Ta Oxide Minerals Research Articles

Chrysoberyl and associated beryllium minerals resulting from metamorphic overprinting of the Maršíkov–Schinderhübel III pegmatite, Czech Republic

AbstractThe Maršíkov–Schinderhübel III pegmatite in the Hrubý Jeseník Mountains, Silesian Domain, Czech Republic, is a classic example of chrysoberyl-bearing LCT granitic pegmatite of beryl–columbite subtype. This thin pegmatite dyke, (up to 1 m in thickness in biotite–amphibole gneiss is characterised by symmetrical internal zoning. Tabular and prismatic chrysoberyl crystals (≤3 cm) occur typically in the intermediate albite-rich unit and rarely in the quartz core. Chrysoberyl microtextures are quite complex; their crystals are irregularly patchy, concentric or fine oscillatory zoned with large variations in Fe content (1.1–5.3 wt.% Fe2O3; ≤0.09 apfu). Chrysoberyl compositions reveal dominant Fe3+ = Al3+ and minor Fe2+ + Ti4+ = 2(Al, Fe)3+ substitution mechanisms in the octahedral sites. Tin, Ga, and V (determined by LA-ICP-MS) are characteristic trace elements incorporated in the chrysoberyl structure, whereas anomalously high Ta and Nb concentrations (thousands ppm) in chrysoberyl are probably caused by nano- to micro-inclusions of Nb–Ta oxide minerals; especially columbite–tantalite. Textural relationships between associated minerals, distinct schistosity of the pegmatite parallel to the host gneiss foliation and fragmentation of the pegmatite body into blocks as a result of superimposed stress are clear evidence for deformation and metamorphic overprinting of the pegmatite. Primary magmatic beryl, albite and muscovite were transformed to chrysoberyl, fibrolitic sillimanite, secondary quartz and muscovite during a high-temperature (~600°C) and medium-pressure (~250–500 MPa) prograde metamorphic stage under amphibolite-facies conditions. A subsequent retrograde, low-temperature (~200–500°C) and pressure (≤250 MPa) metamorphic stage resulted in the local alteration of chrysoberyl to secondary Fe,Na-rich beryl, euclase, bertrandite and late muscovite.

Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China

Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.

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.

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.

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.

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.

Differentiated rare-element mineralization in an ongonite–topazite composite dike at the Xianghualing tin district, Southern China: An electron-microprobe study on the evolution from niobium–tantalum-oxides to cassiterite

Our study characterizes in detail the mineralogical, textural and compositional features of a highly evolved, composite ongonite–topazite dike and its magmatic differentiation history. We present compositional data collected by established techniques, i.e. by electron microprobe and wet-chemical analysis, which provide a detailed framework for future studies that employ state-of-the-art analytical techniques. The studied dike (referred to as the No. 431 dike) crops out within the Xianghualing area in the Nanling Range of southern China, in close spatial association with Jurassic Sn–Nb–Ta granite plutons. The rock samples in the No. 431 dike were collected from a structurally lower drill hole and a trench at higher level. The ongonite is encountered throughout the dike, but the topazite is only revealed along the margin of the upper, near-surface dike. The results of whole-rock major and trace element analyses show that the rocks of the No. 431 dike are strongly peraluminous with an average ACNK value of ~1.5 for ongonite and >3.9 for topazite. They are enriched in F, 1.7wt.% and 5.4wt.% on average for ongonite and topazite, respectively. The rocks have low Zr/Hf and Nb/Ta ratios, and high levels of ore-forming elements including Nb, Ta, Sn, and W. Silicate and oxide mineral assemblages, textures, and compositions are also distinct for the two rock types studied. In the lower ongonite of the dike, there are abundant phenocrysts of K-feldspar, quartz, and albite, and microphenocrysts of topaz and zinnwaldite in a matrix dominated by quartz, K-feldspar, and albite. Characteristic oxides are columbite–tantalite, tapiolite, and microlite, but cassiterite is absent. The upper ongonite of the dike has a silicate assemblage similar to the lower ongonite; columbite-(Mn), uranomicrolite, and limited amounts of cassiterite are the dominant accessory minerals. The topazite is characterized by large amounts of topaz and zinnwaldite intergrown with quartz, while K-feldspar, albite, and quartz phenocrysts have rounded shapes and are relatively rare. Cassiterite is the most abundant ore mineral, while Nb–Ta oxide minerals are less abundant. We interpret the whole-rock compositional trends, mineral textures, assemblages, and compositions to reflect the differentiation of an evolved, initially homogeneous magma that separated into aluminosilicate and hydrosaline melts, corresponding to crystallization of ongonite and topazite, respectively. The crystallization of Nb–Ta- and Sn-bearing ore minerals was strongly controlled by the separation of the two melt phases. We hypothesize that dike propagation/widening subsequent to the initial dike emplacement may have driven the separation of the aluminosilicate and hydrosaline melt phases that crystallized to ongonite in the core and topazite along the margins of the structurally higher part of the dike.

