Rare earth elements and uranium geochemistry in the Al-Kora phosphorite province, Late Cretaceous, northwestern Jordan

Abstract

Sixty-three samples representing the phosphorite deposits of the Al-Kora province in northwest Jordan are analyzed for their major and certain trace elements including the rare earth and uranium. They are collected from the following four sections: Tubna, Dair Abu Sa’id, Wadi Al-Arab, and Wadi Ziglab. The samples studied are mainly phosphorite packstone/grainstone consisting of phosphate intraclasts and vertebrate skeletal fragments (bone and teeth) of varying sizes, associated with minor carbonate wackestones. Laminated, in situ phosphorites, sometimes called pristine phosphorites, are also present. The Al-Kora phosphorites are authigenic, i.e., precipitated from the interstitial solutions enriched with the phosphate ion. Carbonate fluorapatite (CFA) or francolite is the dominant mineral. Geochemical data suggest that the analyzed elements can be grouped into (a) land-derived detrital clay group (Al2O3, Fe2O3, TiO2, K2O, Cr, Ga, Hf, Nb, Rb, Th, and Zr). This group constitutes less than 5 % of the total elemental concentrations in the analyzed samples, (b) marine- or seawater-derived phosphate-carbonate group (P2O5, CaO, MgO, Na2O, Ba, Sr, U, Y, and the 14 rare earth element (REE)), making the bulk of the samples studied, and (c) organic matter/detrital clay group (Cr, Ni, Mo, Cu, Pb, As, Zn, and Sb). Uranium substitutes for Ca in the CFA structure with a range from 1 to 186 ppm for all samples including carbonates, with an average of 58.4 ppm. Average for the phosphate-only samples is 101 ppm. The shale-normalized REE patterns exhibit distinct seawater-derived mineral patterns. The patterns are characterized by an enhanced negative Ce anomaly and an enriched heavy REE. This signal (pattern) seems to have survived the phosphogenesis processes. Average Ce anomaly is −0.76, including the carbonate samples. It indicates the fractionation of Ce3+ into Ce4+ and the deposition of the latter in oxic, or possibly oxygen minimum, seawater. It, thus, confirms the oxic water conditions of the Neo-Tethys Ocean at the time of deposition.