Proterozoic Gremyakha-Vyrmes Polyphase Massif, Kola Peninsula: An example of mixing basic and alkaline mantle melts

The paper presents newly obtained data on the geological structure, age, and composition of the Gremyakha-Vyrmes Massif, which consists of rocks of the ultrabasic, granitoid, and foidolite series. According to the results of the Rb-Sr and Sm-Nd geochronologic research and the U-Pb dating of single zircon grains, the three rock series composing the massif were emplaced within a fairly narrow age interval of 1885 ± 20 Ma, a fact testifying to the spatiotemporal closeness of the normal ultrabasic and alkaline melts. The interaction of these magmas within the crust resulted in the complicated series of derivatives of the Gremyakha-Vyrmes Massif, whose rocks show evidence of the mixing of compositionally diverse mantle melts. Model simulations based on precise geochemical data indicate that the probable parental magmas of the ultrabasic series of this massif were ferropicritic melts, which were formed by endogenic activity in the Pechenga-Varzuga rift zone. According to the simulation data, the granitoids of the massif were produced by the fractional crystallization of melts genetically related to the gabbro-peridotites and by the accompanying assimilation of Archean crustal material with the addition of small portions of alkaline-ultrabasic melts. The isotopic geochemical characteristics of the foidolites notably differ from those of the other rocks of the massif: together with carbonatites, these rocks define a trend implying the predominance of a more depleted mantle source in their genesis. The similarities between the Sm-Nd isotopic characteristics of foidolites from the Gremyakha-Vyrmes Massif and the rocks of the Tiksheozero Massif suggest that the parental alkaline-ultrabasic melts of these rocks were derived from an autonomous mantle source and were only very weakly affected by the crust. The occurrence of ultrabasic foidolites and carbonatites in the Gremyakha-Vyrmes Massif indicates that domains of metasomatized mantle material were produced in the sublithospheric mantle beneath the northeastern part of the Fennoscandian Shield already at 1.88 Ga, and these domains were enriched in incompatible elements and able to produce alkaline and carbonatite melts. The involvement of these domains in plume-lithospheric processes at 0.4–0.36 Ga gave rise to the peralkaline melts that formed the Paleozoic Kola alkaline province.     

Chemical composition and geochemical characteristics of the Koksharovka alkaline ultrabasic massif with carbonatites,primorye

New geochemical data are discussed on the magmatic complexes of the Koksharovka alkaline ultrabasic massif of Late Jurassic age obtained by the ICP-MS method. Based on the first results on rare earth geochemistry of carbonatites and associating pyroxenites and geological observations, the magmatic origin of the Koksharovka carbonatites was substantiated, and the problems of formation of accompanying igneous rocks were considered.     

Composition and conditions of crystallization of zircon from the rare-metal ores of the Gremyakha–Vyrmes Massif,Kola Peninsula

The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.     

Geochemical features of rare-metal granites of the Zashikhinsky Massif,East Sayan

The paper presents detailed geochemical data on the rocks of the Zashikhinsky Massif and mineralogical–geochemical characteristics of the ores of the eponymous deposit. The rare-metal granites are divided into three facies varieties on the basis of the degree of differentiation and ore potential: early facies represented by microcline–albite granites with arfvedsonite, middle facies represented by leucocratic albite–microcline granites, and late (most ore-bearing) facies represented by quartz–albite granites grading into albitites. Microprobe data were obtained on major minerals accumulating trace elements in the rocks and ores. All facies of the rare-metal granites, including the rocks of the fluorite–rare-metal vein, define single compositional trends in the plots of paired correlations of rock-forming and trace elements. In addition, they also show similar REE patterns and spidergrams. The latter, however, differ in the depth of anomalies of some elements. Obtained geological, petrographic, and geochemical data suggest a magmatic genesis of the rocks of different composition and their derivation from a single magma during its differentiation. On the basis of all characteristics, the Zashikhinskoe deposit is estimated as one of the largest tantalum rare-metal deposits of alkaline-granite type in Russia.     

