The determination of rare earth elements (REEs) and associated trace elements (Sr, Ba) in fluorite (CaF2) is essential for geochemical tracing of hydrothermal processes and ore deposit characterization. However, routine multi-element analysis often relies on costly and complex techniques such as inductively coupled plasma mass spectrometry (ICP-MS). This study presents the first application of total-reflection X-ray fluorescence (TXRF) spectrometry to the quantitative analysis of Sr, Ba, and REEs (Y, La, Ce, Pr, Nd, Sm) in fluorite. Samples were digested using a mixture of HF-HNO3-HClO4, and the resulting solutions with Ga as an internal standard were analyzed by TXRF. The method was validated using an in-house synthetic CaF2 standard, whose REE concentrations were independently verified by solution ICP-MS. For the synthetic standard, TXRF showed good agreement with ICP-MS for Y, Ce, and Pr, but systematic underestimation was observed for La, Nd, and Sm due to spectral interferences from overlapping L-lines in the energy-dispersive spectrum. Element-specific correction factors (k = 1.33-1.69) were derived from the synthetic standard to compensate for these biases. Analysis of five natural fluorite samples demonstrated that TXRF results for all elements were consistent with those from ICP-MS for concentrations above similar to 100 mu g/g, with recoveries typically ranging from 85% to 115%. The proposed TXRF method, utilizing a common acid digestion, presents a cost-effective and robust alternative to ICP-MS for the routine multi-element analysis of fluorite, particularly suitable for samples with elevated REE concentrations common in many hydrothermal and magmatic deposits.

The Chuktukon REE-Nb deposit occurs within the carbonatite laterite supergene zone Chadobets upland, South of the Siberian craton. A unique laterite profile up to 600 m thick was identified by deep drilling. The laterite developed upon ultramafic lamprophyres and Ti-Th-REE-rich carbonatites. Here we report new mineralogical and geochemical data on carbonatites and laterites that reveal the evolution of supergene processes in the Chuktukon deposit within vertical zonation: (from top to bottom): 1) laterite horizon or ferruginous cap rock (ferricrete); 2) loose ochreous weathering products (ochre zone or regolith zone); 3) secondary phosphate enrichment horizon (apatite-rich zone); 4) unevenly altered disintegrated carbonatites. The bulk contents of CaO and P2O5 increases from primary carbonatite upwards due to the crystallization of carbonate-fluorapatite, and decrease in near-surface horizon due to the high leaching of rocks. The total REE2O3 content is highest in the rocks of the regolith zone and the enriched zone due to the redistribution and concentration of these elements both in the form of impurities and as a result of the redeposition of their own mineral phases. The variations in compositions of apatite, pyrochlore and monazite from different horizons of laterite profile were chosen as a proxy for the REE distribution in the supergene zone. Fluorapatite is gradually being replaced by hydroxylfluorapatite and carbonate-hydroxyl-fluorapatite, as the degree of weathering of primary carbonatites increases. Pyrochlore-Ca-Na-F is being replaced by the (Ba-Sr)-variety (with Ce and Pb) during the same processes. Monazite changes from (Ce)-dominated to (La)-dominated. This is influenced by the oxidation factor, the consequence of which is also the formation of cerianite in the upper horizon. The study of the mineral and geochemical features of such an object as Chuktukon provides an improved understanding of the patterns of redistribution of the main and ore components of carbonatites in the supergene zone and can be useful in the study of other supergene deposits and delineation of dispersion halos of ore components.

