Understanding the Geology of Mountain Foothills Through Hydrogeochemistry: Evaluating Critical Raw Materials’ Potential for the Energy Transition in the Salsomaggiore Structure (Northwestern Apennines, Italy)

Abstract

The energy transition is an issue of fundamental importance in the current global context, as an increasing number of countries are committed to searching for minerals and elements essential for the storage, distribution, and supply of energy derived from new renewable and sustainable sources. In some countries, these elements (such as boron, lithium, and strontium) are considered to be critical raw materials (CRMs) because of their limited occurrence within their own borders and are commonly found in minerals and geothermal–formation waters, especially in brackish to brine waters. In the Italian territory, CRM-rich waters have already been identified by previously published studies (i.e., with mean concentrations in the Salsomaggiore Terme of 390 mg/L of boron, 76 mg/L of lithium, and 414 mg/L of strontium); however, their extraction is hampered by several knowledge gaps. In particular, a comprehensive understanding of the origin, accumulation processes, and migration pathways of these CRM-rich waters is still lacking. These factors are closely linked to the geological framework and evolutionary history of each specific area. To address these gaps, we investigated the Salsomaggiore Structure that is located at the northwestern front of the Apennine in Italy by integrating geological data with hydrogeochemical results. We constructed new preliminary distribution maps of the most significant CRMs around the Salsomaggiore Structure, which can be used in the future for the National Mineral Exploration Program drawn up in accordance with the European Critical Raw Materials Act. These maps, combined with the interpretation of seismic reflection profiles calibrated with surface geology and wells, allowed us to establish a close relationship between water geochemistry/CRM contents and the geological evolution of the Salsomaggiore Structure. This structure can be considered representative of the frontal ranges of the Northwestern Apennine and other mountain chains associated with the foreland basin systems.

1. Introduction

The energy transition, derived from the awareness of the impacts of climate change and the progressive depletion and phasing out of fossil resources, has forced several countries to achieve a sustainable, reliable, and energy-efficient future, i.e., shift toward renewable energies (e.g., eolic, solar, tidal, etc.) [1,2,3,4]. This shift has intensified the search for minerals and elements that are essential for the storage, distribution, and supply of energy from new sources. Among these, elements such as boron, lithium, and strontium are expected to see a substantial rise in demand in sectors, including electric mobility and digital technologies [2,3].

However, these elements are readily available only in a limited number of countries—most of which are outside the European Union (EU) (e.g., China, Brazil, Australia, South Africa, Russia, and the Democratic Republic of Congo) [2,3,5]. As a result, many other nations have been forced to secure access to these resources beyond their borders, thus classifying them as critical raw materials (CRMs). Since 2008, the EU has developed and continuously updated a strategic program aimed at identifying new supply areas and improving extraction methods, with the purpose of reducing dependency on foreign countries [5]. In Italy, a law and an exploration plan have recently been approved to implement the above-cited regulation [6]. Within this framework, research in recent decades has focused on the extraction of CRMs from unconventional deposits, such as geothermal waters, formation brines, and marine waters [7,8,9]. In Italy, conventional lithium deposits are limited to some pegmatites in the Alps (South Tyrol) and Elba Island [10,11]. However, due to the naturalistic value of these areas, mining activities are not permitted. In contrast, the outlook for unconventional deposits is markedly different. Two major zones that are enriched in lithium and other CRMs, such as boron and strontium, have been identified. The first includes the peri-Tyrrhenian volcanic–geothermal zones (Tuscany–Lazio–Campania), characterized by their high-enthalpy fluids [12,13,14]. The second is located along the front of the Apennine chain, extending from Alessandria to Pescara, where low-enthalpy thermal manifestations occur [15,16,17,18].

Despite the identification of potential CRM-rich zones within the Italian territory, several critical knowledge gaps still hinder their effective extraction. These include the following:

(i)

A lack of detailed regional maps illustrating the distribution and concentration of CRMs in favorable sites;

(ii)

Limited knowledge of the origin, distribution, migration pathways, and outflow of CRM-containing waters, in addition to an insufficient characterization of the lithological units that influence their geochemical compositions;

(iii)

An inadequate understanding of how major stratigraphic units and tectonic structures control fluid leakage zones that contain CRMs;

(iv)

The presence of conflicting geochemical analyses, with some discrepancies between historical and recent results.

Solving the above knowledge gaps requires interdisciplinary work that integrates geological, geophysical, geochemical, and hydrogeological studies aimed at clarifying the origin, accumulation, migration, and final outflow zones of these waters or unconventional deposits of CRMs, which is essentially a reconstruction of the geological evolution of an area and its associated fluids. This case study, focusing on the frontal portion of the Apennine chain in the Salsomaggiore Structure (Northern Apennines) (∼70 km²), aims to shed light on the geological evolution of mountain frontal ranges through a geochemical investigation on subsurface waters (springs and boreholes) enriched in CRMs. To accomplish this aim, new maps of the distribution and concentration of CRMs have been developed, and the interpretation of seismic reflection profiles has been performed, integrating surface and well data. This will provide (1) preliminary estimates of the distribution and geochemical characterization of these natural resources and (2) new constraints on the geological evolution of the Salsomaggiore Structure. These results pave the way for a clearer vision and quantification of the origin, accumulation, migration, and outflow zones of these waters in relation to the structural and geological setting of the study area and mountain frontal ranges associated with foreland basin systems in general. This represents the first needed step toward a more sustainable exploitation of unconventional deposits of CRMs.

2. Geological Setting and Previous Works

2.1. Geological and Structural Framework of the Northern Apennines

The Apennines, including their northwestern segment, originated from the Neogene deformation of the Adria continental crust and the overlying remnants of a former oceanic accretionary wedge [19,20,21,22]. These remnants correspond to the Ligurian units, which formed between the Middle Jurassic and Upper Cretaceous during the Ligurian phase, prior to the formation of the Apennines [21,23,24,25]. Specifically, the Ligurian Units are divided into two main domains: the Internal Ligurian Units, which originated from an intra-oceanic setting, and the External Ligurian Units, related to the ocean–continent transition toward the Adria plate margin [24,26,27,28]. The Internal Ligurian Units consist of a Jurassic ophiolitic sequence overlain by basin plain deposits (pelagic sequence) and a turbiditic succession, with the latter dated from Upper Cretaceous to Lower Paleocene [24]. The External Ligurian Units are subdivided into Western Ligurian Units (e.g., the Ottone-Caio unit), where Helminthoid Flysch overlies sedimentary mélanges containing both oceanic and continental slide blocks, and Eastern Ligurian Units (e.g., the Cassio unit), which feature a basal Triassic–Jurassic coherent sedimentary succession without an ophiolitic component [27,29,30,31,32,33].

