Amphibolitic wallrocks around the Tanco rare-element pegmatite record three episodes of metasomatic alteration by pegmatite-derived fluids. The apparent sequence of alteration was (1) B (± Li) metasomatism (tourmalinization), (2) K-Rb-Cs-F (± Li) metasomatism (formation of metasomatic biotite), and (3) propylitic alteration (Hbl + Pl → Ep + Chl + Ttn + Cal + clay) with concomitant influx of Li and CO2. Holmquistite is present in all three metasomatic assemblages and served as the primary sink for Li. Wallrock metasomatism was preceded by an episode of textural recrystallization at lower-amphibolite-facies conditions. Recrystallization may have been caused by heat loss from the pegmatite, but is more probably an artifact of pre-emplacement metamorphism. All metasomatic alteration that is clearly pegmatite-related took place at greenschist-facies conditions (T ≤ 500-550 °C, P ≤ 300 MPa). The B-rich fluid that caused tourmalinization of wallrocks was generated by the crystallization of albitites within the pegmatite. Fluids responsible for K-Rb-Cs-F (± Li) metasomatism of wall rocks (at T = 450-460 °C) were generated by (1) liberation of acidic, F-rich fluid by the late crystallization of albitites and (2) consequent sericitization of pegmatitic microcline and pollucite. Log (fHF/fH2O) of fluids in equilibrium with exomorphic biotite at the pegmatite contact was -4.9 at T = 400-500 °C and approached negative infinity within 4 m from the pegmatite. Internal pegmatite fluids equilibrated with primary amblygonite-montebrasite (Xamb = 0.40) at 600 °C record log (fHF/fH2O) = -4.8. Propylitic alteration, the most pervasive type of alteration around Tanco, occurred throughout the entire episode of wallrock metasomatism, but became dominant when solute-depleted pegmatite fluids cooled to T ≤ 420 °C.
Through the documentation of the modal abundance and crystal chemistry of metasomatic and retrograde phases, the total masses of B, F, and alkalis lost from the pegmatite have been estimated. The calculated masses represent significant proportions of these components in the original pegmatite magma.