Overview

Deposit type

• Unconformity related

Summary:

Geological setting
The deposit is in the southeastern portion of the Athabasca Basin in northern Saskatchewan, within the southwest part of the Churchill structural province of the Canadian Shield. The deposit is located at or near the unconformity contact between the Athabasca Group sandstones and underlying metasedimentary rocks of the Wollaston Domain.

The deposit is similar to other Athabasca Basin deposits but is distinguished by its very high grade and overall size. Unlike Cigar Lake, there is no development of extensive hydrothermal clay alteration in the sandstone above the uranium mineralization and the deposit is geochemically simple with negligible amounts of other metals.

McArthur River’s geological setting is similar to the Cigar Lake deposit in that the sandstone that overlies the deposit and basement rocks contains large volumes of water at significant pressure.

Mineralization
McArthur River’s mineralization is structurally controlled by a northeast-southwest trending reverse fault (the P2 fault), which dips 40-65 degrees to the southeast and has thrust a wedge of basement rock into the overlying sandstone with a vertical displacement ranging between 60 and 80 metres.

The deposit consists of nine mineralized zones with delineated mineral resources and/or reserves: Zones 1, 2, 3, 4, 4 South, A, B, McA North 1 and McA North 2. These and three under-explored mineralized showings, known as McA North 3, McA North 4 and McA South 1, as well as other mineralized occurrences have also been identified over a strike length of 2,700 metres.

The main part of the mineralization, generally at the upper part of the basement wedge, averages 12.7 metres in width and has a vertical extent ranging between 50 metres and 120 metres.

The deposit has two distinct styles of mineralization:
• high-grade mineralization at the unconformity near the P2 reverse fault and within both sandstone and basement rocks;
• fracture controlled and vein like mineralization that occurs in the sandstone away from the unconformity and within the basement quartzite.

The high-grade mineralization along the unconformity constitutes the majority of the mineralization within the McArthur River deposit. Mineralization occurs across a zone of strongly altered basement rocks and sandstone across both the unconformity and the P2 structure. Mineralization is generally within 15 metres of the basement/sandstone contact with the exception of Zone 2.

Uranium oxide in the form of uraninite and pitchblende (+/- coffinite) occurs as disseminated grains in aggregates ranging in size from millimetres to decimetres, and as massive mineralization up to several metres thick.

Geochemically, the deposit does not contain any significant quantities of the elements nickel, copper, cobalt, lead, zinc, molybdenum, and arsenic that are present in other unconformity related Athabasca uranium deposits although locally elevated quantities of these elements have been observed in Zone B.

Deposit type
McArthur River is an unconformity-associated uranium deposit. Deposits of this type are believed to have formed through an oxidation-reduction reaction at a contact where oxygenated fluids meet with reducing fluids. The geological model was confirmed by surface drilling, underground drilling, development and production activities.

Mining Methods

• Raiseboring
• Blast Hole Stoping

Summary:

The mineral reserves at McArthur River are contained within seven zones: zones 1, 2, 3, 4, 4 South, A and B. There are currently two active mining zones (zone 2 and 4), one with development significantly advanced (zone 1), and one in the early to mid-stages of development (zone 4 South).

Zone 2 has been actively mined since production began in 1999. The ore zone was initially divided into three freeze panels. As the freeze wall was expanded, the inner connecting freeze walls were decommissioned to recover the inaccessible uranium around the active freeze pipes. Mining of zone 2 is almost complete. About 3.1 million pounds of mineral reserves remain secured behind a freeze wall, and we expect to recover them using a combination of raisebore and blasthole stope mining.

Zone 4 has been actively mined since 2010. The zone was divided into four freeze panels, and like in zone 2, as the freeze wall was expanded, the inner connecting freeze walls were decommissioned. Zone 4 has 87.5 million pounds of mineral reserves secured behind freeze walls, and it will be the main source of production for the next several years. Raisebore and blasthole stope mining will be used to recover the mineral reserves.

