Geological and geochemical evaluation of phosphorite deposits in northwestern Saudi Arabia as a possible source of trace and rare-earth elements

The Sirhan-Turayf region in northwestern Saudi Arabia hosts one of the largest Paleogene phosphate-bearing deposits in the Middle East and forms part of the world’s largest accumulation of Neo-Tethys Oceanic phosphorite. The Hazm Al-Jalamid and Umm Wu’al areas in the north and northeastern parts of the Sirhan-Turayf region are the largest and most famous phosphorite deposits in Saudi Arabia. The studied phosphorite localities are essentially carbonaceous phosphorites with similar major constituents (P2O5 and CaO), but different trace element and REE compositions. Umm Wu’al phosphorites are highly enriched in trace and ∑REEs contents compared with Hazm Al-Jalamid phosphorites. In Hazm Al-Jalamid phosphorites, P2O5 as well as most trace elements and REE contents progressively increased from the coarse- to fine-size fractions (<1 mm). The negative correlation between the terrestrial components (SiO2 + Al2O3 + TiO2) and the authigenic components (P2O5 + CaO + Na2O), as well as the very low Th contents of the studied phosphorites indicate pristine formation from seawater rather than derivation from detrital input. The low to moderate ∑REEs contents and their distribution patterns, substantial negative Ce-anomaly, and characteristic redox indices of the studied phosphorites all are indicative of formation from oxygenated seawater under fairly oxic marine conditions during early diagenesis. Compositional characteristics of the studied phosphorites, in terms of trace and REE, are similar to those of the Eastern Mediterranean phosphorites, but clearly different from those of the North African phosphorites. The low to intermediate ∑REEs contents and other redox proxies in the studied phosphorites, along with the Eastern Mediterranean phosphorites, compared to those in the North African phosphorites, are attributed to the relatively shallow water depth of eastern paleo-Tethys and global warming and greater water depth in the western side of paleo-Tethys in the North African countriesduring the Paleocene – Eocene time. Based on the ∑REEs contents and considering the outlook coefficient (Koutl) of REE composition, and the percentage of critical elements in ∑REE (REEdef), an attempt has been made to evaluate the economic potentialof the studied phosphorites as a source for U and REE. Despite of the low to intermediate REE contents and considering the huge phosphorite resources, as well as the big Wa’ad Al-Shamal Phosphorite Project in northern Saudi Arabia, studied phosphorites can be considered as promising to highly promising REE ores representing a valuable and profitable alternative source for critical REE.