Abstract
The Kangan anticline in the Folded Zagros Zone contains phosphate deposits enriched in trace metals. Field observations, petrography, X-Ray Diffraction, Scanning Electron Microscopy, 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 metal enrichment. Phosphatic layers are grainstone–packstone with microfossils and contain green glauconite. Carbonate rocks of the Early–Middle Eocene Pabdeh Formation host the phosphate units. Glauconite, calcite, and fluorapatite are the primary minerals of the marine sedimentary phosphate deposit in the Kangan anticline. Whole-rock compositions of phosphate layers indicate negligible clastic components and show enrichment in U and HREE. Limestone and pelagic limestone units in the Pabdeh Formation do not display enrichment of these elements. Carbonate fluorapatite is the host mineral for REEs and uranium. Cation substitution into carbonate fluorapatite is considered to be the main mechanism of trace element enrichment due to positive correlations between P2O5 and trace metals; 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.