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Journal of Asian Earth Sciences 42 (2011) 83–96

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Journal of Asian Earth Sciences

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Fluid inclusion and H–O isotope evidence for immiscibility during mineralizationof the Yinan Au–Cu–Fe deposit, Shandong, China

Y.M. Zhang a, X.X. Gu a,⇑, L. Liu a, S.Y. Dong a,b, K. Li c, B.H. Li b, P.R. Lv a

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, Chinab College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, Chinac Sichuan Xinye Mining Investment Co., Ltd., Chengdu 610041, China

a r t i c l e i n f o

Article history:Received 12 May 2010Received in revised form 5 April 2011Accepted 6 April 2011Available online 23 April 2011

Keywords:ImmiscibilityBoilingFluid inclusionH–O isotopesYinan Au–Cu–Fe depositShandong

1367-9120/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jseaes.2011.04.011

⇑ Corresponding author. Tel.: +86 010 82322169.E-mail address: [email protected] (X.X. Gu

a b s t r a c t

The fluid inclusion and H–O isotope studies have provided the evidences for the source of ore-formingfluids, and helped to recognize two types of immiscibility and their relationships with mineralization.Hydrogen and oxygen isotopic geochemistry shows that the earlier ore-forming fluids during the anhy-drous skarn stage (I) and the hydrous skarn-magnetite stage (II) were mainly derived from magmaticwater, while the later fluids during the quartz-sulfide stage (III) and the carbonate stage (IV) were mainlyfrom magmatic water mixed with small amounts of meteoric water. Various types of fluid inclusions,including abundant vapor- or liquid-rich two-phase aqueous inclusions, daughter minerals-bearing mul-tiphase inclusions, CO2–H2O inclusions, and less abundant liquid inclusions, vapor inclusions and meltinclusions, are present in hydrothermal minerals of different stages. The liquid–vapor fluid inclusionsare mainly composed of H2O, with significant amounts of CO2 and a small amount of CH4. In the opa-que-bearing fluid inclusions, the hematite and fahlore (tetrahedrite) were identified. The homogenizationtemperature of the aqueous fluid inclusions decreases from Stage I (520–410 �C), through Stage II (430–340 �C) and III (250–190 �C), to Stage IV (190–130 �C). The coexistence of melt inclusions with simulta-neously trapped vapor- or liquid-rich two-phase aqueous inclusions and daughter minerals-bearing mul-tiphase inclusions in garnet, diopside and epidote of Stages I and II suggests an immiscibility betweensilicate melt and hydrothermal fluid. It is an effective mechanism on scavenging and transporting ore-forming components from magmas. The aqueous fluid inclusions with various vapor/liquid ratios (from<10% to >65%) commonly coexist with simultaneously trapped liquid inclusions, vapor inclusions, daugh-ter minerals-bearing multiphase inclusions and CO2–H2O inclusions in the quartz of Stage III, and the dif-ferent kinds of the fluid inclusions have similar homogenization temperatures. This indicates that theboiling – another kind of immiscibility, widely took place during Stage III. It resulted in the precipitationand enrichment of gold, copper and iron.

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1. Introduction

There are three types of mineralization associated with immis-cibility in nature: (1) The separation of immiscible magmatic meltleads to the formation of metallic and silicate melt. For instance,the immiscibility of copper–nickel sulfide melt and silicate meltcan form the copper–nickel sulfide deposits (Naldrett, 1989); (2)during the late stage of magmatic crystallization, the silicate meltimmiscibly separates from the H2O–CO2–H2S bearing volatiles,leading to the formation of ore-bearing gas–water hydrothermalfluids. These fluids can form deposit by means of replacementand filling (Jin et al., 1999; Fulignati et al., 2001; Campos et al.,2002; Reyf, 2004; Cesare et al., 2007); and (3) boiling and phase

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separation occur in various ore-bearing gas–water hydrothermalfluids along with decompression, resulting in the precipitationand concentration of ore-forming minerals (Reed and Spycher,1985; Zhu, 1999; Graupner et al., 2001; Gu et al., 2003; Calagari,2004; Coulibaly et al., 2008). Therefore, immiscibility between hy-drous silicate melt and hydrous saline melt (and aqueous solu-tions) is responsible for many ore deposits in particular (Roedder,1992). The fluid inclusions and H–O isotopes are microscopic re-cords of the sources and evolution of ore-forming fluids, as wellas the immiscibility process during mineralization.

As a typical Au–Cu–Fe polymetallic skarn deposit, the Yinan de-posit has become a serious crisis mine after more than 50 years ofexploitation. From 1957, when it was discovered, to 2006, it wasmined constantly and has produced 6000,000 t of ore whichyielded 20,000 t of copper, 1050,000 t of iron, 300,000 oz of gold,and 3,80,000 oz of silver. The serious shortage of reserves turns

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84 Y.M. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 83–96

successful resource exploration into a top priority. Although themost recent exploration in 2007–2009 (new discovered resourceof 19,000 t of copper, 6000,000 of iron and 4,80,000 oz of gold)indicated a significant progress on ore prospecting in the deepand peripheral mining area (Gu et al., 2008a), the scientific re-search of this deposit is still limited. Therefore, the in-depth under-standing of the mineralization laws and ore-controlling factors hasbeen constrained. The samples of different stages in the Yinan de-posit provide a wealth of material for the study of fluid inclusionsand H–O isotopes. From this, we seek to identify the source of ore-forming fluids, recognize the types of immiscibility, and discuss therelationships between immiscibility and mineralization.

