Assessing the Magnetic Mineralogy of the Pre-Variscan Manteigas Granodiorite: An Unexpected Case of a Magnetite-Series Granitoid in Portugal

Abstract

The Early Ordovician Manteigas granodiorite has been characterized as having a magnetic susceptibility with high heterogeneity and values compatible with the presence of ferromagnetic phases, such as magnetite. However, granitoids with magnetite are rare in the Iberian Massif. Therefore, an in-depth study of mineralogical composition was carried out on the Manteigas granodiorite by evaluating its magnetic susceptibility, analyzing the isothermal remanent magnetization (IRM) and the IRM curve, identifying the magnetic coercivity components, evaluating the magnetic grain size and performing a petrographic study. Low concentrations of very small grains of magnetite and/or titanomagnetite have been found in areas with low magnetic susceptibility and with values not compatible with ferromagnetic phases. The petrographic study has explained this phenomenon as a result of the transformation of magnetite into hematite (martitization), indicating a redox reaction. A high concentration of magnetite and/or titanomagnetite, occurring as multidomain crystals, has been found in areas with high magnetic susceptibility. This study indicates that the Manteigas granodiorite has a deep magma origin, and formed under oxidizing conditions since it has magnetite instead of ilmenite, which is an uncommon phenomenon in the Iberian massif.

1. Introduction

Iron is the most abundant transition element in the solar system. For this reason, most magnetic studies rely on iron-bearing minerals such as iron oxides (e.g., magnetite, hematite, and martite), iron hydroxides (e.g., goethite), and iron sulfides (e.g., pyrrhotite and pyrite) [1,2].

The study of rock magnetism is the study of magnetic minerals in those rocks. The magnetic properties do not require the presence of a magnetic field and depend on factors such as the grain size and shape of the magnetic minerals.

Several methods can be utilized to characterize the magnetic mineralogy and evaluate parameters such as magnetic particle size.

This work focuses on magnetic susceptibility, isothermal remanent magnetization, and frequency-dependent susceptibility.

When a mineral or rock is exposed to a magnetic field, H, it acquires an induced magnetization, M. The induced magnetization and the magnetic field are directly related to magnetic susceptibility, K_m, defined according to [3]:

M = K_m ∙ H

(1)

Magnetic susceptibility is scalar if the body is isotropic, but if the body is anisotropic, K_m is represented by a second-order symmetric tensor, where M_i translates magnetization in the i direction, and H_j represents the effective inducing magnetic field in the j direction [3,4]:

M_i = K_ij ∙ H_j (i, j = 1, 2, 3)

(2)

Knowing that rocks are composed of minerals with different magnetic behaviors and that the K_m intensity results from the contribution of the magnetic susceptibilities of all the constituent minerals, the rock magnetic behavior can be:

(a)

Diamagnetic—in iron-free minerals (e.g., quartz and feldspars), where K_m is negative and around −10⁻⁶ SI;

(b)

Paramagnetic—in minerals with magnetic moments that tend to align along an applied magnetic field but do not have a spontaneous magnetic order (e.g., biotite and ilmenite), K_m is positive with a magnitude between 10⁻⁵ and 10⁻³ SI;

(c)

Ferromagnetic (s.l.)—characterized by the hysteresis curve, occurring in minerals that show spontaneous magnetization even in the absence of an external field (e.g., magnetite, hematite, and martite). In this case, the induced field returns to zero and unlike the diamagnetic and paramagnetic behaviors, residual magnetization is recorded, known as remanent magnetization [4].

Isothermal remanent magnetization (IRM) refers to the remanence acquired by a sample exposed to a direct magnetic field H at room temperature [5]. The analyses of IRM acquisition curves allows for the estimation of the characteristic coercivity of the ferromagnetic minerals [1]. The IRM at 1 T is defined as saturation isothermal remanent magnetization (SIRM).

The IRM acquisition curves can be represented by a cumulative Log-Gaussian function (CLG) [5]. By decomposing the IRM curve into different CLG curves, which are a function of SIRM, mean coercivity, and dispersion index, it is possible to identify the ferromagnetic minerals present in the studied rocks and their relative concentration. The unmixing of the magnetic components was performed by use of two statistical methods that decompose the magnetization curves, namely Cumulative Log-Gaussian (CLG) function with the software developed by Kruiver [6] and MAX UnMix statistical unmixing [7].

