Lithium distribution in argillite-clay sequences of the Northern Apennines, Italy: investigating a potential source of Li-rich fluids

Lithium demand has grown exponentially over the last decade, driven by the ongoing energy transition, and is expected to increase in the EU by up to 12 times by 2030 (European Commission, 2023). To meet this rising demand, efforts must focus on enhancing recycling and developing new unconventional lithium resources. While Italy does not have records of lithium production from conventional deposits such as pegmatite ores or salars, promising sources have been identified in low-enthalpy, lithium-rich waters originating from sedimentary sequences along the Apennine compressional front (Dini et al., 2022). Lithium content in these waters, up to 164 mg/L in the Salsomaggiore Terme area (Boschetti et al., 2011), is likely influenced by fluids expelled from sediments during diagenesis or by subsequent water-rock interactions. Modern Direct Lithium Extraction (DLE) techniques provide an efficient method for lithium recovery from fluids, highlighting the significant potential of these resources. Despite their scientific and economic potential, limited knowledge exists regarding the genetic processes and chemical and isotopic characteristics of these systems.

To address the existing data gap and explore the sources and processes governing lithium distribution, an extensive sampling campaign was conducted on the main argillite-clay sedimentary formations of the Northern Apennines in the Emilia Romagna region. The sampling also covered sediments from mud volcanoes, which serve as proxies for deeper processes, as well as waters from SPA wells and natural springs to investigate water-rock interaction.

The collected samples were analysed for major and trace elements. Lithium concentrations ranged from 13 to 263 mg/kg in rock samples, with several exhibiting values well above the shale average. The mineralogical composition was identified through XRD analysis, while SEM-EDS imaging revealed micro-scale mineralogical and chemical zonations. Selected samples underwent further investigation using LA-ICP-MS, revealing significant micro-scale lithium variability, with concentrations reaching up to 700 mg/kg in phyllosilicate-rich micro-layers. These enriched layers were strongly associated with boron, potassium, aluminium, and iron, in contrast to bordering carbonate-rich layers, which systematically showed lower lithium contents.

Isotope analysis of boron (δ¹¹B), strontium (⁸⁷Sr/⁸⁶Sr), and lithium (δ⁷Li) provided additional insights into the origin of the formations and suggested the potential involvement of multiple provenances. In this regard, a lithium isotope purification and analysis protocol was developed for MC-ICP-MS at the Radiogenic and Unconventional Stable Isotopes Laboratory at IGG-CNR. Leaching experiments conducted on the most Li-rich sample, using various solutions (deionised water, NaCl, CaCl₂, and HCl) and different temperatures, showed the behaviour of Li release from the solid phase, with a significant release occurring during the initial hours of interaction that demonstrated the importance of early-stage water-rock interaction processes.

This research is part of a PhD project funded by the European ITINERIS Project. The results could provide a fundamental basis for understanding the sources and mechanisms influencing lithium distribution in the sedimentary basins of the Northern Apennines and in other similar geological settings. Meeting the goals outlined by the EU in the Critical Raw Materials Act (CRMA 2024), this study could promote interest in the sustainable exploration of lithium resources and DLE techniques.