The formation of the Caosiyao giant porphyry Mo deposit on the northern margin of the North China Craton: Constraints from U?Pb and Re?Os geochronology, whole-rock geochemistry, Hf isotopes, and oxygen fugacity of the magma

The Caosiyao giant porphyry Mo deposit (1.76 Mt), which is the third largest such deposit in China, is located on the northern margin of the North China Craton and associated with the Caosiyao intrusive complex that was emplaced in Archaean metamorphic rocks. The intrusive complex consists of monzogranite and coarse-, medium-, and fine-grained granite porphyries that yield U?Pb ages of 148.5 +/- 1.2, 146.4 +/- 1.0, 144.6 +/- 1.0, and 142.8 +/- 3.2 Ma, respectively. Four molybdenite Re?Os model ages range from 143.1 +/- 2.4 to 145.3 +/- 2.1 Ma, which are consistent with the U?Pb ages for the igneous complex. The geochronological data indicate that the multistage magmatic-hydrothermal processes were important in the formation of the Caosiyao giant deposit. The rocks of the Caosiyao granitic complex have high contents of silica (SiO2 = 71.7-75.6 wt.%) and total alkalis (mostly K2O + Na2O > 8 wt.%), low contents of CaO and MgO, and exhibit peraluminous high-K calc-alkaline to shoshonitic affinities. They are enriched in large-ion lithophile elements (e.g., K, Rb, U, and Th) and light REEs and depleted in Eu, Ba, Sr, Nb, P, and Ti. The geochemical characteristics indicate advanced fractional crystallization and that the rocks are strongly fractionated I-type granites. The negative Hf isotopic compositions of the granitic rocks, combined with old two-stage Hf model ages, indicate the magmas were derived from partial melting of the ancient lower crust of the North China Craton. All the intrusive rocks in the complex have relatively high Ce4+/Ce3+ ratios and oxygen fugacity, which would have facilitated Mo extraction into the melts owing to the much higher solubility of S as sulphate (SO42-) than sulphide (S2-). The magma crystallization temperatures decreased from 882 degrees C to 770 degrees C as the magma evolved. The multistage magmatic-hydrothermal processes, and the prolonged fractional crystallization, combined with high magma oxygen fugacity are the key factors to form the Caosiyao giant Mo deposit. The tectonic setting of deposit formation was one of extension related to rollback of the subducting Palaeo-Pacific Plate during the Late Jurassic to Early Cretaceous.