Borborema Pegmatite Province Research Articles

The Borborema pegmatite Province, NE-Brazil revisited

The Borborema pegmatite Province, NE-Brazil revisited

Mg-rich mica and tourmaline in pink-colored “parelhas quartzite” intercalations in the equador formation, Borborema pegmatite province, NE-Brazil: record of evaporitic protolith?

Light gray, green to pink-colored banded quartzite intercalations in the NeoproterozoicCEquador Formation (Seridó Group), Seridó Fold Belt, Northeast Brazil, were exploited during several decades as dimension stones under the fashion name “Parelhas Quartzite” (PQ). They can be easily sliced along intensively foliated mm-thick laminae with relatively high content of light green or pink-colored mica, in alternation with more massive, a few cm-thick bands with lower mica contents. The variable ratios between pink and light green micas in the partition laminae, define the color of the produced rock plates, used as outdoor floor or wall coating. Massive, quartzites, meta-arcoses and meta-conglomerates of the Equador Formation (EF) are the exclusive host rock of a few Li-bearing rare element class granitic pegmatites of the Borborema Pegmatite Province, mined for the prized gemstone known as “Paraíba Tourmaline”. The EF hosts also nearly 10% of the Be-Li-Ta-bearing pegmatites of the same province. Pink-colored tourmaline in the form of millimeter- to decimeter-long crystals occur as prominent accessory mineral in the pink-colored PQ and are empirically described by miners and field geologists as “rubellite” and the pink-colored mica as “lepidolite”. The PQ could therefore be considered as a potential source of Lithium. The tourmaline crystals occur isolated or as radial arrangements, always flattened and oriented in the foliation planes. Light green micas are dominant in the greenish-gray PQ and are always referred in the literature as common muscovite. Tourmaline with the same textural relations as the pink-colored counterpart may also occur in the gray PQ plates but in this case is always black. Electron microprobe analyses of the pink mica and tourmaline allowed establish that they are respectively Al-rich phlogopite and magnesiofoitite (this one preferentially occurring in the crystal cores) with transition to dravite and uvite (rims). Both, mica and tourmaline present insignificant lithium contents. The light green micas were identified as the Fe-Mg-rich variety of muscovite known as “phengite”). The PQ can therefore be discarded as possible source of Li for economic exploration. They can also be excluded as source for Li-enrichment in pegmatites by “in situ” anatexis of quartzite or metasomatic assimilation of ascending pegmatite forming melts. There is also no support for the hypothesis of the genesis of the pink-colored mica and tourmaline by pegmatite-related exomorphic alteration. The most probable origin of the B- and Mg-enrichment in the PQ is related to syn-sedimentary evaporitic or volcano-exhalative incursions during the deposition of the original sediments.

Phosphate sequence study of Boqueirão pegmatite (Rio Grande do Norte state, Brazil)

