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. 385–360 Ma) 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 490 ppm), apatite (up to 4400 ppm in Kovdor and 3.5 wt.% REE₂O₃ in Khibina), and dolomite (up to 77 ppm), as well as accessory pyrochlore (up to 9.1 wt.% REE₂O₃) and zirconolite (up to 17.8 wt.% REE₂O₃). Late-stage carbonatites, at some localities, are strongly enriched in REE (up to 5.2 wt.% 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.     

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

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

Conditions of accumulation of radioactive metals in the process of differentiation of ultrabasic alkaline-carbonatite rock associations

The distribution of radioactive elements in alkaline rocks from Polar Siberia and Ukraine shows that U and Th are markedly concentrated in carbonatite complex and nepheline syenite as final products of magma fractionation. Peralkaline nepheline syenites from Polar Siberia are characterized by very high contents of radioactive elements, which are close to the economic level. Radioactive elements are also concentrated in rocks of the carbonatite complex. For example, some soevites contain up to 294 × 10^?4%U and 916 × 10^?4% Th. In late dolomite carbonatites, the contents of radioactive elements are appreciably lower. The Th/U ratio in alkaline rocks of Polar Siberia is close to the chondrite value in primary high-Mg rocks and increases in late derivatives: phoscorite, calcite and dolomite carbonatites. The main amount of radioactive elements is contained in rare-metal accessory minerals: perovskite, pyrochlore, calzirtite, and apatite. Rock-forming minerals are distinguished by very low concentrations of radioactive elements. In alkaline series of the Chernigovka massif (Ukraine), U and Th also accumulate in the course of crystal fractionation, especially in phoscorites from the carbonatite complex. Mantle xenoliths and alkaline rocks from Ukraine reveal uranium specialization. Most likely, the discrepancy in fractionation of radioactive elements between Polar Siberia and Ukraine is caused by different geodynamic regimes of these provinces. The Mesozoic alkaline magmatism of Polar Siberia is a part of the Siberian superplume, whereas the Proterozoic alkaline complex in Ukraine is related to subduction of the oceanic crust.     

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.     

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.     

Carbonatite-hosted niobium deposit at Aley,northern British Columbia (Canada): Mineralogy,geochemistry and petrogenesis

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5 m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nb was mobilized only to a very limited extent and sequestered in a variety of minerals (fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F⁻, (PO₄)^3 −, U, Th and rare-earth elements, but not Nb.     

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.Schematic reactions were proposed for zirconolite transformation to secondary phases: (Ca,Th,U)Zr(Ti,Nb)₂O₇ (zirconolite) + SiO₂ + Ca(F,OH)₂ → ZrSiO₄ (zircon) + (Ca,Th,U)₂(Ti,Nb)₂O₆(OH,F) (Th-calciobetafite) and (Ca,Th,U)₂(Ti,Nb)₂O₆(OH,F) + Na₂Si₂O₅ → ThSiO₄ (thorite) + (Ca,Na,Th)₂(Nb,Ti)₂O₆(OH,F) (Th-pyrochlore), where SiO₂, Ca(F,OH)₂, and Na₂Si₂O₅ are the components of melt-solution coexisting with the carbonatite.It was shown that the distribution and behavior of rare and radioactive elements in oceanic carbonatites show the same tendencies as in continental carbonatites. The contents and distribution of Ti, Ta, and Th in zirconolites and pyrochlores from oceanic and continental carbonatites are different: the minerals of oceanic carbonatites are enriched in Ti and Th and strongly depleted in Ta.     

Hafnium–Neodymium Isotope Systematics of Carbonatites from the Guli Massif (Maimecha–Kotui Province,Russia)

Doklady Earth Sciences - This work presents new data on the Hf-isotope systematics of baddeleyite and the Nd-isotope compositions of calcite carbonatites of the Guli massif, located within the...     

