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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)

Chladek S., Breiter K., Uher P.

Geology, Geophysics and Environment

2016

Vol. 42, no. 1

61--62

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) Ti 3 (Fe,Mn) 2+ −1 (Nb,Ta) −2 , (ii) Ti 2 Fe 3+ −1 (Nb,Ta) −1 , (iii) (Nb,Ta) 4 Fe 2+ −1 W −3 . Moreover, a part of minor cations can enter via: (iv) (Fe,Mn) 2+ 1 W 1 (Fe,Sc) 3+ −1 (Nb,Ta) −1 into wolframite lattice, (v) W 1 (Ti,Sn) 1 (Nb,Ta) −2 , (vi) (Sc,Fe) 3+ 3 (Fe,Mn) 2+ −2 (Nb,Ta) −1 , and (vii) W 2 Sc 3+ 1 (Nb,Ta) −3 into columbite lattice. Calculated Fe 3+ can be introduced into rutile lattice predominantly via mechanism (ii), while via (iv) into wolframite lattice and together with Sc 3+ via (vi) into columbite lattice. The last mechanism results in charge imbalance of A and B positions of columbite lattice entering R 3+ cations to. The distinctly varying calculated Fe 3+ values can refer to changing f O 2 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.

Calibration of Portable XRF Spectrometer in Sn-W Ore-Bearing Granites: Application in the Cínovec Deposit (Erzgebirge/Krušné Hory Mts., Czech Republic)

Knesl I., Jandova T., Rambousek P., Breiter K.

Inżynieria Mineralna

2015

R. 16, nr 2

67--72

This study evaluates the suitability of portable X-Ray fluorescence spectrometry (PXRF) in the environment of granites and greisens with Sn-W and secondary mineralization of other important elements (Rb,Y, La, Ce, Nb, Ta and Tl). International certificated reference materials (CRM), historical data from the sixties based on the borehole CS-1, current analyses in accredited laboratory and data measured by a portable X-ray fluorescence spectrometer (PXRF) with factory settings were evaluated in this study. Powder CRMs and samples from borehole CS-1 were analysed by PXRF. The results of PXRF analysis were then tested for reliability, credibility, accuracy and precision. The accuracy was calculated as a relative percent difference (RPD), regression equations and correlation coefficients. A detailed statistical evaluation of the results proves, that PXRF can be a very useful method for a primary determination of main, secondary and trace elements. Comparison of the conventional methods and the PXRF shows a good correlation of different analytical methods and a good possibility of using the PXRF method for a future selection of samples for subsequent more demanding and expensive conventional and special analyses. A calibration of the PXRF spectrometer to the lithological environment of Sn-W ore-bearing granites and greisens was made by the statistical comparison of the methods.

Niniejsze badania sprawdzają przydatność przenośnego spektrometru fluorescencji rentgenowskiej (ang. skrót PXRF) w warunkach występowania granitów i grejzenów z Sn-W oraz wtórnej mineralizacji innych pierwiastków (Rb, Y, La, Ce, Nb, Ta i Tl). W badaniach uwzględniono międzynarodowe certyfikowane materiały odniesienia (ang. skrót CRM), dane historyczne z lat 60 oparte na odwiertach CS-1, bieżące analizy w akredytowanych laboratoriach i dane uzyskane z przenośnego spektrometru fluorescencji rentgenowskiej (PXRF) z ustawieniami fabrycznymi. CRM proszkowe oraz próbki pobrane z odwiertu CS-1 przeanalizowano w PXRF. Wyniki analizy PXRF zostały następnie poddane testom na niezawodność, wiarygodność, dokładność i precyzję. Dokładność została policzona jako względna różnica procentowa (RPD), równania regresyjne i współczynniki korelacji. Szczegółowa ocena statystyczna wyników dowodzi, że PXRF może być pomocną metodą we wstępnym określaniu zawartości pierwiastków głównych, wtórnych i śladowych. Porównanie metod konwencjonalnych z metodą PXRF pokazało pozytywną korelację różnych metod analitycznych i możliwość stosowania metody PXRF do przyszłych doborów próbek w celu dalszych trudniejszych i droższych analiz konwencjonalnych i specjalistycznych. Kalibracja spektrometru PXRF do warunków występowania litu w granitach i grejzenach zawierających rudę Sn-W została sporządzona została na podstawie statystycznego porównania wymienionych metod.