Nb-Ta Oxides Research Articles

Mineralogical characterization and physical upgrading of radioactive and rare metal minerals from Wadi Al-Baroud granitic pegmatite at the Central Eastern Desert of Egypt

Granitic pegmatites of Wadi Al-Baroud area in the Central Eastern Desert of Egypt have appreciated amounts of radioactive (U, Th) and rare metal- (Nb, Ta, Y, Zr, Hf, and rare earth elements—REEs) bearing minerals. A combination of both geochemical and physical methods has been used for investigation of Wadi Al-Baroud granitic pegmatites. The identification, imaging, and chemistry of rare metal-bearing minerals were conducted using the optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM), and electron microprobe analyzer (EMPA). The pegmatites host Nb-Ta oxides (euxenite-Y, fergusonite-Y, and yttrocolumbite-Y), REE minerals (xenotime-Y, monazite-Ce and allanite-Ce), U-Th minerals (thorite and uranothorite), and Hf-rich zircon. In addition, muscovite, quartz, feldspars, and some iron oxide minerals such as magnetite, goethite, and lepidocrocite represent the essential, gangue, and associated minerals. Physical upgrading of Wadi Al-Baroud-mineralized pegmatites was carried out using gravity and magnetic separation techniques. Applying the optimum conditions for both separation techniques, it is possible to attain a good concentrate with an acceptable recovery. Accordingly, the final concentrate contains ~ 0.25% U, 0.46% Th, and 0.31% REE with recovery of 91.17% U, 96.56% Th, and 86.19% REE in a weight of 6.58% out of the original sample assays 0.02% U, 0.04% Th, and 0.03% REE. Therefore, it is well-recommended that the final concentrate should subject to proper hydrometallurgical treatment to extract their valuable metal contents.

Aillikites and Alkali Ultramafic Lamprophyres of the Beloziminsky Alkaline Ultrabasic-Carbonatite Massif: Possible Origin and Relations with Ore Deposits

The 650–621 Ma plume which impinged beneath the Siberian craton during the breakup of Rodinia caused the formation of several alkaline carbonatite massifs in craton margins of the Angara rift system. The Beloziminsky alkaline ultramafic carbonatite massif (BZM) in the Urik-Iya graben includes alnöites, phlogopite carbonatites and aillikites. The Yuzhnaya pipe (YuP) ~ 645 Ma and the 640–621 Ma aillikites in BZM, dated by 40Ar/39Ar, contain xenoliths of carbonated sulfide-bearing dunites, xenocrysts of olivines, Cr-diopsides, Cr-phlogopites, Cr-spinels (P ~ 4–2 GPa and T ~ 800–1250 °C) and xenocrysts of augites with elevated HFSE, U, Th. Al-augites and kaersutites fractionated from T ~ 1100–700 °C along the 90 mW/m2 geotherm. Higher T trend for Al-Ti augite, pargasites, Ti-biotites series (0.4–1.5 GPa) relate to intermediate magma chambers near the Moho and in the crust. Silicate xenocrysts show Zr-Hf, Ta-Nb peaks and correspond to carbonate-rich magma fractionation that possibly supplied the massif. Aillikites contain olivines, rare Cr-diopsides and oxides. The serpentinites are barren, fragments of ore-bearing Phl carbonatites contain perovskites, Ta-niobates, zircons, thorites, polymetallic sulphides and Ta-Mn-Nb-rich magnetites, ilmenites and Ta-Nb oxides. The aillikites are divided by bulk rock and trace elements into seven groups with varying HFSE and LILE due to different incorporation of carbonatites and related rocks. Apatites and perovskites reveal remarkably high LREE levels. Aillikites were generated by 1%–0.5% melting of the highly metasomatized mantle with ilmenite, perovskite apatite, sulfides and mica, enriched by subduction-related melts and fluids rich in LILE and HFSE. Additional silicate crystal fractionation increased the trace element concentrations. The carbonate-silicate P-bearing magmas may have produced the concentration of the ore components and HFSE in the essentially carbonatitic melts after liquid immiscibility in the final stage. The mechanical enrichment of aillikites in ore and trace element-bearing minerals was due to mixture with captured solid carbonatites after intrusion in the massif.

