Hydrothermal lead-zinc-copper mineralizations in sedimentary rocks of Northern Apennines (Italy)

Lead and zinc hydrothermal mineralizations associated with late Oligocene-early Miocene sedimentary rocks (sandstones and marls) were found and investigated for the first time in the Northern Apennines (Northern Italy, province of Reggio Emilia). The mineralizations are located in a structurally complex area involved in the tectonic events forming the Northern Apennines chain. They are stratabound mineralizations mainly in the form of sub-vertical veins, suggesting a strong structural control. The ore minerals are sphalerite and galena (rare chalcopyrite), the gangue minerals are carbonates, barite and quartz, in different proportions in the three investigated sites (Busana, Casalino, Casenove). On the basis of microthermometric data on fluid inclusions in ore and gangue minerals, the ore-forming fluids belonged to an aqueous system containing Na-, Mg-, Ca-chlorides and/or carbonate and had a relatively high salinity, mainly in the range 11–17 wt% NaCl equivalent. The trapping temperatures were relatively high (142–231 °C range), suggesting basins characterized by anomalous geothermal gradients. The sphalerite composition reveals different FeS contents and different Zn/Cd ratios in the three sites. The δ34S values of the ore minerals vary from −7.98/−10.98 ‰ in Busana to 4.38/6.85 ‰ in the other two sites. The barite in Casalino has values of δ34S = 25.56 ‰ and δ18O = 13.81 ‰. The sphalerite composition and the isotopic data suggest complex interactions of the basin fluids involving crossed sediments, evaporite rocks, organic matter, as well as mixing between fluids and Jurassic marine waters. On the basis of their geological, petrographic-mineralogical and geochemical features, these mineralizations can be considered Alpine- or Irish-type MVT mineralizations.