Subsolidus behavior of niobian rutile from the Písek region, Czech Republic: a model for exsolution in W- and Fe2+>>Fe3+-rich phases

Abstract

Niobian rutile with substantial W content is associated with black tourmaline, beryl, zircon, xenotime-(Y), monazite-(Ce), "písekite", and possibly pseudorutile or pseudobrookite in two cogenetic, beryl-columbite-subtype, granitic pegmatites at Písek and Údraž, southern Bohemia, Czech Republic. Crystals, up to 3×1 cm in size, consist of relics of a primary, phase-homogeneous but oscillatorilly zoned niobian rutile phase and of exsolution intergrowths of depleted niobian rutile with granular titanian-tungstenian ixiolite. Primary niobian rutile has the monorutile structure and shows extensive substitution (Fe, Mn, Mg) 2+ +1 (Nb, Ta) 5+ +2 Ti 4+-3 (up to 26.34 wt. % Nb 2 O 5 and 8.60 wt. % Ta 2 O 5 ; 26 to 37 mol. % columbite component), significant (Fe, Mn, Mg) 2+ +1 W 6+ +1 Ti 4+-2 (up to 3.12 wt. % WO 3) and minor (Fe, Sc) 3+ +1 (Nb, Ta) 5+ +1 Ti 4+-2 substitutions. The values of Fe 2+ /(Fe 2+ + Fe 3+) are high and average at ~ 0.8. Oscillatory zoning is controlled by Nb, Ta and Fe; variations in W are independent and erratic. Depleted niobian rutile still contains 8 to (rarely) 32 mol. % ferrocolumbite component. Individual grains of the exsolved orthorhombic ixiolite are commonly heterogeneous in terms of the W content. They display highly variable degree of the substitution Fe 2+ +1 W 6+ + 3 Nb 5+-4 (up to 49.00 wt. % WO 3), moderate but widespread (Ti, Zr, Hf) 4+ + 3 (Fe, Mn, Mg, Ca) 2+-1 (Nb, Ta) 5+-2 (up to 9.83 wt. % TiO 2 , 1.94 wt. % ZrO 2), and subordinate (Fe, Sc) 3+ + 3 (Fe, Mn, Mg, Ca) 2+-2 (Nb, Ta) 5+-1 (up to 1.51 wt. % Sc 2 O 3). Exsolution reveals the preference of Sn 4+ and Fe 3+ for the rutile structure, whereas Fe 2+ , Mn, Mg, Nb, Ta, Sc, Zr, Hf and W are concentrated in the ixiolite. The results confirm the extent of the W substitution in pegmatitic niobian rutile and its significance for the style of subsolidus breakdown, on par with the magnitude of effects exerted by (Nb > Ta + Fe 2+), (Nb > Ta + Sc + Fe 3+), (Nb > Ta + Fe 2+ /Fe 3+~1), or (Fe/Nb > 0.5).

Contrasting Origins of the Mixed (NYF + LCT) Signature in Granitic Pegmatites, with Examples from the Moldanubian Zone, Czech Republic

“…Common W-bearing minerals in both granite-pegmatite systems open a potential use of W as an indicator of the mixed signature of a pegmatite. However, W is a typical trace to minor element in (Nb,Ta,Ti)-oxide minerals in both NYF (Škoda et al 2006(Škoda et al , Škoda & Novák 2007 and in LCT pegmatites (e.g., Novák & Černý 1998b, 2001, Černý et al 2007, Novák et al 2008 in the Moldanubian Zone. The Třebíč pluton and O'Grady Batholith represent granite-pegmatite systems with a similar origin of parental granite and geochemistry.…”

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part I: accessory Sn, Nb, Ta and Ti minerals in leucogranites, aplites and pegmatites

“…Examples in the French Massif Central include the Ta-Sn-Li mineralization of the Beauvoir granite (Cuney et al 1992;Raimbault et al 1995), the columbite-tantalite minerals of the complex lepidolitesubtype pegmatites of Chèdeville (Raimbault 1998), as well as the cassiterite and wolframite-group minerals of the Vienne granite (Cuney et al 2002;Bouchot et al 2005). The rare-metal deposits linked to the LCT pegmatites occur also in the Bohemian Massif, Czech Republic (Johan and Johan 1994;Černý et al 2007;Breiter et al 2007) and the tin and tungsten deposits are known from the Cornwall Massif, south-eastern England (Charoy 1986;Willis-Richards and Jackson 1989). In the Iberian Massif, the pegmatite fields of Lalín-Forcarei (Fuertes-Fuentes and Martín-Izard 1998) and Fregeneda-Almendra (Roda-Robles 1993;Roda-Robles et al 2007;Vieira et al 2011) are some of the best-known rare-metal deposits associated with leucogranites in northern and western Spain.…”

Mineralogical constraints on Nb–REE mineralization of the Zhujiayuan Nb (−REE) deposit in the North Daba Mountain, South Qinling, China

“…In addition, studies have indicated that Nb is highly compatible with rutile during magma crystallization (Brenan, Shaw, Phinney, & Ryerson, ; Linnen & Keppler, ). Here, Fe 2+ + 2(Nb, V) 5+ = 3Ti 4+ has been suggested as one of the main replacement mechanisms for the incorporation of Nb in rutile (Černý, Novak, Chapman, & Ferreira, ). The rutile in the Zhujiayuan deposit is common, such that most rutile deriving from magma genesis contains a certain amount of Nb 2 O 5 (0.75–1.79 wt%) (Supplementary Table S3).…”