Ankeritic Carbonatite Research Articles

New Insights into the Rare Earth Element Mineralization of the Storkwitz Carbonatite, Germany

ABSTRACTThe Storkwitz carbonatite breccia, located near Delitzsch, Germany, is one of the few European domestic rare earth elements (REE) deposits, but is relatively understudied owing to more than 100 m of Cenozoic sedimentary cover. We present the results of a petrological investigation of the recently acquired ∼700 m-deep SES 1/2012 borehole. The Storkwitz breccia is composed of clasts of country rock and carbonatite ranging from <1 mm to ∼30 cm in size, cemented by ankeritic carbonatite. Extensive fenitization and biotitization mainly affects clasts of coarse-grained granitoids and medium-grained dolomite-calcite-carbonatites. An intersection of Storkwitz breccia at 425 m to 542 m contains local REE enrichment up to ∼1.7 wt.%. total rare earth oxides, which is predominantly contained in a REE-fluorcarbonate bearing mineral assemblage. The assemblage locally forms irregularly shaped vug-like features and rare hexagonal pseudomorphs in clasts of fine-grained ankerite-carbonatite. The REE-fluorcarbonate mineral assemblage formed prior to brecciation in the ankerite-carbonatite, which paragenetically fits with recent experimental and fluid inclusion data demonstrating the importance of late magmatic processes in forming carbonatite-hosted REE mineralization, possibly from an evolved ‘brine-melt' phase. Our findings indicate that minor REE recrystallization and redistribution occurred during late-stage hydrothermal or supergene processes, without leading to significant REE enrichment in the upper part of the breccia compared to the lower part. Cross-cutting faults represent the last deformation event and post-date carbonatite intrusion and fenitization. They may represent important conduits for late-stage hydrothermal or supergene fluids responsible for recrystallization of the breccia matrix to a cryptocrystalline oxide mineral assemblage. Our findings highlight the importance of REE enrichment in late-stage ‘brine-melt' phases through magmatic fractionation and in situ hydrothermal replacement.

Evolution of pyrochlore in carbonatites of the Amba Dongar complex, India.

AbstractPyrochlore-group minerals are common accessory rare-metal bearing minerals in the calcite and ankerite carbonatites of the Amba Dongar complex (India). Pyrochlore from the Amba Dongar carbonatites differs from that in other Indian complexes in Ta, Zr, Ti, rare earth element (REE) and Pb contents, but is similar with respect to Ca, Ba and Sr abundances. The evolution of pyrochlore composition was studied to understand the alteration processes and the formation of late-stage pyrochlores enriched in REE and Pb. The early magmatic pyrochlore are calcio- and niobium-dominant types and were replaced by secondary cation-deficient varieties as a consequence of the action of hydrothermal fluids and supergene weathering. These processes produce changes mainly at theAsite, rarely at theBsite, and the original F is replaced by OH–groups. Calcium and Na can be extracted from the structure at the alteration stage and charge balance is achieved by the introduction of REE, Th, U, Ba or Sr. At the latest supergene stages, marginal and fractured zones of pyrochlore grains are altered to Pb-rich, Si-rich and cation-deficient hydrated varieties. The magmatic pyrochlore was crystallised in a highly alkaline environment at a high activity of Ca and at temperatures near 600°C, the alteration of pyrochlore began in a hydrothermal environment at temperatures below 350°C. The major compositional changes that are associated with the alteration are summarised by the following reactions: Ca2++ Nb5+→REE3++ Ti4+; Nb5++ Fe3+→ Ti4++ Zr4+; and 2Nb5++ Ca2+→ Ti4++ Si4++ U4+.