Main minerals of abnormally high-grade ores of the Tomtor deposit (Arctic Siberia)

The Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~ 250 km²) is ~ 20 km in diameter, with a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. All rocks are weathered and covered with eluvium, which is the thickest after carbonatites enriched in phosphates and REE. The weathering profile consists of four layers, from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in “sagging” eluvium. The ores are laminated and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb₂O₅, 7-10% REE₂O₃, 0.75% Y₂O₃, and 0.06% Sc₂O₃). The highest-grade ores are natural Nb and REE concentrates. The total REE content in some layers is > 10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The ore-forming minerals belong to the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals occur mainly as crystals that were completely replaced by barium-strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure, with an unaltered pyrochlore core and a reaction rim. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~ 50 nm particles, which cover the outer part of halloysite tubes (800–3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates. The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal-sedimentary process with a presumably important role of bioaccumulation of REE phosphates.     

New data on carbonatites of the Il’mensky-Vishnevogorsky alkaline complex,the southern Urals,Russia

Carbonatites that are hosted in metamorphosed ultramafic massifs in the roof of miaskite intrusions of the Il’mensky-Vishnevogorsky alkaline complex are considered. Carbonatites have been revealed in the Buldym, Khaldikha, Spirikha, and Kagan massifs. The geological setting, structure of carbonatite bodies, distribution of accessory rare-metal mineralization, typomorphism of rock-forming minerals, geochemistry, and Sr and Nd isotopic compositions are discussed. Dolomite-calcite carbonatites hosted in ultramafic rocks contain tetraferriphlogopite, richterite, accessory zircon, apatite, magnetite, ilmenite, pyrrhotite, pyrite, and pyrochlore. According to geothermometric data and the composition of rock-forming minerals, the dolomite-calcite carbonatites were formed under K-feldspar-calcite, albite-calcite, and amphibole-dolomite-calcite facies conditions at 575–300°C. The Buldym pyrochlore deposit is related to carbonatites of these facies. In addition, dolomite carbonatites with accessory Nb and REE mineralization (monazite, aeschynite, allanite, REE-pyrochlore, and columbite) are hosted in ultramafic massifs. The dolomite carbonatites were formed under chlorite-sericite-ankerite facies conditions at 300–200°C. The Spirikha REE deposit is related to dolomite carbonatite and alkaline metasomatic rocks. It has been established that carbonatites hosted in ultramafic rocks are characterized by high Sr, Ba, and LREE contents and variable Nb, Zr, Ti, V, and Th contents similar to the geochemical attributes of calcio-and magnesiocarbonatites. The low initial ⁸⁷Sr/⁸⁶Sr = 0.7044?0.7045 and εNd ranging from 0.65 to ?3.3 testify to their derivation from a deep mantle source of EM₁ type.     

Petrography and mineral chemistry of Alkaline-Carbonatite Complex in Singhbhum Crustal Province,Purulia region,Eastern India

The variant rock types of an Alkaline-Carbonatite Complex (ACC) comprising alkali pyroxenite, nepheline syenite, phoscorite, carbonatite, syenitic fenite and glimmerite along with REE and Nb-mineralization are found at different centres along WNW-ESE trending South Purulia Shear Zone (SPSZ) in parts of Singhbhum Crustal Province. The ACC occurs as intrusions within the Mesoproterozoic Singhbhum Group of rocks. Alkali pyroxenite comprises of aegirine augite, magnesiotaramite, magnesiokatophorite as major constituents. Pyrochlore and eucolite are ubiquitous in nepheline syenite. Phoscorite contains fluorapatite, dahllite, collophane, magnetite, hematite, goethite, phlogopite, calcite, sphene, monazite, pyrochlore, chlorite and quartz. Coarse fluorapatite shows overgrowth of secondary apatite (dahllite). Secondary apatite is derived from primary fluorapatite by solution and reprecipitation. The primary fluorapatite released REE to crystallize monazite grains girdling around primary apatite. Carbonatite is composed dominantly of Srcalcite along with dolomite, tetraferriphlogopite, phlogopitic biotite, aegirine augite, richterite, fluorapatite, altered magnetite, sphene and monazite. The minerals comprising of the carbonatite indicate middle stage of carbonatite development. Fenite is mineralogically syenite. Glimmerite contains 50–60% tetraferriphlogopite. An alkali trend in the evolution of amphiboles (magnesiotaramite-magnesiokatophorite-richterite) and chinopyroxenes (aegirine augite, aegirine) during the crystallization of the suite of rocks is noted. Monazite is the source of REE in phoscorite and carbonatite. Fluorapatite has low contents of REE, PbO, ThO₂ and UO₂. Pyrochlore reflects Nb-mineralization in nepheline syenite and it is enriched in Na₂O, CaO, TiO₂, PbO and UO₂. Pyrochlore containing UO₂ (6.605%) and PbO (0.914%) in nepheline syenite has been chemically dated at 948 ± 24 Ma by EPMA.     

