Rare-element Class Research Articles

Garnet composition from the Reflejos de Mar LCT-pegmatite, Ancasti district, Argentina and its implication for exploration of primary deposits of lithium

The Reflejos de Mar Li-pegmatite, located in northwestern Argentina, is part of the Villismán pegmatite group, Ancasti District, Pampean Pegmatite Province. Four garnet crystals from the outermost part of the pegmatite were analyzed by major and minor elements (SiO2, TiO2, Al2O3, Cr2O3, MgO, CaO, MnO, FeO) using electron microprobe. The pegmatite belongs to the rare-element class, spodumene type, LCT (Li-Cs-Ta) petrogenetic family. Based on their Mn and Fe contents, the analyzed garnet can be assigned to the spessartine-almandine serie. The cores and rims of the analyzed garnets show significant differences for the divalent components in VIII-coordination, especially, Mn and Fe. The average MnO and FeO contents in the cores of the grains is 34.6 and 8.29 wt%, respectively, while in the rims is 29.31 and 12.95 wt%, respectively. The Fe/Mn ratio at the core of the grains is 0.24 while at the rims it is 0.44. Cr2O3 and TiO2 contents are very low (˂0.17 wt%) and the values of SiO2 and Al2O3 are ~36 and ~21 wt%, respectively. The mean chemical and molecular formulas of the core can be expressed: [(Mn2.40 Fe0.57 Ca0.02 Mg0.02)3.01 (Al1.99 Cr0.002)2.00 (Si2.99 Ti0.01)3.00 O12]; {Sps79.8 Alm18.9 Grs0.7 Prp0.5}; and the rim as: [(Mn2.04 Fe0.89 Ca0.05 Mg0.04)3.02 (Al2.00 Cr0.002)2.00 (Si2.98 Ti0.005)3.00 O12]; {Sps67.6 Alm29.5 Grs1.6 Prp1.3}. The chemical composition of garnet from the Reflejos de Mar pegmatite is similar to other worldwide examples in similar rocks, especially LCT pegmatites, which are highly evolved and associated with Li mineralization. Therefore, its composition could be used as an additional tool in the exploration of Li-bearing pegmatites in the Pampean Pegmatite Province. The differences in Fe-Mn contents between core and rim of the crystals would be controlled by variations in composition of the pegmatitic melt and, in addition, by the simultaneous precipitation of other mineral phases, for example, schorl and Mn-Fe-bearing phosphates.

Genesis and rare metal potential of pegmatites around Makarfi area, North Western Nigeria

• The pegmatites belong to the LCT family, rare element class, beryl subtype, peraluminous, highly differentiated, fractionated and enriched in rare metals. • The rare metal mineralization of the pegmatites is comparable to other mineralized pegmatites in Nigeria but low when compared to highly mineralized Tanco mine. • Fractional crystallization in combination with varying degree of wall rock assimilation is the dominant process involved in the differentiation of the magma . • Prospects for rare metal mineralization is associated with the NW – SE trending pegmatites in the area.

A mineralogia acessória do Amazonita Pegmatito Serra Branca: classificação de um pegmatito NYF na Província Borborema, Nordeste do Brasil

