The metasomatic enrichment of Li in psammopelitic units at San José-Valdeflórez, Central Iberian Zone, Spain: a new type of lithium deposit

A new type of Li mineralisation in hard rock has been found to occur in the Valdeflórez area (Cáceres, Spain), where there is 111.3 Mt of resources and a mean value of 0.61 wt% of Li2O. Lithium is mainly held by very fine-grained micas, important constituents of Ordovician psammopelitic rocks belonging to the Palaeozoic metasedimentary sequence of the Cáceres syncline. The mineralised zone has an elliptical surface shape with a dimension of ~ 700 × 500 m. Lithium-bearing rocks show a characteristic layered appearance, in which light grey quartz-micaceous laminae < 1 mm to some centimetres in thickness, with a variable ratio of quartz to mica, alternate with fine to very fine-grained, dark grey to black tourmalinite laminae parallel to the regional foliation. Subvertical quartz + (montebrasite)-veins that cut the regional foliation at an extremely high angle are also common in this area. Mineralisation and the associated veins are likely to be linked to the intrusion of the nearby Cabeza de Araya pluton. The infiltration of granite-derived Li-, F-, B- and P-rich aqueous fluids into the host rocks through fractures related to shearing processes is considered to be the cause of the formation of Li-rich micas and intense tourmalinisation at the expense of pre-existing phyllosilicates.

Scientific RepoRtS | (2020) 10:10828 | https://doi.org/10.1038/s41598-020-67520-6 www.nature.com/scientificreports/ century, most production came from non-U.S. sources. The largest reserves of Li are found in evaporitic-type deposits, particularly in Bolivia, Chile and Argentina, with ≈ 56% of the world's reserves occurring in these countries 3 . In addition to these types of mineralisation, lithium-bearing clay deposits have been estimated to contain about 7% of the world's lithium resources 3 .
Rare-element granitic pegmatites may also have concentrations that are high enough to make their mining economically profitable. A significant number of pegmatites are being mined or explored for Li 9,10 . Spodumene, lepidolite, petalite, eucryptite and amblygonite-montebrasite are the most common Li-bearing minerals found in economic deposits of this type, with spodumene being the most important. Lithium-enriched rocks are common in the Central Iberian Zone (CIZ), mainly in aplite-pegmatite bodies grouped in pegmatite fields, which are usually spatially related to larger granite intrusions 11,12 . Less commonly, Li occurs in concentrated forms in quartz-rich and greisen-type veins that constitute stockworks at the top of more or less fractionated leucogranitic cupolas. In these veins, the main Li-carrier minerals belong to the amblygonite-montebrasite series 11,12 .
Lithium mineralisation in the Valdeflórez area (Spain) represents a new mode of occurrence of large-scale Li enrichment in metamorphic rocks, which nowadays constitutes the San José-Valdeflórez mining project. This is an important mineralisation of Li in metasedimentary rocks that was presumably induced by granite-derived Li-and B-rich fluids. Here, we focus on the geological characteristics that distinguish this from other deposits of Li, the mode of occurrence and the composition of the lithium-rich minerals, spatial distribution of lithium in the area and the possible genetic model. In this way, we propose a new type of Li deposit in hard rock.

Geological setting
The Valdeflórez area is located in the southern part of the CIZ, in the province of Cáceres, Spain (Fig. 1). The CIZ constitutes the innermost region of the Iberian Variscan belt and represents the westernmost segment of the European Variscides. Two main domains can be distinguished in the CIZ: (i) the northern CIZ, which mainly consists of a thick Neoproterozoic to Early Palaeozoic metasedimentary sequence that is intruded by a set of Cambro-Ordovician metagranites, and (ii) the southern CIZ, which includes a very thick sequence of Neoproterozoic to Lower Cambrian psammopelitic metasediments with some conglomeratic, carbonate and volcaniclastic rock intercalations, the so-called Schist-Greywacke Complex. In some areas, early Ordovician to early Carboniferous sediments occur unconformably over these metasedimentary sequences. The northern and southern CIZ domains show differences in the tectonic style, geochemical features and metamorphic grade 13,14 .
The most prominent structural features of the CIZ were acquired during the Variscan orogeny, which caused a polyphasic deformation (see 15 for an overview). The main phases, D1 and D2, yielded a heterogeneous ductile  www.nature.com/scientificreports/ deformation with a regional low-to high-grade metamorphism that evolved from intermediate-P/high-T to low-P/high-T conditions, reaching partial melting in the lower structural levels 13,16,17 . Subsequent phases of deformation developed under low-grade metamorphic conditions, which are considered as responsible for CIZ's macrostructural models and retrograde effects on previous metamorphic mineral associations. The CIZ is characterised by a huge volume of syn-to postkynematic granitoids that were emplaced between ~ 330 and ~ 290 Ma [18][19][20] , where four S-types (S1, S2, S3 and S4) and one I-type granite series have been documented 21 . Peraluminous granitoids (A/CNK ratios ≥ 1.1) with a crustal isotopic signature ( 87 Sr/ 86 Sr ~ 0.711-0.715 and εNd ~ − 4 to − 8 18 ) constitute an important rock suite, but metaluminous granitoids and basic rocks are scarce in the CIZ [22][23][24] .
Many ore deposits are genetically related to granitic massifs in this geologic domain, particularly in the western part, including pegmatites, aplites, stockworks and quartz-and greisen-type veins. Although most of the pegmatites exhibit a poor degree of evolution, Li is commonly enriched in some fractionated pegmatitic bodies (usually aplite-pegmatites) and in some quartz-rich veins 11,12 . According to field relationships and geochemical affinities, the S1 and S2 granitic series-interpreted as deriving mainly from the partial melting of highly peraluminous, Ca-poor and P-rich Neoproterozoic metasediments during the Variscan orogeny-are related to Li mineralisation in the CIZ 12 .

