Eur. J. Mineral. 2009, 21, 811–816 Published online June 2009
Poorly crystalline Pd–Hg–Au intermetallic compounds from Co´rrego Bom Sucesso, southern Serra do Espinhac¸o, Brazil ´ 2, BERND LEHMANN3, MIGUEL TUPINAMBA ´ 4, ALEXANDRE RAPHAEL CABRAL1,*, ANNA VYMAZALOVA 2 2 5 6 ˇ ˇ ˇ JAKUB HALODA , FRANTISEK LAUFEK , VOJTECH VLCEK and ROGERIO KWITKO-RIBEIRO 1
Department of Geology: Exploration Geology, Rhodes University, Grahamstown, 6140, South Africa *Corresponding author, e-mail:
[email protected] 2 Czech Geological Survey, Geologicka´ 6, 152 00 Prague 5, Czech Republic 3 Institut fu¨r Mineralogie und Mineralische Rohstoffe, Technische Universita¨t Clausthal, Adolph-Roemer-Str. 2A, D-38678 Clausthal-Zellerfeld, Germany 4 Tektos – Geotectonic Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua S. Francisco Xavier 524 s. A4016, 20550-050 Rio Janeiro-RJ, Brazil 5 Faculty of Natural Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic 6 Centro de Desenvolvimento Mineral, Companhia Vale do Rio Doce, Santa Luzia-MG, Brazil
Abstract: Potarite, ideally PdHg, is reported in the literature to have compositions varying from PdHg or Pd(Hg,Au) to Pd3Hg2. Such a Pd3Hg2 phase is unknown in the synthetic Pd–Hg binary system. For the first time, Pd–Hg grains recovered from the historical Bom Sucesso alluvium, regarded as the type locality of Pd, are shown to consist of arborescent and lamellar intergrowths of two intermetallic compounds, compositionally close to empirical Pd(Hg,Au), i.e. auriferous potarite, and (Pd,Au)3Hg2. The Pd–Hg–Au grains have a rim of palladiferous Pt. The otherwise sharp Pd–Hg–Au intergrowths become diffuse at the contact with the palladiferous Pt rim. Both the Pd–Hg–Au compounds and the palladiferous Pt rim did not diffract using the electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction techniques, indicating that they are poorly crystalline. Their poor crystallinity and the diffuse zone between the Pd–Hg–Au core and the Pt-rich overgrowth are suggestive of electrochemical metal precipitation from dilute solutions within the alluvium. Key-words: auriferous PdHg, auriferous Pd3Hg2, electron-microprobe analysis, electron-backscattered diffraction, powder X-ray microdiffraction, Co´rrego Bom Sucesso, Serro, Minas Gerais, Brazil.
1. Introduction The mineral potarite (PdHg) from its type locality, the Potaro River of Guyana, has a slightly divergent columnar or fibrous structure (Spencer, 1928). Two of four samples from the type locality of potarite were determined to be PdHg, but two were found to correspond to Pd3Hg2 (Spencer, 1928). Such a Pd3Hg2 phase is not present in the Pd–Hg synthetic system (Guminski, 1990). However, an alloy of probable Pd3Hg2 stoichiometry was reported in hydrothermally altered chromitite (Yang & Seccombe, 1993). Stoichiometric Pd3Hg2 was indicated within a dendritic Pt–Pd nugget from Co´rrego Bom Sucesso, Serra do Espinhac¸o of Minas Gerais (Cassedanne et al., 1996). Also within Pt–Pd nuggets from Co´rrego Bom Sucesso, Fleet et al. (2002) characterised auriferous potarite as an alloy varying in composition from Pd3Hg2 to near Pd(Hg,Au). Although potarite has been reported from diverse geological settings, its compositional variation deserves more study (Cabri, 2002).
In this contribution, we present results of electronmicroprobe analysis, electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction on Pd–Hg grains from Co´rrego Bom Sucesso. The grains consist of intergrowths of empirical Pd(Hg,Au) and (Pd,Au)3Hg2, and exhibit a divergent fabric similar to that described for the type-locality potarite.