Niobium–tantalum oxide minerals in alluvial placer deposits from the Ngoura area, East-Cameroon

This work presents a preliminary mineralogical characterization of Nb–Ta oxide minerals from alluvial placers located in the Ngoura area, eastern Cameroon. The heavy minerals are characteristic of short hydromechanical transport and highlight the stability of Nb–Ta oxides during the weathering and transport. Nb–Ta mineralization in placer deposits from the Ngoura area may derive from local sedimentation. The assemblage of Nb–Ta oxides in the studied placers consists of tapiolite-(Fe) and tantalite-(Mn). Tapiolite-(Fe) exhibits high concentrations in Ta2O5 (78.07–80.46 wt%) and FeO (12.18–13.66 wt%) and plots within a narrow range of Ta* (0.900–0.933) and Mn* (0.037–0.119), which correspond to the ranges typically observed in tapiolite-(Fe) worldwide. Tantalite-(Mn) shows Ta2O5 contents ranging from 62.17 to 69.45 wt%, Nb2O5 from 12.09 to 17.37 wt%, MnO from 7.63 to 12.49 wt% and FeO from 1.48 to 6.62 wt%. It is also characterized by a wider range of Mn* (0.538–0.891) and relatively homogeneous Ta/(Ta + Nb) (0.683–0.779) ratios. Texturally, the studied Nb–Ta minerals exhibit oscillatory zoning characterized by bright Ta-rich zones alternating with dark Nb-rich zones. This oscillatory zoning is progressive in some minerals and the alternating bands may appear regular to wavy with gradual transitions. Oscillatory zoning in Nb–Ta oxides from the Ngoura placers is considered to be a primary magmatic feature and is tentatively explained as a result of magmas mixing, rapid cooling or degassing/decompression of the igneous system. The geomorphology and the tropical humid climate of the eastern region offer suitable conditions for the deposition of alluvial placers. Therefore, niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of alkaline granites and pegmatites widespread in the study area.

Barren and Li–Sn–Ta Mineralized Pegmatites from NW Spain (Central Galicia): A Comparative Study of Their Mineralogy, Geochemistry, and Wallrock Metasomatism

In Central Galicia, there are occurrences of barren and Li–Sn–Ta-bearing pegmatites hosted by metasedimentary rocks. A number of common and contrasting features between Panceiros pegmatites (barren) and Li–Sn–Ta mineralized Presqueira pegmatite are established in this study. K-feldspar and muscovite have the same trace elements (Rb, Cs, P, Zn, and Ba), but the mineralized one has the highest Rb and Cs and the lowest P contents. The barren bodies show fluorapatite and eosphorite–childrenite replacing early silicates. The mineralized body has primary phosphates (fluorapatite and montebrasite), a metasomatic paragenesis (fluorapatite and goyazite) replacing early silicates, and a late hydrothermal assemblage (vivianite and messelite). Ta–Nb oxides from the Presqueira body show a trend from columbite-(Fe) to tantalite-(Fe) and tapiolite-(Fe). In this body, the Li-aluminosilicate textures support primary crystallization of petalite that was partially transformed into Spodumene + Quartz (SQI) during cooling, and into myrmekite during a Na-metasomatism stage. As a result of both processes, spodumene formed. The geochemical study supports magmatic differentiation increasing from the neighboring granites to the Li–Sn–Ta mineralized pegmatite. In both barren and mineralized bodies, the pegmatite-derived fluids that migrated into the wallrock were enriched in B, F, Li, Rb, and Cs and, moreover, in Sn, Zn, and As.

Magmatic-hydrothermal evolution of rare metal pegmatites from the Mesoproterozoic Orange River pegmatite belt (Namaqualand, South Africa)

The 450 km long Orange River pegmatite belt intruded the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Province, Southern Africa, at ca. 1 Ga. The western part of the belt is characterized by the occurrence of LCT pegmatites locally mineralized in Li, Ta, Nb, Be and Bi. Most of these mineralized pegmatites display zonation with a quartz-feldspar-muscovite-garnet-beryl-bearing aplite border and wall zone, a quartz-feldspar-spodumene-lepidolite-bearing intermediate zone, and a quartz-K-feldspar core. The pegmatites and their granodioritic country rocks commonly show evidence of albitization and greisenization (i.e. secondary muscovitization). In this study, detailed petrographic observations and the compositions of mica, feldspar, spodumene and Nb-Ta oxide minerals are reported. This information is combined with bulk country rock compositions and new U-Pb zircon and monazite ages plus Sm-Nd isotope compositions of monazite to constrain the origin and magmatic-hydrothermal evolution of weakly and strongly mineralized LCT pegmatites.The Li-Ta-Be Kokerboomrand I pegmatite emplaced at ca. 985 Ma during the main stage of pegmatite emplacement in the Orange River belt and regional strike-slip deformation. It has εNd(t)monazite = −14 ± 3 and potentially formed by partial melting of Paleoproterozoic rocks from the Richtersveld magmatic arc. Alternatively, Mesoproterozoic sediments of the Bushmanland Subprovince may also provide a suitable magma source for the Kokerboomrand I pegmatite.The rare metal contents of muscovite and columbite group minerals (CGM) increase from the weakly mineralized to strongly mineralized pegmatites, suggesting early-stage fractional crystallization or variable partial melting conditions. Mica Nb/Ta values of <14 and K/Rb ratios of <40 can be used as geochemical tools for Li-Ta-(Nb)-Be exploration in the region, and possibly in other pegmatite belts worldwide.Rare metal-bearing minerals crystallized throughout the magmatic-hydrothermal evolution of mineralized pegmatites. During the magmatic-dominated stage I, constitutional zone refining likely induced a decrease of spodumene Fe contents and increase of CGM Ta contents toward the pegmatite core. Stage II marks the pervasive circulation of an immiscible hydrosaline melt. This stage is characterized by the replacement of the primary mineral assemblage by lamellar albite (cleavelandite), (fluor)calciomicrolite and Cs-Ta-rich Li-muscovite and zinnwaldite. Stage III represents the progressive transition from a magmatic-hydrothermal system dominated by a Li-Cs-Ta-rich hydrosaline melt toward a system mainly involving (Li)-Cs-(Ta)-Bi-rich aqueous fluids. This stage is characterized by the crystallization of lepidolite and zero-valence-dominant pyrochlore as well as strong localized greisenization and Bi mineralization.Magmatic-hydrothermal alteration of the immediate country rock of the mineralized pegmatites is marked by the crystallization of albite, Li-Ta-Cs-rich mica, various Nb-Ta oxide minerals, and led to a strong decrease of the whole-rock Nb/Ta ratio plus significant rare metal enrichment.Our findings from the Orange River belt demonstrate that an important role can be ascribed to melt-melt and melt-fluid immiscibility and metasomatism in the formation of pegmatite-related rare metal deposits. Greisens and albitites associated with pegmatites and their immediate country rocks can be considered as favorable targets for Li-Ta mineral exploration. Strong and pervasive metasomatism across the mineralized pegmatites and their country rocks suggests involvement of a parental medium that was highly enriched in water.

