Epithermal Au–Ag and Ag Deposits of the Okhotsk Sector of the Okhotsk–Chukchi Volcanic Belt: Metallogeny, Mineral Paragenesis, and Fluid Regime

The Okhotsk sector, located in the central part of the Cretaceous marginal-continental Okhotsk–Chukchi volcanic belt (OСhVB), stretches along the coast of the Sea of Okhotsk (within the borders of Magadan oblast) for a distance of about 1000 km. To date, about 150 t of Au and more than 14 000 t of Ag have been mined from epithermal deposits in the Okhotsk sector. Deposits of the Se-subtype ores predominate; Te-bearing mineralization is rare. Volcanic troughs, depressions, calderas, and dome uplifts, as well as zones of tectonomagmatic activation (TMA) control the location of deposits. A brief description of the main epithermal Au–Ag and Ag deposits of the Okhotsk sector is given. The ore bodies are represented by quartz–adularia and quartz–adularia–carbonate veins, sometimes arranged in an echelon into extended vein zones. The ores of most deposits are characterized by complex, combined brecciated-crustiform structures, widely developed rhythmically and colloform-banded textures, which have a predominantly thin and mottled distribution of ore inclusions, often with their banded accumulation (ginguro). Native gold has a relatively low fineness and a low to high range of this parameter. Native silver is present in significant quantities in the polysulfide ores of the Omsukchan metallogenic zone (OMZ). Fahlore of the tetrahedrite–freibergite group, acanthite, and sulfosols (Ag, Sb, As, and Sn) are widespread in the ores. The fluids that contributed to the formation of the OMZ deposits are characterized by the maximum values of temperature and salinity, while the minimum values are characteristic of the fluids of the eastern (Even Group) and central parts of the sector. The results of textural and mineralogical analyses, and physicochemical parameters of mineral-forming fluids allow the majority of Au–Ag deposits in the Okhotsk sector to be classified as a low sulfidation class of epithermal deposits, and Ag-containing deposits of the OMZ as an intermediate sulfidation class. Thermal metamorphism of ores, rejuvenation of the hydrothermal system, and the formation of exotic mineral parageneses were noted at the Dukat, Nyavlenga, Julietta, and other deposits. The results obtained can be used in regional predictive metallogenic models for searching of epithermal deposits and evaluating of their reserves. Given the fragmentary exploration of the Okhotsk sector of the OСhVB and the feathering zones of the TMA, there are real prospects for discovering of new epithermal deposits.