O pegmatito Boqueirão localiza-se naProvíncia Pegmatítica daBorborema, no estado do Rio Grande do Norte (Nordeste do Brasil), encontra-se encaixado em metaconglomerados de idade neoproterozóicada Formação Equador. Trata-se de um pegmatito zonado concentricamente, exibindo diferentes zonas caracterizadas por suas mineralogias. As zonas de contato e de parede são compostas essencialmente por quartzo, microclina e moscovita exibindo muitas vezes texturas esqueléticas e gráficas, e,tendo como minerais acessórios a turmalina e a granada. A zona intermediária é dividida em duas subzonas distintas e mostra uma mineralogia diversa exibindo uma complexa associação de fosfatos, berilo, columbita-tantalita e zircão. A zona mais interna é principalmente constituída por um núcleo de quartzo. Além disso, observa-se corpos de substituição típicos, constituídos por albita,situados na parte oriental do corpo. Litofilita - triplilita representa a fase de fosfato mais abundante exibindo exsoluções de sarcopsida e substituição por sicklerita, resultante da alteração por Li- lixiviação, mais conhecida como Série de Quensel-Mason. A ausência de membros da série heterosita-purpurita é notável. Fosfatos primários, tais como montebrasita e triplita são também observados. Subsequentemente, a presença de alluaudita e varulita sugere que o pegmatito foi submetido a metassomatismo sódico.Toda esta assembléia de minerais sofreu uma extensa alteração que levou a cristalização de fosfatos secundários. Dados de microssonda eletrônica em trifilita-litiofilita revelam razões Mn/(Mn + Fe) entre 0,21-0,24 e também até 0,72 que sugerem um magma de origem pouco fracionado; consequentemente, a ferrisicklerita mostra valores de Mn/(Mn + Fe)entre 0,69-0,77, característicos de elevado grau de evolução, sugerindo que o pseudomorfismo foi produzido por uma elevada interação entre rocha/fluido. Alluaudita e varulita também revelam conteúdos de Mn e Fe até 0,71 semelhantes aos fosfatos precursores. Além disso, oito diferentes fosfatos secundários foram identificados substituindo fases primárias, quer como pseudomorfos ou cortando-as. Ocorrem como cristais euédricos de grã fina, esferulíticos ou maciços, com cores vivas facilmente identificados. As fases secundárias ricas em Mn são hureaulita, eosphorita, whiteita, jahnsita e serrabrancaita, enquanto que lipscombita, phosphosiderita e mitridatita correspondem a associação secundária rica em ferro (Fe). A origem supergênica tardia e a entrada fluidos ricos Ca, K e Mg procedentes da rocha hospedeira são sugeridos como responsáveis pela formação destes fosfatos. Não há quaisquer feições que os correlacionem a cristalização do pegmatito. O conteúdo de Háfnio (Hf) de zircões encontrados na zona intermediária tem sido analisado por microssonda eletrônica, e, usado como um poderoso indicador do grau de evolução do pegmatito, juntamente com o conteúdo de Mn dos fosfatos. O conteúdo Hf obtido é baixo chegando até 5% em peso de HfO2, que revela um magma pegmatítico pouco fracionado. Além disso, a presença de silicatos ferromagnesianos que constituem a mineralogia de zonas do pegmatito, como a turmalina rica em Fe, a granada e o berilo verde está de acordo com esta hipótese. Portanto, o Pegmatito Boqueirão corresponde a um pegmatito de elementos raros subtipo berilo-columbita-fosfato.

Mineralogy and technological characterization of two kaolin deposits from the Borborema Pegmatite Province, northeastern Brazil

AbstractThis research presents the mineralogical and chemical characterization and beneficiation study of two kaolin deposits from the Borborema Pegmatite Province, Brazil. Seven samples were collected and treated in two different beneficiation routes involving magnetic separation and chemical bleaching. The fractions obtained were studied by X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy, particle-size analyses, electron paramagnetic resonance and determination of optical properties. The samples are composed mainly of subhedral kaolinite in the form of booklets, as well as muscovite, quartz, microcline and illite impurities. The kaolinite structural formulae indicated significant replacement of Si4+by Al3+in tetrahedral sites and low replacement of Fe3+by Al3+in the octahedral sites. The first 30 min of chemical bleaching improved significantly the optical properties of kaolin, indicating that the process is more efficient than magnetic separation.

Formation Conditions and 40Ar/39Ar Age of the Gem-Bearing Boqueirão Granitic Pegmatite, Parelhas, Rio Grande do Norte, Brazil

