Mineralogy

The samples were split into four groups according to the field criteria, including texture and color. This classification included two different altered rock types, veins, and nodules. Although the mineralogy found along the deposit is similar, the mineral proportions vary considerably in these four groups. In some areas, the altered rock preserves the granite texture and consists of a regolith where the feldspars are kaolinized almost completely. Manchado (Reference Manchado2012) referred to them as ZAHO (Fig. 1b). The ZAHE materials did not preserve the original texture of the granitic rock because of the intense alteration. They consist of highly heterogeneous and fractured clayey rocks, demonstrating great color variations owing to the circulation of iron and manganese oxides. The main clay mineral is smectite, in which the bentonite areas are rich (García-Romero et al., Reference García-Romero, Manchado, Suárez and García-Rivas2019; Manchado, Reference Manchado2012). Usually, ZAHE samples appear as large areas included within ZAHO (Fig. 1b). Three-dimensional relationships between ZAHO and ZAHE are frequently very complex owing not only to the combined effect of the two alteration processes and different fracture stages but also to the slides associated with surface tectonics affecting unconsolidated materials. Contacts between ZAHO and ZAHE can be caused by faults, which are rarely gradual. Typically, ZAHE materials present big bags surrounded by ZAHO with net and irregular contacts (Fig. 1b). Nodules and veins, the thicknesses of which range from millimeters to centimeters, are frequent in both ZAHO and ZAHE (Fig. 1c,d,e).

The mineralogical composition of these four material types, namely, ZAHO, ZAHE, veins, and nodules, is similar because all of them are composed of kaolinite, smectite, micas, quartz, scarce feldspar, and occasionally natroalunite. However, their proportions vary drastically, as revealed by the mean, maximum, and minimum contents of each mineral (Table 1). Predominantly, kaolinitic areas correspond to ZAHO; in these areas, kaolinite concentration reaches 70%, with mean values of 43%, quartz concentration is 25%, and smectite concentration is 18%. In these areas, feldspars are scarce, with mean concentrations of <3%; however, in certain cases they can be found in high concentrations in less altered granites. In certain cases, natroalunite can also appear in very small concentrations. The microstructure of the ZAHO samples observed under SEM is characterized by the presence of mica crystals surrounded by clayey material (Fig. 2a) in which the pseudohexagonal morphology of kaolinite is observed easily at greater magnification (Fig. 2b,c). The sizes of these kaolinite crystals range between 100 nm and several microns. In certain cases, restitic feldspar crystals with their surfaces partially dissolved can be observed, as in Fig. 2d, in which kaolinite epigenesis on the feldspar surface is noted.

Table 1 Statistical data of mineralogical composition (%)

Q Quartz, F Feldspars, Al Alunite, Sm Smectite, K Kaolinite, M Mica, BI Biscaye Index, HI Hinckley Index, Max maximum value, Min Minimum value, SD Standard deviation, n Number of analyses

Fig. 2 Scanning electron microscopy images of representative samples. a Sample from ZAHO, b,c massive kaolinite in a sample from ZAHO, d kaolinite crystals growing by epigenesis on the surface of an altered and partially dissolved felspar, e sample from ZAHE, f bentonitic sample from ZAHE in which a very altered crystal of mica appears surrounded by smectites, g kaolinite from a vein in ZAHE, h smectite and alunite–APS minerals in a vein of ZAHE. ZAHO: Homogeneous alteration zones. ZAHE: Heterogeneous alteration zones. APS: Aluminum–phosphate–sulfate

Conversely, ZAHE are bentonitic areas that can host very high concentrations of smectite (87%), with the mean concentration being slightly >60%. The mean concentrations of kaolinite, quartz, micas, and feldspars in ZAHE are less than those in ZAHO. The smectite concentration is greater, owing to the significant impact of the hydrothermal process. As in ZAHO, alunite is found occasionally.

Representative XRD patterns of the ZAHO and ZAHE mineralogical assemblages are shown in Fig. 3. The microstructure observed in ZAHE samples is quite different from those observed in ZAHO samples; furthermore, the mica crystals are altered to a greater extent and exhibit very curved edges (Fig. 2e,f). The clayey material exhibits the typical bentonite texture: laminar crystal aggregates with undulated edges in which it is not possible to observe individual smectite crystals and with frequent edge–face contacts. The co-existence of kaolinite and smectite from the hydrothermal alteration of acid igneous rocks, demonstrating very similar microstructure to the ZAHE materials, was reported by Kadir and Kart (Reference Kadir and Kart2009).

