Barren and Li–Sn–Ta Mineralized Pegmatites from NW Spain (Central Galicia): A Comparative Study of Their Mineralogy, Geochemistry, and Wallrock Metasomatism

In Central Galicia, there are occurrences of barren and Li–Sn–Ta-bearing pegmatites hosted by metasedimentary rocks. A number of common and contrasting features between Panceiros pegmatites (barren) and Li–Sn–Ta mineralized Presqueira pegmatite are established in this study. K-feldspar and muscovite have the same trace elements (Rb, Cs, P, Zn, and Ba), but the mineralized one has the highest Rb and Cs and the lowest P contents. The barren bodies show fluorapatite and eosphorite–childrenite replacing early silicates. The mineralized body has primary phosphates (fluorapatite and montebrasite), a metasomatic paragenesis (fluorapatite and goyazite) replacing early silicates, and a late hydrothermal assemblage (vivianite and messelite). Ta–Nb oxides from the Presqueira body show a trend from columbite-(Fe) to tantalite-(Fe) and tapiolite(Fe). In this body, the Li-aluminosilicate textures support primary crystallization of petalite that was partially transformed into Spodumene + Quartz (SQI) during cooling, and into myrmekite during a Na-metasomatism stage. As a result of both processes, spodumene formed. The geochemical study supports magmatic differentiation increasing from the neighboring granites to the Li–Sn–Ta mineralized pegmatite. In both barren and mineralized bodies, the pegmatite-derived fluids that migrated into the wallrock were enriched in B, F, Li, Rb, and Cs and, moreover, in Sn, Zn, and As.


Introduction
Some rare elements, mainly those that support high-technology industry, have an increasing demand due to their variety of uses in new technologies. Among these elements, also known as high-technology metals, Ta, Li, and Sn are in high demand owing to their use in capacitors for wireless technology, in battery applications, and in liquid crystal displays (LCD) and touch-screen displays. These metals can occur in high concentrations in pegmatites and occasionally form ore deposits. Consequently, data to improve the prospecting criteria for rare metal ores in pegmatites are critical for the localization of undiscovered resources ( [1] and references therein). The presence of Li-Sn-Ta mineralization associated with pegmatites is known in Galicia, northwestern Spain (e.g., [2][3][4]), and, moreover, there are several occurrences of barren pegmatites. This work focuses on a Li-Sn-Ta mineralized body, the Presqueira pegmatite, which is a non-outcropping body that has been recently explored by a mining company, and a swarm of pegmatites that are barren in Li-Sn-Ta bearing minerals, the Panceiros pegmatites. In this research, we have carried out a comparative study of the whole-rock and mineral geochemistry of the barren and the mineralized bodies and their host-rock together with   [9]). temperature-pressure conditions of this episode were 375-425 °C and 2.5-3.5 kbar. The study of the deformation-recrystallization relationships during Variscan events shows that the peak of this metamorphism was reached during D2 [22]. Summarized data from the literature give ages for the main deformation events and metamorphism between 360-320 Ma [23], and the later deformation, which includes shear zones affecting the granites and the thermal aureoles [15], has been dated at about 315 ± 10 Ma [24].  [15]).
In the area where the studied pegmatites outcrop, this medium-pressure metamorphism varies from the chlorite to sillimanite-orthoclase isograd [9] and is coeval with the development of a highly evolved schistosity (S2), which is affected by D3 crenulation [7]. During D3, NNW-SSE trending folds with vertical axial planes, such as the Cotobade Antiform and Forcarei Synform ( Figure 2) fold of the previous structure, which is a major recumbent fold that verges eastwards. Moreover, during D3, a ductile shear zone affected the western limb of the Forcarei Synform [15]. According to Barrera et al. [15], in the investigated area, synkinematic (D3) granitoids are widely exposed and have been subdivided into two suites on the basis of petrology and geochemistry [15]: (1) calc-alkaline biotite  [15]).
The tectonic style of the GTMZ is dominated by the thrust regime related to nappe emplacement during the Variscan Orogeny. There are two Variscan deformation events associated with nappe emplacement [7]: D1, recumbent folds with axial plane cleavage (S1), which are visible only when affecting the quartzite beds; and D2, also consisting of recumbent folds with axial plane crenulation cleavage or schistosity (S2) and having phyllonite fabrics developed in association with the Galicia-Trás-os-Montes basal thrust. After the nappe emplacement, the tectonic evolution becomes predominantly a wrench regime during the last deformation event (D3), which is characterized by folds with subvertical axial planes and shear zones (right folds) at all scales that include crenulation. At the same time, large volumes of synkinematic and late synkinematic granitoids are emplaced [11]. As regards the metamorphism, the Allochthonous Complexes have undergone an early high pressure (HP) metamorphism, which characterizes them but is not observed in the relative autochthon. The end of this HP metamorphism was dated at 374 Ma [18,19]. At the end of the emplacement of the allochthonous complexes to its present position, both allochthon and relative autochthon underwent a metamorphic event under greenschist-facies conditions [20]. Arenas [21] suggested that the temperature-pressure conditions of this episode were 375-425 • C and 2.5-3.5 kbar. The study of the deformation-recrystallization relationships during Variscan events shows that the peak of this metamorphism was reached during D2 [22]. Summarized data from the literature give ages for the main deformation events and metamorphism between 360-320 Ma [23], and the later deformation, which includes shear zones affecting the granites and the thermal aureoles [15], has been dated at about 315 ± 10 Ma [24].
In the area where the studied pegmatites outcrop, this medium-pressure metamorphism varies from the chlorite to sillimanite-orthoclase isograd [9] and is coeval with the development of a highly evolved schistosity (S2), which is affected by D3 crenulation [7]. During D3, NNW-SSE trending folds with vertical axial planes, such as the Cotobade Antiform and Forcarei Synform ( Figure 2) fold of the previous structure, which is a major recumbent fold that verges eastwards. Moreover, during D3, a ductile shear zone affected the western limb of the Forcarei Synform [15]. According to Electron-microprobe analyses in muscovite, according to the Monier and Robert [26] classification ( Figure 4), show F contents between 0.2 and 0.5 wt % (Table A3). The trace elements determined were Rb, Cs, P, and Zn. Rubidium has homogenous contents from 2100 to 3300 ppm, whereas Cs, as in K-feldspar, varies from undetected (or below LOD) up to 2200 ppm. Measurements of P and Zn always reveal contents of between 212 and 430 ppm, and from 280 to 370 ppm, respectively.   [25].
Chemical composition of garnet from Panceiros pegmatites (Table A4) vary in the almandinespessartine solid solution with Mn/(Mn + Fe) ratios between 0.31 and 0.42. The most remarkable feature is the consistent presence of P contents ranging from 0.2 to 0.3 wt % P2O5. Haggerty et al. [27] and Burt [28] suggested these three possible substitutions in garnet: AlP Si−2, MnP(AlSi)−1, and Figure 4. Chemical composition of white mica from the Panceiros and the Presqueira pegmatites plotted on the triangular diagram as a function of Al (IV + VI)-R 2+ -Si (apfu), according to [25]. 7 of 37 Chemical composition of garnet from Panceiros pegmatites (Table A4) vary in the almandine-spessartine solid solution with Mn/(Mn + Fe) ratios between 0.31 and 0.42. The most remarkable feature is the consistent presence of P contents ranging from 0.2 to 0.3 wt % P 2 O 5 . Haggerty et al. [27] and Burt [28] suggested these three possible substitutions in garnet: AlP Si −2 , MnP(AlSi) −1 , and NaP(R 2+ Si) −1 , other potential substitutions have also been proposed by Breiter et al. [29]. The substitution involving sodium is not applicable to the studied garnets due to a lack of this element in the studied samples. Figure 5 unambiguously shows that P enters the garnet of the Panceiros pegmatites by means of the substitution mechanism of AlP Si −2 . Other authors [30][31][32] also propose this substitution in garnets from granitic pegmatites.

