The mantle-crustal ore-forming systems of the Pechenga ore district (Fennoscandian shield)

The mantle-crustal ore-forming systems of the Pechenga ore district is known for large Cu-Ni ores and smaller occurrences of Pt, U, Au, Pb and Zn. It comprises four superposed systems: plutonic PGE deposits, volcano-plutonic Cu-Ni deposits and hydrothermal U and Au deposits which were formed at different geotectonic conditions in the Karelian and Svecofennian (2.4-1.6 Ga) cycles. The continental crust in the Pechenga ore district can be divided into an upper heterogeneous and a lower homogeneous zone which differ by the acoustic velocity parameters. The structure and location of Moho was determined from seismic tomography data. Its upper surface is located at the depth of 34-41 km. A minimum depth of 34-36 km was registered below the northern limb of the Pechenga structure which encloses the Cu-Ni deposits. They correspond to the anomaly of seismic tomography data 35-45 km in diameter which is isometric in horizontal plane and can be interpreted as a relic mantle plume. Anomalies with the maximum values of VpNs are revealed above the plume at the boundary between the upper and lower crust at the depth of 20 km. These anomalies indicate previous existing intermediate magma chambers with mafic and ultramafic melts.