Methods
Electron-microprobe analyses (640) of the micas and tourmaline were obtained using a CAMECA SX100 at the research centre of the University of Oviedo (Spain). The operating conditions included a voltage of 15 kV, a beam current of 30 nA and a beam diameter of about 2 μm. The standards for calibration were as follows: orthoclase (K and Si), wollastonite (Ca), MnTi (Mn and Ti), magnetite (Fe), albite (Na), corundum (Al), forsterite (Mg), Cs glass (Cs), apatite (P), IRX (Rb) and LiF (F). Data were reduced according to the procedure by Pouchou and Pichoir 25 . Analytical errors are in the ranges of ± 1-2% (major elements) and ± 10% (minor elements). Structural formulae for the micas were established based on 22 atoms of oxygens, and tourmaline formulae were normalised based on 15 cations exclusive of Na, Ca and K, according to the scheme by Henry and Dutrow 26 .
Trace elements, including Rb, Cs, Sn, Zn, Ba, Be, Nb, Ta, Zr, Ni, Tl, Pb, P, Sc, V, Ni, Ga, Sr, Y, Zr, Mo, Hf, W, Tl, Th, U and REE, were analysed by Quadropolar Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-Q-ICP-MS) (iCAP-Qc model) in SGIker at the Universidad del País Vasco (UPV/EHU) (see 27 ). The LA-Q-ICP-MS analyses were conducted with a 213 nm New Wave Research UP213 laser coupled with a Thermo Fisher Xseries-II ICP-MS with an Xt interface and a shielded plasma torch using the NIST SRM 612 glass for tuning. The ablation was carried out in a helium atmosphere. The laser beam was fixed to an 80-μm-wide square section. The spot was preablated for 45 s using a laser repetition rate of 10 Hz and 40% output energy. Then, the spot was ablated for 60 s at 10 Hz with a laser output energy of 75%. A typical session of analysis of a single thin section began and ended with an analysis of the calibration standard NIST-612 glass, which was also analysed every five spots to correct for drift. Silicon was used as the internal standard, and the accuracy and precision were based on the measurement of secondary standards (NIST-610). The limits of detection (LOD) were based on three times the standard deviation (3σ) of the measurements. Data reduction was carried out with Iolite 3 software 28 (https ://iolit e-softw are.com). Taking into account the good positive correlation between the Li and F contents in the micas, which is consistent with the empirical relationships offered by Henderson, Martin and Mason 29 , Tindle and Webb 30 , and Tischendorf, Gottesmann, Förster and Trumbull 31 , the Li 2 O values in trioctahedral micas were estimated from microprobe data using the equation tri-4b of 29 , whereas those of the dioctahedral micas have been calculated using the di-1 equation of the same authors.
It is necessary to mention the specific problem of obtaining good chemical data on samples that are mostly very fine-to ultrafine-grained. To overcome this challenge, only those chemical analyses (≈ 80%) that fit sufficiently with the ideal formula have been considered. The others probably do not correspond to analyses of a single crystal, but to a mixture of fine-grained crystals of different minerals.