2. Regional geological setting

The Yinan Au–Cu–Fe deposit is located in the western ShandongProvince (Luxi area) on the west side of Tanlu fault zone, on thesoutheast margin of the North China plate. It is composed of theTongjing and Jinchang Mines which are about 6 km away fromeach other (Fig. 1). The basement formed in the Luxi area afterexperiencing a series of geological processes such as sedimenta-tion, metamorphism, magmatism, magmatic activities, cymatoge-ny and craton consolidation from Archean to Paleoproterozoic.Then the rocks were uplifted and experienced denudation. As a re-

Fig. 1. Simplified geologic–tectonic map of the western Shandong Province (modifiedepression; DX-Dawenkou-Xintai depression; TL-Tailai depression; HM-Huimin depreF1-Fengpei fault; F2-Liaocheng-Lankao fault zone; F3-Qihe-Guangrao fault zone; F4-Mfault; F8-Chennan fault; F9-Enan fault.

sult, the Late Paleoproterozoic to Mesoproterozoic sediments wereremoved from the geologic record resulting in unconformity sur-face. During the Neoproterozoic, the Taishan continental cratoncracked longitudinally, forming the Yishu rift. The Neoproterozoicto Paleozoic strata deposited on the rift basin and formed the caprock above the unconformity surface (Cao, 1995; Song and Li,2001; Hou et al., 2004).

During the Yanshanian, the Tanlu fault system had experiencedsinistral strike-slip movement, and this tension action induced theinvasion and emplacement of intermediate-acidic magmas alongthe composite positions of NE trending faults (branch faults ofthe Tanlu fault zone) and the NW faults in cap rock. Consequently,extensive tectonic magmatic activities occurred in the Luxi area(Zou and Shen, 2001; Zhu et al., 2002; Shen et al., 2003; Wuet al., 2007). Various types of polymetallic deposits (hydrothermalmetasomatic and breccia type) are related to intermediate-acidicmagmatic activities of the Yanshan period in this area (Xu et al.,1999, 2002; Liu et al., 2002).

3. Geology of the Yinan deposit

The deposit is produced in the contact zones between the Ton-gjing and the Jinchang intermediate-acidic complexes of Yansha-nian and the strata from Neo-Archean to Cambrian (Fig. 2). It is

d from Li et al., 2007). Depressions and faults: PY-Pingyi depression; SS-Sishuission; DY-Dongying depression; ZH-Zhanhua depression; CZ-Chezhen depression;engshan fault; F5-Wensi fault; F6-Xintai-Duozhuang fault; F7-Taishan-Tongyedian

Fig. 2. Simplified geologic map of the Tongjing (A) and Jinchang (B) ore districts in the Yinan Au–Cu–Fe deposit (after Gu et al., 2008b).

Y.M. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 83–96 85

controlled by magmatic rocks, strata lithology and structures,showing typical characteristics of skarn deposits (Gu et al., 2008b).

The Tongjing complex (in the Tongjing mining area) outcrops asstocks, and on its edges there is sill intruded into the wallrock,showing irregular shape on the surface area with approximately

4.4 km2. There is mainly quartz diorite in the western part andquartz diorite porphyrite in the eastern part of this complex(Fig. 2A), and the respective K–Ar ages of the whole-rock are125.5 Ma and 117.5 Ma (Zheng and Luo, 1996). The quartz dioriteis intruded by quartz diorite porphyrite or forms xenoliths, indicat-


ing that the emplacement of quartz diorite may be earlier thanquartz diorite porphyrite. Most recent LA–ICP–MS dating of zirconfrom quartz diorite yields a mean age of 135.8 ± 2.7 Ma (Li, 2009).The Jinchang complex (in the Jinchang mining area) is composed ofintrusion and dykes: the Jinchang intrusion is located in its wes-tern part and the Yeguanmu dyke is situated at its eastern part.The ring-like and radial dykes are distributed around the complex(Fig. 2B). The Jinchang intrusion shows round-shaped at the sur-face, with an area of about 0.2 km2. There is porphyritic medium-and fine-grain monzonitic granite in its center, surrounded withgranite porphyry, and gradually transiting to veins and dykes to-wards the outside. The Yeguanmu dyke shows dumbbell shape inthe surface of the NE-direction extension with an area of about0.1 km2. It is mainly composed of granite porphyry, and the K–Arage of the whole-rock is 121 Ma. The geochemical studies indicatethat (Gu et al., 2008b), the calc-alkaline ore-forming complex plu-tons are relatively silicic, alkalic, and magnetite-bearing with highcoefficient of oxidation (Fe2O3/(Fe2O3 + FeO) > 0.5), and they arebroadly similar to the oxidized, Au skarn-related plutons definedby Meinert (1989, 1998, 2000).

In the mining area, the stratigraphic sequences include Neo-archean Taishan Group (Ar3t), Neoproterozoic Tumen Group (Zt),Lower Cambrian Chuangqing Group (�c) and Middle-Upper Cam-brian Jiulong Group (�j) (Fig. 3A). The Archean Taishan Group isthe crystalline basement, and is angular unconformably overlainby the cover composed of the Proterozoic and Cambrian strata.The Archean basement is mainly composed of granitic gneiss, pla-gioclase amphibolite and hornblende granulite. The Neoproterozo-ic Tumen Group is a set of shallow marine sedimentary facies.Gray–white medium- and fine-grain sandstone and pebbly sand-stone are at the bottom, and the gray–yellow and gray–purpleshale intercalated with lamella marl are at its middle and upperparts. The Cambrian Changqing Group is mainly a set of fine clasticrocks, calcareous mudstone and shale interbedded with lamellamarl, oolitic limestone, calcarenite and dolomitic limestone. TheCambrian Jiulong Group is mainly composed of thin-bedded lime-stone, wormkalk, oolitic limestone, bioclastic limestone and argil-laceous dolomite, intercalated with shale in some region. Thestrata above the unconformity plane are gently inclined to the sur-rounding center on the Yanshanian intermediate-acidic complexes(dip angle <15�). Far from the intrusion, the attitude of stratumtends to horizontal, showing the characteristics of a dome struc-ture (Gu et al., 2008a).

There are a total of 14 layers of ores in this deposit. All of themare produced in the contact zones of Yanshanian intermediate-acidic complexes and Neoproterozoic–Cambrian wallrock, inwhich two layers of ores are produced on the unconformity surfaceof Archean basement and in the Neoproterozoic strata, and 12 lay-ers of ores occur in the Cambrian strata (Fig. 3A; Gu et al., 2008a).The outputs of ore-bodies are controlled by the contact zone be-tween intrusion and wall rock and the tectonic weak zone in theNeoproterozoic–Cambrian wall rock (unconformity surface, inter-layer fractured zone, and decollement zone). The shapes of ore-bodies are variable, including bedded-, lenticular- or irregular-shapes (Fig. 3B and C). Generally, the ore-bodies are 140–200 mlong on the direction of strike, and are 100–150 m on the directionof dip; their thicknesses range from 0.5 to 11.6 m. The averagegrade of ore body: Au 1–5 g/t, Cu 0.5–0.8%, TFe 27–33% (Gu et al.,2008a).