The Presqueira Pegmatite
The Presqueira pegmatite is a non-outcropping tabular body with a main dike that forks into several branches and reaches 700 m in length and up to 15 m in width, according to NI 43-101 Technical Report of resource estimate [34]. The pegmatite is hosted by quartz-schists of the Paraño Group and strikes N160E, being concordant with the regional schistosity (S2). The body is subhorizontal at the northern end and dips 40° towards the west at the southern end.
In the exploration adits, it is possible to observe the clearly defined contact between the pegmatite and the host schist, which shows tourmalinization. The body can be referred to as pegmatite-aplite since it has a layered internal structure with coarse-grained (pegmatitic) bands and fine-grained (aplitic) bands. This magmatic layering, which runs approximately parallel to the hostrock contact, is continuous at the macro-and microscopic scale in both aplite and pegmatite bands. However, a mylonitic texture, more noticeable in the pegmatitic bands, is superimposed on the magmatic layering. This tectonic fabric may be associated with the shear zone developed in this area during the third tectonic phase (D3) of the Variscan orogeny.

Mineralogy and Geochemistry
The Presqueira pegmatite is mainly composed of plagioclase (Ab99.6-Ab100), spodumene, quartz, muscovite, and perthitic K-feldspar found chiefly in pegmatitic bands. The accessory minerals are petalite, which mainly occurs associated with K-feldspar in the pegmatitic bands, Sn-Ta-Nb bearing oxides, more abundant in the aplitic bands, and phosphates. Uraninite, sphalerite, and arsenopyrite occur in much lower proportions.
Albite frequently shows myrmekites, and some crystals have a turbid core that is mantled by a The identified phosphates belong to the apatite group and to the eosphorite-childrenite series (Table A5). Despite the lack of X-ray diffraction data, transmission optical microscopy properties, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) data together with electron-microprobe analyses allowed us to identify the phosphates described in this work. According to optical microscopy and backscattered-electron images from SEM-EDS, apatite shows different textures that match different compositions. This mineral appears isolated as colorless subeuhedral crystals or forming irregular crystal aggregates infilling interstices between the main silicates ( Figure 3b). From EPM analyses (Table A5), it can be classified as Mn-rich fluorapatite, using Pasero et al. [33] nomenclature, with up to 5 wt % of MnO. This fluorapatite is partially replaced by brownish masses of a second generation of apatite, which is pervasively argillized (Figure 3c,d). This alteration prevents us from obtaining pristine analyses (Table A5); nevertheless, this apatite has much lower contents in F and Mn, being probably hydroxylapatite. It also occurs as patches replacing feldspars and more scarcely in open cavities and veinlets. Aluminum phosphates with compositions varying from eosphorite to an intermediate member of the eosphorite-childrenite series (Table A5) form similar irregular patches corroding albite and K-feldspar crystals. In fact, it is common to observe a feldspar crystal in which both the aluminum phosphate and the altered hydroxylapatite with fluorapatite relicts form replacement patches (Figure 3c,d). The argillized hydroxylapatite represents an alteration product of previously formed fluorapatite.

The Presqueira Pegmatite
The Presqueira pegmatite is a non-outcropping tabular body with a main dike that forks into several branches and reaches 700 m in length and up to 15 m in width, according to NI 43-101 Technical Report of resource estimate [34]. The pegmatite is hosted by quartz-schists of the Paraño Group and strikes N160E, being concordant with the regional schistosity (S2). The body is subhorizontal at the northern end and dips 40 • towards the west at the southern end.
In the exploration adits, it is possible to observe the clearly defined contact between the pegmatite and the host schist, which shows tourmalinization. The body can be referred to as pegmatite-aplite since it has a layered internal structure with coarse-grained (pegmatitic) bands and fine-grained (aplitic) bands. This magmatic layering, which runs approximately parallel to the host-rock contact, is continuous at the macro-and microscopic scale in both aplite and pegmatite bands. However, a mylonitic texture, more noticeable in the pegmatitic bands, is superimposed on the magmatic layering. This tectonic fabric may be associated with the shear zone developed in this area during the third tectonic phase (D3) of the Variscan orogeny.

