Mineralogy and Petrology

, Volume 101, Issue 3, pp 169–183

A cervelleite-like mineral and other Ag-Cu-Te-S minerals [Ag2CuTeS and (Ag,Cu)2TeS] in gold-bearing veins in metamorphic rocks of the Cycladic Blueschist Unit, Kallianou, Evia Island, Greece

Authors

    • Department of Mineralogy-Petrology, Faculty of Geology & GeoenvironmentNational and Kapodistrian University of Athens
  • Paul G. Spry
    • Department of Geological and Atmospheric SciencesIowa State University
  • Gregory Aarne Sakellaris
    • RWTH Aachen University
  • Constantinos Mavrogonatos
    • Department of Mineralogy-Petrology, Faculty of Geology & GeoenvironmentNational and Kapodistrian University of Athens
Original Paper

DOI: 10.1007/s00710-011-0144-z

Cite this article as:
Voudouris, P.C., Spry, P.G., Sakellaris, G.A. et al. Miner Petrol (2011) 101: 169. doi:10.1007/s00710-011-0144-z

Abstract

A cervelleite-like mineral, two unnamed silver sulfotellurides in the system Ag-Cu-Te-S [Ag2CuTeS, (Ag,Cu)2TeS], Te-rich polybasite and cadmian tetrahedrite occur in gold-bearing quartz veins in metapelites and faults within brecciated marbles of the Cycladic Blueschist Unit in the Kallianou area (southern Evia Island, Greece). The quartz veins and faults are discordant to syn-metamorphic structures and formed during ductile to brittle deformation in the final stages of exhumation of the Styra Nappe extrusion wedge (~21 Ma). Te-rich polybasite (up to 7.4 wt. % Te), cadmian tetrahedrite (up to 12.4 wt. % Cd), together with electrum (23–54 wt. % Ag) and the sulfotellurides, are the main silver carriers in the mineralization. The two unnamed sulfotellurides, Ag2CuTeS and (Ag,Cu)2TeS are believed to be new quaternary minerals in the system Ag-Cu-Te-S. These minerals and the cervelleite-like phase could have exsolved from galena during cooling (below 200°C). Initial temperatures for the formation of the sulfotellurides, in the form of hessite-intermediate solid solution, at Kallianou may be up to 300°C under logfS2 values between ~ −11.5 to −8.3, and logfTe2 from ~ −14.8 to −7.8. The values of logfTe2 and logfS2 during re-equilibration (at ~200°C) were constrained to −19.5 to −15.2 and to −15.8 to −11.5 respectively.

Introduction

Sulfotellurides of the Ag-Cu-Te-S system are trace constituents accompanying native gold and gold-silver tellurides in various deposit types that span the magmatic-hydrothermal spectrum. Similarly to gold-silver tellurides, that have become widely used to help assess conditions of ore formation (Ciobanu et al. 2006), sulfotellurides may provide physicochemical constraint on ore formation. Investigation of Ag-sulfotellurides is important to understanding the trace mineral distribution within volcanic-hosted massive sulfide (VHMS) deposits, to understand genetic processes, changes in physicochemical conditions during ore formation, and the possible genetic relationships between VHMS and epithermal ores (Novoselov et al. 2006). Compositional variation among phases of the cervelleite group may be a source of petrogenetic information. For example, in skarns, these minerals are part of broader ‘exotic’, volatile-rich mineral parageneses tracing changes in physicochemical conditions during retrograde stages (Cook and Ciobanu 2003; Cook et al. 2009).

Cervelleite (Ag4TeS) was first described from the Bambolla deposit, Mexico (Criddle et al. 1989) but has been reported along with cervelleite-like silver sulfotellurides from various ore types including the Shadiitsa epithermal deposit, Bulgaria (Gadzheva 1985), the Ivigtut cryolite deposit, Greenland (Karup-Møller 1976), the Zyranovskoe VHMS deposit, Altay (Aksenov et al. 1969), the intrusion-hosted San Martin deposit, Argentina (Paar and De Brodtkorb 1996), the epithermal Mayflower Au-Ag deposit, Montana (Spry and Thieben 1996), the Um Samiuki volcanogeneic massive sulfide deposit, Egypt (Helmy 1999), epithermal (Larga, Roşia Montană) and skarn (Băiţa Bihor and Ocna de Fier) occurrences in Romania (Cook and Ciobanu 2003; Ciobanu et al. 2004), the Eniovche epithermal deposit, Bulgaria (Dobrev et al. 2002), the intrusion-hosted Funan Au deposit, China (Gu et al. 2003) and several VHMS deposits in the southern Urals (Novoselov et al. 2006). Unnamed Ag-sulfotellurides [(Ag,Cu)6TeS2 - (Ag,Cu)4TeS] were described from the Funan deposit (Gu et al. 2003) and a phase with the composition Ag2Cu2TeS was reported by Cook and Ciobanu (2003) from the Băiţa Bihor and Ocna de Fier skarns in Romania.

In Greece, cuprian cervelleite and unnamed Ag-Cu sulfotellurides [(Ag,Cu)12Te3S2 and (Ag,Au,Cu)9Te2S3] were described from the intrusion-related deposit at Panormos Bay, Tinos Island (Tombros et al. 2004, 2010; Spry et al. 2006). Another member of the system Ag-Cu-Te-S that was reported from Greece is that by Voudouris (2006) who noted the presence of an unnamed silver sulfotelluride (Ag12Te4S3) in the Fakos porphyry-epithermal deposit, Limnos Island. In the Kallianou area (southern Evia Island, Greece) gold-bearing quartz veins contain an exotic ore mineralogy including cervelleite-like sulfotellurides [Ag2CuTeS and (Ag,Cu)2Te] and Te-rich polybasite (Voudouris and Spry 2008).

