Calcite Carbonatite Research Articles

"Rare earth elements in phoscorites and carbonatites of the Devonian Kola Alkaline Province, Russia: Examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes"

The Devonian (ca. 385–360Ma) Kola Alkaline Province includes 22 plutonic ultrabasic–alkaline complexes, some of which also contain carbonatites and rarely phoscorites. The latter are composite silicate–oxide–phosphate–carbonate rocks, occurring in close space-time genetic relations with various carbonatites. Several carbonatites types are recognized at Kola, including abundant calcite carbonatites (early- and late-stage), with subordinate amounts of late-stage dolomite carbonatites, and rarely magnesite, siderite and rhodochrosite carbonatites. In phoscorites and early-stage carbonatites the rare earth elements (REE) are distributed among the major minerals including calcite (up to 490ppm), apatite (up to 4400ppm in Kovdor and 3.5wt.% REE2O3 in Khibina), and dolomite (up to 77ppm), as well as accessory pyrochlore (up to 9.1wt.% REE2O3) and zirconolite (up to 17.8wt.% REE2O3). Late-stage carbonatites, at some localities, are strongly enriched in REE (up to 5.2wt.% REE2O3 in Khibina) and the REE are major components in diverse major and minor minerals such as burbankite, carbocernaite, Ca- and Ba-fluocarbonates, ancylite and others. The rare earth minerals form two distinct mineral assemblages: primary (crystallized from a melt or carbohydrothermal fluid) and secondary (formed during metasomatic replacement). Stable (C–O) and radiogenic (Sr–Nd) isotopes data indicate that the REE minerals and their host calcite and/or dolomite have crystallized from a melt derived from the same mantle source and are co-genetic.

Rare earth elements in phoscorites and carbonatites of the Devonian Kola Alkaline Province, Russia: Examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes

The Devonian (ca. 385–360Ma) Kola Alkaline Province includes 22 plutonic ultrabasic–alkaline complexes, some of which also contain carbonatites and rarely phoscorites. The latter are composite silicate–oxide–phosphate–carbonate rocks, occurring in close space-time genetic relations with various carbonatites. Several carbonatites types are recognized at Kola, including abundant calcite carbonatites (early- and late-stage), with subordinate amounts of late-stage dolomite carbonatites, and rarely magnesite, siderite and rhodochrosite carbonatites. In phoscorites and early-stage carbonatites the rare earth elements (REE) are distributed among the major minerals including calcite (up to 490ppm), apatite (up to 4400ppm in Kovdor and 3.5wt.% REE2O3 in Khibina), and dolomite (up to 77ppm), as well as accessory pyrochlore (up to 9.1wt.% REE2O3) and zirconolite (up to 17.8wt.% REE2O3). Late-stage carbonatites, at some localities, are strongly enriched in REE (up to 5.2wt.% REE2O3 in Khibina) and the REE are major components in diverse major and minor minerals such as burbankite, carbocernaite, Ca- and Ba-fluocarbonates, ancylite and others. The rare earth minerals form two distinct mineral assemblages: primary (crystallized from a melt or carbohydrothermal fluid) and secondary (formed during metasomatic replacement). Stable (C–O) and radiogenic (Sr–Nd) isotopes data indicate that the REE minerals and their host calcite and/or dolomite have crystallized from a melt derived from the same mantle source and are co-genetic.

Halogens (F, Cl and Br) at Oldoinyo Lengai volcano (Tanzania): Effects of magmatic differentiation, silicate–natrocarbonatite melt separation and surface alteration of natrocarbonatite