2. Co´rrego Bom Sucesso Streams, or co´ rregos, containing platiniferous alluvia occur for about 90 km along the eastern border of Serra do Espinhac¸o in south-eastern Brazil, from the village of Serro southwards to Itambe´ do Mato Dentro (e.g., Humboldt, 1826; Hussak, 1904; Freyberg, 1934). Among them, Co´rrego Bom Sucesso is the best known because it is considered the type locality of Pd (Hussak, 1906; Cassedanne & Alves, 1992; Cassedanne et al., 0935-1221/09/0021-1943 $ 2.70
DOI: 10.1127/0935-1221/2009/0021-1943
# 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
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1996; Fleet et al., 2002). A location map of Co´rrego Bom Sucesso is available in Cabral et al. (2008b). Characteristically arborescent, botryoidal and coralloidal, its Pt nuggets generally have a dendritic core of potarite-like composition and do not contain appreciable amounts of base metals (Hussak, 1904, 1906; Cassedanne & Cassedanne, 1974; Cassedanne & Alves, 1992; Cassedanne et al., 1996; Fleet et al., 2002; Cabral et al., 2006). Bom Sucesso is a stream incised into quartzite of the Sopa-Brumadinho Formation, which is part of the 1.7 Ga Espinhac¸o Supergroup (e.g., Martins-Neto, 1996). The Sopa-Brumadinho Formation comprises elsewhere micaceous quartzite, ferruginous quartzite, polymict and diamantiferous metaconglomerate, and muscovite phyllite. The Bom Sucesso alluvium has manually been worked for Au, Pt, Pd and diamonds since the eighteenth century. The historical garimpo is immediately adjacent to a cliff, at the base of a rock fall-style landslide of boulders of quartzite (Cabral et al., 2008b). The alluvial mineralisation is situated within a poorly sorted pebble layer beneath the fallen blocks.
3. Sample material and analytical techniques A local garimpeiro (Mr Geraldo Pereira de Lima) recovered a platiniferous concentrate from the alluvial pebble layer. Coating by palladiferous Pt made the recognition of Pd–Hg grains difficult. A number of Pt grains were thus picked under a binocular microscope for backscatteredelectron (BSE) imaging with a Philips XL30 scanning electron microscope, equipped with an Oxford Inca Suite 4.07 ATW2 energy-dispersive spectrometer (EDS), at Companhia Vale do Rio Doce, Minas Gerais, Brazil. The grains were then embedded in epoxy and polished for reflected-light microscopy and electron-microprobe analysis with a Cameca SX100 at Technische Universita¨t Clausthal, Germany. Operating conditions, spectrometer crystals, X-ray lines and standards (in brackets) are as follows: 20 kV and 40 nA; LPET for Pd-La (Pd) and Hg-Ma (HgTe); LIF for Au-La (Au) and Pt-La (Pt). Electron-backscattered diffraction (EBSD) analysis was performed on the CamScan CS 3200 scanning electron microscope, equipped with a NORDLYS II (HKL Technology) system, at the Czech Geological Survey in Prague, Czech Republic. Polished sections were submitted to chemical polishing with colloidal silica (OP-U, Struers) for 45 min to reduce the extent of surface damage caused by mechanical polishing. The EBSD patterns, collected at 20 kV accelerating voltage and 5 nA beam current, were processed using a Channel-5 software provided by HKL Technology (2004). The centre of Kikuchi bands was automatically detected by a Hough transform routine (Schmidt et al. 1991) with a resolution of 100 (internal Hough resolution parameter in the HKL software). A small fragment of an EBSD-analysed Pd–Hg grain was carefully taken from the polished section and studied by X-ray powder microdiffraction. The X-ray powder microdiffraction
patterns were collected on a PANalytical X’Pert Pro diffractometer, equipped with an X’Celerator detector using a conventional X-ray tube (CuKa radiation, 40 kV, 30 mA) and X-ray monocapillary with diameter of 0.8 mm. The monocapillary acts as a waveguide, which results in collimating the divergent beam into a narrow quasiparallel beam, which is less sensitive to surface irregularities than a divergent beam in the conventional Bragg–Brentano geometry (Simova et al., 2005). Data were acquired in the angular range 8–60 2y in steps of 0.0167 and counting time of 12,500 s per step.
