Abstract
In this study, geochemical and petrographic characteristics of veins from the Permian Boda Claystone Formation (Mecsek Mts., Hungary) are presented. Understanding the connection between these properties is vital in the reconstruction of paleofluid history and tectonic evolution of this potential high-level radioactive waste disposal site. Each of the observed four vein generations consists of several mineral phases and has complex evolution, which can be attributed to multiple diagenetic and tectonic processes. Syntaxial and antitaxial veins have been observed suggesting oscillatory advective and diffusive material transport mechanisms; however, veins associated with mobile hydrofractures have also been detected. The parent fluids with a predominant temperature of 100–150 °C and variable salinity (3.5–13.7% wNaCleq) originated potentially by mixing of connate brine with freshwater released by smectite-to-illite transition and isotope exchange with sedimentary minerals, which may have caused the observed δ18O values (−1.12 to 4.67‰, V-SMOW). In contrast to the most vein-filling mineral phases, a breccia cement phase precipitated from a low-temperature (<50 °C) and salinity (0.0–0.4% wNaCleq) meteoric fluid. In the pores of this breccia veins, late-stage fluid migration is verified. Therefore, these veins are of paramount importance for studying the isolation properties of the rock body.
Highlights
During the operation and future decommissioning of the Hungarian nuclear power plant, a significant volume of high activity radioactive waste is generated
Based on fluid inclusion micro thermometry (Torok, 1994) and H–C–O stable isotope geochemistry, the barite-quartz-dominated veins formed at ~150 ◦C from magmatic parent fluid, while the calcite-dominated veins precipitated at ~70 ◦C from meteoric waters, which, based on δD compositions, are related to warm and cold climates (Arkai et al, 2000)
We show the petrographic characteristics of four vein generations from the BAF–2 well of the Boda Claystone Formation (BCF)
Summary
During the operation and future decommissioning of the Hungarian nuclear power plant, a significant volume of high activity radioactive waste is generated. Arkai et al (2000) classified the veins from 12 different locations (boreholes and tunnels) into calcite-, barite + quartz- (±calcite and sulfidic mineralisation) and anhydrite-dominated generations. Large data scattering of final melting temperatures (Tm) measured in anhydrite-dominated veins suggests mixing fluids of very different salinity. Lenti et al (2010) determined homogenization temperature of ~105 ◦C for inclusions in barite-calcite veins from the tunnel Alfa–1, which veins were precipitated from low salinity (3.2–4.3% wNaCleq) aqueous solutions. A well-defined vein type is the branched calcite-barite vein, which, established on microstructural characteristics such as cone-in-cone arrangement of wall rock inclusions within the veins (VeinCIC), is suggested to be of early, syn-diagenetic origin (Hra bovszki et al, 2020)
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