| Popis: |
Lead isotope ratios from three base metal deposits, Rangpo, Gorubathan in the eastern Lesser Himalaya and Bageswar in the Lesser Himalaya of Kumaun have been studied. The first two deposits, generally stratiform and stratabound, occur within pelitic, psammopelitic/wacke type metasediments which are locally carbonaceous. The Bageswar deposit is hosted by silicified dolomitic carbonate rocks. The Pb-isotope ratios appear to be homogenous in two and heterogenous in one of the deposits. Model Pb–Pb ages obtained for the Rangpo and Gorubathan deposits are 1800 Ma and for the Bageswar deposit, 1550–1700 Ma. Dispersion in isotopic ratios reflect the geochemical heterogeneity, particularly with respect to μ. Larger variation in the composition of Pb-isotope ratios in the Bageswar deposit may be explained by the participation of Pb from heterogenous sources. Pb-isotope characteristics of the Himalayan deposits are compared with those of a number of base metal deposits in the Delhi–Aravalli belt in Rajasthan, NW India. Ore lead from the deposits of both the Himalaya and of Rajasthan are enriched in 206Pb and 207Pb. However, lead in all cases plots on the evolution lines, corresponding to sources with higher U/Pb ratios. But the sources of lead in the Himalayan deposits had a lower Th/U ratio. The geologic observations as well as the Pb-isotope compositions are consistent with a sedimentary-diagenetic origin of the Himalayan base metal deposits. |
Full Text from ScienceDirect