Magnetic mineralogy of Variscan granites from northern Portugal: an approach to their petrogenesis and metallogenic potential

DOI: 10.1344/GeologicaActa2020.18.5  C. Cruz, H. Sant’Ovaia, F. Noronha, 2020 CC BY-SA C . C r u z e t a l . G e o l o g i c a A c t a , 1 8 . 5 , 1 2 0 ( 2 0 2 0 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 0 . 1 8 . 5 Magnetic mineralogy and W(Mo) mineralizations associated with Variscan granites 2 These two types of granites have distinctive wholerock oxygen isotope (δ18O) values. The highest δ18O values indicate a crustal origin, and the lowest values suggest a mantle contribution and/or source region dominated by mafic and meta-igneous rocks (Ellwood and Wenner, 1981; Ishihara, 1977). The δ18O mean values range between 10.65‰ and 12.90‰ for the two-mica granites and 9.75‰ and 12.84‰ for the biotite granites (Antunes et al., 2008; Cruz et al., 2016; Sant’Ovaia et al., 2013a, b; Teixeira et al., 2012). The magnetic susceptibility (Km) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite as well as ferromagnesian silicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and different magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidizedor magnetite-type granites and reducedor ilmenite-type granites, respectively (Ishihara, 1977). Magnetite granites are considered to have been generated at great depth (upper mantle and/or lower crust), whereas the ilmenite-series are considered to have originated at a shallower level (middle to lower continental crust) where small amounts of crustal carbon are present (Ellwood and Wenner, 1981; Ishihara, 1977; Sheppard, 1977). Relative abundances of magnetic minerals in granites can be measured in terms of magnetic susceptibility, with the magnetite-series having Km> 3.0·10-3SI and ilmeniteseries, in general, with Km around 10-6SI (or μSI) (e.g. Ishihara, 1977; Takagi and Tsukimura, 1997). In the Iberian Variscan belt, specifically in the Central Iberian Zone (CIZ), in Spain and Portugal, the magnetic behavior of numerous granites has been analyzed. The Spanish granites are mostly ilmenite-type and have been extensively analyzed by several authors, like Aranguren et al. (1996); Olivia-Urcia et al. (2012); Porquet et al. (2017); Román-Berdiel et al. (1995). These authors found Km values between 22μSI and 467μSI. Recent studies carried out by Villaseca et al. (2017) proved the existence of both ilmeniteand magnetite-type granites in the Spanish Central System. The magnetic susceptibility of these granites yielded Km ca. 15μSI to 180μSI for Sand I-type granites (ilmenite-type granites) and values between 500μSI and 1,400μSI for the leucogranites I-type granites (magnetite-type granites). Magnetic susceptibility studies in several Portuguese granites (Sant’Ovaia et al., 2014) yielded Km mean values between 48μSI and 84μSI for two-mica granites corresponding to ilmenite-type granites and between 72μSI and 11,676μSI for biotite granites corresponding to ilmenite-type and/or magnetite-type granites. Several metallogenic events are identified on the granite bodies: i) Sn-Li pegmatites are essentially associated to syn-D3 with two-mica granites; ii) W (Sn) and W quartz-vein deposits occur in lateand lateto postD3 biotite granites; and iii) W (Mo) deposits are related to post-D3 biotite sub-alkaline granites (e.g. Mateus and Noronha, 2010; Noronha, 2017; Thadeu, 1965). The magnetic susceptibility (Km) and δ18O data allow to establish a relationship between the granite type (ilmeniteor magnetite-type) and the associated mineralizations (Kumar, 2010; Takagi and Tsukimura, 1997). The occurrence of ore deposits associated with ilmenite-type and/or magnetite-type granites has been largely described, for example, in Indonesia (Maulana et al., 2013) and, in southern Korea and southwestern Japan (Ishihara et al., 1981) and, more recently, in Portugal (Cruz et al., 2016; Sant’Ovaia et al., 2014). In Sulawesi, Indonesia, the ilmenite-type granites outcrop in the southern zone of the island while the magnetite-type granites occur in the northern area. Both granites have associated ore mineralization. Cu-Au-Mo mineralizations are associated with magnetite-series granitic rocks, while Sn-W mineralizations related to reduced ilmenite-type granites have not yet been reported (Maulana et al., 2013). Contrastingly, in southern Korea and southwestern Japan (Ishihara et al., 1981), W-Mo occurrences were described in areas with both ilmeniteand magnetite-type granites with no clear separation in the distribution of the two types of granites. In the Portuguese sector of the Central Iberian Zone, the Sn deposits are associated with Sand ilmenite-type granites and the W-(Sn) mineralizations occur in veins that cut the I (peraluminous)and ilmenitetype granites, both with high δ18O values, The W (Mo) occurrences are spatially related to I (peraluminous)and magnetite-type granites (Cruz et al., 2016; Sant’Ovaia et al., 2012). The study of magnetic mineralogy expands our knowledge of the relation between granites and their related mineralizations, since the redox conditions control the specific mineral occurrences. It is very important to understand the relationship between magnetic mineralogy and ore deposits considering the complexity of magma genesis. The main goal of this work is to characterize the magnetic behavior of the Lamas de Olo Pluton (LOP) and compare it with the magnetic behavior of other post-tectonic Variscan biotite plutons (Peneda-Gerês Pluton (PGP), LavadoresMadalena Pluton (LMP) and Vila Pouca de Aguiar Pluton (VPAP). We aim at establishing a relationship between the magnetic behavior of the granites and the occurrence of different kinds of mineralizations.


