Rare earth elements and yttrium (REY) as genetic proxies fordifferent ironstonestypes (BahariyaDepression, Egypt)

Abstract

The Bahariyaironstones (WesternDesert, Egypt) occur either as thin crusts and concretions within the clastic rocks of the Cenomanian Bahariya Formation or as big ore bodies associated with the Eocene carbonatesalong major faults. The two ironstone types show slight similarities in mineralogy and major and trace elements geochemistry displaying some evidence about their formation after carbonate replacement. Even though, rare earth elements and yttrium (REY) show two different genetic ironstone types. The Upper Cretaceous ironstones display negative Y and Ce anomaliesand intermediate Nd concentrations, between 10 and 100 ppm pointing to their diagenetic origin. Whilst, the negative Ce anomaly, positive Y and Eu anomalies, the low Nd concentration (mostly less than 10 ppm) and the very low concentrations of REE in the Eocene ironstones confirm their hydrothermal origin.The presence of positive Y anomalies in the Eocene ironstones suggests that iron precipitation occurred rapidly and immediately after reducing, slightly acidic waters reached a more oxidizing and more alkaline water.