Occurrence of placer gold is reported for the first time in the insitu and colluvial gravels derived from amphibolite/hornblende schist, associated pegmatite and quartz veins of Nagamangala schist belt in Marlagalla –Allapatna areas, Mandya district, Karnataka. Gold occurs in the form of small nuggets, measured up to 3mmx4mm in size and 0.2 gram in weight. The host rock is amphibolite which are intruded by numerous pegmatite and quartz veins. The pegmatites are complex, highly evolved type and contain economic concentration of rare metal bearing minerals viz., columbite-tantalite, beryl and spodumene. Prolonged weathering of these rocks resulted in liberation of gold from host rocks and concentrated in the insitu/eluvial and colluvial gravels. The present finding of gold opens up a new vista for further detailed geological/geochemical exploration to establish and evaluate gold potential in the eluvial, colluvial gravels as well as in the other lithounits of Nagamangala schist belt in Marlagalla-Allapatna areas.
Uranium mineralization in Madyalabodu area, Cuddapah district, Andhra Pradesh, is spatially related to chloritized and brecciated quartzite of the Gulcheru Formation in the immediate vicinity of E-W to ESE-WNW trending basic dyke. Chloritization transgresses the lithological boundaries. Whole-rock geochemical data indicate enrichment of MgO and Al 2 O 3 coupled with depletion of SiO 2 , Na 2 O, K 2 O, CaO and TiO 2 in the chlorite-rich zone. Fe 2 O 3 and FeO do not vary significantly in the altered and the unaltered zones. Electron Probe Microanalysis (EPMA) data reveal that the chlorites in contact with uranium minerals are enriched in MgO and depleted in FeO than in the others. Considering the petrological evidence, geochemical signature and structural constraints, it appears that chlorite acted more as an adsorbent rather than as a reductant in facilitating uranium mineralization. Uraninite crystallized later from the uranium originally adsorbed on chlorites. Chloritization might also have facilitated mineralization through the generation of nascent hydrogen, H 2 S and lowering pH of uranium-bearing solution.
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