High Nb Contents Research Articles

Geology, igneous geochemistry, mineralization, and fluid inclusion characteristics of the Kougarok tin-tantalum-lithium prospect, Seward Peninsula, Alaska, USA

The Kougarok prospect is situated in a Sn-W (+Ta, Nb, Li, Be) metallogenic belt formed in a post-collisional to within-plate tectonic environment. Crystal fractionation of granitic magma, combined with its mixing/mingling with mantle-derived mafic magma, is proposed as the major process causing the formation of the complex B-rich to Li-F-type granitic suite and associated Sn-rare metal (Ta, Nb, Li) mineralization. An intrusion of alkalic (potassic lamprophyric) mafic magma into a crystallizing biotite-tourmaline granite magma reservoir may have supplied a geochemically distinct assemblage of volatiles (F) and associated metals (Li, Ta, Nb) into the boron- and Sn-rich granitic system. An alternative model considers differentiation-driven unmixing of mafic and granitic silicate melts sequestering different volatile and mineralizing species that could also trigger magma-fluid and fluid-fluid unmixing, with separation of gaseous fluid coexisting with immiscible granitic and mafic magmas in the form of magmatic melt-fluid-crystal “suspension.” Biotite-tourmaline granite was accompanied by early quartz-tourmaline-cassiterite greisen at > 430–380 °C followed by quartz-tourmaline-chlorite-cassiterite stockwork at < 350 °C. The immiscibility of F- and B-rich fluids was followed by preferred ascent of F-rich fluids upward in the granitic magma reservoir, with its strong enrichment in F, Li, Ta, and Nb and subsequent crystallization of zinnwaldite granite at a shallower level, with separation of homogenous high-salinity magmatic fluid at ~ 600 °C. Crystallization and fluid exsolution continued at lower temperatures, followed by formation of topaz-quartz stockscheider at 550–500 °C. Ta-Nb mineralization in the uppermost part of the zinnwaldite granite appears to be associated with final episodes of magmatic crystallization, particularly W-Nb-rutile and columbite-tantalite with a higher Nb content. Some Ta-Nb minerals, such as columbite-tantalite with a higher Ta content, appear to be also stable during the post-magmatic stage, in quartz-tourmaline+topaz greisen and higher-temperature quartz-albite-Li mica alteration that replace zinnwaldite granite at ~ 500 to 400 °C. Nearly contemporaneous, lower-temperature quartz-albite-Li mica alteration and quartz-muscovite, quartz-topaz, and quartz-fluorite greisens formed from boiling fluids at 390–350 °C and caused removal of Ta-Nb minerals. Instead, fluid cooling and neutralization of boiling fluids affected Sn solubility and promoted massive cassiterite deposition in the late greisens. The latest phyllic quartz-sericite-carbonate alteration assemblage, comprising arsenopyrite, pyrrhotite, chalcopyrite and other sulfides, native Bi, and Bi tellurides, formed from boiling fluids at < 310 °C.

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally ∼100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally ∼700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean 206Pb/238U age of 416 ± 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have δ18O ranging from +6.5 to +7.3‰. The calculated zircon saturation temperatures (TZr) range from 850 to 900 °C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A1-type granitoids formed in an intra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeOt/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have εNd(t) of −1.5 to 0. There is a positive correlation between εNd(t) and ΔεHf(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have εHf(t) and εNd(t) varying along the “Island arc array”, which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have εNd(t) of −2.4 to −2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting.

Early Carboniferous mafic dike–syenitic granite association in the Atengtao Mountain, Yili Block (NW China): Geochronology, petrogenesis, and tectonic significance

