Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jiří Zachariáš

Interpretation of bidimensional fluid inclusion microthermometric data in terms of pressure and compositional variations in gold-bearing fluids: An example from the Libčice and Kasejovice deposits, Czech Republic

Journal of the Czech Geological Society, volume 47 (2002), issue 3-4, 123 - 132

Systematic mapping of selected fluid inclusion microthermometric characteristics is combined with standard microscopy and various cathodoluminescence techniques to provide detailed information on the evolution of pressure, temperature and compositional variations in hydrothermal fluids. The advantage of such a complex approach is demonstrated on samples from Variscan mesothermal gold deposits of Libčice and Kasejovice in Central Bohemia.
Primary fluid inclusions from remnants of original quartz grains were studied at the Libčice deposit. They display systematic zoning of CO₂-melting and homogenization temperatures [Tm-CO₂: -57.5 to -65.0 °C and Th-CO₂: +24 to -21 °C (always to liquid)], and little variation in the other parameters [salinity: 1-5 wt. % NaCl eq., total homogenization: +340 to +370 °C (to liquid, or critical), degree of fill: ~ 0.5]. These variations reflect an increase in the amount of CH₄ in the gaseous phase of fluid inclusions from 1 to 50 mol. % from the cores towards the rims of quartz grains. In addition, variations in fluid density (0.70-0.88 g/cm³) were observed. The data are discussed in terms of P-T-X variations: the difference in fluid pressure between the grain core and grain rim during quartz gangue precipitation is estimated to equal nearly 2000 bars. The gradual evolution of CH₄/CO₂ ratio is interpreted as being due to mixing between initial CO₂-rich fluids of deep provenance with local CH₄-enriched fluids. The former were probably produced by large-scale regional devolatilization, while the latter are a result of local "in situ" devolatilization of organic carbon-rich Lečice Member black shales in a narrow contact-metamorphic zone of the CBPC.
An example of a trail of secondary aqueous fluid inclusions from the Kasejovice deposit exhibits systematic variations in the total homogenization temperatures (+180 to +130 °C, to liquid). These are interpreted as pressure and temperature variations during sealing of microfractures.