The Boqueirão granitic pegmatite, alias Alto da Cabeça pegmatite, is situated in Borborema Pegmatitic Province (BPP) in Northeast Brazil. This pegmatitic province hosts globally important reserves of tantalum and beryllium, as well as significant quantities of gemstones, including aquamarine, morganite, and the high-quality turquoise-blue “Paraíba Elbaite”. The studied lithium-cesium-tantalum Boqueirão granitic pegmatite intruded meta-conglomerates of the Equador Formation during the late Cambrian (502.1 ± 5.8 Ma; 40Ar/39Ar plateau age of muscovite). The pegmatite exhibits a typical zonal mineral pattern with four defined zones (Zone I: muscovite, tourmaline, albite, and quartz; Zone II: K-feldspar (microcline), quartz, and albite; Zone III: perthite crystals (blocky feldspar zone); Zone IV: massive quartz). Huge individual beryl, spodumene, tantalite, and cassiterite crystals are common as well. Microscopic examinations revealed that melt inclusions were entrapped simultaneously with fluid inclusions, suggesting the magmatic–hydrothermal transition. The magmatic–hydrothermal transition affected the evolution of the pegmatite, segregating volatile compounds (H2O, CO2, N2) and elements that preferentially partition into a fluid phase from the viscous silicate melt. Fluid inclusion studies on microcline and associated quartz combined with microthermometry and Raman spectroscopy gave an insight into the P-T-X characteristics of entrapped fluids. The presence of spodumene without other LiAl(SiO3)2 polymorphs and constructed fluid inclusion isochores limited the magmatic–hydrothermal transition at the gem-bearing Boqueirão granitic pegmatite to the temperature range between 300 and 415 °C at a pressure from 1.8 to 3 kbar.

Mapeamento Geológico do Pegmatito Alto do Urubu, município de Picuí (PB), Faixa Seridó, Província Pegmatítica da Borborema

The Alto do Urubu Pegmatite is located in the municipality of Picuí, northern of Paraíba, in Borborema Pegmatitic Province (PPB), the area that has the greatest concentration of pegmatites in the Brazilian Northeast, corresponding to the eastern part of the Seridó Belt. PPB is known worldwide for the abundance of pegmatites, with some incidence of mineralized bodies. In the region we studied, 42 outcrops were described, in each of them the main textures and mineralogical aspects of rock were described. The geological mapping made it possible to observe the predominance of the rocks of the Seridó Formation. Schists and pegmatites intruded in the form of dikes and sills are very common in the place. The schists of the region present a dark, grayish coloration, fine to medium granulation, and are composed mainly of quartz, biotite, muscovite and granade porphiroblasts, in some cases, there are cordierite centimeter nodules. It is possible to observe in these rocks foliation Sn and Sn+1, as well as crenulations. The studied pegmatite has an elongated shape, it is about 106 meters long and 18 meters wide and N-S direction, in agreement with the foliation of the schist. It shows a well defined mineral zoning, namely: i) border zone (zone I); ii) zone III and iii) quartz core. Zone I corresponds to the outermost part of the body, measuring about 30 cm in width and consisting essentially of muscovite. Zone III is composed of quartz, feldspar and mica and it is where the majority of the minerals of economic interest (especially tantalite and beryl) are concentrated, in a smaller quantity there is the occurrence of arrojadita and espersatita. In the innermost part of the pegmatite, there is the quartz nucleus, composed of milky and rosacea quartz. The observed mineral zoning enables us to classify it as a heterogeneous pegmatite type.

Mapeamento geológico do Pegmatito Alto do Tibiri: aspectos estruturais e mineralógicos

<p>The Alto Tibiri is a pegmatite body located in the southern region of the Rio Grande do Norte, in Parelhas city, geologically inserted in the Borborema Pegmatite Province and occur intruding the mica schists of the Seridó Formation. Currently, both mica and feldspar are commercially exploited in this pegmatite body. In the field missions, 39 outcrops were described, in each one the main texture and mineralogical aspects of the outcropping rock were described. The geological mapping has allowed us to distinguish two types of schists, based on content and size of the mineral phases. There is a garnet rich schists and other with a relevant cordierite content. The studied region shows narrow pegmatite dykes, composed of quartz, feldspar, muscovite and/or biotite and black tourmaline, these dikes were classified as homogeneous. It’s possible to observe in Alto Tibiri a well- defined mineral zoning, as follows: i) border area, marked by the abundance of muscovite; ii) Zone II, composed of quartz, feldspar and mica, in which the main mineral resources occur (tantalite, spodumene, apatite, etc.) and iii) quartz pockets. The observed mineral zoning is similar to that described as mixed traditional pegmatite bodies.</p>

