U-Pb dating of hydrothermal titanite resolves multiple phases of propylitic alteration in the Oyu Tolgoi porphyry district, Mongolia

Oyu Tolgoi is a world-class, Late Devonian porphyry district in southern Mongolia. Because of its age and geodynamic setting, it has undergone a complex geological history that includes major post mineralization magmatic-hydrothermal events in close proximity to the porphyry deposits. The propylitic alteration halos that surround the Cu-Au deposits contain widespread hydrothermal titanite, as do the younger altered volcanic and intrusive rocks. Here, we present a comprehensive laser ablation-inductively coupled plasma-mass spectrometry U-Pb study on in situ, propylitic titanite from the Oyu Tolgoi district. The results identify district-wide hydrothermal alteration episodes that coincide with known magmatic events: Devonian porphyry mineralization ( approximately 372 Ma); the intrusion of granodiorite plutons and andesite dikes in the Carboniferous ( approximately 320-310 Ma); and the emplacement of the Permian Khanbogd Granite alkaline batholith ( approximately 290 Ma). Both Carboniferous and Permian alteration events variably overprint the earlier porphyry alteration halo. Overdispersion in the U-Pb data from some samples, due to Pb and/or U mobility, makes interpretation of some titanite ages more difficult, but further exemplifies the complex alteration history of the district. We conclude that U-Pb dating of propylitic titanite is a viable means by which explorers can identify alteration within a prospect that is synchronous with potentially fertile intrusions of known age. The extent of the coeval propylitic alteration and its mineral chemistry can then be used to assess the likelihood of a major porphyry center being present.