Geochronology and geochemistry of the giant Qian'echong Mo deposit, Dabie Shan, eastern China: Implications for ore genesis and tectonic setting

Abstract

The Qian'echong Mo deposit is a giant porphyry deposit in the Dabie Shan of eastern China. Molybdenum (Mo) mineralization mainly occurs as numerous veinlets in altered schists, with the development of potassic, phyllic, argillic, and propylitic alteration assemblages. A buried granite porphyry stock and locally exposed dikes are spatially and genetically related to the Mo mineralization. Zircon crystals from exposed quartz porphyry, rhyolitic porphyry, and granite porphyry dikes, as well as a buried granite porphyry dike yield weighted average 206Pb/238U ages of 128.9±1.1Ma (MSWD=0.30, 2σ, n=13), 127.42±0.94Ma (MSWD=0.58, 2σ, n=17), 127.44±0.98Ma (MSWD=1.6, 2σ, n=16), and 126.6±1.4Ma (MSWD=0.67, 2σ, n=15), respectively. The buried granite porphyry stock yields a zircon U–Pb weighted average age of 124.7±1.6Ma (MSWD=1.5, 2σ, n=16), which is slightly younger than the exposed dikes, but within error of the age of the buried granite porphyry dike. Five molybdenite samples from the ores yield Re–Os isotope ages of 123.31±1.02 to 128.49±1.40Ma, which are consistent with the U–Pb ages for the igneous rocks These age data constrain magmatic–hydrothermal activity at the Qian'echong Mo deposit to a period of about six million years from 129 to 123Ma. This period overlaps the recognized regional Mo mineralization event in the Dabie Shan of 142–111Ma, which took place during the post-collisional tectonism subsequent to amalgamation of the Yangtze and North China blocks.The Qian'echong granite porphyry stock and dikes have high contents of SiO2, K2O and Al2O3, and low contents of TiO2, MgO and CaO, showing a peraluminous high-K calc-alkaline to shoshonite affinity, with obvious LREE enrichment and a negative Eu anomaly. The rocks have a high initial 87Sr/86Sr of 0.70729 to 0.71788 and highly negative εNd(t) values of −16.2 to −26.1, with TDM2(Nd) ages of 2.24 to 3.03Ga. Their (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values range from 16.017 to 16.701, 15.252 to 15.368, and 37.095 to 37.578, respectively. This Sr–Nd–Pb isotopic signature indicates that the causative granitic intrusions for the Qian'echong deposit were mainly melts with components of both the northern Dabie complex and the Taihua and Xiong'er Groups that are part of the Precambrian basement of the North China block. The melts were generated during the tectonic extrusion of the Dabie Shan and their emplacement onto the basement of the North China block. The Qian'echong Mo deposit is unique based upon the crustal source for the causative porphyry, localization of most mineralization in the surrounding schist country rocks, and the CO2-rich ore-forming fluid. Thus we propose the Qian'echong deposit to represent a new sub-type of porphyry Mo deposit, herein termed collisional- or Dabie-type porphyry deposit.