Las Pegmatitas De Barroso-Alvão, Norte De Portugal: Anatomía, Mineralogía Y Geoquímica Mineral

Cadernos Lab. Xeolóxico de Laxe Coruña. 2011. Vol. 36, pp. 177 - 206 ISSN: 0213-4497

Las pegmatitas de Barroso-Alvão, Norte de Portugal: anatomía, mineralogía y geoquímica mineral

Pegmatites from Barroso-Alvão, Northern Portugal: anatomy, mineralogy and mineral geochemistry

MARTINS, T.1 and LIMA, A.2

(1) Geology Centre-Porto University; Rua do Campo Alegre, 687; 4169-007 Porto, Portugal ([email protected]) (2) Department of Geosciences, Environment and Spatial Planning, Rua do Campo Alegre, 687; 4169- 007 Porto, Portugal ([email protected])

Recibido: 13/12/2010 Revisado: 5/02/2011 Aceptado: 20/02/2011

Abstract

The Barroso-Alvão pegmatite field is located in the Variscan belt, in the western part of the Iberian Peninsula, Northern Portugal and it is recognised for its numerous and varied aplite- pegmatite intrusions. This is a rare-element aplite-pegmatite field with enrichment in Li, Sn, Nb>Ta, Rb, and P. Several hundreds of pegmatite bodies were identified and described in this field area intruding a variety of rock types including different metasedimentary, and granitic rocks. In this study we present the geology and mineralogy, and mineral geochemistry of different types of aplite-pegmatites bodies found at Barroso-Alvão. Their description was based on field observation, mineralogy, emplacement of the bodies and geochemical data. There were identified five different groups: intragranite pegmatites with major quartz, feldspar, muscovite, biotite, and minor tourmaline, beryl and garnet; barren pegmatites with quartz, feldspar, muscovite, and minor biotite, apatite, and beryl, among other accessories; spodumene pegmatites with spodumene, Nb-Ta minerals, and Mn-Fe-Li phosphates, along with other accessory mineral phases; petalite pegma- 178 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

tites with petalite, cassiterite, Nb-Ta minerals, Mn-Fe-Li-Ca-Al-Ce-U phosphates; and lepidolite pegmatites with albite, lepidolite, cassiterite and Sr-Al phosphate minerals.

Keywords: pegmatite, lithium, Barroso-Alvão, Hercinian orogeny, Portugal CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 179

INTRODUCTION its entirety, including its origins, petrogenesis and the distribution of different intru- The Barroso-Alvão pegmatite field has sive phases (MARTINS, 2009). Particular been studied for more than twenty years, interest was given to the following aspects: and it is recognised for a great number of the pegmatite’s relationship with the hosting aplite-pegmatite intrusions. The pegmatite rock, their internal structure and morphol- field, located in the Trás-os-Montes e Alto ogy; petrography and mineralogical descrip- Douro region, Vila Real district of North- tion; mineral geochemistry and evolution of ern Portugal, covers portions of four 1/50 the pegmatite bodies. 000 scale geological maps: 6A-Montalegre, In this paper we divide the pegmatites 6B-Chaves, 6C-Cabeceiras de Basto, and of the Barroso-Alvão area into different 6D-Vila Pouca de Aguiar. groups and describe their anatomy, relation Barroso-Alvão potential was first rec- with the country rock, mineralogy and min- ognised during a regional mapping and eral geochemistry. We use the mineral geo- granite petrology programme where several chemistry to evaluate the pegmatite degree spodumene aplite-pegmatite dykes were de- of fractionation and the evolution trend in scribed (NORONHA, 1987). Several studies this pegmatite field. focussed on the mineralogy, geochemistry, petrology and exploration aspects of these GEOLOGICAL SETTING spodumene-amblygonite bearing intrusions (e.g. CHAROY and NORONHA, 1988; The Barroso-Alvão pegmatite field is DÓRIA et al., 1989; NORONHA and located in the Variscan belt, in the western CHAROY, 1991; CHAROY et al., 1992; portion of the Iberian Peninsula (Figure 1). PIRES, 1995; AMARANTE et al., 1999; It belongs to the Galicia-Trás-os-Montes FARINHA and LIMA 2000; LIMA, 2000; geotectonic zone (FARIAS et al., 1987), CHAROY et al., 2001). close to the thrust that represents the south- Later it was identified a suite of aplite- ern boundary with the Central Iberian geo- pegmatite bodies in which petalite is a tectonic zone. The studied pegmatite field dominant phase (LIMA et al., 2003 a,b,c). is positioned to the West of the Penacova- Following this discovery a more expansive Régua-Verin fault, one of the major NNE- project was undertaken in order to under- SSW Variscan faults that affect the Iberian stand the Barroso-Alvão pegmatite field in Peninsula. 180 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Fig. 1. Simplified geology map of the Barroso-Alvão region according to the 1/50 000 scale geological maps: 6A-Montalegre, 6B-Chaves, 6C-Cabeceiras de Basto, and 6D-Vila Pouca de Aguiar.

Metasediments and deformation Three deformation phases (D1 to D3) were identified in the metasedimentary

The main hosts of the pegmatites from rocks in S1 to S3 superimposed schistosities.

Barroso-Alvão are meta-pelitic, mica-shists, The S1, S2 and S3 foliations observed in the and rarely carbonaceous or graphitic shists field were produced during the three main of upper Ordovician to lower Devonian age. ductile deformation phases of the Hercyni-

RIBEIRO et al., 2000 subdivided the sequence an orogeny (D1, D2 and D3) (NORONHA et into three distinct formations called unit Sa, al., 1981; DIAS and RIBEIRO, 1995). unit Sb, and unit Sc based on metamorphic First deformation phase (D1) generated conditions and association of lithologies: an axial planar foliation (S1) that is well pre- - Unit Sa is composed of phyllites and served at higher levels of the Galicia–Trás– mica schists intercalated with carbonaceous os–Montes Zone. The S1 foliation is parallel or graphitic schists and some quartz phyl- to the stratification (S1//S0) with main orien- lites and calcsilicate rocks. tation N120º.