The ores are mainly with massive, disseminated, vein-stock-work shaped, banded and brecciated textures (Fig. 4). The meta-so-matic relict, poikilitic, skeletal and pseudomorph textures are lesscommon. Magnetite is abundant and it occurs as massive bodies orvein-stockworks cementing fragments of anhedral to subeuhedralgarnet and diopside. Chalcopyrite occurs as disseminations or asveinlets accompanying with quartz. Banded ore with alternating

bands composed of anhedral to subeuhedral garnet and epidote(Fig. 4A). The brecciated ores are pervasively developed, that is, la-ter quartz and sulfide veinlets often cut early skarn minerals andmagnetite, indicating that the Cu mineralization occurred laterthan the deposition of the skarn and magnetite (Fig. 4B–E). The lat-ter assemblage (quartz + sulfide) is then broken and cemented bylater quartz and calcite (Fig. 4C–E). The native Au is mostly associ-ated with quartz and sulfide (Fig. 4F). According to the mineralparagenesis and interspersed relationship between veins, the min-eralization process can be divided into four stages: the anhydrousskarn stage (I, garnet + diopside), the hydrous skarn-magnetitestage (II, epidote + magnetite + specularite ± tremolite ± actino-lite ± quartz), the quartz-sulfide stage (III, quartz + chalcopy-rite + pyrite + native gold ± chlorite), and the carbonate stage (IV,calcite ± quartz).

Wall-rock alteration are widely distributed in the contact zonesbetween complexes and wall rocks, in the areas nearby the inter-layer fractures, and under or above the unconformity surfaces,with the range of alteration up to 200–300 m or even more. Endos-karn and relative altered ores occur within intrusions associatedwith K-feldspar alteration, and consist of garnet, pyroxene andminor vesuvianite. Exoskarn typically occurs in an external zoneassociated with marbles or hornfels, and consists of garnet anddiopside. Retrograde alteration is widely distributed in the frac-tured zone; it consists of quartz, calcite, chlorite and sericite,accompanied by sulfide minerals such as chalcopyrite, pyrite,bornite, and minor amounts of pyrrhotite, galena and sphalerite.

4. Characteristics of fluid Inclusions and H–O isotopes

4.1. Methods of investigation

Hydrothermal minerals of different stages from drill holes andtunnels were selected for thermometric analysis. The microther-mometric studies were carried out in the fluid inclusion laboratoryat Chengdu University of Technology. The phase transitions of thefluid inclusions were observed at temperatures from �196 �C to600 �C using a Linkham THMSG-600 heating/freezing stage. Tem-perature measurement is accurate to ±0.1 �C at �22 �C and the er-ror increases gradually to ±2 �C at 400 �C relative to standardmaterial. Temperatures were controlled at 20 �C/min during heat-ing or freezing process, and less than 4 �C/min near the phase tran-sition point.

Certain inclusion constituents were determined using a Reni-shaw Invia Reflex-type confocal Laser Raman microspectrometry(LRM) in the fluid inclusion laboratory at China University of Geo-sciences (Beijing). The test conditions: 514.5 nm light of an argon–ion laser, 20 mW laser power, 7 mW laser power at sample surface,1800 grooves/mm grating, spectral resolution of 2 cm�1, spatialresolution of 1 lm (�100 lens), 60 s scan time, scan range of 0–4000 cm�1. The wavenumber accuracy was ±1 cm�1.

The H–O isotopes were analyzed in the stable isotope labora-tory of Mineral Resources Institute, Chinese Academy of GeologicalSciences, using the methods of (Clayton and Mayeda, 1963) in aFinnigan MAT253-type mass spectrometer. Oxygen gas was pro-duced by quantitatively reacting the samples with BrF5 in exter-nally heated nickel reaction vessels. Hydrogen was determinedby quantitatively reacting the H2O with zinc at temperature of550 �C. The results adopted the SMOW as their standards, with aprecision of ±2‰ for dD and ±0.2‰ for d18O.

4.2. Characteristics of primary fluid inclusion

Representative host minerals including garnet (Stage I), diop-side (Stage I), epidote (Stage II), quartz (Stage III) and calcite (Stage

Fig. 3. Sketch geological map showing the relationship between strata and ore bodies in the Yinan Au–Cu–Fe ore deposit.


IV) of four stages were selected for analysis. These host mineralscontain both primary and secondary fluid inclusions. The fluidinclusions are usually small in diameter, generally ranging from

5 to 10 lm, with a minimum size of <1 lm and maximum size of25–30 lm. Various shapes of inclusions include negative crystalshaped, spheroidal, ellipsoidal, elongate, square, rectangular and

Fig. 4. Typical ores and mineral association in the Yinan Au–Cu–Fe deposit. (A) Banded garnet–epidote–magnetite ore, underground exposure; (B) Au-bearing magnetite–chalcopyrite ore; (C) fractured garnet–magnetite ore cemented by chalcopyrite and crosscut by later calcite veins; (D) magnetite fractured and cemented by quartz andcalcite, which both crosscut by later calcite veinlets; (E) fragments of chalcopyrite with scattered garnet and magnetite ore is cemented by quartz; (F) Anhedral native gold atthe margin of pyrite or between pyrite and quartz, reflected light, single nicol. Gr-garnet, Ep-epidote, Mt-magnetite, Cpy-chalcopyrite, Py-pyrite, Au-native gold, Qz-quartz,Cal-calcite.


irregular. Only primary inclusions are chosen to analyze and dis-cuss in the following text.