The alkaline-ultrabasic carbonatite complex Arbarastakh is located in the southwestern part of the Siberian craton. In addition to ultrabasic rocks such as pyroxenites and ijolites, various types of carbonatite dikes, phoscorites, and aillikites are present in the massif. Based on their modal and compositional characteristics, as well as the chemical composition of minerals, the rocks of the Arbarastakh complex have been divided into three groups: "aillikite," "phoscorite", and "alkaline-silicate and carbonatite" groups. The chemical compositions of olivines, phlogopites and spinellides indicate that aillikites are the least differentiated rocks in the complex. The compositional differences of micas from the "phoscorite" and "alkaline-silicate and carbonatite" groups support the liquation of the aillikite melt into two immiscible silicates and CPIO (carbonate-phosphate/iron-oxide-rich) melts. After liquation, for the "phoscorite" and "alkaline-silicate and carbonatite" groups, evolution follows fractional crystallization. Pyroxene-phlogopite-calcite and calcite carbonatites probably resulted from metasomatic alteration of silicate phases by apatite-dolomite carbonatites, which is confirmed by structural-textural features and the overlapping compositions of phlogopites, apatites and pyroxenes from calcite carbonatites and those from pyroxenites and ijolites.

The Srednyaya Zima alkaline-ultramafic carbonatite complex is located in the Eastern Sayan and is a part of the area of manifestation of Neoproterozoic rare-metal alkaline-carbonatite magmatism along the southern and southeastern margins of the Siberian craton. Mineralogical studies of calciocarbonatites of the Srednyaya Zima complex have shown the presence of primary magmatic mineral phases of calcite, biotite (annite-phlogopite), ilmenite, and fluorapatite. Pyrochlore, zircon, burbankite, magnetite, rutile, titanite, strontianite, and barite were identified of the accessory minerals. The chemical composition of the magmatic minerals of the Srednyaya Zima carbonatites is similar to the mineral composition of the closely aged carbonatite complexes Belaya Zima and Arbarastakh. The rare-element and structural analysis of zircon from carbonatites showed the presence of two zones – a magmatic core and areas of recrystallization. U-Pb dating of igneous zircon showed the age interval of its crystallization – 637±4 Ma, which coincides with the geochronology of the formation of alkali-ultramafic rare-metal complexes along the southern margin of the Siberian craton. The formation of Neoproterozoic alkaline-carbonatite complexes is associated with tectonic events of the breakup of the Rodinia supercontinent.

Late Neoproterozoic Arbarastakh alkaline ultramafic carbonatite-phoscorite complex in the southern margin of the Siberian craton (Aldan-Stanovoy shield), includes carbonatites and phoscorites closely associated with pyroxenites-ijolites and ultramafic lamprophyres. Major and trace elements data, Sr, Nd and Pb isotope com-positions for the Arbarastakh rocks have been obtained to characterize the sources involved in their formation, primary melt composition and to build the petrogenetic model.All rock varieties, excluding nepheline syenites, are characterized by incompatible elements enrichments, including light rare earth elements, and strong fractionation of REEs. The initial isotope ratios of the analyzed samples, calculated at 645 Ma, display limited variations: epsilon Nd from +5.9 to +6.9 and 87Sr/86Sr from 0.70225 to 0.70272, excluding nepheline syenite with epsilon Nd +5.4. The initial Pb isotope ratios for the most studied samples overlap with each other within uncertainties. They yield an age of 642 +/- 5 Ma. The Nd and Sr isotope data of the Arbarastakh rocks generally fit the patterns of the other Neoproterozoic alkaline ultramafic carbonatite com-plexes of the southwestern and southern margins of the Siberian craton. Observed Sr, Nd and Pb isotope vari-ations indicate mixing of the asthenosphere and the depleted mantle components.It is supposed that the primary melts for the Arbarastakh rocks were generated directly by low-degree melting of metasomatic phlogopite-carbonate veins with apatite and Ti-oxides in garnet peridotite, which formed shortly before the onset of melting. Ultramafic lamprophyre (aillikite) is closest to the primary melt composition in terms of high #MgO, Cr and Ni. The aillikite and latter pyroxenite crystallized from primitive melt by fractionating the olivine, phlogopite and clinopyroxene dominated mineral assemblage that was free of feldspar. This fractionation forced Na-enrichment in the magmas resulted in liquid silicate-carbonate immiscibility. Major and trace element and isotope data indicate that the nepheline syenites are unlikely to be related with other alkaline silicate rocks through fractional crystallization and could have been formed through pre-emplacement interaction with the ambient crustal materials. Emplacement of calcite carbonatites resulted in metasomatism of surrounding py-roxenites with formation of clinopyroxene-phlogopite-calcite carbonatites. Subsequent portions of Fe-P-enriched carbonatite magmas crystallized phoscorites and apatite-dolomitic carbonatites.

