Lithium-cesium-tantalum Research Articles

Characterisation and rare-metal potential of the Winneba-Mankoadze pegmatites, Southern Ghana: Evidence of two pegmatite fields

In southern Ghana, the region along the coast between Accra and Cape Coast hosts a large number of pegmatites mineralized in lithium, niobium-tantalum and tin. The pegmatites occur in many distinct groups, each extending over several kilometers. They intrude metasedimentary units of the Birimian Supergroup, and are associated with early to late orogenic granite intrusions which are metaluminous, sterile, and too old to be potential parental granites for the pegmatites. In this study, we characterized the Winneba-Mankoadze group of geographically coeval pegmatites, using field description, petrography, rare-metal mineralogy and accessory mineral geochemistry on micas, garnet and Nb–Ta–Sn minerals, in order to determine its rare-metal potential and to investigate its origin. The results indicate that the pegmatites are part of the albite-spodumene type of the Lithium–Cesium–Tantalum (LCT) family. The rare metal mineral assemblages are particularly complex and display relevant oxide species such as columbite- and wodginite-group minerals, tapiolite, microlite, cassiterite and rutile, which are evidences of an extremely evolved magmatic system. Based on mineral assemblages, whole-rock geochemistry, and mineral geochemistry on garnet, micas and the CGM, two pegmatite fields are distinguished in the Winneba-Mankoadze group, and an anatectic origin is proposed. For the first time in West Africa, we fully describe a highly fractionated LCT-family pegmatite field comparable to the most evolved pegmatite bodies in the world.

Columbite–Tantalite from Northern Scandinavia (Kaustinen, Kolmozero) Pegmatites: An Optical and Spectroscopic Properties

LCT (lithium–cesium–tantalum) pegmatites from the Kaustinen and Kolmozero regions contain columbite–tantalite mineralization, which has been presented in this study. Crystal structure, Raman microscopy, and optical property analyses of these minerals were performed. As a result of the structural studies and micro-area analyses, it was determined that these minerals in the pegmatites in question constitute a solid solution with numerous Mn-Fe and Nb-Ta substitutions within a single crystal. The ratio between Mn-Fe and Nb-Ta can change from crystal to crystal, which makes it impossible to find precise stechiometry between these ions. The crystallization conditions of these minerals were also determined by studying the associations of other rock-forming minerals and accessory minerals in the discussed rocks.

Lithium, Phosphorus, and rare earth elements in magmatic garnets from granitic Pegmatites: Coupled substitution and petrogenetic implications

The general chemical formula of garnet can be expressed as X3Y2(ZO4)3 where X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites, respectively. Lithium, phosphorus, and rare earth elements (including yttrium) in garnets from the Luanzhuang-Guanpo and Ziyugou (ZYG) pegmatite fields in the East Qinling orogen and the Kalu’an (KLA) Li pegmatites in the Chinese Altay orogen have been investigated both at a mineral scale and in the context of pegmatite types to identify the substitution mechanisms for these elements. Our results show that Y + REE or P can enter the garnet lattice through coupled substitution with Li (+Na) that can compensate the charge differences due to Y + REE in the X-site or P in the Z-site of the garnet. Two substitutions, namely [X(Li,Na)+1X(Y,REE)3+1(R2+)–2] and [X(Li,Na)1ZP1XR2+-1ZSi-1], are likely the most important and can account for incorporation of the majority of Y + REE, P, and Li + Na in pegmatite garnets. Whichever substitution becomes predominant in garnets is strongly dependent on the bulk chemistry of pegmatite-forming melts. Consequently, the first substitution dominates in garnets from NYF (niobium-yttrium-fluorine) pegmatites and mixed-type pegmatites with NYF affinity whereas the other one dominates in garnets from relatively P-rich LCT (lithium-cesium-tantalum) and barren pegmatites. Incorporation of Sc and Ti in garnet may also need Li + Na, although Sc and Ti are not as much as Y and REE in pegmatite garnet. The zonings of Y, REE, and Sc in garnet are mainly controlled by the composition of pegmatite-forming melt and consumption of these elements due to the growth of garnet itself and other minerals enriched in Y + REE (e.g., xenotime, fergusonite, monazite, etc.) or Sc (e.g., biotite). Additionally, our results, combined with the data in the literature, show that garnets from NYF pegmatites and mixed-type pegmatites with NYF affinity commonly contain elevated Y (up to ∼ 20000 ppm), REE (up to ∼ 20000 ppm), and Sc (up to ∼ 2400 ppm) but low P (<100 ppm) compared to those from LCT and barren pegmatites. Nevertheless, individual garnets with low concentrations of Y, REE, Sc, and P can occur in any types of pegmatites. Therefore, the coupled substitution between Li + Na and Y + REE or P along with zonings and concentrations of Y, REE, and Sc in garnet should be investigated on a regular basis to interpret pegmatite petrogenesis.

