Geochronology and geochemistry of Late Cretaceous igneous intrusions and Mo-Cu-(W) mineralization in the southern Yidun Arc, SW China: Implications for metallogenesis and geodynamic setting

摘要

The Sanjiang Tethyan Metallogenic Domain (STMD) is an important part of the Tethyan giant metallogenic belt. The Yidun Arc is a part of the STMD in the eastern Tibetan Plateau. Recently, four newly discovered Mo-Cu-(W) ore deposits related to granitic intrusions were found distributed along the north-south strike in the southern Yidun Arc, which are identified as the Xiuwacu, Relin, Hongshan, and Tongchanggou deposits herein. These four deposits formed along high-angle north-northwest or north-west strike-slip faults, with vein-type and porphyry-type Mo-Cu mineralization developed in the intrusions. Molybdenite Re-Os and zircon U-Pb dating together with zircon Hf isotopes and whole-rock geochemistry of the intrusions were studied to discern the relationship between mineralization and magmatism, metallogenesis, and tectonic settings. Molybdenite from skarn-type mineralization at the Hongshan deposit has a Re-Os isochron age of 81.2 ± 2.6 Ma (MSWD = 1.3, n = 5) consistent with previously published zircon U-Pb ages and Re-Os ages of porphyry-type Mo mineralization. These results indicate that the Hongshan is a Late Cretaceous porphyry-skarn Cu-Mo deposit. Zircon U-Pb ages of the granitic intrusions in the Xiuwacu, Relin, and Tongchanggou deposits varying from -87.4 Ma to -82.7 Ma. Combined with published molybdenite Re-Os age spectrum (-85 Ma to -81.2 Ma), it is proposed that the Mo-Cu-(W) mineralization in the Shangri-La region is spatially, temporally, and probably genetically related to the Late Cretaceous granitic intrusions. The Relin, Hongshan, and Tongchanggou intrusions have high SiO_2 (65.2-70.0 wt.%), Sr (363-905 ppm), Sr/Y (22-72), and La/Yb (37-69) ratios, and low Y (11.6-17.0 ppm) and Yb (0.97-1.59 ppm), which displayed adakitic affinities. Their low MgO (0.66-1.44 wt.%), Mg# (25-46), variable negative zircon ε_(Hf)(t) values (- 7.9 to - 2.3), and Proterozoic two-stages Hf model ages (T_(DM2) = 1.13-1.62 Ga) suggest that they were probably dominantly derived from partial melting of thickened lower continental crust. According to the tectonic evolution of the Bangong Meso-Tethys Ocean during the Late Mesozoic, the Late Cretaceous igneous event and mineralization in the Yidun Arc likely formed under a late- or post-collision extensional environment, probably related to the collision between the Lhasa and Qiangtang terranes during the Late Cretaceous.