Modern and Palaeozoic iron ooids - a similar volcanic origin

The discovery of an unconsolidated deposit of iron ooids of a kind not prev iously described, and in the vicinity of a volcanic island in Indonesia has shed new light on the genesis of many ancient iron oolites. The discovery shows that iron ooid formation is geologically a rapid process. The ooids f orm by chemical precipitation of cryptocrystalline iron oxyhydroxides on av ailable grains on the seafloor, from seawater enriched with Fe, Al and Si. The enrichment can be a result of hydrothermal fluids, volcanic ash falls i nto shallow basins or rapid weathering of fresh volcanic rocks. Ordovician iron ooids from northern Europe are strikingly similar to the modem ooids i n chemical composition, REE distributions and internal structures. The diff erences in mineralogy can be explained as diagenetic transformation reactio ns under different environments. A model for the formation of the Ordovicia n iron ooids in northern Europe from volcaniclastic material is presented, which could be applicable to other ancient oolites in the world. (C) 2000 E lsevier Science B.V. All rights reserved.