Acta Musei Moraviae

Mineralogical characteristics of phosphorites from carbonate breccias, Líšeň Formation near the village of Březina (the central part of the Moravian Karst)

The concretions and angular clasts of phosphorites are hosted in the limestone breccia of the Tournaisian to Viséan age (Hostěnice development, Líšeň Formation of the Moravian Karst). We interpreted angular clasts as fragments of concretions. Black-coloured concretions are ellipsoidal, up to 5 cm in diameter. Both textural types of phosphorites consist of crystalline and cryptocrystalline fluorapatite, pyrite crystals and framboids often replaced by goethite, clay minerals (kaolinite), quartz and calcite. Locally are visible allochems (peloids), fragments of macrofossils (e.g. Brachiopods and/or Bivalves) and microfossils (Foraminifera). We use an optical microscope, X-ray diffraction analysis and electron microprobe to study these phosphorites. Fine-grained needle-like subhedral to anhedral fluorapatite crystals with 0.10–0.22 wt. % FeO and up to 0.01 wt. % MnO are a dominant mineral component. The phosphorite concretions were probably formed in a shallow, Famennian carbonate ramp environment (Upper part Macocha Fm.). Apatite recrystallization (colomorphic or banded) textures and precipitation together with pyrite demonstrate formation concretions during early diagenetic anoxic events in unconsolidated host sediments. The period of phosphogenesis was followed by Tournaisian erosion, reworking, and condensation during the tectonic modification sedimentary basin. These processes were responsible for depositing limestone breccia consisting of angular to subangular clasts of the Devonian limestones (Macocha Fm.), black silicates and phosphorites.