Genesis of Cu-mineralisation in Vinjamuru Group, Nellore Schist Belt, Eastern Dharwar Craton, India Sukanya Chaudhury*, Reshma V. Mohan and Prasun Ghosh Geological survey of India, State unit: Andhra Pradesh, Southern region, Hyderabad-500068, India *Corresponding author’s e-mail:sukanya.chaudhury@gsi.gov.in Abstract: Basemetal occurrences are reported in Nellore Schist Belt since ancient times. One such copper occurrence in Vinjamuru Group of NSB is hosted within meta-tholeiites and quartz veins intruding it, in the form of fine disseminations and pockets of chalcopyrite, covellite, digenite and pyrite. Malachite staining occur along the schistosity and joint planes of host metavolcanics (actinolite-chlorite schist). The hydrothermal fluid responsible for basemetal mineralisation is saline carbonic- Mg rich fluid with temperature <300°C having an average of -2.8 δ 34 SVCDT (‰) value of the primary chalcopyrite and pyrite, indicating magmatic fluid source derived during magmatic differentiation. The chalcophile elements were scavenged by the fluid and were transported in the form of aqueous metal complexes through transcrustal weak planes to a low temperature environment and precipitating them in the form of copper sulphides. The chalcopyrite with time got oxidized to covellite and digenite along grain margin. Continued oxidation in turn resulted in formation of copper carbonates (malachite, azurite). Keywords: Chalcopyrite, Vinjamuru Group, Nellore Schist Belt, stable sulphur isotope, fluid inclusion Introduction The Eastern Dharwar Craton (EDC) is the amalgamation of western, central and eastern tectonic blocks joined during various Archean-Proterozoic events. Nellore Schist Belt (NSB) sited in the eastern periphery of Dharwar Craton is in tectonic contact with the Eastern Ghat Mobile Belt (EGMB) to the east and juxtaposed with Nallamalai Fold Belt (NFB) to the west. The NSB extends from Sirasanambedu, in the south to Vinukonda in the north approximately over 250 km in parts of Nellore, Prakasam, and Guntur districts of Andhra Pradesh(Pascoe, 1950; Satyanarayana, 1994; Srinivasan et al., 1994; Okudaira, 1994; Moeen, 1998; Hari Prasad et al., 1999; Sesha Sai, 2013; Saha et al.,2015; Das and Shukla, 2016;Hrushikesh et al., 2019; Burhanuddin and Kumar, 2022 and the references therein)The NSB is known for basemetal occurrences and old workings are reported along the northern part of the Duttaluru quartzite ridge, east of Somalaregada, south of Garimanipenta village, 3km north of Vinjamuru, south &south-west of Kotepetta, South-west of Tirumalapuram and north & north-east of Somalaregada (Boswell, 1873; M/s. Pripmeyer & Co., Germany, 1933; Roy, 1942; Sen, 1946; Kerr-Cross, 1949; Kannan, 1962and the references therein). Earlier subsurface exploration was carried out by Geological Survey of India (Vasudevan and Rao, 1971) in Garimenipenta area for copper investigation in Vinjamuru Group of NSB. This work indicated scattered and sporadic occurrences of copper sulphides in the form of lenses, pockets and stringers. GSI further carried out subsurface exploration for basemetal mineralization in the Udayagiri Group of NSB near Masayapeta village, Udayagiri taluk of Nellore district (Sukanya et al., 2019). One of the boreholes showed scattered copper mineralization in the form of lenticular, pockets and stringers in five zones with cumulative thickness of 46m with an average grade of 0.4% Cu. The presence of sulphide minerals such as pyrite and chalcopyrite and constant association of the mineralization with quartz veins and the absence of pyrrhotite-pentlandite group of minerals support the contention that mineralization is associated with magmatic fluid (Sankar and Prasad, 2012). Martin et al., 2007 described a model where the metals were derived from deep seated mantle sources and are separated from the basic magma during differentiation, giving rise to younger intrusive. The fluids ascended through transcrustral fractures and got deposited along weak plane, viz. fault and shear developed in older supracrustrals, such as amphibolites, biotite schist, and pegmatite resulting in occurrences like that of Garimanipenta (Vinjamuru Group) and Masayapeta Received: 11-01-2023 Accepted: 15-02-2023 Research Article Indian Journal of Geosciences, Volume 77, No. 3 July - September, 2023; pp. 213-238