Inferred Phase Relations in Part of the System Au–Ag–Te: an Integrated Analytical Study of Gold Ore from the Golden Mile, Kalgoorlie, Australia

Mineralogy and Petrology (2005) 83: 283–293 DOI 10.1007/s00710-004-0065-1

Inferred phase relations in part of the system Au–Ag–Te: an integrated analytical study of gold ore from the Golden Mile, Kalgoorlie, Australia

L. Bindi1, M. D. Rossell2, G. Van Tendeloo2, P. G. Spry3, and C. Cipriani1

1 Museo di Storia Naturale, Sezione di Mineralogia, Universita degli Studi di Firenze, Italy 2 Electron Microscopy for Materials Research (EMAT), University of Antwerp, Belgium 3 Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA, USA

Received May 24, 2004; revised version accepted October 7, 2004 Published online December 7, 2004; # Springer-Verlag 2004 Editorial handling: J. G. Raith

Summary Integrated X-ray powder diffraction, scanning electron microscopy, electron probe, and transmission electron microscopy studies have identiﬁed the rare contact assemblage calaverite–sylvanite–hessite in a sample of gold ore from the Golden Mile deposit, Kalgoorlie, Australia. The presence of coexisting calaverite–hessite at Kalgoorlie is a non-equilibrium assemblage whereby the stable hessite-bearing assemblage is hessite– sylvanite, which formed from the breakdown of the -phase or -phase below 120 C, stuutzite€ þ -phase, or sylvanite þ stuutzite€ þ -phase, as predicted by Cabri (1965).

Introduction Epizonal and mesozonal gold and gold–silver telluride deposits spatially asso- ciated with calc-alkaline, alkaline, and mafic igneous rocks are among the largest resources of gold in the world. They include Golden Mile, Kalgoorlie, Australia (e.g., Clout et al., 1990; Shackleton et al., 2003), Emperor, Fiji (e.g., Ahmad et al., 1987; Pals and Spry, 2003), Cripple Creek, Colorado (e.g. Thompson et al., 1985), and Saacaar^mb, Romania (Alderton and Fallick, 2000). Although these deposits 284 L. Bindi et al. contain a wide variety of tellurium-bearing minerals, the most important group of minerals from an economic standpoint is that in the system Au–Ag–Te. An under- standing of the mineral stabilities in this system is provided by phase relation and crystal chemical studies (e.g., Pellini, 1915; Markham, 1960; Luo and Klement, 1962; Cabri, 1965; Cabri and Rucklidge, 1968; Legendre et al., 1980; Van Tendeloo et al., 1983a, b, 1984; Wagner et al., 1994). Experiments were undertaken on this system by Markham (1960), Cabri (1965) and Legendre et al. (1980) but those of Cabri (1965) are to be preferred because Markham (1960) was unable to synthesize krennerite [(Au, Ag)Te2] and Legendre et al. (1980) could not synthesize krennerite and sylvanite (AuAgTe4). The reasons for the experimental problems encountered by Markham (1960) and Legendre et al. (1980) are discussed by Wagner et al. (1994). Calaverite, krennerite, and sylvanite belong to the group of gold–silver tellurides with the chemical formula Au1 xAgxTe2. Based on the experiments of Cabri (1965), calaverite contains 0 to 2.8 wt.% Ag, krennerite contains 3.4 to 6.2 wt.% Ag, and sylvanite contains 6.7 to 13.2 wt.% Ag. In nature, the assemblages calaverite–krennerite and sylvanite–krennerite are common whereas the assemblage calaverite–sylvanite is rare. There are occasional reports of natural assemblages that are incompatible with the experimental results of Cabri (1965) but, in general, there is remarkable consistency between Cabri’s experiments and natural assemblages. The inconsistencies are likely due to re-equilibration of natural assemblages upon cooling below temperatures conducted in Cabri’s experiments, misidentification of minerals in natural assemblages, and due to the incorporation of trace elements in phases in the system Au–Ag–Te that may alter the stability field of a given mineral. In discussing these potential discrepancies, Geller (1993), for example, pointed out that the contact assemblages calaverite (AuTe2) – sylvanite and native tellurium–stuutzite€ (Ag5 xTe3) from the Boulder County, Colorado, are not predicted by Cabri’s (1965) experiments. Other examples of inconsistencies include the assemblage native gold–krennerite, which was reported by Baker (1958) from the Golden Mile, and the assemblage native tellurium–hessite that was identified by Berbeleac (1980) from Musariu, Romania. In the course of research projects dealing with the characterization of tellurium- bearing minerals from museum collections (Bindi and Cipriani, 2003a, b, 2004a, b, c, d, e; Bindi et al., 2004; Cipriani and Bindi, 2004) and a mineralogical study of tellurides in the Golden Mile, Kalgoorlie, Australia (Pals and Spry, 2003), we analyzed sylvanite in a calaverite-bearing sample (D33193) with an electron microprobe that gave Ag contents of 9.2 wt.%. This sample was not found in situ but came from the collection of the Australian Museum in Sydney where it was labelled ‘‘calaverite – Kalgoorlie, Western Australia.’’ The aim of the current contribution is to characterize minerals in the system Au–Ag–Te from sample D33193 by X-ray power diffraction (XRPD), scanning electron microscopy (SEM), electron probe microanalytical (EPMA), and transmission electron microscopy (TEM) techniques and to discuss the phase relations involving calaverite, petzite (Ag3AuTe2), krennerite, sylvanite, and hessite in light of the experiments of Cabri (1965). Inferred phase relations in part of the system Au–Ag–Te 285

