Vol 60, No 6 (2018)

The role of magmatic differentiation is considered for the formation of the Ulan-Tologoi Ta–Nb–Zr deposit (northwestern Mongolia) related to the eponymous alkali granite pluton. Data are presented on the structure of the pluton, the composition of its rocks, and distribution of rare metal mineralization. The ores of the pluton include alkali granites with contents of ore elements exceeding the normative threshold for Ta (>100 ppm). The rare metal mineralization includes pyrochlore, columbite, zircon, bastnaesite, monazite, and thorite, which are typical of all alkali–salic rocks; however, their amount varies depending on the REE content of the rocks. The pluton was formed ~298 Ma ago under the influence of a mantle-crustal melt source.

The hydrothermal molybdenum–uranium deposits of the Streltsovka ore field are localized in a volcanic caldera of Late Mesozoic age. The paper presents the results of numerical modeling of free thermal convection of fluids in the residual thermal field of the consolidated magma chamber of the Streltsovka caldera, which justifies the idea of thermoconvective mechanism for the formation of the structurally conjugate Streltsovka and Antei deposits. The process of fluid heat and mass transfer in the Antei–Streltsovka oreforming system self-organized into a convection cell with a fluid circulation circuit including a descending convection branch along the ring-fault zone of the caldera, a lateral flow branch from the descending to the ascending convection branch in the caldera’s basement rocks, an ascending convection branch along faults of the Antei deposit, and a lateral return branch from the ascending to the descending convection branch along the caldera’s volcano-sedimentary rock fill, in which the ores of the Streltsovka deposit are localized. At the same time, formation of the total reserves of the Antei–Streltsovka deposit, largest in this ore field, was facilitated by uranium sourced, during the hydrothermal ore-forming process, not only from the uranium-rich rocks of the consolidated magma chamber, but also from the granitic host rocks of the Streltsovka caldera basement.

The Dvoinoe Au-Ag low-sulfide epithermal deposit is located in the Ilirnei ore district (western Chukotka) within the outer zone of the Okhotsk–Chukotka volcanic belt (OChVB). The paper considers the results of geological–structural, mineralogical–geochemical, and thermobarogeochemical studies of the deposit. The ores of the deposit are characterized by dominant colloform-banded (often combined with brecciated) structures; a high Au/Ag ratio (1: 1 to 1: 2); low sulfidity (<0.5%); the presence of hessite, small sulfide segregations (0.005–0.015 mm), and a larger amount of native gold (0.01–0.07 mm); high Au, Ag, and Sb contents; relatively high As, Cu, Pb, and Cd contents; higher Zn, Bi, and Te contents; a low total REE; and negative Eu and positive Ce anomalies. The geochemical features are consistent with the ore mineral composition. The high Rb/Sr, Th/La, Y/Ho, and U/Th ratios of ores are most likely caused by the position of the deposit in the zone of influence of the Ilirnei granitic pluton. A fluid inclusion study showed that the ores were formed from Na–K–Mg low-saline (5.58–0.2 wt % NaCl-equiv.) hydrothermal fluids with a small amount of CO₂, H₂S, and CH₄ with decreasing temperatures from 370 to 130°C and the following fluid parameters: CO₂/CH₄ = 217–53, Na/K = 5–2, and K/Rb = 2722–202. The results are compared with original and published data on the Kupol, Moroshka, and Sentyabr’sky deposits. Our data are important for regional forecasting, interpreting metallogeny, and the search for and evaluation of Au–Ag epithermal deposits.