Beryl is both an accessory and a rock-forming mineral in pegmatites that contain beryl, making it a major source of Be. Beryl-bearing pegmatites of the Shongui deposit, located in the Kola province of the Northeastern Fennoscandian Shield, hold beryl with a yellowish-greenish color. An investigation into the chemical composition of this beryl from pegmatite dike No. 7 has been performed for the first time via the secondary ion mass spectrometry (SIMS) technique, and the chemical composition of the beryl-bearing pegmatites has been analyzed for the first time by the inductively coupled plasma mass spectrometry (ICP-MS) method. These pegmatites have high concentrations (ppm) of Be (11.8), Li (30.9), Rb (482), Nb (50.3), Ta (14.6), Cs (66.8), and Mn (283) and low concentrations of Sr, Y, Ba, rare earth elements (REE), Zr, and Th. In the Shongui pegmatite field, concentrations of Be, Li, Rb, Cs, Nb, Ta, and Mn increase from barren to beryl-bearing pegmatites, whereas concentrations of Ba, Sr, Y, and REE decline. Rb/Ba, Rb/Sr, and Zr/Hf ratios, showing the fractionation degree, change from the barren to beryl-bearing pegmatites: Rb/Ba and Rb/Sr increase from 111 and 0.46 to 1365 and 8.06, respectively, and Zr/Hf decreases from 18.9 to 14.5. The chemical composition of beryl from the Shongui deposit is unique. This mineral has a concentration of 25,300 ppm of alkalis (Li, Cs, K, Rb, Na) and the average Li, Ce, and Na content is 4430, 5000, and 15,400 ppm, respectively. According to its chemical composition, the Shongui beryl belongs to the Li-Cs-Na type, a type that is not recognized in the available classifications. It is supposed that this beryl was mainly crystallized in the magmatic stage rather than in any hydrothermal and metasomatic stages. Two beryl groups have been distinguished in beryl-bearing pegmatite dike No. 7: beryl from the intermediate zone (Brl-I) and beryl from the core zone (Brl-II). These beryls are concluded to have crystallized in the following order: Brl-I and then Brl-II. Compared with Brl-I, Brl-II is depleted in Cs, Na, Cl, and H2O and is enriched in Fe and Mn. The Fe/Mn ratio varies from 9.18 to 16.50 in these beryls and their yellowish-greenish shades are thought to be driven by a large amount of Fe compared to Mn.
Banded iron formations (BIFs) are widespread over the world (3.81 Ga to 1.80 Ga) as the Algoma and Superior types. The paper provides new isotope (Nd-Sr, U-Pb) data on associated noble-metal mineralization in Algoma-type BIFs of the Fennoscandian Shield. U-Pb dating of accessory zircon applied to estimate the formation time of noble-metal-bearing skarn and related rocks in the Olenegorsk and Kirovogorsk BIFs, NW Fennoscandian Shield show that they occurred in the interval between the formation of basalt protolith (2.81 Ga amphibolites) and metamorphism in BIFs (2.75 Ga) to the formation of gold-bearing iron-rich skarn with native Au, Ag, Bi and tellurides (2.65 Ga) and low-temperature quartz-zeolite veins with redeposited Au (2.55 Ga). REE distribution in zircons and corresponding rocks indicated an important role of redox reactions in Fe and REE redistribution affected by reducing fluids on haematite-quartz protolith of BIFs.
