Geochemistry, Mineralogy and Economic Aspects of Phosphatic Rock of The Samchampi Alkaline-Carbonatite Complex, Assam, North East India

Four discontinuous lensoid bodies of secondary phosphatic rock (0.433 sqkm) occur along the curvilinear fault zone in the dominantly soil covered Samchampi Alkaline-Carbonatite Complex (SACC) that was emplaced in close proximity to the Kalyani lineament during the upper Cretaceous (109Ma) in the Mikir Hills massif of Assam. Petrographic studies indicate that fine-grained flourapatite is the dominant phosphate mineral set in a ground mass of calcite and francolite. Crandallite, pyrochlore, magnetite, ilmenite, goethite, biotite and zircon occur as accessory minerals. Major and minor oxides show a wide range of P₂O₅(31.49–38.0%), CaO (41.70–54.73%), Fe₂O₃(1.40–11.16%), and Al₂O₃(0.53–4.47%) and SiO₂(0.64–7.24%). Trace element analysis including REE indicate high REE (1518ppm), U₃O₈ (223ppm), Nb (168ppm), Y (69ppm) and Zr (674ppm). Electron Microprobe studies have revealed the presence of mineral phases such as apatite, crandallite, pyrochlore that account for the chemical constituents like P, REE, U, Nb and Y in the rock. A moderate reserve of 15 million metric tons of phosphatic ore averaging 35% P₂O₅ has been estimated under the resources category down to a depth of 15m. Thus, the phosphatic rock of Samchampi Complex could become a potential source for production of phosphoric acid together with uranium and rare-earth elements as by-products. They can be extracted economically by using a combination of froth flotation to remove calcite and silica as gangue and hydrometallurgical treatment of the beneficiated ore.