Ore Geology Reviews 63 (2014) 498–509

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Ore Geology Reviews

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Fluid evolution of the Tongkuangyu porphyry copper deposit in theZhongtiaoshan region: Evidence from fluid inclusions

Yuhang Jiang a,b, Hecai Niu a,⁎, Zhiwei Bao a, Ningbo Li a,b, Qiang Shan a, Wubin Yang a

a Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Chinab University of Chinese Academy of Sciences, Beijing 100049, China

⁎ Corresponding author.E-mail address: niuhc@gig.ac.cn (H. Niu).

http://dx.doi.org/10.1016/j.oregeorev.2014.05.0180169-1368/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 June 2013Received in revised form 7 May 2014Accepted 8 May 2014Available online 27 May 2014

Keywords:Fluid inclusionTongkuangyu porphyry Cu depositOxygen fugacityBrine transportFluid boiling

The Tongkuangyu porphyry copper deposit is located in the Zhongtiaoshan region, southernmargin of the NorthChina Craton. More than 2.8 million tons of Cu (Average Cu grade: 0.68%) is hosted in the Paleoproterozoicquartz-monzonite porphyry (2122 ± 10 Ma). The ore-forming processes of the Tongkuangyu porphyry Cudeposit are divided into three mineralization stages, namely the early-, main- and late mineralization stages.The majority of Cu ores were formed in the main stage, which is characterized by veinlets and disseminatedchalcopyrite and minor molybdenite, pyrite, magnetite and hematite. Quartz is the major gangue mineral,which occurs as veinlets and associates with metallic minerals.Through systematic petrographic observation, the ore-bearing quartz veins (A-type quartz) and the quartzhosted in chalcopyrite (B-type quartz) were studied in detail to investigate the fluid evolution of theTongkuangyu porphyry Cu deposit. Four types of fluid inclusions were recognized in the A-type quartz, whichinclude (1) pure vapor or vapor-rich (V-type) inclusions, (2) CO2–H2O (C-type) inclusions, (3) liquid-rich(L-type) inclusions, and (4) daughter mineral-bearing (S-type) inclusions. S-type inclusions in the mainstage A-type quartz contain mainly halite and minor sylvite, calcite, hematite, chalcopyrite and anhydrite,with homogenization temperatures of 233–450 °C. V-type inclusions (coexisting with the S-type inclusions)homogenize to vapor phase at 282–375 °C, suggesting that the ore-forming fluids may have experienced boilingin the main mineralization stage. The late stage S-type inclusions have homogenization temperatures of186–245 °C and contain largely halite with minor sylvite. Fluid inclusions in the B-type quartz (intergrownwith chalcopyrite) are closely associated with the ore-forming processes. The main stage B-type quartzcontains predominantly S-type inclusions with homogenization temperatures of 250–412 °C and minorV-type inclusions. On the other hand, the late stage B-type quartz contains abundant S-type inclusionswith homogenization temperatures of 195–230 °C and less L-type inclusions. Our results show that the S-typeinclusions in both theA- andB-type quartz (of the sameore stage) have very similar homogenization temperatures.Therefore, fluid boiling may have promoted the Cu enrichment in the high salinity fluids and the brine phase is themain metal transport medium in the Tongkuangyu porphyry Cu deposit.The common occurrence of hematite and anhydrite daughter minerals in the S-type inclusions further suggeststhat ferrous ion oxidization may have caused sulfate reduction. This process may have played an important roleduring the Cuprecipitation event. The progressive cooling offluids and the incorporation ofmeteoricwater in thelate mineralization stage possibly resulted in the small-scale vein-type mineralization.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Porphyry copper deposits are commonly associated with oxidizedfelsic magmas (Audétat et al., 2004; Ballard et al., 2002; Hedenquistand Lowenstern, 1994; Sillitoe, 2010; Sun et al., 2013). Many studieshave suggested that highly oxidized magmas derived from the mantleor lower crust are crucial for chalcophile element enrichment(Mungall, 2002; Sillitoe, 1997). Actually, most ore-forming porphyries

have abundant SO42− (Liang et al., 2009; Mungall, 2002; Sillitoe, 2010;Sun et al., 2013), since sulfide under-saturation during the mainmagma evolution stage is advantageous for the enrichment ofincompatible Cu (Sun et al., 2004, 2013). Crystallization of magnetiteand hematite may result in sulfate reduction and oxygen fugacityfluctuation, which is considered to be responsible for the formation ofCu mineralization (Sun et al., 2013). However, this hypothesis is stillhighly controversial (Pokrovski, 2014; Richards, 2014; Sun et al.,2014a,b). Our work on fluid inclusions hosted by Cu-bearing quartzveins at the Tongkuangyu porphyry copper deposit provides evidencesupporting the importance of iron oxides in Cu mineralization.

