The insufficient geological literacy, particularly in Portugal, is in part due to the inappropriate and unproportionally teaching of geology through middle and high school, when compared to other natural sciences (Reis et al 2014). This inefficiency leads to a deficient awareness to the role of mineral resources in society. This becomes a problem when it is necessary to grasp environmentally responsible exploration and mining projects, leading to problems with local communities and therefore in acquiring the necessary social license to operate. Geologists have their share in this problem, since normally it is quite difficult to deconstruct some of the concepts of geology (sensu lato), and particularly the ones related to exploration and mining, to a non-specialised public. For this, the assessment of geodiversity and mining heritage by cataloguing and promoting the scientific and educational value of key locations is of major importance.
The Évora – Montemor-o-Novo districts comprise attractive and didactic geosites and geodiversity sites that display a strong relationship with several cultural and historical values of the region that could, and should, be used in informal and formal teaching activities focusing on the local and regional geology. A total of ten sites, both outside and inside the ancient iron mining area (Fig. 2), are outlined as being able to integrate a geological and cultural route between Évora and Montemor-o-Novo (Fig. 2). Limelight will be focussed on the mining heritage located in the Montemor-o-Novo municipality, near the Santiago do Escoural village, since geodiversity characterisation and assessment within these areas raises more concerns in terms of geoconservation due to the rapid deterioration of some of the mining heritage sites.
The description of geodiversity is herein addressed both for the outside and inside of the ancient Montemor-o-Novo mining district, aiming to propose a geodiversity route that interlinks the Évora and Montemor-o-Novo municipalities. A qualitative assessment (Brilha et al 2016, 2018) of individual potential location is provided through the next sections, as well as a tentative quantitative assessment of Geodiversity regarding the Scientific (SV), Educational (EV) and Touristic values (TV) of the locations proposed to integrate the geodiversity route, allied with the degradation risk assessment following the criteria defined by Brilha et al (2016). The quantitative assessment results can be consulted in Supplementary Material A. A score from 1 to 4, is attributed to each site selected to integrate the geodiversity route, with exception of the Almendres Dolmen Complex since it does not meet the criteria to be considered a geodiversity location Nevertheless, the Almendres Dolmen Complex was most probably constructed using local granitic stones, and due to its significant archaeological and cultural heritage is herein considered as an important asset for the route. The numbering of the sites is shown in accordance to Figure 2 and does not represent the sequence of visitation for the potential geodiversity route (see section 6.). The list of the sites both inside and outside of the ancient mining area can be found in Table 1.
4.1. Geodiversity and geosites outside the ancient mining area
Alto de São Bento granitic suite (Ancient Quarry)
The Alto de São Bento area is a geomorphological feature of the Évora landscape, located at around 3 km from the city centre, and corresponds to an elevation (aprox. 360 m) that contrast with the surrounding characteristic Alentejo plains. At the area, outcrops of two-mica leucogranite and porphyritic granites (Fig. 3a) were exposed by quarrying activities during the 20th century and are now accessible to the public. The contrasting altitude of Alto de São Bento with the surrounding landscape was an important factor for the settlement of several windmills used for seed processing.
The geological, biological and cultural assets of the area are currently musealized (Brilha & Carvalho 2010) by the Évora municipality, which developed a museum that offers several educational and touristic activities, allied with the fact that the Alto de São Bento area offers one of the most beautiful sight-seeing sceneries over the Évora UNESCO city (Fig. 3b).
Several geological features are displayed at the outcrop, such as the ubiquitous course-grained porphyritic granite with abundant K-feldspar phenocrysts which display concentric zoning (Fig. 3a) and a medium grained two-mica leucogranite. The porphyritic granite facies also exhibit large mafic enclaves of igneous material (biotite-rich; Fig. 3a) that put in evidence fractional crystallisation processes (Moita et al 2009).
Considering the described characteristics, it is evident that the substantive scientific, educational and even touristic values of the area, with scores of 3.1, 3.3 and 2.9 respectively, flag it as a potential geosites location, benefiting from the musealization and conservation already developed in the area. Furthermore, the Alto de São Bento granitic suite could easily be integrated in the Pre-Mesozoic Granitoids framework of the Portuguese Geosite Inventory (PGI; Brilha et al 2005, 2008, 2010), and for this reason this site is herein considered a mandatory geodiversity stop in the geodiversity route proposed through section 6.