Gamma-rays and heat-treatment conversions of point defects in massive rose quartz from the Borborema Pegmatite Province, Northeast Brazil

An extensive characterization of trace elements and point defects in rose quartz from the Borborema Pegmatite Province (BPP) in the northeast of Brazil was carried out by complementary spectroscopic methods. The aim here was to document the change in the configuration of point defects into the quartz lattice induced by heat-treatment and ionizing radiation. The samples were extracted from the core of two granitic rare element (REL) pegmatites, Taboa (Carnauba dos Dantas, RN) and Alto do Feio (Pedra Lavrada, PB). The contents of Al, P, Ti, Ni, Fe, Ge, Li, Be, B and K were measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Polished plates were heat-treated at 500 and 1000 °C and then irradiated with 50 kGy of γ rays. Point defects were characterized by optical (UV–Vis), infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies. In the as-received condition, [AlO4/H]0 centers, Li- and B-dependent OH defects were observed. Point defects related to Al and Li species were significantly affected by heat-treatment at 1000 °C and/or γ radiation. Paramagnetic centers such as [AlO4]0, [GeO4/Li]0, [TiO4/Li]0 and [O2 3−/Li]0 were created by the diffusion of Li+ ions from their original diamagnetic centers related to substitutional Al3+ and OH-species. The smoky color developed after irradiation and the signal intensities of the paramagnetic centers were independent from the original rose color grade. The samples from the Taboa (TB) pegmatite showed the highest concentration of Al, Ti, Fe and Li elements as well as the highest signal intensities for [AlO4]0, [AlO4/H]0, [GeO4/Li]0 and [TiO4/Li]0 centers. Although TB also showed the higher concentration of B element, the intensity of the 3597 cm−1 IR band related to [BO4/H]0 centers was higher for Alto do Feio (AF) samples. This result suggests that the uptake of B into the quartz core of each pegmatite took place through different mechanisms. It was concluded that the change in the point defect configuration was essentially governed by the motion of Li species whose incorporation into the quartz lattice is closely related to Al concentration.

ASPECTOS TÉCNICOS DA LAVRA DO PEGMATITO “ALTO” FEIO, PEDRA LAVRADA, PARAÍBA

A Província Pegmatítica da Borborema (PPB) notabilizou-se ao final da II Guerra Mundial por ser importante fonte de Nb-Ta e Be para os países aliados. Nessa época de intensa produção de bens minerais definiu-se, pioneiramente, a estrutura interna de pegmatitos graníticos. A partir do entendimento da estrutura interna foi possível determinar métodos de lavra para pegmatitos da região. Normalmente nos pegmatitos homogêneos (sem zoneamento interno e de pequenas dimensões) realiza-se a extração total do corpo. Nos pegmatitos heterogêneos (com zoneamento interno – zonas I, II, III e IV) geralmente extrai-se a zona III (unidade onde ocorrem minerais econômicos). O Pegmatito Alto Feio, localizado em Pedra Lavrada, Paraíba, é um pegmatito diferenciado com grande núcleo (zona IV) de quartzo róseo de excelente qualidade. Por esse motivo a lavra ocorre nas zonas III e IV.

The role of magmatic and hydrothermal processes in the evolution of Be-bearing pegmatites: Evidence from beryl and its breakdown products

Beryl and euclase crystals from the Mina do Santino and the Jacu pegmatites in the Borborema Pegmatite Province in northeastern Brazil contain several generations of melt and fluid inclusions, which allow interpretation of P-T - X conditions responsible for beryl crystallization and for alteration of a primary pegmatitic mineral assemblage to a mixture of hydrothermal minerals (euclase, bertrandite, kaolinite, and quartz). Primary melt and fluid inclusions hosted by beryl were trapped simultaneously. However, their homogenization temperatures are significantly higher (870–900 °C) than the values previously reported for pegmatitic systems (<712 °C) and should be treated with caution. An isobaric drop of temperature resulted in the exsolution of a fluid. A low-salinity CO2-enriched phase and a saline water-rich phase were trapped in pseudosecondary inclusions in beryl at a pressure of 2.1–2.7 kbar and temperature of 390–480 °C. Cooling of the country rocks below 400 °C caused a ductile-to-brittle transition and allowed infiltration of cold groundwater, which further decreased the temperature in the system to 190–240 °C. At the same time, the pressure dropped from a lithostatic (2.1–2.7 kbar) to a hydrostatic value (0.57–0.73 kbar). Consequently, minerals deposited under magmatic conditions (feldspars and beryl) became unstable and a newly formed hydrothermal mineral paragenesis (euclase, bertrandite, kaolinite, and quartz) overprinted the earlier one. The hydrothermal fluids responsible for the alteration differ from the earlier-exsolved fluids in having a lower salinity, lower homogenization temperature, the absence of CO2, and the presence of CH4.

