Geochemistry, facies characteristics and palaeoenvironmental conditions of the storm-dominated phosphate-bearing deposits of eastern Tethyan Ocean; A case study from Zagros region, SW Iran

Abstract

Phosphate deposits in south-western Iran are part of the South Tethyan Phosphogenic Province, a huge carbonate-dominated strata that extends to the Middle East. The Tethyan phosphorites of Iran are dated Eocene-Oligocene (Pabdeh Formation) and categorized as low-grade ore deposits on a global scale. Depositional conditions of the facies indicate that the Pabdeh Formation was deposited on a carbonate ramp setting as a distally steepened ramp. Under such an environment, turbidity currents transported phosphate particles from the back-shoal setting to the deeper middle and outer ramp of the ocean where they were suspended and deposited as shell-lag and phosphate lamination. Microfacies studies demonstrate that all the phosphatic ooids and phosphatized foraminifera, fish scales, bones and phosphatic intraclasts reworked from shallow parts of the Tethyan Ocean to deeper parts with the help of turbidity currents. Analysis and interpretation of the data reveal positive correlation between. REE+Y and P2O5 in all studied sections which attests to their strong coherence as a geochemical group. The shale normalized REE patterns of Mondun phosphorites are characterized by negative Ce anomalies. This anomaly indicate that the depositional environment was oxic and highly reworked, bioturbated with higher energy realm during phosphate deposition, conversely Nill section with Ce enrichment reflect conditions of relatively deeper water sedimentation. These geochemical findings are in accordance with microfacies studies which indicate shallow and high energy condition for Mondun section with negative cerium anomalies and a deep ramp setting for Nill and Siah sections which denote a positive cerium anomalies in REE patterns.