Peralkaline Rhyolite Dikes at the Cape Ashizuri : A New Type of REE and Rare Metal Mineral Resources

Abstract

The final product of the Cape Ashizuri A-type granitic activities of Miocene age in the Outer Zone of Southwest Japan is a few N-S trending dikes of rhyolitic composition. This rock is pale bluish green in color composed mainly of alkali feldspar and alkali amphibole, and some accessory minerals. Its (Na2O+K2O)/Al2O3 (mole) exceeds 1, thus peralkaline in the chemistry. The rock is anomalously rich in the following valuable rare metal elements; Nb 633 ppm (50 times of JG-1 granodiorite reference sample), Ta 52 ppm (31 times), Zr 1567 ppm (15 times), Hf 38 ppm (11 times), Th 104 ppm (8 times), U 25 ppm (7.6 times), total rare earth elements (REE) 751 ppm (6.5 times) and Y 161 ppm (5.4 times). These elements appear to be contained at least four independent accessory minerals. The rock can therefore be a new type of rare metal mineral resources.