Rare earth element deposits are most commonly associated with which rock type?

• A. Alkaline igneous rocks
• B. Sedimentary phosphate rocks
• C. Metamorphic schist
• D. Basalt lava flows

Answer: (A)

Rare earth element deposits are most commonly associated with alkaline igneous rocks. REEs behave as incompatible elements, so they become highly concentrated in late-stage, silica-undersaturated alkaline magmas. These rocks, especially carbonatites and peralkaline granitoids, host minerals such as bastnäsite, monazite, and xenotime that store REEs in economically workable amounts. In contrast, basaltic lava flows are mafic and don’t typically concentrate REEs, metamorphic schist is not a common REE host, and while phosphate rocks can contain some REEs, they are not the primary, most productive source. Thus alkaline igneous rocks best explain the common association with REE deposits.