The Kallianou district (Fig. 1) is part of the Attic-Cycladic-Pelagonian ore belt, which includes base- and precious-metal skarn, intrusion-related and epithermal mineralization in Lavrion/Attika, and the Evia, Sifnos, Mykonos, Tinos, Kythnos and Milos Islands (Vavelidis and Michailidis 1990; Vavelidis 1997; Skarpelis 2002; Tombros et al. 2004, 2007; Neubauer 2005; Alfieris and Voudouris 2006; Spry et al. 2006; Bonsall et al. 2007; Voudouris et al. 2008). These deposits are spatially associated with arc-related magmatic rocks, which were, in part, controlled by extensional kinematic conditions when the metamorphic core complexes were uplifted to near-surface levels (Neubauer 2005). The Kallianou mining district is famous for the exploitation of gold-silver-rich ore during ancient times. The indicated mineral resource of the Kallianou deposit is estimated to be 500,000 tonnes at an average grade of 2–2.4% Pb, 0.7% Zn, 0.5–0.8% Cu, 35–60 g/t Ag and 5 g/t Au (Alexouli-Livaditi 1978; Katsikatsos 1978).
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Fig. 1

(a) Simplified geological map of the Kallianou ore district showing the location of Kallianou district within the Styra Nappe (modified after Ring et al. 2007) (b) Cross-section through the Cycladic Blueschist Unit on Evia Island, showing thrust contact at the base of the Styra Nappe and normal fault contact at its top (modified after Ring and Glodny 2010)

The current study presents new data on the mineralogy and mineral chemistry of the Kallianou deposit. New compositional data are also presented on minerals in the system Ag-Cu-Te-S and compared to previously published compositions. This study also aims to evaluate the stability of sulfotelluride-rich assemblages as a function of sulfur and tellurium fugacities of the hydrothermal fluids and obtain information on the physicochemical conditions of the sulfotelluride formation. A comparison with Te-rich magmatic-hydrothermal systems elsewhere in Greece using specific telluride associations help to constrain the relationship between the distinct magmatic-hydrothermal environments, and to develop a model that could help in prospecting for gold-telluride deposits in Greece.

Geology

In the Attic-Cycladic Crystalline Belt, an eclogite— to blueschist-facies metamorphism (~53 Ma) at 400–500°C and 12–20 kbar was followed by a greenschist— to amphibolite-facies stage (>30– 12 Ma), and extensional deformation (started 23–19 Ma) (Altherr et al. 1982; Lister et al. 1984; Gautier and Brun 1994; Jolivet et al. 2010; Ring et al. 2010). Three major tectono-metamorphic units, separated by thrust/normal faults, can be distinguished in the Attic-Cycladic Crystalline Belt: the Basal Unit, the Cycladic Blueschist Unit and the Upper Unit (Dürr et al. 1978; Papanikolaou 1987; Forster and Lister 2005; Bröcker and Keasling 2006). The lowermost Basal Unit comprises Mesozoic to Eocene platform metacarbonates and is exposed in tectonic windows in the Attica peninsula and in the islands of Tinos and Evia (Papanikolaou 1987; Ring et al. 2007, 2010). The overlying Cycladic Blueschist Unit consists of Mesozoic metasediments (marbles, metapelites) and metavolcanics and is locally intercalated with a Hercynian anatectic basement (Jolivet et al. 2010; Ring et al. 2010). The Cycladic Blueschist Unit is overlain by the Upper Unit, which is a non- to weakly metamorphosed nappe consisting of various sequences of Permian to Tertiary sediments, Jurassic and Cretaceous ophiolite rocks, and low- to medium-grade metamorphic rocks (Dürr et al. 1978). Miocene extension in the Aegean Sea was accompanied by the intrusion of magmatic rocks at upper crustal levels (Altherr and Siebel 2002). Jolivet and Brun (2010), based on the hypothesis that a single subduction of the African slab under the European plate has been active throughout most of the Mesozoic and the entire Cenozoic, discussed the mechanisms that likely control both syn-orogenic exhumation within the subduction channel and post-orogenic exhumation in extensional metamorphic domes in the back-arc region during slab retreat.

The Kallianou mining district covers an area of about 50 square km at the southern end of Evia Island (Fig. 1b). Southern Evia Island is the footwall block of the North Cycladic Detachment System, whereas remnants of the hanging wall block are preserved on the neighboring islands of Andros and Tinos (Jolivet et al. 2010). In southern Evia Island, Cycladic Blueschists are represented by the Styra-Ochi Nappes. The Ochi Nappe represents the ophiolitic melange of the uppermost Cycladic Blueschist Unit and consists of metagabbro, metabasalt, metarhyolite, piemontite-rich chert, quartzite and carbonate-rich schist (Katzir et al. 2000; Ring et al. 2007). The underlying Styra Nappe represents the passive-margin sequence of the Cycladic Blueschist Unit and is composed of metabauxite-bearing marble, quartzite and metapelites, as well as metabasite and serpentinite lenses at its base (Ring et al. 2007). The underlying Almyropotamos Nappe belongs to the Basal Unit. It is mainly composed of Triassic to Middle Eocene marbles. On Evia Island, high-P metamorphism started at about 50 Ma (Maluski et al. 1981). The Styra and Ochi Nappes underwent high P-low T metamorphism at 0.7– 0.9 GPa, 500– 550°C and 1.0– 1.2 GPa, 400– 450°C, respectively (Ring and Glodny 2010). The Styra Nappe was emplaced during a top-to-the-SSW-directed thrusting above the Almyropotamos Nappe (Shaked et al. 2000; Xypolias et al. 2003, 2010; Ring et al. 2007, 2010). Ring et al. (2007) argued that thrusting of the Styra Nappe onto the Basal Unit (Almyropotamos Nappe) was coeval with shortening-related normal faulting between the Styra Nappe and the overlying ophiolitic melange (Ochi Nappe) resulting in an extrusion wedge that was active for about 10 Ma (from ~33– 21 Ma) (Fig. 1b). The exhumation of the Styra Nappe during underthrusting and burial of the Almyropotamos Nappe was largely accomplished by the D2 top-to-the-NNE-displacing Mt. Ochi normal-sense shear zone (Ring et al. 2007). Normal shearing commenced at ~33 Ma that was associated with high-pressure metamorphism of the Styra and Ochi Nappes. High-P conditions of the Styra and Ochi Nappes persisted until ~33 Ma, when the rocks started to be exhumed and finally reequilibrated under greenschist-facies conditions at ~21 Ma (Ring et al. 2007, 2010).

