Phoscorite-carbonatite Complex Research Articles

Phoscorites and Carbonatites: Relations, Possible Petrogenetic Processes, and Parental Magma, with Reference to the Kovdor Massif, Kola Peninsula

The paper presents petrochemical, geochemical, and mineralogical data on rocks of the phoscorite–carbonatite complex in the Kovdor alkaline–ultramafic massif, Kola Peninsula, Russia. In contrast to what is usually thought, trace elements were determined to be concentrated not in the carbonatites themselves but in the related phoscorites. The paper presents data on the evolution of the compositions and zoning of minerals in successive generations of both rock types as a result of the fractional crystallization of their parental magmas, and data on general trends in the concentrations of trace elements. The Fe-rich phosphate–carbonate melt seems to have separated from the primitive alkaline–ultramafic silicate melt during its advanced differentiation. The Fe-rich phosphate–carbonate magma was parental for the phoscorite–carbonatite complex. The possibility of its splitting into immiscible Fe-rich phosphate and carbonatite melts is discussed.

Atmospheric S and lithospheric Pb in sulphides from the 2.06 Ga Phalaborwa phoscorite-carbonatite Complex, South Africa

Lead and multiple sulphur isotope compositions were measured in-situ by SIMS on sulphide minerals from phoscorites and carbonatites of the ca. 2.06 Ga Phalaborwa Complex in South Africa. Additionally, sulphide mineral separates and bulk-rock samples were analyzed with IRMS methods to confirm SIMS data. Lead isotope ratios define a trend stretching from unradiogenic to highly radiogenic ratios corresponding to a Pb–Pb regression date of 2054 ± 99 Ma. This apparent date is consistent with the timing of emplacement and thus provides an age estimate for the sulphide mineralization. The least radiogenic Pb isotope compositions overlap, and the regression line intersects, a hypothetical mixing line between MORB mantle and an upper crustal reservoir at ca. 2.1 Ga, suggesting that either a significant quantity of crustal Pb contributed to sulphide mineralization, or that sulphidic xenomelts were derived from an isotopically enriched mantle source. Sulphur isotope ratios of individual sulphide minerals obtained by SIMS are highly variable (δ34S: −15 to +15‰ V-CDT) and, importantly, reveal the contribution of pre-Great Oxidation Event (GOE) atmospheric sulphur with mass-independent isotope fractionation (Δ33S = δ33S–[(1+δ34S)0.515-1]×1000 ≠0.0‰). Mass-independent sulphur isotope fractionation is also revealed by sulphur isotope ratios measured on sulphide mineral separates (Δ33S: 0.2 to 0.7‰) and bulk rock samples (Δ33S: 0.2 to 0.4‰). Generally, the range of sulphur isotope ratios obtained with SIMS is much larger than that observed in non-SIMS data, possibly reflecting isotopic variability at the μm scale, resolvable only with microbeam measurements. Various sources and mechanisms by which supracrustal material may have been incorporated into mantle-derived carbonatite-phoscorite magmas are assessed, taking into account that geological evidence for the presence of sedimentary material available for assimilation during shallow-level magma emplacement is lacking. Given the variability in S and Pb isotopic compositions, it is inferred that pre-GOE surficial Pb and S were not derived from asthenospheric mantle contaminated with supracrustal materials. Instead, whole rock trace element compositions, in concert with published geochemical and petrological evidence, are consistent with interaction of asthenospheric, plume-derived melt with compositionally heterogeneous lithospheric mantle that was metasomatically modified by fluids and melts released from a subducting slab. Despite geochemical and geochronological similarities with the 2055 Ma Busvheld Complex, lead and sulphur isotope data for both complexes are resolvably different, pointing to distinct lithospheric mantle sources involved in sulphide mineralization.

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: III. Pyrochlore Supergroup Minerals

