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

摘要

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 D_1 to D_5 deformation sequence at Hutti, the proximal biotite alteration zones, containing isoclinally folded quartz veins (D_2) and the laminated fault-fill veins (D_3) are auriferous. The absence of extension or extensional shear veins implies that the pore fluid pressure was <= sigma_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 (P_f >= sigma_3 + T). Metabasites from Hutti record am-phibolite facies conditions (3-5 kbars and approx 650 deg 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 (approx 6 kbars and 650 deg -700 deg 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 H_2O + CO_2. By assuming volatile release between 400 deg and 600 deg C, the total heat required to metamorphose a kilogram of an average mafic rock is approx 235 kJ. Furthermore, by considering 3 percent volatile loss during metamorphism, the maximum rate of volatile production is 28.98 kg centre dot 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 culminated with the formation of auriferous laminated quartz veins at lower greenschist facies. The ore fluid had a SIGMA S content approx 0.1 m, and a decrease in both fo_2 (and pH) caused the precipitation of gold in the proximal biotite zone from a hydrothermal solution containing Au(HS_2~-). Fluid inclusions in D_2 quartz veins within the proximal zone comprise a unique assemblage of five distinct types of carbonic inclusions containing variable proportions of CO_2, CH_4, graphite daughter products, and H_2O. Precipitation of thin films of graphite in the inner walls of carbonic inclusions is interpreted to be the result of reaction between CO_2 and CH_4 (CO_2 + CH_4 = 2C + 2H_2O) within those inclusions that were trapped at >400 deg C and contained sufficient CH_4. 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 SIGMA S 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 temperatures and by several mechanisms. As in the Hutti deposit, mineralization at Uti occurred on the metamorphic retrograde path. Mass-balance calculations indicate introduction of SiO_2, K_2O, 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 densi