Geochronological studies have been made on the main types of rocks from the Burpala alkaline massif and two gabbro and gabbro-diorite dikes located nearby. U-Pb LA-SF-ICP-MS zircon geochronology for the Burpala massif yielded a date of 294±2 Ma for gabbro crystallization, 607±4 Ma for diorite crystallization, 298±2 and 296±2 Ma for melanocratic alkaline and nepheline syenite crustallization, 291±2 and 293±3 for quartz and quartz-containing syenite crystallization, and 293±3 Ma for alkaline granite crystallization. The stage of formation of the massif rocks (298–291 Ma) coincides with the formation period of alkaline rocks from the Synnyr pluton and some alkaline complexes of the Vitim segment of Transbaikalia and confirms a widespread occurrence of post-collisional alkaline magmatism in Western Transbaikalia.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23070401

The mineral fluorite (CaF2) has wide variations in physicochemical conditions of formation and crystallizes in magmatic, hydrothermal and sedimentary processes. Rare Earth Elements in minerals & scy;an indicate parameters of its formation, therefore it can be considered as geochemical markers or indicator of the genetic characteristics of fluorite-bearing rocks, including ore metallogenic specificity. In this work, the behavior of rare earth elements (REE) in fluorites of carbonatites of folded formation belts is studied using the example of polymetallic calciocarbonatites of Eastern Taimyr - ore fields Orientirnoye and Zeleninskoye, and Fe-F- REE siderite carbonatites of Central Tuva (Central Asian folded region) - Karasug and Ulatai ore fields. The objects have a number of similar geological and genetic characteristics: the rocks are located in tectonic fault zones formed under conditions of rifting and intraplate tectonic-magmatic activity. Carbonatite ore bodies take the form of dykes, stockworks, stocks and explosion pipes, and the main rock-forming minerals are calcite and siderite, fluorite, barite and bastn & auml;site-(Ce). The quantitative rare earth composition (La-Lu, Sr, Y, U and Th) of fluorites was determined using the LA-ICP MS and SEM-EPMA methods. Studies of the rare earth composition of fluorites have shown that objects have their own characteristics of REE distribution, reflecting the specifics of crystallization of the mineral. Fluorites of the East Taimyr region are sharply different and have signs of a crustal component - low elemental contents (& sum;REE < 2.8 ppm), weak negative Ce and Yb anomalies. However, the predominance of medium REE over light and heavy REE, namely the dome-shaped configuration of the spectra, can be explained by atypical magmatic genesis associated with interaction with the crustal component. Fluorites of carbonatites of the Karasug and Ulatay fields are characterized by the highest concentrations of rare earth elements (& sum;REE = 502-7,352 ppm). Chondrite-normalized REE values showed that fluorites are divided into two types. The first type is characterized by a sharp predominance of light elements over heavy ones and a positive europium anomaly (Eu/Eu)n* up to 3.7). Depletion of light rare earth elements and a gentle slope of the spectra characterize the second type. Moreover, the only difference between the Karasug and Ulatai fluorites of the second type is the significant depletion of La in the first type. The results of the study showed that fluorites have certain distribution configurations of rare earths, which reflects the characteristics of the mineral crystallization environment for each object and characterizes the specific composition of fluorites of folded formation belts.

