Formation of pretulite (ScPO4) by recrystallization of Sc-rich precursors in Dolní Bory pegmatite: Evidence for different mobility of Sc, Y, REE and Zr in hydrothermal conditions

- Scandium can be selectively mobilized by fluids and separated from REE, Y, Zr, Nb.
- Scandium minerals commonly form by hydrothermal alteration of Sc-rich precursors.
- Hydrothermal processes are essential for concentration of Sc.

Three distinct types (A, B and C) of secondary pretulite (ScPO4) were found in a single andalusite-diaspore nodule from the quartz core of the pegmatite dyke No. 3, Dolní Bory, Czech Republic. Primary crystallization of the nodule containing pretulite precursors - Sc, Y, U, P-rich zircon and Sc,W-rich ixiolite - proceeded at T ≤ 410 °C and P ≤ 2 kbar. Subsequent subsolidus hydrothermal alteration at T ≈ 350-300 °C transformed part of the andalusite to pyrophyllite, and triggered complete dissolution of Sc, Y, U, P-rich zircon I. In-situ reprecipitation produced Y, U-depleted zircon II with common inclusions of xenotime II and pretulite A (0.88-0.92 apfu Sc, 0.04-0.07 apfu Zr, < 0.03 apfu Y). During the same process recrystallization of Sc, W-rich ixiolite produced heterogeneous pretulite B (0.21-0.89 apfu Sc, 0.09-0.78 apfu Zr, ≤ 0.07 apfu Y). The origin of pretulite C (almost ideal ScPO4) closely associated with pyrophyllite replacing andalusite and distant from the potential Sc-precursors is explained by the separation of Sc from other HFSE present in the original precursors, which were immobile probably due to absence of suitable strong ligands in the fluids. Saturation of any Sc-dominant mineral (except thortveitite, Sc2Si2O7) is almost never achieved during primary magmatic crystallization due to the extremely high compatibility of Sc; consequently, the processes of hydrothermal alteration of Sc-enriched precursors and the selective mobilization and fractionation of Sc from other HFSE are important for the formation of scandium minerals.