Se-bearing polybasite-Tac from the Martha mine, Macizo del Deseado, Santa Cruz, Argentina

Miner Petrol (2008) 94:145–150 DOI 10.1007/s00710-008-0001-x

SHORT COMMUNICATION

Se-bearing polybasite-Tac from the Martha mine, Macizo del Deseado, Santa Cruz, Argentina M. F. Márquez-Zavalía & L. Bindi & M. Márquez & S. Menchetti

Received: 18 October 2007 / Accepted: 27 December 2007 / Published online: 18 March 2008 # Springer-Verlag 2008

Abstract Se-bearing polybasite-Tac is associated with galena, chalcopyrite, sphalerite, tetrahedrite, electrum and quartz at the Martha mine, an epithermal silver–gold deposit located adjacent to the Deseado Massif, Santa Cruz province, Argentina. Three samples, with variable chemical composition and showing the 111 unit-cell type, were studied by means of X-ray single crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the

Ag, Sb, and Se contents. The predicted values resulted in excellent agreement with experimental unit-cell parameters. We observed that high contents of selenium in polybasite are associated with relatively low copper contents. This finding corroborates previous studies that the copper content of pearceite–polybasite group minerals can be very low if selenium is present.

Introduction

Editorial handling: P. Spry M. F. Márquez-Zavalía (*) Unidad de Mineralogía, Petrografía y Geoquímica, IANIGLA, CRICYT, CONICET, Av. Ruiz Leal s/n, Parque Gral. San Martín (5500), Mendoza, Argentina e-mail: [email protected] L. Bindi Museo di Storia Naturale—sezione di Mineralogia e Litologia, Università degli Studi di Firenze, Via La Pira, 4-I-50121 Florence, Italy e-mail: [email protected] M. Márquez Delegación Comodoro Rivadavia, SEGEMAR, Ruta Provincial Nro. 1, Kilómetro 8, (9003), Comodoro Rivadavia, Chubut, Argentina e-mail: [email protected] S. Menchetti Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4-I-50121 Florence, Italy e-mail: [email protected]

The members of the pearceite–polybasite group of minerals, general formula [(Ag,Cu)6M2S7][Ag9CuS4] with M = Sb and As, are relatively widespread in nature. Studies by Frondel (1963) proposed two separate solid solution series for this group of minerals: polybasite–arsenopolybasite and pearceite–antimonpearceite, where Sb and As substitute mutually. Subsequently, Hall (1967) experimentally determined the stability range and compositional limits of pearceite and polybasite and pointed out that Cu is a necessary component. Recently, this group of minerals has been the focus of several studies (Bindi et al. 2006a, b, 2007a, b, c; Evain et al. 2006a, b). As a result of these studies, new information about this group of minerals and changes in the nomenclature of its members were made. Bindi et al. (2007b) proposed that only two names should be used: pearceite and polybasite, depending on their As/Sb ratio. If crystallographic data are available, a hyphenated italic suffix is added, indicating the crystal system and the cell-type symbol. This new nomenclature was approved by the International Mineralogical Association (Bindi et al. 2007b). A new name was also added: selenopolybasite (Bindi et al. 2007c) to indicate the Se-dominant analogue of polybasite from the De Lamar mine, Idaho, USA.

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During a mineralogical study of samples from Martha mine, Santa Cruz province, Argentina, a member of this series of minerals was identified and characterized as Sebearing polybasite-Tac. Since there are few papers dealing with Se-rich polybasite (Harris et al. 1965; Barret and Zolenski 1986; Bindi et al. 2007c), the chemical and crystallographic characteristics of specimens from the Martha mine are given in this paper.

