Mineralogy and organic geochemistry of phyllite from the Dewon–Pokrzywna deposit, the Opava Mountains (SW Poland)

Abstract

Phyllites from the Dewon–Pokrzywna deposit in the Opava Mts., SW Poland, were investigated by XRD (Rietveld method), XRF, EPMA, SEM, and ATR-FTIR from the perspective of their potential usage as a buffer and/or backfill material in a geological repository of radioactive waste. Organic matter dispersed in the phyllite matrix was analysed by GC-MS. Fine-grained Mg-Fe-muscovite (13 to 29 wt.%), Fe-ripidolite (10 to 25 wt.%), detrital quartz (20 to 46 wt.%), and albite (7 to 28 wt.%) ± microcline, illite or illite/smectite, and kaolinite are major minerals in phyllite samples. The chlorite/muscovite ratio ranges from 0.65 to 1.1. Mg-annite inherited from the precursor rock is a minor constituent. Detrital ilmenite is a dominant accessory mineral. Ancylite-(Ce) occurs in quartz-calcite-ripidolite veins. Two types of phyllite have been distinguished based on the proportion of phyllosilicates to silt fraction: argillaceous (47 to 55 wt.% phyllosilicates) and silt-rich (28 wt.% phyllosilicates). Argillaceous phyllite shows elevated content of alumina and moderate concentration of silica. It is highly enriched in Fe compared to phyllites from other localities worldwide. The BET specific surface area of argillaceous phyllite ranges from 1.73 to 3.64 m2/g. Whole-rock chemical composition, mineral assemblages, chlorite geothermometry, and the occurrence of aliphatic hydrocarbons suggest that argillaceous phyllite originated from a pelagic pelite protolith under low-temperature (260–370°C) greenschist to subgreenschist facies conditions. Persistent biomarkers are indicative of bacterial degradation of planktonic organic matter suspended in a high water column. Enrichment in Fe-rich chlorite and Mg,Fe-muscovite, low volume of interconnected pores with dominant mesopores suggest that argillaceous phyllite from the Dewon–Pokrzywna deposit is a potential candidate for a buffer and/or backfill material.