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The Canadian Mineralogist Vol. 48, pp. 139-144 (2010) DOI : 10.3749/canmin.48.1.139
MINERALOGICAL DATA ON ANGELAITE, Cu2AgPbBiS4, FROM THE LOS MANANTIALES DISTRICT, CHUBUT, ARGENTINA Dan TOPA§, Werner H. PAAR, Hubert PUTZ and Georg ZAGLER Department of Materials Research and Physics, Paris–Lodron University of Salzburg, Hellbrunnerstrasse 34, A–5020 Salzburg, Austria
Milka K. de BRODTKORB CONICET, University of Buenos Aires, Paso 258-9A, 1640 Martinez, Argentina
Chris J. STANLEY Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
Andrew C. ROBERTS Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8, Canada
Emil MAKOVICKY Department of Geography and Geology, University of Copenhagen, Østervoldgade 10, DK–1350 Copenhagen, Denmark
Abstract Angelaite, ideally Cu2AgPbBiS4, occurs as a hypogene mineral in polymetallic ores at the Ángela groups of veins in the mining district of Los Manantiales, in the province of Chubut, Argentina. The new mineral species is predominantly associated with pyrite, sphalerite, chalcopyrite, hematite, native gold and galena; less common associates are aikinite, wittichenite, miharaite and cervelleite. Angelaite forms subhedral, commonly oriented inclusions in galena; these may attain a size of up to 200 3 50 mm. The mineral is grey in color with a brownish tint, opaque, and lacks internal reflections. It has a metallic luster and a dark grey streak. The VHN10 ranges between 245 and 263 kg/mm3 (mean 253), corresponding to a Mohs hardness of 3½. In planepolarized light, it is strongly bireflectant and pleochroic from light grey with a brownish tint to light cream with a greenish tint. Angelaite is strongly anisotropic, with rotation tints in shades of pale grey, deep green and deep blue. We provide the measured values of reflectance in air and oil. The average of 23 electron-microprobe analyses is: Cu 16.7(3), Ag 13.4(2), Pb 27.8(6), Bi 26.6(5), S 16.0(2), total 100.5(5) wt.%, equivalent to Cu2.07Ag0.97Pb1.05Bi1.00S3.91. The ideal formula (on the basis of nine atoms) is Cu2AgPbBiS4, which requires Cu 16.31, Ag 13.84, Pb 26.58, Bi 26.81, S 16.45, total 100 wt.%. Angelaite is orthorhombic, with a 12.734(5), b 4.032(1), c 14.633(5) Å, V 751.8(5) Å3, space group Pnma and Z = 4. The calculated density is 6.934 g/cm3. It is a homeotype of galenobismutite, with Cu and Ag replacing one of the Bi positions in a complicated way. The strongest eight lines in the calculated powder-diffraction pattern [d in Å(I)(hkl)] are 3.672(100)(032), 3.660(64)(004), 3.407(60)(120), 3.319(62) (121), 3.317(62)(121), 3.111(69)(041), 3.022(72)(113) and 3.017(72)(113). The mineral is named after the location. Both the mineral and its name were approved by the CNMNC (IMA #2003–064). Keywords: angelaite, copper–lead–silver–bismuth sulfosalt, electron-microprobe analyses, X-ray-diffraction data, reflectance data, Los Manantiales district, Chubut, Argentina.
Sommaire L’angelaïte, de composition idéale Cu2AgPbBiS4, est un minéral hypogène découvert dans le minerai polymétallique exploité dans le groupe de veines Ángela, dans le camp minier de Los Manantiales, province de Chubut, Argentine. La nouvelle espèce minérale est surtout associée à pyrite, sphalérite, chalcopyrite, hématite, or natif et galène; plus rarement, on la trouve avec
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l’aikinite, la wittichenite, la miharaïte et la cervelleïte. L’angelaïte se présente en inclusions orientées dans la galène; celles-ci peuvent atteindre jusqu’à 200 3 50 mm. Il s’agit d’un minéral gris avec une teinte brunâtre, opaque, et dépourvu de réflexions internes. Il possède un éclat métallique et une rayure gris foncé. La dureté VHN10 varie entre 245 et 263 kg/mm3 (en moyenne, 253), ce qui correspond à une dureté de Mohs de 3½. En lumière linéairement polarisée, il est fortement biréflectant et pléochroïque de gris pâle avec une teinte brunâtre à crème pâle avec une teinte verdâtre. L’angelaïte est fortement anisotrope, avec teintes de rotation gris pâle, vert foncé et bleu foncé. Nous présentons les valeurs mesurées de la réflectance dans l’air et dans l’huile. Les 23 analyses effectuées avec une microsonde électronique ont donné, en moyenne, Cu 16.7(3), Ag 13.4(2), Pb 27.8(6), Bi 26.6(5), S 16.0(2), pour un total de 100.5(5)% (poids), équivalent à Cu2.07Ag0.97Pb1.05Bi1.00S3.91. La formule idéale, calculée sur une base de neuf atomes, est Cu2AgPbBiS4, ce qui requiert Cu 16.31, Ag 13.84, Pb 26.58, Bi 26.81, S 16.45, total 100%. L’angelaïte est orthorhombique, avec a 12.734(5), b 4.032(1), c 14.633(5) Å, V 751.8(5) Å3, groupe spatial Pnma et Z = 4. La densité calculée est 6.934 g/cm3. Elle est un homéotype de la galénobismutite dans lequel Cu et Ag remplacent un atome de Bi de façon assez compliquée. Les huit raies les plus intenses du spectre de diffraction, méthode des poudres [d en Å(I)(hkl)] sont 3.672(100)(032), 3.660(64)(004), 3.407(60)(120), 3.319(62)(121), 3.317(62)(121), 3.111(69)(041), 3.022(72)(113) et 3.017(72) (113). Le nom du minéral rappelle la localité type. Le minéral et son nom ont reçu l’approbation de la Commission des Nouveaux Minéraux, de Nomenclature et de Classification (IMA #2003–064).
(Traduit par la Rédaction)
Mots-clés: angelaïte, sulfosel de cuivre–plomb–argent–bismuth, données de microsonde électronique, données de diffraction X, données de réflectance, district de Los Manantiales, Chubut, Argentine.
Introduction Angelaite was described by Brodtkorb & Paar (2004) as a new mineral species from the Ángela group of Au-and Ag-bearing vein-type deposits (N 68°58’36” E, 42°01’33”), situated in the district of Los Manantiales, province of Chubut, Argentina (Fig. 1). It has been known as mineral “X” since 1990, when its chemical composition was published by Wiechowski
et al. (1990). Arizmendi et al. (1996) re-analyzed ore material from the same location and confirmed the presence of this phase. Several attempts to extract suitable grains for X-ray study were unsuccessful at that time, mainly because of small grain-sizes and an intimate intergrowth with other sulfides and sulfosalts. During project work in Argentina by the above authors, one polished section prepared from the previously collected material was found to contain angelaite
Fig. 1. a. Location of the Los Manantiales district, province of Chubut, Argentina. b. Geological map of the Los Manantiales district, modified from Marquez (1999).
mineralogical data on angelaite, chubut, argentina
in sufficiently large grains. These allowed the determination of the X-ray properties and the crystal structure (Topa et al. 2004, 2010) and led to its accreditation as a new mineral species (IMA 2003–064). Holotype material (one polished section) was deposited under catalogue number 14934 in the Systematic Mineralogical Collection of the Department of Materials Research and Physics, Division of Applied Mineralogy, University of Salzburg, Austria. Cotype material of angelaite will reside as several polished sections in the reference collection of one of the authors (MKdB). The present contribution describes the occurrence and properties of angelaite in greater scope and detail than the original two-page description by Brodtkorb & Paar (2004).
