluids during the long-lived history of the Yulong deposit requires geochemically similar reservoirs in the upper crustal magma chamber. As is the case for many other porphyry Cu deposits (e.g., Bingham Canyon: Keith et al., 1997; Zhang and Aude´tat, 2017; Grondahl and Zajacz, 2017; Alumbrera: Halter et al., 2005; El Teniente: Stern and Skewes, 2005), the ore-forming fluids at Yulong were probably derived from andesitic magma reservoirs. A model that invokes the input of new batches of andesitic and/or basaltic magmas into the magma chamber can well explain the repeated exsolution of similar ore-forming fluids during a prolonged period of time (Annen, 2009; Gelman et al., 2013; Tapster et al., 2016). The appearance of the late and more mafic QAP dikes indicates that the underlying magma chamber at Yulong behaved as an open system. The widespread coeval potassium-rich basaltic to andesitic volcanic rocks in the Changdu-Simao continental block (Fig. 1A; Jiang et al., 2006) also support the hypothesis that basaltic to andesitic magmas played an important role in the formation of the Yulong deposit.

170 94 130 200

65 50 42 3700 3100 2000 300 220 520 560 270 340 <14 18 16 30 14 39 1000 980 1100 61 98 51 3100 3000 2600 9400 5600 6000 5.1 4.0 3.8 42.2 38.0 38.0 Transitional-stage brine inclusions in BT veins BT vein 0912-302 brine2 15 lm BT vein 1007-266 brine 17 lm 1103-497 brine3 15 lm BT vein

12 9.8 8.7

<1.0 <1.5 0.83

5.0 5.5 8.9

<10 <12 <7.8

0.67 0.61 0.55

4500 3800 7300 5700 A1T vein A1T vein

0908-380B brine1 25 lm 0908-380B brine2 25 lm

42.6 43.2

11 10

0.84 0.82

4.9 6.1

2.4 <3.7

0.86 0.97

5.3 5.9

88 63

790 1100

320 220

21 16

190 270

530 380

4300 4200

42 41

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5.4. Subsolidus evolution of ore-forming fluids and Cu-Mo decoupling The initial ore-forming fluids of the early stage at Yulong were likely exsolved from magmas as a singlephase fluid, considering that ID type fluid inclusions (5– 15 wt% NaClequiv.) are predominant in A1E veins from deep barren zones (Fig. 9). Upon ascent and decompression, the single-phase ID fluids entered the two-phase field, as indicated by more abundant brine and vapor fluid inclusions in shallow parts of the deposit. Ore-forming fluids of the transitional stage probably followed a similar evolution path, as the same fluid inclusion types were observed in A1T and BT veins. Liquid-rich inclusions from a boiling assemblage in DL vein have homogenization temperature of
370 °C and salinity of
6 wt% NaClequiv. (Table A3), which give a hydrostatic pressure estimation of
200 bars taking the NaCl-H2O model system for reference (Driesner and Heinrich, 2007). A similar pressure is also constrained from brine inclusions (apparent salinity = 36.7 ± 0.3 wt% NaClequiv., total homogenization temperature = 383 ± 21 °C) of a potentially well-preserved boiling trail in a A2E vein. Therefore, the fluid pressure of the early and transitional stages likely fluctuated over an interval of 400 bars between lithostatic pressure of
600 bars and hydrostatic pressure of
200 bars during the formation of quartz veins (Fournier, 1999). If a single phase fluid with a salinity of 9 wt% NaClequiv. resides in the two-phase field at 450 °C and 300 bar, then it will evolve to 21 wt% brine with a salinity of 43 wt% NaClequiv. and 79 wt% vapor with a salinity of 0.1 wt% NaClequiv., and the vapor/brine mass ratio would be 4 based on the H2O-NaCl system (Driesner and Heinrich, 2007). This could be one of the common scenarios for the early- and transitional-stage fluid evolution, but the vapor/brine mass ratio can vary in the range of 2–9 by changing the input parameters in geologically realistic conditions.

200

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Fig. 13. (A) Comparison of the least differentiated intermediatedensity (ID) fluid inclusions of the early and transitional stages. The inconsistent of Cu and Ag could be due to post-entrapment diffusion of these elements in or out from the inclusions. The insert ID inclusion is
25 lm. (B) Comparison of the least differentiated brine inclusions of the early and transitional stages. The insert brine inclusion is
20 lm.