Halogen abundances of natrocarbonatites and their alteration products from Oldoinyo Lengai (Tanzania) are compared to those of associated silicate rocks and to various calcite carbonatites from eleven occurrences worldwide. Fresh natrocarbonatites are extremely enriched in F (up to 3.6wt.%), Cl (up to 5.9wt.%) and Br (up to 100μg/g). During meteoric alteration, however, the major Cl- and Br-bearing minerals (mainly sylvite and gregoryite) are easily dissolved, leaving behind residual materials with relatively low Cl (≤1000μg/g) and Br (≤15μg/g) contents. The comparatively low Cl/Br ratios suggest preferential leaching of Cl relative to Br during alteration. At the same time, F is passively enriched as fluorite remains relatively stable during alteration.In the associated silicate rocks at Oldoinyo Lengai concentrations of all three halogens increase from primitive olivine melilitite to evolved combeite–wollastonite nephelinite, demonstrating their incompatible behavior during magmatic differentiation. Relatively constant Cl/Br ratios, but strongly decreasing F/Cl ratios, in these samples are explained by moderately incompatible behavior for F compared to pronounced and very similar incompatibilities for Cl and Br. Our data further imply that during silicate–natrocarbonatite melt separation all three halogens show a strong affinity for the natrocarbonatite. F and Cl are equally enriched in the natrocarbonatite while Cl and Br are slightly fractionated from each other with a preferential partitioning of Cl relative to Br into the natrocabonatite melt.

Preliminary radiogenic isotope study on the origin of the Khanneshin carbonatite complex, Helmand Province, Afghanistan

The REE-mineralized Khanneshin carbonatite complex in southern Afghanistan is an economically light rare earth element (LREE)-enriched carbonatite complex of Quaternary age. Geological studies have suggested that the LREE-enriched rocks at Khanneshin are comparable in grade to many economic REE deposits, including parts of the world-class Bayan Obo (China), and Mountain Pass, California (United States) deposits. The extent of mineralization at Khanneshin is unknown; however, mineralized areas delineated as part of a reconnaissance study are estimated to contain ~1.29Mt REE ore. We have determined for the first time the radiogenic isotope compositions (Pb–Sr–Nd) of bulk rocks and mineral separates (barite, strontianite, carbonate, etc.) of this carbonatite complex and propose the likely source of radiogenic isotope reservoirs associated with the REE mineralization. This study provides an opportunity to establish fundamental isotopic features of this highly mineralized system to aid in regional exploration efforts and to identify other comparable mineralized systems that shared similar source reservoirs, origins, and evolution in southern Afghanistan.The carbonatite complex consists of coarse-grained sövite and brecciated and agglomeratic barite–ankerite alvikite. Small satellitic intrusions of biotite–calcite carbonatite and rare leucite phonolite also crop out. The massif is encircled by alkali metasomatized Neogene sedimentary strata. A peripheral apron of volcanic and volcaniclastic strata extends farther away from the complex. Present-day Pb isotopic compositions of bulk rocks (dikes, welded tuffs, sövite, and mineralized rocks) and mineral separates exhibit a narrow range in 206Pb/204Pb (18.814–18.877), 207Pb/204Pb (15.616–15.674), and 208Pb/204Pb (38.892–39.094). The results show that the source region of the carbonatite had a moderate compositional variation (most samples overlapping within the level of analytical precision). Present-day neodymium isotopic compositions from the Khanneshin carbonatite show a range in 143Nd/144Nd (0.512374–0.512462). Present-day strontium isotopic compositions also have a moderate range in 87Sr/86Sr (0.708034–0.709577); the mineral separates and ore are somewhat less radiogenic in 87Sr/86Sr (0.707954–0.708686) than the whole rocks.A fundamental feature of the Khanneshin carbonatite is the overall high degree of isotopic homogeneity evident for the Nd–Sr–Pb isotopic systems and we regard this consistency as a diagnostic feature related to its evolution. The carbonatite constitutes a unique isotopic target for identification of other mineralized systems that might have shared the same source. The bulk rocks and minerals in the deposit are generally analytically isotopically indistinguishable, regardless of the mineralogical complexities. All rocks and minerals shared radiogenic isotopic reservoirs, a common evolution, and have remained in isotopic equilibrium. Isotope variations eliminate the source of MORB rocks, or depleted mantle (DMM), as a significant reservoir to the Khanneshin carbonatite. Compared to model mantle reservoirs, the Pb and Nd isotopes of the Khanneshin carbonatite roughly suggest source combinations of enriched mantle, type EMI and HIMU. Sr isotopic data highlight the contribution of another source (EMII?) to account for the relatively high values of 87Sr/86Sr. The radiogenic isotope compositions of the Khanneshin carbonatite differ from world-class REE-mineralized systems such as Bayan Obo (China) and Mountain Pass (California).

