Rare-element Granitic Pegmatites Research Articles

Metasomatic effect of Li-bearing aplite-pegmatites on psammitic and pelitic metasediments: Geochemical constraints on critical raw material exploration at the Fregeneda–Almendra Pegmatite Field (Spain and Portugal)

Fluid-assisted mass transfer and re-equilibration of mineral phases are common consequences of metasomatism associated with igneous intrusions. The addition and/or removal of chemical components in these environments may result in the generation of metasomatic aureoles, which can be recognized by their mineralogy and geochemistry. Due to an increasing demand for critical raw materials used in green energy technologies, rare-element granitic pegmatites have seen renewed interest in the mineral exploration industry. Granitic pegmatites represent potential sources of critical commodities and geochemical studies of their related aureoles help to advance techniques in exploration targeting. Moreover, the role and timing of fluid exsolution during magmatic–hydrothermal evolution in granitic–pegmatitic systems and concomitant element mobility remain highly debated.We present a prospect-scale systematic study of geochemical haloes generated by LCT (Li-Cs-Ta) family pegmatite dykes from the Fregeneda–Almendra Pegmatite Field, in the Central Iberian Zone of the Iberian Massif (Spain and Portugal). To understand the magnitude of metasomatic processes linked to these intrusions, we performed whole-rock mass-balance calculation of element gains and losses in variably metasomatized psammitic and pelitic host metasediments. The results show that F, B, Li, Rb, Cs, Sn, Be, Tl, As, W and S (±Mo, Ta) were carried by early exsolved and expelled aqueous fluids. The first evidence of element enrichment is recorded at distances of 4–5 times the thickness of the dykes, with exponentially increasing gains of those fluid-mobile elements proximal to the pegmatite margin. Enrichments that were detected farthest from the pegmatite margins were those of Li and Cs, followed by Rb and, to a lesser extent, Sn, F, B, Be, and Tl. The most evolved (fractionated) aplite-pegmatites generated the broadest haloes, with concentrations higher than 200 ppm Li, 30 ppm Cs, 300 ppm Rb, and 15 ppm Sn in the metasediments indicating proximity to a mineralized dyke. In addition, absolute gains of up to ∼4000 ppm Li, ∼1300 ppm Cs, ∼1300 ppm Rb, and 170 ppm Sn in the host rocks could point to the presence of superimposed haloes from multiple evolved dykes.

Petrogenesis of post-orogenic Pan-African rare-element granitic pegmatites in the western Arabian-Nubian Shield, Aswan area, southern Egypt

• Aswan pegmatites evolved via fractional crystallization of the parental granitic magma . • The rare-element pegmatites were formed in a post-orogenic setting. • Post-magmatic hydrothermal fluids play an important role in their formation. Pegmatites with rare-element mineralization from the Aswan area in southern Egypt intruded the post-orogenic Pan-African granitic rocks and the associated gneisses. The granitic pegmatites show a regular pattern of concentric zoning and display a weak affinity to the Nb-Y-F (NYF) pegmatite family. These pegmatites consist mainly of K-feldspar, albite, quartz, and subordinate muscovite, accessory minerals include Nb-Ta oxide minerals, monazite-(Ce), xenotime-(Y), zircon, rutile, and magnetite. Field observations combined with the petrography, mineralogy and geochemistry of pegmatites and associated rocks in the Aswan area indicate that the pegmatites evolved via fractional crystallization of the parental granitic magma with an effective role for the post-magmatic alterations. The wide compositional variations of Nb-Ta minerals in term of their major and trace elements reflect the pegmatite formation stages. Primary magmatic generation of columbite-(Fe) occurs as homogeneous grains or showing oscillatory zoning with high Nb/Ta and Fe/Mn ratios. Late ixiolite shows patchy and irregular zoning with high Sc and W replacing the earlier regular texture, indicating metasomatic replacement. Electron microprobe U–Th–Pb dating of monazite from the studied pegmatites yielded post-orogenic Pan-African age of 578 ± 5 Ma, comparable with age of the fine-grained granite in this area, but it is younger than the previously estimated U–Pb zircon age of surrounding the Monumental granites and gneisses (∼600 to 620 Ma). The similarity in age and geochemical characteristics of the pegmatites and the fine-grained granite proved their genetic relationship.