Analytical methods

Sixty two polished sections were studied by optical microscopy and a JEOL JSM 5600 scanning electron microscope, equipped with back-scattered imaging capabilities, at the Department of Mineralogy and Petrology, University of Athens. The chemical composition of the sulfides, sulfosalts, sulfotellurides, tellurides and native elements was determined by a Cameca-SX 100 wavelength-dispersive electron microprobe at the Department of Mineralogy and Petrology, University of Hamburg, Germany. Operating conditions were: 20 kV and 20 nA, with a beam diameter <1 μm. The following X-ray lines were used: AgLα, AsLα, AuMα, BiMβ, CuKα, FeKα, HgMα, PbMα, SKα, SbLα, SeLα, TeLα, and ZnKα. Pure elements (for Ag, Au, Bi, Se, Te), pyrite and chalcopyrite (for Fe, Cu and S), galena (for Pb), sphalerite (for Zn), HgS (for Hg), Sb2S3 (for Sb) and synthetic GaAs (for As) were used as standards. Corrections were applied using the PAP online program (Pouchou and Pichoir 1991). Chemical compositions of sulfotellurides and sulfosalts were also determined with a JEOL JXA 8200 electron microprobe at Iowa State University using the following X-ray lines: AgLα, AsLα, AuMα, BiMβ, CuKα, FeKα, PbMα, SKα, SbLα, SeLα, TeLα, and ZnKα. Pure elements (for Ag, Au, Bi, Se, Te), pyrite and chalcopyrite (for Fe, Cu and S), galena (for Pb), sphalerite (for Zn), Sb2S3 (for Sb) and synthetic GaAs (for As) were used as standards. The concentrations of the major and minor elements at Iowa State University were determined at an accelerating voltage of 15 kV and a beam current of 15 nA, with 10 s as the counting time (5 s on each background).

Mineralization

According to Alexouli-Livaditi (1978), Theophilopoulos and Vakondios (1982), Perlikos (1989), and Vavelidis and Michailidis (1990), mineralization in the Kallianou area consists mainly of Fe-Pb-Cu-Zn sulfides. The host rocks to the sulfides are metapelites (mica schists) and marbles of the Styra Nappe (Fig. 2). Ore mineralization occurs either within monomictic tectonic breccias in the hanging-wall marble unit along the contact with the schists (Fig. 2a,b), or as meter thick quartz veins that crosscut the foliation of the footwall schists (Fig. 2c–e). Galleries, shafts and dumps of rock waste suggest intense mining activity since ancient times (Fig. 2d). Breccia in the marble unit consists of disrupted angular fragments of host rock cemented mainly by quartz, chlorite, albite and carbonate. Massive to disseminated porphyroblasts of galena, are scattered through low-angle faults (Fig. 2b), particularly in breccia zones in a calcite-chlorite-quartz matrix. These fault systems within the marble unit involve mm-thick normal faults along which extension has taken place in both the brittle and ductile regimes, where the primary schistosity of the carbonate and the schists is disrupted. In some places, oxidized massive ore forms zones up to 60 cm thick that follow the direction of these faults. According to Theophilopoulos and Vakondios (1982), the supergene zone associated with the hypogene ore contains up to 30 g/t Au. The quartz veins (up to 3 m thick and 100 m long) generally strike NW-SE and are discordant to metamorphic structures (Theophilopoulos and Vakondios 1982; Perlikos 1989; Vavelidis and Michailidis 1990). The veins postdate the pervasive ductile deformation associated with high P-low T metamorphism that formed during the Miocene, just below the shortening-related normal fault separating the Styra Nappe from the overlying ophiolitic melange (Ochi Nappe). Structural and microstructural studies of similar unmineralized discordant quartz veins in metamorphic rocks of the Styra-Ochi Unit of southern Evia Island by Nüchter and Stöckhert (2007) suggested that the quartz veins formed close to the brittle-ductile transition. Sulfide-bearing quartz veins contain up to 52 g/t Au and 242 g/t Ag (Alexouli-Livaditi 1978).
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Fig. 2

Photographs of the different ore styles in the Styra Nappe. a. Contact between marble and schist showing brecciated marble and the NW-SE trending mineralized fault system; b. Multiple mm-thick faults cemented by galena (Gn) + calcite + quartz crosscutting the planar schistosity of the marble; c. Mica schists of the Styra Nappe d, e. Precious metal-bearing quartz veins crosscutting the metamorphic fabrics of the host schist; f. Hand specimen of sulfotelluride-bearing chalcopyrite (Ccp) ± galena mineralization within milky quartz (Qz)

Previous authors argued for both a hydrothermal granitoid-related origin (Vavelidis and Michailidis 1990; Alexouli-Livaditi 1978) for the Kallianou mineralization, as well as for the introduction of metals during retrograde greenschist facies metamorphism (Theophilopoulos and Vakondios 1982; Perlikos 1989). In the absence of fluid inclusion, stable isotope, and radiometric age data, the classification of the Kallianou veins as part of an intrusion-related or orogenic system (according to the definition of Goldfarb et al. 2005; Hart 2007) remains uncertain.

Ore minerals in the Kallianou quartz veins occur in masses (up to 10 vol %) to disseminations, filling fractures (Fig. 2f), or cementing brecciated quartz fragments. The main gangue minerals include quartz and calcite, whereas wallrock alteration consists of chlorite, muscovite, albite, and calcite. Metallic minerals include pyrite, arsenopyrite, löllingite, sphalerite, chalcopyrite, tetrahedrite, tennantite, galena, gold, pearceite, sylvanite, argentite, electrum, native silver, a cervelleite-like phase, two other members of the system Ag-Cu-Te-S [Ag2CuTeS and (Ag,Cu)2TeS], hessite and Te-polybasite (Alexouli-Livaditi 1978; Vavelidis and Michailidis 1990; Voudouris and Spry 2008). Pyrite, galena and chalcopyrite are the most common metallic minerals. Pyrite occurs as euhedral (Fig. 3a), corroded and brecciated crystals that were replaced by chalcopyrite or galena. Chalcopyrite postdates pyrite, is associated with galena (Fig. 3a), and is also included in sphalerite (Vavelidis and Michailidis 1990). Sphalerite is a very minor phase and is closely related to chalcopyrite (Alexouli-Livaditi 1978). Idiomorphic grains of arsenopyrite and löllingite are included in quartz and chalcopyrite (Alexouli-Livaditi 1978). In the marble-hosted mineralization, galena forms monomineralic ores. In both ore types, galena is Ag-, Sb- and Bi-free), suggesting that the high Ag-content of galena-bearing ores is due to the presence of inclusions of Ag-bearing sulfosalts, sulfotellurides and hessite. One single analysis of a galena sample contains 50 ppm Sb, 450 ppm Ag, and 6.5 ppm Bi (Agiorgitis and Becker 1975).
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Fig. 3

