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phyry copper ore-magmatic system have remained debatable.Under the current economic conditions of northeastern Russia, most of the known porphyry Cu-Mo deposits and occurrences are not promising for economic development. However, the metallogenic belts and zones where these deposits are located are extremely important in forecasting of potentially economic highgrade epithermal gold-silver deposits.At different times, we have studied the metallogenic belts and ore districts considered below. The most recent results of thermobarogeochemical studies were obtained only in 2005. In preparing this paper, we pursued the following goals: (i) to consider the current state of the problem and to summarize the previously obtained results, (ii) to outline the geodynamic settings favorable for formation of porphyry-type Au-bearing Cu-Mo deposits in the Northeast of Russia and the main trends in their spatial distribution, (iii) to review the geology of typical deposits and the ore composition therein, (iv) to study ore formation conditions in a comparative aspect, and (v) to clarify the relationships between epithermal Au-Ag and porphyry Cu-Mo ores in the common ore-forming system. GENERAL OUTLINE OF PORPHYRY-TYPE DEPOSITSPorphyry-type deposits are the main worldwide source of Cu and Mo; Au, Ag, and platinum group and some other rare metals are recovered from this ore as by-products. Porphyry deposits are accompanied by Abstract -By analogy with other metallogenic belts of the Circum-Pacific ring, the metallogenic belts in the Northeast of Russia are promising for discovery of large and superlarge porphyry-type Au-Cu-Mo deposits. The spatial distribution of these deposits is controlled by intrusive domes in Middle Paleozoic, Late JurassicEarly Cretaceous, and Late Cretaceous volcanic belts. New data on formation conditions and sources of ore matter are presented in this paper with respect to the deposits of the Baim and Koni-P'yagina ore districts of the Oloi and Uda-Murgal metallogenic belts. Some aspects of a geological and genetic model of the porphyry copper ore-magmatic system are discussed.

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The Shkol'noe deposit is localized in a small granitoid stock, the root portion of which is traced using geophysical data to a depth of 5-8 km. The high grade gold ore (33 gpt Au) is enriched in silver and principally differs in ore composition from the previously studied mesothermal gold-quartz and epithermal gold-silver deposits in the Russian Northeast. The main reserves of the Shkol'noe deposit concentrate in bonanzas (20% of the total volume of orebodies). The internal deformation is related to the rearrangement of matter in freibergite; exsolution structures in fahlore and native gold are related to postmineral metamor phism. It is suggested that the ore of the Shkol'noe deposit occupies a transitional position between porphyry type and epithermal levels of ore deposition.

Geology and mineralogy of the Ulakhan Au-Ag epithermal deposit (northeastern Russia, Magadan Region) are considered. A four-stage scheme of mineral formation sequence is proposed. Concentrations of Au and Ag in minerals of early and late parageneses were determined. It has been established that uytenbogaardtite is associated with native gold and hypergenesis stage minerals — goethite, hydrogoethite, or limonite replacing pyrite. The compositions of uytenbogaardtite (Ag3AuS2), acanthite (Ag2S), and native gold were studied. The composition of the Ulakhan uytenbogaardtite is compared with those of Au and Ag sulfides from other deposits. Thermodynamic calculations in the system H2O–Fe–Au–Ag–S–C–Na–Cl were carried out, which simulate the interaction of native gold and silver with O2- and CO2-saturated surface waters (carbonaceous, sulfide-carbonaceous, and chloride-sodium-carbonaceous) in the presence and absence of acanthite and pyrite at 25 °C and 1 bar. In closed pyrite-including systems, native silver and kustelite are replaced by acanthite; electrum, by uytenbogaardtite, acanthite, and pure gold; and native gold with a fineness of 700–900‰, by pure gold and uytenbogaardtite. Under the interaction with surface waters in the presence of Ag2S and pyrite, Au-Ag alloys form equilibrium assemblages with petrovskaite or uytenbogaardtite and pure gold. The calculation results confirmed that Au and Ag sulfides can form after native gold in systems involving sulfide-carbon dioxide solutions (H2Saq > 10–4 m). The modeling results support the possible formation of uytenbogaardtite and petrovskaite with the participation of native gold in the hypergenesis zone of epithermal Au-Ag deposits during the oxidation of Au(Ag)-containing pyrite, acanthite, or other sulfides.