Based on the components and phase transitions characteristicof inclusions at room temperature (25 �C), several types of fluidinclusions have been identified, including abundant vapor- or li-quid-rich two-phase aqueous inclusions (V + L), daughter min-eral-bearing multiphase inclusions (S + V + L), CO2–H2O inclusions(CO2 + V + L), and less abundant liquid inclusions (L), vapor inclu-sions (V), possibly melt inclusions (M) (Figs. 5 and 6). The combi-nations of inclusions are different in various hydrothermalminerals. The inclusions in garnet (Stage I) are mainly V + L typeinclusions, and occasionally S + V + L, M and L type inclusions.The inclusions in diopside (Stage I) are mainly V + L and S + V + Ltype inclusions (opaques, halite and sylvite), and occasionally Mand L type inclusions. The inclusions in epidote (Stage II) aremainly V + L and opaque-bearing S + V + L type inclusions, followed

by transparent minerals-bearing (halite) S + V + L type inclusions,and occasionally V and M type inclusions. Some M type inclusionsalso occasionally occur in the coexisting quartz (Stage II). Theinclusions in Stage III quartz are various, including V + L, S + V + L,CO2 + V + L, L and V type inclusions. The inclusions in calcite (StageIV) are mainly V + L type inclusions, and the next are L type inclu-sions. Inclusions in Stages II and IV quartz veinlets are too small tobe chosen for micro-thermometric analysis.

It is clear from the inclusion analysis that vapor–liquid two-phase aqueous inclusions are the most widely distributed. Theirvapor–liquid ratios (vapor filling degrees) vary from 1% to 75%and usually are in the range of 5–35%. Their main componentsare composed of H2O, with significant amounts of CO2 and a smallamount of CH4 (Fig. 6A, B and D). CO2–H2O inclusions are com-posed of vapor CO2, liquid CO2 and H2O. The CO2 (mainly vaporCO2) phase accounts for 10–90% of the total volume of inclusions

Fig. 5. Typical fluid inclusions in the Yinan Au–Cu–Fe deposit. (A) Melt inclusions in garnet; (B) multiphase inclusions with different daughter minerals in diopside (Di); (C)vapor–liquid two-phase inclusions in epidote; (D) vapor-rich inclusions in quartz; (E) liquid-rich inclusions in calcite; (F) coexistence of vapor-, liquid-rich two-phaseinclusions with opaque-bearing three-phase inclusions in diopside; (G) multiphase inclusions with two bubbles and different daughter minerals in diopside; (H) multiphaseinclusions with different daughter minerals in diopside; (I) CO2–H2O inclusions, vapor–liquid two-phase inclusions and liquid inclusions in quartz; (J) coexistence of two-phase inclusions of various vapor/liquid ratios with halite-bearing three-phase inclusions in quartz; (K) multiphase inclusions with different daughter minerals in quartz; (L)coexistence of melt inclusions with halite-, sylvite-, opaque- or rod crystal-bearing multiphase inclusions and vapor–liquid two-phase inclusions; (M) coexistence of meltinclusions with halite-bearing multiphase inclusions in quartz, two-phase inclusions with various vapor/liquid ratios, and vapor inclusions. M-melt, V-vapor, L-liquid H2O,VCO2-vapor CO2, LCO2-liquid CO2, H-halite crystal, Ht-hematite crystal, O-opaque crystal, Sy-sylvite crystal, R-rod crystal.


Fig. 6. Representative Raman spectra of fluid inclusions in the Yinan Au–Cu–Fe deposit.


(Fig. 6C). Daughter mineral-bearing multiphase inclusions containliquid, vapor and solid phases, of which the vapor bubble occupies5–10% of the cavity volume and solid minerals account for 3–20%of the inclusion volume. Their liquid and vapor phase are mainlyH2O identified by laser Raman. There are two types of daughterminerals: transparent and opaque. The cubic halite is by far themost common solid phase, accompanied by minor equant to anhe-dral sylvite (Fig. 5). The opaque are difficult to analyze by laser Ra-man, only the hematite and fahlore (tetrahedrite) (Fa) wereidentified (Fig. 6E and F). Hematite (Ht) was readily recognizedby their red color, and its Raman spectra is consistent with thehematite Raman shift near two A1g modes (225 and 498 cm�1)and five Eg modes (247, 293, 299, 412 and 613 cm�1) (de Fariaet al., 1997). The micro-Raman spectra of fahlore occurs between200 and 400 cm�1. Fitting of the spectrum occurs near 325 and352 cm�1, and the appearance of the spectra change (near382 cm�1) may be related to the Sb/(Sb + As) (Mernagh and Trudu,1993; Kharbish et al., 2007). The host mineral of fahlore is diopside(Di) (stageI) whose Raman shift is near 667 and 1012 cm�1 (Xuet al., 1996) (Fig. 6). It cannot exclude the possibility that there ex-ist other opaque like chalcopyrite and magnetite according to thepetrographic observation. Melt inclusions occasionally occur inthe garnet, diopside, epidote and quartz, being composed of severalsilicate daughter minerals and bubbles. The silicate daughter min-erals have different interference colors from the ones of host min-erals under the orthogonal polarizing microscope, and there wasno phase change when they underwent heating up to 600 �C.

It is worth noticing that there are different types of inclusionsthat appear to be coeval in the same domain of the diopside (StageI), epidote (Stage II) and quartz (Stage III) under the microscope. Inthe quartz, the vapor–liquid two-phase aqueous inclusions withvarious vapor/liquid ratios (from <10% to >65%) commonly coexistwith simultaneously trapped other types of inclusions, showingthe characteristics of boiling (immiscibility).

4.3. Homogenization temperature and salinity

Micro-thermometric analysis was performed principally on pri-mary fluid inclusions in the hydrothermal minerals representingdifferent mineralization stages. Two common thermometric proce-dures, freezing and heating, were employed to determine thehomogenization temperatures (th), melting temperatures of CO2

clathrate (tmcl) and approximate salinities (NaCleq) in wt.% respec-tively. Last ice melting temperatures (tice), daughter homogeniza-tion temperatures (tm) or melting temperatures of CO2 clathrate(tmcl) were measured to calculate salinities according to the for-mula proposed by Potter et al. (1978), Collins (1979), Hall et al.(1988) and Bodnar (1993). A summary of data is presented in Ta-ble 1 and Fig. 7.