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499Y. Jiang et al. / Ore Geology Reviews 63 (2014) 498–509

The Paleoproterozoic Tongkuangyu porphyry Cu deposit is locatedin the southern margin of the North China Craton (Fig. 1). Regardingits ore-forming fluids, many works have been carried out on themicrothermometry and hydrogen–oxygen–carbon (H–O–C) isotopecompositions, focusing mainly on statistical interpretations (Chen andLi, 1998; Sun and Ge, 1990; Xu, 2010; Xu et al., 1995; Zhang, 2012;Zhen et al., 1995). A detailed study is absent on fluid inclusion assem-blages and the types of daughter mineral in fluid inclusions, which canindicate the fluid evolution during ore formation. This investigationshows that magnetite and hematite coexist in the ores, and fluid inclu-sions with hematite and chalcopyrite daughter minerals are ubiquitousin the hydrothermal quartz. Through systematic field observation,petrographic, microthermometric and laser Raman spectroscopyanalyses on fluid inclusions, we will discuss the redox potential controlon the fluid evolution of the Tongkuangyu Cu deposit, as well as itsimplications on porphyry Cu mineralization.

2. Regional geology

Various Precambrian rock units are exposed in the Zhongtiaoshanregion, which is located on the southern segment of the Trans-NorthChina Orogen (Liu et al., 2012; Zhao et al., 2001). These Precambrianrock units (from the oldest to youngest) include the Sushui Complexand the Jiangxian-, Zhongtiao-, Danshanshi- and Xiyanghe groups(Bai, 1997; Hu and Sun, 1987; Sun et al., 1990; Fig. 1). The Sushui Com-plex is the oldest complex in the Zhongtiaoshan region, and consistsmainly of Neoarchean (2.8–2.5 Ga) tonalite–trondhjemite–granodiorite(TTG) gneiss formed through periods of granite emplacements (Liuet al., 2012; Sun and Hu, 1993; Tian et al., 2006; Zhao et al., 2012).

Fig. 1. Location and regional geology of the Zhongtiaoshan region. (A). Tectonic framework oZhongtiaoshan region. I—Sushui Complex; II—Jiangxian Group; III—Zhongtiao Group; IV—Dans

During 2.5–1.9 Ga, the Zhongtiaoshan region accepted primarily terrig-enous and volcanic sedimentation and formed the Jiangxian- andZhongtiao groups (Bai, 1997; Sun et al., 1990). During 1.90–1.85 Ga,the region has experienced a series of tectonic events, including crustuplifting, exhumation, granite intrusion and regional metamorphism,which eventually formed the Danshanshi Group molasse sequences.The Danshanshi Group consists of conglomerates and quartzites,which may indicate the end of cratonization of the North China Craton(Sun et al., 1991; Zhai and Santosh, 2013). The Late PaleoproterozoicXiyangheGroup in themiddle-eastern part of the Zhongtiaoshan regionconsists mainly of andesitic volcanic rocks. Its geological setting andlithology are comparable to the Xiong'er Group on the southern marginof the North China Craton (Sun et al., 1990).

The North China Craton experienced an extensional regimeduring 2.2–2.1 Ga (Zhai and Santosh, 2011, 2013; Zhai et al., 2010),and was exposed to abundant extension-related magmatism in theTrans-North China Orogen (Du et al., 2010, 2012). Paleoproterozoicmagmatic activities in the Zhongtiaoshan region are divided intothe Jiangxian- (2166–2115 Ma) and Zhongtiao (2090–2060 Ma)periods (Sun and Hu, 1993). Volcanic activities were intense in theJiangxian Period, resulting in the accumulation of a series of bimodalvolcanic rocks, characterized by potassic mafic volcanic rocks, quartzporphyries, potassic rhyolites and quartz crystal tuffs, indicative of acontinental rift setting (Zhai and Santosh, 2013).

Ores and mineralized porphyries from the Tongkuangyu porphyryCu deposit were dated to be 2108 ± 32 Ma and 2122 ± 10 Ma, respec-tively, using molybdenite Re–Os isotopic isochron and zircon U–Pbmethods (Chen and Li, 1998; Li et al., 2013). The overlapping agesof the porphyry crystallization and Cu mineralization demonstrated

f the North China Craton and the location of Zhongtiaoshan. (B). Regional geology of thehanshi Group; V—Xiyanghe Group (modified after Sun and Hu, 1993).

500 Y. Jiang et al. / Ore Geology Reviews 63 (2014) 498–509

that Cu mineralization may be genetically linked to the porphyryemplacement, and both of themwere formed in a continental exten-sional setting during the Jiangxian Period.

Rock units in the Zhongtiaoshan region are intensively fractured. TheNW- or NE-trending basement fractures have controlled the emplace-ment of granite intrusions and the associated Cu deposits, notably theTongkuangyu, Hujiayu-Bizigou and Luojiahe Cu deposits (Sun and Hu,1993; Fig. 1).

3. Ore deposit geology

The Tongkuangyu porphyry Cu deposit is located in the northernpart of the Zhongtiaoshan region, and the ore bodies are hosted in thePaleoproterozoic quartz-monzonite porphyry and the surroundingTongkuangyu Formation of the Paleoproterozoic Jiangxian Group(Fig. 2A). The rocks exposed in the mining area include quartz-monzonite porphyries, sericite–quartz rocks, meta-potassic maficvolcanic rocks and sericite-schists (Fig. 2A).

The structure of the Tongkuangyu mine is highly complex. TheTongkuangyu Formation is intensely sheared and formed an overturnedanticline, showing intense shear movements. The nearly N–S trendingDabaogou–Tongkuangyu translational normal fault offsets the No. 5 orebody. The formation of amajority of faults is later than Cumineralization;however, its destructive effect to the ore bodies is far from significant(CGGCDZM, 1978).