The Martha ore deposit The Martha Au–Ag deposit (48°41′30″ S, 69°41′58″ W) is located in Santa Cruz province, at the southernmost extreme of continental South America (Fig. 1). It is located on the SW border of the Deseado Massif geological province, an extensive volcanic field of Jurassic age, which hosts several precious metal deposits and occurrences in the Patagonia terrain. The Deseado Massif is restricted geographically by the Cretacic San Jorge Gulf basin to the north, the Cretacic Magallanes basin to the south and southeast, and the Phanerozoic Cordillera Patagónica Austral to the west. Detailed geological data concerning this mining district were reported by Lesta and Ferello (1972) and Guido (2004). The Martha deposit comprises a group of veins and veinlets that are up to 1,500 m long and up to 5 m wide. The veins and veinlets follow pre-mineral faults and

Fig. 1 Location map of the Martha mine

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fractures, and subordinate breccias. The deposit is hosted in rhyolitic ignimbrites of Middle Jurassic age (168– 162 Ma) of the Bahía Laura Group (Pankhurst et al. 2000). The ore formed in multiple stages and occurs in open-space fillings, bands, vugs, stockworks, combs and as masses. The first event produced intense pyritization followed by the development of quartz, adularia, and Cu– Pb–Zn-bearing minerals in veins and veinlets and was followed by the formation of precious metal mineralization. The main ore minerals in the deposit are arsenopyrite, pyrite, marcasite, chalcopyrite, galena, sphalerite, tetrahedrite, pyrargyrite, miargyrite, freibergite, native silver, and argentite. The Ag-bearing minerals developed in nests, veinlets up to 2 mm, and as granular intergrowths with quartz and adularia. In general, weak silicic and argillic alteration grades into moderately strong silicification near and adjacent to the silver-bearing veins. The upper oxidized zone of the deposit is up to 10 s of meters thick, but it locally varies in thickness in close proximity to faults. The vein system has a ratio Ag:Au of ∼1,000:1 (Schalamuk et al. 2002; González Guillot et al. 2004).

Analytical methods The specimens were prepared as conventional polished sections, and studied with polarizing reflected-light microscope. Chemical analyses of the minerals were obtained with a JEOL Superprobe JXA-8200, using the following standards: naumannite (Ag Lα, Se Lα), chalcopyrite (Cu Kα, Fe Kα, S Kα), sphalerite (Zn Kα), arsenopyrite (As Lα), and Sb metal (Sb Lβ). The electron beam diameter was 2 μm; accelerating voltage and beam current were kept at 15 kVand 20 nA on the Faraday cup, respectively. Peak measurement times were 20 s and background measurement times 10 s. The CITZAF method was employed for matrix corrections. The X-ray study was carried out with a Bruker-P4 singlecrystal diffractometer using graphite-monochromatized MoKα X-radiation. Three crystals were hand-picked from the polished sections under a reflected light microscope and used for the structural study. Unit-cell dimensions were determined by least-squares refinement of the setting angles of 38 high-θ reflections (22 < 2θMoKα < 30
) giving: a= 7.425(2), c=11.853(4) Å, V=565.9(7) Å3 (VM1-1), a=7.465 (1), c=11.908(3) Å, V=574.7(4) Å3 (VM1-3/4), and a= 7.521(1), c=11.965(3) Å, V=586.1(4) Å3 (VM2-3/4). The possibility that the unit-cell parameters (unit-cell type 221 and 222) were doubled was carefully checked but not observed. This confirmed that the samples are to be classified as Se-bearing polybasite-Tac. Since the samples are considered to be “normal” polybasites and no peculiar cations were present, we decided not to carry out intensity

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data collections. Hence, the X-ray study was limited to the determination of the unit-cell parameters only.

