Amba Dongar Carbonatite Complex Research Articles

High MREE-HREE solubility in a carbonatite-derived hydrothermal fluid: Evidence from fluorite-hosted fluid inclusions in the Amba Dongar carbonatite complex, India

Recent experimental works suggest that complexation with alkalis, carbonate, and hydroxyl groups enhances rare earth element (REE) transport and HREE/LREE fractionation, seen in carbonatite-hosted REE deposits. The Amba Dongar carbonatite complex in western India hosts fluorite and REE-fluorcarbonate mineralization occurring as vug/vein fillings or as disseminated ores associated with calcite, dolomite, apatite, baryte, and quartz. In this study, we analyzed the major and trace element composition of fluorite-hosted fluid inclusions, and the trace element chemistry of calcite and fluorite, using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) to characterize the nature of the hydrothermal fluid and the processes of mineral-fluid interaction. Fluorite contains aqueous bi-phase (liquid + vapor) fluid inclusions of variable size that furnish low salinities (0.4–2.2 wt% NaCl equivalent) and liquid-vapor homogenization temperatures (130 °C–155 °C). The fluid inclusions have high concentrations of alkalis (Na, K), Si, Al, Mg, and Mn, and their chemistry resembles orthomagmatic fluid derived from evolved carbonatite melts. They also have elevated MREE and HREE concentrations relative to the LREE, that is significantly higher than in many other hydrothermal settings. Such fluids with fractionated LREE/HREE were likely derived from residual brines post the LREE-mineralization stage, when the HREE still remained in solution. This report of hydrothermal fluids with elevated MREE and HREE solubility from natural carbonatites supports experimental findings that alkali‑carbonate-hydroxyl complexation helps to mobilize REE and fractionate the LREE from MREE and HREE. Magmatic calcite has LREE-enriched REE patterns while those of hydrothermally-altered calcite is nearly flat. The secondary calcite also has elevated concentrations of Na and K, which suggests that the alkali-rich fluids scavenged the REE from magmatic calcite and redistributed them into more insoluble REE-fluorcarbonates. The REE patterns of fluorite mimic those of its fluid inclusions, with the inclusions containing higher and more variable REE concentrations, suggesting that the REE signals of fluorite are coming from the ablation of fluid inclusions, and not from fluorite themselves.

Petrography and geochemistry of carbonatite breccia from Amba Dongar carbonatite complex, Gujarat in the Deccan Large Igneous Province suggest mantle origin

Petrography and geochemistry of carbonatite breccia from Amba Dongar carbonatite complex, Gujarat in the Deccan Large Igneous Province suggest mantle origin

Mineralogical and Geochemical Evidence of Dissolution-Reprecipitation Controlled Hydrothermal Rare Earth Element Mineralization in the Amba Dongar Carbonatite Complex, Gujarat, Western India

Abstract The Amba Dongar carbonatite complex in western India comprises an inner ring of carbonatite breccia surrounded by a sövite ring dike. The various carbonatite units in the body include calcite carbonatite, alvikite, dolomite carbonatite, and ankerite carbonatite. The carbonate phases (calcite and ankerite) occur as phenocrysts, groundmass phases, fresh primary grains, and partially altered grains and/or pseudomorphs when hydrothermally overprinted. Rare earth element (REE) enrichment in the groundmass/altered calcite grains compared to the magmatic ones is ascribed to the presence of micron-sized REE phases. Fluorapatite and pyrochlore constitute important accessory phases that are altered to variable extents. Higher concentrations of Sr, Si, and REEs in fluorapatite are suggestive of a magmatic origin. Fresh pyrochlore preserves its magmatic composition, characterized by low A-site vacancy and high F in the Y-site, which on alteration becomes poorer in Na, Ca, and F and displays an increase in vacancy. The C-O isotope compositions of the carbonates also corroborate the extensive low-temperature hydrothermal alteration of the carbonatites. The REE mineralization is the result of interaction of the carbonatite with a sulfur-bearing, F-rich hydrothermal fluid that exsolved from late-stage carbonatitic magmas. The hydrothermal fluids caused dissolution of the primary carbonates and simultaneous precipitation of REEs and other high field strength element (HFSE)-bearing minerals. Complex spatial associations of the magmatic minerals with the REE fluorocarbonates, [synchysite-(Ce), parisite-(Ce), bastnäsite-(Ce)] and florencite-(Ce) point to the formation of these REE phases as a consequence of postmagmatic hydrothermal dissolution of the REEs from fluorapatite, pyrochlore, and carbonates. Ubiquitous association of fluorite and barite with REE minerals indicates transport of REEs as sulfate complexes in F-rich fluids. Precipitation of REE fluorocarbonates/florencite resulted from fluid-carbonate interaction, concomitant increase in pH, and decrease in temperature. Additionally, REE precipitation was aided and abetted by the removal of sulfur from the fluid by the precipitation of barite, which destabilized the REE sulfate complexes.

