Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)

Abstract Nb-Ta-oxide minerals (NTO) are commonly associated with rare-element pegmatites where they interpreted to precipitate at magmatic magmatic-hydrothermal conditions. Although high-temperature experiments show that phosphorus and other fluxing elements (e.g., Li, B, F) can affect the saturation of NTO in pegmatitic systems, it is still uncertain how occurs natural, flux-rich melts whether crystallization multiple stages during or subsolidus The lithium-cesium-tantalum (LCT) family P-rich Buranga granitic pegmatite (western Rwanda) used as a type locality address this question. mineralization dike two mineralogical units: mineralogically complex phosphatic nodules, albitized parts. In Fe-Nb-Ta-rich rutile columbite-group (CGM) observed, while parts, only CGM found. precipitates stage along early primary phosphates (i.e., F-rich montebrasite, wyllieite, fluorapatite). Conversely, association replacement like bertossaite, after rutile, rosemaryite, trolleite) destabilized due crystal-melt-fluid interactions. textures chemical zoning uneven evolution from core rim related localized alteration phenomena surrounding minerals. This indicates local processes element transfers more important than dike-scale fractionation for pegmatites. restricted availability Fe Mn system, which competition between phosphate oxide minerals, identified main control on dike. precipitation possible when leached remobilized Nb-Ta-bearing residual melt by exsolved fluid.