Industriya, Petrozavodsk, Russia:

A. V. Korneev, Chief Geologist, KorneevAV@polimetal.ru
A. S. Vikhko, Chief Project Geologist
N. V. Fatov, Cat 2 Geologist

Institute of Geology, Karelian Research Center, Russian Academy of Sciences, Petrozavodsk, Russia
V. I. Ivashenko, Leading Researcher, Candidate of Geologo-Mineralogical Sciences

The Koikary gabbro-dolerite sill (western flank of the Onega Structure, Karelia), titanomagnetite horizon and a noble-metal reef, which are part of the sill, are briefly described. The data on prospecting, appraisal and exploration within the sill that have led to the discovery of the big Viksha noble-metal (Pd, Pt and Au) deposit are reported. The deposit, consisting of three adjacent sites (Viksheozero, Kenti and Shargi), is located in the northern Koikary sill. Its highest-grade portion with titanomagnetite concentration of up to 50% lies 33–36 m above the bottom of the sill, and the 5–11 m thick noblemetal reef, which coincides partly with the titanomagnetite horizon, is 38–45 m above the bottom as indicated by drilling. Noble-metal mineralization within the reef displays zonal distribution: gold predominates in its upper portion and palladium in the lower one. The ore horizon in natural outcrops has been traced over 20 km. The major ore minerals of the titanomagnetite horizon are titanomagnetite (Ti 3-16%) and ilmenite and those of the noble-metal reef are arsenides, stibioarsenides, stannides and Pt and Pd sulphides, native gold and electrum that occur as micron-sized (0,1-15 μm) grains. PGM and gold concentrations in the ores are low: 0.022–0.8 g/t Pt; 0.026–2.4 Pd; Rh <0.003; ΣPGE-0.9–3; 0.008–2.3 g/t Au. Titanomagnetite ore reserves are 314.1 M t with ~20-47.9% Fe_bulk 3.7–8.1% TiO₂ and 0.13–1.0% V₂O₅. By independent audit conducted by AMC Consultants in accordance with the JORC Code, the mineral noble-metal resources of the Viksha deposit are estimated as 213 Mt with an average concentration of 0.98 g/t of ΣPd,Pt,Au and 0.1% copper. The total noble-metal reserves are estimated as 6.6 M oz., which means that Viksha is a big PGM deposit suitable for open-pit mining. As its mineral composition is technologically “simple” (no nickel minerals), conventional flotation can be used for the production of bulk copper–PGM–sulphide concentrate, which can be processed at affiliated PGMproducing plants or at copper smelters with a technological cycle for extra platinoid dressing.

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