Fluid speciation controls of low-temperature copper isotope fractionation

$: Fluid speciation controls of low-temperature copper isotope fractionation$
Chemical Geology xxx (2009) xxx–xxx

CHEMGE-15646; No of Pages 12

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Chemical Geology

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Fluid speciation controls of low temperature copper isotope fractionation applied tothe Kupferschiefer and Timna ore deposits

Dan Asael a,c,⁎, Alan Matthews a, Slawomir Oszczepalski b, Miryam Bar-Matthews c, Ludwik Halicz c

a Institute of Earth Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israelb Polish Geological Institute, Rakowiecka 4, 00-975 Warsaw, Polandc Geological Survey of Israel, 30 Malchey Israel St., 95501 Jerusalem, Israel

⁎ Corresponding author. Institute of Earth Sciences, H91904 Jerusalem, Israel.

E-mail address: [email protected] (D. Asael).

0009-2541/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2009.01.015

Please cite this article as: Asael, D., et aKupferschiefer and Timna ore deposits, Ch

a b s t r a c t
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Article history:
Mass-balance among reduce Received 2 June 2008Received in revised form 14 November 2008Accepted 16 January 2009Available online xxxx
Editor: B. Bourdon

Keywords:Copper isotopesRedox isotopic fractionationTimnaKupferschieferFluid speciationSediment-hosted Stratiform Copper deposits

d and oxidized solution species exerts a strong control on the isotopic composition ofsulphide minerals. We explore this issue for copper isotopic fractionation in sedimentary copper sulfide depositsthrough fluid speciation-isotopic Eh–pH calculations based on the thermodynamic stability of copper chlorideaqueous solution complexes and experimentally measured copper isotope fractionation factors. Applying thesespeciation diagrams toMC-ICP-MS copper isotope data on the Kupferschiefer (SWpart of the Lubin-SieroszowiceCopper District) and Timna (S. Israel) Sediment-hosted Stratiform Copper deposits (SSC), identifies differencesin ore formation redox conditions. Timna Valley copper sulfides have light Cu-isotopic compositionsofδ65Cu=−2.04±0.44‰ 2σ (relatively to SRM 976 copper standard), which are shown to correspond to Ehvalues of 0.5 to 0.6 V at formation conditions (T=40 °C; pH b6). These Eh values indicate precipitation atrelatively oxidized conditions where the Cu(I) solution complex (CuCl32−) is b10% of the total solutionspecies. In contrast, the Kupferschiefer Cu-sulfides analyzed in this work dominantly show significantlyhigher δ65Cu values=−0.39±0.36‰, corresponding to Eh values of 0.4 to 0.5 V at formation conditions(T=100 °C; pH=6.3). In these Eh conditions, most copper in solution occurs as CuCl32− complexes (~80%).The above observations are in accord with field relations showing that Cu(II) minerals dominate the Timnasystem, but Cu-sulfides are the major minerals of the Kupferschiefer deposits.Fluid speciation–isotopic calculations show that copper isotopes can be used as effective tracers of redoxconditions and this approach can be potentially applied to various hydrothermal ore deposits, such as blacksmokers and volcanic-hosted massive sulphide deposits, and to other metallic isotope systems. However,the isotopic fractionationmay also be strongly influenced by the types of ligand bonding of copper ions andthese effects need to be fully evaluated before the isotope geochemistry of copper ores can be fullyunderstood.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

One of the major controls of stable isotopic fractionation in hy-drothermal systems is complexation and speciation in solution. Theclassic study of Ohmoto (1972) on the sulfur isotope system wasamong the first to demonstrate that mass-balance among reducedand oxidized solution species exerts a strong control on the isotopiccomposition of hydrothermal sulphide ore minerals. Recent experi-mental studies on copper isotope fractionation also suggest that fluidspeciation among reduced and oxidized species can be an importantcontrol of the Cu-isotopic composition of copper sulphides and Cu(II)minerals (Marechal and Sheppard, 2002; Ehrlich et al., 2004; Mathuret al., 2005; Asael et al., 2006). Redox fractionations of copper

ebrew University of Jerusalem,

l rights reserved.

l., Fluid speciation controlsemical Geology (2009), doi:1

isotopes have been demonstrated in a number of important geo-logical environments including: Sediment-hosted Stratiform Copperdeposits (SSC), supergene enrichment deposits, black smokers andporphyry copper deposits (Larson et al., 2003; Graham et al., 2004;Rouxel et al., 2004;Mathur et al., 2005;Markl et al., 2006; Asael et al.,2007).

We report here the results of a new copper isotope study of thecopper sulphideminerals from the Kupferschiefer deposit in the Lubin–Sieroszowice region of SW Poland. This study shows how the isotopicsystematics of the Kupferschiefer and the Cambrian ores of TimnaValley(southern Israel), previously studied by Asael et al. (2007), can beinterpreted in terms of copper isotope–fluid speciation diagrams.Although both the Kupferschiefer and Timna are SSC deposits, a majordifference between the two deposits is that the Kupferschiefer copperis dominated by Cu-sulphideminerals and the Timna is dominated byCu(II) minerals. Additionally, whilst the highly redox cycled coppersystem in the Timna Valley provides a natural laboratory for studying

of low temperature copper isotope fractionation applied to the0.1016/j.chemgeo.2009.01.015

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Fig. 1. Location of the Kupferschiefer deposits within the Lubin-Sieroszowice MiningDistrict (south-western Poland) from which the samples of this study were taken.

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the systematics of copper isotope fractionation, in terms of SSC deposits,it is a marginal sulphide system compared to the economicallysupergiant copper sulphide deposits of the Kupferschiefer in SWPolandor the Central African Copperbelt (Robb, 2005). Thus, the question ofhow such supergiant SSC systems fractionate Cu isotopes on a regionalscale arises. In this study fluid speciation diagrams are shown to providean important key in the interpretation of the thermochemicaldifferences between the two deposits and in achieving greater under-standing of the thermo-chemical processes involving the formation ofsedimentary and hydrothermal copper ore systems.

Our previous study of the copper isotope fractionation in thePrecambrian–Cambrian rocks at the Timna Valley of southern Israel(Asael et al., 2007) showed that large isotopic fractionationsaccompany redox processes. Isotopic mass balance was modeledusing the inorganic experimental Cu-isotope fractionation factors(Marechal and Sheppard, 2002; Ehrlich et al., 2004; Asael et al., 2006).The experimental studies show that similar isotopic fractionations(Δ65Cu(Cu(II)aq-Cu-sulphide)=2.5 to 3‰) occur when copper sul-phides form by direct precipitation during the mixing of Cu(II) so-lutions with sulphide solutions or when Cu(II) solutions react withiron sulphides in anoxic conditions. These contrasting mechanismssuggests that isotopic fractionation takes place during the Cu(II)aq ⇨Cu(I)aq transformation, as previously inferred by (Luther et al., 2002;Ehrlich et al., 2004).

The isotopic fractionation during redox reactions in solution iscritically dependent on the changes of ionic speciation in solution as afunction of Eh–pH conditions, as originally shown by Ohmoto (1972)for H2S–SO4 speciation in the hydrothermal sulphur isotope system.Transport of copper in sedimentary and hydrothermal sulphide oredeposits dominantly occurs as copper chloride solution complexes(Rose, 1976; Robb, 2005). The mass balance fluid speciation controls ofreduced and oxidized copper chloride complexes are applied here forthe first time to copper isotope partitioning.

2. Geological and geochemical background

2.1. General features of SSC deposits

The characteristic geological setting of SSC deposits featuresigneous basement Cu-bearing source rock (copper porphyry) with atypical δ65Cu value of around zero permil, clastic sediments derivedby weathering and erosion of the igneous source, and carbonate/shalestrata which host the copper deposit. Circulating fluids elute copperfrom detrital copper minerals in the clastic sediments under neutralmildly oxidizing conditions and ascend ormove laterally into reducing(typically pyriteferous and organic matter-rich) sediments to pre-cipitate a series of copper minerals. The minerals precipitated at theredox front are predominantly Cu-sulphides (covellite (CuS), chalco-cite (Cu2S), chalcopyrite (CuFeS2), and bornite (Cu5FeS4)), dependingon the concentration of ions in solution and Eh–pH conditions (Brown,1992; Wodzicki and Piestrzynski, 1994; Brown, 1997; Robb, 2005).Subsequent re-oxidation of the sulphides may lead to the formationof Cu(II) minerals such as malachite (Cu2(CO3)(OH)2), paratacamite,((Cu,Zn)2(OH)3Cl), brochantite (Cu4(SO4)(OH)6) and chrysocolla (Cu,Al)2H2Si2O5(OH)4·n(H2O).