INTRODUCTION
The northern area of Portugal is characterized by large volumes of granitic intrusions related to Variscan orogeny (321-290Ma). Synorogenic granites represent the plutonic magmatism and, based on their geological, petrographic and geochemical characteristics, are divided into two main groups (e.g. Chappell and White, 1974;Dias et al., 2010;Ferreira et al., 1987;Noronha et al., 2006). The first group consists of two-mica peraluminous granites, which are dominantly syntectonic (syn-D 3 : 321-312Ma) and considered S-type granites, resulting from the crystallization of wet peraluminous magmas originated at a mesocrustal level. The second group consists of biotite granites considered to have been generated at deep crustal levels and corresponding to dry magmas. The age of their formation is either syn-D 3 (321-312Ma), late-D 3 (312-305Ma), late-to post-D 3 (ca. 300Ma), or post-D 3 (299-290Ma) (Dias et al., 2010).
These two types of granites have distinctive wholerock oxygen isotope (δ 18 O) values. The highest δ 18 O values indicate a crustal origin, and the lowest values suggest a mantle contribution and/or source region dominated by mafic and meta-igneous rocks (Ellwood and Wenner, 1981;Ishihara, 1977). The δ 18 O mean values range between 10.65‰ and 12.90‰ for the two-mica granites and 9.75‰ and 12.84‰ for the biotite granites (Antunes et al., 2008;Cruz et al., 2016;Sant'Ovaia et al., 2013a, b;Teixeira et al., 2012).
The magnetic susceptibility (K m ) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite as well as ferromagnesian silicates such as biotite.
The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and different magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized-or magnetite-type granites and reduced-or ilmenite-type granites, respectively (Ishihara, 1977). Magnetite granites are considered to have been generated at great depth (upper mantle and/or lower crust), whereas the ilmenite-series are considered to have originated at a shallower level (middle to lower continental crust) where small amounts of crustal carbon are present (Ellwood and Wenner, 1981;Ishihara, 1977;Sheppard, 1977). Relative abundances of magnetic minerals in granites can be measured in terms of magnetic susceptibility, with the magnetite-series having K m > 3.0·10 -3 SI and ilmeniteseries, in general, with K m around 10 -6 SI (or µSI) (e.g. Ishihara, 1977;Takagi and Tsukimura, 1997).
In the Iberian Variscan belt, specifically in the Central Iberian Zone (CIZ), in Spain and Portugal, the magnetic behavior of numerous granites has been analyzed. The Spanish granites are mostly ilmenite-type and have been extensively analyzed by several authors, like Aranguren et al. (1996); Olivia-Urcia et al. (2012); Porquet et al. (2017); Román-Berdiel et al. (1995). These authors found K m values between 22µSI and 467µSI. Recent studies carried out by Villaseca et al. (2017) proved the existence of both ilmenite-and magnetite-type granites in the Spanish Central System. The magnetic susceptibility of these granites yielded K m ca. 15µSI to 180µSI for S-and I-type granites (ilmenite-type granites) and values between 500µSI and 1,400µSI for the leucogranites I-type granites (magnetite-type granites). Magnetic susceptibility studies in several Portuguese granites (Sant'Ovaia et al., 2014) yielded K m mean values between 48µSI and 84µSI for two-mica granites corresponding to ilmenite-type granites and between 72µSI and 11,676µSI for biotite granites corresponding to ilmenite-type and/or magnetite-type granites.
Several metallogenic events are identified on the granite bodies: i) Sn-Li pegmatites are essentially associated to syn-D 3 with two-mica granites; ii) W (Sn) and W quartz-vein deposits occur in late-and late-to post-D 3 biotite granites; and iii) W (Mo) deposits are related to post-D 3 biotite sub-alkaline granites (e.g. Mateus and Noronha, 2010;Noronha, 2017;Thadeu, 1965).
