Ore Geology Reviews 56 (2014) 276–291

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Gold mineralization in the Guilaizhuang deposit, southwesternShandong Province, China: Insights from phaserelations among sulfides, tellurides, selenides and oxides

Wen-Gang Xu a, Hong-Rui Fan a,⁎, Fang-Fang Hu a, M. Santosh b, Kui-Feng Yang a,Ting-Guang Lan a, Bo-Jie Wen a

a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinab China University of Geosciences (Beijing), Beijing 100083, China

⁎ Corresponding author. Tel.: +86 10 82998218; fax:E-mail address: fanhr@mail.iggcas.ac.cn (H.-R. Fan).

0169-1368/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.oregeorev.2013.06.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 December 2012Received in revised form 18 June 2013Accepted 21 June 2013Available online 6 July 2013

Keywords:Gold mineralizationSulfideTellurideSelenidePhase relationGuilaizhuang gold depositChina

The Guilaizhuang gold deposit is composed of limestone- and breccia-type ores. In the limestone ores, gold ishosted by pyrite, As-bearing pyrite and arsenopyrite, whereas in the breccia ores, tellurides are the main goldcarriers. Selenium often isomorphously substitutes for sulfur in sulfides in the limestone ores, but occurs asselenides in the breccia ores. Hematite is the only oxide in the breccia ores, associated with barite and othertellurides. In this paper, the thermodynamic parameters, especially the Gibbs free energies of formation andreaction, of related sulfides, tellurides, selenides and oxides are computed to explain the distinct mineral stylesin the limestone- and breccia-type ores. We construct the phase relations among minerals delineated by phasediagrams at 250 °C, the temperature for gold precipitation in the Guilaizhuang deposit. According to the phaserelations between sulfides and selenides, we propose that gold has been transported as [Au(HS,HSe)2]

−, andreleased during rapid formation of pyrite, As-pyrite and arsenopyrite, at logfS2(g) between −12.8 and−11.4 (250 °C). In the breccia ores, logfTe2(g) and logfSe2(g) are constrained within −12.9 to −9.4,and −12.4 to −6.9 (250 °C), respectively, based on the mineral assemblages observed in the ores. Thephase relations between tellurides and selenides indicate that calaverite, the only stable gold telluridemineral in nature, was not stable in the breccia ores at 250 °C during the gold precipitation. Based onEMPA analysis, the so-called “calaverite” revealed from microscopic observations is in fact Au–Ag telluride,probably derived from the decomposition of sylvanite (AuAgTe4) and petzite (AuAg3Te2), the only two stableAu–Ag tellurides in nature at high temperature. However, we do not exclude the possibility that calaverite occursin this deposit, whichmay have precipitated in early relatively high tellurium concentration stage preceding goldmineralization. Selenium was not detected in gold-bearing tellurides, implying that selenide is unrelated withgold precipitation directly in the breccia ores, but can buffer the tellurium fugacity, and thus influence the goldprecipitation indirectly. Furthermore, in logfO2(g) range of N−35.4 at 250 °C, the majority of the telluridesand selenides can be stable in the breccia ores, but residual sulfides are proposed to be oxidized into sulfateminerals, which could also free some gold into ores, although less important relative to that formed fromtelluride decomposition.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Sulfides, mainly pyrite, As-bearing pyrite and arsenopyrite, are themain carriers of gold in most gold deposits (Benning and Seward,1996; Boullier et al., 1998; Clark and Williams-Jones, 1990; Emsboet al., 2003; Fortuna et al., 2003; Gammons and Williams-Jones,1997; Hofstra and Cline, 2000; Jugo et al., 1999; Kesler et al., 2002;Lang and Baker, 2001; Pokrovski et al., 2008; Zezin et al., 2011).However, gold-bearing tellurides, selenides and oxides have alsobeen reported from many epithermal Au–Te deposits, telethermal

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selenide vein-type gold deposits, and epithermal Au–Ag deposits inthe subaerial volcanic environments (Afifi et al., 1988; Alderton andFallick, 2000; Cook and Ciobanu, 2004; Liu et al., 2000; Mills, 1974;Nasar and Shamsuddin, 1990; Simon and Essene, 1996; Simon et al.,1997).

Tellurides and selenides of Au, Ag, Cu, Fe and other elements arecommonly reported as trace minerals associated with gold (Ciobanuet al., 2006; Cooke andMcPhail, 2001). The association among tellurium,selenium and gold has long been recognized by economic geologists,and is themost evident in the prevalence of Au–Ag-tellurides–selenidesin some ore deposits (Ciobanu et al., 2006). A comprehensive study oftellurides, selenides, oxides and sulfides is significant for understandingthe processes of gold mineralization. Previous studies have used

277W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

thermodynamic properties to calculate relative stabilities of nativeelements, sulfides, tellurides, selenides and oxides as a function offugacity of S2(g), Te2(g), Se2(g) and O2(g) (Afifi et al., 1988; Liuet al., 2000; Simon and Essene, 1996; Simon et al., 1997). However,most of these studies principally focus on the thermodynamic charac-teristics of chemical complexes. In this paper,we provide amore simpli-fied approach with potential application to ore-forming processes.

The Guilaizhuang gold deposit, southwestern Shandong Province,China, is primarily composed of sulfidized limestones and oxidizedbreccias (Hu, 2005; Teng, 1994; Yu, 2010). In the limestone ores,gold is invisible and hosted by pyrite, As-bearing pyrite and arsenopy-rite. In the breccia ores, sulfides are absent, and the main gold-bearingminerals are tellurides and oxides, with less selenides (Hu, 2005; Yu,2010). In order to understand the geochemical significance of thesemineral assemblages for gold precipitation, we have calculated relativestabilities of native elements, especially Au, sulfide, selenide, tellurideand oxide minerals as a function of fugacity of S2(g), Te2(g), Se2(g),and O2(g) through thermodynamic approach. Our study provides abetter interpretation of the diversity in gold mineralization betweenthe two type ores in this deposit.

2. Geological setting and ore geology

The Guilaizhuang gold deposit is located in the easternmargin of theLuxi Block affiliated with the Eastern Block of the North China Craton,and is bounded by the continental Tan-Lu Fault to the east (Fig. 1a).The Luxi Block is composed of Neoarchean gneisses, amphibolites andtonalitic-trondhjemitic-granodioritic gneisses (TTGs), Paleoproterozoicgranitoids, Paleozoic marine carbonates interbedded with clastic rocks,Mesozoic and Cenozoic continental clastic rocks, volcaniclastics,intermediate-basic igneous rocks, mafic dykes, carbonatites and al-kaline rocks (Fig. 1b) (Lan et al., 2012; Zhang et al., 2005; Zhaoet al., 2001, 2005). The Tongshi intrusive complex is the most im-portant magmatic body in this region (Fig. 2), composed of thefine-grained quartz monzodiorite, porphyritic quartz monzodiorite,and coarse- to fine-grained porphyritic syenites. LA-ICPMS zircon U–Pbages show that this complex was emplaced at 180.1–184.7 Ma(Lan et al., 2012). Previous researchers inferred that the porphyriticsyenites were genetically related with the Guilaizhuang gold deposit(Hu, 2005; Teng, 1994; Yu, 2010).

The Guilaizhuang deposit is hosted by the Cambrian–Ordoviciancarbonate rocks, limestone and dolomite, and controlled by the W–Etrending fault (Fig. 3a, b). The gold resource is estimated to beN30 tonnes with an average grade of 8.10 ppm (Hu, 2005; Hu et al.,2004, 2005; Shen et al., 2001; Yu, 2010). The ores in the deposit can

Fig. 1. (a) Tectonic setting of the Luxi Block, modified after Zhao et al. (2005) and (SantoGuilaizhuang gold deposit, modified after Lan et al. (2012).

be subdivided into two sub-types, limestone- and breccia-type ores. Inthe limestone ores, gold is predominantly contained by sulfides, mainlyAs-bearing pyrite and arsenopyrite, which occurs as disseminations inlimestone (Fig. 4a). The breccia ores, main gold producer in this deposit,are composed of various kinds of breccias, including porphyritic syenite,monzodiorite, carbonate rocks and sandstone, which are cemented bylithic fragments with the same lithology as the breccias (Fig. 4b).Calaverite (AuTe2) and hessite (Ag2Te) in the breccias are the maingold-bearing minerals in the deposit, except the native gold associatedwith them. Someother telluridemineralswere also found in the depositby previous researchers (Hu, 2005; Yu, 2010), includingweissite, altaiteand coloradoite. Minor hematite, barite, and tiemannite are closelyassociated with tellurides. Gold-related fluoritization is extensivelydeveloped in the deposit, which is followed by carbonatization.Silicification was observed locally, but the relationship betweenthe gold mineralization and silicification is unclear. The parageneticsequence of minerals in the limestone and breccia ores is illustratedin Fig. 5.

3. Methods

3.1. Mineralogical analysis

Electronmicroprobe analysis is used to characterize and identify thechemistry of the sulfides, tellurides, oxides and selenides observed inthe deposit.