Mineralogy and geochemistry of the newly discovered Late Mesozoic granite-pegmatite and associated Sn-Nb-Ta-Be mineralization in the Miao'ershan-Yuechengling composite batholith, northern Guangxi, South China

The Miao’ershan–Yuechengling granitic batholith (MYGB) in the west Nanling Range, South China, contains multiple intrusive phases including the Paleozoic porphyritic biotite granite and Early Mesozoic two-mica and muscovite granite associated with the W–Cu–U mineralization. In this study, the Gedongping granitic rocks, in the central part of the MYGB are shown to be related to Sn–Nb–Ta–Be mineralization. They comprise albite granite, pegmatite, and aplite, and intrude the Paleozoic granite. Sn–Nb–Ta oxide minerals such as cassiterite, columbite-group minerals, microlite and wodginite are common. Beryl occurs in the pegmatite and aplite. Zircon U-Pb isotopic data yielded an age of 153 ± 3 Ma for the formation of the aplite. In situ U-Pb dating of cassiterite from the albite granite and aplite yielded ages of 147 ± 5 and 155 ± 3 Ma, respectively, for the Sn mineralization. Geochemically, theses granitic rocks are rich in SiO2 (~72 wt%), Al2O3 (~15 wt%), and total alkali (~10 wt%), and the alumina saturation index is 1.0 to 1.1. Combined with the high HfO2 contents (up to 15.1 wt%) and low Zr/Hf ratios (6–35) in zircons, it is indicated that the Gedongping granitic rocks are highly evolved. We infer that the Gedongping granitic rocks underwent magmatic fractionation and subsolidus hydrothermal alteration, based on the textural and compositional variations of the Sn–Nb–Ta oxide minerals. Two fine-grained granite stocks within the MYGB, which were described as Late Mesozoic products, have been confirmed to form during the Paleozoic (420–426 Ma).

Formation of columbite and microlite after alteration of Nb- and Ta-bearing biotite from the Lizaizhai pegmatite (Guangning ore district, Guangdong, South China): Identification of a new potential Nb–Ta mineralization type

The Lizaizhai pegmatite is located in Guangning, Guangdong Province, South China. Unlike common rare-element (Li–Sn–Nb–Ta) pegmatites in Guangning, which contain muscovite as the dominant mica, the Lizaizhai pegmatite represents a different type of rare-element mineralization with biotite as the dominant mica. Our detailed study shows that the Lizaizhai pegmatite can be divided into a border zone and a core zone. Biotite in the border zone contains lower contents of Ta (mean 121 ppm) and higher contents of Ti (mean TiO2 = 0.31 wt%) and Nb (mean 409 ppm) compared with biotite in the core zone, which contains slightly more Ta (mean 381 ppm) than Nb (mean 375 ppm) and Ti (mean TiO2 < 0.1 wt%). The primary Nb–Ta oxide minerals consist of Ta–Nb-rich rutile, columbite-group minerals (CGMs), tapiolite, and microlite, which are found as inclusions in quartz and feldspar. Ta–Nb-rich rutile and tapiolite are present only in the core zone of pegmatite. Biotite in the pegmatite dike has been intensely altered by acidic and F-rich hydrothermal fluids, mostly by chloritization. With H+ and F− being consumed continuously with alteration, Nb and Ta complex anions became unstable. By combining with other ions (e.g. Ca, Fe and Ti), alteration-stage Nb–Ta-bearing minerals (titanite, ilmenite, and rutile) and Nb–Ta oxide minerals (CGMs and microlite) were able to be precipitated in chlorite and alteration-formed muscovite. The border and core zones can be distinguished by distinct assemblages of alteration products: Alteration-stage minerals such as titanite, ilmenite, rutile, and Nb–Ta oxides are present in the border zone, whereas only Nb–Ta oxide minerals are found in the core zone. Considering its distinct petrology, mineralogy, and texture, we propose that the Lizaizhai pegmatite represents a new potential type of Nb–Ta mineralization from biotite alteration.