A thermodynamic study was carried out in order to determine the forms of transport for the entire series of lanthanides and their ratio with changing parameters of a hydrothermal fluid of moderate concentrations of chloride, carbonate and fluoride components. Hydrothermal solution, cooling from 500 to 100 ∘C, affected barite and celestine, which are used as a source of sulfate sulfur, monazite as a source of rare earth elements (REE) and phosphorus, and calcite as a source of calcium. It has been established that, under weakly acidic (pH about 4.1) conditions, the equilibrium mineral association is represented by rare earth fluorite, monazite, rare earth fluorapatite, and strontiobarite. In the high-temperature region for light and medium REE, the leading is the first chlorocomplex LnCl+2. For heavy REE, the second fluorine complex LnF+2 takes the first place, except for terbium and dysprosium, for which a sharp predominance of the sulfate complex is revealed. A special picture is observed at 100 ∘C: the leading position is occupied by Ln+3 for both light and heavy REE. In the case of a near neutral weakly alkaline fluid (pH about 7.1), the equilibrium mineral association is represented by calcite, monazite, REE-fluorite, REE-fluorapatite, strontiobarite, and strontianite. The appearance of the latter in natural associations may serve as an indication of the increased alkalinity of the ore-forming environment. In an equilibrium weakly alkaline fluid up to 200 ∘C, hydroxocomplexes are prevalent for all REEs with the ratio Ln(OH)03> Ln(OH)+2. The first chloro complex for light REE at 500–400 ∘C, and the second fluoro complex for medium and heavy REEs follow them. At 100 ∘C, the concentration of hydroxocomplexes sharply decreases, and fluorine and carbonate complexes come to the fore. In general, there is an increased stability of the first chlorocomplex in the high-temperature region, and with decreasing temperature, the role of REE fluorocomplexes increases. Two variants of acidity-alkalinity calculations presumably correspond to modeling of two types of fluids: greisenizing – weakly acidic and carbonatite-forming – weakly alkaline.

The carbonatite complexes of the Central Asian carbonatite province comprise the Siberian carbonatites of the Western Transbaikalia and the Central Tuva regions, as well as those from the Mushugai-Khudag complex in Southern Mongolia. They are confined to Late Mesozoic rift structures and have endured considerable tectono-magmatic processes caused by intense plume activity, which also accompanied their formation. A systematic study of melt and fluid inclusions revealed that these carbonatites formed as a result of immiscibility processes in silicate–carbonate (salt) melts, as well as fractional crystallization. Alkaline–carbonatite rocks crystallized in the presence of brine–melts with different compositions, i.e., alkaline–fluorine, carbonate, sulfate, phosphate, and chloride. These melts are responsible for mineralization during the orthomagmatic stage and the primary phase of Fe-F-P-(Ba)-(Sr)-REE ore formation at temperature ranges of 850–830 °C, 650–610 °C, and 560–440 °C and pressures between 290 and 350 MPa. At a later stage, the brine–melts evolved into saline hydrothermal fluids, which are considered to be the source of the second stage of F-(Ba)-(Sr)-REE ore mineralization. The saline crystal–fluid inclusions consist mainly of fluorine–sulfate–carbonate–chloride and bicarbonate–chloride compositions, with temperatures of approximately 480–250 °C and pressures below 250 MPa. The shift from melt to fluid in carbonatite complexes could occur more frequently in nature than previously believed and could also apply to other F-REE carbonatite complexes that are linked to rifting and plume activity in mountain-building zones.

