Ag2(S,Se) solid solutions in the ores of the Rogovik gold-silver deposit (northeastern Russia)

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Ag2(S,Se) solid solutions in the ores of the Rogovik gold-silver deposit (northeastern Russia) G.A. Pal’yanova a,c,*, R.G. Kravtsova b, T.V. Zhuravkova c a

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia c Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 7 July 2014; accepted 4 March 2015

Abstract The relationships and chemical compositions of silver sulfoselenides in the ores of the Rogovik gold-silver deposit (northeastern Russia) were studied to refine the low-temperature region of the Ag2S–Ag2Se phase diagram and identify contradictions between natural and experimental data. Two types of relationships between the phases of the system Ag2S–Ag2Se have been recognized using optical and scanning electron microscopy: (1) Se-acanthite and S-naumannite occur as monomineral microinclusions or fill cracks in the grains or the interstices of other minerals, and acanthite (free of impurities) forms rims on Fe-sphalerite; (2) Se-acanthite forms rims on S-naumannite. Electron probe microanalysis of silver sulfoselenides from the Rogovik ores revealed 0–7.9 wt.% Se in acanthite and 0–3.2 wt.% S in naumannite, which corresponds to the acanthite series Ag2S–Ag2S0.74Se0.26 and naumannite series Ag2S0.28Se0.72–Ag2Se. The composition ranges of the studied acanthite and naumannite series are wider than those of natural silver sulfoselenides from the Guanajuato (Mexico), Silver City (USA), Salida (Indonesia), and other deposits (Ag2S–Ag2S0.85Se0.15 and Ag2S0.12Se0.88–Ag2Se, respectively) but are significantly narrower than the composition ranges of synthetic samples: Ag2S–Ag2S0.4Se0.6 and Ag2S0.3Se0.7–Ag2Se. The presence of intergrowths of two phases of the Ag2S-Ag2Se series in the form of Se-acanthite rims on S-naumannite in the Rogovik ores and the absence of three-phase intergrowths of silver sulfoselenides Ag2S1-xSex from this and other deposits do not confirm the assumption on the existence of the third solid solution. The results of earlier studies of natural Ag2(S,Se) solid solutions show the existence of two solid solutions (of the acanthite and naumannite series) in the Ag2S–Ag2Se system and confirm the experimental data. It is necessary to carry out a detailed examination of natural silver sulfoselenides falling in the interval from Ag2S0.4Se0.6 to Ag2S0.3Se0.7 in order to identify the limits of two-phase immiscibility. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Ag2(S,Se) solid solutions; acanthite; naumannite; Ag2S–Ag2Se phase diagram

Introduction Acanthite (Ag2S), naumannite (Ag2Se), and silver sulfoselenides Ag2(S,Se) are typical of hydrothermal gold and silver deposits (Sindeeva, 1959; www.mindat.org). In nature their low-temperature modifications or, more seldom, paramorphs are stable. Since silver chalcogenides Ag2(S,Se) are characterized by phase transitions and structural phase transformations, they are of special interest as mineral geothermometers and indicators of the physicochemical conditions of ore formation. There is a contradiction between the natural and experimental data on the low-temperature region of the Ag2S–Ag2Se phase diagram. Petruk et al. (1974), who com* Corresponding author. E-mail address: [email protected] (G.A. Pal’yanova)

prehensively studied the structural relationships and compositions of natural silver sulfoselenides, established limiting contents of Se impurities in acanthite (Ag2S–Ag2S0.85Se0.15) and limiting contents of S impurities in naumannite (Ag2S0.12Se0.88–Ag2Se) as well as variations in the contents of these elements in aguilarite (Ag4S0.95Se1.05–Ag4S1.10Se0.90 (or Ag2S0.475Se0.525–Ag2S0.55Se0.45)). Earlier experiments (Bontschewa-Mladenowa and Vassilev, 1984; BontschewaMladenowa and Zaneva, 1977) revealed two solid solutions, acanthite (Ag2S–Ag2S0.5Se0.5) and naumannite (Ag2S0.5Se0.5– Ag2Se), with a break in miscibility at 50 mol.% Ag2Se, as well as individual phase Ag4SSe. The experimental study of the Ag2S–Ag2Se phase diagram (Pingitore et al., 1992, 1993) at room temperature demonstrated the presence of two solid solutions: acanthite, Ag2S–Ag2S0.4Se0.6 (monoclinic system, space group P21/c), and naumannite, Ag2S0.3Se0.7–Ag2Se

1068-7971/$ - see front matter D 201 5, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2015.11.006

