The Vuoriyarvi Devonian carbonatite–ijolite–pyroxenite–olivinite complex comprises several carbonatite fields: Neske Vara, Tukhta-Vara, and Petyayan-Vara. The most common carbonatites in the Tukhta-Vara and Neske-Vara fields are calciocarbonatites, which host several P, Fe, Nb, and Ta deposits. This paper focuses on the Petyayan-Vara field, in which the primary magmatic carbonatites are magnesian. The least altered magnesiocarbonatites are composed of dolomite with burbankite and are rich in REE (up to 2.0 wt. %), Sr (up to 1.2 wt. %), and Ba (up to 0.8 wt. %). These carbonatites underwent several stages of metasomatism. Each metasomatic event produced a new rock type with specific mineralization. The introduction of K, Si, Al, Fe, Ti, and Nb by a F-rich fluid (or fluid-saturated melt) resulted in the formation of high-Ti magnesiocarbonatites and silicocarbonatites, composed of dolomite, microcline, Ti-rich phlogopite, and Fe–Ti oxides. Alteration by a phosphate–fluoride fluid caused the crystallization of apatite in the carbonatites. A sulfate-rich Ba–Sr–rare-earth elements (REE) fluid (probably brine-melt) promoted the massive precipitation of ancylite and baryte and, to a lesser extent, strontianite, bastnäsite, and synchysite. Varieties of carbonatite that contain the highest concentrations of REE are ancylite-dominant. The influence of sulfate-rich Ba-Sr-REE fluid on the apatite-bearing rocks resulted in the dissolution and reprecipitation of apatite in situ. The newly formed apatite generation is rich in HREE, Sr, and S. During late-stage transformations, breccias of magnesiocarbonatites with quartz-bastnäsite matrixes were formed. Simultaneously, strontianite, quartz, calcite, monazite, HREE-rich thorite, and Fe-hydroxides were deposited. Breccias with quartz-bastnäsite matrix are poorer in REE (up to 4.5 wt. % total REE) than the ancylite-dominant rocks (up to 11 wt. % total REE).

This article is devoted to the geology of titanium-rich varieties of the Petyayan-Vara rare-earth dolomitic carbonatites in Vuoriyarvi, Northwest Russia. Analogues of these varieties are present in many carbonatite complexes. The aim of this study was to investigate the behavior of high field strength elements during the late stages of carbonatite formation. We conducted a multilateral study of titanium- and niobium-bearing minerals, including a petrographic study, Raman spectroscopy, microprobe determination of chemical composition, and electron backscatter diffraction. Three TiO₂-polymorphs (anatase, brookite and rutile) and three pyrochlore group members (hydroxycalcio-, fluorcalcio-, and kenoplumbopyrochlore) were found to coexist in the studied rocks. The formation of these minerals occurred in several stages. First, Nb-poor Ti-oxides were formed in the fluid-permeable zones. The overprinting of this assemblage by residual fluids led to the generation of Nb-rich brookite (the main niobium concentrator in the Petyayan-Vara) and minerals of the pyrochlore group. This process also caused niobium enrichment with of early generations of Ti oxides. Our results indicate abrupt changes in the physicochemical parameters at the late hydro (carbo) thermal stage of the carbonatite formation and high migration capacity of Ti and Nb under these conditions. The metasomatism was accompanied by the separation of these elements.

