MVT-SHMS signature in basement-hosted Zn-Pb-(Cu-Fe) mineralization in the Sila Massif (Calabria, Italy): Evidence from trace elements and fluid inclusions data

Abstract

Typically, Mississippi Valley Type (MVT) and Sediment-hosted Massive Sulphides (SHMS) Zn-Pb-deposits are hosted by sedimentary basins and are originated from fluids sourced from a crystalline basement. Mixing of fluids from crystalline basement and overlying sedimentary basins, through faults and fractures, is a major trigger for the formation of Zn-Pb deposits. In this context, Zn-Pb mineralization hosted in a crystalline basement and preserving a MVT-SHMS geochemical signature are rarely considered, although found. Here we present the results on a Zn-Pb(−Cu-Fe) mineralization associated to fault zones developed within Permian–Carboniferous intrusive bodies in the northern Sila Massif of Calabria (Italy), at Longobucco (LGB) and Fonte Argentila (FAR) localities. The ore-mineral assemblage consists of sphalerite, galena, ± chalcopyrite, and pyrite. We identified four distinct stages and three generations of sphalerite (Sp1, Sp2, Sp3), characterizing the paragenetic evolution of the mineralization. The Fe-content in sphalerite of LGB and FAR increases from Sp1 (medians of 3.34 and 2.46 wt%) to Sp2 (medians of 6.84 and 7.29 wt%), the latter containing the highest amounts of Cu (up to 1023 ppm), Ga (up to 338 ppm), Ge (up to 400 ppm), and Cd (up to 7589 ppm). Sp3 is characterized by the lowest Fe-content (median of 0.43 wt%) and formed after dissolution-precipitation of the earlier sphalerite generations. Based on the trace element signatures, the LGB-FAR sphalerite formed under low-temperature conditions (medians of 150–183 °C), as indicated by the GGIMFis geothermometer and the Ga/In, In/Ge and Zn/Cd ratios. The geochemical features and 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 record evidence from meteoric to high salinity basinal-type ore-forming fluids trapped within fluorite (T_h = 72.2–114.6 °C; salinities from 0 to 21.2 wt% NaCl eq.). The later ore-forming fluids show meteoric and basinal-type with low to moderate salinity, as evidenced by fluid inclusions trapped within second quartz (Qz2) generation (T_h = 111.6–163.8 °C; salinities of 0.5 to 6.1 wt% NaCl eq.). 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 comparable to those related to MVT-SHMS deposits, although we cannot exclude at least an indirect magmatic contribution to the mineralizing fluids.