G.A. Pal’yanova et al. / Russian Geology and Geophysics 56 (2015) 1738–1748

(rhombic system, space group P212121). Recent research (Bindi and Pingitore, 2013) has shown that aguilarite is monoclinic, crystallized in space group P21/n, with a = 4.2478(2), b = 6.9432(3), c = 8.0042(5) Å, β = 100.103(2)º, V = 232.41(2) Å3, and Z = 4, and isostructural with acanthite. Many researchers (Bindi and Pingitore, 2013; Cocker et al., 2013; Novoselov et al., 2009; Pal’yanova et al., 2014) emphasized that it is necessary to study the chemical composition of silver sulfoselenides for revealing S-Se isomorphism in natural solid solutions and refining the low-temperature region of the Ag2S–Ag2Se phase diagram. Data on the chemical composition of these minerals are often given in publications and must be generalized and analyzed (Nekrasov, 1997; Petruk et al., 1974; Sakharova et al., 1993; Shikazono, 1978; Sindeeva, 1959; Vassalo-Morales and Borodaev, 1982). Since aguilarite is a phase of the acanthite series and is not an individual mineral species (Bindi and Pingitore, 2013), it is necessary to re-examine the published data on it (Kazachenko et al., 2008; Nekrasov, 1997; Petruk et al., 1974; Sakharova et al., 1993; Warmada et al., 2003). The goal of this work was to study the compositions of acanthite and naumannite and their relationships in the ores of the Rogovik gold-silver deposit (northeastern Russia) and elucidate the peculiarities of isomorphous substitution of sulfur and selenium in natural solid solutions of the series Ag2S–Ag2Se. One of the tasks was to investigate natural parageneses of silver sulfoselenides, using the Ag2S–Ag2Se phase diagram, and to confirm or refute the results of experimental studies of Ag2(S,Se) (Pingitore et al., 1992, 1993) and the data on the existence of two or three solid solutions at environment temperatures (Petruk et al., 1974). Methods To detect inhomogeneities in natural rocks containing minerals of the system Ag2S–Ag2Se, we applied optical and scanning electron microscopy. Determination of the composition of microinclusions and detection of impurity elements were made by electron probe microanalysis (EPMA), using a JXA-8200 (JEOL Ltd, Japan) microprobe (Institute of Geochemistry, Irkutsk, analyst L.A. Pavlova). The following detailed examination of the chemical composition of minerals was carried out with a MIRA 3 LMU (Tescan Orsay Holding) scanning electron microscope equipped with an INCA Energy 450+ microanalysis system and an INCA Wave 500 (Oxford Instruments Nanoanalysis Ltd) wave spectrometer (Institute of Geology and Mineralogy, Novosibirsk, analyst N.S. Karmanov). Analyses on MIRA 3 LMU (with the use of SEM-EDS in most cases) were performed at 20 kV accelerating voltage and 1.5 nA probe current; the counting time was 15–20 s. Acquisition of spectra was performed in the raster mode with the scanning area from 0.5 × 0.5 μm2, for fine phases, to 2 × 2 μm2, for larger phases, and with a slightly defocused electron beam. This regime makes it possible to reduce the influence of the sample microrelief on the quality of analysis and the destructive effect of electron current on unstable Ag sulfides and selenides.

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Pure metallic Ag (for Ag), CuFeS2 (for S, Cu, and Fe), PbSe (for Pb, and Se), and ZnS (for Zn) were used as standards. We studied mainly coarse grains (>5 μm) to prevent the presence of background contents of elements from the surrounding phases. In these analytical conditions, the detection limits of elements were tenths of percent. The error of determination was ≤1 rel.% for major components (>10– 15 wt.%) and <2 rel.% for minor ones (1–10 wt.%). Several samples of three types of ores from the Rogovik gold-silver deposit were analyzed for Au, Ag, Se, and S in the analytical laboratories of the Institute of Geochemistry (Irkutsk). The contents of Au were measured by the atomicabsorption method (Torgov and Khlebnikova, 1977) (analyst P.T. Dolgikh). The contents of Ag were determined by approximate quantitative spectral analysis (spilling method) and approximate quantitative atomic-emission spectral analysis (evaporation method) (Vasil’eva et al., 1997) (analyst N.E. Smolyanskaya). Analysis for Se was carried out by the fluorometric method (Govindaraju, 1984) (analyst N.N. Bryukhanova). The total contents of S were measured by the volumetric (iodometric) method (Instruction..., 1965) (analyst T.N. Galkina). Brief characteristics of the deposit The Rogovik gold–silver deposit is located in the central part of the Okhotsk–Chukchi volcanogenic belt, in the northern closure of the Balygychan–Sugoi trough, in the area of the largest industrial Omsukchan Au-Ag-ore district. Earlier this deposit was assigned to the gold-argentite type of epithermal Au-Ag ore association (Kuznetsov et al., 1992). The subsequent research showed that the ore mineralization of the deposit is of more complex chemical composition and formed in several stages (Kravtsova et al., 2012, 2015). Epithermal Au-Ag ores of poor mineral and elemental compositions were produced at the early volcanic stage. A predominant ore mineral is pyrite; galena, sphalerite, and chalcopyrite are rare. The main Ag-minerals are acanthite (Ag2S), silver sulfosalts (proustite, Ag3AsS3; pyrargyrite, Ag3SbS3), and kustelite; native silver and naumannite (Ag2Se) are scarcer. The fahlores of the tennantite–tetrahedrite (Cu3AsS3–Cu3SbS3) series sometimes contain Ag. Native gold (fineness of 600–700) occurs as microinclusions and intergrowths with the other above-mentioned ore minerals. At the later volcanoplutonic stage, the intrusion of granitoids led to the formation of Ag ores; at the sites of the coexistence of Au-Ag and Ag mineralization in deeper horizons, polyassociation ores were produced (Kravtsova et al., 2012, 2015). The Ag ores are rich in native silver, kustelite, silver sulfosalts (mainly pyrargyrite), and S-naumannite, whereas Se-acanthite, sternbergite (AgFe2S3), stromeyerite (CuAgS), polybasite ((Ag,Cu)16Sb2S11), and freibergite (Ag6Cu4Fe2Sb4S13) are scarce and miargyrite (AgSbS2) is rare. A predominant simple sulfide is pyrite, whereas galena, sphalerite, and chalcopyrite are scarce. The polyassociation ores are characterized by still more intricate elemental and mineral compositions. They bear all