Structure,age, and ore potential of the Burpala rare-metal alkaline massif,northern Baikal region

The Burpala alkaline massif is a unique geological object. More than 50 Zr, Nb, Ti, Th, Be, and REE minerals have been identified in rare-metal syenite of this massif. Their contents often reach tens of percent, and concentrations of rare elements in rocks are as high as 3.6% REE, 4% Zr, 0.5% Y, 0.5% Nb, 0.5% Th, and 0.1% U. Geological and geochemical data show that all rocks in the Burpala massif are derivatives of alkaline magma initially enriched in rare elements. These rocks vary in composition from shonkinite, melanocratic syenite, nepheline and alkali syenites to alaskite and alkali granite. The extreme products of magma fractionation are rare-metal pegmatites, apatite-fluorite rocks, and carbonatites. The primary melts were related to the enriched EM-2 mantle source. The U-Pb zircon ages of pulaskite (main intrusive phase) and rare-metal syenite (vein phase) are estimated at 294 ± 1 and 283 ± 8 Ma, respectively. The massif was formed as a result of impact of the mantle plume on the active continental margin of the Siberian paleocontinent.     

New data on carbonatites of the Il’mensky-Vishnevogorsky alkaline complex,the southern Urals,Russia

Carbonatites that are hosted in metamorphosed ultramafic massifs in the roof of miaskite intrusions of the Il’mensky-Vishnevogorsky alkaline complex are considered. Carbonatites have been revealed in the Buldym, Khaldikha, Spirikha, and Kagan massifs. The geological setting, structure of carbonatite bodies, distribution of accessory rare-metal mineralization, typomorphism of rock-forming minerals, geochemistry, and Sr and Nd isotopic compositions are discussed. Dolomite-calcite carbonatites hosted in ultramafic rocks contain tetraferriphlogopite, richterite, accessory zircon, apatite, magnetite, ilmenite, pyrrhotite, pyrite, and pyrochlore. According to geothermometric data and the composition of rock-forming minerals, the dolomite-calcite carbonatites were formed under K-feldspar-calcite, albite-calcite, and amphibole-dolomite-calcite facies conditions at 575–300°C. The Buldym pyrochlore deposit is related to carbonatites of these facies. In addition, dolomite carbonatites with accessory Nb and REE mineralization (monazite, aeschynite, allanite, REE-pyrochlore, and columbite) are hosted in ultramafic massifs. The dolomite carbonatites were formed under chlorite-sericite-ankerite facies conditions at 300–200°C. The Spirikha REE deposit is related to dolomite carbonatite and alkaline metasomatic rocks. It has been established that carbonatites hosted in ultramafic rocks are characterized by high Sr, Ba, and LREE contents and variable Nb, Zr, Ti, V, and Th contents similar to the geochemical attributes of calcio-and magnesiocarbonatites. The low initial ⁸⁷Sr/⁸⁶Sr = 0.7044?0.7045 and εNd ranging from 0.65 to ?3.3 testify to their derivation from a deep mantle source of EM₁ type.     

Petrology and composition of rare-metal alkaline rocks in the South Gobi Desert,Mongolia

