2D seismic imaging of the Koillismaa Layered Igneous Complex, north-eastern Finland

Abstract. The Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project is associated with the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The KLIC is characterized by a Bouguer positive gravity and magnetic anomaly zone connecting the two exposed ends of the KLIC, i.e. the Koillismaa intrusion and the Näränkävaara intrusion. The KLIC has the potential to host several critical raw minerals like nickel and cobalt, which are in the European Union's critical raw material list. For this purpose, two reflection seismic profiles were acquired to map the regional reflectivity in the area with a focus on imaging the large-scale geological architecture of the KLIC and the associated mineralization. Seismic imaging delineated reflectivity up to a depth of approximately 5 to 6 km with several distinctive reflective packages at various depths representative of the presence of dykes, faults, and major lithological contacts in the area. Symmetrical orientation of several shallow reflectors indicates systematic fracturing in the area. The deep-seated magma conduit associated with the KLIC was successfully mapped, revealing a complex internal structure with more structural or compositional heterogeneity than was previously assumed to be formed from a single lithological unit. The magma conduit is characterised as a vertically extensive mafic–ultramafic package with the potential of hosting orthomagmatic critical raw material deposits of mostly Ni-Cu-PGE sulphide type. A system of overthrust – steep regional faults was imaged for the first time in the western part of the study area. It is interpreted that such steep fault systems might have played a key role in magma emplacement by acting as pathways for ascending melts and mineralising fluids. Results were compared against the available petrophysical data and a preliminary geological model based on the density model obtained from the gravity inversion with constraints from the drillhole data.