Mineralogy and Geochemistry
The Presqueira pegmatite is mainly composed of plagioclase (Ab 99.6 -Ab 100 ), spodumene, quartz, muscovite, and perthitic K-feldspar found chiefly in pegmatitic bands. The accessory minerals are petalite, which mainly occurs associated with K-feldspar in the pegmatitic bands, Sn-Ta-Nb bearing oxides, more abundant in the aplitic bands, and phosphates. Uraninite, sphalerite, and arsenopyrite occur in much lower proportions.
Albite frequently shows myrmekites, and some crystals have a turbid core that is mantled by a clear myrmekite rim (Figure 6a). This core sometimes has K-feldspar relicts similar to those described for Panceiros pegmatites (Figure 6a). The abundance of textures indicative of the albitization of K-feldspar is a striking petrographic characteristic of the Presqueira pegmatite.
Electron-microprobe analyses of K-feldspar (Table A1) indicate that the more abundant trace elements are Cs, Rb, and, in lower concentrations, P. Rubidium contents are between 4700 and 5800 ppm. Cesium concentrations range from 3200 to 6400 ppm. Phosphorus concentrations vary from 1800 to 3000 ppm. Zinc has contents of around 150 ± 40 ppm, and Ba is seldom detected. In albite (Table A2), the main trace elements are Rb and P. The former shows consistent contents of around 2200 ppm among crystals, in contrast to the latter that shows highly variable concentrations from 195 up to nearly 3000 ppm. In much lower proportions, Ba is also detected above the LOD and varies from 140 to 240 ppm.
Regarding muscovite (Figure 4), EPM analyses (Table A3) show F contents of between 0.13 and 0.33 wt %. The main trace elements are Rb, Cs, and, in lower concentration, Zn. Rubidium has homogenous contents from 3600 ppm to 4800 ppm, whereas those of Cs are more variable, from 2000 up to 4800 ppm, the more frequent interval lying between 3500 and 4500. Zinc is always detected with contents of between 174 ppm and 368 ppm. Barium is sometimes detected with contents ranging from 116 to 170 ppm.
The most abundant lithium aluminosilicate is spodumene, which occurs in the following two different forms. It appears as coarse-grained spodumene-quartz intergrowths (Figure 6b). These resemble the spodumene-quartz intergrowth known as SQI resulting from the pseudomorphic replacement of petalite defined byČerný and Ferguson [35]. In fact, in some samples, it is possible to observe this intergrowth replacing petalite (Figure 6b). The spodumene also occurs as aggregates of subeuhedral crystals in the interface between feldspar grains. In this case, the lithium aluminosilicate often develops a narrow rim consisting of a fine-grained vermicular quartz-spodumene symplectitic intergrowth (Figure 6c). This texture has also been described in other pegmatites in this pegmatitic field [36] and also in the bodies of the Kangxiwa-Dahongliutan pegmatite field in the Western Kunlun Orogen of China [37]. Electron-microprobe analyses of K-feldspar (Table A1) indicate that the more abundant trace elements are Cs, Rb, and, in lower concentrations, P. Rubidium contents are between 4700 and 5800 ppm. Cesium concentrations range from 3200 to 6400 ppm. Phosphorus concentrations vary from 1800 to 3000 ppm. Zinc has contents of around 150 ± 40 ppm, and Ba is seldom detected. In albite Figure 6. Photomicrographs of samples from the Li-Sn-Ta ore-bearing pegmatite of Presqueira: (a) albite crystal with a turbid core showing K-feldspar relicts. The core is mantled by a clear myrmekite rim. Note the presence of spodumene between the albite crystal and the petalite aggregates; (b) spodumene+quartz replacement (SQI) of petalite; (c) spodumene crystals with a fine-grained symplectitic quartz-spodumene intergrowth at the contact with neoformed albite. Note that from the contact rim between both minerals towards the inner parts of the spodumene crystal, the symplectitic intergrowth shows grain coarsening. Note the myrmekites (upper left corner of the image); (d) aggregate of petalite crystals with tiny acicular spodumene replacing them along crystal boundaries; (e) Petalite partially replaced by irregular patches of extremely fine-grained myrmekites. In the image center, one of this patch shows neoformed albite in the core; (f) Petalite replaced by mymekite. Note a rim that coarsens away from the contact with petalite resulting in albite showing coarse-grained myrmekites. Quartz (Qtz), petalite (Pet), spodumene (Spd), albite (Ab) and K-feldspar (Kfs). See text for further explanation.
Petalite occurs as aggregates of euhedral crystals (Figure 6d) or as coarser-grained isolated crystals ( Figure 6b). As previously mentioned, some petalite crystals are replaced by the above-mentioned SQI.
In some samples, petalite aggregates are replaced by tiny acicular spodumene along crystal boundaries ( Figure 6d). In addition, petalite is replaced by extremely fine-grained myrmekites that coarsen away from the contact, with petalite transformed into albite with a fine-grained myrmekite rim (Figure 6e) or, more frequently, albite showing coarse-grained myrmekites (Figure 6f). The latter resembles the common form of albite occurrence in this pegmatite.
As regards phosphates, Mn-bearing fluorapatite is the most abundant one in this pegmatite (Table A6). It often appears as isolated colorless euhedral crystals. Occasionally, it forms intergrowth with silicates or other phosphates. In addition, two different aluminous phosphates have been identified. The more common Al-rich phosphate in this pegmatite is montebrasite with F/(F + OH) < 0.30 (Table A6). It is commonly associated with spodumene and forms isolated euhedral-subeuhedral crystals. The other identified aluminum phosphate belongs to the crandallite series, goyazite being the Sr-rich member (Table A6). It forms irregular acicular crystal aggregates that are occasionally intergrown with fluorapatite. These aggregates fill open spaces or replace K-feldspar (Figure 7a  The Sn-Ta-Nb bearing oxides are cassiterite and members of the columbite-tantalite group. These minerals commonly occur disseminated in the fine-grained aplitic bands of the Presqueira body. Cassiterite often appears as twinned subeuhedral crystals. This oxide is mainly enriched in Ta and Nb (Ta + Nb up to 4.5 wt %) with Ta/(Ta + Nb) = 0.5-1.0, having lower contents of Fe (1000-7500 ppm), In (1000-1500 ppm), and Ti (600-1200 ppm) (Table A7). Under transmitted light optical microscopy, this oxide commonly reveals color zoning with dark reddish-brown and clear yellow growth bands of variable thickness. The dark bands are the richest in Ta and Nb together with the above-mentioned trace elements. Moreover, cassiterite sometimes presents columbite-tantalite inclusions or is intergrown with this mineral.
Columbite-tantalite often forms fine-grained euhedral to subeuhedral crystals with prismatic and tabular habits. Uraninite, which sometimes has thorium contents of up to 5000 ppm, is Finally, the Fe-Mn-Ca phosphates vivianite and messelite (Table A6) were identified, filling cavities and late veinlets that crosscut the whole body ( Figure 7c). These veinlets are very abundant and an outstanding characteristic in hand samples due to the deep blue color of the vivianite. Vivianite is sporadically associated with messelite. In such cases, vivianite is partially replaced by messelite accompanied by sphalerite (Figure 7d).
The Sn-Ta-Nb bearing oxides are cassiterite and members of the columbite-tantalite group. These minerals commonly occur disseminated in the fine-grained aplitic bands of the Presqueira body. Cassiterite often appears as twinned subeuhedral crystals. This oxide is mainly enriched in Ta and Nb (Ta + Nb up to 4.5 wt %) with Ta/(Ta + Nb) = 0.5-1.0, having lower contents of Fe (1000-7500 ppm), In (1000-1500 ppm), and Ti (600-1200 ppm) (Table A7). Under transmitted light optical microscopy, this oxide commonly reveals color zoning with dark reddish-brown and clear yellow growth bands of variable thickness. The dark bands are the richest in Ta and Nb together with the above-mentioned trace elements. Moreover, cassiterite sometimes presents columbite-tantalite inclusions or is intergrown with this mineral.
Columbite-tantalite often forms fine-grained euhedral to subeuhedral crystals with prismatic and tabular habits. Uraninite, which sometimes has thorium contents of up to 5000 ppm, is occasionally present as inclusions or as intergrowths at the border of the columbite-tantalite crystals (Figure 8a,b). SEM backscattered images of discrete columbite-tantalite grains show euhedral cores mainly enriched in Nb and rims of variable thickness enriched in Ta (Figure 8c-e). Cores and rims usually present fine oscillatory zoning ( Figure 8) that sometimes exhibits truncated layers, mainly in the Nb-richer core ( Figure 8e). In addition, the rims are sometimes irregular, resulting in a sub-euhedral to anhedral crystal with a euhedral prismatic core ( Figure 8f). On other occasions, the contact of these Ta-rich rims with the Nb-rich cores is lobate, giving rounded corners, and embayments ( Figure 8a,b,f). In some crystals, the oscillatory zonation of Nb-rich cores is partially disrupted by a patchy Ta-richer zonation (Figure 9a), to use the Lahti [38] nomenclature. Discontinuities in growth marked by corrosion and patchy zoning suggest replacement processes [39]. Superimposed upon the described zonation patterns, anhedral tapiolite-(Fe) (Table A7 and Figure 9) occurs at the margins of the grains or along microfractures, which leads us to interpret it as evidence of this phase representing the last Ta (Table A7 and Figure 10). Rim composition varies between columbite-(Fe) and tantalite-(Fe) with Ta/(Ta + Nb) = 0.37−0.65 and Mn/(Mn + Fe) = 0.20−0.49 (Table A7 and Figure 10). The Ta/(Ta + Nb) and Mn/(Mn + Fe) ratios of the individual crystals ( Figure 10) exhibit significant changes and follow two main evolutionary trends. A trend characterized by an increase in the Ta ratio (Ta/Ta + Nb) at near-constant Mn ratio (Mn/Mn + Fe) (Figures 8b,c,e and 10), and a trend characterized by increasing Ta ratio at decreasing Mn ratio (Figures 9a,b and 10). Figure 8. SEM-BSE images of columbite-tantalite crystals from the Presqueira pegmatite. In these images, the darker the grey color is, the richer is the composition in Nb. Numbers and dots (red, green, and blue) correspond to the zone of the crystal plotted in the compositional columbite-tantalite quadrilateral of Figure 10; (a) euhedral prismatic crystal of columbite-tantalite with an inclusion of a tiny uraninite crystal. The columbite-tantalite grain shows a discontinuous oscillatory-zoned core, with mainly columbite-(Fe) composition partially corroded by Nb-poorer patch with complex zoning, Figure 8. SEM-BSE images of columbite-tantalite crystals from the Presqueira pegmatite. In these images, the darker the grey color is, the richer is the composition in Nb. Numbers and dots (red, green, and blue) correspond to the zone of the crystal plotted in the compositional columbite-tantalite quadrilateral of Figure 10; (a) euhedral prismatic crystal of columbite-tantalite with an inclusion of a tiny uraninite crystal. The columbite-tantalite grain shows a discontinuous oscillatory-zoned core, with mainly columbite-(Fe) composition partially corroded by Nb-poorer patch with complex zoning, surrounded by an irregular thinner rim with embayments of tantalite-(Fe) composition; (b) columbite-tantalite grains with tiny uraninite crystals attached at their borders. Note the left-hand crystal showing a fine-oscillatory zoned core, columbite-(Fe), a Nb-poorer irregular rim with complex zoning, and a lobulated rim of tantalite-(Fe) composition; (c) a prismatic columbite-tantalite crystal with oscillatory zoning in the core, columbite-(Fe), a Nb-poorer irregular rim with complex zoning, and a narrow rim of mostly tantalite-(Fe) composition; (d) a prismatic columbite-tantalite crystal showing a small homogenous core of columbite-(Fe) and a thicker fine-oscillatory zoned rim of chiefly tantalite-(Fe) composition; (e) euhedral oscillatory zoned columbite-tantalite grain exhibiting truncated layers suggesting episodes of partial dissolution and successive crystallization; (f) sub-euhedral columbite-tantalite grains showing fine-oscillatory zoned cores, mostly columbite-(Fe), and thick fine-oscillatory zoned rims of mainly tantalite-(Fe). In the right-hand crystal, note the core containing inward-penetrating embayments of the rim. In the right-hand of the image, the bright tiny crystals are uraninite. and a narrow rim of mostly tantalite-(Fe) composition; (d) a prismatic columbite-tantalite crystal showing a small homogenous core of columbite-(Fe) and a thicker fine-oscillatory zoned rim of chiefly tantalite-(Fe) composition; (e) euhedral oscillatory zoned columbite-tantalite grain exhibiting truncated layers suggesting episodes of partial dissolution and successive crystallization; (f) subeuhedral columbite-tantalite grains showing fine-oscillatory zoned cores, mostly columbite-(Fe), and thick fine-oscillatory zoned rims of mainly tantalite-(Fe). In the right-hand crystal, note the core containing inward-penetrating embayments of the rim. In the right-hand of the image, the bright tiny crystals are uraninite.

