Gold- and silver-containing pyrites of the Tikhii area at the Julietta deposit (Engteri 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 °C. 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.

This paper presents the brief geological and mineralogical characteristics of the Dorozhnoye mesothermal gold ore deposit (Magadan Region, Russia). The gold and silver minerals are represented by electrum (530‰ -700‰), freibergite, stephanite, pyrargyrite and Bi-diaphorite. The homogeneous fluid inclusions enable one to determine that the quartz-molybdenite veins were formed at a temperature of 340˚C to 140˚C, and that the quartz-sulfide gold ore veins were formed at a temperature of 165˚C to 125˚C. It is assumed that the fluid had a mixed composition and evolved from K-chloride and K-carbonate to Na-chloride. The hypergenesis zone contains: high fineness gold (800‰ -950‰), petrovskaite (AgAuS) and uytenbogaardtite (Ag3AuS2) associated with goethite and melanterite. The surface waters contain: Ca ++ up to 293 mg/L, HCO3 up to 125 mg/L, SO4 up to 777 mg/L; рН is 5.8 to 7.2. Thermodynamic calculations have been conducted and Eh-pH diagrams have been built for the Au-Ag-S-H2O system at 25˚C and 100˚C (p = 1 bar) to reveal gold and silver stability fields. Au-Ag sulfides at the Dorozhnoye deposit are formed under oxidizing conditions and an acidic environment with dominating sulfates. KeywordsDorozhnoye Deposit, Au-Ag Minerals, Genesis, Eh-pH Diagrams * Corresponding author.N. E. Savva et al. 479culiarities, such as gently dipping (3˚ -5˚) veins and low-grade gold found in ores containing silver sulfosalts. This deposit was defined by Firsov [1] as a typical gold-quartz formation. Kalinin [2] in the early part of the 1990s determined the following minerals within the ores: petrovskaite (AgAuS), pyrargyrite (Ag 3 AsS 3 ), miargyrite (AgSbS 2 ), native bismuth, bismuthite (Bi 2 S 3 ), and diaphorite (Ag 3 Pb 2 Sb 3 S 8 ). Pyrargyrite and petrovskaite were also detected and described by Al'shevskii [3]. In 2009, we worked at the Dorozhnoye deposit and detected quartz-molybdenite (Q-Mo) and quartz-cassiterite (Q-Cst) veins accompanying the gold mineralization. We also found stephanite and exceeding Bi concentrations in ores. Our objective was to study the genetic peculiarities that had formed the Au-Ag mineralization, which are not typical of a gold-quartz type occurrence. We studied ore minerals of gold-quartz, quartz-molybdenite and quartz-cassiterite veins and oxidation zones, analyzed the composition of the surface waters, and determined homogenizing temperatures and the composition of fluid inclusions. Furthermore we carried out thermodynamical calculations and built Eh-pH diagrams for the Au-Ag-S-H 2 O system at 25˚C and 100˚С (p = 1 bar), in order to determine the Au-Ag sulfides stability fields. General Characteristics of the Dorozhnoye DepositThe Dorozhnoye deposit is located in the apical part of the Sylgytarsky 1 granitoid intrusion. It is situated in the Magadan Region (Russia) near the inhabited locality of Susuman on the right-hand bank of the Dorozhny creek, the left-hand tributary of the Verkhny Neksikan River (Figure 1). The ore field is confined by faults of a north-

Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray single-crystal analysis, compound Ag3Au1Se0.5S1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3¯c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag3AuSe0.25S1.75 and Ag3AuSe0.75S1.25 also show trigonal symmetry. Compounds Ag3AuSeS and Ag3AuSe1.5S0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag3AuSe2 (fischesserite) with space group I4132. These data suggest the existence of two solid solutions: petzite-type cubic Ag3AuSe2–Ag3AuSeS (space group I4132) and trigonal Ag3AuSe0.75S1.25–Ag3AuS2 (space group R3¯c). It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS.

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The mineral and chemical compositions of ores from the Corrida epithermal Au-Ag deposit (Chukchi Peninsula, Russia) were studied using the optical and scanning electron microscopy with X-ray energy-dispersion microanalysis. The deposit was formed at the time close to the period when the basic volume of acid magmas had been emplaced within the Okhotsk–Chukotka belt (84 to 80 Ma). The Au–Ag mineralization is distinguished with Au-Ag sulphides and selenides (uytenbogaardtite-fischesserite solid solution, Se-acanthite, S-naumannite) and Ag halides of the chlorargyrite-embolite-bromargyrite series. The ores were formed in two stages. Using microthermometric methods, it has been established that the ore-bearing quartz was formed in the medium-temperature environment (340–160 °C) with the participation of low-salt (3.55 to 0.18 wt.% NaCl eq.) hydrotherms, mostly of the NaCl composition with magnesium, iron and low-density СО2. According to our results of thermodynamic modeling at temperatures from 300 to 25 °C and data on mineral metasomatic alterations of the host rocks, the Au-Ag-S-Se-Cl-Br mineralization was formed at decreasing temperature and fugacity of sulphur (logƒS2 from −6 to −27), selenium (logƒSe2 from −14 to −35), and oxygen (logƒО2 from −36 to −62), with near-neutral solutions replaced by acid solutions. Analysis of the obtained data shows that the Corrida refers to the group of the LS-type epithermal deposits. This deposit is a new example of epithermal deposits with significant quantities of Au–Ag chalcogenides (acanthite, uytenbogaardtite, fischesserite, naumannite and others).