Genetic linkage between carbonaceous materials and Triassic gold deposits in the Liaodong Peninsula, northeastern North China Craton: Insights from Raman spectroscopy and C-O isotope analysis

The Qingchengzi gold ore-concentration district, in the Liaodong Peninsula, northeastern North China Craton, is dominated by altered rock-type gold deposits and hosted within metamorphic rocks of the Paleoproterozoic Liaohe Group. The deposits are mainly controlled by a low-angle, north-dipping thrust structural system, con-taining numerous carbonaceous materials (CMs) and syn-deformational ore-bearing quartz veins and adjacent clastic rocks. Raman spectroscopy and C-O isotope analysis were performed on CM samples from the fault zone to decipher their possible origins and potential genetic links with gold mineralization. Raman spectra analysis distinguishes three types of CM with low (CML), medium (CMM) and high (CMH) crystallinity and maturity. The estimated Raman-derived temperatures of CML range from 308 degrees C to 401 degrees C corresponding to greenschist-to blueschist-facies metamorphism. Large temperature variations of 363-678 degrees C for CMM and 442-664 degrees C for CMH suggest higher metamorphic grades from low blueschist to amphibolite-facies. The organic carbon isotopic valuess (delta 13Corg =-20.9 %o - -23.1 %o) suggest an organic carbon source, mixing with carbonate wall rocks characterized by the delta 13Ccarb of-1.92 %o to-6.22 %o. Combined with the Raman spectrum, the CMs are formed by the demethanation of the organic matter during the greenschist-to amphibolite-facies metamorphism. The disseminated CML coexists with Au-bearing quartz veins and is inferred to have facilitated the migration of Au in the fault zone as a solid lubricant and the precipitation of Au from hydrothermal fluids as a reductant. CMM and CMH are recognized in medium-to high-grade metamorphism with higher temperatures and occur with fewer Au-bearing pyrite or quartz veins, and thus they are unrelated to gold mineralization. In summary, the disseminated CML with low crystallinity contributes to the formation of the Triassic gold deposits and serves as a potential indicator for further prospecting.