The Mayskoye gold-bearing ore deposit is located at the Eastern wing of the Palyvaamsky synclinorium, related to the Chukotsk Mesozoic plicate system. The stratigraphy of the district is represented by Triassic siltstones and limestones, unconformably overlapped by cretaceous stratificated and sub-volcanic formations, and early-cretaceous continental precipitations. The cretaceous volcanic formation becomes revealed towards the South from the Mayskoye and forms the Chukotka volcanic belt. Intrusive strata are rarely found, and are represented by acid sub-volcanic bodies.

Triassic strata are divided into middle Triassic, undivided, and upper Triassic subdivisions, having a total thickness from 1700 to 2500 m. The lower parts of the subdivisions, are composed, in general, by clayey shales, the middle part being formed by sandstones, the upper part, in general, formed by siltstones and flaky shales. The cretaceous of volcanic origin, and carboniferous continental precipitations overlap the Triassic precipitations with an angular disagreement. The cretaceous complex is composed of acid tufas, tufa-lavas and ignimbrites. The continental complex is represented by sandstones, siltstones, carbonaceous shales and coals. The undivided Quaternary precipitations are found all around the area.

The localization of gold mineralization is controlled by the Northern and North-Eastern tectonic zones, dated to the Mesozoic folding. The containing strata are composed by siltstones and shales, which have been burrowed through by a series of acid dikes. These are up to 100m thick and up to 2 km in length, and occupy approximately 10% of the deposit area. The extension of intrusions is sub-parallel to the axis which controls anticlinals.

The mineralization is controlled by two systems of fractures – with an abrupt dip (>65), and with a mildly sloping dip (<65). The mildly sloping disturbances are represented by linear bodies, with pinches and bulges from 0.6 to 10 m. The contacts are generally sharp and rectilinear. The main factor, influencing the form of mineralized zones – the composition of containing strata and the availability of pre-ore fractures.

All the zones have similar structure and texture peculiarities, and have been the zones of fragmentation with relatively sharp geological boundaries. The zones of mineralization have been subjected to metasomatism with a weak development of sericite and kaolin in cretaceous siltstones, shales, sandstones, and, much more rarely, in quartz-porphyry dikes. These zones are intersected by quartz lodes, which contain well crystallized disseminations of gold-bearing sulphide minerals (pyrite and arsenopyrite). Gold-bearing ore mineralization has been revealed in three formations – gold-pyrite arsenopyrite, quartz-stibnite, and gypsum carbonate.

Current reserves at Mayskoye have an estimated depth of 300 m from the surface, with resources extending to a depth of 1,000 m. The primary ore that makes up more than 90 % of the total reserves, is accessed with three separate sets of twin spiral ramps in the central, eastern and western parts of the deposit. This technique allows for greater mining flexibility in terms of tonnage, grade and metallurgical characteristics (sulphur content and organic carbon content).

Several underground mining methods are used at Mayskoye, but more than 80% of ore is mined by sub-level open stoping. Oxidised ore is mined in a series of shallow open pits, as well as the underground. However, due to persistent problems with ore stability that was partly responsible for fatal accidents at the mine, we accelerated a planned changeover from drift-and-fill mining to sublevel open stoping with backfill. The new mining system was designed to ensure that employees are not exposed to the falling rocks in the partially broken stopes since all development and production drilling will be done from waste openings. This new mining method has demonstrated positive dynamics, ensuring lower dilution from secondary stopes and reduced primary stope length.

Primary ore from Mayskoye will be processed via conventional floation at the 850 Ktpa circuit based on on-site. The circuit comprises single-stage crushing followed by three-stage grinding (in one SAG mill and two ball mills) and three-stage conventional flotation. The flotation concentrate is then thickened, filtered, dried and packed into big bags, which are then transported to the port of Pevek, for further transportation to the Amursk POX hub and third party off-takers.

Oxidised ore, on the other hand, is processed using conventional RIL cyanidation. This process uses the same crushing and milling equipment as the flotation circuit, where slurry is subject to thickening, leaching, resin, resin deactivation, electrolysis and smelting. The cyanidation tailings are then filtered and dry stacked in the lined tailings storage facility.

Due to refractory nature of Mayskoye ore, pre-treatment oxidation processing is required, which is done at the Amursk POX hub that was launched in 2012. In the process of choosing an optimal processing method for refractory ores, autoclave leaching (POX) was selected as the most viable option.