Iron isotope systematics and ore forming processes: first results from the ophiolite-hosted, Tuscan Cu-Zn VMS deposits

Ore forming processes still need to be improved, and new conceptual genetic models must be developed to help exploration of new ore deposits for the energy transition. Decarbonization and the switch to renewable energies will require increasingly large amount of Critical Raw Materials (CRMs) that must be provided by conventional and unconventional ore deposits. The availability of copper, although considered as a common metal, will potentially limit the speed of the energy transition, because large amounts of this metal will be used in electric vehicles and power generation. For this reason, any implementation of genetic models regarding Porphyry-Cu and Volcanogenic Massive Sulphide (VMS) deposits is highly desirable. Ophiolite-hosted Cu-Zn VMS deposits from orogenic settings, with respect Porphyry-Cu deposits, represent a minor provider of copper, but their better understanding could shed new light on the mining potential at seafloor. Stable isotopes of ore-forming metals, like Fe, can be used for the investigation of ore-forming processes, metal source(s), and water-rock interactions. The development of the High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometers (HR-MC-ICPMS), like that hosted in the Neptune-TIMS Laboratory (IGG-CNR, Pisa, Italy), given the opportunity to accurately measure Fe isotope ratios for ore deposit applications. In the framework of the project “TEOREM - Deciphering Geological Processes using Terrestrial and Extraterrestrial ORE Minerals” (PRIN-MUR 2017AK8C32) and the project ITINERIS ISOTOPE VRE (IR0000032; PNRR; Next Generation EU), we developed the methodology for the Fe separation from different matrices, and the determination of Fe isotope ratios using HR-MC-ICPMS (Neptune Plus). Determination of iron isotope composition was performed on samples of massive sulphide ore and gangue minerals from the ophiolite-hosted Cu-Zn VMS deposits of Tuscany (Italy). Samples of both primary chalcopyrite-pyrite-pyrrhotite stockworks and reworked nodular ores (bornite-chalcocite-chalcopyrite), were studied by SEM-EDS (Dini et al., 2024) and selected mineral separates were processed for Fe isotopic analyses. First results show a significant variability of negative δ56Fe, consistent with the available data on copper sulphides from Mid-Atlantic slow-spreading ridge (Rouxel et al., 2004). The new data on Tuscan VMS deposits suggest that the nature of the oceanic crust hosting the ores (ultramafic vs. mafic) is likely to exert some control on the variation of δ56Fe