Herdade Monte das Flores Quarry
The Monte das Flores Quarry is an example of geodiversity in the Évora Municipality, located at circa 6 km from the city centre, it is part of the PGI framed on the Pre-Mesozoic Granitoids (Brilha et al 2008, 2010) and publicised on the ProGEO – Portugal website. The quarry is located at the Évora Massif, and extraction is devoted to the Évora granite/ granodiorite facies for industrial uses (Industrial rocks). The quarry was classified as a geosite due to its geological, economic, cultural, archaeological, and educative value. The most relevant features are associated to the use of the Évora granites as building-stones from the Megalithic period until the present days. Evidence of that is the Almendres Dolmen Complex (see next section).
Almendres Dolmen Complex
The Almendres Dolmen Complex corresponds to a cromlech archeologic structure located at circa 7 km of the UNESCO Évora city and is composed of 95 Neolithic granitic monoliths (Fig. 3c), arranged in a circle. The site was classified as a National Monument in 2015, and besides being an example of archaeological heritage, the geological aspects of the monument, like the nature of the rock utilized in the construction, can also be used for educational purposes.
Although the Almendres Dolmen Complex does not fit the criteria for geological heritage, if inserted in the geodiversity route, this site could be used as an example of the societal and cultural evolution associated with the use of geological resources. This monument is part of the Megalithic Route, promoted by the Évora Municipality, and although tourists intensely visit the area, it lacks proper interpretative communication, better road access, infrastructures, and visitation circuits.
Migmatites at the Almansor river
The Almansor area corresponds to an outcrop that displays the perfect conditions for the observation of the high-grade metamorphic terrains from the Variscan Gneiss-Migmatite Complex, part of the Évora Massif.
The area is located along the left and right margins of the Almansor river that contours the Montemor-o-Novo municipality (ca. 2 km from the city centre). At the outcrop scale several petrogenetic features have been individualised by several authors, such as diatexites, weakly foliated granitoids and trondhjemitic veins (Moita et al 2009). The high-grade metamorphic terrains register the effects of intense migmatization of the metasedimentary country rocks (Escoural Formation), in which partial fusion (crustal anatexy) is evidenced by the presence of diatexites, metatexites, restites and numerous mafic enclaves observed through the area, revealing variable fusion rates.
The diatexites-metatexites mark the migmatization flow (Fig. 3d) commonly exhibiting centimetric to metric restites of metamorphic origin (Moita 2007; Moita et al 2009), possibly reflecting metasedimentary host rock blocks, from the Escoural formation, that did not undergo total fusion and in which pre-migmatization textures are sometimes preserved (Fig. 3e).
The diatexites and metatexites are composed of leucosome and melanosome components (Fig. 3e) and the relation between the GMC and the adjacent Hospitais Tonalite Massif is evidenced by the weakly-foliated granitoids present in the area (Moita et al 2009).
As gathered from the previous description, and reinforced by the SV, EV and TV scores (Supplementary Material A) of 3.6, 3.1, and 2.65, respectively, it is here suggested that the area has the potential to be classified as a geosite framed in the Pre-Mezosoic Framework of the PGI Inventory (Brilha et al 2008 2010). Furthermore, there has been a previous proposition for the creation of two Eco-trails through the Almansor river margins (Dias da Silva et al 2006), which contemplates the geological, cultural and landscape characteristics of the area. The geoconservation strategies that could possibly be implemented should focus on the implementation of interpretative centres focusing on the representative and rare geological processes displayed through the outcrops. Besides the proposals from other authors, we believe that contemplating this site in a geodiversity route is extremely important in interpreting regional geological settings.
4.2. Geodiversity and geosites inside the ancient mining area
Nogueirinha and Serrinha mine
The Nogueirinha and Serrinha mines were two of the main open-pit exploitation areas corresponding to two of the 10 mining concessions part of the ancient Montemor-o-Novo iron district (Fig. 1). Both mines are located approximately at 26 km and 21 km from Évora and Montemor-o-Novo municipalities, respectively. Mining activities were sustained from 1876 until 1929, and the total amount of exploited iron ores are estimated at around 137 406 tons (Andrade et al 1949), which at the face of current societal supply and demand is considered insignificant.
Currently, the open-pit mining areas are abandoned and have not been the target of any geoconservation proposals. They are both located inside private owned lands, although access is usually granted for educational and scientific purposes. From a visitation point of view these proposed locations are the ones that raise more concerns regarding security and accessibility to the outcrops, but nevertheless field work is possible.