The Carboniferous is an important period for understanding tectonic evolutionary history and subduction processes of Chinese western Tianshan orogen. Here, we present results of petrologic, geochemical, and in situ LA‐ICP‐MS zircon U–Pb and Hf isotope data from an Early Carboniferous mafic dike–syenitic granite association in Atengtao Mountain, Yili Block, aiming to constrain their ages and petrogenesis and tectonic significance. Zircon U–Pb dating results reveal that the mafic dikes and syenitic granites were coevally emplaced at ca. 345–349 Ma. Geochemically, the mafic dikes contain high MgO, Nb, TiO2, and Ta contents, low SiO2 contents, and high Na2O/K2O ratios, showing Nb‐enriched affinities. Additionally, they are enriched in LILEs, relatively depleted in HFSEs, and positive εHf(t) values (+1.2 to +8.7), likely representing melts derived from partial melting of a depleted mantle wedge metasomatized by subducted slab‐derived fluids, with minor involvement of asthenospheric components. Compared with mafic dikes, the coeval syenitic granites display calc‐alkaline I‐type features and have high SiO2, Al2O3, and low MgO, Mg# (&lt;40), Ni, and Cr contents, considered to have been derived from the reworking of the Precambrian basement with an input of mantle‐derived components. These mafic dike–syenitic granite associations likely formed in an arc‐related setting rather than postcollision or mantle plume setting. In addition, pronouncedly higher zircon saturation temperature of the syenitic granites, compared with the coeval I‐type granitoids from western Tianshan, imply the occurrence of a thermal anomaly in this period (ca. 345 Ma). With these studied results, together with previously published data, we argue that a slab break‐off model was likely responsible for the mafic dike–syenitic granite association, and related rocks, in the western Yili Block during Early Carboniferous.

Characterization of Ti-xNb (x = 25 – 35) Alloys in as Melt and Annealed States

The effect of Nb content on microstructure, mechanical properties and phase formation in as-melt and annealed binary Ti-Nb alloys were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. The content of Nb varied in the range 25-35 mass % leading to significant changes in the microstructure. The annealed and furnace-cooled binary Ti-Nb samples exhibited HCP martensitic α` phase at a Nb content below 27.5 mass % and metastable BCC β phase at higher contents of Nb. The mechanical properties of alloys depended strongly on the Nb content and type of the dominating phase.

Meso-Neoarchean magmatism and episodic crustal growth in the Kudremukh-Agumbe granite-greenstone belt, western Dharwar Craton, India

Mafic volcanic rocks and dykes associated with Na-rich TTG and K-rich granites in the Kudremukh greenstone belt of western Dharwar Craton provides a window to Precambrian crustal evolution in the southern Peninsular India. Here we present zircon U-Pb geochronology and geochemistry of these rocks to constrain the greenstone-basement relationship and crustal growth events in the western Dharwar Craton. TTGs exhibit fractionated REE patterns [(La/Yb)N = 11.8–50.3; (Gd/Yb)N = 2.1–4.9] whereas the granites with K2O contents of 3.84–5.20 wt% are peraluminous calc-alkaline with moderate enrichment of LREE over HREE and depleted MREE abundances. The metabasalts are characterized as arc basalts with moderate to high Fe2O3, CaO, SiO2, MgO, LILE, LREE and low HFSE contents. The higher Nb contents (6.2–11.4 ppm) in some of them, classified as Nb-enriched basalts, were erupted at 2498 ± 43 Ma, indicating a 2.5 Ga greenstone volcanism in the western Dharwar Craton. These are characterized by slightly fractionated REE along with negative Nb, Zr, Hf and Ti anomalies. The amphibolitic and doleritic dykes have overlapping chemistry with moderate Al2O3, moderate to high Fe2O3 and MgO displaying coherent chondrite normalized REE patterns. The 3068 ± 34 Ga TTG were generated through high pressure melting of mafic crust in a garnet-spinel peridotite mantle regime with the assimilation of upper crustal material in a subduction zone environment. The granites of 2936 ± 23 Ma were the products of partial melting and fractional crystallization of TTG melts which display intrusive relationship with the thickened TTG crust. The mafic volcanism took place at 2498 ± 43 Ma subsequently followed by the dyke emplacement at 2484 ± 29 Ma. Therefore, two major episodes of crustal growth are identified from this region: i) TTG-granite association ∼3.0–2.9 Ga and ii) mafic volcanism – dyke emplacement between 2.49 and 2.48 Ga. The geochemical signatures of arc basalts and Nb enriched basalts indicate variable slab-mantle interactions including fluid-fluxed metasomatism and melt-fluxed hybridization of mantle wedge in a subduction zone environment followed by the emplacement of amphibolitic dykes during post-subduction collision phase. Our results document the episodic growth of continental crust in western Dharwar Craton from 3.0 to 2.5 Ga.