MICROLITE AND TANTALITE IN THE LCT GRANITIC PEGMATITES OF LA CANALITA, NAVASFRIAS Sn-W DISTRICT, SALAMANCA, SPAIN

The La Canalita pegmatites in the Navasfrias Sn–W district, in Spain, are a group of Li-rich LCT rare-element pegmatites hosted by metamorphic rocks. It consists of several dikes up to 1 m thick, 16 m in length and of unknown depth that were mined during the 40s and 50s for tin. Here, we document the Ta-rich oxide minerals, such as those of the columbite–tantalite group, tapiolite-(Fe) and microlite. It is possible to distinguish a primary assemblage, formed of extremely Mn-rich columbite and tantalite, and microlite, the latter forming the host of columbite-(Mn) exsolution lamellae in the intermediate parts of the dikes. The influence of metasomatic fluids led to the formation of a lepidolite–albite late unit and the crystallization of a secondary assemblage consisting of tantalite-(Fe), tantalite-(Mn) and tapiolite-(Fe). Uranium-rich microlite (up to 6% wt. UO 2 ) is present in this assemblage, replacing tantalite. Subsequently, the entrance of hydrothermal fluids, probably mixed with metamorphic fluids, caused a partial greisen-like alteration of the pegmatite bodies and formed a second generation of microlite replacing tapiolite, with high Ca and Ta and low F contents. The lack of a chemically suitable protolith in the surrounding metamorphic rocks, combined with the mineralogical paragenesis containing U-rich microlite, together with the composition of the Sn, Nb and Ta oxide minerals of the pegmatite dikes, suggest that the Cadalso – Casillas de Flores batholith is the most probable source of the melts forming the La Canalita pegmatite dikes.

COMPLEX INTERNAL TEXTURES IN OXIDE MINERALS FROM THE NANPING No. 31 DYKE OF GRANITIC PEGMATITE, FUJIAN PROVINCE, SOUTHEASTERN CHINA

The Nanping No. 31 granitic pegmatite, a highly evolved spodumene-type pegmatite in the Nanping pegmatite district, southeastern China, can be divided into five internal zones from the border inward: zones I–III as the outer zones and zones IV–V as the inner zones. It is strongly mineralized with Nb–Ta–Sn oxide minerals. We investigated these minerals by using back-scattered electron (BSE) imaging and quantitative electron-microprobe analyses (EMPA) to characterize their complex internal textures. The primary Nb–Ta oxide minerals show a sequence from columbite-(Fe) (zone I) through tantalite-(Fe) (zone II) to tantalite-(Mn) (zone III), but distinctly become columbite + wodginite in zone IV and, rarely, columbite in the zone V. Microscopic tapiolite-(Fe) is identified as a product of exsolution in cassiterite from the zones II–III, whereas complex associations of wodginite, columbite–tantalite and tapiolite-(Fe) are characteristic of cassiterite in zone IV. The classic variation of the columbite-group minerals from the zones I to III seems to result from the greater stability of tantalite than columbite components during fractional crystallization of the melt. However, decreasing values of Ta/(Nb + Ta) in columbite-group minerals in the inner zones are more probably compensated by crystallization of abundant primary wodginite-group minerals, controlled by the f (O2) of the magma. Columbite-group minerals commonly exhibit zoning, but complex and multiple-generation phenomena are notable in columbite–tantalite crystals from zones IV–V. In the latter case, later generations of columbite–tantalite overgrow or cross-cut the earlier ones. Irregular patterns of zoning are linked to the activity of hydrothermal fluids at the late stages of pegmatite evolution. The high mobility of Ta in such fluids is supported by the development of veinlets of tantalite. They cross-cut first the earlier-formed columbite + wodginite aggregates, and later fill brittle-deformation-induced fissures in the columbite, forming a network texture. The complex internal textures of Nb–Ta–Sn oxide minerals from the zones IV–V indicate a distinctly fluid-rich environment at the late stage of crystallization of the Nanping No. 31 pegmatite.

