On the geotectonics and ore forming processes in the basic intrusive belts of Kemi-Suhanko, and Syöte-Näränkävaara, northern Finland

Abstract

The intrusive belts from Kemi to Euhanko and from Syöte to Näränkävaara incluse the ore deposit of chromite near Kemi and that of vanadium-bearing magnetite-ilmenite at Mustavaara. In addition to these deposits, there are in many parts of the belts, disseminations of pyrrhotite, chalcopyrite and pentlandite. The intrusives containing the abovementioned ore deposits were formed by the initial igneous activity of the Svecokarelian orogeny. At this stage the emerging magma formed layered intrusions between the old basement and the younger Karelian volcanic and sedimentary formations in the zones between the culmination and depression areas. In the lower areas where clastic sediments were present the magma was characterized by a high water content, which allowed the crystallization of spinel at an early stage, leading to the deposition of chromite ores. It is to be noted that no sulfide or magnetite-ilmenite ores are found in the lower areas. On the contrary, they lie in the higher areas, where magma crystallized in dry conditions. Here the fugacity of oxygen necessary for the formation of spinel was only reached at the final stage of crystallization and thus the crystallizing spinel was titanomagnetite. The layered intrusions are now visible as basic belts on the edges of the depression areas.