Rare earth element (REE)-enriched granitic pegmatite pockets of Lagoa Real Uranium Province, Brazil

Rare-earth-element (REE) bearing minerals have been recognized for the first time within rare-element granitic pegmatite pockets from Lagoa Real Uranium Province (LRUP), Bahia state, NE Brazil. The Province is one of the chief U deposits in the world and the largest in South America. Notwithstanding its economic and strategic importance, there are gaps in the geological knowledge about other mineral resources which could occur in the area. One of them, is the lack of petrographic and chemical data sets on its REE content, especially in pegmatite pockets which occur in the contact between Juazeirinho (1755 ± 6 Ma) and São Timóteo (1741 ± 4 Ma) granites. In this paper, it is reported a petrographic description and geochemical characterization of allanite, bastnäsite, monazite, xenotime, cheralite, and parisite recognized in three spongy mineral assemblages within unzoned granitic pegmatite pockets from Lagoa Real Uranium Province. The REE-bearing minerals present moderate-to-high REE content with significant amounts of Ce, La, Y, and critical REE such as Nd, Er, and Yb, totalling to up to 49 wt%. Petrographical and geochemical evidence arguably indicate that the REE-enrichment in granitic pegmatite pockets of Lagoa Real Uranium Province is a result of late-magmatic and hydrothermal events involving F−, OH−, CO32− and PO43− REE-bearing fluids. The report and characterization of these recently recognized minerals indicate that the Lagoa Real Uranium Province, might hold important REE mineralization as U by-product.

Beryl crystal chemistry and trace elements: Indicators of pegmatite development and fractionation (Damara Belt, Namibia)

The study reveals the relationships between crystal-chemical and genetic properties of beryl from several rare-element granitic pegmatites of the LCT family from the Damara Belt, Namibia. Based on their chemical compositions, the studied beryl samples can be divided into two groups. Beryl from the first group (the Engelbrecht's and Etusis pegmatites) is poor in Fe, Mg and Mn; however, slightly enriched in Zn and alkali elements (Na > Li, Rb, Cs). The samples from the second group (Kuduberge and Tsaobismund) are slightly enriched in Fe and Mg, in which they are mostly charge-balanced by Na, while other octahedral and channel constituents are negligible. The affinity of the first group of beryl to the dominant tetrahedral substitution is manifested in cell dimensions and the c/a ratio. Based on Raman and infrared spectroscopy, the Kuduberge and Tsaobismund samples have a very similar position and intensity of water FTIR bands, indicating the presence of both water types in the channels. The Englebrecht's sample is similar, however, the Etusis sample exhibits a shift of H2O II ν1 band, which could be attributed to a smaller Na content and strong dominance of doubly coordinating water. Our results indicate preferential occupancy of Na in the octahedral Na + (Fe,Mg)2+ = + Al substitution, which is typical for beryl from less fractionated pegmatites (Kuduberge and Tsaobismund), however, Na can also be a part of the complex tetrahedral substitution found in Englebrecht's beryl. Higher concentrations of Mn, Sc and Ti indicate a less fractionated genetic environment due to their affinity to Mg and Fe2+, whereas Cs, Rb and Zn prefer tetrahedral substitution, which is typical in highly evolved pegmatites.

U-Pb dating and composition of columbite from Vishteritsa: Implication for timing of granite magmatism and rare-element granitic pegmatites in the Western Rhodopes, Bulgaria

The economic significance of pegmatites as a source of strategic rare metals for high-tech products and green energy motivated the present study on Ta–Nb oxides from Vishteritsa rare-element beryl–columbite LCT pegmatites of the Rila–West Rhodopes Batholith in the Western Rhodopes, Bulgaria. Here, we present the first U/Pb age data from columbite with application of the LA–ICP–MS U–Pb technique and a new X36 columbite standard reference material. The obtained Concordia age of 47.57 ± 0.32 Ma with a small spread of the individual 206Pb/238U ages between 45 and 51.3 Ma argues for Early Eocene magmatism and pegmatite formation. The host granite of the rare-element pegmatites is dated 51.94 ± 0.61 Ma with LA–ICP–MS U–Pb technique on zircon and suggests a fertile Early Eocene magmatic period in the Western Rhodopes. EPMA data for the composition of the columbite is used to refine the formula of the mineral (Mn0.554Fe0.427U0.006)0.987(Nb1.826Ta0.085Ti0.116)2.03O6 and define it as columbite-(Mn). Application of the in-situ LA–ICP–MS data technique establishes a series of typical trace elements (Ti, U, Zr, Hf, Y, W, and Zn) that are usually found in content above 500 ppm. The studied columbite is enriched in heavy rare earth elements (HREE sum: 306–697 ppm) and depleted in light REE and Eu. These geochemical characteristics are collectively interpreted as evidence for crystallization from highly fractionated fluid-rich magma. High UO2 content reaching 0.89 wt. % is characteristic for the Vishteritsa columbite. The decrease of U proximal to cracks and in outer crystal zones documents U-mobility during overprinting hydrothermal processes.

