Rare earth element (REE) enriched granitic pegmatites associated with alkaline granite complex of southern India: Source characteristics, enrichment mechanisms, and insights into potential ion-adsorption REE deposits

Rare-earth elements (REE) form indispensable components of daily life, as they are essential constituents of the modern high-technology applications, including clean energy, high-tech electronics, and ultimately to achieve the sustainable development goals of the United Nations. With a growth rate of approximately 10–15% per year, the demand for REE has been increased significantly. However, production and supply chains of REEs are very limited, especially due to the rare occurrences and/or discoveries of REE-enriched deposits. It also invokes an alarming situation, since the REE industry is largely controlled by a small number of countries across the globe, with one holding the dominant position in both mining and processing. Consequently, there is an increasing interest in the REE exploration studies across the globe for finding out new potential sources.

Granitic pegmatites are considered as important sources of rare metals, such as REEs, and other high-field strength elements (HSFE) such as U, Th, Y, Zr, Hf, Nb, Ta and large-ion lithophile element (LILE) such as Li, Rb, and Cs. Here, we report the occurrence of rare-metal granitic pegmatites associated with alkaline granite complex of Munnar in the southern Indian shield. The mineralized pegmatites are intruded along and across the shear planes of granites. The pegmatites are composed of quartz, K-feldspar, plagioclase, biotite and muscovite. Several veins also contain magnetite, pyrite and pyrrhotite. They are characterized by high ΣREEs contents ranging from 1318 ppm to 7682 (avge. 3992 ppm). The chondrite-normalized REE patterns of the pegmatites are characterized by a strong enrichment of LREE over HREE, with a (La/Yb)N ratio between 42 and 1000, with characteristic negative Eu anomalies. The ΣREE of host granites ranges between118 and 6502 ppm. The REE patterns of the pegmatites suggest that the pegmatites are formed from LREE enriched melt, generated possibly during the shearing of host granitic rock. During this process the incompatible REEs are concentrated in the melt causing LREE enrichment, which eventually intruded into the lower curst as granitic pegmatites. This indicates enhanced mobility of REE during alteration of host granites. Thus, the study imposes important insights into the sources and enrichment mechanisms of REEs in the parent rocks as well as their remobilization during alteration processes forming ion-adsorption REE deposits in their weathered crusts.