The article discusses the composition of studied ore-forming solutions and the P-T conditions of molybdenum mineralization in the Pervomaisky stockwork deposit which is situated within the Dzhidinsky ore field (South-Western Transbaikalia, Russia). New geochronological data of zircons from granites, muscovite, and molybdenite from the ore zones indicates the association of the granite formation and ore deposition processes which occurred 119–128 million years ago. Quartz-molybdenite veins of the Pervomaisky deposit were formed at the temperature of ≥314–186 °C with some boiling periods. Fluid inclusions in these veins have total salt concentration of 6.3–12.7 wt. % NaCl equivalent (eq. NaCl). The salt solution is composed of chlorides of Na, Ca, K, and Fe. The gas phase contains CO₂, CH₄, and N₂. A series of elements were determined in fluid inclusions by laser ablation (LA)-ICP-MS: Li, Be, B, F, Na, Mg, Al, Cl, K, Ca, Mn, Fe, Cu, Zn, Nb, Mo, Ag, Sn, La, Ce, Ta, W, Au, Pb, Th, U. The Mo content reaches 559 ppm (average of 228 ± 190 ppm) in high-grade quartz-molybdenite veinlets, whereas Mo content is up to 212 ppm (average of 25 ± 29 ppm) in the low-grade veinlets. High-grade veinlets were formed by near-neutral solutions with a higher content of Mo, S, and F, while relatively low-grade veinlets were deposited from alkaline solutions. Our results demonstrate the pH of the solutions as one of the key factors for ore deposition.

The gold-bearing deposits of southeastern East Sayan have been categorized according to mineral composition. The most important classification criterion is the composition of productive ore mineral...

This article discusses the peculiarities of mineral composition and a fluid inclusions (FIs further in the text) study of the Kholtoson W and Inkur W deposits located within the Dzhida W-Mo ore field (Southwestern Transbaikalia, Russia). The Mo mineralization spatially coincides with the apical part of the Pervomaisky stock (Pervomaisky deposit), and the W mineralization forms numerous quartz veins in the western part of the ore field (Kholtoson vein deposit) and the stockwork in the central part (Inkur stockwork deposit). The ore mineral composition is similar at both deposits. Quartz is the main gangue mineral; there are also present muscovite, K-feldspar, and carbonates. The main ore mineral of both deposits is hubnerite. In addition to hubnerite, at both deposits, more than 20 mineral species were identified; they include sulfides (pyrite, chalcopyrite, galena, sphalerite, bornite, etc.), sulfosalts (tetrahedrite, aikinite, stannite, etc.), oxides (scheelite, cassiterite), and tellurides (hessite). The results of mineralogical and fluid inclusions studies allowed us to conclude that the Inkur W and the Kholtoson W deposits were formed by the same hydrothermal fluids, related to the same ore-forming system. For both deposits, the fluid inclusion homogenization temperatures varied within the range ~195–344 °C. The presence of cogenetic liquid- and vapor-dominated inclusions in the quartz from the ores of the Kholtoson deposit allowed us to estimate the true temperature range of mineral formation as 413–350 °C. Ore deposition occurred under similar physical-chemical conditions, differing only in pressures of mineral formation. The main factors of hubnerite deposition from hydrothermal fluids were decreases in temperature.

The paper discusses the geology of Zun-Ospa gold deposit, which is situated near the Ospino ophiolitic nappe in the southeastern part of the Eastern Sayan, and the ore composition therein. The...

The Pionerskoe gold–quartz deposit is the first primary gold deposit discovered in the southeastern part of the Eastern Sayan. The orebodies of the Pionerskoe deposit are low-sulfidation pyrite–quartz and carbonate-pyrite–quartz veinlets and veins that occur in crush, mylonitization, and shear zones. Pyrite predominates among the ore minerals; chalcopyrite, pyrrhotite, galena, tellurides, native gold, and single fahlore grains are less common. In the quartz veins occurring in listvenite bodies, in addition to the listed ones, are Ni and Co minerals: cobaltite, alloclasite, gersdorffite, and pentlandite. Among the tellurides, the following minerals have been identified: altaite, petzite, hessite, calaverite, melonite, tellurobismuthite, pilsenite, coloradoite, rucklidgeite, volynskite, tsumoite, and tetradymite. Native gold is associated with tellurides; single grains of low-grade native gold are found in association with galena and sphalerite. Native gold disseminations are present in ores as small veinlets and irregularly shaped grains in quartz and pyrite; they often form intergrowths with telluride minerals. The ores contain three mineral assemblages: quartz–pyrite, quartz–polysulfide, and gold–telluride. Studies have shown that during the formation of these assemblages, the sulfur fugacity decreased and the tellurium fugacity increased. Fluid inclusion study and mineral thermometry made it possible to establish the general temperature range of mineral formation from 285 to 225°С, and the formation of the gold–telluride assemblage, from 225 to 227°C. Sulfur isotope compositions in pyrite and equilibrium fluid have values typical of juvenile sulfur. Oxygen isotope composition studies indicate that magmatic fluids were involved in ore formation. The interaction of the primary fluid with the host rocks containing increased concentrations of gold and, possibly, tellurium, led to additional enrichment in these elements and the formation of a small (in terms of reserves) but rich gold deposit with a wide range of telluride minerals.

The new results have been represented of mineralogical–geochemical and geochronological studies of rocks of the Yehe-Shigna ophiolite massif located in the Tuva-Mongolian microcontinent in the northern part of the Central Asian orogenic belt (Eastern Sayan, Southern Siberia). The Yehe-Shigna ophiolite massif is part of the Belsk-Dugda ophiolite belt. The structural position, age, and geochemical characteristics of the belt indicate its formation in the setting of the back-arc basin of the Shishkhid intraoceanic island arc, developing in the period of 810–750 million years. It is assumed that together with the same-age formations of the Oka accretion wedge and the Sarkhoi active margin, it formed on the convergent margin of the Gondwana supercontinent. Its basement is represented by the Archean-Early Precambrian crystalline rocks and carbonate cover (“Gargan Glyba”). The gold-bearing Neoproterozoic deposits with dominant gold-telluride assemblages are localization in large ophiolites thrust zones along with the frame of the “Gargan Glyba”. They are allochthonous with respect to the Late Neoproterozoic-Cambrian Tuva-Mongolian island arc of the Siberian continent. A similar type of gold deposit is probably worth looking for ophiolites thrust zones in other Precambrian Gondwana-derived microcontinents.