JultikaUniversity of Oulu repository

Abstract

Igneous activity during the early Palaeoproterozoic (2.4–2.5 Ga) led to the development of layered intrusions, mafic dyke swarms, and volcanic rock in the Earth’s cratonic regions. The layered mafic and ultramafic Paleoproterozoic intrusions in Fennoscandian Shield host Ni-Cu-PGE, titanium, vanadium, chromium, and iron deposits. The Koillismaa-Näränkävaara Complex comprises the Näränkävaara Intrusion, the Koillismaa Intrusion, and a strong positive gravity anomaly, which connects these distant eastern and western parts of the complex.

This thesis focuses on Sr isotopic composition of the plagioclase in the Koillismaa Intrusion. The intrusion is composed mostly of gabbronorites, gabbros, and leucogabbros. It is divided into Marginal series and Layered series. The Marginal Series is Cu-Ni-PGE mineralized (for example Haukiaho and Kaukua), and the Layered Series contains at least two orthomagmatic mineralization which are the PGE-Cu-Ni enrichment at its central part (RT Reef) and the Fe-Ti-V enriched oxide gabbro (Mustavaara deposit) at its upper part. After its solidification, the intrusion was tectonized into several smaller blocks of which the central, up to 2200 m thick Porttivaara block is studied in this thesis.

In situ Sr isotope compositions of plagioclase from the Koillismaa Intrusion shows differences between the Marginal Series and the Layered Series; the Marginal Series show an initial 87Sr/86Sr (2.44 Ga) of 0.7037, and for the Layered Series, the ratio is 0.7027. The isotopic difference indicates isotopically distinct magma types in the genesis of the Layered Series and the Marginal Series or more intense crustal contamination at the margin of the intrusion. Sr isotope ratio throughout the oxide gabbro, which includes the Mustavaara deposit is isotopically homogeneous which doesn’t support involvement of different types of magma in its formation. Similarly, the study suggests that the formation of RT reef did not involve magma replenishment. The highly radiogenic Sr isotopes obtained from the Marginal Series indicate that crustal contamination may have played a role in the genesis of the contact-type sulfide mineralization.

The isotope composition of the Koillismaa Intrusion show affinity to coeval tholeiite group dykes and SHMB (GBNO and SHMB) group dykes. The bulk of the the Koillismaa Layered series is isotopically comparable to the MCUIV of the coeval Penikat Layered Intrusion.