Fig. 3 XRD patterns of representative samples. Upper: powdered samples. Lower: oriented aggregates of the < 2 µm fraction of ZAHO and ZAHE samples where AO is an oriented air-dried aggregate, EG is an oriented aggregate solvated with ethylene glycol, and 550ºC is an oriented aggregate heated at 550ºC for 2 h. Sm = smectite, M = mica, K = Kaolinite, Q = Quartz, F = Feldspar, Al = Natrolunite

The vein thicknesses range from millimeters to centimeters (Fig. 1c,d,e), although they are wider in some cases. They appear embedded both in ZAHO and ZAHE and, as for the massive host rocks, kaolinite and smectite are the most abundant components. No pure kaolinite veins were identified. In ZAHO, vein composition is quite similar, hosting large amounts of kaolinite (Fig. 2g) and variable concentrations of smectite, along with minor amounts of natroalunite or primary minerals, while the veins cutting the ZAHE materials have smectite as the most abundant mineral (56% as mean content) and are rich in alunite. Figure 2h shows the image of a smectite–alunite vein in which isometric crystals of alunite ~1 µm long are surrounded by laminar smectite aggregates. The materials referred to here as natroalunite are, in fact, minerals of the APS group (Reference Manchado, Suárez and García-RomeroManchado et al., 2008). These results concur with those reported by Meunier (Reference Meunier2005), which state that short hydrothermal pulses favor the formation of low-charge expansive clays (such as the smectites appearing in these veins), while non-expansive species crystallize in systems in which fluids circulate for longer periods. Ultimately, nodules found in the deposit have compositions similar to those of the veins in ZAHE, being rich in smectite and alunite and with variable concentrations of quartz and kaolinite.

In the samples that were studied, the 060 reflection of phyllosilicates corresponds to a d ₀₆₀ spacing of 1.49 Å, both in ZAHE and ZAHO, indicating that all of them are dioctahedral. Moreover, smectites have very high crystallinity (Fig. 3). The calculated mean value of the Biscaye Index (BI) of all the studied samples of the deposit is 0.92. Comparing their BI values, ZAHE samples have slightly greater crystallinity than ZAHO; the greater the percentage of smectites in the sample, the greater its crystallinity (Table 1). However, kaolinite crystallinity is low according to the Hinckley Index (HI), showing the kaolinite from the ZAHO (which was formed by feldspar weathering) to have a greater crystallinity with a mean value of HI = 0.58 and the kaolinite from the veins to have lower crystallinity with HI = 0.29. Kaolinites from ZAHE, which have experienced a greater influence of the hydrothermal process, have lower HI values (0.39) than those from ZAHO.

2D and 2.5D graphics of the deposit were obtained from the study by mean of SURFER-8 of the bulk data of the borehole provided by ‘Arcillas y Feldespatos Río Pirón’. They were obtained by the experimental and theoretical variograms calculated from the core data (Fig. S1a,b) that were used to obtain the isocontents maps (Fig. S1c,d) and the 2.5D (Fig. S1e,f) models of smectite and kaolinite concentrations. Lag is the distance or interval considered for the calculations, which were 25 in all the variograms. The direction of the experimental variogram considered was N140E and the tolerance was 20º; therefore, all the data from the soundings found between N120E and N160E were considered. Subsequently, the isocontent maps were obtained from the variograms using kriging as the interpolation method. Unsampled points were interpolated by point kriging.

The map of the smectite isocontents was calculated from the weighted average percentage of smectite with regards to the thickness of altered granites in each borehole, calculating the experimental and theoretical variograms, as previously stated, and the isocontent map was obtained from the theoretical variogram (Fig. S1a). The weighted average percentage of smectite was an anisotropic variable and the experimental variogram was calculated for a lag direction of N115E with an angular tolerance of 20º. In the boreholes, the smectite values varied from 0 to 64.24% and the average was 20.26%. The maximum value in the scale was 55%; therefore, some values were underrated. The highest weighted percentage values of smectite had a distinct preferential direction, approximately N40E (Fig. S1c). These maxima form two parallel and displaced imaginary lines (Fig. 4). This showed that the smectite origin was related to a main zone of fracture, with direction ~N40E. A higher degree of fracture through two conjugated faults produced a movement (with horizontal displacement ranging from 100 to 200 m) of a central block in this area (Fig. 4). Furthermore, the theoretical variogram model that best fitted the experimental data for the weighted average kaolinite contents was obtained, and the isocontent map was also obtained from the theoretical variogram (Fig. S1b,d, respectively). Similar to the smectite model, the maximum of the theoretical model underestimated the percentage of kaolinites and in the minima they are overestimated. The maximum and minimum kaolinite percentage values obtained from the boreholes were 47.83 and 1.72%, respectively, with a mean value of 26.92% and a standard deviation of 11.73, slightly lower than that of the smectite percentage. The kaolinite distribution is relatively homogeneous in the deposit as in a regolith stemming from the meteoric alteration of the original granitic rock, except for the zone in which the greatest smectite concentrations appear close to fault zones.

