Deposition of phosphates in the lower Paleocene-Oligocene Pabdeh Formation, Kangan anticline, Southwest Iran: implications for rare earth element enrichment

Abstract

The Kangan anticline in the Folded Zagros Zone contains sedimentary phosphate deposits enriched in trace elements. Field observations, petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), and whole-rock geochemistry are used to determine the petrogenesis of this phosphate deposit, evaluate the mechanisms of deposition, and assess the implications for trace element enrichment. Phosphatic layers are phosphatic wackestone–packstone with microfossils and contain glauconite. Carbonate rocks of the Early–Middle Eocene Pabdeh Formation host the phosphate units. Calcite, fluorapatite, and glauconite are the primary minerals of the marine sedimentary phosphate deposit in the Kangan anticline. Whole-rock analysis of the phosphate layers indicates negligible clastic components and show enrichment in U and HREE. Limestone and calcareous mudstone units in the Pabdeh Formation do not display enrichment of these elements. Carbonate fluorapatite is the host mineral for REEs and U. Cation substitution into carbonate fluorapatite is considered to be the main mechanism of trace element enrichment due to positive correlations between P₂O₅ and the trace elements; ion adsorption did not play a crucial role in the metal enrichment in these phosphates. Strong negative Ce anomalies, slight positive Eu anomalies and low ΣLREE/ΣHREE ratios of phosphate layers indicate enrichment of the HREE relative to their marine origin. The depositional environment of the phosphate units is interpreted as a basin margin carbonate ramp in the reduced and suboxic-to-anoxic zone that had low detrital input but occasionally high-energy erosional events. Ocean upwelling had an essential role in depositing the sandy glauconite-bearing phosphate layers.