The report presents a modified processing flowsheet for the Fox Mill at the Stock mine site, supported by testwork and past operational performance.
Report context
The NI 43-101 technical report, dated January 2022 (Project No. 246279), describes proposed modifications to the Fox Mill to increase overall tonnage throughput and process harder materials from multiple deposits including Froome, Grey Fox, Stock West, and Fuller. The report is prepared for McEwen Mining Inc. as a preliminary economic assessment (PEA).
Processing route
Existing and Proposed Flowsheet
The overall flow of the existing plant is maintained with modifications to increase tonnage throughput and process harder materials at higher rates. The simplified process flowsheet aligns with recommendations from this PEA.
Mineralized material delivered to the Fox Mill is stockpiled and bucket-blended into a portable crusher. The material continues through a three-stage crusher and is fed to a primary ball mill operating in closed-circuit. Sized material is transferred to regrind with two ball mills operating in parallel in closed-circuit. Final sized product is leached with cyanide and recovered to activated carbon. Gold is stripped from the carbon and recovered with electrowinning cells and poured into doré bars. Stripped carbon is acid washed, regenerated, and returned to the leach circuit.
Historical Operating Data
When the PEA study work started, plant operations processed material on an on/off schedule based on mining rates and typically ran at 1,600 t/d with a target grind of 75 μm. The mill now operates continuously with a 24/7 schedule.
Proposed Design Changes
Operational changes required within the plant include increasing the target production size to 130 μm and reducing the closed side setting on the secondary crusher to 12 mm. The larger product size requires less grinding and the tighter CSS reduces the workload on the tertiary crusher, which is the current plant bottleneck. Since the site visit, the primary crusher has been rebuilt, improving availability.
Recommendations for operational improvements that are not imperative to production but will improve operation and reduce risk of not meeting throughput rates include moving the portable crusher to discharge to the apron feeder and increasing the width of the conveyors around the secondary and tertiary crushers feeding the sizing screen.
Crushing Circuit
Plant feed involves a pre-crush step with a Cedarapids portable crusher. Wood recommends relocating the portable crusher such that the product discharges to the apron feeder. Feed to the crushing circuit first passes over a fines screen to remove undersize material, though the installed screen does not currently operate properly and all feed is transferred to the Kemco C-95N jaw crusher. This screen should be replaced with screens designed to manage wet, sticky material. McEwen is currently replacing the existing screen with a Weir scalping screen.
Material less than 11 mm reports to a bypass conveyor and goes directly to the fine ore bin. Material larger than 11 mm and less than 50 mm bypasses the jaw crusher. Oversize material is crushed in the jaw crusher to a transfer size P100 of 133 mm.
Jaw crusher product and intermediate size material from the fines screen are fed to the Sandvik secondary cone crusher operating in open circuit. Wood recommends the CSS be reduced to 12 mm to reduce the P80 to 15.6 mm. Product from the secondary and tertiary cone crushers combine on conveyor #2 that feeds conveyor #3, which feeds the sizing screen. Wood recommends increasing the width of conveyor #2 and #3 to 762 mm. The sizing screen is a Metso banana screen with undersize as final product to the fine ore bin and oversize fed to the tertiary crusher. The tertiary crusher is a Metso cone crusher and is often overloaded but with crusher circuit modifications should operate at 60 to 70% of installed power.
Grinding Circuit
Crushed material is fed to the primary ball mill via conveyor from the fine ore bin. Wood recommends increasing the diameter of the primary ball mill to 4.3 m to utilize installed power. An engineering study should be completed to confirm that a larger shell will fit without major modifications and that items such as bearings can withstand the additional weight and increased volume of total charge.
The primary ball mill is operated in closed circuit with three hydrocyclones and one stand-by. Underflow is returned to the ball mill and overflow is sent to the secondary cyclone feed pumpbox. Underflow of the secondary cyclone cluster, with five cyclones operating and one stand-by, is split to two regrind mills operating in parallel. Wood recommends increasing the target grind size to P80 130 μm to meet tonnage production targets. With the larger primary ball mill and coarser transfer and product size, Wood recommends using larger size grinding media: 89 mm grinding balls for the primary ball mill and 38 mm grinding balls for the regrind mills.
Leach Circuit
The back end of the process plant is adequate to process the increased production levels. Wood does not recommend any changes to the leach circuit.
Ground slurry is thickened and pumped to the first of two leach tanks. Cyanide is added to the first tank, leach tank #3. Lime is currently added to the primary mill cyclone pumpbox for pH control. There are two concrete pedestals available if additional leach volume is required in the future.
