Both the Eagle River and Mishi deposits are mesothermal lode gold deposits hosted by Archean Greenstone Belts.

The Eagle River deposit has been described as a vein type deposit along a regional deformation zone with discrete brittle-ductile shears localized along lithological contacts (Heather 1986 and 1991).

Gold bearing quartz veins at Eagle River are hosted primarily by subvertical to steeply north dipping east-west striking shear zones within an elliptical quartz diorite stock with dimensions of 2.0 kilometres east-west and 0.5 kilometres north- south.

The quartz diorite stock intrudes a steeply dipping north-facing sequence of thin mafic to intermediate volcanic flows, flow breccias and interflow volcaniclastic rocks.

A number of different ore zones have been distinguished that constitute different segments of the overall shear zone corridor and each has its own gold grade characteristic. Mineable portions of the individual zones form ore shoots that plunge steeply to the east. The bulk of the historic production has come from Zone 8 and Zone 6, which are entirely within the intrusive quartz diorite, while Zone 2 mineralization is hosted in sheared mafic volcanic rocks just east of the stock.

In general, the ore shoots mined to date occur at a spacing of 400 metres along a 2.4 kilometre strikelength. They appear to be spatially related to an array of oblique 110º striking mafic dykes, which pre-date mineralization and deflect into and out of the shear zones.

Gold mineralization is structurally concentrated within highly strained portions of the various quartz veins. Ore microscopy (Clemson, 1989; Johnston, 1990) indicates that 60% of the gold occurs along quartz-sericite grain contacts, 32% along sulphide-gangue contacts and 1.4% within sulphide grains. The grains are generally less than 500 microns, free milling and 40 to 60% recoverable by gravity methods. Gold grains less than 5 microns account for a negligible percent of the total gold. Free gold generally occurs as a multitude of fine grains which result in a relatively low sub sampling variance generating very good assay precision for a vein type gold deposit.

The ERM was initially mined using a shrinkage mining method before converting to its current mining method of captive sub-level open stoping with access to the sub-level provided by Alimak raises, a variant of longhole mining. The site is currently transitioning to mechanized sub-level open stoping with access to the sub-levels being provided by ramp access.

Currently, ERM utilizes a mix of captive and mechanized access to the sub-levels depending on how close the ramps are. Mechanized access is more productive, but also requires more lateral development with some areas being unable to support mechanized access. Minimum mining widths are 1.5 with 1 m of external dilution expected. The ore body strikes eastwest with strike length varying between 50 and 80 m depending on mining zone. Generally, 19% of the mill feed comes from sill development and 81% comes from stope production.

The main access to the ERM is via a portal, which connects to the mines through three primary ramp systems (East, West, and Central ramps). The majority of materials entering the mine and personnel access is through these ramps. Development waste rock is also handled via the ramp systems, generally being hauled to a stope requiring backfill, although some waste must be hoisted to surface.

The East or Main Ramp is the primary ramp for the mining zones. This ramp extends from surface to the deepest levels of the mine and provides access to the majority of the active mining zones. The West Ramp starts at 325 mL (4,675 m elevation) and continues to the 670 mL (4,330 m elevation) where it connects to the East Ramp. The Central Ramp starts at 650 mL (4,350 m elevation) and continues down to the 805 mL (4,195 m elevation). Plans are in place for this ramp to coincide with a shaft extension project in the future.

The mine also has a three-compartment hoisting shaft used for transporting ore. One compartment is equipped with a skip over cage; site procedures limit this cage to 6 person capacity. The main shaft station is located at the 70 mL (4,930 m elevation) with the skip dump located directly above this level. All current ore production is hoisted from the loading pocket located below the 460 mL (4,540 m elevation) with shaft bottom at the 580 ml (4,420 m elevation). Underground trucks then load at a truck chute and haul ore out the portal to a surface pad where it is reloaded into surface trucks for the 16.6 km haul to the mill. The shaft is also used as a fresh air intake for the mine to the 580 mL (4,420 m elevation).

The underground hoisting plant consists of a Canadian Ingersoll Rand (CIR) 2.4 m diameter double drum, double clutch hoist driven by two 400 hp (300 kW) AC motors through a single reduction open (bull) gear and pinions. The electrical power systems and controls were upgraded in 2020, though the loading pocket has not been automated. Maximum hoisting speed is 6 m/s with a certified skip payload of 4 tonnes. Based on a realistic maximum of 16 operating hours per day, the hoisting capacity greatly exceeds the planned production rates from current hoisting depth.

Stockpiles and Crushing Circuit
There are three stockpile floors that can feed the mill with a front-end loader (FEL). The crushing circuit is a standard two stage process (2CB). From the stockpile a FEL deposits the ore into the jaw crusher. Coarse ore is transported by conveyor to be classified on a double deck vibrating screen. Oversize material goes to the secondary crushing and is reclassified after size reduction (cone crusher is in closed circuit with the vibrating screen). Undersize material is directed to the fine ore bin. The fine ore is transported from the fine ore bin by two conveyors to the primary ball mill (the second conveyor is fitted with a weightometer).

Grinding Circuit
The grinding circuit consists of a primary ball mill followed by a secondary ball mill in closed circuit with hydrocyclones (cyclones). The primary ball mill discharges into the cyclone pumpbox, and the slurry is then pumped to cyclones for classification. Process water (mill solutions) is added to the primary ball mill and cyclone feed pumpbox to obtain the required cyclone feed density. Lime slurry is added to the cyclone pumpbox to target the pH for cyanide leaching and downstream Merrill Crowe circuit.

Cyclone undersize material reports to the secondary ball mill, which also discharges into the cyclone pumpbox. The cyclone overflow is sent to the vibrating trash screen and via gravity, the slurry enters the thickener pumpbox to be pumped to the thickener feedbox. The trash screen oversize is collected in drums and periodically removed.

The Eagle River Mill (Mill) is permitted for up to 1,200 tonnes per day (t/d) on a crushing basis.

The Mill comprises the following unit operations:
• Three ore pads for stockpiles, to separate Eagle and Mishi ore;
• Primary and secondary crushing of run-of-mine (ROM) material;
• Primary and secondary grinding of fine ore;
• Gravity recovery of cyclone feed;
• Trash screen and pre-leach thickener;
• Leach circuit;
• Merrill-Crowe circuit;
• Tailings filtration;
• Refinery;
• Cyanide detoxification;
• Metallurgical laboratory.

Gravity Circuit
The gravity circuit includes centrifugal concentration fed via a 2 mm guard screen. Feed to the gravity circuit is a portion of the cyclone feed (varied based on feed tonnage). The guard screen oversize material reports to the primary ball mill.

The guard screen undersize material is fed to the centrifugal concentrator via gravity. Operation of the gravity ........