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

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Highlights

  • Temperatures of Mg-rich fluids during syn-ore stage estimated at 122 °C–258 °C.

  • Raman spectra revealed structural difference in brannerite and surrounding matrix.

  • Brannerite grain core consisted of amorphous mineral with UO22+, UO2.12, and TiO2.

  • Three types of carbonaceous particles were distinguished using light microscopes.

  • No radiation-based alteration of organic matter was documented.

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.

Introduction

The total amount of extracted uranium, approximately 350 000 tons (OECD-IAEA, 2003), makes the Bohemian Massif the most important uranium ore district in the European Variscan belt. The most important genetic type of uranium deposit is represented by vein-type uranium mineralisation (50,000 t U at Příbram, 8500 t U at Jáchymov). The other significant genetic type is shear-zone hosted uranium mineralisation in the Moldanubian Zone (4200 t U at Zadní Chodov, 20,000 t U at Rožná). The shear-zone hosted uranium mineralisation is usually connected with organic matter-enriched filling of these mineralised shear zones (Dill, 1985, Dill, 1986, Kříbek, 1989, Kříbek et al., 2009, Dill et al., 2019).

Biogenic carbonaceous matter is common in many types of sedimentary rocks. During regional metamorphism, it changes in structure from turbostratic amorphous to crystalline hexagonal graphite. Possible alterations in the graphite during retrograde metamorphism or the influence of circulating hydrothermal fluids were documented (Frost, 1988, Wang et al., 1989). Although in several works the authors have characterized graphite as chemically and thermally inert, with stable structural arrangements up to approximately 400 °C, several studies have described alterations of graphite at temperatures around 250 °C (Gautneb and Tveten, 2000, Schiffbauer et al., 2007). Generally, it is believed that alteration of graphite is affected by uranium mineralization, as well as circulation of hydrothermal fluids (Kyser et al., 1989).

The aim of this study is to analyse in detail the organic matter in uranium enriched shear zones of the Zadní Chodov uranium deposit. The petrology and chemical composition of different organic matter in mineralised shear zones were analysed using optical microscopy and spectrophotometry of dispersed organic matter, and gas chromatography/mass spectrometry (GC/MS) and micro-Raman spectroscopy were used to estimate the original organic matter, as well as possible conditions that prevailed during the origin of uranium enriched shear zones. Micro-Raman spectroscopy was also used for a detailed study of brannerite.

Section snippets

Geological setting

The Zadní Chodov uranium deposit is located on the northwestern margin of the Bor pluton, which intruded into the West Bohemian shear zone (Zulauf, 1994) at the boundary between the Moldanubian and Teplá-Barrandian zones during the Variscan magmatic event (337 ± 6 Ma, U/Pb TIMS analyses on zircon; Dörr et al., 1997) (Fig. 1). The Zadní Chodov uranium deposit was mined from 1952 to 1992 and was ranked as a medium-size uranium deposit in the Bohemian Massif. The total mine production of low-grade

Samples and methods

Five representative samples of the mineralised shear zone infill, located in the lowest part of the uranium deposit (23 level), were studied (ZCH-8, ZCH-31, ZCH-94, ZCH-95, ZCH-102).

The distribution, morphology, and reflectance of carbonaceous particles were studied and measured on polished sections of rock slabs oriented perpendicular to banding or foliation of the rock samples. The morphological types of high-reflecting carbonaceous matter, including fine granular, lamella-like and dense

Petrography of metasediments

Metasediments from the Zadní Chodov uranium deposits consisted predominantly of partly migmatised biotite paragneisses, sillimanite-biotite paragneisses, biotite quartzitic paragneisses and cordierite-biotite paragneisses. The dominant biotite and quartzitic biotite paragneisses were medium- to fine-grained rocks, containing quartz, plagioclase, biotite, and K-feldspar. The amount of anorthite in the plagioclases from these high-grade metamorphosed sediments ranged from An20 to An35. Garnet was

Origin of mineralised shear zones and temperatures of hydrothermal alteration

The origin of shear zones is compatible with N–S dextral kinematics. These zones of brittle deformation are associated with phyllosillicate- and organic matter-rich coherent and incoherent cataclastites and fault breccias, from 30 cm to aproximately 1–2.5 m in thickness.

The pre-ore stage was characterised by widespread chloritisation, argillisation, and hematitisation of highly metamorphosed sediments. In comparison to the French deposits (Poty et al., 1986), the altered high-grade

Conclusions

Mineralisation is a process with impact on organic material in sediments, and sedimentary organic matter is a sensitive marker for environmental and alteration processes. Results of the study of samples from uranium ore-enriched shear zones of the Zadní Chodov uranium deposit revealed:

  • Evidence of higher activity of Mg-rich fluids in the study area during syn-ore stage, with temperatures from 122 °C to 258 °C, and the presence of coffinite and brannerite minerals;

  • Structural differences within

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the Czech Science Foundation grant 19-05360S “Radiolytic alteration of organic matter in uraniferous environment”. Our thanks are to the Operational Program Prague - Competitiveness, project “Centre for Texture Analysis” (No. CZ.2.16/3.1.00/21538), and the long-term conceptual development of the research organization RVO: 67985891. Geological Institute, Academy of Sciences of the Czech Republic, is thanked for the interpretation of X-ray diffraction analysis and for

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