Greenstone Metamorphism, Hydrothermal Alteration, and Gold Mineralization in the Genetic Context of the Granodiorite-Hosted Gold Deposit at Jonnagiri, Eastern Dharwar Craton, India
Authors: Chinnasamy Sakthi Saravanan , Mishra Biswajit 
Published: June 2013

Gold mineralization at Jonnagiri is hosted in laminated quartz veins within sheared granodiorite in a typical Archean greenstone-granitoid ensemble. The greenstones and the intrusive granodiorite have been affected by two folding and shearing events, respectively. THERMOCALC-aided P-T calculations suggested that lower amphibolite-grade metamorphism affected the greenstones, involving peak conditions at ~5 kbar and 500°C.

Gold mineralization is characterized by a chlorite + biotite-rich proximal alteration zone and a muscovite-dominant inner zone among laminated auriferous quartz veins. Whereas the proximal zone had significant gain in S, C, K, Si, and Rb and depletion of Ca, Mg, and Fe, the inner alteration zone is typified by major addition of Si, K, S, C, and Au. General depletion of rare earth elements in the altered granodiorites is ascribed to high ambient fluid/rock during hydrothermal alteration. Results of the computed (aMg2+/aH+2) vs. (aK+/aH+) and (aNa+/aH+) vs. (aK+/aH+) diagrams, along with an increase in carbonation and alkali enrichment indices in the altered rocks, explain the observed hydrothermal alteration-induced mineralogical changes and constrain the possible fluid composition. Gold occurs as fracture fillings in the quartz matrix and pyrite grains, together with other sulfides, and associated with the fS2-buffering assemblage pyrite + pyrrhotite. Arsenopyrite compositions in the pyrite-arsenopyrite assemblage yielded temperature and log fS2 ranges of 305° to 335°C and −10.9 to −8.6, respectively. These temperature estimates compare well with the published results of chlorite thermometry (263°–323°C). Mineral-fluid equilibrium computations place narrow constraints on the ore fluid fO2 (and aH2S). Gold precipitation was a consequence of postpeak metamorphic fluid phase separation involving immiscibility of gaseous spies, fluid-rock interaction, and decrease in fO2 of the fluid.

Published δ34S values in pyrite fall in 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; these imply that Au(HS)2− was the dominant gold complex. The narrow range of δ34SPy values is indicative of magmatic (± mantle) source, or involvement of fluid with an average crustal sulfur composition.

The geologic events responsible for Jonnagiri started with accretion of the greenstone package at the subduction boundary between the eastern and western blocks of the Dharwar craton, followed by their deformation, shearing, and metamorphism, with subsequent emplacement of the Pagadarayi granodiorite and continued shearing, culminating with fluid flow focused along the shear fractures, which resulted in postpeak metamorphic hydrothermal alteration and precipitation of gold-bearing quartz lodes. From the observed craton-scale uniformity in the chemical and sulfur isotope compositions of the ore fluids, we propose a model involving contamination of an auriferous metamorphic fluid with crustal sulfur during its upward transport.