Carbonatite Emplacement Research Articles

Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry

The carbonatite-related Nb deposits of the Alto Paranaíba Igneous Province (APIP) in central Brazil, currently account for ∼92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnetite–apatite–tetraferriphlogopite ± carbonate rocks or phoscorites, occurring as interbedded layers with carbonatites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalão II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An examination of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferriphlogopite phoscorite dikes and suggests that alkaline–carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalão II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis.

Petrology and source characteristics of the Arbarastakh alkaline ultramafic carbonatite-phoscorite complex, the Aldan-Stanovoy Shield

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

Geochronology, geochemistry, and Sr–Nd–C–O isotopic composition of the Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, northwest China

Carbonatites are the predominant source of rare earth element (REE) resources globally, but their origins and petrogenesis remain unclear. Currently, >600 carbonatites have been found worldwide, but only a few (<14%) of these carbonatites contain REE deposits. Some carbonatites are rich in REEs but do not host high-grade REE deposits. The Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, China, are ideal for investigating why some carbonatites do not host REE mineralization. We undertook a detailed petrographic, geochronological, and isotopic study of the Qieganbulake carbonatites. In previous studies, a carbonatite at Qieganbulake was dated to ca. 811 Ma. However, in this study, ages of ca. 926 Ma were obtained for phlogopite and baddeleyite in carbonatite dykes rather than pyroxenites or phlogopitelites-bearing carbonatite complex. Such ages of ca. 926 Ma are consistent with a previously published apatite U–Pb age. 40Ar/39Ar dating of phlogopite yielded a plateau age of 928 ± 17 Ma and secondary ion mass spectrometry U–Pb dating of baddeleyite yielded a concordia age of 926 ± 8 Ma, implying the carbonatite was emplaced at ca. 926 Ma. Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of monazite yielded a 206Pb/238U intercept age of 811 ± 27 Ma, indicating that the main REE mineralization stage was coeval with later carbonatite emplacement at ca. 811 Ma. The studied Qieganbulake calcio-carbonatite samples are REE-rich (864–1495 ppm) and consist of calcite (62.18–93.92 vol%), apatite (1.10–24.82 vol%), dolomite (0.37–10.48 vol%), and lesser amounts of phlogopite, olivine, monazite, magnetite, and vermiculite. The calcite, apatite, and monazite (<1 vol%) have REE contents of 757–1111 ppm, 5127–11,556 ppm, and 45.35–59.88 wt%, respectively, and account for the high REE contents of the carbonatites (883–1539 ppm). The small amount of monazite in this carbonatite suggests it is a prospective rather than a confirmed ore deposit. Petrographic studies, oxygen values of 5.74 and 7.51 ‰ and characteristics of trace element for apatites with less monazite around suggest they have magmatic origin with less alteration. The 926 Ma Qieganbulake carbonatite has high Sr isotope ratios (0.70580–0.70639) and negative εNd(t) values (−7.6 to −11.8), indicative of an enriched mantle source. δ13CPDB and δ18OVSMOW values of the carbonatites vary from –3.3‰ to –4.1‰ and 9.7‰ to 12.0‰, respectively, indicative of a contribution from recycled crustal C that was probably derived from a subducted oceanic slab. Above characteristics of carbonatite with ∼926 Ma are different with those of carbonatite occurred at ∼811 Ma, suggesting two carbonatite emplacements in Qieganbulake. Based on our results and the regional geology, we infer that the 926 Ma Qieganbulake carbonatite was derived by partial melting of a subcontinental lithospheric mantle source that had been metasomatized by early Neoproterozoic subduction during circum-Rodinia subduction in the northern Tarim Craton. Early crystallization of apatite sequestered the REEs, resulting in limited REE mineralization. In addition, the limited hydrothermal alteration and ligands available, such as F− and SO42−, hindered REE transportation and precipitation.