Representative reflected light and SEM photomicrographs of the Kallianou ore assemblages. a. Electrum (El) surrounding pyrite (Py) and associated with galena (Gn) and chalcopyrite (Ccp) within quartz (Qz) gangue (plain polarized light,); b. Cadmian tetrahedrite (Cd Ttr) included in galena (Gn) (SEM-BSE image); c. Telluropolybasite (Te-Plb) included in galena (Gn) (plain polarized light); d. Telluropolybasite (Te-Plb) at the interface between pyrite (Py) and galena (Gn) (plain polarized light,)

Zincian tetrahedrite and ferrian tennantite are minor constituents of the metallic mineralization. Tetrahedrite occurs as inclusions in galena, whereas tennantite fills fractures in and around chalcopyrite. Silver substituting for Cu shows a limited compositional range (up to 1.3 wt. % Ag in both tetrahedrite and tennantite). Cadmian tetrahedrite is included in galena (Fig. 3b). It contains up to 12.4 wt. % Cd (Table 1) and is the highest Cd content yet reported for tetrahedrite (Pattrick 1978; Voropayev et al. 1988; Pascua et al. 1997). It also contains up to 5.8 wt. % Ag. The chemical variation of tetrahedrite-group minerals, in terms of CuS2-Ag2S-(Sb2S3 + As2S3), is shown in Fig. 4.
Table 1

EPMA data of Te-polybasite (1–5), cadmian tetrahedrite (6,7), electrum (8–12) and hessite (13,14)

wt%

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Cu

3.52

3.43

2.87

4.85

5.29

29.88

30.80

0.04

0.00

0.00

0.00

0.00

0.05

0.07

Ag

68.27

67.71

67.26

65.50

65.55

5.78

5.55

23.24

23.45

25.63

35.42

53.58

62.53

63.04

Au

0.14

0.21

0.00

0.00

0.09

0.00

0.00

77.21

75.78

73.23

63.78

46.05

0.00

0.00

Bi

0.00

0.00

0.00

0.15

0.00

na

na

0.27

0.44

0.29

0.27

0.07

0.01

0.00

Sb

9.06

9.49

9.75

8.59

8.69

28.29

28.11

0.00

0.00

0.00

0.00

0.00

0.00

0.11

Pb

0.00

0.00

0.21

0.45

0.65

0.00

0.00

0.10

0.00

0.00

0.00

0.10

0.26

0.22

Cd

na

na

na

na

na

12.41

12.10

na

na

na

na

na

na

na

Zn

0.00

0.00

0.00

0.00

0.00

0.26

0.00

0.00

0.00

0.00

0.00

0.02

0.00

0.00

As

0.42

0.40

0.28

0.16

0.20

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.00

Te

6.08

6.36

7.41

6.31

6.28

0.00

0.00

0.02

0.06

0.00

0.09

0.08

37.42

37.98

Se

0.09

0.03

0.00

0.03

0.01

0.00

0.00

0.01

0.02

0.01

0.00

0.00

0.00

0.00

S

12.53

12.43

12.06

12.71

12.24

24.30

23.41

0.06

0.09

0.10

0.27

0.17

0.11

0.12

Total

100.11

100.06

99.85

98.79

100.04

100.92

99.97

100.78

99.73

99.26

99.83

100.07

100.44

101.58

Atoms

29

29

29

29

29

29

29

1

1

1

1

1

3

3

Cu

1.330

1.301

1.101

1.833

1.994

8.376

8.759

0.001

0.000

0.000

0.000

0.000

0.003

0.004

Ag

15.186

15.119

15.225

14.602

14.790

0.954

0.930

0.352

0.358

0.387

0.473

0.674

1.977

1.971

Au

0.017

0.026

0.000

0.000

0.011

0.000

0.000

0.640

0.633

0.605

0.468

0.317

0.000

0.000

Bi

0.000

0.000

0.000

0.017

0.000

0.000

0.000

0.002

0.003

0.002

0.002

0.000

0.000

0.000

Sb

1.785

1.877

1.955

1.697

1.712

4.137

4.172

0.000

0.000

0.000

0.000

0.000

0.000

0.003

Pb

0.000

0.000

0.025

0.052

0.075

0.000

0.000

0.001

0.000

0.000

0.000

0.000

0.005

0.003

Cd

1.967

1.946

Zn

0.000

0.000

0.000

0.000

0.000

0.071

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

As

0.134

0.127

0.093

0.053

0.065

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.001

0.000

Te

1.143

1.201

1.419

1.190

1.180

0.000

0.000

0.000

0.001

0.000

0.001

0.001

1.000

1.004

Se

0.027

0.009

0.000

0.009

0.003

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

S

9.378

9.339

9.183

9.535

9.156

13.495

13.193

0.003

0.005

0.005

0.012

0.007

0.012

0.013

na: not analyzed

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Fig. 4

Ternary diagram of Ag-Cu-(Sb + As) sulfosalts. Theoretical compositions are shown as open circles. Filled circles represent composition of tetrahedrite-group minerals. Diamonds are compositions of Te-polybasite. Compositional ranges of tetrahedrite-freibergite and polybasite solid solution are shown by dashed lines for reference

Native gold (up to 30 μm in size) occurs as inclusions and fracture fillings in pyrite spatially associated with quartz and galena, as well as inclusions in chalcopyrite and galena (Vavelidis and Michaelidis 1990). It contains 6.4 to 19.2 wt. % Ag. The same authors previously noted the presence of native silver and acanthite as inclusions in galena. Sylvanite (up to 9 μm in length) and pearceite are also included in pyrite (Alexouli-Livaditi 1978). Electrum, up to 30 μm in length, is closely associated with galena and chalcopyrite in fracture fills, surrounding and/or occurring as inclusions in pyrite (Fig. 3a) and quartz. It contains 23.2 to 53.6 wt. % Ag (35.2–67.4 apfu Ag) and elevated Bi content (up to 0.44 wt. %) (Table 1, Fig. 5).
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Fig. 5

Ternary Au-Ag-Te diagram (atomic proportions) for mineral compositions analyzed in the present study. Theoretical compositions are shown as open squares, whereas solid lines indicate compositions of coexisting phases

