Regional geology
Outokumpu mining camp is located in eastern Finland within the North Karelia Schist Belt (NKSB). In the east and northeast, NKSB is bordered by Archean Karelian craton. In southwest the delimiting units are Paleoproterozoic island arc complexes accreted by Svecofennian orogeny.

The NKSB itself consists dominantly of folded and imbricated metasedimentary sequences representing two major tectonic-stratigraphic units. The older, 2.5-2.0 Ga, Sariola and Jatuli sequences comprise autochthonous, shallow-water cratonic to epicratonic metasedimentary deposits resting discordantly on the Archaean gneissic basement (Kohonen & Marmo 1992). The younger, 2.0-1.90 Ga, Kaleva sequences contain mainly deeper water turbidite deposits. Lower Kaleva rocks are considered autochtonous, but most voluminous Upper Kaleva rocks, that surround Kylylahti on all sides, are of allochtonous origin with depositional age of 1.95 – 1.92 Ga (Lahtinen et al. 2010). Upper Kaleva rocks were thrust over onto current location from direction that now lies in west 1.92 – 1.87 Ga ago (Peltonen et al 2008).

Kylylahti geology
Kylylahti is a polymetallic sulphide deposit featuring a 1.5 km long, north-northeast elongated group of lenses with a sub vertical attitude, that plunges from surface to the south southwest at approximately 30 degrees. The mineralized lenses have an average sub vertical height of approximately 150 m. Each lens has width ranging from 2 to 60 meters.

The hanging wall of Kylylahti deposit is an ophiolithic mantle fragment thrust onto place with the Upper Kaleva rocks. Mostly this once metaperidotitic ophiolite consists of serpentinite, but has later altered on the sides into talc-carbonate, skarn and quartz-sulphide rocks. These serpentinites, along with its alteration products, form a distinctive association (the Outokumpu Association or Outokumpu assemblage). This rock assemblage is almost thoroughly crosscut by mafic dykes appearing as chlorite schists and less altered metagabbros. These complexly altered ultramafic and mafic rocks nowadays occur as pods or lenses enclosed in the footwall black schists belonging to Upper Kaleva.

Kylylahti mineralization
In Kylylahti, the Outokumpu assemblage, along with the Upper Kaleva rocks are folded into tight synformal fold structure, with the mineralisation located along the near vertical eastern limb. Here, along or close to the carbonate-skarn-quartz rock to black schist interfaces, two main types of Co-Cu-Zn sulphide mineralization are present:

1) semimassive-massive sulphide lenses
2) sulphide disseminations in the carbonate-skarn-quartz rocks immediately paralleling the massive-semimassive lenses.

The semi-massive mineralisation comprises 25% to 60% sulphide. Mineralogy wise this mineralization type consists of predominantly pyrrhotite, pyrite and chalcopyrite, with subordinate local accumulations of cobalt-rich pentlandite, sphalerite, cobaltite and gold. It ranges in thickness from 5 m up to 50 m.

Structurally, semi-massive mineralisation at Kylylahti occurs in three elongated lenses, which strike to the northeast, dip near vertically to the northwest and plunge at between 25° and 40° to the southwest. The total length of the mineralised corridor is defined to approximately 1.5 km. Lenses are named the Wallaby, the Wombat and the Gap.

The semi-massive zone grades sharply into the disseminated ore over one to two meters, although isolated pods of semi-massive mineralisation may occur entirely within the disseminated zone.

The disseminated zone, situated at the hanging wall, contains medium to coarse grained sulphides (5% to 25% sulphides) and veinlets, with pyrrhotite predominating and lesser amounts of chalcopyrite, pyrite, cobalt-rich pentlandite, sphalerite and linnaeite-polydymite. Disseminated zones host Cu-dissemination and Co-dissemination domains. These two domains within the disseminated zone are the main domains hosting ‘NiCo’-ores. Mafic dykes crosscut most of the disseminated zone.

The disseminated zone is locally gold-rich. Three distinct Au-domains can be identified: AuCu-dissemination, Au-Ni-dissemination and Au-Cu-Ni-dissemination. All gold-bearing zones have the Outokumpu assemblage skarn and quartz rocks as host rocks.