The vapor-brine immiscibility can lead to contrasting metal partitioning between these two phases. The higher S/K and As/K ratios in ID and vapor inclusions than in brine inclusions at Yulong (Fig. A3) can be explained by the preferential fractionation of S and As into the vapor phase (Kouzmanov and Pokrovski, 2012). As discussed above, the unusually high Cu/K ratios in ID and vapor inclusions are caused by post-entrapment modification. Therefore, the experimentally determined Cu partitioning coefficient of 0.11–0.15 between vapor and brine (Lerchbaumer and Aude´tat, 2012) are used for a mass balance calculation: using the vapor/brine mass ratio on the order of 4, the total amount of Cu in the brine phase is about 2 times higher than that in the vapor phase. The brine phase is also likely the major Mo carrier after phase separation, considering the very high Mo/K ratios of a group of brine inclusions (Fig. 12B). This interpretation is in agreement with recent experimental studies for Mo partitioning between brine and vapor (Kokh et al., 2016; Zajacz et al., 2017).

For the above reasoning, the total amounts of Cu and Mo in the brine phase are higher than those in the vapor phase. Considering that the brine phase occupy a much smaller volume than the same mass of vapor phase, condensation of the high-density brine phase from a low-density input fluid and its accumulation at the level of the vaporbrine immiscibility could thus be an efficient mechanism to concentrate Cu and Mo in a small rock volume. The following two lines of evidence suggest that brines indeed played a significant role at Yulong. Firstly, the least differentiated brine inclusions with high concentrations of Mo (330 ± 90 ppm) and Cu (9300 ± 3200 ppm) only occur in the barren EBE and A1E&T veins, whereas more than half of brine inclusions in A2E and BT veins that contain molybdenite ± chalcopyrite have similar Cu concentration but much lower Mo concentration (10–100 ppm) (Fig. 14). These trends strongly suggest that (1) the brine phase was an important carrier for both Cu and Mo after phase separation, (2) Mo started to precipitate earlier than Cu, and (3) the bulk of Mo could precipitate either from the single-phase ID fluids or from the brine phase, whereas the bulk of Cu likely precipitated from the brine phase. Secondly, the high-grade Cu and Mo mineralization in shallow parts of the deposit are closely associated with the earlystage quartz veins that contain the highest abundance of brine inclusions (Fig. 9), supporting the accumulation of the brine phase at this level to form the high-grade Cu and Mo mineralization (the vapor phase because of its very low-density tends to escape quickly from the system). It is thus concluded that most Mo and Cu of the early stage precipitated sequentially from the brine phase. Nevertheless, it is unclear whether the transitional-stage Mo mineralization precipitate dominantly from the single-phase ID fluids or from the brine phase. The sequential precipitation of Mo and Cu is consistent with the vein chronology and vein-texture observations. The CL-bright quartz in A2E veins formed coevally with the bulk molybdenite and followed by a volumetrically much smaller CL-dark quartz (Fig. 6A–D). Chalcopyrite in A2E veins occurring as microfracture infillings are frequently in contact with the CL-dark quartz. A3E veins are equivalents of Cu-Fe-sulfides in A2E veins. Minor amount of chalcopyrite in the transitional stage precipitated unequivocally later than molybdenite (occurring along the margins of BT veins; Fig. 3F), because the former commonly occurs in the central voids of BT veins or as A3T veins cutting BT veins. Quartz would precipitate rapidly due to the sudden decrease of fluid pressure from lithostate to hydrostate during hydraulic fracturing in porphyry environment (Fournier, 1999; Rusk and Reed, 2002). Therefore, the precipitation of chalcopyrite paragenetically later than the bulk quartz veins at Yulong suggests that the precipitation of most Cu was probably driven by further cooling of the brine phase at pressure close to hydrostatic (Landtwing and Pettke, 2005). The fluid inclusion data, however, cannot explain why a relatively deep Mo-rich mineralization represented by BT veins formed in the transitional stage (Fig. 2), as the oreforming fluids of the early and transitional stages are very similar in all the analyzed elements (Fig. 13). A possible

J. Chang et al. / Geochimica et Cosmochimica Acta 232 (2018) 181–205

201

Fig. 14. Cu versus Mo concentrations of brine inclusions in various quartz veins. Uncertainties of Cu and Mo concentrations are <15% for most brine inclusions (shown by solid-line error bars), but a few brine inclusions quantified by roughly estimated salinities can reach up to 40% (dotted-line error bars).

