The speciation of mercury in hydrothermal systems, with applications to ore deposition

Hg in hydrothermal systems is generally thought to be transported as Hg-S complexes. However, the abundance of Hg⁰_vap, in geothermal emissions suggests that Hg⁰_eq, is present in the liquid phase of geothermal systems. Calculations for reducing fluids (HS^- dominant over SO⁼₄) in equilibrium with cinnabar indicate that Hg⁰_eq, can be quite abundant relative to other species at temperatures above 200°C. Increasing pH and temperature, and decreasing total S, ionic strength, and pO₂ all promote the abundance of Hg⁰_eq. When a vapor phase develops from a geothermal liquid, Hg partitions strongly into the vapor as Hg⁰_vap. Vapor transport at shallow level then results in the formation of Hg halos around shallow aquifers as well as in a flux of Hg to the atmosphere. Hg deposition may occur in response to mixing with oxidizing or acidic water, turning Hg⁰_eq, into Hg⁺⁺, with subsequent cinnabar precipitation. When pyrite is the stable Fe-sulfide, cinnabar solubility is at its lowest, so cinnabar + pyrite assemblages are common. Cinnabar + hematite ± pyrite can precipitate from more oxidized or S-poor water. Hg⁰_liq, can occur as a primary mineral, in coexistence with all common Fe-sulfides and oxides. Cinnabar ± Hg⁰_liq cannot coexist with pyrrhotite or magnetite at temperatures between 100° and 250°C. Evidence from Hg deposits indicates that many formed from dilute hydrothermal fluids in which Hg probably occurred as Hg⁰_eq. In S-rich systems, Hg may occur as Hg-S complexes, and in saline waters it can occur as Hg-Cl complexes.