The magnetic susceptibility (K m ) and δ 18 O data allow to establish a relationship between the granite type (ilmenite-or magnetite-type) and the associated mineralizations (Kumar, 2010;Takagi and Tsukimura, 1997). The occurrence of ore deposits associated with ilmenite-type and/or magnetite-type granites has been largely described, for example, in Indonesia (Maulana et al., 2013) and, in southern Korea and southwestern Japan (Ishihara et al., 1981) and, more recently, in Portugal (Cruz et al., 2016;Sant'Ovaia et al., 2014). In Sulawesi, Indonesia, the ilmenite-type granites outcrop in the southern zone of the island while the magnetite-type granites occur in the northern area. Both granites have associated ore mineralization. Cu-Au-Mo mineralizations are associated with magnetite-series granitic rocks, while Sn-W mineralizations related to reduced ilmenite-type granites have not yet been reported (Maulana et al., 2013). Contrastingly, in southern Korea and southwestern Japan (Ishihara et al., 1981), W-Mo occurrences were described in areas with both ilmenite-and magnetite-type granites with no clear separation in the distribution of the two types of granites. In the Portuguese sector of the Central Iberian Zone, the Sn deposits are associated with S-and ilmenite-type granites and the W-(Sn) mineralizations occur in veins that cut the I (peraluminous)-and ilmenitetype granites, both with high δ 18 O values, The W (Mo) occurrences are spatially related to I (peraluminous)-and magnetite-type granites (Cruz et al., 2016;Sant'Ovaia et al., 2012).
The study of magnetic mineralogy expands our knowledge of the relation between granites and their related mineralizations, since the redox conditions control the specific mineral occurrences. It is very important to understand the relationship between magnetic mineralogy and ore deposits considering the complexity of magma genesis.
The main goal of this work is to characterize the magnetic behavior of the Lamas de Olo Pluton (LOP) and compare it with the magnetic behavior of other post-tectonic Variscan biotite plutons (Peneda-Gerês Pluton (PGP), Lavadores-Madalena Pluton (LMP) and Vila Pouca de Aguiar Pluton (VPAP). We aim at establishing a relationship between the magnetic behavior of the granites and the occurrence of different kinds of mineralizations.  (Franke, 1989;Kroner and Romer, 2013;Ribeiro et al., 1990). After the collision, during the late Paleozoic, the Pangea amalgamation leads to continuous rock deformation and the C-shaped Cantabrian orocline was formed (e.g. Gutiérrez-Alonso et al., 2012;Weil et al., 2013). Other authors suggest a more complex scenario, with the existence of two oroclines, in S-shaped, the Cantabrian, and Central-Iberian (e.g. Shaw et al., 2012).
Most of the granite intrusions are coeval with D 3 and the others are late-and post-D 3 (e.g. Dias et al., 2010;Ferreira et al., 1987). The development of different tectonic structures, namely shear zones, during D 3 and successive reactivation events controlled the granite emplacement and caused extensive hydrothermal activity throughout the entire crust, involving distinct fluid sources, some transporting significant amounts of ore mineral phases (Mateus and Noronha, 2010;Noronha et al., 2013).

Plutons and associated mineralizations
The LOP is located in the northern part of the CIZ near the limit with the GTMZ (Figs. 1; 2). It is a small post-tectonic pluton with a rhombus shape, controlled by NNW-SSE offsets conjugated with the NNE-SSW fault system, which are parallel to the Verin-Régua-Penacova fault (VRPF). The LOP is composed of distinct outcropping granites (Fig. 2): i) Lamas de Olo (LO), the more representative, a mediumto coarse-grained porphyritic granite (biotite > muscovite); ii) Alto dos Cabeços (AC), a fine-to medium-grained porphyritic granite (biotite > muscovite) and iii) Barragem (BA), a fine-to medium-grained slightly porphyritic leucocratic granite (biotite= muscovite) (Fernandes et al., 2013;Pereira, 1989). 207 Pb/ 235 U dating in monazite yield an age of 297.19±0.73Ma for the LO granite (Fernandes et al., 2013). Field observations show that the contact between the LO and AC granites is generally diffuse, and that the BA granite crosscuts the LO and AC granites. Previous studies of anisotropy of magnetic susceptibility showed that LOP has two types of behavior granites: magnetite-and ilmenitetype (Cruz et al., 2016). W-Mo (Sn) mineralizations occur, mostly in N 80º E sub-vertical quartz veins (Helal, 1992).
