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Russian Geology and Geophysics

2016 year, number 8


G.A. Palyanova1,2, N.E. Savva3, T.V. Zhuravkova1,2, E.E. Kolova3
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3N.A. Shilo Northeastern Complex Research Institute, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia
Keywords: Au- and Ag-containing pyrites, Au-Ag chalcogenides (acanthite, naumannite, uytenbogaardtite, petrovskaite), electrum, genesis, physicochemical parameters of ore formation


Gold- and silver-containing pyrites of the Tikhii area at the Dzhulietta deposit (Engterinskii ore cluster, Magadan Region) were studied by optical and scanning electron microscopy and electron probe microanalysis. One- or two-phase rounded microinclusions consisting of electrum (450-680 ‰) and/or galena or of petrovskaite and/or uytenbogaardtite, galena, and sphalerite have been found in early pyrites. Later As-pyrites (up to 2.6 wt.% As) contain multiphase xenomorphic microinclusions of acanthite, uytenbogaardtite, freibergite, argentotetrahedrite-tennantite, naumannite, petzite, selenopolybasite-selenostephanite, tellurocanfieldite, and other ore minerals localized in pores, cracks, and interstices. Pyrites that underwent hypergene alterations have rims and veinlets formed by acanthite, goethite, anglesite, plattnerite, and native silver. The presence of rounded ore mineral microinclusions and large pores in the early pyrites suggests the participation of volatiles in the mineral formation and the uptake of large amounts of impurities by pyrite under high-gradient crystallization conditions. The thermobarogeochemical studies of fluid inclusions in quartz have shown that the ore zone formed under boiling-up of hydrothermal medium-concentration NaCl solutions at 230-105 ºC. The results of thermodynamic calculations evidence that Ag-Au-S-Se minerals formed under decrease in temperature and fugacity of sulfur (log10fS2 = -22 to -9) and selenium (log10fSe2 = -27 to -14) and change of reducing conditions by oxidizing ones in weakly acidic to near-neutral solutions.