MINERALIZATION OF LONGOBUCCO AND FONTE ARGENTILA (SILA MASSIF, CALABRIA, SOUTHERN ITALY): FROM MINERALOGY TO GENETIC CONTEXT

This study examines, through a multi-scale and multi-methodic approach, the Zn-Pb(-Cu-Fe) mineralization associated with fault zones in the Sila Massif (Calabria, southern Italy), focusing on its paragenetic evolution using samples collected from the historical mining sites of Longobucco (LGB) and Fonte Argentila (FAR). Hosted within Permian–Carboniferous granodiorites of the Sila Batholith, the mineralization exhibits similar mineralogical assemblages from both mining sites and, in addition, shows geochemical features similar to those characteristics of Mississippi Valley Type (MVT) and Sediment-hosted Massive Sulphides (SHMS) systems, despite their vein-like nature in the field. The primary ore mineral, sphalerite, occurs in three distinct generations (Sp1, Sp2, Sp3), reflecting multiple mineralization episodes. The Sp1 is light-colored with relatively low Fe-content, Sp2 is darker and Fe-richer (up to 11.3 wt.%, corresponding to 0.21 mol% FeS), while Sp3 is colorless and with very low Fe-content, and formed through dissolution-precipitation processes. The μ-Raman Spectroscopy evidenced clear dependence on FeS mole fraction in sphalerite through systematic variations in band positions, intensity and area ratios. Based on the trace element signatures, the LGB-FAR sphalerite formed under low-temperature conditions (medians formation temperature of 178–180 °C), as indicated by the GGIMFis geothermometer, by the Ga/In, In/Ge and Zn/Cd ratios, and by single-crystal X-ray diffraction (SCXRD). The reconstructed paragenetic sequence spans four stages. Stage 1 is characterized by the precipitation of sphalerite (Sp1 and Sp2) and quartz (Qz1). Stage 2 is marked by massive precipitation of calcite (Cal), hosting sporadic synchysite crystals (20-60 μm), at LGB and fluorite (Flr) at FAR. Galena and chalcopyrite precipitated during stage 3, while quartz (Qz2) and sphalerite (Sp3) formed in stage 4. The observed geochemical features, as well as the low sulphur fugacity values (log10ƒS2 = 10-17.55–10-17.29 atm), suggest precipitation from an ore-forming fluid of MVT-SHMS/basinal derivation. This model is also supported by fluid inclusions data that register direct evidence of meteoric to high salinity basinal-type ore- forming fluids trapped within fluorite of the stage-2 (Th = 72.2–114.6 °C; salinities from 0 to 21.2 wt.% NaCl eq.). Ore-forming fluids later evolved into meteoric to basinal-type with low to moderate salinity (0.5 to 6.1 wt% NaCl eq.), as registered by fluid inclusions trapped within Qz2 of the stage-4, and higher temperature than Flr (Th = 111.6–163.8 °C), also corroborated by Mg-Li geothermometer (median of 171°C). Rare Earth Element and Yttrium (REY) analyses in Cal and Flr confirm the presence of a diagenetic and basement-derived geochemical signature during their precipitation. These findings suggest that the LGB and FAR mineralization may have formed from fluids of basinal- type derivation, within basement host rocks. The study highlights the importance of fluid mixing and fault-controlled fluid flow in the formation of Zn-Pb deposits in crystalline basement settings, providing a comprehensive model for their paragenetic and geochemical evolution. By comparing our results with those of similar Zn-Pb-deposits, we suggest that the fluids responsible for the peculiar vein-type LGB-FAR mineralization had several characteristics compatible with those of MVT-SHMS deposits, despite an indirect magmatic contribution to the mineralizing fluids cannot be excluded at all.