Geochronology and geochemistry of rare-element pegmatites from the Superior Province of Canada

The crystallisation age and thermochronology of late- to post-orogenic, rare-lithophile element (Li, Cs, Ta) pegmatites that are chemically associated with peraluminous granites have been determined from U-Pb TIMS and LA-MC-ICP-MS analyses of columbite-tantalite, and UV -laser 40Ar/39Ar analyses of mica. In the northwest Superior Province, U-Pb ages reveal that the Pakeagama Lake pegmatite crystallised at 2673 ±8Ma. Rare-element pegmatites in the western Superior Province yield crystallisation ages of 2649 ±4Ma and 2644 ±8Ma for the Separation Rapids group, and 2665 ±lOMa for the Mavis Lake group, showing that they are temporally associated with adjacent peraluminous granites. Replacement zones of columbite-tantalite grains from the pegmatites show disturbance to the U-Pb system, probably caused by interaction with residual fluids and sub solidus metasomatic processes that lasted for -30Ma after crystallisation. Mica from rare-element pegmatites is characterised by distinct compositional zoning with muscovite cores and F-, Li- and Rb-rich zinnwalditellepidolite rims. The zinnwalditellepidolite rims that replace the original muscovite were most likely precipitated from a residual melt or fluid phase during the late-magmatic to hydrothermal period of pegmatite formation. Within these micas, 40Ar/39Ar apparent ages young towards the grain boundary and also towards the core - rim boundary, which suggests that the compositional zones form separate argon diffusion domains. However, similar 40Ar/39Ar apparent ages are found within F-rich zinnwaldite/lepidolite and coexisting muscovite indicating that diffusion rates may be comparable. Mica from rare-element pegmatites of the Superior Province yield intra-grain 40Ar/39Ar apparent age variations of up to 800Ma within single crystals. Theoretical slow cooling models do not match the measured 40Ar/39Ar profiles and cannot account for the large apparent age variations. Thermal history models that produce the best fit to apparent age profiles include two re-heating events; a minimum 350°C re-heating at -2450Ma and circa 300°C re-heating at 1850Ma, both for a period of at least 10Ma. These apparent ages are concordant with two major Proterozoic events affecting the Superior Province; intrusion of the Matachewan - Hearst dyke swarms and the Trans Hudson Orogeny, respectively

The Li-Bearing Pegmatites from the Pampean Pegmatite Province, Argentina: Metallogenesis and Resources

The Li-bearing pegmatites of the Pampean Pegmatite Province (PPP) occur in a rare-element pegmatite belt developed mainly in the Lower Paleozoic age on the southwestern margin of Gondwana. The pegmatites show Li, Rb, Nb ≤ Ta, Be, P, B, Bi enrichment, and belong to the Li-Cs-Ta (LCT) petrogenetic family, Rare-Element-Li (REL-Li) subclass; most of them are of complex type and spodumene subtype, some are of albite-spodumene type, and a few of petalite subtype. The origin of the pegmatites is attributed predominantly to fractionation of fertile S-type granitic melts produced by either fluid-absent or fluid-assisted anatexis of a thick pile of Gondwana-derived turbiditic sediments. Most of the pegmatites are orogenic (530–440 Ma) and developed during two overlapped collisional orogenies (Pampean and Famatinian); a few are postorogenic (~370 Ma), related to crustal contaminated A-type granites. The pegmatites were likely intruded in the hinterland, preferably in medium-grade metamorphic rocks with PT conditions ~200–500 MPa and 400–650 °C, where they are concentrated in districts and groups. Known combined resources add up 200,000 t of spodumene, with variable grades between 5 and 8 wt.% Li2O. The potential for future findings and enlargement of the resources is high, since no systematic exploration program has yet been developed.This research was funded by CONICET grants during several periods and lastly by PIP 1489 from CONICET to M.F.M.-Z. The Spanish Ministry of Economy, Industry and Competitiveness (project no. RTI2018-094097-B-100, with ERDF funds), and the European Union’s Horizon 2020 Innovation Programme (grant agreement no. 869274, project GREENPEG: New Exploration Tools for European Pegmatite Green-Tech Resources) granted E.R.-R