Magnetic susceptibility is affected by all the minerals present in the rock, diamagnetic, paramagnetic, and ferromagnetic (s.l.) [5]. SIRM is affected by the ferromagnetic minerals and also by their grain size.

The frequency-dependent susceptibility (KfD%) is a clear indicator of the presence of ferrimagnetic grains, reflecting the concentration of superparamagnetic (SP) grains and allowing for the identification of SP particles characterized by a dimension of lower than 0.03 μm. Samples in which these particles are present have slightly lower magnetic susceptibility values when measured at high frequency. On the other hand, in samples where these particles are absent, the magnetic susceptibility values are identical at the two frequencies [8]. The ratio SIRM/K_m is a further useful pointer to the magnetic grain size of ferrimagnetic crystals [9,10].

In the studied case, K_m, IRM, and KfD% analytical techniques were applied to representative samples of Manteigas granodiorite. The main goal of this work is to characterize, quantify and identify the magnetic mineralogy present in the studied rocks.

2. Geologic Setting

2.1. Regional Geologic Setting

The Iberian Massif corresponds to the western part of the “Variscan Chain”, being an arched segment of the European Variscan Orogenic Belt that extends for more than 3000 km, from Germany to the Iberian Peninsula. This was divided into the following five main zones with different geological characteristics [11]: Cantabrian Zone (CZ), West Asturic-Leonese Zone (WALZ), Central Iberian Zone (CIZ), Ossa-Morena Zone (OMZ), and South Portuguese Zone (SPZ). Later, the allochthonous and parautochthonous units of the CIZ [12] were included in a newly distinguished zone called Galicia-Trás-os-Montes Zone (GTMZ). According to this classification, the studied area is located in the autochthonous terrains of the Central Iberian Zone (Figure 1).

The Variscan orogeny resulted from the oblique collision between the supercontinents Laurussia and Gondwana during the Lower Devonian to Carboniferous periods [13,14,15]. In the Northwest (NW) of the Iberian Peninsula, three main ductile deformation phases (D₁, D₂, and D₃) were described with ages of 360–337 Ma, 337–320 Ma, and 320–310 Ma, respectively. After D₃, the deformation phases are fragile. In central Portugal and especially in the study area, the CIZ is characterized by the presence of vertical D₃ folds that deform D₁ structures [13,16,17,18,19,20].

Variscan granitoids in Portugal are syn-orogenic and on the basis of geological, petrographic, and geochemical studies, they have been divided into two main groups:

(a)

Biotitic granitoids (biotite >> muscovite)—originated at a high depth of the earth’s crust. If they have muscovite, it is usually of secondary origin. The intrusion and distribution of these granites are mainly controlled by the D₃ shear zones and the late-Variscan tectonic structures, which means that they can be syn-D₃ (320–313 Ma), late-D₃ (311–306 Ma), late- to post-D₃ (300 Ma), or post-D₃ (299–290 Ma);

(b)

Two-mica granitoids (muscovite > biotite)—usually leucocratic granites with primary muscovite and biotite, syn-tectonic concerning the third stage of deformation (syn-D₃) and are usually found in the nucleus of regional D₃ folds [21,22,23,24].

Pre-Variscan granites are also currently well defined in the Iberian Peninsula. U-Pb geochronological dating indicate Ordovician ages for some granite massifs, both in Portugal and Spain: e.g., Manteigas granodiorite (ca. 481 Ma; [25]), Carrascal and Portalegre granites (486–471 Ma, and ca. 493 Ma, respectively; [26,27,28]), Oledo pluton (479–480 Ma; [29]), granites from Penha Garcia-Salvaterra do Extremo sector (ca. 588 Ma; [30]), Ruanes tonalite and peraluminous Plasenzuela two-mica leucogranite (ca. 330 Ma, and ca. 464 Ma, respectively; [20]), and Zarza la Mayor pluton [31]. Most of these authors suggested that these granitoids, mostly of a peraluminous character, have an I-type (or I- transitional to S-type) feature. The geodynamic context is still debatable, but the authors suggested that Ordovician magmatism should be related to a magmatic arc or subduction zone in a continental plate edge (e.g., [31], and references therein; [32]).