- Unit Sb consists of quartzites and The D2 is responsible for a sub-hori- quartz phyllites interlayered with phyllites zontal shear cleavage (S2) that is locally up- and some carbonaceous or graphitic schists. turned vertically by the D3 phase.

Lutites and graphitic schists occur in smaller The D3 produced a locally strong, pen- proportions than in Unit Sa; etrative crenulation cleavage (S3) that is - Unit Sc is characterised by a relatively preferentially associated with ductile shear monotonous sequence of phyllites, mica zones (NORONHA et al., 1981). This was schists and quartz wackes with rare calcsili- produced during the final Hercynian duc- cate rocks. Although rare, these calcareous tile upright folding event that resulted in the rocks are important in differentiating the 120° trending folds, which overprint struc- third unit from the previous units. tures, related to phases D1 and D2. CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 181

Following D3, brittle deformation gener- A biotite isograd was also identified par- ated important faults that affect the entire allel to the andalusite isograde with three study area. The main trends of these fault mineralogical associations: systems vary from N020º - N030ºE to N-S, i) quartz + muscovite + biotite; and E-W to ENE-WSW. ii) quartz + muscovite + biotite + chlorite; Metamorphism iii) quartz + muscovite + biotite + albite +/- chlorite +/- garnet. The Central-Iberian Zone and the Gali- In addition to the regional metamor- cia–Trás–os–Montes Zone have experienced phism, we can distinguish a post-kynematic intense and widespread granitic plutonism contact metamorphism related with post-D3 and metamorphism. MARTINEZ et al. granite magmatism and local, hydrother- (1990) delineated isograds of several meta- mal alteration related to aplite-pegmatite morphic zones for the Iberian Massif. Their or quartz vein emplacement has also been distribution is related to the regional meta- recognised. It is inferred by a decussate tex- morphism, closely related to granite pluton- ture in the phyllosilicates and the develop- ism that has been complicated by subse- ment of aluminosilicate porphyroblasts of quent faulting events. andalusite and rarely sillimanite (fibrolite). Within the study area it is possible to Porphyroblasts of cordierite, garnet and bi- identify a synorogenic prograde metamor- otite are also common. All porphyroblasts phism of medium to low pressure and high are Late- to Post-D3, and some of them are temperature. The metamorphism isograds restricted to the presence of quartz veinlets are parallel to the contact of syntectonic (RIBEIRO et al., 2007). granites and to the lithostratigraphic units.

The isograds result from the Pre to Syn-D3 Granitic rocks regional thermal peak, related to prograde dynamothermal metamorphism (NO- The region hosting the Barroso-Alvão RONHA, 1983; NORONHA and RIBEI- pegmatite field contains several types of gra- RO, 1983; RIBEIRO et al., 2000). nitic rocks. In this work it is going to be used

Close to the contact with the syn-D3 Ca- the classification proposed by FERREIRA beceiras de Basto granite there are two dif- et al. (1987), which takes into account he re- ferent mineralogical associations defining lationship of the granitic rocks and the Her- the andalusite isograde: cynian orogeny. In the field area, it is possi-

i) quartz + muscovite + biotite +/- anda- ble to identify Syn-D3 granitoids and late to lusite +/- staurolite; post tectonic granitoids (Post-D3). Figure 2 ii) quartz + muscovite + biotite +/- anda- shows a very simple and schematic division lusite +/- staurolite +/- garnet +/- plagioclase. of the granites found in Barroso-Alvão. 182 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Fig. 2. Schematic representation of the different types of granitic rocks encountered at the Barroso-Alvão pegmatite field (BA). 1- Syn D3 granitoids; 2- Late to post tectonic granitoids (Post-D3); 3- Régua-Verin fault; 4- Gerês-Lovios fault. Adapted from FERREIRA et al. (1987).

The granitic rocks classified as Syn-D3 ALMEIDA (1994) described the mineral as- define the boundaries of the study area to sociation for this granitic complex: quartz, the north and southwest. In the north we anorthite, perthitic K-feldspar, biotite and find the Barroso granite (Syn to Late D3) muscovite; apatite, monazite, zircon, ilmen- that is a two-mica granite, with a medium ite, rutile, rare sillimanite and tourmaline to coarse grained and porphyroid tendency. occur as accessory minerals. The granites Petrographic study by NORONHA and constituting the complex were subsequently RIBEIRO (1983) describes deformation es- deformed by late- to post-magmatic shear pecially apparent when the granite is close to zones associated with significant hydrother- the contact with the metasedimentary rocks. mal alteration (ALMEIDA et al., 2002). The major mineral phases are orthoclase The geochemistry of the Cabeceiras de (sometimes perthitic), albite-oligoclase, bi- Basto granitic complex indicates almost otite (usually chloritised) and less abundant identical peraluminous compositions (1.2 late muscovite. As accessories, apatite, tour-

Basto is a two mica granitoid that is Syn-D3 The granitic rocks classified as Post-D3 in age (Syn to Late D3). The pluton com- define the limits of the field area to the east. plex intrudes lower Silurian metasediments The Vila Pouca de Aguiar pluton is a com- along the core of a D3 antiform defining a posite massif elongated according to NNE- large NW-SE (N130º) trending structure. SSW direction. This pluton has not been CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 183

deformed and it is discordant relatively to the on field observation, mineralogy, emplace-