The closure of the Ligurian-Piedmont Ocean began in the Lower Cretaceous, driven by the convergence between the European and the Adria plates [22,34,35,36,37,38,39]. This event was followed by the continental collision between the two plates, which occurred between the Upper Eocene and the Oligocene [37,38,39,40,41]. The onset of the Adria plate subduction beneath the European plate triggered the east-vergent migration and tectonic uprooting of the Ligurian Units from their original substrate, leading to overthrusting of the previous accretionary Ligurian wedge onto the Adria plate [21,28,42]. Since the Oligocene, the progressive migration of the Ligurian accretionary wedge and the shortening of the Adria plate led to the formation of the flexural basins and related foredeep basins filled with deep-water sandstone-/clay-rich turbiditic deposits coming from the Alpine domain. These deposits constitute the Macigno, Cervarola, and Marnoso-arenacea Fms., which testify to the progressively eastward migration of main sedimentary depocenters atop the Adria plate [31,32,33,43,44,45]. At the same time, wedge-top basins developed on top of the Ligurian accretionary wedge. These basins, grouped into the Epiligurian units, were established and passively transported atop the “allochthonous” Ligurian units. From the Oligocene to the Lower Miocene, the coeval counterclockwise rotation of the Sardinian-Corsican Block and the progressive opening of the Ligurian-Provençal basin [36,37,38,46,47,48], followed by the opening of the Tyrrhenian Sea since the Upper Miocene [36,49,50,51,52], led to the dismemberment of the earlier Alpine chain and the gradual formation of the Apennine chain as we see today.

The study area is located in the northwesternmost segment of the NW-SE striking Northern Apennines chain. The Northern Apennine front lies beneath the Po plain and is covered by the Messinian to the Pleistocene–Holocene sediments [53,54,55]. It is composed of three distinct arcuate thrust systems that started to develop at different times, from west to east: the Monferrato arc (Lower Miocene), the Emilian arc (Lower Miocene to Messinian), and the Ferrara arc (Messinian to Upper Pleistocene) [54,55,56,57,58]. The study area belongs to the Emilian arc and extends along the foothills and the adjacent Po plain in the Parma province (Figure 1). This sector includes the front of the Ligurian Units, the underlying Tertiary foredeep deposits of the Marnoso-arenacea and related formations that outcrop in the tectonic window of Salsomaggiore (Figure 1), and the Plio-Pleistocene deposits that overlie the outermost buried thrusts of the Emilian arc below the Po plain.

2.2. The Salsomaggiore Structure

The Salsomaggiore Structure is one of the major frontal structures of the Apenninic foothills in the Parma province between the Taro and Ghiara Streams (Figure 1). It is characterized by a NE-verging thrust system associated with an anticline having a NW-SE-striking axis, plunging toward NW and SE. The Salsomaggiore Structure forms the core of a tectonic window. According to the 1:50,000 Geological Map of Italy [59,60,61], it exposes foredeep deposits, comprising the marly Torrente Ghiara Formation (Langhian-Serravallian) and the sandstones of the Rio Gisolo Formation (Serravallian), which are coeval to the Marnoso-arenacea Formation (Figure 1).

The SW backlimb and the NE forelimb of the anticline are overthrusted by the “allochthonous” Ligurian and Epiligurian units. The NE forelimb of the anticline is unconformably overlain by a succession ranging from the latest Messinian to the Pleistocene and the Holocene deposits [55,62,63,64] (Figure 1). The allochthonous units, emplaced onto the Salsomaggiore Structure after the Serravallian and, most likely, during the Messinian, are interpreted to form an Intra-Messinian Chaotic Complex [63,65,66].

The latest Messinian to Pleistocene succession lies above a composite erosional surface that unconformably incised both the Middle Miocene foredeep, the Ligurian deposits, and the Intra-Messinian Chaotic Complex [61,63,64], posing a firm constraint on the final emplacement timing of the allochthonous terranes.

The Salsomaggiore Structure is related to the blind thrust and thrust’s splays, mainly dipping to the SW and cutting down to the Mesozoic and the Tertiary carbonate succession [21,22,61,63,64,67]. Additionally, the backlimb of the anticline is also affected by NW-striking lineaments interpreted either as (1) a hinterland-vergent blind back-thrust dipping to the NE, which deforms the entire Salsomaggiore tectonic window [55,68] (Figure 1A-A’,B-B’), or (2) SW-dipping normal faults related to extension in the anticline hinge zone [64,69] (Figure 1C-C’).

Figure 1. Geological sketch map of the study area modified after [70]; geological cross-sections modified after [68] (A-A’—* Note: in this section the orange unit also includes Lower Pliocene marine deposits) [55] (B-B’), and [64] (C-C’).

Figure 1. Geological sketch map of the study area modified after [70]; geological cross-sections modified after [68] (A-A’—* Note: in this section the orange unit also includes Lower Pliocene marine deposits) [55] (B-B’), and [64] (C-C’).

2.3. Occurrence of Saline and Brine Waters in the Northern Apennines

Saline and brine waters in the Northern Apennines and the adjacent foredeep (Po Basin) have been documented since historical times, primarily exploited for salt production [71,72]. Following World War II, their use shifted mainly to thermal and therapeutic applications, prompting extensive investigations due to their association with hydrocarbons [73] and methane seepages, both during the paleo-era and present day, which are structurally controlled by regional tectonics [53,74,75,76,77]. Hydrocarbon exploration frequently encountered brines in the Po Basin, with the highest salinities concentrated in a relatively small area of the Emilian sector, including the entire thrust top-foredeep setting in the province of Parma. In this area, the most saline brines have been identified near Fontevivo (TDS up to 167 g/L) and Salsomaggiore Terme (TDS up to 190 g/L) [16,78,79].

Geochemical studies have provided key insights into the origin, evolution, and flow paths of these fluids. In particular, Boschetti et al. (2011) [16] proposed the following: (i) the brines originated from the Miocene seawater evaporated to a point between gypsum and halite saturation; (ii) the decomposition of organic matter during burial and diagenesis produced elevated concentrations of iodine and bromine, combined with hydrocarbon generation and sulfate reduction; (iii) diagenetic reactions such as dolomitization/chloritization (e.g., at Salsomaggiore Terme and Cortemaggiore), resulting in decreased Mg and increased Ca, albitization/zeolitization (decreased Na and increased Ca), and clay dewatering/illitization with potassium feldspar precipitation (decreased K) contributed to compositional variability; and (iv) diagenesis also affected isotopic signatures, such as the δ¹⁸O enrichment observed at Salsomaggiore, likely resulting from peak burial temperatures (100–150 °C).