The homogenization temperatures of inclusions in garnet (StageI) vary from 427 to 549 �C, with an average of 487 �C. The ones indiopside (Stage I) vary between 369–484 �C, with an average of433 �C. The tice of fluid inclusions in Stage I minerals were not listedowing to the possibility of large error. The salinity of one halite-


bearing inclusion is 56.7% NaCleq in the garnet. While the salinity ofnine halite-bearing inclusions is 49.6–70.2% NaCleq in the diopside(average 56.3% NaCleq). Sometimes sudden movements of bubbleswere occasionally observed between 18 and 30 �C in the diopsideand could indicate small quantities of CO2 according to Hedenquistand Henley (1985).

The ones in epidote (Stage II) vary between 290 �C and 438 �C,with and an average of 368 �C. The tice of the vapor–liquid two-phase fluid inclusions change in the range of �21.1 �C to �6.4 �C,and the calculated salinity data are 9.7–23.1% NaCleq, (average18.1% NaCleq). The salinity of two halite-bearing inclusions is52.0–53.5% NaCleq (average 52.9% NaCleq).

The ones in quartz (Stage III) range between130 �C and 335 �C,with most of 150–300 �C and an average of 226 �C. The tice ofV + L fluid inclusions change in the range of �13.5 �C to �4.0 �Cand the calculated salinity data are 6.5–17.3% NaCleq (average11.7% NaCleq). The salinity of 23 S + V + L inclusions is 41.5–57.1%NaCleq (average 47.8% NaCleq). The tmcl of CO2 + V + L inclusionsare within the range 4.7–6.9 �C, and the calculated salinity isapproximately 5.9–9.4% NaCleq (average 8.1% NaCleq). It is equalto the estimated salinity using tice of vapor–liquid aqueous inclu-sions (Table 1).

During the freezing–heating process, a small amount of incipi-ent melting of the liquid–vapor two-phase fluid inclusions inquartz can be observed. The first (eutectic) melting temperaturesare in the range of �31.2 �C to �27.5 �C, which are lower thanthe standard eutectic point (�20.8 �C) in the pure H2O–NaCl sys-tem. Low eutectic temperatures suggest that, the hydrothermalfluids are polysaline and multi-cation (K+, Ca2+, Mg2+) in additionto the Na+ (Ruano et al., 2002; Lu et al., 2004). The first eutectictemperature range indicates it could be H2O–NaCl–KCl (�22.5 �C)or H2O–MgCl2 (�33.6 �C) system. The presence of sylvite in somefluid inclusions proves the existence of KCl. Furthermore, the triplepoint of CO2 + V + L inclusions in quartz ranges from �58.2 �C to�57.1 �C, which is lower than the standard triple point(�56.6 �C) of the pure CO2. It suggests that, the fluid inclusionsmay contain CH4, H2S and other volatiles in addition to the CO2.

The ones in calcite (Stage IV) range between 102 �C and 268 �C,with the majority of 100–200 �C and an average of 163 �C. It can beseen that the temperatures of ore-forming fluids decrease duringsuccessive stages of the mineralization process. The tice change inthe range of �11.8 �C to �1.2 �C and the calculated salinity dataare in the range of 2.1–15.8% NaCleq (average 9.1% NaCleq).

What is noteworthy is that the vapor bubbles in diopsidehomogenize before halite or nearly simultaneously disappear withhalite. While the fluid inclusions in epidote and quartz typically

Table 1Microthermometric and salinity data of fluid inclusions from the Yinan Au–Cu–Fe deposit

Host mineral/mineralization stage Inclusion types Homogenizationtemperature (th/�C)

Latem

Range (num.) Mean Ra

Garnet/I V–L 427–549 (32) 487S–V–L 421 (1) 421

Diopside/I V–L 369–484 (23) 433S–V–L 190–515 (9) 442

Epidote/II V–L 290–438 (30) 368 �2S–V–L 340–357 (2) 349

Quartz/III V–L 130–335 (57) 226 �1S–V–L 180–280 (23) 230CO2–V–L th: 261–300 (6) 286 thC

Calcite/IV V–L 102–268 (49) 163 �1

Note: V–L refers to the vapor–liquid two-phase brine inclusions, S–V–L means the halitemagnetite stage, III: the quartz-sulfide stage, IV: the carbonate stage. th: Temperaturtemperature of CO2 clathrate melting.

homogenize by halite dissolution. That is, the halite temperaturesof melting (tm) are greater than or equal to the liquid homogeniza-tion temperatures (th) of S + V + L type inclusions and homogeniza-tion temperatures (th) of coexisting V + L type fluid inclusions(Table 1, Fig. 7). These S + V + L type inclusions may suggestentrapment of halite supersaturated fluid (halite crystal + salinityfluid) (Calagari, 2004). The trapped temperatures of S + V + L typeinclusions should be equal to the homogenization temperaturesof coexisting V + L type inclusions (th) (Chen et al., 2010). Alterna-tively, the simultaneous homogenization of halite and bubble orhomogenization of halite dissolution may also indicate critical tohigh density, as illustrated by Bodnar (1994).

4.4. Hydrogen and oxygen isotopic compositions

Petrographic studies on doubly polished section have shownthat the fluid inclusions in hydrothermal minerals for the H analy-sis are dominated (>95%) by primary inclusions in both numberand volume. They typically have rounded or negative crystalshapes. Therefore, the fluids extracted from the inclusions arethought to represent the primary ore forming solutions. The oxy-gen and hydrogen isotopic data of different stage minerals and cal-culated d18OH2O values of ore-forming fluids are listed in Table 2.The d18OH2O values were calculated based on the fluid inclusionhomogenization temperatures and fractionation equation betweenminerals and water, according to the equations of O’Neil and Ep-stein (1966), O’Neil et al. (1969), Lu and Yang (1982) and Zhengand Chen (2000). There into, the formation temperatures of garnet,quartz and calcite can be basically represented by the mean valuesof their homogenization temperatures. The formation tempera-tures of specularite and magnetite are represented by the meanhomogenization temperatures of their paragenetic epidote.