Earlier, a belt of alkali-granite plutons and a carbonatite province were discovered in the South Gobi Desert, Mongolia. The Lugingol pluton of pseudoleucitic syenites with carbonatites was assigned to the alkali-granite belt. However, new dating showed that it is 40 Myr younger than the Khan-Bogdo pluton and a large fault separates it from the alkali-granite belt. In the same part of the South Gobi Desert, a dike series of alkaline K-shonkinites with a rare-metal carbonatite vein was found by V.I. Kovalenko west of the Lugingol pluton, near Mt. Baruun Hasar Uula, and a dike series of alkali and nepheline syenites was found by us northeast of the Lugingol pluton. These data give grounds to distinguish an intrusive complex of K-alkaline shonkinites and leucitic syenites with Late Paleozoic REE-bearing carbonatites. Thus, three alkaline-rock complexes of different ages are distinguished in the South Gobi Desert. We present refined geological maps of these complexes. The plutons of all three complexes are deposits of trace elements (REE, Nb, Zr, Y, P). The chemical composition of the silicate rocks of the complex, rare-metal agpaitic pegmatites, and carbonatite and apatite rare-metal ores was considered in detail. Shonkinites from Mt. Baruun Hasar Uula and the Mountain Pass mine (United States) and their carbonatites, along with the Lugingol carbonatites, belong to a single association of K-alkaline rocks and carbonatites, as evidenced by their identical chemical, mineral, and geochemical rare-metal compositions. Rare-earth element patterns and spidergrams show similarities and differences between the rare-metal rocks of three complexes as well as paragenetic differences between their rare-metal minerals. A rare process is described—the amorphization of rare-metal minerals, related to their high-temperature crystallization in a medium with abnormal silica contents of the Khan-Bogdo pegmatites. The parental magmas of the alkali-carbonatite complexes were generated from the EM-2 contaminated mantle that had undergone recycling, whereas the parental magmas of the Khan-Bogdo agpaitic alkali granites were produced from depleted mantle.     

On the geochemistry of tantalum in the carbonatite process

The behavior of tantalum in carbonatites and related rocks of alkaline complexes was analyzed. In particular, we considered factors favorable for its accumulation in carbonatites, both in absolute amount and relative to its companion element niobium.The contents of both elements show moderate variations in earlier alkaline silicate rocks and more significant variations in carbonatites; this difference is especially pronounced for tantalum.Their simultaneous accumulation in carbonatites is controlled mainly by the affiliation to certain temperature facies, when tantalate-niobate phases with high Ta₂O₅ contents (up to 26 wt %) are formed. The accumulation of these elements with the formation of almost purely niobian pyrochlores and Ta-U pyrochlores (hatchettolites) occurs efficiently only during the formation of metasomatic zoning with the separation of purely Nb and Nb-Ta mineralization between the zones of the metasomatic column. This process is characteristic mainly of relatively deep-seated massifs, where the metasomatic processes of carbonatite formation are dominant, at least for the given temperature facies.     

Geochemical typification of kimberlite and related rocks of the North Anabar region,Yakutia

The results of geochemical typification of kimberlites and related rocks (alneites and carbonatites) of the North Anabar region are presented with consideration of the geochemical specification of their source and estimation of their potential for diamonds. The content of representative trace elements indicates the predominant contribution of an asthenospheric component (kimberlites and carbonatites) in their source, with a subordinate contribution of vein metasomatic formations containing Cr-diopside and ilmenite. A significant contribution of water-bearing potassium metasomatic parageneses is not recognized. According to the complex of geochemical data, the studied rocks are not industrially diamondiferous.     

Suture zones of the Urals as integral prospective ore-bearing tectonic structures

Rift-related (1.2 Ga) and collision (380–240 Ma) suture zones of the Urals are described. The riftrelated suture zones comprise an ultramafic-gabbro complex with titanomagnetite mineralization, an ultramafic complex with chromite mineralization, and a complex of alkali granitoids with rare-metal (including REE) mineralization accompanied by K-feldspathites, albitites, and calcite metasomatic rocks. The collision suture zones are distinguished by early collision granitoids specialized for tungsten (scheelite) and gold, as well as by raremetal granites and such derivatives of them as pegmatite and greisen with rare-metal and colored-stone mineralization. The suture zones are characterized by long-term (up to 80 Ma or more) continuous-discontinuous periods of ore deposition; heterogeneous sources of ore matter and ore-bearing fluids; a polyelemental composition of lithogeochemical halos and an integral mineral composition of altered wall rocks; and the occurrence of mafic, intermediate, and felsic dikes at large gold deposits, as well as wide variations in PT parameters of the ore-forming process: T = 620?150°C and P = 3.2?0.6 kbar. Collision played a dual role in ore formation. On the one hand, collision led to deformation and metamorphism of precollision massive sulfide deposits and, to a lesser degree, Au-bearing Fe and Cu skarn and porphyry copper deposits, and, on the other hand, to the formation of new gold, rare-metal, quartz, colored-stone, talc, muscovite, and noble serpentine deposits. As a rule, this polygenetic mineralization differs in age and is related to collision volcanic and plutonic complexes. This diversity can be a good basis for metallogenic analysis, forecasting, and prospecting of various metallic deposits and industrial minerals. Polygenetic mineralization of various age known in suture zones is accompanied by integral lithogeochemical and metasomatic halos characterized by a continuous-discontinuous history. The complexity of ore mineralization and often nonlinear metallogeny increase the practical value of suture zones.     

