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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. 相似文献
2.
Arsenic contamination in tube-well water in Ambagarh-Chowki block, central India, is restricted to local areas confined within the N-S trending Dongargarh rift zone. Affected areas are preferentially located in acid volcanics, close to shear zones and also in granites. Dug-wells even in severely contaminated areas generally have As concentration ≤10 μg/l. But in Kaurikasa area, several tube-wells and dug-wells are severely polluted. Weathered rocks and soils are also enriched in As from severely contaminated areas. As preferentially occurs in iron-enriched soil and similarly altered biotite, chlorite in granite. As sorbed in hydrated iron oxide (HFO) that preferably occurs in acid-leachable fraction and possibly as coatings on kaolinite, illite and goethite in soil or as coatings and along cleavage traces on weathered biotite and chlorite. Reductive dissolution of HFO released sorbed As to groundwater and enriched it in Fe. Pyrite in volcanic and shear zone rocks, although locally As-bearing is a minor source of As in groundwater. 相似文献
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