Composition, diagenetic and post-diagenetic alterations of a possible radioactive waste repository site: The Boda Albitic claystone formation, southern Hungary

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Abstract

In southern Hungary a thick Upper Permian claystone formation has been selected as a possible repository of high-level nuclear waste. Detailed petrographic and geochemical characterizations of the formation are given, using the results of microscopic observations, X-ray diffractometric modal and phyllosilicate crystallinity, X-ray fluorescence major element bulk chemical, K-Ar isotope geochronologic and stable C, O and H isotope ratio analyses. On the basis of peculiar bulk chemistries and modal compositions characterized by extremely high, authigenic albite, low quartz, high hematite and moderate carbonate contents, the formation is considered to have been deposited in a shallow-water lacustrine environment, under semi-arid to arid climatic conditions and highly alkaline, strongly oxidative hydrological ones. Illite and chlorite crystallinity as well as vitrinite reflectance data point to late or deep diagenesis (max. 200-250 °C) that - according to the K-Ar ages of the <2 μm grain-size fraction K-white micas - culminated in the Lower Jurassic. Post-diagenetic fracturing caused by repeated brittle deformations of the rigid rock mass is a common phenomenon. In the most frequent fault gouges phyllosilicate retrogression (i.e. chlorite alteration, smectite and mixed-layered clay mineral formation, etc.) occurs. K-Ar ages show that these processes proceeded under disequilibrium conditions. C and O isotope compositions of whole rock carbonates indicate sedimentary origin, while whole rock carbonates with high δ13C and variable δ18O values suggest pervasive diagenesis. Ubiquitous fracture fillings are grouped into calcite-, baryte+quartz- and anhydrite-dominated veins, the barytic one with traces of sulfidic mineralizations. The combined H-C-O isotopic study of fissure-filling carbonates and fluid inclusions suggests three fluid generations acting in fractures, namely: magmatic fluid and meteoric waters related to warm and cold climates, respectively. A significant effect of recent meteoric water in fracture fillings is unlikely.

Original languageEnglish
Pages (from-to)351-378
Number of pages28
JournalActa Geologica Hungarica
Volume43
Issue number4
Publication statusPublished - Dec 1 2000

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Keywords

  • Chlorite crystallinity
  • Diagenesis
  • Fracture fillings
  • Hungary
  • Illite crystallinity
  • K-Ar dating
  • Modal composition
  • Permian claystone
  • Radioactive waste repository
  • Stable isotope geochemistry

ASJC Scopus subject areas

  • Geology

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