A triple oxygen isotope perspective on the origin, evolution, and diagenetic alteration of carbonatites

Carbonatites are unique magmatic rocks that are essentially composed of carbonates, and they usually host a diverse suite of minor and accessory minerals. To provide additional insights on their petrogenesis, triple oxygen isotope analyses were carried out on carbonatites from sixteen localities worldwide in order to assess the behaviour of oxygen isotopes (mass-dependent fractionation) during their formation. The study evaluates the mineralogical differences, i.e., calcite, dolomite, ankerite, and Na-carbonates, and the mode of emplacement (intrusive or extrusive) in the mantle-derived carbonatites to further constrain the triple oxygen isotopic composition (Δ′17O) of the upper mantle. Δ′17O values in the intrusive calcite carbonatites vary between −0.003 to −0.088‰ (n = 20) and −0.024 to −0.085‰ (n = 5) in the dolomite varieties. We surmise that the magnitude of isotopic fractionation in the different carbonate phases during their formation is similar and thus, the observed variations are independent of mineralogy and may be related to alteration in the rocks. Taking the samples that classify as primary igneous carbonatites altogether, the average Δ′17O value of the mantle is estimated as −0.047 ± 0.027‰ (1SD, n = 18) which overlaps those of other mantle rocks, minerals and xenoliths, indicating that the mantle has a relatively homogenous oxygen isotope composition. Two ankerite carbonatites have identical Δ′17O values as calcite but a few samples, together with pyroclastic tuffs have significantly lower Δ′17O values (−0.108 to −0.161‰). This deviation from mantle Δ′17O signature suggests diagenetic alteration (dissolution and recrystallisation) and mixing of carbonate sources (juvenile and secondary carbonates) which is consistent with the high δ18O and clumped isotope (Δ47) values recorded in the pyroclastic and ankeritic rocks. In summary, diagenetic alteration driven by fluid-rock interaction at low temperatures, sub-solidus re-equilibration with magmatic waters, and the incorporation of secondary carbonates altogether facilitate the alteration of original isotopic compositions of carbonatites, obliterating their primary mantle signatures.

Геохимическая и Sr-Nd-Pb изотопная характеристики щелочных пород и карбонатитов Белозиминского массива (Восточный Саян)

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.

Ferrocarbonatites in the Amba Dongar Diatreme, Gujarat, India

In the Amba Dongar diatreme, “ferrocarbonatite” is not a single unit of late differentiate of calciocarbonatite magma but it is a family with variation on field occurrence, mineralogy and chemistry of each unit. The family includes dikes of ankeritic carbonatites (phase I and II), plugs of ankeritic carbonatite within sovite ring dike, dikes of sideritic carbonatite in ankeritic carbonatite plug and rodberg veins. Their intrusive relations are very clear in the field and each phase has characteristic mineralogy and trace and REE geochemistry. According to the nomenclature suggested by Harmer and Gittins (1997) majority of “ferrocarbonatites” of Amba Dongar plot in field of “ferruginous calciocarbonatite” and only siderite and rodberg plot in the field of “ferrocarbonatite”. Within these family members, their trace and REE show clear increase from early phase to last phase of sideritic carbonatite. The present short communication discusses various aspects of “ferrocarbonatites”.

Compositional Evolution of Pyrochlore-Group Minerals in Carbonatites of the Belaya Zima Pluton, Eastern Sayan

Pyrochlore-group minerals are the main concentrators of niobium in carbonatites of the Belaya Zima alkaline pluton. Fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore were identified in chemical composition. Their main characteristics are given: compositional variation, morphology, and zoning. During evolution from early calcite to late ankerite carbonatites, the UO2, TiO2, REE, and Y contents gradually increased. All carbonatite types are suggested to contain initial fluorcalciopyrochlore. However, in calcite–dolomite and ankerite carbonatites, it is partially or completely hydrated due to hydrothermal processes at the late stage of the pluton. This hydration resulted in the appearance of kenopyrochlore and hydropyrochlore due to removal of Ca, Na and F, and input of Ba, H2O, K, Si, Fe, and probably U and REE. At the last stage of the pluton, this hydrated pyrochlore was replaced by Fe-bearing columbite.