Hessite occurs in contact with cervelleite-like phases (Table 1, Fig. 6a,b), both included in galena. Such a spatial relationship is common for these minerals (e.g., Criddle et al. 1989; Spry and Thieben 1996).
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Fig. 6

Representative reflected light and SEM photomicrographs of the Kallianou ore assemblages. a, b. Cervelleite-like mineral (Crv) and hessite (Hs) included in galena (Gn) (plain polarized light); c. Unnamed Ag2CuTeS and cervelleite-like mineral (Crv) included in galena (Gn) ((plain polarized light); d, e. Unnamed Ag2CuTeS included in galena (Gn), chalcopyrite (Ccp) is also present (SEM-BSE image); f. Intergrowth of unnamed Ag2CuTeS and (Ag,Cu)2TeS included in galena (Gn) (SEM-BSE image)

Mineralogy of Te-polybasite and silver sulfotellurides

Te-polybasite

Tellurium-rich polybasite is a major Ag-carrier in Kallianou mineralization. It occurs together with zincian tetrahedrite as small (up to 30 μm) inclusions in galena that formed after pyrite (Fig. 3c,d). Polybasite with contains up to 68.3 wt. % Ag, up to 7.4 wt. % Te, and up to 9.8 wt. % Sb (Table 1). The Cu content is low (<5.2 wt.% Cu) in accordance with the hypothesis of Bindi et al. (2007a, b) that the copper content of pearceite-polybasite group minerals can be very low if selenium and/or tellurium are present. The chemical variation of polybasite in terms of CuS2-Ag2S-(Sb2S3 + As2S3) is shown in Fig. 4.

A cervelleite-like mineral and unnamed Ag2CuTeS and (Ag,Cu)2TeS

Cervelleite-like sulfotellurides (members of the system Ag-Cu-Te-S) are up to 20 μm in length and enclosed in galena. They form isolated grains or are intergrown with hessite (Fig. 6a, b). They are isotropic with a blue-green color in reflected light, in places with a patchy appearance due to minor compositional variation. Ideal cervelleite is absent in the deposit. The analyzed grains contain minor contents of Cu ranging from 0.14 to 2.64 wt. % corresponding to 0.013–0.239 apfu (Table 2). They also contain minor amounts of Bi and Au (<0.15 wt. %). The cervelleite-like mineral has a slight deficiency in the cation site occupancy relative to cervelleite (3.70 to 3.94 apfu). Most grains also have a Te/S ratio of ~1, like cervelleite (Criddle et al. 1989); however, some grains, have a Te/S ratio between 1.3 and 2.8, an excess in total anions (Te + S up to 2.5 apfu) and a significant deficit in cation site occupancy relative to cervelleite (3.48 apfu). Combining our data with previously published data for cervelleite-like phases (Fig. 7a, b), it is apparent that the cervelleite-like mineral from Kallianou plots close to cervelleite from other locations, but deviates from the Cu + Ag = 4 line (Fig. 7a). The variable, non-stoichiometric ratios between Σ(Cu + Ag) and Σ(Te + S) could be due to the presence of very thin intergrowths of different mineral phases such as hessite and/or other sulfotellurides (Novoselov et al. 2006). However, such intergrowths were not observed with a transmitted-light microscope or in back-scattered SEM images.
Table 2

Representative electron microprobe analyses of cervelleite-like minerals

wt%

Ag

Cu

Fe

Pb

Au

Bi

Sb

S

Te

Se

Total

Calculated formula (to 6 atoms)

1

64.75

0.54

0.00

1.02

0.00

0.13

0.00

7.11

27.12

0.05

100.82

(Ag3.429Cu0.050)3.479Te1.214S1.267

2

67.37

2.64

0.02

0.00

0.00

0.00

0.00

5.72

25.24

0.01

101.00

(Ag3.593Cu0.239)3.832Te1.138S1.026

3

68.59

2.54

0.05

0.00

0.00

0.00

0.00

5.82

25.01

0.01

102.02

(Ag3.619Cu0.227)3.846Te1.115S1.033

4

69.06

1.92

0.05

0.00

0.13

0.00

0.03

5.49

25.16

0.04

101.86

(Ag3.690Cu0.174)3.864Te1.136S0.986

5

64.45

0.34

0.01

0.00

0.00

0.00

0.03

5.68

28.84

0.01

99.38

(Ag3.561Cu0.032)3.593Te1.347S1.056

6

69.34

1.94

0.02

0.00

0.01

0.00

0.00

5.40

23.37

0.00

100.09

(Ag3.761Cu0.178)3.939Te1.072S0.985

7

67.75

0.28

0.00

0.24

0.15

0.00

0.01

5.99

24.07

0.00

98.48

(Ag3.732Cu0.026)3.758Te1.121S1.109

8

64.11

0.69

0.00

0.20

0.12

0.05

0.01

2.86

32.17

0.02

100.23

(Ag3.759Cu0.068)3.827Te1.595S0.564

9

63.90

0.14

0.00

0.06

0.00

0.00

0.05

5.70

30.79

0.13

100.76

(Ag3.498Cu0.013)3.511Te1.425S1.050

10

63.69

0.25

0.02

0.00

0.06

0.04

0.04

5.69

31.70

0.05

101.57

(Ag3.464Cu0.023)3.487Te1.457S1.042

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Fig. 7

Compositional plots illustrating variation of cervelleite-like minerals, Ag2CuTeS, and (Ag,Cu)2TeS. All previously published data have been included for comparative purposes (see legend). (a) Plot of Cu vs Ag, (b) Plot of Te/(Te + S+Se) vs Ag/(Ag + Bi + Cu)