2.2. Kupferschiefer geological background

The Kupferschiefer copper deposit of SW Poland (Fig. 1) is a classicSSC and one of the world largest deposits. A stratigraphic column ofthe copper bearing sequence is shown in Fig. 2a. The copper min-eralization is hosted by the first marine sedimentary layer of thePermian transgression of the Zechstein Sea in central Europe. Thismarine sequence lies on the top of Lower Permian (Rotliegendes) redbeds thatwere deposited on a truncated series of the Early Rotliegendeslava flows and pyroclastics, and Carboniferous metasedimentary rocks.

Please cite this article as: Asael, D., et al., Fluid speciation controlsKupferschiefer and Timna ore deposits, Chemical Geology (2009), doi:1

The Kupferschiefer Formation is represented by organic matter-richcalcareous-dolomitic shale with high concentrations of Cu, Ag, Fe, ZnandPb. The ultimate sourceof thesemetals is in the underlying red beds,which were originally derived from the igneous and metamorphicbasement rocks. The Kupferschiefer system is characterized by metalzoning, schematically illustrated in the cross section presented in Fig. 2b(Oszczepalski, 1999). At the contact of the Rotliegendes rocks and theKupferschiefer series, at the point where the metalliferous solutionsentered the Kupferschiefer Formation, black shales were post-deposi-tionally altered to form a zone of oxidizing alteration, referred to as RoteFäule. The metal-rich fluids moved laterally from this oxidized zone intothe reducing environment of the adjacent rocks,where Fe-sulphideswerereplaced by Cu-sulphides (Cu zone) and successively precipitated galena(Pb zone), sphalerite (Zn zone) and pyrite/marcasite to form mineralzonation.

The samples studiedwere taken from several coppermining areas atdifferent distances from the ore fluid source in the oxidized zone. Thestudied transect (SW part of the Lubin–Sieroszowice Copper District)covers a small but very important fragment of the mineralizationsystem, which spreads from Sudetes in the south to Poznan in centralPoland (Fig. 1). Samples categorized as Rote Fäule zone are situatedon or closest to source area, Cu zone samples are found at ca. 15 to50 km from the source, and Pb–Zn zone samples are the farthest, atca. 40–80 km away from the source area. The samples encompass avariety of host rocks (the Kupferschiefer shales, the Weissliegendessandstones and carbonates of the Zechstein Limestone), ore zones(Rote Fäule, Cu and Pb–Zn) and ore types (primary disseminatedshale ores, secondary veinlets in shales) (Figs. 1, 2 and Table 1).

Sulphur isotope compositions define 5 groups of Cu-sulphideassociation: (1) rhythmites in the Weissliegendes δ34S~−40‰; (2) dis-seminated sulphides δ34S~−33‰; (3) veinlets δ34S~−26‰; (4) massivecementations δ34S~−18.1‰; (5) veins δ34S~−14‰ (Wodzicki andPiestrzynski, 1994). Higher values in the latest sulphides (assumingthat disseminated sulphides predated cements and veinlets) wereassumed to indicate a closed bacterial sulphide reduction system(Wedepohl, 1971; Haranczyk, 1986; Sawlowicz, 1989; SawlowiczandWedepohl, 1992). However, Jowett et al. (1991) and Oszczepalskiet al. (2002) suggest that the above trend indicates an additionalsupply of isotopically heavier sulphur from extrinsic sources, thermaldecomposition of organics, or as a result of thermal sulphide re-duction. Wodzicki and Piestrzynski (1994) argue for sulphatesfrom overlying Lower Anhydrite and other evaporites; δ34S=+9.5‰as a source of sulphur supplied via descending solutions. The



Fig. 2. a. Stratigraphic column of the Kupferschiefer deposit (after Oszczepalski (1999)). The symbol ‘Cu’ indicates the rock sequences that are studied in this work for Cu-isotopegeochemistry; b. Schematic model for formation of the Kupferschiefer deposit, showing the Variscan igneous and metamorphic basement, the clastic Rotliegende sequenceunderlying the Kupferschiefer and the overlying Zechstein evaporites. The movements of the copper bearing solutions into the Kupferschiefer are indicated by arrows. Metal zoningreflects the directions of fluid flow (after Oszczepalski and Rydzewski (1997)).

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combination of various sulphur sources and types eliminates a simpleevaluation of the sulphur isotope system in terms of speciationdiagrams.

Maximum palaeotemperatures estimated from vitrinite Ro measure-ments decline from 95–135 °C in the oxidized shales, through 60–126 °Cin Cu-bearing rocks, decreasing to 65–100 °C in Pb–Zn-bearing shales(Oszczepalski et al., 2002; Speczik et al., 2007). These data suggest amaximum palaeotemperature of around 135 °C in the Rote Fäule feederareas. Based on a normal heat flow, the maximum burial temperature ofthe mineralized horizon in the Lubin area would have been in the range60–80 °C, indicating that the higher observed temperatures occurred asthe result of ascendant fluids. High local heat flow during Late Permian-


to-Early Jurassic rifting provided a heat source that influenced thetemperature of the brine and enhanced fluid motion (Blundell et al.,2003).

The mineralogy and location of the samples studied in this workare listed in Table 1 together with sample depths. All samples consistof copper sulphides (covellite, chalcocite, chalocopyrite and bornite)and there is a general trend from a covellite-chalcocite dominatedmineralogy in samples closer to the Rote Fäule in the west to suc-cessive increasing amounts of bornite and chalcopyrite in the moredistal parts in the east. Textural studies show that this mineralogicalsequence forms by progressive reaction of precursor iron sulphideswith Cu-solutions (Oszczepalski, 1999; Fig. 3a).



Table 1δ65Cu (SRM-976) ‰ values of the samples from the Kupferschiefer and Timna deposits

Sample Lithology – ore types Depth[m]

