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Formation of Acid Volcanic Brines through Interaction of Magmatic Gases, Seawater, and Rock within the White Island Volcanic-Hydrothermal System, New Zealand

By
Werner F. Giggenbach
Werner F. Giggenbach
Institute of Geological and Nuclear Sciences, P. O. Box 31312, Lower Hutt, New Zealand
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Hiroshi Shinohara
Hiroshi Shinohara
Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305–8567, Japan
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Minoru Kusakabe
Minoru Kusakabe
Institute for Study of the Earth's Interior, Okayama University, Misasa 682–0193, Japan
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Takeshi Ohba
Takeshi Ohba
Volcanic Fluid Research Center, Tokyo Institute of Technology, Kusatsu 377–1711, Japan
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Published:
January 01, 2005

Abstract

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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Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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