The Blagodatnoye deposit with 340 t gold reserves is one of the most productive mines in Russia. Modern methods of studying fluid inclusions were used to determine the properties of fluids that formed this deposit. A comprehensive study revealed that the Blagodatnoye gold deposit was formed between 120 and 350 °C and at 0.2–2.6 kbar, and from fluids with salinities ranging from 0.5 to 30 wt.% (NaCl–eq.). These fluids are: 1—water–carbon dioxide; 2—carbon dioxide–hydrocarbon; 3—highly saline aqueous. According to Raman spectroscopy and gas chromatography–mass spectrometry, ore–forming fluids contained H₂O, CO₂, hydrocarbons and oxygenated organic compounds, sulfonated, nitrogenated and halogenated compounds. Early oxidized water–carbon dioxide fluids formed barren associations of the deposit. Later reduced carbon dioxide–hydrocarbon fluids had a key role in the formation of gold-bearing quartz veins. The stable isotope data (δ³⁴S = 0.8 to 21.3‰, δ¹³C = −2.8 to −20.9‰, ³He/⁴He = 0.14 ± 0.3 × 10^–6) suggest the ore-forming fluids have a crustal source.

The formation of hydrocarbons (HCs) upon interaction of metal and metal–carbon phases (solid Fe, Fe₃C, Fe₇C₃, Ni, and liquid Fe–Ni alloys) with or without additional sources of carbon (graphite, diamond, carbonate, and H₂O–CO₂ fluids) was investigated in quenching experiments at 6.3 GPa and 1000–1400 °C, wherein hydrogen fugacity (fH₂) was controlled by the Fe–FeO + H₂O or Mo–MoO₂ + H₂O equilibria. The aim of the study was to investigate abiotic generation of hydrocarbons and to characterize the diversity of HC species that form in the presence of Fe/Ni metal phases at P–T–fH₂ conditions typical of the upper mantle. The carbon donors were not fully depleted at experimental conditions. The ratio of H₂ ingress and consumption rates depended on hydrogen permeability of the capsule material: runs with low-permeable Au capsules and/or high hydrogenation rates (H₂O–CO₂ fluid) yielded fluids equilibrated with the final assemblage of solid phases at fH₂^sample ≤ fH₂^buffer. The synthesized quenched fluids contained diverse HC species, predominantly light alkanes. The relative percentages of light alkane species were greater in higher temperature runs. At 1200 °C, light alkanes (C₁ ≈ C₂ > C₃ > C₄) formed either by direct hydrogenation of Fe₃C or Fe₇C₃, or by hydrogenation of graphite/diamond in the presence of Fe₃C, Fe₇C_3, and a liquid Fe–Ni alloy. The CH₄/C₂H₆ ratio in the fluids decreased from 5 to 0.5 with decreasing iron activity and the C fraction increased in the series: Fe–Fe₃C → Fe₃C–Fe₇C₃ → Fe₇C₃–graphite → graphite. Fe₃C–magnesite and Fe₃C–H₂O–CO₂ systems at 1200 °C yielded magnesiowüstite and wüstite, respectively, and both produced C-enriched carbide Fe₇C₃ and mainly light alkanes (C₁ ≈ C₂ > C₃ > C₄). Thus, reactions of metal phases that simulate the composition of native iron with various carbon donors (graphite, diamond, carbonate, or H₂O–CO₂ fluid) at the upper mantle P–T conditions and enhanced fH₂ can provide abiotic generation of complex hydrocarbon systems that predominantly contain light alkanes. The conditions favorable for HC formation exist in mantle zones, where slab-derived H₂O-, CO₂- and carbonate-bearing fluids interact with metal-saturated mantle.