There are seven ore bodies in the Tongkuangyu Cu deposit, ofwhich Nos. 4–5 are economically viable. These two ore bodies contain2.8 million tons Cu metal at an average grade of 0.68% (Xu, 2010),accounting for more than 90% of the total reserve. No. 5 ore bodyforms a lens that is 1100m long and 185m thick. Host rocks are mainlymeta-quartz monzonite porphyries and meta-quartz crystal tuffs(Fig. 2B).

Alteration mineral zonation of the Tongkuangyu porphyry Cudeposit includes quartz–K-feldspar-, quartz–sericite- and carbonatealteration (CGGCDZM, 1978). Unlike the alteration and mineralizationzoning for typical porphyry Cu deposits, the order of the alterationzonation is unclear at Tongkuangyu. This is partly due to the overprintingeffects of the intense regional metamorphism (CGGCDZM, 1978;Chen and Li, 1998; Sillitoe, 2010; Zhang, 2012).

The mineralization processes in the Tongkuangyu porphyry Cudeposit are divided into three stages:

1. The early mineralization stage is characterized by quartz–potassiumfeldspar alteration with minor disseminated chalcopyrite and pyriteores. Quartz veinlets, containing sparsely disseminated sulfides, are

Fig. 2. Ore deposit geology map of the Tongkuangyu porphyry Cu deposit. (A). Geological m(modified after Sun and Hu, 1993; Huang et al., 2001).

visible in some places. Minerals in the quartz–potassium feldsparalteration zone include potassium feldspar, biotite, quartz, magnetite,scapolite and tourmaline.

2. The main mineralization stage, associated with quartz–sericitealteration, is characterized by veinlets (0.2 to 3 cm) containingquartz and disseminated chalcopyrite with minor molybdenite,pyrite, magnetite, hematite, bornite and chalcocite (Fig. 3A, C, and E).Chalcopyrite–pyrite–molybdenite is the main ore mineralassemblage. Three sulfide zones are recognized as part of themain mineralization stage, namely (from the deepest toshallowest occurrences) the pyrite-chalcopyrite, molybdenite–chalcopyrite and bornite zones. Minerals in the quartz–sericitealteration zone include sericite, quartz, magnetite, hematite,carbonate, chlorite and scapolite. Ore bodies in the quartz–sericitezone are mostly located proximal to quartz–potassium feldsparalteration zones.

3. The late mineralization stage is characterized by vein-type ores. Thequartz–calcite veins (3 to 10 cm) associated with this event arethicker than those in the main mineralization stage (Fig. 3B and D).Ores consist of chalcopyrite, pyrite, molybdenite, magnetite andspecularite, with gangue quartz and calcite. Carbonate alterationalso occurred in the late mineralization stage.

4. Analytical methods

4.1. Sampling

Samples used for the fluid inclusion study were collected fromthe No. 5 ore body and comprise ore-bearing quartz of differentmineralization stages. Thirty-six thick sections (300 μm) were usedfor petrographic fluid inclusion studies, among which 21 were furtheranalyzed by microthermometry and laser Raman spectroscopy.

4.2. Microthermometry

Microthermometry studieswere carried out with a LinkamMDS600Heating–Freezing System at the Key Laboratory of Mineralogy andMetallogeny, Chinese Academy of Sciences. Thermocouples were cali-brated in the range of −196 °C to 600 °C using synthetic fluidinclusions. The precision of temperature measurement is ±0.1 °C inthe range of −100 °C to 25 °C, ±1 °C in the range of 25–400 °C, and±2 °C for temperature above 400 °C. The heating rate was generally0.2–5 °C/min during the process of fluid inclusion testing, but reducedto 0.1 °C/min near the freezing point, and 0.2–0.5 °C/min near the

ap of the Tongkuangyu porphyry Cu deposit. (B). Mining tunnel plan of No. 5 ore body

image of Fig.�2

Fig. 3. Petrographic features of A-type quartz and the magnetite–hematite assemblage. (A) and (B). Ore-bearing quartz veins in the main- and late mineralization stages, respectively.(C) and (D). Fluid inclusion samples of A-type quartz veins in the main- and late mineralization stages, respectively. (E) and (F). Coexistence of magnetite and hematite in the main-and late mineralization stages, respectively.

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homogenization temperature to record the phase transformationprocess accurately.

Salinity of aqueous fluid inclusion, expressed as wt.% NaCl equiv.,was estimated using the data of Bodnar (1994) for the NaCl–H2O sys-tem. We estimated the salinities, pressures and densities of halitedaughter mineral-bearing inclusions using the methodology outlinedby Lecumberri-Sanchez et al. (2012), which is based on the temperatureof bubble dissolution and homogenization temperature. Salinities of theCO2-bearing inclusions were calculated using the equation provided byRoedder (1984).

4.3. Laser Raman microspectroscopy

Vapor and solid compositions of individual fluid inclusions weremeasured using the LabRam HR800 Laser Raman microspectroscopyat the Institute of Geology and Geophysics, Chinese Academy ofSciences. The excitation wavelength was the 532 nm line of an Ar+

ion laser operating at 44mW. The scanning range of spectra was set be-tween 100 and 4000 cm−1 with an accumulation time of 10 s for eachscan. The spectral resolution was 0.65 cm−1. The Raman shift of a

monocrystalline silicon piece was measured to be 520.7 cm−1 beforeanalyzing.

