Complex magmatic and subsolidus compositional trends of columbite-tantalite in the beryl-columbite Šejby granitic pegmatite, Czech Republic: role of crystal-structural constraints and associated minerals

Abstract

The simply zoned Šejby pegmatite of beryl-columbite subtype is enclosed in migmatitized gneisses-country rocks of the southern part of the Central Moldanubian Plutonic Complex, Bohemian Massif, Czech Republic. The columbite-group minerals (CGM) occur mostly in blocky K-feldspar unit. Primary magmatic CGM [columbite-(Fe)−tantalite-(Fe)], show an interesting compositional and textural evolution. Subhomogeneous cores of columbite-(Fe)−tantalite-(Fe) (A) are surrounded by heterogeneous intermediate zones (B) and (C) with irregular to oscillatory zoning and mutually comparable compositions. Zone (B) contains grains of tapiolite-(Fe) and inclusions of pyrite decomposed into a mixture of secondary jarosite-group minerals. In the next zone (C), irregular Mn-enriched patches occur and in the outermost thin zone (D) CGM exhibit fine oscillatory zoning typically developed at crystal terminations. The late, subsolidus CGM include patchy-zoned Mn-enriched columbite-(Fe) and late columbite-(Fe) veinlets both formed at the expense of their magmatic precursors in the zone C; the veinlets are also present in the zone D. Similar but fine patches and veinlets are developed around jarosite pseudomorphs after pyrite. The compositional evolution of CGM shows a slight increase in Ta/ Nb and Mn/Fe in the zones A and B followed by a reverse trend to elevated Nb and Fe in the zone C and, in particular, in the zone D. Complex subsolidus fluid-melt interactions that generated patches and veinlets caused only minor changes in Ta/Nb and slight Mn-enrichment. Tantalum depletion in veins close to jarosite pseudomorphs after pyrite indicates higher mobility of Ta in acidic low-temperature fluids.

Non-Metamict Aeschynite-(Y), Polycrase-(Y), and Samarskite-(Y) in NYF Pegmatites from Arvogno, Vigezzo Valley (Central Alps, Italy)

“…The zoned patterns of the examined aeschynite-(Y) and polycrase-(Y) were similar to those described in columbite-group minerals [61], although only aeschynite had the same crystal structure. Except for the sample Fiume1 (Figure 3f), they represent typical primary crystallization [61,62]. Irregular oscillatory zoning as overgrowings on polycrase-(Y) (Figure 3f) did not show any corrosive textures, so it was also very likely primary.…”

“…Zoned patterns of Y-REE-U-Nb-Ta-Ti-oxide minerals from Arvogno pegmatites were not overprinted by later alteration processes and/or weathering as is typical in almost all pegmatites [38]. Hence, we examined in detail zoned patterns (Figure 3) which were studied in detail in other Nb-Ta-Ti-oxide minerals-mainly in columbite-group minerals [61,62]. Aeschynite-(Y) from Bosco1,2 had a simple zoned pattern with a rather homogeneous core and regular perfectly parallel oscillatory to irregular oscillatory zoning (Figure 3a,b).…”

Komplexní magmaticko-hydrotermální vývoj columbitu, mikrolitu a fersmitu z beryl-columbitového pegmatitu D6e u Maršíkova, Česká republika

“…Uvedené fenomény v CGM byly popsány na mnoha vzácnoprvkových pegmatitech (např. Lahti 2000;Van Lichtervelde et al 2007;Rao et al 2009;Chudík et al 2011;Alfonso et al 2018;Novák et al 2018), včetně beryl-columbitových pegmatitů studované oblasti Maršíkova (Černý et al 1992;Novák et al 2003), nicméně mechanismus jejich vzniku je stále předmětem diskuse (např. Holten et al 2000;Rudolph 2015).…”

The Distribution of Rare Metals in the LCT Pegmatites from the Giraúl Field, Angola

“…According to [60], this zonation can be due to an incomplete miscibility of the solid solution. In this case the total composition would be located in the immiscibility gap [61] of the CGM quadrilateral, and the minerals in contact should be columbite or tantalite and tapiolite [62].…”