Characteristics of Gases in Fluid Inclusions from the Nurukawa Kuroko Deposit and Submarine Sulfide Deposits of the Izu-Ogasawara Arc
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Gas compositions of fluid inclusions from the Nurukawa kuroko deposit of north Japan and the Myojinsho and Kita-Bayonnaise submarine sulfide deposits of the Izu-Ogasawara Arc, northwestern Pacific, are reported. The Nurukawa deposit was formed in a back-arc depression near a Tertiary volcanic front whereas the sulfide deposits at Myojinsho and Kita-Bayonnaise are presently forming near a ridge along a Quaternary volcanic front. Samples used in this study include quartz from the siliceous ores of the Nurukawa kuroko deposit and hydrothermally precipitated barite from the Myojinsho and Kita-Bayonnaise deposits.<BR>Homogenization temperatures (Th) of fluid inclusions from the Nurukawa deposit range from 105 to 305°C with apparent salinities between 3.2 and 7.0 wt.% NaC1 equivalent. Fluid inclusions from the Myojinsho deposit range in Th from 245 to 305°C with salinities between 2.2 and 4.2 wt.% NaCl equivalent. Fluid inclusions from the Kita-Bayonnaise deposit have Th ranging from 150 to 215°C with salinities between 3.0 and 5.0 wt.% NaCl equivalent. Most of apparent salinities for the Nurukawa and Kita-Bayonnaise deposits are slightly higher than the salinity of the normal seawater (3.5 wt.% NaCl equivalent) whereas most of apparent salinities for the Myojinsho deposit are slightly lower than the salinity of the normal seawater. The apparent salinities higher than the salinity of the normal seawater for the Nurukawa and Kita-Bayonnaise deposits may be attributed to CO<SUB>2</SUB> dissolved in the fluid inclusions. The apparent salinities lower than the salinity of the normal seawater for the Myojinsho deposit may be the result of sub-surface boiling.<BR>The H<SUB>2</SUB>O, CO<SUB>2</SUB>, N<SUB>2</SUB>, CH<SUB>4</SUB> and Ar contents in fluid inclusions from the Nurukawa, Myojinsho and Kita-Bayonnaise deposits all have similar ranges, i.e., 99.4 to 99.7 mol% H<SUB>2</SUB>O, 0.24 to 0.54 mol% CO<SUB>2</SUB>, 0.024 to 0.067 mol% N<SUB>2</SUB>, 0.0011 to 0.0074 mol% CH<SUB>4</SUB> and less than 0.0035 mol% Ar. The similarity of gas contents might be one of features for submarine sulfide deposits generating in island-arc settings.
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