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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2014“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2014”

Trace-element signature in pyrite from the Shaumyan gold-base metal deposit, Lesser Caucasus, SE Armenia – рreliminary dataЕлементи-следи в пирит от златно-полиметалното находище Шаумян, Малък Кавказ, ЮИ Армения – предварителни данниIrina Marinova1, Georgi Georgiev 2, Valentin Ganev1, Mihail Tarassov1

Ирина Маринова1, Георги Георгиев2, Валентин Ганев1, Михаил Тарасов1

1 Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences; E-mail: irimari@gmail.com2 Dundee Precious Metals Kapan

Key words: Shaumyan deposit, intermediate sulfidation, low sulfidation, gold, base metals, Lesser Caucasus.

IntroductionThe Shaumyan deposit (Kapan gold-base metal field), SE Armenia is related to a large Middle Jurassic vol-canic edifice (Matveev et al., 2006) developed in island-arc setting (Mederer, Moritz, 2011). All eco-nomic ore bodies are hosted by a subvolcanic dacites to andesites in the central part of the volcanic edifice and are dominated by veins (Fig. 1a) and relatively thin veinlets often accompanied by zones of stringer-disseminated mineralization. The main warlock altera-tions are of phyllic and propylitic types. Typically, the ore veins display intensive sericitic halos. When the composition of the host rock is less siliceous (tends to andesitic) chloritic alteration occurs. The dykes in the southern part of the deposit are strongly carbon-atized. Pyrite, sphalerite, chalcopyrite and galena are the main ore minerals with minor fahlore. Gold and silver present in the form of tellurides; Bi and Pb tel-lurides occur too. The main gang minerals are quartz and calcite, while rhodochrosite and manganese cal-cite occur at places. The sulfide and sulfosalt mineral

assemblages indicate intermediate-sulfidation (IS) en-vironment in the deposit (Hedenequist et al., 2000). At the same time we observed amygdales in dacites filled mainly with milky silica accompanied by pyrite, chal-copyrite, sphalerite, fahlore and gold (Fig. 1b). The silica under optical microscope appears quartz with typical textures, characteristics of the low-sulfidation (LS) deposits: rosettes, spherical shapes of aggre-gates, chalcedony-like extinction (Dong et al., 1995; Marinova et al., 2014).

Material and methodA trace-element study was carried on 2 polished sec-tions: the first one from vein pyrite-chalcopyrite-spha-lerite-galena-fahlore-quartz assemblage (Fig. 1a) The other one is presented by amygdales in lava breccia filled with milky quartz, minor pyrite and scarce chal-copyrite, brown sphalerite, fahlore and gold (Fig. 1b). We studied major, minor and trace elements contents by means of LA-ICP-MS in 4 pyrite grains, by two

Fig. 1. The mineralizations in this study: a, an underground base metal-quartz-calcite vein. Inset, photomicrograph of the base metal mineralization in a polished section; b, an amygdale filled with milky quartz and pyrite (drill hole core). Inset, photomicrograph of a polished thin sectionch, chalcopyrite; py, pyrite; fah, fahlore; qz, quartz

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in each polished section (designated as Py1 for the first mineralization type and Py2 for the second one). The applied LA-ICP-MS methodology is described in Marinova et al. (2014). The Py2 grains were of 2 types: homogenous (Py2h) and inhomogenous one with submicron-sized ore inclusions (Py2inh). The Fe contents, obtained by EPMA were used as internal standards for pyrite during the LA-ICP-MS determinations. 23Na, 25Mg, 27Al, 29Si, 39K, 42Ca, 49Ti, 51V, 55Mn, 57Fe, 59Co, 62Ni, 65Cu, 66Zn, 75As, 77Se, 79Br, 97Mo, 107Ag, 111Cd, 115In, 121Sb, 125Te, 127I, 137Ba, 182W, 197Au, 202Hg, 205Tl, 208Pb and 209Bi were determined by their isotope contents. The abundances of In, Sb, Hg and Bi on Fig. 2 were multiplied by 10, while these for Au – by 100.

Results and discussionThe studied pyrite grains clearly differ by some trace element grades: Py1 appears with higher contents of Te, Au, Ag, Sb, Bi, Pb, Zn and Cu. Both Py2 grains dis-play higher contents of Co, while the inhomogeneous one in addition to higher contents of In. Hg content is slightly higher in Py1 (Fig. 2a, b). The wide content range of the studied trace elements in both pyrite types could be due to mineral inclusions. Cu, Zn, Pb, Au, Ag, Bi, Te, Hg, Sb in Py1 correspond to mineral as-sociation of chalcopyrite, sphalerite, galena, Au-, Ag-, Au-Ag-, Bi- and Pb-tellurides, coloradoite (HgTe) and fahlore as known from published data (Matveev et al., 2006; Mederer et al., 2014). The higher variations of Co and In in Py2 could also indicate presence of Co- and In-bearing submicro-inclusions.

Conclusions1. The studied pyrite grains from IS and LS miner-alizations in the Shaumyan deposit display specific trace-element signatures as follows: the first one is characterized by higher contents of Au, Ag, Sb, Bi, Pb, Te, Hg, Zn and Cu, while Co and In are higher in the second one; 2. The IS mineralization seems to be major career of precious metals in the Shaumyan de-posit that has also been noted in previous studies.

Acknowledgements: The authors thank Dundee Precious Metals Kapan for permission to publish this study.

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Hedenequist, J., A. Arribas, E. Gonzalez-Urien. 2000. Explora-tion for epithermal gold deposits. – SEG Reviews, 13, 245–277.

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Matveev, A., E. Spiridonov, S. Grigoryan, S. Tabatabaei, S. Filimonov. 2006. Mineralogical and geochemical charac-teristics and predicted reserves of gold-base metal ore min-eralization in Southern Armenia and Northwestern Iran. – Geochem. Intern., 44, 814–824.

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Fig. 2. Charts of trace-element abundances in pyrite from the two styles of mineralization: Py1, pyrite 1-st type; Py2h, pyrite 2-nd type – a homo-geneous grain; Py2inh, pyrite 2-nd type – an inhomogeneous grain