Felsic Volcanics in South Khetri Copper Belt, Rajasthan and their Metallogenic Significance

Felsic volcanics in the south Khetri Copper Belt (KCB) occur interlayered with fine grey banded phyllitic quartzite which often contains fine carbonaceous matter. The volcanics include crystal tuff, ultrafinely laminated grey tuff and pumice. They have been recognised in field on the basis of vesicles and rare amygdules; and under microscope by embayed and resorbed quartz phenocrysts. Glass is altered and incipiently recrystallised. Devitrification textures like spherulites are intact in rare cases. The relatively unaltered tuffs fall in dacitic field of Total Alkali-Silica (TAS) diagram. The tuffs are intimately associated with baryte and ironstone layers at places. Recent work in the south KCB indicates that sulfide mineralisation is polymetaHic. It is stratabound hosted by felsic volcanics and fine carbonaceous phyllite and quartzite. The mineralised zones show wall-rock alteration in the form of profuse silicification and sericitisation. These features when seen in conjunction with the occurrence of concordant baryte and ironstone layers, indicate that the sulfide mineralisation is volcanic-exhalative type.