Uranium oxidation state and related variations in geochemistry of phosphorites from the Negev (southern Israel)

Study of the U(IV) fraction in Mishash phosphorites of the Negev reveals an apparently paradoxical trend. The U(IV) fraction is very low (less than 10%) in the pristine, organic-rich laminated phosphorites while this proportion considerably increases (up to 60%) in the recycled (mechanically reworked) granular phosphorites which deposited under more oxidizing conditions. This U(IV) trend seems to differ from that reported in other phosphate sequences elsewhere, and is also quite unexpected in view of the sediment patterns and the specific formational conditions of the two phosphorite facies. In addition, a lower %U(IV) in the analyzed granular phosphorites is coupled with a lower rare earth elements (REE)/P2O5 and Y/P2O5 ratio, a higher Cd and Zn concentration, less negative Ce (Ce/Ce*) and Eu (Eu/Eu*) anomalies, a lower HREE (Dy-Er) enrichment, and a lower F/P2O5 and higher CO2/F ratio in francolites. This U(IV) trend in the two Negev phosphorite facies may be explained by varying time-spans of apatite growth, changes in sediment burial rates, and principally, different residence times of the phosphate fraction in the suboxic apatite-forming diagenetic zone near the sediment–water interface. The associated varying concentrations of REEs, Y, Cd and Zn in phosphorites and of F and CO2 in francolites in the two granular phosphorite facies are apparently collectively regulated by the same sedimentary dynamics during phosphate deposition. It is presumed that the U(IV) trend observed in the Negev phosphorites could be detected in other phosphate sequences as well if pristine phosphorites are carefully identified and selectively analyzed.