GEOCHEMISTRY AND GENESIS OF TONGDE FLAKE GRAPHITE DEPOSIT IN THE WESTERN MARGIN OF YANGTZE PLATE

Tongde flake graphite deposit, located in the western margin of Yangtze Plate with over 500×10⁴ t reserves, is one of the largest and most representative flake graphite deposits in the graphite metallogenic belt of Panxi area. The study shows that the SiO₂ content in ores is 55.65%-61.68%, with the SiO₂/Al₂O₃ ratio of 4.59-5.42, the Ni/Co ratio of 6.23-12.88, characterized by enrichment of LILEs (Ba, Rb and Sr) and HFSEs (Nb, Zr, Hf, Th and U), ΣREE of 149.13×10^-6-195.37×10^-6, and weak negative Ce and Eu anomalies, indicating an anoxic marine sedimentary environment. The microscopic size measurement of graphite shows the flake diameter is 0.08-0.9 mm, mainly ranged in 0.15-0.4 mm, averagely counting for 53%, belonging to the medium-large flake graphite. The δ¹³C_V-PDB of carbon isotopes is -25.0‰ to -23.5‰, indicating that the carbon is derived from organic matter. After the carbon is deposited and formed into graphite by regional metamorphism, the heat from the intrusion of magmatic rocks would contribute to the further enrichment of graphite and then form large graphite flakes. Based on previous studies and newly available data, a three-stage genetic model of Tongde graphite deposit is proposed, including carbon source deposition, regional metamorphism and late magmatic-hydrothermal transformation.