The paper concerns the geochemical analysis of rocks from the ore-bearing layered intrusions that belong to two age groups of the Monchepluton and the Imandra–Umbarechka Complex (2.50 and 2.44 Ga) and the largest gabbro-anorthosite of the Main Ridge Complex (2.51–2.45 Ga). The intrusion of these complexes happened at different depths when the endogenous and geodynamic settings changed at the beginning of the Paleoproterozoic Era. Five megacycles are distinguished in a generalized cross-section of the two-chamber Monchepluton. The megacycles differ in rock composition, rock geochemical features, and mineralization types, i.e., the chromite, sulfide Cu–Ni–PGE and low-sulfide PGE types. The abrupt changes in isotope indicators (εNd, ⁸⁷Sr/⁸⁶Sr) mark their boundaries. At a depth of 2037–2383 m, the M-1 borehole intersects a standalone intrusive body that is essentially a magma feeder channel. The intrusive body’s geochemical characteristics and U–Pb isotope age correlate to the Monchepluton rocks. The gabbro-anorthosite massifs united in the Main Ridge Complex were intruded in the following order: the Monchetundra, Chunatundra, Volchetundra, and Losevo–Medvezhye tundras. The largest Monchetundra massif was formed as a result of multiple intrusions of mafic magmatic melt from the deep reservoirs. The melts intruded in two stages, i.e., 2.51–2.49 Ga and 2.48–2.47 Ga, and their composition changed gradually. The gabbro-pegmatites and coeval harrisite dykes are more recent ones (2.46–2.45 Ga). The summarized results of the U–Pb, Sm–Nd, and Re–Os systems research allowed us to establish genetic relations between the studied geological objects. We proposed a model where there was an uplift of a mantle plume to the lower crust area at the age of 2.5 Ga, the deep mantle reservoirs were formed, and a large-scale interaction happened between the parental magma and granulite–eclogite complex rocks. Local contamination and assimilation processes took place during the uplifting of magmas in areas where the magmatic feeding system contacted the host amphibolite–gneiss Archean complexes.

Large-scale layered intrusions of a peridotite–pyroxenite–gabbronorite complex, to which Cr, Ni, Cu, and PGE deposits and ore occurrences are confined, were emplaced into the Baltic paleocontinent 2.50–2.45 Ga. Layered intrusions in the Monchegorsk Ore District, including the Monchepluton and Imandra–Umbarechka Complex, as well as the gabbro-anorthosite complex of the Main Ridge, were analyzed earlier geochemically and isotopically. In the present paper, the authors analyze layered intrusions in the Kola region (Mount Generalskaya) and Karelia (Kivakka, Kovdozero, and the Burakovsky Pluton). The primary composition of mantle magmas for the layered intrusions is assumed to be identical to that of the komatiitic basalts making up the volcanogenic units of the Vetreny Belt and the Imandra–Varzuga zone. A general model for the formation of layered intrusions includes superplume uplift in the early Paleoproterozoic, the generation of mantle magmas and their injection into the lower portion of the earth crust, the formation of deep-seated and intermediate magma chambers, and the intense contamination of the granulite–metamorphic complex followed by the generation of magma chambers provoked by single or multiple injections.

