ПЕРСПЕКТИВЫ ТОМТОРСКОГО РУДНОГО ПОЛЯ НА МЕСТОРОЖДЕНИЯ МАРГАНЦА

Pb/Pb age determinations on the Newania and Sevattur carbonatites of India: evidence for multi-stage histories

The configuration of the Indian shield — Precambrian tectono-thermal events and constraints on the thermal history of Gondwana

The Proterozoic carbonatites and associated pyroxenites and syenites of Tamil Nadu are emplaced in the Precambrian gneissic rocks. The carbonatite complexes are located along a major NE trending lineament. In Sevathur, dolomitic carbonatite is predominant over sovite and ankeritic carbonatite whereas in Samalpatti, sovite and silico-sovite form the major bulk of carbonatite mass with small dykes of dolomitic and ankeritic carbonatites. In different types of carbonatites, in addition to calcite, dolomite and ankerite, phlogopite, amphibole, magnetite, and apatite are found in varying amounts between 2% and 10% with accessory amounts of pyrochlore, perovskite, monazite, NbiImenorutile, zircon, baddeIeyite, pyrite, ilmenite, galena and thorite. Both mica and amphibole show change in composition from Fe-rich in syenite and sovite to Mg-rich in the dolomitic carbonatite. The carbonatites are enriched in Ba, Sr, Nb, REE's, Th and U in comparison to the primitive mantle values. Pyroxenites are characterized by high abundances of Mg, Cr and Ni and, low abundances of Nb and Zr. The petrographic evidence, with globules silicate rocks surrounded by carbonatitic matrix is sufficiently clear to suggest a genetic link between syenite and carbonatite by liquid immiscibility.

Abstract:The Siriwasan carbonatite-sill along with associated alkaline rocks and fenites is located about 10 km north of the well-known Amba Dongar carbonatite-alkaline rocks diatreme, in the Chhota Udaipur carbonatite-alkaline province. Carbonatite has intruded as a sill into the Bagh sandstone and overlying Deccan basalt. This resulted in the formation of carbonatite breccia with enclosed fragments of basement metamorphics, sandstone and fenites in the matrix of ankeritic carbonatite. The most significant are the plugs of sövite with varied mineralogy that include pyroxene, amphibole, apatite, pyrochlore, perovskite and sphene. REE in sövites is related to the content of pyrochlore, perovskite and apatite. The carbon and oxygen isotopic compositions of some sövite samples and an ankeritic carbonatite plot in the “mantle box” pointing to their mantle origin. However, there is also evidence for mixing of the erupting carbonatite magma with the overlying Bagh limestone. The carbonatites of Siriwasan and Amba Dongar have the same Sr and Nd isotopic ratios and radiometric age, suggesting the same magma source. On the basis of available chemical analyses this paper is aimed to give some details of the Siriwasan carbonatites. The carbonatite complex has good potential for an economic mineral deposit but this is the most neglected carbonatite of the Chhota Udaipur province.

Carbon and Oxygen Isotope Geochemistry of the Amba Dongar Carbonatite Complex, Gujarat, India

The lens-shaped Miaoya carbonatite complex, located west of the Wudang massif, Hubei Province, consists of several types of carbonatites associated with syenites. According to their composition, the carbonatites can be classified into four types, i. e., sovite, alvikite, carbon-bearing alvikite and ankeritic carbonatite. Their average composition is in agreement with the abundance values for carbonatites compiled by Gold (1966). There are three stages recognizable: the earliest sovite and alvikite followed by carbon-bearing alvikite, with ankeritic carbonatite being the latest. Some rules dominating the distribution of major and rare elements are observed with respect to the evolution of these carbonatites, for example, Nb is essentially enriched in sovite. Except for niobite and ilmenorutile, there are also pyrochlore, nioboeschynite and fersmite. RE are concentrated mainly in ankeritic carbonatite, within which bastnasite, parisite and monazite are found. In general, Ca, Nb, and Sr decrease, while Fe, Mg, Mn, and RE increase from earlier to later stages. It is suggested that the carbonatites are genetically connected with syenitic magma.