The CIZ is characterized by large amounts of granitic rocks (60–70%) in the outcrops [33]. The syn-D₃ granitoids are represented by two-mica peraluminous granites, granodiorites, and biotitic granites. The late-D₃ and post-D₃ granitoids are mainly represented by biotitic-rich granites, granodiorites, diorites, quartz-monzodiorites, and gabbros. For the study area, most Variscan granitoids are late-D₃ and late- to post-D₃ that intruded in metasedimentary rocks corresponding to the autochthonous formations of the CIZ. In addition, pre-Variscan granitoids such as Manteigas massif [34] are also present.

2.2. Geographic and Geologic Setting

Manteigas granodiorite (Figure 2) is located in the municipality of Manteigas, which is part of the Central mountain range, “Beira Interior Norte” [35], and it is an integral part of the Estrela Geopark [36].

The area is essentially characterized by the occurrence of metasedimentary formations belonging to the ante-Ordovician “Schist-Greywacke Complex”, intruded by coarse-grained biotite porphyritic granite (Seia granite) of late-D₃ Variscan age [22,37,38].

The Manteigas granodiorite is a small intrusion that occurs between the Schist-Greywacke Complex, to the northeast, and the porphyritic biotite granite, to the southwest. The Manteigas intrusion is a medium to coarse-grained, slightly porphyritic biotite granodiorite, that presents deformation patterns in outcrop.

The Manteigas intrusion is a calc-alkalic, weakly peraluminous granodiorite of Lower Ordovician age, 481.1 ± 5.9 Ma, with a source from a depleted mantle or from the meta-igneous lower crust on the basis of (⁸⁷Sr/⁸⁶Sr)₄₈₂, εNd_T and δ¹⁸O. Therefore, due to the particular characteristics of the Manteigas granodiorite, namely the presence of ferromagnetic phases in its composition [22], which is not frequent in other granites from the CIZ, several analytical techniques were performed in this study to assess its magnetic mineralogy.

The granitic rocks and the metasediments are both cut by subvertical aplite and pegmatite veins (NE–SW and NNE–SSW), subhorizontal Sn-bearing aplite-pegmatite veins (NW–SE and NE–SW), locally subvertical quartz veins containing cassiterite and wolframite (N–S and NW–SE), and NE–SW and NNE–SSW faults that cut both the rocks and the quartz veins [25].

3. Materials and Methods

3.1. Fieldwork and Sampling

To carry out the rock magnetism studies it was necessary to collect oriented and uniformly distributed samples in the granitoid in study, which had to be fresh and in situ. The samples were taken from the outcrops with a portable drill machine, for which the sampler is made up of a non-magnetic tube that ends in a diamond crown. The sampler was cooled down and lubricated by pressurized water by the use of an external manual water pump. At each sampling site, before removing the core, with approximately 25 mm in diameter, the orientation was determined by the use of a compass. In each sample, the values of the azimuth contained in the hole, and its dip were noted and marked in the sample with indelible black or blue ink by the use of oriented arrows to indicate the top of the cylinder. Then, in the laboratory, the cylinders were sawed to obtain samples with a height of approximately 25 mm. Afterward, the exact measurement of the dimensions (diameter and height) of each sample was noted to calculate its volume, which is required to determine the magnetic susceptibility [39]. To perform the magnetic studies, 21 samples from four sampling sites (Figure 2) were obtained.

All the sampling equipment belongs to the Institute of Earth Sciences—Porto Pole (ICT-Porto) and the Department of Geosciences, Environment and Spatial Planning from the Faculty of Sciences of the University of Porto (FCUP).

3.2. Petrographic Studies

The petrographic studies took place at the Department of Geosciences, Environment and Spatial Planning facilities (FCUP), with the use of a petrographic polarizing microscope coupled with a digital camera. Therefore, with the transmitted light, it was possible to study the thin section of the four sites (ES18, ES19, ES21, and ES87), and with the reflected light it was possible to identify different opaque minerals (ES18, and ES87).

3.3. Magnetic Susceptibility (K_m) Measurements

The magnetic susceptibility measurements took place at the Department of Geosciences, Environment, and Spatial Planning facilities (FCUP), with the use of a KLY-4S Kappabridge susceptometer Agico model (Czech Republic) from the ICT-Porto, in 21 core samples from the four sampling sites.

