Phosphorus concentration mechanisms and phosphorite formation, case study: the formation of francolite in Akashat Phosphorite of Eastern Tethys, Iraqi Western Desert

Abstract

The increasing in P₂O₅ content from 70 ppb in seawater to more than 30 wt% in marine phosphorites is a multi-stage process, which includes: the biological trapping of P from the sea, the enrichment of P by decay, the reverse fluxing of P to seawater, the upwelling currents, the formation of phosphatic phases, the early and late diagenetic processes (lithification, mechanical, chemical and biological) which lead to the formation and discrimination of the phosphatic grains (peloids, coprolites, ooides, cortoides, and fish teeth and bone remnants…etc.). The final stage represents the redistribution of phosphatic facies in depositional environments along the continental shelf. Depending upon the suggested views, one can describe the general mechanism as the biogenic – diagenetic phosphorite formations. Akashat phosphorites have mainly consisted of francolite, which depends on the chemical composition of interstitial water. P, Ca and Sr are related to the organic activity of the decay by bacteria in the interstitial water within the upper part of the sea floor mud. The organic material contributes in the collection of some elements like V, Pb, As and HREE from seawater and fixed them on phosphatic grains during the accumulative growth of the phosphatic grains., whereas Na, S, U, Y, and LREE reflect the chemical properties of the seawater in the shallow continental shelf. This environment represents the Francolite formation area which is exposed to partial isolation from the open sea by the local submarine bars, reflected by a slight increase in salinity and alkalinity. The mole/formula of carbonate in Francolite related to the multi diagentic processes toward the more stable phase (fluorapatite). In addition to the association of positive and negative anomalies of Ce and Eu, respectively, which indicated non-oxidizing conditions and without sub-marine volcanic activity through francolite formation. Both roles of primary formation and the diagenetic process play similar effects on the crystal chemistry of francolite.