Genetic model of carbonatite hosted rare earth elements mineralization from Ambadongar Carbonatite Complex, Deccan Volcanic Province, India

Carbonatites and associated alkaline rocks are the primary sources for REE mineralization. The Ambadongar Carbonatite Complex (ADCC) from NW Deccan Volcanic Province (DVP) constitutes the largest Carbonatite Associated REE Deposits (CARD) in India. ADCC belongs to the final stages of the Late Cretaceous alkaline-carbonatite magmatism associated with main Deccan basalt volcanic activity. The ADCC is an envisioned diatreme structure in which four carbonatitic phases are recognized, mainly calcio-carbonatites and ferro-carbonatites. Each successive carbonatite phase shows higher REE enrichment. The primary REE mineralization with bastnäsite as the dominant REE phase is hosted by pervasive hydrothermally altered ferro-carbonatite plugs. The secondary mineralogy formed with barites in the main orebody during late- to post-magmatic hydrothermal fluid alteration is fluorite, quartz, ankerite, and other REE-bearing minerals like bastnäsite, parisite, synchysite, strontianite, florencite, monazite and columbite.Carbonatite samples contain 18.61% to 52.42% of CaO, and the LOI varies from 5.28% to 38.79%. Most can be classified as calcio-carbonatites. Since all the samples also contain an appreciable amount of Fe2O3 (4.13% to 20.20%) and MnO (0.07% to 5.46%), some may be classified as ferro-carbonatites. Total REE content varies from 0.6 to 4%, with a high Ce concentration and LREE/HREE ratio. The highest values for La, Ce, Pr, and Nd are 1.95%, 1.56%, 0.16%, and 0.45%, respectively.Metasomatism of SCLM from asthenospheric melts followed by the low degree partial melting of the SCLM region is responsible for fertile carbonatite generation in ADCC. The multiphase liquid immiscibility of carbonatite melts from carbonate-silicate magma followed by immiscibility of REE rich carbonatite melt and REE deficient fluoride-rich aqueous fluids explain the higher level of REE enrichment in each successive phases of carbonatites in ADCC.The mineralizing fluids were probably the result of residual magmatic volatiles that brought mainly REE and later SiO2 into the overprinted rocks. Ambadongar carbonatites' stable isotopic compositions agree with a magmatic origin (δ13C = −4.1 ± 1.9‰ [PDB] and δl8O = 10.3 ± 1.7‰ [SMOW]). The C–O stable isotopic modeling indicates re-equilibration under hydrothermal conditions between 180 °C and 70 °C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited during re-equilibration. The REE fluorocarbonate bastnäsite-(Ce) occurs as late individual crystals, overgrown on the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into rare-earth elements' mobility during fluid-rock interaction. Early crystallization of synchysite/parisite indicates the high activity of Ca2+, OH−, (SO4)2−, Al and Si in the fluid. Later, the fluid was characterized by increased activity of F−, (SO4)2−, REE and Si, and decreased activity of Ca2+ as reflected in the association of barite, fluorite, quartz, and bastnäsite typical of strongly overprinted ferro-carbonatites.Re-equilibration and recrystallization of the primary minerals in the presence of OH−, (SO4)2−, F−, REE, Al, and Si carried in solution by the hydrothermal fluid is the leading cause behind the refixing of REE in the form of REE fluorocarbonate in REE rich ferro-carbonatites.