Timing of rare-elements (Li-Be-Ta-Sn-Nb) magmatism in the European Variscan belt

International audienceHigh-phosphorus peraluminous rare-elements granites and rare-elements LCT (Lithium, Caesium, Tantalum) pegmatites are the most important sources of raw materials for some critical metals like tantalum (1,2)represent important economic storehouses for industrial minerals like feldspar, quartz, mica or kaolin. They principally emplace in orogenic settings (3). Afast overview of three mainEuropean Variscan districts, i.e. the Moldanubian domain of the Bohemian massif, the French Massif Central (FMC) and the NW Iberia provides a basis for questioning the origin of rare-elements magmatism and the actual classification of rare-elements pegmatites, in particular the LCT pegmatites. Granitic pegmatites are widespread in most of the Bohemian Massifbut LCT pegmatites are most common in the Moldanubian domain. In this area, their emplacements seem mainly controlled by migmatitic domes and shear zones and correspond to two events(4). The older at ~ 333 ± 3 Ma just follow HT-MP event of the end of the Moravo-Moldanubian phase and the younger at ~ 325 ± 4 Ma is contemporaneous with beginning of the Bavarian phase (U-Pb ages on colombite and tantalite). In the FMC, most of the actually known rare-elements magmatic bodies form a province in the North Limousin area, which represents the northwestern part of the FMC.U-Pb dating of columbite-group minerals from Beauvoir, Montebras and Chèdeville rare-elements magmatic bodies leads to emplacement ages at 317 ± 6 Ma, 314 ± 4 Ma and 309 ± 5 Ma respectively. The contemporaneous Marche fault system (5), which crosscuts in a general E-W trend all the northern part of the Limousin, seems to be a key-structure for the rare-elements magmatism of the area

Granitoids and rare-element pegmatites of the Georgia Lake area, northwestern Ontario

The Georgia Lake pegmatite field is located in the Quetico Gneiss Belt of the Superior Province. Spodumene-bearing and subordinate beryl-bearing pegmatites of the Georgia Lake area are flanked to the south and east by an extensive granitoid terrain, which previously has not been subdivided. Granitoids of the immediate Georgia Lake area were investigated in conjunction with rare-element pegmatites to determine the character of the granitoids as parental intrusions to rare-element pegmatites. The granitoids include two-mica leucogranites occurring as a large plutonic mass south of the pegmatite field and as smaller satellitic intrusions, the Kilgour Lake Group granitoids centered on a small gabbroic-metagabbroic unit near Kilgour Lake and tonalitic sills dispersed throughout the pegmatite field. The distinction of the three types of granitoids was made on the basis of field observations, petrography and analytical geochemistry. Two-mica leucogranites and tonalitic sills were derived as partial melts of politic metasediments and metagreywacke, respectively. The Kilgour Lake Group granitoids were presumed to be the products of fractional crystallization of a mafic melt generated in the upper mantle or lower crust. Mineralogical studies were carried out on perthitic microcline, tantalite-columbite and Sn oxide minerals from rare-element pegmatites. Results indicate that perthitic microcline in all pegmatites is of the maximum microcline structural state, tantalite-columbite minerals occur in a partly to completely disordered structural state and the dominant Sn oxide mineral is staringite. Division of spodumene-bearing rare-element pegmatites into Southern, Central and Northern Groups was made on the basis of internal textural variations, mineralogy and differences in geochemistry of perthitic microcline and muscovite. The Southern Group consists of one pegmatite which is unique to the Georgia Lake pegmatite field with respect to development of mineralogical zones and strong internal fractionation of Rb and Cs. Central Group pegmatites are linked by a fractionation trend, with respect to Rb and Cs, across the group. A similar fractionation trend is not observed across the Northern Group pegmatites. The pegmatite groupings reflect different modes of source fluid derivation, although all pegmatites of the Georgia Lake area originated as the result of a common anatectic event responsible for the intrusion of two-mica leucogranites. Central and Southern Group pegmatites were derived from low viscosity fluids differentiated from granitic melts, while Northern Group pegmatites are presumed to be the products of fluids generated by direct anatexis of metasediments

Geochemistry of granitic aplite-pegmatite veins and sills and their minerals from Cabeço dos Poupos, Sabugal, central Portugal

Comunicação apresentada ao V International Symposium on Granitic Pegmatites : PEG 2011, Mendoza, Argentina, 20-27 Fev. 201

Lithium and tantalum mineralization in rare-element pegmatites from southern Africa