LA-ICP-MS U[sbnd]Pb columbite ages and trace-element signature from rare-element granitic pegmatites of the Pampean Pegmatite Province, Argentina

Crystals of the columbite-group minerals (CGM) from 23 rare-element granitic pegmatites of the Pampean Pegmatite Province (PPP) were analyzed by LA-ICP-MS technique. The resulting database shows weighted mean 206Pb/238U ages between 530 and 335 Ma defining two time windows, T1 and T2. T1, from 490 to 440 Ma, spans 75% of the ages of LCT rare-element pegmatites formed during the Famatinian collisional orogeny of peri-Gondwana location. T2, from 370 to 340 Ma, comprises postorogenic pegmatites with lithium-caesium‑tantalum (LCT) and niobium‑yttrium‑fluorine (NYF) or mixed NYF-LCT signatures. The CGM ages of the PPP orogenic pegmatites show a shifting from older in the north to younger in the south.The major elements composition of the CGM shows preference for high #Mn at variable but usually moderate to high #Ta. The normalized, with the Global CGM median, trace elements composition of the CGM of most orogenic LCT (Li-Cs-Ta) pegmatites shows an enrichment in Bi, Pb and U, a remarkable depletion in Sn and sometimes a slight decrease in W (Y ± Yb). The postorogenic pegmatites show CGM with a general enrichment in Y (±Yb), W, Pb, U, and occasionally in Sn. The REEN diagrams of CGM from the northern districts of the PPP show patterns with LREEN < MREEN > HREEN and strong negative Eu anomalies. In the southern districts, there is a bimodal behavior in the total content and distribution of the normalized patterns of REE. The CGM from some pegmatites of the Conlara Pegmatite district have similar features to the ones from the northern districts. It seems that the kind of pegmatite has no definitive control in the type of pattern. The other group shows low contents of REEN mostly comprised between 0.01 and 10 times the chondrite values, flattened patterns, weak to moderate negative anomalies of Eu, and occasionally a weak Ce anomaly. In the southern Totoral district, the CGM of the different kinds of pegmatites also have approximately similar, though relatively lower, total REEN contents and patterns with LRREN < MREEN < HREEN. The petrogenesis of the orogenic LCT pegmatites is linked to the differentiation of S-type leucogranites, likely of low temperature, generated by a low degree of partial melting of the upper crustal sequences of micaceous protoliths. The postorogenic pegmatites are of NYF (Nb-Y-F) or mixed signatures, linked likely to the A-type monzogranitic batholiths. The combined data of ages, trace element and REE of the CGM from the PPP give a typical signature to its Ta ore minerals.

Petalite postdating spodumene in pegmatite as a consequence of the ~2.02 Ga meteorite impact in the Vredefort structure, southern Africa

Rare-element granitic pegmatites enriched in lithium are economically important hosts for the lithium aluminosilicates, spodumene and petalite. Most spodumene and petalite crystallize from the pegmatite melt, depending on pressure and temperature. This paper reports the rare occurrence of petalite postdating the subsolidus stage in spodumene pegmatites within the ~2.02 Ga Vredefort impact structure, the largest, oldest and most deeply eroded meteorite impact structure currently known on Earth. The studied spodumene-bearing pegmatites postdate the deformation fabric in the surrounding amphibolites of the granitoid-greenstone terrane. Impact-related veins of fragment-rich pseudotachylites cut across the pegmatites. Spodumene, albite and quartz constitute the primary mineral assemblage of the pegmatites. Muscovite is late magmatic and contains inclusions of spodumene and albite. During the subsolidus stage, K-feldspar replaced spodumene and albite during pervasive metasomatic alteration. Samples collected closer to the pseudotachylites show reaction textures between spodumene and quartz without the involvement of muscovite. Raman spectroscopy and microprobe analyses show that the reaction textures consist of quartz and petalite. Unlike spodumene, petalite does not show a replacement by K-feldspar. The 40Ar/39Ar muscovite age of 2027 ± 8 Ma coincides with the timing of the meteorite impact and documents its effect on the spodumene pegmatites. We suggest that the formation of petalite after spodumene was related to heating, which was generated in the target rocks by the post-shock thermal event. Thus, the unique conditions associated with the meteorite impact can not only result in the formation of rare minerals in the target rocks, but can also account for rare scenarios resulting in phases that would not otherwise form in nature.

