High salinity fluid inclusions in the Yinshan polymetallic deposit from the Le–De metallogenic...

1 (2007) 247–260www.elsevier.com/locate/oregeorev

Ore Geology Reviews 3

High salinity fluid inclusions in the Yinshan polymetallic depositfrom the Le–De metallogenic belt in Jiangxi Province, China:

Their origin and implications for ore genesis

Dehui Zhang a, Guojian Xu b,⁎, Wenhuai Zhang c, Suzanne D. Golding b

a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, People's Republic of Chinab Department of Earth Sciences, The University of Queensland, Brisbane, Qld. 4072, Australia

c Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, People's Republic of China

Received 5 January 2003; accepted 20 November 2004Available online 5 June 2006

Abstract

A fluid inclusion investigation of the polymetallic mineralization at Yinshan from the Le–De metallogenic belt in JiangxiProvince of China has been carried out using petrographic and microthermometric techniques. The data obtained here indicate thatthree major types of fluids were involved during the formation of the deposit. They are type I vapor-rich, type II liquid-rich andtype III halite-bearing inclusions within the H2O–NaCl system. The high salinity fluids represented by type III inclusions, beingunusual to the distal part of an intrusion-centered ore-forming system such as Yinshan, have been interpreted as the product ofdirect exsolution of a crystallizing magma, rather than a result of fluid immiscibility from a low salinity fluid. Evidence used tosupport such an interpretation includes the mode of homogenization of type III inclusions exclusively via halite dissolution, spatialseparation of type I and type III inclusions on microscopic scale, the consistent phase ratios within the inclusions concerned, andconsiderable deviation in homogenization temperature for both type I and type III inclusions. Trapping conditions for type Iinclusions were estimated to be around 440 °C and 260 bars, while type III inclusions were constrained to be trapped at least above900 bars and >500 °C. The formation temperatures for type II inclusions range from 270 to 390 °C if a lithostatic pressure of260 bars is assumed. Pressure fluctuation determined by this fluid inclusion study coupled with decreases in salinity andtemperature as result of the potential fluid mixing are supposed to have played an important role in triggering the precipitation ofore minerals from the hydrothermal solution.© 2006 Elsevier B.V. All rights reserved.

Keywords: Polymetallic mineralization; High salinity fluid inclusions; Magmatic exsolution; Yinshan; China

⁎ Corresponding author. Xstrata Copper Exploration Pty Ltd.,Locked Mail Bag 100, 102 Oban Road, Mount Isa, Qld. 4825,Australia. Tel.: +61 7 47443096; fax: +61 7 47443998.

E-mail address: [email protected] (G. Xu).

0169-1368/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.oregeorev.2004.11.002

1. Introduction

High salinity fluid inclusions, characterized by thepresence of a halite crystal at room temperature, arecommon in many intrusion-centered mineralizationsystems, particularly in porphyry (Bodnar and Beane,1980; Roedder, 1984; Cline and Bodnar, 1994; Ulrich etal., 2001) and skarn (Kwak, 1986; Meinert et al., 1997;

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248 D. Zhang et al. / Ore Geology Reviews 31 (2007) 247–260

Xu and Lin, 2000) deposits. In contrast, fluid inclusionsassociated with intrusive-related mineralization thatformed distal to the plutonic sources are normallydominated by low salinity liquid–vapor inclusions, astypified by those found in epithermal Au environments(Bodnar et al., 1985; Shimizu et al., 1998). A generalunderstanding of the relationship between fluid salinityand depth of trapping can be deduced from the study byHedenquist et al. (1998) on the Far Southeast porphyryCu–Au and the Lepanto high sulfidation epithermaldeposits in the Philippines. In any case, the presence ofhigh salinity fluid inclusions and their homogenizationmodes during microthermometry are important indica-tors to the estimation of pressure–temperature condi-tions and to the sources of mineralizing fluids (Bodnar,1994; Cline and Bodnar, 1994; Meinert et al., 1997).

In line with the above generalization, we report forthe first time the occurrence of high salinity fluidinclusions in the Yinshan polymetallic deposit inDexing County, East China (Fig. 1). The region hasbeen well known for the discovery of the largestporphyry copper deposit in East China, which isconventionally called the Dexing porphyry copperdeposit and is located 20 km northeast of Dexing City(Khin Zaw et al., 2007-this volume). It actually consistsof three large porphyry Cu (Mo) deposits, namely theTongchang Cu deposit in the middle, Fujiawu Cu–Modeposit to the southeast of Tongchang, and the

Fig. 1. Regional location of the Yinshan polymetallic deposit and oth

Zhushahong copper deposit to its northwest (Fig. 1).These deposits are of the same genetic type, andtogether form the largest porphyry copper resource inEast China with over ten million tons of containedcopper. The detailed geological characteristics of theDexing porphyry Cu deposit have been described byZhu et al. (1983).