5. Results

5.1. Fluid inclusion petrography

5.1.1. Types of fluid inclusionsFour fluid inclusion types have been identified based on the phases

present at room temperature (20 °C), phase transitions during total ho-mogenization and laser Raman spectroscopy. These four fluid inclusiontypes include (Table 1):

1) V-type (pure vapor or vapor-rich) inclusions: these inclusions(diameter: 3 to 20 μm) are rounded isometric or elliptic inshape (Fig. 4D). They consist mainly of pure vapor or occasionallyb15 vol.% liquid. In the main mineralization stage, V-typeinclusions occur either randomly or in clusters.

2) C-type (CO2–H2O) inclusions: these inclusions (diameters: 5to 16 μm) are mainly elliptic in shape. They consist of two(liquid H2O + CO2-rich supercritical fluid) or three phases

image of Fig.�3

Table 1Summary of fluid inclusion types of the Tongkuangyu porphyry Cu deposit.

Type Fluid phases Daughter minerals Size (μm) Vol.% Shape Occurrence Location

C VCO2 þ LH2O or VCO2 þ LCO2 þ LH2O 5–16 15–80 Elliptic Isolate M-AV VH2O or VH2O þ LH2O 3–20 N85 Rounded isometric

or ellipticIsolate, random distributions or inclusters

M-A; M-B

L VH2O þ LH2O 1–15 b30 Irregular, elliptic Random distributions, in linear orin clusters

M-A; L-A; L-B

S VH2O þ LH2O Halite, sylvite, calcite, anhydrite,hematite, chalcopyrite, opaque

3–25 15–30 Irregular, elliptic Random distributions or in clusters E-A; M-A; M-B;L-A; L-B

Abbreviations: Vco2: CO2 vapor; Lco2: CO2 liquid; VH2O:H2O vapor; LH2O:H2O liquid; vol.%: volume proportion of the vapor phase; E-A: early stage A-type quartz;M-A:main stageA-typequartz; M-B: main stage B-type quartz; L-A: late stage A-type quartz; L-B: late stage B-type quartz.

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(vapor CO2 + liquid CO2 + liquid H2O) at room temperature(20 °C). Carbonic phases occupy 15–80% of the total inclusionvolume (Fig. 4F). C-type inclusions are rare in the main stageand absent in the late mineralization stage.

3) L-type (liquid-rich) inclusions: these inclusions (diameter: 1 to15 μm) are irregular to elliptical in shape (Fig. 4C). They consist ofliquid and b30% vapor, with H2O being the major component.

Fig. 4.Photomicrographs of thefluid inclusions from the TongkuangyuporphyryCudeposit. (A)stage, S-type inclusionwith only halite daughterminerals. (C). Late stage, L-type inclusionwithinclusionwith anhydrite and opaque daughterminerals. (F).Main stage, C-type inclusionwith V(K). Coexistence of V- and S-type inclusions (with halite, hematite, chalcopyrite and opaque dauH2O vapor; LH2O: H2O liquid; Hal: halite; Cal: calcite; Hem: hematite; Cp: chalcopyrite; Anh: an

L-type inclusions mainly occur in the late mineralization stageand appear either randomly, in linear trails or in clusters.

4) S-type (daughter mineral-bearing) inclusions: these inclusions(diameter: 3 to 25 μm) mainly occur as an elliptical shape, withvapor phases occupying 15–30 vol.%. They contain abundantdaughter minerals such as halite (dominant), sylvite, calcite,anhydrite, hematite and chalcopyrite (Fig. 4A, B, E, H, I and J).

. Main stage, S-type inclusionwith halite, hematite and opaque daughterminerals. (B). LateVH2O and LH2O. (D).Main stage, V-type inclusionwith VH2O and LH2O. (E).Main stage, S-typeco2, Lco2, and LH2O. (G).Main stage, coexistence of S- andV-type inclusions. (H), (I), (J), andghterminerals in the field of (G)). Abbreviations: Vco2: CO2 vapor; Lco2: CO2 liquid; VH2O:hydrite.

image of Fig.�4

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S-type inclusions occur in all the three hydrothermal mineraliza-tion stages.

5.1.2. Occurrence and temporal relation of fluid inclusionsSystematic petrographic observations can be used to distinguish dif-

ferent fluid inclusion assemblages and infer the relative timing of fluidentrapment (Goldstein and Reynolds, 1994; Li et al., 2012; Ulrich andHeinrich, 2001). In the Tongkuangyu porphyry Cu deposit, we havefound that fluid inclusions in the quartz from the ore-bearing quartzveins (hereafter referred to as A-type quartz) and those in the quartzenclosed in chalcopyrite (hereafter referred to as B-type quartz) are dif-ferent in occurrence and inclusion types (Fig. 3C and D; Fig. 5A and B).

