The Grey Fox project in northeastern Ontario will rely on a simple, proven whole-ore cyanidation flowsheet, with ore crushed at the mine and processed 34 km away at the existing Stock Mill.
Overview
The Grey Fox project is an underground gold mine development located near Matheson, Ontario, within the established Timmins mining district. The project is being developed by McEwen Inc. (McEwen) of the Fox Complex, a portfolio of gold properties that includes the operating Froome Mine and Stock Mill, the former Black Fox and Stock mines, and several development-stage assets. Grey Fox consists of two separate underground operations, Grey Fox North and Grey Fox South, utilizing longhole mining. Ore will be processed at the existing Stock Mill, approximately 30 km away. The project is based on a prefeasibility study (PFS) dated July 17, 2026. Mining will occur in two phases, with Phase 1 ramping up to approximately 1,500 t/d by 2030, and Phase 2 commencing in 2035 to reach a combined rate of approximately 2,700 t/d before ramping down through 2041.
Ore will be sourced from underground mining operations only. A new modular primary crushing plant at the Grey Fox portals will reduce ore to sub-120 mm before trucking it to the Stock Mill. The flowsheet uses the existing crushing, grinding, and leaching circuits at the Stock Mill, with leach feed ground to a P80 of 75 µm. The design incorporates a whole-ore cyanidation circuit, followed by recovery of gold onto activated carbon, elution, electrowinning, and smelting to doré.
Metallurgical testwork has demonstrated that the ore is hard and competent, and available grinding power at the Stock Mill is the primary constraint on overall throughput. The design case accounts for this with a throughput of 1,123 t/d at the 85th percentile of ore hardness.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Expected throughput (Nominal) | 1,470 | t/d | 50th percentile ore hardness |
| Expected throughput (Design) | 1,123 | t/d | 85th percentile ore hardness |
| ROM Feed Grade | 3.42 | g Au/t | Design value |
| Gold Extraction (Overall) | 88 | % | Design value, based on testwork average |
| Bond Ball Mill Work Index (Nominal) | 23.2 | kWh/t | 50th percentile |
| Bond Ball Mill Work Index (Design) | 27.2 | kWh/t | 85th percentile |
| Grinding Section Product P80 | 75 | µm | Design value |
| Fine Ore Bin Capacity | 1,360 | t | Approximately one day surge capacity |
| Number of Large Leach Tanks | 2 | # | Live capacity 1,166 m³ each |
| Number of CIL Tanks | 6 | # | Live capacity 277 m³ each; first two used for leaching |
| Slurry Residence Time Required | 34 | h | Design value |
| Slurry Residence Time Provided | 45 | h | At nominal case |
| NaCN Consumption | 0.42 | kg/t | Testwork result |
| Hydrated Lime Consumption | 2.35 | kg/t | Testwork result |
| Loaded Carbon Grade | 3,700 | g/t | Expected value |
| Carbon Batch Size | 3 | t | Elution circuit |
| Expected Strips per Week (Nominal) | 2.5 | per week | Calculated, 3-t circuit |
| Target WAD CN Concentration | 0.1 | ppm | Effluent target |
| Overall Gold Recovery (Testwork) | 88 | % | Average, whole-ore cyanidation range 70–95% |
Overview
The Grey Fox process flowsheet was developed from metallurgical testwork and industry best practices. The selected flowsheet keeps the existing whole-ore cyanidation process at the Stock Mill unchanged. Flotation followed by fine grinding of the concentrate could improve recoveries for certain ore types, but retrofitting those stages into the existing Stock Mill was judged costly and operationally disruptive, so that option was not adopted.
Ore will be processed through the existing Stock Mill facility at an average rate of 536,550 dry tonnes per year. The primary crushing front end at Grey Fox consists of new equipment purchased specifically for the project. All subsequent processing is conducted through existing facilities. The installed grinding power at the Stock Mill represents the primary processing bottleneck and determines the overall achievable throughput. Due to ore hardness, the mill will operate at reduced throughput relative to its current capacity, which provides a longer cyanidation residence time that has been shown to be required for some samples tested.
Key Process Stages
The primary crushing section at the Grey Fox site is staffed for 12 h/d but operates for 8 h/d. This schedule avoids nighttime ore transportation and allows 4 h/d for delays and maintenance. Ore is fed to a modular crushing plant consisting of a ROM feed bin, a vibrating grizzly feeder, a fixed rockbreaker, a C106 jaw crusher, and a crushed product conveyor. Crushed rock is loaded by front-end loader onto road trucks and transported to the Stock Mill.
At the Stock Mill, primary crushed ore is reclaimed from stockpiles and fed into the load-in bin. Material is conveyed to an open-circuit Sandvik CH430 secondary cone crusher. Secondary crushed product at -40 mm is conveyed to a double-deck banana screen measuring 6.1 m by 2.05 m. Screen oversize at +10 mm reports to a Metso Nordberg HP300 tertiary cone crusher. Screen undersize at -10 mm is conveyed to the fine ore bin, which has a capacity of 1,360 tonnes.