The analyses were performed on MonoCL equipment (Gatan,Germany), scanning electron microscope (SEM) (Leo145VP, Germany)equipped with BSD detector, energy dispersive spectrometer (EDS)(Inca Energy 300, Oxford Instruments, UK) and EMPA (JXA-8100, JEOLCompany, Japan) equipped with five X-ray wavelength dispersivespectrometers (WDS), respectively, at the Institute of Geology andGeophysics, Chinese Academy of Sciences. For CL analysis, BSE imag-ing, and X-ray energy spectroscopic determination, the voltage was10–20 keV, and the beam current was adjusted to the voltage(from 0.1 to 10 nA). For EMPA analyses, Fe, S, As, Sb, Se, Bi, Ag, Cu,Te, Hg and Au were detected in sulfide, telluride and selenide min-erals using a 20 keV accelerating voltage, 10–20 nA beam currentand a 10s-counting time. Standards used were natural marcasitefor Fe and S, GaAs for As, stibnite for Sb, galena for Pb, cinnabar forHg, bismuthite for Bi, and native gold, silver, tellurium, copper,zinc, nickel, cobalt, and selenium for Au, Ag, Te, Cu, Zn, Ni, Co, andSe, respectively. The detection limits for the target elements in thisstudy are: Fe 0.03%, S 0.05%, Au 0.04%, Ag 0.04%, As 0.03%, Te 0.05%,

sh (2010). (b) Geological and tectonic map of the Luxi Block and the location of the

Fig. 2. Geological map of the Tongshi intrusive complex and the location of the Guilaizhuang gold deposit, modified after Lan et al. (2012).

278 W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

Cu 0.03%, Pb 0.03%, Zn 0.04%, Hg 0.04%, Ni 0.04%, Co 0.03%, Bi 0.03%,Se 0.04%, and Sb 0.04%.

3.2. Phase relations construction methods

The chemical potential equilibrium method was applied in con-struction of phase relations among various minerals, which hasbeen described by Afifi et al. (1988) and Simon and Essene (1996).Here we provide the salient details with specific relevance togeological applications.

3.2.1. Gibbs free energy of formationAll Gibbs free energies of formation used in this study for sulfide,

telluride, selenide and oxide minerals are those from elements intheir stable form at the temperature of interest (T) and pure idealS2(g), Te2(g), Se2(g) and O2(g), with notation of ΔfGT

o kJ/mol. The ΔfGTo

values have been partially compiled from related works (see tables inAppendices 1 and 2). The Gibbs–Helmholtz equation was used asthere was no information on the standard Gibbs free energies of forma-tion from elements in their stable state (particularly in solid state) at1 bar and the temperature of interest:

∂ Δf GoT

T

� �∂T ¼ ΔfH

om

T2

where ΔfHmo represents the molar enthalpy of formation at room tem-

perature (298 K), considered to be constant in a wide temperature

range for a solid phase, and ΔfGTo value at room temperature can be

found in thermodynamics handbook. Hence, we can calculate ΔfGTo

value of compound in solid phase at interesting temperature T:

ΔfGoT ¼ T� ΔfG

o298 K

298þ ΔfG

o298 K � 298−T

298 T

� �=103:

Previous microthermometric data on ore-stage fluid inclusions inthe Guilaizhuang deposit indicate that gold was precipitated at ca.250 °C (Hu, 2005; Hu et al., 2005; Shen et al., 2001). We thereforeset the temperature of interest to be 250 °C (=523.15 K) in thisstudy. Based on the EMPA results, the metallic elements selected forphase relation studies include Au, Ag, As (semi-metal element), Bi,Cu, Fe, Hg, Ni, Pb, Sb, Sn and Zn.

The Gibbs free energies of formation for sulfide, telluride, selenideand oxide minerals in this study are listed in Appendix 1.

3.2.2. Fugacity calculationsThe fugacities of S2(g), Te2(g), Se2(g) and O2(g) are useful mea-

sures of the relative stability of solid phase, and is independent ofwhether we use aqueous solutions or gases to derive the equilibriumreactions for solid phase (Simon and Essene, 1996).

For a given balanced reaction, the equilibrium constant (lnK) canbe calculated from Gibbs free energy of reaction (ΔfGT

o):

lnK ¼ ΔrGoT

−RT

Fig. 3. Geological map of the Guilaizhuang gold deposit, modified after Yu (2010).(a) Map view. (b) No. 30 prospecting profile with section line A–B in (a).

279W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

where R refers to the gas constant, 0.008314 kJ/mol·K, T temperaturein Kelvin, and:

ΔfGoT ¼ ∑ ϑiΔfG

oT

� �res−∑ ϑjΔfG

oT

� �rea

where ‘res’ and ‘rea’ represent resultant and reactant of the reaction,respectively, and ϑi and ϑj denote the stoichiometric number ofcorresponding resultant and reactant.

Fig. 4. Photographs of (a) the limestone ores, with gold-bearing sulfid

Assuming that the solid phases are pure, the equilibrium constantwill be a function of temperature. Therefore, we can obtain the fugacityfor this reaction as form of:

logF ¼ lnK=−2:303

Afifi et al. (1988) and Simon and Essene (1996) proposed that becausethe change in volume for the solids is generally small, the effect of confin-ing pressure on the equilibriumconstant is not significant for a pressure ofup to 1 kbar formost reactions in this study. Previousfluid inclusion stud-ies have estimated themineralization pressure at Guilaizhuang deposit islower than 500 bar (Hu, 2005; Shen et al., 2001; Yu, 2010), and therefore,the influence of pressure can be neglected in this study.

Here we subdivide the equilibrium mineral systems into twosub-systems, univariate and bivariate one, for fugacity calculation.

In univariate system, the fugacity of the gaseous member in agiven reaction can be calculated according to the method above.Taking the balanced reaction below for example:

2AsSþ 0:5S2 gð Þ ¼ As2S3:

The fugacity of S2(g) of this reaction can be calculated as follows(T = 523.15 K):

ΔrGoT¼523:15 K ¼ ΔfG

oT¼523:15 K As2S3ð Þ

− 0:5� ΔfGoT¼523:15 K S2 gð Þð Þ þ 2� ΔfG

oT¼523:15 K AsSð Þ�

:

The Gibbs free energies of formation of As2S3, S2(g) and AsSare −83.52, 45.64 and −32.58 kJ/mol, respectively, so we obtainthe Gibbs free energy of this reaction, which equals to −41.18 kJ/mol.So the equilibrium constant lnK:

lnK ¼ ΔrGoT¼523:15 K

−RT¼ 9:47

and

logf S2 gð Þ ¼ 2 lnK=−2:303 ¼ −8:22:

However, in bivariate system, the functional relationship between thefugacity of gaseous species should be expressed as the following form:

logfA ¼ a logfBþ b

where A and B represent gaseous species, whereas a and b denotecalculation coefficients. The phase relation between compound withA and compound with B can be constructed accordingly. Taking thebalanced reaction below as an example:

Ag2Teþ 0:5Se2 gð Þ ¼ Ag2Seþ 0:5Te2 gð Þ:

es mainly disseminating in the dark bands; and (b) breccia ores.

Fig. 5. Paragenetic sequence of minerals in the limestone and breccia ores.

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The equilibrium constant lnK can be written as:

lnK ¼ ΔrGoT¼523:15 K

−RT¼ −1:37

and

log fTe2 gð Þ½ �0:5= f Se2 gð Þ½ �0:5� �

¼ lnK=2:303

then the functional relationship between fTe2(g) and fSe2(g) will be:

logfTe2 gð Þ ¼ logfSe2 gð Þ−1:19:

Lastly, the phase relation between hessite (Ag2Te) and naumannite(Ag2Se) can be constructed.

Using the methods mentioned above, we have obtained the fugac-ities of S2(g), Te2(g), Se2(g) and O2(g) of the equilibrium reactions inunivariate and bivariate systems, and have listed them in Appendix 2.

4. Results

4.1. Texture and composition

Backscattered electron (BSE) images of the sulfides, tellurides,oxides and selenides are shown in Figs. 6 and 7. In limestone-typeores, the sulfides include pyrite, As-bearing pyrite, arsenopyrite andtetrahedrite. BSE images show that pyrite is commonly surrounded byAs-pyrite which is in turn cemented by tetrahedrite. Tetrahedrite exso-lution is noticed in pyrite (Fig. 6). The precipitation of As-pyrite impliesthat As concentration in the hydrothermal fluids increased graduallywith the decrease of sulfur concentration, and therefore arsenopyritelikely formed after As-pyrite. The above observation indicates thatAs-pyrite and arsenopyrite were preceded by pyrite and followed bytetrahedrite, from the evolving sulfur-rich fluids. In breccia-type ores,tellurides, calaverite, Cu-telluride and hessite, the main gold producer,are paragenetically associated, as shown in Fig. 7a and b. Tiemannite(HgSe) is scattered in the assemblages, locally accompanied by barite(Fig. 7c and e). Tiny grains of native gold, 2–4 μm in size, are embeddedin calaverite (Fig. 7f). Hematite is also found in the colloidal and grainforms (Fig. 7g and h), which is considered to be a primary phase byprevious researchers (Hu, 2005; Hu et al., 2005; Shen et al.,2001). Previous studies also reported native silver, tellurium, elec-trum, frohbergite, altaite, clausthalite, coloradoite, melonite andCu–Au telluride (Hu, 2005; Hu et al., 2005; Yu, 2010).

The EMPA results from this study and previous work are listed inTables 1 and 2. The EMPA analysis of the sulfides in limestone-typeores shows that gold widely occurs in pyrite, As-pyrite, arsenopyrite,and locally tetrahedrite. The As-pyrite usually contains higher concen-trations of Au than other sulfides, ranging from 600 to 2500 ppm. Sbmainly concentrates in tetrahedrite (12.36 to 15.21 wt.%) with minorin As-pyrite and arsenopyrite. No tellurium can be detected in thistype ore. Gold grains in the breccia ores always host minor silver(b6 wt.%). Tellurides in the breccia ores are mainly composed ofAu–Ag-, Ag-, Cu-, Fe-, Pb-, and Hg-tellurides. Native Te(s) andAg(s) can also be detected in previous studies.