Detecting Li-bearing pegmatites using geospatial technology: the case of SW Konar Province, Eastern Afghanistan

Lithium-cesium-tantalum (LCT) pegmatites are widely distributed in northeastern, eastern, and central Afghanistan. The largest rare-metal pegmatites having Lithium (Li)-bearing minerals occur in the Nuristan pegmatite belt in Eastern Afghanistan. The main aim of this study is to detect and highlight the Li-bearing pegmatites using spectral analysis within the Southwest Konar province in the east of Afghanistan and to establish self-proposed spectral indices for the detection of Li-rich minerals. To achieve the study’s objective, we apply several algorithms, for example, false color composite, band ratio, and spectral angle mapper on ASTER, Landsat 8 OLI/TIRS, and Sentinel-2MSI data. Considering the spectral characteristics of key mineral spodumene and other diagnostic minerals, for example, lepidolite, amblygonite, and cookeite, which are considered diagnostic minerals of LCT pegmatites, particularly lithium, we utilize the ECOSTRESS Spectral Library-Version 1.0 for the interpretation of reflectance and absorption determinants of these minerals. Based on the diagnostic characteristics of the selected Li-bearing minerals, we examine several BRs and thresholding and consequently, propose new spectral indices which show a high concentration of spodumene, cookeite, and lepidolite over the northern regions of the study area. According to the obtained results, ASTER data and thresholded BRs present effective results compared to Landsat 8 OLI/TIRS. Based on the final outputs, we define nine zones having Li-bearing pegmatites, particularly over the Chapa Dara and Dara-e-Pech districts of southwestern Konar province, which could be promising areas of interest for further detailed exploration. Lastly, we compare the 118 known Li-bearing points collected from previous studies and field observations with the newly-defined zones to validate accuracy. The study’s overall accuracy is 71.1%, indicating a good match between our findings and the reference points.

Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)

Abstract Nb-Ta-oxide minerals (NTO) are commonly associated with rare-element pegmatites where they are interpreted to precipitate at magmatic to magmatic-hydrothermal conditions. Although high-temperature experiments show that phosphorus and other fluxing elements (e.g., Li, B, F) can affect the saturation of NTO in pegmatitic systems, it is still uncertain how NTO saturation occurs in natural, flux-rich pegmatitic melts and whether crystallization occurs at multiple stages during magmatic or subsolidus conditions. The lithium-cesium-tantalum (LCT) family P-rich Buranga granitic pegmatite (western Rwanda) is used as a type locality to address this question. NTO mineralization in the Buranga dike occurs in two mineralogical units: in mineralogically complex phosphatic nodules, and in albitized parts. In the phosphatic nodules, Fe-Nb-Ta-rich rutile and columbite-group minerals (CGM) are observed, while in the albitized parts, only CGM is found. Fe-Nb-Ta-rich rutile precipitates at the magmatic stage along with early primary phosphates (i.e., F-rich montebrasite, wyllieite, and fluorapatite). Conversely, CGM mineralization occurs at the magmatic-hydrothermal stage in association with replacement phosphates like bertossaite, after primary minerals (i.e., rutile, wyllieite, rosemaryite, and trolleite) are destabilized due to crystal-melt-fluid interactions. NTO textures and chemical zoning show uneven evolution from core to rim and are related to localized alteration phenomena of the surrounding minerals. This indicates that local processes and element transfers are more important than dike-scale fractionation processes for NTO mineralization in P-rich granitic pegmatites. The restricted availability of Fe and Mn in the system, which is related to the competition between phosphate and oxide minerals, is identified as the main control on the CGM mineralization in the Buranga dike. CGM precipitation is only possible during the magmatic-hydrothermal stage when Fe and Mn are leached from the primary phosphates and remobilized to the Nb-Ta-bearing residual melt by the exsolved fluid.

