In this paper, we present a new point of view on the deposit geology and genesis of the largest gold deposit in Eastern Sayan, Zun-Kholba. Quartz–sulfide replacement ores and shallow quartz veins are of great economic importance. At the deposit, quartz–pyrite ore bodies are dominant, whereas paragenetically late base metal-bearing quartz veins occur only at shallow levels. The study of the fluid inclusions, stable isotopes (C–O–S), and mineral geothermometry allowed us to determine the P–T conditions of ore deposition. It is established that the temperature of ore formation changed from 380 to 433 °C at the deepest levels of the deposit to 316 °C at the shallowest levels. The pressure estimates for gold deposition at 570–950 bar were calculated for the 1490-m level in the center of the deposit. The determined P–T conditions can help estimate the average depth of deposit formation at about 3.6 km. Ore mineral assemblages were formed from homogeneous fluids of low to moderate salinity (2.4–7.9 wt.% eq. NaCl). The sulfur isotope composition of sulfide minerals (δ³⁴S = 0–4.6‰), as well as pyrite geochemistry, corresponds to that of older volcanogenic massive sulfide (VMS) occurrences, which are widespread within the Eastern Sayan ophiolitic belts. Lens-like fragments of metamorphosed VMS-type sulfide ores are also present in the Zun-Kholba deposit. The oxygen isotope data indicate a metamorphic origin for the ore-forming fluids. Migrating metamorphic fluids may have become enriched in gold, sulfur, and other metals during the flow through the complex ore-hosting lithologies, including ophiolitic bodies with sulfide-rich VMS fragments that are characterized by a relatively high content of gold and related ore elements. The obtained data allow us to suggest a metamorphic devolatilization model for the genesis of the Zun-Kholba gold deposit.

The paper presents the results of studying the mineral composition of the main quartz–sulfide ores of the Zun-Kholba gold-ore deposit, the largest gold deposit in Eastern Sayan. Three mineral ore types are distinguished: compact pyrite–pyrrhotite, quartz–carbonate–sulfide, and quartz–polymetallic. Native gold occurs as isolations in pyrite-2. It develops along fractures and crystal edges or deposits in association with other ore minerals such as chalcopyrite, sphalerite, and galena. Gold deposited at the quartz–carbonate–sulfide stage has grade varying from 634 to 926‰; at the polymetallic stage, 467–860‰; at the Bi-telluride stage, 327–693‰. In addition, the dependence of the composition of impurity mineral associations on ore-bearing rock complexes was revealed. The source of bismuth could be a lamprophyre dike, and nickel minerals appear in ores in the areas of development of listvenitized rocks, products of tectonometasomatic transformations of ultramafic rocks. In terms of mineral relationships, four ore associations are established: pyrite–pyrrhotite, quartz–carbonate–sulfide, quartz–polysulfide, and Bi-telluride corresponding to the pre-ore stage and three successive ore-formation stages.