Formation of Acid Volcanic Brines through Interaction of Magmatic Gases, Seawater, and Rock within the White Island Volcanic-Hydrothermal System, New Zealand
Werner F. Giggenbach, Hiroshi Shinohara, Minoru Kusakabe, Takeshi Ohba, 2005. "Formation of Acid Volcanic Brines through Interaction of Magmatic Gases, Seawater, and Rock within the White Island Volcanic-Hydrothermal System, New Zealand", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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A series of acid volcanic brines was collected at White Island, New Zealand, largely during the period 1974 to 1977, when the acid brine discharge rate was high. The chemical and isotopic compositions of these brines were compared to the compositions of fumarolic gases, seawater, and volcanic and the altered rocks in order to determine their origins. Isochemical dissolution of volcanic host rocks appears to account for the relative concentrations of many rock-forming elements in the acid brines with the exception of the enrichment in Na, which most likely derives from the influx of the seawater, and the depletion in Al, K, Ca, and Fe, which reflects formation of alunite, anhydrite, and pyrite during acid alteration. Magmatic gas is the major source of B, NH3, Cl, and S in the brines. As it passes through the hydrothermal liquids, B and NH3 are quantitatively condensed into brine, whereas the HCl and S are partially absorbed into the brine. Brine sulfate shows a large variation in δ34S values, ranging from 0 to 17 per mil; the heavy sulfate comes from disproportionation of magmatic SO2, whereas light δ34S sulfate is derived from near-surface bacterial oxidation of elemental sulfur and pyrite. The δD and δ18O values of acid brines and fumarolic condensates reflect the formation of the volcanic-hydrothermal system, mainly by the seawater-magmatic water mixing. However, a significant amount of meteoric water is likely incorporated during brine formation, but its proportion cannot be quantitatively evaluated because δD and δ18O compositions of meteoric water are close to that of the seawater. Mass-balance calculations replicating the average White Island brine composition require dissolution and alteration of 0.15 kg of the host volcanic andesite by a solution made up of 1 kg of the seawater and 3 kg of the magmatic gas. The detailed examination of the brine compositions shows that infiltrating the seawater into the White Island hydrothermal system and nonisochemical dissolution of the source volcanic rocks are responsible for their formation.
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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.