D3 structures of the region: the two-mica syn- ment of the bodies and geochemical data tectonic granite and the upper Ordovician to (MARTINS 2009). There were identified lower Devonian metasedimentary sequence five different groups: intragranite pegmatites (RIBEIRO, 1998). The elongated shape of with major quartz, feldspar, muscovite, bi- this pluton extends in the same direction as the otite, and minor tourmaline, beryl and gar- Penacova-Régua-Vein fault suggesting that its net; barren pegmatites with quartz, feldspar, emplacement was controlled by this regional muscovite, and minor biotite, apatite, and tectonic structure (MARTINS et al. 2009). beryl, tourmaline, chlorite, zircon, pyrite, and monazite-(Ce); spodumene pegmatites PEGMATITES FROM BARROSO- with spodumene, Nb-Ta minerals, apatite- ALVÃO (F), montebrasite, triphylite, phosphoferrite, dufrenite, fairfieldite, chlorite, tourmaline, Several hundreds of pegmatite bodies zircon, uraminite and sphalerite; petalite were identified and described in this field pegmatites with petalite, cassiterite, Nb-Ta area (CHAROY et al. 1992, LIMA 2000, minerals, apatite-(F), montebrasite, fer- MARTINS 2009). They intrude a variety of risicklerite, eosphorite, pyrite, sphalerite, rock types including several metasedimen- uraninite, monazite-(Ce), autrenite, and tary units, and different granitic rocks. In xenotime; and lepidolite pegmatites with this study we present the different types of albite, lepidolite, cassiterite and Sr-Al phos- aplite-pegmatites bodies found at Barroso- phate minerals. Alvão (Table 1). Their description was based 184 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Table1: Different types of pegmatites that can be found at Barroso-Alvão pegmatite field.

Relation with the hosting Morphology and Type Mineralogy rock/ Other features structure Metamorphism

Major Minor

T o u r m a l i n e Qtz Ab Internal zoning; layering in the Intra Intrude the Cabeceiras de Basto Kfs Bt Main orientations: contacts and granite granitic complex. Ms Tur N010ºE and N130ºE crossing the pegmatite. Intrude Sa, Sb and Sb unit; located in the andalusite Located close Qtz Bt and biotite isograde; No obvious zoning to the granite Kfs Chl Barren tourmalinisation occurs in the although quartz-rich but intruding Ab Tur metasedimentary rocks near zones are observed. metasedimentary Ms Ap the contacts with the pegmatite rocks. bodies.

No internal zoning; Major study Intrude Sb unit; located Ab Ap-(F) aplite and pegmatite done by previous in the andalusite isograde; Kfs Mnt unit are a textural authors based Spodumene tourmalinisation occurs in the Qtz Pet feature; Discordant in outcrop and metasedimentary rocks near the Spd CT with D ; usually extensive drill contacts. 3 concordant with D2. core.

The maximum Intrude Sb and Sc unit; located No obvious internal length for in the andalusite and biotite zoning although o b s e r v a t i o n Pet Mnt isograde; tourmalinisation couplets of aplite of exposed Ab Spd without preferential orientation and pegmatite are outcrop was Petalite Ms Cst in the contacting hosting rock; observed; usually 2.80 m; Nigerite

Qtz CT “pinched and swelled” quartz discordant with D3 described on veins occur in the metasediments structures although contacts drill core

near the contact. concordant with D2. description and sampling. I n t e n s e No complicated Intrude Sb unit; located w e a t h e r i n g ; Lpd Ms internal zoning in the biotite isograde; sugary albite Lepidolite Ab Cst observed. Discordant tourmalinisation in the host is the major Qtz CT with D ; usually rocks. 3 component of the concordant with D . 2 pegmatitic body.

Sa, Sb, and Sc are metasedimentary units defined by RIBEIRO et al. (2000). (For detailed description see Geological setting). Qtz- quartz, Kfs- K-felspar, Ms- muscovite, Ab- albite, Bt- biotite, Tur- tourmaline, Chl- chlorite, Ap- apatite, Spd- spodumene, Mnt- montebrasite, Pet- petalite, CT- columbite-tantalite, Cst- cassiterite, Lpd- lepidolite. CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 185

Intragranite pegmatites ly shows internal zoning with concentration of quartz in the centre (an incipient quartz The studied pegmatite bodies outcrop in core), and K-feldspar and muscovite in the the Cabeceiras de Basto granite and in the margins. The muscovite and K-feldspar usu-

Barroso batholith (all Syn-D3 granitic bod- ally develop comb structure, growing per- ies). Intragranite pegmatites are barren and pendicular to the walls of the pegmatite, as composed of quartz, K-feldspar, Na-rich it is possible to see in Figure 3. Tourmaline, plagioclase, muscovite and biotite as major when present, can either define a layering in mineral phases; tourmaline, apatite, chlo- the margin of the pegmatitic body or in the rite, beryl, and garnet as accessory mineral host rock or can appear completely spread phases; and trace amounts of zircon, and without any order or alignment. iron oxides. This type of pegmatite frequent-

Fig. 3. Polished surface of an intragranite pegmatite. It is possible to observe muscovite (Ms), K-feldspar (Kfs) and quartz (Qtz) growing perpendicular to the walls of the pegmatite. Qtz is accumulated in the middle of the pegmatite. (The coin is used for scale.)

Barren pegmatites pegmatite field. The metamorphic conditions of the metasedimentary rocks corre- Barren pegmatites are intruded into spond to the andalusite or biotite isograde. phyllites and mica schists metasedimentary These types of pegmatites frequently show units. They can be located close to the con- associated aplitic facies and rarely present tact with the Cabeceiras de Basto massif but a crude layering. The aplites are composed they are also found spread throughout the of quartz, K-feldspar, plagioclase and mus- 186 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

