Chemical composition and evolution of tourmaline-supergroup minerals from the Sb hydrothermal veins in Rožňava area, Western Carpathians, Slovakia

Abstract

Tourmaline-supergroup minerals are common gangue minerals in Sb-hydrothermal veins on Betliar – Strakova, Cucma – Gabriela and Rožňava – Peter-Pavol vein deposits in the Rožňava area, Slovakia. Tourmaline-supergroup minerals form relatively large prismatic to radial aggregates of parallel black to greyish-black crystals. Tourmaline-supergroup minerals from Betliar – Strakova and Rožňava – Peter-Pavol are almost homogeneous with intermediate schorl-dravite composition. Cucma – Gabriela tourmaline have distinct zoning with massive core of the schorlitic-to-feruvitic shifting to schorlitic-to-dravitic composition, and dravitic to magnesio-foititic rim. The tourmaline composition is influenced by two main substitutions, namely Ca(Mg,Fe)Na−1Al−1 and X □AlNa−1(Mg,Fe)−1. Betliar – Strakova and Rožňava – Peter-Pavol tourmaline-supergroup minerals exhibit only small extents of the X □AlNa−1(Mg,Fe)−1 substitution. This substitution shifts the composition to magnesio-foitite in Cucma – Gabriela tourmaline. The decrease of Al in the core of Cucma – Gabriela tourmaline crystals is caused by extensive Ca(Mg,Fe)Na−1Al−1 substitution. The unit-cell dimensions of all investigated tourmaline-supergroup minerals indicate an octahedral disorder with the Z (Fe3++Mg) proportion calculated from empirical equations varying between 0.85 and 0.87 apfu (atoms per formula unit). Based on Mossbauer spectra, the Z Fe3+ content varied between 0.25 apfu in Betliar – Strakova tourmaline and 0.45 apfu in Cucma – Gabriela sample. Based on Fe/(Fe + Mg) ratio, Betliar – Strakova tourmaline is slightly enriched in Fe compared to Rožňava – Peter-Pavol, suggesting the impact of the host-rock composition; first are grown in Fe-richer acidic metarhyolitic rocks, latter in metapelites. In Cucma – Gabriela, the variations in Fe/(Fe + Mg) are very likely reflecting the change in fluid composition. Magnesio-foitite is the product of second-stage crystallization forming rims and crack fills. The relatively low Fe3+/Fe2+ ratio suggests only minor proportion of meteoric fluids forming tourmaline.