The Açdif gold deposit is located in the Zenaga Inlier (central Anti-Atlas), approximately 120 km southwest of the city of Ouarzazate. It hosts gold mineralization associated with a shear zone affecting the metamorphic and magmatic formations of the Eburnean basement. It mainly consists of alternating micaschists, augen gneisses, and orthogneiss. These crystalline rock assemblages are intruded by a variety of granitoids. These are the Azguemerzi granitoids, which are locally affected by Eburnean deformation. Subsequently, these facies are intersected by a swarm of mafic dykes, which show a variety of rocks ranging from microgabbro-dolerite to quartz diorite. Detailed mapping, petro-mineralogical investigations, and geochemistry of the major and trace elements of these magmatic intrusions suggests the following: (i) for the granodiorite rocks (deposited before the basic dykes), a calc-alkaline affinity, with a chemical signature similar to a syn-collisional context; (ii) for mafic dykes, a contemporaneous emplacement of these mafic intrusions with an evolutionary process controlled by fractional crystallization of the same magma of continental tholeiites, whose chemical composition is comparable to that of enriched MORBs (EMORBs). These continental tholeiites are related to a distensive tectonic context that would have affected the Zenaga inlier prior to the Pan-African orogeny.

Research subject. This paper presents the results of a study of minerals of the apatite group from aillikites of the Zima alkaline-ultramafic carbonatite complex. Aim. To determine the composition of apatites to characterize the studied rocks and to elucidate the patterns of element distribution in the magma of aillikites in the late crystallization stages. Materials and Methods. Minerals of the apatite group from 6 samples (4 from dikes of the Bol'shetagninsky massif, 1 from the Bushkanai dike, 1 from the Beloziminskaya pipe) were studied. The samples showed similar textural-structural features, differing in the mineral composition of the groundmass. Results. Minerals of the apatite group are widespread in the groundmass of all the studied rocks. They are characterized by idiomorphic, subidiomorphic crystals of homogeneous composition, 10-100 microns in size. Minerals of the apatite group were identified as fluorapatite and hydroxyapatite characterized by a significant SiO2 content (up to 5 wt %), low SrO content (up to 1.5 wt %) and REE2O3 (up to 2 wt %). The studied apatites also include MgO, FeO, Na2O, SO4, and CO3. Fluorapatites from dyke aillikites are characterized by a higher silicon content as compared to apatites from the Beloziminskiy pipe. Apatites from fresh aillikites of the Bushkanai dyke have a relatively higher content of Sr, REE, and F than minerals from serpentinized samples. Conclusions. The composition of the studied apatites has a lower Sr and Ba content than orangeite and lamproite apatites. In terms of Sr, Si and REE contents, the studied apatites are similar to kimberlite and ailikite apatites. Such a difference allows minerals of the apatite group to be used as an indicator mineral for classification of kimberlites and related rocks, but only in combination with petrography and composition of other minerals of the groundmass.

Carbonatites of the Central Taimyr region, formed during the early Triassic (246.5 +/- 1.9 Ma), are coeval with other occurrences of alkaline-carbonatite magmatism of the Siberian LIP and related to the Siberian superplume activity. Petrological characteristics indicate that Central Taimyr carbonatites were derived from an alkaline-carbonatitic melt source with remarkably high concentrations of chloride-phosphate-fluoride-sulfate brines. Magmatic minerals (bearing F-Ba-Sr-REE ores), represented by calcite, fluorite, barite, bastnaesite-(Ce) and fluorapatite, were formed at a temperature range of 720-550 degrees C by brine-melts. Whereas, hydrothermal mineral assemblages (bearing F-Ba-Sr-REE ores), including magnetite, hematite, monazite-(Ce), parisite-(Ce), fluorite, roscoelite, rutile (Nb), barite, quartz, calcite, dolomite, as well as sulfides and uraninite, occur within a late-stage ore-forming processes, for which mineralized fluids of sulfate-fluoride-chloride-carbonate and chloride-hydrocarbonate compositions were responsible. Significant changes in the composition of fluid inclusions occurred as the homogenization temperature varied from 460 to 230 degrees C, and concentrations from 55 to 25 wt% NaCl-eq, respectively.