The Yinshan polymetallic deposit has attractedconsiderable research interest in the last 20 yearsowing to its complex metal content, distinct ore zoningand the transitional nature in mineralization type fromtypical porphyry to epithermal categories. Most of theprevious work carried out by Chinese scholars has beenfocused on the origin of the deposit through investiga-tions of the genetic link between volcanic–subvolcanicrocks and mineralization (Wang et al., 1984; Hao, 1987;Liu, 1994; Ye and Mo, 1998), emphasis on the structuralcontrol of mineralization (Qiu, 1991; Liu et al., 1994;Mo et al., 1995, 1996; Ye et al., 1998), documentation ofore zoning and alteration (Ye, 1984, 1987; Hao, 1988;Mo et al., 1996; Zhang et al., 1997, 1998), constraints onthe sources of metals and ore-forming fluids (Hua et al.,1993, 1995; Shen et al., 1991; He and Lin, 1992; Zhanget al., 1996) and modeling heat transfer and fluid flow(Zhang et al., 1998).

Despite this volume of research, the fluid inclusionaspect of the deposit, which is critical to a comprehen-sive understanding of the ore genesis, is much less

er major base metal occurrences in the Dexing mineral district.

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249D. Zhang et al. / Ore Geology Reviews 31 (2007) 247–260

studied. As a result, the physicochemical conditions andformation mechanism of mineralization, as well aspossible sources of fluids, are poorly constrained. Thecurrent research is directed toward documentation of thefluid inclusions trapped during the formation of theYinshan polymetallic deposit. In addition, specialattention has been paid to the evaluation of themechanisms by which the saline fluids were generatedthrough approach of fluid inclusion microthermometryand phase equilibrium.

2. Regional geology and mining history

The Yinshan polymetallic deposit is situated inJiangxi Province, East China, and is located in theinner zone of the circum-Pacific metallogenic belt.Tectonically, it occurs on the southern margin of theeastern extension of the Jiangnan anticline within theYangtze meta-platform. Regionally, the deposit is

Fig. 2. The geology of the Yinshan polymetallic deposit (modi

located in the central part of the NE–SW trending Le–De Cu–Pb–Zn–Au–Ag belt, and next to the northeast-ern margin of the Le–De Mesozoic volcano–sedimen-tary basin (Fig. 1). The whole belt is defined by theLeanhe crustal fault in the northwest and by theNortheast Jiangxi crustal fault in the southeast (Fig.1). The Northeast Jiangxi crustal fault also acts as aboundary separating the South China cratonic upliftfrom the Qiantang cratonic depression (Zhu et al.,1983). The Le–De base–precious metal metallogenicbelt is about 100 km long and 12 to 15 km wide and isthe most significant accumulation of Cu–Pb–Zn–Au–Ag in China. It comprises, from NE to SW, primarily theTongchang copper deposit, Fujiawu copper–molybde-num deposit, Zhushahong copper deposit, Jinshan Audeposit, Yinshan polymetallic deposit and Lehua Pb–Zn–Mn–Ag deposit (Fig. 1).

The regional stratigraphy is dominated by Middleand Upper Proterozoic metamorphic basement and a

fied from Jiangxi Geological Exploration Bureau, 1996).


Mesozoic cover sequence (Fig. 2). The Middle andUpper Proterozoic rocks are mainly continental clasticsinterbedded with intermediate to mafic volcanics andhad been metamorphosed to lower greenschist facies.Mesozoic rocks include the Upper Jurassic and LowerCretaceous volcano–sedimentary formations which aredacite–rhyolite and volcanoclastic lithologies. Thebasement rocks have been subject to strong deformationwith the development of folds and faults and theintrusion of the Middle Jurassic to Early Cretaceous(167 to 139 Ma) granitoids into the study area. Dating ofa suite of rock samples collected from the quartzporphyry, dacite porphyritic dikes and pyroclasticsyields a Rb–Sr age of 164 Ma, a U–Pb age of 167to139 Ma and a K–Ar age of 159 to 142 Ma for theigneous activity in the deposit area (Jiangxi GeologicalExploration Bureau, 1996).

The Yinshan deposit is reported to have beendiscovered in 668 BC and mining activity in ancienttimes continued for about 400 years with annualproduction of silver up to 883,000 oz during the boomyears. More recent underground mining started in 1958by the state-owned Jiangxi Copper Limited. By the endof 2001, a total of 2.3 Mt of ore grading 0.65% Cu and16.4 Mt of ore grading 1.8% Pb and 1.93% Zn has beenmined. In addition, 3560 kg of Au with a grade of 0.2 g/tand 1825 tonnes of Ag with a grade of 110 g/t have beenproduced. Mining continues at Yinshan with provenreserves of 120 Mt of Cu ore at 0.54%, 5 Mt of Pb ore at1.8% and 5.2 Mt of Zn ore at 2.3%, plus 125.4 Mt of Auore at 0.63 g/t and 127.3 Mt of Ag ore at 11 g/t.