Formation of the world's largest REE deposit through protracted fluxing of carbonatite by subduction-derived fluids

Rare Earth Elements (REE) are essential to modern society but the origins of many large REE deposits remain unclear. The U-Th-Pb ages, chemical compositions and C, O and Mg isotopic compositions of Bayan Obo, the world's largest REE deposit, indicate a protracted mineralisation history with unusual chemical and isotopic features. Coexisting calcite and dolomite are in O isotope disequilibrium; some calcitic carbonatite samples show highly varied δ26Mg which increases with increasing Si and Mg; and ankerite crystals show decreases in Fe and REE from rim to centre, with highly varied REE patterns. These and many other observations are consistent with an unusual mineralisation process not previously considered; protracted fluxing of calcitic carbonatite by subduction-released high-Si fluids during the closure of the Palaeo-Asian Ocean. The fluids leached Fe and Mg from the mantle wedge and scavenged REE, Nb and Th from carbonatite, forming the deposit through metasomatism of overlying sedimentary carbonate.

Geology of the Monapo Klippe, NE Mozambique and its significance for assembly of central Gondwana

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35×40km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637±5Ma and metamorphism of the Metachéria Complex at 634±8Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Tinderet volcano, Kenya: an altered natrocarbonatite locality?

AbstractThe Tinderet volcano (19.9 to 5.5 Ma), located within the Kavirondo rift in Kenya, contains blocks of carbonatite lavas with calcite, minor apatite, fluorite, spinel-group minerals, accessory perovskite and 'plumbopyrochlore'; nyerereite is present as inclusions in the perovskite. At least four types of calcite are present in the carbonatite lavas; they differ in morphology, composition and origin. The dominant variety is secondary type-II calcite, which is enriched in sodium (up to 1.1 wt.% Na2O) and strontium (up to 1.3 wt.% SrO). The spinel-group minerals are manganese-bearing and include Mn-rich magnetite, magnesioferrite and jacobsite. Oxygen isotope data for bulk carbonatite samples (δ18O = +16.2 % to +22.6 % VSMOW) support a low crystallization temperature for the secondary calcite. Petrographic, mineralogical and isotopic data indicate that the Tinderet carbonatites are similar to natrocarbonatites from the Oldoinyo Lengai and Kerimasi volcanoes that have altered and recrystallized to form calcite carbonatites. These data support the hypothesis that some of the Tinderet carbonatites were originally alkali-rich rocks which contained primary nyerereite.

Evolution of chemical composition of pyrochlore group minerals from phoscorites and carbonatites of the Khibina alkaline massif

Phoscorites and carbonatites of the Khibina alkaline massif, Russia contain three minerals of the pyrochlore group. They are, in order of crystallization: uranpyrochlore and pyrochlore in the phoscorites, and pyrochlore and bariopyrochlore in late calcite carbonatites. Early calcite carbonatites also contain uranpyrochlore and pyrochlore, but they are xenocrysts derived from the phoscorites. Alteration of the pyrochlore group minerals led to increasing U, Ti and water contents and decreasing Na, Ca, Nb and F contents. Crystallization of zoned uranpyrochlore to pyrochlore crystals in the phoscorites is explained by the experiments of Ryabchikov and Hamilton (1993, 1994) on the interaction of carbonate-phosphate melts with mantle peridotites.