covite. The pegmatitic facies of the barren swells in accordance with the ductility of pegmatites is composed of quartz, K-feld- their hosting rocks and a large majority of spar, Na-rich plagioclase and muscovite. these bodies exhibits a structural control. Accessory mineralogy consists of biotite, The orientation of the pegmatites is chlorite, tourmaline, apatite, garnet, zircon, mainly controlled by the S2 foliation, which pyrite, monazite, and Fe oxides. has been locally deformed by D3 (striking at Some pegmatites from this group fre- 120°E on average). The internal structure quently show incipient internal zoning with of the pegmatitic bodies is not easy to es- quartz concentration that resemble an incip- tablish and they are considered internally ient quartz core, and growing of K-feldspar unzoned. Textures were locally described and muscovite perpendicular to the walls. as aplite-pegmatite couplets that alternate Different textural units with predominant rhythmically. A contemporaneous aplitic quartz and muscovite are observed. Units unit is randomly and intimately mixed with with K-feldspar as the dominant mineral a largely granular pegmatitic unit. The con- phase are frequently observed as well, al- tacts between the pegmatite bodies and en- though they are not present in all the studied closing schists are sharp. Tourmalinisation pegmatites. K-feldspar goes up to 20 cm in was observed in the meta-pelitic units but length and it is fractured. Crude layering of there is no visible metasomatic alteration of garnet is often observed. the walls (CHAROY et al., 1992). LIMA (2000) describes and characterises Spodumene pegmatites three of these spodumene pegmatite bodies. K-feldspar, quartz, spodumene and musco- Pegmatites with spodumene were the vite are the main mineral phases. Montebra- first lithium-bearing bodies described at site, triphylite, fluorapatite, dufrenite, phos- the Barroso-Alvão pegmatite field (NO- phoferrite, fairfieldite, interstitial petalite, RONHA, 1987). The initial discovery report tourmaline, eucryptite, illite, montmorillon- was followed by several studies focused on ite, chlorite, cookeite, sulphides, and organic spodumene aplite-pegmatite bodies (e.g. matter are the accessory phases. Abundant CHAROY and NORONHA, 1988; NO- petalite, beryl, columbite-tantalite, pyrite RONHA and CHAROY, 1991; CHAROY and uraninite were later described by MAR- et al., 1992; FARINHA and LIMA 2000; TINS (2009). LIMA, 2000). According to CHAROY et al. (1992, 2001) this type of pegmatite bod- Petalite pegmatites ies is unevenly distributed. They form local swarms of several bodies of various sizes Petalite pegmatites are intruded in a up to 100 meters in outcrop width. Their metasedimentary sequence composed of average thickness is variable, from less than phyllites, mica schists and quartz wackes. a few meters up to ten meters across. All The petalite pegmatites are composed of pegmatites display irregular patterns in out- an aplitic and pegmatitic unit. Like the crop: some are flat lying; others have gently spodumene pegmatites, internal structure is or steeply dipping attitudes along strike. difficult to establish, so they are considered The largest occurrence usually pinches and internally unzoned with randomly and inti- CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 187

mately mixed texture of the aplite and the columbite-tantalite, SQUI (isochemical pegmatite units. Locally it was described breakdown of petalite to spodumene plus a vertical zoning of aplite-pegmatite cou- quartz described by ČERNÝ and FER- plet texture that alters rhythmically. Pet- GUSON, 1972), beryl, eucryptite, cookeite, alite pegmatites are composed of petalite, ferrisicklerite, and Fe and Mn oxides are Na-rich plagioclase, quartz, K-feldspar observed; trace amounts of zinnwaldite, and muscovite as major mineral phases montmorillonite, pyrite, zircon, monazite, (Figure 4). As for accessories, montebra- uraninite, sphalerite, bastnaesite, eospho- site, spodumene, fluorapatite, cassiterite, rite, thorite, and loellingite.

Fig. 4. Hand sample of a petalite pegmatite. 1- petalite crystals; 2- mass of feldspars; 3- white masse of cryp- tocrystalline petalite; 4- Mn oxide. (The pen cap is used for scale)

Lepidolite pegmatites tin and with an outcrop reduced to barely a dump and it is exposed to severe weath- In Barroso-Alvão, pegmatites with ering. Samples collected from this outcrop lepidolite are limited to two occurrences: reveal the pegmatite is composed of fine- to CHN26 and CHN27, described by CHA- medium-grained weathered (almost sugary) ROY et al. (1992). The brief reference to albite, fine- medium- to coarse-flaked pale these pegmatites mainly focuses their strong violet to pink lepidolite, and quartz. Musco- enrichment in Li, Rb and F, based on whole vite, cassiterite, apatite and beryl are acces- rock geochemistry. MARTINS (2009) stud- sories. Trace amounts of columbite-tantal- ied a body with lepidolite mined out for ite, zircon and goyazite are identified. 188 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

METHODOLOGY determinations as described (Donavan and Tingle, 1996). A 1-2 micron beam diameter Petrographic study was carried out on was used for feldspars, micas and phos- a Nikon Eclipse E 400 POL polarising mi- phates. For beam sensitive phosphates the croscope equipped with transmission and beam diameter was adjusted to 3-4 microns. reflection light (objectives LU Plan 5x/0.15 Standards used for analyses of feldspars, POL ∞/0 EPI; 10x/0.30 POL ∞/0 EPI; micas and phosphates included: Clinopy- 20x/0.45 POL ∞/0 EPI; and 50x/0.80 POL roxene (Si Kα, Fe Kα, Ti Kα, Ca Kα, Mg ∞/0 EPI). Software Axiovision 3.1. was used Kα), Albite (Si Kα , Al Kα, Na Kα), Fibbia for photographic recording. adularia (Si Kα, Al Kα, K Kα), Plagioclase Representative samples of each type An50 (Si Kα, Al Kα, Ca Kα), Cerro de Mer- of pegmatite were crushed to centimetre cado apatite (P Kα, Ca Kα, F Kα), Fayalite size and then reduced to millimetre size. (Si Kα, Fe Kα), Rhodonite (Si Kα, Mn Kα), Crushing was followed by a heavy mineral Andalusite (Si Kα, Al Kα), Fluortopaz (Al separation using lithium metatungstate. Kα, Si Kα, F Kα), Strontium sulfate (Sr Lα), The corresponding heavy (> 2.80 g/ml) and Barium sulfate (Ba Lα), Pollucite (Cs Lα) light fractions (< 2.80 g/ml) were examined and Rubidium Leucite (Rb Lα). Standards under a binocular microscope, mounted in for feldspars, micas, and phosphates includ- round disks, polished and analysed. Scan- ed: Periclase, hematite, vanadium (V) oxide, ning Electron Microscope Energy dispersive zinc oxide, quartz, fluorite, as well as any of spectral analysis, and BSE images of pol- the above where applicable. ished mounts and mineral samples affixed Operating conditions for Nb-Ta oxides to conductive putty were collected using a included: 25 kV acceleration potential, 20 digital AMRAY 1820 scanning electron mi- nA beam current and 60 seconds counting croscope. Operating conditions for energy time. MAN was used for background deter- dispersive spectral analysis, and BSE im- minations. A 2-3 micron beam diameter was agery included an acceleration potential of used for analyses of Nb-Ta oxides. Stand- 15-25 kV, 400 micron final aperture, 18 mm ards used for analyses of Nb-Ta oxides: Zir- working distance, 0 to 35 degrees sample tilt, conium (IV) oxide (Zr Lα), manganotanta- and 3.0 spot size. BSE images were collected lite (Ta Mα, Mn Kα), yttrium niobate (Nb using a frame size of 512x512 pixels and X- Lα), Tin (IV) oxide (Sn Lα), Hematite (Fe ray maps collected at 256x256 pixels with a Kα), rutile (Ti Kα), Bismuth germanate (Bi 20-50 ms dwell time per pixel. Data and im- Mα) and Lead (II) oxide (Pb Mα). MAN ages were acquired using Iridium/EDS2008 standards for Nb-Ta oxides included: Peri- software by IXRF Systems, Inc. clase, hematite, vanadium oxide, quartz, Electron microprobe analyses were car- fluorite, zinc oxide, Harding microlite, as ried out on a ARL SEMQ Electron Micro- well as any of the above where applicable. probe. Operating conditions for feldspars, Data were processed via Probe for Windows micas, and phosphates included: 15 kV ac- by MicroBeam, Inc. celeration potential, 15 nA beam current Lithium was measured in muscovite, and 60 seconds counting time. Mean Atomic K-feldspar and lepidolite separates using a Number (MAN) was used for background Beckman Spectraspan V, Direct-Coupled CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 189