3. Geology of the ore deposit

3.1. Stratigraphy

The stratigraphy in the Yinshan deposit consists oflow-grade metamorphic rocks of the Middle ProterozoicShuangqiaoshan Group and pyroclastic rocks of theunconformably overlying the Upper Jurassic EhulingFormation. The lithology of the Shuangqiaoshan Groupis dominated by various phyllites with a thickness of2500 m. The Shuangqiaoshan Group, the major hostrocks to the orebodies, has been dated at 1410 Ma by theRb–Sr method (Zhu et al., 1983). The Upper JurassicEhuling Formation comprises rhyolitic and daciticpyroclastics and occurs in the west and south of thedeposit. It also hosts some orebodies and varies from 260to 1100 m in thickness. The Lower Cretaceous ShixiFormation is mainly observed in the southern margin ofthe deposit and characterized by continental clastic rocksof conglomerate, sandstone and tuffaceous shale (Fig. 2).

3.2. Structure

The major structures developed in the Yinshandeposit are the Yinshan anticline and related faults.The axis of the Yinshan anticline is oriented at 45° to50° and steeply plunges NE (Fig. 2). Both limbs arecomposed of the Shuangqiaoshan metamorphic rocksand are steeply dipping, commonly with parasitic folds.Axial plane-parallel faults are well developed in thedeposit area. Most are nearly vertical in occurrence andsome are filled with subvolcanic intrusions. The otherimportant structure is the Xishan edifice or caldera in thewestern part of the deposit (Fig. 2). It is ellipsoidal andNW-oriented. The associated ring-like and radiatingfaults control the emplacement of ore-related subvolca-nic intrusions and subsequent mineralization.

3.3. Magmatic rocks

The magmatic rocks exposed in the deposit area aredominated by a variety of porphyries intruded during thesecond stage of the Yanshanian orogeny (167 to 139Ma),which is an important tectono-magmatic event affectingthe bulk of eastern China. Three cycles of magmaticactivity have been identified at Yinshan with the earliestbeing quartz porphyry, through dacitic porphyry, to thelatest andesitic porphyry. Inmost cases, they are present asstocks and dikes next to the caldera and along faults. Wallrock alteration and mineralization are spatially associatedwith felsic intrusions, whereas late mafic intrusions arelittle altered and are typically non-mineralized.

3.4. Yinshan orebodies

Over 100 orebodies have been defined in the Yinshandeposit where they occur as veins, tabular bodies andlenses mainly in phyllites and to a lesser extent inpyroclastics and porphyries (Fig. 3). The length ofindividual orebodies varies mainly between 300 and600 m, with a maximum of 1050 m and thickness variesfrom 1 to 15 m. The strike of the orebodies is primarilynear EW and NS with a steep dip (Fig. 3). The mineralcomposition at Yinshan is complex and 77 mineralspecies have been identified so far in the deposit. Theore minerals include pyrite, chalcopyrite, galena,sphalerite, enargite, tennantite, arsenopyrite, nativegold, native silver and pyrargyrite, while gangueminerals are dominated by quartz, calcite, siderite,chlorite and sericite. The deposit is a product of multi-stage hydrothermal events, with early Cu–Au mineral-ization succeeded by late Pb–Zn–Ag mineralization.Based on crosscutting relationship observed in the field,

Fig. 3. Cross section through the majority of orebodies in the Yinshan deposit, showing localities of samples used in this study (modified from JiangxiGeological Exploration Bureau, 1996; the location is indicated in Fig. 2).


four different stages of mineralization can be recognizedat Yinshan and correspond to four different mineralassemblages in hand specimen, i.e., barren quartz,pyrite–quartz, pyrite–chalcopyrite–quartz and pyrite–sphalerite±galena–quartz from early to late stages. Onthe other hand, zonal distribution of mineralization typesis evident with Cu–Au in the center, Cu–Zn–Pb in themiddle, and Pb–Zn–Ag at the periphery of the deposit(Zhang et al., 1997). Pervasive and fracture-controlledwall rock alteration is common and characterized bypyritization, sericitization and silicification for Cu–Aumineralization and chloritization, carbonation andbaritization for Pb–Zn–Ag mineralization.

4. Sampling and analytical procedures

The samples examined in this study were exclusivelycollected from underground exposures along the maintunnels at three different levels (designated as −60 m,−105 m and −150 m) crosscutting the majority oforebodies in the Yinshan deposit (Fig. 3). A total of 84samples, representative of all mineralization typesencountered at Yinshan, were used for reconnaissancesurvey of fluid inclusions. However, suitable fluidinclusions were only found in 55 samples. Of these, 3samples are quartz porphyries, 14 samples are barrenquartz veins, 15 samples are pyrite-bearing quartz veins,

7 samples are pyrite–chalcopyrite quartz and 16samples are pyrite–sphalerite±galena quartz veins.

Doubly polished sections, approximately 200 μmthick, were prepared for microthermometric determina-tion. Measurements were made using a Linkam THMS600 heating–freezing stage. Reproducibility of thehomogenization temperature is within 2 °C and of thetemperature of final ice melting within 0.2 °C.

5. Classification and distribution of fluid inclusions

Fluid inclusions suitable for microthermometricstudy were mainly found in quartz, with rare exceptionsobserved in calcite for which no freezing and heatinghas been done due to the problem of stretching. Quartzgrains hosting fluid inclusions are either clear pheno-crysts in quartz porphyries or subhedral crystalsinterstitial to other metallic minerals in veins, such aschalcopyrite, pyrite and sphalerite. The primary, sec-ondary and psuedosecondary criteria suggested byRoedder (1984) were used as a standard for distinguish-ing the origin of inclusions but microthermometricmeasurements were mainly carried out on inclusionswith a primary or most likely primary origin. In terms ofthe number and the volumetric proportions of phasespresent in the fluid inclusion at room temperature, threemajor types of inclusions can be recognized at Yinshan.


