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Abstract

Equilibrium thermodynamic reaction pathways of Hg have been calculated for new analytical data from a number of hydrothermal environments in the Taupo Volcanic Zone, using reaction path simulation algorithms. The calculations suggest that Hg is present in the near-neutral chloride reservoir fluids of the Kawerau, Ohaaki, and Rotokawa geothermal systems principally as Hg°(aq) but in concentrations that are uniformly undersaturated with respect to Hg mineral phases. Simulated conductive cooling of these fluids (without gas loss) leads to the precipitation of cinnabar, whereas adiabatic cooling strongly partitions Hg into the vapor phase. Simulated condensation of Hg-bearing geothermal vapors into shallow (T = 100°C), oxygenated ground waters is predicted to lead to near-quantitative precipitation of Hg as cinnabar, whereas unhindered passage of the Hg(g)-bearing vapors to the surface may result in condensation of Hg(g) to liquid Hg (quicksilver) at temperatures <5°C. The modeled results are consistent with observed Hg enrichment in production well cuttings from the upper 100 m of the Kawerau reservoir.

Speciation calculations show that Hg in 270°C fumarolic gases at the White Island volcano is present predominantly as gaseous Hg°, with gas species HgCl2 and HgS being some four and five orders of magnitude less abundant, respectively. Simulated conductive cooling of the fumarolic gas leads to the precipitation of elemental sulfur at 265°C and cinnabar at 187°C along the fumarolic conduits, whereas condensation of these vapors into either acidic brines or shallow acidic condensates at 100°C also leads to the precipitation of cinnabar.

Hg in the Ruapehu Crater Lake is calculated to be present mainly as Hg(HS)°2 and, at 0.13 μg/l concentration, the lake is supersaturated with respect to cinnabar. Simulated heating of the lake water shows that this phase undersaturates at about 105°C, suggesting that the 0.1-μg/l Hg concentration in the lake probably represents quenched equilibrium conditions from the vent. Collectively, these investigations suggest that Hg behaves largely as a conservative constituent in the high-temperature portions of the magmatic and hydrothermal Taupo Volcanic Zone environments.

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