The concentration and redistribution of ore components from a primary melt to hydrothermal fluids are important for understanding ore formation. The Mushgai-Khudag complex is a typical example of an ...intrusion where hydrothermal processes are widespread and where we can observe the redistribution of ore components during hydrothermal processes. In this study, we use mineralogical, melt and fluid inclusion data to trace element characteristics of apatite from the Mushgai-Khudag complex to reconstruct the formation of the magnetite-apatite rocks and their magmatic-hydrothermal evolution and to clarify the origin of the REE mineralization. We conclude that the magnetite-apatite rocks crystallized from salt melt with a high content of phosphate and sulfate components at a temperature of approximately 830–850 °C. The origin of magnetite-apatite rocks probably can be explained by the silicate-salt immiscibility that occurred at the alkaline syenite crystallization stage. Further evolution of the salt melt to the brine of the carbonate-(fluoride)-chloride-sulfate composition was accompanied by the barite, celestite and monazite-Ce formation at the temperature of approximately 500–580 °C. The dissolution of apatite and the crystallization of gypsum, phosphosiderite and monazite-Ce pseudomorphs after apatite took place at the hydrothermal stage after a reaction with a fluid that evolved from carbonate-chloride-sulfate (at 250–350 °C) into a predominantly chloride composition (at 150–250 °C). The high activity of the sulfate component and a significant enrichment of the rocks in REE also occurred at the late hydrothermal stage.
•The magnetite-apatite rocks have been formed from the phosphate-sulfate salt melt.•The rocks are the product of the silicate-salt liquid immiscibility.•High activity of the SO42— was revealed at the magmatic and hydrothermal stages.
The Mushgai-Khudag alkaline‑carbonatite complex, located in southern Mongolia within the Central Asian Orogenic Belt (CAOB), comprises a broad range of volcanic and subvolcanic alkaline silicate ...rocks (melanephelinite-trachyte and shonkinite-alkaline syenite, respectively). Magnetite-apatite rocks, carbonatites, and fluorite mineralization are also manifested in this area. The complex formed between 145 and 133 Ma and is contemporaneous with late Mesozoic alkaline–carbonatite magmatism within the CAOB. Major and trace element characteristics of silicate rocks in the Mushgai-Khudag complex imply that these rocks were formed by the fractional crystallization of alkaline ultramafic parental magma. Magnetite-apatite rocks may be a product of silicate-Ca-Fe-P liquid immiscibility that took place during the alkaline syenite crystallization stage. The Mushgai-Khudag rocks have variable and moderately radiogenic Sr (87Sr/86Sr(i) = 0.70532–0.70614), ƐNd(t) = −1.23 to 1.25) isotopic compositions. LILE/HFSE values and SrNd isotope compositions indicate that the parental melts of Mushgai-Khudag were derived from a lithospheric mantle source that was affected by a metasomatic agent in the form a mixture of subducted oceanic crust and its sedimentary components. The δ18OSMOW and δ18CPDB values for calcites in carbonatites range from 16.8‰ to 19.2‰ and from −3.9‰ to 2.0‰, respectively. CO covariations in calcites of the Mushgai-Khudag carbonatites can be explained by the slight host limestone assimilation.