The Twihinate carbonatite (Moroccan Sahara): Evidence for compositional mantle heterogeneity and northwestern African plate geodynamics

The Cretaceous Twihinate carbonatite in the Moroccan Sahara is a ∼ 5 km diameter ring-shaped intrusion made of an inner core preserving sparse occurrences of medium- to coarse-grained calcite carbonatite encircled by a ring of vuggy siliceous breccia. The Twihinate carbonatite is enriched in large ion lithophile elements (Cs, Rb, Ba, U and Th) and light rare earth elements (LREE), but shows negative anomalies in high field strength elements (particularly Ta, Zr, Hf and Ti). Stable and radiogenic isotope ratios vary in the range of δ13Cv-PDB = −10.5 to −1.6‰, δ18OV-SMOW = 6.4–28.3‰, initial 87Sr/86Sr = 0.7034–0.7043 (εSri between −14.5 and − 1.8), 143Nd/144Nd = 0.51282–0.51283 (εNdi between 2.8 and 3.6), 206Pb/204Pbi = 19.52–23.78, 207Pb/204Pbi = 15.56–15.69 and 208Pb/204Pbi = 38.69–39.02). Altogether, these isotopic compositions reflect compositional mantle heterogeneity, and are interpreted to reflect partial melting of heterogenous mantle sources with a potential eclogite component in an intraplate, rift-controlled tectonic setting. From a geodynamic perspective, the time span ascribed to age emplacement of Twihinate carbonatite shortly follows the Upper Jurassic hyper-extension event which ultimately resulted in mantle exhumation and subsequent onset of drifting in the Central Atlantic Ocean and Maghrebian Tethys.

Stable isotopic study of carbonatites from the Pakkanadu Alkaline Complex, southern India: Constraints on carbonatite melt evolution and sub-solidus, high temperature fluid-rock interaction

Carbonatites of the Pakkanadu Alkaline Complex (PAC) of the Dharmapuri Rift Zone (DRZ), in the Northern Granulite Terrain (NGT) of southern India, were intruded as small concordant and discordant bodies into pyroxenites and syenites during the Neoproterozoic (∼759‐–750 Ma). Carbonatite samples from the PAC were grouped into three major groups for oxygen and carbon isotope analysis: (A) calcite-dominated (88–100%), (B) dolomite-dominated (83–97%) and (C) admixtures of calcite and dolomite. End-member isotopic compositions were ẟ 13 C = −5.7 ± 0.1‰ and ẟ 18 O = +8.4 ± 0.2‰ for calcite and ẟ 13 C = −6.0‰ and ẟ 18 O = +6.9‰ for dolomite. The carbonatite isotopic compositions lie within the range known for primary igneous carbonatites (PIC), indicating their origin from a mantle source. The higher ẟ 18 O of the calcite end-member than the dolomite end-member indicates oxygen isotopic disequilibrium in the carbonatites. This disequilibrium was the product of sub-solidus oxygen isotope exchange with fluids derived from carbonatite melt during the brine-melt stage (600–400 °C). The narrow range of ẟ 13 C of the carbonatites and its similarity to the ẟ 13 C of depleted upper mantle (−6 ± 2‰) suggests that very little CO 2 was associated with the carbonatite melt. We infer that this type of carbonatite melt evolved from the upper asthenosphere or the base of the lithosphere (> 3 GPa, i.e., >100 km). Similar observations were made based on compiled ẟ 13 C and ẟ 18 O data for carbonatites from Salem-Attur fault, Sevathur, and Hogenakkal regions of the NGT. The ẟ 13 C and ẟ 18 O compositions of carbonatites from Samalpatti, India, however, are distinctly different and suggest two sources for carbonatite melts, one containing a significant amount of CO 2 and possibly evolved from somewhere in the middle of the lithospheric mantle (∼ 2–3 GPa, i.e., 60–100 km) and the other associated with crustal sedimentary or metasedimentary carbonates, which were possibly incorporated during emplacement of carbonatite melts originating from the lithospheric mantle. This study shows that carbonatite melts in the NGT mostly evolved from different levels of the upper mantle and were emplaced to crustal levels, perhaps via a rifting-induced fracture network. • Carbonatites of the Pakkanadu Alkaline Complex from southern India were investigated. • ẟ 13 C and ẟ 18 O of the carbonatites indicate their origin from a mantle source. • O isotope exchange occurred between the carbonatite and carbonatite-derived fluids. • CO 2 production was insignificant during partial melting of the mantle source. • The carbonatite was emplaced to crustal levels via a rift-related fracture system.

