McArthur River is an unconformity-associated uranium deposit.

In general, the high grade mineralization, characterized by botryoidal uraninite masses and subhedral uraninite aggregates, constitutes the earliest phase of mineralization in the deposit. Pyrite, chalcopyrite, and galena were also deposited during this initial mineralizing event. Later stage, remobilized uraninite occurs as disseminations, veinlets, and fracture coatings within chlorite breccia zones and along the margins of silt beds in the Athabasca sandstone.

Primary Mining method: raiseboring.
Secondary Mining method: blasthole stoping, boxhole boring.

Raisebore mining is an innovative non-entry approach that we adapted to meet the unique challenges at McArthur River. It involves:
- drilling a series of overlapping holes through the ore zone from a raisebore chamber in waste rock above the mineralization;
- collecting the broken ore at the bottom of the raises using line-of-sight remote-controlled scoop trams, and transporting it to a grinding circuit;
- once mining is complete, filling each raisebore hole with concrete;
- when all the rows of raises in a chamber are complete, removing the equipment and filling the entire chamber with concrete;
- starting the process again with the next raisebore chamber.

Similar to raiseboring, blasthole stoping requires establishing drill access above the mineralization and extraction access below the mineralization. We begin each stope with a single raisebore hole (explained above). The stope is then formed by expanding the circumference of the raise by drilling longholes around the raisebore hole and blasting the ore. The blasted material funnels into the raisebore hole and drops to the extraction level below. The broken rock is collected on the lower level and removed by line-of-sight remote-controlled scoop trams, then transported to the grinding circuit. Once a stope is mined out, it is backfilled with concrete to maintain ground stability and allow the next stope (or raise) to be mined.

We continue to employ blasthole stoping only in areas where the longholes can be accurately drilled, and where stable stopes can be excavated without jeopardizing the integrity of the freezewall.

Our use of blasthole stoping as an ore extraction method has increased as a result of the significant productivity improvements we have achieved with this method. The amount of ore extracted from a single stope can be equivalent to four to eight raisebore holes, resulting in more efficient mining, less waste rock handling, less backfill placement and lower backfill dilution in the ore shipped to Key Lake.

Boxhole boring is similar to the raisebore method, but the drilling machine is located below the mineralization, so development is not required above the mineralization.

Test mining to date has identified this as a viable mining option; however, only a minor amount of ore is scheduled to be extracted using this method.

McArthur River ore is processed at two locations. Size reduction is conducted underground at McArthur River and the resulting finely ground ore is pumped to surface and transported in purpose-built containers to Key Lake as a 50% solids slurry at an average grade of 15% U3O8. Blending down with mineralized waste to a nominal 4% U3O8 mill feed grade and all remaining uranium processing, tailings disposal and effluent treatment steps occur at Key Lake. The final uranium product is a calcined yellowcake grading 98.7% U3O8 on average.

Sulphuric acid produced on site by burning/converting sulphur is used to leach uranium, along with various impurities, from the host ore in a single stage atmospheric leach circuit.

The slurry is heated to a target temperature of 60° C and nearly pure oxygen, produced on site, is injected into the leach vessels to oxidize the uranium minerals and thereby permit uranium dissolution. Approximately 99.5% of the uranium and varying percentages of the impurities contained in the ore enter the leach solution during leaching.

Counter current decantation (CCD) consists of eight thickeners and a clarifier located outdoors on the mill terrace beside the leach plant. Acidic water is introduced at the tail end of the circuit and advanced from thickener to thickener in the opposite direction to the leached solids flow. The result is that the dissolved components are washed away from the leached solids. The washed leach residue is sent to the Deilmann TMF for neutralization and disposal. Pregnant solution containing 10 to 15 g/L dissolved uranium and varying levels of impurities is clarified to remove residual solids and pumped through sand filters to the solvent extraction plant.

In the solvent extraction plant, the filtered pregnant solution is mixed with an organic solvent consisting of isodecanol and amine dissolved in kerosene. The uranium transfers from the aqueous solution to the organic phase leaving behind most of the dissolved impurities. The waste solution (raffinate) containing the impurities is pumped to the bulk neutralization plant for treatment and disposal. The organic solvent, loaded with uranium, is contacted with ammonia in ammonium sulphate solution, causing the uranium to transfer back to a highly concentrated aqueous phase known as loaded strip solution. Special treatment circuits are available to deal with problem impurity elements such as arsenic, molybdenum and selenium that tend to follow the uranium through the solvent extraction process.

Using ammonia, uranium is precipitated from loaded strip solution in the yellowcake plant as ammonium diuranate. The precipitate is dewatered in a thickener followed by a centrifuge then calcined to U3O8 in a multi hearth furnace at 840 oC. The final calcined product is packed in 200 litre drums for shipment to refineries around the world. Excess ammonium sulphate is recovered in a crystallization circuit by evaporating the water and drying the resulting by-product, which is sold locally for use as a high purity fertilizer.