A weak radioactive anomaly (0.080mR/hr) was detected at Yuya in 1959, but since then no furthur efforts in prospection have been undertaken in the area, since at that time it was not customary to attach much value to such occurrences. In November, 1966, significant radioactivity (1.7mR/hr, 0.6% U3O8) was detected 35m south of the previous location, in the course of a systemafic car-borne radiometric survey undertaken by the Atomic Fuel Corporation of Japan. To date, several uranium occurrences have been discovered in formations belonging to the Cretaceous Period. This is the first instance of such occurrence in Cretaceous formation discovered in Japan.
The Yuya uranium deposits occur mainly in dark gray to brown, hard compact sandstone and/or sandy shale. The deposits appear related to rhyolitic rock intrusions. A secondary uranium mineral, believed to be autunite, is sporadically found in the black silicified sandstone closely associated with pyrite and chlorite. Little uranium would appear to be contained in such clay minerals as chlorite according to radioluxograph and X-ray fluorescent examination, since primary uranium minerals have not yet identified.
So far prospection for uranium deposits in Japan have been directed mostly to the Neogene Tertiary just above the granitic host rocks. The discovery of the Yuya deposits in Cretaceous formation is highly significant in that it has opened up a vast new area to prospection in strata other than Tertiary formation, and this urges us to continue our efforts in gathering further information that should contribute to a better assessment of the domestic uranium potentiality.