Analysis of Garnet by Laser-Induced Breakdown Spectroscopy—Two Practical Applications

Laser-induced breakdown spectroscopy (LIBS) is a simple and straightforward technique of atomic emission spectroscopy that can provide multi-element detection and quantification in any material, in-situ and in real time because all elements emit in the 200–900 nm spectral range of the LIBS optical emission. This study evaluated two practical applications of LIBS—validation of labels assigned to garnets in museum collections and discrimination of LCT (lithium-cesium-tantalum) and NYF (niobium, yttrium and fluorine) pegmatites based on garnet geochemical fingerprinting, both of which could be implemented on site in a museum or field setting with a handheld LIBS analyzer. Major element compositions were determined using electron microprobe analysis for a suite of 208 garnets from 24 countries to determine garnet type. Both commercial laboratory and handheld analyzers were then used to acquire LIBS broadband spectra that were chemometrically processed by partial least squares discriminant analysis (PLSDA) and linear support vector machine classification (SVM). High attribution success rates (&gt;98%) were obtained using PLSDA and SVM for the handheld data suggesting that LIBS could be used in a museum setting to assign garnet type quickly and accurately. LIBS also identifies changes in garnet composition associated with increasing mineral and chemical complexity of LCT and NYF pegmatites.

Columbite-Tantalite Group Mineral U-Pb Geochronology of Chaqiabeishan Li-Rich Granitic Pegmatites in the Quanji Massif, NW China: Implications for the Genesis and Emplacement Ages of Pegmatites

The Chaqiabeishan area is characterized by small Li-rich granitic pegmatites in the Quanji Massif (QM), northwest China. In this study, the columbite-tantalite group minerals (CGMs) from a typical Li-rich pegmatite dike were analyzed for major element contents using an EMPA (electron microprobe analyzer), for trace element contents using LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometry), and for ages using LA-ICP-MS U-Pb dating, respectively. The CGMs from the sample can be divided into two types, i.e., magmatic Type 1 and metasomatic Type 2. Although these two types of CGMs do not exhibit distinct major and trace element variations from core to rim within an individual grain, the Ta# values, Mn# values, and some trace element contents (such as Zr, Hf, W, and Sr) of Type 1 CGMs are distinct from those of Type 2 CGMs. The overall compositional changes from Type 1 CGMs to Type 2 CGMs are consistent with the typical evolutionary trend described for many lithium-cesium-tantalum (LCT) pegmatites and the complex spodumene trend described by Černý and Ercit (Bull. Mineral., 1989, 108, 499–532). The Type 2 CGMs have formed later and must be a metasomatic product of Type 1 CGMs. Eighteen Type 1 CGMs yielded a weighted mean 206Pb/238U age of 240.6 ± 1.5 Ma. The slight oscillatory zoning and/or sector zoning suggest that the dated Type 1 columbites have a magmatic origin. Thus, the crystallization ages of Type 1 columbites represent the emplacement ages of Li-rich pegmatites. One of the Type 2 CGMs yielded a 206Pb/238U age of 211.0 ± 4.7 Ma, which is hardly interpreted to be an age representing the later hydrothermal metasomatism, because one dataset has no apparent statistical significance. Therefore, our dating results can only indicate that the Li-rich pegmatite-forming melts were emplaced at approximately 240.6 Ma. Based on these results and previous studies of the 240–254 Ma granitoids in the QM, we conclude that the 240.6 Ma Li-rich granitic pegmatites, as well as 240–254 Ma granitoids in the QM, were both emplaced during the southward subduction of the Zongwulong Ocean Plate in the Late Permian to Middle Triassic.

