Uranium and Polymetallic Sulphide Mineralisation in the Banded Iron Formation at Udaisagar, Udaipur District, Rajasthan

摘要

Thinly laminated, pelagic, Banded Iron Formation (BIF) of Lower Proterozoic Aravalli Supergroup lies close to basement granite and phyllites over a stretch of 5 km along the Udaisagar-Kalamagra tract, Udaipur district, Rajasthan. BIF is characterised by rhythmic stratification of meso- and micro-bands of iron oxides with light coloured bands of silica/chert. Banded Haematite Quartzite (BHQ), a part of BIF, is 5-30 m in width and is intercalated with carbonaceous shale/phyllites. Primary sedimentary structures, viz., bedding, banding, laminations, convolute laminations and synsedimentary breccia and fault, coupled with low value for Co/Ni (0.18; n = 7) suggest its deposition as chemogenic sediment in a calm, tranquil and deep-sea environment. BIF analysed on an average (n=5) 35.81% Fe_2O_3 and 34.5% SiO_2 with enrichment in sulphides and uranium (138 ppm). Correlation matrix shows strong positive correlation between uranium and copper (r = 0.80) and negative correlation of U with V, Ni, Co and Mo. Uranium content varies from 100 to 470 ppm (n = 37) and appears to be in adsorbed state on ferric oxide. Field relationship, coupled with ternary plots of Al_2O_3 - (CaO + MgO) - SiO_2, Fe_2O_3 - (CaO + MgO) - SiO_2 and Eu/Sm ratio (av. 0.275; n = 10) confirms its Proterozoic age. BIF exposed at Udaisagar-Kalamagra tract forms a linear belt and resembles the early Proterozoic Lake Superior type in its geochemical signatures.