MODERN IRON OOIDS FROM A SHALLOW-MARINE VOLCANIC SETTING - MAHENGETANG, INDONESIA

An unconsolidated submarine deposit of iron ooids and pisoids, Located offshore the volcanic island Mahengetang, Indonesia, is described her ein. The deposit occurs in a shallow-marine setting, in an area charac terized by venting of hydrothermal fluids and expulsion of gas across the sediment-water interface. The ooid deposit is essentially pure to a minimum depth of 0.5 m and covers an area of at least 1000 m(2). Ooi ds are composed of concentric accretionary layers of limonite admired with amorphous silica, precipitated around andesitic rock fragments. T he smallest layers forming the ooids are about 10 mu in thickness. The iron-rich precipitate is similar to 50% iron by weight. Ooid formatio n is likely promoted by precipitation of iron and silica from exhalati ve fluids rising up through the substrate. Concentric layering is thou ght to result from constant agitation of ooids associated with current s and expulsion of gas from the sediment.