The Phosphatic chalk of the Mons Basin, Belgium : petrography and geochemistry of the Ciply Phosphatic Chalk and implications on its genesis
Jacquemin, Damien (2018-05-07)
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The Ciply Phosphatic Chalk (CPC) has been exploited for its enrichment in phosphorus in the early part of the twentieth century to produce fertilisers. Regained interests stimulated new research to characterise the potential for rare earth elements endowment and propose a genetic model for the formation of this phosphate deposit. This work studied the CPC using scanning electron microscopy, X-ray diffraction, cathodoluminescence and geochemical analyses. New insights into the formation of the deposits have been obtained regarding the mode of formation of the deposit. First, the deposit clearly shows evidence of alternating phase of phosphatisation and reworking giving rise to the formation of phosintraclasts which are the dominant phosphatic grains of the deposit. Weak or moderate upwellings brought nutrients to the Mons Basin during a period of sea-level highstand. Negative Ce-anomaly and the presence of bioturbation strongly argue against the development of an important oxygen-minimum zone indicating a low-productivity system. In these conditions, Fe-oxyhydroxides might have played an important role in scavenging phosphorus from the water column to the sediment. Phosphatisation seems to have occurred in the sediment when supersaturation relative to francolite was reached in the sediment pore water. Reworking processes probably consisted of wave action during storms. The REE are hosted in francolite and possess an average ΣREE of 350 ppm for the Hyon borehole. Their shale-normalised patterns are similar to other Cretaceous phosphate deposits but also to Cambrian deposits of China which typically display negative Ce-anomaly and HREE depletion. Post-depositional processes are only evident for the “phosphatic sands” which are strongly enriched in both phosphorus and REE. Further studies should aim to better constrain the paleoenvironmental conditions of deposition using stable isotopic studies. LA-ICP-MS studies should be able to give interesting insights into the compositional variations of the phosintraclasts thus giving elements to better characterise the genetic model of the CPC.
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