The main mining methods used in Kylylahti are transverse and longitudinal open stoping (or longhole stoping) with cemented rock fill (CRF) and/or waste rock used as backfill. Transverse stoping is used in the wide parts of the orebody and longitudinal stoping in the narrow parts. The typical spacing between production levels is 30 m. Due to the gentle dip of the orebody, stoping in the main production areas have started from the bottom at the northern end of the orebody and advanced upwards (see Figure 16). Smaller production areas are located at the top of the orebody, and ore pillars have been left below the levels where mining has started. Most of the ore in the horizontal pillars will be mined before closing the operation. Some stopes are located near or within existing access tunnels and these will be mined out at the end of mine life.

Transverse stoping advances in two stages. First, a 10 m wide block perpendicular to the orebody, so called primary stope, is mined and backfilled with CRF. Later, the remaining 15 m wide block, so called secondary stope, is mined and backfilled with waste rock (see Figure 11: Example of secondary transverse stoping.Figure 11). In longitudinal stoping, the stoping advances along the orebody. When a longitudinal stope is mined out, the end towards the next stope in mining sequence is backfilled with CRF while the rest of the stope is backfilled with waste rock. Then the next stope is blasted against the CRF backfill (see Figure 12).

Stopes are drilled with longhole drill rigs. Both upwards and downwards drilling is used due to the high level spacing and curvy shape of the ore. Charging is normally done with emulsion, which is pumped into the blast holes. While impulse detonators are used in development blasts and small stope blasts, electronic detonators are often used in large/complex stope blasts.

Once a stope has been mined out, it will be scanned using a laser scanner (or Cavity Monitoring System, CMS, see examples of scanned stopes in Figure 11 and Figure 12). The backfilling of the stope will not start before the scanned stope shape is checked and it is considered safe and ready for backfill. The result of the stope scan is also used to obtain dilution and ore loss parameters. The combined parameters from all stopes are used to analyze and update the planned dilution and ore loss parameters for future stopes.

Ground conditions at Kylylahti are generally very good. However, as the mining proceeds, the rock stresses concentrate into certain areas within the mine, causing a need for additional rock support. Similarly, when re-accessing old development drifts, the original support is typically outdated and needs to be re-installed. This may cause additional costs and delays in production. The high rock stress and the damage it causes to production tunnels could in some cases result in having to abandon stopes or partial stopes. Most of the mine is in final mining stage and ground conditions are closely reviewed on stope by stope basis.

Some of the remaining stopes are located near the decline and exhaust air shafts. A new decline tunnel will be built to access the deeper parts of the mine after the stopes near the current decline have been mined out. The exhaust air route will, however, be cut off by the nearby stopes. Therefore, at the final stage of the mine exhaust air will move mostly along the decline, which will increase the time to ventilate the mine after blasts. The removal of fixed equipment and cables will start from the bottom of the mine and advance upwards when they are no longer needed for production. Similarly, water pumping from the bottom of the mine will cease once production levels are mined out.

Ore from the Kylylahti mine is transported by trucks 42 km to the Luikonlahti concentrator. The comminution steps at the concentrator involve crushing by jaw crusher, two cone crushers and grinding in three stages, first with a rod mill in the primary stage and ball mills in the subsequent secondary and tertiary stages. The third stage grinding is conducted in closed circuit, with classification by hydrocyclones and cyclone underflow fed to the mill.

Gravity separation is performed on the tertiary mill product using a Knelson concentrator, in order to separate coarse grained gold at an early stage in the process. Flotation is carried out in a three-stage process: copper-gold flotation, zinc or nickel flotation and sulphur flotation to avoid deposition of acid forming solids in the low sulphur tailings pond. The second cleaner tail stream from the copper circuit is fed back to the tertiary ball mill for regrinding.

Ores which are processed to produce a zinc concentrate generate a sulphur concentrate in the third stage. This concentrate also contains some cobalt and nickel. The sulphur concentrate and the first zinc cleaner tailings streams are combined and deposited in the high sulphur tailings pond. There is an ongoing project to find an economically feasible process to recover refractory cobalt and nickel from these tailings. In addition, there is an increasing volume of ores with floatable nickel-cobalt. For su ........