The Messinian salt deposits as a possible resource of trace elements: prospects for industrial applications

In an increasingly technologically advanced and industrially developed world, the depletion of current reservoirs of critical elements poses an important risk. Consequently, identifying new exploitable reservoirs, while ensuring environmental sustainability, is of paramount importance. The evaporite deposits of Realmonte, Racalmuto, and Petralia, located in Sicily (provinces of Agrigento and Palermo), formed during the Messinian Salinity Crisis acme (5.97-5.33 Ma), represent a vast reservoir of evaporitic minerals that currently mined for halite, which is used in both the food and industrial sectors. However, in addition to halite, these deposits contain clay-rich layers and potassium and magnesium sulphates and chlorides, which precipitated under extreme evaporitic conditions when brine concentration reached 70–90 times that of seawater, which potential as a source of critical raw materials (CRMs) remains largely unexplored. This doctoral research aims to provide a detailed geochemical characterization of these saline deposits, exploring their potential as alternative reservoirs for trace elements of significant technological and industrial interest. A total of 28 saline and 10 clayey samples were collected from the three deposits, along with an additional sample from the Trapani saltworks. Saline samples were processed to analyse only the soluble fraction, whereas clayey samples were treated to separately investigate both the soluble and insoluble fractions. Mineralogical investigations, through XRPD technique, confirmed that halite is the predominant phase in most saline samples, although some are primarily composed of kainite, leonite, or carnallite. Additionally, traces of polyhalite and sylvite were identified. In contrast, the clayey samples exhibit a more varied mineralogical composition, including phyllosilicates such as illite, kaolinite, montmorillonite, antigorite, biotite, muscovite, and clinochlore. Carbonate phases consist of calcite, magnesite, and dolomite, while two samples are particularly enriched in Ca-sulphates, mainly polyhalite and anhydrite. Due to the significant matrix effects that complicate trace element analysis, a novel chromatographic separation method using AG® 50W-X8 cation-exchange resin (Bio-Rad®) was developed. This new method effectively isolates Li, V, Fe, Cu, As, Rb, Sr, Y, Cd, Sn, Cs, Ba, REEs, Tl, and Bi from the major matrix elements (Na, Mg, K, and Ca). The validity and reliability of this method were assessed by comparing results of trace metals and REE with those obtained using two conventional separation and preconcentration techniques. These comparisons demonstrated the high efficiency of the newly developed chromatographic method, with observed discrepancies primarily attributed to differences in sample preparation procedures. Geochemical analysis of saline samples revealed Na, K, Mg, and Ca concentrations in line with the mineralogical composition identified by XRPD. Trace elements such as Sr (400 ppm) Fe (up to 7 ppm), Rb (up to 100 ppm), Cs (up to 4 ppm), Ba (up to 1 ppm), and Li (up to 1 ppm) were notably enriched in saline samples from Realmonte and Racalmuto, whereas soluble fraction of clayey samples shows an higher concentration of Sr (up to 2900 ppm), Ba (up to 45 ppm), Li (up to 23 ppm), and Rb (up to 15 ppm). The results suggest that these deposits could represent a viable alternative source of CRMs of economic relevance, in particular for Li, Rb, Cs Sr and K and Mg, contributing to the diversification of raw material supply in the European Union. This aligns with the goals of EU Regulation 2024/1252, which aims to enhance CRM self-sufficiency within the EU and mitigate geopolitical supply risks. This study provides a comprehensive geochemical characterization of the Messinian salt deposits and introduces an innovative analytical approach for trace element separation in highly saline matrices. The results contribute to a broader understanding of these deposits' composition and their potential exploitation for sustainable resource management.