Manganese pebbles from Hochschartehöhlesystem (the Hoher Göll Massif, Austria): insight into potential genesis and provenance

Abstract

Heavy, black manganese pebbles have been found in the clastic sediments of the Hochschartehöhlensystem (the Northern Calcareous Alps). Six selected pebbles were subjected to X-ray diffraction, optical microscopy and optical and electron microscopy analysis. The results reveal that the main component of the pebbles is manganese silicate, braunite, Mn2+Mn3+6(SiO4)O8. Braunite is a mineral formed at elevated temperatures, mainly through hydrothermal, metamorphic or diagenetic processes. This means that the manganese pebbles were formed outside the caves. However, manganese rock was not found in situ on the surface of the Hoher Göll Massif. This probably indicates that their origin is from eroded parts of Mesozoic rocks. The first studies of pebbles and their mineralogy, by analogy with contemporary marine sediments, indicate that their genesis is related to spreading zones and accompanies hydrothermal vents. The analysed material highlights two important issues: (1) the manganese pebbles are significant arguments for the occurrence of hydrothermal vents in the Northern Calcareous Alps; and (2) the importance of cave sediments studies, which provide relevant evidence for palaeogeographic reconstruction.