Ni-(Cu-PGE) deposits in Finland – Geology and exploration potential

Abstract

Several types of nickel sulphide deposits occur in Finland. Some of them are also important sources of Cu, Co and PGE. The main deposit types are related to Archaean and Palaeoproterozoic mafic to ultramafic rocks, Palaeoproterozoic metamorphosed black shales of the Talvivaara type, or the ultramafic and metasomatic calc-silicate rocks of the ophiolitic Outokumpu association. Several komatiite-hosted Ni-Cu-PGE deposits have been discovered in the Archaean and Paleoproterozoic greenstone belts in northern and eastern Finland, but only one has been briefly mined, in 1989. Many 2.44Ga Palaeoproterozoic mafic-ultramafic layered intrusions in Finland host contact-type PGE-Ni-Cu deposits and significant reef-type PGE deposits; typically the Ni resources are of low grade but large tonnage. The mafic-ultramafic magmatism at ca. 2.06Ga produced significant Ni-Cu-PGE resources in the Central Lapland greenstone belt, including the Kevitsa and Sakatti deposits. The Svecofennian domain in central and southern Finland contains hundreds of small, ca. 1.88Ga, mafic-ultramafic orogenic intrusions; many of these host nickel mineralization. Nickel exploration potential was evaluated in general, based on identification of critical factors within a mineral system framework and a quantitative assessment of the potential occurrence of undiscovered nickel resources. The MgO content of the parental magma was estimated for 28 deposits: 6 komatiite-hosted occurrences and 22 Svecofennian orogenic intrusions. The estimated MgO content of the parental magmas for the selected deposits varies from around 7% to 15% in basaltic to komatiitic basalt systems and up to 30% in more magnesian komatiitic systems; the Ni/Cu value varies mostly between 0.7 and 12, correlating in general positively with the MgO content of the magma. Exceptionally high or low Ni/Cu values in some deposits may result from, for example, copper loss during post-magmatic processes (serpentinization and/or metamorphism) or part of the sulphide ore (zoned by the Ni/Cu value) still being unknown. Recent advances in understanding of komatiites in Finland indicates high exploration potential for nickel within the Paleoproterozoic komatiitic formations, which typically are also significantly enriched in Cu and PGE. However, the quantitative assessment of undiscovered nickel resources in Finland suggests that the contact-type Ni-Cu-PGE deposits hosted by 2.44Ga mafic-ultramafic layered intrusions in northern Finland have the greatest potential for hosting undiscovered resources among the assessed nickel deposit types; these occur within restricted areas, resulting in a high deposit density. Finally, considerable nickel resources might exist in undiscovered Talvivaara-, Kevitsa- or Sakatti-type deposits. These deposit types should be assessed as soon as more geological information becomes available.