Abstract
Strongly deformed phosphorite layers enriched with U-Be-Mn-REE occur in a weathered Triassic metarhyolite tuff near the town of Bükkszentkereszt, NE Hungary. The phosphorite is massive or earthy, has an especially fine-grained 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 Bükkszentkereszt 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.
Highlights
Phosphorite is a marine sedimentary rock type containing at least 18 wt% P2O5 and is the main ore of the element phosphorous (Pufahl and Groat, 2017)
In accordance with the previous data, a volcanicpyroclastic hydrothermal system can be proposed as the host rock of the P-Mn-U-Be-HREE-enrichment in phosphorite bodies (Csáki and Csáki, 1973; Kubovics et al, 1989; Polgári et al, 2000; Szabó and Vincze, 2013; Zajzon et al, 2014)
The previous and most appropriate formation model is a hydrothermal vent precipitation deposited in a marine, volcanoclastic environment accumulating elements (Be, U, Rare Earth Elements (REE), etc.) into the apatite structure (Szabó and Vincze, 2013; Zajzon et al, 2014)
Summary
Phosphorite is a marine sedimentary rock type containing at least 18 wt% P2O5 and is the main ore of the element phosphorous (Pufahl and Groat, 2017). Phosphates are the main ore-forming minerals of phosphorous, apatite (commonly in the form of francolite – carbonaterich fluorapatite, general formula: Calo-a-bNaaMgb(PO4)6-x(CO3)x-y-z (CO3·F)y (SO4)F2). The lattice structure of this mineral is conducive to various major and trace element substitutions (Jarvis, 1995). Due to its tendency to contain substituting trace elements, phosphate rock is on the list of critical raw materials all over the world. Phosphorite deposits can be the source of U and REE elements (typically 50–200 ppm U content and 500–2000 ppm REE), so phosphorite is a significant high-tech resource (Pufahl and Groat, 2017)
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