They are designated as vapor-rich (type I), liquid-rich(type II) and halite-bearing multiphase (type III)inclusions. A detailed discussion of each type ofinclusion is presented below.

5.1. Type I inclusions

Type I inclusions are dominated by more than 50 vol.%(typically around 80%) of H2O vapor at room

Fig. 4. Microphotographs showing different types of fluid inclusionsobserved in the Yinshan polymetallic deposit.

temperature and were locally observed in the examinedsamples (Fig. 4A). Fluid inclusions of this type usuallyoccur as isolated individuals with rare occasions whichcan be attributed to specific growth zones in veincrystals, and hence bear an unambiguous primary origin.Fluid inclusions within this group are generally between4 and 10 μm in size and show ellipsoidal to roundedmorphologies. Type I inclusions are found rarely in thequartz phenocrysts in quartz porphyry and in the quartzgrains in the four stages of mineralization describedabove, i.e., barren quartz vein, pyrite–quartz vein,pyrite–chalcopyrite–quartz vein and pyrite–sphalerite±galena quartz vein. These mineralized veins occureither in contact with porphyritic dykes or in thephyllite.

5.2. Type II inclusions

Type II inclusions, characterized by a gas bubblewithin an aqueous liquid, are commonly found in allselected samples (Fig. 4B). This is the most abundantinclusion type and accounts for more than 90% of allfluid inclusions observed during this study. In mostcases, such inclusions are relatively small in size (5–20 μm) with ellipsoidal, negative crystal and irregularshapes. At room temperature, a variable vapor to liquidratio can be seen for this type of inclusions with amajority ranging from 10% to 20%. The distribution ofthese inclusions in the host quartz varies from randomisolations, clusters and trials along microfractures,giving rise to a complex origin embracing primary,pseudosecondary and secondary according to the criteriasuggested by Roedder (1984).

5.3. Type III inclusions

Fluid inclusions of this type consist, at roomtemperature, of aqueous liquid, a vapor bubble and ahalite crystal, and in some circumstances, additionalsolid phases such as sylvite and opaque minerals (Fig.4C). They are interpreted to be true daughters ratherthan accidental crystals trapped during the formation offluid inclusions according to their consistent phaseratios (ca. 20 vol.%) when present. Overall, fluidinclusions with two or more solid phases are commonlyfound in the quartz phenocrysts and early barren andpyrite-bearing quartz veins, while inclusions with halitealone tend to occur in quartz grains in mineralizedveins. Inclusions of this type exhibit ellipsoidal toirregular shapes and are 5 to 30 μm in diameter. Mostof such inclusions probably bear a primary origin asdemonstrated by their isolated distribution pattern


though some inclusions do occur as planar trailscrosscutting grain boundaries and hence show anapparent secondary nature.

Halite exhibits a light green color in all of these inclusionsand has been identified based on its cubic crystal shape andabsence of birefringence. The opaque phases cannot beidentified with certainty in this study, but they could bechalcopyrite or pyrite based on their morphologies.

6. Microthermometric measurements

6.1. Type I inclusions

The small inclusion size coupled with limited amountof liquid hindered the observations of initial ice melting,final ice melting and even homogenization for vapor-richtype I inclusions so that only 34 heating–freezingmeasurements were carried out. Final ice meltingoccurred at temperatures above −1.6 °C for all measuredtype I inclusions, indicating a consistently low salinity(<2.7 wt.% NaCl equiv.). Upon heating, all type Iinclusions measured showed homogenization via theexpansion of vapor phase. Homogenization tempera-tures vary from 277 °C to 560 °C and average at elevatedtemperatures for all five different occurrences of hostminerals, i.e., 488 °C for quartz phenocrysts, 487 °C forbarren quartz veins, 459 °C for pyrite–quartz veins,427 °C for pyrite–chalcopyrite–quartz veins and 436 °Cfor pyrite–sphalerite–galena quartz veins (Table 1).

6.2. Type II inclusions

Fluid inclusions of this type freeze to ice uponcooling below −70 °C. During warming, initial melting

Table 1Microthermometric results of fluid inclusions from the Yinshan polymetallic

Mineral assemblage Inclusiontype

Number ofsamples

Number ofmeasurements

Th r(°C)