Mining area Mineralogy Zone δ65Cu(SRM-976) Mean

I II

POLK 1 Kupferschiefer – disseminated shale ore 740 Polkowice Bn (Dg+Cv) RF −0.63 −0.72 −0.67POLK 2 Kupferschiefer – disseminated shale ore 740 Polkowice Bn (Dg+Cv) RF −0.29 −0.30 −0.30PZ7/2-1 Kupferschiefer – disseminated shale ore 734 Polkowice Cc RF −0.11 −0.02 −0.07PZ7/3-1 Kupferschiefer – disseminated shale ore 734 Polkowice Bn RF −2.71 −2.76 −2.73S-106/47-1 Kupferschiefer – disseminated shale ore 921 Sieroszowice Cc Cu 0.19 −0.13 0.03S-115/100-1 Kupferschiefer – disseminated shale ore 918 Lubin Bn (Dg+Ccp) Pb Zn −0.39 −0.17 −0.28S-115/100-2 Kupferschiefer – disseminated shale ore 918 Lubin Bn (Dg+Ccp) Pb Zn −0.33 −0.12 −0.23S-146/111-1 Kupferschiefer – disseminated shale ore 744 Polkowice Cc RF −0.45 −0.34 −0.39S 300/78-1 Kupferschiefer – disseminated shale ore 954 Rudna Cc Cu −1.65 −1.80 −1.73S 300/78-2 Kupferschiefer – disseminated shale ore 954 Rudna Cc Cu −1.78 −1.78S-355/54 Kupferschiefer – disseminated shale ore 1128 Rudna Bn+Cc Cu −0.50 −0.47 −0.48S 336/2-1 Kupferschiefer – disseminated shale ore 995 Sieroszowice Cc RF −2.02 −2.07 −2.05S 337/44-1 Kupferschiefer – disseminated shale ore 1025 Sieroszowice Bn+Cc Cu −0.94 −1.53 −1.23S 381/53-1 Kupferschiefer – disseminated shale ore 1149 Gaworzyce Cc RF −1.06 −1.06S 381/53-2 Kupferschiefer – disseminated shale ore 1149 Gaworzyce Cc RF −0.22 −0.22S 81/31-1 Kupferschiefer – disseminated shale ore 882 Sieroszowice Cc RF −2.25 −2.25S 329/4-1 Kupferschiefer – disseminated shale ore 908 Lubin Ccp Pb Zn −0.66 −0.66S 159/95-1 Kupferschiefer – disseminated shale ore 609 Lubin Ccp Cu −2.05 −2.05LW 6/6-1 Kupferschiefer – ore veinlets in shales 565 Lubin Bn+Ccp Cu −0.40 −0.45 −0.42LW 6/6-2 Kupferschiefer – ore veinlets in shales 564 Lubin Bn+Ccp Cu 0.55 0.74 0.65LZ-5/5-1 Kupferschiefer – ore veinlets in shales 711 Lubin Ccp+Bn+Cv Cu −1.05 −0.90 −0.98LZ-5/5-2 Kupferschiefer – ore veinlets in shales 711 Lubin Ccp+Bn +Cv Cu −1.84 −1.64 −1.74PW 1/5-1 Kupferschiefer – ore veinlets in shales 585 Polkowice Cc RF −0.42 −0.45 −0.44PW 1/5-2 Kupferschiefer – ore veinlets in shales 585 Polkowice Cc RF −0.32 −0.29 −0.31S-159/109-1 Kupferschiefer – ore veinlets in shales 610 Lubin Ccp, Bn Cu −0.90 −0.90 −0.9S-159/109-2 Kupferschiefer – ore veinlets in shales 610 Lubin Ccp, Bn Cu −0.53 −0.57 −0.55S-159/109-3 Kupferschiefer – ore veinlets in shales 610 Lubin Cu Cu −0.79 −0.83 −0.81S 205/75-1 Kupferschiefer – ore veinlets in shales 808 Polkowice Cc RF −0.72 −0.79 −0.76S 335/74-1 Kupferschiefer – ore veinlets in shales 1089 Rudna Bn+Cc Cu −2.09 −1.92 −2.00S 335/74-2 Kupferschiefer – ore veinlets in shales 1089 Rudna Bn+Cc Cu −0.29 −0.65 −0.47S-106/48-1 Weissliegendes – disseminated sandstone ore 920 Sieroszowice Cu 0.03 −0.01 0.01S-106/48-2 Weissliegendes – disseminated sandstone ore 921 Sieroszowice Cv Cu 0.06 −0.07 0.00S-106/48-3 Weissliegendes – disseminated sandstone ore 921 Sieroszowice Cv Cu −0.07 0.07 0.00S 206/42-1 Weissliegendes – disseminated sandstone ore 611 Sieroszowice Cv Cu −0.07 0.07 0.00S 262/119-1 Weissliegendes – disseminated sandstone ore 537 Malomicae Bn+Dg+Cc Cu −0.19 −0.33 −0.26S 262/119-2 Weissliegendes – disseminated sandstone ore 537 Malomicae Bn+Dg+Cc Cu −0.34 −0.68 −0.51S 381/55-1 Weissliegendes – disseminated sandstone ore 1149 Gaworzyce Cv RF −1.73 −1.73PZ 15/6-1 Zechstein Limstone – carbonate ore 729 Polkowice Cc RF −0.47 −0.47 −0.47PZ 15/6-1 Zechstein Limstone – carbonate ore 729 Polkowice Cc RF −0.63 −0.68 −0.65S 304/111-1 Zechstein Limstone – carbonate ore 595 Lubin Cc Cu −0.49 −0.49

Sample Stratigraphy Mineralogy δ65Cu(SRM-976) Mean

I II III

⁎CMDO 1 Cambrian Timna Formation Mal+Par −1.22 −1.23 −1.23⁎CMDO 2 Cambrian Timna Formation Mal+Par −1.02 −0.91 −0.96⁎CMDO 3 Cambrian Timna Formation Mal+Par −1.18 −1.22 −1.20⁎CMDO 4 Cambrian Timna Formation Mal+Par −0.56 −0.71 −0.64⁎CMDO 5 Cambrian Timna Formation Mal+Par −1.10 −1.19 −1.14⁎CMDO 6 Cambrian Timna Formation Mal+Par −0.89 −0.89 −0.89⁎CMDO 7 Cambrian Timna Formation Mal+Par −1.16 −1.22 −1.19⁎CMDO 8 Cambrian Timna Formation Mal+Par −1.16 −1.16 −1.16⁎CMDO 9 Cambrian Timna Formation Mal+Par −0.82 −0.66 −0.74⁎CMDO 10 Cambrian Timna Formation Mal+Par −0.21 −0.26 −0.24⁎CMDO 11 Cambrian Timna Formation Mal+Par −0.38 −0.11 −0.25⁎CMDO 15 Cambrian Timna Formation Mal+Par −0.86 −1.06 −0.96⁎CMDO 12 Cambrian Timna Formation Mal+Par −0.64 −0.66 −0.65⁎CMDO 16 Cambrian Timna Formation Mal+Par −0.20 −0.17 −0.19⁎CMDO 17 Cambrian Timna Formation Mal+Par −0.16 −0.03 −0.09⁎CMDO 13 Cambrian Timna Formation Mal+Par −0.56 −0.57 −0.56⁎CMDO 14 Cambrian Timna Formation Mal+Par −0.69 −0.79 −0.74CMDO 19 Cambrian Timna Formation Mal+Par −1.77 −1.50 −1.63CMDO 18 Cambrian Timna Formation Mal+Par −1.75 −1.48 −1.62⁎CMDS 1 Cambrian Timna Formation Cu-sulphides −1.74 −1.73 −1.73⁎CMDS 2 Cambrian Timna Formation Cu-sulphides −3.26 −3.10 −3.18⁎CMDS 3 Cambrian Timna Formation Cu-sulphides −1.39 −1.45 −1.42⁎CMDS 4 Cambrian Timna Formation Cu-sulphides −1.82 −2.00 −1.91⁎CMDS 5 Cambrian Timna Formation Cu-sulphides −1.22 −1.32 −1.27⁎CMDS 6 Cambrian Timna Formation Cu-sulphides −1.24 −1.24⁎CMDS 7 Cambrian Timna Formation Cu-sulphides −2.82 −2.51 −2.66⁎CMDS 8 Cambrian Timna Formation Cu-sulphides −2.80 −2.80⁎CMDS 9 Cambrian Timna Formation Cu-sulphides −2.52 −2.53 −2.81 −2.62⁎CMDS 10 Cambrian Timna Formation Cu-sulphides −2.06 −2.04 −2.05⁎CMDS 11 Cambrian Timna Formation Cu-sulphides −2.20 −2.22 −2.21⁎CMDS 15a Cambrian Timna Formation Cu-sulphides −1.98 −1.90 −1.94


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Please cite this article as: Asael, D., et al., Fluid speciation controls of low temperature copper isotope fractionation applied to theKupferschiefer and Timna ore deposits, Chemical Geology (2009), doi:10.1016/j.chemgeo.2009.01.015


Table 1 (continued)