3.4. Isothermal Remanent Magnetization (IRM) Studies

Isothermal Remanent Magnetization (IRM) can be induced in the laboratory by the application of magnetic fields. In this case, it was measured in a magnetically stable environment, at the Laboratory of Paleomagnetism at the Department of Earth Sciences at the University of Coimbra.

For this process, selected samples (ES18-A1 and ES18-B2 from ES18 site; ES19-A1 from the ES19 site and ES87-C2 from ES87 site) were chosen and first subjected to demagnetization by the use of an alternating field (AF) demagnetizer coupled to an anhysteretic field magnetizer (Molspin). Thus, the samples were exposed to an alternating field comparable to a sinusoid, whose magnitude decreased over time. Then, the magnetic field (H) was applied to the sample, always in the same direction, by the use of a strong field magnetizer to obtain the acquisition curves of the isothermal remanent magnetization (IRM) (IM-10, ASC Scientific). The resulting remanent was measured at each induction step until saturation was reached, using a Minispin (Molspin) magnetometer.

3.5. Frequency-Dependent Susceptibility (KfD%)

The frequency-dependent susceptibility (KfD% = (Klf − Khf)/Klf × 100%) measurements were achieved with a Bartington MS2 System at the Department of Geosciences, Environment, and Spatial Planning facilities (FCUP). Two field frequencies with 0.46 kHz and 46 kHz were applied for the measurements of the low-field magnetic susceptibility, at low frequency, Klf, and at high frequency, Khf. The percentage of frequency-dependent susceptibility was acquired in 12 samples [40].

4. Results and Discussion

4.1. Petrographic Studies

The studied samples are mostly composed of quartz, plagioclase, K-feldspar, and biotite (Figure 3a). As accessory minerals, apatite, chlorite, magnetite, hematite, goethite, and zircon were identified. Muscovite is rare and mostly of secondary origin (muscovite II). Monazite, sphene and rutile are also present but in minor amounts (Figure 3b,c). Tectonic fabrics are visible under the microscope, namely undulose extinction in quartz and feldspar, curved micas, fluid inclusion planes in quartz, and microfractures in feldspar. Sometimes, the microfractures are filled with Fe-oxide/hydroxide.

The plagioclase appeared frequently zoned in all samples, with oscillatory zoning, and were also altered by sericitization (Figure 3a). It is possible to find myrmekitic intergrowths with quartz.

Quartz shows undulose extinction, was particularly evident in some samples. The edges of quartz crystals are mostly wavy to sutured, but linear or curved in some cases. The inclusion of quartz in feldspars were also observed.

Biotite is brownish to greenish in the thin section and usually appears to be partially to completely chloritized. It shows pleochroic halos associated with zircon inclusions. Curved (folded) biotites were also observed (Figure 3d).

The presence of pure magnetite was observed in some samples (Figure 3e), although hematite crystals and martite (magnetite partial oxidized to hematite) were also present (Figure 3f).

No significant modal differences between the thin (for transmitted light) or the thick sections (for light reflected microscopy) were observed.

4.2. Magnetic Susceptibility (K_m)

Magnetic susceptibility values fall between 191.03 × 10⁻⁶ SI and 18,837.00 × 10⁻⁶ SI. The K_m values for the ES87 and ES18 sites indicate a ferromagnetic behavior and the other two samples from the ES19 and ES21 sites suggest the presence of mainly paramagnetic minerals.

According to Sant’Ovaia [22], the strongest degrees of magnetic anisotropy are found in samples with ferromagnetic behavior, as a consequence of the anisotropic shape of ferrimagnetic grains. On the other hand, in the samples with a paramagnetic behavior, the degree of magnetic anisotropy is an expected value for deformed granites [22].

4.3. IRM Studies

The acquisition curves of the isothermal remanent magnetization were obtained for the detailed study of the magnetic mineralogy at the Manteigas granodiorite. Four samples were selected, namely ES18-A1 (Figure 4), ES18-B2 (Figure 5), ES19-A1 (Figure 6), and ES87-C2 (Figure 7). The interpretation of these curves allowed for the estimation of several parameters, such as the (i) saturation isothermal remanent magnetization value (SIRM); (ii) S-ratio parameter to 300 mT (IRM_{300 mT}/IRM_{1000 mT}) which, when close to the unity, indicates the presence of a low coercivity magnetic phase; (iii) the coercivity (Bh); and (iv) dispersion parameter (DP) of each ferromagnetic mineral present in the rocks. As each ferromagnetic mineral is characterized by different parameters, this technique allows for the identification of several components present in the studied samples [41]. SIRM values are very different for the three samples, ES18-A1, ES18-B2, and ES87-C2 (18.0, 16.4, and 12.4 A/m, respectively), compared to the sample ES19-A1 (5.7 A/m).