The diversity of B-isotope variations in tourmaline from rare-element pegmatites in the Borborema Province of Brazil

Tourmaline chemical and B-isotope compositions have been determined from 4 rare-element pegmatites and 1 barren pegmatite in the Borborema Pegmatite Province of NE Brazil, and also from a leucogranite considered to be parental to the pegmatites. The rare-element pegmatites are of the LCT (lithium–cesium–tantalum) class and they display a classic zoning structure with tourmaline present in all units but the quartz core. Tourmaline major-element composition varies systematically from schorl–dravite in the border and wall zones to varicolored elbaite in the inner zones, consistent with increasing differentiation of a pegmatite magma. Previous studies supposed a continuous variation in Li contents from schorl–dravite to elbaite but the SIMS data instead show a major gap between 0.07wt.% and 0.72wt.% Li2O, followed by a continuous variation to the maximum value of 1.8wt.%. The barren pegmatite and leucogranite contain schorl–dravite only.The full range of δ11B values in tourmaline from the Borborema pegmatites is from −20.2 to +1.6‰ (n=62), which comes close to the total known diversity of pegmatite tourmalines worldwide and is much broader than the range of the associated leucogranite (−15.1 to −13.4‰, n=3). However, more than 80% of the pegmatite tourmalines define a “main range” between −17 and −9‰ δ11B. The average value of the latter (−13.5‰, s.d.=2.3‰, n=50) is identical within error with the leucogranite, supporting suggestions of a genetic relationship. The B-isotope compositions of barren and mineralized pegmatites overlap within the main range. We find no correlation between chemical composition and B-isotope ratios. Famous for the Borboreman province are gem-quality crystals of blue “Paraíba type” elbaite. The δ11B values of blue tourmaline from the Capoeira pegmatite also fall within the main range (−16.4 to −11.9‰, n=5).Four of the 5 pegmatites studied are heterogeneous in δ11B, with typical variations of 6‰. However, there are no consistent trends in these examples from outer to inner units or from core to rim of zoned crystals that would fit any one scenario of melt evolution. Instead, the factors controlling B-isotope evolution, e.g., crystallization, fluid exsolution or mixing with external B reservoirs, appear to vary locally and can be specific to the individual pegmatite or internal zone within it. For example, the Carrascão pegmatite shows a shift from higher to lower δ11B values in tourmaline from the outer to inner zone (−14.9 to −9.1‰ vs. −20.2 to −13.9‰, resp.), and there is a parallel trend of lighter rims in zoned elbaite from the inner intemediate zone. These trends are consistent with 11B removal by tourmaline crystallization and/or fluid exsolution. An opposite trend in isotopic variation was found in zoned tourmaline from the wall zone of the Boqueirão pegmatite, whose core compositions are in the main range (−15.1 to −14.9‰) and whose rims are much heavier (−5.5 to −1.7‰). Similarly high δ11B values, about 10‰ beyond the main range, were found in wall-zone tourmaline in the barren Fazenda Turmalina pegmatite (−6.0 to +1.6‰). The patchy distribution of heavy B-isotopic compositions, found only in outer zones of pegmatites, and the strong isotopic contrast with the main range suggest the possibility of mixing with heavy B from the country rocks, a potential source being layers of marble and calc-silicate rocks.