Phenocrysts in quartzporphyry

I 1 4 449–II 3 6 256–III 2 4 320–

Barren quartz vein I 1 3 385–II 13 31 213–III 5 9 320–

Pyrite quartz vein I 5 13 314–II 18 38 202–III 4 7 305–

Pyrite–chalcopyritequartz vein

I 2 4 389–II 8 18 214–III 2 3 285–

Pyrite–sphalerite±galenaquartz vein

I 3 10 277–II 18 36 200–III 4 8 311–

was observed for a few inclusions between −19.4 and−21.2 °C, indicating an H2O–NaCl system for the fluidstrapped. On subsequent heating, final ice melting occursover a relatively wide range of temperatures for differentoccurrences of quartz hosts, with −4.6 to −8.8 °C forquartz phenocrysts from porphyries, −3.7 to −15.8 °Cfor barren quartz veins, −4.1 to −10.3 °C for pyrite-bearing quartz veins, −1.5 to −10.7 °C for pyrite–chalcopyrite–quartz veins and −4.1 to −10.1 °C forpyrite–sphalerite±galena quartz veins (Table 1). Sali-nities determined by freezing point depression (Bodnar,1993) were 7.3 to 12.7 wt.% (mean=9.6 wt.%) NaClequivalent for inclusions trapped within quartz pheno-crysts, 6.0 to 19.4 wt.% (mean=10.5 wt.%) NaClequivalent for barren quartz veins, 6.6 to 14.3 wt.%(mean=9.9 wt.%) NaCl equivalent for pyrite-bearingquartz veins, 2.5 to 14.7 wt.% (mean=10.7 wt.%) NaClequivalent for pyrite–chalcopyrite–quartz veins and 6.6to 14.1 wt.% (mean=10.8 wt.%) NaCl equivalent forpyrite–sphalerite±galena quartz veins. The salinity dataindicate a slight variation from low to moderatesalinities recorded by the fluid inclusions present ineach type of mineral assemblage but a rather constantaverage value for different modes of quartz host grains(Table 1).

Upon heating, total homogenization of type IIinclusions was observed through the disappearance ofvapor in the liquid. Homogenization temperatures varyfrom 256 to 356 °C (mean=308 °C) for quartzphenocrysts in quartz porphyries, 213 to 386 °C(mean=288 °C) for barren quartz veins, 202 to 369 °C(mean=292 °C) for pyrite-bearing quartz veins, 214 to391 °C (mean=324 °C) for pyrite–chalcopyrite quartzveins and 200 to 388 °C (mean=296 °C) for pyrite–

deposit

ange AverageTh (°C)

Tm ice

range (°C)Salinity range(wt.% NaCl)

Average salinity(wt.% NaCl)

548 488 −0.5 to −0.8 0.9–1.4 1.2356 308 −4.6 to −8.8 7.3–12.7 9.6515 395 39.4–58.1 46.2400 487 −0.4 to −1.2 0.7–2.0 1.2386 288 −3.7 to −15.8 6.0–19.4 10.5530 403 39.4–60 46.7560 459 −0.6 to −1.5 0.7–2.6 1.7369 292 −4.1 to −10.3 6.6–14.3 9.9580 507 37.3–65.8 57.7475 427 −0.4 to −1.3 0.7–2.2 1.6391 324 −1.5 to −10.7 2.5–14.7 10.7520 415 36.9–58.8 48.6560 436 −0.3 to −1.6 0.5–2.7 1.5388 296 −4.1 to −10.1 6.6–14.1 10.8490 389 38.7–55.1 45.4


sphalerite±galena quartz veins (Table 1). Similar tothe salinity data reported above, a reasonably widerange but constant average in homogenization tem-perature exists for different types of fluid inclusion-containing materials at Yinshan, which, combinedwith the salinities, may indicate repeated episodes ofhydrothermal activities with similar physicochemicalproperties throughout the veining and mineralizationhistory.

6.3. Type III inclusions

As mentioned above, fluid inclusions of this typeusually contain an NaCl crystal and, on rare occasions,one to two other solid phases comprising sylvite andpossibly chalcopyrite. Upon heating, vapor disappear-ance normally occurs at a temperature of 4 °C to 260 °C,prior to complete homogenization which was exclu-sively achieved via the dissolution of salt crystals at upto 580 °C (Fig. 5). Opaque phases, assumed to bechalcopyrite, remain undissolved until 600 °C in thisstudy. A total of 31 measurements on type III inclusionsof primary or most likely primary origin shows a widerange of homogenization temperatures of 320 to 515 °C(mean=395 °C) for quartz phenocrysts in quartzporphyries, 320 to 530 °C (mean=403 °C) for barrenquartz veins, 305 to 580 °C (mean=507 °C) for pyrite-bearing quartz veins, 285 to 520 °C (mean=415 °C) for

Fig. 5. Halite dissolution temperature as a function of vapor disappearance temwhich inclusions homogenize by halite dissolution (upper left) from that wh

pyrite–chalcopyrite quartz veins and 311 to 490 °C(mean=389 °C) for pyrite–sphalerite±galena quartzveins (Table 1). Due to the limited number ofmeasurements, it is difficult to compare the range ofhomogenization temperatures among different types ofmineral assemblage though a large variation exists forthe overall homogenization temperatures of type IIIinclusions (Fig. 5). Salinities determined from thedissolution temperatures of halite average at 46.2 wt.% NaCl equivalent for quartz phenocrysts, 46.7 wt.%NaCl equivalent for barren quartz veins, 57.7 wt.%NaCl equivalent for pyrite-bearing quartz veins,48.6 wt.% NaCl equivalent for pyrite–chalcopyritequartz veins and 45.4 wt.% NaCl equivalent for pyrite–sphalerite±galena quartz veins. Relatively small varia-tions in average homogenization temperature (389 to507 °C) and salinity (45.4 to 57.7 wt.% NaClequivalent) are noted for type III inclusions, which areindependent of the occurrence of host minerals andconsistent with the pattern observed for type IIinclusions above.