3. Materials and Methods

To fully understand the origin, distribution, migration, and outflow zones of waters containing CRMs in relation to the geological context of the study area, different investigation methodologies were carried out, as outlined in the following subsections.

3.1. Water Analysis Data Collection

The chemical analyses concern water samples from both springs and boreholes in the Province of Parma, sampled between the 1930s and the 2000s. The data were retrieved from scientific studies primarily focused on the study area [16,80,81,82].

3.2. Subsurface Data

Three NE–SW-oriented multichannel seismic reflection profiles are integrated in this study to better define the structural and stratigraphic framework of the Salsomaggiore Structure. The two profiles (Line 1 and Line 2) were provided by Eni S.p.a. (Milano, Italy) (Figure 1) and addressed hydrocarbon exploration. Thus, they focused on deep targets, which implies low resolution for superficial portions and variable vertical and horizontal resolutions within seismic profiles. The profiles have been interpreted by tracing the higher amplitude reflections and seismic facie changes based on already published data [53,55,64,68,76,83] and were finally presented as artwork using Adobe Illustrator^® v. 24.0.1 software. The third profile (Line 3), originally published by Rizzini (2007) [84] and redrawn by Artoni et al. (2010) [66] (Figure 1), was also acquired by Eni S.p.a. for hydrocarbon exploration. It was partially interpreted and redrawn similarly to the other two seismic profiles. However, its stratigraphy was harmonized with the same two seismic reflection profiles interpreted and reported in this study to ensure stratigraphic consistency across all three profiles. Two exploration wells (Noceto_001 and Campore_001), located nearby the Salsomaggiore Structure, publicly available in VIDEPI (Visibility of petroleum exploration data in Italy—https://www.videpi.com (accessed on 25 July 2025)), were used to calibrate the stratigraphy of the selected seismic lines, allowing us to define the main seismic stratigraphic sequences, thus establishing a robust stratigraphic architecture of the study area (Figure 1 and Figure 2). The Noceto_001 and Campore_001 wells, which have no sonic log and velocity information, were projected on Line 1 and Line 3, respectively, and were used to define the interval velocities needed for the conversion of the stratigraphic unit thicknesses (wells) into two-way times (TWT) (seismic reflection profile) (Figure 3A,C). The depth-to-time conversion was pursued considering that the major lithological changes in the wells correspond to higher amplitude reflections and unconformity surfaces in the seismic reflection profiles.

The stratigraphic calibration of the seismic lines included three different phases:

(i) Identifying the thicknesses (in meters) of each stratigraphic unit reported in the Noceto_001 and Campore_001 wells, and for each unit, assigning interval velocities based on similar lithologies or stratigraphic units documented in the literature [53,86,87] (Figure 2);

(ii) Manually transforming the unit thickness (in meters) into time (s/TWT) using the standard interval velocities for different stratigraphic units and employing the formula:

s = Vint × twt/2

where twt = double travel time of the seismic waves in milliseconds; s = thickness in meters between the top and base of the unit considered; and Vint = interval velocity in m/s of the unit;

(iii) Cross-checking and averaging the resulting time (s/TWT) in (ii) with the time at which the base and top of a given stratigraphic unit were interpreted on the seismic reflection profiles.

3.3. Construction of CRM Distribution–Concentration Maps

Based on the main geochemical characteristics of the waters in the area (Figure 4 and Table S1) and to assess whether the concentrations of CRMs are present in a significant number of samples, we constructed concentration maps only for CRMs such as boron, lithium, and strontium. To define whether their concentrations are high or low in the study area, a range of values was established according to the maximum and minimum concentrations measured for each analyzed CRM (

Table 1. Maximum and minimum concentrations for each CRM in the most relevant areas of the study area.

Location	Water Type	Sample Type	B (mg/L)	$\overline{\mathbf{x}}$ ± σ	Li (mg/L)	$\overline{\mathbf{x}}$ ± σ	Sr (mg/L)	$\overline{\mathbf{x}}$ ± σ
Salsomaggiore Terme	Br-I-rich brine	B	434–358	390 ± 35	96–59	76 ± 17	414–343	383 ± 36
Tabiano	Sulfurous brackish	B, S	5.24–0.25	2.74 ± 3.53	1.7–0.003	0.47 ± 0.53	10.04–0.1	6.47 ± 4.37
		B	5.24	=	1.7–0.03	0.63 ± 0.66	10.04–0.1	7.14 ± 4.75
		S	0.25	=	0.5–0.003	0.28 ± 0.26	3.82	=
Contignaco	Sulfurous fresh	S	=	=	0.1	=	=	=
Sant’Andrea Bagni	Mineral fresh	B	3.89–2.13	2.71 ± 1.02	0.3–0.02	0.11 ± 0.12	1.7–0.4	0.83 ± 0.37
	Sulfurous brackish	B	55.98–1.92	23 ± 26	0.6–0.1	0.4 ± 0.3	28.9–0.3	16.9 ± 14.7
	Br-I-rich saline	B	65.33–1.5	33 ± 45	1.1	=	=	=

B: borehole; S: spring; $\overline{\mathrm{x}}$: mean; σ: standard deviation; = no data or not calculated.

and Figure 5). The three types of ranges (high, medium, and low) were created by subtracting the minimum value from the maximum value; the result was divided by three, thus obtaining the value of each range (Figure 5). This was carried out for all three elements.

The ranges of plotted values are fitted to those derived from the analyzed dataset, and they do not imply exploitability of these elements. As a matter of fact, studies focused on the selective extraction of CRMs from water are largely lacking. In contrast, due to the exponential increase in lithium demand, several innovative extraction methods have been developed and published for recovering this element from waters containing concentrations as low as 1 mg/L, even in the presence of very high TDS and Mg/Li ratios [88].

The first phase of this work, map construction, involved the creation of a QGIS (v. 3.34.3) project that includes the geological map used as a basis, in this case, the structural map of the Northwestern Apennines by Vescovi et al. (1997) [70] and other geological maps [59,60,61]. In addition, to facilitate data entry, the multiscale CTR downloaded from the Emilia-Romagna Region’s geoportal (WMS services; https://geoportale.regione.emilia-romagna.it/ (accessed on 1 May 2025)) was loaded into the GIS project. In the next step, the data were added by creating a layer for each element. By georeferencing wells and springs derived from bibliographic data (including the Emilia Romagna Geographic Data Catalogue), it was possible to map points corresponding to analyses. An exception is the Sant’Andrea Bagni area, where multiple water types with distinct geochemical characteristics are represented as a single water type. This choice was made because the maps developed in this study are intended as preliminary tools for an initial assessment of the area, rather than detailed representations of the different water types present within a single location. To this end, the concentrations of each CRM in the different waters were averaged into a single value. The dots (spring/borehole location) were then colored according to the range and the element they represented.