Fig. 4 Deduced fracture zones from the smectite isocontents map. Abscissa and ordinate are UTM (Universal Transversal Mercator) coordinates. Red circles containing crosses indicate the borehole positions. The vertical scale bar indicates the values of smectite isocontents in %

Borehole data revealed that the least variability of the deposit is found in the direction N40E. All the diagrams obtained showed a similar trend in this direction, being more evident in the graph corresponding to the smectite distribution. This direction corresponds to the reactivated Late Variscan fractures (Anton, 2003) and, in turn, coincides with that of the faults limiting the deposit. These data allow the deduction that, in the area studied, a N40E-direction fault occurred with the uplifted block to the NW (the base of the tertiary in this area was slightly higher). One could, therefore, deduce that the smectites from the Tamame de Sayago deposit, which present maximum concentrations in the same direction, could have an origin related to this fault related to the circulation of hydrothermal fluids. Two other conjugate faults displace this fault zone and, therefore, the smectite maximums. These faults (both the N40E and the two conjugates) appeared before the sedimentation of the Tertiary. These conclusions are summarized graphically in Fig. 4, and they agree with field observations as the bentonitic zones of ZAHE coincide with the maximum in the isocontents in smectite lines.

Geochemistry

Major element analyses of representative samples show that the most abundant oxides are SiO₂ and Al₂O₃ because they correspond to the mineralogy of rocks that are composed principally of dioctahedral phyllosilicates and quartz (Table S1). The Pearson correlation coefficients (r) among the major elements and the main minerals of the deposit (Table S2) show interesting correlations, in addition to the obvious correlations such as quartz with SiO₂. MgO and CaO are highly correlated with smectites, with r = 0.931 and 0.878, respectively, while natroalunite is correlated with Na₂O (r = 0.857), as expected. Moreover, P₂O₅ (r = 0.744) is in good agreement with the XRD patterns and the presence of some solid solutions with aluminum–phosphate–sulfate (APS) minerals that were referred to in this deposit (García-Romero et al., Reference García-Romero, Manchado, Suárez and García-Rivas2019; Reference Manchado, Suárez and García-RomeroManchado et al., 2008).

Significant variations were found among the ZAHO and ZAHE compositions. SiO₂, MnO, Na₂O, and K₂O percentages were similar in ZAHO and ZAHE, the Al₂O₃ concentration is higher in ZAHO, while Fe₂O₃, MgO, and CaO were found in greater concentrations in ZAHE (Table 2, Fig. 5). These variations are related to the lower concentrations of kaolinites and the higher smectite concentrations in ZAHE compared to those found in ZAHO. The standard deviations are large (Table 2) because of the material heterogeneity. According to the variations in chemical composition in the clayey materials, the hydrothermal process involved the mobilization of the main octahedral cations of the phyllosilicates and Ca²⁺ as an exchangeable cation in smectites. A priori, ZAHE materials were expected to host SiO₂ concentrations owing to the different crystal chemistry of kaolinites and dioctahedral smectites. Kaolinite, as a 1:1 phyllosilicate, has a smaller amount of silica than smectites, which have a 2:1 layer. However, the mean concentrations of this oxide are very similar in the kaolinitic (ZAHO) and bentonitic (ZAHE) areas. The additional silica needed to transform a 1:1 (kaolinite formed by weathering of the granite) into a 2:1 phyllosilicate (smectite formed by the hydrothermal alteration of the weathered granite) comes from a new alteration of the primary minerals, the restitic micas, feldspars, and mainly from quartz, the concentrations of which are lower in ZAHE (Table 2).

