1974 Volume 24 Issue 124 Pages 157-168
The Tono uranium deposits were first detected by a carborne survey team of the Geological Survey of Japan in 1962. Following this discovery, exploration in the area has been carried out by the Power Reactor and Nuclear Fuel Development Corporation. There are four major uranium deposits so far discovered in the Tono district, namely Tsukiyoshi, Misano, Jorinji and Utozaka.
The basement of the area is composed of late Cretaceous to early Tertiary granites and Paleozoic rocks. The Neogene sedimentary rocks overlying the basement are stratigraphically divided into two groups, the Mizunami group of Miocene age and the Seto group of Pliocene age.
Uranium deposits are exclusively found in the Toki lignite-bearing formation of the Miziinami group, and their distribution is generally controlled by the so-called "channel structure". The "channel" is defined as shallow trough excavated on the erosional surface off the basement rocks by pre-Miocene streams. The most favourable position in the channel for the deposition of uranium is the bottom part, where conglomerate and/or coarse-grained sandstone are well developed as the permeable bed.
The principal uranium minerals found in the Tono district are as follows.
(1) Tetravalent uranium minerals; uraninite, coffinite.
(2) Hexavalent uranium minerals; autunite, uranocircite.
(3) Minerals adsorbing uranium; zeolite, montmorillonite, carbonaceous matter.
At the prospecting tunnel of the Tsukiyoshi ore deposits, uranium has been recognized mainly in such mineral as coffinite and uraniferous zeolite. Most of, the uranium in the ores are considered to be adsorbed by zeolite (heulandite-clino ptilolite group). Coffinite has been found as a main mineral of high grade ores, and usually accompanying pyrite and carbonaceous matter.