Petrological and Geochemical Studies of Lepidolite (LCT Type) and Non-Lepidolite Pegmatite’s from Chakrasila, Dhubri District, Assam, North East India

Lepidolite pegmatite occurs as intrusive within biotite gneiss and amphibolite of Assam Meghalaya Gneissic Complex (AMGC) or Precambrian Gneissic Complex in the Dhubri district, Assam. AMGC is the north western extension of the Proterozoic rocks of Meghalaya Plateau or Shillong plateau. In the field it occurs as small to large veins and scattered boulders. Lepidolite pegmatite is later intruded by non lepidolite pegmatite. Pegmatites are medium to coarse grained with quartz and K-feldspar. It also contains lepidolite, which occurs in the form of flakes and clusters varying from pink to purple in colour. Petrography of lepidolite pegmatite reveals lepidolite as major constituents with quartz, K-feldspar and muscovite as minor constituents. XRD analysis reveals lepidolite (muscovite) is major mineral phase with kaliophilite in minor amount. Geochemically, they are calc-alkaline to high calc-alkaline and per-aluminous in nature. On the basis of Alumina Saturation Index (ASI), these pegmatites resemble Lithium-Cesium-Tantalum (LCT) family and compositional affinity with S-type granites of orogenic environments. Trace element compositions (Rb, Sr, Ba) indicate crystal fractionations, variable degrees of fractionation, highly evolved nature of pegmatite’s and strongly differentiated granites protoliths as source. The different tectonic discrimination diagrams indicate S-type and I-type melt for pegmatite derivations. Therefore, both the studied pegmatites could be an evolved variety of granitic rocks that originated from the same magma. The REE is relatively low to moderate.

The Minerals of Lithium

Lithium is rare in the cosmos, but the formation of continental crust has concentrated lithium into economic deposits. The 124 recognized Li mineral species occur largely in four geologic environments: (1) lithium–cesium–tantalum (LCT) granitic pegmatites and associated metasomatic rocks; (2) highly peralkaline pegmatites; (3) metasomatic rocks not directly associated with pegmatites; (4) manganese deposits. The geologically oldest Li minerals are reported from LCT pegmatites and date to 3,000–3,100 Ma, a critical period in the evolution of the continental crust and the rate of its generation. This suggests a link between the earliest appearance of LCT-family pegmatites and the onset of plate tectonics, consistent with the correlation between the observed abundance of LCT-family pegmatites and supercontinent assembly.

An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites

AbstractNiobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e.g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same P-T conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry's Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

U-Pb and reconnaissance Lu-Hf isotope analysis of cassiterite and columbite group minerals from Archean Li-Cs-Ta type pegmatites of Western Australia

Lithium–Cesium–Tantalum pegmatites represent a valuable global source of rare metals, however the crustal processes that facilitate the generation of Li-Cs-Ta pegmatite melts are not fully understood. Knowledge of the timing, melt sources and tectonic setting of this class of rare element pegmatite is limited in particular by the availability of geochronological and other isotopic data. This study presents new geochronological data from Li-Cs-Ta pegmatites across Western Australia, that vary in age, host lithology and geodynamic setting, by applying in situ LA-ICPMS U-Pb methods to date columbite group minerals and cassiterite samples. In a novel approach, reconnaissance Lu-Hf isotope data obtained from cassiterite samples by LA-MC-ICPMS are also reported providing insights into the source of Li-Cs-Ta pegmatite melts. The crystallisation ages and Lu-Hf isotopic data collected from the Li-Cs-Ta type pegmatites are interpreted within the existing magmatic and tectonic framework of their host terranes, and commonalities are identified. Li-Cs-Ta pegmatites are found to have been emplaced late in regional tectonic history and to follow periods of extended crustal reworking.

Tectonic and paleoclimatic controls of lithium-cesium-tantalum (LCT) pegmatite genesis, exhumation, and preservation in the Appalachians

Tectonic and paleoclimatic controls of lithium-cesium-tantalum (LCT) pegmatite genesis, exhumation, and preservation in the Appalachians

The Jurassic tourmaline–garnet–beryl semi-gemstone province in the Sanandaj–Sirjan Zone, western Iran

