NIOBIUM-TANTALUM OXIDE MINERALS FROM COMPLEX GRANITIC PEGMATITES IN THE MOLDANUBICUM, CZECH REPUBLIC : PRIMARY VERSUS SECONDARY COMPOSITIONAL TRENDS

Abstract

Compositional trends of Nb, Ta-oxide minerals in four complex granitic pegmatites (lepidolite and petalite subtypes) in the Moldanubicum, Czech Republic, covering primary (magmatic) and secondary (hydrothermal replacement) stages of crystallization, were investigated. The primary stage is characterized by ferrocolumbite to manganocolumbite, followed after Fe- and Mn-depletion by Ca- or Sb-rich minerals: microlite (Nova Ves), rynersonite (Chvalovice) and stibiotantalite (Dobra Voda, Lastovicky). Compositional paths in columbite are similar to those of lepidolite pegmatites; the other primary Nb, Ta-oxide minerals exhibit Ta/(Ta+Nb) values significantly higher relative to columbite. Decreasing Nb/Ta and the cation sequence Fe-Mn-(Sb,Ca) are typical of the primary stage. The secondary stage displays a broad spectrum of mainly fracture-filling secondary phases, such as diverse microlite-group minerals, cesstibtantite, manganotantalite, ferrotantalite and ferrotapiolite, which replace stibiotantalite, microlite and rynersonite; columbite in outer pegmatite units remains unaffected. The Ta/(Ta+Nb) values of the secondary phases are comparable to those of their precursors. The general sequence of major A-site cations in hydrothermal Nb, Ta-oxide minerals, (Sb)-Ca-Mn-Fe, is reversed relative to that of the primary stage, and it may represent a universal pattern of hydrothermal replacement of primary Nb, Ta-oxide minerals in comparable granitic pegmatites. The composition of microlite-type minerals and textural relations indicate that the secondary hydrothermal stage includes a broad range of P-T-X conditions from early subsolidus replacement at approximately 500-350 degrees C, approximately 2.5-2.0 kbar (Nova Ves, Dobra Voda) to near-surface weathering at