Age, geochemistry, and Sr–Nd–Hf–Pb isotopes of the Caosiyao porphyry Mo deposit in Inner Mongolia, China

Abstract

The giant Caosiyao porphyry Mo deposit (Yinshan area, central Inner Mongolia) is recently discovered in the northern margin of the North China Craton. The mineralization is newly molybdenite Re–Os dated to be Late Jurassic (ca. 148Ma). LA-ICP-MS zircon U–Pb dating of the two syenogranite porphyry intrusions in the Caosiyao deposit yielded ca. 148.5Ma and ca. 140.9–140.1Ma, respectively, and the Caosiyao monzogranite porphyry yielded ca. 145.0Ma. These Late Jurassic to Early Cretaceous granitic porphyries are high-K calc-alkaline and featured by high Rb, Th, U, and Zr contents and low TiO2, P2O5, Nb, and La contents, with high K2O/Na2O and medium to strong negative Eu anomalies, all resembling post-orogenic granitoids. The Caosiyao granitic porphyries have high whole-rock initial 87Sr/86Sr (0.710417–0.718568), low zircon εHf(t) (−18.9 to −12.5) and whole-rock εNd(t) (−17.8 to −13.7), old single-stage and two-stage model ages for zircon Hf (1282–1552Ma and 1989–2393Ma, respectively) and for whole-rock Nd (1766–2431Ma and 2046–2375Ma, respectively), as well as low K-feldspar 206Pb/204Pb (16.603–17.177), 207Pb/204Pb (15.319–15.374), and 208Pb/204Pb (36.841–37.225) values. We suggest that the Late Jurassic and Early Cretaceous syenogranite porphyry may have been sourced from a highly and moderately mature lower crust, respectively, whereas the Early Cretaceous monzogranite porphyry may have been sourced from an immature lower crust. The Caosiyao porphyries may have formed during the post-orogenic extension after the Mongol–Okhotsk Ocean closure and coeval with the Paleo-Pacific plate subduction.