In the limestone ores, selenium is widely contained by sulfides,especially enriched in As-pyrite, arsenopyrite and tetrahedrite, rangingfrom n × 100 to n × 1000 ppm with S/Se ratio of n × 103. It has beenwidely accepted that the concentrations of some trace elements in pyritemay serve as good genetic indicators (Hawley and Nichol, 1961; IvorRoberts, 1982; Liu et al., 2000; Loftus-Hills and Solomon, 1967;Raymond, 1996). In hydrothermal pyrite, the concentration of Se ishigh (20–50 ppm), with S/Se ratios between 1 × 104 and 2.67 × 104,and in pyrite associated with volcanics, the Se concentration is evenhigher (up to 1000–2000 ppm), with an S/Se ratio of the order ofn × 103. Thus pyrite in the limestone ores at Guilaizhuang probably isof magmatic origin. Other sulfides cogenetic with pyrite should be alsoof volcanic origin. All of these sulfides were likely precipitated from asulfur-rich magmatic vapor containing significant amount of gold.

4.2. Phase relations among sulfides, selenides, tellurides and oxides

4.2.1. Phase diagram constructionFugacity–fugacity diagrams are regarded as a useful tool to investi-

gate the phase relations between paired compounds and predict equi-librium assemblages for a given system and the occurrence forms ofelements such as Au, Ag and Cu. Four types of fugacity–fugacitydiagrams were constructed for the Guilaizhuang deposit based on acommon logarithm form at 250 °C: logfS2(g)–logfSe2(g), logfTe2(g)–logfSe2(g), logfTe2(g)–logfO2(g), and logfSe2(g)–logfO2(g).

logf S2 gð Þ– logf Se2 gð Þ diagram:

The logfS2(g)–logfSe2(g) diagram (Fig. 8) shows the stability fieldsof binary selenides and sulfides, and provides useful information on

Fig. 6. Backscattered electron images of sulfides in the limestone ores. a: arsenopyrite; b: pyrite grain in perfect pyritohedron shape; c: pyrite (Py), As-bearing pyrite (As–Py),arsenopyrite (Apy), and tetrahedrite (Td), noting that the As–Py surrounds the Py and is cemented by Td in the last stage; (d) exsolution of Td from Py.

281W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

chemical evolution of the hydrothermal fluids and gold precipitationin the limestone ores. Selenides in this diagram are selected based onthe EMPA results, including Fe–Se, Ni–Se, Ag–Se, Sb–Se, Au–Se, Cu–Se,Hg–Se, Pb–Se and As–Se compounds, most of which are stable in na-ture at 250 °C, and have been reported from many selenide deposits(Afifi et al., 1988; Ciobanu et al., 2006; Echmaeva and Osadchii,2009; Liu et al., 2000; Mills, 1974; Simon et al., 1997). Althoughmany natural minerals occur in the Cu–Se system, only Cu2Se–CuSebinary is constructed in our study due to their stability in the temper-ature range of gold mineralization in the Guilaizhuang deposit. In theFe–Se system, the only stable compounds at room temperature areβFe1 + xSe and FeSe2 (Mills, 1974), and above 200 °C, the phasediagram for this system contains five compounds, βFe1 + xSe (x =0.04–0.06), δFe1 – xSe (x = 0.00–0.28), γFe1 – xSe (x = 0.18–0.37),FeSe2 and Fe7Se8 (Schuster et al., 1979). Based on the fugacity ofSe2(g) and temperature in our study, only FeSe1.333, the δ and γ phase,and FeSe2 were selected. The upper and lower limits of logfSe2(g)were set at Se2(g)–Se(l) and Ag-naumannite binaries, respectively.Sulfides in this diagram include bornite (Bn), chalcopyrite (Cp), pyrrho-tite (Po, Fe0.89S), pyrite (Py), arsenopyrite (Apy), argentite, cinnabar,stibnite, realgar and orpiment, which are stable in nature and commonin many hydrothermal deposits (Archibald et al., 2002; Franchini et al.,2011; Halter et al., 2002; Landtwing et al., 2005; Redmond et al., 2004;Richards, 2011; Young et al., 2003; Zajacz et al., 2011).

In this diagram, native gold is stable and coexists with almostall selenides and sulfides in the common range of logfSe2(g) andlogfS2(g), except native selenium and krutaite (CuSe2) that existonly at very high logfSe2(g), close to selenium saturation line. WhenlogfSe2(g) is lower than−16.2, native silver will be stable at this tem-perature, coexisting with Cu2Se likely, and to form eucairite (CuAgSe)

in the metal-rich part of the system. The stable field of sulfides isbroader than that of selenides, explaining the prevalence of sulfidesover selenides in most hydrothermal ore deposits.

The diagram also shows the selenide and sulfide assemblages.For example, naumannite can occur in equilibrium with commonsulfides in ore deposits, such as pyrite, arsenopyrite, galena, stibnite,chalcopyrite and bornite. These mineral assemblages can be usedto constrain fugacity conditions. For example, berzelianite (Cu2Se) +pyrrhotite (Fe0.89S) − chalcopyrite represents an equilibrium as-semblage at logfSe2(g) between −10.1 and −14.2 and logfS2(g)between −16.4 and −18.8 at 250 °C.

logfTe2 gð Þ– logf Se2 gð Þ diagrams:

Telluride minerals are the main gold-bearing species in the brec-cia ores at Guilaizhuang. The common telluride compounds involvedin this study include Au–Te, Ag–Te, Fe–Te, Cu–Te, Ni–Te, Hg–Te, Bi–Te, Sb–Te, As–Te, Pb–Te and Zn–Te. In Fe–Te system, the only irontelluride mineral is frohbergite (FeTe2), which occurs in a variety oftelluride deposits (Afifi et al., 1988; Simon and Essene, 1996; Simonet al., 1997). However, in this study, the β phase (FeTe0.9) was alsoconsidered for phase equilibrium analysis. The minerals in Ni–Tesystem contain two end compounds of δ (NiTe2 – x; x = 0–0.9) phase,NiTe1.1 and NiTe2. The phase diagram for Cu–Te is complicated by thepresence of compounds containing both Cu+ and Cu2+ in defect struc-tures (Afifi et al., 1988), and the three principal compounds used in thestudy are the minerals weissite (Cu2Te), rickardite (Cu4Te3) andvulcanite (CuTe).

The stability of telluride and selenide minerals is shown in Figs. 9and 10, as a function of logfTe2(g) and logfSe2(g) at 250 °C. The limits of

Fig. 7. Backscattered electron images of tellurides, selenides, oxides and native gold in the breccia ores. See text for details.

282 W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

Table 1EMPA results (wt.%) of sulfides in the limestone ores from the Guilaizhuang deposit.a

Sample S Fe As Se Au Ag Sb Te Cu Total

Py 1 52.90 45.59 u.db 0.05 0.12 u.d u.d u.d u.d 98.662 53.18 45.84 u.d u.d 0.06 0.05 u.d u.d u.d 99.133 52.81 44.88 u.d u.d 0.05 u.d 0.08 u.d 0.04 97.864 53.35 45.20 u.d u.d 0.10 u.d 0.04 u.d u.d 98.695 52.62 45.68 u.d 0.06 0.07 u.d u.d u.d 0.03 98.466 52.65 45.65 u.d u.d 0.18 u.d u.d u.d 0.03 98.517 52.69 45.71 u.d 0.06 0.11 u.d u.d u.d u.d 98.478 52.70 45.83 u.d 0.16 0.04 u.d 0.05 u.d u.d 98.789 52.28 44.82 u.d 0.04 0.11 u.d 0.04 u.d u.d 97.2910 51.43 45.44 u.d u.d 0.20 u.d 0.08 u.d u.d 97.15

As–Py 1 46.60 44.98 7.80 0.12 0.25 0.09 0.75 u.d u.d 99.842 47.84 44.38 5.30 0.16 0.15 0.11 0.58 u.d 2.24 100.653 45.26 45.15 6.40 0.10 0.17 0.09 0.24 u.d 2.42 100.344 46.29 45.27 4.61 0.06 0.14 0.10 0.26 u.d 1.70 98.415 45.02 45.45 6.88 0.09 0.17 0.08 u.d u.d 1.47 99.426 45.35 45.45 6.30 0.12 0.11 0.09 u.d u.d 0.23 97.667 53.12 44.19 2.10 0.11 0.07 u.d 12.54 u.d 0.03 99.618 51.15 44.48 2.32 0.13 0.06 0.11 u.d u.d 2.10 100.759 47.55 44.42 5.24 0.10 0.11 u.d u.d u.d 0.03 97.4810 52.24 43.84 3.20 0.10 0.09 u.d u.d u.d u.d 99.4611 47.84 45.78 5.25 0.09 0.12 0.05 u.d u.d u.d 99.1112 52.71 45.57 1.34 0.05 0.09 u.d 12.03 u.d u.d 99.7913 51.38 44.51 3.45 0.21 0.10 u.d 13.24 u.d u.d 99.50

Apy 1 21.35 30.01 47.03 0.05 0.12 u.d 0.11 u.d 0.12 98.792 22.45 30.04 46.09 0.07 0.09 u.d 0.09 u.d 0.23 99.063 25.63 30.26 43.03 0.12 0.06 u.d 0.12 u.d 0.09 99.314 24.65 28.84 45.63 0.16 0.11 u.d 0.08 u.d 0.11 99.585 24.34 29.93 44.02 0.10 0.12 u.d 0.09 u.d 0.13 98.736 25.64 31.02 44.67 0.04 0.09 u.d 0.10 u.d 0.09 100.057 25.23 30.42 44.12 0.06 0.11 u.d 0.07 u.d 0.14 100.05

Tdc 1 25.08 0.44 10.24 0.16 0.07 0.10 14.01 u.d 18.75 66.932 25.13 1.96 9.64 0.17 0.05 u.d 12.36 u.d 38.96 89.123 25.30 3.02 10.26 0.15 0.08 0.05 15.21 u.d 18.27 71.134 25.96 2.81 9.20 0.09 0.06 0.05 13.37 u.d 19.81 70.395 25.74 2.18 8.21 0.11 0.06 0.05 u.d u.d 19.05 70.586 26.09 1.74 10.20 0.08 0.14 0.05 u.d u.d 18.36 70.63

a Data source: this study.b u.d = under detection limit.c The rest component of Td is zinc, which has not been determined in EMPA but can

be detected by energy dispersive spectrometer (EDS).

