The hydrothermal mineralization at Suiyo Seamount, in the central part of the Izu-Ogasawara Arc
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概要
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Some sea-floor hydrothermal ore deposits have been identified on the mid-oceanic ridge and in the back-arc basin. This time at the top of a seamount on the volcanic front in Izu-Ogasawara Arc we have discovered the modem occurrence of hydrothermal activity. The active hydrothermal field with the occurrences of sulfide mineralization has been discovered by the JAMSTEC 'Sinkai 2000' in the caldera at the top of Suiyo Seamount in Sitiyo Seamounts.<BR>The volcanic rocks sampled at the top and the foot of the caldera wall are the calc-alkaline rock series dacites, and the low potassium andesites. This is in contrast to the tholeiitic basalts from some seamounts around Suiyo Seamount.<BR>The sulfides of the vent chimneys from this hydrothermal field is characterized by the enrichment in Cu. The Cu contents of many ores exceed 10wt%, suggesting that the temperature of these hydrothermal fluids is enough high to dissolve copper in them. The highest Au content of sulfide samples is 71 ppm, whose average of 12 samples is 25 ppm. The highest Ag content is 337.1 ppm, whose average of 12 sumples is 176 ppm. The average Au/Ag ratio is 0.142. This ratio is very high, compared with the sulfides from some sea-floor hydrothermal ore deposits.<BR>The high Au content of hydrothermal ore deposits at the Suiyo Seamount is worthy to note because the geological setting resembles that of the hydrothermal system at Osore-zan that was discovered in recent year in the most northern part of Japan.
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