Magmatic evolution and related W-Mo mineralization in the Larong deposit, eastern Tibet: Evidence from zircon U-Pb ages, geochemistry and Sr-Nd-Hf isotopes

摘要

The Late Cretaceous (ca. 92 Ma) Larong porphyry W-Mo deposit (39.44 Mt WO3 @ 0.1541%) is the first-discovered giant W-Mo deposit in the Leiwuqi-Zogang metallogenic belt, Sanjiang region. Multi-stage granitoids including the dacite porphyry (DP), granodiorite porphyry (GP), biotite granite porphyry (BGP), monzogranite porphyry (MP), biotite granite (BG), and granite aplite (GA) have been identified in the Larong deposit. However, the magmatic evolution and related W-Mo mineralization in this deposit are still poorly understood. LA-ICP-MS zircon U-Pb dating of the DP, GP, BGP, MP, BG, and GA yield weighted mean Pb-206/U-238 ages of 214.0 +/- 0.7 Ma, 213.8 +/- 1.3 Ma, 104.4 +/- 0.9 Ma, 93.9 +/- 1.3 Ma, 91.7 +/- 0.5 Ma, and 36.2 +/- 0.6 Ma, respectively. The Late Triassic DP and GP should be classified as peraluminous (A/CNK values of 0.95-1.57) S-type granitoids. They are characterized by pronounced negative epsilon(Nd)(t) values (-10.01 to -13.92) and epsilon(Hf)(t) values (-8.5 to -16.6) with old T-DM2(Nd) and T-DM2(Hf) ages (1.81-2.12 Ga and 1.79-2.30 Ga, respectively), indicating that the DP and GP were mainly derived from partial melting of ancient crustal materials after break-off of the Paleo-Tethyan oceanic slab. The Late Cretaceous BGP, MP, and BG and Late Eocene GA have low A/CNK ratios (0.92-1.06), but high SiO2 (70.57%-77.34%) and differentiation index (DI = 87.55-98.06), similar to the characteristics of highly fractionated I-type granite. Detailed elemental (e.g., depletion in Nb, Ta, Ti, Sr, Ba, Eu, and P) and isotopic data (Sr-87/Sr-86((i)) = 0.71192-0.72932; epsilon(Nd)(t) = -11.81 to -9.11; epsilon(Hf)(t) = -10.4 to 1.4) demonstrate that they were the result of magma mixing between ancient crust-derived melts and mantle-derived melts. The BGP was produced during the final stage of Bangong-Nujiang Tethyan ocean (BNTO) subduction, the MP and BG were formed under a post-collisional tectonic setting related to slab break-off of the BNTO, and the GA was likely associated with delamination of the thickened eclogitic lower crust together with lithospheric mantle. The W-Mo mineralization in the Larong deposit is genetically associated with the MP, with four favorable factors that can account for the formation of this deposit: (1) high W background, (2) multi-stage magmatism, (3) highly evolved, relatively oxidized, and F-rich magmatic system, and (4) sufficient calcium obtained by intense fluid-rock interaction.