The pyrochlore supergroup minerals (PSM) are typical secondary phases that replace (with zirconolite–laachite) earlier Sc-Nb-rich baddeleyite under the influence of F-bearing hydrothermal solutions, and form individual well-shaped crystals in surrounding carbonatites. Like primary Sc-Nb-rich baddeleyite, the PSM are concentrated in the axial carbonate-rich zone of the phoscorite-carbonatite complex, so their content, grain size and chemical diversity increase from the pipe margins to axis. There are 12 members of the PSM in the phoscorite-carbonatite complex. Fluorine- and oxygen-dominant phases are spread in host silicate rocks and marginal carbonate-poor phoscorite, while hydroxide-dominant PSM occur mainly in the axial carbonate-rich zone of the ore-pipe. Ti-rich PSM (up to oxycalciobetafite) occur in host silicate rocks and calcite carbonatite veins, and Ta-rich phases (up to microlites) are spread in intermediate and axial magnetite-rich phoscorite. In marginal (apatite)-forsterite phoscorite, there are only Ca-dominant PSM, and the rest of the rocks include Ca-, Na- and vacancy-dominant phases. The crystal structures of oxycalciopyrochlore and hydroxynatropyrochlore were refined in the Fd3¯m space group with R1 values of 0.032 and 0.054 respectively. The total difference in scattering parameters of B sites are in agreement with substitution scheme BTi4+ + YOH‒ = BNb5+ + YO2‒. The perspective process flow diagram for rare-metal “anomalous ore” processing includes sulfur-acidic cleaning of baddeleyite concentrate from PSM and zirconolite–laachite impurities followed by deep metal recovery from baddeleyite concentrate and Nb-Ta-Zr-U-Th-rich sulfatic product from its cleaning.

Approach of automatic 3D geological mapping: the case of the Kovdor phoscorite-carbonatite complex, NW Russia

We have developed an approach for automatic 3D geological mapping based on conversion of chemical composition of rocks to mineral composition by logical computation. It allows to calculate mineral composition based on bulk rock chemistry, interpolate the mineral composition in the same way as chemical composition, and, finally, build a 3D geological model. The approach was developed for the Kovdor phoscorite-carbonatite complex containing the Kovdor baddeleyite-apatite-magnetite deposit. We used 4 bulk rock chemistry analyses – Femagn, P2O5, CO2 and SiO2. We used four techniques for prediction of rock types – calculation of normative mineral compositions (norms), multiple regression, artificial neural network and developed by logical evaluation. The two latter became the best. As a result, we distinguished 14 types of phoscorites (forsterite-apatite-magnetite-carbonate rock), carbonatite and host rocks. The results show good convergence with our petrographical studies of the deposit, and recent manually built maps. The proposed approach can be used as a tool of a deposit genesis reconstruction and preliminary geometallurgical modelling.

Kampelite, Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O, a new very complex Ba-Sc phosphate mineral from the Kovdor phoscorite-carbonatite complex (Kola Peninsula, Russia)

Kampelite, Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O, is a new Ba-Sc phosphate from the Kovdor phoscorite-carbonatite complex (Kola Peninsula, Russia). It is orthorhombic, Pnma, a = 11.256(1), b = 8.512(1), c = 27.707(4) A, V = 2654.6(3) A3 and Z = 4 (from powder diffraction data) or a = 11.2261(9), b = 8.5039(6), c = 27.699(2) A, V = 2644.3(3) A3 (from single-crystal diffraction data). The mineral was found in a void within the calcite-magnetite phoscorite (enriched in hydroxylapatite and Sc-rich baddeleyite) inside the axial zone of the Kovdor phoscorite-carbonatite pipe. Kampelite forms radiated aggregates (up to 1.5 mm in diameter) of platy crystals grown on the surfaces of crystals of quintinite-2H in close association with pyrite, bobierrite and quintinite-3R. Kampelite is colourless, with a pearly lustre and a white streak. The cleavage is perfect on {001}, the fracture is smooth. Mohs hardness is about 1. In transmitted light, the mineral is colourless without pleochroism or dispersion. Kampelite is biaxial + (pseudouniaxial), α ≈ β = 1.607(2), γ = 1.612(2) (589 nm), and 2V calc = 0°. The calculated and measured densities are 3.28 and 3.07(3) g·cm−3, respectively. The mean chemical composition determined by electron microprobe is: MgO 4.79, Al2O3 0.45, P2O5 31.66, K2O 0.34, Sc2O3 16.17, Mn2O3 1.62, Fe2O3 1.38, SrO 3.44, and BaO 29.81 wt%. The H2O content estimated from the crystal-structure refinement is 7.12 wt%, giving a total of 96.51 wt%. The empirical formula calculated on the basis of P = 6 apfu (atoms per formula unit) is (Ba2.62Sr0.45K0.10Ca0.06)Σ3.23Mg1.60Mn0.28(Sc3.15Fe3+ 0.23Al0.12)Σ3.50(PO4)6(OH)2.61·4.01H2O. The simplified formula is Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O. The mineral easily dissolves in 10% cold HCl. The strongest X-ray powder-diffraction lines [listed as d in A (I) (hkl)] are as follows: 15.80(100)(001), 13.86(45)(002), 3.184(18)(223), 3.129(19)(026), 2.756(16)(402), 2.688(24)(1010). The crystal structure of kampelite was refined to R 1 = 0.092 on the basis of 2620 unique observed reflections. It is based upon complex [MgBa2Sc4(PO4)6] layers consisting of the Ba-PO4 zigzag sheet inserted between two Mg-Sc-PO4 sheets. Raman spectrum of kampelite contains characteristic bands of vibrations of the PO4, ScO6 and H2O groups. Kampelite formed as a result of low-temperature hydrothermal alteration of Sc-bearing baddeleyite, which also produces Sc-rich pyrochlore and juonniite. The structural complexity parameters for kameplite are equal to 5.272 bits/atom and 1244.304 bits/cell, which points out that the mineral is structurally very complex, in agreement with its late-stage hydrothermal origin. The mineral is named in honour of Russian mining engineer Felix Borisovich Kampel’ (b. 1935) for his contribution to the development of technologies of mining and processing of complex magnetite-apatite-baddeleyite ores of the Kovdor deposit.

