Fluid mixing and organic carbon-mediated sulfate reduction in the formation of Alpine-type Zn-Pb sulfide deposits (Gorno district, Southalpine domain, N Italy)

Alpine-type Zn-Pb deposits are a subclass of Mississippi Valley-type (MVT) deposits, hosted in Anisian-Carnian carbonate rocks of the European Alps. They represent the second-largest province of Zn-Pb deposits in Europe in terms of resources and historic production. The Gorno mining district in Italy is a prime example of an Alpine-type district, with stratabound Zn-Pb-Ag (± fluorite ± barite) mineralization hosted in lower Carnian clay-rich sediments and peritidal limestones. Despite scientific and economic interest, there is no consensus on the genesis of the Gorno mineralization. This study sheds light on the ore-forming processes involved in Alpine-type deposits: metallogenesis at Gorno was related to hydrothermal activity associated with multiple stages of diagenesis and mineralization, including dolomitization, silicification, brecciation, dissolution, and cementation. Petrographic evidence, showing replacive and displacive growth of hydrothermal sulfides, dolomite, and quartz within unlithified sediments, indicates that mineralization occurred at shallow burial conditions, linked to Late Triassic extension, as confirmed by recent radiometric data. Microthermometry of primary fluid inclusions in sphalerite and fluorite, alongside sphalerite trace-element geothermometry—both never before applied in Gorno—indicates fluid temperatures between 70 and 140 ◦C (mean: ~100 ◦C) and salinities up to ~25 eq.wt% NaCl. Micro-Raman spectroscopy documents the involvement of gaseous hydrocarbons (e.g., CH4). Carbon, oxygen, and strontium isotopes suggest that the ore fluid was modified seawater that interacted with underlying Permian sandstones and basement. Ore-mineral precipitation was controlled by the distribution of organic-rich lithologies in the host-sequence and occurred through a combination of fluid mixing and in situ reduction of the ore fluids.