Bulletin Mineralogie Petrologie

Chládek Štěpán, Zimák Jiří

Ročník 24, číslo 1 (2016), strana 25-32

beryl-columbite pegmatite, Nb-Ta minerals, columbite group, rutile, aeschynite-(Ce), pyrochlore supergroup, pegmatite evolution, Maršíkov, Czech Republic

Accessory Nb-Ta-Ti-REE oxide minerals were detected in recently described granitic pegmatite of the beryl-columbite subtype at Maršíkov – Lysá hora, northern Moravia, Czech Republic. Primary magmatic mineral assemblage is represented by columbite-group minerals, rutile and aeschynite-group minerals in the blocky and albite (cleavenlandite) textural-paragenetic units. Columbite-(Fe) is the most common Nb-Ta mineral whereas rare columbite-(Mn) occurs only in the cleavenlandite unit. Columbite-group minerals shows a lower degree of fractionation; Ta↔Nb and Mn↔Fe are dominant major element substitution mechanisms, minor Ti enters into the columbite lattice via rutile substitution: 3Ti⁴⁺ ↔ (Fe,Mn)²⁺ + 2(Nb,Ta)⁵⁺. Very rare aeschynite-(Ce) occurs in association with rutile and in Nb-rich rutile. The magmatic Nb-Ta-Ti assemblage underwent a significant transformation during post-magmatic to hydrothermal stage of the pegmatite evolution. Breakdown products of primary precursors are minerals of pyrochlore supergroup, they occur on the tiny cracks of the zonal columbite-(Fe) and rutile. Secondary phases of betafite and pyrochlore group-minerals show significant uranium enrichment. High contents of Si and Al in the pyrochlore-supergroup minerals are consistent with their hydrothermal-metasomatic origin and crystallization of late alpine-type alpine paragenesis on cracks in the pegmatite body.

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