Foordite-thoreaulite, Sn2+Nb2O6Sn2+Ta2O6: compositional variations and alteration products

The foordite-thoreaulite series of Sn 2+ -bearing Nb, Ta-oxide phases is a rare constituent of complex, rare-element, Li-Cs-Ta-rich (LCT-family) granitic pegmatites with local low f O 2 environment. In this study, detailed electron-microprobe analyses (EMPA) reveal a broad range of nearly continuous foordite-thoreaulite solid solution: at. Ta/(Ta+Nb) =0.23–0.92. Valence equilibration of the formulae suggests up to 6 at.% of total Sn in Sn 4+ state occupying the octahedral Nb, Ta-populated B -site. Three substitution mechanisms dominate the foordite-thoreaulite chemistry: NbTa −1 , Pb 2+ Sn −1 2+ and Sb 3+ Sn 4+ Sn −1 2+ (Nb, Ta) −1 5+ ; Pb 2+ and Sb 3+ occupy ⩽ 21.5 at. % and ⩽ 7.6 at. % of the A -site position, respectively. Sb shows positive correlation with Ta/(Ta+Nb). Primary large foordite-thoreaulite crystals are compositionaly homogeneous, only areas of secondary alteration show small zones of recrystallized foordite-thoreaulite and grains with diffuse or patchy zoning, variable Nb/Ta ratio and locally increased Pb content. Influx of late-magmatic to hydrothermal fluids and/or alkali elements under higher f O 2 causes breakdown of foordite-thoreaulite and production of cassiterite and numerous Nb, Ta-oxide minerals. At Lutsiro pegmatite, foordite is replaced by mosaic fine-grained aggregate of secondary foordite-thoreaulite + columbite-tantalite + Ta-rich cassiterite. Simpsonite is present in Manono and Maniema pegmatites. Local replacement of foordite-thoreaulite by alkali-bearing Nb-Ta phases is widespread; irregular veinlets and zones of lithiotantite, calciotantite, irtyshite/natrotantite, cesplumtantite, a mineral with composition (Na, Cs) 2 (Pb, Sb 3+ ) 3 Ta 8 O 24 , rankamaite, fersmite, and pyrochlore-group minerals occur.