O campo pegmatítico de Vieirópolis é caracterizado pela ocorrência de pegmatitos com amazonita e/ou berilo. Esse campo contém pegmatitos do tipo NYF (Nb-Y-F) inseridos no Domínio Rio Grande do Norte (DRGN), fora do contexto da Província Pegmatítica do Seridó (PPS). O principal pegmatito desse campo, o Amazonita Pegmatito Serra Branca, destaca-se pela mineralização de megacristais de amazonita e por sua exploração como rocha ornamental de elevado valor econômico. Para caracterizar a paragênese e a química mineral das fases acessórias do Amazonita Pegmatito Serra Branca, além de elucidar a classificação e a natureza petrogenética desse pegmatito, foram utilizados dados de campo, petrográficos, difração de raios X e química mineral. O Amazonita Pegmatito Serra Branca ocorre como dique tabular com aproximadamente 3 m de espessura, de direção NW-SE e caimento 45° WSW, caracterizado por apresentar uma zona composta por megacristais de amazonita e quartzo, e outra, por albita e quartzo sacaroidais. Apresenta vasta mineralogia acessória que compreende anglesita, biotita, bismutita, cerussita, columbita-(Mn), espessartina, helvina, fenaquita, fluorita, hematita, ilita, ilmenita, magnetita, montmorilonita, muscovita, piromorfita, pirocloro, rutilo e zircão. Destacando-se, também, atípicas mineralizações de sulfetos e a presença de beríliossilicatos reportados pioneiramente no contexto geológico da Província Borborema (PB). A paragênese mineral e a química dos minerais acessórios do Amazonita Pegmatito Serra Branca o classificam como pertence à família NYF da classe dos elementos raros, tipo da gadolinita, possivelmente associado a um magmatismo do tipo I.

Geochemistry of the Villismán granite, associated aplite-pegmatites and surrounding Li-pegmatites, Sierra de Ancasti, Argentina

The Villismán pegmatite group belonging to the Ancasti district of the Pampean Pegmatite Province is geologically formed by banded schists of the Ancasti Formation which are intruded by the homonymous granitic stock and Li-pegmatites. It is located in the northern sector of the Sierra de Ancasti, Catamarca province, Argentina. The Villismán stock -VG- is a small, sub-rounded in shape two-mica-syenogranite. The VG hosts felsic aplitic and pegmatitic dykes -FDVG- with muscovite, black-tourmaline (schorl) ± garnet as main accessory minerals. The Li-pegmatites contain giant crystals of spodumene and are classified as of the Rare-Element class (REL) and REL-Li subclass pegmatites and belong to the LCT (Li–Cs–Ta) pegmatite family. They are more or less symmetrically zoned with outward zones composed of felsic aplite and coarse-grained to very coarse-grained granite -OZLiP-. The minerals of OZLiP are quartz, feldspars (secondary albite ‘cleavelandite’ predominant over K-feldspar), muscovite and scarce black-tourmaline (schorl), garnet and F-apatite. The VG, FDVG and OZLiP have high and similar SiO2 concentrations (72–78 wt %). They are peraluminous (ASIVG 1.10–1.16; ASIFDVG 1.23–1.77, ASIOZLiP 1.00–1.14). The VG has subalkaline-calc alkaline character with K2O > Na2O. Ferromagnesian elements (included Ti) and CaO have low concentrations in VG and the FeOt/FeOt + MgO ratio is low (0.80–0.82) classifying it as magnesian granite. High Rb/Sr (FDVG ~16; OZLiP ~19) and Rb/Ba (FDVG ~41; OZLiP ~ 7) and low K/Rb (FDVG ~107; OZLiP ~56) ratios clearly indicate that aplites and pegmatites are geochemically more evolved than the Villismán Granite (Rb/SrVG ~ 3; Rb/BaVG ~ 0.8; K/RbVG ~ 161). The Zr-saturation temperature in the granites, as expected, is higher than in the dykes and the outward zones of the Li-pegmatites. The VG is a post-orogenic granite developed in the easternmost sector of the Ordovician Famatinian active arc. All geological, mineralogical and geochemical features of the VG are comparable to typical S-type granites, probably originated from partial melting of the metasedimentary rocks of the Ancasti Formation. FDVG could have been formed by fractional crystallization from the VG parental-magma.