Estudo da suscetibilidade ao escurecimento por radiação gama de quartzo róseo-leitoso da província pegmatítica da Borborema

This work has investigated the changes induced by γ-radiation on impurity-related point defects in massive rose quartz from one deposit located at The Borborema Pegmatite Province (Northeast Region, in Brazil). Samples extracted from rose and colorless (milky) quartz blocks were irradiated with doses of 60Co, from 0.5 to 96 kGy. Point defects related to Al, Ge, Li and OH were measured by optical, infrared, and electron paramagnetic resonance spectroscopy, prior and after irradiation. The contents of Al, Li, Ge, Fe, Ti and other impurities were measured by inductively-coupled plasma mass spectrometry in quartz fragments exhibiting rose, pale-rose, and milky colorations. It was found that [AlO4]0, [AlO4/H]0 and [GeO4/Li]0 were generated by the dissociation of [AlO4/Li]0 and [Li-OH] centers with doses as lower as 0.5 kGy. Above 8 kGy, the electron paramagnetic resonance signal related to [GeO4/Li]0 decreases due to the intense mobility of Li species throughout the quartz lattice, giving rise to E’1 centers perturbed by Ge. The increase in [AlO4]0 content with γ doses and the consequent rise in the intensity of smoky color were similar for both rose and colorless quartz. Scanning electron microscopy carried out in insoluble residues obtained after chemical dissolution of each type of quartz revealed the presence of nanometric fibers only in rose specimens. These results suggested that the cause of rose color in massive quartz from Borborema Pegmatite Province is probably related to the presence of dumortierite inclusions.

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 &gt;250 ppm), contrasting with 7—25 ppm (with Al &lt;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.

Tantalite-(Mn) from the Borborema Pegmatite Province, northeastern Brazil: conditions of formation and melt- and fluid-inclusion constraints on experimental studies

Carbon dioxide-rich fluid and carbonate-rich aluminosilicate melt inclusions in tantalite-(Mn) from the Alto do Giz pegmatite in the Borborema Pegmatite Province, northeastern Brazil were investigated to constrain the formation of the host crystals. The results demonstrate that in the Alto do Giz pegmatite, water- and alkaline carbonate-rich fluids and melts are responsible for the transport and deposition of tantalite-(Mn) at temperatures around 600°C and about 4 kbar. Moreover, evidence is presented to show that during crystallization of the tantalite-(Mn), three different components coexisted, which are now trapped as separate inclusions: two immiscible silicate melts (types A and B melt inclusions) and a CO2-rich aqueous fluid. We hypothesize that immiscible fluid separation may have been a critical factor in producing the water- and alkaline carbonate-rich fluids and melts necessary for Ta and Nb transport. Since the tantalite-(Mn) crystallized during pegmatite formation, this mechanism must also have implications for pegmatite genesis in general.

Geochemical and geological controls on the genesis of gem-quality "Paraiba Tourmaline" in granitic pegmatites from northeastern Brazil

