The Cigar Lake uranium deposit, which has no direct surface expression, is located at the unconformity between the middle Paleoproterozoic Wollaston Group metasediments and the late Paleoproterozoic to Mesoproterozoic Athabasca Group, between 410 and 450 metres below surface. It has the shape of a flat, elongated lense, approximately 1,950 metres in total length, 20 to 100 metres in width, and ranges up to 13.5 metres thick, with an average thickness of about 5.4 metres. It shows longitudinal and lateral geological continuity. It has a crescent-shaped cross-sectional outline that closely reflects the topography of the unconformity. The deposit is subdivided into the eastern Phase 1 and western Phase 2 zones. Phase 1 is further divided into the east and west pods.

The deposit and host rocks consist of three principal geological and geotechnical elements:
• the deposit itself;
• the overlying sandstone;
• the underlying metamorphic basement rocks.

The Manitou Falls Formation is 420 to 445 metres thick. The basement lithological domains consist of:
• a variably graphitic pelite unit located directly below the deposit;
• a calc-silicate rich unit labeled as “Meta-Arkose”;
• a biotite pelite unit located to the south of the deposit area within which most of the mine access infrastructure is located.

The graphitic pelite unit has been further divided into two sub-domains, including a graphite and sulphide-rich portion located directly below the uranium mineralization that has undergone variable and locally significant shear deformation, and a lesser graphite-rich portion that contains significantly less sulphides and shows less shear deformation.

The structural framework in the Phase 1 area is dominated by an east-west trending protomylonitic to locally mylonitic zone containing numerous steeply dipping, east-striking fault zones. Directly below the Phase 1 zone, these east-striking faults consist of graphitic breccia zones that are up to several metres wide and largely coincide with the 20-metre basement high, along which the uranium mineralization is located. Several steeply dipping northwest- and northeast- striking fault zones are located in this area and intersect the east-striking fault corridor in the central area of the Phase 1 mineralization. This area of intersecting faults controls the most extensive clay alteration observed within the Phase 1 area, both at the mineralized horizon and down to the 480 metre mining level.

The Phase 1 deposit is surrounded by a strong alteration halo affecting both sandstone and basement rocks, characterized by extensive development of Mg-Al rich clay minerals (illite- chlorite). This alteration halo in the sandstone is centred on the deposit and reaches up to 200 metres in width and 250 metres in height, tapering with elevation. In the basement rocks, this zone extends in the range of 200 metres in width and as much as 100 metres in depth below the deposit. The mineralization is hosted principally by the Athabasca Group and consists mainly of uraninite and pitchblende along with nickel and cobalt arsenides.

Mineralization
Two distinct styles of mineralization occur within the Cigar Lake deposit:
• high-grade mineralization at the unconformity (“unconformity” mineralization), which includes all of the mineral resources and mineral reserves;
• fracture controlled, vein-like mineralization which is located either higher up in the sandstone (“perched” mineralization) or in the basement rock mass.

The high-grade mineralization located at the unconformity contains the bulk of the total uranium metal in the deposit and currently represents the only economically viable style of mineralization, in the context of the selected mining method and ground conditions. It is characterized by the occurrence of massive clays and very high-grade uranium concentrations.

The unconformity mineralization consists primarily of three dominant rock and mineral facies occurring in varying proportions. These are quartz, clay (primarily chlorite with lesser illite) and metallic minerals (oxides, arsenides, sulphides). In the relatively higher grade Phase 1 area, the ore consists of approximately 50% clay matrix, 20% quartz and 30% metallic minerals, visually estimated by volume. In this area, the unconformity mineralization is overlain by a weakly mineralized contiguous clay cap 1 to 10 metres thick. In the relatively lower grade Phase 2 zone, the proportion changes to approximately 20% clay, 60% quartz and 20% metallic minerals.

While pre- and post-mineralization faulting played major roles in creating preferential pathways for uranium-bearing fluids and, to some extent, in remobilizing uranium, the internal distribution of uranium within the unconformity mineralization has likely been largely controlled by geochemical processes. This is reflected in the continuity and homogeneity of the mineralization and its geometry, particularly in the eastern part of the deposit. A very sharp demarcation exists between well mineralized and weakly mineralized rocks, both at the upper and particularly at the lower surface of the deposit.

Uranium oxide in the form of uraninite and pitchblende occurs in both a sooty form and as botryoidal, metallic masses. It occurs as disseminated grains in aggregates ranging in size from millimetres to decimetres, and as massive metallic lenses up to a few metres thick floating in a matrix of sandstone and clay. Coffinite (uranium silicate) is estimated to form less than 3% of the total uranium mineralization. The mineralized rock is variably black, red and/or green in colour.

Uranium grades of the unconformity mineralization range up to 82% U3O8 for a 0.5 metre interval from a drillhole intersection within the mining area. Geochemically, the deposit contains quantities of the elements nickel, copper, cobalt, lead, zinc, molybdenum and arsenic, but in non-economic concentrations. Higher concentrations of these elements are associated with massive pitchblende or massive sections of arseno-sulphides. Primary age of the unconformity mineralization has been estimated at 1.3 billion years.

Cameco use the JBS method to mine the Cigar Lake deposit.

Bulk ground freezing
The permeable sandstone that overlays the deposit and basement rocks contains large volumes of water under significant pressure. From surface, Cameco freeze the ore zone and surrounding ground in the area to be mined to prevent water from entering the mine, to help stabilize weak rock formations, and meet our production schedule. This system freezes the deposit and underlying basement rock in two to four years, depending on water content and geological conditions. Cameco have identified greater variation of the freeze rates of different geological formations encountered in the mine, based on information obtained through surface freeze drilling. To manage our risks and to meet our production schedule, the area being mined must meet specific ground freezing requirements before we begin jet boring. Bulk freezing reduces but does not eliminate the risk of water inflows.

