0361-0128/08/3752/801-27 801 Introduction LATE ARCHEAN granite-greenstone terranes, worldwide, char- acteristically host orogenic gold deposits, amazingly efficient products of crustal-chemical processes of that time. Genetic models of these deposits must address the following key com- ponents: (1) structural styles and tectonic setting; (2) the re- lationship between metamorphism of host (and associated) rocks and mineralization; (3) the alteration geochemistry in- cluding gains and/or losses of elements and changes in mass and/or volume of the shear zones; and (4) the fluid evolution, with emphasis on transport and deposition of gold (Goldfarb Metamorphism, Fluid Flux, and Fluid Evolution Relative to Gold Mineralization in the Hutti-Maski Greenstone Belt, Eastern Dharwar Craton, India* BISWAJIT MISHRA † Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India AND NABARUN PAL Reliance Industries Ltd. Petroleum Business (E & P), RCP 5a 2nd Floor, Ghonsoli, Mumbai 400701, India Abstract We evaluate the metamorphic conditions (P, T, and fluid composition), resultant fluid flux, and gold miner- alization in the Hutti-Maski greenstone belt, eastern Dharwar craton, southern India, from the integrated study of three working mines (Hutti, Uti, and Hira-Buddini). In the observed D1 to D5 deformation sequence at Hutti, the proximal biotite alteration zones, containing isoclinally folded quartz veins (D2) and the laminated fault-fill veins (D3) are auriferous. The absence of extension or extensional shear veins implies that the pore fluid pressure was ≤σ3 + T, where T is the tensile strength of the greenstones. Important auriferous vein struc- tures of the Hira-Buddini mine are the fault-fill veins that run along the steeply dipping, reverse, brittle-duc- tile shear zone and shallow-dipping, sigmoidal extension veins (Pf ≥σ 3 + T). Metabasites from Hutti record am- phibolite facies conditions (3–5 kbars and ~650ºC) as the probable peak metamorphic P-T. A clockwise P-T-t path could be established for the Uti region, from garnetiferous amphibolite and garnet biotite schist, with peak P-T reaching amphibolite facies conditions (~6 kbars and 650º–700ºC). The deduced path can be ex- plained by a subduction-related compressional to transpressional tectonic setting, invoked for the Dharwar cra- ton. The estimated average enthalpy change for the relevant dehydration and decarbonation is about 75 kJ, which is necessary to release one mol of H2O + CO2. By assuming volatile release between 400° and 600°C, the total heat required to metamorphose a kilogram of an average mafic rock is ~235 kJ. Furthermore, by con- sidering 3 percent volatile loss during metamorphism, the maximum rate of volatile production is 28.98 kg·cm –2 my –1 . Gold mineralization at Hutti took place on the metamorphic retrograde path beginning with initial alteration (and sulfidation) at upper greenschist facies. After a protracted phase of fluid evolution, the mineralization cul- minated with the formation of auriferous laminated quartz veins at lower greenschist facies. The ore fluid had a ΣS content ~0.1 m, and a decrease in both fO 2 (and pH) caused the precipitation of gold in the proximal bi- otite zone from a hydrothermal solution containing Au(HS – 2). Fluid inclusions in D2 quartz veins within the proximal zone comprise a unique assemblage of five distinct types of carbonic inclusions containing variable proportions of CO2, CH4, graphite daughter products, and H2O. Precipitation of thin films of graphite in the inner walls of carbonic inclusions is interpreted to be the result of reaction between CO2 and CH4 (CO2 + CH4 = 2C + 2H2O) within those inclusions that were trapped at >400ºC and contained sufficient CH4. Entrapment of these carbonic and aqueous inclusions followed phase separation of the initial aqueous-carbonic fluid dur- ing a decompression event. Simultaneous entrapment of coeval and cogenetic aqueous and carbonic inclusions in D3 auriferous laminated quartz veins was due to phase separation of fluid of broadly similar composition but at lower temperature. Accordingly, gold precipitation in these veins may have been a result of decrease in ΣS content of the aqueous fluid rather than the wall-rock sulfidation and fO 2 decrease, as in the biotite zone. Thus, gold precipitation at Hutti, in the proximal alteration zone and laminated veins occurred over a range of tem- peratures and by several mechanisms. As in the Hutti deposit, mineralization at Uti occurred on the meta- morphic retrograde path. Mass-balance calculations indicate introduction of SiO2, K2O, S, As, and Zr and de- pletion of CaO in the mineralized portion. Gold at Hira-Buddini is recovered both from the wall-rock mylonites and from the fault-fill and sigmoidal extension veins. The sigmoidal extension veins contain numerous aqueous as well as carbonic inclusions in close association, showing large density variation within single clusters. Such variation points to pressure cycling or operation of a fault-valve during the formation of these extensional veins. Occurrence of gold within these veins implies that phase separation during sudden pressure drops caused gold precipitation. Thus, a striking charac- teristic of this belt is the variability in the style of mineralization and fluid evolution among the deposits. † Corresponding author: e-mail, bmgg@iitkgp.ac.in *A digital supplement to this paper is available at <http://www.geoscience world.org/> or, for members and subscribers, on the SEG website, <http:// www.segweb.org>. ©2008 Society of Economic Geologists, Inc. Economic Geology, v. 103, pp. 801–827