Occurrence and Associated Minerals The mining district of Los Manantiales is located approximately 50 km north–northeast of Gastre and 125 km south of a location called Ingenio Jacobaci. The mineral deposits occur at the northwestern edge of the Somuncurá massif, and are structurally controlled and hosted within andesitic to dacitic rocks of the Taquetrén Formation of Late Jurassic age. Two belts, each composed of numerous veins, are distinguished. The more important is Grupo Ángela with the Cobre, Platífero and Susana Beatriz veins, the second is Clara Natividad with the Diseminado San José mineralization (Marquez 1999). The veins trend between N 30° and
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N 60°, have a total extent of less than 1000 m along strike, and show a typical pinch-and-swell structure. The rich ore occurs in ore shoots of greatly variable dimensions. Open-space filling is indicated by banding, cockade textures and widespread vugs. Wallrock alteration is common, with phyllic and argillic alteration near and propylitic alteration farther away from the veins. The Ángela and Clara Natividad groups of veins were exploited between 1970 and 1990. The average grade of the ore, mined at different levels down to a depth of 245 m below surface, was 4.76% Zn, 2.1% Pb, 0.6% Cu, 3.21 ppm Au and 47.57 ppm Ag. Three different stages of mineralization are documented (Brodtkorb & Paar 2004). The first stage is characterized by abundant base-metal sulfides, such as sphalerite, galena and chalcopyrite, generally associated with pyrite and lesser amounts of arsenopyrite, bornite, betekhtinite, matildite and acanthite. Angelaite is present as microscopic inclusions in galena (Figs. 2a, b, c), and may occur with trace amounts of aikinite, miharaite, wittichenite and a silver-bearing variety of the latter. Very rare and small grains of an unnamed sulfosalt (mineral “Z”), compositionally different from angelaite, was observed in the same assemblage by Arizmendi et al. (1996), but they are not suitable for X-ray analyses. The chemical composition of this sulfosalt is (wt.%) Pb 18.7, Bi 39.5, Cu 17.4, Ag 7.0, Te 1.0, and S 17.6, sum 101.1, approximately corresponding to (Cu, Ag)4PbBi2S6. A similar compound was mentioned
Fig. 2. Angelaite-bearing sulfosalt association of from the Los Manantiales district. a) Polarized-light image; b) and c) BSE images, showing angelaite (ang), galena (gn), aikinite (aik), miharaite (mih), wittichenite (wit) and pyrite (py).
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by Kovalenker et al. (1993) from polymetallic veins of the Štiavnica–Hodruša ore field, in Slovakia. It was not found in the material available to us during this study. The second stage is dominated by hematite and a precious-metal mineralization composed of native gold, native silver, as well as cervelleite and native tellurium. A reopening of the veins during a third stage was accompanied by the precipitation of a similar assemblage of sulfides and native gold, all of which occur in fractures and open spaces. Supergene enrichment, with the formation of minor digenite and covellite, is of minor importance. The gangue consists of quartz (dominant), calcite and K-feldspar of adularia habit. The mineralization is of epithermal type and was deposited in an environment of intermediate degree of sulfidation (Sillitoe & Hedenquist 2003). Fluidinclusion studies revealed temperatures of homogenization for the Platífero Oeste and the Susana Beatriz veins ranging between 230 to 340°C and 270 to 390°C, respectively, at a pressure of approximately 120 bars (Bengochea et al. 1984, Bengochea & Varela 1988).
Appearance and Physical Properties Angelaite is opaque and occurs as subhedral to anhedral inclusions in galena. Therein, it generally forms trails of grains that may be arranged parallel to {100} or irregularly distributed in the hosting sulfide. The grains range in size from a few mm up to 50 3 50 mm. The maximum size of very rare elongate inclusions may attain 200 3 50 mm. The color very much resemble that of the commonly associated aikinite, i.e., white with a distinct creamy tint. The sulfosalt has a metallic luster and a dark grey streak. It is brittle, with an uneven fracture, and without observable cleavage. The density could not be measured because of the small size of grains and their intimate intergrowth with other minerals. A value of 6.934 g/cm3 was calculated on the basis of the empirical formula. This density is very similar to the densities of galenobismutite and aikinite (both 7.1 g/cm3). A Leica Miniload VMHT tester was used to determine the microhardness. As a consequence of the size of the grains, a load of 10 g was chosen. The Vickers hardness, established with 10 measurements, ranges between 245 and 263 (mean 253) kg/mm2. This corresponds to a Mohs hardness of about 3½. No twinning was observed in any of the grains studied.
