Hydrothermal mineralization on the Sitito-Iozima Ridge in the Izu-Ogasaware Arc
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AbstractThe Sititō-Iōzima Ridge exists on the volcanic front in the northern part of the Izu Ogasawara Arc. Recently on some submarine volcanoes along the Sititō－Iōzima Ridge, active hydrothermal mineralizations have been discovered as a result of sea-floor surveys by the JAMSTEC's submersible ’Shinkai 2000'. From the Kita-Beyonese Submarine Caldera, hydrothermal sulfides, sulfates, manganese oxides have been collected. In the Myōzin syō Caldera, the fragments with hydrothermal sulfides and barites were discovered. In the caldera at the western top of Suiyō Seamount, some active sulfide chimneys gushing the hydrothermal water were discovered. Hydrothermal pyrites at a hydrothermal alteration zone in the Kaikata Caldera and possible recent hydrothermal manganese oxides on the flank of the Kaikata Seamount are observed respectively. The Kaikata Caldera is consisted of andesites or basalts. But other three submarine calderas are consisted of dacites and they are at similar geological setting. Two reasons why some hydrothermal sulfides exist in the submarine caldera consisted of dacites are supposed as follows. 1) The dacitic magma with the high viscosity stays steadily at the shallow part below the sea-floor. 2) The activity of the dacitic magma is easy to make the concave sea-floor (submarine caldera), to protect sea-floor hydrothermal sulfides from the oxidization by deep-sea waters. The characteristics about hydrothermal mineralizations on the Sititō－Iōzima Ridge are arranged as follows. 1) Sea floor hydrothermal sulfides exist in submarine calderas consisted of dacite. 2 ) Hydrothermal sulfides have lower Fe content, in comparison with those from mid-oceanic ridges. 3) Contents of As, Cd and Au on minor elements are comparatively high in hydrothermal sulfides. 4) The Au/ Ag ratio is relatively high, because the Ag content is nearly equal to that in hydrothermal sulfides of other sea areas. 5) It seems that the gashing hydrothermal water doesn’tr eact on marine sediments beneath the sea-floor. 6) Manganese oxides at the Kaikata Seamount were resulted from the low-temperature hydrothermal activity of the submarine volcano on a island-arc. 7) Almost of hydrothermal oxides from the Kaikata Seamount consist of manganese oxides and poor in iron oxides.
JournalReport of Hydrographic and Oceanographic Researches
Page Rangepp. 1-16