Late Cretaceous ultramafic lamprophyres and carbonatites from the Delitzsch Complex, Germany

The Delitzsch Complex consists of late Cretaceous ultramafic lamprophyres and carbonatitic rocks. They form dikes and diatremes, emplaced into Palaeozoic to lower Permian volcanic and sedimentary rocks, and are covered by up to 120m thick sequences of Tertiary sedimentary rocks. The complex includes a diversity of magmatic and subvolcanic rocks. The lithologies range from monchiquites and alkali picrites to dolomite– and calcite–carbonatites (rauhaugites, beforsites, and alvikites), and ultramafic lamprophyres (alnöites, aillikites). Contact relationships and the distribution of xenolithic material indicate that phases of carbonatitic and ultramafic lamprophyre magmatism overlapped. New U–Pb ages (72±1Ma on baddeleyite) from a dolomite–carbonatite (beforsite), Rb–Sr ages (73±2Ma on phlogopite) from an ultramafic lamprophyre (alnöite) in combination with modeling of the effect of the initial 87Sr/86Sr of phlogopite on the isochron ages of dolomite– and calcite–carbonatites demonstrate: (1) a short duration of magmatic activity for the main phases of the subvolcanic emplacement of the Delitzsch Complex; (2) phlogopite crystals in carbonatites of the Delitzsch Complex are xenocrysts; (3) the calculated initial 87Sr/86Sr composition of xenocrystic phlogopite equals the initial Sr composition of phlogopite from the alnöite.

Geochemistry of radioactive elements in the rocks of the Guli Massif, Polar Siberia

The study of radioactive element distribution in the rocks of the Guli Complex revealed an increase of uranium and thorium contents in the final products of magmatic differentiation. In the carbonatite complex, the radioactive elements are mainly accumulated in the early rocks—phoscorites, while their contents in the late phases, dolomitic carbonatites, decrease. The Th/U ratio increases from near-chondritic values in the weakly differentiated highly-magnesian primary magmas to the late rocks—phoscorites, calcitic carbonatites, and dolomitic carbonatites. The majority of radioactive elements are hosted in rare-metal accessory minerals: perovskite, pyrochlore, calzirtite, and apatite. Rock-forming minerals are characterized by extremely low contents of radioactive elements.

Fluorine-, yttrium- and lanthanide-rich cerianite-(Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosion craters, Gregory Rift, northern Tanzania

AbstractCerianite-(Ce), ideally CeO2, occurs as rounded grains up to 5 μm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 μm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) Å) and Raman spectroscopy (minor vibration modes at 184 and 571 cm—1 in addition to a strong signal at 449 cm—1) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.9-15.5 wt.% LREE2O3 and 4.9-9.7 wt.% HREE2O3), and that it is enriched in yttrium (7.1 — 14.5 wt.% Y2O3) and fluorine (2.2—3.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cation—anion distance of 2.3471(6) Å. The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks.

Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: Constraints for the mechanism of super accumulation of rare earth elements

The Bayan Obo mine in North China contains the largest rare-earth element (REE) resources in the world. The mechanism of giant REE enrichment in such a restricted region has been the focus of several studies. Carbonatites are known to contain high concentrations of REE. Three types of carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite-dolomite carbonatite varieties. The contact relations show that the intrusion of calcite carbonatite dykes post-date the dolomite dykes during the process of evolution of the carbonatite magma. The geochemical data show that the content of Sr and light (L) REE in these dykes gradually increases from dolomite [(La/Yb) N values range from 1.6 to 3.8], through calcite-dolomite [(La/Yb) N ranging from 51 to 57], to calcite type [(La/Yb) N ranging from 85 to 4617]. This evolutionary trend suggests the crystal fractionation of the carbonatite magma, which might have played a critical role for the giant REE accumulation in the Bayan Obo region. The ε Nd(t) values of the carbonatitic dykes show a tight cluster between − 0.47 and 0.65, whereas the initial Sr isotope values show a broad range from 0.703167 to 0.708871. The massive ore-hosting dolomite marbles show comparable element content and Nd isotope composition, and a Sm–Nd isochron age similar to that of the carbonatite dykes, implying a close relationship during their magmatic origin. This interpretation is also supported by the intrusive contact between the ore-hosting dolomite marble and Mesoproterozoic Bayan Obo group, as well as the presence of wall-rock xenoliths in the dolomite marble.