The unnamed minerals with compositions approximating Ag2CuTeS and (Ag,Cu)2TeS (grain sizes are up to 20 μm in length), occur at the grain boundaries between galena-chalcopyrite or as inclusions in galena (Fig. 6c to f). The phase with the composition of Ag2CuTeS forms either as isolated inclusions in galena (Fig. 6c), or is spatially associated with the cervelleite-like mineral or unnamed (Ag,Cu)2TeS (Fig. 6f). The latter phase is optically similar to the cervelleite-like mineral since it possesses a blue-green color in reflected light and is isotropic. Representative electron microprobe compositions of the unnamed Ag2CuTeS phase are given in Table 3 and are plot together with all available data on minerals in the system Ag-Cu-Te-S in Figs. 7 and 8. The Ag2CuTeS phase contains minor amounts of Au, Bi, Sb, Fe and Se (Table 3) and shows very little variation in the Te/S ratio (~1) and Ag + Cu (~3). A limited solid solution exists between Ag and Cu (Fig. 7a). The phase (Ag,Cu)2TeS is intergrown with Ag2CuTeS, but appears to have been replaced by the phase Ag2CuTeS (Fig. 6f). Its optical properties and compositional data with Te/S ratio ~1 and inverse correlation between Ag and Cu are similar to those of the phase Ag2CuTeS (Fig. 7a,b). For both Ag2CuTeS and (Ag,Cu)2TeS, the Te:S remains fixed at 1:1 while the Ag + Cu varies from 3 to 2, and are distinguished from most cervelleite-like phases and the unnamed Ag3CuTeS and Ag2Cu2TeS phases described by Novoselov et al. (2006) and Cook and Ciobanu (2003), respectively, where Ag + Cu is close to 4 (Fig. 7).
Table 3

Representative electron microprobe analyses of Ag2CuTeS (1-17) and (Ag,Cu)2TeS (18-25)

wt%

Ag

Cu

Fe

Pb

Au

Bi

Sb

S

Te

Se

Total

 

Ag2CuTeS

Calculated formula (to 5 atoms)

1

47.92

14.03

0.01

0.40

0.00

0.00

0.00

7.12

29.91

0.00

99.47

Ag1.975Cu0.982Te1.045S0.988

2

47.64

14.29

0.13

0.40

0.01

0.08

0.00

7.71

28.38

0.08

98.73

Ag1.945Cu0.990Te0.979S1.060

3

48.30

13.80

0.12

0.48

0.00

0.00

0.00

7.11

29.21

0.02

99.08

Ag1.997Cu0.969Te1.021S0.989

4

48.81

14.53

0.87

0.31

0.00

0.00

0.00

7.34

29.58

0.04

101.47

Ag1.951Cu0.986Te0.999S0.987

5

49.32

13.20

0.56

0.58

0.00

0.02

0.00

6.86

30.08

0.04

100.69

Ag2.026Cu0.921Te1.044S0.948

6

48.16

13.73

0.01

0.43

0.00

0.00

0.00

7.19

28.57

0.04

98.14

Ag2.005Cu0.970Te1.005S1.007

7

47.91

13.83

0.00

0.71

0.09

0.05

0.05

7.04

28.99

0.03

98.72

Ag1.993Cu0.977Te1.020S0.985

8

46.59

14.80

0.03

0.67

0.00

0.00

0.00

8.04

29.41

0.03

99.59

Ag1.877Cu1.012Te1.002S1.089

9

47.46

16.92

0.00

0.00

0.02

0.00

0.07

6.67

28.39

0.00

99.53

Ag1.934Cu1.171Te0.978S0.915

10

46.71

18.21

0.00

0.00

0.02

0.02

0.06

6.53

27.99

0.00

99.54

Ag1.894Cu1.253Te0.959S0.891

11

49.39

13.07

0.00

0.20

0.10

0.00

0.06

7.06

29.98

0.00

99.86

Ag2.043Cu0.918Te1.048S0.983

12

49.28

13.56

0.73

0.00

0.00

0.00

0.04

7.03

30.44

0.00

101.09

Ag2.001Cu0.935Te1.045S0.960

13

48.23

12.99

0.09

0.00

0.00

0.00

0.11

6.98

29.98

0.00

98.41

Ag2.019Cu0.923Te1.061S0.984

14

49.00

13.35

0.03

0.27

0.07

0.03

0.05

7.19

29.78

0.00

99.78

Ag2.019Cu0.933Te1.037S0.997

15

48.80

13.48

0.02

0.05

0.00

0.00

0.08

7.03

29.94

0.00

99.39

Ag2.019Cu0.947Te1.048S0.979

16

48.72

14.55

0.00

0.00

0.00

0.02

0.03

7.46

29.72

0.00

100.48

Ag1.970Cu0.998Te1.016S1.016

17

48.49

13.75

0.02

0.00

0.08

0.00

0.07

7.03

29.88

0.00

99.33

Ag2.009Cu0.964Te1.045S0.978

(Ag,Cu)2TeS

Calculated formula (to 4 atoms)