Optical properties In reflected-light, polybasite from the Martha mine is grey and its hue changes depending on the adjacent minerals; it has a greenish hue next to galena and is brownish where surrounded by tetrahedrite. In air, the bireflectance is subtle and observed almost only at grain boundaries. This mineral is soft and its relative hardness is softer than galena. Its anisotropy, observed in air, is difficult to see but in some grains it is possible to observe variations from greenish grey to grey with a violet tint; internal reflections were not

Fig. 2 Textural and paragenetic features of Se-bearing polybasite. a Se-bearing polybasite (pol) in quartz (qtz). b Se-bearing polybasite with sphalerite (sp), galena (gn) and chalcopyrite (ccp). c Se-bearing polybasite in quartz. d Se-bearing polybasite with sphalerite, galena, chalcopyrite and electrum. e Polybasite in quartz. Scale bar in the photomicrographs represents 100 μm

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observed. Most of the grains are dark in color and locally show reddish internal reflections. These grains contain a slightly higher Se content. Polybasite is spatially associated with galena, chalcopyrite, sphalerite, tetrahedrite and quartz. It occurs as plates, tabular to irregular isolated crystals (Fig. 2a), and as rosettelike textures (Fig. 2b). It also occurs as elongated subhedral crystals generally in the centre of the host grains, in myrmekitic textures (Fig. 2b) or it follows a crystallographic pattern (Fig. 2c,d) in galena or sphalerite, and is locally associated with chalcopyrite. Polybasite also occurs as patches in galena, generally along the borders of the crystals (Fig. 2e). Where polybasite is spatially associated with chalcopyrite ± galena, it is locally associated with irregular grains of electrum (Fig. 2d), which shows a fairly constant composition (∼60 wt.% Ag and 40 wt.% Au).

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Chemical composition

Crystalchemistry

Selected chemical analyses of Se-bearing polybasite and polybasite from Martha deposit are compared with analyses of equivalent minerals from other localities (Table 1). All the analyses from the Martha deposit correspond to polybasite, even though some of them have high Se contents. However, these contents are not high enough to have two independent sulfur structural positions dominated by selenium, according to the chemical formula introduced by Bindi et al. (2007c) for the selenopolybasite endmember: [(Ag,Cu) 6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2]. Hence we refer to this mineral as Se-bearing polybasite.

To compare samples belonging to pearceite–polybasite group minerals, Bindi et al. (2007b) studied the variation of the hexagonal subcell parameters (i.e., a∼7.5, c∼12 Å). They found that both the a and c parameters are strongly influenced by the Ag content whereas the influence of the Sb content is very minor. Moreover, to take into account all the effects of any chemical substitutions in these minerals (i.e., Ag ↔ Cu, Sb ↔ As, S ↔ Se), a multiple regression model was conducted and yielded the following equations:

apred ¼ 6:67ð9Þ þ 0:009ð1ÞAgð%Þ þ 0:0005ð2ÞSbð%Þ þ 0:002ð1ÞSeð%Þ cpred ¼ 11:35ð5Þ þ 0:0073ð6ÞAgð%Þ  0:0007ð1ÞSbð%Þ þ 0:0045ð8ÞSeð%Þ Vpred ¼ 426ð15Þ þ 1:8ð2ÞAgð%Þ þ 0:05ð3ÞSbð%Þ þ 0:5ð2ÞSeð%Þ

Based on the chemical data reported in Table 1 (anal. 1, 2, and 5) and by using the equations reported above, we obtained the predicted values of the unit-cell parameters for the three samples. They are: a=7.431, c=11.881 Å, V= 574.0 Å3 (VM1-1), a=7.485, c=11.937 Å, V=585.4 Å3 (VM1-3/4), and a=7.524, c=11.980 Å, V=593.6 Å3 (VM23/4). The values obtained are in excellent agreement with the experimental parameters (Fig. 3).