•The Mushgai-Khudag alkaline rocks formed between 145 and 133 Ma.•The rocks formed by fractional crystallization and liquid immiscibility processes.•They originated from heterogeneous lithospheric mantle source.•The mantle source was affected by metasomatic agent.•This agent could be a mixture of subducted oceanic crust and sediment components
Rippite K2(Nb,Ti)2(Si4O12)(O,F)2, a new K-Nb-cyclosilicate, has been discovered in calciocarbonatites from the Chuktukon massif (Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, ...Russia). It was found in a primary mineral assemblage, which also includes calcite, fluorcalciopyrochlore, tainiolite, fluorapatite, fluorite, Nb-rich rutile, olekminskite, K-feldspar, Fe-Mn–dolomite and quartz. Goethite, francolite (Sr-rich carbonate–fluorapatite) and psilomelane (romanèchite ± hollandite) aggregates as well as barite, monazite-(Ce), parisite-(Ce), synchysite-(Ce) and Sr-Ba-Pb-rich keno-/hydropyrochlore are related to a stage of metasomatic (hydrothermal) alteration of carbonatites. The calcite–dolomite coexistence assumes crystallization temperature near 837 °C for the primary carbonatite paragenesis. Rippite is tetragonal: P4bm, a = 8.73885(16), c = 8.1277(2) Å, V = 620.69(2) Å3, Z = 2. It is closely identical in the structure and cell parameters to synthetic K2Nb2(Si4O12)O2 (or KNbSi2O7). Similar to synthetic phase, the mineral has nonlinear properties. Some optical and physical properties for rippite are: colorless; Mohs’ hardness—4–5; cleavage—(001) very perfect, (100) perfect to distinct; density (meas.)—3.17(2) g/cm3; density (calc.)—3.198 g/cm3; optically uniaxial (+); ω = 1.737-1.739; ε = 1.747 (589 nm). The empirical formula of the holotype rippite (mean of 120 analyses) is K2(Nb1.90Ti0.09Zr0.01)Si4O12(O1.78OH0.12F0.10). Majority of rippite prismatic crystals are weakly zoned and show Ti-poor composition K2(Nb1.93Ti0.05Zr0.02)Si4O12(O1.93F0.07). Raman and IR spectroscopy, and SIMS data indicate very low H2O content (0.09–0.23 wt %). Some grains may contain an outermost zone, which is enriched in Ti (+Zr) and F, up to K2(Nb1.67Ti0.32Zr0.01)Si4O12(O1.67F0.33). It strongly suggests the incorporation of (Ti,Zr) and F in the structure of rippite via the isomorphism Nb5+ + O2− → (Ti,Zr)4+ + F1−. The content of a hypothetical end-member K2Ti2Si4O12F2 may be up to 17 mol. %. Rippite represents a new structural type among Si4O12-cyclosilicates because of specific type of connection of the octahedral chains and Si4O128− rings. In structural and chemical aspects it seems to be in close with the labuntsovite-supergroup minerals, namely with vuoriyarvite-(K), K2(Nb,Ti)2(Si4O12)(O,OH)2∙4H2O.
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 SO
3
, Ba and Sr. In addition to the presence of SO
3
, calculated
f
O
2
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 SO
3
-rich and characterized by high oxygen fugacity.
The Arbarastakh ultramafic carbonatite complex is located in the southwestern part of the Siberian Craton and contains ore-bearing carbonatites and phoscorites with Zr-Nb-REE mineralization. Based on ...the modal composition, textural features, and chemical compositions of minerals, the phoscorites from Arbarastakh can be subdivided into two groups: FOS 1 and FOS 2. FOS 1 contains the primary minerals olivine, magnetite with isomorphic Ti impurities, phlogopite replaced by tetraferriphlogopite along the rims, and apatite poorly enriched in REE. Baddeleyite predominates among the accessory minerals in FOS 1. Zirconolite enriched with REE and Nb and pyrochlore are found in smaller quantities. FOS 2 has a similar mineral composition but contains much less olivine, magnetite is enriched in Mg, and the phlogopite is enriched in Ba and Al. Of the accessory minerals, pyrochlore predominates and is enriched in Ta, Th, and U; baddeleyite is subordinate and enriched in Nb. Chemical and textural differences suggest that the phoscorites were formed by the sequential introduction of different portions of the melt. The melt that formed the FOS 1 was enriched in Zr and REE relative to the FOS 2 melt; the melt that formed the FOS 2 was enriched in Al, Ba, Nb, Ta, Th, U, and, to a lesser extent, Sr.