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the above ore minerals as well as hessite (Ag2Te), argyrodite (Ag8GeS6), canfieldite (Ag8SnS6), mercurian gold and silver, imiterite (Ag2HgS2), chalcostibite (CuSbS2), and Se-stephanite (AgSbS4). This type of ores is the richest in silver sulfoselenides. The polyassociation Au-Ag ores are of complex composition and are specific for the deposits of the Omsukchan ore district. The latter encloses the world-largest Dukat Au–Ag deposit, whose ores are similar in mineral composition and enrichment in silver sulfoselenides to the analogous ores of the Rogovik deposit (Konstantinov et al., 1998, 2003; Kravtsova, 2010; Kravtsova et al., 2012, 2015; Tauson et al., 2014). The contents of Ag, Au, S, and Se in different types of ores of the Rogovik deposit are listed in Table 1. The Au-Ag ores and polyassociation ores are sulfide-poor (0.2 to 4 wt.% S), whereas the predominantly silver ores contain up to 15 wt.% S and must be referred to as sulfide ores. The content of Se in the ores increases from 8–11 in the Au–Ag ores to 16–133 ppm in the predominantly silver ores, reaching a maximum (23–440 ppm) in the polyassociation ores. Accord-

ing to Ag contents (up to 10,000 ppm), the ores of the three types can be assigned to the Ag-complex-metal association (Konstantinov et al., 2003) widespread in the Omsukchan ore district (Konstantinov et al., 1998; Kravtsova, 2000, 2010; Kravtsova et al., 1996). The largest silver chalcogenide grains (2–200 μm) have been found in the polyassociation ores. The grains of Au–Ag ores and predominantly silver ores measure <10 μm. The polyassociation ores and silver sulfoselenides in them will be considered in detail in the next section.

Study of the compositions and relationships of silver sulfoselenides in the ores of the Rogovik deposit The comprehensive studies of Se-enriched polyassociation Au–Ag ores made it possible to study the variations in S and Se contents in Se-acanthite and S-naumannite and to elucidate the relationships between silver sulfoselendies as well as the compositions of minerals in paragenesis with them. We have

Table 1. Contents of Au, Ag, Se, and S in ores of the Rogovik gold-silver deposit Sample

Abs. level, m*

Au, ppm

Ag(Ag)**, ppm

Se, ppm

S, %

Gold-silver ores of early volcanic stage 500/11

300

1.50

20(10)

8.2

0.81

724/11

310

0.57

20(20)

7.4

0.11

155003

320

2.21

10(10)

9.2

0.25

155025

300

1.05

30(25)

10.3

0.17

155033

295

2.52

20(15)

10.6

0.44

155035

295

4.33

30(10)

10.1

0.50

155037

295

2.16

30(15)

10.2

0.57

155045

290

1.18

30(15)

8.8

0.50 2.13

Predominantly silver ores of late volcanoplutonic stage 220/11

250

0.07

30(500)

21.7

222/11

240

0.18

50(100)

132.8

15.43

223/11

240

0.10

30(50)

67.2

10.71

750/11

260

0.11

> 100(1000)

71.3

1.65

781/11

200

0.03

> 100(1000)

17.1

2.51

808/11

150

0.06

> 100(600)