6.4. Homogenization temperature–salinity relationship

If the salinities (S) of type II liquid-rich inclusionsare plotted against their homogenization temperatures(Th), a very weak positive correlation can be recog-nized for inclusions related to quartz phenocrysts in

peratures for type III inclusions. The diagonal line separates the field inere inclusions homogenize by vapor disappearance (lower right).

Fig. 6. Plot of homogenization temperatures versus salinities of type II liquid rich inclusions at the Yinshan polymetallic deposit. Note the weakpositive correlation between them, being characteristic of a cooling magmatic hydrothermal system.


quartz porphyries, barren quartz veins, pyrite quartzveins, pyrite–chalcopyrite veins and pyrite–sphalerite±galena quartz veins in the Yinshan polymetallicdeposit (Fig. 6). Such a trend in Th and S ischaracteristic of many intrusion-centered mineralizingsystems around the world (Meinert et al., 1997; Xu andLin, 2000). On the other hand, an almost similar rangein Th and S exists for the above five different mineralassemblages sampled for this investigation. Given thestrain-free nature of the host quartz grains examined, itis highly unlikely that such a widespread distribution isdue to post-trapping modification of fluid inclusions.Alternatively, the wide variation in values of Th and Smight suggest that the inclusions measured, andpossibly the sampled quartz, were not contemporane-ous. Petrographic observations do show at least threegenerations of quartz precipitation within each mode ofmineral assemblage involved in the microthermometricexperiments of this study. The earliest quartz is usuallycoarsely crystalline with little sulfides associated. Theintermediate quartz is fine-grained and intergrown withsulfide minerals while the latest quartz is generallyassociated with carbonates and sulfides such assphalerite and galena. Finally, microthermometric datacollected from different mineral assemblages or

different mineralization types do not show anyobservable differences in Th and S that can be usedto distinguish them from one another. Conversely,strong similarities in microthermometric data suggestthat all different mineralization types recognized in thefield may have experienced an almost identicalevolution history.

7. Discussion and conclusions

7.1. Origin of the high salinity fluid inclusions

Over the past decade or so, the origin of high salinity(NaCl-oversaturated) fluid inclusions has been a themeof much attention. Rational interpretation of the originof these inclusions has been important to achieving acomprehensive understanding of the evolution historyand trapping conditions of ore-forming fluids (Roedderand Bodnar, 1980; Roedder, 1984; Cline and Bodnar,1994; Wilkinson, 2001). As summarized by Bodnar(1994), high salinity fluids can be formed either throughimmiscibility of a low salinity aqueous fluid or via directexsolution from a crystallizing melt, depending on thedensity of the inclusion fluid, which is in turn dependenton the P–T conditions of entrapment.


As presented in the preceding sections, threedifferent types of fluid inclusions occur in the Yinshandeposit, i.e., vapor-rich, liquid-rich and halite-bearinginclusions. Such a fluid inclusion assemblage ischaracteristic of many porphyry systems and hencemay favor a boiling mechanism for the formation ofsupersaline inclusions. However, several lines ofevidence obtained from this study suggest an alternativeinterpretation of the origin of the high salinity inclusionsencountered at Yinshan. Firstly, the mode of homoge-nization of all halite-bearing inclusions was via halitedissolution, which requires them to have been trapped inthe liquid-stable but vapor-absent field, and militatesagainst the boiling hypothesis (Roedder and Bodnar,1980; Bodnar, 1994; Cline and Bodnar, 1994). Second-ly, the contemporaneity of vapor-rich and halite-bearinginclusions is the critical evidence for boiling, which isnot supported by the spatial separation of type I and typeIII inclusions in the examined samples, where they arenormally present in different quartz grains. Thirdly, thewide variation in phase ratio within fluid inclusions, ascharacterized by a boiling system, is at odds with the

Fig. 7. Pressure–Temperature diagram showing the possible fluid trapping cdeposit (modified from Bodnar, 1994 and Cline and Bodnar, 1994). Note tentrapment of type III halite-bearing inclusions. L–V–H: liquid–vapor–halitephase stability field; L+V: liquid and vapor phase stability field; L: liquid phahomogenization by halite melting; ThL: total homogenization by vapor disapwhich vapor dissolves.

relatively consistent phase ratios demonstrated by thefluid inclusions observed in this study. Finally, if thevapor-rich and halite-bearing inclusions did result fromboiling of a low salinity aqueous fluid, then they shouldyield a very similar range of homogenization tempera-tures. Actual measurements of type I inclusions,however, show a homogenization temperature of 314to 560 °C, averaging 459 °C, which is about 50 °Chigher than the corresponding type III inclusions.