In this paper, we study the geochronology, mineral chemistry, and whole-rock elemental, stable (O, C, D) and Sr-Nd isotopic data for alkaline ultrabasic–basic massifs of the Vitim alkaline province ...(Sayzhenski complex) in the Central Asian Orogenic Belt, near the boundary with the Siberian craton, to evaluate their petrogenesis and geodynamic significance. U-Pb zircon dating results in Early Paleozoic (520–486Ma) and Late Paleozoic (306–294Ma) stages of alkaline rock formation. The mineralogy and geochemistry exhibit a wide range of SiO2 (38–73wt.%), enrichment in Sr, Ba, LREE and Ta and, most significantly, in Na and Al. The rocks crystallized from a parental CO2- and H2O-rich silica-undersaturated melt. Isotopically, the rocks are highly variable, with (87Sr/86Sr)i – 0.705595–0.707729 and (143Nd/144Nd)i – 0.512237–0.512643. The geochemical and isotope data suggest that the rocks were derived from a source composed of three distinct components: PREMA, EM II and marine carbonate. Additionally, stable (O, C, D) isotope data display the shifting influence of assimilated organic sediments in the source of melts and a partial secondary isotope exchange between the late-magmatic fluids and minerals.
► There are Early and Late Paleozoic stages of the rocks formation. ► The rocks were derived from a source composed of three components PREMA, EM II and marine carbonate. ► Contamination was responsible for change in the composition in the system.
•The fluorite mineralization is associated with carbonatite magmatism.•Fluorites are extremely enriched in REE, especially light.•Formation temperatures of fluorites reach more than 500 °C.
The ...Mushgai-Khudag complex is part of the Late Mesozoic Central Asian carbonatite province. Fluorite mineralization is manifested throughout the province, including the Mushgai-Khudag complex. We have investigated the geochemical features and fluid inclusions of fluorites from different types of fluorite-bearing rocks. Fluorite from quartz-fluorite rocks has rare earth element (REE) concentrations in the range of 10500−144300 ppm and the highest light REE contents, with (La/Yb)N = 56−960. Fluorite from the fluorite-apatite-celestine rocks has slightly lower REE enrichment, especially light REE content, with concentrations of 200−5900 ppm and (La/Yb)N = 18−204. Fluorite from the fluorite-calcite rocks is characterized by REE contents of 22−1100 ppm and a variable (La/Yb)N of 0.6−59. These variations in the fluorite REE composition from different types of rocks were probably caused by the fact that at elevated temperatures, fluorine-containing light REE complexes are more stable than fluorine-containing heavy REE complexes. The progressive enrichment of medium and heavy REEs in the latter fluorite is related to fluid evolution. The homogenization temperature and salinity values of fluid inclusions in the Mushgai-Khudag fluorites vary between 550 and 185 °C and from rather high to 2 wt.%, respectively. The parental fluids of the fluorite-bearing rocks evolved from quartz-fluorite rocks to fluorite-apatite-celestine rocks to fluorite-calcite rocks. The key component was changed from sulfate to carbonate-chloride along with the high to medium temperature decrease (∼500−245 °C).
The world-class Katugin deposit (Eastern Siberia, Russia) in the high-F alkaline granite of the Katugin complex is located within the Early Proterozoic Stanovoy orogenic belt on the southeastern ...periphery of the Siberian craton. The deposit stores economic amounts of Nb, Y, Zr, REE, Ta, Th, U, and cryolite. Its formation was previously interpreted in terms of a single-stage model, but new zircon data reveal an additional stage in its history. That stage was separated in time from the magmatic activity and led to a redistribution of REE + Y and, probably, additional enrichment of rocks in these components.