Constraints of the formation of carbonatites in the Chernigovka Massif,Azov Region,Ukraine

Data on compositions of coexisting minerals in the graphite-bearing carbonatites of the Chernigovka massif are reported. Thermodynamic analysis of these results made it possible to establish that the temperature of equilibrium between graphite, dolomite, calcite, magnetite, and olivine for silica activity buffered by the (zircon + baddeleyite) assemblage is approximately 600°C. The minimal pressure of formation of these mineral assemblages is approximately 0.2 GPa, which is consistent with estimates of the erosion depth for the Chernigovka massif. The oxygen fugacity typical of the graphite-bearing carbonatite is 0.6–0.8 log units below the quartz-magnetite-fayalite buffer. Such values are typical of magmatic systems, e.g., basalts of the mid-ocean ridges (MORB). At 600°C, the gas phase in the C-H-O system equilibrated with the mineral assemblage of the carbonatite studied is dominated by CO₂ and H₂O, whereas methane-rich fluids appear at lower temperatures.     

REE-bearing minerals in a Ti-rich vein from the Adamello contact aureole (Italy)

Zirconolite, aeschynite-(Ce), titanite and apatite have been found as minor or accessory minerals in a Ti-rich (TiO₂=2.1–4.5 wt.%) hydrothermal vein occurring in dolomite marbles at the contact with a tonalite intrusion of the Tertiary Adamello batholith (northern Italy). The vein consists of four distinct mineral zones, comprising from margin to center: (1) forsterite+calcite, (2) pargasite+calcite+titanite+sulfides, (3) phlogopite +calcite+titanite+sulfides, and (4) titanian clinohumite +spinel+calcite+sulfides. Zirconolite occurs in two vein zones only: in the phlogopite zone it is invariably anhedral, often corroded, and exhibits complex chemical zonation patterns. In the titanian clinohumite zone zirconolite is idiomorphic and characterized by a pronounced discontinous chemical zoning, but shows no evidence of corrosion. The considerable compositional variation observed for zirconolite (in wt.%: (REE₂O₃)=0.74–16.8, UO₂=0.59–24.0, ThO₂=0.67–17.1) is due to the zoning, and may be attributed to four major substitutions described by the exchange vectors:

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO₃, Ba and Sr. In addition to the presence of SO₃, calculated fO₂ for magnetites indicate a high oxygen fugacity and that Fe⁺³>Fe⁺² in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO₃-rich and characterized by high oxygen fugacity.     

The fluid phase evolution during the formation of carbonatite of the Guli massif: Evidence from the isotope (C,N, Ar) data

The first data on variations of the isotope composition and element ratios of carbon, nitrogen, and argon in carbonatites of different generations and ultrabasic rocks of the Guli massif obtained by the method of step crushing are reported. It is shown that early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotope compositions of carbon (CO₂), argon, and hydrogen (H₂O). The data obtained allow us to conclude that the mantle component predominated in the fluid at the early stages of formation of rocks of the Guli massif, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon, and most likely a high portion of carbon dioxide with isotopically heavy carbon.     

In situ LA-ICPMS Isotopic and Geochronological Studies on Carbonatites and Phoscorites from the Guli Massif,Maymecha-Kotuy,Polar Siberia

