Contribution to the origin of Mn-U-Be-HREE-enrichment in phosphorite near Bukkszentkereszt, NE Hungary

Strongly deformed phosphorite layers enriched with U-Be-Mn-REE occur in a weathered Triassic metarhyolite tuff near the town of Bukkszentkereszt, NE Hungary. The phosphorite is massive or earthy, has an especially finegrained texture, and forms inhomogeneous bands with different Mn-oxide contents. High-resolution optical rock microscopy investigations helped to distinguish mineralized biosignatures, filamentous, vermiform structures, and coccoid-like aggregates. These microstructures encompass almost the whole phosphorite. Based on in situ FTIR and Raman spectroscopy, the phosphorites contain ferrihydrite and pyrite, and also different types of embedded organic matter. These structures can be interpreted as series of Fe rich biomats, forming microbialites. On a larger scale, these microstructures can play a role in shaping the stratiform structure by evolving stromatolite-like bands. Results presented here propose a new interpretation of the origin of the P-Mn-U-Be-HREE-enrichment. The Bukkszentkereszt occurrence claim a mineralized microbially mediated deposit, and our results support this scenario based on the mineralized microbial structures, the embedded minerals, and the presence of organic matter.