Burbankite-bearing ferrocarbonatite and its alteration to LREE ore at Swartbooisdrift, NW Namibia

Based on compiled mineralogical and geochemical data, we show that burbankite-rich ferrocarbonatite is altered to calcite–bastnäsite–monazite ore at Swartbooisdrift, NW Namibia. The magmatic carbonatite assemblage comprises ankerite, magnetite, pyrochlore, fluorapatite, and locally abundant burbankite. All of these minerals can be enriched in layers that define a magmatic flow banding. Burbankite formation can be restrained to ~ 650 °C. Late-magmatic to hydrothermal alteration proceeds in situ from burbankite to carbocernaite (I to II) ± calcite ± barite ± strontianite, then cordylite-(Ce), monazite-(Ce) and finally ancylite-(Ce). Pyrochlore alters to columbite-(Fe) and fluorapatite to secondary monazite-(Ce). The calcite–bastnäsite–monazite ore forms below ~ 460 °C as veins, lenses and bands confined to the main ferrocarbonatite dikes. Its pinkish color results from minor hematite. Mass-balance calculations indicate fluid-assisted removal of Sr, Na, Mg, Ba and addition of Ca, P, Fe, Mn and LREE with modest volume loss to reach ore grades. C-O stable isotope analyses for the ferrocarbonatite, burbankite and LREE ore show an increasingly heavier δ18O signature linked to the hydrothermal alteration with an interpreted rising crustal influence and decreasing temperatures. Additionally, in-situ trace element analysis further provides arguments for the hydrothermal character of LREE redistribution through fluid-sensitive element ratios (Th/U; Y/Ho; Pb/Th). Our data suggest that formation of the Swartbooisdrift LREE ore results from instability and reaction of early Na-carbonates, apatite, and pyrochlore with progressive fluid evolution (decreasing Na and increasing oxygen fugacity with decreasing T), finally leading to a transformation of burbankite-rich zones of ferrocarbonatite into pink ore along carbonatite-hosted structures. This establishes a coherent magmatic–hydrothermal pathway from primary burbankite to economic LREE mineralization.