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Umber Deposit

Chemical Process (Liquid-phase Growth) - Under water

Required Geological Setting

Neogene and Quaternary sediments


Fe/Mn-rich sediments associated with basaltic pillow lavas erupted on seafloors. Umbers are observed in the interstices of pillows or conformably overlay pillow lavas. Umbers conformably underlay sediments such as volcanic clasts, shale, and chert. Major components are Fe2O3 (30-50 wt5) and MnO (10-15 wt%) with minor V, Pb, Zn and REE. Basalt associated with umber is Mid-ocean ridge basalt (MORB). It is considered that Fe/Mn dissolved into seawater from basalt lava erupted on seafloors near the Mid-ocean ridge, and deposited as Fe/Mn hydroxide and/or oxide.

Mineral Assemblages

Goethite, Pyrolusite, Hematite, Caryopilite/Bementite

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