Experimental studies were conducted to identify the physical and chemical features of gold’s behaviour in hydrothermal processes linked to ore formation and involving CO2 in oxidized deposits. With the aid of the autoclave method, in a temperature range of between 200 and 400 °C, the isochoric dependences of the PVT parameters of concentrated sulphate chloride fluids were plotted, both in the presence and absence of CO2. Our experiments established that concentrated sulphate–chloride fluids (22 wt % Na2SO4 + 2.2 wt % NaCl) that lack CO2 are characterized by a wide supercritical temperature range, with homogenization temperatures of between 250 and 325 °C. In the presence of CO2, the same type of fluids showed heterogenization at a molar fraction of XCO2 = 0.18 (t = 192 °C, P = 176 bar). The process of homogenization for these low-density and high-salinity fluids was impossible at temperatures between 375 and 400 °C and at pressures between 600 and 700 bar. The behaviour of gold was studied during its interaction with a basic composition fluid of sulphate–chloride. We applied the autoclave method under the conditions of a simultaneous synthesis of pyrite and gold dissolution (metallic Au), at a temperature of 340 °C and at a pressure of 440 bar. High Au concentrations (up to 4410 ppm of Au in CO2-bearing fluids) were attained at high gold solubilities (up to 13.5 ppm in the presence of CO2), owing to the process of Au reprecipitation within the pyrite phase. We did not detect Au in the pyrite when we used the XRD or SEM methods, which suggested that it might be present as invisible gold. High values of the distribution coefficient (KD = CAu(solid)/CAu(solution)) in the fluids lacking (KD = 62) and bearing CO2 (KD = 327) empirically confirmed the possibility that gold concentrates in pyrite in structurally non-binding forms.

The Chadobets alkaline-carbonatite complex (Siberian craton) is a natural laboratory for all varieties of ultramafic lamprophyres, including damtjernites formed by fluid-explosion mechanisms, which contain a large number of pelletal lapilli. Data obtained from comprehensive mineralogical, structural and chemical studies of these pelletal lapilli show strong similarity with the main magmatic mineral assemblage of damtjernites, suggesting a juvenile composition for them. The composition of phlogopite, carbonate and fluorapatite in pelletal lapilli is mostly constrained toward primitive compositions (beginning) of mineral crystallization trends in ultramafic lamprophyres. According to the petrographic and mineralogical features found in pelletal lapilli from damtjernites, these can be divided into three types based on the conditions and depth of formation. Estimated late magmatic temperatures of pelletal lapilli mica and fluorapatite formation vary from 815 to 990 °C.

The article presents new data on the age and isotopic (Sr, Nd) characteristics of the Yuzhnoe and Ulan-Ude REE-fluorite occurrences, paragenetically related to alkaline carbonatite magmatism. Age estimates of the fluorite-containing rocks were obtained from bastnaesites using U-Th-Pb (LA-ICP-MS) method and are 130.2±1.1 and 136.6±1.9 Ma for the Yuzhnoe and Ulan-Ude occurrences, respectively. The Ɛ Nd (T) values of the bastnaesites vary from –7.41 to –6.08 for the Yuzhnoe occurrence and from –4.28 to –2.67 for the Ulan-Ude occurrence. The Yuzhnoe carbonatites are characterized by 87 Sr/ 86 Sr (I) ratios ranging from 0.705883 to 0.706011, and 87 Sr/ 86 Sr (I) ratios obtained for the Ulan-Ude bastnaesite-fluorite rocks are ranging from 0.70683 to 0.70687. The age estimates are consistent with the published geochronological data on alkaline carbonatite magmatism of the Central Asian orogenic belt related to Late Mesozoic intraplate magmatism and rifting. Isotopic Sr-Nd signatures of bastnaesite, as well as of the Yuzhnoe carbonatites and the Ulan-Ude bastnaesite-fluorites, indicate that their source rocks came from the enriched lithospheric mantle.

