Origin and significance of high-grade phosphorite in a sediment core from the continental slope off Goa, India

Abstract

A phosphorite crust was found at 380-390 cm depth interval of a sediment core collected from the topo­graphic high occurring on the continental slope off Goa. This crust is fragile and grey to light brown in colour. Carbonate fluorapatite is the predominant mineral followed by minor pyrite. Thin section stud­ies indicate that it is mostly homogeneous with a few bone fragments and shows porous microstructure. SEM studies show that it contains agglomerated 1- 2 µm size apatite globules resembling phosphatized bacteria and coalesced bacteria. Microprobe geo­chemistry of the phosphorite indicates that it con­tains 33% P₂O₅ and 50% CaO with other major elements (Si, AI, Ti, Fe, Mg, Na and K) amounting to only 1.8%. The mode of the phosphorite crust for­mation is discussed in relation to Quaternary phos­phorites in upwelling and non-upwelling regions. It is suggested that the initial substrate for the phos­phorite crust was most probably a fish coprolite which phosphatized under lower rates of terrigenous sedimentation and calm environmental conditions during the Pleistocene.