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.
