The McArthur River deposit is an unconformity-related uranium deposit. Deposits of this type are believed to have formed through an oxidation-reduction reaction at or near an unconformity where oxygenated fluids transporting uranium in a U6+ state interacted with reducing fluids and/or lithologies along fault zones resulting in precipitation of U4+ minerals. Within the McArthur River deposit, the unconformity surface is a contact between Athabasca Group sandstones and underlying metasedimentary rocks of Wollaston Domain (basement rocks). Graphitic faults such the P2 fault provided a conduit for interaction of oxygenated fluids from the sandstones with reducing fluids and/or lithologies in the basement.

Uranium mineralization has been delineated from surface drilling over a strike length of 2,700 metres and occurring at depths ranging between 500 metres to 640 metres below surface. Mineralized widths are variable along strike but the most consistent, high-grade mineralization occurs proximal to the main graphitic reverse fault by the upthrust basement block. Less consistent and generally lower grade mineralization occurs downdip along this fault contact between basement rock and sandstone. 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 consists of nine distinct mineralized areas and three under explored surface defined mineralized showings over a strike length of 2,700 metres, from 7225N to 9925N. The mineralized areas with reported mineral resources are named Zone 1, Zone 2, Zone 3, Zone 4, Zone 4 South, Zone A, Zone B, McA North 1 and McA North 2. The mineralized showings are designated as McA North 3, McA North 4 and McA South 1.

Mineralization within the McArthur River deposit is primarily controlled by the P2 graphitic fault and occurs in two styles:
- 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 occur 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. Less significant areas of mineralization occur further from the contact, usually in the sandstone associated with subsidiary fracture/fault zones or along the margins of flat lying siltstone beds. The majority of the mineralization occurs on the footwall side of the thrust except in one area where more mineralization is on the hangingwall side of the thrust.

At McArthur River, there are three approved mining methods: raisebore mining, blasthole stope mining, and
boxhole mining.

Raisebore mining
Raisebore mining is suitable for massive high-grade zones where there is access both above and below the ore zone. The raise opening created by mechanical cutting has proven to be very stable making this method favourable for mining the weaker rock mass areas of the deposit. In addition, holes can be drilled accurately over long distances when compared to traditional production drilling, eliminating the need for sublevel access.

A raisebore chamber is typically developed in waste above the ore zone and an extraction chamber is typically developed in waste below the ore zone. A raisebore drill is set up in the raisebore chamber and a standpipe is installed. A pilot hole is then drilled to breakthrough into the lower extraction chamber. All cuttings from pilot drilling are contained and piped away to avoid radiation contamination of the work area. Once breakthrough occurs, the reamer is installed and the face is “sumped in” (establish a flat face in the back perpendicular to the drill string). Reaming continues through waste and into the ore. Raisebore cuttings are mucked remotely as required. All cuttings from production raises are scanned for ore grade estimates and delivered to the appropriate dump locations.

Reaming stops at the upper ore contact below the raisebore chamber. The reamer is lowered to the brow of the open hole and final muck cleanup of the chamber is carried out. The reamer is then lowered to the sill and the backfill gantry is installed for head cover protection. The chamber and reamer are washed down and then the reamer and rods are removed. Once all the rods are out, the raisebore is moved to the next scheduled production raise. Once the rods are removed, backfilling can begin. This is done in three stages (plug, second and final) using concrete as backfill material. The bottom of the raise is sealed with a backfill gantry and the initial plug pour is placed from the bottom of the raise using a portable concrete pump. After the first pour has set, the second pour is placed from the raisebore chamber through the pilot collar.

After the second pour is set, the plug is bolted for ground stability purposes. The final pour is then completed from the raisebore chamber through the pilot hole. Production raises are designed to overlap each other in order to maximize recovery of the high-grade ore at the expense of an average cement dilution of approximately 17%. Recoveries are typically 97.5% with a small amount of the ore lost in the cusps between the circular raises.

Cameco’s plan is to continue to use raisebore mining as one of the main extraction methods over the mine life, specifically for the creation of slots for blasthole stoping, for mining the Zone 4 clay area and for mining the more massive vertical ore areas of Zone B.

Blasthole stope mining
After successfully completing six test stopes in ore, blasthole stoping was approved by the CNSC as an extraction method in November 2013. Since approval, the use of this method has expanded to the point where the majority of the ore is now extracted using this method. Up to the end of 2017, 70 stopes have been successfully mined.

Blasthole stoping is planned in areas where blastholes can be accurately drilled and small stable stopes excavated without jeopardizing freeze wall integrity. Blasthole stope mining has shown an advantage over raisebore mining on overall extraction efficiency by reducing underground development, concrete consumption, mineralized waste generation, and improving extraction cycle time. Use of this method has significantly reduced McArthur River’s overall operating costs.

Drill access is developed in waste above the ore and undercut mucking access is developed in waste below the ore. A raisebore slot is excavated and drillholes placed around the slot. Drill standpipes are used to contain drill cuttings to avoid radiation contamination of the work area. The drillholes are then blasted into the slot, typically as several small blasts. Blasting takes place in the ore zone only to minimize dilution. A waste cap is left at the top of the stope.

Blasted ore is remotely mucked from the raise draw point and scanned for ore grade estimates and delivered to the appropriate dump locations. Once blasting is complete, the stope is backfilled in the same manner as a production raisebore hole described above.

Boxhole mining
After successfully completing six test raises, boxhole boring was approved by the CNSC as an extraction method in July 2013. A total of thirteen 1.5 metre and 2.1 metre diameter raises were completed and approximately 0.5 million pounds were extracted. Originally, boxhole mining was planned for the some of the more challenging upper mining areas, but following the success in development of Zone 2 - Panel 5, mine designs were revised and boxhole mining was replaced with more productive and cost effective methods. In 2015, the boxhole program was discontinued. No further use of this mining method is planned.

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.

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. As much water as possible is re-used for mining and process activities. Excess water from both underground and surface is sent to surface collection ponds which act as surge capacity for the water treatment plant. The water treatment plant facilities include the primary/secondary water treatment plant and the contingency water treatment system. A total treatment capacity of up to 1,500 m3 /h is available, 750 m3 /h in the secondary treatment plant and 750 m3 /h in the contingency water system. Primary water treatment features chemical treatment to control molybdenum, selenium, and arsenic while secondary water treatment includes chemical treatment to control uranium, radium and other metals. Flocculation and clarification are provided in a lamella thickener in primary water treatment and a conventional clarifier in secondary water treatment. Clarified water from the secondary thickener is polished in sand filters then the pH of the water is adjusted before reporting to the monitoring ponds. Precipitated solids from the water treatment process are dewatered in a filter press then transferred to the low-grade mineralization pads where they are mixed with low-grade mineralization and hauled to Key Lake. Treated water is re-used where possible on surface and only excess water is released to the environment. 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 water treatment plant until the treated effluent becomes suitable for release. The contingency water treatment system is designed to handle and treat inflow water that exceeds the treatment capacity of the secondary water treatment plant. It is a pond-based chemical precipitation contingency treatment system. This is a contingency system only and is tested on a yearly basis to ensure operational readiness.

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.