Geochronology and geochemistry of the Tianmugou Mo deposit, Dabie Shan, eastern China: Implications for ore genesis and tectonic setting

Abstract

The Tianmugou molybdenum (Mo) deposit is a newly discovered porphyry deposit in the Dabie Shan of eastern China. The Mo-mineralization is spatially and genetically related to the Tianmushan granite batholith, and mainly occurs as thin molybdenite-bearing pegmatite dykes and numerous hydrothermal veins, with the development of potassic alteration, silicification, sericitization, propylitization, and fluoritization. The Tianmushan batholith is composed of outermost fine-grained K-feldspar granite (Phase 1), medium-grained K-feldspar granite (Phase 2), coarse-grained biotite monzogranite (Phase 3), porphyritic granite (Phase 4), and innermost fine-grained K-feldspar granite (Phase 5). Zircon crystals from the biotite monzogranite yield a weighted average 206Pb/238U age of 123.0±0.8Ma, but those from the porphyritic granite do not yield a good age because of their high contents of Th, U and Pb. Seven molybdenite samples from the ores yield individual Re–Os isotopic ages of 120.5±1.7Ma to 122.5±1.9Ma, which are slightly younger than the U–Pb age for the biotite monzogranite. These ages constrain the ore-forming magmatic-hydrothermal activity at the Tianmugou Mo deposit that took place during the post-collisional tectonism subsequent to the collision between the Yangtze and North China continents, consistent with the 142–111-Ma regional Mo mineralization event in Dabie Shan. The biotite monzogranite and porphyritic granite have high contents of SiO2, K2O and Al2O3, and low contents of TiO2, MgO and CaO, showing a metaluminous to peraluminous high-K calc-alkaline affinity, with significant depletion in Eu, Ba, Sr, P and Ti, and enrichment in Rb, Th, U, Nb, Ta, Hf and Y. The rocks have high initial 87Sr/86Sr ratios of 0.73546–0.89204, and negative εNd(t) values of −11.6 to −11.0, with TDM2(Nd) of 1.81–1.86Ga. Their (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t are 17.337–17.554, 15.450–15.465 and 37.992–38.147, respectively. The Sr-Nd-Pb isotopic signatures indicate that the causative granitic intrusions for the Tianmugou deposit mainly originated from partial melting of the Qinling Group during Early Cretaceous post-collisional tectonic extension. The Tianmugou Mo deposit belongs to a collisional- or Dabie-type porphyry Mo system.