Figure 4. Langlier–Ludwig (A) and Brine Differentiation (B) diagrams of the water samples considered in this study [89,90,91]. All chemical ratios are expressed on an equivalent basis. In both diagrams, the pentagon represents the composition of present-day seawater. In (A), the dark-gray field represents the composition of formation brines from the Northern Apennines/Po Basin; the arrows indicate general mixing between two end-members or water–rock interaction processes (e.g., sulfate dissolution), originating from a Ca-bicarbonate field (meteoric recharge and shallow groundwater); and the dashed lines indicate the typical geochemical variations in formation waters from sedimentary basins [89,92]. In (B), points G and H mark the theoretical precipitation thresholds of gypsum and halite from residual seawater during static evaporation (light-gray, “ev” circle: evaporites).

Figure 4. Langlier–Ludwig (A) and Brine Differentiation (B) diagrams of the water samples considered in this study [89,90,91]. All chemical ratios are expressed on an equivalent basis. In both diagrams, the pentagon represents the composition of present-day seawater. In (A), the dark-gray field represents the composition of formation brines from the Northern Apennines/Po Basin; the arrows indicate general mixing between two end-members or water–rock interaction processes (e.g., sulfate dissolution), originating from a Ca-bicarbonate field (meteoric recharge and shallow groundwater); and the dashed lines indicate the typical geochemical variations in formation waters from sedimentary basins [89,92]. In (B), points G and H mark the theoretical precipitation thresholds of gypsum and halite from residual seawater during static evaporation (light-gray, “ev” circle: evaporites).

Figure 5. Distribution map of CRMs (boron, lithium, and strontium).

Figure 5. Distribution map of CRMs (boron, lithium, and strontium).

4. Results

The obtained results will first show the interpretation of seismic reflection profiles. Then, the distribution maps of CRMs, based on the collected bibliographic data, will be compared to the evolution of geological structures of the study area. The Salsomaggiore Structure comprises four main zones that present specific compositional characteristics of the waters. The comparison of the geological and waters’ characteristics of the four main zones will allow us to discuss the relationships between the evolution of mountain foothills and water compositions.

4.1. Interpretation of Structural and Stratigraphic Architecture from Seismic Reflection Profiles

The interpreted seismic profiles, calibrated by two wells, reveal two important structures belonging to the Emilian arc, i.e., the Cortemaggiore (to the NE) and the Salsomaggiore (to the SW) structures, with the latter being overthrust by the Ligurian allochthonous unit and Intra-Messinian Chaotic Complex (Figure 1, Figure 2 and Figure 3A–C). The Ligurian allochthonous unit is recognized by SW-dipping, high-amplitude reflections at the top of the seismo-stratigraphic units of the Miocene foredeep that likely correspond to the Rio Gisolo and T. Ghiara Fms. (Figure 3A–C).

The stratigraphic architecture from the Upper Miocene to Quaternary is observed in the Campore_001 well (Figure 2), which is representative of the Messinian stratigraphy within the wedge top basins of the Emilian arc as defined in previous studies [66,67,84,93,94,95]. At the base of the well, the Tortonian unit (Upper Miocene) is identified and consists of fine-grained turbidites deposited in deep basins [66,94,96,97,98,99]. Above the Tortonian deposits is the 805 m thick Intra-Messinian Chaotic Complex unit, derived from destabilized allochthonous Ligurian units located to the south [66,67,95,100]. Above this unit is a continental formation, made of transitional environments between the fluvial and the lagoonal, overlaying an erosional regressive surface that marks an abrupt shallowing event [101,102,103]. These uppermost Messinian deposits are vertically stacked in a transgressive trend; they progressively seal the topography created by the chaotic complex and the erosional surface toward SW (landward) [101,102,103]. These deposits belong to the Colombacci Fm. and are mainly formed by clays, marls with sandstones, and channelized conglomerates that locally reach 100 m in thickness [103]. These Colombacci Fm. is covered by the Lower Lugagnano clays, deposited in a deep basin (850 m), and in turn overlain by shelfal to fluvial deposits (200 m) dated Middle-Upper Pliocene in the composite well logs (Figure 2). Note that the Middle-Upper Pliocene contains the base of the Quaternary, which recently changed according to the International Commission of Stratigraphy [85].

The Noceto_001 well, located NE of the Salsomaggiore Structure and 45 km E of the Campore_001 well (Figure 1 and Figure 2), does not cross the Intra-Messinian Chaotic Complex unit and reaches the Middle Miocene unit, characterized by marl with alternating clays and sandstones attributable to the Marnoso-arenacea Fm., below the Tortonian stratigraphic unit (290 m) (Figure 2). The well progresses upward with the Messinian unit (710 m), the Lower Pliocene unit (250 m), and the Middle-Upper Pliocene deposits (150 m), which are covered by Quaternary deposits, mainly consisting of clay and cobble and sand sediments, with a thickness of 100 m (Figure 2). Note that Quaternary refers to an older boundary, which recently changed and moved to the base of Gelasian according to the International Commission of Stratigraphy [85].