Petrogenesis and geochemical characteristics of Early Carboniferous sanukitic high‐Mg andesite from Atengtao Mountain, Yili Block: Implications for the tectonic setting during Late Palaeozoic in Chinese West Tianshan

As an important part of the Central Asian Orogenic Belt (CAOB), Chinese western Tianshan contains voluminous Late Palaeozoic volcanic rocks. Here, we present geochronological, geochemical, and in situ Lu–Hf isotopic data from the Atengtao sanukitic high‐Mg andesites (HMAs) of the western Yili Block. LA‐ICP‐MS zircon U–Pb analyses indicate that the Atengtao high‐Mg andesites were generated in the Early Carboniferous (346 ± 2 Ma). All the studied rock samples are characterized by high MgO (3.30–5.45 wt.%), SiO2 (53.25–60.10 wt.%), Cr (90.3–157 ppm), and Ni (31.0–46.7 ppm) contents, with geochemical characteristics analogous to those of sanukitoids of the Setouchi Volcanic Belt (SVB), SW Japan, and Bieluagaxi area, West Junggar. The Atengtao sanukitic HMAs are enriched in LILE and LREE but depleted in HFSE, similar to those of typical arc‐related magmatic rocks. They are also characterized by high Nb (11.4–15.8 ppm) concentrations, high Th/La (0.3) and Th/Yb (2.6–4.0) ratios, and varying zircon εHf(t) values (−2.3 to +4.3). These features are suggested to result from the partial melting of a relatively enriched mantle wedge metasomatized by subducting oceanic slab‐derived sediments. Considering the occurrence of sanukitic HMAs, combined with the contemporaneous special rock assemblages (such as slab‐derived adakite, Nb‐enrich basalt, A‐type granite/rhyolite, and bimodal volcanic rocks) found in the western part of Yili Block, we propose that the generation of the Carboniferous sanukitic HMAs in Atengtao Mountain were most likely related to the slab break‐off following the southern Tianshan Ocean slab northward subduction beneath the Yili–Central Tianshan Block. This mechanism not only produces intense magmatism with compositional diversity but also gives rise to a local extension in the western part of Yili Block. The slab break‐off model may play an important role in the crustal growth of the CAOB in Phanerozoic time.

Sayan–Biryusa Volcanoplutonic Belt (Southern Siberian Craton): Age and Petrogenesis

Abstract —Geological, geochronological, and geochemical isotope studies are carried out for metamorphosed volcanic rocks and dolerites of the Maltsevka sequence of the Elash Group in the Biryusa block of the Siberian craton. It is found that mafic igneous rocks (dolerites and basaltic andesites) are close in composition to intraplate basalts. Flat or slightly fractionated REE patterns ((La/Yb)n = 1.3–2.3) and positive εNd(T) values of +3.7 and +4.1 are observed. It is assumed that the depleted asthenospheric mantle and, possibly, plume mantle were the sources of these rocks, while the lithospheric mantle had no significant effect. Meta-andesites of the Maltsevka sequence belong to the tholeiitic series and have high La, Th, and U contents. Pronounced negative Nb and Ti anomalies are observed in the multielement patterns of these rocks, along with negative εNd(T) values of –4.6. It is assumed that meta-andesites were resulted from the late Archean crustal melting with the participation of the mantle material. Metarhyolites prevalent in the Maltsevka sequence are divided into two groups similar in REE composition to A-type and I-type granites. A-type metarhyolites show high contents of Zr, Y, Nb, Th, and REE (except for Eu) and positive εNd(T) values of +2.2 and might have resulted from the melting of the source with geochemical isotope parameters close to those of mafic igneous rocks of the Maltsevka sequence. I-type metarhyolites have low contents of Y, Yb, Zr, and Nb but high contents of Th and show negative εNd(T) values of –3.7. They might have resulted from the melting of lower crustal diorite–tonalite rocks with addition of juvenile mantle material to the magma generation area. U–Pb zircon dating of metarhyolites of the Maltsevka sequence corresponding to A- and I-type granites showed that they are close in age, 1872 ± 10 and 1874 ± 10 Ma, respectively, which agrees with the age estimated earlier for granitoids of the Sayan complex of the Biryusa block. The similar ages and structural positions, along with the localization within the same structure, made it possible to unite volcanic rocks of the Elash Group and granitoids of the Sayan complex of the Biryusa block into a Paleoproterozoic volcanoplutonic association. The rocks of the association form the Sayan–Biryusa volcanoplutonic belt stretching for about 300 km along the zone of junction of the Biryusa block of the Angara fold belt and the Archean Tunguska superterrane of the Siberian craton. The belt is part of the large Paleoproterozoic South Siberian postcollisional magmatic belt formed at the final formation stage of the Siberian craton, when it was possibly part of the Paleoproterozoic Columbia supercontinent.

