Distribution Features and Species of Finely Dispersed and “Invisible” Gold in Arsenopyrite and Pyrite from the Natalka Deposit (Northeastern Russia)

The distribution features and species of finely dispersed and “invisible” gold in arsenopyrite and pyrite of the Natalka gold deposit (Northeastern Russia) have been studied in detail using phase chemical analysis based on atomic absorption spectrometry (PCA-AAS), light microscopy (LM), and X-ray electron probe microanalysis (EPMA) methods, as well as atomic absorption spectrometry with analytical data selections for single crystals (AAS-ADSSC). According to LM and EPMA, the proportion of finely dispersed and “invisible” gold, most of which is incorporated in sulfide minerals, mainly in arsenopyrite and pyrite, may account for up to 20% of the total gold in the ores. The main composition of finely dispersed gold inclusions differs from that of coarse gold (550–850‰) with a greater fineness (750–990‰). According to PCA-AAS, the maximum Au contents were measured in arsenopyrite monofractions of up to 1383 ppm, with less high contents in pyrite monofractions, up to 158.2 ppm. The use of the AAS-ADSSC method allowed us to reveal species of “invisible” gold—evenly distributed structural and surficial, corresponding to chemically bound element in the structure of mineral and in the structure of a nanosized nonautonomous phase located on the crystal surface. The latter occurs and prevails in the very thin surface layer (∼100–500 nm). The occurrence of micromineral species of native gold in crystals and in the composition of their surface is evidence to assume partial transformation and directed aggregation of released Au with the formation of its nanoscale to submicron particles. This makes it possible to extract such Au species during ore enrichment. Accordingly, this significantly increases the quality and gross recoverable value.