Remnants of the ancient mining works are ubiquitously denounced by large volumes of tailings in both Nogueirinha and Serrinha mines. At the Serrinha mining area, the access to the outcrop is easier, and primary magnetite ± primary pyrite mineralisation can be observed.
The quantitative assessment of both locations revealed SV; EV and TV of 1.75/1.9, 2.2/2.2 and 2.2/2.2 respectively.
The Escoural caves is a geosite framed in the “Karst Systems of Portugal” classified by the PGI (Brilha et al 2008, 2010; Brilha and Pereira 2020) and corresponds to a typical carbonate karst system, located at approximately 15 km from the Montemor-o-Novo municipality (Fig. 2). The cave was accidentally discovered in 1963 during quarrying works in the area, which focused operations on marble extraction for ornamental purposes. The marbles belong to the Cambrian Monfurado Formation (Chichorro 2006). Allied with the karst other geological features are identified, such as stalactites and stalagmites (Fig. 3f).
Besides the described geological features, that classify the Escoural cave as a geosite, carvings and paintings found on the cave walls demonstrate that the karst system was used as a shelter for human settlements since the Palaeolithic (Silva et al 2017). The discovery of these findings has classified the cave as a national monument due to its major archaeological value.
Hence, the Escoural cave has been the aim of several conservation interventions and academic research in a wide range of scientific fields (e.g. Caldeira et al 2021). Visitation is currently possible, although prior booking with the Montemor-o-Novo municipality is mandatory. The guided tour through the small cave takes approximately 30 minutes and can be organised in groups of up to 10 persons (pre-COVID). Although the cave displays interesting geodiversity features, the visitation currently only focuses on the pre-historic archaeological features of the monument. The geodiversity quantitative assessment of the Escoural cave revealed SV, EV and TV scores of 3.1, 3.3 and 3.25 respectively, supporting the geological significance of the monument. We believe that since the area is already classified as a geosite, the guided tour in the Escoural cave should also focus on the geological features of the karst system, and if so, should integrate the herein suggested geodiversity route.
Serra do Conde Quarry
The Serra do Conde area corresponds to a differential erosion relief (439 m) part of the Monfurado Mountain range, located at approximately 23 km from Évora and 21 km from the Montemor-o-Novo municipalities.
At the Serra do Conde area outcrops of amphibolites from the Carvalhal Formation (Fig. 1; Fig. 4a) occurring in the core of a synclinal structure can be observed (Fig. 4c; Chichorro 2006). The amphibolites display a nematoblastic texture and two main metamorphic-deformation fabrics can be seen. One parallel to an S0 foliation with a N325º direction and sub-vertical dipping, strongly marked by the development of epidote (Fig. 4b). Primary foliation is transposed by a generally folded S1 mylonitization (N145º direction; Fig. 4b). The rock quality and weak fracturing fomented the extraction of several blocks as ornamental stone, and therefore the outcrops at the Serra do Conde area were exposed by the quarrying activity.
The area contributed to scientific research that focused on the interpretation of the regional and structural geological settings (Chichorro 2006) reinforcing the scientific value score of 3.2 (Supplementary Material A). Access to the area can be done by foot or by SUV through an earth road (approximately 1.5 km from the nearest paved road). The described characteristics of the Serra do Conde Quarry resulted in EV and TV scores of 1.95 and 2.05 respectively, although we believe that the educational value is far greater than the touristic value.
