Abstract:

The Gema Pb polymetallic deposit was discovered by our team in 2010. It is currently the only medium-sized Pb-Zn-Ag polymetallic deposit in the eastern section of Bangong Lake-Nujiang metallogenic belt. The deposit is located in the eastern section of the Anglonggangri-Bange-Tengchong magmatic arc zone. The ore bodies are produced in the surrounding rocks of the outer contact zone of the Early Cretaceous granodiorite in the form of veins. The shapes and locations of the ore bodies are strictly controlled by two groups of faults in the nearly east-west and northwest directions. The ore-bearing surrounding rocks are sandstone and siltstone of the Mali Formation (hornfelization) of the Middle Jurassic. Based on the geological characteristics of the Gema Pb polymetallic deposit and the studies of the geochemistry of the surrounding rocks and ores, and the LA-ICP-MS zircon U-Pb chronology of granodiorite, this paper examined the source of ore-forming materials, the ore-forming age and the ore-forming geological background, also discussed the ore-forming process of the deposit, and established a preliminarily metallogenic geological model of the Pb-Zn-Ag polymetallic deposit in the eastern section of the Bangong Lake-Nujiang  metallogenic belt. The results show that the granodiorite in the area has low silicon, high aluminum and relatively rich potassium, which is the product of magmatism in the subduction stage before plate collision. The LA-ICP-MS zircon U-Pb dating of granodiorite yields an age of (119.86 ± 0.45) Ma, indicating an Early Cretaceous diagenetic age. The rare earth element (REE) of granodiorite, ore and surrounding rocks show enrichment in light REEs, depletion in heavy REEs, and a negative Eu anomaly, suggesting a common origin. Compared to the abundance values of Chinese continental crust, the granodiorite contains relatively enriched Pb and Ag, while Zn is deficient. In the metamorphic siltstone of Mali Formation, Pb and Ag are extremely strong enriched elements, and Zn is also strongly enriched. The ore-forming materials of this deposit are derived not only from granodiorite but also from the surrounding rocks of Mali Formation, indicating obvious “double source” feature. The deposit was formed in an island arc environment before the closure of the Bangong Lake-Nujiang Ocean basin. The ore forming age is Early Cretaceous, and the genetic type of the deposit is a post-magmatic hydrothermal deposit.

Key words: Bangong Lake-Nujiang metallogenic belt, Gema lead polymetallic deposit, chronology, metallogenic geological setting, Tibet