Mineralogy, geochemistry and depositional environment of phosphates in the Pabdeh Formation, Khormuj anticline, SW of Iran

Abstract

Phosphate deposits are found in the Khormuj anticline at the end of the Folded Zagros Zone of Iran and are enriched in REE and trace metals. Field survey, petrography, X-ray diffraction and whole-rock geochemistry were used to determine the petrogenesis of these phosphate deposits and evaluate the mechanisms of trace metal enrichment. Khormuj anticline phosphate layers are hosted by carbonate rocks of the Pabdeh Formation (Lower Paleocene-Oligocene). The phosphatic layers are composed of phosphorus grainstone–packstone with microfossils and contains green glauconite. Whole-rock compositions of phosphates indicate a minimal detrital component and enrichment in U and HREE. These elements are not enriched in the limestone units that overlie and underlie the phosphate layers. Overall, the textures and trace element compositions of phosphate layers are interpreted to represent accumulation on a basin margin carbonate ramp, in the reduced and suboxic-to-anoxic zone, with low detrital input but occasional high-energy erosional events. Upwelling process played a fundamental role in the deposition of the sandy glauconite-bearing phosphate layers. Phosphate mineralization has syngenetic, diagenetic, and epigenetic components. Positive correlations between P2O5 and REE, U and other trace elements suggest that cation substitution into carbonate fluorapatite and not ion adsorption is the dominant mechanism for metal enrichment in these phosphates. REE patterns in these phosphate layers show strong negative Ce anomalies, positive Eu and Y anomalies and high La/Yb ratios (> 10). Yttrium versus (La/Nd)N ratios are in the seawater range and have been affected by diagenesis process. These elevated ratios suggest that the phosphates are relatively enriched in both the LREE and HREE. This enrichment is related to their marine origin, and weathering had no effect on the phosphate horizons. This research show that marine phosphates have high potential to preferentially fractionate the HREE and U and could represent a future source of these metals.