Artificial ground freezing is accomplished by drilling a systematic grid of boreholes through the orebody from surface. A network of supply and return pipes on surface convey a calcium chloride brine to and from each hole. The warm brine returning from each hole is chilled to a temperature of approximately -30ºC at the surface freeze plant and recirculated.

JBS mining
After many years of test mining, Cameco selected jet boring, a non-entry mining method, which was developed and adapted specifically for this deposit. This method involves:

- drilling a pilot hole into the frozen orebody, inserting a high pressure water jet and cutting a cavity out of the frozen ore;
- collecting the ore and water mixture (slurry) from the cavity and pumping it to storage (sump storage), allowing it to settle;
- using a clamshell, transporting the ore from sump storage to an underground grinding and processing circuit;
- once mining is complete, filling each cavity in the orebody with concrete; and
- starting the process again with the next cavity.

This is a non-entry method, which means mining is carried out from headings in the basement rock below the deposit, so employees are not exposed to the ore. This mining approach is highly effective at managing worker exposure to radiation levels. Combined with ground freezing and the cuttings collection and hydraulic conveyance system, jet boring reduces radiation exposure to acceptable levels that are below regulatory limits.

The mine equipment fleet is currently comprised of three JBS units plus other equipment to support mine development, drilling and other services, and is sufficient to meet production requirements for the remainder of the mine life.

Cameco have divided the orebody into production panels. At least three production panels need to be rozen at one time to achieve the full annual production rate of 18 million pounds. One JBS machine will be located below each frozen panel and the three JBS machines required are currently in operation. Two machines actively mine at any given time while the third is moving, setting up, or undergoing maintenance.

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Cigar Lake ore slurry is processed in two locations:

Cigar Lake – The ore slurry produced by the JBS is pumped to Cigar Lake’s underground crushing, grinding and thickening facility. The resulting finely ground, high density ore slurry is pumped 500 meters to surface to one of the two slurry holding tanks. It is blended and thickened, removing excess water. The final slurry, at average grade of approximately 14%, is pumped into transport truck containers like the ones used at McArthur River.

Water from this process, including water from underground operations, is treated on the surface. Any excess treated water is released into the environment.

McClean Lake – Containers of ore slurry are trucked to Orano’s McClean Lake mill, 69 kilometres to the northeast for further processing (Leaching to Yellowcake Packaging).

In accordance with the JEB Toll Milling Agreement, the McClean Lake mill was expanded to process and package all of Cigar Lake’s current mineral reserves. Originally, the mill had a production capacity of 12 million pounds U3O8 per year. In order to process all of Cigar Lake’s current mineral reserves and other ores at McClean Lake, projects were identified to increase the total production capacity at the mill to 22.3 million pounds U3O8 per year. Construction of the expanded facility began in 2012.

Finely ground ore slurry is trucked from Cigar Lake by B-trains carrying four 5 m3 slurry containers to the receiving facility located at McClean Lake. This facility was modelled on the Key Lake ore slurry receiving facility, with enhancements. The slurry is off-loaded by vacuum, thickened and pumped to storage pachuca tanks.

The previous two-stage near-atmospheric pressure leach circuit was reconfigured to a one-stage atmospheric leach circuit to allow ore to be leached to the target leach recovery (99.5%). Leach cooling and hydrogen gas concentration control have been added to deal with the exothermic reaction and potential for hydrogen to be produced from leaching Cigar Lake high-grade ore. The leached solution is fed to the primary thickener. The overflow reports directly to the clarification area, and the underflow is washed in the counter current decantation (CCD) circuit. Additional wash capacity is available in the new counter current cyclone (CCC) circuit if required. Additional clarification and storage capacity is provided for pregnant leach solution. The washed and clarified uranium solution is fed to two parallel solvent extraction (SX) plants. The old 12 million pound U3O8 per year circuit capacity is supplemented by a new 14 million pound U3O8 per year SX circuit to provide a total capacity of 26 million pounds U3O8 per year. The pregnant strip solution from the SX circuits is fed to two parallel molybdenum removal carbon column circuits comprised of two new columns in addition to the old six. An additional precipitation reaction tank and barren strip sand filter to increase retention times and improve barren strip clarification have been constructed and commissioned. A new centrifuge provides yellowcake dewatering requirements, with the old centrifuge available as a backup spare if required. Existing calciner and new packaging facilities are used to provide the packaged, calcined product. The new packaging system was installed in 2013 to accommodate increased production rates with improved control of fugitive dust.

Further to the changes mentioned above, a third ammonia reagent supply tank was added for solvent extraction and precipitation. An additional ammonium sulphate crystallization plant similar in size to the existing plant was installed as well. Modifications to the existing acid plant were partially completed. The remaining scope of this project was deferred until additional acid is required. A new tailings neutralisation circuit is being constructed to provide the retention times required for full production rates. Cameco believes the McClean Lake mill will have access to sufficient water, power and process supplies necessary to process all of Cigar Lake’s current mineral reserves.

In 2022, Cameco plan to:
- continue production activities focused on bringing one new production panel online and closing out a completed one;
- continue surface freeze drilling and complete construction and commissioning of freeze distribution infrastructure expansion in support of future production;
- continue underground mine development on two new production tunnels as well as expand ventilation and access drifts in alignment with the long-term mine plan;
- continue upgrades to process water handling circuits and the surface backfill batch plant to support ongoing operations.