Carbon isotope and geochemical characteristics of the Paleoproterozoic graphite deposits in the Jiao-Liao-Ji belt, North China Craton: Implications for genesis and depositional environment

Abstract

As the northern segment of the Jiao-Liao-Ji Paleoproterozoic graphite metallogenic belt, the Liaohe Group contains a significant amount of phaneritic graphite deposits, which are of great significance to the exploration of graphite resources in China. However, few studies have investigated the metallogeny of these deposits. To unravel the carbon source and metallogenic process of the graphite deposits in the Liaohe Group, we performed detailed petrological, zircon U-Pb dating, Raman spectroscopy, and carbon isotope studies for the graphite deposits in the North (MG-Magou graphite deposit) and South (WLG-Wuligou graphite deposit) Liaohe Groups. The results show that the graphite-bearing rocks of the two groups are located in the sandstone, shale, wacke, and arkose regions in the log (Fe2O3/K2O) – log (SiO2/Al2O3) sediment classification diagram. They have similar trace element characteristics and exhibit similar rare earth element features with post-Archean Australian average shales (PAAS). Geochemical data show that these samples are compositionally immature and underwent a relatively low degree of weathering, which are the products of rapid accumulation of sediments. The carbon isotopic values of graphites from the WLG and MG both show a wide range of variation (δ13CWLG = −13.69 to −17.82‰, δ13CMG = -16.49 to −25.72‰), which are caused by the demethanation of organic matter during metamorphism. Raman spectroscopy indicates that the peak metamorphic temperature of the graphite from WLG is higher than 650 ℃, which is higher than that of MG graphite deposits (T = 551–627 ℃) from the North Liaohe Group. Combined with previous geochronology data, we suggest that a large amount of organic matter was deposited in an active continental margin environment at 2130–2175 Ma. During the collisional orogeny between the Longgang and LiaonanNangrim blocks (1950 to 1800 Ma), organic matter gradually transformed into graphite through prograde metamorphism and accumulated to form the graphite deposits in the Liaohe Group.