Weathered crust of granitic rocks of two geochemically similar plutons that differ in age of magmatism were investigated in the Philippines. These are the Cretaceous Daroctan Granite and the Middle Miocene Kapoas Granitoid located in Palawan Island, a part of the Palawan Continental Block derived from the continental margin of southeastern Eurasia. Both plutons are composed of biotite granodiorite and biotite granite and are geochemically characterized as ilmenite series, high-K calc-alkaline, intermediate I- and S-types, and peraluminous. Light rare earth element (LREE) content of the weathered granitic crusts in the Kapoas Granitoid is higher than that in the Daroctan Granite, and LREE are enriched in the Daroctan Granite compared to the parent granitic rocks. In both plutons, the residual REE-bearing minerals are mostly monazite. Allanite is only present in the Kapoas Granitoid while the Daroctan Granite contains xenotime. The Kapoas Granitoid pluton shows potential for placer-type LREE deposit as observed in the elevated LREE content of the stream and beach sediments containing La, Ce, Nd and Sm up to >10,000 ppm. Sequential leaching extraction of REE using eight reagents which selectively dissolved REE with different mode of occurrence (i.e. water soluble, ion-exchangeable salts, Pb-displaceable, acid soluble, Mn-oxide occluded, organically-bound, amorphous Fe-oxide occluded, and crystalline Fe-oxide occluded forms), show that the Daroctan Granite has a potential for ion-adsorption type REE resource with proportion of extracted REE ranging from 53 to 74 % among ΣREE. Formation of secondary REE-bearing minerals, such as cerianite formed on the surface of clay minerals, results in enrichment of REE.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology