Here's a general overview of the formation process of REE-rich deposits in central Sweden:
1. Mantle Melting: The process begins with the partial melting of the Earth's mantle. During mantle melting, certain minerals, such as garnet and pyroxene, retain REE because they are not easily incorporated into the molten material. As a result, REE become enriched in the remaining solid mantle material.
2. Magma Generation and Differentiation: The enriched mantle material eventually undergoes further melting, generating magma. As the magma rises towards the Earth's surface, it undergoes fractional crystallization. REE are incompatible elements, meaning they tend to concentrate in the remaining liquid magma as minerals crystallize and settle out. This process leads to further enrichment of REE in the magma.
3. Emplacement and Crystallization: The REE-enriched magma intrudes into the continental crust and crystallizes to form igneous rocks, such as granites and syenites. During the crystallization process, REE are incorporated into certain minerals, such as allanite, monazite, xenotime, and apatite. These minerals act as REE-bearing phases and form the main REE-rich zones within the igneous rocks.
4. Crustal Contamination and Assimilation: As the magma intrudes into the crust, it may interact with the surrounding country rocks. This can lead to crustal contamination, where the magma assimilates material from the host rocks. The assimilation of crustal material can modify the composition of the magma, potentially enriching it further in REE.
5. Hydrothermal Alteration: After the igneous rocks have crystallized, they may undergo hydrothermal alteration. Hydrothermal fluids, which are hot, water-based solutions, can circulate through the rocks and interact with the REE-bearing minerals. This can result in the redistribution and concentration of REE, forming veins, fractures, and mineralized zones with higher REE concentrations.
6. Weathering and Erosion: Over time, the REE-rich igneous rocks are subjected to weathering and erosion. These processes can break down the rocks and release the REE-bearing minerals, which can be transported and concentrated by surface water and sedimentary processes. This can lead to the formation of secondary REE-rich deposits, such as placer deposits or sedimentary REE-enriched horizons.
The combination of these geological processes, including mantle melting, magmatic differentiation, crustal contamination, hydrothermal alteration, and weathering, results in the formation of REE-rich deposits in central Sweden. These deposits are of great economic importance as they contain valuable rare earth elements used in various high-tech applications, including electronics, magnets, and batteries.