18

42.58

12.40

0.01

0.42

0.00

0.05

0.01

8.28

36.09

0.00

99.83

(Ag1.393Cu0.688)2.081Te0.998S0.912

19

42.85

12.98

0.03

0.22

0.00

0.07

0.00

8.28

35.40

0.00

99.85

(Ag1.394Cu0.718)2.112Te0.974S0.907

20

43.61

10.89

0.02

0.61

0.07

0.00

0.00

7.74

35.10

0.01

98.10

(Ag1.474Cu0.625)2.099Te1.003S0.881

21

40.31

13.33

0.10

1.41

0.14

0.00

0.11

8.69

35.21

0.00

99.30

(Ag1.310Cu0.735)2.045Te0.968S0.951

22

40.90

12.58

0.04

0.00

0.23

0.02

0.14

8.58

36.07

0.00

98.57

(Ag1.341Cu0.700)2.041Te0.999S0.947

23

42.98

12.24

0.00

0.00

0.00

0.00

0.14

8.20

36.74

0.00

100.34

(Ag1.409Cu0.679)2.088Te0.989S0.897

24

41.30

12.95

0.05

0.00

0.03

0.00

0.12

8.63

36.52

0.00

99.60

(Ag1.339Cu0.713)2.052Te1.000S0.941

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Fig. 8

Ternary plot of Cu-(Ag)-(Te + S). Symbols as in Fig. 7

Discussion

Compositional variation

This study of the Kallianou deposit demonstrates that there are two new minerals in the system Ag-Cu-Te-S with the formula Ag2CuTeS and (Ag,Cu)2TeS. The first quaternary phase reported in the system Ag-Cu-Te-S was that by Cook and Ciobanu (2003) who described the mineral Ag2Cu2TeS from the Băiţa Bihor and Ocna de Fier skarns in Romania. The unnamed Ag3CuTeS phase described by Novoselov et al. (2006) is possibly another quaternary phase in this system. The compositions of these phases, which are plotted in Fig. 7, demonstrate that there is a wide range of Cu-substitution correlating with Ag extending from pure cervelleite (Ag4TeS) to Ag3.2Cu0.8TeS as analyzed by Cook and Ciobanu (2003) and Gu et al. (2003). A cervelleite-like phase from Panormos Bay, Tinos Island, plots within this range and is cuprian cervelleite. It is probable that this compositional range extends up to Ag2.9Cu1.2TeS (e.g. cervelleite-similar phases analyzed by Novoselov et al. 2006). Although this solid solution could further extend to the unnamed Ag2Cu2TeS phase (Cook and Ciobanu 2003), a considerable compositional gap between cuprian cervelleite and this phase, probably suggests that it is a distinct mineral. The coexistence of Ag2CuTeS and (Ag,Cu)2TeS with the cervelleite-like mineral in the Kallianou deposit indicates they are distinct and new minerals in the system Ag-Cu-Te-S. Their small size prevents any crystallographic studies to determine their structure. Moloshag and Gulyaeva (1990) and Maslennikov et al. (1997) identified a mineral with the formula close to Ag3TeS from the Guyskoe and Yaman-Kasy deposits, Urals (see Novoselov et al. 2006). It is probable that a solid solution also exists between Ag3TeS and the unnamed Ag2CuTeS, as is the case for Ag4TeS-Ag3CuTeS, but this remains to be proven, since no other phases until now have been described with Ag + Cu = 3 apfu. However, it is unclear why the Te:S ratio in the studied sulfotellurides remains fixed while the Ag + Cu varies from 4 to 3 to 2.

Tellurium-rich polybasite and cadmian tetrahedrite in the Kallianou deposit are also too small for crystallographic investigation. Cadmian tetrahedrite was defined as a new mineral by Voropayev et al. (1988) from the Ushkatin deposit, central Kazakhstan, but this contained much lower Cd (5.8 wt. %) than the Kallianou sample. Te-rich polybasite (up to 5.5 wt. % Te) was firstly described from the Pongkor epithermal gold-silver deposit, west Java, Indonesia (Warmada et al. 2003). Tellurium-rich polybasite from Kallianou deposit contains up to 7.4 wt. % Te (Voudouris and Spry 2008) and is only the second known occurrence. The Te-bearing polybasite from Kallianou, with the highest Te content ever reported, is also believed to be a new member of the polybasite-pearceite group of minerals, similar to selenopolybasite (Bindi et al. 2007b).

Conditions of sulfotelluride formation

Cervelleite-like sulfotellurides can form from hydrothermal fluids over a wide range of temperatures and fTe2 values: <100°C, logfTe2 from -26 to −17 (Novoselov et al. 2006), <230°C (Karup-Møller 1976), 160–260°C, logfTe2 from −23 to −17 (Helmy 1999), 250°C, logfTe2 from −17.4 to −13.4 (Gu et al. 2003), 195–250°C (Spry and Thieben 1996), ~260°C (Tombros et al. 2004) and up to 420°C (Cook and Ciobanu 2003). At Ocna de Fier, cervelleite and coexisting Ag2Cu2TeS appear to belong to a distinct, post-peak Ag-Te-Bi-bearing assemblage formed at lower temperatures (Cook and Ciobanu 2003). In the Larga epithermal deposit, cervelleite is stable in pyrite-rich ores buffered by intermediate fTe2 in which co-existing Bi-tellurides/tellurosulfides have Bi/Te(+S) ≤ 1 (Cook and Ciobanu 2003). Relationships among Ag-sulfotellurides and associated minerals indicate that may be formed in different ways (Novoselov et al. 2006): as isolated subhedral grains, as reaction products between Ag-sulfides and tellurides, and as decomposition products of a higher-temperature solid solution phase.

Even in the absence of fluid inclusion data, it is considered that maximum temperature of formation of the Kallianou quartz veins was about 300°C. This temperature estimate is based on that of Nüchter and Stöckhert (2007) who proposed that microfabrics in the discordant quartz veins in the metamorphic rocks of the broad Kallianou area, indicate deformation at temperatures around 300°C, at a depth just below the brittle-ductile transition. Similar temperatures (290–310°C) were also estimated on the basis of chlorite geothermometry for the formation of extensional quartz veins hosted within retrograde metamorphic rocks of the Cycladic Blueschist Unit in the central part of Evia Island (Voudouris et al. 2005).

The assemblage fahlore + polybasite + sphalerite at Kallianou was further used to estimate temperatures of formation based on the polybasite-pearceite and tetrahedrite-group minerals geothermometer of Sack (2005), which uses the composition of fahlores in the system Ag2S-Cu2S-ZnS-FeS-Sb2S3-As2S3. Fahlore and polybasite compositions from Kallianou are plotted in terms of Cu/(Cu + Ag) vs. As/(As + Sb) in Fig. 9 and suggest that fahlore and polybasite at Kallianou formed below 200°C. This low temperature suggests either re-equilibration between a solution and pre-existing fahlores, precipitation (at low temperatures) from an evolving fluid, or retrograde reactions without changing fluid composition (Chutas and Sack 2005). Polybasite from Kallianou has Cu/(Cu + Ag) ratios less than those of the calculated polybasite-pearceite curves and is believed to reflect the nonquenchable nature of polybasite-pearceite compositions (Sack 2005). However, if the fahlores analyzed were not in equilibrium with polybasite a temperature different from that at which precipitation occurred would be obtained (Chutas and Sack 2004). Fahlores coexisting with polybasite-perceite and (Ag,Cu)2S solid solutions are usually enriched in Ag/(Ag + Cu) (Sack 2005), whereas Kallianou fahlores have low Ag content (<6 wt % Ag).
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Fig. 9

Fahlore (circles) and polybasite (diamonds) compositions from Kallianou samples plotted in the Cu/(Cu+Ag) vs As/(As+Sb) diagram (after Sack 2005). The Kallianou assemblages are saturated with respect to sphalerite. Dark curves represent compositions of fahlore (Fah) and polybasite-pearceite (Plb-Prc) calculated for the assemblage Fah + Plb-Prc + (Ag,Cu)2S solid solution (Argcal) + Sphalerite at 200 and 300°C using the updated parameters of Sack (2005). Dashed curves for 300 and 350°C are for the Fah + Plb-Prc + pyrargyrite-proustite (Prg-Prs) +Sph assemblage with molar Zn/(Zn+Fe) in Fah of 0.75