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logfTe2(g) and logfSe2(g) can be determined by mineral assemblagesobserved in SEM analyses, which will be discussed in a later section.

logfO2 gð Þ– logfTe2 gð Þand logfO2 gð Þ– logf Se2 gð Þ diagrams:

Hematite is the only oxide in the breccia ores, which is an importantindex to determine the oxygen fugacity. Barite is associatedwith hema-tite in the ores (Fig. 7g), which can be used to construct the phase dia-grams in this study. Other oxides involved in these diagrams includeCu–O, Bi–O, Pb–O, As–O, Ni–O, Zn–O, Sn–O and Sb–O. Tellurides andselenides included in these diagrams are the same as those in Figs. 9and 10. The diagrams are shown in Figs. 11 and 12.

The phase diagrams shown in Figs. 11 and 12 indicate that most sel-enides, except SnSe2 and SnSe, coexist with hematite over a large rangeof logfO2(g) and logfSe2(g), with an exception for Sn and Fe. It is clearthatmostmetallic oxides are unstable with respect to tellurides, consis-tent with the absence of oxides except cassiterite, magnetite, or hema-tite from hypogene telluride ores (Afifi et al., 1988).

5. Discussion

5.1. Limitation of fugacity ranges of S2(g), Te2(g), Se2(g) and O2(g)

5.1.1. LogfS2(g)Sulfides in the limestone ores include pyrite, As-pyrite, arsenopyrite

and tetrahedrite. The phase diagrams of logfS2(g) show that arsenopy-rite is only stable at the logfS2(g) range between −14.9 and −11.4,

which can be used to limit the fugacity of S2(g) in the limestone ores.Because of the absence of pyrrhotite, the logfS2(g) for pyrrhotite–pyriteequilibrium must be higher than −12.8. Thus we obtain the logfS2(g)range between−12.8 and −11.4.

5.1.2. LogfTe2(g)Tellurides are widely developed in the breccia ores in the

Guilaizhuang deposit (Hu, 2005; Yu, 2010), including calaverite,hessite, Au–Ag-telluride, Fe-telluride, Cu-telluride, altaite, coloradoiteand melonite (see Table 2). The presence of native gold indicates thatthe upper limit of logfTe2(g) can be defined by the phase equilibriumof Au–AuTe2 binary, which equals to−9.4. The lower limit of logfTe2(g)is supported by the EMPA results of the tellurides in the breccia ores.Fe-telluride of num. 19 in Table 2 was determined to be FeTe2.24, closeto FeTe2 in Fe–Te binary, for which the equilibrium tellurium fugacityis −12.9. Therefore, the logfTe2(g) is constrained at between −12.9and −9.4. In this fugacity range, telluride minerals, except calaveritethat will be discussed later, from the Guilaizhuang deposit can be linkedtogether, and are also consistent with the fact that native gold is widelydistributed in ores.

5.1.3. LogfSe2(g)Based on microprobe analysis, HgSe is stable in the breccia ores.

Because selenium is extremely low in the Guilaizhuang deposit, it isreasonable to assume a constant selenium concentration during theevolution of the hydrothermal system (Echmaeva and Osadchii, 2009;Okamoto, 1990; Okamoto and Massalski, 1986; Schuster et al., 1979;Simon and Essene, 1996; Simon et al., 1997), based on which we canfix the lower limit of logfSe2(g) at Hg(l)–HgSe binary, equal to −12.4.Based on thermodynamic and experimental results, Okamoto andMassalski (1986) pointed out that native gold and native selenium areunlikely to coexist at any temperature, and AuSe is the only known binary compound in the Au–Se sys-tem, although it has not yet been identified as a discrete mineral. Thusthe equilibrium logfSe2(g) for Au–AuSe (−6.9, at 250 °C) should beused to constrain the Se fugacity in this study. The logfSe2(g) rangecan be defined as−12.4 to −6.9.

5.1.4. LogfO2(g)Hematite is the only metallic oxide in the breccia ores and can be

used to limit the oxygen fugacity. The lower limit of logfO2(g) is esti-mated to be −35.4, the equilibrium oxygen fugacity for magnetite–hematite at 250 °C. The upper limit of logfO2(g) is defined by thefugacity of atmospheric oxygen, although this condition is generallynot attained during telluride and selenide deposition.

The limits of fugacities of S2(g), Te2(g), Se2(g), and O2(g) in phasediagrams are shown in Figs. 8 to 12.

5.2. Interpretation of the different gold precipitation types

5.2.1. Gold precipitation in the limestone oresIn logfS2(g)–logfSe2(g) phase diagram, the S2(g) fugacity range

is extremely limited, indicating that sulfides were precipitated rapidlyin a short period under reducing condition. Selenium is highlychalcophile, and often substitutes for sulfur in sulfide minerals dueto the similarity of their crystallochemical properties, especially inreducing environment (Liu et al., 2000; Simon and Essene, 1996).Many available experiments and calculations have proven that goldcan form a stable complex with sulfur in a vapor phase (Benningand Seward, 1996; Gammons and Williams-Jones, 1997; Jugo et al.,1999; Kamenetsky et al., 1999; Kesler et al., 2002; Metrich andClocchiatti, 1996; Pokrovski et al., 2008; Richards, 2011; Simmonsand Brown, 2006; Simon and Ripley, 2011; Tosdal et al., 2009;Williams-Jones and Heinrich, 2005; Williams-Jones et al., 2009;Zezin et al., 2011). According to the phase diagram in Fig. 8, the sele-nium fugacity is not high enough to form AuSe, thus Au is probably

Table 2EMPA results (wt.%) of gold related minerals in the breccia ores, Guilaizhuang deposit.a

