Geochemical and mineralogical characterization of phosphatic crusts developed on the basement carbonatites of Sri Lanka: towards a better understanding of the weathering process

Abstract

The diversity and complexity of basement rocks at Eppawala are accompanied by complicated matrices of phosphate deposits. Detailed field, geochemical and mineralogical studies were carried out on phosphatic weathering crusts developed on carbonatite occurrences to elucidate the weathering process and to explore mineralization of possible economic minerals. The presence of markedly distinguished weathering horizons at the Eppawala southern quarry (ES) site provided an advantage for studying the sequence of alteration and weathering processes occurred on the parent bodies. Leaching of carbonate minerals in the early stages had resulted in the alteration of primary apatite. Under diverse conditions, several generations of supergene phosphate minerals have formed. All major and trace elements in the profile are mobile and displaced due to weathering of the principal minerals, with the exception of Mg, Sr, Na and Ba. The lower leached zone and higher-saprolitic layers are enriched with Rare Earth Elements (REEs) due to the presence of a high amount of secondary phosphates, such as crandallite, gorceixite, florencite and goyazite. Anomalies of REEs in upper lateritic layers are inherited by the presence of smectite group clay minerals. The presence of a high amount of silicate minerals and high SiO2 content indicates the interactions between carbonatites and surrounding granitic rocks during the weathering process. Moreover, geochemical results confirm that the several phosphate matrices are highly enriched with Fe2O3. Therefore, iron bearing phases must be removed during the production of phosphate fertilizer. Understanding of detailed mineralogical and geochemical variations of exposed phosphate orebodies along with the weathering profiles at Eppawala is critical to mining processes and beneficial for the production of phosphate fertilizer and REEs.