7.2. Trapping conditions of the fluid inclusions

Fluid inclusions investigated in this study indicatethat a relatively wide variation in temperature and fluidcomposition in the binary system of H2O–NaCl hadoccurred during the formation of the Yinshan poly-metallic deposit. In order to establish the P–T conditionsduring mineralization, both isochores and liquidi wereconstructed for fluid inclusions using the computerprogram MacFlinCor (Brown and Hagemann, 1995)and the experimental data produced by Bodnar (1994).Calculated isochores and the related liquidi for halite-

onditions for the fluid inclusions observed in the Yinshan polymetallichat the shaded area represents the most likely P–T conditions for thecurve; L–V: liquid–vapor curve; LIQ: liquidus; V+H: vapor and halitese stability field; L+H: liquid and halite phase stability field; ThH: totalpearance; TmH: temperature at which halite melts; TdV: temperature at


bearing inclusions are plotted in Fig. 7, from which fourP–T fields are defined by the liquid–vapor–halite curve(L–V–H), liquid–vapor curve (L–V) and the liquidus(LIQ) of the H2O–NaCl system. They are vapor andhalite phase stability field (V+H), liquid and vaporphase stability field (L+V), liquid phase stability field(L) and liquid and halite stability field (L+H). Withinthe liquid stability field, the isochore for halite-bearinginclusions experiencing simultaneous disappearance ofboth vapor phase and halite crystal divides the space intotwo subregions designated as ThL and ThH (Fig. 7).Inclusions trapped in the liquid stable field on the hightemperature side of this isochore will show finalhomogenization by vapor disappearance while thoseinclusions trapped on the low temperature side of thisisochore will show final homogenization by halitedissolution (Cline and Bodnar, 1994).

As previously noted, all type III inclusions encoun-tered in the Yinshan deposit exclusively homogenizedthrough halite dissolution. Phase equilibria constraintsrequire such inclusions were trapped in the liquid-stablebut vapor-absent field in the P–T space (Bodnar, 1994;Cline and Bodnar, 1994). Nevertheless, trapping condi-tions for these high salinity fluid inclusions can beconfined by their isochores and liquidi. Primaryinclusions of this group normally have vapor to liquidhomogenization at temperatures from 250 °C to 450 °Cand complete homogenization by halite melting mainlyat temperatures from 350 °C to 530 °C (see Fig. 5),corresponding to salinities of 40 to 60 wt.% NaClequivalent. Inclusions representing the lower salinitywill undergo vapor disappearance at 250 °C (point A inFig. 7) on the three phase (L–V–H) curve. Withcontinued heating, it will then move into the L+H fielduntil halite melting at the intersection between theisochore and the corresponding liquidus (point A′).Inclusions of this kind can be trapped anywhere alongthe isochore at temperatures above the halite meltingtemperature in the liquid stable field (ThH). In the sameway, inclusions representing the upper salinity limit willbe subject to vapor disappearance at 450 °C (point B inFig. 7) and final halite melting at the intersectionbetween the isochore and the liquidus for the 60 wt.%NaCl equivalent salinity fluid (point B′ in Fig. 7). Takenaltogether, the trapping conditions for type III inclusionscan be constrained with the shaded area that representsthe most likely overlap in P–T conditions for suchinclusions examined by this study (i.e., >500 °C and>900 bars). One of the most remarkable features of thisdetermination is that type III inclusions must have beentrapped under pressure conditions at least above900 bars (see Fig. 7). However, stratigraphic reconstruc-

tions at Yinshan indicate that the maximum thickness ofthe overlying rocks for the orebodies is 1000 m, givingrise to a lithostatic pressure estimate of 260 bars (JiangxiGeological Exploration Bureau, 1996). Such a differ-ence in lithostatic pressure estimated from the thicknessof overlying rocks and fluid pressure determined fromfluid inclusion study implies that the mineralizingsystem at Yinshan was overpressured, which issupported by the development of implosive brecciationalong the contacts of quartz and dacitic porphyries(Zhang et al., 1997).

On the other hand, previous studies have indicatedthat the salinities of aqueous fluids exsolved fromcrystallizing magmas are a function of system pressureand crystallization degree and the increase or decrease ofthe salinities of the exsolved fluids mainly rests with thepressure or depth at which magmas crystallize (Candelaand Holland, 1984; Cline and Bodnar, 1991, Webster,1992). Numerical modelling and experimental resultsdemonstrate that the first fluids exsolved from a typicalcalc–alkaline melt at 2 kbar will have the highest salinityup to 53 wt.% NaCl equivalent but with the continuationof crystallization the last fluids will have the leastsalinity. In contrast, if a calc–alkaline melt crystallizesunder 500 bars, the first exsolved fluids will have thelowest salinity but with the advance of crystallization upto 90% of the melt being solidified, the final fluids willdramatically increase their salinities up to 80% NaClequivalent (Cline and Bodnar, 1991; Webster, 1992).The quartz porphyry and dacitic porphyry at Yinshanwere emplaced at a shallow depth, corresponding to arelatively low pressure condition. It is therefore deducedthat the final fluids exsolved from such a silicate melt arecharacterized by very high salinity, which attests to theinterpretation that halite-bearing inclusions were trappedfrom the late exsolved magmatic fluids rather thanformed by fluid immiscibility.