SHRIMP U-Pb zircon geochronology of the carbonatite-hosted REE deposit of Kamthai, Late Cretaceous polychronous Sarnu Dandali alkaline complex, NW India: Links to plume-related metallogeny and CO2 outgassing at the K-Pg boundary

The carbonatite plug at Kamthai in the Late Cretaceous polychronous (66–89 Ma) Sarnu- Dandali alkaline complex (SDAC), NW India, is India’s only known carbonatite-hosted rare- earth-element deposit. The SDAC intrudes the Neoproterozoic (∼750 Ma) Malani silicic igneous suite in the Aravalli raton. The Kamthai carbonatite is a calcio-carbonatite dominated by calcite along with barite, strontianite, quartz, zircon and REE-bearing minerals. Textural features suggest an open-system exchange of the magma chamber at varying depths of crystallization. REE patterns are smooth and characterized by strong LREE enrichment, as typical of carbonatites. SHRIMP dating of 12 spots on three magmatic zircons from the carbonatite yielded a concordia age of 66.1 ± 1.4 Ma and an indistinguishable average 206Pb/238U mean age of 68.4 ± 1.8 Ma. The obtained U-Pb ages unambiguously link the carbonatite emplacement to the Deccan Large Igneous Province, i.e. the Réunion hotspot near the K-Pg (Cretaceous-Paleogene) boundary in the polychronous set-up of the SDAC. These ages are also indistinguishable from the 68.53 ± 0.16 Ma magmatic pulse recorded from the nearby Mundwara polychronous alkaline complex and the 69.7 ± 0.2 Ma alkaline sills from the Tethyan suture zone in Pakistan, which are regarded to be earliest manifestations of the Réunion hotspot in the Indian sub-continent. Our results in the context of REE mineralization in the Kamthai deposit have also implications with respect to CO2 outgassing, induced by at least four near-synchronous carbonatitic eruptions in NW India, and to triggering the mass extinction near the K-Pg boundary.

Emplacement of Carbonatite and Associate Rocks of Siriwasan Carbonatite Complex, Gujarat: Evident by Field Relationship and Petrology

The Siriwasan carbonatite complex is situated in the western part of Deccan Basalt Province and belongs to Chhota Udepur carbonatite-alkaline sub-province. The area consist a variety of rock types including sedimentary rocks of Bagh Group (sandstone, limestone and shale), Deccan basalts, carbonatized sandstone, pyroclastic rocks, carbonatite breccia, carbonatites (Alvikite and Sovite), tinguaite, trachyte and dolerite. The detailed field studies and petrography of the carbonatites and associated rocks of the area concluded that there should be at least two episodes of carbonatitic magmatism of different ages. The older episode was extrusive coupled with intrusive dykes and the later event was intrusive only. The tinguaite is older than Deccan basalts while the trachyte and dolerite are younger rock units.

Enrichment of heavy REE and Th in carbonatite-derived fenite breccia

AbstractEnrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite.A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid–rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite.At a maximum Y content of 3100 µg g−1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid – country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.

Geochemistry and origin of high-Sr carbonatite from the Nuba Mountains, Arabian-Nubian Shield, Sudan

Abstract Carbonatite from the Arabian-Nubian Shield of Sudan occurs as dykes in the Nuba Mountains. It is composed of calcite with some feldspars, quartz and fluorite. CaO is the major constituent in this carbonatite and accordingly, it is classified as calico-carbonatite. The studied carbonatite shows exceptionally high concentrations of SrO (4.4 to 5.9 wt%). Ba, Pb and Y occur in relatively higher concentrations compared to other trace elements. Concentration of rare earth elements (ΣREEs) is relatively low (average 1550 ppm) compared to many primary igneous carbonatites. The chondrite-normalized REE patterns display higher light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) with slight negative Ce/Ce* and Eu/Eu* anomalies. The δ18OV-SMOW values range between 7.48 and 10.05‰, while δ13CV-PDB values vary from −6.24 to −7.38‰, which is close to the primary carbonatites values. Occurrence of carbonatite as dykes with cumulate and triple junction textures, plot of the carbonatite in the true carbonatite fields of the Ba-Sr and Ba + Sr-REE + Y diagrams, igneous-derived δ13CV-PDB and δ18OV-SMOW values and high (La/Yb)N ratios indicate its primary igneous origin. The strong positive correlation between REEs and Sr suggests the occurrence of these elements as secondary strontianite, which was confirmed by SEM and EDX analyses. This might indicate that the enrichment of REEs and Sr in the studied carbonatite is not from the primary magma and most probably took place during a sub-solidus metasomatic process after the carbonatite emplacement.