A-type quartz contains abundant primary and secondary fluid inclu-sions. The primary inclusions occur as isolated inclusions, distributedrandomly or in clusters, whereas secondary inclusions occur as trailsalong healed fractures. However, unrecognized microfractures in theA-type quartz may lead to the misinterpretation of some secondaryinclusions to be primary ones. In contrast, B-type quartz is rather trans-parent and contains very few secondary inclusions. It occurs as isolatedgrains and commonly shows a straight contact with chalcopyrite

Fig. 5. Photos of ores and photomicrographs showing petrographic features of B-type quartz anmineralization stages, respectively (scanned specimen). (C) and (D). B-type quartz with a relhematite and calcite daughter minerals in the main mineralization stage. (F). S-type inclusiovapor; L: H2O liquid; Hal: halite; Cal: calcite; Hem: hematite; Cp: chalcopyrite.

(Fig. 5C and D). Therefore, B-type quartz is typical syn-ore quartz, andthus theirfluid inclusionsmay be safely taken as a proxy for the primaryore-forming fluids. Furthermore, characteristics of B-type quartzfluid inclusions may be used to identify the primary fluid inclusionsin A-type quartz.

Microscopically, inclusion types and distributions are significantlydistinct in different mineralization stages. S-type inclusionsdominate the early stage A-type quartz (with halite being thepredominant daughter mineral), indicating high salinity primaryore-forming fluids.

S-type inclusions also dominate the main stage A-type quartz.Daughter minerals are mainly halite and sylvite, calcite, hematite, chal-copyrite and minor anhydrite. This also suggests the ore-forming fluidsto be highly saline and metal rich. Remarkably, S-type inclusions arecommon in quartz that contains also V-type inclusions, and explicittrails of both types being trapped simultaneously have been recognized,pointing to the occurrence of fluid boiling (Fig. 4G and I). Furthermore,main stage S-type inclusions predominate in the B-type quartz, withpetrographic features identical to those present in A-type quartz(Fig. 5E).

d their fluid inclusions. (A) and (B). Quartz grains inside chalcopyrite in themain- and lateatively straight contact with chalcopyrite. (E). S-type inclusions with halite, chalcopyrite,ns with halite daughter mineral in the late mineralization stage. Abbreviations: V: H2O

image of Fig.�5

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The late stage A-type quartz still contains many S-type inclusions,but L-type inclusions are more predominant. Different from the daugh-ter minerals in the main stage S-type inclusions, those in the late stageS-type inclusions (in A-type quartz) contain only halite and sylviteand no hematite or chalcopyrite (Fig. 4B). Unlike fluid inclusions inthe late stage A-type quartz, inclusions in the late stage B-type quartzare predominantly S-type with subordinate L-type (Fig. 5F).

In conclusion, S-type inclusions in either A- or B-type quartz fromthe same mineralization stage contain very similar petrographicfeatures. S-type inclusions are the dominate type in the main- and latemineralization stages, and they are likely to be highly saline and metalrich.

5.2. Microthermometric results

The microthermometric results of fluid inclusions are summarizedin Table 2 and illustrated in Figs. 6 and 7, which show the links betweenthe physico-chemical conditions of the ore-forming fluids and the threemineralization stages.

In the early stage A-type quartz, S-type inclusions homogenized byhalite disappearance. The homogenization temperatures of theseinclusions range from 479 °C to 560 °C, with corresponding salinitiesof 53.3–61.4 wt.% NaCl equiv. (Fig. 6A and D). The S-type inclusionsmay thus represent the initial high temperature and high salinity ore-forming fluids.

In the main stage A-type quartz, S-type inclusions homogenizedat temperatures between 233 and 450 °C, with corresponding salin-ities of 34.3–50.9 wt.% NaCl equiv. (Fig. 6B, E). S-type inclusions thatcontain hematite and/or chalcopyrite did not homogenize, becausethese daughter minerals do not normally disappear with increasingtemperature. Our study shows that halite daughter minerals inthese S-type inclusions (that contain only halite) disappeared at atemperature close to the homogenization temperature, suggestingthat the inclusions and their daughter minerals were entrapped inidentical conditions. V-type inclusions homogenized to the vaporphase between 282 °C and 375 °C. Since aqueous solution is rare inthe V-type inclusions, phase change was not observed and thus no sa-linity result was obtained. Given that the coexisting S- and V-type inclu-sions have approximately the same homogenization temperature range(Fig. 6B) but homogenize into different phases, fluid boiling was likelyto have occurred at this stage. C-type inclusions contain liquid H2Oand supercritical CO2 at room temperature (25 °C), but CO2 bubblesoccur in cooling processes. The measured melting temperatures forsolid CO2 varied from−62.0 °C to −56.6 °C, and melting of CO2 clath-rate occurred at temperatures between−9.6 °C and−2.6 °C. Themelt-ing temperatures of CO2 solid are somewhat lower than the triple pointfor pure CO2 (−56.6 °C), suggesting that the presence of minor othergases. However, the gases were not detected in our laser Raman spec-troscopy analyses (Fig. 9B). The partial homogenization temperaturesof CO2 varied considerably from 9.1 °C to 23.2 °C, corresponding to

Table 2Microthermometric data of fluid inclusions in the Tongkuangyu porphyry Cu deposit.

