Distribution and speciation features of finely dispersed and “Invisible” gold in arsenopyrite and pyrite of the Natalka Deposit (Northeastern Russia)

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.