Stable isotope composition of minerals in the Belaya Zima plutonic complex, Russia: Implications for the sources of the parental magma and metasomatizing fluids

Belaya Zima alkaline carbonatite complex (East Sayan, Russia) is a multiphase intrusion of the central type. The carbonatite stock is exposed in the central part of the complex, which is composed of different mineralized carbonatite. Alkaline silicate rocks form ring-shaped bodies surrounding the central carbonatite and are represented by melteigites and ijolites, dyke-shaped bodies of alkaline syenites. The article presents the results of Pb/Pb dating, petrological and geochemical studies, including radiogenic (Nd, Pb and Sr) isotopic data. The age value of 631 ± 11 Ma, obtained from the Pb/Pb isochron line for all rocks of intrusion, is consistent with the previous age determinations [643 ± 3 Ma, Yarmolyuk et al. 2005 and 645 ± 6 Ma, Doroshkevich et al., 2016; 645 ± 6 Ma, Salnikova et al., 2019]. The age is in the range of 700-600 Ma, the interval of the formation of alkaline carbonatite complexes located along the edge of the Siberian craton (Aldan-Stanovoy shield, Yenisei Ridge, East Sayan). Emplacement of the complexes is associated with the final stage of the break-up of the Rodinia supercontinent [Yarmolyuk et al., 2005]. Melteigites are more magnesium (Mg # = 28–35) in comparison with ijolites (Mg # = 24–30) and alkaline syenites (Mg # = 15–29), which proves the lowest degree of differentiation of the former. The low total Mg # of the Belaya Zima alkaline silicate rocks, their low nickel and chromium concentrations confirm the fact that olivine crystallization has completed. Early crystallization of perovskite in melteigites led to depletion of the residual melt in REEs and Nb and formation of REE-depleted ijolites and nepheline syenites. There are the evolution trend of HFSE and REEs from early calcite carbonatite to calcite-dolomite and ankerite carbonatites. The calcite and calcite-dolomite carbonatites have the highest contents of Nb-Ta and Zr-Hf and reflect the early crystallization of pyrochlore and zirconolite. Late ankerite carbonatites are enriched in REEs in comparison with calcite and calcite-dolomite carbonatites, are REEs are concentrated in rare earth carbonates and monazite (Ce). Isotopic data (87Sr/86Sr = 0.702672–0.703125 and eNd (Т) = 3.14–4.97) for rocks of the Belaya Zima complex indicate that primary melts were formed from a heterogeneous, moderately depleted mantle source. The high concentration of incompatible elements in the rocks (Sr and Nb, La/Sm ratio), the presence of carbonatites in the complex indicate that the possible mantle metasomatic agent had a carbonate initial composition. This conclusion is confirmed by the results of melt inclusions study in minerals from the Belaya Zima ijolites [Andreeva et al., 2007]. A high Gd/Yb ratio (up to 7) in the rocks indicates that they were derivatives of primary melts formed by low degree of partial melting of a garnet-bearing source.

Hydrothermal rare earth mineralisation in carbonatites of the Tundulu complex, Malawi: Processes at the fluid/rock interface

Boron isotopic investigation of the Bayan Obo carbonatite complex: Insights into the source of mantle carbon and hydrothermal alteration

The Early Cretaceous (~132 Ma) Anitapolis stock is made up of cumulate pyroxenites, minor nepheline syenites and ijolites and scarce dykes of nephelinitic composition. Carbonatites form a small body, but are also widespread as dykes and veins. The complex is entirely surrounded by Late Proterozoic granitic-gneissic rocks (570–630 Ma). The Late Cretaceous (70–77 Ma) Lages complex constitutes a proeminent dome structure underlined by a concentric arrangement of Permian to Triassic sediments. It consists mainly of peralkaline phonolites and nepheline syenites with subordinate ultramafic rocks (melilitite, olivine nephelinite, basanite and tephrite). Other important rock-types are kimberlitic breccias and carbonatites as a minor intrusion. Chemical data suggest that the Anitapolis carbonatites are essentially Ca-carbonatites and the Lages ones Fecarbonatites. Normalized IE and REE diagrams for both carbonatites display different patterns. d 18 O and d 13 C isotopes for the Anitapolis carbonatites plot near the primary carbonatite box, whereas the Lages rocks point to a large spreading of d 18 O values. Radiogenic isotope data indicate that the carbonatites and the associated silicate rocks present similar Sr i and Nd i values, spanning from time integrated depleted to enriched types. Nd-model ages for the Early (Anitapolis) and Late Cretaceous (Lages) carbonatites are 1.3 ± 0.1 Ga and 1.2 ± 0.2 Ga, respectively, but the Lages rocks seem to have been affected by two different Proterozoic mantle metasomatic events. Considerations on the geodynamic implications of the alkaline and alkaline-carbonatitic magmatism are made on the basis of models other than mantle plume.

