Geological setting and intrusion tectonics of the Kotalahti nickel-copper deposit, Finland

The wall rocks of the nickel bear ing maf ic and u l t r amaf ic plutonic rocks of Kotalaht i a re Archean banded granitoidic gneisses containing inliers of Archean metasediments and metavolcanics. The zircon ages of the granitoidic gneisses are a round 2800 Ma. The zircons of the maf ic plutonic rocks of the Kotalaht i deposit have been dated 1883 ± 6 Ma. The Archean rocks are exposed in the oval-shaped Valkeinen brachyant i form, sur rounded by veined mica gneisses of probable Proterozoic age. Five deformat ion phases have been dist inguished by s t ruc tura l analysis. Deformat ion of the f i rs t phase took place dur ing Archean times. The t iming of the second deformat ion phase is uncer ta in but for the th i rd deformat ion phase a Proterozoic age has been established. During the f i rs t two phases granitoidic rocks wi th supracrus ta l inliers and amphiboli t ic dikes were t r ans fo rmed into banded gneisses by strong deformat ion. The third deformat ion phase refolded these rocks into NNW trending synforms and ant i forms, which were s imultaneously in t ruded by the maf ic and u l t r amaf ic plutonic rocks. The subver t ical axial planes of the Fs folds were refolded into S shapes dur ing the f o u r t h deformat ion phase in a NW-SE t rending le f t la te ra l duct i le-shear belt. This shear belt is regarded as the upper pa r t of a deep-seated faul t , along which the nickel bear ing magma ascended f r o m the upper mant le . Na r row NE-ENE t rending deformat ion zones crossing the shear belt a re in terpre ted to belong to the f i f t h deformat ion phase. The largest occurrence of maf ic and u l t ramaf ic plutonic rocks in the vicinity of Kotalaht i is the dome-shaped massif of the Valk ia jä rv i gabbro in the core of the Valkeinen brachyant i fo rm. Ni, Cu, S, TiOo and P contents of the Valkea jä rv i gabbro are d i f fe ren t f r o m those of the maf ic to u l t r amaf ic plutonic rocks of Kotalaht i .