Final remarks In Fig. 4, the composition of the samples studied here are plotted on a diagram originally constructed by Hall (1967) together with all the samples studied by Bindi et al. (2007b). It is evident that two samples (i.e., VM1-3/4 and VM2-3/4) showing higher Se contents (i.e., 3.00% and 2.44% for VM1-3/4 and VM2-3/4, respectively) and lower

Table 1 Electron microprobe analyses (in wt.% of elements) and atomic ratios (calculated on the basis of 29 atoms p.f.u.) Se-bearing polybasite-Tac

Ag As Zn S Sb Cu Se Fe Total Ag As Zn S Sb Cu Se Fe

polybasite-Tac

1

2

3

4

5

6

61.80 0.57 0.00 16.17 10.32 7.91 3.00 0.07 99.84 12.46 0.17 0.00 10.97 1.84 2.71 0.82 0.03

67.47 0.97 0.00 15.26 8.22 4.95 2.44 0.00 99.31 14.06 0.29 0.00 10.69 1.52 1.75 0.69 0.00

67.90 0.00 0.03 15.44 10.93 3.46 2.03 0.02 99.81 14.24 0.00 0.01 10.89 2.03 1.24 0.58 0.01

66.17 0.60 0.03 11.36 9.47 3.19 8.42 0.07 99.31 14.68 0.19 0.01 8.47 1.86 1.21 2.55 0.03

60.36 0.73 0.00 18.08 10.07 10.89 0.31 0.00 100.44 11.67 0.20 0.00 11.76 1.72 3.57 0.08 0.00

64.64 0.59 0.03 16.23 9.64 8.46 0.00 0.03 99.62 13.11 0.17 0.01 11.06 1.73 2.91 0.00 0.01

1 Se-bearing polybasite, this paper; 2 Se-bearing polybasite, this paper; 3 Se-bearing polybasite, Pachuca, Hidalgo, Mexico, 17004/80: Bindi et al. (2007b); 4 selenopolybasite, De Lamar mine, Idaho, USA, 2453/1: Bindi et al. (2007c); 5 polybasite, this paper; 6 polybasite, Eagle mine, Colorado, USA, 171541: Bindi et al. (2007b)

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Fig. 3 Relationship between the observed unit-cell parameters of the hexagonal subcell and the predicted values (see text for explanation)

7.56

apred subcell (Å)

7.52

7.48

7.44

7.40

7.36 7.32 7.32

7.36

7.40

7.44

7.48

7.52

7.56

7.60

aobs subcell (Å)

cpred subcell (Å)

12.10 pearceite-Tac (Bindi et al., 2007b) polybasite-Tac (Bindi et al., 2007b) pearceite-T2ac and M2a2b2c (Bindi et al., 2007b) 12.05 polybasite-T2ac and M2a2b2c (Bindi et al., 2007b) polybasite-Tac (this study) 12.00

11.95

11.90

11.85 11.85

11.90

11.95

12.00

12.05

cobs subcell (Å) 610 605

Vpred subcell (Å)

600 595 590 585 580 575 570 565 560 555 545

550

555

560

565

570

575

580

585

Vobs subcell (Å)

590

595

600

605

150

Miner Petrol (2008) 94:145–150 40

1

atomic % of Cu in (Ag,Cu)

35

2

30 25 VM1-1

20 L2283

15

K1422

4

3

10

VM1-3/4 VM2-3/4

5

2453/I 17004/80

0 0

10

20

30

40

50

60

70

80

90

100

atomic % of Sb in (Sb,As) Fig. 4 Relationship between the atomic % of Sb in (Sb,As) and the atomic % of Cu in (Ag,Cu) (after Hall 1967). The numbers 1, 2, 3 and 4 (inside the circles) indicate the fields of: pearceite-Tac, polybasite-

Tac, arsenpolybasite-T2ac and -M2a2b2c, and polybasite-T2ac and -M2a2b2c, respectively. Symbols are the same as in Fig. 3

Cu contents (i.e., 7.91% and 4.95% for VM1-3/4 and VM23/4, respectively), fall in the polybasite-T2ac and -M2a2b2c field (221–222 unit-cell type). This finding corroborates the hypothesis of Barrett and Zolensky (1986) and Bindi et al. (2007c) that the copper content of pearceite–polybasite group minerals can be very low if selenium is present.

