Metamorphism, Fluid Flux, and Fluid Evolution Relative to Gold Mineralization in the Hutti-Maski Greenstone Belt, Eastern Dharwar Craton, India

Abstract

We evaluate the metamorphic conditions (P, T, and fluid composition), resultant fluid flux, and gold mineralization 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 ≤σ 3 + T, where T is the tensile strength of the greenstones. Important auriferous vein structures of the Hira-Buddini mine are the fault-fill veins that run along the steeply dipping, reverse, brittle-ductile shear zone and shallow-dipping, sigmoidal extension veins (Pf ≤σ 3 + T). Metabasites from Hutti record amphibolite facies conditions (3–5 kbars and ~650°C) 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 650°–700°C). The deduced path can be explained by a subduction-related compressional to transpressional tectonic setting, invoked for the Dharwar craton. 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 considering 3 percent volatile loss during metamorphism, the maximum rate of volatile production is 28.98 kg·cm−2my−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 culminated with the formation of auriferous laminated quartz veins at lower greenschist facies. The ore fluid had a ∑S content ~0.1 m, and a decrease in both f O2 (and pH) caused the precipitation of gold in the proximal biotite zone from a hydrothermal solution containing Au(HS2−). 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 >400°C and contained sufficient CH4. Entrapment of these carbonic and aqueous inclusions followed phase separation of the initial aqueous-carbonic fluid during 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 ∑S content of the aqueous fluid rather than the wall-rock sulfidation and f O2 decrease, as in the biotite zone. Thus, gold precipitation at Hutti, in the proximal alteration zone and laminated veins occurred over a range of temperatures 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 depletion 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 characteristic of this belt is the variability in the style of mineralization and fluid evolution among the deposits.