69.4

1.97

38399

185

0.06

100(80)

16.0

2.51

MN-157914

220

0.14

> 100(500)

57.0

1.36

260.10

> 100(6000)

25.1

3.81

Polyassociation gold-silver ores of late volcanoplutonic stage MN-628/11

240

820/11

125

3.10

> 100(600)

29.1

1.08

822/11

120

31.09

> 100 (4000)

141.5

1.11

830/11

105

88.13

> 100(10000)

23.1

2.60

156125

240

2.16

> 100(4000)

320.2

1.12

156572

220

1.88

> 100(2000)

120.1

1.62

MN-156572

220

0.36

> 100(2000)

128.3

1.76

MN-156954

240

0.47

> 100(3000)

119.4

1.50

MN-159425

100

2.61

> 100(6000)

436.1

1.59

Note. We conventionally accepted the depth range 290–320 m as upper ore, 150–260 m as middle ore, and 100–240 m as lower and middle ore intervals. *Absolute levels of the sampling horizons. **Ag(Ag), Ag was measured by electron probe microanalysis (Ag was measured by approximate quantitative atomic-emission spectral analysis).

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established that acanthite and naumannite in the Rogovik ores are present as xenomorphous monomineral microinclusions in pyrite and argyrodite and as veinlets in the grain cracks and interstices of pyrite, quartz, muscovite, and K-feldspar crystals (Figs. 1 and 2). As seen from Fig. 1, naumannite occurs as microinclusions in argyrodite and pyrite present in the Q–Mu–Kfs1 matrix. Naumannite contains sulfur impurities (0.3–0.4 wt.%), and argyrodite (Ag8GeS6) contains selenium (3.1 wt.%), whereas pyrite lacks it. Figure 1b shows that naumannite (containing up to 0.3 wt.% S) and imiterite (Ag2HgS2) are present as rims on argyrodite (containing 4.4 to 4.8 wt.% Se), and acanthite (lacking impurities of Se and other elements) occurs as veinlets in the argyrodite core. Acanthite (containing 2.6 wt.% Se) is porous and is often localized at the contact of Se-argyrodite with mercurian silver (Ag0.85Au0.05Hg0.10) (Fig. 1c). Study of the composition of coarse naumannite grains in the core, in the rim, and in the interstices of pyrite crystals (Fig. 1d) revealed variations in the contents of S from 0.3 to 3.2 wt.% and of Se from 13.6 to 23.4 wt.% as well as impurities of Fe (1.2 wt.%) and Te (1.4 wt.%). S-naumannite (containing 0.9 to 1.8 wt.% S) is intergrown with euhedral pyrite microcrystals (Fig. 1e, f) or mercurian kustelite (Ag0.7Au0.2Hg0.1) (Fig. 1e) localized in quartz and K-feldspar interstices. S-naumannite (containing 0.4 to 0.8 wt.% S) intergrown with galena (Fig. 2a–c), pyrargyrite (Ag3SbS3) (Fig. 2d, e), and Se-stephanite (Fig. 2e) composes veinlets and fills voids in pyrite. Impurities of Se and Fe are found in galena (~2.6 and ~0.7 wt.%, respectively) and proustite (~1.3 and ~5 wt.%), whereas stephanite contains only Se (3.8– 6.8 wt.%). Fe-acanthite (containing 2.3 wt.% Fe) is one of late minerals; it forms continuous 5–20 μm thick rims on Fesphalerite (containing 1.2 wt.% Fe) (Fig. 2f). The S-naumannite grains measure 2–30 or, more seldom, 50–100 μm. Note that the phases of the acanthite and naumannite series are not in contact with each other (Figs. 1 and 2). In addition to monomineral microinclusions of silver sulfoselenides, the Rogovik ores were found to contain intergrowths of two mineral phases of the Ag2S–Ag2Se system in the form of Se-acanthite rims on S-naumannite. As well seen from Fig. 3a, naumannite (bluish-gray, with blue to brown-gray anisotropy) has an acanthite rim (metallic-gray) and is intergrown with pyrite and chalcopyrite. Figure 3c–i shows the distribution of Ag, Se, S, and other elements over the scanning area of this polished-section site (Fig. 3a). In Fig. 3b, e, g, one can see galena lamellae (3–5 μm) in naumannite, and in Fig. 3f, i, As-chalcostibite (1.7 wt.% As) microinclusions. A grain fragment (outlined by a red line, Fig. 3b) containing S-naumannite with Se-acanthite rim was thoroughly examined for variations in S and Se contents in both silver sulfoselenides 1

Ac, acanthite; Ag(Hg,Au), mercurian kustelite; Nmt, naumannite; Argd, argyrodite; Ccp, chalcopyrite; Ga, galena; Sp, sphalerite; Pr, proustite; Py, pyrite; Imt, imiterite; Stph, stephanite; Csb, chalcostibite; Q, quartz; Kfs, K-feldspars; Mu, muscovite.