5. Discussion

5.1. The source of ore-forming fluids

The hydrogen and oxygen isotope of fluid at different mineral-izing stages in the Yinan Au–Cu–Fe deposit provide importantinformation on the source and the evolution of ore-forming fluids.The d18OH2O value and dDH2O value (�73‰) of one garnet of theanhydrous skarn stage (I) are 6.8‰ and �73‰ respectively, fallingin the range of magmatic fluids (Taylor, 1974) (Fig. 8). The isotopiccompositions of magnetite and specularite of the hydrous skarn-magnetite stage (II) are relatively changeable. The d18OH2O valuesof three samples vary in the range of 7.9–8.3‰, except for one

.

st ice meltingperatures (tice/�C)

Melting temperature ofhalite (tm/�C)

Salinity w (NaCleq)%

nge (num.) Mean Range (num.) Mean Range (num.) Mean

477 (1) 477 56.7 (1) 56.7

419–574 (9) 473 49.6–70.2 (9) 56.3

1.1 to �6.4 (4) -15.2 9.7–23.1 (4) 18.1440–452 (2) 446 52.0–53.5 (2) 52.9

3.5 to �4.0 (20) �8.1 6.5–17.3 (20) 11.7344–480 (23) 401 41.5–57.1 (23) 47.8

O2: 25.4–29.3 (5) 26.8 tmcl: 4.7–6.9 (5) 5.8 5.9–9.4 (5) 8.1

1.8 to �1.2 (12) �6.1 2.1–15.8 (12) 9.1

-bearing multiphase inclusions; I: the anhydrous skarn stage, II: the hydrous skarn-e of CO2 homogenization; thCO2: temperature of CO2-phase homogenization; tmcl:

Fig. 7. Histogram of the final homogenization temperatures and salinities of fluid inclusions from the Yinan Au–Cu–Fe deposit. TD-transparent daughter minerals;OD-opaque daughter minerals; Di-diopside; Grt-garnet.


example with 11.6‰ value. The oxygen isotopes still have the char-acteristics of magmatic fluid generally. The dDH2O values are lowand within the range of �112‰ to �82‰. The wide range may re-cord the degassing of parent magma chamber, as this isotopic frac-tionation results in lighter dD values of magmatic water (Suzuokiand Epstein, 1976). The data are also similar to the H–O isotopiccompositions (dD �110‰ to �65‰, d18O 6.0–9.0‰) of initial mix-

ing magmatic water in the granite of Au–Cu and Fe–Co series de-fined by Zhang (1985), indicating the ore-forming fluids at thisstage are still dominated by the magmatic water, but the possibil-ity of mixing with a minor amount of meteoric water cannot be ru-led out. According to Zhang (1985), the initial mixing magmaticwater refers to the magmatic water exsolved out by the initialmagma which is formed by the anataxis of crust affected by the

Table 2Hydrogen and oxygen isotopic compositions (SMOW) of hydrothermal minerals from the Yinan Au–Cu–Fe deposit.

Sample no./deposit Mineral/mineralization stage d18O (‰) dDH2O (‰) d18OH2O (‰) t (�C) Data source

KG16-9 Garnet/I 5.4 �73 6.8 430KS10-12 Magnetite/II 4.6 �107 8.3 340MW280-B1 Magnetite/II 4.6 �112 8 330KG16-9 Specularite/II 2.2 �82 7.9 300KS10-2 Specularite/II 6 �84 11.6 290

5 Quartz/III 8.1 �87 �0.4 260 aQiu et al. (1996)

KS10-10 Calcite/IV 10.55 �72 �2.9 140S10-8 Calcite/IV 12.7 �67 �2.4 120

Mashan Au–S deposits in Tongling, Anhui Province Quartz/III �69 to �62 6.9–10.7 380 aTian et al. (2007)

Xiaotongguanshan Cu–Au deposits in Tongling Garnet etc./I 7.0–15.4 �50 to �108 5.1–10.7 260–720 aTian et al. (2005)Magnetite /II 6.6–8.4 13.5–15.9 350–400Quartz, Calcite /III 14.6–17.4 �54 to �63 2.0–5.1 150–247Calcite/IV 13.4–13.5 �77 0.8–0.9 150

Elkhorn gold deposit in America Skarn/I, II �63 to �54 7.6–12.9 525 aBowman et al. (1985)

Copper Canyon Cu–Au deposits in America Quartz/III 11.3–15.1 �102 to �76 2.7–9.2 280–395 aBatchelder (1977)

Note: The calcite samples are tested by Analysis and Testing Center of Beijing Institute of Geology Research, Nuclear Industry, and the other sample analysis is carried out byMineral Resources Institute of Chinese Academy of Geological Sciences; the ‘t’ refers to the average homogenization temperatures of fluid inclusions.

a Refers to stable isotope data of other skarns for comparison.

Fig. 8. Plot of dD vs d18O for the ore-forming fluids in the Yinan Au–Cu–Fe deposit.Isotopic compositions of magmatic and metamorphic waters after Taylor (1974),meteoric water line after Craig (1961), the Xiaotongguanshan Cu–Au deposit afterTian et al. (2005), the Mashan Au deposit in Tongling after Tian et al. (2007), theCopper Canyon Cu–Au deposit after Batchelder (1977), and the Elkhorn Au depositafter Bowman et al. (1985).


interaction with surface water (atmospheric water, sea water). Itleaches/exchanges the (sub-) solidified rocks and forms ore-bear-ing rebalanced-mixing magmatic hydrothermal fluids (Zhang,1985). The rebalanced mixing magmatic water is usually rich inD compared with the initial mixing magmatic water. Therefore,there is a relatively large variation on the dDH2O values of magne-tite and specularite precipitated from the initial- and rebalanced-mixing magmatic hydrothermal fluids. Until the quartz-sulfidestage (III) and the carbonate stage (IV), the dDH2O values (�87‰

to �67‰) of ore-forming fluids still remain the characteristics ofmagmatic water. However, the d18OH2O values (�2.9‰ to �0.4‰)are significantly low, showing the hydrothermal characteristics ofmeteoric water with the d18O drift. All the above-mentioned fea-tures indicate the ore-forming fluids in this stage were from mag-matic water mixed with meteoric water.

