2006 Volume 40 Issue 3 Pages 147-165
Rare earth element (REE) contents and mineral assemblages of weathered granitic soil (crust) in the Ashizuri-misaki area, have been investigated in order to evaluate potential REE resources. The weathered soil with maximum thickness of about 5 meters formed at the surface of the Middle Miocene granitic rocks which consist mainly of quartz syenite and alkaline and fluorine-rich granite with high content of light REE such as La and Ce. In the weathered soil, chlorite, gibbsite, and kaolinite are pronounced when compared with the underlying granitic rocks, which mainly resulted from the decomposition of biotite and plagioclase during a weathering process. Almost all the quartz and K-feldspars in granitic rocks are, however, preserved during a weathering. The weathered soil in the Ashizuri-misaki area records the average Σ REE + Y value of 342 ppm with maximum contents of ΣREE + Y 701 ppm. The REE contents, especially light REE with exception of Ce and Eu, of the weathered soils are slightly depleted than those of the underlying granitic rocks. The depletion is considered to be caused by a leaching of LREE from LREE-bearing minerals such as chevkinite and fluocerite. The REE contents within the weathered soils reflect mainly residual REE-bearing minerals such as zircon and Nb-bearing multiple oxides which are originated in the granitic rocks. Enrichment of REE which is equivalent to the ion-adsorption style REE deposits in southern China is not expected to occur in the weathered soil within the granitic rocks in the Ashizuri-misaki area.