Characterization of Nb-Ta oxides associated with the aplopegmatites from Tres Arroyos (Badajoz, Spain)

Characterization of Nb-Ta oxides associated with the aplopegmatites from Tres Arroyos (Badajoz, Spain)

The Distribution of Rare Metals in the LCT Pegmatites from the Giraúl Field, Angola

The Giraúl granitic pegmatite field in Angola is composed of five pegmatite types, the most evolved belong to the beryl-columbite, beryl-columbite-phosphate and spodumene types. Pegmatites are concentrically zoned with increased grain size toward a quartz core; the most evolved pegmatites have well-developed replacement units. These pegmatites are rich in Nb-Ta oxide minerals and the field has a moderate interest for critical elements such as Ta and Hf. Tourmaline, garnet and micas occur as accessory minerals. The abundance of Zr and Nb-Ta minerals increases with the evolution of the pegmatites, as well as the proportions of beryl and Li-rich minerals. The Ta/(Ta + Nb) ratio in Nb-Ta oxide minerals and the Hf/(Hf + Zr) ratio in zircon also increase with the evolution of the pegmatites and within each pegmatite body from border to inner zones, and especially in the late veins and subsolidus replacements. Textural patterns and occurrence of late veins with Ta-rich minerals suggest that Nb and especially Ta can be enriched in late hydrothermal fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.

The geochemical evolution of Nb–Ta–Sn oxides from pegmatites of the Cape Cross–Uis pegmatite belt, Namibia

AbstractThe Cape Cross–Uis pegmatite belt, Damara Orogen, north-central Namibia hosts multiple Ta–Nb- and Sn-oxide-bearing pegmatites. Columbite-group minerals, tapiolite, cassiterite and minor ixiolite and wodginite occur in abundance within pegmatites and display various compositional and internal structural mineralogical variations. Ta–Nb oxides display various zonation patterns indicative of multiple crystallisation phases, whereas cassiterite is dominantly homogeneous with minor euhedral columbite-group mineral inclusions. Ta–Nb oxides are mostly rich in Fe, with fractionation patterns in the columbite quadrilateral being sub parallel to the Ta/(Ta + Nb) axis; increasing Ta/(Ta + Nb) with little change in Mn/(Mn + Fe), which is consistent with classical trends in beryl-to-spodumene rare-element pegmatites. In addition, these trends suggest that co-crystallising minerals compete with Ta–Nb oxides for elements such as Mn, preventing Ta–Nb oxides from attaining Mn-rich compositions during the fractionation process. Cassiterite shows similar fractionation patterns with Fe &amp;gt; Mn and notable increases in the Ta content. Minor-element substitution in Ta–Nb oxides shows sharp decreases with increasing fractionation supporting the hypothesis that newly stabilised co-occurring minerals compete with columbite-group minerals for certain elements. Tapiolite shows the same minor-element trend, however, only for Sn and Ti suggesting cassiterite was a dominant competing mineral. Although crystallisation of Ta–Nb oxides from an aqueous fluid at the late-stages of pegmatite genesis is highly debated, significantly elevated Ta contents in metasomatised country rock, compared to unaltered country rock, may give new insight, suggesting that Ta may indeed partition into, and be transported by, an exsolved aqueous fluid. However, further studies of the country rock metasomatic contacts are required as currently the dataset is limited. The degree of fractionation as depicted by Ta–Nb and Sn oxides within pegmatites, indicate that a zonation from primitive to evolved pegmatites surrounding granites is not present and that pegmatites are probably not related to granites in the typical parent–daughter relationship.

