西南日本の足摺岬地域及び山陽帯と中国南部の高REE花崗岩類における風化に伴う希土類鉱化作用

抄録

Rare earth elements (REE) are attractive mineral resources for modern high-technology industries. Here, we report the potential of REE mineralization as ion-adsorption type deposits, which are now known only in southern China, in weathered granitic crust and clay sediment in SW Japan, i.e., Ashizuri-misaki, Tanakami (Shigaraki), and Naegi areas.
The weathered crust in the Ashizuri-misaki area records an average ΣREE of 310 ppm (N=17); this reflects the higher REE content of the original granitic rocks with an averageΣREE of 366 ppm. The weathered crusts in the Tanakami and Naegi areas record an averageΣREE of 182 ppm (N=20) and 171 ppm (N=15), respectively. The weathered crust is depletedslightly more in REE than in the bedrock granite (211 ppm and 224 ppm, respectively). In the Tanakami and Naegi areas, it was discovered that halloysite-rich clay sediments have an average ΣREE of 288 ppm (N=6) and 341 ppm (N=4), respectively; In the Naegi area, the halloysite-rich clay sediments have higher I REE contents (> 400 ppm); they are equivalent to those of the ion-adsorption type REE deposits (ca. ΣREE 400-650 ppm) in southern China. Therefore, in Japan, REE mineralization tends to occur not in the weathered crust but reworked clay sediments derived from the weathering product of granitic rocks.