Phase relations in the Hg-Zn-(Fe)-S system as a tool in geothermometry: an example from the Hg Levigliani deposit (Apuane Alps, Tuscany)

A number of minerals of the Hg-Zn-(Fe)-S system occur in the Hg deposit at Levigliani (Apuane Alps, northern Tuscany). The deposit is hosted within phyllitic and metavolcanic rocks of Paleozoic age, which were intensely deformed and metamorphosed to the greenschist facies (about 3-4 kbar and 350°-370°C) during the Tertiary Apenninic orogeny. Mineralization is mostly synchronous to the first deformation stage (Di). Specifically, an earlier, quantitatively more abundant cinnabar I + pyrite + zincian metacinnabar assemblage was deposited at conditions not far from the metamorphic peak. The zincian metacinnabar shows fairly constant chemical composition, with average 7.3 ± 1.2 (lα) mol% ZnS and 1.4 ± 0.5 mol% FeS. Extrapolation to 3.5 kbar of available experimental and thermodynamic data in the system Hg-Zn-(Fe)-S indicates that a zincian metacinnabar with the above composition can coexist at equilibrium with cinnabar at a temperature of 375°-385°C. Such a range is in good accordance with the above reported estimates of peak metamorphic temperatures, and with calculated trapping temperatures for fluid inclusions in vein quartz (about 370°C at P « 3.5 kbar). The earlier assemblage was later overprinted and partially replaced by cinnabar II + mercurian sphalerite ± pyrite. Except for a few optically zoned crystals, mercurian sphalerite has fairly constant HgS and FeS contents (on average 25.3 ± 0.6 mol% and 6.4 ± 0.2 mol% respectively). P-T conditions for this late assemblage are less constrained; however, by combining phase equilibria with recently proposed post-metamorphic P-T paths of Apuane Alps, indicative upper temperature and pressure limits of, respectively, ~ 200°C and 1.5 kbar can be suggested. The reasonable agreement between extrapolated conditions and independent P-T estimates suggests that the available experimental and thermodynamic data, although limited, may be cautiously used to model phase equilibria in the HgS-ZnS-(FeS) system in geological environments. The persistence of zincian metacinnabar as a metastable phase through the post-peak metamorphic path down to room temperature further confirms that Zn (and Fe) severely inhibit the transition from metacinnabar to cinnabar.