Stage Type Number Tm,ice (°C) Tm,cla (°C)

Early stage S 23Main stage C 11 –9.6 to 2.1

V 10A-S 55B-S 14

Late stage L 23 −0.1 to −3.9A-S 19B-S 7

Note: Tm, ice (°C): temperature of ice point; Tm,cla (°C):melting temperature of CO2 clathrate; Tminclusion in B-type quartz; –: no temperature data has been obtained.

salinities in the range of 13.1 to 20.5 wt.% NaCl equiv. The homogeniza-tion temperatures of the inclusions, ranging from 280 to 372 °C, weresimilar to those of V-type inclusions (Fig. 6B). This suggests that thepresence of CO2 in ore-forming fluids promotes immiscibility betweenthe vapor and saline liquid phases of the solution and thus metalprecipitation (Lowenstern, 2001). CO2 effervescence would also in-crease the pH in the remaining fluids, further influencing ore-formingprocess (Robb, 2005).

In the late stage A-type quartz, homogenization temperatures ofthe S-type inclusions were relatively low, varying from 186 °C to245 °C with corresponding salinities of 31.6–35.2 wt.% NaCl equiv.(Fig. 6C and F). The homogenization temperatures of the L-type in-clusions range from 141 °C to 230 °C, with salinities of 0.2–6.3 wt.%NaCl equiv. This suggests that meteoric water may have beenpresented during the late mineralization stage. Xu et al. (1995)reported H and O isotopic data for the ore forming fluids of theTongkuangyu deposit, which showed that the ore forming fluidswere predominantly magmatic with meteoric water influx duringthe late mineralization stage.

Microthermometric results from S-type inclusions in the main-and late stage B-type quartz show that the homogenization temper-atures varied in the range of 250–412 °C and 195–230 °C (Fig. 6B andC; Fig. 7), respectively, which are similar to their counterparts in thecorresponding stage A-type quartz. Microthermometric data fromthe S-type inclusions may reflect the physico-chemical conditionsof the ore-forming fluids in the different mineralization stages(Fig. 7).

Fluid inclusions that homogenize by halite disappearance havebeen commonly reported in hydrothermal ore deposits (Baker andLang, 2003; Bodnar and Beane, 1980). At Tongkuangyu, almost allS-type inclusions show consistent microthermometric behavior,characterized by homogenizing by halite disappearance (Fig. 8).This suggests that NaCl in fluids was supersaturated before the en-trapment, and the initial fluids may have been trapped at higherpressure (Rusk et al., 2008).

5.3. Laser Raman spectroscopy analysis

Representative inclusions were systematically measured by laserRaman spectroscopy, and the results show that the vapor phase compo-sition of the main stage V-type inclusions was primarily H2O (Fig. 9A).C-type inclusions showed distinct characteristic peaks for CO2 (1284and 1386 cm−1; Fig. 9B), and H2O was the major composition in thevapor phase of the abundant late stage L-type inclusions. The vaporphase composition of the different stage S-type inclusions is alsoprimarily H2O. In addition, hematite (1320 cm−1; Fig. 9C), chalcopyrite(288 cm−1; Fig. 9D), anhydrite (1017 cm−1; Fig. 9E) and calcite(1086 cm−1; Fig. 9F) daughter minerals have been recognized in themain stage S-type inclusions.

Tm,hal (°C) Homogenization state Th (°C) Salinity (wt.% NaCl)

479–560 Liquid 479–560 53.3–61.4Liquid 280–372 13.1–20.5Vapor 282–375 –

233–450 Liquid 233–450 34.3–50.9250–412 Liquid 250–412 35.4–47.9

Liquid 141–230 0.2–6.3186–245 Liquid 186–245 31.6–35.2195–230 Liquid 195–230 31.6–34.0

,hal (°C):melting temperature of halite; A-S: S-type inclusion in A-type quartz; B-S: S-type

Fig. 6. Histograms of salinities and homogenization temperatures of fluid inclusions at the different mineralization stages.

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6. Discussion

6.1. Metal transport in fluids

Despite decades of research, themetal transport agents in magmatic–hydrothermal ore deposits are still controversial (Williams-Jones andHeinrich, 2005). For more than a century, most researchers believedthat the metals are transported as complexes in aqueous liquids(Crerar and Barnes, 1976; Holland, 1972; Roedder, 1971). Despitethat, Henley and McNabb (1978) have proposed a model that a plumeof vapor may play an important role as a metal transport medium inporphyry ore deposits, which was accepted by very few researchers atthat time.With the continuous development of experimental technology,increasing evidence shows that the vapor phase may indeed be thedominant metal transport agent in some hydrothermal systems(Audetat et al., 1998; Heinrich et al., 1999; Lai and Chi, 2007). Hightemperature and pressure simulated experiments indicate that Cu is

preferentially partitioned into low-salinity magmatic vapor bycomplexation of Cu with sulfur-bearing ligands in sulfur-rich magmatic–hydrothermal systems (Heinrich, 2007; Williams-Jones and Heinrich,2005).

