Acta Musei Moraviae

Desilicated pegmatite with OH-rich fluorapatite and niobian rutile from Utín near Havlíčkův Brod

Mineral assemblages of strongly desilicated granitic pegmatite at Utín near Havlíčkův Brod were examined. The pegmatite crosscuts a small serpentinite body. Prevailing mineral of the pegmatite is a massive chlorite; it encloses relics of plagioclase, fresh phlogopite and accumulations of phlogopite with macroscopically red apatite, colored by thin veinlets of Fe-oxides. Hyalophane (K, Ba-feldspar), scheelite, Nb-rutile, uraninite and zircon were found in microscopic grains. Chlorite occurs in at least three generations, chlorite I and II replaces plagioclase, chlorite III fills fissures in older minerals. Plagioclase is preferentially replaced along twinning lamellae. Phlogopite is a primary mineral; it remained stable during replacement processes. Apatite is a member of fluorapatite-hydroxylapatite (FAp_0.6–0.7HAp_0.3–0.4) solid solution with slightly elevated content of Fe, Mn and Mg, and low content of Y+REE. Nb-rutile contains 7.33 wt. % of Nb₂O₅, 2.03 wt. % Ta₂O₅ and 1.49 wt. % SnO₂. Desilication of the pegmatite is most probably related to the hydrothermal alteration processes in the host serpentinite.