The Gross deposit is a structurally controlled zone of potassic metasomatism, hosted by Proterozoic sandstones. Mineralisation is associated with iron and manganese oxides, that follow from primary sulphides, particularly pyrite. The main part of the mineralised zone is truncated by surface topography, strikes east-west for approximately 1 km, dips shallowly to the south with a down-dip extent of 1.5 km, and is up to 400 m thick.

The deposit is situated in the western part of the Aldan shield, in the southwestern corner of the Uguskiy Graben. The major regional structural features are: 1) the N-S striking Tokkinsky Fault Zone (along the western margin of the Uguskiy Graben); and 2) WSW-ENE striking faults of the Kondinsky Fault System, which cut across the Tokkinsky Fault Zone, and are broadly parallel to the southern margin of the graben.

Gross mineralisation is hosted by the Olonnokonskiy Formation, and in the project area this formation comprises up to 20-30 m of basal conglomerates and gravels, overlain by 300-400 m of fine to medium grained sandstones. The sediments are shallow-dipping to the north, northeast or east.

The Olonnokonskiy Formation host rocks have undergone multiple tectonic and magmatic activation events, dominated by deformation and metasomatism associated with the intrusion of Mesozoic dykes and sills.

The main part of the mineralisation is located between two major south-dipping reverse faults: the Bottom Thrust and the Granichniy Fault. Between these faults are many secondary structures, dipping both north and south. To the south of the main mineralisation is a thinner zone of mineralisation, concentrated around a shallow north dipping structure.

The thickness of alluvial and glacial Quaternary cover sediments is negligible; usually no more than 5 m.

Mineralisation was introduced by potassic metasomatism, and is controlled by structural features within the fault-bound wedge of sandstone host rock. Gold and silver are the only elements of economic significance. Gold is strongly correlated with potassium and arsenic. Sulphide content on average is less than 1%. The dominant sulphide mineral appears to have been pyrite. The sulphides have been oxidised to limonite, down to a depth of approximately 400 m.

The gold is seldom visible, and mainly occurs as micron-scale particles.

The main part of the mineralised zone is truncated by surface topography, strikes east-west for approximately 1 km, dips shallowly to the south with a down-dip extent of 1.5 km, and is up to 400 m thick. The overall orientation and continuity of mineralisation is aligned with the south-dipping reverse faults that bound the deposit. Within the deposit, there are local variations in the orientation of the mineralisation, corresponding to the secondary structures between the bounding faults.

Gross is a conventional open pit gold mine which has been operational since September 2018 mining low grade gold ore. Current mining operations at Gross are focused on a single open pit.

The mine is owner operated with a mining cycle consisting of drilling, blasting, loading, hauling, dumping & stockpiling.

Open Pit Geotechnical Considerations
The Gross open pit mine is located in a mid-mountainous relief, with flat watersheds and intersected by river valleys. The elevations of the watershed surfaces are up to 1400 mRL, the relative elevations usually do not exceed 300-400 m. Natural topographic slope angles are up to 25°- 30°. The open pit is advancing to the south (cutting into the topographic high point) and at its deepest is some 230 m. The maximum depth of the ore bodies from surface is 450 m. The rock mass of the ore zone and the ore itself is highly fractured, while the host rocks are represented by fine and medium grained red sandstones with this material being less fractured and more competent.

The deposit is located within a series of faulted blocks in addition to thrust faults that dip shallowly to moderately to the south. Bedding is strongly developed and in general dips between 30° and 50° towards 350° to 030°. Bedding dip and dip direction is variable, and in addition to an orthogonal joint set and other variable jointing, blocky conditions are prevalent. Two rock mass domains have been defined as a function of the level of weathering and alteration. Group 1 has a GSI 57 and Group 2 a GSI 48.

Mine Water Management
Surface water flow measurements on the mine site are undertaken as legally required, for discharge permits and every five years. The mine’s measurements are limited to spot measurements and a recent study by SRK (SRK, 2020) recommended further surface water flow gauging be carried out in streams and rivers in the vicinity of both pits. Contact surface water run-off from industrial sites will soon be collected prior to discharge from site. Collection facilities are planned and approved in the discharge permit.

The Gross open pit is currently mining at around approximately 150 m below ground level (“bgl”), within the permafrost, working towards a final pit depth of around 450 m. Permafrost depth is variable, being deepest under the ridges and shallowest under the river valleys where taliks are present. The exact depth of permafrost is not fully understood although it is thought that the final Gross pit might extend as far as 250 m below the base of the permafrost. Groundwater inflows are negligible and limited to localised melting. There is a strong correlation between localised seepage in the pit (where groundwater melting has occurred), geological structures, and small-scale failures. One hydrogeological borehole was recently drilled, logged for temperature and pump tested near Gross open-pit in 2020. Other groundwater monitoring is limited to some sampling boreholes near the landfill and leached ore dump which have been dry to date.

Crushing
Ore is crushed to a nominal -40 mm using a single Nordberg (Metso) Superior MkII 50-65 gyratory crusher followed by two Nordberg (Metso) GP500S cone crushers operating in parallel. Ore is direct tipped into the gyratory crusher. Product from this crusher is screened using double deck (75 mm and 40 mm) screens, with the +40 mm then crushed using the cone crushers operating in open circuit.

The ore from Gross is fed into a heap leach processing facility on site for the extraction of gold with the current crushing facility designed for a maximum throughput of 12 Mtpa, constrained by primary and secondary crusher capacities. Since 2020, processing throughput was increased by direct “truck dumping” onto the heap leach pads (without crushing) to approximately 16 Mtpa in 2020 and is set to be increased to 18 Mtpa from 2021 onwards (12 Mtpa Crushed + 6 Mtpa Truck dumped). Current productivity rates for the primary fleet are aligned with benchmark data and a ramp up to 18 Mtpa is well underway.

The Gross process plant treats low grade non-refractory sandstone hosted gold ore by heap leaching to produce gold doré on site.

Heap Leaching
The crushed ore is transport to the leach pad using a series of overland conveyors, feeding onto mobile grasshopper conveyors and final to a radial stacker. The original maximum lift height was 12 m; this was increased to ........