explanation is that molybdenite precipitated preferentially from the single-phase ID fluids during the transitional stage. Because Cu-Fe sulfides tend to precipitate from the brine phase as discussed above, the apparently deep Morich mineralization of the transitional stage can be caused by a low degree of phase separation to produce Cu-rich brines or by the condensation of a brine phase at a higher level of the deposit. The early precipitation of molybdenite from the transitional-stage single-phase fluids could be triggered by their higher fS2, acidity and/or lower fO2 than the earlystage single-phase fluids, as such conditions can significantly decrease the solubility of Mo (Zhang et al., 2012). A recent experimental study suggests that the increase of CO2 concentration in aqueous fluids to up to
4 mol% may decrease the solubility of Mo as well (Kokh et al., 2017). 5.5. Formation and evolution of the late-stage fluids The formation of widespread DL veins and closely associated sericitic alteration assemblages (pyrite-quartzmuscovite ± illite) after the intrusion of QAP dikes marks a waning stage of the metal-charged magmatichydrothermal system (Chang et al., 2017). The DL veins formed from fluids with relatively low temperatures (370 °C) and low salinity (3–7 wt% NaClequiv.). Liquidrich inclusions in DL veins locally coexisting with a much smaller number of vapor inclusions suggest that the latestage fluids intersected the solvus as liquids at relatively low temperature and pressure, and boiled to form minor low-density vapors (Driesner and Heinrich, 2007). Liquidrich inclusions in DL veins have higher Cs/K and As/K than the other types of fluid inclusions in the early- and transitional-stage quartz veins (Fig. 12), suggesting that the late-stage fluids are more evolved than the oreforming fluids. Furthermore, the similar element/K ratios of the liquid-rich inclusions to ID inclusions in magmatic apatite implies a common origin for them, i.e., being derived from highly evolved melt fractions. The formation

of the low-temperature late-stage fluids at the deposit level could be caused by the progressive solidification of the underlying magma chamber without the input of new batches of magmas. The relatively low salinity of the liquid-rich fluids was probably aided by the dilution of meteoritic water (Hedenquist and Richards, 1998). 5.6. An integrated model and comparison with other porphyry Cu deposits Based on the above discussion, an integrated model of the formation and evolution of the multistage magmatichydrothermal fluids at Yulong is developed (Fig. 15). The input of new batches of andesitic and/or basaltic magmas into the long-lived upper crustal magma chamber maintained the repeated exsolution of compositionally similar single-phase ID, metal-charged fluids over a long period of
1.4 m.y. (Fig. 15A and B; Chang et al., 2017). Following the intrusion of MGP and KGP, the singlephase ID fluids decompressed and cooled upon ascending from the underlying magma chamber, and separated at the deposit level into a less abundant brine phase enriched in Cu ± Mo and a more abundant vapor phase (vapor/brine mass ratio of
4). The high-density brine phase condensed at the deposit level, whereas the lowdensity vapor phase escaped rapidly from the hydrothermal system. During the early-stage mineralization, Mo and Cu precipitated successively from the brine phase after further decompression, cooling, and/or fluid-rock reaction, which caused the locally decoupled Cu-Mo mineralization (Fig. 15A). However, the deeply located Morich mineralization of the transitional stage could start to precipitate from the single-phase ID fluids before phase separation (Fig. 15B). After the intrusion of QAP, the metal-deficient late-stage fluids that formed the DL veins and associated sericitic alteration exsolved from highly evolved rhyolitic melts during the solidification of magma chamber, probably due to the lack of new batches of input magmas (Fig. 15C).

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Fig. 15. Schematic illustration of the formation and evolution of the multistage magmatic-hydrothermal fluids at Yulong. (A) and (B) The input of new batches of andesitic to basaltic magmas into the magma chamber contributed to the repeated exsolution of similar single-phase intermediate-density (ID) ore-forming fluids during the early and transitional stages. The single-phase ID fluids of the two stages decompressed and cooled upon ascending, and condensed into a less abundant Cu ± Mo-rich brine phase and a more abundant vapor phase after the intrusion of MGP and KGP, respectively. The locally decoupled Cu-Mo mineralization in the early stage was caused by the sequential precipitation of Mo and Cu from the brine phase, whereas the deep Mo-rich mineralization of the transitional stage could largely precipitate from the single-phase ID fluids before phase separation. (C) The initial metal-deficient late-stage fluids exsolved from highly evolved rhyolitic melts, and evolved to a low-temperature and low-salinity liquid-rich fluids at the deposit level following the intrusion of QAP. The spatial distribution and temporal relations of different vein types are shown briefly.