Plasma (DCP). The mineral separates were can be found in Table 2. Representative ground into powder. A sample of 200 mg analyses are available in Table 3 and 4. The was diluted in 0.035 l of hydrofluoric acid analysed K-feldspar from the studied pegm- (HF). The wavelength used for Li was atites has very similar composition and it 610.362 nm and the counting time was 10 is almost pure orthoclase (%Or= 93 – 99). seconds; detection limit for Li= 0.002 mg/l. As it is possible see in Table 4, the studied albite from the different types of pegma- RESULTS AND DISCUSSION tites also has a similar chemical composition. It is very close to the albite end mem- The mineralogy of the different groups ber. Anorthite percentage variation for the of pegmatites is variable and becomes more studied pegmatites is as follow: intragranite complex from intragranite pegmatites to pegmatites- (An0-1), spodumene pegmatites- lepidolite pegmatites. Quartz, feldspars and (An0-3) (CHAROY et al., 2001; LIMA 2000), muscovite are always present, but minerals petalite pegmatites- (An0-1) lepidolite pegm- such as tourmaline, garnet, and phosphates, atites- (An0). spodumene, petalite and lepidolite, are only We anticipated that the trace element present in certain groups of pegmatites. The content of these feldspars would allow us petrographic characteristics of the identified to trace fractionation and evolution in the minerals are listed in Table 2. In this section pegmatitic field. We predicted that the feld- we present the mineral geochemistry of var- spars associated with the less evolved in- ious mineral phases of the studied pegma- tragranite pegmatites were going to be the tites. poorest in Rb and Cs, and show higher K/ Rb ratios than those from the Li-rich (spo- Feldspars dumene, petalite, and lepidolite pegmatites). The Li-enriched ones were predicted Feldspars are present in all the studied to have the feldspars richest in Rb and Cs, pegmatitic bodies. In intragranite, and bar- and one of the poorest in Ba and with the ren pegmatites, K-feldspar is dominant over lowest K/Rb ratios, as showed in the litera- albite. On the other hand, in spodumene, ture by many authors (e.g. RODA-ROB- and petalite pegmatites, albite is dominant LES 1993; NEIVA 1995; LARSEN 2002; over K-feldspar. In lepidolite pegmatites, K- ALFONSO 2003). P2O5 in feldspars was feldspar is not present and albite is the only also measured once it can be considered identified feldspar. a sign of evolution and can represent the Petrographic characteristics for the feld- amount of this element in the pegmatitc spars from the different types of pegmatites melt. 190 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Table 3. Representative chemical composition of K-feldspar from the different types of pegmatites. Intra Intra Spodu Spodu Pegmatite type Barren Barren Petalite Petalite granite granite mene mene

Al2O3 wt.% 18.41 18.44 18.81 18.79 18.51 18.50 18.58 18.70

SiO2 64.52 64.49 64.66 64.62 64.54 64.41 64.40 64.40

TiO2 0.03 0.02 0.01 0.02 0.05 0.04 0.01 0.00 FeO 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00 CaO 0.02 0.02 0.00 0.01 0.01 0.00 0.00 0.00

K2O 15.28 15.44 15.48 15.45 15.50 15.51 15.56 15.49 MgO 0.00 0.00 0.00 0.01 0.01 0.04 0.00 0.00 MnO 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00

Na2O 0.32 0.34 0.28 0.32 0.32 0.22 1.05 0.67

P2O5 0.01 0.02 0.02 0.02 0.01 0.02 0.00 0.00 BaO 0.02 0.02 0.01 0.02 0.01 0.01 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Rb2O 0.36 0.38 0.31 0.33 0.28 0.29 0.39 0.41

Table 4: Representatitve chemical composition for the albite from the studied pegmatites.

