Trace Elements Geochemistry in the Zagros Phosphorite Horizon: New Approach on Deposition and Genesis

Phosphorites are marine sediments of biogenic origin containing 15–20 wt% P2O5 and between 50 and 120 ppm U (Boggs, 2009; Tzifas et al., 2014; Zarasvandi et al., 2019). The study of phosphorites, especially trace elements geochemistry, confirms the importance of deposition conditions and diagenesis on the elemental composition of phosphatic minerals. Even more importantly, marine phosphorites are considered to have an economic potential for elements such as REE, Sc, U and Th (Altschuler, 1980). Some trace elements, including Sr, Ba, Se, Mo, Ag, Pb, Zn, V, Cr, Ni, Cu, Cd, and U are commonly found in phosphorites and sediments rich in phosphorus related to the crystal structure of apatite and carrier organic ligands (Tzifas et al., 2014; Zarasvandi et al., 2019). In general, more than seven horizons with an extent of ∼400 to 100km have been delineated in the Zagros Mountains. The Zagros phosphorite horizon of Eocene-Oligocene age hosted by the Pabdeh Formation is located in the Zagros fold belt with NW-SE trend (Halalat and Bolourchi, 1994; Zarasvandi et al., 2019). The aim of this study is to investigate the geochemistry of trace elements in order to obtain the deposition and genesis conditions of these elements in the Zagros phosphorite horizon.

A total of 29 samples were taken from phosphorite, hydrocarbon-bearing shale, phosphorite and limestone and oxide zone of the studied phosphorites of Zagros. Hence, based on stratigraphy, different samples from Zagros phosphorite horizons were collected from the phosphorites of Kuh-e-Sefid (n=9), Kuh-Rish (n=12) and Sheykh-Habil (n=8). Mineralogical and geochemical studies were carried out using ICP-MS analyses. 20 polished-thin sections were prepared. Mineralogy and petrography of the samples was determined and examined using polarizing-reflected light optical microscopy at the Shahid Chamran University of Ahvaz in Iran. Geochemical studies on mineralized and host rocks of Zagros phosphorite horizon were performed by the ICP-MS technique (Thermo Scientific- X Series II; DL= 0.001 ppb) at the Department of Earth Sciences, Pondicherry University in India.

According to the petrographic studies, phosphorite components and non-phosphorite components mainly consist of pellets, Ooids, intraclasts, fish skeletal fragments, micro-fossils, glauconite, calcite, pyrite, iron-oxide and quartz. Several elements that substitute Ca including rare earth elements and trace elements are suitable for contribution in the carbonate-rich fluorapatite (francolite) crystalline structure. Thus, some oxo-anions such as VO4, As2O4, SO2, SO4 and CO3 can be substituted into PO4 structure in apatite group lattices (Tzifas et al., 2014; Zarasvandi et al., 2019). Consistently, the Zagros phosphorite horizon exhibits different concentrations of elements such as Sr, REE, Zn, V, Mo, Cr, Cd, Se, As and U. Trace element distribution patterns in the studied phosphorites are similar to phosphorite in Iran and worldwide, especially in terms of concentration of U, Se, and Cd that can be related to apatite group minerals crystal lattice (Tzifas et al., 2014; Zarasvandi et al., 2019). Due the low entrance rate of detrital components from continental to the basin, the most probable source for trace elements is hydrocarbon-bearing shale in the stratigraphic column as a result of activities of microorganisms.

Field observation and microscopic studies showed that the phosphorite components occur as authigenic apatite with sparite cement, abundant pellets, ooids of symmetrical to elongated shape due to pressures caused by diagenesis, oval shape intraclasts, fish skeletal fragments and abundant microfossils. In additions to phosphorite and biogenic components, non-phosphorite minerals such as calcite, glauconite, pyrite, iron oxide, and microcrystalline quartz are present.There are many indications of change in conditions. They include bituminous shale in stratigraphic sequence, presence of abundant framboidal pyrites, PAAS-normalized patterns of REEs, negative Ce anomaly of all samples and positive Eu anomalies of all samples except bituminous shale sample of Kuh-e Rish phosphorite, the Ni/Co ratio and also the diagram of V/(V+Ni) vs. Ni/Co. These indicate changes in conditions from oxides during phosphate deposition into dysoxic to anoxic due to degradation and decomposition of organic compounds by microorganisms and the entry of trace elements such as uranium into the crystalline structure of apatite in the Zagros Basin.The significant economic potential of organometallic elements especially U and REE is observed in the Zagros phosphorite horizon due to favorable conditions of dysoxic to anoxic as a result of decomposition of organic compounds and then the entry of the elements into the apatite crystal structure.