The Paleoproterozoic Monchegorsk Complex, located in the Russian part of the Fennoscandian Shield, constitutes one of the largest mafic-ultramafic layered intrusions in Europe. The complex hosts extensive contact-style PGE-Ni-Cu sulfide mineralization along its margin, irrespective of the host lithology, which ranges from peridotite to pyroxenite and gabbronorite. The mineralized intervals reach up to 3 ppm Pt + Pd and attain a thickness of up to 50 m in the central portions of the intrusion, thinning towards the periphery. Our study shows that the key process controlling the size and grade of a contactstyle deposit in the Monchegorsk Complex, was the efficiency of sulfide collection in distinct zones of the intrusion. Strongly mineralized basal contacts are always associated with intense brecciation and the presence of large amounts of felsic pegmatite, suggesting a multi-stage emplacement of the mafic-ultramafic succession. Thermal modeling demonstrates that multiple episodes of magma influx are required to allow for significant partial melting of the basement. Moreover, the interaction between magma and basement led to the local addition of water and potentially carbon dioxide to the magma, resulting in partial melting of cumulus phases and a reduction in viscosity of the interstitial melt. This increased the porosity of the mush in the vicinity of the lower intrusion contact, which promoted preferential sulfide liquid accumulation at the base, while the local decrease in magma viscosity facilitated gravitational settling of sulfide droplets. These factors led to an efficient collection of sulfide liquid, especially in the center of the complex, where permeability was maintained the longest due to slower cooling relative to more peripheral parts.

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The South Sopchinsky massif (SSM), Gabbro-10 (G-10) massif, and Moroshkovoe Lake (ML) target Monchegorsk area, Kola Peninsula, are located at the junction of the Monchepluton and Monchetundra layered intrusions. The intrusions were studied in detail as they are targets for platinum-group element (PGE) mineralization. The rocks in these targets comprise medium-to coarse-grained mesocratic to leucocratic gabbronorites, medium-grained mesocratic to melanocratic norites and pyroxenites, and various veins mainly comprising norite, plagioclase-amphibole-magnetite rocks, and quartz-magnetite rocks. The veins contain Ni-Cu-PGE mineralization associated with magnetite and chromite. In all targets, the contacts between gabbronorite and noritepyroxenite are undulating, and the presence of magmatic (intrusive) breccias suggests that these rocks formed through mingling of two distinct magmatic pulses. In places, the gabbronorites clearly crosscut the modal layering of the norites and pyroxenites. Trace element data indicate that the gabbronorites have similar compositions to rocks of the upper part of the Monchetundra intrusion, whereas the norites and pyroxenites resemble rocks from the lower to intermediate stratigraphic levels of the Monchepluton, such as in the Nude-Poaz and Sopcha massifs. Sulfide mineralization in the studied targets principally consists of secondary bornite, millerite, and chalcopyrite. In contrast, the primary sulfide assemblage within the layered sequence of the adjacent Monchepluton is characterized by pentlandite, chalcopyrite, and pyrrhotite. Therefore, the mineralization in the studied targets is interpreted to be of a contact style. We argue that the studied area represents the contact zone between gabbronorites of the Monchetundra intrusion and norites and pyroxenites of the Monchepluton. In addition, the rocks were overprinted by postmagmatic veining and remobilization of contact style sulfide and PGE mineralization.