It could be a common phenomenon in long-lived porphyry Cu(-Mo-Au) systems that pulses of ore-forming fluids derived from the magma chamber are compositionally similar over a prolonged period of time, considering that the other two well-studied giant porphyry Cu(-Mo-Au) deposits (i.e., Bingham Canyon and El Teniente) also show similar composition in different stages of ore-forming fluids that bracket a ore-forming history of
1 m.y. and
4 m.y., respectively (Klemm et al., 2007; Seo et al., 2012; Spencer et al., 2015). Consequently, the formation of a late Morich mineralization in many porphyry Cu(-Mo-Au) deposits (Sillitoe, 2010 and reference therein) is likely caused by subsolidus hydrothermal processes rather than the progressive increase of Mo/Cu ratio in the residual parental melt as crystallization proceeds (Candela and Holland, 1986). In addition, this study provides a new model for the origin of pyrite + quartz veins (D veins) and associated sericitic alteration that form after the bulk ores in nearly all porphyry Cu(-Mo-Au) deposits (Harris et al., 2005; Sillitoe, 2010). Notably, a similar magmatic origin has recently been proposed to explain the formation of the coarse muscovite veins and alterations in the roots of a porphyry copper system related to the Yerington batholith (Runyon et al., 2017). 6. CONCLUSIONS

(1) The initial single-phase intermediate-density (ID) ore-forming fluids of the early and transitional stages that formed over a prolonged period of time (up to
1.4 m.y.) were compositionally similar; these fluids

were likely derived from similarly evolved andesitic (?) magma reservoirs that were replenished by the input of new batches of andesitic to basaltic magmas into the long-lived upper crustal magma chamber. (2) The single-phase ID fluids of the early and transitional stages (9.1 ± 3.0 wt% NaClequiv.;
1000 ppm Cu and 22 ± 19 ppm Mo) entered the two-phase field upon ascent and separated into a small amount of metal-rich brine phase (42.0 ± 4.5 wt% NaClequiv.; 9300 ± 3200 ppm Cu and 330 ± 90 ppm Mo) and a large amount of low-density vapor phase with a vapor/brine mass ratio of
4; the condensation and accumulation of metal-rich brines at the deposit level was likely critical for the concentration and precipitation (during subsequent cooling of these brines) of Cu ± Mo in a small rock volume. The sequential deposition of Mo and Cu from the condensed brine phase of the early stage caused the local Cu-Mo decoupling at shallow levels of the deposit, whereas the mechanism that formed the deeply seated Morich but Cu-poor mineralization of the transitional stage is less clear. (3) Sulfur-rich apatite (0.3–1.4 wt% SO3) in oxidized felsic magmas related to porphyry Cu systems and arc volcanos can crystalize from highly evolved melts in pegmatitic environment during the solidification of felsic magma reservoirs. (4) The metal deficient late-stage fluids (temperatures 370 °C, salinity = 3–7 wt% NaClequiv.) that formed the DL veins and associated sericitic alteration were derived from highly evolved rhyolitic melts during the solidification of the magma chamber.

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ACKNOWLEDGEMENTS This research is funded by the National Natural Science Foundation of China (Grant No. 41325007), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan), the GPMR State Key Laboratory (MSFGPMR03), and National Demonstration Center for Experimental Mineral Exploration Education at China University of Geosciences (Wuhan). Jia Chang is financially supported by the China Scholarship Council. Shen-Tai Liu, Yong-Gang Liu, Jia-Cheng Liu and XiaoDong Deng are appreciated for helping us in the field. We also thank Xin-Fu Zhao, Li-Ping Zeng, and Zi-Ye Lu for their help with Optical-CL microscopy. Three journal reviewers B. Konecke, Z. Zajacz and S. Salvi provided insightful comments and constructive suggestions, for which we are grateful. Our thanks extend to associate Editor G. Pokrovski for handling the manuscript. This is contribution 5 from CUG Center for Research in Economic Geology and Exploration Targeting (CREGET).

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