The physical and chemical evolution of fluids in rare-element granitic pegmatites associated with the Lacorne pluton, Québec, Canada

Co-magmatic rare-element (Be, Li, Nb, Ta) granitic pegmatites associated with the Lacorne monzogranite pluton of the Preissac-Lacorne batholith (Québec) range from a least evolved beryl-type in the pluton through a transitional beryl + spodumene-type to a most evolved spodumene-type in mafic country rocks. They possess internal lithologic fabrics and primary fluid inclusions that provide evidence for saturation of the magma with aqueous fluids immediately after the formation of aplite in the beryl pegmatite, and pene-contemporaneously with formation of the beryl + spodumene and spodumene pegmatites. The early fluids are represented by aqueous liquid-vapor (Type 1a) and aqueous liquid-vapor-solid (Type 2) inclusions in beryl from beryl and beryl + spodumene pegmatites and are estimated to have been trapped between 500 and 550 °C and at 3.5 Kb. On the basis of microthermometric measurements, analyses of the compositions of the precipitates from decrepitated inclusions and the nature of the solids (trapped), these fluids are interpreted to have been NaCl-dominated (≤16 wt% NaCl eq.) and to have contained appreciable Fe and dissolved CO2. In spodumene pegmatite, the equivalent fluid, which was trapped at 450 to 500 °C, contained significant Mn, Li and Cs in place of Fe, consistent with corresponding trends in the chemical evolution of the host rock.Volatile-rich, mainly secondary, aqueous fluid inclusions (Type 1b) trapped in beryl are interpreted to reflect temporary sharp drops in pressure later in the crystallization history of the beryl-bearing pegmatites due to local fluid overpressures that led to brecciation and consequent pressure release. These inclusions have high salinity, are CaCl2-rich and are considered to represent magmatic hydrothermal fluids that acquired their calcium as a result of their interaction with the host Lacorne pluton. Incursion of CaCl2-rich fluids into the spodumene pegmatites is interpreted to have occurred earlier in their crystallization history, i.e., upon emplacement of the pegmatites, as shown by the fact that a high proportion of primary inclusions in spodumene in these pegmatites contains considerable calcium, and calcium contents are generally higher in inclusions in spodumene than in beryl. The inclusions in the spodumene pegmatites are thought to have acquired their calcium through interaction of the fluids with the amphibolites (metabasalts) that host these intrusions.The crystallization of the pegmatites terminated with the entrapment of low-salinity aqueous‑carbonic fluids (Type 3 inclusions) in quartz (6–10 wt% NaCl eq.), which most likely formed as a result of the unmixing of the magmatic fluid into separate aqueous and carbonic fluids, probably at temperatures between 250 and 350 °C.The results of this study show that aqueous fluid saturation can take place at an early stage of pegmatite emplacement, and that magma evolution is matched by changes in the solute chemistry of the exsolving aqueous fluid. They also show that evolution to a more volatile-rich composition (H2O + F + Li) may have contributed to the increased mobility of later magma batches, thereby helping to explain the regional zonation of the pegmatites, from beryl-type within the pluton to spodumene-type in the adjacent amphibolite.

Rare-element granitic pegmatite of Miocene age emplaced in UHP rocks from Visole, Pohorje Mountains (Eastern Alps, Slovenia): accessory minerals, monazite and uraninite chemical dating

Abstract The granitic pegmatite dike intruded the Cretaceous UHP rocks at Visole, near Slovenska Bistrica, in the Pohorje Mountains (Slovenia). The rock consists mainly of K-feldspar, albite and quartz, subordinate muscovite and biotite, while the accessory minerals include spessartine-almandine, zircon, ferrocolumbite, fluorapatite, monazite- (Ce), uraninite, and magnetite. Compositions of garnet (Sps48-49Alm45-46Grs + And3-4 Prp1.5-2), metamict zircon with 3.5 to 7.8 wt. % HfO2 [atom. 100Hf/(Hf + Zr) = 3.3-7.7] and ferrocolumbite [atom. Mn/(Mn + Fe) = 0.27-0.43, Ta/(Ta + Nb) = 0.03-0.46] indicate a relatively low to medium degree of magmatic fractionation, characteristic of the muscovite - rare-element class or beryl-columbite subtype of the rare-element class pegmatites. Monazite-(Ce) reveals elevated Th and U contents (≤11 wt. % ThO2, ≤5 wt. % UO2). The monazite-garnet geothermometer shows a possible precipitation temperature of ~495 ± 30 °C at P~4 to 5 kbar. Chemical U-Th-Pb dating of the monazite yielded a Miocene age (17.2 ± 1.8 Ma), whereas uraninite gave a younger (~14 Ma) age. These ages are comtemporaneous with the main crystallization and emplacement of the Pohorje pluton and adjacent volcanic rocks (20 to 15 Ma), providing the first documented evidence of Neogene granitic pegmatites in the Eastern Alps. Consequently, the Visole pegmatite belongs to the youngest rare-element granitic pegmatite populations in Europe, together with the Paleogene pegmatite occurrences along the Periadriatic (Insubric) Fault System in the Alps and in the Rhodope Massif, as well as the Late Miocene to Pliocene pegmatites in the Tuscany magmatic province (mainly on the Island of Elba).