Intra Pegmatite type Barren Petalite Lepidolite granite

MgO (wt.%) 0.00 0.00 0.00 0.00

Al2O3 19.62 19.70 19.59 19.64

SiO2 68.55 68.65 68.64 68.73

TiO2 0.00 0.00 0.01 0.00 FeO 0.00 0.00 0.00 0.00 CaO 0.14 0.14 0.08 0.06

K2O 0.04 0.03 0.04 0.03 MnO 0.00 0.00 0.00 0.00

Na2O 11.40 11.40 11.55 11.53

P2O5 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 Total 99.75 99.79 99.92 99.99

Apfu on the basis of 8 atoms of oxygen

Mg (apfu) 0.000 0.000 0.000 0.000 Al 1.022 1.024 1.019 1.021 Si 2.985 2.984 2.986 2.986 Ti 0.000 0.000 0.000 0.000 Fe2+ 0.000 0.000 0.000 0.000 Ca 0.007 0.006 0.004 0.003 K 0.002 0.002 0.002 0.002 Mn 0.000 0.000 0.000 0.000 Na 0.977 0.975 0.988 0.986 P 0.000 0.000 0.000 0.000 Ba 0.000 0.000 0.000 0.000 Zn 0.000 0.000 0.000 0.000

%An 0.68 0.63 0.39 0.28 %Ab 99.07 99.09 99.39 99.56 %Or 0.33 0.28 0.23 0.16

apfu- atoms per formula unit; An- anorthite; Ab- albite; Or- orthoclase CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 193

Fig. 5. Charts representing the variation of K/Rb ratio with Ba, Cs, and Rb for the different types of pegmatites (a-c). Plotted with data from EPMA. All values in ppm. Pegmatite types as described previously. (continued...) 194 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

As it is possible to observe in Table 3 and Table 5 presents representative chemi- Figure 5, our predictions were not observed. cal composition of the studied micas for the Unfortunately it is not possible to see geo- different types of pegmatites. It was selected chemistry evolution taking into consid- primary muscovite from all of the different eration the concentration of incompatible types of pegmatites. The different studied elements or the decrease in the K/Rb ratio muscovite has chemical compositional vari- (Figure 5 and Table 3). This may be due to ation, as it is possible to observe in Figure K-feldspar incapacity of keeping its primary 6 and Table 5. BSE imaging of the studied chemical attributes or they were modified or muscovite does not show chemical zoning lost as argued by LONDON (2008). and quantitative analyses show constant chemical composition either on their cores Micas and rimes. It is possible to observe that the mus- Muscovite is found in all the pegmatites covites from intragranite, barren, and pet- types described for Barroso-Alvão, whereas alite pegmatites present a variation in their other type of micas such as biotite, chlo- Al content. Micas from lepidolite pegma- rite, and lepidolite are exclusive of certain tites are the most impoverished in FeO and groups of pegmatites. Biotite and chlorite MgO. Trace element content shows differ- were only observed in intragranite, and bar- ences between the micas from the different ren pegmatites; lepidolite is only observed in types of pegmatites (Figure 6). Lepidolite lepidolite pegmatites. Detailed petrographic is the richest in Rb (4682 – 5596 ppm), Cs characterization can be found in Table 2. (424 – 565 ppm), F (44460 – 55520 ppm), CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 195

and Li (12246 – 21842 ppm). Intragranite and MnO (0.03 – 0.05 wt.%), and the most pegmatites are the most enriched in TiO2 depleted in Rb (2094 – 3145 ppm), Cs (170 – (0.02 – 0.03 wt.%), FeO (1.32 – 1.46 wt.%), 311 ppm) (MARTINS, 2009).

Table 5: Representative chemical composition of the EMPA analyses in muscovite from the studied pegmatites. Intra Barren Spodumene Petalite Lepidolite granite

SiO2 (wt.%) 47.36 46.85 45.86 46.83 50.22

TiO2 0.02 0.02 0.01 0.01 0.00

Al2O3 34.90 35.97 37.89 36.63 27.11 FeO 1.42 0.93 1.08 1.19 0.01 MnO 0.04 0.03 0.02 0.04 0.21 MgO 0.02 0.03 0.02 0.02 0.00 CaO 0.03 0.03 0.03 0.03 0.10

Li2O * 0.78 0.38 0.05 0.27 3.61

Na2O 0.11 0.11 0.09 0.09 0.30

K2O 9.45 9.42 9.88 9.92 9.60

Rb2O 0.32 0.35 0.44 0.44 0.57

Cs2O 0.03 0.03 0.03 0.03 0.05 F 1.73 1.62 1.47 1.59 2.48 Total 1 96.21 95.76 96.63 97.10 94.26 O=F 0.73 0.68 0.62 0.67 1.05 Total 2 95.48 95.08 96.01 96.43 93.22 Apfu in the basis of 22 oxygens Si (apfu) 6.191 6.052 5.901 6.001 6.485 Al (IV) 1.809 1.948 2.099 1.999 1.515 Sum Z 8.00 8.00 8.00 8.00 8.00 Ti 0.002 0.002 0.001 0.001 0.000 Al (VI) 3.568 3.529 3.647 3.533 2.611 Fe 2+ 0.156 0.101 0.116 0.127 0.001 Mn 0.004 0.003 0.003 0.005 0.023 Mg 0.005 0.005 0.004 0.005 0.000 Li 0.412 0.201 0.027 0.168 1.917 Sum Y 4.15 4.17 3.96 4.01 4.80 Ca 0.004 0.004 0.003 0.004 0.014 Na 0.028 0.027 0.024 0.023 0.074 K 1.576 1.552 1.621 1.622 1.581 Rb 0.027 0.029 0.036 0.037 0.048 Cs 0.002 0.001 0.002 0.002 0.003 Sum X 1.64 1.61 1.69 1.69 1.72 F 0.714 0.661 0.596 0.645 1.014

K (ppm) 78403 78170 81954 82304 79620 Rb 2926 3160 3992 4056 5234 K/Rb 27 25 21 20 15 Cs 255 252 315 262 509 Li 3640 1768 238 1275 16758 apfu- atoms per formula unit; *- calculated using the equation 2 Li2O = (3.735*F) – 5.6669 (R = 0.9705; n= 6) according to TISCHENDORF et al. (1999) 196 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Fig. 6. Variation of the K/Rb ratio with Rb for micas from the different types of pegmatites. Pegmatite types: square- In- tragranite pegmatites; diamond- Barren pegmatites; cross- Spodumene pegmai- tes; circle- Petalite pegmatites; triangle- Lepidolite pegmatites.