The allochthonous Ligurian unit and the Intra-Messinian Chaotic Complex partially cover the T. Gisolo Formation (Middle Miocene) to the southeast, along Seismic Line 1 (Figure 3A). In contrast, moving northwest along Seismic Line 2, the Middle Miocene deposits—specifically the Rio Gisolo and T. Ghiara Formations—crop out at the surface (Figure 3B), whereas in the NW area (Line 3), the allochthonous Ligurian unit and Intra-Messinian Chaotic Complex completely cover the Middle Miocene foredeep units (Figure 3C). Below the Ligurian units, three seismo-stratigraphic units constitute the Salsomaggiore Structure ascribed to the Oligo-Miocene foredeep turbiditic deposits (from Paleogene to Middle Miocene seismo-stratigraphic units). These units are marked by discontinuous reflectors in Lines 1 and 2 (Figure 3A,B) and are more continuous in Line 3 (Figure 3C) [63,66]. These three units, as well as a slice of the Cretaceous deposit (Mz3 seismo-stratigraphic unit), are affected by two NE-verging blind thrusts characterized by a flat detachment at 4 sec TWT (Figure 3A), which allowed us to investigate the deeper portions of the Salsomaggiore Structure. Below the detachment of the Salsomaggiore Structure, Paleogene deposits and part of the Cretaceous unit (Mz3 in Figure 3A) are affected by three NE-verging blind thrusts that depart from 5 sec TWT and reach the Lower Miocene seismo-stratigraphic unit (Figure 3A). In Lines 1 and 2, the two blind thrusts of the Salsomaggiore Structure are tipping out in the Middle Miocene units, i.e., the sand-rich deposits of the Gisolo Fm. These blind thrusts bind the Tortonian units encountered in the Campore_001 well; partially cut the Intra-Messinian Chaotic Complex (Messinian); and are sealed by the Colombacci Fm. (Upper Messinian) and the Lugagnano clays (Lower Pliocene), and locally by the Middle-Upper Pliocene and Quaternary deposits (Figure 3A,B). In contrast, in Line 3, only one thrust is interpreted; it dies out in the Intra-Messinian Chaotic Complex and is completely sealed by the Colombacci Fm. and younger deposits (Figure 3C). The Intra-Messinian Chaotic Complex, characterized by chaotic reflections, fills the Messinian wedge top basin between the Salsomaggiore and Cortemaggiore Structures [63,95,104]. It rests above the Tortonian deposits, which are characterized by low continuity reflectors in Lines 1 and 2 (Figure 3A,B) and more continuous reflectors in Line 3 (Figure 3C). The Colombacci Fm. (Upper Messinian) generally shows discontinuous reflectors with local progradational geometries, as shown in published higher-resolution seismic reflection profiles [101,105,106]. The Lugagnano clays (Lower Pliocene) are characterized by partially continuous reflectors. The above Upper-Middle Pliocene deposits display lateral continuity of the reflectors, while Quaternary units show progradational geometries (Figure 3A–C). Integrating the interpreted seismic profiles with the surface geology (Figure 1), it is clear that the forelimb of the Salsomaggiore Structure folds the Colombacci Fm., the Lugagnano Fm., and the Upper-Middle Pliocene deposits. The diverging geometries in the reflections suggest syn-folding sedimentation (Figure 3). The folding is less clear in the Quaternary units, which is mainly characterized by the continental depositional environments, and tends to erode the Salsomaggiore Structure and to seal the Cortemaggiore Structure. Anyhow, in the Po plain, folding of Quaternary deposits has been highlighted, and thrusts’ slip rates have been calculated [107,108].

Moving northeastward, the Cortemaggiore Structure is characterized by a NE-verging blind thrust that has deformed the Oligo-Miocene foredeep deposits, while the overlying strata are attributed to the Upper Miocene through the Middle to Upper Pliocene. The Quaternary deposits, marked by overall continuous and sub-parallel reflectors, seal the Cortemaggiore Structure and lie above a marked, basal erosional surface (Figure 3A–C). Similar to the Salsomaggiore Structure, the Cortemaggiore detachment penetrates the deeper levels of Mz3, reaching the basement, as illustrated in Line 1 (Figure 3A). Moreover, it is important to point out that the deformed recent deposits (from the Upper Miocene to the Upper-Middle Pliocene) suggest possible activation in more recent times with respect to the Salsomaggiore Structure, where these deposits are folded (Figure 3A–C). Below the Cortemaggiore detachment, three blind structures located between 5 and 6.5 s TWT involve the deep Mesozoic carbonate units, as well as the Paleogene and Lower Miocene deposits (Figure 3A). The outermost structure corresponds to a reverse fault developed from a flat detachment at 6/7 s TWT, while the innermost one represents extensional structures, revealing a complex deep structure (Figure 3A) that cannot be verified in the other two seismic profiles (Figure 3B,C).

4.2. Chemical Analyses of Water in Four Sectors of the Salsomaggiore Structure: A Review

The chemical analysis data collected during this study involved the review of historical data (Table S1), which is presented here along with results from a detailed literature review on the types of water found in various parts of the study area. The table includes 47 geochemical analyses, the locations of sampling points, the dates of analysis, the concentrations of selected CRMs, and whether each analysis was performed on water from a spring or a borehole. This information will be essential to assess the pathways of CRM-bearing waters from their origin to the surface, their entrapment/accumulation in the subsurface, and the evolutionary stages of the Salsomaggiore Structure that contributed to their distinctive chemical compositions.

In addition, this section presents the results of in-depth research on these waters that led to the identification of different water types present in the study area and in certain specific sectors of the Salsomaggiore Structure. In particular, four distinct sectors could be identified: Salsomaggiore Terme, Contignano, Tabiano, and Sant’Andrea Bagni.

4.2.1. Salsomaggiore Terme Area

Salsomaggiore Terme is characterized by a so-called Br-I-rich brine (average TDS: 150 g/L; [16]), considered syngenetic between the Serravallian and the older Miocene units in which it is currently hosted. In the area, 29 water wells were drilled between 1864 and 1964, encountering Br-I-rich brines at depths ranging from 350 to 650 m [81].

The high salinity of these waters is thought to be linked to a Miocene marine basin that underwent extreme evaporation up to hypersaline conditions. These brines were subsequently modified during diagenesis—particularly in their cationic ratios—due to processes such as albitization, chloritization, and sulfate reduction [16]. In contrast, the elevated bromine and iodine concentrations are likely related to the decomposition of organic-rich marine deposits from the Miocene sea.

Previous studies suggest that the brines, originally formed and trapped within Middle Miocene deposits—mainly the Rio Gisolo Formation [100]—may have migrated upward into the overlying latest Messinian ortho-/para-conglomerates characterized by a sandy matrix (Colombacci Fm. [103]) and subsequently sealed by the Lower Pliocene clays of the Lugagnano Fm. [100].

Strontium and oxygen isotopic signatures clearly differentiate the Salsomaggiore brines from those in the Parma province (e.g., Monticelli Terme and Messinian in age), supporting the hypothesis of interaction with Lower Miocene (Aquitanian–Burdigalian) units and diagenetic evolution under relatively high temperatures (100–150 °C; [16]). Although these fluids may have originated during the Messinian, they likely interacted with the deeper and older formations during tectonic thrusting, concurrently with diagenetic processes. The three seismic profiles presented in this work confirm the involvement in thrusting of the Lower Miocene and even older stratigraphic units (Figure 3).

Regardless of the precise genetic model, tectonic stresses associated with thrusting and the emplacement of Ligurian allochthonous units played a fundamental role in promoting the vertical migration of these brines through the stratigraphic sequence. Inside the allochthonous units, the particular geological significance is the presence of the Antognola and Contignaco Formations—marly to siliceous lithologies of the Epiligurian succession [103], referred to as the “marl/tripoli complex” by Mezzadri (1995) [81]. These formations favor osmosis by delivering desalinated waters and consequent enrichment in salts for deeper waters.