At the beginning of the 1960s, a large body of funnel-shaped ore pegmatite composed of Cu–Ni sulfide ores and a gabbronorite matrix was discovered at the Nittis–Kumuzhya–Travyanaya ore vein field of the Northern Chamber of Monchepluton (Kola Peninsula). In terms of its localization conditions, size, structure, and enrichment in sulfides and platinum-group elements (PGEs), it is a unique formation that has no analogues among other Paleoproterozoic layered complexes of the Fennoscandian Shield. Ore pegmatite occurs in the upper part of the layered zone of Mt. Nittis, which is composed of harzburgites and orthopyroxenites. Its horizontal size is 9 × 16 m, and the vertical one is 15 m. Three zones are distinguished in the structure of the body: I, a core of continuous sulfides; II, coarse- and giant-grained gabbronorites, enriched in interstitial sulfides; and III, a contact zone with sideronite sulfides with gradual transitions into host orthopyroxenites. Petro- and geochemical, mineralogical, and isotope studies of rocks and ores were performed using modern analysis methods. Ore pegmatite is most recent product in the processes of fractional crystallization of a magmatic melt, which derived as a large schlier with a sulfide core and a silicate matrix with a high concentration of fluids. The hypsometric level of the termination of the melt upwelling was determined by the equilibrium of the internal pressure of the volatiles and the external pressure of the overlying rocks. The δ¹⁸O values (+4.9…+6.1‰) are close to mantle marks (δ¹⁸O = +5.7‰) that correspond to basic igneous rocks. Early, late, and postmagmatic stages of mineral formation with a consistent increase in the role of fluid components (H₂O, CO₂, Cl, F) are distinguished in the formation history of the ore pegmatite. According to the results of calculations performed using various mineral geothermometers, the melt crystallization at the magmatic stage occurred in the range of ∼1100–900°C at a pressure of about 5 kbar. The separation (liquation) of the immiscible sulfide liquid began at temperatures of 1100–1000°C. As the temperature decreases, the main silicate minerals (clino- and orthopyroxenes, plagioclase) were first to crystallize. In the interstitial space between these minerals, residual melt and sulfide liquid enriched in PGEs, Au, Ag, and chalcophile elements (As, Sn, Sb, Te, Bi, Pb, Zn) accumulated. A late magmatic association (pargasite, magnesian hornblende and phlogopite) was formed from the residual melt. During the cooling of the sulfide liquid to a temperature of ∼1000°C and below, a copper-bearing monosulfide solid solution (Mss) is detached. As a result of its solid-phase transformations, pyrrhotite, pentlandite, and chalcopyrite were formed. An intermediate solid solution (Iss) was formed from the residual sulfide liquid, which was enriched with Cu and noble metals. Under a temperature below 550°C, this solution was successively decomposed into chalcopyrite, pyrrhotite, and cubanite. The dominant PGE minerals are the following: michenerite PdBiTe, sobolevskite Pd(Bi,Te), froodite PdBi₂, merenskyite PdTe₂, and moncheite PtTe₂. Au and Ag minerals are represented by electrum (AuAg) and hessite (Ag₂Te). A rare mineral—cervelleite (Ag₄TeS)—has been discovered. The Pd content in ore pegmatite varies in the range of 64.13–0.09 ppm, and that of Pt in the range of 2.70–0.004 ppm. The ore potential of Monchepluton is far from exhausted. Therefore, clarifying the genetic features of the origin of ore pegmatites and their connection with copper–nickel mineralization is of not only petrological, but also important practical significance, including for the purpose to develop prospecting indicators for the vein type of PGE–Cu–Ni ores.

Detrital zircons with an age of 3.65–3.87 Ga have been found earlier in Jatulian terrigenous rocks from the eastern Fennoscandian Shield, i.e., the Karelian and Kola regions (Kozhevnikov et al., 2010; Smol’kin et al., 2011, 2019), while rocks of this age are not reported in the inferior Sumian–Sariolian complexes and in the Archean basement. To establish the provenance area and composition of the sources, the first study of geochemical composition (rare-earth and trace elements) of detrital zircons from Jatulian red gravelstones of the Luchlompol’skaya Fm., Pechenga structure, quartzites of the Volomskaya syncline, and cement of conglomerates in the western Onega Trough, which are located at considerable distances from each other, has been carried out. The age of detrital zircon grains ranges mainly within 2.70–3.23 Ga. Some of the detrital zircon grains are of igneous type. Grains and outer envelopes of zonal grains having the youngest age (2.70–2.72 Ga) are referred to the metamorphic type. “Porous” zircon that underwent a fluid effect is also present. The main sources of igneous zircon grains were tonalite and trondhjemite gneisses and acid granulites, which are widespread in the vicinities of the studied structures and were revealed in the lower part of the Kola Superdeep Well (VSD-3), as well as gneisses and amphibolites of the Vodlozero block. The source of detrital zircon grains with an age of 3.65–3.87 Ga is the Siurua trondhjemite gneiss (northern Finland). Their erosion and transportation of zircons took place 2.2–2.1 Ga ago along the western margin of the Svecofennian–Pre-Labradorian Ocean that existed at the initial stage of the Columbia Supercontinent assemblage.

