Cu-Mo infertile granite: Insights from the late cretaceous plutons in the Northern Yidun Terrane, eastern Tibetan Plateau

Abstract

The Yidun Cu-Mo polymetallic metallogenic belt, located in the eastern Tibetan Plateau, is well known for numerous Mesozoic porphyry Cu–Mo deposits. The Cu-Mo deposits are only concentrated in the southern part of the belt while the granites situated in the north are generally barren in Cu-Mo mineralization. To understand the cause of Cu and Mo infertility, geochronology and petro-mineral geochemistry of three Cu- and Mo-barren granite plutons (Cilincuo, Rongyicuo, and Hagela) in the Northern Yidun Terrane were investigated. Petrogeochemical characteristics and zircon U-Pb dating (95–85 Ma) along with the tectonic evolution history confirmed that the three plutons formed in a Late Cretaceous intracontinental setting. The whole-rock geochemical compositions showed that these plutons can be classified as A-type granites and have undergone extensive magmatic differentiation. Based on the calculated magmatic oxygen fugacity using zircon composition and a set of geochemical criteria including whole-rock Sr/Y ratios, Eu/Eu* and Fe3+/Fe2+ values as well as apatite Cl-H2O contents, the parental magma of the Cilincuo pluton was identified as more oxidized and H2O-Cl-rich than those of the Rongyicuo and Hagela plutons. These features enable the Cilincuo pluton to have an elevated capacity of Cu mineralization compared to the other two plutons. However, the causative magmas of the three plutons are still not enough hydrous and oxidized for typical porphyry Cu ± Mo mineralization. The whole-rock Sr-Nd isotope values of these three plutons are consistent with Late Triassic Cu and Mo barren granites in the same region such as Dongcuo, Cuojiaoma and Sucuoma, suggesting that they were derived from a similar magma source which might lack the oxidized and hydrous Neoproterozoic arc root remnant.