Post-entrapment modification of fluid inclusion properties is acommon concern for the interpretation of fluid inclusion data from por-phyry systems. Recently, experiments confirm that Cu concentration inquartz-hosted melt- and fluid inclusions can be modified by selectivediffusional exchange with the surrounding fluids (Li et al., 2009; Seoand Heinrich, 2013; Zajacz et al., 2009, 2011). Lerchbaumer andAudétat (2012) have demonstrated that sulfur in quartz-hosted fluidinclusions has played an important role in determining the Cu diffusionfrom the external hydrothermal fluids to the internal fluid inclusions,resulting in the precipitation of CuFeS2 in the fluid inclusions. Since sul-fur fractionates preferentially into the vapor phase (Drummond andOhmoto, 1985; Suleimenov and Krupp, 1994), sulfur-rich vaporinclusions may incorporate more Cu than the coexisting sulfur-poor

image of Fig.�6

Fig. 7.Homogenization temperatures vs. salinities of fluid inclusions in different stages. The regions inside the dotted line show the entire range of salinities and homogenization temper-aturesmeasured for each inclusion type. Since no salinity resultwas obtained for V-type inclusions, salinity of V-type inclusions has been estimated to be 10wt.%NaCl andmark the pointswith the homogenization temperatures of V-type inclusions. Fluid boiling and the appearance of S-type inclusions with metal daughter minerals occur in the main mineralization stage,corresponding to the major phase of Cu deposition.

Fig. 8. Temperature of bubble homogenization vs. temperature of halite dissolution in theS-type inclusions. Along the diagonal line, both bubble and halite homogenize at the sametemperature. Besides, almost all S-type inclusions show consistent microthermometricbehavior, marked by homogenizing with the halite disappearance (with exception oftwo isolated inclusions that are homogenized by bubble disappearance). This suggeststhat NaCl in fluids were supersaturated before entrapment, and initial fluids may havebeen trapped at higher pressure.

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brine inclusions after entrapment, leading to the partition coefficientDCuvap/brine to reach 4 to 9 (Lerchbaumer and Audétat, 2012). In con-

trast, realistic DCuvap/brine values are between 0.11 and 0.15 in porphyryCu environments (Lerchbaumer and Audétat, 2012).

From our results on the Tongkuangyu porphyry Cu deposit,particularly those on fluid inclusions, we demonstrate that thebrine phase may have been the dominant Cu transport medium,because:

1. S-type inclusions dominate the main- and late stage B-type quartz(Fig. 5E and F). Given that B-type quartz is closely associated withore minerals (Fig. 5A, B, C and D), the predominance of S-type inclu-sions provides direct evidence that the brine was the main metaltransport medium.

2. S-type inclusions (with chalcopyrite and hematite daughterminerals) are ubiquitous in both themain stage A- and B-type quartz(Fig. 4G; Fig. 5E). In contrast, chalcopyrite daughter minerals are ab-sent in the synchronic V-type inclusions, and similarly the early- andlate stage S-type inclusions have no chalcopyrite daughter mineralseither. Hence, chalcopyrite was unlikely to be derived by selectiveCu diffusion. As a result, we propose that the brine phase was thedominant Cu transport medium in the Tongkuangyu porphyry Cudeposit.

6.2. Fluid evolution of the Tongkuangyu porphyry Cu deposit

S, Fe, H and C are the only elements that can exist in variable oxida-tion states, as well as the only elements that are present in sufficientabundance to affect the redox state of the silicate magmas (Mungall,2002; Simon and Ripley, 2011; Simon et al., 2006). In highly oxidizedporphyry magma, sulfur is typically present in the form of sulfates,which provide a favorable condition for magmas to incorporatechalcophile elements (Gaetani and Grove, 1997; Mungall, 2002;Sillitoe, 1997).

Different from the oxidized magma, sulfur in ore minerals mainlyexists in the reduced form of S2−. Therefore, the sulfur species musthave been reduced from sulfates to sulfides during the Cumineralizationevent (Liang et al., 2009; Sun et al., 2004). During the earlier magmaticstage, Cu may be extracted by highly oxidized porphyry magma fromthe source region. In the later main mineralization stage, sulfatesmay have been reduced to sulfides, with Cu precipitated fromsulfur-supersaturated hydrothermal fluids (Liang et al., 2009; Mosset al., 2001).

In addition, ferrous ion plays a pivotal role in influencing the reduc-tion of sulfates, as it is the most effective and abundant reducing agentin magmas (Mungall, 2002). Magnetite crystallizing from the magmasmay lead to the reduction of SO42− to S2−, providing an ideal conditionfor Cu to separate from the magma to enter the hydrothermal fluids(Sun et al., 2004), as illustrated by the following equation (Liang et al.,2009):

12 FeO½ � þH2SO4 ¼ 4Fe3O4 þ H2S ð1Þ

Quartz–potassium feldspar alteration mainly occurs in the earlymineralization stage, and the homogenization temperatures of S-type

image of Fig.�7image of Fig.�8

Fig. 9. Laser Raman spectra of fluid inclusions. (A). H2O-spectrum of the liquid in the V-type inclusion. (B). CO2-spectra of the liquid in the C-type inclusions. (C). Hematite-spectra of thedaughter mineral in the S-type inclusion. (D). Chalcopyrite-spectra of the daughtermineral in the S-type inclusion. (E). Anhydrite-spectra of the daughtermineral in the S-type inclusion.(F). Calcite-spectra of the daughter mineral in the S-type inclusion.

