The Triassic Pulang porphyry Cu-Au deposit, located in the South Yidun terrane, is the oldest and one of the largest porphyry deposits in the southeastern Tibetan Plateau. The mineralization occurs mostly in the potassic alteration zone of the Pulang intrusive complex. U-Pb-He triple dating, namely apatite (U-Th)/He, zircon U-Pb, and zircon (U-Th)/He dating, together with inverse thermal modeling, reveals that the Pulang complex was emplaced at a paleodepth of ~5.0 to 6.5 km at 215 ± 2 Ma. The deep-level emplacement of the complex, coupled with the episodic replenishment of the magma chamber, gave rise to the establishment of a prolonged magmatic-hydrothermal system at Pulang. Although a range of single-grain zircon and apatite (U-Th)/He ages were obtained on each sample, the weighted mean zircon and apatite (U-Th)/He ages vary systematically with elevation, defining a multistage cooling/denudation history at Pulang. Specifically, three phases of cooling were recognized from inverse thermal modeling, including rapid cooling (80°–120°C/m.y.) in the Late Triassic, moderate cooling (3°–5°C/m.y.) from the Late Triassic to Early Cretaceous, and a protracted slow cooling period (50 m.y.) of extremely slow cooling and erosion at Pulang could be related to the formation and preservation of a peneplain on the southeastern Tibetan Plateau since the Late Cretaceous. A relict peneplain thus signifies a favorable tectonic environment for the preservation of ancient porphyry systems worldwide.

The Yidun arc, located in the southeastern Tibetan Plateau, was formed by the westward subduction of the Ganze-Litang Paleo-Tethys ocean in Late Triassic. It is well-known for the formation of numerous Mesozoic porphyry-skarn Cu-Mo-(Au) deposits in the arc. To date, more than 20 Cu-Mo-(Au) deposits (>10 million tonnes Cu resources) have been discovered in the southern Eastern Yidun arc. However, few Cu-Mo deposits have been discovered in the northern Eastern Yidun arc. In recent years, some Cu-Mo deposits or occurrence are successively discovered in the northern Eastern Yidun arc, but their ore-forming ages are not well constrained. It remains unclear whether such Cu-Mo mineralization formed by similar metallogenic event and geodynamic setting as the Cu-Mo-(Au) mineralization in the south. In order to determine the metallogenic age and shed light on potential links between Cu-Mo mineralization and regional magmatic events, we present molybdenite Re-Os and zircon U-Pb ages to constrain the timing of two types of Cu-Mo mineralization in the northern Eastern Yidun arc (type I and type II). Molybdenite ICP-MS Re-Os dating results show that type I mineralization was formed at 217.7 ± 3.6 Ma, which is highly consistent with the formation ages of the host granite (218.1 ± 1.5 Ma, 2σ, n = 15, MSWD = 0.92) and aplite dyke (217.3 ± 1.3 Ma, 2σ, n = 16, MSWD = 0.50) within error. While the type II mineralization has a relatively younger formation age of 211.8 ± 4.7 Ma than the host granite (217.1 ± 1.5 Ma, 2σ, n = 14, MSWD = 0.96) and type I Cu-Mo mineralization. These data indicate that the Cu-Mo mineralization in the northern Eastern Yidun arc was temporally and spatially related to the Late Triassic magmatism in the region. Rhenium (Re) concentrations in the molybdenite from type I mineralization, ranging from 12.77 to 111.1 ppm (typically > 100 ppm), indicate that the ore-forming metals were derived mainly from a mantle source. However, Re contents in molybdenite from the type II mineralization, ranging from 7.983 to 10.40 ppm, indicate that the ore-forming metals were derived from a mixed mantle and crustal source with a predominantly crustal component. This study confirms that the northern Eastern Yidun arc exists Late Triassic Cu-Mo metallogenesis, and thus much attention should be paid on this region to find more Late Triassic Cu-Mo resources.