Microanalytical investigation of K-rich fenites from the Catalão II alkaline‑carbonatite complex in Central Brazil: Implications for ore-forming processes within the world's largest niobium province

Alkaline‑carbonatite complexes are the main source of critical raw materials such as niobium (Nb) and rare earth elements (REE), which concentrate through a combination of magmatic and carbohydrothermal processes. These systems typically occur in close spatial connection with altered country rocks resulting from metasomatic alteration by exsolved K-Na-rich fluids, a process known as fenitization. Thus, the association between Nb-REE-rich carbonatites and fenites provides a unique opportunity for the investigation of carbohydrothermal alteration processes leading to critical metal enrichment. In this study, we focused on K-rich fenites associated with a shallow dike swarm system in the Boa Vista niobium deposit, the second-largest global producer of Nb, which is hosted within the Catalão II alkaline‑carbonatite complex in Central Brazil. We used a combination of micro-analytical techniques including EBSD, EMPA, and μ-EDXRF to unravel the complex micro-textural features of the fenites. Our data suggest that alkaline fluids exsolved vigorously from the carbonatite melts upon dike emplacement and pervasively metasomatized the country rocks, forming a melanocratic proximal fenite (phlogopitite), and distal fine-grained orthoclase-phlogopite-calcite fenites. Fluid alteration mobilized soluble cations (K, Fe, Mg, Ba, and Sr) plus S, CO2 and OH−, whereas Nb and REE were retained in the carbonatite dikes. The consistent dike-orthogonal orientation of metasomatic phlogopite and orthoclase grains in the proximal fenite, determined by EBSD, suggests that newly formed metasomatic minerals precipitated from the fenitizing fluids along the flow path. These orientations differ from the isotropic textures defined by primary (magmatic) minerals like pyrochlore, tetraferriphlogopite, dolomite, and calcite. However, and despite the microstructural differences between magmatic and carbohydrothermal phlogopite, their mineral chemistry is similar, suggesting that fenites are almost synchronous to the emplacement of carbonatites. These data indicate that fenites provide a rich archive of alkali-rich fluid infiltration around Nb-REE-rich carbonatite intrusions.