Редкоземельные минералы карбонатитовых жил массива Соустова (Кольский полуостров) — индикатор его рудной специализации

The article presents first results of geological, isotope-geochemical and mineralogical studies of carbonatite veins previously unknown in the Soustov massif. Veins studied have the Sm-Nd isotope composition and model age similar to rocks of the massif, indicating the common source of their formation. High contents of light lanthanides, Sr, and Nb in carbonatite veins were revealed. These elements are concentrated in bastnaesite, strontianite, monazite, and pyrochlore. The data obtained significantly expand the geochemical and ore specialization of the massif.

X-ray fluorescence determination of Ni, Ti and Nb in memory alloys using solution and thin film samples with an internal standard

Specimen for XRF analysis of high content of Ni, Ti and Nb in memory alloys, prepared by solution (a) and a filter paper disk (b).

Interaction of crustal and mantle materials, sources of trace elements during the formation and evolution of Early Paleozoic Li-rich granite-pegmatite systems in southeastern Tuva

We present new data on the age, composition, and environments of formation of granites of the Kystarys complex and the associated Li-rich rare-element pegmatites of the South Sangilen pegmatite belt including the large Tastyg lithium deposit. It has been established that they formed during the Early Paleozoic collisional orogeny in the Tuva-Mongolian massif at the Cambrian-Ordovician boundary. The granites of the Kystarys complex are moderately alkaline high-K rocks and are enriched in Zr, Nb, Y, and REE; therefore, they are classified as postcollisional, transitional to within-plate (A-type). The spodumene pegmatites of the South Sangilen pegmatite belt are similar to the above granites in age and isotopic and geochemical parameters, which suggests a paragenetic relationship between these rocks. Pegmatites form several pegmatite fields within the belt, which differ in trace-element signatures. In addition to predominant Li, Cs, and Ta, specific to all spodumene pegmatites (LCT family), pegmatites of two fields have high contents of Nb, Y, REE, and Zr, which are indicator elements of NYF family pegmatites. It has been established that the formation of spodumene pegmatites with combined LCT-NYF geochemical signatures was preceded by the intrusion of dikes of monzogabbro with the geochemical characteristics of OIB and of alkali aegirine granites and by the formation of associated metasomatites enriched in Zr, Nb, Y, and REE. Based on the geological, mineralogical, and geochemical data, we substantiate the hypothesis of the formation of Li-bearing granite-pegmatite melts from a mixed source resulted from the influence of fluids of an alkaline igneous complex of mantle genesis on the crustal protolith.