Experiments conducted in the olivine–serpentine–anthracene–metal (FeNi) system have shown that the recrystallization of olivines occurs under substantially reduced conditions with active participation of hydrocarbons, especially paraffins; moreover, their amount increases with increasing pressure and temperature. During the decomposition of serpentine, a large amount of water is released; therefore, the fluid at relatively low P-T parameters (2 GPa, 1100 °C) has mainly water–hydrocarbon composition. With an increase in pressure up to 3–4.5 GPa and temperature up to 1300–1400 °C, the composition of the fluid changes greatly towards an increase in the relative amount of hydrocarbons, while the main share is occupied by light (C1–C4) aliphatic hydrocarbons. Therefore, a biogenic material with a carbon–hydrogen composition can make a certain contribution to the carbon budget in subduction processes when falling into the subduction zones and may affect the oxygen fugitivity in the subducted slab.

The Dobroe deposit with 10 t gold reserves is one of the gold mines located within the Yenisei Ridge Orogenic Belt. The ore-forming conditions of orogenic gold deposits are have recently been widely discussed. A comprehensive study of fluid inclusions revealed that the Dobroe gold deposit was formed by water–carbon dioxide and carbon dioxide–hydrocarbon fluids within a temperature range of 180 to 360 °C, a pressure range of 0.8 to 1.3 kbar, and a salinity range of 1.5 to 15.0 wt.% (NaCl-equiv.). Gas chromatography–mass spectrometry showed that ore-forming fluids consisted of H₂O, CO₂, hydrocarbons, nitrogenated, sulfonated, and chlorinated compounds. The distribution patterns of δ¹³C in fluid inclusions (−11.3‰–−3.6‰) and δ³⁴S in sulfides (1.9‰–17‰) of the Dobroe deposit indicate a crustal source for ore-bearing fluids.

C- and N-bearing species in reduced fluids weree studied experimentally in C–O–H–N and muscovite–C–O–H–N systems and in natural carbonate-bearing samples at mantle P–T parameters. The experiments reproduced three types of reactions leading to formation of hydrocarbons (HCs) at 3.8–7.8 GPa and 800–1400 °C and at hydrogen fugacity (fH₂) buffered by the Fe–FeO (IW) + H₂O or Mo–MoO₂ (MMO) + H₂O equilibria: (i) Thermal destruction of organic matter during its subduction into the mantle (with an example of docosane), (ii) hydrogenation of graphite upon interaction with H₂-enriched fluids, and (iii) hydrogenation of carbonates and products of their reduction in metamorphic clayey rocks. The obtained quenched fluids analyzed after the runs by gas chromatography-mass spectrometry (GC–MS) and electronic ionization mass-spectrometry (HR–MS) contain CH₄ and C₂H₆ as main carbon species. The concentrations of C₂-C₄ alkanes in the fluids increase as the pressure and temperature increase from 3.8 to 7.8 GPa and from 800 to 1400 °C, respectively. The fluid equilibrated with the muscovite–garnet–omphacite–kyanite–rutile ± coesite assemblage consists of 50–80 rel.% H₂O and 15–40 rel.% alkanes (C₁ > C₂ > C₃ > C₄). Main N-bearing species are ammonia (NH₃) in the C–O–H–N and muscovite–C–O–H–N systems or methanimine (CH₃N) in the fluid derived from the samples of natural pelitic rocks. Nitrogen comes either from air or melamine (C₃H₆N₆) in model systems or from NH₄⁺ in the runs with natural samples. The formula CH₃N in the quenched fluid of the C–O–H–N system is confirmed by HR–MS. The impossibility of CH₃N incorporation into K-bearing silicates because of a big CH₃NH⁺ cation may limit the solubility of N in silicates at low fO₂ and hence may substantially influence the mantle cycle of nitrogen. Thus, subduction of slabs containing carbonates, organic matter, and N-bearing minerals into strongly reduced mantle may induce the formation of fluids enriched in H₂O, light alkanes, NH₃, and CH₃N. The presence of these species must be critical for the deep cycles of carbon, nitrogen, and hydrogen.