Bicarbonate-rich fluid inclusions and hydrogen diffusion in quartz from the Libčice orogenic gold deposit, Bohemian Massif

Unusual paleofluid composition is reported for the Libčice orogenic-type gold deposit located in the Central Bohemian Plutonic Complex (CBPC), Czech Republic. Unexpected bicarbonate-rich fluids and their complex chemistry variations characterize primary fluid inclusions from the main gold-bearing quartz vein. A detailed microthermometry, Laser Raman Micro Spectroscopy (LRMS) and SEM cathodoluminescence study was used to understand the complex chemistry and P–T–X history. The results indicate the presence of H2O and H2O–CO2–CH4 (± N2; H2S) fluids, the latter displaying variations of the CO2/CH4 ratio in the gaseous phase from 6.8 to 0.06. Variation of the CH4 content across single grains and between different levels of the mine was recorded. The presence of nahcolite, H2 (up to 6 mol%) and ethane (0–0.2 mol%) in the fluids were also discovered by LRMS. Potential models for the formation of different types of fluids present in the deposit are discussed, including the genesis of HCO3− rich fluids as well as H2 and C2H6 presence in the primary fluid inclusions. The potential influence of organic matter-bearing sediments, as well as the impact of the intrusion of CBPC, re-equilibration and/or re-speciation of fluid inclusions during the post-entrapment history is considered to have the main impact on the complex paleofluid chemistry. Based on the thermodynamic modeling, H2 diffusion into the fluid inclusions was shown to be the main reason for the CH4 variation on the scale of a single grain, as well as across the whole vein. Although the exact processes of production/formation of HCO3− and H2 at the Libčice deposit remain open to discussion, reactions in the C–O–H system are considered to be a possible formation mechanism. This work also contributes to our understanding of the importance of post-entrapment modifications and reactions in the C–O–H system on interpretation/deciphering the processes in orogenic-type deposits.

Research Highlights
► Unusually bicarbonate rich paleofluids were reported from Libčice gold deposit.
► HCO3− was directly measured by Raman for the first time in natural fluid inclusions.
► Systematic variations in CO2/CH4 in fluids were recorded on grain and mine scales.
► H2 diffusion represents the main mechanism responsible for observed variations.