Том 65, № 2 (2023)

The Birkachan deposit in the Middle Paleozoic Kedonsky Volcanic Belt (KVB) is unique in Northeast Russia for its stockwork epithermal Au–Ag ores. The deposit is located in the northeastern part of the Gurnik volcanotectonic depression, in the Kedonsky segment of the KVB. The deposit is localized in the lying side of a large sublatitudinal thrust fault, due to which it was overlain in the Jurassic–Early Cretaceous by allochthon rocks and, therefore, was slightly eroded, as evidenced by the preserved fragments of the argillisite cap. The ore-bearing Gurnik sequence is composed of felsic tuffs and lavas with intercalations of ignimbrites. The main ore-bearing zone of the Birkachan deposit is 4.5 km long and 200–300 m wide consists of several echelon-like vein zones of northeast strike, falling to the southeast at angles of 55°–70°. Most of the ore bodies are linear stockworks. Single, lenticular rich ore bodies represented by mineralized breccias have been found at deep horizons. In the section, the system of ore bodies of the Birkachan deposit forms a fan-shaped structure. The main textures of ores are vein-disseminated, breccia, and rhythmically banded. The ores are enriched in comparison with the upper continental crust in a rather narrow range of elements (Au, Ag, Sb, As, Mo, W, and Li) and are poor in rare-earth elements, among which light lanthanides predominate. The sulfide grade of ores is 0.1–0.5%. The vein minerals in ores are dominated by quartz, sericite, and siderite. Among the ore minerals, pyrite dominates, with fahlore, native gold and chalcopyrite, and minerals of the series acanthite → Se–acantite → naumannite, pearceite, and Se–pearceite being less common. The average fineness value of native gold is 643‰. The parameters of mineral-forming solutions correspond to typical medium-temperature fluids of epithermal low-sulfidized deposits: homogenization temperature of 93–291°С, salt concentration of 0.2–7.0 wt % eq. NaCl, and fluid density of 0.71–0.99 g/cm3. The main indicators of fluid composition are CO2/CH4 = 15.8–23.6, Na/K = 2.3–4.3, and K/Rb = 2007. The prospects for increasing the reserves of Au and Ag are associated with further study and exploration of the flanks and deep horizons of the deposit.

A new technology of stress inversions is proposed on the basis of data of mute fracturing with measured amplitudes of dilative opening of fractures (tension shear), which are registered by mineralization (vein bodies). The involvement of a basic law of geomechanics of supercritical deformation, which determines the interrelation between amplitudes of shear deformations and deformations of transverse extension (dilatancy), allows us to link the amplitudes of opening of shear fractures with the level of Coulomb stresses. This approach can be used for a conjugated fracture system and fault zones with a system of similarly oriented fractures. Our algorithm and technology of stress calculation consist of consecutive analysis of a geometrical form and characteristic lineaments of density zone of fracture poles on the stereospheres and stress conditions of these fractures on Mohr’s diagram. The algorithm allows the calculation of the parameters of stress tensor both on the basis of graphical analysis on the stereonets and Mohr’s diagram and using a group of formulas. The algorithm is applied for the Nadvigovaya zone of the Badran deposit. The stress values are calculated for a fault area with measured fracturing, and a forecast for promising areas is given for a deeper fault part in areas with possible most intense dilatancy of fracture flow.