The hydrogen and oxygen isotopic compositions of the hydro-thermal minerals from the Yinan deposit reflect the complexityof the ore-forming process about the skarn deposits to a certain ex-tent. However, the above characteristics are consistent with thegeneral laws of typical skarn-type Cu–Au deposits whose ore-forming fluids are dominated by magmatic water at the early stageand mixed with the meteoric water during the late stage (Table 2,Fig. 7). For example, the d18OH2O values of ore-forming fluids rangebetween 7.6‰ and 12.9‰, and the dDH2O values range between�63‰ and �54‰ in the US Elkhorn skarn-type gold deposit duringthe early skarn stages (Bowman et al., 1985). In the Mashan Au–Sdeposit in Tongling, Anhui Province, the d18OH2O values are 6.9–10.7‰, and the dDH2O values are �69‰ to �62‰ at the quartz-sul-fide stage (Tian et al., 2007), indicating that the hydrothermal flu-ids of these two above-mentioned deposits are dominated inmagmatic water without or with very small amount of meteoricwater during the forming process of skarn and ore bodies. In theXiaotongguanshan Cu–Au deposit, Tongling, from 5.1‰ to 10.7‰

at the early skarn stage, the d18OH2O values of ore-forming fluidsare downgraded as 0.8–5.1‰ at the late quartz-sulfide and carbon-ate stages, showing that the ore-forming fluids are dominated bymagmatic water at an early stage, whereas meteoric water enteredthe mineralizing system at a late stage (Tian et al., 2005). Thehydrogen and oxygen isotopic compositions (d18O 2.7–9.2‰, dD�102‰ to �76‰) of fluid inclusion water in quartz from the Cop-per Canyon Cu–Au deposits, United States, suggest that the ore-bearing fluids are most likely composed of magmatic water thathad mixed with some meteoric water during mineralization(Batchelder, 1977).

5.2. Immiscibility and mineralization

Melt inclusions are the magmatic droplets (silicate melts) cap-tured by various types of igneous minerals during their formingprocess. As the host minerals cool, they quenched and condensedinto glass, or further crystallized to an assemblage of silicatedaughter minerals and fluid phases (liquid- and vapor-phases).Within garnet, diopside and epidote, occasionally in parageneticquartz (Stage II) of the skarn stage in the Yinan Au–Cu–Fe deposit,the melt inclusions composed of felsic solids and vapors had nophase changes when being heated to 600 �C, indicating that theywere captured in the high-temperature magmatic melts. Interest-ingly, these melt inclusions often coexisted with the vapor- or li-quid-rich two-phase aqueous inclusions of various vapor–liquidratios and daughter minerals-bearing multiphase inclusions, evencoexisting within the same tiny view (Fig. 5L and M). Meanwhile,there is no petrographic evidence to demonstrate that they werecaptured in the different stages. Thus, the above-mentioned phe-


nomenon implies an immiscible state coexisting of melt and fluid(Roedder and Commbs, 1967). The calculated salinities of the coex-isting hypersaline inclusions in diopside are up to 70.2 wt.% NaCleq,implying a magmatic origin (Harris et al., 2003). There are no meltinclusions in the post-skarn stages (the quartz-sulfide stage andthe carbonate stage). The above facts further emphasize the geneticrelationship between mineralization and magmatism. That is, theore-forming fluids came from gas–water hydrothermal fluids ex-solved by the magmas; the diagenesis and the mineralizationexperienced the evolution of melt ? melt + fluid ? fluid. It is usu-ally considered that, with the crystallization of silicate mineralsand a progressive decrease of temperature and pressure duringthe magma evolution, the volatile/silicate melt ratio gradually in-creases. When the volatiles reach the supersaturation, immiscibil-ity occurs, forming the volatile-rich, high-salinity hydrothermalfluids and the residual silicate melts. At this point, the melt andfluid were trapped by crystallizing minerals simultaneously. Previ-ous literatures have also shown that, silicate melts and multiphasesalt-rich inclusions are interpreted as having originally formed asimmiscible phases at magmatic temperatures during the mag-matic-hydrothermal transition stage (Kamenetsky et al., 1999).There into, volatile-rich highly saline hydrothermal fluids repre-sented by the salt-rich inclusions are effective in scavenging ore-forming components from magmas (Candela, 1989), and have thepotential to transport metals (Davidson and Kamenetsky, 2001).They are often the main contributors of hydrothermal fluids andmetals in the ensuing skarn- and/or porphyry-mineralization sys-tems (Eastoe, 1978; Candela, 1989; Bodnar, 1995; Meinert et al.,1997, 2003; Hedenquist et al., 1998; Kamenetsky et al., 1999; Har-ris et al., 2003). The wide occurrence of salts-rich, opaque-bearing(magnetite, chalcopyrite) multiphase fluid inclusions in diopsideimplies that the fluids are exsolved from the magma at elevatedtemperature during the early skarn stage (Campos et al., 2002),confirming the important role of the magmatic hydrothermal flu-ids mentioned above.

There is another kind of immiscibility widely spread in the Yi-nan Au–Fe–Cu deposit. As already noted, the boiling inclusionsare developed in the diopside (Stage I), epidote (Stage II) andquartz (Stage III), particularly prevalent in quartz. In the same zoneof one mineral, the aqueous fluid inclusions with various vapor/li-quid ratios (from <10% to >65%) commonly coexist with liquidinclusions, vapor inclusions, daughter minerals-bearing multi-phase inclusions and CO2–H2O inclusions trapped simultaneously.Petrographic observations show that they are primary inclusionsand have similar homogenization temperatures. However, thesalinity of vapor–liquid two-phase aqueous fluid inclusions is verydifferent from the one of the paragenetic daughter minerals-bear-ing multiphase inclusions (varies in the range of 9.7–23.1% NaCleq

and 52.0–53.5% NaCleq in epidote, 6.5–17.3% NaCleq and 41.5–57.1% NaCleq in quartz, respectively), indicating that they werecaptured in two different fluids with dissimilar natures, namely,one is the low-salinity and low-density fluid, while the other ishigh-salinity and high-density fluid. This is interpreted as evidencefor boiling (immiscibility) (Lu et al., 2004). Sometimes, in some va-por–liquid two-phase inclusions with medium vapor/liquid ratios(40–60%), there was no obvious change of the bubbles beingheated above 400 �C, indicating that they were captured in the het-erogeneous fluid phase (liquid-phase + vapor-phase) at the boilingstate. Petrographic observation and micro-thermometric analysisreveal that, the boiling occurred in a wide range of temperaturesfrom 130 �C to 430 �C, and most of the temperatures were concen-trated in the range of 190–280 �C at the quartz-sulfide stage.

