The Palaeoproterozoic (1.9 Ga) Rytky and Kotalahti mafic-ultramafic intrusions are located in the contact zone between the Archaean craton and Proterozoic supracrustal rocks. During the second deformation event (D2) the surrounding country rocks were subjected to intensive metamorphism and deformation associated with the Svecofennian orogeny; the Archaean/Proterozoic boundary controlled both D2 thrusting and magma ascent. Emplacement of the Rytky and Kotalahti intrusions took place at the culmination of D2, as shown by the gneiss inclusions with S2 schistosity within the intrusions. Overthrusting continued after emplacement, with detached fragments of the bodies incorporated into the Archaean gneisses. During the third deformation event (D3) the originally subhorizontal intrusions were rotated into a subvertical position, so that they now have their stratigraphic top towards the west. The Rytky intrusion is composed mainly of medium- and coarse-grained lherzolite, websterite and gabbronorite. The nickel deposit with pentlandite as the main nickel mineral is associated with the lherzolite and websterite. The coarse-grained lherzolite, websterite and melagabbro represent the first rocks to form, and they contain the nickel sulphide mineralisation. Country rock contamination, as indicated by high TiO2, P2O5, Rb, Zr and light rare earth element contents (LREE), is most pronounced in the marginal part of the intrusion, which was the first to form. The variation in olivine composition (Fo 78.6-84.77 mole %; Ni 630–2386 ppm) and the metal ratio of the sulphide (Ni/Co 19.3 – 50.3) along with the internal stratigraphy of the intrusion indicate an in-situ process of sulphide ore formation.
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We are grateful to Peter Lightfoot for his many valuable comments on the manuscript. We thank Olavi Kontoniemi, Jouni Luukas, Erkki Luukkonen, Peter Sorjonen-Ward and Olli Äikäs for their comments on the early version of the manuscript and Malcolm Hicks for checking the English.
Editorial handling: P. Lightfoot
An erratum to this article can be found at http://dx.doi.org/10.1007/s00126-004-0445-9
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Mäkinen, J., Makkonen, H.V. Petrology and structure of the Palaeoproterozoic (1.9 Ga) Rytky nickel sulphide deposit, Central Finland: a comparison with the Kotalahti nickel deposit. Miner Deposita 39, 405–421 (2004). https://doi.org/10.1007/s00126-004-0413-4
- Structural geology
- Ultramafic intrusion