The Julianna pegmatite vein system at the Piława Górna Mine, Góry Sowie Block, SW Poland – preliminary data on geology and descriptive mineralogy

The newly discovered Julianna pegmatitic system from the Piława Górna Quarry (the Góry Sowie Block, Sudetes Mts., NE margin of the Bohemian Massif) is described in terms of geological setting, petrography and descriptive mineralogy. The system represents the largest pegmatitic occurrence in the Polish Sudetes and consists of a complex network of cogenetic rare-element granitic pegmatites that intruded into tectonized amphibolite as discordant dikes. The pegmatites range from barren and weakly zoned to texturally well-differentiated ones that are composed of a fine-grained border zone, coarse-grained wall zone, graphic and blocky feldspar intermediate zones and a quartz core. Unidirectional and skeletal solidification textures are well-developed. The Julianna pegmatites consist of rock-forming plagioclase (ŁAn39), microcline, quartz and biotite accompanied mostly by accessory to minor muscovite, tourmaline, garnet and beryl. They crystallized from anatectic melt of hybrid NYF (niobium-yttrium-fluorine) + LCT (lithium-cesium-tantalum) geochemical characteristics. Pegmatites with a low to moderate degree of fractionation, that dominate in the Julianna system, bear NYF-signature accessory minerals, such as allanite-(Ce), columbite-, euxenite- and samarskite-group minerals, fergusonite-(Y) and gadolinite-(Y). However rare dikes that attained a very high degree of fractionation contain typical minerals of LCT-signature including tourmalines of the elbaite-olenite-rossmanite series, lepidolite, lithiophilite, spodumene, Cs-rich beryl and pollucite.

COMPOSITIONAL AND STRUCTURAL VARIATIONS IN COLUMBITE-GROUP MINERALS FROM GRANITIC PEGMATITES OF THE BRUNSWICK AND OXFORD FIELDS, MAINE: DIFFERENTIAL TRENDS IN F-POOR AND F-RICH ENVIRONMENTS

The Coastal Lithotectonic Block of the Brunswick pegmatite field and the Central Maine Synclinorium of the Oxford pegmatite field, in Maine, are intruded by numerous rare-element granitic pegmatites that carry columbite-group minerals (CGM) as the principal (Nb,Ta)-bearing phases. Generally, the CGM from Maine are hosted by pegmatites characterized by a LCT (Li–Cs–Ta) geochemical signature and are infrequent in pegmatites with NYF (Nb–Y–F) characteristics ( e.g ., Topsham area of the Brunswick pegmatite field). Significant chemical-compositional and structural differences are observed between the CGM from the Brunswick and Oxford pegmatite fields. Columbite-group minerals from the Brunswick pegmatite field mainly consist of columbite-(Fe), commonly with elevated Ti and Mg contents. By comparison, pegmatites from the Oxford field carry columbite-(Fe), columbite-(Mn) and tantalite-(Mn) with low Ti and Mg contents. The structural state of CGM from the Brunswick pegmatite field ranges from highly cation-disordered to cation-ordered, in contrast to predominantly highly ordered CGM in the Oxford pegmatite field. Limited concurrent fractionation of Mn from Fe and Ta from Nb observed in the Brunswick pegmatite field is characteristic of F-poor environments in general, and of F-enriched pegmatites with high μHF/μalkF (stability field of topaz). However, the extensive Fe–Mn fractionation preceding moderate to strong enrichment in Ta is typical of the Oxford pegmatite field, and apparently promoted by lepidolite-generating environments with prominent chemical potential of alkali fluorides and low μHF/μalkF. This observation finds support in data on CGM from numerous other pegmatite populations worldwide. In contrast, the structural state of CGM does not seem to be linked to the activity of different forms of fluorine. The overall abundance, diversity and temporal extent of the volatile components were undoubtedly different in the two pegmatite fields under study, and were apparently enhanced in the more fractionated pegmatites of the Oxford pegmatite field.

Granitic Pegmatites as Sources of Strategic Metals

Rare-element granitic pegmatites are well recognized for the diversity and concentrations of metal ores that they host. The supply of some of these elements is of concern, and the European Commission recently designated metals such as tantalum and niobium as “critical materials” or “strategic resources.” Field relationships, mineral chemistry, and experimental constraints indicate that these elements are concentrated dominantly by magmatic processes. The granitic melts involved in these processes are very unusual because they contain high concentrations of fluxing compounds, which play a key role at both the primary magmatic and metasomatic stages. In particular, the latter may involve highly fluxed melts rather than aqueous fluids.