The electric blue Paraiba Tourmaline (PT) occurs in northeastern Brazil only in four deposits of zoned and highly fractionated granitic pegmatites of the Borborema Pegmatitic Province (BPP). These PT-bearing pegmatites are rich in spodumene or lepidolite, and are emplaced in iron-poor quartzites or metaconglomerates of the Neoproterozoic Equador Formation. Black comb-textured dravitic tourmaline in the border zone indicates crystallization from a boron-bearing granitic magma. The PT occurs at the transition between the albite-rich inner intermediate zone (IIZ) and the quartz core of the pegmatites, in primary wedge-shaped crystals of near-end-member color-zoned elbaite, arranged in fan-like, radial or comb-textured groups of crystals. The PT most commonly forms the intermediate growth-zone of color-zoned elbaite crystals having a pink to red core and a green to dark green rind. The PT-bearing pegmatites are distinguished by intensive late replacement of primary K-feldspar, spodumene, elbaite and Li-phosphates by lepidolite. The lepidolite replacement of zoned elbaite is highly selective; it begins in the core, then affects the light blue intermediate zones, and preferentially leaves intact the green rim. PT crystals or crystal zones preserved from lepidolite replacement are rare, and occur enveloped in quartz of the pegmatite core or in vug fillings. According to electron-microprobe data on color-zoned elbaite crystals, PT crystal-zones are distinguished not only by high Cu contents, but also by higher values of Li and F (mostly fluor-elbaite), Fe concentrations near zero, and lower concentrations of Mn than the contiguous zones with other colors. Increasing Fe + Mg concentrations are responsible for the green color in the rims enveloping PT, even if Cu values remain elevated. Values of Al, X □ and Mn are slightly elevated in the pink zones in comparison with PT in the same crystal. Textural relations indicate that PT formed by direct crystallization from a pegmatite-forming melt rich in B, Li and F, at the early stage of formation of the quartz core, and prior to the appearance of secondary lepidolite and other late hydrothermal minerals. The difference between PT-bearing and other elbaite-rich pegmatites may not necessarily be the failure to produce primary PT, but also the failure to preserve it from late conversion to lepidolite. The formation of PT is linked with low content of mafic minerals in the wallrock and in the pegmatite-forming melt itself.

Espectrometria de raios gama de granitos pegmatíticos da Província Pegmatítica da Borborema (PPB), nordeste do Brasil

O presente estudo tem como objetivo a caracterização de granitos pegmatíticos localizados na região sul da Província Pegmatítica da Borborema (PPB) usando dados de aerogamaespectrométricos e de gamaespectrometria terrestre. Medidas gamaespectrométricas terrestres, dosagens radiométricas e determinações químicas por ICP-MS indicaram concentrações de urânio variando entre 0,4 ppm e 7,8 ppm, concentrações de tório entre 0,1 ppm e 21 ppm e concentrações de potássio entre 2% e 5,2%. Com base nessas características, e utilizando imagens dos canais individuais, das razões de canais e da composição ternária dos radioelementos foi possível diferenciar das rochas encaixantes regionais um conjunto de granitos pegmatíticos e também diferenciar quartzitos das regiões norte e sul da área investigada com base nas concentrações mais elevadas de tório (Th) nestes últimos. Dados de gamaespectrometria terrestre de detalhe coletados em um desses granitos pegmatíticos conhecido como granito Galo Branco, revelaram irregularidades na distribuição espacial de radioelementos, sugerindo mudanças de conteúdo e concentração de minerais acessórios durante a diferenciação e cristalização magmática da rocha. Pegmatitos encaixados em quartzitos ao sul do granito Galo Branco foram individualizados com base nas razões eU/eTh e eU/K. Os resultados revelam grande potencial dos métodos para a caracterização e mapeamento geológico de granitos e pegmatitos da PPB.

COMPOSITIONAL VARIATION OF TOURMALINE-GROUP MINERALS IN THE BORBOREMA PEGMATITE PROVINCE, NORTHEASTERN BRAZIL

Various species of the tourmaline-group are common accessory minerals in the Boqueirao, Capoeira and Quintos Li–Be–Ta–Nb-bearing, LCT-family, rare-element-class granitic pegmatites from the Borborema Pegmatite Province (BPP), northeastern Brazil. Tourmaline from the border and wall zones of the pegmatites is enriched in Mg and Fe, crystallizing as Mg-rich members of the dravite–schorl solid-solution series. Tourmaline from the intermediate zone is richer in Fe and Al+Li, crystallizing as schorl. The Fe–Mg-free and Li–Al-rich elbaite is typical of the transition between the intermediate zone and the quartz core, and of replacement pockets. The bright turquoise blue elbaite known as “Paraiba tourmaline” is produced in the Capoeira 2 and Quintos pegmatites and is distinguished by high Cu contents, up to 1.07 wt.% CuO. In other occurrences of the “Paraiba tourmaline” in the BPP, up to 2.37 wt.% CuO were reported. Differences in the geochemical evolution trend of tourmalines among the pegmatite bodies investigated (vacancy at the X site, Fe, Mg, Zn, Li and F contents), suggest that they reached variable degrees of fractionation. This observation agrees with chemical data on white mica, feldspar, garnet and gahnite, and therefore permits the use of tourmaline composition as an indicator of the degree of evolution of the hosting pegmatite. According to these data,”Paraiba tourmaline”-producing pegmatites share the following characteristics: 1) they are the most evolved pegmatites so far known in the BPP; 2) they are hosted by quartzites or metaconglomerates (iron-poor host rocks); 3) they exhibit comb-textured dravite in the border zone (early saturation in tourmaline) and 4) the elbaitic “Paraiba tourmaline” is found in the most evolved parts of the pegmatites.