The K/Rb ratio, and the enrichment in CHAROY et al. (1992) describes lepidolite incompatible elements such as Li, Cs, and aplite-pegmatite veins exhibiting low-grade Cs, are one of the most valuable indica- tin mineralization (0.1 to 0.3 vol. %). Those tors of fractionation in micas (ČERNÝ et veins were mined, wherever argilitic altera- al. 1985). With fractionation, the K/Rb ra- tion made digging by hand possible. This tio tends to decrease. On the other hand, type of cassiterite was not found thus it is the enrichment of incompatible elements not characterized in this study. The studied tends to increase. In this study we observe cassiterite is observed in petalite pegmatites decreasing of the K/Rb ratio in the studied and lepidolite pegmatites. It is considered micas in intragranite pegmatites towards primary based on petrographic and micro- the lepidolite pegmatites (Figure 6). Thus, textural observation. using this indicator it is possible to observe In petalite pegmatites, cassiterite is that the intragranite pegmatites are the most chemically homogenous (SnO2 content var- primitive and the lepidolite pegmatites, the ying from 94.24 to 99.01 SnO2 wt.%) even most evolved. Spodumene pegmatites, and when close to the smallest identified inclu- petalite pegmatites have similar evolution sion of columbite group minerals (CGM). indicators. Table 6 shows representative analyses of cassiterite from petalite pegmatites and lepi- Nb-Ta-Sn oxides dolite pegmatites. It is possible to observe that cassiterite from petalite pegmatites is

In the area it was reported frequent�� oc- enriched in Nb2O5, Ta2O5, MnO, FeO, and currence of cassiterite in the greisens sur- WO3 when compared with the cassiterite rounding the pegmatites, exploited for tin on from lepidolite pegmatites that is almost 100 a small scale after the Second World War. wt.% SnO2. CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 197

Table 6. Representative chemical composition of electron microprobe (EPMA) analyses of cassiterite from petalite pegmatites and lepidolite pegmatites.

Pegmatite type Petalite Lepidolite

SnO2 97.61 98.74 FeO 0.33 0.21 MnO 0.16 0.04 MgO 0.03 0.05 CaO 0.00 0.00 Total 100.05 99.28

Atoms per formula unit calculated based on 4 oxygens

W (apfu) 0.002 0.000 Ta 0.018 0.002 Nb 0.006 0.001 Ti 0.007 0.001 Sn 1.948 1.987 Fe 0.014 0.009 Mn 0.007 0.002 Mg 0.002 0.003 Ca 0.000 0.000 n- number of analyses; apfu- atoms per formula unit

Columbite group minerals (CGM) are only appear as discrete grains in the aplitic only present in the Li-enriched pegmatites: and pegmatitc ground mass. In the petalite spodumene, petalite, and lepidolite (Figure pegmatites the CGM grains are found in 7). CGM are considered of primary crystal- the ground mass as well, and as inclusions lisation based on petrography and micro- in cassiterite. These minerals are inclusions textures observation. CGM show complex and not exsolutions because it is not ob- internal chemical composition with Nb- and served a decreasing in the Nb2O5 and Ta2O5 Ta-rich cores and rimes. This suggests crys- content of the host cassiterite when close to tallisation at disequilibrium. In spodumene the CGM grains. and lepidolite pegmatites CGM minerals 198 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

Fig. 7. BSE imaging of different aspects of CGM from the Li-enriched pegmatites. CGM has complex chemical zoning. In the CGM inclusions, the darkest areas correspond to Nb enrichment, and the bright- est areas correspond to the Ta enrichment. (a) Elongated CGM with an oscillatory growth pattern. Num- bers 1 to 4 represent the analysed areas of the crystal. In this example we have a Ta rich core, surrounded by Nb rich concentric zones, and a Ta enriched rime. (b) Skeletal CGM evidencing sharp contacts with the host cassiterite (light grey colour). (c) CGM from a lepidolite pegmatite.

All of the CGM of the three types of very close to 0%wt. Except for the CGM pegmatites exhibit variable and distinct minerals described as inclusions in cassiter- chemical composition expressed in Table 7, ite from the petalite pegmatites; these are the and can be classified as columbite-(Fe), tan- most enriched in SnO2. The CGM minerals talite-(Fe), and columbite-(Mn). The SnO2 from spodumene pegmatites are the most content in all the discrete CGM from all the enriched in TiO2 that gradually decreases to- different types of pegmatites is constant and wards the lepidolite type (Table 7). CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 199

Table. 7. Representative analyses of discrete grains of columbite-(Fe) from spodumene pegmatites; columbite– (Fe) grain included in cassiterite (Cst) and discrete columbite grains from petalite pegmatites; discrete grains of strongly Mn–enriched columbite–(Mn) and tantalite–(Mn) from lepidolite pegmatites.

Pegmatite type Spodumene Petalite Lepidolite

discrete grains discrete grains incl. in Cst discrete grains

MnO wt.% 4,57 4,98 4,78 4,43 15,81 17,59 FeO 13,38 13,89 12,54 12,44 0,17 0,18

Ta2O5 43,80 32,21 49,65 38,54 60,93 41,62

Nb2O5 34,56 43,96 31,90 42,68 23,75 40,45

SnO2 0,03 0,03 0,02 0,00 0,01 0,01

TiO2 3,22 3,12 0,79 1,68 0,00 0,00

WO3 0,31 0,29 0,16 0,19 0,25 0,20 Total 99,87 98,49 99,90 100,01 100,92 100,07 A site Mn 1,046 1,098 1,134 0,988 3,928 4,012 Fe2+ 3,025 3,026 2,940 2,738 0,041 0,041 B site Ta 3,221 2,281 3,784 2,758 4,861 3,049 Nb 4,224 5,175 4,041 5,077 3,150 4,925 Sn 0,003 0,003 0,002 0,000 0,001 0,001 Ti 0,656 0,612 0,166 0,332 0,000 0,000 W 0,022 0,019 0,012 0,013 0,019 0,014 Mn/(Mn+Fe) 0,26 0,27 0,28 0,27 0,99 0,99 Ta/(Ta+Nb) 0,43 0,31 0,48 0,35 0,37 0,38 Cation formula based on 24 atoms of oxygen. Oxides in wt.%