The LOP intrudes Lower Paleozoic formations (Armorican quartzite and schists of Upper Silurian to Middle Ordovician age), the Douro Group metasediments (Upper to Middle Cambrian) and two-mica syntectonic granites in the Vila Real Massif (Fig. 2). The Vila Real Massif is a syn-D 3 two-mica granite composite massif with associated Sn mineralizations in pegmatites, and W-Sn in hydrothermal quartz veins (Pereira, 1989). Monazite and zircon analysis yield a weighted average 207 Pb/ 235 U age of ca. 311±1Ma (Almeida et al., 1998).
Vila Pouca de Aguiar and Peneda-Gerês plutons are located in the GTMZ and Lavadores-Madalena Pluton on the NW border of the CIZ.

MATERIAL AND METHODS
Representative samples of different Variscan granites were examined. The sampled granites were from the Lamas de Olo Pluton, Vila Pouca de Aguiar Pluton, Peneda-Gerês Pluton and Lavadores-Madalena Pluton (Table 1; Fig. 1). Special attention was given to the LOP (Fig. 2), where 67 sites were sampled for this study.

Microscopy
A Leica petrographic polarizing microscope with a digital camera was used for the reflected and transmitted light petrographic studies. The Raman spectra of different opaque minerals were obtained using a Raman LabRAM HORIBA Jobin Yvon Spex spectrometer interfaced with an Olympus microscope with 50x objective lens, diffraction gratings with 1,800 lines mm -1 and equipped with a 632.8nm emission line of a HeNe laser at a power of 20mW. The incident beam perpendicular to the plane of the sample was focused through the microscope lens, which also collected the Raman scattered radiation in back-scattering geometry. A highly sensitive Charge-Coupled Device (CCD) camera was used to collect the Raman spectra. Extended scans were performed on a spectral range from 50 to 1,200cm -1 . The time of acquisition and the number of accumulations varied in order to obtain an optimized spectrum for each analyzed mineral. Both equipment belong to the Departamento de Geociências, Ambiente e Ordenamento do Território (DGAOT) of the Faculdade de Ciências da Universidade do Porto (FCUP) and Instituto de Ciências da Terra-Polo Porto (ICT-Porto).
Complementary studies to identify minor mineral amounts and characterize the magnetic mineralogy were carried out using a Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) X-ray microanalysis at the Centro de Materiais da Universidade do Porto (CEMUP). The SEM/EDS studies were performed using a high-resolution JEOL JSM 6301F SEM coupled with an Oxford INCA Energy 350 system. Samples were carbon-coated using a JEOL JEE-4X Vacuum Evaporator.
A total of 13 thin sections from the LOP were used for petrographic studies (Table 1). For complementary studies, some representative samples were selected: 4 for Raman (3 from LO and 1 from BA) and 5 for SEM/EDS (3 from LO and 1 from BA). These samples were selected taking into  Helal, 1992;Pereira et al., 1987;Pereira, 1989)  Magnetic mineralogy and W(Mo) mineralizations associated with Variscan granites 6 account the presence of magnetite, hematite, and ilmenite, which was previously identified under a microscope or by magnetic susceptibility studies. The results from other plutons were compiled from the literature (Table 1).

Magnetic susceptibility and isothermal remanent magnetization
The relationship between the magnetization induced in a material M and the external field H is defined as M=KH, where K is the magnetic susceptibility. The K reflects the whole-rock mineral composition, comprising the diamagnetic, paramagnetic and ferromagnetic minerals (Rochette, 1987). K measurements were performed using a KLY-4S Kappabridge susceptometer Agico model (Czech Republic) from the DGAOT-FCUP and ICT-Porto. Measurements were undertaken in a 300A/m field at room temperature. For each site, the ANISOFT 4.2 program package (Chadima and Jelinek, 2009) enabled us to calculate the mean susceptibility K m . The magnetite is an important ferromagnetic mineral (s.l.), easily detected because increases the K m values to >10 -3 SI (Bouchez, 1997(Bouchez, , 2000Tarling and Hrouda, 1993).
The magnetic susceptibility data were mostly compiled from the literature, and only a few samples from the Peneda-Gerês Pluton were measured in this study (Table 1).