Occurrence of Liddicoatite-Bearing LCT Pegmatites in Sirohi Region, Northwest India and their Rare Metal Potentiality

Rare and trace element study of pegmatites around Sibagaon area of Sirohi region, Rajasthan, India revealed that the pegmatites are of rare element class and LCT (Li–Cs–Ta) type. Rare metal concentration in these pegmatites is notably high against crustal abundance, with Li ranging from 1007 to 10785 ppm, Rb from 1285 to 9147 ppm, Cs from 36 to 1142 ppm, Ta from 12 to 386 ppm, Sn from 54.22 to 2283.61 ppm and F from 2724 to 48,275 ppm. Integration of EPMA and LA-ICPMS analyses indicated that the main Li-bearing mineral in pegmatites is lepidolite with Li ranging from 21,599 to 28,178 ppm, which is economically worthwhile to process for Li, followed by elbaite and rare mineral liddicoatite, which has not been reported so far in India. Rare earth element (REE) distribution pattern shows enrichment of LREEs than HREEs, indicating that pegmatites are fractionated and thus causing enrichment of rare metals. This reporting of liddicoatite-bearing LCT pegmatites from Sirohi region with high anomalous values of rare metals can therefore be a good prospect for rare metal exploration in India in the present economic scenario. These pegmatites have indicated a syn-collisional signature and may be related to the accretion of Marwar craton with greater Indian land mass along the Phulad Shear zone.

Evaluation of the petrogenetic significance of melt inclusions in pegmatitic schorl-dravite from graphic tourmaline-quartz assemblages: Application of LA-ICP-QMS analyses and volume ratio calculations

The composition of melt inclusions in tourmaline was studied from graphic quartz-tourmaline intergrowth assemblages of common, muscovite – rare-element class pegmatites belonging to the lithium-caesium-tantalum family. This study applied laser ablation inductively coupled quadrupole mass spectrometry in combination with 2D and 3D microscopic volume calculation to analyse these unhomogenised melt inclusions. The determination of the analytical precision of the method is performed by repeated microscope-scanning and laser ablation analyses of tourmaline mineral inclusions in muscovite hosts for which the compositions have been independently measured by electron microprobe.The analysed melt inclusions are hosted in schorl-dravite series tourmaline from the early-Neoproterozoic pegmatite dykes of the Gatumba-Gitarama field in Rwanda. Multiple dykes have been studied, representing three regional pegmatite zones showing an increasing bulk regional degree of differentiation. The sampled graphic quartz-tourmaline assemblages are the only manifestation of an internal mineralogical zonation within these pegmatite dykes.The melt inclusions dominantly contain muscovite, quartz, dickite, feldspars, jeremejevite and childrenite minerals and variable amounts of Cs-rich, interstitial fluid. The major element composition of the ablated melt inclusions is variable but mainly plot on the low Ca-granitic compositional trend. The inclusions show, moreover, a significantly higher normative quartz content compared to bulk pegmatitic rock counterparts. The melt inclusions demonstrate a near-constant Rb concentration (100 up to 160 ppm) whereas Cs concentrations vary between 10 and 1000 ppm. Although Rb and Cs enrichment trends are accurate proxies to deduce pegmatite melt differentiation in general, both the Rb and Cs concentrations of the melt inclusions are remarkably unrelated to the regional degree of differentiation deduced from the constituent bulk rock and rock-forming minerals of the Gatumba-Gitarama pegmatites. A comparison of the alkali metal content of melt inclusions and cogenetic fluid inclusions with previously modelled bulk melt concentration indicates that this deviating Rb and Cs behaviour is caused by heterogeneous trapping of variable portions of the Cs-enriched aqueous fluid phase together with the residual melt and this during melt inclusion formation.The melt inclusions in tourmaline represent local and residual, hydrous boro-aluminosilicate melt compositions. The formation of graphic intergrowths inside the pegmatite dykes indicates rapid tourmaline growth rates causing in turn quartz saturation and nucleation in the surrounding residual melt. Fast tourmaline crystallisation contributed directly to the formation of these “melt pools” with distinct residual compositions. Our study shows that already for less-differentiated common pegmatites, melt inclusions from internal mineralogical zones represent closed melt subsystems that cannot be directly linked to the bulk system. These findings imply, moreover, that the use of tourmaline trace element compositions as an indirect geochemical proxy for the bulk melt composition of pegmatites and evolved granites, has to be considered with caution.