Num. Mineral Au Ag Te Fe Cu Pb Zn S Hg Ni Co As Bi Se Sb Total

1 Native Au(s) 96.81 4.31 u.db 0.09 0.07 u.d u.d u.d u.d u.d u.d 0.06 u.d u.d 0.15 101.492 Native Au(s) 96.38 4.36 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.743 Native Au(s) 96.49 0.98 0.14 0.07 0.27 u.d 0.06 0.05 u.d 0.09 0.07 u.d 0.68 0.16 0.09 99.154 Native Au(s) 93.08 5.43 u.d 0.14 0.33 u.d 0.08 u.d u.d 0.18 0.12 0.07 0.40 0.17 0.18 100.175 Native Au(s) 94.01 3.27 1.2 u.d u.d u.d u.d u.d u.d u.d u.d 0.36 u.d u.d 1.14 99.986 Native Au(s) 90.97 3.91 4.1 u.d u.d u.d u.d u.d u.d u.d u.d 0.37 u.d u.d 1.02 100.417 Electrum 84.14 13.53 0.17 0.14 0.08 0.10 0.14 0.05 u.d 0.06 0.10 0.09 0.89 0.16 0.18 99.718 Calaveritec 28.88 8.57 62.52 0.31 0.24 u.d 0.14 0.05 u.d u.d u.d u.d u.d u.d u.d 100.709 Calaverite 28.96 8.99 61.97 u.d 0.31 u.d 0.04 u.d u.d u.d u.d u.d u.d u.d u.d 100.2710 Calaverite 29.62 8.32 61.97 0.03 u.d 0.08 u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0211 Calaverite 29.80 8.23 61.32 u.d u.d 0.03 0.08 u.d u.d u.d u.d 0.05 u.d 0.06 u.d 99.5912 Calaverite 25.82 10.76 62.40 u.d 0.49 0.31 u.d 0.06 u.d u.d u.d 0.16 u.d u.d u.d 100.0013 Calaverite 28.76 9.78 61.22 0.16 u.d u.d 0.10 u.d u.d u.d u.d u.d u.d u.d u.d 100.0214 Calaverite 28.60 8.63 61.64 u.d u.d 0.91 u.d u.d u.d u.d u.d 0.20 u.d 0.04 u.d 100.0215 Calaverite 25.78 10.37 63.14 0.07 0.64 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0016 Calaverite 32.54 1.24 61.49 3.26 0.78 u.d u.d u.d u.d u.d u.d u.d u.d u.d 0.62 99.9317 Hessite 0.65 59.89 37.77 0.07 u.d u.d u.d 0.16 u.d 0.18 0.11 u.d u.d 0.19 0.33 99.3818 Petzite 25.78 40.58 33.15 u.d u.d u.d 0.12 u.d u.d u.d u.d u.d u.d u.d u.d 99.6319 Frohbergite 0.28 0.36 82.37 16.13 0.35 u.d 0.30 0.11 u.d u.d u.d u.d u.d 0.10 u.d 100.0020 Altaite u.d 1.32 42.38 0.18 0.34 53.17 0.74 1.87 u.d u.d u.d u.d u.d u.d u.d 100.0021 Altaite u.d 0.87 36.42 1.21 0.06 59.21 0.16 1.43 u.d u.d u.d u.d u.d u.d u.d 99.3522 Altaite 4.26 0.74 15.26 u.d 0.22 64.00 0.12 7.14 u.d u.d u.d u.d u.d 8.16 u.d 99.9023 Altaite 4.32 0.61 14.93 u.d u.d 64.62 0.10 7.36 u.d u.d u.d u.d u.d 7.92 u.d 100.0024 Clausthalite u.d 0.64 2.65 u.d u.d 70.26 u.d 0.50 u.d u.d u.d u.d 1.46 20.23 u.d 95.7425 Clausthalite u.d 0.47 2.88 u.d u.d 69.54 u.d 0.47 u.d u.d u.d u.d 2.10 20.13 u.d 95.6026 Cu–telluride 38.13 0.11 60.27 u.d 1.49 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0027 Cu–telluride 30.02 0.53 64.55 u.d 4.89 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 99.9928 Cu–telluride 28.43 0.65 65.90 u.d 5.02 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0029 Cu–telluride 27.47 0.67 65.76 u.d 6.11 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0030 Cu–telluride 26.35 0.22 66.92 u.d 6.52 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0131 Cu–telluride 25.41 0.84 70.22 u.d 3.53 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0032 Cu–telluride 23.05 0.51 72.10 u.d 4.34 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 100.0133 Cu–telluride 21.63 0.14 71.90 u.d 6.34 u.d u.d u.d u.d u.d u.d u.d u.d u.d u.d 99.9934 Au–Ag–telluride 26.33 40.78 32.68 u.d 0.07 u.d u.d u.d 0.12 u.d u.d u.d u.d u.d u.d 99.3835 Coloradoite 0.25 0.15 37.44 u.d 0.04 u.d 0.27 u.d 63.64 0.16 0.08 0.21 0.14 0.21 0.32 102.9136 Melonite u.d u.d 78.07 0.16 u.d 0.13 0.12 u.d u.d 18.01 0.98 0.22 0.13 0.19 0.61 98.6937 Melonite 0.06 0.19 85.54 0.19 u.d u.d u.d u.d u.d 13.83 0.09 u.d 0.09 u.d u.d 99.9938 Tiemannite u.d u.d 0.14 0.12 u.d u.d u.d u.d 64.34 u.d u.d u.d u.d 34.56 u.d 99.1639 Tiemannite u.d u.d 0.23 u.d u.d u.d u.d u.d 65.32 u.d u.d u.d u.d 35.42 u.d 100.9740 Tiemannite u.d u.d 0.16 u.d u.d u.d u.d u.d 63.78 u.d u.d u.d u.d 36.20 u.d 100.1441 Native Te(s) u.d 0.42 99.04 0.03 u.d 0.31 0.04 u.d u.d u.d u.d u.d u.d u.d u.d 99.8542 Native Te(s) u.d u.d 99.67 0.13 u.d u.d u.d 0.05 u.d u.d u.d 0.06 u.d u.d u.d 99.9143 Native Ag(s) u.d 97.63 2.13 u.d u.d u.d u.d u.d u.d u.d u.d 0.22 u.d u.d u.d 99.98

a Data source: Yu (2010) and this study.b u.d = under detection limit.c The calaverite listed in this table is identified by microscopic observation and inherited from previous studies, which will be discussed detailed in text.

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complexed by S–Se. This allows us to assume the existence of a Au–S–Se complex probably in the form of [Au(HS,HSe)2]−. Fig. 8 also showsthat FeSe2, Sb2Se3 and As2Se3 are stable in this condition, in equilibriumwith FeS2 (Py), Sb2S3 and arsenopyrite (Apy). In Carlin-type gold de-posits, iron in pyrite, As-pyrite and arsenopyrite are considered tohave derived from the ferriferous carbonaceous country rock (Cline etal., 2005; Heitt et al., 2003; Hofstra and Cline, 2000; Mao, 1991;Mumin et al., 1994; Palenik et al., 2004; Simon et al., 1999; Su et al.,2008). Therefore, gold precipitation can be caused by the followingchemical reactions:

Au HS;HSeð Þ2� − þ 2Fe2þ→Au0 þ FeS2 þ FeSe2 þ 0:5H2 gð Þ þ 3Hþ

Au HS;HSeð Þ2� − þ 2Fe2þ þ 10=3As→Au0 þ 2FeAsSþ 2=3As2Se3

þ 0:5H2 gð Þ þ 3Hþ

Au HS;HSeð Þ2� − þ 8=3SbþHþ→Au0 þ 2=3Sb2S3 þ 2=3Sb2Se3

þþ2:5H2 gð Þ:

As and Sb were probably carried in the vapor phase, forming arse-nopyrite and Zn-bearing tetrahedrite, postdating the precipitation ofpyrite. Gold hosted by pyrite, As-pyrite and arsenopyrite is invisible,and probably exists as nano-particles in the crystal lattice of sulfides,similar to the occurrence of gold in many Carlin-type deposits (Cabriet al., 2000; Hofstra and Cline, 2000; Palenik et al., 2004).

5.2.2. Gold precipitation in the breccia oresIn the breccia ores, the oxygen fugacity is generally higher, and

most selenides and tellurides, except Sn selenides and Fe telurides,coexist with hematite over a large range of logfO2(g), logfSe2(g) andlogfTe2(g) (Figs. 11 and 12), consistent with the observation frommany telluride ores (Afifi et al., 1988). Under such oxidizing condi-tion, sulfides are no longer stable and would be oxidized to SO4

2−,and selenium would be released from the complex and form sele-nides (Liu et al., 2000):

Au HS;HSeð Þ2� − þ 5O2 gð Þ þ 2Ba2þ þ 2Hg2þ→Au0 þ 2BaSO4 bariteð Þþ 2HgSe tiemanniteð Þ þ 2H2O:

Fig. 8. LogfS2(g)–logfSe2(g) diagramshowing the relative stability of selenides and sulfidesat 250 °C. The shaded area represents the fugacity range of S2(g) and Se2(g) in this study.Abbreviations: Apy = arsenopyrite, Bn = bornite, Cp = chalcopyrite, Lo = loellingite,Po = pyrrhotite.

Fig. 10. LogfTe2(g)–logfSe2(g) diagram showing the relative stability of selenides andtellurides at 250 °C. The shaded area represents the fugacity range of Te2(g) andSe2(g) in this study.

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However, sulfur concentration was dramatically reduced in ore-forming fluids by the sulfidization process in the limestone ores,thus the oxidization reaction shown above is not significant for goldprecipitation in the breccia ores.

Gold-bearing tellurides (and selenides) are commonly presentas trace minerals in many gold deposits, indicating that tellurium(and selenium) can be a significant gold carrier (Cepedal et al.,2006; Ciobanu et al., 2006; Hu et al., 2006; Novoselov et al., 2006;

Fig. 9. LogfTe2(g)–logfSe2(g) diagram showing the relative stability of selenides andtellurides at 250 °C. The shaded area represents the fugacity range of Te2(g) andSe2(g) in this study.

Plotinskaya et al., 2006; Spry and Scherbarth, 2006; Vikentyev,2006; Voudouris, 2006). Gold is probably complexed by tellurium inthe form of [Au(HTe)2]− and/or [HAu(HTe)2]. In the Guilaizhuang de-posit, telluride is the main gold-bearing mineral, as shown in Table 2.However, according to the phase diagrams (Figs. 9 and 10), nativegold cannot coexist with calaverite, because of the high telluride

Fig. 11. LogfO2(g)–logfTe2(g) diagram showing the relative stability of selenides andtellurides at 250 °C. The shaded area represents the fugacity range of O2(g) andTe2(g) in this study.

Fig. 13. Phase relations in the system Au–Ag–Te at 250 °C, modified after Cabri (1965)and Afifi (1988). EMPA data of Au–Ag tellurides in Table 2 (num. 8–18 and 34) areplotted in the diagram denoted by the solid dots.

Fig. 12. logfO2(g)–logfSe2(g) diagram showing the relative stability of selenides andoxides at 250 °C. The shaded area represents the fugacity range of O2(g) and Se2(g)in this study.

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fugacity for calaverite formation. Based on the EMPA results, the“calaverite” identified from microscopic observations is in fact aAu–Ag telluride as shown in Fig. 13, except num. 16 in Table 2,which approximates to AuTe2 in stoichiometric composition.

As reported in previous studies (Afifi et al., 1988; Cabri, 1965;Echmaeva and Osadchii, 2009; Voudouris, 2011; Voudouris et al.,2011), Au and Ag can be complexed by Te to form Au–Ag–Te ternarysystem. In this system, minerals stable along the binary Au–Te andAg–Te joins include electrum, krennerite ((Au,Ag)Te2), sylvanite(AuAgTe4) and petzite (AuAg3Te2) (Fig. 13).

