鳥取県東郷鉱山ウラン鉱床の基盤岩ならびに母岩の変質 (1)方面･麻畑鉱床

東郷鉱山,方面･麻畑ウラン鉱床の基盤岩および母岩は粘土化が著しく,その特徴として次のことが挙げられる. (a)粘土鉱物の種類,存在量は,鉱床の酸化帯･非酸化帯にはゞ規制されている.(b)粘土化の程度は原岩の種類により異なり,同一種数の岩石では酸化帯における変質が特に顕著である.(c)粘土量とウラン含有量との間には明確な相関は見出せないが,モンモリロナイトの量についてのみ観察すると,おおよそ負の相関がみられる.また,粘土化の主体をなすモンモリロナイトは,結晶度が良好で熱水変質が示唆きれる.化学分析の結果,SiO(2)の量が少ないので珪ばん比が異常に小さいが,ウラン鉱床近辺においてモンモリロナイト化に伴なう珪化帯が認められず,珪酸の逸脱が考えられる.鉱量的にみて,両鉱床の主体をなすウラン鉱物が燐顧塩鉱物であり,珪酸塩鉱物が少ないことなどから,モンモリロナイト化作用と鉱化作用との間には,直接関連がないものと考察される.

The Katamo and Asabatake Uranium Deposits, Togo Mine, about 5 Km south east of the lake Togo, central Tottori prefecture, is investigated to understand the nature of the bed rock and wall rock alterations. These alterations are remarkable and characterized as follows. (1) Both the clay mineral species and their relative amounts are closely related to the "oxidation-reduction enviroment" of the deposits. (2) The degree of alteration depends on the nature of the mother rocks and differs from an area to another. The most remarkable alteration is realized in the secondary oxidation zone. (3) Although the secondary uranium minerals are confined in the oxidation zone, there is no apparent relationship between the total amounts of clay minerals and uranium contents of the deposits. But the amounts of montmorillonite seems to decrease as uranium content goes up within some limited areas. The principal clay mineral in the deposits is montmorillonite. Kaolinite is also present in a small amount. Sericite is found only in some of the veins as an accessory mineral. Hydrothermal solutions seems to have played an important role for the formation of montmorillonite, as it is found in a well crystallized form in altered acidic pyroclastic sediments. Kaolinite, on the other hand, has a tendency to increase in its amount in arkose sediments which are sandwiched in volcanic sediments. This seems to show that kaolinite is a weathered product of the bed rocks of this region. The results presented in this paper could be a help in considering the origin of the Uranium deposits in this area.

Original Papers