The Age and Composition of Alkaline Rocks of the Dugda Massif (Eastern Sayan): Patterns of the Manifestation of the Late Paleozoic Rare-Metal Magmatism in the Southwestern Fold Frame of the Siberian Platform

Doklady Earth Sciences - The age and geochemical characteristics of alkaline rocks of the Dugda massif (Eastern Tuva), attributed to the East Sayan Late Paleozoic rare-metal magmatic zone, have...     

Metasomatites of the Kara gold deposit (Eastern Transbaikalia)

Wall-rock metasomatites of the Kara gold deposit, a high-temperature medium-depth pneumatolytic-hydrothermal formation, have been studied. Gold mineralization is associated with the intrusion of granitoids of the Kara-Chacha massif (J3) and dikes of alkaline rocks (J3-K1), which include hybrid porphyries, “grorudites”, etc. They are characterized by telescoping of ores, expressed best of all on joints of ore-bearing sites.The origin of the Kara-Chacha massif (Amudzhikan-Sretensk complex) is connected with pre-ore areal propylitization. The propylites demonstrate a zonal pattern relative to the massif and ore veins. A composite metasomatic column of propylitized rocks has been compiled.The thickness of intensely altered wall rocks does not exceed 1.5–2.0 m and the structure of these zones is very heterogeneous. Syn-ore metasomatites are found in propylitized rocks. The major factor of syn-ore alteration of host rocks is the active behavior of alkaline elements. Albitization, silicification (in separate sites), tourmaline and pyrite alteration occur at the early quartz-pyrite-tourmaline stage of mineralization. Sodium is supplied at this stage. During the next quartz-actinolite-magnetite stage sodium and potassium are active. The host rocks demonstrate albitization, feldspar alteration, silicification, actinolitization, biotite alteration, and magnetite impregnation. Aegirine in veins is accompanied by occurrence of aegirine, alkaline amphibole, green biotite and, locally, quartz in host rocks. Potassium becomes more significant later, reaching the maximum activity at the quartz-sulfide stage. The development of quartz-arsenopyrite assemblage was accompanied by K-feldspatization, sericitization of host rocks, formation of green and tan biotites, and arsenopyrite impregnation. The formation of K-feldspar, sericitization, silicification, and sulfide impregnation are associated with quartz-sulfide ore. The final quartz-carbonate-polymetallic stage is accompanied by silicification and carbonate alteration of host rocks. Potassium becomes increasingly more active from outer zones of metasomatic columns to inner ones. The gold contents tend to increase with the potassium contribution in zones of hydrothermal alterations.The propylite alteration and syn-ore changes become more intense veinward. It can indicate that hydrothermal solutions with dissolved minerals penetrated through the most reworked zones. However, hydrothermal solutions during propylite alteration and later syn-ore changes of host rocks not always penetrated through the same zones of weakness, such as tectonic dislocations, contacts of various rocks, etc. The rocks, comprising inner zones of the metasomatic column of propylites are quite often observed at a certain distance from veins and accompanied inner zones of metasomatic columns of later syn-ore metasomatites. They sometimes are not associated with ore veins. However, they are demonstrate later superimposed threads and separate impregnations of syn-ore minerals.Abundant telescoping of mineralization and inheritance of mineralization stages complicate the structure of zones with syn-ore metasomatites. In the sites with telescoped mineralization the metasomatites contain minerals intrinsic to all stages of mineralization found at the deposit.     