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(Fig. 4a). As seen from Fig. 4b, the contents of these elements in each mineral are constant within the accuracy of electron probe microanalysis. Selenium impurities in acanthite amount to 4.8–5.2 wt.% (11.3–11.7 wt.% S), and sulfur impurities in naumannite amount to 1.7–2.3 wt.% (21.8–22.4 wt.% Se). No other silver sulfoselenide at the contact between acanthite and naumannite has been revealed. In the optical photo (Fig. 3a) one can see a distinct boundary between the two phases. Figure 3d, e shows the distribution of S and Se over the scanning area and a sharp boundary between the silver sulfoselenides. The performed analyses confirm that the transition of S-naummanite into Se-acanthite is not smooth and gradual and there is no intermediate phase between them. Most likely, acanthite formed later than naumannite, and these silver sulfoselenides are not exsolved products of high-temperature Ag2(S,Se). Figure 5 shows the earlier determined compositions of silver sulfoselenides from the Rogovik deposit (Kuznetsov and Livach, 2005) and our research data on the contents of S and Se in its acanthite and naumannite. The maximum content of S in naumannite is 3.2 wt.%, and the maximum content of Se in acanthite is 7.9 wt.%.

Discussion Based on the above research data on the composition of the Rogovik silver sulfoselenides, two ranges of isomorphous substitution of sulfur and selenium in acanthite and naumannite have been recognized (calculated for 3 f.u.): Ag2S– Ag2S0.74Se0.26 and Ag2S0.28Se0.72–Ag2Se (Figs. 5 and 6). These composition ranges are broader than those of natural silver sulfoselenides from the Guanajuato (Mexico), Silver City (USA), Salida (Indonesia), and other deposits (Ag2S– Ag2S0.85Se0.15 and Ag2S0.12Se0.88–Ag2Se, respectively) (Petruk et al., 1974) but are significantly narrower than the composition ranges of synthetic samples (Ag2S–Ag2S0.4Se0.6 and Ag2S0.3Se0.7–Ag2Se). There are only two types of relationships between the phases of the acanthite and naumannite series: (1) with no contact or intergrowth of two different silver sulfoselenides and (2) with contact and mutual replacement of two silver sulfoselenides (Se-enriched and S-enriched ones). Three-phase intergrowths of silver sulfoselenides have not been revealed in the Rogovik ores. Petruk et al. (1974) observed the following intergrowths of three phases in the ores of the Silver City and Guanajuato deposits: (1) Massive naumannite containing aquilarite lamellae (<2 μm) with acanthite inclusions and (2) acanthite with aguilarite inclusions (<2 μm), surrounded by naumannite. These data, however, are based on the degree of etching (acanthite is strongly etched, aguilarite is moderately etched, and naumannite is not etched) and on the results of electron microprobe analysis performed with a probe of large diameter (15–80 μm) exceeding the grain size (i.e., these results characterize the bulk compositions). The above authors ignored the variant that one of the phases taken for silver sulfoselenide might have been native silver or selenium. The

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Fig. 1. Forms of acanthite and naumannite in the Rogovik ores. a, S-naumannite as separate microinclusions in Se-argyrodite and pyrite; b, Se-argyrodite intergrown with naumannite, acanthite, and imiterite in quartz; c, Se-acanthite at the contact with Se-argyrodite and mercurian kustelite; d, coarse and fine S-naumannite grains in the core, rim, and interstices of pyrite crystals; e, S-naumannite and pyrite intergrown with mercurian kustelite and K-feldspar; f, S-naumannite intergrown with pyrite and K-feldspar.

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Fig. 2. Forms of naumannite in the Rogovik ores. a–c, S-naumannite as separate microinclusions and veinlets in pyrite intergrown with galena; d, S-naumannite and proustite filling pyrite cracks; e, S-naumannite intergrown with proustite and Se-stephanite in quartz; f, acanthite rims on Fe-sphalerite and acanthite veinlets in pyrite in quartz–K-feldspar aggregate.

