The ca. 230 Ma gold mineralization in the Fengtai Basin, Western Qinling orogen, and its implications for ore genesis and geodynamic setting: A case study of the Matigou gold deposit

The Matigou gold deposit, located in the Fengtai Basin, is newly discovered and contains more than 30 t Au. The gold ore bodies are mainly hosted in shear structures as auriferous quartz veins and in the upper Devonian Luohansi Formation as disseminated ores. Granite porphyry and dacite porphyry dykes have been recognized in the ore zone. The granite porphyry dykes normally share the same structure as gold ore bodies and are locally mineralized. Dacite porphyry dykes likely intruded later, as they occasionally cut gold ore bodies. Pyrite is common in all kinds of rocks in the deposit. Multiple isotope (H-O-S) analyses of different minerals (quartz and pyrite), together with the geological characteristics, suggest that the Matigou deposit should be classified as an orogenic gold deposit like others in the Fengtai Basin. Zircon U-Pb dating of magmatic dykes and 40Ar/39Ar dating of hydrothermal ore-stage sericite both indicate that the Matigou deposit formed at similar to 234 Ma. It is coeval with the Pangjiahe deposit (231.2 Ma) 20 km to the west but much older than previously published orogenic gold ages (216 to 200 Ma). The ca. 230 Ma gold mineralization occurred between two episodes of magmatism (245-235 Ma and 227-205 Ma) in the Qinling orogen. The later orogenic gold mineralization is consistent with the later episode of magmatism, which is widely believed to have formed in a postcollisional extensional regime. The early episode of magmatism is still debated as related to either subduction or an early postcollisional stage. Because the ca. 230 Ma gold mineralization displays no strong deformation or metamorphism after formation, and the orogenic gold deposits imply a relatively extensional background. Hence, the early episode of magmatism may have intruded in a compressional regime after continental collision. The ca. 230 Ma gold mineralization occurred at the beginning of the transition in a tectonic setting from compression to transpression. Otherwise, the gold ore bodies would have been destroyed to some extent during the late collision. Due to the continuous extension background, more orogenic gold deposits with ages of 216 to 230 Ma are expected to be discovered in the Qinling orogen.