Petrology,geochemistry and source characteristics of the Burpala alkaline massif,North Baikal

The Burpala alkaline massif contains rocks with more than 50 minerals rich in Zr,Nb,Ti,Th,Be and rare earth elements(REE).The rocks vary in composition from shonkinite,melanocratic syenite,nepheline and alkali syenites to alaskite and alkali granite and contain up to 10%LILE and HSFE,3.6%of REE and varying amounts of other trace elements(4%Zr,0.5%Y,0.5%Nb,0.5%Th and 0.1%U).Geological and geochemical data suggest that all the rocks in the Burpala massif were derived from alkaline magma enriched in rare earth elements.The extreme products of magma fractionation are REE rich pegmatites,apatite-fiuorite bearing rocks and carbonatites.The Sr and Nd isotope data suggest that the source of primary melt is enriched mantle(EM-Ⅱ).We correlate the massif to mantle plume impact on the active margin of the Siberian continent.     

Abdar-Khoshutula intrusive-dike series: Evolution and origin of granitoids in Early Mesozoic magmatic area (Central Mongolia)

The dike belt and separate intrusive bodies of the Abdar–Khoshutula series were formed in the NE-trending linear zone, southwest of the Daurian–Khentei batholith, in the peripheral part of the Early Mesozoic magmatic area, on the western termination of the Mongol–Okhotsk belt. The granitoids of this series are subdivided into following geochemical types: anatectic granitoids of the calc-alkaline and subalkaline series, alkaline rocks, and plumasite rare-metal leucogranites (Li–F granites). The entire series was formed within approximately 12–15 Ma. Its geochemical evolution follows two trends, which correspond to two stages of the granitoid magmatism. The early stage was responsible for the formation of granitoids of two phases of the Khoshutulinsky Pluton and alkaline syenites with similar trace element distribution patterns. However, syenites, as agpaitic rocks, are significantly enriched in Ba, Zr, and Hf. The late stage of the intrusive- dike series resulted in the formation of the dike belt and Abdar Massif of rare-metal granites. These rocks show enrichment in Li, Rb, Cs, Nb, Ta, Sn, and Y, and deep negative anomalies of Ba, Sr, La, and Ce, which are best expressed in the late amazonite–albite granites of the Abdar intrusion and ongonites of the dike belt. The intrusive-dike series in the magmatic areas of different age of Mongolia and Baikal region are characterized by the wide compositional variations, serve as important indicators of mantle-crustal interaction and differentiation of granitoid magmas, and could highlight the nature of zonal areas within the Central Asian Fold Belt. Obtained geochemical data indicate a potential opportunity to concentrate trace and ore components during long-term evolution of the intrusive-subvolcanic complexes, which could be indicators of the evolution of the ore-magmatic systems bearing rare-metal mineralization.     

The Il’mensky-Vishnevogorsky miaskite-carbonatite complex,the Urals,Russia: Origin,ore resource potential,and sources