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Fig. 3. Optical (reflected light) (a) and SEM (b) photos of Se-acanthite rims on S-naumannite and chalcopyrite and distribution of elements over the scanning area of the polished section in characteristic AgLα1, SeLα1, SKα1, SbKα1, PbKα1, FeKα1, and CuKα1 radiation (c–i, respectively).

presence of metastable phases AgSe2, AgSe, and Ag3Se2 is also possible (Kienel, 1960). The conclusion about the existence of the third (aguilarite) solid solution in the system Ag2S–Ag2Se is doubted. The absence of this solution is confirmed by the data of Vassalo-Morales and Borodaev (1982) and Vassalo-Morales et al. (1982), who comprehensively studied the mineral phases Ag2S–Ag2Se at the Guanajuato deposit and established the ranges and discontinuity of the acanthite series Ag2S–Ag2S0.5Se0.5. These authors also observed naumannite Ag2Se in ores. In terms of phase equilibria, three-phase intergrowths of silver sulfoselenides in the system Ag2S–Ag2Se are forbidden and cannot exist in nature. According to the Ag2S–Ag2Se phase diagram (Fig. 6), only one silver sulfoselenide either of the acanthite series Ag2S– Ag2S0.4Se0.6 or the naumannite series Ag2S0.3Se0.7–Ag2Se

can crystallize under equilibrium. The composition of monomineral silver sulfoselenides is determined by the physicochemical conditions of ore formation: temperature, redox potential, pH, and S and Se concentrations in the ore-forming solution. When the crystallization temperature is higher than the temperature of phase transition (Tph.t.), but lower than the temperature of melting (Tm), a high-temperature phase is produced, which becomes a low-temperature one on cooling. Differential scanning calorimetric study showed that the synthetic samples of different compositions intermediate between Ag2S and Ag2Se undergo rapid reversible phase transformations in the temperature range from 70 to 178 ºC (Pingitore et al., 1992, 1993). These temperatures depend on the phase composition and are the maximum for the terminal phases Ag2S (178 ºC) and Ag2Se (134 ºC) and the minimum for the phase range from Ag2S0.4Se0.6 to Ag2S0.3Se0.7 (70 ºC).

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Fig. 4. Intergrowths of two silver sulfoselenides. a, Magnified fragment (red outline, Fig. 3b) with Se-acanthite rim on S-naumannite; b, section demonstrating a change in element contents in silver sulfoselenides along the line with points a(1)–a(11) in Fig. 4a.

The temperature of α ↔ β phase transition for aguilarite Ag4SSe (or Ag2S0.5Se0.5) is within 75–100 ºC, according to different authors: 100 ± 5 °C (Bontschewa-Mladenowa and Zaneva, 1977), 95 °C (Vassilev and Ivanova, 2003), and 75 °C (Alekperova et al., 2007). Low-temperature modifications of silver sulfoselenides are more stable in nature; sometimes, their paramorphs occur (Godovikov, 1983; Obushkov et al., 2010). For example, the fahlores from the Engteri Au-Ag deposit bear silver sulfide, sulfoselenide, and selenide as fine isometric grains (0.001–0.10 mm) intergrown with pyrite, sphalerite, and galena (Obushkov et al., 2010). The isometric shape of the grains suggests their crystallization at temperatures above Tph.t. (75–178 ºC). Co-formation of two different silver sulfoselenides is possible only in the two-phase region of the Ag2S-Ag2Se diagram (Fig. 6), i.e., in the narrow composition range Ag2S0.4Se0.6–Ag2S0.3Se0.7, but the phases must have the terminal compositions of this range, Ag2S0.4Se0.6 and Ag2S0.3Se0.7. In this case, exsolved structures or two-phase intergrowths can form. In fact, we did not observe such structures or intergrowths of the above terminal phases in the Rogovik ores. The second type of relationships, i.e., formation of Se-acanthite rims on S-naumannite or vice versa, is quite reasonable. According to the phase diagram, any silver sulfoselenide of the acanthite series (Ag2S–Ag2S0.4Se0.6) can be in assemblage with any silver sulfoselenide of the naumannite series (Ag2S0.3Se0.7–Ag2Se) because they are phases of solid solutions of different structures. As follows from the research data on mineral intergrowths and ore structures at other deposits (Betekhtin et al., 1958; Ramdohr, 1969), the rims and veinlets of silver sulfoselenides and their localization at the edges of

grains of pyrite and other minerals from the Rogovik deposit point to their later deposition during the ore-forming process. The regularities revealed for the silver sulfoselenides of the Rogovik deposit are also specific for other objects. Se-acanthite and S-naumannite occur as xenomorphous monomineral inclusions in other minerals at many deposits. Xenomorphous acanthite grains from the jarosite breccias of the Kupol deposit contain up to 5.5 wt.% Se (Savva et al., 2012). At the Krutoe deposit (northeastern Russia), acanthite poor in Se (0.1 wt.%) occurs in the zone of secondary sulfide enrichment (Savva et al., 2010). Naumannite containing 2.7 to 5.3 wt.% S

Fig. 5. Variations in S and Se contents in silver sulfoselenides from the Rogovik deposit (data of electron microprobe analysis). 1, ideal compositions of acanthite, aguilarite, and naumannite; 2, data from Kuznetsov et al. (1992); 3, results of this study.

