Depositional Environments and Carbonaceous Sources of the Cheng-Gang Crystalline Graphite Deposit Revealed by Elemental and Isotopic Evidence

The Cheng-gang crystalline graphite deposit is a recently discovered medium-to-large-sized deposit within the Tan-Lu Fault Zone (TLFZ), East China. However, the knowledge on this deposit remains limited, resulting in a poor understanding of its genesis. In this study, this deposit is chosen to elucidate the degree of graphite mineralization, the nature and depositional environments of the protoliths, and the carbon source of graphite through geochemical and stable isotope investigations, and mineralogical analysis. The fixed carbon contents in the graphite-ore-bearing layers range from 2% to 3%. X-ray diffraction analyses reveal a high degree of graphitization. Analyses of elemental ratios indicate that the protoliths of metamorphic rocks predominantly consist of felsic rocks derived from the upper crust and deposited in brackish-water and reducing environments (anoxic to dysoxic). Stable carbon isotope analyses show that CH4 with lighter carbon isotopes released from the decomposition of pristine organic matter was trapped into adjacent inorganic reservoirs and the residual fraction with heavy carbon isotopes evolved to become graphite under metamorphism. Assuming the existence of isotope exchange between carbonate minerals and graphite, the temperature of peak metamorphism is estimated to be 580–860 °C, corresponding to amphibolite–granulite facies during regional metamorphism. The direct mixing of organic fluids and adjacent inorganic reservoirs may have contributed to graphite ore formation and needs to be further explored in future studies. The findings shed light on the genesis of the TLFZ graphite deposits, providing practical implications for local mineral exploration.