Three-dimensional textural investigation of sulfide mineralisation from the Loolekop carbonatite–phoscorite polyphase intrusion in the Phalaborwa Igneous Complex (South Africa), with implications for ore-forming processes

AbstractCopper-sulfides within carbonatites and phoscorites of the Phalaborwa Igneous Complex, South Africa, have been investigated since the middle of the 20th Century. However, aspects of ore formation have remained unclear. This study examines the mechanisms involved in Cu-sulfide mineralisation by micro-focus X-ray computed tomography as applied to sulfide-rich drill core samples. Several texturally distinct assemblages of magmatic sulfides can be identified, including: (1) &lt;500 μm rounded bornite and chalcopyrite grains disseminated within the gangue; (2) elongated mm-scale assemblages of chalcopyrite and bornite; and (3) mm-to-cm thick chalcopyrite cumulates. Chalcopyrite veins were also observed, as well as late-stage valleriite, documenting late-stage fluid circulation within the pipe, and alteration of magmatic and hydrothermal sulfides along fractures within the gangue, respectively. The results of micro-focus X-ray computed tomography indicate that magmatic sulfides are sub-vertically aligned. Spatial variability of the sulfide assemblages suggests that textural changes within sulfide layers reflect fluctuating magma flow rate during emplacement of carbonatite–phoscorite magmas, through coalescence or breakup of sulfide liquid droplets during ascent. Modal sulfide abundances, especially for disseminated assemblages, differ from one carbonatite–phoscorite layer to another, suggesting a strong control of the mechanical sorting in the formation of Cu-sulfide textures within the Loolekop carbonatite. The alternation of carbonatite and phoscorite within the intrusion suggest that the Loolekop Pipe was emplaced through a series of successive magma pulses, which differentiated into carbonatite and phoscorite by melt immiscibility/progressive fractional crystallisation and pressure drop. Three-dimensional textural analysis represents an effective tool for the characterisation of magma flow and is useful for the understanding of magmatic processes controlling sulfide liquid-bearing phoscorite–carbonatite magmas.

Two discrete stages of fenitization in the Lizhuang REE deposit, SW China: Implications for REE mineralization

Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnäsite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO42−, CO32− and F−. This is also supported by the elevated F and SO3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnäsite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit.

Anomalous Concentration of Uranium in Groundwater in Parts of Salem District, Central Tamil Nadu, India

Anomalous concentration of Uranium in groundwater has been identified around Virakkal, Veerapanpalayam, Pakkanadu, Erithottam, Kammampatti, Kumarappalyam, Idappadiand Moolakkadu areas in parts of Salem district, Tamil Nadu. In this study, an area of 970 sq. km from Mettur in the north to Kumarapalayam in the south parallel to the Koratti shear zone has been examined for uranium content in groundwater. Systematic sampling of groundwater (n=78) from bore well, open well and hand pump have been collected. The samples were analysed for uranium concentration using LED Fluorimeter. Analysis resulted higher concentration of uranium along the southern extension of Koratti Shear Zone where the emplacement of younger granites, syenite and carbonatites has been reported. These lithounits might have played a vital role in the contribution of uranium to the groundwater. Based on the result, two areas have been identified such as North Block and South Block for detailed investigation.58 systematic samples from North block and 52 samples from South block have been collected and analysed. Uranium values assayed in these areas are many times higher than the prescribed limit of World Health organisation and Atomic Energy Regulatory Board of 30ppb and 60ppb respectively. In northern block, among 58 samples 18 samples were assayed higher than 30ppb in which the samples from Veerappanpalayam assayed upto 545.81 ppb. Apart from Veerappanpalayam, many areas such as Mosakumarapalayam (273.90), Arasiramani (250.49ppb), Vellarivalli (131.32ppb), Kodarapalayam (106.52ppb), Savuthanur (174.79ppb), Veerapanpalayam-2 (373.43ppb) and Kotamppalayam (100ppb) were assayed more than 100ppb. Similarly, in the southern block, 52 samples were assayed in which 15 samples were higher than the prescribed limit in which Kuppampatti analyzed 402.29ppb, Virakkal and Thannikuttampatti assayed more than 100ppb. Among the total samples (n=188), 28% of samples were falling under non-potable category. The Present study has spatially brought to light significant areas where groundwater enriched by uranium concentration. It is also provided indirect evidences for uranium exploration in the study area.

