Mineralization age and geodynamic background for the Shangjiazhuang Mo deposit in the Jiaodong gold province, China

Abstract

The Shangjiazhuang Mo deposit is located on the Jiaodong Peninsula in eastern China, which is famous for the ca. 120Ma “Jiaodong-type” Au deposits with total Au endowment of over 3000t. In this paper, we discuss the deposit geology, mineralization age, and geochemical features of the host granodiorite of the Shangjiazhuang Mo orebody. Using this information, we aim to clarify the time and geodynamic mechanism for the Mo deposit, which is another constraint to understand the genesis of Au deposits. The Mo mineralization generally occurs as quartz–sulfide veins within the medium-grained Yashan granodiorite. The alteration consists of potassic alteration, silicification, sericitization, chloritization, and carbonatization with a weak unclear zonation. The ore minerals mainly include molybdenite, chalcopyrite, and pyrite. We measured Re–Os isotopes of molybdenite grains, which yielded a weighted mean model age of 116.9±0.81 (MSWD=1.03) and a well-constrained 187Re–187Os isochron age of 117.1±1.4Ma (MSWD=1.6). These ages are slightly younger than the age of Au mineralization on the Jiaodong Peninsula. Rhenium contents of 5.84–29.99ppm with an average of 16.4ppm in molybdenites indicate a crustal source. Whole-rock geochemical compositions show that the granodiorite is high-K calc-alkaline and metaluminous to peraluminous. The samples show low Y contents from 8.2 to 10.5ppm and Sr/Y ratios from 48.2 to 58.8, displaying an adakitic affinity. The Yashan granodiorite has high initial 87Sr/86Sr ratios of 0.7101 to 0.7104, low εNd(t) values of −17.6 to −16.7, and zircon εHf(t) values from −24.8 to −17.1, with corresponding Hf model ages of 2.7 to 2.2Ga. These isotopic data, together with the adakitic affinity of the granodiorite, indicate that the parental magma was derived from ancient crust. Mafic microgranular enclaves (MME) that are contemporaneous with the host granodiorite show SiO2 contents of 57.98–58.41wt% and depletion in Nb–Ta. The MMEs show enriched initial 87Sr/86Sr ratios of 0.7102 to 0.7106 and low εNd(t) values of −17.3 to −16.3. The MMEs are the products of mixing between the metasomatized lithospheric mantle-derived mafic magma and the ancient crust-derived felsic magma. The Early Cretaceous Mo mineralization (120–110Ma) is slightly younger than the peak time of Au mineralization (126–120Ma) on the Jiaodong Peninsula, but have a different spatial distribution which suggests different sources of Au and Mo. The “Jiaodong-type” Au deposits were probably related to the upwelling of metasomatized lithospheric mantle, while the Mo mineralization on the Jiaodong Peninsula may delineate a 120–110Ma Mo metallogenic belt along the southern margin of the North China Craton with the East Qinling, which is related to the melting of ancient crustal sources. The subduction of the Paleo-Pacific slab and accompanying asthenospheric upwelling triggered upwelling of metasomatized lithospheric mantle, forming “Jiaodong-type” Au deposits. Subsequently, the ponding of mantle-derived magmas resulted in partial melting of ancient crust and associated Mo deposits.