Sylvanite and petzite are stable in higher temperature (b354 °C),which represent the main form of gold precipitating primarily fromthe ore-forming fluids in the oxidized and acidic condition:

Au;Agð Þ HTeð Þ2� − þ 1:75O2 þ 3Hþ þ 2HTe−→AuAgTe4 sylvaniteð Þ

þ 3:5H2O

Au;Agð Þ HTeð Þ2� − þ 2 Ag OHð Þ2

� − þ 3Hþ

þ 2:25O2→AuAg3Te2 petziteð Þ þ 4:5H2O:

Sylvanite and petzite are unstable as temperature decreases,and decompose into hessite and calaverite at temperature below~120 °C (Afifi et al., 1988; Cabri, 1965; Echmaeva and Osadchii,2009; Voudouris et al., 2011). At the relatively low temperature ofgold precipitation estimated in this study (ca. 250 °C), the decompo-sition process should be buffered by other stable tellurides such asPbTe, Bi2Te3, FeTe2 and HgTe (Figs. 9 and 10), producing nativegold, hessite and Au–Ag tellurides with composition on the tie linesamong calaverite, sylvanite and petzite, as the plotted dots shownin Fig. 13:

AuAgTe4→Aux¼0e1AgTe4 þ 1−xð ÞAu0

AuAg3Te2→Auy¼0e1Agz¼2e3Te2z−4 þ 1−yð ÞAu0 þ 3−zð ÞAgTe2:

Besides, as indicated by Afifi et al. (1988) and Ciobanu et al.(2004), the γ- and χ-phases of Ag–Au–Te system can be stable athigh temperature and decompose rapidly on cooling below 120 °C

and 50 °C into a mixture of petzite, hessite, stutzite, krennerite andsylvanite. The plotted dots on the tie lines among calaverite, sylvaniteand petzite in Fig. 13 could probably represent the solid solutionsproduced by the decomposition of the γ- and χ-phases, but due tolimited thermodynamic data, the decomposition reactions of thesetwo phases could not be provided here.

According to the phase relations between selenides and tellurides,pure calaverite cannot coexist with native gold produced by the above-mentioned decomposition process. Therefore, we interpret that thedata num. 16 in Table 2, represented by the solid dot close to AuTe2end member in Fig. 13, is actually a supergene alteration product ofAu–Ag tellurides lying on the tie lines, with byproducts of native silver.However, we do not exclude the possibility that this data point mayrepresent the occurrence of calaverite which precipitated before thegold under a relatively high tellurium fugacity, similar with the casesin other Au–Te deposits, such as the Sandaowanzi deposit of China(Han et al., 2011; Yu et al., 2012). Native tellurium found in this depositshould have precipitated in the early extremely high tellurium fugacitystage preceding gold mineralization.

As shown by EMPA results, selenium is absent in Au–Ag telluridesat Guilaizhuang, implying that selenium is not directly related withgold precipitation in the breccia ores. The equilibrium between sele-nides and tellurides can buffer tellurium fugacity, which could proba-bly influence the decomposition process of Au–Ag tellurides and goldprecipitation indirectly.

As shown in Fig. 9, no Cu-tellurides are stable in our study, be-cause of the high selenium fugacity. Cu-tellurides, num. 26 to 33(Table 2), are not pure (e.g. kostovite), which should be incorporatedby gold and/or other metallic elements to form a Au–Cu–Te systemsimilar to the case of the Au–Ag–Te system discussed above (Afifiet al., 1988; Simon and Essene, 1996; Simon et al., 1997; Voudouriset al., 2011), consistent with the EMPA results in this study.

5.3. Potential mineral predictions in ores

Because of isomorphic substitution of selenium for sulfur in sul-fides, independent selenide will not form under high sulfur fugacity.

In the breccia ores, except for Fe–Te compounds, Sb2Te3, SnTe,FeSe1.333 and Sn–Se compounds, themajority of tellurides and selenidesare stable in logfO2(g) range. For frohbergite (FeTe2) (num. 19 inTable 2), we propose that some other metallic elements, such as Au,

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Ag, Cu and Zn, were also incorporated, which probably changed thethermodynamic property of Fe–Te system, maintaining stability in thislogfO2(g) range, although the precise change remains unknown. Asconstrained by logfSe2(g), NiTe2 (melonite) turns to be the only stablecompound in theNi–Te system in this study. The Cu–Te system is unsta-ble only if some other metallic elements are incorporated as discussedpreviously. As and Bi can be complexed by tellurium and selenium inthe form of As2Te3 and Bi2Te3, and As2Se3 and Bi2Se3, respectively, butas shown in Fig. 10, Bi2Se3 and As2Se3 should be the predominantphases.

6. Conclusions

The fugacities of S2(g), O2(g), Te2(g) and Se2(g) have played sig-nificant roles on the different gold mineralization types in theGuilaizhuang deposit. The volcanic vapor phase containing significantamounts of S, Se, As, Sb and Au, in which gold was complexed by Sand Se in the form of [Au(HS,HSe)2]−, led to sulfidization of the near-by limestone along the main fault zone in the deposit. During thissulfidization process, gold complex reacted with the iron in lime-stone, liberating nano-particles of gold in the crystal lattices of pyrite,As-pyrite and arsenopyrite. As the sulfur-rich vapor phase was

Appendix 1The standard Gibbs free energy of formation ΔfGT = 523.15 K

o (kJ/mol) of compounds used in tnot listed in this table).

Component Mineral species ΔfGT =o

S2(g) 4Se2(g) 5Te2(g) 7O2(g)Ag2S Argentite −4AsS Realgar −3As2S3 Orpiment −8Cu2S Chalcocite −9CuS Covellite −5CuFeS2 Chalcopyrite (Cp) −19Cu5FeS4 Bornite (Bn) −40FeS Troilite −10Fe0.89S Pyrrhotite (Po) −10FeS2 Pyrite (Py) −14FeAs2 Loellingite (Lo) −5FeAsS Arsenopyrite (Apy) −11HgS Cinnabar −3MoS2 Molybdenite −25PbS Galena −9Sb2S3 Stibnite −14SnS Herzenbergite −10Sn2S3 Ottemannite −24SnS2 Vaesite −13ZnS Sphalerite −19Ag2Te Hessite −4Ag1.64Te −4As2Te3 −4AuTe2 Calaverite −1Bi2Te3 Tellurobismuthite −7Cu2Te Weissite −4Cu4Te3 Rickardite −9CuTe Vulcanite −2FeTe0.9 −3FeTe2 Frohbergite −5HgTe Coloradoite −2MoTe2 −9NiTe1.1 −5NiTe2 Mellonite −8PbTe Altaite −6Sb2Te3 Tellurantimony −6SnTe −6ZnTe −11Ag2Se Naumannite −5

Appendix A

exhausted, tellurium-rich fluids from the underlying magmatic cham-ber moved into the fault and cemented the breccias. Oxygen fugacitywas higher enough to oxidize the residual sulfides into sulfate min-erals such as barite and anglesite. Gold was complexed by telluriumin the form of Au–Ag tellurides, probably sylvanite (AuAgTe4) andpetzite (AuAg3Te2). As temperature decreased, the sylvanite andpetzite became unstable, and decomposed into native gold, hessiteand more stable Au–Ag tellurides at 250 °C. Calaverite, native silverand tellurium found in the deposit are considered to be the supergenealteration products of Au–Ag tellurides decomposed from sylvaniteand petzite. However, minor calaverite is not excluded which precip-itated prior to the main gold mineralization stage under relativelyhigh tellurium fugacity.

Acknowledgments

We are grateful to Qian Mao and Yu-Guang Ma for their help duringEMPA and BSE analyses. Dr. P. Voudouris and an anonymous referee arethanked for their constructive and valuable comments which greatlycontributed to the improvements of the manuscript. This study wasfinancially supported by the Natural Science Foundation of China(41172083) and 100 Talents Program of Chinese Academy of Sciences.

his study (the ΔfGT = 523.15 Ko of simple substance in solid form is defined to be zero and

523.15 K References

5.64 Barin (1989)6.59 Barin (1989)7.93 Barin (1989)06.44 Barin (1989)2.58 Craig and Barton (1973)3.52 Barin (1989); O'Hare (1993)1.77 Calculated2.70 Calculated3.41 Robie and Hemingway (1995); Robie et al. (1994)9.31 Robie and Hemingway (1995); Robie et al. (1994)3.67 Grønvold and Stølen (1992)0.10 Grønvold and Stølen (1992)8.84 Robie and Hemingway (1995)8.84 Barton and Skinner (1979)0.95 Barton and Skinner (1979)8.82 Barin (1989); Mills (1974)3.47 Robie and Hemingway (1995)4.64 Barin (1989)5.47 Robie and Hemingway (1995); Seal et al. (1992)3.19 Barin (1989); Mills (1974)1.67 Barin (1989); Mills (1974)6.27 Barin (1989); Mills (1974)5.00 Robie and Hemingway (1995)8.10 Barin (1989); Mills (1974)0.65 Afifi et al. (1988)1.43 Afifi et al. (1988)6.02 Afifi et al. (1988); Mills (1974)5.89 Barin (1989)0.69 Afifi et al. (1988)5.90 Afifi et al. (1988)5.91 Afifi et al. (1988)0.40 Mills (1974); Shukla et al. (1990)8.40 Afifi et al. (1988); Mills (1974)5.34 Mills (1974); Nasar and Shamsuddin (1990)1.25 Mallika and Sreedharan (1988)7.18 Afifi et al. (1988)1.24 Afifi et al. (1988)6.16 Afifi et al. (1988); Mills (1974)0.31 Afifi et al. (1988); Mills (1974)1.03 Afifi et al. (1988); Mills (1974)2.12 Mills (1974); Nasar and Shamsuddin (1990)2.81 Barin (1989)

