Copper metallogeny in the Jogimardi volcanics, Chitradurga greenstone belt

Abstract

Massive volcanic-hosted sulfide deposits, within the Jogimardi volcanic suite (JVS) are Cu-rich with minor Zn and Pb and form an important early phase in the evolution of the Late Archean Chitradurga belt. The deposits are concordant stratabound lenses in the bedded mafic tuffs emplaced as submarine volcaniclastic rocks. The stratiform massive and vein-type sulfide deposits of the JVS are confined to the eastern limb of the antiform. The volcanic rock succession in the JVS consists of 85 tholeiitic basalts, 12$SiO_2$-poor variolitic andesites, and 3% of rhyolites. The basalts are quartz-normative tholeiites, not known to occur in the mid-ocean ridge basalt (MORB) environment. Unlike those of MORB, in their major, minor and REE (rare earth element) geochem., the basalts related to massive sulfide deposits are almost identical to back-arc basalts generated in an ensialic marginal basin environment. The thermal and chem. environment of ore formation are detd. based on fluid inclusions data and 34S pyrite values of individual sulfide phases. The ores were deposited in an anoxic chem. environment at $350^o$ within a pH range of 5-7, corresponding to proximal facies conditions. The stratiform sulfide ore body in the area is located in the vicinity of a fracture-controlled stringer/stockwork ore zone, a hydrothermal discharge vent in the pillowed basalts of tholeiitic compn. which confirms the proximal nature of the deposit. Uranogenic and thorogenic Pb isotope studies on the galenas of Ingladhal stratiform sulfide bodies indicate that they are not of mantle origin but derived from the basement gneisses, dated at 3 Gyr. Geol. setting, preservation of quench plagioclase textures in the pillowed basalts, and volcanogenic synsedimentary nature of the stratiform sulfide deposits indicate that the sulfides were deposited initially in the JVS prior to folding and regional greenschist-facies metamorphism