A modern terrestrial phosphorite-an example from Sri Lanka

Abstract

At Eppawala in Central Sri Lanka, a thick weathering profile is mined for use as phosphate fertilizer. Recent tropical subaerial weathering processes have produced a phosphate-enriched horizon in the weathering profile which has developed on a Precambrian apatite marble formation. The enriched horizon with P 2O 5 compositions ranging from 10 to 40% is formed essentially of laminar phoscretes occurring in a network of sinkholes. Detailed examination of the laminar phoscretes reveals that primary metamorphic apatite crystals occur in association with phosphatic allochems such as coated grains, ooids, pisoids and intraclasts in a finely layered groundmass. It has been observed that the primary apatite grains released from the parent apatite marble due to tropical weathering processes have been deposited in sinkholes where nutrient-rich percolating meteoric waters were precipitating secondary apatite. Subsequent diagenetic processes active in such sinkhole sedimentary environments have produced the phosphatic allochems characteristic of secondary phosphate mineralization. The allochems together with their mineralized groundmass and the associated primary apatite grains have given rise to the Eppawala phosphorite deposit.