Abstract
In 1961–63 the Czechoslovakian Geological Survey drilled a 1596 m deep borehole in the Sn-W-mineralized Cinovec (Zinnwald) granite cupola. The hole traversed zinnwaldite granite (ZG) to 730 m, then protolithionite granite (PG). The boundary between the two granites is a transition zone (TZ) about 10 m thick. The oxides of Nb, Ta and Ti, present in accessory amounts, are columbite, ilmenorutile, rutile and pyrochlore. The columbite occurs in both granites, but in the PG only below 1558 m depth. Its crystals are strongly zoned, the zoning representing variations in Nb/(Nb + Ta) on the one hand, and non-uniform distribution of W on the other. The columbite in the TZ is strongly enriched in W, up to 32.6 wt% WO3. The columbites with W ΣM4+, tungsten is introduced by the substitution W6+ + M4+ → 2(Nb, Ta)5+, but also through the appearance of Fe3+ in the B site according to the replacement 2W6+ + Fe3+ → 3(Nb, Ta)5+. The ratio Fe/(Fe + Mn + Ca) increases with depth, and Nb/(Nb + Ta) is higher in the PG.
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