The article presents new Sm–Nd and U–Pb geochronological data on rocks of the poorly studied Pados-Tundra Cr-bearing complex. It is part of the Notozero mafic–ultramafic complex (western Kola Peninsula) and occurs at the border of the Paleoproterozoic Lapland Granulite Belt and the Archean Belomorian composite terrain. The Pados-Tundra complex hosts two major zones, the Dunite and Orthopyroxenite Blocks. Dunites are associated with four levels of chromite mineralization. Isotope Sm–Nd studies of dunites, harzburgites, and orthopyroxenites from the central part of the complex have been carried out. The isochron Sm–Nd age on 11 whole-rock samples from a rhythmically layered series of the complex is 2485 ± 38 Ma; the mineral Sm–Nd isochron for harzburgites shows the age of 2475 ± 38 Ma. It corresponds with the time of large-scale rifting that originated in the Fennoscandian Shield. When the rhythmically layered series of the intrusion and its chromite mineralization were formed, hornblendite dykes intruded. The U–Pb and Sm–Nd research has estimated their age at ca. 2080 Ma, which is likely to correspond with the occurrence of the Lapland–Kola Ocean. According to isotope Sm–Nd dating on metamorphic minerals (rutile, amphibole), the age of postmetamorphic cooling of rocks in the complex to 650–600 °C is 1872 ± 76 Ma. The U–Pb age on rutile from a hornblendite dyke (1804 ± 10 Ma) indicates further cooling to 450–400 °C. The conducted research has determined the early Proterozoic age of rocks in the rhythmically layered series in the Pados-Tundra complex. It is close to the age of the Paleoproterozoic ore magmatic system in the Fennoscandian Shield that developed 2.53–2.40 Ga ago. Later episodes of alterations in rocks are directly related to main metamorphic episodes in the region at the turn of 1.9 Ga. Results of the current study expand the geography of the vast Paleoproterozoic East Scandinavian Large Igneous Province and can be applied for further studies of similar mafic–ultramafic complexes.
The Monchetundra massif is located in the north-eastern Fennoscandian Shield and refers to Paleoproterozoic massifs of the East-Scandinavian Large Igneous Province. The general section of the massif comprises two parts, the lower norite-orthopyroxenite and the upper mafic zones. The lower zone is of great interest due to its associated industrial platinum group elements (PGE) mineralization. The structure and peculiar features of rocks in the lower zone were studied using a drill core from the borehole MT-70 in the south-eastern slope of the Monchetundra massif intersecting the ore zone 1 of the Loypishnun deposit (according to the CJSC Terskaya Mining Company data). A comparison of the barren and ore-bearing varieties of norites and pyroxenites in the Loypishnun deposit shows that the ore samples have the lowest negative εNd values, a relatively more differentiated distribution spectrum with the Light rare earth elements (LREE) dominating over the Heavy REE (HREE), Eu/Eu* ≥ 1, and a higher mean content of alkali and large-ion lithophile elements (Ba, Rb, and Cs). New geochemical data indicated an origin of magmas for rocks from a layered series in the Loypishnun deposit by a high degree of melting of a LREE-rich source with a low mean content of REE. Negative εNd values, low ISr values, and a marked negative Nb indicate that the crustal material affected the evolution of rocks in the lower zone of the massif more than in the upper zone. The formation of ore bodies in the Loypishnun deposit was governed by the crust-mantle interaction, magmatic differentiation, and association with the most differentiated varieties, and by further concentration of the ore at the late and post-magmatic stages in a highly permeable environment for fluids in the Monchetundra fault zone.
Genesis of the oldest continental crust retains a marked trace in the Earth’s evolution over its 4.5 Ga history. Despite ample isotope data on the role of the continental crust in the Earth’s evolution, there has been much debate on the origin of grey gneisses and tonalite-trondhjemite-granodiorites (TTG). Precise U-Pb (ID-TIMS) and SHRIMP data on single zircon for paragneisses and TTG (3158.2 ± 8.2 Ma) have indicated the Central-Kola and Belomorian (White Sea) megablocks of the Fennoscandian Shield to be 3.16 Ga and 3.70 Ga, respectively. The newly obtained ages of zircon from these megablocks indicate the origin of the discrete continental crust to be 3.16 and 3.70 Ga. It is close to the Nordsim zircon data on the Siurua TTG (Finland), which are 3.45 and 3.73 Ga in the core. The new summarized data on the Earth’s oldest rocks (basement and continental crust) indicate the younger age of the rocks in the Fennoscandian Shield as compared to those in Australia (Kronendonk et al., 2019).