4. Results Palladium–Hg grains are completely or partially coated with palladiferous Pt, which gives a massive appearance. Cross sectioning, nonetheless, reveals Pd–Hg cores exhibiting an arborescent fabric (Fig. 1a) and a lamellar intergrowth (Fig. 1b). The Pd–Hg cores have their fabrics defined by different Pd:Hg ratios and Au contents. In Fig. 1, BSE-dark zones vary in composition from 40.2 to 46.3 wt% Pd, from 53.1 to 57.3 wt% Hg and from 0.7 to 3.1 wt% Au, corresponding to auriferous Pd3Hg2 (Table 1). BSEbright zones contain 36.5–39.3 wt% Pd, 52.2–57.0 wt% Hg and 4.5–12.3 wt% Au, being similar to auriferous PdHg, i.e. potarite (Table 1). Element correlations (Fig. 2) suggest that Au replaces Hg in potarite, i.e. Pd(Hg,Au) (cf., Fleet et al., 2002), and PdHg dissolves up to about 12 wt% Au, but Au exchanges for Pd in Pd3Hg2, i.e. (Pd,Au)3Hg2. The Pd vs. Au plot (Fig. 2) denotes that a change from (Pd,Au)3Hg2 to Pd(Hg,Au) occurs at about 4 wt% Au. The contact between the palladiferous Pt coating and the (Pd,Au)3Hg2–Pd(Hg,Au) core is marked by a reaction-like zone, ,50 mm across, which is compositionally close to Pd(Hg,Au). This is particularly well illustrated in Fig. 1b: under BSE imaging, the sharp, (Pd,Au)3Hg2–Pd(Hg,Au) lamellar intergrowth becomes diffuse in the reaction-like zone. The Pd–Hg–Au compounds and the palladiferous Pt did not diffract under the EBSD technique. Also, the X-ray powder microdiffraction pattern of the same grain studied by EBSD did not show any significant diffraction lines. However, an X-ray powder diffraction pattern of a Pt–Pd grain from the alluvium confirmed the presence of Pt, likely with admixture of PdHg. For the sake of comparison, the EBSD technique was applied to grains of hongshiite, PtCu, and associated Cu-depleted, Pt-rich zones. The hongshiite grains, which come from hydrothermal veins rich in specular haematite, and their alteration are described elsewhere (Kwitko et al., 2002; Cabral et al., 2008a). The hongshiite and the Cu-depleted, Pt-rich zones produced discriminating diffraction patterns.
5. Discussion and conclusion The composition of auriferous PdHg from Co´rrego Bom Sucesso has been considered to extend from Pd3Hg2 to
Poorly crystalline Pd–Hg–Au intermetallic compounds
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Fig. 1. BSE images. (a) Arborescent Pd–Hg grain with X-ray mapping for Pd, Hg and Pt (upper right). The Pd–Hg grain has an overgrowth of palladiferous Pt (arrows). (b) Palladium–Hg grain of lamellar fabric. A diffuse, reaction-like zone between the lamellar core and the palladiferous Pt rim is particularly well developed. All numbers in Fig. 1 refer to electron-microprobe analyses in Table 1.
43.42 1.96 54.95 100.33
2.949 0.072 1.979 5
45.08 0.83 54.53 100.44
3.027 0.030 1.943 5
3.092 0.027 1.881 5
46.31 0.74 53.12 100.17
3
2.870 0.081 2.049 5
41.91 2.18 56.41 100.50
4
2.783 0.115 2.102 5
40.24 3.08 57.28 100.60
5
2.867 0.070 2.063 5
41.76 1.89 56.64 100.29
6
2.826 0.083 2.091 5
41.04 2.22 57.23 100.49
7
2.860 0.059 2.081 5
41.69 1.59 57.19 100.48
8
1.093 0.067 0.840 2
39.32 4.45 57.01 100.77
9
1.034 0.178 0.788 2
36.54 11.66 52.49 100.69
10
1.038 0.156 0.806 2
36.73 10.22 53.78 100.73
11
1.030 0.188 0.782 2
36.47 12.34 52.21 101.02
12
1.056 0.134 0.810 2
37.46 8.77 54.20 100.44
13
1.038 0.177 0.785 2
36.93 11.69 52.62 101.24
14
Analysis numbers refer to spots indicated in Fig. 1. All spots were measured for Pt, which is below the detection limit of 0.3 wt%. Analyses: 1–8, (Pd,Au)3Hg2; 9–14, Pd(Hg,Au). apfu ¼ atoms per formula unit.
Pd Au Hg Total (wt%) apfu Pd Au Hg apfu
2
1
Table 1. Electron-microprobe analyses of auriferous Pd3Hg2 and auriferous potarite (PdHg).
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Fig. 2. Diagrams of Pd vs. Hg, Au vs. Hg and Pd vs. Au from microanalyses listed in Table 1 and indicated in Fig. 1.