Optical Properties In a polished section, in plane-polarized light (~3200 K), angelaite is both strongly bireflectant and pleochroic. Where associated with galena and chalcopyrite, the color appears to be a light grey (R1) with a slight brownish tint and light cream with a slight greenish tint (R2). Between crossed polars, angelaite is strongly anisotropic, with rotation tints in shades of pale grey, deep green and dark blue.
Reflectance measurements were made within the visible spectrum (400–700 nm) both at the Department of Material Research, University of Salzburg, using the equipment and techniques described in Paar et al. (2005), and at the Natural History Museum (London). The reflectance of angelaite is slightly lower than that of galena. The Rmax values are almost identical for all wavelengths, whereas Rmin values decrease marginally toward the red end of the spectrum, which explains the tint of the mineral in reflected light, described above. The results of the measurements (in air and oil: Salzburg) are summarized in Table 1.
Chemical Composition Quantitative chemical data for angelaite and the associated phases were obtained with an electron microprobe (JEOL Superprobe JXA–8600, controlled by the Probe for Windows system of programs operated in wavelength-dispersion (WDS) mode, at 25 kV and 35 nA, 15 s for peak and 5 s for background), installed at the Department of Geography and Geology, University of Salzburg. The following standards and X-ray lines were used: natural CuFeS2 (chalcopyrite, CuKa), natural PbS (galena, PbLa), synthetic Bi2S3 (BiLa, SKa) and Ag metal (AgLa). Selenium and Te were sought, but not detected. The raw data were corrected with the on-line ZAF–4 procedure. The results, obtained from eight aggregates of grains in two polished sections of the holotype specimen, show only minor variation of the chemical composition of angelaite (Table 2).
mineralogical data on angelaite, chubut, argentina
On the basis of Me + S = 9, the ideal formula of angelaite is Cu2AgPbBiS4 and requires Cu 16.31, Ag 13.84, Pb 26.58, Bi 26.81 and S 16.46, for a total of 100.00 wt.%.
X-Ray Crystallography Single-crystal and crystal-structure study Single-crystal studies of angelaite extracted from a polished section were performed using a singlecrystal diffractometer with a CCD detector (Topa et al. 2010). They indicate that angelaite is orthorhombic, a 12.734(5), b 4.032(1), c 14.633(5) Å, space group Pnma. Based on a crystal-structure determination, Topa et al. (2010) demonstrate that angelaite is a homeotype of galenobismutite and of its isotypes (listed in Olsen et al. 2007). The crystal structure contains one independent Pb site, one Bi site, three distinct Cu sites, of which two are partially occupied, and one Ag position. The monocapped trigonal prism of Pb and the slightly asymmetric coordination octahedra of Bi resemble closely such polyhedra in the structure of galenobismutite. The linearly coordinated Cu1 site and the trigonal-planarcoordinated Cu2a, Cu2b and Ag sites are concentrated in a relatively small structural volume, which in galenobismutite host a pair of Bi2 sites. As a result of this substitution, the unit-cell parameters of angelaite are larger than those of galenobismutite, and the axial ratio is modified (Topa et al. 2010). Being so different in chemical composition, the structural analogy between angelaite and galenobismutite is remarkable.
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X-ray powder-diffraction study X-ray powder data (Table 3) were obtained from a powder admixed with galena that was dug out of the polished section. Unit-cell parameters were refined on 19 powder reflections between 3.659 and 1.806 Å. Of these, only ten were unambiguously indexed, and eight of these are 0kl reflections. Indexing is based on the calculated powder-diffraction data derived from the successful crystal-structure refinement. The refined unit-cell parameters are: a 12.733(8), b 4.022(3), c 14.585(10) Å, V 746.9(9) Å3, and a:b:c 3.1658:1:0.2748, in accord with the single-crystal study of Topa et al. (2010). Diffraction lines ascribable to the admixed galena have been deleted from the powder dataset for angelaite. On the basis of the calculated data, there is no overlap of the major powder-diffraction reflections of angelaite with those of galena.
