The Aitik Cu-Au ore is interpreted as a deformed and metamorphosed porphyry Cu-Au deposit with an IOCG-type hydrothermal overprint (Wanhainen et al. 2012).

Footwall rocks include two rock types, namely, feldspar–biotite–amphibole gneiss and the porphyritic intrusion of quartz monzodiorite. Both rock types show porphyritic texture, the former having amphibole phenocrysts while the latter is plagiophyric. The rocks within the ore zone transition from biotite gneiss, to biotite schist, to muscovite schist towards the hanging wall. Biotite and muscovite schist/gneiss both contain garnet porphyroblasts and is mainly differentiated by the amount of ferromagnesian minerals. Tourmaline occurs as fine-disseminations and as porphyroblasts. The hanging wall is comprised of three rock types, namely, feldspar–biotite–amphibole gneiss, metadolerite and occasionally amphibolite. Several pegmatite dykes cut the deposit, primarily in the hanging wall and ore zone. The dykes range from 0.5 to 20 m in width, which may be mineralized if situated within the ore zone.

Chalcopyrite, pyrite and pyrrhotite are the main sulphide minerals in Aitik, whilst bornite, chalcocite and molybdenite constitute minor components. Oxides such as magnetite and ilmenite are also found throughout the deposit. Ore minerals occur as disseminations, veinlets, patches and clots, and also in several types of veins together with varying amounts of other minerals such as quartz, biotite, amphibole, garnet, tourmaline, barite, zeolites (stilbite and chabazite) and thaumasite (Wanhainen and Martinsson, 2003).

Mineralization exhibits a north-northwesterly plunge, which reaches up to 800m below surface in the northern part and a depth of 400 meters in the southern part (Bergmann, 2001; Wanhainen 2005). Zonation of copper and gold distribution is observed within the deposit. Wanhainen et al. (2003) reported copper enrichment (>0.6% Cu) within 100-300 m, which decreases at deeper levels, i.e., 0.4–0.6% Cu at 400–600 m depth. On the other hand, gold-rich areas (>0.6 ppm) only exist at the deeper levels (400–600 m depth) and typically associated with amphiboles (Wanhainen et al. 2003).

Several alteration minerals are present in the ore body and are probably associated with several pulses of hydrothermal activity. Most extensive within the ore zone is biotite and K-feldspar alteration, commonly accompanied by garnet porphyroblasts of spessartine-almandine composition. Biotite is altered to sericite towards the hanging wall. Pervasive K-feldspar- epidote alteration is mainly restricted to the footwall and hanging wall contacts but may occur sporadically within the ore zone (Monro 1988). Albite-scapolite alteration associated with amphibole-pyroxene veins sporadically occurs within the deposit (Bergman et al. 2001).

Two other minor deposits are present within the Aitik mine area. The Salmijärvi deposit, found in the southern end of the Aitik open pit, have a similar mineralization style as the main deposit but of lower grade. Amphibole-biotite gneisses and local diorite hosts the mineralization. On the other hand, Liikavaara deposit is located in the eastern part of the mine area. Mineralization in Liikavaara is hosted by volcanosedimentary rocks of mainly andesitic composition and dominantly controlled by quartz-(calcite)-(tourmaline) veins. Ore minerals are chalcopyrite, pyrite and pyrrhotite, with significant amounts of magnetite, molybdenite and scheelite (Warlo et al. 2017).

The ore is mined in two open pits along the same mineral deposit. The main pit is called Aitik and measures roughly 4 km by 1.1 km at surface, with the deepest point currently at 480 meters from surface (Figure 8). In 2010, mining commenced in a second pit called Salmijärvi which has currently reached a depth of 225 meters below surface, with a surface foot print of roughly 0.9 by 0.6 km. The main pit will be expanded in all directions with five new pushbacks. Mining of pushback S3 in the southern part of the main pit started in 2016. Prestripping of pushback N7 commenced in 2019 to allow mining to start during 2020.

The ore and waste rock is blasted in 15 meter high benches and is loaded on 300 tons capacity size trucks by large rope shovels or hydraulic excavators. Ore from the deeper parts of the main pit is fed to one of two in-pit-crushers, while ore from pushbacks near surface and from the Salmijärvi pit is transported to a surface crusher situated between the two pits. Waste rock is separated in the loading process and hauled by trucks to dumps at the surface, where potentially acid forming waste is dumped separately from non-acid forming waste.

Holes for blasting are drilled with electric drills from Bucyrus and Atlas Copco. Hole depth: 16-17 m Hole diameter: 311 mm Drill attern: 7x9 m Between 200-300 holes are drilled for each round of blasting. A normal blast round produces around 700 000 tonnes of rock for extraction.

Boosters are loaded into the holes followed by emulsion explosives. Explosives: Emulsion explosives (Kemitti-610) from Forcit, about 1 tonne of explosive is used in each hole.

With the help of shovels from Bucyrus, P&H, Komatsu and wheelloaders from Caterpillar, the blasted rock is loaded on to trucks for further tranportation. The largest trucks, CAT 795F, holds about 313 tonnes. A bucket can hold 45 m³ rock (Komatsu 26 m³). The ore is transported to the crusher and the waste rock to a waste rock dumpsite. The machines are equipped with the positioning system Minestar.

In the processing plant the ore is ground in two stages, with autogenous grinding in the primary stage and pebble mill grinding in the second.

Ore handled by the crushers in pit is transported on conveyor belts to an intermediate storage on surface, where it is mixed with ore from the surface crusher fed in by a separate conveyor line. From the intermediate storage another conveyor belt transports the ore up to the main ore storage beside the processing plant. The main ore storage has a storage capacity corresponding to about one day’s production, providing buffer for the production.

The crushing of ores is done using three gyratory crushers with a combine throughput of 120,000 tonnes/day. The broken ore is transported via trucks to the grinding circuits. Two grinding lines are present in the mine, both of which are fully autogenous. The grinding lines are operated in a closed circuit consisting of a primary AG mill, a secondary pebble mill and a screw classifier. The AG primary mills are the largest in the world, having a power rating of 22.5 MW, diameter of 11.6 m, length of 13.7 m and volume of 1400 cm3 (June 2011). Grinding capacity ranges from 2,500-3,200 ton/hr while operating work index ranges from 9.7 to 12.4 kWh/ton. These values are highly dependent on the hardness of the ore. The grinding line targets a P80 size of 175 µm for the downstream process. This particle size is based on the liberation analysis of sulphides, mainly chalcopyrite, in the ore.

The milled ore is classified using a spiral classifier. The milled ore is sent for concentration through a series of froth flotation stages, namely, rougher, scavenger and four cleaning stages.

The copper concentrate is dewatered using thickeners and air pressure filters. The precious metals are reported in the copper concentrate. The copper concentrate is trucked to on-site railway terminal and reloaded for further transport by rail to the Boliden Rönnskär smelter in Skelleftehamn.

The plant produces a final concentrate containing 20-23% copper with gold, silver and molybdenum. Copper recovery is at 85-95% while silver and gold is at 65% and 50%, respectively. Molybdenum is currently not being recovered.