• V Rajamani

      Articles written in Journal of Earth System Science

    • Significance of Sm-Nd isotope systematics in crustal genesis: A case study of Archaean metabasalts of the eastern Dharwar Craton

      J K Zachariah S Balakrishnan V Rajamani

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      Determination of the age of rocks by whole rock Sm-Nd isochron method has several limitations imposed by petrogenetic processes. If the age of the rocks can be determined by other independent methods, the Sm-Nd system provides a wealth of information to understand crustal genesis. Sm-Nd isotopic studies of metabasaltic rocks of the Archaean Kolar and Ramagiri Schist belts in the eastern Dharwar Craton indicate that the system was disturbed by postmagmatic fluid alteration processes associated with terrane accretion.

    • REE geochemistry of ore zones in the archean auriferous schist belts of the eastern Dharwar Craton, south India

      T S Giritharan V Rajamani

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      The eastern Dharwar Craton of southern India includes at least three ∼ 2700Ma supracrustal belts (schist belts) which have mesothermal, quartz-carbonate vein gold mineralization emplaced within the sheared metabasalts. In the Hutti and the Kolar schist belts, the host rocks are amphibolites and the ore veins have been flanked by only a thin zone of biotitic alteration; in the Ramagiri belt, however, the host rocks to the veins have been affected by more extensive but lower temperature alteration by fluids. The rare earth element (REE) geochemistry of the host metabasalts, alteration zones, ore veins and the bulk sulfides separated from the ore veins and the alteration zones suggest that

      •the REE chemistry of the immediate host rocks has been modified by fluids which added LREE,

      •the REE abundance of the ore veins vary with the amount of host rock fragments included in the veins,

      •the sulfides formed during mineralization have significant REE concentration with patterns nearly identical to the ore veins and alteration zones and

      •therefore the ore fluids involved in gold mineralization here could be LREE enriched.

      Because alteration and mineralization involved addition of REE, more LREE compared to HREE, the fluids could be of higher temperature origin. The initial Nd isotope ratios in the host rocks (εNd calculated at 2700 Ma) showed a large variation (+8 to -4) and a deep crustal source for the fluid REE seems likely. A crustal source for Pb and Os in the ore samples of Kolar belt has previously been suggested (Krogstadet al 1995; Walkeret al 1989). Such a source for ore fluids is consistent with a late Archean (2500Ma) accretionary origin for the terrains of the eastern Dharwar Craton.

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