For a detailed magnetic mineralogy study, the acquisition of Isothermal Remanent Magnetization (IRM) curves were performed. The IRM acquisition curves are important to estimate the characteristic coercivity of the ferromagnetic minerals (Butler, 1992). The IRM refers to the remanence acquired by a sample exposed to a direct magnetic field (H), at ambient temperature. These measurements were performed in the Laboratório de Paleomagnetismo at the Instituto Dom Luís, Lisboa. Beforehand, standard samples were demagnetized through an alternating field cleaning at ca. 100mT, with an LDA-3A (Agico) demagnetizer. Then, the magnetic field (H) was imparted using an Impulse Magnetizer IM-10-30 (ASC Scientific) and the resulting remanence was measured using a spinner magnetometer JR-6A (Agico). Samples were magnetized in the same direction with increasing magnetic fields from ca. 3.51mT up to 1.15T in different steps (ca. 40 steps) and the induced magnetization acquired by the specimens was measured after each step of induction. Data were analyzed using a Cumulative Log-Gaussian (CLG) function (Robertson and France, 1994) with the software developed by Kruiver et al. (2001). For the IRM studies, 4 granite samples from the LOP were selected according to their magnetic behavior: 1 from the LO-magnetite-type granite, 1 from the LOilmenite-type granite, 1 from the BA granite and 1 from the AC granite (Table 1).
Additionally, measurements of frequency-dependent susceptibility were done with a Bartington MS2 System in the Laboratório de Paleomagnetismo at the Instituto Dom Luís, Lisboa. The low-field magnetic susceptibility was measured at two applied field frequencies (0.46kHz and 46kHz). The percentage of frequency-dependent susceptibility (KfD%) (Dearing et al., 1996) was obtained in 10 samples from the LOP granites (Table 1).

Thermomagnetic experiments
The thermomagnetic experiments allow the identification of ferromagnetic minerals (s.l.) based on    their Curie/Néel temperature. The Curie temperature (T C ) is the temperature above which a ferromagnetic material (s.s.) becomes paramagnetic, and the Néel temperature (T N ) is analogous to the T C but for antiferromagnetic materials.
The Curie/Néel temperature of ferromagnetic minerals (s.l.) can be determined in low-field thermomagnetic experiments where the magnetic susceptibility of a sample is monitored while temperature is increased or decreased, and it is defined as the point of major decrease in magnetic susceptibility of the sample during the heating cycle (Butler, 1992). The temperature of this drop in magnetic susceptibility corresponds to the temperature at which a magnetic mineral loses its spontaneous magnetization. In contrast, if magnetic susceptibility, with values much lower than ferromagnetic values, decreases regularly with increasing temperature, the sample has the typical behavior of paramagnetic minerals (Dunlop and Özdemir, 1997).
The Curie/Néel temperature is typical for a particular mineral and is therefore commonly used to identify its composition. For example, the Curie/Néel temperature is 580ºC for magnetite, 680ºC for hematite, 320ºC for pyrrhotite and 120ºC for goethite. However, the rocks can have other iron minerals with titanium in their composition, corresponding to the titanomagnetite and titanohematite series. It should be noted that, in the titanomagnetite series, the Curie point decreases as the Ti content increases; therefore, the T C can range between 580ºC (T C of pure magnetite) and -150ºC (T C of pure ulvospinel) (Dunlop and Özdemir, 1997).
The temperature dependence of low field magnetic susceptibility was monitored with a CS-2 furnace apparatus attached to the KLY-3 susceptometer (Agico) from the Laboratoire Géosciences Environnement Toulouse, Université de Toulouse III -Paul Sabatier, using samples obtained from 19 rock specimens (Table  1). Fifteen samples were selected in order to represent all LOP granites and their different magnetic behavior according to previous magnetic susceptibility studies. Regarding the other post-tectonic plutons, 4 samples were selected (Table 1).
The samples were exposed to increasing temperatures up to 700°C in heating/ cooling cycles. The measurements were made in an argon flux in order to minimize oxygen fugacity, which reduces the oxidation of the samples and the consequent formation of new minerals with the increase and/or decrease of temperature. However, a small amount of oxygen is always present in the sample holder, so although the probability is lower, some mineralogical changes may still occur.
The plagioclases occur frequently zoned in all granites (Fig. 3C) and are albite-oligoclase in the BA granite, and oligoclase-andesine in the AC and LO granites. Occasionally, the plagioclases exhibit myrmekitic intergrowths with quartz, common mostly in the LO granite (Fig. 3F), and are altered by sericitization. The K-feldspar, perthitic orthoclase and microcline occur as heterogranular crystals. It is sometimes possible to observe some K-feldspar megacrystals with small plagioclase inclusions. The quartz has sometimes undulatory extinction, subgrain boundaries and fluid inclusion planes (Fig. 3G).
The biotite -brownish to greenish (Fig. 3E) in all granites-sometimes appears partially to completely chloritized. Occasionally, biotite is altered to muscovite, mainly in the leucocratic granite from BA, where biotite is observed in minor amounts (Fig. 3H). Frequently, biotite has pleochroic halos associated with zircon inclusions (Fig.  3A) and curved biotites are rare (Fig. 3H).
