Bulletin Mineralogie Petrologie

Dolníček Zdeněk, Fojt Bohuslav, Mašek Vlastimil

Volume 26, issue 1 (2018), pages 64-73

fluid inclusions, H2O-CO2 fluids, methane, metamorphic remobilization, ore shoots, stratiform sulphidic deposits, Zlaté Hory Ore District

Genesis of massive base-metal sulphidic ores from discordant ore shoots of the Zlaté Hory-South deposit has been characterized using fluid inclusion petrography, microthermometry, and Raman analysis. The studied sample is composed of quartz, pyrrhotite (partly replaced by pyrite), galena, chalcopyrite, native bismuth, bismuthinite, and younger siderite and chlorite hosted by veinlets. Quartz-hosted fluid inclusions belong to three compositional types: H₂O-CH₄, H₂O-CO₂ and H₂O. Continuous evolution of carbonic phase from methane-dominated compositions to CO₂-dominated ones has been documented. The studied ore assemblages originated during retrograde phase of Variscan metamorphism at temperatures between ~220 and 300 °C and pressures between 0.7 and 2.0 kbar. Significant fluctuations of pressure recorded in the fluid inclusions indicate involvement of crack-seal mechanism during formation of ore shoots. The early phase of ore-forming process was characterized by high CH₄/CO₂ ratio and the presence of ethane, which is incompatible with source of fluids in local overmatured greenschist-facies rocks of the Vrbno Group. We thus interpret this portion of fluids to be externally derived from adjacent unmetamorphosed Culmian siliciclastic sediments. The influence of methane-rich fluids diminished rapidly during further evolution as the role of local metamorphic H₂O-CO₂ fluids with low CH₄/CO₂ ratios increased. The changes of fluid composition, pressure and temperature were the key factors causing the crystallization of ores in the studied ore shoots.

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