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Appendix 1 (continued)

Component Mineral species ΔfGT = 523.15 Ko References

AsSe −36.13 Mills (1974)As2Se3 Laphamite −96.18 O'Hare et al. (1990)AuSe −6.06 Barin (1989); Mills (1974)Bi2Se3 Guanjuatite −137.66 Barin (1989); Mills (1974)Cu2Se Berzelianite −78.64 Barin (1989); Mills (1974)CuSe Klockmannite −42.66 Mills (1974)CuSe2 Krutaite −43.90 Mills (1974)FeSe0.961 −67.69 Mills (1974)FeSe1.333 −75.68 Mills (1974)FeSe2 Ferroselite (Fo) −90.51 Mills (1974)HgSe Tiemannite −34.08 Barin (1989); Mills (1974)MoSe2 Drysdallite −217.78 O'Hare et al. (1987)Ni3Se2 −170.98 CalculatedNiSe1.05 −74.84 Barin (1989); Mills (1974)NiSe2 −102.06 Barin (1989); Mills (1974)PbSe Clausthalite −96.99 Barin (1989); Mills (1974)Sb2Se3 Antimonselite −123.07 Barin (1989); Mills (1974)SnSe −85.70 Barin (1989); Mills (1974)SnSe2 −103.10 Barin (1989); Mills (1974)ZnSe Stilleite −151.45 Barin (1989); Mills (1974)As2O3 −476.98 CalculatedBaO −505.96 CalculatedBaSO4 Barite −1268.40 CalculatedBi2O3 Bismite −433.71 Barin (1989)CuO Tenorite −106.04 CalculatedCu2O Cuprite −123.87 CalculatedFe2O3 Hematite −679.63 CalculatedFe3O44 Magnetite −936.45 CalculatedHgO Montroydite −34.50 Barin (1989); Mills (1974)NiO Bunsenite −190.53 CalculatedPbO Litharge −166.72 CalculatedPbSO4 Anglesite −730.28 CalculatedSb2O3 Valentinite −565.02 Barin (1989)SnO2 Cassiterite −469.05 Robie and Hemingway (1995)ZnO Zincite −298.04 CalculatedZnSO4 −790.74 Calculated

Appendix 2Equilibrium constant (lnK) and gaseous substance fugacity (logf) of the reactions in each mineral pair at 523.15 K.

Mineral pair Reactions lnK Equilibrium fugacity

Univariate systemSulfides LogfS2(g)

Cp–Bn + Po 400/89Po + Bn + (3–200/89)S2(g) = 5Cp 32.70 −18.9Apy + AsLo 2FeAs2 + S2(g) = 2FeAsS + 2As 34.47 −14.9Sb2S3–Sb 4/3Sb + S2(g) = 2/3Sb2S3 32.79 −14.2Bi2S3–Bi 4/3Bi + S2(g) = 2/3Bi2S3 31.13 −13.5Ag2S–Ag 4Ag + S2(g) = 2Ag2S 31.85 −13.8Po–Fe 0.89Fe + 0.5S2(g) = Po 28.26 −24.5Py–Po Po + 0.39S2(g) = 0.89Py 11.53 −12.8Cu2S–Cu 2Cu + 0.5S2(g) = Cu2S 26.35 −22.9CuS–Cu2S Cu + 0.5S2(g) = CuS 17.36 −15.1HgS–Hg(l) 2Hg(l) + S2(g) = 2HgS 28.34 −12.3Apy + As–AsS + Py FeAsS + As + 1.5S2(g) = 2AsS + FeS2 39.43 −11.4As2S3–AsS 2AsS + 0.5S2(g) = As2S3 9.47 −8.2AsS–As As + 0.5S2(g) = AsS 12.74 −11.1Bn + Py–Cp 5CuFeS2 + 7S2(g) = Cu5FeS4 + 5FeS2 116.32 −7.2S(l)–S2(g) S2(g) = 2S(l) 10.49 −4.56

Tellurides LogfTe2(g)SnTe–Sn 2Sn + Te2(g) = 2SnTe 45.98 −20.0ZnTe–Zn Zn + 0.5Te2(g) = ZnTe 34.74 −30.2PbTe–Pb Pb + 0.5Te2(g) = PbTe 24.17 −21.0Sb2Te3–Sb 4/3Sb + Te2(g) = 2/3Sb2Te3 27.16 −11.8Bi2Te3–Bi 4/3Bi + Te2(g) = 2/3Bi2Te3 29.55 −12.8As2Te3–As 4/3As + Te2(g) = 2/3As2Te3 24.27 −10.53Cu2Te–Cu 2Cu + 0.5Te2(g) = Cu2Te 19.05 −16.5Cu2Te–Cu4Te3 2Cu2Te + 0.5Te2(g) = Cu4Te3 12.30 −10.7Cu4Te3–CuTe Cu4Te3 + 0.5Te2(g) = 4CuTe 10.74 −9.3NiTe2–NiTe1.1 NiTe1.1 + 0.45Te2(g) = NiTe2 13.59 −13.2NiTe1.1–Ni Ni + 0.55Te2(g) = NiTe1.1 23.00 −18.2FeTe2–FeTe0.9 FeTe0.9 + 0.55Te2(g) = FeTe2 16.29 −12.9FeTe0.9–Fe Fe + 0.45Te2(g) = FeTe0.9 15.05 −14.5As2Te3–As 4/3As + Te2(g) = 2/3As2Te3 24.27 −10.5Ag2Te–Ag 4Ag + Te2(g) = 2Ag2Te 40.03 −17.4Ag1.64Te–Ag2Te 0.82Ag2Te + 0.09Te2(g) = Ag1.64Te 1.89 −9.1AuTe2–Au Ag + Te2(g) = AuTe2 21.60 −9.4

288 W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

Appendix 2 (continued)

Mineral pair Reactions lnK Equilibrium fugacity

HgTe–Hg(l) 2Hg(l) + Te2(g) = 2HgTe 29.57 −12.8Te2(g)–Te(s) Te2(g) = 2Te(s) 17.92 −7.8

Selenides LogfSe2(g)CuSe–Cu2Se Cu2Se + 0.5Se2(g) = 2CuSe 8.04 −7.0Cu2Se–Cu 2Cu + 0.5Se2(g) = Cu2Se 24.59 −21.4AuSe–Au 2Au + Se2(g) = 2AuSe 15.80 −6.9FeSe2–FeSe1.333 FeSe1.333 + 0.3335Se2(g) = FeSe2 7.75 −10.1FeSe1.333 + As2Se3–Lo Lo + 2.167Se2(g) = FeSe1.333 + As2Se3 54.18 −10.9FeSe1.333–Fe Fe + 0.667Se2(g) = FeSe1.333 26.08 −17.0HgSe–Hg 2Hg + Se2(g) = 2HgSe 28.68 −12.4Sb2Se3–Sb 4/3Sb + Se2(g) = 2/3Sb2Se3 31.87 −13.8As2Se3–As 2As + 1.5Se2(g) = As2Se3 41.63 −12.1Ag2Se–Ag 4Ag + Se2(g) = 2Ag2Se 37.29 −16.2SnSe2–SnSe SnSe + 1/2Se2(g) = SnSe2 10.51 −9.1SnSe–Sn Sn + 1/2Se2(g) = SnSe 26.21 −22.8Bi2Se3–Bi 4/3Bi + Se2(g) = 2/3Bi2Se3 34.11 −14.8NiSe2–NiSe1.05 NiSe1.05 + 0.475Se2(g) = NiSe2 12.44 −11.4NiSe1.05–Ni3Se2 Ni3Se2 + 0.575Se2(g) = 3NiSe1.05 19.79 −14.9Ni3Se2–Ni 3Ni + Se2(g) = Ni3Se2 52.32 −22.7ZnSe–Zn Zn + 0.5Se2(g) = ZnSe 41.32 −35.9PbSe–Pb Pb + 0.5Se2(g) = PbSe 31.26 −27.1Se2(g)–Se(l) Se2(g) = 2Se(l) 13.01 −5.6

Oxides LogfO2(g)NiO–Ni Ni + 0.5O2(g) = NiO 43.81 −38.0SnO2–Sn Sn + O2(g) = SnO2 114.05 −49.5HgO–Hg Hg + 0.5O2 = HgO 7.93 −6.89Cu2O–Cu 2Cu + 0.5O2(g) = Cu2O 28.48 −24.7CuO–Cu2O Cu2O + 0.5O2(g) = 2CuO 21.05 −18.3PbO–Pb Pb + 1/2O2(g) = PbO 38.32 −33.3ZnO–Zn Zn + 1/2O2(g) = ZnO 68.52 −59.5Bi2O3–Bi 4/3Bi + O2(g) = 2/3Bi2O3 66.48 −28.9Sb2O3–Sb 4/3Sb + O2(g) = 2/3Sb2O3 86.60 −37.6As2O3–As 4/3As + O2(g) = 2/3As2O3 73.11 −31.7Fe2O3–Fe3O4 2Fe3O4 + 0.5O2(g) = 3Fe2O3 40.80 −35.4