Sample Stratigraphy Mineralogy δ65Cu(SRM-976) Mean

I II III

⁎CMDS 15b Cambrian Timna Formation Cu-sulphides −1.89 −1.89 −1.89⁎CMDS 15c Cambrian Timna Formation Cu-sulphides −1.85 −1.86 −1.85⁎CMDS 15d Cambrian Timna Formation Cu-sulphides −1.84 −1.82 −1.83⁎CMDS 15e Cambrian Timna Formation Cu-sulphides −1.74 −1.73 −1.73⁎CMDS 12 Cambrian Timna Formation Cu-sulphides −1.83 −1.75 −1.79⁎CMDS 16a Cambrian Timna Formation Cu-sulphides −2.35 −2.27 −2.31⁎CMDS 16b Cambrian Timna Formation Cu-sulphides −1.78 −1.74 −1.76⁎CMDS 16c Cambrian Timna Formation Cu-sulphides −1.58 −1.42 −1.50⁎CMDS 17a Cambrian Timna Formation Cu-sulphides −2.22 −2.26 −2.24⁎CMDS 17b Cambrian Timna Formation Cu-sulphides −2.18 −2.18 −2.18⁎CMDS 17c Cambrian Timna Formation Cu-sulphides −2.07 −2.16 −2.11⁎CMDS 17d Cambrian Timna Formation Cu-sulphides −2.04 −1.92 −1.98⁎CMDS 13 Cambrian Timna Formation Cu-sulphides −2.06 −2.06 −2.06⁎CMDS 14 Cambrian Timna Formation Cu-sulphides −2.22 −2.22 −2.22CMDS 19 Cambrian Timna Formation Cu-sulphides −2.36 −2.15 −2.25CMDS18 Cambrian Timna Formation Cu-sulphides −2.42 −2.26 −2.34CMDOZ 1 Cambrian Timna Formation oxidized Cu-sulphide −2.20 −2.88 −2.54CMDOZ 2 Cambrian Timna Formation oxidized Cu-sulphide −2.01 −1.98 −2.00

Bn – Bornite, Dg – Digenite, Cv – Covellite, Cc – Chalcocite, Ccp – Chalcopyrite, Mal – Malachite, Par – paratacamite, RF – Rote Fäule.⁎ Previously published at Asael et al., 2007.


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2.3. Timna geological background

The Timna Valley deposit is located in a half-crater shaped valley,truncated on its eastern side by the Dead Sea Transform (Fig. 4a). Astratigraphic section of the Precambrian–Cambrian rocks of the TimnaValley is given in Fig. 4b. Intenseweatheringof copper porphyrygranites

Fig. 3. Photographs of hand specimens of the copper-bearing rocks studied. a. Chalcociteband from the Cu zone of the Weissliegendes disseminated sandstone ore; b. Cu-sulphides (djurleite and anilite) with oxidized margins of malachite in the MiddleCambrian Timna Formation dolomite.


and quartz-porphyry dikes of the Timna Intrusive Complex during theLate Precambrian and Early Cambrianprovided the source of copper intothe Timna basin (Wurzburger, 1967; Beyth, 1987). The Timna IntrusiveComplex is unconformably overlain and onlapped by a sequence ofsedimentary Cambrian rocks consisting of the Amudei-ShelomoFormation fluvial sandstones, the Lower Cambrian Timna Formation,and Middle Cambrian continental sandstones of the Shehoret Forma-tion. Copper mineralization predominantly occurs in the sandstones,dolomites and shales of the Sasgon Member of the Timna Formation(Fig. 4b), aswell as in the Shehoret Formation sandstones (Bentor,1956;Wurzburger, 1967; Garfunkel, 1970; Bartura and Wurzburger, 1974;Segev, 1986; Shlomovitch et al., 1999). Cu-sulphides occur in dolomitesas dispersed mm to cm sized spherules (Fig. 3b) and are interpreted tohave formed in reduced pore waters during early diagenesis. The Cu-sulphides were then oxidized as a result of changing Eh–pH conditionsduring diagenesis, to form malachite rims and paratacamite veins(Shlomovitch et al., 1999). The Cu-bearing rock samples studied in thiswork are from the Cambrian dolomites, and consist of spherules of Cu-sulphides, partially or completely altered tomalachite and paratacamite.The samples were taken from surface exposures of the Cambrian units,located at the southern Timna valley.

2.4. Copper isotope fractionation factors

Ehrlich et al. (2004) studied the anoxic precipitation of covellite(CuS) from aqueous 0.01M CuSO4 solution, showing fractionationfactors of Δ65Cu(Cu(II)aq–CuS) varying from 3.47‰ at 2 °C to 2.72‰ at40 °C. This temperature dependence was interpreted as reflectingkinetic effects. However, similar copper isotope fractionation factors(3.02 to 2.66‰) were measured for Cu(I) sulphides (chalcopyrite,covellite, chalcocite) formed by the reaction of pyrite and pyrrhotitecrystals with CuSO4 solution under anoxic conditions at 40 °C and100 °C (Asael et al., 2006). It is noteworthy that these latterexperiments conform to the mechanism described for the formationof copper sulphides in the Kupferschiefer. Where there is noreduction, as during the precipitation of malachite or copperhydroxide from Cu(II) solutions, small fractionations of ~0.2 to 0.3‰are measured (Marechal and Sheppard, 2002; Ehrlich et al., 2004).Fractionations of about 2.5 to 3.5‰ thus characterize redox processesamong copper minerals and solutions at temperatures of up to 100 °C.

An important additional factor influencing fractionations is thetype of Cu complex forming in saline hydrothermal solutions. Seo et al.(2007) used Density Function Transfer theory to calculate the reducedpartition function ratios of different copper solution complexes.



Fig. 4. a. Map showing the location of the Timna Valley; b. Stratigraphic column of the Timna Valley (after Segev (1986)) showing the Precambrian and Cambrian rocks of the TimnaValley. The symbol ‘Cu’ indicates the rock sequences studied for Cu-isotope geochemistry.


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Significant fractionation differences occur between different ligandspecies of Cu(I), where complexes with longer bond length and largercoordination number tend to be more 65Cu depleted. Similarcalculations for Cu(II) species are not available. Thus, an additionalfactor that needs to be considered is type of ligand complex formed,which is in turn dependent on temperature, activity of chloride ion, Ehand pH conditions. For the calculations used in this study involvingcopper chloride complexes, the following conditions were applied;

Kupferschiefer: T=100 °C, activity of chloride ion (aCl−)=1.5 andpH=6.3 (Wodzicki and Piestrzynski, 1994).

Timna: T=40 °C, activity of chloride ion (aCl−)=1, pH=4–6 (Bar-Matthews and Matthews, 1990; Shlomovitch et al., 1999; Sass E.personal communication).

The fractionation between Cu(II)–Cu(I) aqueous solution speciesand the precipitation of Cu-sulphides from the Cu(I) complexes arekey processes in this study. There are conflicting views regarding thevalency of copper in Cu-sulphides, though several recent mineralo-gical studies have inferred that copper only exists as the Cu(I) ion inCu-sulphides (Luther et al., 2002; Goh et al., 2006). However, even ifthe Cu(II) ion is also present in Cu-sulphide phases, this would noteffect modeling in this study, which is based on the proposition thatthe dominant control of isotopic fractionation in the experimentalsystems is the Cu(II)⬄ Cu(I) transformation in solution and that there is

Table 2The set of equilibrium reactions applicable for the thermo-chemical conditions of the Kupfe

Reaction a

CuOH++Cl−+H+ ⬄ H2O+CuCl+ 7.375CuOH++e−+3Cl−+H+ ⬄ H2O+CuCl--3 K= 15.37CuCl++2Cl−+e− ⬄ CuCl--3 7.996

⁎This set of reactions apply for T=40°C, aCl=1.5 and P=1atm.


a near-zero fractionation between the Cu(I) solution ion and coppersulphide minerals.

3. Methodology

3.1. Sample preparation

Samples were first sawn into thin slices and drilled using a 1 mmdiameter diamond drill. Some samples that could not be handled witha driller were separated from the host rock by crushing and handpicking. Mineral phase identification of the Timna samples wasperformed using X-ray powder diffraction (XRD). The petrographicexaminations of ores and iron oxides from the Kupferschiefer depositwere performedwith a Leitz microscope in Polish Geological Institute.Mineralogical characterization and hand specimens photographs arepresented in Table 1 and Fig. 3, respectively.