Geochemical and spectroscopic investigation of apatite in the Siilinjärvi carbonatite complex: Keys to understanding apatite forming processes and assessing potential for rare earth elements

The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution–reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite.

The Upper Devonian Aley carbonatite, NE British Columbia: a product of Antler orogenesis in the western Foreland Belt of the Canadian Cordillera

Paleozoic continental margin strata in the western Foreland Belt of the Canadian Cordillera are characterized in part by alkaline volcanic sequences, carbonatite intrusions, coarse clastic sedimentary units, and erosional unconformities. These strata also contain a record of mid-Paleozoic contractional deformation unseen in coeval passive margin strata in the eastern Foreland Belt. In order to test potential genetic links between Paleozoic alkaline igneous activity, active margin sedimentation, and deformation in the western Foreland Belt, and better understand their implications for the evolution of the Foreland Belt as a whole, we have undertaken a detailed mapping and structural study of the Aley carbonatite intrusion and its host strata in the western Foreland Belt of NE British Columbia. Our work demonstrates that carbonatite emplacement was coeval with a Late Devonian contractional nappe-forming tectonic event. Interpreting tectonism as associated with continental collision along a long-lived active margin provides the best explanation for our structural and stratigraphic observations, and suggests that the western Foreland Belt is far-travelled and exotic relative to coeval passive margin strata in the eastern Foreland Belt. Deformed alkaline-carbonatite intrusions that characterize continental suture zones in Africa may provide an analogue for the Aley carbonatite and correlative alkaline-carbonatite complexes in the western Foreland Belt.

C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA

The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late-stage hydrothermal fluids that added Sr, Ba, S, F, and REE, analogous to the replacement processes described for some carbonatite complexes of Russia's Kola Peninsula. Carbon and oxygen stable isotope ratios indicate that the primary carbonatite mineralogy experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO2/H2O ratios, and/or meteoric water content. Isotopic differences of matrix calcite between Group 1 carbonatites (avg. δ13C = −7.3‰; δ18O = 9.1‰) and Group 2 carbonatites (avg. δ13C = −9.9‰; δ18O = 10.2‰) are consistent with loss of CO2 during degassing. The open-system alteration of burbankite caused a pronounced positive δ18O-shift in bulk ancylite pseudomorphs (δ18O: 14.3–25.7‰) relative to matrix calcite (δ18O: 8.7–11.2‰). Oxygen isotope compositions of biotite (δ18O: 4.5–5.9‰) and K-feldspar (δ18O: 7.3–7.9‰) in unoxidized carbonatite are typical of primary magmatic silicates and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in 13C and 18O, oxidation, matrix carbonate dissolution, and the replacement of REE carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite) suggest interaction with late-stage, low temperature (<250 °C) fluids characterized by lower CO2/H2O ratios, and an increasing meteoric water component. The first 40Ar/39Ar ages from carbonatite-hosted biotite and K-feldspar at the BLAC are between 51.45 ± 0.08 and 51.89 ± 0.14 Ma. Although carbonatite is commonly observed as the final intrusive phase in alkaline igneous complexes, relative-age relationships and previously published geochronology for Bear Lodge rocks indicate that alkaline silicate magmatism both preceded and followed carbonatite emplacement.

On the Chemical Composition and Possible Origin of Na–Cr-Rich Clinopyroxene in Silicocarbonatites from Samalpatti, Tamil Nadu, South India