Whole Rock Geochemistry
For whole-rock geochemical analyses, pegmatite outcrops of five bodies were sampled in the Panceiros area. In the sampling, an effort was made to take representative samples of both aplitic and pegmatitic bands for each body. In the Presqueira pegmatite, four samples were taken from the footwall to the hanging wall in an exploration adit that traversed the body. Furthermore, representative samples of the main granites outcropping in the area, i.e., GRP, GRM, GRT (Figure 2), Figure 10.
Compositions of Ta-Nb oxides from the Presqueira pegmatite plotted in the columbite-tantalite quadrilateral, Mn/(Mn + Fe) vs. Ta/(Ta + Nb). The compositional variations of the individual crystals of Figure 8b,c,e, and Figure 9a,b are also plotted (numbers and dots correspond to those of the crystals from these figures).

Whole Rock Geochemistry
For whole-rock geochemical analyses, pegmatite outcrops of five bodies were sampled in the Panceiros area. In the sampling, an effort was made to take representative samples of both aplitic and pegmatitic bands for each body. In the Presqueira pegmatite, four samples were taken from the footwall to the hanging wall in an exploration adit that traversed the body. Furthermore, representative samples of the main granites outcropping in the area, i.e., GRP, GRM, GRT (Figure 2), were taken.

Igneous Rock Geochemistry: Pegmatites and Granites
All samples from the Panceiros bodies and the Presqueira pegmatite ( Figure 11, Table A8) are characterized by similarly high contents in SiO 2 , higher than 73 wt %, and low in CaO and Fe 2 O 3 (<1 wt %), although the latter clearly shows the lowest concentration in the Presqueira body. MgO, MnO, and TiO 2 contents are low and also similar among the pegmatites bodies (<0.1 wt %). The Na 2 O/K 2 O ratio is comparable in the Panceiros bodies from 0.9 to 1.5 (mode close to 1) but lower than that of the Presqueira pegmatite, which reaches a maximum of two. All of them have very similar ASI and A/CNK indexes due to their low Ca contents. ASI and A/CNK values are comparable among Panceiros bodies, ranging from 1.26 to 1.36 and from 1.18 to 1.32, respectively, whereas the Presqueira body reveals higher values of 1.54 and 1.49, respectively. According to these indices, the studied pegmatites are peraluminous, but the Presqueira pegmatite has a stronger peraluminous character. Phosphorus concentrations are quite similar in all the studied bodies, varying between 0.2 and 0.6 wt % P 2 O 5 . Regarding the sampled granites ( Figure 11), they have high silica contents (72-73 wt % SiO 2 ) ( Table A8). The Fe 2 O 3 , CaO, MgO, TiO 2 , and MnO are low but higher than those of the pegmatites. They are peraluminous, their ASI and A/CNK indexes lying between 1. 35   With regard to the analyzed trace elements (Table A8), the Presqueira pegmatite has strikingly higher contents than those of the Panceiros bodies in elements such as Li (>2000 ppm), Be (145 ppm), Rb (999 ppm), Cs (59 ppm), Nb (66 ppm), Ta (64 ppm), Sn (845 ppm), Sr (94 ppm), and Zn (99 ppm). Figure 11. Plot of the whole-rock compositions from the studied pegmatites and the granites neighboring the pegmatites in the A-B diagram of Debon and Le Ford [40]. Granitic facies: GRP moderately leucocratic, medium-grained two-mica granite, GRM moderately leucocratic, fine-grained two-mica granite, and GRT inhomogeneous granite. I, II, III = peraluminous series; IV, V, VI = metaluminous series.
With regard to the analyzed trace elements (Table A8), the Presqueira pegmatite has strikingly higher contents than those of the Panceiros bodies in elements such as Li (>2000 ppm), Be (145 ppm), Rb (999 ppm), Cs (59 ppm), Nb (66 ppm), Ta (64 ppm), Sn (845 ppm), Sr (94 ppm), and Zn (99 ppm). The high contents in elements such as Li, Sn, Ta, Nb, Sr, and Zn are supported by the previously described mineralogy, i.e., spodumene, montebrasite, petalite, cassiterite, columbite-tantalite, goyazite, and Zn-bearing muscovite together with sphalerite. On the other hand, the Panceiros bodies have higher contents in Ba, Y, and, remarkably, in As. In fact, one of the sampled pegmatites reaches 593 ppm As despite the fact that As-bearing phases, such as arsenopyrite, have not been recognized in any of the studied samples. As regards the trace element amounts in the granites of the area, they have lower concentrations than pegmatites in some incompatible elements such as Be, Rb, Nb, Ta, and Sn (Table A8). However, Cs concentration in granites is similar, or even higher, and Li is clearly higher than in Panceiros pegmatites. Other incompatible elements, such as Zr and Y, have noticeably higher contents in granites than in pegmatites, whereas Hf, U, and W contents are quite similar. Barium is significantly higher in granites than in pegmatites, and Sr does not show a variation in concentration between granites on the one hand and pegmatites on the other.
Rare earth element (REE) concentrations from the studied pegmatites and granites are given in Table A9. REE contents are much lower in pegmatites than in granites. In fact, concentrations of some particular REEs (mostly Eu and HREEs) in the pegmatites are below the limit of detection (LOD) of the analytical technique. Presqueira pegmatite presents the lowest concentration in REEs, showing various rare earth elements below LOD (Table A9). The chondrite-normalized REE patterns [40] of these rocks are shown in the spider diagram of Figure 12 Considering the more complete REE patterns (i.e., larger number of REE contents above LOD), which are those of the Panceiros pegmatites, a common characteristic is that they are flatter than those of the granites. However, they show very variable LREE enrichment (La/Sm from 1.71 up to 6.87) and mostly flat HREE patterns (Gd/Yb from 0.64-1.90), the Eu anomaly being predominantly negative.

