Genesis of the hot spring mercury deposits of the Mt. Amiata geothermal system (Italy) constrained by Hg and S isotope geochemistry

The world-class geothermal system of Mt. Amiata, Italy, comprises a district of 14 hot-spring cinnabar ± stibnite deposits. We determined the Hg isotopic compositions of cinnabar and basement rocks of the geothermal system, coupled with S isotopic compositions of cinnabar, marcasite and stibnite, to constrain the source rocks in this geological environment. Most cinnabar, stibnite, and marcasite samples have δ34S between − 0.9 and + 5‰. Cinnabar shows a relatively large range of δ202Hg from − 3.64 to + 0.17‰ and Δ199Hg from − 0.43 to + 1.06‰, although samples from individual deposits show much narrower δ202Hg ranges. We interpret these values as the products of hydrothermal fractionation and mixing of magmatic, metamorphic, and marine sedimentary sources of Hg (Sb) and S, from which the samples inherited their signature. Comparison between the Hg isotopic signatures of the Mt. Amiata cinnabar and those of the world-class submarine geothermal systems of Almadén (Spain) and Idrija (Slovenia) shows significant differences. At Almadén and Idrija, the range of δ202Hg values are typically < 2‰, while the corresponding values for Mt. Amiata have a much larger spread, similar to other geothermal systems in the western USA. This indicates notable differences between the range of fractionation processes occurring in submarine and continental geothermal systems.