Numerical and experimental study on peritectic transition in TiAl-Nb alloys

The solidification pathway and peritectic transition characteristic related to the solute concentration in TiAl-Nb system are calculated using the Thermo-Calc software. The results show that there are four typical solidification modes in the TiAl-Nb system, which are single β, hypo-peritectic, hyper-peritectic and single α solidification. The mole fraction of peritectic growth αp can be used to describe the undergoing extent of the peritectic tranformation (EPT) directly for the alloys with the peritectic transition, and βp/βmax can also be used to describe the EPT for the hypo-peritectic and Lp/(1 − βmax) for the hyper-peritectic. The mole fraction of αp shows a parabolic shape variation on a plot of solute Al at constant Nb content. The composition of the complete peritectic transformation (PT) is around 46.5 at.% Al for the TiAl binary alloy, and it gradually shifts to the direction of high Nb content with increase of solute Al. A temperature gap (△Tp) corresponding to the peritectic transition exists in Nb containing TiAl alloys and the △Tp shows a parabolic shape on a plot of solute Al at constant Nb content. The experimental results are generally in accordance with the theoretical rules.

Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea

The Karadag Massif in south-eastern Crimea hosts a thick sequence of volcanic rocks, which underwent significant secondary alteration. Here we deduce the effects of alteration, remove their mask and estimate the primary chemical composition and age of these volcanic rocks using their bulk chemical analyses, as well as chemical data from plagioclase, ortho- and clinopyroxene-hosted melt inclusions, and Ar isotope analyses of plagioclase and groundmass from the freshest andesite. Our data reveal two distinct magmatic series in the Karadag Massif. The first defines a subalkaline, calcic, medium-K series with high enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, which is typical of supra-subduction volcanic rocks. According to previously published trace element data, most of the Karadag volcanic rocks belong to this series. We use Ar isotope data to argue that the age of these rocks is 172.8 ± 4.5 Ma, which is consistent with the biostratigraphy. No reliable major-element chemical data and no isotopic data were obtained from the second series, although its trace-element signature is similar to that of some Nb-enriched supra-subduction volcanic rocks. It is characterised by less pronounced enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, and relatively high contents of Nb and Ta. Typical supra-subduction series of the Karadag Massif is similar to other pre-Cretaceous magmatic rocks exposed in Crimea, and hence likely formed in a similar environment. Literature data on Middle Jurassic magmatic rocks of the Pontides Belt in northern Turkey also allow to distinguish two magmatic series, which closely correspond to the magmatic rocks in Crimea. This is consistent with palaeotectonic reconstructions which suggest that the Pontides Belt was joined to Crimea prior to the opening of the Black Sea, and they formed a single volcanic arc.

The effect of phosphorus on the formation of grain boundary laves phase in high-refractory content Ni-based superalloys

The effect of phosphorous on the microstructure of a powder-processed, high Nb-content, Ni-based superalloy was studied and compared to a P-free variant. The P-doped sample had a lower incipient melting temperature, forming a (Ni, Co, Cr, P)2(Nb, Mo) laves phase along the grain boundaries. Transmission electron microscopy and atom probe tomography were used to identify the crystal structure, assess phase compositions, and determine any elemental segregation to phase boundaries. The results revealed strong partitioning of P to the laves phase and a slight Mo gradient towards the γ′ phrase. Finally, a simplified laves phase was successfully modeled via density functional theory calculations.

The Neoproterozoic volcanic complex of the Boumalne inlier (Saghro massif, Eastern Anti-Atlas; Morocco): petrological and geochemical characteristics

Neoproterozoic volcanic series are exposed in the northern edge of the Saghro massif (Eastern Anti-Atlas, Morocco). Four volcanic rock types (basalt, andesite, dacite, and rhyolite) were distinguished in the Boumalne inlier within the so-called Saghro volcanic sequences based on petrographic and geochemical observations. Boumalne volcanic rocks contain high Al2O3, Fe2O3, Ba, Sr, Zr, Rb, and Nb contents, including calc-alkaline affinity in composition. Boumalne volcanic rocks are similar to other lower-Neoproterozoic volcanic rocks such Agouiniy formation in Sirwa inlier and in other parts of Bou-Azzer inlier. Indeed, they indicate an active subduction signature. The geochemical data show a LREE enrichment compared to HREE. The fractional crystallization has played a major role during the evolution of the magma. The less-siliceous dacitic rocks could have been formed after a low degree of partial melting of mafic parental magma source, whereas the rich-siliceous rhyolite may have been derived from dacitic magma source by a higher degree of fractional crystallization.

