Acta Musei Moraviae

Petrography, mineralogy, geochemistry and genetic aspects of the Lahn-Dill type deposit of Malý Děd (= Leiterberg) in the Devonian Vrbno Group (Silesicum), Czech Republic

The Lahn-Dill type deposit of Malý Děd located in the Silesian domain is a part of the Devonian metasedimentary and metavolcanic cover rocks (Vrbno Group) that contain about 30 small Lahn-Dill type iron-ore occurrences. The Malý Děd deposit occurs in close association with muscovite-biotite gneiss of the Precambrian Desná Dome. The investigations involved detailed petrographic-petrological studies supported by whole-rock geochemical, mineral (amphibole, biotite, chlorite, carbonate, feldspar, magnetite and ilmenite) and fluid inclusion studies.

The following metamorphosed rocks were observed: chlorite-amphibole(-magnetite-ilmenite) schist originating from submarine intrusions of a basaltic volcanite. The schist consists predominantly of chamosite (chlorite group) and subordinately ferroanthophyllite/grunerite which are arranged in the crenulation-cleavage texture. Other rock constituents, totally not higher than 20 vol. % are magnetite, ilmenite, graphite, albite, quartz and relics of ferrotschermakite. The other rocks are inhomogeneous and consist of layer-arranged mineral assemblages consisting of: (I) magnetite (mostly higher than 50 vol. %), chamosite, ferroanthophyllite/grunerite, rarely biotite and quartz; and (II) ankerite-dolomite solid solutions and calcite. Between (I) and (II) various transitions occur.

The basaltic volcanite is the source of the Lahn-Dill type iron-mineralization. The parental material of the assemblages (I) and (II) and mixtures of (I) + (II) (= III), identical with the ore body, was derived from iron-rich carbonates that precipitated from submarine volcanogenic exhalations. These precipitations were admixed with pyroclastic material of the basaltic volcanic activity. During the Variscan orogeny, the parental material of the ore body and the basaltic volcanite underwent metamorphic changes lying just at the border between low-grade to high-grade greenschist-facies conditions. The temperatures estimated from the system CaCO₃-FeCO₃ lie close to 450 °C, from the chlorite compositional geothermometer lying on average between 403 °C and 419 °C and from early “peak” CO₂-H₂O fluid inclusions leading to ~ 450 °C (and ~ 3.5 kbars), fit well to the established conditions of our petrological investigations. The metamorphic overprint led to the formation and/or recrystallization of amphibole (in the form of felty aggregates consisting of tiny ferroanthophyllite/grunerite crystals), rarely biotite (siderophyllite), chamosite, magnetite, quartz, albite, calcite and ankerite-dolomite solid solutions which occur in varying proportions within the magnetite-rich ore body and the carbonate-free chlorite-amphibole(-magnetite-ilmenite) schist. In addition, multiple late-metamorphic to post-metamorphic fluid events have been distinguished in secondary fluid inclusions, characterized by contrasting compositions (H₂O-CO₂, pure aqueous, methane). Of particular significance are ankerite-dolomite solid solutions that contain up to 85 mol. % ankeriteendmember. Such high concentrations were reported for the first time. Hitherto known ankerite concentrations do not exceed 70 mol. % in solid solution with dolomite.