Pegmatitic Nb-Ta oxide minerals in alluvial placers from Limbach, Bratislava Massif, Western Carpathians, Slovakia: compositional variations and evolutionary trend

*Corresponding author Niobium-tantalum mineral assemblage of granitic pegmatite origin was found in alluvial placers near Limbach, the Bratislava granitic massif, Male Karpaty Mountains, Central Western Carpathians, southwest Slovakia. The most widespread mineral is ferrocolumbite to ferrotantalite, rarely manganocolumbite to manganotantalite I [at. Mn/(Mn + Fe) = 0.17–0.52 and Ta/(Ta + Nb) = 0.19–0.70] shows primary magmatic, fine to coarse, regular oscillatory zoning with an evolutionary trend indicating a moderate degree of fractionation, analogous to beryl–columbite subtype of rare-element granitic pegmatites. On the contrary, irregular oscillatory and patchy zoning of ferrocolumbite to manganotantalite II [Mn/(Mn + Fe) = 0.17–0.66, Ta/(Ta + Nb) = 0.16–0.81] together with precipitation of anhedral Ta-rich intergrowths and inclusions of ferrotapiolite [Mn/(Mn + Fe = 0.05–0.07, Ta/(Ta + Nb) = 0.87–0.93], Ta>Nb-rich rutile [Mn/(Mn + Fe) = 0.02–0.03, Ta/(Ta + Nb) = 0.80–0.86], Sn-rich ixiolite [Mn/(Mn + Ta) = 0.40–0.41, Ta/(Ta + Nb) = 0.81] and uraninite + uranmicrolite [Ta/(Ta + Nb) = 0.81–0.94] indicates their subsolidus origin.

Voloshinite, a new rubidium mica from granitic pegmatite of Voron’i Tundras, Kola Peninsula, Russia

Voloshinite, a new mineral of the mica group, a rubidium analogue of lepidolite, has been found from the rare-element granitic pegmatite at Mt. Vasin-Myl’k, Voron’i Tundras, Kola Peninsula, Russia. It is closely associated with pollucite and lepidolite and commonly with muscovite, albite, and quartz; K,Rb-feldspar, rubicline, spodumene, montebrasite, and elbaite are among associated minerals as well. Voloshinite, a late mineral that formed after pollucite, commonly fills polymineralic veinlets and pods within the pollucite aggregates. It occurs as rims up to 0.05 mm thick around lepidolite, as intergrowths of tabular crystals up to 0.25 mm in size, and occasionally replaces lepidolite. The new mineral is colorless, transparent, with vitreous luster. Cleavage is eminent parallel to {001}; flakes are flexible. The calculated density is 2.95 g/cm3. The new mineral is biaxial (−), with 2V = 25°, α calc = 1.511, β = 1.586, and γ = 1.590. The optical orientation is Y = b, Z = a. The chemical composition of the type material determined by electron microprobe (average of five point analyses; Li has been determined with ICP-OES) is as follows (wt %): 0.03 Na2O, 3.70 K2O, 12.18 Rb2O, 2.02 Cs2O, 4.0 Li2O, 0.03 CaO, 0.02 MgO, 0.14 MnO, 21.33 Al2O3, 53.14 SiO2, 6.41 F, -O = F2 2.70, total is 100.30. The empirical formula is: (Rb0.54K0.33Cs0.06)Σ0.93(Al1.42Li1.11Mn0.01)Σ2.54(Si3.68Al0.32)Σ4O10 (F1.40(OH)0.60)Σ2. The idealized formula is as follows: Rb(LiAl1.5□0.5)[Al0.5Si3.5O10]F2. Voloshinite forms a continuous solid solution with lepidolite. According to X-ray single crystal study, voloshinite is monoclinic, space group C2/c. The unit-cell dimensions are: a = 5.191, b = 9.025, c = 20.40 A, β = 95.37°, V= 951.5 A3, Z = 4. Polytype is 2M1. The strongest reflections in the X-ray powder diffraction pattern (d, A-I[hkl]) are: 10.1-60[001]; 4.55-80[020, 110, 11\( \bar 1 \)]; 3.49-50[11\( \bar 4 \)]; 3.35-60[024, 006]; 3.02-45[025]; 2.575-100[11\( \bar 6 \), 131, 20\( \bar 2 \), 13\( \bar 4 \)], 2.017-50[136, 0.0.10]. The mineral was named in honor of A.V. Voloshin (born in 1937), the famous Russian mineralogist. The type material is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.