Pegmatite-Wallrock Interaction
In order to obtain information on the interaction between pegmatite-derived fluids and host-rock, representative samples from the schist adjacent to two pegmatitic bodies from Panceiros (Santa Baia schist) and to the Presqueira pegmatite (Paraño schist) were collected. In addition, samples of the corresponding host-schist were also collected, two samples from the Santa Baia group and two from the Paraño group. In order to collect these samples beyond the alteration halos of the pegmatites, the Paraño group samples were taken in the eastern limb of the Forcarei Synform, in areas where there are no pegmatite outcrops, and pegmatite occurrences are not documented in the literature. In the same way, samples from the Santa Baia group were taken at the southern end of the Cotobade Synform (Figure 2), where there are no pegmatite outcrops. enrichment and negative europium anomaly. Pegmatites have complex, very variable chondritenormalized REE patterns among sampled bodies. Considering the more complete REE patterns (i.e., larger number of REE contents above LOD), which are those of the Panceiros pegmatites, a common characteristic is that they are flatter than those of the granites. However, they show very variable LREE enrichment (La/Sm from 1.71 up to 6.87) and mostly flat HREE patterns (Gd/Yb from 0.64-1.90), the Eu anomaly being predominantly negative. Chondrite-normalized rare earth element abundances [41] of representative whole-rock samples from the barren pegmatites of Panceiros and the Li-Sn-Ta mineralized pegmatite of Presqueira, and from the granite facies (GRP, GRM, and GRT) of the studied area. Note that REE contents are much higher in granites than in pegmatites, and the latter show incomplete patterns because some REEs are below the limits of detection of the analytical technique used.