3.2. Copper isotope measurements

Isotopic compositions are reported in this work using the δ65Cunotation in permil relative to the SRM 976 standard and fractionationsare given as: Δ65Cu(A–B)=δ65Cu(A)–δ65Cu(B). Sample powders werefirst dissolved: Cu-sulphide and Cu-carbonate samples were dissolved

rschiefer and Timna deposits

b·t c·t2 d·t3 e·t4

- 0.0309 0.0001785 -5.34E-07 3.38E-10-0.0374 0.0002048 -6.43E-07 6.52E-10-0.0065 2.64E-05 -1.09E-07 3.14E-10




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in 10 ml of 0.1N HNO3 and Cu-silicates were dissolved in a mixtureof 40% hydrofluoric acid mixed with 7N HNO3. The solutionswere chemically analyzed by ICP-AES, using Sc as an internal standard.The appropriate amounts of copper solution were taken for chroma-tographic separation and copper isotopic analysis. In this work, weused the chromatographic separation procedure of Marechal et al.(1999) Average yields were 95±5%. The copper isotopemeasurementswere made on a Nu Plasma™ MC-ICP-MS at the GSI. Samples wereintroduced to the MC-ICP-MS as 1 ppm solutions in 0.1N HNO3, usingan Aridus introduction system with aerosol desolvation. The isotopicmeasurements were made using standard-sample-standard bracket-ing technique relative to the SRM 976 copper standard. Precision wasroutinely tested by running the SRM 976 Cu standard, an internal labstandard, and artificially contaminated malachite solutions through-out the whole protocol, giving a 2σ error of ±0.06‰. Further details ofthe chromatographic separation procedures and mass spectrometryare to be found at Asael et al. (2007).

3.3. Calculated isotopic speciation diagrams

Reactions and equilibrium constants among the different coppersolution species were calculated with the Geochemist's Workbench4.04 software, which gives reactions constants as a 4th orderpolynomial function of temperature. All calculations were made at astandard state pressure of 1 bar. The effect of pressure on the fluidspeciation calculations in the range relevant for the two sites isnegligible. The Eh–pH phase diagrams in this work were calculatedwith Matlab software to give the stable solution complexes andcontour diagrams of the species relative abundance and their isotopiccomposition. Table 2 presents the different reactions and reactions

Fig. 5. Frequency diagrams of the copper isotopic compositions of: (a) the KupferschieferCu-sulphides; (b) Timna Cu-sulphides; (c) Timna Cu(II) minerals.

Fig. 6. Copper isotopic compositions of the Kupferschiefer samples plotted according totheir geological setting as a function of: a. stratigraphically-related host-rock type(bottom to top in stratigraphic order); b. the mineralization zone in which the samplesare located (bottom to top in order of distance from the feeder area); c. the location ofthemining areas fromwest to east (bottom to top). All data show awide spread of δ65Cuvalues and no discernable trends with respect to stratigraphy or distance from thesource can be inferred.


constants for a representative set of calculated diagrams. The choice ofconditions and parameters will be discussed in the results.

4. Results

4.1. Copper isotope compositions

The copper ore deposit of the Kupferschiefer is dominated bycopper sulphides (mainly chalcocite, chalcopyrite and bornite)whereas the Timna ore deposit is dominated by Cu(II) minerals(mainly copper silicates, malachite and paratacamite) with a minoroccurrences of Cu-sulphide minerals.

The copper isotopic compositions of the Kupferschiefer and Timnasamples are summarized in Table 1 and plotted as histograms in Fig. 5.The histogram of the Kupferschiefer Cu-sulphides (Fig. 5a) showsbimodal distributionwhereby the dominant higher peak shows δ65Cuvalues of −0.39±0.36‰ and the smaller peak shows values of −1.93±0.40‰. The smaller peak mostly consists of samples from primarydisseminated shale ores; whereas the dominant higher δ65Cu samplepeak encompasses all units and rock-types. The distribution of theKupferschiefer copper isotopic data with respect to host rock type,mine location and genetic zone is presented in Fig. 6. The coppersulphides and Cu(II) minerals from Timna show average δ65Cu valueof −2.04±0.44‰ and −0.85±0.46‰, respectively (Fig. 5b and c).




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There are clearly significant differences in the δ65Cu valuesbetween the Cu-sulphides of the two deposits. In Timna, there is noequivalent of the high δ65Cu range of values found in the Kupferschie-fer Cu-sulphides. Both groups of rocks are derived from igneoussource rocks with initial δ65Cu of 0±0.5‰ (Zhu et al., 2000; Larsonet al., 2003; Mason et al., 2005; Mathur et al., 2005; Asael et al., 2007).

The Cu(II) minerals of Timna replacing copper sulphides showsignificantly higher δ65Cu values than the copper sulphides (Fig. 5band c). These differences were ascribed by Asael et al. (2007) to apositive fractionation accompanying oxidation of precursor coppersulphides. However, the Timna Cu(II) samples also include severalminerals that have undergone complete oxidation of the originalcopper sulphide. These oxidized Cu-sulphides show δ65Cu valuessimilar to the values of the Cu-sulphides from the same location(δ65Cu=−2.27‰).

4.2. Fluid speciation diagrams

Fig. 7 presents Eh–pH mineral stability diagrams calculated forthe Cu–S–O–H–C system using the GWB 4 software. The specified

Fig. 7. Calculated Eh–pH phase diagrams for the temperature and aCl− conditions of theTimna deposit (a) and the Kupferschiefer deposit (b). Both diagrams were calculatedwith ∑Cu=10−4, ∑S=10−2 and pCO2=10−2. See text for details of the calculations andchoice of parameters values. Note that the stable solution species are the same in eachdiagram: Cu(I) species, CuCl32− and Cu(II) species CuCl+. The boundary between thestability fields of the solution species is defined where [CuCl32−]= [CuCl+].


conditions at each site are: aCl=1, T=40 °C and P=1 atm for Timna;and, aCl−=1.5, T=100 °C and P=1 atm for the Kupferschiefer. For bothsites the calculations were made under representative activityconditions of aCu=10−4, aSO4=10−2 and aHCO3=10−2. Importantsolution speciation features revealed by the diagrams are the widestability field of the Cu(I) complex CuCl32− in mildly reducing Ehconditions and acidic to neutral pH values. The field of stability ofCuCl32− is larger at the Kupferschiefer conditions. In both diagrams thestable Cu(II) species is CuCl+. Another feature identifiable is the widerstability field for malachite under Timna conditions. Copper sulphideminerals analyzed in this study and featured in the GWB software(covellite and chalcocite) are in equilibrium with the CuCl32− species.

Eh–pH fluid speciation for the above thermo-chemical conditionsat the two sites are given in Fig. 8. All calculations were made for anactivity of copper in solution aCu=10−4, it should be noted that thechoice of aCu does not affect the contours and fields shown in thespeciation diagrams.

Fig. 8a, c and e shows relative abundance of the copper chloridespecies, whereas Fig. 8b, d and f shows the δ65Cu values as a functionof Eh–pH conditions.

The Eh–pH diagrams relevant to the Kupferschiefer are shown inFig. 8a and b. The stable Cu(I) species is CuCl32− and the Cu(II) speciesare CuCl+ and CuOH+. Stability fields labeled 100% indicate where therelevant species are totally dominant. The contours give percentageisopleths of the reduced copper species (CuCl32−) during the transitionto the fields of the oxidized species. Fig. 8b plots the calculated δ65Cuvalues of the reduced CuCl32− species for a bulk solution of δ65Cuvalue=0‰ (i.e., that of fluid formed by congruent solution of anigneous source) and a fractionation factor Δ65Cu(CuCl32—CuCl+)=2.5‰taken from the inorganic experiments for Cu(II)–Cu-sulphide fractio-nation (Section 2.4). As previously noted, a basic assumption of themodeling is that there is a near-zero fractionation between the Cu(I)solution ion and Cu-sulphide and that the experimental fractionationfactors reflect isotopic fractionation in solution (Zhu et al., 2002;Ehrlich et al., 2004). Similar assumptions govern the work of Ohmoto(1972). The case for isotopic fractionation occurring during theaqueous Cu(II)–Cu(I) transformation was presented in the Introduc-tion. Evidence for near-zero fractionation factors between a solutionion and solid minerals is given by experiments for Cu(II) phases(Section 2.4) and is a viable assumption for Cu(I) phases. The questionof influences of chloride complexing on isotope fractionation will bediscussed later.

