3:00 PM - 3:15 PM
[HRE16-10] Characteristics and genesis of weathering REE deposits
★Invited Papers
Keywords:rare earths, weathering, deposits, resources, adsorption
The world rare earth deposits can be classified into carbonatite, alkaline-rocks related, placer and ion-adsorption types. Rare earth elements (REE) which are present as trace elements in the mantle and crusts can be geologically condensed by either or both of the formation of alkaline magma and weathering. This talk reviews on the characteristics and genesis of weathering REE deposits consisting of weathered carbonatites, placer and ion-adsorption type deposits.
Weathered carbonatite deposits are characterized by high REE grades. Carbonatites are high in REE (>1%, approximately) and these elements are condensed by dissolution of carbonate minerals during chemical weathering. The dominant REE minerals are primary and secondary phosphate minerals including monazite. The ores are rich in LREE (light REE) and Th and poor in HREE (heavy REE), reflecting compositions of the unweathered carbonatites. REE minerals are generally fine-grained compared to unweathered carbonatites and this may influence the recovery of REE during mineral processing.
Placer deposits are formed by weathering and erosion of igneous and metamorphic rocks and deposition of heavy minerals in alluvium and beach sands. Placer ore minerals are mainly Ti minerals (ilmenite and rutile), zircon and cassiterite. REE-bearing placer deposits contain monazite and lesser amount of xenotime. Although the bulk REE grade is low, the ore consisting of sand and gravels can be mined and heavy minerals are processed easily. This deposit type was the leading world REE producer in the world until 1965. Since monazite is the dominant REE mineral, placer deposits are rich in LREE and Th and poor in HREE. Zircon is the common HREE-bearing mineral in placer deposits, however they are not economically extractable.
Contrasting to the two types of residual deposits mentioned above, ion-adsorption type deposits are characterized by REE adsorbed electrostatically on the surface of clays (not in the crystal structure of minerals) and they are extractable by ion exchange from the REE ores. The ores, called ion-adsorption ores, are formed mainly from granite by chemical weathering. Although these deposits are generally low in REE, they can be rich HREE. Parent granites contain weatherable REE bearing minerals such as fluorocarbonates (e.g., bastnaesite, synchysite) and REE-bearing silicates (e.g., allanite, titanite) and LREE/HREE ratios of these minerals constrain those of ion-adsorption ores. In contrast, parent granite rich in phosphate minerals (e.g., monazite and apatite) tends to form weathered granites with low ion-exchangeable REE because most REE are incorporated in phosphate minerals which resistant to chemical weathering. Kaolinite, halloysite, illite and vermiculite appear to have ion-exchangeable REE. Since the ores and extracted solutions are generally low in Th, ion-adsorption type deposits are poor in Th compared with the other REE deposits.
Weathered carbonatite deposits are characterized by high REE grades. Carbonatites are high in REE (>1%, approximately) and these elements are condensed by dissolution of carbonate minerals during chemical weathering. The dominant REE minerals are primary and secondary phosphate minerals including monazite. The ores are rich in LREE (light REE) and Th and poor in HREE (heavy REE), reflecting compositions of the unweathered carbonatites. REE minerals are generally fine-grained compared to unweathered carbonatites and this may influence the recovery of REE during mineral processing.
Placer deposits are formed by weathering and erosion of igneous and metamorphic rocks and deposition of heavy minerals in alluvium and beach sands. Placer ore minerals are mainly Ti minerals (ilmenite and rutile), zircon and cassiterite. REE-bearing placer deposits contain monazite and lesser amount of xenotime. Although the bulk REE grade is low, the ore consisting of sand and gravels can be mined and heavy minerals are processed easily. This deposit type was the leading world REE producer in the world until 1965. Since monazite is the dominant REE mineral, placer deposits are rich in LREE and Th and poor in HREE. Zircon is the common HREE-bearing mineral in placer deposits, however they are not economically extractable.
Contrasting to the two types of residual deposits mentioned above, ion-adsorption type deposits are characterized by REE adsorbed electrostatically on the surface of clays (not in the crystal structure of minerals) and they are extractable by ion exchange from the REE ores. The ores, called ion-adsorption ores, are formed mainly from granite by chemical weathering. Although these deposits are generally low in REE, they can be rich HREE. Parent granites contain weatherable REE bearing minerals such as fluorocarbonates (e.g., bastnaesite, synchysite) and REE-bearing silicates (e.g., allanite, titanite) and LREE/HREE ratios of these minerals constrain those of ion-adsorption ores. In contrast, parent granite rich in phosphate minerals (e.g., monazite and apatite) tends to form weathered granites with low ion-exchangeable REE because most REE are incorporated in phosphate minerals which resistant to chemical weathering. Kaolinite, halloysite, illite and vermiculite appear to have ion-exchangeable REE. Since the ores and extracted solutions are generally low in Th, ion-adsorption type deposits are poor in Th compared with the other REE deposits.