Mineralogy and geochemistry of heavy concentrate from the Malaya Nesterovka gold placer (Primorsky krai): electron microprobe data

Background. The Fadeevsky ore-placer cluster (Primorsky Krai) is a promising gold-bearing area; however, the bedrock sources of its numerous placers remain a subject of debate. For the first time, a comprehensive microprobe study of the composition of native gold and its associated heavy fraction minerals has been conducted on the Malaya Nesterovka River placer, located within this cluster.

Aim. To perform a mineralogical and geochemical study of native gold and accessory minerals in the magnetic, electromagnetic, and heavy fractions of the Malaya Nesterovka placer using microprobe analysis and scanning electron microscopy for reconstructing potential types of primary source rocks and assessing subsequent post-sedimentary (supergene) processes within the placer system.

Materials and methods. Samples of natural concentrates were studied. Preliminary mineralogical inspection was carried out using MBS-10 and Nikon SMZ 465 binocular microscopes. For a detailed microchemical research, individual grains of gold and accessory minerals were selected and analyzed by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). The analyses were performed at the Center for Collective Use of the Far East Geological Institute FEB RAS using a Jeol JXA-8100 electron probe microanalyzer (Japan) and a Carl Zeiss EVO 50 XVP scanning electron microscope (Germany) equipped with an INCA Energy 350 spectrometer.

Results. The concentrates were found to have a complex polymictic composition. Four geochemical types of native gold were identified: high-fineness, uranium-rubidium-bearing, mercury-bearing,  and carbonaceous/organomineral-coated gold. Heavy mineral concentrates yielded platinum- group minerals (sperrylite), high-chromium spinels (Cr# = 0.95–0.99), ferrian baddeleyite, sulfides (bornite, pyrrhotite), and supergene Cr-M n-Si formations. The surface of gold particles was established to be susceptible to sorption of organic matter and clay minerals forming organomineral films containing up to 14.5 and 12.7 wt % of C and O, respectively.

Conclusion. As a result of the paragenetic analysis, a heterogeneous/diverse model of placer source rocks was proposed, including: (1) ultramafic-mafic massif (source of sperrylite, chromite, and baddeleyite); (2) mesothermal gold-quartz formation; (3) low-temperature gold-mercury mineralization; and (4) specialized gold-uranium-rare metal mineralization. The preservation of chemically unstable sulfides indicates the proximity of primary source rocks. Mineralogical and geochemical criteria for predicting concealed mineralization within the Fadeevsky Ore Cluster were developed.

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