Vale da Arca Mine
Vale da Arca Mine is located at circa 16 km and 25 km from the Montemor-o-Novo and Évora municipality and contributed with a minor amount of ore production to the overall tonnage of the Montemor-o-Novo iron district, although concrete numbers are not known (Andrade et al 1949). The mine was abandoned during the early 20th century, although it was latter used for quarrying activities that focused on the extraction of the marbles. The quarry is presently abandoned, but both the mining and quarrying activities exposed outcrops that allow for the characterisation of geodiversity features related to the geodynamic and structural settings of the MNSZ
For the present work the Digital Elevation Model (DEM) was constructed using drone imagery collection in the Vale da Arca area (Fig. 5), and five geological features of the outcrops are outlined (Fig. 5): 1) Observation marbles with olivine and disseminated magnetite ± pyrite mineralisation. This lithotype belongs to the Monfurado Formation marbles and here it is possible to understand that the disseminated textures and low tonnage were probably the reasons why the mine was abandoned; 2) Boudinage structures of a silicious layer interbedded within the olivine marbles (Fig. 6a) resulting from the different mechanical/rheological competence between the marbles, with a ductile behaviour, and the silicious layer which has a brittle behaviour, generated due to the overall NW-SE deformation associated to the MNSZ activity. Boudinage structures such as chocolate tablet boudinage and inter-boudin (Fig. 6c) and necking structures (Fig. 6d) are observed in this site; 3) At the area, beds of massive pyrite ± magnetite layers can be observed (Fig. 6b) and usually exhibit surficial oxidation (Fig. 6b). These beds are over thrusted (N355º; 40ºE) by pristine course-grained marble lithotype, which correspond to point 4 in Figure 5; 5) The upper unit of the Monfurado Formation marks the initial stages of Cambrian basic volcanism, with abundant intercalations of metavolcanic coarse-grained rocks, which can be observed in this point. These rocks exhibit large crystals of amphibole, epidote and feldspar (Fig. 6e).
The pin-pointed geological features (Fig. 2) show the relevance of these outcrops for a possible geodiversity route, which favour the accessibility conditions of the Vale da Arca Mine. The SV, EV and TV scores (2.1, 2.5, 2.4; Supplementary Material A) reinforce the proposition of the Vale da Arca area as a geosite that would easily be integrated in a geodiversity route.
Ongoing research focusing on trace element composition of magnetite (Maia et al. in prep.) found evidence for the deposition of sphalerite (ZnS) along late stages of magnetite deposition. The textures found on magnetite crystals indicate that sphalerite is associated with the porous rims (Fig. 6f), where they occur as inclusions along with pyrite (Fig. 6g – green Zn distribution). Such findings might be good indications for future mineral exploration in the area, which should not be constrained by future geoconservation strategy because the mining potential of the area should be an ally of the use of the area for scientific, formal and informal education activities.
Monges Mining Complex
The Monges Mining Complex correspond to the biggest mining works part of the Montemor-o-Novo iron district. The area is located approximately 10 km from the Montemor-o-Novo and 35 km from the Évora cities. Mining works were mostly performed in open pit, although locally underground mining was adopted, with ore production estimated around 206 783 tons, which correspond to 60% of the iron production in the sector (Andrade et al 1949).
The Monges mine (Monges = Monks) owe its name to the existence of an Abbey (Fig. 7a) which construction dates to 1738 and served hermit monks that inhabited the Abbey until 1834. After the abandonment, and with the discovery of the iron ores, the Abbey was used to lodge many of the miners that worked at the mine site (Andrade et al 1949). Currently, the beautiful building is in ruins (Fig. 7a) due to an unfortunate lack of architectonic conservation strategies.
The remnant open-pit mining activities are currently dominated by intense vegetation which outlines nature restoration promoted by natural processes combined with human abandonment. Even though access is difficult, at the area large mine tailings can be observed, as well as in-situ ore bodies. As previously described, mining activities in the region are thought to date back to the Roman period, and at the time iron ores were found due to the intense leaching capping (gossan zone) that are an evidence of intense iron-oxide surficial alteration (Fig. 7c).
The massive ore bodies are mainly constituted by magnetite (± pyrite ± chalcopyrite) and perfect crystallisation of magnetite is ubiquitously observed (Fig. 7b). The effects of pyrite exposure to atmospheric conditions are observed at the outcrop scale, and the oxidation of pyrite results in sulphates and native sulphur formation, denounced by its characteristic yellow colour (Fig. 7d). Microscopic examination of magnetite samples collected at the Monges open pits (Fig. 7e and f) revealed euhedral magnetite crystals with porous textures and homogenous chemical compositions (Fig. 7f), but with sparse silicate inclusions.
The examination of the Monges Mining Complex indicates that this is one of the prime examples of mining heritage in the region and is herein proposed to integrate the Portuguese geosites list, although a framework that considers the mining heritage of Ossa-Morena Zone would have to be proposed. This proposition is supported by the obtained SV and EV quantitative assessments (Suplementaty Material A). Although some accessibility conditioning is identified, such as the access by an earth road and by foot, the area also displays the necessary conditions for a trekking route, taking advantage of the Monfurado Mountain habitats and characteristic vegetation (Quercus Suber).