Carbon Isotope Composition and Raman Geothermometry of Graphite From the Pestpaksha Deposit (Kola Region): Conditions of Ore Mineralization

A comprehensive study of graphite from the Pestpaksha deposit (Lapland Granulite Belt, Kola Region) has been conducted, including Raman-based temperature determination and carbon isotopic characterization. C and O isotopic compositions of carbonates from rocks adjacent to graphite ores have also been studied. The structural control of the graphite mineralisation indicates that it was crystallised from the fluid phase. The crystallization of graphite was found to have taken place at a temperature of about 600°C at the retrograde stage of the metamorphic event. Large variations in carbon isotopic composition (δC from –18.5‰ to –29.0‰) can be explained either by the mechanism of Rayleigh depletion of C-O-H fluid with a high CO fraction in a closed system relative to this fluid, or by mixing of carbon from two isotopically contrasting sources. These sources may be the C-poor methane-bearing fluids produced during degassing of metamorphic rocks and the anomalously enriched C carbonates (δC ~ +14‰) observed in this study, which are similar in isotopic characteristics to carbonate rocks of the Lomagundi-Jatulian event. In the Rayleigh depletion model, these specific rocks are considered to be the product of crystallization of the residual fluid. Irrespective of the formation model adopted, the available evidence suggests that Pestpaksha graphite inherited its isotopic signature (δC of about –25‰) from hydrocarbons and is most likely derived from a metamorphic event.