The Origin of Carbonatites from Amba Dongar within the Deccan Large Igneous Province

Abstract There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr–Nd–Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan–Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (∼2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.

A geochemical and Nd, Sr and stable Ca isotopic study of carbonatites and associated silicate rocks from the ~65 Ma old Ambadongar carbonatite complex and the Phenai Mata igneous complex, Gujarat, India: Implications for crustal contamination, carbonate recycling, hydrothermal alteration and source-mantle mineralogy

Major, trace element concentrations and Nd, Sr and Ca stable isotopic compositions (δ44/40Ca and δ44/42Ca w.r.t. NIST SRM915a) of carbonatites and associated igneous silicate rocks from the ~65 Ma old Ambadongar carbonatite complex and the surrounding Phenai Mata igneous complex of western India are reported. Samples of fluorspar from Ambadongar and the Bagh Limestone and Sandstone, which are part of the country rocks at Ambadongar, have also been analysed. The Ambadongar carbonatites are primarily calcio- and ferro-carbonatites while the silicate rocks from these two complexes are alkaline and tholeiitic in composition. The δ44/40Ca values of the carbonatites (0.58–1.1‰, n = 7) and the associated igneous silicate rocks (0.50–0.92‰, n = 14) show a broad range. The low K/Ca values of the carbonatites (<0.2) and silicate rocks (<2) along with their young eruption age (~65 Ma) rule out any effect of radiogenic 40Ca ingrowth due to decay of 40K on the δ44/40Ca values. The lack of correlations between δ44/40Ca and Mg# as well as La/Yb(N) values suggest that the variability in δ44/40Ca is not controlled by the degree of partial melting. The δ44/40Ca values of the carbonatites (0.58–1.1‰) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the δ44/40Ca value of the Bagh Limestone (0.66‰) suggesting that assimilation of these crustal limestones by the magma is unlikely to have caused the variability in δ44/40Ca of the carbonatites. In plots of δ44/40Ca versus εNd(t) and 87Sr/86Sr(t), the igneous silicate rocks from the Ambadongar and Phenai Mata complexes plot on a mixing trend between a primitive (plume) mantle source and the continental crustal basement suggesting the role of continental crustal contamination during eruption of the Reunion plume. While simple binary mixing calculations yield unrealistically high amounts of crustal contamination (40%), assimilation and fractional crystallization (AFC) models suggest up to 20% contribution from a heterogeneous basement for these igneous silicate rocks. The role of continental crustal contamination in the genesis of the igneous silicate rocks is further supported by their unradiogenic εNd(t), radiogenic 87Sr/86Sr(t) and low Ce/Pb values. In contrast, carbonatites plot away from the mixing trend between a primitive mantle (plume) source and continental crust in Ca-Sr-Nd isotopic diagrams suggesting that the Ca isotopic variability of carbonatites is not caused by continental crustal contamination. In contrast, the isotopic composition of the carbonatites can be explained by mixing of the plume end-member with up to 20% of ~160 Ma-old recycled carbonates suggesting their derivation from a highly heterogeneous, recycled carbonate-bearing plume mantle source. The composition of one carbonatite sample showing unusually high δ44/40Ca and highly radiogenic 87Sr/86Sr(t) is explained by hydrothermal alteration which is also invoked for the formation of massive fluorspar deposits with high δ44/40Ca (1.44‰) at Ambadongar. In a plot of δ44/40Ca versus K/Rb, the carbonatites plot towards the phlogopite end-member (δ44/40Ca = 1‰, K/Rb = 40–450) while the igneous silicate rocks plot towards the amphibole end-member (δ44/40Ca = 0.44‰, K/Rb >1000). Phlogopite, especially if F-rich, is stable at greater depths in the mantle compared to amphibole. Hence, the correlated δ44/40Ca and K/Rb values of the carbonatites and associated igneous silicate rocks suggest the derivation of these carbonatites from a relatively deeper mantle source compared to the silicate rocks, both within the Reunion mantle plume. The origin of the carbonatites from the F-rich phlogopite-bearing mantle is also consistent with the occurrence of large fluorspar deposits within the Ambadongar carbonatite complex.