It is suggested that the Ag-bearing phases observed at Kallianou may be the products of retrograde solid-state re-equilibration during cooling from a higher temperature galena phase (Chutas et al. 2008). According to Chutas and Sack (2005) and Sack et al. (2005), it appears that significant Ag is concentrated in exsolution products of galena such that all of the Ag now contained in fahlores, polybasite-pearceite, pyrargyrite-proustite, and (Ag,Cu)2S phases in many Ag-Pb-Zn deposits could have been derived from the galena originally deposited by hydrothermal solutions. As suggested by Foord et al. (1988), during cooling, primary Bi-Ag-bearing galena should decompose into Ag-free galena, pyrargyrite and Bi-bearing sulfosalts. Bismuth-bearing sulfosalts are not spatially associated with galena from Kallianou, but this is believed to reflect high Ag + Sb + Te/Bi ratio of the ore fluid, rather than a low temperature of galena deposition (from 100 to 160°C, based on the Sb:Bi = 7.7 ratio in one galena sample and the geothermometer of Foord et al. 1988).

Ciobanu et al. (2004) interpreted binary blebs of hessite-cervelleite in galena from Roşia Montană, Romania as evidence of formation of the two minerals at eutectic conditions in the system Ag-Te-S system. They argue that the presence of cervelleite indicates that the hessite-forming droplets originally contained sulfur.

The telluride and sulfide mineralogy is used to constrain fTe2 and fS2 conditions of the ore-forming solutions at Kallianou (Fig. 10). Thermodynamic data for sulfides and tellurides used for calculating fTe2 and fS2 were derived from Mills (1974), Barton and Skinner (1979) and Afifi et al. (1988). Telluride stabilities were calculated for an average temperature of 300°C. Since no thermodynamic data are available for sulfotellurides, logfTe2 values of the hydrothermal solutions from ~−14.8 to −7.8 can be estimated by the boundaries of the stability fields of hessite/argentite and γ-phase/hessite (Fig. 10a). These values are consistent with the presence of galena, instead of altaite, and electrum in the Kallianou deposit. The deposition of sulfotellurides occurred under logfS2 values of ~ −11.5 to −8.3 and is consistent with the presence of chalcopyrite, pyrite and tennantite instead of enargite.
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Fig. 10

LogfTe2 - logfS2 diagram indicating equilibria between tellurides and sulfides at 300° and 200°C at vapor saturation. The shaded area indicates approximate conditions of sulfotelluride deposition at Kallianou deposit, estimated from the observed mineral assemblages. Abbreviations: γ = gamma-phase, Ccp = chalcopyrite, Clv = calaverite, Eng = enargite, Hs = hessite, Po = pyrrhotite, Py = pyrite, Sz = stützite, Ttn = tennantite

In order to show some of the low temperature relations that are likely to have taken place as the telluride system cooled, we also calculated equilibria at 200°C (Fig. 10b). The logfTe2 and logfS2 values during re-equilibration were constrained to −19.5 to −15.2 and to −15.8 to −11.5 respectively, on the basis of equilibrium assemblages involving hessite, acanthite and native silver. As discussed by Gu et al. (2003), the close spatial association of silver sulfotellurides to hessite and acanthite, indicates a hypothetical field close and parallel to the Ag2Te/Ag2S line on a plot logfTe2 versus logfS2. However, according to Helmy (1999), cervelleite will form at lower values of fTe2 than hessite. The low fTe2 values during sulfotelluride formation at Kallianou are consistent with the occurrence of native silver, Ag-rich electrum and the absence of other tellurides (e.g. Au, Pb) in the mineralization (Afifi et al. 1988).

Silver sulfotellurides are not yet observed in the telluride-enriched northeastern Greek magmatic-hydrothermal systems (Voudouris 2006), but was found in the Fakos porphyry-epithermal prospect (Limnos Island), where an unnamed silver sulfotelluride (Ag12Te4S3) coexists with hessite as tiny blebs within galena. The close affinity among galena, hessite and silver sulfotelluride at Fakos is similar to that described from Kallianou and strongly resembles textures from Roşia Montană (Ciobanu et al. 2004). Blebs of hessite, electrum and petzite (but without sulfotellurides) within galena are also described from the epithermal Au-Ag-Te Profitis Ilias deposit, Milos Island (Alfieris and Voudouris 2006), thus emphasizing the role that galena plays in hosting hessite intermediate solid solutions (Hs-iss) and the distribution of precious metals in several deposits (Ciobanu et al. 2004). A cuprian cervelleite-hessite-galena assemblage also occurs in the nearby Tinos Island, where marble-hosted Au-Ag-Te-Sn-bearing quartz veins at Panormos Bay are considered to be genetically related to the Sn-W-bearing Tinos leucogranite (Tombros et al. 2004, 2007, 2010; Spry et al. 2006).

Conclusions

  1. 1.

    A cervelleite-like mineral, two unnamed silver sulfotellurides, Te-rich polybasite and cadmian tetrahedrite are described for the first time in the Kallianou area, Evia Island, and are accompanied by native gold/electrum, native silver, sulfides and sulfosalts in the deposit.

     
  2. 2.

    The unnamed sulfotellurides with compositions approximating Ag2CuTeS and (Ag,Cu)2TeS are new minerals in the system Ag-Cu-Te-S.

     
  3. 3.

    The Te-bearing polybasite from Kallianou, with the highest Te content ever reported (up to 7.4 wt. % Te), is believed to be a new member of the polybasite-pearceite group of minerals, similar to selenopolybasite.

     
  4. 4.

    The Ag-Te-Cu-S assemblage observed at Kallianou may be formed from the breakdown of a higher temperature galena phase during cooling, thus emphasizing the role that galena played in the distribution of precious metals in the deposit.

     
  5. 5.

    Initial deposition of sulfotellurides at ~300°C occurred under logfS2 values of −11.5 to −8.3, and logfTe2 values of −14.8 to −7.8. The logfTe2 values during re-equilibration at ~200°C were constrained to −19.5 to −15.2 close to the acanthite/hessite/native silver boundaries.

     

Acknowledgements

For assistance with EPMA and SEM work, we thank Stefanie Heidrich and Evangelos Michailidis, respectively. Drs. Luca Bindi and Iain Pitcairn are greatly acknowledged for their helpful and constructive comments. Editor Prof. Dr. R. Abart is especially thanked for editorial handling.

Copyright information

© Springer-Verlag 2011