Geochemical and mineralogical characteristics of the Yonghwa phoscorite–carbonatite complex, South Korea, and genetic implications

The Yonghwa phoscorite–carbonatite complex occurs as an isolated individual body with an inclined pipe shape within the Precambrian Gyeonggi Massif, South Korea. The phoscorite consists mainly of olivine, apatite, magnetite, carbonates, amphibole, and phlogopite, and can be subdivided into two types, olivine-rich and apatite-rich. The carbonatite is composed of calcite, Mg-rich dolomite, Fe-rich dolomite, magnetite, apatite, and/or siderite. Intensive fenitization occurred along the boundary between the complex and the wall rocks of leucocratic banded gneiss and garnet-bearing metabasite. The paragenetic sequences of the phoscorite–carbonatite complex demonstrate that the early crystallization of silicate minerals was followed by the crystallization of carbonates as the carbonatitic melt cooled. Magnetite occurs within the complex, and the carbonatites have Fe contents that are higher than typical ferrocarbonatites, due to the high magnetite contents. The rare earth elements (REEs) in the phoscorites and carbonatites are weakly fractionated and show enrichments of LREEs and Nb relative to HREEs. Furthermore, the apatites reflect the fractionated trends of LREEs relative to HREEs. Phoscorite apatites are enriched in Sr and show substitutions between Ca and Sr. Mica chemistry reflects the evolutionary trend of Fe2+ and Mg2+ in the phoscorite–carbonatite melt without Al substitution. Micas exhibit high values of Mg# in the phoscorite–carbonatite complex, but lower values in fenites. Via thermodynamic analysis, the early stability fields of magnetite–pyrrhotite–graphite–carbonate assemblages indicate that the Yonghwa phoscorite and carbonatite crystallized under conditions of 600°C, 2kbar, and XCO2=0.2. Afterward, melts underwent an evolution to the late stability fields of magnetite–pyrite–pyrrhotite–ilmenite assemblages. The δ13C and δ18O isotopic compositions of carbonates in the Yonghwa phoscorite–carbonatite complex are −8.2‰ to −3.4‰ and 6.6 to 11.0‰, respectively, and together with the sulfur isotope compositions of the sulfides (δ34S values of about 0.2‰ to 2.2‰) indicate a primary mantle source of the magmas. Phlogopites from the fenites yielded K–Ar ages of 193.4±4.9 and 195.0±5.1Ma, which demarcate the timing of the cooling of the phoscorite–carbonatite intrusion, and indicate that the phoscorite–carbonatite may be related to a post-collisional magmatic regime. The discovery of this complex marks the first known occurrence in Korea, of Fe and Nb–REE mineralization related to phoscorite–carbonatite igneous activity.

Economic minerals of the Kovdor baddeleyite-apatite-magnetite deposit, Russia: mineralogy, spatial distribution and ore processing optimization

The comprehensive petrographical, petrochemical and mineralogical study of the Kovdor magnetite-apatite-baddeleyite deposit in the phoscorite–carbonatite complex (Murmansk Region, Russia) revealed a spatial distribution of grain size and chemical composition of three economically extractable minerals — magnetite, apatite, and baddeleyite, showing that zonal distribution of mineral properties mimics both concentric and vertical zonation of the carbonatite-phoscorite pipe.The marginal zone of the pipe consists of (apatite)-forsterite phoscorite carrying fine grains of Ti–Mn–Si–rich magnetite with ilmenite exsolution lamellae, fine grains of Fe–Mg-rich apatite and finest grains of baddeleyite, enriched in Mg, Fe, Si and Mn. The intermediate zone accommodates carbonate-free magnetite-rich phoscorites that carry medium to coarse grains of Mg–Al-rich magnetite with exsolution inclusions of spinel, medium-grained pure apatite and baddeleyite. The axial zone hosts carbonate-rich phoscorites and phoscorite-related carbonatites bearing medium-grained Ti–V–Ca-rich magnetite with exsolution inclusions of geikielite–ilmenite, fine grains of Ba–Sr–Ln-rich apatite and comparatively large grains of baddeleyite, enriched in Hf, Ta, Nb and Sc. The collected data enable us to predict such important mineralogical characteristics of the multicomponent ore as chemical composition and grain size of economic and associated minerals, presence of contaminating inclusions, etc. We have identified potential areas of maximum concentration of such by-products as scandium, niobium and hafnium in baddeleyite and REEs in apatite.

