Geochemistry of Archaean and Palaeoproterozoic mafic-ultramafic rocks in NE Fennoscandia and implications for their Ni-Cu-PGE prospectivity

Northeastern Fennoscandia contains multiple generations of extrusive and intrusive mafic-ultramafic magmatism in the Archaean greenstone belts and Palaeoproterozoic rift-related basins, resulting in the formation of many mineral deposits. This study is focused on the geochemistry of the mafic-ultramafic rocks to constrain the petrogenesis and ore potential of different magmatic events and to identify key factors in ore formation which have potential implications for exploration. The study is sub-divided into three parts focusing on 1) Archaean komatiites in Russian Karelia, 2) Palaeoproterozoic dyke swarms in NE Fennoscandia, and 3) Palaeoproterozoic basaltic, komatiitic and picritic metavolcanites in several rift-related supracrustal belts in NE Fennoscandia. The potential of the Archaean greenstone belts in Russian Karelia to host Ni-Cu sulphide mineralisation is relatively low. During the early stage of rifting the lithosphere was still relatively thick, permissive to the formation of large, slowly cooling magma chambers in which magmas could fractionate extensively to form reef-type PGE sulphide deposits. Progressive extension during the middle stage of rifting weakened and thinned the lithosphere, preventing the formation of large magma chambers in the upper crust. During the late-stage rifting, interaction of magmas with sulphur-rich sediments deposited in deep rift basins allowed the formation of Ni-Cu-dominated sulphide deposits in magma conduits.