ABSTRACT Deposits of semi-gemstones tourmaline, beryl, and garnet associated with Jurassic granites are found in the northern Sanandaj–Sirjan Zone (SaSZ) of western Iran, defining a belt that can be traced for about 400 km. Granitic magmas strongly interacted with or were derived from melts of continental crust and/or sediments. Based on morphologies, size, mineral assemblage, and contact relationships with host granite and associated metamorphic aureoles, these deposits are categorized into six types: (1) garnet in skarns, (2) tourmaline, beryl, and garnet in pegmatite and aplitic dikes, (3) disseminations and patches of tourmaline in leucogranites, (4) quartz-tourmaline veins in granite, (5) tourmaline and garnet in metamorphic aureoles, and (6) tourmaline orbicules in aplite. Tourmalines are mostly schorl and dravite, and garnets are mostly almandine, spessartine, and grossular. Tourmaline, beryl, and garnet from pegmatites in the contact aureole of Jurassic granites reflect segregations of Be, B, Mn, and Al bearing melts from the Jurassic peraluminous granites. Quartz-tourmaline veins and hydrothermal garnets in skarns reflect fluids exsolved from the surrounding metasediment and pegmatite melt. In contrast, tourmaline patches and orbicules developed from boron-rich aqueous fluids exsolved from cooling granitic magma. Distribution of semi-gemstones in the SaSZ shows that these are mostly related to pegmatites associated with Jurassic granitic intrusions. Mineral equilibrium considerations indicate that SaSZ semi-gemstones crystallized at P = 3.5–7.5 kbar (11.5–25 km deep) and temperatures of 550–650°C. SaSZ pegmatites fall in the muscovite (MS) and MS-rare element classes. They are Lithium Cesium Tantalum (LCT)-type pegmatites. Fluids responsible for gem mineralization were exsolved from cooling granite bodies and released by metamorphosed sediments. Further studies are needed to better understand the northern Sanandaj–Sirjan tourmaline–garnet–beryl semi-gemstone Province.

Columbite-Group Minerals from New York Pegmatites: Insights from Isotopic and Geochemical Analyses

Columbite crystals from niobium-yttrium-fluorine (NYF) pegmatites lacking zircon or containing metamict cyrtolite were analyzed for major and minor elements (Electron Microprobe (EMP)), trace elements (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)), and U-Pb geochronology (Laser AblationMulti-Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS)). All four pegmatite localities sampled are hosted by the Proterozoic Fordham gneiss and/or Paleozoic Bedford gneiss (Columbite-(Fe); Kinkel and Baylis localities) and the Manhattan Schist of Lower Paleozoic age (Columbite-(Mn); Fort George and Harlem River Drive localities) and yield Neoacadian ages. The weighted average U-Pb ages are 372.2 ± 8.2 Ma (Baylis Quarry), 371.3 ± 7.3 and 383.4 ± 8.9 Ma (Kinkel Quarry); 383 ± 15 Ma (185th St. and Harlem River Drive); and 372 ± 10 Ma (Fort George). A partial metamict zircon (“cyrtolite”) from the Kinkel Quarry yielded a weighted average U-Pb age of 376.9 ± 4.3 Ma. The Neoacadian ages obtained agree with those determined by thermal ionization mass spectrometry (TIMS) for zircon from Lithium-Cesium-Tantalum (LCT) pegmatites from Connecticut and Maine. No pegmatites temporally associated with the Taconic orogeny were found. The size, lack of common Pb, uniform U concentrations across crystal cross-sections, sufficient but moderate uranium concentrations, lack of metamictization, and consistency in U-Pb isotopic ratios for columbite samples BCB-COL, NYSM #25232, and NYSM #525.8 suggests they show promise as potential standards for oxide mineral LA-MC-ICP-MS geochronological analyses, however, additional characterization using ID-TIMS would be necessary to develop as such.

Fluor-elbaite, lepidolite and Ta–Nb oxides from a pegmatite of the 3000 Ma Sinceni Pluton, Swaziland: evidence for lithium–cesium–tantalum (LCT) pegmatites in the Mesoarchean

