本邦の黒鉱式鉱床に認められる放射能異常について

抄録

Radioactivity in so-called “Kuroko” deposits was first reported by Mr. I. ASAI in 1920. He described: “…The radloactivity of Kuroko may have been related to the presence of rodiobarite in the ore”. Recently, in 1960, radiobarite, occurring in close association with uraniferous clayey rocks, was confirmed by the author from the Iwami gypsum mine, Shimane Prefecture. The author recognized that the radioactivity was closely related to the clay minerals of hydrothermal origin. Then the author took geologic reconnaissances at the Hanaoka copper mine, Akita Prefecture, and at the Asahi gypsum mine, Fukushima Prefecture, and he detected radioactivity from clays of similar occurrences at the Iwami mine.
Radioactive anomaly was detected at marginal part, lower grade gypsum ores, and pyrite are body as much as 220μr/h. The clays of some 50μr/h expose at a level for about 5-20m in length, and the intensitivepart for about 1 to 2m across. Average uranium content of grabed samples is 0.01 to 0.08 per cent containing minor amounts of Zr, Mo, Sr, Rb, TI, and others. Uranium is fixed in sericite, chlorite, nacrite (a kaolin mineral), and in montmorillonite. It is nortworthy that at the Iwami mine, honey yellow, transparent radiobarite is closely associated with chlorite or montmorillonite. The barite is not plumbian like hokutolite, but contains some quantity of strontium.
The envelope of these, in general, is constituted of a zonal distribution of concentric zones of clay minerals of hydrothermal origin, they are kaolin-sericite-chlorite-montmorillonite, from center to margin, respectively.
It has been revealed by the author that the uranium is mostly enriched at the clays of sericite-chlorite zone, especially of tuffaceous rocks origin.