LREE–Nb Mineralization in the South Western Part of Ambadongar Carbonatite Complex, Chhota Udepur District, Gujarat, India

The Ambadongar sub-volcanic carbonatite alkali complex is located about 140 km east of Vadodara, Chhota Udepur district, Gujarat, India and falls in Survey of India Toposheet No. 46 K/1. The complex intrudes Bagh sandstone (Cretaceous) and overlying Deccan basalts (Eocene) and is situated in the Narmada rift zone. The carbonatites came into prominence in the early 1960s with significant discoveries of fluorite and conspicuous radioactivity located near Chhota Udepur, at Ambadongar, the erstwhile Baroda district, Gujarat. Earlier workers have clarified many aspects of the carbonatite complex.

Origin of the Amba Dongar carbonatite complex, India and its possible linkage with the Deccan Large Igneous Province

AbstractThe genetic connection between Large Igneous Province (LIP) and carbonatite is controversial. Here, we present new major and trace element data for carbonatites, nephelinites and Deccan basalts from Amba Dongar in western India, and probe the linkage between carbonatite and the Deccan LIP. Carbonatites are classified into calciocarbonatite (CaO, 39.5–55.9 wt%; BaO, 0.02–3.41 wt%; ΣREE, 1025–12 317 ppm) and ferrocarbonatite (CaO, 15.6–31 wt%; BaO, 0.3–7 wt%; ΣREE, 6839–31 117 ppm). Primitive-mantle-normalized trace element patterns of carbonatites show distinct negative Ti, Zr–Hf, Pb, K and U anomalies, similar to that observed in carbonatites globally. Chondrite-normalized REE patterns reveal high LREE/HREE fractionation; average (La/Yb)N values of 175 in carbonatites and approximately 50 in nephelinites suggest very-low-degree melting of the source. Trace element modelling indicates the possibility of primary carbonatite melt generated from a subcontinental lithospheric mantle (SCLM) source, although it does not explain the entire range of trace element enrichment observed in the Amba Dongar carbonatites. We suggest that CO2-rich fluids and heat from the Deccan plume contributed towards metasomatism of the SCLM source. Melting of this SCLM generated primary carbonated silicate magma that underwent liquid immiscibility at crustal depths, forming two compositionally distinct carbonatite and nephelinite magmas.

Fluid Inclusion Study of Quartz Xenocrysts in Mafic Dykes from Kawant Area, Chhota Udaipur District, Gujarat, India

Abstract Unusual mafic dykes occur in the proximity of the Ambadongar Carbonatite Complex, Lower Narmada Valley, Gujarat, India. The dykes contain dense population of quartz xenocrysts within the basaltic matrix metasomatised by carbonate-rich fluids. Plagioclase feldspars, relict pyroxenes, chlorite, barite, rutile, magnetite, Fe-Ti oxides and glass were identified in the basaltic matrix. Quartz xenocrysts occur in various shapes and sizes and form an intricate growth pattern with carbonates. The xenocrysts are fractured and contain several types of primary and secondary, single phase and two-phase fluid inclusions. The two-phase inclusions are dominated by aqueous liquid, whereas the monophase inclusions are composed of carbonic gas and the aqueous inclusions homogenize to liquid between 226°C and 361°C. Majority of the inclusions are secondary in origin and are therefore unrelated to the crystallization of quartz. Moreover, the inclusions have mixed carbonic-aqueous compositions that inhibit their direct correlation with the crustal or mantle fluids. The composition of dilute CO2-rich fluids observed in the quartz xenocrysts appear similar to those exsolved during the final stages of evolution of the Amba Dongar carbonatites. However, the carbonates are devoid of fluid inclusions and therefore their genetic relation with the quartz xenocrysts cannot be established.

