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 À 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 À 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: Æ0.40 for Au and Te; Æ0.10 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]Ã and [311]Ã are shown in Fig. 2. The corresponding ED patterns show a cubic structure.

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