A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits

Research Article| January 01, 2005 A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits JULIE B. SELWAY; JULIE B. SELWAY Precambrian Geoscience Section, Ontario Geological Survey 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Search for other works by this author on: GSW Google Scholar FREDERICK W. BREAKS; FREDERICK W. BREAKS Precambrian Geoscience Section, Ontario Geological Survey 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Search for other works by this author on: GSW Google Scholar ANDREW G. TINDLE ANDREW G. TINDLE Department of Earch Sciences, Open University Milton Keynes, Buckinghamshire, UK MK7 6AA Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2005) 14 (1-4): 1–30. https://doi.org/10.2113/gsemg.14.1-4.1 Article history received: 16 Feb 2004 accepted: 20 Sep 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JULIE B. SELWAY, FREDERICK W. BREAKS, ANDREW G. TINDLE; A Review of Rare-Element (Li-Cs-Ta) Pegmatite Exploration Techniques for the Superior Province, Canada, and Large Worldwide Tantalum Deposits. Exploration and Mining Geology 2005;; 14 (1-4): 1–30. doi: https://doi.org/10.2113/gsemg.14.1-4.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyExploration and Mining Geology Search Advanced Search Abstract Rare-element pegmatites may host several economic commodities, such as tantalum (Ta-oxide minerals), tin (cassiterite), lithium (ceramic-grade spodumene and petalite), and cesium (pollucite). Key geological features that are common to pegmatites in the Superior province of Ontario and Manitoba, Canada, and in other large tantalum deposits worldwide, can be used in exploration.An exploration project for rare-element pegmatites should begin with an examination of a regional geology map. Rare-element pegmatites occur along large regional-scale faults in greenschist and amphibolite facies metamorphic terranes. They are typically hosted by mafic metavolcanic or metasedimentary rocks, and are located near peraluminous granite plutons (A/CNK > 1.0). Once a peraluminous granite pluton has been identified, then the next step is to determine if the pluton is barren or fertile. Fertile granites have elevated rare element contents, Mg/Li ratio < 10, and Nb/Ta ratio < 8. They commonly contain blocky K-feldspar and green muscovite. Key fractionation indicators can be plotted on a map of the fertile granite pluton to determine the fractionation direction: presence of tourmaline, beryl, and ferrocolumbite; Mn content in garnet; Rb content in bulk K-feldspar; and Mg/Li and Nb/Ta ratios in bulk granite samples. Pegmatite dikes with the most economic potential for Li-Cs-Ta deposits occur the greatest distance (up to 10 km) from the parent granite.Metasomatized host rocks are an indication of a nearby rare-element pegmatite. Metasomatic aureoles can be identified by their geochemistry: elevated Li, Rb, Cs, B, and F contents; and by their mineralogy: presence of tourmaline, (Rb, Cs)-enriched biotite, holmquistite, muscovite, and rarely garnet.Once a pegmatite dike has been located, the next step is to assess its potential to contain Ta mineralization. Pegmatites with the highest degree of fractionation (and thus the most economic potential for Li-Cs-Ta) contain blocky K-feldspar with >3,000 ppm Rb, K/Rb < 30, and >100 ppm Cs; and coarse-grained green muscovite with >2,000 ppm Li, >10,000 ppm Rb, >500 ppm Cs, and >65 ppm Ta. Pegmatites with Ta mineralization usually contain Li-rich minerals (e.g., spodumene, petalite, lepidolite, elbaite, amblygonite, and lithiophilite) and may contain Cs-rich minerals (e.g., pollucite, Cs-rich beryl). The ore minerals of Ta are commonly manganotantalite, manganocolumbite, wodginite, and microlite; Ta-rich cassiterite is also commonly present. Tantalum mineralization tends to occur in albitic aplite, mica-rich (lepidolite, cleavelandite ± lepidolite), and spodumene/petalite pegmatite zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Nature and Distribution of Tantalum Mineralization in Pegmatite Dikes, Lilypad Lakes Property, Fort Hope, Northwestern Ontario

The Lilypad Lakes property is host to a group of Ta-mineralized dikes that occur over an area of at least 10 km2. Field relationships are consistent with a syn- to late syn-tectonic timing for their emplacement (with respect to the major regional D1 episode of deformation), with several of the dikes exhibiting a steeply dipping, open, S- to M- to Z-shaped fold geometry. Individual dikes range up to 30 m wide (Rubellite Dyke, Pollucite Dyke), have strike lengths of up to 750 m (e.g., JJ Dyke), and are continuous to depths of greater than 250 m (e.g., Rubellite Dyke, Pollucite Dyke). Typically, they display sharp intrusive contacts characterized by the development of thin, holmquistite-rich haloes. The Ta-mineralized dikes are comprised of silica- and alumina-rich, sodic, granitic pegmatites that display extreme levels of geochemical fractionation (K/Rb typically 0.045 wt.% Ta2O5) in both the Rubellite and the Pollucite Dykes. Three Ta-oxide minerals (microlite, wodginite, manganotantalite) of primary origin predominate in the Ta-mineralized dikes, where they occur as fine, disseminated grains in the albite-rich pegmatite matrix. The sequence of crystallization is typically manganotantalite wodginite microlite, which conforms to a well-established paragenetic sequence for rare-element granitic pegmatites in general. All three of the primary Ta oxides are characterized by a high degree of chemical purity (e.g., average Ta concentrations in microlite from 77–79 wt.% Ta2O5), a compositional feature consistent with the extreme levels of geochemical fractionation characteristic of the host granitic pegmatites, and one that indicates that the Lilypad Lakes pegmatites are likely to produce Ta mineral concentrates of the very highest quality. Locally, a second generation of Ta minerals is present, restricted to small vugs in the pegmatite matrix that are surrounded by fine aggregates of K-rich mica or clay. The secondary Ta minerals are dominated by microlite, which is typically intergrown with fluorite and Fe-sulphides, and is distinguished from its primary counterpart by its lower Ta content (around 70 wt.% Ta2O5) and much higher concentration of uranium (up to 9 wt.% UO2). Also, secondary microlite consistently yields low analytical totals from electron microprobe analysis, suggesting that it is H2O-rich. The presence of this second, vug-related generation of microlite, together with the occurrence of localized networks of crosscutting fractures that host high concentrations of Cs and Rb (in the minerals pollucite and Rb-K-feldspar), suggests that a late-stage, fluid-dominated metasomatic/hydrothermal event may have been important in certain sections of the dikes, and has effected a redistribution of the economically important elements Ta and Cs.

