Vol 67, No 2 (2025)

In this paper, ore-containing amphibolized gabbroids from the new low–sulfide ore occurrence of noble metals (Pt–Au-Pd) Vasilinovskoe, located near the Kharp town in the Yamalo-Nenets Autonomous okrug, are studied in detail. These rocks presumably belong to the Kershorsky complex, which is dated mainly to the Late Ordovician. Due to metamorphism, up to the appearance of amphibolites, a number of generations of amphiboles and plagioclases occurred; then epidote, chlorite, and some other minerals crystallized. Peak PT parameters probably reached ~6 kbar and ~700 °C; for later paragenesis, ~4 kbar and ~650 °C, decreasing to 1 kbar and ~550 °C, thus decompression occurred. For the late low-temperature transformations, chlorite geothermometer estimations reveal t = 275–100 °C. Mineralization zones are developed in these rocks (from the first cm to 50 m, sulfides 1–3 vol.%). The platinum group elements in them are represented by micron–sized palladium minerals – tellurides (merenskiite, temagamite, kotulskite, sopcheite), antimonides (stibiopalladinite, sadberite) and arsenoantimonides (arsenic stibiopalladinite, isomerthite), as well as others – moncheite, native osmium and others. The formation of noble-metal parageneses was associated with late-magmatic processes, as well as with subsequent redistribution by magmatogenic hydrothermal fluids, at temperature decreasing down to ~250 °C; pressure decreased from ~0.9–1.3 to ~0.4–0.5 kbar. The sulfur isotopic composition δ³⁴S (‰) in pyrite varies from -4.2 to +6.3, in chalcopyrite from –1.6 to +4.2. In pyrite of southern flank of the occurrence, δ³⁴S = –2.02 ... +2.72 %; in chalcopyrite, δ³⁴S = –1.74 ... +0.29. According to the Pb-Th-U isotopic parameters and the sulfur isotopic composition of sulfides, the sources of the studied low-sulfide mineralization are similar to mantle-type sources.

The distribution and speciation features of finely dispersed and “invisible” gold in the arsenopyrites and pyrites of the Natalka gold deposit (Northeastern Russia) were examined in detail using a “phase” chemical analysis based on atomic absorption spectrometry (PCA-AAS), light microscopy (LM), electron probe microanalysis (EPMA), and atomic absorption spectrometry with analytical data selections for single crystals (AAS-ADSSC). According to the LM and EPMA data, the finely dispersed and invisible forms (< 0.01 mm) amount to 20% of total gold quantity, and mostly enclosed in sulfide minerals, mainly arsenopyrite and pyrite. The main composition (fineness) of finely dispersed inclusions often differs from the composition of coarse gold by a greater fineness: 750–990‰ and 550–850‰, respectively. The PCA-AAS showed that arsenopyrites and pyrites of the Natalka deposit are the gold concentrators with the highest concentrations in the monofractions of arsenopyrite – up to 1383 ppm, lesser in the monofractions of pyrite – up to 158.2 ppm. The AAS-ADSSC method revealed two forms of uniformly distributed “invisible” Au corresponding to the chemically bound element in the structure of the mineral, and in the superficial non-autonomous phase (NAP). The superficially bound form dominates over the structural form and presumably exists in a very thin surface layer of the crystal (~100–500 nm). The occurrences of micromineral forms of native gold in sulfide crystals and on their surfaces give evidence of the transformation of NAP resulting in the formation and oriented aggregation of nano-to-micron-sized Au particles. This can point the way to “invisible” Au extraction under ore processing which allows enhancing the quality and value of primary products.