Serving as one of the main ways of natural fluid immiscibility,boiling of ore-forming fluids is considered as an important mecha-nism in the metal precipitation and enrichment in many hydro-thermal deposits (Roedder, 1984; Reed and Spycher, 1985; Logan,

2000; Shen et al., 2000, 2001; Yao et al., 2001; Ruano et al.,2002; Gu et al., 2003; Calagari, 2004; Zhou et al., 2007). Speciationand reaction progress calculations confirm that boiling would be amore efficient depositional mechanism than cooling or mixing ofthe ore fluid with groundwater (Graney et al., 1991). Due to boil-ing, the vapor-phase H2O, as well as H2S, CO2, HCl and other acidiccomponents is constantly escaping from the original homogeneousfluid phases. The increasing pH values and the decreasing temper-atures will promote the instability of metal complexes and lead tothe precipitation of metal oxides (magnetite), sulfides (chalcopy-rite, pyrite) and native gold (Reed and Spycher, 1985):

3Fe2þ þ 4H2O ! Fe3O4 ðmagnetiteÞ þ 6Hþ þH2

Fe2þ þ Cu2þ þ 2HS� ! CuFeS2 ðchalcopyriteÞ þ 2Hþ

Fe2þ þ 2HS� ! FeS2 ðpyriteÞ þ 2Hþ

AuðHSÞ�2 þHþ þ 0:5H2 ! Au ðnative goldÞ þ 2H2S

The opaque-bearing (magnetite, chalcopyrite) multiphase fluidinclusions abound in epidote and quartz indicating that the boilingwas the main mechanism leading to the precipitation of metallicminerals. It is generally accepted that boiling is mainly caused bythe sudden pressure decrease (lithostatic pressure converts intohydrostatic pressure). The rapid pressure release is either relatedto fracturing or resulted from the hydrofracturing occurring whenthe fluid pressure exceeds the resistance of overlying rocks. There-fore, in addition to the micro-geological evidences (occurrence ofplenty of boiling fluid inclusions), lots of brecciated structures of-ten developed in the ore deposit are the macro-geological marksof the boiling fluids. During the hydrous skarn-magnetite stage(II) and the quartz-sulfide stage (III) in the Yinan deposit, earlyfractured skarn minerals and magnetite are often cemented by la-ter quartz and sulfide, and then the latter are broken and cementedby later quartz, calcite (Fig. 4C–E). Meanwhile, there is also brec-cia-type ore developed on the edge of the intrusion in somewhere.All these are important characterization of mineralizing fluid boils.

Multiple boiling events are well characterized by the occur-rence of multistage brecciated ores and wide range of boiling tem-peratures (Roedder, 1971; William and Nash, 1974; Calagari,2004). At the beginning of each shattering event, fluid inclusionswere trapped with relatively low salinity (unsaturated). As theboiling process continued, the liquid phase became increasinglymore saline. Hence, successive generations of halite-bearing inclu-sions were trapped. Fractures or hydraulic fractures were healedby the precipitation of quartz and sulfides. As a result of chokingof the fluid conduits, permeability was reduced and the ascendingfluids experienced lithostatic pressure once again. By rising inter-nal pressure of the fluid, another episode of shattering and con-comitant release of fluid and boiling took place.

In summary, we believe that there may have been two differentkinds of immiscibility during the whole evolution process from themagmatic crystallization differentiation to hydrothermal mineral-ization in the Yinan Au–Cu–Fe deposit. The early immiscibility ofmagma melt and the vapor–liquid phase fluids led to the unmixingand separation of ore-bearing water–gas hydrothermal fluids;while the later immiscibility (boiling) of vapor- and liquid-compo-nents resulted in the precipitation and enrichment of gold, copperand iron.

6. Conclusion

The Yinan Au–Cu–Fe deposit is a typical skarn deposit producedin the contact zone between the Yanshanian intermediate-acidicintrusive rocks and the Neoarchean–Cambrian strata. H–O isotopestudies have shown that, the ore-forming fluids of early unhydrousskarn stage (I) and hydrous skarn-magnetite stage (II) were mainly


derived from magmatic water; while late quartz-sulfide stage (III)and carbonate stage (IV) indicated the fluids were dominantlymagmatic origin with a component of meteoric water. The fluidinclusion studies have shown that, the early skarn minerals wereformed from an immiscible mixture of silicate melt and vapor–li-quid phase fluids. This immiscibility directly led to the ore-bearinggas–water hydrothermal fluids separating out from the magmaticsystem, laying the material foundation for the later hydrothermalmineralization; at the later hydrothermal stage, the immiscibility(boiling) occurred in ore-forming fluids as an indicator of the vaporand liquid phase separation. Furthermore, the decompression andboiling may occur repeatedly, thus resulting in a large number ofprecipitations and enrichment of gold, copper and iron.

Acknowledgements

This research was supported by the National Natural SciencesFoundation of China (NFSC) under Grants of 40930423 and40873036, the 973 National Basic Research Priorities Program(2009CB421003-01), the Program for Changjiang Scholars andInnovative Research Team in University (PCSIRT), the Program ofMineral Prediction of the Yinan Au–Cu–Fe deposit in ShandongProvince and the Programme of Introducing Talents of Disciplineto Universities under Grant B07011. Dr. Z.S. Chang and one anony-mous reviewer are thanked for their constructive comments. Ourthanks are also given to J.G. Liou and S.J. Alexander for their effortin reviewing and improving the presentation.

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