Geochemistry of shear zone-hosted uranium mineralisation at the Zadní Chodov uranium deposit (Bohemian Massif)

Abstract

Samples from the Zadní Chodov uranium deposit of the Bohemian Massif were analysed using optical and chemical methods. Organic matter and brannerite were studied in detail to estimate conditions that prevailed in association with uranium-enriched shear zones. Four major events are known in the high-grade metasediments. The pre-ore stage is associated with the formation of shear zones. During the uranium ore deposition stage, temperatures ranged from 122 °C to 258 °C, and coffinite and brannerite appeared. Two generations of post-ore stage minerals are distinguishable. A detailed study of a brannerite mineral grain revealed variations in its interior and surroundings, but these are related to the composition of brannerite, which is not a nominal mineral. Si and Al are presented as chlorites, but the main mineral phases are mixtures of anatase, oxidized brannerite with non-stoichiometric uranium oxide, and carbonaceous matter. Carbonaceous matter in the samples studied appeared as a mixture of substances of different composition, origin, and age. Optical and structural characteristics showed variations in three coexisting types of metamorphosed carbonaceous matter, including fine-grained and larger lamella-shaped particles that represent anthracite, meta-anthracite and semi-graphite stages, lamellas of crystalline graphite, and pyrobitumen, corresponding to impsonite. The structure of organic particles was not affected by uranium minerals, which is an interesting difference from changes that minerals undergo in, for example, coal. Biomarkers, compounds used in assessing sources of organic matter, were present in extracts, but they were not a remnant of the original shear zone organic material. They are likely products from a later impact, probably of Tertiary and younger sediments.