The Uranium Deposits in the Vicinity of Nakatsugô-Ombara, Nigyô-tôge Mine, Okayama Prefecture

Abstract

The Ningyô-tôge area is one of the richest uranium areas in Japan. Uranium deposits of the eastern extension of the Ningyô-tôge area are called the Nakatsugô-Nambu body, the Nakatsugô-main body, and the Ombarabody, respectively. The average uranium content throughout these ore bodies is about 0.04% U₃O₈ or less, while high grade ore body of the Nakatsugô-Nambu averages over 0.5% U₃O₈. The ore reserve of the largest Nakatsugô main body are estimated to be about 430, 000 tons (0.035% U₃O₈).
The uranium deposits occur in a thin bed (0-90m thick) of river or lake formations consisting of conglomerates, sandstone and shale, successively from bottom to top. They overlie unconformably over various granitic rocks of the Chugoku Batholith, of which K-Ar dating using biotite ages from 33 to 64 million years. These sediments of the Miocene to Pliocene epoch, are covered by Pliocene andesite and/or andesitic pyoclastics.
The uranium deposits are located at the northern edge of the main channel structure of a paleo-current which flew eastward. Since the trend of the major sheared zone and joints in granites are almost identical to that of the main channel, these weak zones of the granites might have favored the formation of the main channel.
The highly minerallized horizon of the Nakatsugo-Nambu body occurs in the basal conglomerates above the granite. The ningyoite, which is the major uranium mineral of the ore body, fills the matrix of the conglomerates. It is usually associated with pyrite. On the other hand, the major uranium minerallization in the Nakatsugo main body are present in the basal conglomerates, and the minor one in alternations of sandstone and shale over the conglomerates although ningyoite is widely distributed, uraninite is present only locally in this ore body. The low grade ore body of the Ombara consists of uraniferous conglomerates, sandstone, and alternations of sandstone and shale. No uranium mineral has been identified at present.
These ore bodies are the Neogene bedded-type uranium deposits. It appears that the important geologic factors for the formation of these uranium deposits are surface and internal structures of the basement granite, amounts and kinds of the host sedimentary rocks, and presence of a cap rock.