Some Observations on the Phosphatic Rocks of the Lower Gondwana Sequences of the Ib River Coal Field, Sundergarh District, Orissa, India

The phosphate occurs as discontinuous patches forming massive compact claystone-like rocks. Submicroscopic grains of quartz, feldspar and flakes of mica, and grains of cryptocrystalline rock fragments and chert occur in a phosphatised ground mass. Coarsely crystalline calcite and siderite of secondary origin are commonly associated. The cores of nodular masses show altered. olivine, pyroxene, prismatic calcic plagioclase, rock and chert fragments, and vein-quartz in a coarsely crystalline carbonate cement, while the outer shells of the nodular bodies are of amorphous phosphatic material. Trace-element analyses show high values of Ba, Y, and La in phosphatic rocks compared to non-phosphatic ones. An igneous source for the clasts in the cores of nodular bodies and secondary phosphatisation through late hydrothermal solutions are suggested.