Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Zdeněk Dolníček, Jana Ulmanová, Jaroslav Havlíček, Karel Malý, Ivana Jačková

Fluorite-bearing vein mineralization from Bartoušov near Havlíčkův Brod: a complex fluid history related to post-Variscan uplift of the Moldanubian basement (Bohemian Massif, Czech Republic)

Journal of Geosciences, volume 68 (2023), issue 4, 261 - 280

DOI: http://doi.org/10.3190/jgeosci.379

A subvertical NW-SE trending fluorite-bearing vein at Bartoušov cut small body of two-mica granites hosted by high-grade metamorphic rocks of the Moldanubian Zone of the Bohemian Massif, in the northern part of the tectonic structure of the Jihlava Graben. The vein mineralization features open-space filling and multiple episodes of mineral precipitation. Early fine-grained quartz containing rare Nb- and W-enriched rutile is overgrown by crystals of late quartz and greenish to violet fluorite, followed by veinlets of dolomite-ankerite and calcite with pyrite and siderite. Quartz and fluorite crystallized from heterogeneous aquo-carbonic fluid with highly variable both bulk density and composition. Early CO₂-dominated aquo-carbonic fluids probably represent local retrograde-metamorphic fluids, whose variations in density likely reflect the pressure decrease during uplift of the basement during the Carboniferous. Late N₂-dominated low-density aquo-carbonic fluids were trapped at ~135-150 °C and ~60-100 bars. Raman analysis proved minor occurrence of CH₄, C₂H₆, H₂, and/or O₂ in the N₂-rich carbonic fluid. The composition of N₂-dominated carbonic fluid cannot be explained by the local basement sources only, but it could be derived from the now eroded immature Permo-Carboniferous sediments of the Jihlava Graben. In this concept, N₂, hydrocarbons, and possibly some H₂ could have been formed due to thermal alteration of immature organic matter, whereas O₂ and rest of H₂ were likely formed due to radiolysis of water within bodies of synsedimentary uranium ores present in this sedimentary basin. The infiltration of basinal fluids into basement was allowed due to an extensional tectonic regime. Paragenetically late carbonates originated from low- to moderate-salinity aqueous fluids derived from meteoric water and containing elevated amount of carbon from the oxidized organic matter.