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| Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata-700019, Indi E-mail: sansong@rediffmail.com ABSTRACT The sulphide deposit of Sargipali, in the Proterozoic fold belt of Gangpur, is strikingly lead dominant in character. It has an ore reserve of 2.06 million tonnes with an average grade of 6.73% Pb and 0.33% Cu, with a little Ag (about 50 ppm). The Zn content in the ore is insignificant (<0.5%), below recoverable limit. The unusual compositional character of the deposit will be evident when compared with that of many known sulphide deposits. Pb- and S- isotope studies reveal extreme uniformity of Pb isotope composition, along with high source m ( 238U/ 204 Pb ) value of Pb,which suggest a single- stage lead, derived from isotopically homogeneous, uranium enriched, felsic upper crustal source, though sulphur was derived from reduction of contemporary sea water sulphate source. However,the Pb- isotope study may well indicate that the Sargipali is a 1682-1695- Ma- old sedimentary- exhalative ( SEDEX ) deposit. The petrographic and chemical studies of ore and host rocks indicate the metamorphosed synsedimentary- exhalative genesis of the deposit.The abnormal enrichment of Pb in the residual fluid is suggested to have caused by the buffering of metal containing hydrothermal fluids by mica present in the felsic rocks or sediments, to a low pH at relatively low temperature. Pb-Pb ages of ores ( ranging between 1682-1695 Ma ) suggest that mineralisation occurred during the closing phase of sedimentation in the Gangpur basin. The present study may help in throwing new light about the genesis of similar sediment- hosted Pb-rich deposists in terms of the SEDEX model. |
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