4.2.2. Contignaco Area

The Contignaco area is characterized by sulfurous fresh groundwater linked to the leaching of the marly-arenaceous formation of the above-mentioned Middle Miocene units [81,103]. The recharge area of these waters is located toward the south in the portion of the Salsomaggiore Structure where there are sulfur springs with water composition very similar to those of the Contignaco area [81]. The meteoric recharge percolates into the Rio Gisolo Formation (Serravalian—Middle Miocene) and flows toward lower altitudes [81]. In this area, river incision has exposed the heads of the arenitic strata from which mineralized waters flow and characterize the Contignaco springs [81]. In the early 1900s, sulfur waters were found in the artesian wells at undefined depths in the available documents, while later, in the 1960s, it was necessary to drill wells reaching depths of 10–20 m from the ground level to intercept the sulfur-rich aquifer [81].

4.2.3. Tabiano Area

The Tabiano area is characterized by waters of meteoric origin circulating primarily within the permeable levels of the Messinian succession, which is composed of alternating conglomerates, sands, and clays interbedded with gypsum and anhydrites. The Tabiano waters are classified as cold mineral, sulfureous/sulfate–calcium–magnesian, and are among Europe’s richest in H₂S [109,110]. The presence of hydrogen sulfide in the water requires specific conditions: (i) interaction with sulfate-rich rocks (i.e., gypsum and anhydrite); (ii) the presence of organic matter at all depths, typically hydrocarbons; and (iii) input from meteoric water recharge [109,111]. Faults and permeable formations present in this area allow meteoric water to percolate to depths estimated at around 400 m, where they can dissolve sulfate minerals and interact with hydrocarbons (or fossil marine waters enriched in iodine and bromine—elements typical of oil field formation brines). An analysis of the lithologies associated with four springs and the well in the study area allows for the definition of the hydrogeological basin [109]. The Pergoli spring, the earliest exploited spring, emerges at the base of a chalky lens in contact with impermeable Messinian–Pliocene clays. Here, the hydrogeological basin is formed by an Upper Miocene monocline composed of clastic rocks interbedded with gypsum according to Venzo (1959) [112] in Fuganti et al. (1993) [109]. The Passelle Sotto spring issued from a deeper chalky level flows inside a conglomeratic layer situated between the impermeable Tortonian and Messinian clays. As these units lack significant gypsum, the waters are only weakly sulfurous [109]. The hydrogeological basin of both springs is given by terraced alluvial deposits, where infiltrating meteoric waters pass through the porous gypsum-bearing sediments and recharge two distinct aquifers. Finally, the Arvè borehole taps into the same chalky lens that feeds the water of the Pergoli Castello springs, at a depth of approximately 180 m [109]. Based on lithological logs from the Arvè borehole, the depth of the aquifer hosting these sulfurous brackish waters was estimated to be between 150 and 180 m [81].

4.2.4. Sant’Andrea Bagni Area

The Sant’Andrea Bagni area hosts waters with highly variable chemical composition. Low-salinity waters were attributed to meteoric infiltration, with an estimated residence time of at least 10–15 years [78]. In addition, more saline waters are present, likely resulting from the mixing of high- and low-salinity endmembers. Chemical composition and the stable isotope ratios of oxygen and hydrogen in the water molecule indicate that these intermediate-salinity (brackish) waters originate from a mixture of distinct saline endmembers—such as the so-called “Purgativa Forte” water—and meteoric low-salinity waters [78]. However, the saline end-member appears to be chemically distinct from those identified in Salsomaggiore Terme and Monticelli. The saline component of these waters derives mainly from the leaching of old fossil deposits that are gradually exposed to runoff by percolation water [81]. Also, in this area, there are (i) sulfurous waters in the middle-lower parts of the Marnoso-arenacea Formation (Rio Gisolo Fm.), linked to the presence of lignite and plant remains, which are decisive in the mineralization of sulfurous waters [81], and (ii) mineral-alkaline waters (such as Lydia, Ducale, and Fonte Nuova) that formed as a result of a significant interaction with Middle Miocene units. Therefore, in the Sant’Andrea bagni area, it is possible to define three types of waters: mineral fresh water, sulfurous brackish water, and Br-I-rich saline water. The location of the different water types is attributable to the geological evolution of the area [81]. The axial portion of the structure and the Middle Miocene foredeep units were the first to be put in direct contact with the atmosphere due to the combined effect of tectonics and erosion. Here, any gaseous presence immediately flowed away, and the exposed porous rocks were the first to be washed away by surface infiltration water. These waters contributed to the continuous leaching that took place in this area, which led to the formation of mineral waters. The rock units framing the Salsomaggiore Structure, especially those towards the north of the Sant’Andrea Bagni Area, have remained for a considerably longer time below the Pliocene clay seal. In addition, the synfolding sedimentation has led to the formation of progressive unconformities and numerous mixed (stratigraphic–structural) traps, which likely formed WPU (Wedge-margin Progressive Unconformities) and HDU (Hinged-margin Drowning Unconformities) [101,105,106]. Around the hinge of the Salsomaggiore anticline, these types of unconformities are now eroded by younger unconformities, and they could not be detailed with the available data. In this sector, the salty and sulfurous waters described above are more frequent, leading to mineral waters being found in the central area of the structure [81]. It is not possible to determine a precise depth of origin for each water type in this area. However, the various types of water are encountered at depths ranging from approximately 20 to 450 m below the ground surface. The very complex geological context has led to having different types of water within the same layers both vertically and laterally.

4.2.5. Overview of the Main Geochemical Characteristics

The Langelier–Ludwig diagram (Figure 4A) and the Brine Differentiation Plot (Figure 4B) based on the main chemical water composition highlight several distinctive features of the waters examined in this study [89]. The Na-Cl brines from Salsomaggiore exhibit a composition consistent with formation waters of marine origin typically associated with hydrocarbon reservoirs. The high Ca/(Ca⁺SO₄) ratio—approaching 1—is attributed to the reduction of SO₄ to H₂S (likely thermogenic), coupled with an increase in Ca concentration driven by diagenetic processes such as plagioclase albitization. The latter is supported by a Na/(Na⁺Cl) ratio below 0.5, which testifies to a Na deficiency [16].

In contrast, the waters from Tabiano show a Ca-SO₄ composition primarily resulting from the dissolution of sulfate evaporites, accompanied by bacterial sulfate reduction to sulfide.

Finally, the waters from Sant’Andrea Bagni display a broader spread across all four quadrants of the Langelier–Ludwig diagram. The more saline samples closely resemble the composition of the Salsomaggiore brines, while the Ducale and Lidia waters exhibit a distinct Na-HCO₃ signature typical of sedimentary aquifers at intermediate depths. Finally, the Ca-HCO₃ composition observed in the less saline samples from Sant’Andrea Bagni and Contignaco is characteristic of shallow and meteoric-derived aquifers.