It can be seen that the transition from one dominant species in thesolution to another occurs over restricted Eh–pH ranges. Correspond-ingly, the isotopic composition of that species also changes rapidly. Forexample, the copper isotopic composition of the CuCl32− (in Fig. 8b)species, and consequently of the equilibrium Cu-sulphide precipitate,ranges in δ65Cu from −0.1‰ to −2‰ (CuCl32− varying from 95% to b10%of the total copper in the solution) when Eh increases from 0.45 V to0.6 V at pH=6.

The speciation diagrams representative of Timna conditions(Fig. 8c and d) are similar to the diagrams for the Kupferschiefer,except for the previously noted contraction of the CuCl32− field andexpansion of the Cu(OH)+ and CuCl+ fields. Consequently, there is aslight contraction of the transition zones and the Eh–pH ranges overwhich the dominant species change.

The final two diagrams in Fig. 8 explore the isotopic changes of theCu(II) species for the representative Timna conditions. The calcula-tions are made for two bulk copper isotope compositions: δ65Cu=0‰and −2‰. The latter bulk composition represents the isotopiccomposition of the copper sulphide from which Cu(II) minerals formin the Timna Cambrian dolomites. The isopleths in Fig. 8e show thepercentage of the CuCl+ species with Eh–pH change and as is evident,change from 100% to b1% with decrease of Eh into the CuCl32− field andincrease in pH. Correspondingly the δ65Cu values of the CuCl+ speciesincrease from the bulk value within the CuCl+ field (δ65Cu=0‰ and



Fig. 8. Calculated Eh–pH diagrams showing contours of relative abundance and copper isotopic composition for each of the sample groups studied in this work: a and bKupferschiefer Cu-sulphide system; the contours show isopleths of relative abundance of the CuCl32− ion and its δ65Cu value in the transition zone between the CuCl32 and CuCl+ fieldsfor a bulk solution with δ65Cu=0‰. c and d Timna Cu-sulphides; contours of relative abundance of the CuCl32− ion and its δ65Cu value in the transition zone between the CuCl32 andCuCl+ fields for a bulk solution with δ65Cu=0‰. e and f Timna Cu(II) minerals; contours a show isopleths of relative abundance of the CuCl+ ion and its δ65Cu value in the transitionzone between the CuCl+ and CuCl32− fields for bulk solutions with δ65Cu=0‰ and −2‰. Refer to text for methods of calculation and choice of parameters.


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Please cite this article as: Asael, D., et al., Fluid speciation controls of low temperature copper isotope fractionation applied to theKupferschiefer and Timna ore deposits, Chemical Geology (2009), doi:10.1016/j.chemgeo.2009.01.015


Fig. 9. Eh–pH diagram showing the changes in the copper-chlorine speciation insolution and the position of the equi-concentration boundaries between the differentcopper complexes at varying values of the chlorine activity aCl−.


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−2‰) to +2.5 and +0.5, respectively when CuCl+ is less than 0.1% of thetotal solution species. Decreasing Eh therefore brings about anincrease in the δ65Cu of the CuCl+ species and the Cu(II) mineralsthat form in equilibrium with this species.

The thermodynamic calculations show that copper chloridespecies are the dominant solution ions, even though sulphate ispresent in the model system. This is consistent with the study of Rose(1976) showing that CuCl3− complexes are the major solution ionstransporting copper in SSC systems. We also performed a number ofadditional calculations for a wide range of aCl− and aSO4

2− values. Theyreveal that chloride is the dominant ligand, and only at extremely highvalues of sulfur activity (aSO4/aCl N200) copper sulfate complexes areimportant component in solution. This allows that sulphate to bepresent in the saline solution, but will not affect the modeling in thisstudy.

5. Discussion

5.1. Copper isotope compositions of the Kupferschiefer

The δ65Cu values of the Kupferschiefer samples exhibit bimodaldistribution. The main peak of analyses, showing higher δ65Cu valuesin the histogram frequency diagram of Fig. 5a, clearly represents Cu-sulphide precipitation processes at relatively uniform thermo-chemi-cal conditions. These samples comprise all zones, rock types andmining areas (Fig. 6 and Table 1). However, the apparent tendency forδ65Cu values from earlier formed disseminated shale ores to dominatethe low δ65Cu peak in the Kupferschiefer histogram (Figs. 5a and 6)could suggest that different thermo-chemical conditions wereexperienced during their formation.

5.2. Eh–pH controls of copper sulphide isotopic composition

When compared with the Cu-isotopic compositions for the coppersulphides of Timna (−2.04±0.4‰), it is clear that the main groupof Kupferschiefer samples (−0.39±0.36‰) is isotopically heavier(Fig. 5). Only the small group of Kupferschiefer samples with δ65Cuvalues ~−1.9‰ (Fig. 5) corresponds to the Cu-sulphide samples ofTimna, Cu(II) minerals are rare in the Kupferschiefer, but commonlyreplace Cu-sulphide minerals observed in the Cambrian dolomites ofTimna. The mean δ65Cu value of the Timna Cu(II) minerals (mainlymalachite) is −0.85±0.45‰.

The Eh–pH diagrams shown in Fig. 8 show that in the redoxtransition zones from one dominant solution species to another, smallchanges in the Eh or pH can produce a significant change in therelative abundance of a species and its Cu-isotopic composition. Thus,a straightforward explanation can be put forward to explain thedifferences in Cu-isotopic composition of the copper sulphides. Theisotopic composition of the Timna deposit copper sulphides (−2.04±0.44‰) corresponds to Eh values of 0.5 to 0.6 V at pH b6, aCl−=1 andT=40 °C (Fig. 8d). These Eh values indicate precipitation at relativelyoxidized conditions where most of the copper exists in the solution(~90%) as the divalent Cu chloride complex, and the CuCl32− complex isb10% of the total solution species (Fig. 8c). On the other hand, theisotopic composition of the main group of Kupferschiefer Cu-sulphides (−0.39±0.36‰) corresponds to Eh values of 0.4 to 0.5 V atpH=6.3, aCl−=1.5 and T=100 °C (Fig. 8b). These Eh values representrelatively reducing conditions where most of the copper in thesolution (~80%) occurs as the CuCl32− complex (Fig. 8a).

The correspondence between the minor peak of low δ65Cu valuesin the Kupferschiefer samples and δ65Cu values of Timna coppersulphides suggests that these samples could have formed undersimilar redox conditions. If, as previously indicated, the low δ65Cuvalues are more prevalent in the disseminated shale ores, this couldimply that slightly more oxidizing conditions occurred during theirformation. The relatively small number of samples taken in this work,


and the fact that the sampling was confined to a spatially limitedtransect of the Kupferschiefer deposits, does not allow us to assess thisinference more fully.

The proposal of the modeling, presented in Section 4.2, that Δ65Cu(CuCl32−–Cu-sulphide) fractionation factors are close to zero, dictatesthat the Cu isotopic composition of the CuCl32− complex is closelyreflected in the copper sulphide. However, a full application of thismodel to natural system data also requires that mass balance stronglyfavours the solution ion. This assumption appears to be valid for theKupferschiefer where the lack of regional δ65Cu variation suggeststhat the major Cu-sulphide precipitation processes occurred from ahomogeneous reservoir at relatively uniform thermo-chemical con-ditions. At Timna, the lack of significant isotopic zoning in the coppersuphide spherules (Asael et al., 2007) also suggests relatively uniformsolution conditions. This situation contrasts with that of low-temperature hydrothermal ore deposits of southern Germany, whereMarkl et al. (2006) infer both fluid speciation and Rayleigh fractiona-tion controls of isotopic composition in a closed system.