Reflected light and electron microprobe studies Tiny fragments (approximately 100 mm in size) were hand-picked under a reflected light microscope from sample D33193 and mounted in epoxy and polished. In reflected light, the sample appeared to be homogeneous and to consist almost entirely of calaverite. However, minute inclusions of optically unidentifiable phases were locally interspersed within calaverite. The fine-grained phase in this sample was analyzed with a Jeol JXA-8600 electron microprobe in the Department of Earth Sciences at the University of Florence. Major and minor elements were determined at 20 kV accelerating voltage and 40 nA beam current, with 30 s as counting times. For the wave-length dispersive analyses the following lines were used: AuL,TeL, and AgL. The estimated analytical precision (in wt.%) is: 0.40 for Au and Te; 0.10 for Ag. The standards employed were: Au-pure element (Au), Ag-pure element (Ag), and synthetic Sb2Te3 (Te). At the microprobe scale (2 mm beam diameter), the minute grains were found to be homogeneous within analytical error. The average chemical compositions (8 analyses on different spots), together with ranges of wt.% of elements, are reported in Table 1 and indicate that inclusions of sylvanite (9.2 wt.% Ag) also occur in sample D33193.

X-ray diffraction analyses Single-crystal X-ray studies were conducted on three crystal fragments from sample D33193 by Weissenberg film techniques and with a Nonius CAD4 four-circle diffractometer at the University of Florence. The fragments gave extremely broad X-ray diffraction profiles, thus indicating the powder study as the only possible X-ray investigation. Fully indexed 114.6 mm Gandolfi camera X-ray powder data (Ni-filtered CuK) for this samples are presented in Table 2. The intensities were measured with an automated densitometer. Detailed examination of the observed patterns shows the absence of diffraction peaks belonging to any crystalline phase other than calaverite. The refined unit-cell parameters of calaverite based on 21

Table 1. Chemical composition (means and ranges of elements in wt.%) of sylvanite from Kalgoorlie D33193 Range Au 31.67 31.02–32.17 Ag 9.18 8.98–9.30 Te 59.13 58.85–59.37 Total 99.98 No. of atoms 6 Au 1.360 Ag 0.720 Te 3.920 D33193 Golden Mile, Kalgoorlie 286 L. Bindi et al.

Table 2. X-ray powder diffraction pattern for calaverite from sample D33193 hkl Sample D33193

I dmeas dcalc 0 0 1 20 5.07 5.0679 1 1 0 10 3.76 3.7568 1 1 1 100 3.02 3.0187 2 0 1 35 2.93 2.9284 0 2 0 20 2.205 2.2041 3 1 0 35 2.106 2.1035 1 1 2 5 2.102 2.1012 1 1 2 25 2.099 2.1002 2 0 2 15 2.073 2.0712 2 0 2 5 2.069 2.0693 3 1 1 8 1.941 1.9422 2 2 1 15 1.760 1.7610 4 0 1 8 1.692 1.6927 0 0 3 5 1.690 1.6893 1 1 3 5 1.542 1.5404 2 2 2 11 1.510 1.5094 2 2 2 3 1.508 1.5086 1 3 1 8 1.385 1.3848 4 2 1 7 1.343 1.3425 0 2 3 5 1.340 1.3408 3 1 3 6 1.317 1.3177

reﬂections between 5.07 and 1.317 A˚ for sample D33193 are: a ¼ 7.181(3) A˚ , b ¼ 4.408(2), c ¼ 5.068(2) A˚ , ¼ 89.95(3).