Tourmaline, an indicator of external Mg-contamination of granitic pegmatites from host serpentinite; examples from the Moldanubian Zone, Czech Republic

Dominant primary solidus and minor subsolidus tourmalines from a variety of granitic pegmatites enclosed in serpentinites of the Moldanubian Zone, Czech Republic were examined, mainly by electron probe micro-analyser, to reveal the degree of external Mg(Ca)-contamination from their host rocks. The rocks include: (i) homogeneous to slightly heterogeneous nests of plagioclase-tourmaline rocks (group A) of anatectic or metasomatic origin, (ii) subhomogeneous to simply zoned barren pegmatite dikes (group B), and (iii) Li-bearing zoned pegmatite dikes of rare-element class (group C). The plagioclase-tourmaline rocks (group A) show spatial relation to pegmatites of the group B. Mostly black primary tourmalines (dravite, oxy-dravite, uvite, schorl, oxy-schorl, fluor-schorl) show extensive Mg- and Ca-contamination (group A), moderate Mg- and locally minor Ca-contamination (group B plus the locality Věžna I of the group C) and weak Mg-contamination of the tourmaline solely from outermost pegmatite units (group C); tourmalines from internal units of the pegmatites are typically Mg-free. The substitution mechanisms include MgR2+ −1 (R2+ = Fe2+ > Mn2+) in all groups, NaR2+ (□Al)−1 and R2+ (OH) (AlO)−1 in Ca-poor tourmalines and CaO (NaOH)−1 combined with the substitution CaR2+ (NaAl)−1 in Ca-enriched tourmalines (group A). Both Mg- and Ca-contamination events were very likely contemporaneous. The extent of contamination is higher in small and texturally simple plagioclase-tourmaline rocks (group A). Larger and more highly evolved Li-bearing pegmatites (group C) with zoned internal structure show a high degree of undercooling; consequently, rapid crystallization of outer zones with biotite and/or tourmaline depleted melt in almost all Mg and isolated the pegmatite body from further external contamination during solidus crystallisation. The granitic pegmatites (group B and group C) were open to the host serpentinite during early solidus crystallization immediately after emplacement of melt and then in early and/or late subsolidus crystallization (hydrothermal stage); in plagioclase-tourmaline rocks (group A) the system was likely continuously open to host serpentinite. This study affirms tourmaline as a very useful indicator of external contamination and elevated contents of Mg in tourmaline or in other minerals are the most reliable sign of external contamination in granitic rocks.

PEGMATITIC BERYL AS INDICATOR OF MELT EVOLUTION: EXAMPLE FROM THE VELASCO DISTRICT, PAMPEANA PEGMATITE PROVINCE, ARGENTINA, AND REVIEW OF WORLDWIDE OCCURRENCES

Fil: Sardi, Fernando Guillermo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Tucuman. Instituto Superior de Correlacion Geologica; Argentina

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).

AN ASSEMBLAGE OF BISMUTH-RICH, TELLURIUM-BEARING MINERALS IN THE EL QUEMADO GRANITIC PEGMATITE, NEVADOS DE PALERMO, SALTA, ARGENTINA

