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, ........

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. 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.

Stopes are drilled with longhole drill rigs. Both upwards and d ........

- subscription is required.

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 such ores, ........