Mineral evolution is concerned with the timing of mineral occurrences, such as the earliest reported occurrences in the geologic record. Minerals containing essential Li have not been reported from rocks older than ca. 3000 Ma, thus the lithian tourmaline (fluor-elbaite) and mica (lepidolite) assemblage from a pegmatite near Zishineni associated with the ca. 3000 Ma Sinceni Pluton presents unusual interest. Fluor-elbaite (0.75–0.98 F per formula unit) forms green crystals up to 50 mm long. Spindle stage measurements give ω = 1.652(1), e = 1.627(1) (589.3 nm). Optical absorption spectroscopy shows Fe and Mn are divalent; infra-red spectroscopy demonstrates the presence of Li and indicates the presence of (OH) at both the (OH) sites. Electron microprobe analysis of 330 points on several prisms, the largest of which is zoned in Fe and Ca, gives the following average and standard deviations in wt%: SiO2 37.29 (0.26), TiO2 0.05 (0.05), Al2O3 38.14 (0.35), Cr2O3 0 (0.02), MgO 0.02 (0.01), MnO 3.57 (0.25), FeO 2.48 (0.60), Na2O 2.48 (0.09), K2O 0.03 (0.12), CaO 0.77 (0.21), F 1.80 (0.11) wt%; Cl 0 (0.01). Nuclear reaction analyses gave Li2O 0.91 (0.04) and B2O3 10.55 (0.45). The empirical formula of fluor-elbaite was determined by integrating crystal-chemical data from electron microprobe analysis, nuclear reaction analysis, crystal structure refinement using X-ray diffraction, infra-red and optical absorption spectroscopy: X(□0.09Na0.77K0.01Ca0.13)Σ1.00Y(□0.35Li0.59Mn 2+ 0.49 Fe 2+ 0.33 Mn0.492+Fe0.332+Al1.23Ti0.01)Σ3.00Al6(Si6O18)(BO3)3O3(OH)3O1[F0.92(OH)0.08]Σ1.00. The crystal structure of fluor-elbaite was refined to statistical indices R1 for 1454 reflections ∼2% using MoKα X-ray intensity data. Structural data confirm the presence of significant vacancies at the Y site. Micas include lepidolite in flakes several millimeters across that are veined and overgrown by fine-grained muscovite. Silica and (FeO + MnO) increase, and Al decreases with F, all giving tight linear fits for both micas taken together, suggesting both micas can be regarded as interstratified muscovite and lithium mica consisting of 35.2 wt% masutomilite containing nearly equal amounts of Mn and Fe, 52.8 wt% polylithionite and 11.9 wt% trilithionite. Muscovite and lepidolite contain Ta, hubnerite(?) and a possible Pb-bearing microlite (Ta > Nb). The oxides, together with the muscovite, are interpreted to be related to later hydrothermal reworking of the primary lepidolite–fluor-elbaite assemblage. Given the 2990 ± 43 Ma Rb–Sr isochron and 3074 ± 4 Ma evaporation Pb–Pb ages reported for the Sinceni Pluton and Rb/Sr mineral ages ranging from 2906 ± 31 Ma to 3072 ± 33 Ma reported for the pegmatites, the fluor-elbaite–cesian lepidolite–fluorcalciomicrolite-bearing pegmatite is the first reported occurrence of a lithian tourmaline and lepidolite in the geologic record, as well as one of the two earliest known examples of the lithium–cesium–tantalum (LCT) family of pegmatites. The Sinceni magma is most plausibly derived from a metasedimentary source by intrusion of hot mantle melts into the crust from below, thereby indicating that a “mature” continental crust existed in the Kaapvaal craton at ca. 3000 Ma. (Less)

The Sveconorwegian Pegmatite Province – Thousands of Pegmatites Without Parental Granites

Abstract The Late-Proterozoic Sveconorwegian pegmatite province in southern Norway and southwest Sweden hosts seven rare-element pegmatite districts with more than 5000 rare-element pegmatites. Most of these pegmatites with Niobium-Yttrium-Fluorine (NYF) signature are not related to a parental granite, but instead occur in areas of high-grade metamorphism and are the result of migmatization and local melt collection. There are three groups of pegmatites: (1) rare-element pegmatites related to H P -H T high-grade metamorphism associated with the assembly of the Sveconorwegian orogen; (2) rare-element pegmatites related to post-orogenic extension with L P -H T granulites; and (3) rare-element pegmatites related to granite magmatism during post-orogenic extension. The pegmatite formation principally comprises four periods restricted to certain tectono-metamorphic domains: (I) 1094–1060 Ma (Bamble sector); (II) 1041–1030 Ma (Idefjord terrane); (III) 992–984 Ma (Idefjord terrane, Rogaland-Hardangervidda-Telemark sector); and (IV) 922–901 Ma (Rogaland-Hardangervidda-Telemark and Bamble sectors). The observed relationships between pegmatite formation and regional high-grade metamorphism reveal that the majority of Sveconorwegian pegmatites are formed by anatexis, either by crustal stacking during different stages of continental/terrane collision (H P metamorphism) (periods I to III), or by mafic magma underplating (H T metamorphism) during orogenic extension (period IV). In several provinces that have been affected both by early H P metamorphism during continental collision and by late H T metamorphism during crustal extension, there may occur several generations of pegmatites that show mineralogical and geochemical affinity, even though they formed during several different periods. In addition, the results imply that the majority of Sveconorwegian NYF pegmatites are not necessarily formed in an anorogenic setting in relation to A-type magmatism, but in compressional or extensional orogenic settings unrelated to pluton-scale magmatism. In light of this, the genetic criteria of the pegmatite family classification scheme [NYF versus Lithium-Cesium-Tantalum (LCT)] will have to be re-evaluated.

