Rare Earth Elements and Bed Types

Abstract

Rare earth element (REE) mineralization occurs in primary and secondary deposits. The primary deposits are volcanic, granitic, pegmatitic, alkaline-ultramafic and carbonatite-associated deposits, while the secondary deposits are alluvial and coastal placer deposits. Rare earth elements associated with volcanism are generally observed as cracks or fine-grained fillings in volcanic rocks that have been subjected to hydrothermal alteration. Volcanic rocks are especially in the form of trachyte, rhyolite and tuff. Granitic rocks contain rare earth elements formed during the crystallization of magma. Granite pegmatites, which are pegmatitic rocks, constitute important sources for lithium, tantalum and other rare earth elements. The rocks associated with alkaline-ultramafic and carbonatites generally contain rare earth element and niobium mineralizations. Carbonatite rocks form important source rocks for minerals such as phylogopite, apatite, vermiculite, provskit, magnetite, apart from rare earth elements. Carbonatite rocks important for the rare earth elements are rocks associated in origin with dolomite, calcite and alkali ultrabasic rocks. Carbonatites are formed when the rare earth element concentration increases by hydrothermal dissolution or chemical weathering. Carbonatite magma may be formed from mantle-derived carbonate-silicate melt by very low-grade crystal fractionation or partial melting of carbonate peridotite in the upper mantle. Secondary deposits are generally formed as a result of weathering and transport of carbonatites, pegmatites and granitic rocks containing rare earth elements. Alluvial (fluvial) placers consist of geological materials carried by rivers. The rare earth elements that have economic importance in the transported material are zircon, monazite, euxenite, brannerite, xenotime, tantalum and niobium minerals. These minerals are found in varying amounts in magnetite, cassiterite, chromite, ilmenite, garnet, tourmaline and rutile, which are common in placer deposits. Coastal placer deposits are widespread formations in Australia, Brazil, USA, and Sierra Leonna and Egypt. In general, rare earth elements are found in sands together with heavy minerals.