The origin and sources of the Il’mensky-Vishnevogorsky miaskite-carbonatite complex, one of the world’s largest alkaline complexes, with unique rare-metal and colored-stone mineralization and Nb, Zr, and REE deposits, are discussed in this paper. Geochemical and isotopic studies, including of Nd, Sr, C, and O isotopes, as well as estimation of PT formation conditions, of miaskites and carbonatites from various deposits of the Il’mensky-Vishnevogorsky Complex have been carried out. The Vishnevogorsky, Potaninsky, and Buldym Nb-REE deposits and the Il’mensky, Baidashevo, and Uvil’dy occurrences related to carbonatites were investigated. Their geological setting, composition, and ore resource potential are characterized. The genetic models and typical features of the Il’mensky-Vishnevogorsky Complex are considered. The rocks of the Il’mensky-Vishnevogorsky Complex were formed at T = 1000?230°C and P = 2–5 kbar. Carbonated miaskite melt was divided into immiscible silicate and carbonate liquids at T = 1000°C and P = 5 kbar. Miaskite crystallized at T = 850?700°C and P = 3.5–2.5 kbar. The formation temperature of carbonatite I of the Vishnevogorsky pluton was close to the temperature of miaskite crystallization (700–900°C). The crystallization temperature of carbonate-silicate rock and carbonatite I in the Central alkaline tract was 650–600°C. The formation temperature of carbonatite II varied from 590 to 490°C. Dolomite-calcite carbonatite III and dolomite carbonatite IV of the Buldym massif were formed at T = 575?410°C and T = 315?230°C, respectively. The geochemical features of carbonatites belonging to the Il’mensky-Vishnevogorsky Complex differ from those of carbonatites related to alkaline ultramafic rocks and are close to those of carbonatites related to nepheline syenite or carbonatites localized in linear fracture zones. A high Sr content in early carbonatites along with relatively low Ba, Nb, Ta, Ti, Zr, and Hf contents and a certain enrichment in HREE (a low La/Yb ratio) in comparison with carbonatites of the alkaline ultramafic association are typical. The geochemistry of carbonatites of the Il’mensky-Vishnevogorsky Complex corresponds to the trend of geochemical evolution of carbonatitic melts and their fluid derivatives. The Sr, Nd, C, and O isotopic compositions indicate a mantle magmatic source of the Il’mensky-Vishnevogorsky Complex and participation of moderately depleted mantle (DM) and enriched mantle EM1 in magma generation. Carbonatite and miaskite of the Vishnevogorsky pluton are related to the DM magma source, and carbonatite of the Buldym massif, to the EM1 source, probably, involved in the plume ascent.     

Uranium and thorium in carbonatitic minerals from the Guli massif, Polar Siberia

This paper reports a geochemical and mineralogical study on carbonatites from the Guli massif, which hosts rare-metal mineralization. The principal carriers of radioactive elements in the carbonatites are pyrochlore-group minerals, zirconolite, and thorianite, which are described here. They are characterized by elevated concentrations (wt %) of radioactive elements: up to 17.89 UO₂ and 20.01 ThO₂ in pyrochlore, up to 6.49 UO₂ and 94.29 ThO₂ in thorianite, and up to 6.74 ThO₂ in zirconolite. The pyrochlore-group minerals, zirconolite, and thorianite from the early calcite carbonatites occur in intimate association with Ti-Zr oxides calzirtite, perovskite, and baddeleyite. Significant radioactive element fractionation in early-stage derivatives results in the depletion of the residual magmatic products in these elements. The dolomite carbonatites are reported to contain only trace amounts of pyrochlore-group minerals. It was shown that the distribution of U, Th, Nb, and Ta in the calcite and dolomite carbonatites is correlated with the evolutionary trends of pyrochlore composition. Typical schemes of isomorphic substitution are proposed for pyrochlore-group minerals and zirconolite. The pyrochlore-group minerals show an apparent evolutionary trend from U-rich towards more Th- and Ta-rich varieties, and Ba-Sr cation-deficient varieties originate during the latest stage of the evolution. The pyrochlore-group minerals, zirconolite, and thorianite may also accumulate in placers, together with gold. Because of the relative ease of extraction of the accessory minerals, the carbonatites of the Guli massif can be considered as commercial sources of radioactive raw materials.     

Genesis and crystallization of ultramafic alkaline carbonatite magmas of Siberia: ore potential,mantle sources,and relationship with plume activity

This paper discusses the genesis of large Siberian alkaline massifs hosting major ore deposits. These reference massifs are grouped based on the predominance of alkalies (K or Na) and their agpaitic index (miaskitic and agpaitic). We proposed new emplacement schemes for the Tomtor, Murun, Burpala, Synnyr, and Bilibino massifs supported by petrochemical and geochemical data, as well as new age estimates. Types of their ore potential and genesis of rare-metal mineralization are discussed. The formational types of carbonatites as the main ore-bearing rocks are given. The depth of magma generation and types of mantle sources are determined using isotopic data from previous studies. A model of plume-related generation of ultramafic alkaline magmas is proposed.     