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.

Kerimasite, Ca3Zr2(Fe23+Si)O12, a new garnet from carbonatites of Kerimasi volcano and surrounding explosion craters, northern Tanzania

AbstractKerimasite, ideally is a new calcium zirconium silicate-ferrite member of the garnet group from the extinct nephelinitic volcano Kerimasi and surrounding explosion craters in northern Tanzania. The mineral occurs as subhedral crystals up to 100 μm in size in calcite carbonatites, and as euhedral to subhedral crystals up to 180 μm in size in carbonatite eluvium. Kerimasite is light to dark-brown in colour and transparent with a vitreous lustre. No cleavage or parting was observed and the mineral is brittle. The calculated density is 4.105(1) g/cm3. The micro-indentation, VHN25, ranges from 1168 to 1288 kg/mm2. Kerimasite is isotropic with n = 1.945(5). The average chemical formula of the mineral derived from electron microprobe analyses (sample K 94-25) and calculated for O = 12 and all Fe as Fe2O3 is (Ca3.00Mn0.01Ce0.01Nd0.01)Σ3.03(Zr1.72Nb0.14Ti0.08Mg0.02Y0.02)Σ1.98(Ti0.09)Σ3.00O12. The largest Fe content determined in kerimasite is 21.6 wt.% Fe2O3 and this value corresponds to 1.66 a.p.f.u. in the tetrahedral site. Kerimasite is cubic, space group with a = 12.549(1) Å, V = 1976.2(4) Å3 and Z = 8. The five strongest powder-diffraction lines [d in Å, (I/Io), hkl] are: 4.441 (49) (220), 3.140 (91) (400), 2.808 (70) (420), 2.564 (93) (422) and 1.677 (100) (642). Single-crystal structure refinement revealed the typical structure of the garnet-group minerals. The name is given after the locality, Kerimasi volcano, Tanzania.

The Yungul carbonatite dykes associated with the epithermal fluorite deposit at Speewah, Kimberley, Australia: carbon and oxygen isotope constraints on their origin

The Yungul carbonatite dykes at Speewah in the Kimberley region of Western Australia were emplaced along a north-trending splay from the northeast-trending Greenvale Fault located at the western boundary of the Halls Creek Orogen. The Yungul carbonatite dykes intrude a thick composite sill of the Palaeoproterozoic Hart Dolerite (~1,790 Ma), consisting of tholeiitic dolerite and gabbro with its felsic differentiates that form the Yilingbun granophyres and associated granites. The carbonatite dykes consist of massive, calcite carbonatite that host very coarse, pegmatitic veins and pods of calcite, and have largely replaced (carbonatitized) and fenitized the country rock Hart Dolerite suite in a zone up to 150 m wide. Dykes of red-brown siliceous fluidized-breccia and epithermal-textured veins consisting of bladed quartz, adularia and fluorite are closely associated with the carbonatite dykes. The Yungul carbonatites are closely associated with fluorite occurrences with resources currently reported as 6.7 Mt at 24.6% CaF2. The precise age of the Yungul carbonatite is not known, although it is believed to be post early Cambrian. The total REE content of the Yungul carbonatite is low (174.0–492.8 ppm; La/Yb 2.28–10.74) and thus atypical for calciocarbonatite. Chondrite-normalized REE patterns for the carbonatite are relatively flat compared to average calciocarbonatite, and show small negative Eu anomalies. These unusual geochemical features may have been acquired from the Hart Dolerite suite during emplacement of the carbonatite, a process that involved extensive replacement and fenitization of country rocks. Carbon and oxygen isotope compositions of massive calcite carbonatite and the coarse calcite veins and pods from the carbonatite suggest a deep-seated origin. The C and O isotope compositions show an overall positive correlation that can be attributed to both magmatic and magmatic-hydrothermal processes in their evolution. The magmatic δ13C-δ18O trend is also indicative of crustal contamination and/or low-temperature water/rock exchange. The carbon isotopic compositions have δ13C values that range from about −5.2‰ to −6.3‰ that support a mantle-derived origin for the Yungul carbonatites and are consistent with earlier conclusions based on whole-rock geochemistry and radiogenic isotopes studies.