Geology and mineralogy of the host rocks and Li mineralisation
The Valdeflórez area is located on the northern flank of the Cáceres syncline, near the Cabeza de Araya pluton ( Fig. 1). This pluton is the most representative of the granitoids of the so-called "serie mixta" 22 and belongs to the S2 granitic series in the classification of Villaseca 21 . It consists of three main units with an emplacement sequence from 308 to 305 Ma 30 ( Fig. 1): (i) cordierite-bearing two-mica granite with feldspar megacrysts and biotite > > muscovite in the outer zone; (ii) a coarse-grained two-mica granite in the intermediate zone; and (iii) aplitic granite and leucogranite in the inner zone, with a mineral assemblage similar to (ii) but with albite and a higher proportion of muscovite 31 . Andalusite, sillimanite, tourmaline and cordierite are usually among the accessory minerals in these granites. Granites of the outer and intermediate zones appear to be related by a fractional crystallisation process, as illustrated by their geochemical trends 31 . In addition, some two-mica granites display relatively high Na, Rb and Li contents that can be attributed to enrichment by late-magmatic fluids. In contrast, the granites in the inner zone do not seem to form part of the differentiation series defined by the other granites, as evidenced by their discontinuous geochemical trend 31 .
The Li mineralisation consists of metasomatised metasedimentary units, including a psammopelitic sequence with some interlayered quartzitic levels (Fig. 1). Metapelites are fine-grained (< 100 μm) and, in general, display a spaced to continuous foliation on a thin section scale defined by the micas. Post-S1 partially chloritised biotite, which developed during contact metamorphism, appears at some locations. Metapsammites and quartzites show a massive aspect with a granoblastic texture, in which variable proportions of quartz and micas and minor feldspar, constitute the main mineral association. Tourmaline, zircon, rutile and ilmenite occur as accessory phases. The mineralised zone has an elliptical surface shape with a dimension of ~ 700 × 500 m (Fig. 2) www.nature.com/scientificreports/ www.nature.com/scientificreports/ programme revealed that mineralisation extends at least 450 m in depth, dipping subvertically to the north west; it largely inherits the primary fabric and is characterised by the occurrence of a repetitive lithological sequence of a millimetre to centimetre thickness, where the following alternate: (i) very fine-to fine-grained, light grey predominantly micaceous layers; (ii) very fine-to fine-grained, dark grey to black tourmalinite; and (iii) layers with intermediate compositions in which quartz, micas and tourmaline show variable proportions. Quartz-and greisen-type veins that strike mainly NE-SW, dip subvertically to the NW and cut the regional foliation at a very high angle are also common in this area. Vein thicknesses vary from around 1 cm to a few metres in places, with average values of 2-5 cm. Locally, the veins branch and cross-cut the host rock parallel to and across the regional foliation, in places forming stockworks. No clear evidence of compositional gradients with proximity to the veins has been observed. Li-rich quartz-micas rocks. Overall, the quartz-micaceous layers consist of micas, quartz in variable proportions, minor tourmaline and apatite, zircon, ilmenite, rutile, pyrite and arsenopyrite as accessory minerals. White mica predominates over dark mica, and they commonly are intimately associated at a microscopic scale. In these rocks, the micas display a variety of sizes, colours and textures 33 but mostly occur (i) as compact layers and laminae with variable amounts of disseminated very fine-grained quartz clasts (Fig. 3a) and (ii) as filling the fine-grained tourmaline network in tourmalinite (Fig. 3b). White micas locally exhibit yellowish, beige or pinkish hues. Unlike the metamorphic reddish-brown biotite, the dark mica of the metasomatic zone is optically heterogeneous with a variable colour from pale grey or orange to pinkish brown. White mica flakes (< 200 μm) inside quartz-rich augen containing opaque minerals can also be observed (Fig. 3c). Tourmaline-rich rocks. These rocks are very fine-to fine-grained, dark green to black and contrast sharply with the light-coloured quartz-mica rocks. A characteristic feature is a layered appearance where tourmalinerich laminae < 1 mm to some centimetres in thickness alternate with quartz-mica laminae (Fig. 3a,d,e and see Fig. 3a,b in 35 ). The percentage of tourmaline is variable, from < 20% to ≈ 80% by volume, with subordinate white micas, quartz and ore minerals. In the thin sections, tourmaline appears as pale green-colourless to dark green or yellowish brown, euhedral to subhedral crystals (< 50 ~ μm generally), without a distinctive lineation, forming a more or less dense network filled by a fine-grained quartz-micaceous matrix (Fig. 3b). However, mylonitic textures with very fine-grained psammitic lenses, σ-type psammitic to quartzitic porpyroclasts and boudinage are also common (Fig. 3e,f) (Fig. 4b). Amounts of MnO are very low (< 0.15 wt%), and Li contents (< 80 ppm) are negligible.

Quartz-and greisen-type veins.
A set of white mica-bearing quartz-and greisen-type veins with cassiterite and minor amounts of K-feldspar ± amblygonite-montebrasite ± apatite ± fluorite ± topaz traverses the mineralised area. Micas from veins are mostly prismatic muscovite with 0.4-0.8 wt% F and 0.1-0.3% wt% Li 2 O and very fine-grained Li micas with 0.1-7.8 wt% F and up to 5.1 wt% Li 2 O. A detailed petrographic and mineralogical description of these veins can be found in Pesquera, Torres-Ruiz, Gil-Crespo and Velilla 33 , and Torres-Ruiz, Pesquera, Gil and Casas 35 .