Table 2 Statistical data for the main oxides (%) of the analyzed ZAHO, ZAHE, and veins and nodules

Max maximum value, Min: Minimum value, SD: Standard deviation, n: Number of analyses

Fig. 5 Histograms of major element contents (% oxide) of representative samples from ZAHO and ZAHE materials. The relative enrichment in Mg and Ca of ZAHE relative to ZAHO can be observed. ZAHO: Homogeneous alteration zones. ZAHE: Heterogeneous alteration zones

The CIA (Reference Nesbitt and YoungNesbitt & Young, 1982) was calculated for the clayey materials of ZAHO and ZAHE. CIA could have a value of 100 when all Ca, K, and Na are leached from the original granite. In the Tamame deposit, the mean value of CIA is 89.31, indicating a high degree of rock alteration. However, significant differences were observed among the ZAHO and ZAHE materials. In ZAHO, the CIA index ranges between 89.62 and 96.69, with 93.21 as the mean. The values obtained for the kaolinitic areas of ZAHO are like those found by Galán et al. (Reference Galán, Aparicio, Fernández-Caliani, Miras, Márquez, Fallick and Clauer2016), who studied the kaolin from Burela (Spain) and by Bedassa et al. (Reference Bedassa, Getaneh and Hailu2019) at the Belessa kaolin (Ethiopia). However, the CIA values in the samples from ZAHE are lower; the mean value is 85.16 and ranges between 69.34 and 88.38. This does not imply that ZAHE are less altered; in fact, it implies just the opposite because the superimposed hydrothermal alteration implies an increase in the Ca content.

When the major element compositions of the samples were normalized with fresh granite data provided by López-Plaza et al. (Reference López-Plaza, López-Moro, Vicente-Tavera and Vicente-Villardón2008) for the Domo de Tormes pluton in which the deposit is located (Fig. 6), both ZAHO and ZAHE altered granites were significantly depleted in MnO and Na₂O, which was more pronounced in ZAHO than in ZAHE and to a smaller extent in K₂O, Fe₂O₃, and TiO₂. This is because Ca, Na, and K from the K-feldspars and plagioclases were released, in addition to the remobilization of Fe, Ti, and Mn oxides during the weathering. Conversely, ZAHO materials are significantly depleted in MgO and CaO, while they are enriched in ZAHE with regard to the granite. This is because these elements have been remobilized from the weathered granite and concentrated in the ZAHE areas most affected by the hydrothermal process. The main chemical variations that occur in the deposit are related to the degree of alteration, the combined effect of both weathering and hydrothermal processes and element remobilization. According to this comparison, K, Na, Ti, and Mn were lixiviated with the first alteration process, while Mg and Ca were remobilized and concentrated by the hydrothermal fluids. Similar variations were observed in the meteoric alteration of granite by Duddy (Reference Duddy1980), Galán et al. (Reference Galán, Fernández-Caliani, Miras, Aparicio and Márquez2007), Mongelli (Reference Mongelli1993), and Panahi et al. (Reference Panahi, Young and Rainbird2000).

Fig. 6 Representation of the contents of the major elements, as oxides, normalized to the regional fresh granite (López-Plaza et al., Reference López-Plaza, López-Moro, Vicente-Tavera and Vicente-Villardón2008)

The chemical composition of veins and nodules is quite similar (Table 2). The SiO₂ percentage is lower than that in altered granites because the quartz concentration is much lower, while Al₂O₃, Na₂O, and especially K₂O are found in high concentrations in veins and nodules and are influenced mainly by the alunite proportions. As for the whole rock, veins from ZAHE are richer in Mg and Ca than veins from ZAHO.

Minor and trace elements (Sc, Be, V, Co, Ga, Rb, Sr, Y, Zr, and Nb) (Fig. S2) for altered granite normalized data with respect to those of fresh granite (López-Plaza et al., Reference López-Plaza, López-Moro, Vicente-Tavera and Vicente-Villardón2008) show that, throughout the alteration, enrichment in Sc, V, and Sr has occurred which agrees with observations by Condie et al. (Reference Condie, Dengate and Cullers1995) and Galán et al. (Reference Galán, Aparicio, Fernández-Caliani, Miras, Márquez, Fallick and Clauer2016). Co, Rb, and Be were depleted. According to previous observations, weathered granites lost Rb compared with the fresh rock (Galán et al., Reference Galán, Fernández-Caliani, Miras, Aparicio and Márquez2007, Reference Galán, Aparicio, Fernández-Caliani, Miras, Márquez, Fallick and Clauer2016; Mongelli, Reference Mongelli1993). Ga remained constant with approximately the same values as those in fresh granite, as in Panahi et al. (Reference Panahi, Young and Rainbird2000). Y shows a great variation in ZAHO altered granites and maintains approximately the same concentration values of this element compared to those in fresh granite, while ZAHE show a depletion, as it occurs in the alunite-bearing kaolin deposit of Mudamköy (Reference Kadir, Atres, Erkoyun, Külaj and EsenliKadir et al., 2022).

