Micromagnetic survey experience in the mapping of the South Sopcha platinum-bearing intrusion, Monchegorsk ore district

The norite-pyroxenite zone hosting platinum group element (PGE) mineralization is located at the base of the South Sopcha intrusion being a part of the Monchegorsk layered complex. The complex geological structure of the norite-pyroxenite zone, dipping at angles from 20° to 80°, is defined by frequent alternation of pyroxenites and taxitic gabbroids of mafic composition. There is an assumption that the first group of rocks of this zone, represented by pyroxenites with peridotites interlayers, was formed before the rocks of the second group, including mineralized taxitic mafic gabbroids distributed throughout the whole zone in the form of lenticular vein-like bodies. Micromagnetic survey used for the first time in the Kola Region is an effective tool for studying objects with such complex structures. The results of the micromagnetic survey at two detailed sites located within the Arvarench PGE occurrence, surveyed in 2011 and 2024 using 5×1 m and 4×1 m grids, respectively, are presented in our paper. The analysis based on a comparison of the loss of information when using less detailed grids shows that the loss in the case of grid discharge is significant and the effectiveness of the magnetic survey for mapping is greatly reduced. The results of petrophysical measurements and micromagnetic survey together lead to the following assumptions about the magnetic field over the norite-pyroxenite zone: 1) high-amplitude positive anomalies are associated with peridotites; 2) mineralized taxitic mafics containing a high proportion of magnetite along with disseminated sulfides are characterized by medium-amplitude anomalies; 3) the lower contact of the intrusion with less magnetic Archean gneisses is fixed in the magnetic field by a negative curve-shaped anomaly; 4) tectonic zones probably consisting of remagnetized rocks are marked by linear negative anomalies. It is argued that micromagnetic survey in combination with detailed magnetometry can be used for geological mapping of layered intrusions and prospecting for PGE mineralization.