Sr–Nd–Pb isotopic compositions of the Kovdor phoscorite–carbonatite complex, Kola Peninsula, NW Russia

The Sr, Nd and Pb isotopic compositions for the Kovdor phoscorite–carbonatite complex (PCC), Kola Peninsula, NW Russia, have been determined to characterize the mantle sources involved and to evaluate the relative contributions of a plume and subcontinental lithospheric mantle in the formation of the complex. The Kovdor PCC is a part of the Kovdor ultramafic–alkaline–carbonatite massif, and consists of six intrusions. The initial isotopic ratios of the analyzed samples, calculated at 380 Ma, display limited variations: ε Nd, + 2.0 to + 4.7; 87Sr/ 86Sr, 0.70319 to 0.70361 ( ε Sr, − 12.2 to − 6.2); 206Pb/ 204Pb, 18.38 to 18.74; 207Pb/ 204Pb, 15.45 to 15.50; 208Pb/ 204Pb, 37.98 to 39.28. The Nd and Sr isotope data of the Kovdor PCC generally fit the patterns of the other phoscorites and carbonatites from the Kola Alkaline Province (KAP), but some data are slightly shifted from the mixing line defined as the Kola Carbonatite Line, having more radiogenic 87Sr/ 86Sr ratios. However, the less radiogenic Nd isotopic compositions and negative Δ7/4 values of Pb isotopes of the analyzed samples exclude crustal contamination, but imply the involvement of a metasomatized lithospheric mantle source. Isotopic variations indicate mixing of at least three distinct mantle components: FOZO-like primitive plume component, EMI-like enriched component and DMM-like depleted component. The isotopic nature of the EMI- and DMM-like mantle component observed in the Kovdor samples is considered to be inherited from metasomatized subcontinental lithospheric mantle. This supports the previous models invoking plume–lithosphere interaction to explain the origin of the Devonian alkaline carbonatite magmatism in the KAP.

Pyrochlore chemistry from the Sokli phoscorite-carbonatite complex, Finland: Implications for the genesis of phoscorite and carbonatite association

The phoscorite-carbonatite complex in the Sokli alkaline-carbonatite massif, northern Finland, comprises five stages of intrusions of phoscorites and carbonatites (P1-C1, P2-C2 and P3-C3 for phoscorites and calcite carbonatites; D4 and D5 for dolomite carbonatites). The phoscorites and calcite carbonatites at Sokli usually occur as pairs with the same mineral assemblages. Pyrochlore is found in the majority of rock types in the Sokli phoscorite-carbonatite complex, shows wide compositional variation and seems to preserve evolution trends of host rocks. Crystallization of pyrochlore begins from the P2-C2 phoscorite and calcite carbonatite and continues up to the latest D5 dolomite carbonatite. Pyrochlore in the early stage P2-C2 rocks has high U and Ta contents. These elements suddenly decrease from the P3-C3 rocks, on the other hand, Th and Ce contents increase. The compositions of the late generations from the D4 and D5 rocks are close to that of an ideal end-member pyrochlore with formula (Ca,Na)2Nb2O6F. The Nb/Ta ratio and F content of pyrochlore increase from P2-C2 to the latest D5 dolomite carbonatite. The composition and evolutionary history of pyrochlore from the phoscorites are distinguished from those of the associated calcite carbonatites. Pyrochlore from the calcite carbonatites shows larger A-cation deficiencies compared to those from the paired phoscorites. Ta and Zr contents are slightly higher in pyrochlore from the calcite carbonatites, whereas Ti is generally higher in pyrochlore from the associated phoscorites. Moreover, pyrochlore from the phoscorites always shows a longer and more complex crystallization history compared to that of the same stage carbonatites. This indicates that the chemical condition was clearly different in the two systems during the crystallization of pyrochlore. Based on these results, together with the previous mineralogical and geochemical studies on the Sokli phoscorite-carbonatite complex, we propose a liquid immiscibility process as the most possible segregation mechanism of the two associated rocks. The composition of pyrochlore in the late dolomite carbonatites is distinct and always lies on the evolutional trend of the earlier varieties. This implies that the dolomite carbonatites are the final magmatic products of the Sokli phoscorite-carbonatite system.

