Geochemistry and Genesis of Fe-Mn Mineralization in Island Arcs in the West Pacific Ocean

摘要

This paper presents materials on the chemical and mineralogieal composition of Fe-Mn mineralization in island arcs(Kurile,Nampo,Mariana,New Britain,New Hebrides,and Kermadec)in the western part of the Pacific Ocean.The mineralization was proved to be of hydrothermal and/or hydrogenic genesis.The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments.Some Fe oxi-hydroxides can be derived via the halmyrolysis of volcaniclastic material.Crusts of this stage are characterized by fairly low concentrations of trace and rare elements,and their REE composition is inherited from the volcanic-terrigenous material.The minerals of the Mn oxi-hydroxides are todorokite and"Ca-birnessite."The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions.The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group(Co,Ni,Tl,and Mo)and the Fe group(REE,Y,and Th).The hydrogenic crusts consist of Fe-verna-dite and Mn-feroxyhyte.Some of the hydrothermal crusts originally had a hydrothermal genesis.The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc,which contained anomalously high concentrations of Co(2587 ppm)and other Mn-related trace elements in the absence of hydrogeneous Fe oxi-hydroxides.