Zircon in the deposit area occurs ubiquitously in alkaline granite and forms zones at granite contact with cryolite veins and lenses. Four types of zircon have been identified based on their microstructure. Type I zircon with magmatic signatures makes relict cores in zircon grains and has an age of 2064 ± 5 Ma in granite and 2080 ± 10 in ores (> 20 vol% of zircon), which is coeval with the emplacement of the Katugin granite. This type of zircon exhibits positive ɛHf(t) values, from +3.3 to +1.1, testifying for a juvenile source. Type II zircon (1921 ± 11 Ma) occurs within a core at the granite-cryolite contact. Type III zircon (1900 ± 6 Ma), heterogeneous in cathodoluminescence (CL) images, overgrows Type I and II zircon grains. The zircon zones with low or no CL response are characterized by high concentrations of REE2O3, Y2O3 and P2O5. This type of zircon contains abundant multiphase (solids + fluid) and polycrystalline inclusions, with a large percentage of REE + Y fluoride and fluorocarbonate among solids and a high-density CO2-enriched fluid phase. Type IV homogeneous zircon (1909 ± 10 Ma) rims are observed in most zircon grains. Types II, III and IV zircon have negative ɛHf values, from −2.2 to −3.8. They are coeval and crystallized during high-grade metamorphism and magmatism, which accompanied the final amalgamation of the Siberian craton at 1.93–1.88 Ga. The formation of Type III zircon occurred in the presence of high-F CO2-rich fluids, possibly, coming from a mantle source, which supposedly led to redistribution and additional inputs of REE in the granite.
•The world-class Katugin rare metal deposit was formed in two stages: magmatic and metasomatic.•The economic Zr mineralization was formed during the magmatic stage (2064 ± 5 Ma).•REE + Y and cryolite ores result from metasomatic stage (1904 ± 6 Ma).•Nb mineralization developed during all stages, including metamorphic events.
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•The Seligdar carbonatites were overprinted by hydrothermal and metamorphic events.•Carbonatites are dominated by a component derived from an enriched mantle source.•The enriched ...mantle source was separated from the depleted mantle reservoir at the Archean period.
The Paleoproterozoic Seligdar magnesiocarbonatite intrusion of the Aldan-Stanovoy shield in Russia underwent extensive postmagmatic hydrothermal alteration and metamorphic events. This study comprises new isotopic (Sr, Nd, C and O) data, whole-rock major and trace element compositions and trace element characteristics of the major minerals to gain a better understanding of the source and the formation process of the carbonatites. The Seligdar carbonatites have high concentrations of P2O5 (up to 18 wt%) and low concentrations of Na, K, Sr and Ba. The chondrite-normalized REE patterns of these carbonatites display significant enrichments of LREE relative to HREE with an average La/Ybcn ratio of 95. Hydrothermal and metamorphic overprints changed the trace element characteristics of the carbonatites and their minerals. These alteration processes were responsible for Sr loss and the shifting of the Sr isotopic compositions towards more radiogenic values. The altered carbonatites are further characterized by distinct 18O- and 13C-enrichments compared to the primary igneous carbonatites. The alteration most likely resulted from both the percolation of crustal-derived hydrothermal fluids and subsequent metamorphic processes accompanied by interaction with limestone-derived CO2. The narrow range of negative εNd(T) values indicates that the Seligdar carbonatites are dominated by a homogenous enriched mantle source component that was separated from the depleted mantle during the Archean.
Here we present an experimental study of the distribution of a broad range of trace elements between carbonatite melt, calcite and fluorite. The experiments were performed in the CaCO.sub.3 + ...CaF.sub.2 + Na.sub.2CO.sub.3 ± Ca.sub.3(PO.sub.4).sub.2 synthetic system at 650-900 °C and 100 MPa using rapid-quench cold-seal pressure vessels. Starting mixtures were composed of reagent-grade oxides, carbonates, Ca.sub.3(PO.sub.4).sub.2 and CaF.sub.2 doped with 1 wt% REE-HFSE mixture. The results show that the distribution coefficients of all the analyzed trace elements for calcite and fluorite are below 1, with the highest values observed for Sr (0.48-0.8 for calcite and 0.14-0.3 for fluorite) and Y (0.18-0.3). The partition coefficients of REE gradually increase with increasing atomic number from La to Lu. The solubility of Zr, Hf, Nb and Ta in the synthetic F-rich carbonatitic melts, which were used in our experiments, is low and limited by crystallization of baddeleyite and Nb-bearing perovskite.
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