Data were analyzed on the basis of a cumulative log-Gaussian function, as Robertson, D.J. [5] demonstrated that the remaining isothermal magnetization can be fitted to a Gaussian curve since the magnetic distribution of grain size is also logarithmic. If there are no magnetic interactions, the set of grains of a single magnetic mineral can be characterized by saturation isothermal remanent magnetization (SIRM), by the field in which half the SIRM is reached, or by the standard deviation of the logarithmic distribution. The IRM curve must be fitted with more than one component as the data do not have a linear projection. Thus, we resorted to the software developed by Kruiver et al. [6], which adjusts the remaining isothermal magnetization curve versus the logarithm of the field applied to a linear scale and a probability scale, as well as expressing it as a gradient. The combined analysis of these three fits is referred to as a cumulative log-Gaussian analysis [6].

From the IRM curves, we can see that the samples ES18-A1 (Figure 4), ES18-B1 (Figure 5), and ES87-C2 (Figure 7) were saturated when the applied field reached approximately 300 mT. However, the sample ES19-A1 (Figure 6) does not seem to saturate for the applied fields. Regarding the S ratio parameter to 300 mT, samples ES18-A1, ES18-B2, and ES87-C2 also show similar values (0.966, 0.968, and 0.969, respectively), and sample ES19-A1 has a lower ratio (0.943).

For a better fit of the IRM curve, the MAX UnMix software [7] was also used to create the IRM curves presented in Figure 8. This software was used to complement the previously performed IRM curve adjustments because, in addition to using the parameters already used by Kruiver et al. [6], it also uses coercivity division methods that take into account the asymmetry (S) of the distributions [7].

The number of magnetic components necessary for an ideal fit to an IRM curve is obtained by completing this statistical analysis with the use of both techniques.

Table 1. Isothermal remanent magnetization data from the first component of the studied samples. The Cumulative log-Gaussian (CLG) treatment allows the definition for each magnetic component: K_m: magnetic susceptibility; SIRM: saturation of isothermal remanent magnetization; Bh: coercivity (log(B1/2)); B1/2: field at which half of the SIRM was reached; DP: dispersion parameter; S: skewness. S-ratio (IRM_{300 mT}/IRM_{1000 mT}); n.a.—not attributed value.

Site	Km (μSI)	Method	Component 1—Magnetite and/or Titanomagnetite							S-Ratio 300
			SIRM A/m	SIRM/Km (kA/m)	Bh (mT)	B1/2 (mT)	DP	S	Contribution (%)
ES18-A1	16,826.6	MAX UnMix	n.a.	n.a.	1.60	39.81	0.42	0.86	100	n.a.
		IRM-CLG	18.00	1.070	1.61	40.74	0.41	n.a.	100	0.966
ES18-B2	14,739.5	MAX UnMix	n.a.	n.a.	1.60	39.81	0.41	0.90	100	n.a.
		IRM-CLG	16.40	1.113	1.60	39.81	0.41	n.a.	100	0.968
ES19-A1	327.8	MAX UnMix	n.a.	n.a.	1.80	63.10	0.32	1.02	83	n.a.
		IRM-CLG	5.70	17.387	1.78	60.26	0.33	n.a.	85	n.a.
ES87-C2	18,837.0	MAX UnMix	n.a.	n.a.	1.39	24.55	0.49	0.70	100	n.a.
		IRM-CLG	12.35	0.656	1.45	28.18	0.48	n.a.	100	0.969

and

Table 2. Isothermal remanent magnetization data from the second component of the studied samples. The Cumulative log-Gaussian (CLG) treatment allows the determination for each magnetic component: K_m: magnetic susceptibility; SIRM: saturation of isothermal remanent magnetization; Bh: coercivity (log(B1/2)); B1/2: field at which half of the SIRM was reached; DP: dispersion parameter; S: skewness. S-ratio (IRM_{300 mT}/IRM_{1000 mT}); n.a.—not attributed value.

