Volume 66, Nº 5 (2024)

The Khangalas orogenic gold deposit is located in the central part of the Yana-Kolyma metallogenic belt. The structure of the deposit is determined by several mineralized crush zones with a thickness of up to 70 m and a length of up to 1400 m in the arch and on the southwest wing of the anticline of the northwest strike. Host rocks – Upper Permian terrigenous deposits. Ore bodies are characterized by massive, banded, veined, disseminated and breccia structures. The main vein minerals are quartz, carbonates, sericite is less common. The main ore minerals are pyrite, arsenopyrite; minor – galena, sphalerite, chalcopyrite, native Au; rare – Fe-gersdorffite, tetrahedrite, argentotennantite. Hypergenic minerals – sulfates, phosphates, arsenates and hydroxides – are widely manifested in the linear oxidation zone. Mineral formation occurred in two stages – gold-sulfide-quartz and silver-quartz ones. Quartz veins with visible Au were formed with the involvement of low-concentrated (about 5.0 wt.% eq. NaCl) of hydrocarbonate hydrotherms with CO₂ in the gas component, at a temperature of 330–280 °C and a pressure of about 0.8 kbar. Disseminated gold-bearing pyrite-3 (up to 39.3 g/t Au) and arsenopyrite-1 (up to 23.8 g/t Au) from sericite-carbonate-quartz metasomatites have a non-stoichiometric composition, Fe excess and S lack (and As in Ару), Fe/(S+As)=0.47–0.52 (Py3) and 0.47–0.50 (Ару1). The predominant form of “invisible” gold in Py3 and Apy1 is structurally related Au⁺. Isotopic composition of oxygen δ¹⁸O quartz (from +15.2 to +16.1‰), oxygen in the fluid δ¹⁸O_H2O (from +8.4 tо +9.2‰)‰), sulfur δ³⁴S in sulfides (from –2.1 to –0.6‰); isotopic ratio ¹⁸⁷Os/¹⁸⁸Os (from 0.2212 to 0.2338) in native gold and Pb in galena (²⁰⁶Pb/²⁰⁴Pb=18.0214, ²⁰⁷Pb/²⁰⁴Pb=15.5356, ²⁰⁸Pb/²⁰⁴Pb=38.2216), as well as the geochemical features of Py3 and Apy1 suggest the participation in ore formation mainly of sources from the subcontinental lithospheric mantle and, to a lesser extent, crustal reservoirs. The formation of the gold ore bodies of the deposit is related to the completion of reverse and thrust fault progressive deformation of the stage D1, which occurred in the Valanginian of the Early Cretaceous (about 137 million years ago) during late-orogenic processes in the Yana-Kolyma belt with regional south-western transport of rocks. The results obtained are important for predictive metallogenic and prospecting work aimed at identifying large-volume gold mineralization of orogenic belts.

The Davenda-Klyuchevsky ore cluster contains a large number of gold placers and ore occurrences, deposits of gold-sulfide-quartz and gold-bearing Mo-porphyry formations of stockwork and vein types. Judging by the morphology of gold, most placers are closely spatially related to ore sources, but data on the chemical composition of gold from placers and ores are very limited in the literature. A detailed mineralogical and geochemical study of placer gold, which is the subject of this work, provides important genetic information that can be used to predict and search for gold mineralization. It has been established that the fineness of placer gold is well comparable to the fineness of ore gold. In the autochthonous placers, that formed a distant aureole around the Klyuchevskoye deposit, are dominated by gold with a fineness of 900–950‰. Gold often contains inclusions of ore minerals and forms intergrowths with them. First of all, these are pyrite, galena and Bi minerals (tetradymite, Bi tellurides, Bi-containing sulfosalts, native Bi, bismuthine), less often – arsenopyrite, chalcopyrite, fahlores. In the autochthonous placers, located around the Davenda deposit, gold with a fineness of more than 950‰ predominates; inclusions of ore minerals (pyrite, galena, tetradymite, PbBiCuS mineral, arsenopyrite) in the gold of these placers are rare. The third type of gold (850–900‰) is less common in the above-mentioned placers, but predominates in the autochthonous placers of the Maly and Levy Amundzhikan rivers. In essentially allochthonous placers on the periphery of the Davenda-Klyuchevsky ore cluster and the Cherny Uryum River, lower grade gold is also common. In this group of placers, the mineral association of Au changes somewhat. Galena and Bi minerals are almost never found together in gold grains; hessite, acanthite, and petzite appear, associated with Bi minerals or pyrite. The heavy concentrates contain cinnabar. The fineness of placer gold and its associations with ore and nonmetallic minerals indicate telescoping mineralization in ore sources and the superimposed nature of placer-forming gold mineralization.

The age restrictions for productive mineral associations of the two largest gold deposits of the Southern Urals — Svetlinsk and Kochkar’, which are located in the East-Uralian megazone, are discussed. The ⁴⁰Ar/³⁹Ar dating method was performed for the first time for potassium-containing hydrothermal minerals (micas and amphiboles) from the ore veins and ore-host alteration, as well as marbles. The age estimates obtained are in the range of 290–276 Ma; the weighted average value for the Svetlinsk deposit is 284 ±2 Ma, for the Kochkar’ field 276 ±2 Ma. It is assumed that the studied mineral assemblages of the Svetlinsk and Kochkar’ deposits were formed in the beginning of the post-collision stage corresponding to the tectonic relaxation regime. Age of hydrothermal mineral formation in the gold fields of the Kochkar anticlinorium are consistent with the time of post-tectonic plutonic activity, which was expressed in the Middle–Southern Urals in large-scale granitization (~ 300 million years ago). The ore-bearing alteration of a femic profile (of the same age for two deposits), which are most likely basificates, are near-synchronous to the granitization.