Geneze fosfátu v jeseneckých vápencích konicko-mladečského pruhu (drahanský faciální vývoj moravskoslezského paleozoika na Moravě)

Abstract

Jesenec Limestone of the Famennian to Tournaisian age belongs to the Drahany Facies Domain (Development) of the Moravian-Silesian Paleozoic sedimentary sequence. Massive micritic and biosparitic limestone layers occasionally alternate with sandy limestone and limestone with an admixture of pyroclastic material.Several mm to 25 mm angular and rounded fragments of black phosphorites were found in the investigated limestones crop out in the Konice-Mladeč Belt (sediments and volcanites) north of the villages of Dzbel and Jesenec. We use an optical microscope and electron probe microanalysis to classify and interpret these phosphorites. Based on shape, phosphorite particles we divided into two principal groups: angular intraclasts and rounded concretions. Fine-grained brecciated intraclasts are fragments of phosphorite layers formed on the surface of carbonate sediment (probably near the volcanoes) in a shallow-water environment. The destruction of these phosphate crusts may have been due to waves associated with storm activity or volcanic eruptions.Fine-grained phosphorite concretions comprise predominantly dark-colored subhedral to anhedral fluorapatite (0.07–0.14 wt. % FeO; up to 0.04 wt. % MnO) with interspersed calcite (0.06–0.14 hm. % FeO; up to 0.43 wt. % MgO) and quartz crystals. Apatite spherulites up to 0.2 mm in diameter are locally visible. Rare present pyrite crystals and their relics indicate polyphase evolution, including partial replacement of pyrite by apatite. We assume the crystallization of pyrite in an anoxic environment as the first phase of the process. The low pH of pore waters due to pyrite oxidation led to the dissolution of the biogenic phosphate and its subsequent precipitation in the form of concretions.