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Composition of Nb-Ta-Ti-Sn-W oxide minerals - indicators of magmatic to hydrothermal evolution of the Cínovec granite intrusion and Sn-W deposit (Czech Republic).

Štěpán Chládek, Karel Breiter, Pavel Uher

Vol. 42, 1 (2016), s. 61-62

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The Cínovec (Zinnwald) Sn-W ore deposit is genetically linked to intrusion of late Variscan, highly fractionated granite which expresses the latest evolutionary stage of a volcano-plutonic system of the Teplice caldera. Whole intrusion is relatively highly fractionated and from bottom (~1500 m) to top part of cupola-shaped deposit is obviously following succession from biotite (annite) granodiorite-granite-zinnwaldite granite, with the partly greisenized uppermost part at 300–500 m depth (Štemprok 1965, 1971). In 1961−63 the Czechoslovakian Geological Survey (CGS) drilled a 1596 m deep borehole in the Sn-W-mineralized Cínovec granite cupola (Štemprok 1965, Štemprok & Šulcek 1969). All studied rock types include W- and Sn-bearing minerals (wolframite series, scheelite and cassiterite) and disseminated accessory Nb-Ta-Ti-W-Sn minerals (Štemprok & Šulcek 1969, Štemprok 1989, Johan and Johan 1994) which were obtained from the collection of CGS in Prague and studied by BSE and electron microprobe. They crystallized in following succession: rutile + columbite + cassiterite (biotite granodiorite) → rutile + columbite + W-rich ixiolite + cassiterite + scheelite in zinnwaldite granite. Textural relationships of these Nb-Ta-Ti-Sn-W minerals indicate predominantly their magmatic origin and part of them (e.g., cassiterite and columbite) show minor post-magmatic alteration phenomena like distinctly inhomogeneous mixtures of secondary pyrochlore-group minerals (“oxykenopyrochlore” and oxycalciopyrochlore). Nb/Ta and Fe/Mn fractionation trends led to characteristic Mn and Ta enrichment from bottom (biotite granite) to uppermost zinnwaldite granite, especially in columbite-group minerals. While Nb/Ta fractionation is limitedly applied, effective Fe/Mn fractionation led to significant Mn – enrichment of late-magmatic phases [columbite-(Mn) and W-rich ixiolite]. Post-magmatic to hydrothermal metasomatic fluids caused partial greisenization of the granites and this stage is represented by latest columbite + scheelite + cassiterite + wolframite assemblage. The last two minerals were objects of extensive mining in the past. Although the hydrothermal system was enriched in F and Li (presence of topaz and zinnwaldite), there are only relatively limited Nb/Ta and Fe/Mn fractionations in post-magmatic columbite. Similarly to primary fractionation, both Nb/Ta and Fe/Mn ones take place and overlap characteristic primary Mn-enrichment. Effective Mn-redistribution is predominantly controlled by crystallization of Mn-dominant wolframite like hübnerite in the hydrothermal stage. Scandium is typical rare element in primary (magmatic) and secondary (hydrothermal) mineral assemblage. While primary Sc-fractionation continues the ongoing Sc-enrichment mostly in columbite to uppermost parts of intrusion, the hydrothermal Sc-redistribution is controlled by crystallization of main ore mineral – wolframite, which consumed a major part of scandium. Main substitution mechanisms in rutile-cassiterite-wolframite-columbite assemblage include following heterovalent substitutions: (i) Ti3(Fe,Mn)2+-1(N b,Ta)-2, (ii) Ti2Fe3+-1(N b,Ta)-1, (iii) (Nb,Ta)4Fe2+-1W-3. Moreover, a part of minor cations can enter via: (iv) (Fe,Mn)2+1W1(Fe,Sc)3+-1(N b,Ta)-1 into wolframite lattice, (v) W1(Ti,Sn)1(N b,Ta)-2, (vi) (Sc,Fe)3+3(Fe,Mn)2+-2(N b,Ta)-1, and (vii) W2Sc3+1(N b,Ta)-3 into columbite lattice. Calculated Fe3+ can be introduced into rutile lattice predominantly via mechanism (ii), while via (iv) into wolframite lattice and together with Sc3+ via (vi) into columbite lattice. The last mechanism results in charge imbalance of A and B positions of columbite lattice entering R3+ cations to. The distinctly varying calculated Fe3+ values can refer to changing fO2 during columbite, rutile, W-rich ixiolite and wolframite crystallization. Therefore, the textural and crystallo-chemical features of studied Nb-Ta-Ti-Sn-W oxide minerals in the Cínovec granite cupola reflect a complex geochemical development of this granite system and ore mineralization from primary magmatic stage, through late-magmatic to subsolidus conditions, and ending in distinct hydrothermally – metasomatic overprint of pre-existing phases.