The gold mineralization at Macassa is located along the breaks and subordinate splays as individual fracture fill quartz veins, from several centimetres to a few meters thick. Veins may be of single, sheeted, brecciated or stacked morphology. Several generations of quartz deposition are evident from colour and textural variability and quartz veins are generally fractured. Also found are sulphide rich (pyrite) zones.

The presence of a fault splay is often a prerequisite for gold deposition. Broader zones of mineralized, brecciated and fragmented quartz are found in the footwall and hanging wall of major faults.

Gold is usually accompanied by 1% to 3% pyrite and sometimes is associated with molybdenite and/or tellurides of lead, gold, gold-silver, silver, nickel and mercury (altaite, calaverite, petzite, hessite, melanite, coloradoite). Silver is present amalgamated with the gold and in the minerals petzite and hessite.

The presence of pyrite and silicification does not guarantee gold; however, higher grade gold is generally accompanied by increased percentages of pyrite and silica.

Hematization or bleaching with carbonatization and silicification are common alterations of the wall rocks. Sericitization is a more local feature. The alteration has enriched the rocks in K2O and depleted them in Na2O.

The new discoveries in the South Mine Complex (SMC) generally are of a different style of mineralization with wide sulphide systems rather than the quartz vein mineralization that is found in the Main Break complex. Tellurides appear to be more prevalent in the SMC, compared to the historical mineralized systems, in particular the occurrence of the gold telluride mineral calaverite. These new, wide, hydrothermally altered zones could represent a new plumbing system for a southern mineralized part of the Camp parallel to the Main Break, fed by a deep porphyry body. The gold mineralization is found in carbonate altered conglomerate, tuff and porphyry, mineralized with up to 10% disseminated pyrite. Quartz veining and silicification when hosted within the porphyry may also characterize the SMC.

Panterra Geoservices (Rhys 2017) has proposed a new conceptual mineralizing model for the ’04/Main Break and SMC zones. Here the Amalgamated Break is interpreted as the main structure off which the ‘04 Break, SMC and AK zone splay and link between. Reduced, sericite- carbonate-chlorite alteration is developed extensively along the Amalgamated Break in association with largely barren, white quartz veins and may feed into the subsidiary faults. Fluids originally flowing along the Amalgamated Break may have fed into splaying structures such as the ‘04 Break and SMC. Most ore deposition has occurred in areas where carbonate-pyrite alteration is interspersed with more oxidized reddish-orange tinted alteration assemblages that occur more distally to the feeder structures, and regional magnetite-biotite-amphibole assemblages are altered to K-feldspar-hematite carbonate. The Amalgamated and ‘04 Break are interpreted to merge near the -9000 foot elevation (depth from surface) in the #3 Shaft area.

There are currently three active mining areas in Macassa Mine: Main Break (MB), Lower North (LN) and New South (NS). The areas LN and NS are both part of the SMC. Access to the mining areas is through the #3 Shaft and connecting lateral development within the MB and SMC zones. The main mining methods at Macassa Mine include Underhand Cut and Fill, Longhole stoping and Mechanized Overhand Cut and Fill. The selection of mining method depends on several factors including ore geometry, grade and the need for locations to deposit waste fill. There are also several geomechanical considerations, such as structure and stresses, which impact the mining method selection.

Paste fill is the main material used to backfill stopes, although unconsolidated rockfill is also used where possible. Material hoisted to surface via #3 Shaft, which has an average capacity of 2,200 tpd.

Once the ore is hoisted to surface, it is then trucked to the crushing facilities. After crushing and grinding (95% passing, 45 microns), the ore is processed by conventional cyanide leaching with a carbon-in-pulp recovery system.

Macassa Mine has been on the forefront in the use of Battery Electric Vehicles (“BEVs”) and was the first mine in Ontario to implement BEVs as the standard for the LHD and truck fleet. Kirkland Lake Gold has partnerships with battery equipment manufacturers to develop and design BEVs, as opposed to retrofitting diesel powered equipment. Macassa Mine will continue to replace its fleet of underground diesel equipment with BEVs as required.

The ore is crushed down to 11mm at a maximum throughput rate of 80 tph and then ground to 40-45 microns; cyanide is added at the grinding stage. It is then delivered to two pre-oxidation tanks before being pumped to the thickener. The overflow reports to the carbon columns (where over 75% of the gold is recovered) and the underflow to the leach circuit. Leaching takes place in seven tanks during a retention time of 100 hours. The carbon-in-pulp circuit (CIP) consists of six tanks. Following electrowinning, the concentrate is melted in an induction furnace to produce doré grading 85% to 88% gold and 8% to 10% silver. The capacity of the plant is 2,000 tpd.

The company’s mill was built in 1986 at a capacity of 725 tpd. Modifications over the years increased the throughput capacity to 2,000 tpd in 2013.