Altered rocks of the Onguren carbonatite complex in the Western Tansbaikal Region: Geochemistry and composition of accessory minerals

The paper discusses the mineralogy and geochemistry of altered rocks associated with calcite and dolomite–ankerite carbonatites of the Onguren dyke–vein complex in the Western Transbaikal Region. The alteration processes in the Early Proterozoic metamorphic complex and synmetamorphic granite hosting carbonatite are areal microclinization and riebeckitization; carbonates, phlogopite, apatite, and aegirine occur in the near-contact zones of the dolomite–ankerite carbonatite veins; and silicification is displayed within separated zones adjacent to the veins. In aluminosilicate rocks, microclinization was accompanied by an increasing content of K, Fe3+, Ti, Nb (up to 460 ppm), Th, Cu, and REE; Na, Ti, Fe3+, Mg, Nb (up to 1500 ppm), Zr (up to 2800 ppm), Ta, Th, Hf, and REE accumulated in the inner zone of the riebeckitization column. High contents of Ln Ce (up to 11200 ppm), U (23 ppm), Sr (up to 7000 ppm), Li (up to 400 ppm), Zn (up to 600 ppm), and Th (up to 700 ppm) are typical of apatite–phlogopite–riebeckite altered rock; silicified rock contains up to (ppm): 2000 Th, 20 U, 13000 Ln Ce, and 5000 Ва. Ilmenite and later rutile are the major Nb carriers in alkali altered rocks. These minerals contain up to 2 and 7 wt % Nb2O5, respectively. In addition, ferrocolumbite and aeschynite-(Ce) occur in microcline and riebeckite altered rocks. Fluorapatite containing up to 2.7 wt % (Ln Ce)2O3, monazite-(Ce), cerite-(Ce), ferriallanite-(Ce), and aeschynite-(Ce) are the REE carriers in riebeckite altered rock. Bastnasite-(Ce), rhabdophane-group minerals, and xenotime-(Y) are typical of silicified rock. Thorite, monazite-(Ce), and rhabdophane-group minerals are the Th carriers.

Metamict U-rich Pyrochlore of Newania Carbonatite, Udaipur, Rajasthan

ABSTRACT U-rich pyrochlore (UO2 up to 18.63%) occurs along the magmatic bands as well as disseminated grains within rauhaugites of the Newania complex. UO2 appears to be primary as it seems mostly evenly distributed all over the grains, though in one grain it does show concentration in parts. Ta is also an important element in some pyrochlore grains and its concentration reaches up to 17.15%. High U and Th are responsible for bringing metamictization in the Newania pyrochlore. Newania carbonatite has a complex emplacement history; rauhaugite seems to have been replaced at 2200 Ma years followed by emplacement of ankeritic carbonatite at around 1500 Ma. Later during tectonic event pyrochlore was involved in reaction with hydrothermal fluid and at this stage Fe and FeS were deposited on the rims of pyrochlore grains. At the same time pyrochlore was subjected to hydro-thermal alteration resulting in removal of Na, Ca and F leaving large vacancy in its A-site. Very few grains have escaped such alteration.

Trace Elements and REE Geochemistry of Siriwasan Carbonatite, Chhota Udaipur, Gujarat

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.

New carbonatite complex in the western Baikal area, southern Siberian craton: Mineralogy, age, geochemistry, and petrogenesis

A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar–39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.

