Genesis of the Tianbaoshan Polymetallic Ore District, Yanbian, NE China: Constraints from Geochronology and Isotopic Analysis

The large-scale Tianbaoshan polymetallic (Cu, Pb, Zn, and Mo) ore district in the Yanbian area of China hosts the Xinxing Pb-Zn, Lishan Pb-Zn, Dongfeng Zn-Cu (Pb), and Dongfengbeishan Mo deposits. We report zircon U-Pb and Re-Os age data, geochemical data, and zircon Hf isotope compositions of granitic rocks associated with the Tianbaoshan ore district. Zircon U-Pb dating of the Xinxing, Lishan, Dongfeng, and Dongfengbeishan granitic rocks, all of which are associated with polymetallic mineralization, yielded weighted mean Pb-206/U-238 ages of 272.1 +/- 2.1, 272.7 +/- 1.8, 272.5 +/- 2.7, and 199.6 +/- 1.4Ma, respectively. The Dongfengbeishan Mo deposit yields molybdenite Re-Os model ages of 200.3 and 190.3Ma, an isochron age of 196.5 +/- 3.1Ma, and a weighted mean model age of 194.6 +/- 2.6Ma. This suggests that mineralization took place in the ore district in two stages during the middle Permian and Early Jurassic. The intrusive rocks of these two stages are identified as medium- to high-K calc-alkaline I-type granites. They are enriched in large ion lithophile elements, depleted in high field-strength elements, and contain positive zircon epsilon Hf(t) values (+4.6 to +13.9), indicating the primary magma originated from the partial melting of juvenile lower crust. The granitic rocks show geochemical characteristics similar to those of coeval rocks in NE China. Based on the new geochemical data and other regional geological studies, we propose that the mineralization of the two stages of granitic rocks in the Tianbaoshan polymetallic ore district occurred in active continental margin settings, but was controlled by different geotectonic mechanisms: the subduction of the Paleo-Asian Oceanic Plate in the middle Permian and the subduction of the Paleo-Pacific Plate in the Early Jurassic.