High iridium concentration of alkaline rocks of Deccan and implications to K/T boundary

We report here an unusually high concentration of iridium in some alkali basalts and alkaline rocks of Deccan region having an age of about 65Ma, similar to the age of the Cretaceous-Tertiary boundary. The alkali basalts of Anjar, in the western periphery of Deccan province, have iridium concentration as high as 178pg/g whereas the alkaline rocks and basalts associated with the Amba Dongar carbonatite complex have concentrations ranging between 8 and 80 pg/g. Some of these values are more than an order of magnitude higher than the concentration in the tholeiitic basalts of Deccan, indicating the significance of alkaline magmatism in the iridium inventory at the Cretaceous-Tertiary boundary. Despite higher concentration, their contribution to the global inventory of iridium in the Cretaceous-Tertiary boundary clays remains small. The concentration of iridium in fluorites from Amba Dongar was found to be <30 pg/g indicating that iridium is not incorporated during their formation in hydrothermal activity.

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.

Rayleigh fractionation of stable isotopes from a multicomponent source

A formulation of the Rayleigh equation for the stable isotopic evolution of a multicomponent source reservoir is presented. Its applicability to the carbon and oxygen isotopic evolution of a fluid-rich carbonate magma and the crystallizing calcite carbonatite is demonstrated using data from the Amba Dongar carbonatite complex, Deccan province, India. The initial δ 13C of the parent magma for this complex has been estimated to be −5.3 ± 0.2‰ relative to V-PDB.

Pb, Sr and Nd Isotopic Systematics of the Carbonatites of Sung Valley, Meghalaya, Northeast India: Implications for Contemporary Plume-Related Mantle Source Characteristics

A Pb/Pb–Pb/Pb age of 134± 20 Ma with a model INTRODUCTION l1 of 8·19 ± 0·02 has been obtained for the carbonatites from Carbonatites are carbonate-rich igneous rocks of mantle Sung Valley, Meghalaya, placing their time of emplacement very origin. They are found mainly in continental settings and close to the time assigned for the break-up of Gondwana. Initial their emplacement ages range from Late Archaean to eSr (+5·3 to+7·8), eNd (+1·7 to+2·3), Pb/Pb (19·02), Quaternary (Woolley, 1989). Their high abundances of Pb/Pb (15·67) and Pb/Pb (39·0) indicate a mantle Sr (average 7000 ppm) and Nd (average 250 ppm) source region with a somewhat higher Rb/Sr ratio than Bulk Earth, preclude significant changes in Sr and Nd isotopic ratios minor light rare earth element (LREE) depletion, and a timecaused by crustal contamination and provide a unique integrated enhancement of U/Pb. The isotopic characteristics of the means of characterizing the evolution of the mantle from Sung Valley carbonatites, however, differ significantly from those of Archaean to Recent times (Bell et al., 1982; Bell & basalts from Rajmahal and the Ninetyeast Ridge, which have a Blenkinsop, 1987). In the Indian Peninsula, carbonatites relatively more depleted upper-mantle source component, indicating have been reported from various geological settings with involvement of different mantle sources. The mantle source region of ages ranging from Early Proterozoic to Tertiary (Krishnathe carbonatites is characterized by an EM2–HIMU signature, murthy, 1988; Natarajan et al., 1994). Petrological, minsimilar to that observed for Ambadongar, western India, but different eralogical and chemical data are available for most of from the carbonatites of East Africa, which have an EM1–HIMU the Indian carbonatite complexes (Krishnamurthy, 1988). signature. It is speculated that the Sung Valley carbonatites were derived from a parental magma generated by partial melting of the However, isotopic data remain scarce and are confined sub-continental lithosphere, which was previously metasomatized to Sr/Sr analyses from Sevattur (Deans & Powell, by fluids derived from an EM2–HIMU type mantle plume. The 1968; Kumar & Gopalan, 1991), an Rb–Sr study on data suggest that the pre-130 Ma sub-continental mantle exhibited carbonatites of Hogenekal (Natarajan et al., 1994), an preferentially an EM2 type signature. isotopic study of Sr, Nd and Pb from the Ambadongar carbonatite complex (Veena et al., 1993; Simonetti et al., 1995) and a recent Pb/Pb age determination on the Newania and Sevattur carbonatites of India (Schleicher et al., 1997). This study attempts to define the combined Sr, Nd and Pb isotopic systematics of the Sung Valley