Lithium and tantalum mineralization in rare-element pegmatites has been studied in 4 field areas. Three field areas are within a pegmatite belt which stretches for 450 km from Steinkopf, Namaqualand in the west, to Kenhardt in the east along the Orange River in South Africa, incorporating Tantalite Valley, Namibia in the central area. This Belt is considered to be of 1200 my age. The 4th field area is in central Namibia in the Karibib-Usakos region of 500 my age. Lithium mineralization involves primary minerals, petalite and spodumene (crystallizing< 650&deg; C) and amblygonite which crystallize from a magma +/- an aqueous fluid, and lithian mica which along with cleavelandite is one of the last mineral assemblages to form, probably these last two assemblages are replacement in origin. Petalite is dominant in the Karibib area and spodumene in Steinkopf, Namaqualand and Tantalite Valley. The Kenhardt area is poor in lithium in comparison with the western and central portions of the Pegmatite Belt. Amblygonite-montebrasite is present in Karibib and Tantalite Valley usually in association with cleavelandite and lithian mica. Hydrothermal low temperature replacements, < 400&deg;C occur in spodumene in the Steinkopf and Tantalite Valley pegmatites, being pseudomorphed by albite and mica +/- sericite. Amblygonite-montebrasite in Karibib displays replacements of natromontebrasite (the first occurrence in Karibib, Namibia), crandallite, brazilianite and possibly cookeite. Apatite is always prominent at the contact. An unusual occurrence of Mn-tantalite lamellae, primarily parallel, lying in microlite, is intergrown with montebrasite at the Rubicon pegmatite, Karibib, suggesting simultaneous crystallization of these three minerals, i.e. Ta-dominated tantalite and microlite and LiAl(F/P04) involving late fluids rich in F, P and Ta. Mn-tantalite and Ta-rich microlite are the dominant Ta-minerals in the rare-element Li-rich pegmatites of Namaqualand, Tantalite Valley and Karibib. In contrast, columbite (Nb-rich) is prevalent in the Li-poor, less differentiated pegmatites in the eastern Pegmatite Belt near Kenhardt. Microlite replaces Mn-tantalite in Li-rich rare-element pegmatites in all three field areas. A uranmicrolite from Karibib, Namibia contains 14.35% UO2, 1.03% PbO, 56.12% Ta205, 13.18% Nb205, 0.58% Fe203, 6.87% CaO, 0.54% SrO, 0.59% MnO, 0.86% Na2O and 0.47% F. U-plumbomicrolite or Pb-uranmicrolite is intergrown with manganotantalite from the same pegmatite. Throughout one aggregate of microlite PbO varied from 21.98 to 1.57% and UO2 from 12.89 to 16.20%. Pb appears to be concentrated around the periphery of the crystal. Backscattered electron images reveal metamict textures in radioactive microlites and distinctive subspheroidal features. A uranoan microlite from Tantalite Valley, Namibia, revealed two essentially different compositions; a more hydrated rim area of 200 mum radius containing 7% higher Ta2O5, 10% lower CaO and 1.3% lower F than a main central area of slightly variable composition. Crystals of uranoan microlite from Steinkopf, Namaqualand contain remnants of a bismuth phase. Bismuth intergrowths with quartz reveal the presence of two rare-minerals, pyromorphite [Pb5(PO4) 3 C1] and m0ttramite[PbCu(VO4)OH], new data is given for these minerals. Ferro tantalite occurs at Rubicon mine. A schematic diagram is produced for the paragenetic sequence of mineral assemblage in each of the pegmatite areas in Karibib, Tantalite Valley, and Steinkopf, Namaqualand in relation to T and P of formation, and the magma and fluids effecting the crystallization sequence. Finally different fractionation trends of Ta-Nb, Mn-Fe, Rb-K and Cs-K in columbite-tantalites and lithian mica have highlighted variable paths of differentiation in contrasting rare-element pegmatites which may reflect different sources of original parental magma

Fluid inclusion studies of rare element pegmatites, South Platte District, Colorado.

Over fifty granite-hosted pegmatites occur in the South Platte district, located in the northern portion of the 1.01 Ga Pikes Peak batholith in the Rocky Mountain Front Range of central Colorado. Many of these pegmatites are concentrically zoned and enriched in fluorite, REE, Y and Nb, relative to the host granite, and may be classified as NYF pegmatites. Rare-element mineralization principally occurs in a core-margin zone as vein and replacement assemblages comprising albite, fluorite, hematite, muscovite and a variety of rare-element minerals, including samarskite, allanite, monazite, bastnaesite and gadolinite. Laser-excited emission (fluorescence) spectra indicate early, massive, core-margin fluorite (probably magmatic) in many instances contains higher concentration of REE than later hydrothermal white, clear and purple fluorite which replace it. Fluid inclusion studies of both magmatic and hydrothermal phases reveal that four compositionally distinct hydrothermal fluid types have permeated the pegmatites. Data from primary inclusions within hydrothermal fluorite indicate that the rare-element mineralization was formed from low salinity (${\u3c}10$ equiv. wt. % NaCl + CaCl\sb2), orthomagmatic fluids at temperatures of at least 340 to 500\sp\circC. The other fluid types occur exclusively in secondary inclusions, and post-dated the early, low-salinity fluid. (Abstract shortened by UMI.) Source: Masters Abstracts International, Volume: 37-01, page: 0211. Adviser: Iain Samson. Thesis (M.Sc.)--University of Windsor (Canada), 1997