Plotting of the isochore for type I inclusions plus theknowledge of formation depth of the mineralizationwill constrain the trapping conditions of the vapor-richinclusions to a temperature of about 440 °C under apressure of 260 bars (see Fig. 7). Given the difficultiesrelating to microthermometric measurements of thevapor rich inclusions, such a P–T estimate is lessaccurate with regard to the actual trapping conditions oftype I inclusions. On the other hand, type II inclusionsobserved at the Yinshan polymetallic deposit show arelatively large variation in both homogenizationtemperature and salinity. Intersections of the isochoresfor the lower and upper ends of the primary inclusionsof this type with the lithostatic pressure of 260 barsyield a formation temperature range from 270 to


390 °C but there will be about 20 °C correspondingdeduction if hydrostatic conditions are assumed (seeFig. 7).

7.3. Evolution and source of mineralizing fluids

The fluid inclusion data presented so far indicatethat a multi-staged fluid entrapment with variablecomposition in the H2O–NaCl system occurred duringthe formation of the Yinshan polymetallic deposit. Thecoexistence and overprinting of different types ofprimary inclusions in the same host mineral and morecommonly in the same paragenetic assemblage make itdifficult to characterize the fluid chemistry andevolution history without consideration of their relativechronology. Petrographic examination indicates thattype I vapor-rich inclusions are the earliest inclusions,trapped mainly in the first stage of coarsely crystallinequartz grains without significant sulfide mineralsassociated under microscopic scale, whereas type IIIhalite-bearing inclusions tend to be trapped within thesecond stage of fine-grained quartz accompanied bycertain ore mineral precipitation. In contrast to thelimited distribution of both type I and III inclusions,type II liquid-rich inclusions were not only observed inthe first and second stages of quartz, but are also seenin the latest quartz grains mosaic to calcite associatedwith galena and sphalerite. It is nevertheless envisagedthat the mineralizing system at Yinshan starts with theemplacement of quartz and dacitic porphyries at ashallow depth. The early fluids exsolved from suchsilicate melts will have a very low salinity due to thelow pressure conditions, being represented by type Iinclusions in this study. Such a dilute hot fluid ismainly responsible for the development of early barrenand possibly some pyrite-bearing quartz veins. Withcontinued crystallization saline fluids were thenexsolved from the crystallizing magmas. Sealing ofthe fractures by precipitation of minerals led to increasein pressure and the system became overpressured. Itwas under such conditions that type III halite-bearinginclusions were trapped, exhibiting final homogeniza-tion characteristic of high pressure conditions(>900 bars). Collapse of the overpressured systemthrough implosion and accompanied introduction ofmeteoric water resulted in the generation of low tomoderate salinity fluids characterized by type IIinclusions. Such fluids, together with part of the salinefluids, may have been responsible for the deposition ofore minerals at Yinshan, during which pressurefluctuation as deduced from fluid inclusion data plusdecreases in salinity and temperature as result of the

potential fluid mixing are supposed to have played animportant role in triggering the precipitation of oreminerals from the hydrothermal solution.

The source of the mineralizing fluids has not been welldefined yet. However, a common feature of the Yinshandeposit is its close relationshipwith porphyritic intrusions.In addition, oxygen and hydrogen isotope analyses ofseven quartz separates from sulfide-bearing veins atYinshan indicate a δ18O value of fluids from 6.6 to 9.5‰at temperatures from 270 to 390 °C, and δD value ofinclusion fluids from −48 to −34‰, suggesting amagmatic derivation for at least some of the mineralizingfluids (Zhang et al., 1997). Given the argument that thehigh salinity fluids encountered at Yinshan cannot be aproduct of fluid immiscibility, but rather result from directexsolution from amelt at low pressure, it is concluded thatearly to intermediate fluids responsible for all the barren,pyrite and chalcopyrite-bearing veins are magmatic inorigin. However, late fluids related to galena and calciteveining may be a mixed product of magmatic andmeteoric waters as illustrated by a shift of oxygen andhydrogen isotope composition towards themeteoric waterline (δ18OH2O=0.5‰ and δDH2O=−70‰; Zhang et al.,1996). The fluid inclusion characteristics of the Yinshanpolymetallic deposit share similarities withmost porphyrytype deposits, and this suggests a possibility to explore forporphyry style Cu–Aumineralization in the deeper levelsof the deposit area. However, fluid inclusion parameterssuch as homogenization temperature and salinity failed todifferentiate different types of veining and mineralizationdemonstrated in the field outcrops.

Acknowledgements

We wish to register our appreciation for theassistance offered by Jinzhang Zhang and ShengxiangLiu from the Yinshan mine of the Jiangxi CopperCorporation during fieldwork. Thanks are also due toWei Liu for his help in microthermometric analysis ofinclusions. Edwin Roedder and Thomas Ulrich arethanked for their constructive and insightful reviews ofthe paper. The encouragement of Khin Zaw is greatlyappreciated. This project was financially supported byNational Natural Science Foundation of China (Nos.49173021 and 40573033) and State Major BasicResearch Program of China (2002CB4126).

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