Genesis of the carbonatite-hosted fluorite deposit at Amba Dongar, India; evidence from fluid inclusions, stability isotopes, and whole rock-mineral geochemistry

The Amba Dongar carbonatite complex is located approximately 400 km northeast of Bombay, India, and consists of a carbonatite ring dike and a number of syenitic intrusions which were emplaced into Late Cretaceous Bagh sandstones and Early to Late Eocene Deccan volcanic rocks. Hydrothermal activity associated with intrusion of the carbonatite was responsible for fenitization of the surrounding sandstones, and deposition of economic quantities of fluorite (11.6 Mt of 30% CaF 2 ). Fluorite mineralization occurs as veins and vug fillings, localized along fractures within the calcite carbonatite, near its contact with the sandstone.Fluid inclusions in fluorite indicate a low temperature-low salinity ( -42 and <3.5, respectively. The presence of Al and S in the fluid, the molar equivalence of Na and Cl, and the positive deviation of 18 O and D from the meteoric water line point to a small contribution from orthomagmatic fluids.A model is proposed in which the intrusion of a carbonatite magma at high crustal levels caused faulting and fracturing of the surrounding country rocks, and was accompanied by the release of orthomagmatic fluids, expressed as extensive K and Na metasomatism of the surrounding sandstones. With the decline of orthomagmatic activity, a meteoric water-dominated hydrothermal system was initiated by the heat of the intrusion. The interaction of Ca-bearing meteoric fluids with the last vestiges of F-bearing orthomagmatic NaCl brines caused deposition of large quantities of fluorite at the site of mixing.

Nd, Pb, and Sr isotope systematics of fluorite at the Amba Dongar carbonatite complex, India; evidence for hydrothermal and crustal fluid mixing

The Cretaceous Amba Dongar Complex contains one of the largest fluorite deposits (11.6 million tons at 30 wt % CaF 2 ) associated with any carbonatite. Fluorite mineralization postdates carbonatite emplacement and takes the form of replacement hydrothermal veins concentrated at the carbonatite-country rock contact. Sr isotope ratios from the Amba Dongar fluorites reveal large isotopic variations that can only be attributed to reaction between a carbonatite-derived, F-rich fluid and continental crust (or crustal-derived fluid). The extremely variable initial 87 Sr/ 86 Sr ratios (0.70910-0.71729) are intermediate between those of the carbonatites (0.70549-0.70628) and the host Bagh sandstones (0.75359-0.78274); they correlate negatively with Sr abundances- dances. Sr isotope ratios from intrasample fluorites show large isotopic variations (0.70910-0.71425) that cover almost the entire range of values found for all of the fluorites measured from Amba Dongar. The Pb isotope ratios from the fluorites are also variable and differ from those of the country rocks. Compared to the carbonatites, most contain similar 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios but more radiogenic 206 Pb/ 204 Pb values. Galena and pyrite coeval with fluorite mineralization have Pb isotope ratios which overlap those from most of the fluorites, but which are isotopically distinct from one another. In contrast, initial 143 Nd/ 144 Nd isotope ratios from most of the fluorites are relatively uniform (0.51240-051247), quite different to those from the surrounding Bagh sandstones (0.51122-0.51149), but similar to those from the carbonatites (0.51248-0.51253). The new isotopic results are partly consistent with a model for fluorite deposition involving interaction between ground waters within the surrounding country rocks and an F-rich, carbonatite-derived fluid (Deans and Powell, 1968). The variation in the Sr isotope composition for the fluorites suggests that certain physio-chemical parameters of the mineralizing fluids were changing with time of deposition.

