The Khangalas orogenic Au deposit, Yana-Kolyma metallogenic belt (Northeast Russia): structure, ore mineral and isotopic (O, S, Re, Os, Pb, Ar, He) composition, fluid regime and formation conditions

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 δ18OH2O (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.