Pegmatite-Wallrock Interaction
In order to obtain information on the interaction between pegmatite-derived fluids and hostrock, representative samples from the schist adjacent to two pegmatitic bodies from Panceiros (Santa Baia schist) and to the Presqueira pegmatite (Paraño schist) were collected. In addition, samples of the corresponding host-schist were also collected, two samples from the Santa Baia group and two from the Paraño group. In order to collect these samples beyond the alteration halos of the pegmatites, the Paraño group samples were taken in the eastern limb of the Forcarei Synform, in areas where there are no pegmatite outcrops, and pegmatite occurrences are not documented in the literature. In the same way, samples from the Santa Baia group were taken at the southern end of the Cotobade Synform (Figure 2), where there are no pegmatite outcrops.
The mineralogy of these samples was characterized by optical microscopy. The schist adjacent to the pegmatites shows a metasomatic mineralogical assemblage that overprints the metamorphic assemblage [42]. In both the Santa Baia and the Paraño schist, the dominant prograde metamorphic assemblage consists of quartz, biotite, muscovite, ±andalusite, ilmenite partially replaced by rutile and, in the Paraño schist, ± chlorite. The main metasomatic paragenesis of the schist at the contact of The mineralogy of these samples was characterized by optical microscopy. The schist adjacent to the pegmatites shows a metasomatic mineralogical assemblage that overprints the metamorphic assemblage [42]. In both the Santa Baia and the Paraño schist, the dominant prograde metamorphic assemblage consists of quartz, biotite, muscovite, ±andalusite, ilmenite partially replaced by rutile and, in the Paraño schist, ± chlorite. The main metasomatic paragenesis of the schist at the contact of the Panceiros pegmatites is mainly made up of quartz, muscovite, biotite, and, in minor proportions, fluorapatite, oligoclase (Ab 85-87 ), and tourmaline. Some sulphides, pyrite, chalcopyrite, and galena occur as accessory minerals. The dominant metasomatic assemblage of the schist at the contact of the Presqueira pegmatite is similar. It mainly consists of quartz, muscovite, biotite, and, in less proportion, tourmaline, rutile, and Mn-bearing ilmenite (around 6 wt % MnO). Of special note is the presence of tiny (40-100 µm) crystals of a Zr-bearing phosphate, identified by SEM-EDS, adjacent to or as an overgrowth on rutile crystals [42]. Electron microprobe analyses (47.3 wt % P 2 O 5 , 29.9 wt % ZrO 2 , 15.7 wt % FeO) allowed us to identify it as malhmoodite FeZr(PO 4 ) 2 4H 2 O. A wider variety of sulphides occurs as accessory minerals: pyrite, chalcopyrite, galena, sphalerite, Ni-Co bearing löllingite, and arsenopyrite.
The whole-rock compositions of the sampled schists are shown in Table A10. To quantify the amounts of individual elements added to and subtracted from the wallrock schists overcoming the distortions of the closure effect, we have used the single precursor mass transfer technique [43]. The calculations have been performed for the major elements and, also, for the main trace elements that characterize the whole-rock compositions of the studied pegmatites. The first step, as in any mass transfer calculation, is to establish which elements act as immobile ones in the schist during the pegmatite-wallrock interaction. For this, we have plotted pairs of potentially immobile elements (i.e., Al, Ti, Zr, and Y) from the metasomatized schist of the pegmatite wallrock and the corresponding non-metasomatized schist on x-y bivariate diagrams with origins at zero. Several authors indicate that a highly correlated trend that passes through the origin with correlation coefficients exceeding 0.85 permits the selection of that pair of elements as immobile elements [44,45]. In the schist that hosts the Panceiros pegmatites, Ti vs. Zr gives an R 2 = 0.99. In the schist that hosts the Presqueira pegmatite, the highest correlation coefficients were that of Al vs. Y (R 2 = 0.85). Taking into account these values, we have done the mass transfer calculations for the metasomatized wallrock of Panceiros pegmatites and of Presqueira pegmatite, using as immobile elements Ti and Al, respectively, in the following calculation [43] ∆ a = (Z p /Z a × C a ) − C p (1) where ∆ a is absolute mass change of element a; C a concentration of component in metasomatized host-schist (wallrock); C p concentration of component in non-metasomatized host-schist (precursor schist); Z a concentration of immobile element in metasomatized host-schist (wallrock); Z p concentration of an immobile element in non-metasomatized host-schist (precursor schist).
In addition, we have verified that similar results are obtained using the other possible immobile elements for the Panceiros pegmatite wallrock, i.e., Zr, and for the Presqueira pegmatite wallrock, i.e., Y. The results are shown in Table A11 and Figure 13. does not document a gain in Be or Ta-Nb. In both wallrock schists, there is an addition, in lower proportions, of W, Tl, and Ga. Several authors (e.g., [48] and reference therein) point out that late magmatic fluids export W more efficiently than Ta and Nb. Barium and strontium show variable behavior, being added or leached during the pegmatite-wallrock interaction. Other trace elements, such as uranium and hafnium, do not record a mass transfer high enough to be taken into account. Figure 13. Bar diagrams showing estimated absolute mass changes (calculations according to [43]) of major and minor elements from the wallrock of two pegmatites from Panceiros (black bars) and from the wallrock of the Presqueira pegmatite (grey bars). The net mass change of major elements (Net) is also plotted.
In order to check where these gained trace-elements were accommodated in the wallrock schist of the pegmatite bodies, EMP analyses were carried out on micas (biotite and muscovite) on samples from the non-metasomatized Paraño and Santa Baia schist, and samples from the metasomatized schists of the pegmatite wallrocks. The composition of the studied micas is plotted in the Monier and Robert [26] classification of Figure 14. Moreover, the neoformed oligoclase in the wallrock schists of the Panceiros pegmatites was also analyzed. As was shown in Table A12, the main trace elements in biotite are F, Cs, Rb, Ba, and Zn, whilst Cl, Ga, Sn, and Sr were below the detection limit. The data show an enrichment in Rb, Cs, and F in biotite from the wallrock schist of the Panceiros and Presqueira pegmatites, whereas the Ba content tends to be lower. The variations in the Zn contents are not important (Table A12). Figure 14 shows these variations in trace-element content in biotite from the non-metasomatized Santa Baia schist and from wallrock schist. In the latter case, the analyses are plotted in two separate fields, each corresponding to the wallrock of two different pegmatite bodies (samples ESQ-C and ESQ-4 in Table A12); the concentration of Li2O in the biotite according to Tindle and Webb [49]. Taking this into account, the F-richer biotite from the wallrock may have had a higher Li contents (from 0.23 to 0.70 wt % Li2O in the case of Panceiros pegmatites; from 0.24 to 0.54 wt % Li2O in the case of Presqueira pegmatite) than that of the non-metasomatized Figure 13. Bar diagrams showing estimated absolute mass changes (calculations according to [43]) of major and minor elements from the wallrock of two pegmatites from Panceiros (black bars) and from the wallrock of the Presqueira pegmatite (grey bars). The net mass change of major elements (Net) is also plotted.
From these data, we note that the wallrock schist of Panceiros pegmatites has a net mass gain mainly due to the addition of SiO 2 and, in minor proportions, Al 2 O 3 and K 2 O (Figure 13). This addition is consistent by the presence of neo-formed quartz, biotite, and muscovite in these alteration halos. However, the wallrock schist of the Presqueira pegmatite has a net mass loss, mainly due to the loss of SiO 2 (Figure 13). This silica loss could be associated with the strong ductile deformation affecting these rocks during D 3 shear that, as previously mentioned, developed mylonitic textures in the Presqueira pegmatite. Removal of large amounts of silica during shear zone formation has been reported by other authors [46,47]. Thus, this silica loss would not be related to the interaction between pegmatite-derived fluids and host-rock.
The most noteworthy results concern the trace-elements ( Figure 13 and Table A11), the wallrock schists of both the Panceiros, and the Presqueira pegmatites showing a distinct gain in Li, Rb, Cs, Sn, Zn, and As despite the lack of Li-and/or Sn-bearing mineralization in Panceiros pegmatites. Similarly, the wallrock of the latter shows a gain in beryllium and a slight gain in tantalum and niobium though Ta-Nb or Be-bearing minerals have not been recognized in the studied samples of the bodies. On the other hand, the wallrock schist of the Li-Sn-Ta mineralized body of Presqueira does not document a gain in Be or Ta-Nb. In both wallrock schists, there is an addition, in lower proportions, of W, Tl, and Ga. Several authors (e.g., [48] and reference therein) point out that late magmatic fluids export W more efficiently than Ta and Nb. Barium and strontium show variable behavior, being added or leached during the pegmatite-wallrock interaction. Other trace elements, such as uranium and hafnium, do not record a mass transfer high enough to be taken into account.
In order to check where these gained trace-elements were accommodated in the wallrock schist of the pegmatite bodies, EMP analyses were carried out on micas (biotite and muscovite) on samples from the non-metasomatized Paraño and Santa Baia schist, and samples from the metasomatized schists of the pegmatite wallrocks. The composition of the studied micas is plotted in the Monier and Robert [26] classification of Figure 14. Moreover, the neoformed oligoclase in the wallrock schists of the Panceiros pegmatites was also analyzed. As was shown in Table A12, the main trace elements in biotite are F, Cs, Rb, Ba, and Zn, whilst Cl, Ga, Sn, and Sr were below the detection limit. The data show an enrichment in Rb, Cs, and F in biotite from the wallrock schist of the Panceiros and Presqueira pegmatites, whereas the Ba content tends to be lower. The variations in the Zn contents are not important (Table A12). Figure 14 shows these variations in trace-element content in biotite from the non-metasomatized Santa Baia schist and from wallrock schist. In the latter case, the analyses are plotted in two separate fields, each corresponding to the wallrock of two different pegmatite bodies (samples ESQ-C and ESQ-4 in Table A12); the concentration of Li 2 O in the biotite according to Tindle and Webb [49]. Taking this into account, the F-richer biotite from the wallrock may have had a higher Li contents (from 0.23 to 0.70 wt % Li 2 O in the case of Panceiros pegmatites; from 0.24 to 0.54 wt % Li 2 O in the case of Presqueira pegmatite) than that of the non-metasomatized schist (from 0.0 to 0.33 wt % Li 2 O). Microprobe analyses of muscovite from schist (Table A13) indicate that the main trace elements in this mineral are Rb and Ba, which were always detected, whereas Zn, Cs, and F were found in lower concentrations, sometimes below their respective LOD. In addition, Cl, Ga, Sn, and Sr were always below the LOD. As in the case of biotite, the muscovite from the metasomatized schist alongside Panceiros pegmatites are richer in Rb, Cs, and F and poorer in Ba than muscovite from the non-metasomatized Santa Baia Schist (Figure 15). The concentration of Li2O in the muscovite was estimated using the empirical equation 0.3935 × F 1.326 [50], giving values of between 116 and 664 ppm for the F-richer muscovite from the wallrock. The electron-microprobe data for muscovite from the non-metasomatized schist of the Paraño Group and from the metasomatized wallrock schist of the Presqueira pegmatite did not show differences in trace-element contents, which in the binary diagrams, are plotted together. For this reason, they are not included in Figure 15.
We also analyzed the trace elements in the oligoclase present in the metasomatized wallrock schist of the Panceiros pegmatites. The microprobe data showed that the main trace elements are Rb Microprobe analyses of muscovite from schist (Table A13) indicate that the main trace elements in this mineral are Rb and Ba, which were always detected, whereas Zn, Cs, and F were found in lower concentrations, sometimes below their respective LOD. In addition, Cl, Ga, Sn, and Sr were always below the LOD. As in the case of biotite, the muscovite from the metasomatized schist alongside Panceiros pegmatites are richer in Rb, Cs, and F and poorer in Ba than muscovite from the non-metasomatized Santa Baia Schist (Figure 15). The concentration of Li 2 O in the muscovite was estimated using the empirical equation 0.3935 × F 1.326 [50], giving values of between 116 and 664 ppm for the F-richer muscovite from the wallrock. The electron-microprobe data for muscovite from the non-metasomatized schist of the Paraño Group and from the metasomatized wallrock schist of the Presqueira pegmatite did not show differences in trace-element contents, which in the binary diagrams, are plotted together. For this reason, they are not included in Figure 15.