507Y. Jiang et al. / Ore Geology Reviews 63 (2014) 498–509

inclusions in this stage range from 479 °C to 560 °C, consistent with thetemperature of typical potassic alteration (420–700 °C) in the porphyryCu environment (Rui et al., 1984). These S-type inclusions (withoutopaqueminerals) correspond to theweak ore-mineralization of the po-tassic alteration zone. The ubiquitous presence of magnetite in thequartz–potassium feldspar alteration zones indicates that reduction ofsulfates occurred in this stage and may have promoted the formationof the weak Cu mineralization.

Ore bodies formed during the main mineralization stage aremainly hosted in the quartz–sericite alteration zone, where thecoexistence of magnetite and hematite is common (Fig. 3E). Thefluid inclusion study shows that hematite and chalcopyrite daughter

minerals are ubiquitous in the S-type inclusions, whose homogenizationtemperatures vary in the range of 233–450 °C. It is noteworthy that thecrystallization temperatures of chalcopyrite and pyrite in the porphyryCu environment are in the range of 250–350 °C and 180–410 °C,respectively (Rui et al., 2003). Thus, the temperature decrease of theore-forming fluids apparently plays an important role in Cu deposition.On the one hand, fluid boiling in the temperature range of 233–450 °Cpromotes further Cu enrichment in the fluids with high salinity. Onthe other hand, the coexistence of magnetite and hematite indicatesthat oxygen fugacity of the hydrothermal system fluctuates near themagnetite–hematite (MH) buffer line (Sun et al., 2013). With a fluidtemperature decrease, magnetite is further oxidized to hematite by

image of Fig.�9

508 Y. Jiang et al. / Ore Geology Reviews 63 (2014) 498–509

the reduction of SO42−. The findings of S-type inclusions (with hematite,chalcopyrite and anhydrite daughterminerals) also show that hematitecrystallization may lead to the reduction of sulfates and promote theoversaturation of S2− and deposition of metal sulfides (modified afterSun et al. (2004)):

8 Fe2þ þ 3SO2−4 ¼ 4Fe2O3 þ 3S2− ð2Þ

Magnetite and specularitewere also observed in the latemineraliza-tion stage, indicating that the oxygen fugacity of hydrothermal systemwas still near the magnetite–hematite (MH) buffer line. However, thetemperature of the late mineralization stage dropped to the range of186–245 °C, and B-type quartz contains some L-type inclusions. Thissuggests that the salinity of the late stage ore-forming fluids may havedecreased with the influx of meteoric water. Lowering of the fluids'salinity may have led to the reduction of its Cu transport capacity,corresponding to the absence of chalcopyrite daughter minerals in thelate stage S-type inclusions. Furthermore, the absence of hematite andanhydrite in the late stage S-type inclusions indicates that the reductionof SO42− to S2− was much less significant, as demonstrated by the verysmall-scale late stage vein-type mineralization.

7. Conclusions

(1) The pervasive occurrence of S-type inclusions in A- and B-typequartz implies that high salinity fluids have played an importantrole in the Cu metallogenesis. Microthermometric data for theS-type inclusions shows that the main mineralization stageoccurred at 233–450 °C and the late mineralization stagemainly occurred at 186–245 °C.

(2) Fluid boiling during the main mineralization stage may havepromoted Cu enrichment in the brine phase. S-type is thedominant inclusion type in B-type quartz of the main stageand late stage alike, indicating that the brine phase is themain Cu transport medium in the Tongkuangyu porphyry Cudeposit.

(3) The ubiquitous coexistence of magnetite and hematite in theTongkuangyu Cu deposit indicates that the oxygen fugacity ofhydrothermal system fluctuates near the magnetite–hematite(MH) buffer line. Daughter minerals in S-type inclusions andmineral assemblage in the different alteration zones show thatthe process of sulfate reduction commenced during the earlymineralization stage. This process was particularly significantin the main mineralization stage and diminishes in the latemineralization stage, suggesting that ferrous ion oxidizationwas responsible for Cu deposition.

Acknowledgements

This study was financially supported by the National Basic ResearchProgram of China (No. 2012CB416603). We gratefully acknowledge theZhongtiaoshan Non-Ferrous Metals Group Co., Ltd. for providing fieldwork assistance. We thank two anonymous reviewers for constructivecomments. Profs. Weidong Sun, Huaying Liang and Huayong Chenfrom GIGCAS are thanked for their encouragements and suggestionsduring the preparation of the article. We also thank the editor-in-chief, Prof. Franco Pirajno, for handling this article and polishing theEnglish language. This is contribution No. IS-1889 from GIGCAS.

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Fluid evolution of the Tongkuangyu porphyry copper deposit in the Zhongtiaoshan region: Evidence from fluid inclusions1. Introduction2. Regional geology3. Ore deposit geology4. Analytical methods4.1. Sampling4.2. Microthermometry4.3. Laser Raman microspectroscopy

5. Results5.1. Fluid inclusion petrography5.1.1. Types of fluid inclusions5.1.2. Occurrence and temporal relation of fluid inclusions

5.2. Microthermometric results5.3. Laser Raman spectroscopy analysis

6. Discussion6.1. Metal transport in fluids6.2. Fluid evolution of the Tongkuangyu porphyry Cu deposit

7. ConclusionsAcknowledgementsReferences