Mercury and Sulphur Isotope Compositions of Cinnabar and Stibnite Constrain the Source Rocks of a World-Class Hot Spring Mine District (Mt. Amiata, Italy)

The cinnabar (±stibnite) deposits of the Mt. Amiata geothermal system (Italy) and the associated hot springs and gas vents form a world-class hot spring ore district. Here, we combine 22 δ34S ratios of cinnabar, marcasite, and stibnite with their δ202Hg and Δ199Hg ratios, as well as a Bayesian mixing model, to constrain the source rocks in this geological environment. 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 a much narrower δ202Hg range. Most cinnabar, stibnite, and marcasite samples yielded δ34S ratios between -0.9 and +5 ‰. We interpret these data as the products of mixing of dominantly magmatic, metamorphic, and marine sedimentary sources of Hg (Sb) and S. Although Rayleigh fractionation likely played a role at Mt. Amiata, it was not explicitly modelled in our study. A comparison between the Hg isotopic signatures of the Mt. Amiata and those of world-class submarine geothermal systems (Almadén, Spain; Idrija, Slovenia, with typically: δ202Hg<2‰; Δ199Hg<0.3‰) show significant differences. This indicates that the range of fractionation processes occurring in submarine and continental geothermal systems are substantially different.