The values of large-ion lithophile elements, namely Sn, Cs, Ba, Hf, Ta, Tl, Pb, Th, and U normalized with respect to those of fresh granite (Fig. S2), reveal a depletion in Cs and Tl with respect to the concentrations in fresh granite, as was also observed by Galán et al., (Reference Galán, Fernández-Caliani, Miras, Aparicio and Márquez2007, Reference Galán, Aparicio, Fernández-Caliani, Miras, Márquez, Fallick and Clauer2016). Sn, Ba, and Pb maintain approximately the same concentrations as those in fresh granite or slightly lower. Hf and Ta have similar behavior with analogous values to the fresh granite in ZAHO while in ZAHE they are lower. Panahi et al. (Reference Panahi, Young and Rainbird2000) affirmed that Hf is an immobile element under meteoric conditions. The Zn and Ni concentrations in samples are below the detection limit (30 ppm for Zn and 20 for Ni); the deposit samples are depleted in these elements with respect to the concentrations in the fresh granite, in good agreement with Turekian (Reference Turekian and Wedepohl1978) who affirmed that Ni is mobilized easily during the weathering process.

REE analyses are also normalized with respect to the values obtained by López-Plaza et al. (Reference López-Plaza, López-Moro, Vicente-Tavera and Vicente-Villardón2008) (Fig. S2). Most samples from ZAHO are enriched in REEs while those from ZAHE show very different concentrations, although most of them present normalized values of <1. The samples from veins of ZAHE from Navalacruz quarry show notable impoverishment in Tm (below the detection limit). Therefore, a certain fractionation of HREE would be more immobile, but not of the LREEs.

Nesbitt (Reference Nesbitt1979), Duddy (Reference Duddy1980), Middleburg et al. (Reference Middleburg, Van der Weijden and Woittiez1988), Marsh (Reference Marsh1991), Prudencio et al. (Reference Prudencio, Braga and Gouveia1993), Mongelli (Reference Mongelli1993), and Morey and Setterholm (Reference Morey and Setterholm1997) studied systems in which weathering caused REE mobilization and fractionation. Thus, the results of Nesbitt (Reference Nesbitt1979) and Duddy (Reference Duddy1980) indicated that the REEs are redistributed within the profile of meteoric alteration; i.e. no element gains or losses occur when considering the entire profile of meteoric alteration. Nesbitt (Reference Nesbitt1979) justified general variations in the profile with respect to the original rock, considering that part of the profile may have been eroded. However, other authors justified the REE fractionation, emphasizing that they form more soluble complexes than LREEs (Reference Cantrell and ByrneCantrell & Byrne, 1987, among others). Thus, Mongelli (Reference Mongelli1993) verified that in the meteoric alteration of granite, the LREEs show a greater tendency to concentrate than the HREEs, which is partially because a greater solubility of the HREEs is favored by being in non-acids.