The article presents the results of study of the Ilmeno-Vishnevogorsky and Buldym carbonatite complexes in the Urals. It has been established that the carbonatites of the Ilmeno-Vishnevogorsky complex are represented by high-temperature calciocarbonatites (sövites I and II) with pyrochlore ore mineralization. U-Ta-rich populations of uranium pyrochlores (I) and fluorocalciopyrochlores (II) crystallize in miaskite-pegmatites and sövites I; fluorocalciopyrochlores (III) and Sr-REE-pyrochlores (IV) of late populations form in sövites II. In the Buldym complex, along with high-temperature calciocarbonatites containing fluorocalciopyrochlore (III), medium-temperature varieties of magnesiocarbonatites with REE-Nb mineralization (monazite, niobo-aeschynite, columbite, etc.) are widespread. Miaskites and carbonatites of the Urals are characterized by high contents of LILE (Sr, Ba, K, Rb) and HFSE (Nb, Ta, Zr, Hf, Ti), which are close to the contents in rift-related carbonatite complexes of intraplate settings and significantly differ from synorogenic collisional carbonatite complexes. The Ural carbonatite complexes formed on continental rift margins during the opening of the Ural Ocean at the time of transition from extensional to compressional tectonics. Later on, they were captured and deformed in the suture zone as a result of collision. Plastic and brittle deformations, anatexis, recrystallization of rocks and ores of carbonatite complexes in the Urals are associated with orogenic and post-collision settings.

The Loe Shilman carbonatite sheet complex comprises an extensive amphibole sovite which is intruded by minor biotite sovite and amphibole ankeritic carbonatite. The carbonatites have fenitized the country rocks to form a metasomatic zone c. 100 m wide of alternating mafic and felsic mica-bearing banded fenites which grade into slates and phyllites. Phlogopite-rich micas occur nearest to the carbonatite contact. The biotites occur in K-feldspar + albite ± Na-amphibole ± aegirine and ± phengite fenites produced by the intrusion of the early amphibole sovite. Aegirine buffered the iron distribution and the biotites became more magnesian. Veins cross-cutting the fenites consist of biotite and/or Ba-bearing K-feldspar, and are interpreted to result from solutions emanating from the biotite sovite. The ankeritic carbonatite is responsible for the formation ofphlogopite in the fenites in a c. 3 m wide zone adjacent to the carbonatite, and evidently are the result of fenitizing fluids rich in Mg. Chemical equations calculated to balance the reactions interpreted to have taken place in the fenites suggest that about 10% of the Al and Si in the protolith was mobilized and moved towards the carbonatites during fenitization, and that the fenitizing solutions were strongly alkaline and oxidizing.

Sövite and ankeritic carbonatite are the main source of rare earth elements in the Amba Dongar carbonatite complex. The mineralogy and mineral chemistry of a single vein of ankeritic carbonatite invading sövite is presented. Along with REEminerals – bastnäsite and synchysite, apatite, pyrochlore, barite and carbonates are also analyzed.

Trachytic rock and its altered rock—fenite—in the Bayan Obo ore district, Inner Mongolia, China, were referred to as slate or feldspar rock before, and identified by the authors for the first time (in 1992). In the paper the mineral assemblages, structures and textures and petrochemical compositions of the rocks, as well as the electron microprobe analysis of feldspars in the rocks are described. The Sm‐Nd isochron age of the trachytic rock is 1096 ± 56 Ma, with INd=0.51100±4 (2 s̀) and ɛNd(t)= −4.4 ± 0.7. Alterations of the trachytic rock, including microclinization, riebeckitization, aegirinization and biotitization, and accompanied rare element and REE mineralizations are discussed. Based on the occurrence of the trachytic rock and associated fenitization it is deduced that the Bayan Obo Fe‐Nb‐REE ore deposit is genetically related to magmatic‐hydrothermal activity of an alkali carbonatite complex.