The Vuoriyarvi Devonian alkaline–ultramafic complex (northwest Russia) contains magnesiocarbonatites with rare earth mineralization localized in the Petyayan-Vara area. High concentrations of rare earth elements are found in two types of these rocks: (a) ancylite-dominant magnesiocarbonatites with ancylite–baryte–strontianite–calcite–quartz (±late Ca–Fe–Mg carbonates) ore assemblage, i.e., “ancylite ores”; (b) breccias of magnesiocarbonatites with a quartz–bastnäsite matrix (±late Ca–Fe–Mg carbonates), i.e., “bastnäsite ores.” We studied fluid inclusions in quartz and late-stage Ca–Fe–Mg carbonates from these ore assemblages. Fluid inclusion data show that ore-related mineralization was formed in several stages. We propose the following TX evolution scheme for ore-related processes: (1) the formation of ancylite ores began under the influence of highly concentrated (>50 wt.%) sulphate fluids (with thenardite and anhydrite predominant in the daughter phases of inclusions) at a temperature above300–350 °C; (2) the completion of the formation of ancylite ores and their auto-metasomatic alteration occurred under the influence of concentrated (40–45 wt.%) carbonate fluids (shortite and synchysite–Ce in fluid inclusions) at a temperature above 250–275 °C; (3) bastnäsite ores deposited from low-concentrated (20–30 wt.%) hydrocarbonate–chloride fluids (halite, nahcolite, and/or gaylussite in fluid inclusions) at a temperature of 190–250 °C or higher. Later hydrothermal mineralization was related to the low-concentration hydrocarbonate–chloride fluids (<15 wt.% NaCl-equ.) at 150–200 °C. The presented data show the specific features of the mineral and fluid evolution of ore-related late-stage hydrothermal rare earth element (REE) mineralization of the Vuoriyarvi alkaline–ultramafic complex.

The Precambrian rocks of the Keivy Terrane reveal five types of carbonaceous matter (CM): Fine-grained, flaky, nest, vein, and spherulitic. These types differ in their distribution character, carbon isotope composition, and graphitization temperatures calculated by the Raman spectra of carbonaceous material (RSCM) geothermometry. Supracrustal rocks of the Keivy Terrane contain extremely isotopically light (δ¹³C_PDB = –43 ± 3‰) carbon. Presumably, its source was a methane–aqueous fluid. According to temperature calculations, this carbon matter and the host strata underwent at least two stages of metamorphism in the west of the Keivy Terrane and one stage in the east. The CM isotope signatures of several samples of kyanite schists (δ¹³C_PDB = –33 ± 5‰) are close to those of oils and oil source rocks, and they indicate an additional carbon reservoir. Thus, in the Keivy territory, an oil-and-gas bearing basin has existed. Heavy carbon (δ¹³C_PDB = −8 ± 3‰) precipitated from an aqueous CO₂-rich fluid is derived from either the lower crust or the mantle. This fluid probably migrated from the Keivy alkaline granites into the surrounding rocks previously enriched with “methanogenic” carbon.

This paper reports the results of an investigation of metasomatic alterations in basement gneisses within the contact aureole of the Ozernaya Varaka alkaline ultrabasic massif. Based on mineralogical and geochemical data, the metasomatic rocks were classified, variations in major and trace elements were evaluated along a cross-section through the contact zone, the mobility of components during the metasomatic transformation of rocks was estimated, and the gain or loss of components was quantified. The obtained results provided insight into the trends of changes in mineral assemblages, the mechanisms of migration of fluid flows, and, most importantly, the scale of input or removal of major and a number of trace components at different stages of the evolution of the metasomatic system within the contact aureole and border zones. It was shown that a fluid phase was released directly from a magma chamber filled with alkaline ultrabasic melt, which is indicated by the high ³He/⁴He value (5.9 × 10⁻⁶) of the fluid, corresponding to 50% of the mantle component. It was shown that the release of a considerable amount of fluid phase related to the alkaline ultra-basic melt was not accompanied by a significant input of high field strength and rare earth elements. In contrast, the fluids of the subsequent carbonatite stage were enriched in volatile components (F, Cl, P, and S), which resulted in the migration of REE into the fenitized zones and removal of Ta, Zr, and Hf from it. The low ³He/⁴He value of this fluid suggests that its noble gases were contaminated by a crustal component, and solutions percolated along fracture systems and shatter zones from the root of the magmatic system. During the final postmagmatic stage, alkaline metasomatism was caused mainly by aqueous fluids and was accompanied by the removal of all rare earth elements and most of the high field strength elements (Nb, Ta, Zr, and Hf). A comparison of the fenitized zone of the Ozernaya Varaka complex with those of the Khibiny and Lovozero agpaitic syenite massifs showed that the metasomatic reworking of gneisses in the multiphase alkaline ultrabasic complex occurred in several stages and was accompanied by the input and removal of components, whereas the single-stage fenitization in the agpaitic syenite plutons involved minor amounts of heterogeneous water-fluoride fluids and resulted in extensive migration of rare earth and high-field strength elements in the contact zone.