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

The Belaya Zima alkaline–carbonatite complex in the East Sayan Mountains, Russia, is a multiphase concentric intrusion, which comprises diverse alkaline silicate rocks and carbonatites. Melteigites are the earliest products of crystallization and they are followed by ijolites–urtites, nepheline syenites, and finally, calcite, calcite–dolomite and ankerite carbonatites. In this paper, we present new geochronological data (Ar–Ar method), chemical and stable isotope (H, C and O) compositions of the main rock-forming minerals in carbonatites and the silicate rocks of the complex. The Ar–Ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic. The evolutionary trends of pyroxene, fluorapatite, amphibole and phlogopite are consistent with fractional crystallization from a single parental magma. Primary Nb mineralization is represented by pyrochlore, whereas columbite-(Fe) is a replacement product of pyrochlore at the post magmatic, hydrothermal stage. At that stage, interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite–ankerite, monazite-(Ce), ancylite-(Ce), minerals of burbankite group and Ca–REE fluorcarbonates. The hydrothermal processes were also responsible for cation leaching from pyrochlore, the replacements of phlogopite by tetraferriphlogopite and early Ca-rich pyroxene by aegirine. The stable isotope data suggest that the rocks crystallized from a 18O-depleted magma. Shifts toward higher δ18O values relative to the primary compositions are commonly observed in minerals from carbonatites, which is attributed to hydrothermal processes.

Fluid-rock interaction in the Kangankunde Carbonatite Complex, Malawi: SEM based evidence for late stage pervasive hydrothermal mineralisation

Abstract The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO2-rich and alkali chloride-carbonate-bearing fluid at ∼250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis.

ASTER spectral analysis and lithologic mapping of the Khanneshin carbonatite volcano, Afghanistan

Advanced Spaceborne Thermal and Reflection Radiometer (ASTER) data of the early Quaternary Khanneshin carbonatite volcano located in southern Afghanistan were used to identify carbonate rocks within the volcano and to distinguish them from Neogene ferruginous polymict sandstone and argillite. The carbonatitic rocks are characterized by diagnostic CO3 absorption near 11.2 μm and 2.31–2.33 μm, whereas the sandstone, argillite, and adjacent alluvial deposits exhibit intense Si-O absorption near 8.7 μm caused mainly by quartz and Al-OH absorption near 2.20 μm due to muscovite and illite. Calcitic carbonatite was distinguished from ankeritic carbonatite in the short wave infrared (SWIR) region of the ASTER data due to a slight shift of the CO3 absorption feature toward 2.26 μm (ASTER band 7) in the ankeritic carbonatite spectra. Spectral assessment using ASTER SWIR data suggests that the area is covered by extensive carbonatite flows that contain calcite, ankerite, and muscovite, though some areas mapped as ankeritic carbonatite on a preexisting geologic map were not identified in the ASTER data. A contact aureole shown on the geologic map was defined using an ASTER false color composite image (R = 6, G = 3, B = 1) and a logical operator byte image. The contact aureole rocks exhibit Fe2+, Al-OH, and Fe, Mg-OH spectral absorption features at 1.65, 2.2, and 2.33 μm, respectively, which suggest that the contact aureole rocks contain muscovite, epidote, and chlorite. The contact aureole rocks were mapped using an Interactive Data Language (IDL) logical operator. A visible through short wave infrared (VNIR-SWIR) mineral and rock-type map based on matched filter, band ratio, and logical operator analysis illustrates: (1) laterally extensive calcitic carbonatite that covers most of the crater and areas northeast of the crater; (2) ankeritic carbonatite located southeast and north of the crater and some small deposits located within the crater; (3) agglomerate that primarily covers the inside rim of the crater and a small area west of the crater; (4) a crater rim that consists mostly of epidote-chlorite-muscovite–rich metamorphosed argillite and sandstone; and (5) iron (Fe3+) and muscovite-illite–rich rocks and iron-rich eolian sands surrounding the western part of the volcano. The thermal infrared (TIR) rock-type map illustrates laterally extensive carbonatitic and mafic rocks surrounded by quartz-rich eolian and fluvial reworked sediments. In addition, the combination of VNIR, SWIR, and TIR data complement one another in that the TIR data illustrate more laterally extensive rock types and the VNIR-SWIR data distinguish more specific varieties of rocks and mineral mixtures.