Selected ore deposits of Brazil

This series of papers on a variety of ore deposits in Brazil is the outgrowth of the “First Brazilian Symposium on Metallogeny” held in Gramado, Rio Grande do Sul, in May 2005. The selection of papers mirrors quite well the range of commodities and fascinating diversity of ore deposit types Brazil has to offer. The thematic issue starts with a paper by Monteiro et al. who use detailed petrography to establish a consistent paragenetic sequence of alteration and mineralization throughout the Sossego iron–oxide–copper–gold (IOCG) deposits in the giant Carajas Mineral Province. Combined with stable isotope data, it allows the authors to constrain the spatial and temporal zoning of hydrothermal alteration and mineralization. They conclude that the Pista-SequerinhoBaiano and Sossego-Curral ore bodies formed at a deep and high structural level of an IOCG system, respectively. Significantly, the copper–gold mineralization was late in the alteration history and broadly synchronous in both ore bodies. Another contribution to the ore deposit geology of the Carajas Mineral Province is by Dreher et al. who use new geologic, fluid inclusion and stable isotope data to establish that the Igarape Bahia Cu–Au deposit displays both characteristic syngenetic VHMS and epigenetic IOCG style geological features. This controversial paper provides evidence via recent B isotope data on tourmaline that there is the distinct possibility of an involvement of a marine evaporitic source in the hydrothermal system of Igarape Bahia. The authors carefully evaluate evidence for both genetic models and correctly agree that more detailed work needs to be conducted to resolve this fascinating genetic conundrum. Souza Neto et al. provide an overview of W–Au skarns in the Serido mobile belt in the Borborema Province in northeast Brazil. Special emphasis is put on the reduced Bonfim Au–Bi–Te skarn deposit. The majority of skarns in the Serido mobile belt are oxidized tungsten skarns even though the Itajubatiba and Bonfim gold-bearing skarns display reduced features, such as pyrrhotite as predominant sulfide, garnet with high almandine plus spessartite component, and elevated gold content. PIXE data revealed that significant amounts of F (up to 9,490 ppm) are present in prograde titanite, and retrograde clinozoisite–zoisite and gold-related epidote, suggesting significant F activity in the skarn forming hydrothermal fluids. Beurlen et al. in a second paper on mineralization in the Borborema Province use Nb–Ta–(Ti–Sn)–oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the pegmatite-rich Borborema Province. The Borborema Pegmatite Province, with over 750 registered mineralized rare-element granitic pegmatites, is a historically important tantalum province and is currently well known for top-quality gems, such as the Paraiba Elbaite. Detailed electron microprobe analyses of pegmatite Miner Deposita (2008) 43:127–128 DOI 10.1007/s00126-007-0174-y

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraiba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Serido Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Serido Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.

Textural Features and Chemical Evolution in Tantalum Oxides: Magmatic Versus Hydrothermal Origins for Ta Mineralization in the Tanco Lower Pegmatite, Manitoba, Canada

Tantalum, a key element in the electronics industry, is produced mainly from rare element granitic pegmatites. Although their internal structure, mineralogy, and petrogenesis have been extensively investigated, the processes that control tantalum mineralization remain poorly understood, in particular the role of fluids in the crystallization of tantalum ore. One of the major problems arises from the difficulty in distinguishing primary magmatic from secondary, hydrothermal textures in such complex rocks. In the Tanco pegmatite, Manitoba, Canada, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Eight different families of tantalum oxides occur in various associations and compositions. The Tanco Lower pegmatite is an isolated body beneath the main pegmatite body that contains abundant tantalum associated with mica alteration. Tantalum mineralization in the Tanco Lower pegmatite occurs as three different styles. Facies 1 is hosted by the wall zone and hosts primary magmatic Ta oxides with simple textures (progressive and oscillatory zoning). Facies 2 is hosted by the lower and upper intermediate zones where most mineralization occurs with dendritic amblygonite. Facies 3 is hosted by the central zone, which is affected by mica alteration. In this latter facies, the oxides show particularly complex textures evident from X-ray mapping. By combining information obtained from textural observations and chemical analyses, we are able to determine the paragenesis for each Ta oxide-bearing mineral assemblage and hence to evaluate the relative contributions of magmatic versus hydrothermal processes. In complex associations, we observe relics of primary Ta phases that are replaced or overgrown by secondary Ta phases. We propose the paragenetic sequence: columbite group minerals + microlite (early primary magmatic) → columbite + wodginite group minerals + microlite (late primary magmatic) → wodginite group minerals + microlite (secondary magmatic) → ferrotapiolite (secondary magmatic). In addition, chemical variations were identified in the columbite and wodginite group minerals, both at the crystal scale and at the pegmatite scale. Columbite and wodginite group minerals show the typical Ta* = Ta/(Ta + Nb) and Mn* = Mn/(Mn + Fe) (atomic ratios) increase from earlier to later zones. At the crystal scale, the increase in Ta* and Mn* with fractionation in the columbite group is explained by the higher solubility of the Ta end member relative to the Nb end member and the crystallization of other minerals that shift the melt composition toward Mn* enrichment. The fractionation trend in the wodginite group shows Fe enrichment, which is consistent with experimental results that show a higher solubility of the Fe end member relative to the Mn end member in columbite group minerals. Considering the available experimental data, as well as the intimate association of Ta oxides with zircon observed in this study, we conclude that tantalum mineralization is a product of direct crystallization from the melt rather than hydrothermal in origin. Fluids are attributed an indirect role only, as they could have brought minor elements (Fe, Mn, or Ca) into the melt, which resulted in the crystallization of secondary Ta phases. Textural evidence shows that the late secondary phases were formed at the same time as mica alteration. We suggest that mica alteration was due to a late melt, rather than a hydrothermal fluid, that reacted with the blocky K-feldspar of the central zone. During the alteration event, tantalum was remobilized from primary phases and incorporated in new, secondary Ta oxide species.

