Graphite deposits of Sri Lanka: a consequence of granulite facies metamorphism

The vein graphite deposits of Sri Lanka are located in a Precambrian high grade metamorphic terrain dominated by granulite facies rocks. The vein graphite has been interpreted as being of solid phase lateral secretion origin, derived by hydrothermal solutions or of biogenic origin. Based on what is known on the composition of the fluids under granulite facies conditions and the role of these fluids in their transport through the crust, the origin of the graphite is proposed to be the direct consequence of granulite facies metamorphism in the presence of a CO₂ rich fluid under low fO₂ conditions. This CO₂ rich fluid could promote hydraulic fracturing and precipitation of vein graphite. Textures and structures of the vein graphite indicate syntectonic deposition by a crack-seal process under granulite facies metamorphic conditions. This model is supported by temperature estimates on graphite based on XRD data and stable carbon isotopes of graphite that suggest a deep-seated crustal origin.