Vein-type graphite deposits in Sri Lanka: The ultimate fate of granulite fluids

Abstract

Hydrothermal graphite vein deposits hosted in granulite facies metamorphic rocks in Sri Lanka are unique because of their large size and high crystallinity. In this paper, we present a review of the structural-metamorphic setting of the graphite veins and of the graphite stable carbon isotope data, and we present fluid inclusion data from quartz in the graphite veins and in the host rocks from several graphite deposits in Sri Lanka. The studied host rocks show decompression rims of plagioclase and orthopyroxene after garnet. The quartz in these decompression rim textures comprise, in order of abundance, high-salinity brine, H2O-CO2, and low-salinity aqueous fluid inclusions. The brine fluid is responsible for metasomatic features observed in garnet decompression rims, including feldspar leaching and re-precipitation. Quartz cogenetic with vein graphite comprises, in order of abundance, low-salinity aqueous, CO2, high-salinity brine, and H2O-CO2 fluid inclusions. Published graphite carbon isotope data indicates a dominant mantle source, mixed with small amounts of carbon-bearing fluids of supracrustal origin. We propose that large quantities of mantle-derived CO2 fluid are temporarily stored in the lower crust during the final stage of Gondwana supercontinent amalgamation. The CO2 is subsequently released from the lower crustal rocks during decompression associated with fast uplift. The graphite veins in Sri Lanka were formed during this uplift stage and represent as such paleofluid channels. In this respect, they are comparable to the quartz‑carbonates mega-shear zones found in other granulite terranes. Depending on the redox conditions, mantle CO2 and brines may either result in the formation of graphite or quartz‑carbonate veins.