An assemblage of bismuth-rich, tellurium-bearing minerals in the El Quemado pegmatite, Salta, Argentina, represents late-stage mineralization in a spodumene-subtype, rare-element class granitic pegmatite. The minerals occur in an irregular, volumetrically minor core-margin association, dominated by fine-grained muscovite and quartz, with accessory U-rich fluornatromicrolite, zircon, bismuth, bismuthinite, emplectite, hodrusite, and the sulfotellurides joseite-A, joseite-B, tetradymite and ingodite. Early bismuth is replaced by abundant bismuthinite, which also locally contains symplectitic intergrowths of emplectite (Cu 1.02 Bi 0.98 S 2.00 ). A single euhedral crystal of hodrusite [(Cu 4.04 Fe 0.12 Ag 0.03 ) ∑4.19 (Bi 5.87 Pb 0.02 ) ∑5.89 S 10.92 ] has Pb-bearing tips that contain 3.62 to 4.39 wt.% Pb (0.33 to 0.40 apfu respectively). Joseite-A (Bi 4.04 Te 0.95 S 2.01 ) forms an intergrowth with ingodite, enclosed in emplectite. Joseite-B (Cu 0.02 Bi 4.01 Te 1.93 S 1.04 ) is intergrown with joseite-A and with bismuthinite in emplectite. Tetradymite [(Bi 2.00 Cu 0.02 ) ∑2.02 Te 1.95 S 1.03 ] is included in emplectite and bismuthinite. Ingodite is usually enclosed in bismuthinite as very small grains with a composition Bi 1.98 Te 1.00 S 1.02 . Late-stage or supergene alteration of this association generated fairly abundant bismutite and rare bismite. On the basis of experimental data, the equilibrium association of bismuth, bismuthinite, emplectite and hodrusite, typical of hydrothermal ore deposits but uncommon in granitic pegmatites, is restricted to relatively low temperatures and a pressure in the range 2–3 kbar.

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part I: accessory Sn, Nb, Ta and Ti minerals in leucogranites, aplites and pegmatites

*Corresponding author Sn–Nb–Ta–Ti oxides occur as accessory minerals in the granitic facies of the External Unit in the Jalama Batholith (Salamanca, Spain) and in the related LCT pegmatite dikes of the rare-element class. Moreover, abundant cassiterite and columbite-group minerals crystallized in the Cruz del Rayo peribatholithic pegmatite dikes, cutting the pre-Ordovician low-grade metasedimentary rocks of the surrounding Schist-Graywacke Complex. Cassiterite, rutile, ilmenite and tantalite-(Fe) occur disseminated in the border facies of the External Unit, especially throughout the tourmaline-bearing leucogranite and the apical aplites. Additionally, cassiterite, rutile and Ta-rich rutile developed locally in the intragranitic pegmatite dikes. Two types of peribatholithic pegmatites can be distinguished at Cruz del Rayo: (i) granite-like pegmatites, with columbite-(Fe) I and II and cassiterite, in which the influence of metasomatic fluids led to formation of the late albite unit and crystallization of tantalite-(Fe), and (ii) greisen-like bodies, which contain high amounts of columbite-(Fe), columbite(Mn), tantalite-(Mn) I and II as well as cassiterite. The primary oxide assemblage in both types of the peribatholithic pegmatite dikes would have crystallized as a result of magmatic differentiation of the residual melts coming from the Jalama granitic cupola. However, crystallization of the secondary assemblage, richer in Fe and Ta, is interpreted mainly as a consequence of interaction with external fluids coming from the metamorphic host rocks, more or less mixed with meteoric fluids, although partial dissolution and re-precipitation could have played an important role, as well.

Petrogenetic significance of LA-ICP-MS trace-element data on quartz from the Borborema Pegmatite Province, northeast Brazil

AbstractQuartz from different zones within five granitic pegmatites of the rare-element class from the Borborema Pegmatite Province in northeast Brazil were analysed for fourteen trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The concentrations of Li (6—150 ppm), B (1—9 ppm) and Ge (1—23 ppm) in quartz show a positive correlation with Al (30—770 ppm). The concentrations of these elements increase from the border zone to the quartz core of pegmatites of the spodumene or lepidolite subtypes. The Ge concentrations in the quartz core are the highest so far reported in igneous quartz. In the less evolved pegmatites of the beryl-columbite subtype, the Al, Li, B, and Ge concentrations in quartz from all zones remain at the same level as the border and wall zones. The Ti concentrations in quartz from the core of the more evolved pegmatites are below 3 ppm (with Al >250 ppm), contrasting with 7—25 ppm (with Al <280 ppm) in samples from the border and wall zones of the less evolved and more evolved pegmatites. The concentrations of Al. Li, B, Ge, and Ti in quartz are therefore confirmed as good indicators of the degree of magma fractionation and analyses of pegmatite quartz cores can be used for exploration purposes to distinguish pegmatites with high metallogenic potential. Atoms of Li and Al are incorporated into quartz such that Li/Al ranges between 0.75 and 1.0. This suggests a coupled substitution of the form Si4+ ↔ (Li+ + Al3+). The other elements analysed either showed an erratic distribution (e.g. Be and P) or were below the respective limits of detection (Na, K, Rb, Ca, Sr, Mn, Fe) in most samples.