Zone 1 is the next planned mine area to be brought into production. Freeze hole drilling was completed in 2023 and brine distribution construction and commissioning was completed in 2024. All freeze walls are actively freezing and it is predicted that an active freeze wall will be in place in the second quarter of 2025. Once an active freeze wall has been established, drill and extraction chamber development will need to be completed prior to the start of production (first production expected late 2025). Once complete, an additional 48.0 million pounds of mineral reserves will be secured behind freeze walls. Blasthole stope mining is currently planned as the main extraction method in zone 1.

Zone 4 South remains in the early development stages. Development for the freeze drifts is in progress on the lower levels and freeze drilling continues on the completed upper freeze drifts. Brine distribution work is scheduled to begin on the upper levels in 2025.

All the mineralized areas discovered to date at McArthur River are in, or partially in, water-bearing ground with significant pressure at mining depths.

There are three approved mining methods at McArthur River: raisebore mining, blasthole stope mining and boxhole mining. However, only raisebore and blasthole stope mining remain in use. Before begin mining an area, Cameco freeze the ground around it by circulating chilled brine through freeze holes to form an impermeable frozen barrier.

Ground freezing
All the mineralized areas discovered to date at McArthur River are in, or partially in, water-bearing ground with significant pressure at mining depths. This high pressure water source is isolated from active development and production areas in order to reduce the inherent risk of an inflow. To date, McArthur River has relied on pressure grouting and ground freezing to successfully mitigate the risks of the high pressure ground water.

Chilled brine is circulated through freeze holes to form an impermeable freeze barrier around the area being mined. This prevents water from entering the mine, and helps stabilize weak rock formations. Ground freezing significantly reduces, but does not fully eliminate, the risk of water inflows.

Blasthole stope mining
Blasthole stope mining began in 2011 and is the main extraction method planned for future production. It is planned in areas where blastholes can be accurately drilled and small stable stopes excavated without jeopardizing the freeze wall integrity. The use of this method has allowed the site to improve operating costs by increasing overall extraction efficiency by reducing underground development, concrete consumption, mineralized waste generation and improving extraction cycle time.

Raisebore mining
Raisebore mining is an innovative non-entry approach that we adapted to meet the unique challenges at McArthur River, and it has been used since mining began in 1999. This method is favourable for mining the weaker rock mass areas of the deposit and is suitable for massive high-grade zones where there is access both above and below the ore zone.

Backfill
When a raise has been extracted of ore, it is filled with a concrete backfill that is produced at McArthur River. Components of the backfill include cement, sand, potentially acid generating (PAG) fines, aggregates and chemical admixtures.

Comminution

Crushers and Mills

Summary:

McArthur River handling and processing of mineralized material
Mined material is scanned and transported in load-haul-dump vehicles and depending on grade and production requirements fed directly to the underground grinding circuit, stockpiled underground in coarse ore storage or hoisted to surface.

Low-grade mineralization is transferred by load-haul-dump vehicles to a loading pocket at the base of Shaft 1. A rock breaker mounted over a storage bin is used to break the oversize material until it passes through the grizzly screen. The grizzly undersize is hoisted to surface and hauled to low-grade mineralization pads. This ore is then hauled to the Key Lake low-grade mineralization blend pads.

The high-grade ore is transferred by load-haul-dump vehicles to another grizzly covered hopper. A rock breaker mounted over the hopper is used to break the oversize material until it passes through the grizzly screen. Grizzly undersize material is then fed by belt conveyor to the semi-autogenous (SAG) mill.

The grinding circuit consists of a 4.6 metre long by 2.9 metre diameter SAG mill in closed circuit with hydrocyclones and a safety screen. The mill discharges through grates onto a blind trommel which removes tramp metal via a magnet. The trommel discharge is pumped to the hydrocyclones and the hydrocyclone underflow is returned to the SAG mill for further grinding.

