A note on radioactive riverine placers around Midtul,Bastar craton,Central India

Reconnaissance survey in the Midtul area (toposheet no. 65F/1; 18°52′30″N: 81°09′55″E) of the Bastar district, Chhattisgarh, revealed well-developed, 1 to 4 m thick riverine placers, derived from granitic terrain in the environs of the Mari River. The sediments comprise largely unconsolidated, cross-stratified gravels, sands, and silts, with low flat-topped surfaces which, at places, correspond to the morphology of terraces that are preserved on convex banks of the present day meanders, indicating their deposition by fluvial processes as channel bars. Available field data suggest deep-seated secular weathering of granites of the area, extensive reworking of such weathered products by fluvial processes, and their deposition as riverine placers in the vicinity of the streams draining the area. The granitic rocks of the area record radioactivity of 0.01–0.02 milli Roentgen per hour (mR/hr), with spots of higher radioactivity of 0.04–0.08 mR/hr, whereas riverine placers derived from them show radioactivity up to 0.25 mR/hr. Hand-panned polymineral concentrates from such placer deposits, analyzed by wavelength-dispersive x-ray fluorescence (WDXRF) spectrometry, showed 0.006–0.67 (n = 47; av. 0.092) % Y₂O₃ and 0.019–4.729 (n = 46; av. 0.834) % Ce₂O₃, with low values of Nb (41–467; n = 15; av. 197 ppm), Ta (115–598, n = 13; av. 230 ppm), and Sn (17–468; n = 14; av. 104 ppm). From the observed heavy mineral assemblage of monazite, zircon, ilmenite, epidote, hematite, and magnetite, as revealed by x-ray diffraction (XRD) studies, it is clear that the radioactivity is contributed by cerium, besides monazite (a phosphate of La, Ce, Th), and yttrium, possibly by zircon. Small amounts of niobium, tantalum, and tin are considered to be associated with ilmenite, epidote, and biotite. An occasional higher value of cerium (∼5% Ce₂O₃), together with restricted higher radioactivity, indicates only localized and uneconomic concentration of light REE-bearing mineral, monazite in isolated patches in such riverine placers.