4.3. Distribution Maps of Critical Raw Material Concentrations

This section describes the distribution maps of CRM concentrations developed following the method described in Section 3.3. The data used for the construction of the maps were extracted from the chemical analyses described in Section 4.1 (Supplementary Material Table S1), using only the results for the CRMs considered in this work. For the construction of the maps, several factors were considered, such as the presence of thermal waters, lithological units that influence the composition of the water, and the presence of geological structures that contribute to the distribution of the outflow areas and delineate the areas with different concentrations of CRMs.

In addition, a table has been constructed (Table 1) in which the subsurface waters of the most important areas described in the previous section (Salsomaggiore Terme, Tabiano, Contignaco, and Sant’Andrea Bagni) have been categorized according to (i) the characteristics of the waters, (ii) source type (well or spring), and (iii) maximum and minimum concentrations of each CRM.

The construction of the maps focused on the study area of the Salsomaggiore Structure, where the elements show concentrations and distributions peculiar to area. The maps display the concentration ranges for each element, represented by colors: green (high concentration), yellow (medium concentration), and red (low concentration). The maps also indicate the locations of the four main zones, and the legend reports the number of chemical data points used to construct them. The main sources of uncertainty are two-fold: the limited number of data points and the inclusion of some older/outdated results. These latter analyses may not have detected all CRMs present in the samples (e.g., Contignaco), primarily due to the analytical methods used being less reliable than those currently available. The distribution map of boron (16 chemical data) generally displays low concentrations (red area), with one exception at Salsomaggiore Terme in the Br-I-rich brine water; here, it has been identified at a depth ranging from 350 m to 650 m deep, with high values above 290 mg/L (green area) (Figure 5 and Table 1).

Lithium (28 chemical data) shows a different distribution within the Salsomaggiore Structure (Figure 5), with the area of Salsomaggiore Terme showing the highest values compared to the axial portion of the structure (Figure 5). In fact, in the Salsomaggiore Terme area, the values are from 96.4 to 58.93 mg/L (green area) (Table 1). The lower values (red area) from 0.5 to 0.02 mg/L are associated with the waters of Sant’Andrea Bagni (the sulfurous brackish and Br-I-rich waters present the first medium and the second high concentrations). Conversely, to the NE, there is a belt with average values (yellow area) between 1 and 0.5 mg/L passing through Tabiano (Figure 5).

Finally, strontium (19 chemical data) has a high concentration only in the Salsomaggiore Terme, linked to the presence of Br-I-rich brine water with values greater than 276 mg/L (green area). In the other part, it has a low concentration under 138 mg/L (red area) (Figure 5 and Table 1).

Based on a comparison with exploration threshold values for lithium and boron recovery from Canadian formation brines (75 mg/L and 100 mg/L, respectively; [113,114]), the concentrations measured in the waters of Salsomaggiore Terme exceed both thresholds.

Currently, no comparable threshold values exist for barium and strontium, likely because, in hydrocarbon exploration, these elements are primarily monitored as potential scaling agents—particularly when waters rich in sulfates mix with those containing high concentrations of barium or strontium.

5. Discussion

5.1. Tectono-Stratigraphic Evolution of the Salsomaggiore Structure

In this section, we discuss the tectono-stratigraphic evolutionary phases of the Salsomaggiore Structure from the Upper Miocene to the present in relation to the geochemical characteristics of the previously described waters (Table 1) and the interpreted seismic reflection profiles interpreted (Figure 3). Keeping in mind the uncertainties described in Section 4.3, new sampling campaigns are planned for future work, including chemical and isotopic analyses, with the latter focusing specifically on Li, B, and Sr, as well as ¹⁴C for fluid dating.

Starting from a common geological setting immediately after the deposition of the Intra-Messinian Chaotic Complex (Messinian) across the three areas of interest within the Salsomaggiore Structure, i.e., the NW zone (Salsomaggiore Terme), the central zone (Tabiano on the forelimb of the structure and Contignaco on the backlimb of the structure), and the SE zone (Sant’Andrea Bagni), it is possible to recognize different evolutionary phases for the different parts of the structure over time (Figure 6).

During the Messinian, the three areas were characterized by an ancient Apenninic-verging thrust, disrupting the Cretaceous to the Middle Miocene units, associated with a low-amplitude anticline (Figure 6A). The Tortonian sediments were deposited in a deep marine basin environment and as a syn-tectonic unit during the fold formation in front of the Salsomaggiore Structure. Above the deposits of the Middle Miocene and Tortonian, the Intra-Messinian Chaotic Complex was emplaced by the dismantlement and re-sedimentation of the allochthonous Ligurian unit, already present behind the Salsomaggiore Structure [66,67]. During this time, some of the waters of interest acquired most of their geochemical characteristics, such as those of Salsomaggiore Terme (Section 4.2.1) and the salty waters of Sant’Andrea Bagni (Section 4.2.4). Presumably, Tabiano (Section 4.2.3) and Contignaco (Section 4.2.2) had similar geochemical characteristics to Salsomaggiore Terme and Sant’Andrea Bagni because Miocene rocks were buried underneath the Intra-Messinian Chaotic Complex without any interaction with meteoric water.

From the end of the Messinian until the Upper Pliocene, the progressive folding uplift and enhanced erosion of the Salsomaggiore anticline generated variable geological settings along the Salsomaggiore Structure. Starting from the SE zone (Sant’Andrea Bagni area), the Intra-Messinian Chaotic Complex was eroded, and the transgressive conglomerate of the Colombacci Fm. was deposited directly on top of the Rio Gisolo Fm., and Lower Pliocene to Upper Pliocene marine deposits that likely sealed the whole structure (Figure 6B). In the central zone (Tabiano area), the Intra-Messinian erosional surface reached the T. Ghiara Fm. (Langhian), after eroding the Intra-Messinian Chaotic Complex and the Rio Gisolo Fm. (Figure 6B). This is interpreted as due to (i) the higher tectonic uplift rate of these zones linked to a greater shortening of the Salsomaggiore Structure; (ii) a major incision of the Messinian erosion related to the abrupt shallowing above the Intra-Messinian Chaotic Complex [101,102,105,106]; or (iii) a combination of these two processes (uplift and erosion). On the contrary, in the NW area (Salsomaggiore Terme area), the Miocene foredeep units remained buried underneath the Ligurian unit and the Intra-Messinian Chaotic Complex (Figure 6B). This has allowed the brine waters of the Salsomaggiore Terme area to preserve their original geochemical characteristics, and the mixing with other meteoric or (Pliocene) marine waters was hampered. Instead, in the Tabiano and Sant’Andrea Bagni areas, the water trapped in Middle Miocene units could interact with fresh/lower salinity waters present during the fluvio-deltaic deposition of the Colombacci Fm. (Late Messinian) before being overlain by marine clays and marls of the Lugagnano Fm. (Lower Pliocene) (Figure 6B). The latter formation acts as a seal for the waters of Salsomaggiore Terme, Sant’Andrea Bagni, and partly Tabiano (Section 4.2). The Tabiano area is located at the front of the Salsomaggiore Structure and likely affected by a density of the fracture greater than the other areas [81,109], which favored the increase in permeability of the seal; the formation of springs; and a stronger interaction between meteoric waters, evaporitic rocks inside the Intra-Messinian Chaotic Complex, and deep Br-I-rich brine waters. This suggests a possible earlier mixing of saline and fresh waters, although of short duration, that corresponds to the latest Messinian period before the reflooding of the Po Plain during the Pliocene. During the Pliocene, the synfolding deposition and the overall shallowing upward trend of the depositional environments favored the formation of progressive unconformities (Figure 6B) and, consequently, the mixing of the earlier saline water with new marine and, possibly, fresh waters around the hinge of the Salsomaggiore anticline. However, the precise reconstruction of the sedimentary environments around the hinge zone cannot be determined because of the following Quaternary erosional events.

