Niobium and tantalum in rocks of the Wiśniówka area (south-central Poland)

This niobium–tantalum study was conducted in the Wiśniówka massif (Holy Cross Mts., south-central Poland) hosting Upper Cambrian hydrothermal stratiform pyrite–goethite–hematite–REE mineralization. The distinctly higher Nb and Ta enrichments were found in organic-rich clayey-silty shales (7.52–28.99 and 0.54–1.84 mg/kg) than in interbedded quartzites/quartzitic sandstones (0.10–2.63 and <0.01–0.12 mg/kg). Bulk chemical analysis revealed the highest determination coefficients (R ²) for the following chemical pairs: Nb/Ta–Rb (0.95, 0.95), Nb/Ta–TiO₂ (0.93, 0.94), Nb/Ta–V (0.94, 0.95), Nb/Ta–K (0.93, 0.93), and Nb/Ta–Al₂O₃ (0.91, 0.92) pointing to the substitution of these two elements for titanium in the TiO₂ polymorph crystal structure and partly to their adsorption on clay minerals and organic matter (distinct affinity to vanadium). The relationship of Nb and Ta to TiO₂ mineral phases was confirmed by electron microprobe analysis exhibiting mean contents of Nb (0.18 wt%) and Ta (0.02 wt%). The Nb/Ta ratio shows a narrow dispersion of values (∼16.0) for carbonaceous clayey-silty shales, tuffites/bentonites, and pyrite FeS₂ regardless of Nb concentrations, representing intermediate values between the upper continental crust (∼13.3) and the Earth’s mantle (∼17.5). This narrow Nb/Ta ratio may also suggest lower fractionation of both metals in hydrothermally-affected sediments.