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A 2 cm long and 0.8 cm thick, single graphite crystal embedded in quartz, feldspar, and orthopyroxene of a granulite-facies metamorphic rock from southern India exhibits unique homogeneity in δ[sup13]C values along layers parallel to the (0001) surface and smooth unidirectional variation of about 1‰ along the c-crystallographic axis. The fluid-precipitated graphite shows isotope homogeneity indicative of crystal growth along the (0001) plane. The C-isotope data along with the textural features suggest that the formation of graphite initiates in a layer perpendicular to the c-axis, possibly by a spiral growth mechanism. The initial layer is followed layer after layer, until the final stages of graphite growth. A second stage of graphite growth unrestricted by surrounding minerals is inferred from the un-deformed idiomorphic hexagonal form of the overgrown crystals. The C-isotope values in the overgrown rim of the crystal are lowered by about 2‰, suggesting a multistage precipitation. Morphologic and stable isotope studies thus confirm uniform crystal growth of graphite recording an isotope evolution pattern relating to fractionation from a fluid in an infinite reservoir during the main crystal formation. Spiral growth parallel to the (0001) face can explain the perfect uniformity of stable isotope composition in a single layer, whereas the variation across the (0001) surface gives insights into the time-integrated fluid evolution attending the crystal growth. [ABSTRACT FROM AUTHOR]

In southern India, major gold deposits occur in the Archean granite–greenstone terrain of Kolar, Hutti, Ramagiri, and within Proterozoic granulite terrain in Wynad. The Wynad Gold Field (WGF) which is located within the Moyar–Bhavani lineament—a Proterozoic transcrustal shear zone hosts gold within quartz veins, which are emplaced along the mesoscopic faults and shears in the region. The quartz veins cut the regional fabric of the country rocks, suggesting that the veins were formed in post-tectonic and post-metamorphic episode of crustal shearing. Gold as visible specks or interstitial grains and veinlets are seen within quartz and associated sulfide minerals, indicative of multiple mineralizing events. Fluid inclusion studies carried out on gold–quartz veins showed the frequent occurrence of bi-phase primary inclusions of H2O–CO2, pseudosecondary inclusions of H2O–CO2 and aqueous secondary inclusions. Microthermometric studies of different samples revealed a near pure CO2 composition, associated with the aqueous phase, with melting temperatures close to −56.6 °C. Clathrate melting temperatures of primary and pseudosecondary inclusions show that the fluids were low to moderately saline (2–14 wt% NaCl equiv.). Homogenization temperatures of primary and pseudosecondary inclusions range between 250–330 °C and 230–300 °C, respectively. The secondary aqueous inclusions have much lower homogenization temperatures between 130 and 220 °C. Fluid densities calculated from the microthermometric data were between 0.8 and 1.0 g/cm3. The isochores pertaining to these fluid densities intercept, the trapping temperatures, within pressure ranges of 0.5 and 2.9 kbars. Fluid inclusion gas analyses were carried out on selected samples using a quadrupole mass spectrometer. The analysis shows that the fluid inclusions were mainly of H2O and CO2 with minor quantities of N2 and CH4, which was in close correlation with our visual analytical data. We envisage that the fluids responsible for the Wynad gold mineralization evolved from an H2O–CO2 fluid and precipitated gold at temperatures of 300–350 °C, due to the sudden pressure and temperature decrease as the fluids entered the shear zone. [Copyright &y& Elsevier]