Exploring critical and strategic raw materials via stream sediment geochemistry in Emilia-Romagna (Northern Italy)

This study examines the distribution, concentration mechanisms and original provenance of critical and strategic raw materials (C/SRMs) in sediments from three Apenninic rivers of Emilia-Romagna (Nure, Taro and Scoltenna), which drain mixed sedimentary–ophiolitic catchments. Although ophiolitic rocks represent only 1–4% of basin areas, they exert a strong geochemical influence on sediment composition. Grain-size-resolved analyses (sand, fine sand, mud), combined with XRF, ICP-MS measurements and multivariate statistics, reveal that sediment chemistry is primarily controlled by lithological inputs and hydraulic sorting rather than by in-channel alteration. Carbonate fragments dominate coarse fractions, diluting most heavy trace elements, whereas muds concentrate phyllosilicates and display higher weathering indices. Mafic–ultramafic signatures are strongest in the Nure and Taro basins, where Cr, Ni and Mg show significant enrichments linked to detrital spinels, serpentine and chlorite. Downstream increases in Cr and Ni reflecting hydraulic concentration of dense minerals rather than chemical processes. In contrast, chalcophile elements associated with VMS (Volcanogenic Masses Sulphides) mineralization such Cu and Zn show limited enrichment due to rapid sulphide weathering. Lithophile elements such as Li and B display catchment-scale anomalies unrelated to hydraulic upgrading, with B likely sourced from datolite within ophiolitic units. Overall, the rivers faithfully record upstream lithologies, but they do not function as efficient concentrating systems for most C/SRMs such as Ni, Cr, Zn, Cu, B. Their economic potential is therefore selective and largely restricted to coarse ultramafic detritus, whereas the grain-size-resolved geochemical approach remains a powerful reconnaissance tool for identifying mineralised units in mixed geological terrains.