Geochemistry and paleoenvironment of the phosphorites from the Ameki Formation, Niger delta, Nigeria

Abstract

The phosphorites of the Ameki Formation occur as nodules, pellets as well as primary phosphatic shales and siltstones. Geochemical analysis of the phosphate samples was carried out to determine its chemical composition as well as its depositional environment. The methodology applied include XRF, INAA, XRD and thin section petrography using Polarizing Microscope. The XRF result identified CaO- P₂O₅- F as the major mineral group, SiO₂, Al₂O₃, Fe₂O₃, MgO and TiO₂, which show minor occurrences and Cr, U, Pb, V, Cu, Zn, Se and Cd that occur in trace amounts. The nodules and pellets are of medium to high grade (25- 34wt% P₂O₅) whereas the primary phosphatic shales and siltstones are of low to medium grade (4.5– 22wt% P₂O₅). The phosphorites comprise mostly of francolites. The mean index of refraction estimated by Becke-line method using Standard Polarizing Microscope gave 1.634 and1.636.XRD analysis yielded an average values of 9.243 (± 0.002) A° and 6.715 (± 0.002) A° for a and c crystallographic axes respectively with an axial ratio (c/a) of 0.726for unit cell parameter, suggestive of low degree of carbonate substitution. An increase in P₂O₅ content is found to be accompanied by increase in CaO, CO₂ and F contents, but by a decrease in H₂O, organic carbon, SiO₂ and Fe contents indicative of amorphous solid phase of calcium phosphate. The phosphorite is interpreted to form under high biologic productive, shelf setting. High nutrient availability in the ancient sea is linked to upwelling along the West African Coastline during the Eocene.

Keywords: Phosphorites; Geochemistry; Paleoenvironment; Ameki; Francolites; Nodules