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.

Chemical evolution of Nb-Ta oxides and zircon from the Koktokay No. 3 granitic pegmatite, Altai, northwestern China

AbstractThe Koktokay No. 3 granitic pegmatite, Altai, northwestern China, is a strongly zoned rare-element granitic pegmatite, where the petrographic zones were distinguished into two groups: outer zones (I to IV) and inner zones (V to IX). Nb-Ta oxides and zircon are investigated in this paper by using quantitative electron-microprobe analyses (EMPA) and backscattered-electron (BSE) imaging. Columbite-tantalite and zircon occur in most textural zones, whereas tapiolite and uranmicrolite are mainly restricted to zone VII. Manganocolumbite and zircon from the outer zones (zones II and IV) are homogeneous except for a few exceptions, whereas manganotantalite and hafnian zircon from the inner zones (V–VII) are obviously heterogeneous and strongly zoned. Chemically, Ta/(Nb+Ta) in columbite-tantalite and Hf/(Zr+Hf) in zircon increase from the outer to the inner zones on one hand, and from core to rim in single zoned crystals on the other hand. Observations of intra-zonal variations of the chemical composition of Nb-Ta oxides and zircon in the Koktokay No. 3 granitic pegmatite may suggest that the outer zones crystallize under magmatic conditions, whereas the inner zones crystallize under fluid-rich magmatic conditions, and locally under hydrothermal conditions. The extreme enrichments of columbite-tantalite in Ta, and of zircon in Hf, as well as the occurrence of uranmicrolite and tapiolite, indicate an elevated evolution of the Koktokay No. 3 granitic pegmatite.

GRANITIC PEGMATITES OF THE O'GRADY BATHOLITH, N.W.T., CANADA: A CASE STUDY OF THE EVOLUTION OF THE ELBAITE SUBTYPE OF RARE-ELEMENT GRANITIC PEGMATITE

The first confirmed Canadian occurrence of the elbaite subtype of rare-element granitic pegmatite has been encountered within the O’Grady batholith, approximately 100 km NNW of Tungsten, N.W.T. The batholith, part of the Selwyn plutonic suite, is a mesozonal, hornblende-bearing, metaluminous composite intrusion with lesser amounts of pegmatitic granite and felsitic satellite dykes. On the whole, pegmatite bodies show a mildly NYF-type geochemical and mineralogical signature (allanite- and magnetite-bearing), but in the region of the pegmatitic granite, they grade toward a distinct LCT-type signature (elbaite-bearing). The melts parental to the pegmatite bodies were more voluminous, more alkaline, less aluminous and less reduced than is the norm for melts parental to LCT-type pegmatite. Differentiation of these melts sequentially produced hornblende granite, pegmatitic leucogranite and pegmatite, as shown by major-, minor- and trace-element geochemistry of K-feldspar, plagioclase, micas, tourmaline, amphibole and a variety of accessory phases. Lithium-mineralized pegmatite is recognized by the presence of multicolored elbaite, by lepidolite, and in the diversity of characteristic accessory phases such as danburite, hambergite, stibiocolumbite, pollucite, a nanpingite-like mineral, and bismutite. The various stages of the evolution of the batholith can be well documented in similar variations in mineral chemistry, and result in one of the few well-characterized case studies of the elbaite subtype.