Major- and trace-element compositional variation of phlogopite from kimberlites and carbonatites as a petrogenetic indicator

Ferromagnesian trioctahedral micas are a common macrocryst/phenocryst phase in kimberlites and carbonatites. We explored the possibility of using the chemistry of ferromagnesian micas to discriminate between these rocks in order to assess their economic potential. We determined the extent of major- and trace-element variations of micas from 14 selected kimberlite and carbonatite localities worldwide representing several different kimberlite fields and alkaline–carbonatite provinces. Phlogopite macrocrysts from kimberlites follow two principal evolutionary trends: primary macrocrystic (involving an increase in Ba, Al, Ti, Cr, Zr, Nb and Sr contents at decreasing Mg towards the rim) and transitional macrocryst-groundmass trend (involving an increase in kinoshitalite component at decreasing Fe, Ti and Cr contents). Micas from (dolomite–)calcite carbonatites cover a much greater compositional range than those from kimberlites, defining evolutionary trends towards annite, tetra-ferriphlogopite and kinoshitalite. Our data confirm that macrocrystic micas from kimberlites cannot be reliably distinguished from carbonatitic phlogopite on the basis of their major-element compositions. Exceptions are Fe–Al-rich [Fe/(Fe + Mg) ≥ 0.2] and Na-rich (> 0.3 wt.% Na 2O) micas that seem to be virtually restricted to carbonatites. The generally higher levels of Na in carbonatitic micas relative to those from kimberlites can be explained by the lower Na abundances in the latter relative to carbonatites. Our data indicate that the trace-element composition of mica determined by LA-ICP-MS can be used as a reliable petrogenetic indicator. Kimberlitic phlogopite is characteristically enriched in Cr (up to 12030 ppm), Ni (up to 1600 ppm) and Co (40–70 ppm), and depleted in Mn (≤ 280 ppm), Nb (≤ 28 ppm), Sr (≤ 55 ppm), Sc (≤ 8 ppm), Zr (≤ 12 ppm) and, to some extent, Cs (≤ 7 ppm) and Ta (≤ 3 ppm) with respect to carbonatitic micas (≤ 315 ppm Cr, ≤ 470 ppm Ni, ≤ 34 ppm Co, up to 17200 ppm Mn, 1080 ppm Nb, 830 ppm Sr, 80 ppm Sc, 105 ppm Zr, 27 ppm Cs and 19 ppm Ta). With the exception of Zr, the observed differences in trace-element composition of mica probably reflect the contrasting trace-element geochemistry of kimberlitic and carbonatitic magmas. The depletion of kimberlitic phlogopite in Zr (and possibly, Nb + Ta) can be explained by sequestration of these elements in the earlier-crystallized ilmenite. Variations in Ba, Rb and V contents are similar in mica from both rock types and cannot be used as a petrogenetic indicator.

Rare metal mineralization of calcite carbonatites from the Cape Verde Archipelago

Rare metal mineralization of oceanic carbonatites was studied for the first time by the example of calcite carbonatite from Fogo Island in the Cape Verde Archipelago. The following evolutionary sequence of rare metal minerals was established: zirconolite-Th-calciobetafite-betafite + Th-pyrochlore-thorite + Ti-Zr-Nb silicates + zircon.