TEM and SEM investigations Transmission electron microscopy studies, SEM, and energy dispersive X-ray (EDX) analysis were also carried out on sample D33193. EDX analysis was carried out with a Philips CM20 microscope equipped with a LINK-2000 attachment at the University of Antwerp. For the EDX analysis, the results were based on the Au(L), Ag(L) and Te(L) lines of the spectra. Over 20 spectra were recorded. The main phase present in the sample is calaverite. Together with the EDX analysis we also performed electron diffraction (ED) from sub-micron areas (Fig. 1). The recorded ED patterns are typical for calaverite. A combination of EDX and ED studies reinforces the XRPD and electron microprobe studies that the main phase in sample D33193 is calaverite. However, other minor phases were also detected. ED patterns of petzite along [101] and [311] are shown in Fig. 2. The corresponding ED patterns show a cubic structure. In addition, another phase with the composition Ag2Te was detected by EDX and is likely to be hessite rather than stuutzite.€ SEM studies were carried out with a JEOL JSM-5510 microscope equipped with an INCAx-sight attachment at the University of Florence. Secondary Inferred phase relations in part of the system Au–Ag–Te 287

Fig. 1. ED patterns of the calaverite phase along [010], ½101, ½131 and ½111. Satellite reﬂections in the [010] pattern are outlined by two arrowheads

Fig. 2. ED patterns of the petzite phase along [101] and [311] electron images and X-ray maps of Au, Ag and Te were recorded (Fig. 3). Color phase maps, which were derived from the X-ray maps, show the inhomogeneous distribution of Au (in blue), Ag (in red) and Te (in green). The images in Figs. 3 and 4 are taken from three distinct zones and clearly show that the sample is not homogeneous. Figure 3 shows four energy dispersive X-ray spectra acquired in four distinct points (indicated in the top left ﬁgure). Spectra 2 and 4 only show Au and Te, whereas spectra 1 and 3 show the presence of Au, Ag, and Te. The sets of spectra correspond to calaverite and sylvanite, respectively. Care- ful examination of the images in Fig. 3 also shows the additional presence of a phase containing only Ag and Te, which is hessite, based on the EDX analysis. 288 L. Bindi et al.

Fig. 3. Secondary electron image (top left), color phase map (top right), X-ray maps of Au, Ag and Te (middle) and four energy dispersive X-ray spectra (bottom) acquired in four distinct zones indicated in the secondary electron image of sample D33193. In the color phase map Au is represented in blue, Ag in red and Te in green Inferred phase relations in part of the system Au–Ag–Te 289

Fig. 4. Back-scattered electron image of calaverite, sylvanite, and hessite in sample D33193. Note the intergrowth between sylvanite and hessite and the absence of inclusions of these two minerals in calaverite. The texture suggests the assemblage sylvanite–hessite was derived from the breakdown of the -phase or -phase below 120 C from stuutzite€ þ - phase or sylvanite þ stuutzite€ þ -phase. The black scale bar is 40 mm in length

The presence of the contact assemblage calaverite–sylvanite–hessite is best seen in Fig. 4.

Discussion Nineteen tellurium-bearing minerals have been identiﬁed previously in the Golden Mile deposit (e.g., Stillwell, 1931; Markham, 1960; Travis, 1966; Golding, 1978; Shackleton et al., 2003). Of these minerals, all stable phases in the system Au–Ag–Te (calaverite, petzite, krennerite, sylvanite, hessite, stuutzite,€ muthmannite [(Au, Ag)Te], native tellurium and native gold) occur except for muthmannite and empressite (AgTe). Recent detailed studies of tellurides from the Golden Mile by Shackleton et al. (2003) report the following contact assemblages: calaverite– native gold, calaverite–krennerite, calaverite–sylvanite, calaverite–petzite, native gold–hessite, native gold–petzite, hessite–petzite, sylvanite–hessite, krennerite– stuutzite,€ calaverite–native gold–petzite, and hessite–petzite–sylvanite (Fig. 5). Shackleton et al. (2003) reported Ag contents of sylvanite from the Golden Mile ranging from 11.7 to 12.9 wt.% Ag; however, Golding (1978) found sylvanite in the assemblage krennerite–sylvanite–petzite–coloradoite (HgTe) with a very wide range of silver contents (6.1 to 13.1 wt.% Ag). This range of compositions (6.7 to 13.2 wt.% Ag) is compatible with that reported by Cabri (1965) in his experiments. Sylvanite with a low Ag content of 8 wt.% was also found by Travis (1966) from the Doolette lode in the Golden Mile. Sylvanite showing the full range of silver contents (6.3 to 13.5 wt.% Ag) was reported previously by Stumpﬂ (1970) and Pals and Spry (2003) from the Emperor deposit. Therefore, the low 290 L. Bindi et al.