Stable Isotope Analysis δ¹⁸O and δ³⁴S

Stable isotope analysis of δ¹⁸O and δ³⁴S was carried out in 12 representative samples that contain exclusively silicate minerals and in four samples with the highest alunite contents, respectively. The δ¹⁸O values were compared with the Mean Ocean Water Standard range between 17.5 and 25.8‰, with significant differences among samples derived from ZAHO, ZAHE, and veins (Fig. 7). In any case, these δ¹⁸O values are similar to those obtained by Savin and Epstein (Reference Savin and Epstein1970) for kaolin from Arkansas and South Carolina (~20‰), by Clauer et al. (Reference Clauer, Fallick, Galán, Aparicio, Miras, Fernández-Caliani and Aubert2015) for the kaolin deposits from weathered Variscan granitoids of Galicia (16.5–22.4‰), and by Galán et al. (Reference Galán, Aparicio, Fernández-Caliani, Miras, Márquez, Fallick and Clauer2016) (19.8–21.0‰) from the Burela (Spain) kaolin deposit. Those authors found that these values were consistent with a supergene origin, indicating that the most likely scenario was that formation occurred in equilibrium with meteoric waters at near surface temperatures (between 15 and 36ºC). However, these values are high compared with those in fresh granite proposed by Faure (Reference Faure1977) for S type granites (from 13 to 18‰) and those obtained by López-Plaza et al. (Reference López-Plaza, López-Moro, Vicente-Tavera and Vicente-Villardón2008) for the unaltered granite in the Tamame area (12.9‰). According to Murray and Janssen (Reference Murray and Janssen1984), when kaolins have a hydrothermal origin, the δ¹⁸O values range between 15 and 19‰, whereas kaolins from weathered igneous rocks range between 2 and 14‰. Finally, sedimentary kaolins demonstrate the highest values with δ¹⁸O > 19‰. Only three of the samples from the Tamame deposit have values that correspond to kaolins from weathered granites, and the other samples have values >19‰. This is due to the fact that the isotopic signal in these materials is related to the circulation of surface waters that percolate and then go up through fractures. According to that, the highest values are those of veins (24.5‰). In the kaolin deposits of western Sardinia (Italy), Simeone et al. (Reference Simeone, Dilles, Padalino and Palomba2005) found δ¹⁸O values ranging between 16.6 and 21.4‰ for clayey rocks formed from steam-heated meteoric waters.

Fig. 7 Box diagram with the δ¹⁸O values grouped by sample type: ZAHO and ZAHE materials and veins. ZAHO: Homogeneous alteration zones. ZAHE: Heterogeneous alteration zones

The samples analyzed for δ³⁴S were all derived from ZAHE veins; they do not have restitic minerals and S is present only in alunite. The values obtained are referred to Canyon Diablo Troilite and range between 12.8 and 13.5‰. The mean analyzed value of δ³⁴S was 13.2‰. Dill (Reference Dill2001) proposed δ³⁴S values for alunite of hydrothermal origin that range between 16 and 28‰; for alunite of supergene origin, values between 4 and 10‰; and for alunite of evaporitic origin the δ³⁴S ranges from 11 to 15‰. Bird et al. (Reference Bird, Chivas and McDougall1990) obtained δ³⁴S values of between 1.5 and 6.9‰ in alunite formed in a supergene environment with a sulfate derived from the marine cycle. According to this, Field and Lombardi (Reference Field and Lombardi1972) determined the supergene origin of alunite with δ³⁴S values ranging between 5.5 and 6.3‰, and higher δ³⁴S values corresponding to hydrothermal waters. The values found in the Tamame samples are not typical of meteoric water because they are too high. They are clearly far from the δ³⁴S values corresponding to the marine environment and do not reach the values of hydrothermal waters. This observation can be explained, as for oxygen isotopes, by the fluids coming from meteoric waters that percolated through the weathered granite, enriching themselves with the most mobile elements, to later ascend through fractures and precipitate into new minerals (smectite and alunite), then percolate again through the granite, modifying its mineralogical composition in a low-temperature hydrothermal process.

K/Ar Geochronology

Four samples from veins with high alunite concentration and no mica, which ensures that the age obtained is not influenced by this restitic mineral, were chosen for dating the fracture process (Table S3). The ages obtained ranged between the Maastrichtian (Upper Cretaceous) and the Selandian (Paleocene) (Fig. 8). The veins analyzed were formed between 66.4 ± 1.7 and 58.8 ± 1.5 Ma, indicating their formation in several hydrothermal pulses. Considering the measurement errors, the Nav9bis and Nav36 samples may correspond to the same process at ~59 Ma. These results indicate that the meteoric alteration was prior to 66.4 ± 1.7 Ma and that the Tertiary sediments covering the deposit were formed after 58.8 Ma, and in the middle, three fracture steps occurred at least. These results agree with those reported by Meunier (Reference Meunier2005) when he stated that short hydrothermal pulses favor the formation of low-charge expansive clays (such as the smectites that appear in these veins), while non-expansive species crystallize in systems in which fluids circulate for longer periods.

Fig. 8 Box diagram with the ages of representative samples of veins dated by K/Ar geochronology

The ages obtained coincide with the first stages of formation of the Duero Basin, in which the deposit located at the western edge occurred in the uppermost Cretaceous–Paleogene time according to Molina et al. (Reference Molina, Cantano, Vicente and García Rodríguez1990) and Granados et al. (Reference Granados, Ferrer, Butillé, Muñoz, Roca, Ballesteros, Giménez, Vallejo and Gonzále2012). The first sediments in the southeast areas of the basin could be Maastrichtian in age.