The effect of P2O5 on the viscosity of haplogranitic liquid

The effect of P2O5 on the viscosity of a haplogranitic (K2O-Na2O-Al2O3-SiO2) liquid has been determined at 1 atm pressure in the temperature interval of 700 - 1650°C. Viscosity measurements of a haplogranite, haplogranite + 5.1 wt.% P2O5 and haplogranite + 9.5 wt.% P2O5 have been performed using the concentric cylinder and micropenetration methods. The viscosity of haplogranite liquid decreases with the addition of P2O5 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongest decrease exhibited at low P2O5 concentration. The temperature-dependence of the viscosity of all the investigated liquids is Arrhenian, as is the case for P2O5 liquid. The Arrhenian activation energy is slightly lower in the P2O5-bearing liquids than in the P2O5-free haplogranite with the result that the effect of P2O5 on viscosity is a (weak) function of temperature. At temperatures corresponding to the crystallization of phosphorus-rich granitic and pegmatitic systems the addition of 1 wt.% of P2O5 decreases the viscosity 0.2 log10 units. The effect of P2O5 on haplogranitic melt viscosity is much less than that for B2O3, F2O−1 on the same melt composition (Dingwell et al., 1992 and this study). This implies that P2O5 concentration gradients in high-silica melts during, for example, phosphate mineral growth or dissolution in granitic magmas, will not significantly influence melt viscosity

Composition and probable ore igneous rocks source of columbite from alluvial deposits of Mayoko district (Republic of the Congo)

The article presents the results of optical, electron microscopic and electron microprobe studies of columbite group minerals, collected during heavy mineral concentrate sampling of alluvial deposits in the Mayoko region (Republic of the Congo). The aim of the study is revealing tantalum niobates ore body in this region. We found that these minerals in loose deposits are represented by two grain-size groups: less than 1.6 mm (fine fraction) and 1.6-15 mm (coarse fraction). The grains of both fractions belong mainly to columbite-(Fe), less often to columbite-(Mn), tantalite-(Mn) and tantalite-(Fe), contain impurities of Sc, Ti, and W. The crystals have micro-scaled zoning (zones varies slightly in the Ta/Nb ratio values) and contains a lot of mineral inclusions and veins represented by zircon, pyrochlore supergroup minerals and others. Columbite-(Fe) and columbite-(Mn) are characterized by an increased content of Ta2O5 up to the transition to tantalite-(Fe) and tantalite-(Mn). This allows us to exclude the formation of subalkaline rare-metal granites, their metasomatites (albitites and greisenes) and carbonatites, from the list of possible columbite ore rocks source in the Mayoko district. Thus, beryl type and complex spodumene subtype rare-element pegmatites of the mixed petrogenetic family LCT-NYF (according to P.Černý) should be considered as a probable root source. The results of the research should be taken into account when developing the methodology for prospecting in this area

Geochemistry of granitic aplite-pegmatite veins and sills and their minerals from the Sabugal area, central Portugal

Granitic beryl-columbite-phosphate subtype aplite-pegmatite veins and sills from the Sabugal area intruded a biotite > muscovite granite which is related to another two-mica granite. Variation diagrams of major and trace elements of whole rocks show fractionation trends. REE patterns and δ18O of whole rocks, BaO and P2O5 contents of K-feldspar, anorthite and P2O5 contents of plagioclase, major element and Li contents of muscovite and lithian muscovite support this series. Least squares analysis of major elements indicate that the biotite > muscovite granite and aplite-pegmatite veins and sills are derived from the earlier two-mica granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Modelling of trace elements shows that magmatic fl uxes and fl uids controlled the Rb, Sr and Ba contents of aplite-pegmatites, probably also lithium micas (zinnwaldite, polylithionite and rare lepidolite), cassiterite, columbite-tantalite, fl uorapatite and triplite. In aplite-pegmatites, lithian muscovite replaces primary muscovite and late lithium micas replace lithian muscovite. Complexely zoned columbite crystals are chemically characterized and attributed to disequilibrium conditions. Relations of granites and aplite-pegmatites and pegmatites from other Portuguese and Spanish areas are compared. The granitic aplite-pegmatites from Sabugal are moderately fractionated and the granitic complex type aplite-pegmatites from Gonçalo are the richest in Li and Sn, derived from a higher degree of fractional crystallization and fl uxes and fl uids control the Ba and Rb behaviours and Li, Sn, F and Ta concentrations