Geochemistry of hydrothermal Mn-oxide deposits from the S.W. Pacific island arc

Hydrothermal Mn-oxide crusts have been removed from the Tonga-Kermadec Ridge, the first such hydrothermal deposits to be reported in the S.W. Pacific island arc. In several respects the deposits are similar to hydrothermal Mn-crusts from oceanic spreading centre settings. They are limited in areal extent, comprise well-crystalline birnessite and generally display extreme fractionation of Mn from Fe. They are strongly depleted in many elements compared to hydrogenous Mn deposits but are comparatively enriched in Li, Zn, Mo and Cd. The Group IA and Group IIA metals show strong intercorrelations and the behaviour of Mg in the purest samples may indicate the extent to which normal seawater has influenced the composition of the deposits.Certain aspects of the deposits are not typical of hydrothermal Mn deposits. In particular at least some of the crusts have developed on a sediment or unconsolidated talus substrate. Some crusts, or specific layers within some crusts, display a chemical composition which suggests a significant input from normal seawater.