Reappraisal of the petrologic status of Newania ‘carbonatite’ of Rajasthan, western India

Considerable data exist on the geology, petrochemistry and isotopic compositions of the Newania carbonatite body (NCB) of Rajasthan in western India. The NCB consists of dolomitic carbonatite and ankeritic carbonatite while the ‘sovite’ is present only as late stage fine veins intruding the dolomitic and ferroan phases. The absence of any associated alkaline activity, low Sr, Ba and Σ REE contents, wide variations in δ 13C and δ 18O values and an unusual sequence of ‘carbonatite emplacement’ represented by dolomitic→ankeritic→calcitic carbonatite characterize the body. None of these criteria, whether considered individually or collectively, provide incontrovertible evidence in favour of an exclusive magmatic origin for these carbonatitic rocks. We present a critical review of the Newania ‘carbonatite’, including the metasomatic aureole, and put forward the possibility that the body may not represent only magmatic carbonates. Instead, it probably appears to be an example of post-magmatic selective metasomatism of magnesio carbonatite affected by ferruginous solutions. Our inferences are based upon critical evaluation of existing database, C and O isotopic data, and observations at NCB.

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

A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in 13C coupled with a sizeable increase in 18O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ 13C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ 18O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ 13C and δ 18O over the δ 18O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of 18O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of 18O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic 13C/ 12C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.

U–Pb zircon ages from a Devonian carbonatite dyke, Kola peninsula, Russia: a record of geological evolution from the Archaean to the Palaeozoic

Zircon from a xenolite-rich carbonatite dyke on Telyachi island, near Kandalaksha on the Kola Peninsula, northwest Russia, have been analysed for U–Th–Pb to determine the age of the dyke and study the geological evolution of the Belomorides. Zircon from the ankeritic carbonatite, analysed by conventional isotope dilution, yielded a concordant U–Pb age of 380±7 Ma. This age is confirmed by SIMS U–Pb analysis of zircon from thin carbonatite veinlets in xenoliths, which gave a concordant 238 U/ 206 Pb age of 378±8 Ma. These results demonstrate that the dyke belongs to the 360–380 Ma old Kola Alkaline Province. The xenoliths represent a variety of rock types. We have analysed zircon from xenoliths of amphibolite, gneissic granite and granite. Most of these crystals have distinct core–rim structures which are clearly shown by cathodolumenescence (CL) imaging. The majority of SIMS analysed spots in cores are close to concordant at c. 2.65–2.75 Ga, which reflects the ages of the rocks from which the xenoliths were derived. Some cores lost a large fraction of their Pb during the carbonatite magmatism. One analysed, CL-dark rim appears to reflect a high grade metamorphic event which is slightly older than 2.75 Ga. This result requires that the xenoliths also include pre-2.75 Ga rocks. Most rims developed during high grade metamorphism 1.7–1.9 Ga ago. These are CL-bright and characterized by very low Th/U ratios, and formed by a rejuvenation process in solid state which resulted in partial or total loss of radiogenic Pb. Most rims have also lost Pb during the Devonian carbonatite magmatism. The ages obtained from xenolith zircons are in excellent agreement with the independently known geological evolution of the Belomorides.