Two reclaim belt feeders extract finely crushed ore from the fine ore bin onto the primary ball mill feed conveyor. The primary ball mill has a diameter of 3.66 m and an effective grinding length of 5.49 m, driven by a 1,250 kW motor. Slurry is pumped to a 4-way cyclopack fitted with 380 mm hydrocyclones. Cyclone underflow returns to the primary mill, and overflow proceeds to two parallel secondary ball mills. Secondary Ball Mill #1 has a diameter of 2.89 m and an EGL of 3.66 m with a 448 kW motor. Secondary Ball Mill #2 has a diameter of 2.74 m and an EGL of 3.48 m with a 336 kW motor. These mills operate in closed circuit with a common cluster of three 250 mm cyclones.
Cyclone overflow passes through a trash screen and is pumped to a 19.8 m diameter high-rate thickener. Flocculant is added at approximately 15 g/t solids. Thickener underflow at approximately 50% w/w solids is pumped to the first of two leach tanks. Sodium cyanide at 0.42 kg/t and lime at 2.35 kg/t are added primarily into the first leach tank, with additional doses downstream to maintain free cyanide and a slurry pH above 10.7. The two large leach tanks have a live capacity of 1,166 m³ each, providing about 27 hours of residence time. Slurry flows by gravity to six CIL tanks, each with a live capacity of 277 m³. The first two operate as leach tanks with no carbon. The remaining four hold a minimum carbon concentration of 10 g/L, targeting a solution concentration of 0.01 mg/L. A carbon safety screen intercepts any gold-bearing carbon that may pass through the retention screens.
Loaded carbon is stripped using the pressure Zadra process in 3-t batches. Under nominal conditions, about 2.5 strips per week are required. The acid wash column contacts carbon with dilute nitric acid at 3% w/w to remove calcite buildup, with a cycle duration of about 10 h. The elution cycle lasts approximately 15 h. Carbon regeneration is performed in a horizontal, electrically heated kiln with a minimum power rating of 143 kW and a capacity of 100 kg/h, operating between 650°C and 700°C.
The pregnant solution is collected and pumped to two electrowinning cells, each containing 30 stainless steel cathodes. Gold sludge is dislodged, dewatered by pressure filter, dried in an oven, and smelted to produce doré bars. On average, approximately one smelt is required per week.
CIP tailings are pumped to the tailings management facility. A dam lift is planned for the 2026–2027 period to expand capacity. Supernatant solution is reclaimed and treated in dedicated detoxification tanks using the SO₂/air process, with sodium metabisulphite as the sulphur dioxide source and hydrated lime for neutralization. Treated solution is returned to replenish the process water system. Reagents used across the plant include hydrated lime, sodium cyanide, activated carbon, caustic soda, nitric acid, flocculant, smelting fluxes, antiscalant, copper sulphate, and sulphur dioxide.
Reclaim water is recovered from the tailings management facility supernatant pond and detoxified before reuse. One of the decommissioned CIP tanks is used for hydrated lime mixing. Existing compressors supply compressed air for leaching and detoxification aeration, filter presses, and pneumatic equipment.
Additional Interesting Data and Summary
The Grey Fox ore is relatively competent and requires high grinding energy. Bond Ball Mill Work Index values range from approximately 18 kWh/t to 27 kWh/t, with the majority of the orebody at the higher end. The process design is based on conservative hardness assumptions, shown in the design case throughput of 1,123 t/d versus the nominal 1,470 t/d. The difference between the Nominal and Design cases corresponds to the average (50th percentile) and design (85th percentile) ore hardnesses, respectively.
Whole-ore cyanidation recoveries from testwork ranged from 70% to 95%, with an average of approximately 88%. Mineralogical analysis confirmed gold is primarily associated with pyrite. Flotation testwork on open pit samples achieved gold recoveries of 91% to 97%, but this option was not incorporated into the flowsheet. Testwork on underground samples from 2025 showed lower gravity recoverable gold values compared to open pit samples.
The project, as described in the PFS, represents a key growth asset within the Fox Complex. The operation will be developed as two underground mines, with ore hauled approximately 34 km to the Stock Mill. Metallurgical testwork has shown whole-ore cyanidation is viable for the ore types present, and the existing mill infrastructure has been adopted unchanged for the project.
Key Processes
- Primary crushing at Grey Fox using a new modular jaw crushing plant
- Truck haulage of crushed ore to the Stock Mill
- Secondary and tertiary crushing to -10 mm product
- Two-stage grinding and classification to a P80 of 75 µm
- Densification in a high-rate thickener
- Cyanidation in leach and CIL tanks
- Recovery of gold onto activated carbon
- Pressure Zadra elution, electrowinning, and smelting to produce doré
- Tailings detoxification using the SO₂/air process and disposal in the tailings management facility
Source: Grey Fox Project Prefeasibility Study NI 43-101 Technical Report, July 17, 2026. Project website: Grey Fox Project