In the Quaternary period, uplift within the Salsomaggiore Structure still varies across different sectors, with the central zone (Tabiano area) experiencing higher uplift rates according to tilted straths created by the streams crossing the structure [108] (Figure 6C). Furthermore, the NW zone (Salsomaggiore Terme area) has the lowest uplift rates in the structure [108]. These observations lead to the conclusion that Quaternary erosion also varies in the different zones depending on the uplift rate. In the Contignaco area, located behind the central zone of the Salsomaggiore Structure (Figure 1), the lack of impermeable units has led to the development of springs with a very low salt concentration, likely due to limited interaction between meteoric waters and the hosting Middle Miocene rocks. These springs, along with the mineral waters of Sant’Andrea Bagni, represent the highest flow rates observed within the study area. In the zones of Contignaco, Tabiano, and partly Sant’Andrea Bagni (for the fresh mineral waters), the percolation of meteoric waters continues even today, feeding the hydrogeological basins and mixing with the fossil waters, modifying their composition.

5.2. Challenges in Mapping CRM Distributions Across Different Water Types in the Sant’Andrea Bagni Area

The distribution maps of the CRMs should be considered preliminary maps. One of the challenges encountered during this study concerns the representation of areas containing multiple water types with distinct geochemical characteristics. In the current work, the preliminary maps were constructed by averaging the concentrations of each CRM for all water samples within the main areas of interest. In the case of Sant’Andrea Bagni, this approach resulted in the amalgamation of three distinct water types (Section 4.2.4; Table 1) as a single representative sample. While this method provides a general overview of CRM concentrations in the area, it does not capture the geochemical variability between water types at specific sites (e.g., Sant’Andrea Bagni). To obtain more accurate representation of maps and resolve the uncertainty described in Section 4.3, future work should involve the construction of separate maps for each salinity range (i.e., fresh, brackish, saline, and brine waters) and geochemical facies (Figure 4A). This will allow for both a preliminary regional overview and more detailed geospatial analyses based on water type, improving the interpretative power of the geochemical data, and obtaining detailed distribution maps of CRM concentrations.

6. Conclusions

The assessment of hydrogeochemical data from previous studies provided a preliminary overview of the source zones (primarily Middle Miocene units), accumulation zones (Middle to Upper Miocene units), and migration pathways (fracture/fault zones and permeable formations) of these waters in relation to the local geological setting. Based on this framework, preliminary maps of CRM concentrations (boron, lithium, and strontium) were developed. These maps indicate that the Br-I-rich brine waters of Salsomaggiore Terme represent the most CRM-enriched waters in the study area.

The geological evolution of the area, water–rock interaction, and mixing with the meteoric waters lead to the formation of chemically different waters in response to the combined effects of differential uplift along the axis of the Salsomaggiore structure and depth of erosion of the intra-Messinian and younger Quaternary erosional unconformities.

Specifically, the central zone (Tabiano area) shows a larger uplift and deeper erosion of the intra-Messinian unconformity (eroding down to the Langhian Ghiara Formation that is directly overlain by the Colombacci Fm.) as well as of the Quaternary erosional surfaces.

The SE perianticlinal termination (Sant’Andrea Bagni area) shows less uplift and shallower erosion of the intra-Messinian (Colombacci formation eroded down to the Gisolo Formation) and Quaternary unconformities.

The NW perianticlinal termination (Salsomaggiore Terme area) also shows less uplift but also the shallowest erosion of the intra-Messinian and Quaternary unconformities, which were not able to remove the mass transport deposits (the Intra-Messinian Chaotic Complex). The latter, still at present day, overlays the Middle Miocene foredeep units containing CRM-enriched waters.

The differential uplift and different geological units exhumed along the Salsomaggiore tectonic window show a strict correspondence with the distribution of the different types of water and the CRM abundances. This correspondence becomes clearer when taking into account that the original reservoir (Gisolo Fm.—Middle Miocene) of the fossil marine brine water was suddenly buried by the Intra-Messinian Chaotic Complex. Afterward, this original reservoir was progressively exhumed by tectonic uplift and folding coeval to two major erosional events (intra-Messinian and Quaternary, with the latter obliterating the Pliocene erosive events) that favored mixing with meteoric and diagenetic waters circulating inside the Salsomaggiore Structure.

Vice versa, the distribution of different types of water and the CRM abundances mark the differential exhumation of the Salsomaggiore tectonic window since the latest Messinian period, with the unmixed (fossil) or less mixed waters marking the portion of the Salsomaggiore Structure not or less exhumed.

Considering the limitations in the resolution of the subsurface data (Section 3.2), the uncertainties in the CRM distribution maps (Section 4.3), and the planned future sampling campaigns (Section 5.1), fluid pathways and CRM trapping mechanisms will be addressed as new data become available, thereby providing more up-to-date validation of the relationships between geological processes and the chemistry of CRM-bearing waters.

The case study of the Salsomaggiore Structure can be used as a natural laboratory and reference case study for other structures present along the Emilia-Romagna Apennine front where chaotic complexes and Pliocene deposits either are sealing or were previously sealing marine deep-water foredeep deposits. This case study can also be useful and applied to tectonic windows and foothill thrust fronts of other mountain chains associated with foreland basin systems. Therefore, we believe that the Salsomaggiore Structure can provide useful guidelines for the preliminary assessment and localization of critical raw materials (CRMs) in similar geological settings and contribute to the energy transition, an issue of fundamental importance in the current global context.