Mineoka Umber: A Submarine Hydrothermal Deposit on an Eocene Arc Volcanic Ridge in Central Japan

  1. John Parnell,
  2. Ye Lianjun and
  3. Chen Changming
  1. A. Iijima1,
  2. Y. Watanabe1,
  3. S. Ogihara1 and
  4. K. Yamazaki2

Published Online: 3 APR 2009

DOI: 10.1002/9781444303872.ch7

Sediment-Hosted Mineral Deposits: Proceedings of a Symposium Held in Beijing, People's Republic of China, 30 July-4 August 1988

Sediment-Hosted Mineral Deposits: Proceedings of a Symposium Held in Beijing, People's Republic of China, 30 July-4 August 1988

How to Cite

Iijima, A., Watanabe, Y., Ogihara, S. and Yamazaki, K. (1990) Mineoka Umber: A Submarine Hydrothermal Deposit on an Eocene Arc Volcanic Ridge in Central Japan, in Sediment-Hosted Mineral Deposits: Proceedings of a Symposium Held in Beijing, People's Republic of China, 30 July-4 August 1988 (eds J. Parnell, Y. Lianjun and C. Changming), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303872.ch7

Editor Information

  1. Department of Geology, The Queen's University of Belfast, Belfast BT7 INN, UK

Author Information

  1. 1

    Geological Institute, University of Tokyo, Hongo 7–3–1, Tokyo 113, Japan

  2. 2

    Mitsui Coal Mining Company, Muromachi 2–1–1, Nihonbashi, Tokyo 103, Japan

Publication History

  1. Published Online: 3 APR 2009
  2. Published Print: 27 SEP 1990

ISBN Information

Print ISBN: 9780632028818

Online ISBN: 9781444303872

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Keywords:

  • Mineoka Umber - lower metalliferous and upper argillaceous segments;
  • manganese oxides and hydrated ferric oxides, occurring in Mineoka Umber;
  • instrumental neutron activation analysis (INAA);
  • heavy rare earth element (HREE)-depleted pattern;
  • pelitic to psammitic biotite schist, consisting biotite, quartz, microcline and plagioclase;
  • Kamogawa Formation;
  • Mineoka-Kobotoke-Setogawa Tectonic Belt

Summary

A strata-bound iron–manganese umber, extending for 2 km along strike, overlies the top surface of an Eocene submarine tholeiitic pillow lava of the Mineoka terrain in the southern Boso Peninsula, 70 km southeast of Tokyo. The Mineoka Umber is divided into lower metalliferous and upper argillaceous segments, changing upwards gradually to ferruginous, spicule-rich shale. The metalliferous umber contains 41−54 wt% Fe2O3 and 8.0−16.8 wt% MnO2 + MnO, comprising massive aggregates of very fine goethite, amorphous calcic manganese oxides and pyrolusite. The Mineoka Umber formed during submarine hydrothermal activity following basaltic volcanism. Direct evidence of the hydrothermal origin is the discovery of an umber-filled pipe, a conduit for hydrothermal water, within the pillow lava beneath the strata-bound umber. Away from this conduit, the MnO: MnO2 ratio in the strata-bound umber decreases, reflecting the redox potential of the umber formation, and thus the content of pyrolusite increases. The high concentrations of rare earth elements and the strong negative cerium anomaly in the umber are also explained by the submarine hydrothermal origin. Calcites filling pores and contraction cracks in the pipe umber have δ13C and δ18O values of −8.5 to −10.6‰ and −14.3 to −20.4‰ PDB, respectively. These values suggest precipitation from heated Eocene seawater at 55−68°C. The metal-carrying hydrothermal water was probably much hotter than the calcite-precipitating water. Abundant spicules in the overlying shale are thought to represent a school of sponges that fed on the nutrient-rich water. The Mineoka Umber accumulated at the foot of an Eocene arc volcanic ridge in the supra-subduction zone off the Japanese continental arc.