Abstract
Numerous phosphate occurrences are located in the Bohemian Cretaceous Basin (BCB) of the Czech Republic, within the Cenomanian–Turonian sequences. Small phosphate occurrences have been reported in the Upper Cenomanian, Lower Turonian, and Upper Turonian marine glauconitic siliciclasts. The phosphates are generally <1 m thick, present as phosphatized hardgrounds, nodules, coprolites, skeletal remains, phosphatized shells, peloids, sponges, and tube-fills, associated with black mudstone and other siliciclasts. Only recently the critical elements have been highlighted in these phosphates. The present study covers eight of these occurrences and provides information on petrography, mineralogy, and chemical composition of major elements, trace elements, and stable isotopes. The phosphate mineralogy is comprised of carbonate-fluorapatite, associated with quartz, glauconite, smectite, kaolinite, and pyrite. Most of the phosphates are rich in organic matter. The phosphate chemistry is dominated by P2O5, CaO, F, Na2O, SO3, and CO2. Minor amounts of SiO2, Al2O3, K2O, and MgO are found, related to quartz and alumino-silicate impurities. Evidence of fossil microbial structures is revealed. The indices derived from rare earth elements (REE) indicate phosphogenesis at various redox conditions, ranging from anoxic to oxic, whereas the carbon stable isotopes of the apatite suggest generally reducing conditions. The critical and other valuable elements found in these Mid-Cretaceous phosphates include P2O5 (18.9–26.76 wt. %), F (1.67–3.25 wt. %), REE (325–1338 ppm), Y (74–368 ppm), and U (10.4–37.9 ppm). The investigation of the Turonian phosphate occurrences show that those located at the base of the Bílá Hora Formation (earliest Turonian) are the most persistent in the southern margins of the BCB, and found in localities extending for about 200 km. They were developed at the onset of the Early Turonian global transgression and are strata-bound to the base of the Bílá Hora Formation. Future exploration for marine sedimentary phosphorites should focus on thicker and better developed deposits at the base of the Turonian sediments as the main target.
Highlights
Phosphate resources are limited in Europe and occur as small deposits hardly sufficient to meet the industrial demands of European countries
It is well known that phosphate rocks are important sink for rare earths, uranium, and many other critical and valuable elements, but little is done in the European Union (EU) countries to extract these metals as by-products during fertilizers’ production
The previous and new findings show that the phosphate-bearing horizon at the base of the Bílá Hora Formation, dated as Early Turonian, is encountered in all the localities investigated along the southern margins of the Bohemian Cretaceous Basin (BCB) [3,4,5,6,8,10,13,22]
Summary
Phosphate resources are limited in Europe and occur as small deposits hardly sufficient to meet the industrial demands of European countries. Phosphate components are reported at the base of the Bílá Hora Formation (Lower Turonian) in many locations in the southern part of the BCB. They are present in various forms, such as phosphatized hardgrounds, nodules, sponges, skeletal bones, shark teeth, and tube-filled deposits [3,4,5,9,13]. The phosphate occurrences of the BCB can be compared with numerous marine sedimentary phosphate occurrences reported in the Mid-Cretaceous (Albian–Turonian) marine sedimentary sequences in several European countries including Russia, France, Belgium, Germany, Poland, Hungary, Portugal, and Spain [15,16]. Petrology, geochemistry, metallogenesis, and assessment of the Cenomanian–Turonian phosphates in the BCB as potential resources of REE and other critical elements and to suggest a specific stratigraphic level for future exploration
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