Compositional variation of Fe−Ti oxides from the Sokli complex, northeastern Finland

The phoscorite-carbonatite complex at Sokli, northeastern Finland, is composed of five stages of intrusions of phoscorites and carbonatites (P1-C1, P2-C2, P3-C3 phoscorites and calcite carbonatites, D4 and D5 dolomite carbonatites) which are cut by numerous lamprophyric dikes. Magnetite is ubiquitous in all constituent rock units of the complex and frequently associates with ilmenite. Most ilmenite intergrowths from the Sokli phoscorite-carbonatite complex occur as internal and/or external granules (composite type exsolutions) or sandwich type exsolution lamellae in the host magnetites. Discrete ilmenite crystals are found only in P3 and C3. On the other hand, ilmenites in the ultramafic lamprophyres occur as discrete crystals as well as trellis and sandwich type exsolution lamellae or composite type exsolutions in magnetite. Electron microprobe analyses reveal that magnetite of the Sokli complex belongs essentially to the magnetite-ulvospinel solid solution series with a small proportion of magnesioferrite. The compositions of magnetite from the Sokli phoscorite-carbonatite complex vary systematically from stage to stage with increasing Fe2+/(Fe2++Mg) ratio, and decreasing Al, Mn and Ti contents. Magnetite from the ultramafic lamprophyre is characterized by large compositional ranges owing to the large amount of mantle-derived xenocrysts. Some grains are particularly high in Cr (up to 21.0 wt.% Cr2O3). Aluminous magnesian titanomagnetites are also found, but most magnetites in the ultramafic lamprophyres are pure magnetite. The compositional variation of ilmenite from the Sokli complex is mainly caused by the substitution of Fe2+ into Mg and Mn, and partly Ti into Nb. Mg-and Mn-rich ilmenites in the early stage P1-C1 rocks evolve towards pure FeTiO3 composition in the latest D5 dolomite carbonatite. Ilmenites from the ultramafic lamprophyres are relatively poor in Mn compared to those from the phoscorite-carbonatite complex. In the coexisting magnetite and ilmenite, Mg and Mn cations preferentially partition into ilmenite rather than magnetite. In spite of the mineralogical and geochemical contrasts between the phoscorites and associated carbonatites, compositions of Fe−Ti oxide minerals from the two paired rocks at a given stage are basically the same, and evolve systematically from stage to stage. This supports the hypothesis that the phoscorites and conjugate carbonatites are derived from common parental melts. Furthermore, the intrusions of the Sokli phoscorite-carbonatite complex are considered to have either crystallized from successive batches representing a continuum in time and magmatic evolution, or that the P1-C1 parental magma has simply differentiated to form the later stage rocks.

Phlogopite and tetraferriphlogopite from phoscorite and carbonatite associations in the Sokli massif, Northern Finland

The phoscorite—carbonatite complex (PCC) in the Sokli massif, northern Finland, is divided into 5 stages according to mineral assemblages and occurrences. The earlier three stages comprise phoscorites and calcite carbonatites (P1 to P3 and C1 to C3, respectively), and they usually occur as pairs with the same mineral assemblage (calcite, apatite, megnatite, olivine, and mica). The latter two stages consist of only dolomite carbonatites called D4 and D5. All micas investigated from the Sokli PCC fall in the range of the phlogopite-tetraferriphlogopite series. Tetraferriphlogopite begins to crystallize from late stage 2 and becomes a dominant silicate in the P3C3 rocks and D4-D5 dolomite carbonatites. Although tetraferriphlogopites occur as primary or secondary products, discrete and euhedral (magmatic) tetraferriphlogopites are considered to have crystallized from a melt strongly depleted in aluminum and saturated in Ti-bearing phases under low temperature condition. The chemical variation of phlogopites shows that Fe and F contents increase, whereas Al, Ba, Ti and Mg contents decrease from stage 1 to stage 5. The progressive depletion in aluminum and the enrichment in fluorine towards the later stages seem to be a specific feature of the Sokli phlogopite. The difference of phlogopite rim compositions between phoscorites and paired carbonatites indicates that there was a slight difference of elemental partitioning into the interstial melts during the segregation of the rocks from a parental magma.