Greenstone metamorphism, geochemistry, ore mineralogy and fluid evolution of the granitoid-hosted gold mineralization at Jonnagiri,eastern Dharwar Craton

Abstract

Gold mineralization at Jonnagiri is hosted in laminated quartz veins within sheared granodiorite in a typical Archean greenstone-granite ensemble. Effects of two folding- and shearing events, are respectively observed in the greenstones and the intrusive granodiorite. Geochemical considerations point toward magmatism at subduction zone tectonic setting, for the greenstones (metabasites, garnet-chlorite schist, meta- tuffs) and the granodiorite. The greenstones were metamorphosed at ∼5 kbar and 500 °C. Hydrothermal alteration resulted in formation of the proximal zone within the sheared granodiorite and the inner zone of auriferous laminated quartz veins. Compositions of chlorites and arsenopyrites, from these zones, yielded comparable temperature ranges of 263 to 335 °C. Substantial mobility of elements (including the REEs), took place during alteration, possibly at high fluid/rock ratio. The computed (aMg2+/aH+2) vs. (aK+/aH+) and (aNa+/aH+) vs. (aK+/aH+) diagrams explain the observed alteration-induced mineralogical changes, in accordance with the isocon plot and constrain the possible fluid composition. The gold occurs both as fracture fillings in quartz and enclosed within sulfides. Pyrite-δ34S values furnished a narrow range of +1.4 to +7.1 ‰, and the calculated δ34SH2S varies from +0.2 to +5.8 ‰, at log fO2 = −32.6 and pH = 5.15 to 5.95; implying that Au(HS)2− was the dominant gold complex. The narrow δ34Si values are indicative of magmatic (± mantle) source, or involving an average crustal sulfur composition. Fluid inclusion microthermometric and Raman spectroscopic studies in quartz veins from the proximal and the inner zones reveal common existence of a low salinity metamorphic aqueous-gaseous fluid, entailing negligible fluid evolution between alteration and gold precipitation. Although the estimated P-T window (1.39 to 2.57 kbar at 263 to 323 °C), broadly compare with the P-T conditions of other orogenic gold deposits, significant pressure fluctuation characterize the ore fluid evolution at Jonnagiri. Gold precipitation was a consequence of fluid phase separation, fluid-rock interaction, and decrease in fO2. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits elsewhere in the world reaffirms the metamorphic nature of the ore fluids; thus underplaying the role of granitic fluid, in the formation of the Archean lode gold systems.