2011 年 61 巻 1 号 p. 1-11
The Aki Fe-Mn umber deposit exists in the Northern Shimanto Belt, central Shikoku, Japan. The ore deposit is strata-bound and the orebodies are generally sandwiched between greenstone (originated from basalt) and red chert. In some cases, the umber layers are also distributed among red chert beds. The Aki umber ores are typically enriched in rare earth elements and Y (REY) as well as P, V, Co, Ni, and Zn. The REY contents of the umber ores range from 414 to 1,120 ppm, which are comparable to those of ion-adsorbed-type REY deposits in the southern China. The post-Archean average Australian shale-normalized REY patterns of the umbers show flat patterns with conspicuous negative Ce anomalies and are very similar to those of modern hydrothermal Fe-Mn sediments in the periphery of mid-oceanic ridge (MOR). In addition, the elemental ratios of REY, Mn, P, V, Co, Ni, and Zn relative to Fe in the Aki umbers are generally identical to those ratios in the MOR Fe-Mn sediments. These geochemical lines of evidence strongly indicate that the Aki umber deposit is an ancient counterpart of the hydrothermal Fe-Mn sediments near MOR. The scavenging of REY by plume fall-out Fe-Mn precipitates from ambient seawater was responsible for very high REY contents in the Aki umber ores.
The ore resource of the Aki umber deposit is estimated to be ca. 1,430,000 tons, based on the field survey of the present study. The contained REY and Dy of the Aki umber deposit are calculated to be 1,200 tREO and 50 tREO, respectively. These REY resource abundances correspond to only less than 5 % of the annual REY consumption in Japan. Hence, it is concluded that the Aki umber deposit does not have good potential as a source of REY.