Site	Km (μSI)	Method	Component 2—Hematite							S-Ratio 300
			SIRM A/m	SIRM/Km (kA/m)	Bh (mT)	B1/2 (mT)	DP	S	Contribution (%)
ES18-A1	16,826.6	MAX UnMix	not observed							n.a.
		IRM-CLG								n.a.
ES18-B2	14,739.5	MAX UnMix	not observed							n.a.
		IRM-CLG								n.a.
ES19-A1	327.8	MAX UnMix	n.a.	n.a.	2.23	169.82	0.16	0.90	0.17	n.a.
		IRM-CLG	1.00	3.050	2.24	173.78	0.18	n.a.	0.15	0.943
ES87-C2	18,837.0	MAX UnMix	not observed							n.a.
		IRM-CLG								n.a.

compare the values obtained by both software. The similitude of the parameters’ values gives us confidence in the analysis.

Concerning the ES18-A1 sample, only one component was identified. With the use of the Kruiver approach, a coercivity parameter (B1/2) of 40.74 mT and a dispersion parameter (DP) of 0.41 mT were obtained. By employing the MAX UnMix software, a coercivity of 39.18 mT and a DP of 0.42 mT were registered.

A similar interpretation was achieved for sample ES18-B2, where only one component was identified with a coercivity of 39.81 mT and a DP of 0.41 mT calculated by the use of both IRM unmix analyses.

As to the ES19-A1 sample, two components were identified. The first, in accordance with the Kruiver program, had a coercivity of 60.26 mT and a DP of 0.33 mT, while with the MAX UnMix software, a coercivity of 63.10 mT and a DP of 0.32 mT were obtained. The second component was characterized by a higher coercivity and a lower DP. The Kruiver program pointed out a coercivity value of 173.78 mT and a DP value of 0.18 mT, and the use of the MAX UnMix software indicated a coercivity value of 169.82 mT and a DP value of 0.16 mT.

Regarding the ES87-C2 sample, only one component was identified with a coercivity parameter (B1/2) of 28.18 mT and a dispersion parameter (DP) of 0.48 mT, obtained with the Kruiver approach and a coercivity of 24.55 mT and a DP of 0.49 mT were obtained with the MAX UnMix software (Figure 9).

The coercivities (e.g., [6,42]) of the determined components indicates the presence of a single low-coercivity magnetic phase of magnetite/titanomagnetite in samples ES18-A1, ES18-B2, and ES87-C2. In sample ES19-A1 beyond magnetite/titanomagnetite, a higher coercivity component such as hematite is also present (Figure 9). As the coercivity increases with the replacement of Fe³⁺ by Ti in the titanomagnetite [43], the differences in coercivity parameter in component 1 are justified as a different titanium content in the magnetites.

4.4. Magnetic Grain Size Determinations

The frequency-dependent susceptibility evaluates the presence of ferrimagnetic grains and the concentration of superparamagnetic (SP) grains. According to Dearing et al. [40], a KfD% of lower than 2% indicates that virtually no superparamagnetic grains are present. On the other hand, if KfD% is between 2% and 10%, an admixture of SP grains and coarser non-SP grains are present. The determination of KfD%, performed in 21 samples of the Manteigas granodiorite, highlighted that values comprised between 0.1% and 3.8%, indicating that magnetic grains are probably composed of a mixture of fine and coarser non-SP grains (Figure 10).

Additionally, a ratio of the saturation isothermal remanent magnetization to the mean magnetic susceptibility (SIRM/K_m) was also obtained in ES18-A1, ES18-B2, ES19-A1, and ES87-C2 samples. According to several authors (e.g., [9,10]), when ferrimagnetic structures are present, SIRM/K_m can be used as an indicator of magnetic mineral grain size. This is due to the ratio numerator being dependent on the magnetic mineral grain size, although the denominator is independent. The saturation remanence versus susceptibility plot of the four selected samples indicates that the magnetic grain size might be between 2 and 256 μm (Figure 11).

