Superimposed mineralization in the Tongbai composite orogen, central China: Revealed from geological and geochronological data of the Yindongpo gold deposit

Abstract

Multi-period mineralizations commonly form superimposed deposits in the orogenic belt, resulting in some disputes about the ore geneses and metallogenic background. Here, we present a comprehensive geological and geochronological study on the Yindongpo super-large gold deposit in the Tongbai composite orogen, central China, to reveal the ages of multi-period gold mineralizations and their links to the complex tectonic evolution framework of the Tongbai orogen. The Yindongpo gold deposit is located in the Weishancheng ore field in the northern segment of the Tongbai orogen and is mainly hosted in the carbonaceous schist of the Waitoushan Formation. Zircon U-Pb dating of leptynite (445 ± 3 Ma) from the Waitoushan Formation shows that it was deposited in the Ordovician, in an island arc or back-arc basin setting during the accretionary orogeny. Orebodies at Yindongpo are strictly controlled by the Zhuzhuang anticline and related fault systems, and are mainly distributed in the turning end, the collapsed part of the dipping end, and the conjugate thrust shear zone of the limbs. Based on field and petrographic observations, two periods of mineralization and four paragenetic stages (stage I to IV) are recognized: the early period consists of the laminated quartz-molybdenite-pyrite stage (I) and laminated-disseminated pyrite stage (II), whereas the late period consists of the quartz-pyrite vein stage (III) and the quartz-polymetallic sulfide vein stage (IV). Molybdenite from stage I has a Re-Os model age of 409 ± 6 Ma, which broadly matches the U-Pb age of intergrown rutile grains (396 ± 4 Ma). These two ages, in combination with the mineralization characteristics, indicate that the early-period gold mineralization was generated by the metamorphism of the accreted terranes during the Early Devonian northward subduction of the oceanic plate. The timing of the late-period mineralization is constrained by the U-Pb age of monazite (127 ± 2 Ma) intergrown with gold-bearing pyrite from stage III and the 40Ar/39Ar ages of ore-related sericite (131 ± 1 Ma) from stage IV and altered sericite (133 ± 1 Ma) from the carbonaceous schist. Combined with the extensive development of coeval granitoids and porphyry Mo deposits in the region, the late-period gold mineralization at Yindongpo belongs to a unified large-scale metallogenic event related to the Early Cretaceous intensive granitic magmatism in an extensional tectonic setting. The results presented here thus reveal two discrete mineralization processes at Yindongpo related to the Tongbai composite orogeny: the early mineralization of Devonian accretionary orogeny and the later mineralization of Cretaceous intracontinental orogeny. Our study contributes to the knowledge of superimposed metallogenesis in composite orogen, and also highlights the decisive role of multi-mineral isotopic dating in revealing the metallogenic epoch of superimposed deposits.