The Al (Nb, Ta) Ti(in−2) substitution in titanite: the emergence of a new species?

The highest (Nb, Ta) content ever encountered in titanite is reported from the Marsikov 11 pegmatite in northern Moravia, Czech Republic. This dike is a member of a pegmatite swarm of the beryl-columbite subtype, metamorphosed under conditions of the amphibolite facies. The pegmatite carries, i.a., rare tantalian rutile intergrown with titanian ixiolite, titanian columbite-tantalite, fersmite and microlite. Fissures generated in the Nb, Ta oxide minerals during deformation are filled with titanite, formed by reaction of the oxide minerals with metamorphic pore fluids. The titanite displays limited degrees of substitutions Na(Ta > Nb)(CaTi)−1, (Ta > Nb)4□Ti−4Si−1 and AI(OH, F)(TiO)−1, but an extensive (and occasionally the sole significant) substitution (Al > Fe3+)(Ta > Nb)Ti−2, responsible for widespread oscillatory zoning. This substitution reduces the proportion of the titanite componentsensu stricto, CaTiSiO4,O, to less than 50 mole % in many analyzed spots. The extreme composition corresponds to (Ca0.994Na0.011)(Ti0.436Sn0.007Al0.280Fe3+ 0.006Ta0.199Nb0.079)Si0.988O4(O0.974F0.026). However, so far this substitution fails to generate compositions that would define a new species.

Pristine vs. contaminated trends in Nb,Ta-oxide minerals of the Jihlava Pegmatite District, Czech Republic

Columbite-tantalite is widespread in the lepidolite-subtype rare-element pegmatites of the Jihlava pegmatite district, western Moravia, Czechoslovakia. In most pegmatites, fractionation of columbite-tantalite shows initial enrichment in Mn followed by increasing Ta, in accordance with the usual trend from ferrocolumbite through mangano-columbite to manganotantalite, typical of the lepidolite pegmatites. Many dikes, however, show local deviations toward Fe- and Ti-rich compositions. In extreme cases, all columbite-tantalite is strongly enriched in Fe and Ti, and is associated with ixiolite and tantalian rutile. The degree of enrichment of the Nb,Ta oxide-mineral assemblage in Fe and Ti is proportional to tectonic introduction of wallrock xenoliths of mafic pyroxene-biotite syenite into the pegmatites during late stages of their consolidation. Extensive reaction of the residual pegmatite melt with the xenoliths contaminated the near-by melt, and generated Nb,Ta oxide minerals and tourmaline of non-typical chemistries. Late fersmite probably formed after thermal equilibration of the pegmatites with their syenitic country rocks, from Ca-bearing regional interstitial fluids pervading through the solidified pegmatites.

Exsolution intergrowths of titanian ferrocolumbite and niobian rutile from the Weinebene Spodumene Pegmatites, Carinthia, Austria

Titanian ferrocolumbite is a rare accessory mineral in the spodumene-bearing pegmatites at Weinebene, Carinthia, Austria. It contains abundant exsolved niobian rutile and scarce inclusions of cassiterite that may be primary. The titanian ferrocolumbite is relatively homogeneous with Mn/(Mn + Fe) 0.24–0.33, Ta/(Ta + Nb) 0.09–0.13 (atomic ratios) and 0.47–0.88 Ti per 12 cations (2.7–5.0 wt.% TiO2). Natural specimens are considerably disordered but become more ordered on heating. Niobian rutile has Mn/(Mn + Fe) 0.00–0.04 and Ta/(Ta + Nb) 0.26–0.38; it concentrates Fe, Ta, Ti and Sn relative to the Mn- and Nb-enriched ferrocolumbite. The overall scarcity of Nb, Ta-oxide minerals in the spodumene-bearing pegmatites of southern Ostalpen conforms to their general features ranking them with the albite-spodumene type of rare-element pegmatites.