Проанализировано распределение ресурсов хрома в геологической истории Земли. Наиболее крупные ресурсы сосредоточены в стратиформных месторождениях кенорского (> 2.25 млрд лет) и колумбийского (1.35–2.25 млрд лет) суперконтинентальных циклов, локализованных в дифференцированных и расслоенных мафит-ультрамафитовых интрузиях. Значимых месторождений любого типа в родинийском цикле (1.35–0.75 млрд лет), не известно. В пангейском (0.75–0.19 млрд лет) и амазийском (<0.19 млрд лет, не завершен) циклах промышленное значение имеют только подиформные месторождения в ультрамафитах офиолитовых комплексов. Такое распределение месторождений в геологическом временикоррелируется с модельными представлениями о теплофизической эволюции Земли.

At the beginning of the 60s of the last century at the Nittis-Kumuzhya-Travyanaya (NKT) ore vein field of the Northern Chamber of Monchepluton (Kola Peninsula), was discovered a large body of funnel-shaped ore pegmatite, composed of Cu-Ni sulfide ores and a gabbronorite matrix. In terms of localization conditions, its size, structure, enrichment in sulfides and PGE, it is a unique formation that has no analogues among other layered complexes of the Paleoproterozoic age of the Fennoscandian shield. Ore pegmatite occurs in the upper part of the layered zone of Nittis mont., composed of harzburgites and orthopyroxenites. Its horizontal size is 9 × 16 m, vertical – 15 m. Three zones are distinguished in the structure of the body: I – core of continuous sulfides, II – coarse- and giant-grained gabbronorites, enriched in interstitial sulfides, and III – contact zone with sideronite sulfides with gradual transitions into host orthopyroxenites. Petro- and geochemical, mineralogical and isotope studies of rocks and ores were performed using modern analysis methods. Ore pegmatite is most recent product in the processes of fractional crystallization of a magmatic melt, which separated in the form of a large schlier with a sulfide core and a silicate matrix with a high concentration of fluids. The hypsometric level of melt stop was determined by the equilibrium of the internal pressure of the volatiles and the external pressure of the overlying rocks. The δ18O values (+4.9–+6.1‰) are close to mantle marks (δ18O = +5.7‰) and correspond to basic igneous rocks. In the history of the formation of the ore pegmatite, early magmatic, late- and post-magmatic stages of mineral formation with a consistent increase in the role of fluid components (H2O, CO2, Cl, F) are distinguished. According to the results of calculations performed using various mineral geothermometers, crystallization of the melt at the magmatic stage occurred in the range of ~1100–900оC at a pressure of about 5 kbar. At temperatures of 1100–1000оC, separation (liquation) of the immiscible sulfide liquid began. As the temperature drop, the main silicate minerals (clino- and orthopyroxenes, plagioclase) initially crystallized, in the interstices of which residual melt and sulfide liquid enriched in PGE, Au, Ag and chalcophile elements (As, Sn, Sb, Te, Bi) accumulated, Pb, Zn). A late magmatic association (pargasite, magnesian hornblende and phlogopite) was formed from the residual melt. When the sulfide liquid cooled at a temperature of ~1000оC and below, a copper-containing a monosulfide solid solution (Mss) separated from it. As a result of its solid-phase transformations, pyrrhotite, pentlandite, chalcopyrite, precious metal minerals and an intermediate solid solution (Iss) were formed, which at temperatures below 550оC successively decomposed into chalcopyrite, pyrrhotite and cubanite. The dominant PGE minerals are: maychenerite PdBiTe, sobolevskite Pd(Bi,Te) frudite PdBi2, merenskite PdTe2 and moncheite PtTe2. Au and Ag minerals are represented by electrum (AuAg) and hessite (Ag2Te). A rare mineral in nature – servelleite (Ag4TeS) – has been discovered. The Pd content in ore pegmatite varies in the range (64.13–0.09 ppm), Pt – (2.70–0.004 ppm). The ore potential of Monchepluton is far from being exhausted, therefore, elucidating the genetic features of the origin of ore pegmatites and their connection with copper-nickel mineralization is not only petrological, but also of important practical significance, including to develop search signs on the vein type of PGE–Cu–Ni ores.