Geological setting and intrusion tectonics of the Kotalahti nickel-copper deposit, Finland
Bulletin of the Geological Society of Finland
The wall rocks of the nickel bearing mafic and ultramafic plutonic rocks of Kotalahti are Archean banded granitoidic gneisses containing inliers of Archean metasediments and metavolcanics. The zircon ages of the granitoidic gneisses are around 2800 Ma. The zircons of the mafic plutonic rocks of the Kotalahti deposit have been dated 1883 ±6 Ma. The Archean rocks are exposed in the oval-shaped Valkeinen brachyantiform, surrounded by veined mica gneisses of probable Proterozoic age. Five
... age. Five deformation phases have been distinguished by structural analysis. Deformation of the first phase took place during Archean times. The timing of the second deformation phase is uncertain but for the third deformation phase a Proterozoic age has been established. During the first two phases granitoidic rocks with supracrustal inliers and amphibolitic dikes were transformed into banded gneisses by strong deformation. The third deformation phase refolded these rocks into NNW trending synforms and antiforms, which were simultaneously intruded by the mafic and ultramafic plutonic rocks. The subvertical axial planes of the Fs folds were refolded into Sshapes during the fourth deformation phase in a NW-SE trending left-lateral ductile-shear belt. This shear belt is regarded as the upper part of a deep-seated fault, along which the nickel bearing magma ascended from the upper mantle. Narrow NE-ENE trending deformation zones crossing the shear belt are interpreted to belong to the fifth deformation phase. The largest occurrence of mafic and ultramafic plutonic rocks in the vicinity of Kotalahti is the dome-shaped massif of the Valkiajärvi gabbro in the core of the Valkeinen brachyantiform. Ni, Cu, S, TiOo and P contents of the Valkeajärvi gabbro are different from those of the mafic to ultramafic plutonic rocks of Kotalahti.