Leached slurry overflows to the carbon-in-leach circuit with six tanks. The first two tanks do not contain carbon and loaded carbon is removed from CIL tank #3. If additional carbon and residence time is required in the future, interstage screens and carbon transfer pumps could be installed to the first two tanks to convert them to CIL. Tailings pass over a safety screen before being pumped to the tailings dam.
Carbon Stripping and Electrowinning
Carbon from the loaded carbon screen is transferred to the loaded carbon tank in three tonne batches to the carbon strip vessel. There is a parallel system to strip and acid wash a one tonne batch that is not in use. Gold is removed from the carbon using a Zadra process with a concentrated solution of caustic and cyanide at 142 °C. Hot pregnant solution is cooled through a heat exchanger before going into an electrowinning cell where gold plates onto stainless steel cathodes.
Stripped carbon is not currently being acid washed. Wood recommends reinstating this practice. The circuit has the capability to acid wash carbon in three tonne batches with 3% nitric acid. Acid washed carbon is split to two carbon regeneration kilns that can each process 1.5 t/d. Regenerated carbon is cooled in a quench tank, passed over a carbon sizing screen into a conditioning tank before being pumped back to the CIL circuit.
Tailings and Water Management
A new 16 m tailings thickener is presented as an addition to the current flowsheet to send thickened tails to the tailings facility and return overflow water back to the mill solution tank. Underflow will target 60 to 65% solids to the tailings facility. Additional reclaimed water is recovered from tailings via a barge-mounted pump. Water can also be pumped to water treatment tanks for cyanide destruction using air, sodium metabisulphite, and copper sulphate. Ferric sulphate can be added to precipitate arsenic when required. Treated water flows to a polishing pond before discharging to the environment.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Feed Characteristics | |||
| Bond Work Index – Froome | kWh/t | 20.9 | Design |
| Bond Work Index – Grey Fox | kWh/t | 25.9 | Design |
| Bond Work Index – Other feed | kWh/t | 16.0 | Design |
| Run of Mine F80 | mm | 264 | Design |
| Run of Mine F100 | mm | 500 | Design |
| Operating Schedule | |||
| Rotation | – | 24/7 | Design |
| Shifts | shifts/day | 2 | Design |
| Operating days | days | 365 | Design |
| Operating hours | hours | 24 | Design |
| Crusher availability | % | 70 | Design |
| Plant availability | % | 93 | Design |
| Production Rate | |||
| Plant feed rate, 100% Grey Fox/Froome | t/d | 1,950 | Design |
| Plant feed rate, 50% Grey Fox | t/d | 2,350 | Design |
| Plant feed rate, 0% Grey Fox | t/d | 2,500 | Design |
| Grind size P80 | μm | 130 | Design |
| Gold recovery | % | 85-94 | Design |
| Historical Operating Data | |||
| Historical throughput | t/d | 1,600 | Actual |
| Historical target grind | μm | 75 | Actual |
| Primary Ball Mill | |||
| Diameter | m | 4.3 | Proposed design |
| Length | m | 5.5 | Proposed design |
| Available power | kW | 1,500 | Proposed design |
| Ball size | mm | 89 | Proposed design |
| Regrind Ball Mill #1 | |||
| Diameter | m | 2.9 | Existing |
| Length | m | 3.66 | Existing |
| Available power | kW | 371 | Existing |
| Ball size | mm | 38 | Proposed |
| Regrind Ball Mill #2 | |||
| Diameter | m | 2.74 | Existing |
| Length | m | 3.51 | Existing |
| Available power | kW | 296 | Existing |
| Ball size | mm | 38 | Proposed |
Project website: https://www.mcewenmining.com/operations/black-fox-complex/default.aspx
Technical qualifications
The report states that testwork and past operational performance supports the flowsheet as appropriate to process mineralized material from all the deposits. The design criteria were used to determine recommendations for this PEA. Wood recommends that an engineering study be completed to confirm that a larger primary ball mill shell will fit without major modifications and that bearings can withstand the additional weight and increased volume of total charge.
Source: Fox Complex Expansion, NI 43-101 Technical Report, McEwen Mining Inc., Project No. 246279, January 2022, Section 17.0 Recovery Methods.
Consolidated report summaries
The following former standalone pages about this operation were consolidated here on October 25, 2026. They are retained for source traceability; dates and figures reflect each report’s own effective date and may be superseded by the current information above.
Grey Fox PFS: Hard Ore Shapes a Two-Site Gold Flowsheet
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