Characterized by its copper-silver sulfide mineralization, the Tagmout deposit (Eastern Saghro, Anti-Atlas, Morocco) is hosted in plutonic magmatic rocks (quartz-monzodiorite and granodiorite). The sulfide mineralization occurs in the form of disseminations, veinlets/stockworks crosscutting the granitoids, and is associated with quartz and carbonate gangue and adularia, sericitic and argillic alteration of the host rocks. Some silver occurs in the form of the silver-copper sulfides stromeyerite, mckinstryite and jalpaite. These minerals occur sporadically in the copper sulfide ores in association with other copper-bearing phases (mainly chalcopyrite II, bornite II and chalcocite II). Textural relationships show that these minerals are derived from pre-existing argentiferous copper sulfides (bornite I, chalcocite I and chalcopyrite I) as reaction products. Electron probe microanalyses indicate that the structural formula of the Ag-Cu sulfides from Tagmout deposit is slightly different from the theoretical composition and from that of Ag-Cu sulfides elsewhere. Stromeyerite, mckinstryite and jalpaite from Tagmout have average compositions Cu1.02Ag0.89Fe0.05S1.02, Cu3.24Ag4.35 Fe-0.34 S-4.06 and Cu1.17Ag2.70 Fe-0.10 S-2.01, respectively. These compositions show a deficiency in Ag and a slight excess in Cu and S, compared to these minerals reported elsewhere. This Ag deficiency reflects the significant Fe contents of these sulfides. Iron content reach values of up to 7.61 wt%, Fe likely substituting for Ag by Ag++Cu+ => Fe2+ in the crystal structure. The Tagmout deposit is to our knowledge the fifth occurrence in the world where all three Ag-Cu sulfides occurs close to each other. Silver at Tagmout deposit was not incorporated in the early Pb and Zn sulfides, but was transported in the ore-forming fluid, where it formed solid solutions in the early copper sulfides. It was later remobilized by lower temperature-high salinity fluids resulting in the deposition of primary silver-copper minerals associated with secondary silver-copper sulfides.

This paper is devoted to a new geochronological data for the main rocks that make up the Arbarastakh alkaline ultramafic carbonatite complex, and includes information on the mineralogical and petrographic features of these rocks. The Arbarastakh complex is located in the Republic of Sakha (Yakutia) in the southeast of Russia, within the basin of the Arbarastakh River, in the Lena River basin. Tectonically, the complex is located in the eastern part of the Aldan shield on the southeastern margin of the Siberian craton. The main structural feature of the Arbarastakh complex is its concentric-zonal structure, expressed in the regular alternation of rocks with different compositions and different occurrence conditions. The main phase of the complex composes a stock-like body of alkaline pyroxenites, which has a rounded-ellipsoid shape in plan view. Between the body of pyroxene rocks and the ring fault there is a continuous zone of fenites - contact-metasomatic rocks. Dikes of alkaline syenites, ijolite-urtites and melteigites, are found in the area of the alkaline complex. Dikes of alkaline syenites are localized in the endocontact zone of the complex. The carbonatites of the Arbarastakh complex are concentrated within the core of pyroxene rocks and occur as a series of incompletely conical dike-like bodies. The most common are calcite varieties of carbonatites with pyrochlore and phlogopite, as well as silicocarbonatites. Veins of dolomitic and ankeritic carbonatites occur much less frequently in pyroxenites. Geochronological studies of alkaline rocks of the Arbarastakh alkaline-ultramafic carbonatite complex reflect a rather long history of the formation of the complex and characterize the multi-stage intrusion of rocks. The results of Ar-Ar dating showed the time of crystallization of the main phase of the complex - pyroxenites - 632.5 +/- 6 Ma; alkaline syenites - 645.9 +/- 6.4 Ma. The emplacement time of silicocarbonatites is 642.6 +/- 6.6 Ma; the formation of carbonatites occurred in the ranges of 651 +/- 6 and 657.8 +/- 6.5 Ma. U-Pb dating of pyroxenites showed an age of 638.2 +/- 3.1 Ma; time of intrusion of carbonatites is 650.3 +/- 9.8 Ma; and the crystallization age of alkaline syenite is 641.7 +/- 5.6 Ma. Thus, a certain age range of formation of rocks of the Arbarastakh complex is similar to 657-636 Ma, which is within the range of values for other alkaline carbonatite complexes, which structurally gravitate towards the marginal parts of the Siberian Craton and are characterized by similar types of rare-metal deposits.