In what respect to fractionation, there that the trend starts from the region Mn-poor is an overlapping of the CGM for the spo- columbite-(Fe) towards a slightly Mn- and dumene pegmatites and the ones hosted in Ta- enriched columbite-(Fe) and tantalite- cassiterite from the petalite pegmatites (Fig- (Fe). The Ta/(Ta+Nb) values are maintained ure 8). The Mn and Ta content increases and enters the field of columbite-(Mn) and from the CGM minerals of the spodumene tantalite-(Mn) as the Mn/(Mn+Fe) ratio ap- pegmatites and the inclusions from the petal- proaches 1.0 for the lepidolite pegmatites. ite pegmatites towards the lepidolite pegma- CGM from Barroso-Alvão can be used tites. From Figure 8, it is possible to observe to observe regional geochemical fractiona- 200 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

tion taking into account the Li-enriched based on the Mn/(Mn+Fe) and Ta/(Ta+Nb) pegmatites. We propose a higher degree of ratios observed in this pegmatite’s CGM. fractionation for the lepidolite pegmatites

Fig. 8. Compositions of CGM from the three different pegmatite types plotted in the quadrilateral diagram columbite-(Fe), columbite-(Mn), tapiolite, tantalite-(Mn). The dashed arrow indicates the evolution compositional trend. Open squares: discrete CGM from spodumene pegmatites; Open circles: CGM inclusions in cassiterite from petalite pegmatites; Filled circles: discrete CGM from petalite pegmatites; Triangles: discrete CGM from lepidolite pegmatites.

CONCLUDING REMARKS resentations in a plan view compared to a three-dimensional approach. For example, The pegmatite bodies of the Barroso- just by having a different position of the ero- Alvão pegmatite field were studied taking sion level it is possible to observe different into account several parameters, which al- fractionated patterns outcropping. Another lowed a division into different groups. The example to observe this, and considering the pegmatite field has a rare-element enrich- same erosion level, will be the existence of ment with major enrichment in Li, Rb, Be, a basement of granitic massifs structures in Sn, Nb, and Ta. Enrichment in Ga, B and Cs domes not observed outcropping (RODA- is not observed. ROBLES et al., 2007). Regional zoning in this pegmatite field Muscovite was the mineral where the is not easy to observe but it seems to be degree of fractionation was evaluated more related with the syn-D3 granitic intrusions efficiently amongst all the studied pegmatite in the area. One cannot forget the two di- types. In muscovite, the K/Rb ratio decreas- mensional limitations of schematically rep- es with fractionation. From the mica study, CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 201

it is possible to observe that the evolution ROY & NORONHA (1999). This is in part increases from intragranite pegmatites to- due to the severe weathering that affects the wards the lepidolite pegmatites. Muscovites studied lepidolite pegmatite. Further studies from intragranite pegmatites are the most are required to get a better understanding of primitive with the highest K/Rb ratio. The the fractionation process in Barroso-Alvão lepidolite from the lepidolite pegmatites has and trying to define the relationship between the lowest K/Rb value of the studied pegm- the pegmatites and the larger granite bodies atites, thus they are considered the most that outcrop in the region. evolved. Columbite group minerals can be used to ACKNOWLEDGMENTS observe regional geochemical fractionation for the Li-enriched pegmatite bodies of the This work is part of a PhD thesis and Barroso-Alvão pegmatite field. CGM from benefited from financial support from the spodumene pegmatites and CGM inclu- Fundação para a Ciência e Tecnologia, sions in cassiterite from the petalite pegma- Portugal. The first author would like to ac- tites have a similar chemical composition. knowledge all opportunities created by her However, the CGM discrete grains from the adviser, and all of the shared knowledge and petalite pegmatites are more MnO enriched. time spent abroad with scientists from differ-

Even if there it is observed a similar Ta2O5 ent Universities. The authors would like to content for the CGM in pegmatites, it is express appreciation to William Simmons, proposed a higher degree of fractionation Karen Webber and Al Falster at Univer- for petalite pegmatites based on the MnO sity of New Orleans, USA for the analyses enrichment. Mn/(Mn+Fe) and Ta/(Ta+Nb) and numerous discussions. Comments from ratios are higher for lepidolite pegmatites Fernando Noronha are truly acknowledged than for the spodumene pegmatites and petalite pegmatites. These observations allow REFERENCES proposing a higher degree of evolution for the lepidolite pegmatite. ALFONSO P., MELGAREJO J., YUSTA Based on mineral geochemistry it is pos- I. and VELASCO I. (2003). Geochemis- sible to observe a trend between intragranite try of feldspars and muscovite in granitic pegmatites, barren pegmatites, spodumene pegmatite from the Cap de Creus Field, pegmatites, petalite pegmatites, and lepido- Catalonia, Spain. Canadian Mineralogy lite pegmatites. We consider the intragranite 41, 103-116. and barren pegmatites less evolved whereas ALMEIDA A. (1994). Geoquímica, petrogé- the Li-bearing ones are highly evolved, and nese e potencialidades metalogénicas dos rare-element rich.�� Nonetheless the fraction- granitos peraluminosos de duas micas do ation process is not completely understood complexo de Cabeceiras de Basto. ��(Geo- in Barroso-Alvão. It is undeniable there is chemistry, petrogenises and metalogenetic evolution in this pegmatite field but there are potencial of the two mica peraluminous not enough data to sustain that the observed Cabeceiras de Basto granitic complex). evolution is only related with a single frac- Ph. D. thesis. University of Porto, Portu- tionation process as already stated by CHA- gal. 305 p. (in Portuguese). 202 ��Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011)

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