Genesis of carbonate-hosted Sb (±Au) deposits in southern Tuscany (Italy): Tracing ore-forming processes through stibnite and pyrite trace elements composition and stable isotopes of quartz and calcite

Antimony (Sb) is a critical raw material of worldwide strategic importance. Thus, understanding the processes by which Sb becomes concentrated in the Earth's crust, forming ore deposits, is pivotal in developing innovative exploration approaches and a more secure and sustainable mining. In central Italy, hydrothermal Sb (±Au) mineralization developed at the fringes of the Larderello, Mt. Amiata and Latera geothermal systems in an extensional tectonic setting. They are associated with the silicification of carbonate rocks beneath low permeability sedimentary formations, resulting in the formation of jasperoids. In this work, we used stibnite and pyrite trace element composition coupled with stable isotope composition of quartz (δ18O) and calcite (δ18O, δ13C) to constrain the source of ore-bearing fluids and the mechanisms of ore formation. Stable isotope data suggest that ore forming fluids are the results of mixing between meteoric water and about 25-50% of a metamorphic-magmatic deep fluid component. In the proposed model, Sb, along with As, Tl and possibly Au were scavenged from Paleozoic metasedimentary rocks of the Tuscan basement predominantly by fluids generated in situ by dehydration of the metasedimentary sequence, during contact metamorphism induced by the emplacement of magmatic intrusions during extensional tectonics. These fluids mixed with meteoric waters and were focused into structural traps created by the interface between permeable carbonate and impermeable sedimentary formations. Here, they formed jasperoids and the drop in temperature of the fluids triggered stibnite deposition.