In this study we present a fresh isotopic data, as well as U–Pb ages from different REE-minerals in carbonatites and phoscorites of Guli massif using in situ LA-ICPMS technique. The analyses were conducted on apatites and perovskites from calcio-carbonatite and phoscorite units, as well as on pyrochlores and baddeleyites from the carbonatites. The ⁸⁷Sr/⁸⁶Sr ratios obtained from apatites and perovskites from the phoscorites are 0.70308–0.70314 and 0.70306–0.70313, respectively; and 0.70310–0.70325 and 0.70314–0.70327, for the pyrochlores and apatites from the carbonatites, respectively.Furthermore, the in situ laser ablation analyses of apatites and perovskites from the phoscorite yield εNd from 3.6 (±1) to 5.1 (±0.5) and from 3.8 (±0.5) to 4.9 (±0.5), respectively; εNd of apatites, perovskites and pyrochlores from carbonatite ranges from 3.2 (±0.7) to 4.9 (±0.9), 3.9 (±0.6) to 4.5 (±0.8) and 3.2 (±0.4) to 4.4 (±0.8), respectively. Laser ablation analyses of baddeleyites yielded an eHf(t)d of +8.5 (± 0.18); prior to this study Hf isotopic characteristic of Guli massif was not known. Our new in situ εNd, ⁸⁷Sr/⁸⁶Sr and eHf data on minerals in the Guli carbonatites imply a depleted source with a long time integrated high Lu/Hf, Sm/Nd, Sr/Rb ratios.In situ U–Pb age determination was performed on perovskites from the carbonatites and phoscorites and also on pyrochlores and baddeleyites from carbonatites. The co-existing pyrochlores, perovskites and baddeleyites in carbonatites yielded ages of 252.3 ± 1.9, 252.5 ± 1.5 and 250.8 ± 1.4 Ma, respectively. The perovskites from the phoscorites yielded an age of 253.8 ± 1.9 Ma. The obtained age for Guli carbonatites and phoscorites lies within the range of ages previously reported for the Siberian Flood Basalts and suggest essentially synchronous emplacement with the Permian-Triassic boundary.     

Composition and conditions of crystallization of zircon from the rare-metal ores of the Gremyakha–Vyrmes Massif,Kola Peninsula

The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.     

Initial Os-isotopic composition of Os-Ir-Ru alloys from ultramafic massifs of the Polar Siberia

This study firstly presents chemical and initial Os-isotopic compositions of Os-Ir-Ru minerals of two ultramafic formations of Polar Siberia, which are exemplified by Guli clinopyroxene-dunite massif of the Maimecha-Kotui Province and the Kunar dunite-harzburgite massif from the Chelyuskin ultramafic belt of the Taimyr Peninsula. The study employed a range of methods, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and laser ablation attached to an inductively coupled plasma mass spectrometry (LA MC-ICP-MS). The majority of platinum-group minerals (PGM) from the Guli massif are Os-(Ir-Ru) solid solutions or Os-rich minerals. At Kunar, minerals of Ru-Os-Ir system (i.e., osmium, ruthenium, iridium and rutheniridosmine) dominate the PGM assemblage. The ruthenium trend in the mineral compositions is due to the formation of these minerals under high pressures and temperatures at considerable depths. The ¹⁸⁷Os/¹⁸⁸Os values of Os-rich minerals from the Guli massif range from 0.12309 ± 0.00002 to 0.12606 ± 0.00003 (n = 168). The initial Os-isotopic composition of PGM from the central block of the Guli massif is characterized by the ¹⁸⁷Os/¹⁸⁸Os values, varying in the range 0.12404–0.12606. Osmiumrich minerals from the southwestern block of the Guli massif are characterized by the least “radiogenic” ¹⁸⁷Os/¹⁸⁸Os values (i.e., 0.12309–0.12341). Low relative to the chondritic universal reservoir (CHUR) ¹⁸⁷Os/¹⁸⁸Os values are indicative of a near-to-chondritic source of platinum-group elements (PGE). The most “productive” stage of PGM formation at Guli (n = 121) is recorded in the time interval of 545–615 Ma. The older model ¹⁸⁷Os/¹⁸⁸Os ages of osmium minerals are characteristic of the southwestern block of the Guli massif (e.g., 745–760 Ma). The results of the initial Os-isotopic composition for Os-rich alloys are consistent with a model, in which PGM were formed during multi-stage melt depletion events in the mantle. This agrees well with the suggestion that the Guli massif consists of heterogeneous blocks of ultramafic rocks. The ¹⁸⁷Os/¹⁸⁸Os ratio in the investigated PGM from the Kunar massif varies in a wider range (0.1094–0.1241, n = 28). For the dominant set of PGM samples (n = 25), regardless of their chemical composition, four groups of the initial osmium isotopic compositions can be estimated, with average ¹⁸⁷Os/¹⁸⁸Os values of 0.1217 ± 0.0002 (n = 7), 0.1223 ± 0.0002 (n = 7), 0.1230 ± 0.0002 (n = 6) and 0.1238 ± 0.0003 (n = 6), respectively. The average model Re-Os ages for the defined groups of the Kunar massif are consistent with Late Riphean age interval (e.g., 975 ± 42 Ma, 892 ± 42 Ma, 791 ± 28 Ma and 681 ± 42 Ma, respectively). Significant variations in the ¹⁸⁷Os/¹⁸⁸Os values and model ages for Ru-Os-Ir alloys at Kunar are close to those from other duniteharzburgite massifs of the Earth, pointing out for their prolonged multi-stage evolution within the upper mantle.     