Mineral Geochemistry: Fractionation Trends
Černý et al. [54] established, and several authors (e.g., [55][56][57]) corroborated, the utility of the K/Rb and K/Cs of K-feldspar and micas as guides for chemical fractionation among pegmatites and between pegmatites and associated granites. Figures 16 and 17 show K/Rb vs. Rb and K/Cs vs. Cs in K-feldspar and in muscovite, respectively, from the Panceiros bodies, from the Presqueira pegmatite, and from the granites outcropping in the area. The plot of K/Rb vs. Cs in K-feldspar is also shown in Figure 16. The K/Rb and K/Cs ratios decrease, and Rb and Cs concentrations increase, either in Kfeldspar or in muscovite, from the granites to the Presqueira pegmatite, indicating a fractionation trend. Thus, the degree of magmatic differentiation is lower in Panceiros pegmatites than in the Presqueira pegmatite and is close to that of the granites. Finally, the plot of K/Rb vs. Cs in K-feldspar shows a trend from the granites to the studied pegmatites. This is consistent with a fractional crystallization in which the magmatic differentiation increases in this direction, Presqueira pegmatite having the higher degree of differentiation. This is also supported by the whole-rock compositions of the pegmatites and associated granites shown in Figure 11. We also analyzed the trace elements in the oligoclase present in the metasomatized wallrock schist of the Panceiros pegmatites. The microprobe data showed that the main trace elements are Rb (around 2000 ppm Rb 2 O), Sr (from 570 to 1200 ppm SrO), P (from 340 to 2570 ppm P 2 O 5 ), and Zn (from 300 to 440 ppm ZnO). Only in one case was Cs found to be above the detection limit. The rest of the trace elements measured (Ba, F, Cl, Sn, Ga) were always below their detection limits.
In the walkrock of the Presqueira pegmatite, electronic microprobe analyses in the rutile and in the manganese-bearing ilmenite constantly detected niobium above the LOD, with concentrations between 320 ppm and 1800 ppm, the highest contents being in rutile. The associated Zr-bearing phosphate, malhmoodite, showed W contents constantly above the LOD (from 500 to 1169 ppm).
Taking into account the mineral geochemistry of the metasomatic assemblage from the wallrock schists and the mass transfer results of the host-schist lithogeochemistry, the prominent gain in Li, Rb, Cs, and Zn of the pegmatite wallrocks is supported by the trace-element content of, mainly, biotite, but also of muscovite. In the wallrock of the Presqueira pegmatite, the addition of As, and also Zn, is clearly correlated with by the presence of sulfides such as arsenopyrite, löllingite, and sphalerite. Arsenic is also added to the wallrock of Panceiros pegmatites. We consider that As occurs in some of the phases forming the sulfide assemblage of the wallrock (e.g., pyrite). As regards Sn, this is constantly and noticeably added during the metasomatism of the wallrock by the pegmatite-derived fluids. However, we have not found the Sn-bearing mineral phase. Rutile can accommodate a wide range of highly charged trace elements up to percent levels, e.g., Sn, Nb, Ta, or W, among others [51][52][53].
We have checked for the presence of these trace elements in rutile or in ilmenite, but only niobium was detected. We propose that Sn could be a trace-element in rutile or micas, but in concentrations below the limits of detection of the EMP attained in this study. Hence, micro-analytical techniques attaining lower limits of detection would be necessary to assert that these minerals are the carriers of the mentioned trace element.

Mineral Geochemistry: Fractionation Trendš
Cerný et al. [54] established, and several authors (e.g., [55][56][57]) corroborated, the utility of the K/Rb and K/Cs of K-feldspar and micas as guides for chemical fractionation among pegmatites and between pegmatites and associated granites. Figures 16 and 17 show K/Rb vs. Rb and K/Cs vs. Cs in K-feldspar and in muscovite, respectively, from the Panceiros bodies, from the Presqueira pegmatite, and from the granites outcropping in the area. The plot of K/Rb vs. Cs in K-feldspar is also shown in Figure 16. The K/Rb and K/Cs ratios decrease, and Rb and Cs concentrations increase, either in K-feldspar or in muscovite, from the granites to the Presqueira pegmatite, indicating a fractionation trend. Thus, the degree of magmatic differentiation is lower in Panceiros pegmatites than in the Presqueira pegmatite and is close to that of the granites. Finally, the plot of K/Rb vs. Cs in K-feldspar shows a trend from the granites to the studied pegmatites. This is consistent with a fractional crystallization in which the magmatic differentiation increases in this direction, Presqueira pegmatite having the higher degree of differentiation. This is also supported by the whole-rock compositions of the pegmatites and associated granites shown in Figure 11.

Discussion and Conclusions
The mineralogical and geochemical studies of these selected pegmatites from Central Galicia point to a number of common and contrasting features between barren and Li-Sn-Ta mineralized pegmatites from this part of northwestern Iberia. These features are presented and discussed below.
As regards possible distinctive trace elements in major minerals, i.e., K-feldspar and muscovite, for use as a tool to prospect for Li-Sn-Ta mineralized pegmatites, our study shows that these minerals have the same type of trace elements in both barren and mineralized bodies, these trace elements being Rb, Cs, and P and, to a lesser degree, Zn and Ba. However, K-feldspar and muscovite of the Li-Sn-Ta mineralized body show the highest contents in Rb and Cs together with the lowest contents in P. It is noticeable that the muscovite with the highest F contents is in the barren bodies. Thus, high F contents may be characteristic of less evolved barren pegmatites genetically associated with Li-Sn-Ta mineralized bodies having spodumene as the main Li-bearing phase. Taking into account the