Mineralogical and chemical data are presented for a suite of Na–Cr-rich clinopyroxenes associated with chromite, winchite (sodium-calcium amphibole), titanite and calcite in Mg-Cr-rich silicocarbonatites from the Samalpatti carbonatite complex, Tamil Nadu, South India. The Mg-Cr-rich silicocarbonatites occur as 10–30 cm large enclaves in pyroxenites. The chemical composition of the pyroxenes differs among individual enclaves, with variable proportions of diopside, kosmochlor and jadeite-aegirine end-members. These compositions fill a previously unoccupied space in the kosmochlor-diopside-jadeite+aegirine ternary plot, indicating a distinct origin of kosmochlor-rich pyroxene compared with previous findings from diverse settings. The Na–Cr-rich clinopyroxene has low ΣREE = 9.2 ppm, with slight enrichment in LREE (LaN = 7), coupled with low HREE (YbN = 0.6), and flat HREE, paralleled by a significant fractionation of Nb/Ta (2408) and Th/U (26.5). Sodic metasomatism (fenitization) associated with either carbonatite emplacement at shallow levels or during carbonatite ascent through the upper mantle most likely was the major process operating in the area. We suggest two scenarios of the formation of Na–Cr-rich pyroxene: (1) from mantle-derived chromian mineral phases (spinel and/or garnet) through fenitization, with subsequent corrosion by growing winchite due to volatile influx; (2) via metasomatic reaction of Cr-rich garnet in mantle peridotite due to reaction with Na-rich carbonatite melt. Collectively, the appearance of kosmochlor may play an important role in deconvolving metasomatic processes, and fenitization in particular. If combined with petrologic experiments, it could improve our understanding of the origin and subsequent history of chemical signatures of carbonate-rich materials in the mantle.

Rare-earth mobility as a result of multiple phases of fluid activity in fenite around the Chilwa Island Carbonatite, Malawi

AbstractCarbonatites are enriched in critical raw materials such as the rare-earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and rare-earth minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study. In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, rare-earth minerals and pyrochlore in vein networks. Petrography using a scanning electron microscope in energy-dispersive spectroscopy mode showed that the rare-earth minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by laser ablation inductively coupled plasma mass spectrometry) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallization of plagioclase and co-crystallization with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm ≈ 1.3 monazite, ≈ 1.9 bastnäsite, ≈ 1.2 parisite) than their counterparts in the carbonatites (La/Sm ≈ 2.5 monazite, ≈ 4.2 bastnäsite, ≈ 3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few micrometres in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and baryte were found in the inclusions. Decrepitation of inclusions occurred at ∼200°C before homogenization but melting-temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of ∼24 wt.% NaCl equivalent was determined. Among the trace elements in whole-rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Yand REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct rare-earth mineral microassemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.

Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities

The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ∼100–170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene–spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ∼90–110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590–550 Ma extended to >210 km depth, and suggests loss of ∼40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (∼90–110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.

Surface alteration of a melilitite-clan carbonatite and the potential for remote carbonatite detection

The varied lithologic facies and mineralogy resulting from emplacement of syngenetic alkaline, ultramafic and carbonatite (AUC) intrusions are made more diverse by variable weathering and alteration. Ultramafic-carbonatite intrusive complexes are a source for many valuable minerals including diamonds and rare earth element minerals. The intrusive bodies are often difficult to detect in the field due to their paucity, weathering, vegetation, and, in some instances, similarity to country rock, especially in the case of carbonatites among sedimentary carbonates. Remote spectroscopic detection is used extensively for geologic mapping yet has not been applied to differentiating sedimentary and igneous carbonate weathering profiles. Here we document the alteration mineralogy of a newly authenticated melilitite-clan carbonatite occurrence in the Avon Volcanic District in southeast Missouri, USA. The presence of lizardite, vermiculite, phlogopite, and andradite in the weathered crust of calcic and dolomitic carbonatites differentiate them from sedimentary dolomites. We apply field and laboratory spectral measurements to determine the feasibility of humid region AUC remote sensing and classification. Automated humid region detection and classification of carbonatites among sedimentary carbonates is shown to be possible using ratios of absorption features in the 2000–2400 nm range as well as features centered near 680, 900, and 1100 nm due transition metal charge transfer and crystal field splitting in garnet, sheet-silicates, and spinel.

Southwestern Africa on the burner: Pleistocene carbonatite volcanism linked to deep mantle upwelling in Angola

The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A tectonic control, i.e., decompression melting associated with far-field stress, is suggested by correlation with lithospheric sutures, repeated magmatic cycles in the same areas over several million years, synchronicity across the plate, and lack of clear age progression patterns. Conversely, a dominant role for mantle convection is supported by the coincidence of Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies, and slow seismic velocities. To improve constraints on the genesis of African volcanism, here we report the first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite 40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ca. 500–800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-μ (HIMU)–like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bie Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia. This is strong evidence that intraplate late Cenozoic volcanism, including the Catanda complex, resulted from the interplay between mantle convection and preexisting lithospheric heterogeneities.