Geochemistry and Hf–O–Pb isotopes of the Hongniangshan complex, Inner Mongolia, China: Constrains on petrogenesis and mineralization of the Quanzigou porphyry Mo deposit

Abstract

The Quanzigou porphyry molybdenum deposit is located in Fengzhen County, Inner Mongolia, China, belonging to the northern margin of the North China Craton. A Yanshanian granitic complex, namely the Hongniangshan complex, is exposed in the deposit, mainly composed of medium–coarse-grained granite, porphyritic granite, and quartz porphyry. The Mo orebodies occur mainly in the porphyritic granite as ore-bearing quartz veins or veinlets. Previous study on molybdenite Re–Os and zircon U–Pb dating suggests that the Hongniangshan complex emplaced during 173–160 Ma and that the Quanzigou Mo deposit formed at ca. 160 Ma. Geochemical analyses in this study show that all these Middle–Late Jurassic granites are peraluminous, high-K calc-alkaline granite, featured by enrichment in Rb, Th, U, Hf, and Zr and depletion of Ba, P, and Ti, with high K2O/Na2O ratios. Nearly all the samples from the medium–coarse-grained granite and the porphyritic granite show weakly negative Eu anomalies, whereas the negative Eu anomalies of the quartz porphyry are moderate or strong. The Hongniangshan complex has low zircon εHf(t) values (−14.4 to − 6.7), old single-stage and two-stage model ages (1099–1471 Ma and 1637–2124 Ma, respectively), and low K-feldspar Pb isotopic compositions (206Pb/204Pb = 17.019–17.590, 207Pb/204Pb = 15.365–15.489, 208Pb/204Pb = 37.002–37.670), with K-feldspar δ18O values of 8.0–10.4‰, suggesting that the granitic complex was predominantly originated from a Paleoproterozoic lower crust source. Geochemical characteristics, combined with results from previous studies, show that the Quanzigou Mo mineralization was closely related to the quartz porphyry. The Middle Jurassic medium–coarse-grained granite was likely formed in the back-arc extension environment triggered by the subduction of the Mongol–Okhotsk Ocean and the Paleo-Pacific plate, and the formation of the Late Jurassic porphyritic granite and the quartz porphyry was controlled by both the post-collisional process of the Mongol–Okhotsk orogenic belt and the coeval subduction of the Paleo-Pacific plate.