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Fig. 6. Ag2S–Ag2Se phase diagram based on experimental data (1–5) and supplemented by our and published data on the compositions of natural silver sulfoselenides of the deposits (6–27): 1, Bontschewa-Mladenowa and Zaneva (1977), 2, Pingitore et al. (1993), 3, Vassilev and Ivanova (2003), 4, Alekperova et al. (2007), 5, Xiao et al. (2012); 6, Rogovik (this study), 7, Dzhul’etta (Savva and Fidrya, 1996), 8, Sergeevskoe (Botova et al., 1981); 9, Ol’cha (Savva and Shakhtyrov, 2011); 10, Kubaka (Sakharova et al., 1993; Stepanov and Shishakova, 1994); 11, Yunoe (Pal’yanova and Savva, 2009; this study); 12, Ulakhan (Savva and Pal’yanova, 2007); 13, Kupol (Savva et al., 2012); 14, Karamken (Sakharova et al., 1993; Shilo et al., 1992); 15, Krutoe (Savva et al., 2010); 16, Dukat (Shilo et al., 1992); 17, Valunistoe (Novoselov et al., 2009; Sakharova et al., 1993); 18, Agatovskoe (Shilo et al., 1992); 19, Ametistovoe (Nekrasov, 1996); 20, Al’fa (Neksrasov, 1997); 21, ore occurrence in the Dal’negorsk region (Kazachenko et al., 2008); 22, Guanajuato (Mexico) (Vassalo-Morales and Borodaev, 1982; Petruk et al., 1974); 23, Silver City (USA) (Petruk et al., 1974); 24, Broken Hill (New Zealand) (Cocker et al., 2013); 25, Ikuno, Ochmidani, Seigoshi, Yatani, Toicha (Japan) (Shikazono, 1978); 26, Pongkor (Indonesia) (Warmada et al., 2003); 27, Don Sixto (Argentina) (Mugas-Lobos et al., 2011).

(Ag2S0.4Se0.6–Ag2S0.2Se0.8) was found in the ores of the Murzinskoe deposit (Rudny Altai) (Naumov, 2007). Acanthites with Se ≤ 9 wt.% are present at Au–Ag deposits in Japan (Shikazono, 1978). Se-acanthite (0–5.19 wt.% Se, Ag2S–Ag2S0.83Se0.17) and S-naumannite (0.96–1.4 wt.% S, Ag1.98Se0.90S0.11) were revealed at the Don Sixto low-sulfide epithermal gold deposit located in the Mendoza province (Argentina) (Mugas-Lobos et al., 2011), but their intergrowths were not observed. Acanthite forms xenomorphous grains measuring 8 to 50 μm and is usually intergrown with native gold and uytenbogaardtite, and naumannite occurs in paragenesis with native silver. Study of the composition of silver sulfoselenides from the Broken Hills deposit (Cocker et al., 2013) showed a continuous acanthite series Ag2S0.97Se0.03–Ag2S0.75Se0.25 and the presence of S-naumannite Ag2Se0.8S0.2. Until recently, aguilarite has been regarded as an individual mineral (Fleisher and Mandarino, 1995; Petruk et al., 1974; www.mindat.org) rather than an isostructural phase of the acanthite series (Bindi and Pingitore, 2013; Pingitore et al., 1992, 1993); therefore, silver sulfoselenides compositionally similar to Ag4SSe were interpreted as aguilarite or phases of aguilarite solid solution. For example, Sakharova et al. (1993) studied Se mineralization at the deposits of Au-Ag-ore asso-

ciation in the Okhotsk–Chukchi volcanogenic belt (Kubaka, Karamken, Dukat, Dzhul’etta, and Valunistoe) and established three mineral series: naumannite, aguilarite, and acanthite. We corrected their data, uniting the aguilarite and acanthite compositions into one series, and obtained the composition ranges for the phases of the acanthite series, Ag1.87–2.02(S0.53–1.11Se0.02–0.64)0.93–1.11, and naumannite series, Ag1.89–2.01(Se0.79–0.99S0.01–0.24)0.8–1.23, lying within the error of experimental determination of the corresponding series (Pingitore et al., 1992, 1993). No intergrowths of two or three silver sulfoselenide phases (Sakharova et al., 1993) were found at the above deposits. Acanthite and aguilarite were also discovered at the Pongkor deposit (Warmada et al., 2003). The content of Se varies in the range 2.1–13.8 wt.%, and the content of S, in the range 6.2–12.6 wt.%, which corresponds to the acanthite series Ag2S–Ag2S0.54Se0.46 and agrees with the experimental data (Pingitore et al., 1992, 1993). Silver sulfoselenides of the acanthite series, namely, Se-acanthite Ag1.91(S0.94Se0.15)1.09 and aguilarite Ag1.90(S0.49Se0.61)1.10 (i.e., enriched in Se and depleted in Ag relative to the accepted ideal composition Ag2S0.5Se0.5 (or Ag4SSe)), are present in manganese rocks in the Dal’negorsk region (Kazachenko et al., 2008).