Fig. 5. Ternary plot showing phase relations among precious metal tellurides and native elements in the Golden Mile deposit, in terms of Au–Ag–Te (after Shackleton et al., 2003). The early gold-rich assemblages preceded deposition of more silver-rich assemblages in stage 3 of three stages in Fimiston-style ore. The ﬁlled circles are the compositions obtained by Shackleton et al. (2003). Note the absence of a tie-line between calaverite and hessite silver content of 9.2 wt.% Ag that was obtained herein for sylvanite in sample D33193 is within the range of values observed in natural samples and synthesized in experiments. Figure 3 show the presence of hessite in contact with both calaverite and sylvanite although it appears to be mainly enclosed in sylvanite in Fig. 4. Graphic intergrowths between sylvanite and hessite are common at Golden Mile (Stillwell, 1931; Baker, 1958) and it should be noted that the assemblages sylvanite–empressite and sylvanite–stuutzite€ have never been reported in the deposit. Furthermore, based on the experiments of Cabri (1965), Ag-rich sylvanite (>10 wt.% Ag) would be expected to be in equilibrium with stuutzite,€ thus supporting the suggestion that the Ag–Te phase is hessite rather than stuutzite.€ Although not synthesized in Cabri’s experiments, sylvanite with Ag contents close to ideal sylvanite (i.e., 13.2 wt.% Ag) should be in equilibrium with empressite. Assuming the Ag–Te phase is hessite, it is likely that the sylvanite–hessite intergrowth formed as a result of the breakdown of the -phase or -phase below 120 C from the following assemblages: stuutzite€ þ -phase, or sylvanite þ stuutzite€ þ -phase (Cabri, 1965). If this is the case, then the rare contact assemblage calaverite–hessite as is observed in Figs. 3 and 4 is likely to be a non-equilibrium assemblage. This is compatible with the experiments of Cabri (1965), which show that there is no tie-line between calaverite and hessite at any temperature. Furthermore, the assemblage calaverite– sylvanite suggests that krennerite is metastable at room temperature, also consistent with the experiments of Cabri (1965). Inferred phase relations in part of the system Au–Ag–Te 291

We suspect that the difficulty in identifying the nature of possible fine-grained intergrowths between tellurides in the system Au–Ag–Te from Kalgoorlie is not unique to this study because Henley et al. (2001) in a study of flotation concentrate from the Golden Mile identified a phase which they referred to as ‘‘gold-bearing hessite.’’ Analyses of this mineral showed that it contained 9.0 to 11.3 wt.% Au, which would be the highest known gold content of any hessite reported in the literature. By contrast, Markham (1960) and Shackleton et al. (2003) reported hessite with up to only 0.8 and 1.1 wt.% Au, respectively. We suggest that the phase analyzed by Henley et al. (2001) was either petzite that yielded Ag-rich compositions as a result of the diffusion of Au away from the electron microbeam as it melted petzite, as has been described by Rucklidge and Stumpfl (1968), or it is hessite with a minute amount of an incorporated gold-bearing phase. Based on assemblages reported previously from the Golden Mile (e.g., Shackleton et al., 2003), this phase is probably sylvanite. The assemblage described in sample D33193 is a sylvanite–hessite intergrowth with a high sylvanite to hessite ratio whereas the Au-rich hessite described by Henley et al. (2001) was also probably a sylvanite–hessite intergrowth with a low sylvanite to hessite ratio. The presence of minor amounts of petzite in calaverite is in keeping with the stable assemblage calaverite–petzite that was reported previously by Markham (1968), Travis (1966), Golding (1978) and Shackleton et al. (2003).

Conclusions Fine-grained intergrowths of silver-poor sylvanite with calaverite from Kalgoorlie are consistent with the experimental results of Cabri (1965) and natural assemblages elsewhere. The study reafﬁrms that the assemblage calaverite–sylvanite, while rare in nature, does exist and that the contact assemblage calaverite– hessite–sylvanite at Kalgoorlie is unlikely to be in equilibrium. In particular, calaverite–hessite is a non-equilibrium assemblage. Instead, the stable assemblage is sylvanite–hessite, which was derived from the breakdown of the -phase or -phase below 120 C from stuutzite€ þ -phase, or sylvanite þ stuutzite€ þ -phase.

Acknowledgements The authors wish to thank F. O lmi (CNR – Istituto di Geoscienze e Georisorse – sezione di Firenze) for his help during the electron microprobe analyses. Financial support was provided by the University of Florence (60% grant) and by M.I.U.R., coﬁnanziamento 2003, project ‘‘crystal chemistry of metalliferous minerals’’ issued to C. Cipriani. R. Pogson from the Australian Museum in Sydney is also thanked for kindly providing sample D33193. L. Cabri and Editor J.G. Raith are gratefully acknowledged for their helpful and constructive comments.

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Authors’ addresses: L. Bindi (corresponding author; e-mail: [email protected]ﬁ.it) and C. Cipriani, Museo di Storia Naturale, Sezione di Mineralogia, Universita degli Studi di Firenze, Via La Pira 4, 50121 Firenze, Italy; M. D. Rossell and G. Van Tenderloo, Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerpen, Belgium; P. G. Spry, Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, IA 50011-3210, USA