Restrictions on Niobium enrichment by alteration of Niobium-rich biotite in pure water, acid, alkaline and fluoride-bearing solutions at 200 MPa and 300–600 °C

Biotite plays an important role in controlling Nb/Ta fractionation during subduction-related, dehydration reactions. It is probable that Nb–Ta-bearing minerals and Nb–Ta oxides could form from Nb–Ta-rich biotite as a result of fluid-aided alteration. However, the mechanism by how this alteration occurs is not well understood. In this study, a Nb-rich, Li-bearing biotite (up to 1232 ppm Nb) with REE-bearing inclusions (e.g., bastnäsite) is altered using pure H2O, as well as HCl-, NaOH-, NaF-, KF-, and MnF2-bearing solutions in a rapid-quench, cold-seal pressure vessel at 200 MPa, and 300 to 600 °C. A set of Nb-Ta-Ti alteration products (titanomagnetite, ilmenite, Nb-rutile, fergusonite, and aeschynite) formed successively over a series of temperatures, i.e., Nb-rutile + aeschynite formed first at 600 °C, followed by fergusonite + titanomagnetite ± ilmenite at 400 to 500 °C, and lastly titanomagnetite at 300 °C. The results from this study indicate that Nb–Ta-bearing oxides (titanomagnetite and ilmenite) and Nb–Ta oxides (Nb-rutile, fergusonite and aeschynite) can form via metasomatic reactions between Nb-rich, Li-bearing biotite and a variety of low pH, high pH, and F-bearing fluids. The formation of Nb-rutile with much higher Nb/Ta ratios (76.60) compared with Nb-rich, Li-bearing biotite (Nb/Ta: 24.79 in average), provides new insight into explaining the “missing high Nb/Ta reservoir”. Formation of Nb-rutile (Nb2O5: up to 31.71 wt%), fergusonite (Nb2O5: up to 42.62 wt%), and aeschynite (Nb2O5: up to 36.48 wt%) from Nb-bearing biotite indicates that metasomatic processes can lead to Nb mineralization in the form of oxides.