New petrological, mineralogical and palaeontological research in the quarry Podhůra (Maleník Block, Moravo-Silesian Culm)Article PDF
New petrological, mineralogical and palaeontological research in the quarry Podhůra (Maleník Block, Moravo-Silesian Culm)
The quarry Podhůra is situated near the contact of the Bohemian massif and Western Carpathians. There are exposed Lower Carboniferous sedimentary rocks belonging to Culm facies of the adjacent parts of the Bohemian massif.
Within the shales and fine-grained greywackes, fossils of Archaeocalamites scrobiculatus and Archaeocalamites sp. have been discovered recently.
A swarm of steep NW–SE-trending dykes of strongly altered igneous rock was found. The rock is composed mainly of albite, K-feldspar, chlorite (clinochlore), and amphibole (tschermakite to magneziohastingsite). Minor constituents are quartz, pyroxene (diopside-augite), titanite, ilmenite, apatite, pyrite, calcite, prehnite and epidote. The modal and chemical compositions are equivalent to alkalinefeldspar syenite and basaltic trachyandesite, respectively. The rock is enriched in both the litophile elements (Ba, Sr, Rb, Zr) and the transitional metals (V, Ni, Cr, Zn). Chondrite-normalized REE pattern shows LREE enrichment and no Eu anomaly. The near-zero δ34S value of accessory pyrite from the least altered part of the dyke suggests an orthomagmatic source of sulphur. Based on geological position and geochemical features, the magmatic rock may be easily parallelized with Late Variscan post-tectonic lamprophyric dykes of the Bohemian massif. However, it was subject of extensive post-magmatic hydrothermal alkaline metasomatism which altered mafic silicates and plagioclase into hydrous mineral phases and alkaline feldspars.
Products of three different hydrothermal systems were recognized to be hosted within the igneous rock:
1) Amygdule paragenesis composed of quartz, chlorite, prehnite, epidote and calcite is the oldest one. The origin of this mineralization was probably connected with above mentioned post-magmatic hydrothermal alteration of the host rock, and was possibly related to activity of meteoric waters (fluid δ18O around -5 ‰ SMOW, salinity around 2 wt. % NaCl equiv.). 2) The superimposed monomineralic calcite mineralization forms NW–SE and N–S striking steep veins. The fluid inclusions are characterized by very low homogenization temperatures (35–81 °C), Ca–Na–Cl nature and high fluid salinities (22–27 wt. % salts). Both the field and fluid inclusion parameters are comparable with post-Variscan vein-type mineralizations occurring along the eastern margin of the Bohemian massif. However, the calculated fluid
δ18O values are very low (-10 ‰ SMOW). Several attempts are introduced to explain the latter, but the problem is not fully resolved. 3) The youngest monomineralic calcite vein mineralization strike NW–SE and E–W. Low homogenization temperatures (54–87 °C), low fluid salinities (up to 3.7 wt. % NaCl equiv.), low δ13C values and near-zero fluid oxygen isotope signature are typical features. The formation conditions can be compared to the Tertiary mineralization in this area, originating from fluids of marine and/or meteoric provenance.
Zdeněk Dolníček, Kamil Kropáč, Tomáš Lehotský, Department of Geology, Palacký University, tř. Svobody 26, 771 46 Olomouc, Czech Republic, e-mail: firstname.lastname@example.org
Radek Škoda, Institute of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, e-mail: email@example.com
Ivana Jačková, Czech Geological Survey, Stable Isotope Lab, Geologická 6, 152 00 Praha 5 – Barrandov, Czech Republic.