Mineral pair Reactions lnK Equilibrium fugacity

Bivariate systemSulfides–Selenides logfSe2(g) = alogfS2(g) + b

a bCu2Se + Po–Bn 2.225Cu2Se + Po + 1.78S2(g) = 0.89Bn + 1.1125Se2(g) 19.48 1.15 7.60Cu2Se + Po–Cp 0.445Cu2Se + Po + 0.39S2(g) = 0.89Cp + 0.2225Se2(g) 9.71 1.75 18.96Cu2Se + FeSe2–Cp 2FeSe2 + Cu2Se + 2S2(g) = 2Cp + 2.5Se2(g) 17.69 0.80 3.07Sb2Se3–Sb2S3 Sb2Se3 + 1.5S2(g) = Sb2S3 + 1.5Se2(g) 1.37 1.00 0.40HgSe–HgS HgSe + 0.5S2(g) = HgS + 0.5Se2(g) −0.17 1.00 −0.15As2Se3–As2S3 As2Se3 + 1.5S2(g) = As2S3 + 1.5Se2(g) −6.69 1.00 −1.94FeSe1.333 + As2Se3–Apy + As 0.5As2Se3 + FeSe1.333 + 0.5S2(g) = Apy + As + 1.417Se2(g) −27.19 0.35 −8.33As2Se3–AsS As2Se3 + S2(g) = 2AsS + 1.5Se2(g) −16.15 0.67 −4.68FeSe1.333–Po 0.89FeSe1.333 + 0.5S2(g) = Po + 0.59Se2(g) 5.10 0.85 3.75FeSe2–Py FeSe2 + S2(g) = Py + Se2(g) 10.89 1.00 4.73Bi2Se3–Bi2S3 Bi2Se3 + 1.5S2(g) = Bi2S3 + 1.5Se2(g) −4.46 1.00 −1.29Ag2Se–Ag2S Ag2Se + 0.5S2(g) = Ag2S + 0.5Se2(g) −2.72 1.00 −2.37PbSe–PbS PbSe + 0.5S2(g) = PbS + 0.5Se2(g) −1.80 1.00 −1.56

Tellurides–Selenides LogfTe2(g) = alogfSe2(g) + bAg2Te–Ag2Se Ag2Te + 0.5Se2(g) = Ag2Se + 0.5Te2(g) −1.37 1.00 −1.19Ag1.64Te–Ag2Se Ag1.64Te + 0.41Se2(g) = 0.82Ag2Se + 0.5Te2(g) −3.01 0.82 −2.62HgTe–HgSe HgTe + 0.5Se2(g) = HgSe + 0.5Te2(g) −0.44 1.00 −0.39AuTe2–AuSe AuTe2 + 0.5Se2(g) = AuSe + Te2(g) −13.70 0.50 −5.95Cu2Te–Cu2Se Cu2Te + 0.5Se2(g) = Cu2Se + 0.5Te2(g) 6.27 1.00 5.45Cu4Te3–Cu2Se Cu4Te3 + Se2(g) = 2 Cu2Se + 1.5Te2(g) 0.25 0.67 0.07CuTe–Cu2Se 2CuTe + 0.5Se2(g) = Cu2Se + Te2(g) −5.25 0.50 −2.3FeTe0.9–FeSe1.333 FeTe0.9 + 0.667Se2(g) = FeSe1.333 + 0.45Te2(g) 11.03 1.48 10.64FeTe2–FeSe1.333 FeTe2 + 0.667Se2(g) = FeSe1.333 + Te2(g) −5.27 0.67 −2.29FeTe2–FeSe2 FeTe2 + Se2(g) = FeSe2 + Te2(g) 2.48 1.00 1.08SnTe–SnSe SnTe + 0.5Se2(g) = SnSe + 0.5Te2(g) 3.22 1.00 2.80NiTe1.1–Ni3Se2 3NiTe1.1 + Se2(g) = Ni3Se2 + 1.65Te2(g) −16.68 0.61 −4.39NiTe1.1–NiSe1.05 NiTe1.1 + 0.525Se2(g) = NiSe1.05 + 0.55Te2(g) 1.04 0.95 0.82NiTe2–NiSe1.05 NiTe2 + 0.525Se2(g) = NiSe1.05 + Te2(g) −12.56 0.525 −5.45NiTe2–NiSe2 NiTe2 + Se2(g) = NiSe2 + Te2(g) −0.12 1.00 −0.05Bi2Te3–Bi2Se3 Bi2Te3 + 1.5Se2(g) = Bi2Se3 + 1.5Te2(g) 6.84 1.00 1.98Sb2Te3–Sb2Se3 Sb2Te3 + 1.5Se2(g) = Sb2Se3 + 1.5Te2(g) 7.07 1.00 2.05As2Te3–As2Se3 As2Te3 + 1.5Se2(g) = As2Se3 + 1.5Te2(g) 5.23 1.00 1.51ZnTe–ZnSe ZnTe + 0.5Se2(g) = ZnSe + 0.5Te2(g) 6.59 1.00 5.72PbTe–PbSe PbTe + 0.5Se2(g) = PbSe + 0.5Te2(g) 4.64 1.00 4.03

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Appendix 2 (continued)

Mineral pair Reactions lnK Equilibrium fugacity

Oxides–Selenides LogfSe2(g) = alogfO2(g) + bZnO–ZnSe ZnSe + 0.5O2(g) = ZnO + 0.5Se2(g) 27.20 1.00 23.62HgO–HgSe HgSe + 0.5O2(g) = HgO + 0.5Se2(g) −6.41 1.00 −5.57SnO2–SnSe SnSe + O2(g) = SnO2 + 0.5Se2(g) 81.63 2.00 70.89SnO2–SnSe2 SnSe2 + O2(g) = SnO2 + Se2(g) 71.15 1.00 30.89NiO–Ni3Se2 Ni3Se2 + 1.5O2(g) = 3NiO + Se2(g) 79.09 1.50 34.34NiO–NiSe1.05 NiSe1.05 + 0.5O2(g) = NiO + 0.525Se2(g) 19.77 0.95 16.35NiO–NiSe2 NiSe2 + 0.5O2(g) = NiO + Se2(g) 7.33 0.50 3.18Sb2O3–Sb2Se3 Sb2Se3 + 1.5O2(g) = Sb2O3 + 1.5Se2(g) 82.09 1.00 23.76Bi2O3–Bi2Se3 Bi2Se3 + 1.5O2(g) = Bi2O3 + 1.5Se2(g) 48.55 1.00 14.05As2O3–As2Se3 As2Se3 + 1.5O2(g) = As2O3 + 1.5Se2(g) 68.06 1.00 19.70PbO–PbSe PbSe + 0.5O2(g) = PbO + 0.5Se2(g) 9.52 1.00 8.27Cu2O–Cu2Se Cu2Se + 0.5O2(g) = Cu2O + 0.5Se2(g) 3.89 1.00 3.38Fe3O4–FeSe1.333 3FeSe1.333 + 2O2(g) = Fe3O4 + 1.99Se2(g) 137.33 1.01 29.97Fe3O4–FeSe2 3FeSe2 + 2O2(g) = Fe3O4 + 3Se2(g) 113.96 0.67 16.50Fe2O3–FeSe2 2FeSe2 + 1.5O2(g) = Fe2O3 + 2Se2(g) 89.58 0.75 19.45

Oxides–Tellurides LogfTe2(g) = alogfO2(g) + bZnO–ZnTe ZnTe + 0.5O2(g) = ZnO + 0.5Te2(g) 33.79 1.00 29.34HgO–HgTe HgTe + 0.5O2(g) = HgO + 0.5Te2(g) −6.85 1.00 −5.95SnO2–SnTe SnTe + O2(g) = SnO2 + 0.5Te2(g) 84.85 2.00 73.69PbO–PbTe PbTe + 0.5O2(g) = PbO + 0.5Te2(g) 14.15 1.00 12.29NiO–NiTe1.1 NiTe1.1 + 0.5O2(g) = NiO + 0.55Te2(g) 20.80 0.91 16.42NiO–NiTe2 NiTe2 + 0.5O2(g) = NiO + Te2(g) 7.21 0.50 3.13Sb2O3–Sb2Te3 Sb2Te3 + 1.5O2(g) = Sb2O3 + 1.5Te2(g) 89.16 1.00 25.81Bi2O3–Bi2Te3 Bi2Te3 + 1.5O2(g) = Bi2O3 + 1.5Te2(g) 55.39 1.00 16.03As2O3–As2Te3 As2Te3 + 1.5O2(g) = As2O3 + 1.5Te2(g) 73.26 1.00 21.21Cu2O–Cu2Te Cu2Te + 0.5O2(g) = Cu2O + 0.5Te2(g) 10.17 1.00 8.83Fe3O4–FeTe0.9 3FeTe0.9 + 2O2(g) = Fe3O4 + 1.35Te2(g) 170.27 1.48 54.77Fe3O4–FeTe2 3FeTe2 + 2O2(g) = Fe3O4 + 3Te2(g) 121.39 0.67 17.57Fe2O3–FeTe2 2FeTe2 + 1.5O2(g) = Fe2O3 + 2Te2(g) 94.53 0.75 20.52

Appendix 2 (continued)

290 W.-G. Xu et al. / Ore Geology Reviews 56 (2014) 276–291

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