Isotopic data from the Amba Dongar Carbonatite Complex, west-central India: Evidence for an enriched mantle source

Initial 206 Pb 204 Pb (19.05–19.19), 207 Pb 204 Pb (15.72–15.74) and 208 Pb 204 Pb (39.72–40.16) ratios, and ϵ Sr ( + 15.1 to + 19.2) and ϵ Nd (−1.6 to −0.8) -values for the calciocarbonatites from the Cretaceous ( ∼60 Ma) Amba Dongar complex, Gujarat (India) are relatively uniform. The isotopic ratios for the ferrocarbonatites, however, are slightly more variable, in particular the initial 206 Pb 204 Pb (18.94–19.28), 207 Pb 204 Pb (15.66–15.81) and 208 Pb 204 Pb (39.55–40.05) ratios. δ 13C (−4.20 to −3.58‰) and δ 18O ( + 8.80 to + 10.02‰) isotopic ratios for the calciocarbonatites plot within or close to the field defined for primary magmatic carbonatites, whereas the ferrocarbonatites are characterized by much higher δ 13C (−4.29 to −1.62‰) and 8'80 ( + 13.73 to +24.67%o) -values. Variations in Pb, C and O isotopic data from ferrocarbonatites are attributed to fluid activity that is probaby related to formation of associated massive fluorite deposits, known to be of low-temperature hydrothermal origin. In contrast, the high abundances of Nd (average 1191 ppm) and Sr (average 4261 ppm) from the calciocarbonatites, which buffer their initial Nd and Sr isotopic ratios against crustal contamination, and their “mantle-like” C and O isotopic ratios suggest that their isotopic ratios are probably inherited from their mantle source. The initial Nd and Sr isotopic ratios from the Amba Dongar calciocarbonatites indicate derivation from a Rb/Sr- and Nd/Sm-enriched mantle source quite different to most young ( < 200 Ma) carbonatites which have signatures that lie between HIMU and EM I mantle components. The isotopic ratios from the Amba Dongar calciocarbonatites are similar to proximal alkaline complexes of similar age and the least contaminated Deccan flood basalts. We propose, therefore, that the Réunion hot spot responsible for the Deccan flood basalts may have had some bearing on producing the parental melt that generated the Amba Dongar carbonatite.

Economic importance of some carbonatites in India and the relation of Amba Dongar carbonatite complex to plateau basalts

The igneous apatite deposits which are of considerable importance in the fertiliser industry, are associated with three inter-related types of intrusive rocks, namely, carbonatites, nepheline syenites and alkaline ultrabasic rocks. The economic significance of the four carbonatite alkalic complexes is indicated, and the feasibility of utilising the Amba Dongar carbonatite complex for multipurpose chemical industry is discussed. Attention is drawn to the genetic relation of carbonatite alkalic complexes to the alkaline and ultrabasic rocks. In the context of Amba Dongar carbonatite, the differentiation trends in the tholeiitic basalts (involving the earlier separation of pyroxenes), and the relation of the carbonatites, nephelinites and phonolites are delineated with the help of differentiation diagrams and chemical data.