In this study, we discuss mineral chemistry data, melt inclusion study results, and report Ar-Ar phlogopite age for the aillikite dikes of the Arbarastakh alkaline-carbonatite complex on the Aldan-Stanovoy shield, Russia. Aillikite was crystallized at 631 +/- 8.5 Ma, coeval with the intrusion age of the Arbarastakh rocks. The Arbarastakh complex was formed during the late Neoproterozoic epoch of rare earth element-Nb ore-bearing alkaline-carbonatite magmatic activity that was widespread on the southwestern and southern margins of the Siberian craton, related to rifting processes during the breakup of the supercontinent Rodinia. The aillikites show mineralogical characteristics of primitive magmas such as highly forsteritic olivine, Mg-ilmenite, and Cr-rich spinel. The variance in olivine zonation, morphologies, and chemical element distribution indicate that olivine in the aillikites is represented by several genetic types: xenogenic olivines (Fe-poor cores) from the sheared peridotite, olivine antecrysts (Fe-rich cores) related to mantle metasomatism by preceding proto-aillikite melt, and olivine phenocrysts formed during crystallization of aillikite melt. The latter shows decreasing Ni and Cr due to fractional crystallization of olivine, ilmenite, and chromite, along with increasing Mn and Ca concentrations that are consistent with enrichment of these elements in the residual melt. The olivine phenocrysts chemistry shows variations that are characteristic of the presence of phlogopite and carbonate in the mantle source (low 100*Ca/Fe (0.4-1.2) and 100*Mn/Fe (1-2), moderate 100*Ni/Mg (1.4-0.4)). Spinel shows a wide compositional variation with two compositional zoning trends, one of which follows the titanomagnetite trend, while the other follows the qandilite-rich magnesio-ulvospinel-magnetite one. The latter trend indicates an increase in fO(2) and attendant Fe oxidation to Fe3+ during crystallization. Ilmenite composition evolution (from Mg-rich to Mn-rich) also reflects the carbonate-rich nature of aillikite melt. We identify primary melt inclusions hosted in phlogopite and secondary melt inclusions in olivine; both melt inclusions types have daughter minerals dominated by dolomite, calcite, Na-Ca carbonates, phosphates, and phlogopite, consistent with the carbonate-rich nature of aillikite melt. The calculated temperatures reflect the early stage of aillikite crystallization, with values ranging from 1169 to 1296 degrees C and fO(2) values (olivine-spinel pair) varying from +0.40 to +1.03 Delta FMQ and from Delta NNO -0.9 to Delta NNO -2.0 (perovskite oxygen barometer); in contrast, the homogenization temperature of the secondary melt inclusions in olivine (700-720 degrees C) characterizes late-stage aillikite melt evolution. The carbonate-rich nature of the Arbarastakh aillikite and its similar age to the carbonatites are consistent with a genetic relationship between them.

Thermodynamic calculations on the impact of carbonate–bicarbonate fluids cooling from 500 to 100°C on a monazite + calcite association with a simultaneous decrease in pressure from 2000 to 125 bar are performed. It is shown that the REE carbonate–bicarbonate complexes gain importance only at low temperatures. In this case, the total concentration of lanthanides in the fluid turns out to be rather low. This indicates that, in the presence of even insignificant amounts of fluorine and calcium in the hydrothermal system, carbonate–bicarbonate fluids do not lead to the removal of REEs but contribute to their accumulation in the form of precipitating fluorocarbonates. Thus, the role of carbonate-bicarbonate hydrothermal fluids in the transport and deposition of rare earth elements has been assessed for the first time for a wide range of TP-parameters and for the entire series of lanthanides.