Sedimentary–Metamorphic Evolution of Mudstone-Derived Graphite-Bearing Gneiss in the Datong–Xinrong Graphite Belt, North China Craton

Regional metamorphic graphite deposits in the North China Craton (NCC) are important crystalline graphite resources, but the links among sedimentary protolith, metamorphic evolution, and graphite mineralization remain insufficiently constrained in many ore belts. In the Datong–Xinrong graphite belt of northern Shanxi Province, the studied ore is hosted by graphite-bearing gneiss of the Paleoproterozoic Huangtuyao Formation. The graphite-bearing rocks are interpreted as mudstone-derived paragneiss and contain graphite, plagioclase, quartz, biotite, diopside, garnet, alkali feldspar, pyrrhotite, and minor sillimanite. To clarify the ore-controlling factors and metallogenic evolution of these rocks, we integrated whole-rock geochemistry, Raman spectroscopy of carbonaceous material (CM), and zircon U-Pb geochronology. Geochemical indicators and discrimination diagrams show that the protolith was deposited in a continental-margin setting under dry, brackish-water, and generally oxygen-rich conditions. Raman CM thermometry yields peak metamorphic temperatures of 445.9–534.0 °C, and mineral assemblage constraints indicate pressures of 6.10–7.13 kbar. Zircon U-Pb ages constrain the maximum depositional age of the sedimentary protolith to 2102 ± 42 Ma and the age of mineralization-related metamorphism to 1888 ± 25 Ma. These results support a sedimentary–metamorphic model in which carbon-bearing mudstone was deposited along an early Paleoproterozoic continental margin and was later transformed into graphite-bearing gneiss during late Paleoproterozoic orogenic metamorphism.