The differences in redox conditions found here are consistent withthe field relations showing that copper sulphides are a minor mineralcomponent of the Timna ore deposit but most of the coppermineralization in the Kupferschiefer occurs as Cu(I) sulphideminerals.One possible explanation for the different conditions between thetwo deposits is because the Kupferschiefer is an organic matter richunit. High organic matter contents potentially can maintain reducingconditions through extensive bacterial sulphate reduction as indi-cated by the sulphur isotope data (Haranczyk, 1986; Sawlowicz, 1989;Sawlowicz and Wedepohl, 1992), whereas the low organic mattercontent of the Timna deposit favours only localized copper sulphideprecipitation.

5.3. Effects of chloride complexing

Changing the activity of chloride in solution may affect the Eh–pHcalculations in several ways. Fig. 9 shows the changes of boundariesbetween the dominant Cu(I) and Cu(II) solution species as a functionof aCl− at 40 °C. One can see that as the chlorine activity increases, thedominance of Cu(I) species in the solution increases as well (i.e., thetransition from Cu(I) to Cu(II) species appears at increasingly higherEh values). As a consequence, and independent of any Eh change, thedominance of the Cu(I) species and hence the δ65Cu of this speciesincreases as a function of aCl−. The proposed differences in aCl− for the




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Kupferschiefer and Timna (1.5 and 1, respectively) are relatively smallbut favour higher δ65Cu values in the Kupferschiefer. It should benoted however that the calculations in Fig. 8 take into account thedifferences in aCl− and thus do not alter the conclusion regarding thevalues of the Eh conditions.

A major assumption of the previous section was that the isotopicfractionation between the Cu(I) and Cu(II) species is represented bythe value found in experiments made in dilute solutions (0.01M) andpotential effects of chloride complexing on fractionation factors hasbeen neglected. However, changing chlorine activity generatesdifferent copper-chloride complexes, which have different reducedpartition function ratios (Seo et al., 2007). In this study the thermo-chemical conditions for the two deposits are sufficiently similarthat the same Cu(I) and Cu(II) chloride complexes are stable in both,CuCl32− and CuCl+, respectively (Fig. 8). However, at lower chlorinity(aCl b0.5) the stable Cu(I) species is CuCl2− and at aCl− b0.1 thedominant Cu(II) species becomes Cu2+. Seo et al.'s (2007) calculationsshow that there will be a significant difference in the fractionationproperties of the CuCl32− and CuCl2− complexes, with Δ65Cu(CuCl32−–CuCl2−)=−1.4 at 50 °C and −0.94 at 100 °C. Calculations for Cu(II)chloride complexes have not yet been published and thus it isimpossible to estimate the isotopic impact of a change from a CuCl2−–Cu2+ to a CuCl32−–CuCl2− fractionation pair. Despite this uncertaintyon the actual value of the Cu(I)–Cu(II) fractionation factors, the factthat the same species occur for both the Kupferschiefer and Timnasystems indicates that the relative differences in redox conditions willbe preserved.

5.4. Formation of Timna Cu(II) minerals

While the precipitation of the Cu-sulphides in both locations canbe simply explained with Eh–pH fluid speciation diagrams, theformation of the Cu(II) minerals surrounding the Cu-sulphides ofthe Timna deposit is more complicated. A given copper solution canonly precipitate Cu(II) minerals with δ65Cu value equal or higher(δ65Cu(CuII)≥δ65Cu(ΣCu)) than the source solution value (Fig. 8e and f).Taking this into account and the fact that the Cu(II) mineralssurrounding the Cu-sulphides of the Timna deposit show δ65Cu valuesof −0.85±0.46‰, a source solution value of 0‰ is not possible. On theother hand, a copper solution that originated from congruentdissolution of the parent Cu-sulphides will have the same δ65Cuvalue as the Cu-sulphides. As the calculations in Fig. 8f show, such asolution with δ65Cu ~−2‰ is capable of precipitating Cu(II) mineralswith the observed δ65Cu values. Eh conditions, however, would haveto be roughly the same as for the formation of the Cu sulphideminerals (ca 0.5 V). In view of the dominance of the Cu(II) minerals inthe Timna mineralization and the fact that they postdate the coppersulphides, this scenario is considered unlikely. It is more likely that thesource solution for the Cu-sulphides was a mixture of two solutions,one from congruent dissolution of Cu-sulphides and the secondsimilar to the source solution with δ65Cu=0‰.

6. Conclusions

This study interprets the copper isotopic compositions of mineralsfrom two sediment-hosted sedimentary copper ore deposits usingcalculated Eh–pH fluid speciation diagrams. This fluid speciationmodeling provides a thermodynamic insight into redox conditionsduring the formation of copper sulphides in the ore deposits of Timna,Israel and the Kupferschiefer deposits of the SW part of the Lubin–Sieroszowice Copper District of Poland. As originally shown for thehydrothermal sulphur isotope system by Ohmoto (1972), the keyarguments underlying the modeling are that the copper isotopiccomposition of copper sulphides reflects the isotopic composition ofthe Cu(I) solution species, and that the isotopic composition of this Cu(I) species depends on its relative amount in solution, which in turn is


a function of redox conditions (Eh–pH) and thermodynamic para-meters such as T and aCl. The principal findings can be summarized asfollows:

a. Significant differences in the copper isotope composition occurbetween Cu-sulphides of the Kupferschiefer and the Timna samples.Using Eh–pH fluid speciation diagrams, these differences areinterpreted to reflect contrasting redox conditions during mineralformation. The conclusion that most of the sampled Kupferschiefercopper sulphides were precipitated at more reducing conditionsthan in the Timna deposits is generally consistent with fieldobservations showing that the Kupferschiefer mineralogy is domi-nated by Cu-sulphides, whereas at Timna, Cu(II)–minerals dominateover copper sulphides.

b. The effects of chloride activity (aCl−) on the stabilityfields of coppersolution species are shown to be important. At higher aCl− values,Cu(I) species become increasingly dominant components of thesolution, with the result that the copper isotopic composition ofprecipitating Cu-sulphides will become closer to that of the sourcesolution. Moreover, due to the relatively narrow range in Eh–pHspace in which Cu(I) species transform to Cu(II) species, small aCl−

changes can bring about marked shifts in the Cu(I)/Cu(II) ratio,and hence marked variations in the copper isotope composition ofminerals.

c. Although published studies have shown that bacterial reduction ofsulphate was a significant control of the sulphur isotope geochem-istry, its role in the copper isotope system was to produce thereducing conditions in which copper sulphides could form and thecontrols of copper isotope fractionation were abiogenic.

Fluid speciation modeling has wide potential applicability tohydrothermal copper ore deposits. At higher temperatures (N150 °C),bacterial sulphate reduction ceases and sulphur isotope behaviourbecomes increasingly equilibrium thermodynamic; thus fluid specia-tion modeling can be simultaneously applied to the both copper andsulphur isotope systems. This combination of isotope systems offers avery powerful tool for interpreting ore genesis; particularly so for theinsight it can give into processes as source mixing and secondaryoxidation–reduction recycling of primary ores where changes in bulkisotopic composition can occur. Finally, the fluid speciation approachcan be applied to other metallic isotopic systems where redoxand other ligand species changes reactions play an important role.This includes the important molybdenum isotope system, wherewide ranges of δ98/97Mo values are observed in suboxic and anoxicsediments (e.g., Poulson et al., 2006).

Acknowledgements

This research was supported by grant number 186/03 from theIsrael Science Foundation, Jerusalem Israel. AM acknowledges thereceipt of the Raymond F Kravis Chair in Geology. We are thankful toNathalya Teplyakov, Dr Sara Ehrlich, and Dr. Irena Segal for their helpwith the chemical separations and plasma mass spectrometry. Wethank the two anonymous reviewers for their critical comments.

References

Asael, D., Matthews, A., Butler, I., Rickard, A.D., Bar-Matthews, M., Halicz, L., 2006. (CU)-C-65/(CU)-C-63 fractionation during copper sulphide formation from ironsulphides in aqueous solution. Geochimica et Cosmochimica Acta 70 (18), A23-A23.

Asael, D., Matthews, A., Bar-Matthews, M., Halicz, L., 2007. Copper isotope fractionationin sedimentary copper mineralization (Timna Valley, Israel). Chemical Geology243 (3–4), 238–254.

