資源地質
Online ISSN : 2185-4033
Print ISSN : 0918-2454
ISSN-L : 0918-2454
61 巻, 1 号
選択された号の論文の5件中1~5を表示しています
論説・報告
  • 藤永 公一郎, 野崎 達生, 中山 健, 加藤 泰浩
    2011 年 61 巻 1 号 p. 1-11
    発行日: 2011年
    公開日: 2013/07/27
    ジャーナル フリー
    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.
  • 小林 孝男
    2011 年 61 巻 1 号 p. 13-28
    発行日: 2011年
    公開日: 2013/07/27
    ジャーナル フリー
    The characteristics of three sub-types of the unconformity-type uranium deposits in the Athabasca Basin, Thelon Basin, Kombolgie Basin, and Rudall River area are summarized. A new ore fluids flow model is proposed to explain the mechanisms that produce such deposits. It is proposed that these deposits are formed by the combination of basinal brines lateral flows driven by hydraulic gradients, and the downward flow of heavy brines through brittle faults which were reactivated by continent-wide tectonic events that postdated the basin deposition. The role of basinal brines was to leach uranium from basin sediments and transfer uranium to the fault planes. The role of heavy brines with high oxygen fugacity and high salinity, derived from evaporite-hosted surficial sediments, was to capture uranium along the fault planes and to transfer it down to the unconformity and farther down into the basement rocks. Ore fluids that flowed into the basement and reacted with rocks rich in graphite and/or mafic minerals, formed monometallic type deposits along the faults, or formed sub-unconformity post-metamorphic type deposits within specific strata-structures. In the case of basement rocks with thick and deep seated graphitic fault zones, the thermally-driven basement brines in reduced condition and rich in Ni-Co-As flowed upwards to the unconformity and reacted with oxidizing ore fluids to form polymetallic type deposits.
    This model was investigated quantitatively by calculating the uranium balance in the simplified ore fluids flow system. It was found that uranium deposits with 10's to 100's of thousands tU can be formed within 1 Ma when basinal brines bearing 1-10 ppmU and flowing at a rate of 0.1m/y cross the fault planes.
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