Conclusions Angelaite is a recently defined mineral species which most probably was formed as a part of an exsolution assemblage from originally copper–silver–bismuth-rich galena. It is a new species of Cu–Ag–Bi–Pb sulfosalt with structural affinities very different from the other known species of this compositional group. At present, it appears to be the best characterized Cu–Ag–Bi phase produced by exsolution in galena; the other minerals of this type, e.g., larosite (Cu,Ag)21PbBiS13 (Petruk 1972), arcubisite CuAg6BiS4 (Karup-Møller 1976) and the
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B engochea , A.L., M as , G., Maiza , P. & Varela , M.E. (1984): Petrografía y termometría de las inclusiones fluidas de la veta Platífero W, mina Ángela, prov. del Chubut. Rev. Asoc. Geol. Argentina 39, 430-442. Bengochea, A.L. & Varela, M.E. (1988): Las inclusiones fluidas del sector Susana Beatriz, mina Ángela, prov. del Chubut. Rev. Asoc. Geol. Argentina 43, 462-473. Brodtkorb, M.K. de & Paar W.H. (2004): Angelaíta, en la paragénesis del distrito Los Manantiales, provincia de Chubut: una nueva especie mineral. Rev. Asoc. Geol. Argentina 59, 787-789. Karup-Møller, S. (1976): Arcubisite and mineral B – two new minerals from the cryolite deposit at Ivigtut, south Greenland. Lithos 9, 253-257. Kovalenker, V., Jeleň, S. & Sandomirskaya, S. (1993): Minerals of the system Ag–Cu–Pb–Bi– S from the polymetallic veins of the Banska–Štiavnica–Hodruša ore field (Slovakia). Geologica Carpathica 44, 409-419. Marquez, M. (1999): Los sistemas hidrotermales del distrito Los Manantiales, Chubut. In Recursos Minerales de la Republica Argentina (E. Zappettini, ed.). SEGEMAR, Anales 35, 1167-1175. Olsen, L.A., Balić-Žunić, T., Makovicky, E., Ullrich, A. & Miletich, R. (2007): Hydrostatic compression of galenobismutite (PbBi2S4): elastic properties and high-pressure crystal chemistry. Phys. Chem. Minerals 34, 467-475. Paar, W.H., Topa, D., Makovicky, E. & Culetto, F.J. (2005): Milotaite, PdSbSe, a new palladium mineral species from Předbořice, Czech Republic. Can. Mineral. 43, 689-694. Petruk, W. (1972): Larosite, a new copper–lead–bismuth sulphide. Can. Mineral. 11, 886-891.
unnamed (Cu,Ag)4PbBi2S6 of Arizmendi et al. (1996), require extensive additional investigation.
Acknowledgements This present study was financed by the Christian Doppler Research Society (Austria) to DT and by the grant no. 272–08–0227 of the Research Council for Nature and Universe (Denmark) to EM. The manuscript benefitted from comments of Maria Florencia MárquezZavalía and Allan Pring, as well as from the editorial care of Robert F. Martin.
References Arizmendi, A., Brodtkorb, M.K. de & Bernhardt, H.J. (1996): Paragénesis mineral de la mina Ángela, dpto. Gastre, Chubut. 3° Reunión de Mineralogía y Metalogenía, Universidad Nacional de la Plata 5, 1-7.
Sillitoe, R.H. & Hedenquist, J.W. (2003): Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious metal deposits. In Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses within the Earth (S.F. Simmons & I. Graham, eds.). Soc. Econ. Geol., Spec. Pap. 10, 314-343. Topa, D., Makovicky, E., Paar, W.H. & Brodtkorb, M.K., de (2004): The crystal structure of angelaite, Cu2AgPbBiS4, a new mineral species from Angela mine, Province of Chubut, Argentina. 32nd Int. Geol. Congress (Florence), Abstr. Topa, D., Makovicky, E. & Putz, H. (2010): The crystal structure of angelaite, Cu2AgPbBiS4. Can. Mineral. 48, 145-153. Wiechowski, A., Arizmendi, A. & Brodtkorb, M.K. de (1990): Estudio analítico de las sulfosales de la mina Ángela, dpto. Gastre, prov. del Chubut, Argentina. Asociación Arg. Geol. Ec., Publ. Esp. (Homenaje al Ing. Victorio Angelelli), 41-43. Received February 20, 2009, revised manuscript accepted November 30, 2009.