Potassium alkaline lamproite-carbonatite complexes: petrology,genesis, and ore reserves

This paper studies the petrology of K-alkaline lamproite-carbonatite complexes, which are widespread in Siberia. They are exemplified by the Murun and Bilibino massifs in West and Central Aldan. In these massifs, the entire range of differentiates was first found, from K-ultrabasic-alkalic rocks through basic and intermediate ones to alkali granites and unique residual calc-silicate rocks (benstonite Ba-Sr carbonatites and charoite rocks). Also, intrusive equivalents of lamproites occur in these massifs, and the Murun massif was probably formed from highly differentiated lamproite magmas. In many K-alkaline complexes, silicate and silicate-carbonate magma layering takes place. Stages of magmatism are described for both massifs. Binary and ternary petrochemical diagrams exhibit the same compositional trend from early to late rocks.In this paper, lamproites are considered from the chemical point of view; their diagnostic properties are described in terms of chemical and mineral composition. From geological, petrological, and geochemical data, formational analysis of alkaline complexes was performed, four formational types of world lamproites were first identified, and diamond content criteria were developed for them.The carbonatite problem was studied from the petrological point of view, and four formational types of carbonatites were identified using geological, geochemical, and genetic criteria. It has been suggested that for dividing carbonatite complexes into four formational types the following criteria be used: the alkalinity type (Na or K) of alkalic rocks in the complex and the time when the carbonatite liquid separates from silicate melts in different stages of primary magma differentiation. These linked parameters influence the ore content type of carbonatite complexes.A formation model for K-alkaline carbonatite complexes is given, and the Tomtor alkaline carbonatite massif with tuffaceous rare-metal ores is described to prove that they have ore reserves. The geochemistry of C, O, Sr, and Nd isotopes shows that K-alkaline complexes, depending on their geotectonic setting, can originate from three types of mantle sources: depleted mantle, enriched mantle 1 (EM1), and enriched mantle 2 (EM2). It is concluded that ore-bearing ultrabasic-alkaline complexes of lamproites and carbonatites can melt out of different types of mantle, whose composition only slightly influences their ore content. Apparently, the main factors are the low degree of selective mantle melting (less than 1%) and plumes supplying fluid and alkaline components, which stimulate this melting. Later on, the processes important for the accumulation of ore and trace elements are long-term magma differentiation and its layering during crystallization.     

Main minerals of abnormally high-grade ores of the Tomtor deposit (Arctic Siberia)

The Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~ 250 km²) is ~ 20 km in diameter, with a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. All rocks are weathered and covered with eluvium, which is the thickest after carbonatites enriched in phosphates and REE. The weathering profile consists of four layers, from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in “sagging” eluvium. The ores are laminated and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb₂O₅, 7-10% REE₂O₃, 0.75% Y₂O₃, and 0.06% Sc₂O₃). The highest-grade ores are natural Nb and REE concentrates. The total REE content in some layers is > 10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The ore-forming minerals belong to the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals occur mainly as crystals that were completely replaced by barium-strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure, with an unaltered pyrochlore core and a reaction rim. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~ 50 nm particles, which cover the outer part of halloysite tubes (800–3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates. The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal-sedimentary process with a presumably important role of bioaccumulation of REE phosphates.     

Gahnite in Rare-Metal Metasomatic Rocks of the Verkhneurmiysky Ore Cluster,Amur River Region

A new finding of Zn-spinel (gahnite) in biotite metasomatic rock of the Verkhneurmiysky copper–tungsten–tin ore cluster is described in this paper, including the chemical composition of gahnite, its paragenesis, and place in the regional evolution history of hydrothermal processes. Gahnite is commonly localized in biotite–microcline metasomatic rocks at the lower subore stage of the greisen metasomatic column in proximity to the exocontact of the rare-metal granite massif. Gahnite from rare-metal ore occurrences belongs to the hercynite–gahnite–franklinite isomorphic series and contains significant isomorphic admixtures of iron (24.3%) and manganese (0.65%). Based on our results and earlier published data, it is suggested that gahnite can be a mineral-indicator of rare-metal metasomatism.