Some geological and geochemical attributes of the phosphate deposit at Ridigama, N.W. Sri Lanka

Abstract

Ridigama phosphate deposit occurs in the form of apatite rich veins that have intruded into the Precambrian granitic gneisses. Weathering processes active in the tropical Sri Lankan environment have given rise to a residual deposit. This deposit which is a weathering profile contains large primary apatite crystals in a fine matrix composed of ferruginous, siliceous, aluminous secondary phosphate minerals. Chemical studies have revealed that the primary apatite crystals of Ridigama have total P2O5 contents varying from 39% to 41% whereas it is less than 35% in the matrix. The 2% citric acid and water solubilities of primary apatite cystals are around 7% P2O5, and 0.05% P2O5 respectively; for the matrix the values were approximately 4% P2O5 0.04% P2O5 . Neutral ammonium citrate (NAC) solubility exceeds 2.5% P2O5, for crystals and less than 2% P2O5 for the matrix. The primary apatite crystals of this deposit showed higher solubility than those of Eppawala. However, the matrix at Eppawala was more soluble than that of Ridigama. The major components in the primary crystals are hydroxyl chlorapatite and carbonate fluor-apatite. Both these apatite types are associated with crandallite, quartz, feldspar, hematite and magnetite in the fine matrix.