Characterization and Mineralization Potentials of Granitic Pegmatites of Komu area, Southwestern Nigeria

AbstractThe geology of the Komu area, south‐western Nigeria was investigated in order to determine the nature of occurrence, petrography and chemical composition, and to evaluate the mineral potential of granitic pegmatites. The pegmatites are hosted by para‐gneisses, amphibolites and granitic rocks. These host rocks are characterized by low pressure mineral assemblages of upper greenschist to lower amphibolite facies grades. The pegmatites consist of four lithological units: alkali feldspar unit, graphic granite unit, quartz‐albite‐muscovite‐tourmaline unit and saccharroidal albite‐muscovite‐tourmaline‐garnet unit.A K/Rb ratio of below 100 is indicative of a mineralization potential in most pegmatites, except for the saccharroidal albite‐muscovite‐tourmaline‐garnet unit which has exceptionally high ratios. The discrimination diagrams Rb versus K/Rb, Zn versus K/Rb and Th versus K/Rb show a limited range of fractionation, suggesting that the pegmatites belong to the barren muscovite and rare element classes. The rare element class is moderately enriched in beryl, tourmaline and niobium–tantalum minerals. The pegmatite has affinity for beryl columbite sub‐type and lithium–cesium–tantalum (LCT) family. The ores plot mainly in the field of mangano‐tantalite of tantalite‐columbite quadrilateral diagram. These results indicate that the pegmatites are promising hosts for Nb‐Ta ores.

THE TANCO PEGMATITE AT BERNIC LAKE, MANITOBA. XVI. ZONAL AND BULK COMPOSITIONS AND THEIR PETROGENETIC SIGNIFICANCE

The Tanco pegmatite deposit at Bernic Lake, in southeastern Manitoba, is a virtually completely hidden, essentially undeformed, subhorizontal, bilobate, saddle-shaped body ~1520 m long, 1060 m wide and up to ~100 m thick. The highly fractionated pegmatite of the LCT family, rare-element class, petalite subtype, consists of nine internal zones. The outer zones are concentric, whereas the layered inner zones are segmented and locally complex in shape. A 3D computer model representation of the pegmatite assisted in calculation of volumes and compositions of individual zones and of the whole pegmatite, based on 102 km of 1355 drill-hole intersections, underground observations, measured and estimated mineral modes of the zones, zone-specific compositions and densities of minerals, and ore grades. The early outer zones tend to show granitic modes, but the inner zones strongly deviate in proportions of feldspars, or by high enrichment in lithium minerals, and some of the zones are virtually monomineralic. The bulk mode of Tanco is granitic, except ~9 vol.% petalite (including spodumene + quartz pseudomorphs). Despite also being economically significant for Cs, Ta and Li, the pegmatite contains less than 1 vol.% of any other non-micaceous minerals, and less than 0.1% of most of them. The bulk chemical composition closely reflects the modal relationships: it is peraluminous, and granitic, with prominent enrichment in Li, Rb, Cs and F, moderate contents of Tl, Be, B, Ga, Sn, Nb and Ta, and remarkable depletion in Fe, Mn, Mg, Ca, Ba, Sc, Ti and Zr. A very high degree of fractionation is shown for the bulk pegmatite by the values K/Rb 4.7, K/Cs 9.3, Rb/Cs 2.0, Rb/Tl 137, Fe/Mn 0.63, Mg/Li 0.02, Al/Ga 917, Zr/Hf 2.6, Zr/Sn 0.21 and Nb/Ta 0.19. Zonal and bulk chemical composition is in rough agreement with the experimental thermal trough in the system Ab + Qtz + Ecr at 2 kbar P(H2O), but shifted toward the Ab apex by elevated B, P and F, and by the voluminous wall-zone. The composition of the wall zone (20) is remarkably similar to that of bulk Tanco, except for the elevated contents of rare elements in the latter that are concentrated in the intermediate and inner zones. In turn, zone (20) is very close to the composition of nearby, somewhat less fractionated peraluminous leucogranites that are parental to local genetically related pegmatite groups. The wall zone (20), which composes 44 vol.% of Tanco, represents by analogy a petrogenetic link between an undisclosed parent leucogranite and the Tanco deposit. These relationships document derivation of the Tanco pegmatite and its analogues elsewhere by igneous differentiation from a fertile leucogranitic magma.