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

TITANIAN IXIOLITE - NIOBIAN RUTILE INTERGROWTHS FROM THE BORBOREMA PEGMATITIC PROVINCE, NORTHEASTERN BRAZIL

Intergrowths of orthorhombic and tetragonal Nb–Ta–Ti–(Sn) oxides were at first recognized by conventional X-ray diffraction and electron-microprobe analyses in six pegmatite occurrences of the Borborema Pegmatitic Province in northeastern Brazil. In three of these cases, electron-diffraction patterns (HRTEM) on chips 0.15 μm thick obtained by focused ion-beam (FIB) sampling were necessary to confirm the preliminary identification of titanian ixiolite. Electron-microprobe analyses reveal very variable bulk-compositions and extents of compositional gaps between the coexisting tetragonal and orthorhombic phases in the Nb,Ta – Ti,Sn – Fe,Mn triangular plot. This behavior is difficult to explain as a result of a simple solvus along a pseudobinary section between rutile and tapiolite – columbite – ixiolite – wodginite. In two of six cases, the data obtained plot exactly within the compositional gaps observed between these phases in other pegmatitic provinces. This apparent violation of the gap is due to abnormally high Ti-contents in ixiolite and high levels of Ta + Nb (up to 55 wt.% Nb + Ta oxides) observed in the tetragonal phase. Therefore, in the cases studied, the observed trends and gaps seem to be the result of disequilibrium (or incomplete equilibrium) between the coexisting phases, despite the exsolution-like, cuneiform to vermicular intergrowths observed in three of the occurrences studied. Compositional growth-zoning observed in the other cases agrees with the hypothesis of disequilibrium. In five of the six cases, a partition of Ta and Fe in favor of the tetragonal phase and of Nb and Mn in the coexisting orthorhombic phase is observed; it agrees with the behavior of similar intergrowths elsewhere. The sixth case is noteworthy because of the higher Ta:Nb ratio in the ixiolite in comparison with the coexisting niobian rutile. This enrichment of Nb and Fe from core to rim represents an anomalous pattern of fractionation behavior.

Chemical composition of gahnite and degree of pegmatitic fractionation in the Borborema Pegmatitic Province, northeastern Brazil

Gahnite electron probe microanalyses from three pegmatites (Mirador, Capoeira and Quintos) of the Borborema Pegmatitic Province in northeastern Brazil allowed to determine molar compositions ranging from 86.22 to 95.41 mol% gahnite (ZnAl2O4), 1.51 to 6.62 mol% hercynite (Fe2+Al2O4), 0.11 to 5.89 mol% spinel (MgAl2O4), 1.48 to 2.74 mol% galaxite (MnAl2O4), and Zn/FeTotal atomic ratios ranging from 11.53 to 60.87. These data compared with gahnite compositions from albite subtype pegmatites from Cap de Creus, Spain (Zn/FeTotal between 10 and 60) and from the source granite of the Separation Rapids pegmatite field, Ontario, Canada (Zn/FeTotal =3.5) indicate a high degree of fractionation of the studied pegmatites. This interpretation agrees with conclusions based on compositional data of tourmaline (vacancy in X-site 0.42 to 0.49), white mica (K/Rb between 8 and 33), feldspar (K/Rb between 8 and 109), garnet (Mn/(Mn+Fe2+)between 0.93 and 0.97) and in Nb-Ta oxides (Mn/(Mn+Fe2+)&gt;0.5) in the same pegmatites.