For a detailed magnetic mineralogy characterization, complementary studies of opaque minerals were conducted to point out the presence of magnetite, hematite and/or ilmenite. The SEM/EDS analyses showed the presence of different Fe-Ox minerals (Fig. 4) and other minerals in small amounts, like monazite, xenotime (Fig. 4B) and columbo-tantalite. Figure 4E and F are examples of some EDS spectra performed in LOP thin sections, showing the presence of Fe-Ox and Fe-Ox-Ti minerals. Figure 5 shows the micro-Raman spectrum of the principal magnetic minerals present in the LOP, namely magnetite (Fig. 5A), hematite (Fig. 5B) and ilmenite (Fig. 5C).
A summary of the petrographic descriptions of other Variscan granites made in previous studies is presented in Table 2. The Vila Pouca de Aguiar Pluton is located to the NE of the LOP, in the same NNE-SSW alignment, paralell to the Verin-Régua-Penacova Variscan fault. This pluton has two main granite facies with similar mineralogical composition. Both granites are composed G e o l o g i c a A c t a , 1 8 . 5 , 1 -2 0 ( 2 0 2 0  of quartz, perthitic K-feldspar (orthoclase and microcline) and plagioclase with normal zoning. The Vila Pouca de Aguiar granite contains also oligoclase-andesine and the Pedras Salgadas granite albite-oligoclase. The biotite, the only ferromagnesian phase, is more abundant in the Vila Pouca de Aguiar granite. Accessory minerals include zircon, apatite, allanite, xenotime, ilmenite, sphene and rare monazite (Martins and Noronha, 2006;Sant'Ovaia et al., 2000).

Magnetic susceptibility and isothermal remanent magnetization data
The magnetic susceptibility study in the post-D 3 Variscan granites showed the presence of two types of magnetic behavior : ilmenite-and magnetite-type, with the predominance of the ilmenite-type (K m < 1,000µSI) ( Table  3; Fig. 6). Previous studies in the LOP (Cruz et al., 2016) indicated that: i) the K m values in the LO granite have a huge variability (ranging from low to high K m ), ii) the AC granite has intermediate K m values, and iii) the BA granite has the lower K m values (Table 3). This data show that LO granite is the most heterogeneous granite of the pluton, suggesting that both ferromagnetic and paramagnetic behavior are present. On the other hand, the AC and BA granites have exclusively a paramagnetic behavior. So, the LOP is the most heterogeneous pluton, showing three K m classes, below 50µSI, between 50µSI and 2,000µSI, and higher than 2,000µSI (  Fig. 6B). In the Peneda-Gerês Pluton, two granite facies are ilmenite-type and one is magnetite-and ilmenite-type. The granites from the Lavadores-Madalena Pluton have both minor K m dispersion and the highest K m mean values, always with K m higher than 1,550µSI (Martins et al., 2011;Sant'Ovaia et al., 2014; Table 3; Fig. 6B).
The IRM data from the LOP are presented in Table  4 and Figure 7. The LM 5 sample shows saturation at   et al. (2000) and Martins and Noronha (2006) for VPAP; Mendes and Dias (2004) and Cottard (1979) for PGP; Martins et al. (2011) andSant'Ovaia et al. (2014) for LMP G e o l o g i c a A c t a , 1 8 . 5 , 1 -2 0 ( 2 0 2 0    After analising the data using the CLG function (Kruiver et al., 2001;Robertson and France, 1994), best fits of the raw IRM curves are obtained by considering two or three components (Table 4; Fig.  7). All specimens have, at least two components: component 1 with low coercive phase and B 1/2 (the field at which half of the SIRM is reached) ca. 17mT and component 2, showing an intermediated coercive phase and B 1/2 ca. 59mT. Two samples show a third component, with higher coercivity with B 1/2 ca. 466mT.
The Dispersion Parameter (DP) ranges between 0.25 and 0.42 for all components, with mean values of 0.36, 0.31 and 0.34 for component 1, 2 and 3, respectively. According to Dunlop and Özdemir (1997), the coercivity increases with the replacement of Fe 3+ by Ti in titanomagnetite. Taking this into account, the components 1, 2 and 3 have been interpreted as magnetite, Ti-poor magnetite and hematite, respectively (Abrajevitch and Kodama, 2011;Font et al., 2014;Kruvier et al., 2001;Maxbauer et al., 2016;Robertson and France, 1994). Ti-poor magnetite (component 2) is dominant in all samples (contributes to more than 50% of the total remanence), with exception of LM 7 with 13% of Ti-poor magnetite and 87% of magnetite (component 1; Table 4).
