Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Novák, M. Articles by Veselovsky, F. Search for Related Content GeoRef GeoRef Citation

Primary oxide minerals in the system WO₃–Nb₂O₅–TiO₂–Fe₂O₃–FeO and their breakdown products from the pegmatite No. 3 at Dolní Bory-Hat, Czech Republic

Milan Novák^1,*, Zdenk Johan², Radek koda¹, Petr ern³, Vladimír rein⁴ and Frantiek Veselovsk⁵

¹ Department of Geological Sciences, Masaryk University, Kotlárská 2, 611 37 Brno, Czech Republic
² BRGM, 3, avenue Claude Guillemin, BP 6009, 45060 Orléans, Cedex 2, France
³ Department of Geological Sciences, University of Manitoba, Winnipeg, R3T 2N2 Manitoba, Canada
⁴ Institute of Rock Structure and Mechanics, Academy of Sciences of Czech Republic, v.v.i. V Holeovickách 41, 182 09 Praha 8, Czech Republic
⁵ Czech Geological Survey, Klárov 3, 118 21 Praha, Czech Republic

^* Corresponding author, e-mail: mnovak{at}sci.muni.cz

Symmetrically zoned barren pegmatite dike No. 3 cuts granulite at Dolní Bory-Hat in the Czech Republic. It contains minor to accessory biotite, schorl, muscovite, sekaninaite, andalusite, diaspore, apatite and several rare accessory minerals. Black, tabular crystals of ferberite-"wolframoixiolite" occur almost exclusively in an andalusite-diaspore aggregate with sequential accumulations of late pyrophyllite, kaolinite, and muscoviteillite. Complex zoning of the individual crystals of ferberite-"wolframoixiolite" shows primary, coarse oscillatory zoning consisting of narrow zones of ferberite (5.01–6.00 apfu W) and dominant niobian ferberite I (5.00–3.01 apfu W) to "wolframoixiolite" (< 3.00 apfu W), the latter two phases with very fine oscillatory internal zoning. The primary phases were locally replaced by a fine-grained mixture of ferberite to niobian ferberite II > tungstenian-titanian ferrocolumbite >> niobian and tungstenian rutile > ScPO₄ phase > scheelite. Primary and secondary phases are characterized by large variation in W/(W+Nb+Ta) and calculated Fe²⁺/(Fe²⁺+Fe³⁺) but low and almost constant Mn/(Mn+Fe_tot) and Ta/(Ta+Nb). The contents of P, U, Ti, Zr, Si, Sc, Al and Ca vary from negligible in ferberite to minor in "wolframoixiolite" and its U-rich variety: UO₂ 2.71 (19.82 U-rich variety), P₂O₅ 0.30, TiO₂ 4.81, ZrO₂ 2.84, SiO₂ 0.93, Sc₂O₃ 4.36, Al₂O₃ 0.87, CaO 1.02 (all in wt.%). The combined exchange vector: (R³⁺)₈(R⁴⁺)₆(R⁵⁺)₁₆(R²⁺)_–13(R⁶⁺)_–17, where R²⁺ = Fe²⁺ > Mn, Ca; R³⁺ = Fe³⁺ > Sc, Al; R⁴⁺ = Ti > Zr, Si, U; R⁵⁺ = Nb > Ta, P; R⁶⁺ = W, seems to be the best expression of the actual substitution mechanism from ferberite to "wolframoixiolite" with the theoretical end composition (R²⁺_1.1R³⁺_3.4R⁴⁺_1.9R⁵⁺_5.6)₁₂O₂₄. The homovalent substitutions expressed by the exchange vectors: Fe²⁺Mn_–1, TaNb_–1 and ScFe³⁺_–1 are rather negligible. Formation of primary W, Nb, Fe-oxide minerals is closely related to the assemblage andalusite+diaspore, formed at T < ~ 400 °C for P = 2 kbar. The breakdown process probably proceeded at slightly lower temperatures of about 350–300 °C. Textural relations of the breakdown products characterized by the total absence of any depleted primary phase in the BSE images indicate that the secondary assemblage did not originated from an exsolution-type process. Complete recrystallization and reconstitution of the primary minerals seems to be responsible for its origin. The chemical composition of the primary niobian ferberite to "wolframoixiolite" from Dolní Bory-Hat is distinct from all other W-rich Nb,Ta-oxide minerals described to date except niobian wolframite from the granitic pegmatite at Nuaparra, Mozambique.

Key-words: tungsten, ferberite, "wolframoixiolite", columbite, rutile, electron microprobe, miscibility, subsolidus breakdown, granitic pegmatite, Dolní Bory-Hat, Czech Republic.