Poorly crystalline Pd–Hg–Au intermetallic compounds
near Pd(Hg,Au), which is within the compositional range of the type-locality potarite (Fleet et al., 2002). The Pd–Hg grains investigated in this study are, nevertheless, made up of chemically distinct zones that are intricately grown into each other in a variety of forms, from arborescent to lamellar fabrics (Fig. 1). Compositionally, two types can be distinguished: auriferous potarite and (Pd,Au)3Hg2. However, these compounds, together with their palladiferous Pt rim, were not responsive to the EBSD technique, a fact suggestive of poor crystallinity. Such poor crystallinity can be an artefact due to polishing, i.e. the Beilby layer. The chemical polishing applied before the EBSD is expected to have removed the Beilby layer (e.g., Newbury et al., 1986). In addition, hongshiite grains and their Cu-depleted, Pt-rich zones showed characteristic diffraction patterns. Because the palladiferous Pt rim around the Pd–Hg–Au grains is compositionally similar to the Pt-rich zones within hongshiite, the poorly crystalline character of the former is most likely natural. This poorly crystalline character was further confirmed by the X-ray microdiffraction technique. Crystalline PdHg, i.e. potarite, can be prepared either by reaction of Hg with Pd(NO3)2 solution at room temperature or by direct synthesis from powdered Pd and Hg at 400 C (Terada & Cagle, 1960). A Pd3Hg2 compound is absent in the Pd–Hg binary system (Guminski, 1990; Baranski et al., 1993). Nevertheless, the absence of Pd3Hg2 phase in the synthetic system does not exclude the existence and formation of this phase under natural conditions. There are naturally occurring platinum-group minerals, such as plumbopalladinite, whose synthetic analogues have not been reported. In the case of plumbopalladinite, Pd3Pb2, which is stoichiometrically similar to Pd3Hg2, a Pd3Pb2 compound is not known in the Pd–Pb system (Durussel & Feschotte, 1996; Massalski, 1990). A Pd3Hg2 phase would likely be stabilized by some Au, up to about 4 wt% Au in the present case (Table 1). While experiments in the Pd–Hg binary system attained chemical equilibrium, the poorly crystalline Pd–Hg–Au compounds may represent a case for a natural, perhaps rapid, precipitation of metals under non-equilibrium conditions. The clearly defined, (Pd,Au)3Hg2–Pd(Hg,Au) lamellae become diffuse over a sharp boundary (Fig. 1b), in such a way that they appear to be homogenised towards Pd(Hg,Au). Such a diffusion of metals would have been facilitated by the poor crystallinity of the compounds. Petrographical evidence indicates that this reaction-like zone was induced by the palladiferous Pt on the pre-existing, poorly crystalline substratum. Due to its high electrochemical cell potential, Pt would be fixed on a metallic, Pd–Hg– Au surface, establishing an electrochemical gradient that would induce diffusion of metals through a narrow zone within the non-crystalline substratum, along the contact with the palladiferous Pt, to form the observed reactionlike zone. Metallic overgrowths have been recorded from other alluvia, such as Au films on Pt–Fe alloy from the Fadeevka platiniferous placer in Russian Far East (Shcheka & Lehmann, 2007) and, pertinently, Hg-bearing Pt coating on auriferous potarite from drainage at
815
Brownstone, southern Devon, England (Leake et al., 1991). Such metallic overgrowths can be explained by electrochemical reactions in dilute solutions within alluvia. The palladiferous Pt coating on the Pd–Hg–Au grains should be distinguished from the Pt-rich halo on dendritic and botryoidal nuggets of Pt–Pd from the Bom Sucesso alluvium (Cassedanne et al., 1996; Fleet et al., 2002; Cabral et al., 2006). The Pt-rich halo is derived from the supergene leaching of Pd from Pt–Pd in an analogous manner that Ag is leached from detrital Au–Ag grains to ultimately result in pure Au (Cabral et al., 2007). One century ago Hussak (1904, 1906) proposed a secondary (supergene) origin for the botryoidal Pt–Pd, after a primary platiniferous mineral, within the Co´rrego Bom Sucesso alluvium. Later, Cassedanne et al. (1996) and Fleet et al. (2002) speculatively advanced a low-temperature hydrothermal origin for the Bom Sucesso Pt–Pd nuggets. Their origin remains unclear, but the poorly crystalline character of the Pd–Hg–Au grains and their Pt overgrowth is consistent with non-equilibrium metal precipitation from dilute solutions within the Co´rrego Bom Sucesso alluvium.
Acknowledgements: ARC and MT sincerely thank Mr Geraldo Pereira de Lima for taking them to the Bom Sucesso garimpo and for making available a platiniferous concentrate. This contribution is part of a research project financed by Deutsche Forschungsgemeinschaft (Project LE 578/29-1). ARC is grateful to the Rudolf-Vogel-Preis (Technische Universita¨t Clausthal) for financing his early travels to the Serro region. Elisabetta Mariani and an anonymous reviewer, as well as Fernando Nieto Garcı´a, provided us with pertinent and thoughtful comments that greatly improved the manuscript. Prof. H.V. Eales kindly revised the final version of the manuscript.
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