Hydrocyclone overflow is pumped to the safety screen and the screen undersize is thickened in a 13-metre diameter thickener. The thickener underflow is pumped to and stored in the underground ore slurry storage tank before being pumped to surface. Safety screen oversize is returned to the SAG mill for further grinding while the thickener overflow is stored in a dam before being pumped to the surface ore loadout thickener.

Contaminated water from collection points throughout the mine as well as drill cuttings are pumped to two underground overflow-type surge tanks where the settled solids are intermittently re-slurried and transferred to the SAG mill by tank bottom-mounted solids recovery systems. The tank overflow is pumped to a second 13 metre diameter thickener for clarification and the thickener overflow is stored in a second dam before being pumped to surface for treatment. The thickener underflow is mixed with the ore slurry and pumped to surface.

The high-grade slurry, after pumping to surface, is stored in four air agitated pachuca storage tanks. Slurry discharged from the pachucas is blended to a maximum grade of 25% U3O8 in the ore mix tank. After excess water is removed from the blended ore slurry in the 15 metre diameter ore loadout thickener, the slurry is pumped into truck-mounted containers for shipment by road to Key Lake. Each truck train carries four 5 m3 containers. Typically 12 to 20 truck loads are required daily to meet target production rates. The ore loadout thickener overflow is pumped to surface collection ponds prior to water treatment.

Key Lake activities
McArthur River low-grade mineralization and legacy low-grade mineralized waste stored at Key Lake on the low-grade mineralization blend pads is delivered to the grinding circuit grizzly by loader. The grinding circuit consists of a SAG mill in open circuit with a sizing screen. The screen oversize reports to the ball mill in closed circuit with two sizing screens. The undersize from all three screens report to the neutral thickener. The neutral thickener overflow is combined with industrial water to be re-used throughout the circuit.

Processing

• Calcining
• Sulfuric acid (reagent)
• Solvent Extraction
• Crush & Screen plant
• Autoclave
• Counter current decantation (CCD)
• Agitated tank (VAT) leaching
• Acid tank leaching
• High Pressure Acid Leach (HPAL)
• Dewatering

Summary:

McArthur River produces two product streams, high-grade slurry and low-grade mineralization, which both report to the Key Lake mill to produce calcined uranium ore concentrate.

High-grade ore is slurried, ground, and thickened underground at McArthur River. The resulting slurry is pumped to surface and, after blending and further thickening, is transported to Key Lake in slurry trucks.

Low-grade mineralization is hoisted to surface and stored on the low-grade mineralization pads. This material is then hauled to the Key Lake low-grade mineralization blend pads.

McArthur River low-grade mineralization, including legacy low-grade mineralized waste stored at Key Lake, is slurried, ground and thickened at Key Lake and then blended with the McArthur River high-grade slurry to a nominal 5% U3O8 mill feed grade. All remaining uranium processing (leaching through to calcined uranium ore concentrate packaging) and tailings disposal also occur at Key Lake.

The Key Lake mill saw notable improvements in its operational performance in 2024, with the site becoming more familiar and experienced with new equipment and control system upgrades. In addition, the systematic understanding of process bottlenecks and efforts to remove or decrease their impacts allowed Key Lake to optimize the mill throughput rates.

Key Lake activities
High-grade ore slurry arriving at the Key Lake ore slurry receiving plant is unloaded from the truck-mounted containers by a vacuum aspiration system and pumped to one of four large air-agitated slurry storage pachucas.

High-grade ore slurry is withdrawn from the ore storage pachucas and pumped to the blending tank where it is mixed with the neutral thickener underflow. The resulting slurry is pumped to one of three storage pachucas located in the leaching plant. Blending is necessary as the original Key Lake processing facilities were not designed from a radiation protection perspective to accommodate the high ore grades found at McArthur River. In addition to reducing the radiation exposure in the mill, the dilution of the high-grade ore serves two other purposes: recovery of uranium from the low-grade mineralized material; and final disposal of the low-grade mineralized waste.

