Copper metallogeny in the Jogimardi Volcanics, Chitradurga Greenstone Belt

Abstract

Massive volcanic-hosted sulphide 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 Archaean Chitradurga belt. The deposits are concordant stratabound lenses in the bedded mafic tuffs emplaced as submarine volcaniclastites. The JVS is an integral part of the Chitradurga antiform flanked by synclines on either side. The stratiform massive and vein-type sulphide deposits of the JVS are confined to the eastern limb of the antiform.

The volcanic rock succession in the JVS consists of 85% tholeiitic basa1ts; 12% low silica variolitic andesites and 3% of rhyolites. The basalts are quartz-normative tholeiites, not known to occur in MORB environment. Unlike those of MORB, in their major, minor and REE geochemistry, the basalts related to massive sulphide deposits are almost identical to back-arc basalts generated in an ensialic marginal basin environment.

The thermal and chemical environment of ore formation are determined based on fluid inclusions data and δ³⁴S pyrite values of individual sulphide phases. These data indicatce that the ores were deposited under anoxic chemical environment at a temperature of 350°C within a pH range of 5 to 7 corresponding to proximal facies conditions. The stratiform sulphide ore body in the area is located in the vicioity of a fracture controlled stringer/stock-work ore zone - 'a hydrothermal discharge vent' in the pillowed basalts of tholeiitic composition which confirms the proximal nature of the deposit.

Uranogenic and thorogenic lead isotope studies on the galenas of Ingladhal stratiform sulphide bodies indicate that they are not of mantle origin but derived from the basement gneisses, dated at 3 b.y.

Geologic setting, preservation of quench plagioclase textures in the pillowed basalts and volcanogenic synsedimentary nature of the stratiform sulphide deposits indicate that the sulphides were deposited initially in the JVS prior to folding and regional greenschist facies metamorphism.