Summary:
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.