U–Pb and Re–Os isotopic systematics and zircon Ce4 +/Ce3 + ratios in the Shiyaogou Mo deposit in eastern Qinling, central China: Insights into the oxidation state of granitoids and Mo (Au) mineralization

Abstract The newly-discovered Shiyaogou molybdenum deposit is located in the eastern Qinling metallogenic belt in central China. The deposit contains at least 152,000 t of Mo metal and bears typical porphyry-type features in terms of its concentric alteration zonation, quartz vein-hosted Mo mineralization, veining sequence and the spatial association with concealed granite porphyries. Re–Os isotope analyses of molybdenite from the deposit yield an ore-forming age of 132.3 ± 2.8 Ma. LA-ICP-MS U–Pb zircon dating of ore-related porphyries yields crystallization ages from 135 Ma to 132 Ma, indicating a temporal link between granitic magmatism and Mo mineralization. A population of captured magmatic zircons indicates another pulse of magmatism at ~ 143 Ma. A barren granite intrusion near the deposit gives a zircon U–Pb age of 148.1 ± 1.1 Ma. These magmatic activities were concurrent with the emplacement of the nearby Heyu granitic batholith, a largely ore-barren intrusive complex formed from ~ 148 Ma to ~ 127 Ma. Zircon Ce 4 + /Ce 3 + ratios of ore-related porphyries are obviously higher than those of contemporaneous barren granitoids, implying an affinity between Mo mineralization and highly oxidized magmas. Moreover, zircons from these granitoids overall have decreasing Ce 4 + /Ce 3 + ratios from 148 Ma to 132 Ma, reflecting decreasing oxygen fugacities during magma evolution. Available geological, radiometric and stable isotopic evidence suggests that the decrease of magma oxygen fugacity was probably associated with an increase of mantle contribution to granitic magmatism and metallogenesis, which probably gave rise to successive mineralization of Mo and Au in the eastern Qinling. The intense magmatic–metallogenic events in the eastern Qinling during Late Jurassic to Early Cretaceous times are interpreted as a response to the large-scale lithosphere thinning and subsequent asthenosphere upwelling beneath the eastern part of the North China Craton.