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

Abstract

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 ("Nd 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.