Evidence for Nb-Ta Occurrences in the Syn-Tectonic Pan-African Mayo Salah Leucogranite (Northern Cameroon): Constraints from Nb-Ta Oxide Mineralogy, Geochemistry and U-Pb LA-ICP-MS Geochronology on Columbite and Monazite

The Mayo Salah pluton, which is located in the North-Cameroon domain of Central African Bold Belt (CAFB), is emplaced as a laccolith in volcano-sedimentary schists of Poli series, and displays features of Rare-metal Granite (RMG). It is made of two main rock groups: (1) the metaluminous barren muscovite granite (MsG) and (2) the Nb-Ta bearing peraluminous leucogranite (MsL) which expresses four subtypes. The evolved Rare-element MsL is subalkaline, slightly peraluminous (ASI = 1.01–1.21), and it displays flat REE chondrite-normalized patterns with a strong negative Eu anomaly (Eu/Eu* = 0.02–0.20). It belongs to the peraluminous low phosphorus Rare-element Granites and L-type igneous rocks, as shown by the relatively low Zr/Hf (4.8–14) and Nb/Ta (1.4–9.0) ratios and the positive slope of the Zr-Hf-Nb-Ta profile in spider diagrams. The rare-element-bearing mineral is represented by columbite-group minerals (CGM) and other Nb-Ta-oxides (Nb-rutile and pyrochlore supergroup minerals). The CGM is classified as Mn-columbite, with Ta# and Mn# ratios increasing from core to rim. Two stages of mineralization are identified; the earliest stage (CGM-I) consists in scattered tabular or prismatic euhedral grains that were related to magmatic fractionation. The latest stage (CGM-II) is expressed as a Ta-rich Mn hydrothermal CGM episode represented as rims and/overgrowths around and/or as veinlet crosscutting CGM-I or in cleavage planes of muscovite. The U-Pb dating of columbite and monazite of the Mayo Salah leucogranite indicates a late-Neoproterozoic magmatic-hydrothermal mineralization event from 603.2 ± 5.3 to 581.6 ± 7.2 Ma, as consistent with both late D2 to D3 events that were recorded in the CAFB in Cameroon, and the associated continental collision environment. The Nb-Ta mineralization of the Mayo Salah pluton provides evidence for the presence of RMG in Northern Cameroon of CAFB, and its temporal association with the youngest period of metallogenic epoch of Nb-Ta-ore formation in Africa associated to Pan-African times.

Mineralogical constraints on the genesis of W–Nb–Ta mineralization in the Laiziling granite (Xianghualing district, south China)

The Laiziling granitic pluton, located in the Xianghualing district, Hunan Province, China, comprises typical rare-metal granites with a pronounced vertical lithological zonation. This pluton includes three granitic phases comprising, from base to top, medium-grained (MG) alkali feldspar granite, fine-grained (FG) alkali feldspar granite and albite granite. Here we report the results of textural and chemical analyses of W–Nb–Ta oxide minerals from the Laiziling granitic pluton. The chemical compositions of whole-rock samples, rock-forming minerals and zircon have allowed us to identify the magmatic–hydrothermal processes that operated during the formation and rare-metal mineralization of the Laiziling pluton. The granites are slightly peraluminous to peraluminous, highly evolved and fractionated, as inferred from whole-rock geochemical and mineralogical (micas and zircon) signatures. All the micas in different granitic phases are classified into the zinnwaldite group. Li concentration in the primary micas (∼2.6–4 wt% Li2O) and Hf concentration (∼1–8 wt% HfO2) in the zircon increase for the base to the top granitic phases. Both alkali feldspar granites contain oxide mineral aggregates including columbite-group minerals, wolframoixiolite, qitianlingite, wolframite and cassiterite. The columbite-group minerals, wolframite and cassiterite also occur as isolated grains within the albite granite. The presence of the oxide minerals suggests that the W–Nb–Ta mineralization developed during the latter stages of the magmatic evolution, related to fluids that were compositionally evolved from the granites. The composition of the fluids controls the aggregate of the oxide minerals. The wolframite and cassiterite that precipitated within the Laiziling skarn-type deposit are chemically different from those within the granite, indicating a hydrothermal origin for the oxide minerals that formed as a result of pervasive alteration.

Textural and mineral-chemistry constraints on columbite-group minerals in the Penouta deposit: evidence from magmatic and fluid-related processes