Approaches for Extractive Hydrometallurgy of Niobium and Tantalum from Ethiopian Kenticha Ores

A Review Approaches for Extractive hydrometallurgy of niobium and tantalum from Ethiopian Kenticha Ores The Kenticha rare-element pegmatite, a globally important tantalite source in the Neoproterozoic of southern Ethiopia, is a highly fractionated, huge, subhorizontal, sheet-like body, discordantly emplaced in ultramafic host rock. It corresponds to the spodumene subtype of the rare element pegmatite class and belongs to the lithium–cesium–tantalum petrogenetic family. The Kenticha pegmatite is asymmetrically zoned from bottom to top into granitic lower zone, spodumene-free intermediate zone, and spodumene-bearing upper zone. The Kenticha pegmatite is rich in Ta-mineralized rare-element pegmatites. Thus, need to conduct research in enrichment of Ta2O5 and Nb2O5 and extraction of Nb and Ta into metallic form. This paper reviews the successful extraction of Nb and Ta from tanatlite ores. Its success normally depends on the physiochemical properties of the ore, choice of leaching extraction solvent. Currently, HF and methyl-isobutyl Ketone has been used for decomposition and separation of Nb and Ta from tantalite ore. Therefore Kenticha atantalite ore containe vaiaty compositions of elements and higher concentration of Li and U. Thus needs further investigation in extraction process in the sence of removing the use of HF for leaching and replacement of traditional organic solvents by “Green Solvents”. This review gives directions for future development of extraction of Kenticha tantalite ore. Keywords: Kenticha; leaching; magnetic separation; pegmatite, rare-element; solvent extraction, sopdumene; tantalite.

Crystallographic control on lithium isotope fractionation in Archean to Cenozoic lithium-cesium-tantalum pegmatites

Abstract The age distribution of lithium-cesium-tantalum (LCT) pegmatites largely overlaps with major phases of collisional orogenic events and assembly of supercontinents. Some of the largest known LCT pegmatite deposits formed in very short intervals, 2.7–2.5 Ga and 1.9–1.8 Ga, corresponding to two major pulses of continental crust growth. However, the exact process of generation and segregation of large volumes of Li-bearing pegmatite liquids, perhaps involving disequilibrium fractional crystallization and leaving residual melts enriched in fluxing elements such as B, F, H2O, Li, and P, remains obscure. The new data on Li contents and isotope compositions in major mineral phases from temporally and geographically separated pegmatite bodies document extreme variations in δ7Li values among individual large LCT pegmatite bodies, in particular Archean occurrences. The observed &amp;gt;10‰ variations in δ7Li values for the same mineral phases from different localities (i.e., beryl, petalite, spodumene, lepidolite, amblygonite, muscovite) contrast with globally homogeneous Li isotope systematics of major mineral phases from unmodified mantle rocks. Consistent Li isotope offsets between coexisting mineral phases are best explained by Li isotope fractionation as a function of the bond length between Li and neighboring ions (O, OH, F). We suggest that spatially distinct Li isotope patterns act as fingerprints for different pegmatites and can be explained by the preexisting Li isotope differences of their crustal sources at the time of pegmatite formation owing to differences in crustal age and evolution. This would imply secular evolution of the continental crust over Earth history toward present-day globally broadly uniform crustal 7Li/6Li ratios (δ7Li ∼0‰). The differences among Archean occurrences could reflect possible Archean paleogeography and could be linked with different thermal regimes of individual cratons as a consequence of variations in crustal thickness. One possible application of the new data set may be in source verification of gem-quality stones by using Li isotopes.

