Tourmalines from the siderite-quartz-sulphide hydrothermal veins, Gemeric unit, western Carpathians, Slovakia: crystal chemistry and evolution

Abstract

Tourmaline is an important gangue mineral in a large number of Cretaceous siderite-quartz-sulphide hydrothermal veins in the Gemeric Unit, Slovak Ore Mountains, Slovakia, such as Dobsina, Vlachovo, Rožňavske Bystre, Hnilcik, Rakovnica, Novoveska Huta, Gretla, Rudňany, and Bindt. In this study we combine by electron microprobe analysis, powder X-ray diffraction, Mossbauer and optical emission spectroscopy to determine the range of tourmaline compositions in the deposits and constrain the mechanisms of its precipitation. Selected samples from the mentioned deposits belong mostly to the alkali group, schorl to dravite series, rarely dominant X-site vacant foititic tourmaline (Vlachovo and Bindt) and oxy-dravite compositions (Hnilcik) were detected. Rim zones of some schorlitic tourmalines show high concentrations of Ti (up to 2.35 wt.% TiO2, 0.30 apfu; Rožňavske Bystre). The chemical composition is mostly controlled by alkali-deficient X □AlNa−1(Mg,Fe2+)−1 and proton-deficient AlO(Mg,Fe2+)−1(OH)−1 substitutions. Titanium is incorporated into the structure by Y Ti Y (Mg,Fe) Y Al−2, Y Ti Z Mg Y Al−1 Z Al−1, Y TiO( Y AlOH), and X Ca Y Ti Z MgO2 X □−1 Y,Z Al−2(OH)−2 substitutions. Along trace elements, Sr and V attain concentrations of ~80–450 and ~70–320 ppm, respectively. The unit-cell parameter a varies between 15.960 and 15.985 A; variations in c are larger, between 7.177 and 7.236 A indicating the presence of Fe3+ and Mg2+ at Z site. Mossbauer spectroscopy has shown variable Fe3+ proportions (0.17–0.55 apfu) in all samples. The gathered dataset suggests some qualitative considerations on the mechanisms controlling tourmaline compositions at the regional scale. The highest Fe3+ concentrations occur in samples from Rudňany and Gretla in the external part of Gemeric unit, suggesting higher oxidation during longer transport of fluids. We propose that the determined XFe in the samples are correlated with the compositions of the host rocks, as schorlitic to foititic tourmalines occur in veins located in the meta-rhyolites host, and tourmalines with the highest Mg contents occur in metabasalts.