Mineralogy and Genesis of Fine-Scale Crystalline Graphite in the Cambrian Hetang Formation Hornfels in Eastern Segment of the Jiangnan Orogenic Belt

The Cambrian Hetang Formation is widely distributed in the Anhui section of the eastern Jiangnan orogenic belt, a large number of crystalline graphite was found in the hornfels near the contact zone between the stratum and the rock mass, its mineralogical characteristics showed that the crystalline graphite was finely squamose, with a diameter of about 2~60μm and a relatively flat edge; 2H-type graphite characteristic; The thickness of the scale is 30.6~40.7nm, the degree of graphitization is 67~89, the estimated metallogenic metamorphism temperature of graphite is 494.4~564.8°C, and the content of 3R polytype is 10.85%~15.98%. The genetic analysis showed that the δ13C value of graphite in the ore was between -33.218‰ ~ -24.957‰, and the value was obviously negative, indicating that the source of graphite carbonaceous matter was mainly bio-organic carbon deposited in the strata, which was closer to the level of peat, asphalt and modern organic matter. The age of the ore-bearing strata and its surrounding rock mass is 151~132Ma, which is the product of magmatic activity from the Middle-Late Jurassic to Early Cretaceous, and reaches the peak at 140~130Ma; magmatic hydrothermal fluid acts on carbonaceous strata, which is the cause of contact metamorphism.