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The intergrowths of phases of the naumannite and aguilarite series discovered by Nekrasov (1997) in the ores of the Al’fa Au-Ag deposit formed at different time. Naumannite contains impurities of S (0.5–0.8 wt.%) (i.e., has composition Ag2Se0.97S0.03), and aguilarite contains impurities of S (7.4 wt.%) and Se (11.8 wt.%) (i.e., has composition Ag2S0.6Se0.4 corresponding to a phase of the acanthite series). Shilo et al. (1992) found naumannite as fine irregular-shaped segregations in early acanthite. Note that literature data on two-phase acanthite-naumannite segregations are much scarcer than data on one-phase silver sulfoselenides. The Ag2S–Ag2Se phase diagram (Fig. 6) presents the compositions of acanthite and naumannite taken from published data and converted by us from wt.% to 3 f.u. It is seen that they form a continuous trend. The range Ag2S0.4Se0.6– Ag2S0.3Se0.7, marking the two-phase region, includes silver sulfoselenides from the Dzhul’etta (Savva and Fidrya, 1996), Sergeevskoe (Botova et al., 1981), and Kubaka (Shilo et al., 1992; Stepanov and Shishakova, 1994) deposits. The absence of data on the homogeneity of silver sulfoselenide microinclusions and the presence of exsolved structures makes it impossible to draw conclusions about this region. The difference in S contents in the terminal phases of the above range, Ag2S0.4Se0.6 (4.65 wt.% S) and Ag2S0.3Se0.7 (3.43 wt.% S), is 1.22 wt.%, i.e., is close to the limit of accuracy of electron probe microanalysis. Hence, additional comprehensive studies of silver sulfoselenides and the homogeneity of their near-terminal phases are required. We think that more precise data will be obtained for the Se contents in silver sulfoselenides because they are higher than the S contents and cover the range 17.14–19.70 wt.%. Probably, the two-phase region on the Ag2S–Ag2Se diagram is very narrow, as for the similar system Ag3AuS2–Ag3AuSe2, which was shown to have a composition region of morphotropic transition, Ag3AuSeS– Ag3AuSe0.75S1.25 (Seryotkin et al., 2013a,b). The variations in S and Se contents in monomineral inclusions of silver sulfoselenides of the Rogovik ores confirm sulfur and selenium isomorphism, and Se-acanthite rims on S-naumannite testify to the presence of two experimentally established solid solutions (acanthite and naumannite) on the Ag2S–Ag2Se phase diagram (Pingitore et al., 1992, 1993). Analysis of the relationships and compositions of silver sulfoselenides from other deposits did not support the conclusions about the presence of the third solid solution (aguilarite) (Petruk et al., 1974).

Conclusions The determined contents of S in naumannite and of Se in acanthite and the established relationships between these silver chalcogenides in the polyassociation ores of the Rogovik deposit show the presence of two solid solutions in the system Ag2S–Ag2Se: acanthite Ag2S–Ag2S0.74Se0.26 and naumannite Ag2S0.28Se0.72–Ag2Se. Se-acanthite rims on S-naumannite, the wide variations in S contents in naumannite and in Se contents in acanthite, and the absence of intergrowths of three

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different silver sulfoselenides testify to the existence of two rather than three (as was erroneously determined earlier (Petruk et al., l974)) solid solutions on the Ag2S–Ag2Se phase diagram. The results of study of natural Ag2(S,Se) solid solutions from the Rogovik deposit and the review of data on the types of intergrowths and variations in S and Se contents in silver sulfoselenides of other deposits agree with the experimental Ag2S–Ag2Se phase diagram (Pingitore et al., 1992, 1993). If there were a two-phase region on this diagram, natural parageneses would contain exsolved structures and, correspondingly, regularly arranged intergrowths of minerals, with terminal compositions. However, such intergrowths of Se-acanthite with S-naumannite have not been detected in the Rogovik ores and are not described in scientific literature. It is necessary to study in detail natural silver sulfoselenides of the range Ag2S0.4Se0.6–Ag2S0.3Se0.7 in order to reveal a two-phase immiscibility region and establish its limits. We thank N.S. Karmanov (Institute of Geology and Mineralogy, Novosibirsk) and L.A. Pavlova (Institute of Geochemistry, Irkutsk) for electron probe microanalysis of minerals as well as Dr. M.V. Borisov (Moscow State University) for valuable critical remarks. This work was supported by grant 14-05-00361a from the Russian Foundation for Basic Research and Integration project 48 from the Siberian Branch of the Russian Academy of Sciences.

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