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 platinum group element-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 toward the periphery. Our study shows that the key process controlling the size and grade of a contact-style deposit in the Mon-chegorsk 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 multistage 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 local small-scale dissolution 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.

This paper compares the geological, geophysical, and isotopic geochemical data on the Paleoproterozoic East Scandinavian Pd-Pt province in the Baltic Shield and the Late Paleozoic Noril’sk Pd-Pt province in the Siberian Craton. Both provinces contain large magmatic PGE deposits: low-sulfide in the Baltic Shield and high-sulfide in the Siberian Craton. Multidisciplinary evidence shows that the East Scandinavian mafic large igneous province, which has a plume nature, is intracratonic and was not subjected to the crucial effect of subduction-related and other contamination processes, whereas the Noril’sk province is pericratonic with substantial crustal contamination of the intrusive processes. Low-sulfide Pd-Pt deposits dominate in the East Scandinavian province, while high-sulfide Ni-Cu-PGE deposits play the leading role in the Noril’sk province. The U-Pb, Sm-Nd, and Rb-Sr isotopic data indicate multistage and long-term (tens of millions of years) geological history of mafic large igneous provinces. The plume magmatism with specific geochemistry and metallogeny is probably related to lower mantle sources.

The Paleoproterozoic Monchegorsk Complex in northwest Russia represents one of the largest layered intrusions in Europe and hosts several examples of broadly stratiform platinum group element (PGE) mineralization at different stratigraphic levels of the intrusion that have been suggested to represent reef-style mineralization. The Sopcha reef occurs in the ultramafic lower portion of the complex and constitutes an up to 6-m-thick succession of layered, mineralized dunite, harzburgite, and olivine-orthopyroxenite, with peak grades of 3.4 ppm Pt + Pd and 1.1 wt % Ni. Another PGE occurrence is hosted by the leucogabbronoritic to anorthositic Vuruchuaivench intrusion, which represents part of the mafic upper portion of the Monchegorsk Complex. The disseminated sulfide mineralization reaches up to 7.3 ppm Pt + Pd and is concentrated in several lenticular bodies over a strike length of ~5 km, rather than in a laterally continuous reef as previously suggested. Moreover, our work identified a previously unreported minor enrichment in precious metals of up to 0.2 ppm Pt + Pd in leucogabbroic rocks of the Monchetundra intrusion, which represents the uppermost portion of the Monchegorsk Complex and belongs to the more than 60-km-long mafic Main Ridge. Detailed lithophile and chalcophile element data, coupled with mineral chemistry, indicate that the PGE mineralization at Sopcha and Vuruchuaivench does not represent classic reef-style mineralization, which is commonly narrow and marked by a sharp increase in Cu/Pd ratios, reflecting the in situ sulfide saturation within a large magma chamber. Instead, it is more likely that the Sopcha reef was emplaced as a crustally contaminated and sulfide-saturated, olivine-rich crystal mush that was sourced from a deeper chamber. The Sopcha mineralization is characterized by Pd/Pt > 5 and Pd/Ir > 55, similar to contact-style mineralization elsewhere in the complex, possibly suggesting a common origin of the sulfides. The mineralized Vuruchuaivench rocks have similar Pd/Pt but much higher Pd/Ir ratios of up to 600, whereas the unmineralized host rocks, below as well as above the mineralization, have Pd/Ir ratios <100 and Pd/Pt ratios <2. These data indicate that the PGE-rich sulfides did not segregate in situ from the same magma that crystallized the host gabbronorites and anorthosites at Vuruchuaivench. Considering R factor and sulfide fractionation modeling results, we suggest that the mineralized Vuruchuaivench rocks represent a sill-like intrusion of gabbroic crystal mushes, which have entrained fractionated sulfide liquid that is related to an earlier sulfide saturation event. In contrast, the mineralized leucogabbroic rocks from the Monchetundra intrusion are characterized by a sharp increase in Cu/Pd ratios, which is consistent with a classic PGE reef model, in which sulfide saturation was triggered in situ by extensive fractionation and possibly affected the entire magma chamber. Furthermore, the Pd/Ir and Pd/Pt ratios of the mineralized horizon are distinctly lower at <66 and <1 in comparison to all other types of mineralization in the Monchegorsk Complex. The potential of this mineralization style elsewhere in the Main Ridge remains to be evaluated.

Several deposits of low-sulfide Pt–Pd ores have been discovered in recent decades in the Paleoproterozoic Fedorova–Pana Layered Complex located in the Kola Region (Murmansk Oblast) of Russia. The deposits are divided into two types: reef-style, associated with the layered central portions of intrusions, and contact-style, localized in the lower parts of intrusions near the contact with the Archean basement. The Kievey and the North Kamennik deposits represent the first ore type and are confined to the North PGE Reef located 600–800 m above the base of the West Pana Intrusion. The reef is associated with a horizon of cyclically interlayered orthopyroxenite, gabbronorite and anorthosite. The average contents of Au, Pt and Pd in the Kievey ore are 0.15, 0.53 and 3.32 ppm, respectively. The North Kamennik deposit has similar contents of noble metals. The Fedorova Tundra deposit belongs to the second ore type and has been explored in two sites in the lower part of the Fedorova intrusion. Mineralization is mainly associated mainly with taxitic or varied-textured gabbronorites, forming a matrix of intrusive breccia with fragments of barren orthopyroxenite. The ores contain an average of 0.08 ppm Au, 0.29 ppm Pt and 1.20 ppm Pd. In terms of PGE resources, the Fedorova Tundra is the largest deposit in Europe, hosting more than 300 tons of noble metals.

This report contains the results of the authors’ studies on geology and isotope geochronology, which allowed them to formulate the thesis of a two-phase mechanism of the formation of rock associations in the Fedorova Tundra massif. During the former phase (2526–2507 My), a laminated series of the massif was formed, with shows of platinum-metal mineralization of reef type. During the later phase (2493–2485 My), a taxitic zone of the massif appeared, with a highly developed basal platinum-metal mineralization.

The West-Pana intrusion belongs to the Paleoproterozoic Fedorova-Pana Complex of the Kola Region in NW Russia, which represents one of Europe’s most significant layered complexes in terms of total platinum group element (PGE) endowment. Numerous studies on the age of the West-Pana intrusion have been carried out in the past; however, all published U-Pb isotope ages were determined using multi-grain ID-TIMS. In this study, the mineralized Main Anorthosite Layer from the upper portion of the intrusion was dated using SHRIMP-II for the first time. High Th/U (0.9–3.7) zircons gave an upper intercept age of 2509.4 ± 6.2 Ma (2σ), whereas the lower portion of the intrusion was previously dated at 2501.5 ± 1.7 Ma, which suggests an out-of-sequence emplacement of the West-Pana intrusion. Furthermore, high-grade PGE mineralization hosted by the anorthosite layer, known as “South Reef”, can be attributed to (1) downward percolation of PGE-enriched sulfide liquid from the overlying gabbronoritic magma or (2) secondary redistribution of PGEs, which may coincide with a post-magmatic alteration event recorded by low Th/U (0.1–0.9) zircon and baddeleyite at 2476 ± 13 Ma (upper intercept).