Petrogenesis, Geochemistry and REE Mineralization of the Ilmenite-series Granitoids in Northern Palawan, Philippines

Abstract:

A sliver of the Palawan Continental Block associated with the Late Yanshanian magmatism was recently recognized in northern Palawan, Philippines. This intrusive unit is the Late Cretaceous Daroctan granites. This granitic intrusive body, as well as the Middle Miocene Kapoas granitoids, were studied for their petrogenesis. The REE mineralization was also examined because of the I-type and ilmenite-series character of the granitoids and the close spatial and temporal relationship with the granitic units in southern and eastern China, which are viable sources for ion-adsorption type REE mineralization.

The granitic intrusive rocks being investigated in this study are composed of biotite granodiorite and biotite granites. Both intrusive units are characterized by high-K calc-alkaline, intermediate I and S-types, and peraluminous. Despite the difference in ages, the geochemistry of both intrusive units shows distinct similarities which suggest same source materials, the partial melting of middle to lower continental crust. The granitoids in the Kapoas show higher light rare earth element (LREE) enrichment than that in the Daroctan but the weathered granitic crusts show higher enrichment in the Daroctan granites compared to the parent granitic rocks.

Sequential leaching extraction for REE, Y, and Nb using 8 reagents which selectively dissolved REEs with different mode of occurence (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) show that the Daroctan granites has a potential of ion-adsorption type REE resource with 60 – 73% adsorbed ΣREE. On the other hand, about 18 – 28% adsorbed ΣREE was extracted in the Kapoas granitoid samples. Only monazite occurs as the residual REE-bearing mineral in the Kapoas granitoids, while monazite and xenotime occur in the Daroctan granite. In both samples, mica minerals were altered to vermiculite, which further resulted in a loss of K and addition of P and Cl in the vermiculite grains. Cerianite with Fe and Mn occurs at the edges of the vermiculite in the sample of Daroctan granite.