5. Conclusions

Studies of magnetic susceptibility on Variscan granites of the Central Iberian Zone have been carried out in several works (e.g., [22,39,44,45,46]). Their objective was to determine whether the granites belong to the ilmenite-series or the magnetite-series from Ishihara [47], since the former is linked to reduced and the latter to oxidizing conditions. This indication of the redox conditions of the granites genesis is very important because different mineralizations are spatially associated with each of these types of granites [48].

In the magnetic susceptibility study performed on the granites of Serra da Estrela area, Sant’Ovaia [22] pointed out high magnetic susceptibility values in Manteigas granodiorite are concordant with the presence of ferromagnetic minerals in some areas.

However, there are other sectors in which the magnetic susceptibility was mainly controlled by paramagnetic minerals. This apparent heterogeneity in terms of mineral contributions to the magnetic susceptibility led to this in-depth study on magnetic mineralogy including IRM curves analysis and magnetic grain size determination, complemented by petrographic studies.

Effectively, the K_m values showed us that two sites (ES87 and ES18) have ferromagnetic behavior and the other two (ES19 and ES21) have the presence of mainly paramagnetic minerals. The SIRM values (min: 12.35 A/m–max: 18.00 A/m), S-ratio (min: 0.966–max: 0.969) and magnetic coercivities (min: 28.18 mT–max: 40.74 mT) obtained confirm the presence of magnetite or titanomagnetite in the samples ES87 and ES18. Differently, on ES19 site, a SIRM of 5.7 A/m and an S-ratio of 0.943 confirm a complex magnetic behavior whereby other magnetic phases are present besides ferrimagnetic ones, which explains the lower values of the magnetic susceptibility. This statement is further reinforced by the IRM unmix which pointed to the presence of two components with different coercivities, 60.26 mT, and 173.78 mT. Due to its coercivity, the first can be considered magnetite or titanomagnetite, while the second has to be interpreted as hematite.

The observation of opaque minerals, a frequently difficult task given the size of these minerals, also revealed the presence of magnetite and martite (magnetite partially oxidized to hematite), which corroborates the results obtained by the magnetic studies.

Concerning the magnetic grain size, two approaches have been carried out. One was the frequency-dependent susceptibility (KfD%) determination and the other was the ratio of the SIRM to the mean magnetic susceptibility (K_m) calculation. A KfD% of lower than 2% is found in samples ES18 and ES87 and are projected into the field of “stable single domain magnetite” and “multidomain magnetite” (Figure 10). Most of the samples of sites ES19 and ES21 (no IRM studies have been performed on samples from this site, but the studies from Sant’Ovaia et al. [22] have shown that they have paramagnetic susceptibility) have (KfD%) between 2% and 4%, indicating that the small percentage of magnetite grains are probably composed by a mixture of fine and coarser non-SP crystals (Figure 10). The SIRM/K_m analysis (Figure 11) demonstrated that samples of ES18-A1, ES18-B2, and ES87-C2 have magnetite crystals with dimensions of around 256 μm, which is confirmed by petrographic observations (Figure 3e) and also pointed out by KfD% indicating that magnetite was mainly multidomain. On the other hand, sample ES19-A1 has magnetite crystals with a size of ca. 2 μm in amounts of around 0.01 vol%.

In summary, this study shows that even in samples where magnetic susceptibility values preclude the presence of magnetite, this ferrimagnetic mineral may be present in small grains and at low concentrations, when it is partially oxidized to hematite (martite pseudomorphism).

The pervasive presence of magnetite in the Manteigas granodiorite suggests a deep magma origin and the presence of melt-oxidized conditions, which is not commonly found in the Iberian massif. During the process of magma ascending and/or after granodiorite emplacement, redox reactions occurred, leading to the oxidation of magnetite into hematite (martitization process), and minerals with a lower magnetic susceptibility.

Previous studies in magnetite-series granites (e.g., [21,49]) suggested that the presence of oxidizing conditions is a metallogenic indicator of the presence of W(Mo) mineralizations. However, there is no evidence of this mineralization associated with the Manteigas granodiorite. Nevertheless, our results corroborate the findings of Noronha [50] and Cruz et al. [49], which proposed that the presence of W(Mo) is mostly related to the post-tectonic granites and also that the mineralization is related to the convective circulation of fluids from different origins, triggered by the heat of several granite intrusions. The older age of this granite, syn-D₃, or Ordovician may explain the absence of associated mineralizations.