What triggers fertile porphyritic Mo magmas in subduction setting: A case study from the giant Daheishan Mo deposit, NE China

Abstract

The Daheishan porphyry Mo deposit in Jilin Province, NE China, is classified as a giant porphyry deposit with a total reserve of 1.87 Mt. @0.07% Mo. It is located within the Lesser Xing'an-Zhangguangcai Ranges at the eastern part of Central Asian Orogenic Belt, which was tectonically shaped by both the Paleo-Asian Ocean and the Paleo-Pacific Ocean regimes. The Mo mineralization (167 Ma) is related to the Daheishan intrusive complex which comprises of the barren Changgangling biotite granodiorite (177.9 Ma), the fertile Qiancuoluo biotite granodiorite (169.9 Ma) and the Qiancuoluo granodioritic porphyry (166.6 Ma). The Daheishan intrusive complex is high-silica, alkali-rich, metaluminous to slightly peraluminous in composition, belonging to high-K calc-alkaline series. In addition, they are characterized by enrichment in large ion lithophile elements (LILEs, e.g. K, Rb, Ba, Sr, U and Th) and light rare earth elements (LREEs), and relative depletion in high-field strength elements (HFSEs, e.g. Nb, Ta, P and Ti), suggesting the formation in oceanic arcs above subduction zones and an involvement of the oceanic component. The Sr-Nd-Pb isotopic compositions (87Sr/86Sr = 0.705413–0.707889, εNd(t) = −1.28 to +0.92, and 206Pb/204Pb = 18.44–18.73) of the Daheishan intrusive complex are consistently comparable to values of the regional mantle derived rocks, suggesting a source of depleted mantle wedge for the magmas. The trace element signatures, i.e. Th/Ce, Th/Sm, Th/Yb and Ba/La ratios, identify a minor (~4%) contribution of slab melts that metasomatized the mantle magma source. Geochemical particularity, i.e. high water content and oxygen fugacity, has been recorded at both bulk rock and mineral scales to account for the fertility of magmas of the Qiancuoluo granodioritic porphyry and Qiancuoluo biotite granodiorite relative to the barren Changgangling biotite granodiorite. The mineralization of the Daheishan supergiant porphyry Mo deposit, in association with the intrusion of contemporaneous granitoids in the region, was formed in an active continental margin environment due to the subduction of Mudanjiang Ocean between the Jiamusi and Songliao blocks in the early Jurassic, which, in a larger framework, was driven by the westward subduction of the Palaeo-Pacific Ocean. Under the subduction environment, an input of slab melts, which secured abundant molybdenum from the shales overlying the oceanic plate, is proven to have improved the fertility of primary magmas by comparing the geochemical compositions of the fertile and barren granitoids (porphyry Mo mineralization related-) in the Lesser Xing'an-Zhangguangcai Ranges. To conclude, factors such as contribution of slab melts, high water content and high oxygen fugacity, are regarded to be crucial for generating fertile magmas for porphyry Mo mineralization in the Lesser Xing'an-Zhangguangcai Ranges.