Discussion
It is obvious that the micas of the psammopelitic metasediments are the main host for Li in the Valdeflórez area. Outside the mineralised zone, the micaceous minerals include muscovite and minor biotite. Petrographic observations, in conjunction with chemical and experimental data, indicate the following: (i) micas underwent a  36 . In the case of Valdeflórez, Al, Fe and Mg are supplied by the metasedimentary rocks, while fluids would be provided by Li, F, B, P and, to a lesser extent, Rb and Cs. Temperature estimates with the Ti-in-biotite thermometer 37 for biotites with an average of Mg/ (Mg + Fe + Mn) = 0.32 ± 0.034 and Ti = 0.14 ± 0.03 apfu yield temperatures of 400-450 °C, which are considered the highest temperatures at which the metasomatic processes developed. Likewise, the use of the calibrations by Munoz and Ludington 34 to calculate the activity ratio aH 2 O/aHF of aqueous fluids in equilibrium with biotite suggests that the log (fH 2 O/fHF) values in the fluid phase are around 4.4 for the temperature and biotite compositions indicated above. A high activity of Fin the hydrothermal fluids may be of paramount importance for the transport of Li because Li + and F -(as well as PO 4 3− ) are considered to be hard metal and ligands, respectively 38 , and tend to form strong complexes in hydrothermal fluids 39 .
Stockworks constituted by montebrasite ± cassiterite quartz veins, which are very similar to those of Valdeflórez, are observed in other localities of the CIZ and are related to fractionated granitic rocks in cupolas (e.g., Golpejas, Barquilla 40 ; Massueime 41 ; El Trasquilón 42 ). The spatial distribution of these quartz veins and the extensive associated Li-B metasomatism in the Valdeflórez area suggest that Li enrichment, tourmalinisation and mineralised quartz-and greisen-type veins are closely related to fluids derived from the same granitic source in a tectonically active regime. Drilling works down to 450 m in depth have not found any granitic body under the (1) Li mineralisation is located near the pluton, with the occurrence of some Araya-type granitic apophysis in the Valdeflórez area (Fig. 1). In addition, gravimetric data reveal the pluton extends largely in depth to the south east 43 . (2)  To assess the potential of the different granitic units from the Cabeza de Araya pluton to fractionate and exsolve Li-rich fluids, a geochemical modelling using the Rayleigh equation for fractional crystallisation has been carried out (Fig. 5): where C 1 is the weight concentration of a trace element in the differentiated melt, C 0 is the weight concentration of that element in the parent melt, F is the fraction of melt, and D is the bulk distribution coefficient. Modelling has been done starting from the Li and Ba contents of the three main petrographic units described in the pluton (Fig. 1). Modal proportions of the minerals of these granitic units have been used as fractional crystallisation components. It should be noted that to apply this equation, it is needed to know the distribution coefficients of Li and Ba between solid and melt. Inasmuch as the aim of this modelling is just to check the potential of the three Cabeza de Araya pluton-constituting units to generate Li-rich melts and fluids, two different sets of distribution coefficients have been used: one being the most favourable for Li fractionation based on the given modal proportions (Fig. 5a) and the other one being the less favourable for it ( Fig. 5b) (data compiled by Jolliff, Papike and Shearer 45 and London 46 ). Degrees of fractionation of granitic melts up to 99% are proposed to obtain the concentrations of Li observed in the aplite-pegmatites of some fields and in some leucogranitic cupolas in the CIZ (e.g., [47][48][49] ). According to the results obtained for the Cabeza de Araya pluton, the concentration of Li in the fractionated melts would be in the order of thousands of ppm for the three granitic units. Accordingly, the fractional crystallisation of a melt with the composition of any of the three units of the Cabeza de Araya pluton can lead to significant Li concentrations, presumably high enough to exsolve Li-(B-F-P-Sn)-enriched hot fluids during a hydrothermal stage. Experimental work with hydrous granitic melts indicates that the liquidus and solidus may be lowered with the presence of B, Li, F and P, which also enhances the solubility of H 2 O and controls the behaviour of incompatible lithophile elements (e.g., [50][51][52][53].
The shearing processes related to D3 deformation may have triggered an abrupt change in the evolution of the system from closed to open. Fluorine is generally considered to be a compatible element in fluid-saturated melts 54 and is not lost into the volatile phase until there are very low pressures 55 . Accordingly, a sharp drop in pressure when the system is suddenly opened through shear processes is expected to provoke the exsolution of the Li-F-B-P-rich fluids responsible for the Li mineralisation of Valdeflórez.

conclusions
The salient points of the present study are as follows: www.nature.com/scientificreports/