关键词: 荆山群, 石墨矿床, C同位素

Abstract: Graphite deposits in China are widely distributed in ore-bearing khondalite series around the North China Craton. The massive accumulation of graphite deposits from multiple sources has significant implications for the palaeoenvironment. Here, our compilation of previously published isotopic ages for the Jingshan Group suggests a depositional age of 2.11.9 Ga. We also report new geochemical data, including major elemental and C/O isotope data, and Raman characteristic frequencies for graphite ore and related rocks from the Douya Formation, Jingshan Group in the Zhangshe mining area. The δ13Cgrap values of 8 gneiss samples ranged from -18.6‰-21.7‰, with an average of -20.7‰±0.8‰. The δ13Cgrap values for two diopside marbles were -16.3‰ and -19.3‰, respectively. All the δ13Cgrap values were slightly lower than the average (-26‰±7‰) for the worlds organic matter. The δ13Ccarb and δ18Ocarb values of four diopside marbles were -8.9‰0.2‰ and 12.5‰16.6‰, respectively. Raman microspectroscopy of graphites showed that the highest rock temperature reached 700800 ℃. The above mentioned geochemical characteristics indicate the carbon source of graphite mainly came from organic matter that subsequently mixed with inorganic carbon (homogenization) to yield heavier carbon isotopes. And high temperature metamorphism and fluid influence likely contributed to carbon homogenization. The occurrence of large amounts of graphite deposits in the Paleoproterozoic, accompanied by stromatolite expansion, accumulation of phosphorite deposits and perturbation of marine carbon cycles, suggests a flourishing of marine microbial life and high rates of biological productivity and carbon burial during this period.

Key words: Jingshan Group, graphite deposit, C isotope