Mineralogical and isotopic characterization of graphite deposits in the western part of the North Qaidam Orogen and East Kunlun Orogen, northeast Tibetan Plateau, China

Abstract

Medium to high-grade metamorphic rocks comprising numerous graphite deposits are widely distributed in the periphery of the Qaidam Basin, Northern Tibetan Plateau, China. We present textural, XRD, and Raman spectroscopy characteristics of graphite, δ13C results of graphite and cogenetic calcite, and δ18O results of calcite in three graphite occurrences located at the periphery of Qaidam Basin. Graphite displays hexagonal platy, flaky, irregular polygonal, ovate morphologies with occasional spherulitic textures. Graphite morphologies, combined with XRD and Raman spectroscopy, indicate that graphite has relatively high crystallinity. Raman microspectroscopy of graphite shows that peak metamorphic temperatures vary between 690 and 801 °C. Graphite δ13CV-PDB values range between –26.3 and –21.6‰, indicating organic carbon as the major carbon source for graphite. Calcite δ13CV-PDB and calcite δ18OV-SMOW values range between –10.8 and +3.9‰, and between +6.6 and +29.5‰, respectively. Calcite δ13CV-PDB results show that organic carbon was mixed with inorganic carbon, which resulted in an increase of δ13CV-PDB. Calcite δ18OV-SMOW values indicate that the graphite deposits experienced late-stage water–rock interaction. The positive and negative anomalies of calcite δ13CV-PDB indicate that organic matter oxidation and recycling locally occurred during the final stage of the Great Oxidation Event. The wide distribution of regional metamorphic organogenic graphite deposits provides evidence for large-scale Paleoproterozoic biological activity in the periphery of the Qaidam Basin.