UNIVERZITA KOMENSKÉHO V BRATISLAVE VZÁCNOPRVKOVÁ ...

Abstract
CHUDÍK, Peter: Rare-element Nb-Ta mineralization in granitic pegmatites and granites of
the Western Carpathians. [Ph.D. thesis]. Comenius University, Bratislava. Faculty of Natural
Sciences. Department of Mineral Deposits. Supervisor: doc. RNDr. Pavel Uher, CSc. FNS
CU. 2010. 135 p.
Nb-Ta phases, especially members of columbite group, tapiolite and wodginite, represent
most characteristic accessory minerals of the LCT-family rare-element granitic pegmatites.
Detailed study of their textural and compositional variations reveals a complex evolution of
their parental rare-element granitic pegmatites and granites.
The Jezuitské Lesy granitic pegmatite exhibits an example of moderately evolved dike of the
beryl-columbite subtype, where several generations of the Nb-Ta minerals portrayed their
magmatic to post-magmatic evolution. All members of columbite group minerals,
ferrotapiolite, ferrowodginite and microlite were identified in the Jezuitské Lesy pegmatite.
Moreover, the columbite-tantalite, ferrotapiolite and ferrowodginite form three generations
with distinct chemical composition and evolution. The first generation of Nb-Ta minerals
(ferrotantalite I, ferrotapiolite I and ferrowodginite I) originated during the crystallization of
coarse-grained quartz-microcline-muscovite zone and it is characterized by the relatively
high Ta/(Ta+Nb) ratio. The second generation of Nb-Ta minerals (ferrocolumbite to
ferrotantalite II, ferrotapiolite II, ferrowodginite II) formed during the crystallization of finegrained
saccharoidal albite zone and it is characterized by the decrease of Ta/(Ta+Nb) ratio.
Low Ta/(Ta+Nb) values in columbite-tantalite II can be explained by crystallization from the
Ta-depleted melt, which was significantly impoverished in Ta, after crystallization of Ta-rich
phases (ferrotapiolite I, ferrowodginite I). The third generation of Nb-Ta minerals
(manganotantalite III, manganocolumbite III, ferrotapiolite III and ferrowodginite III) most
likely formed during the final magmatic stage of the pegmatite consolidation and it is
characterized by increase of Mn/(Mn+Fe) ratio, as a consequence of fluid composition
change, connected with increased activity of F. Microlite represent the youngest Nb-Ta phase
which formed probably during a post-magmatic, hydrotermal overprint of the Jezuitské lesy
granitic pegmatite by Ca, Na and F rich fluids.
Accessory ferrocolumbite to ferrotantalite and ferrotapiolite were identified in pegmatitic
leucogranite near Duchonka, Považský Inovec Mts.. The composition of columbite-tantalite
shows a relatively constant Mn/(Mn+Fe) ratio (0,20-0,27; locally 0,35-0,40), but extreme
range of Ta/(Ta+Nb) ratio (0,18-0,72). Ratios higher than 0,63 plot inside the tantalitetapiolite
miscibility gap. On the basis of experimental works, boundaries of the miscibility
gap are controlled by temperature, oxygen fugacity and phase structural state. An empirically
determined miscibility gap is based only on data from granitic pegmatites. In a case of
Duchonka leucogranite, the temperature is probably the key factor. The Duchonka pegmatitic
leucogranite crystallized most likely at higher temperature compared to rare-element granitic
pegmatites. For that reason, the tantalite-tapiolite miscibility gap has most probably smaller
range at such conditions.
The accessory Nb-Ta-W mineralization represented by ferrocolumbite, manganocolumbite,
tungstenian ixiolite, pyrochlore, microlite, uranmicrolite was identified in the highly
fractionated leucogranite at Dlhá dolina, near Gemerská Poloma. Characteristics feature of
leucogranite from Dlhá dolina is elevated content of W, expressed also by increased content
in Nb-Ta phases. The highest concentration are in tungstenian ixiolite (up to 35 wt.% WO3)
and manganocolumbite (up to 10 wt.% WO3). Another specific feature is decrease of
Mn/(Mn+Fe) ratio with increasing value of Ta/(Ta+Nb) ratio in the outer parts of some
manganocolumbite crystals. This feature can be connected with crystallization of apatite,