Bar-Matthews, M., Matthews, A., 1990. Chemical and stable isotope fractionation inmanganese oxide phosphorite mineralization, Timna Valley, Israel. GeologicalMagazine 127 (1), 1–12.

Bartura, Y., Wurzburger, U., 1974. The Timna copper deposit. Geol. Belgique: GisementsStratiformes et Provinces Cupriferes Liege Soc. Geol. Belg. pp. 277–285.

Bentor, Y.K., 1956. The manganese occurrences at Timna (Southern Israel). A lagoonaldeposit. 20th Int. Geol. Congr., vol. 2, pp. 159–172.




ARTICLE IN PRESS

Beyth, M., 1987. Precambrian magmatic rocks of Timna valley, southern Israel.Precambrian Research 36, 21–38.

Blundell, D.J., Karnkowski, P.H., Alderton, D.H.M., Oszczepalski, S., Kucha, H., 2003.Copper mineralization of the Polish Kupferschiefer: a proposed basement fault-fracture system of fluid flow. Economic Geology and the Bulletin of the Society ofEconomic Geologists 98 (7), 1487–1495.

Brown, A.C., 1992. Sediment-hosted stratiform copper-deposits. Geoscience Canada19 (3), 125–141.

Brown, A.C., 1997. World-class sediment-hosted stratiform copper deposits: character-istics, genetic concepts andmetallotects. Australian Journal of Earth Sciences 44 (3),317–328.

Ehrlich, S., Butler, I., Halicz, L., Rickard, D., Oldroyd, A., Matthews, A., 2004. Experimentalstudy of the copper isotope fractionation between aqueous Cu(II) and covellite, CuS.Chemical Geology 209, 259–269.

Garfunkel, Z., 1970. The tectonics of the margins of the southern Arava: a contributionto the understanding of rifting. Ph.D. Thesis, Hebrew University of Jerusalem,Jerusalem, 204 pp.

Goh, S.W., Buckley, A.N., Lamb, R.N., 2006. Copper(II) sulfide? Minerals Engineering 19(2), 204–208.

Graham, S., Pearson, N., Jackson, S., Griffin, W., O'Reilly, S.Y., 2004. Tracing Cu and Fefrom source to porphyry: in situ determination Of Cu and Fe isotope ratios insulfides from the Grasberg Cu–Au deposit. Chemical Geology 207, 147–169.

Haranczyk, C., 1986. Zechstein copper-bearing shales in Poland. Lagoonal environmentsand sapropelmodel of genesis. In: Friedrich, G., et al. (Ed.), Geology andmetallogenyof copper deposits. Springer, pp. 461–476. Berlin-Heidelberg-New York.

Jowett, E.C., Rye, R.O., Oszczepalski, S., Rydzewski, A., 1991. Isotopic evidence for theaddition of sulfur during formation of the Kupferschiefer ore deposits in Poland.Zentralblatt für Geologie und Paläontologie, Teil I 4, 1001–1015.

Larson, P.B., Maher, K., Ramos, F.C., Chang, Z., Gaspar, M., Meinert, L.D., 2003. Copperisotope ratios in magmatic and hydrothermal ore-forming environments. ChemicalGeology 201, 337–350.

Luther, G.W., et al., 2002. Aqueous copper sulphide clusters as intermediates duringcopper sulphide Formation. Environmental Science Technology 36, 394–402.

Marechal, C.N., Sheppard, S.M.F., 2002. Isotopic fractionation of Cu and Zn betweenchloride and nitrate solutions and malachite or smithsonite at 30 degrees and 50degrees C. Geochimica Et Cosmochimica Acta 66 (15A), A484-A484.

Marechal, C.N., Telouk, P., Albarede, F., 1999. Precise analysis of copper and zinc isotopiccompositions by plasma-source mass spectrometry. Chemical Geology 156, 251–273.

Markl, G., Lahaye, Y., Schwinn, G., 2006. Copper isotopes as monitors of redox processesin hydrothermal mineralization. Geochimica et Cosmochimica Acta 70, 4215–4228.

Mason, T.F.D., et al., 2005. Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia. Chemical Geology221 (3–4), 170–187.

Mathur, R., et al., 2005. Cu isotopic fractionation in the supergene environment withand without bacteria. Geochimica et Cosmochimica Acta 69 (22), 5233–5246.

Ohmoto, H., 1972. Systematics of sulfur and carbon isotopes in hydrothermal oredeposits. Economic Geology 67 (5), 551–578.


Oszczepalski, S., 1999. Origin of the Kupferschiefer polymetallic mineralization inPoland. Mineralium Deposita 34 (5–6), 599–613.

Oszczepalski, S., Rydzewski, A., 1997. Metallogenic Atlas of the Zechstein Copper-bearing Series in Poland, Państwowy Instytut Geologiczny – WydawnictwoKartograficzne Polskiej Agencji Ekologicznej SA, Warszawa.

Oszczepalski, S., Nowak, G.J., Bechtel, A., Zák, K., 2002. Evidence of oxidation of theKupferschiefer in the Lubin-Sieroszowice deposit: implications for Cu–Ag and Au–Pt–Pd mineralisation. Geological Quarterly 46 (1), 1–23.

Poulson, R.L., Siebert, C., McManus, J., Berelson, W.M., 2006. Authigenic molybdenumisotope signatures in marine sediments. Geology 34, 617–620.

Robb, L., 2005. Introduction to Ore-Forming Processes. Blackwell Publishing Company.239 pp.

Rose, A.W., 1976. The effect cuprous chloride complexes in the origin of red-bed copperand related deposits. Economic Geology 71 (6), 1036–1048.

Rouxel, O., Fouquet, Y., Ludden, J.N., 2004. Copper isotope systematics of the LuckyStrike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-AtlanticRidge. Economic Geology and the Bulletin of the Society of Economic Geologists99 (3), 585–600.

Sawlowicz, Z., 1989. On the origin of copper mineralization in the Kupferschiefer: asulphur isotope study. Terra Nova 1, 339–343.

Sawlowicz, Z.,Wedepohl, K.H.,1992. The originof rhythmic sulfidebands from thePermiansandstones (Weissliegendes) in the footwall of the fore-sudetic Kupferschiefer(Poland). Mineralium Deposita 27 (3), 242–248.

Segev, A., 1986. Lithofacies relations and mineralization occurrences in the TimnaFormation, Timna Valley. Ph.D. Thesis, Hebrew University of Jerusalem, Jerusalem,131 pp.

Seo, J.H., Lee, S.K., Lee, I., 2007. Quantum chemical calculations of equilibrium copper(I)isotope fractionations in ore-forming fluids. Chemical Geology 243 (3–4), 225–237.

Shlomovitch, N., Bar-Matthews, M., Matthews, A., 1999. Sedimentary and epigeneticcopper mineral assemblages in the Cambrian Timna Formation, southern Israel.Israel Journal of Earth Sciences 48, 195–208.

Speczik, S., Oszczepalski, S., Karwasiecka, M., Nowak, G.J., 2007. Kupferschiefer – a huntfor new reserves. In: Andrew, C.J., et al. (Ed.), Digging deeper. Ninth Biennial SGAMeeting. Irish Association for Economic Geology, Dublin, pp. 237–240.

Wedepohl, K.H., 1971. Zinc and lead in common sedimentary rocks. Economic Geology66 (2) 240-&.

Wodzicki, A., Piestrzynski, A., 1994. An ore genetic model for the Lubin–SieroszowiceMining District, Poland. Mineralium Deposita 29 (1), 30–43.

Wurzburger, U.S., 1967. The occurrence of sulfides in the Timna massif and secondarycopper minerals in the Timna copper deposit, Negev (southern Israel). M.Sc. Thesis,Imperial College of Science and Technology, London.

Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination ofnatural Cu-isotope variation by plasma-source mass spectrometry: implications foruse as geochemical tracers. Chemical Geology 163, 139–149.

Zhu, X.K., et al., 2002. Mass fractionation processes of transition metal isotopes. Earthand Planetary Science Letters 200, 47–62.



Fluid speciation controls of low-temperature copper isotope fractionation

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