Bindi L, Evain M, Menchetti S (2007c) Selenopolybasite, [(Ag, Cu)6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2], a new member of the pearceite–polybasite group from the De Lamar mine, Owyhee County, Idaho, USA. Can Mineral 45:1525–1528 Evain M, Bindi L, Menchetti S (2006a) Structural complexity in minerals: twinning, polytypism and disorder in the crystal structure of polybasite, (Ag,Cu)16(Sb,As)2S11. Acta Crystallogr B62:447–456 Evain M, Bindi L, Menchetti S (2006b) Structure and phase transition in the Se-rich variety of antimonpearceite, [(Ag,Cu)6(Sb,As)2(S, Se)7][Ag9Cu(S,Se)2Se2]. Acta Crystallogr B62:768–774 Frondel C (1963) Isodimorphism of the polybasite and pearceite series. Am Mineral 48:565–572 González Guillot M. De Barrio R, Ganem F (2004) Mina Martha: un yacimiento epitermal argentífero en el Macizo del Deseado, Provincia de Santa Cruz. 7° Congreso de Mineralogía y Metalogenia 199–204 Guido D (2004) Subdivisión litofacial e interpretación del volcanismo Jurásico (Grupo Bahía Laura) en el este del Macizo del Deseado, provincia de Santa Cruz. Revista Asociación Geológica Argentina 59:727–742 Hall HT (1967) The pearceite and polybasite series. Am Mineral 52:1311–1321 Harris DC, Nuffield EW, Frohberg MH (1965) Studies of mineral sulphosalts: XIX-Selenian polybasite. Can Mineral 8:172–184 Lesta P, Ferello R (1972) Región Extraandina de Chubut y Norte de Santa Cruz. In: Leanza, A. (ed.) Geología Regional Argentina, Academia Nacional de Ciencias 1:602–687 Pankhurst RJ, Riley TR, Faninng CM, Kelley SP (2000) Episodic silicic volcanism in Patagonia and the Antartic peninsula: chronology of magmatism associated with the break-up of Gondwana. J Petrol 41:605–625 Schalamuk I, De Barrio R, Zubia M, Genini A, Valvano J (2002) Mineralizaciones auro-argentíferas del Macizo del Deseado y su encuadre metalogénico. In: Haller MJ (ed.) Geología y recursos naturales de Santa Cruz. Relatorio 15° Congreso Geológico Argentino, 6-2:679–713

Acknowledgements The paper benefited by the official reviews made by Filippo Vurro and Paul G. Spry. MFMZ is grateful to Christoph A. Heinrich for the access to the electron microprobe facilities at the Institute of Isotope Geochemistry, ETH-Zürich, Switzerland, and for the grants received from PIP 5907-CONICET and from Minera Alumbrera Ltd. LB and SM acknowledge C.N.R. (Istituto di Geoscienze e Georisorse, sezione di Firenze) and M.I.U.R., P.R.I.N. 2005 project “Complexity in minerals: modulation, modularity, structural disorder”.

References Barrett RA, Zolensky ME (1986) Compositional-crystallographic relations for polybasite. GSA Meeting Abstr Progr 18:535 Bindi L, Evain M, Menchetti S (2006a) Temperature dependence of the silver distribution in the crystal structure of natural pearceite, (Ag,Cu)16(As,Sb)2S11. Acta Crystallogr B62:212–219 Bindi L, Evain M, Pradel A, Albert S, Ribes M, Menchetti S (2006b) Fast ionic conduction character and ionic phase-transitions in disordered crystals: the complex case of the minerals of the pearceite–polybasite group. Phys Chem Min 33:677–690 Bindi L, Evain M, Menchetti S (2007a) Complex twinning, polytypism and disorder phenomena in the crystal structures of antimonpearceite and arsenopolybasite. Can Mineral 45:321–333 Bindi L, Evain M, Spry PG, Menchetti S (2007b) The pearceite– polybasite group of minerals: Crystal chemistry and new nomenclature rules. Am Mineral 92:918–925