Application of geochemical techniques for prospecting unconventional phosphate sources: A case study of the lake sediments in Eppawala area Sri Lanka

Abstract

Jaya-Ganga is a water canal constructed bisecting the phosphate deposit in Eppawala, Sri Lanka. It nourishes three lakes around the vicinity of Eppawala Phosphate Deposit (EPD). In this study, the lake sediment samples were analyzed for major oxides and phosphate solubility for prospecting unconventional phosphate sources. Analyses of the geochemical content of the surface sediments in both upstream and downstream lakes revealed higher contents of P2O5, TiO2, Al2O3, Fe2O3 and MnO and, low levels of SiO2, MgO, Na2O, K2O and CaO compared with the norms of Upper Continental Crust (UCC) values. Log plots of (Na2O/K2O) vs log (SiO2/Al2O3) and (Fe2O3/K2O) vs (SiO2/Al2O3) showed chemical immaturity and mineralogical instability of the sampled sediments. The average Chemical Index of Alteration (CIA) values of 93 for upstream and 92 for downstream portrayed extreme chemical weathering conditions in the sediment source area, facilitating the breakdown of the EPD under prolonged exposure. Besides the transportation of detrital sediments, the above could convert residual phosphate into a more soluble form enabling transportation through Jaya-Ganga and re-deposition in downstream sediments. Thus, positive anomalies of P2O5 and high phosphate solubility on surface and core sediments in the downstream lake indicated the contribution of the EPD. The results of the study reflect the potential of secondary phosphate enrichment in downstream sediments and its applicability as a low-grade phosphate fertilizer. Continuous nourishment of phosphate-bearing materials, into lake sediments, increase the quantity of the phosphate content in downstream sediments and the minable quality of the lake sediments as an economically viable phosphate source.