The crystal chemistry of roméite

Roméite (Ca, Fe, Mn, Na)₂(Sb⁵⁺, Ti⁴⁺)₂(O, OH, F)₇ is a rare mineral found in metamorphic iron-manganese deposits and in hydrothermal Sb-bearing veins. It is isostructural with the pyrochlore-group minerals of the general formula A_2–mB₂X_6–wY_1–n · pH₂O. The pyrochlore-group minerals are important Nb and Ta ores, and are also used as an actinide host phase in␣radioactive waste. The crystal chemistry of roméite from the type locality Praborna (Italy), from Massiac (France), and from four newly discovered localities in␣the Swiss Alps, and of “lewisite”, a questionable species related to roméite from Tripuhy (Brazil), is compared to that of pyrochlore. A wide range of substitutions has been observed including (1) independent substitutions on the A- and B-sites, and (2) coupled substitutions between the A- and B- and between the A- and Y- sites. Only the roméite from Massiac, derived from weathering of stibnite, contains significant H₂O (up to 14 wt %). The A-site vacancies in roméite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of roméite varies from locality to locality; it can be dominated by F, OH, or be fully vacant. The “lewisite” octahedral crystals studied are a sub-microscopic mixture of roméite with a mineral structurally related to pyrochlore, which grows at the expense of roméite. Received: 5 March 1996 / Accepted: 18 October 1996     

REE mineralization in the carbonatites of the sung valley ultramafic-alkaline-carbonatite complex,Meghalaya, India

The Early Cretaceous Sung Valley Ultramafic-Alkaline-Carbonatite (SUAC) complex intruded the Proterozoic Shillong Group of rocks and located in the East Khasi Hills and West Jaintia Hills districts of Meghalaya. The SUAC complex is a bowl-shaped depression covering an area of about 26 km² and is comprised serpentinised peridotite forming the core of the complex with pyroxenite rim. Alkaline rocks are dominantly ijolite and nepheline syenite, occur as ring-shaped bodies as well as dykes. Carbonatites are, the youngest intrusive phase in the complex, where they form oval-shaped bodies, small dykes and veins. During the course of large scale mapping in parts of the Sung Valley complex, eleven carbonatite bodies were delineated. These isolated carbonatite bodies have a general NW-SE and E-W trend and vary from 20–125 m long and 10–40 m wide. Calcite carbonatite is the dominant variety and comprises minor dolomite and apatite and accessory olivine, magnetite, pyrochlore and phlogopite. The REE-bearing minerals identified in the Sung Valley carbonatites are bastnäsite-(Ce), ancylite-(Ce), belovite-(Ce), britholite-(Ce) and pyrochlore that are associated with calcite and apatite. The presence of REE carbonates and phosphates associated with REE-Nb bearing pyrochlore enhances the economic potential of the Sung Valley carbonatites. Trace-element geochemistry also reveals an enrichment of LREEs in the carbonatites and average ΣREE value of 0.102% in 26 bed rock samples. Channel samples shows average ΣREE values of 0.103 wt%. Moreover, few samples from carbonatite bodies has indicated relatively higher values for Sn, Hf, Ta and U. Since the present study focuses surface evaluation of REE, therefore, detailed subsurface exploration will be of immense help to determine the REE and other associated mineralization of the Sung Valley carbonatite prospect.