Mineralogical and geochemical investigation of seafloor massive sulfides from Panarea Platform (Aeolian Arc, Tyrrhenian Sea)

Panarea seafloor hydrothermal system is associated with a range of mafic to felsic volcanic rocks. The hydrothermal system is active at present and discharges magmatic-hydrothermal fluids and precipitates massive sulfides. The sulfides exhibit multi-stage deposition, evident in the alternation of several mineral generations that follow a general temporal precipitation sequence: marcasite → alunite → opal. Sr-Nd-Pb isotope data indicate that most of the metals in the sulfides are derived predominantly from the Panarea volcanic rocks with some contribution from ambient seawater and/or local sediments. A remarkable feature of these sulfides is their chondrite-normalized rare earth element (REE_N) distribution pattern with a pronounced negative Eu anomaly, which has not been observed previously. Our study demonstrates that this REE_N pattern reflects the REE fractionation during sulfide deposition. The ionic radius mismatch between Eu²⁺ (the main form of Eu in reduced, high-temperature hydrothermal fluids) and the only possible site for REE substitution in the marcasite, that of Fe²⁺, suggests a crystallographic control on the REE_N pattern. Apparently, marcasite precipitation can generate a sulfide deposit with a negative Eu anomaly due to discrimination against Eu²⁺ relative to REE³⁺ in the Fe²⁺ crystallographic site.