Paleo-tethys late triassic orogenic gold mineralization recorded by the Yidi’nan gold deposit, West Qinling, China

Abstract

The Tongren-Xiahe-Hezuo polymetallic district in the West Qinling Orogen of central China has emerged as a promising new gold belt. However, significant debate exists as to whether the numerous Triassic intrusion-hosted lode gold deposits in the district represent intrusion-related or orogenic mineral systems. The Yidi’nan gold deposit is an example of a Triassic intrusion-hosted gold deposit endowed with an estimated resource of over 20 tonnes gold. The gold mineralization at Yidi’nan is fault-controlled and hosted by granite breccias associated with cataclastic zones. Its large size and intrusive host rocks make Yidi’nan an ideal site for attempting to resolve the origin of the intrusion-hosted gold systems in the West Qinling. Zircon U-Pb ages obtained as part of this study indicate that the ore-hosting quartz diorite pluton and a later dacite dike formed at 240.3 ± 2.5 and 239.3 ± 2.4 Ma. Hydrothermal sericite from fault gouge from main gold orebody returned much younger 40Ar/39Ar plateau and normal isochron ages of 220.21 ± 0.44 and 220.42 ± 7.69 Ma. Optical microscopy investigation revealed that the sericite coexists with gold-bearing sulfides, indicating that the sericite is coeval with gold mineralization. Our new age data reveal that gold mineralization at Yidi’nan postdated the Triassic magmatism by ca. 20 million years. Such a temporal framework is consistent with the observed geologic relationships, namely mineralized breccia ore crosscutting the quartz diorite host. Such evidence also suggests that the orogenic gold mineralization in West Qinling was not genetically linked to the Triassic magmatism associated with the subduction of the Paleo-Tethys Ocean. Instead, the formation of the Yidi’nan gold deposit appears to have been part of a mineralization event associated with a Late Triassic collisional event with the release of gold-bearing fluids possibly related to crustal thickening processes. High-angle reverse shear zones produced during ongoing continental collision probably facilitated the movement of hydrothermal fluids along major structures, eventually leading to the formation of plentiful orogenic gold deposits in West Qinling.