According to some authors (e.g. Font et al., 2009;Font et al., 2014;Kruiver et al., 2001), the differences in B 1/2 and DP values can be interpreted as: varying degrees of oxidation of magnetite for component 1; different Ticontent in the Ti-poor magnetite for component 2; and different magnetic grain sizes for component 3. Sant'Ovaia (1993) in the Vila Pouca de Aguiar Pluton showed that the Vila Pouca de Aguiar and Pedras Salgadas granites have similar magnetic behavior. The S 0.3T ranges between 0.624 and 0.968, with minor values obtained in the Pedras Salgadas granite. Both granites show an absence of saturation, however, the Pedras Salgadas granite presents an acquisition curve with a high slope, showing the depletion of ilmenite (Table 4; Fig. 8). Previous studies in the Lavadores-Madalena Pluton showed saturation fields under 300mT and S 0.3T values higher than 0.955 (Table 4; Fig. 8), pointing out high ferromagnetic mineral content (Martins et al., 2011;Sant'Ovaia et al., 2014).

IRM studies carried out by
Frequency-dependent susceptibility (KfD%) studies were made in LOP samples. In the BA granite, KfD% was higher than 10% (LM 7 ~ 35%; LM 58 ~29% and LM 62 ~30%), which have been interpreted by some authors (e.g. Dearing et al., 1996) as due to the presence of superparamagnetic fine grains, a weak magnetic signal or a high alteration degree. On the other hand, the values acquired for the LO samples (KfD%< 2%) indicated absence of superparamagnetic grains, and for the AC samples (4% <KfD%< 7%) an admixture of superparamagnetic and coarser non-superparamagnetic grains (Dearing et al., 1996). The presence of superparamagnetic minerals in samples from the BA granite could explain the lower values of the magnetic susceptibility (mean K m ca. 27µSI) and the presence of low coercive minerals.   o l o g i c a A c t a , 1 8 . 5 , 1 -2 0 ( 2 0 2 0  Magnetic mineralogy and W(Mo) mineralizations associated with Variscan granites 12 of the Variscan granites. It should be noted that in the K t vs T charts the susceptibility data, on the y-axes, is referred to as total or bulk susceptibility (K t ), and it is not corrected by volume or mass. These values are, therefore, quite different from the K m values measured in the magnetic susceptibility experiments, where the K m values are corrected for the specimen volume or mass.

Thermomagnetic experiments
The K t vs T curves from selected LO granite samples show a significant fall of K t at ca. 580ºC, indicating magnetite T C (Fig. 9A). Other curves have a K t drop just before 580ºC corresponding to a Ti-poor magnetite (Fig.  9B, C, D). Some samples have a slighter drop than the others because they have a smaller Ti-poor magnetite content (Fig.  9D). The K t vs T curves from BA samples show the presence of hematite (Fig. 9E, F, G), and some traces of Ti-poor magnetite. The K t values in the AC granite are, in general, lower than in the LO granite, but some K t vs T curves show a slight drop just before 580ºC, pointing to an ilmenite-type granite, but probably with some traces of Ti-poor magnetite (Fig. 9H, I). This could be related to the circulation of postmagmatic fluids capable of changing the characteristics of the primary magnetic minerals with the formation of new, less magnetic minerals, like hematite, maghemite or goethite (Lagoeiro, 1998;Nédélec et al., 2015). All the LOP curves show the presence of hematite and, in some A B     (Martins et al., 2011;Sant'Ovaia et al., 1993;Sant'Ovaia et al., 2014).   (Robertson and France, 1994). The CLG treatment was made using the software developed by Kruiver et al. (2001). n.a.: not attributed Magnetic mineralogy and W(Mo) mineralizations associated with Variscan granites 16 cases (Fig. 9E, J), during the cooling cycle, below 580ºC, K t progressively increases, surpassing the values attained in the heating curve, suggesting the formation of a magnetic mineral, probably Ti-poor magnetite.
In some K t vs T curves from the LO and AC granites, an inflection around 300ºC was observed on the heating curves ( Fig. 9A, C, I). This inflection may correspond to pyrrhotite or to maghemite. The pyrrhotite is a ferromagnetic mineral, with T C at ca. 320ºC (Butler, 1992), and is very common in granites (e.g. Jover et al., 1989); however, the monoclinic pyrrhotite is chemically stable up to its Curie point (Dekkers, 1989), which cannot explain the disappearance of the inflection in the cooling cycling. The presence of maghemite in the analyzed samples, may be due to the oxidation of magnetite with increased furnace temperatures and the presence of oxygen in the system (Evans and Heller, 2003). Maghemite is normally destroyed before 350ºC, which could explain the inflection around 300ºC in the heating curves and its absence in the cooling cycle. For that reason, maghemite is probably the magnetic mineral responsible for the inflection at 320ºC.