Discussion and Conclusions
The mineralogical and geochemical studies of these selected pegmatites from Central Galicia point to a number of common and contrasting features between barren and Li-Sn-Ta mineralized pegmatites from this part of northwestern Iberia. These features are presented and discussed below.
As regards possible distinctive trace elements in major minerals, i.e., K-feldspar and muscovite, for use as a tool to prospect for Li-Sn-Ta mineralized pegmatites, our study shows that these minerals have the same type of trace elements in both barren and mineralized bodies, these trace elements being Rb, Cs, and P and, to a lesser degree, Zn and Ba. However, K-feldspar and muscovite of the Li-Sn-Ta mineralized body show the highest contents in Rb and Cs together with the lowest contents in P. It is noticeable that the muscovite with the highest F contents is in the barren bodies. Thus, high F contents may be characteristic of less evolved barren pegmatites genetically associated with Li-Sn-Ta mineralized bodies having spodumene as the main Li-bearing phase. Taking into account the empirical equations [50], the concentration of Li is proportional to the F contents in muscovite; muscovite with the highest F contents would, therefore, have the highest Li contents. Smeds [56] points out that in less fractionated pegmatites, i.e., Panceiros bodies in the current study, muscovite is probably the only Li-bearing mineral. This author also reports a strong correlation between elevated amounts of Sn in muscovite (>500 ppm), especially when coupled with low Mg and Fe, and the presence of cassiterite in Li-Sn bearing pegmatites in Sweden. Our study does not support this observation since with Mg and Fe being very low, the Sn contents obtained in the studied muscovite are always below the limit of detection (233 ppm).
Sn-Ta-(Nb) oxides have only been recognized in the Li-Sn-Ta mineralized Presqueira pegmatite, and two main trends of compositional evolution are shown by the Ta-Nb bearing oxides. The trend characterized by the increasing Ta ratio at near-constant Mn ratio indicates that the magmatic fractionation progressed from columbite-(Fe) to tantalite-(Fe) (Figure 8b,c,e and Figure 10). This fractionation trend is thought to be in accordance with low F activity [58][59][60][61][62][63][64][65]. The Presqueira pegmatite may represent an F-poor system, which it is also supported by the lack of lepidolite, as the Li-bearing phase, in this pegmatite. However, the presence of amblygonite may indicate conditions of not so low mPFO 2 , leading to the stabilization of both Li-aluminosilicates and amblygonite-montebrasite [66]. The other trend, characterized by increasing the Ta ratio and decreasing Mn ratio (Figure 9a,b and Figure 10), is shown by crystals with oscillatory zonation of Nb-richer cores partially disrupted by patchy Ta-richer zonation, with anhedral tapiolite-(Fe) occurring at the margins of the grains or along microfractures. We consider that this tapiolite-(Fe) represents a later event superimposed on the earlier columbite-tantalite. We interpret this trend related to the replacement of early Nb-richer columbite-tantalite by a late generation richer in Ta. This is a common feature in rare-element granitic pegmatites (e.g., [38,48,64,65,67,68]). Some of these authors propose that reactive fluids that separated from highly evolved residual albitic melts in the last stages of pegmatite crystallization are involved in these replacements. Since the albitization is a striking characteristic of the Li-Sn-Ta mineralized body in this study, a fluid separated from a Na-rich residual melt may have played an important role in the subsolidus evolution of this pegmatite. Hence, the re-equilibration of the earlier columbite-tantalite with this late fluid could have produced its partial resorption followed by the later crystallization of a Ta-Fe-richer oxide. This fluid could have also been the means of transport for the elements into the wallrock in a late magmatic subsolidus stage [69].
Both the barren and the Li-Sn-Ta mineralized pegmatites of this study have phosphates, though the paragenetic associations are different in each case. In the barren bodies, the textural relationships between phosphates and silicates support a metasomatic paragenesis of fluorapatite and eosphorite-childrenite replacing early silicates (Figure 3c,d). In the Li-Sn-Ta mineralized body, however, not all the phosphates are metasomatic in origin. In fact, the textural features allow us to establish that during the pegmatite evolution, P behaved compatibly during the final stages of the main silicate crystallization. Roda-Robles et al. [70] point out that P combines with Fe, Mn, and/or other elements to form primary Fe-Mn-rich and Al-rich phosphates. In the Li-Sn-Ta mineralized Presqueira pegmatite, these primary phosphates are fluorapatite and montebrasite. Later on, a new generation of fluorapatite and hydrated Al-bearing phosphate, goyazite, formed to replace early silicates, i.e., K-feldspar (Figure 7a,b). The mineralized body of Presqueira could be an open-system at the end of the crystallization, in a later subsolidus stage, allowing the penetration of elements, such as Sr, from host-rocks into the igneous body. A similar situation has been described in some Beauvoir granites [71]. Nevertheless, an internal origin due to the liberation of this element from the magmatic feldspars during their replacement/alteration cannot be discarded despite the measured Sr in feldspar being mainly below the LOD of the electronic microprobe (519 ppm). The above-mentioned fluid, which separated from a Na-rich residual melt, would also have been the means of transport for these elements. Finally, this body has veinlets filled with a paragenesis of hydrated phosphates (vivianite and messelite), together with sulfides such as sphalerite (Figure 7d). This paragenesis seems to represent an even later post-magmatic hydrothermal event.
A noticeable textural feature of the Li-Sn-Ta mineralized body is the previously described textures of the lithium aluminosilicates, the petalite replacement by both myrmekites and the spodumene + quartz intergrowth "SQI", and the quartz-spodumene symplectitic intergrowth at the contact with feldspars (Figure 6b,c). On the basis of these textures, we suggest a primary crystallization of petalite that was partially transformed into SQI during the cooling of the pegmatite, an evolution commonly observed by several authors (e.g., [57] and references therein). In addition, the residual albitic melt that caused the sodium metasomatism in the pegmatite also led to the partial transformation of petalite into albite + quartz (myrmekites) (Figure 6e,f) and the release of the lithium in the residual albitic melts. This favored spodumene crystallization during the albitization, and probably the development of quartz-spodumene symplectitic rims at the contact with the coeval albite. Other authors [72,73] also reported the coexistence of Li-aluminosilicates displaying interesting textures in some Li-rich aplite-pegmatites of northern Portugal. Hence, this is a stimulating point to encourage investigation of the pegmatites with Li-aluminosilicates in northwestern Iberia.
Whole-rock and mineral geochemistry of the studied pegmatites and the neighboring granites show fractionation trends that are consistent with magmatic differentiation increasing from the granites to the barren pegmatites and to the Li-Sn-Ta mineralized pegmatite. The latter attained the highest degree of magmatic differentiation (Figures 11, 16 and 17). The neighboring granites are peraluminous and strongly differentiated ( Figure 11). The two-mica granite facies labeled GRM (Figure 2) would be the most evolved of them, a fact which is supported by the strongest europium anomaly of its REE pattern ( Figure 12). As regards REE contents, there is REE depletion from the granite facies to the studied pegmatites. In fact, there is a remarkable disruption that disconnects the chondrite-normalized REE patterns and REE abundances of the neighboring granites from those of the studied pegmatites, even from the less-evolved barren pegmatites of Panceiros ( Figure 12). A distinctive feature of the Li-Sn-Ta mineralized Presqueira pegmatite is that it has the lowest concentrations in rare earth elements. The chondrite-normalized REE patterns of the studied pegmatites tend to be flat, variable, and complex, and the europium anomaly can be either negative or absent. These features agree with those described by several authors who compare the REE distribution patterns of pegmatites and their parental granites [74,75]. According toČerný [76], similar characteristics to those shown for the studied pegmatites are distinctive of granite-pegmatite sequences of the LCT family. This author proposes that these features are due to partial melting of metamorphic protoliths leaving REE-rich accessory minerals in the residua, to progressive REE-depletion of the differentiating melts by crystallization of some minerals (e.g., monazite, xenotime, apatite, garnet) and to a change in the oxidation state of europium. Complex variable chondrite-normalized REE patterns in pegmatites have been documented by several authors (e.g., [74][75][76][77][78][79][80]). As a possible explanation for the REE-patterns of the studied pegmatites, we suggest that late-magmatic to post-magmatic (hydrothermal) processes, and interaction with external metamorphic fluids, affected the studied bodies and probably remobilized some rare earth elements to produce the complex variable patterns. These stages involved the described replacement of primary silicates, such as K-feldspar or petalite by albite, and feldspars by phosphates.
From the current geochemical study of the pegmatite-wallrock interaction, the pegmatite-derived fluids that migrated into the surrounding host-rock were enriched in alkali metals such as Li, Rb, and Cs and, moreover, in Sn, Zn, and As ( Figure 13). The presence of tourmaline in these alteration halos suggests that B also migrated to the country rock. According to London ([81] and references therein), tourmaline halos of pegmatite wallrocks are explained by mixing of two reservoirs: B-rich, hydrous peraluminous granitic melts (and vapor equilibrated with these melts) and typical host rocks (or fluids equilibrated with them) that contain sufficient Fe and Mg to exceed the solubility product of the tourmaline. This supports that the tourmalinization of host rocks adjacent to the studied pegmatites is due to the egress of B-rich aqueous fluids from pegmatites during the late stages of their consolidation. In addition to B, F also transferred from the pegmatite system into the host-schist since the metasomatic biotite of the wallrock is enriched in this halogen. Rubidium, Cs, F, and probably Li, enter the biotite more than the muscovite, as has been documented by other authors (e.g., [82]). London [57,83] points out that the formation of a biotite metasomatic aureole results in well-developed Li-Rb-Cs dispersion patterns around LCT pegmatites, and alkali metasomatism is a consequence of relatively high fluid-melt partition coefficients for Li, Rb, and Cs. Tin, Zn, and As also have relatively high fluid-melt partition coefficients, so they probably behaved in the same manner as did the alkali metals. Our results show that there is no difference in the suite of elements, i.e., B-F-Li-Rb-Cs-Sn-Zn-As, between the metasomatized wallrocks of the Panceiros pegmatites, less evolved and barren, and the Li-Sn-Ta mineralized body of Presqueira. In fact, only Li is clearly added in significant higher amounts to the wallrock of the ore-bearing Presqueira pegmatite. From the current results, the wallrock signature would not seem to be a useful tool for the exploration, at least for pegmatites from Central Galicia (northwestern Spain). However, London [83] states that wallrock alteration could provide clues for the exploration of internal zones. This author comments that "whereas the border zones of most pegmatites possess rather simple granite mineralogy and petrology, the chemistry of wallrock alteration reflects that of the innermost and most chemically fractionated portions of a pegmatite". Taking this into consideration, the possibility of the existence of more chemically fractionated zones carrying Li-Sn-Ta mineralization in the apparently barren bodies of Panceiros would be given, and, at the present level of erosion, these evolved zones would not outcrop.

Acknowledgments:
The authors thank Solid Mines España, S.A.U for access to sample the Presqueira pegmatite and the data provided.

Conflicts of Interest:
The authors declare no conflict of interest.
Appendix A Table A1. Representative chemical compositions (major and trace elements) of K-feldspar from the barren pegmatites and the Li-Sn-Ta mineralized pegmatite.