The Gorno mining district is an example of Mississippi Valley-type (MVT) deposits in the Italian Orobic Alps. Spanning an area of ~100 km2, it consists of stratabound Zn-Pb-Ag (± fluorite ± barite) mineralization hosted in a lower Carnian stratigraphic succession. Despite Vedra Metals S.r.l.´s acquisition of the exploration license for the deposits has reignited an economic interest, an updated metallogenic model has yet to be developed.
In the carbonate host rocks at Gorno, a complex series of dolomitization, silicification, brecciation, dissolution, and cementation occurred. Microthermometry of primary fluid inclusions in sphalerite and fluorite, alongside sphalerite trace-element geothermometry, indicates formation temperatures ranging from 80 to 140 °C (mean value: ~100 °C). Moreover, fluid inclusion microthermometry and micro-Raman spectroscopy document the involvement of high-salinity brines (up to ~25 eq.wt% NaCl) and gaseous hydrocarbons (e.g. CH4) in ore deposition. Isotopic signatures from ore-related carbonates for carbon (0.5 to 2.5 ‰ PDB), oxygen (-6.6 to -12.1 ‰ PDB), and strontium (0.70840-0.70943) indicate that the ore fluid was likely seawater modified through interaction with the underlying Permian clastic sediments and/or with the metamorphic basement.
The presence of sulfide bodies in association with organic-rich shales implies a notable role of organic carbon in ore deposition. Organic matter and associated hydrocarbons likely served as reactive barriers, leading to the reduction of the ore fluid and initiating the precipitation of sulfide minerals.