Potential Reaction Pathways of Hg in Some New Zealand Hydrothermal Environments
B. W. Christenson, E. K. Mroczek, 2005. "Potential Reaction Pathways of Hg in Some New Zealand Hydrothermal Environments", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.