Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: Evidence from Ore Petrography and Fluid Inclusion Studies

Gold mineralization at Ajjanahalli is BIF-hosted and structurally controlled. Ore petrographic studies revealed that gold was introduced as a consequence of focussed fluid flow and sulfidation of an oxide facies BIF horizon. Ubiquitous syn-kinematic quartz veins within the BIF as well as in the associated metasedimentary units lay testimony to the high fluid flux during deformation. Fluid inclusion studies involving microthermometry and laser Raman microspectroscopy on inclusions from quartz veins from different litho-units were carried out. The primary fluid preserved as aqueous-carbonic inclusions within quartz veins in carbonaceous phyllites is found to be H2O-CO2-CH4-NaCl type. Near-isothermal decompression led to splitting of this fluid at greenschist facies conditions (0.8 kb/263°C to 1.15 kb/309°C) and resulted in a low saline aqueous and a carbonic (+CH4) fluid. The auriferous aqueous part reacted with the oxidic BIF, leading to epigenetic replacement of oxides to pyrite and concomitant precipitation of gold.