Mapping lithology of the Sarfartoq carbonatite complex, southern West Greenland, using HyMap imaging spectrometer data

The Sarfartoq carbonatite complex occurs in the southern West Greenland in a transition zone between Archaean gneiss complex to the south and a Proterozoic mobile belt to the north. The Sarfartoq carbonatite complex consists of a core zone composed of dolomite carbonatite and minor søvite (calcite carbonatite) surrounded by a fenite zone and a marginal zone of gneisses frequently altered due to hydrothermal activity. High spatial and spectral resolution imaging spectrometer data recorded by the HyMap imaging system were used to map lithology of the Sarfartoq carbonatite complex. A careful analysis of the spectral reflectance properties of the carbonatite lithology preceded the HyMap data analysis stage. The spectral reflectance measurements showed that the various lithologic units including dolomite carbonatite, søvite, fenite and the marginal alteration zone have distinct spectral reflectance characteristics. The analysis of the HyMap data was based on an unsupervised clustering algorithm, the Self Organizing Maps (SOM), for the mapping of the main lithology and a hierarchical tree for the mapping of sparsely occurring søvite rocks. Spectral mixture analysis was applied to map fractional abundances and compare with the SOM results. The resulting lithological map shows the spatial distribution of dolomite carbonatite, søvite, fenite with abundant carbonatite dykes (representing the outer core of the carbonatite complex), fenite and hematized gneiss (marginal alteration zone). The results compare well with the field data collected for the assessment of the mapping accuracy and due to the spatially contiguous nature of the hyperspectral data could be used to better map the outcropping carbonatite lithology. The spectral reflectance measurements and the mapping results provide information of petrological importance for the carbonatite core zone.

Isotopic composition of Sr, Nd, and Pb in pirssonite, shortite and calcite carbonatites from Oldoinyo Lengai volcano, Tanzania

Isotopic composition of Sr, Nd, and Pb in pirssonite, shortite and calcite carbonatites from Oldoinyo Lengai volcano, Tanzania

Cymrite as an indicator of high barium activity in the formation of hydrothermal rocks related to carbonatites of the Kola Peninsula

Cymrite, BaAl2Si2O8 · nH2O, is a rare mineral formed during low-grade dynamothermal metamorphism (T = 250–300°C, P = 1–3 kbar). Cymrite has been described from many metasedimentary ores and hydrothermal rocks. In carbonatites, it has been found for the first time. Cymrite has been identified in the Kovdor and Seblyavr massifs, Kola Peninsula. In Kovdor, this mineral has been described from one of the hydrothermal veins cutting the pyroxenite-melilitite-ijolite complex at the Phlogopite deposit; cymrite is associated with thomsonite, calcite, and stivensite. In the Seblyavr pluton, cymrite occurs in thin veins of calcite carbonatite that cut pyroxenite contacting with ijolite. Cymrite from the Seblyavr pluton is associated with calcite, natrolite, pyrite, and chalcopyrite. The mineral is optically negative and uniaxial, with extinction parallel to elongation; ω ∼ 1.607(1). According to X-ray diffraction data, cymrite from Seblyavr is monoclinic, space group P1m1; unit-cell dimensions are: a = 5.33, b = 36.96, c = 7.66 A, β = 90°, V = 1510.55 A3. According to the results of IR spectroscopy, in the series of samples from different massifs (in the running order Kovdor-Voishor-Seblyavr), the double-layer deformation is enhanced and accompanied by a decrease in the Si-O-Si angle and weakening of hydrogen bonds of interlayer water. The empirical formulas of cymrite calculated from electron microprobe analyses are Ba0.93–0.95Ca0.01–0.02K0.00–0.05Na0.02–0.04Al1.97–2.01Si1.99–2.03O8(H2O) and Ba1.00–1.02Ca0.00–0.01Sr0.00–0.01Fe0.00–0.01Al1.94–2.00Si1.98–2.03O8(H2O) at Seblyavr and Kovdor, respectively. Cymrite from the carbonatite massifs of the Kola Peninsula was formed under hydrothermal conditions at low temperature (200–300°C), high activity of Ba and Si, and high water pressure. At Kovdor, the mineral crystallized directly from the residual solution enriched in Ba. The sequence of mineral deposition is as follows: thomsonite-cymrite-calcite-stevensite. Cymrite from the Seblyavr pluton is a product of hydrothermal alteration of primary Na-K-Ba silicates of ijolite: nepheline, feldspar, and probably celsian. Natrolite replaces cymrite indicating high alkalinity of late hydrothermal fluids.