Timing of the Yuchiling giant porphyry Mo system, and implications for ore genesis

The Yuchiling Mo deposit is a recently discovered giant porphyry system in the East Qinling Mo belt, China. Its apparent causative intrusion, i.e., the Yuchiling granite porphyry, is the youngest intrusion (phase 4) of the Heyu multiphase granite batholith, which was emplaced between 143 and 135 Ma. New robust constraints on the formation of the Yuchiling porphyry Mo system are provided by combined zircon U-Pb, biotite 40Ar/39Ar, and molybdenite Re-Os dating. Zircon grains from the Mo-mineralized granite porphyry yield weighted Pb-206/U-238 age of 134.0 +/- 1.4 Ma (n = 19, 2 sigma error, MSWD = 0.30). Magmatic biotite from the same sample yield a 40Ar/39Ar plateau age of 135.1 +/- 1.4 Ma (2 sigma error), and an inverse isochron age of 135.6 +/- 2.0 Ma (n = 7, 2 sigma error, MSWD = 10.8), which are effectively coincident with the zircon U-Pb age within analytical error. Three pulses of mineralization can be deduced from the molybdenite Re-Os ages, namely: similar to 141, similar to 137, and similar to 134 Ma, which agree well with the zircon U-Pb ages of granitic phases 1, 2, and the Yuchiling porphyry (phase 4), respectively. These well-constrained temporal correlations indicate that Mo mineralization was caused by pulses of granitic magmatism, and that the ore-forming magmatic-hydrothermal activity responsible for the Yuchiling porphyry Mo system lasted about 8 Ma. The Yuchiling Mo deposit represents a unique style of porphyry Mo system formed in a post-collision setting, and associated with F-rich, high-K calc-alkaline intrusions, which differ from convergent margin-associated porphyry Mo deposits.