The uranium is leached from the ore in the atmospheric leach pachuca and three continuous stirred tank reactors, while uranium-bearing solution is separated from waste solids in the counter current decantation (CCD) wash circuit. The high pressure autoclave secondary leaching circuit is on stand-by as the current ore is amenable to leaching at atmospheric pressure. Sulphuric acid, steam and oxygen are injected into the leach vessels to promote uranium extraction.

The CCD circuit consists of eight thickeners in series. The slurry flow is counter current to the wash water. The slurry moves from thickener one to thickener eight, while the wash water moves from thickener eight to one. The uranium-rich CCD overflow is pumped to the clarifier whilst the CCD underflow, with minimal residual uranium, is sent to the Deilmann tailings management facility.

In the solvent extraction plant, the clarified overflow pregnant solution is concentrated and purified by mixing with an organic solvent. The uranium transfers from the aqueous solution to the organic phase leaving behind most of the dissolved impurities. The organic solvent, loaded with uranium, is contacted with ammonium sulphate solution causing the uranium to transfer back to a highly concentrated aqueous phase known as loaded strip solution. A molybdenum removal circuit treats the loaded strip solution to remove molybdenum, an undesirable impurity in the final product.

Using ammonia, uranium is precipitated from the loaded strip solution in the precipitation tank as ammonium diuranate. The precipitate is dewatered in the yellowcake thickener followed by a centrifuge then calcined to U3O8 in a multi-hearth furnace. The final calcined uranium ore concentrate is packed in 200 litre drums for shipment to refineries around the world.

Excess ammonium sulphate is recovered from the yellowcake thickener overflow by evaporating the water and drying the resulting product, which is sold locally for use as a high purity fertilizer.

Water Supply

Summary:

Surface Water Supply
Fresh water supply for potable water use is drawn from Toby Lake and is regulated through the Saskatchewan Watershed Authority. The water is pumped from the lake and stored in two tanks on a centrally located drumlin and gravity fed to the site’s distribution system.

Shaft water is pumped to surface and stored in a tank for either industrial or firefighting requirements with Toby Lake water used as a backup if shaft water is not available.

These two water sources are sufficient to meet the current and future surface water requirements.

Underground Mine Water Supply
Shaft water (water leaking into the shafts from the sandstone formation) provides the underground operation with its water supply. No surface water is sent underground. The water is collected via shaft water rings in Shafts 1 and 3 and at the bottom of Shaft 2. The water is pumped or directed to the mine water distribution dam where it is pumped throughout the mine for various uses. This water supply is sufficient to meet current and future underground water supply needs.

Water Treatment
The current water treatment capacity of 1,500 m3 /hr is expected to be sufficient to meet future operation and contingency requirements.

Contaminated water from the dewatering system associated with the depleted Gaertner and Deilmann open pits at Key Lake is treated in a reverse osmosis plant with the permeate used as industrial water.

Reject water from the reverse osmosis plant along with waste solvent extraction solution (raffinate) is sent to the bulk neutralization plant where the streams are neutralised with lime and other reagents are added to precipitate dissolved impurities. The resulting solids are combined with the CCD underflow and pumped to the Deilmann tailings management facility for final disposal. The treated water is sampled and released to one of four monitoring ponds. If all federal and provincial regulations are met, the treated water is released to the environment. If not, the pond is recycled through the bulk neutralization plant until the treated effluent becomes suitable for release.

Production

Operational metrics

Personnel

Job Title	Name	Profile	Ref. Date
General Manager - Processing	Daley McIntyre		Feb 26, 2025
Maintenance Superintendent	Paul Baker		Feb 26, 2025
Mill Manager	Keith Perry		Feb 26, 2025
Plant Operations Manager	Nathan Rolston		Feb 26, 2025
Sr. Mine Engineer	Austin Roberts		Feb 26, 2025
Technical Manager	Drew Williams		Feb 26, 2025