Geochronology and Tectonic Context of Lithium-Cesium-Tantalum Pegmatites In the Appalachians

Abstract Lithium-cesium-tantalum (LCT) and niobium-yttrium-fluorine (NYF) pegmatites occur at intervals along the Appalachian orogen from Alabama to Nova Scotia. We report new U-Pb zircon (CA-TIMS), 40 Ar/ 39 Ar muscovite, U-Pb apatite (LA-ICP-MS), and apatite fission track ages for 14 pegmatites. Based mostly on these new results, we recognize a minimum of four ages of LCT pegmatites in the Appalachians. From oldest to youngest, these are: (1) Brazil Lake, Nova Scotia, dated at ca . 395 (U-Pb columbite) by Kontak et al . (2005); (2) Clark Ledge, Massachusetts, at ca . 371–339 Ma; (3) Palermo, New Hampshire, at ca . 326–294 Ma; and (4) Mount Mica, Maine, at ca . 264–260 Ma. The NYF pegmatites in the Appalachians must be of at least three ages: Topsham, Maine, dated at ca . 273 Ma (U-Pb monazite) by Tomascak et al . (1996); Morefield, Virginia, at ca . 253 Ma; and Hurricane Mountain, New Hampshire, which cross-cutting relations prove can be no older than ca . 183 Ma. The CA-TIMS method yielded high-resolution, concordant to near-concordant ages from most zircons, but at least 25 of the 41 zircons dated are interpreted to be xenocrysts or to have an inherited component. Thus a set of zircon ages from an LCT pegmatite must be treated as analogous to detrital zircon ages from a metasedimentary rock, which can be no younger than the depositional age of the protolith. All of the dated LCT pegmatites of the New England Appalachians are significantly younger than the two main orogenies in this sector of the orogen, the Taconic (Ordovician) and Acadian (Late Silurian to Middle Devonian). The pegmatites broadly correlate with the two younger orogenies, namely the Late Devonian to Mississippian Neoacadian orogeny and the Pennsylvanian to Permian Alleghanian orogeny. Thus the New England LCT pegmatites were emplaced during the latter stages of the assembly of Pangea. 40 Ar/ 39 Ar muscovite ages from 11 LCT and one NYF pegmatites range from ca . 339 to ca . 243 Ma and are interpreted to record cooling of the various pegmatites through ca . 425 °C. These ages are broadly consistent with the pattern of muscovite argon cooling ages from metasedimentary and granitic rocks across the region. Apatite fission-track ages for the pegmatites range from ca . 139 to ca . 68 Ma and are broadly consistent with regional results from plutonic and metamorphic rocks.

Sediment and weathering control on the distribution of Paleozoic magmatic tin–tungsten mineralization

The formation of major granite-hosted Sn and/or W deposits and lithium–cesium–tantalum (LCT) type pegmatites in the Acadian, Variscan, and Alleghanian orogenic belts of Europe and Atlantic Northern America involves weathering-related Sn and W enrichment in the sedimentary debris of the Cadomian magmatic arc and melting of these sedimentary source rocks during later tectonic events, followed by magmatic Sn and W enrichment. We suggest that within this, more than 3,000-km long late Paleozoic belt, large Sn and/or W deposits are only found in regions where later redeposition of the Sn–W-enriched weathered sediments, followed by tectonic accumulation, created large volumes of Sn–W-enriched sedimentary rocks. Melting of these packages occurred both during the formation of Pangea, when continental collision subjected these source rocks to high-grade metamorphism and anatexis, and during post-orogenic crustal extension and mantle upwelling. The uncoupling of source enrichment and source melting explains (i) the diachronous occurrence of tin granites and LCT pegmatites in this late Paleozoic orogenic belt, (ii) the occurrence of Sn and/or W mineralizations and LCT pegmatites on both sides of the Rheic suture, and (iii) the contrasting tectonic setting of Sn and/or W mineralizations within this belt. Source enrichment, sedimentary and tectonic accumulation of the source rocks, and heat input to mobilize metals from the source rocks are three unrelated requirements for the formation of Sn and/or W granites. They are the controlling features on the large scale. Whether a particular granite eventually generates a Sn and/or W deposit depends on local conditions related to source melting, melt extraction, and fractionation processes.