Genetic Framework for the Classification and Distribution of Archean Rare Metal Pegmatites in the North Pilbara Craton, Western Australia

The Pilbara pegmatite province in the Archean North Pilbara Craton contains at least 120 pegmatite deposits in over 27 pegmatite groups and fields, including the giant Mount Cassiterite tantalum orebody in the Wodgina pegmatite district. Generally, rare metal pegmatites in the province are hosted by mafic-ultramafic volcanic-dominated supracrustal sequences of predominantly greenschist facies, adjacent to, and rarely within, domal multiphase granitoid-gneiss complexes. The most fractionated pegmatites are hosted by tongues or inliers of greenstone belts protruding into or within the larger granitoid complexes. Most of these are within two of the six tectonostratigraphic domains of the East Pilbara granite-greenstone terrane of the North Pilbara Craton. They tend to be clustered along and within 5 km (at surface) of major faults and craton-scale lineaments that coincide with or are parallel to domain boundaries. All of the major tantalum deposits (Wodgina, Mount Cassiterite, Tabba Tabba, Strelley, Pilgangoora) are along a north-northeast–trending corridor that is within one tectonostratigraphic domain. Most rare element pegmatites are within 5 km (at surface) of their apparent parent pluton, and all are within 10 km where the pluton has been identified. Although granitic rocks of the North Pilbara Craton were emplaced over an 800 m.y. period (3600–2800 Ma), rare element class pegmatites can be tied to a number of post-tectonic plutons of a younger granite suite that were emplaced into most of the granitoid complexes and adjacent greenstone belts at 2890 to 2830 Ma. Subtle differences in the composition of these granites that have given rise to different petrogenetic pegmatite suites can be identified in processed radiometric data from airborne surveys. These parental younger granites are enriched in lithophile and volatile elements and have a highly fractionated character that permitted formation of significant pegmatite mineralization. They represent the culmination of the crustal evolution of the North Pilbara Craton and the onset of cratonization, as part of a global appearance of rare metal pegmatite mineralization post-3 Ga.

Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts

The distribution of F between amblygonite (Amb, LiAlPO 4 F)-montebrasite (Mbr, LiAlPO 4 OH) solid solutions and metaluminous haplogranitic melt has been calibrated at 585 °C and 200 MPa H 2 O. The partition coefficient for F between the crystalline phase and melt, D F Mbr/melt , is linear between 0 to ~10 wt% F in amblygonite, which contains 13 wt% F at the end-member: C F Mbr = 3.65C F melt + 0.07, r 2 = 0.995, n = 6. Values of D F Amb/melt decrease sharply above 10 wt% F in amblygonite as the amblygonite reaches saturation in F at 200 MPa H 2 O. In natural occurrences, however, the vast majority of primary amblygonite- montebrasite solid solutions contain ~4-7 wt% F, well within the linear range of the calibrated exchange reaction, and the montebrasite-bearing assemblages are among the last to crystallize. If the F contents of the montebrasite are magmatic, then these most-fractionated residual melts of the LCT (Li-Cs-Ta, and mostly peraluminous S-types) rare-element class generally contained up to ~1.0-1.8 wt% F near the end of their crystallization. The modest F contents of pegmatites are consistent with the common association of Li aluminosilicates and with the general paucity of topaz in these occurrences. In topaz-bearing granites of Western Europe, however, high-F amblygonite (~10- 11 wt% F) reflects >3 wt% F in melt during crystallization of these magmatic phases.