ABSTRACTThe Penouta Sn-Ta deposit, in the northwest of Spain, is a greisenized granitic cupola where Ta minerals occur mainly as disseminations in a leucogranite body intruded in Precambrian–Lower Cambrian gneisses and mica-schists. This leucogranite is a medium- to fine-grained inequigranular rock consisting mainly of quartz, albite, K-feldspar and muscovite. Accessory minerals are mainly of spessartine, zircon, cassiterite, Nb-Ta oxides, monazite, xenotime, native bismuth and pyrite. The alteration processes were mainly albitization, muscovitization and kaolinitization.This leucogranite is peraluminous and P-poor, with 0.03–0.07 wt.% P2O5, 900–1500 ppm Rb, 30–65 ppm Cs, 120–533 ppm Li, 80–140 ppm Ta, 51–81 ppm Nb and up to 569 ppm of Sn.Mineralogical characterization of Nb-Ta oxide minerals was determined by X-ray diffraction, scanning electron microscopy, electron microprobe analysis and mineral liberation analysis. Mn-rich members of the columbite-group minerals (CGM) are the most common Ta-bearing phases, but microlite, wodginite, tapiolite and Ta-rich cassiterite occur also. CGM crystals are commonly zoned concentrically, with a Nb-rich core surrounded by a Ta-rich rim, with a sharp boundary between them. Convoluted zoning occurs also. Dissolution textures resulting from the corrosion of columbite and tantalite rims, in particular, are common. The Mn/(Mn + Fe) ratio varies between 0.33 and 0.97 and the Ta/(Ta + Nb) ratio between 0.07 and 0.93. Wodginite and microlite formed as late replacements of CGM and occur associated with tantalite and cassiterite. Subhedral to anhedral cassiterite crystals, usually up to 200 μm across, occur in two generations: the earlier one is Nb,Ta-poor whereas in the later generation, the Ta content can reach &gt;9 wt.% of Ta2O5and 1.7 wt.% of Nb. The presence of a fluid phase in the apical zone of the granite, probably related to the separation of a fluid/vapour of the melt, could explain the sponge-like textures, the Ta enrichment associated with these textures, the occurrence of Ta-enriched mineral phases (microlite and wodginite) and their common interstitial character.

Diversity of Ti–Sn–W–Nb–Ta oxide minerals in the classic granite-related magmatic–hydrothermal Cínovec/Zinnwald Sn–W–Li deposit (Czech Republic)

The diversity of Ti–Sn–W–Nb–Ta oxide minerals within a complex granite-related magmatic–hydrothermal system was studied by electron microprobe on the example of the Cinovec (Zinnwald) granite cupola containing a well-known Sn–W–Li deposit. The Cinovec granite cupola is a late Variscan ( ca . 322 Ma), strongly fractionated A-type pluton composed of upper zinnwaldite granite and lower biotite granite. The zinnwaldite granite contains numerous subhorizontal quartz veins and greisen bodies, both composed of quartz, zinnwaldite and topaz and minor amounts of fluorite and Ti–Sn–W–Nb–Ta oxide minerals. Common Nb–Ta-rich rutile and rare cassiterite, columbite-(Fe) and -(Mn), and minerals of the pyrochlore-supergroup were found in biotite granite in deeper parts of the pluton, whereas common columbite occurs with rare W-rich ixiolite, qitianlingite, pyrochlores, and scheelite in some varieties of zinnwaldite granite. In the uppermost part of the cupola, Ta-rich cassiterite, microlite and pyrochlore are frequently observed. The greisens host predominantly wolframite, scheelite and Ta-poor cassiterite, which are accompanied by columbite-group minerals, rare W-rich ixiolite and qitianlingite. Throughout the pluton, the molar Mn/(Fe + Mn) ratio in columbite increases from 0.15–0.40 in the deeper part of the zinnwaldite granite (depths of 500–635 m) to 0.5–0.9 in granite and greisen in the uppermost part of the pluton (depths of 0–180 m), but no systematic changes in the Ta/(Nb + Ta) ratio were found. On the scale of individual zoned crystals, the rims are always Ta-enriched, whereas the Mn/(Fe + Mn) values in the rims are equal or lower. The Nb,Ta-rich rutile, interiors of the columbite crystals and Ta-rich cassiterite disseminated in the granites are interpreted as products of magmatic crystallization. The Ta-rich rims of columbite-(Fe) to -(Mn), Ta-poor cassiterite and all wolframite found in the greisen should be attributed to the early hydrothermal (greisenization) stage, whereas W-rich ixiolite, qitianlingite, and pyrochlore-supergroup minerals appear to belong to the later, hydrothermal event at comparatively lower temperature.

Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa

Tantalum is an important metal for high-technology applications of the modern world. It is recovered from oxide minerals that are present as minor constituents in rare-metal granites (RMG) and granitic rare-element pegmatites (REP). Significant potential exists as a by-product of spodumene mining from pegmatites, and from Nb-Zr-rare earth element (REE) mining from carbonatites and peralkaline igneous rocks. Columbite-group minerals (CGM) account for the majority of the current Ta production; other Ta-Nb oxides such as tapiolite, wodginite, ixiolite, rutile and pyrochlore-supergroup minerals are of minor importance. The estimated Ta resources of Africa are >50,000 tons of contained Ta 2 O 5 , representing 16% of the world resources. Currently, the economically most important mining districts in Africa are located within the Kibara Belt (mainly Rwanda and the DR Congo), around the Kenticha mine (Ethiopia) and in the Alto Ligonha Province (Mozambique). Production from pegmatites and related placer deposits is mostly from hundreds of artisanal and small-scale workings with only a handful of semi-industrial Ta mines. Collectively, African operations are currently responsible for more than half of the world’s primary Ta. Rare-metal granite and REP districts in Africa are part of Archaean, Palaeoproterozoic, Neoproterozoic, Palaeozoic and Mesozoic provinces. Each period of Ta ore formation is characterised by specific mineralogical and geochemical features. Compositions of CGM are variable: Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (DR Congo), the Kamativi Belt (Zimbabwe) and the Jos Plateau (Nigeria); Mn-rich columbite-tantalite is typical of the Alto Ligonha Province (Mozambique), the Arabian-Nubian Shield (Egypt, Ethiopia) and the Tantalite Valley pegmatites (southern Namibia) and Large compositional variations through Fe-Mn fractionation, followed by Nb-Ta fractionation are typical for pegmatites of the Kibara Belt of Central Africa, pegmatites associated with the Older Granites of Nigeria and many pegmatites in the Damara Belt of Namibia. Trace element concentrations in CGM and other Ta-Nb oxides are highly variable, with Ti, W, Sn, Zr, Hf, Y, Sc and REE displaying significant regional differences that may be used to discriminate Ta provinces.

New data on REE and rare-metal mineralization in pegmatites of the Slyudyanogorsk muscovite deposit in the Southern Urals

The Slyudyangorsk muscovite deposit in the southern Urals was explored and mined in 1926–1957. By the mid-1950s, 104 veins of quartz–feldspar pegmatites including 21 muscovite-bearing veins have been found. Pegmatites with giant black Y-bearing epidote crystals are crosscut by veins with giant muscovite crystals, which, in turn, are intersected by veins of two-mica–quartz–two-feldspar pegmatites with rare-metal and REE mineralization. Microprobe data on compositions of complex Ti–Ta–Nb oxides [fergusonite-(Y), samarskite-(Y), euxenite-(Y), polycrase-(Y), columbite-(Fe), pyrochlore supergroup] are characterized, as well as of uraninite, ilmenorutile, scheelite, Y-bearing epidote, certain sulfides and rock-forming minerals from the Slyudyanogorsk deposit. The morphology and interrelation of minerals indicate that they are the result of crystal growth in cavities rather than of metasomatic replacement of gneisses, as has been suggested earlier. Thus, it is more promising for rare-metal and REE minerals in the Slyudorudnik area to be found in igneous rocks (granitic muscovite–quartz–feldspar pegmatites with the Nb–Ta–Ti–Y–U–W–Mo mineralization) than in metasomatic rocks.

Ta and Sn concentration by muscovite fractionation and degassing in a lens-like granite body: The case study of the Penouta rare-metal albite granite (NW Spain)

The Penouta peraluminous low-phosphorous granite is the most important low-grade, high-tonnage Sn-Ta-Nb-bearing albite granite from the Iberian Massif. A sheet or laccolith shape, instead of a stock, is inferred for the Penouta granite, maybe in relation with the low viscosity and high mobility of a fluorine-bearing melt. Subhorizontal lateral extension of the magma is also inferred via vertical and horizontal geochemical variations. The absence of compositional gaps in variation diagrams, coupled with continuous evolutionary trends of compatible and incompatible elements with height, discard a multi-pulse intrusion and point to a single magma pulse. Mineral chemistry, trace element and least-squares mass balance modelling support a differentiation process from bottom to top in the emplacement place. The absence of switch from incompatible to compatible behaviour (bell-shaped trends) in Sn, Nb and Ta variation diagrams, coupled to experimental constraints on tantalite and cassiterite saturation, suggest that Nb-Ta oxides and probably cassiterite were not fractionated mineral phases, their crystallisation being related to concentration gradients within a trapped intercumulus melt. Major and trace element modelling support that the concentration upwards of Ta and the Ta/Nb ratio could be a consequence of mineral fractionation, with a key role of muscovite (mainly primary) for the Ta/Nb ratio, as this mineral has a higher partition coefficient for Nb than Ta. Our results suggest that fluorine and peraluminosity had a limited effect in the Ta/Nb ratio variations. Hence, Ta enrichment is mainly controlled by fractional crystallisation processes. In most cases, Sn enrichment was also concomitant with Ta, indicating that crystal-melt fractionation processes also played an important role in Sn concentration. Nevertheless, the strongest Sn enrichment in the granite (e.g., central part of the granite body) does not correspond to a significant Ta enrichment. The high affinity of Sn for fluids and the high partitioning of Ta for melt could explain this decoupling. Nevertheless, the magmatic signature of cassiterites in these strongly Sn-enriched zones (central part of the granite body) rules out a hydrothermal subsolidus origin for this fluid. By analogy with models carried out in sill-like bodies it seems likely that the Sn enrichment in the central part of the granite body is related to fluid saturation/degassing occurred in the lower margin, as a consequence of cooling and crystallisation of mostly anhydrous minerals (i.e. second boiling). The vapour exsolved migrated into the hotter melt up to the central part, where it probably was reabsorbed, yielding cassiterite with a magmatic signature. Moreover, we suggest that heat loss in the upper margin of the granite body might also contribute to the formation of a second fluid-saturated zone. As a result, pegmo-aplites and greisen were developed.