The thermomagnetic behavior of other Variscan granites is quite similar to the LOP. With the exception of the Lavadores-Madalena Pluton, the thermomagnetic curves show a slight drop before 580ºC, indicating a small quantity of magnetic minerals. The K t drops around 570ºC and 565ºC in the Vila Pouca de Aguiar Pluton (Fig. 10A) and Peneda-Gerês Pluton (Fig. 10B) samples, respectively. These values indicate the presence of Ti-poor magnetite, although Ti content in the sample from Peneda-Gerês Pluton is slightly higher (lower T c ) than the Vila Pouca de Aguiar Pluton. Regarding the Lavadores-Madalena Pluton experiments, the well-marked drop at 580ºC demonstrates the presence of magnetite and the inflection at 300ºC in the heating curve suggests the presence of maghemite in the Lavadores (Fig. 10C) and Madalena (Fig. 10D) granites. All K t vs T curves also revealed the presence of hematite.

FINAL REMARKS AND CONCLUSIONS
The magnetic mineralogy studies in the Portuguese posttectonic Variscan granites allowed to conclude that these are ilmenite-and magnetite-type-granites. Thermomagnetic curves are quite similar and show the presence of magnetite/ Ti-poor magnetite and hematite in all the plutons, though in different proportions. When the magnetite content is compared, a sequence can be established: Lavadores- Previous studies in the LOP granites, demonstrated the co-occurrence of ferromagnetic and paramagnetic behavior in the pluton (Cruz et al., 2016). Alteration associated with the circulation of post-magmatic fluids, could explain the presence of other oxides such as Ti-poor magnetite and hematite in the granites.
The complexity of the magnetic mineralogy is not recognized in the magnetic susceptibility measurements, because this parameter only expresses the combined magnetic behavior of all minerals. For that reason, other methodologies (e.g. IRM, thermomagnetic curves, and frequency-dependent susceptibility) should be performed to identify magnetic minerals and their complex magnetic behaviors.
In the Vila Pouca de Aguiar Pluton, the magnetic mineralogy is simpler when compared with other plutons. Studies carried out in the Peneda-Gerês and Lavadores-Madalena plutons pointed to a complex mineralogy, with the presence of both magnetite and ilmenite mineralogy in their granites.
The Lamas de Olo, Peneda-Gerês and Lavadores-Madalena plutons are all post-tectonic and composed of several granites of complex magnetic mineralogy, with magnetite/Ti-poor magnetite and ilmenite. Then we conclude that the presence of ilmenite-and magnetite-type in the same pluton, results from complex redox reactions, that occurs during the ascend and granite emplacement. The presence of magnetite pointing to a deep magma origin and oxidizing conditions in the magma chamber. This is an important metallogenic indicator of the W (Mo) mineralizations associated with post-tectonic biotite granites. Thus, the magnetic mineralogy can be a useful pathfinder for the W (Mo) mineralization exploration in the Iberian Peninsula.
The measurement of magnetic susceptibility is an unexpensive and rapid technique that can be used to characterize the magnetic behavior. The presence of magnetite and/or ilmenite allows to infer the redox condition of the granitic magma and to assess the metallogenic potential. However, the low field magnetic susceptibility measurements have some limitations when complex magnetic mineralogy is present, and for that reason, other magnetic studies should be performed.

ACKNOWLEDGMENTS
The first author was financially supported by SFRH/ BD/109693/2015 (Fundação para a Ciência e Tecnologia Portugal). The authors acknowledge funding from COMPETE 2020 through the ICT (Institute of Earth Sciences) project (UID/GEO/04683/2013) with POCI-01-0145 reference-FEDER-007690 and from ESMIMET, an INTERREG Spain-Portugal POCTEP project. The authors also thank Philippe Olivier and Sonia Rousse for the thermomagnetic measurements carried out in Toulouse and their helpful comments and explanations during the advanced training in the Université de Toulouse III -Paul Sabatier. Acknowledgments are given to Alexandra Guedes for the follow-up and explanations given during the micro-Raman analyses. We thank Eric Font for the helpful guidance in paleomagnetism and explanations during the IRM and KfD% analysis carried out in the Instituto Dom Luís facilities. The authors are grateful to Emilio L. Pueyo and another anonymous referee whose comments greatly helped to improve the manuscript.