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Modern iron ooids from a shallow-marine volcanic setting: Mahengetang, Indonesia

¹ Department of Geology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
² The Ecology of Indonesian Seas, EMDI Project, School for Resource and Environmental Studies, Dalhousie University, 1312 Robie Street, Halifax, Nova Scotia B3H 3E2, Canada

An unconsolidated submarine deposit of iron ooids and pisoids, located offshore the volcanic island Mahengetang, Indonesia, is described herein. The deposit occurs in a shallow-marine setting, in an area characterized 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 m². Ooids are composed of concentric accretionary layers of limonite admixed with amorphous silica, precipitated around andesitic rock fragments. The smallest layers forming the ooids are about 10 ± in thickness. The iron-rich precipitate is 50% iron by weight. Ooid formation is likely promoted by precipitation of iron and silica from exhalative fluids rising up through the substrate. Concentric layering is thought to result from constant agitation of ooids associated with currents and expulsion of gas from the sediment.

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