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Intensive low-temperature tectono-hydrothermal overprint of peraluminous rare-metal granite: a case study from the Dlhá dolina valley (Gemericum, Slovakia)

Abstract A unique case of low-temperature metamorphic (hydrothermal) overprint of peraluminous, highly evolved rare-metal S-type granite is described. The hidden Dlhá dolina granite pluton of Permian age (Western Carpathians, eastern Slovakia) is composed of barren biotite granite, mineralized Li-mica granite and albitite. Based on whole-rock chemical data and evaluation of compositional variations of rock-forming and accessory minerals (Rb-P-enriched K-feldspar and albite; biotite, zinnwaldite and di-octahedral micas; Hf-(Sc)-rich zircon, fluorapatite, topaz, schorlitic tourmaline), the following evolutionary scenario is proposed: (1) Intrusion of evolved peraluminous melt enriched in Li, B, P, F, Sn, Nb, Ta, and W took place followed by intrusion of a large body of biotite granites into Paleozoic metapelites and metarhyolite tuffs; (2) The highly evolved melt differentiated in situ forming tourmaline-bearing Li-biotite granite at the bottom, topaz-zinnwaldite granite in the middle, and quartz albitite to albitite at the top of the cupola. The main part of the Sn, Nb, and Ta crystallized from the melt as disseminated cassiterite and Nb-Ta oxide minerals within the albitite, while disseminated wolframite appears mainly within the topaz-zinnwaldite granite. The fluid separated from the last portion of crystallized magma caused small scale greisenization of the albitite; (3) Alpine (Cretaceous) thrusting strongly tectonized and mylonitized the upper part of the pluton. Hydrothermal low-temperature fluids enriched in Ca, Mg, and CO2 unfiltered mechanically damaged granite. This fluid-driven overprint caused formation of carbonate veinlets, alteration and release of phosphorus from crystal lattice of feldspars and Li from micas, precipitating secondary Sr-enriched apatite and Mg-rich micas. Consequently, all bulk-rock and mineral markers were reset and now represent the P-T conditions of the Alpine overprint.

Kaolinization — a tool to unravel the formation and unroofing of the Pleystein pegmatite–aplite system (SE Germany)

The Hagendorf–Pleystein Pegmatite Province, SE Germany, is known for the largest feldspar–quartz pegmatite in Central Europe and renowned for its rare elements, e.g., Li, Nb, and Ta, giving rise to a spate of exotic minerals, mainly phosphates. Argillaceous rocks are scarce and eclipsed by the numerous mineralogical investigations on rare phosphates. These phosphate pegmatites are for the first time subjected to a clay mineralogical study, rendered possible by the newly discovered strongly kaolinized aplite near the Kreuzberg Quartz Pegmatite at Pleystein. The supergene kaolin akin to the residual kaolin deposit at Tirschenreuth, SE Germany, was analyzed for its major and minor elements by XRF and micro-chemically by EMPA. Mineralogical investigations involved XRD, IR spectroscopy, thermoanalytical studies, CEC analyses and SEM-EDX. Supergene kaolinization forms a repository for heavy minerals critical for the interpretation of the emplacement of the Late Paleozoic pegmatites as well as a matrix for pegmatite-related trace elements and thereby may be used as an ore guide during exploration of these rare metal pegmatites. The resultant kaolin is also the protagonist in the story of exhumation and destruction of a pegmatite by weathering and erosion. Irrespective of the strength of kaolinization, Nb–Ta–Ti heavy minerals can be identified in the regolith atop the host pegmatite or aplite and used for genetic interpretation of the primary mineralization and the origin of the felsic intrusive. Nb–Ta solid solution series (s.s.s.) have to be treated cautiously because of the disposition of Ta-enriched Nb–Ta oxide s.s.s. to undergo corrosion in their tantalite lamellae more easily than in their niobium-enriched zones. Kaolinization may alter the primary Nb/Ta ratio but not to the extent that Ta is released completely. The most strongly kaolinized new aplite is the youngest member of a series of felsic intrusive rocks in the Pleystein pegmatite–aplite system. The supergene kaolinization extending from the Miocene through the Pliocene can easily be correlated by “minero-stratigraphy” with the larger Tirschenreuth kaolin deposit. The four stages established in the area furnish evidence of gradual alkalinization of the meteoric pore fluids throughout the Neogene and the Quaternary. Youngest stages are found at Tirschenreuth, the oldest regolith stage is present at Pleystein.