REE Mineralogical Evolution in a F-Rich Peralkaline System: A Review on the REE Mineralization Associated with the Madeira Sn-Nb-Ta-Cryolite (REE, U, Th, Zr, Li) Deposit (Amazonas, Brazil)

This study is centered on REE distribution in several minerals exhibiting exceptionally rare mineralogical and chemical compositions in the 1.8 Ga Madeira albite-enriched granite (AEG). This is a peralkaline A-type granite and corresponds to the Madeira Sn-Nb-Ta-cryolite (REE, Th, U, Zr, Li) world-class deposit (195 Mt) (Amazonas, Brazil). The REE mineralization ranks among the major deposits associated with alkaline and peralkaline magmatism in intracontinental and extensional anorogenic environments in terms of tonnage and grades. However, with respect to REE paragenesis and structure, it differs from all other known REE deposits. The REE mineralization (xenotime, gagarinite, fluocerite, thorite, pyrochlore, zircon, fluorite, and cryolite) is disseminated and zoned. In addition, in the central part of the deposit, there is a massive hydrothermal cryolite body, whose feasibility for REE extracting has been demonstrated. The evolution of rare earth minerals followed a precise order, with minimal formation of compound minerals and minerals with compositions distinct from their typical occurrences. Small pegmatites very rich in xenotime and gagarinite occur in the core AEG. These characteristics are due to the very high F activity in the magma, buffered by cryolite crystallization, to progressive, undisturbed crystallization from the margins toward the center, and to minimal CO2 activity. The alteration of primary REE minerals by F-rich hydrothermal fluids, the origin of these fluids, and the formation of secondary REE minerals are also discussed.