The behaviour of metals and sulphur during the formation of hydrothermal mercury–antimony–arsenic mineralization, Uzon caldera, Kamchatka, Russia

Uzon caldera, located in the eastern volcanic belt of the Kamchatka peninsula, is a complicated structure of Middle Pleistocene age. The composition of the co-existing solid and fluid phases, temperature and pH were determined with the aim of establishing the distribution of sulphur species, As, Sb and the main ore-forming metals. In the solid samples, the following sulphur-bearing minerals were identified: pyrite, realgar, orpiment, alacranite (As₈S₉), uzonite (As₄S₅), amorphous As-sulphide, stibnite, cinnabar and native sulphur. The following sulphur-bearing species H₂S, H₂S₂+S₅²⁻(aq)(aqueous polysulphanes), S⁰(aq), SO₃²⁻(aq), S₂O₃²⁻, SO₄²⁻ and total concentration of sulphur were determined in solutions. Eh, pH and H₂S concentration were measured potentiometrically in situ. Zero-valent sulphur (S⁰(aq)+H₂S₂+S₅²⁻(aq)) predominates in Uzon solutions. The pair H₂S–S_colloidal is Eh-determining in Uzon solutions up to 75–85°C. A quantitative thermodynamic model of the mineral deposition process at Uzon was constructed using the collected data. It was obtained that the composition of the hydrothermal solution and the precipitation of Sb–As–Hg species can be described using two only main factors: the initial composition of fluid and the temperature variation.