The Nature and Distribution of Tantalum-bearing Minerals in Newly Discovered, Rare-Element Pegmatites at the Musselwhite Mine, Northwestern Ontario

Newly discovered pegmatites distributed throughout the sub-surface environs of the Musselwhite Archean gold deposit occur as narrow dikes that intrude most of the major lithological units. The mineralogy and textures of the pegmatite dikes are characteristic of granitic pegmatites of the “Rare-Element Class”; more specifically, they closely resemble varieties of the “Complex-type” granitic pegmatites that are commonly associated with zones of tantalum mineralization. The essential minerals in the dikes comprise albite, quartz, and K-feldspar, together with lesser amounts of mica (including lepidolite), spessartine, tourmaline, and locally, spodumene. Fine-grained (30 μm to 300 μm), tantalum-bearing minerals occur in the quartzo-feldspathic matrix of the dikes as disseminations, in interstices, and in small vugs. The nature of the tantalum-bearing mineral assemblage present in individual pegmatite dikes varies from a simple mineral assemblage composed of ferrocolumbite (with around 39 wt% Ta2O5) in the most northerly dike, to more complex assemblages comprised of manganotantalite (with up to 75 wt% Ta2O5), ferrotapiolite (with up to 79 wt% Ta2O5), and microlite (with up to 76 wt% Ta2O5) that are present in dikes located several kilometers to the south and southwest. The compositional characteristics and textural relationships of the tantalum-bearing minerals in the dikes provide evidence of igneous fractionation trends that are typical of rare-element granitic pegmatites. Such trends suggest that the newly discovered pegmatites represent part of a more extensive group of cogenetic, tantalum-mineralized dikes.

Tin mineralization in the Archean Sinceni rare element pegmatite field, Kaapvaal Craton, Swaziland

The Sinceni pegmatite field constitutes a large group of 3.0 Ga granitic pegmatites in the exocontact of the Sinceni pluton in south-central Swaziland. Field relations, age dating, and geochemical evidence indicate a genetic relationship between the pegmatites and the Sinceni pluton. The Sinceni pegmatites are moderately fractionated granitic pegmatites belonging to the beryl type of the rare element class. The pegmatite field is compositionally zoned, with the highest fractionation and strongest mineralization in a restricted area referred to as Sinceni West. Only the alluvial and eluvial tin concentrations are of economic interest, and the largest of these is the Sinceni West deposit. Primary tin mineralization at the Sinceni West deposit consists of cassiterite in late-stage pegmatite units of sugary albite, associated with mica-tourmaline metasomatic selvages in the host rock. The albitic units are interpreted as the products of late residual melt, and the metasomatic selvages reflect fluid exsolution from the melt at the latest stage of pegmatite consolidation. Several factors contributed to the development of primary tin mineralization in the Sinceni West area. First, the pegmatites in this area are the most highly fractionated, as shown by Ta, Rb, and Cs concentrations in blocky K feldspar. Second, the richest mineralization occurs in pegmatites that lack tourmaline, whereas tourmaline is abundant in the barren pegmatites. A lack of tourmaline allows the buildup of boron in residual melts, which promotes extended fractionation by increasing water solubility and depressing the solidus temperature. Third, the best tin values occur in pegmatites that exhibit boudinage and other signs of ductile deformation caused by an adjacent shear zone. Microstructures show that at least some cassiterite predates the deformation and that the albitic units and wall-rock alteration are synkinematic. The ductile deformation may have influenced the distribution of residual melt and its interaction with preexisting pegmatite and wall rocks, but it was probably of minor importance for the mineralization itself.