Surface tension convection caused by free surface deformation in KTa1−xNbxO3 melts

Experiments on surface tension driven convection of high temperature potassium tantalate niobate (KTa1−xNbxO3, KTN) melts have been carried out in an in-situ observation system. There is a double loop axially symmetric vortex pattern. With the temperature increases, the convection can be transferred from one stable state to oscillatory state and then to another stable state. The oscillatory surface tension convection, which is related to the free surface deformation, can be characterized by the oscillatory main trunk and two unequal branches. The branches depress the large pressure gradient and compensate the return flow to stabilize the circular current. There is a linear decreasing relationship between the deformation angle θ and the oscillatory frequency f. For KTN melts with higher Nb content, the convective pattern is not various, but a smaller degree of deformation can occur accompanied by a weaker oscillation process. The values of critical deformation coefficient S parameter for both the onset and the offset of oscillations decrease with the increase of the content of Nb. The critical values of oscillation satisfy with Son · x ≈ 2.3 × 10−3 and Soff · x ≈ 4.0 × 10−3, respectively.

Break-away of South China from Gondwana: Insights from the Silurian high-Nb basalts and associated magmatic rocks in the Diancangshan-Ailaoshan fold belt (SW China)

The Diancangshan-Ailaoshan fold belt (Yunnan, SW China) represents the remnants of the once vast Ailaoshan Ocean, a branch or back-arc basin of the Eastern Paleotethys. However, when and how this ocean/back-arc basin opened, and rifted South China away from Gondwana are still unresolved. In this study, Ordovician-Silurian high-Nb basalts (HNBs; 420 Ma), Mg-rich andesite (435 Ma), high-Mg basalt (446 Ma) and I-type granite (423 Ma) were newly identified in the Diancangshan-Ailaoshan fold belt. The HNBs are alkaline with high TiO2 (1.8–1.9 wt%) and Nb (16.9–23.1 ppm) contents, high Nb/U (36.8–39.1) and (Nb/Th)PM (1.17–1.21) ratios, positive Nb-Ta anomalies (primitive mantle-normalized) and positive εNd(t) values (+3.2). The HNBs were likely derived from an OIB/arc-mixed mantle source in the garnet stability field. The Mg-rich andesites are medium-K calc-alkaline andesite with negative εNd(t) values (−10.1) and negative Nb-Ta anomalies. The magma was likely derived from a mixture of depleted mantle peridotites and subducted sediments under low pressure and high H2O conditions. The high-Mg basalts are high-K calc-alkaline and characterized by high MgO contents (9.3–9.4 wt%; Mg# = ~70), high Cr (369–379 ppm) and low Al2O3 (13.6–13.9 wt%) contents. They were likely derived from an enriched mantle source metasomatized by marine sediment-derived fluids. The I-type granites show medium SiO2 content (63.6 wt%), low A/CNK ratio (0.9) and negative εNd(t) value (−9.8), and may have formed by the partial melting of meta-basalts. These HNB, Mg-rich andesite, high-Mg basalt and I-type granite were likely formed in an intracontinental back-arc rift setting. Integrated with our published detrital zircon UPb age and LuHf isotope evidence, we propose that South China may have commenced its break-up from Gondwana in response to the Late Ordovician-Silurian (446–420 Ma) Proto-Tethyan subduction, which opened a back-arc basin along the Ailaoshan fold belt.

Comendite Melts of the Early Mesozoic Bimodal Sant Association (Central Mongolia) and Mechanisms of Their Formation

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas producing the the comendites of the Sant bimodal volcanic association (Central Mongolia). The mechanisms of the formation of melts were determined. The primary melt and coexisting crystalline inclusions in quartz from three samples of comendites collected from different parts of the volcanic section were studied. Among the crystalline inclusions, sanidine, zircon, and the REE diortosilicate–chevkinite were identified. The phenocrysts of the comendites were determined to crystallize at temperatures of 880–960°C. The homogeneous glasses of melt inclusions have both trachydacite and rhyolite compositions. They are characterized by high concentrations of Zr, Nb, Rb, Y, Th and REE. Significant differences were determined in concentrations of Li and volatile component (H2O and F) in the glasses: some of the melts are enriched in these components, whereas other are depleted in them.