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

The ages of Indian carbonatites are still controversial. Most of the earlier datings were done by K/Ar methods. We therefore analysed Pb/Pb ratios in carbonatites from carbonatite-alkalic complexes of Newania (NW India, Rajasthan State) and Sevattur (SW India, Tamil Nadu State) to constrain the age and geological history of these rocks. Newania carbonatites are intrusive into Precambrian Untala granite-gneiss and mainly dolomitic in composition (rauhaugite) followed by a later phase of ankerite carbonatite, while thin calcite carbonatite (sövite) dykelets are the youngest in the sequence. The analysed whole-rock samples are characterised by 206Pb/ 204Pb ratios between 60 and 176 and 207Pb/ 204Pb ratios between 22 and 40, which are extremely high in comparison to common igneous rocks and even for carbonatite compositions. One sample, New 37, shows the extreme ratios of 206Pb/ 204Pb = 574 and 207Pb/ 204Pb = 73. This requires a μ-value of about 2000 for the last 1550 Ma. If the samples are classified according to their petrographic/geochemical characteristics this results in an isochron age of 1551 ± 46 Ma for the ankerite carbonatites (six samples). The dolomites (6 samples) yield an isochron age of 2.27 Ga. Although these results fit quite well into the geological evolution scheme of the area, the extreme long age hiatus between dolomite carbonatite and ferrocarbonatite formation events raises severe problems for their petrologic interpretation. The Proterozoic Sevattur carbonatite complex (SCC, Tamil Nadu) was emplaced contemporaneously with a large number of carbonatite complexes within the Precambrian gneissic terrane of the Eastern Ghats Mobile Belt. The main mass is composed of dolomite carbonatite (rauhaugite) with a few dikes of calcite carbonatite (sövite) and ankerite carbonatite within it. All eight samples together yield an isochron of 805 ± 10 Ma. This isochron is mainly determined on ankerite carbonatites with μ-values up to 1900 for the last 800 Ma. Taking only ankerite carbonatites into account, the resulting age is 801 ± 11 Ma. The 206Pb/ 204Pb and 207Pb/ 204Pb ratios of these samples are similar to the main group of Newania and far beyond the isotopic composition of common igneous rocks. Our investigations show that in carbonatitic rock systems extremely high lead isotopic ratios can be established due to the crystallization of uranium-rich mineral phases. In both cases the observed high to extremely high initial Pb isotope ratios require the residence of the lead in intermediate high-μ reservoirs either within the upper mantle or the crust prior to the carbonatite formation. A high-temperature event, which completely reset the Rb/Sr and K/Ar isotopic systems of Nevania carbonatites, seems to have no influence on the lead isotopic systematics.

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

The Tundulu carbonatite complex in southeastern Malawi was intruded during the late Jurassic to early Cretaceous over three episodes. During the first and second episodes, the major rock types were calcite carbonatites, ankerite carbonatites, and apatite carbonatites. These rocks experienced hydrothermal alteration at the close of the second episode, during which quartz-baryte veins containing a significant level of rare earth fluorocarbonates were deposited. Veins in calcite carbonatites contain abundant synchysite [(La, Ce, Nd)Ca(CO 3) 2)F] with subordinate amounts of parisite [(La, Ce, Nd) 2Ca(CO 3) 3F 2] and bastnäesite [(La, Ce, Nd)CO 3F], crystallising in the order synchysite parisite → bastnäesite. Some of the parisite is retrograde in origin, having formed as an alteration product from synchysite. Only synchysite has been identified in veins hosted by apatite carbonatites. However, bastnäesite predominates the hydrothermal assemblage in ankerite carbonatites. Parisite and synchysite have been found only as a fibrous fringe between wallrock ankerite and bastnäesite. The crystallisation sequence seen in calcite carbonatites represents a progressive depletion of both Ca 2+ and CO 2− 3 in the fluids from which this mineral suite precipitated, with Ca 2+- and CO 2− 3-poor phases precipitating last. Clearly, the predominance of bastnäesite in veins hosted by ankerite carbonatites suggests insufficient Ca 2+ and CO 2− 3 in the fluids for the ubiquitous precipitation of synchysite and parisite. These observations are consistent with a model in which hydrothermal fluids reacted with the various wallrocks, which then released different amounts of Ca 2+ and CO 2− 3, that subsequently reacted with REEs in the fluid to form the various fluorocarbonates. As such, the Tundulu carbonatites provide a natural laboratory in which compositional phase relationships of rare earth fluorocarbonates can be related to variations in the activities of Ca 2+ and CO 2− 3 within the Ln(CO 3)F-CaCO 3-F 2(CO 3) −1 system.

The biotite-phlogopite series in fenites from the Loe Shilman carbonatite complex, NW Pakistan

AbstractThe 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.

Geochemical features and petrogenesis of Miaoya carbonatites, Hubei Province

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.