Trace element composition of iron oxides from IOCG and IOA deposits: relationship to hydrothermal alteration and deposit subtypes

Trace element compositions of magnetite and hematite from 16 well-studied iron oxide–copper–gold (IOCG) and iron oxide apatite (IOA) deposits, combined with partial least squares-discriminant analysis (PLS-DA), were used to investigate the factors controlling the iron oxide chemistry and the links between the chemical composition of iron oxides and hydrothermal processes, as divided by alteration types and IOCG and IOA deposit subtypes. Chemical compositions of iron oxides are controlled by oxygen fugacity, temperature, co-precipitating sulfides, and host rocks. Iron oxides from hematite IOCG deposits show relatively high Nb, Cu, Mo, W, and Sn contents, and can be discriminated from those from magnetite + hematite and magnetite IOA deposits. Magnetite IOCG deposits show a compositional diversity and overlap with the three other types, which may be due to the incremental development of high-temperature Ca–Fe and K–Fe alteration. Iron oxides from the high-temperature Ca–Fe alteration can be discriminated from those from high- and low-temperature K–Fe alteration by higher Mg and V contents. Iron oxides from low-temperature K–Fe alteration can be discriminated from those from high-temperature K–Fe alteration by higher Si, Ca, Zr, W, Nb, and Mo contents. Iron oxides from IOA deposits can be discriminated from those from IOCG deposits by higher Mg, Ti, V, Pb, and Sc contents. The composition of IOCG and IOA iron oxides can be discriminated from those from porphyry Cu, Ni–Cu, and volcanogenic massive sulfide deposits.

PETROGENESIS AND STRUCTURAL POSITION OF THE EARLY PROTEROZOIC CHARNOCKITES OF THE TATARNIKOVSKY MASSIF IN THE SOUTH SIBERIAN POST-COLLISIONAL MAGMATIC BELT OF THE SIBERIAN CRATON

The article reports on the geological, mineralogical, geochemical and isotope-geochemical studies of granitoids (charnockites) from the Tatarnikovsky massif located in the northern part of the Baikal uplift of the Siberian craton basement. The age of the studied granitoids is 1.85 Ga. Like other unmetamorphosed granitoids and associated volcanic, the granitoids dated 1.88–1.84 Ga are abundant in the southern area of the Siberian craton. These rocks are a part of the South Siberian post-collisional magmatic belt. The Tatarnikovsky granitoids form a series of small massifs confined to the Davan tectonic zone. However, unlike the rocks of the Davan zone, these granitoids have not been subjected to dynamometamorphism, mylonitization and metasomatism, and seem younger than the geologic structure of this zone. The formation of granitoids coincides in time with the youngest formations in the North Baikal volcanoplutonic belt (1.85–1.84 Ga). The Tatarnikovsky granitoids have two facies varieties – coarse-grained and medium-fine-grained porphyric, the transition being gradual. Considering the mineral composition of the granitoids, specifically the presence of orthopyroxene, these rocks can be classified as charnockites. The research results presented in this article are based on the study of charnockites in the Tatarnikovsky massif, the largest in the Tatarnikovsky complex. The chemical composition of the Tatarnikovsky coarse-grained granitoids corresponds to monzonite and syenite, and fine-grained porphyry granitoids are granosyenite. All the studied granitoids are close to alkaline and calc-alkaline, metaluminous (ASI=0.83–0.97), ferrous (FeO*/(FeO*+MgO)=0.86–0.89) granite, with high concentrations of Nb, Y, Zr, and Ba, and low concentrations of Sr. According to their geochemical characteristics, the Tatarnikovsky granitoids correspond to A-type granite. These rocks show negative values eNd(t)=–1.4…–3.5 and model age ТNdDM=2.4–2.5 Ga. The temperature estimated for the initial stages of crystallization of granitoid melts suggests that granitoids formed at high temperatures, 890–960°С (i.e. the zircon saturation temperature). The granitoid melts crystallized in hypabyssal conditions at the pressure of 2.2–2.9 kbar, as well as in conditions of low or moderate oxygen fugacity. According to the mineralogical, geochemical and isotope-geochemical data, the Tatarnikovsky charnockite could have resulted from melting of mafic rocks from the lower crust (gabbroid, and ferrodiorite) which are products of differentiation of the tholeiitic mantle magmas that had intruded into the base of the continental crust. Taking into account the high concentrations of Ba and the positive anomalies of Eu in the distribution spectra of rare-earth elements (REE) of the coarse-grained granitoids, it can be suggested that these granitoids are the products of partial melting of the crustalmafic source. The fine-grained porphyry granitoids with higher silica contents and lower Ba and Zr contents than those in the coarse-grained granitoids, as well as the negative anomalies of Eu in the REE distribution spectra, are the products of fractional crystallization of the granitoid melt. With regard to formation of the unified structure of the Siberian craton, the geodynamic setting for formation of the Tatarnikovsky charnockite is considered as postcollisional extension due to the fact that these rocks belong to the South Siberian post-collisional magmatic belt. However, on a more local scale of the Baikal uplift of the Siberian craton basement, we suggest that the intercontinental rifting setting was in place during the intrusion of the Tatarnikovsky granitoids, the rocks of the North Baikal volcanoplutonic belt, the granitoids of the Primorsky and Achadsky complexes that cross the rocks of the Akitkan fold belt, collision events in which ceased 1.98–1.97 Ga ago.

In-situ LA-ICP-MS trace element analyses of scheelite and wolframite: Constraints on the genesis of veinlet-disseminated and vein-type tungsten deposits, South China

Veinlet-disseminated and vein-type tungsten deposits are important tungsten resources in South China and show remarkable diversity in dominant tungsten minerals. To better understand their genesis, scheelite from the Shimensi veinlet-disseminated deposit and wolframite from the Xihuashan and Piaotang vein-type deposits were selected to conduct in-situ laser ablation-inductively coupled plasma-mass spectrometry trace element analyses.The Shimensi tungsten mineralization occurs in intensively altered granitic rocks and is characterized by scheelite. Two generations of scheelite in a single grain can be identified by cathodoluminescence (CL) imaging. The early scheelite (dark domains in CL images) is characterized by nearly flat chondrite-normalized REE (REEN) patterns with significantly negative Eu anomalies, whereas the late one (bright domains in CL images) shows light rare earth element (LREE)-enriched REEN patterns and obviously positive Eu anomalies. The former has higher REE, Na, Nb and Ta and lower Sr contents than the latter. The Xihuashan and Piaotang wolframite-quartz veins are developed in greisenized granite (type I) and metasedimentary rocks (type II). Both types of wolframite show LREE-depleted patterns, but type I exhibits strongly negative Eu anomalies and type II positive Eu anomalies. Type I contains higher REE, Nb and Ta concentrations, and lower FeO/MnO ratios than type II.Variations of Eu anomalies and trace element compositions in both scheelite and wolframite can be used to decipher the origin and processes of tungsten mineralization. Both the early scheelite and wolframite (type I) display significantly negative Eu anomalies and have high REE, Nb and Ta contents, suggesting that the initial ore-forming fluids were of magmatic origin. Precipitation of tungsten minerals and alteration would effectively modify the composition of ore-forming fluids. Deposition of the early tungsten minerals would lower REE, Nb and Ta in the mineralizing fluids, leading to depletion of these elements in the late ones. Although both the type I and II wolframite have different REE contents and Eu anomalies, they show similar left-dipped REEN patterns, implying that compositional variation of fluids is likely driven by crystallization of wolframite during the processes of fluid evolution. In contrast, the elevated LREE/HREE (heavy rare earth element) ratios, δEu (EuN/EuN∗) values and Sr abundances in the late scheelite are possibly caused by the decomposition of plagioclase and K-feldspar. Alteration plays an important role in the formation of veinlet-disseminated scheelite deposits. It can be concluded that vein-type wolframite mineralization is mainly formed by filling and that veinlet-disseminated scheelite mineralization is associated with metasomatism.